Experimental study on the characterization of the self-starting capability of a single and double-stage Savonius turbine

The Savonius hydrokinetic turbine is a viable alternative for rural electrification despite its relatively low power coefficient. There are many ways to improve power performance, including the use of multiple-stage turbines. Although many studies have investigated the effects of multi-staging, few have captured their effect on three performance parameters combined: power efficiency, flow structure, and self-starting capability. Therefore, the current study explored and analyzed the extent of these parameters using a multi-stage conventional Savonius turbine in a wind tunnel facility with a varying flow speed of 5 m/s to 9 m/s. The findings suggested that adding a stage increased the power coefficient to a maximum of 138% at 5 m/s, but reached a plateau as the wind speed increased to 9 m/s. The double-stage was discovered to reduce the self-starting speed of the turbine at all rotor angles for self-starting capabilities. The coefficient of static torque results indicated that the maximum static torque for the single and double-stage configurations occurred at 45° and 45°/135° rotor angles, respectively. The qualitative study revealed that the double-stage turbine has a larger wake size at higher wind speeds, resulting in a reduced wake intensity at the rear side of the rotor. The reduced torque variation caused by the additional stage has improved the overall performance of the turbine, making the double-stage configuration more preferable than the single-stage, particularly for low speed, high depth, and narrow river applications.