typically operate at about 200˚F (93˚C) to deliver water at about 120˚F to 140˚F (49˚C to 60˚C ). Efficiency is highest with a tem- perature difference of about 60˚F (33˚C) between input to the solar collector field and output from the field to the thermal storage tank.
In addition to the collectors, IST's para- bolic-trough systems include an electric tracking system (to keep troughs facing the sun or turned downward when it’s dark or too windy), a nonpressurized hot-water storage tank, a circulating pump to drive the hot transfer fluid from the troughs through a closed-coil heat exchanger in the storage tank to heat that tank (pressurized loop), a loop that draws cold intake potable water through a second heat exchanger in the storage tank on its way to the conventional water heater, and various sensors to trigger the proper operation. (See Figure 7.) The system acts as a preheating system for the facility's conventional water-heating system.
If the solar-heated water is too hot, a tempering valve mixes in cold water. If the water is not hot enough, the conventional system heats it to the proper temperature. As hot water is used from the conventional water heater, incoming cold water flows through a heat exchanger, drawing heat from the hot-water storage tank, and then to the conventional water-heating system. In locations with no freeze danger, water can be used instead of antifreeze because it’s cheaper and is a better heat transfer fluid. In the Southwestern United States, there is sufficient sunlight for parabolic-trough col- lector systems to operate about 30% to 35% of the time.
The systems will generally be most cost effective if sized so that on the best summer days they are just able to meet the demand—that is, there is no excess capacity. Such a system will provide about 50% to 60% of annual water-heating needs. If conventional energy costs are particularly high, it may be economical to increase sys- tem size somewhat beyond this point. Warren Gretz, NREL/PIX05640 Figure 9. Parabolic-trough collectors at Paul Beck Recreation Center in Aurora, Colorado, heat water for a swimming pool and related facilities. industrial-size water heating that uses nat- ural gas, propane, or fuel oil is typically only about 60% efficient and 75% to 80% efficient at best.
Electric water heating is typically about 90% efficient and can be 98% efficient. However, producing that electricity from fossil fuels is generally only 30% to 40% efficient. Furthermore, reduc- ing fossil-fuel use for water heating not only saves fossil fuel stock, but eliminates the air pollution and climate-changing gas emis- sions associated with burning those fuels. heat source. One type of system that can be operated efficiently using solar energy is the water/lithium bromide absorption chiller. Water at very low pressure is used as the refrigerant and lithium bromide is used as the absorbent.
Absorption chillers can be either direct-fired (usually by natural gas) or indirect-fired (by hot water or steam). They are produced commercially by heat- ing, ventilation, and air-conditioning (HVAC) equipment manufacturers in the United States and abroad. Several major air-conditioning equipment manufacturers make indirect-fired absorption chillers that can run on solar-heated water or steam. (See manufacturer’s information on page 20.) Two varieties of indirect-fired absorption chillers are available—single-effect and double-effect.
(See Appendix C.) Single- effect absorption cooling systems can use solar-heated fluid (usually water) at temper- atures of 77˚C–99˚C (170˚F–210˚F) to drive absorption refrigeration cycles. Double- effect systems are more efficient but require temperatures of about 177˚C (350˚F), usu- ally in the form of steam. Parabolic-trough collectors efficiently heat water to these required temperatures. Double-effect absorption coolers usually have coefficients of performance (COPs) of about 0.9 to 1.2, compared to about 0.6 to Variations As described in this FTA, domestic water heating is the principal application for para- bolic-trough collectors.
They can, however, be used for other water-heating needs. At the Paul Beck Recreation Center in Aurora, Colorado, these collectors provide water for an indoor swimming pool, showers, and related facilities. (Smaller swimming pools— particularly outdoor ones—can generally be more effectively heated by inexpensive, low-temperature collectors that are noncon- centrating and uninsulated.) Parabolic-trough solar collectors can also power air-conditioning. Air-conditioning systems are typically based on a compres- sion cycle driven by an electric motor.
But an alternative, mechanically simpler system is based on an absorption cycle driven by a Benefits Solar water heating reduces the amount of water that must be heated by a facility's conventional water-heating system; it thereby directly substitutes renewable energy for electricity or fossil-fuel energy and cuts utility bills. Each unit of energy delivered to heat water with a solar heating system yields an even greater reduction in use of fossil fuels. For example, 11