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Electricity from Renewable Resources: Status, Prospects, and Impediments Summary Renewable resources—the sun, wind, water, and biomass—were the first to be tapped to provide heat, light, and usable power. But throughout the 20th century and today, the dramatic increase in energy use for industrial, residential, transportation, and other purposes has been fueled largely by the energy stored in fossil fuels and, more recently, supplied by nuclear power. Linked to the exploitation and development of high-energy-density resources such as coal and oil at the scales required for powering the modern U.S. energy system are potentially significant environmental and other impacts. Concern about greenhouse gases released by the combustion of these fuels, for example, and awareness of eventual limits on the supply of fossil-fuel resources have strengthened interest in expanding the use of renewable energy resources. Escalations in energy prices, increasing worldwide demand for energy, and the need to ensure U.S. energy security have also combined to put energy in the headlines, increasing policy makers’ interest in domestically produced renewable energy. As part of the America’s Energy Future (AEF) project initiated by the National Academy of Sciences and the National Academy of Engineering (Appendix A), the National Research Council convened the Panel on Electricity from Renewable Resources (Appendix B) to examine the technical potential for development and deployment of renewable electricity technologies. The full statement of task is provided in Box P.1 in the preface. As a result of its study, the panel found that technologies for generation of electricity from renewable resources represent a significant opportunity—with attendant challenges—to provide low carbon dioxide (CO2)–emitting electricity generation from resources available domestically and to generate new economic
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Electricity from Renewable Resources: Status, Prospects, and Impediments opportunities for the United States. Sufficient domestic renewable resources exist to allow renewable electricity to play a significant role in future electricity generation and thus help confront issues related to climate change, energy security, and the escalation of energy costs. Generation of electricity from renewable resources has increased substantially over the past 20 years. As shown in Chapter 1, some sources have sustained a 20 percent or higher compound annual growth rate in capacity expansion and electricity generation over the past decade. However, non-hydroelectric renewable resources still provide only a small percentage of total U.S. electricity generation (about 2.5 percent of all electricity generated), even with these large recent growth rates. The most recent U.S. Energy Information Administration projections, which are presented in Chapter 1, indicate that under a “business as usual” scenario, the share of electricity generated from non-hydroelectric renewable resources in 2030 would be only 8 percent of the total U.S. electricity generated. The panel concluded that sustained actions involving the coordination of policy, technology, and capital investment will be essential to achieving a greatly increased market penetration of renewable electricity. All three of these factors are important because improvements in the economics of renewable electricity generation, large increases in the scale and rate of deployment, and the establishment of consistent long-term policies are all required in order for renewables to make a material contribution to the nation’s energy supply. Although continued technological advances are critical, the degree of penetration by renewable electricity will also be determined by actions that collectively center on sustainably improving the economic competiveness of electricity generated from renewable versus other resources and on policy initiatives that have a positive impact on competitive balance and the ease of deployment of renewable electricity. CHALLENGES AND OPPORTUNITIES AHEAD FOR THE USE OF RENEWABLE ELECTRCITY Immense challenges are presented by the need to reduce the vulnerabilities associated with climate change, energy supply interruptions, and volatile fossil-fuel markets. Reducing electric-sector CO2 emissions by significant levels will require major changes in how we use and produce electricity. Cutting energy imports and substantially reducing our dependence on fossil fuels also will involve major changes. Reliance on a greater amount of renewable energy, particularly renewable
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Electricity from Renewable Resources: Status, Prospects, and Impediments electricity, can help address these challenges. Renewable energy is an attractive option because renewable resources available in the United States, taken collectively, can supply significantly greater amounts of electricity than the total current or projected domestic demand. These renewable resources are largely untapped today. There are, however, important disadvantages to the use of non-hydropower renewables for electricity generation. The energy available from renewable resources is less concentrated than that provided by fossil-fuel or nuclear power, posing significant challenges to the development of renewable resources for electricity generation on a large scale. Generation must occur at the site of the resource and accommodate the temporal fluctuations characteristic of some non-hydropower renewable resources. At high penetrations of non-hydropower renewable sources, electricity system operators must deal with spatial and temporal constraints to integrating the generated electricity into the electric grid in ways that ensure a reliable, controllable supply of electricity. Large penetrations also will result in land-use requirements that in turn can lead to instances of local opposition to the siting of generation and transmission facilities. In turn, the use of renewable electricity provides some significant advantages over the use of fossil-based electricity. Many types of renewable electricity-generating technologies can be developed and deployed in smaller increments, and constructed more rapidly, than large-scale fossil- or nuclear-based generation systems, thus allowing faster returns on capital investments. Generation of electricity from most renewable resources also reduces vulnerability to increases in the cost of fuels and mitigates many environmental impacts, such as those associated with atmospheric emissions of greenhouse gases and emissions of regulated air pollutants. Further, distributed renewable electricity generation located at or near the point of energy use, such as solar photovoltaic systems installed at residential, commercial, or industrial sites, can offer operational and economic benefits while increasing the robustness of the electricity system as a whole. FINDINGS Shown in bold text are the most critical elements of the panel’s findings based on its consideration of the material presented in Chapters 2 through 7 of this report.
