Movement of air is called wind. About 1 to 3% of solar energy falling on earth gets converted into wind energy. Wind energy is dilute form of energy like solar energy. Uneven heating of the earth's atmosphere is the main cause of wind. The cause of uneven heating of earth's surface is the equatorial regions which receive more solar radiation than the polar region. Warm air at the equatorial region rises and comes down at about 30° North and 30° South latitude.
During day time, air above the land mass heats up more than air above water bodies likes lakes, seas, oceans. Hot air expands and rises up while cool air over water bodies rushes to fill the space and is called local winds. At night, the process is reversed.
HARNESSING WIND ENERGY
Harnessing of wind energy is not new. Wind energy has been used for centuries to sail vessels, pump water and grind grain. At present, Germany, USA, Denmark, Spain and India account for about 80% of worlds wind power with installed capacity of 7,93,000 MW. India ranks fifth in world and has wind potential of 45,000 MW. States harnessing wind energy are Tamil Nadu, Gujarat, Andhra Pradesh, Maharastra, Karnataka, Kerala, Lakshadweep, Rajasthan, Madhya Pradesh, Odisha, Uttar Pradesh, Andman and Nicobar. Among these states, Tamil Nadu has highest potential of wind energy. Asia's largest wind power plant named Pawan Shakti is located at Lamba near Porbandar in Gujarat.
POTENTIAL WIND AREAS
Wind potential of any geographical region depends on wind speed and wind energy density. Power can be extracted from a Wind Turbine Generator (WTG) if wind speed lies between cut in speed and cut out speed. However rated, output power obtained if the wind speed range lies between 14 m/s to 25 m/s. Areas having these speed range wind potential. According to Beaufort wind scale it is evident that areas with code 4, 5 and 6 have good wind potential. Above cut out speed, pitch control comes into action to produce rated output power.
Tropical regions are at 30° north and south of the equator. These regions have seasonal wind systems, like the monsoon and trade winds. These are high pressure belts. India is dominated by monsoon type of flow. Equator is the high temperature and high humidity region. Wind blows from sub-tropical belts towards the equator and are known as trade winds. The areas in these regions with rich wind potential are open sea, coastal areas, hills, valleys, terrace, saddle and khals (low depression).
In coastal regions, sea breezes and land breezes are the prevailing winds which occur due to temperature difference. During day times, land is hotter than sea and air above the land is heated and rises upward. The hot air over land has low pressure. The cooler air above the sea has high pressure. This causes a cooler air called sea breeze to blow from sea to land. At night, the land cools off more quickly than the sea. When the temperature onshore cools below the temperature offshore, the pressure over the water will be lower than that of the land. This causes a cooler air called land breeze to blow from land to sea.
Hilly areas have high wind potential. Hill experiences high wind speeds due to acceleration over the hill. Monsoon winds from Arabian sea and the bay of Bengal strike Himalaya mountain ranges and have high wind potential.
WIND TURBINE
To harness the energy of the wind for production of electric power we need a wind turbine. A wind turbine is a device which converts wind energy into mechanical energy. When wind blows, low pressure air forms on the downward side of blade. This air pulls the blade towards it causing the rotor to rotate. This is called lift force. Thus, lift force exerted by wind in a direction perpendicular to the direction of wind flow. The force exerted by wind on blade in the direction of wind is called drag force. The lift and drag causes rotor to rotate.
Main Parts of Wind Turbine
2. Hub: The blades of wind turbines are bolted to the hub which is mounted on a shaft.
3. Nacelle: It houses the generator, gear box and yawing mechanism. Nacelle is placed on the top of the tower.
4. Power Transmission System: The main parts of transmission system are main shaft, gear box, bearing and coupling. Gear box is placed between main shaft and generator. The mechanical power of rotor blades is transmitted to generator by a two stage gear box. It increases the slow speed of rotor blades to generator speed of 1000 or 1500 RPM.
