Solar-Reports:
Solar Thermal Technologies in the United States
By Joseph Philip
17.04.2001
Solar Thermal energy has been used in the United States
for a long time. Clarence Kemp patented the first commercial water heaters
in 1891 and the idea caught on quickly in areas that had to import fuel
for water heating. In 1897, nearly 30% of the houses in Pasadena, California
had solar water heaters. Solar thermal energy became popular again
during the 1970's when the oil crisis occurred. Unlike photovoltaics,
which have resembled the stock markets in boom and bust, the solar thermal
industry has been slowly growing since the 1970's. Although solar thermal
energy is mostly used for heating water, it can also be used to turn water
into steam and produce electricity. First appearing in the early 1980's,
currently 400 peak megawatts of solar thermal electricity are available
in the United States.
Solar Thermal Collector Production
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Over the past years, shipment of solar thermal collectors has been
steadily increasing. According to the Energy Information Agency,
between 1993 and 1999, annual growth from 6,557 square feet of collector
to 8,046 square feet has occurred. Over this period roughly 51,278
square feet of solar collector has been shipped within the United
States.
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The absorber, the "heart" of
a
solar thermal system. Foto: dbu |
While the United States exported 0.5 million square feet, it imported
2.5 million square feet. 95% of the total shipped products were
low temperature solar collectors. Less than 1% was high temperature
solar collectors and was used mostly experimentally by utilities.
Prices dropped significantly from 1998 for low temperature collectors.
The price dropped 25% from $2.83 to $2.06 per square foot.Most of
the current solar thermal systems are installed in California and
Florida. These two states account for roughly 73% of the total solar
thermal systems installed in 1999.
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Solar Water Heating Systems
Approximately, 3% of the total energy consumed by the United States
is used in heating water both in residential and commercial applications.
Houses in the United States usually have standard tanks between
60 to 120 gallons. They can be easily retrofitted for solar thermal
systems. Approximately 1.5 million homes have been fitted with solar
thermal systems and 6000 are added each year. According to the United
States Department of Energy, solar hot water systems are expected
to avoid 0.9 million tons of carbon emissions as of 2000.
However, this represents only 2% of the total homes. The payback
period of solar water thermal systems is only 5 to 10 years when
replacing electric hot water systems. With the price of natural
gas rising, these systems are becoming more attractive to replace
hot water heated by gas.
Over 250,000 commercial and industrial buildings in the United
States use solar thermal systems to provide hot water or space heating.
Industries, in which these systems are most common, include laundry,
food service, food processing, metal plating, and textiles.
Solar Thermal Electricity
Solar thermal electric power plants generate heat by using lenses
and reflectors to concentrate the sun's energy. The possibility
of storing heat allows for the possibility of generating power when
the sun is not shining but the energy is needed. Ranging in size
from power for single-family homes to 10 MW and potentially larger
systems, solar thermal electricity can provide electricity on a
small and large scale.
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Parabolic trough
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A parabolic trough is made of long rows of concentrators only curved
in one direction. They track the sun from east to west with a reflective
surface that focuses the sun's energy. A heat transfer fluid is
run through a pipe that is at the focus of the reflective troughs.
The heat is then transferred to a working fluid (usually water)
that can be used to drive a turbine. Natural gas can also be used
to suplement the solar energy.
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Parabolic troughs are the most commercialized solar-thermal electric
technologies on the market today. Proven in the field more than
354 MW have been operating in California for more than a decade.
Parabolic trough power plant in Spain (Plataforma
Solar de Almeria) Foto: ZSW.
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Solar Power Towers
Although most people consider solar technologies mostly on the
small scale, large-scale generation of electricity is possible to
produce electricity. The first large scale electricity project in
the United States began in 1982 and was a collaboration between
the United States Department of Energy and corporations. Known as
Power Tower One, the solar plant was a field of mirrors that reflected
sunlight towards a central tower where the heat was used to produce
steam. The steam turned a turbine as in a conventional power plant.
Power Tower One operated between 1982 to 1988 and was located near
Barstow, California.
The first power tower proved the idea was feasible and paved the
way for Power Tower Two located at the same site. Power Tower Two
is a 10 MW second-generation demonstration power station. Solar
Power Tower Two is made of 1926 heliostats and a 300 foot tower.
It is capable of providing power for 10,000 homes. Molten salt is
used for heat exchange and storage of 3 hours of power. The system
operated for three years until 1999 and proved that larger systems
are feasible. Currently, discussion is underway on putting a 30
- 100 MW system in Nevada. Boeing and Bechtel are also considering
putting a system in Spain where regulations would be favorable for
such a system.
Solar Dish/Engine Systems
Solar dish/engine systems are still under development, but represent
a new opportunity for solar-thermal electricity. Parabolic dish
generating systems consist of parabolic-shaped concentrators that
usually track the sun in two-axis. A cycle heat engine mounted on
the receiver can generate the electricity, or the sunlight can be
used to heat a fluid that is transmitted to a central engine.
Science Applications International Corporation is currently working
on commercializing a stirling/dishengine. The current prototype
can produce between 20-25 peak kW of standard 3-phase power. If
there is inadequate power or at night, the power system can operate
in hybrid mode in which an alternative fuel can be burned to provide
heat. The engine works on the stirling induction cycle and powers
a 40-horsepower induction generator.
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One dish was demonstrated at the Pentagon in Washington, D.C. in
1998. Currently, they are in the testing stage but have the potential
to offer the cheapest form of solar electricty.
Testing of Dish Concentrating Solar Collector
Components at Sandia National Laboraties. National Solar Test Facility.
Foto:www.sandia.gov.
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Solar Thermal Energy in the Future
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Foto: MBWNRW
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Although the technology is not seen as high tech as the photovoltaics
industry, solar thermal energy will play an important role in providing
clean energy. From providing hot water to a family to creating megawatts
of electricity from the desert of Nevada for the electricity hungry
west, solar thermal energy could indeed lead the United States into
a renewable future.
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Related Solar-Report: An Overview
of Photovoltaics in the USA
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Additional Solar-Reports:
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Solar Magazine
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