Energy: The Quick Tour
Energy sources, energy choices

What are the best energy sources? "Best" depends on many factors-how the energy is being used, where it is being used, what energy sources are available, which sources are most convenient and reliable, which are easiest to use, what each costs, and the effects on public safety, health, and the environment. Making smart energy choices means understanding resources and their relative costs and benefits.

Some energy sources have advantages for specific uses or locations. For example, fuels from petroleum are well suited for transportation because they pack a lot of energy in a small space and are easily transported and stored. Solar photovoltaics are well suited for heating or electricity generation in desert climates or other sunny places with lots of flat, open space. Small hydroelectric installations are a good solution for supplying power or mechanical energy close to where it is used. Coal is widely used for power generation in many fast-developing countries-including China, India, and many others-because domestic supplies are readily available.

Efficiency is an important factor in energy costs. How efficiently can the energy be produced, delivered, and used? How much energy value is lost in that process, and how much ends up being transformed into useful work? Industries that produce or use energy continually look for ways to improve efficiency, since this is a key to making their products more competitive.

The ideal energy source-cheap, plentiful, and pollution-free-may prove unattainable in our lifetime, but that is the ultimate goal. The energy industry is continuing to improve its technologies and practices, to produce and use energy more efficiently and cleanly. A future source may be hydrogen-based.

Energy resources are often categorized as renewable or nonrenewable.

Renewable energy resources are those that can be replenished quickly-examples are solar power, biomass, geothermal, hydroelectric, wind power, and fast-reaction nuclear power. Renewable energy sources supply about seven percent of energy needs in the United States; the other 93 percent comes from nonrenewables. The two largest categories of renewable energy now in use in the U.S. are biomass-primarily wood wastes that are used by the forest products industry to generate electricity and heat-and hydroelectricity.

In most cases, fossil energy resources are currently more affordable and easier to store and transport than renewable sources. For renewables to become more widely used, many hurdles must be overcome-most related to producing and distributing renewable energy more economically.

How far into the future will energy resources be available to supply our needs? The sustainability of any particular energy resource is an important consideration in determining where to invest in energy technology and infrastructure. All energy resources, whether renewable or nonrenewable, must be used efficiently and sustainably in order to safeguard the future for ourselves and our children.

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Nonrenewable energy resources include coal, oil, natural gas, and uranium-235, which is used to fuel slow-reaction nuclear power. Projections of how long a nonrenewable energy resource will last depend on many changeable factors. These include the growth rate of consumption, and estimates of how much of the remaining resources can be economically recovered. New exploration and production technologies often increase the ability of producers to locate and recover resources. World reserves of fossil energy are projected to last for many more decades-and, in the case of coal, for centuries.

World oil reserves are greater now than ever. Even if no new oil is discovered, proven reserves are projected to sustain current consumption levels for 40 years.

Since the beginning of the industrial revolution, significant progress has been made in the efficient use of fossil fuels. For example, new gas-powered fuel cells are 40 percent to 80 percent efficient, with no combustion or emissions involved in the energy conversion process. Likewise, the next generation of hybrid-fuel cars will improve efficiency by capturing kinetic energy from the wheels to power the battery.

Hybrid cars that use electricity from batteries together with gasoline are providing new transportation options. Automakers also are developing fuels cells that extract hydrogen from gasoline or methanol. Like batteries, fuel cells rely on chemical reactions rather than combustion.

Hydrogen, a simple, abundant element, has very high energy potential. Hydrogen is the primary ingredient in water, which covers 70 percent of the earth. On this planet, hydrogen is an unstable element-it exists in stable form only in combination with other molecules. When pure hydrogen gas is formed, it will immediately evaporate, then condense with oxygen to become water.

Many believe that hydrogen gas is the fuel of the future. It is clean; when burned, its only byproduct is water. It is also a renewable and sustainable resource. But making hydrogen fuel in large quantities is not economical or efficient-currently it takes more energy to make it and stabilize it than it's worth.

Today's methods include "reforming," which uses heat to separate hydrogen from hydrocarbons, typically natural gas; electrolysis, in which an electrical current run through water separates the hydrogen and oxygen molecules; and algae farming, in which algae excrete hydrogen gas under certain conditions. Major efforts are now under way to make these processes more efficient and cost effective.

