What are the major sources and users of energy in the United States?

The major energy sources in the United States are petroleum (oil), natural gas, coal, nuclear, and renewable energy. The major users are residential and commercial buildings, industry, transportation, and electric power generators. The pattern of fuel use varies widely by sector. For example, oil provides 93% of the energy used for transportation, but only about 1% of the energy used to generate electric power. Understanding the relationships between the different energy sources and their uses provides insights into many important energy issues.

Primary energy includes petroleum, natural gas, coal, nuclear fuel, and renewable energy. Electricity is a secondary energy source that is generated from these primary forms of energy.

Primary energy sources are commonly measured in different units: barrels (= 42 gallons) of oil, cubic feet of natural gas, tons of coal. To compare across fuels, we need to use a common unit of measure. The United States uses Btu, or British thermal units, which measure fuel use by the energy content of each fuel source.

Total U.S. energy use in 2011 was about 97.5 quadrillion (=1015, or one thousand trillion) Btu. One quadrillion Btu, often referred to as a "quad," therefore represents about 1% of total U.S. energy use.

In physical energy terms, 1 quad represents 172 million barrels of oil (about 10 days of U.S. oil use), 50 million tons of coal (enough to generate about 3% of annual U.S. electricity use), or about 1 trillion cubic feet of natural gas (equal to 4% of annual U.S. natural gas use in 2011).

The number of quads used in 2011 from each primary energy source is shown in the pie chart on the left. Petroleum (oil) provides the largest share of U.S. primary energy, followed by natural gas, coal, nuclear energy, and renewable energy (including hydropower, solar, geothermal, wind, and biomass).

Primary energy is used in residential and commercial buildings (including homes, businesses, schools, and churches), in transportation, and by industry. Primary energy is also used to generate electricity. The bar chart shows the amount of primary energy used in each of these sectors. As you can see, electric power generation is the largest user of primary energy, followed by transportation.

The electric power sector uses primary energy to generate electricity, which makes electricity a secondary, rather than a primary, energy source. Nearly all electricity is then used in buildings and by industry. This means that the total levels of energy used by residential and commercial buildings, industry, and transportation are actually higher than the amounts shown on the graphics when electricity is added in.

The lines in the figure below connecting the primary-energy-sources on the left with the demand-sectors on the right summarize the source-sector linkages in the U.S. energy system. For example, because all nuclear energy is used in the electric power sector to generate electricity, and nuclear represents 21% of the primary energy used by that sector, the line between nuclear energy and the electric power sector shows 100% on the nuclear (supply source) side and 21% on the electric power (demand sector) side.

The mix of primary energy sources varies widely across demand sectors. Energy policies designed to influence the use of a particular primary fuel for environmental, economic, or energy security reasons often focus on sectors that are major users of that fuel.

For example, because 71% of petroleum (oil) is used in the transportation sector, where it provides 93% of the total energy used, policies to reduce oil consumption have tended to focus on the transportation sector. These policies usually seek to increase fuel efficiency or promote alternative fuels. Ninety-one percent of coal, but only 1% of oil, is used to generate electricity, suggesting that policies affecting electricity generation are likely to have a much larger impact on coal use than oil use.

Some primary energy sources, such as nuclear and coal, are entirely or predominately used in one sector. Others, like natural gas and renewables, are more evenly distributed across sectors. Similarly, while transportation is almost entirely dependent on one fuel (oil), electric power uses a variety of fuels.

Linkages between fuels and sectors can and do change over time, but the change tends to occur slowly. For example, coal was once used extensively as a fuel for heating homes and commercial buildings, but that use has dwindled to almost nothing in the United States over the past half-century. Although renewable energy is still relatively small as a share of total primary energy in the transportation and electric power sectors, its role has been growing.

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What is the role of coal in the United States?

The United States holds the world's largest estimated recoverable reserves of coal and is a net exporter of coal. In 2011, our nation's coal mines produced more than a billion short tons of coal, and more than 90% of this coal was used by U.S. power plants to generate electricity. While coal has been the largest source of electricity generation for over 60 years, its annual share of generation declined from 49% in 2007 to 42% in 2011 as some power producers switched to lower-priced natural gas.

