Thermal Energy Storage for Space Cooling--FTA

F E D E R A L E N E R G Y M A N A G E M E N T P R O G R A M

Application DomainThe potential for cost-effective applica-tion of cool storage systems of one type oranother exists in most buildings with a spacecooling system. Originally, cool storage technol-ogy was developed for integration with chilledwater cooling systems that typically serve largerbuildings. More recent cool storage develop-ments have included technologies designed forintegration with roof-mounted, direct-expansion(DX) cooling systems. Residential-sized coolstorage technologies, including smaller versionsof the equipment designed for the roof-mountedDX application, have also been developed, butcost economies-of-scale have been difficult toovercome in the residential market.

Although originally developed to shift electricaldemand to off-peak periods (from an electricutility’s perspective) and to take advantage oflow-cost off-peak electric rates (from an end-user’s perspective), many applications can alsoresult in lower first costs and/or higher systemefficiency compared to non-storage systems.Therefore, while a large differential betweenon-peak and off-peak kWh charges or a highdemand charge definitely improves cool storageeconomics, cost-effective applications also exist

without these ben-efits. Still, not everycooling system pre-sents a cost-effectiveapplication, so care-ful consideration ofsite-specific condi-tions is warranted todetermine whethercool storage makessense or not, whichcool storage technol-ogy is best, and theoptimum configu-ration for a specifictechnology.

Thermal Energy Storage for Space CoolingTechnology for reducing on-peak electricity demand and cost

Internet: http://www.eren.doe.gov/femp

FederalTechnologyAlert

A publication seriesdesigned to speed theadoption of energy-efficient and renewabletechnologies in theFederal sector

Prepared by theNew TechnologyDemonstrationProgram

DOE/EE-0241

No portion of this publi-cation may be alteredin any form without priorwritten consent from theU.S. Department of Energyand the authoring nationallaboratory.

Thermal energy storage for space cooling, alsoknown as cool storage, chill storage, or cool ther-mal storage, is a relatively mature technologythat continues to improve through evolutionarydesign advances. Cool storage technology canbe used to significantly reduce energy costs byallowing energy-intensive, electrically drivencooling equipment to be predominantly oper-ated during off-peak hours when electricity ratesare lower. In addition, some system configurationsresult in lower first costs and/or lower operatingcosts compared to non-storage systems.

A survey of approximately 25 manufacturersproviding cool storage systems or componentsidentified several thousand current installations,but less than 1% of these were at Federal facili-ties. With the Federal sector representing nearly4% of commercial building floor space and 5% ofcommercial building energy use, Federal utiliza-tion would appear to be lagging. The potentialcost savings resulting from the application ofcool storage systems in the Federal sector is esti-mated to be $50 million per year. Thus, thisFederal Technology Alert has been written to rein-troduce the concept and make Federal energymanagers aware of the latest technologies andenergy- and cost-saving opportunities.

Modular ice-on-coil storage tanks with condenser units above

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Design VariationsThere are many different types of coolstorage systems representing differentcombinations of storage media, chargingmechanisms, and discharging mecha-nisms. The basic media options are water,ice, and eutectic salts. Ice systems can befurther broken down into ice harvesting,ice-on-coil, ice slurry, and encapsulatedice options. Ice-on-coil systems may beinternal-melt or external-melt and maybe charged and discharged with refrig-erant or a single-phase coolant (typicallya water/glycol mixture). Independentof the technology choice, cool storagesystems can be designed to providefull storage or partial storage, with load-leveling and demand-limiting options forpartial storage. Finally, storage systemscan be operated on a chiller-priority orstorage-priority basis whenever the cool-ing load is less than the design conditions.

Chilled water storage systems rely solelyon the sensible (i.e., no phase change orlatent energy) heat capacity of waterand the temperature difference betweensupply and return water streams goingto and from the cooling load. Ice-on-coilsystems come in several variations, asnoted above. In all variations, ice isformed on a heat transfer surface (generi-cally referred to as a “coil,” whatever theactual configuration or material) with-out being released during the chargingmode and melted away during the dis-charge mode. Ice-harvesting systemsform ice on coils or other refrigerantevaporating surfaces and periodicallyrelease the ice into a storage tank thatcontains a mixture of ice and water. Iceslurry systems produce small particlesof ice within a solution of glycol andwater, resulting in a slushy mixture that


DisclaimerThis report was sponsored by the United States Department of Energy, Office of Federal Energy ManagementPrograms. Neither the United States Government nor any agency or contractor thereof, nor any of their employees,makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, complete-ness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would notinfringe privately owned rights. Reference herein to any specific commercial product, process, or service by tradename, mark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommenda-tion, or favoring by the United States Government or any agency or contractor thereof. The views and opinions ofauthors expressed herein do not necessarily state or reflect those of the United States Government or any agency orcontractor thereof.

can be pumped. Encapsulated ice sys-tems consist of water contained in plas-tic containers surrounded by coolant, allcontained within a tank or other storagevessel. Eutectic salt systems are similarto encapsulated ice systems, but theplastic enclosures contain a eutectic saltinstead of water.

Full storage systems are designed tomeet all on-peak cooling loads fromstorage. Partial storage systems meetpart of the cooling load from storage andpart directly from the chiller during theon-peak period. Load-leveling partialstorage is designed for the chiller tooperate at full capacity for 24 hours onthe peak demand day. Demand limit-ing partial storage represents a middleground between full storage and load-leveling partial storage where chilleroperation is reduced but not eliminatedduring the on-peak period. Storage pri-ority and chiller priority are two alterna-tive operating strategies for cool storagesystems with partial storage designs. Asthe names imply, cooling is preferentiallyprovided from storage with storage pri-ority operation and directly from thechiller with chiller priority operation.

Where to ApplyCool storage will reduce the averagecost of energy consumed and can poten-tially reduce the energy consumptionand initial capital cost of a cooling systemcompared to a conventional cooling sys-tem without cool storage. While mostbuilding space cooling applications arepotentially attractive candidates, theprospects will be especially attractiveif one or more of the following condi-tions exists.

• Electricity energy charges vary sig-nificantly during the course of a day.

• Electricity demand charges are highor ratcheted.

• The average cooling load is signifi-cantly less than the peak cooling load.

• The electric utility offers other incen-tives (besides the rate structure) forinstalling cool storage.

• An existing cooling system is expanded.• There is new construction.• Older cooling equipment needs

replacing.• Cold air distribution benefits can be

captured.

What to AvoidIn general, applications lacking theconditions identified above should beavoided. In addition, the followingconditions should also be avoided.

• Lack of operation and maintenanceexperience or training with systemequipment, especially where built-uprefrigeration systems are used ratherthan packaged chillers.

• Lack of operator training on operat-ing and control strategies for mini-mizing cooling system life-cycle costs.

• Sites where the space available forcool storage equipment is limitedor has other, more valuable uses.

• Limited resources for engineeringfeasibility studies and system design.Cool storage systems are inherentlymore complicated than non-storagesystems and extra time will berequired to determine the optimumsystem for a given application.

There are many differenttypes of cool storage sys-tems representing differentcombinations of storagemedia, charging mecha-nisms, and dischargingmechanisms. The basicmedia options are water,ice, and eutectic salts. Icesystems can be furtherbroken down into ice har-vesting, ice-on-coil, iceslurry, and encapsulatedice options. Ice-on-coil sys-tems may be internal meltor external melt and maybe charged and discharged

with refrigerant or a single-phase coolant (typi-cally a water/glycol mixture). Independent ofthe technology choice, cool storage systems canbe designed to provide full storage or partialstorage, with load-leveling and demand-limitingoptions for partial storage. Finally, storage systemscan be operated on a chiller-priority or storage-priority basis whenever the cooling load is lessthan the design conditions.

The first section describes the basic types of coolstorage technologies and cooling system integra-tion options. The next three sections define thesavings potential in the Federal sector, presentapplication advice, and describe the performanceexperience of specific Federal users. A step-by-stepmethodology illustrating how to evaluate coolstorage options is presented next, followed by acase study of a GSA building using cool storage.Latter sections list manufacturers, selected Federalusers, and reference materials. Finally, the appen-dixes give Federal life-cycle costing proceduresand results for a case study.

Thermal Energy Storage for Space CoolingTechnology for reducing on-peak electricity demand and cost

FederalTechnologyAlert

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A stratified chilled water storage tank with cooling towers on the left

AbstractCool storage technology can be used to signifi-cantly reduce energy costs by allowing energy-intensive, electrically driven cooling equipmentto be predominantly operated during off-peakhours when electricity rates are lower. In addi-tion, some system configurations may result inlower first costs and/or lower operating costs.Cool storage systems of one type or anothercould potentially be cost-effectively applied inmost buildings with a space cooling system. Asurvey of approximately 25 manufacturers pro-viding cool storage systems or components iden-tified several thousand current installations, butless than 1% of these were at Federal facilities.With the Federal sector representing nearly 4% ofcommercial building floor space and 5% of com-mercial building energy use, Federal utilizationwould appear to be lagging. Although currentapplications are relatively few, the estimatedpotential annual savings from using cool storagein the Federal sector is $50 million.

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Contents

Abstract ........................................................................................................................... 1

About the Technology .................................................................................................... 5Application DomainEnergy-Saving MechanismCold Air DistributionOther BenefitsVariations: Storage Media and MechanismsVariations: Design and Operating StrategiesInstallation

Federal Sector Potential .................................................................................................. 12Estimated Savings and Market PotentialLaboratory Perspective

Application ..................................................................................................................... 13Application ScreeningWhere to ApplyWhat to AvoidEquipment IntegrationMaintenance ImpactEquipment WarrantiesCostsUtility Incentives and Support

Technology Performance ................................................................................................ 16Evaluating Cool Storage Systems .................................................................................. 17

Cooling RequirementsIdentifying AlternativesScreening AlternativesRefining the Evaluation

Case Study ...................................................................................................................... 22Facility DescriptionExisting Technology DescriptionNew Technology Equipment SelectionSavings PotentialLife-Cycle CostImplementation and Post-Implementation Experience

The Technology in Perspective ....................................................................................... 23The Technology’s DevelopmentTechnology Outlook

Manufacturers ................................................................................................................ 24

Who is Using the Technology ......................................................................................... 25

For Further Information ................................................................................................. 26AssociationsDesign and Installation Guides

References ....................................................................................................................... 26

Appendix A: Federal Life-Cycle Costing Procedures and the BLCCSoftware ................................................................................................ 31

Appendix B: QuickBLCC Results for Case Study .................................................... 32

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About the TechnologyThermal energy storage for space cool-ing, also known as cool storage, chillstorage, or cool thermal storage, is a rela-tively mature technology that continuesto improve through evolutionary designadvances. Cool storage technology canbe used to significantly reduce energycosts by allowing energy-intensive,electrically driven cooling equipmentto be predominantly operated duringoff-peak hours when electricity ratesare lower. In addition, some systemconfigurations result in lower first costsand/or lower operating costs. Unfortu-nately, cool storage technologies havebeen underutilized in the Federal sectorcompared to the private sector. Thus,this Federal Technology Alert has beenwritten to reintroduce the concept andmake Federal energy managers awareof the latest technologies and energy-and cost-saving opportunities.

Cool storage technologies come in manydifferent forms, each with their pros andcons. The storage media is most com-monly water (with “cold” stored in theform of ice, chilled water, or an ice/waterslurry), but other media (most notablyeutectic salts) have also been used. Stor-age media can be cooled (charged) byevaporating refrigerant or a secondarycoolant (typically a water/glycol mix-ture). Discharge is usually accomplisheddirectly via circulating water or indirectlyvia secondary coolant. At least one sys-tem has been developed that dischargesstorage via circulating refrigerant.

Application DomainCool storage systems of one type or an-other could potentially be cost-effectivelyapplied in most buildings with a spacecooling system. Originally, cool storagetechnology was developed for integra-tion with chilled water cooling systemsthat typically serve larger buildings.More recent cool storage developmentshave included technologies designed forintegration with roof-mounted, direct-expansion (DX) cooling systems. Resi-dential-sized cool storage technologies,

including smaller versions of the equip-ment designed for the roof-mounted DXapplication, have also been developed,but cost economies-of-scale have beendifficult to overcome in the residentialmarket.

Although originally developed to shiftelectrical demand to off-peak periods(from an electric utility’s perspective)and to take advantage of low-cost off-peak electric rates (from an end-user’sperspective), many applications canalso result in lower first costs and/orhigher system efficiency compared tonon-storage systems. Therefore, whilea large differential between on-peakand off-peak kWh charges or a highdemand charge definitely improvescool storage economics, cost-effectiveapplications also exist without thesebenefits. Still, not every cooling systempresents a cost-effective application, socareful consideration of site-specificconditions is warranted to determinewhether cool storage makes sense ornot, which cool storage technology isbest, and the optimum configurationfor a specific technology.

A survey of approximately 25 manufac-turers providing cool storage systems orcomponents identified several thousandcurrent installations, but less than 1% ofthese were at Federal facilities. Withthe Federal sector representing nearly4% of commercial building floor spaceand 5% of commercial building energyuse, Federal utilization would appearto be lagging.

