Exploring the impact of flexible membranes on the performance of H- and S-type hydrokinetic turbines: An experimental investigation

Vertical axis hydrokinetic turbines (VAHTs) have the potential to harness deep-sea current energy without the need for a yaw mechanism. However, H-VAHTs have weak self-starting capabilities, and S-VAHTs also face startup dead point issues. To address the self-starting challenges of VAHTs, this study proposes attaching flexible membranes to the blades of VAHTs. The impact of flexible membranes on the performance of both types of VAHTs is evaluated through Particle Image Velocimetry (PIV) water flume experiments. The results demonstrate that attaching flexible membranes to the blade surfaces resolves the self-starting dead point issue for both turbines, with a greater improvement in self-starting performance for H-VAHTs than for S-VAHTs. The study found that for both turbines, attaching flexible membranes with a large opening on the blade's outer side offers the best self-starting performance and power output. However, the flexible membranes reduced the C_P of H-VAHTs, while slightly enhancing the performance of S-VAHTs. PIV tests showed that the flexible membranes form a cup-shaped structure when facing the flow. The pressure difference between the inside and outside of the membranes generates additional static torque, improving the self-starting performance of both turbines. Furthermore, even with one or two membrane blades missing, the self-starting capability of H-VAHTs was still enhanced; flexible membranes with a certain installation angle can also be attached to the endplates of S-VAHTs to address startup dead issues. In summary, attaching flexible membranes to VAHTs improves their self-starting performance and ensures power output, thereby facilitating the application of such turbines in deep-sea areas with low flow velocities.