Spatially and Temporally Resolved Analysis of Environmental Trade

Mar 11, 2016 - (10) The model, implemented in Energy Exemplar's PLEXOS software, simulates the dispatching algorithm utilized by ERCOT to coordinate t...
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Spatially and Temporally Resolved Analysis of Environmental TradeOffs in Electricity Generation Rebecca A. M. Peer,*,† Jared B. Garrison,‡ Craig P. Timms,¶ and 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 ‡ Research Center for Energy Networks, ETH Zürich, Sonneggstrasse 28, SOI C 1, 8092 Zürich, Switzerland ¶ Department of Electrical and Computer Engineering, University of Dayton, Kettering Laboratories 341, 300 College Park, Dayton, Ohio 45469-0232, United States S Supporting Information *

ABSTRACT: The US power sector is a leading contributor of emissions that affect air quality and climate. It also requires a lot of water for cooling thermoelectric power plants. Although these impacts affect ecosystems and human health unevenly in space and time, there has been very little quantification of these environmental trade-offs on decision-relevant scales. This work quantifies hourly water consumption, emissions (i.e., carbon dioxide, nitrogen oxides, and sulfur oxides), and marginal heat rates for 252 electricity generating units (EGUs) in the Electric Reliability Council of Texas (ERCOT) region in 2011 using a unit commitment and dispatch model (UC&D). Annual, seasonal, and daily variations, as well as spatial variability are assessed. When normalized over the grid, hourly average emissions and water consumption intensities (i.e., output per MWh) are found to be highest when electricity demand is the lowest, as baseload EGUs tend to be the most water and emissions intensive. Results suggest that a large fraction of emissions and water consumption are caused by a small number of power plants, mainly baseload coal-fired generators. Replacing 8−10 existing power plants with modern natural gas combined cycle units would result in reductions of 19−29%, 51−55%, 60−62%, and 13−27% in CO2 emissions, NOx emissions, SOx emissions, and water consumption, respectively, across the ERCOT region for two different conversion scenarios.

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INTRODUCTION Electricity generation in the US impacts the environment in time and space in regards to air quality, climate, water quality, and water quantity. In the Electric Reliability Council of Texas (ERCOT, the independent system operator that governs the provision of electricity for the majority of Texas) region, fossil fueled electricity generating units (EGUs) supplied approximately 77% of total generation in 2014, while nuclear and renewable sources supply about 12% and 11%, respectively, which is slightly more fossil fueled and slightly less nuclear generation than the national average.1,2 The majority of the EGUs in Texas are thermoelectric (approximately 90% of generating capacity); that is, a heat source is used to create steam or high enthalpy air that spins a turbine. Power generation has various water trade-offs that depend on power plant configuration. Rankine cycle thermoelectric EGUs utilize cooling systems to cool the closed steam loop exiting the turbine in order to reduce its backpressure, which helps maintain the efficient operation of the facility. Once-through and recirculating cooling systems form approximately 31% and 53% of total thermoelectric cooling systems by percentage of generation in the US, respectively.1,3 Dry and hybrid cooled EGUs represent about 2% of generation, and the remaining 14% of EGUs require no cooling (e.g., hydropower, solar, and wind).1 Water is used as the cooling fluid in once-through (i.e., open loop) and recirculating (i.e., closed loop) cooling systems. In these systems, the fluid either is used once and discharged © 2016 American Chemical Society

into a receiving water reservoir (once-through) or is recycled back into the cooling system for subsequent cooling cycles (recirculating). Dry cooling systems, on the other hand, use ambient air as the primary cooling fluid, reducing or eliminating the need for water in the cooling system. However, dry cooled systems generally increase capital costs and are less efficient than wet cooled systems, which can translate to higher emissions per unit of electricity produced.4−7 Hybrid cooling systems that use both dry and wet cooling technologies are available but are typically cost prohibitive compared to current cooling technologies in use.8,9 Water use for each EGU can be defined in terms of water withdrawals and water consumption. Withdrawals refer to the total volume of water removed from a source for use; consumption refers to the subset of withdrawn water that is not returned to its original source after use. Water withdrawals and consumption for thermoelectric generation in Texas represent 40−60% and 2.5−4.2% of the state’s total withdrawals and consumption, respectively.10−12 Water use is influenced by a generator’s fuel type, prime mover type, cooling system type, and environmental controls. Oncethrough cooling systems have high withdrawal with relatively Received: Revised: Accepted: Published: 4537

