1.2.1 Power

Power is the rate at which energy is produced or consumed. When we refer to an electricity bill, we call it as a power bill, which actually is the electrical energy consumption.

Power is measured in watts. One watt is defined as the usage of energy at the rate of one joule per second. Thus, we see here that electricity is commonly measured as watt-hours or kilowatt-hours. These are represented by the symbol Wh or kWh.

1.2.2 Conversion of energy units

We have seen that different forms of energy have different measuring units. For designing a system, it is very important that a common measure of unit is used. Different forms of energy should be converted into one unit before performing any calculations for a design. As an example, let us see a conversion between joules and kilowatt-hours.

Convert 1000 kilojoules into kilowatt-hours

Often, conversions may not be as simple as this. Hence, conversion tables are used. For the benefit of delegates attending this course we have provided some conversion software, which is very easy and fast to use.

1.3.1 First law of thermodynamics

This is also known as the law of conservation of energy. The law states that,

“Energy can neither be created nor destroyed. Whenever energy is transformed from one form to another, the total quantity of energy remains the same”.

Let us see this with an example.

The input energy shown here is the electricity. The output is in the form of light energy, which is also called useful work. While doing the useful work, the bulb also looses energy in the form of heat.

However, the input energy will always be equal to the sum of output energy and the energy loss. This can be represented by the following equation

E_i = E_o + E_l

Where

E_i    input energy in joules or watt-hours

E_o    output energy in joules or watt-hours

E_l    energy loss in joules or watt-hours

1.3.2 Second law of thermodynamics

The second law of thermodynamics can be stated in a number of ways as follows based on the observations.

• Heat flows from a body at higher temperature to a body at lower temperature irrespective of the mass and material of the body participating in the heat transfer. This heat flow is possible without the addition of external work
• Work has the tendency to convert into work but the heat cannot be converted into work
• Every engine rejects heat to the surrounding

The second law of thermodynamics implies that energy can always be converted from its high quality form (e.g. electricity) into lower quality energy ( e.g. heat). However the reverse is not true.

For example, energy in the form of high-pressure steam is converted into mechanical energy in a turbine and further converted into electrical energy. Here the high quality energy is transforming a part of its energy into more high quality energy and the remaining is being transformed into low quality energy in the form of low-pressure steam or hot water.

The sun’s energy, for example, is of a very high quality. Most of this energy is directly converted into heat, which is not used by mankind. Heating up the earth’s surface is simply wasting the high quality energy.

The use of a renewable energy system can help to get some useful work done by converting the sun’s energy into heat.

1.4.1 Applications of the technology

Wind
It captures the energy of air currents using turbine blades. As the blades rotate, electricity is generated. Wind turbines can be used as stand-alone applications, or they can be connected to a utility power grid or even combined with an alternative power generating system like photovoltaic (solar cell) system. For utility-scale sources of wind energy, a large number of wind turbines are usually built close together to form a wind plant. Several electricity providers today use wind plants to supply power to their customers.

Solar
There are many ways in which solar energy is effectively used today. Solar energy is used in solar heating, solar refrigeration, solar electricity to power traffic lights etc. Solar technologies include: photovoltaic cells, which convert sunlight directly into useable energy. The solar concentrators use mirrors to focus the sun’s light and generate intense heat. This turns water to steam and generates electricity in the process. Solar thermal heating devices such as solar water heaters and even solar ovens are used for smaller applications.

Hydro
It captures the energy generated by water movement and converts it into electricity. While hydro is the largest source of renewable energy in California and the U.S, it can be controversial. In the context of renewable energy, low impact, small hydro, and micro hydro (those installations producing less than 20 megawatts of electricity) projects are considered by some as more environmentally sensitive and appropriate than traditional large-scale projects.

Biopower
It releases the energy trapped in organic material or biomass. Biopower uses biomass energy to generate electricity. Biopower has diverse applications from diverse sources: from creating gas that is used to fire electric plants, to recycling cooking oil and using it to power buses and cars. Biopower applications include co-firing with coal, collecting methane and landfill gases and burning urban wood waste to generate electricity.

1.5.1 Cost

The generation cost of wind energy is calculated based on the following parameters:

• Total project cost
• Operation and maintenance cost
• Average annual wind speed at the site
• Wind turbine efficiency
• Amortization period and interest rate

Total project cost

The total project cost includes

• Cost of the wind energy system
• Transportation and erection cost
• Project preparation cost (permit etc)
• Cost of land and infrastructure

The total project cost can be expressed as a function of the wind system’s rated electrical capacity. A grid-connected residential-scale system (1-10 kW) generally costs between $2,400 and $3,000 per installed kilowatt. That is $24,000-$30,000 for a 10 kW system. A medium-scale, commercial system (10-100 kW) is more cost-effective, costing between $1,500 and $2,500 per kilowatt. Large-scale systems of greater than l00 kW cost in the range of $1,000 to $2,000 per kilowatt, with the lowest costs achieved when multiple units are installed at one location. In general, cost rates decrease as machine capacity increases.

Remote systems with operating battery storage typically cost more, averaging between $4,000 and $5,000 per kilowatt. Individual batteries cost from $150 to $300 for a heavy-duty, 12 volt, 220 amp-hour. Larger capacity batteries, those with higher amp-hour ratings, cost more. A 110-volt, 220 amp-hour battery storage system, which includes a charge controller, costs at least $2,000.

Operation and maintenance cost
The operation and maintenance cost include the servicing and repair of the equipment, insurance and other operating expenses.. As a rule of thumb, one can estimate annual operating expenses of about 2% to 3% of the initial system cost. Another estimate is based on the system’s energy production and is equivalent 1 to 2 cents per kWh of output.

Average annual wind speed
The average annual wind speed on the site is of paramount importance to the cost of energy. As a rule of thumb the annual energy output of modern wind turbines can be estimated by means of the expression

e = 0.625V3XA(kWh/m2 swept rotor area)

where V (m/s) is the annual average wind speed at the hub height and A is the rotor swept area

Wind turbine efficiency
The efficiency is defined as the ability to operate when the wind speed is higher than the wind turbine’s cut-in wind speed and lower than its cut-out speed, is typically higher than 98% for modern wind farms.

Amortization period
The amortization period or economic life time depends upon the technical life time of the wind energy system. The technical lifetime for modern wind systems is typically 20 years. As a rule amortization period is taken to be equal to the technical life time of the system. Interest rate again depends upon the type of loan and the amortization period. Lending institutions base their interest charges upon a number of factors, including the amount of risk involved in a particular loan and current economic conditions. For this example we will assume that the interest rate is 5%.

The cost of wind energy in $/kwh can now be calculated as follows:

c = ( a.I_tot/ E.e) + m

where,

c cost of the wind energy in $ / kwh

a annuity factor

Itot total project cost per m2 rotor area

E wind turbine efficiency

e annual energy output (kwh/m2)

m operation and maintenance cost

The annuity factor a is calculated as follows:

a = i(1+i)ⁿ / (1+n) ⁱ-1

where

i is rate of interest

n is amortization period

e = 0.625 x pi/4 x D² V³ W

where

V (m/s) is the annual average wind speed at the hub height and

D is rotor diameter in meters.

For this example lets consider a domestic wind energy system which has a installed power capacity of 10 kw . The wind energy system cost for such a system would be approximately US$ 30000.

The total project cost is approx. 125 % of the wind energy system cost . The cost per kw will be $3000

Itot = 3000x 1.25 = 3750 $ / kw

E = 98%

m = 1 US cent/ kwh

i = 5%

n = 20 years

Assuming a wind speed of 7.5 m/s at a 50 m hub height and the rotor diameter to be 5 m let us calculate the annual energy output

e = 0.625X pi/4 x 5²X 7.5³ X24X365= 45352 kwh/year

a = 0.05 x (0.05+1)²⁰/{(1+20)^.05 -1} = 0.808

c = (0.808X 3750/.98X 45352) + .01) = 0.070 $/kwh

From this example we can see that compared to the cost of conventional power which is about 20 cents /kwh, wind energy is very cost effective.

1.5.2 Payback period

The payback period is the period in which the initial investment is recovered through the energy cost savings. To calculate the payback period of a residential and commercial wind energy system we need to first calculate the annual energy cost savings and annual operating cost of the wind energy system.

Dividing total project cost by the difference between annual energy cost savings and annual operating costs gives the payback period:

For example, consider the total initial cost of a 15 kW residential system and a 100 kW commercial system: Generally the cost of wind energy system is approximately $3000 per KW. Hence following will be the total project costs :

Residential 15 kW system = $45,000
Commercial 100 kW system = $300,000

Annual electric savings is the retail value of electricity from the wind energy system that would otherwise have been bought from the utility company. It is determined by multiplying the retail cost of electricity by the number of kilowatt-hours the wind turbine is supposed to produce in a typical year. A manufacturer or dealer can provide an estimate of the wind system’s annual output as a function of a specific location’s average wind speed. Assume the cost of electricity to be 20 cents per kWh and the annual output from the residential and commercial systems at a 14 mph site to be 30,000 kWh and 300,000 kWh, respectively. The annual energy-cost savings from both systems would be:

Residential $ 0.20 / kWh x 30,000 kWh = $ 6000
Commercial $0.20/kWh x 300,000 kWh = $ 60,000

Annual operating costs are estimated by multiplying the wind system’s energy output by a typical operations and maintenance cost, such as 5 cents per kWh. The annual operating costs are:

Residential $0.05/kWh x 30,000 kWh = $1500
Commercial $0.05/kWh x 300,000 kWh = $15,000

Now that all components of the payback equation are defined, the payback period can be calculated as.

Residential payback period:

Commercial payback period:

You will see above, that the larger the system is, the lesser is the payback period.

1.5.3 Other economic factors

Apart from the energy cost savings, there could be other factors also which influence the cost savings. For example the costs of conventional electrical energy keep increasing over a period of time. This increase in energy cost can be seen as an additional cost saving in case of a wind energy system.

In many countries, if the wind energy system is connected to the grid, the excess energy produced by it, is bought out by the utility. However, the rate at which the utility companies buy back the excess power is generally very low compared to the rate of a KW of conventional energy. Therefore it is advised that the system capacity should be properly matched with the load demand. In case if the buy back rate is equal or more than the purchased power rate, the system can be designed for higher output.

In some states in the USA, the government offers several incentives in terms of tax benefits and low interest loans. This can further reduce the cost of the wind energy system.

Finally the resale value at the end of the technical design life time of a wind turbine also will contribute towards the return on investment. Typically the resale value of a wind turbine is assumed to be 10% of its initial cost.

2.1.1 Introduction

Photovoltaics (photo=light, voltaic=electricity or voltage) is a semiconductor-based technology that converts energy from sunlight directly into an electric current that can either be used immediately or stored, for example in a battery, for later use. Some materials possess a property, known as the photoelectric effect, which causes them to absorb photons of light and release electrons. When these free electrons are captured, an electric current results that can be used as electricity. Photovoltaic cells are made of semiconductor materials, usually silicon. For solar cells, a thin semiconductor wafer is specially treated to form an electric field, positive on one side and negative on the other. The following diagram illustrates the basic PV cell functioning.

The current produced by photovoltaic cells is direct current (DC), which can be converted into alternating current (AC) with the help of appropriate power conversion equipment. The principal difference between the PV and other types of solar energy based equipment is that the former uses the sun’s light, rather than its heat. Photovoltaic cells are extremely low-maintenance and have very long lifespan, as there are no moving parts and the components are solid-state electronics. That makes photovoltaic systems usable at remote areas, where technical resources are scarce.

In general, the cost of energy from PV is relatively higher, nevertheless there are certain applications for which PV is most economical. In some cases PV is preferred over other technologies due to its unique environmental benefits. PV systems generate electricity without polluting the environment or creating noise. PV systems can also be combined with other types of electric systems for instance wind, hydro and diesel, to provide power on demand.

There are a number of photovoltaic technologies available in the market today that use a wide range of manufacturing methods and products. For a system designer to make choices, he needs to understand the basic differences, advantages and disadvantages of each of these types of products and methods and the latest technological developments.

2.1.2 Applications

The first real use of photovoltaic cells in the late 1950s, and throughout the 1960s, was for earth-orbiting satellites. In the 1970s, due to the advancements in the manufacturing, performance and quality of PV modules, the cost reduced considerably and this resulted in exploration of the usage of the PV in remote terrestrial applications. These applications include battery charging for navigational aids, signals, telecommunications equipment and other critical, low power needs such as street lighting, signboards, etc.

Subsequently, photovoltaics found a usage as a power source for consumer electronic devices, including calculators, watches, radios, lanterns and other small battery charging applications. At about the same time the energy crisis pushed scientists and technologists to channel their efforts into developing PV power systems for residential and commercial uses, both for stand-alone remote power as well as for utility-connected applications. Today, the production of PV modules is growing at approximately 25 percent annually (refer Figure 2.2). This has resulted in considerable cost reduction, accelerating the implementation of PV systems in buildings and interconnection thereof to utility networks.

2.1.3 Photovoltaic system components

Photovoltaic cells are connected electrically in series and/or parallel circuits to produce higher power levels. Photovoltaic modules consist of PV cell circuits sealed in an environmentally protective laminate, and are the fundamental building block of PV systems. Photovoltaic panels include one or more PV modules assembled as an independent unit. A photovoltaic array is the complete power-generating unit, consisting of any number of PV modules and panels.

All components other than the PV modules or panels are known as Balance of System (also known as BOS). The BOS requirements depend on the function of PV systems and also whether the system will be stand-alone or grid connected. We will learn about system configuration in detail in the coming chapters.

Since Photovoltaic arrays can only collect power during the day, batteries are required to store energy. Since batteries have to be charged and discharged in a certain way, battery charge regulators or battery charge controllers are required. As the system is expected to supply AC power, inverters to convert the DC power from the PV and batteries into AC power become essential. The fuses or circuit breakers also form an essential part to protect the systems. As the direction of the sunlight keeps changing during the day, the use of a PV tracker is a good option. However, its cost effectiveness should be taken into consideration. Apart from this, certain other types of power sources such as generators are often used as backup in some cases. Other PV system components comprise of racks and mounting systems, roof attachments, wiring and interconnection components and monitoring systems. In summary, all or some of the following components can be used in photovoltaic systems:

• PV cells (In the form of PV modules and arrays)
• Storage batteries
• Inverters
• Battery charge regulators or controllers
• PV trackers
• Generator sets
• Fuses or circuit breakers
• Wiring and interconnection components
• Monitoring systems
• Mechanical components for mounting of the system

Each of these system components is discussed in detail in the following chapters.

Photovoltaic cells
Photovoltaic cells (solar cells) are energy conversion devices used to convert sunlight to electricity by the use of photovoltaic effects. The photovoltaic effect can be defined as the generation of electromotive force as a result of absorption of ionizing radiation. When the incident ray of the sunlight falls on the solar cells, the solar radiation (light energy) is converted into electricity (direct current) without the involvement of any mechanical generators. The photovoltaic effect doesn’t involve any thermo-dynamics.

Photovoltaic solar energy is a reliable and proven non-conventional energy source. The following are its advantages:

• Micro- to mega-watt level energy generation
• Can work in normal room temperature and pressure conditions
• Simple solid state devices used to convert the energy
• Absence of moving parts
• Can function unattended for a long period
• Long effective life
• Pollution free
• Ease of operation
• Modular in nature – desired currents, voltages power levels can be achieved by series and parallel connections
• Simple construction and equally simple operating principles
• Highly reliable

Nevertheless, it is not free from disadvantages. Key disadvantages are:

• Solar energy is available only during the day time
• Useful solar energy varies a lot depending on the seasons and the location (latitude)
• High capital cost
• Additional energy storage requirements (batteries, etc.)

The Photovoltaic effect can be seen at any junction between two materials with different electrical characteristics. In addition, the best performance is observed in cells that use semiconductor materials. It therefore becomes important to understand the principles of construction and functioning of semiconductors.

Basics of Semiconductors
The electrical conductivity of materials, especially solids, vary widely. The good conductors, for example silver, have a conductivity as high as 5.98×10⁷ mho/m whereas insulators such as certain quartz materials have a conductivity of less than 1.8×10^-17 mho/m. Materials with certain intermediary conductivity values are grouped under semi-conductors. The so-called ‘band theory’ or the material energy band concepts may be used to explain the differences between the conductors, insulators and semi-conductors.

Band theory
The Band theory may be explained by the following.

The electrons in an atom are accommodated at different energy levels, also called shells. In other words, there are only specific energy bands or regions in which electrons are allowed to exist within an atom. Outermost shells of some materials are partially filled and some others are fully filled. Based on where the electrons are distributed within the allowable bands, the electrical characteristics of the material are determined. These energy levels are represented by one-dimensional energy diagrams, see Figure 2.3.

These band representations only give us an indication of where the electrons are allowed to exist and do not give the value of the energy levels. Fermi functions f(E) are used to find out the probability of an electron occupying an energy state E.

f(E) = 1/exp((E-E^f)/kT)+1

Where

In a semiconductor, the electrons occupy one of the two levels of energy or bands. They are the ‘Valence Band’ and the ‘Conduction Band’.

The Valence Band has electrons at a lower energy level and this is the energy range in which bonding between atoms takes place. The outermost shell in the atom is only partially filled.

The Conduction Band has electrons at a higher energy level and this is the energy range in which electrons can contribute to conduction. The outermost shell in the atom is fully occupied.

The most important thing to note is that there is a gap between the valence and conduction bands, called band gap energy or Eg (as seen in Figure 2.4).

If the electrons at the lower energy levels are excited by photons (or any other form of energy) with an energy E, higher than the band gap energy Eg, (E>Eg) then the electron jumps across the band gap from the valence band to the conduction band. The movement of this electron leaves a hole in the valence band. Thus an electron-hole pair is formed.

This band gap energy is extremely high in the case of good insulators. In addition the outermost allowed energy band (the outermost shell) is fully filled. Good electrical conductors have a partially filled outer shell.

In the case of semiconductors (see figure below), although they have the same band configuration as the good insulators, the band gap is narrow at room temperatures. For instance, a good insulator may have a forbidden energy gap of about 9eV, whereas a semiconductor may have a forbidden gap of less than 1eV.

As semiconductors have such a small forbidden band gap, electrons can be easily excited into the conduction band. The excited electron leaves behind a hole, which can be filled by a nearby electron. Another nearby electron occupies the hole left by the previous electron and this flow of electron gives rise to the current.

When the conduction of current in a semiconductor is due to the native electrons excited from valence to the conduction band, the material is called as intrinsic semiconductor. Moreover, when conduction happens due to the impurities (see below) then the material is called an extrinsic semiconductor.

Photovoltaic materials
The most common PV cell material is silicon. It is one of the most abundant elements on earth. Silicon and materials like germanium, carbon have some special chemical properties, especially in their crystalline form. An atom of silicon has 14 electrons, arranged in three different shells. The first two shells, those closest to the center, are completely full. The outer shell, however, is only half full, having only four electrons, (see Figure 2.7).

These four electrons form covalent bonds with four neighboring atoms, forming a lattice In silicon, the crystalline form is a metallic, silver-like substance. The above-mentioned crystalline structure turns out to be important to this type of PV cell.

Pure silicon is a poor conductor of electricity because the electrons are all locked in the crystalline structure. To modify the structure of this silicon, certain impurities are added to it. These impurities in the form of an atom of phosphor or boron, serve in making it a good conductor of electricity.

Consider silicon with an atom of phosphor. A phosphor atom has five electrons in its outer shell, which bonds with its silicon neighbor atoms. However, the phosphor has an extra electron that can be easily removed and made a conduction electron.

The silicon with phosphor atoms require a lot less energy to release one of the “extra” phosphor electrons because they aren’t tied up in a bond. As a result, there are a lot more free carriers compared to pure silicon. The process of adding impurities on purpose is called doping, and when doped with phosphor, the resulting silicon is called N-type (“n” for negative) because of the prevalence of free electrons. N-type doped silicon is a much better conductor than pure silicon.

The other part of the cell is doped with boron, which has only three electrons in its outer shell instead of four, to become P-type silicon. Instead of having free electrons, P-type silicon (“p” for positive) has free holes.

In its simplest form, the solar cell consists of a junction formed between n-type and p-type semiconductors, either of the same material (homojunction) or different materials (heterojunction). The atomic structure of the two differently doped sides of silicon can be seen in Figure 2.8.

An electric field is formed when the N-type and P-type silicon are in contact.

This electric field acts as a diode, allowing electrons to flow from the P side to the N side. As seen earlier when light, in the form of photons strikes the solar cell, its energy frees electron-hole pairs. The electric field acts as a stimulus to these electrons, which are released, and a flow of electron takes place. The electron flow provides the current to an external electrical load connected to the cell.

These cells absorb around 15-25 percent of sunlight. Moreover, silicon is a very shiny material, which means that it is very reflective. The cell can’t use photons that are reflected. For that reason, an antireflective coating is applied to the top of the cell to reduce reflection losses to less than 5 percent. To protect the cells from external elements, a glass plate is used to cover the PV modules.

The other forms of silicon used in PV cells are Polycrystalline silicon and amorphous silicon. Materials other than silicon include gallium arsenide, copper indium diselenide and cadmium telluride.

The technologies used in PV cell manufacturing can be classified as Crystalline Silicon technologies and Thin Film technologies.

Types of photovoltaic cells

Single crystal silicon cells
These cells are made from very pure single crystals of silicon. The silicon has a single and continuous crystal lattice structure with almost no defects or impurities. The main advantage of monocrystalline cells is their high efficiencies, typically around 15%. The cost of the manufacturing process required to produce monocrystalline silicon is higher compared to other technologies.

Poly-crystalline and amorphous silicon cells
Polycrystalline cells are produced using several grains of monocrystalline silicon. Polycrystalline cells are manufactured by casting molten polycrystalline silicon into ingots. These ingots are then cut into very thin wafers and assembled into complete cells. Polycrystalline cells are cheaper to produce than monocrystalline ones, due to the simpler manufacturing process. However, they tend to be slightly less efficient, with average efficiencies of around 12%, because of the defects in the structure, which occur at the boundaries between grains.

Amorphous silicon cells are composed of silicon atoms in a random order rather than a crystal structure. Amorphous silicon absorbs light more effectively than crystalline silicon. For this reason, amorphous silicon modules require only a thin layer of silicon and can be mass-produced. Amorphous silicon can be deposited on a wide range of substrates, both rigid and flexible, which makes it ideal for curved surfaces and “fold-away” modules. Amorphous cells are, however, less efficient than crystalline based cells, with typical efficiencies of around 6%, but they are easier and therefore cheaper to manufacture. They are ideally suited for applications where high efficiency is not required and low cost is important.

Cadmium Telluride cells

These are a type of thin film cells which are now being used for PV modules. The cell consists of a 10 micron layer of Cadmium Tellluride deposited on a 12 micron layer of Cadmium Sulphide. The main advantage of these technologies is that the manufacturing process is relatively low cost, compared to crystalline silicon technologies, yet they typically offer higher efficiencies than amorphous silicon. The laboratory efficiency of 8.7% is reported, which will increase further in the commercial modules. The quantity of materials used in these technologies is expected to further decrease which will help in the reduction of the cost.

Copper Indium Diselenide cells

In the last few years thin film cells made from Copper Indium Diselenide are gaining popularity. The reason behind it is they are less sensitive to variations in composition and thickness, than other thin film cells. The module efficiency of 11% is achieved using these cells.

Photovoltaic principles
As per quantum physics, light energy is not transmitted continuously but in small discrete packets called photons. Max Planck established the relationship between the energy of photon and frequency of radiation of the sunlight as:

Ep=hl = hc/y

Where

As we have seen earlier, when photons of sunlight have energy E higher than the band gap energy Eg, (E>Eg), then the cell materials absorb this energy and excite some electrons from the n-type side. In other words, there is a jump in the energy levels at the junction interface.

Output power and conversion efficiency
The current to voltage relationship can be given by:

Ii = Io[exp (Ve/kT)-1]

Where

Pmax=Vmp x Imp

The behavior of the cells may be explained by plotting the characteristic current-voltage curve. (see Figure 2.14)

The point where the curve intersects the X-axis is called the ‘short circuit current’ I_sc and the point where it intersects the Y-axis is called open circuit voltage V_oc. This point on the curve corresponds to the maximum useful power.

The ratio Im x Vm/ I_sc x V_oc is called as the Fill Factor (FF) and it is a measure of how well the junction was made in a cell and how low the series resistance is. Typically, a silicon cell will have a fill factor of 0.8 and a voltage factor of about 0.5.

Conversion efficiency
Efficiency is defined as the ratio if the electrical power output to the power content of the sunlight over its exposed area.

For a variety of reasons, the theoretical efficiencies are higher than the practical efficiencies.

The conversion efficiency may be given as:

Emax = Im x Vm/ I_t x Ac

Or

Emax = FF I_sc x V_oc / I_t x Ac

Where

The cell efficiencies may be increased by adding concentrators (cells will be at the foci of the parabola), thermo-photovoltaic systems and cascade systems (multi-functional systems).

Photovoltaic modules and arrays
A typical monocrystalline cell of 4-inch diameter produces 1 to 1.5 watts of power at a voltage of 0.5 to 0.6 V. These electrical characteristics are not suitable for most loads. For this reason the photovoltaic cells are connected in series to form a module. The number of cells in a module depends on various factors such as the required operating voltage. This voltage has to be matched with the voltage of the storage battery, which is usually 12V. Typically, a module consists of 33 to 36 photovoltaic cells connected in series. The following figure shows the relationship between cell, module and array.

The module construction
After testing each cell under standard conditions to match the current and voltage, approximately 33 to 36 cells are interconnected and encapsulated in an inert filler with a transparent, ultraviolet resistant plastic cover and a backing plate. The encapsulant used should satisfy a number of conditions, since they are designed to last for about 20 years. The material used for this purpose should be transparent, heat resistant and possess insulation capabilities. It should also provide electrical isolation and protection from environmental and chemical attacks. The most widely used encapsulant is a thermoplastic polymer EVA(Ethyl Vinyl Acetate).

