Solar One, a first-generation solar central-receiver power plant, began the generation of electricity in 1982, and its successes led to the development of improved designs.
Solar One, the world’s largest solar central-receiver electric power plant, was initiated in 1976, came online on April 12, 1982, and was dedicated on November 1, 1982. Solar One was a first-generation pilot plant to test the feasibility of the large central-receiver solar-generating station concept. The plant produced 10.8 megawatts of electrical power (MWe) for approximately eight hours at summer solstice and four hours at the winter solstice. During the first two years of operation, the plant underwent extensive testing. This was followed by four years of successful operation, during which the plant delivered a total of 37,000 MWe hours of energy to Southern California Edison’s electrical power grid. The plant was decommissioned on September 27, 1988.
The plant design of Solar One contained six major components: the collector field, which included the heliostats (reflecting mirrors) and supporting hardware; the receiver, which was placed on top of a central tower; a thermal storage system; master control systems; the electric power-generation system, which included the turbine and generator; and the plant-support systems. Heliostats in the collector field reflected and concentrated sunlight onto a receiving tower 298 feet high. The collected solar energy converted liquid water to superheated steam of about 960 degrees Fahrenheit. Focusing sunlight onto the central receiving tower were 1,818 heliostats that tracked the Sun during the daylight hours. These heliostats were arranged in a series of concentric arcs around the central receiving tower. The collector field had a total reflecting area of 72,538 square meters and was divided into four quadrants. Each steerable heliostat had a reflecting area of 40 square meters and was made of twelve concave mirror panels. These heliostats were controlled by computer to track the Sun during the daylight hours and were designed to operate effectively in winds up to 36 miles per hour (mph). In the stowed position, the heliostats were designed to survive in winds up to 90 mph.
In order to extend the day-to-day output of the plant, a thermal storage system was included in the design. The storage tank contained 4,532 tons of granite, 2,266 tons of sand, and 239,600 gallons of Caloria HT-43 oil. When fully charged, the temperature of the storage mixture was approximately 572 degrees Fahrenheit. This storage system was designed to generate steam at 525 degrees Fahrenheit and deliver it to the turbine for up to four additional hours of electrical generation per day.
Solar One was launched as a pilot project for testing the feasibility of developing large central-receiver generating stations. Its design features were tested as a prototype to be extrapolated for the design of plants up to ten times its size that is, in the range of 100 MWe, which approached commercial interest and feasibility. Researchers collected data on the facility’s development, production, and operation over a period of several years to compare the economics of similar commercial power plant designs with more traditional technologies.
Solar One represented a combined venture by the U.S. Department of Energy
Although the solar energy-collecting portion of Solar One represented new technology, the heat-exchange and generation parts of the project were based on traditional steam electricity-generating technology. This choice of technologies had two strategic advantages. The heat-exchange and generation parts of the project were well known and similar to those of more traditional electric power plants; consequently, the performance of the solar energy-collecting portion of the project could be more easily isolated and evaluated. Second, the application of solar energy had the potential to repower existing fossil-fuel plants as well as to develop hybrid plant systems that use solar in conjunction with traditional technologies such as fossil fuels, which would be able to use preexisting systems of steam-turbine electricity generation.
An array of large mirrors at the Solar One facility near Barstow, California. The mirrors reflect sunlight onto the central tower in the background, which collects the sun’s heat and converts it to electricity.
The advantages of the successful development of a central-receiver solar-generating station included the fact that there would be minimal environmental degradation in comparison with other forms of commercial generation of electricity. Not only would there be less impact on the environment caused by the acquisition of plant fuels, but there also would be no thermal runoff, as was characteristic of the traditional steam-electric power stations. Further, traditional electric power plants had no economically effective means for the storage of their product. Solar One, on the other hand, could store the heat energy that it collected during daylight hours to extend electric energy generation into the evening hours of peak demand. The principal costs associated with central-receiver solar-generating stations were those of construction and operation. Because there were no fuel costs, the production costs were not dependent on fluctuating fuel costs. Finally, solar-electric energy generation had far more widespread public support than other forms of steam electricity generation, particularly in light of the electric industries’ experience with construction and operation of nuclear electricity-generating facilities.