The United States is home to the largest estimated recoverable reserves of coal in the world. In fact, we have enough coal to last more than 200 years, based on current production levels. Coal is produced in 25 states spread across three coal-producing regions. In 2011, approximately 72% of production originated in five states: Wyoming, West Virginia, Kentucky, Pennsylvania, and Texas.

Over 90% of U.S. coal consumption is in the electric power sector. The United States has more than 1,400 coal-fired electricity generating units in operation at more than 600 plants across the country. Together, these power plants generate over 40% of the electricity produced in the United States and consume more than 900 million short tons of coal per year.

Although coal-fired generation still holds the largest share among all sources of electricity, its use has declined since 2007 due to a combination of slow growth in electricity demand, strong price competition with natural gas, and increased use of renewable technologies. See related article — Today in Energy, July 6, 2012

While the share of our electricity generated from coal is expected to decrease by 2035, the amount of coal used to meet growing demand for power is expected to increase in the absence of new policies to limit or reduce emissions of carbon dioxide and other greenhouse gases. Revised emissions policies could significantly change the outlook for domestic coal use. See related article — Today in Energy, May 4, 2012

Besides its role in generating electricity, coal also has industrial applications in cement making and conversion to coke for the smelting of iron ore at blast furnaces to make steel. A small amount of coal is also burned to heat commercial, military, and institutional facilities, and an even smaller amount is used to heat homes.

Between 2000 and 2010, about 5% of the coal produced in the United States, on average, was exported to other countries. Coal exports come in two forms: metallurgical coal, which can be used for steel production, and steam coal, which can be used for electricity generation. In 2011, U.S. coal exports climbed to 10% (the highest level in two decades), partly because flooding disrupted coal mining in Australia, which is normally the world's largest coal exporter. Metallurgical coal dominated U.S. coal exports in 2011 with Europe the largest importer, followed by Asia. See related article — Today in Energy, June 19, 2012

The United States also imports a small amount of coal; some power plants along the Gulf Coast and the Atlantic Coast find it cheaper to import coal by sea from South America than to have it transported from domestic coal mines.

Although some natural gas plants are more efficient than coal plants at generating electricity, in the past the fuel cost of generating one kilowatthour of electricity from natural gas had typically been higher than that of coal. In 2009, coal began losing its price advantage over natural gas for electricity generation in some parts of the country, particularly in the eastern United States as a surge in natural gas production from domestic shale deposits (made possible by advances in drilling technologies) substantially reduced the price of natural gas. See related article — Today in Energy, July 13, 2012

Coal is plentiful and fairly cheap relative to the cost of other sources of electricity, but its use produces several types of emissions that adversely affect the environment. Coal emits sulfur dioxide, nitrogen oxide, and heavy metals (such as mercury and arsenic) and acid gases (such as hydrogen chloride), which have been linked to acid rain, smog, and health issues. Coal also emits carbon dioxide, a greenhouse gas. In 2011, coal accounted for 34% of the energy-related carbon dioxide emissions in the United States. On the production-side, coal mining can have a negative impact on ecosystems and water quality, and alter landscapes and scenic views.

The economics of burning coal may change if the U.S. adopts policies that restrict or otherwise control carbon dioxide emissions. For example, a cap-and-trade program to regulate carbon dioxide emissions would likely increase the cost of burning coal because of its carbon content, and thereby cause power companies to consider using less carbon-intensive generating technologies such as nuclear, renewables, and natural gas. In March 2012, the U.S. Environmental Protection Agency proposed a new source performance standard for emissions of carbon dioxide (CO2) that would establish an output-based emission limit of 1,000 pounds of CO2 per megawatthour for new fossil-fuel-fired power plants. This emission limit would effectively require that new coal-fired generating units employ carbon capture and sequestration (CCS) technologies to reduce uncontrolled emissions of CO2 by approximately 50%.

Researchers are working on ways to lower the costs and improve the efficiency of various CCS technologies with a goal of capturing approximately 90% of the carbon dioxide from coal plants before it is emitted into the atmosphere and then storing it below the Earth's surface. CCS would theoretically address much of coal's carbon dioxide emissions; however, substantial economic and technological hurdles remain.

In 2011, Wyoming produced 438 million short tons of coal, or 40% of the coal mined in the United States. West Virginia was the second largest producer, with 135 million short tons (12%).

Coal is the largest source of U.S. electricity generation.

