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Renewable Energy

Renewable Energy

Renewable energy is energy that is regenerative or, for all practical purposes, virtually inexhaustible. It includes solar energy, wind energy, hydropower, biomass (derived from plants), geothermal energy (heat from the earth), and ocean energy. Renewable energy resources can supply energy for heating and cooling buildings, electricity generation, heat for industrial processes, and fuels for transportation. The increased use of renewable energy could reduce the burning of fossil fuels (coal, petroleum, and natural gas), eliminating associated air-pollution and carbon dioxide emissions, and contributing to national energy independence and economic and political security.

Historical and Current Use

Before the 1900s, the world as a whole used wood (including wood converted to charcoal) for heat in homes and industry, vegetation for feeding draft animals, water mills for grinding grain and milling lumber, and wind for marine transportation and grain milling and water pumping. By the 1920s, however, coal and petroleum had largely replaced these energy sources in industrialized countries, although wood for home heating and hydroelectric power generation remained in wide use. At the end of the twentieth century, nearly 90 percent of commercial energy supply was from fossil fuels.

Renewable energy, however, makes important contributions to world energy supplies. Hydroelectric power is a major source of electrical energy in many countries, including Brazil, Canada, China, Egypt, Norway, and Russia. In developing countries many people do not have access to or cannot afford electricity or petroleum fuels and use biomass for their primary energy needs. For example, most rural people in Africa use wood, scrub, grass, and even animal dung for cooking fuel. Small-scale renewable energy technologies are often the only practical means of supplying electricity in rural areas of these countries. The table indicates the relative consumption of energy sources in the United States.

Major Types of Renewable Energy Sources

Biomass. Biomass includes wood, agricultural crops and residues, municipal refuse, wood and paper products, manufacturing process waste, and human and livestock manure. It can be used to heat homes and buildings, produce electricity, and as a source of vehicle fuel. Wood and paper manufacturers and sugar mills use biomass residues for process heat and electricity production. There are power plants that burn wood, agricultural residues, and household trash to produce electricity. Biogas (composed of methane, carbon dioxide, and other gases) produced by decomposing biomass in anaerobic conditions is captured from landfills, municipal sewage treatment plants, and livestock waste management operations. This gas can be used for heat or to generate electricity.

Ethanol is used as a transportation fuel in the United States, Brazil, and a few other countries. Nearly all the fuel ethanol in the United States is made from corn, although it can also be produced from other sources, including wastepaper. There is a small but growing consumption of "biodiesel" made from grain oils and animal fats.

Geothermal systems. Geothermal energy (heat from the earth) created deep beneath the earth's surface is tapped to produce electricity in twenty-two countries, some of which include the United States, Iceland, Italy, Kenya, and the Philippines. Geothermal hot springs can also heat buildings, greenhouses, fish farms, and bathing pools.

Hydropower. Hydropower, produced from flowing water passing through hydroelectric turbines , is the leading renewable energy source, contributing to approximately 9 percent of the electricity generated in the United States. Most hydropower is produced at large dams, although there are many small systems operating around the world, such as the small hydropower plant in Namche Bazar, Nepal, which provides power for the tourist and market town near Mt. Everest. The production of hydroelectricity from year to year varies with precipitation.

Ocean energy. The world's oceans are a vast and practically untapped source of energy. There are a few operating wave and tidal power plants around the world, and several experimental ocean thermal energy conversion (OTEC) plants have also been built. A small wave power plant in Norway captures water from waves in a dam and lets the water out through a turbine. A 240-megawatt tidal power facility on the Rance River in France produces electricity as tidal flows move back and forth through turbines located at the mouth of the river. In Hawaii, a small OTEC plan was built which uses the temperature of warm surface water to evaporate cold seawater in a vacuum to produce steam that turns a turbine and generator.

Solar energy systems. The simplest uses of solar energy are for drying crops, and heating buildings and water. Solar-heated homes and solar water heaters can be found in nearly every country around the world. Crops can be simply laid in the sun to dry, or more sophisticated collectors can be used to heat air to dry food stored on drying racks. Solar water heaters use collectors to heat water that is stored in a tank for later use. Homes can be heated by using a masonry floor to absorb sunshine coming through windows, or by using solar collectors to heat a large tank of water than can be distributed for heating at night.

