FROM giant hydraulic oysters that sit on the sea floor, to long rubber snakes that writhe in the ocean swell, there’s no shortage of creatures designed to harness the power of the waves. If wave power is to emerge as a viable form of green energy, we need to put them to the test and only the most reliable can expect to survive.
While there’s a veritable menagerie of strange beasts taking to the sea, most of them can expect a humdrum life, says , a marine engineer at the University of Southampton in the UK. “The fundamental problem facing wave-power devices is that most of the time the water is moving with rather low velocities,” he says.
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Just as wind turbines grind to a halt on a quiet day, wave power machines generate little power in quiescent conditions. That’s the challenge for wave power – how to extract energy from lifeless waters. “Such a wide-open brief has led to an enormous range of inventions,” says Chaplin.
Budding wave-power designers are getting ample opportunity to find ways to turn gently bobbing waves into energy, with new projects hitting the water with metronomic regularity. For example, last month, New Jersey-based confirmed it was to start work on a project to deploy 10 of its PowerBuoy machines 4 kilometres off the coast of Reedsport, Oregon. They ride on the surface, converting the up-and-down motion of the waves into electrical power.
This project and others like it will add to the growing throng of wave-power systems already in the water.
of Southend-on-Sea, UK, chose a system of floats that bob up and down in the waves to drive generators on a platform above. Aquamarine Power and , both based in Edinburgh, UK, have opted for more unusual solutions. has developed the Oyster, a hinged metallic shell that sits on the sea floor and opens and closes as waves wash over it – hydraulic cables link it to onshore hydroelectric generators. A farm of 20 Oysters would power 9000 homes.
The Pelamis device is a jointed mechanical snake that floats near the surface, flexing in the waves to drive hydraulics systems that power electrical generators. It has already been tried out in the North Sea off Orkney – the proximity of a populated region to an area where waves are relatively strong makes it a popular choice. It has also been tested at the world’s first commercial wave farm off the Portuguese coast.
Early experience with these devices shows how difficult it is to set up a viable wave power system. The three Pelamis snakes tried out in Portugal’s wave farm in 2008 had to be towed back to shore after barely two months because of buoyancy issues and a lack of cash. Trident Energy, meanwhile, is still struggling to start sea trials. Last September, the company’s prototype rig capsized as it was being towed to its test site, setting the project back several months.
Although early setbacks are to be expected, says , a renewable energy engineer at Durham University, UK, wave power must learn lessons from the more established wind-power industry if it is to thrive.
Unlike solar energy, where efforts have focused on making designs more efficient, successful wind-power designs are based on reliability. The ubiquitous three-blade wind turbine has a history stretching back at least 50 years to Gedser in Denmark. The Gedser Mill, as the turbine is called, hardly represents the pinnacle of turbine efficiency, says Tavner. Others in the industry, particularly engineers in the US, thought a two-blade turbine offered better aerodynamics, he says.
The trouble was that the hub at the centre of a two-blade turbine is under severe strain as the two blades rotate. That strain reduces the turbine’s life expectancy; adding a third blade eases the problem.
As tweaks to the Gedser Mill design further improved its reliability, it became clear even in the US that the Danish turbine would win out. “Is a three-blade turbine optimal? No,” says Tavner. “But is it functional and has the production volume risen to make it a good solution? Yes.”
“I think the three-blade Gedser turbine design is a bit like a four-stroke petrol engine,” Tavner says. “It worked and it was rugged.”
There are signs that the wave-power industry has grasped the importance of reliability. For example, the Trident Energy system has abandoned hydraulics, which are prone to rusting, so its bobbing floats, whose movement is used to generate energy, should stay operational, says company founder, Hugh-Peter Kelly.
The Pelamis snakes are easily towed into dry dock for cheap and easy onshore maintenance, points out Tavner. Aquamarine Power claims that maintenance problems with the Oyster are minimal because it pumps pressurised water onshore to drive easy-to-reach generators there.
Chaplin, meanwhile, is a member of the team behind perhaps the most outlandish of all wave-power converters, the 200-metre-long Anaconda. Floating just below the surface, the water-filled rubber tube is squeezed by the waves. As a swell hits the front of the snake, it squeezes the tube creating a bulge that ripples along its length to power a turbine in its tail. The unusual material and general lack of moving parts should give it a long life, Chaplin claims.
At this stage in the nascent wave-power industry, no one actually knows which designs will prove to be up to the job, says Tavner. To some extent, the industry will have to “throw the machines out there” and see how long they survive.
“The wave-power industry will have to throw the machines out there and see how long they survive”
It’s only through a period of very public failures that the wave-power industry will arrive at its own version of the Gedser Mill, he suggests. When that happens, perhaps the industry will become known for its utility, rather than its oddity.
