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Figure 7-42 Illustration of Power Tower System.

Receiver

Concentrator

Figure 7-42 Illustration of Power Tower System.

projects that produce steam to drive a conventional steam-turbine generator and provide electricity to the grid. They may also be used for more localized district steam systems or other specialized heat process applications. Concentrating solar thermal systems can absorb and concentrate 1,000 times or more normal intensity of the sun. Currently these systems use three types of concentrators:

• Central receiver (or power tower) systems (Figure 7-42) that use a circular field array of heliostats (individually tracking, highly reflective mirrors) that track the sun and focus it on a central receiver.

• Parabolic dish systems (Figure 7-43) that use an array of parabolic dish-shaped reflectors (mirrors) to focus/concentrate solar energy onto a receiver located at the focal point of the dish. Fluid in the receiver is heated to a typical temperature of about 1,380°F (750°C). This can be used to produce superheated steam, or, in some systems currently under prototype development, operate an integral, small-capacity engine.

Figure 7-43 Illustration of Parabolic Dish System.

• Parabolic trough systems (Figure 7-44) that use parabolic trough-shaped reflectors (mirrors) to focus/ concentrate sunlight on thermally efficient receiver tubes, running the length of the trough, that contain a heat transfer fluid. The fluid is heated to a typical temperature of about 735°F (390°C) and pumped through a series of heat exchangers to produce superheated steam. The troughs are situated in parallel rows, aligned on a north-south axis that enables the single-axis troughs to track the sun continuously from east to west.

When applied to electric power generation, the solar trough and power tower rely on conventional steam turbine generators, while the parabolic dish systems use integral Stirling or Brayton-cycle type engines. Currently, the parabolic trough systems are the most commercially advanced high-temperature systems and can be practical in a wide range of applications and capacities. Figure 7-45 shows a parabolic trough-based solar hot water system applied at a correctional facility in Colorado. This system provides about 20,000 gallons (75,700 liters) of hot water per day for laundry and other domestic uses. The power

Concentrator

Figure 7-44 Illustration of Parabolic Trough System.

tower has been proven effective in demonstration projects for grid-feeding electric power generation, while the parabolic dish systems are still in prototype development with models in operation that can generate up to 25 kW of electricity.

Figure 7-46 shows a parabolic dish concentrator system featuring stretched membrane heliostats. With this self-contained system design, the receiver absorbs energy reflected by the concentrator and transfers it to the engine's working fluid. In addition to the dish module itself, continued research and development is focused on the engine technology and associated interface issues. Figure 7-47 shows the renowned "Solar Two" power tower project in Southern California. This 10 MW electric generation system features a 295-ft (90-m) tower with a (43 MW) thermal rating with 800-sun peak-flux capability. The tower is surrounded by 1,181 420-ft2 (39-m2) and 108 1,020-ft2 (95-m2) heliostats. The 16.7ft (5.1-m) diameter by 20.3-ft (6.2-m) high receiver has 32 1-in. (25-mm) diameter tubes in each of 24 panels. Its steam generation system features separate pre-heater, evaporator and superheat vessels that combine to produce 121 million Btu/h (35.5 MW) of steam at 1,450 psi (100 bar) and 1,000°F (538°C) for use in the steam-turbine electric generator set.

These solar thermal generation systems are limited to about 25% availability, even when located in the most ideal sites. Availability and reliability can be enhanced through a variety of means, including the addition of thermal storage and fossil fuel boilers for steam generation or battery storage and back-up combustion engines for electricity production. The addition of thermal storage can greatly increase output availability. One currently used thermal storage system

Figure 7-45 Parabolic Trough-Based Solar Hot Water System Source: Warren Gretz, DoE/NREL
Figure 7-46 Parabolic Dish Engine System Prototype. Source: Warren Gretz, DoE/NREL

features two storage tanks. Liquid salt is pumped from a "cold" storage tank through the receiver, where it is heated and then pumped to a "hot" tank for storage. When steam is needed, hot salt is pumped to a steam generation system. From the steam generator, the salt is returned to the cold tank, where it is stored and eventually reheated in the receiver. Shown to the right of the central tower in Figure 7-47 are two large storage tanks. These tanks contain molten nitrate salt, which is a clear liquid with properties similar to water at temperatures above its 464°F (240°C) melting phase point. The salt is pumped from the cold storage tank to the receiver, where it is heated in the tubes to about 1,050°F (565°C). The salt is then pumped to the hot storage tank, where is remains available for dispatch to the steam generator. After producing steam it is then pumped back to the cold storage tank at a temperature of about 545°F (285°C). While such a high-temperature storage system allows availability to be increased dramatically, it remains very costly.

In addition to steam generation, high-powered solar furnaces can be used directly for process applications, replacing conventional furnaces or very costly laser furnaces. Current prototype applications range from high-temperature coatings on metals and ceramics (where it is advantageous to heat only the surface of the material without affecting the base material) to providing effective decontaminating of hazardous wastes. The solar furnace's ability to quickly generate temperatures in excess of 5,500°F (3,000°C), focus it with great precision, and selectively heat the surface of a sample have led to studies of phase transformation hardening, thin-film deposition, rapid thermal tempering, and a wide variety of treatments

Figure 7-47 Solar Two Power Tower Demonstration Project. Source: Joe Flores, Southern California Edison; DoE/NREL

for metal, ceramic, and composite materials to obtain higher-value materials with desired properties, such as superconductivity or greater resistance to corrosion, friction, and oxidation. Such solar furnaces are also well suited to the destruction of hazardous wastes. Focusing a beam of concentrated light onto hazardous wastes breaks down numerous toxic chemicals, including dioxin and polychlorobiphenyls (PCBs). The ultraviolet portion of the solar radiation breaks the bonds holding together the hazardous components.

Figure 7-47 Solar Two Power Tower Demonstration Project. Source: Joe Flores, Southern California Edison; DoE/NREL

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