The University of Southampton
Engineering

Research project: The hydrodynamics of deformable flexible fabric structures for wave energy conversion

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A joint project with Plymouth University.

Project Overview

A significant drawback of wave energy converters acting as heaving (i.e. oscillating vertically) point absorbers is the mismatch between the typical wave period of the wave climate and the resonant natural period of motion response. This means that devices have to be large in order to operate at resonance in swell waves. For a semi-submerged floating sphere, oscillating in heave, the diameter required for resonance in sea waves of 10 seconds period is approximately 52 m. A vertical cylinder would need a draught of 25m for the same period. Devices of this size would be impractical and uneconomic.

To overcome these limitations, control systems may be used in order to modify the motion response to suit the wave climate, but this can be complex and expensive. In this project, we investigate a simple alternative approach in which the device’s geometry responds to the hydrodynamic loading on it. In its simplest form the concept consists of a floating wedge-shaped air-filled body with a spring-loaded hinge at its submerged apex. If the spring is very soft, a small downwards displacement will cause the wedge to close and sink to the seabed. If the spring is very stiff, it will behave in waves almost like a rigid body of the same profile and mass distribution. Between these extremes there is a critical stiffness at which for small displacements, changes in hydrostatic and spring moments are equal and opposite, so that the wedge is neutrally stable -- or in other words it has an infinite natural period of oscillation. By increasing the stiffness above this critical value it is possible to set the natural period much lower than that of a rigid body of the same dimensions. And for the purposes of wave energy conversion, the breathing motion of the wedge in waves is ideal for a power take-off inside it.

But a simple wedge with a spring is probably not the best way to apply this principle in a wave energy converter. In this project we are studying other configurations, using flexible materials, with the restoring force provided mainly by the compressibility of the internal air. Both laboratory experiments and numerical modelling will help to determine whether the concept of variable geometry can be usefully applied to the difficult problem of converting wave energy into electricity.

Related research groups

Energy and Climate Change

Staff

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