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Designing Next Generation of Floating Wind Turbines

Matthew Lackner (pictured) and Yahya Modarres-Sadeghi of the Mechanical and Industrial Engineering Department have received a three-year, $167,000 grant from the state-run Massachusetts Clean Energy Center to develop a design and economic analysis for the next generation of two-bladed, downwind, offshore, floating, wind turbines. If properly engineered, floating turbines, which can be anchored out of sight and out of mind far at sea, could well prove the only viable alternative for wind energy in New England, which has little room on land for wind farms and has experienced a fierce public backlash against bottom-mounted wind turbines located within sight of shorelines.

One star-crossed project of the Cape Wind company, trying to build America’s first offshore wind farm between Cape Cod and Nantucket Island, has been stymied by negative public opinion for a decade.

The mechanical engineers will be working on the cutting edge of floating wind turbines, a technology that, according to MIT’s prestigious Technology Review, “holds the key to exploiting” the powerful offshore winds blowing steadily off the Northeastern coast. In order to turn that “key,” Lackner and Modarres-Sadeghi must design and analyze turbines that are lighter and more economical than current turbines, and yet maintain their stability in face of powerful offshore winds.

Floating turbine farms can be sited far from land, but have built-in engineering problems to be solved first. One is that, in offshore floating turbine design, engineers must minimize mass in the tower and blades.

“That really helps in the design of the floating platform down below,” explains Lackner. “So light, flexible blades would be imperative in these designs.”

The necessity for light turbines has engineers studying how to bring back two-bladed, downwind machines, which were common 25 years ago, but were replaced by the three-bladed, upwind design that is now popular. Downwind turbines have blades in back of the tower, facing away from oncoming wind, while blades in upwind turbines are in front, facing the wind. In downwind machines the blades can be quite light and flexible without the danger of bending backwards and hitting the turbine tower, which would be the prohibitive problem of flexible blades on upwind machines.

“But one of the issues when you make blades very flexible is that they can become unstable,” notes Lackner. “There is always the potential that the combination of the aerodynamics and the properties of the structure itself will create oscillations that grow until they’re basically out of control. If you didn’t take this kind of instability into account during your wind turbine design stage, you might get an unpleasant surprise.”

Lackner and Modarres-Sadeghi will be designing small-scale, two-bladed, wind turbines with various blade lengths and designs, and test scale models in either a wind tunnel or water tunnel. “We want to find the common ground between light, cheap, flexible, and stable two-bladed turbines,” says Lackner. “We’re looking for the best of all possible worlds.” (September 2011)