November 4, 2015 February 3, 2016 March 11, 2016 March 11, 2016 DOI: 10.1021/acs.est.5b05419 Environ. Sci. Technol. 2016, 50, 4537−4545

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Environmental Science & Technology

stringent carbon reductions by 2050, with varied water savings, carbon emissions reductions, and public health benefits depending on the technologies used (i.e., a focus on nuclear increases water consumption but reduces emissions; a focus on renewables reduces both water consumption and emissions, with the potential for reduced electricity costs). Pacsi et al.36 studied the feasibility and air quality impacts of drought-based dispatching in the ERCOT footprint using a power flow model, finding that dispatching power plants based on water availability, although expensive, could result in coupled water savings and air pollution reductions in drought-stricken areas of the state. Although previous studies have analyzed the systems-level water or air emissions impacts of the electricity sector under a variety of scenarios,4,8,10,25,31,39 to date, no analysis has evaluated the trade-offs in terms of water consumption, GHG, and air quality pollutant emissions with high spatiotemporal resolution.10,15,16,38 This analysis fills this existing research gap by quantifying water, air, and climate trade-offs with high temporal (i.e., hourly) and spatial resolution, which will be important to consider as the US makes new investments in power infrastructure to meet different and often competing environmental priorities.

low consumption, whereas recirculating cooling systems have relatively low withdrawals but high consumption because they lose the majority of withdrawn water to evaporation.4,10,13,14 In the ERCOT region, once-through cooling systems represent approximately 54% of generating capacity, whereas closed-loop cooling systems represent approximately 44% of generating capacity. This analysis focuses on water consumption for environmental impact assessment, as consumption represents a physical alteration to the water system via losses. Other environmental concerns for the electric power sector include air quality pollutants and greenhouse gas (GHG) emissions associated with carbon-based fuel (i.e., fossil fuels and biomass) combustion at the point of generation. Increases in atmospheric concentrations of GHGs such as carbon dioxide (CO2) emitted from combustion sources are known to contribute to climate change.15 Since the 1990s, electricity generation has been the largest contributor to CO2 emissions worldwide, accounting for approximately 40% of all emissions.16 Emissions of air pollutants such as sulfur oxides (SOx), which contribute to the formation of sulfate aerosols, and nitrogen oxides (NOx), a precursor for ozone formation, are also a concern for climate change and air quality.15 As the ERCOT region generates more electricity from carbon-based fuels than the average US electricity mix, new environmental regulations are placing added pressure on this region to reduce emissions. In the past decade, proposed, newly implemented, and revised policies (e.g., EPA Clean Power Plan, Clean Water Act 316 a and b, Clean Air Act) have effectively reduced the generation of high GHG emitting and water intensive EGUs. In combination with economical natural gas prices, these policies have led to the retirement of a significant amount of coal-fired capacity, which has been largely replaced with natural gas-fired combined cycle EGUs.3 The expansion of natural gas-fired capacity is largely a result of economical domestic hydraulic fracturing production, which has led to cheap natural gas fuel costs.5,17−24 Natural gas is also attractive because it has reduced environmental impacts at the point of generation, compared to coal.20,25−27 Many previous analyses have evaluated how changes in grid configurations and demand profiles impact cooling water requirements3,4,10,12−14,29−39 and GHG emissions,16,28,34,40,41 as well as air pollutant emissions.19,42,43 However, very few analyses have addressed air emissions and water use simultaneously.8,25,36 Webster et al.8 found that a restriction on CO2 emissions (e.g., 75% reduction below business as usual) at ERCOT power plants would also reduce water withdrawals. However, more stringent restrictions on CO2 emissions (e.g., greater than 75% below business as usual) would likely increase water withdrawals due to the implementation of pollution controls (which often require large amounts of water); restrictions on both CO2 emissions and water withdrawals would force the ERCOT region to shift toward different (and sometimes more expensive) technologies (e.g., hybrid or dry cooling systems, carbon capture, and sequestration). Clemmer et al.25 used the National Renewable Energy Laboratory’s Regional Energy Deployment System (ReEDS), a long-term capacity expansion model, to assess the impact of various lowcarbon electricity futures on national water use. The researchers found that the 2050 reference case scenario (projected business as usual) had the lowest electricity costs but at the expense of high carbon emissions, water consumption, and a poorly diversified electricity mix; conversely, they found that significant technical changes would be required to meet

