Integrating wind-generated power into the UK system is not just about shutting down conventional power stations when the wind is blowing. Starting up and shutting down power plant can take minutes or hours, depending on the type of plant, while power may be needed within seconds or minutes. Any calculation of the CO₂ emission reduction must take into account the amount of conventional generating capacity that has to be kept in varying states of readiness to meet demand. In practice, significant capacity has to be operating and synchronised to be available for rapid response, which in turn means fossil-fired capacity operating in parallel with wind. This supporting capacity will be operating below its optimum efficiency and such operations will produce more CO₂ per kWh than if plant operations were optimised. The more wind capacity that is introduced, the more of this lower efficiency conventional capacity will be required to operate on part-load - with increased CO₂ emissions.

Data is available from some of the power generators (ESB in Ireland, Elsam in Denmark and Innogy in the UK) and equipment manufacturers (Siemens and Matsui Babcock) showing the increase in CO₂ emissions as a result of any decline in generating efficiency with reduced load. Indeed, in a paper given to the Institute of Mechanical Engineers, Innogy described the actual UK operating experience of accommodating a modest amount of electricity generated from wind and stated: “… it has been estimated that the entire benefit of reduced emissions from the renewables programme has been negated by the increased emissions from part-loaded plant…” (Tolley D; 2003).

 The Electric Power Research Institute in California supports this position. They agree that it is technically incorrect to assume that wind power will displace fossil generated power and decrease CO₂ emissions on a kWh for kWh basis and that in a real operating situation (where storage of electricity is not possible) any CO₂ saving will be small.

In 2005, the Environmental Change Institute (ECI), of the University of Oxford, produced a report for the Department of Trade and Industry on the characteristics of the UK wind resource and the implications of these for security of electricity supply (Wind power and the UK wind resource). This report confirmed the average UK capacity factor for wind to be 27%. Despite this, the authors of this report then used a figure of 35% capacity factor in their subsequent calculations. This figure of 35% for the UK seems dubious – given that it is 25% higher than the average UK capacity factor for wind! The authors stated that an electricity network with a peak demand of 70 Gw needs 84 Gw of conventional generating capacity to guarantee meeting demand (this model is in excess of the current UK situation: peak demand of 61 Gw with generating capacity of 80 Gw in 2004).

This is equivalent to 6500 wind turbines of 2 Mw maximum output. In this ‘best case scenario’, the ECI calculated that this would allow 3 Gw of conventional generating capacity to be retired – meaning that 81 Gw of conventional generating capacity would need to be retained and kept in operation. In other words, even with this huge number of wind turbines in operation, 96.4% of conventional power stations would need to be retained.                                                                                                    This assessment is overly optimistic. Firstly, the ECI calculation is in respect of an electricity supply network with greater generating capacity than currently available in the UK. Secondly, if a capacity factor of 27% is used (the actual figure achieved in the UK), then nearly 20 Gw of wind power capacity would be required (over 9000 turbines of 2 Mw maximum output). This would require even more conventional generating capacity to be maintained in operation.

A considerable amount of this conventional capacity would be operating at low output/efficiency for much of the time, with frequent powering up and down to meet electricity demand. In this study from the ECI, no attempt was made to analyse CO₂ emissions. However, if nearly all of our current conventional power stations have to be kept running, despite all these wind turbines, to guarantee maintaining supply and if some of these power stations are running at low efficiency (thus increasing CO₂ emissions), then it seems very unlikely that this massive wind power programme would lead to any real reduction in CO₂ emissions from electricity generation in the UK.