Wind speed and power
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The following shows the wind speeds around Cape Town, and corresponding power available for various size rotors.


Bear in mind that actual windspeed at any locality is very dependant on local terrain, especially trees and buildings. The higher the mounting tower, the less these obstructions will affect the wind. Even in completely flat terrain, wind power increases approximately linearly with height.

The rule of thumb is that turbulence from an obstruction reaches up to twice the height of the obstruction. But if your site has obstructions, remember that wind energy is 'free', so its Ok to 'waste' some of it by not having a high enough tower.

This map shows very approximate areas of wind power density class, where class 1<100 W/m2, class 2<150, 3<200, 4<250, 5<300, 6<400, 7<1000, and class 8 >1000 W/m2.

The maximum gust speed above shows what a turbine must be able to withstand.

The graphs below show the maximum power available to turbines of 1, 2 and 5 metre diameters. Note how power increases dramatically with rotor size - rotor size is by far the best single method of rating wind turbine power. Don't be fooled by a 'kilowatt' rating of the generator - this inlcudes a host of hidden assumptioms. The power on these graphs is calculated using an air density of 1.2kg/m3, the Betz limit, and a Rayleigh distrubution factor of 1.9.
Note that the above plots maximum power available. This means that (barring some unprecedented breakthrough in engineering and physics) no wind turbine can possibly generate more power than shown. Actual turbines may operate at anywhere from 20% to 80% efficiency.

The 'farm windmills' typically used to pump water extract only about 15% of wind energy.

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