PumplessThermalStorageSystemAbstract One of the biggest consumer of energy is the HVAC industry, catering to the cooling demand, both residential and industrial consumer. At majority sites over sized units are installed to meet the peak load, which is not only economically disadvantageous but invariably leads to wastage of energy. Thermal Storage Systems (TSS) are a solution to such problems. TSS stores cold thermal energy in Phase Change Materials (PCM), during off peak hours and utilize it during peak load time. Pump less TSS have a unique pump less operation. ThispaperexplainsthisuniqueoperationanddiscussesthetestperformedwithPTSSusingdifferentPCMs.Key words Pumpless Thermal Storage System (PTSS), Thermal Storage Systems (TSS), Phase Change Materials (PCM),peakload.

I.INTRODUCTION

1.1TheNeedofThermalStorageSystems(TSS)

The peak load during the entire day exists for only a small duration. To counter it most sites install over sized units which run at part load for most of the day at lower efficiency. Moreover the unit supplier also has to install over sized units to cater to the peak demand of the customers. One of the major contributors to the peak load is the cooling demandofthebuildings.The power consumption at any site is charged on two fronts. One is the demand charge which corresponds to the peak demand at the site. Second is the consumption rate. The demand charge is a hefty price which contributes to a chunk of expenses on power. If the peak demand is flattened (Fig1) or the peak load is shifted to the lower tariff time of the day, substantial savings can be made. Also suppliers sometimes have a variable tariff rates for the different time of the day, for instance the Thane district of Mumbai has substantially lower night time tariff of Rs 2.6 as compared to thatduringdayofRs7[1].Thisvariabletariffplancanalsobeexploitedtocutcostspentonpower.A thermal storage system is an innovative way to store the thermal energy, either cold or hot, in order to utilize it at a later time. Since economically storing electricity is not a viable option today, thermal storage systems are an attractive way of saving energy for future use. A thermal storage system use power during off peak periods to store cold thermal energy in a phase change material as latent heat, which is then recovered during the peak load hours. This flattens the peak demand and in places where the night time tariffs are lower, thermal storage system can help make great savings! Installation of thermal storage system at the site of end user also benefits the utility supplier. This is so because of the flatteningofthepeakdemand,thesurpluspower

Fig1.FlatteningofthepeakdemandusingTSS.supplycanbedistributedtoothersitesorusedforotherpurposes.Henceitisawinwinsituation!The scope of thermal storage system does not end here. It can also be utilized as a backup to meet cooling loads during power cuts. This in turn eliminates the need for DG sets. This not only reduces the dependence on fossil fuels but also helpstheindustrialusertomeetthevariousenvironmentalnormsimposedontheusageofDGsets.1.2WhatisPumplessTSS(PTSS)?PTSS is a unique thermal storage system which is designed to have a pump less operation. PTSS stores cold thermal energy by freezing PCM within its system during the night time off peak hours. This cold thermal energy is harvested during the day time peak load by melting the PCM, as seen in figure 2. Although PTSS was originally developed to work with water as the PCM, it works equally well with other PCMs. In either case it offers an extremely simple and flexible design. As shown in the Fig 3 there are two refrigeration circuits. In the red circuit the condensing unit is connected to PTSS. The condensing unit works during night or off peak period to freeze the PCM. This process is termed as charging. The blue circuit illustrates the process of melting PCMs during the peak hour to meet the cooling load. This process is called Discharging. This circuit connects the PTSS with a room unit. During Discharging the condensing unit is not required and hence remains in the OFF state. The PTSS unit is installed at an elevated level to use gravity as an aid for the refrigerant flow in the discharging. circuit. This simple but effective installation facilitates apumplessoperationandthussavesvaluableenergy.ThecapacityofPTSSsystemisestimatedinTRHrs.

