Critical Review: Uncharted Waters? The Future of the Electricity-Water

Critical Review pubs.acs.org/est

Critical Review: Uncharted Waters? The Future of the ElectricityWater Nexus Kelly T. Sanders* Sonny Astani Department of Civil and Environmental Engineering, University of Southern California, 3620 S. Vermont Avenue, Los Angeles, California 90089-2531, United States ABSTRACT: Electricity generation often requires large amounts of water, most notably for cooling thermoelectric power generators and moving hydroelectric turbines. This so-called “electricity-water nexus” has received increasing attention in recent years by governments, nongovernmental organizations, industry, and academics, especially in light of increasing water stress in many regions around the world. Although many analyses have attempted to project the future water requirements of electricity generation, projections vary considerably due to differences in temporal and spatial boundaries, modeling frameworks, and scenario definitions. This manuscript is intended to provide a critical review of recent publications that address the future water requirements of electricity production and define the factors that will moderate the water requirements of the electric grid moving forward to inform future research. The five variables identified include changes in (1) fuel consumption patterns, (2) cooling technology preferences, (3) environmental regulations, (4) ambient climate conditions, and (5) electric grid characteristics. These five factors are analyzed to provide guidance for future research related to the electricity-water nexus.

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INTRODUCTION The power generation sector requires significant volumes of water for the safe and reliable generation of electricity. With the exception of hydropower, which requires water to move hydroelectric turbines, the majority of the water withdrawn for power generation is used to condense steam exiting turbines at thermoelectric power facilities.1,2 Thermoelectric power generation represented nearly 40% of annual U.S. freshwater withdrawals in 2010.3,4 Of the volume of water withdrawn for power generation, approximately 3% was consumed.3 Although water might also be also used for turbine inlet cooling, handling ash, washing, wastewater reclamation, and flue gas desulferization, power plant cooling is typically the largest water requirement for thermoelectric generators.5,6 While projections of the water use of the electricity sector in the future exist,7−17 large regulatory, social, and technological uncertainty make it difficult to anticipate future changes, especially over the longterm.8,18,19 However, these changes will be important for water managers, electric power utilities, and policy makers to understand moving forward. This manuscript is intended to (1) provide a critical review of the recent literature addressing the electricity-water nexus, with focused attention on future projections and (2) define the factors that will impact the water requirements of the electric grid moving forward in order to inform future research.

controls (and other auxiliary systems) and ambient climate characteristics.1,2,20 Water use is characterized in terms of withdrawals, the total volume of water removed from a reservoir or river, and water consumption, the subset of water withdrawals that is not returned to the original point of extraction.1,2,21 A summary of tradeoffs is provided below. Fuel and Prime Mover Considerations. “Thermoelectric power” refers to electricity derived from a heat source.22 The type of fuel used to generate heat (e.g., coal, uranium, natural gas, petroleum, biomass, solar, or geothermal energy) in a thermoelectric power plant impacts its water requirements due to variations in the efficiency with which a primary fuel is converted into finished electricity. Less efficient EGUs convert less thermal energy from the boiler to mechanical energy via the turbine, thus requiring more steam and more cooling water per unit of electricity generated.23 For example, nuclear facilities tend to operate with lower thermal efficiencies than average coal or natural gas facilities because they operate at lower temperatures and require more circulating steam per unit of power output thereby increasing water use.24,25 Additionally, while most steam boiler plants are able to release some heat through flue gases, nuclear and concentrating solar power plants (CSP) plants do not.26,27 Although steam boiler EGUs utilize a closed loop steam cycle for power generation, the volume of external cooling water required to condense steam (i.e., the working fluid within this closed loop) into water is

BACKGROUND The water required for power production varies according to an electric generating unit’s (EGU) fuel type, generation technology (i.e., prime mover), cooling system, environmental

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© 2014 American Chemical Society

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September 1, 2014 November 26, 2014 December 8, 2014 December 8, 2014 dx.doi.org/10.1021/es504293b | Environ. Sci. Technol. 2015, 49, 51−66

Environmental Science & Technology

Critical Review

Table 1. Water Consumption and Water Withdrawal Factors for Selected Thermoelectric Power Plant Technologiesa power plant configuration fuel type CSP CSP CSP CSP CSP CSP CSP CSP geothermal geothermal geothermal geothermal geothermal biopower biopower biopower biopower biopower natural gas natural gas natural gas natural gas natural gas natural gas natural gas coal coal coal coal coal coal coal coal coal coal coal coal coal nuclear nuclear nuclear

cooling system N/A dry dry hybrid hybrid tower tower tower dry dry dry hybrid tower dry tower tower once-through pond dry tower tower tower once-through once-through pond tower tower tower tower tower tower tower once-through once-through once-through pond pond pond tower once-through pond

generation technology b

stirling power towerb troughb power towerb troughb power towerb troughb fresnelb flashb binaryb EGSb Binaryb Flashb biogasb biogasb steam steam steam combined-cycle combined-cycle combined-cycle with CCS steam combined-cycle steam combined-cycle IGCC subcritical supercritical IGCC with CCS generic supercritical with CCS subcritical with CCS supercritical subcritical generic supercritical generic subcritical generic generic generic

water consumption rate (gal MWh−1) median

min

5 26 78 170 338 786 906 1000 5 270 505 461 15 35 235 553 300 390 2 205 393 826 100 240 240 380 479 493 549 687 846 921 103 113 250 42 545 779 672 269 610

4 26 43 102 117 751 725 1000 5 270 290 221 5 35 235 480 300 300 0 130 378 662 20 95 240 318 394 445 522 480 815 900 64 71 100 4 300 737 581 100 560

max 6 26 79 302 397 912 1109 1000 5 270 720 700 361 35 235 965 300 480 4 300 407 1170 100 291 240 439 664 594 604 1100 907 942 124 138 317 64 700 804 845 400 720

n 2 1 11 2 3 4 18 1 1 1 1 2 4 1 1 4 1 1 2 6 2 4 3 2 1 8 7 8 4 5 3 2 3 3 4 3 2 3 6 4 2

water withdrawal rate (gal MWh−1) median 5 26 78 170 338 786 906 1000 5 270 505 461 15 35 235 878 35 000 450 2 255 506 1203 11 380 35 000 5950 393 587 634 642 1005 1147 1329 22 590 27 088 36 350 15 046 12 225 17 914 1101 44 350 7050

min 4 26 43 102 117 751 725 1000 5 270 290 221 5 35 235 500 20 000 300 0 150 487 950 7500 10 000 5950 358 463 582 479 500 1098 1224 22 551 27 046 20 000 14 996 300 17 859 800 25 000 500

max 6 26 79 302 397 912 1109 1000 5 270 720 700 361 35 235 1460 50 000 600 4 283 544 1460 20 000 60 000 5950 605 714 670 742 1200 1157 1449 22 611 27 113 50 000 15 057 24 000 17 927 2600 60 000 13 000

n 2 1 11 2 3 4 18 1 1 1 1 2 4 1 1 2 1 1 2 7 3 2 2 1 1 12 8 9 7 4 4 3 3 3 4 3 2 3 3 4 2

a

Withdrawal and consumption rates within a technology category might be from different samples of power plants. (Adapted from Tables 1, 2, and 3 in J Macknick et al. (2012) Environ. Res. Lett. 7 045802, ©IOP Publishing Ltd. CC BY-NC-SA.28). bWithdrawal rates for CSP, Geothermal, and Biogas facilities are assumed to be equal to consumption rates for the sample of facilities evaluated in each category.

