Modelling and testing of a pressure-differential wave energy converter with flexible membranes

We investigate a wave energy converter composed of two submerged, bottom-fixed air-filled cylindrical chambers, separated by a distance of ideally half a wavelength, and closed on top by flexible membranes. A passing wave creates a pressure differential between the two chambers which drives the membranes and causes air to flow through a turbine. Using linear analysis and physical model testing, we show that the device exhibits broad-banded power absorption. Our linear model successfully predicts the pressure differential and absorbed power measured in the tests, except at low frequencies, where the differences are attributed to unstable membrane conditions due to a negative total stiffness. Using a simple model, we show that the flat-membrane equilibrium, in the absence of additional stiffness, is unstable. By introducing an additional stiffness to the membrane, this instability can be avoided and the natural period of the device can theoretically be tuned to any value. With a matching flow resistance, it is possible to attain absorbed power values close to the theoretical limit.