Optimum wave regime for lift-based wave energy converters

Note: This is a preprint of paper #1914 presented at the 14th European Wave & Tidal Energy Conference (EWTEC) 2021 in Plymouth, UK. The final version of paper with the same title can be found in the EWTEC 2021 proceedings. Abstract: Lift-based Wave Energy Converters (WECs) have a number of attractive features, including the potential for unidirectional rotation, simplifying power take-off and reduction in wave loads by reducing generation of circulation, increasing survivability. The common assumption of small body, small amplitude response, together with the Haskinds Relationship is used to determine the optimum motion for a lift-based WEC to maximise power capture. It is shown that whilst for a 2D hydrofoil in deep water the optimum motion is circular, the optimum motion for a finite-width hydrofoil is generally elliptical due to differences in the hydrodynamic damping coefficients associated with the vertical and horizontal motions of the hydrofoil. It is shown that more circular hydrofoil motion can be achieved by utilising the elliptical motion of the water particles in shallow water. This occurs because the increased horizontal water particle motion in shallow water results in an increase in the wave-induced lift force associated with horizontal fluid particle motions, and thus a reduction in the optimum amplitude of motion in this direction. Preliminary calculations suggest that for a 30 metre wide hydrofoil in wave periods of about 10 seconds, the ideal water depth (where the optimum hydrofoil motion is circular) occurs at around 25 metres, which is a highly utilisable water depth. Other advantages of deployment in shallower water include an improvement in the alignment of the waves parallel to the hydrofoil and a reduction in the structural task associated with reacting against the seabed.This work was produced as part of the LiftWEC Project. This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 851885. This output reflects the views only of the author(s), and the European Union cannot be held responsible for any use which may be made of the information contained therein