Energy Sources: History, Selection, and
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
| Photo 1. Lignite coal mine in Poland.
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.
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.
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
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.
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
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
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,
|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.
Photo 5. Hoover
Dam on the Colorado River between Arizona and Nevada.
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,
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
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
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.
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,
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
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
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
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,
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
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
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
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
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
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
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
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
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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