2- thermal energy storage systems

Copyright © May 1997, Pacific Gas and Electric Company, all rights reserved. Revised 4/25/97

APPLICATION NOTEAn In-Depth Examination of an Energy

Efficiency Technology

Thermal EnergyStorage Strategies

for CommercialHVAC Systems

Summary

Thermal energy storage (TES) systemsshift cooling energy use to non-peaktimes. They chill storage media such aswater, ice, or a phase-change materialduring periods of low cooling demandfor use later to meet air-conditioningloads. Operating strategies are gener-ally classified as either full storage orpartial storage, referring to the amountof cooling load transferred from on-peakto off-peak.

TES systems are applicable in mostcommercial and industrial facilities, butcertain criteria must be met for eco-nomic feasibility. A system can be ap-propriate when maximum cooling load issignificantly higher than average load.High demand charges, and a significantdifferential between on-peak and off-peak rates, also help make TES sys-tems economic. They may also be ap-propriate where more chiller capacity isneeded for an existing system, or whereback-up or redundant cooling capacityis desirable.

Besides shifting load, TES systems mayalso reduce energy consumption, de-pending on site-specific design, notablywhere chillers can be operated at fullload during the night. Also, pumpingenergy and fan energy can be reducedby lowering the temperature of the wa-ter, and therefore the air temperature,affecting the quantity of air circulationrequired.

Capital costs tend to be higher than aconventional direct-cooling system, butother economic factors can reduce suchcosts. In new construction, ductwork

Summary .............................................1

How This TechnologySaves Energy ......................................2

Types of Demand and EnergyEfficiency Measures ...........................3

Applicability ........................................7

Field Observations to AssessFeasibility............................................7

Cost and Service Life .......................11

Laws, Codes, and Regulations........13

Definitions of Key Terms .................13

References to More Information......14

Major Manufacturers ........................14

PG&E Energy Efficiency Information© “Thermal Energy Storage”

could be smaller, allowing more usablespace. Or a TES system may enablereduction in electrical capacity, reducingthe cost of electrical service for a new orexpanding facility.

How This TechnologySaves Energy

In a TES system, a storage medium ischilled during periods of low coolingdemand, and the stored cooling is usedlater to meet air-conditioning load orprocess cooling loads.

The system consists of a storage me-dium in a tank, a packaged chiller orbuilt-up refrigeration system, and inter-connecting piping, pumps, and controls.The storage medium is generally water,ice, or a phase-change1 material(sometimes called a eutectic salt); it istypically chilled to lower temperaturesthan would be required for direct coolingto keep the storage tank size withineconomic limits. Figure 1 illustrates thebasic operation of a system that useschilled water.

Load shifting is typically the main rea-son to install a TES system. Cool stor-age systems can significantly cut oper-ating costs by cooling with cheaper off-peak energy, and reducing or eliminat-ing on-peak demand charges.

These systems have a reputation forconsuming more energy than nonstor-age systems. This has often been truewhere demand reduction was the pri-mary design objective. Cool storage 1 Bold italicized words are defined in the sectiontitle “Definition of Key Terms.”

does require the chiller to work harderto cool the system down to the requiredlower temperatures (for ice storage);and energy is needed to pump fluids inand out of storage.

But a number of design options canmake TES systems more energy-efficient than nonstorage systems. Stor-age systems let chillers operate at fullload all night , versus operating at full orpart load during the day.

Depending on the system configuration,the chiller may be smaller than would berequired for direct cooling, allowingsmaller auxiliaries such as cooling-tower fans, condenser water pumps, orcondenser fans. Pumping energy canbe reduced by increasing the chilledwater temperature range; fan energycan be cut with colder air distribution.Storage systems can also make in-creased use of heat recovery and wa-terside economizer strategies.

Chiller

Building Load

DistributionPump

Primary Pump

Warm

Cool

Discharging

Charging

Storage

Figure 1: Thermal Energy StorageSystem with Stratified Chilled Water

Storage (Source: ASHRAE)*


Types of Demand andEnergy Efficiency Measures

TES systems can be characterized bystorage medium and storage technol-ogy. Storage media include chilled wa-ter, ice, and phase-change materials,which differ in their operating charac-teristics and physical requirements forstoring energy. Storage technologiesinclude chilled water tanks, ice systems,and phase-change materials.

Chilled Water Storage

These systems use the sensible heatcapacity of water (1 Btu per pound perdegree Fahrenheit) to store cooling.Tank volume depends on the tempera-ture difference between the water sup-plied from storage and the water re-turning from the load, and the degree ofseparation between warm and cold wa-ter in the storage tank. Where mostconventional nonstorage HVAC systemsoperate on temperature differentials of10° to 12°F, chilled water systems gen-erally need a differential of at least 16°Fto keep the storage tank size reason-able. A difference of 20°F is the practi-cal maximum for most building coolingapplications, although a few systemsexceed 30°F.

Chilled water is generally stored at 39°Fto 42°F, temperatures directly compati-ble with most conventional water chillersand distribution systems. Return tem-peratures of 58° to 60°F or higher aredesirable to maximize the tank tem-perature difference and minimize tankvolume.

Tank volume is affected by the separa-tion maintained between the stored coldwater and the warm return water. Natu-ral stratification has emerged as thepreferred approach, because of its lowcost and superior performance. Colderwater remains at the bottom andwarmer, lighter water remains at the top.Specially designed diffusers transferwater into and out of a storage tank at alow velocity to minimize mixing.

