LUZ SYSTEMS

Nine commercial Solar Electric Generating Systems {SEGS}, designed and constructed by Luz International Limited, are operating in the Mojave Desert of southern California. These plants arc based on large parabolic hough concentrators providing steam to Ran – kine power plants. They generate peaking power which is sold to the Southern California Edison utility. Located at the Daggct site are the first two of these plants owned and operated by Sunray Energy. SEGS I is a I4-MW electric (MWe) plant and SEGS H is a 30-MWe plant. The next live are all 30-MVVe plants built at the Kramer Junction site and the Inst two are 80-MWc plants located at the Harper Lake site. SEGS III to IX are partially owned and operated by FPL Energy. Basic data on the nine operatiomd plants are shown in Table 17,3.1. An aerial view’ of SEGS III, IV. and V (Kramer Junction site) is shown in Figure 17.3.1.

Plant	First Year	Turbine Size (MWcl	Solar Temperature ГС1	Field Aren (mh	Turbine Efficiency	Annual Output (MVVh)
Solar	Gas
1	1985	ITS	307	82,960	31.5*	_	30.100
If	1986	30	316	190,33$	29.4	37.3	80.500
Ш	і 987	30	349	230,300	30.6	37.3	91.311
IV	1987	30	349	230.300	30.6	373	91,311
V	1988	30	349	250,560	30.6	373	99,182
vt	1989	30	390	188,000	37.5	39.5	90.850
Vll	1989	30	390	194,280	375	39.5	92,646
VIH	‘ 1990	80	390	464,340	37.6	37.6	252,842
IX	1991	80	390	483,960	37.6	37.6	256.125

Table 173.1 Characteristics of SEGS 1 to IXе

‘Data courtesy FPL Eacrgy. ‘includes itniurut gas superheating.

*

Figure 173-1 Aerial view of SEGS ЛІ, IV, and V, three of the Luz plants located on the Mojave Desert of southern California. Photo courtesy of Luz International, Inc. •

The design and economics of these plants are substantially influenced by U. S. federal law. The plants qualify under the Public Utilities Regulatory Policies Act (PURPA) as small power producers and are allowed under this law to supplement solar output of the plant by fuels (natural gas) to the extent of 25% of the annual output. The plants can supply peaking power, using alt solar energy, all natural gas, or a combination of (he two, regardless of time or weather, within the constraint of the annual limit on gas use. The most critical time for power generation and delivery (and tire time in which the selling price of the power per kilowatMtour is highest) is between noon and 6 pm in the months of June through September. Operating strategy is designed to maximize solar energy use and depends on gas to provide power during cloudy periods early in the year and late in the summer when solar output drops off before the end of the peak-power period. The turbine-generator efficiency is best at full load, and use of gas supplement to allow full-load operation maximizes plant output. The plants do not have energy stor­age facilities. A schematic of a typical plant is shown in Figure 17.3.2; the solar and natural gas loops are in parallel to allow operation with either or both of the energy sources. Data and experience with these plants have been reported by Jaffe et al. (1987), Kearney and Gilon (1988), Kearney el nl. (1988). Jensen et al. (1989), and Harats and Kearney (1989).

The major components in the systems are the collectors, the fluid transfer pumps, the power generation system, the natural gas auxiliary subsystem, and the controls. Three collector designs have been used in these plants, LS-l in SEGS I. LS-2 in U to VII, and LS-3 in part of VII and in subsequent plants. Data for LS-2 and LS-3 are shown in Thble

17.3.2. Photographs of the collector field are shown in Figures 17.3.3(a) and (b). The reflectors are made up of back-silvered, low-iron float-glass panels which are shaped over parabolic forms. Metallic and lacquer protective coatings are applied to the back of the silvered surface, and no mensurable degradation of the reflective surface has been ob­served. Hie glass is mounted on truss structures, with the position of large arrays of modules adjusted by hydraulic drive motors. The receivers are 70-mm-diameter steel

Figure 17.3.2 Schematic process diagram for SEGS systems. Adapted from Kearney and Gilon (198S).

tubes with cornet selective surfaces surrounded by a vacuum glass jacket. The surfaces’ have an absorptance of 0.96 and an omittance of 0.19 at 350”C.

