Wave power capture performance of an over-actuated power take-off system

For oscillating-body wave energy converters (WECs), converting the kinetic energy of the floater's multi-degree-of-motion (multi-DOF) motions to electricity using a single power take-off (PTO) system may be attracting due to high power capture efficiency and reliability. In this work, a novel PTO system is proposed to gather the floater's two-DOF motions, i.e. surge and heave, using a single output shaft via a total of four rope transmission systems. Since the output shaft is simultaneously driven by two-independent-DOF motions, the PTO system is over-actuated. It is found that, when the surge restoring coefficient is rather small or large, the floater's surge velocity is too low to catch up with the output shaft, hence only the heave motion of the floater contributes to wave energy harvesting. Otherwise, both surge and heave motions engage in wave energy harvesting, and the power capture efficiency peaks when the surge motion is brought into resonance. Compared to traditional PTO systems that each can only extract the kinetic energy of the floater's single-DOF motion, this over-actuated PTO system can significantly improve power capture performance, demonstrated by the maximum capture width ratio exceeding the theoretical limit of either DOF.