The figure of merit (FOM) is a measureof a tank’s ability to maintain such sepa-ration; it indicates the effective percent-age of the total volume that will beavailable to provide usable cooling.Well-designed stratified tanks typicallyhave FOMs of 85 to 95 percent.

The practical minimum storage volumefor chilled water is approximately 10.7cubic feet per ton-hour at a 20°F tem-perature difference.

Chilled Water System ExampleApplication

Needing an additional 3,750 tons ofpeak cooling capacity for a 250,000-square-foot addition to its Dallas head-quarters, Texas Instruments chose achilled water thermal storage system:Adding chiller capacity would have costabout $10 million, while the TES systemcost about $7 million—and a $200/kWutility rebate reduced this to about $5.75million. The new system also has cutoperating costs about $1.5 million peryear and allowed postponement of anexpansion of the facility’s high-voltagesubstation.

The TES system uses a 5.2-million-gallon, thermally stratified chilled waterstorage tank, built under a parking lot.


This pre-stressed, cylindrical concretereservoir, 140 feet in diameter and 45feet in height, has a design dischargerate of 7,500 tons—delivering 40°F wa-ter at 12,000 gallons per minute (gpm)which returns to the tank at 55°F.

This system shifts 5.1 MW (35 percent)of existing electric chiller load to off-peak hours—about 8.5 percent of totalfacility demand. Five existing chillerswith total capacity of 6,500 tons nowremain off during the day. Mechanicalcooling systems now operate more effi-ciently because they are more fullyloaded (0.85 kW/ton versus the previ-ous 0.95 kW/ton) and produce 13 per-cent more annual ton-hours of cooling.

Ice Storage

Ice thermal storage uses the latent heatof fusion of water (144 Btu per pound).Storage volume is generally in therange of 2.4 to 3.3 cubic feet per ton-hour, depending on the specific ice-storage technology.

Thermal energy is stored in ice at 32°F,the freezing point of water. The equip-ment must provide charging fluid attemperatures of 15° to 26°F, below thenormal operating range of conventionalcooling equipment for air-conditioning.Depending on the storage technology,special ice-making equipment is used orstandard chillers are selected for low-temperature duty. The heat transfer fluidmay be the refrigerant itself or a secon-dary coolant such as glycol with wateror some other antifreeze solution.

The low temperature of ice can alsoprovide lower temperature air for cool-ing. The lower-temperature chilled watersupply available from ice storage allows

a higher temperature rise at the load, upto 25°F. The following technologies areused:

• Ice harvesting. Ice is formed on anevaporator surface and periodically re-leased into a tank partially filled withwater. Cold water is pumped from thetank to meet the cooling load. Returnwater is then pumped over ice in thetank. Refer to Figure 2.

• External melt ice-on-coil. Ice isformed on submerged pipes or tubesthrough which a refrigerant or secon-dary fluid is circulated. Storage is dis-charged by circulating the water thatsurrounds the pipes, melting the icefrom the outside.

• Internal melt ice-on-coil. Ice isformed on submerged pipes or tubes,as in the external melt system. Coolingis discharged by circulating warm cool-ant through the pipes, melting the icefrom the inside.

Ice HarvesterChiller

BuildingLoad

ChilledWaterPump

Ice-waterPumpIce / Water

Mixture

Figure 2: Ice Harvesting (Source: ASHRAE)*


• Encapsulated ice. Water insidesubmerged plastic containers freezesand thaws as cold or warm coolant iscirculated through the tank holding thecontainers.

• Ice slurry. Water in a water/glycolsolution is frozen into a slurry like theice in a “Sno-Cone” and pumped to astorage tank. Slurries can be pumpedfrom the tank to heat exchangers or di-rectly to cooling coils, resulting in highenergy transport rates.

The most common commercial technol-ogy today is internal melt ice-on-coil.External melt and ice-harvesting sys-tems are more common in industrial ap-plications, and can also be applied incommercial buildings. Encapsulated icesystems are also suitable for manycommercial applications.

Ice Storage Example Application

The Seafirst Building in Bellevue,Washington uses an ice storage systemand cold air distribution with smaller-than-typical ducting. An Electric PowerResearch Institute comparison showedthat this increased gross constructioncosts but reduced construction cost persquare foot by about $4, because re-duced floor-to-floor heights allowed 21stories within a height that would nor-mally accommodate only 20—thusadding 13,000 square feet of rentablespace. Smaller mechanical roomsadded another 4,000 square feet. Thisprovides about $340,000 per year ofadditional income.

Phase-Change Material Storage

Phase-change materials, or eutecticsalts, are available to melt and freeze at

selected temperatures. Most common isa mixture that stores 41 Btu per poundat its melting/freezing point of 47°F.This material is encapsulated in rectan-gular plastic containers, which arestacked in a storage tank through whichwater is circulated. The net storage vol-ume of such a system is approximatelysix cubic feet per ton-hour.

The 47°F phase-change point of thismaterial allows the use of standardchilling equipment. Discharge tempera-tures are higher than the supply tem-peratures of most conventional coolingsystems, so operating strategies may belimited.

Phase-change materials are also avail-able for lowering the storage tempera-tures of ice systems. Additives on themarket reduce freezing temperatures to28° and 12°F in ice storage tanks; theyreduce the latent heat capacity of water,as well as lower the freezing point. Thematerial is highly corrosive, so caremust be used in applying it.