The front cover is usually made of low-iron tempered glass. The material used for the back plate in most cases is either Tedlar or Mylar. The metal interconnects used for connecting the photovoltaic cells should be provided with thermal stress relief loops. All these layers are then laminated by applying heat and pressure under vacuum. The edges are sealed with a neoprene gasket and then framed with anodized aluminium. The final step is the testing and rating of the modules according to their power outputs. The following figure shows the cross section of a typical module.

Electrical characteristics of the module
The power of a typical silicon module ranges from 40 to 60 W under standard test conditions. The most important electrical characteristics of a module are:

• Short circuit current
• Open-circuit voltage
• Nominal peak power
• Current and voltage at nominal peak power

These parameters are measured as the functions of the temperature (at 25°C) and irradiance(1000W/m²). The following I-V curves show the relationship between module characteristics and temperature and irradiance. The short circuit current occurs on a point on the curve where the voltage is zero. The open circuit voltage occurs where the current is zero. The maximum power point is located at the knee of the curve. At this point, the power output is maximum. This output is called peak power output.

The following equations are used to determine the short circuit current, open-circuit voltage and the maximum power point under the operating conditions:

Isc(G) = Isc(at 1kW/m²) X G(in kW/m²)

Where

Isc   short circuit current

G     irradiance under operating conditions

dVoc/dT = -2.3 X Nc mV/⁰C

Where

Voc     open circuit voltage

Nc      number of cells in a module

The maximum power point is defined as the point on the I-V curve where the output of the module (the product V X I) is maximum. At this point the voltage is called Vmp and the current is called Imp. The power at this point is the peak power of the module.

There is a considerable difference between an ideal cell and an actual cell output, because of various factors. This degree of difference between the ideal cell and the actual cell is called as the Fill Factor. It is the ratio of:

FF= Pmp / (Voc X Isc)

The peak power under the operating conditions can then be calculated as follows:

The Pmax = FF X Isc X Voc

Photovoltaic arrays
A single module does not supply sufficient power required for most of the applications. Hence it becomes essential to connect the individual modules in series or parallel to fulfill the system requirements. The power output of a single module is anywhere between 40 and 60 W. A typical domestic system of 1.5 – 2 kW may therefore comprise some 20 – 30 modules. The interconnected modules form a photovoltaic array. One square metre of a fixed array kept facing north or south (depending on the hemisphere) yields nearly 0.5 kWh of electrical energy on a normal sunny day. If the array is oriented towards the sun throughout the day, the output can be increased by 30%.

The nominal output voltage of a single module is 12V. However, most of the systems require a voltage much higher than that. In order to achieve higher output of voltage, the modules are connected in series. For example, when 4 modules of 12V each are connected in series, the nominal voltage is 48 V. Here we are assuming that the characteristics of all the modules are identical. But in practice this is not the case.

Their characteristics vary due to variations in the quality of the individual cells. It mainly affects the current output. The change in voltage is negligible or due to variation in the operating conditions in different parts of the array. For example if one part of the array is covered by a cloud, it may lead to dissipation of energy, resulting in overheating and cell damage

To alleviate this situation, by-pass diodes are used within an array or a module. These diodes provide the low resistance path for the excess current produced by the unshaded module, and limit the dissipation of power to the shaded module.

In order to get higher current output, the photovoltaic modules are connected in parallel. For example, when 2 strings of 4 modules each are connected in parallel, the nominal voltage output will be 48V and the output current will be double the current output of its component modules. The characteristics of these modules must be matched.

In parallel configuration there is a possibility of short circuit occurring in one of the strings. If this happens, it may lead to the loss of output of the array. To avoid this scenario, blocking diodes are used in each of the strings.

Batteries
Energy storage is an essential part of stand-alone PV system design, which uses batteries to take care of the energy requirements during the periods of low or no sunlight. Even during the daytime, there can be situations when there is a peak in power demand. In such cases, batteries are needed to supply the excess power and also to avoid overloading of PV Systems. For grid-connected applications, batteries provide backup electricity in the event of grid failure.

The choice of battery size and type is an important design consideration, particularly for systems that have no backup power source. The batteries alone can represent 25 to 50 percent of total system cost, so it is essential to select the right type. Since a PV system’s life exceeds 30 years, the battery will require replacement two, three, or more times over this time period, making it the most expensive system component. The type of battery and how it is cycled in the system will affect both system performance and battery lifetime. Different types of rechargeable batteries can be used depending on the system’s requirements. Batteries with a long life expectancy have higher initial costs but are economical in the long run.

A Battery is defined as an energy storage device that converts stored energy into electrical energy through chemical reactions. Batteries are broadly classified as primary cells and secondary cells.

A primary cell is meant for one time use only. The reaction inside the cell only occurs once, after which the cell has to be disposed of. The examples of this type of cells are zinc-carbon, alkaline and lithium batteries. This makes the primary batteries unusable in PV systems. As charging and recharging is bound to happen every day rechargeable batteries, is the only option available for PV systems.

A secondary cell stores electrical energy in a reversible chemical reaction, enabling the battery to produce repeated current. The process of reversing however is not 100%, since there is some loss of energy due to heating and voltage difference. Examples of these types of battery are:

• Nickel-Cadmium (NiCad)
• Lead acid
• Nickel-Iron (Ni-Fe)
• Metal Hydride

Most PV systems use lead-acid batteries such as lead-calcium or lead-antimony batteries.

Lead-acid batteries
The lead-acid battery is made up of two plates, namely a lead (negative) plate and a lead plate coated with lead dioxide (positive plate) with a 35% sulphuric acid (H₂SO₄) and 65% water solution. This solution is called the electrolyte, and causes a chemical reaction that produces electrons. In the fully charged state the acid solution is concentrated, and the specific gravity is about 1.260 to 1.285.

When the battery is in use (discharging), the SO₄ in the electrolyte combines with the lead dioxide on the positive plate to form PbSO_4. This reaction dilutes the acid. Acid in the diluted form, if left for a longer period, reacts with the negative plate and forms a hard coating of lead sulphate on it. This makes recharging difficult.

The reaction that occurs in discharging the cell can be reversed, and it can be restored to its former charged condition by sending direct current through it in an opposite direction to the current flow on discharge. The active materials are restored to their respective conditions, and the electrolyte again becomes a more concentrated sulfuric acid solution. Cell voltage rises as the two plates become increasingly different in composition and the specific gravity of the electrolyte increases. If recharging is done long enough, a stage is reached when the water breaks down into hydrogen and oxygen, giving rise to gassing (bubbles floating on the surface) and loss of water. In this case, it is necessary to top up the battery with distilled water, to compensate for the evaporated water. There are 2 types of lead acid batteries that are available: Valve regulated and Gel type.

Valve Regulated Lead Acid (VRLA) batteries
VRLA Batteries are the variation of the lead acid batteries which operate as a starved electrolyte system in which the quantity of electrolyte is limited to the amount that either is absorbed in the plates or wets the fibers in the separator. In the resulting system, the electrolyte is immobilized and the gases produced during overcharge recombine to form water within the battery. These batteries are sealed and addition of water is not required in these type of batteries.

To prevent the battery casing from rupturing due to battery overcharging, a valve system is used, which keeps the battery casing under slight positive pressure.

The advantages of lead acid batteries are their low cost and fairly known operating characteristics. The basic drawback of these batteries is a poor cycle life. VRLA batteries are commonly used in the distributed power applications

Gel type Batteries
In this type of cell, a thick gel is formed by mixing the electrolyte with a gelling agent (like silica flour). They have the advantage of being completely sealed. They can operate in any position, even sideways or upside down, and will not leak acid or gas. The disadvantage of gelled cells is that these batteries cannot tolerate high rates of charging or discharging for extended periods, although their thinner plates will allow high rates for a short time. The higher voltage charging commonly used in PV systems will cause gassing and eventual premature cell failure due to water loss. Gel type batteries are very robust and can take more heat and charge abuse than traditional lead-acid batteries.

Gel batteries specifically tailor-made for use in PV systems are available. Although they are more expensive compared to the equivalent flooded cells, they are more compact, easily transportable and require less maintenance.

Deep cycle solar batteries
These batteries are designed specifically for the PV systems with higher rates of charge/discharge cycling. The electrolyte in these batteries can be either flooded type or gelled type. They have larger and thicker plates and transparent casing for the visual inspection. These batteries can withstand deep discharge cycles and can survive hundreds or even thousands of 80% discharge cycles. They are available in different sizes and types, usually in 6 volt and 2 volt configurations. These batteries can then be connected in series or parallel as per the requirements. For best results and longer life, it is recommended that these batteries be discharged to 50% of their capacity.

Battery capacity
Depending upon the energy requirements and the amount of reserved capacity needed for a particular system, an appropriate battery with a certain capacity is selected.

Battery storage capacity is defined as the amount of current that a battery will deliver over a given number of hours at its normal voltage and at a temperature of 25°C.

It is generally rated in Ampere-hours (Ah). Ah are the product of current in amperes (amperes are coulombs per second) and time in hours. For example, the capacity of a battery in which 1.0A of current is flowing for 100 hrs is same as the battery in which 10A of current is flowing for 10 hours.

Ah (ampere-hours) capacity of a battery
The total ampere-hours that a battery is capable of delivering, starting from the fully charged state and ending at the state when it is completely discharged, is called Ah capacity. Since it is practically impossible for the lead acid batteries to be completely discharged, the manufacturers usually specify a higher minimum cell voltage, which is based on the fully discharged condition to avoid any damage to the battery. Amp-hour capacity is generally given as the “20 hour rate” of the battery. Therefore, the number given as the amp-hour capacity for a deep cycle battery will be the number of amp-hours the battery can deliver over a 20 hour period at a constant draw. A 105 amp-hour battery can deliver 5.25 amps constantly over a 20-hour period before it’s voltage drops below 10.5 volts, at which point the battery is discharged.

Batteries capacity is defined in terms of Ah and rate of discharge The rating of the battery therefore is in terms of number of Ah, which it can deliver over a number of hours, typically 100,20 or 5. . The symbol C is used for capacity. So the above ratings are defined as C_100, C_20, and C₅

The discharge rate of the battery is also affected by the change in temperature. With the decrease in temperature, the cell capacity also decreases. For this reason, the cell capacity is usually given at the standard temperature of 25°C.

We also need to gauge the capacity of the battery available at any given time. There are two ways to estimate this capacity: the amount of charge remaining is called state of charge (SOC) and the amount that has been used is called depth of discharge (DOD). They are measured as a percentage of the total Ah capacity.

SOC = (Ah capacity – Ah used)/ Ah capacity

DOD = Ah used/ Ah capacity

Ah capacity = SOC + DOD

There are several methods of measuring SOC. SOC can be estimated from specific gravity (SG), open circuit voltage, charging and discharging voltages and from net Ah flow. The following graphs show the relationship between SOC, SG, and charging and discharging volts.

Specific gravity reading

Specific gravity measurements can only be done for the flooded electrolyte cells. A hydrometer is used to measure the specific gravity of the electrolyte. The hydrometer has a glass cylinder attached to a suction bulb on one end and a rubber nozzle at the other end. A sample of battery fluid is sucked into the glass cylinder with the help of the suction bulb. The calibrated float inside the glass cylinder indicates the reading of the specific gravity.

Temperature correction is needed because specific gravity changes with temperature. Hydrometers are calibrated at 80’F (26.7’C). Electrolyte temperatures above or below 80’F must be adjusted. For every 10°F increment below 80°F, subtract 0.004 to the hydrometer readings, and for each 10’F increment above 80’F, add 0.004 to the readings.

The characteristics of the particular battery should be used for determination of SOC. These data and graphs are usually provided by the manufacturer.

Open circuit voltage

Measurement of the voltage of the battery during charging and discharging does not give an accurate value of the battery voltage. The open circuit voltage of the battery is defined as the voltage of the battery when the battery is disconnected from the charging current and the load. This voltage is measured over a period of 15 to 20 minutes during which time period the battery is disconnected. This open circuit voltage has a steady value and is proportional to SOC. Therefore it can be used to determine SOC. A digital voltmeter must be used to check the battery’s open-circuit voltage. Analog meters are not accurate and cannot be used.

A fully charged battery will have an open-circuit voltage of 12.6 volts. On the other hand, a totally dead battery will have an open-circuit voltage of less than 12.0 volts.

Estimation of SOC from net Ah flow can be carried out by monitoring the Ah of charge into the battery and the Ah of discharge out of the battery, over a period of 20 to 30 days. The net Ah (charge-discharge) thus calculated allows the user to determine the net change in SOC over the same time period.

Cycle life of a battery

The cycle life of a battery is inversely proportional to the depth of discharge. Deep discharge reduces the battery life. Different types of batteries have different DOD/cycle life characteristics. Gelled type batteries have a cycle life of about 2900 (8 years), whereas at the lower end are the Lead Calcium Batteries having a cycle life of about 300 (0.8 years) at 40 % depth of discharge.

Battery efficiency
There will be a certain loss of energy in batteries due to various reasons: due to difference in voltages during charging and discharging, heating, gassing and due to loss of charge over a period of time. There are two types of efficiencies defined for the battery:

• Ampere-hour efficiency
• Watt-hour efficiency

Ampere-hour efficiency
Ah efficiency is defined as the ratio of Ah delivered by the battery during discharge to the Ah supplied to the battery during charge.

Ah efficiency = Ah out/ Ah in

Watt-hour efficiency
Watt hour efficiency is a true measure of the battery efficiency because it takes into account the electrical potential at which the current is flowing. It is the ratio of number of watt hours delivered by the battery to the number of watt hours going in the battery. Due to the difference between charging and discharging voltages, Wh efficiency will be lower than Ah efficiency. Wh efficiency is also highly variable due to the considerable changes in voltages during charging and discharging of the battery. Over a period of time the Wh efficiency decreases considerably and towards the end of the battery life it drops to 2/3 of its original efficiency. The watt hour efficiency of VRLA batteries is about 85% and that of Nickel cadmium batteries is 65%.

Series or parallel connection of batteries
The required voltage and capacity can only be achieved by connecting the individual cells in series or parallel. The battery bank thus formed consists of either single cells (2V) or blocks (6V). The system voltages are generally either 24V or 48V.

Series connection
Batteries are connected in Series by connecting the positive terminal of one battery to the negative terminal of the other, and so on. If we connect two 12 V batteries with 100 amp-hour capacity each, we get a total voltage of 24V and the total capacity remains the same as that of one battery i.e. 100 amp-hours. The disadvantage of series wiring is that if one of the cells is defective then the whole battery bank may fail prematurely. To avoid this, the bank should be closely monitored.

Parallel connection
Batteries are connected in Parallel by connecting the positive terminal of one battery to the positive terminal of the other, and the same with the negative terminal. If we connect two 6 volt batteries with the rated capacity of 300 amp-hours, we will get the total capacity of 600 amp-hours and the voltage remains the same as that of one battery i.e. 6V.

If there are too many parallel strings in a battery bank then there is a possibility of voltage differences occurring in individual strings, resulting in dissimilar capacities within the bank. Hence, it is recommended that there should be as few parallel strings as possible.

Inverters
The usage of inverters becomes essential to convert the Direct Current (DC) into Alternating Current (AC). As we have seen earlier PV cells generate direct current (DC), and batteries store electricity as DC, but most common appliances require alternating current (AC). AC is most common as AC wiring, components and appliances are more available and generally less expensive than similar DC products. Consequently, inverters are convenient for many systems. So, in cases where you need AC power, the inverter is used to change low-voltage DC (12, 24, 32, 36, 48, 96, 120) to higher voltage AC (120 or 240). This conversion uses up some energy so there is some amount of power loss. On average, the inverters are about 80- to 95-percent efficient.

Inverters became commonly available from 1980. Since then, they have become increasingly sophisticated and reliable. Commercially available inverters these days have become exclusively electronic devices.

The figure below shows clearly, where inverter is required in a PV system.

Operating principles of a basic inverter
Basic inverters operate on the basis of commutation (we will discuss the principle of modern inverters later). The single-phase inverter consists of 4 switches. The arrangement is such that when switch X is “on” and switch Y is” off”. DC voltage is applied to the load in one polarity and in case when switch Y is “on” and X is “off” the polarity is reversed (refer to figure).

A number of switching devices such as bipolar transistors are used.

Apart from converting current from DC to AC, most inverters have to convert from one voltage level to the other. This means a transformer has to be included in the circuit to step up or step down the voltage.

Based on the circuitry the inverters may be classified into two major categories.

One kind of inverters converts DC into AC by using push-pull circuitry, driving the two primary windings of a transformer alternatively, to provide the secondary winding with a two-phase AC output. The output generated is a square waveform.

Some modification is necessary to control the width of the square wave in order to obtain an effective voltage (VRMS) equal to that of the sine wave and peak voltage.

Such inverters are simple but very heavy because they use bulky, low frequency transformers. In addition, such inverters are not suitable for use in critical equipment that is sensitive to high-frequency interference (more detail in the later part of this chapter) because the rise or fall time of a square wave is very fast. It is also difficult to control because it contains high-frequency components. Therefore, these circuits are mostly used in small UPSs.

The other kind of inverter performs DC to AC inversion in a full bridge circuit having four switches as discussed earlier. Switching the two diagonal switches on and off alternately produces a bi-directional output pulse between the two middle terminals of the bridge.

This kind of inverter does not have a large transformer, and the output waveform either may be produced as a sine or stepped waveform, by switching the full-bridge transistors at high frequency.

The disadvantage of this second type of inverter is the difficulty of driving the upper two switches with a floating voltage.

Another problem with this circuit is its inability to protect against overload and short-circuit conditions.

Many modern inverters come with a powerful microprocessor, which controls the working and the signals. These inverters are highly reliable and have safety features against short circuits and overloads. DC/AC conversion is accomplished through a MOSFET power stage in the circuit, which drives the transformer. The input circuit is separated from the output circuit by the transformer. A control board controls the MOSFETs and stabilizes the output voltage when there is a surge. Inverters cover a wide range of power capacity, and the right one has to be chosen depending upon the application.

Broadly, there are three categories of inverters:

• Synchronous Inverters

Synchronous inverters change DC power into AC power to be fed into the utility grid. A power system with this type of inverter uses the utility company as a storage battery. When the sun is shining, the power comes from the PV array, via the inverter. If the PV array is making more power than is used, the excess is sold to the utility power company through a second electric meter. If power consumption is more than the PV array can supply, then the utility makes up the difference. It makes the most sense to use this type of inverter in conjunction with the utility power, because there are no batteries to maintain or replace, but it has a very long payback period and may not be cost-effective at today’s electric rates. Synchronous Inverters and utility companies deliver a pure sine wave.

• Multifunction inverters

Multifunction inverters can operate as stand-alone inverters and as synchronous inverters at the same time.

A multifunction inverter allows you to sell excess power to the utility, and maintain a battery bank for stand-by power in the event of a utility power failure.

This type of inverter is typically connected to a battery bank and the utility power lines (sometimes to a standby generator). When batteries are in a charged condition, the inverter supplies AC power to the house from the batteries. If the batteries become discharged, the inverter supplies the house loads from the utility.

• Stand-alone inverters

Stand-Alone inverters convert DC power stored in batteries to AC power that can be used as needed. High quality stand-alone inverters are available in sizes from 100 watts, for powering notebook computers and fax machines from your car, to 8000 watts, for powering an entire house or small commercial operation. On the basis of the waveforms produced by the inverters they can be further classified into :

• Square wave inverters
• Modified sine wave inverters
• True sine wave inverters

Square wave
Low-cost inverters produce square wave. This is not good as utility power. Many electrical appliances do not run well on this type of inverter. These square wave inverters may operate fluorescent lamps, which are not affected by the waveforms. In this form there is no provision for regulating the voltage. The Total Harmonic Distortion (THD) is worse than 30%. The quantity Total Harmonic Distortion indicates the variation of the output voltage waveform from the ideal sine wave. The higher the harmonic content, the more adverse it is for the electrical appliances as this leads to increased heating and electro-magnetic interferences (EMI). Today these inverters are very rare.

Modified sine wave (a.k.a. Modified square wave or Quasi sine wave)
Some low-cost but relatively high efficiency inverters have a modified square wave output with harmonic distortion of around 30%. These popular inverters represent a compromise between the square wave and sine wave inverter and are an economical choice in power systems where waveform is not critical. Their high surge capacity allows them to start large motors while their high efficiency makes them economical with power when running small loads like a stereo or a small light. They can power most lighting, televisions, and appliances. But some of them can’t power computer systems with laser printers, some variable speed tools etc. Some audio equipment will have a background buzz that may be annoying to music buffs. The main disadvantage of a modified sine wave inverter is that the peak voltage varies with the battery voltage. Inexpensive electronic devices with no regulation of their power supply may behave erratically when the battery voltage fluctuates. This type of inverter may also destroy some low cost rechargeable tools and flashlights.

True sine wave
True sine wave inverters are the latest in inverter technology. Harmonics are virtually eliminated. Sine wave inverters have a slightly higher cost, but they can operate almost anything that can be operated on utility power. Sine wave units produce power, which is almost identical to the utility grid or at times better, but cost more per watt of output. Sine wave inverters are available in sizes from 150 watts to 5500 watts, and a pair of them can be synchronized to deliver up to 11,000 watts. They are an excellent choice for power systems running audio equipment and other electronics that are waveform-sensitive. Larger Sine wave inverters are available in sizes up to 180,000 watts, which can supply even a small village.

An inverter with appropriate waveform should be chosen based on the commercial aspects and the appliances that are to be powered. The following chart could be handy in choosing the inverter.

Compatibility chart

Inverter Stacking: Some inverters can be linked together. This is generally done either to produce twice the power output or to produce power that is out of phase from one inverter to another in order to produce 240VAC power. Some modern inverters can be stacked up to 12 together. In this way systems can be scaled from 300W (maximum AC) output to 3600W.

Interference

The electronic circuitry in inverters may, in some cases, cause problems with radio and television reception, noise on telephones and buzz in audio equipment. Sine wave inverters cause the least amount of interference. Interference can be minimized by:

• Locating the inverter close to the batteries
• Twisting together cables that connect the inverter to the battery and
• Locating the inverter away from appliances that are susceptible to interference

All inverters cause interference on AM radio.

Battery Charger/transfer relay
Many modern inverters come with built-in battery charging circuitry. This is easy and economically accomplished because of the design of most inverters. Inverters step up low voltage and change DC power to AC power. Battery chargers do the reverse of this. Additional circuitry is all that is required to add a whole second function and economically create a battery charger- sometimes called an Inverter/Charger. When an inverter is used as a back up power system, the battery charger plays an important role. If a significant amount of the power being delivered to the battery is coming from a generator, battery charger size may be very important. Having a very small charger will require running the generator for too long under a low load to achieve a proper charge. It is most efficient to use the generator to its maximum capacity in order to minimize the running time.

Transfer relays are also incorporated into these Inverter/Chargers. This allows a unit to operate AC loads directly from an external source (such as a generator or grid power). These units would be able to switch from generator or grid power to inverter power fast enough to prevent computers or electronics from crashing allowing them to function as an uninterruptible power supplies. From a reliability, performance and economical standpoint, built-in battery chargers are the way to go.

Apart from this it is important to have a few safety features in an inverter, such as Automatic shutdown and start-up in the event of low or high voltage, short circuit protection, thermal overload protection etc. It is also desirable to have features like front panel metering and reverse polarity protection in the inverter

Battery charge regulators
Battery charge regulators control the amount of current entering the battery and protect it from overcharging and from completely discharging. They can also measure battery voltage to detect the state of charge. Regulators range from 2 to 300 A for voltages from 12 to 48 volts DC. The use of a charge regulator is essential for long battery life.

Different types of controllers exist, the switch and the pulse-width modulation controls being the most common types. More sophisticated controllers are more efficient, but the cost effectiveness of these controllers should be evaluated before using them. For example, some controllers include a maximum power point tracker (MPPT) feature. It allows a PV module or array to work at its highest power point depending on solar intensity, even if the battery is recharged at a constant voltage. This feature provides about 10 percent more power in the summer and roughly 30 percent more in the winter. These gains are generally higher for panels with high voltage peak (V_p) values.

There are different types of regulators. The most widely used in PV systems are series regulators and smart regulators.

Series regulators
A series regulator is an electronic switch that senses when the battery is full and either disconnects the panel from the battery or diverts the energy away from it. Most modern regulators are of this type, which disconnect the panel with a switch in series with it.

Voltage controlled switch regulators
Almost all regulators use the battery’s terminal voltage to determine how fully charged it is. As a battery gets charged up, it’s terminal voltage increases. Single stage regulators simply allow the battery voltage to increase until it reaches a set limit (in the range 14.5-15 Volts for a 12V lead acid battery) and then turn the charge current off until the battery voltage drops below a lower limit. The charge current is then turned back on and the cycle repeats.

Two-stage regulators improve on this by allowing the voltage to rise to a high voltage initially (typically 15V) and then change to a second mode, which maintains the battery voltage at a lower level. The first stage is referred to as a boost charge and the second as floating the battery.

Both these single and two-stage regulators use only the immediate battery voltage to determine the battery’s state of charge. The basic disadvantage of such regulators is that they make a decision about the state of charge based on only one piece of information – the battery voltage at the time. This leads to the consequences, stated below:

• Undercharged batteries
• They can boost charge a battery too often if an occasional heavy load is applied
• Under some conditions, the charge current can oscillate rapidly on and off causing flickering if any lighting or other voltage sensitive equipment is connected. This fast switching can also cause radio interference

PWM
Pulse Width Modulation (PWM) is the most effective means to achieve constant voltage battery charging by switching the solar system controller’s power devices. The current switch configuration is same as the voltage controlled switch regulator, but the working of the switch is very different.

