Correspondence/Rebuttal pubs.acs.org/est
Response to Comment on “Effects of Environmental Temperature Change on the Efficiency of Coal- and Natural Gas-Fired Power Plants” e respectfully but firmly refute Yearsley et al.’s comments on three grounds. First, their argument that our conclusions are only valid if environmental regulations are ignored and water availability is unchanged disregards the broader significance of our findings.1,2 Prior to our paper, previous studies including that of van Vliet et al. only modeled how changes in air and/or water temperature affect power plant cooling and thus plant performance.3 Even though van Vliet et al. used empirically derived plant-specific characteristics (e.g., rated capacity), they applied these to thermodynamic equations of plant cooling.3 We, on the other hand, used publicly available time series data to assess how the efficiency and thus useable capacity of individual power plants actually relates to the temperatures of the air and/or water used by the plants for cooling.2 We did this by statistically analyzing multiple years of hourly plant input, output, and local temperature data for tens of individual power plants located throughout the U.S.2 Thus, we constrained not only the unique sensitivity of each plant’s efficiency to air and/or water cooling temperatures, but also the uncertainty associated with these sensitivities, an important issue not addressed by van Vliet et al.2,3 Furthermore, our temperature sensitivities applied to all levels of output achievable by the plants, not just to plants running at full capacity as modeled by van Vliet et al.2,3 Second, Yearsley et al. incorrectly argue that our efficiency results cannot be compared to van Vliet et al.’s usable capacity because they are obtained using different equations and methods.1 While our equations and methods are indeed different, our estimates of changes in power plant efficiency, represented by the regression coefficients d, e, and p in eqs 2 and 3 of Henry and Pratson (2016),2 are in fact directly related the plant’s usable capacity, that is, when efficiency decreases, output or usable capacity decreases holding all else constant. This can be seen by rearranging eq 1 for efficiency in Henry and Pratson2 and then converting it into change in useable capacity, ΔUC (MW), for the type of 40-plant region modeled by van Vliet et al.:3
W
Eout(MWh) = η × Fin(mmBtu) ×
1MWh 3.412mmBtu
fuel energy input per hour, which we used here because van Vliet et al.’s equations for required water withdrawal (q) are based on the plants’ installed or maximum rated capacity (KW).3 This calculated megawatt change in usable capacity was then converted to a percentage change. Finally, Yearsley et al. misrepresent aspects of our study in ways that imply we overstated the relevance of our findings. For instance, they characterize our suggestion that droughts may have a greater impact on plant performance than temperature as being made “without further analysis”.1 In fact, by this section in our paper, the Discussion section, we had already demonstrated through thorough statistical analysis that the temperature impact on power plant efficiency (and thus usable capacity) was minor.2 Given the difference between this result and the larger impacts estimated by van Vliet et al. and Barton & Chester, both of which were based on the combined modeled effects of water temperature and drought,3,4 we simply suggested that the difference may be because droughts affect power plant performance more than temperature does. Yearsley et al. also use an incomplete quote from our study to conclude that because we highlighted a multitude of instances in our data when temperature regulations were not observed, we “assume that these regulations will also be ignored in the future”.1 However, we did not make any assumptions about the future state of regulations. Rather, our full statement reads, “Thus, studies that have assumed open-loop plants reduce output at 27−30 °C will have overestimated the impact of water temperature on efficiency, unless f uture enforcement of these regulations becomes more stringent”.2 They further misrepresent the relevance of our findings by stating that we recommended the switch from once-through to closed-loop cooling systems “despite [our] finding on the low impacts of climate change on electricity production.”1 We did not recommend a switch but simply stated that our finding that “the plants least affected by and thus least vulnerable to environmental temperature change are closed-loop plants” provides “additional support to the EPA’s Clean Water 575 Act 316(b)”.2 It appears to us, having spent considerable time revisiting van Vliet et al.’s paper and Supporting Information, that useable capacity in their analysis declines with rising water temperatures less so because of the temperature impacts on plant thermodynamics (e.g., reduced heat exchange capacity between the boiler and cooling cycles decreases plant efficiency), and more because temperatures that approach environmental regulatory limits (encapsulated by parameters Tlmax and ΔTlmax in eqs 3A and 3B of van Vliet et al.) require operators to reduce plant output toward zero.3,5 This leads us to make the following converse argument to that of Yearsley et al.1 The conclusions of van Vliet et al. are only valid under the
(1)
ΔEout(MWh) Δη(%) = × ΔT̅ (°C) h ΔT (°C) F max(mmBtu/h) 1Mwh × in × region size(40Plant) × 3.142mmBtu plant
ΔUC(MW) =
(2)
where Δη/ΔT is the change in a power plant’s efficiency per degree increase in temperature, that is, our regression coefficients d, e, and p, and ΔT̅ is the future summer average water temperature increase estimated by van Vliet et al.,3 such that Δη/ΔT × ΔT̅ is the total change in efficiency due to future temperature increase. Fmax in is each individual plant’s maximum © 2017 American Chemical Society
Published: April 11, 2017 5345
DOI: 10.1021/acs.est.7b01587 Environ. Sci. Technol. 2017, 51, 5345−5346
Environmental Science & Technology
Correspondence/Rebuttal
assumptions that (1) current environmental regulations are strictly observed, (2) current regulations will not change in the future, (3) plant operations are not optimized to deal with temperature and water availability impacts (which, as we argued in our paper, could be a significant reason why many of the plants we studied are not very sensitive to environmental temperature change), and (4) future cooling water availability will undergo significant change. While we acknowledge the very real possibility of the last assumption, we suggest caution about accepting assumptions (1) − (3).
Candise L. Henry* Lincoln F. Pratson
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Nicholas School of the Environment, Duke University, 9 Circuit Drive, Box 90328, Durham, North Carolina 27708, United States
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Yearsley, J.; van Vliet, M.; Lettenmaier, D.; Ludwig, F.; Vogele, S. Comment on “Effects of Environmental Temperature Change on the Efficiency of Coal- and Natural Gas-Fired Power Plant”. Environ. Sci. Technol. 2017, DOI: 10.1021/acs.est.7b00561. (2) Henry, C.; Pratson, L. F. Effects of Environmental Temperature Change on the Efficiency of Coal- and Natural Gas-Fired Power Plants. Environ. Environ. Sci. Technol. 2016, 50, 9764. (3) Van Vliet, M. T. H.; Yearsley, J. R.; Ludwig, F.; Vögele, S.; Lettenmaier, D.; Kabat, P. Vulnerability of US and European electricity supply to climate change. Nat. Clim. Change 2012, 2, 676−681. (4) Bartos, M. D.; Chester, M. V. Impacts of climate change on electric power supply in the Western United States. Nat. Clim. Change 2015, 5, 748−752. (5) Koch, H.; Vögele, S. Dynamic modelling of water demand, water availability and adaptation strategies for power plants to global change. Ecological Economics 2009, 68, 2031−2039.
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DOI: 10.1021/acs.est.7b01587 Environ. Sci. Technol. 2017, 51, 5345−5346
