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Home » Clean Energy » Clean Energy 101
The Sources of Energy

Most of our energy comes from fossil fuels—coal, oil, and natural gas supply about 85 percent of US primary energy consumption. Although the supplies of these fossil fuels are vast, they are not unlimited. And more important, the earth's atmosphere and biosphere may not survive the environmental impact of burning such enormous amounts of these fuels. Carbon stored over millions of years is being released in a matter of decades, disrupting the earth's carbon cycle in unpredictable ways.

But fossil fuels are not the only source of energy, and burning fuel is not the only way to produce heat and motion. Renewable energy offers us a better way. Some energy sources are "renewable" because they are naturally replenished, because they can be managed so that they last forever, or because their supply is so enormous that they can never be meaningfully depleted by humans. Moreover, renewable energy sources have much smaller environmental impacts than fossil and nuclear fuels.

Biomass energy, from plants, is a rich source of carbon and hydrogen, and one that can be used within the natural carbon cycle. Fast-growing plants, such as switchgrass and willow and poplar trees, can be harvested as "power crops." Biomass wastes, including forest residues, lumber and paper mill waste, crop wastes, garbage, and landfill and sewage gas, can be used to produce heat, transportation fuels, and electricity, while at the same time reducing environmental burdens.

Solar energy, power from the sun, is free and inexhaustible. Converting sunlight into useful forms is not free, but the fuel is. Sunlight has been used by humans for drying crops and heating water and buildings for millennia. A twentieth-century technology is photovoltaics, which turns sunlight directly into electricity.

Wind power is another ancient energy source that has moved into the modern era. Advanced aerodynamics research has developed wind turbines that can produce electricity at a lower cost than power from polluting coal plants.

Geothermal energy taps into the heat under the earth's crust to boil water. The hot water is then used to drive electric turbines and heat buildings.

Hydroelectric power uses the force of moving water to produce electricity. Hydropower is one of the main suppliers of electricity in the world, but most often in the form of large dams that disrupt habitats and displace people. A better approach is the use of small, "run of the river" hydro plants.

Coal is the largest source of fuel for electricity production, and also the largest source of environmental harm. Coal provides 54 percent of the US electricity supply.

Oil is used primarily for transportation fuels, but also for power production, heat and as a feedstock for chemicals. The US imports over half of the oil we use, more than ever before.

Natural gas is a relatively clean burning fossil fuel, used mostly for space and water heating in buildings and running industrial processes. Increasingly, natural gas is used in turbines to produce electricity.

Nuclear power harnesses the heat of radioactive materials to produce steam for power generation. Nuclear power provides about 21 percent of US power, but is expected to decline as old plants retire.
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