Monday, December 01, 2008

A Design for Cheaper Wind Power

A design that draws on jet engine technology could halve the cost of generating electricity from wind.

By Kevin Bullis

Wind jet: A rendering of a new turbine design, which can increase the energy captured from wind. Credit: FloDesign Wind Turbine

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FloDesign Wind Turbine, a spin-off from the aerospace company FloDesign based in Wilbraham, MA, has developed a wind turbine that could generate electricity at half the cost of conventional turbines. The company recently raised $6 million in its first round of venture financing and has announced partnerships with wind-farm developers.

The company's design, which draws on technology developed for jet engines, circumvents a fundamental limit to conventional wind turbines. Typically, as wind approaches a turbine, almost half of the air is forced around the blades rather than through them, and the energy in that deflected wind is lost. At best, traditional wind turbines capture only 59.3 percent of the energy in wind, a value called the Betz limit.

FloDesign surrounds its wind-turbine blades with a shroud that directs air through the blades and speeds it up, which increases power production. The new design generates as much power as a conventional wind turbine with blades twice as big in diameter. The smaller blade size and other factors allow the new turbines to be packed closer together than conventional turbines, increasing the amount of power that can be generated per acre of land.

The idea of enshrouding wind-turbine blades isn't new. But earlier designs were too big to be practical, or they didn't perform well, in part because the blades had to be very closely aligned to the direction of the wind--within three or four degrees, says Stanley Kowalski, FloDesign's CEO. The new blades are smaller and can work at angles of up to 15 to 20 degrees away from the direction of the wind.

From the front, the wind turbine looks something like the air intake of a jet engine. As air approaches, it first encounters a set of fixed blades, called the stator, which redirect it onto a set of movable blades--the rotor. The air turns the rotor and emerges on the other side, moving more slowly now than the air flowing outside the turbine. The shroud is shaped so that it guides this relatively fast-moving outside air into the area just behind the rotors. The fast-moving air speeds up the slow-moving air, creating an area of low pressure behind the turbine blades that sucks more air through them.

It's plausible that such a design could double or triple a turbine's power output, says Paul Sclavounos, a professor of mechanical engineering at MIT. Part of the increase comes simply from guiding the air to the turbine with the shroud. But Sclavounos notes that it also helps to use the wind surrounding the turbine to speed up the airflow, because the power produced by a wind turbine increases with the cube of the wind speed. The key question is whether the new turbines can be built and maintained at a low-enough cost, Sclavounos says.

FloDesign has already built a small prototype for wind-tunnel tests. Its next step is to build a 12-foot diameter, 10-kilowatt system for field tests. The prototype will be finished by the end of next year or early in 2010, with commercial wind turbines to follow. (The company is not yet taking orders.) Eventually the company plans to make turbines as large as one megawatt.