Anti-wind lobby restarts intermittency debate

The intermittency of wind power has been underestimated and will cause problems managing the electricity network, according to the Renewable Energy Foundation. The report marks a step-change in the tactics of the anti-wind lobby.

The Renewable Energy Foundation - a not-for-profit organisation that aims to promote renewables while "safeguarding the landscapes of the UK" - has been labelled an "anti-windfarm" group since it formed in 2004.

It has been prominent in campaigns against individual windfarms (ENDS Report 372, p 21 ).

However, in December the group departed from previous tactics and issued two reports - one looking at the current performance of wind power,1 the other at the effect of adding 25GW of wind to the UK electricity network2, based on work by the Oswald Consultancy. Both got widespread media coverage.

According to the first report, the UK’s wind farms had an average "capacity factor" of 28.4% in 2005 - varying from 23.8% in mid-Wales to 32.6% for the offshore windfarms at North Hoyle and Scroby Sands.

The capacity factor of a wind farm is the amount of electricity it generates compared to its potential. The figures came from Ofgem’s renewables obligation certificates database.

The UK’s wind farms are the best performing in Europe, the report says - Germany’s average capacity factor was only 17.8% in 2005. However, the REF’s take on the data - and that picked up by the media - was that many windfarms had been built on the incorrect assumption of a 30% capacity factor.

The group highlighted the poor performance of wind turbines at Ford’s plant in Dagenham, Essex, and at GlaxoSmithKline’s plant at Barnard Castle, County Durham. These have capacity factors of only 19% and 9% respectively, although they are located in urban areas with poor wind resource.

When questioned, the REF was unable to provide ENDS with evidence of windfarms being incorrectly granted planning permission due to misleading capacity factors.

"Thirty percent is really a rule of thumb," said Gordon Edge, director of policy at the British Wind Energy Association. "But 28% isn’t that different and it’s still the best in Europe, so what’s the problem?" However, the media did not reflect BWEA’s view.

Although it received less publicity, the second report is the more interesting piece of work. It calculates the hourly power output of wind farms in eight regions across the UK using information from Ofgem’s ROC register and wind speed data from the Met Office.

During January 2006, the output from wind farms varied by 97% due to changes in wind speed, it says. The minimum power output in the month was only 1% of installed capacity. "Power swings of 70% in 30 hours are commonplace in January," it adds.

The report then uses a model to extrapolate that data to an electricity system with 25GW of installed wind capacity. It finds the effect on residual generators would be "unpredictable" and "random".

Base load power plants would have to vary output between 5.5GW and 56GW to match demand, it says. Increased cycling would lead to a higher risk of mechanical failure.

The report does not calculate the risk, but recommends the government invest in a diverse range of renewables to curtail the effects of intermittency.

"The report provides new evidence of the rate of change of wind power in the UK and the effect it could have on thermal plant," said Mr Oswald, its author. "It worries me that we’ll need to cycle thermal plant more frequently than is currently thought."

Mr Oswald’s view is at odds with that of the UK Energy Research Centre, which undertook a review of over 200 reports on the intermittent generation of renewables last year (See ENDS Report 375, pp 13-14 ).

"None" of the studies reviewed suggested that intermittent generation would compromise the reliability of the electricity system. Conventional generation equivalent to 5-10% of installed wind capacity would be needed to cope with short-term fluctuations in wind speed, it said.

Mr Oswald commented that these figures are an under estimate.

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