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Current power: New tide turbines tap oceans of energy
20 September 2011 by Fred Pearce
Rising tide (Image: Rob Jones III/Flickr/Getty)
The huge potential of ocean currents could at last be tapped, thanks to the latest generators now being trialled on the seabed
OFF the northernmost tip of Scotland, where the turbulent waters of the Atlantic Ocean meet those of the North Sea, sits a chain of 70 mostly uninhabited islands collectively known as Orkney. Best known for its wildlife and Neolithic historical sites, it isn't the first place you think of as a centre for cutting-edge science. Yet it is at the heart of what could soon be a renewable energy renaissance.
The strong tides around these islands have led the European Union-funded European Marine Energy Centre (EMEC) to use Orkneys' waters as the world's largest testbed for a renewable energy source that has been stalled for years: tidal power.
If new turbine designs being tested here can survive the pitiless Scottish swells, they will finally tame one of the world's most promising sources of renewable energy. But that's not all. Some of these designs could go far beyond tapping tides along the coast, reaping energy from the most powerful phenomena in the oceans - the world's major ocean currents. Harness these powerhouses, and tidal power would help smash all previous predictions of how much energy the oceans can provide.
Tidal power has lagged behind other forms of renewable energy for almost 30 years. Not even the Intergovernmental Panel on Climate Change has managed to muster much enthusiasm for the technology: it recently estimated that the ocean's maximum contribution to the global energy budget would not exceed a paltry 1 to 2 per cent by 2050.
Tidal power fell out of favour for good reason. Initial attempts to generate electricity from tides did so with big, dam-like structures called barrages. The first such barrage, built in Brittany, France, in the 1960s, remains the largest tidal-energy generating station in the world. Few are eager to build more barrages, and those that have been suggested - such as a recently proposed project across the Severn estuary in the UK - have met with resistance. That's because building these monsters is hugely expensive, and concerns have been raised about their impact on the environment. Soon, wave power began to offer a more attractive alternative (New Scientist, 27 August, p 17).
And yet the tides have proved too alluring to abandon. They are predictable, renewable, non-polluting and powerful - everything other renewable sources aren't. So engineers tried a different tactic. Instead of tapping the vertical motion of the tides with costly barrages, they refocused their efforts on the horizontal tidal currents, which are the water's response to the rise and fall of the tides.
The best way to tap these currents is with underwater turbines - vertical piles anchored to the seabed, with blades at their top end set to rotate around a horizontal axis. They work just like wind turbines, the tidal current spinning the blades; the resulting mechanical energy is converted to electricity in the turbine and taken ashore by cable.
Underwater turbines offer a host of benefits. For one thing, the environmental concerns that plagued tidal barrages appear to be largely irrelevant for tidal turbines (see "Stirring up controversy").
Need for speed
More importantly, tidal turbines can in principle generate impressive amounts of energy, as long as the speed of the current exceeds about 2.5 metres per second. Because the density of sea water is more than 800 times that of air, in power generation terms that speed is the equivalent of a wind speed of more than 260 kilometres per hour, 10 times the average encountered by a typical wind turbine.
However, though tidal currents are everywhere in the ocean, they are seldom fast enough to generate a worthwhile amount of energy. They only get up to the required speed in places where water is forced into a channel between islands, like in the Minas Passage into the Bay of Fundy in Nova Scotia, Canada, or surges round headlands (see map). The scarcity of locations suited to tidal turbines has meant that technical progress in the past 10 years has been much slower than many in the industry were hoping. It is hard to test prototypes properly without a fast current, and the few existing demonstration projects, such as a small turbine that powers a grocery store on New York's East river, have only been tried in relatively weak currents that do not replicate conditions in the open ocean.
Tidal currents could theoretically provide up to 5 per cent of the UK's power, as compared to wind, which now contributes just over 3 per cent. That's according to The Carbon Trust, which advises the government on low-carbon energy.