cycle; the remaining two-thirds is generated via a combustion turbine, reducing the overall water requirements of combinedcycle power plants.1,29 While traditional natural gas combustion EGUs have negligible water requirements, they tend to be smaller and expensive to operate per unit of electrical output (but more responsive to quick changes in demand) than steam boiler or combined-cycle EGUs. Consequently, combustion turbines are generally used to provide peaking and ancillary services that improve the reliability and performance of the grid.1,30 Some newer natural gas combustion turbine units (as well as combustion turbines within combined cycle units) use a relatively small amount of water to prechill turbine inlet air to increase EGU efficiency when ambient air temperatures are

typically orders of magnitude higher than the volume of water contained within the closed loop itself.28 (Table 1 summarizes water withdrawal and water consumption factors for selected thermoelectric technologies based on the literature.) Power generation requiring water for cooling represented approximately 87% of U.S. generation in 2012 including 62% by steam turbine facilities and 24% by combined cycle facilities (See Table 2).29 Coal and natural gas represented the majority of this generation, and thus, the majority of the water-cooled generation in 2012.29 Natural gas combined cycle (NGCC) EGUs, which offer efficiency, cost, and operational benefits over other forms of thermoelectric generation, also have water benefits over other water-cooled facilities.20 Only one-third of total combined-cycle electrical output is generated via a steam 52

dx.doi.org/10.1021/es504293b | Environ. Sci. Technol. 2015, 49, 51−66


Critical Review

Table 2. Approximately 87% of U.S. Electricity Generation Required Water for Cooling in 2012 (Data from Ref 29) all values in terawatt hours (TWh)

coal

natural gas

steam turbine combinedcycle (combustion turbine part) (steam part) hydraulic turbine wind turbine combustion (gas) turbine single-shaft combinedcycle internal combustion engine photovoltaic binary cycle turbine fuel cell pumped storage other

1512.19

108.28

0.62

660.21

1.38

321.10

0.04

0.02

fuel total

1514.23 37.4%

1225.89 30.3%

a

nuclear

hydroelectric

wind

biomass

geothermal

22.24

13.14

769.33

solar 0.76

2488.29

4.22

665.19

2.80

325.39 276.24

6.8%

N

5.33

140.82 103.87

3.5% 2.6%

N Na

0.00

35.73

0.9%

Y

8.04

10.41

0.3%

N

0.17

3.45 2.59

0.1% 0.1%

N Y

0.01

0.15 -4.95

0.0% −0.1%

N N

0.03

0.49

0.58

0.0%

4.33 0.1%

83.40 2.1%

4047.77 100.0%

100.0%

140.82 0.09

35.73 1.99

0.39 3.45 2.43

0.12

0.03 −4.95

769.33 19.0%

271.29 6.7%

140.82 3.5%

22.91 0.6%

15.56 0.4%

cooling required? Y Y

0.09

62.35

276.24

98.45

prime mover total 61.5% 24.4%

0.14 0.02

other

Some combustion turbine EGUs use a relatively small amount of water for turbine inlet cooling.

high.20 As ambient temperatures rise, air density is reduced. Since turbines are constant volume flow devices, less dense air translates into less mass flow through the turbine, reducing net power output.31,32 Thus, by cooling incoming air with water at the inlet of the turbine, the generation unit can maintain its efficiency during the hottest hours of the year. Inlet air-cooling is now incorporated into the majority of combustion turbine and combined cycle power plants in California.32 Independent of prime mover, natural gas EGUs tend to be more efficient at converting primary fuel into electricity than coal plants. Coal has a high moisture content and contaminants that decrease its combustion properties.33 Coal-fired EGUs also require more pollution controls, most notably SOX scrubbers, which require water and electricity for operation.27 For example, an average NGCC power plant with recirculating cooling consumes approximately 205 gallons per megawatt hour (gal MWh−1), whereas the most efficient integrated gasification combined cycle (IGCC) coal facility with a similar cooling system consumes more than 300 gal MWh−1.28 An IGCC EGU requires water for gasification of the coal, which offsets some of its energy conversion efficiency gains compared to a pulverized coal plant.23 Thus, an NGCC EGU generally achieves a higher thermal efficiency than IGCC and it uses less water for auxiliary processing. The replacement of coal-fired by natural gas-fired generation in the power sector is likely to continue based on economic, technical and regulatory factors, most notably through increases in economical shale and tight gas extraction that have resulted from advances in hydraulic fracturing and horizontal drilling.27,34−43 Between 2005 and 2012 U.S. natural gas production increased by one-third. (In 2005, shale and tight gas extraction represented 34% of natural gas production; in 2012, they

collectively represented 60% of U.S. production.) During that same time period, generation from natural gas in the electric power sector increased by 112%, while coal generation in the electric power sector decreased by 23%.44 Natural gas-fired power generation is also favorable to coalfired plants due to its attractive operational characteristics (i.e., predictable and dispatchable), flexibility (i.e., serving baseload or peak power loads), and comparatively low carbon emissions.14,30,36,45 Increased regulatory compliance costs and age also have also played a role in the retirement of coal plants.27 The Energy Information Administration (EIA) projects that natural gas for electric power will continue to increase in the upcoming decades.44 Replacing coal-fired with NGCC EGUs offers substantial water benefits.28,46 Although hydraulic fracturing (i.e., “fracking”) has higher water requirements (2−13 million gallons per well) than conventional oil and gas (1 million gallons per well),39,47 shifting from coal to natural gas-fired (from shale gas) power generation still offers significant water reductions from a life-cycle perspective. In Texas, replacing a kilowatt-hour of coal-fired generation with a kilowatt-hour of NGCC generation (from shale gas) reduces water consumption by about 60%.27 However, these water impacts might be shifted in space and time, so local regions where fracking occurs might have a net increase in water consumption since primary fuel production is generally spatially decoupled from electricity generation.48 (Hydraulic fracturing also poses water quality management challenges in terms of managing wastewater produced at the well.39,47,49) Nuclear power generation is the third most utilized form of power production (behind coal- and natural gas-fired generation), representing about 19% of total U.S. generation 53



Critical Review

(Table 2).29 The nuclear power generation technologies utilized within the U.S. include pressurized water reactors (65 power plants) and boiling water reactors (35 power plants), which use light water for cooling (i.e., H2O).50 Approximately 80% of the world’s nuclear power reactors are cooled by light water. Other coolants include heavy water (D2O), carbon dioxide, helium, sodium, sodium-potassium, lead, lead-bismuth, organic fluids, and molten salts; however, these alternative coolants are not utilized by U.S. power plants.51 The reliable cooling of nuclear power reactors is critical to maintaining the safe operation of the nuclear power plant. Unlike other types of power plants, heat continues to be released through the radioactive decay of fission products, even after the fission process is stopped after the shut-down of the plant. Thus, if the cooling system fails during shut-down, the residual heat production will overheat the core.51,52 Eventually, the core will begin to melt and create the potential for an explosion due to the hydrogen generated during melting and the release of fission products into the environment. The Three Mile Island (1979) and Fukushima (2011) nuclear power plant accidents were both provoked by the cessation of cooling water to reactor cores due to operator error and the failure of backup cooling systems, respectively.52 To date no U.S. plant license applications have been accepted that utilize dry-cooling systems and it is unlikely that large dry-cooled nuclear power plants will be built in the future, since reliable cooling systems are so critical to safe plant operation.25 However, several new small modular reactor (SMR) designs include dry cooling systems, suggesting that SMRs might offer more water-lean nuclear power in the future.25,53 There are several thermal renewable electricity technologies that require water for cooling including biomass, CSP and geothermal systems. Accordingly, the large scale expansion of CSP, which is most feasible in sunny, hot, and arid regions, might be restricted if dry cooling is not employed in water constrained regions.54,55 Nonthermolectric power plants that use fuel to generate electricity without the use of a steam-loop, typically have negligible water requirements. Hydroelectric power plants are the exception. However, reporting the water use impacts of hydropower is contentious, since natural evaporation (subject to various climatic conditions) is difficult to quantify, reservoirs often serve multiple purposes or are located on existing lakes, and temporal and spatial analysis boundaries are difficult to establish.56−59 Additionally, while the presence of a hydropower dam might increase evaporation as compared to the natural run on the river, it might increase the water supply through water storage.58 Estimates of water consumption by hydroelectric facilities in the literature range from 10.6 gallons to 55 200 gallons (i.e., 0.04−209 m3) per MWh.58 In addition to climate and regional variability, the ratio of reservoir area to gross static head (i.e., vertical height that water falls through turbine) is one indicator of evaporative losses. Generally, facilities that have large surface area to gross static head ratios have relatively large evaporative losses per unit of electricity generated.56 In addition to evaporative losses, water quality and ecosystem impacts are a concern for hydropower facilities.56 Wind turbines and solar photovoltaics (with the exception of cleaning) require no water for power generation.28,46,60 Consequently, increasing renewable energy generation has offered an attractive alternative to large centralized power facilities that require water for cooling, especially in water scarce regions.61,62 An analysis by Johnst and Rothstein (2014)

