Improvement of sinusoidal pitch for vertical-axis hydrokinetic turbines and influence of rotational inertia

Abstract

In previous study (Chen et al., 2018), performances of a fixed pitch and a sinusoidal pitch H-Darrieus vertical-axis turbine (VAT) are compared by numerical simulations. With an appropriate amplitude of sinusoidal pitching, the power efficiency could be improved by 33%, and the fluctuation in power output, rotation speed and shaft torque are suppressed meanwhile. In this study, to reduce the hydrodynamic resistant torque, the original sinusoidal pitching is modified to keep the pitch angle at zero or at a small initial angle in the downstream half of rotation cycle. Numerical simulation results show this modification increases power efficiency by 20% over that of an original sinusoidal pitch turbine. Furthermore, systemically study confirms that as rotational inertia of rotor increases, the power coefficient and tip-speed ratio do not change much, and the fluctuation in output torque can be dramatically reduced. However, a very large rotational inertia turbine would perform very poor unless the rotor be accelerated over a threshold speed with exterior assistance.