Quantifying the challenge of reaching a 100% renewable energy power system for the United States

Abstract

•100% RE can be reached in the US using existing technologies•Marginal costs rise nonlinearly for the last few percent of RE penetration•Average costs remain much lower, even at 100% RE This study evaluates pathways and quantifies the costs of transitioning to a 100% renewable energy (RE) power system for the contiguous United States. That cost depends on future system conditions (e.g., low versus high RE costs), the definition of the 100% requirement (e.g., whether the requirement applies to end-use demand or total generation), and the time frame for reaching 100%. Under base case conditions, the least-cost buildout has RE penetration reaching 57% in 2050 with average levelized costs of $30/MWh. Requiring penetration of 100% by 2050 raises these costs to less than $10/MWh (29%). Incremental costs increase nonlinearly for the last few percent to 100% RE. The scenarios examined improve the understanding of the implications of transitioning to higher penetrations of renewable energy. We simulate pathways for achieving up to 100% renewable energy (RE) electric power systems for the contiguous United States. Under base conditions, the least-cost buildout has RE penetration growing up to 57% in 2050. Relative to this base scenario, average CO2 abatement costs of achieving 80%, 90%, 95%, and 100% RE are $25, $33, $40, and $61/ton, respectively, with system costs growing from $30 to $36/MWh at 95% (achieved in 2040) and $39/MWh at 100%. Incremental abatement costs from 99% to 100% RE reach $930/ton, driven primarily by the need for firm RE capacity. In addition to the base conditions, we also examined 22 alternative conditions for a buildout of up to 100%. These sensitivities capture different technology trajectories, compliance requirements, requirement timing, electrification, and transmission availability. Nonlinear marginal costs for the last few percent approaching 100% RE were found for all sensitivities, which might motivate alternative nonelectric-sector abatement opportunities. We simulate pathways for achieving up to 100% renewable energy (RE) electric power systems for the contiguous United States. Under base conditions, the least-cost buildout has RE penetration growing up to 57% in 2050. Relative to this base scenario, average CO2 abatement costs of achieving 80%, 90%, 95%, and 100% RE are $25, $33, $40, and $61/ton, respectively, with system costs growing from $30 to $36/MWh at 95% (achieved in 2040) and $39/MWh at 100%. Incremental abatement costs from 99% to 100% RE reach $930/ton, driven primarily by the need for firm RE capacity. In addition to the base conditions, we also examined 22 alternative conditions for a buildout of up to 100%. These sensitivities capture different technology trajectories, compliance requirements, requirement timing, electrification, and transmission availability. Nonlinear marginal costs for the last few percent approaching 100% RE were found for all sensitivities, which might motivate alternative nonelectric-sector abatement opportunities. Over the last decade, there has been growing discussion about, and interest in, the possibility of transitioning electric systems to be powered by 100% renewable energy (RE). This interest is evident in state, local, and corporate RE goals and mandates,1Sierra Club Committed. Sierra Club, 2020https://www.sierraclub.org/ready-for-100/commitmentsGoogle Scholar,2RE100 Companies - RE100.https://www.there100.org/companiesDate: 2020Google Scholar and in the increasingly large set of literature that examines the benefits and challenges of moving to 100% RE.3Deason W. 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Sims R. et al.Energy storage in long-term system models: a review of considerations, best practices, and research needs.Prog. Energy. 2020; 2: 032001Crossref Google Scholar Although some aspects of 100% RE are fairly well established (e.g., wind, solar, storage, and transmission all play an active role in 100% RE systems), many aspects remain ambiguous as have been discussed in review articles on this topic.3Deason W. Comparison of 100% renewable energy system scenarios with a focus on flexibility and cost.Renew. Sustain. Energy Rev. 2018; 82: 3168-3178Crossref Scopus (51) Google Scholar, 4Hansen K. Breyer C. Lund H. Status and perspectives on 100% renewable energy systems.Energy. 2019; 175: 471-480Crossref Scopus (287) Google Scholar, 5Diesendorf M. Elliston B. The feasibility of 100% renewable electricity systems: a response to critics.Renew. Sustain. Energy Rev. 2018; 93: 318-330Crossref Scopus (87) Google Scholar,7Denholm P. Arent D. Baldwin S.F. Bilello D.E. Brinkman G.L. Cochran J. Cole W.J. Frew B. Gevorgian V. Heeter J. et al.The challenges of achieving a 100% renewable electricity system in the United States.Joule. 