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Here are key characteristics of renewable energy resources.

Hydropower comes from moving water and ocean waves. Hydropower systems use the energy in flowing water for mechanical purposes or to produce electricity. Hydroelectric plants use the kinetic energy of moving water to spin the turbine generator. Most sites suitable for large-scale hydroelectric plants in the U.S. have already been developed. New developments are small-scale, which can efficiently supply local needs. The U.S. and Canada have the greatest number of hydroelectric plants. Virtually every other country in the world has some plants-hydroelectricity is the power source of choice for many developing countries. There are concerns over the detrimental environmental effects of hydroelectric power, which include siltation and erosion, soil and water salinity, the breaking up of the free passage between oceans and rivers, weed growth, floods due to dam failures, and disease spread by small organisms that live in stagnant water.

Biomass materials such as wood, agricultural crop wastes, fast-growing willow and switchgrass crops, animal wastes, and even garbage can be used as renewable sources of energy to generate heat and power. They also can be used as alternatives to petrochemicals in making plastics and other products. Today, biomass energy systems are very small-scale; some examples include ethanol in gasoline, and use of municipal waste to produce methane gas. The biggest user of biomass energy is the forest products industry, which burns much of its waste to make heat and electricity. Biomass fuels contain a lot of carbon; using them requires high-tech burners that reduce smoke. Still, biomass energy can productively use wastes that would otherwise go to landfills or incinerators.

Passive solar heating for buildings is a common application of renewable energy. A passive solar heating system collects energy from the sun. It uses this energy to heat a space directly, or to heat a fluid that later radiates heat to a space. A sunroom, an example of a passive solar approach, uses double-layered windows trap heat in the room and reduce the amount of heat loss by convection. Solar systems are dependent on the weather conditions and number of daylight hours. Their effectiveness is greatly affected by climate, season, building orientation, and site conditions.

Active solar systems-or photovoltaic systems-are another way of capturing the sun's energy. These systems use solar cells to directly produce electricity from solar radiation. The solar cell is made of two semiconducting materials-generally silicon-based-with a boundary between them. When a photon of electromagnetic energy from the sun strikes an electron near the boundary between the semiconductors, it starts a series of reactions that separates electrons and "holes"-the unoccupied spaces left behind when the electrons leave. The electrons move in one direction through the conductor, and the holes move in the other direction. This creates an electrical current when the object being powered is connected in a circuit to the semiconductor. Some homes and businesses use solar cells to reduce the amount of power they buy from the electric utility. Some experimental cars also use photovoltaic cells to power electric motors.

At this time, photovoltaic systems are relatively expensive to build and maintain. They also require a back-up source of power, or batteries, to provide power when sunshine is inadequate. Because semiconductors contain toxic materials, the environmental impacts of manufacturing and disposing of solar cells and their batteries are also a concern.

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Solar thermal electricity is another variation on solar energy. These plants use a highly curved mirror to focus sunlight onto a pipe, concentrating the heat to boil water and create steam. That steam is then used to turn a turbine generator to make electricity.

Wind power uses energy from the moving air to turn large blades on windmills. In the past the motion of the blades was used to grind flour or pump water, but now the blades turn turbines, which rotate generators in order to produce electricity. Very large, wide open windy spaces are needed for this system to be efficient. Although wind energy makes noise and can harm birds, it produces no air or water pollution. The major constraint to wind power is the limited availability of sites with steady wind. Today, costs of electricity from wind are generally higher than costs of power from other sources. These costs have been declining as wind turbines are made more efficient and long-lived.

Geothermal energy systems use hot water from below ground to heat homes and buildings during the winter. The hot water circulates through insulated pipes placed hundreds of feet into the ground. Some regions, such as areas of Iceland, use active springs and geysers to heat buildings. A few places have so much steam and hot water that it can be used to generate electricity. Even though vast amounts of energy are available within the earth, our ability to use it is limited by site considerations. Today's geothermal heat pumps are also more costly than conventional heating systems. A variation on geothermal power systems uses ocean thermal energy. Ocean thermal energy conversion is currently being used in Japan and Hawaii in some demonstration projects.

Here are key characteristics of nonrenewable energy resources currently available.