5. Generator: It converts mechanical power input into electrical power. AC or DC generator is used depending on type of service rendered. Direct current can be generated by a brushless DC generator attached to the rotor shaft. Large induction generator type of wind turbine generators (WTG) are used for grid connected system. These are asynchronous AC generators. They draw reactive power from grid and supply active power to grid in a grid connected system, during the period of no wind, the induction generator will operate as a motor and this will draw power from the system and drags the rotor blades as a large electric fan. Therefore, the wind turbine is disconnected from the grid during calm period. Likewise, it is also disconnected during low wind speed.
6. Yaw Control: The yaw control turns the rotor and nacellee to face the wind. Modern large wind turbines are controlled to face the wind direction measured by a wind vane situated on the back of the nacelle. By minimizing the aw angle (the misalignment between wind and turbine pointing direction), the power output is maximised. This is done by yawing motor which continuously tracks and keeps the rotor axis in the direction of wind.
7. Controller: The controller is basically a microcomputer. A series of sensors measure the conditions in wind turbine. Mechanical and electrical parameters are measured by these sensors. For example, wind speed is measured by anemometer which helps in taking decision by microcomputer to stop as soon as wind speed exceed rated speed. Similarly, other parameters are measured constantly by sensors and help to control the operation of wind turbine.
8. Braking: It becomes necessary to stop wind turbine during adverse situations by applying brakes. The mechanical brake is a disc brake placed on the gearbox on the high speed shaft.
9. Tower: Tower provides stuctural support to wind turbine. It should be capable to withstand gravity load and wind loads of turbine. Tower height depends on wind speed and rotor design. It costs about 15% of turbine.
Classification of Wind Turbine
Wind turbine are generally grouped into two types:
1. Vertical Axis Wind Turbine (VAWT): The axis of rotation of rotor is vertical, i.e., perpendicular to the stream of wind. Savonius and Darrius wind turbines are vertical axis type. The Darrius turbine works on lift force and the Savonius turbine works on drag force. The vertical type turbine systems need no special infrastructure and can be kept on the ground.
2. Horizontal Axis Wind Turbine (HAWT): The axis of rotation of this turbine is horizontal, i.e., parallel to the stream of wind. This type of turbine requires special infrastructure to keep generator, gearbox, etc. at the hub height. As the height from ground increases the wind speed increases. Thus high power can be generated using horizontal axis wind turbine.
This type of turbine has five - systems :
- Rotor: Consisting of two or three blades mounted on a hub and also has pitch control system.
- The Transmission System: Including gearbox, hydraulic system, shafts, braking system and naccelle.
- Yaw System: This helps in positioning of rotor blade perpendicular to wind stream.
- Electrical and Electronic System: Consisting of generator, protective relays, circuit breakers, cables, control and electronic devices and sensors.
- Tower: This supports nacelle.
CLASSIFICATION OF WIND POWER PLANT
Different types of wind power plant are classified according to rating and are given in Table 1. Also type of generator generally used in each case is indicated.
Table 1
| Rating | Type | Generator Used |
|---|---|---|
| 0.5 to 1 kW | Very Small | PM Type DC Generator |
| 1 to 15 kW | Small | PM Type DC Generator |
| 15 to 200 kW | Medium | PM Type DC Generator Induction Generator |
| 200 kW to 1 MW | Large | Induction Generator (three phase) Synchronous Generator (three phase) |
| 1 to 6 MW | Very Large | Induction Generator (three phase) Synchronous Generator (three phase) |
TERMS AND DEFINITIONS
Cut in Speed: The speed at which turbine starts to rotate and generate power is called cut in speed. It is about 3 to 4 m/s.
Rated Output Speed: Wind speed at which output power reaches rated power is called rated output speed. It lies between 12 to 17 m/s. Above this speed there is no further increase in output power. This is done by adjusting the blade angle.
Cut Out Speed: At a speed there is risk of damage to the rotor and braking system is employed to bring the rotor to stand still. This speed is called cut out speed. It is around 25 m/s.
Wind Farm: A wind farm is an area where a large number of wind turbine generators (WTGs) are installed for developing wind power. Generally, a wind farm has 5 to 50 units of wind turbine generator (WTG) sets. These areas must have steady wind speed range of 6 m/s to 30 m/s with average annual wind speed of 10 m/s.
