Energy Sources: History, Selection, and Transitions

Energy Sources

Fossil Fuels

Coal. Coal is formed from the compressed remains of terrestrial plants that grew 100 to 300 million years ago. Made up of primarily carbon and hydrocarbons, coal is a combustible rock that has been a source of heat for thousands of years. It has been used in China since 4000 BC and in Europe since the Bronze Age, 2000 – 3000 B.C. (13). Coal was first collected from exposed seams and along shorelines and later dug from underground shafts and open pits. Today, coal is mined commercially in 100 countries. Of the 7.04 billion short tons of coal extracted throughout the world in 2007, 40 percent was mined in China and 16 percent in the U.S. Other top coal producing countries include India, Australia, and Russia (14).

The U.S. currently has the largest known coal reserves in the world (14) (Figure 5). These reserves include coal of the major types, including: anthracite (the hardest, least volatile coal; used for heating), bituminous coal (a dense form used for heating and in manufacturing), sub-bituminous coal (an intermediate type used in electric power generation), and lignite (the softest and most volatile; also used in electrical generation) (13). Lignite is the type found in western North Dakota. In fact, North Dakota has the largest known deposit of lignite in the world, of which 25 billion tons is considered mineable (15).

Figure 5. Coal mines in U.S.
  Photo 1. Lignite coal mine in Poland.

The depth and thickness of the coal seam determines the type of mining operation used. Underground mining is required when the coal is several hundred feet below the surface. About 60 percent of the world’s coal (and 31 percent in the U.S.) is from underground mines (16, 17). Open pit mines may cover many square miles, and huge draglines are used to remove the earth and rock, or overburden, to expose the coal (Photo 1). Topsoil is removed and set aside, and then the same is done with the subsoil. After the coal is removed, the land is contoured and the subsoil is replaced, followed by the topsoil. A seedbed is prepared and the land is seeded to annual crops, hay, or native pasture (18). In a modern open pit mining operation, land reclamation is an important factor and may account for 30 percent of the cost of coal production (18).

Coal can be used in several ways, but in the U.S., 90 percent of it is used to generate electricity (13). Nearly half of the electricity used in this country comes from coal. Of the 2,788 fossil-fuel power plants in the U.S., 620 are coal-fired (19).

In a power plant, the coal is crushed, dried, and in some plants, pre-treated to remove impurities. The pulverized coal is blown into a furnace where it burns at 2,300 to 3,000°F, depending upon the type of coal (20). This heat turns water in a boiler into steam that is pressurized, superheated, and then used to turn a series of turbines and an electrical generator. In modern plants, the flue gas is sent through stack gas scrubbers to remove sulfur dioxide and nitrogen oxides. Fly ash is also removed using filters or electrostatic precipitators (18, 21). Fly ash is usually treated as a waste product and is stored in surface ponds or buried, but it may be used in high-quality concrete (22).

Another product of a coal-fired plant is carbon dioxide (CO2), thought to be the primary cause of global warming. The use of coal produces 40 percent of CO2 emissions in the U.S., which is more than the CO2 emissions produced by petroleum use. Techniques to capture CO2 from flue gas are being developed, and one technique using an ammonia-based solution will be tested for the first time on a commercial-scale at Basin Electric’s Antelope Valley Station in North Dakota, starting in 2012 (23). The CO2 will be sold for use in oil recovery (see below).

Lignite is also used to produce synthetic natural gas through gasification. This gas can be used for heating and electrical generation, or it can be processed into liquid fuels. The Great Plains Synfuels plant near Beulah, North Dakota, is the only large-scale plant in the U.S. that produces natural gas from coal (21). Using coal from the nearby Freedom Mine, this plant has been in operation since 1984. By-products from the operation are also refined and marketed, including anhydrous ammonia, krypton, xenon, phenol, and naphtha (white gas). Nearly 50 percent of the CO2 produced at the plant is also captured and compressed, and then delivered through a pipeline to heavy-oil fields in Saskatchewan. There the CO2 is injected into the ground to increase oil recovery; it is stored permanently underground (21).

The world’s known supply of coal is estimated to last for at least 155 to 200 more years at current production levels (24, 14). Concerns over CO2 emissions have slowed the construction of coal-based power plants in some countries. However, new types of cleaner coal technology and methods of land restoration are being developed, and it is likely that coal will continue to play a major role in our energy mix for years to come.

Petroleum. Petroleum, also known as crude oil, is the most important source of energy in the industrial world. Not only is it the primary source of many products, it is easily transported and has a high energy content. The petroleum industry—production, refining, and retailing—is the single largest industry on earth in terms of dollar value (25). Crude oil has been both beneficial and a source of conflict around the world.

