Solar Futures Study
September 2021 (U.S. Department of Energy Office of Energy Efficiency and Renewable Energy)
Dramatic improvements to solar technologies and other clean energy technologies have enabled recent rapid growth in deployment and are providing cost-effective options for decarbonizing the U.S. electric grid. The Solar Futures Study explores the role of solar in decarbonizing the grid. Through state-of-the-art modeling, the study envisions deep grid decarbonization by 2035, as driven by a required emissions-reduction target. It also explores how electrification could enable a low-carbon grid to extend decarbonization to the broader energy system (the electric grid plus all direct fuel use in buildings, transportation, and industry) through 2050.
The Solar Futures Study uses a suite of detailed power-sector models to develop and evaluate three core scenarios. The “Reference” scenario outlines a business-as-usual future, which includes existing state and federal clean energy policies but lacks a comprehensive effort to decarbonize the grid. The “Decarbonization (Decarb)” scenario assumes policies drive a 95% reduction (from 2005 levels) in the grid’s carbon dioxide emissions by 2035 and a 100% reduction by 2050. This scenario assumes more aggressive cost-reduction projections than the Reference scenario for solar as well as other renewable and energy storage technologies, but it uses standard future projections for electricity demand. The “Decarbonization with Electrification (Decarb+E)” scenario goes further by including large-scale electrification of end uses. The study also analyzes the potential for solar to contribute to a future with more complete decarbonization of the U.S. energy system by 2050, although this analysis is simplified in comparison to the grid-decarbonization analysis and thus entails greater uncertainty.
Even under the Reference scenario, installed solar capacity increases by nearly a factor of 7 by 2050, and grid emissions decline by 45% by 2035 and 61% by 2050, relative to 2005 levels. That is, even without a concerted policy effort, market forces and technology advances will drive significant deployment of solar and other clean energy technologies as well as substantial decarbonization. The target-driven deep decarbonization of the grid modeled in the Decarb and Decarb+E scenarios yields more extensive solar deployment, similarly extensive deployment of wind and energy storage, and significant expansions of the U.S. transmission system. In 2020, about 80 gigawatts (GW) of solar, on an alternating-current basis,1 satisfied around 3% of U.S. electricity demand. By 2035, the decarbonization scenarios show cumulative solar deployment of 760–1,000 GW, 2 serving 37%–42% of electricity demand, with the remainder met largely by other zero-carbon resources, including wind (36%), nuclear (11%–13%), hydroelectric (5%– 6%), and biopower/geothermal (1%). By 2050, the Decarb and Decarb+E scenarios envision cumulative solar deployment of 1,050–1,570 GW, serving 44%–45% of electricity demand, with the remainder met by wind (40%–44%), nuclear (4%–5%), hydropower (3%–5%), combustion turbines run on zero-carbon synthetic fuels such as hydrogen (2%–4%), and biopower/geothermal (1%) (Figure ES-1). Sensitivity analyses show that decarbonization can also be achieved via different technology mixes at similar costs.
Although the Solar Futures Study emphasizes decarbonizing the grid, the Decarb+E scenario envisions decarbonization of the broader U.S. energy system through large-scale electrification of buildings, transportation, and industry. In this scenario, electricity demand grows by about 30% from 2020 to 2035, owing to electrification of fuel-based building demands (e.g., heating), vehicles, and industrial processes. Electricity demand increases by an additional 34% from 2035 to 2050. By 2050, all these electrified sectors are powered by zero-carbon electricity. In this scenario, the combination of grid decarbonization and electrification abates more than 100% of grid CO2 emissions relative to 2005 levels (Figure ES-2).
In terms of the broader U.S. energy system, the Decarb+E scenario reduces CO2 emissions by 62% in 2050, compared with 24% in the Reference scenario and 40% in the Decarb scenario. The 38% residual in the Decarb+E scenario reflects emissions from direct carbon-emitting fossil fuel use, primarily for transportation and industry. We do not model elimination of these remaining emissions in detail, but a simplified analysis of 100% decarbonization of the U.S. energy system by 2050 shows solar capacity doubling from the Decarb+E scenario—equating to about 3,200 GW of solar deployed by 2050—to produce electricity for even greater direct electrification and for production of clean fuels such as hydrogen produced via electrolysis.
The Solar Futures Study is the most comprehensive review to date of the potential role of solar in decarbonizing the U.S. electricity grid and broader energy system. The study was initiated by the U.S. Department of Energy’s Solar Energy Technologies Office and led by the National Renewable Energy Laboratory.
