Energy supply is the total amount of usable energy available to people for doing work. This energy heats and cools our homes and offices. It powers our lights, electronics, factories, and farm equipment. Energy moves our automobiles, trains, ships, and airplanes. Modern society could not exist without a plentiful and stable supply of energy.
Energy exists in several different forms. People burn fuel to produce heat energy. Batteries produce electric energy. Both batteries and fuel store energy as chemical energy. Mechanical energy is associated with the motion of objects, such as a spinning wheel or a thrown ball. In physics, power—as in electric power—is the rate at which energy is used or delivered.
In some cases, energy from a particular source is used directly. For example, people may burn wood to heat their homes. But energy is often transformed before use. At many power plants, for example, fuel is burned to boil water, creating steam. The energy of the expanding steam spins a device called a turbine. An electric generator then transforms the turbine’s mechanical energy into electric energy. Wood, oil, and other fuels are called primary sources of energy. Electric energy, in contrast, is an energy carrier. It is used to move energy quickly over long distances, from power plants to homes and offices. Electric energy can easily be transformed into other kinds of energy, such as the mechanical energy of a spinning motor or light energy from a computer screen. Energy that is produced by businesses and governments and sold to the public is called commercial energy.
This article discusses the various sources of energy that people use, problems and challenges associated with the energy supply, and the history of humanity’s use of energy. For information on the physics of energy, see the article Energy.
Sources of energy
Coal, oil, and natural gas account for about 85 percent of the world’s primary energy. They account for about 80 percent of the U.S. energy use. These sources are called fossil fuels because they are the remains of long-buried ancient life. Coal comes from the remains of plants. Oil and natural gas—both forms of petroleum—come from the remains of plantlike plankton that lived on the ocean surface. Over millions of years, heat and pressure transformed the remains of these organisms into fuel. Fossil fuels are a nonrenewable source of energy, which means that they cannot be readily replaced. People burn fossil fuels much faster than they form by natural processes, and their supply may eventually run out. The nuclear fuel used in today’s power plants is another nonrenewable energy source. Such fuel comes from a limited supply of the rare element uranium.
Renewable energy sources, in contrast, have potentially unlimited supplies. The energy that comes from sunlight, wind, rain, and tides, for example, need never run out. People can also regrow trees and crops after using them as fuel. Scientists and engineers are working to develop renewable energy sources to replace fossil fuels.
Fossil fuels
store chemical energy that can be burned to produce great amounts of heat. This heat is used in power plants, in automobile engines, and in such home appliances as stoves and furnaces. Fossil fuels store a large amount of energy, and this energy can be released at any time. These two qualities make fossil fuels attractive for any number of uses. But the burning of fossil fuels is a major source of air pollution and other environmental problems.
Coal
provides about 30 percent of all the primary energy used in the world. It provides about 10 percent of the energy used in the United States. Much of the heat energy in coal is used to produce steam in boilers. The steam, in turn, is used to generate electric power or to produce motion in steam engines. Coal is also widely used in the manufacture of steel. In many Asian and European countries, people use coal to heat homes and other buildings.
There are two primary methods for mining coal. Surface mining removes coal less than 200 feet (60 meters) below Earth’s surface. The removal is generally done by completely stripping away the material above the coal. Underground mining involves digging deep tunnels to reach coal buried at deeper levels.
Oil
furnishes about 30 percent of the primary energy used in the world. It also accounts for about 35 percent of primary energy used in the United States. Most of the energy in oil is used to produce transportation fuel—such as gasoline and diesel fuel—and heating oil.
Most oil is removed from deep underground as a liquid called crude oil. Workers pump crude oil out of the ground through wells drilled into oil-bearing formations called reservoirs. Because crude oil is a liquid, it is relatively inexpensive to transport long distances using pipelines. Tanker ships also carry huge quantities of oil overseas. Refineries process crude oil into gasoline and other petroleum products. Refining removes many impurities from crude oil.