2021; https://doi.org/10.1016/j.joule.2021.03.028Abstract Full Text Full Text PDF Scopus (18) Google Scholar Additionally, the definition of 100% RE can vary. Some definitions expand eligibility to include non-RE clean energy technologies such as nuclear or carbon capture and sequestration (CCS), while others limit it to explicitly renewable technologies. Similarly, some definitions use sales as a basis for the target16Barbose G. U.S. Renewables Portfolio Standards: 2019 Annual Status Update. Lawrence Berkeley National Laboratory, 2019Google Scholar while others use total generation—inclusive of transmission and distribution losses—which increases the stringency of the target. In this work, we explore both the eligibility and the basis for the target by considering a variety of scenarios, including RE-only and low-carbon targets and by applying the target to both generation and to sales. While there is a large body of work that evaluates high RE electricity systems for many geographic extents,4Hansen K. Breyer C. Lund H. Status and perspectives on 100% renewable energy systems.Energy. 2019; 175: 471-480Crossref Scopus (287) Google Scholar,5Diesendorf M. Elliston B. The feasibility of 100% renewable electricity systems: a response to critics.Renew. Sustain. Energy Rev. 2018; 93: 318-330Crossref Scopus (87) Google Scholar,17Jacobson M.Z. Cameron M.A. Hennessy E.M. Petkov I. Meyer C.B. Gambhir T.K. Maki A.T. Pfleeger K. Clonts H. McEvoy A.L. et al.100% clean and renewable wind, water, and sunlight (WWS) all-sector energy roadmaps for 53 towns and cities in North America.Sustain. Cities Soc. 2018; 42: 22-37Crossref Scopus (49) Google Scholar, 18Zozmann E. Göke L. Kendziorski M. Rodriguez del Angel C. von Hirschhausen C. Winkler J. 100% renewable energy scenarios for North America—spatial distribution and network constraints.Energies. 2021; 14: 658Crossref Scopus (10) Google Scholar, 19Cochran J. Denholm P. Mooney M. Steinberg D. Hale E. Heath G. Palmintier B. Sigrin B. Keyser D. McCamey D. et al.LA100: The Los Angeles 100% Renewable Energy Study. National Renewable Energy Laboratory, 2021Crossref Google Scholar there are few studies of the full United States power system that consider 100% RE. Furthermore, even fewer studies have addressed the question of how much it will cost to transition from our current power system to one with 100% RE for the United States; instead, they typically present snapshots of systems in a future year without considering the evolution needed to arrive there. Jacobson et al.20Jacobson M.Z. Delucchi M.A. Cameron M.A. Frew B.A. Low-cost solution to the grid reliability problem with 100% penetration of intermittent wind, water, and solar for all purposes.Proc. Natl. Acad. Sci. USA. 2015; 112: 15060-15065Crossref PubMed Scopus (244) Google Scholar,21Jacobson M.Z. Delucchi M.A. Bazouin G. Bauer Z.A.F. Heavey C.C. Fisher E. Morris S.B. Piekutowski D.J.Y. Vencill T.A. Yeskoo T.W. 100% clean and renewable wind, water, and sunlight (WWS) all-sector energy roadmaps for the 50 United States.Energy Environ. Sci. 2015; 8: 2093-2117Crossref Google Scholar and Frew et al.22Frew B.A. Becker S. Dvorak M.J. Andresen G.B. Jacobson M.Z. Flexibility mechanisms and pathways to a highly renewable US electricity future.Energy. 2016; 101: 65-78Crossref Scopus (101) Google Scholar were among the first to publish high-profile analyses of 100% systems for the full United States using high-resolution wind and solar datasets. Aghahosseini et al.6Aghahosseini A. Bogdanov D. Barbosa L.S.N.S. Breyer C. Analysing the feasibility of powering the Americas with renewable energy and inter-regional grid interconnections by 2030.Renew. Sustain. Energy Rev. 2019; 105: 187-205Crossref Scopus (69) Google Scholar and Zozmann et al.18Zozmann E. Göke L. Kendziorski M. Rodriguez del Angel C. von Hirschhausen C. Winkler J. 100% renewable energy scenarios for North America—spatial distribution and network constraints.Energies. 2021; 14: 658Crossref Scopus (10) Google Scholar built on this work and extended it to geographic extents larger than just the United States Brown and Botterud23Brown P.R. Botterud A. The value of inter-regional coordination and transmission in decarbonizing the US electricity system.Joule. 2020; 5: 115-134Abstract Full Text Full Text PDF Scopus (30) Google Scholar added additional temporal detail work by examining transmission in 100% RE scenarios over seven years of historical weather and load data. Other researchers, such as Shaner et al.,14Shaner M.R. Davis S.J. Lewis N.S. Caldeira K. Geophysical constraints on the reliability of solar and wind power in the United States.Energy Environ. Sci. 2018; 11: 914-925Crossref Google Scholar have modeled 100% RE systems but without considering costs and while focusing on systems that rely exclusively on wind and solar power. Further work has examined important aspects of 100% RE systems using subsections of the United States (e.g., Budischak et al.24Budischak C. Sewell D. Thomson H. Mach L. Veron D.E. Kempton W. Cost-minimized combinations of wind power, solar power and electrochemical storage, powering the grid up to 99.9% of the time.J. Power Sources. 