3. Chapter1: BACKGROUND
1.1 ENERGIEDAK
SolartechInternationalcurrentlyproducessolarpoweredheatingsystemsaimedtowardsawide varietyof
buildings:residential,commercial aswell asindustrial.The productisstandardizedandappliedonflatroofsasa
buildingintegratedthermalcollector.The thermal collectoris,essentially,anetworkof pipesrunningthrough
an insulatedbulkwithadarksurface.Thusthe surface,whenexposedtosunlight,achievesahightemperature.
The pipeslocatedadjacenttothe surface,collectthe heatinthe fluidrunningthroughthem.Thisthermal
collectorcan alsobe usedto rejectheaton summernights.
On itsown,the systemdescribedabove shallprovideclimaticcomfortfora verysmall fractionof the year.Thus,
incurrent installations,thissystemiscoupledwithundergroundsensible storage systems:ATES&BTES. In such
systems,twostorage regionsare createdunderground:hotwell/aquiferforwinteruse andcoldone forsummer
use.Predictably,these systemssufferfrom:
I. Highfootprint area
II. Highinstallationcosts
III. Lossesto ambience athighstorage temperatures
IV. Lots of governmentregulationsince itmayinterfere withwatersupplyorneighbouringstorages.
A lotof smallerdwellingsandhighrise buildingsare thenleftwithouta conventional storage option.Inthe
followingsectionswe will assessthe storage demandanddiscussthe optionswhichcantake the place of ATES
and BTES.
1.2 STORAGE REQUIREMENT
A roughestimationof the storage size requirementwasdone inordertodecide the size of the labsetup.Thisinvolved
calculatingthe supplyof heatfromthe energydakfor everyhourof an year andcombiningitwiththe heatconsumption
profile of aheat pumpof 6kW capacityoperatingina typical Dutchhome.The data for the energydakwas gathered
fromthe installationonthe roof of VertigoinTU Eindhovenandwascombinedwiththe publicallyavailable weather
data. The data for the heatpump wassuppliedbyMartanvan Meurs of NRGTEQ BV (heatpumpmanufacturers).
For all the calculationswe have assumedaroof installationof 20m2
. We combine the weatherdatawiththe
correlationsdeducedfromthe testinstallationonvertigo.The heatiscollectedonlyif the calculatedoutlettemperature
fromthe collectoris1o
C higherthanthe inlettemperature.The inlettemperature isassumedtobe fixedat10o
C. Fora
storage that islarge enough,thisisa safe assumptionsince the PCMstorage will alwaysbe intransition:neverfully
frozenor fullymolten.
Nextwe calculate the dailyenergyconsumptionforasingle familydwellingthroughoutthe year.The energy
consumptionfromthe storage hastwo maincustomers:space heatingandtapwater heating.The tapwaterheating
requirementisconstantthroughoutthe year,whereasthe space heatingrequirementismodelledmonthwise.
The resultsof the aforementionedsimulationsof heatproductioninthe collectorandconsumptionbythe heatpump
are as follows:
4. Figure 1. Daily heatproduction at
the multi-energy panel and daily
heat consumption atthe heat
pump.
From figure 1 we can observe
that the average production
is8 to 9 timesthe
consumption.However,
summertime (days150to
300) experience ahigh
productionandlow
consumption,withthe
reverse beingtrue for
wintertime.Another
significantobservationisthe
day to dayvariationinheat
collectionbecause of cloud
covervariation.This
necessitatesthe use of
storage inorder to
meaningfullyutilize the heatcollectionfromthe energiedak.
Usingthe data above,we canestimate the size of storage requiredtosupplythe heatpumpcontinuously.Toachieve
thisgoal,we can test the performance of eachstorage.The parameterchosentoevaluate aparticularsize of storage is
“supplyfraction”,whichgivesthe fractionof time the storage isable toprovide the heatpumpsystem, the restof the
time itis emptyandisthus unable toprovide heat.Since wintertime performance ismuchmore sensitive,we have
expressedthe supplyfractionintotal,winterandJanuarysupplyfractions. WinterencompassesmonthsfromOctober
to April.Followingare the results:
Figure 2. Supply fraction of storage of
different capacities annually,in winter and in
January.The storage is modelled as
lossless.
From the above calculationswe have
foundthat the storage size requiredto
achieve unitsupplyfraction
(uninterruptedsupply) is294 kWh (or
about1 GJ). Thistranslatestoa vessel
size of about5000 Litres(assuming
meltingenthalpyof 200kJ/kgand the
volume fractionof 0.5 for PCMin
vessel). Thissizeistoobigfordomestic
applications.Butfromthe calculations
performedabove,anacceptable
compromise betweenvessel size and
supplyfractioncanbe achievedbasedonthe costof the vessel perunitvolume.
