A.H. Clement

Wave energy in Europe: current status and perspectives

The progress in wave energy conversion in Europe during the past ten years is reviewed and current activities and initiatives in the wave energy sector at National and Union level are described. Other important activities worldwide are summarized. The technical and economical status in wave energy conversion is outlined and important wave energy developments are presented.

Optimization and Time-Domain Simulation of the SEAREV Wave Energy Converter

This paper introduces a new second generation wave energy converter concept named SEAREV [Systeme Electrique Autonome de Recuperation d’Energie des Vagues]. The working principle and linearized equations of the device are described. It is shown how energy absorption depends on the shape of the external floating body and on the mechanical characteristics of the moving mass. This allows to numerically optimize the geometry of the device. Latching control is used to further improve the capture width of the system, with success in regular waves.Copyright

Effect of non-ideal power take-off on the energy absorption of a reactively controlled one degree of freedom wave energy converter

In this paper, the effect of non-ideal actuators on the performance of reactive control for a heaving wave energy converter is studied. The concept of the control is to cancel all or part of the reactive terms in the equation of motion. The proposed control is causal, thus it may be applied in practice. Actuators efficiencies from 50 to 100% are considered. The methodology used in the study relies on mathematical and numerical modeling. Control performance is investigated in regular waves and in irregular waves, and also from the perspective of the annual mean absorbed power at a typical Western Atlantic site. Motion constraints are not taken into account in the analysis for sake of simplicity. As already shown in previous work, it is found that reactive control can increase the mean annual power absorption at the considered site by a factor 10 in case of ideal actuators. However, it is shown that actuators efficiency is critical to control performance, because of the large amount of reactive power involved in the control strategy. Thus, for low efficiencies actuators (<80%), control performance is a fraction of what it can be with ideal actuators (approximately 10%). Even with 90% efficiency, control performance is less than 30% of the ideal case. In the range 90–100%, every percent of increase in efficiency leads to significant increase in control performance.

On the Numerical Modelling of the Non Linear Behaviour of a Wave Energy Converter

Wave energy converters of the wave activated body class are designed to have large amplitudes of motion, even in moderate sea states, because their efficiency is directly related with the amplitude of their motion. Hence, classical seakeeping numerical tools based on linear potential theory, which are widely used in the design process of offshore structures, are not accurate enough in the case of wave energy conversion. So, large differences between numerical predictions and wave tank experiments are often observed. On the other hand, the use of CFD models theoretically able to provide more accurate results is still difficult for wave energy applications, mainly because this requires a huge computation time. Moreover, it is well known that viscous solver have difficulties in propagatating gravity waves accurately. In this paper, we assess the potential of two advanced hydro-dynamic numerical models in the numerical modelling of wave energy converters. These numerical models are expected to provide more accurate results than classical linear theory based numerical models and faster results than CFD models. Particularly, these tools are expected to be able to deal efficiently with large motions of wave energy converters. In the first one, the hydrostatic forces and the Froude-Krylov forces are computed on the exact wetted surface of the wave energy converter, whereas radiation and diffraction forces are computed using the standard linear potential theory. Using this model, it is shown that we were able to predict the parametric roll phenomenon in the case of the SEAREV wave energy converter. In the second one, a Navier Stokes solver, based on RANS equations, is used. Comparisons are made with experiments and it is showed that this tool is able to model quite accurately viscous effects such as slamming. However, computation time is found to be long with this last tool.Copyright

Influence of an Improved Sea-State Description on a Wave Energy Converter Production

Sea-states are usually described by a single set of 5 parameters, no matter the actual number of wave systems they contain. We present an original numerical method to extract from directional spectra the significant systems constituting of a complex sea-state. An accurate description of the energy distribution is then given by multiple sets of parameters. We use these results to assess the wave climatology in the Bay of Biscay and to estimate the power harnessable in this area by a particular Wave Energy Converter, the SEAREV. Results show that the fine description of sea-states yields a better assessment of the instantaneous device response. The discrepancy between the classical and multi-sets descriptions show that the new one is preferable for the assessment of harnessable power and for device design.© 2007 ASME

