Interactions of a submerged membrane with water waves and its use in harnessing nearshore wave power

Abstract

Developed and developing countries need electricity, and this usage is increasing everyday. This constant increase cannot be satisfied with the current ways of electricity generation that have shown themselves to be out of phase with environmental concerns. Oceans yield great amount of energy that could be converted into electricity and the current research deals with one portion of ocean energy, wave energy. The wave energy converter studied in this thesis is a bottom-mounted, liquid-filled rectangular duct, covered with a rubber membrane and aligned head to the waves propagation direction. Two types of membrane were tested. The behaviour of this device beneath waves was investigated with two configurations: one with both its ends closed, the other one with a power take-off connected at its stern. The pressure at both ends was characterised by means of pressure transducers and the pressure inside the duct by means of laser sensors measuring the membrane displacement. Results from experiments carried out on the closed version of the duct pointed out a resonant behaviour of the system for wave frequencies at which bulges, propagating in the rubber membrane, could travel an integer number of times along the duct’s length. This resonance was characterised by pressure magnitudes at the stern up to 2.8 times that acting on the membrane from the incident wave. Moreover, the membrane displacement was for the first time mapped and the profile obtained showed characteristic nodes and antinodes. The performance of this device in harnessing wave power was evaluated by connecting a linear dashpot at its stern. Capture widths of up to 2.2 times the device width were obtained and the bandwidth of maximum power capture not limited to a single frequency. The pressure behaviour in both configurations was explained with a one dimensional theory of bulges propagating in distensible tubes with good agreement for the thicker tested membrane. On the contrary, this was not the case for the thinner membrane, suggesting that this model could not be used for configurations where bulge wavelengths are much shorter than that of the incoming wave.

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ORCID for J.R. Chaplin: orcid.org/0000-0003-2814-747X

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