Natural gas
accounts for about 25 percent of the world’s primary energy use. It accounts for about 35 percent of U.S. energy use. Natural gas is chiefly made up of methane. Most of the heat energy contained in natural gas is used to generate steam for electric power or steam engines, to heat buildings, and for cooking and other household needs. Like oil, natural gas comes from underground deposits and can be transported long distances through pipelines. Natural gas lacks many of the pollutants associated with coal and oil.
Bituminous sands and oil shale
have become important energy sources. Bituminous sands are also called tar sands. Each grain of sand is covered with an oil-producing substance. Oil shale is a type of rock that can be processed to yield crude oil and natural gas. The cost of obtaining fuel from tar sands and oil shale is higher than that of mining natural reservoirs of crude oil and natural gas. Since the 2000’s, however, new technologies have made extracting fuel from these sources much more profitable in the United States and Canada.
Renewable energy
sources include energy from the sun and from wind, water, and the heat beneath Earth’s surface. They also include biomass (biological materials) that—unlike fossil fuels—can be quickly regrown.
Solar energy
can provide a clean and almost unlimited source of energy. People capture solar energy with two basic kinds of devices. Solar concentrators capture or focus the sun’s energy to generate heat. This heat can be used to heat water or air or to generate electric power. Solar cells, also called photovoltaic cells, on the other hand, convert solar energy directly into electric energy. Solar energy’s use is limited by the high cost of such equipment and the fact that it only works when the sun is shining. But the cost of solar devices is falling. Solar plants generate significant electric power in a variety of countries, even in some such as Germany that are not particularly sunny.
Wind energy
uses the spinning motion of windblown turbines to generate electric power. Such turbines generate about 7 percent of the electric power in the world. They produce about 10 percent of the electric power of the United States. In Spain, wind turbines generate about 25 percent of the country’s electric power. China and Germany are also leading consumers of wind energy. Wind turbines are relatively cheap, and wind energy could potentially supply a large portion of the world’s total energy demands. But areas with high winds are often remote, and winds themselves do not blow continuously. Thus, like solar energy, wind energy cannot generate a continuous supply of electric power.
Water power,
or hydropower, furnishes about 7 percent of the primary energy in both the world and the United States. Water power is mostly used to generate electric power in hydroelectric power plants. Such plants are often built within dams. As water falls through the dam, its energy spins turbines connected to electric generators. Water power supplies energy without air pollution. The water itself is renewed over time by rainfall and snowmelt.
Some dams rely on tidal energy, the up-and-down motion of water due to the gravitational attractions of the moon and sun. Other forms of water power use energy from ocean waves or currents to power generators. But such technology is difficult to maintain in remote, saltwater environments.
Geothermal energy
makes use of Earth’s natural interior heat to turn water into steam. In some areas, underground water naturally comes into contact with hot rocks deep beneath the surface. The resulting steam can be captured to power generators. In other areas, engineers can pump water into the ground to be heated by hot rocks.
Geothermal electric plants have historically been limited to areas near the edges of tectonic plates. About 30 such plates float on a deep, fluid layer of hot rock, covering Earth’s surface like the pieces of a jigsaw puzzle. Newer technology known as hot dry rock (HDR) systems or enhanced geothermal systems (EGS), however, can be used almost anywhere in the world, even far from tectonic boundaries. Such technology pumps water into deep wells. The wells reach a layer of hot rock that lies virtually everywhere beneath the surface. The water captures the rock’s heat and can be used continuously. But high costs of drilling and concerns about causing earthquakes have limited the development and use of such systems.
Biomass
includes wood, crop residues, dung, solid waste, sewage, landfill gas, and other forms of biological materials. Biomass is an increasingly important source of renewable energy. It provides around 10 percent of the world’s primary energy supply. But most of this energy is used inefficiently for cooking and heating in less developed countries.