Angle of Attack (AOA): or α: It is the angle between a reference line on a body and the vector representing the relative motion between the body and the fluid through which it is moving. Angle of attack is the angle between the body's reference line and the oncoming flow.
Tip Speed Ratio (λ): It is the ratio of speed of outer blade tip to speed of wind
λ = Tip speed of blade / Wind speed
λ = ωr / v
Where,
ω = angular velocity of blade in radian per second
r = radius of blade
v = wind speed
Swept Area: It is the ar area of the circle created by rotating blade.
Swept area A = πr²
Solidity (σ): It is the ratio of blade area to swept area. Solidity represents fraction of swept area covered by metal part of blade. Turbine has low solidity if the number of blade is less. In such case, rotor should moves faster to capture wind energy otherwise major part of the wind energy would be lost through the large gap between blades. Turbine with low solidity must operates at high tip speed ratio. On the other hand, if the number of blades is more then the turbine has high solidity. These turbines are operated at low tip speed ratio.
It is evident that number of blade decreases with increase of tip speed ratio and is given in the Table 2.
| Tip Speed Ratio | Number of Rotor Blades |
|---|---|
| 1 | 8-18 |
| 2 | 6-12 |
| 3 | 4-10 |
| 4 | 3-8 |
| 5 | 3-5 |
| 8 | 3-4 |
| 10 above | 1-2 |
Table 2
Pitch Angle: It is the angle between blade and the plane of blade
Pitch Control of Blade: A system where the pitch angle of blade changes according to wind speed for efficient operation. Any change in speed is sensed by a governor and blade pitch angle is automatically adjusted to get constant frequency power with different speed.
Yaw Angle: The angle between wind direction and turbine pointing direction is called yaw angle.
Yaw Control: As the direction of wind changes frequently, the yaw control is provided to steer the axis of the turbine in the direction of wind. It keeps the turbine blade in the plane perpendicular to wind. There is a wind vane at the back of nacelle which monitors the wind direction and gives signal to the control system so that rotor blade face the direction of wind.
Anemometer: An instrument used to measure wind speed.
WIND ENERGY AND POWER
Wind Energy (E)
Kinetic energy of wind is given by
E = 1/2 mv²
where m = wind mass and v = wind velocity
where A = swept area through which wind is flowing, ρ = air density and t = time
E = 1/2 (Avtρ)v² = 1/2 Atρ v³
Air Density (ρ)
Air density (ρ) is directly proportional to air pressure and inversely proportional to air temperature in degree Kelvin and is expressed as
where R is gas constant, P is air pressure temperature in degree Kelvin.
R = 287 J/kg.K
For 15°C temperature
T = 15 + 273 = 288° Kelvin
Substituting the above values in the expression of air density gives ρ= 1.226 kg/m³
Air density is maximum increase of altitudes.
Wind Power (P)
Wind power P is energy per unit and is given by
P = E / t
= 1/2 Aρv³
Wind power is proportional to third power of wind speed. Therefore, wind turbines need to be efficient at greater wind speeds. If the wind speed doubles, then wind power increases to eight times. Also wind power is proportional to swept area (the area covered by rotating blades). Hence, the rotor blades should be long to make swept area large to get more wind power. Wind power is also proportional to air density (ρ). A wind generator will produce less power in summer than in winter at the same wind speed as air has low density in summer than in winter. Similarly, a wind generator will produce lesser power at higher altitudes as air density decreases gradually with increase of altitudes.
The air density is 1.22 kg/m³ under standard temperature (25°C) and pressure (760 mm of Hg), therefore the simple formula for the power is.
P = 0.6 Av³ watts
Wind speed increases at higher altitudes. It is proportional to 1/7th power of altitude. In an open flat area away from cities and forests, wind speed is given by
V ∝ H^1/7
where V = wind speed and H= height
Doubling the tower height of a wind turbine increases wind speeds by 10% and the expected power by 34%. Modern wind turbines have tower height of 50m to capture more power.
For a generator of given rated power, a low average wind speed requires a large turbine rotor, whereas a high average wind speed requires a small turbine rotor.
