Refrigeration Cycle TES Technology Overview

For refrigeration and air conditioning applications, all TES systems operate on the same fundamental concept: the cooling equipment produces a refrigeration effect

Fig. 35-10 Schematic Representation of Indirect Water-Side Free-Cooling System.

(direct or indirect), either to meet the load or to be added to storage. The storage system either accepts excess cooling capacity or supplies it to meet the load. The load may be served directly by the cooling system or the storage system. Figure 35-11 is a simplified schematic representation of a basic TES system. Descriptions of the three most common cool storage systems follow, and a fourth is briefly mentioned.

Liquid Systems

Liquid (water) storage is usually the simplest for heating and cooling applications. In cooling applications, liquid storage may be applied with a wider range of refrigeration systems than can ice systems, including LiBr-type absorption chillers. The high specific heat of water, which is approximately 1 Btu/lbm • °F (1.16 Wh/kg • °K), makes it well suited for TES. Above ground tanks are commonly made of steel and buried tanks are commonly made of concrete.

The principle advantages of chilled water storage systems are simplicity and the ability to operate chillers at or close to normal supply (suction) temperatures. The ability to operate at relatively high supply temperatures results in greater chiller performance.

A principle disadvantage of water storage is the immense storage equipment size and space requirement. Given the temperature differential range between stored and supplied chilled water, which is limited to 10 or 20°F (6 or 11°C), chilled water storage requires a very large amount of storage volume. With hot water storage systems, however, a far greater temperature differential can be used, resulting in a lesser volume requirement for equivalent amounts of stored energy.

Another disadvantage of water storage systems is the tendency for mixing or temperature blending of the water returning to storage and the colder stored water. There are

Fig. 35-11 Schematic Representation of Basic TES System.

several techniques to limit mixing, although they add to system cost. Water storage systems are also subject to problems associated with lower than desired return water temperatures and the potential for leaks in large concrete installations. They also require chemical treatment and filtering systems.

Ice Systems

The high heat of fusion of ice — 144 Btu/lbm (0.093 kWh/kg) — makes it an excellent storage medium. Three common types of ice storage systems are ice builder systems, ice harvesting systems, and ice slurry systems.

• Ice builder systems may operate with DX systems or brine (typically 25% ethylene glycol/75% water) chiller systems. The refrigerant or brine coils, which are submerged in a water storage tank, freeze the water in their immediate vicinity. The water then melts when warm return water is introduced to the tank. Primary concerns with any type of refrigerant coil system are 1) minimizing bridging of ice between coils to promote water circulation and increase exposed ice surface area and 2) creation of the minimal necessary ice thickness to reduce compressor loading during the build cycle. Typical storage systems use plastic containers filled with deionized water and an ice nucleating agent placed in a steel, concrete, fiberglass, or polyurethane tank. The inventory of available ice can be determined by measuring the water volume and adjusting for the expansion to ice (about 9%).

• Ice harvesting systems consist of an ice producing section and an ice/water storage section. Ice is accumulated on the outside of the evaporator in the ice producing section and then drops to storage as hot gas is passed through the evaporator to break the bond between the ice and the evaporator wall. Typical cycle intervals are about 30 minutes. Since the ice floats on water and does not completely displace its own volume, inventory of available ice must be determined by water conductivity or heat balance methods.

• Ice slurry systems use a brine solution, which is cooled to its freezing point where ice crystals form in its fluid film. The slurry portion is pumped to storage, where it forms a floating porous ice pack, and the remaining concentrated solution is re-cooled. The process is continuous, with the brine solution continually circulating through the evaporator until the desired stored capacity is reached as indicated by temperature measurement of the solution in storage at equilibrium conditions. An advantage of this system is that no defrost cycle is required.

A primary advantage of ice versus water storage is that size requirements are much lower. Storage space requirements for ice systems are generally less than one fifth those of chilled water systems. Ice systems are also less subject to problems associated with lower than desired return water temperatures.

A major disadvantage of ice systems is decreased cooling system performance due to the need to produce the lower refrigerant (suction) temperatures required for ice making. As discussed in Chapter 37, the performance of a vapor compression system depends on the saturated suction and discharge pressures of the compressor. The suction temperature is a function of the inlet and outlet supply temperature and the discharge is a function of ambient conditions. As the entering water temperature drops, the capacity of the system drops and the temperature differential increases. This results in a requirement for more energy input per ton-h (or kWhr) output. Another disadvantage is that ice storage tends to present more complications than water storage with respect to generation, storage, discharge, and control.

Eutectic Salts

Eutectic salts change phase at various temperatures. The most commonly used eutectic salt consists primarily of sodium sulfate, which has a heat of fusion of 41 Btu/lbm (0.026 kWh/kg) and freezes and melts at about 47°F (8°C). Other types of eutectic salts may have a higher heat of fusion. Systems typically consist of containers stacked in a tank with small clearances for chilled water to circulate. A principal advantage is that cool storage can be generated with typical chilled water temperatures in excess of 40°F (4°C). Their advantage versus ice is that they can operate more efficiently because they do not have to produce such low temperatures. Their advantage over water is that they require half of the space (significantly more space than ice). Their primary disadvantage is significantly higher capital cost than conventional water or ice storage systems.

Carbon Dioxide (CO2)

Although not common, CO2 systems are sometimes useful for very cold applications, such as commercial food freezing. CO2 has a triple thermodynamic equilibrium point of -70°F (-57°C) at about 60 psig (5.2 bar), which makes it a somewhat unique storage medium because it is used as a solid, liquid, or gas. It can be used as a vapor compression refrigerant, a liquid refrigerant, and a solid storage medium. Though not as dense a storage medium as ice, CO2 typically requires less than one-third the storage space of chilled water. The advantage of a CO2 system is its compatibility with very low temperature refrigeration applications. The disadvantages are complexity of operation and significantly higher capital cost than conventional water or ice storage systems.

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