Different types of coal have different characteristics including sulfur content, mercury content, and heat energy content. Heat content is used to group coal into four distinct categories, known as ranks: anthracite, bituminous, subbituminous, and lignite (generally in decreasing order of heat content).

There are far more bituminous coal mines in the United States than the other ranks (over 90% of total mines), but subbituminous mines (located predominantly in Wyoming and Montana) produce more coal because their average size is much larger.

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What is the role of hydroelectric power in the United States?

The importance of hydropower as a source of electricity generation varies by geographic region. While hydropower accounted for 8% of total U.S. electricity generation in 2011, it provided over half of the electricity in the Pacific Northwest. Because hydroelectric generation relies on precipitation, it varies widely from month to month and year to year.

Conventional hydroelectric generators of varying capacity operated in 48 states in 2011. Operating expenses for hydroelectric generators are lower than for most other forms of electricity generation but facilities are limited by geography and operations are subject to seasonal constraints. There is a large concentration of capacity in the Pacific Northwest, contributing to low wholesale and retail electricity prices in that region, especially in the spring runoff season.

Conventional hydroelectric generators were among the oldest of the Nation's power plants operating in 2011. The vast majority of hydroelectric generators were built before 1980 and recent changes to hydroelectric capacity have been small.

Conventional hydroelectric plants come in two broad categories: run-of-river and storage. A run-of-river plant utilizes the flow of a waterway (usually a river) to turn a turbine, while a storage plant creates a reservoir using a dam that controls water flow over a turbine.

A run-of-river plant has little control over generator output. A storage plant has some control over generation by controlling spillway water flow at intake through the dam, but is still constrained by total reservoir water levels.

There are several other types of non-conventional hydroelectric generators including pumped-storage, hydrokinetic axial flow and wave buoy turbines. Pumped-storage generators represent the only non-conventional form of hydroelectric generation currently in wide commercial use. These systems pump water to high elevations during low load periods then run the same water through the turbines to produce electricity during high demand times. Other hydroelectric technologies, such as wave buoys, are being developed and demonstrated but not in wide use at this time.

Depending on the season and precipitation, the hydroelectric share of total generation varies from 4% to 10%. Precipitation, snowpack, drought conditions, and other meteorological factors contribute to water availability for generation through hydroelectric dams. For example, early snow melt runoff in the Pacific Northwest, elevated snowpack levels throughout much of the Western river basins, and significant rainfall in March in areas of high hydropower capacity resulted in a large increase in hydroelectric generation in 2011.

Most hydroelectric generators in the United States were co-located at dams originally built for other purposes, like flood control, municipal water supply, and irrigation. Operations are affected by environmental considerations associated with water use, fish populations, and impact on wildlife in surrounding areas. For example, fish ladders and lifts have been constructed at many dams to help protect migrating populations.

The Grand Coulee Dam, operated by the U.S. Bureau of Reclamation, is the fifth-largest power plant operating in the world and the largest in the Nation, with a net summer capacity of 7,079 Megawatts.

The U.S. Army Corps of Engineers was the largest operator of U.S. conventional hydroelectric generating capacity in 2011, followed by the U.S. Bureau of Reclamation.

The Nation's oldest power facilities are hydroelectric plants.

The Nation's 25 oldest operating power facilities are hydroelectric, the oldest of which began operating in 1891.

Hydroelectric generation is highly variable because it depends on precipitation.

Source: U.S. Energy Information Administration, Electric Power Monthly, (July 2012).

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What are greenhouse gases and how much are emitted by the United States?

Greenhouse gases trap heat from the sun and warm the planet's surface. Of all U.S. greenhouse gas emissions, the majority are related to energy consumption, and most of those are carbon dioxide(CO2). From 1990 to 2012, energy-related carbon dioxide emissions in the United States increased by about 0.3% per year. The United States produced about 17% of the world's total energy-related carbon dioxide in 2011 — the most recent year for which comparable data are available.

Because greenhouse gases trap radiation (heat) from the sun and warm the planet's surface, a certain amount of these gases is beneficial (see "Did You Know?"). But as concentrations of these gases increase due to human activity, more warming occurs than would happen naturally. According to the U.S. Environmental Protection Agency, about 6.7 billion metric tons of carbon dioxide equivalent (CO2e) greenhouse gases were emitted by the United States in 2011 (the last year the full inventory is available).1 Other countries with significant emissions include China, the countries of Europe, Russia, and Japan.

