Effect of the number of stages on the startup performance of an equivalent combined vertical axis tidal turbine

Vertical Axis tidal turbines demonstrate significant advantages in small-scale marine energy generation due to high reliability and the independence of flow direction. Although adopting a combined turbine enhances self-starting performance under low flow velocities, the influence of the number of Savonius turbine stages on the comprehensive performance of system requires further investigation. In this study, an equivalent combined turbine model is established by integrating three-dimensional computational fluid dynamics with the dynamic fluid body interaction method. The effects of the inflow velocity, load damping, and number of stages on self-starting characteristics are systematically analyzed. Results indicate that increasing the number of stages effectively eliminates static dead zones and suppresses fluctuations in instantaneous torque and angular velocity during the startup process. However, an increase in the number of stages decreases the power coefficient; specifically, the two- and three-stage configurations exhibit reductions of 3.35% and 8.60%, respectively, compared to the single-stage turbine. Based on a comprehensive evaluation of the startup time and power coefficient, the two-stage Savonius configuration yields the optimal performance. Consequently, the combined turbine reduces the startup velocity threshold, overcoming stall risks to achieve a maximum power coefficient of 0.311 at 0.3 m/s, demonstrating superior self-starting capability in low-velocity environments.