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Battle of the bulge
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09/04/2008
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A hydraulic effect is being harnessed to build up massive amounts of energy – and may well be the key to producing cheap power from sea waves
The movement of bulge waves, first noticed in human arteries, may prove the means to turn mechanical motion into hydraulic pressure for storage in accumulators.
By encouraging the effect in a 7m diameter, 150m long rubber sausage, which could be moored in the sea and allowed to weathercock with tides and currents, it should be possible to generate an average of about 1MW of power. On a smaller scale, an oil-filled version could be used to extract energy from cars or walking people, to supply power to items such as signs on roads or at railway stations.
The idea is still very much in the research stage, but the Carbon Trust appears to be very keen, EPSRC funding has been found and Checkmateuk – headquartered in Sheerness, Kent – has taken it on board with a view to commercialising and manufacturing the sausages.
Paul Auston, chairman and managing director of the company – which makes lifting and lashing, and height safety equipment – has a wealth of experience with flexible materials. He believes he has identified the solution to the nation’s energy needs in the form of a project called ‘Anaconda’.
Eighteen months ago, his company was approached by inventors Francis Farley FRS and Rod Rainey – the latter being head of technology for floating structures at Atkins Oil and Gas. They had latched on to an effect described by Sir James Lighthill in his book, ‘Waves in Fluids’, in which bulge waves travelling along an elastic tube can accumulate energy. This is why pressure pulses in human arteries travel at much less than the speed of sound in blood. If an elastic tube, on the other hand, is filled with water and is just beneath the surface of the sea – and the speed of the bulge wave propagation is similar to the speed of the waves – the bulge waves build up and travel just in front of the wave, similar to a surfboard.
Auston said they had applied to the Carbon Trust and are now working with the Checkmate Group to develop the device. Checkmate Group has formed specialist subsidiary Checkmate Seaenergy, which has worldwide exclusive rights to exploit the technology.
The project has also been boosted by a £430,000 EPSRC grant awarded to Professor John Chaplin, professor of Applied Fluid Mechanics at the University of Southampton. This started on 1 April and is scheduled for completion by 31 March 2010.
Laboratory-scale experiments undertaken by Chaplin’s team, using a model tube about 2.5m long and 78mm in diameter, and made from rubber sheet 0.15mm thick, show the basic idea works. In a published paper, he and his co-authors state that they installed the tube just below still water level on the centre line of a wave flume, pressurised the tube to an excess of about 70mm and sealed it. A transducer at the trailing end then showed the pressure pulses.
The challenges now are to scale up and test ideas for extracting power from the system on a steady basis. The Chaplin paper describes a scheme whereby the pressure waves pump water into water accumulators, where gas is compressed in internal bladders. Water first enters an upper, high-pressure, accumulator through a one-way ‘Duck bill’ valve and generates power as it passes to a lower, low-pressure, accumulator, which connects again with the main tube through a second one-way valve. The gas-filled bladders also communicate with two external bladders, which provide the necessary pressurisation and allow for volume fluctuations.
Chaplin’s grant abstract states the concept of the Anaconda has been proved at a scale of about 1:85 and that, in the new study: “Experiments will be carried out at scales of 1:28 and 1:14 with tubes of diameter 0.25m and 0.25mm, at which rubber hysteresis losses will be proportionately much lower than at smaller scale. Three types of experiments are planned, to provide measurements of internal pressures, tube displacements, radiated waves, mooring forces and absorber power: (1) measurements in still water with bulge waves generated mechanically at one end of the tube and absorbed at the other; (2) measurements in regular and irregular waves; (3) measurements in extreme waves.”
According to inventor Farley, the capital cost per megawatt is likely to be about £2-3million, which is “much less than existing wave power converters”.
The idea has other potential applications. For example, if an oil-filled sausage were to be built into a slightly flexible road surface, it could take some of the energy from the passing traffic to power safety equipment or traffic signs. It would even be possible to extract energy from people or animals as they walked by, provided they always walked in the same direction, as in the passages on the London Underground.
Pointers
* Moving a compressive force along a fluid containing elastomeric tube can accumulate energy
* The compressive force can be applied mechanically or by hydrostatic pressure
* It is currently being researched as a means of making large, relatively low-cost, wave energy power generators
* The rubber technology required has already been demonstrated as being extremely durable in marine and other applications
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Author
Tom Shelley
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