Influences of Greenhouse Gas Emission Reduction Options on Water ...

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Sustainability and Energy Development: Influences of Greenhouse Gas Emission Reduction Options on Water Use in Energy Production D. Craig Cooper*,† †

Earth & Water Resource Sciences, Idaho National Laboratory, P.O. Box 1625, MS 2107, Idaho Falls, Idaho 83415-2107, United States

Gerald Sehlke‡ ‡

Earth & Water Resource Sciences, Idaho National Laboratory, P.O. Box 1625, MS 2213, Idaho Falls, Idaho 83415-2213, United States S Supporting Information *

ABSTRACT: Climate change mitigation strategies cannot be evaluated solely in terms of energy cost and greenhouse gas (GHG) mitigation potential. Maintaining GHGs at a “safe” level will require fundamental change in the way we approach energy production, and a number of environmental, economic, and societal factors will come into play. Water is an essential component of energy production, and water resource constraints will limit our options for meeting society’s growing demand for energy while also reducing GHG emissions. This study evaluates these potential constraints from a global perspective by revisiting the climate wedges proposal of Pacala and Socolow (Science 2004, 305 (5686), 968−972) and evaluating the potential water-use impacts of the wedges associated with energy production. GHG mitigation options that improve energy conversion or use efficiency can simultaneously reduce GHG emissions, lower energy costs, and reduce energy impacts on water resources. Other GHG mitigation options (e.g., carbon capture and sequestration, traditional nuclear, and biofuels from dedicated energy crops) increase water requirements for energy. Achieving energy sustainability requires deployment of alternatives that can reduce GHG emissions, water resource impacts, and energy costs.

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INTRODUCTION The United Nations Framework for Convention on Climate Change states that GHG emissions must be reduced to a level sufficient to prevent “dangerous anthropogenic interference with the climate systems”.2 The exact meaning of this phrase is still a subject of debate. The scientific community recommends that “dangerous climate change” would likely occur if global average temperature exceeds a maximum 2 °C/3.5 °F increase as compared to preindustrial levels.3 To avoid dangerous change, the global average for GHG concentrations should remain below a target range of 400−450 ppm (ppm) CO2 equivalents (CO2-eq), with 500 ppm as an absolute maximum.3 However, the IPCC clearly states that dangerous climate change cannot be defined by science alone.4 Policymakers must make value judgments that include scientific findings on the impacts of climate change on ecosystems and human societies as just one of multiple considerations. As such, mitigating climate change involves more than just reducing GHG emissions. It is also a journey toward greater sustainability of human practices. Pursuing sustainability requires consideration of additional dimensions. If mitigation efforts become too focused on GHG emission reductions at the expense of other considerations, then such efforts run the risk of creating new problems that © 2012 American Chemical Society

may have a greater negative impact on sustainability. Consider the Brundtland Commission’s definition of sustainability, which includes two key concepts: (i) meeting the needs of the world’s poor and (ii) that the combination of technology and social organization place limits on the environment’s ability to meet present and future needs.5 One of the greatest needs of the world’s poor is water for food production and domestic use.6,7 Furthermore, water resources place an important limit on our ability to harvest primary energy resources and convert them into secondary energy services.8−11 Climate change is expected to alter the availability of water resources for ecosystems and human use, with the resulting impacts expected to be greatest in the developing world.4 When climate impacts are considered in context of increased strain on water resources from population growth and economic development,12,13 it becomes vital that we focus global clean energy development efforts toward strategies that both reduce GHG emissions and systemic energy impacts on regional water resources. Received: Revised: Accepted: Published: 3509

June 3, 2011 January 17, 2012 January 25, 2012 January 25, 2012 dx.doi.org/10.1021/es201901p | Environ. Sci. Technol. 2012, 46, 3509−3518

Environmental Science & Technology

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Pacala and Socolow’s climate wedges are presented in terms of GHG emissions rather than energy consumption. The water requirements of the energy generation wedges can only be calculated from the energy change associated with the proposed transition. The first step in estimating the water impact for each wedge is to calculate the energy change associated with each mitigation option. A summary of the energy transitions associated with Pacala and Socolow’s climate wedges is provided in Table 1. These calculations are approximate, as a number of factors need to be assumed when converting these GHG emissions wedges to energy transition wedges. All calculations assume that the specified transitions for the BAU and climate wedge cases occur over 50 years and that the total change for each case is defined by the area of a right-triangle with a base equal to the time period of change and a height equal to the magnitude of the annual consumption of primary energy. The difference in primary energy consumption between each wedge and its corollary BAU scenario is calculated as the difference in the area of these two right triangles. Additional assumptions associated with these calculations are summarized in the Supporting Information. One key observation from the results presented in Table 1 is that “all climate wedges are not created equal”. The size of the energy shift required to reduce GHG emissions reductions by 25 GtC from BAU varies significantly based on which energy technology wedge is selected. In some cases, such as with carbon capture and sequestration, measures to reduce GHG emissions increase the consumption of primary energy resources. In other cases, such as with energy efficiency, measures to reduce GHG emissions decrease net energy consumption. These trends hold even if alternate assumptions are used to calculate the energy transitions in Table 1. For example, capturing CO2 at coal-fired power plants will introduce a parasitic load that increases primary resource usage whether the plant operates at 50% thermal efficiency assumed by Pacala and Socolow or the approximately 32% thermal efficiency that is the current fleet-wide average of U.S. coal plants.33 Current nuclear technology requires more water than coal, though water consumption could be reduced by deployment of high temperature nuclear reactors that could reach higher thermal efficiencies.34−37

