Harnessing the Power of Waves
July 07, 2008
As long as people have turned their eyes from the shore to the sea, they have seen and recognized the power of waves as they relentlessly hurl themselves toward the shoreline. I assume that for all those eons someone has also dreamed of harnessing that power. In the modern age, serious efforts to harness wave power remain in their infancy [“The coming wave,” The Economist, 7 June 2008 print edition].
“The first patents for wave-power devices were issued in the 18th century. But nothing much happened until the mid-1970s, when the oil crisis inspired Stephen Salter, an engineer at the University of Edinburgh, in Scotland, to develop a wave generator known as Salter’s Duck. His design contained curved, floating canisters, each the size of a house, that would be strung together and then tethered to the ocean floor. As the canisters, known as Ducks, were tossed about by the waves, each one would rock back and forth. Hydraulics would convert the rocking motion to rotational motion, which would in turn drive a generator. A single Duck was calculated to be capable of generating 6 megawatts (MW) of electricity—enough to power around 4,000 homes. The plan was to install them in groups of several dozen. Initial estimates put the cost of generating electricity in this way at nearly $1 per kilowatt hour (kWh), far more than nuclear power, the most expensive electricity at the time. But as Dr Salter and his team improved their design, they managed to bring the cost-per-kWh down to the cost of nuclear power. Even so, the research programme was shut down by the British government in 1982.”
Interest in wave power has never waned and with skyrocketing energy prices it appears to be rising like the incoming tide.
“As soaring oil prices and concern over climate change give added urgency to the search for new, renewable sources of energy, the sea is an obvious place to look. In theory the world’s electricity needs could be met with just a tiny fraction of the energy sloshing around in the oceans. Alas, harnessing it has proved to be unexpectedly difficult. In recent years wind farms have sprouted on plains and hilltops, and solar panels have been sprinkled across rooftops and deserts. But where the technology of wind and solar power is established and steadily improving, that of wave power is still in its infancy. The world had to wait until October 2007 for the first commercial wave farm, consisting of three snakelike tubes undulating with the Atlantic swell off the coast of Portugal.”
Although trying to harness the power of nature found in open stretches along the coastline might sound like a no-brainer, people living along the coast often object to such projects. They pay big bucks for those ocean views and don’t want them spoiled by manmade objects, especially objects the size of wind-powered generators. One would think that harnessing waves would have less visual impact on the coastline, but, as The Economist notes, harnessing wave power is extremely difficult.
“The story of wave power … has been one of trials and tests followed by disappointment and delays. Of the many devices developed to capture wave energy, none has ever been deployed on a large scale. Given wave power’s potential, why has it been so hard to get the technology to work—and may things now be about to change?”
The article notes that in “December  Pacific Gas & Electric, an American utility, signed an agreement to buy electricity from a wave farm that is to be built off the coast of California and is due to open in 2012. Across the world many other wave-power schemes are on the drawing board.” It also reports that Dr. Salter’s legacy lives on.
“One example [of the next generation of wave power generators] is the Pelamis device, designed by some of Dr Salter’s former students, who now work at Pelamis Wave Power, a firm based in Scotland. Three such devices, each capable of generating up to 750kW, have been deployed off the coast of Portugal, and dozens more are due to be installed by 2009. There are also plans for installations off Orkney in Scotland and Cornwall in England. As waves travel along the 140-metre length of the snakelike Pelamis, its hinged joints bend both up and down, and from side to side. This causes hydraulic rams at the joints to pump hydraulic fluid through turbines, turning generators to produce electricity. Pelamis generators present only a small cross-section to incoming waves, and absorb less and less energy as the waves get bigger. This might seem odd, but most of the time the devices will not be operating in stormy seas—and when a storm does occur, their survival is more important than their power output.”
Since the Pelamis device is submerged beneath the ocean’s surface, it is aesthetically more acceptable than some other designs now being considered (pictures are available at the article’s link). Surprisingly, the design selected to be installed the California coast one of the semi-submerged designs.
