Aerodynamic analysis of a novel pitch control strategy and parameter combination for vertical axis wind turbines

Abstract

The performance of a vertical axis wind turbine (VAWT) deteriorates at low tip speed ratios (TSR) and it is mainly characterized by flow separation and dynamic stall. Several mitigating techniques have been developed recently based on flow separation and dynamic stall research activities. One of such techniques is the use of blade pitch angle control, which shows very promising optimal performance in VAWTs. However, its adaptation for periodic variation of the angle of attack remains an important issue that needs to be addressed urgently. Therefore, this paper proposes a novel pitch control strategy based on the VAWT-shape pitch motion to achieve blade dynamic pitch with the rotational parameters (TSR and azimuth angle). The pitch scale factor (μ) is introduced to proportionally vary the angle of attack. High accuracy computational fluid dynamics (CFD) methods are used to simulate dynamic changes in pitch angle, flow field and vortex shedding vorticity, with the turbulence modelled using the SST k-ω model. The results show that a 146% increase in power coefficient can be achieved using a μ of 0.3 at TSR of 1.25. Additionally, the use of dual pitch scale factors (dpsf) in the windward and leeward regions causes intense transient torque fluctuations at 0° (360°) and 180° azimuths due to a breaking distance in pitch angular velocity at these azimuths. Adding a weight function into the fitting process of the dpsf pitch curve effectively minimize these fluctuations.