Hydrodynamic performance evaluation of a wave energy converter with two concentric vertical cylinders by analytic solutions and model tests

In this paper, the hydrodynamic performance of a two-concentric-cylindrical-body WEC (Wave Energy Converter) is investigated through a systematic parametric study by using analytical solutions and model tests. The two-body WEC generates power by LEG (linear electric generator) through the relative heave motion between the inner and outer buoys. In order to maximize the relative heave motions between the two buoys, resonance of each buoy was used. As a means of finding its maximum hydrodynamic efficiency, the matched eigenfunction expansion method (MEEM) was applied to obtain the analytic solutions under the assumption of linear potential theory. The numerical results are validated through comparisons with a series of model tests conducted by authors in a 2-D wave tank at Jeju National University. Based on the case study, several design strategies that can further enhance the PTO (Power take-off) efficiency are proposed, including the optimal PTO damping and intentional mismatching of heave natural frequencies of the two buoys and the peak frequency of target wave spectrum. The intentional mismatching strategy, in particular, can increase high-quality extracted power for broader wave conditions, which is a big advantage in designing the proposed WEC-LEG system.