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Power systems are responding to climate change both through efforts to reduce greenhouse-gas emissions and through adaptive responses to the effects of climate change, such as changes in hydroelectric generation. These changes, experienced together, may cause new patterns in plant operations or pollutant emissions. Various scenarios representing future climate change mitigation and adaptation in the power sector were explored using the PLEXOS production-cost model, with a focus on plant utilization and behaviour, total system-operating costs and total CO emissions. Further, the effect of introducing widespread utility-scale energy storage into these scenarios was quantified in terms of these same parameters. Large increases in variable renewable penetration combined with extreme reduction of hydroelectric generation in the American Southwest (based on climate modelling in the Colorado River basin and actual drought experience in California) caused significant increases in thermal plant start-up/shutdown cycling. The introduction of storage significantly reduced this cycling, without a material increase in CO emissions. Storage introduced on even a modest scale can provide considerable flexibility under a range of future power-system circumstances that might be experienced due to climate change. Keywords: renewable energy integration; power-system decarbonization; production-cost modelling; energy storage; climate change resources . The increased use of variable resources and Introduction reduced use of dispatchable fossil-fuelled power plants re- Human-caused climate change, largely attributable to sults in changes in the historical approach to managing the combustion of carbon-based fossil fuels, is one of the the electrical system [4–6]. greatest challenges facing humans today . Efforts to At the same time that electrical systems are under - mitigate emissions of the greenhouse gases that cause going changes in order to reduce greenhouse-gas emis- climate change have resulted in rapid increases in the sions, these systems must also begin to adapt to the deployment of renewable energy sources for electricity effects of climate change, including changing and more generation . The most-utilized new resources are wind extreme weather patterns . This paper focuses on the and solar energy, which are variable because they are de- climate change effects of reduced hydroelectric generation pendent upon the availability of the underlying natural Received: 6 March, 2019; Accepted: 14 June, 2019 © The Author(s) 2019. Published by Oxford University Press on behalf of National Institute of Clean-and-Low-Carbon Energy This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http:// creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, 241 provided the original work is properly cited. For commercial re-use, please contact firstname.lastname@example.org Downloaded from https://academic.oup.com/ce/article-abstract/3/4/241/5554400 by DeepDyve user on 10 December 2019 242 | Clean Energy, 2019, Vol. 3, No. 4 in the southwestern USA, and the consequences for affect the emissions efficiency of individual power plants power-system operation and power-plant behaviour, using [18–20]; researchers investigating this effect have recom- the state of Arizona as a focal point. Hydroelectric and mended that additional modelling of part-load operation, renewable energy changes were modelled in the Desert ramping and start/stop cycling be incorporated into indi- Southwest and California, and the results presented focus vidual power-plant air-quality protection measures [21 15]. , on Arizona, with some results presented for California due to its regional importance. This research used production 1.1.2 Effects of storage with high levels of variable cost modelling to examine the role for utility-scale energy renewable generation storage under circumstances in which the power system is Integration of storage along with the variable generation attempting to mitigate climate change through increased can provide needed system operational flexibility to re- variable renewable energy deployment and is adapting to duce the ramping and starting/stopping of thermal plants the climate change effects of extremely reduced regional [13, 22]. Research exploring the effects of deploying storage hydroelectric generation. in combination with high levels of renewables has found that it can reduce thermal plant cycling [22 13, , 23] and the consequent emissions  but, depending on the system 1 Background makeup, may cause other operational changes such as a heavier reliance on coal-fired generation that increase In the western USA, drivers for rapid increases in renew- emissions . able energy penetration include increases in the renew- able energy standards in California, Colorado, Nevada and 1.1.3 Climate change and hydroelectric capacity New Mexico, as well as utility-led moves to retire coal A contribution of the current work is to examine the im- plants  and incorporate thousands of megawatts of new pacts of storage on a system under the operational stress wind and solar generation . Utilities across the West, of high levels of variable renewable