Thermal Solar Collector Designs SEGS and DSG

Thermal collectors on the roofs of private homes are usually flat and stationary units that serve to provide residences with heat and hot water. The yearly energy needed for heat and hot water of a well-insulated home in the Northeast United States is about 150 million Btus (44,000 kWh), which is about the same as the energy content of 1,000 gal of oil. According to a 2007 study by the McKinsey Global Institute, the residential energy consumption in the United States is 21.3 Q, or 21% of the total energy used. Of the 21.3 Q total, 8.5 Q is used to operate appliances, 7.5 Q for heating/cooling, 3.2 Q for the generation of hot water, and 2.1 Q for lighting.

Today thermal-solar plants are the most popular. Besides their lower cost and fairly good efficiency (compared to photovoltaic designs), their main advantage is the ability to store energy for continued nighttime operation. One such 354 mW plant in the Mojave Desert has been in operation for several decades. Ten new plants are planned for Arizona, California, and Nevada, with an electricity generating capacity equaling that of three nuclear plants. Other thermal-solar plants are under construction in Spain, Algeria, and Morocco and another nine are planned in Israel, Mexico, China, South Africa, and Egypt.

In the United States solar-thermal-electric generating system power plants have been in operation since 1985. In the Southwest they generate up to 1,000 kWh/yr/m2 of electricity. They include SEGS I (1985, Daggett, CA, 14 mWe, 5,000 m2/mWe); SEGS II (1986, Daggett, CA, 30 mWe, 6,000 m2/mWe); SEGS III, IV, V, VI, VII (1987-1989, Kramer Junction, CA, 30 mWe, 6,500 to 8,500 m2/ mWe); SEGS VIII and IX (1990-1991, Harper Lake, CA, 80 mWe, 6,000 m2/ mWe); and Solar One (2007, Boulder City, NV, 64 mWe (5,500 m2/mWe).

The first costs of the plants are competitive with conventional fossil or nuclear plants while the time needed for construction is one fifth (3 to 4 instead of 10 years). One reason why solar power is competitive is the increase in the costs of oil and natural gas, which makes solar electricity competitive (12^/kWh) and actually less expensive than fossil or nuclear during peak periods.

In solar power plant applications, the largest solar energy generating system (SEGS) in the world is operated by Southern California Edison. The photograph in Figure 1.31 shows part of that solar power plant. It has been in operation at Kramer Junction in the Harper Valley of the Mojave Desert in California since 1985. This facility consists of nine solar electric plants, with a combined capacity of 354 mW. The facility has 400,000 mirrors, which are distributed over 1,000 acres (4 km2). It was built by Luz International and is owned and operated by FPL Energy, a subsidiary of Florida Power and Light.

In this design, parabolic mirror reflectors (troughs) are used to track the trajectory of the Sun and to concentrate the sunlight onto absorber tubes that are located at the focal line of the parabolic mirrors. Inside the absorber tubes, heat-resistant oil is circulated. This heat-transfer fluid serves to transport the collected heat into boilers that generate steam to drive the turbine generators.

In this particular installation, the hot oil circulates at 400°C (752°F), but at other solar power plants (for example, Boulder City, Nevada, by Solargenix Energy, using molten salt), the operating temperature is even higher. One advantage of the SEGS design is that the solar energy can be conveniently

Dsg Parabolic Troughs

figure 1.31

A 354 mW thermal solar plant at Kramer Junction in the Harper Valley in California that has been in operation since 1985 (courtesy of NREL/DOE). (Top) This is one of nine solar electric energy-generating plants at Kramer Junction, California and it uses parabolic troughs to collect the Sun's energy. (Courtesy of National Recoverable Energy Laboratory-NREL/DOE.) (Bottom) In the receiver tubes, hot oil transports the concentrated solar heat to steam boilers, which drive the turbine generators.

figure 1.31

A 354 mW thermal solar plant at Kramer Junction in the Harper Valley in California that has been in operation since 1985 (courtesy of NREL/DOE). (Top) This is one of nine solar electric energy-generating plants at Kramer Junction, California and it uses parabolic troughs to collect the Sun's energy. (Courtesy of National Recoverable Energy Laboratory-NREL/DOE.) (Bottom) In the receiver tubes, hot oil transports the concentrated solar heat to steam boilers, which drive the turbine generators.

figure 1.32

A 10 mWe central receiver concentrator plant with molten salt-based thermal storage, located at Daggett, California. (Courtesy of Sandia National Laboratories.)

figure 1.32

A 10 mWe central receiver concentrator plant with molten salt-based thermal storage, located at Daggett, California. (Courtesy of Sandia National Laboratories.)

stored (as thermal energy) and used later as needed to compensate for diurnal-, seasonal-, and weather-related variations in the availability of solar energy (insolation).

Thermal solar collectors are also available in so-called central concentrator or solar concentrator designs. In these configurations, a large number of independently movable flat mirrors (heliostats) are used to reflect the solar radiation onto a central receiver on the top of a tower. Each heliostat moves about two axes. The receiver typically is a vertical bundle of tubes in which the heat-transfer fluid (water or oil or molten salt) is heated by the reflected and concentrated insolation. The molten salt technology also provides thermal energy storage.

Designed by the DOE, such a design was implemented at Daggett, California, and named Solar One. In 1995 this plant was converted and expanded to a total area of 82,750 m2, and is referred to as Solar Two. A 10-mW central receiver-type generating plant located at Daggett, California is shown in Figure 1.32.

In some of the more recent SEGS designs, the circulating fluid temperature has been more than doubled, whereas other designs generate steam directly. These improvements have increased the amount of power produced by about 15%. The main limitation of direct steam generation (DSG) is the difficulty of providing energy storage, which is not a limitation in SEGS.

The main advantage of DSG is its improved efficiency in comparison with the SEGS design. This is because the two-circuit system (hot oil collection and water/steam Rankine power circuit) of the SEGS design is replaced by a single circuit in which the collector circuit is directly coupled to the

Steam Turbine

Steam Turbine

Dsg Solar Power Plant Design

Condenser

Low Pressure Deaerator Preheater figure 1.33

(See color insert following page 140.) The integrated solar/combined cycle system (ISCCS) design, a thermal solar collector field is integrated with a steam- and gas turbine-based electric power generation. (Courtesy of FLAGSOL, Cologne.)

Condenser

Low Pressure Deaerator Preheater figure 1.33

(See color insert following page 140.) The integrated solar/combined cycle system (ISCCS) design, a thermal solar collector field is integrated with a steam- and gas turbine-based electric power generation. (Courtesy of FLAGSOL, Cologne.)

power circuit. This eliminates the oil-to-water heat transfer equipment and the associated loss in efficiency. With DSG systems, superheated steam at 100 bar pressure and 400°C temperature can be generated.

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