Electric power generation in the United States is currently dominated by the use of fossil fuels. Coal is the major fuel used to produce electricity from steam turbine-generators employing basic principles in use since the Industrial Revolution. However, burning coal results in environmental costs as emissions of nitrogen and sulfur oxides contribute to formation of smog and acid rain. Today, carbon dioxide emissions from the burning of coal and other fossil fuels are also widely believed to be contributing to global climate change and its potentially damaging effects. Renewable energy has been used since long before the Industrial Revolution, but not on the scale of steam power generation. Renewable energy technologies use the power of the sun, wind, water, and heat from the earth, offering the possibility of producing electricity on a large scale without many of the environmental and climate consequences of electric power generation using fossil fuels. If harnessed by the right technologies, renewable energy offers the possibility for achieving inexpensive, almost limitless electricity with minimal adverse environmental impacts. Much of the modern impetus for renewable energy development in the United States came from the Arab oil embargo of the 1970s, and the resulting energy crisis. A focus on national security and energy independence emerged with a goal of reducing dependence on foreign supplies of oil. While petroleum is not a major fuel source for electric power generation, the importance of energy to the national economy was underscored. Energy security and independence concerns, combined with growing reliability and environmental concerns led to the development of the renewable energy research and development programs at the U.S. Department of Energy (DOE). Today, added concerns over the global impacts of anthropogenic climate change and a desire for a lasting recovery from the recent recession have increased calls for a comprehensive national strategy making renewable energy a cornerstone of a policy for continuing U.S. economic development and jobs growth. Congress is currently considering legislation to address these concerns. This report discusses current ideas for a federal Renewable Electricity (or Energy) Standard (RES) and a broader Clean Energy Standard (CES). The RES concept would require certain retail electric providers to obtain a minimum percentage of the power they sell from renewable energy sources or energy efficiency. The CES concept would extend the eligible technologies meeting the requirement to include advanced coal and nuclear energy, while also targeting retirement of existing polluting fossil generation. This report refers generally to both concepts as an RES unless otherwise stated. The goal of this report is to explore how such policies could potentially increase the amounts of renewable electricity generated in the United States, discussing other related public policy goals and rationales for renewable energy development, and the challenges/drawbacks of RES policy. The choice of power generation technology in the United States is heavily influenced by the cost of fuel. Historically, the use of fossil fuels has provided some of the lowest prices for generating electricity. Figure 1 shows that, as of 2009, coal accounts for approximately 45% of net generation by the electric power sector, followed by natural gas at 23%, and nuclear power at 20%. Electric power generation is responsible for 37% of U.S. domestic carbon dioxide emissions (the primary anthropogenic greenhouse gas (GHG)), and over one-third of all U.S. GHG emissions. Growing concerns over GHG emissions, other environmental costs associated with burning fossil fuels, and existing or anticipated state and federal policies addressing these issues are leading some utilities and energy providers to deploy more renewable energy technologies to meet power demands. As of 2009, hydropower represented 7% of all U.S. electric power industry net generation with all other renewable energy accounting for a further combined total of 4%. A summary of the status of current major renewable energy technologies follows, using figures for domestic estimated growth based on projections in the DOE's Energy Information Administration's (EIA's) Annual Energy Outlook for 2010 . Renewable energy technologies are at different stages of maturity in the technology development cycle, and are still being optimized from an engineering viewpoint. Some of the major perceived barriers the technologies must overcome to achieve a greater share of the electricity generation market are presented. Biomass for electric power is arguably the most conventional of all renewable electricity technologies and has a potentially large future. Biomass is currently the largest non-hydropower source of renewable energy consumed in the United States. Approximately 53% of all renewable energy used comes from biomass represented by biofuels, landfill gas, biogenic municipal solid waste, wood, wood-derived fuels, and other biomass such as switchgrass and poplar trees. Agricultural wastes (such as corn stover) are another potential feedstock. With wood and biomass net summer capacity reported at 7 Gigawatts (GW) for 2009, DOE estimates that 29 GW of domestic biomass generation could be available by 2030. Sustainable management of biomass resources, especially