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MATERIALS AND METHODS This work utilizes a unit commitment and dispatch model (UC&D) model of the ERCOT grid developed and described by Townsend,44 Garrison et al.,45 and Sanders et al..10 The model, implemented in Energy Exemplar’s PLEXOS software, simulates the dispatching algorithm utilized by ERCOT to coordinate the reliable provision of electric power services. The UC&D framework simulates EGU dispatch and reserve supply according to a least cost optimization method, solving each day using load and available supply conditions defined for the previous, current, and following day (using 4 h look-ahead windows averaging grid requirements for the following day) such that the short run marginal cost (SRMC) is minimized. Thus, capital costs are excluded. User inputs include generator type definitions, associated generator information (e.g., EGU capacity, ramp rates, minimum operating level, maintenance and downtime, minimum runtime requirements, etc.), systemwide hourly load profiles, and ancillary generation requirements. Model constraints limit overall generator operations to reflect realistic EGU performance (see Garrison et al. for full model documentation).45 SRMC (eq 1) is a function of variable operation and maintenance costs (CO&M), fuel costs (Cfuel), and power plant heat rate (HR). SRMC = CO&M + Cfuel × HR

(1)

Thus, the model’s algorithm determines the optimal generation fleet dispatch required to meet a prescribed load for every hour of the year such that the total dispatch cost of generating electricity is minimized. Model results are quantified on hourly time steps for a 2011 ERCOT load curve using 2011 fuel prices and the 2011 ERCOT generation fleet. Eq 2 was used to calculate EGU specific environmental impact, Xk,t, based on generation, Gk,t, for each EGU, k, at time, t; EGU-specific emissions rate, Ek (in gallons per MWh generated or pounds of emissions per MMBTU of fuel consumption); time-varying, EGU-specific heat rate, HRk,t. Xk,t was calculated for water consumption, CO2, NOx, or SOx emissions, respectively. 4538

DOI: 10.1021/acs.est.5b05419 Environ. Sci. Technol. 2016, 50, 4537−4545

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Environmental Science & Technology

Figure 1. Electricity generation profiles for EGUs in the ERCOT region are generally flat for coal and nuclear EGUs (i.e., baseload EGUs) and variable for natural gas EGUs, which vary according to fluctuations in demand. CO2 and NOx emissions follow the shape profiles of coal and natural gas generation, while SOx emissions are primarily associated with coal generation. Water consumption is skewed toward recirculating cooled thermoelectric generators. Large seasonal and daily fluctuations reflect energy use patterns and climatic characteristics. Data reflect 2011 load characteristics. Daily profiles are for August 1, 2011. Fuel and prime mover type: nuclear (Nucl), coal (Coal), natural gas combined cycle (NGCC), natural gas boiler (NGBlr), open-cycle natural gas (OCGT), internal-combustion natural gas (NGIC), and hydroelectric (Hydro). Cooling technology: once-through (OT), recirculating (RC), and no cooling (NA).