Nuclear fuel-Uranium-235, found in certain rock formations, is mined for use as the fissile material in slow-reaction nuclear power generation. Rare in its natural state, uranium-235 is a nonrenewable resource, although small quantities go a long way. For example, the fission of one pound of uranium releases more energy than burning three million pounds of coal.

Nuclear fission reactors split atoms to release the energy from the nucleus of enriched uranium. In this process, the fuel is placed in rods in the reactor core, and a chain reaction is started by bombarding the fuel with slow neutrons. Heat from the chain reaction is absorbed by the water in the reactor. The water then turns into steam, which, in turn, drives a turbine and a generator to produce electricity. Control rods-made of cadmium or boron-are introduced to slow down or stop the chain reaction.

In fast-reaction nuclear power generation (breeder reactors), high-velocity neutrons cause the fissions, using plutonium or uranium-233. Breeder reactors produce more fuel (enriched uranium and plutonium) than they consume. Thus, fast-reaction nuclear power fuel is considered renewable and sustainable. Nuclear power plants do not release carbon dioxide (a contributor to global climate change) or sulfur dioxide (a contributor to acid rain).

One obstacle to nuclear fission power is that radioactive nuclear waste is generated. High-level radioactive waste-the fission products in the used fuel rods-will be dangerous for the next 100 to 1,000 years. There is no known way to speed the removal of radioactivity from waste. Spent fuel rods are first cooled in large tanks, then encapsulated in ceramic or glass containers. These containers are then placed in stainless steel containers and stored. Very little high-level waste is made by a reactor in a year-enough to occupy a volume of about half a cubic yard.

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Fossil fuels-including coal, oil, and natural gas-are sources of energy that humans have taken advantage of over thousands of years. About 90 percent of the world's energy consumption comes from fossil fuels. These were created by the decomposition of primitive organisms, buried in sand and mud, and compressed under the weight of accumulating layers. Over millions of years, temperatures and pressures changed the organic matter into coal, oil, and gas. Deposits of these resources are now found below ground in many areas of the world.

Combusting fossil fuels with oxygen releases water, carbon dioxide, and other substances into the environment. In the case of coal, these substances include sulfur dioxide and nitrogen oxides, which have been shown to be responsible for acid rain. To control these emissions, today's coal-fired power plants are equipped with scrubbers, filters, collectors, electrostatic precipitators, and other devices. Natural-gas-fired power plants release virtually no sulfur dioxide, but require controls to limit their nitrogen oxide emissions.

Smog is created by a photochemical reaction of sunlight with hydrocarbons, carbon monoxide, and other molecules emitted in car exhaust. To control smog, oil companies have reformulated gasoline, and automakers have designed cars that burn gasoline more cleanly and efficiently, with better filtering mechanisms. The results: tailpipe emissions from the average new car contain 95 percent less hydrocarbons than they did in the 1960s.

Global climate change is another environmental issue linked to fossil fuel use. As a greenhouse gas, the carbon dioxide released in the combustion of fossil fuels traps infrared radiation from the earth that would otherwise radiate out to space. This effect is believed to raise the heat of the earth's atmosphere. Planting more trees is one way to remove more carbon dioxide from the atmosphere, since trees need carbon dioxide as part of photosynthesis. Other greenhouse gases-like methane and carbon dioxide-come from animals and industry practices.

Oil spills in the oceans can damage coastal and marine plants and wildlife. Double-hulled tankers and rigorous safety practices are highly effective in preventing spills and limiting damage. Since 1990, more than 99.999 percent of oil delivered by tankers to the U.S. reached its destination without incident.

Coal is recovered from the earth by surface mining or deep mining. Surface mining, or strip mining, is less expensive and usually occurs on flat land. Deep mining requires digging shafts and tunnels to get to the coal seam. Automation of deep mining has helped to counter its safety and health hazards. Coal can be gasified to form a synthetic fuel similar to natural gas. It can also be liquefied to make a synthetic crude oil. To date, it has not been economical to make synthetic fuels from coal on a large scale. As processes become more efficient, the use of synthetic fuels may become more economical.