Energy-Saving MechanismCool storage systems are not commonlythought of as energy-saving technolo-gies. No matter how well insulated,thermal storage systems inevitably suf-fer some losses as energy flows fromwarmer bodies to cooler bodies. In addi-tion, both cool and warm water is com-monly stored in the same storage tankin chilled water systems to save on tankcosts. Mixing is minimized by injectingand removing water from different halvesof the tank via specially designed piping

to take advantage of natural differencesin water density and buoyancy at differ-ent temperatures. Still, some mixing andloss of cooling capability are inevitable.

Historically, the driving force for devel-oping cool storage has been reduction ofon-peak electric demand and the corre-sponding reduction of electricity costs.While this is still important, and maybe the most important factor affectingapplication cost-effectiveness, energysavings are possible, and can be a sig-nificant benefit when the entire coolingsystem, and not just the storage mediaand vessel are considered.

Besides heat gain by the storage media,chillers in cool storage systems operateat lower evaporator temperatures, whichincreases energy consumption if otherconditions remain the same. This isparticularly true for ice storage systems,which require the lowest evaporatortemperatures. The impact of lowerevaporator temperatures is partially ortotally offset, however, by the lower con-densing temperatures generally experi-enced when operating a chiller at nightrather than during the day. In most partsof the country, dry-bulb temperaturesare about 20°F lower and wet-bulb tem-peratures 5°F lower at night than duringthe day (MacCracken 1993). Thus, night-time operation improves the efficiencyof all chillers, but especially improvesthe efficiency of air-cooled chillers, wherethe condensing temperature is controlledby ambient dry-bulb temperature. Chillerefficiency is also improved with storageby allowing more continuous operationat outputs closer to full capacity, thusminimizing part-load losses. In retrofitsituations, adding storage to meet peakcooling demands allows the least effi-cient chillers to be left off or run muchless, further increasing savings.

Cool storage systems, with separatecharge and discharge cycles, will gener-ally require more pumping. This poten-tial disadvantage can be minimized,however, by increasing the differencebetween water supply and return tem-perature by a few degrees, thus reducing

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the volume of water that must be circu-lated. Pumping energy may also beminimized with variable-speed drives.

The energy savings possible with coolstorage will vary significantly from siteto site, depending on the load profileand the specific cooling system equip-ment employed. For example, Caldwelland Bahnbleth (1997) reported energysavings ranging from 1–27% for coolingsystems with chilled water storage,depending on the load profile. Addi-tional discussion of the energy impactsof cool storage can be found in Bahnblethand Joyce (1995), Strutz (1995), andDuffy (1992).

Cold Air DistributionIce storage systems also present anopportunity for energy savings viacold air distribution. The supply ofnear-freezing water to air-handlingunits allows return air to be cooled toa lower temperature. Primary air isdistributed at 45°F in a cold air systemcompared to 55°F in a conventionalsystem, which allows air flow to bereduced by about 40% (ASHRAE 1993).The colder primary air is fully mixedwith a portion of the return air to achievethe desired room delivery temperature.Thus, smaller, less costly air handlersand ducting may be installed, with pro-portional reductions in fan power con-sumption. Where growing cooling loadshave exceeded the capacity of existingair distribution systems, cold air distri-bution could be implemented to increasecapacity without significant renovationto the ducting and air-handling system.

Cooling the primary air to 45°F willalso lower conditioned space relativehumidity from 60% to 35%, which gen-erally improves the perceived comfortof occupants. This effect may allow a3°F increase in the dry-bulb temperatureset point with the same perceived com-fort (MacCracken 1994).

Reducing the size of air-handling equip-ment lowers construction costs for multi-story buildings by reducing the height

system. Included here are costs associ-ated with refrigerant replacement andcooling tower cleaning and water treat-ment (ASHRAE 1993).

Variations: Storage Mediaand MechanismsThere are many different types of coolstorage systems representing differentcombinations of storage media, charg-ing mechanism, and discharging mecha-nism. The basic media options are water,ice, and eutectic salts. Ice systems can befurther broken down into ice harvesting,ice-on-coil, ice slurry, and encapsulatedice options. Ice-on-coil systems may beinternal melt or external melt and maybe charged and discharged with refrig-erant or a single-phase coolant (typicallya water/glycol mixture). Independent ofthe technology choice, cool storage sys-tems can be designed to provide full stor-age or partial storage, with load-levelingand demand-limiting options for partialstorage. Finally, storage systems can beoperated on a chiller-priority or storage-priority basis whenever the cooling loadis less than the design conditions.

Chilled water storage systems rely solelyon the sensible (i.e., no phase change orlatent energy) heat capacity of waterand the temperature difference betweensupply and return water streams goingto and from the cooling load. As a result,the storage volume required is greaterthan for any of the ice or eutectic saltoptions. However, using water elimi-nates the need for secondary coolantsand heat exchangers and standard waterchillers can be used without signifi-cantly degraded performance or capac-ity. Water is typically cooled to between39 and 44°F, or slightly lower than for astandard chilled water system withoutstorage. The return water temperaturemay be increased slightly as well, butmust remain low enough to ensureadequate indoor humidity control.Maximizing the difference between cool-ing water supply and return tempera-tures maximizes the sensible energystorage capacity per unit of water and

required per floor for HVAC systems.The cumulative height savings allowedthe installation of an extra floor withinthe same total building height at ahigh-rise office building in Bellevue,Washington (Hasnain 1998).

Early problems with cold air distributionincluded fan-powered mixing boxesthat negated much of the energy savings,poor air diffusers that created comfortproblems, and condensation problemson some surfaces. Improved designshave essentially eliminated the formertwo problems and condensation prob-lems can be minimized by locating ductsin air-conditioned space. A bibliographyof references describing cold air distri-bution is presented in the Design Guidefor Cool Thermal Storage (ASHRAE 1993).

Other BenefitsIn addition to reducing the average costof electricity consumed and possiblyreducing energy consumption, coolstorage can reduce overall cooling sys-tem capital and maintenance costs. Fornew construction, partial storage designs(where the chiller and storage combineto meet peak cooling loads) reduce chiller(and cooling tower and cooling waterpiping for water-cooled chillers) capac-ity and cost. Savings in chiller and relatedcosts are often greater than the incre-mental costs of the partial storage unit.Similarly, adding storage is a way toincrease a cooling system’s peak capac-ity without adding new chillers in situa-tions where cooling load is growing.Retrofit of old rooftop air-conditioningsystems with cool storage systems canalso be less expensive than replacementwith new rooftop units. Placement ofthe cool storage system on the groundavoids expensive crane or helicoptercharges associated with replacing theold rooftop unit, which is left in placeand modified slightly to work with thestorage system. Rooftop replacementsmay also require structural modifica-tions which can be expensive. Finally,maintenance costs will be less for thedown-sized components of the storage

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minimizes the size of the storage tank.A single tank is usually used to storeboth the chilled water and the warmwater returning from the cooling load.Separation of the two water bodies ismaximized by placing the cooler, denserwater at the bottom of the tank and thewarmer water at the top of the tank.Specially designed piping networkscalled diffusers allow water to enter andleave the tank without causing signifi-cant mixing. The result is a layer of coldwater separated from a layer of warmwater by a thermocline, as shown inFigure 1. Chilled water systems tend towork best in retrofit situations (no chillermodifications required) and/or highercapacity systems where size economies-of-scale lower the unit cost of the tank.A typical chilled water storage systemconfiguration is shown in Figure 2.Chilled water storage tanks may alsobe used as a reservoir for fire-protectionwater, reducing total facility costs and/or fire insurance premiums.

Ice-harvesting systems form ice on coilsor other refrigerant evaporating surfacesand periodically release the ice into astorage tank that contains a mixture ofice and water. A typical ice harvestingstorage system configuration is shownin Figure 3. Water is pumped from thebottom of the tank and passed over therefrigerant evaporating surface duringthe charging cycle. During discharge,water is pumped from the tank to theload. Warm water returns from the loadand is sprayed onto the top of the icewater mixture to facilitate mixing andheat transfer between ice and water.Compared to ice-on-coil systems, iceharvesters have much less ice-makingsurface, but the surface is a specializeddesign to facilitate ice release, so thepotential cost savings is not as great asa comparison based on area would sug-gest. The average thickness of ice on theheat transfer surface is generally less,however, which improves performance.

Figure 2. Typical chilled water configuration.(a)

Figure 1. Chilled water stratification.(a)

On the other hand, ice harvesters mustgo through a defrost cycle to release icefrom the heat transfer surface, which

results in a significant performancepenalty. Ice harvesting refrigerationequipment tends to be more expensive

(a) Copyright 1994, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. www.ashrae.org. Reprinted by permis-sion from Design Guide for Cool Thermal Storage.

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than other cool storage options whilethe storage capacity itself is generallythe least expensive. Thus, ice-harvestingsystems are most attractive for applica-tions requiring high storage capacityand relatively low refrigeration capacity.

Ice-on-coil systems come in severalvariations, as noted above. In all varia-tions, ice is formed on a heat transfersurface (generically referred to as a“coil,” whatever the actual configura-tion or material) without being releasedduring the charging mode and meltedaway during the discharge mode. Coilsare packed in various arrangementswithin a tank and surrounded by water.Ice is formed by transferring energyfrom the water to an evaporating refrig-erant or secondary coolant (generally aglycol/water mixture) passing throughthe coils. Discharge is accomplished bycirculating warm water past the out-side of the ice on external-melt systemswhile secondary coolant is usually pastthrough the coils on internal-melt sys-tems. Charging and discharging ofexternal-melt and internal-melt systemsare illustrated in Figures 4 and 5. At leastone internal-melt system designed forretrofit of direct-expansion rooftop cool-ing equipment is discharged by condens-ing a refrigerant, but this is an exception

to the general use of a secondary coolant.Some external-melt systems bubble airthough the water to facilitate uniformfreezing and melting of ice. This is notrequired on internal-melt systems thatare frozen solid. Freezing all of the wateralso results in slightly higher chill stor-age density for the internal-melt design.External-melt systems are able to avoidusing a secondary coolant and coolant/water heat exchangers and also benefitfrom direct-contact heat exchange. How-ever, if not fully discharged, remainingice on the coil will result in an efficiencypenalty during the subsequent chargingcycle. Care must also be taken to avoidFigure 3. Typical ice-harvesting configuration.(b)

Figure 4. External-melt ice-on-coil.(b)

Figure 5. Internal-melt ice-on-coil.(b)

(b) Copyright 1994, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. www.ashrae.org. Reprinted by permis-sion from Design Guide for Cool Thermal Storage.

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overcharging the external-melt storageunit solid as it will become increasinglydifficult to discharge without adequatewater flow passages. Charging withrefrigerant is more efficient than with asecondary coolant because one less heattransfer step is involved. On the otherhand, charging with a secondary cool-ant uses much less refrigerant and therefrigeration system is generally lesscomplicated. Typical ice-on-coil systemconfigurations are shown in Figures 6,7, and 8.

Ice slurry systems produce small par-ticles of ice within a solution of glycoland water, resulting in a slushy mixturethat can be pumped. Like ice harvesters,ice slurry generators are dynamic ice-making machines, in contrast to thestatic ice-on-coil systems. Thus, iceslurry generators do not suffer from theefficiency degradation that occurs asice builds up on an evaporator surface.However, unlike ice harvesters, nodefrost cycle is required for ice slurrygenerators, which avoids another effi-ciency loss. In ice slurry systems, ice par-ticles are generated by passing a weakglycol/water solution (~ 5-10% glycol)through tubing that is surrounded byan evaporating refrigerant containedwithin a shell (i.e., the evaporator unitis a shell-and-tube heat exchanger). Asthe glycol/water solution is cooled bythe evaporating refrigerant, ice particlesform. Depending on the system configu-ration, the resulting slush can eitherdrop directly into a storage tank or bepumped into a storage tank. The latterconfiguration is illustrated in Figure 9.Ice-free glycol/water solution is pumpedfrom the storage tank. Discharge isaccomplished by pumping the coolsolution from the tank either directlythrough the cooling load or throughan intermediate heat exchanger that iso-lates the cooling load from the ice slurrysystem. Warm solution is returned to thetop of the tank and distributed over theice slurry via multiple spray nozzles.

Figure 6. Direct refrigerant external-melt ice-on-coil configuration.(c)

Figure 7. Secondary coolant external-melt ice-on-coil configuration.(c)

(c) Copyright 1994, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. www.ashrae.org. Reprinted by permis-sion from Design Guide for Cool Thermal Storage.

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Figure 9. Typical ice slurry configuration.(d)Figure 8. Typical internal-melt ice-on-coilconfiguration.(d)

Figure 10. Encapsulated ice balls.(d)

Figure 11. Typical encapsulated iceconfiguration.(d)

The small size of the particles results inbetter heat transfer between the solutionand the ice than is possible for either iceharvesting or ice-on-coil systems. Like anice harvester, ice slurry systems have rela-tively high fixed costs associated withthe evaporator or ice generator compo-nent, but relatively low incremental costsas storage capacity is added. Thus, iceslurry systems will look their best in rela-tively high storage capacity applications.

Encapsulated ice systems consist ofwater contained in plastic containerssurrounded by coolant, all containedwithin a tank or other storage vessel.During the charging cycle subfreezingcoolant from a chiller is circulatedthrough the storage tank and past theplastic containers, freezing the ice. Dis-charge is accomplished by circulatingwarm coolant through the tank and pastthe containers, melting the ice. These twoprocesses are shown in Figure 10. Thecoolant may be routed directly to theload or be isolated from the load viaa heat exchanger. The most commonform of plastic container is a dimpledball about 4 inches in diameter. Thespherical shape creates a relativelyhigh heat transfer area per unit of waterbeing frozen, while the dimples allowfor expansion and contraction whilecycling between liquid and solid states.

