Earth is like a car slowly running out of gas as it speeds down the road to the future. Although the supply of petroleum, the source of fuel oil and gasoline, is now stable, it is only a matter of time before worldwide reserves are exhausted. Experts predict that there will be an inevitable shift away from fossil fuels before the end of the 21st century. Growing concerns about global warming and other environmental issues have also prompted efforts to develop alternate sources of power that are Earth-friendly, abundant, and cost-effective.
The world's supply of coal, our most abundant fuel, is expected to last about 250 years. This has prompted research into methods of converting it into a liquid that can replace petroleum, or into a gas that can replace natural gas. In a typical gasification process, powdered coal is mixed with steam and oxygen under high pressure to produce a gaseous mixture of carbon monoxide, hydrogen, and methane.
Coal liquefaction processes convert coal into a gas or slurry (thin slush). Then hydrogen is added to break down the coal molecules and turn them into a liquid. Coal also can be gasified and converted into methyl alcohol, or methanol, a substitute for gasoline.
There have been a number of efforts and research projects to develop viable alternatives to the dwindling resources of petroleum and natural gas. Synthetic fuels, or synfuels, can be made from oil shale, tar sands, and biomass. Oil shale, a soft, fine-grained rock, contains kerogen, from which oil and gas can be obtained by heating. Tar, or oil, sands contain bitumen, a thick black substance that can be made into oil or gas. Biomass refers to plant or animal matter from which energy is released by heating or chemical reaction.
Projects are now under way to make coal a more environmentally friendly energy source. In the United States, as part of President George W. Bush's Clean Coal Power Initiative, there is a concerted effort to develop new ways of reducing harmful emissions and improving the efficiency of coal-fired power plants. Gasification is still a primary focus, but plans are also in the works to build a new prototype plant that converts coal into a hydrogen-rich gas instead of burning it directly. Such a facility would not only generate electricity, but could also produce hydrogen for use in fuel-cell motor vehicles.
Scientists from the Department of Energy (DOE), from industry, and from various research institutions are studying ways to wrest energy from biomass—land and water plants, farm crops, garbage, manure, and sewage. They employ heat and chemical changes produced by bacteria—a process called fermentation—to convert these materials into steam, liquid fuels, and gaseous fuels.
A result of biomass conversion familiar to many people is gasohol—a fuel that is a mixture of 90 percent gasoline and 10 percent alcohol. Sugar, corn, wheat, potatoes, farm wastes, and other materials can be fermented and distilled to produce ethanol, or ethyl alcohol.
In the United States, gasohol surged in popularity during the fuel shortages of the late 1970s. When gasoline became more plentiful and prices fell in the early 1980s, the gasohol industry sputtered. Interest was revived again in the 1990s thanks to tax incentives, amendments to the Clean Air Act, and a large decrease in production costs. Ethanol is now the most commonly used biofuel, and its popularity is growing. In 2003, U.S. production of ethanol hit 2.8 billion gallons (10.6 billion liters). In 2006, the U.S. had 101 plants in operation with an annual capacity of 4.8 billion gallons (18.2 billion liters). The Energy Policy Act of 2005 offers benchmarks for increased ethanol production and use. But environmentalists caution that ethanol production from corn, switchgrass, soybeans, and sunflowers requires between 29 and 118 percent more fossil energy than the ethanol fuel produced.
Financial help offered by governments can encourage alcohol-fuel production, particularly in places rich in source materials. Brazil is the world's largest producer of ethanol, thanks to the country's abundant supply of homegrown sugarcane. In 2005, Brazil's ethanol production reached 4.8 billion gallons (15.9 billion liters). That is expected to increase to 6.87 billion gallons (26 billion liters) by 2010. Critics warn that expanding the large-scale sugarcane plantations for this effort could irreversibly harm Brazil's rainforests.
In the United States, alcohol fuel is produced primarily from corn. But corn, sugar, and other crops usually have more value as food than as fuel. This is not the case with garbage. Many cities around the world solve their waste-disposal problem by burning trash to make steam for heating and for generating electricity. About 100 such facilities exist in the United States. Wastes can produce almost as much energy as can burning coal. Coal produces heat energies of 28 million to 38 million joules per 2.2 pounds. (A joule is a unit of energy.) Old, dry newspapers can generate 20 million; and cooked meat scraps, 29 million.