The need to address the water resource impacts of climate change is broadly acknowledged,4,12,14 and the water resource impacts of biofuels production and thermoelectric generation are coming under closer scrutiny with regards to both quality and quantity issues.9,11,15−26 However, despite the acknowledged need to consider the water impacts of climate change and energy production, few of the studies that contrast options for GHG emissions reductions include an analysis of the impact on water resources. Jacobsen27 has ranked potential global warming solutions for the transportation sector according to eleven different criteria (including water supply) and has generated four different tiers based on multiple impacts to environmental quality, energy security, and human health and finds that electricity and hydrogen from wind and solar resources provide the most secure and sustainable options for providing transportation services under these constraints. Ethanol, electricity from nuclear, hydroelectricity, and coal with CCS score significantly lower than these renewable options. Outside of Jacobsen’s work, we are not aware of other studies that explicitly investigate the impact of climate change mitigation options on water resources. This study addresses that gap by revisiting the influential climate wedges paper of Pacala and Socolow1 and identifying which of the wedges associated with energy production also result in a net reduction in water demand. This study focuses on evaluating the impacts of the climate wedges on consumptive water requirements and only assesses the climate wedges associated with energy production and use. Only water requirements associated with the extraction and conversion of primary energy resources into secondary energy services are considered. The consumptive water requirements for biofuel crops are included as a source of water demand, regardless of whether that water is provided to the plants via irrigation or natural precipitation. Impacts associated with the manufacture of energy infrastructure are specifically excluded. The water impact of global energy deployment in the coming decades is estimated based on the performance of current technologies by (i) estimating the energy changes associated with each wedge, (ii) determining the consumptive water requirements per unit of primary energy consumed, and (iii) estimating the changes in consumptive water use for each wedge. The potential impacts of emerging technologies on water requirements for energy production are subsequently discussed relative to the performance of current technology.

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WATER REQUIREMENTS FOR ENERGY PRODUCTION Energy production requires water during three general stages: (i) manufacture and deployment of infrastructure, (ii) energy resource extraction, and (iii) conversion of primary energy resources into secondary energy services. During all stages, water use is either consumptive or nonconsumptive. Nonconsumptive uses are in-stream uses and withdrawals that are returned to the same source water or hydrologic basin. Consumptive uses are either not discharged back to the same source water or water is removed from the basin − either as water vapor, a trans-basin diversion, or as an energy product. Water withdrawals include both consumptive and nonconsumptive use; and withdrawals often provide the primary basis for regulation of water use. However, consumptive use is more impactful in water-constrained basins because it removes water from the basin. Consequently, these calculations only evaluate water consumption. Limited water withdrawal information is provided for reference purposes. Consumptive water requirements for the extraction and conversion of primary energy resources into electricity, with the associated values for water withdrawals using current technologies,

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ENERGY CHANGE ASSOCIATED WITH CLIMATE WEDGES Pacala and Socolow1 describe a set of GHG emissions reduction wedges, each of which would result in a cumulative emissions reduction of 25 gigatons of carbon (GtC) over 50 years, relative to a “business as usual” (BAU) scenario. Pacala and Socolow report that 7−10 wedges would be needed to stabilize atmospheric GHG concentrations below 500 ppm CO2-eq relative to a BAU scenario where GHG emissions continue to grow at historic rates. These climate wedges provide a firstorder analysis of options for reducing GHG emissions. A number of more detailed assessments have been made for the energy options, using a variety of econometric energy systems models.28−32 However, the climate wedges approach of Pacala and Socolow1 is easier to replicate and provides a more broadly accessible method for assessing the potential water impacts of GHG mitigation options. Their method is adopted here. 3510

dx.doi.org/10.1021/es201901p | Environ. Sci. Technol. 2012, 46, 3509−3518

cut carbon emissions associated with buildings by 25% by 2050

produce twice todays coal output at 60% efficiency instead of 40% build 1,400 GW of natural gas instead of 50% efficient coal

efficient buildings

efficicent baseload coal plants gas baseload power for coal baseload power capture CO2 at baseload power plant

