ELECTRICAL POWER POTENTIAL OF A WAVE ENERGY CONVERTER USING AN ACTIVE MECHANICAL MOTION RECTIFIER POWER TAKE-OFF

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 its optimal power. This paper introduces a power evaluation method that can approximate the optimal power with high accuracy. 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. 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, AMMR PTO increases electrical power output by 10-30 % over 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 an average 60% power enhancement compared to a conventional mechanical PTO. This suggests the AMMR PTO can be particularly useful when reactive power is not available.