When a battery voltage reaches the regulation set point, the PWM algorithm slowly reduces the charging current to avoid heating and gassing of the battery, yet the charging continues to return the maximum amount of energy to the battery in the shortest time. The result is a higher charging efficiency, rapid recharging, and a healthy battery at full capacity. This charging methodology helps in increasing the battery capacity, and in counteracting the effects of sudden voltage drops or temperature changes. The PWM technology benefits to the solar system user are:

• Longer battery life
• Lower costs of the solar system
• Lesser battery disposal problems
• More battery reserve capacity:
• More reliable solar system
• Opportunity to reduce battery size to lower the system cost
• Greater use of the solar array energy

Smart regulators
Smart controllers are implemented using a microcontroller integrated circuit. This single chip microprocessor controls all the operations of the regulator. It allows all the pieces of information about the battery state and charge/discharge history to be easily gathered, stored and interpreted. The controller is programmed with a sophisticated set of rules to help it to decide what to do. The data used in the programming requires an in depth understanding of the working of PV system and its storage requirements.

Battery charger
A battery charger is used in stand-alone PV systems to convert AC power into DC power for the purpose of charging the batteries. The AC to DC conversion is carried out to provide the required current and voltage for the battery charging. The main types of battery chargers are:

• Unregulated charger
• Electronically regulated charger
• Switch mode charger

Unregulated charger
This type of charger consists of transformer and a rectifier. The battery with a low State Of Charge (SOC), will receive the full rated output current. But as the battery voltage increases, the charging voltage will reduce and stabilize at a final voltage. The efficiency of an unregulated charger is 60% to 85%.

Electronically regulated charger
In this type of charger, a constant current is provided to the battery until it is fully charged at the required voltage. Subsequently the current is tapered off to a float level, where the battery can be maintained at full charge.

A modified version of an electronic battery charger is a charger which uses switch mode technique. This type of charger does not require a transformer, because of which it is less bulky and provides a more stable output.

Generators
Generators are used as a back-up power option for the PV system. They provide a cost- effective and reliable solution for the back up power. The fuel used in generators is either propane, diesel or gasoline.

Generator ratings are based on the standard conditions and generally specified in KVA. Their ratings drop significantly with the increase of operating temperature and altitude. Hence, the specified ratings should be adjusted to take into account these factors.

PV trackers
PV Trackers allow the array to follow the path of the sun, in order to maximize the amount of solar radiation falling on the cell surface. PV Trackers are economical means of capturing maximum the amount of sunlight without increasing the number of cells. There are two types of trackers: automatic and manual.

Automatic trackers are of two types:

• Single axis
• Two-axis trackers

Single axis tracker allows the array to rotate around a single axis, which is fixed to the ground. Two axis trackers allow the array to rotate around two different axes. The increase in the total power when using this tracker is about 50%.

Manual Trackers require the array to be adjusted manually several times during the day as the sun moves across the sky.

Monitoring equipment
Monitoring current and voltage in various parts of the system is essential with PV systems. Typically, such monitoring equipment will include a voltmeter to monitor the batteries and an ammeter to monitor power output from the PV module(s). Because meters consume system power when in use, meters in general, and especially those in smaller PV systems, should be equipped with “on-off” switches so the meters can be switched off when they are not in use.

Fuses and circuit breakers
All electrical loads in the PV system should be fused or have circuit breakers. Fuses and circuit breakers provide over current protection. The typical locations where fuses and circuit breakers are used are listed below:

• Array fuse for each string of the array
• Battery fuse (before the other equipment connected to the battery)
• Load fuses or circuit breakers

Apart from this, circuit breakers are sometimes also used for voltmeter and inverters. The rating of all the fuses should not exceed the maximum current carrying capacity of the cable to which they are connected.

Blocking diodes
Blocking diodes are used to prevent the reverse discharge of the battery through the PV array when there is little or no sunlight. Blocking diodes are usually integrated into the PV modules or the battery charge regulator. As with battery charge regulators and other power conditioning equipment, blocking diodes consume some power, dropping the current coming from the module. Such losses in system efficiency should be taken into account by the system designer.

2.1.4 Typical system configurations

The classification of the PV systems is carried out on the basis of the functionality and operating characteristics and depending upon the energy sources involved. The two main types of Photovoltaic system configurations are Grid-connected PV systems and Stand-alone PV systems.

Grid-connected or utility-interactive PV systems
In the grid-connected systems, the load is connected to both a photovoltaic power system and an electricity grid. The photovoltaic array powers the load during the periods of sufficient sunlight; otherwise, the load is powered by the electricity grid. The most important component of grid-connected PV systems is the inverter or power conditioning unit, which converts the DC current from the solar array to AC current of voltage, frequency and phase to match that needed to integrate with utility grid. A power conditioning unit is also required for preventing the overcharge or discharge of the battery. When the PV system output is more than the load demand, the power generated by PV arrays is lead to the electricity grid. This is known as back-feeding. In situations when the load demand exceeds the power output of the PV array (for example during nights or periods of low sunlight), the electricity grid provides the required balance of power. Another important issue of the installation of grid connected system is to work out the metering of the energy consumed and the energy fed back in the grid. There are several metering systems that are used depending on the energy policies in different countries. Usually a dual metering system is used in which one meter is installed to record the Power fed back from PV system to the grid and another meter is installed to record the power fed by the utility to the customer. This is because in certain countries the power tariffs are calculated differently. Single metering systems are also in use in some countries like for example in US, where the tariffs for the renewable energy produced by the PV system and the electricity supply companies are the same. This allows the use of single meter which operates in both the directions. When the energy is supplied by the customer to the grid the meter winds back and when the energy is consumed from the grid the meter winds forward. As a result the meter will read the net energy consumed by the customer subtracting the amount of energy fed back in the grid. The important considerations for the grid connected system are:

• Output synchronization : This is done using a synchronous inverter.
• Auto shut down of the PV system in the event of grid shutdown: This is done by connecting synchronization breakers between the grid and the inverter. In the event of grid shut down, they use the inbuilt sensors to close the PV system.
• To effectively use these equipments in a grid connected system it is necessary that their frequency, phase angle, phase sequence and the terminal voltage match the grid.

Stand-alone photovoltaic systems
These types of systems are powered by PV array only and they are independent of the utility grid. There are various types of stand-alone photovoltaic systems:

Direct-coupled DC PV systems
The simplest type of stand-alone PV system is a system where the PV module is directly connected to the load. In this case, the system only operates during the day because there is no provision for the energy storage. This type of configuration is suitable only for certain non-critical common applications such as small circulation pumps, ventilation fans etc.

DC PV systems with battery storage
In this type of system, the PV array charges the battery and the battery provides the power to the loads during low or no sunlight hours. The most common applications of such type of systems are RV systems, rural electrification, telecom, lighting, etc.

AC PV systems with inverter
This type of PV system is used in remote homes or industrial applications.. The basic components of the system are a PV array, batteries, a charge controller and an inverter. The inverter converts the DC power produced by the PV array into AC. This system with a large PV array can serve as a reliable source of energy in the absence of the utility grid.

Hybrid PV systems
These are special type of systems in which a back-up power supply is present in the form of a generator set (genset) or any other alternative energy resources such as wind or hydro energy. Hybrid systems with the back up generators are the most commonly used systems, because of their low cost and reliability.

2.1.5 Some applications of stand-alone systems

The PV power systems have numerous applications in the various areas. Some of the important applications are listed below:

• Telecommunications: Microwave, TV, and radio repeater stations, radiophones etc
• Residential power supply: Grid connected residential systems and off-grid remote homes
• Health care systems: Vaccine storage, sterilizers, blood storage etc
• Transportation signaling: Aircraft warning lights, road traffic warning systems etc
• Water pumping, lighting and heating in remote areas

Discussed below are the examples of few of the above applications.

PV water pumping system
PV systems can be used to pump water in remote areas, where they provide a maintenance-free solution for water supply, irrigation etc without the need of any human intervention. PV pumps are used to pump the water from bore wells, open wells rivers, etc.

The PV water pumping system consists of a PV array, power conditioning equipment and the pump with the electrical motor. The electrical characteristics of this motor do not match with the PV array characteristics; hence, the power-conditioning unit is required. This unit is located between the PV array and the pump motor. The PV array powers the pump motor and the water is pumped up through a pipe to a storage tank. The schematic diagram of the PV water pumping system is shown below.

Health care system (Vaccine storage)
There are several vaccination programmes that are carried out in developing countries on a regular basis for the prevention of common diseases. These vaccines have to be handled as per the strict regulations regarding the storage temperature etc. The temperature range recommended by the WHO (World Health Organisation) is from 0° to 8°C. This temperature range must be maintained at all times. The solar PV refrigerators are a reliable alternative to the kerosene refrigerators. Solar PV refrigerators also offer better temperature control and longer life of the equipment. The main disadvantage of the PV refrigeration system is its complex nature. Skilled technicians are required for the planning and installation of this system. However, because of the high reliability and the cost effectiveness in terms of the reduction in vaccine spoilage, PV refrigerators are the preferred option. Shown below is the schematic diagram of the vaccine storage system.

3.1.1 Load data calculation

The first step in designing the system is to determine the daily load demand of the particular application. The total load demand is calculated on the basis the power rating(in Watts) of all the appliances that are used everyday. This power rating is multiplied by the number of hours each appliance is going to run per day. In addition to the electricity used by appliances, the system itself also consumes some power. Some amount of energy is lost due to wire resistance and due to the conversion from DC to AC. This loss of energy is taken into account while designing the PV system using a factor in the final calculations (see Chapter 5 for details). Usually a safety factor of 1.2 is used to take into account the system losses. An example of a load data worksheet for a stand-alone domestic system is given in Table 3.1.

The next step is to calculate the maximum load demand. The maximum load demand can be calculated as the sum of the loads that are likely to operate simultaneously.

Apart from this, the annual and daily load profiles of the system should be known in order to determine the peak load demand and the storage capacity required.

The surge demand for the system is calculated based on the surge power needed for some appliances for a brief period. For example to start a motor, more power is required. This surge demand should match the rated surge power of the inverter and the generator.

3.1.2 Radiation data

Radiation data is available in most countries in the form of monthly averaged daily data. This data is represented in the form of global irradiance on a horizontal surface. The radiation, in both magnitude and structure depends upon the geographical location and the tilt angle of the PV array. Sunlight intensity is measured in KWh/m². A typical radiation data table looks like the one given below. Daily irradiance in KWh/m², tilt angle of the PV array 60°.

The amount of electricity produced by the PV array is affected by various factors such as the local weather patterns, the shading of the array or the seasonal changes of the region. The local weather pattern may vary greatly even within a small geographic region. Even a brief shading of the PV array may affect the output current greatly. There can be a significant reduction in energy production during winter or rainy season. That is why the measured irradiance data needs to adjusted to take into account all these factors.

3.1.3 System voltage calculation

The term system voltage is used pertaining to the battery voltage. The maximum peak load determines the system voltage. The limit for the maximum current is kept at 120A.

The value of system voltage is either 12V, 24V or 48V. The battery closest to the required system voltage is chosen accordingly.

3.1.4 Selection of system configuration

Depending upon the application and criticality of the loads, a suitable system configuration is selected. For stand-alone systems, a genset may be used to cater for the critical loads in case of low or no power from PV array. A battery is also used in a stand alone system to provide power back up for the periods of insufficient solar radiation. If the system is connected to the grid, then there are other considerations like net metering etc (see ‘Typical system configurations’). The utility grid serves as a back up and there is no need of a battery or a genset in this case.

3.1.5 Selection and sizing of PV array and other balance of system components

The number of modules and the size of the battery bank, wires, controller, fuses, inverter, etc., mainly depend on the maximum power demand, and the amount of solar radiation available at the given location on a daily and seasonal basis. System sizing is usually based on the maximum energy demand during the month of lowest solar radiation intensity.

Selection of the PV module
There are a wide range of modules available for PV systems. The differences between them are in the number of cells, the size of each cell, and the manufacturing technology used. There are a number of factors that affect the module selection. The most important of them are listed below:

• The number of modules used depends upon the power output of each module. The higher the power output of each module the fewer will be the number of modules in an array. This will result in reduction of the PV array size and mounting and fixing costs.
• Architectural requirements such as a requirement for a building integrated PV system. These modules are designed to be part of the building structure. They come in the form of solar shingles, roof tiles or metal tiles.
• The operating temperatures on the selected site can affect the module performance considerably. The efficiency of the modules (especially that of polycrystalline silicon) may reduce drastically in extreme temperatures. If the operating temperatures are very high, it is recommended to use special type of modules designed to withstand high temperatures.
• The current –voltage characteristics of each type of module should be compared. For example, the I-V performance of the crystalline silicon module is better compared to the amorphous silicon module.
• The efficiency of the module is an important criteria for the selection of the module when the availability of space is limited. High efficiency modules should be selected to reduce the array size (especially for the pole mounted arrays).
• The cost of the module is an important factor to be considered for the selection. The low efficiency low cost modules would not necessarily translate into cost savings. The costs of mounting and fixing the arrays will be more than the high efficiency modules. A thorough cost analysis of various modules taking into account these hidden costs should be conducted.
• It is vital to make sure that the module manufacturer has a first-rate reputation and supplies the modules with proper warranty.

PV array sizing
The selection criteria for the PV array size depend on the type of the PV system.

If the system is a stand-alone PV only system, then the array sizing is done based on the energy demand and the irradiation data for the ‘worst month’. The load is calculated taking into account the system losses for the entire year. For stand-alone PV systems with a generator, the array sizing is done only for the fraction of the total load demand as the generator provides for the remaining energy requirements. The proportion of the load provided for by the array and the genset depends upon the type of the system.

Generally speaking, for the residential systems the PV array caters for about 90% of the load.

If the system is grid connected, then the array size is calculated based on the annual load demand. The use of a PV array is not so critical in this case, as the continuous power supply is always available. Hence, the economic consideration is the most important determining factor for the array size.

PV array tilt angle and orientation
The tilt angle and the orientation of the PV array affects the amount of irradiation and the energy output of the array. The orientation of the PV array should be true north for the southern hemisphere and true south for the northern hemisphere. If the deviation from true north (or true south) is more than +-5°, it will affect the energy output during the winter months.

Establishing true north (or south)
A magnetic compass will show the magnetic north (or south) instead of true north (or south). The deviation of magnetic north from true north can be obtained from the geological survey organizations in most of the countries for any particular location. This information can be used to estimate the true north (or south) at that particular location.

Tilt angle adjustment
The tilt angle of the array is adjusted throughout the year depending on the load demand pattern. If the load demand peaks in summer the tilt angle in case of latitude (L)<25° should be L-10° and for latitude>25°, it will be L to L-5°. If the load demand peaks in winter, the tilt angle should be L+10° to L+20°. For example in case of latitude = 45°, with the load peaking in winter can have tilt angles as follows:

In some special cases like vehicles and yachts, the array is mounted horizontally.

The number of series and parallel connected modules is calculated on the basis of the voltage and current demand of the PV system. The operating voltage and nominal current of one module is obtained from the manufacturer’s specifications after deciding on the type of modules to be used. The methodology to determine the number of module in series and parallel is as follows:

Number of series connected modules
The number of modules (N_S) connected in series is determined by the DC operating voltage (V_DC).

N_S = V_DC / Vm

Where

Vm   operating voltage of one module.

Number of parallel strings
N_{P =} (E_tot X f₀)/ V_DC X I_d X H_tilt X n_c

Where

N_P     number of parallel strings

E_tot    total daily energy demand in watt-hours

f₀      oversupply coefficient (values range from 1.3 to 2)

I_d   nominal current of the module

H_tilt   daily irradiation on the tilted array in peak sun hours

n_c    Amp hour efficiency of the battery

Selection and sizing of the balance of system components
The selection of the storage battery, inverter, charge regulator, battery charger and generator is discussed below.

Selection and sizing of the battery
The important points to be considered while selecting a battery are:

• Voltage and current characteristics
• Storage capacity
• Maximum depth of discharge (different for each type of battery)
• Mechanical robustness
• Battery cycle life
• Maintenance requirements: some batteries are almost maintenance-free
• Self discharge
• Cost
• Autonomy (number of days a battery is required to deliver energy in the absence of any input from the PV modules)

The steps involved in selecting a battery are:

• Determine daily energy requirements
• Determine system voltage
• Calculate Ah per day and select the required autonomy
• Select a battery as per above factors
• Check all the battery parameters based on supplier’s data
• Re-select battery, if necessary

Battery sizing
Determination of daily energy requirements

Depending upon the power requirements of the various loads, daily energy requirement is calculated in Wh per day. A sample of a load chart for calculating daily energy requirements is given below.

Total daily energy requirement is 7310 Wh.

Determination of system voltage
Depending on the maximum continuous power requirement, the appropriate system voltage is selected from the following table. The recommended value of maximum continuous current is 100A.

For example, if the maximum continuous power requirement for the above system is 4400W, then the battery with 48V voltage will be required.

Calculation of Ah per day and selection of the required autonomy

The Ah per day is calculated as follows:

Ah per day = Wh per day / system volts

Ah per day = 7310/48 = 152.29 Ah/day

The maximum autonomy required for the system depends on various factors such as weather conditions, type of PV system (with or without backup), cost considerations etc.

For domestic systems, the typical autonomy requirement is about 3-5 days. On the other hand, for hybrid systems, which use alternate power supply, the requirement is around 1-3 days.

If an autonomy of 3 days is specified and DOD_MAX= 70% then the battery capacity at 100h discharge rate is calculated as below:

C₁₀₀ = (E_tot X N_aut ) / (V X DOD_MAX) = (7310X3)/ (48X.7) = 652.67 Ah

Where

Selection of the battery
The battery selection can be done using manufacturer’s data sheets. The battery capacity closest to the calculated Ah capacity is chosen. Based on this Ah capacity, a number of batteries from different manufacturers can be short-listed.

The number of batteries required is calculated as per the nominal voltage of the system. For example in our case the system voltage is 48V, hence the required number of batteries will be eight 6V batteries or twenty-four 2V batteries.

A number of other considerations like maintenance requirements, cost, operating temperatures and cycle life of the battery are then taken into consideration for the selection of the appropriate battery.

Given below are the examples of a typical datasheets provided by the manufacturers.

Lead acid battery specifications

Gel type battery specifications

Selection of the Inverter

The following parameters should be considered while choosing the inverter.

Inverter efficiency
It takes energy to operate the inverter and to invert DC to AC electricity. This is simply energy in and energy out. If you input 100 watts and 95% comes out of it then the inverter loses 5% of the energy.

It is also important to understand how the DC energy converts to AC. For example if the inverter were 100% efficient, an inverter would yield 1 amp of AC power if 10 amps of DC current were taken from the battery. At 50% efficiency, the 10 amps drawn gives only 0.5 amp AC power.

Modern inverters can achieve efficiencies as high as 95% and typically operate above 85% at full power. Efficiency varies from model to model and according to the relative load on the inverter.

True sine wave inverters are usually less efficient than modified sine wave inverters. Overall efficiency may be similar due to the energy lost in the harmonics of a modified sine wave, especially when running motor loads.

Surge and overload capacity
Most inverters deliver two to three times their continuous rated power depending on the design. The surge capacity allows the inverter to deliver enough power to start electronic equipments; large electric motors such as pump motors and power tools. All electronic equipments have a filter capacitor and rectifier as their first stage. Current is drawn from the supply only when the supply voltage is higher than the capacitor voltage. On switching-on, the capacitor is initially fully discharged. A very large current is drawn in order to charge the capacitor. In certain equipments, this surge may be 10-20 times higher than the normal operating current.

There could be certain times when we would be sucking up excessive load through the inverter. Examples could be as simple as starting up two or more appliances simultaneously, or starting some heavy-duty motors. In these times of overloading the waveform becomes very different. Quasi sine wave and sine wave inverters will produce a waveform with flat top. So the waveform tends to get closer to the square waveform or in certain cases actually becomes a square waveform. Both cases are bad for appliances for the reasons discussed above. Good inverters employ appropriate voltage control method and are capable of supplying this load without distorting the waveform.

Idle current
All inverters use some power even when no loads are turned on. This is the idle current of the inverter. It is a fixed amount and is always present as a load on the battery when the inverter is turned on. Some inverters have a search mode, which reduces idle current by turning the inverter on and off every second to search for a load. If no load is present the inverter stays in search mode. So, a good inverter would have a low value of current for this characteristic.

Sizing of inverters
Quite a few things need to be considered while sizing the inverters. Sizing of an inverter starts with load assessment. This includes the maximum demand as well as the surge loads. The inverter should use the appropriate input voltage and produce the required output voltage waveform. Inverter specifications vary from one manufacturer to another. The critical specifications are the efficiency vs. power function and power output vs. duration function. It also depends on factors like the latitude the panel location etc.

Let us use the load assessment chart given above for the battery selection as an example for the selection of the inverter. The first step is to calculate the total energy required to run the critical appliances simultaneously for a certain time period. It is very unlikely that all the appliances will be running simultaneously all the time, so for this example lets us assume that the total energy required will be 4000 WH/day.

In addition to this surge requirement for certain appliances like electric motors have to be taken into account. Lets assume that the total surge requirement(for all the appliances) is 1400 Wh.

The total Watts of power needed can now be calculated by adding the two (4000+1400 == 5400 Wh).

This value is then multiplied by the inverter loss factor of 1.2(5400 x 1.2 = 6480 Wh) This is the power rating which is used for the selection of the inverter. Generally, an inverter with a higher power rating specification is selected, to take care of any unexpected increase in the load demand or addition of any new loads.

The other things that need to be considered would be the ambient operating temperatures, which may be determined by the latitude information. The standard specification in general would be given for 25^oC. So, for a higher operating temperature the inverters have to be de-rated.

Given below is a typical inverter data sheet provided by the manufacturer.

Typical inverter specifications

Standards
In order to overcome the electromagnetic induction problems various countries have come out with standards like the Australian standard-AS1044and European standard-C-Tick. Generally, these standards recommend using sine wave inverters within certain specified EMI levels.

Selection and sizing of the charge regulator
The main function of a charge controller is to prevent batteries from overcharging. However there are additional features desirable in a charge controller . Most charge controllers come with these special features such as deep discharge protection, Temperature compensation, Voltage and current indicators / meters, Fuses, circuit breakers, blocking diodes, Automatic equalization routines, Different set points for different battery types (or user adjustable set points). The following parameters are used for the selection of the appropriate charge regulator:

• System Dc voltage (nominal typically 12, 24 and 48 V) must match the rated voltage of the controller.
• The charge controller should be sized for the maximum load currents.
• Voltage drop at rated current
• Charging method (voltage controlled, smart regulators etc.)
• Sensor range for battery temperature
• Operating temperature
• Optional feature requirements like energy metering (daily charge and discharge), alarm (low or high battery voltage) etc
• The cost and the ability to work in the specific environment should be determined.

Selection and sizing of battery charger
The selection of the battery charger depends on many factors. The most important of them are output current, charging method (regulated or unregulated) for the type of application, cost and efficiency. Unregulated chargers are cheaper but less efficient. In the system where cost is a limiting factor, unregulated chargers are used. However, it is recommended to use regulated chargers for greater efficiency and lesser generator runtime.

The generator and battery charger are closely related. The size of the battery charger will affect the runtime of the generator. If the charger is undersized, the genset runtime will increase and if the charger is oversized, the battery will be overheated. That is why the sizing of generator should take into account the battery charger size and vice versa.

Battery charger sizing
The 10-hour rate capacity (C₁₀) of the battery is used to calculate the maximum charge rate of the battery (I_bc ).

I_bc = C₁₀/ 10

The recharge time of the battery is calculated as follows:

T_R = {Cx X (DOD_max -(1-SOC_fin))}/( I_bc X f _util)

If the recharge time is already specified then the above equation can be rewritten to obtain the battery charger size for the given recharge time.

I_bc = {Cx X (DOD_max -(1-SOC_fin))}/( T_R X f _util)

The upper and lower limit of the charger output current can be calculated as follows:

C₂₀/20 < I_bc < 0.25 X C₁₀

Generator sizing
Including a generator as an additional power source can increase system reliability and may reduce life cycle cost. But, it increases system complexity. The need of the back up power however is reduced and the battery capacity can be smaller in this case.

Generators are generally selected depending on the design of the system and the client’s needs. For example, if the generator is required to be used only occasionally and the major fraction of the load demand is met by the PV array then the generator size can be smaller. In case if the customer needs the generator to take larger loads then the generator capacity should be higher.

Gensets can be used in different ways:

• For battery charging only (series system)
• For battery charging and additional AC loads (switched system)
• For large AC loads

The estimated power requirement is calculated as follows:

For series system (battery charging only)

S_gen = S_bc X f_go

For the switched system (battery charging + load)

S_gen = {(S_bc + S_max)X f_go }/ R_surge

Where

S_max      Maximum AC load demand, in VA

R_surge    surge ratio, depends upon the surge requirements of the selected loads (typically 2, 3)

The generator output is then further derated to take into account the operating temeperature. Generally, for every 5° increase in the temperature above 25⁰C, the output is derated by 2.5%.

3.1.6 Selection and sizing of wiring, fuses, circuit breakers and monitoring equipment

Once the system components are selected and their location fixed, the next step is to select the wiring, fuses, circuit breakers etc.

PV wires interconnecting the PV arrays with batteries and other equipment should be sized to carry the peak current. For low voltages, system larger wire sizes are required to prevent overheating and voltage losses. All the wires must be attached securely to junction boxes. Loose connections may lead to poor system performance.

The wires from the battery bank to the individual appliances must be sized to carry the appliance load and must be properly fused for protection against short circuits in the wiring or the appliances. Switches can be installed for certain parts of the system that need to be switched off for maintenance purposes. Here are the basic guidelines for wire selection :

• Conductor type

  – Material (copper, aluminum, etc.)

