On using Helmholtz-type resonance to reduce the size of dual-purpose offshore oscillating water column wave energy converters

Oscillating water column (OWC) devices with Helmholtz resonator features are analyzed with semi-analytical, numerical, and experimental techniques. Conventionally, an OWC is a type of wave energy converter that consists of a duct with one end submerged, where the device's peak frequency is tuned by the length of the submerged duct. This leads to large devices when the desired peak frequency is low. The size of a conventional OWC can be reduced significantly by synthesizing features of Helmholtz resonators into it, such as using a narrow entrance. When integrated with a breakwater, it has been shown that the Helmholtz-type OWC can generate power while protecting the coastline from low-frequency ocean waves, making them dual-purpose. A systematic study of dual-purpose offshore Helmholtz-type OWC is still wanting. This study considers offshore "buoy-like" Helmholtz-type OWCs by a fast and heuristic semi-analytical model, which incorporated viscosity-related damping by empirical terms. The model shows that the Helmholtz-type OWCs have the potential to protect the coastline from low-frequency (long wavelength) waves, by producing a wide wave shadow behind them. The semi-analytical model is validated against the boundary element method (BEM), as well as experiments. The first experiment compares a Helmholtz-type OWC and a conventional OWC of the same size but different peak frequencies; the second compares a larger conventional OWC with a smaller Helmholtz OWC, which is tuned to have the same peak frequency as the former. In both cases, the semi-analytical model and the results from BEM agree well with the experiments.