Numerical Design Methods and Tools of Wave Energy Converters – MONACOREV

Because of their specifics, wave energy converters requires developing new methods and innovative algorithms in the context of numerical modeling in hydrodynamics. Regarding large amplitude motion, non linear and weak scatterer approaches have been identified as promising candidates, provided that suitable empirical corrections are provided to take into account viscous effects (such as vortex shedding). CFD will be used to calibrate these empirical corrections.
Regarding wave interactions in arrays, and the effect of arrays on the wave climate at regional scale and particularly at shore, coupling between phase averaged or phase resolved wave models and far field approximations for the modeling of the each wave energy converters are promising.

Regarding the modeling at machine scale, a data base of viscous damping coefficients in oscillatory flow has been generated for common wave energy converters' shapes using CFD softwares. Progress in the development of the full and weakly non linear numerical models is satisfactory.

Regarding the modeling of arrays and their impacts on the regional wave climate, it was quicly realized that the fast multipole approach is a dead end because of the slow convergence of the series expansion for high wave numbers. Therefore, an alternative approach based on the plane wave approximation has been selected. It allows a considerable speed up in computational time while maintaining the accuracy. It also allows a relatively easy coupling with wave models be means of domain decomposition: fictitious island are created in the wave model mesh on which the far field approximation of the velocity potential calculated with BEM is imposed. First resultats are promising and further work should confirm the efficiency of this innovative approach.

Eventually, the project will result in a new class of numerical tools and methods dedicated to the modeling of wave energy converters.

1. J. Singh, A. Babarit (2013) A fast approach coupling BEM and plane wave approximation for hydrodynamic analysis of distant spaced multiple bodies. Submitted for publication in Ocean Engineering.
2. O. Thilleul, A. Babarit, A. Drouet, S. Le Floch (2013) Validation of CFD for the determination of damping coefficients for the use of wave energy converters modelling. In Proc of the ASME 32nd International Conference on Ocean, Offshore and Arctic Engineering (OMAE2013), 9-14 Jube 2013, Nantes, France
3. A. Babarit, M. Folley, F. Charrayre, C. Peyrard, M. Benoit (2013) On the modellling of WECs in wave models using far field coefficients. In Proc. of the 10th European Wave and Tidal Energy Conference (EWTEC’2013), 2-5 September 2013, Aalborg, Danemark
4. L. Letournel, P. Ferrant, A. Babarit, G. Ducrozet (2012) Développement d’un outil de simulation numérique basé sur l’approche Weak-Scatterer pour l’étude des systèmes houlomoteurs en grands mouvements : expression analytique des équations intégrales pour une discrétisation linéaire de la géométrie. Actes des 13èmes Journées de l’Hydrodynamique, 21-23 Novembre 2012, Chatou, France.

Among the different renewable marine energies, wave energy has a significant potential resource. However, although the large number of Wave Energy Converter (WEC) technologies, which have been proposed and which are currently under development, there is not yet a leading technology or an acknowledged best system design. This is due to several reasons, a good part of them arising from new and specific hydrodynamic issues. These fundamental problems depend on the scale at which they are considered.
• At the scale of a single machine (~100m), the issue consists of modelling the large motions of WEC actuators both for sea-keeping purposes and for the system operation.
• At the WEC farm scale (~1 km), due to the large number of machines, the wave interactions must be correctly taken into account.
• At regional scale (~10 kms), the crucial aspect is the impact of the WEC farm on the nearshore wave climate and the farm energy yield evaluation.
At the moment, usual computer aided design tools in the field of Ocean Engineering cannot deal with these issues in a satisfactory way. At machine scale, uncertainties are too large. At farm and regional scale, the tools do not work. The purpose of this project is to provide a new class of design tools fitted for these problems. Depending on the scale, the numerical approaches will be based on non linear potential theory, the Fast Multipole Algorithm, Berkhoff’s equations or the wave action density evolution equation. These approaches are existing in their principles, but their numerical implementations are not mature, particularly in the context of wave energy conversion. In addition, the coupling between these tools is mandatory to be able to deal with all scales of the problem.
The numerical tools will be validated by comparison with experimental datas. The realisation of the experiments is out of the scope of this project but the consortium will have access to the results through external collaborations with Ghent University in Belgium and Edinburgh University in the United Kingdom.
The numerical simulation methodologies and tools developed in the framework of the project will represent a useful tool for wave energy project developers and for WEC farm design. Providing specific tools for wave energy applications, which allow to analyze and compare performances and impact of wave energy converters and wave energy farms at specific locations, will contribute to reduce duration and costs of preliminary and detailed feasibility studies.