Operating and ControlStrategies

TES operating strategies are generallyclassified as either full storage or par-tial storage, referring to the amount ofcooling load transferred from on-peakperiods. Strategies for operation at lessthan design loads include chiller priorityand storage priority control.

The period during which a system mustreduce electric demand is generallycalled “on-peak,” often but not neces-sarily synonymous with on-peak hoursdefined by the electric utility. In somefacilities the period may actually beshorter than the utility on-peak period.


Peak electric demand from cooling alsomay not occur simultaneously with thepeak facility demand.

Cool storage systems are usually sizedto generate enough cooling in 24 hoursto meet all the loads occurring duringthat period, but some applications uselonger cycles.

Full Storage

Full-storage, or load-shifting (shown inFigure 3), shifts the entire on-peakcooling load to off-peak hours and usu-ally operates at full capacity to chargestorage during all non-peak hours. On-

peak, all cooling loads are met fromstorage, and the chiller does not run. Afull-storage system requires relativelylarge chiller and storage capacities andis most attractive where on-peak de-mand charges are high or the on-peakperiod is short.

Partial Storage

With this strategy chiller capacity is lessthan design load. The chiller meets partof the on-peak cooling load and storage

meets the rest. Such operating strate-gies can be further subdivided into load-leveling and demand-limiting, Figures 4and 5.

In a load-leveling system, the chillertypically runs at full capacity for 24hours on the design day. When the loadis less than the chiller output, the ex-cess charges storage. When the loadexceeds chiller capacity, the additionalrequirement is discharged from storage.

LoadChiller On

Chiller Charging StorageChiller Meets Load Directly

Storage Meets Load

24-hour Period

Ton

s

Figure 3: Full Storage OperatingStrategy (Source: ASHRAE)*

LoadChiller OnChiller Charging StorageChiller Meets Load Directly

Storage Meets Load

24-hour Period

Ton

s

Figure 4: Partial-storage Load-Leveling Operating Strategy

(Source: ASHRAE)*

LoadChiller On

Chiller Charging Storage

Chiller Meets Load Directly

Storage Meets Load

24-hour Period

Ton

s Reduced On-peakDemand

Figure 5: Partial-storage Demand-Limiting Operating Strategy

(Source: ASHRAE)*


This approach minimizes requiredchiller and storage capacities and isparticularly attractive where peak cool-ing load is much higher than averageload. In many systems, cooling loadsduring off-design periods are smallenough that partial-storage systemsmay be operated as full-storage sys-tems. This can increase savings butcare must be taken to not deplete stor-age before the on-peak period is over.

A demand-limiting partial-storage sys-tem operates the chiller at reduced ca-pacity on-peak. The chiller may be con-trolled to limit the facility demand at thebilling meter. This strategy falls betweenload shifting and load leveling. Demandsavings and equipment costs are higherthan for load-leveling, and lower thanfor load-shifting. Here too care must betaken not to deplete storage before theend of the on-peak period.

Additional variations on the full- andpartial-storage strategies are possibleby scheduling the operation of multiplechillers.

Partial-storage systems use one of twocontrol strategies to divide the loadbetween chiller and storage. A chiller-priority strategy uses the chiller to di-rectly meet as much of the load as pos-sible. Cooling is supplied from storageonly when load exceeds chiller capacity.Storage-priority meets as much of theload as possible from stored cooling,using the chiller only when daily loadexceeds total stored cooling capacity.Some systems use combinations ofthese strategies. For example, chillerpriority during off-peak daytime hours,and storage priority during on-peakhours.

Applicability

Thermal energy storage can be used invirtually any building; the merits arecompelling in the right situations. Themain issues are the type of storagesystem and the amount of cooling loadto be shifted. Before embarking on acool storage project, however, oneshould consider the field observationguidelines below. In addition, alternativeapproaches should be considered foreach project.

Field Observations toAssess Feasibility

This section discusses observationsand checks that can ensure a TES sys-tem is appropriate and is installed andworking properly.

Related to Applicability

The many successful TES systems op-erating today demonstrate that thetechnology can provide significantbenefits. However, many cool storagesystems have failed to perform as pre-dicted because they did not meet thecriteria for applicability cited below.

Cool storage systems are most suitablewhere any of the following criteria apply:

• The maximum cooling load of thefacility is significantly higher than theaverage load. The higher the ratio ofpeak load to average load, the greaterthe potential.


• The electric utility rate structureincludes high demand charges,ratchet charges, or a high differentialbetween on- and off-peak energy rates.The economics are particularly attrac-tive where the cost of on-peak demandand energy is high.

• An existing cooling system is be-ing expanded. The cost of adding coolstorage capacity can be much less thanthe cost of adding new chillers.

• An existing tank suitable for coolstorage use is available. In some ret-rofits, particularly in industrial applica-tions, using existing tanks can reducethe cost of installing cool storage.

• Electric power available at the siteis limited. Where expensive trans-formers or switchgear would otherwisehave to be added, the reduction inelectric demand through the use of coolstorage can mean significant savings.

• Backup or redundant cooling ca-pacity is desirable. Cool storage canprovide short-term backup or reservecooling capacity for computer roomsand other critical applications.

• Cold air distribution can be used,is necessary, or would be beneficial.Cool storage technologies using icepermit economical use of lower-temperature supply water and air. Engi-neers can downsize pumps, piping, airhandlers, and ductwork, and realizesubstantial reductions in first cost.