penetration combined recognizing that these system changes require additional with severely constrained hydroelectric capacity. For the flexibility, have worked to develop operational tools such extreme-drought scenarios considered, literature was re- as the Energy Imbalance Market and demand response viewed on the impacts of climate change on surface-water programmes and energy-storage-deployment goals. availability and related hydroelectric-generation potential In the American Southwest, changes in precipitation in the Colorado River basin and across California. Other re- and snowfall patterns are predicted to cause seasons of search [26, 27] has specifically quantified the impact of cli- extreme drought, in addition to earlier and greater snow- mate change on surface-water availability and consequent melt run-off in the north-west and mountain regions changes in hydroelectric-generation capacity in California. . These hydrologic changes may cause changes in the This work has not examined the impacts of those changes availability or operation of hydroelectric facilities, which on neighbouring states or system parameters across the are currently a great source of flexibility on the power region and thus this bears further investigation. Additional system in many regions . In the past several years, research explores the effects of reduced water availability California has experienced situations of severely reduced on the thermal-power-plant fleet [28, 29]; this is not the hydroelectric generation due to drought and situations subject of the research described in this paper. in which, due to increased spring run-off, hydroelectric The combination of increased variable generation and plants must run continuously at maximum capacity in changes in hydroelectric availability has been explored order to avoid flooding . These changes can conse- at a high level ; a contribution of this research is to quently create additional demands for operational flexi- provide a regional focus and specific results for power- bility from other plants on the power system. system behaviour, costs and emissions. Further, this re- search investigates the effects of storage on system and power-plant behaviour under the combined circumstances 1.1 Literature of climate change mitigation and adaptation within the 1.1.1 Renewable energy, thermal plant operations and power system. emissions Large increases in renewable energy penetration on elec- trical systems are well established to reduce system-wide 2 Methods emissions of carbon dioxide and other air pollution . 2.1 Study area Nevertheless, increased variable generation can cause additional thermal plant ramping or start/stop cycling The r egion of interest for this study was the American [12–16] and can reduce the number of full-load hours (in Southwest. Arizona, as the largest load in the Desert which they are running at design capacity) of fossil plants Southwest region, was considered as a laboratory and focal [14, 17]. While the overall effect of renewable energy inte- point for this research. A primary research goal was to ex- gration is reduced system-wide emissions, this additional plore a ‘bookend’ scenario of extreme drought in the region. ramping, part-load operation and start/stop cycling can Because California is such a relevant actor in the regional Downloaded from https://academic.oup.com/ce/article-abstract/3/4/241/5554400 by DeepDyve user on 10 December 2019 Wadsack and Acker | 243 hydroelectric system, and the Arizona and California sys- retirements that are known or will meet compliance with tems are very interconnected, the study modelled changes current regulations, as well as additions that are known or in renewable energy capacity and hydroelectric generation must take place to meet compliance with current regula- across Arizona and California. tions, such as Renewable Portfolio Standards. All wind- and solar-power plants in the TEPPC 2024 dataset have fixed-dispatch electricity generation rep- 2.2 Dataset resented by an hourly time series based on the natural resource in the plant’s geographical location. The wind- This research used the PLEXOS production cost model generation data employed in the dataset were hourly- from Energy Exemplar with a 2024 dataset from WECC’s averaged time-series data created for the Western Wind Transmission Expansion Planning Policy Committee and Solar Integration Study . These power time series (TEPPC) . The modelling included all of the WECC were generated using the Weather Research and Forecast balancing areas (Fig. 1); the analyses of the results were fo- mesoscale weather model to produce wind-speed datasets, cused on Arizona, with some additional exploration of re- then run through an algorithm to convert wind-speed sults for California. The TEPPC 2024 Common Case dataset data into wind-power output of wind-power plants com- was developed to represent the WECC bulk power system posed of Vestas V-90 3-MW turbines (details included in in the year 2024 for use in production-cost modelling and ). Solar irradiance and power measurements were de- other transmission and electrical system planning ana- rived from a satellite cloud-cover model to simulate the lyses. The dataset serves as