Wind turbines function best in wide-open spaces where they can capture airflow unobstructed by buildings or mountains. Unfortunately, these same conditions are also optimal for aircraft takeoffs and landings, creating tension between wind energy utilities and airports in a number of locations worldwide. Utility-scale wind turbines, many of which stand more than 100 meters tall, can interfere with the radar used to safely guide aircraft.

Radar works by emitting radio waves in a particular direction and gathering data about waves reflected back to the radar's position that can be used to identify the range, altitude, direction and speed of nearby objects. Wind turbines can defeat radar either by blocking signals or by creating unwanted reflections of the signals, resulting in clutter on radar maps.

Aging radar technology and the demand for renewable sources of energy have complicated the situation, slowing and in some cases stopping the construction of new wind farms. The British Wind Energy Association (BWEA) estimates that 6 gigawatts of planned new wind capacity are being held back by objections over radar. (Britain's overall installed wind-power capacity as of the end of 2009 was 4.1 gigawatts.)

In the U.S., new wind farms are threatening to interfere with surveillance radars used by the North American Aerospace Defense Command (NORAD), the U.S. Northern Command and the Department of Homeland Security, said Deputy Under Secretary of Defense Dorothy Robyn in June at a House of Representatives Armed Services Committee hearing on the impact of wind turbines on military readiness (pdf). Long-range radars managed by NORAD and Northern Command to maintain airspace surveillance and air defense are decades old, and many still use analog signal processors, which are inherently less effective at removing wind turbine clutter, according to Robyn.

Concerns over the impact of wind farms and aircraft radar must be resolved if the U.S. Department of Energy is to reach its goal of using wind energy to provide 20 percent or more of the nation's electricity (pdf), according to the American Wind Energy Association (AWEA), a trade association representing business in the wind-energy industry.

One approach to the problem is upgrading radar systems, which have been used to track ships and aircraft since before World War II, with advanced digital signal processors so they can manage larger amounts of data and thereby identify and filter out the signal scrambling caused by wind turbines.

Typically a radar system will send and receive a single beam of radio waves—either high or low radio frequency—that can be deciphered with a minimal amount of computer processing power. Concerned that wind farms would create disturbances that prevent conventional radar systems from distinguishing between signal clutter and aircraft in need of assistance, Britain's National Air Traffic Services (NATS) began working with Raytheon Company in 2006 to upgrade systems with advanced digital signal processors and data-processing software. The upgraded systems were designed to handle both high and low radio frequency beams concurrently, providing a wealth of data to better map signal clutter and distinguish between the Doppler signals (indicating movement) created by turbines and by aircraft.

In July and August, Raytheon and NATS worked with the Royal Netherlands Air Force to test an enhanced radar system at that country's Soesterberg Air Base and determine whether the system was effective at keeping the nearby wind turbines from cluttering air traffic control displays with false targets and obfuscating real aircraft. Once the results of these tests are analyzed, NATS plans to further test the new radar system at a civilian airport with nearby turbines in northern Scotland later this year.

Whereas Raytheon advocates the upgrade of radar systems, others propose ways to make the wind turbines themselves less visible to radar. Denmark's Vestas Wind Systems, which makes wind turbines with blades as long as the wings of a Boeing 747, is working with QinetiQ Group (formerly part of the U.K.'s Defense Evaluation and Research Agency) to develop radar-absorbing coatings and composite materials containing conductive particles like iron and carbon for Vestas's turbines and towers. Vestas began testing prototype "stealth" blades about a year ago and plans to begin selling them next year. Although the company acknowledges that it cannot make its turbines invisible to radar, these radar-absorbing efforts could have an impact on whether companies get a green light from the government to build wind turbine fields.