Barstow, California, and by design studies for solar thermal repowering of existing oil- and gas-fired plants in the Southwest. In repowering, a solar thermal field provides supplementary power, using existing turbines and generators. Construction of the Barstow pilot plant is expected to cost $123 million, or over $10,000 per kilowatt (electric) (kWe). (This is about $1 kWh, assuming a load factor of 0.2 and an annual fixed charge rate of 15 percent.) A commercial solar thermal electric plant built with present technology and operational in 1985 would probably cost about one fourth this much and have generating costs about 5 times the current average. Such plants would probably be designed with more energy storage than Barstow. Increasing the plant load factor, which would reduce the cost per kilowatt-hour, depends on the development of a practical technology for storing high-temperature heat or electricity. Early experiments conducted on storage systems for the 10-MWe pilot plant are encouraging, but much needs to be done before high load factors can be achieved. An alternative to storage is the use of oil and gas for backup generating capacity, as in repowering.

One promising extension of solar thermal electric conversion designs is the so-called solar total energy system, in which the waste heat from the generator is used near the power plant. No commercializable technology for this has been demonstrated, but an experimental system is being built at Shenandoah, Georgia. The electricity from these early demonstrations will probably have costs of the same order of magnitude as the first electricity from central station solar thermal plants, or several hundred mills per kilowatt-hour. However, until the first solar thermal total energy system demonstrations are completed, it will be difficult to estimate costs precisely.

Environmental Health, and Social Considerations

These impacts have been considered by the Risk and Impact Panel,25 and the following material makes use of that analysis. An environmental assessment of solar energy is also being performed by the Department of Energy.26,27 The most significant environmental impacts of large solar thermal conversion plants would be their land and material requirements. Studies suggest that such a plant, with enough storage to generate 100 MWe at a 40 percent load factor, if located optimally (e.g., in the southwestern desert) would take up at least 1 square mile. This is about as much land as would be taken during the entire expected life of an equivalent coal plant (including underground mining) and an order of magnitude greater than that for the equivalent nuclear plant using high-grade ore (including mining, milling, etc.). Depending on assumptions



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