But tidal power's time is at hand, says Neil Kermode, who manages EMEC. He says tidal turbines have taken the lead in the race to develop marine energy sources, surging past both barrages and wave power. That's why the seas off Orkney have never been busier, having hosted five of the world's dozen new tidal turbine designs over the past few months. Out here, the currents reach 3 metres per second.
Tapping these fast tidal flows requires highly reliable equipment - it costs millions of dollars to install underwater turbines, and just as much to retrieve them for repairs. "Metal fatigue will be critical," says Kermode. "Ocean flow is very chaotic."
Failures are inevitable, says Dave Elliott, professor of technology policy at the Open University in Milton Keynes, UK. He points out that shortly after being installed for a test in the Bay of Fundy last year, a turbine built by the Irish company OpenHydro lost all its blades.
A winning design must survive the hostile environment on the ocean floor, where the possibility of undertaking repairs is limited. And so many companies have focused on robustness. For example, OpenHydro is now testing a new turbine whose rotor blades are nested inside a doughnut-shaped ring. The company argues the design is stabler and therefore better equipped to survive rough waters (see diagram).
Instead of trying to build fail-safe designs, other companies have opted to make their turbines more easily retrievable in the event of a problem. Scotrenewables, based in Stromness in Orkney, has built a free-floating turbine mounted on a buoyant vehicle that resembles a yellow submarine, with propeller-like rotors for tapping the tidal currents.
However, tidal power's biggest problem remains the shortage of fast currents. Some have tried to get round this with turbines that can exploit currents racing through very shallow waters. These are inaccessible to most turbine designs, which are so tall they would jut helplessly out of water less than 20 metres deep.
How low can you go?
Pulse Tidal, based in Sheffield, UK, has created one such shallow-water turbine, consisting of a pair of vertical poles that hold hydrofoils. The flow of water around the hydrofoil causes it to see saw, which in turn drives a generator. Because the design is squat and rectangular instead of round like most turbines, it can generate electricity even in shallow water.
Others are trying to design turbines that can tap slower currents, which is anything but straightforward. Reducing the minimum working current speed from 2.5 to 1.8 metres per second, for example, might not seem like a major change, but it turns out that the energy output of a turbine is proportional to the cube of the flow speed, decreasing the water speed by a factor of 2 decreases the energy output eightfold. In other words, even a small dip in the flow speed may mean it is no longer cost-effective to build and install the turbine in the first place.
Nonetheless, a phenomenal bounty awaits those who can make low flows work. Reducing the minimum flow to 1.8 metres might double the number of usable sites, says Richard Willden, an engineer at the University of Oxford's Future Energy research group, though he is careful to add that no one knows the exact number.
One way to harness slower currents is to increase the flow speed artificially. Two of the turbines in Orkney do just that. A turbine designed by Lunar Energy, based in Glasgow, UK, uses blades enclosed inside a tapered duct, which forces the water flowing through it to accelerate.
Minesto, a spin-off of the Swedish carmaker Saab, has approached the problem from a different angle. Instead of attempting to increase the flow of water through the turbine, its engineers thought of a way to move the turbine through the water, against the flow. They did so by mounting their turbine on an underwater "kite" tethered to the sea bed by the cable that carries away the electricity generated. The same cable is linked to a mechanical control system that steers the kite according to a preset trajectory. When the kite is "flown" into the tidal currents, the tide creates a lifting force, just like wind hitting an aerial kite. This causes the attached turbine to travel at speed against the current, increasing the speed of the water flowing through it. Minesto claims their method increases a stream's water flow, and the resultant energy output, by a factor of 10.
The slower the flows you can tap, the more attractive and potentially lucrative tidal power becomes. "I'd think 1 to 1.5 metres per second is doable," says Gareth Davies of Aquatera, a Stromness-based environmental consultancy that advises several tidal power companies.
A study for the European Commission estimated that the 106 sites with flows above 1.5 metres per second could generate 12,000 megawatts of power, the equivalent of 12 conventional power plants.
