"We have been learning about things that we thought were a major problem for us," said Wing Commander Nicky Loveday from the MOD last month. "We have had to step away and say: actually it really isn't a problem for the air defence community." Until now, the MOD has raised concerns with nearly 50% of all wind farm proposals in Britain each year (Windpower Monthly, October 2005).
The trials have revealed the extent of the impact of wind turbines on the MOD's seven radars around Britain. Meantime, with funding from the Department of Trade and Industry (DTI), BAE Systems has been looking for a way to resolve the MOD's objections to six large offshore wind power stations proposed in the Greater Wash off England's east coast. The wind farms, part of the "round two" permitting process, would be within line of sight of air defence radar -- either at Trimingham in Norfolk or Staxton Wold in North Yorkshire -- which automatically triggers an objection from the MOD.
Presenting the results of the combined efforts of MOD and BAE Systems at the British Wind Energy Association conference in Cardiff, Loveday offered the industry a choice between two possible solutions for the Wash: a package of software measures or installation of an additional radar.
She explained that a solution need not apply just to the wind farms in the Wash. "Whilst we set out to find mitigations for round two wind farms, we think we have solved all offshore and onshore problems with wind farms within the air defence environment." Moreover, proliferation of several projects within a specific area is no longer an issue, she added. "So you can build as many wind farms as you like within sight of air defence radar." Only a very small exclusion zone, five to ten kilometres, say, would need to be retained around each radar, she said.
Better understanding of the interaction of wind turbines and radar as a result of the trials has led the MOD to reassess its position on wind farms, Loveday explained. One of the MOD's concerns has been the "dead zone" above a wind farm which obscures small aircraft on radar screens. "We believe now it is due to the signal processing techniques of the radar," she said.
"We were also concerned about the sizeable shadow effect around the turbines. That too is also largely due to the way the radar processes the clutter and the returns coming back." The trials showed that the shadow was smaller than originally thought.
One of the issues that has concerned the MOD is clutter, but it is not such a problem for defence systems as for civil aviation, where clutter is a major concern. "But from an MOD perspective, I like to see everything [on radar screens]. We find our operators can work through clutter, so it is not an issue for me at all," said Loveday.
The solutions found by BAE Systems are three separate software mitigation techniques that change the way that radars signal processing works. None of the techniques provides adequate mitigation on its own, she warned. "All three need to be implemented for it to be a hundred per cent solution." A single 100% solution could be provided, however, by an additional radar employing e-tilt (electronic tilting of the beam) technology to bring obscuration down to a manageable level, she said.
It is now up to the wind industry to weigh up the two choices. The software measures may take years to develop, while the e-tilt radar could provide a near term solution. But e-tilt is geographically limited and expensive -- one radar expert estimates the cost to be similar to the cost of a wind turbine.
Loveday pledged to withdraw the MOD's six objections to development in the Greater Wash provided the wind industry commits to deploy one of the solutions before the plants become operational. "If I was going to recommend a way forward, I would suggest you spend those two to three years and your resources in funding the software solutions," she said. "They solve a far bigger problem [than the additional radar] and are not geographically limited. But it is your call; you are the developers, you have to get value for money."