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United States

Windtech: Supersize turbines - Windtec floats 10MW offshore ambition

UNITED STATES: AMSC Windtec is working on the SeaTitan, a 10MW offshore machine that will include a direct-drive, high-temperature superconductor (HTS) generator and 164-metre rotor. Windtec also plans a 10MW sister model, the PrairieTitan, for onshore application. Strategic supply chain manager Christian Santner says a prototype is expected within a few years.

By replacing copper winding with HTS, manufacturers can build generators smaller and lighter. Superconductors have already made their way into ship motors, power cables, high-speed trains, transformers and defence technologies. Windtec claims the SeaTitan's nacelle and rotor will weigh less than 500 tonnes - which is comparable to today's 5MW wind turbines. A 10MW unit using a conventional generator would be three times larger and heavier, it says. Another potential benefit of HTS generators is that they eliminate the need for rare-earth materials, such as the neodymium-iron-boron increasingly used in permanent-magnet wind power generators. Because motors for hybrid and fully electric automobiles, along with other applications use these magnetic materials, there is increasing concern about future availability. China controls 95% of the market.

Offshore wind is pushing the boundaries of turbine design, as developers need not worry about visual impact or noise. Most offshore developers deploy or are planning for turbines between 2.5 and 5MW, but deeper water developments, such as the UK's Crown Estate Round 3, create opportunities for turbines 10MW or bigger.

Clipper Windpower Marine, a fully owned subsidiary of US wind turbine manufacturer Clipper Windpower, and Norwegian state utility Enova, are in the early stages of developing 10MW units. "It's all about economies of scale," says Nicolas Fichaux, head of policy analysis at the European Wind Energy Association. "The assumption is that it's cheaper to install one 10MW turbine than two 5MW turbines. It's been proven that when you increase the size of the turbines you decrease your costs per megawatt significantly."

Enova is partnering with Norwegian firm Sway to design the 10MW behemoth. Sway and Clipper have different approaches, and developers will be watching to see which proves more successful. Sway's upwind turbine with 145-metre blades will be mounted initially on fixed foundations, anchored to the seabed using a single support and a suction anchor, or on a more traditional jacket foundation - a pylon-like structure within which the base of the mast sits. The construction of the prototype should be complete by 2012 and testing will continue for two to five years after that.

Sway is running a concurrent project focusing on foundation design. This is looking at mounting downwind 5MW turbines on a floating structure that can tilt at angles of up to six degrees, and when the wind direction changes, can pivot via an undersea bearing. This floating wind turbine will be loaded with ballast, with the mast supported by a single tension leg and anchored to the seabed by a flexible stay that allows the structure to withstand the fatigue load on the slender foundation.

This approach, Sway claims, reduces stresses on the structure, meaning that the mast can carry a larger turbine and be deployed in deeper water. By 2017, Sway is hoping to mount 10MW turbines on these floating structures.

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