Last month Danish wind turbine company Vestas and U.K. defense contractor QinetiQ demonstrated the first "stealth" wind-turbine blade--their solution to the aviation radar interference problem holding up the installation of gigawatts-worth of proposed wind farms worldwide. Vestas composites specialist Steve Appleton says the firm is eager to test a complete stealth turbine and begin limited production by the end of 2010. "Clearly this technology, if proven fully and then adopted by Vestas, would give us a competitive advantage," says Appleton.

Lingering doubts over how stealthy turbines can be, especially when it comes to long-range military radars, are prompting continued research on alternate solutions. Just last month the U.K. government launched an $8.5 million research project with Calgary-based radar system maker Raytheon Canada to make existing air-traffic control systems capable of recognizing and discounting the radar signature from a wind farm.

Wind turbines can interfere with radar in several ways. The turbines can reflect the radar systems' microwave signals, creating a shadow that erases airplanes from radar operators' screens and clutters those screens with the turbines' signature. The signature is also always changing, as blades accelerate and decelerate with the wind, reaching speeds of well over 200 kilometers per hour. Aviation safety and military authorities insist that the potential for confusion and accidents is real.

Such concerns are stalling more than 10 gigawatts of wind power in the United Kingdom. Last year the U.S. Department of Homeland Security commissioned a study on wind power and radar from the JASON Defense Advisory Panel, a science and technology policy advisory group managed by the Mitre Corporation. This study determined that U.S. authorities had halted development of several gigawatts of wind energy over radar concerns, calling it "a serious impediment to the nation's mandated growth of sustainable energy." Radar concerns raised by the U.S. Federal Aviation Administration (FAA) are among the final hurdles holding up Cape Wind from installing 130 turbines in Massachusetts's Nantucket Sound.

The solution offered by QinetiQ and Vestas relies partly on materials analogous to those added to stealth aircraft to absorb some of the radar signal. A five-millimeter coating takes care of the towers, but this coating would add 1,200 kilograms to the large turbine blades. So instead, as was demonstrated in the 44-meter blade installed by the companies on a turbine at a Norfolk, U.K., wind farm last month, two layers of radar-absorbing sheets consisting of glass-reinforced epoxy and plastic foam are laid into the blade's composite structure.

Testing with a mobile radar installation showed that, as expected, the stealth blade produces a markedly smaller signature relative to the turbine's two conventional blades, according to Appleton. He anticipates that subsequent structural testing will confirm that there is no net weight or structural change, since the stealth material simply displaces some of the composite's reinforcing fibers.