Renewable energy comes from sources that can be regenerated or naturally replenished. The main sources of renewable energy are:
Renewable energy is used for electric power generation, space heating and cooling, and transportation fuels. All sources of renewable energy are used to generate electric power. In addition to generating electricity, geothermal steam is used directly for heating and cooking. Biomass and solar sources are also used for space and water heating. Ethanol and biodiesel are the renewable transportation fuels with gaseous biomethane also fueling transport to a much lesser extent.
Renewable energy sources are considered to be zero (wind, solar, and water), low (geothermal) or neutral (biomass) with regard to greenhouse gas emissions during their operation. A neutral source has emissions that are balanced by the amount of carbon dioxide absorbed during the growing process. However, each source’s overall environmental impact depends on its overall lifecycle emissions, including manufacturing of equipment and materials, installation as well as land-use impacts.
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Water power captures the energy of flowing water in rivers, streams and waves to generate electricity. Conventional hydropower plants can be built in rivers with no water storage (known as “run-of-the-river” units) or in conjunction with reservoirs that store water, which can be used on an as-needed basis. As water travels downstream, it is channeled down through a pipe or other intake structure in a dam (penstock). The flowing water turns the blades of a turbine, generating electricity in the powerhouse, located at the base of the dam.
Figure 1. Hydroelectric Power Generation
Source: Environment Canada , 2012
Large conventional hydropower projects currently provide the majority of renewable electric power generation. With 1,000 gigawatts (GW) of global capacity, hydropower produced an estimated 3,750 terawatt  hours (TWh) of total global electricity in 2013. Note that in 2013, total global electricity generation was 22,865 TWh. Hydropower operational costs are relatively low, and it generate little to no greenhouse gas emissions. The main environmental impact is to local ecosystems and habitats; a dam to create a reservoir or divert water to a hydropower plant changes the ecosystem and physical characteristic of the river.
The United States is the fourth-largest producer of hydropower after China, Canada and Brazil. In 2011, a much wetter than average year in the U.S. Northwest, the United States generated 7.9 percent of its total electricity from hydropower. The quantity of electricity generated each year depends on the amount of precipitation that falls over a particular area.
Small hydropower, generally less than 10 megawatts (MW), and micro-hydropower (less than 1 MW) are less costly to develop and have a lower environmental impact than large conventional hydropower projects. In 2011, the total amount of small hydro installed worldwide was 106 GW – China had the largest share  at 55.3 percent, followed by India at 9 percent and the United States at 6.9 percent. Many countries  have renewable energy targets that include the development of small hydro projects. In the United States, the Federal Energy Regulatory Commission (FERC) approved more than 50 project permits in 2009.
Hydrokinetic electric power , including wave and tidal power, is a form of unconventional hydropower that captures energy from waves or currents and does not require dam construction. These technologies are in various stages of research, development and deployment. In 2011, a 254 MW tidal power plant  in South Korea  began operation, doubling the global capacity to 527 MW.
Low-head hydro  is a commercially available source of hydrokinetic electric power that has been used in farming areas for more than 100 years. Generally, the capacity of these devices is small, ranging from 1kW to 250kW.
Pumped storage hydropower plants use inexpensive electricity (typically overnight during periods of low demand) to pump water from a lower-lying storage reservoir to a storage reservoir located above the power house for later use during periods of peak electricity demand. Since this technology uses more electricity than it generates, it is not considered to be renewable energy. Note that it is economical to do this since the revenues that a generator receives during times of peak electricity generation far exceed the costs that they pay to pump the water during times of low electricity demand.
Figure 2. Pumped Storage Power Generation
Source: U.S. Geological Survey , 2012
Water is one of the most widely used renewable energy sources worldwide. For more details about these technologies, see Climate Techbook: Hydropowe r and Hydrokinetic Electric Power .
