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Stealth-Mode Wind Turbines

Coatings and composites ease the air-traffic worries dogging wind power.
November 2, 2009

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.

Stealth mode: This turbine was fitted with a “stealth” blade last month.

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.

Blade switch: Composites built into this 44-meter blade installed on a U.K. wind turbine can absorb microwave signals, reducing radar signatures that can disrupt air-traffic safety control systems.

Appleton says that the stealth technology should be suitable for any type of wind turbine, and that “any cost increase will prove acceptable to our customers.” What is less clear is what proportion of blocked wind farms could be freed up by the stealth technology. “Each site is different and needs assessing to see what the problem is and whether our technology will help,” says Appleton. Crucial factors include the type of radar in use, distance from radar towers, and type and distribution of wind turbines.

The JASON report concluded that stealth technology showed “considerable” potential to deal with short-wave radar but is not suitable for reducing interference for long-wavelength L-band radars employed by U.S. air security–a critique that Appleton rejects. “We have already demonstrated an L-band absorber,” he claims.

Nevertheless, the U.K. is financing development of Raytheon’s alternative solution: using signal-processing algorithms to distinguish stationary targets such as wind turbines and erasing them from air-traffic control radar screens. Brian Smith, Raytheon Canada’s general manager, says the key is teaching the systems to recognize wind turbines as false targets, despite their spinning blades. “As the radar spins, it sends out scans every few seconds,” Smith says. “Our solution will use an algorithm in tracker software to say this can’t really be a plane, because it’s standing still.”

Smith projects that by 2011 Raytheon will have algorithms in place for both short and long-range radar systems, and will have demonstrated not only that they can erase wind farms from radar screens but also that they can retain valid stationary targets such as hot-air or weather balloons. He estimates that it will take another year to add the algorithms to any of the 250 Raytheon radar systems operating worldwide–about 40 percent of the market.

The JASON report proposed one more approach to addressing radar interference problems: replacing aging analog radar stations with modern digital equipment that is amenable to upgrades such as Raytheon’s. “Current circumstances provide an interesting opportunity for improving the aging radar infrastructure of the United States, by replacing radar that inhibits the growth of wind farms with new, more flexible and more capable systems, especially digital radar hardware and modern computing power. Such improvements could significantly increase the security of U.S. airspace.”

The FAA has proposed precisely that as a means of allowing the Cape Wind offshore wind farm to be built. It says that adding digital radar at Otis Air Force Base on Cape Cod, one of three radar stations expected to be affected by the project, would help the region’s air-traffic controllers see through the anticipated signal clutter. Negotiations between the FAA and Cape Wind are said to be nearing completion. But the price tag, according to the FAA, could be $1.5 million to $15 million.

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