Dual-function flapping hydrofoil: Energy capture and propulsion in ocean waves

Abstract

Bionic flapping hydrofoils can be utilized for either energy extraction or propulsion, which has attracted the interest of many researchers. This paper proposes a semi-active flapping hydrofoil device connected to a spring damper power takeoff (PTO) that can simultaneously capture wave energy and propel under ocean wave excitation. The effects of various PTO stiffnesses, PTO damping, and torsional stiffnesses on energy capture and propulsion are investigated using CFD simulation. The performance maps of power generation and propulsion are presented. Compared with the traditional semi-active flapping hydrofoil, the study discovered that the proposed dual-function flapping hydrofoil system could achieve higher overall efficiency. With increasing PTO damping, the overall efficiency initially rises and then falls. After optimizing the PTO parameters, it is possible to increase the overall efficiency by 32 %–86 %. By adjusting the PTO damping, the proportion of energy absorbed for electricity generation and propulsion can be redistributed. Larger torsional stiffness can compensate for the reduction in energy capture and has slight influence on propulsion power. The proposed flapping hydrofoil is expected to be implemented in small wave-powered marine robots, enabling wave propulsion and wave power generation simultaneously.