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Wind power harnesses the energy generated by the movement of air in the earth’s atmosphere to drive electricity-generating turbines. Although people have used wind power for hundreds of years, modern turbines reflect significant technological advances over early windmills and even over turbines from just 10 or 20 years ago. Generating electric power using wind turbines creates no greenhouse gases, but since a wind farm includes dozens or more turbines, widely-spaced, it requires thousands of acres  of land. For example, Lone Star is a 200 MW wind farm located in Texas on approximately 36,000 acres.
After hydropower, wind was the next largest renewable energy source used for power generator with 318 GW of global capacity  at the end of 2013, producing 2.9 percent of global electricity. Capacity is indicative of the maximum amount of electicity that can be generated when the wind is blowing at sufficient levels for a turbine. Because the wind is not always blowing, wind farms do not always produce as much as their capacity. With more than 91 MW, China  had the largest installed capacity of wind generation in 2013, and the United States  with 62 GW had the second-largest capacity; Texas, Iowa, California, Oklahoma and Illinois were the top five wind power producing  states.
Average turbine size has been steadily increasing over the past 30 years. Today, new onshore turbines are typically in the range of 1.5 – 3.5 MW. The largest production models, designed for off-shore use, are capable of generating more than 7.5 MW; some innovative turbine models  under development are expected to generate more than 15 MW in offshore projects in the coming years. Due to higher costs and technology constraints, off-shore capacity , approximately 3 GW in 2010, is only a small share of total installed wind generation capacity. For more information on wind power, see Climate TechBook: Wind .
Figure 3. Size and Power Evolution of Wind Turbines Over Time
Solar power harnesses the sun’s energy to produce electricity as well as solar heating and cooling. Solar energy resources are massive and widespread, and they can be harnessed anywhere that receives sunlight. The amount of solar radiation, also known as insolation, reaching the earth’s surface  every hour is more than all the energy currently consumed by all human activities each year. A number of factors, including geographic location, time of day, and current weather conditions, all affect the amount of energy that can be harnessed for electricity production or heating purposes.
Solar energy can be captured for electricity production using solar photovoltaics and concentrating solar power. A solar or photovoltaic cell converts sunlight into electricity using the photoelectric effect . Typically, photovoltaic is found on the roofs of residential and commercial buildings. Concentrating solar power uses lenses or mirrors to concentrate sunlight into a narrow beam that heats a fluid, producing steam to drive a turbine which generates electricity. Concentrating solar power projects are larger-scale than residential or commercial PV and are often owned and operated by electric utilities.
Figure 4. Concentrating Solar Power
Source: NextEra Energy, 2012 
Solar hot water heaters, typically found on the roofs of homes and apartments, provide residential hot water by using a solar collector, which absorbs solar energy, that in turn heats a conductive fluid, and transfers the heat to a water tank. Modern collectors are designed to be functional even in cold climates and on overcast days.
Electricity generated from solar energy emits no greenhouse gases. The main environmental impacts  of solar energy come from the use of some hazardous materials (arsenic and cadmium) in the manufacturing of PV and the large amount of land required, hundreds of acres, for a utility-scale solar project. For more information on solar energy, see Climate TechBook: Solar .
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Biomass energy sources are used to generate electricity, provide direct heating and can be converted into biofuels as a direct substitute for fossil fuels used in transportation. Unlike intermittent wind and solar energy, biomass can be used continuously or according to a schedule. Biomass is derived  from wood, waste, landfill gas, crops and alcohol fuels. Traditional biomass , including waste wood, charcoal and manure has been a source of energy for domestic cooking and heating throughout human history. In rural areas of the developing world, it remains the dominant fuel source. Globally in 2010, traditional biomass accounted for about 8.5 percent of total energy consumption. The growing use of biomass has resulted in increasing international trade  in biomass fuels in recent years; wood pellets, biodiesel, and ethanol are the main fuels traded internationally.
In 2013, global biomass electric power capacity stood at 88 GW . In 2011, the United States had 11.5 GW of installed biomass-fueled electric generation capacity . In the United States, most of the electricity from wood biomass is generated at lumber and paper mills  using their own wood waste; in addition, wood waste is used to generate the heat for drying wood products and other manufacturing processes. Biomass waste is mostly municipal solid waste , i.e., garbage, which is burned as a fuel to run power plants. On average, a ton of garbage  generates 550 to 750 kWh of electricity. Landfill gas contains methane that can be captured, processed and used  to fuel power plants, manufacturing facilities, vehicles and homes. In the United States, there is currently nearly 2 GW of installed landfill gas-fired generation capacity  at more than 600 projects.
