Published online 11 October 2005 | Nature | doi:10.1038/news051010-7

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Water lens' might help to harness the ocean's energy.

Focussing wave energy to a point could help power generators suck the most from the sea. © Punchstock

Attempts to harness wave power for clean energy generation could be helped by building posts at sea, say researchers who have taken a lesson from optics and applied it to the ocean.

Xinhua Hu and Che Ting Chan of the Hong Kong University of Science and Technology say that a forest of pillars standing in orderly rows in shallow water can bend ocean waves by refraction - the same phenomenon that makes a pencil immersed in a beaker of water look kinked.

If the pillars are arranged in a lens-shaped array, the researchers report in Physical Review Letters1, they act as a glass lens does on light, to focus the waves into a point.

This would allow wave energy to be collected from a stretch of coastline several hundred metres long and focused onto a single device that converts their mechanical energy into some other form, such as electricity (see 'Swell magnet stokes support for wave power'). Such devices often work best when the waves are large, as they would be after this kind of focusing.

"It's a very interesting idea," says John Pendry, a physicist at Imperial College London. "There's a lot of power in water waves."

Andrew Scott, project development engineer with the Edinburgh-based wave-power company Ocean Power Delivery, says that there is plenty of interest in focusing ocean waves for power generation. He says that existing focusers, like that used in the Wave Dragon device developed in Denmark, simply use floating arms to channel the waves.

A lot like light

“There's a lot of power in water waves” John Pendry

Imperial College, London.

Pendry and other researchers are exploring ways of using regular arrays of very tiny objects to control other kinds of waves, particularly light. Microscopic pillars carved in silicon wafers, or stacks of minuscule plastic beads, can act as so-called 'photonic crystals' that reflect light perfectly, making them useful in light-based information technology.

This behaviour happens when the spacing of the 'scattering' objects is about the same as the wavelength of the waves concerned. Thus, researchers in Spain have shown that arrays of metal bars a few centimetres apart can be used as barriers to deflect sound waves. They have even speculated about using ranks of holes drilled in the ground to protect buildings from seismic waves during earthquakes.

Extending the principle to ocean waves is "a very clever idea," says Pendry.

In focus

Hu and Chan have added to the idea by considering what happens when the wavelength is much longer than the spacing of the pillars. Ocean waves in shallow waters typically have wavelengths of hundreds of meters, whereas the researchers consider pillar spacings of about a fifth of that.

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They find that ten rows of such pillars will reflect shallow water waves completely when the waves strike at certain angles. At other angles, the water waves are bent by refraction. A lens-shaped array of about 650 pillars can focus nearly half of the incident wave energy into an area less than five times the pillar spacing: a few tens of metres, perhaps.

If successful in trials, such devices could also in principle be used to disperse rather than focus wave energy, which would be useful for coastal defences against erosion. "Maybe you could deflect the energy away," muses Pendry. "Yes, why not?"

Imperial College, London.