The effects of predominant nonlinear hydrodynamics on the power absorption performance of a heaving point absorber adopting linear control principles

The power performance capability of a point absorber (PA) wave energy converter (WEC) is highly dependent on the design of the power take-off (PTO) controller, which plays a significant role in maximising power absorption efficiency. The wave-body interaction hydrodynamics are usually assumed to have a linear behaviour in the design optimisation of point absorbers, including PTO controllers. However, unavoidable nonlinearities in reality may violate the commonly used linear assumptions in the design of optimal WEC PTO controllers. This paper aims to introduce/include predominant nonlinear hydrodynamics, including the nonlinear Froude–Krylov (NLFK) forces experienced by a semi-submerged spherical heaving PA (HPA). The impacts of these nonlinear forces on the power absorption of the most commonly used linear controller is investigated in regular waves to validate the efficacy of applying linear control principles in a close-to-real environment. The main findings of this study are: for a semi-submerged PA under medium to high sea states in regular waves (i) the linear power maximising control principles underestimate the absorbed power by up to 54 % under NLFK forces; (ii) both dynamic NLFK and viscous drag forces have significant impacts (up to 57 %) on the power absorption efficiency of WEC systems; and (iii) tuning of traditional linear spring-damper control can achieve maximum power absorption under predominant nonlinear hydrodynamics, similar to that under multi-resonant control.