2013; 225: 60-74Crossref Scopus (364) Google Scholar) or using representative systems (e.g., Sepulveda et al.25Sepulveda N.A. Jenkins J.D. de Sisternes F.J. Lester R.K. The role of firm low-carbon electricity resources in deep decarbonization of power generation.Joule. 2018; 2: 2403-2420Abstract Full Text Full Text PDF Scopus (160) Google Scholar), and we note connections between our results and their findings throughout this paper. Our work builds on this prior work, but extends it in several important ways by:(1)Modeling the evolution of the current system to a 100% system for the full contiguous United States rather than taking a snapshot of a steady-state system in some future year,(2)Considering a wide range of sensitivities (see Table 1 in the experimental procedures) in both system conditions and application of the 100% requirement that highlights how costs can vary,(3)Capturing the ability to retrofit existing fossil plants to serve needs under 100% scenarios,(4)Evaluating whether inertial response can be maintained at 100% RE,(5)Using detailed production cost modeling with unit commitment and economic dispatch to verify the results of the capacity expansion modeling, and(6)Representing the power system with higher spatial and technology resolution than previous studies in order to better capture differences in technology types, RE resource profiles, RE siting and land-use constraints, and transmission challenges. We focus primarily on estimating the cost of achieving a 100% RE power system for the contiguous United States. We consider how that cost changes under a wide range of future conditions and definitions for 100%, including with systems that allow non-RE low-carbon technologies to participate. Our objective is to robustly quantify the cost of a transition to high penetration RE system in a way that provides electric sector decision makers with the information they need to assess the cost and value of pursuing higher penetration RE systems. Furthermore, given the frequent emphasis on using the electricity sector as the foundation for economy-wide decarbonization,7Denholm P. Arent D. Baldwin S.F. Bilello D.E. Brinkman G.L. Cochran J. Cole W.J. Frew B. Gevorgian V. Heeter J. et al.The challenges of achieving a 100% renewable electricity system in the United States.Joule. 2021; https://doi.org/10.1016/j.joule.2021.03.028Abstract Full Text Full Text PDF Scopus (18) Google Scholar,26Larson E. Greig C. Jenkins J. Mayfield E. Pascale A. Zhang C. Drossman J. Williams R. Pacala S. Socolow R. et al.Net- zero america: potential pathways, infrastructure, and impacts interim report. Princeton University, 2020https://wiresgroup.com/wp-content/uploads/2021/01/Princeton_NZA_Interim_Report_15_Dec_2020_FINAL-1.pdfGoogle Scholar,27Williams J.H. Jones R.A. Haley B. Kwok G. Hargreaves J. Farbes J. Torn M.S. Carbon-neutral pathways for the United States.AGU Adv. 2021; 2 (e2020AV000284)Crossref Google Scholar we believe this work extends understanding of what is required to decarbonize this sector. We use total cumulative system cost as the primary metric for assessing the challenge of increased RE penetration for the contiguous United States power system. This system cost is the sum of the cost of building and operating the bulk power system assets over the model horizon to 2050, discounted to 2020 using a real discount rate of 5%. We begin by considering the system cost of increasing RE penetrations for base conditions, which use the mid-range projections for all model inputs such as capital costs, fuel prices, and demand growth (see Section S3 for details). The reference scenario, which has no RE requirement and only existing state and federal policies as of June 2020, results in a least-cost system that grows RE penetration from 20% today to 57% in 2050 and has a discounted system cost of $2,617 billion. This 57% RE scenario serves as the baseline for comparing higher RE penetration scenarios (see Section S7 for a counterfactual scenario that holds the 2020 generation mix constant over time). To enforce higher RE penetrations, we use a trajectory that scales RE penetration linearly from 20% in 2020 to 95% in 2040, and linearly again from 95% to 100% in 2050. Scenarios with less than 100% RE follow the same pathway but hold their final level constant through 2050 (see experimental procedures and Section S3 for details of the RE penetration trajectory). As we require higher RE penetration requirements—80%, 90%, 95%, 97%, 99%, and 100% as a fraction of total generation by 2050—the system cost increases as shown in Figure 1 (left). The corresponding levelized system