Rival designs race to harness ocean energy

www.newscientist.com
10 July 2009

A bout of gawky prototypes have taken to the water for the first time in recent weeks, signalling a new assault on a decades-old problem: how to generate power from the oceans.

While most wind turbines look much the same, the contest to tap that power is more like wacky races than Formula 1. A suite of varied designs are under development in an attempt to work out the most efficient way to generate juice in the harsh chemical and physical environment of the waves and tides.

Extreme environment
Dedicated research into renewable energy at sea truly began during the oil crisis of the 1970s. But progress towards harnessing the estimated 1 million GW-hours of globally available ocean power has been slow, according to the World Energy Council.

Pelamis, the world's first commercial wave "farm" utilises three 150-metre-long jointed steel structures that flex to drive hydraulic generators and produce 750 kWs of power each. Perhaps surprisingly, this renewable-power milestone off the north coast of Portugal only began operations only last year. Now, though, a suite of rivals are hard on its tail.

Sea snake
One even looks similar. The Anaconda, designed by UK firm Checkmate Seaenergy, is a giant snake-like device made of rubber rather than steel. Waves passing over the water-filled tube squeeze it and generate a "bulge wave" inside that spins a generator as it reaches the tail. The final design is intended to be 7 metres wide and 200 metres long; trials with a 1/25th-scale recently took place. Anaconda's developers say a full-sized device could power 1000 average homes when operational. This should be around 2014, they say.

Pole dancing
Another class of wave-harvesters are based around an entirely different concept – floats. "The easiest way to get power from waves is to use a float attached to pole that drives a linear generator," says Hugh-Peter Kelly of UK-based Trident Energy, who is sceptical that Pelamis-style joints and hydraulics can last long in a corrosive ocean environment.

But even this basic idea has been developed into a surprising range of final designs. Kelly's firm uses float with a hydrofoil shape that produce lift and an extra boost of power as waves pass by. "That helps us extract 50 per cent more energy than using a similar sized traditional float," Kelly says.

The buoys each contain a linear generator that moves magnets past one another to make electricity. Each Trident Energy rig could generate up to 1 MW, says Kelly, and a test installation will begin its first sea trial off the Suffolk coast, UK, this summer.

Dive for cover
Other buoys are designed to stay underwater, where they can avoid the roughest sea conditions. A float system installed near Fremantle, in Western Australia last year, pumps seawater through pipes and to onshore hydroturbines where they are out of reach of the corrosive, damaging sea. The CETO system has performed well so far and the first commercial plant is planned for later in 2009. The company say a 5-hectare array of the buoys could generate 50 MWs of power.

Back in the northern hemisphere Archimedes Waveswing is another underwater buoy, designed by UK firm AWS Ocean Energy. Each stays at least 6 metres below the surface and has an upper section that can move up and down like a piston relative to the lower section. That compresses the gas inside the hollow buoy as a wave peak passes, releasing it when the trough arrives. The gas inside is squeezed through a generator inside each buoy, and 100 could power 55,000 typical homes, says the firm.

Go with the flow
Several new tidal power schemes have also benefited from a surge in interest to enter the water in recent months. A fixed platform mounted with two turbines was installed in the tidal power currents of Strangford Lough, Northern Ireland, in 2008 and is already supplying 1.2 MWs of power to local houses. The operators Marine Current Turbines plan an installation 10 times as powerful off the Welsh coast by 2011.

UK company TidalStream recently tested a 3-metre-tall scaffold bearing six turbines designed to be used in a similar way, in a tank at the Ifremer energy test centre in Brest, France. Also designed for tidal power channels, a full-sized 60-metre-tall platform is projected to generate 10 MWs of power

Beating waves
John Armstrong of TidalStream points out that the reliable ebb and flow of the tides compares favourably with the fluctuating output of wind and solar installations, which have hitherto garnered more attention, and funding. The reliability of wave power has also been under appreciated, according to a recent report by UK government agency the Carbon Trust.

That could help sustain the growth in interest in extracting energy from the sea, as the challenges of introducing fluctuating energy sources like wind into existing energy grids become apparent. Only long-term trials, though, will determine whether one of the various designs in testing today will emerge to become the industry standard.