Time-Varying Damping Coefficient to Increase Power Extraction from a Notional Wave Energy Harvester

Energy efficiency and renewable energy generation is attracting increasing attention, due to the established understanding that a new sustainable approach to power human activities is compulsory, on the one hand, and thanks to technological advances, on the other hand. Energy harvesters, at different scales, are promising tools, but their performance and economic viability depend on advanced energy-maximisation control techniques. This paper borrows a control strategy from the vibration energy harvesting field, and implements it to a notional wave energy conversion application. A linear time variant damping coefficient of the power take-off unit is considered, oscillating at twice sinusoidal excitation force frequency, inducing parametric resonance in the system and significantly expanding the response bandwidth. Active and semi-active solutions are investigated, allowing or preventing bi-directional power flow, comparing results with traditional passive and impedance-matching control. Results show that, although both active and semi-active control successfully increase the mean power extraction, much larger than passive control, semi-active control ensures better power quality and lower control forces.