Stealthy wind turbines aim to disappear from radar screens

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. ...

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.

via Stealthy wind turbines aim to disappear from radar screens - tech - 23 October 2009 - New Scientist.

Ah, this may be the reason for UFO sightings around windmills.