concludes that achieving the German target of 80% renewable energy penetration by 2050 (mainly wind and solar PV) will result in a 7% decrease in annual water consumption across the country.17 Cooling System Considerations. The vast majority of thermoelectric power facilities in the U.S. utilize either oncethrough cooled or recirculating cooling systems. Once-through cooled facilities (also referred to as open-loop) withdraw a large volume of water, use it once to condense steam exiting the turbine during power production, and discharge the warmed water back into the environment. Recirculating (i.e., “closedloop”) cooling systems, on the other hand, recycle cooling water continuously, typically through the use of cooling towers.1,2 Thus, recirculating systems typically withdraw much smaller volumes of water from reservoirs for cooling. However, their water use is not trivial as the majority of water withdrawn in recirculating cooling systems is lost to the environment via evaporation.5 Furthermore, since salts build up as hot water evaporates from the towers, water concentrated with salts (i.e., the “blowdown”) must be periodically discharged and replaced with fresh makeup water to reduce scaling.63 Nationally, EGUs with once-through cooling represented approximately 30% of water-cooled generating capacity across the U.S., yet represented 70% of the power sector’s 2010 withdrawals (which were 75% freshwater and 25% saline) and only 20% of its freshwater consumption. Recirculating pondand tower-cooled power plants represented approximately 50% of water-cooled generation in 2010 and 56% of power-related freshwater consumption, but only 2% of power-related water withdrawals. “Complex plants”, which include power plants with multiple cooling system types, generation technology types, or use multiple fuels, represented approximately 20% of 2010 water-cooled generation and 28% and 24% of powerrelated water withdrawals and freshwater consumption, respectively. Since these complex plants might couple oncethrough and recirculating cooled systems, the ratio of total withdrawals to total freshwater consumption across these facilities is closer to unity. These data reflect U.S. Geological Survey (USGS) heat budget estimates for power-related water use in 2010.4 Generally, shifting from once-through cooling to recirculating cooling can reduce vulnerability to water disruptions thereby increasing power reliability; however, the large-scale shift toward recirculating cooling could increase the net evaporation in a watershed thereby undermining water availability.1 While increasing water availability for oncethrough cooled power plants might improve power reliability, analysis by Stillwell and Webber (2013) suggests that increasing reservoir storage for power plants in water scarce regions such as Texas can undermine water availability for other users and is not generally beneficial for resiliency (i.e., how quickly the system recovers) or vulnerability (i.e., the severity of failure consequences).64 Also prudent to the discussion is the fact that physical water availability is not always reflected in the allocation of water rights in water constrained states. For example, the switching from once-through cooled EGUs toward recirculating EGUs has been shown to improve water reliability for junior water rights holders by reducing water diversions across watersheds in Texas.65 While less water would be diverted by power plants due to the proposed cooling technology shifts, more water might ultimately be consumed across Texas’ watersheds. 54



Critical Review

the efficiency of the system, but increases water consumption.74 Hybrid systems that combine wet and dry cooling can increase cooling efficiency, but these systems are expensive, reduce regulatory benefits, and suffer from the same shortcomings as dry cooling when water resources are not available.75 In a longterm scenario analysis of the arid Western U.S. through 2100, Ackerman and Fisher conclude that the extensive use of drycooling is not a likely to be a cost-effective mode of achieving water sustainability in the west because of its high economic costs in regards to other water conservation options.10 In addition to alternative cooling technologies, alternative cooling supplies are an option for decreasing freshwater use.61,75−78 Unconventional water supplies such as treated municipal wastewater effluent, brackish groundwater, industrial wastewater, mine water, and produced water from oil and gas operations can be used for cooling when sufficient volumes are remediated to sufficient quality.75,79 The Palo Verde nuclear power facility in Arizona, for example, uses reclaimed wastewater for cooling.61 While purchasing treated wastewater effluent is often cheaper than purchased freshwater per unit volume, the associated increases in condenser scaling, corrosion and fouling can lead to cleaning events that add to operational costs and decreased power plant efficiency (translating into higher fuel costs and lost revenues).75,77 (It should also be noted that many power plants pay little or nothing to withdraw freshwater from adjacent water sources for cooling.1) Reclaimed water quality standards vary by state and most states do not require effluent to be treated to a standard high enough for use in cooling systems.67,75,78,80 Typically additional chemical or membrane treatment after wastewater treatment is required to remove excess salts and solids from treated effluent to reduce fouling.67,75 Many plants that currently use alternative cooling supplies cite mineral scaling, stress cracking, excessive biological growth as negative impacts.67 Despite these tradeoffs, an analysis by Walker et al. (2013) concludes that utilizing treated municipal wastewater for use in recirculating cooling towers is a cheaper way to conserve freshwater than using dry-cooling systems.81 Furthermore, if the price differential between freshwater and treated municipal wastewater is more than $0.14 per kL, than it is economically advantageous to use recycled water over freshwater, even with the added costs of condenser fouling.81 A 2013 assessment of nontraditional water supplies for thermoelectric power generation concludes that unconventional water supplies are generally available, especially when brackish groundwater is included, and will likely be an important source of cooling water to thermoelectric power plant cooling in the future.76 An analysis by Tidwell et al. offers regional water availability and water cost for five categories of potential water sources for the power sector (i.e., unappropriated surface water, unappropriated groundwater, appropriated water, municipal water and brackish groundwater) across 1200 watersheds of the western U.S., which can serve to facilitate integrated planning decisions. 82 Wastewater reuse in the U.S. is growing approximately 15% per year marking increasing interest in this resource.83 Although the regulatory impact of the 2014 316(B) provision on existing generating facilities (discussed in detail below) in the short term is not yet clear, the net outcome will be a large reduction of once-through cooled facilities across the U.S. in the future. A large shift toward recirculating systems might decrease net water withdrawals (and the associated ecological

