Cooling thermal energy storage

Here's a strategy to reduce your cooling energy costs while maintaining a cool, comfortable working environment.

Summer air conditioning bills are composed of two components; an electric demand charge and an electric usage charge. The usage and demand charges are further divided into on- and off-peak periods. The peak operating period of electric air conditioning systems falls in the high cost demand and usage periods (i.e., the summer afternoon). Thermal energy storage systems are designed to shift the peak operating period of electric air conditioning systems to the low cost nighttime.

How Does it Work?

Air conditioning systems produce cooling by removing heat from a chilled water network or directly from an air steam. Most air conditioning systems produce a cooling effect at precisely the time that cooling is needed in a building or room.

Thermal energy storage systems operate like air conditioning systems except that they remove heat from an intermediate substance at a time when the building doesn't actually need cooling. In this way, cooling or the absence of heat is actually stored before there is a need for cooling. The intermediate substance is normally water, ice or eutectic salt solutions. The most popular thermal storage medium is ice.

When 144 BTUs of heat is removed from one pound of water at 32°F, one pound of ice at 32°F is formed. If 144 BTUs of heat is subsequently added to the ice, one pound of water at 32°F is formed. Cooling thermal energy storage using ice as the storage medium works in just this fashion. At night, heat is removed from water to produce ice at a time when the cost of electricity is lower and the building doesn't require cooling. This period is referred to as the changing cycle. During the day when the building requires cooling, heat is removed from the building and added to the ice. This period is referred to as the discharge cycle. The heat addition melts the ice and forms water which will be reused during the next charging cycle.

The advantage of this cooling scheme is the energy consumer of cooling systems, namely the compressor motor, is operated at night when electrical costs are lowest instead of during the afternoon when electrical costs are highest.

The previous example illustrates one design possibility. Another common design calls for the compression system to operate 24 hours per day to provide stored cooling at night and to partially meet the cooling load during the daytime. This design requires the least investment.

How Can it Be Used?

Cooling thermal storage systems are confined to chilled water systems since a fluid is required to get the cooling capacity from the storage system to the building air distribution system. Although chilled water systems are usually confined to large buildings and conventionally designed air conditioning systems, chilled water systems as part of a thermal storage system are now being designed for smaller buildings.

Is a Cooling Thermal Storage System Right For You?

Cooling thermal storage systems are generally good for any new facility that has a large cooling load during the daytime and little or no cooling load at night. For retrofit situations, cooling thermal storage is almost never justified unless it is necessary to purchase a new cooling system because of the age or inadequacy of the current air conditioning unit. For both situations, it is also necessary for your facility to be on some type of electric demand meter.