Biomass can be used to generate electricity and to produce solid, liquid, and gaseous fuels that are less harmful to the environment than are fossil fuels. Biomass can also be turned into chemicals for making plastics and other products typically made from petroleum. In the United States, biomass ranks as the largest source of renewable energy. It is used to heat buildings, to produce biofuels for transportation, and to generate steam and electricity. Many cities throughout the world produce usable energy by burning trash. Some cities also process liquid organic wastes, such as sewage, to produce methane gas.
Nuclear energy
provides about 5 percent of the primary energy used in the world and about 10 percent of the energy used in the United States. Nuclear power plants generate huge amounts of heat from controlled fission—that is, the splitting of atomic nuclei (cores) of certain elements, generally uranium. Uranium supplies are limited, however, and scientists and engineers are exploring methods to use uranium more effectively. Scientists also hope to eventually produce virtually unlimited nuclear energy from fusion. Fusion is a more powerful reaction in which atomic nuclei combine. For a detailed discussion of fission and fusion, see Nuclear energy.
Problems and challenges
Modern living requires massive amounts of energy for transportation, the use of electric and electronic devices, heating and cooling, and the manufacture of goods. But no single source of energy can meet the world’s needs without significant drawbacks. The use of some energy sources—notably fossil fuels—results in large amounts of pollution. Other sources, such as wind and solar energy, cannot generate continuous power and so must be combined with other energy sources. Balancing the demand for cheap energy with long-term supply and environmental protection is a top concern for government officials, scientists, and engineers
Supply and demand.
Wealthy countries use far more energy than do other countries. For example, the United States has less than 5 percent of the world’s population. But it uses about 15 percent of the world’s energy. Many developed countries engage in energy conservation. They seek to decrease—or at least stabilize—their energy consumption. But developing countries can do little to limit their energy consumption. To become developed, they need more factories, farm machinery, and transportation facilities. People in developing countries demand more heating and cooling, electric lighting, and other comforts that use energy. Thus, raising living standards around the world will likely raise demand for energy.
Even as demand increases, fossil fuel reserves are dwindling. The coal supply is expected to last about 200 years. Oil and gas supplies could run out even sooner.
As fuels become scarce—or as demand for fuels increases—the price of fuel rises. Rising energy costs can raise the cost of virtually any good or service, threatening living standards. High oil and gas costs may lead energy companies to produce more fuel from unconventional and potentially polluting sources, such as oil shale, tar sands, and low-grade deposits of coal. On the other hand, high fossil fuel costs can drive people to conserve energy or switch to renewable sources.
International conflict.
Many countries lack sufficient energy resources to meet local demand. They must import fuel from abroad. A number of less developed countries, by contrast—notably in the Middle East—produce much more oil than they consume. The desire to ensure a stable supply of oil or gas from such countries can be a key factor in international conflict. Energy concerns played a role in such wars as the Iran-Iraq War (1980-1988), the Persian Gulf War of 1991, and the Iraq War (2003-2011). Such wars had causes beyond energy. But experts believe that the need to secure petroleum supplies contributed to the decisions to engage in war.
Conflicts may also arise over renewable energy technology. For example, the Chinese government heavily subsidizes (funds) wind and solar technology companies. The subsidies give these companies an artificial advantage in the global marketplace. These tactics risk provoking other countries to heavily tax or bar Chinese energy technology exports, potentially sparking major trade conflicts.
Environmental pollution.
The use and extraction of many energy sources damages the environment. Fossil fuels cause a great deal of air pollution and other environmental problems. But other energy sources can cause environmental damage as well. Energy producers, power companies, manufacturers, and governments take various steps to limit pollution from energy use.
From fossil fuels.
When burned, coal releases carbon dioxide, coal ash, sulfur oxides (compounds with oxygen), nitrogen oxides, mercury, and other substances that cause widespread pollution. Sulfur and nitrogen oxides can react with moisture in the air. They may eventually fall to the ground as acid rain, polluting lakes and rivers (see Acid rain). Vehicle fuels derived from oil also rank as a leading source of air pollution.
