Betzel

Meet Betzel, a wind turbine who knows he can never catch all of the wind.

A turbine makes power by slowing the wind down and taking some of its kinetic energy. But if it stopped the air completely, the air would pile up behind the blades and no new wind could flow through. So some speed must be left in the air as it leaves.

In 1919 Albert Betz worked out the best possible balance. No turbine can capture more than 16/27, about 59 %, of the power in the wind passing through its blades. The wind's power is ½ρAv³. With sea-level air density ρ = 1.225 kg/m³, blades 40 m long in a 10 m/s wind receive about 3.1 MW, and at most about 1.8 MW could ever be captured.

The best design slows the wind to about one third of its original speed behind the turbine.

Because the power goes as the cube of wind speed, doubling the wind speed gives eight times the power, which is why turbines are put on hilltops and out at sea where the wind is strong and steady.