Petroleum is a flammable liquid formed from the remains of plankton that grew in lakes and oceans 300 to 400 million years ago. Oil found in conventional oil fields is trapped under an impermeable layer of rock. Oil sands contain oil of a heavier type and are found in Canada and Venezuela (25). In Alberta, these oil sands are surface-mined (26).

  Photo 2. Oil drilling rig in Spitsbergen, Norway.

The first wells were drilled in China during the Han dynasty (207 B.C. – 220 A.D.) using bamboo poles for percussion drilling. Today, wells are drilled using a rotating drill bit, and fracturing fluids are pumped in the well to help start the flow of oil (Photo 2). At first, the oil flows to the surface due to natural underground pressure. Later, a pump or injection with water or gas is used to maintain the flow of oil. When a well is no longer productive, the well is filled and capped to prevent contaminants from entering the aquifer (27).

Most oil wells are vertical wells, but in thin shale formations such as the Bakken Formation in North Dakota, Montana, and Saskatchewan, horizontal drilling is used to drill wells that drop vertically from the surface and then turn horizontally through the bed. In the Elm Coulee Field in Montana, the formation is only 45 feet thick, but the wells run through 3,000 to 5,000 feet of reservoir rock (28). Several lateral well bores may be placed from a single site (29). Fracturing with steam, solvents, and sand that keeps the cracks in the shale open increases productivity. This process is expensive: $6-8 million per well. However, the well may be productive for 20 years. Although the oil is of high quality, the price of oil must be at least $50-60 per barrel for this type of mining to be profitable (30). Because horizontal wells can extract oil from a wide area, far fewer wells and therefore fewer surface acres are required than for typical vertical wells.

Crude oil is a complex mixture of hydrocarbons. At the refinery, crude oil is processed by fractional distillation into light distillates (liquid petroleum gas, butane, gasoline, and naptha), middle distillates (kerosene and diesel), heavy distillates and residuum (fuel oil, lubricating oils, wax, petroleum coke, asphalt, and tar) (31). Another process called "cracking" is used to break the long-chain hydrocarbons of the heavier distillation fractions into simpler, more useful molecules, producing even more lighter-weight fuels (32). A barrel of oil (42 gallons) will produce 19 gallons of gasoline and 7 gallons of diesel.

Crude oil is classified by its location of origin, its density (API gravity), and its sulfur content (sweet or sour), with light crude and sweet crude oils being higher grades and therefore more expensive.

Oil refineries are very complex and expensive. Even a small plant would cost $3 billion to build in the U.S. today (33). Emissions, wastewater, and siting concerns have halted the construction of new major, complex refineries in the U.S. since 1976. Expansion of and upgrades to the 149 current refineries (Figure 6) have been more economical.


      Figure 6. Oil refineries in U.S.


Crude oil production in the U.S. has declined an average of 2.5 percent each year since 1985. Still, the U.S. is by far the biggest consumer of petroleum, using 24 percent of the world’s output. Of the 20.7 million barrels consumed per day in the U.S. in 2008, 58 percent was imported (34). The major suppliers include Canada (23 percent of the total imports), Saudi Arabia (17 percent), Venezuela (11 percent), Mexico (11 percent), and Iraq (6 percent) (35). The total world oil supply during 2008 was 85.5 million barrels per day, while oil consumption was slightly higher (36). OPEC (the Organization of Petroleum Exporting Countries), with 11 member nations, supplies 33% of the current oil production and holds two-thirds of the world’s reserves (37).

Estimates of oil reserves are difficult to determine, however (38). Figures may include not only proven reserves but also "probable" reserves. Governments may not wish to reveal accurate numbers for political reasons (39). Also, up to two-thirds of the world’s reserves may be found in oil sands (40), and the recovery rates from these sources vary widely. Still, how long might oil supplies last? The Energy Information Administration estimates that oil supplies will last 49 years at the current rates of production and use (41). Other estimates range from 15 to 283 years (42, 43). The amount of oil from unconventional sources is huge, but production costs, environmental costs, and the future demand for oil will determine how much and long they will be utilized.

Natural Gas. Another important fossil fuel is natural gas. This flammable gas is made up of hydrocarbons, primarily methane (80 to 95 percent). Other components include propane, butane, ethane, pentane, CO2, nitrogen, hydrogen, helium, and sulfur. Natural gas may be found in gas-only fields, associated with oil, or in coal beds. Synthetic natural gas, or "syngas," is made from coal. The Great Plains Synfuel plant in North Dakota is the largest of the two producers of synthetic natural gas in the U.S. (44) [see "Coal", above]. Natural gas is also produced by decaying biomass and is termed "biogas".

  Photo 3. Natural gas power plant in Illinois.

