Wind power may be one of the obvious solutions to the world's renewable energy conundrum, but the actual generators can create as much noise as they do electricity - and not only from those who question the environmental friendliness of erecting massive towers in formerly scenic vistas. Turbines, in fact, cause visual clutter of more than aesthetic impact, as their whirling blades play havoc with air traffic control radar.
According to Raytheon, whose radar systems form the basis of ATC at many civil and military airfields, the wavelength of signals reflected back to source in normal two-dimensional systems is distorted by the rotating blades.
As motorists caught speeding by hand-held police radar know all too well, this Doppler effect can be used to measure the difference in speed between source and target. However, when the source is an ATC installation, wind turbine blades can cause noise and false targets - amounting to a radar blackout zone.
If a radar installation must look through a wind turbine - or, more typically, an array of turbines installed in the open spaces which may surround an airfield - aircraft can disappear from its view. The UK Royal Air Force has even expressed concern that offshore wind farms pose a national security threat, by offering radar cover for approaching enemy aircraft.
The scale of the problem is, literally, massive - and getting bigger. Lengthening the blades is a good way to improve the low-wind performance of a turbine, but clearly increases the size of the radar target.
A new class of 3MW onshore turbine from market leader Vestas - that will undergo its first installation, in Denmark in 2013 - sits on a 119m (390ft) tower and the blades sweep out a 126m diameter circle, nearly 27% bigger than its predecessor.
And, UK air traffic services provider NATS says it received more than 2,000 wind turbine development applications in 2011 - more than double the previous year. Some 98% raised no objection from NATS, but the others were at least held up over radar "clutter" concerns.
There are, however, several promising approaches to solving this problem. In 2009, NATS asked Raytheon to develop a system capable of overcoming wind turbine interference. Trials of the resulting wind-farm mitigation package of system upgrades began in the Netherlands at the end of 2010, and flight tests have also been made in the USA.
The site acceptance test was successfully completed in October 2012 on the first production kit, at Woensdrecht air force base in Holland, where operational acceptance is expected imminently.
The combination of improved signal analysis algorithms and some equipment upgrades has also been tested at Soesterberg, an airfield some 40nm (74km) southeast of Amsterdam and which is in the vicinity of the 227-turbine Flevoland Polder wind farm.
Brian Cross, Raytheon UK's head of air traffic management systems, says the before and after effect of a combination of improved signal analysis algorithms and some equipment upgrades has been "phenomenally good".
By adding a second receiver ahead of the signal processor, Raytheon has been able to add target detection to the radar's weather channel task, resulting in a "substantial" improvement in radar performance generally; the resulting two-beam system improves low-level performance - that is, at the level where wind turbines are an issue - by a factor of seven, says Cross.
And, he adds, all the upgrades sit within the existing system infrastructure. No new power supply is needed, or other changes that might demand recertification of the system, so the solution is a "straightforward upgrade" that for many locations could be entirely adequate - places, for example, with only a few turbines.
However, Cross notes that depending on local geography and system configuration, this advanced solution can be expensive to install. Critically, he says, it has to be recognised that the wind farm problem is a "multi-layered" one, and any cost-effective solution needs to reflect that complexity by dipping into a "toolbox" of appropriate measures. One obvious answer might be to move the radar installation away from a wind farm. Another is to upgrade the hardware and/or software.
Yet another possibility that may be called for in some locations is to add a "gap-filling" radar system to directly address the area obscured by a wind farm. Real-world tests of one such concept are now beginning following a late-2012 installation near the UK's Cambridge airport, in the vicinity of Wadlow wind farm. This "3D holographic radar", developed by technology incubator Cambridge Consultants and being refined and marketed by a specially created spin-off company, Aveillant, promises to reliably distinguish between wind farm Doppler returns and aircraft of all sizes and speeds.
Chief executive David Crisp says the system does away with the familiar radar sweep in favour of a flat panel phased array that effectively stares in all directions continuously, with "hundreds" of beams. The result - made possible by computing power that 20 years ago could only have been achieved using a Cray supercomputer - is to produce a 3D image that characterises targets and gives range, direction and altitude.
Crisp describes the technology, originally devised to track shells on military gunnery ranges, as "disruptive" but, he says, holographic radar will not replace standard ATM systems; rather, he adds, it should be deployed as an extra layer of detection in difficult areas.
The chemicals industry is coming from another angle - stealth. Osgram, a consortium of European companies working with EU funding, hopes to develop a radar-absorbing material that could be applied to blades, nacelles and towers to a thickness of 3mm at a target cost of €200 ($260) per square metre. The challenge is to develop a suitable process to make the conductive polymer - polyaniline - but Osgram believes the concept could be effective against both air traffic control and marine radar signals.
Making wind turbine blades invisible to radar would be a particularly elegant answer to one of the main obstacles to expanding wind power generation. To overcome aesthetic objections, however, somebody is probably going to have to figure out how to make the turbines invisible to the naked eye.