latent system thermal with energy storage operational Analysis performance of of a mechanical ventilationcooling Energy and Buildings

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Energy and Buildings

jo u r n al h om ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / e n b u i l d

Analysis of operational performance of a mechanical ventilation cooling system with latent thermal energy storage

ThiagoSantosâ^,¹,ChrisWinesâ,NickHopper^b,MariaKolokotroniâ^,∗

aInstituteofEnergyFutures,BrunelUniversityLondon,KingstonLane,Uxbridge,UB83PH,UK

bMonodraughtLtd.,HalifaxHouse,HalifaxRoad,HighWycombe,UK

a r t i c l e i n f o

Articlehistory:

Received6October2017 Receivedinrevisedform 29November2017 Accepted29November2017 Availableonline1December2017

Keywords:

ActiveLTES

Operationalperformance Cooling

Ventilation

a b s t r a c t

LatentThermalEnergyStorage(LTES)isapromisingsolutiontoreducecoolingenergyconsumptionin buildings.Laboratoryandcomputationalstudieshavedemonstrateditscapabilitieswhilecommercial passiveandactivesystemsareavailable.Thispaperpresentsdataandanalysisoftheperformanceof anactiveLTESventilationsystemintwocase-studies,aseminarroomandanopenplanofﬁceinthe UK.Analysisusingenvironmentaldatafromthesystem’scontrolaswellasadditionalspacemonitoring indicatesthat(a)internaltemperatureismaintainedwithinadaptivethermalcomfortlimits,(b)accept- ableIndoorAirQualityisalsomaintained(usingmetabolicCO2asindicator)and(c)energycostsare lowcomparedtoair-conditionedbuildings.ThermalandCFDcomputationalstudiesindicatethatpurg- ingandchargingdurationandassociatedset-pointsforroomtemperatureaswellasairﬂowrateare theimportantparametersforoptimisedperformanceforagivenLTESdesign.Theseparametersshould beoptimisedaccordingtotheuseofthespaceandprevailingexternalconditionstomaintaininternal thermalcomfortwithinupper(usuallyintheafternoon)andlower(usuallyinthemorning)limits.

1. Introduction

Energystorageisaveryactiveareaofresearchinrecentyears as it provides a sustainable solutionto energy demand fluctu- ationsand increases energyefficiency. Differentenergy storage methodssuchasmechanicalenergystorage(gravitationalenergy, flywheels);electricalstorage(e.g.batteries);thermalstorage(sensible,latent)and thermochemicalheatstorage [1]canbeused.

Thermalenergy storage(TES)is particularlysuited tobuildings becauseahighpercentageoftheirenergydemandrelatestoheating andcoolingneeds.Sensible TESutilizestheheatcapacityprop- ertiesofmaterialswhilelatentTESusesheatexchangesviathe phasechangeof materials,usually betweensolid andliquid for buildingapplications.Latentthermal energy storage (LTES)can providemore energy per volumethan a sensible thermal stor- agesystem,makingLTESapromisingsolutionforbuildingseither integratedintobuildingenvelope(passiveLTES)orinventilation systems(activeLTES)toreducecoolingdemand[2]orreduceheat- ingdemand[3].

∗Correspondingauthor.

E-mailaddress:maria.kolokotroni@brunel.ac.uk(M.Kolokotroni).