  – Stranded vs. solid core conductor

• Wire insulation type

  – Different insulation types for different environments and application types

• Color coding
• Wire size (ampacity and voltage drop)

PV modules, inverters and other exposed equipment should be properly grounded and protected against lighting. Conduits should be used for cables to protect them from high temperatures and radiation. Separate meters can be used to monitor the PV system performance.

3.3.1 Orientation angle

In a day the output from ‘our’ 100W module will start from zero at dawn, rise to a peak at noon and drop back to zero at dusk. This is because of two factors namely the sun’s intensity changes during the day and the angle of the Sun’s rays relative to the module.

Similarly, the output will be very different between summer and winter. As we have seen in the earlier chapters, the orientation angle and direction would drastically affect the output. For instance, a roof with a pitch of 7:12 facing true north in Australia will give a maximum output of 100% whereas an almost vertical orientation facing East-West will yield close to 50% output. The output ratio corresponding to various pitches and orientation angle for different countries may be obtained from the respective country standards.

3.3.2 Conversion losses

Some amount of power is lost in the DC to AC load conversion process. This is necessary as many electrical appliances work on AC.

As we have seen in the previous chapter, the inverter has peak efficiency of 90-95%. The efficiency varies depending upon the methodologies adopted by the manufacturer and generally given by the manufacturer himself. Therefore, the conversion loss factor (correction) C_f =95% or 0.95.

3.3.3 Wiring losses

Some amount of power is lost due to resistance in system wiring. One should always try to minimize the length of the wire between different electrical modules. It is tough to maintain the power loss due to wiring at below 3%. On an average, a loss of 5% is generally taken for calculation. This means a wiring loss correction W_f =95% or 0.95.

3.3.4 Module mismatch

The maximum power output of PV array would always be lesser than the maximum output of the individual modules, independently tested. This difference is due to the inconsistencies in performance between one module and the other. This is called the module mismatch. The loss due to module mismatch (M_mf) generally amounts to around 2%, in which case M_{mf =} 98% or 0.98.

3.3.5 Influence of temperature

As the temperature increases, the output power of the module decreases. It depends a lot on the type of attachment used for mounting the PV arrays. When operating on the roof because of poor ventilation below the PV array, the inner temperatures might reach up to 70-75⁰C. This result in the reduction of the power output and generally, a factor of 90% is taken. In this case T_f =90% or 0.9.

3.3.6 Battery

If the system includes battery backup then the output may be reduced further by 6-10%.

3.3.7 Miscellaneous factors

Miscellaneous factors (M_f) like dust, dirt etc. also adversely affects the output of a PV system. They reduce the output by another 8-10%, in which case M_f =90% or 0.9. Therefore, the system output is a product of STC power and all the above-mentioned efficiencies.

S_o = P_STC x M_mf x C_f x T_f x Tp x W_f x M_f

4.1.1 Module construction

Cells are usually connected together to form a module in order to provide a suitable voltage for application. The PV cells are very thin and fragile, hence a protective front glass sheet and a backing plate of either aluminium or plastic is provided on both the sides of the cells. The modules are hermetically sealed to avoid any physical damage due to weather, moisture, pollution or corrosion. There are two types of module designs:

• Framed module
• Frameless laminates

Framed modules consist of a front sheet of low-iron glass, which has high transmission efficiency while protecting the front surface material. To protect the glass, modules are framed around the edge. An aluminium frame can be used around the PV module, to enable easy fixing to a support structure.

Frameless modules or laminates use heat and shatter resistant glass. These modules act like double-glazing and can be used in skylights or conservatories in place of glass. These laminates are available in a variety of colours and sizes selection of which depends upon the application.

Apart from this, the modules are also available in the form of tiles, slates, shingles etc. The selection of the appropriate module construction depends upon the mounting method used. The various mounting methods are discussed below in detail.

4.1.2 Temperature effects on array performance

High operating temperatures reduce the efficiency of PV arrays, especially in case of roof mounted PV arrays; proper ventilation should be provided to avoid high temperatures.

4.1.3 Array orientation and shading

As discussed in the previous chapter, slight deviation of the array from the true north (or south), affects the power output significantly. The deviation from true north (or south) should not exceed +/-5⁰.

Any kinds of obstructions should be avoided, as even a small amount of shading will affect the system output drastically.

4.1.4 Structural characteristics of the roof

The structural aspect of the roof should be taken into account while designing the roof mounted PV array systems. The roof should be weather-tight and the structural integrity of the roof should be maintained.

4.1.5 Ease of installation, access and maintenance

The design should allow for easy installation of the PV array. The access to the PV array for maintenance purposes should be taken into account.

4.1.6 Cost-effectiveness

The cost considerations from the client side should be taken into account. The cost effectiveness of various mounting methods should be studied before selecting the appropriate method.

4.2.1 Ground mounting

This is the most popular method of PV mounting. Installations range from very simple racks to large, adjustable racks capable of holding multiple rows and columns of PV modules. Ground mounts are generally the least expensive mounting option if a sturdy, flat surface is available or can easily be prepared.

The ground mounting frames (racks) have adjustable legs for adjusting the tilt angle seasonally. This mounting style can be used for water pumping or telecommunication systems. These frames are designed to be fixed on the ground with the help of concrete. These frames can withstand wind speeds of up to 120 mph. The main drawback of these frames is that they cannot be used for tracking. In addition, there are chances of damage caused due to their location for example (rocks hitting the array surface etc.). For this reason roof or pole mounting is preferred over ground mounting wherever possible.

4.2.2 Roof mounting

Roof mounting is the most preferred option for the domestic PV system because there is no need to allocate separate space for the array mounting. This also allows the use of building integrated PV materials. The shading of the PV array is also avoided in case of roof mounting. In the case of buildings under construction, even the tilt angle and orientation of the PV array can be taken into account while constructing the roof. This will help in mounting the PV arrays directly on the roof without using any special stand-off structures.

Roof attachment methods
There are various methods of PV mounting on the roof. The most common of them are:

• Flush mounting
• Stand off frame mounting
• Building integrated PV mounting

Flush mounting
Flush mounting of the array is done by attaching the array directly to the roof tiles. This is normally done by mounting PV modules to an aluminium or galvanized steel framework lying a few centimeters above the roof. This framework is fixed to the roof with the help of mounting slates or roof hooks. The design of the frame is such that it can be tilted at an angle for the maintenance purpose. The main disadvantage of such an arrangement is the lack of ventilation and air circulation beneath the modules, which results in higher operating temperatures.

Stand off frame mounting
The stand-off frame is attached to the roof at an angle, which enable the modules to receive maximum sunlight. The increased air circulation ensures the optimum operating temperatures. Some of these frames can be tilted at various angles seasonally.

Building integrated PV mounting (BiPV)

The PV system can be integrated either partially or fully with the roof itself. In case of a fully integrated system, the PV panels are mounted directly on the roof battens in place of the tiles. This enhances the appearance of the building as well as reduces the roofing cost. In new buildings, renovations, or where aesthetic considerations are particularly important, fully integrated systems are recommended. In this case, the panels must also be designed to withstand weather conditions and also maintain water tightness and allow for drainage. Semi-transparent and custom sized/shaped modules can be used in skylights in place of glass. Although BiPV systems are appealing architecturally, the roof’s load factor, the increased operating temperatures and structural integrity of the roof materials should be considered while designing such a system.

There are various types of roofing systems available in the market that can be used for BiPV system installations. The most widely used are:

Tiled roof system
This roof system makes use of special tiles or slates on which the galvanized steel frames are mounted. These steel frames are then fixed with the PV modules in the form frames or laminates. The tiles are also provided with roof hooks, which are fastened to the roof battens. The laminates can be fastened with clamps directly on to the tiles. It enables the installation to be faster and easier, for the reason that there are no large subassemblies to be transported to the roof.

Metal roof system
This type of roof system uses aluminum mounting bars, designed in the form of a grid for the installation of PV laminates. The edges of the laminates are overlapped to give it a conventional roof appearance.

Solar shingles integrated with the roof tiles
Solar shingles incorporate PV cells into conventional roof tiles. Once installed the solar shingles have the appearance of a traditional shingle or plain tiled roof. They allow easy access to the back of the tiles for ventilation and maintenance purposes. In most of the roofs the tilt angle is approximately the same as the tilt angle required for the PV array. Hence the use of solar shingles on such roofs not only enhances the appearance but also eliminates the need for special array mounting frames.

Panel assembly method
The panel assembly consists of two components: PV panels and the assembly set for the PV panels. The PV panels have a hardened glass surface that is assembled with the help of plugs and sockets to form a PV array. By-pass diodes are also provided to protect the panels in case of partial shading. The panels are fixed in place of the tiles maintaining the appearance of the roof. The panels should be provided with proper ventilation to avoid panel overheating. The overheating of the panel will cause a drop in the power output resulting in low efficiency.

4.2.3 Pole mounting

Pole mounting is preferred for areas where the ground or roof mounting of an array causes excessive shading of the array. Sometimes, the roof structure itself is not suitable for array mounting. Pole mounts are also ideal for sloping, uneven sites or other locations where ground mounting is not possible. They avoid shading from growing vegetation (in case of ground mounting) as well as keeping the array out of reach so that it cannot be tampered with. The disadvantage of pole mounting is that it cannot be used for large size arrays due to the load factor. Pole mounts are designed to withstand winds up to 120 mph. The snow cannot accumulate on their surface and for this reason they are most suitable for cold climates. For small or remote systems, pole mounts are the least expensive and simplest choice. These mountings are usually used for one- or two-module systems, like street lighting, water pumping, etc.

There are two types of pole mounting methods:

• Top of pole mounting
• Side of pole mounting

Top of pole mounting
Top of pole mounting is done by fixing the PV array mount on the top of a standard steel pipe. The pipe is fixed in a concrete footing in the ground. The size and length of the pipe depends on the PV array size and the location. Generally, a 10 feet long pipe with 2-3 inch thickness is sufficient. The PV array orientation and tilt can be adjusted and the array can be tracked manually. The mounting rails used for the array are usually made of galvanized steel sections & aluminiuim.

Side of pole mounting
The PV module is clamped on the side of a steel pipe with the help of U-bolts or band clamps. This mounting method is used when the pole is being used for some other purpose like lighting antenna or wind turbine pole. This mounting can also be implemented for wood or concrete poles using various types of fasteners. The tilt angle achievable in this type of mounting is from 10 to 70 degrees. The material used for the mounting structure is galvanised steel.

4.2.4 PV Array Trackers

In a fixed rack mounting there is no provision for the PV array to be facing the sun at all times. Hence, the power generation takes place only when the array is directly facing the sun for a few hours a day. This reduces its power output significantly. A tracker follows the sun throughout the day maximizing the output power of the PV array all day long. There are two types of trackers available:

• Passive trackers
• Active trackers

Passive trackers
Passive trackers use the heat energy of the sun for tracking. There are two freon tanks situated on both the sides of an array. As the sun moves from east to west, the fluid in the tanks heats up unevenly, which results in the fluid moving from one tank to the other. The equilibrium is reached only when the array faces directly towards the sun. As a result the array always faces the sun. The north-south axis can be adjusted manually depending upon the season.

This is a simple method, which doesn’t draw any additional electricity from the system and requires minimum maintenance. The main disadvantages of passive tracking are the wind disturbances and slow warm-up in cold weather. Particularly at the time of sunrise, it takes few hours to warm up and start rotating towards east.

The wind speed also affects the functioning of passive tracker. At the wind speeds higher than 85 mph, these trackers do not work well.

Active trackers
Active trackers are more accurate and efficient than passive trackers, as they use light sensing devices for tracking the movement of sun. A low power motor is used in active trackers to allow them to follow the direction of the sun. As a result, these trackers can follow the sun with an accuracy of around half a degree in any climatic condition. The warm up time required for the active trackers is much less than that of passive trackers and hence the power output is much more than the passive trackers, throughout the day. The small amount of power consumed by the motor is compensated by the higher efficiency of these trackers.

4.2.5 Recreational Vehicle (RV) mounts

The mounting arrangement for a RV is required to be horizontal due to the wind factor and because the direction of the RV is not fixed. Most RV systems are attached flat on the roof of the RV at a height of one or two inches from the roof for cooling. The simplicity and low cost of these systems make them suitable for small, simple domestic systems. The material used for the mounts is mostly corrosion resistant aluminium. Some of the RV mounts have the provision for tilting the module when the vehicle is not moving for higher solar gain.

5.10.1 Visual check

Visual inspection of the components for any physical damage like cracks, etc:

• Check mechanical installations such as mounting frames, mountings for the electrical equipment etc for structural integrity and weather proofing
• Check electrical installation to ensure proper continuity of circuit, fusing and polarity

5.10.2 PV array performance check

The testing of the PV array performance should be carried out at the irradiance level of at least 400 W/m².

The following parameters should be measured and checked against the manufacturer’s specifications:

• Open circuit voltage
• Maximum power point voltage
• Maximum power point current
• Short circuit current
• Rated power
• Mounting details, orientation and tilt angle

5.10.3 Batteries and other BOS performance check

• Measurement of the battery voltage
• Measurement of the inverter voltage and frequency
• Measurement of the genset voltage and frequency
• Testing the functioning of all regulators

5.10.4 Overall system performance test

After completing the above tests and measurements, the complete system test is conducted. The major test procedures are listed below:

• Measurement of voltages at all the critical points, to ensure that there is no significant voltage drop at any of the places
• Verify start up and shutdown functionality. The start up and shut down procedures should be provided to the user
• Overall system functionality. The operation of each component and the system as a whole should be checked against the rated characteristics
• Check the cables and the interconnection wiring between components

The final step is to supply the appropriate documentation, drawings and safety instructions to the user. The documentation should include:

• The operating instructions
• The components list
• Startup and shutdown procedures
• System components ratings and system performance warranty
• Maintenance and diagnostic procedures
• Safety instructions associated with the operation and maintenance of the system
• Mechanical and electrical drawings of the system

6.2.1 Wind energy is an indirect form of solar energy. How is wind generated?

The earth’s surface comprising of deserts, oceans and hilly terrain absorb the solar energy in different magnitudes. Due to this the earth’s surface gets unevenly heated The regions around equator, at 0° latitude are heated more by the sun than the rest of the globe. Since the hot air is lighter than the cold air, it rises in the sky to the height of approximately 10 km and starts spreading towards the North and the South. If the earth did not rotate, this hot air will just turn up at the North pole and the South pole and then gradually return to the equator. However, due to the rotation of the earth, the wind is deflected towards right in the northern hemisphere and towards the left in the southern hemisphere. This bending force is known as Coriolis force. For example this phenomenon can also be observed in railroad tracks. Imagine the track going from the south pole to the north pole. In the northern hemisphere the track on the eastern (right) side will wear out faster than the one on the other side.

This bending (Coriolis) force affects the formation of the global winds. It prevents the hot air rising at the equator from moving too far. At the latitude of about 30 degrees in both the hemispheres the air begins to sink down thus forming a high pressure zone in these areas. At the equator there will be a low pressure zone near the ground level due to the rising hot air, and at the poles there will be a high pressure area due to the cooling of the air. Following are the results of the direction of the wind at various latitudes.

Prevailing Wind Directions

The predominant winds in the Northern hemisphere are called north easterly trade winds while in the southern hemisphere they are called south easterly trade winds.

The winds discussed above are also called geostrophic winds. They depend largely on the temperature differences on the earth. They are not notably affected by the uneven surface of the earth. The geostrophic wind is found at altitudes above 1000 meters above ground level. At the lower altitudes (up to 100 meters) the earth’s surface roughness and obstacles come into picture. The winds at this level are called as surface winds. In the following figure we can see how wind flows over the earth’s surface.

Local winds also play a major role in determining the wind direction and speed in the given area. The direction of the wind in any given location is the combination of prevailing global winds and local winds in that area. In some cases local wind could play a bigger role than the global (large scale) winds. We will study in detail the formation and types of global and local winds in various parts of the world.

6.2.2 Formation of global winds

As we discussed above, due to the curvature of the earth and its rotation around an inclined axis, the amount of solar energy reaching the earth’s surface varies at different locations. In addition to this the amount of irradiation varies throughout the season. The earth’s surface absorbs this heat energy radiated and re-radiates it into the atmosphere in at much longer wavelengths.

The atmosphere gets heated mostly due to the release of the latent heat of water vapour combined with vertical turbulent mixing. This happens mostly in the tropical regions where the temperature and vapour levels are higher. To maintain a heat balance, heat is transported from the latitudes near the equator to the latitudes near the poles. Large-scale wind systems act as the transport media for this heat and help maintain a global energy balance. This phenomenon is known as Hadley circulation. Hadley circulation is nothing but the average heat circulation in the earth’s atmosphere caused due to the movement of air from lower latitudes towards the higher latitudes, and its descent to the lower latitudes closer to the earth’s surface. Hadley circulation plays a major role in maintaining the heat balance.

The large-scale circulation of air can be understood better with the help of the following figure.

Gravity also affects the wind speed and direction. When the atmosphere is thermally stable, the air tends to move down towards the earth due to its weight.

For wind energy applications it is very important to understand the most useful prevailing winds and the damaging extreme winds. We shall be studying this in the next section.

6.2.3 Prevailing large scale winds

The trade winds

The Hadley circulation together with the Coriolis force produces the characteristic trade winds at latitude 30 degrees North or South. These trade winds are termed south-easterly trade winds in the southern hemisphere and north-easterly trade winds in the northern hemisphere. On a tropical island these winds provide a nearly constant wind flow throughout the day and night.

The average wind speeds here can exceed about 7 to 8 m/s at 40 meters elevation. These are very useful for wind energy.

The westerly winds

The winds prevailing in the latitudes 30 to 70 degrees north or south are termed as the westerly winds. These are formed due to the presence of high-pressure systems. These winds are associated with large-scale circulation at high elevations in the form of jet streams moving from west to east.

The westerly winds are strong at latitudes 40 to 50 degrees north and south and hence are called as “Roaring forties”. These are also excellent energy winds and have recorded an average wind speed of 10 m/s.

Doldrums

The doldrums are usually located between 5° north and 5° south of the equator. They are also known as the Intertropical Convergence Zone or ITCZ. Around the equator, the northeast and southeast trade winds converge to form the doldrums. Doldrums are characterized by the stillness of the rising air and convectional storms.

In this region, there are small temperature and pressure differences due to which the speed of the winds is light and variable.

A wind energy application here will not be highly effective as the utilization of wind energy would be highly seasonal. This is because; very little wind energy is available during the doldrums periods.

The monsoons and tropical cyclones

The effects of monsoons are extended to the south and north of the equator. The monsoons are a large-scale sea breeze effect (to be discussed in the next section) resulting from the differential heating of land and sea masses.

In summer, the Asian continent is heated producing a low-pressure region over the land as the air here gets heated and it expands and rises upwards. This forces the air from the colder regions over the oceans, which are under high pressure to move in and take its place. Since this air flows over the water mass, it is moisture laden and causes heavy precipitation on the land masses. These are termed as southwest monsoons in the northern hemisphere. In winter, it happens exactly the reverse. Due to cold climate the prevailing north to northeast winds descend over the landmasses forming high-pressure zone, which forces the airflow southwards from the land to the water towards the equator. This dry air produces clear skies and little rain.

These winds bring the most wind energy potential to the island and the coastal areas.

Tropical cyclones are normally formed close to the north and south of the equator during the late summer and fall. They are cased due to the evaporation and condensation of the water vapour formed on the surface of the ocean. The clouds formed through this process reach a high altitude and form huge thunderheads. This rising air causes the wind to spin and form cyclones. When inward spiralling wind reaches a velocity of 40 mph, it is termed as tropical cyclone. If the wind reaches a speed of 74 mph or above, it is called as hurricane or typhoon. These wind speeds are destructive and not a useful wind energy.

6.2.4 Local winds

There is a large interaction of the large-scale winds that we have seen above and the local winds such as the katabatic winds in the hilly regions or the sea breezes in the coastal areas. The local topographical features such as cities, forests, mountains and valleys affect the magnitude and direction of these local winds. Let us see the influence these local winds over the large-scale winds.

Katabatic and anabatic winds

The winds that blow downhill from any inclined topographical surface such as mountains or hills are called katabatic winds. There are two types of katabatic winds; winds that are warmer than the surrounding are called as Foehn, and the winds that are cooler, such as the Mistral in the Mediterranean or Bora in the Adriatic. However the term Katabatic wind is generally associated with the cold form of the winds. These winds are formed due to the cooling of air at the top of the mountain or glacier. This results in a higher density of air, because of which the air tends to move downwards. This phenomenon is observed more prominently in the night, since the warming of the air due to the solar radiation ceases to exist in the night. Prominent cold katabatic winds exist in Antarctica and Greenland almost throughout the year. At certain places like the mountainous areas in the east coast of Australia, the southwest winds from large-scale pressure gradients reinforce the katabatic winds in winter.

Winds which blow up an inclined slope are called as Anabatic winds. These winds are formed due to the heating of air on the mountain slopes by the solar radiation. The air over these slopes becomes warmer and lighter and tends to move up the slope. This draws cooler air to the center of the valley. The speed of these winds is generally low averaging around 3-4 m/s.

The sea breeze effect

This is caused by the difference in the heating and cooling of the surfaces with land and sea masses. Let us see in the figure below how a sea breeze is formed.

The land and sea masses heat at different rates. The land heats and cools quickly unlike sea, which takes longer time to cool once, heated. The sea is hence thermally more stable.

During the day, the land mass gets heated faster, resulting in the heating of the air over it. This heated air expands, becomes lighter and moves upwards. Thus a low-pressure zone is formed and the cool and denser air over the sea forms a high-pressure zone. This difference in the pressure causes the air from high-pressure zone that is cold and dense to move towards the low-pressure zone replacing the light and warmed up air over the land. The speed of these sea breezes varies form 5 to 8 m/s and can sometimes reach up to 10 m/s.

These sea breezes can either be reinforced or weakened by the presence of the large-scale winds, high and low pressure system and troughs on their way. The following figure shows a typical sea breeze circulation.

In the above figure you will observe that:

• Winds typically do not extend more than 20 km inland
• Sea breeze carries moisture from the ocean
• The wind converges at the point where the cool moist air meets the inland hot air

Thunderstorms and tornadoes

Thunderstorms are formed when the warm, moist air from the surface of the earth is lifted vertically upwards into the atmosphere. This is caused due to the uneven heating of the earth’s surface or due to the topographic obstruction of the air flow. This warm, moist air, starts cooling as it rises up, resulting in a formation of a cloud at a certain elevation. This cloud develops into a thunderstorm when the uplift continues in an unstable atmosphere. These clouds can reach up to the height of 20 km, above the earth’s surface.

Tornadoes are very intense small cyclones very often called as twisters in the inland parts. The wind speed is high and in rare cases may exceed 50 m/s. These are not useful for wind energy systems as the velocities are very high.

6.3.1 Power

Before understanding the dynamics of wind flow, lets see the difference between wind speed and wind power.

Wind Speed

Wind speed is the rate at which air flows past a point above the earth’s surface. Wind speed can be quite variable and is determined by a number of factors which will be discussed in this section.

Wind Power

Wind power is a measure of the energy available in the wind. It is a function of the cube (third power) of the wind speed. If the wind speed is doubled, power in the wind increases by a factor of eight (2³). This relationship means that small differences in wind speed lead to large differences in power. Even a minor change in the wind speed can have a considerable change in the wind power. For this reason, a thorough and accurate wind study is conducted before designing a wind energy system. Following is the methodology to calculate the wind power available in the wind:

The kinetic energy of the wind can be given by the following equation:

K.E.= ½ . m . v^2,

Where m is the mass of the air , which is moving at a velocity v.

The amount of power available in the wind is determined by the equation

w = 1/2 r A v³

where w is power,

r is air density,

A is the rotor area,

and v is the wind speed.

Air density varies with the change in temperature and pressure. It can be calculated by using the following equation :

p = (1.325 x P) / T

where T is the temperature in Fahrenheit + 459.69 and P is the pressure in inches of Mercury adjusted for elevation.

A standard value for air density at sea level is used. This value is then corrected for the particular site, depending on the temperature and pressure data obtained from the site The simplified power equation using the sea level values for temperature and pressure can be given as:

In metric units    w = 0.625 A v³

where w is power in watts, and A is the cross-sectional area in square meters swept out by the wind turbine blades, and v is the wind speed in meters per second.

However for the sites at high elevations the temperature and pressure values can significantly change the value of air density. The combined effect of the temperature and pressure on the air density can be given by the following equation:

r = r₀.e ^{–{0.297Hm/3048}} ;

where Hm is the elevation of the site in meters.

The area swept by the rotor for the horizontal axis turbine is calculated as

A= pi/4(D²), where D is the diameter of the rotor

6.3.2 Mean Wind speed and actual power

As seen earlier the energy available in the prevailing wind largely depends on the power in the wind and the time duration during which the wind blows at a particular speed. The speed of the wind is never steady at any given site. It keeps on changing with time and thus the power also varies largely. Because of the wind’s normal variability, and the effect of this variability on the cube of the wind speed, the power equation should only be used for instantaneous or hourly wind speeds and not for long-term averages. To illustrate why, consider two places where the average speed is 15 mph. At the first location the wind always blows at 15 mph, giving a power density of 189 W/m². At the second site the speed fluctuates. The wind is at 10 mph half the time (power density = 56 W/m²), and at 20 mph the other half (power density = 448 W/m²). The mean power density here is (56 W/m² x 1/2) + (448 W/m² x 1/2) or 252 W/m². At both locations the mean speed is exactly 15 mph, but there is 33% more power at the site with varying speeds.

Hence the actual wind power density at most sites can range from 1.7 times to 3 times greater than that calculated from the mean wind speed. The following figure shows a sample graph of the recorded wind speed.

This method of recording data is called the “method of bins”. We shall discuss this method in next section below.