Xk , t = Gk , t × Ek × HR k , t

modeled in UC&D simulations with piece-wise linear segmented second-order polynomial function to better represent generators’ actual fuel consumption during all stages of operation. Results from the UC&D model were postprocessed to interface with ArcGIS to perform spatial analysis. The UC&D modeling approach facilitates high temporal and spatial resolution of environmental criteria results. Although emissions data are available on an hourly time scale from the EPA, to date, cooling water use data have only been available with yearly resolution. The model also allows for the evaluation of peak-shifting scenarios and their impact on EGU dispatch resulting from any prescribed load scenario. Most importantly, this framework allows for the realistic simulation of power plant retrofitting and replacement, since dispatch order moderates

(2)

Next, the hourly environmental intensity of the entire ERCOT grid, Xt, (i.e., the total grid-wide impact per hourly electricity generation) was calculated for each environmental impact, Xk,t. 252

intensity, X t =

∑k = 1 Xk , t 252

∑k = 1 Gk , t

(3)

Water consumption rates were assigned according to a report published by King et al. for the Texas Water Development Board in 2008 for each EGU in the ERCOT region.46 Average annual emissions rates were calculated from the Environmental Protection Agency’s Air Markets Program.47,48 Heat rates are 4539

DOI: 10.1021/acs.est.5b05419 Environ. Sci. Technol. 2016, 50, 4537−4545

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Figure 2. Averaged CO2, NOx, SOx, and water consumption intensity rates generally trend with the fraction of baseload coal-fired power on the grid at any given hour.

Natural gas combined cycle plants are able to follow load more than baseload power plants but are unable to react to rapid changes in demand. They are also dispatched before other older, more expensive or inefficient natural gas EGUs, such as steam boiler (NGBlr) plants, since EGUs are generally dispatched in order of least to highest cost to meet demand. Natural gas combustion turbines (i.e., OCGT and internalcombustion natural gas (NGIC)) are much smaller and can react much quicker than baseload and combined cycle units. Combustion turbine EGUs are often used for ancillary services to maintain the reliable operation of the grid by quickly ramping up and ramping down to balance supply with demand. However, these EGUs are typically expensive to operate and emissions intensive, since they are not as efficient as larger combined cycle units. Electricity demand in the ERCOT region is the highest during the hottest months of the year, due largely to the use of space cooling systems (Figure 1).49,50 The weekly generation profile shows a slight decrease in electricity demand during the weekend due to less business and industrial activities.16 Daily electricity demand peaks in the late afternoon and early evening as people generally arrive home from work and perform energy intensive tasks. This observation is particularly pronounced during the hottest months of the year, when space cooling usage is highest. The emissions profiles of CO2 and NOx are similar in shape to the electricity generation profile (except for the obvious

overall grid operation. This is to say that retrofitting and/or replacing power plants with identical generating capacity produces nonlinear environmental impacts. Additionally, this methodology provides a framework for computing emissions and water use at hourly time scales for projected scenarios.

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RESULTS AND DISCUSSION Temporally Resolved Environmental Impacts. Hourly generation (row 1), CO2 (row 2), NOx (row 3), and SOx (row 4) emissions, as well as water consumption (row 5) data were aggregated by EGU fuel type, cooling technology, and prime mover, as displayed in Figure 1. Each resulting data type is shown for the entire year (column 1), as well as for a sample summer week (column 2) and a sample summer day (column 3) in August 2011 to represent seasonal, weekly, and daily variability, respectively. Coal and nuclear EGUs provide relatively constant baseload generation throughout the year to ensure grid reliability, minimize the SRMC, and avoid ramping (these EGUs typically have slow ramp rates, large minimum down times, and high minimum operating levels). The majority of this baseload generation in the ERCOT footprint is provided by EGUs that use once-through (OT) cooling systems, typically older units that withdraw large volumes of water. While baseload nuclear generation does not generate air or GHG emissions, baseload coal generation is responsible for a large portion of air quality and GHG emissions in the ERCOT region. 4540

