Specific output of windmills—A discovery

Because the wind varies widely from point to point on the earth it has generally been assumed that the specific output of a wind turbine generator, the kilowatthours generated in a year per kilowatt of rating, could only be determined from the particular wind pattern involved. However, it was discovered empirically that the specific output of windmills is practically independent of their location on the earth, or of the mean annual wind velocity. It depends instead on the ratio of the rated speed Vr, the wind speed at which full rating is realized, to the mean annual wind velocity Vm. This is demonstrated, by data from many designs and wind-power sites. The resulting curve of specific output versus Vr/Vm, together with the fundamental formula for power extracted from the wind by a windmill (1) constitutes a useful approximate design and optimizing method. Examples are given.