Effect of pitch angle on power performance and aerodynamics of a vertical axis wind turbine

- For the studied turbine, a small negative pitch angle β = −2° increases turbine CP for 6.6% compared to β = 0°.
- Fixed pitch angle can affect the instantaneous and averaged loading and power conversion for VAWTs.
- Adding a fixed bound circulation (fixed β) can change the strength of shed vortices and wake generation for VAWTs.
- Fixed pitch angle shifts the instantaneous moment (Cm) on turbine blades between the fore and aft halves.
- The shift in Cm proposes individual blade dynamic pitching as a promising power enhancement method for VAWTs.

Due to growing interest in wind energy harvesting offshore as well as in the urban environment, vertical axis wind turbines (VAWTs) have recently received renewed interest. Their omni-directional capability makes them a very interesting option for use with the frequently varying wind directions typically encountered in the built environment while their scalability and low installation costs make them highly suitable for offshore wind farms. However, they require further performance optimization to become competitive with horizontal axis wind turbines (HAWTs) as they currently have a lower power coefficient (CP). This can be attributed both to the complexity of the flow around VAWTs and the significantly smaller amount of research they have received. The pitch angle is a potential parameter to enhance the performance of VAWTs. The current study investigates the variations in loads and moments on the turbine as well as the experienced angle of attack, shed vorticity and boundary layer events (leading edge and trailing edge separation, laminar-to-turbulent transition) as a function of pitch angle using Computational Fluid Dynamics (CFD) calculations. Pitch angles of −7° to +3° are investigated using Unsteady Reynolds-Averaged Navier-Stokes (URANS) calculations while turbulence is modeled with the 4-equation transition SST model. The results show that a 6.6% increase in CP can be achieved using a pitch angle of −2° at a tip speed ratio of 4. Additionally, it is found that a change in pitch angle shifts instantaneous loads and moments between upwind and downwind halves of the turbine. The shift in instantaneous moment during the revolution for various pitch angles suggests that dynamic pitching might be a very promising approach for further performance optimization.