A comprehensive analysis of blade tip for vertical axis wind turbine: Aerodynamics and the tip loss effect

Vertical axis wind turbines are receiving renewed attention due to the challenges faced by horizontal axis wind turbines in recent years. To achieve commercial applications, a key aspect is to improve the relatively low power coefficient of vertical axis wind turbines. Among several factors that affects output efficiency, the threedimensional tip loss effect is essential because each vertical axis wind turbine blade has two tops (e.g., the Htype wind turbine). Although several blade tips have been designed, a comprehensive and fair analysis is necessary to find a suitable one. Therefore, in this paper, a total of 20 blade tips have been analyzed using the three-dimensional computational fluid dynamic method. The results indicate that the improved winglet and endplate we proposed are relatively optimal when the synthetic performance of aerodynamic efficiency, structural loads and start-up torque is considered. However, the improvement mechanisms of these two tips are different. Based on the performance curves and the fluid fields at specific tip speed ratios, we find that the endplate increases the blade torque in the upwind region by reducing the spanwise flow, but its negative effect in the downwind region partially counteracts the improvement. For the improved winglet, it reduces the tip loss in the upwind region and suppress separation in the downwind region at the appropriate tip speed ratio, thus significantly improving its performance in one rotor revolution. The winglet also contributes to the reduction of rotor thrust. In addition, the influence of the blade tip on wake is not remarkable and is mainly concentrated in the tip area affected by the tip vortex.