Energy Conversion OWC Devices With Additional Vertical Ducts

In breakwaters embodying an OWC connected to the sea through a vertical duct or through a small opening, the oscillations of the water column are due to the wave pressure acting on the outer opening of the vertical duct or on the small opening. In fact, in neither of the two cases waves can enter the plant, like it happens in conventional OWCs. The additional vertical duct extends along the wave-beaten wall, giving the device the characteristic form of a U-conduit; for this reason they were also named U-OWC. Experiments on a small-scale breakwater embodying a U-OWC were carried out in the natural laboratory of Reggio Calabria. The plant is a 1:10 scale model of a breakwater suitable for the North-East Pacific coast. The paper describes new experiments on the U-OWC device connected to a monoplane Wells turbine. During an intensive measurement campaign, more than 260 sea states, 5 min long were recorded in order to characterize the energy conversion process. From the experiments, the analysis of the energy conversion shows that: 1) the system is able to absorb a share up 80% of the incident wave energy; this result is similar to that obtained in previous experiments carried out without the turbine; 2) a large fraction of the energy entering the U-OWC is converted in pneumatic power acting on the turbine, being head losses in the water flow limited; 3) the efficiency of conversion of the pneumatic power in turbine power is relatively low (about 36% of the pneumatic power) due to the small dimensions of the turbine that lead to low Reynolds number and large influence of secondary losses.