Concentrated sunlight can be used to produce high-temperature heat and electricity. Nine concentrating solar parabolic trough power plants, with a combined generation capacity of 354 megawatts, are located in the Mojave Desert in California. (A megawatt is 1 million watts, or 1,000 kilowatts.) The U.S. Department of Energy built and tested a ten-megawatt solar thermal central receiver power plant near Barstow, California, which operated successfully for about seven years. Another type of concentrating solar thermal power system is a parabolic dish. Systems with a capacity of up to twenty-five kilowatts have been developed.

Photovoltaic (PV) systems are based on solar electric cells, which convert sunlight directly to electricity. They can be used to power hand calculators or in large systems on buildings. Many PV systems are installed in remote areas where power lines are expensive or unfeasible, although the number of systems connected to electricity transmission systems is increasing, and range in size from 1 to several kilowatts on houses, to systems over one hundred kilowatts on large buildings. PV systems are very suitable for use in developing countries where people have no electricity from electric power lines.

Wind energy systems. Water-pumping and grain-milling windmills have evolved into electric power turbines. There are now tens of thousands of wind turbines operating around the world. They range in size from tiny turbines on the back of sailboats to very large units that can produce as much as

energy source (quads*) (%total) (bill. kwh**) (%total)
*a quad is quadrillion british thermal units (btus), and is the equivalent of about 180 million barrels of crude oil.
**bill. kwh = a billion kilowatt-hours; one kilowatt-hour (kwh) is the equivalent of running a 100-watt lightbulb for 10 hours.
note: values are rounded.
source: Energy Information Administration, U.S. Department of Energy.
total 96 85 3,641  
coal/coal coke 22 23 1,891 52
petroleum 38 39 116 3
natural gas 22 23 546 15
nuclear 8 8 674 19
renewables (total) 7.2 7.5 419 12
hydro 3.5 3.6 339 9.4
biomass/biofuels 3.2 3.3 58 1.6
geothermal 0.4 0.4 17 0.46
solar 0.07 0.07 0.85 0.02
wind 0.05 0.05 4.5 0.12

2 to 3 megawatts of electricity, with 100-foot (30-meter) blades. They can be installed on land and in shallow water in coastal areas.

The Future for Renewable Energy

Renewable energy has many advantages that will help to maintain and expand its place in world energy supply:

  • Renewable energy resources are enormoushundreds of times beyond the needs of world energy consumption in 2000.
  • Advances in technologies are reducing manufacturing costs and increasing system efficiencies, thereby reducing the cost of energy from renewable resources.
  • Negative environmental and health impacts of renewable energy use are much fewer than those of fossil fuels and nuclear power.
  • Many renewable energy technologies can produce energy at the point of use, allowing homeowners, businesses, and industry to produce their own power.
  • There is strong support for renewable energy from people around the world.
  • Many governments have programs that support renewable energy use to limit the emission of greenhouse gases and thereby reduce the threat of global warming.

As fossil fuels such as oil and natural gas become scarce, they will become more expensive. Some experts believe that demand for oil will exceed production capability within the next twenty years.

Using energy conservatively and efficiently, no matter how it is produced or where it comes from, is the most economical way to consume energy. Simply turning off lights and computers when they are not in use can save an individual household or business money and reduce the environmental impact associated with producing electricity.


U.S. Energy Information Administration. (2001). Annual Energy Review 2000. Washington, D.C.: U.S. Department of Energy.

U.S. Energy Information Administration. (2001). International Energy Annual 1999. Washington, D.C.: U.S. Department of Energy.

U.S. Energy Information Administration. (2001). Renewable Energy Annual 2000, with Data for 1999. Washington, D.C.: U.S. Department of Energy.

