Overlap Ratio as the Design Variable for Maximizing the Efficiency of a Savonius Wind Rotor: An Optimization Approach

Abstract

The application of Savonius wind rotor is increasing worldwide to provide electricity without contributing to global warming and promoting the small-scale power production. However, its lower performance remains a major problem due to high unproductive torque produced by the returning blade. In this paper, an optimum blade profile is obtained by maximizing the power coefficient (CP) considering the overlap ratio (OR) as an optimization parameter. This is done by coupling computational fluid dynamics (CFD) simulation to the rotor blade profile developed through the simplex search method. The blade profile is symmetric about the x-axis, where half of the blade geometry is formed by a natural cubic spline curve using three points. Two end points are retained fixed, while x and y of the third point is taken as a variable in addition to OR in the simplex search process throughout its iteration using the MATLAB platform. In all the iterations, the blade profile is meshed by using ANSYS ICEM CFD platform. The rotor performance analysis is carried out by ANSYS Fluent using the shear-stress transport (SST) k-ω turbulence model. The finite volume method (FVM) is used as a solver setup to solve the transient 2D flow around the rotor blade. The optimum blade profile is compared to a conventional semicircular blade profile over a wide range of tip speed ratio (TSR). The present study demonstrates the superior performance of the optimum blade profile showing CPmax that is 23% higher than the conventional semicircular blade profile at TSR = 0.8. Further, at OR = 0.154, the CP is found to be maximum. The velocity magnitude contours, total pressure and turbulence intensity contours are generated to analyse the effect of the optimal design approach.