parabolic dish systems that concentrate light in two directions instead of one, but nearly all existing solar thermal electrical genera- tion is from parabolic-trough systems. Because they use only direct-beam sunlight, parabolic-trough systems require tracking systems to keep them focused toward the sun and are best suited to areas with high direct solar radiation. (See Figures 5 and 6). Most systems are oriented either east-west or north-south with single-axis tracking during the day. East-west orienta- tion is better in winter and more constant throughout the year, while north-south orientation is better in summer and provides greater annual output.
The systems are pro- grammed to turn the collectors upside down at night or during high winds to reduce stress on the structure and help keep the mirrors clean. Because they transmit structural stress from wind loading and require large areas for installation, parabolic-trough collectors are usually ground mounted. IST is develop- ing a smaller, lower-profile version of its collector system that will be more suitable for roof mounting, but its standard troughs are 20 feet by 7.5 feet (6.1 meters by 2.3 meters). IST typically installs sets of either 16 troughs (4 rows of 4 for 2400 square feet of collector) or 24 troughs (4 rows of 6 for 3600 square feet of collec- tor).
Each set can run on a single-drive- motor tracking system. These installations require about 6,000 and 9,000 square feet (557 and 836 square meters), respectively, of level or slightly south-facing, unshaded space with the latter being the smallest size generally viable project. The collector sys- tem should be fenced and within 1000 feet of the conventional water heater for the building where the water is being used (the closer the better). Parabolic-trough systems will usually be installed by the manufac- turer. IST generally assembles the collectors onsite or nearby. Installation Figure 10. Schematic of trough cooling system. 0.8 for single-effect systems.
COP is the ratio of the heat energy removed divided by the work required to remove it. That work is defined differently for absorption cooling than for compression cooling (based on heat input rather than electrical input). Therefore, these COPs cannot be directly compared to those for electrical compression cooling systems (generally 2.0 to 4.0). Solar heat is well suited to heat water or generate steam for indirect-fired absorption cooling because of the close time match between solar heat generation and the need for cooling. Because air-conditioning fre- quently drives peak electrical demand, the time match of parabolic-trough energy collection to air-conditioning needs is sig- nificant.
Electric utilities frequently charge commercial facilities two to three times as much for power during peak demand periods and may even subsidize customer projects that reduce peak demand. Absorption cooling offers a critical additional benefit for hospitals and other facilities that cannot tolerate being without air-conditioning—only minimal electricity is necessary to keep a parabolic-trough system operating during a power outage. Facilities with parabolic-trough collector/absorption cooling systems need far smaller backup generators than those with electric compres- sion cooling. Nearly all solar absorption cooling sys- tems include subsystems for space and water heating.
This allows the expensive solar equipment to be used throughout the year, rather than just during the cooling season. Figure 10 shows how parabolic- collector and absorption cooling systems work together. As with parabolic-trough water heating, the economic viability of parabolic-trough air-conditioning will largely be a function of the system cost and conventional utility costs. Absorption chillers will cost from 1.5 to 2.5 times as much as electric- compression air conditioners or chillers. But if the electricity cost is high enough relative to solar heat or fuel such as natural gas or propane, the extra equipment cost for absorption cooling may be a good invest- ment.
Similarly, if the conventional fuel cost is high enough, the cost of a parabolic- trough collector system to drive the absorp- tion cooler may be a good investment. With a 9-month or longer cooling season and electricity costing about 10ў/kWh or more, a parabolic-trough/absorption chiller system is worth investigating. Because the para- bolic-trough collector portion will usually account for more than half the cost of such a system, it is important for economic reasons that the system also be used for water heat- ing and space heating. Parabolic-trough systems can also be used to heat industrial or industrial-like process water or other fluids to high temper- atures.
(IST's systems can generate tempera- tures as high as 520˚F [271˚C]). Federal facility activities that might need such high- temperature water include cleaning engines or other equipment, industrial laundries, and certain hospital uses. For higher-temperature uses, oil is often used instead of antifreeze, both in the collector loop and in the thermal storage tank. In addition, parabolic-trough collectors can be used to generate electricity. During the 1980s Luz International (no longer in business) built nine plants for solar thermal electrical generation. All are still operating.
These systems, which encase the collector tube in a vacuum, use oil as a transfer fluid and reach temperatures of 730˚F (388˚C). Electrical generation facilities can also use Federal-Sector Potential Technology-Screening Process The FTA series targets technologies that appear to have significant untapped Federal- sector potential and for which some Federal installation experience exists. Many of the 12 Sunlight sensor Controller Chilled water to air conditioning system Parabolic troughs Cooling One or water two-stage Thermal Heater absorption storage chiller Solar pump Hot water pump 0227092820769m806m 0227092820769m806m t a One or wo-stage bsorption chiller Hea ter