Fig2.Assemblyofpipeswhoserecurrenceproducestheentiremodule

A system is designed to give the desired cooling for a fixed interval of time. For instance a unit for storage of thermal energytomeet5kWloadfor4hrswillhaveacapacityof20kWhrsor6TRHrsPTSS is essentially an insulated network of flexible pipes filled with phase change material. Fig 2 shows an assembly ofpipeswhosenetworkproducesthePTSS

Fig3.AbasiclayoutforPTSSinstallationmodule. The outer pipe is corrugated plastic pipe of 2 mean diameter. The plastic expands when the PCM within it changes phase from liquid to solid. The other two pipes are part of the two refrigerant circuits which have been mentioned aboveCircuit 1 being part of the charging process and Circuit2 that of the discharging. R22 is used in both thecircuits.During charging the condensing unit freezes the PCM by circulating refrigerant in Circuit 1. During discharging, the refrigerant flows in the circuit between the HIMODER and the room unit. The refrigerant vapors from the room unit/evaporator rise up into the HIMODER where it condenses by rejecting heat to the solid phase of PCM. The liquid refrigerant then flows back to the evaporator. The PCM in the PTSS in turn absorbs latent heat form the refrigerant.DesiredcoolingintheroomisobtainedtillallthePCMinPTSSmelts.1.3PTSSvs.OtherTSSComparingFig.3andFig4[2]theadvantagesofHIMODERareevident:

Fig4.AtypicallayoutofotherTSS

ItoffersaPUMPLESSoperationwhichisnotavailableinanyotherthermalstoragesystem(TSS).

MostTSSproduceiceinalargetankanduseittochillwater.ThischilledwateristhencirculatedinAHUstocoolthespace.Thisaddstothecostofpipingandothersophisticatedcontrols.PTSSprovidesdirect

coolingtothespaceusingasimplerefrigerantcircuitwithoutanysecondaryrefrigerant.Thusavoidinganyneedofsophisticatedcontrolsorcomplexnetworkofpiping.

TheconceptofstoringPCMinflexiblepipesallowsPTSStobeextremelyflexibleinitsinstallation.Forthesamecapacity,thesizeandarrangementofpipescanbemodifiedtofitintodifferentspaceconstraints.Afeatwhichisnearlyimpossiblewithbulkystoragetanks.

Italsohastheflexibilitytobeinstalledasonelargeunitandtherebytomeetcoolingloadsatdifferentlocations.

II.EXPERIMENTSCONDUCTED2.1SpecificationsoftheComponentsoftheTestSetupA.PTSS

32corrugatedPolyethylene(PE)pipesof2meandiameterand24inlength.

Thetotalinternalvolumeofthenetworkofpipesis42liters.

The overall dimensions of the pipe network with thermocol insulation 770 x 320 x 65 (All dimensions in mm).

IDCupipeusedfortherefrigerantcircuitconnectedwiththecondensingunit(Circuit1).

3/8IDCupipeusedfortherefrigerantcircuitconnectedtotheroomunit.(Circuit2).

The PE pipes are set up on thermocol bases, stacked one over the other. These bases hence also perform theroleoffirstlayerofinsulationforthepipes.

2 thick rubber padding is then used to cover the stack of thermocol bases. This forms the second layer of insulation.

B.CondensingUnit:

KCJ511HAE,Kirloskarmadereciprocatingcompressor.C.RoomUnit:

Standard1TRHighwallroomunitwith350CFMairhandlingcapacity.

D.PipingandInsulation

3/8IDCupipeForCircuit1and7/8IDCupipeforCircuit2.3/8IDRubberInsulationForCircuit1and7/8IDRubberInsulationforCircuit2.

Remark: The test apparatus has been set up with standard parts and not with an aim to optimize the results. The tests

conducted are qualitative in nature and not quantitative. The tests are an attempt to determine how well the assembly respondsanduptowhatextentaretheexpectationsaremetintermsofdesignedsystemcapacity.2.2DifferentPCMstested

Originally PTSS was developed to work with water as the PCM. However tests have also been conducted with inorganic salt solutions as PCM. The system has been tested with Acme Tele Power Ltd., made PCM 25. With water as PCM, it was possible to maintain lower temperatures in the room. The properties of both PCM have been enlisted in TableI.

TABLEIPCM FreezingPt.MeltingPt. Latent

Heatoffusion

Specificgravity

Water 0C 0C 330kJ/kg 1PCM25

[3]19C 25C 120kJ/kg 1.48

PCM 25 has been tested specifically for Acme Tele Power Ltd., for the application in Telecom Shelters. With water lower supply temperature (20C) in the room were targeted whereas with PCM 25 it is required to maintain a room temperatureofnotmorethan38Cintheshelter.2.2.1TestwithwaterasPCMExperimentSetup:

PTSS has been placed at an elevated level as compared to the room unit. The elevation has been kept tonotmorethanafoottotestthesystemunderminimumpossibleheadavailable.

Thecondensingunithasbeeninstalledontheoutside.

Therefrigerantcircuitsaremade.