Either atmospheric or pressurized stor-age tanks can be used, but a screen mustbe used near the top of an atmospherictank to keep the balls below the coolantlevel. Installation is relatively simple;the balls are simply poured into a tankand naturally conform to whatevershape the storage vessel may be. A typi-cal encapsulated ice system configura-tion is shown in Figure 11.

Eutectic salt systems are similar toencapsulated ice systems, but the plasticenclosures contain a eutectic salt insteadof water. One type of stacked eutecticsalt containers is shown in Figure 12.Eutectic salts made for cool storageapplications are typically a combina-tion of inorganic salts, water, and nucle-ating and stabilizing agents that freeze

(d) Copyright 1994, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. www.ashrae.org. Reprinted by permis-sion from Design Guide for Cool Thermal Storage.

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at 47°F and have a latent heat of 41 Btuper pound (E Source 1998). This com-pares to a latent heat of 144 Btu perpound for water. Eutectic salt systemsoffer higher energy density than chilledwater systems and like chilled watersystems can be charged with standardchillers without the efficiency penaltyof a lower evaporator temperature. Theeutectic salts are more expensive thanwater, of course. In addition, the watertemperature leaving a eutectic salt sys-tem during discharge will be warmerthan normally supplied to a cooling load.This will generally require downstreamoperation of a chiller (see Figure 13) to

larger and, therefore, more expensivechiller and storage units compared topartial storage systems. However, fullstorage also captures the greatest sav-ings possible by shifting electricitydemand from on-peak to off-peak. Fullstorage systems are relatively attractivewhen demand charges are high, the dif-ferential between on-peak and off-peakenergy charges is high and/or when thepeak demand period is short.

Load leveling and demand limitingversions of partial storage systems arealso illustrated in Figure 14. In general,partial storage systems meet part ofthe cooling load from storage and partdirectly from the chiller during theon-peak period. Load leveling versionsare designed for the chiller to operateat full capacity for 24 hours on the peakdemand day. Storage is charged whenthe load is less than the output of thechiller and discharged when the load isgreater than the output of the chiller.Load leveling designs minimize the sizeand cost of chiller and storage compo-nents, but achieve less electricity costsavings than full storage systems. Loadleveling systems are relatively attractivewhen electric rate incentives for loadshifting are moderate, the ratio of peakto average load is high, and/or theon-peak period is long. Demand limit-ing partial storage represents a middleground between full storage and loadleveling partial storage where chilleroperation is reduced, but not eliminatedduring the on-peak period. Thus, systemsize and cost, and electricity cost savingstend to fall between that for the othertwo design options. Chiller operation indemand-limiting systems may also becontrolled to minimize site peak demand,resulting in variable chiller output dur-ing the peak demand period.

Storage priority and chiller priority aretwo alternative operating strategies forcool storage systems with partial storageFigure 13. Typical eutectic salt

configuration.(e)

further cool the water, unless humiditycontrol is not a concern in the application.

Variations: Design and OperatingStrategiesFull storage systems, also known asload shifting systems, are illustratedin Figure 14. Full storage systems aredesigned to meet all on-peak coolingloads from storage. On the peak demandday, the chiller in a full storage systemoperates at its capacity during off-peakhours to charge storage and meet cool-ing loads occurring during off-peakhours. This type of system results in

Figure 14. Cool storage design options.(e)

Figure 12. Eutectic salt containers.(e)

(e) Copyright 1994, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. www.ashrae.org. Reprinted by permis-sion from Design Guide for Cool Thermal Storage.

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designs. As the names imply, cooling ispreferentially provided from storagewith storage priority operation anddirectly from the chiller with chiller pri-ority operation. The preference is mostlydriven by the relative cost of providingcooling via either mode. The cost com-parison must consider average chillerefficiency and electricity costs per kWhfor both modes plus storage efficiencyif cooling from storage. Storage prioritygenerally requires a more complex con-trol scheme to ensure that adequatecooling capacity will be available late inthe day as storage is being preferentiallydepleted. Predictions of remaining cool-ing load must be combined with mea-surement of remaining cooling capacityand knowledge of chiller capacity todetermine the appropriate mix of stor-age and chiller cooling. Additional dis-cussion of storage priority operatingstrategies can be found in ASHRAE(1993). Chiller priority control is muchsimpler. When the cooling load exceedsthe capacity of the chiller, storage is dis-charged to meet the residual demand.No predictions of remaining coolingload for the day are necessary.

InstallationInstallation requirements vary signifi-cantly among the alternative cool stor-age systems and depending on whethera new construction or retrofit scenariois being considered. Chilled water oreutectic salt storage systems are the easi-est to retrofit because a standard waterchiller can be used and secondary cool-ant loops coupled with additional heatexchangers are not usually employed.Modification or replacement of currentchillers is generally required for any ofthe ice storage systems. This is particu-larly true for ice harvesting and ice slurrysystems because they require special-ized evaporators. Ice-on-coil systemscharged via evaporating refrigerant orsecondary coolant below 18°F will alsorequire new equipment; standard waterchillers are usually adequate for chill-ing a secondary coolant as low as 18°F,albeit at reduced capacity and efficiency

compared to chilling water to about40°F (ASHRAE 1993). Thus, minimalchiller modifications are required forcool storage systems designed to becharged by 18+�°F coolant, whichincludes many ice-on-coil systemsplus encapsulated ice systems. Manyvendors offer integrated chiller andstorage systems that are particularlyattractive when replacing conventionalcooling systems that are worn out.

Federal Sector PotentialCool storage technologies have beensuccessfully applied in thousands ofnon-Federal facilities, but only a fewdozen Federal facilities. The potentialfor successful Federal application wouldappear to be much greater, however,because Federal facilities tend to haveseveral characteristics that should makecool storage generally more attractivethan in other sectors. These characteris-tics include:

• Relatively large cooling systems thatcan take advantage of storage systemeconomies-of-scale.

• A preponderance of chilled watercooling systems that are generallyeasier to integrate with cool storagethan cooling systems served bydirect-expansion equipment.

• Rate structures characterized byhigh-demand charges and/or largevariation in hourly energy charges.

• Older equipment that needsreplacement.

Estimated Savings and MarketPotentialDespite the qualitative advantagesnoted above, it is difficult to quantifythe Federal potential (or the non-Federalpotential, for that matter) of cool storagesystems because of the significant impactthat site-specific factors (such as electric-ity demand profile, electric rate structure,cooling demand profile, existing coolingsystem equipment, etc.) have on the cost-effectiveness. A truly accurate assessment

needs to consider each site on anindividual basis. Nevertheless, themarket potential for cool storage hasbeen estimated for the U.S. Army bythe Construction Engineering ResearchLaboratory (Sohn and Cler 1990).

The work by Sohn and Cler focusedon the potential savings in electricitydemand and energy charges from shift-ing chiller use to off-peak hours. Theoverall efficiency of the cooling systemwas presumed to be unaffected by thecool storage system, i.e. there was nonet increase or decrease in energy con-sumption. Incremental capital costswere estimated for two scenarios: newconstruction or equipment replacementand retrofit. The first scenario allowscredit for chiller downsizing, but notthe second. Simple rules of thumb wereused to establish the size of the cool stor-age system required to reduce peakelectricity demand by either 5% or 10%.Incremental system costs were assumedto be $80/ton-hour for the new construc-tion or equipment replacement scenarioand $150/ton-hour for the retrofit sce-nario. Thus, the only site-specific inputsto the estimate were the electric rates.

The results of the study indicated thatcost-effective application (paybackperiod of 10 years or less for govern-ment investment) of cool storage sys-tems designed to reduce peak electricaldemand by 10% would result in annualsavings of about $12 million for the newconstruction/equipment replacementscenario. This figure was reduced toabout $4 million per year for the retrofitscenario. The Army represents about25% of the total Federal floor space, soa rough estimate of the total Federalpotential for cool storage systems wouldbe annual savings of about $50 million.

Laboratory PerspectiveThermal energy storage for space coolingis a relatively mature technology experi-encing evolutionary improvements toolder concepts, innovation with newerconcepts, and extension of applicability

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from chilled water systems to packagedrooftop systems. The impetus for con-sidering cool storage systems (from theend-user’s perspective rather than theelectric utility’s perspective) was origi-nally driven by high-demand chargesand/or on-peak energy charges andthe opportunity to save on energy costs.While electric rates are still a significantmotivation for implementing cool stor-age, many systems are being installedtoday on the basis of lower first cost and/or lower energy consumption as well.

Cool storage systems have become rela-tively common in the commercial sector,particularly in applications such asschools that have high ratios of peak toaverage cooling loads. Federal applica-tions have lagged, however, represent-ing only about 1% of the totalpopulation of installed systems. Withmany Federal facilities having similarcharacteristics to commercial facilitieswhere cool storage has been success-fully installed, broader consideration ofcool storage at Federal facilities seemswarranted. The rough estimate of Fed-eral sector potential described abovealso suggests that significantly greaterutilization would be beneficial. Still,there are no simple rules-of-thumb thatwill always identify where cool storagecan be cost-effectively applied. The useof cool storage or not and selection of thebest cool storage system must be care-fully considered via screening studieson a site-specific basis.

ApplicationThis section addresses the technicalaspects of applying cool storagetechnology. The conditions in whichcool storage can be best applied areaddressed. The advantages, limita-tions, and benefits of each applicationare enumerated. Design and integrationconsiderations for the technology arediscussed, including equipment andinstallations costs, installation require-ments, maintenance impacts, and utilityincentives and support.

Application ScreeningHistorically, cool storage has been morecommonly applied in buildings withrelatively high cooling loads, usuallyserved by central chillers coupled withchilled water distribution systems. Themajority of applications served peakcooling demands of 100 tons or more andrequired storage capacities of 500 ton-hours or more (Potter 1994; E-Source1998). Several manufacturers now offerpackaged ice storage systems as smallas 100 ton-hours at unit costs that areessentially the same as larger sizes. Thesesmaller storage systems are also beingcoupled with chillers to retrofit direct-expansion (DX) rooftop cooling sys-tems. One manufacturer has developeda 42 ton-hour storage unit specificallydesigned for integration with DX cool-ing systems. Thus, cool storage equip-ment is available for practically all typesof buildings. Cost-effectiveness mustbe considered on a case-by-case, site-specific basis, however.

Where to ApplyCool storage will reduce the average costof energy consumed and may potentiallyreduce the energy consumption andinitial capital cost of a cooling systemcompared to a conventional coolingsystem without cool storage. While mostbuilding space cooling applications arepotentially attractive candidates, theprospects will be especially attractiveif one or more of the following condi-tions exists.

• Electricity energy charges vary sig-nificantly during the course of a day.

• Electricity demand charges are highor ratcheted.

• The average cooling load is signifi-cantly less than the peak cooling load.

• The electric utility offers other incen-tives (besides the rate structure) forinstalling cool storage.

• An existing cooling system isexpanded.

• There is new construction.

• Older cooling equipment needsreplacing.

• Cold air distribution benefits canbe captured.

What to AvoidIn general, applications lacking theconditions identified above should beavoided. In addition, the following con-ditions should also be avoided.

• Lack of operation and maintenanceexperience or training with systemequipment, especially where built-uprefrigeration systems are used ratherthan packaged chillers.

• Lack of operator training on operat-ing and control strategies for mini-mizing cooling system life-cycle costs.

• Sites where the space available forcool storage equipment is limitedor has other, more valuable uses.

• Limited resources for engineeringfeasibility studies and system design.Cool storage systems are inherentlymore complicated than non-storagesystems and extra time will berequired to determine the optimumsystem for a given application.

Equipment IntegrationThe specific integration requirementsvary for the different types of cool stor-age systems. In some cases, multipleintegration options exist for a singletype of cool storage system. Fundamen-tally, the storage device separates thegeneration of chilled coolant from itsdelivery to air handling units. Thus, anextra piping loop (one for charging andone for discharging storage) with pumps,valves, and controls is required com-pared to a conventional system. Typicalsystem configurations were shown in

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ice slurry systems requires different skillsthan for standard packaged chillers.

Equipment WarrantiesA one-year warranty is commonlyoffered by cool storage system manu-facturers on all equipment, parts, andmaterials. Labor is not usually covered.Specific system components (e.g., thecoil in ice-on-coil systems) are war-ranted for a longer period by somemanufacturers. Other manufacturerswill guarantee performance in terms ofavailable storage and discharge capaci-ties or the maximum rate of heat gainthrough the walls of the tank.

CostsCosts will vary significantly for coolstorage systems because of the manydifferent technology options and signifi-cant size economies-of-scale for somecomponents, in addition to variation insite-specific conditions and individualvendor offerings. In general, all systemshave a chill-generating device and a chillstorage device, even if the two devicesare closely integrated (e.g., external-melt,ice-on-coil systems). The following costequations can be used to prepare roughestimates of system costs, suitable forinitial screening of alternatives. A morerigorous approach based on site-specificsystem integration requirements andvendor quotes for major componentsshould be used when refining the initialscreening evaluation (see “Refining theEvaluation” below) and preparing thefinal estimate.