To reduce pollution, many large factories and power plants that burn coal have installed scrubbers, filters, and other cleaning devices. Coal can also be converted into a gas or a liquid. Burning either of these results in less pollution than does burning solid coal. Also, car manufacturers build cars with filtering technology. Such technology reduces the amount of pollutants released when gasoline and diesel are burned.
It is not just the burning of fossil fuels that causes environmental problems. The extraction and transportation of such fuels can also damage the environment. Both surface mining and deep underground mining of coal can contaminate air and water supplies and destroy nearby habitats. Underground coal mines may cave in and release dangerous gases. Strip mining—the main method of surface mining—has exposed large areas of land to erosion. The drilling of offshore oil fields and the shipment of petroleum by tankers sometimes result in oil spills. Such spills pollute the ocean, contaminate beaches, and kill wildlife. Burying oil and gas pipelines requires changes in the environment, such as the clearing of trees along the route. Coal mining and petroleum companies have taken a number of measures to limit such environmental damage.
From nuclear power.
Nuclear power plants do not release air pollution. But they may cause thermal pollution by heating up nearby waters, threatening wildlife that depends on them. Nuclear power plants also create radioactive wastes. Such wastes release harmful energy for many years as they break down. They must be kept safely sealed away. An accident at a nuclear plant can spread radioactive material far and wide in the form of fallout. Modern nuclear plants have numerous safety measures designed to prevent a release of radioactivity in case of an accident.
From renewable energy sources.
Renewable sources do not result in air pollution or radioactive wastes and in general are much cleaner than fossil fuels. But they can also harm the environment. The construction and operation of large hydroelectric dams changes river conditions and disturbs local ecosystems. Geothermal power plants release heat and gases into the atmosphere. They may also cause earthquakes. Wind turbines may kill birds. Used widely enough, they may have the potential to change weather conditions in unpredictable ways.
Global warming.
Burning fossil fuels produces carbon dioxide, a greenhouse gas that accumulates in Earth’s atmosphere. Like the glass in a greenhouse, such gases allow sunlight through to warm Earth’s surface. But they prevent heat from escaping back into space. Scientists think that the steady buildup of greenhouse gases from human activity has resulted in global warming. Global warming is an observed increase in Earth’s average temperature that threatens to shift the world’s climate in destructive ways (see Global warming).
All fossil fuels produce carbon dioxide when they are burned. Natural gas lacks many of the pollutants associated with coal and oil and releases about half as much carbon dioxide when burned. But natural gas is composed chiefly of methane, a greenhouse gas that traps heat nearly 20 times as effectively as does carbon dioxide. Thus, unburnt natural gas can greatly contribute to global warming if it leaks into the air during extraction, transport, or use.
Burning biomass generates about the same amount of carbon dioxide as does burning fossil fuels. But as a living plant grows, it removes carbon dioxide from the atmosphere. Thus, the total carbon dioxide emissions from biomass can be controlled if it is grown and transported sustainably.
History
Human beings learned to use fire over 1 million years ago. Until then, their only sources of energy had been their own strength and direct sunlight. With the heat energy released by burning wood, people warmed themselves, cooked food, and hardened pottery. By 3000 B.C., the Egyptians invented sails and used the wind to drive their boats. Water wheels, developed in ancient times, harnessed the energy of falling water.
Wood, coal, and steam.
Until the late 1700’s, wood ranked as the most important fuel. People used so much timber that it began to grow scarce, and coal gradually took its place as the chief fuel source. The growing demand for coal brought a search for better mining methods, including ways to keep mine shafts from flooding.
In 1698, an English inventor named Thomas Savery patented an improved pump to drain mines. The pump was powered by the first practical steam engine. People now had a device that could change heat energy into mechanical energy to do work.
The Industrial Revolution
was a period of great technological and social change that began in the late 1700’s. The steam engine became the chief source of power for industry and transportation during this time. People’s use of energy increased tremendously during the Industrial Revolution. Power-driven machinery largely replaced hand labor, and steamboats replaced sailing ships. New uses of energy made work easier and more productive, and increased production brought greater wealth. This prosperity helped bring about a growth in population, and so there were more people to consume energy. At the same time, people could afford to buy more energy-consuming conveniences.