6.3.3 Measurement of wind speed and direction

The wind speed in the vicinity of flora and fauna determines the rate of convective heat and mass transport between these organisms and their environment. The basic objectives of measuring the wind speed are to determine if the location is worth installing a wind mill and to estimate the output power of Wind Energy Conversion System. There are many ways to measure the speed of wind. A few common methods of measuring wind speed could be observing the effect of the wind on the plant growth habits, on site measurement using wind monitoring equipments.

Vegetative indicators

The trees in the vicinity of site consideration could be a rough indicator of the wind speed and direction.

If the trees are permanently deformed, then we can assume that the wind speed is high in that area. Severe deformation to the tree trunk happens at wind speeds of 24 to 30 kmph. Brushing and flagging can be observed at localities with wind speeds of about 10 to 15 kmph.

However the absence of deformation does not necessarily imply that the wind resource is weak. Some tree species are more sensitive to the wind than others. Trees inside a thick forest are too sheltered and strong winds may blow from more than one major direction. Therefore tree deformation cannot be used as a primary tool in selecting a wind turbine site.

For more precise information about the wind speed at any given location wind speed measuring instruments are used

Wind speed measurement methods

Wind speed measuring instruments are known as anemometers. The wind direction is measured either using a weather wane or by the anemometer itself. In addition, proper monitoring also requires that measurements of the air temperature and the atmospheric pressure be taken since this information is also important in determining the energy content in the wind and the appropriateness of the equipment being considered. Air density is only slightly affected by common fluctuations in barometric pressure; so this variable is really used only when analyzing details.

There are several types of anemometers. The most commonly used anemometers and their working principles are discussed below.

Anemometers based on Drag

When wind blows on a surface, it exerts a force against the surface proportional to the area perpendicular to the wind. The amount of drag depends on the shape and orientation of the object. This principle is the basis for several types of anemometer, including the pressure plate, the cup anemometer and the propeller.

Apart from drag based anemometers, lift based anemometers are also available.

It should be noted that the effect of variations in air density (due to temperature and/or pressure conditions) are accounted for differently in the lift devices (for example, propeller anemometers) and drag devices (for example, cup anemometers).

Pressure Plate Anemometer

A pressure plate anemometer has a hinged plate suspended on a frame. It records the force of the wind against a single ball or plate.

Graph paper can be used for a scale. The anemometer is held level with the plate facing in the direction of the wind. The average deflection of the plate is estimated.

The intensity of the wind speed is measured by the extent to which the angle of the plate deviates from the perfectly perpendicular position it took on during still wind conditions.

A pressure plate anemometer is not accurate and has low sensitivity at low wind speeds.

Cup anemometer

This is the most widely used anemometer. The anemometer has three or six light, metallic cups mounted on a shaft which turns on low-friction bearings. As the drag of the wind on the open side of the cups is greater than the other side the shaft rotates. The faster the wind blows, the greater the difference in drag between the two cup faces, so the greater the speed of rotation. The light shaft rotations are counted by a magnet and reed switch by a small tachometer generator. The speed of rotation varies with the magnitude of the wind speed. It is known as a “drag” device because its rotation is caused by the drag of the cups against the wind. Usually, a little arrow provides information on the direction of the wind. The little arrow has a vane at its end and is attached to the top of the tower holding the cup anemometer; this arrow rotates freely around the anemometer’s center (like a compass needle).

The two most important parameters that characterize the performance of cup anemometers are their threshold, and their linearity.

Threshold

The threshold is the wind velocity below which the cups won’t move. This is also called as the sensitivity of the anemometer.

Linearity

If the rotation speed of an anemometer is linearly related to wind speed, then it is said to be properly designed.

Wind speed calculation

Single rotation of the anemometer corresponds to the passage of a certain set distance of wind travel, irrespective of the wind speed. The number of rotations in a given time period can therefore be counted using the counters mentioned above and directly converted to total wind travel. For any particular period of time the average wind speed is calculated by dividing the total wind travel by the time period over which rotations are counted.

Propeller anemometer

Another increasingly popular type of anemometer is the propeller anemometer. This anemometer is simply a very small two or three-blade propeller, calibrated so that the revolutions per minute (rpm) corresponds to specific wind speeds.

By using two propellers held fixed perpendicular to each other, the wind speed and direction can be measured simultaneously with the help of vector analysis. By including a third propeller positioned vertically relative to the other two; vertical wind movements can also be measured. Propeller anemometers have excellent linearity.

Dynamic Pressure Anemometer

Dynamic pressure anemometer functions like a pitot tube and works by the following principle.

A tube facing the wind will experience an increase of pressure on its open end as compared to the closed end. The increase in pressure would be in proportion to the velocity of the wind normal to the open end of the tube. The difference between the dynamic pressure in the tube and the static pressure (measured perpendicular to flow) can be measured with a pressure transducer. The velocity of the wind is deduced from this pressure differential.

Reliability of Anemometers

It is the probability that the instrument will work to its defined limits. Besides the errors commonly associated with the specifications above, there are other errors that result during wind measurements of which you should be aware.

First, there is the already mentioned problem of calibration. This is very important because with improper calibration all, not just some, of the readings will be inaccurate. Besides, as mentioned above, inaccurate calibration has been found to be pervasive in all anemometers. Some need to be calibrated more often than others, but all need to be monitored periodically for calibration errors.

Reliability of anemometer data

Normally the wind speed is measured at a height of 10m. For precise assessment of the wind power potential it is imperative that the data is acquired exactly at the site and the proposed hub height of the WECS.

Tips for the location and the duration of the measurements

As mentioned earlier the instruments should ideally be placed at the intended location of the wind turbine and the wind speed should be measured at this location. If it is not possible to place the instruments there, it is recommended to take measurement at a nearby location with similar environment. Measurements should be taken in an open area. Wind instrument towers can be used to take measurements.

The longer the monitoring period, the more accurate will be the results. Ideally, one full year, and preferably several years, of wind measurement is recommended. This will provide data for all the seasons, and on interannual changes, should the period be more than one year. Because of cost or scheduling problems, it is sometimes not feasible to make lengthy measurements. Shorter-term measurements can still be of value, but they also will have greater limitations. The next section provides some tips for short-term monitoring.

Tips for Short Term Monitoring

The measurement period should be no less than three months, and preferably 6 to 12 months. Measurements should be targeted to spring or fall when winds can be measured from both southerly and northerly wind directions.

Because only part of the year will be measured, the data for the rest of the year must be estimated. This can be done by correlating the data obtained from a nearby meteorology center with the data from the site. This method works well for large areas of flat terrain; however in case of mountainous terrain this method is not very accurate.

The accuracy of the wind instruments should be checked at the end of the measurement program. Anemometers may lose accuracy with prolonged operation.

The Analysis of wind data

The wind data can be analyzed in many different ways. Discussed below are some of the methods of wind speed analysis:

Using the method of bins

We have briefly seen in the earlier section how the wind speed measurement is done by the method of bins.

This method provides information about how many hours during the measurement period the wind speed was within the wind speed bin range. The data recorded in this method of measurement constitutes a wind speed frequency distribution, which means, it shows the frequency of occurrence of wind speed within each range.

The wind speed frequency distribution can also be represented graphically on a histogram as follows.

With the available data on the various wind speeds over a period of time, the mean

wind speed for the whole measurement period can be calculated in the following manner.

V_m = {( V₁ * N₁ ) + ( V₂* N₂ ) + ……..( V_n * N_n ) } / (N₁ + N₂ ….+ N_n )

Where

V_m    mean wind speed in m/s

V_n     n^th wind speed bin in m/s

N_n    number of hours counted in the n^th bin range

This type of statistical distribution model is called a “Weibull distribution” The shape of the Weibull distribution is controlled by a variable called shape factor, k.. The mean wind and the value of k at a particular site together determine the usefulness of the site for wind energy utilization. Normally, k tends to increase with increasing mean wind speed and height above the ground level.

The site is considered best suited for effective energy utilization if the mean wind speed is high and the value of k is lower. The table below gives the typical values of k for various sites

6.3.4 Vertical velocity profile

We have seen in the earlier sections that the wind speed increases as it moves upwards from the ground. This variation in the speed with the height follows a logarithmic law. The following graph shows the logarithmic law up to an elevation of 100 meters.

According to the logarithmic law, the theoretical height at which the velocity profile commences is called the roughness length (Zo). This is actually the height that is measured but by international convention, it is termed as length. It is nothing but the measure of mean height of surface irregularities.

Usually all meteorological departments measure wind speed at a height of 10 meters above the ground level. When the terrain is smooth, the wind speed starts at ground level and increases rapidly up to 10 meters height. For rough terrain the wind speed does not gain momentum very fast up to a height of 10 meters. So in rough terrain, an adjustment has to be made to the height used to estimate where the wind profile begins. The height considered here is from a new plane above the true ground level. This displacement in the height is termed as the displacement length (d), as the whole wind speed profiled is displaced upwards.

This can be illustrated with the help of the following figure given below.

6.3.5 Power law and log law

Log law

The wind speed variation between the two elevations at a site of known surface roughness, given by Z_o is as follows,

V_Z / V_H = In ( Z / Z_O ) / In ( H / Z_O )

To simplify it further let us see this with an example.

Example

The wind speed ( V_H ) at a height ( H ) of 10 meters has been measured at 3.5 m/s . The surface is a smooth terrain and hence and roughness length (Z_O) is taken as 0.012. Find the wind speed at a height of 18 meters.

V_H = 3.5 m/s

H = 10 mtrs.

Z = 18 mtrs

Z_O = 0.012

V_Z = ?

V_Z / V_H = In ( Z / Z_O ) / In ( H / Z_O )

V_Z    = [ In ( 18 /0.012 ) / In ( 10 / 0.012 ) ] * 3.5

         = 1.087 * 3.5

         = 3.81 m/s

Thus with the above equation, we can find the wind speed at various altitudes with the given data.

Power law

This is an alternative to the log law that is often used for engineering purposes and it known as Sverdrup’s power law. This law uses the typical meteorological measurement height of 10 meters as the basis for calculations.

(V_Z / V₁₀ ) = ( Z / 10 ) ⁿ ……

With the known roughness length ( Z_O ) , the power law exponent can be derived using the following equation.

Z_O = 15.25 e ^{( -1 / n )}

The following table gives the power law exponent for various terrains. This has been extracted from the Australian standards AS1170.2.

6.4.1 Surface roughness

This refers to the condition of the earth’s surface. A rough surface is a surface which has obstructions in the form of buildings and trees .These obstructions interferes with the smooth flow of air. The wind speed reduces substantially near the ground due to this surface roughness. The speed of the wind depends on the height of different surfaces. The greater the height of obstructions, the greater is the height up to which the wind speed is slowed by the surface. However, the effect of the surface roughness is observed only up to a certain height, above which the effect does not exist.

6.4.2 Trees and buildings

Trees and buildings are the most common obstacles to wind in the vicinity of a wind turbine site. They slow down both upwind and downwind and increase the turbulence.

Following are the rules used while selecting the location for the wind turbine in view of the obstacles:

1. The wind turbine should be sited upwind at a distance of more than two times the height of the obstruction.

2. The wind turbine should be sited downwind at a minimum distance of 10 times, and preferably 20 times, the height of the obstruction.

3. If the wind turbine is located immediately downwind of the obstacle, then the wind turbine hub should be sited at least twice the height of the obstruction above ground.

The upwind and downwind directions can be defined as being aligned with the prevailing wind direction.

The width to height ratio of the building determines the influence on wind speed. When this ratio is 3 or more(very wide structures), decrease in the wind power is about 10% at a distance 20 times the building height downwind, whereas tall and narrow buildings(width to height ratio < 3) , reduce the wind power less than 10% at the distance of only 5 times the building height. Most residential structures, such as houses, barns and garages have a width to height ratio of 1 ore more so generally the abovementioned rules of thumb can be used to site a wind turbine.

6.4.3 Ridges

Ridges are very useful sites for installation of wind energy systems since the wind power increases with the height above the ground level. However, the flow of air around the ridge is quite complicated due to the shape of the ridge and the surrounding terrain.

The following figure shows the effect of shape of the ridges on the airflow around these ridges. As the air flows over the crest of the ridge the air layer gets squeezed and the velocity increases. The phenomenon is similar to that of a venturi effect. The air accelerates maximum when it flows perpendicular to the ridgeline.

The following table gives typical values of the increase in the speed as it flows over various barriers.

6.4.4 Cliffs

Cliffs are also barriers but generally are at least 10 times longer than their height. Cliffs force the air over them rather than around them. These also create turbulence in the airflow at the base. The following figure shows the effect of cliff on the airflow.

While selecting a site on the cliff, choose a location where the wind is perpendicular to the cliff face. Care should also be taken to locate the energy system above the turbulent zone.

6.4.5 Temperature inversion layer

During cold winter nights a thick layer of cold air might settle down just above the earth’s surface. This happens especially over flat terrain where trees are scanty or in valleys. This layer of dense cold air dampens the transfer of wind energy from air at higher altitudes to air close to the ground. Hence the turbulence is affected.

If a phenomenon like this occurs at the site of consideration for WECS, then the height of the hub should be more than 20m.

The reduction in power due to this layer can be calculated by comparing the data acquired at the site at different heights (say between 10 and 25 m) with the commercially available data.

The following figure illustrates the effect of temperature inversion layer on the wind turbines.

6.5.1 Effect and estimation of roughness length

In the earlier sections we have seen what is roughness length and its derivation. For selecting a site in the region with sparsely spread vegetation, the roughness elements may be considered to be uniformly distributed. The following steps can be followed while estimating the roughness length for such regions:

• Measure and estimate the average height and cross sectional area of the potential roughness elements in the region
• Estimate the average horizontal area available between each of these potential roughness elements
• Estimate the porosity of the potential roughness element. Porosity is the opening in the roughness elements through which air can flow. The porosity for different objects has been tabulated below:
  

  Appearance of potential roughness element	Porosity
  Solid	0
  Very dense	0.35 or less
  Dense	0.35 to 0.5
  Open	0.5 or more

  Using the above data in the equation below, we can get Z_O

  Z_O = { 0.5 ( h * A₁ ) * ( 1 – P ) } / A_H ….

  Where
  ZO	roughness element in m
  A1	cross section area of the roughness element in m2
  h	average height of the roughness element in m.
  P	porosity of the roughness element
  AH	average horizontal area in m2

6.5.2 The effects of obstructions

The local terrain at the site should be analyzed as the trees or grass or vegetation present can destructure the wind. In such cases the hub height of the rotor needs to be higher , which will add to the cost. Ideally the site should be in an open area. It should be free from any obstacles, which could cause turbulence, increased stress and reduce the energy output.

6.5.3 Mean wind speed and distribution

The amount of power in the wind is a function of the wind speed cubed. A small fluctuation in the wind speed can cause a considerable change in the wind power. Since the wind speed keeps changing constantly, the economics of the wind energy system will highly dependant on the annual distribution of wind speed. Areas with the annual wind speed of 6.5 m/s and above at 30 m height are considered economically viable for wind energy production. The usefulness of a site for energy production can also be determined by using energy pattern factor (EPF). It can be derived by the wind data obtained from the method of bins.

6.5.4 The correction of wind speed data for the hub height

Wind speed generally increases with height above the ground. Most weather stations measure the wind at a height of 10 meters (33 feet), whereas the hub height of new utility wind turbines of 500 to 750 kW capacities is typically 50 meters or more. To estimate the wind speed available for power generation, it is necessary to correct the measured data to the appropriate height. Remember that changing the hub height can have a large impact on wind speeds, and thus on power output.

Correct for air density if necessary. The amount of power in the wind also depends on the air density. Air density, in turn, is a function of temperature and height above sea level. The turbine manufacturers normally provide rating data based on the density at “standard atmosphere” (1.225 kg/m³), that is, air at sea level and at 15°C. The wind speed at the potential site location can be corrected to an “effective wind speed” that corresponds to the density assumed in manufacturers’ rating data. Monthly average temperature data can be obtained from the local weather service.

6.7.1 Equipment needed for on site assessment

The following equipment is generally needed to carry out an on-site wind assessment.

• A telescopic, guyed tower. (10 to 30 m towers can be used according to the capacity of the wind turbine to be installed)
• One anemometer
• One data logger to store the wind speed measurements
• One wind direction sensor

8.1.1 Stand-alone power system

The following figure shows a typical configuration of this system.

As shown in the block diagram above this type of wind energy system configuration consists of wind turbine, generator, inverter and the batteries. The energy generated by the wind turbine is converted into DC power with the help of a generator. The excess energy generated by the wind turbine is collected by the batteries during the periods when the wind is blowing hard. This excess energy is used in case of no wind or slow wind speeds. The energy in the batteries is stored in the form of DC power. Connecting an inverter to the DC power source gives the AC power, which can be utilized for AC loads.

In order to ensure a continuous energy supply and its availability it is necessary to oversize the wind turbine to meet the worst conditions. This type of configuration typically comes in the range of 4 to 20 kWh per day of energy generation. Typical applications of this configuration are stand-alone lighting systems, remote telecommunications and navigational aid.

8.1.2 Inverter system with auto/manual switching from inverter to back-up genset

The following figure shows a typical system configuration.

As seen in the above figure, this is similar to a simple stand-alone system with an addition of a back-up generator. This back up generator provides power to a set of batteries during poor wind conditions. The DC power thus obtained from the batteries during such lean periods is then converted into AC power with the help of inverter, since most of the electrical appliances use energy in the form of AC power. The generator used here is a small diesel generator.

Along with a wind turbine, the energy conversion can also be done through other renewable energy sources as a stand-by arrangement during lean periods.

This type of configuration can produce 4 to 20 kWh of energy per day. The wind turbine operates at variable speeds. The ratio of rotor diameter to the rated power used for this type of configuration is 1.5 m: 300 watts. Typical applications of this configuration are stand-alone lighting systems, medium sized homes and telecommunication.

8.1.3 Inverter system running parallel with the back-up genset

A typical block diagram for this configuration is similar to the configuration explained in section 8.1.2 above. The only difference here is that the primary source of energy in this case may or may not be the wind turbines. The wind turbines act as a supplementary resource. The generator runs full time and the excess energy requirement is met by the wind turbines. This configuration can also be used in a reverse order, i.e. the wind turbine can be the primary source and the generator can act as a supplement. This configuration requires a short-term battery storage capacity as the generator is run most of the time. Such systems often generate excess energy that can be utilized for loads such as water heating, refrigeration etc.

Another variation of this configuration is to use a wind turbine in tandem with the local electricity grid. An interactive inverter is used in such systems. In normal conditions the wind turbine acts as the primary source of power and meets the demand through the inverter and the excess energy generated is sent to the grid. During poor wind conditions, when the wind turbines cannot meet the power demand, the power is supplemented from the grid.

A typical configuration connected to the grid is explained with the following block diagram.

This configuration is capable of providing energy output ranging from 70 to 350 kWh per day. Typical applications of this configuration are farms and large homes.

8.1.4 Direct power to the local electricity grid

This system basically provides power directly to the local electricity grid. This approach is used where the diesel generators are the primary source of power in normal conditions. In order to reduce the diesel consumption, wind turbines are operated along with the generators. Say for example, there are 10 gensets providing power to a particular load. The power output here can be modulated using the wind energy system wherein few of the gensets can be switched on and off according the fluctuating power demand. These systems do not require larger battery storage capacities.

Such systems are available in the range of 200 to 1000 kWh per day. These systems are connected to the local electricity grid.

8.2.1 Momentum theory

Before we addressing the aerodynamic theories, let us study the terms used.

Actuator disk

We have seen in earlier sections that the swept area is the area swept by the turbine blades through which the wind passes, intercepting the blades. This area forms a disk shape while being swept. A set of rotating blades form a plane called the actuator disk.

Stream tube

When the air flows through the actuator disk it forms a moving column also known as a stream tube. In order to have the same mass flow rate everywhere in the stream tube, it is necessary to change the cross section area of the stream tube.

Bernoulli’s equation

It states that, under steady state condition, the total energy in the airflow comprising of the kinetic energy, static energy and the gravitational energy remains constant as long as no work is being done.

In a wind turbine, there is no change in the gravitational and the potential energy in the airflow. Due to the energy absorbed by the rotor the kinetic energy and the momentum of air will change, resulting in a change in pressure across the rotor from upstream to downstream. In other words, as the energy is extracted from the moving air, its momentum and velocity decreases.

The following figure shows the stream tube and the change in pressure and velocity as the wind passes through the actuator disk.

Change in the momentum can be given by the equation,

T = ( ρ * A₁ * V₁² ) – ( ρ * A₁ * V₂² )

Where

T   change in the momentum or the thrust

ρ   density of air

A    area of the stream tube at the rotor.

V₁    velocity upstream of the actuator disk

V₂    velocity downstream of the actuator disk.

8.2.2 General momentum theory

An important extension of the axial momentum theory is the introduction of an angular momentum balance equation for the slipstream, to take into account the rotation imparted to the air by the actuator disk.

The theory finds that the optimal load on the actuator disc is constant, and that there is a small efficiency loss for lightly loaded blades. The efficiency increases with rotor diameter, and decreases with increasing disc loading. The rotor should be as large and as lightly loaded as possible, because it is more efficient to move a given mass of fluid through a large stream tube at low speed, rather than through a small stream tube at high speed.

This is one of the reasons why marine propellers are so large and operate at such a slow speed.

8.2.3 Aerodynamic forces acting on the turbine blades

The most common type of rotor blade is the simple curved plate. Wind turbines use airfoil shaped blades as used for aircraft wings. The following figure shows a typical cross-section of an airfoil shaped blade.

Due to the curvature of the blade, the air flowing along the upper portion of the plate takes a longer path and the velocity of airflow is higher here. Because of this the pressure on the top of the blade is lower. While at the lower part of the blade, due to the slower movement of airflow, the pressure is higher. This difference in pressure causes the lift force.

Lift and drag

Objects in the path of a stream of air experience a downwind force called drag. This force always acts along the direction of the relative airflow. Due to the pressure difference across the blade, a lift force is produced at right angles to the relative airflow. This is the force primarily used to drive wind turbines. The rotors on modern wind turbines have very high tip speeds (tip speed ratio is discussed below) for the rotor blades, usually around 75 m/s (270 km/h, 164 mph). In order to obtain high efficiency, it is therefore essential to use airfoil shaped rotor blades with a very high lift to drag ratio, i.e. rotor blades which provide a lot of lift with as little drag as possible. This is particularly necessary in the section of the blade near the tip, where the speed relative to the air is much higher than close to the centre of the rotor. The following figures show the lift and drag forces produced.

Angle of attack

The angle of attack is the angle at which the wind strikes the blade. The value of the lift and drag forces largely depends on the angle of attack and the wind speed.

The maximum values of lift to drag ratio of a blade and the angle of attack for this ratio is analyzed to determine the optimum operating condition of the blades. The higher the lift to drag ratio, the better the performance of the turbine blades.

As the angle of attack increases, the pressure on the blade reduces, producing a higher lift and steeper pressure gradient.

Lift and drag co-efficient

The measure of lift and drag forces acting on an airfoil is termed as a lift or a drag co-efficient. This defined as the ratio of the lift force to the fore of the wind on the blade.

C_L = L / (0.5 * ρ * V² * A )

Where

C_L   lift co-efficient

L     lift force

ρ     air density in kg/m³

V     air velocity in m/s

A     projected blade area in m²

Similarly, the drag co-efficient can be given by the following equation.

C_D = D/ ( 0.5 * ρ * V² * A )

The values of lift and drag coefficients depends upon the angle of attack and the cross section of the blade used. These coefficients are measured in a wind tunnel and recorded .

Power from airfoils

The airflow around the airfoils is complex. As the air flows over a rotating airfoil blade, the speed and the direction of the airflow felt relative to the blade’s surface is a sum of two airflows. This resultant velocity is called the relative velocity.

When the blade is rotating at right angles to the true wind velocity the relative velocity generated will be higher than the true velocity. This will vary from point to point across the length of the airfoil.

Since the wind velocity at any point across the blade depends on the speed of rotation of the blades and the distance form the center of rotation, the relative velocity will also vary in magnitude. The blade speed increases towards the tip and so does the relative velocity. In order to keep the angle of attack, lift and drag constant across the length of the blade, it is necessary to vary the angle of the blade relative to the plane of the rotation. This angle is called the pitch angle.

The power extracted from the airfoils is the product of the resultant force in the direction of airfoil or blade motion and the blade velocity.

P = F_b * V_b

Where

P     power extracted

F_b    resultant force to rotate the blade

V_b   blade velocity

The resultant force to rotate the blade can be derived by the following formula.

F_b = (L * Sinθ) – (D * cos θ )

Where

L   lift force

D   drag force

θ   flow angle between the relative air velocity and the plane of rotation of the blade

Tip speed ratio

The tip speed ratio is a dimensionless value. This is the ratio of the highest occurring circumferential velocity at the rotor, i.e. at the rotor tip, to a certain wind speed, for example the wind speed concerning to the rated output. In simple terms, it is the measure of how fast the turbine is turning compared to the wind speed.

Wake rotation

When the air flows between the blades of the wind turbine, the blades start to move towards the left as illustrated in the figure below. At the same time, the airflow is diverted towards the right. This results in the rotation of wake. During this phenomenon, there is a loss in the kinetic energy of the wind resulting in reduction in the power co-efficient and the output co-efficient. This effect is quite small for blades with higher tip speed ratios.

Blade tip losses

When the wind flows from the upwind to the downwind side across the airfoil, we have seen that there is a pressure gradient across its length. Due to this, there is some amount of air leakage from the tips of the blades. This is called blade tip loss as it reduces the power for the wind.

The blade tip losses increase if the tip speed ratio is less and the chord length is more.

Yaw Control

The wind machine can achieve maximum efficiency, when the blades face into the wind. That’s when the area of the windstreams swept by the wind turbine is maximum. So in order to achieve this, a control arrangement in which a motor rotates the turbine slowly about the vertical axis, so as to face the blade into the wind, is attached.