1 Permanentaddress:FederalInstituteofPernambuco,Av.ProfLuizFreire,500, Recife/PE,Brazil.

ActiveLTESintegratedinmechanicalventilationsystemshas receivedattentionduringthelasttwodecades.Aroomventilation systemincorporatingheatpipesembeddedinPCMthermalbat- terywastestedexperimentallyforapplicationsintheUK20years ago[4,5].Heattransferratesofupto200Wweremeasuredunder simulated UK summerconditions comparingthesystem favor- ablytoconventionalairconditioningandothertechnologiessuch ascooledbeams.Sincethen,manyinvestigationsthroughexper- imentsand simulationsfollowed. A recent review [6] critically discussesexperimentalstudiesofPCMapplicationsinbuildings dividingtheminfreecoolingpassiveandactivemethods,active andpassiveheatingmethodsandhybridapplications.Itdescribes developments in ventilationand air-conditioned systemsbased onPCMsaswellasnano-enhanced PCMs.Theextensivelitera- turereviewhasrevealedthatactiveLTESincorporatedwithinthe ventilationsystemcanovercomeheatexchangelimitationsofpas- sivesystemsbecauseoftheincreasedheattransferbyconvection andLTESisanappropriatesolutiontoincreaseenergyefﬁciency ofcoolingsystemsinbuildings.Areview[7]focusingoncooling LTESapplicationssummarisesexperimentalresultsofactiveLTES anddiscussestheimportanceofPCMselectionaccordingtocool- ingneedsduetointernalheatgainsandclimaticconditions.The PCMmeltingtemperatureisoneofthemostinﬂuencingparame- tersforthesuccessoftheapplication.Suchrecentreviewshavealso highlightedthatlimitedanalysishasbeenpublishedfromoper- ationalbuildingswithcommerciallyinstalledLTESsystems;such https://doi.org/10.1016/j.enbuild.2017.11.067

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Nomenclature Symbols

yi Measureddata yˆi Simulateddata N Samplesize

Ys Samplemeanofthemeasureddata T1 Coolexternalintakeairtemperature T₂ Recirculatedtemperature

T₅ Temperaturebeforethermalbatteries T7 Temperatureafterthermalbatteries T_min Minimumtemperature

T_max Maximumtemperature T_rm Runningmeantemperature Superscripts

N Samplesize Abbreviations

AC AirConditioning

ASHRAE AmericanSocietyofHeating,RefrigeratingandAir- ConditioningEngineers

CFD ComputationalFluidDynamics

CIBSE CharteredInstitutionofBuildingServicesEngineers COP Coefﬁcientofperformance

CVRMSE CoefﬁcientofVariationoftheRoot-Mean-Square Error

EPP ExpandedPolypropylene EWY ExampleWeatherYear HUI InternalSpaceHumidity

HVAC Heating,VentilationandAirConditioning IAQ IndoorAirQuality

EWY IESVE

LTES LatentThermalEnergyStorage MBE MeanBiasError

PCM PhaseChangeMaterial ppm Partspermillion RH RelativeHumidity TES ThermalEnergyStorage LTES LatentThermalEnergyStorage TUI InternalSpaceTemperature

resultsareimportanttoaccelerateinclusionindesignsfornewand refurbishedbuildings.

Thispaperattemptstoﬁllpartofthisgapbypresentingﬁeld measurementsfromtwooperationalcase-studyroomsventilated andcooledthroughacommerciallyavailablemechanicalventila- tionsystemincorporatinganactiveLTES.Section2describesthe ventilationunit,thetwocase-studiesandhowthesystemworks.

Section3presentstheoperationalperformanceofthesystemin termsofthermalcomfort,indoorairqualityandenergyuse.Sec- tion4presentsvalidatedthermalandCFDmodelswhichwereused tocarryoutparametricanalysisforperformanceimprovements.

Finally,Section5includestheconclusions.

2. Descriptionofthesystemandthecase-studybuildings 2.1. LTESventilationunit

TherearefewtechnologiesavailableonthemarketofLTESfor freecoolingandheating;oneisCool-Phase^®byMonodraughtLtd.

Fig.1showsthesysteminvestigatedinthispaperindicatingits componentsconsistingofaG4ﬁlter,recirculationdamper,fan,LTES anddiffusers.TheLTESphysicalsizevariesaccordingtocapacity(6,

Fig.1.DiagramofCool-Phase^®Unit[source:MonodraughtLtd].

8and10kW) andmodel;3995mmto5805mmwidth,966mm depthand 400mmheight.The PCM is encapsulatedin analu- miniumpanelavailablecommercially[8].Eachpanelholds2kg ofPCMabletoprovide88Wh(317kJ);eachmoduleoftheLTES studiedhas18panels.Modulesareputtogetherdependingonthe requiredcapacity;forthe10kWmodel,theunitconsistsof18pan- elsx6modules(3modules perside)whichequals to9.5 kWh (34.2MJ);the8kWmodelconsistsof18panelsx4modules(2 modulesperside)whichequalsto6.37kWh(22.8MJ).Anelec- tronicsystemcontrolsthedamperanddirectstheairﬂowthrough theLTESorbypassesitthroughanEPPExpandedPolypropylene duct.Thefanprovides260l/smaximumduringcoolingmodeand 300l/sduringchargingmode.