3511

wind H2 in fuel-cell car for gasoline in hybrid car biomass fuel for fossil fuel

PV power for coal power

capture CO2 at coalto-synfuels plant conventional nuclear power for coal power wind power for coal power

capture CO2 at H2 plant

decrease VMT for 2 billion cars from 10,000 to 5,000 miles/yr

reduced use of vehicles

add 100 times the current Brazil or U.S. ethanol production

add 2 million 1-MW-peak windmills; balance with energy storage add 2 million 1-MW-peak windmills; balance with storage and natural gas add 2000 GW peak solar; balance with energy storage add 2000 GW peak solar; balance with storage and natural gas add 4 million 1-MW-peak windmills to electrolyze water into H2

introduce CCS at 800 GW of 50% efficient coal plants introduce CCS at 1600 GW of 60% efficient natural gas plants do CCS at plants producing 250 Mt H2/yr from coal do CCS at plants producing 500 Mt H2/yr from natural gas do CCS at synfuels plants producing 30 mmbd from coal if 50% of feedstock can be captured build 700 GW nuclear instead of 700 GW of 50% efficient coal

increase fuel efficiency for 60 billion cars from 30 mpg to 60 mpg

description of effort

efficient vehicles

climate wedge (Pacala and Socolow)

gasoline and diesel fuel


coal electricity (50% efficient)

corn or sugar cane ethanol

solar PV (30%) + batteries (30%) plus gas peaking (40%) H2 from electrolysis (assume H2 in water is primary energy)

wind electricity (30%) + batteries (30%) plus gas peaking (40%) solar PV electricity + batteries

coal electricity (50% efficient) coal electricity (50% efficient)

wind electricity + batteries

nuclear electricity


additional coal

additional natural gas

coal

natural gas

additional coal

additional natural gas

natural gas electricity coal

additional coal

gas fired electricity



none

none

none

replacement technology


fuel and electricity consumption in buildings (45% fuels, 55% electricity)



old technology

primary energy new technology (EJ)

1,180

1,210

1,240

990

1,240

990

990

6,540

2,010

1,260

1,890

1,130

1,990

1,180

1,210

490

0

490

0

1,510

7,860

2,430

1,520

2,520

1,510

1,650

reduce transportation fuel use by enough gasoline and diesel to emit 25GtC reduce transportation fuel use by enough gasoline and diesel to emit 25GtC reduce energy use in buildings by enough to energy to emit 25 GtC using the energy distribution in the IPCC AR3 Mitigation report, baseline year = 1995 1,050 120

primary energy old technology (EJ)

Table 1. Approximate Energy Shift Associated with Energy Related Greenhouse Gas Reduction “Wedges” Proposed by Pacala and Socolow

0

0

(740)

(990)

(740)

(990)

510

1,320

420

260

630

380

(330)

(930)

(1,580)

(1,210)

(1,210)

estimated change in primary energy use (EJ)

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Table 2. Approximate Range and Median Water Demand for Electricity Production Using Technology Options Discussed by Pacala and Socolow1a consumptive use, current technology (gallons/ MMBtu primary energy) activity Coal coal mining coal washing coal slurry subtotal coal liquefaction coal gasification total-liquefaction total-gasif ication Natural Gas extraction and processing pipeline operations storage in salt cavern subtotal natural gas electricity (open-loop cooling) natural gas electricity (cooling tower) natural gas, combined cycle (cooling tower) total (open loop) total (all, cooling tower) total (comb. cycle, tower) Nuclear uranium mining uranium processing subtotal nuclear electricity (open-loop cooling) nuclear electricity (cooling tower) total (open loop) total (cooling tower) Renewable solar-PV wind

withdrawals, current technology (gallons/MMBtu primary energy)

minimum

median

maximum

minimum

median

maximum

1 1 10 2 40 60 42 62

5 2 12 14 60 80 74 94

8 3 14 25 80 100 105 125

n/a n/a n/a -n/a n/a ---



4 1 700 5 6 40 40 11 45 45

5 4 750 6 60 180 60 60 190 70

5 2 800 7 110 320 82 117 327 89

n/a n/a n/a -220 40 40 ----

n/a n/a n/a -9,890 240 70 ----

n/a n/a n/a -17,600 440 90 ----

1 6 7 30 120 37 127

5 8 12 70 180 190 200

8 9 17 120 250 137 267

n/a n/a -7,330 230 ---

n/a n/a -12,500 500 ---

n/a n/a -17,600 760 ---

0 0

5 0

10 0

n/a n/a

n/a n/a

n/a n/a

a

Data are presented for energy extraction, processing, transport, and power generation. Values estimated from Figure A-1 and Table B-1 of Energy Demands on Water Resources,9 and amended with data from more recent studies.11,23,38,51 1. All values limited to a maximum of 3 significant figures. For n/a, data are not available. 2. For subtotals, minimum values exclude steps that are optional (e.g., coal transport by slurry pipeline), and maximum values include these steps. Median values are the average of the minimum and maximum. Values used in the subtotal calculations are in boldface. 3. For electricity, values originally reported in gal/MWhe are converted to gal/MMBtu primary energy by dividing by 3.412 and accounting for thermal efficiency (nuclear 33%, gas 60%, current coal 32%).

