Tech

For all their environmental appeal, wind turbines have few fans in the military or among air traffic controllers. Strange as it might seem, radar systems easily confuse the turbines' rotating blades with passing aircraft. Now a company has developed a "stealthy", radar-invisible blade that could see many more wind farms springing up across the UK and elsewhere.

The concern over wind turbines is delaying their deployment. According to the UK Government's Department of Energy and Climate Change, plans for over 5 gigawatts of wind power are currently stalled by aviators' objections .

It is no less of an issue elsewhere: last month the German army blocked investment in some new wind power projects because of radar interference concerns. Even in the US, where the population density is lower, the Department of Defense is investigating whether turbines could weaken radar defences.

At the root of the problem is the fact that traditional radar sweeps across an area, scanning a particular location only once every few seconds. It detects moving objects by looking for a Doppler shift in the reflected radar signals. If an aircraft flies low over a wind farm, it becomes difficult to distinguish whether those shifts have come from the aircraft's movement or the rotating turbine blades.

Concealed blades

One solution would be to require all aircraft that fly over wind farms to carry transponders to identify themselves, says Nicola Vaughan, head of aviation at the British Wind Energy Association. Indeed, she says transponder-mandatory zones already operate over two offshore wind farms in UK waters. However, this is more problematic for land-based farms as large numbers of light aircraft, which generally don't have transponders, fly over them.

Vestas, a wind turbine manufacturer, may have a better answer. Last week it carried out the first test of a "stealthy" turbine blade, built in collaboration with defence technology company Qinetiq.

It is relatively easy to hide objects from radar by applying an absorbent coating – an approach used to disguise vehicles. This is no good for wind turbines, though. "The mass of the blade is critical," says Steve Appleton of Vestas. Blades can be up to 45 metres long, so slapping on a thick, radar-absorbent coating of paint would add substantial extra weight. "The blades flex in the wind too," Appleton adds, so any coating is likely to crack and flake off.

Instead, a radar-absorbing blade has been designed from scratch, and in such a way that Vestas' existing manufacturing process isn't significantly altered – so it doesn't push up costs. Although Vestas is not releasing details of the design as yet, Appleton says the blade behaves like a Salisbury screen – incoming radar waves bounce off two surfaces that are precisely spaced so that the reflections interfere and cancel each other out.

Perfect replicas

To achieve like-for-like generating power, the stealth blade will have to resemble a standard blade as closely as possible. That must have been a huge challenge for the designers, says Lee Ford, a specialist in radar-absorbing materials at the University of Sheffield in the UK. "It's not just a case of sticking a Salisbury screen onto the outside," he says. "They will have had to redesign the internal structure, which is already complicated – the blades aren't just a piece of metal."

Appleton says the new blade successfully reduced radar reflection, but as the test was of a single blade, it is too early to give an exact figure on how much reduction will be achieved in practice. Still, he says the blades could reduce radar reflectance by an order of magnitude or even more. "We could be talking about a reflectance reduced to a few per cent," he says.

Stealth blades are not the only potential solution. Last week the UK government, various wind-power companies and the Crown Estate announced a £5.2 million ($8.5 million) fund to investigate how to bring turbine-spotting technology to radar operated by the National Air Traffic Services – the UK's main air navigation service provider.

US contractor Raytheon will work with NATS to develop the technology over the next 19 months, which will include "enhanced software algorithms for radar processing and tracking" according to Ginny McAdams of Raytheon.

Sweeping improvements

Technology firm Cambridge Consultants thinks it already has a radar solution to turbine-spotting. Its "holographic infill radar" generates a 3D image of the wind farm 10 times per second, so it can track the movement of reflective objects in the area and distinguish a rotating blade from a fast-moving aircraft (New Scientist, 25 October 2008, p 24). "We completed a successful trial for the UK Ministry of Defence last year," says Gordon Oswald, creator of Holographic Radar. Aviation authorities in the UK are studying those trials to see whether the technology is suited to widespread use.

Vaughan says that having so many options on the table gives grounds for optimism. "It's a really big issue, but it's very much on the government's radar now."

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