Bitz Limit
Total wind power available could be captured only if the wind speed is reduced to zero. But this is practically impossible as the captured air must leaves the turbine. The maximum theoretical efficiency is the ratio of maximum power output to total wind power available. It is also called power coefficient Cp.
Cp = Pmax / Ptotal = 0.593
The factor 0.593 is called Bitz limit.
Therefore, the maximum achievable extraction of wind power by a wind turbine is 59% of the total theoretical power.
The efficiency of wind turbine is further reduced due to the following factors:
Wind Power Density (WPD)
Wind power density (WPD) is the ratio of wind power per unit area of wind through which it is passing. WPD is the yardstic used to determine the best locations for wind energy.
= 1/2 ρv³
Maximum power density = Pmax / A
Actual power density = Efficiency of turbine × Total power density
Power output of turbine P = Actual power density × Swept area
Torque
Torque (T) on rotor shaft depend on angular velocity (w) of rotor and power output (P) of turbine.
P = ωT
T = P / ω
= ηt . 1/2 Aρv³ . 1/ω
Where ηt is efficiency of turbine.
LIFT AND DRAG
Energy of wind can be extracted by wind turbine by two forces acting on it, namely, lift force and drag force. When wind passes over the rotor blade it produces a pressure difference across its surface. The force acting from a high pressure side to low pressure side due to pressure difference is called lift force. This force acts perpendicular to the direction of air flow. A lift force based wind turbine has medium torque and high rotational speed (rpm). So these type of turbines are mainly used for electricity generation.
Drag is the force which is exerted on the turbine blade due to airflow across them. Drag force acts in the same direction as that of air flow. Drag force causes axial thrust on the blades. Wind turbines based on drag force provide high torque and low rotational speed (rpm). So they are used for water pumpling and grinding type of applications.
WIND ENERGY STORAGE
Wind energy can be stored in the following ways:
Pumped Water Storage
This system requires two reservoirs, one at the lower side and other at the upper side with sufficient head. Water from the lower reservoir is fed to the upper one by pump operated by wind turbine generator (WTG). The stored water in the upper reservoir is used to run a micro hydro turbine generator which produces electricity as and when required. So this type of storage is a hybrid of WTG and micro hydro system. It is suitable where both wind and water are available sufficiently. The size of this system is less than 100 kW.
Compressed Air Storage
Wind energy can be stored in the form of compressed air. When wind is not blowing energy stored in the storage tank can be used to drive the wind turbine to generate electricity by an electric generator as and when required.
Battery Storage
In this system, electrical energy generated by WTG is given to charge a battery bank which stores electrical energy in the form of chemical energy. The stored energy can be utilized at any time. However, an inverter system is required to convert de supply of battery to required ac voltage.
Lead acid batteries are commonly used for storage because they have high efficiency and low cost. Since high capacity batteries are not available at present, so a large number of batteries is required to store energy. Huge amount of energy cannot be stored because of high capital cost and high maintenance cost.
LIMITATIONS IN USING WIND ENERGY
There are certain limitations in using wind energy as an alternative energy source.
1. The locations of wind energy generators are limited to the areas where strong and dependable winds are available most of the time.
2. Since, the sufficiently strong winds are not available all the time, the energy produced by the wind energy generators is also intermittent. So, the wind energy generators are required to be supported by a back-up supply. This enhances the overall cost of the system.
3. Because of the great heights of turbine tower and large rotor blades, it is difficult to repair and maintain them.
4. The wind turbines must be strong to withstand storms and lightning.
BLOCK DIAGRAM OF CONTROL PANEL OF WIND ENERGY SYSTEM
Wind energy in large scale can be obtained from a group of wind turbine generator system working in an area is called wind farm.
It is microprocessor based control system of a grid connected Wind Energy Generator (WEG). It is provided to change setting and to adjust parameters to get maximum output. The controller receives input of wind speed and direction along with load requirement at given voltage and frequency. It gives signal to the turbine for yaw control blade pitch control and to apply brake in case of high wind.