Shifts in cooling technologies have been occurring gradually over time due to increased attention to the ecological impacts of thermoelectric power plant cooling. Prior to the 1970s, oncethrough cooling was the pervasive cooling technology for cooling power plants in the U.S. However, with the implementation of provision 316(B) of the EPA’s Clean Water Act (CWA), there has been a shift toward recirculating cooling, which has contributed to a 3-fold reduction in the water withdrawn per unit of electricity generation as well as a reduction in ecological impacts from power plants.5 Oncethough cooled systems discharge large volumes of water back to the environment at temperatures higher than ambient water temperatures. Additionally, water intake systems that pump large volumes of water into an EGU can cause entrainment (the drawing in of fish and shellfish larvae and eggs into the cooling system) and impingement (the trapping of fish against intake screens).66 During times of low flow, generators with oncethrough cooled systems, might have to extend their intake pipes to get access to sufficient volumes of cooling water, which can exacerbate this ecological disruption.67 Recirculating systems generally withdraw water in lower volumes and flow rates and return negligible amounts of water back into the environment, decreasing environmental impacts. However, large visible evaporation plumes (i.e., water vapor) released from recirculating cooling towers are often a nuisance and can contribute to undesirable chemical drift deposition, especially in urban areas.68,69 (More discussion of the 316(B) is included in the Environmental Control Considerations section below.) In the U.S., the majority of once-through cooled generation units are located in eastern states where water is more abundant so increases in water consumption due to the shift to recirculating cooling is not likely to negatively impact water supply.10,70,71 An analysis by Stewart et al. (2013) concludes that the collective thermoelectric stress (e.g., habitat degradation, thermal pollution, evaporation losses) on watersheds in the northeast is driven by thermal pollution imposed by oncethrough cooling rather than the increased evaporation imposed by recirculating systems since they consume a very small percentage of summer flow.71 However, limited water availability is likely to constrain the new development of highly water consumptive power plants in water scarce regions.72 Dry air-cooling systems have a number of shortcomings that have reduced the large-scale deployment of the technology. (Dry-cooled systems only represent approximately 1% of thermoelectric generation in the U.S.73) They are generally more expensive, less efficient, and typically have a larger land footprint than once-through cooled facilities of comparable capacity.74 There are a few characteristics that impact a dry cooled facility’s efficiency. First, dry cooled facilities utilize fans to condense water, which require electricity for operation. Consequently, EGUs with dry cooling are generally less efficient since they have a net reduction in electricity output, compared to similar units with wet cooling.42 Second, a dry cooling system’s efficiency varies inversely with ambient air temperature, as higher temperatures result in increases in turbine backpressure. Typical plant efficiency losses are approximately 1% per 0.55 °C (1 °F) increase in the condenser, which can result in higher greenhouse gas emissions.2,42 On hot days, a power plant’s efficiency can be reduced 8−25%, but average approximately 2% losses per year compared to wetcooled systems.42 Consequently, dry cooled systems are also largely ineffective in hot, arid climates where they are needed most.67 Spraying water within a dry cooled tower can increase 55



Critical Review

recirculating cooling auxiliary system with water requirements comparable to a full power plant.97 Similarly, pollution controls to reduce SOX emissions in coal-fired power plants consume approximately 68 gal MWh−1, which can significantly increase a power plant’s net consumption.27 A large scale shift toward emissions free nuclear power could also increase water consumption in the power sector.10,12 In addition to air-quality and greenhouse gas considerations, the ecological impacts of power generation have received more attention in recent years. In particular, ensuring that enough water is left in natural freshwater or estuarine ecosystems to support native habitats has become increasingly important.99,100 The federal Endangered Species Act (ESA) requires that the endangered species be protected regardless of cost.101 Thus, it requires, among other provisions, that sufficient water be available to ecosystems to ensure ecological security.5,101−103 The ESA has a volatile history of provoking tension between development and environmental priorities, and will continue to impact water availability for competing uses moving forward, especially in the water scarce west, where it has already prompted water conflicts.5,102,103 The development of wind power, solar PV, hydropower, and associated transmission lines has been constrained by the ESA because of negative impacts to wildlife and ecosystems.104−108 Thus, the ESA will affect the build-out of water-lean and transmission-heavy generation in the future. One of the most sweeping environmental controls currently affecting the power sector is the CWA 316(B) provision, which regulates the cooling systems used for thermoelectric power plants. The 316(B) provision mandates that “the location, design, construction, and capacity of cooling water intake structures (CWIS) at regulated Facilities...reflect the best technology available (BTA)” to minimize environmental impact.109 The original 316(B) provision applied to intake structures at new power facilities. In 2004, an amended version extended the verbiage to those existing facilities withdrawing over 50 million gallons of water per day, but the rule was ultimately suspended by the U.S. second Circuit Court of Appeals in 2007. Since 2007, several new rules have been proposed but have remained controversial and highly contested since the definition of “adverse environmental impact” remains vague.66 However, in May 2014, a final 316(B) ruling states that existing power generation and industrial facilities designed to withdraw water in excess of 2 million gallons per day and use at least 25% of this water for cooling purposes would be required to implement one of seven compliance options to reduce environmental impact.109 The seven available compliance options include

impacts of water intakes and return), but would likely increase net evaporation (i.e., water consumption) across water-cooled power generation facilities in the power sector. However, since once-through cooled EGUs are more sensitive to low stream flows and high water temperatures, this shift will likely improve the power sector’s resilience to drought.21,84 Other improvements in efficiency, technological advancements in cooling technologies, and technology shifts will contribute to incremental tradeoffs in terms of water withdrawals and consumption that are difficult to anticipate.63 Environmental Control Considerations. There are growing tensions between new electricity generation development, greenhouse gas emissions and air pollution reduction goals, water scarcity, and environmental impacts.16,85 Nationally, U.S. thermoelectric power generation accounts for nearly 40% of total freshwater withdrawals, 3−4% of water consumption, 40% of annual carbon dioxide (CO2) equivalent emissions, 73% of sulfur oxides (SOX), and a significant proportion of nitrous oxide (NOx) emissions making the power sector an important target for water and emissions reductions.3,86,87 Pollution controls typically use auxiliary systems that require power to operate, introducing parasitic losses. In addition to reducing net output and efficiency, pollution controls often consume water during operation.23,88 While some emissions-lean generation technologies require negligible amounts of water for electricity production, other low-emissions technologies require large amounts of water that can constrain development in water-scarce areas.6,12,28,89 Likewise, some water-lean generation technologies can have negative emissions consequences. Strategies for achieving either water reductions or air emissions reductions in the power generation sector have been explored independently in the literature,1,65,84,90−92 but to a lesser-extent together.9,16,93 These strategies include technological mechanisms such as retrofits to existing technologies or the expansion of low carbon (i.e., renewables, nuclear, or carbon capture and sequestration) and/or low freshwater technologies (i.e., alternative cooling systems, nonfreshwater cooling supplies, or fuel-switching). Developing holistic strategies to reduce greenhouse gas emissions and air pollutants without exacerbating regional water scarcity or electricity reliability will be important to future development moving forward. In some cases, there might be synergistic reductions of both water and air emissions (e.g., wind, solar PV, shifts from coal to natural gas combined-cycle) due to technological shifts. In other cases a strategy to achieve a water reduction can have negative emissions consequences (e.g., dry-cooled fossil fueled plants), or a strategy to achieve an emissions reduction can increase the water requirements of the average generation fleet (e.g., nuclear power plants, concentrating solar power, SOX scrubbers, CCS, etc.).2,16 Policy mechanisms, such as emissions pricing or cap and trade schemes, can affect environmental impacts at the electric power grid level. For example, the implementation of strict carbon regulations might incentivize CCS expansion, which could have negative water consequences.8,94,95 CCS requires water for CO2 scrubbers and supplemental power generation to run the added parasitic load,6,9,23,96,97 which typically consumes 20−30% of a power plant’s total output.88 Cooling water requirements increase with the percentage CO2 capture.98 A pulverized coal plant equipped with current postcombustion CCS technology would double its water demand. Even if the plant was dry-cooled, the CCS system itself would require a

1. Achieving a withdrawal intake rate commensurate with closed cycle cooling 2. Designing through-screen velocities of less than 0.5 feet per second (fps) 3. Retrofitting existing offshore intake with a submerged velocity cap 4. Reducing actual through-screen velocity to less than 0.5 fps 5. Implementing an active screen with a modern fish handling and return system 6. Implementing a system of impingement mortality reduction measures that the discharge permitting agency deems BTA 56



