The hydrodynamics of deformable flexible fabric structures for wave energy conversion

Whereas tidal stream technology is relatively advanced, the wave energy industry is still in a nascent stage of development with a wide range of different design concepts for extracting wave energy being developed. A winning technology has yet to be identified and there may well be more than one. It is within this framework that the theme of 'Novel, future, concepts for marine energy generation' is listed within the EPSRC SUPERGEN Marine challenge 2 Marine energy (Wave and Tidal) technology for 2050. Addressing this theme, we propose a new concept for wave energy conversion that uses a novel hydrodynamic action and is constructed from low cost material leading to significant reduction in cost and size of device in comparison with others.

A significant drawback of wave energy converters acting as heaving 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 optimally in swell waves. 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 an alternative approach in which the device's geometry responds to hydrodynamic loading. In its simplest form, the concept is a floating wedge-shaped body that has a spring-loaded hinge at its apex that closes as the device sinks and opens as it rises in a breathing action. In the proposed project an axisymmetric form will be investigated; comprising a sealed bag that shrinks and swells without hinges. This breathing action makes it possible to install a power take-off inside the device that requires no external reference. This distinguishes it from other heaving point absorbers. The breathing action can be used to pump air through a reversible flow (Wells) turbine into a second container of fixed volume. No other mechanical parts are needed because the pressure change in the fixed volume generates a spring force which contributes to the restoring force on the device. The concept can be designed so that the breathing system resonates at the heaving frequency.

The proposed project will assess the hydrodynamics of the breathing action within a new device concept called the Squid, by developing an optimisation tool using semi-analytical and numerical models and by performing a series of physical experiments. Both still water tests in the flume at the University of Southampton and wave tests in the ocean basin within the new COaST Laboratory at Plymouth University will be carried out and used to investigate the characteristics of the new concept and to validate the numerical model optimisation tool.

The research project proposed here will investigate a novel concept for wave energy conversion: the hydrodynamics of deformable flexible fabric structures and in particular the hydrodynamics of freely floating structures using breathing action to enhance the resonance characteristics in waves. Distinguishing features of the new breathing wave energy device concept are that it needs no external reaction point, and that its natural frequency in heave can be adjusted without changing its geometry. It is very simple, has no sliding seals, bearings, hinges, or valves and is constructed from low cost material. Thus it is anticipated that this new concept will exhibit significant reductions in cost and size in comparison with others and will make a significant contribution to the wave energy industry.

There is clearly a long term benefit in research developments that lead to improvements and cost reductions in the wave energy industry, enabling the expectations and predictions for 2050 to be realised. The benefit is to society at large in contributing to the development of the wave energy industry, providing a renewable source of energy, reducing carbon emissions, providing greater energy security for the UK, in an environmentally sensitive manner.

Research developments made in this project will also add to the UK expertise in the new industry, with potential to develop into a significant export industry for the UK, generating economic benefit to the UK through export of technology and expertise. Research training in the field of wave energy, wave tank testing and numerical modelling for the PDRA researcher will also benefit the industry and academic community.

The main benefit of the research is likely to be gained within the wave energy sector, particularly within the wave energy technology development community and the academic community concerned with wave energy. The new design concept of using a deformable structure to naturally change hydrodynamic motion response characteristics and the idea of using this feature to tune the response may be incorporated within other wave energy device concepts. The benefits of the new design concept may also be transferred to other applications; for example in naval architecture where control of motion response is an advantage in design of roll stabilisation systems. The new knowledge and advances in hydrodynamics that will be gained through this research project will benefit the wider marine hydrodynamics industry, including naval architecture, coastal and offshore engineering where other applications of wave interaction with deformable structures are found.

The results of this research will be disseminated through the usual channels by presentation at international conferences, such as ICOE and EWTEC, at national professional body conferences and special interest meetings, such as RINA and ICE, and by publications in peer reviewed journals. In addition, the project findings will be disseminated to the academic and industry community both nationally and internationally through the SuperGen UKCMER. The project will be reported at quarterly UKCMER progress meetings, the Industrial Advisory Panel and at the Annual Assembly.

In order to make the impact happen, we propose to carry out the following activities:
1) Involve key stakeholders, academics and key industrialists through the Supergen research Advisory Forum and the broader Supergen III management group;
2) Create a website, accessed through the Supergen Web portal, to allow open access to the information produced;
3) Disseminate results at the Supergen III annual assemblies to widen the awareness of these achievements;
5) Arrange public awareness events so that the interested public is well informed.