5. 1.3 STORAGE OPTIONS
Thermal storage systemsare designedtostore variousquantitiesof heat,atdifferenttemperatures,fordifferingperiods
of time.Theyare usedtolevel outthe differencesbetweenthe heatsupplyandheatdemandof a system.These
differencesinsupplyanddemandmaybe hourly,diurnalorevenseasonal.The workingprinciple behindthese systems
can be broadlyclassifiedintothree categories:
I. Sensible Storage:Asthe name suggests,these systemsutilize the sensible heatingof the material tostore heat.
These systemshave averylowenergydensityasthe specificheatcapacityismuch lowerthanphase change and
chemical change enthalpies.But,theyenjoyextremelylow installationcostsperunitcapacityandare thus
widelyused.Thesesystemsinclude pitstorage,boreholestorage,aquiferstorage etc.
II. Phase change storage:Phase change materialsprovideavarietyof temperature-specificstorage options.These
materialsremainattheirmelting/freezingtemperaturesuntil the entire storage haschangeditsphase.The
phase change materialsthemselvescanbe incorporatedintothe storage viadifferentencapsulations:
a. Microencapsulation:Inthis method,pcmiscontainedinaplasticcapsulesof sizes1to 30 µm. These
capsulesare theneitherincorporatedintobuildingmaterials(Kosny,2013) to act as passive storagesor
suspendedinfluidstoformslurriesforactive storage systems(Delgado,2012).
b. Macroencapsulation:MacroencapsulationreferstoPCMsencapsulatedinanytype of containersuchas
tubes,spheresorpanelswhichcanbe incorporatedintobuildingmaterialsorserve asheatexchangers
by themselves(Kalnaes,2015).In thispaper we will focusonspherical encapsulationsimmersedinheat
transferfluid(HTF).
III. Thermochemical storage
Thermochemical storage utilizesthe reversiblechemical reactionstostore the heatforlongperiodsof time.A simple
diagramaticexplanationisasfollows:
Figure 3. Schematic explanation of thermochemical storage. The two blocks of different
colours represent different chemicals. (Zondag, 2009)
Normally,the chemicalsusedforsuchstoragesare hygroscopicinnature,
i.e.theycan absorbhuge quantitiesof water.Thusheatisusedtorelease
waterfrom these materialsandwhentheyare exposedtomoisture again,
theyabsorbwaterintotheirlattice inan exothermicreaction.There is
extensive researchbeingcarriedonbyECN and TNO on salt-hydratesand
zeolitestocreate seasonal storage (Zondag,2009). The storage densityfor
these materialsare several timeshigherthanthose forphase change
materials.
These materialsare still underresearchandhave nocommercial
availability.
1.4 THERMOCLINE TWO STAGE PCM STORAGE
Of the optionsstatedabove,sphericallyencapsulatedPCMstorage wasselecteddue toreasonsof easyavailability(from
Global E systemsB.V.) andthe extensive researchavailable forsuchsystems.
Thermocline storagesare widelyusedinconjunctionwithCSPpowerplantsandwithdomesticsolarheaters.Insuch
systemsthe hot- andcold-temperatureregionsare separatedbyatemperature gradientor“thermocline”.A
thermocline PCMstorage hasPCMs of differentmeltingtemperaturesinthe same tankasshowninfigure 4. Such
storagesare alsocalledmulti-stage PCMstorages(Aldoss,2014).
6. Figure 4. Schematic diagram ofa thermocline PCM (or
multi stage PCM) storage during winter time.PCM-A has
a higher melting temperature than PCM-B, thereby
creating a thermocline.
The advantage of a thermocline systemsare three-
fold:
1. Higherchargingand dischargingrates:Asper
Aldoss(2014), boththe charging anddischarging
ratesof multi-stage storagesare higherforupto 3
stages,afterwhichgainsare minimal.
2. Lowerreturnwatertemperature:Due tolow
meltingPCMplacedat outlet,returnwater
temperature tothe heatsource islower.This
increasesthe heatcollectionrate fromthe heat
source.
3. Higherstabilityof thermocline:Since the
encapsulatedPCMisnotdisplacedwiththe
movementof HTF,the thermocline achievedis
much more stable thanina sensiblestorage.
Besidesthe advantagesstatedabove,the inclusion
of a PCMwithlow meltingtemperature alsoopens
up the possibilityof usingitasa cold
storage for summerseason. Figure 4
representsthe wintertime scheme of
the storage.Whereasfigure 5 depicts
the usage of the storage inthe summer
time where itisdividedintotwoparts,
heatstorage and coldstorage usingthe
same two PCMs.The heatstorage
servesthe tapwater heatingdemand
and ischargedduringthe day.The cold
storage servesthe space cooling
demandandis chargedduringthe night
usingthe same Energie dak.