If one or more of the following are true,TES may not be an appropriate tech-nology:

• The maximum cooling load of thefacility is very close to the averageload. A TES system would offer littleopportunity to downsize chilling equip-ment.

• On-peak demand charges are lowand there is little or no difference be-tween the costs of on- and off-peak en-ergy. There is little economic value forcustomers to shift cooling to off-peakperiods.

• The space available for storage islimited, there is no space available, thecost of making the space available ishigh, or the value of the space for someother use is high.

• The cooling load is too small tojustify the expense of a storage sys-tem. Typically, a peak load of 100 tonsor more has been necessary for coolstorage to be feasible.

• The design team lacks experienceor funding to conduct a thorough de-sign process. The design team shouldbe capable of TES design, which differsfrom standard HVAC system design. Ifthis is not the case, or if funding for de-sign fees is limited, the chances for asuccessful system are reduced.

Performance testing of a new thermalstorage plant is particularly important.Each system should be tested forcharge capacity, discharge capacity,and scheduling and control sequences.

The charge capacity test verifies thatthe system can fully charge storagewithin the available time. The dischargecapacity test verifies its ability to pro-vide the required cooling, at or below


the maximum usable supply tempera-ture, for each hour over the design loadprofile. Of primary concern is that storedthermal energy is used at a rate con-sistent with the design. Especially inchilled water systems, if oversizedpumps circulate water at higher thandesign flow rates, a smaller supply-return temperature difference will re-duce the capacity of the storage system.

A test of scheduling and control se-quences confirms the proper operationof valves, resetting of setpoints, andstarting and stopping of equipment, ac-cording to scheduled operating modes.

A TES system operates differently froma nonstorage system, in that the inven-tory of stored cooling must be properlymanaged. Operators of TES systemsmust receive training in basic conceptsas well as the intended operating se-quences and specific operating proce-dures for their systems. As operatorsgain experience they can improve sys-tem performance and minimize operat-ing costs by refining the design operat-ing strategies and control setpoints.

Related to Energy Savings

In new construction, TES systems canreduce overall energy consumptioneven though there may be increaseduse in the chiller: distribution of colderchilled water can allow use of smallerpumps and less fan horsepower to cir-culate a smaller quantity of air throughthe cooling coils.

In retrofits it also is possible to reduceenergy consumption but generally lessso than in new construction. Here en-ergy savings result from chillers oper-ating at full load nearly all the time, po-

tentially at higher efficiency. And opera-tion is generally at night when lowerambient temperatures make for coolercondenser temperatures, reducing en-ergy use.

However, TES systems do not alwayssave energy. A retrofit using ice storagethat fails to take advantage of the colderwater available from the ice system mayconsume more energy than a directcooling system. Yet this system canmake economic sense—consumingmore off-peak, lower cost energy atnight can still significantly reduce ex-pensive electrical demand and expen-sive on-peak electricity use.

Related to Implementation Cost

Most TES systems cost more up frontand (if cost-effective) pay off throughreduced electricity bills. But carefullydesigned systems needn’t cost more—or much more—than conventionalHVAC systems.

Sometimes cool storage systems cancost less and save energy. The TexasInstruments’ case study discussed ear-lier describes one of three chilled-waterstorage installations in Dallas that havereduced energy consumption by morethan 10 percent compared to nonstor-age alternatives. The TI installation alsoreduced capital costs millions of dollarsby deferring the need for added chillercapacity. Other factors that may helpreduce cost of implementation include:

• Smaller air flows will provide ade-quate cooling in new construction ormajor retrofits, especially for ice stor-age. This can decrease the duct di-ameter, reducing not only the cost of the


ductwork, fans, and pumps but also theinstallation labor cost.

• Smaller ducts may increase rent-able space in new construction.

• System sizing is critical. For ex-ample, a careful assessment of thenumber of hours that peak load must bemet with stored cooling could show thata considerably smaller storage systemmay only minimally affect demand re-duction.

Estimation of Energy Savings

Demand costs are the main considera-tion in determining the economics of aTES system. Energy savings may beachieved, particularly in new construc-tion or major renovation projects, buttypically are a small percentage of op-erating cost savings. In fact, energyconsumption may increase and still al-low for an economically viable project.

To determine economic feasibility, accu-rate cooling load data from the existingchiller plant is always best. If data arenot available, not considered reliable orincomplete, or if only a short time periodof data has been collected, an hourlycomputer simulation model must be de-veloped by a reputable energy analysisprofessional. Available data, even foronly a short period, can be used to helpcalibrate the simulated building modeland improve its accuracy.

Back-of-the-envelope calculations cangive a rough estimate of possible sav-ings but must not be used for the finaleconomic analysis, or chiller or storagetank sizing.