Critical Review

Diablo Canyon nuclear power facility will comply with the regulation.116 In addition to the regulations discussed above, there are several new environmental provisions that will impact the U.S. generation mix in the short to medium-term. A 2011 report by the National Energy Regulatory Commission (NERC) anticipates that the collective impact of the 2014 CWA 316(B) provision, the final 2011 Mercury and Air Toxics Standards (MATS) for Utilities, the final 2011 CrossState Air Pollution Rule (CSAPR) and the 2010 proposed Coal Combustion Residuals (CCR) rule will require 234−258 GW of U.S. power generation capacity to be retrofitted by 2015 to remain compliant. Another 36−59 GW of capacity is anticipated to be retired or derated by 2018.34 Climate change legislation will also impact the future of the electricity-water nexus. The 2014 Clean Power Plan, which aims to cut carbon emissions at existing power plants, is anticipated to result in the retirement of 30−49 GW of existing capacity by 2020 (however, this capacity is not entirely independent of the capacity expected to retire due to the aforementioned environmental regulations).117 The tradeoffs between the water and carbon impacts of various technologies warrant strategic coordination of climate, water, and other environmental priorities.12,79,83,89,118 Chandel et al. (2011) forecast that freshwater withdrawals decline 2−14% in climate mitigation scenarios relative to a business-as-usual scenario with no climate policy in 2030.9 Generally, they find that water use decreases as carbon price increases, unless CCS is widely deployed. By 2030, the large-scale implementation of CCS could increase freshwater consumption in the power sector by 52−55% and withdrawals by 2−3%, nationally.9 Climate Considerations. Ironically, energy production impacts climate by increasing greenhouse gas emissions,43,79,119,120 yet climate change can negatively impact energy production,33,58,118,120−126 creating a positive feedback loop. Climate change is anticipated to increase air and water temperatures, reduce precipitation, increase sea level and cause more frequent and intense extreme events in many regions of the world, which will affect global electricity production and electricity demand.33,94,120,124,127−131 The largest increases in water temperatures are projected in the U.S., Europe, eastern China, and regions across Africa and Australia.132,133 Various power producers are likely to suffer efficiency reductions as a result of climate change due to lower stream flows and higher air and water temperatures. The efficiency of a power plant is largely dictated by the ambient temperature of its cooling reservoir, as warmer water is less effective in removing heat from power plants.134 Consequently, as cooling supplies increase in temperature, power plant efficiency will decrease, warranting more water per unit of electricity generated. Furthermore, as air and water temperatures rise above standard design criteria in thermoelectric power facilities, steam condensate temperatures and turbine backpressure will increase, thereby reducing power output.124,127,134 Some work has been done to assess the sensitivity of thermal power plant efficiency to changes in temperature and precipitation,32,127 but more research is needed to assess regional sensitivity. Combustion turbine natural gas facilities (including NGCC) are also subject to efficiency losses. As temperature rises, the air mass to volume intake ratio of the turbine decreases, since warm air is less dense, reducing turbine work output. Higher temperatures reduce the pressure ratio of the gas turbine, reducing mass flow through the turbine, and consequently

7. Meeting the average annual impingement mortality standard of no more than 24% It is difficult to estimate the net impact that the 2014 CWA 316(B) amendment will have nationally, as every EGU has flexibility in how it might address compliance. Plants that withdraw over 2 million gallons of water per day represent upward of one-third of the total operational power generation capacity in the U.S.110 In total, the EPA estimates that approximately 544 power plants and 521 industrial facilities are affected by the 2014 amendment.111 Before the final ruling, the National Energy Regulatory Commission projected that the 316(B) amendment would likely cause 25−39 GW of generation capacity to be vulnerable to retirement.34 The state of New York recently ruled that the Indian Point nuclear power facility must retrofit to cooling towers to reduce fish mortality or shut down during fish migration season, suggesting a changing precedent for existing once-through cooled power facilities.112 Despite its unclear federal regulatory future prior to 2014, the State of California’s State Water Board mandated that existing facilities comply with Section 316(B) of the federal CWA through the establishment of its technology-based Policy on the Use of Coastal and Estuarine Waters for Power Plant Cooling. The 2010 regulation applies to 19 coastal power plants that collectively withdraw over 15 billion gallons of seawater per day and represent approximately 17.8 GW of generation capacity.113 (The majority of the 19 affected OTC generation units are older natural gas steam boiler plants, with the exception of two nuclear facilities and a few modern NGCC power plants.) Since the announcement, the San Onofre nuclear facility (approximately 2.3 GW) has retired, and 5.0 GW of the remaining OTC plants are now in compliance with the 316(B) rule. The remaining 10.5 GW of capacity, which includes the Diablo Canyon nuclear facility, will have to retire or implement an acceptable compliance plan to remain operational within the required time frame proposed by the state.114 In California, the majority of the power plants that will remain operating will repower with dry-cooling systems due to limited freshwater supplies. (Retrofitting to recirculating systems that are able to use seawater is typically prohibitively expensive.) For several California EGUs, switching to dry cooling systems was not economically viable, prompting retirement.115 (Dry-cooled systems typically cost 3−4 times more than similarly sized wet-systems.42) Since dry-cooled systems reduce water use by 75−90% depending on the system and power plant,42 the retirement of once-through cooled coastal facilities could cause a small net increase in freshwater consumption (in the absence of reclaimed water usage).15,74 However, the state of California has strongly encouraged the use of recycled water sources for new thermoelectric power development to decrease pressure on freshwater resources. The Diablo Canyon nuclear facility currently uses oncethrough cooling and represents nearly once-third of total average annual water withdrawals by the once-through cooled power facilities in California. Although the California State Water Resources Control Board has considered whether the facility should be exempted from the once-through cooling phase out policy, it concluded in September 2014 that the environmental impacts have note been sufficiently addressed to warrant exemption. Thus, to date it remains unclear how the 57



Critical Review

varies by state but is typically 32 °C (89.6 °F).141 Thermal pollution can alter the chemical composition of an ecosystem by inducing eutrophication, a process that promotes excessive plant (namely algae) growth and decay. As plants and algae decompose, they deplete the oxygen content of the aquatic system, which can have devastating ecological affects.67 Between 2001 and 2005, approximately 57% of 403 oncethrough cooled EGUs that reported thermal discharge temperatures, reported at least one annual peak discharge temperature that exceeded the limit of 32 °C (89.6 °F) each year.141 The siting of several new power plants across the Northwest, Southeast, and Southwest have been restricted because of water concerns.72 In 2006, Idaho placed a moratorium on the construction of new thermoelectric facilities based on concerns regarding water availability and environmental impacts.5 These trends are likely to persist, especially in regions that are already water constrained. A 2013 analysis by Averyt et al. conclude that collective water demands already exceed water availability in 9% of 2103 U.S. watersheds examined.142 The Colorado River, which serves nearly 30 million people across seven states, will suffer runoff reductions of 10−30% by 2050.143 Insufficient water availability and/or high water temperatures in the U.S. has shut down or threatened the shut down of several coal and nuclear power plants in recent years, including the Browns Ferry nuclear facility (Athens, AL), the coal-fired Navajo Generating Station (Page, AZ), the Millstone Nuclear Power Station (Waterford, CT), the Joseph M. Farley Nuclear facility (Dothan, AL), and Turkey Point nuclear power plant (Miami-Dade County, FL).5,5,42,144,145 However, these issues are not isolated to nuclear and coal-fired power units. In its 2014 summer resource assessment, the California independent system operator (CalISO) predicted that up to 1150 MW of thermoelectric power in California might be shut down due to inadequate cooling water supplies.114 One of the most dramatic examples of power curtailment due to hot temperatures occurred in 2003, when a European heat wave spurred an aggregated loss of 5.3 Terawatt-hours in nuclear power generation as generators had to dial back to comply with thermal discharge limits.120 A few studies have offered systems-scale analyses of these water temperature impacts of regional power generation. Flores-Lòpez and Yates (2013) have developed a systemslevel, climate-driven model to anticipate increases in water temperatures due to thermoelectric power generation in the southeastern U.S. to improve system planning.146 Förster and Lilliestam (2009) conclude that anticipated decreases in stream flows and increases in reservoir temperatures due to climate change can significantly threaten power reliability.26 An analysis presented by Koch and Vögele (2013) simulates climate impacts (i.e., higher air and water temperatures, lower flows, increased evaporation, etc.) through the development of hydrological models to assess future disruptions to power plants.147 An analysis on 17 nuclear power plants in Germany reveals that curtailment to avoid thermal discharge limit exceedance causes power production disruptions at all power plants across all climate change scenarios assessed.148 Climate change will impact the future freshwater intensity of the grid by influencing new power sector build-out decisions (e.g., climate mitigation and drought management policies) and changing the water requirements of existing infrastructure (e.g., higher evaporation rates, lower cooling efficiencies). Thus,