In smaller turbines the yaw control is taken care by a tail vane. If the prevailing wind is in only one direction then a yaw-fixed turbine is deployed.

8.4.1 Main components of a wind turbine

The following figure is a typical large wind turbine showing all its working components.

To simplify the explanation of the working of each important component, the following figure has been simplified to show the cross-section of a wind turbine.

Based on their operation the wind turbines may be classified into fixed speed and variable speed turbines.

Fixed speed turbines

The traditional electricity generators produce electricity at the grid frequency of 50Hz or 60Hz only when it rotates at an exactly constant (synchronous generator) speed, or nearly constant (induction generator) speed. Matching the rotor to this requires an additional mechanical system like a gearbox. For instance, a gearbox with a ratio of 50:1 is required with a generator speed of 1800 rpm, and a rotor shaft at 45 rpm. This is discussed in greater detail later in the chapter. If the gearbox has only one speed ratio, then the designer has to design for one wind speed probably, the ‘most probable wind speed’, based on the statistics on that particular location. If there are two speed ratios, it is possible to reduce the rotor speed to match low wind speeds too. When the wind is at speeds other than the design (rated) wind speed, generation can still occur, but the efficiency of energy would be less.

Variable speed turbines

Constant tip-speed ratio TSR gives the best performance. To achieve a constant TSR the rotor speed should change accordingly, when the wind speed changes. This is possible, in two main ways. a) The wind turbine is entirely ‘decoupled’ from the grid by either generating AC, with a synchronous generator, rectifying all the power to direct current (DC), inverting (converting) the DC to standard 50 HZ or 60 Hz AC for grid connection or by a ‘doubly fed’ induction generator. Such generators allow variable speed rotation by controlling and changing the currents in the rotor using power electronics.

8.4.2 Rotor

Rotor forms one of the key components in wind turbines. A wind turbine’s rotor may have one, two or more number of blades.

Rotor Design for Horizontal Axis Machines

In general the rotor is mounted on a horizontal (or close to horizontal) shaft which is connected to either a generator (in most of the cases) or some other mechanical device such as a water pump or a heat generator. And this whole assembly is mounted on a tower so that there is no interference between the blade and the ground. From the performance point of view taller tower is preferred as the wind speed increases with height. A turbine can have either a single rotor or multiple rotors. They can be constructed with cups, paddles or sails made out of metal or wood and can be mounted upwind or down wind of the support tower.

As we saw earlier the horizontal axis rotors can be either lift or drag devices. Lift devices are preferred over the drag devices as relatively more output power can be developed by the former. In fact a lift device can move faster than the wind speed, while a drag device cannot. Thus a lifting surface can obtain a higher top to wind speeds, which results in a higher output to weight ratio and lower cost of power generation.

Lift devices use slender blades with an aerofoil section that generates aerodynamic lift when placed in an air current, whereas drag devices are those that rely on drag forces to extract energy from the wind. Small lift rotors can spin at around 300 to 450 rpm with the blade tip speeds of several hundred kilometres per hour, which could be as much as 10 times faster than the wind speed. Very large lift rotors turn at roughly around 20 to 50 rpm but the blade tip speeds could be around 250 kilometers per hour.

Drag devices on the other hand turn more slowly than the wind and hence they work best in less windy inland locations. The ratio of the power extracted by a lift device to that of a drag device is usually greater than 3:1 of the same swept area.

Often the drag devices generate high torques and hence are ideal for work like water pumping, etc.

The efficiency would be maximum when the angle of incidence approaches the angle of attack prior to stalling. As the rotor’s tip travels faster than the points nearer to the axis, the angle at which the wind meets the plane of rotation decreases linearly with the radius. Hence the blades of efficient wind rotors are twisted so that the angle of incidence is closer to that of attack, at all radii.

Rotor Design for Vertical Axis Machines

The VAWT rotor consists of a propeller and generator inside a duct which flares outwards at the back. The air is accelerated inside the duct and the propeller spins quickly. The main virtue of the VAWT rotor is that it can operate in a wide range of winds. The VAWT can operate in a narrow range of TSR under all conditions, so its rotational speed is closely proportional to the wind speed.

The simplest type of wind generator is the savonius rotor. It consists of two vertical curved airfoils mounted between two disks. They are positioned in such a way that they have almost an S-shaped cross-section. These two semi-circular drums are mounted on a vertical axis perpendicular to the wind direction with a gap at the axis between the two drums. Whenever wind blows horizontally through this device, the disk turns driving a generator. This has become popular as it requires a relatively low velocity wind for operation.

The aspect ratio (height to diameter) of the machine can be varied, but is generally less than 3:1.

The main advantages of this type are:

1. the device itself doesn’t have to be turned into the wind, so no set of slip rings or yaw mechanism is necessary
2. the heavy generator is on the ground
3. the airfoils can be made from a pipe section and there is only one moving part
4. it has low cut-in wind speed (the speed required to generate power). It produces power at wind speeds as low as 8kmph, whereas a typical HAWT requires about 16kmph. In fact larger HAWT needs more speed for effective operation. This also means that the Savonius machines are useful most of the time and the cost of storage equipment can be considerably lowered
5. Simpler structure, hence easy to manufacture and commission

The disadvantages of this type are:

1. its low efficiency the wind profile can’t be reduced in high winds

Darrieus wind turbines look like a wire supported eggbeater. It consists of thin vertical airfoils which meet at their tips and bend out at the middle. It has two or three thin curved (egg beater) blades with airfoil cross-section and constant length (fig.).The force in the blade is pure tension as both ends of blades are attached to a vertical shaft. The blades thus are lighter as compared to HAWT. When the airfoil blades rotate they provide a torque to the central shaft, which in turn is transferred to the generator at the base and thus the power is generated.

Darrieus machines are lift devices. They have low starting torques and high TSRs, which explains their high output for a given rotor cost and weight. The following rotor configurations have been worked out: Y, triangle, square and Phi type. The main advantages of this type are:

1. their operation doesn’t depend on the direction of the wind
2. the device itself doesn’t have to be turned into the wind, so no set of slip rings or yaw mechanism is required
3. the heavy generator can be mounted on the ground. No need for tower structures and complex erection procedures can be avoided
4. easier serviceability

The disadvantages of this type include

1. low efficiency
2. need for a starter
3. huge side loads at the top which usually has to be braced with wires
4. availability of low velocity wind as compared to the conventional tower mounted systems
5. need for a special high torque braking system

8.4.3 Blades

Blades are like the wings of an aircraft, housed together in the rotor. These blades may be made out of wood, metal or composites of several materials including glass reinforced plastics. The blades are attached to the hub at their root (Figure 8.14a), which in turn is attached to the main shaft. The portion of the wind turbine that collects energy from the wind is called the rotor. A wind turbine usually consists of two or more blades that rotate about an axis (horizontal or vertical) at a rate determined by the wind speed and the shape of the blades. The blades are slightly twisted to reduce the tendency for the rotor to stall. Though the blades can have the same chord edge lengths on both the sides, they are made narrower at the tip than at the root in order to get better performance (Figure 8.14b).

At the tip of the blades the velocity of the blade could be up to six times the wind velocity. This means that the blades are designed to be flat at a small angle with the plane of rotation and more or less at right angle to the wind direction. This is done so that the effective wind properly approaches from ahead of the leading edge. At the other parts of the blade the airfoil is at a greater angle to the plane of rotation.

Although the wind power of the turbine is proportional to the square of the swept diameter of the blade, the mass and the drag also increase proportionately. This puts a practical limit to the size of the blades.

The blades are expected to be as light as possible still have adequate strength to withstand the vagaries of climate. Hence material selection plays an important role.

The smaller blades may be made out of laminated wood, possibly covered with a thin skin of aluminium. Rotors up to 34m diameter may be fabricated with FRP (fibre reinforced plastic). The large ones need to be made out of steel to provide adequate strength. The following figure shows a typical cross-section of a blade.

The following are the advantages and the disadvantages of different materials used for blade construction:

• Blades made out of wood and laminates:

  Only wooden blades are used in turbines of unto 10KW, whereas laminated ones can be used for wind turbines up to several hundred kW and up to 40 m diameter.

  Advantages:

  • Light and strong
  • Easily available
  • Easy to carve out any shape
  • Fatigue resistance
  • Low shrinkage & warpage

  Disadvantages:

  • Limitation to the size of the blades
  • High maintenance

• Aluminium:

  Used in small turbines.

  Advantages:

  • Light and strong

  Disadvantages:

  • Expensive
  • Can cause interference to communication signals
  • Fatigue prone

• Steel:

  Used in large turbines. Being used lesser and lesser these days.

  Advantages:

  • Excellent strength

  Disadvantages

  • Requires heavy structural support
  • Reflects transmission signals

• Fibre glass reinforced plastic:

  Used in modern turbines.

  Advantages:

  • Fatigue resistance
  • Low cost
  • Light and strong

  Disadvantages

  • Toxic chemicals used in manufacturing

The turbines reach their maximum, rated, power in strong winds. Therefore the blade has to be designed and adjusted to become inefficient in wind speeds greater than the rated wind speed (usually about 12 metres/second). This value, however, depends on the wind conditions of the site. Therefore, each turbine has to be designed and tuned to the expected wind conditions of its site.

While designing the blade all these factors need to be considered as they largely affect the performance of the wind energy system.

Apart from those, a designer also needs to take care of the following conditions while designing the blade.

• Cyclic stresses causing fatigue
• Variations in temperature, humidity, wind velocity
• Abrasion from air borne dust particles
• Bird impacts

Working principle of Turbine Blades

As discussed earlier the turbine blades are designed based on either of the two principles, the principle of drag or lift.

Drag design

In the drag design, the blades are pushed out of the way by the wind. These types of turbines are primarily used for pumping, grinding or sawing work, as they produce a high torque at slower rotational speed. They can be particularly useful for pumping of water from a deep well where the initial torque required is high.

Lift design

The lift blade design is based on the aerodynamic principles used for the design of the airplane wings. The blade is very similar to an airfoil, or wing. The wing profile makes the air pass over the top of the wing faster than it passes under the wing. This wind speed and pressure differential between the upper and lower blade surfaces, causes the blade to lift. This lift is translated into rotational motion by the rotor. The rotational speeds in the lift design are much higher than in the drag design. For this reason lift powered blades are well suited for the purpose of electricity generation.

The number of blades that make up a rotor and the total area they cover affect wind turbine performance. For a lift-type rotor to function effectively, the wind must flow smoothly over the blades.

Hub

The hub is the component on which the blades are cantilevered. The main function of the hub is to attach the blades to the drive shaft of the rotor. The blades can either be fixed or have a variable pitch.

Most small wind energy systems use fixed pitch blades to reduce the cost and maintenance. However, modern large wind turbines use variable pitch blades for better speed control and optimization of power output.

8.4.4 Turntable foundation for HAWT

It is the component that houses the rotor, main drive shaft, gearbox, brake and generator. It can rotate along the vertical axis in order to allow the rotor to face the wind. It is also known as the nacelle foundation (Figure 8.14). The enclosure that surrounds the turntable protects the generator and transmission from weather.

Tower

The tower on which a wind turbine is mounted is not just a support structure. It also raises the wind turbine so that its blades safely clear the ground and so it can reach the stronger winds at higher elevations. Maximum tower height is optional in most cases, except where zoning restrictions apply. Different types of towers are available. The decision of what height tower to use will be based on the cost versus the increase in energy production resulting from the height. A detailed discussion on the tower types and the selection is found in Chapter 9.

Transmission

Mechanical transmission

Mechanical transmission is the transmission of energy from the rotor in the form of its torque and speed to the generator. Direct drives are used at times. As the generators need higher speeds than that generated by the turbines to effectively generate AC-type electricity, often direct drives are not used. Mechanical systems involving fixed ratio gears, belts and chains, singly or in combination or hydraulic systems involving fluid pumps and motors are used to step up the angular velocity. Most common of the above mentioned is gearbox.

Gear Box

A gear box transmission is generally used to step up the angular velocity fed to the generator. This is essential as there is a big difference between the speed of the turbine blades and the speed at which the generators generate electricity efficiently.

Generators typically require 1,800 revolutions per minute (rpm). Whereas the number of revolutions per minute (rpm) of a wind turbine rotor can range between 40 rpm and 400 rpm, depending on the model and the wind speed. Even at speeds as low as 40 rpm, electricity needs to be generated. For example large turbines have blades with diameters of up to 60m. If these are to turn with speeds much more than 40rpm, the velocities of the tips, and subsequently the stresses in the blades, become very high. If the speeds of the blades were to be kept low, a direct drive wind turbine would require a generator with 800 poles to generate electricity at 50Hz. As a result, most wind turbines require a gearbox transmission to increase the rotation of the generator to the speeds necessary for efficient AC-type electricity production. But DC-type wind turbines do not use transmissions. These are known as direct drive systems, which have a direct link between the rotor and the generator. Without a transmission, wind turbine complexity and maintenance requirements are reduced.

The speed of the blades is limited by efficiency and also by safety factors governed by the material properties of the turbine and supporting structure.

Typically the gear ratio in a gearbox used in a wind turbine is of the order of 45:1. This means the generator spins 50 times faster than the blades that rotate at between about 40 rpm. The rotational speed of the high-speed shaft is dependent on the number of poles, and type, of the generator, although it usually rotates at about 1800 rpm. The shaft connecting the blades and the gearbox is tubular, and contains hydraulics, mechanical brake, aerodynamic braking system, and other actuators on the blades.

Mechanical brake

A disk brake is usually located between the gearbox and the generator. This is the most suited location as the torque at this point is lower than before the gearbox. This brake is also used to lock the mechanical components of the turbine during servicing and maintenance. This brake is activated in the event of failure of aerodynamic brake.

Aerodynamic brake

Aerodynamic brake is typically a spring operated system, which is employed to turn the blades or the blade tips in order to resist the aerodynamic forces which rotate the blades. They are generally spring-operated so that they can work under any conditions. The brake is activated in the event of other components of the system failing, for example hydraulic pressure drop. The aerodynamic brake is the preferred brake for stopping the turbine as the work is done by the air resulting in less stress on the working components as compared to a mechanical brake. An aerodynamic brake can stop a turbine in a few revolutions. However, once the turbine has stopped, a mechanical brake must be engaged.

Electrical transmission

The power from the generator has to be transferred to the grid or any storage device. This transmission takes place through the cable connected to the generator and a bank of batteries or an inverter. This transmission cable is usually run along the tower height through the turntable. When the turntable rotates according to the wind direction, the cable can progressively twist and eventually break. Hence Slip rings are used to transfer the power past the turntable.

8.4.7 Electrical Generator

The turbines convert the wind energy to rotary mechanical energy and this rotating motion is converted into electricity by the electrical generators. An electrical generator consists of coils of wire rotated in a magnetic field to produce electricity. The inner rotary member called the Rotor is mounted on bearings fixed to the stationary member called Stator. Both the stator and the rotor are made of magnetic iron of high permeability. They have conductors embedded in slots distributed on the core surface.

The mechanical energy, therefore, is supplied to the generator through the rotor, and electrical energy is gleaned from the stator windings. The stator is constructed by laminating many thin, insulated, ferrous sheets. This reduces the likelihood of eddy currents, and hence reduces electrical inefficiency.

The generators could be used to produce either DC or AC current. DC generators are normally used in battery charging applications and for operating DC appliances and machinery. They also can be used to produce AC electricity with the use of an inverter, which converts DC to AC. The choice of the electrical generator is decided based on the following factors:

1. the basis of operation of the wind machine – constant tip or constant tip speed ratio
2. the wind power rating of the turbine and
3. the type of load connection – battery connected or grid connected
4. length of the wind turbine’s blades as longer blades capture more energy

Small generators are used for wind turbine power ratings of up to 1KW. These small generators could be permanent magnet, DC generators.

Medium generators are used for wind turbine power ratings of 1KW to 50KW. These medium generators could be either permanent magnet, DC generators or induction generators.

Large generators are used for wind turbine power ratings of above 50KW. These large generators could be synchronous generators or induction generators

Schemes for electrical generation

Control of turbine speed by flaps etc. is costly and inefficient thus rotational frequency is best controlled by varying the electrical load on the turbine

Several electrical generation schemes are possible. Following are the most commonly used schemes:

1. Constant Speed Constant Frequency systems (CSCF)
2. Variable Speed Constant Frequency systems (VSCF)
3. Variable Speed Variable Frequency systems (VSVF)

Let’s look at how these systems work.

1. Constant Speed Constant Frequency systems (CSCF)
Wind turbine efficiency is greatest if rotational frequency varies with the wind velocity in order to maintain a constant tip speed ratio. However, for most wind turbines the generator must operate at a constant or nearly constant frequency in order to supply grid-compatible electricity

Constant speed drives are used for large generators connected to the grid where constant frequency operation is essential. Either of the following two types of generators can be used for this purpose.

a) Synchronous generators

This is the most established machine as most of the electrical power consumed by the World is generated by this type. The basic characteristic of a synchronous generator is that, when it is driven by the wind rotor, the output voltage varies in proportion to the speed of the rotor. Altering the torque applied to the rotor does not affect the speed of the rotor, but affects the amount of electrical power supplied.

So for such machines the constant speed is very much essential and only minor fluctuations about 1% for short durations probably less than a second could be allowed.

The synchronous speed is calculated by a simple formula N=120xf/p,

Where, N is the speed

f is the frequency of the alternating current (line frequency)
p is the number of poles for which the stator winding is made

Synchronisation of wind driven generator with power grid also would pose problems at gusty winds.

The rotor of a synchronous generator is either a permanent, or an electromagnet. Permanent magnet, synchronous generators are rarely used for two primary reasons. The life of a permanent magnet is greatly reduced if they are subjected to a strong magnetic field (i.e. they are demagnetized). Large rare earth magnets (used for their high field strength to size ratio) are expensive, although the cost is falling. These components wear, and therefore require maintenance. The advantages of these machines include least loss due to excitation current and power, torque characteristics are well matched with the rotor characteristics. The disadvantages include higher relative cost, difficulty in controlling the generator output quality and higher starting torque. An induction or Asynchronous generator eliminates these disadvantages.

b) Asynchronous or Induction generators

Although the concept of asynchronous operation is not unique to wind turbines, they are mainly used as motors, not generators. The stator of the induction machine is connected to the power grid and the rotor is driven above the synchronous speed. The induction machine becomes a generator and delivers constant line frequency power to the grid.

It consists of a rotating constant frequency magnetic field that is created in the stator by the power from the grid. The rotor consists of copper or aluminium bars arranged in a cylindrical fashion, and connected electrically by rings at the ends. A rotating magnetic field, created by the stator, induces currents in the bars. Due to the low resistance of the rotor, a large magnetic field is generated.

The output of the wind driven induction generator is determined by the operating speed.

Advantages

• High reliability
• low cost
• has overload capacity
• the rotor need not be changed if the number of poles is changed. This means that a generator may be designed as two generators in one, by prudently arranging the electromagnets of the stator. The same turbine may spin slowly, or quickly, depending on the wind velocity

2. Variable speed constant frequency scheme (VSCF)

These are typical of small wind generators used in autonomous applications. The variable speed operation of the WECS yield higher output for both high and low speed winds. This results in higher annual energy yield per rated installed capacity. Both HAWT and VAWT have better performance under variable speed operations.

Constant frequency output is obtained either through AC-DC_AC link or through AC commutation generator.

AC-DC-AC link uses a high power thyristor. The AC output of the 3 phase alternator is rectified using a bridge rectifier and then converted back to AC using line commutated inverters.

The AC commutation generator also known as Scherbius system employs two polyphase windings in the stator and a commutator winding on the rotor. Cost and additional maintenance are the disadvantages in using these types of devices.

3. Variable speed Variable frequency scheme (VSVF)

These systems are gaining importance for stand alone wind power applications. The varying output voltage can be converted to constant DC using choppers or controlled rectifiers on constant AC with the help of force-commutated inverters.

8.4.8 Inverters

Inverters are attached to the WECS when the energy is first converted into DC type electricity through the DC generator or the energy is stored in batteries and it has to be converted into AC type electrical energy.

An inverter is a system that converts DC into AC. It takes the DC output from the battery storage system and through a conversion process changes the waveform to alternating current at the desired voltage and frequency.

Depending on the overall AC requirements and uses of the wind system, either, none, some or all of the power is inverted to AC. Since all inverters use up some power to operate themselves, only those appliances which operate solely on AC should operate through the inverter.

There are three basic categories of inverters available and each differs widely in cost and efficiency ratings. The three categories are:

• Rotary
• Vibratory
• Solid State

Rotary-type Inverters

This inverter system utilizes a DC motor to operate a governed AC generator to produce the desired AC power. The system generally exhibits excellent waveform output and voltage, but its efficiency may run as low as 50%, using considerable power to operate the inverter. Furthermore, since the rotary inverter is a rotating device with brushes, it requires regular maintenance and repair.

Vibrator-type Inverters

These inverters are also electromechanical devices but contain less moving parts than the rotary inverter. The vibrator assembly is driven by a DC power source, which commutates the waveform through a transformer-filter network into an AC signal at constant frequency. The efficiencies of these types run up to approximately 75%. These inverters generally require little maintenance. The vibrator assembly is replaced once every 1,000 to 1,500 hours. Vibrator-type inverters are a good choice for low power applications and are relatively inexpensive, compared to the solid-state inverters.

Solid state inverters

Solid-state inverters are generally either constant frequency output inverters or synchronous inverters. Constant frequency output inverters provide a nominal 120V, 50 Hz signal when used independently of the utility grid. A synchronous inverter matches the voltage level, frequency and power output to the utility lines to provide AC output. A synchronous inverter allows the wind plant to interface with the utility line to sell excess electricity back to the utility company. Because it is interfaced with the utility grid, there

is no need for a storage system.

There are two general types of solid-state inverters, one of which produces a square (or semi-square) wave output, and the other a sine wave. The square (or semi-square) wave units are less expensive and in many applications work very well. However, they should not be used to power devices where a good quality sine wave is required (stereos, television, etc.)

Modern, high quality sine wave units are generally more expensive but will be very sensitive to sine wave characteristics where a high quality AC signal is essential to performance of the load.

Though the true-sine-wave, solid-state inverter is generally more expensive than the other two types, it has the highest efficiency rating (up to 90%) when operated at rated capacity, and is virtually maintenance-free.

Various types of inverters are explained in detail in Chapter 2 of this manual.

8.6.1 Integration of wind turbines on weak rural networks

Lower prices for wind-generated electricity tend to concentrate wind farm developments at high wind speed sites, which, in many regions, are areas with low population density, remote from a strong electrical connection point. The high capital costs of reinforcing the network, together with the difficulty of obtaining planning permission for new overhead lines, encourage maximum use of the existing network infrastructure.

Long medium voltage lines to distributed loads, which are predominantly single phase, characterize rural electricity networks in most countries. This leads to a low fault level at the point of connection. These conditions, combined with the high wind turbulence intensity that is often associated with upland terrain, provide the least favourable circumstances for the quality of output power from wind turbines.

Problems encountered where the penetration of wind-generated power in such a rural network is significant include:

• Surges in reactive power as each wind turbine comes on-line until the turbine is synchronized with the grid. This can lead to a voltage drop of up to 5% on a 33kV line at each start-up
• Voltage flicker, which can be significant due to synchronizations, wind gusting and tower shadow
• Harmonic factors, which can be 3% for a 33kV grid

The allowable peak output, harmonics, flicker, power factor and switching operations are all clearly stated in relevant engineering recommendations. Extremely low fault levels typical of wind farm grid connections increase the necessity of predicting any potential disturbances to the network. Methods to prevent possible disturbances such as staggering turbine “start ups” and the use of fast acting voltage control equipment may need to be investigated as the penetration of wind generated electricity into weak networks increases.

9.2.1 Site Selection

At a height of around three fourths a kilometre, from the ground level, the wind is hardly influenced by the surface of the earth. But wind flowing over the surface of the earth is slowed by the friction of the ground. Objects such as trees and houses contribute to the turbulence. This reduces the energy a turbine can extract from the wind and also stresses the turbine components. The higher a turbine is placed the more power it generates. A wind turbine should be at least 10 meters above any object.

There shouldn’t be any blowing dust or icing problems as these would potentially damage the blades and the rest of the mechanisms.

9.2.2 Planning approval

Irrespective of the owner and the manufacturer of a wind energy system, its installation requires prior approvals from the planning and building Department of the local governing body.

The following are factors that are generally considered in obtaining an approval for installation of a wind energy system:

• Location of the wind energy system
• Height of the tower
• Proximity to airports to take care of the low flying zones
• Potential interference with the communication signals
• Environmental aspects such as noise and impact on the flora and fauna

Once an approval for installation has been obtained, the preparatory work for installation could be started.

9.3 Installation

For the purpose of ease in understanding, let us segregate the installation requirements into two categories:

• Mechanical system
• Electrical system

9.3.1 Mechanical system

Tower

The tower is built to support the structure as well as to raise the wind turbine so that its blades safely clear the ground. Wind speed increases with the height of the turbine. Doubling the tower height increases the available wind power by about 40%. Most often it is more economical to install a higher tower rather than purchasing a larger generator. A wind generator should be installed at least 10 meters above any obstruction. There are two types of towers:

• Self supporting tower
• Guyed tower

Self supporting towers

These types of towers are constructed when the terrain is rough and the clear area available is less. These towers are also low on maintenance.

Some of the disadvantages of these types of towers include expensive transportation, heavier structure and hence requires large capacity cranes to erect them. The maintenance of these towers is a little difficult as it is not easy to climb them. Self Supporting Towers are of two types:

a) Self-supporting tubular (steel) tower

b) Self supporting steel lattice tower

a) Self-supporting tubular (steel) tower

These types of towers are deployed for small wind turbines. Short self-supporting tubular tower are good enough to support small wind turbines. Wooden poles, rods, and angle iron may also be used to support small turbines.

b) Self supporting steel lattice tower

Larger wind turbines require lattice tower.

