Introduction

There are three solar thermal power systems currently being developed by U.S. industry: parabolic troughs, powe r towers, and dish/engine systems. Because these technologies involve a thermal intermediary, they can be readil y hybridiz ed with fossil fuel and in some cases adapted to utilize thermal storage. The primary advantage o f hybridizatio n and thermal storage is that the technologies can provide dispatchable power and operate during period s when solar energy is not available. Hybridization and thermal storage can enhance the economic value of the electricity produced and reduce its average cost. This chapter provides an introduction to the more detailed chapters on each of the three technologies, an overview of the technologies, their current status, and a map identifying the U.S. regions with best solar resource.

Parabolic Trough systems use parabolic trough-shaped mirrors to focus sunlight on thermally efficient receiver tubes that contain a heat transfer fluid (Figure 1). This fluid is heated to 390 oC (734oF) and pumped through a series of heat exchangers to produce superheated steam which powers a conventional turbine generator to produce electricity. Nin e trough systems, built in the mid to late 1980's, are currently generating 354 MW in Southern California. These systems, sized between 14 and 80 MW, are hybridized with up to 25% natural gas in order to provide dispatchable power when solar energy is not available.

Cost projections for trough technology are higher than those for power towers and dish/engine systems due in larg e part to the lower solar concentration and hence lower temperatures and efficiency. However, with 10 years of operating experience, continued technology improvements, and O&M cost reductions, troughs are the least expensive, most reliable solar technology for near-term applications.

Figure 1. Solar parabolic trough.

Power Tower systems use a circular field array of heliostats (large individually-tracking mirrors) to focus sunlight onto a central receiver mounted on top of a tower (Figure 2). The first power tower, Solar One, which was built in Southern California and operated in the mid-1980's, used a water/steam system to generate 10 MW of power. In 1992, a consortium of U.S. utilities banded together to retrofit Solar One to demonstrate a molten-salt receiver and thermal storage system.

The addition of this thermal storage capability makes power towers unique among solar technologies by promisin g dispatchable power at load factors of up to 65%. In this system, molten-salt is pumped from a "cold" tank at 288°C

(550oF) andcycledthroughthereceiverwhereitisheatedto 565°C(1,049°F) and returned to a "hot" tank. The ho salt can then be used to generate electricity when needed. Current designs allow storage ranging from 3 to 13 hours .

"Solar Two" first generated power in April 1996, and is scheduled to run for a 3-year test, evaluation, and power production phase to prove the molten-salt technology. The successful completion of Solar Two should facilitate th e early commercial deployment of power towers in the 30 to 200 MW range.

Power Towers

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Figure 2. Solar power tower.

Figure 2. Solar power tower.

Dish/Engine systems use an array of parabolic dish-shaped mirrors (stretched membrane or flat glass facets) to focus solar energy onto a receiver located at the focal point of the dish (Figure 3). Fluid in the receiver is heated to 750 oC (1,382oF) and used to generate electricity in a small engine attached to the receiver. Engines currently unde r consideration include Stirling and Brayton cycle engines. Several prototype dish/engine systems, ranging in size from 7 to 25 kWe, have been deployed in various locations in the U.S. and abroad.

High optical efficiency and low startup losses make dish/engine systems the most efficient (29.4% record solar t o electricity conversion) of all solar technologies. In addition, the modular design of dish/engine systems make them a good match for both remote power needs in the kilowatt range as well as hybrid end-of-the-line grid-connected utilit y applications in the megawatt range. If field validation of these systems is successful in 1998 and 1999, commercia l sales could commence as early as 2000.

Figure 3. Solar dish/engine system.
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