A computational study of the effects of the radius ratio and attachment angle on the performance of a Darrieus-Savonius combined wind turbine

Small wind turbines as energy saving devices can be applied in many fields and are in great demand. There are two common types of rotors in wind turbines. Savonius rotors require a low torque for starting, but their efficiency is low. Darrieus rotors have high efficiency, but they are difficult to start up. A Darrieus-Savonius combined rotor aiming at a high aerodynamic performance with a low start-up requirement has drawn the attention of many researchers. The radius ratio and the attachment angle between the two types of rotors are important factors, but have not been studied systematically. In this paper, the effects of these two parameters on the efficiency and starting performance are investigated by a computational fluid dynamics approach. Steady simulations solving the Reynolds averaged Navier-Stokes equations with realizable k-ε turbulence model were conducted to obtain the static torque. Unsteady simulations were performed to calculate the power coefficient of the combined rotor at different tip-speed-ratios. A number of configurations were tested and the optimum attachment angle and radius ratio for the combined rotor was obtained with a maximum power coefficient of 0.363 and required starting torque below 0.1 Nm at a wind speed of 2 m/s.