The earth's resources are commonly divided into renewable and nonrenewable resources . Some renewable resources are perpetual, meaning that they are not affected by human use, such as solar energy or wind energy . Other renewable resources are organic and inorganic materials that are replenished by physical and biogeochemical cycles. Examples of organic renewable resources are all plant and animal species that people use for food, building materials, drugs, leisure, and so on. Examples of inorganic renewable resources are water and oxygen, which are replenished in the hydrological and oxygen cycles, respectively. The main sources of renewable energy in the United States are biomass (wood and waste burned for fuel), hydroelectric power (energy produced from flowing water), geothermal sources (energy from heat sources in the earth's surface), solar (energy from the sun), and wind energy. Nonrenewable resources are those materials that are present in the earth in limited amounts (minerals) or are produced only over many millions of years (fossil fuels ). Nonrenewable resources may be recyclable so that their usefulness to human beings can be extended, but often they are transformed during use into useless matter such as waste gas.
Taking advantage of renewable energy sources became important in the United States after the OPEC (Organization of Petroleum Exporting Countries ) oil embargo during the 1970s. Until that time, coal and oil easily supplied nearly 90% of energy to the United States, but sudden shortages and huge price increases prompted the United States government to invest in researching alternative and renewable energy sources. The 1990s were a decade of cheap and readily available fossil fuel, and United States development of renewable energy grew relatively slowly. In the early twenty-first century renewable energy became important again, with an energy crisis in California, the nation's most populous state, and with increasing concern over pollution and global warming. Political issues related to energy production, such as growing United States dependence on Middle Eastern oil reserves and the protection of wildlife and offshore areas that contain United States oil reserves, also brought attention and hope to renewable energy.
According to figures released by the Energy Information Administration (EIA) of the U.S. Department of Energy (DOE), renewable energy sources accounted for about 7% of all United States energy used in 2000. Biomass contributed about 48% of that renewable energy total. Biomass includes wood used to heat homes, as well as the waste burned in municipal waste recovery plants and converted to electricity. Other biomass sources of energy include ethanol and methanol , which are alternative fuels made from fermented plant material. Ethanol is made from corn and often added to gasoline for use in internal combustion engines. Methanol is an alternative fuel made from wood, and like ethanol, is expensive to produce and thus at a disadvantage when compared to fossil fuels and to other renewables. Another biomass energy source is landfill gas, which is methane that is captured and used for fuel as waste landfills decompose.
Hydropower accounted for 46% of United States renewable energy in 2000, and was the world's largest renewable energy source. Hydropower uses the force of dammed water to turn turbines, which produce electricity. Future United States development of hydropower is not projected to increase, as the most productive sites have already been used. Several Third World countries have viewed hydropower as a cheap source of future energy and are developing new projects, while environmentalists have strongly protested the damming of more rivers and the habitat destruction associated with large hydropower facilities.
Geothermal sources produced 5% of United States renewable energy in 2000. Geothermal energy is produced by converting the heat of the earth into electricity, using steam-powered turbines, or by using the heat directly for buildings and industrial purposes. The United States was the world's largest producer of geothermal power, accounting for about 44% of the world total. Like hydropower, U.S. geothermal power is not projected to grow in the future, as the most advantageous sites are already taken. Other countries including the Philippines, New Zealand, and Iceland are developing significant thermal energy resources.
Solar power accounted for 1% of United States renewable energy consumption in 2000, although the solar power industry grew by 20% for five straight years in the late 1990s. Solar energy systems include passive ones, such as greenhouses or building designs utilizing the sun's heat and light, and active systems, which use mechanical methods to convey the sun's energy. Photovoltaic conversion (PV) systems use chemical cells that convert the sun's energy to electricity. The solar energy industry illustrates how technology affects renewable energy development and use. Between 1975 and 1999, the cost of electricity produced by PV systems declined from a prohibitively expensive $100 per watt to less than $4 per watt, making the costly technology more accessible to consumers.
Wind energy accounted for less 1% of United States renewable energy consumption in 2000. Wind energy is produced when wind forces the movement of turbines on windmills, and generators convert that movement into electricity. Technology improvements helped wind energy to become the world's fastest growing alternative energy source by 2001, lowering its price and making it more competitive with other energy sources. During the 1990s, the United States lost its edge in wind energy technology, as countries such as Germany, Denmark, Spain, and Japan significantly invested in wind energy production. Global wind electric production capacity doubled between 1995 and 1998, and doubled again by 2001, reaching 23,300 megawatts in 2001.
Scientists are busy researching other renewable energy sources. Some countries are attempting to develop means to capture the energy in the ocean's waves (tidal power ) or the energy in warm ocean water (ocean thermal energy conversion ). Some scientists believe that hydrogen may be the energy source of the future, as hydrogen gas can be made from water and other compounds. However, no cheap or efficient method has yet been discovered for taking advantage of highly combustible hydrogen, the most plentiful chemical element in the universe.
Fossil fuel prices and environmental laws are predicted to affect the development and use of renewable energy in the future. Furthermore, technology improvements may make the cost of producing renewable energy competitive with fossil fuel energy. Research and development of alternative energy technology in the future may depend upon government-sponsored initiatives, as they have in the past.
[Douglas Dupler ]
Asmus, Peter. Reaping the Wind: How Mechanical Wizards, Visionaries, and Profiteers Helped Shape Our Energy Future. Washington, DC: Island Press, 2001.
Dupler, Douglas. Energy: Shortage, Glut, or Enough? Farmington Hills, MI: Gale, 2001.
Hazen, Mark E., and Michael Hauben. Alternative Energy. Clifton Park, NY: Delmar Learning, 2001.
Schaeffer, John, and Doug Pratt, eds. Real Goods Solar Living Source Book: The Complete Guide to Renewable Energy Technologies and Sustainable Living. Broomfield, CO: Real Goods, 2001.
Gelbspan, Ross. "A Modest Proposal to Stop Global Warming." Sierra (May/June 2001): 63.
Energy Information Administration. Renewable Energy Consumption by Source, 1989–2000. May 7, 2002 [cited July 6, 2002]. <http://www.eia.doe.gov/emeu/aer/renew.html>.