In the late 1800s, natural gas produced from coal was piped into cities and used first for lighting and later for heating. Today, natural gas heats 52 percent of the homes in the U.S. and generates 22 percent of the country’s electricity (11) (Photo 3). Due to its low density, natural gas is usually transported by pipeline (Figure 7). Compressed natural gas is stored in tanks and used in vehicles and rural homes. Liquid natural gas is gas that has been cooled to -260° F, greatly reducing its volume and allowing it to be shipped by tankers and trucks.

The largest producers of natural gas are Russia, the U.S., and Canada, although the largest gas fields are found in the Middle East (45). Natural gas production from shale reserves in the U.S. and Canada has increased recently due to horizontal drilling (46). Natural gas produces less CO2 per unit of energy than either coal or petroleum. For that reason, Texas oilman and financier T. Boone Pickens (47) and others think that natural gas could replace these fuels, at least in part, in the future (48).


Figure 7. Distribution in U.S.: Pipelines and transmission lines.


Nuclear Energy

Nuclear power plants utilize the energy released when uranium-235 undergoes the process of fission. Heat released in this reaction is used to produce steam or superheated water that powers turbines similar to those in coal-fired plants. The nuclear chain reaction is controlled with a moderator, such as graphite or beryllium, and waste heat is dissipated by water that is sent to cooling towers, ponds, rivers, or the ocean. Nuclear power plants do not release CO2 or other emissions (49, 50).

Photo 4. Nuclear power plant in California.

Nuclear fission was discovered in 1934 and was first used to generate electricity in 1951 at Arco, Idaho. In 1954, the first nuclear power station came on line in the Soviet Union. Today, there are 104 reactors in the U.S. (Figure 3) and 443 worldwide (51) (Photo 4).

The major producers of uranium in 2007 were Canada (23 percent of the world’s total), Australia (21 percent), and Kazakhstan (16 percent) (52). Uranium ore is mined from open pits, underground mines, and from hillsides using in-situ leaching (52). The ore is crushed and then processed into a more stable form known as "yellowcake." After transport to another processing facility, it is formed into fuel rods. These rods are placed in the nuclear reactor and are productive for about six years. Spent fuel rods are cooled in pools for several years and are eventually re-processed for further use or stored in steel and concrete-lined storage facilities. This radioactive waste takes 10,000 years to decay to a safe level (49).

Development of nuclear energy has been slowed by the high cost of plant construction, the abundance of fossil fuels, and the concerns about waste storage and the release of radiation. In the U.S., the placement of a permanent disposal site is still controversial. The plan to develop a geologic repository at Yucca Mountain, Nevada, has been stalled by opposition due to concerns regarding geologic stability, transportation, and cultural issues (53). This storage problem, along with the accident at the Three Mile Island power plant in 1979, swayed public opinion in the U.S. against the construction of new nuclear power plants (54). Views have changed recently due to improved safety features and concerns regarding CO2 emissions from other fuel sources (55). Nineteen percent of the country’s electricity comes from nuclear power. (In France, that figure is 79 percent.) The U.S. now produces 30 percent of the world’s total nuclear-generated electricity, followed by France (16 percent of the total), Japan (11 percent), and Germany (6 percent) (51). Overall, 14 percent of the world’s electricity is produced by nuclear energy.

Uranium resources are predicted to last for at least 100 years at the current rate of use. The International Atomic Energy Agency predicts that nuclear power capacity could increase by 80 percent by the year 2030 (56). Many new "Generation IV" reactor designs are being developed (50), which will be more economical and safer than current models. The problem of permanent waste processing and storage has yet to be solved.

Renewable Sources

Photo 5. Hoover Dam on the Colorado River between Arizona and Nevada.

Hydropower. Water has been a source of energy for hundreds of years. Water wheels have been used to grind grain, saw wood, cut stone, raise water levels for irrigation and barge traffic, and, beginning in 1880, generate electricity. The modern version of the water wheel is the turbine. Working with an electrical generator, a turbine turns kinetic energy (energy of motion) or potential (gravitational) energy into electrical energy. The "run of the river" system utilizes the river flow (kinetic energy), while in a storage system, water is held in a reservoir and released (potential energy) when electrical demand is high (57) (Photo 5).

Hydroelectric power is much less expensive than power from fossil fuels, and its generation releases very few pollutants. Construction costs and overall impacts are high, however. This includes the loss of valuable farmland, wetlands, and areas with important cultural and scenic values. Millions of people worldwide have been forced to leave their homelands when dams were constructed and land was flooded for these projects. Dams also affect the river flow and water quality, which permanently alter the aquatic ecosystem, hydrology, and geologic features of the area, both upstream and downstream.

