Computational fluid dynamics investigations over conventional, modified, and twisted Savonius wind turbines

Abstract

Wind turbines are devices that convert the kinetic energy present in the wind to clean, sustainable, and effectively renewable energy that could be used to generate electricity. A Savonius wind turbine is a drag-based vertical axis wind turbine (VAWT) that is simple, easy to construct and maintain, has low noise levels and good starting characteristics even at low wind speeds. Its only disadvantage lies in its low efficiency or low coefficient of performance. There are many design parameters that affect this efficiency. Therein, in order to increase the coefficient of performance, numerical investigations were carried out on different modified Savonius VAWT configurations, having either different curvatures, different overlap percentages, different angles of twist or a combination of the three. These investigations were computationally executed on Ansys FluentTM using the sliding mesh technique. Two dimensional simulations show that for a wind speed of 5 m/s and a tip speed ratio of 0.8, the half circle blade curvature combined with an overlap of 20% performs best, giving the highest net coefficient of moment, equal to 0.3065. Addition of mini blades to this optimal configuration was applied and a slight improvement in the coefficient of moment was observed. In contrary, the addition of extended surfaces on the blades caused a decrease in the minimum Cm reached during the rotation which resulted in a drastic decrease in the net average Cm extracted. Three dimensional simulations show that for a TSR of 0.8, Cm values are relatively low. A helical Savonius wind turbine with an angle of twist equal to 90º is considered an optimal configuration for these conditions (wind speed, tip speed ratio, aspect ratio, etc.…) with a Cm of 0.0701. Further three-dimensional simulations were carried out, which show that for a TSR of 0.45, the coefficient of moment values increase. The optimal Cm value is found to be equal to 0.2114 for a zero angle of twist.