The advantages include easier transportation and maintenance.

Guyed towers

As the name indicates guyed Towers are towers that are erected using guy wires anchored to the ground on three or four sides. For small wind systems Guyed Towers are used.

The advantages of these towers include easier installation, lower cost and material, easier transportation and lower maintenance. Just a winch is enough to erect the tower with the turbine mounted on it as compared to the requirement of cranes to erect the self-supporting steel towers.

Some of the disadvantages of these towers include the requirement of larger ground space to anchor the guys and the climbing difficulty.

Which ever tower type you choose, at the end of the day, it must be strong enough to support the wind turbine, sustain vibration, wind loading and the overall weather conditions for the lifetime of the wind turbine. Tower costs vary largely as a function of design and height.

The following figure depicts a typical construction of each of the towers discussed above.

Tower design

Whichever tower type you choose, at the end of the day, it must be strong enough to support the wind turbine and to sustain vibration, wind loading and the overall weather conditions for the lifetime of the wind turbine. Tower costs vary largely as a function of design and height.

The following are the other factors to be considered for erecting a tower:

• Type of tower
• Site access
• Soil type
• Ease of maintenance

The selected type of tower should have the following characteristics:

• Resistance to fatigue stress
• Strong enough to withstand wind gusts
• Rigid enough not be bend or vibrate or damage any components of the wind energy system
• Long lasting
• Low maintenance
• Should consume less space
• Relatively inexpensive

Site access

As we saw earlier the erection of self-supporting steel towers require cranes and the capacity of cranes is determined by the size of the tower. Most often large capacity cranes are required for erection. Crane maneuvering at the site is another aspect to be taken into consideration. If enough space is not available for maneuvering then either the space needs to be upgraded or steps should be taken to make provision for easy access. At times trees might have to be cut to pave way for the passage of crane.

Construction

Before starting the erection of the tower following work might have to be carried out.

• Widening works to existing road network
• Construction of site entrances and temporary compound
• Construction of site access tracks with field gates and temporary fencing if required
• Clearing up of the trees where ever required

The actual erection work starts with digging the turbine foundation. Then the wind turbine foundation is constructed. Sub-station is built and the grid connections installed. The power and instrumentation cables are laid. Turbines are commissioned.

Best practices on construction and environmental safety should be adopted.

Health-and-safety is another important aspect.

Nature of ground

The ground condition should be such that the foundation is strong and secure. The soil should be checked for its load bearing capacity and stability. The nature of its strata, if known, would help in making a good foundation for the tower. Erosion problems should not be there as this could destroy the whole system.

Foundation

Appropriate amount of concrete should be calculated and poured in the foundation pit. The foundation surface should lie up to 1m below the ground surface. A central column would protrude upwards from the base and would contain the bolts onto which the turbine tower would be attached.

Erection

The erection of the tower can be done in two ways. These are:

• On-site construction

  If it is steel lattice tower, it is sent in pieces to site from its place of manufacture. On site, the pieces are assembled one by one from the ground upwards. This is a time consuming activity. The assembly of the higher sections of the towers is risky as it involves assembly at larger heights from the ground. Generally this method is adopted as a last alternative and only for towers up to 15 metres in height

• On-site assembly

  In this method the various sections of the tower are assembled on the ground. After the entire assembly is made, the tower is erected with the help of a crane at one time. This is the preferred way of erecting a tower. Smaller wind energy systems with output up to 10 kW can be erected with the other components assembled on the tower

The following photograph will give you a better idea of this method of erection.

Ease of maintenance

One should make sure that there is sufficient access for easy maintenance if required to various parts of the tower.

Erection of other components

The erection of the tower is one of the major activities in the installation of the wind energy system. The tower acts as the structure on which the wind turbine components including the hub and rotor are assembled. These components can be mounted by various methods:

• The assembly can be raised with the help of the crane to the top of the tower and then mounted on the tower properly fastened as recommended by the manufacturer
• The assembly can be raised to the top of the tower by means of a chain pulley block, which is hung on a temporary structure that is supported by the tower itself
• The assembly can be mounted and secured thoroughly to the tower on the ground and then together raised

The following sketches illustrate these methods.

Operation

Generally the wind farms are operated remotely from a central computer system. Hence a large amount of infrastructure is not required on-site. There is also no need to man the wind farm site.

Each turbine will have its own internal control system interfaced to a central control system located in the sub-station.

9.3.2 Mechanical Safety

Overload Protection

To prevent the hazards of overload two revolution counters are mounted on the shaft. These counters operate independent of each other. In case the RPM of the counter exceeds the critical speed, the emergency stop is activated. Generally the counter is set to 24RPM.

Blade detachment

Blade detachment or blade fragmentation could be the most serious failure. Either of the above could cause considerable damage to the living beings or properties. Generally the turbine designer carries out the Failure Mode Engineering Analysis during the design process and controls these kinds of failures. Nevertheless a very reliable control system should be in place. This control system should have the capability to quickly identify the fault situation and once identified should be able to bring the rotor to halt. Routine inspection and a good safeguard against overloading should minimize the hazard.

Noise

The Windmills produce two types of noise: a) Mechanical noise generated by the links and the gear box, b) Aerodynamic noise, which is produced by the movement of the turbine blades. But in the case of wind energy systems noise is not of any major concern as the frequencies are either very high or very low. This means the aerodynamic noise could be heard only in a very short range from the installation.

9.3.3 Electrical

Lightning Protection

One of the biggest threats to a wind turbine is a lightning strike. Tall towers as we know are vulnerable to lightning risks. The local weather conditions specifically the number of thunder storms per annum determine the amount of risk involved.

The enormous amount of energy generated by the lightning is diverted and dissipated to the ground. This is done by providing the least resistance path of a conductor leading it to the ground.

Tower grounding rods and wires provide lightning protection and assure a low resistance path to ground. The grounding wire should be straight, without sharp angles and should run from the top of the tower to the bottom and buried into the ground for about one metre. A free-standing tower poses a special problem because its base acts as an insulator.

A coordinated protection scheme using lightning arrestors and spark gaps (see below for details) are used in this case to avoid any damages. The transformers are placed inside the tower to minimize risk. The electrical generator the transformer and other electrical components are sufficiently insulated to reduce the risk of lightning.

Here are some of the general electrical procedures to be adopted for the lightning protection of a wind energy system.

Current diverters

Lightning induces heavy surges on the voltage and current in the transmission cables running from the wind turbine to the battery storage or the sub station. A detachable plug and socket may be added to isolate the transmission lines from the storage device. The plug is unplugged in the event of a storm. As it is a manual method it’s hardly used in the modern wind energy systems.

Spark Gaps

The spark gaps are widely used to divert the current in the transmission lines.

The spark gap arrangement allows current to jump to the earthing strip preventing the surge current to flow into the system i.e. the power loads.

The following figure gives a view of a typical spark gap arrangement on the transmission cable.

Surge protectors

A surge protector diverts current and clips voltage spikes that may damage the system. A surge protector is a DC power storage device consisting of a series MOV (Metal Oxide Varsistors), diodes, circuit breakers and inductors. Surge protectors can either divert the current surges to the ground or short-circuit them.

9.5.1 Scheduled maintenance

Scheduled maintenance includes all preventive maintenance work, including inspections to determine whether any repairs are required.

Routine checks

As a part of routine inspection each machine should be inspected regularly (at least monthly) with the help of a checklist. A checklist is very useful during preventive maintenance, as it ensures nothing is missed. The manufacturer normally provides the maintenance schedule. Typical activities needed for the preventive maintenance are:

• Check the oil levels of the gearbox and hydraulic system
• Check for any oil leakages
• Inspect the cables running down the tower
• Check any unusual noise or vibrations during operation
• Carry out the Inspection of brakes

9.5.2 Preventive maintenance

During preventive maintenance, the following activities are carried out:

• Check whether all the fixations are secured, e. g. blade attachment, gearbox hold down, jaw bearing attachment, tower base bolts
• Check the alignment of the high-speed shaft
• Check the wear and tear on the brake pad
• Carry out the greasing of the bearings
• Check and secure of cable terminations
• Check oil filters and replace if necessary
• Test the over speed protection system at regular intervals as specified by the manufacturer
• The blades should be cleaned at least once in a year to maintain the consistent performance of the wind energy system

During preventive maintenance it is also important to replace parts that are subject to wear and tear and have a certain life in terms of operating hours. This can be more cost-effective than suffering downtime waiting for a repair to be effected after a failure has occurred.

9.5.3 Electrical maintenance

As the wind energy system also deals with electricity, a regular electrical maintenance is a vital activity for a trouble-free operation of the system.

Electrical safety

A systematic approach to electrical maintenance practices, combined with effective risk assessment procedures minimizes the risk to employees. The health and safety regulations that must be observed during electrical maintenance vary from country to country. However, general guidelines exist as follows:

• The transformer and switchgear compound should be protected against unauthorized entries
• Switchgear equipment should have locking arrangements in required positions
• Relevant signs should be used during maintenance
• The switching tasks should be carried by authorized staff only
• The health and safety regulations at the workplace should be observed
• Danger signs should be posted in high voltage zones
• Protective equipment to be available during maintenance

Maintenance schedule

An effective schedule for the maintenance of the electrical system ensures reliable operation and minimizes the risk of breakdowns. A typical schedule can be planned in the following manner.

Quarterly checks

• Visual inspection of the blade condition
• Security arrangement
• Oil levels
• Electrical connections
• Battery voltages

Half- yearly checks

• Overload relay settings
• Transmission oil change
• Earthing connections
• Cable corrosion

Yearly checks

• Entire wind system
• Tower
• Storage devices
• Circuit breakers
• Entire Wiring system

Biannual checks

• Complete switchgear overhauling
• Change of tap

Spares

Availability of emergency spares at site is very important in order to avoid long breakdowns. The entire wind farm can shut down for want of even a small important component. Thus, it is essential that a comprehensive stock of spares be carried. Components to be stocked include:

• Full set of overload relays
• Grid and turbine transformer bushings
• Grid transformer windings
• Temperature indicators
• Tap changer fixed and moving contacts
• HRC fuses
• Turbine transformer gaskets
• Breather oils

10.2.1 Active Systems

Active systems use electric pumps, valves, and controllers to circulate water or other heat-transfer fluids through the collectors. They are usually more expensive than passive systems but are also more efficient. Active systems are usually easier to install than passive systems because their storage tanks do not need to be installed above or close to the collectors. The main disadvantage of active system is that it uses electricity and in case of power failure, the system will not work.

Open-Loop Active Systems

Open-loop active systems use pumps to circulate water through the collectors. This type of system is more efficient and reduces operating costs but is not appropriate if the water is hard or acidic because scale and corrosion quickly disable the system. These systems should only be installed where the climate is moderate without any prolonged periods of freezing temperatures.

The places where mild freezes occur about once or twice a year, recirculation systems with a freeze protection can be used. They use the system pump to circulate warm water from storage tanks through collectors and exposed piping when temperatures approach freezing. However using the freeze protection more frequently will waste electricity and stored heat. In addition to that if there is a power failure the pump will stop working and the system will freeze. In order to avoid this, a freeze valve can be provided for additional protection.

Closed-Loop Active Systems

These systems pump heat-transfer fluids (usually a glycol-water antifreeze mixture) through collectors. Heat exchangers transfer the heat from the fluid to the water stored in the tanks.

Closed-loop glycol systems are popular in the climates with extended freezing periods because they offer good freeze protection. However, these systems are more expensive and require regular replacement of the glycol used in the system depending on the quality of the antifreeze fluid and the operating temperatures.

Another modified form of these systems is drainback systems which use water as the heat-transfer fluid in the collector loop. A pump circulates the water through the collectors. The water drains by gravity to the storage tank and heat exchanger. When the pumps are off, the collectors are empty, which assures freeze protection and also allows the system to turn off if the water in the storage tank becomes too hot.

The pumps in solar water heaters have low power requirements, and some systems use direct current (DC) pumps powered by small photovoltaic panels.

10.2.2 Passive Systems

Passive systems circulate water or a heat-transfer fluid through the system without pumps. Passive systems have no electric components, which makes them more reliable, easier to maintain, and possibly longer lasting than active systems.

Passive systems are less expensive than active systems, but also less efficient. There are two types of passive systems: batch heater and thermosiphon system.

10.6.1 Energy efficiency

Energy efficiency means using your building’s individual components to do the same job as less efficient components for less money over the long-term. Energy-efficient building components applies to everything from the building envelope, which includes energy efficient windows, lighting, insulation, foundation, and the roof, to office equipment that doesn’t waste energy sitting idle and equipment with built-in power management features. It also applies to space heating and cooling, which are aided through the use of automated controls, ventilation, improved duct systems, and other advanced technologies. Energy efficiency can also apply to water heating when combined with water-efficient appliances and fixtures that will save water, energy, and money.

10.6.2 Integrated building design

Integrated building design is a process of design in which multiple disciplines and specialists come together to design a system which gives high performance at a lower cost. This process often includes integrating green design strategies into conventional design criteria for building form, function, performance, and cost. A key to successful integrated building design is the participation of people from different specialties of design: general architecture, HVAC, lighting and electrical, interior design, and landscape design. By working together at key points in the design process, these participants can often identify highly attractive solutions to design needs that would otherwise not be found. In an integrated design approach, the mechanical engineer will calculate energy use and cost very early in the design, informing designers of the energy-use implications of building orientation, configuration, mechanical systems, and lighting options.

10.7.1 What is a hybrid energy system?

The systems that combine two or more power sources to form a usable energy system are called hybrid energy systems. They are generally independent of large centralized electric grids and are used in remote areas. Hybrid energy systems range from small systems designed for one or several homes to very large ones for remote island grids or large communities The advantage of a hybrid system is that it provides uninterrupted power supply, by optimizing the use of one of the resources. For example in the case of a solar and wind energy hybrid system, when the weather is cloudy and windy, the low level of solar energy can be compensated by the higher production of the wind energy and vice versa.

Given below are a few examples of hybrid energy systems:

• Photovoltaic + diesel generators
• Wind Energy systems + diesel generators
• Wind energy + solar energy
• Solar energy + electricity
• Photovoltaic + fuel cells

The advantages of a hybrid energy systems are as follows:

• The optimum use of energy resources
• Reduced energy storage requirement
• Maximized power output.

However, there are certain disadvantages of hybrid systems:

• Balancing of several sources of energy is quite complex.
• Additional system requirements for load sharing.

10.7.2 Design considerations

Before designing a hybrid system, it is essential to do a thorough analysis of the load requirements. In addition to this, a study of potential energy resources (such as wind, solar, or other resources), available the site needs to be done. A careful matching of the available energy source should be done with each load so that system is operated with the least costs. It is very important that a life cycle cost analysis is done to evaluate the actual costs to be incurred throughout the life of the system.

10.9.1 Combination of photovoltaic and wind energy

Ideally the combination of photovoltaic and wind energy systems are suitable for the locations where there are longer periods of sunlight for the certain duration of the year and there are periods of windiness during the rest of the year. In such cases, this combination will provide uninterrupted power supply throughout the course of the year. The cost effectiveness in such cases is quite high. However in many areas, during the wet season, the radiation and the wind speed both are low. Hence this type of system is designed with higher margins, to take care of variations in the output power. In addition to this a thorough weather analysis should be conducted before installing this hybrid system.

These systems are not suitable for areas where the wind speeds are low for longer period of the year. In such areas the PV systems have to be designed to cater to the full load, which is not cost effective.

10.9.2 Combination of photovoltaic and hydro hybrid systems.

This system uses a stream of water to generate hydro electricity during the rainy season and uses solar energy during non-rainy season. This system is more suitable at sites where the rainwater forms a flowing stream while the non-rainy seasons are sunny.

This system also requires a historical record of stream flow to assess the success of this system.

10.9.3 Combination of photovoltaic and genset systems.

The diesel generator hybrid system is the most commonly used hybrid system. The diesel generators are reliable source of energy with a long life. Batteries can also be used in these systems along with the generator. Batteries take care of the daily load demand and diesel generators take care of the long term load demands like extended overcast or rainy season. The disadvantages of this hybrid system are the high cost and noise and air pollution. Generally a connection and control unit is installed in a central place to organize the connections to most of the system components. This control unit houses battery charge and discharge regulators, transfer switches, circuit breakers, power meters and a mode controller. The mode controller is required to be included in the connection and control unit in order to switch between the genset and the PV system.

10.9.4 Combination of photovoltaic system and a fuel cell

In urban areas the gensets are increasingly being replaced by fuel cells due to their environmental issues. The fuel cell is an electrochemical device that generates electricity by chemical reaction without altering the electrolyte or the electrodes. Fuel cell is a static device that converts chemical energy to electrical energy. They differ from the electrochemical batteries in that they don’t alter the electrolyte material. The fuel cells are also different from the diesel generators in that they bypass the thermal to mechanical conversion which takes place in diesel generators. The chemical reaction that takes place in the fuel cell is the reverse of electrolysis of water. In fuel cells hydrogen and oxygen are combined to produce electricity and water.

Fuel cell construction and features

The hydrogen is combined with oxygen of the air to produce electricity. Hydrogen in this case does not burn as in the IC engines instead produces electricity by the chemical reaction. The byproducts of this reaction are water and heat, if the fuel is pure hydrogen. In case of methanol or ethanol, the byproducts consist of carbon dioxide, carbon monoxide, hydrocarbons and nitrogen oxides in a very small quantity compared to the diesel generators. Another advantage of fuel cells over the diesel generators is that they don’t have moving parts, which translates into their higher reliability.

Fuel cells can be used in the combination of series and parallel just like the electrochemical batteries to obtain the required voltage and current.

There are two types of fuel cells available, low temperature and high temperature fuel cells. The low temperature fuel cell uses phosphoric acid as the electrolyte. It costs roughly twice as much as the diesel generator.

The high temperature fuel cell is used only in special applications due to their high cost. They have higher power generation capacity, which makes them suitable for use in utility power plants. The development work is being conducted for the production of solid oxide, solid polymer, molten carbonate, and proton membrane fuel cells.

Mode Controller

The mode controller is used to monitor and control the switching of hybrid system. The mode controller is designed to accommodate the characteristics of different devices that are used in the hybrid system like diesel generator, PV system , and battery. It consists of a microcomputer based software system to monitor and select the source of energy at any given time, the battery management and the load shedding strategy. The mode controller also monitors the health of the hybrid system, controls the state of charge of the battery, starts up and shuts down the diesel generator when required and sheds the low priority load as per the preset priorities.

Load Sharing

The key design consideration for the hybrid system is the aspect of load sharing between the various sources of energy used in these systems. The electrical properties of these systems must match in order to enable the load sharing according to their rated capacities. The load sharing strategy varies depending on the cost of the power from the different sources and the priority of loads. The essential loads are given the first priority. The power demand of such loads is met by the PV system as far as possible. If the power supply from the PV system is more than the demand for high priority loads, then the PV system supplies part of the low priority loads. If the supply drops at any given point, then the diesel generator supplies the power to the loads.

10.10.1 Assess the character of the load demand

It is important that the system is designed to meet the actual energy requirement of the customer. The daily and yearly load demand profile should be obtained before designing a hybrid power system. Depending upon the variation in demand, the hybrid system is designed. For example if the load demand varies throughout the day, a hybrid system consisting of a renewable resource and a storage battery along with the fueled generator would be ideal. The peak loads can be met by the battery while the renewable energy generator can support the load demand at other times. There are some areas where the load demand is steady most of the year, increasing only in certain seasons. In such cases a renewable energy resource can be sized to provide baseload demand, and a diesel genset can be used during the peak load seasons. When assessing the load demand, the increase in future load demand should also be considered. For example, if the hybrid system is being designed for a village, there may be an increase in the number of appliances people use over the years. A good hybrids system should be able to supply the required load at any point of time. An incorrect load assessment may result in over-sizing or under- sizing a system and hence the assessment becomes a very critical design activity.

10.10.2 Establish design criteria and required specifications

It is recommended that the initial design be undertaken based on the design criteria specified by the customer.

However after the initial design is completed see whether this meets the standard design criteria, which include:

• High reliability
• Low maintenance and operating costs
• Minimum effects on the environment
• Must conform to the local governing laws
• Spares availability and service backup
• Automatic control of the load
• Type of inverter
• Good aesthetics
• Low capital costs

10.10.3 Assess the quality of available resources at the selected site

A renewable energy resource is assessed on the basis of two criteria – magnitude and consistency. If the site receives more than 1800 kwh/year of solar radiation most of the year, it is considered a good site for the PV system installation. A site with a wind speed of about 13 miles/hr is considered a good location for wind power generation. Depending upon the available energy resource at the site, a suitable hybrid system can be designed.

10.10.4 Work out the system configuration

This step involves a critical design of which system combination should be adopted to form a hybrid system configuration. This is done after a thorough assessment of the available energy resources.

10.10.5 Inspect the available equipments and work out the cost.

Every designer should be aware of the available makes of the selected components or should have resources for selecting the right make of components in the system. The factors to be considered while selecting the equipments are:

• Provide reliable equipment to the owner
• Ensure the cost of the equipment is within the budget of the customer
• Spares and service backup is readily available

10.10.6 Carry out sizing of the components

The main components that need to be sized properly are the renewable energy system, the generator and the batteries. While sizing the energy system, it is important that the designer balances the capital cost and the operating cost of the system. We shall see this in detail in the next chapter. Once these three components are sized, the balance of plant sizing becomes easy.

10.10.7 Decide on the control system

In deciding the method of control to be adopted on the system, the user of the system plays a very important role. It is the level of comfort required by the customer and kind of automation required that determines the selection of the control system.

Having understood the operating strategy of the user the designer can then work out the control system by considering the following factors:

• The components that need to be controlled
• The parameters that need to be captured for affecting these controls
• The instrumentation required for this control system

10.10.8 Finalize the scheme and optimize to match the budget

Not always is the system, that has been designed by following the steps outlined so far, accepted by the user. One of the major constraints is the budget. Hence it is very important that the designer compares the estimated cost with the budget. In most of the cases the system needs to be optimized to suit the user’s pocket. The best method to optimization is based on the life cycle cost analysis, which we will study in the next chapter.

10.10.9 Documentation

Finally, the entire design needs to be put on paper. The following documents should be prepared at the end of a good system design. This would help in the installation, commissioning, operation and maintenance of the system:

• The instruction manual giving the details of the selected components
• Installation checklist
• Drawings such as the P & I, Electrical, etc.
• Operating and maintenance manual of the system
• Equipment manufacturers documentation and test certificates
• Logbook/ log sheets
• Troubleshooting guidelines

11.3.1 Dams or Reservoirs

Dams raise the water level of a stream or river to an elevation needed to create water pressure or “head.” Dams can be constructed of earth, concrete, steel or a combination of such materials. Dams may create secondary benefits such as flood control, recreation opportunities and water storage. The most common type of hydropower plant uses a dam on a river to store water in a reservoir. Water released from the reservoir flows through a turbine, spinning it, which, in turn, activates a generator to produce electricity. But hydropower doesn’t necessarily require a large dam. Some hydropower plants just use a small canal to channel the river water through a turbine.

11.3.2 Run-of-river plants

Run-of-river plants are characterized by the absence of either a reservoir or a diversion canal. The power is generated by the river’s natural flow continuously throughout the year. Typically this type of plant consists of a weir running throughout the river, which also acts as a flow regulator; and a powerhouse. The selection of the site for the run of river plant should be done in such a way as to allow easy access and stable bed for the power house. The sediment in the water should be low and the anticipated flooding should have low value.

11.3.3 Diversion canal plants

In diversion canal plant the water from the river is diverted through a canal to the powerhouse. The water from the canal is again diverted back to the river at the downstream point. The forebay (weir) is constructed at the end of the canal. Through the penstock (pipe), the water reaches the powerhouse which is located at the lower end of the river.

11.3.4 Pumped storage plants

In Pumped storage plants the power is sent from a power grid into the electric generators. The generators then spin the turbines backward, which causes the turbines to pump water from a river or lower reservoir to an upper reservoir, where the power is stored. To use the power, the water is released from the upper reservoir back down into the river or lower reservoir. This spins the turbines forward, activating the generators to produce electricity. The pumped storage plants generate electricity during the peak load hours. During off peak hours, they pump back the water to the reservoir. The main advantages of the pumped storage plant are their high cost effectiveness, dependability and adaptability to automation. The principle drawback of this type of plant is that it takes about six hours to fully load the system after a complete shutdown and the turbine remains idle for several hours. The operating cycle of a pumped storage plant can be daily, monthly or seasonal.

On the basis of the head height, the hydro power plants can be further classified into Low head, Medium head, and High head plants.

Low Head plants (<15 meters)

In low head plants, the amount of water required to generate power is higher than the high head plants. For this reason the catchment areas for such plants are very large. Typical examples of a low head plant are run-of river, tidal or midget plants.

Medium head plant (15 to 70 meters)

Compared to high head plants, a larger volume of water is required in such plants, therefore a large reservoir is needed to store water. Water is first fed to the forebay and then directed to the powerhouse using short penstocks in these plants.

High head plant (71 meters and above)

A small amount of water is sufficient for producing a significant amount of power in these plants due to high head. The dams or reservoirs are located at the top of a mountain and the powerhouse is at the foot. This level difference helps in creating water pressure. The water is channelised through a tunnel to the surge tank and then to the powerhouse. The catchment area is usually small in such plants and sometimes the water from neighbouring streams needs to be diverted to the reservoir using pipelines or tunnels.

11.5.1 Measurement of flow

The measurement of the flow is a more complex process. As the flow varies from day to day and season to season, a one off measurement would not be of any use in this case. The study of the flow data needs to be conducted over a long time period. Also it is important to measure the lowest flow at any given site, since this value will affect the maximum power output of the plant. There are various study methods which can be used depending upon the type of the site. They are as follows:

Measuring weirs

A weir is like a small dam over which the water flows. To measure the flow, special weirs can be built with a notch in it, through which the stream flows. The flow rate will be the difference between the upstream water level and the bottom of the notch.