Additional key findings of the study include the following:
• Achieving the decarbonization scenarios requires significant acceleration of clean energy deployment. Compared with the approximately 15 GW of solar capacity deployed in 2020, annual solar deployment doubles in the early 2020s and quadruples by the end of the decade in the Decarb+E scenario. Similarly substantial solar deployment rates continue in the 2030s and beyond. Deployment rates accelerate for wind and energy storage as well.
• Continued technological progress in solar—as well as wind, energy storage, and other technologies—is critical to achieving cost-effective grid decarbonization and greater economy-wide decarbonization. Research and development (R&D) can play an important role in keeping these technologies on current or accelerated cost-reduction trajectories. For example, a 60% reduction in PV energy costs by 2030 could be achieved via improvements in photovoltaic efficiency, lifetime energy yield, and cost. Higher-temperature, higherefficiency concentrating solar power technologies also promise cost and performance improvements. Further advances are also needed in areas including energy storage, load flexibility, generation flexibility, and inverter-based resource capabilities for grid services. With the requisite improvements, solar technologies may proliferate in novel configurations associated with agriculture, waterbodies, buildings, and other parts of the built environment.
• Solar can facilitate deep decarbonization of the U.S. electric grid by 2035 without increasing projected 2035 electricity prices if targeted technological advances are achieved. In the Decarb and Decarb+E scenarios, 95% decarbonization is achieved in 2035 without increasing electricity prices (compared with Reference scenario marginal system costs of electricity), because decarbonization and electrification costs are fully offset by savings from technological improvements and enhanced demand flexibility.
• For the 2020–2050 study period, the benefits of achieving the decarbonization scenarios far outweigh additional costs incurred. Cumulative (2020–2050) power-system costs are one measure of the long-term economics of the decarbonization scenarios, helping to capture the impact of long-lived generating technologies. These costs are about $225 billion (10%) higher in the Decarb scenario than in the Reference scenario—reflecting the added cost of capital investments in clean generation, energy storage, and transmission; operations and maintenance of these assets; and the reduced fuel and other expenditures for fossil fuel technologies. Power-system costs are $562 billion (25%) higher in the Decarb+E scenario, but this higher estimate reflects the costs of serving electrified loads previously powered through direct fuel combustion. Using central estimates for electrification costs, the net incremental cost of the Decarb+E scenario is about $210 billion after factoring out offset fuel expenditures. However, avoided climate damages and improved air quality more than offset those additional costs, resulting in net savings of $1.1 trillion in the Decarb scenario and $1.7 trillion in the Decarb+E scenario.
• The envisioned solar growth will yield broad economic benefits in the form of jobs and workforce development. The solar industry already employs around 230,000 people in the United States, and with the level of growth envisioned in the Solar Futures Study’s scenarios, it could employ 500,000–1,500,000 people by 2035.
• Challenges must be addressed so that solar costs and benefits are distributed equitably. Low- and medium-income communities and communities of color have been disproportionately harmed by the fossil-fuel-based energy system, and the clean energy transition presents opportunities to mitigate these energy justice problems by implementing measures focused on equity. This study explores measures related to the distribution of public and private benefits, the distribution of costs, procedural justice in energy-related decision making, the need for a just workforce transition, and potential negative externalities related to solar project siting and disposal of solar materials.
• Solar can help decarbonize the buildings, transportation, and industrial sectors. In the Decarb+E scenario, electrification of fuel-based end uses enables solar electricity to power about 30% of all building end uses and 14% of transportation end uses by 2050. For buildings, rooftop solar can increase the value of batteries and investments in load automation systems; distributed batteries and load automation can, in turn, increase the grid value of solar. For transportation, rooftop solar could increase the value of electric vehicle adoption to consumers through a combination of low-marginal-cost electricity and managed charging—and thus could accelerate electrification of the transportation sector. The longterm role of solar electricity in industry is less certain, but industrial process heat from concentrating solar thermal plants can help decarbonize this sector as well. In all three sectors, solar can play a long-term role in producing zero-carbon fuels.
• Diurnal energy storage enables high levels of decarbonization, but additional clean firm capacity is needed to achieve full grid decarbonization. In the Decarb+E scenario, storage with 12 hours or less of energy capacity expands by up to 70-fold, from 24 GW in 2019 to more than 1,600 GW in 2050. This diurnal storage complements renewable energy deployment by storing energy when it is less useful to the grid and releasing it when it is more useful. However, because solar and wind occasionally provide insufficient supply for several days, advances in technologies that can provide clean firm capacity at any time are needed to reliably meet demand as full decarbonization is approached.