is either released to the environment or upgraded prior to disposal or use. Most of this “produced water” is not currently captured and put to use and thus is not included in our evaluation. However, this water could become a valuable future resource for energy production in water-constrained regions.

are presented in Table 2. Consumptive water requirements for the extraction and conversion of primary energy resources into transportation fuels are presented in Table 3. These values have been gathered from the literature and are reported as a range and estimated median value, as data allow. Only the water requirements associated with the energy resource wedges discussed by Pacala and Socolow1 are presented in this study. Corollary data for other energy sources, such as hydropower, solar thermal, and geothermal are available; but consideration of the water impact of these resources is beyond the scope of this study. The data provided in Table 2 and Table 3 do not account for “new” water produced through extraction and conversion of fossil fuels. In some cases, this water production can be significant. Using typical values for energy content provided by the U.S. Department of Energy, simple combustion produces approximately 5−10 gallons per mmBtu for coal and approximately 15 gallons per mmBtu for natural gas. Coal also contains water, with moisture contents typically ranging from 3−30% by mass, depending upon coal rank. Oil and natural gas extraction can also bring low quality water to the surface, which

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WATER IMPACT OF CLIMATE WEDGES Once the energy transition associated with each climate wedge and the amount of water required for the extraction and conversion stages of that transition is known, the water requirements associated with each climate wedge can be estimated. The results calculated for consumptive use are presented in Figure 1. These results are presented as likely ranges, reflecting variability in the amount of water required to generate energy. Options that reduce consumptive water requirements are shown in blue, and options that increase consumptive water requirements are shown in red. When assessing these trends, five assumptions should be highlighted. First, all calculations are based on the values presented in 3512



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Table 3. Approximate Range and Median Water Demand for Production of Transportation Fuels Using Technology Options Discussed by Pacala and Socolow1a consumptive use, current technology (gallons/MMBtu primary energy) activity Biofuels soy growth biodiesel refining total rapeseed, jatropha growth biodiesel refining total corn growth ethanol processing total sugar cane growth ethanol processing total hydrogen electrolysis hydrogen reforming total Conventional Oil petroleum extraction (primary) enhanced oil recovery (secondary, water flood) oil storage in salt cavern refining subtotal subtotal (10% EOR) Unconventional Oil oil sands oil shale in-situ oil shale surface retort

minimum

median

maximum

14,000 4 14,000 100,000 4 100,000 2,500 20 2,520 n/a 20 30,100 30 60 90

61,500 6 61,500 130,000 6 130,000 15,750 110 15,900 30,000 110 30,200 35 65 100

109,000 8 109,000 160,000 8 160,000 29,000 200 29,000 n/a 200 30,300 40 70 110

0.8 14

1.4 1,260

2.0 2,500

70 7 8 9

75 10 14 139

80 18 20 270

20 1 20

260 6 40

500 10 60

a

Data are presented for energy extraction, processing, storage, and transport. Values estimated from Figure A-1 of Energy Demands on Water Resources9 and amended with data from more recent studies.11,23,38,51 1. All values limited to a maximum of 3 significant figures. For n/a, data are not available. 2. For subtotals, minimum values exclude steps that are optional (e.g., enhanced oil recovery) and maximum values include these steps. Median values are the average of the minimum and maximum. Values used in the subtotal calculations are in boldface. 3. For sugarcane ethanol, no data are available for the minimum and maximum water requirements for sugarcane growth. The median value is used to calculate the subtotal. 4. The subtotal for the case where 10% of oil comes from enhanced oil recovery (EOR) only adds 10% of the consumptive water requirements for EOR to the subtotal.

Figure 1. Approximate change in net water consumption for energy related greenhouse gas reduction “wedges” proposed by Pacala and Socolow.1 Data presented as a range of likely values. Note that results include an evaluation of two options for replacing coal with electricity from wind or solar-PV.

of primary energy consumed (this study). Our calculations estimate water requirements per unit primary energy consumed while also accounting for the reductions in primary resource usage that would come from improved efficiency. Changes in thermal efficiency should not bias the results. This said, future plants may convert low temperature steam into condensate with greater water efficiency than current technology, and these calculations may slightly overstate the water consumption associated with Pacala and Socolow’s assumptions. Third, it is assumed that all electricity generation will use evaporative cooling towers. This introduces inaccuracies that may tend to overstate the future water consumption, while understating withdrawals. However, a larger portion of future electricity generation will need to use cooling towers to reduce demand for water withdrawals; as large withdrawals have been shown to negatively impact aquatic ecosystems.39 Fourth, it is assumed that the water consumption reported to be associated with the production of biofuel raw materials11 refers to the

Tables 1−3 and the assumptions provided in the Supporting Information. Second, we use water consumption data for currently deployed coal-fired power plants (∼32% efficient on average) to estimate water requirements for future plants. Pacala and Socolow1 assumed an average of ∼50% thermal efficiency for their wedges, without specifying a technology. Different power plant designs have different water use profiles (e.g., refs 9−11,23, and 38), and the consumptive water requirements for the future global deployment of thermoelectric generation is unknown. More thermally efficient plants consume less water per unit of electricity produced because they convert a greater portion of their primary resource into electricity (e.g., Supporting Information), but this does not necessarily translate into reduced water requirements per unit 3513