the expected future for the USA at a 10-km hourly resolution [31 33 , ]. The wind- and Western Interconnection, given all known planned devel- solar-power-generation hourly time series were produced opments and policies. It includes all existing electricity- using input weather data from the same calendar year generation resources, transmission and load, and includes BCHA AESO SCL TPWR DOPD AVA PSEI NWMT CHPD WAUW GCPD PGE BPAT IPTV PAID IPMV PAWY IPFE PACW WACW CIPV SPPC PAUT BANC PSCO TIDC VEA CIPB NEVP WALC AZPS CISC PNM SRP LDWP CISD IID TEPC CFE HGMA GRMA EPE DEAA Fig. 1 Map of the TEPPC 2024 power-system dataset region including balancing areas  Downloaded from https://academic.oup.com/ce/article-abstract/3/4/241/5554400 by DeepDyve user on 10 December 2019 244 | Clean Energy, 2019, Vol. 3, No. 4 that serves as the basis for the TEPPC dataset load data, a starting point but, to more accurately reflect the current thus preserving the correlation between the load and the regional power-system configuration, updated some of the wind and solar generation. renewable energy and coal-plant details for Arizona and re- The hydroelectric-generating facilities in the dataset duced the natural-gas fuel price in the reference case. Two are set up in three different ways: fixed dispatch estab- central test cases included high levels of regional renew- lished for each hour of the year, represented as a .csv file ables and, further, the reduction of regional hydroelectric fed into the model for the facility’s operation; monthly capacity. To each of these test cases, different configur - constraints on total generation with a minimum and ations of 100-MW storage units were added to the TEPPC maximum allowed in each hour to represent real-world 2024 model of the transmission system in order to test their seasonal or environmental constraints but to allow the effects. Results from 100-MW, 2-hour-capacity storage units dams to dispatch economically and optimize within the deployed in each of the balancing areas across the West are monthly range; or a pure economic dispatch based on included in this paper as a sensitivity scenario added to the facility’s total generating capacity. Further details re- both the high-renewables and the renewables-plus-drought garding the TEPPC dataset’s specific efforts to ensure ac- test cases. These five cases are named in Table 1 and the de- curate representation of hydroelectric generation in the tails of each case are provided below. model are described in detail in . 2.4.1 Reference and central test cases The reference case (TEPPC+) is based on the default values 2.3 Production-cost modelling in the TEPPC 2024 dataset, with updates to Arizona renew- able and coal generation, in response to regionally an- Production cost models such as PLEXOS are used to com- nounced plans, and a WECC-wide change in natural-gas pare scenarios of the power system in order to evaluate pricing. In Arizona, solar generation was increased by 342 the operational effects of the specific differences between MW. The Navajo Generating Station and Cholla coal-fired scenarios. They model the economic operation of the power plants were shut down and Coronado Generating system and do not take into account life-cycle costs such Station was turned off seasonally. The natural-gas price as capital investments, nor are they used for modelling- was reduced from the dataset values of $4–$6/MMBTu to capacity expansion decisions. $3.50/MMBTu ($3.32/GJ) in order to more accurately reflect PLEXOS is a mixed-integer linear programming current gas prices and projections. production-cost model that allows representation of the The first central test case (AZ30) adds solar- and wind- electricity grid including all load, generation, transmission generating capacity in Arizona and California to meet 30% and ancillary service constraints, and uses an optimiza- and 40% of these states’ load from renewables, respect- tion algorithm to mimic the function of the actual elec- ively. These values were selected based on proposed policy trical system operation. For this research, we used PLEXOS in Arizona and the California mandate to achieve 50% re- to model a security-constrained economic dispatch of newables by 2030. the Western electricity grid using an hourly time step, The second central test case (AZXD) adds an extreme- yielding results for each electricity-generation unit cost drought circumstance to the first central test case. Thus, and dispatch, transmission-line flows and prices across the differences in results between the AZ30 case and the the system. Because of the Arizona study focus, Arizona’s AZXD case isolate the effects of extreme drought. Changes balancing areas (BAs) were modelled nodally, while the in hydroelectric generation in the Colorado River basin and rest of the WECC was modelled zonally. The zonal model- California were selected to represent a regional extreme- ling of the rest of the WECC meant that the precise elec- drought scenario because of the importance of their hydro- trical location of introduced storage or variable generation, electric facilities to the region. The Colorado River system is or hydroelectric facilities that were altered, was not critical