Equations 1 and 2 can be used forestimating the costs of conventional air-cooled reciprocating chillers and water-cooled centrifugal chillers, respectively(Means 1999). As indicated, Equation 1is generally applicable at capacities

Figures 2, 3, 6, 7, 8, 9, 11, and 13. In addi-tion, a common variation to consider iswhether to install the chiller upstreamor downstream of the storage unit asshown in Figure 15 for an ice-on-coilsystem. Installing the chiller in theupstream position increases chiller effi-ciency because the coolant temperatureis higher, but reduces the usable storagecapacity because storage is dischargedwith a lower temperature coolant. Theopposite effects occur when the chilleris installed in the downstream posi-tion. The two choices present a trade-off between storage capital costs andchiller operating costs that should beconsidered in a detailed evaluation ofsystem options.

Maintenance ImpactChiller and cooling tower maintenanceactivities with cool storage are essen-tially the same as for a conventionalcooling system. However, the chiller

and cooling tower are usually smallerwith cool storage, which should gener-ally reduce the cost of replacementparts and specifically reduce refrigerantreplacement and cooling tower clean-ing and water treatment costs. Chillerand cooling tower maintenance canalso be conducted while cooling is pro-vided from storage, which benefitsmaintenance scheduling.

The addition of a cool storage tank is themost obvious difference from a conven-tional cooling system, but incrementalmaintenance requirements tend to beminimal. Water treatment requirementsare the same as for non-storage systems,but the volume of water to be treated,hence cost, is greater. Water levels shouldbe checked at least once a year, or moreoften for open tanks. Special attentionneeds to be given to the water chemistryin ice harvesting systems where warmreturn water is highly aerated as it’ssprayed on the ice/water mixture withinthe tank. Detailed discussion of watertreatment requirements can be found inAhlgren (1987).

Systems using glycol must use a versionintended for HVAC applications thatinclude corrosion inhibitors and otheradditives that allow contact with air.Glycol chemistry should be checkedannually to ensure that proper concentra-tion of the inhibitors and other additivesexist, as well as to ensure maintenanceof the intended water/glycol mixture.

Cool storage systems usually have morepumps, control valves, and possibly heatexchangers than conventional coolingsystems, with periodic maintenancerequired for each. Tank inventory sensorswill also require periodic calibration.Finally, maintenance of refrigerationequipment such as that used in ice har-vesting, external melt ice-on-coil, andFigure 15. Upstream and downstream

chiller configurations.(f)

(f) Copyright 1994, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. www.ashrae.org. Reprinted by permis-sion from Design Guide for Cool Thermal Storage.

15


less than 200 tons, while Equation 2 isgenerally applicable at sizes greaterthan 200 tons.1 Both equations are basedon the equipment capacity for produc-ing chilled water at standard ARI ratingconditions. Equations 1 and 2 can beused directly to estimate the costs ofwater chillers used in conventional cool-ing systems and for systems using chilledwater or eutectic salt storage.

(1) Air-Cooled Reciprocating ChillerInstalled Cost = $11,900 + 591*Twc

Where Twc = nominal water chillercapacity in tons, from 20-200 tons.

(2) Water-Cooled Centrifugal ChillerInstalled Cost = $57,700 + 307*Twc

Where Twc = nominal water chillercapacity in tons, from 200-1500 tons.

Equations 1 and 2 can also be used forestimating the cost of chillers used tomake ice in ice-on-coil storage systemsby applying an adjustment factor.Operation at ice-making evaporatingtemperatures reduces capacity by about1/3 compared to water chilling; theactual ratio depends on the evaporat-ing temperature required (varies by asmuch as 10°F depending on the typeand specific model of ice-on-coil stor-age). Alternatively, the equivalent waterchilling capacity is about 50% higherthan the ice generating capacity. There-fore, the cost of chillers for ice-on-coilstorage systems can be estimated usingequations 1 or 2 by first multiplying therequired ice-generating capacity by 1.5.

Water-cooled systems require coolingtowers to cool condenser water. Cool-ing tower costs can be estimated usingEquation 3 (Means 1999). The sameequation applies whether the coolingtower is applied to a water-cooling orice-generating “chiller.”

(3) Cooling Tower Installed Cost =$982*Thr

0.64

Where Thr = heat rejection capacityin tons, from 60-1000 tons2

Dynamic ice-generators are generallymore expensive than the chillers usedwith static ice-on-coil systems. Ice slurrygenerators are currently offered inmodular units resulting in an installedcost of about $1000/ton of ice at capaci-ties of 100 tons or more. Ice-harvestinggenerators are typically slightly moreexpensive as indicated by Equation 4(ASHRAE 1993).

(4) Ice-Harvesting Generator InstalledCost = $195,000 + 990*Tig

Where Tig = ice generating capacityin tons, from 200-1000 tons

Chilled-water storage costs depend onthe difference between water supply andreturn temperature in addition to the sizeof the storage unit. For example, the sameequipment would have twice the stor-age capacity if operated through a 20°Fdifferential compared to a 10°F differen-tial. Alternatively, the same capacitycould be achieved with a tank that is50% smaller. Equations 5 through 7 canbe used for estimating chilled water stor-age costs for three alternative supplyand return water temperature designassumptions (EPRI 1992). Similarly,Equation 8 can be used for estimatingstorage-related costs for dynamic ice sys-tems using ice-harvesting or ice slurrygenerators. Again, the vessel is essen-tially the same as for chilled water stor-age, but costs per ton-hour are lowerbecause of the higher cooling densityof ice compared to water.

(5) 10°F (T Chilled Water Storage InstalledCost = 802*(TH)0.686

Where TH = storage capacity in ton-hours, ton-hours from 600-6000

(6) 15°F ∆T Chilled Water Storage InstalledCost = 616*(TH)0.686

Where TH = storage capacity in ton-hours, ton-hours from 900-9000

(7) 20°F ∆T Chilled Water Storage InstalledCost = 498*(TH)0.686

Where TH = storage capacity in ton-hours, ton-hours from 1200-12,000

(8) Dynamic Ice Storage Installed Cost =211*(TH)0.686

Where TH = storage capacity in ton-hours, ton-hours from 4,000-40,000

Ice-on-coil and encapsulated ice storagesystem costs are dominated by the heattransfer surface, which is the piping orcoils for the former and the flexibleencapsulating material for the latter.Storage capacity, hence cost, is directlyproportional to the heat transfer areaand the amount of ice that can be gener-ated and stored at full charge per unit ofheat transfer area. The installed cost forice-on-coil and encapsulated ice storageis about $70/ton-hour.3 Although therecould be economies-of-scale associatedwith the tank or containment vessel,most ice-on-coil systems come in pre-assembled tank and coil packages ofmoderate individual capacity with largercapacity needs met via multiple tanks.Thus, the cost per ton-hour for thesetypes of systems is usually independentof the number of ton-hours required.

Similar to ice-on-coil and encapsulatedice systems, eutectic salt system costs aredominated by components (the salt andits enclosure) that vary directly in sizeand cost with the required system capac-ity. Again, the tank or vessel representsa relatively small portion of the total stor-age system cost, so its economies-of-scaleare overshadowed by the cost of the saltand its enclosure. The installed cost of

1 In this section and throughout this FTA, a “ton” means 12,000 Btu per hour. This rating is derived from the average hourly cooling rateachieved from melting one ton (2000 pounds) of ice over a 24 hour period (2000 pounds * 144 Btu/pound / 24 hours = 12,000 Btu/hour). Thus,a 100-ton ice generator has a cooling capacity of 1.2 million Btu per hour and would be able to produce roughly 100 tons (200,000 pounds) ofice in 24 hours.2 Note that cooling tower capacity is greater than chiller capacity because the energy input to the chiller must be rejected along with the coolingload being served by the chiller.3 Based on information collected from Baltimore Air Coil, Calmac, Chester-Jensen, Cryogel, Dunham-Bush, Fafco, and Girton Manufacturing.

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eutectic salt storage is about $125/ton-hour (ASHRAE 1993).

In addition to the chiller and storagecomponents, installation of a cool stor-age system will require miscellaneouspipes, valves, pumps, instrumentation,controls, and possibly heat exchangers.The requirements for this miscellaneoushardware, hence costs, vary significantlydepending on site-specific conditions.The costs for these components can beignored when conducting initial screen-ing studies, but should be estimatedfrom an assessment of specific require-ments when preparing the final designevaluation.

Utility Incentives and SupportUtilities offer various forms of financialand technical support for cool storagesystems. Examples include rebates spe-cific to cool storage, rebates for peakload reduction, and cost-sharing of fea-sibility studies. The following utilitiesprovided incentives and/or supportspecific to cool storage systems as ofspring 1999, according to a survey con-ducted by Energy User News (CahnersBusiness Information 1999a and 1999b).Other programs, such as those targetingchillers, HVAC, load management, etc.,may also apply. Incentive and supportprograms are subject to change, par-ticularly as the electric utility industryderegulates. Therefore, prospectiveusers of cool storage systems shouldcontact their electric utility to see whatincentives are available and applicable.

Alabama Power Company: $100/kWdeferred; $5000 for feasibility studies

Baltimore Gas and Electric Company:$200/kW shifted; $15,000 for feasibilitystudies

Columbia Water and Light: Rate inven-tive; no specific information available

Delmarva Power and Light: $140/kWshifted with $100,000 maximum

Florida Power and Light: No specificinformation available

Gainesville Regional Utilities: Rebatebased on tonnage; no specific informa-tion available

GPU Energy: No specific informationavailable

Houston Power and Lighting: $300/kWshifted

Lansing Board of Water and Light:$100/kW of installed equipment

Northern States Power Company: Nospecific information available

Pasadena Water and Power Company:$5000 for feasibility studies

Riverside Utility District: $5000 for feasi-bility studies; $200/kW shifted off peak

South Carolina Public Service Authority:$200/kW shifted off-peak; co-funding offeasibility study

United Illuminating Company: $400/kWshifted or $400/ton shifted; feasibilitystudy grants

Technology PerformanceSeveral thousand cool storage systemshave been installed in the United States,but only about 1% of these have been atFederal facilities. The majority of sys-tems have used ice-on-coil technology,but stratified chilled water systems havealso been moderately popular. Both ice-on-coil and stratified chilled water aremature technologies, but evolutionaryimprovements are continuously imple-mented by manufacturers. The experi-ences with cool storage at three Federalfacilities are summarized below; see sec-tion on “Who is Using the Technology”for contact information.

An ice-on-coil system installed at theRalph H. Johnson VA Medical Center inCharleston, South Carolina, was origi-nally designed to provide the entireon-peak cooling load from storage, butsubsequent building expansions nowrequire partial chiller operation onpeak cooling days. The facility wasdriven to consider cool storage as ameans of reducing its energy costs.

A $1 million rebate offered by the serv-ing electric utility provided a hugeincentive and served to ensure projectcost-effectiveness. Initial problems wereexperienced with the chiller, but not thestorage system itself. Chiller problemswere probably caused by disassemblyof the chiller during installation, whichwas required to fit the chiller into themechanical room. These problems havesince been ironed out. Other minor per-formance problems developed initiallybecause no operating guidelines werein place. Standard operating procedureshave now been established for eachmonth and performance has been con-sistently good. Electricity cost savingsare typically around $5000 per monthduring the cooling season at this 500,000square foot facility.

A 10,000 ton-hour stratified chilled watertank was tied into an existing districtcooling system serving 10 buildings atthe Sandia National Laboratories inAlbuquerque, New Mexico. Installationof the storage system allowed extensionof the district cooling system to a new150,000 square foot building withoutadding additional chiller capacity. Inaddition, annual energy savings areexpected to be $200,000 per year. Thesystem was installed and has operatedwithout any unexpected problems. Thesystem was designed to allow severaldifferent operating strategies, but didnot require the installation of any newpumps. The biggest problem experiencedwas overcoming internal resistance tochanges of any kind. Therefore, sitepersonnel advise working hard to get“buy-in” from management and main-tenance personnel. Also suggested isthorough consideration of alternativedesign options before committing to asingle approach.

A full-storage, ice-on-coil system wasinstalled at the U.S. Army ReserveCenter in Monclova, Ohio, when this54,000-square-foot facility was builtin 1996. The facility received a 1998Engineered Systems Engineering TeamAward for the cool storage system and

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other energy management features. Esti-mated energy savings for the cool stor-age system are about $1000 per monthduring the cooling season. In general,the system works well, but they haveexperienced a few minor problems.Occasionally, the building is occupiedat night, which increases the coolingload and doesn’t allow the storage sys-tem to be fully charged. Basically, thesystem wasn’t designed to handle thistype of occupancy pattern. As a result,the chillers had to be operated duringthe day and it takes a few days to fullyrecharge and return to normal opera-tion. As site personnel point out, “Ifyour cooling load is nearly constant,24 hours a day, it won’t work.”

Evaluating Cool StorageSystemsThe process of identifying and evaluat-ing alternatives is more complicatedfor cooling systems using cool storagethan for those without. The increasedcomplexity is driven by the plethoraof alternative storage types and systemconfigurations plus the need to considercooling loads and cooling system opera-tion for a complete charge and dischargecycle rather than just a single designpoint. The evaluation process consistsof the following steps:

• determine cooling requirements

• identify alternative storage types andsystem configurations to be evaluated

• conduct screening evaluation ofalternatives

• refine screening evaluation results forpreferred alternative(s).

