Broadband wave conversion by a “Pan Flute”-type multi-oscillating-water-column (M−OWC) breakwater system

Abstract

Majority of wave energy converters (WEC) have a relatively narrower capture due to their design philosophy, which focus on matching WEC’s natural frequency to a single peak frequency of a seastate spectrum. This paper introduces an innovative “Pan Flute”-type WEC embedded into a breakwater. This design consists multiple oscillating water column (OWC) units, each with a distinguished natural frequency. By strategically tuning these frequencies to span a broader sea-state spectrum rather than merely matching the peak frequency, the system achieves a wider capture bandwidth. An experimentally validated Computational Fluid Dynamics (CFD) methodology was adopted to assess the hydrodynamic performance of the proposed design in irregular waves. This new concept possesses different resonant periods of internal water column to reasonably absorb short-, moderate-, and long-period components of irregular waves by facing-wave, central and rear chambers, respectively. Additionally, the multi-chamber design transforms the sloshing motion of water column into the piston-type motion, amplifying the wave elevation inside sub-chambers as well as accelerating the vortex detachment from the chamber-wall end. Consequently, the hydrodynamic efficiency is guaranteed to be higher than 0.5 for all wave periods, and its maximum value achieves 0.82. The wave attenuation is also improved, especially for long-period waves where there is a maximum 47.8% of reduction compared with S-OWC system. The varying-draft M−OWC system adopting separate PTO units is found to be superior to adopt a corporate PTO. It is practically possible to design the optimised number and draft of sub-chambers to obtain a broad harvesting width of wave energy. These findings promote the WEC-breakwater systems to be deployed in extensive sea areas regardless of energy density.