Electrical power potential of a wave energy converter using an active mechanical motion rectifier based power take-off

Abstract

For wave energy converters (WECs), power take-off (PTO) design used to be all about increasing efficiency. Recently, more emphasis has been placed on the control execution capabilities of PTOs. The active mechanical motion rectifier (AMMR) is such a design that balances efficiency and controllability. However, the intrinsic nonlinearity brought by switching of its active clutches makes it difficult to evaluate the optimal power the PTO can achieve. This paper introduces a power evaluation method that can approximate the optimal power within tractable time. A larger control space is explored by making the control state-independent as a polynomial function of time. Periodical states are solved analytically under a symmetric switching scheme, leading to an analytical expression of the power in terms of the polynomial coefficients, which significantly speeds up the optimization process. This new method also enables the direct evaluation of electrical power output based on a linear modelling of the PTO drivetrain and the generator. A complete WEC model including an oscillating surge flap, the AMMR PTO, and a generator with a controllable load is represented as an equivalent circuit to analyze various mechanical and electrical responses of the device under regular waves. Particle swarm optimization is employed to find the optimal polynomial coefficients leading to the upper bound power potential. It is found that for the flap structure, an AMMR PTO increases electrical power by 10–30 % over a conventional PTO near the resonance period, where motion rectification is the most beneficial. Hardware-in-loop tests were performed on a small-scale PTO prototype, with damping control of the generator. Experimental results show 10–120 % power enhancement compared to a conventional mechanical PTO. This suggests the AMMR PTO can be particularly useful when reactive power is not available.