Thermal Energy StorageDescription: Thermalenergy storage (TES)systems cool a storagemedium and then usethat cold medium tocool air at a later pointin time. Using thermalstorage can reduce the size and initial cost of cooling systems, lowerenergy costs, and reduce maintenance costs. If electricity costs moreduring the day than at night, thermal storage systems can reduce utilitybills further. Systems can be sized to eliminate compressor energy useduring periods when electricity is most expensive, but most systems aredesigned to simply augment mechanical cooling in order to limit peakdemand. The latter type of system can be considerably smaller than theformer.

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

Applications: TES may be economical if one or more of the following condi-tions exist:

♦ High utility demand costs

♦ Utility time-of-use rates (some utilities charge more for energy use duringpeak periods of day and less during off-peak periods)

♦ High daily load variations

♦ Short duration loads

♦ Infrequent or cyclical loads

♦ Capacity of cooling equipment has trouble handling peak loads

♦ Rebates are available for load shifting to avoid peak demand

Effective applications of thermal energy storage include:

♦ Electrical power use management by shifting the cooling load to off-peakhours and reducing peak load

♦ Reducing required capacity of building and process cooling systems, orhelping existing cooling equipment to handle an increased load

Energy Efficiency

FactsheetThermal Energy Storage

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Water storage systems are often used in new large cooling system applications in conjunction withcogeneration and/or district energy systems. Water-ice storage is the most common cooling storagein smaller applications. Because latent heat storage (phase change between water and ice) has asmaller 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.The Dallas Veterans Affairs Medical Center installed a 24,628 ton-hours chilled water TES thatresulted in a reduction in demand of 2,934 kW and a reduction in annual electricity cost of$223,650. The local utility provided $500,000 of the total cost of $2.2 million required for designand installation. Savings resulting from installation of the thermal storage technology will allowthe VA to recoup its investment within 7 years.

Performance/Costs: Thermal energy storage systems are installed for two major reasons: lowerinitial project costs and lower operating costs. Initial cost may be lower because distribution tem-peratures are lower and equipment and pipe sizes can be reduced. Operating costs may be lowerdue to smaller compressors and pumps as well as reduced time-of-day or peak demand utility costs.

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

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

FactsheetThermal Energy Storage

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

Table 1 shows the benefit of using thermal storage to shift chiller use to a time when the rest of thefacility is unoccupied. The chiller load (in terms of ton-hours delivered) is the same, but demandcharges for the chiller would be eliminated. The table shows the effect of two different utilitydemand rates ($6 and $12 per kW).

Availability: Thermal energy storage equipment and installations are available from a number ofsuppliers. For a list of equipment manufacturers, contact the Thermal Storage Applications Re-search Center (TSARC) or go to the Energy User News Buyers Guide website (see “For AdditionalInformation” below).

For Additional Information:Heating, Ventilating, Air Conditioning, and Refrigerating Center and Thermal Storage Appli-cations Research Center (TSARC)These centers, located at the University of Wisconsin, perform research to develop technologies thatincrease the efficiency of systems, and offer technical assistance and education to industry.http://www.engr.wisc.edu/centers/tsarc/tsarc.html

Energy User News Buyers GuideThis list of manufacturers can be searched by technology type, e.g. search for Thermal EnergyStorage.http://www.energyusernews.com/buyers.htm

Thermal storage and deregulationFrom a paper published in the April 1998 ASHRAE Journalhttp://www.pwi-energy.com/main/whitepapers/tsdereg.htm

Benefits and savings of chiller replacement & various thermal storage projectsU.S. Army case studieshttp://www.cecer.army.mil/ul/gascool/storage.htm

Thermal Energy Storage at a Federal FacilityDescribes the benefits of a project in which the Dallas Veterans Administration Medical Center andTexas Utilities Electric Company join in an unprecedented partnership to lower energy costs.http://www.eren.doe.gov/femp/financing/fed_facility_439.htmlIce Storage Retrofit for Rooftop Air ConditioningFederal Energy Management Program (FEMP) New Technology Demonstration Program evaluatesice storage retrofit for rooftop air conditioning.http://www.eren.doe.gov/femp/prodtech/icesum.html © 2003 Washington State University Cooperative

Extension Energy Program. This publicationcontains material written and produced for publicdistribution. You may reprint this written material,provided you do not use it to endorse a commercialproduct. Please reference by title and creditWashington State University Cooperative ExtensionEnergy Program. WSUCEEP00-127