the most significant river system in the region; it supplies in those BAs. This simplified the scenario setup and saved drinking water to 27 million people and irrigates 3.5 million considerable computational time. acres of farmland . While most of the roughly 50 dams on the river system are used for irrigation and flood control, the 2.4 Dataset modifications and scenario selection 9 hydroelectric dams provide about 10 000 GWh of electri- In order to evaluate the cost, emissions and operational im- city annually (with Hoover Dam providing an average of 3800 pacts of storage, this study used the 2024 TEPPC dataset as GWh and Glen Canyon Dam providing 3700 GWh annually) Table 1 Production-cost modelling scenario abbreviations Abbreviation TEPPC+ AZ30 AZXD WECCST2 WECCST2XD Case details Reference case High-renewables High renewables High renewables with High renewables and (updated central case and drought in 2-hour storage in all drought in AZ and CA plus dataset) AZ and CA balancing areas (BAs) 2-hour storage in all BAs Downloaded from https://academic.oup.com/ce/article-abstract/3/4/241/5554400 by DeepDyve user on 10 December 2019 Wadsack and Acker | 245 . The electricity produced serves many public power 2.4.2 Storage cases customers, primarily in Arizona, California and Nevada. For the storage cases (WECCST2 and WECCST2XD), a These dams each have storage reservoirs and are capable, system-wide distribution of 100-MW energy-storage units, to varying degrees, of providing ancillary services and re- with 2 hours of energy capacity, were introduced into each serves. In California, hydroelectric generation has provided of the central test cases (AZ30 and AZXD). The storage units an average of 35% of the state’s net electricity generation were developed with parameters representing the char - over the last 18 years, ranging from a high of 47% in 2006 to acteristics of a utility-scale lithium-ion battery system; a low of 18.6% in 2015 . round-trip efficiency of the storage units was 83.329%. The The low-hydro details for Arizona (plant reductions in units were placed in each of the balancing areas across the the Colorado River basin and shutdowns of Hoover and Glen West (depicted in Fig. 1), with the exception of four BAs Canyon Dam) were derived from analysis of the Colorado in California into which large amounts of battery storage River Basin Water Supply and Demand Study  and devel- had been introduced in the TEPPC dataset to represent oped by researchers at Northern Arizona University in col- California storage-policy mandates. laboration with the National Renewable Energy Laboratory and the US Bureau of Reclamation, using Reclamation’s 2.5 Analysis Colorado River Simulation System model (further details provided in , ). The drought circumstances were mod- The results of each of the production cost model runs were elled using downscaled General Circulation Model (GCM) analysed to visualize and compare specific parameters details (under the Coupled Model Intercomparison Project of interest: generation mix, provision of reserves, power- Phase 3, from a suite of 16 multi-model dataset GCMs total- plant starts and stops, and overall carbon dioxide emis- ling 112 future climate projections; additional details in ) sions associated with generation. The results are largely and represent an extended drought scenario under which focused on the Arizona balancing areas, as this was the flow rates were at the tenth percentile of probability, after a target study area. For power-plant start-up/shutdown series of dry years; thus, the Arizona changes modelled rep- cycles and system-operation carbon dioxide emissions, resent a plausible, but very low-probability, circumstance. California results are included in order to illustrate points For Arizona dams in the drought scenarios, the generation about interconnected power-system behaviour. Any re- was altered in dams in the Colorado River basin by changing sults discussed for the balance of the modelled WECC re- the monthly totals allowed to reflect the water availability gion outside the area of focus are described as being for derived from the Reclamation simulation (details provided the rest of the WECC. in ) and allowing the dams to dispatch economically while honouring all given constraints. In California, reductions were based on the California 3 Results and discussion Department of Water Resources California Climate Science 3.1 Electricity-generation mix & Data Report, which summarizes historical indicators The Arizona electricity-generation mix for the selected and future implications and strategies for water-resource cases is shown in Fig. 2. California results are not shown, managers , and a Pacific Institute 2016 report  on the as they exhibit similar patterns. drought years 2012–15. Exact details for the reduction of Arizona is a net exporter of electricity; total load in generation by 15 000 GWH were selected using data from the Arizona BAs is 107 000 GWh. When wind and solar the California Energy Commission. In total, hydroelectric generation was reduced by nearly 80% in the Arizona–Colorado River basin system and by nearly 40% in California. These two sets of circum- Plant stances (Colorado River basin and