During the 1800’s, inventors learned about several new sources of energy—and ways to use this energy. In the early 1830’s, two physicists—Michael Faraday of England and Joseph Henry of the United States—independently discovered a way to turn mechanical energy into electrical energy. They found that magnets could be used to produce electric current in a coil of wire. Based on this principle, called electromagnetic induction, generators could produce electrical energy from the mechanical energy of a spinning water wheel or steam turbine.
In 1860, Jean Joseph Étienne Lenoir, a French inventor, built one of the first workable internal-combustion engines. These engines produce power from the explosion of a mixture of air and flammable vapors. Gasoline, which is made from oil, proved to be the most convenient fuel for such engines because it easily turns into vapor. In 1885, Karl Benz, a German engineer, built one of the first gasoline automobiles. The demand for oil soared as automobiles came into use.
The 1900’s.
During the 1900’s, the use of energy almost doubled every 20 years. Much of the increased use stemmed from a steadily increasing—and increasingly wealthy—population. For example, the population of the United States increased about 25 percent from the late 1950’s to the late 1970’s. But during that period, the use of energy increased about 90 percent. The standard of living improved, and people could afford to buy such energy-using conveniences as air conditioners and cars. Such new inventions as televisions and microwave ovens consumed even more energy. People also used more of such materials as aluminum and plastic, which required huge amounts of energy to manufacture.
Many other countries followed a similar pattern during this period, putting huge demands on the world’s energy supply. For most of the century, the supply of fossil fuels remained relatively plentiful, and their price remained stable—and cheap. During the late 1960’s and 1970’s, however, Middle Eastern oil producers acted in concert to reduce oil production and raise prices, sparking a worldwide “energy crisis.” Many developed nations responded by seeking to conserve more energy. But in the 1980’s, the global supply of oil stabilized, and prices remained relatively low until the 2000’s.
The 2000’s.
During the first decade of the 2000’s, energy use leveled off and then began to decline. Part of the decline was driven by increasing oil prices. In the United States, the use of hydraulic fracturing greatly increased the supply of natural gas. Hydraulic fracturing involves forcing fluids into the ground to break up underground rock formations, releasing petroleum trapped within. Because of hydraulic fracturing, cheap natural gas began displacing coal in U.S. power plants.
Demand for energy also decreased, in part because of a number of measures that made existing technologies more efficient. Automakers greatly increased the efficiency of modern vehicles. Consumers purchased more hybrid and electric vehicles that could use rechargeable batteries in addition to—or instead of—a gasoline engine. Highly efficient compact fluorescent lights (CFL’s) and light-emitting diodes (LED’s) also became popular alternatives to traditional incandescent light bulbs, which waste a great deal of electric power.
Even with such conservation measures, the goal of a reliable, clean, and affordable energy supply remained far off. Experts believe a number of steps are necessary to accomplish this goal. They emphasize developing alternatives to the modern fossil fuel-based transportation system. Such alternatives include cars that run on sustainably produced biofuels and on electric power. Another alternative is the use of hydrogen as a fuel. When burned, hydrogen emits much heat and one harmless byproduct—water. But separating hydrogen from water requires much energy. Hydrogen is also highly explosive, making it difficult to store and transport.
Solar and wind energy remain the most promising renewable energy technologies. Scientists and engineers are working to develop energy storage systems, such as advanced batteries, that would help stabilize the supply of energy from these sources. For example, an array of batteries could store the energy produced on windy or sunny days and release it during calm or cloudy periods.
Finally, increasing efficiency and conservation remains a major goal in energy research. New technologies, such as batterylike fuel cells, convert stored chemical energy into electric energy much more efficiently than do engines and power plants that use fossil fuels. In architecture and engineering, new designs and materials can cut down the energy required to heat, cool, and light buildings. Much energy can also be conserved by expanding the use of traditional methods, such as turning off lights when not in use and recycling used metal, glass, and plastic.
