Experimental investigation of a dual-pontoon WEC-type breakwater with a hydraulic-pneumatic complementary power take-off system

Floating offshore multi-purpose structures have recently attracted much attention worldwide in terms of sharing costs, enhancing efficiencies and reducing environmental impacts. In this paper, hydrodynamic experiments are conducted to examine the accessibility of wave energy absorption by a dual-pontoon floating breakwater integrated with hybrid-type wave energy converters (WECs). The main structure and the air chamber between the two pontoons are designed as an Oscillating Buoy (OB) and an Oscillating Water Column (OWC), respectively. Two power take-off (PTO) systems, including a hydraulic system and a Wells turbine, are selected to extract wave energy from structural motion and chamber airflow. To investigate the hydraulic-pneumatic complementary effects, PTO damping coefficients of two systems are adjusted both individually and synchronously. Besides, the hydrodynamic comparison among different geometrical parameters, are further studied. The breakwater-integrated hybrid WEC system led to a larger optimal pneumatic damping for the OWC while the optimal hydraulic damping for the OB is not affected. Additionally, the internal air pressure is acting as a resistive force to pontoons, hence the pontoon motion is smaller, and part of the kinematic energy from pontoons is extracted by the OWC. Correspondingly, the wave attenuation capacity is enhanced by multiple harvesting manners, especially for long-period waves.