The objective of the solar central-receiver power plant program in the United States was to demonstrate the commercial feasibility of that system for utility-scale power production. Solar One was the first of three generations in the development of commercially feasible solar-generated electricity. Solar One was important because the results exceeded expectations and provided the data necessary for subsequent commercialization of solar power. The facility operated successfully as a power plant for more than four years and demonstrated the feasibility of generating electricity with solar central-receiver power plants.
The stage that followed the testing of Solar One was the incorporation of improvements into the design, based on the extensive data collected during its operation. The most significant of the improvements that were retrofitted into Solar One was the conversion of its system of water-steam heat transfer to one based on a molten nitrate salt mixture. This project was named Solar Two.
A nitrate salt system had several advantages over a water-steam system. The salt solution was a single-phase fluid that operated at a low vapor pressure. Consequently, the receiver system could be of a simpler, more compact design. The steam conditions and flow rates were easier to control, which resulted in more efficient turbine operation. In addition, hot salt could be stored in tanks, which allowed for a more efficient energy storage system. This allowed Solar Two to operate into the evening hours with excess energy collected and stored during the optimum daylight hours. This was expected to be a major improvement over the energy storage system of Solar One. Once successful energy storage was demonstrated, it was hoped that central-receiver power plants could be designed with an excess collection capacity specifically for the purpose of storing energy for later use. This would also smooth over the effects of cloud-induced transient periods of lower solar energy collection.
Solar Two was designed to be a large-scale demonstration facility. It was expected that successful operation of Solar Two would lead to the development of a third generation of the design, Solar 100, a 100-MWe commercial demonstration plant. Construction on Solar Two was completed in 1995. After proving the feasibility of solar molten salt storage technology, the plant was decommissioned in 1999.
The funding for Solar Two survived the large government budget cuts characteristic of the early and mid-1990’s. This attested the success of Solar One as a pilot plant to demonstrate the technological feasibility of central-receiver solar-generating stations. The data collected during the operation of Solar One continued to show significant promise, particularly for application in appropriate areas of the United States at a time when other technologies did not live up to the expectations projected for them during and after the energy crises of the 1970’s.
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Down, Norman. “Solar One.” Solar (January/February, 1983): 10-27. Describes the technical specifications and projections of Solar One shortly after the plant began operation. Suitable for the general reader. -
Kraushaar, Jack J., and Robert A. Ristinen. Energy and Problems of a Technical Society. 2d ed. New York: John Wiley & Sons, 1993. Designed primarily for nonscience and nonengineering college students, hence, reasonably accessible to the general reader. Discussion of Solar One is brief, but it is part of a comprehensive overview of energy that includes alternative energy sources and technologies. -
ReVelle, Charles, and Penelope ReVelle. “Solar Energy.” In The Environment: Issues and Choices for Society. 3d ed. Boston: Jones & Bartlett, 1988. Environmental science text for college students. Describes applications of solar energy, including Solar One. -
Ristinen, Robert A., and Jack P. Kraushaar. Energy and the Environment. 2d ed. Hoboken, N.J.: John Wiley & Sons, 2006. A good textbook for students with little or no background in science or mathematics. Chapter 4 discusses solar power. -
Shefter, Jim. “Solar One: Sun, to Heat, to Electricity.” Popular Science 221 (October, 1982): 114. Brief discussion of the operating principles of Solar One. Accessible to the general reader. -
Van Herik, Ed. “Solar Two: The Hot Button in Solar Energy.” Solar Industry Journal (Third Quarter, 1993): 25-29. Describes Solar One and the transition to the next phase, Solar Two. Brief discussion of the renovations and of projected economics for commercialization of the technology by Southern California Edison and the Sacramento Municipal Utility District.
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