Each of these steps is briefly discussedon the following pages.

For a more detailed discussion the readeris referred to the list of design and instal-lation guides provided on page 26.

Cooling RequirementsCool storage system evaluation anddesign requires knowledge of hourlycooling loads for the peak design day(for daily storage cycles or for the peakdesign week for weekly storage cycles)in addition to the peak design hour.Note that the peak design hour maynot necessarily occur during the peakdesign day, but the cool storage systemmust be sized to meet both require-ments. In addition to requiring morecooling load data than for a non-storagedesign, there is a greater need for thatdata to be accurate. For example, if thechiller in a non-storage system is under-sized, the building it is serving is likelyto be too warm for a few hours a day fora few days a year. However, as ambientconditions cool during the evening andearly morning hours, the non-storagesystem will be able to catch up. Mostcool storage systems have smallerchillers, however, and rely on storageto provide part or all of the cooling loadduring the peak afternoon hours. If astorage system is undersized, it will notbe able to catch up at night and havestorage adequately charged for thefollowing day. If the storage system isundersized and there are several con-secutive days of weather near peakdesign conditions, overheating prob-lems will likely accumulate. In short,the smaller chiller sizes associated withstorage systems provide less reservecapacity compared to non-storage sys-tems, which puts a premium on correctlyspecifying the design conditions. Practi-cal approaches for dealing with thisconcern include selecting more conser-vative design weather conditions (e.g.,design for “99%” conditions rather than“97.5%” conditions) or applying a moreconservative safety factor when sizingthe chiller and storage components of acool storage system. However, spendingadditional effort to accurately define the

design cooling conditions is the bestform of design insurance.

Cooling load profiles developed forgeneric buildings similar to the applica-tion being considered are adequate forthe initial screening evaluation, butmore accurate load estimation proce-dures are required for the revised evalu-ation and preparation of system designspecifications. Cooling load calculationsare discussed in detail in ASHRAE’sHandbook on Fundamentals (ASHRAE1997). Note that the cooling load mustaccount for heat gains from fans, duct-ing, piping, and pumps, as well as theload delivered to the conditionedspace. In addition, heat gain throughthe wall of the storage vessel and avail-ability losses within the vessel must beaccounted for when sizing storage andthe chiller4. In retrofit situations, mea-surement of cooling loads at designconditions is preferred. If measurementsat design conditions are not available,measurements at other conditions couldbe used to calibrate building load simu-lation models and the simulation mod-els used to predict cooling loads atdesign conditions.

Identifying AlternativesWith a plethora of storage unit andsystem configuration possibilities, thenumber of alternatives to be evaluatedquantitatively should be minimized byjudicious, qualitative, pre-screening.With this objective in mind, the follow-ing rules-of-thumb are offered for pre-screening purposes.

Chilled water storage is more compat-ible with standard chilled water coolingsystems than the various ice storagesystems. In general, this makes chilledwater storage relatively attractive forretrofits, and particularly in coolingcapacity expansion situations. Chilledwater systems will look their best where

4 Mixing and/or conduction across the thermocline within a chilled water storage tank significantly increases the chilled water discharge tem-perature near the end of each storage cycle. Similarly, the coolant discharge temperature rises at an increasing rate in ice storage systems nearthe end of each discharge cycle due to ineffective heat transfer between the discharge coolant and the dwindling amount of stored ice.

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relatively large storage requirementscan take advantage of tank economies-of-scale and where the availability ofspace is not a significant concern.

The applicability of eutectic salt storageis similar to chilled water. However, itshigher energy density makes it more at-tractive than water where limited spaceis a greater concern than storage cost ora slightly higher discharge temperature.

Ice storage systems are required to takeadvantage of cold air distribution ben-efits and where limited space is avail-able. Ice storage systems minimize tanksize and cost, so are generally more eco-nomical at smaller capacities where tankcosts are a substantial portion of the totalsystem cost.

Ice-harvesting and ice slurry systemsseparate ice generation from ice storage,resulting in lower storage-related coststhan other systems. However, the ice-generators for both of these types ofsystems are more expensive than othersystems. Thus, ice harvesting and iceslurry systems look their best in applica-tions with a large ratio of storage capac-ity to storage charging capacity. Thiswould suggest considering a weeklystorage cycle as well as a daily storagecycle for these two options. Ice harvestingand ice slurry systems are also capableof providing high discharge rates.

Most ice-on-coil and encapsulated icestorage systems use standard packagedchillers and secondary coolants for charg-ing, which minimizes ice-generatingcosts and ice storage system costs formost applications. Internal-melt, ice-on-coil storage and encapsulated icestorage allow partial storage withoutincurring an efficiency penalty duringthe subsequent ice-building period ashappens with external-melt, ice-on-coilsystems that are partially discharged.External-melt systems, like ice harvest-ing and ice slurry systems, offer thehighest discharge rates. External-meltsystems may also be directly chargedwith refrigerant, which offers efficiencyadvantages, but refrigeration equipment

complexities compared to packagedchillers with secondary coolants.

The three basic configuration optionsare full storage and partial storage inload-leveling or demand-limiting ver-sions. Full storage results in the greatestsavings in on-peak electricity charges,but requires larger, more expensive,chillers and storage units. Thus, fullstorage should be considered wherethere are high demand charges, annu-ally ratcheted demand charges, and/ora large differential between peak andoff-peak electricity energy charges.Short discharge periods are also par-ticularly beneficial to the economics offull storage system designs.

Equipment sizes and capital are mini-mized with partial storage, load-levelingdesigns, but on-peak electricity chargesare reduced the least of the three basicconfigurations. Thus, situations with ahigh ratio of peak to average coolingloads are attractive for partial storageconfigurations as are situations withminimal incentive from the electricrate structure. Partial storage, demand-limiting designs are a hybrid of the twoother configurations and are probablybest considered as a possible variant ofone of the other two principal options.

Screening AlternativesThe initial screening of alternatives fol-lows the steps bulleted below, startingwith the design load profile and appli-cable utility rate schedule. Note that dif-ferences in annual energy consumptiongenerally have a smaller economicimpact than differences in equipmentcosts and electricity demand when com-paring alternatives, so is usually ignoredin the screening process.

The initial screening steps are:

• size chiller and storage

• estimate chiller and storage capitalcost

• estimate annual demand savings

• calculate system life-cycle cost

• select preferred system(s) for detailedanalysis.

For non-storage systems, the chiller issized to meet the peak hourly load. Withstorage, the chiller is sized to meet thecooling load over the storage cycle, typi-cally a 24-hour period. Thus, for partial-storage systems, the chiller will alwaysbe smaller than for a non-storage sys-tem, while for full-storage systems, thechiller may be smaller or larger than fora non-storage system, depending on thedesign load profile and the length of theon-peak period.

At the simplest level, chiller capacityequals the design-day cooling loaddivided by the number of chiller operat-ing hours. For a partial-storage system,the number of chiller operating hours is24, i.e., the chiller operates at full capac-ity for 24 hours on the design day. Fora full-storage system, the number ofchiller operating hours is 24 minus thelength of the peak-demand period.

For greater accuracy the calculation ofnominal chiller capacity should con-sider the relative chiller capacity whencharging storage, direct cooling duringthe on-peak period, and direct coolingduring the off-peak period. Capacitymay be different than the nominal ratingdue to differences in evaporating or con-densing conditions and/or because ofdifferent assumptions regarding selec-tion of full storage or partial storage(with load-leveling or demand-limitingoptions) designs. Thus, the nominalchiller capacity equals the design-daycooling load divided by the sum of thenumber of chiller operating hours ineach mode, with the number of hoursin each mode multiplied by the aver-age capacity in that mode relative tothe nominal capacity (see Equations 9and 10).

(9) Nominal Chiller Capacity = Total Cool-ing Load/Adjusted Chiller OperatingHours

(10) Adjusted Chiller Operating Hours =H1CR1 + H2CR2 + H3CR3

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Where H1 = hours charging storage

CR1 = capacity relative to nominalconditions while charging

H2 = hours direct cooling duringon-peak period

CR2 = capacity relative to nominalconditions while cooling duringon-peak period

H3 = hours direct cooling duringoff-peak period

CR3 = capacity relative to nominalconditions while cooling duringoff-peak period

Note that if the nominal chiller capacitycalculated via Equations 9 and 10 isgreater than the load for any hour dur-ing the direct cooling mode, then thechiller capacity must be recalculated viaan iterative procedure illustrated in theDesign Guide for Cool Thermal Storage(ASHRAE 1993).

The required storage capacity is equalto the total cooling load minus anyload provided directly by the chilleror from storage while storage is beingcharged. Similar to the calculation ofnominal chiller capacity, cooling pro-vided directly by the chiller or fromstorage while storage is being chargedmust consider the variation in chillercapacity while operating in these dif-ferent modes, as shown in Equations 11and 12.

(11) Nominal Storage Capacity = Total Cool-ing Load - Directly Served Load

(12) Directly Served Load = NCC*(H2CR2 +H3CR3) + CAPCH

Where CAPCH = capacity provided fromstorage while simultaneously chargingstorage and NCC = the nominal chillercapacity.

Note that in some cases the actual loadmet from direct cooling will be lessthan NCC*H2CR2 or NCC*H3CR3 if thedemand for direct cooling is less than thechiller’s capacity. In this case, the actualcapacity provided must be used. In short,care must be taken to keep track of thecooling loads and expected provision of

cooling from the chiller and/or storageon an hourly basis. Also note that equa-tions 11 and 12 calculate the nominalstorage capacity, suitable for a screen-ing analysis. The actual required storagecapacity must be determined via anhour-by-hour simulation of performancethat captures the interactions of the chiller,storage, and the load. In particular, allstorage systems suffer from an increas-ing rise in discharge temperature nearthe end of a discharge cycle that effec-tively reduces the useful capacity fromits theoretical maximum.

Sample CalculationsChiller and storage sizing for a screen-ing evaluation is illustrated throughthe following example. The example isbased on a partial storage, load-levelingsystem design with ice storage. Com-pared to its rated capacity at standardARI conditions, the relative chiller capac-ity is presumed to be 0.8 in the storagecharging mode and 0.9 in the direct cool-ing mode. Design cooling day coolingloads, chiller operation, and storageoperation are presented in Table 1 andFigure 16. The cooling load builds in thelate morning, peaks in mid-afternoonand decays to its minimum daily valuein the early morning. The utility on-peakperiod is shown as running from noonuntil 6:00 p.m., but this has no impact ona partial storage, load-leveling design.

The total daily load is 48,500 ton-hours.Thus, if the chillers were able to providetheir nominal capacity while chargingstorage and direct cooling, the requiredcapacity would be 2021 tons (48,500 ton-hours/24 hours = 2021 tons). This is theaverage actual capacity the chillers willneed to provide to meet the daily cool-ing demand. The required nominal ca-pacity will be higher because the chilleroperates at less than nominal capacity.In order to achieve an average net capac-ity of 2021 tons the average net capacitywhile charging (where the relative capac-ity is 0.8) will be less than 2021 while

the average net capacity while directcooling (where the relative capacity is0.9) will be more than 2021. These ap-proximations of net capacity allow forreasonable assumptions regarding thechiller and storage operating modesduring each hour of the day that will beverified or refuted after determining thenominal chiller capacity. For example,with a direct cooling capacity of at least2021 tons, the chillers are assumed to becharging storage during hours 1-9 and21-24 and directly cooling for hours10-20. The cooling load is met by dis-charging storage alone during hours 1-9and 21-24, while storage discharge anddirect cooling are required to meet theload for hours 10-20.

The nominal chiller capacity is calcu-lated by plugging the assumptions pre-sented above into Equations 9 and 10.

Nominal Chiller Capacity = 48,500/((13*0.8)+(6*0.9)+(5*0.9) = 2389.2 tons5

The actual chiller capacity is 1911.3 tons(2389.2 * 0.8) in the storage chargingmode and 2150.2 tons (2389.2 * 0.9) inthe direct cooling mode. 2150.2 is lessthan the cooling load for all hours pre-sumed to be in the direct cooling modeand 1911.3 is greater than the coolingload for all hours where storage is beingcharged and discharged. Thus, no fur-ther adjustments of the initial assump-tions are necessary.

The nominal storage capacity is calcu-lated by plugging the assumptions pre-sented above into Equations 11 and 12.

Nominal Storage Capacity = 48,500 -{2389.2*[(6*0.9)+(5*0.9)]+17,200} = 7,647ton-hours

The chiller and storage capacity calcu-lations are confirmed by the figuresshown in Table 1, which illustrate thecharging and discharging of storagefrom no stored energy in hour 20 to amaximum of 7,647 ton-hours in hour 9.

5 The capacity and cost figures used in this section have not been rounded off so that the reader can more easily duplicate the calculations.