ThePCM’ssolidifyingtemperatureusedinLTESislimitedby thetemperature usedto charge thematerial (nightoutside air temperature).Thisselectionisalsobasedonthecoolingdemand (lowermeltingpointforhighdemandandhighermeltingpointfor lowerdemand)[9].Duetothis,theSalt-hydratedPCMSP21Eby Rubitherm(melting:22–23^◦C;solidifying:21–19^◦C;heatcapac- ity:170kJ/kgforatemperaturerangeof13–28^◦C)suitsthestudied systemanditsapplications.Salthydratesworkbyarrangingand breakingthereactionsalt-water(hydrate-dehydrate).Theyhave highlatent heat perunit volume,high conductivity(double of parafﬁn)andlittlevolumechangeduringmelting.However,salts haveadensityhigherthanwater andstayatthebottomofthe container,makingthefreezingprocessmorecomplicated[1].

Thestudiedsystemisessentiallyademandcontrolventilation andcoolingsystem,controlledbytemperature,relativehumidity andCO₂levelsinsidetheconditionedspace.Asummaryoftheoper- ationmodesispresentedinTable1.Fig.2presentsaschematicof thesystemindicatingthelocationofthesensorsusedtocontrol itsoperation.Temperatureiscontrolledbyeitherairdrawnfrom outsideorrecirculatedfromtheroom;dependingonthetempera- tureandrelativehumidityintheconditionedspace,externalairis useddirectlyormixedwithrecirculatedairby-passingtheLTES.If coolingisneededtheairisdirectedtotheLTES.Iftheroomexceeds metabolicCO2concentrationlimit,freshairfromoutsideisintro-

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Fig.2. SchematicoftheLTESsystemindicatingthelocationofthesensorsanalysedinthispaper.

Table1

Briefdescriptionofoperationmodes.

Directoutsideairventilation Usedwhentheoutsidetemperatureiscoolerthaninside,theairissuppliedintotheroombypassingtheLTESuntilit reachesasetpointtemperature.

Outsideventilationandcooling Usedwhentheoutsidetemperatureislowerthaninsidebutisnotlowenoughtocoolthespace;theaircrossesthe LTESbeforeenteringtheroom.

Recirculationandcooling Whenthetemperatureoutsideishigherthaninside,recirculatingairpassesovertheLTESbeforeenteringtheroom.

SummerCharging Duringunoccupiedhours,thefansuppliesoutsidecoldairtochargetheLTESandreleasethebuild-upheat.Whenthe LTESisfullcharged,thesystemturnsoffautomatically.

Heatrecoverycycle Inwintertimes,whentheroomisunoccupiedorwarm,theairisre-circulatedthroughtheLTEStochargeanduseit toreducetheheatingsystemload.

CO2control WhenCO2concentrationinsideishigherthanaspeciﬁcsetpoint,outsideairissupplied.

Humiditycontrol Whentheroomrelativehumidityislowerorhigherofapresetsetpoint,thesystemwillchangeoutsideairsupply untilthesetpointrangeisachieved.

Forallmodesduringoccupiedhours,anoutsideminimumvolumeﬂowisprovidedtoensureaminimumairﬂowrateaccordingtoregulations

Table2

Defaultcontrolstrategyparameters.

Season Autumn Winter Spring Summer

StartDay 01-Oct 01-Dec 01-Mar 01-May

Temperature(^◦C) 23 24 23 22

DesiredCO2(ppm) 900

HighCO2(ppm) 1200

Chargingmode 1:00to6:59

Coolingmode 8:00to20:59

Boostchargemode 00:00to00:59

FlowRate(l/s) Temperature^◦C CO2Level(ppm) Notes Enhancedﬂowrate(l/s)

300 >26 Purge 300

240 26 1800 Maxundernormalconditions 300

210 25 1600 260

175 24 1300 240

140 23 1000 210

100 22 900 Defaultminlevel 175

0 off off off

duced.Thecontrolsystemhasdefaultset-pointsbuttheuseris abletoadaptthemaccordingtoneeds.Table2presentsthedefault set-points;airtemperaturevariesaccordingtotheseasonandair- ﬂowrateaccordingtometabolicCO₂ andairtemperaturewhile maintainingrelativehumiditywithinarangeof30–70%.

