Thermal energy storage (TES) systems cool a storage medium and then use that cold medium to cool air at a later point in time. Using thermal storage can reduce the size and initial cost of cooling systems, lower energy costs, and reduce maintenance costs. If electricity costs more during the day than at night, thermal storage systems can reduce utility bills further. Systems can be sized to eliminate compressor energy use during periods when electricity is most expensive, but most systems are designed simply to augment mechanical cooling in order to limit peak demand. The latter type of system can be considerably smaller than the former.

Two forms of TES systems are currently used. The first system uses a material that changes phase, most commonly water and ice. The second type just changes the temperature of a material, most commonly water.

TES may be economical if one or more of the following conditions exist: High energy demand costs Energy time-of-use rates High daily load variations Short duration loads Infrequent or cyclical loads Capacity of cooling equipment has trouble handling peak loads

Effective applications of thermal energy storage include:

• Electrical power use management by shifting the cooling load to off-peak hours and reducing peak load
• Reducing required capacity of building and process cooling systems, or helping existing cooling equipment to handle an increased load.

Water storage systems are often used in new large cooling system applications in conjunction with cogeneration and/or district energy systems. Water-ice storage is the most common cooling storage in smaller applications. Because latent heat storage (phase change between water and ice) has a smaller volume, it is often chosen for retrofit applications with limited space.

In general, the buildings that offer the highest potential are offices, retail, and medical facilities.

Thermal energy storage systems are installed for two major reasons: lower initial project costs and lower operating costs. Initial cost may be lower because distribution temperatures are lower and equipment and pipe sizes can be reduced. Operating costs may be lower due to smaller compressors and pumps as well as reduced time-of-day or peak demand energy costs.

The economics of thermal storage is very site- and system-specific. A feasibility study is generally required to determine the optimum design for a specific application. Several examples exist of effective TES systems that were installed for less cost than conventional alternatives and that also provided significant energy and energy cost reductions.

TES projects often profit from unexpected benefits that are secondary to the primary reason for an action. For example, a well designed TES air conditioning application may experience reduced chiller energy consumption, lower pump horsepower, smaller pipes, high reliability, better system balancing and control, and lower maintenance costs.

Determining electrical cost savings from shifting chiller operation from daytime to night-time (or off-peak operation) can be complicated, depending upon the rate structure. Your rate may have time-of-day peak rates as well as differing peak demand rates. Without thermal storage, a chiller will typically operate during times of peak electrical demand, and thus be included in the peak monthly demand charge. Your greatest savings will occur when you shift chiller operation to off-peak times.

Thermal energy storage equipment and installations are available from a number of suppliers. For a list of equipment manufacturers, contact the Thermal Storage Applications Research Center (TSARC).

Source: Pacific National Laboratories