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The nominal chiller capacity (2389 tons)required for the storage system wouldprobably be served by two 1195-tonunits, with each unit costing $424,565based on Equation 2. Assuming ice-on-coil storage (at $70/ton-hour; see priorsection on costs), the cost of the storageunit would be $535,290. Thus, the totalstorage system cost would be $1,384,420.A water chiller for a conventional systemwould need to have an actual capacityof 3500 tons or a nominal capacity of3888.9 tons (3500/0.9) to provide 3500tons during the on-peak period. A totalcapacity of 3888.9 tons would probably

Table 1. Chiller and Storage Sizing Example

Load From Load From Charge to StorageUtility Load, Chiller Storage Chiller, Storage, Storage, Balance,

Hour Period Ton-Hours Mode Mode Ton-Hours Ton-Hours Ton-Hours Ton-Hours

1 off-peak 1200 CH CH/DCH 0 1200 1911 1,9572 off-peak 1100 CH CH/DCH 0 1100 1911 2,7683 off-peak 1000 CH CH/DCH 0 1000 1911 3,6794 off-peak 900 CH CH/DCH 0 900 1911 4,6915 off-peak 800 CH CH/D CH 0 800 1911 5,8026 off-peak 1000 CH CH/DCH 0 1000 1911 6,7137 off-peak 1300 CH CH/DCH 0 1300 1911 7,3258 off-peak 1600 CH CH/DCH 0 1600 1911 7,6369 off-peak 1900 CH CH/DCH 0 1900 1911 7,64710 off-peak 2200 DC DCH 2150 50 0 7,59811 off-peak 2500 DC DCH 2150 350 0 7,24812 off-peak 2800 DC DCH 2150 650 0 6,59813 on-peak 3100 DC DCH 2150 950 0 5,64814 on-peak 3300 DC DCH 2150 1150 0 4,49915 on-peak 3500 DC DCH 2150 1350 0 3,14916 on-peak 3300 DC DCH 2150 1150 0 1,99917 on-peak 3100 DC DCH 2150 950 0 1,04918 on-peak 2800 DC DCH 2150 650 0 40019 off-peak 2500 DC DCH 2150 350 0 5020 off-peak 2200 DC DCH 2150 50 0 021 off-peak 1900 CH CH/DCH 0 1900 1911 1122 off-peak 1700 CH CH/DCH 0 1700 1911 22323 off-peak 1500 CH CH/DCH 0 1500 1911 63424 off-peak 1300 CH CH/DCH 0 1300 1911 1,245

Totals 48,500 23,653 24,847

CH = ChargingDCH = DischargingDC = Direct Cooling

Figure 16. Chiller and storage sizing example.

0

1000

2000

3000

4000

5000

6000

7000

8000

0 6 12 18 24

Hour of the Day

Cooling LoadChiller OutputStorage Balance

on-peakTon-

Hou

rs o

f C

oolin

g

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require three 1296.3-ton units, with eachunit costing $455,663 according to Equa-tion 2 for a total conventional chiller costof $1,366,989. Thus, the initial capitalcost of the storage system would be$17,431 more expensive than the con-ventional system. Differences in coolingtower costs should also be consideredwhen refining the initial screening results.

The screening evaluation must also esti-mate the reduction in electricity costsassociated with the cool storage system.Electricity costs can be reduced from areduction in demand charges and/orenergy charges. The importance ofevaluating one or the other or both de-pends on the site-specific electric ratestructure.

The evaluation of demand charge sav-ings starts by estimating the reductionin on-peak demand when comparingconventional and storage cooling sys-tems. In the example, the peak coolingdemand of 3500 tons would create apeak cooling system electric demandof 3076 kW for a water chilling system[including the electrical demand of thechiller compressor, cooling water pumpsand cooling tower fan (if water-cooled)or condenser fan (if air-cooled)] with aCOP of 4.0. The cool storage systemchillers need only provide 2150 tons ofcooling during the on-peak period, sothe electrical demand of its water chillingsystem components would be 1890 kWat a COP of 4.0 or a reduction of 1186 kW.6

Note that the peak electrical demand forthe chilled water pumps will be approxi-mately the same for the two systemsand any difference can be ignored forthe screening study. Differences insupply and return water tempera-tures, flow rates, and pumping energyshould be considered when refiningthe analysis for alternatives passing theinitial screening.

For a typical monthly demand chargeof $10/kW, the 1186 kW demand reduc-tion translates into savings of $11,860 forthe peak month. Calculation of demandsavings in other months requires addi-tional assumptions or knowledge ofthe variation in peak cooling loads frommonth to month, the type of demandcharge, and whether the partial storagesystem is operated in chiller priority orstorage priority mode. By definition, thepeak cooling loads will be less duringthe other 11 months of the year. In addi-tion, the on-peak period often changesand demand charges are often lower inthe winter than in the summer. In short,the annual demand charge savings willusually be considerably less than 12 timesthe peak monthly savings. Lackingany better information, assuming thatannual demand savings are equiva-lent to 8 months like the peak demandmonth for partial storage systems and6.5 months for full storage systems isreasonable (ASHRAE 1993). Thus,annual demand charge savings wouldbe estimated at $94,880 ($11, 860 * 8) forthe example cooling system.

Clearly, these rules-of-thumb for esti-mating the annual demand charge sav-ings should only be applied for initialscreening purposes and not for the morerefined analyses that follow. Even for theinitial screening, additional analysis ofcooling loads is required where the dif-ferential between on-peak and off-peakenergy charges is thought to be signifi-cant. At a minimum, the annual coolingload must be segregated into that occur-ring during peak and off-peak periodsfor each alternative cooling system. Sys-tem COPs should be estimated for bothperiods to determine kWh consumptionand the applicable energy charge appliedto estimate energy costs and the savingsrelative to the reference non-storage sys-tem. For selecting the best system con-figuration and operating strategy from

similar options or finalizing componentsizing and system design specifications,an evaluation based on hourly coolingloads for the full length of the coolingseason is a requisite.

The results of the screening evaluationshould determine which, if any, of thecool storage system alternatives is wor-thy of further investigation. Unless theleast costly system also incurs the low-est electricity costs, the life-cycle cost ofeach alternative should be calculated todevelop a ranking. Although the abso-lute accuracy of the screening results isrelatively poor, the relative accuracyfor comparative purposes is generallyadequate thanks to the use of commonground rules and assumptions. Still,judgment must be applied regardingwhat constitutes a significant differenceand how many alternatives should becarried forward.

Refining the EvaluationA more detailed evaluation is requiredto select the best system from thoseretained from the screening evaluationand to prepare the final system design.The most important refinement may beto develop better estimates of coolingloads and ambient air conditions whileoperating the prospective cooling sys-tems. The quality of the evaluation andresulting design can be no better thanthe quality of the underlying coolingload and weather data.

Typical building load profiles, whileadequate for the screening evaluation,are not adequate for selecting the bestsystem and preparing its design. Hourlycooling load data allow much greateraccuracy, but may be difficult to obtain.Metered chilled water production orchiller power input may be availablein some cases. Alternatively, buildingenergy simulation models could beused to estimate hourly cooling loads.

6 The peak electric demand for the cooling system usually coincides with the peak electric demand for the entire facility, but this is not neces-sarily true. More generally, the peak electric demand for the alternative cooling systems should be compared for the hour creating the peakbilling demand.

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Another option would be to repeatcalculations used to determine hourlyloads for the peak annual demand dayfor the peak demand days in each of theother 11 months. This will allow a moreaccurate assessment of demand chargereduction (remember that its not neces-sarily the peak cooling system demandthat’s important, but the cooling systemdemand coincident with peak demandat the utility metering point). Barringthe availability of metered or simulatedhourly load data, reasonable assumptionswill need to be made to estimate monthlycooling loads and the portions occurringduring on-peak and off-peak periods.Accurate knowledge of hourly loads isrequired where the difference betweenon-peak and off-peak electricity energycharges is important to the economicjustification of the cool storage system.

The refined analysis needs to specificallyconsider how the cool storage systemwill be integrated and operated withthe rest of the cooling system and theimpact of integration details on perfor-mance. Operation and control schemes(e.g., chiller priority or storage priority)must be selected. Flow diagrams identi-fying the requisite piping, pumps, valves,and heat exchangers must developed.System performance should be simu-lated for the design day at a minimumor for entire year if possible. The simula-tion should evaluate supply and returnfluid temperatures while charging anddischarging, supply air temperatureto the conditioned space, and energyinputs to the cooling system. Specialattention should be paid to the rise instorage discharge and supply air tem-peratures as storage is discharged toensure the cooling load can be comfort-ably met. The actual storage capacityrequired will be greater than the theo-retical storage capacity by a margin thatvaries depending on the storage technol-ogy and discharge rate required whenthe storage system is nearly discharged.

Vendor quotes should be obtained formajor equipment components if notalready done for the initial screening

evaluation. The costs of all ancillaryequipment (e.g., pumps, piping, valves,heat exchangers) need to be estimatedand included in the economic evalua-tion. Maintenance cost differencesshould also be evaluated and incorpo-rated into the analysis.

Case StudyAn internal-melt ice-on-coil thermalenergy storage system was installedat a GSA office building in Pittsburgh,Pennsylvania, as part of a projectupgrading the entire chilled watercooling system. Originally driven bythe need to replace CFC refrigerants,the project eventually evolved to includereplacement of the chillers and coolingtower and installation of the cool storagesystem and variable speed drives onthe chilled water and condenser waterpumps. The system was installed dur-ing the winter of 1995-1996 and hasbeen operating successfully since.

Facility DescriptionThe William S. Moorhead FederalBuilding is a 23-story, 788,000 squarefoot structure constructed in 1963.Various Federal agencies occupy thebuilding that is managed by the GSA.Occupancy is concentrated on week-days from 8:00 a.m. till 5:00 p.m. withoccasional usage during other hours.

Existing Technology DescriptionThe existing cooling system consisted oftwo 990-ton centrifugal chillers with anefficiency of about 0.90 kW/ton. Bothchillers used CFC-12 refrigerant andhad a history of leaking. Two constant-speed 125 hp chilled water pumpsprovided 2866 gpm at 125 feet of head.The 75 hp condenser pumps provided2487 gpm at 80 feet of head. Heat rejec-tion was served by a single 1980 tonroof-mounted cooling tower.

New Technology EquipmentSelectionInitial investigations examined the feasi-bility of retrofitting the existing chillers

with a non-CFC refrigerant and install-ing variable speed drives (VSDs). Whileit was possible to simply change therefrigerant and installing the VSDs wouldimprove efficiency, replacement withnew, high-efficiency non-CFC chillerswas more cost-effective. At this pointin the project development process,Duquesne Light Company, the servingutility, encouraged the GSA to considera cool storage system. By using cool stor-age, the cooling capacity of the replace-ment chillers was reduced by nearly40%, to two 600-ton units. The newchillers have a full load efficiency of0.60 kW/ton at standard rating condi-tions and 0.75 kW/ton when operatingin the ice-making mode. Constraints onphysical space dictated an ice storagesystem rather than chilled water; the icestorage units were installed in a base-ment space previously used for storageand shops. Thirty-nine modular icestorage units were installed with a totalcapacity of 7410 ton-hours.

Savings PotentialInstallation of the cool storage systemreduced the size and cost of the newchillers and also resulted in reduceddemand charges by minimizing chilleroperation during the peak demandperiod. During a typical summer week-day, the ice storage system is chargedfrom 6:00 p.m. until 6:00 a.m. the follow-ing morning. Storage is discharged fromnoon until 4:00 p.m., the utility’s peakdemand period, to minimize on-peakchiller operation in this partial storagetype system. The chillers are operatedas needed during the other hours of theday to directly meet the building coolingload. On relatively mild summer daysand when cooling during late springand early fall months, the chillers don’tneed to be run during the peak demandperiod at all. The lower chilled waterdelivery temperature possible with anice storage system also makes it possibleto consider the benefits of cold-air distri-bution in an anticipated future replace-ment of the building’s airside systems.

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Life-Cycle CostThe project cost $1.6 million, whichincluded removal and replacement ofexisting chillers and cooling towers, thecool storage system, plus pumps, heatexchangers, and other miscellaneousminor equipment. Monthly savingswere estimated to range from $60,000to $80,000 in the feasibility study, whichwas based on a simulation of buildingloads and equipment performance.Actual total energy use has been higherthan expected, but building use andcooling loads have also increased fromthat assumed in the feasibility study, sothe savings estimate may still be valid.Life-cycle cost savings were calculatedto be $10,447,177 using QuickBLCC. Theresults of the QuickBLCC calculation arepresented in detail in Appendix B. BLCC,the Building Life-Cycle Cost Softwaredeveloped by NIST, is described inAppendix A.

Implementation and Post-Implementation ExperienceThe major components of the new cool-ing plant (chillers, cooling tower, andice storage units) were installed first,followed by piping, pump, and controlrevisions. A new electronic control sys-tem replaced the old pneumatic systemto enhance energy savings and improvezone comfort. After installation, the newsystem was subject to a commissioningprocedure by the GSA. According toJerry Bower, Maintenance and Opera-tions Foreman, the system has generallyworked well. They have been able tokeep the building just as cool as the oldsystem, despite having downsized thechillers by 40%. During peak coolingperiods, the two chillers operate whilethe storage system discharges. “The sys-tem acts like three 600-ton units,” saysJerry. They have experienced a fewminor problems. The first control sys-tem was not Y2K compliant, so had tobe upgraded. Other component up-grades (e.g., valves, pumps) have alsobeen implemented over time. “These

things should have been done up front,but we didn’t have enough money,”said Jerry. Jerry would also like to havemore storage capacity, but they wereshort on space as well as money. Insummary, Jerry notes, “We were allskeptical at first, but it’s been provenand is working fine.”

