Following publications are available

International Conference on Ocean Energy, October 2006, Bremerhaven, Germany

Title: A model to investigate interacting wave power devices
Authors: C. Beels, P. Troch, G. De Backer, J. De Rouck, T. Moan and A. Falcão

Abstract: The contribution of Wave Energy Converters (WECs) to the renewable energy supply is rising. A single WEC, with a capacity comparable to a classic power plant (e.g. 400 MW), is technologically impossible. Therefore arrays of WECs, placed in a geometric configuration or 'farm', are needed. WECs in a farm will interact and the overall power absorption will be affected. An optimal pattern of WECs in order to maximise the power absorption is of major importance in the design of a wave farm. An existing mild-slope wave propagation model, developed at the Department of Civil Engineering, Ghent University, is adapted to investigate the interaction between wave power devices.
In this paper the basic characteristics of the mild-slope, phase-resolving model to study interacting WECs are presented.
Power absorption is implemented through sponge layers. Relations between the absorption characteristics of the sponge layers and the reflected, respectively transmitted wave pattern are derived. By adapting the coefficients of the sponge layers the absorption, reflection and transmission are affected and arrays of WECs can be modelled and studied in a relatively fast and accurate way during the design of a wave farm.

17th International Offshore and PolarEngineering Conference & Exhibition ISOPE 2007, July 2007, Lisbon, Portugal

Title: Numerical modelling of wave energy absorption by a floating point absorber system
Authors: G. De Backer, M. Vantorre, R. Banasiak, C. Beels and J. De Rouck

Abstract: Point absorbers are wave energy converters (WECs) consisting of small (floating) bodies oscillating with either one or more degrees of freedom. They can either move with respect to a fixed reference, or with respect to a floating reference. Different buoy geometries are evaluated to obtain the ideal values of size, shape and draft with regard to power absorption for a given moderate wave climate. In this paper, the performance of a heaving point absorber in a floating platform is analysed in a linear way.

7th European Wave and Tidal Energy Conference EWTEC 2007 - September 2007, Porto, Portugal

Paper 1 Title: Performance of a point absorber heaving with respect to a floating platform
Authors: G. De Backer, M. Vantorre, R. Banasiak, J. De Rouck, C. Beels and H. Verhaeghe

Abstract A linear frequency domain model has been developed to simulate the behaviour of a heaving point absorber, moving with respect to a floating reference. A linear external damping coefficient is applied to enable power absorption and a supplementary mass is introduced to allow for tuning the point absorber to the incoming wave conditions [2]. Two motion restrictions are applied on the buoy. The first restriction decreases the occurrence probability of slamming; the second one limits the stroke of the relative motion between the buoy and the platform. The influence of the motion restrictions on power absorption is examined. Optimal values for the external damping and motion control parameters are determined under the given conditions. A sensitivity analysis on these values is conducted to know the effects of less optimal tuning and/or damping on power absorption. The forces associated with these parameters have to be realised by the power take-off and motion control system respectively. The sensitivity analysis allows for assessing the influence on the power extraction of restrictions applied on these forces.

Paper 2 Title: Power Smoothing by Aggregation of Wave Energy Converters for Minimizing Electrical Energy Storage Requirements
Authors: M. Molinas , O.Skjervheim, B. Sørby, P. Andreasen, S. Lundberg, T. Undeland

Abstract The degree of reduction of required electrical energy storage for smoothing the power output in a wave farm is investigated by considering spatial power smoothing by a particular choice of aggregation of wave energy converters (WEC). Several possible arrays are analyzed considering that no energy storage is provided locally at each WEC unit and that all energy storage required will be electrical and provided at the point of connection to the electrical network. This paper focuses on a case study of direct drive WEC for near shore devices and the power output data implemented in the study derives from a linear hydrodynamic model developed in Matlab with input forces obtained by solving the radiation problem with the finite element simulation tool Comsol. The results from simulations indicate that there is a significant smoothing effect for sinusoidal waves and quite a good short term smoothing effect is achieved in an irregular sea.

Paper 3: Title: Direct generation wave energy converters for optimized electrical power production
Authors: H. Lendenmann, K-C. Strømsem, M. Dai Pre, W. Arshad, A. Leirbukt, G. Tjensvoll, T. Gulli

Abstract: New concepts for the direct conversion of the movement of a vertical heaving buoy point absorber to electricity are presented. The low loss direct conversion and generation system without intermediate hydraulic system permits hereto unprecedented speed and force control flexibility of the buoy movement. Three solutions are compared: an induction generator with a rack-pinion linear to rotary motion conversion and a step-up gear, a high pole application-specific permanent magnet (PM) generator with a belt-pulley linear to rotary conversion, and a linear PM generator mounted directly on the guide of the buoy. The linear generator is the least robust and most challenging in design, while the induction generator solution is found best due to its cost and maintenance advantages. The PM rotating generator solution shows the highest efficiency. Control strategies considering practical limitations (motor size, inverter rating) are explored and compared to linear damping and latching control. The peak power handling capability is optimized while diminishing the annual energy take-off only marginally. Simulations for irregular wave traces and the efficiency consequences are discussed. A 1:5 scaled test system using this direct conversion was installed at shore in real sea waves. Experimentally measured energy capture widths for wave-to-electricity in the range of 25% are reported.

International Conference on Offshore Mechanics and Arctic Engineering, June 2008, Estoril, Portugal

Title: Investigation of vertical slamming on point absorbers
Authors: G. De Backer, M. Vantorre, J. De Rouck, S. Victor and C. Beels

Abstract: 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.

International Conference on the Application of Physical Modelling to Port and Coastal Protection, July 2008, Bari, Italy

Title: Experimental study of bottom slamming on point absorbers using drop tests
Authors: G. De Backer, M. Vantorre, J. De Pre, J. De Rouck, P. Troch, C. Beels, J. Van Slycken and P. Verleysen

Abstract: Bottom slamming of point absorbers has been studied experimentally by performing free fall drop tests. Three rigid axisymmetric bodies are dropped from several heights in a water basin. The bodies are a hemisphere and two cone shapes with a deadrise angle of 20° and 45°. The pressure time history and deceleration of the body are recorded during impact. The setup is instrumented with high frequency piezoelectric pressure sensors as well as a shock accelerometer and a high speed camera. A guiding structure is installed to increase the accuracy and reproducibility of the tests. The results are compared to a theoretical formulation based on asymptotic theory, which is found to be rather conservative. The influence of the body shape on the peak pressures is discussed. The hemisphere encounters very high impact pressures at the bottom where the local deadrise angle is very small. The 45° cone experiences significantly smaller impact pressures compared to the hemisphere and 20° cone.

International Conference on Ocean, Offshore and Arctic Engineering, June 2009, Honolulu, Hawaii

Title: Experimental investigation of the validity of linear theory to assess the behavior of a heaving point absorber at the Belgian Continental Shelf.
Authors: G. De Backer, M. Vantorre, J. De Rouck, K. De Beule and C. Beels

Abstract: 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.

Specific Targeted Research Project
Priority 6-1 : Sustainable Energy Systems
Scientific officer : Mr T. Langlois d'Estaintot

The SEEWEC project is funded under the 6th Framework Programme.
The financial support from the European Community is gratefully acknowledged.