DOI: 10.1021/acs.est.5b05419 Environ. Sci. Technol. 2016, 50, 4537−4545

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Figure 3. A geographic representation of power plants contributing the largest fraction of total CO2, NOx, SOx and emissions, and water consumption across the ERCOT region is shown overlain with Texas state boundaries, river basins, and state population density. Colored dots represent power plant type and physical location, as well as generation (by size). Percent of total CO2, NOx, and SOx emissions, water consumption, and generation for each power plant in the ERCOT region is displayed in the bar graph.

than less water-consumptive (but emissions intensive) oncethrough cooled plants. However, these older open-loop cooled baseload plants represent the majority of water withdrawals in ERCOT, which can have negative ecosystem impacts within the water reservoirs that they pump water to and from during the cooling cycle through impingement, entrapment, and thermal discharge issues.7,10,51,52 Grid-Wide Environmental Emissions and Water Consumption Intensities. Total grid-wide CO2, NOx, SOx, and water consumption intensities (i.e., total impact per unit of hourly electricity generated) were calculated using eq 3 and are presented in Figure 2. The variations in each profile are largely a function of the fraction of generation represented by coal and/or natural gas-fired EGUs (Figure 2, top row). Because coal and natural gas-fired EGUs both emit CO2 and NOx and represent a large fraction of total generation across all hours, the profiles of these two emissions intensities remain relatively flat. For short periods during the day and during the summer months, when increased demand is met with more emissionslean generating technologies, the emissions profiles decrease

omission of nuclear) because carbon based fuel (primarily coalfired and natural gas-fired) EGUs are responsible for all CO2 and NOx emissions and also represent a large fraction of generation. However, the shape of these emissions profiles is skewed toward older, open-loop cooled coal-fired EGUs and away from newer combined-cycle natural gas-fired EGUs when compared to the generation shape profile (Figure 1). The SOx emissions profile is flat as SOx emissions are primarily a result of coal combustion. When summed over the entire year, coalfired EGUs contributed 76.1%, 85.2%, and 99.9% of the total CO2, NOx, and SOx emissions, respectively, in the ERCOT region. Baseload coal-fired and nuclear EGUs represent a significant fraction of water consumption. However, the variability in seasonal, weekly, and daily profiles is driven primarily by closed-loop cooled combined-cycle natural gas-fired EGUs, which also consume water at higher rates than once-through cooled plants. The water consumption profile varies slightly from the profiles of CO2 and NOx as it is skewed toward EGUs with closed-loop systems, which are often newer and cleaner 4541

