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Renewable Energy Sources in the United
States |
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Hydropower
Wind
Power
Solar
Power
Geothermal
Power
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Biomass
Power
References
Related
Links
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Renewable energy sources are energy sources that are continually
replenished. These include energy from water, wind,
the sun, geothermal sources, and biomass sources such as energy
crops. In contrast, fuels such as coal, oil, and natural
gas are non-renewable. Once a deposit of these fuels is depleted
it cannot be replenished – a replacement deposit must be
found instead. Both renewable and non-renewable energy sources
are used to generate electricity, power vehicles, and provide heating,
cooling, and light.
Renewable sources of energy vary widely in their cost-effectiveness
and in their availability across the United States. Although
water, wind, and other renewables may appear free, their cost comes
in collecting, harnessing, and transporting the energy so that
it can do useful work. For example, to utilize energy from
water, a dam must be built along with electric generators and transmission
lines.
Renewables themselves are non-polluting, while the structures
built to harness them can have positive or negative environmental
impacts. For example, dams may affect fish migration but
may also create wildlife habitat.
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Hydropower |
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An impoundment hydropower plant dams water
in a reservoir.Hydropower refers to using water to generate electricity. Water
is the most common renewable source of energy in the United States
today.
Many hydroelectric power plants use a dam on a river to store water.
Water released from behind the dam flows through a turbine, spinning
it, which then turns a generator to produce electricity. Electricity
generated this way is known as hydroelectricity, and it accounts for
about 7% of the electricity used by the nation. Hydroelectric power
doesn't necessarily require a large dam – some hydroelectric power
plants just use a small canal to channel the river water through a
turbine. A small or micro-hydroelectric power system can produce enough
electricity for a home, farm, or ranch.
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The Tazimina project in Alaska is an example
of a diversion hydropower plant. No dam was required.Dam sites for hydropower plants are limited both by available
rivers and by competing uses for those rivers, such as recreation,
tourism, industry, and human settlements. Because of such limitations,
water power could never generate all the electricity used in the
United States. In addition, environmental impacts are considered
when locating dams.
While all hydroelectric dams have some environmental impact, the
impacts vary widely, and current regulations and policies attempt
to address environmental concerns. A dam may either create a reservoir
or may be a run-of-river project that does not store large amounts
of water but simply takes advantage of a river's natural flow.
A dam that
Fish ladder.creates a reservoir may flood a large
area upstream, and can change flow patterns and impact flooding
downstream with resulting environmental consequences, either positive
or negative. Fish migration, which has long been a concern associated
with dams, is often addressed with fish ladders and other structures
to ensure the successful movement of fish both upstream and downstream.
In addition to power, dams often provide other benefits such as
recreation opportunities on upstream reservoirs, habitat for a
wide variety of aquatic and terrestrial species, diversion of water
for irrigation, and control of destructive flooding and environmental
damage downstream.
Hydropower is one of the least expensive sources of electricity and
areas with good sources of hydropower tend to attract industries
with large needs for electricity. Major hydroelectric dams in the
United States are found in the Northwest, the Tennessee Valley, and
on the Colorado River. |
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Click image for larger view.
Existing
hydroelectric plants (yellow) and potential high head/low power
energy sites (orange) in the conterminous United States. Purple
represents areas excluded from hydropower development due to
Federal statutes and policies.
Source: Water
Energy Resources of the United States with Emphasis on Low Head/Low
Power Resources (p. 47), U.S. Department of Energy |
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Wind Power |
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Turbines at Martinsdale Hutterite Colony.For hundreds of years, humans have used wind to pump water or
grind grain, usually with small windmills. Large, modern
wind turbines are used to generate electricity, either for individual
use or for contribution to a utility power grid. Wind turbines
usually have two or three blades and, because winds above the ground
tend to be faster and less turbulent than those near the surface,
the turbines are mounted on tall towers to capture the most energy. As
the blades turn, the central shaft spins a generator to make electricity.
In recent years, wind has become an increasingly attractive source
of renewable energy – wind energy is the world's fastest-growing
energy technology. Wind turbines placed at sites with strong,
steady winds can economically generate electricity without producing
pollutants. The power in wind increases rapidly with its
speed, which means that locating windmills in areas of strong winds
is critical. The strongest winds in the United States tend
to be in Alaska, the western United States, and the Appalachians.
Wind power currently supplies about 1% of United States electricity
needs, but capacity is expanding rapidly. Although wind will
contribute more to the United States electric supply in the future,
like hydropower it cannot be expected to supply all of our electric
needs.
