THERMAL ENERGY STORAGE THE NATURAL WAY

THERMAL ENERGY STORAGETHE NATURAL WAY

Klaus Schiess P.E., CEMKSEngineers, La Jolla

ABSTRACI'This paper introduces Thermal Energy Storage (TES),also known as Off-Peak Air Conditioning (OPAC) to thenovice and the engineer. It draws the attention ofdesigners or users to the many challenges (readproblems) that some TES installations have encounteredwhen "natural" occurrences have been ignored. Almostall TES systems are addressed and somerecommendations are made for prospective TES users.

INTRODUCTIONThese days it is in vogue to do things the "natural" way.How about TES the natural way? Let's accept the lawsof nature and design systems accordingly. We know thathot air rises or water expands when it freezes, so let usnot fight nature. If we keep that in mind, we wouldhave less challenges (read "problems") in commissioningTES systems. .

BASIC CONCEPTSWe need to be very specific about the two majorcomponents of TES:

1. The total storage capacity (Ton-Hours).

2. The rate of discharging and charging. Thestorage tank must perform as a chiller, thismeans it must satisfy cooling on a per timebasis. The system must provide a certaincooling rate right to the end of its dischargingcycle and it must also be charged at a givenrate.

We can have enough storage capacity (ton-hours), but ifwe cannot get the cooling out at the desired rate andalteratively back in fast enough, then we have someunnatural challenges or "OS"problems. "OS"problems?Well, we have all seen that poster which shows the leftrail track being connected to the right track and the twoword caption under it "Oh, s...",

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TYPICAL TESDISCHARGE CURVES

CHILLER CURVECONSTANT RATE

TIME (PEAK PERIOD)

CHII.J .ED WATER STORAGEWhen storing chilled water, we attempt to separatewater at different temperatures, preferably with aconstant temperature difference and the larger thebetter. Usually air conditioning considerations limit thistemperature difference (delta-Tv) to about 20°F.

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The challenge nature gives us is to separate the twobodies of water at different temperatures. We havegone through labyrinths, cascading tanks, empty tanksystem, separating membrane, but today diffuser headerdesign is such that stratification is the "natural" methodused. If jet velocities are kept below certain values thenthe thermocline (layer of blending) can be kept to anacceptable depth. But there are other factors thatinfluence the efficiency of chilled water systems.

To achieve proper stratification, we should have aconstant delta-TO between the in-coming and out-goingwater. If we allow the return temperature to vary, weget the "natural" phenomenon of blending which willdestroy the thermocline and thus reduce storagecapacity. Also during the charging cycle, the chiller willnot "see" full charging delta-TO and will not be able toload fully in the required time available. To achieve aconstant delta t.TO,a constant return water temperatureis required, which leads to the use of variable flowpumping at least during the discharging cycle with 2-wayvalve controls and balancing to ensure a commonconstant return temperature.

Hydraulic AspectAn open tank (open to atmosphere) constitutes an opensurface and if located below the highest point in thedistribution system it is "naturally" just a plain hole inthe system. Worse there are two holes, one on thesupply and one on the return. Backflow in the supply isusually prevented by the check valve associated with thepump that provides system pressure in addition to anysystem pipe friction to be overcome. Backflow in thereturn line must be equal to the supply, otherwise somenatural things can happen to the tank like overflowing.Especially at no flow there must be no flow, otherwisethe any water in the system will drain if the system ishigher than what the atmospheric pressure can provide(approx. 33 feet in theory at sea level). A pressuresustaining valve is required to keep the system pressure.But a pressure sustaining valve is nothing else but a"natural" way of creating resistance or pressure drop.Here goes the energy that we needed to provide systempressure.

Often this extra energy loss due to having to pump thewater "up to the roof' is neglected. We all want TES tosucceed but the feasibility study must provide realisticinformation. In high rise buildings this energy loss canbe considerable and bite into the projected savings.

Worse things can happen too if this extra head is notincluded in the determination of the head for the supplypump. Lots of "natural" things happen when this pumpis a variable speed pump. As flexible as variable speed

pumps are, they still adhere to the "natural" laws ofreaction machines. The pump characteristics and thesystem characteristics still obey their relationship. In ourcase the variable speed pump will have to reach acertain speed before actual flow occurs. We allremember using the bicycle pump. We had to pushpretty hard before we got some air into the tire. Theharder the tire, the higher the pressure required beforethe little valve opened.

At the moment when we achieve flow, again "natureslaw determine what happens. We can't just getanywhere on the pump curve. The flow and pressurerelationship is fixed by the pump and the systemscharacteristics.