2.2. Descriptionofcase-studies

Themaincase-studywiththesysteminstalledisaseminarroom atauniversitycampusinWestEngland.Similarunitshavebeen installed inmany rooms (typicalclassrooms and ofﬁces)ofthe buildingbutaseminarroomwaschosenbecauseofitsuse(com-

puterlaboratory)withhigherinternalheatgains.Inaddition,an openplanofﬁcebuildingispresentedasasecondcase-study.This secondcase-study showsthesystem’sperformance(basedonly onthesystem’smonitoringdata)toindicatearangeofapplica- tions.

Theclimateinbothlocations(UK,BristolandCambridge)istem- peratemaritimewith2684/2024HeatingDegreeDaysand196/319 Cooling Degree Days(base 15.5^◦C) [10] indicating low cooling requirementsduetoexternalconditionssocoolingloadismainly determinedbyinternalheatgains.

Themaincase-studywasrenovatedintoaseminarroombyjoin- ingthreepre-existingrooms.Theexistenceofaplenumfavoured

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Table3

Spacemonitoringsensorsdescription.

ParameterMeasured Logger Range Accuracy Resolution

TemperatureandRH HOBOUX100-003 −20^◦to70^◦C15%to95%RH ±0.21^◦Cfrom0^◦to50^◦C,±3.5%from25%to85%RH 0.024^◦C,0.07%RHat25^◦C Temperature ibuttonDS1922L –40^◦Cto+85^◦C ±0.5^◦Cfrom−10^◦Cto+65^◦C 0.5^◦C

CO2 Telaire7001 0to2500ppm ±50ppmor5%ofreading ±1ppm

Fig.3.Floorplanoftheseminarroomcase-studyalsoshowingthepositionoftheLTESunitandthepositionofpurposeinstalledsensors.

theinstallationofthesuspendedceilingsystemmodelofthesys- tem.TherefurbishedseminarroomfloorplancanbeseeninFig.3 wherethepositionofspacemonitoringsensors(forthepurpose ofthisstudy)areshown.Theroomhasafloorareaof117m²and includes29desktopcomputers,peakoccupancyof29students, andartificiallightingcomprisingof24luminaireseachequipped withone48Wlamp. Thetotal maximuminternal heat gain in theroomis60W/m².Theroomhasoneexternalwallfacingwest withU-valueof0.56W/m²Kwhile23%isglazing(overallU-value 1.82W/m²K)withinternalblinds.Ventilationandcoolingispro- videdviathe10kWLTESunitwhichispositionedinthemiddleof theroomabovethesuspendedceiling.Heatingisprovidedthrough perimeterhotwaterradiatorsandwindowsareopenable.

Thesecondcase-studyisagroundflooropenplanofficewitha floorareaof535m²,housing50officeworkerswithinternalmax- imumheatgainsof45W/m².Five8kWunits(twomasterand3 slaves)havebeeninstalled andcanbeseenonFig.4.Themas- terunitsaredirectlyconnectedtothecontrolsensorswhilethe slavesfollowtheoperationofthemasterunit.Theopenplanoffice isonestoreywithexternalwallsfacingnorth,eastandsouthand apitchedroofwithafalseceiling.WallU-valueis 0.46W/m²K, floor0.75W/m²K,roof0.187W/m²Kandwindow3.3W/m²Kwith internalblinds.Theareaofwindowsis30%ofexternalwallarea.

Heatingis providedthrough perimeterhot water radiators and windowsareopenable.