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U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy. Available from

Paul Philip Hesse

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Energy, Renewable


ENERGY, RENEWABLE. Wood, wind, water, and sun power have been used for cooking, heating, milling, and other tasks for millennia. During the Industrial Revolution of the eighteenth and early nineteenth centuries, these forms of renewable energy were replaced by fossil fuels such as coal and petroleum. At various times throughout the nineteenth and twentieth centuries, people believed that fossil fuel reserves would be exhausted and focused their attentions on sources of renewable energy. This led to experiments with solar steam for industry and solid wood, methanol gas, or liquid biofuels for engines. Attention has refocused on renewable energy sources since the 1960s and 1970s, not only because of concern over fossil fuel depletion, but also because of apprehension over acid rain and global warming from the accumulation of carbon dioxide in the atmosphere.

Acid rain is clearly the result of the use of fossil fuels, and most authoritative climatologists also believe that these fuels are contributing to global warming. Many scientists and environmentalists have, therefore, urged a global switch to renewable energy, which derives from the sun or from processes set in motion by the sun. These energy forms include direct use of solar power along with windmills, hydroelectric dams, ocean thermal energy systems, and biomass (solid wood, methane gas, or liquid fuels). Renewable energy thus differs not only from fossil energy sources such as petroleum, gas, and coal, but also from nuclear energy, which usually involves dividing uranium atoms.

In the early 1990s, one-fifth of worldwide energy use was renewable, with by far the largest portion of this coming from fuel wood and biomass. Hydroelectric dams made up most of the rest. More than half the world's population relied on wood for cooking and heating, and although wood is generally considered to be renewable, excessive reliance has long been recognized as a cause of deforestation. Forests disappear faster than they can be renewed by natural processes. Energy "crops" —for example, fast-growing acacia or eucalyptus trees planted for fuel wood in the Third World—and more efficient wood stoves may be useful to poor, wood-reliant nations.

Solar energy is a term for many techniques and systems. The sun's energy can be trapped under glass in a greenhouse or within solar panels that heat water. It can also be concentrated in a trough or parabolic collector. In arid climates a small version of a concentrator is sometimes used to substitute for wood. Although economical, it is unreliable, hard to transport, and difficult to operate. Larger concentrators can produce steam economically for industry or for electric utilities in some climates. Another form of solar energy comes from photovoltaic cells mounted on panels. These panels are economical for all kinds of remote power needs, from cheap hand calculators to mountaintop navigational beacons to orbiting satellites. Costs have dropped dramatically since the mid-1970s, from hundreds of thousands of dollars to several thousands per installed kilowatt, and are expected to drop to under a thousand dollars early in the twenty-first century. At some point they may become competitive with nuclear and fossil energy.

Water power has been well known since its use in the Egyptian and classical Greek civilizations, and at the outset of the Industrial Revolution, it was widely used in Europe and the Americas to grind grain and run looms and in other small-scale industrial processes. Today water power is by far the cheapest of all fossil, nuclear, and renewable forms of energy for producing electricity, but the ecological disruptions caused by hydroelectric dams have caused many environmental controversies. Ocean energy takes advantage of the movement of water in tides or waves or of the temperature difference between sun-heated surface water and cold deep water. A few tidal energy projects have been built, but this form of energy production is expensive and remains largely experimental. Like tidal energy, geothermal energy is produced by continuous natural processes not directly related to solar cycles. Geo-thermal energy takes advantage of hot water trapped deep inside the earth to produce electricity or heat for homes and industry.

Wind power has been used for grinding grain, pumping water, and powering sawmills since the Middle Ages, and thousands of windmills once dotted coastal areas of northern Europe. Water-pumping windmills were a fixture in the American Midwest well into the twentieth century. Windmills are returning in a high-tech form in places like Altamont Pass in California, where they produce electricity. They are widely used for pumping water in the Third World.

Biomass energy involves a wide range of low and high technologies, from wood burning to use of manure, sea kelp, and farm crops to make gas and liquid biofuels. Brazil leads the world in use of pure ethyl alcohol derived from sugarcane as a replacement for petroleum. A common fuel in the United States is corn-derived ethyl alcohol, which is used as a low-pollution octane booster in a 10-percent blend with gasoline called "gasohol." Another form of renewable energy used in the rural Third World is the gas-producing biogas digester. Human and animal wastes are mixed with straw and water in an airless underground tank made of brick or cement. Methane gas is siphoned from the tank to a cooking stove. Meanwhile, the tank gets hot enough to kill disease-causing bacteria, which is an important sanitary improvement in many countries. Over the past few decades, 5 million biogas tanks have been built in China and half a million in India.