• Maintaining reliability in a grid powered primarily by renewable energy requires careful power system planning. In the decarbonization scenarios, the grid becomes increasingly reliant on weather-dependent inverter-based resources (IBRs) such as PV, representing a dramatic change from the current grid based primarily on synchronous electricity generators. A grid dominated by IBRs will require new approaches to maintain system reliability and exploit the ability of IBRs to respond quickly to system changes. New approaches may also be required for high-solar grids to maintain resilience (defined as the ability of grids to respond to critical events such as natural disasters). Small-scale solar, especially coupled with storage, can enhance resilience by allowing buildings or microgrids to power critical loads during grid outages. In addition, advances in managing distributed energy resources, such as rooftop solar and electric vehicles, are needed to integrate these resources efficiently into electricity distribution systems.
• Demand flexibility plays a critical role by providing firm capacity and reducing the cost of decarbonization. Demand flexibility shifts demand from end uses, such as electric vehicles, to better utilize solar generation. In the Decarb+E scenario, demand flexibility provides 80–120 GW of firm capacity by 2050 and reduces decarbonization costs by about 10%.
• Developing U.S. solar manufacturing could mitigate supply chain challenges, but different labor standards and regulations abroad create cost-competitiveness challenges. Global PV supply chains can be constrained by production disruptions, competing demand from other industries or countries, and political disputes. A resilient supply chain would be diversified and not over reliant on any single supply avenue. To enhance the domestic supply chain, American solar technology manufacturers may improve competitiveness by increasing automation and exploiting the advantages of domestically manufacturing certain components. Policies can help promote domestic solar manufacturing.
• Material supplies related to technology manufacturing likely will not limit solar growth in the decarbonization scenarios, especially if end-of-life materials displace use of virgin materials via circular-economy strategies. Under the decarbonization scenarios, demands for important PV materials are small relative to global production of these materials, even when assuming use of virgin materials only and accounting for simultaneous growth in PV deployment worldwide. Displacing virgin material use through circular-economy strategies would enhance material supplies. However, breakthroughs in technologies and participation in what is currently a voluntary recycling and circular-economy landscape in the United States will be required to maximize use of recoverable materials—yielding benefits to energy and materials security, improved social and environmental outcomes, and opportunities for the domestic workforce and manufacturing sectors.
• Although land acquisition poses challenges, land availability does not constrain solar deployment in the decarbonization scenarios. In 2050, ground-based solar technologies require a maximum land area equivalent to 0.5% of the contiguous U.S. surface area. This requirement could be met in numerous ways including use of disturbed lands. The maximum solar land area required is equivalent to less than 10% of potentially suitable disturbed lands, thus avoiding conflicts with high-value lands in current use. Various approaches are available to mitigate local impacts or even enhance the value of land that hosts solar systems. Installing PV systems on waterbodies, in farming or grazing areas, and in ways that enhance pollinator habitats are potential ways to enhance solar energy production while providing benefits such as lower water evaporation rates and higher agricultural yields.
• Water withdrawals decline by about 90% by 2050 in the decarbonization scenarios. The water savings result from the low water use of solar and other clean energy generation technologies, compared with fossil fuel and nuclear generators.
• Achieving the Solar Futures Study’s vision requires long-term policy and market support in addition to continued innovation. Decarbonization targets set by policy are critical to decarbonizing more quickly than would occur owing to market conditions alone. Policy also accelerates cost reductions and technological innovations through R&D investments as well as through driving deployment and reducing costs through learning-bydoing. Even with significant cost and technology improvements, policy will be crucial for promoting decarbonization as the marginal costs of decarbonization increase. In addition, wholesale electricity markets must adapt to the increasingly dominant roles of zero-marginalcost renewable energy, and retail markets must adapt with rates that reflect the changing grid and an increased role for distributed energy resources. Nascent markets such as those for demand-side services and enhanced energy reliability may need to evolve to optimize the roles of distributed energy resources, and efforts are needed to expand the use of these resources to traditionally underserved groups.
A dramatically larger role for solar in decarbonizing the U.S. electricity system, and energy system more broadly, is within reach, but it is only possible through concerted policy and regulatory efforts as well as sustained advances in solar and other clean energy technologies…