Quasi-passive reactance control for two-body axisymmetric heaving WECs

Ocean wave energy has the potential to play a key role in a low-carbon future due to its vast untapped resources. However, Wave Energy Converters (WECs) remain at a pre-commercial stage and must improve competitiveness by enhancing reliability, lowering LCOE, and increasing energy generation. The latter can be achieved by shifting from damping controls to reactance controls, optimizing WEC-wave mechanical resonance to boost energy output. In this context, a novel quasi-passive reactance control concept is introduced, incorporating a physical mass transfer system rather than relying solely on control algorithms to supply reactive energy to the WEC. The paper presents the concept, a phenomenological discussion and the supporting mathematical theory applied to a spar-type axisymmetric two-body heaving point absorber that incorporates a mass transfer mechanism to control the reactance. Point absorbers, by their nature, usually have very narrow frequency responses and quite often their peak frequency response is much higher than the typical wave frequencies. The proposed quasi-passive reactance control significantly broadens the frequency response bandwidth to align it with typical ocean wave frequencies, enhancing energy extraction to optimal reactance levels and thereby increasing annual energy yield. The key finding of this analysis is that, when well controlled, the proposed mass transfer mechanism can increase the power capture bandwidth by more than three times compared to the baseline case without the mechanism.