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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

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.
Absorber
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.


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.


Parabolic trough

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.
Parabolic trough power plant

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.


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.

 
Solar Dish System

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.


Solar Thermal Energy in the Future

Sunrise

Foto: MBWNRW

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.

Related Solar-Report: An Overview of Photovoltaics in the USA

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