DOI: 10.1021/acs.est.5b05419 Environ. Sci. Technol. 2016, 50, 4537−4545

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Environmental Science & Technology slightly. SOx emissions are almost entirely a result of coal-fired EGUs, which maintain a fairly stable baseload generation (Figure 1). Therefore, when the fraction of generation represented by coal-fired EGUs is high (i.e., during the colder months and during the night), the SOx emissions intensity is also high. Conversely, when the fraction of generation represented by coal-fired EGUs decreases as other generating units are used to meet demand, the SOx emissions intensity falls. Water consumption is largely a function of coal-fired and natural gas combined cycle EGUs with closed-loop cooling systems. During the night, the water consumed per MWh generated is high; during the day, it decreases as more waterlean generators come online. Thus, when considering eq 3, the numerator changes much less than the denominator for SOx emissions and water consumption intensities across the year, translating to shape profiles that have much more defined peaks than the shape profiles of CO2 and NOx emissions intensities in Figure 2. As a result, electricity generation during the night and during cooler months of the year is more emissions- and waterintensive per MWh generated as compared to electricity generated during the peak of the day or the hottest months of the year. However, it is important to note that total emissions and water consumption are highest when demand is greatest because of high electricity load. Consequently, reducing a unit of electricity generated during the night or when temperatures are cooler is actually more advantageous in terms of emissions and water consumption reductions than to reduce a unit of electricity generated during the day or when temperatures are hottest. (However, typically peak demand reductions are advantageous for grid reliability and cost standpoints.) To evaluate the grid-scale impacts of shifting generation from the peak of the day to nighttime hours, two additional simulations (50% peak shift and flat load) were executed to quantify shifts in environmental externalities. Both scenarios result in minor grid-wide increases in all evaluated environmental externalities. (Full results are available in the Supporting Information.) Spatially Resolved Environmental Impacts. Results from the UC&D model were also analyzed on an individual power plant level, by evaluating the percentage of CO2, NOx, and SOx emissions as well as water consumed by each power plant relative to total ERCOT-wide emissions and water use for the year (Figure 3). Fractional contributions of each power plant are organized from highest to lowest percentage value, where each section of the figure corresponds to a power plant with one or many EGUs of the same type. Figure 3 illustrates that large fractions of water use and emissions from the grid are attributable to a handful of EGUs, mostly coal-fired or nuclear generation facilities with open-loop cooling systems (as these tend to be the oldest). As seen in Table 1, emissions in the ERCOT region are disproportionately skewed toward coal-fired EGUs and water consumption is disproportionately skewed toward closed-loop cooled EGUs when compared to total relative generation. Therefore, a simultaneous reduction in GHG, air pollution emissions, and water consumption could be achieved if EGUs producing the largest environmental externalities were targeted. Figure 3 also shows the locations of the 10 power plants representing the largest contributions of air, water, and GHG emissions, as well as the total fractional contribution of each power plant to emissions and water consumption across the ERCOT footprint. The geographic context is particularly important for discussion surrounding NOx and SOx emissions,

Table 1. Relative Distribution of Generation, Emissions, and Water Consumption Varies by Generating Technologya

a

EGU type

% total generation

biomass wind hydro NGIC-NA OCGT-NA NGBlr-OT NGBlr-RC NGCC-OT NGCC-RC Coal-OT Coal-RC Nucl-OT

0.06 9.3 0.44 0.13 1.5 1.4 0.05 0.68 28 33 12 13

% total CO2 emissions

% total NOx emissions

NA

NA

0.12 1.9 1.2 0.05 0.52 20 56 20 NA

0.27 5.7 0.99 0.08 0.17 7.6 57 28 NA

% total SOx emissions

% total H2O consump. 0.02

0.02

0.07 79 21

0.02 0.22 1.1 0.08 0.82 18 20 37 23

NA = not applicable.

as well as water consumption. NOx and SOx emissions are important to consider in terms of regional air quality, as these emissions contribute to the formation of localized air pollution via particulate matter (PM).15 Therefore, power plants located in proximity to areas with larger population density can contribute to visual pollution and health impacts via PM formation. The state of Texas suffers from air pollution, particularly in urban areas, and has struggled with nonattainment status for national ambient air quality standards (NAAQS), particularly ozone, in the Houston−Gavelston− Brazoria and Dallas−Fort Worth areas.54 Although CO2 is a globally well-mixed pollutant, total magnitude concentrated across a small subset of generators is substantial. Collectively, the 10 highlighted power plants represent 46%, 50%, 63%, 67%, and 60% of the generation, CO2, NOx, SOx, and water consumption in the ERCOT region, respectively. Spatial trends for water use are important because shifts in water use can impact drought resiliency for a power plant and for regions sharing a watershed with water consumptive generators. Retrofitting power plants from once-through cooling to recirculating cooling systems could reduce water diversions, making more water available to junior water rights holders.11 While decreased reliance on water withdrawal is important for drought resiliency, retrofitting to recirculating cooling can increase water consumption, actually decreasing net water availability in the basin.11,36 However, if newer RC plants displace generation from older, more expensive RC plants in a UC&D regime, there still might be net water consumption benefits. Environmental Implications of Natural Gas Combined Cycle Retrofitting. The impact of natural gas combined cycle retrofitting was evaluated by investigating two conversion scenarios, considering the replacement of the EGUs with the greatest environmental impacts on the grid mentioned in the previous section. • Scenario 1: conversion of 8 coal-fired power plants (2 recirculating cooled and 6 open-loop cooled facilities), identified in Figure 3, to natural gas combined cycle facilities with equal capacities. • Secnario 2: conversion of 8 coal-fired power plants and 2 nuclear power plants (both open-loop cooled facilities), identified in Figure 3, to natural gas combined cycle facilities with equal capacities. 4542