Hydropower is the most widely used form of renewable energy in the world other than wood. In 2007, hydropower accounted for six percent of the world’s total primary energy production and 19 percent of the world’s electrical production (57, 58). In the U.S., seven percent of the total electricity produced is from hydroelectric plants. In fact, the single largest electric power facility in the U.S. is the Grand Coulee Dam in Washington State, with a generating capacity of 6,800 MW (59). As far as future hydropower resources in the U.S., most of the more productive locations have been developed (Figure 3), but there are many other places where small hydro plants producing up to 30 MW would be feasible (60, 61).

Major hydropower development continues in Canada and, in particular, in China. The largest hydroelectric power station in the world, the Three Gorges Dam on the Yangtze River in China, was completed in 2008. When fully operational in 2011, its capacity will be 22,500 MW using 32 generators (62).

 Photo 6. Tidal turbines in
 British Columbia.

The term "hydropower" includes power from the oceans as well as from rivers. Built in estuaries or bays, tidal power systems utilize the stream flow (in both directions), or hold back water at high tide to release it at low tide. Tidal power plants have been built in France, Canada, and Russia (61). Tidal turbines similar to wind turbines are also being developed (Photo 6). They can be installed anywhere with a strong tidal flow (63). Progress has also been made with wave power using both stationary and floating structures. The first commercial wave farm opened in Portugal in 2008 (64). In Australia, a wave-powered system with an air-driven generator located on shore has been successful (61).

Biomass. The term "biomass" encompasses a wide range of organic materials, from both plants and animals. The combustion of biomass in the form of wood, peat, and animal dung is the world’s oldest source of energy other than the sun. A "biofuel" is fuel made from biomass. The source may be harvested from nature, a crop, or a by-product of another use or process.

For thousands of years, wood was, and continues to be for many cultures, the primary energy source for heat and light. In industrial countries, wood was used to power the early steam engines. By 1890, wood was replaced by coal due to availability, cost, and convenience. In the 1970s, some electrical power plants in the U.S made the reverse switch, from coal to readily available waste wood (65). Wood, wood chips, pellets, and residues continue to be used for space heating in homes and manufacturing plants (66), and in small-scale electrical generating facilities (67).

Early liquid biofuels included animal and vegetable oils, and alcohols. Ethanol, an alcohol, was originally used for lighting. In 1826, a precursor to the internal combustion engine was developed that ran on ethanol. Henry Ford’s 1908 Model T, the first flexible-fuel automobile, was designed to run on ethanol or gasoline (and ran for 25 miles on a gallon of fuel) (68). Today, many types of biofuels are processed from many sources. Liquid fuels include ethanol, butanol, biodiesel, and methanol.

Photo 7. Ethanol plant in Iowa.

Ethanol is produced from plant products (feedstocks) high in starch (corn, barley, sorghum) or sugar (sugar cane, sugar beets). After the feedstock is ground, the starches are turned into sugars that are then fermented by microorganisms to produce alcohol. This product is then distilled into fuel-grade ethanol. The residues are sold as distillers grain for livestock feed. Ninety percent of the 9.0 billion gallons of ethanol produced in the U.S. in 2008 was from corn, and almost 30 percent of the U.S. corn crop was used in the process (Photo 7). In Brazil, ethanol is made from sugar cane, and 6.5 billion gallons were produced there in 2008 (69).

Ethanol was found to increase the octane level in gasoline in the 1920s, and "gasohol" was sold in the Midwest into the 1940s, but little was produced after that time. When lead in gasoline was phased out from 1973 to 1986, alcohol-blended fuels were introduced back into the market. Tax benefits, subsidies, and loans were made available to ethanol producers starting in the 1980s. The Energy Policy Act of 1992 included mandates and tax breaks for the use of alternative fuel vehicles, including those that use E85, a blend of 85 percent ethanol and 15 percent gasoline. Ethanol is also used as a replacement for MTBE, an oxygenating gasoline additive that is now banned in some states. The Renewable Fuel Standard, as revised in 2007, requires that 36 billion gallons of ethanol and other biofuels be produced and utilized yearly by 2022. From 2001 to 2006, U.S. ethanol output increased by 300 percent (176). In 2008, there were 1,921 stations across the country (mostly in the Midwest) that sold E85, a 45 percent increase from 2007 (70). Due to falling gasoline prices and the credit freeze at the end of 2008, however, new ethanol plant construction halted, and one of the largest ethanol producers, VeraSun Energy Corporation, declared bankruptcy.