Oil comes from crude oil, which is a mix of hydrocarbons with some oxygen, nitrogen, and sulfur impurities. One barrel of oil (42 U.S. gallons) can provide about 6 million Btu. Crude oil reserves are found all over the world, but the Middle East alone has about 63 percent of the known reserves. Of the oil consumed in the United States, most is used in transportation, and much of the rest goes to industrial, commercial, and residential uses. Crude oil is used to produce not only a range of fuels, but also petrochemical ingredients for plastics, inks, tires, pharmaceuticals, and a host of other products.

High-tech oil exploration technology and practices have led to the discovery of as many new reserves as have already been used. To make the most of this valuable resource, energy producers are developing more efficient refining methods, product makers are finding more efficient ways to use petrochemicals, and manufacturers are developing more efficient cars. New techniques of locating and extracting oil from the earth are also making it possible to recover oil that was once too expensive to produce.

Oil is usually recovered by drilling wells through the non-porous rock barrier that traps the oil. In general, about 30 percent of the oil trapped can be economically recovered by pumping. "Secondary" recovery can remove another 10 percent, by flooding the well with high-pressure water or gas. Another 10 percent can sometimes be recovered with "tertiary" methods that heat the oil to scrub it out. About half of the oil is left trapped in the rock. Oil producers are continually seeking economical ways to recover more of this oil.

The oil refining process separates crude oil into different hydrocarbons and removes impurities such as sulfur, nitrogen, and heavy metals. The first step is fractional distillation, a process that takes advantage of the fact that different hydrocarbons boil at different temperatures. In a tall tower called a fractionating column, crude oil is heated until it boils. Horizontal trays divide the column at intervals. As the oil boils, it vaporizes. Each hydrocarbon rises to a tray at a temperature just below its own boiling point. There, it cools and turns back into a liquid.

The lightest fractions are liquefied petroleum gases (propane and butane) and the petrochemicals used to make plastics, fabrics, and a wide array of consumer products. Next come gasoline, kerosene, and diesel fuel. Heavier fractions make home heating oil and fuel for ships and factories. Still heavier fractions are made into lubricants and waxes. The remains include asphalt.

The refining process then continues, with heavy fractions converted into lighter fractions. In most cases, "cracking" processes are used to transform large (heavy) hydrocarbon molecules and make the smaller, lighter molecules such as gasoline and jet fuel. Better refining technologies have made it possible to produce over 21 gallons of gasoline from a 42-gallon barrel of crude oil-a remarkable advance over the industry's early days, when a barrel of oil yielded just 11 gallons of gasoline.

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Oil shale was never buried deeply enough or heated enough to form crude oil. Its hydrogen content is between that of coal and crude oil. Concentrations of oil are low, so that, at most, one barrel of oil can be recovered from 2.4 tons of sand or 1.5 tons of rock. Huge amounts of oil shale are found all over the world. In fact, the total global resource is 1,000 times greater than crude oil reserves. But extracting the energy value of oil shale is not practical today. Scientists and engineers continue working on ways to recover oil shale for a reasonable cost.

Natural gas is the gas component of coal and oil formation. It is used in industrial and commercial heating and cooking, and, increasingly, to fuel electricity generation. In a compressed form, natural gas can also be used as a transportation fuel. Natural gas is either found mixed in oil or is released from coal.

Energy in 6,000 cubic feet of natural gas is equivalent to one barrel of oil. World reserves of natural gas are greatest in Russian, Iran, Qatar, Saudi Arabia, United Arab Emirates, and the U.S. The U.S. consumed 19.7 million cubic feet of natural gas in 1999, nearly all of which came from domestic production. Five states-Texas, Louisiana, Alaska, New Mexico, and Oklahoma-hold more than 85 percent of U.S. natural gas reserves.

Wells for natural gas are drilled in underground reservoirs of porous rock. When it is removed from a reservoir, natural gas can either be pumped to the processing station for removal of liquid hydrocarbons, sulfur, carbon dioxide, and other components, or stored in large caverns underground until it is needed. Pipelines are the main method of transporting natural gas. Natural gas can also be liquefied and shipped overseas, but this process is complex and expensive.

Electrical generation by natural gas has been improved by the development of combined-cycle systems. These systems put together a natural-gas-fueled combustion turbine with a heat-recovery steam generator and steam turbine, to produce electricity in two ways rather than just one. The result: roughly 60 percent of the heat from the natural gas is harnessed to make electricity, creating a more energy-efficient system.

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