Effects of internal rotor parameters on the performance of a two bladed Darrieus-two bladed Savonius hybrid wind turbine

Abstract

The hybrid vertical axis wind turbines (VAWTs), which are constructed by a combination of the Savonius and Darrieus rotors, have recently been proposed to improve the wind energy extraction from the free-wind flow. This study focuses on the impact of internal rotor parameters on the aerodynamic performance of the resultant hybrid rotor. For this goal, a two-blade Savonius rotor constructed by conventional semicircle blades with a constant overlap ratio of 0.2 is attached to a two-blade Darrieus rotor constructed by straight NACA 0018 blades. The resultant hybrid rotor has a constant diameter ratio of 0.2. To assess the Savonius wind turbine parameters, three various attachment angles of 0°, 45°, and 90° are examined under various wind potentials having average wind speeds of 5 and 10 m/s. A set of computations is performed at different tip speed ratios (TSR) of 1.5, 2.5, and 3.5. The obtained results have been validated against the available data for both Savonius and Darrieus rotors and good agreements are obtained from these comparisons. It is revealed that selecting an appropriate attachment angle is strongly dependent on the free-wind speed and TSR. Additionally, regardless of the free-wind speed, the hybrid rotor with an attachment angle of 45° presents the maximum power coefficient among the hybrid and non-hybrid rotors for TSR = 1.5 and 2.5. However, at TSR = 3.5, the hybrid rotor with ϕ = 0° and Darrieus rotor exhibit optimal performances among the considered cases at U∞ = 5 m/s and 10 m/s, respectively.