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The power coefficient [9] C P (= P/P wind) is the dimensionless ratio of the extractable power P to the kinetic power P wind available in the undistributed stream. [ citation needed ] It has a maximum value C P max = 16/27 = 0.593 (or 59.3%; however, coefficients of performance are usually expressed as a decimal, not a percentage).
The coefficient of power is the most important variable in wind-turbine aerodynamics. The Buckingham π theorem can be applied to show that the non-dimensional variable for power is given by the equation below.
The power coefficient, , expresses what fraction of the power in the wind is being extracted by the wind turbine. It is generally assumed to be a function of both tip-speed ratio and pitch angle. Below is a plot of the variation of the power coefficient with variations in the tip-speed ratio when the pitch is held constant:
For a wind turbine, the power harvested is given by the following formula: = where is the aerodynamic power and is the density of the air. The power coefficient is a representation of how much of the available power in the wind is captured by the wind turbine and can be looked up in the graph above.
Energy harnessed by wind turbines is variable, and is not a "dispatchable" source of power; its availability is based on whether the wind is blowing, not whether electricity is needed. Turbines can be placed on ridges or bluffs to maximize the access of wind they have, but this also limits the locations where they can be placed. [ 116 ]
The advance ratio is the inverse of the tip speed ratio, , used in wind turbine aerodynamics: [6] μ = λ − 1 {\displaystyle \mu =\lambda ^{-1}} . In operation, propellers and rotors are generally spinning, but could be immersed in a stationary fluid.
An example of a wind turbine, this 3 bladed turbine is the classic design of modern wind turbines Wind turbine components : 1-Foundation, 2-Connection to the electric grid, 3-Tower, 4-Access ladder, 5-Wind orientation control (Yaw control), 6-Nacelle, 7-Generator, 8-Anemometer, 9-Electric or Mechanical Brake, 10-Gearbox, 11-Rotor blade, 12-Blade pitch control, 13-Rotor hub
The wind profile power law relationship is = where is the wind speed (in metres per second) at height (in metres), and is the known wind speed at a reference height .The exponent is an empirically derived coefficient that varies dependent upon the stability of the atmosphere.
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