The Technology in PerspectiveCool storage technologies of one type oranother have been successfully appliedin several thousand locations to reduceenergy costs, reduce chiller capacity andcost, and save energy. Relatively few ofthese applications have been in the Fed-eral sector, however. This is unfortunate,because Federal facilities tend to haveseveral characteristics that should makecool storage generally more attractivethan in other sectors. These characteris-tics include:

• Relatively large cooling systems thatcan take advantage of storage systemeconomies-of-scale.

• A preponderance of chilled watercooling systems that are generallyeasier to integrate with cool storagethan cooling systems served bydirect-expansion equipment.

• Rate structures characterized by highdemand charges and/or large varia-tion in hourly energy charges.

• Older equipment that needsreplacement.

On the other hand, Federal facilities oftensuffer from the following conditions thattend to make cool storage less attractive:

• Lack of operation and maintenanceexperience with refrigerationequipment used in some cool stor-age systems.

• Lack of training on operating andcontrol strategies for minimizingcooling system life-cycle costs.

• Limited resources for engineeringfeasibility studies and system design.

Recognition of these constraints sug-gests that Federal energy managersmay need to lean toward selection ofrelatively simple cool storage systemsthat are easier to design and operate, butshould ensure that facility staff are prop-erly trained on the operation and main-tenance of cool storage systems.

The Technology’s DevelopmentCool storage is not a new concept; infact, its first use came in the 1940sshortly after the development of vaporcompression cooling systems (Knebel1995; Hasnain 1998). Early usage wasfocused on applications with excep-tionally high ratios of peak to averagecooling demand, such as in theaters,churches, arenas, and dairies. Ice-on-coilstorage systems were often used withthe principal motivation being to reducechiller size. As cooling systems spread toother building space cooling applicationsin the 60s and 70s, cool storage was notoften used, resulting in significant elec-tric load growth concentrated duringthe daytime hours of summer. The sub-sequent low utilization of power gener-ating and delivery assets caused utilitiesto offer various incentives promotingcool storage as well as other demandmanagement technologies. The resultwas a second wave of cool storagedevelopment and use. The develop-ment of effective water stratificationtechnologies made chilled water stor-age more popular. Ice-on-coil technol-ogy improved through the developmentof non-metal coils and “packaged” sys-tems. Eutectic salt and encapsulated icestorage systems were developed to pro-vide latent heat storage alternatives toice-on-coil and ice-harvesting technolo-gies. More recent developments includeice slurry generators and chilled watersystems employing additives to decreasethe minimum storage temperature inchilled water storage systems.

Technology OutlookCool storage technology is approxi-mately 50 years old, but innovations

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continue as indicated by the recentdevelopments described above. Prod-uct innovation is driven by a competi-tive market (see manufacturers list) andchanging economic conditions. Deregu-lation of the electric utility industry hasreduced or eliminated many demandside management programs. As a result,utility incentives for cool storage are notas common or generous as they were inthe past. On the other hand, deregula-tion seems likely to spur electricity pric-ing structures (e.g., real-time pricing)that will enhance the need for load-control technologies such as cool stor-age. On net, the financial benefits ofshifting load to off-peak hours are stillvery important, but greater emphasisis being placed on system designs thatreduce chiller size and cost and/orimprove building system efficiency.

ManufacturersCool storage system manufacturers wereidentified by combining lists from prod-uct directories published by ThomasRegister, Energy Products, Heating/Piping/Air-Conditioning, Energy UserNews, Consulting-Specifying Engineer,International Thermal Storage AdvisoryCouncil, E-Source, and the InternationalDistrict Energy Association. We also con-ducted searches of Internet web sitesand library databases. Each manufac-turer was contacted to determine thetype of cool storage equipment offeredand its characteristics.

Despite our efforts, it is practically impos-sible to ensure that all manufacturers ofcool storage equipment have been iden-tified. To those, we extend our apologies.This list is provided as a service forthose interested in obtaining informa-tion on specific cool storage products.No endorsement or other judgmentregarding qualification of any manufac-turer listed is given or implied.

Applied Thermal TechnologiesHydro-Miser Division906-B BoardwalkSan Marcos, CA 92069Phone: 760-744-5031Fax: 760-744-5031Principal TES Product: chiller integratedwith external-melt, ice-on-coil storage

Baltimore Aircoil Company7595 Montivides RoadJessup, MD 20794Phone: 410-799-6200Fax: 410-799-6416Principal TES Products: chiller integratedwith internal-melt, ice-on-coil storagefor rooftop HVAC retrofit; external-melt,ice-on-coil storage; ice-on-coil tube bundles

Berg Chilling Systems, Inc.51 Nantucket Blvd.Toronto, ON, Canada M1P 2N5Contact: Walter LangillePhone: 416-755-2221Fax: 416-755-3874www.berg-group.comPrincipal TES Product: chiller integratedwith ice harvester

Caldwell Energy and Environmental, Inc.4000 Tower RoadLouisville, KY 40219Contact: Drew WozniakPhone: 502-964-6450Fax: 502-966-8732Principal TES Products: chiller integratedwith ice harvester; chilled water or ice/water storage tanks; external-melt, ice-on-coil storage

Calmac Manufacturing Corporation101 West Sheffield Ave.P.O. Box 710Englewood, NJ 07631-0710Contact: Roy NathanPhone: 201-569-0420Fax: 201-569-7593www.calmac.comPrincipal TES Products: internal-melt,ice-on-coil storage; internal-melt, ice-on-coil storage for rooftop HVAC retrofit

Chester-Jensen Company, Inc.P.O. Box 908Chester, PA 19016Contact: Steve MillerPhone: 610-876-6276Fax: 610-876-0485Principal TES Product: external-melt,ice-on-coil storage

Chicago Bridge and Iron Company601 W. 143rd Street, P.O. Box 9Plainfield, IL 60544-0009Contact: Rich HornPhone: 815439-3100Fax: 815-439-3130www.chicago-bridge.comPrincipal TES Product: chilled waterstorage tank

Cristopia Energy Systems165 Via CatarinaSan Dimas,CA 91773Contact: Moudood A. AslamPhone: 909-305-0463Fax: 909-305-0463Principal TES Product: eutectic salt latentheat storage systems

CryogelP.O. Box 910525San Diego, CA 92191Contact: Bruce McDavidPhone: 619-792-9003Fax: 619-792-2743Principal TES Product: encapsulatedwater/ice storage balls

Dunham-Bush101 Burgess RoadHarrisonburg, VA 22801Contact: Nathan HathawayPhone: 540-434-0711Fax: 540-434-4595www.dunham-bush.comPrincipal TES Product: chiller integratedwith internal-melt, ice-on-coil storage

Evapco, Inc.P.O. Box 1300Westminster, MD 21158-0399Contact: Craig GoralskiPhone: 410-756-2600Fax: 410-756-6450Principal TES Product: external-meltice-on-coil storage

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FAFCO, Inc.2690 Middlefield RoadRedwood City, CA 94063-3455Contact: Tyler BradshawPhone: 650-363-2752Fax: 650-363-8423www.fafco.comPrincipal TES Product: internal-melt,ice-on-coil storage

Girton Manufacturing Company, Inc.P.O Box 900 TRMillville, PA 17846-0900Contact: Rich PuterbaughPhone: 570-458-5521Fax: 570-458-5589Principal TES Product: chiller integratedwith external-melt, ice-on-coil storage;external-melt, ice-on-coil storage

Integrated Ice Systems Inc.Woodinville, WAContact: R.A. RolandPhone: 425-488-1877Principal TES Product: chiller integratedwith internal-melt, ice-on-coil storage

Matrix Service, Inc.San Luis Tank Division825 26th Street, P.O. Box 245Paso Robles, CA 93447-0245Contact: Lorin ToddPhone: 805-238-0888Fax: 805-238-2724Principal TES Product: chilled waterstorage tank

Morris and AssociatesP.O. Box 1046Raleigh, NC 27602Contact: Dan CaswellPhone: 919-779-1250Fax: 919-779-3466Principal TES Product: chiller integratedwith ice harvester

Paul Mueller Company1600 W. Phelps, P.O. Box 828Springfield, MO 65801-0828Contact: Duke GaultPhone: 417-831-3000Fax: 417-862-9008www.muel.comPrincipal TES Product: ice slurry generator

Natgun CorporationContact: Chris HodgsonPhone: 800-662-8486Principal TES Product: chilled waterstorage tank

North Star Ice Equipment CorporationP.O. Box 80227Seattle, WA 98108-0227Phone: 800-959-0875Fax: 206-763-7323Principal TES Product: chiller integratedwith ice harvester

Pitt-DesMoines, Inc.3400 Grand Ave.Pittsburgh, PA 15225Contact: Gary WildmanPhone: 412-331-3000Fax: 412-331-3188Principal TES Product: chilled waterstorage tank

Preload, Inc.5710 LBJ Freeway, Suite 140Dallas, TX 75240Contact: Bill DevittPhone: 972-385-0550Fax: 972-385-0557www.preload.comPrincipal TES Product: chilled waterstorage tank

Powell Energy Products, Inc.3041 Home Road, P.O Box 203Powell, OH 43065-0203Contact: Michael McRellPhone: 614-881-5596Fax: 614-881-5989Principal TES Product: internal-melt, ice-on-coil storage for rooftop HVAC retrofit

Sunwell Technologies, Inc.180 Caster Ave.Woodbridge, ON, Canada L4L 5Y7Contact: Jamie-Lee WilsonPhone: 905-856-0400Fax: 905-856-1935www.sunwell.comPrincipal TES Product: ice slurry generator

Tampa Tank, Inc.5205 Adamo DriveTampa, FL 33619Contact: Jim DanielsPhone: 813-623-2675Fax: 813-626-1641www.tampatank.comPrincipal TES Product: chilled waterstorage tank

Thermal Technologies, Inc.1827 Wehrli Road, Suite 105Naperville, IL 60565Contact: John AndrepontPhone: 630-357-2666Fax: 630-527-2349Principal TES Product: chilled waterstorage tank with chemical additives toallow lower temperature storage

Trane Company3600 Pammel Creek RoadLa Crosse, WI 54601Phone: 608-787-2000www.trane.comPrincipal TES Product: internal-melt,ice-on-coil storage

Waffle-Crete International, Inc.2500 East 9th Street Road, P.O. Box 1008Hays, KS 67601Contact: Linda McLainPhone: 785-625-3486Fax: 785-625-8542www.waffle-crete.comPrincipal TES Product: chilled waterstorage tank

Who is Using the TechnologyThousands of cool storage systems havebeen installed in the United States. Asurvey conducted for ASHRAE resultedin an estimated population of 1500–2000systems in the early 1990s (Potter 1994).The vast majority of these systems havebeen installed in non-Federal facilities.Applications cover a wide range offacility types, but most commonly areoffices, schools, retail stores, places ofworship, refrigerated food storage facili-ties, and hospitals.

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Between 80% and 85% of the systemsinstalled use one of the several kinds ofice storage. Another 10–15% use chilledwater storage, with eutectic salt systemsrepresenting about 5% of the systems inthe survey conducted by Potter (1994).Based on data collected from cool stor-age equipment manufacturers for thisFTA, only about 1% of the systemshave been installed at Federal facilities.Selected Federal sites using cool storagesystems are identified below.

Moorhead Federal Building1000 Liberty AvenuePittsburgh, PA 15222Jerry Bower, Maintenance and Operations Foreman412-395-5436

Ralph H. Johnson VA Medical CenterCharleston, SCJim Brennen, Energy Manager843-577-5011 ext. 7229

Dallas VA Medical Center4500 S. Lancaster RoadDallas, TX 75216Larry Stevenson, Energy Manager214-372-7020

Brookhaven National LaboratoryBuilding 134 CUpton, NY 11973Mark Toscana, Energy Manager516-344-2599

Sandia National LaboratoriesAlbuquerque, NMGerald Savage, Construction Management Engineer505-844-9403

U.S. Army Reserve Center9825 Garden RoadMonclova, OH 43542Gary Smith, Facility Manager419-868-3921 ext. 109

For Further Information

AssociationsAir Conditioning and RefrigerationInstitute4301 North Fairfax Drive, Suite 425Arlington, VA 22203Phone: 703-524-8800Fax: 703-528-3816www.ari.org

American Society of Heating,Refrigeration, and Air ConditioningEngineers1791 Tullie CircleAtlanta, GA 30329Phone: 404-636-8400Fax: 404-321-5478www.ashrae.org

Electric Power Research Institute3412 Hillview Avenue, P.O. Box 10412Palo Alto, CA 94303Phone: (650) 855-2000Fax: (650) 855-2263www.epri.com

HVAC&R Center150 East Gilman Street, Suite 2200Madison, WI 53703Phone: 800-858-3774Fax: 608-262-6209www.engr.wisc.edu/centers/tsarc/tsarc.html

International District Energy Association1200 19th Street, N.W. Suite 300Washington, D.C. 20036-2412Phone 202-429-5111Fax: 202-429-5113www.energy.rochester.edu/idea/

Design and Installation GuidesDesign Guide for Cool Thermal StorageAmerican Society of Heating, Refrigera-tion, and Air Conditioning Engineers1791 Tullie CircleAtlanta, Georgia 30329

Study of Operational Experience withThermal Storage SystemsAmerican Society of Heating, Refrigera-tion, and Air Conditioning Engineers1791 Tullie CircleAtlanta, Georgia 30329

Successful Cool Storage Projects: FromPlanning to OperationAmerican Society of Heating, Refrigera-tion, and Air Conditioning Engineers1791 Tullie CircleAtlanta, Georgia 30329

Source Energy and Environmental Impactsof Thermal Energy StorageCalifornia Energy Commission1516 Ninth StreetSacramento, CA 95814-5504

Commercial Space Cooling and AirHandling Technology Atlas Cool ThermalStorage ChapterE SOURCE, Inc.4755 Walnut StreetBoulder, Colorado 80301-2537

ReferencesAhlgren, R.M. 1987. Water TreatmentTechnologies for Thermal StorageSystems. EPRI EM-5545. Electric PowerResearch Institute. Palo Alto, California.