Biofuels from crops that are also used for food are called "first generation" biofuels. "Second generation" fuels are thought to be an improvement because they use less valuable non-food sources. One promising fuel is cellulosic ethanol, which is made from perennial plants like switchgrass (71, 72, 73), miscanthus, cattails (74, 75), poplar, and alfalfa. These are called "dedicated" crops that can be grown specifically for biofuel production. The advantage of these feedstocks is that they are perennial species requiring less fertilizer, water, and pesticides than annual crops. Their use also reduces soil erosion, increases CO2 sequestration, and they may be grown on marginal land that would not support a food crop. A study headed by Ken Vogel of the USDA-ARS and based at the University of Nebraska–Lincoln found that switchgrass produces five times more energy in the form of ethanol than it takes to grow and process, and yields as much ethanol per acre as corn (about 300 gallons per acre), even on marginal land (76). Crop residues, hay, sawdust, and paper pulp may also be used.

Cellulosic ethanol is made from cellulose, a sugar found in the walls of plant cells. Typically, an acid steam bath, followed by the addition of enzymes specific to the feedstock, is required to extract the sugars. This process is expensive (185), and researchers are looking for ways to improve the process including the use of microbes found in the digestive tracts of wood-eating beetle larvae (77). Another study involves using recombinant DNA to enable corn stocks to produce a digesting enzyme found in cattle stomachs (78). A cellulosic ethanol plant using current methods is estimated to cost up to five times more than a corn-based plant of similar size, a high investment risk (79). Several cellulosic ethanol plants are currently being built, some funded in part by the U.S. Department of Energy (76, 80). The first commercial cellulosic ethanol plant is expected to go on line in Iowa in 2011 (81). Due to the large variety and amount of potential feedstocks, it is projected that up to 30 percent of petroleum consumed in the U.S. could be replaced by cellulosic ethanol (76).

Like ethanol, biobutanol is an alcohol made by the fermentation of sugars. The difference is either in the fermentation process, which uses genetically modified microorganisms, or in processing using solid catalysts that produce the long-chain hydrocarbons found in gasoline (81, 78). Unlike ethanol, which requires truck transport, butanol is not corrosive and may be sent through pipelines.

Biodiesel is produced from animal fats and vegetable oils. The major biodiesel source in the U.S. is soybean, but other crops may be used: sunflower, canola, camelina, crambe, lesquerella, several mustards, jatropha, cuphea, hemp, and palm (82). The raw oil may be used as a fuel, but fuel of a higher quality is obtained by further refining. Biodiesel production in the U.S. was about 91 million gallons in 2005 (79). Jatropha is a tree grown in India, where it is estimated to produce 20 percent of the country’s diesel by 2011 (83). When grown in Florida, jatropha may yield as much as 1,600 gallons of biodiesel per acre per year (83).

Microscopic algae were studied as a biodiesel source from 1978 to 1996 as a part of the Aquatic Species Program by the U.S. Department of Energy (84). Some strains of algae contain up to 60 percent oil by weight. Production estimates have been made of 5,000 to 150,000 gallons per acre per year (in a greenhouse), as compared to soybeans with 60 to 100 gallons per acre per year (85, 86, 87). One type of alga has been genetically engineered to release oil continuously, thus eliminating the need to dry and extract the oil from the growth medium (81). Water-filled tubes or plastic bags hold the algae in a flowing suspension, and CO2 and nutrients are added to the water (87). Wastewater may be used, for example, from an ethanol plant. The CO2 may come from the stacks of a coal-fired power plant. The difficulty is in building a large-scale system, now known as a "bioreactor." The first commercial plant may go on line in 2012 (81).

Methanol, or wood alcohol, is now produced primarily from natural gas, but also from coal and by the gasification of wood and black liquor, a by-product from paper and pulp mills. Methanol is used as a fuel, solvent, and antifreeze, and as an additive to ethanol. It was used in the 1980s and 1990s in flex-fuel vehicles until ethanol became more available (88).

Photo 8. Peat-fueled power plant in Finland.

Methane is a gas produced by the anaerobic (without oxygen) decomposition of biomass. This is the same "natural gas" found in gas and oil fields. Methane is considered the second greenhouse gas after CO2, and is far more potent in its global warming potential by volume (91). Any form of biomass may produce methane: solid landfill waste, animal manure, or crop residue. Landfill gas is captured by drilling wells and contains about 50 percent methane. This gas is used for heating or to generate electricity at the landfill. Methane capture also reduces CO2 emissions and odor (89). Where manure is the methane source, a digester is used to produce a biogas that is 65 percent methane. The process that turns farmyard slurry into gas takes about 16 days. The final products include liquid fertilizer, livestock bedding material, and heat and electricity from the burning of the biogas (90). Cargill has invested in an $8.5 million plant in Idaho, and other plants are up and running. Only 130 years ago, wood supplied 90 percent of the energy in the U.S. (92). Today, wood, pellets, and wood by-products are burned to generate two percent of the nation’s electricity, primarily at manufacturing plants (2007) (Figure 8) (93). Peat, another form of biomass, is also burned in power plants in Ireland and Finland (Photo 8). Wood and wood by-products are also used to produce ethanol and methanol.