Click image for larger view.
United States
wind resource map.While
wind power helps the environment by producing electricity without
producing pollution, there can be negative environmental impacts
of wind power generation, including wildlife deaths. However, recent
studies suggest that the number of birds and bats killed by collision
with wind turbines is far lower than the number killed by collisions
with other tall structures such as buildings. Appropriate siting
of wind farms and individual turbines can reduce the impact on
wildlife. Noise, which was a problem with older turbine designs,
has mostly been eliminated through improved engineering.
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Solar Power |
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Annual average daily solar radiation per
month, using a flat-plate collector facing south at a fixed tilt
equal to the latitude of the site. Capturing the maximum amount
of solar radiation throughout the year can be achieved using
a tilt angle approximately equal to the site's latitude.Solar technologies use the sun's energy
to provide heat, light, hot water, electricity, and even cooling,
for homes, businesses, and industry. Despite sunlight's
significant potential for supplying energy, solar power provides
less than 1% of U.S. energy needs. This percentage is expected
to increase with the development of new and more efficient solar
technologies.
Different types of solar collectors are used to meet different
energy needs. Passive solar building designs capture the
sun's heat to provide space heating and light. Photovoltaic
cells convert sunlight directly to electricity. Concentrating
solar power systems focus sunlight with mirrors to create a high-intensity
heat source, which then produces steam or mechanical power to run
a generator that creates electricity. Flat-plate collectors
absorb the sun's heat directly into water or other fluids
to provide hot water or space heating. And solar process
heating and cooling systems use specialized solar collectors and
chemical processes to meet large-scale hot water and heating and
cooling needs.
Solar technologies produce few negative environmental impacts
during collector operation. However, there are environmental
concerns associated with the production of collectors and storage
devices. In addition, cost is a great drawback to solar
power. Although sunlight is free, solar cells and the equipment
needed to convert their direct-current output to alternating current
for use in a house is expensive. Electricity generated by
solar cells is still more than twice as expensive as electricity
from fossil fuels. Part of the problem with cost is that
solar cells can
The parabolic troughs that make up this
concentrating solar power system generate power from the sun on
a large scale in California.only operate during daylight hours. In contrast,
a coal or natural gas plant can run around the clock, which means
the cost for building the plant can be spread over many more hours
of use.
Around the United States, available sunlight varies considerably
as a result of differences in cloud cover and latitude, and also
varies with the seasons. In the summer, longer daylight hours and
a higher sun angle provide more solar power, compared to the winter
when the sun is up for fewer hours and at a lower position in the
sky. These variations must be taken into consideration when planning
solar collection facilities.
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Geothermal Power |
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Geothermal power plant at The Geysers,
California.Geothermal
power uses the natural sources of heat inside the Earth to produce
heat or electricity. Currently, most geothermal power is generated
using steam or hot water from underground. Geothermal power generation
produces few emissions and the power source is continuously available.
There are three geothermal technologies currently in use in the
United States: direct-use systems, use of deep reservoirs to generate
electricity, and geothermal heat pumps.
In direct-use geothermal systems, a well is drilled into a geothermal
reservoir to provide a steady stream of hot water. The water is
brought up through the well, and a mechanical system—piping,
a heat exchanger, and controls—delivers the heat directly
for its intended use. A disposal system then either injects the
cooled water underground or disposes of it in a surface storage
pond. Geothermal hot water is used for heating buildings, raising
plants in greenhouses, drying crops, heating water for fish farms,
or for industrial processes, at hundreds of sites around the country.
Geothermal reservoirs appropriate for direct-use systems are widespread
throughout the western United States.
Geothermal power plants convert hydrothermal fluids (hot water
or steam) to electricity. The oldest type of geothermal power plant
uses steam, accessed through deep wells, to directly drive a turbine
to produce electricity. Flash steam plants are the most common
type of geothermal power plants in operation today. They use extremely
hot water (above 300 degrees F (149 degrees C)), which is pumped
under high pressure to the generation equipment at the surface.
The hot
Estimated subterranean temperatures at a
depth of 6 kilometers.water
is vaporized and the vapor in turn drives turbines to generate
electricity. Binary-cycle geothermal power plants use moderate-temperature
water (100-300 degrees F (38-149 degrees C)). The water is used
to vaporize a second fluid that has a much lower boiling point
than water. The vapor from this second fluid is then used to drive
the turbines to produce electricity. California, Hawaii, Nevada,
and Utah currently have operating geothermal power plants.
Geothermal heat pumps are used for space heating and cooling as
well as water heating, for residential and commercial applications.
