In March last year, hundreds of millions of dollars of investment and five years of planning for a turbine wind farm in north central Oregon screeched to a halt.
Concerns by the US Department of Defense, formalised by the Federal Aviation Administration in a "notice of presumed hazard", centred on fears that a 338-turbine portion of a renewable energy wind farm that, when combined with 1,800 wind turbines already built in the area, would interfere with a nearby long-range surveillance radar.
The green roadblock is an example of a growing number of aviation-related compatibility issues arising from renewable energy projects, many of which are located at or near aviation infrastructure. Although solutions are usually found, an expected rise in the number of proposed projects is prompting regulators to be proactive in setting out ground rules.
Denver is one airport that has installed a solar collection system after analysis showed there was no problem for air navigation
Along with the possible distracting or eye-damaging glare from solar cell farms and solar towers for renewable energy, there is concern that the growing number of wind turbines will increase the risk to airborne surveillance as radar microwave signals get blocked by turbine blades. The rotating blades can also cause "clutter" in a radar output because of reflections of the microwave signals.
Dangers can also precede the installation of turbines. Low-level operators for firefighting and other missions in the USA are being warned of 122-152m (400-500ft)-high temporary meteorological test towers that are being installed at potential sites for wind farms to determine whether the site could be profitable.
Aviation officials in Idaho say the slim towers are difficult to spot if not marked, are installed in a matter of days, and can be gone in 12-18 months. "Many pilots have 'close call' stories to relate," say officials.
The turbine farm work delay in Oregon was short-lived. Two months after the FAA order, the DoD reversed its objections. "Internal DoD analysis indicated the impact of the additional turbines would not be as severe as initially thought," said US deputy undersecretary of defence Dorothy Robyn last June.
Also boosting optimism that such interference could be overcome was a Massachusetts Institute of Technology study showing mitigation measures could be developed in the 18 months before the turbines are operational.
Robyn says newer digital radar systems can be modified to combat interference through software, but the FAA radars the DoD uses for long-range surveillance are "decades old and many still use analogue processors" that are less effective at removing turbine clutter.
Last November, Flight International reported that the UK's Newcastle airport had a software fix for surveillance problems caused by radar reflections from wind turbines, but the UK Civil Aviation Authority said this was temporary. "The CAA and others in the industry have made a concerted effort to find a long-term solution to this issue, but no emerging technologies have yet been proven."
Concerns about solar projects are largely tied to the possibility of temporary blindness or eye damage to pilots in a critical phase of a flight. But some airports, including the international airports in Denver and San Francisco, have installed solar collection systems on open portions of their properties after analysis showed no hazard to air navigation.
GLINT OR GLARE
In a study of possible glint or glare impacts from a photovoltaic panel farm in central California, engineering firm Power Engineers said: "The possible glare and reflectance from PV systems are at safe levels and are usually decisively lower than other standard residential and commercial reflective surfaces. Glint and glare to aircraft should not be an issue, and in recent years several large-scale solar projects have been completed and constructed at or near major airports without incident."
Included are the 1.6MW system at Denver International airport's fuel facility, which has more than 7,300 solar panels. Since 2005, FedEx Express has been operating a solar farm with more than 5,700 solar panels that generate more than 900kW of peak energy power at its Oakland International airport hub.
As for wind turbines, Robyn says the problems are "serious, but solvable" and suggests that, as well as improving the process for reviewing renewable energy projects to deal with issues earlier, the US government should realign its research and development priorities "to give greater attention to the issue".