DOI: 10.1021/acs.est.5b05419 Environ. Sci. Technol. 2016, 50, 4537−4545

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Environmental Science & Technology

Table 2. Converting from Once-through Cooled to Recirculating Cooled NGCC EGUs Increases Water Consumption (WC) in Texas Water Basins Where Replacements Occur, Regardless of Fuel Conversion. Water Consumption is Reported in Units of Trillion Gallons baseline

scenario 1

scenario 2

basin name

basin electric WC

WC

share of electric WC, %

WC

share of electric WC, %

WC

share of electric WC, %

Brazos San Antonio Colorado Cypress Red Sabine Trinity

54.9 17.0 23.5 10.8 2.02 18.4 11.0

13.1 6.13 17.9 3.49 1.62 7.19 2.90

24 11 33 6.4 3.0 13 5.3

20.8 10.5 24.6 9.79 7.37 9.71 10.3

38 19 45 18 13 18 19

23.4 12.3 20.8 11.1 12.4 11.8 11.8

43 22 38 20 23 22 22

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IMPLICATIONS Developments in policy and technology over the past decade are pushing the U.S. electric sector toward an important state of transition.20,22,29,34,51,55−58 Separate, and often conflicting, policies have been issued to reduce pollution, greenhouse gas emission, water usage, or ecosystem impacts.3 However, this analysis demonstrated that environmental priorities are not always aligned. Some power plants that are water lean have emission penalties (e.g., dry-cooled fossil fueled EGUs), while some emissions lean technologies are water intensive (e.g., nuclear or concentrating solar power plants). There are also win−win scenarios. Solar PV panels and wind turbines require little to no water. Retrofitting the most environmentally taxing power plants to more emissions-lean and water-lean natural gas combined cycle facilities offers significant benefits in terms of overall reductions in CO2, NOx, and SOx emissions and water consumption for the region. However, new and retrofitted recirculating cooled power plants may individually consume more water than older, once-through cooled power plants. Grid-wide reductions of environmental impact could result in significant improvements in regional air quality and water availability for other users that share airsheds and watersheds with these generators, relatively cost-effectively. Therefore, new policies must consider the multifaceted spectrum of environmental impacts to avoid creating unintended consequences to environmental or human health. Retrofitting coal-fired EGUs results in significant changes in air pollution emissions and, therefore, significantly impacts the overall socioeconomic impacts of these EGUs. In conversion Scenario 1, approximately $600 Million (2011 USD) in social costs of air pollution impacts would be mitigated by retrofitting of 8 coalfired power plants. (See the Supporting Information for full results of socioeconomic analysis.) This highly temporally resolved evaluation of the environmental trade-offs between water consumption, GHG emissions, and air pollutant emissions in ERCOT serves as a foundation for more holistic analyses of the electricity grid and the associated environmental impacts moving forward. Many of the trends identified in this analysis were driven by the fact that baseload generation is provided almost exclusively by coal-fired and nuclear EGUs, while seasonal and daily peaks in demand are generally accommodated by flexible natural gas-fired EGUs, which are able to ramp up or down to follow load. We found that water consumption follows a similarly shaped profile to generation, since flexible combined-cycle natural gas-fired EGUs require a significant fraction of water in addition to baseload nuclear and coal-fired EGUs. When compared to generation profile, water consumption is distorted toward

