tidal power

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> >From New Scientist Magazine 26 January 02

>

> Power surge

>

> By Rob Edwards

>

> Renewable energy's latest contender is lurking by a coast near you

>

> A TIDAL power station that taps the surging power of strong underwater

> currents will be tested for the first time this summer. In Britain, there

> are about 40 key locations around the coastlines where, in theory, there's

> enough energy in tidal streams to generate up to a quarter of the nation's

> electricity.

>

> Wind creates waves on the sea surface, but as wind blows intermittently,

> wave power is quite unpredictable. But tides are regular, as they are caused

> by the gravitational pull of the Moon and Sun on large masses of water. If

> the local geography is right, ocean channels create fast-moving " tidal

> streams " , where vast masses of rising or falling water are squeezed into a

> restricted space. But no one has proved that extracting energy from tidal

> streams is practical.

>

> That could soon change. The British government is now looking for new energy

> sources to help cut carbon emissions. And this month, British offshore

> equipment company Engineering Business was given a $1.5 million government

> grant to build a 150-kilowatt prototype tidal power station.

>

> Dubbed Stingray, the machine should be installed between May and September

> on the seabed to the south-east of the Sound of Yell, off mainland Shetland.

> A pair of 15-metre-long hydroplanes, mounted on a stand, will oscillate with

> the tide to drive a hydraulic motor that generates electricity (see

> animation on the Web at www.engb < TARGET= " _blank " >http://www.engb>.

> com/Pages/animation.htm).

>

> Hydraulic pistons control the angle at which Stingray's hydroplanes face the

> tidal current to make the most of the onrushing water. Like an aircraft

> wing, their angle of attack-the angle at which it bites into the

> current-changes to create " lift " , which pulls the hydroplane up and down. As

> they move, the hydroplanes yank on an attached arm that pumps high-pressure

> oil through a hydraulic motor, which turns an electric generator.

>

> Although the design is still being finalised, the structure is expected to

> weigh 35 tonnes, rise to 20 metres above the seabed and work in currents of

> between 2 and 3 metres per second (4 to 6 knots). Most of it will be made of

> steel, though the hydroplanes could be cast in glass-reinforced plastic.

>

> Stingray may only work with the tide flowing in one direction, but its

> successors will swivel round or flip over four times a day so they can catch

> the tidal stream in both directions. Depending on the site, this should mean

> they generate power at least three-quarters of the time.

>

> Engineering Business's managing director Tony Trapp told New Scientist he is

> " quietly confident " that Stingray will work, but stresses that its economics

> are less certain. He estimates that it will generate electricity for between

> 4.7 and 12 pence per kilowatt-hour. Although this is more expensive than

> wind and nuclear power, it is comparable to wave power-and much more

> predictable.

>

> Another contender in the tidal-stream game is an underwater windmill

> developed by Marine Current Turbines of London. A prototype that generates

> power using the circular motion of propeller-like turbines was originally

> due for installation off the coast of south-west England in 2000 (New

> Scientist, 20 June 1998, p 38). But according to MCT's Peter Fraenkel, it

> was postponed because of delays in getting almost $1 million in government

> funding.

>

> Now he has the money, Fraenkel is planning to install a 300-kilowatt

> underwater windmill north-east of Lynmouth, Devon, in September. But both

> Fraenkel and Trapp insist they are not racing to be the first to generate

> tidal power. It is likely to be a multibillion-pound industry, predicts

> Fraenkel. " There's room for both of us " .