Figure 8. Renewables in U.S.: Potentials and power plants.


As of 2008, the U.S. was importing ethanol (69). However, by 2030, biofuel production in the U.S. is expected to increase to 1.2 million barrels per day (94). Other big biofuel producers include Brazil, China, and India (69).

Wind. Like hydropower, wind energy utilizes a natural source of kinetic energy to turn electrical generators. And like the early waterwheels, windmills were first used to grind grain, pump water, and saw wood. Charles F. Brush built the first windmill for electrical generation in Ohio in 1888 (95). In rural areas of the U.S., windmills were used for pumping water and generating electricity into the 1950s (96, 97). Newer, small wind turbines with a capacity from one to 100 kW have been used for homes and small communities since the 1970s.

The modern utility-scale wind turbine is made up of a rotor with three blades, each blade shaped like an airplane wing. Differential wind speeds flowing past the two sides of the blade cause the air pressure to be higher on one side of the blade. This forces each blade to "lift." Due to the angle of the blade with respect to the rotor, the rotor turns, thus turning the horizontal drive shaft (96). This shaft enters a gearbox inside the nacelle, the housing at the top of the tower. A generator within the nacelle produces the electricity. After its voltage has been increased to a distribution level by a transformer, the electricity flows through underground lines to nearby farms and towns, or on to substations and transmission lines (98). Towers may be 200 to 300 feet tall with individual blades 65 to 130 feet long. A turbine this size could produce up to 2.3 MW (2,300 kW) of power and cost up to $1.5 million (98). Larger turbines capable of producing 3 MW have been built. Turbines for offshore projects are even larger: 3.6 MW, and plans have been made for a 7.5 MW turbine (99).

Other types of turbines have been developed. The Darrieus turbine has a vertical axis and two vertical blades attached at the top and bottom of the axis (96). The Wind Amplified Rotor Platform is a vertical column of disk-shaped modules that increase the speed of the wind flowing into the central turbines (100). Another system uses a large number of small generators in an array. This Multi-Axis Turbosystem is able to utilize low wind speeds as well as turbulent wind (101).

Photo 9. Wind turbines in Germany.

Although small turbines have been used for many years, wind farms are a recent development (Photo 9). The largest onshore wind farms are in the U.S.: Horse Hollow, Texas (736 MW installed capacity), Tehachapi Pass, California (685 MW), Capricorn Ridge, Texas (662 MW), and San Gorgonio Pass, California (619 MW) (Figure 8). Two large wind farms have also been built in Romania, each with an installed capacity of 600 MW. New installations and increased capacity at existing wind farms in the range of 1,000-5,000 MW have been proposed in China, Australia, Sweden, and the U.S. (North Dakota, South Dakota, Texas, and California) (102).

Wind energy does not release emissions or impact water supplies, but there are drawbacks. The intermittent nature of the wind requires that wind energy be coupled with another source such as coal or natural gas, to compensate for fluctuations in production. Another option is to store wind-produced energy in batteries, pumped water storage, or to run geothermal heat pumps. It may also be used to produce hydrogen on site (103) (see Hydrogen).

Another current challenge to wind energy is in transmission: sending power from high wind areas to population centers. A new high-voltage line dubbed "The Green Power Express" has been proposed to cross seven states in the Upper Midwest at a cost of up to $12 billion (104). Opponents say that states should use local sources of energy and not depend upon huge transmission lines.

Other environmental and social concerns regarding wind energy have emerged. Large wind farms may have over 1,000 turbines. In order to reduce the impact on wildlife populations, guidelines for tower design and placement have been developed (105). New blade and tower designs have reduced the risk of collisions, but harm to birds and bats still may occur in critical areas and seasons. The low-frequency vibrations, shadow and reflection flicker produced by the rotors, and blinking lights are known to affect some people (106). In addition, concerns over the visual impact of wind towers in areas of high scenic value have halted plans for some wind projects, including one off the coast of Cape Cod, Massachusetts. These environmental and social aspects must be researched and weighed before siting new projects.

Wind power contributed one percent (25,000 MW) of the total electrical consumption in the U.S. in 2008, more than ten times that of 1999 (109). The U.S. wind energy industry grew 30 percent annually from 2003 to 2008.

There is a great potential for further wind power development (Figure 8). A Department of Energy study states that wind farms in North Dakota, South Dakota, Kansas, and Texas alone could supply the electrical needs of the entire U.S. (110). Federal loan guarantees, production tax credits, and provisions for transmission line construction (111) will encourage continued development of this important energy resource.

