Investigations on self-starting and performance characteristics of simple H and hybrid H-Savonius vertical axis wind rotors

Abstract

With recent surge in fossil fuel prices and demands for renewable energy sources, vertical axis wind turbine (VAWT) technologies have emerged out as one of the prime growing sector for small-scale power generation in the built environment. In such an environment, self-starting and high performances are of utmost importance. Amongst all VAWT designs, H-rotor, being a lift-driven device, exhibits a high power coefficient. However, it suffers from poor starting behavior due to its conventional symmetrical NACA airfoil blades. The objective of the present study is to design a VAWT rotor that possesses both self-starting and high power coefficient simultaneously. For this, a three bladed H-rotor with unsymmetrical cambered S818 airfoil blades is investigated, which shows self-starting characteristics at many of the azimuthal angles. However to make the rotor completely self-starting, the same H-rotor is incorporated in a hybrid system with Savonius rotor as its starter. It is found that the hybrid design fully exhibits self-starting capability at all azimuthal positions, signified by the positive static torque coefficient values. For improving power performance of the hybrid rotor, the same is subjected to rigorous experimentations on the wind tunnel at different Reynolds numbers (Re) between 1.44×105 and 2.31×105 for five different overlap conditions in the Savonius rotor part. The performance coefficients of the hybrid rotor are compared with the simple H-rotor. Out of all the designs investigated, the maximum Cp of 0.34 is obtained for the hybrid rotor at tip speed ratio (TSR) of 2.29 and Re of 1.92×105 for the optimum 0.15 overlap, followed by a maximum Cp of 0.28 at TSR of 2.42 and at same Re for the simple H-rotor. Furthermore insights of the performances of the optimum VAWT rotor are also obtained under different wind velocity conditions as prevail in the built environment. The optimum hybrid H-Savonius rotor demonstrates better power performance as compared to many of the existing VAWT rotors.