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The larger the thermal storage system and the capacity of refrigeration compressors, the greater the savings. Actually, selection of the size of the storage system is better determined according to life-cycle cost, and system reliability should also be considered.
Ice Storage and Chilled Water Storage
In the late 1970s, engineers began experimental thermal storage applications in buildings. Throughout the late 1990s, there are several thousand thermal storage systems operated in various commercial buildings including offices, shopping centers, schools, and hospitals and industrial applications in the United States.
Two thermal storage media are widely used for air conditioning systems: ice storage and chilled water storage. At a temperature difference of 18°F (10°C), 1 lb (2.2 kg) of chilled water can store 18 1 18 Btu (19 kJ) of thermal energy, whereas 1 lb of ice can store 1 144 60 35 169 Btu (178 kJ). If the density of water is 62.3 lb / ft3 (997 kg /m3) and the density of ice is 57.5 lb / ft3 (920 kg /m3), for the same stored cooling capacity, the storage volume for ice is only about 0.12 that of the chilled water. In addition, ice storage systems generally provide chilled water at a temperature of 34 to 35°F (1.1 to 1.7°C) to produce cold supply air between 42 and 49°F (5.6 and 9.4°C). Ice storage systems incorporating cold air distribution significantly reduce the volume flow rate of supply air, so air-side fan energy consumption and initial investment drop accordingly.
Generally, ice storage systems have a rather low incremental capital cost compared with
conventional air conditioning systems without thermal storage. Ice storage systems can be easily incorporated with cold air distribution. A lower incremental capital cost than that of chilled water storage systems and the incorporation with cold air distribution are two main benefits of the ice storage system. In addition to the ice storage and chilled water storage systems, phase-change material storage systems are sometimes used. The most common phase-change material used for cool thermal storage is a mixture of inorganic salts, water, and nucleating and stabilizing agents which melts and freezes at 47°F (8.3°C). Phase-change materials have high discharge temperatures and a high storage tank volume of 6.0 ft3 /ton h (0.048 m3 /kWh) instead of 2.4 to 3.3 ft / ton h (0.019 to 0.027 m3 /kWh) for other ice storage systems; therefore, they have limited applications in commercial buildings.
Currently, ice storage and chilled water storage systems can be classified into the following categories:
Ice-on-coil, internal-melt ice storage system (IMISS)
Ice-on-coil, external-melt ice storage system (EMISS)
Encapsulated ice storage system (EISS)
Ice-harvesting ice storage system (IHISS)
Stratified chilled water storage system (SCWSS)
Chilled water storage systems are often large-capacity storage systems. In newly installed chilled water storage systems, the stratified chilled water storage system is the most widely used.
31.2 ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEMS
System Description
An ice-on-coil, internal-melt ice storage system uses brine flowing inside coils to make ice and to melt ice in the water that surrounds the coil. The central plant (cooling) and a chilled water or brine-incorporated ice storage system of an ice-on-coil, internal-melt ice storage system consists of the following components: chillers, ice storage tanks, chiller pumps, building pumps, controls, piping, and fittings as well as AHUs, terminals, return air system, smoke control systems, and mechanical exhaust systems.
Centrifugal, screw, and reciprocating chillers are usually used in ice-on-coil internal-melt ice storage systems depending on the size of the plant and types of condenser (water-cooled, aircooled, or evaporatively cooled) used. In locations where the outdoor air temperature during nighttime off-peak hours drops 20°F (11.3°C) lower than the daytime maximum temperature, air-cooled chillers may sometimes be more efficient than water-cooled chillers. Figure 31.2 is a schematic diagram of a typical ice-on-coil, internal-melt ice storage system for an office building near Dallas, Texas.