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Response to Comment on “Air Emissions Due to Wind and Solar Power” We welcome the perspective brought to the issue by Mills and coauthors (1). Because they do not question the accuracy of the data we collected from natural gas turbines in power company operation (2), 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?” Mills et al. (1)first dispatch a single natural gas generator that ramps up and down to cover the variability and then start additional generators as required. We operate all generators as spinning reserve. Both groups spread the ramping requirement equally over the running natural gas generators (five in the multiple turbine analysis in our paper). In either dispatch method, there are air emissions penalties to be paid. The first is the penalty associated with starting the generator (for example, see the upper branch of NOx emissions in Figures S1 and S7 of our original Supporting Information) (2). The second is the penalty associated with operating at partial power (both groups minimize this in multiple turbine operations). The third is the penalty that arises from keeping a generator operating at idle power so that it can quickly be ramped up when the wind or solar power falls off (the f0 that the dispatch method in Mills et al. (1) is designed to minimize). We agree that we pay this penalty for all generators operating as spinning reserve. The model presented by Mills and coauthors does not appear to take into account the startup penalty, which we observe to be significant for NOx. We illustrate the issues by drawing on data from the Bonneville Power Authority (BPA) control area (3) for January 2009 (Figure 1), over a thousand wind turbines. The BPA experience does not support the statement made by Mills et al. (1) that “just 10% operating reserves may be required.” In January 2009, wind supplied a maximum of 23.4% of the power required by Bonneville’s load, and the output from the thousand wind turbines dropped to nearly zero for periods of 17 days that month. Adding all the wind turbines in the balancing area does not smooth the output enough to avoid deep and fast power drops. During this period, a maximum of 313 MW of spinning reserve was needed to counteract the fluctuations observed within 10 min (there were 73 occasions on which the 10 min fluctuations in wind were >100 MW). Thus, significant spinning reserves must be on line, and the idle power emissions cannot be neglected. Of course, in BPA, spinning reserves are largely hydro; Texas and California use natural gas. The requirement for spinning reserves significantly modifies the results of Mills et al. (1). Even under the best case quasi-static wind assumption they make (they do not use real wind data), the CO2 emissions reductions for renewables penetration levels of up to 20% are in the range of 76-94% of those expected when one spinning reserve turbine is accounted for, even in a system with 20 fill-in power turbines (Supporting Information). We agree that the jump in emissions of the dry low NOx (DLN) control system when run at less than half power can be controlled in a number of ways and noted in our paper 6108
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FIGURE 1. Five minute time resolution wind output from Bonneville Power Authority balancing area for January, 2009.
that dispatch was one (we also noted that firing modes can be optimized). Both groups calculate that η is significantly different from 1 for DLN. The steam injection turbine exhibits no threshold effect, and we conclude that NOx reductions from such systems will not be observed except at very high renewable penetrations (or unless a high NOx emission coal plant is being displaced). We note that a number of the points made by Mills et al. (1) are applicable in systems with only a small amount of variable renewable generation. These include their statement that “every change in wind or solar power output does not need to be countered” and that “operators are required to balance only the net variations of all loads and all generators.” We are interested in looking ahead to the day when wind or solar power makes up a large proportion of generation. With a 25% renewable energy standard, it is perfectly possible on a windy night for variable generation to make up over half of all power generated and for its fluctuations to overwhelm the variations in load. No power generation sources, even renewables, are without problems. Previous wind integration studies such as those cited by Mills et al. (1) have not considered air emissions from fill-in power generators. As we wrote in our paper, “If system operators recognize the potential for ancillary emissions from gas generators used to fill in variable renewable power, they can take steps to produce a greater displacement of emissions. By limiting generators with GE’s DLN system to power levels of 50% or greater, ancillary emissions can be minimized. Operation of DLN controls with existing (but rarely used) firing modes that reduce emissions when ramping may be practical. On a time scale compatible with RPS implementation, design and market introduction of generators that are more appropriate from an emissions viewpoint to pair with variable renewable power plants may be feasible.” There is more observational data to be gathered on the air emissions of gas generators that provide fill-in power in areas with large penetration of variable renewables and in measurements on shoulder coal and peaking oil units similar to the ones we report for gas generators. It is only by addressing the issues early that industry can make technological and market responses.
Supporting Information Available Multiple turbines dispatch emissions η(R) and heat rate. This material is available free of charge via the Internet at http:// pubs.acs.org. 10.1021/es901485d CCC: $40.75
2009 American Chemical Society
Published on Web 07/07/2009
Literature Cited (1) Mills, A.; Wiser, R.; Milligan, M.; O’Malley, M. Comment on “Air emissions due to wind and solar power”. Environ. Sci. Technol. 2009, DOI:1021/es900831b. (2) Katzenstein, W.; Apt, J. Air emissions due to wind and solar power. Environ. Sci. Technol. 2009, 43 (2), 253–258. (3) Total Load & Wind Generation in the BPA Control Area (Balancing Authority Area) Beginning 1/1/09, at 5-min Increments, Updated Monthly. SCADA 79687; Bonneville Power Transmission, 2009. http://www.transmission.bpa. gov/business/operations/wind/.
Warren Katzenstein and Jay Apt* Carnegie Mellon Electricity Industry Center, Tepper School of Business and Department of Engineering and Public Policy, 254 Posner Hall, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213
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