Hybrid structure combining a wave energy converter and a floating breakwater

This experimental study investigates on a hybrid structure consisting in an "active" floating breakwater (FB), coupled with a new type of wave energy converter, named ShoWED. The hybrid structure achieves the double purpose of generating electrical energy and of protecting marinas. The specific objective of the tests is to evaluate the performance of the ShoWED when installed in front of a FB and the effects of the wave energy device on the performance of the FB. Physical model tests were carried out at two different scales: 1) in scale 1:20, necessary to evaluate the performance and dynamics of the FB in the absence of the ShoWED. 2) in scale 1:1, in order to evaluate the efficiency of the ShoWED, at different distances from a rear reflective vertical wall, simulating the presence of the FB. A peculiarity of these latter tests is that the real PTO was tested, allowing to measure the produced electrical energy, as a function of the real external electrical impedance. It is concluded that the ShoWED is able to harvest electrical energy if the incident wave height is larger than 0.05 m, a limit possibly given by some friction threshold in the PTO, and if the wave has a period longer than 1.0 s, a limit possibly caused by the finite width of the floater, 70 cm, not negligible compared to the wavelength associated to periods smaller than 1 s. Maximum excursion of the floater are achieved when the floater location takes advantage of the total reflection of the rear wall: for T=2 s, a 26% efficiency was obtained (measured with a "wave to wire" approach), so that a 10 cm wave height produced 7 W in the laboratory. The reflection and transmission characteristics of the hybrid structures were evaluated indirectly, and the benefits compared to a traditional FB should be appreciable especially for long waves.