The major greenhouse gases the United States emits as a result of human activity and that are included in U.S. and international emissions estimates are

Carbon dioxide (CO2) Methane (CH4)Nitrous oxide (N2O) High-GWP gases, which are: Hydrofluorocarbons (HFCs) Perfluorocarbons (PFCs) Sulfur hexafluoride (SF6)

There are other greenhouse gases that are not counted in U.S. or international greenhouse gas inventories:

Water vapor is the most abundant greenhouse gas, but most scientists believe that water vapor produced directly by human activity contributes very little to the amount of water vapor in the atmosphere, and therefore EIA does not estimate emissions of water vapor. Research by NASA suggests a stronger impact from the indirect human effects on water vapor concentrations.Ozone is technically a greenhouse gas because it has an effect on global temperature. However, at higher elevations in the atmosphere (stratosphere), where it occurs naturally, it is needed to block harmful UV light. At lower elevations of the atmosphere (troposphere) it is harmful to human health and is a pollutant regulated independently of its warming effects.

Of the total amount of U.S. greenhouse gases emitted in 2011, about 86% were energy related and 92% of those energy-related gases were carbon dioxide from the combustion of fossil fuels.

Petroleum is the largest fuel source of carbon dioxide emissions from energy consumption in the United States, followed by coal and natural gas. The amounts and share of total U.S. energy-related carbon dioxide emissions from these fuels in 2012 include

Petroleum — accounting for 2.3 billion metric tons (43%)Coal — accounting for 1.7 billion metric tons (31%)Natural gas — accounting for 1.4 billion metric tons (26%)

Of the non-CO2 gases that contribute to energy-related greenhouse gas emissions, methane contributes the most at 4% — mainly from emissions that leak out of natural gas systems, coal mines, and petroleum exploration and production facilities. Nitrous oxide contributes another 1% — from mobile and stationary combustion of fuels and waste.

Electric power generation and transportation are the two biggest sources of energy-related CO2 emissions in the United States, with respective shares of 39% and 34% of our total energy-related CO2 emissions in 2012. Taken together, emissions in power generation and transportation increased at an average annual rate of 0.6% between 1990 and 2012. The rest of our CO2 emissions result from direct use of fossil fuels in homes, commercial buildings, and industry. These emissions declined on average by 0.4% per year since 1990.

Because electric power is ultimately used in homes, commercial buildings, and industial facilities, emissions associated with electricity generation can be allocated to each end-use sector based on their electricity consumption to obtain another perspective. Using this approach, the transportation sector is currently the largest emitter. Our cars, trucks, planes, trains, ships, and barges produced 1.8 billion metric tons CO2 in 2012. Emissions of CO2 from this sector have grown at an average rate of 0.7% since 1990.

The industrial sector, which consists of activities such as manufacturing, construction, mining, and agriculture, is the next biggest source of energy-related CO2 (including electricity use-related emissions). This sector produced a total of 1.5 billion metric tons in 2012. Its emissions have been declining since 1990 due primarily to the loss of energy-intensive industries such as steel.

The commercial sector, which includes office buildings, schools, stores, and street lighting, accounts for a total of 0.9 billion metric tons of energy-related CO2 emissions. About 78% of these emissions are from the power plants that provide the electricity used by the sector. Its CO2 emissions have grown the fastest since 1990, at an average annual rate of 0.8%.

The residential sector, which includes the houses and apartments we live in, accounts for 1.1 billion metric tons of energy-related CO2, 71% of which is produced at power plants providing homes with electricity. Residential sector emissions have grown at an average annual rate of about 0.5% since 1990.

1Values expressed as carbon dioxide equivalents (CO2e) are calculated based on their global warming potential (GWP). GWP is the ratio of the warming that would result from the emission of one kilogram of a greenhouse gas to the warming from the emission of one kilogram of carbon dioxide over a fixed period of time, such as 100 years.

If it were not for naturally occurring greenhouse gases, the Earth would be too cold to support life as we know it. Without the greenhouse effect, the average temperature of the Earth would be about -2 degrees Fahrenheit rather than the +57 degrees Fahrenheit we currently experience.

Petroleum is the fossil fuel that accounts for the most carbon dioxide emissions.

Electricity generation and transportation are the biggest sources of energy-related greenhouse gases.

