ABSTRACT
Betz's Theorem is as fundamental for understanding wind-driven machines as Carnot's theorem is for thermal machines. Two big families of wind turbines exist: those featuring a horizontal axis and those having a vertical axis. Each family is composed of different types. Each family and type has its specific characteristics, advantages, and disadvantages. In practice, vertical axis turbines always feature lower aerodynamic efficiency than horizontal axis turbines. The reason for such apparent contradiction with the extension of Betz's Theorem is that the blades' path of a vertical axis turbine is circular; consequently, the trajectory of the blade is orthogonal to the direction of the wind in only two points. The calculation model employed for the extension of Betz's Theorem does not represent accurately the operation of a vertical axis turbine, but instead it represents well the situation in which two horizontal axis turbines are placed in series along an airflow.