The reflectance of the mirrors Is 0.94 when clean. Maintenance of high reflectance is critical to plant operation. With a total of 2.31 X 106 m2 of minor area, mechanized equipment has been developed for cleaning the reflectors, which is done primarily m tire summer. These results have led to average reflectance maintained at 92% year round.

Tracking of the collectors is controlled by sun sensors that utilize an optical system to focus radiation on two light-sensitive diodes, with imbalance causing corrections in the positioning of the collectors. There is a sensor and controller on each collector as­sembly; the resolution of the sensor is about 0.5°. The collectors rotate about horizontal north-south axes, an arrangement which results in slightly less energy incident on them over a year but wltich favors summertime operation when peak power is needed and its sale brings the greatest revenue.

Table 17,3.2 Characteristics of LS-2 and LS-3 Collector Modules"

Parameter	LS-2	LS-3
Area, m*	235	545
Mirror segments	120	224
Aperture, m	5.0	5.76
Length, m	47.1	95.2
Focal length, ni	1.84	2.12
Concentration ratio	71	S2
Distance between rows, in	12.5-15	17.3
Optical efficiency	0.74-0.76	0.80

Adapted from HaraU and Kearney {1989).

of mirrors moved through the day to keep beam radiation focused on the central receiver. More recent design concepts call for heliostats on mounts at fixed locations and movable about two axes of rotation to accomplish concentration. Major research-and-de velopmenl efforts in the United States are aimed at solving the range of optical, ihermal, and me­chanical problems associated with the development of electric power generation systems based on these concepts. These early efforts were reported in the Proceedings of (he 197S DOE Workshop on Systems Studies for Central Solar Thermal Electric (1978). A review of general design considerations is provided by Winter et al. (1991), A more recent review of solar central-receiver systems is provided by Romero et al. (2002).

The major components in the system are the heliostat field, the heliostat controls, the receiver, the storage system, and the heat engine which drives the generator. Heliostat design concepts were briefly outlined in Section 7.9. The objective of heliostat design is to deliver radiation to the receiver at the desired flux density at minimum cost. For an extensive discussion of optical problems, see the Proceedings of the EHDA Solar Work’ shop (1977) on optical analysis.

A range of receiver shapes has been considered, including cavity receivers and cy­lindrical receivers. The optimum shape is a function of intercepted and absorbed radia­tion, thermal losses, receiver cost, and design of the heliostat field. Vant-Hul! suggests that for a large heliostat field a cylindrical receiver has advantages when used with Rankine cycle engines, particularly for radiation from heiiostats at the far edges of the field. If higher temperatures are required for operation of Brayton cycle turbines, it may be necessary to use cavity receivers with larger tower height-heliostat field area ratios.

It has been observed in many solar power studies – that the solar collector represents the largest cost in the system. Under these circumstimccs, an efficient engine is justified to obtain maximum useful conversion of the collected energy. Several possible thermo­dynamic cycles have been considered. First, Brayton or Stirling gas cycle engines op­erated at inlet temperatures of 800 to 1000°C provide high engine efficiencies but are limited by low gas heat transfer coefficients, by flic need for recouperaiors, and by the practical constraints on collector design (t. e., the need for envity receivers) imposed by the requirements of 10QCFC temperatures. Second, turbines driven from steam generated in the receiver would operate at 500 to 55G"C and have several advantages over the Brayton cycle. Heat transfer coefficients in the steam generator are high, allowing the use of high energy densities and smaller receivers with energy absorption on the outer surface. Cavity receivers are not needed and cylindrical receivers permit larger heliostat fields to be used. Use of reheat cycles improves steam turbine performance but entails mechanical design problems. It is also possible to use steam turbines with steam gen­erated from an intermediate heat transfer fluid circulated lino ugh the collector and boiler. The fluids could be molten salts or liquid metals, and cylindrical receivers could be operated at around бОО^С with such systems. These indirect systems are Die only ones that readily lend themselves to the use of storage. Low-temperature steam or organic fluid turbines may also be used, with collector temperatures around 300°C in receiver with tower solar radiation flux densities.

In the next section, a large iO-MWe central-receiver system. Soiar One (inter mod­ified and called Solar TWo), Is described. Other experimental plants have been built and operated, including a 5-MWe plant in Russia, a 2.5-MWe plant in France, and 1-MWe plants in Japan, Italy, Spain, and the United States.