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Energy and the Environment
Differentiating the Effects of Climate ChangeInduced Temperature and Streamflow Changes on the Vulnerability of Once-through Thermoelectric Power Plants Candise L. Henry, and Lincoln F. Pratson Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b05718 • Publication Date (Web): 08 Mar 2019 Downloaded from http://pubs.acs.org on March 8, 2019
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Environmental Science & Technology
Differentiating the Effects of Climate Change-Induced Temperature and Streamflow Changes on the Vulnerability of Once-through Thermoelectric Power Plants Candise L. Henry1* and Lincoln F. Pratson1 1 Nicholas
School of the Environment, Duke University. 9 Circuit Drive, Box 90328, Durham, NC 27708 USA * Corresponding author: 9 Circuit Drive, Box 90328, Durham, NC 27708 USA. (919) 684-1788.
[email protected] 1 ACS Paragon Plus Environment
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Abstract Thermoelectric power plants with once-through cooling systems generated 35% (~300
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GW) of U.S. electricity in 2016. Factors that reduce once-through cooling capacity and thus
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power output are environmental regulations, warming surface waters, and drought. The latter two
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may become more frequent as global climate changes. Previous research indicates that reduction
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in power plant capacity caused by environmental regulations can be significant, while that by
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surface water warming minor. Here, we address the effect of droughts on power output, which
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until now has remained conflated with temperature impacts. We do this using a widely-used
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electricity generation model alongside hourly operational and meteorological data for 52 once-
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through plants located across the U.S. The effect of drought on plant output is examined for
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different water-availability and temperature scenarios, with and without regulations on plant
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water discharge. We find that if surface waters warm 3 °C and river discharges drop 20%,
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droughts would account for ≤20% of total capacity reduction depending on the plant, warming
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surface waters ≤2.3%, and environmental regulations up to 80%. This suggests that maintaining
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environmental regulations will require the continued conversion of plant cooling systems from
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once-through to recirculating, and mitigating climate impacts will require more stringent
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drought-specific watershed management.
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Keywords: Thermoelectric power plants, power plant efficiency, efficiency loss, once-through
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and recirculating cooling, cooling fluid temperature, climate change, droughts.
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Environmental Science & Technology
1. Introduction For once-through power plants, usable capacity is expected to decrease in the future as a
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result of: (1) thermal efficiency loss caused by the reduced ability of cooling water to remove
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heat from the steam load when the cooling water’s temperature increases, (2) forced reduction in
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power output due to a drop in available cooling water during droughts, and (3) forced reduction
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due to environmental regulations on the maximum discharge temperature of cooling waters
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(Figure 1). In the past several years, once-through plants across the U.S. have been forced to
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curtail output or even shut down as a consequence of these impacts. Particularly vulnerable are
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nuclear plants, which tend to face particularly stringent safety-related regulations. In 2012 for
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example, a heat wave in the Northeast forced several reactors in the region to be shut down1,
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while drought conditions across the Southeast in 2008 brought many nuclear plants there within
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days of shutting down.2 Coal and natural gas plants are also exposed to such shutdowns. A
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drought in Texas in 2011 forced one coal plant to curtail output while decreased reservoir levels
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caused another to rush building new cooling capacity so that the plant could maintain full power
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output.3 These incidents suggest that if drought and high temperature extremes worsen in the
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future, meeting electricity demand with thermoelectric power plants will become more
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challenging.
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Initial studies investigating climate impacts on thermoelectric power plant output are
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based on thermodynamic modeling. These include studies that limited their examination to the
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impacts of changes in air and water temperature on the thermal efficiency of particular types of
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power plants (e.g., refs 4-7) as well as others that incorporated the effects of temperature-induced
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efficiency loss (TIEL), drought-induced capacity loss (DICL), and regulation-induced capacity
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loss (RICL) on usable capacity by integrating climatological, hydrological, and electricity
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production models.8-10 In this paper, we focus on the latter studies because they include a wider
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range of potential climate change impacts, including increased droughts and warming air and
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surface water temperatures. Comparisons between studies that use integrated models and studies
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that use historical data to examine variations in power plant output versus ambient climatic and
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hydrologic conditions reveal that the modeling-based studies tend to predict more severe climate
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impacts on power generation than do empirically-based studies. For instance, van Vliet et al.8
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used an integrated model and projected that with an average 2.4 °C increase in summer river
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temperatures and 15-19% decrease in low flows across the U.S. by late-century, greater 16% of
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the generation capacity from the 61 power plants in their study will be lost. Bartos and Chester9
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estimated similar outcomes using the same integrated model for power plants in western United
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States. Meanwhile, Henry and Pratson11 analyzed hourly historical data of power plant
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efficiencies versus water and air temperatures at 20 once-through and 19 recirculating, coal and
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natural-gas power plants across the U.S. and estimated that 3 °C increase in average water
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temperature would result in 0.1), confirming that this model can be used to constrain the
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individual impact of each environmental factor through our scenario analyses.22
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3.2 Impacts of TIEL, DICL, and RICL on power generation Among the modeling results of the scenarios are changes in usable capacity across all
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power plants examined (Figure 3). Comparisons of these modeled changes in usable capacities
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reveal several interesting results. First, we find that climate-related impacts on power generation
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are enhanced when power plants are required to strictly adhere to environmental protection
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regulations (RICL), even when only thermal regulations (and not streamflow regulations, which
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Liu et al.10 also included) are considered. Comparing scenarios with and without the impacts of
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RICL, absolute reductions in median summer usable capacity intensify on the order of 10-100
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times depending on the scenario, e.g., from 0-2.2% decrease in scenario SA2(i) to 7.6-91% in
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SA2(ii) or 0-11.8% in SB1(i) to 0-60% in SB1(ii) (Figure 3). (Note that we report summer usable
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capacity here because summer conditions result in the maximum expected impact that climate
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will have on power generation.) This indicates that the greatest impact on plant usable capacity is
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exceedance of regulatory temperature limits and not the direct impact of droughts and/or warmer
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surface water temperatures on plant cooling efficiency, a point of confusion for readers of the
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previous integrated modeling studies (i.e., van Vliet et al.8,21 and Bartos and Chester9) because
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the sum of the two impacts was presented rather than separated as we have done here. And while
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discharge temperature regulations may become more strictly adhered to in the future as assumed
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in the integrated studies, at present, provisional exemptions from such regulations appear to be
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commonly granted to power plants across the U.S.11,12 However, if thermal regulations become
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more stringent in the future and fewer provisional waivers are granted, then power plants will
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benefit from switching to recirculating cooling systems that do not discharge waste heat into
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vulnerable water bodies, as it would allow the power plants to continue generating full electricity
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output without curtailment.
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Second, we find that regardless of whether thermal regulations are in place, DICL
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impacts are more severe than those due to TIEL. As shown in Figure 4a, when temperature
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regulations are strictly observed (scenario B2(ii)), absolute impacts of all three factors combined
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on the useable capacity of the 52 once through plants (scenario B2(ii)) ranges from 10% mean reduction
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in most scenarios because its cooling source is a river with average streamflow