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Reducing end-use energy demand or increasing power generation efficiencies results in significant reductions in consumptive water demand. End-use energy demand can be reduced through conservation (i.e., choosing to use less) or by increasing efficiency (e.g., more efficient appliances). Decreased demand for electricity or transportation fuels translates into reduced consumption of water and primary energy resources. For electricity production, the greatest reductions in water demand for energy would come from reducing the amount of primary fossil and nuclear energy needed to produce a megawatthour of electricity. For transportation fuels, the greatest reductions in water demand for energy would come from fuel efficiency and transportation efficiency innovations that reduce demand for transportation fuels; thereby reducing demand for dedicated biofuel crops and reductions in the amount of water needed for enhanced oil recovery (EOR). The amount of oil being produced by EOR methods is increasing globally, but the amount of water needed for EOR is highly variable. There are few good estimates for how much future oil will need to be produced via EOR techniques, and there is a large range in the amount of water needed per barrel of oil recovered (e.g., Table 3). Thus, it is difficult to estimate how much water demand for EOR could be reduced through technological innovation. However, improved fuel efficiency, reduced vehicle miles traveled, and use of electrolytic H2 all reduce oil demand, equating to less water needed for EOR. Figure 2 shows water consumption for extraction, transport, and upgrading of energy resources. Figure 3 summarizes water consumption for conversion of energy resources into energy services. This breakdown is important, as the entire cycle of energy extraction, upgrading, and conversion rarely occurs within a single hydrologic unit. Most of the water impacts associated with extraction, transport, and upgrading are associated with production of a primary resource. These processes will largely impact water resources in the source areas for oil and crop production, while having lesser impact on water resources in the areas where these resources are consumed. Conversely, options for conversion of primary energy resources into secondary energy services might predominantly impact water resources in areas closer to where energy is consumed. This discrepancy may be important when evaluating policy and research results and when developing options for sustainable energy production. Some of the most effective options may require a change in consumer habit, yet the primary water-beneficiary may reside in a different location from the person making decisions about where and how to acquire fuels, siting of conversion facilities, and/or the use of various fuels.

amount of water consumed by the biofuel crops and not the amount of water supplied during cultivation. Fifth, the water cost of building salt caverns for storage of oil and natural gas is not included in the water requirements for extraction of these resources. This assumption is made because the water cost associated with storage of oil and natural gas in salt caverns is a one-time cost from building the storage cavern.9 The results shown in Figure 1 show that there is a distinct difference in the water impacts of the different climate wedge technologies. Some options decrease net water consumption, while others increase net water consumption (Note: replacement of base-load coal generation with wind or PV is considered a single wedge, even though multiple replacement strategies are considered in this study). The greatest reduction in water consumption comes from the replacement of coal-fired electricity generation with natural gas-fired electricity generation. This counterintuitive result occurs because natural gas produces CO2, and transitioning 25 GtC of coal-fired emissions to natural gas requires that more coal plants be “turned off” than transitioning to wind and solar-PV. This creates larger reductions in the use of coal as a primary resource. The greatest increase in water requirements comes from replacing gasoline with ethanol produced from dedicated bioenergy crops. The dominant component of this increase comes from the water consumed by plants during the production of corn and sugar cane crops, assuming that this wedge involves simple expansion of U.S. and Brazilian ethanol production using the current technology mix (e.g., Supporting Information). Shifting to different bioenergy crops might reduce this demand, but producing ethanol from dedicated crops still involves plant growth and would require more water to be consumed per unit primary energy than producing fuel from oil. Producing biofuels from forest and agricultural waste products could mitigate this effect. If it is assumed that the water required for plant growth is allocated to forest and agricultural products (e.g., the purpose for which the crop is grown), then producing lignocellulosic biofuels from the remainder (e.g., waste products not currently utilized) would reduce water requirements by over 95%. However, harvesting crop residues reduces the soil’s ability to retain water and can deplete nutrients from the soil. It is unclear how water should be allocated between the various agricultural and forest products and residues. Pacala and Socolow’s wedge is based on expansion of current methods for ethanol production (e.g., dedicated corn and sugar cane crops), and implementation of their wedge would significantly increase water demand for transportation fuels. This conclusion does not necessarily extend to biofuels produced from lignocellulosic materials in forest and agricultural waste. Replacement of coal-generated electricity with nucleargenerated electricity or carbon capture at fossil-based energy facilities also increases consumptive water demand. The increased consumptive requirements for traditional nuclear technology arises from the fact that current generation nuclear power plants operate at lower steam temperatures than coal- and gas-fired plants and reject more low temperature heat. The increased consumptive requirements associated with deployment of carbon capture technology at coal or natural gas facilities comes from the increased primary energy demand that arises from the additional energy needed to power carbon capture and storage operations. This parasitic load ranges from 20% to 30% of the total energy input, depending upon the technology used.40 Higher energy demand equates to higher water demand.

