Power Sources

US Energy Consumption by Source and Sector

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A power source is a fuel that returns more energy than is invested to produce it.

Oil, natural gas, and coal are classified as non-renewable, fossil fuel power sources. They are carbon-based and are the remains of organisms that lived millions of years ago. Their use permanently depletes the natural reservoirs or other deposits from which they are extracted. Currently, these power sources account for about 80% of the U.S. energy supply.

Nuclear energy is based on materials, such as uranium, that have atomic nuclei which can be split in a controlled way, releasing large amounts of energy. Nuclear fuels are non-renewable, since their use does permanently deplete their supply; however, they are not carbon-based and are therefore not classified as fossil fuels. Nuclear energy accounts for about 10% of the U.S. energy supply.

Wind, solar, hydroelectric, geothermal and biomass are classified as renewable power sources. Their use does not permanently deplete the primary sources of energy used to generate usable energy products (e.g. electricity). Currently, these power sources account for about 10% of the U.S. energy supply.


Wind is now producing 21% of renewable energy. That's just .51% of the total energy yield, but wind energy has nearly tripled in production since 2007. It works by using the wind to turn tall propeller-like turbines, whose spinning hubs connect to generators which convert mechanical to electrical energy. Wind is relatively cheap, very clean and a wind farm takes about 2 years to build. But it is intermittent as a power source, hurts birds, and neighbors say it's annoyingly noisy.

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Solar Power

Solar power currently comes in two forms: concentrated solar power (CSP) and photovoltaic (PV). Together, these sources of solar energy account for about 6% of renewable energy.

Concentrated solar power (CSP) concentrates the sun's heat on an absorbant surface, which transfers the heat to water. The resulting steam drives a turbine linked to an electric generator. CSP plants employ large reflective surfaces, which tilt with the movement of the sun to focus solar rays on absorber tubes, which contain a fluid that transfers the heat to the power plant's boilers. For smaller applications like heating and cooling buildings or heating water, flat heat-absorbent panels pass heat to tubes of fluid which transfer it to an insulated water tank. The world's largest solar plant, built in the 1980's in the Mohave Desert of California, uses the CSP method.

Photovoltaic (PV) cells, when exposed to the radiant energy of the sun, produce an electrical voltage. At a photovoltaic power station, multiple panels of silicon-based PV cells covered by non-reflective glass collect electromagnetic energy directly from the sun. Electric fields within the cells force electrons to move in a given direction, establishing direct current (DC) electricity.

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Hydroelectric Power

Hydroelectric is the leading renewable power source, responsible for 6% of overall electrical generation, and 24% of renewable power. A hydroelectric station is built by damming a river and installing turbines. Water is allowed to flow from the dam through the turbines. The turbines are connected to generators that create electrical current.

The energy produced by the hydroelectric process is cheap, clean and constant. But dams are costly to build, may cause flooding and can disrupt ecological plant and animal systems.

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Geothermal power stations tap into reservoirs of deep underground water heated by surrounding hotbeds of rock and volcanic magma. Steam from that water is what ultimately powers geothermal generators to produce 2% of renewable energy or .35% of total electrical generation. Small geothemal systems can heat and cool buildings efficiently, though at considerable installation cost.

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Biomass accounts for 5% of renewable power. It is now used mostly for the transportation fuels ethanol and biodiesel. In addition, other biomass fuels (i.e. fast-growing "energy" crops, agricultural wastes and methane from landfills) may be burned, fermented or "digested" by bacteria to provide an inexpensive, relatively clean electrical power source.

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