TIDAL CURRENTS EFFECT ON THE PERFORMANCE OF A HEAVING POINT ABSORBER

Abstract

A point absorber is the most common wave energy converter, and is a relatively small device that could potentially extract a significant amount of power. This study follows the vertical motion of a point absorber, which may be simplified as a mass-spring damper system. Previous studies have shown that strong tidal currents have a significant impact on the wave power resource. No previous study has investigated the performance of a point absorber in the presence of tidal currents. This study, starting from linear wave theory and following the conservation of wave action, calculates the hydrodynamic forces on a point absorber, and evaluates how the performance of the point absorber is affected by wave-tidal current interactions using a semi-analytical method. A numerical approach using OpenFOAM computational fluid dynamics software was also used to assess the semi-analytical approach. However, more research is required in the development of the numerical methodology, as this study was unable to formulate wave-current interactions with OpenFOAM. However, in the absence of tidal currents, the two approaches were found to perform similarly. Discrepancies between the approaches are present, especially near the surface due to the semi-analytical methodology's neglection of diffraction and viscous effects, and so percent error is taken into account at all methodological comparisons taken. For the specific device under consideration, a tidal current made an absorbed power increase by 54% when comparing an opposing current of -0.75 m/s to no current, and a 30% decrease in absorbed power when comparing a following current of 0.75 m/s to no current. Although this study finds an increase in extracted energy for a point absorber due to wave-current interactions, there are issues that require further investigation. Impacts of currents on mooring were not explored in this study, and may lead to differences in extracted energy. Furthermore, a very strong tidal current may cause wave breaking, or if a stopping current velocity is reached, the propagation of waves may be restricted entirely. These are limitations of the semi-analytical approach.