To avoid all this there is a "natural" way to separate theopen tank from the closed system with a flat plate heatexchanger. This works well if a large delta-TO isavailable (ice systems). With chilled water storagesystems however it cuts deeply into the efficiency of thetank. Even if we take an expensive flat plate heatexchanger and allow only a 2° drop, the drop occurstwice, so we lose 4°. The 4° in 20° represents a 20%loss.

Energy RecoveryA quick word about recovering energy on the returnwith a turbine or a pump running in reverse. I haveread an article of a pump manufacturer who claims arecovery of as close as 5% below the efficiency of thepump when running backwards as a turbine. This maybe true for one operating point on a pump iso-efficiencycurve. The chance to hit that point in practice isminute, practically impossible with a variable pumpingsystem. The real recovery rate in practice is in the 25%to 40% region.

Economics are improved for chilled water TES systemsif the water in the tank can be used to provide stand-bywater for fire fighting purposes. If the cost of a firereservoir can be off-set, payback periods are reduced.

EUTECTIC SALT STORAGE SYSTEMSThe open tank principle applies. When used as a newinstallation, we can specify the chiller for the chargingconditions normally cooling water from eutectic saltfreezing point of 47° down to 40°.

It is important to realize that this is a delta-TO of 7° andnot the lOOP- 18°P usually specified for chillers. For aretrofit application this fact must be kept in mind toestimate charging periods. If we push the design waterquantity through the evaporator with a delta-TO of 7°P,the chiller will run on part load and the "natural"

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charging period may take longer than one has planned.Also a chiller operating at part load of 7°/18°F toCharge the tank is not such a good idea.

ICE-ON-COIL DX TYPEVarious ice-on-coil systems are available. The ice-on-coil with direct expansion (DX) has faded out becauseof the problems with measuring ice thickness. Also, themelting and refreezing occurs at a point farthest awayfrom the freezing medium. To counter the efficiencyloss due to higher energy consumption req uired by lowersuction temperatures, computerized control sequencesare proposed to predict to freeze only as much ice asexpected for the next day. How do you know how muchto freeze the night before? There will always be aweather forecast factor input to predict loads ahead. Amargin for error is always evident.

The problems with uneven burning off and unevenfreezing makes it difficult to tell the chiller when to stop.To counter this effect, air is bubbled through the tank.Increasing the oxygen content in -the water is anundesirable "natural" ingredient within a distributionsystem containing corrodible surfaces.

Also note that usually the ice-on-coil systems arecontained in an open tank.

ICE-ON-COIL GLYCOL TYPEIf a mixture of glycol and water is used as the coolingmedium inside the coil, various "natural" occurrencesimprove matters. The water to be frozen (storagemedium) is stored in an open tank but with the coolingmedia (glycol mixture) inside the coils which can bepressurized. Efficiency is improved as the water willfreeze and melt first closest to the coil transporting theglycol. The glycol inside the tube also affords thebenefit of allowing a certain pressurization up to thelimits of the plastic material used. Direct connection tothe distribution system is then possible.

Tanks that can be frozen solid without the danger ofbursting are preferable to those where the controls haveto protect the structural integrity of the tank. Controlsdo have a habit of failing.

Other "natural" occurrences with glycol systems: Glycolis used at temperatures as low as 20°F. Extra insulationmay be required on piping. Valves may have to beheated to avoid freezing of condensate on the outside,which may physically hamper valve action.

Similarly, if it is possible that glycol below a temperatureof 32°F may get through a flat plate heat exchanger,water may freeze on the other circuit and cause damage.

This can occur towards the end of the charging cyclewith very low load being supplied to the system.

ENCAPSULATED ICEThere are systems that encapsulate the water incontainers usually made of plastic. Two important"natural" factors need to be considered:

1. Water expands considerably (approx. 8%) whenfreezing.

2. Flowing water chooses the path of leastresistance.

The containers are usually placed into pressurized tanks.If the glycol flows in the vertical over the containers,equal flow patterns and even distribution seems assuredif the containers expand or should move in relation toeach other. However, if the flow is in the horizontal, thebuoyancy of the ice can lead to short circuiting of someof the glycol that is supposed to flow over encapsulatedice containers. This affects the charging and dischargingrate of the tank. The total ton-hour capacity is notaffected, but as mentioned at the beginning, a TESsystem is also a chiller which has to deliver and absorbcooling energy at a certain rate. Also the containersshould make allowance for expansion within thecontainer, otherwise the plastic can be subjected toconsiderable cycling internal pressures.