The major drawback of waste-burning plants is air pollution. Some plants, such as a refuse-burning power plant in Saugus, Massachusetts, reduce air-pollution production by using electrostatic devices that "scrub" dust and other particles out of exhaust gases by means of electric attraction. Other types of scrubbers use water to wash particles out of smoke.
Garbage can also be chemically converted to a gas fuel by the action of bacteria. This occurs naturally in landfills. Anaerobic bacteria, which do not need oxygen to survive, convert the waste to methane gas. Companies extract this gas, treat it, and then sell it. The process is made more efficient by removing metals and glass and allowing the refuse to ferment under controlled conditions.
Sewage is another source of methane. A number of projects have explored the use of sewage as an energy source. At sewage-treatment plants, methane gas can be collected to power fuel cells that generate electricity.
In 2005, Smithfield Foods, the world's leading pork producer, was developing a Utah plant that converts swine feces into methane gas. The facility refines the gas into a liquid called methanol, generating up to 3 million gallons (11 liters) per year. From there, the methanol is shipped to another site and combined with biodegradable waste oils (such as cooking oils) to produce biodiesel, a fuel that can be used in diesel engines. Although more costly to produce than traditional petroleum-based diesel fuel, biodiesel does have advantages: it produces fewer pollutants; it has been found to extend engine life because of its lubricating properties; and it could ultimately help to reduce U.S. dependence on foreign oil.
Crop residues, such as cornstalks and wheat straw, also offer enormous potential. Crops and wastes with high fat or oil content can be converted into biodiesel fuel.
Other novel ideas involve converting seaweed into liquid or gas fuels. Some scientists think that the giant brown kelp, which lives off the coast of California, would be ideal for this purpose. The kelp grows about 3 feet (1 meter) per day; special vessels already harvest the tops of these plants for use in food processing. The DOE is also studying the possibility of raising oil-rich varieties of algae on special "farms." The oil from the algae would be extracted for conversion to liquid fuel, and the plants returned to the farm ponds for regrowth.
Biomass accounts for more than half of all renewable electricity generation, and about 2 percent of the total U.S. electricity supply. Wood is by far the largest source of biomass, and has been used by humans to provide heat for many thousands of years, long before the eventual discovery of coal.
Wood is now being considered as a partial replacement for coal and other fossil fuels—without cutting down more forests! In the process of converting a tree into lumber, 20 to 60 percent of each tree is wasted. This waste can be used as a source of steam instead of liquid and gas fuels. Also, acres of fast-growing trees can be planted close together in rows on "energy plantations." The trees are cut down every few years, and new trees are allowed to sprout from the stumps or roots. Eucalyptus, which attains a useful size in just five to seven years, is especially well suited for so-called energy plantations. Since 1979, stands of eucalyptus trees have been cultivated on an 850-acre (340-hectare) energy plantation in Hawaii. Each year, the tallest trees are reduced to chips, which are then burned to generate electricity.
Liquid fuel can also be extracted from wood chips or sawmill wastes. Using a process that combines chips with chemicals at high temperatures and pressures, a DOE-supported experimental plant in Albany, Oregon, has produced wood-chip oil. It requires about 900 pounds (400 kilograms) of chips to produce 1 barrel of oil.
Wood will probably never replace oil or natural gas for large-scale energy production. Nevertheless, it can provide a substantial portion of the energy needs of people who live in heavily forested areas.
Synthetic fuels represent only one solution to dwindling reserves of oil and natural gas. Those involving coal provide only temporary relief, since coal resources will not last indefinitely. To burn fossil fuels (oil, natural gas, and coal) more efficiently and thus prolong their supply, scientists are experimenting with magneto-hydrodynamic (MHD) generators. These devices convert fuel into a hot, electrified gas. When passed through a powerful magnetic field, the gas produces an electric current that can be fed into power lines. This eliminates the boilers, turbines, and generators that are otherwise used to produce electricity. Because of its efficiency, an MHD generator could produce up to 65 percent more energy from 1 ton of coal than does a conventional-power plant. Unfortunately, no one has yet devised a way to build parts that can withstand the extremely high temperatures needed to produce the hot, electrified gas.
