Investigation into the relationship of the overlap ratio and shift angle of double stage three bladed vertical axis wind turbine (VAWT)

This study presents the experimental test results of Savonius-type vertical axis wind turbines (VAWT) with different overlap ratios and shift angles. Each wind turbine was tested under four different wind speeds. The power coefficients (CP) of vertical axis wind turbines are governed by several key factors, e.g. the number of blades, the shape of the blades, the overlap ratio (OL) and the phase shift angle. The overlap ratio and the phase shift angle (PSA) are the most decisive ones since the measured data shows that their power coefficient drops or rises with them significantly. Three turbines with the overlap ratios of 0, 0.16 and 0.32 were designed and constructed. The wind turbines were then adjusted to the phase shift angles (PSA) of 0, 15, 30, 45 and 60 degrees before testing them in an open wind tunnel under the air velocities of 4 m/s, 6 m/s, 8 m/s and 10 m/s.The results show that a higher overlap ratio has a higher impact on improving the starting characteristics of the Savonius wind turbine than any phase shift angle changes. This investigation shows that a specific phase shift angle in relation to a specific air velocity will increase the power coefficient significantly.Besides the increase of the power coefficient seen at specific air velocities and phase shift angles, did the recorded data show an unexpected second performance peak which appeared at higher tip speed ratios. This is surprising since the Savonius turbine is considered as being a drag driven turbine, and suggests that the lift characteristics of Savonius turbines might be more significant than commonly considered.

► The overlap ratio and the phaseshift angle are the most important features to regulate the power coefficient of the VAWT.
► The measured data shows that the generated power coefficient drops or rises with them significantly.
► Results show that higher overlap ratio can improve the starting characteristics more than any phase shift angle change.
► A specific phase shift angle in relation with a specific air velocity can increase the power coefficient.