Renewable energy resources are cleaner and far more abundant than fossil resources, but they tend to be dispersed and more expensive to collect. Many of them, such as wind and solar energy, are intermittent in nature, making energy storage or distributed production systems necessary. Therefore, the direct cost of renewable energy is generally higher than the direct cost of fossil fuels. At the same time, fossil fuels have significant indirect or external costs, such as pollution, acid rain, and global warming. How to account for these external costs and assign the savings to renewable energy is a matter of continued policy debate. Another policy issue is research and development support. Conventional forms of energy, such as fossil fuels and nuclear power, receive more financial support from the federal government than does renewable energy. U.S. government policy toward renewable energy has been a roller coaster of support and neglect. By the end of President Jimmy Carter's administration in 1981, federal contributions to research in solar photovoltaics, solar thermal energy, solar buildings, biofuels, and wind energy research had soared to almost $500 million, but by 1990 the figure was only $65 million. A global transition to renewable energy will have to include developing nations, where energy use in proportion to the world total grew from 20 percent in 1970 to 3l percent in 1990.


Berinstein, Paula. Alternative Energy: Facts, Statistics, and Issues. Westport, Conn.: Oryx Press, 2001.

Blackburn, John O. The Renewable Energy Alternative: How the United States and the World Can Prosper Without Nuclear Energy or Coal. Durham, N.C.: Duke University Press, 1987.

Butti, Ken, and John Perlin. A Golden Thread: 2,500 Years of Solar Architecture and Technology. New York: Van Nostrand Reinhold, 1980.

Flavin, Christopher. Beyond the Petroleum Age: Designing a Solar Economy. Washington, D.C.: Worldwatch Institute, 1990.

Kovarik, Bill. Fuel Alcohol: Energy and Environment in a Hungry World. London: International Institute for Environment and Development, 1982.

Sørensen, Bent. Renewable Energy: Its Physics, Engineering, Use, Environmental Impact, Economy and Planning Aspects. San Diego, Calif.: Academic Press, 2000.

BillKovarik/h. s.

See alsoAcid Rain ; Air Pollution ; Conservation ; Energy Policy ; Energy Research and Development Administration ; Global Warming ; Hydroelectric Power ; Nuclear Power ; Water Pollution ; andvol. 9:Address on Energy Crisis .

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renewable energy

renewable energy (alternative energy) Energy from a source that can be replenished or that replenishes itself, and is more environmentally safe than traditional energy forms such as coal, gas, or nuclear energy. Solar energy harnesses the rays of the Sun. Tidal power stations use the gravitational force of the Sun and Moon on the ocean. Wave power harnesses the natural movement of the sea. The power of rivers and lakes can be tapped by damming the flow and using turbines to generate hydroelectricity. Wind power schemes have existed for centuries in the form of windmills. Another, less well-known, renewable source is the geothermal energy produced in the Earth's crust.

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renewable resource

renewable resource Resource produced as part of the functioning of natural systems at rates comparable with its rate of consumption, e.g. food production by photosynthesis. Limits to renewable resources are determined by flow rate and such resources can provide a sustained yield. Compare NONRENEWABLE RESOURCE.

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renewable resource

renewable resource A resource produced as part of the functioning of natural systems at rates comparable with its rate of consumption (e.g. food production by photosynthesis). Limits to renewable resources are determined by flow rate and such resources can provide a sustained yield. Compare finite resource.

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renewable resource

renewable resource A resource produced as part of the functioning of natural systems at rates comparable with its rate of consumption, e.g. food production by photosynthesis. Limits to renewable resources are determined by flow rate and such resources can provide a sustained yield. Compare FINITE RESOURCE.

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alternative energy

alternative energy See renewable energy

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