Modelling of hydraulic performance and wave energy extraction by a point absorber in heave

The feasibility of energy extraction from sea waves has been investigated, with special attention to potential applications in the Belgian coastal area of the North Sea. The performance of heaving point absorbers in wave conditions that are representative for the considered area is calculated by means of a linear theory. The geometry of the heaving buoy, the external damping and a supplementary inertia are considered as variable parameters to optimise the absorption system. Further, the numerical results are validated by means of physical model testing. The resulting power absorption performance appears to be wave height dependent. For regular waves of relatively small amplitude the absorption length significantly exceeds the absorber diameter and diminishes with larger wave heights. In irregular waves, the absorption length is estimated to reach 60% of the buoy diameter. The comparison between experimental and numerical data shows discrepancies that are primarily related to two issues that inversely affect the absorber effectiveness. Firstly, significant vortex shedding and viscous losses occurring with significant buoy motion reduce the power extraction. Secondly, the actual decrease in absorbed power due to the mistuning effect in irregular waves is smaller as revealed from computational prediction. Regarding the significant motion, restrictions must be included in order to avoid slamming of the buoy. This may be achieved by increasing the external damping compared to the theoretical optimal values or, alternatively, by increasing the draft of the buoy. The adaptation of the draft is also shown as a measure to increase the overall absorber efficiency in natural seaways. In the end, considering the frequency of occurrence of the wave spectra, optimal characteristics for the point absorber are determined. An optimal absorber is evaluated as rated to a 100 kW, with an annual average power capture of about 30 kW or 263 MWh per year per point absorber unit. (C) 2004 Elsevier Ltd. All rights reserved.