electricity cost is $30/MWh for the least-cost baseline of 57% RE in 2050, increasing to $39/MWh for 100% RE (see Table S10). The cost increases observed under the higher penetration requirements result primarily from greater capital expenditures for RE technologies which outpace reductions in fuel costs and other power system operational costs, particularly for the addition of RE-fueled combustion turbines (RE-CTs) from 95% to 100% RE. Associated with the high RE penetration targets are substantial reductions in CO2 emissions. From the counterfactual constant mix scenario to the least-cost 57% RE scenario, both the cost of power and CO2 emissions decline, effectively resulting in a negative cost to reduce emissions (see Figure S37). From the 57% least-cost scenario, Figure 1 (right) translates the changes in system cost and CO2 emissions between scenarios into an average and incremental levelized abatement cost. The average value is the abatement cost relative to the reference scenario with 57% RE, while the incremental value is the abatement cost between adjacent scenarios, e.g., between 80% RE and 90% RE. These values are calculated as the difference in system cost between scenarios divided by the difference in emissions between scenarios, both discounted at 5% (see Figure S22 for other discount rates). The average levelized abatement costs from the reference scenario of 80%, 90%, 95%, 97%, 99%, and 100% RE are $25, 33, 40, 48, 56, and 61/ton, respectively. The incremental abatement cost is nonlinear and reaches a cost of $930/ton (using baseline technology assumptions for the technologies evaluated) when comparing the 99% RE and 100% RE scenarios. The average values are much lower because the costs are spread across a larger reduction in emissions. The system cost and CO2 abatement costs increase with RE penetration above the 57% baseline in the base scenarios for many reasons. The reference scenario (see Figure 2) is a least-cost scenario, so by definition, any requirement to increase or decrease RE penetration relative to that least-cost solution will increase total system costs. In going from 20% RE to the reference scenario, thermal units with lower efficiency or higher costs have already been pushed out of the system, so increasing the penetration beyond that 57% penetration level means renewables are displacing more efficient or lower cost generators, some of which have not yet been paid off. Additionally, after wind and PV are deployed on lower cost or higher resource quality sites, sequentially higher cost (lower quality) sites must be leveraged to achieve the higher penetration levels. Furthermore, because marginal RE curtailment rates increase with penetration, RE utilization will decrease as higher targets are achieved, requiring more nameplate capacity to serve the same load.28Hirth L. The market value of variable renewables.Energy Econ. 2013; 38: 218-236Crossref Scopus (474) Google Scholar Rapid scale-up in deployment (see Figure S16) could also impact the cost of reaching 100% RE, but was not considered in this work. Another related reason for the increasing cost at higher RE penetration is the need to serve load during periods of low RE availability and/or high demand. To ensure the system is resource adequate (i.e., sufficient capacity is available to meet anticipated demands), we model seasonal firm capacity requirements that are equivalent to the North American Electric Reliability Corporation (NERC) reference reserve margin levels.29NERC2018 long-term reliability assessment. North American Electric Reliability Corporation, 2018https://www.nerc.com/pa/RAPA/ra/Reliability%20Assessments%20DL/NERC_LTRA_2018_12202018.pdfGoogle Scholar Figure 3 shows the sources of firm capacity in the seven base scenarios in 2050 for the summer and winter (spring is not shown because there was always excess firm capacity in spring, and fall firm capacity is similar to summer). Even at 95% RE penetration, approximately half of the firm capacity is procured from nonrenewable, non-storage sources. These units have very low capacity factors (they generate just 5% of the annual energy). In the winter, solar and storage contribute at their lowest level for the year. Wind, solar, and diurnal storage have effectively maxed out their ability to deliver more firm capacity (see Figure S27 for hourly dispatch plots of the peak and net peak periods), which follows the well-established trend of declining capacity credit as a function of penetration.30Mills A.D. Wiser R.H. Changes in the economic value of photovoltaic generation at high penetration levels: a pilot case study of California.IEEE J. Photovoltaics. 