SOLAR TODAY - January/February 2009

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Offshore Wind Turbine Far offshore, where the wind blows free, there’s where your wind farm wants to be.

By SETH MASIA

Thanks to Walter Musial of the National Renewable Energy Laboratory for help in preparing this article.

Any sailor knows that once you’ve moved a few miles offshore the wind is steadier and stronger than it can ever be on land, where hills, forests and buildings create turbulence. The continental shelves offer wide expanses for the development of wind farms. They’re close

to major electricity markets, eliminating the need for long-distance transmission lines. Several large wind farms totaling 700 megawatts (M W) are planned for the Mid-Atlantic Bight, about 12 miles off the beaches of New Jersey, Delaware and Maryland.

Turbines produce 34 kilovolts AC, and power is carried via undersea cables, 4 inches thick. Up to eight turbines on a string feed power to an offshore substation on its own platform, resembling an oil rig. Here the power from several strings is boosted to 150 megawatts at 138 kV and sent ashore via a 10-inch diameter cable. The cable, right, has a copper core, armored with galvanized steel and sheathed in polyethylene. At the shore station, power is stepped down for local distribution.

NExANS CABLE

Right, in water less than 30 meters (100 feet) deep, with a stable sandy bottom, a steel monopole tower can be driven into the soil with a gigantic pile driver.

Because it gets less turbulence, and has no nearby neighbors to complain about noise, an offshore turbine can be built with lighter, faster-turning rotor blades. The turbine lives in a corrosive salt-air environment and is visited less frequently for maintenance, so it must be built with more robust electromechanical components.

Footings, towers, turbines and rotors are assembled at onshore shipyards, transported by barge and lifted into place with giant cranes. Towers and turbines must stand up to hurricane winds and waves.

Far right, in deeper water, three shafts can be driven to make a tripod foundation.

Where the bottom is of soft clay or silt, it can be dredged out to take a massive concrete gravity base foundation. This is a hollow concrete platform, built ashore and dropped into place from a barge. It’s then filled with a cheap, heavy ballast, typically pig iron or rock, and backfilled.