How much power can you get from a windmill? Windmills have long been used to pump water from wells, and are now being used to generate electricity on a somewhat wide scale. The most efficient windmills are a little under 50% efficient, and it is physically impossible to exceed 59%. For an excellent windmill (50% efficiency) the power available is given by the following formula: 

where the R is the radius of the propeller (1/2 of the diameter), and the v is the wind speed. The symbol means that the power is approximately equal to the value calculated on the right; however, the formula is correct for only the very best wind turbines, and only for moderate wind speeds.

Most wind systems are designed to produce constant power above a certain wind speed.  For example, a 200-kW system will produce power according to that formula until the wind speed reaches, say, 14 m/s, after which the wind turbine produces 200 kilowatts for all wind speeds until, say, 25 m/s, after which the machine must be shut down lest it blow apart.

The formula is an overestimate for all but the very best wind turbines.  For example, one popular 1-kW unit produces only half as much power as the formula predicts.


How much power will be produced by a very high quality windmill whose diameter is 7 meters (about 20 feet) when the wind velocity is 5 meters per second (about 11 miles per hour)?

Note:  The radius is 3.5 meters!

  <== The answer
California ... California has some huge windmills ---some 3200 of them --- covering mountain sides in their windy areas. (Tehachapi, Altamont Pass, San Gorgonio) All together, they produce --- at a rare full wind --- about 300 MW, which is about 1/4 as much power as a moderately large nuclear power plant produces, and is less than 10% of the electricity the small state of Connecticut consumes. 
Learn More  Ths Solar Fraud has a full chapter and part of an appendix devoted to wind turbines.

  To main Energy Advocate Index 
The Energy Advocate
Copyright © The Energy Advocate 1997. All rights reserved.