2013; 3: 1394-1402Crossref Scopus (32) Google Scholar, 31Holttinen H. Kiviluoma J. Forcione A. Milligan M. Smith C.J. Dillon J. Dobschinski J. van Roon S. Cutululis N. Orths A. et al.Design and operation of power systems with large amounts of wind power: final summary report, IEA wind task 25, phase three 2012-2014. VTT Technical Research Centre of Finland, 2016https://community.ieawind.org/HigherLogic/System/DownloadDocumentFile.ashx?DocumentFileKey=87ada8ed-5a95-20bd-2bd8-5174e6e065d6%20&%20forceDialog=0Google Scholar, 32Frazier A.W. Cole W. 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A major cost of moving from 95% RE to 100% RE is the cost of replacing the thermal generators that operate at very low capacity factors with other technologies that can fill that role. Existing natural gas generators can be retrofitted to become RE-CT generators by paying an upgrade cost, and 252 GW of gas capacity is upgraded in the 100% RE scenario, with the remaining 251 GW of RE-CT capacity built as new capacity. Because of the RE-CT deployed in the highest RE penetration scenarios, the capacity mix pathway among these scenarios is different (see Figure 2). Other developing technologies such as fuel cells have the potential to fill this role at lower cost if research and manufacturing targets are realized.34Dubois A. Ricote S. Braun R.J. Benchmarking the expected stack manufacturing cost of next generation, intermediate-temperature protonic ceramic fuel cells with solid oxide fuel cell technology.J. Power Sources. 2017; 369: 65-77Crossref Scopus (60) Google Scholar,35Langholtz M. 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The storage losses shown in Figure 4 reflect only efficiency losses from batteries and pumped-storage hydropower (PSH), and they do not account for energy losses in the renewable fuel production used by RE-CTs, which might be addressed in part by using curtailed power to produce RE-CT fuel. To avoid stability and certain reliability challenges associated with 100% inverter-based systems,36Holttinen H. Kiviluoma J. Flynn D. Smith C. Orths A. Eriksen P.B. Cutululis N.A. Soder L. Korpas M. Estanqueiro A. et al.System impact studies for near 100% renewable energy systems dominated by inverter based variable generation.IEEE Trans. Power Syst. 2020; https://doi.org/10.1109/TPWRS.2020.3034924Crossref PubMed Scopus (10) Google Scholar these scenarios rely on the commercially demonstrated frequency responsive capabilities of inverter-based resources,37Loutan C. Klauer P. Chowdhury S. Hall S. Morjaria M. Chadliev V. Milam N. Milan C. Gevorgian V. Demonstration of Essential Reliability Services by a 300-MW Solar Photovoltaic Power Plant. National Renewable Energy Laboratory, 2017https://www.nrel.gov/docs/fy17osti/67799.pdfCrossref Google Scholar but they also rely on synchronous machines to provide inertial response, system strength and fault current, without the need for grid-forming inverters. Along with the remaining synchronous RE (hydropower, geothermal, and biopower), we assume 25% of the RE-CTs (approximately 125 GW in the base 100% RE scenario) are clutched to allow their generators to act as synchronous condensers so they can provide grid support services without running at partial load and consuming fuel.38Kroposki B. Johnson B. Zhang Y. Gevorgian V. Denholm P. Hodge B.M. Hannegan B. Achieving a 100% renewable grid: operating electric power systems with extremely high levels of variable renewable energy.IEEE Power Energy Mag. 2017; 15: 61-73Crossref Scopus (575) Google Scholar This technology provides inertial response and system strength, and the amount deployed exceeds the required inertial response; details are provided in Section S5. The prior section described the cost of moving to 100% RE using base case conditions and a 100% definition that required all generations to come from RE technologies. Changing the system conditions or the 100% requirement can change these costs. Figure 5 (left) summarizes the system cost results for the base scenarios and for 22 variations of system conditions or 100% requirements. The 23 lines in the left plot of Figure 5 are composed of 154 scenarios, with each of the 23 scenario groupings running with requirements of no, 80%, 90%, 95% 97%, 99%, and 100% penetration requirement. This figure shows how system costs increase with the requirement. Scenario names indicate the sensitivity; specific definitions for these scenarios are given in the experimental procedures section, and they are summarized in Table 1. For the scenarios that reached 100% requirement by 2050, we found that system costs for 100% systems are 21%–50% higher than the corresponding least-cost scenario with no penetration requirement. Lower penetration requirements have lower cost increases, with a 3%–17% system cost increase for the 80% requirement, a 5%–23% increase for the 90% requirement, and a 13%–33% for the 97% requirement. Average and incremental abatement costs at 100% RE range from $60–106/ and 766–2,447/ton, respectively. Abatement costs are presented in Figures S23 and S24 and Tables S7 and S8. The scenarios have d