power cycle efficiency. Furthermore, the specific volume of air fluctuates according to temperature, which affects the compression work (and consequently, the power required by the compressor). Higher temperatures result in higher specific volumes, which increase compressor work.124,135 Increasing temperatures and drought in many regions of the U.S. have already led to appreciable decreases in hydropower generation and these trends are likely to persist. In California, hydropower has served as much as 28% of all the state’s needs in 1995 (a wet year) and as little as 11% in 1992 (a dry year).136 California’s hydropower forecast in 2014 suggests that generation will average 50% of normal due to severe drought conditions.137 The Pacific Northwest is particularly vulnerable to future drought due to its large reliance on hydropower and a relatively small ratio of storage to flow, as compared to other regions.125 Results suggest that the region might lose 18−22% of total generation depending on the severity of the drought (based on historical drought simulations).137 In the Colorado River Basin, historical precipitation and streamflow data confirm similar trends. For every 1% decrease in precipitation across the Basin, streamflow is reduced by 2−3%; for every 1% decrease in streamflow, hydropower generation drops 3%.138 Impacts will are not isolated to thermoelectric and hydropower generation technologies. Increasing temperatures also reduce the efficiency of solar PV. An increase of 1 °C (1.8 °F) reduces thermoelectric and solar PV output by 0.3−0.7% and 0.65%, respectively.127 Increased temperatures also affect electricity transmission and distribution (T&D) infrastructure by aging transformers, decreasing the carrying capacity of T&D infrastructure and substations, and decreasing overall reliability.121,124 Increased wild fire frequency will also impact electricity infrastructure such as critical high-voltage transmission lines.31,114 Thus, as T&D infrastructure becomes more vulnerable, increased demand will add stress to the system. Power demand is also projected to increase as temperatures rise. It is estimated that the energy requirements for cooling rise 5−20% for every 1 °C (1.8 °F) in warming, while space heating energy use drops by 3−15%.138,139 In addition to increasing water-scarcity in some regions, increasing precipitation and sea-level rise will also present challenges to the power sector in the future. In the event of the projected 1.4 m sea level rise, for example, 25−30 coastal power plants in California are at risk of inundation from a 100-year flood event,31,140 as well as many electricity substations and one natural gas storage facility.31 While the focus of this manuscript is largely devoted to analyzing the power-water nexus within the U.S., these trends will affect many regions around the world. One of the regions expected to be most devastated by climate change, for example, is Bangladesh. Bangladesh’s entire power generation fleet is projected to face either total inundation and salinity from flooding in coastal regions or severe drought in the northwest of the country.123 In the absence of climate change, the grid is already under thermal stress. The combination of rising cooling water temperatures and decreased cooling water supplies is affecting power generation across the U.S. Section 316(A) of the CWA requires the states to set thermal effluent discharge limits to protect ecosystems. In order to obtain a permit to discharge cooling water via the National Pollution Discharge Elimination System, a power plant must comply with the state’s thermal limit. A power plant is typically required to curtail operations if surface water is heated above the thermal threshold, which 58


timeframe

region

electricity projection model

global WECC

U.S U.S. U.S.

U.S. ERCOT U.K.

2005−2095 2008−2100

2010−2050 2011−2050 2012−2030

2050 2050 2050

Global demand forecasted according to scenario Demand driven model to anticipate trade-offs among future power deployment ReEDS ReEDS and Gridview NEMS modified to include CCS

59

2070−2099 2031−2060 2012−2050 2010−2040

California Europe U.S. eastern U.S

Existing fleet analyzed Existing fleet analyzed EPA-IPM GCEM

NA NA

from literature calculated via a linear model of current water use and fleet characteristics from literature from literature

climate change model

climate scenarios based on literature geo-spatial ArcGIS models built based on IPCC climate change projections AOGCM GCM based on IPCC emission scenarios MIT-IGSM climate scenarios based on literature

NA NA NA NA NA NA

WEAP WEAP GCM based on IPCC emission scenarios

NA NA

from literature from literature from literature from literature from literature

NA NA NA

from literature from literature plant specific water use factors calculated from historical data assigned via WiCTS from literature from literature

GCAM temperature increase projections based on IPCC to anticipate demand growth NA NA NA

NA

from literature

EGU water use rates

124 132 94 127

123 120

167 13,168 11

74 73

15,166 16 8

12,14 165 9

164 10

19 89

163

study

AOGCM: Atmospheric-Ocean General Circulation Models - developed by the North American Regional Climate Change Assessment Program (NARCCAP) to simulate global climate conditions based on IPCC projections. EPA-IPM: US EPA Clean Air Markets Division’s Base Case Integrated Planning Model − a power dispatch and capacity planning model that incorporates climate considerations. ERCOT: Electric Reliability Council of Texas. GCAM: Global Climate Assessment Model − integrated model developed by Pacific Northwest National Laboratory that simulates climate scenarios based on emissions scenarios. GCEM:Generation capacity expansion model − general class of optimization models that utilize a mixed integer linear program to minimize the cost of future investments to predict probable build-out. GCM: General circulation model − general classification of climate model used by132 to simulate daily river flows and temperature produced via a physically based hydrological water temperature modeling framework. Gridview − commercially available unit commitment and dispatch simulation software. IPCC: International Panel on Climate Change − provides emissions, temperature, and precipitation forecasts that are utilized in climate models. MIT-IGSM: Integrated Global System Model − developed by Massachusetts Institute of Technology to estimate global environmental change and costs projections from anthropogenic causes. NEMS: National Energy Modeling System - model developed by the US Energy Information Administration for its Annual Energy Outlook that assumes “current laws and regulations remain generally unchanged throughout the projection period”. ReEDS: Regional Energy Deployment System − linear cost optimization model developed by the National Renewable Energy Laboratory used to forecast electricity generation and capacity build-out. WEAP: Water Evaluation and Planning − model developed by the Stockholm Environment Institute used to analyze future drought and climate change scenarios. WECC: Western Electric Coordinating Council. WiCTS: Withdrawal and Consumption for Thermoelectric Systems − empirically based model developed in GAMS programming language by the MIT Joint Program on the Science and Policy of Global Change that calculates water use rates at the electric generation unit level based on historical data to achieve regional specificity.

electricity costs future fleet development

ReEDS GCEM Electricity generation pathways assumed by authors based on UK Climate Change policy 2005−2020 California Technology deployment scenarios designed by authors other environmental 2006−2050 U.S. Modified EIA AEO 2006 reference case to assess various regulations cooling technology build-outs climate change 2010−2050 southeastern U.S. ReEDS 2008−2050 southwestern U.S. ReEDS 2006−2050 U.S. EIA AEO 2009 reference case Studies That Anticipate the Impact of Climate Change on the Grid physical grid 2100 Bangladesh Demand projections/growth rates from the literature infrastructure 2008−2108 France and U.S. Existing fleet analyzed

climate change mitigation

Studies That Quantify the Water Required for the Power Sector under Various Futures business as usual 2004−2025 U.S. EIA AEO 2004 reference case growth scenario, with additional scenarios to reflect uncertainties. 2005−2030 U.S. EIA AEO 2007 reference case growth scenario 2005−2030 Spain Spanish Electrical Industry Association reference case

description

Table 3. Future Assessments of the Electricity-Water Nexus Vary Considerably According to Temporal and Spatial Boundaries, Modeling Frameworks, and Scenario Definitions

