Hollow Blades for Small Wind Turbines Operating at High Altitudes

Abstract

Since the air density reduces as altitude increases, operation of small wind turbines (SWTs), which usually have no pitch adjustment, remains challenging at high altitudes due largely to the reduction of starting aerodynamic torque. By reducing the moment of inertia through the use of hollow blades, this study aims to speed up the starting while maintaining the structural integrity of the blades and high output power. A horizontal axis turbine with hollow blades was designed for two sites in Iran with altitude of 500 m and 3000 m. The design variables are the distributions of the chord, twist, and shell thickness and the improvement of output power and starting are the design goals. Blade-element momentum (BEM) theory was employed to calculate these goals and beam theory was used for the structural analysis to investigate whether the hollow timber blades could withstand the aerodynamic and centrifugal forces. A combination of the goals formed the objective function and a genetic algorithm (GA) was used to find a blade whose output power at a predetermined tip speed ratio (TSR) and the starting performance were high while the stress limit was met. The results show that hollow blades have starting times shorter than solid ones by approximately 70%. However, in the presence of generator resistive torque, the algorithm could not find a blade for an altitude of 3000 m. To solve that problem, the tip speed ratio was added to other design variables and another optimization was done which led to the optimal blades for both altitudes.