Globally, wind power capacity quadrupled between 2000 and 2006, from 14,600 MW to 85,000 MW. Europe led the way starting in the 1980s, with Germany producing the most (24,000 MW) in 2007. Output has grown most dramatically in the U.S., China, and India (107). Worldwide wind energy production is estimated to grow 215 percent by 2012 (108).

Geothermal. One energy source that is often overlooked is the one underfoot. Geothermal energy utilizes the endless supply of heat stored in the Earth due to pressure and radioactive decay. Hot springs have been used throughout history for bathing, heating, and cooking. Today, hot water from these sources (called direct use) is piped into buildings and greenhouses for heat and aquaculture and under roads to melt ice. In places where steam or pressurized hot water from an underground reservoir is readily available, electricity can be generated. This was first tested in Italy in 1904. The geothermal resource at the same location has been providing electricity since 1911 (112).

Photo 10. Geothermal plant in Iceland.

There are several types of geothermal electrical plants, and each one uses steam-driven turbines (Photo 10). Dry steam plants utilize naturally occurring geothermal steam. Flash steam plants draw up high-pressure geothermal water (above 350°F) that is turned into steam, condensed back into water and returned to the ground. Binary cycle power plants have pipes filled with a secondary liquid in a closed system that draw heat from cooler (below 350°F) geothermal water (113). When this liquid reaches its boiling point, it turns into a vapor in a heat exchanger and the vapor turns the turbines (113, 112, 114). Wells for this purpose may be one to two miles deep (114). In places without geothermal water reservoirs, water from another source is injected into hot, dry rock. This is known as an enhanced geothermal system.

The U.S. is the biggest producer of geothermal energy, with 33 plants in California, 14 in Nevada, and plants in Utah, Alaska, and Hawaii. The largest, at The Geysers in California, was built in 1960 and has a capacity of 750 MW. Geothermal power is important in Iceland, where geothermal plants generate 19 percent of the electricity, and 87 percent of homes are heated by direct geothermal heat (112). Plants have also been built in about 20 other countries, including New Zealand, the Philippines, Mexico, Kenya, and Turkey (112).

Emissions from geothermal power plants are very low. Scrubbers are used to capture the hydrogen sulfide that is often present in the geothermal steam and hot water (114). The injection of water in the enhanced system may cause land instability, however (114).

In areas without hydrothermal reservoirs, geothermal heat may still be used to heat homes and businesses by means of geothermal heat pumps. Typically this is a system of vertical wells 150 to 200 feet deep that contain U-shaped pipes filled with a liquid in a closed system. Because of the steady temperatures below ground regardless of season, this liquid transfers heat from underground in the winter, and releases heat below ground in the summer, providing a stable temperature to buildings year round. A home in North Dakota, for example, would require five to seven wells, while a large commercial building may need several hundred (115). In spite of the high initial cost, $20,000 for a home, this system is the most cost-effective, clean, and energy-efficient system for heating and cooling, according to the EPA (114). As of 2007, two million ground-source heat pumps were in operation in 30 countries (116).

A National Renewable Energy Laboratory study estimates that geothermal systems in the U.S. could produce 100,000 MW by 2025, with another 70,000 MW from direct use and heat pumps (117) (Figure 8). Globally, geothermal power contributes to less than one percent of the electricity consumed. However, estimates for potential development are high. According to an MIT-led study in 2007, geothermal energy could become a major source of energy in the future (118).

Solar. Capturing energy directly from sunlight has long been a goal of scientists. Converting the simple energy used to heat homes and water into a source of electricity is difficult. Two techniques have been developed, one using solar thermal systems and the other using photovoltaic devices.

Photo 11. Solar power plant in California.
Photo 12. Heliostats for solar energy tower in Colorado.

Concentrating the direct heat from the sun, solar collectors typically heat a liquid that is used to produce steam or fluid pressure. A turbine or piston then powers an electrical generator. Solar collectors are of several types. Linear parabolic troughs, the collectors most commonly used, concentrate the sun’s energy 30 to 100 times its normal intensity. These are arranged in huge arrays and are connected to one another by liquid-filled pipes that conduct the heat to a central location (Photo 11). The parabolic solar dish, often with an engine at its center, concentrates the energy even more, over 2,000 times, and has a power generating capacity of 2 to 25 kWh (119). The third type of solar collector is the solar power tower. Hundreds of flat mirrors, called heliostats (Photo 12),track the sun and reflect light onto a central tower that holds a heat exchanger and a steam turbine. This system is highly efficient because heat is not lost in transport to the turbine (120).

The largest solar power system in the world is located in the Mojave Desert in California. It utilizes parabolic troughs over a 1,600-acre area, and produces 354 MW of electricity from nine plants, each with 14-80 MW of production (121). Solar towers have been built in California and Spain.

