Deliberate introduction of a nonlinear restoring element to point wave energy absorbers: a review and assessment

Due to their simplicity, relatively high efficiency, and scalability, point wave energy absorbers (PWAs) have emerged as one of the most popular and promising solutions for wave energy harvesting. Their fundamental operation principle is based on activating linear resonance to pump energy from the waves to the PWA. Thus, their effective operation bandwidth is restricted to the range of incident wave frequencies that are close to the natural frequency of the PWA. This key constraint is very challenging to meet given the high stiffness of the hydrostatic buoyancy force and the variability of the wave conditions. A direct result of not meeting this constraint is a substantial reduction in the efficacy and power transduction capabilities of the PWA. To overcome this problem, deliberate introduction of nonlinearities into the PWA design has been recently proposed and exploited in two forms: (i) introduction of a nonlinear multi-stable restoring force, and (ii) introducing and/or exploiting parametric instabilities. The premise is that such approaches may be able to (i) shift the response frequency of the PWA towards the energetic low-frequency waves, and (ii) reduce the sensitivity of the PWA to the uncontrollable spatio-temporal variations in the incident waves. This review critically assesses the feasibility of leveraging nonlinear phenomena to improve the performance of PWAs. Our findings strongly point towards the conclusion that a nonlinear restoring element does not improve the capture width ratio or effective bandwidth of the PWA when compared to an optimal linear design. Furthermore, since the nonlinearity often results in aperiodic and coexisting competing responses, it adds additional layers of complexity during performance optimization and full-scale implementation. Nonetheless, the nonlinearity can be utilized as an effective means to passively shift the effective bandwidth of the PWA towards the energetic low-frequency wave content, and to decrease the sensitivity of the PWA to the wave parameters under irregular waves whose parameters drift with time.