The electric power industry currently emits the most energy-related greenhouse gas.

 

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What are greenhouse gases and how much are emitted by the United States?

Greenhouse gases trap heat from the sun and warm the planet's surface. Of U.S. greenhouse gas emissions, the majority are related to energy consumption, and most of those are carbon dioxide. From 1990 to 2011, energy-related carbon dioxide emissions in the United States increased by about 0.4% per year. The United States produced about 18% of the world's total energy-related carbon dioxide in 2010 — the last year for which comparable data are available.

Because greenhouse gases trap radiation (heat) from the sun and warm the planet's surface, a certain amount of these gases is beneficial (see "Did You Know?"). But as concentrations of these gases increase due to human activity, more warming occurs than would happen naturally. According to the U.S. Environmental Protection Agency, about 6.8 billion metric tons carbon dioxide equivalent (CO2e) of greenhouse gases were emitted by the United States in 2010 (the last year the full inventory is available).1 Other countries with significant emissions include China, the countries of Europe, Russia, and Japan.

The major greenhouse gases the United States emits as a result of human activity and that are included in U.S. and international emissions estimates are:

Carbon dioxide (CO2) Methane (CH4)Nitrous oxide (N2O) High-GWP gases, which are: Hydrofluorocarbons (HFCs) Perfluorocarbons (PFCs) Sulfur hexafluoride (SF6)

There are other greenhouse gases that are not counted in U.S. or international greenhouse gas inventories:

Water vapor is the most abundant greenhouse gas, but most scientists believe that water vapor produced directly by human activity contributes very little to the amount of water vapor in the atmosphere, and therefore EIA does not estimate emissions of water vapor. Research by NASA suggests a stronger impact from the indirect human effects on water vapor concentrations.Ozone is technically a greenhouse gas because it has an effect on global temperature. However, at higher elevations in the atmosphere (stratosphere), where it occurs naturally, it is needed to block harmful UV light. At lower elevations of the atmosphere (troposphere) it is harmful to human health and is a pollutant regulated independently of its warming effects.

Of the total amount of U.S. greenhouse gases emitted in 2010, about 87% were energy-related and 91% of those energy-related gases were carbon dioxide from the combustion of fossil fuels.

Petroleum is the largest fuel source of carbon dioxide emissions from energy consumption in the United States. Other important fossil fuel sources of carbon dioxide emissions include:

Petroleum — accounting for 2.3 billion metric tons (42%) in 2011 Coal — accounting for 1.9 billion metric tons (34%) in 2011 Natural gas — accounting for 1.3 billion metric tons (24%) in 2011

Of the non-CO2 gases that contribute to energy-related greenhouse gas emissions, methane contributes the most (6%) — mainly from emissions that leak out of natural gas systems, coal mines, and petroleum exploration and production facilities. Nitrous oxide contributes another 1% — from mobile and stationary combustion of fuels and waste.

Electric power generation and transportation are the biggest sources of energy-related CO2 emissions in our nation, with respective shares of 40% and 34% of our total energy-related CO2 emissions in 2011. Taken together, emissions in power generation and transportation increased at an average annual rate of 0.8% between 1990 and 2011. The rest of our CO2 emissions result from direct use of fossil fuels in homes, commercial buildings, and industry. These emissions declined on average by 0.4% per year since 1990.

Since electric power is ultimately used in homes, commercial buildings, and industry, emissions associated with power generation can be allocated to each end-use sector based on their electricity consumption to obtain another perspective. Using this approach, the transportation sector is currently the largest emitter. Our cars, trucks, planes, trains, ships, and barges produced 1.8 billion metric tons CO2 in 2011. Emissions of CO2 from this sector have grown at an average rate of 0.7% since 1990.

The industrial sector — which consists of activities such as manufacturing, construction, mining, and agriculture, is the next biggest source of energy-related CO2 to the transportation sector — a total of 1.5 billion metric tons in 2011. Its emissions have been declining since 1990 due primarily to the loss of energy-intensive industries such as steel.

The commercial sector — which includes such sources as schools, office buildings, and shopping malls — accounts for a total of 1.0 billion metric tons of energy-related CO2 emissions, with about 77% of it coming from the power plants providing the electricity used in the buildings. Its CO2 emissions have grown the fastest since 1990, at an average annual rate of 1.1%.