forests, will be critical to this future. Biomass combustion is a relatively mature technology, but it is not widely used and is generally most efficient from a heat input-to-energy produced perspective when used in a combined heat-and-power application. Large scale co-firing of biomass with coal is a higher efficiency, lower per unit cost option. Technologies for biomass gasification could potentially result in higher efficiencies when used to produce synthesis gas or hydrogen for heat and/or power production. Demonstration and deployment of newer industrial gasification technologies is needed to scale-up plants and provide economical designs with high degrees of availability. Huge potential exists for the biomass category, as it is generally regarded as a carbon-neutral source of energy. With about 10 GW of capacity installed, wind power was second only to natural gas in U.S. capacity additions in 2009. The total installed and grid-connected wind power capacity as of 2009 in the United States reached approximately 35 GW. DOE estimates that domestic wind power could reach a capacity of 68 GW by 2030 in its baseline scenario. Wind turbines are increasing in generating capacity, with domestic turbines in 2009 averaging 1.74 Megawatts (MW) in capacity. When the wind is blowing at speeds which can be harnessed, electricity can be generated at prices nearly competitive with conventional fossil fuels. Since the wind doesn't blow all the time and varies in strength, average costs are higher and integration of large amounts of wind into an electricity grid has often been raised as an issue. Backup generation in the form of natural gas combustion turbines has been the standby choice in some instances. Energy storage (using batteries or other means) is often suggested as a potential answer to deal with intermittency concerns. Since many of the best wind resource areas in the United States are far from population centers where the electricity will be used, the development of transmission facilities to carry power to population centers has been discussed as a prerequisite for wider development. Given these factors, DOE projects (with a deliberate and sustained national effort) that as much as 20% of the nation's electrical supply could be provided by wind energy by 2030. This would require wind power capacity to reach 300 GW, or a growth of over 280 GW over the next 21 years. Achieving such a prodigious goal would mean addressing significant challenges in technology, manufacturing, employment, transmission and grid integration, markets, and siting strategies. Offshore wind power in the United States is a fledgling industry, having just received federal authority in 2010 to go ahead with the first U.S. offshore wind farm in Nantucket Sound, off the Massachusetts coast. Known as the Cape Wind project, it is designed to operate 130 turbines from the German firm Siemens AG with a total capacity of 420 MW. Permits for more than 2,476 MW capacity for offshore projects are pending as of 2009. The overall potential for U.S. offshore wind power production capacity was estimated at 908 GW in 2005. Another renewable resource with enormous potential is sunlight. Sunlight is converted directly into electricity using solar photovoltaic (PV) cells which today are largely made from crystalline silicon. Research is underway to reduce the cost of PV cells using base materials other than silicon (such as cadmium telluride) and to improve manufacturing techniques which may increase the efficiency of solar cells. Technologies to increase or concentrate the amount of sunlight in PV cells could raise the efficiency of the light-to-electricity conversion. Solar PV is widely used in a number of off-grid applications where distributed energy resources are useful, and in peaking power applications to reduce power usage from the electric utility grid. Battery storage is important to off-grid usage to extend hours of usage past peak daylight. While solar PV installations only represented 1.37 GW of cumulative capacity in 2009, DOE estimates generating capacity from solar PV could reach almost 12 GW by 2030 in the United States. Since the amount of electricity that can be produced from solar PV generally depends on the intensity of sunshine (and the angle at which PV panels face the sun), the best potential exists for applications in sunny regions and lower latitudes. But the relative success of solar PV installations in Germany (with a total capacity of 8.9 GW in 2009) prove the wider applicability of the technology in less-than-optimal climes. Integration of PV cells and materials into building structures and designs could be a major step for the technology. Concentrating solar thermal technologies use mirrors to concentrate sunlight and generate heat usually for steam production. This steam is then used to generate electricity or provide high-temperature hot water for industrial or other process uses such as heating and cooling. Fairly large areas of land are needed, plus access to water since it is used for steam generation and cooling. Some novel applications heat air directly to generate thermal gradients which are harnessed to produce electricity. Solar thermal technologies are currently used for utility-scale power generation, but costs in the United States are considerably higher than prices for fossil-fuel power generation.