Ocean wave is one of renewable energy sources, of which the theoretical energy potential is estimated to be 32,000 TWh/yr according to Mørk et al. (2010), however, that is highly underutilized up to recently. In this study, a new system is proposed, that can enhance not only the efficiency of the system by using resonant phenomena in wave channel but also the economic feasibility by using existing caisson–type breakwater as support structure.
To evaluate the performance of proposed system, numerical analyses by using Galerkin’s finite element model based on the linear potential theory were carried out for various damping ratio which is directly related to extracting wave power. Numerical results reveal that the performance of the proposed system is fairly good compared with that of the conventional one, and the resonance of the oscillating water column in wave channel is dominated.
Hydraulic model tests were carried out for three different wave channels including floaters by using regular waves. To evaluate viscous damping effects of PTO system, it was measured the floater motions, free surface fluctuations, and wave reflections. To quantify the viscous damping, numerical experiments were also performed for the hydraulic model cases. The comparative results from hydraulic model tests and numerical analysis show that the viscous damping is about 5% of the critical damping of the vertical movement of the floater in the wave channel. It was also found that the viscous damping increases, as the width of the channel decreases. Viscous damping might come from mainly flow separations at the gap between side wall of wave channel and moving floater. Friction between them may also be contributed to attenuation of floater heave motion.
In addition, on the basis of reflection coefficients obtained from experiment, the closest viscous damping ratio of floater can be found to be about 20% through a numerical analysis, and the reflection coefficients was investigated according to the existence of floater to examine the trend of nonlinearities due to the wave channel and indicated by the energy loss coefficients of incident wave. it was evaluated efforts to minimize the attenuation as needed because viscous damping occurring in the channel and around the moving floater is significant in generation efficiency. Viscous damping can be reduced significantly if two damping sources aforementioned can be removed. However, it is impossible to remove the damping sources completely, therefore the PTO system should be designed considering the viscous damping properly through further optimization.