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BWEA Briefing on UK Wind Capacity Factors

Many critics of wind energy focus on the perceived 'efficiency' of wind turbines and suggest that the technology underperforms against expectations. This is very much a 'glass half-empty rather than half-full' approach.

The capacity factor of any power plant is the percentage of generation of its generation against its theoretical maximum output. It is unrealistic to expect any power plant to operate permanently at 100% of its capacity, and in the case of wind energy, where there are no fuel costs, capacity factor is not a reliable measure of productivity.

If taken in isolation, capacity factor is an irrelevant statistic. A wind turbine with a 2 megawatt (MW) generator installed could be power limited to 1 MW; calling this a 1 MW machine, the capacity factor would go up from an average 30% to around 50%. The turbine however would deliver less energy, displace less carbon and have the same energy cost and environmental impact to build.

The UK has one of the best wind regimes in the world and has considerably higher capacity factors than many of the European countries where wind already makes a significant contribution to electricity supplies.

The Digest of UK Energy Statistics, compiled annually by the Department of Trade and Industry, reports an average capacity factor for onshore wind of 28.2% in 2005. This compares favourably with the commonly applied industry average of 30% and is expected to improve year on year as greater numbers of modern wind turbines are brought online. Wind turbine technology is improving with each new model produced, and tower heights are increasing, giving higher capacity factors than with smaller older machines on small towers.

It is a well established industry fact that wind turbines located in built-up areas will generate less electricity than their more windswept counterparts, but it is important not trivialise turbines such as those installed at factories or company headquarters, and providing part or all of their electricity needs: this is carbon-free generation which incurs no additional transmission or distribution charges, and also has additional value in hedging against rising power prices.

The simple fact that the wind doesn't blow all the time also gives rise to other criticisms of the technology; namely wind's variability and system instability, with various claims made of system back up and plant on stand by needed to accommodate less windy periods.

Wind turbines in fact generate electricity for 70-85% of the time, and a report from the Environmental Change Institute research team at Oxford University, the first methodical investigation of Britain's wind resource, shows that there has never been a time over the past 35 years when the entire country has been without wind, and that the wind always blows strongly enough to generate electricity somewhere in Britain. The study also showed that wind tends to blow more strongly when demand is highest, during the day and winter months.

The UK Energy Research Council stated in their report The Costs and Impacts of Intermittency that 100% 'back up' for individual renewable sources is definitely not needed. The extra capacity required is very small and only a small part of the total cost of renewables. It is possible to work out what is needed and plan accordingly. Similarly none of the 200+ studies UKERC reviewed suggested that the introduction of significant levels of intermittent renewable energy would lead to reduced reliability.

Even for wind power to provide 10% of our nation's electricity needs, only a small amount of additional conventional back-up would be required, in the region of 300-500 MW. This would add only 0.2 pence per kilowatt hour to the generation cost of wind energy and would not in any way threaten the security of the grid.

Various commentators from the International Energy Agency to National Grid themselves have frequently said that there are no technical issues associated with accommodating larger amounts of wind generation into electricity systems, and in the case of the UK, National Grid report that the impact of accommodating wind is unlikely to become a significant issue until wind generates over 20% of total electricity supply.

As the UK approaches the landmark of 2 gigawatts of installed wind power plant, the most important consideration is that this is a carbon-free source of generation which is entirely sustainable, contributes significantly to the UK's energy security and Government targets for renewables, and is leading the way in establishing a long-term market for the other renewable technologies to enter in due course.

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