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== Introduction ==

[[File:Flow diagram for wind turbine plant.jpg|thumb|Figure 1. Flow diagram for wind turbine plant]]
A '''wind turbine power plant''' uses the kinetic energy in wind for delivering the mechanical power. [[Wind power]] is a renewable source of energy. Since the prehistoric days wind is being used as a natural source of power and is being utilized for performing variety of mechanical work. It is one of the cleanest form of energy for producing power. There is no considerable waste materials from wind turbines after operations as compared to other energy source. The total annual [[wind energy]] available on earth is approximately about 13*10<sup>12</sup> kWh which is equivalent to 1500 power stations each delivering 1000 MW of energy<ref>Turbines,Compressors and Fans|author= S M Yahya|edition= Fourth edition|isbn=0070707022|year= 2011|Publisher= Tata McGraw Hill Education Private Limited,New Delhi</ref>.

There are different classes of wind turbines ranging from small to large having corresponding suitable applications. The small one having power production less than 10 kW are used in homes, farms and remote applications whereas intermediate wind turbines (10-250kW ) are having their utilities in village power, [[hybrid systems]] and [[distributed power]].  The larger wind turbines (660kW – 2+MW) are used in central station wind farms, distributed power and community wind<ref>http://www.alternative-energy-news.info/what-factors-affect-the-output-of-wind-turbines/</ref>. 


[[File:Economic comparison of wind energy.jpg|thumb|Figure 2. Economic comparison of wind energy]]

== Resources ==

[[Wind]] is natural source for [[wind turbines]]. The moving air is known as [[wind]]. The wind energy is one of the earliest forms of energy which is used for delivering mechanical power from  a [[renewable]] source. The working fluid of wind turbine plants is flowing air or wind which is freely available and thus leading to the reduction of fuel costs whereas other expenditures may be  involved. As similar to other [[power plants]], the cost per unit energy generated decreases with the increase in size of wind turbines<ref>Turbines,Compressors and Fans|author= S M Yahya|edition= Fourth edition|isbn=0070707022|year= 2011|Publisher= Tata McGraw Hill Education Private Limited,New Delhi</ref>.  [[Energy storage system]] must be employed in wind turbine plants in order to compete with [[anisotropic]] or random nature of wind energy<ref>Fundamentals of turbomachinery|author= B.K. Venkanna|Third Printing|isbn= 9788120337756|year= 2009|Publisher= PHI Learning Private Limited, New Delhi</ref>. Wind turbines can be set up anywhere if sufficient wind velocities were available for an adequate time.

== Available energy calculation ==

It is obvious that wind contains some energy. Therefore, the available energy in wind during time T is given by <ref>Turbines,Compressors and Fans|author= S M Yahya|edition= Fourth edition|isbn=0070707022|year= 2011|Publisher= Tata McGraw Hill Education Private Limited,New Delhi</ref>
:<math>E = k\int_{0}^{T} c^3\ dt.\,</math>, where;
k=constant (depends on the [[air density]], [[efficiency]], and size of wind turbine, c=instantaneous wind velocity

Now, maximum [[power]] that a wind turbine can develop is

'''''P<sub>max.</sub> = 0.593ρAv<sup>3</sup> /2''''', where; v = mean wind velocity, A = [[propeller]] diameter. 

Therefore, after considering efficiency factor (ἠ = 0.59)

'''''P = 0.000206Av<sup>3</sup>''''' kW

=== Calculations for different design of wind turbines ===



==== [[Horizontal axis]] ====

Axial [[thrust]] due to [[static pressure]] difference across the disc,

'''''F<sub>x</sub>=A(P<sub>1</sub> - P<sub>2</sub>) = 1/2ρA(c<sub>u</sub><sup>2</sup>  - c<sub>d</sub><sup>2</sup>)''''' , 

where; P<sub>1</sub> - P<sub>2</sub> = Pressure difference across the [[blade]]

c<sub>u</sub> = upwind velocity, c<sub>d</sub> = downwind velocity.

Now, power developed by the wind turbine propeller is 
'''''P<sub>i</sub> = 16/27(1/2ρAc<sub>u</sub><sup>3</sup>)''''', where;
P<sub>i</sub> = Maximum power ignoring [[aerodynamic]] and mechanical losses.

Also, '''''Power of upwind stream(P<sub>u</sub>) = 1/2ρAc<sub>u</sub><sup>3</sup>'''''

So the power coefficient is
'''''C<sub>Pmaximum</sub> = (P<sub>i</sub>) / (P<sub>u</sub>)'''''

==== [[Vertical Axis Wind Turbine]] ====

Maximum power, '''''P<sub>maximum</sub> = 2/27c<sub>F</sub>ρAc<sup>3</sup>''''', where; c<sub>F</sub> = coefficient depends on the type of blade and size of the machine, c = wind velocity

Maximum energy input to the machine is

'''''E<sub>maximum</sub> = 2/9c<sub>F</sub>ρAc<sup>3</sup>'''''

Also, maximum power coefficient is

'''''C<sub>Pmaximum</sub> = 0.333'''''

== Components of wind turbine ==

* [[Rotor]] - It consists of blades. The number of blades depends on the speed of rotor. The blade required will be more for low speed and less for high speed. The number of blades will also be decided after the calculation of vibration. The blades are often subjected to high and alternating stresses so should have high [[strength]] and light weight.