OFF-GRID WIND TURBINE SYSTEM
When the wind turbine is operated in isolation from the grid system, then it is called off-grid or stand alone system. It supplies to a group of local consumers. The generating capacity must be matched with the demand of the consumer. This system is mostly used for. (i) power supply for domestic use, (ii) battery charging, (iii) power supply for water pump for irrigation and drinking for purposes.
The variable ac supply obtained from WEG is first rectified into de in control unit (CU) and then given to charging of battery. With the help of inverter de is again converted into ac at desired voltage and frequency and fed to the local consumers. In the absence of wind, the battery can give supply to the consumer. It has the advantage of supplying load to the local consumers mainly in remote area where no grid connectivity is available. Further, it has no transmission losses as load is near to the WEG.
VOLTAGE REGULATION IN WIND TURBINE GENERATOR (WTG)
Wind energy is one of the most rapidly growing sources of electricity generation all over the world. It is predicted that 12% of the total world electricity demands will be supplied from wind energy by 2020. This encourages researcher to develop various control techniques for better performance of wind turbine within a wind speed range and to generate constant frequency power and to keep the terminal voltage of generator within the prescribed limit. WTG based on induction generators have no inherent voltage control. WTGs installation is connected to a grid substation. This substation has a variety of equipment to control voltage such as capacitor, tapping transformers, etc. Some WTG use power electronic converters which regulate reactive power to achieve desire voltage control.
WIND TURBINE NOISE
Wind turbine generates two types of noise: aerodynamic and mechanical. Aerodynamic noise is generated when blades are rotating through air. Noise level depends on turbine size and wind speed. Estimated noise level in dB of some standard make turbine is given in Table 3.
| Turbine Size | Wind Speed (m/s) | Noise Level (dB) |
|---|---|---|
| 900 W | 5 | 83 |
| 10 | 91 | |
| 10 kW | 5 | 87 |
| 7 | 96 | |
| 10 | 105 |
Table 3
Mechanical noise is generated by turbine's internal gear. It is irritating. Excessive exposure to noise causes hearing loss and sleep disorder.
SUITABLE SITE FOR WIND TURBINE GENERATOR (WTG)
The main factors governing selection of site of WTG are:
1. Availability of Wind: Site of WTG should be selected considering the wind data for that location. For that data from wind observatories may be used. The site should be such that it has high wind potential and wind power density. The area for this are open seas, coastal areas, hilly areas, etc. Wind speed range for power production lies between 14 m/s to 25 m/s.
2. Availability of Land: The height of the wind turbine should be between 10 m to 100 m above the ground. Therefore, the land area for installation of wind turbine should be free from obstacles buildings, towers, poles, trees, etc. Also the terrain and soil condition should be suitable for establishment of wind farms.
3. Accessibility: The site should be accessible for transport of machinery and construction material. Also the site should be accessible for maintenance.
4. Grid Proximity: The site should be near the power grid so that its power generated from WTG can be fed to the grid without transmission losses.
ADVANTAGES AND DISADVANTAGES OF WIND ENERGY
Advantages of Wind Energy
1. The wind is free and with modern technology it can be captured efficiently.
2. This energy does not produce green house gases or other pollutants.
3. Wind turbines mounted over tall tower which occupies a small plot of land. Thus land below can still be used in the case of agricultural areas where farming can still continued.
4. Electricity generated by wind turbines can be used to supply to remote areas that are not connected to grid.
5. Conventional energy sources will be exhausted in future. Wind energy is a renewable and will play main role in future.
6. Wind turbines are available in a range of size which benefit a wide varieties of consumers.
Disadvantages of Wind Energy
1. The wind speed is not constant and it varies from zero to that of storm. This means that wind turbines do not produce the same amount of electricity all the time. There will be times when they produce no electricity at all.
2. Many people feel that these large structures create a bad landscape.
3. Wind turbine are noisy. So the wind power plants should be located away from residential areas.
4. There is some pollution at the time of manufacturing of wind turbines.
5. Large wind farms are needed to provide power supply for entire area.
6. The limitation of wind power is that no electricity is produced when the wind is not blowing.
7. Limited to windy areas.
8. May affect endangered birds, however tower design can reduce impact.
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