Temperature sensors from data logger are connected to record the return and supply dry and wet bulb temperatures.

Temperature sensors are also used to determine the temperature of water inside the HIMODER and thatofcompressortopcell.

During the charging phase a power meter is used to estimate the power consumed by the condensing unit.

Thesystemischargedto7Cafterwhichthecondensingunitiscutoff.

Thecompressortopcelltemperatureislimitedto55C.

Readings for dry and wet bulb temperatures for supply and return are recorded after a time interval of 1minute.

Test1:The condensing unit is run and PTSS is charged. The variation of water temperature, inside PTSS, with time is shown inFig5.

Fig5.Temperaturechangesinwaterduringcharging

TABLEIIDATA VALUE

WaterTemperatureinPTSS 69.8F(21C)RoomTemperature 84.2F(29C),77F(25C)

WaterTemperatureattheendofcharging

5C

Timetakenforcharging 185minsMassofWaterinHIMODER

39kg

The variation in water temperature while discahrging are shown in Fig.6 The variation in the enthalapies of the return and supply air of the room are shown in Fig 7. The capacity obtained per minute (TRmins) is plotted vs time in Fig 8. TheresultsaretabulatedinTableIII.

TABLEIIIDATA RESULT

CHARGINGTIME(from21Cto7C)

195MINS

DISCHARGINGTIME 165MINSCAPACITY 1.17TRHRS

POWERCONSUMEDBYCOMPRESSOR

0.8kW

UNITSCONSUMEDBYCOMPRESSOR

2.2kWhrs

COP 1.87

Fig6.Temperaturechangesinwaterduringcharging

Fig7.Differenceinenthalpiesofreturnairandsupplyairoftheroomduringtheprocessofdischarging.

Fig8.TRMinsrecordedduringthedischargingprocess

Test 2: One of the mode of operation for PTSS could be a situation where simultaneous charging and discharging are required. This test uses the same experiment setup as was used in Test 1 to simultaneously charge and discharge the PTSS. The plot of enthalpies of return and supply air is shown in Fig 9. The variation in TRMins is plotted in plotted in Fig 10. The temperature of water inside the HIMODER stabilized to 5.4C. A continuous capacity of

approximately 0.47TR was obtained. At this point the charging and discharging rates were equal and the system settled intoequilibrium.

Fig9.Differenceinenthalpiesofreturnandsupplyairduringsimultaneouscharginganddischarging.

Fig9.TRMinsrecordedduringsimultaneouscharginganddischarging.RemarksonResults:

1. With water as PCM a COP of 1.87 is obtained which is slightly on the lower side. However the system has not been optimized to enhance the COP. With a more efficient setup a higher COP is definitely achievable.

2. The charging and discharging rate of PCM can be varied depending upon the system requirement. One way of doing this is to vary the sizes of the refrigerant piping in the PTSS module. It has been established that therateofformationoficeoverrefrigerantcoilincreaseswithincreaseinpipediameter[4].

3. In view of the above information optimum size can be established which will freeze the water in the pipesatthedesiredrate.

4. It takes around 270 mins to equalize the rate of charging and discharging. This is the case when the cooling is obtained directly from the compressor. Hence in case where direct cooling is desired and PTSS is not charged a smarter arrangement can be made to provide refrigerant directly to the room unit without entering the

PTSS.

TABLEIVData RESULT

CHARGINGTIME(from29Cto0C)

120MINS

Dischargingtime 100MINSCapacity 0.6TRHRS

Powerconsumedbycompressor

0.8kW

Unitsconsumedbycompressor

1.6kWhrs

COP 1.3Mass 63kg

2.2.2TestwithPCM25Experiment Setup: The requirement for using PCM 25 in PTSS is different from that of water as stated earlier. Hence a certain modifications in the experimental setups are made. However one test is conducted with the original setup to observe the capability of setup to work in an environment different from the one in which it is designed to perform. The PCMfreezesat19C.HoweverforthereasonsmentionedabovePCMisfrozentill0C.The difference of enthalpies is plotted in Fig 10. The TRMins recorded are plotted in Fig. 11. The results are tabulated inTABLEIV.For the actual application in telecom shelters however the cooling obtained from the melting of PCM 25 is required to maintain a temperature of 38C. Moreover since the PCM freezes at a temperature of 19C the next test is conducted byfreezingthePCMto13Cthedesiredtemperatureandsimulatingtheconditionsinsideatelecomshelter.Theexperimentalsetupishencemodifiedasfollows:

Fig10.Differenceinenthalpiesofreturnandsupplyairfromthehighwallroomunit

Fig11.TRMinsrecordedwiththeoldsetup

TheAHUisnowenclosedinaninsulatedcasing.