Photovoltaic devices, also known as solar cells, convert photon energy into electrical current. Made of semiconducting material sandwiched between a plate and glass, these cells are placed into modules, which are arranged in arrays on rooftops or over many acres. The first solar cell was developed in the 1880s. Early cells contained selenium, and later cells made in the 1940s and 1950s contained silicon, which is commonly used today (119). The latest development has been the thin-film solar panel made with indium, gallium, and deselenide. Although thin-film panels generate one-half the power of the heavier crystalline-silicon cells, they are cheaper and more flexible (122).

Solar cells were first used on satellites and are now used for telecommunications and for signaling and alarm systems in remote locations throughout the world. They also provide back-up power for hospitals and gas stations and in disaster-relief situations (123). Large photovoltaic systems have been built in 26 countries, including Spain, Portugal, Germany, South Korea, and the U.S., with generating capacities ranging from 1 to 60 MW (123). In the U.S., photovoltaic power plants are now operating in Nevada, Arizona, and California.

Currently, solar power generates less than one percent of the world’s electrical energy. Solar water heating is the most common use of solar energy, with an estimated capacity of 154,000 MW in 2007 (124).

As with wind power, intermittent power generation and energy storage are problems to be considered with solar power. Heat storage using liquid sodium and potential energy storage using water are used to maintain power supply (120). Like wind farms, the siting of large arrays of collectors is challenging. Proponents of solar energy note that due to the free fuel supply, the lack of emissions, and the fact that no water is required for cooling, sacrificing desert areas for energy production is better than the impact from CO2 output, mining, and drilling of fossil fuels (125).

Hydrogen. Hydrogen has been called the "fuel of the future." Because it rarely occurs in its elemental form on Earth, hydrogen must be produced from another source. Hydrogen has the highest energy content by weight of all the fuels but the lowest energy content by volume (126). To make its use more practical, it must be condensed. Liquid hydrogen is used as a rocket fuel, and condensed hydrogen is used to generate electricity through the use of fuel cells. In both of these processes, the only by-product is water, making it the cleanest of all fuels.

One difficulty with hydrogen as a fuel is in its production. The least expensive and most common method, steam reforming, breaks down natural gas, thus producing hydrogen and CO2. Ninety-five percent of the hydrogen produced in the U.S. is from natural gas and most is used in industry (126). Electrolysis of water for hydrogen production is a much cleaner process, but it is currently much more expensive than producing hydrogen from natural gas. The electricity required for electrolysis can be supplied by wind or solar power, using hydrogen as a way to store energy or to turn electrical energy into a portable fuel. The wind-to-hydrogen process is being studied in Colorado and North Dakota (103). A method of high-temperature electrolysis using solar energy without electricity is also under investigation (127). Hydrogen is also a product of biomass and coal gasification. Several species of algae and bacteria are known to release hydrogen under certain conditions, and this is currently under study as a possible source (85).

In most vehicles that run on hydrogen, a hydrogen fuel cell is required to generate electricity. A fuel cell is like a battery, with an anode and a cathode separated by an electrolyte membrane. Hydrogen enters on the anode side and oxygen (pure oxygen or from the air) enters on the cathode side. In an electrochemical reaction catalyzed by a layer of platinum, the hydrogen atoms split into protons and combine with the oxygen to form water at the cathode. The electrons released in the reaction must flow through a circuit before reaching the cathode, thus creating an electric current (128). The platinum required raises the price of the fuel cell substantially, about $3,000 per car (129). Large fuel cells have been developed to supply hydrogen-generated power to spacecraft, submarines, hospitals, and communities in remote locations (128).

Proponents of a future "hydrogen economy" to replace the current dependence on fossil fuels believe that technology and innovation will solve the problems with production and fuel cell cost. They point out that hydrogen is a clean fuel and eventually, small, efficient electrolyzers will be found at every home (126) and vehicles will run on pure hydrogen without fuel cells, using hydrogen bound in a solid storage material (68, 130). Critics point out that hydrogen production requires a large amount of electricity and unless only carbon-free energy is used, one is simply generating CO2 elsewhere. Today’s battery-powered vehicles are more efficient than hydrogen-powered cars, which require three times the energy (126). Also, cars using compressed hydrogen require fuel tanks that are two to three times larger than those on gasoline-powered cars.

About 400 to 500 hydrogen-powered buses, trucks, and cars are currently on the road in the U.S. (126), including car models recently released by Honda and General Motors. Most use fuel cells; only a few burn hydrogen directly. A flex-fuel car that uses either gasoline or hydrogen is possible and would help with the transition to a hydrogen-based transportation system (68). However, whether such a system comes to pass, and when, is up for speculation.

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