Environmental Science & Technology Critical Review



Critical Review

descriptions for recent projections of the water intensity of the power grid across various regions and time periods. These analyses include a range of assumptions including business-asusual growth scenarios,19,89,163 high renewable penetration and/or climate mitigation policies,8−10,12,14−16,164−166 other environmental policies,73,74 and climate change scenarios.11,13,167,168 While most studies included are focused in the U.S., a few non-U.S. examples are presented to highlight a variety of methodologies. While climate change mitigation (i.e., reducing emissions) is the focus of many electric power studies,10,79,119,164,169,170 not enough attention has been devoted to adaptation (i.e., adjustment to climatic changes),33,55,75,90,118,171−174 and often adapation-mitigation strategies conflict.58,120 There is a growing body of literature projecting the impact that climate change will have on the grid through impacts to physical grid infrastructure due to sea-level rise and extreme events,31,123,140 increased electricity demand,10 future electricity costs,132 and future power capacity build-out.94,127 While these analyses do not address future water for the power sector directly, their conclusions are extremely pertinent to consider in future assessments, as they anticipate future supply, demand, and climatic characteristics. The twenty-first century has marked the onset of a transformative period for the U.S. electric power grid. In the past 10 years increasing environmental priorities, technical innovation, modern computing, and large-scale data management have converged to create a precedent for change that will transform the grid in ways that are difficult to anticipate. Here, I summarize five categories of changes that are likely to impact the freshwater requirements of the grid moving forward, with focused attention on the US. However, these five factors will provide guidance for future research related to the electricitywater nexus across other regions of the world. Changing Fuel Preferences in the U.S. Decreases in Coal Generation. The confluence of economical shale gas production, increasing compliance costs due to environmental and climate regulations (e.g., CWA 316(B), CASPR, MATS, CCR, and the 2014 Clean Power plan), and an aging coal-fired generation fleet have created a regulatory environment that will stifle growth in the coal industry without major technological advancements in pollution controls. CCS, while uneconomical at this point in time, might offer an option for future coal capacity in the future. The large scale deployment of current CCS technologies could be water-intensive. Increases in Natural Gas Production. Hydraulic fracturing and horizontal drilling have contributed to the increased production of shale gas in the United States, relieving price volatility and high costs that historically constrained gas expansion. Natural gas also offers environmental benefits over coal, making it an attractive replacement. Increases in Renewable Energy Generation. New carbon regulations will continue to drive renewable energy expansion. Future improvements in storage and other smart grid technologies, in conjunction with efficiency and cost improvements will allow much higher penetrations of intermittent renewable energy. While most renewable energy is water-lean, a few forms (e.g., CSP and geothermal) can be water-intensive. Unclear Nuclear Regulatory Environment. While nuclear power is attractive because it provides stable, dispatchable baseload power with zero emissions, its public acceptance is mixed due to a handful of nuclear accidents, the most recent being Fukushima Daiichi in 2011.52 A high nuclear power

there are many nonlinear relationships that dictate how much water will be required across the U.S. grid in a changing climate. Grid-Scale Considerations. In addition to regulatory and environmental changes impacting power generation technology decisions, the electric grid itself is changing. These changes will impact its future water requirements. Today the grid is inefficient; 20% of electric generation capacity exists to meet peak demand occurring 5% of the year, and approximately 8% of electricity is lost during transmission.149 Wasted energy translates to wasted water in today’s grid regime. Moving forward, the onset of the “smart grid” will enable the integration of a variety of generation options (e.g., centralized, distributed, intermittent, and mobile) and optimization platforms (e.g., demand management, real time pricing, and storage), as well as a two-way flow of information and electrons between producers, consumers, and devices to enable electricity users and providers to interact in new ways with the grid. Selfhealing capabilities will also reduce energy losses, disruptions, and other inefficiencies.149−151 These shifts toward a more adaptive and interconnected grid infrastructure will require substantial investment, technological innovation and regulatory changes. Increased network connectivity and situational awareness raises cyber and physical security concerns that will require innovation to address new vulnerabilities and threats.152 Due to the complexity of these grid-scale transformations, anticipating the time horizon and scale of these changes is beyond the scope of this review. However, these changes will fundamentally change the characteristics of the grid in ways that will impact water resources. For example, the large-scale electrification of the transportation sector could change power demand, and potentially the prospect of storage if vehicles are used to provide a two-way flow of power.153−155 Depending on the water intensity of the grid, this shift in electricity in the transportation sector might have positive or negative water consequences compared to baseline transportation fuels. (See King and Webber, Scown et al., and Harto et al. for discussions regarding the water-intensity of transportation sector fuels.156−158) More generally, intermittent (and water-lean) renewable energy sources such as solar PV and wind currently raise grid stability concerns; however, integrating storage into the grid can potentially relieve these issues allowing high penetrations of distributed renewables in the future and displace the need for large baseload power plants that tend to be most water-intensive.62,159−161

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FUTURE SHIFTS IN THE POWER-WATER NEXUS Multibasin, multisector, and grid-scale models have facilitated systems-level assessments of the electricity-water nexus across a wide range of future scenarios. While many analysts converge on the opinion that U.S. withdrawals will continue to decline as plants with once-through cooled systems retire, future estimates of water consumption vary considerably in the literature.162 Most analysts agree that the large-scale integration of CCS or nuclear would increase water consumption (and possibly water withdrawals) depending on the scale of build-out and the cooling technologies employed.12,16,162 However, the results of each study is subject to its assumptions regarding future fuelmix, cooling and generation technologies, efficiency improvements, regulatory scenarios, and climatic conditions. Since all studies are subject future uncertainty, developing an understanding of assumptions and modeling techniques is critical to comparing and interpreting the results of multiple studies. Table 3 highlights the overall scope and model 60



Critical Review

future could increase water use significantly.8,16 New SMR designs suggest that these smaller units might be able to offer water-lean nuclear power in the future. Changing EGU Cooling Practices in the U.S. Decreases in Once-through Cooling Systems. The 2014 CWA 316(B) amendment for existing generation facilities is anticipated to have large impacts on the U.S. power generation fleet. Whether existing facilities choose to modify their operations, repower, retrofit, or retire will largely be a function of economics. However, the long-term trend in cooling technologies will continue to trend away from water withdrawal intensive oncethrough cooled systems. A large increase in recirculating cooling could increase water consumption across the thermoelectric power sector significantly. Increases in Dry Cooling. Water scarcity (and the phasing out of coastal once-through cooled power plants in California) have resulted in the expansion of dry-cooling, despite its efficiency and cost penalties. This trend is likely to continue as climate change and water constraints hinder the expansion of water consumptive technologies in dry regions. Increases in the Use of Alternative Cooling Supplies. Despite scaling and corrosion issues, alternative water supplies for cooling are becoming increasingly popular for power plant cooling. Advancements in cooling technologies that can tolerate water with higher levels of solids, in conjunction with increasing potable water costs, will continue to increase the economic viability of alternative cooling supplies. Changing Environmental Regulations. Increases in Greenhouse Gas Reduction Regulations. Climate change has prompted concern regarding the CO2 intensity of electricity generated with fossil fuels. The decarbonizing of the grid, through policy and technology levers, will modify its water requirements. Increases in Other Pollution Controls. Auxiliary systems aimed to reduce greenhouse gas emissions, air pollutants, and other forms of waste typically reduce EGU efficiency and might require more water per unit of electricity generated compared to similar systems without such systems. SOX scrubbers and CCS systems are examples of pollution controls that can increase water requirements. Increased Water Competition due to the Endangered Species Act. The ESA has provided a significant barrier to both conventional power plants, as well as renewable energy projects that tend to be land intensive and require the siting of large transmission lines. As water becomes more scarce, the ESA will likely become an increasing constraint on future energy development. Changing Climate. Increases in Climate-related Disruptions. Warmer water temperatures will become an increasing burden for existing once-through cooled facilities to remain in compliance with the thermal discharge limits defined in CWA 316(A). Increasing storm frequency and strength will also increase the frequency of disruptions. Increases in Evaporative Water Losses. Rising air and water temperatures will increase the evaporative losses of water during some forms of power generation, most notably for hydropower and recirculating cooled thermoelectric power generators. Decreases in Existing EGU Efficiencies. As ambient air and water temperatures increase, thermoelectric power plant efficiency will decrease, requiring more water for cooling. This trend will be especially pronounced as power plants that