Standard Savings Calculation

The following equation can be used inestimating demand savings from TESsystems. While energy consumptionmay increase or decrease, the changegenerally has only a minor impact onproject economics. The time periodwhen the energy is consumed has asignificant impact; i.e. on-peak demandreduction or displacing expensive on-peak energy with less expensive off-peak energy use. In general, systemeconomics depend heavily on the de-mand savings and/or on-peak/off-peakrate differential. Demand savings mustbe calculated for each month of chilleroperation.

kWsavings = # tonsshifted

× (kW/ton)chiller performance

Energy shifted to off-peak is more diffi-cult to calculate without monitored fielddata or a calibrated, hourly computersimulation. A rough estimate can becalculated as follows:

kWhshifted = # tonsshifted

× (kW/ton)chiller performance

× on-peak hours× load shape factor

The load shape factor is a needed mul-tiplier because peak cooling load typi-cally is not constant. This factor, used inthe above equation, is for the on-peakperiod only (the time when cooling loadwill be shifted) and for the peak coolingload for that day. Typical load shapefactors are in the range of 60 to 90 per-cent for a variety of building types andclimates. Annual energy shifted is thesum of daily energy shifted. At thispoint, an estimate can be made using


an average cooling load for each monthand the number of cooling days in themonth, then summing the monthly to-tals.

Cost and Service Life

Factors That Influence ServiceLife and First Cost

Typically, a TES system increases costscompared to those for a direct coolingsystem. But a much larger picture needsto be looked at. Additional issues in-clude:

• Floor Height: In new construction,can low-temperature air distribution—which uses smaller, less expensiveductwork—reduce floor-to-floor height?

• Electrical Capacity: Can usingTES reduce the capacity and thereforethe cost of the electrical service to anew project, or avoid increasing theservice in the case of a building expan-sion?

• Useable Space: Can using TES,with an underground storage tank, suchas under a parking lot, free up space inan existing chiller plant or reduce thesize of a new structure?

Costs given in Table 1 are generalguidelines for initial economic evalua-tions of storage systems. They includethe cost of storage tanks and any re-quired internal diffusers, headers, orheat transfer surface. Costs will varydepending on the size of the project andsite-specific considerations, amongother things. Accurate cost estimates for

a specific application can be obtainedfrom contractors or vendors.

Costs for chilled water tanks are basedon volume, so the cost per ton-hour de-pends on the chilled water temperaturerange. Unit storage costs decrease astank size increases. The cost of chillersor refrigeration equipment must be con-sidered along with the cost of storagecapacity. Chilled water and phase-change material storage are compatiblewith typical conventional HVAC tem-peratures, and can often be added toexisting systems with no chiller modifi-cations. For ice-harvesting systems, lowstorage cost is offset by a relatively highcost for the ice-making equipment.

Typical Service Life

In addition to all equipment in a tradi-tional cooling system, a TES systemhas a storage tank, pumps, piping andpossibly an interface heat exchanger.The service life of each component(except the actual storage tank) hasbeen estimated and can be found inReference 1. All TES-specific compo-nents are rated at a 20-year minimumservice life. Storage tanks, generallyconcrete or steel, also have servicelives of at least 20 years.

Operation and MaintenanceRequirements

Factors that tend to increase mainte-nance costs for cool storage systemscompared to nonstorage systems in-clude:

• Annual tests to ensure solutionscontain proper coolant concentration,levels of corrosion inhibitors, and other


additives for ice storage systems usingglycol or other secondary coolants.Some glycol manufacturers provide freelaboratory analysis of samples. Theremay also be an expense associatedwith replacing glycol lost during mainte-nance procedures and through leaks.

• Increased water treatment ex-pense in chilled water storage andsome ice storage systems, which con-tain large volumes of chilled water in thetanks.

• Added maintenance for cool stor-age system components, such as addi-tional pumps, heat exchangers, andcontrol valves.

Factors that tend to decrease mainte-nance costs for cool storage systemsinclude:

• Smaller components, such as chill-ers, pumps, and cooling towers, for typi-cal systems.

Chilled Ice External Internal Encapsulated Phase-changeWater Harvester Melt Ice Melt Ice Ice Material

Chiller Cost Standard water

Pre-packaged or built-up ice-

making equipment

Low-temperature coolant or built-up refrigeration plant

Low-temperature secondary

coolant

Low-temperature secondary

coolantStandard water

Chiller Costa

$/ton 200-300,or use existing

1,100-1,500 per ice-making ton

200-500 200-500 200-500200-300,

or use existing

$/kW 57-85 313-427 57-142 57-142 57-142 57-85

Tank Volume

ft³/ton-hr 11-21 3.0-3.3 2.8 2.4-2.8 2.4-2.8 6.0

Storage Installed

Costb $/ton-hr 30-100 20-30 50-70 50-70 50-70 100-150

$/kWh 8.50-28 5.70-8.50 14-20 14-20 14-20 48-43

Charging Temperature (oF) 39-42 15-24 15-25 22-26 22-26 40-42

Chiller Charging

Efficiency kW/ton 0.60-0.70 0.95-1.3 0.85-1.4 0.85-1.2 0.85-1.2 0.60-0.70

COP 5.9-5.0 3.7-2.7 4.1-2.5 4.1-2.9 4.1-2.9 5.9-5.0

Discharge Temperaturec (oF)

1-4 above charging

temperature34-36 34-36 34-38 34-38 48-50

Discharge Fluid Water Water WaterSecondary

coolantSecondary

coolantWater

Tank Interface Open tank Open tank Open tank Closed systemOpen or closed

systemOpen tank

StrengthsUse existing chillers; fire

protection duty

High instantaneous

discharge rates

High instantaneous

discharge rates

Modular tanks good for small or large installations

Tank shape flexible

Use existing chillers

Comments

Storage capacity increases with

larger temperature range.

Requires clearance above

tank for ice maker.

Separate charge and discharge

circuits. Charge with coolant or

liquid refrigerant.