Scientists hold high hopes for hydrogen as a potential source of abundant, nonpolluting energy. Hydrogen-fueled aircraft and motor vehicles are being developed. Hydrogen is clean-burning, eliminating the air pollution caused by coal and oil. In the burning process, hydrogen recombines with oxygen, yielding water, not ash or smoke, as a combustion product. The cycle can then be repeated. The primary obstacle to the widespread use of hydrogen is the cost of producing, storing, and transporting it.
Scientists are experimenting with different ways of producing hydrogen. One method is by electrolysis. This involves splitting water into its component parts, hydrogen and oxygen gas, with an electric current. U.S. researchers are seeking ways to extract hydrogen from coal, which plays a major role in U.S. energy production. In 2005, Israeli and European researchers generated hydrogen using solar energy. Sunlight heats a mixture of zinc oxide and charcoal, releasing oxygen and a gas that condenses into zinc powder. When combined with water, the powder reacts to form hydrogen—and zinc oxide, which is recycled by the solar plant.
Wind energy accounts for less than 1 percent of the total electricity in the United States. California and Texas produce most of the nation's wind energy. The largest U.S. wind farms are located in New Mexico, New York, Oregon, Texas, and Washington. As of 2007, 40 states had wind-energy projects either planned or in operation. Cape Wind, a proposed facility off Cape Cod, Massachusetts, would entail 130 wind turbines in Nantucket Sound. A similar project is proposed for the coast of Long Island, New York. A 28-turbine facility is in operation off the coast of Maine.
Tides, like the wind, will always be with us, and people have long utilized the energy of their ebbs and flows. For hundreds of years, the moving waters of the tides turned mills that ground grains. In 1966, France began full operation of the world's first tidal-power plant, located on the Rance River.
The ideal site for a tidal-power plant requires that a large difference exist in the volume of water flowing in and out with each tide, and that there be a narrow bay or river that can be closed off by a dam. High tides raise water in the bay or river, which is closed by the dam before the water begins to ebb. During low tide, the water level outside the dam drops below the level in the bay or river. Gates are opened, and as the stored water falls, it propels the turbines that generate electricity.
North America's first and only tidal-power plant was built at the Annapolis Basin in Nova Scotia. There, tides bring water into the Annapolis River. As it flows out again into the Bay of Fundy, the water spins turbine generators to produce 20 megawatts of electricity.
Another innovation to capture tidal power is the water turbine. This looks like a submerged windmill, with the blades slowly rotating as the tides ebb and flow. A pilot project was installed at the mouth of the East River in New York City in 2006. Four more turbines were added the next year. Ultimately, the project will involve 200 turbines.
In 2005, Portugal installed the world's first "wave farm" demonstration project, called Pelamis, off its shores. Wave farms consist of floating devices that contain hydraulics. As waves buffet the devices, hydraulic fluid is pumped through a cylinder into a generator that produces electricity. The power flows through a single cable tethered to a junction on the ocean floor. Floating devices can be either cylindrical or buoylike, and both designs are low-profile. Pelamis started with three cylinders, and an additional 27 are planned. Wales has approved a wave farm for 2007, and the U.S. has scheduled a pilot wave farm for 2008 off the coast of Oregon. If successful, an array of 300 buoys will eventually supply the electricity needs of 40,000 homes nearby. Offshore wave-energy facilities could also bring thousands of jobs and reduced greenhouse gases.
The United States faces several energy challenges: electricity shortages; antiquated energy infrastructure; unstable gasoline prices; strained natural resources; and dependence on foreign supplies. The growing needs for alternative fuels and cleaner sources of energy have emerged as compelling issues. As nonrenewable energy sources are depleted, alternative sources are becoming increasingly important to keep the lights on and the wheels turning.William J. Cromie