As the wind industry meets for a major conference in Wales today, a new report was published this week claiming that wind power faces "hidden costs" and "reliability issues".

It called on the government to limit the development of wind farms in the UK to 10GW more capacity. The Renewable Energy Foundation, a group set up by the anti-wind TV celebrity Noel Edmonds to assess alternatives to wind power in the UK, commissioned the study led by James Oswald, an engineering consultant and former head of research and development at Rolls Royce Turbines.

The study claimed that wind farms produce "very little energy" during January, a time of high demand for power to heat homes and businesses, and would see power swings of 70% within 12 hours as the wind drops, the study said, requiring It said that if 25GW more wind power was used in the UK, fossil fuel plants would have to be switched on up to 23 times a month to cover the wind farms facing lower winds in winter high pressure weather systems. "Doubts" The Renewable Energy Foundation said the new study "confirms doubts as to the wisdom of a large wind fleet", and "supports REF's long-standing recommendation that the contribution of wind should be limited for technical and economic reasons, to about 10 GW, mostly offshore where winds are stronger and more reliable".

Mr Oswald said: "Wind energy is fine on a small scale, but it works less well on a large scale because British weather and wind is too variable. Wind power does not obviate the need for fossil fuel plants, which will continue to be indispensable. The problem is that wind power volatility requires fossil fuel plant to be switched on and off, which damages them and means that even more plants will have to be built." Carbon savings will be less than expected, the researcher suggested, "because cheaper, less efficient plants will be used to support these wind power fluctuations".

Mr Oswald warned that the government was not taking the issue into account in its predicted figures within the new Renewable Energy Strategy. BWEA As the British Wind Energy Association holds its BWEA Cymru conference in Cardiff today, the report authors attacked the wind industry trade group's claims for the reliability of wind power.

Mr Oswald said: "The British Wind Energy Association sometimes claims that wind produces power for 80 to 95% of the time, but the number of hours wind produces some power is irrelevant, what matters is how much power for these hours and how much it fluctuates hour to hour."

"Times of zero wind output are rare, but can be significant. In fact new data shows that wind output was zero at the moment of peak demand in 2006. This was when a high pressure system sat squarely over Britain for ten consecutive days which lead to high demand and very little wind," Mr Oswald added.

The Renewable Energy Foundation argues that the government's Renewables Obligation support scheme unfairly distorts the market to favour technologies like wind power. Commenting on the findings of the new report, John Constable, director of policy and research for the Renewable Energy Foundation, said: "Poorly thought-out political enthusiasm for windpower has propelled us towards levels of deployment which are irrational and very expensive; this study is a helpful corrective, and will enable us to plan how to get the best from wind technology at reasonable cost."

The Renewable Energy Foundation report comes just days after a separate anti-wind report from the Centre for Policy Studies. "Myths" The BWEA called the assertions in that report "myths", saying claims were "unsubstantiated". It accepted that wind energy is variable, however the trade association said the typical wind farm operates 85-90% of the time - 28% of the time at full power onshore, and 45% of the time offshore - compared to a 61% load factor seen in nuclear power stations in 2006.

Maria McCaffery, BWEA chief executive, insisted: "Wind energy could be delivering 33GW, nearly 30% of the UK's electricity by 2020. There are already 19GW at one stage or another in the system, either in operation, being built or waiting for planning." She added "This reports is little more than rehashing of old myths, half truths and misconceptions". The wind industry group said there was no form of energy that was 100% available, but suggested "there is always wind somewhere in the UK, and a lack of wind in one area is cancelled out by wind in others".