ICE HARVESTERSThe ice harvester is a very simple method of storingenergy. Ice is produced to fall into a tank and the heattransfer area is large. The ice manufacturing industryquickly saw the opportunity to enter the TES market.They have been making ice for a long time, but thereare subtle differences. The ice produced was removedto be used elsewhere. With TES we recirculate water,possibly introducing impurities from the distributionsystem. A heat exchanger reduces this problem and willalso avoid problems associated with the open-tankprinciple.

Another of those "natural" effects that may causeproblems is the fusion effect of ice under pressure. Weall know how pressure makes a snowball. Some methodof keeping the ice in the original small pieces should beprovided. Incomplete burning of ice during extendedperiods of low cooling load requirements can lead toconditions where the fusion effect makes a solid lump ofice. "Channeling" occurs and the heat exchange surfaceis then reduced drastically. This is another potential"OS" problem where we have the storage capacity butwe cannot get it out at the required rate.

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SLUSH SYSTEMSIt is probably due to the problems with this fusion effectthat has caused the slush system to disappear from themarket. The idea of slush is great as we could pumpslush, which is water with increased heat content due tothe latent energy of the ice particles.

Recently a manufacturer has ventured again into makinga slush TES system. The interesting part is that theslush is used as the storage medium only and floats ontop of the storage tank. The water used in transportingthe cooling energy to the heat exchanger is drawn fromthe bottom of the tank which does not contain any slush.

CHIT.I,ED WATER DISTRIBUTION SYSTEMSChilled water systems are very forgiving systems. Inspite of that or perhaps because of it, chilled watersystems are often unbalanced and not tuned formaximum efficiency. This occurs not only from thepoint of view to friction but also the balance betweenchiller capacity and water quantities being pumpedaround the system. If a chilled water system isunbalanced, the chances are very small that the TESsystem will solve balancing problems at the same time.So, beware when adding a TES system to an existingsystem. One must do a lot of homework to check outthe existing system. Even the best designed TES systemwill from that moment on be "naturally" blamed for allthe problems that existed before.

CONCLUSION - NATURAL CAUSESWhy are we experiencing problems or challenges withTES systems? A lot of these challenges occur becausewe neglect these natural occurrences. There is alearning curve as well for designing TES systems, eventhough it may look easy at the beginning. It is thesenatural ingredients that make TES so spicy.

CONCLUSIONS - HUMAN CAUSESHumans are also a natural ingredient to TES. Oftenthere is a natural tendency of making things difficult.Some of us engineers have trouble making designs andoperational sequences simple. Control sequences mustbe simple. The person who operates the system mustunderstand it. Automatic controls have a tendency tofail or go out of calibration. Then the operation of thesystem reverts to the level of the competence of theoperator. Another slogan that can affect TESperformance: Temporary measures tend to becomepermanent.

Often problems are caused by a lack of a singleauthority in the line of responsibility to get the system tofunction as designed. Where is the final authority toensure the system will perform as expected by the client:Consultant, general contractor, storage manufacturer,chiller manufacturer, automatic controls or theOperator? It is our opinion that the design engineershould be in charge of the all the various trades until thesystem is commissioned.

It is our experience that TES systems must be designedby an engineer experienced in TES design. Too oftencapable engineers under-estimate the natural ingredientsof TES. Support of manufacturers is given up to thepoint where the equipment is sold. It is then alwaysexpected that the automatic controls, (the famous blackmagic box) will sort out everything. The control peoplespeak "controls", but it still takes the experiencedengineer to speak "TES" and instruct what and how theblack control box is supposed to control. Controlsequences must be simple.

It is our recommendation that the TES engineercommissions the system and even provides or thoroughlychecks the "as-built" drawings with the final set pointsand values achieved at the take-over clearly marked.We have repeatedly encountered projects where theoperators had to struggle through years of trials anderrors before reaching satisfactory results. Operators dochange jobs. Clear directions with actual values must beavailable to the owner at a later stage stating exactlyhow the system is supposed to operate.

Prospective owners can save themselves considerableexpense if they are prepared to employ an experiencedTES engineer on a direct basis for design andspecification, design-build, contract management andcommissioning or whatever it takes to implement a TESsystem.

Presentation given at 16th World Energy Congress,October 26-28, 1993, Georgia World Congress Center,Atlanta, Georgia.

and

International District Energy Association IDEA, 88thAnnual Conference, San Diego, CA, June 17, 1997.

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