Betz Limit

The Betz limit says there is an upper bound on how much kinetic energy a wind turbine can extract from moving air: at most 59.3% of the wind’s power can be captured by an ideal rotor. It comes from conserving mass and momentum in the airflow, because a turbine must slow the wind down to take energy from it, but it cannot reduce the wind speed to zero or the air would stop flowing through the rotor. The remaining energy stays in the air as motion downstream, so any real turbine must be below this ideal limit because of drag, blade losses, wake rotation, and mechanical/electrical inefficiencies.

Use this formula when evaluating or comparing wind turbine performance, especially when calculating the theoretical maximum power coefficient or checking whether a reported efficiency is physically plausible. It is also useful in wind-energy design, renewable-energy coursework, and discussions of the theoretical limits of energy extraction from a fluid stream.

• Treating 0.593 as the efficiency of every real turbine instead of the theoretical upper bound
• Using the Betz limit to find wind power itself rather than the fraction of wind power a turbine can capture
• Ignoring that the actual power coefficient of a real turbine is always lower because of practical losses