In field measurements on small scale OWC device

A 1:10 scale model of an ocean OWC breakwater was realised and put at the sea off the beach of Reggio Calabria (Italy). The breakwater (about 16 m long and 3.5 m height) was placed on a 2.1 m bottom depth, and embodied the REWEC wave energy converter (Boccotti et al.,2007, J. of Ocean Engineering, 34, 820-841). A small scale Wells turbine was installed onto the central caisson of the breakwater. Two different experiments were carried out. The first one aimed to test the energy absorption capabilities of the system and to analyse the waves-absorber interaction mechanism. The second experiment aimed to test the Wells turbine under oscillating randomly varying flows produced into the plant by sea waves. The goals of these experiments involved some measurements made critical because ofthe harsh environment, the high variability of randomly oscillating flows and unavoidable scale effects. Here, we show how these measurement difficulties were overcome by developing “ ad hoc” measurements techniques. In particular, in this paper some critical measurements are described: the measurement of the water discharge through the plant, necessary to estimate the absorbed power; the instantaneous value of the torque exerted by the air flow on the turbine, necessary to analyse the actual unsteady behaviour of the turbine. Both measurements were carried out by using two different techniques, in order to make them robust. Water discharge were calculated by integrating the acceleration of the water column, by recording the pressure simultaneously in different point along the streamline. This punctual measurement was compared with measurements carried out by an ultrasonic probe located on the roof of the plenum chamber. Because of the width of the sound beam, this measurement is space averaged. Torque measurements were carried out by using the DC motor coupled with the Wells turbine as an electro-mechanical transducer. The procedure proved to be accurate once calibrated. A not trivial task was to separate, in the unsteady randomly varying reverse flow, the share of power transferred to the blade by the air current, from the shaft power driving the turbine, being necessary to sustain the rotation of the wheel many times in the cycle, because of high incidence of friction power losses in the small scale model.