A blade pitching approach for vertical axis wind turbines based on the free vortex method

This paper presents a variable pitch (VP) approach for a vertical axis wind turbine running at rated or higher tip speed ratios (TSRs) in order to improve the blade aerodynamic performance at the azimuth angle with a low angle of attack (AoA) and the peak power coefficient (C-p) of the wind turbine. The four largest pitch angles occurred in each quadrant in the proposed VP approach, whereas the two largest pitch angles occurred at azimuth angles of 90 degrees and 270 degrees n the traditional VP approaches. The free vortex model was used to predict the aerodynamic performances of the blades in the proposed VP approach and the fixed pitch (FP) blade at the designed TSR. The circulation of different vortices, the AoA, the induced velocity, and the lift coefficient obtained from the modeling results were compared for the different methods. The results indicated differences in the strength of the bound vortex and the shed vortex between the VP and FP blades at different azimuth positions. The range of the largest AoAs in the VP blade was in two big scopes, whereas the largest AoAs in the FP blade occurred only at two azimuth positions. The torque and the C-p increased at different azimuth angles, and similar trends of the AoA were observed in the azimuth angle range of 0 degrees-360 degrees. The differences between the VP and FP blades provided in-depth information on the performance of the proposed VP approach. The integration of the proposed VP approach and the traditional approach provides a VP strategy to control the pitch angle of the blade based on the azimuth angles and the TSRs and results in better self-starting ability at low TSRs and greater C-p at various TSRs.