Renewable energy is energy that can be replenished on a time scale appropriate to human use. Solar energy for growing plants for food is renewable because light flows continuously from the sun, and plants can be reproduced on a time scale suitable for human needs. Coal is produced continually in some geologic formations, but the time scale is on the order of hundreds of thousands of years. Accordingly, coal is considered a nonrenewable energy.
The Department of Energy categorizes sources of nonrenewable energy production as fossil fuels, nuclear electric, and pumped hydroelectric. Sources of renewable energy are delineated as conventional hydroelectric, geothermal, biofuel, solar, and wind. Conventional hydroelectricity and energy from biofuels are considered mature sources of renewable energy; the remaining types are thought of as emerging. The chart in Figure 1 depicts energy production for 1997.
More electric energy is invested in pumping water for pumped storage hydroelectricity than is produced by the electric generators. Therefore, the net production of electricity from pumped hydro sources is slightly negative and does not appear on the chart in Figure 1. A breakdown of the 9.57 percent contribution from renewable sources is shown in Figure 2. The great bulk of the energy produced from renewable sources is from biofuels and conventional hydroelectric systems, both of which are considered mature.
Nonrenewable energy such as coal, petroleum, and natural gas are vast but not inexhaustible. A nonrenewable resource can be preserved if it can be replaced with a renewable one. This is the prime attraction of renewable energy. Sometimes a renewable resource offers an environmental advantage. If a house heated by burning fuel oil could instead use a solar heating system, then products from burning that might contribute to environmental problems would not be released into the atmosphere. On the other hand, damming a river to make a reservoir for a hydroelectric power plant might inflict damage to the environs by destroying plant and animal habitat.
Most use of renewable energy involves the sun. Solar energy is converted to thermal energy for space heating, heating hot water for domestic use and, to some extent, for generating electricity. A photovoltaic cell, which functions like a battery, converts solar energy directly to electric energy. Biomass energy has its origin in plants grown with the help of solar radiation. Wind energy is due to unequal heating of Earth's surface by the sun. A hydroelectric plant converts the gravitational energy of water into electricity, but the mechanism that replenishes the water is powered by the sun. Roughly, 95 percent of all renewable energy is solar in origin. The remainder is from geothermal energy. Tidal energy, having its origin mainly in the gravitational force between Earth and Earth's moon, is used in some parts of the world (France, for example), but is yet to be exploited in the United States.
Figure 2 shows that biomass and hydroelectricity account for about 93 percent of renewable energy. Biomass includes wood, ethanol, and municipal solid waste. Wood pellets, manufactured from finely ground wood fiber, represent a growing market for biomass fuels for specially designed stoves and furnaces. Paper products, a biomass fuel, are the major component of municipal solid wastes. Disposal of these wastes is a major societal problem. Burning the wastes for their energy content was a popular option during the 1980s because of federal, state, and local policies promoting the construction of waste-to- energy facilities. Since then, environmental policies encouraging recycling and requiring costly pollution control have made the economics of operating a waste-to-energy facility far less favorable. Accordingly, the use of waste-to-energy facilities has declined. The technology for converting corn to ethanol is very well developed, but the degree to which ethanol becomes a substitute for gasoline will depend strongly on economics.
Solar and wind energy account for less than 2 percent of the use of renewable energy (see Figure 2). Nevertheless, there are some important developments. The greatest increase in wind energy is outside the United States; however, in 1997 the United States led the world for new wind generating systems by adding 1,620 megawatts of capacity. A power output of 1,620 megawatts is the equivalent of the power output of two large electric power plants. However, wind systems do not operate as continuously as large electric power plants, so in the long run they would not deliver nearly as much energy as two large electric power plants. Most wind energy projects are in California, but significant projects are in Texas and Minnesota.
When assessing ways to take advantage of solar energy, one should not necessarily think of sophisticated technological schemes such as roof-top collectors, photovoltaic cells, and wind-powered electric generators. Designing a house or a building to maximize the input of solar radiation during the heating season can produce significant savings in energy. If in the southern United States all the worn-out, dark- colored roofs were replaced with white ones, ten times more energy would be saved in air conditioning than is produced by all wind generators in the United States (Rosenfeld 1997).
The major facility for generating electricity from geothermal sources is at The Geysers in Northern California. Generation at The Geysers is declining both for economic reasons and because of reduced steam pressure. However, other facilities continue to produce steady quantities of electricity.
Energy Efficiency and Renewable Energy Network (EREN) Home Page. Energy Efficiency and Renewable Energy Network. August 1, 2000 <http://www.eren.doe.gov/>.
National Renewable Energy Laboratory (NREL) Home Page. National Renewable Energy Laboratory. August 1, 2000 <http://www.nrel.gov/>.
Oak Ridge National Laboratory Energy Efficiency and Renewable Energy Program Web Site. Oak Ridge National Laboratory. August 1, 2000 <http://www.ornl.gov/ORNL/Energy_Eff/Energy_Eff.html>.
Renewable Energy Technologies. Sandia National Laboratories, Renewable Energy Technologies Division. August 1, 2000 <http://www.sandia.gov/Renewable_Energy/renewable.html>.
Rosenfeld, A. H. et al. (1997). "Painting the Town White—And Green." Technology Review 100:52–59.