Albert Betz (1885-1968), a German physicist noted for his path-breaking theoretical studies of wind turbines. Betz' Law (1919) states that a wind turbine can convert 16/27 (or 59%) of the kinetic energy in the wind to mechanical energy. However, in practice, wind turbine rotors convert much less than this theoretical maximum. This is caused by energy losses in transmission, generation, and power conditioning. Energy loss is also caused by changes in wind speed, changes in wind direction, and changes in temperature. Using early wind tunnels, Betz also tested swept wing models for the German airplane manufacturer Messerschmitt, demonstrating that this wing design would allow airplanes to reach higher speeds. Betz's theoretical contributions are still the foundation for today's rotor theory.
Betz's Law (Danish Wind Industry Association)