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IMPLICATIONS FOR GREENHOUSE GAS MITIGATION STRATEGIES All energy-related options for global GHG mitigation have water resource impacts. Some would reduce water consumption, while others would increase consumption. Water resource impacts will need to be accounted for if GHG mitigation efforts are to be successful and sustainable. Freshwater resources are severely constrained in many regions internationally,12 and strategies that increase strain on these resources are likely to be more costly and contentious than strategies that concomitantly mitigate GHG emissions and water constraints. To that end, this study highlights five broad lessons. First, there are a number of energy development options that could mitigate both GHG emissions and water resource constraints, even in the absence of technologies specifically designed 3514



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Figure 2. Approximate change in net water consumption for extraction, transport, and upgrading of primary energy resources for energy related greenhouse gas reduction “wedges” proposed by Pacala and Socolow.1 Data presented as a range of likely values.

to reduce water consumption. Second, energy conservation and efficiency reduces both GHG emissions and consumptive water demand. Third, both coal with carbon capture and nuclear power wedges consume more water than the corollary BAU scenarios. Fourth, producing biofuels according to Pacala and Socolow’s wedge greatly increases water consumption as compared to fossil fuels or biofuels produced from forest and agricultural waste products. Fifth, these findings are consistent with prior studies (e.g., refs 10, 41, and 42) showing that water impacts for electricity generation occur at the power plant, while most impacts for transportation fuels occur at the extraction and upgrading sites. Note that increased water consumption for a given wedge does not mean that the technology option should not be deployed. Sufficient water may be available locally to satisfy increased demand. Water consumption could also be managed either through deployment of water reduction technologies, or by balancing the increased water demand of some generation technologies with the reduced water demand

Figure 3. Approximate change in net water consumption for production of energy services from primary energy resources for energy related greenhouse gas reduction “wedges” proposed by Pacala and Socolow.1 Data presented as a range of likely values.

of others. Water resource impacts need to be assessed systematically, as part of a holistic approach to evaluating the sustainability of GHG mitigation strategies. Water Management Strategies for Electricity Generation. There are five general strategies that can reduce the impact of electricity generation on freshwater resources. These are (i) replacing thermal electricity production with technologies that utilize less water, such as wind and/or solar-PV; (ii) reducing usage of primary energy resources by increasing energy conversion efficiency in thermoelectric generation, (iii) improving the efficiency of steam condensation, (iv) replacing wet cooling with dry or hybrid cooling, and (v) using degraded or nontraditional water supplies.23 Of these, Pacala and Socolow 3515



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significant production potential for lignocellulosic materials. For example, a recent study has found that U.S. forests have the potential to produce approximately 370 million dry tons of cellulosic feedstock annually, with an additional 370−530 million dry tons potentially available from crop and process residues.47 If refined into cellulosic ethanol, these wastes could potentially offset ∼2.25 million barrels of oil use per day (∼0.1 GtC/yr). Data from the U.N. Food and Agricultural Organization (FAO) indicate that the U.S. produces less than onefifth of total annual global carbon harvests from agricultural and wood products, and thus the 0.1 GtC/yr of emissions savings possible by using U.S. agricultural waste and forest products for biofuels may be less than one-fifth of the global potential. Consequently, it may be feasible to replace Pacala and Socolow’s ethanol wedge with biofuels produced from agricultural waste and forest products, significantly reducing water resource impacts. Improved technologies for monitoring and managing subsurface fluid flow in oil/water mixtures could reduce the water impacts associated with EOR, thereby reducing the amount of water needed per barrel of oil produced. Such work could also be synergistic with use of degraded water and utilization of CO2-based EOR.48−50 These strategies could reduce water demand for oil production while also reducing the environmental impacts of oil production by (i) removing less freshwater from surface resources and (ii) releasing less polluted water into terrestrial and aquatic systems. Strategies for Reducing GHG Emissions, Water Resource Impacts, and Energy Costs. If results from this study are contrasted with the widely discussed GHG abatement cost curves provided by Creyts et al.,28 then it becomes apparent that several strategies that reduce GHG emissions would also reduce water resource impacts and energy costs. These are building efficiency, efficient vehicles, and efficient coal-fired electricity plants. For building efficiency, estimated economic costs for GHG abatement range from -$90/metric ton CO2 abatement (building electronics and lighting) to -$5/ metric ton CO2 abatement (e.g., residential water heaters and commercial building power control systems). For efficient vehicles, estimated economic costs for GHG abatement range from -$70 to -$80/metric ton CO2 abatement (fuel economy in cars and light trucks). For efficient coal fired electricity plants, estimated economic costs for GHG abatement range from -$10 to -$20/metric ton CO2 abatement (existing power plant conversion efficiency). If each of these options is assumed to reduce water demand for energy (from BAU) by ∼500−1,000 million acre-feet over 50 years, then the combination of these options could reduce global water consumption for energy production by ∼30−60 million acre-feet per year over the same time period. This is approximately 6−14% of the annual flow of the Mississippi River or about 0.5−1% of the annual flow of the Amazon River. In addition to these “win-win-win” options, there are some water-saving GHG options that have a moderate economic cost (e.g., lower than ∼$30/metric ton CO2 abatement). These include replacement of coal base-load power with onshore wind and distributed solar-PV. Deployment of new nuclear plants and retrofitting coal-fired power plants with CO2 capture and using the captured CO2 for enhanced oil recovery also fall within this cost category. However, the nuclear and EOR options would increase water demand relative to a BAU scenario with coal-fired electricity. Water consumption for nuclear energy could be reduced, while also increasing the safety of nuclear plants, by deploying new high temperature gas reactors that