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Electricity from Renewable Resources: Status, Prospects, and Impediments Timeframes and Prospects for Renewable Technologies To better assess the prospects for individual renewable electricity technologies, the panel separated its consideration of these technologies and their characteristic costs, performance, and impacts into three time periods: an initial period that considers present technologies out to the year 2020; a second that considers current and potential renewable electricity technologies over the 2020 to 2035 time period; and a third period that looks at technologies beyond 2035. For the time period from the present to 2020, there are no current technological constraints for wind, solar photovoltaics and concentrating solar power, conventional geothermal, and biopower technologies to accelerate deployment. The primary current barriers are the cost-competitiveness of the existing technologies relative to most other sources of electricity (with no costs assigned to carbon emissions or other currently unpriced externalities), the lack of sufficient transmission capacity to move electricity generated from renewable resources to distant demand centers, and the lack of sustained policies. Expanded research and development (R&D) is needed to realize continued improvements and further cost reductions for these technologies. Along with favorable policies, such improvements can greatly enhance renewable electricity’s competitiveness and its level of deployment. Action now will set the stage for greater, more cost-effective penetration of renewable electricity in later time periods. It is reasonable to envision that, collectively, non-hydropower renewable electricity could begin to provide a material contribution (i.e., reaching a level of 10 percent or more, with trends toward continued growth) to the nation’s electricity generation in the period up to 2020 with such accelerated deployment. Combined with hydropower, total renewable electricity could approach a contribution of 20 percent of U.S. electricity by the year 2020. In the period from 2020 to 2035, it is reasonable to envision that continued and even further accelerated deployment could potentially result in non-hydroelectric renewables providing, collectively, 20 percent or more of domestic electricity generation by 2035. In the third timeframe, beyond 2035, continued development of renewable electricity technologies could potentially provide lower costs and result in further increases in the percentage of renewable electricity generated from renewable resources. However, achieving a predominant (i.e., >50 percent) level of renewable electricity penetration will require new scientific advances (e.g., in solar photovoltaics, other renewable electricity technologies, and storage technologies) and dramatic changes in how we generate, transmit, and use electricity. Scientific advances are anticipated to improve the cost, scalability,
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Electricity from Renewable Resources: Status, Prospects, and Impediments and performance of all renewable energy generation technologies. Moreover, some combination of intelligent, two-way electric grids; scalable and cost-effective methods for large-scale and distributed storage (either direct electricity energy storage or generation of chemical fuels); widespread implementation of rapidly dispatchable fossil-based electricity technologies; and greatly improved technologies for cost-effective long-distance electricity transmission will be required. Significant, sustained, and greatly expanded R&D focused on these technologies is also necessary if this vision is to be realized by 2035 and beyond. Resource Base Solar and wind renewable resources offer significantly larger total energy and electricity potential than do other domestic renewable resources. Although solar intensity varies across the nation, the land-based solar resource provides a yearly average of more than 5 × 1022 J (13.9 million TWh) and thus exceeds, by several thousand-fold, present annual U.S. electrical energy demand, which totals 1.4 × 1019 J (~4,000 TWh). Hence, at even modest conversion efficiency, solar energy is capable, in principle, of providing enormous amounts of electricity without stress to the resource base. The land-based wind resource is capable of providing at least 10–20 percent, and in some regions potentially higher percentages, of current electrical energy demand. Other (non-hydroelectric) renewable resources can contribute significantly to the electrical energy mix in some regions of the country. Renewable resources are not distributed uniformly in the United States. Resources such as solar, wind, geothermal, tidal, wave, and biomass vary widely in space and time. Thus, the potential to derive a given percentage of electricity from renewable resources will vary from location to location. Awareness of such factors is important in developing effective policies at the state and federal levels to promote the use of renewable resources for generation of electricity. Renewable Technologies Over the first timeframe through 2020, wind, solar photovoltaics and concentrating solar power, conventional geothermal, and biomass technologies are technically ready for accelerated deployment. During this period, these technologies could potentially contribute a much greater share (up to about an additional 10 percent of electricity generation) of the U.S. electricity supply than they do today. Other technologies, including enhanced geothermal systems that mine the heat