The residential sector — the homes we live in — accounts for 1.2 billion metric tons of energy-related CO2, 71% of which is produced at power plants providing homes electricity. Residential sector emissions have grown at an average annual rate of about 1.0% since 1990.

1Values expressed as carbon dioxide equivalents (CO2e) are calculated based on their global warming potential (GWP). GWP is the ratio of the warming that would result from the emission of one kilogram of a greenhouse gas to that from the emission of one kilogram of carbon dioxide over a fixed period of time such as 100 years.

If it were not for naturally occurring greenhouse gases, the Earth would be too cold to support life as we know it. Without the greenhouse effect, the average temperature of the Earth would be about -2 degrees Fahrenheit rather than the +57 degrees Fahrenheit we currently experience.

Petroleum is the fossil fuel that accounts for the most carbon dioxide emissions.

Electricity generation and transportation are the biggest sources of energy-related greenhouse gases.

The electric power industry currently emits the most energy-related greenhouse gas.

 

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What is the role of coal in the United States?

The United States holds the world's largest estimated recoverable reserves of coal and is a net exporter of coal. In 2011, our nation's coal mines produced more than a billion short tons of coal, and more than 90% of this coal was used by U.S. power plants to generate electricity. While coal has been the largest source of electricity generation for over 60 years, its annual share of generation declined from 49% in 2007 to 42% in 2011 as some power producers switched to lower-priced natural gas.

The United States is home to the largest estimated recoverable reserves of coal in the world. In fact, we have enough coal to last more than 200 years, based on current production levels. Coal is produced in 25 states spread across three coal-producing regions. In 2011, approximately 72% of production originated in five states: Wyoming, West Virginia, Kentucky, Pennsylvania, and Texas.

Over 90% of U.S. coal consumption is in the electric power sector. The United States has more than 1,400 coal-fired electricity generating units in operation at more than 600 plants across the country. Together, these power plants generate over 40% of the electricity produced in the United States and consume more than 900 million short tons of coal per year.

Although coal-fired generation still holds the largest share among all sources of electricity, its use has declined since 2007 due to a combination of slow growth in electricity demand, strong price competition with natural gas, and increased use of renewable technologies. See related article — Today in Energy, July 6, 2012

While the share of our electricity generated from coal is expected to decrease by 2035, the amount of coal used to meet growing demand for power is expected to increase in the absence of new policies to limit or reduce emissions of carbon dioxide and other greenhouse gases. Revised emissions policies could significantly change the outlook for domestic coal use. See related article — Today in Energy, May 4, 2012

Besides its role in generating electricity, coal also has industrial applications in cement making and conversion to coke for the smelting of iron ore at blast furnaces to make steel. A small amount of coal is also burned to heat commercial, military, and institutional facilities, and an even smaller amount is used to heat homes.

Between 2000 and 2010, about 5% of the coal produced in the United States, on average, was exported to other countries. Coal exports come in two forms: metallurgical coal, which can be used for steel production, and steam coal, which can be used for electricity generation. In 2011, U.S. coal exports climbed to 10% (the highest level in two decades), partly because flooding disrupted coal mining in Australia, which is normally the world's largest coal exporter. Metallurgical coal dominated U.S. coal exports in 2011 with Europe the largest importer, followed by Asia. See related article — Today in Energy, June 19, 2012

The United States also imports a small amount of coal; some power plants along the Gulf Coast and the Atlantic Coast find it cheaper to import coal by sea from South America than to have it transported from domestic coal mines.

Although some natural gas plants are more efficient than coal plants at generating electricity, in the past the fuel cost of generating one kilowatthour of electricity from natural gas had typically been higher than that of coal. In 2009, coal began losing its price advantage over natural gas for electricity generation in some parts of the country, particularly in the eastern United States as a surge in natural gas production from domestic shale deposits (made possible by advances in drilling technologies) substantially reduced the price of natural gas. See related article — Today in Energy, July 13, 2012

Coal is plentiful and fairly cheap relative to the cost of other sources of electricity, but its use produces several types of emissions that adversely affect the environment. Coal emits sulfur dioxide, nitrogen oxide, and heavy metals (such as mercury and arsenic) and acid gases (such as hydrogen chloride), which have been linked to acid rain, smog, and health issues. Coal also emits carbon dioxide, a greenhouse gas. In 2011, coal accounted for 34% of the energy-related carbon dioxide emissions in the United States. On the production-side, coal mining can have a negative impact on ecosystems and water quality, and alter landscapes and scenic views.

