Performance Measurement of a Three-Bladed Combined Darrieus-Savonius Rotor

Abstract

Combination of Savonius and Darrieus type Vertical Axis Wind Turbine (VAWT) rotors possess many advantages over their individual designs, like low starting torque, high power coefficient, low cut-in wind speed etc.  However, there is still a need to do more research on such combination of rotors for their viability in the built environment where wind speed is low. In this paper, an attempt is made to measure the performance of a three-bladed combined Darrieus-Savonius rotor, with Darrieus mounted on top of Savonius rotor, for overlap variations from 10.8% to 25.8%. Power coefficients (C_p) and torque coefficients (C_t) were calculated in a low range of Tip Speed Ratio for each overlap condition. It is found that C_p increases with the increase of overlap. However, there is an optimum value of overlap for which, C_p is maximum, beyond this, C_p starts decreasing. The similar trend is observed for C_t as well. The maximum C_p of 0.53 is obtained at 0.604 Tip Speed Ratio (TSR) for an optimum 16.8% overlap. The performance of the rotor is also compared with another version of this hybrid design with Savonius mounted on top of Darrieus rotor. The present Darrieus-Savonius rotor can be suitably placed in the built environment where it can harness more power from wind and, at the same time, would self-start in low wind condition prevalent in such environment.