Notes:

a: Costs are for chiller or refrigeration plant only, and do not include installation. All costs, except ice harvesters, are per nominal ton.

Derating for actual operating conditions may be required.

b: Costs are for storage only, and include tank, internal diffusers, headers, and heat transfer surface.

c: Typical minimum temperatures, with appropriate sizing of storage capacity. Higher temperature can be obtained from each medium.

Table 1: Comparative Costs and Performance of Cool Storage Systems (Source: ASHRAE )*


• Glycol or other secondary cool-ants in the system provide coil freezeprotection, and eliminate the need todrain the cooling system in the winter,or to use special controls to prevent coilfreeze-ups.

• Cooling loads can be met fromstorage while some equipment is takenout of service for maintenance in somestorage systems.

Laws, Codes, andRegulations

All equipment and components used fora TES system should conform with thesame laws, codes, and regulations re-quired for traditional cooling systems.Large tanks may pose a problem. Zon-ing requirements, particularly height re-strictions, should be checked early. Ifheight is an issue, it is possible to com-pletely or partially bury it. It is also pos-sible that a levee will be requiredaround the tank in case of a rupture.

If a fire-protection storage tank is re-quired on site, as it would be at somemanufacturing facilities, it may be pos-sible to use this tank to store chilledwater for the cooling system. If this isavailable, it should be carefully checkedto ensure code compliance.

Definitions of Key Terms

• Condenser: The container in acooling system where gas changesphase to liquid, releasing heat to thesurroundings.

• Demand Charge: A tariff added toa customer’s electric bill that increasesin proportion to maximum kilowattsused.

• DX (direct expansion): Refers to aheat exchanger that contains the refrig-erant inside its tubing rather than water,antifreeze, or other fluid. Heat from thesurroundings is directly absorbed intothe refrigerant, which is “pumped” bythe compressor.

• Figure of Merit (FOM): A measureof a storage tank’s ability to maintainseparation between warm and cool wa-ter.

• Full Storage: Refers to a TES sys-tem that stores sufficient cooling to meetan entire peak day cooling capacity, al-lowing chillers to be off during the on-peak period.

• Off-Peak: A time period, defined bythe utility, when the cost of providingpower is relatively low, because thesystem demand for power is low. Theoff-peak period is often characterized bylower costs to the customer for energycosts, and either no or low demandcharges.

• On-Peak: A time period, defined bythe utility, when the cost of providingpower is high because the system de-mand for power is high. The on-peakperiod is typically characterized byhigher costs to the customer for energyand/or demand charges.

• Partial Storage: Refers to a TESsystem that contains sufficient coolstorage to meet part of the cooling loadof a facility. Such systems are used in


conjunction with a chiller system to pro-vide required cooling capacity.

• Phase Change: As a substancechanges between its solid, liquid, andgaseous forms, it is said to changephase. During transitions in phase—freezing, melting, condensing, boiling—the material releases or absorbs largeamounts of thermal energy withoutchanging temperature. The energy as-sociated with this is called latent heat. Amaterial that can store thermal energyas latent heat is called a phase-changematerial.

• Sensible Heat: Heat that can beperceived by the human senses, as op-posed to latent heat (see phase changedefinition).

References to MoreInformation

1. American Society of Heating Refrig-eration, and Air-Conditioning Engi-neers, “ASHRAE Handbook HVACApplications,” June 1995.

2. American Society of Heating Refrig-eration, and Air-Conditioning Engi-neers, “Design Guide for CoolThermal Storage,” 1994.

3. Electric Power Research Institute(EPRI) HVAC & R Center (formerlythe Thermal Storage Air Condition-ing Center), University of Wisconsin,150 East Gilman Street, Suite 2200,Madison, WI 537093, Tel: (800) 858-3774, Fax: (608) 262-6209.

4. E Source, “State of the Art Technol-ogy Atlas: Commercial Space Cool-ing and Air Handling,” Chapter 11,1995.

5. International Thermal Storage Advi-sory Council (ITSAC), 3769 EagleStreet, San Diego, CA 92103, Tel:(619) 295-6267.

Major Manufacturers

Chicago Bridge & Iron Co.1501 N. Division StreetPlainfield, IL 60544-8929Tel (815) 439-6000 Fax (815) 439-6010E-mail: www.chicago-bridge.com

San Luis Tank & Piping Co.825 26th StreetPaso Robles, CA 93447Tel (805) 238-0888Fax (805) 238-5123

CryogelP.O. Box 910525San Diego, CA 92191Tel (619) 792-9003Fax (619) 792-2743E-mail: [email protected]

For more information on companies whomake TES components see Reference4 above. In addition, you may contactthe International Thermal Storage Advi-sory Council (Reference 5) or the EPRIHVAC & R Center (Reference 3).

*Reprinted with permission. Copyright, 1994 byASHRAE. All rights reserved.

1

November 6, 2009

GP #8

Cut the Tip off the ENERGYberg: Thermal Ice Storage and the LEED®v3 Green Building Rating System

Peak Electrical Utility Peak Electrical Load Flattened

Load Profile using Thermal Ice Storage

Green Piece #8 specifically addresses Thermal Ice Storage and its LEED® credit opportunities. In

addition to energy cost savings credits in Energy and Atmosphere, there are categories in Water Efficiency and

Indoor Environmental Quality for schools that may have the potential to earn additional credits.

