Wind Energy Laboratory

Amirkabir University of Technology


FM Modulation of Wind Sound (modified for human hearing range)

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Wind Speed and Wind Turbines

Drag type wind turbines  have high starting torques. 

As soon as the rotor accelerates, the relative speed between wind and blades becomes lower and hence lower forces on the blades will be available . 

This phenomena causes the rotor to have blade speeds of not higher than the wind speed itself. 

They are rarely being used for electricity generation purposes, but their high starting torque have been utilised in other applications. 


a drag type wind turbine.


Lift type wind turbines are widely used in modern wind energy conversion systems


Horizontal and vertical axis wind turbines. Examples of lift type blades.


The blades in these types of wind turbines are very similar to the wings of an air plane. 

The lift forces are generated by different air pressures between the upper side and the lower side of the blades, and rotate the blades. 


The drag forces do not in general contribute to rotate the blades in the same way as the lift forces (the same phenomena is used to lift an airplane from the ground)

The Energy and Power in the Wind

The power curve of a wind turbine represents power versus wind speed.    This power curve can be modified (improved) by 5 different operation modes; 1) constant rotor speed, constant pitch.  2) constant rotor speed, variable pitch, 3) variable rotor speed, constant pitch, and, 4) variable rotor speed, variable pitch.

In practice the power vurve of a wind turbine is much more variable than straight line powers

The power output can fluctuate due to wind speed variations

The wind speed variations are stochastic (not deterministic).  Therefore the variations can be represented by statistical distributions.  Weibull distribution can best describe the statistical behaviour of wind speed variations.


The larger the wind turbine, the more the rotational moment of inertia.  The larger inertia can filter out the unwanted variations (harmonics) in the wind speed.  This can be studied using Van der Hoven spectrum.



Wind speed is higher at higher altitudes.  The rate of decay of wind speed (descending from higher altitudes) depends on the surface 'roughness'.

Due to the roughness, the higher the tower of the turbine, the higher the wind speed. Also...

...The higher the tower (height) of a wind turbine, the more accurate the wind speed measurements by the anemometer at hub height.  More importantly the measured value would be the average wind spatial distribution over the rotational area (A).

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