Energy Sources and Production
Modern societies rely on a variety of energy sources to heat homes, propel transportation vehicles, and produce goods for shelter, food, health care, and entertainment. Some of these sources are renewable, whereas others are nonrenewable. A renewable energy source, for example, solar energy, is one that is virtually inexhaustible on the human time scale. A nonrenewable energy source, for example, natural gas, is one that can be either completely consumed (during a lifetime, or during several lifetimes) or depleted to such an extent that it is no longer economical for humankind to obtain it. Figure 1 shows energy consumption in both the world and in the United States in 2001 by source.
About 80 percent of commercial energy is obtained from three kinds of fuel: oil, coal, and natural gas. These fuels burn in air with the release of energy. They are called fossil fuels because they are believed to have formed from the remains of plants and animals subject to heat and pressure for millions of years.
Natural gas is a mixture of methane (CH 4 ), 60 to 90 percent, and smaller amounts of other gaseous hydrocarbons, including ethane (C 2 H 6 ), propane (C 3 H 8 ), and butane (C 4 H 10 ). It is valued because it burns hotter and produces less air pollution than other fossil fuels. Complete combustion of a hydrocarbon substance produces carbon dioxide and water.
CH 4 ( g ) + 2O 2 ( g ) → CO 2 ( g ) + 2H 2 O( g ) Δ H = 50.1 kJ/g
In 2001, 2.39 trillion cubic meters of natural gas were consumed worldwide, with estimated remaining reserves of 150 trillion cubic meters.
Oil (also referred to as petroleum) is a complex liquid mixture of organic substances, principally of hydrocarbons containing five to sixteen carbon atoms. Most crude oil, once removed from a well, is sent by pipeline to a refinery, where it is distilled to separate it into gasoline, heating oil, diesel oil, and asphalt. The use of catalysts during the refining process increases the yield of gasoline. In 2001, 25.7 billion barrels of oil were used worldwide, with estimated reserves of 1.05 trillion barrels. (One barrel contains 159 liters.)
Coal is the most plentiful fossil fuel, comprising 80 percent of the fuel reserves of the United States and 90 percent of those of the world. It is a complex mixture of organic compounds and is anywhere from 30 to 95 percent carbon by mass. It also contains sulfur compounds. When coal is burned, the sulfur is converted to sulfur dioxide, a troublesome air pollutant. The description of coal as being of high quality is based on its having a low sulfur content and a high carbon content. Lignite coal (brown coal) has a low carbon content and produces the least energy upon combustion (about 15 kJ/g). Bituminous coal (soft coal) has a higher carbon content and produces more energy. It is the most extensively used coal. Anthracite coal (hard coal) has the highest carbon and heat content (about 30 kJ/g), but supplies of it are limited in most places. In 2001, 4.41 billion metric tons of coal were consumed worldwide, with estimated reserves of 985 billion metric tons. (A metric ton is 1,000 kilograms [2,679 pounds].)
The combustion of fossil fuels produces carbon dioxide gas, a heat-trapping gas. For the past 250 years (since the beginning of the Industrial Revolution ), the increased use of fossil fuels has caused the atmospheric concentration of carbon dioxide to increase by a factor of about 25 percent. It is now generally believed that this increase has produced higher global temperatures—a phenomenon called the greenhouse effect .
Commercial nuclear power is generated by nuclear fission reactions. When slow-moving neutrons strike nuclei of uranium-235 or plutonium-239, these nuclei are split, releasing energy. The energy is used to heat water and drive a turbine, in turn producing electrical energy. Currently nuclear power supplies more than 16 percent of the world's total electricity.
A typical nuclear reactor utilizes uranium oxide, whose uranium content is approximately 3 percent uranium-235, and 97 percent uranium-238, by mass. During the fission reaction, the uranium-235 is consumed and fission products form. As the amount of uranium-235 decreases and the amounts of fission products increase, the fission process becomes less efficient. At some point, the spent nuclear fuel is removed and stored. Some of the radioactive fission products, because of their radioactivity and long half-lives, must be stored securely for thousands of years. Thus, nuclear waste management poses a tremendous challenge.
Scientists hope to someday use controlled nuclear fusion to produce energy. Nuclear fusion, which involves the coming together of light nuclei to form heavier ones, is the process by which stars generate energy. In order for nuclear fusion to occur, the nuclei must have extremely high temperatures. Research has focused on the fusion of deuterium (hydrogen-2) nuclei and tritium (hydrogen-3) nuclei, a process that requires about 50 million degrees Celsius.
Renewable Energy Sources
The principal renewable energy sources are biomass from crops such as trees and corn, hydropower from flowing rivers, geothermal power from heat stored in Earth, wind energy from the movement of winds, and solar energy from the Sun.
Wood is part of an array of plant matter referred to as biomass that can be burned to produce energy. The combustible substances in biomass are primarily carbohydrates (and of these, primarily cellulose). Cellulose, whose simplest or empirical formula is CH 2 O, undergoes combustion to form carbon dioxide and water.
CH 2 O( s ) + O 2 ( g ) → CO 2 ( g ) + H 2 O( g ) ΔH = −14 kJ/g
Wood fuels continue to be used in the rural areas of developing countries.
Hydroelectric power is a well-developed energy source. Today, hydropower provides about 19 percent of the world's electricity supply. Because it is a clean, renewable source of energy, hydropower should continue to serve as a vital energy source.
There has been a rapid growth in the use of wind turbines to generate electricity. In 2001 the amount of electricity generated in this way worldwide corresponded to the amount that would have been obtained from burning 15 million barrels of oil. Although this represents only about 0.05 percent of worldwide energy production in 2001, this fraction will increase.
Solar energy is the most significant and promising renewable energy source. Solar energy is converted to electricity by solar cells (also known as photovoltaic cells). A great deal of solar energy is used currently in what is known as passive heating (which can be directly experienced as the heat gain in a greenhouse caused by sunlight).
H. Eugene LeMay Jr.
Scientific American 263 (September 1990). A special issue devoted to energy and society.
British Petroleum. "Statistical Review of World Energy." June 2002. Available from http://www.bp.com .
World Energy Council. "Survey of Energy Sources." October 2001. Available from http://www.worldenergy.org .