California hydroelectric- Wind generation reduction) were combined in order to present a 100 000 Nuclear bookend of extreme drought for the region, including the Steam gas very-low-probability climate-modelling results for Arizona Hydro/PSH and the extreme stresses experienced by the California Storage Solar system in the last year of a multi-year drought. Details on 50 000 Coal each of the changes made to the hydroelectric system are CT gas provided in . The drought cases do not include modifi- CC gas cations to the thermal fleet to reflect reductions in water Biomass availability for cooling, as the prioritization of water for this use would be subject to societal choices and tradeoffs, and TEPPC+ AZ30 WECCST2 AZXD WECCST2XD Scenario the quantification of these decisions is beyond the scope of this research. Fig. 2 Total annual electricity generation in Arizona BAs by power source and scenario PSH, pumped storage hydro; CT, combustion tur - See http://www.energy.ca.gov/almanac/renewables_data/hydro/. bine; CC, combined cycle Generation (GWh) Downloaded from https://academic.oup.com/ce/article-abstract/3/4/241/5554400 by DeepDyve user on 10 December 2019 246 | Clean Energy, 2019, Vol. 3, No. 4 generation are added in large amounts, as shown going dispatch either a total amount of energy per month or from case TEPPC+ to AZ30, two things are clear: Arizona within their operating range. Their dispatch of generation increases its total generation of electricity, due to the fact and their ability to offer reserves are optimized for each that this zero-marginal-cost resource displaces other more operating hour to achieve the overall lowest cost to the expensive generation in the region; and the wind and solar system, while honouring all constraints (e.g. ramp rates), generation also displace combined cycle and combus- within the following limits: plants with one generating tion turbine (CT) natural-gas generation, as well as some unit were limited to allocating 50% of their capacity to re- coal-fired generation. When hydroelectric resources are re- serves; and plants with multiple units were limited to al- duced, as shown in cases AZXD and WECCST2XD, natural- locating the greater of 15% or a fraction representing one gas-fired generation increases slightly to meet load. In the over the number of units in the plant of their capacity in two cases including storage, WECCST2 and WECCST2XD, reserves (e.g. a plant with four units could provide 1/4 of its storage can be seen to be used for a small slice of gener - capacity as reserves) . ation but is not a major player; this is expected given the The increase in renewable generation in AZ30, com- relative size of capacity introduced and the inherent eco- pared to TEPPC+, corresponds to an increase in utilization nomics due to the energy-efficiency losses of storage. of combined cycle (CC) natural-gas plants for reserves. In the AZ30 case, these plants are running, but are being dis- placed by renewable capacity in the generation stack, so 3.2 Provision of reserves they are an affordable source of reserve capacity. When storage is introduced into the high-renewables scen- The total annual usage of different power plants in the ario (WECCST2), storage provides considerable reserves Arizona balancing areas for providing operating reserves and a significant decrease in CC-plant utilization is ob- is shown in Fig. 3, arranged by power-plant type and scen- served. In this case, where storage is deployed across the ario. Planning reserve margins are not considered in this West, Arizona also provides lower total reserves capacity analysis. The bars on this figure represent the generation throughout the year, because generators in other regions that was scheduled as available reserves to meet required can meet the reserves requirements at lower cost. In the margins during each hour of the year as the system was drought cases (AZXD and WECCST2XD), the hydroelectric scheduling its dispatch; this does not represent reserves fleet still provides significant reserves, showing that it is that were actually converted to generation. Consequently, an important source of flexibility, even when it is not being the aggregated total reserves offered is counted by the used for as much electricity generation, as described above. model in the energy unit of gigawatt-hours (GWh) instead In both drought cases, CC plants are being used less for re- of in capacity units (GW). In all cases, reserves require- serves; they are being used for more generation to offset ments were met. the loss of hydroelectric energy. In both storage cases, The hourly operating reserves requirements within the storage is utilized for considerable reserves and Arizona WECC TEPPC dataset load areas and reserve regions range provides fewer total reserves; storage deployed throughout from 1% to 4% of daily peak load, with the requirement in the West is being utilized for reserves in similar patterns in the Arizona and California regions set at 4% of daily peak other BAs. Reserve results for California exhibited similar plus a flexible reserve adder based on the contribution of patterns in terms of the proportion of reserves provided by variable wind and solar in any given hour . The hydro- CC, CT and hydroelectric plants in each scenario. The total electric power plants that are capable of providing reserves provision of reserves provided by California generators, in the model are those that are set up to economically however, was lower when reserves provision by Arizona generators was higher in the AZ30 and AZXD cases. 