The technology relies on the fact that beneath the surface, the
Earth remains at a relatively constant temperature throughout the
year, warmer than the air above it during the winter and cooler
in the summer. A geothermal heat pump takes advantage of this by
transferring heat, stored in the ground, into a building during
the winter, and transferring it out of the building and back into
the ground during the summer. The heat pump consists of a series
of pipes, buried in the ground near a building to be conditioned
or where water is to be heated. Fluid is circulated through the
pipes to either absorb heat from the ground or distribute heat
to the ground. Geothermal heat pumps can be used in most areas
of the United States.
While geothermal energy use is efficient, reliable, and environmentally
friendly, it currently meets less than 1% of U.S. power needs.
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Biomass Power |
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McNeil Generating Station, Burlington,
Vermont, the country's only utility-owned and operated wood-fired
power plant.Biomass
power is power obtained from the energy in plants and plant-derived
materials, such as food crops, grassy and woody plants, residues
from agriculture or forestry, and the organic component of municipal
and industrial wastes. Biomass power provides two valuable services:
it is the second most important source of renewable energy in the
United States and it is an important part of our waste management
infrastructure. In the future, farms cultivating high-yielding
energy crops (such as trees and grasses) will significantly expand
our supply of biomass. These energy crops, coupled with high-efficiency
conversion technologies, can supplement our consumption of fossil
fuels and help us respond to global climate change concerns.
Wood has been used for energy longer than any other biomass source
and today is still the largest biomass energy resource. The largest
source of energy from wood is pulping liquor or "black liquor," a
waste product from processes of the pulp, paper, and paperboard
industry. Biomass energy can also be derived from waste and from
alcohol fuels. Waste energy is the second-largest source of biomass
energy. The main contributors of waste energy are municipal solid
waste, manufacturing waste, and landfill gas.
Biomass can be used for direct heating (such as burning wood in
a fireplace or wood stove), for generating electricity, or can
be converted directly into liquid fuels to meet transportation
energy needs.
Truck unloading wood chips that will fuel
the Tracy Biomas Plant, Tracy, California.Electricity generated from biomass is also called biopower. Biopower
facilities use many different technologies; the most common is
burning of wood or other biomass feedstocks to produce steam which
then is used to drive turbines and produce electricity. Some generators
use a mix of biomass and fossil fuels to generate electricity,
while others burn methane, a product of the natural decay of organic
materials. In the United States, the pulp and paper industries
are major producers of biopower, using residues from paper production
to produce electricity for industrial plant use.
Biomass power is close to a carbon-neutral electric power generation
option — biomass absorbs carbon dioxide from the atmosphere during
its growth and then emits an equal amount of carbon dioxide when
it is processed to generate electricity. Thus, biomass fuels "recycle" atmospheric
carbon, and may reduce global warming impacts. Biopower facilities
produce fewer other pollutants than equivalent fossil fuel power
facilities.
Biofuels are liquid fuels produced from plants. The two most common
types of biofuels are ethanol and biodiesel. Ethanol is an alcohol,
the same as in beer and wine. It is made by fermenting any biomass
high in carbohydrates through a process similar to beer brewing.
The majority of ethanol produced in the United States is made from
corn. Current research is exploring ways to efficiently convert
cellulose (agricultural waste, forest residue, municipal solid
waste, and energy crops) to ethanol. Ethanol is mostly used as
a fuel additive for vehicles to increase octane and cut down carbon
monoxide and other smog-causing emissions. Biodiesel is made by
processing vegetable oil, animal fat, or recycled cooking grease
with alcohol or other chemicals. It can be used as an additive
(typically 20%) to reduce vehicle emissions or in its pure form
as a renewable alternative fuel for diesel engines.
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Biomass and biofuels resource potential in the conterminous United
States. |
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Because biomass power is produced from plant sources, it can potentially
be produced almost anywhere in the United States.
While biomass is a renewable energy resource, it can have both
negative and positive environmental impacts. It may reduce emissions
and pollutants, but factory farming of biomass crops can reduce
biodiversity and negatively impact wildlife habitat. Municipal
solid waste may contain toxins which could cause pollution if it
is used as a biomass feedstock. As with other renewable resources,
use of appropriate technology will promote the most positive environmental
impacts.
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References |
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Renewable
Energy
National
Hydropower Association
American
Wind Energy Association
Solar
Electric Power Association
Renewable
Energy Policy Project, Solar Power FAQs
State
Environmental Resource Center
Texas
State Energy Conservation Office, Renewable Energy Maps
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Related Links |
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