Experimental and numerical analysis of a hybrid WEC-breakwater system combining an oscillating water column and an oscillating buoy

Integrating multi-type Wave Energy Converters (WECs) enables wave energy to be extracted from multiple harvesting manners simultaneously, which presents a more competitive energy conversion technology. In this study, an Oscillating Water Column (OWC) and an Oscillating Buoy (OB) are combined to devise a hybrid WEC system that serves as a floating breakwater. The two devices are aligned in line with the incident wave direction, where the OB is deployed upstream from the OWC. A series of physical experiments are conducted to understand the working principle of the hybrid WEC system. After a fully non-linear time-domain model is validated by the measured data, the comparisons of the hybrid system and its respective isolated devicesare emphasised. Then, the hydrodynamic dependence of the hybrid system is comprehensively tested. The results indicated that the hybrid system design provides higher energy conversion as well as better wave attenuation performance compared to the isolated OWC and OB devices. Additionally, the OB outperforms the OWC in terms of wave energy conversion. The OWC-OB gap resonance has a negative impact on wave energy extraction but has a negligible effect on wave attenuation. The OWC with a deeper draft and symmetric wall performs better both in terms of wave energy extraction and wave attenuation in long-period waves. The overall conversion efficiency decreases with the increase of the OB draft, but the transmission coefficient follows an opposite trend. The PTO dampings of the OB and the OWC dominate the overall conversion efficiency and the transmission coefficient, respectively.