Take the initiative and go for those hard to obtain credits that push the LEED® rating from Gold to

Platinum! Ice thermal storage provides an energy efficient cooling system that is economically responsible to the

end-user and diminishes the demand on the utility’s electrical infrastructure. The benefits of ice storage include:

Reduced greenhouse gas emissions and source fuel required to generate electricity.

Lower energy costs made possible by shifting power usage to night time when rates are lower.

Smaller chillers required for partial ice storage systems-resulting in lower refrigerant charge on site.

Quiet building operation with a full ice storage system-towers and chillers turned off during the day.

Zero water consumption when Thermal Ice Storage is combined with air-cooled chillers.

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Water Efficiency

Credit

Category

Title LEED

NC

LEED

for

Schools

LEED

Core &

Shell

EVAPCO Product

Contribution

WE Credit 4

Process Water Use

Reduction

NA

1 Point

NA

EVAPCO Thermal Ice

Storage with Extra-Pak® Ice

Coils and Air Cooled

Chillers.1

Energy & Atmosphere

EA Credit 1

Optimize Energy

Performance

1-19 Points

1-19 Points

3-21 Points

EVAPCO Thermal Ice

Storage with Extra-Pak® Ice

Coils help earn credits in

this category by reducing

energy costs – ice is made

during off-peak power rates.2

EA Credit 4

Enhanced Refrigerant

Management

2 Points

1 Point

2 Points

Partial ice storage systems

utilize 40% smaller chillers

compared to conventional

chilled water plants,

therefore holding a smaller

refrigerant charge.3

Indoor Environmental Quality-Schools Only

IEQ Prerequisite 3

Minimum Acoustical

Performance

N/A

Required

N/A

Design a full storage system

to provide chilled water

during school hours and

special events, allowing the

school to turn off their

chillers and cooling towers.4

IEQ Credit 9

Enhanced Acoustical

Performance

N/A

1 Point

N/A

Design a full storage system

to provide chilled water

during school hours and

special events, allowing the

school to turn off their

chillers and cooling towers.4

Innovation in Design

ID Credit 1

Innovation in Design

1-5 Points

1-4 Points

1-5 Points

Thermal Ice Storage

reduces carbon emissions,

peak demand and the need

for new power plants.5

3

Foot Notes:

1. EVAPCO Thermal Ice Storage Systems that utilize air cooled chillers to build ice will reduce

water and energy used for air conditioning schools. If the design consultant prefers to use air-

cooled chillers, Thermal Ice Storage will provide the end-user, not only with significant water

savings, but with incremental energy savings when compared to a conventional system design

with air cooled chillers. Energy is saved by operating the chiller at night to build ice when dry

bulb temperatures are typically 20˚F to 30˚F lower than during the day.

2. “The primary benefit of thermal storage is its ability to substantially reduce total operating costs,

particularly for systems using electricity as the primary energy source. Thermal storage systems reduce

the demand for expensive on-peak electric power, substituting less expensive nighttime power to do the

same job.” ASHRAE Handbook-2008 HVAC Systems and Equipment

To receive Energy & Atmosphere points in the Optimize Energy Performance credit category,

LEED® guidelines specify increasing percentages of energy cost savings in the proposed building

design when compared to a baseline building. For an Optimize Energy credit, the HVAC system

design must show a 12% energy cost savings for new buildings and an 8% energy cost savings for

existing buildings, using the performance rating method in Appendix G of ASHRAE Standard

90.1-2007. This energy cost savings calculation must be supported using a computer simulation

model such as E-Quest, DOE-2, Visual DOE-4 or DOE Energy Plus.

For example, Figure 1 below is a current rate graph (September 2009) for Southern California

Edison (SCE) using the Real Time Pricing (RTP-2) schedule for customers with a demand greater

than 500kW. On days defined as “Very Hot Summer Weekday” (91˚F to 94 ˚F day time

temperature), the average cost of electricity is 42.4 cents/kWh. However, on those same days

the average night time cost of electricity is 5.6 cents/kWh…. a 7.5 to 1 energy cost

differential!

Data for Graph Courtesy of Southern California Edison

Figure 1- Typical Rate Structure Beneficial to Thermal Ice Storage

4

2 continued)

Thermal ice storage reduces peak electrical cost therefore reducing the overall electric cost for

the entire building. This electrical cost saving is recognized by the USGBC and the LEED rating

system. Energy and Atmosphere Credit 1 has up to 21 points available for energy cost saving

strategies based on the energy cost budget method as defined by ASHRAE 90.1.

Energy and Atmosphere, Optimize Energy Performance Credit 1

Figure 2

There are other tangible material cost and energy saving benefits of a Thermal

Ice Storage system. For example, an ice storage system design using chilled water

at 36˚F and an 18˚F delta t compared to a standard chiller system with a 45˚F

chilled water temperature and 10˚F delta t, will have smaller pumps, piping, air

handling cooling coils and ductwork!

5

3. Ice storage systems may be sized for full or partial storage. A partial storage system is

defined as “A cool storage sizing strategy in which only a portion of the on-peak cooling

load is met from thermal storage, with the rest being met by operating the chilling

equipment.” (ASHRAE-2008 HVAC Systems and Equipment Chapter 50 Thermal

Storage).

A partial storage system will typically require a chiller that is 10-40% smaller in size than

a conventional chilled water system. Smaller chillers equate to lower refrigerant

charges, resulting in less impact to the environment from potential leakage during the

operating span of the equipment.

