The Hydrodynamic Efficiency of an Oscillating Water Column

An oscillating water column device enables the conversion of wave energy into electrical energy via wave interaction with a semi-submerged chamber coupled with a turbine for power take off. This present work concentrates on the wave interaction with the semi-submerged chamber, whereby a shore based oscillating water column (OWC) is studied experimentally to examine energy efficiencies for power take-off. The wave environment considered consists of plane progressive waves of steepnesses ranging from kA = 0.01 to 0.22 and water depth ratios varying from kh = 0.30 to 3.72, where k, A and h denote the wave number, wave amplitude and water depth respectively. The key feature of this experimental campaign is a study on the influence of geometrical parameters of the front wall on the OWC’s performance. More specifically, these parameters include: front wall draught; thickness; and aperture shape. We make use of a two-dimensional inviscid theory for an OWC for comparative purposes and to explain trends noted in the experimental measurements. The work undertaken here has revealed a broad banded efficiency centred about the natural frequency of the OWC. The magnitude and shape of the efficiency curves are influenced by the geometry of the front wall. Typical resonant efficiencies of the OWC are in the order of 70%.