where um is the rotor speed.
The power coefficient data is obtained by experimental measurements or by using blade-element moment theory, and it depends on the size and geometry of the blades. Some works have been proposed to characterize the C p (l) expression, like the work presented in Ref. [27] . Although the general characterization of C p depends not only on l but also on the blade pitch q, the present work aims to increase the ef fi ciency of WTS in region 2, and q will be considered as fi xed, i.e. q ≡ 0. The following characterization taken from Ref.[28] is used in this work.