The insulated casing consists of a 1kW heater. The current to the heater is controlled using a modulatingcontroller.Thecurrentismonitoredusingapowermeter.

The output of the heater is so modulated that the temperature in the casing should not exceed 38C.

Akilowattmeterisusedtoobservethepowerconsumedbytheheaterduringtheentireoperation.

Fig 12 shows the variation in temperature of PCM 25 w.r.t time during the charging phase. Fig 13 shows the readings from the kilo watt meter which signifies the power consumed by the heater while a temperature of NMT 38C was maintained in the insulated chamber. The capacity of the system thus is the cumulative of the readings from the kilo watt meter taken during the discharging phase. Table V enlists the initial conditions while Table VI shows the results andendconditionsofthetest.

TABLEVDATA VALUE

PCM25TemperatureinPTSS

82.4F(28C)

Temperatureofairinsidetheenclosure.

84.74F(29.3C)

PCM25Temperatureattheendofcharging

19.0C

Evaporatingpressureforcondensingunit

85psi

Refrigerantused R22Timeintervalforrecording

chargingdata5min

Timeintervalforrecordingdischargingdata

1min

AmbientDry&WetBulbTemperatures

85F(Dry),78F(Wet)

Mass 63kg

Fig12.VariationinTemperatureofPCM25duringCharging

Fig13.TRMinsrecorded

Remarks:1. The tests for PCM 25 have been conducted using the same demonstration unit which was built to work with water as PCM. The condensing unit used in the current unit is for low temperature air conditioning. This condensing unit can be replaced by a new unit which works at higher evaporating temperatures. Thus, with the implementationofthischangewecanexpectlowerpowerconsumptionandahigherCOP.

2. Moreover it has been established using the test results that the efficiency obtained from PCM 25 is of the order of 7075%. This is in sync with the manufacturer's provided efficiency of 80%. Hence by making the changessuggestedabovethehigherefficiencycanbeachieved.

3. While using inorganic salts as PCM it is observed that it freezes over a range of temperature. One of the reasons attributed to this phenomenon is the separation of solid particulate of the salt solution. This causes the solid and liquid phase to crystallize at different temperature. Hence sometimes a nucleating agent is added to the salt solution to sharpen the freezing point. In absence of such a nucleating agent complete latent heat of the solutioncannotbetapped.Henceapartoftheheatenergyharvestedisthatofsensiblenature.[5].

IIICONCLUSION

Using TSS does not only make sense economically but also from the point of view of energy conservation. It helps flatten the peak demand and hence avoiding hefty demand charge. A simple layout and an extremely flexible design allows HIMODER to be custom made for the desired applications. The condensing unit working at night works more efficiently due to the presence of cooler night time air available to the condenser. Furthermore there is reduce dependence on DG sets. The noise and air pollution created by DG sets can be controlled significantly using TSS. It not only benefits the end user but also the utility supplier. This is so since because in the a wake of flattening of peak load, thesurpluspowercanbedistributedtoothersitesorcanbeusedforotherpurposes.The test results indicate that the PTSS design does perform to the meet the desired capacity to a certain extent. The PTSS module was designed to give a capacity of 1TRHr with water which was successfully obtained. The same unit working for PCM 25 extracts latent heat from PCM 25 upto 75%. This can be further enhanced by optimizing the setup.At the heart of PTSS is a pump less operation. It avoids any unnecessary pumping of water for chilling purpose. In particular this is an added advantage for telecom shelters since this pump less operation safeguards the shelter from any waterseepingintothesensitivecircuitswithintheshelter.

IVREFERENCES[1] Maharashtra Electricity Tariff Plan Listing, 200506, Publication of the Govt. of Maharashtra, Maharashtra Power Supplydivision.[2]CourtesyEnergeticConsultingPvt.Ltd.[3]CourtesyAcmeTelePowerPvt.Ltd.[4] Adam CristopheModeling of Ice Storage System Proceedings of the Eighth International IBPSA Conference, Netherlands,August2003.[5] M.YamahaStudy of the Thermal Characteristic of the PCM for Buildings use. Takasago Thermal Engineering Co.Ltd.,TokyoJapan.