But there might be an even more ambitious prize on the horizon. Even the sunniest projections of what tidal currents can generate pale when compared to the potential rewards of tapping into the ocean currents that circulate round the planet.
"Ocean currents have the advantage that they don't reverse and have tidal slacks," says Howard Hanson, who directs a project at Florida Atlantic University in Boca Raton to work out how to tap the Florida current, the part of the Gulf Stream that flows in the channel between Florida and the islands of the Bahamas. If turbine designs could be optimised to work at the high end of slower tidal flows - say between 1.8 and 2 metres per second - they could exploit the ocean current, which reaches speeds of 2 metres per second.
An array of such turbines along the Florida straits could, Hanson says, produce as much as 200,000 megawatts of energy, equivalent to the output of 200 large power stations. There's just one problem. Trying to tap something as vital to the world's climate as the Gulf Stream, which is responsible for moving huge amounts of heat round the planet, would provoke howls of outrage. Some critics speculate that a vast array of tidal turbines planted in the Florida current could shut down this arm of the Gulf Stream, which is already believed to be threatened by global warming.
But Hansen thinks we could generate more than enough power without disrupting the Gulf Stream. A modest installation of turbines could generate about 4000 megawatts, he says, enough to replace a very large power station in south Florida, and with "little effect on the overall flow of the Gulf Stream".
The Gulf Stream isn't the only current that could be harvested. Taiwan is looking into tapping the Kuroshio current, the equivalent of the Gulf Stream off the coast of east Asia, which promises a staggering 60 gigawatts. "We believe it may become Taiwan's biggest asset in terms of a new energy source, more so than solar or wind power," says Chen Chin-te, at Taiwan's Ministry of Economic Affairs. Other candidates include the Humboldt current off South America, and the Agulhas and Benguela currents off South Africa.
First, the new turbines being tested at Orkney have a far more important task ahead. Before these new designs can wean us off one combustible liquid, they will underwrite our addiction to another. By 2013, the Norwegian company Hammerfest Strong plans to install 10 of its 1-megawatt turbines in the narrow channel between Islay and Jura, two islands in south-west Scotland. The islands are home to distilleries that produce some of the world's best-loved malt whiskies, and they will soon be powered exclusively by tidal currents.
Kermode is hopeful that a research enterprise that has been dipping its toes in the water for three decades has finally learned how to swim. For now, the future of tidal power - and of all ocean power - depends on what happens in Orkney.
Stirring up controversy
One reason tidal barrages, and tidal power in general, has been out of favour until recently was their impact on the marine environment. "Tidal streams tend to be valuable wildlife areas," says Gareth Davies of Aquatera, an environmental consultancy based in Stromness, Orkney, UK.
Marine biologists still cite a study published in 1994 by Michael Dadswell of Acadia University in Wolfville, Nova Scotia, Canada, who found that a tidal barrage built across Canada's Bay of Fundy killed between 20 and 80 per cent of fish that passed through it. Other concerns have been expressed over the new tidal turbine designs, which some fear could slow the fast currents that nurture the growth of plankton and the species that feed on it.
Do tidal turbines really pose a threat to marine wildlife? It doesn't look that way, at least, not directly. Barrages and turbines are very different in their environmental effects. Neil Kermode of the European Marine Energy Centre points out that barrages draw nearby fish into their turbines, whereas stand-alone tidal turbines give fish an ample amount of room to swim around them.
Most research so far has concentrated on how turbines might have a direct impact on marine wildlife, but the UK's Environment Agency says there could be subtler effects. Turbines alter the current, which could cause scouring of the seabed or a build-up of silt that might destroy sea grasses, for instance. And there has been little investigation of what happens while turbines are being installed: heavy engineering could damage the seabed.
Ultimately, how much this matters will depend on how big the business of generating electricity from tidal currents becomes.
Fred Pearce is a consultant for New Scientist
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