Wednesday, April 02, 2008

Wind Power That Floats

Continued from page 1

By Peter Fairley

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Turbine structure: The steel superstructure below this wind turbine is called a tension-leg platform, similar to what’s used in offshore oil and gas platforms. Once positioned offshore, the platform is held rigidly in place by chains running to a steel and concrete counterweight on the seabed.
Credit: Blue H Technologies

Faster rotation also means less torque, meaning that the entire structure can be built lighter. (See "Wind Power for Pennies.") The rotor, gearbox, and generator of Blue H's 2.5-megawatt turbine will weigh 97 tons--53 tons lighter than the lightest machine of the same power output on the market. "This is a big advantage," says Jakubowski. "For us, weight on top is something we have to push up." The turbine and platform are correspondingly cheaper to build, he says. The net result, says Jakubowski, should be a highly competitive energy source. He estimates that Blue H's wind farms will deliver wind energy for seven to eight cents per kilowatt-hour, roughly matching the current cost of natural gas-fired generation and conventional onshore wind energy.

And it will be out of sight and thus, the company hopes, out of mind for competing local interests such as tourism. The site off Cape Cod where Blue H intends to install a test platform next summer for its first U.S. wind farm will be 23 miles off the coast.

Blue H's Norwegian competitor SWAY is using a different combination of offshore platform technology and turbine design. SWAY's platform is, in essence, a spar buoy that can rise and fall gently with wave action, thus requiring less anchoring than the tension-leg platform. The buoy, a column nearly 200 meters tall, will be held in place by a 2,400-ton gravel ballast on the seabed. Its turbine is three-bladed, but in contrast to conventional onshore turbines, it is allowed to face downwind rather than held upwind to better accommodate heeling of the tower.

Paul Sclavounos, a mechanical engineer and a specialist in naval architecture at MIT, whose lab is designing both kinds of structures for offshore turbines, says that both companies have chosen viable flotation methods, although he believes that the spar approach taken by SWAY will be better adapted to rougher waters. He says that Blue H's platform may work off the Italian coast, but anchoring it to handle the 30-to-40-meter waves that New England's storms can whip up may not be economical. "The cost that really drives this business is primarily the foundation," says Sclavounos.

Where he questions both firms is in their decision to redesign the wind turbines. Sclavounos says that his group is designing both spars and platforms to carry conventional five-megawatt turbines designed for onshore or shallow-water offshore applications. "You don't want to redesign the turbines for offshore deployment because that's going to be very expensive, and it's probably not necessary early on," he says.

In Sclavounos view, the economics of the power industry are already approaching a tipping point that will drive rapid adoption of floating turbines. "The technology is essentially proven," he says. "We know we can design [platforms] and spars that are not going to move in big storms. What is going to lead to this industry taking off will be the economics. When carbon-emissions trading markets start maturing, you're going to see this industry take off, even without state subsidies. We're not far from it."

Wind Power That Floats

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