Abstract
Ocean waves are a huge resource of renewable energy for utilisation. Wave energy converter (WEC) devices are being developed to enable capture of this energy resource. In the late 1980s, the principle of extraction energy from waves was studied (Falnes, 2007) and it showed that reactive control can increase significantly wave power extraction for a heaving point-absorber. Since then, studies on the control strategies and their implementations have been a focus in the field of wave energy research (Ringwood, 2014). Majority of the wave energy control studies used linear potential flow theory and Cummins equation as the basis for modelling hydrodynamics arising from wave-body interaction, which is essential for the implementation of control simulation as well as for the formulation of model-based control strategies (Penalba, 2016). Although this simplifies/linearises the control problem as well as speeds up the simulation process, the utilisation of linear solver (which assumes that the wave steepness and body motion are both small) contradicts the large motion arising from the reactively controlled wave energy converter and thus does not ensure the fidelity of the control simulation results in medium to large wave conditions. On the extreme opposite, the Navier-Stokes equation based CFD captures the full nonlinear hydrodynamics in the wave-body interaction problem leading to high fidelity simulation results regardless of the system operational conditions, however, is barely used in the study of wave energy converter control due to its high computational requirement. Compromise between the linear method and the Navier-Stokes solver also exists (Wuillaume, 2019). A typical example is the weak-scatterer potential flow method proposed for seakeeping analysis of ship with forward speed (Pawlowski, 1991), which is formulated based on the assumption that the perturbation wave field generated by the body oscillation is small compared to the incident wave field, such as the free surface conditions can be linearised at the incident wave elevation level. The weak-scatterer method takes into account the unsteady and nonlinear hydrodynamic loads associated with dynamic wavebody interaction as long as the aforementioned assumption is satisfied and that viscosity effects remain negligible. However, it remains questionable if the weak-scatterer method is suitable for solving wave energy converter control problem where large perturbation wave field is expected given the large resonant motion of the body. A further simplified version of the weak-scatterer method is the body-exact potential method that assumes the free surface conditions can be linearised around the mean free surface elevation ? = 0. This solver was proposed to account for the body motion induced nonlinearities but is only valid when small steepness waves are present. It is a compromise between the weak-scatterer method and the linear method. The proposed study intends to conduct a comparison study on the aforementioned four wave-body interaction modelling methods in terms of their fidelity in solving the reactive control problem of a submerged point-absorber (PA) WEC.