In addition to landfill gas, biofuels can be synthesized from dedicated crops, trees and grasses, agricultural waste and algae feedstock; these include renewable forms of diesel, ethanol, butanol, methane and other hydrocarbons. Corn ethanol is the most widely used biofuel in the United States. Roughly 40 percent of the U.S. corn crop  was diverted to the production of ethanol for gasoline in 2010, up from 20 percent in 2006. Gasoline with up to 10 percent ethanol (E10) can be used in most vehicles without further modification, while special flexible fuel vehicles can use a gasoline-ethanol blend that has up to 85 percent ethanol (E85).
Closed-loop biomass  ,where power is generated using feedstocks grown specifically for the purpose of energy production, is generally considered to be carbon dioxide neutral because the carbon dioxide emitted during combustion of the fuel was previously captured during the growth of the feedstock. While biomass can avoid the use of fossil fuels, the net effect of biopower and biofuels on greenhouse gas emissions will depend on full lifecycle emissions for the biomass source, how it is used, and indirect land-use effects. For more information, see Climate Techbook: Biofuels  and Biopower . Overall, however, biomass energy can have varying impacts on the environment. Wood biomass , for example, contains sulfur and nitrogen, which yield air pollutants sulfur dioxide and nitrogen oxides, though in much lower quantities than coal combustion.
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Traditional geothermal energy exploits naturally occurring high temperatures, located relatively close to the surface of the earth in some areas, to generate electric power and for direct uses such as heating and cooking. Geothermal areas are generally located near tectonic plate boundaries, where there are earthquakes and volcanoes. In some places, hot springs and geysers naturally rise to the surface. These have been used for bathing, cooking and heating for centuries. At least 78 countries  used direct geothermal power in 2011.
Generating geothermal electric power typically involves the drilling of well, perhaps a mile or two in depth, in search of rock temperatures in the range of 300 to 700°F. Water is pumped down this well, where it is reheated by hot rocks. It travels through natural fissures and rises up a second well as steam, which can be used to spin a turbine and generate electricity or it can be used for heating or other purposes. Note that drilling a suitable injection well is by no means a certain task; several wells may have to be drilled before a suitable one is in place and the size of the resource  cannot be confirmed until after the drilling takes place. Additionally, some water is lost to evaporation in this process, so new water is added to maintain the continuous flow of steam. Like biopower and unlike intermittent wind and solar power, geothermal electricity can be used continuously. Note that very small quantities of carbon dioxide trapped below the earth’s surface are released during this process.
Figure 5. Geothermal Power Station
Source: BBC Science 
Globally, geothermal provided an estimated 205 TWh  in 2011, one third in the form of electricity (with an estimated 11.2 GW of capacity) and the remaining two-thirds in the form of heat. Note that in 2009, total global electricity  generation was 18,979 TWh. In 2011, the 16.7 billion kWh of geothermal electricity  generated in the United States constituted 8.6 percent of the non-hydroelectric, renewable electricity generation, but only 0.4 percent of total electricity generation. The same year, five states  generated electricity from geothermal energy , California, Hawaii, Idaho, Nevada and Utah. Of these, California accounted for 80 percent of this generation. For more information, see Climate TechBook: Geothermal .
Enhanced geothermal systems use advanced, often experimental drilling and fluid injection techniques to augment and expand the availability of geothermal resources. They are being studied by the U.S. Department of Energy. For more on this topic (see Climate TechBook: Enhanced Geothermal Systems ).
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Figure 6. Renewable Energy Indicators 2013
Source: Renewable Energy Policy Network for the 21st Century (REN21) , 2014
There are several factors that determine which renewable technologies are adopted. These include market drivers (cost, diversity, proximity to demand or transmission, resource availability, and others), policy decisions (tax credits and renewable portfolio standards) as well as specific regulations. At least 144 countries , more than half of which are developing, had renewable energy policy targets in place by the end of 2013.
