Renewable Energy from the Tides
THE MAGAZINE DEVOTED TO NICKEL AND ITS APPLICATIONS
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THIS 130-TONNE PROTOTYPE undersea electric turbine (seen here prior to being lowered under the
surface) looks and operates like an underwater windmill.
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A PILING BEING ASSEMBLED off the coast of England in June 2003 to support the 300-kilowatt
turbine pictured above.
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CORROSION-RESISTANT stainless steel cladding protects the guides and runners used to lift and submerge
the rotor assembly.
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THIS 22-TONNE PROTOTYPE of an undersea electric turbine was designed by engineers at The Robert
Gordon University in Aberdeen, Scotland.
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CALLED THE SEA SNAIL, it is capable of generating up to 150 kilowatts of electricity.
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CONSTRUCTED OF TUBULAR steel, the Sea Snail squats on the ocean bottom like an insect.
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OCEAN CURRENTS flow around the wing-shaped foils to create downward pressure much like the spoilers
on a racing car.
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PDF of this article (615 kB)
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For other Nickel Magazine articles on the topic of nickel-containing materials in tidal
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The race to harness the tides using submerged turbines will need corrosion-resistant stainless
steels. By Dean Jobb
Nickel magazine, March, 2004 -- The Pentland Firth is a 23-kilometre-long channel that
separates the Scottish mainland from the Orkney Islands and connects the North Sea to the North
Atlantic. The channel is as little as 10 kilometres wide in places and, when the tidal flow peaks,
it generates more energy than the combined output of the world's oil wells at any given moment.
Scientists and engineers are racing to devise underwater turbines that can harness the power of such waterways, providing access to a clean, renewable and almost unlimited supply of electricity. And nickel alloys will play a key role in ensuring their innovations can withstand years of exposure to seawater.
"Our chief driver (in selecting materials) is clearly the corrosive sub-sea environment," notes George Gibberd, engineering manager for Marine Current Turbines Ltd (MCT). "We are considering the use of corrosion-resistant materials for key selected critical items. For long-term reliability, we can't use anything else."
The British company has produced a 130-tonne prototype turbine that looks, and operates, like an underwater windmill. An 11-metre-long rotor blade is attached to a piling embedded in the seafloor. Dubbed the Seaflow Project, the 300-kilowatt turbine was installed off the south coast of England in June 2003. The rotor and gearbox are attached to a collar that can be lowered into the ocean, leaving only the top of the pylon protruding above the waves.
The prototype incorporates stainless steel components and coatings in "selected specialist areas, where its use is absolutely critical," Gibberd reports. About a tonne of stainless steel plating protects the guides and runners used to lift and submerge the rotor assembly. Stainless plugs, access hatches and seals were used where corrosion was a concern.
MCT is designing a larger, megawatt-sized version as it strives to commercialize the technology within a couple of years. "We do have a variety of stainless steels and nickel-based, high-strength fasteners in mind as potential candidates," Gibberd says. "We need highly pre-tensioned bolts for fatigue purposes, and the standard steels are prone to embrittlement in this environment. We may be using stainless steel wire ropes for lifting mechanisms, but we also have to consider the implications of using noble metals in a plant having a high volume of plain carbon structural steel."
Another, Competing Design
Engineers at The Robert Gordon University in Aberdeen are developing a smaller, moveable turbine to produce
electricity on the ocean floor. A 22-tonne prototype capable of generating up to 150 kilowatts, known as the
Sea Snail, will be tested in those swift-flowing waters off the Orkney Islands in April 2004.
Built on a slim budget and designed only for a short-term test, the Sea Snail contains no stainless steel. But future versions are sure to contain nickel alloys, says engineer Alan Owen of the university's Sustainable Energy and Environmental Research Institute.
"Corrosion at this stage is not a major issue," says Owen, "but should the idea prove viable, then we will be looking to some fairly serious materials work to be involved. These things will be expected to have a working life (of 15 to 20 years) and they'll need to be built for it."
The Sea Snail's tubular steel frame squats on the ocean bottom like an insect. Surrounding the central turbine are wing-shaped foils that create downward force as currents pass over them -- much like the spoilers of a racing car. "The faster the (tidal) stream flows, the more powerfully it's pushed onto the seabed," Owen explains.
The foils must flip as the tide reverses. The reversible hub that allows this to happen will be made out of stainless steel to ensure trouble-free operation. Designers have rejected the idea of an enclosed gearbox system. "From an engineering point of view, simplicity is always the preferred angle," Owen says. "Therefore I'm inclined to the idea that we leave the mechanism open but make it out of materials that are corrosion resistant"
The foils, now made of glass fibre, may be re-engineered with a stainless steel skin, allowing seawater to be pumped in and out to maintain buoyancy. "That would obviously be a prime candidate for stainless steel," Owen says, "it creates a cavity that you can't get into to treat in any other way, so you have to make sure it can't corrode internally," Owen says. Corrosion-resistant stainless steels are also likely to become the material of choice for turbines as they are perfected for undersea use, he adds.
Like MCT's prototype, Sea Snail is designed to be hoisted to the surface for maintenance. Tidal turbines will be installed where currents are strongest, and diving is too dangerous. And since the structures are partly or fully submerged, they will not clutter the landscape like offshore wind farms.
The potential for underwater turbines is vast. Owen's team foresees the installation of more than 100,000 units around the world over the next two decades, generating electricity from large-scale tidal farms or powering water purification or desalination plants.
"It taps into a huge, predictable and clean energy source," MCT's managing director, Martin Wright, says of the technology. "It has the potential to make a major contribution to future energy needs without causing pollution or any significant environmental harm."
Dean Jobb is a Halifax, Nova Scotia-based freelance writer.
PHOTOS: Marine Current Turbines Limited and The Robert Gordon University
Marine Current Turbines Limited Press Office |
