Enhancing Self-Starting Capability and Efficiency of Hybrid Darrieus–Savonius Vertical Axis Wind Turbines with a Dual-Shaft Configuration
Farzad Ghafoorian, Sina Hosseini Rad, Mahdi Moghimi
Abstract
Self-starting capability has consistently presented a significant challenge for Darrieus vertical axis wind turbines (VAWTs). One advantageous approach to addressing this problem is the design of a hybrid Darrieus–Savonius VAWT. The hybrid VAWT enhances self-starting capability by increasing the power coefficient (Cp) within the low tip speed ratio (TSR) range and the torque coefficient (Cm) at initial azimuth angles, when the blades transition from windward to upwind position. A significant challenge associated with conventional hybrid VAWTs, in which both rotors are mounted on a single shaft, is the decline in efficiency at the high-TSR range. This inefficiency is due to the performance limitations of the inner Savonius rotor, which is designed to function at low angular velocities. In the high-TSR range, the vorticity generation around Savonius rotor buckets adversely impacts the Darrieus rotor performance and the hybrid VAWT. A dual-shaft configuration is proposed to mitigate this issue, which utilizes a drivetrain transmission system to prevent the Savonius rotor from exceeding its optimal angular velocity, thus acting as a control mechanism. The findings indicate that implementing the dual-shaft rotor resulted in a 35% improvement in Cp within the low-TSR range and a 25% enhancement in the high-TSR range. This improvement is achieved when the inner rotor’s angular velocity is maintained at 19.79 rad/s, which has been determined to be the optimal value for the inner rotor.