Experimental study of the effect of gap between the stages of a double-stage Savonius Hydrokinetic turbine with end plates and blade overlapping at low water currents

In the past, performance of two-bladed Savonius turbine was improved by double-staging. However, very few works considered its three-bladed design in a double-stage configuration (although it has more uniform average static torque) and attempted to improve its efficiency (which is less than the two bladed design). The focus of this research is to investigate gap as a parameter (from 0 to 20 mm) between the stages of a three-bladed double-stage SHKT having end plates and overlapping condition, and thus investigate its performance under various low water currents (0.45 to 0.65 m/s) and applied brake weights (100 to 1500 g) on the turbine shaft. Brake weights are considered to emulate the actual operation of the loaded turbine. The results demonstrate that the turbine produces more torque as more weights are put on, reaching its maximum hydrodynamic torque (0.084 N-m) at 1250 g brake weight. The stage-gap 5 mm, brake weight 1250 g, tip-to-speed ratio (TSR) 0.259, and upstream water current 0.55 m/s yield the highest coefficient of power (0.087), which is higher than a few of the published works (including two-bladed SHKT) under low TSR conditions. Further, Taguchi-based optimization yields almost the same performance as above.