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Electricity from Renewable Resources: Status, Prospects, and Impediments stored in deep low-permeability rock and hydrokinetic technologies that tap ocean tidal currents and wave energy, require further development before they can be considered viable entrants into the marketplace. The costs of already-developed renewable electricity technologies will likely be driven down through incremental improvements in technology, “learning curve” technology maturation, and manufacturing economies of scale. Despite short-term increases in cost over the past couple of years, in particular for wind turbines and solar photovoltaics, there have been substantial long-term decreases in the costs of these technologies, and recent cost increases due to manufacturing and materials shortages will be reduced if sustained growth in renewable sources spurs increased investment in them. In addition, support for basic and applied research is needed to drive continued technological advances and cost reductions for all renewable electricity technologies. In contrast to fossil-based or nuclear energy, renewable energy resources are more widely distributed, and the technologies that convert these resources to useful energy must be located at the source of the energy. Further, extensive use of intermittent renewable resources such as wind and solar power to generate electricity must accommodate temporal variation in the availability of these resources. This variability requires special attention to system integration and transmission issues as the use of renewable electricity expands. Such considerations will become especially important at greater penetrations of renewable electricity in the domestic electricity generation mix. A contemporaneous, unified intelligent electronic control and communications system overlaid on the entire electricity delivery infrastructure would enhance the viability and continued expansion of renewable electricity in the period from 2020 to 2035. Such improvements in the intelligence of the transmission and distribution grid could enhance the whole electricity system’s reliability and help facilitate integration of renewable electricity into that system, while reducing the need for backup power to support the enhanced utilization of renewable electricity. In the third time period, 2035 and beyond, further expansion of renewable electricity is possible as advanced technologies are developed, and as existing technologies achieve lower costs and higher performance with the maturing of the technology and an increasing scale of deployment. Achieving a predominant (i.e., >50 percent) penetration of intermittent renewable resources such as wind and solar into the electricity marketplace, however, will require technologies that are largely unavailable or not yet developed today, such as large-scale and distributed cost-effective energy storage and new methods for cost-effective, long-distance electricity transmission. Finally, there might be further consideration of an inte-
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Electricity from Renewable Resources: Status, Prospects, and Impediments grated hydrogen and electricity transmission system such as the “SuperGrid” first championed by Chauncey Starr, though this concept is still considered high-risk. Economics A principal barrier to the widespread adoption of renewable electricity technologies is that electricity from renewables (except for electricity from large-scale hydropower) is more costly to produce than electricity from fossil fuels without an internalization of the costs of carbon emissions and other potential societal impacts. Policy incentives, such as renewable portfolio standards, the production tax credit, feed-in tariffs, and greenhouse gas controls, thus have been required, and for the foreseeable future will continue to be required, to drive further increases in the use of renewable sources of electricity. Unlike some conventional energy resources, renewable electricity is considered manufactured energy, meaning that the largest proportion of costs, external energy, and materials inputs, as well as environmental impacts, occur during manufacturing and deployment rather than during operation. In general, the use of renewable resources for electricity generation involves trading the risks of future cost increases for fossil fuels and uncertainties over future costs of carbon controls for present fixed capital costs that typically are higher for use of renewable resources than for use of fossil fuels. Except for biopower, no fuel costs are associated with renewable electricity sources. Further, in contrast to coal and nuclear electricity plants, in which larger facilities tend to exhibit lower average costs of generation than do smaller plants, for renewable electricity the opportunities for achieving economies of scale are generally greater at the equipment manufacturing stage than at the generating site itself. The future evolution of costs for generation of electricity from renewable resources will depend on continued technological progress and breakthroughs. It will also depend on the potential for policies to create greater penetration and to accelerate the scale of production—largely an issue of long-term policy stability and policy clarity. Markets will generally exploit the lowest-cost resource options first, and thus the costs of renewables may not decline in a smooth trajectory over time. For example, in the case of wind power, the lowest-cost resources are generally available at the most accessible sites in the highest wind class areas. Development of these prime resources will thus entail significant resource cost shifts as markets adjust to exploit next-tier resources. At present, onshore wind is an economically favored option relative to other (non-hydroelectric) renewable