* Step-up [[gear]] - It is used to obtain required speed between the driving and driven shaft. It consists of [[gear train]], which should be highly efficient and light weight.

* Speed regulating mechanism -  From the point of view of aerodynamics, a constant blade to wind speed ratio IS required. A propeller type of [[pump]] and a [[hydraulic brakes]] are good speed [[regulators]] for both fixed and variable [[pitch]] blades.The mechanism for variable pitch blades which is used to make efficient operation of rotor at variable wind velocities is same as [[Kaplan]] hydroturbines.The relationship between power developed by various wind turbines of different propeller diameter at different wind velocitiescan be seen from figure 3.
[[File:Power developed by various wind turbines at different wind velocities.jpg|thumb|Figure 3.Power developed by various wind turbines at different wind velocities]]

* [[Electric generator]] - Both direct and [[alternating current]] machines are used as electric generators. [[Direct current]] generator can work at a wide range of speed but a. c. generator requires fixed speed.

* Orientation mechanism - This mechanism is needed in horizontal axis wind turbine to turn the rotor in the wind stream but vertical axis tubine doesn’t need this because they don’t depend on the wind direction.Some of the most common modern orientation mechanisms are :-
         
# By incorporating a [[wind vane]] which is used to orient small windmills in the direction of wind. It is the simplest and widely  used method.
# By incorporating an automatic direction finder and orientation mechanism.It is faster mechanism.
# By incorporating a fan tail mechanism.This is relatively slower mechanism.

* [[Tower]] - Rotor is to be mounted at high altitude because of high wind velocity and for this purpose a tower is used. Economic and vibration problem are the major factor in designing the tower.<ref>Turbines,Compressors and Fans|author= S M Yahya|edition= Fourth edition|isbn=0070707022|year= 2011|Publisher= Tata McGraw Hill Education Private Limited,New Delhi</ref>

== Analysis of Suitable Place ==

[[File:Fluctuations in hourly wind velocities.jpg|thumb|Figure 4.Fluctuations in hourly wind velocities]]

It should be noted that the wind cannot be stored elsewhere or in a way it should be pointed out that wind is available in a highly diluted form.The prediction of characteristic of wind due to the wide variations with time during the year and location is quite difficult and thus proper study of both the long and short range behavior of wind of a given location are required  for the economic installation of a wind power plant at the corresponding site.

[[File:Wind velocity duration curves.jpg|thumb|Figure 5.Wind velocity duration curves]]

[[File:Power duration trend.jpg|thumb|Figure 6.Power duration trend]]

As the wind power is proportional to the cube of wind velocity,the power output of wind turbine increases rapidly with its height above the ground.
[[File:Variation of wind velocity with altitude.jpg|thumb|Figure 7.Variation of wind velocity with altitude]]
Although installation and maintenance cost increases with the height above the ground therefore there should be balance between them.An important factor is the wind direction at that site, the more preferential site is that which has prevailing wind. The wind that blows preferentially in one direction than the other directions are known as prevailing wind.The locations which has prevailing wind does not require any orientation mechanisms.The effect of wind energy on wind energy is shown by [[Wind rose]]<ref>Turbines,Compressors and Fans|author= S M Yahya|edition= Fourth edition|isbn=0070707022|year= 2011|Publisher= Tata McGraw Hill Education Private Limited,New Delhi</ref>. 

[[File:Variation of annual specific output with mean wind velocity.jpg|thumb|Figure 8.Variation of annual specific output with mean wind velocity]]

== Selection of site ==

Major considerations for selecting the site are :-
* High mean wind velocity.
* Open area, not surrounded by mountain, buildings, forest etc. as presence of these retard the wind.
* Good land quality for such a huge foundation.
* Climatic conditions or [[topography]]. 
* Should be located at a distance from the site of application.
* Local labor and building materials should be easily available.
* Low temperature problem<ref>Turbines,Compressors and Fans|author= S M Yahya|edition= Fourth edition|isbn=0070707022|year= 2011|Publisher= Tata McGraw Hill Education Private Limited,New Delhi</ref>.

== Efficiency of wind turbines ==

A wind turbine is an exclusive form of [[turbo machine]] which often operates at relatively lower speeds. Efficiency of wind power plant is maximum at its rated (design) wind velocity and thus efficiency decreases with the fluctuations in wind. The lowest velocity at which the turbine develops its full power is known as rated wind velocity. 8 kmph is the minimum wind velocity below which no useful power output can be produced from wind turbine. There is limitation on both minimum and maximum wind velocity for the efficient operation of wind turbines and 56 kmph is the upper limit on wind velocity above which wind power plants are uneconomical. Therefore, they should work in these velocity ranges<ref>Large wind turbines|author=  Hau, E.--(Erich),Snel, Herman|edition=|isbn= 0471494569|year=2000|[[Wiley]],Chichester,New York</ref>.

