Extended Blade Element Momentum Theory for the Design of Small-Scale Wind Turbines

Abstract

Blade element momentum theory (BEMT) has been widely used in the design of the small scale wind turbine (SSWT). However, the original BEMT has weaknesses in providing the final design values of the wind turbine blade partly due to inaccurate assumptions of infinite number of blades made in deriving the theory. As such, the theory has to be amended in certain areas to form the so called the extended BEMT. In this study, a SSWT blade is designed using the extended BEMT method considering 3 factors: tip loss, low thrust at high axial induction factor, and high angle of attack in post-stall region where 5 correction models applying also the tip loss correction factor have been compared to the original BEMT model. The SSWT rotor has a diameter of 3m. An airfoil, SG6043 known for its suitability for SSWTs is used in this study. Furthermore, the blade geometry prior to the conduct of the shape optimization process are calculated using polynomial obtained from experimental procedures. The effect of infinite number of blades can be seen here to change the axial induction factor, especially at the tip of the blade and as a result increases the lift coefficient, and overpredicts the overall power coefficient and power of the wind turbine. With the Prandtl’s tip loss factor along with the correction model, the corrected final values of aerodynamic performances have been determined.