All renewable energy sources are used to generate electric power. When selecting new electricity capacity additions utility planners often look at levelized costs  as a convenient summary measure of the overall competitiveness of different technologies. Total system levelized costs here (Figure 7) do not include policy-related factors like tax credits, and assumptions about future fuel prices and financing costs in particular can significantly affect these cost projections. Also, dispatchable technologies, i.e., those that can be controlled by an operator, are more desirable than non-dispatchable or intermittent technologies.
Table 1. U.S. Average Levelized Costs (2012 $/MWh) for Plants Entering Service in 2019
Capacity Factor (%)
Levelized Capital Cost
Variable O&M (Including fuel)
Total System Levelized Cost
Integrated Coal-Gasification Combined Cycle (IGCC)
IGCC with Carbon Capture & Storage (CCS)
Conventional Combined Cycle (CC)
Advanced CC with CCS
Conventional Combustion Turbine (CT)
Source: U.S. Energy Information Agency Annual Energy Outlook . April 2014
In the absence of policy mandates and incentives, a utility planner would be inclined to select the least-cost, dispatchable generation technology, which today is a natural gas-fired combined cycle. Additionally, planners often consider the mix or diversity of the generation under their control, so as to minimize exposure to any one particular technology. Also, planners must consider the environmental impacts and regulatory rules, e.g. land and water use, ecosystems, wild-life impacts and pollution mitigation.
An renewable portfollio standard is a state mandate , which specifies that electric utilities deliver a certain amount of electricity from renewable or alternative energy sources by a given date. State standards range from modest to ambitious, and qualifying energy sources vary. Some states also include "carve-outs" (requirements that a certain percentage of the portfolio be generated from a specific energy source, such as solar power) or other incentives to encourage the development of particular resources. Although climate change may not be the prime motivation behind these standards, the use of renewable or alternative energy can deliver significant greenhouse gas reductions. Increasing a state’s use of renewable energy brings other benefits as well, including job creation, energy security, and cleaner air. Most states allow utilities to comply with the renewable portfolio standard through tradeable credits. These credits can be sold in addition to the electricity generated to gain additional revenues for the utilities.
In states where a renewable portfolio standard exists , utilities must consider renewable technologies that satisfy this requirement. Cost is typically a key driver of the selected technology, but intermittency and resource availability have to be taken into account. In the case of wind, it is a lower-cost renewable technology, but it is intermittent, i.e., the wind is not always blowing hard enough to generate electricity. Moreover, many onshore locations in the United States (Figure 8), particularly in the east and south are not well-suited for wind generation. In these areas, many counties have biomass resources (Figure 10) greater than 55,000 tons/year. Since biomass is not an intermittent resource (Figure 7), it might be an attractive option to meet a renewable portfolio standard requirement. Note that roughly 25,000 to 45,000 tons  of biomass is needed to support 5 MW of generation for one year at 70 percent utilization rate (~30,000 MWh/year), depending on the condition and type of biomass. Note also that wind’s intermittency issue can be lessened to an extent by grid connecting individual wind farms from many geographically diverse areas, so if the wind is not blowing in one area, it is likely blowing in others.
At the federal level, there are two tax credits that have served to encourage the adoption of renewable energy sources: the production tax credit and the investment tax credit. First enacted in 1992 and subsequently amended, the production tax credit is a corporate tax credit available to a wide range of renewable technologies  including wind, landfill gas, geothermal and small hydroelectric. For wind, geothermal and closed-loop biomass, the utility receives a 2.2 ¢/kWh ($22/MWh) credit for all electricity generated during the first 10 years of operation. For wind, with an average total system levelized cost of $96/MWh (Figure 7), the production tax credit represents a 23 percent cost reduction. The investment tax credit is earned when qualifying equipment , including solar hot water, photovoltaics, small wind turbines, is placed into service. The credit functions to reduce installation costs and shorten the payback time of these technologies. In addition to these federal incentives, states  offer added incentives, making renewables even easier to implement from a cost perspective.