=== Factors affecting efficiency ===

The efficiency of wind turbines are mostly dependent on five important factors:

* [[Wind power]]- The amount of power generated by a wind turbine would be greater if the wind blows faster and thus applying stronger force. Wind speed vary with the variation in locations and thus relationship between wind velocity with power production should be properly understood<ref>http://www.solarpowerwindenergy.org/2010/03/04/what-factors-affect-the-output-of-wind-turbines/</ref>.         P=Kvf, where  P= wind power, V= wind speed, F=wind force and k is a constant

* [[Altitude]]- Due to various atmospheric factors, wind speeds are higher at higher altitudes there would be lesser obstructions from the surrounding hills, trees and buildings. Adequate study of relationship between wind velocity and altitude shows that if the distance between turbine and ground is doubled then there would be approximately 12% increase in wind speed.
* [[Obstructions]]- The various factors such as trees, buildings, hills etc. restrict the free air flow by creation some form of friction which eventually slows down the wind and thus reduces the efficiency.
* Blade aerodynamics- The angle of airfoil with the direction of relative wind which is known as angle of attack has a significant effect on amount of lift. Turbulence develops and drag increases when angle of attack becomes too large which results in loss of lift.
Thus in order to get maximum efficiency the shape of blades should be accurately selected<ref>Aerodynamics of Wind Turbines|author= Martin O.L. Hansen|edition= Second edition|isbn= 9781844074389|year= 2008|Publisher= [[Earthscan]] in the UK and USA</ref>.
* [[Air temperature]]- The cold air has higher [[density]] and thus increase power output. It has been observed that power output from a wind turbines increases by approximately 16 % when the temperature of air is lowered from +20 deg celsius to  -20 deg celsius  for any given wind speed.

== Advantages and Disadvantages ==



=== Advantages of Wind turbines ===

* Wind is a renewable source of power and thus available in plenty.
* It is eco friendly therefore, no carbon-dioxide emission thus no [[greenhouse effect]].
* Wind turbine plants are easier to set-up with easily available materials in a very short time.
* Installation of wind turbine is labour intensive so also good for employment.
* Fuel expenditure vanishes and thus saves [[fossil fuels]].
* After installation no cost for producing electricity except maintenance.
* Wind turbines can also build on the sea.
* The wind turbine has a operating life of over 30 years and therefore long lasting.

=== Disadvantages of Wind turbines ===

* Wind energy is available in highly diluted form thus unsuitable for large power generation.
* Setting or capital cost for wind turbine plant is high.
* It requires large area of land for the set-up of wind turbine plants therefore disturbing the habitations of the local people and thus disturbing the environment.
* Wind turbines requires complicated mechanism when there is varying wind velocity.
* Limited number of places where there is [[prevailing wind]].
* [[Turbines]] cannot operate continuously for a large part of the year.

== See also ==

* [[Wind farm]]
* [[Wind power]]
* [[Wind turbine]]
* [[Wind turbine design]]
* [[Renewable energy]]
* [[Renewable energy commercialization]]
* [[Environmental impact of wind power]]
* [[History of wind power]]
* [[Wind power in India]]
* [[List of wind farms in the United States]]

== Further readings ==

*Wind Power Plants: Fundamentals, Design, Construction and Operation|edited by Robert. Gasch, Jochen Twel|edition=second edition|ISBN=978-3-642-22938-1|year=2007|Original German edition published by Teubner-Verlag
*Wind Power Generation and Wind Turbine Design|edited by Wei Dong|ISBN=978-1-84564-205-1|year= 2010|publisher= WIT Press
*Wind Power: Renewable Energy for Home, Farm, and Business|edited by= Paul Gipe| edition= second edition|ISBN=978-1-931498-14-2|year=2004|publisher=Chelsea Green Publishing Company

== References ==

{{reflist}}
<!--- After listing your sources please cite them using inline citations and place them after the information they cite. Please see [[Wikipedia:REFB]] for instructions on how to add citations. -

*Turbines,Compressors and Fans|author= S M Yahya|edition= Fourth edition|ISBN=0070707022|year= 2011|Publisher= Tata McGraw Hill Education Private Limited,New Delhi
*Fundamentals of turbo machinery|author= B.K. Venkanna|Third Printing|ISBN= 9788120337756|year= 2009|Publisher= PHI Learning Private Limited, New Delhi
*Aerodynamics of Wind Turbines|author= Martin O.L. Hansen|edition= Second edition|ISBN= 9781844074389|year= 2008|Publisher= [[Earth scan]] in the UK and USA
*Large wind turbines|author=  Hau, E.--(Erich), Snel, Herman|edition=|ISBN= 0471494569|year=2000|[[Wiley]],Chichester,New York
*http://www.articlesbase.com/environment-articles/wind-turbine-physics-factors-affecting-performance-1450642.html
*http://www.alternative-energy-news.info/what-factors-affect-the-output-of-wind-turbines/
*http://www.solarpowerwindenergy.org/2010/03/04/what-factors-affect-the-output-of-wind-turbines/
*http://www.istockphoto.com/file_thumbview_approve/49112563/2/istockphoto_4912563-design-elememts-wind.jpg

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== Wind turbine plants ==

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