High-fidelity numerical modelling of a two-WEC array with accurate implementation of the PTO system and control strategy using DualSPHysics

In order for Wave Energy Converters (WECs) to become economically viable they will have to be installed closely together in so-called WEC arrays. When WECs are placed closely together they influence each other’s motion, which is challenging to implement in a numerical model. The accurate modelling of WECs in WEC arrays is crucial for their further development into cost-effective renewable energy converters. This paper presents the study of numerical modelling of an array of two heaving WECs (a two-WEC array) placed in close proximity of each other in order to increase their interaction effect. The numerical model uses DualSPHysics, a Smoothed Particle Hydrodynamics (SPH) based software to model wave-WEC interaction, coupled to Project Chrono for the modelling of the Power Take-Off (PTO) system and the friction. First, a series of numerical system identification tests are simulated: a free-decay test of an isolated WEC, radiation tests and an excitation test. During the radiation test one WEC is moved by an external force with a chirp-up noise signal, while the other WEC is kept still. This allows the computation of the impedance matrix, and thereby also the hydrodynamic coefficients of the two-WEC array, which has not been done before in SPH. Results were validated with experimental data from the WECfarm test campaign. Secondly, a series of numerical dynamic tests are simulated of the two-WEC array in regular and irregular waves. In the experimental test campaign a friction force caused by the motor and gearbox was counteracting the WEC’s motion, so a friction compensation (FC) force was added to cancel out this friction. This friction and FC force was numerically modelled by adding a novel implementation in Project Chrono. Furthermore, a novel global control strategy was implemented using Project Chrono, in which the PTO force on each WEC depends on the velocity of both WECs. Overall, good to very good agreement was found in terms of heave and free surface elevation when comparing numerical to experimental results.