4. In another LEED® for Schools strategy, using a full storage ice system sized for

complete on-peak cooling capacity would allow a school to run silent during the day or

special off hour event by melting ice without turning on noisy chillers and condenser

cooling equipment. The only components of the chilled water cooling plant that would

operate during the day are the small chilled water pumps.

Ice storage will enable HVAC system designers to use lower temperature air and water

to size smaller air handlers with reduced HP fan motors and slower duct velocities

resulting in lower sound levels, a requirement of IEQ Prerequisite 3 and IEQ Credit 9

shown below:

IEQ Prerequisite 3

“Achieve a maximum background noise level from heating, ventilating and air

conditioning (HVAC) systems in classrooms and other core learning spaces of 45 dBA.”

IEQ Credit 9:

“Reduce background noise level to 40 dBA or less from heating, ventilating and air

conditioning (HVAC) systems in classrooms and other core learning spaces.”

5. Thermal ice storage systems have received positive recognition and LEED® points from

the USGBC for its ability to reduce green house gas emissions as a result of lower

energy use at the power plant. It has been well documented that power plants run

more efficiently and electricity is easier to distribute to the grid at night due to lower

ambient temperatures.

In order to receive the LEED® Innovation Point, according to the USGBC’s latest

Credit Interpretation Request dated 2/23/2005, the ice storage system must be sized to

shift 5% of the total building energy use. In addition, the designer must prove that there

is a reduction in air emissions compared to a standard chilled water system.

6

There are two studies available that quantify air emission reductions by shifting electrical

demand to off peak: one by the California Energy Commission (CEC) completed in

February 1996 (P500-95-005) and a Florida Light and Power (FPL) Study presented at

the 2008 ASHRAE Summer Meeting in Salt Lake City (Seminar 69).

The CEC study evaluated the two largest electricity suppliers in California: Pacific Gas &

Electric (PG&E) and Southern California Edison (SCE). The fuel source energy savings

depends on air conditioning usage patterns, the design and operating strategy for the

thermal storage system-Full or Partial and the source of the electricity supplied (i.e.

Hydro, Coal, Natural Gas or Nuclear).

In the study, Thermal Energy Storage systems in the regions served by PG&E and SCE

shifted 40-80% of the annual kWh’s of electricity used for A/C from Day to Night.

The result was an 8 to 43% reduction in source fuel use. Since the fuel source was

reduced, it also has the added benefit of reducing greenhouses gas emissions.

In the FPL study, it was shown in their evaluation of its power and Thermal Energy

Storage (ice and chilled water) plants in their portfolio, that by shifting power to off

peak, it reduced CO2 output by .43 lbs per kWh shifted, based on 500 tons per peak

day per average installation

FPL was able to achieve this level of carbon reduction because Thermal Ice Storage

allowed them to shift from Oil and Gas fired power plants (34% efficient) to more

efficient Combined Cycle plants using natural gas (50% efficient).

The benefits to the environment and energy costs are significant with Thermal Ice

Storage. Incorporate Thermal Ice Storage ice into the chilled water plant design on your

next LEED® project.

EVAPCO will present the benefits of ice storage at GreenBuild 2009 in Phoenix. Our Thermal

Ice Storage systems are designed for commercial HVAC, industrial cooling and district energy

projects using our Extra-Pak® Ice Coils for internal or external melt applications.

EVAPCO Thermal Ice Storage Installation

7

To learn more about EVAPCO’s Thermal Ice Storage systems go to

http://www.evapco.com/ice-video.asp to view the Thermal Ice Storage video and to download

our Bulletin 401F-Ice Coils. For information on how to incorporate Thermal Ice Storage into

your new or existing system design contact me at EVAPCO or send an e-mail to

[email protected].

Keep it Green!

Daryn S. Cline

Senior Manager,

Environmental Technologies

References: 1) Source: ASHRAE Handbook-HVAC Systems and Equipment-2008 Chapter 50 Thermal Storage

2) Source: Department of Energy E-Quest software-free energy evaluation download at: http://www.doe2.com/

3) Source: ANSI/ASHRAE 90.1-2007, Energy Standard for Buildings Except Low-Rise Residential Buildings

4) Source: USGBC’s LEEDv3 http://www.usgbc.org/DisplayPage.aspx?CMSPageID=1970

5) Source: Southern California Edison RTP-2 Rate Structure

6) Source: Source Energy & Environmental Impacts of Thermal Energy Storage, California Energy Commission (CEC),

P500-95-005, Feb 1996.

7) Source: FPL Study 2008 ASHRAE Summer Meeting in Salt Lake City, UT, Seminar 69.

8) For large scale District Energy applications of Thermal Ice Storage, consult these documents published by the USGBC

District Energy and LEED NC/Schools: http://www.usgbc.org/ShowFile.aspx?DocumentID=4176

Guidance on Combined Heat and Power: http://www.usgbc.org/ShowFile.aspx?DocumentID=1354

LEED NC Guide for Multiple Buildings: http://www.usgbc.org/ShowFile.aspx?DocumentID=1097

http://www.evapco.com/ice-video.asp

mailto:[email protected]

http://www.doe2.com/

http://www.usgbc.org/DisplayPage.aspx?CMSPageID=1970

http://www.usgbc.org/ShowFile.aspx?DocumentID=4176



2- thermal energy storage systems

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