Performance enhancement of submerged wave energy device using bistability

Abstract

The performance of a submerged cylindrical point absorbing wave energy converter was explored under the addition of different nonlinear stiffness (bistable) conditions. The limitations of previous studies were addressed by incorporating higher-fidelity modelling. Devices employing bistability in other energy harvesting applications, have improved the amount of power generated. For wave energy converters, most theoretical models with bistability were limited to one-degree-of-freedom, neglect nonlinearities such as viscous drag, and are excited by unrealistic sinusoidal waves. Such simplifications lead to neglecting features such as modal interactions. The presented model investigated a three-degree-of-freedom submerged point absorber with bistability subjected to regular and irregular waves. The bistable mechanism was an adjustable magnetic model such that a range of potential profiles were examined and parameterised, for generality, by features common between mechanisms. For this device, bistability may be used to obtain near optimal results and was suitably robust for changing ocean conditions. Regions of improvement were identified in terms of the changing natural frequency due to a nonlinear stiffness, and a phase matching property. In varying sea-states, a selected bistable condition demonstrated a 10–20% improvement in power production. The consistency implies that semi-active elements may be able to adjust the bistability to enhance power production.