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Electricity from Renewable Resources: Status, Prospects, and Impediments resources, and hence wind power is expected to continue to grow rapidly if recent policy initiatives continue into the future. Although some forecasts show that biopower will play an important role in meeting future renewable portfolio standards targets, the degree of competition with and recent mandates for use of liquid biofuels for providing transportation fuel and, of course, the use of biomass for food, agricultural feed, and other uses will impact the prospects for greater use of biomass in the electricity market. The future of distributed renewable electricity generation from sources such as residential photovoltaics will depend on how its costs compare to the retail price of power delivered to end users, on whether prices fully reflect variations in cost over the course of the day, and on whether the external costs of fossil-based electricity generation are increasingly incorporated into its price. Formulation of robust predictions about whether the price of electricity will meet or exceed the price required for renewable sources to be profitable and what their resulting level of market penetration will be remains a difficult proposition. Comparisons between past forecasts of renewable electricity penetration and actual data show that, while renewable technologies generally have met forecasts of cost reductions, they have fallen short of deployment projections. Further, the profitability and penetration of electricity generated from renewable resources may be sensitive to investments in energy efficiency, especially if efficiency improvements are sufficient to meet growth in the demand for electricity or lower the market-clearing price of electricity. If the financial operating environment for fossil-fuel and other in-place sources of electricity remains unchanged, then the competitiveness of renewable electricity may be affected more than that of other electricity sources. However, at this time, the deployment of renewable electricity is being driven by tax policies, in particular by the renewable production tax credit, and by renewable portfolio standards. Environmental Impacts Renewable electricity technologies have inherently low life-cycle CO2 emissions as compared to fossil-fuel-based electricity production, with most emissions occurring during manufacturing and deployment. Renewable electricity generation also involves inherently low or zero direct emissions of other regulated atmospheric pollutants, such as sulfur dioxide, nitrogen oxides, and mercury. Biopower is an exception because it produces NOx emissions at levels similar to those associated with fossil-fuel power plants. Renewable electricity technologies (except biopower,
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Electricity from Renewable Resources: Status, Prospects, and Impediments high-temperature concentrated solar power, and some geothermal technologies) also consume significantly less water and have much smaller impacts on water quality than do nuclear, natural-gas-, and coal-fired electricity generation technologies. Because of the diffuse nature of renewable resources, the systems needed to capture energy and generate electricity (i.e., wind turbines and solar panels and concentrating systems) must be installed over large collection areas. Land is also required for the transmission lines needed to connect this generated power to the electricity system. But because of low levels of direct atmospheric emissions and water use, land-use impacts tend to remain localized and do not spread beyond the land areas directly used for deployment, especially at low levels of renewable electricity penetration. Moreover, some land that is affected by renewable technologies can also be used for other purposes, such as the use of land between wind turbines for agriculture. However, at a high level of renewable technologies deployment, land-use and other local impacts would become quite important. Land-use impacts have caused, and will in the future cause, instances of local opposition to the siting of renewable electricity-generating facilities and associated transmission lines. State and local government entities typically have primary jurisdiction over the local deployment of electricity generation, transmission, and distribution facilities. Significant increases in the deployment of renewable electricity facilities will thus entail concomitant increases in the highly specific, administratively complex, environmental impact and siting review processes. While this situation is not unique to renewable electricity, nevertheless, a significant acceleration of its deployment will require some level of coordination and standardization of siting and impact assessment processes. Deployment Policy, technology, and capital are all critical for the deployment of renewable electricity. In addition to enhanced technological capabilities, adequate manufacturing capacity, predictable policy conditions, acceptable financial risks, and access to capital are all needed to greatly accelerate the deployment of renewable electricity. Improvements in the relative position of renewable electricity will require consistent and long-term commitments from policy makers and the public. Investments and market-facing research that focuses on market needs as opposed
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Electricity from Renewable Resources: Status, Prospects, and Impediments to technology needs are also required to enable business growth and market transformations. Successful technology deployment in emerging energy sectors such as renewable electricity depends on sustained government policies, at both the project and the program level, and continued progress requires stable and orderly government participation. Uncertainty created when policies cycle on and off, as has been the case with the federal production tax credit, can hamper the development of new projects and reduce the number of market participants. Significant increases in renewable electricity generation will also be contingent on concomitant improvements in several areas, including the size and training of the workforce; the capabilities of the transmission and distribution grids; and the framework and regulations under which the systems are operated. As with other energy resources, the material deployment of renewable electricity will necessitate large and ongoing infusions of capital. However, renewable energy requires a greater allocation of capital to manufacturing and infrastructure requirements than do the conventional fossil-based energy technologies. Integration of the intermittent characteristics of wind and solar power into the