currently use cool seawater for cooling transition to other cooling systems to comply with CWA 316(B). Decreases in Hydropower Generation. Declining streamflows will continue to decrease hydropower generation in water-scarce regions of the U.S. as temperatures rise and droughts become more frequent. Changing Electric Power Grid. •. Increasing Electrification. There is a lot of uncertainty regarding the growth trajectory of power usage in the U.S. Changes in demand, such as increasing the electrification of the transportation sector or increasing desalination,175,176 might also impact the water requirements of the grid. However, these impacts are difficult to assess. The large scale deployment of electric cars, for example, would increase the electricity demand of transport, but it might also allow for larger penetrations of intermittent renewable energy, which might offset water impacts.177 •. Increasing Electricity Storage. Increasing the amount of electric storage on the grid will be important for increasing the penetration of intermittent (and water-lean) renewable energy sources on onto the grid without compromising stability. Expanding the “Smart Grid”. Distributed energy management and demand management strategies will decrease reliance on large centralized, baseload power plants in the future, likely relieving some water-intensive generation. Building microgrid communities will decrease vulnerability to disruptions and losses, increasing the overall efficiency of power generation and distribution.

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MOVING FORWARD In addition to developing a better understanding of the changes in the future U.S. power sector, more research is needed to • improve the collection and reporting of energy and water data, • integrate energy and water management and planning decisions, and • develop innovative policy frameworks to incentivize synergistic energy and water conservation strategies. Current methods of data collection and reporting are imprecise and often fraught with omissions, errors, and uncertainty that make it difficult to track water use in the thermoelectric power sector.60,178 However, both EIA and USGS have been working to improve data collection and reporting of energy and water-related data, so newer data sets have markedly improved over time.179 Despite some advancements, the water requirements of new energy development, even in the arid west, are often overlooked due to the fragmentation or complete absence of water regulators and stakeholders in energy decisions.61,83,180 Integrating dual resource planning and/or coupling the provision of water and power services might increase resource efficiency, reduce vulnerability to disruptions and/or reduce costs. Coordinating energy and water planning has been cited as critically important to both energy and water security by multiple governmental organizations in recent years.121,178,182−185 The development of innovative policy interventions could placate some of the tension between energy and water resources. For example, changes to electric power unit commitment and dispatch regimes172 or water pricing strategies1 have been proposed to decrease water competition and ecosystem impacts from power plant cooling water demand. Although several bills were proposed to integrate energy and water planning and decision-making in 61



Critical Review

previous Congresses,186−188 all eventually failed to pass. The latest bill, S.1971: Nexus of Energy and Water for Sustainability Act of 2014 (113th Congress), was introduced by Senator Lisa Murkowski (Republican-Alaska) but has not been voted on as of publication of this manuscript.189 While these bills acknowledge growing awareness regarding the electricitywater nexus, firmer commitments will be necessary to reduce the tension between energy and water resources moving forward.

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(16) Webster, M.; Donohoo, P.; Palmintier, B. Water-CO2 tradeoffs in electricity generation planning. Nat. Clim. Change 2013, 3, 1029− 1032. (17) Johst, M.; Rothstein, B. Reduction of cooling water consumption due to photovoltaic and wind electricity feed-in. Renewable Sustainable Energy Rev. 2014, 35, 311−317. (18) Carter, N. T. Energys Water Demand: Trends, Vulnerabilities, and Management; Congressional Research Service, 2010; p 61. (19) Elcock, D. Future U.S. water consumption: The role of energy production. J. Am. Water Resour. Assoc. 2010, 46, 447−460. (20) Scanlon, B. R.; Reedy, R. C.; Duncan, I.; Mullican, W. F.; Young, M. Controls on water use for thermoelectric generation: case study Texas, US. Environ. Sci. Technol. 2013, 47, 11326−34. (21) Stillwell, A. S.; King, C. W.; Webber, M. E.; Duncan, I. J.; Hardberger, A. The energy-water nexus in Texas. Ecol. So. 2011, 16, 1−20. (22) Strohl, G. R. Thermoelectric Power Generator. 2014. http:// www.britannica.com/EBchecked/topic/591615/thermoelectricpower-generator. (23) Gerdes, K.; Nichols, C. Water Requirements for Existing and Emerging Thermoelectric Plant Technologies; U.S. Department of Energy, 2009; 2008, p 26 (24) EPRI. Water & Sustainability (Volume 3): U.S. Water Consumption for Power ProductionThe Next Half Century; Electric Power Research Institute, 2002; Vol. 3. (25) Cooling Power Plants; World Nuclear Association, 2014. (26) Forster, H.; Lilliestam, J. Modeling thermoelectric power generation in view of climate change. Regional Environ. Change 2009, 10, 327−338. (27) Grubert, E. A.; Beach, F. C.; Webber, M. E. Can switching fuels save water? A life cycle quantification of freshwater consumption for Texas coal- and natural gas-fired electricity. Environ. Res. Lett. 2012, 7, 045801. (28) Macknick, J.; Newmark, R.; Heath, G.; Hallett, K. C. Operational water consumption and withdrawal factors for electricity generating technologies: A review of existing literature. Environ. Res. Lett. 2012, 7, 045802. (29) U.S. Energy Information Administration, 2012 Form EIA-923 Data; 2014. (30) Eichman, J. D.; Mueller, F.; Tarroja, B.; Schell, L. S.; Samuelsen, S. Exploration of the integration of renewable resources into California’s electric system using the Holistic Grid Resource Integration and Deployment (HiGRID) tool. Energy 2013, 50, 353− 363. (31) Sathaye, J. A.; Dale, L. L.; Larsen, P. H.; Fitts, G. A.; Koy, K.; Lewis, S. M.; de Lun-cena, A. F. P. Rising temps, tides, and wildfires. IEEE Power Energy Mag. 2013, 33−45. (32) Maulbetsch, J. S.; Consultant, M. N. D.; Maulbetsch, J. S.; Haganm J. O. Cost and Value of Water Use at Combined-Cycle Power Plants; California Energy Commission, 2006; p 134 (33) Sieber, J. Impacts of, and adaptation options to, extreme weather events and climate change concerning thermal power plants. Clim. Change 2013, 121, 55−66. (34) Potential Impacts of Future Environmental Regulations; North American Electric Reliability Corporation, 2011; p 62. (35) Venkatesh, A.; Jaramillo, P.; Griffin, W. M.; Matthews, H. S. Implications of changing natural gas prices in the United States electricity sector for SO2, NOX and life cycle GHG emissions. Environ. Res. Lett. 2012, 7, 034018. (36) Lu, X.; Salovaara, J.; McElroy, M. B. Implications of the recent reductions in natural gas prices for emissions of CO2 from the U.S. power sector. Environ. Sci. Technol. 2012, 46, 3014−21. (37) Chen, J.; Al-Wadei, M. H.; Kennedy, R. C. M.; Terry, P. D. Hydraulic fracturing: Paving the way for a sustainable future? J. Environ. Public Health 2014, 2014, 656824. (38) Hughes, J. D. Energy: A reality check on the shale revolution. Nature 2013, 494, 307−8. (39) Vengosh, A.; Jackson, R. B.; Warner, N.; Darrah, T. H.; Kondash, A. A critical review of the risks to water resources from

AUTHOR INFORMATION

Corresponding Author

*Phone: (717) 329-5392; fax: (213) 744-1426; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Critical Review: Uncharted Waters? The Future of the Electricity-Water

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