The economics of burning coal may change if the U.S. adopts policies that restrict or otherwise control carbon dioxide emissions. For example, a cap-and-trade program to regulate carbon dioxide emissions would likely increase the cost of burning coal because of its carbon content, and thereby cause power companies to consider using less carbon-intensive generating technologies such as nuclear, renewables, and natural gas. In March 2012, the U.S. Environmental Protection Agency proposed a new source performance standard for emissions of carbon dioxide (CO2) that would establish an output-based emission limit of 1,000 pounds of CO2 per megawatthour for new fossil-fuel-fired power plants. This emission limit would effectively require that new coal-fired generating units employ carbon capture and sequestration (CCS) technologies to reduce uncontrolled emissions of CO2 by approximately 50%.

Researchers are working on ways to lower the costs and improve the efficiency of various CCS technologies with a goal of capturing approximately 90% of the carbon dioxide from coal plants before it is emitted into the atmosphere and then storing it below the Earth's surface. CCS would theoretically address much of coal's carbon dioxide emissions; however, substantial economic and technological hurdles remain.

In 2011, Wyoming produced 438 million short tons of coal, or 40% of the coal mined in the United States. West Virginia was the second largest producer, with 135 million short tons (12%).

Coal is the largest source of U.S. electricity generation.

Different types of coal have different characteristics including sulfur content, mercury content, and heat energy content. Heat content is used to group coal into four distinct categories, known as ranks: anthracite, bituminous, subbituminous, and lignite (generally in decreasing order of heat content).

There are far more bituminous coal mines in the United States than the other ranks (over 90% of total mines), but subbituminous mines (located predominantly in Wyoming and Montana) produce more coal because their average size is much larger.

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What is the role of hydroelectric power in the United States?

The importance of hydropower as a source of electricity generation varies by geographic region. While hydropower accounted for 8% of total U.S. electricity generation in 2011, it provided over half of the electricity in the Pacific Northwest. Because hydroelectric generation relies on precipitation, it varies widely from month to month and year to year.

Conventional hydroelectric generators of varying capacity operated in 48 states in 2011. Operating expenses for hydroelectric generators are lower than for most other forms of electricity generation but facilities are limited by geography and operations are subject to seasonal constraints. There is a large concentration of capacity in the Pacific Northwest, contributing to low wholesale and retail electricity prices in that region, especially in the spring runoff season.

Conventional hydroelectric generators were among the oldest of the Nation's power plants operating in 2011. The vast majority of hydroelectric generators were built before 1980 and recent changes to hydroelectric capacity have been small.

Conventional hydroelectric plants come in two broad categories: run-of-river and storage. A run-of-river plant utilizes the flow of a waterway (usually a river) to turn a turbine, while a storage plant creates a reservoir using a dam that controls water flow over a turbine.

A run-of-river plant has little control over generator output. A storage plant has some control over generation by controlling spillway water flow at intake through the dam, but is still constrained by total reservoir water levels.

There are several other types of non-conventional hydroelectric generators including pumped-storage, hydrokinetic axial flow and wave buoy turbines. Pumped-storage generators represent the only non-conventional form of hydroelectric generation currently in wide commercial use. These systems pump water to high elevations during low load periods then run the same water through the turbines to produce electricity during high demand times. Other hydroelectric technologies, such as wave buoys, are being developed and demonstrated but not in wide use at this time.

Depending on the season and precipitation, the hydroelectric share of total generation varies from 4% to 10%. Precipitation, snowpack, drought conditions, and other meteorological factors contribute to water availability for generation through hydroelectric dams. For example, early snow melt runoff in the Pacific Northwest, elevated snowpack levels throughout much of the Western river basins, and significant rainfall in March in areas of high hydropower capacity resulted in a large increase in hydroelectric generation in 2011.

Most hydroelectric generators in the United States were co-located at dams originally built for other purposes, like flood control, municipal water supply, and irrigation. Operations are affected by environmental considerations associated with water use, fish populations, and impact on wildlife in surrounding areas. For example, fish ladders and lifts have been constructed at many dams to help protect migrating populations.