consider five options that either replace coal-fired thermoelectric generation with wind/solar-PV or natural gas or reduce primary resource usage by improving the thermal efficiency of power conversion. Deployed globally, each of these five options would reduce water consumption by ∼5−30 million acre-feet per year. The options for electricity production would reduce consumptive water demand by ∼0.2−5 million acre-feet per exajoule over 50 years (∼1−30 gallons per mmBtu per year). In comparison, water consumption projections provided by the U.S. Department of Energy (DOE)38 suggest that deployment of new fossil-fired generation over the next 20 years would increase U.S. water consumption from ∼160 gallons per mmBtu to ∼170−200 gallons per mmBtu. The DOE report did not list the changes in water intensity of electricity generation. These estimates are derived from their data assuming a 90% average capacity factor and a 50% thermal efficiency for coal. The increase in water consumption likely arises from the report’s use of scenarios that include deployment of carbon capture and storage (CCS) while not assessing potential water savings that could be gained from deployment of various dry and hybrid cooling technologies. Dry and hybrid cooling technologies use air or air/water mixes to cool low temperature steam into liquid condensate. These systems are typically 2%−5% less energy efficient at converting primary energy into electricity, and efficiency can be reduced by 15%− 25% on the hottest days in summer.43,44 Use of unconventional water resources (e.g., saline groundwater, industrial wastewater, mine water, and produced water from oil and gas operations) would decrease the amount of freshwater taken from streams, aquifers, and reservoirs; but this water may be contaminated by excess salts, nutrients, low dissolved oxygen, toxins, bacteria, heat, or a combination of the above. Use of these resources would incur an energy penalty, as energy would be needed to upgrade the water and manage the associated contaminant stream. In addition, these resources may require pumping from greater depths/distances and are more likely to be from more limited sources that may not be sustainable over the life of a plant (e.g., low-recharge aquifers). It has been suggested that unconventional water from deep saline aquifers could help offset the increased water demand from deployment of CCS, but the magnitude of such savings has not been firmly established. Water Management Strategies for Transportation Fuels. There are three general strategies that can reduce the impact of transportation fuel production on water resources: (i) producing biofuel from forest and agricultural waste products rather than dedicated crops, (ii) improving technologies for EOR, and (iii) increased utilization of degraded/nontraditional water. The first two strategies improve the water efficiency of producing transportation fuels through reduced water demand for extraction/production of primary energy resources. The third strategy decreases the amount of freshwater taken from streams, aquifers, and reservoirs. All of these strategies could potentially reduce releases of pollutants into terrestrial and aquatic ecosystems. Biofuels can be produced from cellulosic feedstock from forest products, agricultural wastes, or native grasses rather than from traditional water-intensive crops. Biofuels from lignocellulosic crops have not yet been commercialized, and the consumptive water requirements for broad deployment are still unknown.11 However, recent studies suggest that water impacts from lignocellulosic crops should be lower than from current generation biofuels (e.g., refs 19,45, and 46) and that there is 3516



