A pair of posts at Master Resource (part I, part II) explore the degree to which variable wind power leads to lower efficiency and increased air emissions when natural gas generators are used to provide energy balancing and back-up reserves (and except when and where sufficient hydropower is available, natural gas generation usually is the low cost provider of these services). Depending on assumptions, Kent Hawkins finds that adding wind power could result in no reduction of fossil fuel use and perhaps even an increase in related emissions.
The results are more dramatic than found by Warren Katzenstein and Jay Apt and published in Environmental Science & Technology, but the difference is primarily in the degree of the effect and not the nature of the issue. (Katzenstein and Apt found under certain circumstances that wind and solar power added to a natural gas-based power system acheived about 80 percent of expected CO2 reductions but no more than half of the expected NOx reductions.)
NOTE: Environmental Science & Technology has published a comment on the Katzenstein and Apt article by Andrew Mills and co-authors and then a reply by Katzenstein and Apt. The Mills et al. comment asserts that the methodology used overstates the need for backup power supplies and so at best indicates a possible upper bound on indirect emissions. The comment suggests that geographic diversity of widespread wind power facilities helps to smooth out some of the variability from individual sites, so studies based on a few units exaggerate the actual effects in larger power systems. In addition, unit commitment and dispatch practices by power system operators can accommodate some variability without contributing to added emissions.
In the reply, Katzenstein and Apt say the central issues are, “how are the fill-in generators to be dispatched?” and “what are the emissions from those generators in that dispatch method?” They note that Mills et al. adopt a different approach than the original article, but with either approach their are incremental emissions associated with the dispatch of the required fill-in generators.
UPDATE: The November/December issue of IEEE’s Power and Energy Magazine is devoted to wind power issues. Included is “Wind Power Myths Debunked,” which claims among other things, “the notion that wind’s variations would actually increase system fuel consumption does not withstand scrutiny.” Unfortunately, subscription required so the link only goes to an abstract. The article also reports another analysis saying that adding up to 20 percent wind may extract an efficiency penalty of no more than 7 percent (i.e., emission reductions may be 7 percent lower than expected).
One thing clear from these discussions is that answers to questions about the effect of wind power on emissions will depend very much on what else is going on on the power system to which the wind power is added.
UPDATE 2009/12/04: Kent Hawkins responds to some of these issues in another post up today at Master Resource.