In the current environment of high production of natural gas and low natural gas prices in Texas, natural gas-fired combined cycle EGUs are an attractive option for new or retrofitted generation due to economic, regulatory, and environmental benefits over other generation technologies, especially coal.17,20−26,43,53 For Scenario 1, if more modern natural gas combined cycle facilities replaced the selected coal-fired power plants, total ERCOT-wide reductions of approximately 29%, 55%, 62%, and 13% in CO2 emissions, NOx emissions, SOx emissions, and water consumption, respectively, would be achieved when considering optimum dispatch based on 2011 fuel costs. In Scenario 2, ERCOT wide reductions in CO2 emissions, NOx emissions, SOx emissions, and water consumption are approximately 19%, 51%, 60%, and 27%, respectively. Scenario 1 results show a greater impact on grid-wide emissions reductions, while Scenario 2 results show a lesser impact on reducing emissions and a greater impact on reducing water consumption compared to the baseline (Table 2). These results are expected, as retrofitting old, inefficient coal plants will have a greater impact on emissions than retrofitting emissions-free nuclear plants. Similarly, retrofitting once-through cooled thermal power plants to recirculating cooled power generators actually increases the basin-wide water consumption where new facilities are installed (although water withdrawals and thermal pollution would be significantly reduced). While some basins have increased water consumption where new recirculating NGCC plants replace once-through cooled EGUS, grid-wide water consumption savings are achieved via offsetting generation from older, less efficient recirculating cooled power plants in the least cost dispatching regime. For example, conversions of once-through cooled coal-fired power plants resulted in increased annual water consumption at the plant level on the order of 35−307% across the two scenarios. The Sabine, Cypress, Brazos, Trinity, Colorado, San Antonio, and Red river basins would all experience increases in total electric water consumption (Table 2) as a result of conversion to natural gas combined cycle facilities with recirculating cooling systems. Although the Brazos river basin incurs one of the highest water consumption penalties from retrofitting the conversion scenarios, the water withdrawals saving that would be incurred would likely be most meaningful because it contains the longest section of river in the state and services many large urban areas, agricultural water users, and industrial water users.11 (Full results of conversion scenarios are available in the Supporting Information.) 4543

DOI: 10.1021/acs.est.5b05419 Environ. Sci. Technol. 2016, 50, 4537−4545

Article

Environmental Science & Technology

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EGUs with closed loop cooling systems, which reflects the fact that these systems are very water consumptive per MWh of electricity generated compared to other types of cooling. CO2 and NOx emissions in the ERCOT region also follow a similarly shaped profile as electricity generation, with coal and natural gas-fired EGUs being the primary contributors to CO2 and NOx emissions. Conversely, coal-fired EGUs are shown in this analysis to be almost entirely responsible for SOx emissions, demonstrated by an almost flat profile. When normalized over the grid, average water consumption, CO2, NOx, and SOx emissions intensities (i.e., output per MWh) are highest when electricity demand is the lowest, being provided by primarily baseload EGUs. A large fraction of total CO2, NOx, and SOx emissions and water consumption across the ERCOT region were found to be provided by only a handful of power plants, mainly baseload coal-fired generators. The results of this analysis, which offer results with unprecedented spatiotemporal fidelity, can be used as proxies to evaluate the trade-offs of power generation in other regions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b05419. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; phone: (213) 292-3016. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge Dr. Michael E. Webber at the University of Texas and Dr. Aaron Townsend at the National Renewable Energy Laboratory for their contributions in the development of the unit commitment and dispatch model that facilitated this work. We would also like to thank Mr. Kevin Conde, who helped produce the figures for the report.

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REFERENCES

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DOI: 10.1021/acs.est.5b05419 Environ. Sci. Technol. 2016, 50, 4537−4545