Energy conversion and hydrodynamic analysis of multi-degree-of-freedom wave energy converters integrated into a semi-submersible platform

The hybrid concept of multi-type wave energy converters provides viable solutions to improve the wave energy exploitation per-unit area and reduce the Levelised Cost of Electricity. In this paper, a multi-degree-of-freedom hybrid system combining an oscillating wave surge converter and two oscillating buoys, is proposed and inte-grated into a semi-submersible platform to be suitable for both nearshore and offshore zones. The hydrodynamic characteristics of the hybrid system is investigated by establishing a three-dimensional numerical wave tank in the context of computational fluid dynamics theory including dynamic overset grid scheme, with emphasis on its overall characteristic and respective characteristic of each device. The corresponding physical experiments are also performed to cross-check the numerical solutions, and help to further explain un-simulated phenomenons. By comparing with the single-degree-of-freedom wave energy system, the total power capture efficiency of the hybrid system increases over a broader range of wave periods due to the combination of different resonant periods. The optimal power take-off damping for the oscillating buoy and the oscillating wave surge converter decreases and is insensitive with wave height, respectively. The oscillating buoy device with larger radius and deeper draft demonstrates higher energy absorption which reduces wave loads on the platform. Both the total capture efficiency and the effective frequency bandwidth increase initially to maximum values and then decrease with increasing the geometric radius of the oscillating wave surge converter. The findings of this paper validate the feasibility of different-type wave energy converters integrated into floating platforms and show their synergy.