
First up, we need to understand wind power. If we double the wind speed, we don't double the power we can harness from it ( or push through it ). The power is actually cubed. To put it simply...
Power = Windspeed^{3}
So if a hypothetical windmill can get 10 watts from a 10kmh wind, then it could get 80 watts from a 20kmh wind, or 270 watts from a 30kmh wind. Please note, all figures used on this page are simplified and don't represent real world data.
Our typical windmill alternator, on the other hand, has a linear output. If the windmill RPM doubles, so does the output power. The wind power curve and alternator power curve mismatch, so the alternator is built to run in a "band" of wind speed where it can make the most power. There is more power in high winds, but these happen less often than low winds. If we built a windmill to only use high winds, we would miss out on the more common low wind power. If we build a windmill to use low winds, we miss out on the big power in high winds.
So how do we take advantage of high and low winds? One way is by using different alternators, each designed to run in a different band of wind speed.