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(3) In Executive Summary of the Conference Report, Avoiding Dangerous Climate Change. Scientific Symposium on Stabilisation of Greenhouse Gases, Exeter, United Kingdom, Department for Environment, Food, and Rural Affairs: Exeter, United Kingdom, 2005; p 6. (4) IPCC Climate Change 2007 - Impacts, Adaptation and Vulnerability; Intergovernmental Panel on Climate Change: Geneva, Switzerland, 2007. (5) Our Common Future, Report of the World Commission on Environment and Development; World Commission on Environment and Development: 1987. (6) Investing in Development. A Practical Plan to Achieve the Millennium Development Goals; United Nations Millenium Project: London, UK, 2005. (7) Ecosystems and Human Well-Being. Synthesis; Millenium Ecosystem Assessment: Washington, DC, 2005. (8) Einfeld, W.; Cameron, C. P.; Pate, R. C.; Hightower, M. S. Emerging Energy Demands on Water Resources; Sandia National Laboratory: Sandia, NM, 2007. (9) U.S. Department of Energy Energy Demands on Water Resources. Report to Congress on the Interdependency of Energy and Water; U.S. Department of Energy, Sandia National Laboratory: Washingon, DC, 2006. (10) Gleick, P. H. Water and Energy. Annual Review of Energy and the Environment 1994, 19, 267−299. (11) McMahon, J. E.; Price, S. K. Water and Energy Interactions. Annual Reviews in Environment and Resources 2011, 36 (17), 1−17.29. (12) World Water Assessment Programe The Untied Nations World Water Development Report 3: Water in a Changing World; Paris, FR, 2009. (13) Gleick, P. H. Water in crisis: Paths to sustainable water use. Ecological Applications 1998, 8 (3), 571−579. (14) Bates, B. C.; Kundzewicz; Wu, S.; Palutikof, J. P. Climate Change and Water; IPCC Secretariat: Geneva, 2008; p 210. (15) Blanco-Canqui, H. Energy Crops and Their Implications on Soil and Environment. Agron. J. 2010, 102 (2), 403−419. (16) de Vries, S. C.; van de Ven, G. W. J.; van Ittersum, M. K.; Giller, K. E. Resource use efficiency and environmental performance of nine major biofuel crops, processed by first-generation conversion techniques. Biomass Bioenergy 2010, 34 (5), 588−601. (17) Delucchi, M. A. Impacts of biofuels on climate change, water use, and land use. Year in Ecology and Conservation Biology 2010 2010, 1195, 28−45. (18) Gerbens-Leenes, W.; Hoekstra, A. Y.; van der Meer, T. H. The water footprint of bioenergy. Proc. Natl. Acad. Sci. U.S.A. 2009, 106 (25), 10219−10223. (19) Hoogeveen, J.; Faures, J. M.; Van de Giessen, N. Increased Biofuel Production in the Coming Decade: To What Extent Will It Affect Global Freshwater Resources? Irrigation and Drainage 2009, 58, S148−S160. (20) Huffaker, R. Protecting water resources in biofuels production. Water Policy 2010, 12 (1), 129−134. (21) Payne, W. A. Are biofuels antithetic to long-term sustainability of soil and water resources? Adv. Agron. 2010, 105 (105), 1−46. (22) Yang, H.; Zhou, Y.; Liu, J. G. Land and water requirements of biofuel and implications for food supply and the environment in China. Energy Policy 2009, 37 (5), 1876−1885. (23) Gerdes, K.; Nichols, C. Water requirements for existing and emerging thermoelectric plant technologies; National Energy Technology Laboratory: Morgantown, WV, 2009. (24) Carney, B. Water Vulnerabilities for Existing Coal-Fired Power Plants; National Energy Technology Laboratory: Morgantown, WV, 2010. (25) Conzelman, G.; Koritarov, V.; Poch, L.; Thimmapuram, P.; Veselka, T. Power Systems Simulations of the Western United States Region; Argonne National Laboratory: Argonne, IL, 2010. (26) U.S. Climate Change Science Program Effects of Climate Change on Energy Production and Use in the United States. A Report by the U.S. Climate Change Science Program and the Subcommittee on Global

potentially allow for internal operating temperatures in excess of 900 °C and combined cycle turbine designs.35,37 In summary, the impacts of different energy production technologies on freshwater resources have not been broadly discussed in the climate change debate. This study assesses the global impacts of selected technologies and seeks to stimulate debate that would further improve our understanding these relationships. The results demonstrate that not all major GHG mitigation options are equivalent; some options will decrease demand on energy and water resources, while others will increase the strain on these resources. There are a number of potential synergies between reducing GHG emissions, managing the cost of energy services and reducing water resource impacts. The best strategies for reducing both GHG emissions and water resource impacts of energy production and use include increasing energy conservation and efficiency and fuel switching to renewable energy and natural gas. Eight of Pacala and Socolow’s climate wedge strategies would reduce both GHG emissions and consumptive water demand (e.g., ten of the analyses presented here), and at least three of these would also help reduce energy costs. Greater emphasis should be placed on developing and deploying these “win-win-win” solutions to our global energy sustainability challenges.

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ASSOCIATED CONTENT

S Supporting Information *

Summary of the assumptions made in the calculations, both for the energy change and the associated water impacts. References specific to the Supporting Information are provided in the Supporting Information but not in the reference section of the primary document. The assumptions are listed and described according to the associated climate wedge of Pacala and Socolow. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 208-426-5395. Fax: 208-526-0875. E-mail: craig. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported through the Idaho National Laboratory (INL) Laboratory Directed Research & Development (LDRD) Program under DOE Idaho Operations Office Contract DE-AC07-05ID14517. We also acknowledge the work of the U.S. Department of Energy’s Energy-Water Nexus initiative and Craig Zamuda and Robert Marley at the U.S. Department of Energy Climate Change Technology Program for the helpful discussions that allowed us to better understand the issues associated with climate change, energy production, and water resources. Finally, we thank Robert Cherry at the Idaho National Laboratory and our anonymous reviewers for their comments and insights that improved this work.

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REFERENCES

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