ASHRAE. 1997. ASHRAE Handbook –Fundamentals . American Society ofHeating, Refrigeration, and Air Condi-tioning Engineers. Atlanta, Georgia.

ASHRAE. 1993. Design Guide for CoolThermal Storage. American Society ofHeating, Refrigeration, and Air Condi-tioning Engineers. Atlanta, Georgia.

Bahnfleth, W.P. and W.S. Joyce. 1995.“Stratified Storage EconomicallyIncreases Capacity and Efficiencyof Campus Chilled Water System.”ASHRAE Journal (March, 1995).

Caldwell, J.S. and W.P. Bahnbleth. 1997.“Chilled Water Thermal Energy Storagewithout Electric Rate Incentives orRebates.” Journal of ArchitecturalEngineering (September, 1997).

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Cahners Business Information. 1999.Energy User News. Vol. 24, No. 5.

Cahners Business Information. 1999b.Energy User News. Vol. 24, No. 8.

Duffy, G. 1992. “Thermal StorageEmphasis Shifts to Saving Energy.”Engineered Systems (July/August, 1992).

Electric Power Research Institute. 1992.Water-Thermal Energy Storage Fact Sheet.Palo Alto, California.

E Source, Inc. 1998. Commercial SpaceCooling and Air Handling TechnologyAtlas; Cool Thermal Storage Chapter.Boulder, Colorado.

Hasnain, S.M. 1998. “Review on Sustain-able Thermal Energy Storage Technolo-gies, Part II: Cool Thermal Storage. EnergyConversion Management. Vol. 39, No. 11.

Knebel, D.E. 1995. “Current Trends inThermal Storage.” Engineered Systems(January, 1995).

MacCracken, C.D. 1993. “Off-peak airconditioning: A major energy saver.”ASHRAE Journal (May 1993).

MacCracken, C.D. 1994. “Cold AirSystems: Sleeping Giant.” Heating/Piping/Air Conditioning (April 1994).

Means, R.S. Company. 1999. MechanicalCost Data 1999. Kingston, Massachusetts.

Potter, R.A. 1994. Study of the OperationalExperience with Thermal Storage Systems.ASHRAE Research Project 766. AmericanSociety of Heating, Refrigeration, and AirConditioning Engineers. Atlanta, Georgia.

Sohn, C.W. and G.L. Cler. 1990. “Assess-ment of Market Potential in StorageCooling Systems for Army Facilities.”ASHRAE Transactions, Vol. 96, Part 1.

Strutz, M.J. 1995. “Chilled water storage:It’s more than just shedding peaks.”Proceedings, 86th Annual Conferenceof the International District EnergyAssociation. Washington, D.C.

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29


Appendixes

Appendix A: Federal Life-Cycle Costing Procedures and the BLCC Software

Appendix B: QuickBLCC Results for Case Study

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31


Appendix A

Federal Life-Cycle Costing Procedures and the BLCC Software

Federal agencies are required to evaluate energy-related investments on the basis of minimum life-cycle costs (10 CFR Part 436).A life-cycle cost evaluation computes the total long-run costs of a number of potential actions, and selects the action that mini-mizes the long-run costs. When considering retrofits, sticking with the existing equipment is one potential action, often called thebaseline condition. The life-cycle cost (LCC) of a potential investment is the present value of all of the costs associated with theinvestment over time.

The first step in calculating the LCC is the identification of the costs. Installed Cost includes cost of materials purchased and thelabor required to install them (for example, the price of an energy-efficient lighting fixture, plus cost of labor to install it).Energy Cost includes annual expenditures on energy to operate equipment. (For example, a lighting fixture that draws 100 wattsand operates 2,000 hours annually requires 200,000 watt-hours (200 kWh) annually. At an electricity price of $0.10 per kWh,this fixture has an annual energy cost of $20.) Nonfuel Operations and Maintenance includes annual expenditures on parts andactivities required to operate equipment (for example, replacing burned out light bulbs). Replacement Costs include expendituresto replace equipment upon failure (for example, replacing an oil furnace when it is no longer usable).

Because LCC includes the cost of money, periodic and aperiodic maintenance (O&M) and equipment replacement costs,energy escalation rates, and salvage value, it is usually expressed as a present value, which is evaluated by

LCC = PV(IC) + PV(EC) + PV(OM) + PV(REP)

where PV(x) denotes “present value of cost stream x,”IC is the installed cost,EC is the annual energy cost,OM is the annual nonenergy O&M cost, andREP is the future replacement cost.

Net present value (NPV) is the difference between the LCCs of two investment alternatives, e.g., the LCC of an energy-savingor energy-cost-reducing alternative and the LCC of the existing, or baseline, equipment. If the alternative’s LCC is less than thebaseline’s LCC, the alternative is said to have a positive NPV, i.e., it is cost-effective. NPV is thus given by

NPV = PV(EC0) – PV(EC

1)) + PV(OM

0) – PV(OM

1)) + PV(REP

0) – PV(REP

1)) – PV(IC)

or

NPV = PV(ECS) + PV(OMS) + PV(REPS) – PV(IC)

where subscript 0 denotes the existing or baseline condition,subscript 1 denotes the energy cost saving measure,IC is the installation cost of the alternative (note that the IC of the baseline is assumed zero),ECS is the annual energy cost savings,OMS is the annual nonenergy O&M savings, andREPS is the future replacement savings.

Levelized energy cost (LEC) is the break-even energy price (blended) at which a conservation, efficiency, renewable, or fuel-switching measure becomes cost-effective (NPV >= 0). Thus, a project’s LEC is given by

PV(LEC*EUS) = PV(OMS) + PV(REPS) – PV(IC)

where EUS is the annual energy use savings (energy units/yr). Savings-to-investment ratio (SIR) is the total (PV) savings of a mea-sure divided by its installation cost:

SIR = (PV(ECS) + PV(OMS) + PV(REPS))/PV(IC).

Some of the tedious effort of life-cycle cost calculations can be avoided by using the Building Life-Cycle Cost software, BLCC,developed by NIST. For copies of BLCC, call the FEMP Help Desk at (800) 363-3732.

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Appendix B

QuickBLCC Results for Case Study

QuickBLCC (QBLCC 2.7-00) 06-30-2000/11:38:42

QBLCC filename = COOLSTOR.QI

Analysis type = Federal Analysis—Energy Conservation Projects

Project name = Cool Storage Case Study

Base date of study = 2000

Service date = 2000

Study period = 20 years

Discount rate = 3.4%

Annually recurring costs and energy costs discounted from end of year.

Number of alternatives in file = 2Number of groups in file = 1

Note: Project alternatives displayed in increasing order of investment cost

Group code: Present-Value CostsAlternative Investment OM&R Energy Total Life-

Name Costs* Costs Costs Cycle Costs

Conventional $0 $12047177 $0 $12047177Cool Storage $1600000 $0 $0 $1600000<—MIN LCC

Comparative measures are only calculated for the alternative with lowest

LCC relative to alternative with the lowest present-value investment cost.

Comparative economic measures for Cool Storage relative to Conventional:

NET SAVINGS = $10447177; SIR = 7.53; AIRR = 14.38%

Ratio of present-value energy savings to total savings = 0.00

* Investment costs include capital replacements (if any).

Residual values are not calculated.


About FEMP’s New Technology Demonstration ProgramThe Energy Policy Act of 1992, and sub-sequent Executive Orders, mandate thatenergy consumption in Federal build-ings be reduced by 35% from 1985 levelsby the year 2010. To achieve this goal,the U.S. Department of Energy’s FederalEnergy Management Program (FEMP)is sponsoring a series of programs toreduce energy consumption at Federalinstallations nationwide. One of theseprograms, the New Technology Demon-stration Program (NTDP), is tasked toaccelerate the introduction of energy-efficient and renewable technologiesinto the Federal sector and to improvethe rate of technology transfer.

As part of this effort FEMP is sponsor-ing a series of publications that aredesigned to disemminate informationon new and emerging technologies.New Technology DemonstrationProgram publications comprise threeseparate series:

Federal Technology Alerts—longersummary reports that provide detailson energy-efficient, water-conserving,and renewable-energy technologiesthat have been selected for furtherstudy for possible implementation inthe Federal sector. Additional informa-tion on Federal Technology Alerts (FTAs)is provided in the next column.

Technology Installation Reviews—concise reports describing a new tech-nology and providing case study results,typically from another demonstrationprogram or pilot project.

Technology Focuses—brief informationon new, energy-efficient, environmen-tally friendly technologies of potentialinterest to the Federal sector.

More on FTAsFederal Technology Alerts, our signaturereports, provide summary informationon candidate energy-saving technolo-gies developed and manufactured in theUnited States. The technologies featuredin the FTAs have already entered themarket and have some experience but arenot in general use in the Federal sector.

The goal of the FTAs is to improve therate of technology transfer of newenergy-saving technologies within theFederal sector and to provide the rightpeople in the field with accurate, up-to-date information on the new technolo-gies so that they can make educatedjudgments on whether the technologiesare suitable for their Federal sites.

The information in the FTAs typicallyincludes a description of the candidatetechnology; the results of its screening

tests; a description of its performance,applications and field experience to date;a list of manufacturers; and importantcontact information. Attached appen-dixes provide supplemental informa-tion and example worksheets on thetechnology.

FEMP sponsors publication of the FTAsto facilitate information-sharing betweenmanufacturers and government staff.While the technology featured promisessignificant Federal-sector savings, theFTAs do not constitute FEMP’s endorse-ment of a particular product, as FEMPhas not independently verified perfor-mance data provided by manufacturers.Nor do the FTAs attempt to chart marketactivity vis-a-vis the technology featured.Readers should note the publication dateon the back cover, and consider the FTAsas an accurate picture of the technologyand its performance at the time of publi-cation. Product innovations and theentrance of new manufacturers or sup-pliers should be anticipated since thedate of publication. FEMP encouragesinterested Federal energy and facilitymanagers to contact the manufacturersand other Federal sites directly, and touse the worksheets in the FTAs to aidin their purchasing decisions.

Federal Energy Management ProgramThe Federal Government is the largest energy consumer in the nation. Annually, in its 500,000 buildings and 8,000 loca-tions worldwide, it uses nearly two quadrillion Btu (quads) of energy, costing over $8 billion. This represents 2.5% of allprimary energy consumption in the United States. The Federal Energy Management Program was established in 1974 toprovide direction, guidance, and assistance to Federal agencies in planning and implementing energy management pro-grams that will improve the energy efficiency and fuel flexibility of the Federal infrastructure.

Over the years several Federal laws and Executive Orders have shaped FEMP's mission. These include the Energy Policyand Conservation Act of 1975; the National Energy Conservation and Policy Act of 1978; the Federal Energy ManagementImprovement Act of 1988; and, most recently, Executive Order 12759 in 1991, the National Energy Policy Act of 1992(EPACT), Executive Order 12902 in 1994, and Executive Order 13123 in 1999.

FEMP is currently involved in a wide range of energy-assessment activities, including conducting New TechnologyDemonstrations, to hasten the penetration of energy-efficient technologies into the Federal marketplace.


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For More Information

FEMP Help Desk (800) 363-3732International callers please use (703) 287-8391Web site: www.eren.doe.gov/femp

General ContactsTed CollinsNew Technology Demonstration Program ManagerFederal Energy Management ProgramU.S. Department of Energy1000 Independence Ave., SW, EE-92Washington, D.C. 20585Phone: (202) 586-8017Fax: (202) [email protected]

Steven A. ParkerPacific Northwest National LaboratoryP.O. Box 999, MSIN: K5-08Richland, WA 99352Phone: (509) 375-6366Fax: (509) [email protected]

Technical ContactDaryl R. BrownPacific Northwest National LaboratoryP.O. Box 999, MSIN: K8-07Richland, WA 99352Phone: (509) 372-4366Fax: (509) [email protected]

Produced for the U.S. Departmentof Energy by the Pacific NorthwestNational Laboratory

DOE/EE-0241

December 2000

Log on to FEMP’s New Technology Demonstration ProgramWebsitehttp://www.eren.doe.gov/femp/prodtech/newtechdemo.html

You will find links to• An overview of the New Technology Demonstration Program

• Information on the program’s technology demonstrations

• Downloadable versions of program publications in Adobe PortableDocument Formats (pdf)

• A list of new technology projects underway

• Electronic access to the program’s regular mailing list for new productswhen they become available

• How Federal agencies may submit requests for the program to assessnew and emerging technologies

Thermal Energy Storage for Space Cooling--FTA

Documents

Transcript of Thermal Energy Storage for Space Cooling--FTA