Giant Floating Wind Turbines

GE has announced a $27 million partnership with the U.S. Department of Energy to develop 5-7 megawatt turbines by 2009, each of which could power well over 1,000 homes. Supplanting the company's current 3.6 megawatt turbines, these giant energy factories should make wind power more economical, since the major cost of building and installing offshore wind farms does not depend primarily on a turbine's size, but on the number of them that need to be erected. By 2015, GE could have even bigger, 10-megawatt turbines, according to Jim Lyons, leader of advanced technology for GE's wind energy business.

Making the turbines larger, however, comes with technical challenges. The new turbines will be mounted to towers rising 90 to 95 meters and will have rotors measuring 140 meters in diameter. Imagine a structure larger than a football field rotating at a leisurely ten to twelve revolutions per minute. To decrease the weight of the massive rotor blades and tower, GE plans to use composite fibers, as well as alternatives to the weighty gearboxes now used to transfer energy from the rotor to the electrical generator.

The new turbines will also need to be more reliable than their onshore counterparts, because maintenance will be far more difficult and expensive. GE is developing new ways to deal with the extreme battering the turbines will receive from the wind.

Today's turbines compensate for changes in wind speed by actively turning their blades to catch less wind. The new turbines will adapt to gusts by using sensor-based technology that will quickly angle the blades out of the wind to reduce the wear and tear on the turbine. These sensors could include basic accelerometers, embedded fiber-optic sensors that detect shape changes in the blades in response to gusts, and forward-looking, laser-based "radar" that allows the turbine to anticipate wind-speed changes.

None of these technological advances will make a difference, however, if erecting monstrous turbines is blocked by shoreline residents who see them as visual pollution. A potential solution is floating platforms that allow the turbines to be located farther out in the sea -- and out of sight. Current projects locate wind turbines in waters less than 20 meters deep. Going farther out on the continental shelf, which extends several hundred kilometers from the U.S. East Coast, would mean locating them at depths up to 50 meters, which is probably too deep to build towers or trusses that support turbines standing on the sea floor, at least at an affordable cost.

MIT researchers recently demonstrated the feasibility of "tension-leg" platforms, a technology that oil companies have recently adopted for deep-water rigs. The wind turbines and towers would be assembled at a shipyard and placed on top of large floating cylinders (see images). The canisters would be ballasted on the bottom with high-density concrete to keep the structure from tipping over, and the whole turbine assembly would be tugged out to sea.

There, four steel cables would be attached to the platform, anchoring it to the sea floor. First, though, some water would be allowed into the cylinder, causing the structure to sink more into the water. Once the cables are attached, the water is pumped back out again, allowing the turbine to rise, tightening the cables, and preventing the turbine from bobbing up and down, yet allowing some lateral movement that would help cushion the impact of storm waves on the tower. (The blades themselves would be high enough to avoid even waves from hurricanes.) The cable tension can be adjusted for different weather conditions, says Paul Sclavounos, professor of mechanical engineering and naval architecture at MIT.

Based on wind-speed measurements, researchers at MIT, led by Stephen Connors, director of the Analysis Group for Regional Electricity Alternatives, calculated that large turbines located far offshore could ultimately cost less per power generated than either land-based turbines or near-offshore ones, even factoring in extra costs, such as much longer underground electricity transmission cables. The upside: much more fast and steady wind, which would allow the turbines to generate power at 50 percent capacity on average throughout the year, compared with 30 percent or less with on-land turbines.

Offshore wind farms could also have the advantage of being close to big cities, unlike wind farms in remote areas, which require significant power grid upgrades to transport the power to places where it's needed. "I personally see this as the endgame," says GE's Lyons. "We'll see gigawatt-scale projects delivering clean energy to the East Coast.

But making the technology cheap enough to be feasible will not be easy. "You've got to push all the buttons to get the costs down," Lyons says. Using a combination of far-offshore and land-based farms, however, one day it may be possible to provide 20 percent of U.S. energy from wind, he says.