A linearized model for estimating the performance of submerged resonant wave energy converters

A realistic performance analysis of OWC wave energy converters addresses to a set of non-linear differential equations that need to be integrated in time, by using a stochastic approach, under the hypothesis of random wind-generated sea waves, for all the sea states which characterize the location of the system. Non-linearities of the differential equations have several origins: minor and major losses of the unsteady flow of water and air; compressibility of air and heat exchange with the walls of the air chamber; non-linear characteristics of the turbine. Under the hypothesis of random sea waves with gaussian distribution, the authors propose an original methodology for linearizing the differential equations that describe the system behaviour. Under such hypothesis, the linearized model can be used for predicting the power output by means of the calculations in the frequency domain and for control design. The developed methodology has been applied to the estimation of the performance of the new “Resonant Sea Wave Energy Converters”, called REWEC, patented by Boccotti in 1998, and consisting of several caissons, characterised by a structure similar to the caissons of the traditional breakwaters and placed on the seabed, close one to each other, to form a submerged breakwater. Each caisson is connected to a vertical duct wholly beneath the sea level, where a hydraulic Wells turbine is placed. The matching between turbine and resonance characteristic of the system is carefully analysed in order to maximize the energy conversion efficiency.