Stealthy wind turbines aim to disappear from radar screens
* 15:22 23 October 2009 by Colin Barras


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."