Wind Power Generation | Operation of Wind Turbines
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Wind energy is a form of renewable energy. No doubt, wind energy has been used by the human beings millennia before the invention of electricity. Earlier, wind energy was used in shipping, grinding, pumping water etc. Today wind energy is used for the generation of electricity. The utilization of wind energy for power generation is increasing day by day due to the increase in the coast of oil, gas and coal. Also these resources are depleting fast and will exhaust in few years. One of the serious issues with these conventional energy resources is their impact over the environment. But on the other hand, wind energy is plentiful, inexhaustible and pollution free. These advantages encourage the developing as well as developed countries to trust wind energy for the production of electricity.
What is wind energy?
Wind energy can be considered as a form of solar energy. It is caused due to the uneven heating of Earth’s atmosphere by the Sun. The air near the earth surface gets heated up and moves upwards and the cold air replaces it. The motion of wind is affected by the rotation of the Earth and its geographical variations. The kinetic energy of the wind can be utilized in power production.
Over view of operation
Wind turbines are now primarily used for Electric power generation. The simplified operating principle of a wind turbine is as follows:
The fast moving wind strikes the turbine blades of the wind mill and rotates it. The turbine is coupled with the generator shaft through coupling gears. These gears are used to multiply the rpm of the shaft in order to match the rated rpm of the generator. The rated rpm of the generator can be calculated using the following formula:
Ns = 120f/p
P = number of stator poles.
Ns = Speed of the rotor in rpm.
F = frequency of the output power.
The output of the generator may be AC or DC depending on the type of generator used. If the generator used is an AC generator (alternator or induction generator then the output of the generator is directly supplied to the grid provided the output frequency and voltage of the generator remaining the same as that of the gird or the output of the generator is again rectified, stored in batteries, inverted and supplied to the grid. In case if DC generator is used then the output of the generator is inverted and fed to the grid.
The speed of wind generally increases with height. It is calculated that the wind at a speed of 26 mph contains 1 kW per square meter of cross-section. That is more than 1.3 horsepower. The output power of a wind mill depends upon the following factors:
The area swept by the blades of the turbine.
The Speed of wind
The wind density of air
The output power of a windmill can be given by the following formula:
Developed power P = 2.14 x Adv3 watts
A - The area swept by the blades of the turbine.
d - The wind density of air.
v – Velocity of wind.
Parts of a wind power plant
Parts of a wind power plant
A wind power plant consists of the following parts:
- Gear box
Wind turbines are classified based on their axis of rotation into two general types: horizontal axis and vertical axis. In a horizontal axis machine, the blades rotate on an axis parallel to the ground. In a vertical axis machine, the blades rotate on an axis perpendicular to the ground. Horizontal axis turbines are more commonly used than the vertical axis turbines.
Horizontal axis wind turbines
In a Horizontal-axis wind turbine (HAWT), the main rotor shaft and electrical generator are mounted at the top of a tower. Most of the turbines have two or three blades rotating around a horizontal axis. Horizontal axis turbine is provided with yaw control in order to keep the turbine always pointing towards the direction of wind.
Advantages of horizontal axis turbines.
- Horizontal-axis wind turbines are capable of collecting the maximum amount of wind energy, since the area swept by the turbine is more.
- Horizontal-axis wind turbines can withstand strong winds
- Horizontal-axis wind turbines can be more effectively controlled.
- High efficiency since the turbine always moves perpendicular to the direction of wind.
Vertical axis wind turbines
The vertical axis turbine (VAWT) has its blade rotating in an axis perpendicular to the ground. These turbines are mounded near the ground. Vertical axis turbines are not affected by the change in wind directing. But it is less stable than the horizontal axis wind turbines.
- Advantages of vertical axis wind turbines
- Since this type of turbines is placed at less height from the ground, it is easy to maintain.
- Yaw control is not necessary.
- Electricity can be generated at lower wind speeds as low as 6 m.p.h. (10 km/h).
- They may have a lower noise signature.
Asia's Largest wind farm
The largest wind farm in Asia is Muppandal wind farm in Kanyakumari, India. It has the total generating capacity of more than 1500MW.
2. Gear box
The turbine is coupled with the generator shaft through coupling gears. These gears are used to multiply the rpm of the shaft in order to match the rated rpm of the generator.
Generator is a device used to convert mechanical energy into electrical energy. The turbine is coupled with the generator through coupling gears. When the turbine is rotated by the wind the generator shaft is rotated and electricity is generator due to electro magnetic induction. The generators used for power generation can be classified into three categories. They are:
- Induction generator
- Permanent magnet Synchronous Alternators
- DC generator (Brushed)
1. Induction Generator
It is an ordinary three phase induction machine operating at generating mode. When the shaft of an induction moor is rotated at a speed greater that its synchronous/ rated sped, it acts as an induction generator. In the wind turbines where induction generators are used, initially the turbine is driven using the same induction machine, by drawing power from the grid. When the velocity of wind is sufficient enough to drive the shaft at a speed higher than its rated speed, the machine starts operating in generating mode and supplies the excess power back to the grid. While operating in generating mode, induction machine requires reactive power for magnetization that is drawn from the grid. Hence once it starts generating power, capacitor banks can be connected across the supply terminals in order to supply reactive power.
2. Permanent magnet Synchronous Alternator
Permanent magnet Synchronous alternators are widely used in wind power generation. This type of alternators has permanent magnet in its rotor. They have a very high efficiency (up to 90%). When the shaft of the alternator is rotated, due to electromagnetic induction, emf is induced in the stator of the machine. The output of this type of machines is alternating in nature, having its frequency varying with the speed of rotation of its shaft. Therefore the output of the alternator is rectified and either stored in batteries or again inverted and supplied to the grid.
3. DC generator (Brushed)
Brushed DC generators are used only in home wind turbines. It has very poor efficiency. Output is DC and it is stored in batteries.
Nacelle covers the key components such as generator, gear box etc. It protects them from external environment.
The whole set up of turbine, generator, gear box, control equipments are supported by huge towers. As the speed of wind increases with height, the heights of the tower are usually more than 30 meters.
Advantages and disadvantages of wind power
- Wind is energy is “free”. Hence the fuel cost for the production electricity from wind is “zero”.
- No environmental pollution.
- Wind turbines have low operating costs.
- Wind energy is a renewable form of energy and is inexhaustible.
- Very high initial cost.
- Causes noise pollution.
- It is difficult to predict their energy production in advance that reduces its reliability.
- Covers a vast area of land.