“The Aquabuoy, designed by Finavera Renewables of Vancouver, takes a different approach. (This is the device that Pacific Gas & Electric hopes to deploy off the California coast.) Each Aquabuoy is a tube, 25-metres long, that floats vertically in the water and is tethered to the sea floor. Its up-and-down bobbing motion is used to pressurise water stored in the tube below the surface. Once the pressure reaches a certain level, the water is released, spinning a turbine and generating electricity. The design is deliberately simple, with few moving parts. In theory, at least, there is very little to go wrong. But a prototype device failed last year when it sprang a leak and its bilge-pump malfunctioned, causing it to sink just as it was due to be collected at the end of a trial. Finavera has not released the results of the trial, which was intended to measure the Aquabuoy’s power output, among other things. The company has said, however, that Aquabuoy will be profitable only if each device can generate at least 250kW, and that it has yet to reach this threshold.”
According to the article, the “bobbing” idea seems to be popular among designers, but it is not the only idea being tested.
“Similar bobbing buoys are also being worked on by AWS Ocean Energy, based in Scotland, and Ocean Power Technologies, based in Pennington, New Jersey, among others. The AWS design is unusual because the buoys are entirely submerged; the Ocean Power device, called the PowerBuoy, is being tested off the coast of Spain by Iberdrola, a Spanish utility. The Oyster, a wave-power device from Aquamarine Power, another Scottish firm, works in an entirely different way. It is an oscillating metal flap, 12 metres tall and 18 metres wide, installed close to shore. As the waves roll over it, the flap flexes backwards and forwards. This motion drives pistons that pump seawater at high pressure through a pipe to a hydroelectric generator. The generator is onshore, and can be connected to lots of Oyster devices, each of which is expected to generate up to 600kW. The idea is to make the parts that go in the sea simple and robust, and to keep the complicated and delicate bits out of the water. Testing of a prototype off the Orkney coast is due to start this summer. The logical conclusion of this is to put everything onshore—and that is the idea behind the Limpet. It is the work of Wavegen, a Scottish firm which is a subsidiary of Voith Siemens Hydro, a German hydropower firm. A prototype has been in action on the island of Islay, off the Scottish coast, since 2000. The Limpet is a chamber that sits on the shoreline. The bottom of the chamber is open to the sea, and on top is a turbine that always spins in the same direction, regardless of the direction of the airflow through it. As waves slam into the shore, water is pushed into the chamber and this in turn displaces the air, driving it through the turbine. As the water recedes, air is sucked back into the chamber, driving the same turbine again. The Limpet on Islay has three chambers which generate an average of 100kW between them, but larger devices could potentially generate three times this amount, according to Wavegen. Limpets may be built into harbour breakwaters in Scotland and Spain.”
Those designs are the ones closest to deployment, but The Economist indicates that other designs are on the drawing boards. The reason there are so many designs, the article laments, is that finding one that maximizes the power of waves remains illusive.
“Dozens of wave-energy technologies are being developed around the world: ideas, in other words, are not what has held the field back. So what has? Tom Thorpe of Oxford Oceanics, a consultancy, blames several overlapping causes. For a start, wave energy has lagged behind wind and solar, because the technology is much younger and still faces some big technical obstacles. ‘This is a completely new energy technology, whereas wind and photovoltaics have been around for a long time—so they have been developed, rather than invented,’ says Mr Thorpe. … Once interest in wave power revived earlier this decade, practical problems arose. A recurring problem, ironically enough, is that new devices underestimate the power of the sea, and are unable to withstand its assault. Installing wave-energy devices is also expensive; special vessels are needed to tow equipment out to sea, and it can be difficult to get hold of them. ‘Vessels that could potentially do the job are all booked up by companies collecting offshore oil,’ says Trevor Whittaker, an engineer at Queen’s University in Belfast who has been part of both the Limpet and Oyster projects. ‘Wave-generator installation is forced to compete with the high prices the oil industry can pay.’ Another practical problem is the lack of infrastructure to connect wave-energy generators to the power grid. The cost of establishing this infrastructure makes small-scale wave-energy generation and testing unfeasible; but large-scale projects are hugely expensive. One way around this is to build a ‘Wave Hub’, like the one due to be installed off the coast of Cornwall in 2010 that will provide infrastructure to connect up wave-energy arrays for testing.”
Thorpe goes on to warn that we should expect some spectacular and costly failures going forward and he fears that such failures could set back development another generation. Despite all of these challenges, however, The Economist notes that big utility companies are starting to consider wave power seriously and venture capitalists are beginning to jump on board. There doesn’t appear to be a single design destined to sweep the field, which means that R&D is likely to continue for the next several decades before any true breakthrough is achieved.