This paper proposes a novel power maximising control strategy for wave energy conversion applications and investigates its performance against linear spring-damper control on a fully submerged heaving point absorber wave energy converter (WEC) under both regular and irregular wave conditions. Inspired by the Phi method, the developed WEC control strategy presents an analytical solution which involves a quadratic damping term accounting for the nonlinear viscous drag in the WEC hydrodynamics. Therefore, its optimal control parameters can be analytically determined without optimisation and/or linearisation, which, however, usually accompanies conventional linear control strategies such as linear spring-damper control. Simulation results show that the proposed analytical control solution can absorb almost the same power as the optimised linear spring-damper control does in both regular and irregular wave conditions.
Due to the shortage of fossil fuel and the environmental impact caused by excessive carbon emission, renewable energy has become an emerging research field nowadays. There are various categories of renewable resources such as sunlight, wind, rain, tides, waves, and geothermal heat, among which the wave energy has great potential because of its consistency, high energy density and predictability. A wave energy converter (WEC) is a type of devices which can convert the ocean wave energy into useful electricity.
Although wave energy has several superior characteristics as mentioned above, compared with other renewable energy such as solar and wind, the main challenge in commercialising WEC technologies remains in reducing their costs by taking manufacturing, installation, and maintenance into consideration. The role of WEC control accounts for not only maximizing the WEC's power absorption efficiency but also ensuring its safety operation in harsh sea environment. Therefore, indepth understanding on WEC control can help to increase the efficiency of power absorption, lower the maintenance costs, and thus guarantee competitive energy costs.
According to linear wave theory, which assumes inviscid, irrotational and incompressible fluid (Falnes and Perlin, 2003), complex conjugate control (sometimes called impedance matching control) can achieve the maximum power absorption of a WEC but requires complex linearisation, optimisation and prediction procedures in its design due to the presence of nonlinear WEC hydrodynamics. Furthermore, in practice, the wave energy conversion process is accompanied with a resonant motion and, thus, the influence of nonlinear effects in the hydrodynamics such as the viscous drag may become significant violating the linear optimal control principles. In this case, conventional linear control strategies may no longer provide optimal solutions for the WEC power absorption leading to the emerging need of nonlinear control methods. However, the concept of WEC nonlinear control and its power absorption performance against traditional linear optimal control still requires in-depth study.