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A Wind Farm in Deep Water off the U.S. Coast

A new type of wind-turbine platform can be placed much farther from shore.
December 20, 2010

Deepwater Wind, a company based in Providence, Rhode Island, has drawn up plans for what could be the largest wind farm in U.S. waters, the company announced last week. The proposed farm would generate a huge 1,000 megawatts of power and would be located 18 to 27 miles off the coast of Rhode Island and Massachusetts at a depth of 52 meters—considerably deeper than any other large scale wind project to date. By moving into deeper waters, turbines can harness stronger, more sustained winds. And the massive turbines the company plans to use—each capable of generating more than 5 megawatts of power, with blades rising 150 meters above the water’s surface—will be nearly invisible from shore, thereby avoiding potential legal battles with coastal communities that perceive the turbines as eyesores.

Four-legged steel platforms rising from the seafloor will allow Deepwater Wind to operate in depths more than twice those of conventional steel “monopole” wind turbine platforms. As water depth increases, the diameter of monopoles must increase exponentially, making them uneconomical in water deeper than about 20 meters. By using a four-legged design, company officials say they will be able to work in depths that were previously prohibitively expensive.

The four-legged platform design is already commonly used for offshore oil and gas rigs. It was first adapted for offshore wind turbines in a pilot project in the North Sea, known as the Beatrice Wind Farm Demonstrator Project, in 2007. Two 5-megawatt turbines were each mounted on a four-legged tower in 45 meters of water. Although the towers were never connected to the electricity grid, they remain the world’s deepest offshore wind platforms.

Paul Sclavounos, a professor of mechanical engineering at MIT, says four- or even three-legged towers offer a ready way for offshore wind to expand into deeper waters. However, truly deepwater deployments—platforms in hundreds of meters of water—will require floating platforms unattached to the seafloor.

“For depths much larger than [45 meters], the only solution is floaters, but we still have to do more development on these designs,” Sclavounos says. Only two small pilot floating turbines have so far been deployed anywhere in the world.

Deepwater Wind’s proposed project would cost $4 billion to $5 billion, and state and federal approval would be required. The company is also proposing a $500 million to $1 billion offshore high-voltage transmission line to connect the wind farm to the electricity grid in Massachusetts, Rhode Island, and New York.

Deepwater Wind officials say they plan to sell electricity at a rate that is in the “mid teens” per kilowatt-hour. That is roughly two-thirds the $0.24 per kilowatt-hour that the company hopes to charge for electricity from another proposed wind farm, of 29 megawatts, off Rhode Island. The 1,000-megawatt project would make possible economies of scale, the company says. Even so, its electricity would still cost twice as much as electricity from fossil-fuel power plants.

Offshore wind may nonetheless be a worthwhile investment, says Stephen Connors, director of the Analysis Group for Regional Energy Alternatives at MIT. “Fossil-fuel plants are cheaper now, but there is the structural risk of what happens if and when you have to pay for their C02 emissions,” he says. “A better comparison of Deepwater Wind’s costs is with nuclear power or coal plus carbon capture and sequestration. When you do that, it starts to look much more equivalent.”

Connors says offshore wind is also attractive compared with other renewable energy sources. It produces electricity at half the cost of photovoltaic solar and is only slightly more expensive than onshore wind.

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