3.3 Power-plant start-up and shutdown cycles Plant The total annual number of start-up/shutdown cycles by Steam gas power-plant type for each scenario is shown in Figs 4 and 4000 Hydro/PSH 5. The first figure shows the total numbers for all of the Storage fossil-fuelled power plants in the Arizona BAs and the Coal CT gas second shows the totals for the same types of plants in the CC gas California BAs. Biomass It is noteworthy that, in Arizona, where there was a much lower percentage of renewable energy in the TEPPC+ reference case, the introduction of renewables to meet TEPPC+ AZ30 WECCST2 AZXD WECCST2XD 30% of load in the AZ30 case decreases the total number of Scenario power-plant starts. In this case, thermal generation is being displaced and the system is not at a point of variability to Fig. 3 Provision of reserves, shown by power-plant type and scenario, in the Arizona BAs cause an increased need for flexibility that could be met Reserves (GWh) Downloaded from https://academic.oup.com/ce/article-abstract/3/4/241/5554400 by DeepDyve user on 10 December 2019 Wadsack and Acker | 247 by thermal plant cycling. In the results for California, the gas-plant cycling to within a few percent of the number of base level of renewables in the TEPPC+ is already close to start/stop cycles experienced in the reference case. 30% by energy and the AZ30 case brings that to 40% re- The results presented here for annual gas-plant cycling newables. In these circumstances, this increase in variable suggest that the combined factors of increased variable generation results in the thermal plants experiencing an generation and reduced flexibility from the hydroelectric increase in start-up and shutdown cycles by almost 7%. fleet could be of concern for individual gas-plant operators In this comparison, natural-gas CT plants increase their and for local air-quality regulators. These results high- start-up/shutdown cycles by nearly 9%. light the value, in terms of balancing changes in system In both Arizona and California, the introduction of sig- makeup, of the flexibility provided by the existing hydro- nificant drought in the AZXD case causes natural-gas (CC electric fleet. Further analysis of the air-quality impacts and CT) plant start-up/shutdown cycling to increase. The and the policy implications of these results are provided increase in cycling from the AZ30 to AZXD cases (repre- in . senting the effect of the drought alone) for CT plants is an additional 33% and 30%, respectively, for the Arizona and 3.4 System-operating costs California BAs. Introducing utility-scale storage into the system across the West—with or without the drought— The total system-operating costs are shown in Table 2. reduces this cycling by more than 20% for California CT These costs include the cost to generate and transmit plants and by more than 30% for Arizona CT plants. Without electricity to meet the system load at all time periods in drought, the existence of storage reduces gas-plant cyc- the modelled year; they do not reflect any capital costs or ling below the level experienced in the reference case. the prices that would be paid to the generators (locational Even with the drought, the storage deployment reduces marginal prices). The total cost for operating the system under the refer - ence case is $18.27 million. Adding significant volumes of renewable energy-generating capacity to this system re- duces the total operating cost because the renewable en- ergy displaces more expensive generation in the mix for each hour. The advent of extreme drought increases the 3000 Plant system-operating cost by more than 4%, because inexpen- Steam gas sive hydroelectric generation is replaced by more expen- Coal sive sources of electricity, and because thermal plants are CT gas experiencing more start-up/shutdown cycling, with its at- CC gas tendant cost. In both the non-drought and drought cases, the introduction of storage across the West decreases the total operating costs by ~0.5%. TEPPC+ AZ30 WECCST2 AZXD WECCST2XD Scenario 3.5 Carbon dioxide emissions Fig. 4 Total annual count of start-up and shutdown cycles by plant type The total volume of annual CO emissions resulting from and scenario for Arizona BAs power-plant operation (not including start-up/shutdown cycles), in million metric tons, is shown in Table 3. It is worth noting that, because production-cost models do not include any life-cycle analysis, the embedded emis- 20 000 sions in production of any of the equipment or in fuel ex- traction and transportation are not counted in this type of research. The emissions shown are based on the emissions 15 000 Plant associated with the operation of each of the thermal plants Steam gas dispatched in the model. They do not include start-up/ Coal 10 000 CT gas shutdown emissions, which are typically relatively minor CC gas for CO compared with other