Figure 5. Schematic diagram ofa thermocline PCM (or multi stage PCM) storage during winter
time.PCM-A has a higher melting temperature than PCM-B, thereby creating a thermocline.
7. Chapter2: PROJECT GOAL
The goal of thisprojectis to achieve the highestpossible supplyfraction from the energy storage at the lowest
possible electrical inputfor a two stage encapsulatedPCM storage. Thistranslatesinto twoseparate goalsforthe
entire project:firstlyestablishthe correlationsbetweenthe parametersandattributes1
of the thermal storage in
questions;secondlyconductasystemanalysistosuggestthe optimal designforthe two-stage storage.The breakdown
of these goalsintoaresearchprocessisdiscussedinthe nextsection.
Accordingto A.Felix Regin (2008) variablesthatdeterminethe performanceof apacked bedthermal energystorage
unitcan be dividedintothreegroupsasfollows:
1. Those connectedwiththe bedconstructionlike size,shape andpackingof the material elements,bedlength
and the geometricconfigurationof the container.
2. Those describingthe characteristicsof the flowinglike fluidpropertiesandthe massvelocity.
3. Those associatedwiththe transientresponse of the bedmaterial like initialthermal state of the bed,the inlet
temperature of the fluid,the physical andthermal propertiesof the bedmaterial andconvective heattransfer
coefficient.
To sum up,the aforementionedvariableswillbe variedinanexperimental setupinordertominimize the electric
consumptionandmaximizethe supplyfractionof the energystorage,giventhe average Dutchweatherconditionsand
the average Dutch single familyhouseholdthermal consumption. A brief summaryof the processwhichwill be
employedtoachieve the goal isdescribedasfollows:
Figure 6. Brief overview of the process which will be employed to achieve the goal stated above.
1 “Parameters” will bevaried to observe their impacton “attributes” of the storage, example in figure 6.
8. Chapter3: RESEARCH PLAN
The goal of achievingthe highestpossible supplyfraction from the energystorage at the lowestpossible electrical
input for a two stage encapsulatedPCM storage has to be carriedout inseveral stages.Since mostof the important
data for achievingthe goal hasto be collectedexperimentally,the firststepistodesignthe storage andassemble it.
Several testruns,thenhave tocarriedon the saidsetupto ensure the accuracy of instruments,establishingthe material
properties(of PCMsandHTF) and verifyingresultswiththose achievedbyprevioussuchstudies.The main
experimentationcanthenbe carriedoutin whichthe parametersof the energystorage will be variedtoyieldthe
importantoperatingattributesof the storage.Fromthe results,the correlationswill be derivedbetweenthe variables
and the operatingattributes.Thesecorrelationswill be usedtorunbasicsystemanalysismodelstofindoutthe supply
fractionsandelectricconsumptionof the storage underdifferentweatherandoperatingconditions toyieldthe design
of the storage best suitedfor a single familyDutch dwelling. The stepsdescribedabove are detailedasfollows:
3.1 DESIGN
The designof the experimental setupmustfulfillall the objectivesof the experiments.Thismeansthatthe setupmust
be able to vary and monitorall the parametersplannedtobe variedandmonitorall the attributestobe measured.
N.A.M.Amin (2012) providesanexampleof sphericallyencapsulatedPCMsetupforexperimentation.A verysimilar
setupis
suggestedas
follows:
Figure 6. Schematic
diagram of the
experimental setup
proposed in line
with the one
described in N.A.M.
Amin (2012).
9. The parameterswhichwill be manipulatedinsuchasetupare the massflow rate,charging/dischargingtemperature
and the PCMmeltingtemperature(capsulescanbe replacedusingthe removable cover).The parametersmeasuredby
the setupare the pressure dropacrossthe storage,the massflow rate and temperaturesatthe inlet,outletandat4
cross sectionsinthe storage.Whetherthe sensorscanbe insertedinside the capsuleswillbe determinedaccordingthe
capsule andsensordesign.The diffusorhose at the top distributesthe HTFflow acrossthe cross section.The coldand
the hot HTF circuitneednotbe separate if the same tankcan be maintainedatextremelycoldandhottemperatures.