Biofuels have been gaining attention as a way to lessen dependence on petroleum-based fuels and reduce greenhouse gas emissions. To that end, the United States has adopted a renewable fuel standard.
The Energy Policy Act of 2005 created a Renewable Fuel Standard in the United States that required 2.78 percent of gasoline consumed in the U.S. in 2006 to be renewable fuel. With the Energy Independence and Security Act of 2007, Congress created a new Renewable Fuel Standard, which increased the required volumes of renewable fuel to 36 billion gallons by 2022 or about 7 percent of expected annual gasoline and diesel consumption above a business-as-usual scenario. For more information, see the C2ES overview: Renewable Fuel Standard .
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Renewable resource availability and location are key considerations in the adoption of renewable energy sources.
Idaho National Laboratory  recently estimated that there is approximately 83 GW of mostly small hydropower available in the U.S. Pacific Northwest. While the DOE found that the untapped generation potential at existing dams  in the United States that were designed for purposes other than power production, i.e., water supply and inland navigation represents 12 GW, roughly 15 percent of the current hydropower capacity.
The following maps from the DOE National Renewable Energy Laboratory depict the relative availability of renewable energy resources throughout the United States.
Figure 7. U.S. Wind Resource Map
Source: U.S. National Renewable Energy Laboratories , 2009.
Figure 8. U.S. Photovoltaic Solar Resources
Source: U.S. National Renewable Energy Laboratories , 2008.
Figure 9. U.S. Biomass Resource
Source: U.S. National Renewable Energy Laboratories , 2008
Figure 10. U.S. Geothermal Resource
Globally, 19 percent of world energy  came from renewable sources in 2012. A little less than one half of this was from traditional biomass sources used in residential heating and cooking in developing countries. In 2013, renewable energy accounted for 10 percent  of total U.S. energy use (9.3 quadrillion Btu out of a total of 97.5 quadrillion Btu). In the United States, renewable energy is used across economic sectors (Figure 11).
Figure 11. U.S. Sector Demand for Renewable Energy
Source: U.S. Energy Information Administration, 2014 .
Renewable energy sources made up 12.9 percent of total electricity generation in 2013; hydro, wind and biomass made up the majority of U.S. renewable electricity generation (Figure 12). In the industrial sector , biomass makes up 98 percent of the renewable energy use with nearly 60 percent derived from biomass wood, 33 percent from biofuels, and nearly 8 percent from biomass waste.
Figure 12. U.S. Renewable Electricity Generation (2013)
Source: U.S. Energy Information Administration , 2014.
World energy consumption is expected to grow 56 percent to 820 quadrillion Btus from 2010 to 2040 with most of this growth coming from developing countries (Figure 13). Renewables are projected to be the fastest-growing source of energy  with consumption of hydroelectricity and other renewables set to increase from 11 percent of total energy consumption in 2010 to 15 percent in 2040.
Figure 13. Projected Total Global Energy Consumption
Renewable energy’s share of global electricity generation  is forecast to increase from 21 percent to 25 percent; hydroelectric power is expected to contribute 52 percent of added renewable generation and wind is expected to contribute 28 percent. Large hydro projects are being constructed and planned in China, Canada and Brazil among others. According to the International Energy Agency , the development and market deployment of renewable energy technologies will depend heavily on government policies to make renewable energy cost-competitive.
In the United States over the next 25 years, non-hydro renewable energy consumption , excluding ethanol and biomass, is expected to grow at an average annual rate of 2.5 percent, higher than the overall growth rate in energy consumption (0.4 percent per year), under a business-as-usual scenario. E85 (ethanol transportation fuel) is expected to be the fastest growing renewable energy type, growing at an average annual rate of 11.9 percent over the same period, but it starts from a very low base. For renewable electricity sources, solar is expected to grow the most rapidly, followed by wood and other biomass. Uncertainty about federal tax credits, fuel prices and economic growth will influence the pace of renewable energy source development.