Weirs can be built of various materials such as wood, metal or concrete. The crest of the weir should be built with sheet metal(brass or stainless steel) to avoid corrosion.

Weirs are built perpendicular to the stream flow and sited at a place where the stream is straight. Weirs should also be perfectly sealed to avoid leakages and the distance between the point of measurement and the crest of the weir should be at least twice the maximum head to be measured. Weirs are usually used to measure flow rates in small streams.

Stage discharge method

In this method a curve is plotted showing a relationship between the water level and flow at different water levels. To get an instant flow rate at any point of time, this curve is used. However this curve needs to be updated every year. The method of determining the flow rate in this case can be a temporary weir installed upstream or downstream. This method is used for the larger sites.

Bucket method

For very small streams this method is used to measure flow rate. A bucket or barrel is used in this method in which the flow of the stream is diverted. The time taken by the stream to fill the container is recorded. The volume of the container is then divided by the time, to obtain the flow rate. Flows of up to 20 l/s can be measured using a 200-litre oil barrel.

Flow meters

These consist of a shaft with a propeller or revolving cups connected to the end. The propeller is free to rotate and the speed of rotation is related to the stream velocity. A simple mechanical counter records the number of revolutions of a propeller placed at a desired depth. By averaging readings taken evenly throughout the cross section, an average speed can be obtained which is more accurate than with the float method.

11.6.1 Impulse turbines

In impulse turbine the force of the jet stream coming out of a nozzle strikes the turbine runner, which absorbs the momentum of this jet and starts rotating. After hitting the turbine runner, the water simply flows out of the turbine housing. A pressure casing is not required in impulse turbines since the pressure is already built beforehand in the form of the jet stream. That is why impulse turbines are cheaper than the reaction turbines. But they are generally only suitable for high head, low flow applications. The main advantages of impulse turbines are that they require low maintenance and are easier to fabricate. Accessibility of different parts is enhanced and the influence of sand particles and other impurities in water is not very high on the operation of the turbine. The main disadvantage of the impulse turbines is their unsuitability for the low head sites, due to their low specific speeds.

There are many types of impulse turbines:

Pelton turbine

A Pelton turbine has a runner consisting of buckets mounted on its periphery. The runner rotates when the jet stream coming from one or more nozzles hits the buckets. These buckets are designed to allow a smooth rotation of the turbine. They deflect the jet stream by 165 degrees, which ensures that the return jet does not interfere with the next jet stream on the following bucket. Pelton turbines can either be horizontal shaft type or vertical shaft type. Horizontal shaft type can further be classified into single wheel single nozzle, single wheel double nozzle and double wheel with either 2 or 4 nozzles. Single wheel type Pelton turbines are supported by the bearings of the generator shaft. In case of double wheel turbines the generator is accommodated in between the two wheels. Vertical shaft type turbines can accommodate up to six nozzles which helps in better regulation of the flow.

The casing of the Pelton turbine is a steel cover with no hydraulic function to perform. Its main functions are to prevent water from splashing, to lead the water to the tailrace and to protect against accidents.

A Pelton turbine is also provided with the hydraulic brake or a brake nozzle. It is used to stop the turbine from rotating after the jet is removed. It directs the jet of water at the back of the bucket to bring the rotating runner to rest.

Pelton turbines are best suited for heads above 300 m. The largest Pelton turbine in the world is located at Cimego (Italy) which produces 110 MW power at the head of 780 meters. The Pelton turbine in Reissel (Austria) is the highest head plant for the Pelton turbine with the head of 1765 meters.

The speed regulation in Pelton turbine is done with the help of a needle valve inside the nozzle and the deflector mounted outside the nozzle. When there is a sudden decrease in the load demand, the jet deflector deflects the jet away from the blades, fully or partially depending upon the required load reduction.

The needle valve is then adjusted to obtain the required flow rate. The deflector is then withdrawn.

Turgo Turbines

A Turgo turbine is a turbine with a fan shaped blades which are closed on the outer edges. The jet stream is directed at one side and it exits from the other side of the blade without interfering with the incoming jet stream. The specific speed of the Turgo turbines is higher than the Pelton turbines and a smaller runner diameter can be used to obtain same power as the corresponding Pelton turbine. High specific speeds also make it possible to do away with the expensive transmission systems used to increase the speed. Turgo turbines can often be directly connected to the generator.

Cross-Flow

A cross-flow turbine has a cylinder shaped runner consisting of curved blades and two parallel discs on either side of the cylinder. The runner shaft of the cross-flow turbine is always horizontal. The water jet in cross flow turbines strikes the blades twice, while entering the blade and exiting the blade. A rectangular nozzle is used which is directed at the full length of the runner blade. The main advantage of the cross flow turbine is its relatively high efficiency even at low flow rates. Also it is more suitable for higher flows and lower heads than the Pelton turbine.

11.6.2 Reaction Turbine

The operation of a reaction turbine is more complicated than the impulse turbine. The energy generation in a reaction turbine takes place from the collective action of pressure and moving water. The runner of the reaction turbine is positioned directly in the flowing water stream. This results into rotation of the turbine runner due to the moving water. The pressure of the water changes as it passes through the turbine. A pressure casement is needed to contain the working fluid as it acts on the turbine runner. Reaction turbines are generally used for sites with lower head and higher flows than compared with the impulse turbines. Reaction turbines have higher specific speeds which allow them to be coupled directly to the generator, thereby avoiding the speed-increasing transmission costs. However the manufacturing costs for the reaction turbines are a little higher than the impulse turbines, because of the complex blade design and pressure casing design. But these extra costs are compensated by high specific speeds and high efficiency of these turbines. They are compact in nature and can work efficiently at low heads. All reaction turbines have a draft tube which is used to facilitate the discharge of the water from the runner blades into the tail race. There are many types of reaction turbines available. They are:

Francis Turbines

A Francis turbine has a runner with fixed blades, which are shaped in such a way that the water is directed to exit from the center of the runner. The pressure casing is designed to dispense water uniformly along the entire perimeter of the runner. The runner of a Francis turbine is made of mild steel or stainless steel. The guide vanes are used in the Francis Turbines to regulate the flow and guide the water along the blades at the appropriate angle. The guide vanes have an airfoil section which allows the water to flow through them without forming the eddies and with minimum friction. Guide vanes are usually made of steel or stainless steel. Water enters in the turbine from the top and passes on most of its energy to the runner before exiting through a draft tube. The draft tube is a tapered tube which is fitted at the exit of the runner. The other end of the draft tube is connected to the tailrace. Draft tube is always kept airtight and its lower end is always submerged below the tailrace. The main functions of the draft tube are to create a negative head by allowing the installation of the turbine above the tailrace. This simplifies the inspection and maintenance of the turbine. Secondly it creates a pressure energy, which increases the effective head of the turbine, thus increasing the efficiency of the turbine. Francis turbine is suitable for medium heads in the range of 50 meters to 450 meters and medium flows. The specific speed of Francis turbines is in the range of 60 to 300.

Propeller Turbines

A propeller type turbine consists of a marine type propeller which is fitted into a pipe. It usually has three to six blades. The water constantly contacts all the blades at a constant pressure. The other main components of the turbine are swivel or wicket gates, scroll case and a draft tube. The flow of the water is regulated by the wicket gates. This type of turbine has fixed blades and the flow of the water is in axial direction. Due to this the part flow efficiency of propeller turbine is very low.

Kaplan Turbines

The Kaplan turbine is a variation of a propeller turbine. The turbine blades’ pitch angle can be adjusted to handle variation in the flow in this case. The angle of the blades can be changed with the help of a link mechanism provided inside the runner hub. The efficiency of these turbines is very high (above 90%) even at partial or design power and also at the power higher than the design power. The runner of the Kaplan turbine is fixed with 3, 4 or 6 blades. It differs from the Francis turbine in the following ways:

• The water flows axially in Kaplan turbine runner, unlike the Francis turbine runner where water flows radially.
• Kaplan turbine runner has 3 to 6 blades whereas the Francis turbine runner has 16 to 24 blades.
• Blades of a Francis turbine runner are not adjustable, while the runner blades of Kaplan turbine can be adjusted to control the flow area.

The adjustable wicket gates also help in maintaining a good efficiency even in part flow conditions. They are very expensive and are used principally in large installations. They are best suited for low heads (up to 60 m) and large flow rates. The specific speeds are in the range of 300 to 1000.

11.6.3 Reverse Pump Turbines

Pumps can be used as turbines by reverse engineering the pump curve. The water is passed through them in reverse. The main advantage of the reverse turbines is their low cost and high efficiency (85% to 90%). Besides the efficiency and cost effectiveness of using pumps as turbines, there are numerous other advantages as well. By installing multiple pumps, full efficiencies can be obtained from each on-line unit. Depending upon water flows, different pump units can be turned on or off to match inflows. Apart from this another advantage is the availability of spare parts and a wide support network. The efficiency of the pump turbine also depends upon the gross head height. The higher the head, the lower the cost per KW of power. The pumps are most efficient at the head height of 40 to 250 feet. Research is currently being done to enable the performance of pumps as turbines to be predicted more accurately.

Bulb turbine

This is a recent development where the turbine and generator are sealed together and placed directly in the water stream. The turbine is usually a Kaplan type or a propeller type which is enclosed in a watertight housing supported within a horizontal water channels. The generator can be directly coupled to the turbine shaft or a transmission drive can be used for increasing the speed to reduce the generator size. Bulb turbines are used for low head applications in the range of 3m to 18m. The power output from the bulb type turbines can be in the range of 40 to 200 MW. The efficiency of this type of turbine is very high due to the minimization of hydraulic losses.

Tube Turbine

The tube type turbine is a modification of a Kaplan turbine. It is designed for the heads below 15 meters. The turbine is mounted inside a tube which is inclined to enable the connection of the generator to the turbine shaft.

Straflo Turbine

This is a bulb type turbine in which a separate generator is not provided; rather the poles are mounted directly on the perimeter of the turbine runner. They are sealed to ensure that the water does not enter into the poles. The shaft is horizontal in this type of turbine and the flow of water is always axial. The main advantage of this type of turbine is that the size of the power house can be smaller and it can be used for a larger range of output and head.

Kinetic Energy Turbine

Kinetic energy turbines, also called free-flow turbines, generate electricity from the kinetic energy present in flowing water rather than the potential energy from the head. The systems may operate in rivers, man-made channels, tidal waters, or ocean currents. Kinetic systems utilize the water stream’s natural pathway. They do not require the diversion of water through manmade channels, riverbeds, or pipes, although they might have applications in such conduits. Kinetic systems do not require large civil works; however, they can use existing structures such as bridges, tailraces and channels.

Turbine characteristics

The main characteristics of the turbine are :

Turbine Efficiency

Large modern water turbines operate at mechanical efficiencies greater than 90%. The design of the turbine system is carried out mainly on the basis of the net head and the net flow rate. Nearly all the turbine types can be designed to meet these criteria. But even a minor difference in the specifications can have a high impact on turbine efficiency. Maximum value of turbine efficiency is obtained when the speed of the turbine is exactly enough to consume all the power of the water entering into it. To achieve this, the velocity of the water entering into the turbine should be such that it allows the turbine to run at its maximum efficiency. This velocity of the water is determined by head pressure.

To optimize the water velocity for different flow conditions, a needle nozzle is used in many turbines. It changes the flow of water to the turbine, and regulates the water velocity to attain the maximum efficiency. Apart from this, the quality and design of other components of the turbine also affect the efficiency. For example the diameter of the runner, the shape of the blade, turbine casing, etc. play an important role in determining the turbine efficiency.

Specific speed

Specific speed of a turbine is defined as the speed at which the turbine rotates at its maximum efficiency at a particular power out put and flow rate. Based on the value of the specific speed a turbine design can be selected for a particular hydro plant. The turbine performance can be accurately determined for a range of flow rates and heads using the specific speed. The specific speed rating is supplied by the manufacturer.

Runaway speed

The runaway speed of a hydro turbine is defined as the maximum speed it attains under no load conditions and at full flow. This situation may arise when the governing system fails to close. This will result into an increase in voltage and will cause an electrical breakdown. This can cause damage to the whole system. For this reason the turbine is designed to survive the mechanical forces and also the generator design is done using this speed. The runaway speed rating is supplied by the manufacturer. The value of runaway speed for Pelton and Francis turbines is about 1.6 to 1.8 times of its normal speed. For Kaplan turbines the value is between 1.9 to 2.2 times of the normal speed.

Characteristic curves

These curves represent the variation in speed, flow, and output power of the turbine. They help in predicting the performance of the turbine in various working conditions. These curves are plotted by keeping one or more parameters as constants. For example the constant head curve is plotted by keeping the head constant and by changing the values of speed and flow output power is calculated.

Based on the various criteria turbine selection can be done as shown in the table below.

Cavitation

Cavitation is the phenomenon of formation of bubbles or pits in the inner surface of the pipe through which the water flows under pressure. Cavitation generally occurs at the bends or throat of the pipe. At these locations the pressure of the water decreases below a certain limit, which causes the bubbles to be formed near these areas. As the water flows, the pressure increases again and the bubbles burst causing damage to the inner surface of the pipe. Cavitation causes damage to the inner surface of the pipe. It also causes lot of vibration and noise. To avoid cavitation following measures must be taken:

• The inner surface of the pipe should be made from tougher materials to resist cavitation: such as stainless steel or chrome nickel.
• When using reaction type turbines, cavitaiton is minimum if the turbine is installed below tailrace.
• The pressure is inversely proportional to the velocity of the flowing water. As the velocity increases the pressure drops. For this reason the water velocity should not be allowed to exceed beyond a limit.
• The draft tube should be designed in such a way that the absolute pressure does not fall below the vapor pressure of water at the given temperature.

11.7.1 Direct drive

A direct drive system is one in which the turbine shaft is connected directly to the generator shaft. Direct drive systems are used only for cases where the shaft speed of the generator shaft and the speed of the turbine are compatible. The advantages of this type of system are low maintenance, high efficiency and low cost. The only disadvantage is that the alignment is far more critical than with an indirect drive.

11.7.2 Indirect drives

Flat belt and pulleys

This is the most common choice for micro-hydropower systems. Belts for this type of system are made of high friction material such as rubber and possess higher efficiencies than the V- belts. They also produce less rubber dust when in operation. The main disadvantage of this drive system is it requires higher tension to keep the belt in position, because of which the bearings experience higher loads. This leads to the use of heavy duty bearings. Also the availability of flat belts is poorer in some places than the V-belts. The maximum speed ratio which can be achieved by the flat belt pulley drive is around 5:1.

V-belts and pulleys

The difference between the flat belt and the V-belts is that the frictional grip on the pulley is caused by wedging action of the side walls of the belt within the pulley grooves. Because of this the tension required to keep the belt in position is much less, as a result the load on the bearings is reduced. As a rule a set of multiple V-belt pulleys are run side by side. But multiple pulley sets can become to bulky at higher powers and torques. Compared to the flat belts the tolerance for misalignment is more in V-belts. The maximum speed ratio is 5:1 and the efficiency is about 85 to 95%.

Timing belt and sprocketed pulley

These drives are primarily used for smaller output systems of less than 3 kW. Special toothed belts are used along with the pulleys in these drives which help in reducing the belt tension and the loads on the bearings. The efficiency of these drives is very high (about 98%). The main disadvantages are the high costs of belt and pulleys and the availability. The maximum speed ratio can go upto 10:1 in these drives.

Chain and sprocket

Chains can have a very high efficiency but only at some sacrifice of lifetime. Long-life chain drives tend to be similar in efficiency to belt drives. Chain drives are not recommended because of their high cost, poor availability, the need to replace sprocket wheels periodically and the difficult lubrication requirements. Very high speed ratios of greater than 20:1 can however be achieved.

Gearbox

Gearboxes are suitable for use with larger machines when belt drives would be too cumbersome and inefficient. Gearboxes have problems regarding specification, alignment, maintenance and cost, and this rules them out for micro-hydropower systems except where they are specified as part of a turbine-generator set.

11.8.1 Dam or Barrage

Large hydroelectric power plants require high water heads. The high heads can be achieved by constructing dam or barrage across the river. Dams are also built on top of the hills in the case of pump storage power plants. Gates and valves are provided at the dam site to control the level and flow of water. The size of the dam depends on the amount of the water available and the head that needs to be produced. Depending upon the function, shape and design the dams can be classified into a few types.

Based on the function the dams can be classified as i) Storage dams, ii) Diversion dams

Shape classification would include i) Trapezoidal dams, ii) Arch dams

Based on the Design they can be classified into i) gravity dams, ii) Buttress dams, iii) Embankment dams and iv) Spillway dams

Diversion Structure

This is mostly used in the case of small hydro power plants.

At times a diversion structure/canal needs to be built from the main river to the reservoir. This is required either in order to just divert the flow to the desired location or to regulate the head. A hydro system must extract water from the river in a reliable and controllable way. The water flowing in the channel must be regulated during high river flow and low flow conditions. Either a permanent pool or a weir is constructed to control the water intake. A weir can be used to raise the water level and ensure a constant supply to the intake.

The river might carry small or big boulders during rainy seasons. Where ever the river stream can carry boulders a trench type weir is constructed across the river. The trench type weir consists of a trapezoidal trough located below the bed of the river with top kept at the bed level of the river. The intake is located at the end of the trench weir. It is constructed in reinforced concrete or masonry. A gate is also provided with the intake to release the water in desired volume.

The diversion structure should be simple in design and construction. The maintenance and repairs required must be as less as possible.

The intake should allow desired flow. The peak flow of the river must be able to pass through the intake and weir without causing damage to them. It should have provision to remove the sediments at regular intervals.

Reservoir

The main purpose of a reservoir is to store water which can be either used for irrigation or to generate electricity. The water is stored during rainy season. The size of the reservoir is dictated by the yearly water level pattern in the river, the dependence on rains, etc.

Desilting tank

Desilting tank is provided to trap the suspended silt and pebbles. The silt will be composed of hard abrasive materials such as sand which can cause expensive damage and rapid wear to turbine runners. This is an important equipment as this would minimize the erosion damages to the turbine runner. The size of the silt particles to be trapped varies from 0.1 to 0.5mm. To remove this material the water flow must be slowed down in the desilting tank so that the silt particles will settle on the basin floor. The horizontal flow velocity must not exceed 0.4 m/s to 0.6m/s. The depth of the tank is usually kept between 0.5m to as high as 4m. The deposit formed is then periodically flushed away.

Forebay

Forebay works as a regulating reservoir, which can be used to store water when the load on the turbine is reduced and provide water when the load is increased. It forms the connection between the channel and the penstock. The main purpose is to allow the finer particles to settle down before the water enters the penstock. It is usually constructed in reinforced concrete or stone masonry.

Trash rack

It is placed across the intake. It helps to stop the debris from entering the channel or the penstock or the turbines.

Penstock

The penstock is the tubular structure (pipe) which transfers water under pressure from the forebay tank to the turbine. The penstock often constitutes a major expense in the total cost of a micro hydro power plant. As much as 40 % of the cost is not uncommon in high head installations. It is therefore worthwhile optimizing the design.

The design of penstock involves two aspects via, the hydraulic design and structural design. The hydraulic design consists of determining the diameter of the penstock and the structural design in determining the thickness.

The trade-off is between head loss and capital cost. Head loss due to friction in the pipe decreases dramatically with increase in pipe diameter. Conversely, pipe costs increase steeply with diameter. Therefore a compromise between cost and performance is required.

The available pipe options are first identified and then a target head loss, for example 5 % of the gross head is chosen. The details of the pipes with losses close to this target are then tabulated and compared for cost effectiveness. A smaller penstock may save on capital costs, but the extra head loss may account for lost revenue from generated electricity year after year.

Spillway

In the case of load rejection, the water level may rise and flood the area. Therefore, a spillway is provided keeping the crest at the permissible water level so that the water level may not rise above the maximum permissible levels. The spillway is a flow regulator for the channel. In addition it can be combined with control gates to provide a means of emptying the channel. If the excess water is not discharged, the water level of the reservoir will be raised and the water may start flowing over the dam. This phenomenon is called overtopping.

Flood flows through the intake can be twice the normal channel flow, so the spillway must be large enough for diverting this excess flow. The spill flow must be fed back to the river in a controlled way so that it does not damage the foundations of the channel. The spillways may be classified into i) overflow spillway, ii) side channel spillway or iii) emergency spillway.

Water conductor systems (Channels)

The Water conductor systems (channel) conduct the water from the intake to the forebay tank or the reservoir. The length of a channel can be as long as a few kilometres in order to create a head of 10 to 20 metres. The length of the water conductor system depends on the site conditions and the materials available. In one case a long channel combined with a short penstock can be cheaper, while in other cases a combination of short channel with long penstock would be more suitable. Most channels are excavated, while sometimes structures like aqueducts are necessary. Size and shape of a channel are often a compromise between costs and reduced head. The most commonly used channel section is trapezoidal.

As water flows in the channel, it loses energy in the process of sliding past the walls and bed material. The rougher the material, the greater the friction loss and the higher the head drop needed between channel entry and exit.

The design of the channels should ensure minimum head loss, adequate flow velocity and minimal seepage. To reduce friction and prevent leakages channels are often sealed with cement, clay or polythene sheet or LDPE film.

Powerhouse Structure

The Power house structure accommodates turbine, generator, control panels, auxiliary equipments etc. It may be underground or open type. Water is brought to the power house through penstocks and fed to the turbines. The location of the power house is dictated by the maximum possible head at the turbine. The size of the power house depends upon the size and the number of machines placed inside. The spacing between the machines should be kept at the minimum permissible levels in order to keep the power house size low. The height of the building also should be optimized. The cranes may be replaced by the pulley blocks where ever possible in order to control costs. The structure must be simple, constructed either in RCC or stone masonry.

Tailrace

Water after flow through the machine is discharged into the river through the tailrace. It is an open channel or a tunnel depending upon the location of the power house. The discharge form all the turbines is collected in the tailrace at the beginning by means of branch channels. The tailrace may be trapezoidal or rectangular channel constructed in brick or stone masonry.

11.9.1 Generator Efficiency

The efficiency of the generator is a very important characteristic, as it affects the overall efficiency of the hydropower system. The efficiency also depends upon the size of the generator, the larger generators being more efficient. As a rule three phase generators are more efficient than the single phase generators. Given below are the efficiencies of different types of generators for full and part load conditions.

11.9.2 Generator speed regulation

Speed regulation of the generator is a very critical issue in the design of the hydro power system. It is vital for the generator to maintain a constant speed in all conditions. For example if there is an increase in load the generator will slow down and the voltage and frequency will also drop. Conversely if a load is suddenly switched off, it would cause the generator to spin faster, thereby increasing the voltage and frequency. For this reason governors are used to maintain the exact speed of the generator to get the proper voltage and frequency. The electronic load governors achieve this by controlling the load on the generator. The load at which the generator runs at an optimum speed is known as the design load. Design load depends upon the design flow. It should be increased or decreased depending on the flow. If the flow is more the design load should be increased an vice-versa. The function of the electronic load governors is to constantly adjust the load in such a way that the speed of the generator is always the same. To ensure this the electronic load governors use a ballast load (which can be any resistive device such as a heater), to add or subtract the loads as required depending on the human consumption. For example if the design load of a particular system is 3 KW, to ensure the correct voltage and frequency the power consumption should always be 3 KW. Now supposing that one or more appliances is switched off , then the excess power available is directed to a ballast load. A variable switch constantly regulates the amount of power being shunted to the ballast load. The electronic governor in this case must have a direct control over 3 KW of load, so that in the event of complete shutdown it could direct the total power to the load. Also it should be able to make small wattage adjustments for smallest of the load fluctuations. Electronic load governors are suitable for synchronous generators. For induction generators, induction generator controller (IGC) is used. IGCs work on a similar principle as used by electronic load controllers, but an IGC monitors the generated voltage and diverts the surplus power to the ballast load.

An enhanced version of the electronic load governor is a Load management system. Apart from regulating the load, it also can prioritize the loads. It can directly be connected to a number of loads through relays. Excess power is then directed to the loads in order of their priority. Load management systems can be used even with a small hydro system. They typically have six or more loads which can be prioritized and controlled using the relays.

11.10.1 Emergency Shutdown System

An emergency shutdown system is system that protects the generator from over speed. In cases of sudden breakdown of the transmission line, the generator will experience extremely high load or a zero load. To avoid the damage caused by these conditions, an emergency shutdown system can be installed. It is usually done by stopping all the water supply to the turbine. However it should be kept in mind that an sudden close down of the water supply can cause damage to the pipelines due to the increased pressure in them. Therefore the curt down of water supply should be done slowly, especially in high flow turbines such as Francis and Cross-flow. In impulse turbines this can be done faster, as the only thing that needs to be done is to divert the water jet away from the turbine.

11.10.2 Grid connected hydro systems

A small hydro system can be interconnected with the utility grid. However, proper control and safety mechanisms are essential for such interconnections. During peak power demand the grid connection can be used to draw power, while excess power can be fed into the grid during low power demands.

The control mechanisms used in the grid connected systems monitor the voltage, frequency and phase of the hydro system. If they are not compatible with the grid, they automatically disconnect the system from the grid.

11.10.3 Transmission/Distribution lines

Transmission lines are used to transport electricity from the plant to homes. Generally overhead lines are used for the transfer of power, but depending on the location and other regional factors even underground cable lines can be used. There are various factors affecting the design of a transmission line. The main factors are the amount of power to be transmitted, voltage level efficiency, type and size of the cable.

For micro-hydro systems, single-phase power lines are generally used.

For bigger hydro systems, a three- phase transmission is better suited, to reduce transmission losses.

Each country has its own national and local rules for the electrical transmission. These rules should be followed, while deciding on the transmission system.