Wind energy is the kinetic energy of air, which is partially recovered, by the VAWT. In other words, the power output of the VAWT cannot be entirely recovered for mechanical power. The coefficient of power is known as the fraction of the power output extracted from the power in the wind by VAWT. In the theory of Betz, the coefficient of power is always not greater than 16/17 (0.59)(Abea et al. 2005). This inherent characteristic of a given VAWT can be used as efficiency of the VAWT. The coefficient of power is defined as (Kim and Gharib, 2013): สมการ