electricity system is critical for large-scale deployment of renewable electricity. Advanced storage technologies will play an important role in supporting the widespread deployment of intermittent renewable electric power above approximately 20 percent of electricity generation, although electricity storage is not necessary below 20 percent. Storage tied to renewable resources has three distinct purposes: (1) to increase the flexibility of the resources in providing power when the sun is not shining or the wind is not blowing, (2) to allow the use of energy on peak when its value is greatest, and (3) to facilitate increased use of the transmission line(s) that connect the resource to the grid. The last is particularly relevant if the resource is located far from the load centers or if the system output does not match peak load times well, as is often the case with wind power. However, wind power’s development is occurring long before widespread storage will be economical. Although storage is not required for continued expansion of wind power, the inability to maximize the use of transmission corridors built to move wind resources to load centers represents an inefficient deployment of resources. Several parties are currently exploring the co-location of natural-gas-fired generation and other types of electricity generation with wind power generation to bridge this gap between storage technology and asset utilization. The co-siting of conventional dispatchable generation sources (such as natural-gas-fired combustion turbines or combined cycle plants) with renewable resources could serve as an interim mechanism to increase the value of renewable electric power until advanced storage
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Electricity from Renewable Resources: Status, Prospects, and Impediments technologies are technically feasible and economically attractive. The location of such natural-gas-fired generation could be at or near the wind resource, or at an appropriate site within the control area. Another possibility is the co-siting of two (or more) renewable resources, such as wind and solar resources, which might on average interact synergistically with respect to their temporal patterns of power generation and needs for transmission capacity. Finally, it is important to note that the deployment needs and impacts from renewable electricity deployment are not evenly distributed regionally. Development of solar and wind power resources has been growing at an average annual rate of 20 percent and higher over the past decade. Overall electricity demand is forecasted to continue to grow at just under 1 percent annually until 2030, with the southeastern and southwestern regions of the United States expected to see most of this growth. Although some of this growth may correspond to areas where renewable resources are available, some of it will not, indicating the possible need for increases in electricity transmission capacity. Scale of Deployment An understanding of the scale of deployment necessary for renewable resources to make a material contribution to U.S. electricity generation is critical to assessing the potential for renewable electricity generation. Large increases over current levels of manufacturing, employment, investment, and installation will be required for non-hydropower renewable resources to move from single-digit- to double-digit-percentage contributions to U.S. electricity generation. The Department of Energy’s study of 20 percent wind penetration by 2030 discussed in Chapter 7 demonstrates the challenges and potential opportunities—100,000 wind turbines would have to be installed; up to $100 billion worth of additional capital investments and transmission upgrades would be required; 140,000 jobs would have to be filled; and more than 800 million metric tons of CO2 emissions would be eliminated. In the panel’s opinion, increasing manufacturing and installation capacity, employment, and financing to meet this goal by 2030 is doable, but the magnitude of the challenge is clear from the scale of such an effort. Integration of Renewable Electricity The cost of new transmission and upgrades to the distribution system will be important factors when integrating increasing amounts of renewable electricity. The nation’s electricity grid needs major improvements regardless of whether renewable electricity generation is increased. Such improvements would increase
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Electricity from Renewable Resources: Status, Prospects, and Impediments the reliability of the electricity transmission system and would reduce the losses incurred with all electricity sources. However, because a substantial fraction of new renewable electricity generation capacity would come from intermittent and/or distant sources, increases in transmission capacity and other grid improvements are critical for significant penetration of renewable electricity sources. According to the Department of Energy’s study postulating 20 percent wind penetration, transmission could be the greatest obstacle to reaching the 20 percent wind generation level. Transmission improvements can bring new resources into the electricity system, provide geographical diversity in the generation base, and allow improved access to regional wholesale electricity markets. These benefits can also generally contribute positively to the reliability, stability, and security of the grid. Improvements in the system’s distribution of electricity are needed to maximize the benefits of two-way electricity flow and to implement time-of-day pricing. Such improvements would more efficiently integrate distributed renewable electricity sources, such as solar photovoltaics sited at residential and commercial units. A significant increase in renewable sources of power in the electricity system would also require fast-responding backup generation and/or storage capacity, such as that provided by natural gas combustion turbines, hydropower, or storage technologies. Higher levels of penetration of intermittent renewables (above about 20 percent) would require batteries, compressed air energy storage, or other methods of storing energy such as conversion of excess generated electricity to chemical fuels. Improved meteorological forecasting could also facilitate increased integration of solar and wind power. Hence, though improvements in the grid and related technologies are necessary and valuable for other objectives, significant integration of renewable electricity will not occur without increases in transmission capacity as well as other grid management improvements. FUTURE PROSPECTS FOR RENEWABLE ELECTRICITY Currently, use of renewable resources for electricity generation generally incurs higher direct costs than those currently seen for fossil-based electricity generation, whose price does not now include the costs associated with carbon emissions and other unpriced externalities. Some form of market intervention or combination of incentives is thus required to enable renewable resources to contribute substantially to the national electrical energy generation mix. Sustained, consistent, long-term policies that provide for production tax credits, market incentives,
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Electricity from Renewable Resources: Status, Prospects, and Impediments streamlined permitting, and/or renewable portfolio standards are essential to support significant growth of the market for renewable electricity. With such policies and economic incentives in place, up to 20 percent of additional domestic electricity generation could come from non-hydropower renewable technologies within approximately the next 25 years. In turn, significant technological and scientific barriers must be surmounted if renewables are to provide upward of 50 percent or more of domestic electricity generation in a reliable, controllable system that also has a low-carbon-emissions footprint. The barriers include those related to transmission as well as system integration and flexibility, including storage and other enabling technologies. Specifically, large-scale and distributed electrical energy storage, and/or large capacities for rapidly controllable low-carbon-emission generation, would be required to reach such a goal. Further, a systemwide intelligent, digitally controlled grid could reduce the need for backup power and storage and further facilitate the penetration of renewable electricity into the marketplace. Significant R&D is required now if such technologies are to be available in time to facilitate deployment of renewable electricity at a level of 50 percent or higher. Research is also needed to ensure that large-scale deployment of renewable electricity will not lead inadvertently to undesirable environmental consequences. CRITICAL UNKNOWNS The panel notes that many major unknowns will affect the future of electricity from renewable resources. Several are highlighted below. Technologies—The prospects for reducing manufacturing costs and improving the efficiencies of renewable electricity technologies, including the potential for solar photovoltaics to bring the installed system cost down to less than $1 per watt with at least 10 percent module and system efficiency to enable widespread deployment without subsidies; Economics—The price of electricity in the future, how prices will be structured, and the explicit or implicit price of CO2 imposed by any future climate policy; Policy—The structure of renewables portfolio standards, tax policies (production and/or investment tax credits), and other policy initiatives directed at renewable electricity;
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Electricity from Renewable Resources: Status, Prospects, and Impediments Biomass—The contribution of biomass to electricity production versus the use of the biomass energy resource base for the production of liquid fuels; Transmission—The mechanisms and responsibilities for increases in transmission capacity and other upgrades for the electricity grid; and Transportation—The degree to which renewable electricity can influence the transportation sector and reduce dependence on imported oil and liquefied natural gas through, in the near term, charging vehicle batteries and, in the long term, producing non-petroleum-based fuels. CONCLUSION A future characterized by a large penetration of renewable electricity represents a paradigm shift from the current electricity generation, transmission, and distribution system. There are many reasons why renewable electricity represents such a shift, including the spatial distribution and intermittency of some renewable resources, and issues related to greatly increasing the scale of deployment. Wind and solar, renewable energy resources with the potential for large near-term growth in deployment, are intermittent resources that have some of their base located far from demand centers. The transformations required to incorporate a significant penetration of additional renewables include transformation in ancillary capabilities, especially the expansion of transmission and backup power resources, and deployment of technologies that improve grid intelligence and provide greater system flexibility. Further, supplying renewable resources on a scale that would make a major contribution to U.S. electricity generation would require vast investment in and deployment of manufacturing and human resources, as well as additional capital costs relative to those associated with current generating technologies that have no controls on greenhouse gas emissions. The realization of such a future would require a predictable policy environment and sufficient financial resources. Nevertheless, the promise of renewable resources is that they offer significant potential for low-carbon generation of electricity from domestic sources of energy that are much less vulnerable to fuel cost increases than are other electricity sources. Overall success depends on having technology, capital, and policy working together to enable renewable electricity technologies to become a major contributor to America’s energy future.