CFD modelling of breakwaters embedding wave energy converters

In recent years the raising supply and environmental problems related to traditional fossil fuel exploitation for energy production have pushed the research on renewable sources. Among these, sea waves have a high potential, but still poorly used. Different technologies have been developed in order to harness wave energy, and the Oscillating Water Column (OWC) devices are the most accredited for an actual diffusion. This work is focused on the performance analysis of particular OWC-type devices, namely the REWECs (REsonant sea Wave Energy Converter). Two different versions have been considered, the REWEC1 and the REWEC3, constituting a submerged and an emerged breakwater, respectively. The most interesting aspect of REWECs is the possibility to operate them under resonant conditions with that sea state which is the one that gives the highest yearly energy contribution. Both REWEC1 and REWEC3 dynamic behavior can be approximated by means of a mass-spring-damper system. According to this approximation, a criterion for evaluating the oscillating natural frequency of the REWECs has been derived. A CFD model has been developed in order to verify the resonance behaviour of a scaled REWEC1 device. The water-air interaction has been taken into account by means of the Volume Of Fluid (VOF) model implemented in the commercial code Ansys Fluent. Both air and water fow felds have been assumed to be unsteady. The CFD model has been validated against both analytical and experimental results. Simulation results showed a good agreement with both measurements and predictions, particularly when the Standard k-ω turbulence model is implemented inside the REWEC. The CFD model has been applied also to the simulation of both conventional and REWEC1 submerged breakwater placed into a two-dimensional wave fume, obtaining a novel method for the valuation of the REWEC1 absorption coefcient. In fact, when considering the interaction of waves with a conventional submerged breakwater, the incident wave energy is shared into: a refected fractiona a dissipated fraction, due to the wave breaking and the friction losses on ita and a transmitted fraction to the shore. If the submerged breakwater embeds a REWEC1, a further reduction of the transmitted energy can be achieved due to device absorption, improving the coast protection performance of the structure. Actually, a scaled REWEC1 breakwater without PTO-system (Power Take-Off) has been considered. Then, the simulation results showed only a small difference between the transmission coefcients of the two breakwaters, allowing however to calculate the absorption coefcient of the REWEC1 due to the water losses inside the device. Finally, a full-scale REWEC3 breakwater has been simulated by means of CFD, taking into account the characteristics of the air turbine adopted as power take-off device. The breakwater replicated the one installed in the Civitavecchia harbour (Italy). A typical sea state has been reproduced by means of regular waves generated by a piston-type wave-maker. As in the case of the scaled REWEC1, CFD unsteady simulation have been carried out in a two dimensional (2D) reference frame, reducing the huge need of computational resources. In order to simulate the oscillating fow across the air turbine, a new method has been also proposed here. The zone of the domain corresponding to the duct where the air turbine is embedded has been modelled as a porous zone. Analytical equations able to model the exchange of mass and energy across the air duct and the turbine have been implemented. The simulation results are in good agreement with the ones registered during the operation of the Civitavecchia plant. A further outcome of the work has been a deep insight into the fuid-dynamic behaviour of the water fow inside the REWEC3, highlighting the design improvements needed to reduce friction losses.