pollutants. In order to accur - ately represent the different emissions rates of thermal Table 2 Total annual system-operating costs ($ million) by scenario. TEPPC+ AZ30 WECCST2 AZXD WECCST2XD Scenario Scenario TEPPC+ AZ30 WECCST2 AZXD WECCST2XD Fig. 5 Total annual count of start-up and shutdown cycles by plant type Total costs $18.27 $17.58 $17.48 $18.33 $18.24 and scenario for the California BAs Plant Starts Plant Starts Downloaded from https://academic.oup.com/ce/article-abstract/3/4/241/5554400 by DeepDyve user on 10 December 2019 248 | Clean Energy, 2019, Vol. 3, No. 4 Table 3 Total annual carbon dioxide emissions (million metric tons) by scenario TEPPC+ AZ30 WECCST2 AZXD WECCST2XD Arizona BAs 46.85 42.86 42.79 44.65 44.48 California BAs 69.01 65.27 64.55 71.15 70.39 Other 240.4 235.4 235.8 239.1 239.6 Total 356.3 343.56 343.2 354.9 354.5 plants operating at different load levels, the plants are set research are useful for considering changes in other re- up in the model with a series of heat-rate bands that rep- gions that demand additional system flexibility. resent the plant’s operational profile and fuel use. As such, The changes in system makeup examined here, the dispatch of the plant at different levels during different including increased renewable energy, but in particular the hours in the model is captured in terms of the effect on extreme decrease in hydroelectric generation in Arizona CO emissions. 2 and California, caused an increase in thermal-generator, For each scenario (listed in columns), the total annual CO particularly CT-plant, start-up and shutdown cycles in emissions for the Arizona BAs, the California BAs, the sum those states. The effect of the drought alone was to in- of the balance of the WECC and the total for the WECC are crease CT-plant start-up/shutdown cycling by more than shown in the rows. It is clear that the reduction of hydroelec- 30% in each state. While increased renewable generation tric generation in the drought scenarios increases system- is well established to decrease system-wide emissions, operating emissions, because the hydroelectric power is being the cases explored here point to a concern for individual replaced by generation from the fossil-fuel fleet (comparing gas-plant owners or for local air-quality regulators under the results in Columns 3 and 5, or 4 and 6, in Table 2). Notably, potential future circumstances of periods of extreme re- emissions increase in these drought scenarios, even in areas duction in regional hydroelectric generation. Importantly, of the system (‘other’ in the table) where the hydroelectric- introducing 100-MW energy-storage units with 2 hours generation fleet was not altered. This highlights another value of generating capacity in each balancing area in the West of hydroelectric generation to the Western electrical system. nearly eliminated this increase in start-up/shutdown cyc- In the scenarios where storage is introduced (comparing ling, without causing a significant rise in local or system- WECCST2 to AZ30 and comparing WECCST2XD to AZXD), emissions. wide CO emissions actually decrease in Arizona and California, but in- Notably, the historical availability of hydroelectric crease in the rest of the WECC. Because of the interconnected nature of the power system, changes in one area can and will plants provides valuable service to the power system affect another area, necessitating regional coordination to in terms of flexibility and being an important source plan or manage major system changes. of low-CO -emissions generation. Likely changes in the hydroelectric fleet across the West due to climate change should be modelled in greater detail in order to under - 4 Summary/conclusions stand the implications for system dynamics and poten- tial consequent infrastructure or operational needs at The goals of this modelling work were two-fold: first, to examine future scenarios of the Western power system a regional or local level. In planning for a future power working to simultaneously mitigate and adapt to cli- system where hydroelectric-plant generation or flexi- mate change and, second, to investigate the effects of bility may be compromised, system operators may need introducing large-scale energy storage into these altered both other and additional sources of flexibility, which system dynamics. The interaction of energy storage and could take many different forms. The relative cost-effect- renewables has been thoroughly explored [2244 , ] and the iveness and environmental impacts of these should be effects of climate change on hydroelectric generation have explored and compared, and the strategies to provide been quantified in some parts of the American electrical these sources of flexibility with lowest cost and impact system ; this work took the next step in combining should be planned regionally. the system operation of the high penetration of renew- ables and changes in hydroelectric generation and testing the introduction of widespread energy storage, in order to Acknowledgements quantify the effects on specific power-system parameters The authors wish to thank Energy Exemplar for the use of an aca- of interest. 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