Systematic Dynamic Modeling and Simulation of Multibody Offshore Structures: Application to Wave Energy Converters

This article presents a framework to model and perform time domain dynamic simulations of offshore structures presenting several interconnected rigid bodies. Both fixed and 6 degree of freedom floating structures are considered. It uses a robotics formalism to parameterize the kinematic chain of the structures and is robust with respect to the number of bodies involved. Direct dynamics algorithms are given, using a consistent notation across offshore engineering and robotics fields. They use efficient recursive techniques based on Newton-Euler equations. The advantage of this framework is that tedious analytical developments are no longer needed. Instead of that, it is sufficient to provide a data parameter table as well as principal inertia parameters of each body to entirely describe the mechanical structure. An example of simulation is given, based on the 7 degree of freedom SEAREV Wave Energy Converter.Copyright

Power take off damping optimisation with regard to wave climate

Considered as a source of renewable energy, wave is a resource which exhibit high variability at all time scales. Furthermore wave climate also changes significatively from place to place. Wave energy converters are very often tuned to suit the more frequent significant wave period of the project site. In this paper we show that optimizing the device necessitates to account for all possible wave conditions weighted by their occurrence frequency, as generally given by the classical wave climate scatter diagrams. Instead of a real device, a generic and very simple wave energy converter was considered here. We show how the optimal parameters can be different considering whether all wave conditions are accounted for or not, whether the device is controlled or not, whether the vertical motion is limited or not. We also show how they depend on the area where the device is to be deployed, by applying the same method to three very different sites.Copyright

Computation of the Diffraction Transfer Matrix and the Radiation Characteristics in the Open-Source BEM Code NEMOH

Until now, widely available boundary element method (BEM) codes did not allow the calculation of two non-conventional hydrodynamic operators, which characterize the way a body diffracts and radiates waves, known as Diffraction Transfer Matrix and Radiation Characteristics respectively. When embedded into the finite-depth interaction theory developed by [1], they drastically speed up the computation of the added mass, damping and excitation force coefficients of a group (“farm”) of floating bodies. This paper presents the implementation of their computation in the open source BEM solver NEMOH using the methodology proposed by [2]. Results for two different geometries, a cylinder and a square box, are presented and compared to an alternative computational approach developed by [3]. A very good agreement between them is found. In addition, the hydrodynamic operators of the cylinder are compared to a semi-analytical solution available in the literature showing a good match. Results obtained using the finite-depth interaction theory are shown for a generic multi-body wave energy converter (WEC) demonstrating how the capabilities added to the BEM software NEMOH can facilitate the numerical modeling of the hydrodynamic interactions in large arrays of bodies.

Reduction of the Non-Causal Horizon of the Optimal Wave Energy Converter Control

In our days, wave energy still remains an important resource of renewable energy that has not been yet completely exploited and fully understood. Various prototypes of point absorbers have already been tested numerically and experimentally in wave-tanks or real sea, but only a few of them has reach the full scale prototype stage. For the family of wave absorbers based on oscillating bodies principle, the energy production may be enhanced by motion control. The choice of a particular mode of control remains decisive in the design of point absorbers and is closely linked to the mechanism architecture. It has been shown [1] that the theoretical maximum absorption can be reached by bringing the system into resonance applying a so called “complex-conjugate” control. Several sub optimal control strategies have been derived from this observation, trying to overcome the draw-backs of this method, mainly the non-causality of the optimal control [3]. Non-causality implies that one needs to predict the excitation signal in the near future to optimize the control command. The aim of the present study is to propose a new methodology to reduce the prediction horizon needed to apply a complex-conjugate control. Afterwards, a simplification is made leading to a causal non-adaptive control. In this study, a cylindrical buoy constrained to move in heave only is employed to test numerically the aforementioned control. Numerical comparisons are made under regular and irregular waves with the performance of control based on the classical complex-conjugate method. The new method shows a good energy absorption capacity for a broad range of frequency without having to adapt the control regulator unit to the incident waves.Copyright