3.2 TEST RUN
The experimental setupneedstobe rigorouslytested before actual experimentationcanbegin.Tothisend,there have
to be several trial runswithPCMcapsulesof same meltingpoint(since the resultsforsingle-stagespherically
encapsulatedPCMare widelyavailable).The charging/dischargingratesandthe temperature profilesof the storage
shouldmatchthose statedinliterature if the equipmentisworkingperfectly.The capsulesof differentcapsuleshasto
be each testedseparatelyforitsexactfreezingtemperature(since there canbe undercooling) and meltingtemperature
range.Besidesthese tests,the measuringequipmentmustbe testedforanyerrorsinreadingand recording.Once the
testrun issuccessful,we cancontinue withthe actual experimentation.
3.3 PARAMETER VARIATION
There are several physical parameterswhichwill affectthe workingof the storage considerably,thusinfluencingthe two
majorattributesof the storage:its supplyfractionanditselectricconsumption.The detailsof the parametervariations
to be performedforthe twoare as follows:
a) Supplyfraction:
The supplyfraction,asdefinedinthe introduction,dependsona hostof otherattributesandparametersof the
storage.Since the lowermeltingPCMhas dual usage as heatand coldstorage,all the correlationswill be established
for summeraswell aswintermode.Followingisthe listof attributesandvariablesaffectingthe supplyfraction:
I. Chargingtemperature
II. Dischargingtemperature
III. Flowrate
IV. Initial state of the storage (measuredbytemperaturesensorsinsidethe capsules)
V. PCMmeltingtemperatures(orranges)
VI. PCMconfiguration(placementof low andhightemperature meltingPCMscanbe reversed,dispersed,etc.)
VII. Capsule size
The correlationsof the above witheachotherand withmeasurablessuchasreturnwatertemperature,will make it
possible forthe systemanalysismodeltopredictthe performance of the storage.
b) Electricconsumption
The electricconsumptionof the systemisadirectfunctionof the pressure dropandthe flow rate of the thermal
storage.The pressure dropisin turn,affectedbythe following:
I. The depthof the capsule bed
II. Capsule size
III. Flowrate
Thus,varyingthe above can yieldthe correlationswhichwill predictthe electrical consumptionof the thermal
storage (inconjunctionwiththe Energie dak).
3.4 PRELIMINARY SYSTEM ANALYSIS
The preliminarysystemanalysiswilllinkupthe correlationsyieldedbythe experimentationtoprovide amodel for
functioningof the energystorage undernumericallysimulatedreal conditions.There are three setsof dataon whichthis
model will work
I. Energysupplyfromthe Energie dak(whichisdependentonweatherdata)
10. II. Thermal energyconsumptionpatternsof asingle-familyhousehold
III. Physical parametersof the energystorage
All these togethershall yieldthe supplyfraction foraparticularsetof physical parametersfordifferenttimesof the
year.Thisshouldhelpgreatlyindesigningthe optimumstorage aswell assettingabase fordeepersystemanalysisand
maybe furtherexperimentationonthe setup.
The planningof thisprocessisin the nextsection.
11. Chapter4: PROJECT PLANNING
The projectis to be finishedinthe durationof 6to 8 monthsas perthe guidelinesof the Mechanical EngineeringDepartment atTU Eindhoven. There are several lead
timesinvolvedinthe procurementof components(especiallyPCMcapsuleswithaleadtime of 8-10 weeks) andassemblyof the setup.Thismaycause the projecttobe
organizedinanorder completelydifferentfromwhatisplannedasfollows:
12. BIBLIOGRPHY
A. Felix Regin,S.S.(2008). Heat transfercharacteristicsof thermal energystorage systemusingPCMcapsules:A review.
Renewableand SustainableEnergy Reviews.
N.A.M.Amin,F.B. (2012). Effectiveness-NTUcorrelationforlow temperature PCMencapsulatedinspheres. Applied
energy.
Aldoss,R.(2014). Comparisonbetweenthe single-PCMandmulti-PCMthermal energystorage design. Energy conversion
and management.
Concentrating SolarPowerThermalStorageSystemBasics.(2013, August).Retrievedfromenergy.gov:
http://energy.gov/eere/energybasics/articles/concentrating-solar-power-thermal-storage-system-basics
Delgado,L.M. (2012). Experimental analysisof amicroencapsulatedPCMslurryasthermal storage systemandas heat
transferfluidinlaminarflow. Applied thermalEngineering.
Kalnaes,J.(2015). Phase change materialsandproductsforbuildingapplications:A state-of-artreview andfuture
researchopportunities. Energy and Buildings,154.
Katiyar,M. (n.d.).consultation.
Kosny,S.F. (2013). CostAnalysisof Simple PhaseChangeMaterial-Enhanced Building Envelopesin Southern U.S.
Climates. The National RenewableEnergyLaboratory.
Zondag.(2009, June). Salthydratesasheaters.Retrievedfromecn.nl: https://www.ecn.nl/nl/nieuws/newsletter-
en/2009/june-2009/seasonal-storage/
