Broadband wave energy extraction by a dual-PTO hybrid system of a comb-type breakwater and an oscillating flap

In this study, a novel hybrid system combining a vertical pile-supported comb-type breakwater (CTB) and an oscillating flap (OF) is proposed, to absorb effectively long-period waves and simultaneously extend period range of wave energy extraction. The OF is hinged in the chamber of the CTB, and the hybrid system is collectively called as CTBOF. As waves interact with the CTBOF, the heaving motion of the CTB is excited to drive one power take-off (PTO) system. Meanwhile, the back-forth oscillation of the OF propels another separate PTO system. The two PTO systems are considered in an accurate CFD model which couples the dual degree-of-freedom (DOF) motions of two devices in a rotating joint simultaneously. The matched fluid–structure information is updated adopting an overset dynamic mesh approach. Numerical results demonstrate better wave energy conversion and wave attenuation performance of the CTBOF in a broader wave period range compared with the stand-alone CTB and OF due to the constructive coupled effect. A larger CTB caisson generates adverse effect on the wave energy conversion but beneficial effect on the wave energy attenuation. When the OF is installed at the opening the CTB, water motions with the maximum volume occur in the enclosed chamber, resulting in the highest energy absorption. The complementary effect between two PTO systems in short-wave and long-wave periods are dominated by the OF and CTB, respectively. The CTBOF deployed in shallow water is found to be more advisable compared with the finite water-depth conditions in the principle of the OF. The workings of this paper reflect the potential for synergies between sustainable ocean exploitation and friendly environmental protection.