Charlotte Beels

Wave energy on the Belgian Continental Shelf

The inexhaustible wave energy resource is at the basis of the increasing research on wave energy exploitation. Till now the resource is studied in detail for regions with a high wave energy density. As the problems to guarantee the survivability of wave energy converters (WECs) in a severe and energetic wave climate are not solved yet, the possibilities of wave power conversion in a less aggressive wave climate should be investigated. This paper describes the wave climate and correspondent wave power resource in a rather sheltered area i.e., the Belgian Continental Shelf (southern part of the North Sea). The available wave power is studied on six buoy locations. Furthermore characteristic sea states and design conditions are derived for the most suitable locations for wave energy production. The available wave power is compared with other sites in the North Sea. Near shore (< 30 km off the coast) less than 10 kW/m is available.

Experimental investigation of the validity of linear theory to assess the behaviour of a heaving point absorber at the Belgian Continental Shelf

The results of an experimental investigation on a heaving point absorber are presented. The physical tests are used to validate numerical simulations of the behaviour of the point absorber based on linear theory in the frequency and time domain. Floater response and power absorption are evaluated in regular and irregular waves representing a mild wave climate. A good correspondence is found between the physical and numerical test results. In irregular waves the difference between numerical and experimental power absorption is generally smaller than 20%. In regular waves the correspondence is good as well, except in the resonance zone; i.e. when the natural frequency of the buoy is tuned towards the resonance frequency of the incident wave. In this case, non-linear effects such as viscosity and a non-linear hydrostatic restoring force become important due to the high velocities and motion amplitudes of the point absorber. However, because of these large amplitudes, pure resonant cases are often not preferred in practical applications. In general it is concluded that the numerical results are in good accordance with the experimental results and can be used to predict the point absorber behaviour in mild energetic waves in non-resonance conditions.Copyright

Investigation of vertical slamming on point absorbers

This paper focuses on the impact of vertical slamming on floating point absorber systems and the associated pressures that might be expected when these phenomena occur. In a first part it will be shown how the occurrence probability of slamming can be reduced by implementing a slamming restriction, i.e. by controlling the motion of the point absorber. The impact of these slamming restrictions on power absorption will be discussed. Secondly an investigation is made of the pressures that occur when the buoys are subject to vertical bottom slamming. Analytical results are presented, which give a pressure prediction of an impacting body with conical and hemispherical shape, using Wagner theory. Laboratory experiments have been carried out at Ghent University. Impact pressures were measured during drop tests with both hemispherical and conical buoy shapes. These pressures were measured by ICP pressure sensors with a range up to 345 kPa with small membrane and very high resonance frequency (> 250 kHz). Analytical and physical results are compared and conclusions are drawn.Copyright