A. Van de Sijpe – PhD Thesis, Ghent University, 2012
The development of renewable energy resources is strongly required due to the increasing energy demand, the shrinking reserves of fossil fuels and the effect of greenhouse gas emissions on the change of the wave climate. At Ghent University, study around the extraction of energy from ocean waves is being performed, more specifically with the aid of point absorber wave energy converters (WECs). To deliver a considerable amount of energy output at one location, large numbers of such devices need to be arranged in arrays or farms at sea. Several performed numerical and experimental studies around point absorbers and WEC-arrays are mentioned, indicating the knowledge gap of large scale physical model tests on WEC-farms, which are necessary to study the near- and far-field effects and to verify and improve numerical models. Within the HYDRALAB IV European programme in the frame of the project WEC wakes, large farms of point absorbers will be tested in the Shallow Water Wave Basin of DHI (Denmark). The aim of this master thesis is to develop and optimise such a point absorber wave energy converter in order to ensure behaviour which is in accordance with the real impact of a wave energy converter on the wave climate. The developed WEC has a hemisphere-cylindrical shape with a draft equal to its diameter. It is restricted to heave motion along a vertical axis connected to a foot. The power take-off is simulated by a mechanical brake, whereby the extracted energy is lost through friction. The internal friction within the device, the friction coefficient of the damping system and the influence of the measurement instrumentation are examined out of the water by tests in the tensile testing machine of the Department of Materials Science and Engineering of Ghent University. The performance of the foot and axis for several installation techniques and of the different measuring instruments is verified through experiments in the large wave flume of the Department of Civil Engineering of Ghent University. To check the efficiency of the power take-off system and to determine the influence of the buoy on the wave field, tests are performed in the wave basin of Queen’s University Belfast in Portaferry, North Ireland, as the wave flume is not appropriate for tests with a large oscillating device. The buoy motion is in advance predicted by using the BEM package WAMIT for deriving the hydrodynamic parameters for different incident wave periods and then processing these results by linear point absorber theory.