Improving the Energy Conversion Efficiency of a Savonius Rotor Using Automatic Valves

Savonius wind turbines are popular for their easy fabrication and high starting capabilities. Nevertheless, they suffer from low power coefficients, which are mainly due to a negative torque resulting from the blade moving against the upcoming wind. Numerous methods have been proposed to alleviate the negative torque, among them are modified blade profiles (twisted blades), adding flow deflectors, and valve-aided blades. In this study, the effects of adding automatic valves to a two-bladed Savonius rotor on its energy conversion efficiency are investigated numerically and experimentally. The valves are placed at three different positions: close to the rotor axis, at the blade center, and at the tip of the rotor. Results show that although adding valves can decrease the negative torque of the returning blade, they can also lead to a considerable reduction in the positive torque of the advancing blade. For the rotors in the current study, the maximum power coefficient is increased 20.8% when the valves are at the tip of the blades, while the two other cases have decreased the power coefficient of the rotor. Adding the valves to the blades does not change the tip speed ratio corresponding to the maximum power coefficient of the rotor.