2010年5月11日火曜日

Plenty of wave energy to be harvested close to shore

【出展リンク】:

http://www.newscientist.com/article/mg20627595.700

Closer to home (Image: Pelamis Wave Power)

Closer to home (Image: Pelamis Wave Power)

Plenty of wave energy to be harvested close to shore

SINCE the oil crisis of the 1970s kick-started interest in wave power, it has become received wisdom that only offshore waves are worth tapping into. As a result, the varied designs competing to rule the waves have been banished to distant, deep waters where conditions are rougher and engineering costs highest.
Now a reassessment suggests that waves closer to shore are not such puny prospects after all, raising the hope that harvesting energy from wavesMovie Camera will become easier and more economical than previously imagined.
Statisticians took to the seas in the 1970s. They used data from weather ships and buoys and calculated that the waves passing a 1-metre-wide slice of water at a 50-metre-deep "offshore" location carry an average power of 40 kilowatts, double the power density of waves in 10-metre-deep waters nearer the shore.
Ever since, the wave-power industry has focused its attention 2 to 10 kilometres offshore in the belief that the greater energy yield would offset the higher price of maintenance and of relaying the power back to land.
Now Matthew Folley at Queen's University Belfast, UK, says wave power can be brought closer to home. He calculated how much energy could realistically be extracted from waves using contemporary computer models of the world's oceans. He found that nearshore waves 0.5 to 2 kilometres from the coast carry 80 to 90 per cent of the usable energy of waves further out. Folley reckons offshore waves carry exploitable power at a density of about 18.5 kilowatts per metre-slice, compared with about 16.5 kW for nearshore ones.
Nearshore waves carry 80 to 90 per cent of the usable energy found in offshore ones
The standard figures overestimated the utility of offshore waves for two reasons, he says. They allowed severe storms to push up the average power figures, although wave-power devices generate little power in such storms because they would often have to switch into a self-preservation mode.
They also assumed that offshore waves have a prevailing direction, just as nearshore waves tend to move towards the coast. It is now understood that the latter is due to the shallowing sea floor refracting waves, says Folley - an effect not at work offshore. Instead, offshore waves come from all directions, meaning that some harvesters in an offshore wave farm would be blocked by others. Nearshore farms could be strung out in lines to avoid that.
The figures are convincing, says Ian Bryden at the University of Edinburgh, UK. "But many developers are now committed to deeper water and are unlikely to change their approach, given their investment," he says.
Few designs can simply be towed closer to shore, Folley notes, because offshore waves are best exploited by bobbing up and down, while nearshore ones have most energy in their back-and-forth movements. Some types of energy harvester are ready for shallow waters right now, he says, such as theOyster, developed by Edinburgh-based Aquamarine Power, and theWaveRoller from Finnish firm AW-Energy.
Issue 2759 of New Scientist magazine
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