The Grand Coulee Dam, operated by the U.S. Bureau of Reclamation, is the fifth-largest power plant operating in the world and the largest in the Nation, with a net summer capacity of 7,079 Megawatts.

The U.S. Army Corps of Engineers was the largest operator of U.S. conventional hydroelectric generating capacity in 2011, followed by the U.S. Bureau of Reclamation.

The Nation's oldest power facilities are hydroelectric plants.

The Nation's 25 oldest operating power facilities are hydroelectric, the oldest of which began operating in 1891.

Hydroelectric generation is highly variable because it depends on precipitation.

Source: U.S. Energy Information Administration, Electric Power Monthly, (July 2012).

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What are the major sources and users of energy in the United States?

The major energy sources in the United States are petroleum (oil), natural gas, coal, nuclear, and renewable energy. The major users are residential and commercial buildings, industry, transportation, and electric power generators. The pattern of fuel use varies widely by sector. For example, oil provides 93% of the energy used for transportation, but only about 1% of the energy used to generate electric power. Understanding the relationships between the different energy sources and their uses provides insights into many important energy issues.

Primary energy includes petroleum, natural gas, coal, nuclear fuel, and renewable energy. Electricity is a secondary energy source that is generated from these primary forms of energy.

Primary energy sources are commonly measured in different units: barrels (= 42 gallons) of oil, cubic feet of natural gas, tons of coal. To compare across fuels, we need to use a common unit of measure. The United States uses Btu, or British thermal units, which measure fuel use by the energy content of each fuel source.

Total U.S. energy use in 2011 was about 97.5 quadrillion (=1015, or one thousand trillion) Btu. One quadrillion Btu, often referred to as a "quad," therefore represents about 1% of total U.S. energy use.

In physical energy terms, 1 quad represents 172 million barrels of oil (about 10 days of U.S. oil use), 50 million tons of coal (enough to generate about 3% of annual U.S. electricity use), or about 1 trillion cubic feet of natural gas (equal to 4% of annual U.S. natural gas use in 2011).

The number of quads used in 2011 from each primary energy source is shown in the pie chart on the left. Petroleum (oil) provides the largest share of U.S. primary energy, followed by natural gas, coal, nuclear energy, and renewable energy (including hydropower, solar, geothermal, wind, and biomass).

Primary energy is used in residential and commercial buildings (including homes, businesses, schools, and churches), in transportation, and by industry. Primary energy is also used to generate electricity. The bar chart shows the amount of primary energy used in each of these sectors. As you can see, electric power generation is the largest user of primary energy, followed by transportation.

The electric power sector uses primary energy to generate electricity, which makes electricity a secondary, rather than a primary, energy source. Nearly all electricity is then used in buildings and by industry. This means that the total levels of energy used by residential and commercial buildings, industry, and transportation are actually higher than the amounts shown on the graphics when electricity is added in.

The lines in the figure below connecting the primary-energy-sources on the left with the demand-sectors on the right summarize the source-sector linkages in the U.S. energy system. For example, because all nuclear energy is used in the electric power sector to generate electricity, and nuclear represents 21% of the primary energy used by that sector, the line between nuclear energy and the electric power sector shows 100% on the nuclear (supply source) side and 21% on the electric power (demand sector) side.

The mix of primary energy sources varies widely across demand sectors. Energy policies designed to influence the use of a particular primary fuel for environmental, economic, or energy security reasons often focus on sectors that are major users of that fuel.

For example, because 71% of petroleum (oil) is used in the transportation sector, where it provides 93% of the total energy used, policies to reduce oil consumption have tended to focus on the transportation sector. These policies usually seek to increase fuel efficiency or promote alternative fuels. Ninety-one percent of coal, but only 1% of oil, is used to generate electricity, suggesting that policies affecting electricity generation are likely to have a much larger impact on coal use than oil use.

Some primary energy sources, such as nuclear and coal, are entirely or predominately used in one sector. Others, like natural gas and renewables, are more evenly distributed across sectors. Similarly, while transportation is almost entirely dependent on one fuel (oil), electric power uses a variety of fuels.

Linkages between fuels and sectors can and do change over time, but the change tends to occur slowly. For example, coal was once used extensively as a fuel for heating homes and commercial buildings, but that use has dwindled to almost nothing in the United States over the past half-century. Although renewable energy is still relatively small as a share of total primary energy in the transportation and electric power sectors, its role has been growing.

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