2.3. Descriptionofsystemoperation

Indicativeoperationof theLTESunitsin thetwocase-study spacesduringsummerisshowninFigs.5and6.Thecontrolsystem isverysimilarapartfromdifferencesinthetimesof‘daymode’due

tothedifferentuseofspace.Adescriptionoftheoperationmodes ispresentedbelow:

OperationMode2:NightPurge-Midnightto01:00–thefanrunsat thehighestspeedsettingfor1hwiththeaimofflushingtheroom ofstaleair.Fig.5showsthatalltemperatureswithinthisperiod decreaseincludingtheInternalSpaceTemperature,TUI.Itshould alsonoticedthatT1(externalintakeduct)isthetemperatureof thesensorplacedatthefaceoftheintakeduct.Thissensorcan beinfluencedbyinfiltrationfromtheroomiftheplenumisnot wellsealed.Thisiswhathappenedinthiscaseandhighlightsthe importanceofsensorpositionforoptimumoperationofthesystem.

Intheopenplanofﬁce(Fig.6)theexternalintakeductsensoris betterpositioned.

Operationmode3:SummerNightCharge-01:00to07:00–During thisperiod,theinletandoutlettemperaturesoftheLTES(T5and T7,respectively)canbeseentodecreaseindicatingthattheLTES isbeing‘charged’astheexternalintakeairisbeingpassedthrough them.

Operationmode4:System SwitchedOff −07:00 to 08:00–The systemisturnedofffor1hinthemorninginaperiodbeforethe occupancyschedulebegins.Acrucialobservationhowever,isthat theinternalspacetemperature(TUI)risesduringthisperiodwhich effectivelynegatessomeoftheworkdonetocoolthespaceduring thenight.Theinternalspacetemperatureat7:00waswellwithin thethermalcomfortrangeandtherewasnoriskofovercooling.A similarincreaseofinternalspacetemperatureisobservedinthe openplanofﬁce(Fig.6);inthiscasebymorethan1^◦Canditmay inﬂuencethetimingofpeakinternaltemperatureslaterintheday.

Operationmode1:SummerDayMode–08:00-21:00(seminar room)08:00to18:00(openplanofﬁce).Intheseminarroom(Fig.5) thecoolingmodestartsat8:00until21:00.Inthebeginningof theday(8:00–∼12:00)theLTESisby-passedastheoutsideairis

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Table4

CO2concentrationintheseminarroomfrom8:00to21:00.

Month 2014 2015 >1500ppmformorethan20min

Avg.CO2 Max.CO2 Avg.CO2 Max.CO2

Jan 601 1963 563 1312 0

Feb 719 2019 671 1358 1

Mar 695 2019 645 1204 0

Apr 559 2019 549 1236 0

May 469 885 443 978 0

Jun 412 607 420 531 0

Jul 409 663 420 575 0

Aug 423 580 418 826 0

Sep 493 1199 541 1326 0

Oct 599 1229 689 1268 0

Nov 701 1317 752 1480 0

Dec 551 1463 586 1551 0

Avg. 553 – 558 – –

Fig.4.Openplanofﬁceﬂoorplan.

coolerandroomtemperatureisbelowsummerset-point(22^◦C).At

∼12:00,theexternaltemperaturemeasuredbytheexternalintake duct(T1)reachesthesummerset-pointandairisdirectedthrough theLTESmaintainingairtemperatureintheroombelowexternal temperature.DifferencesbetweenT5andT7from12:00to21:00

conﬁrmLTESabsorbingheatanddeliveringcoolairintotheroom, althoughcompletemeltingmusthavebeenreachedtowardsthe endofthecoolingmode.

Intheopenplanofﬁce(Fig.6),theinternalspacetemperatureis abovetheset-point(23^◦C)at8:00,sothecoolingmodeisinitiated.

Theoutsideairtemperatureisbelowtheset-pointsorecirculated air is by-passing theLTESuntil about10:40 when theinternal temperaturereachestheset-point.Duringthisperiod,T5andT7 readingsaremisleadingasthereisnoairﬂowthroughtherouteof thesesensorsandsoT1iseffectivelythesupplyairtemperature.At 10:40theairisdirectedtotheLTESandtheinletair(T7)iscooled bythemeltingPCM;thisisshownbythesuddenriseofT5anddrop ofT7.ThedifferencebetweenT5andT7reachesitsmaximumat 17:15witha7.2^◦Cdifference.Internalspacetemperatureismain- tainedtobelowadaptivethemalcomfortupperlimitcalculatedto 27.5^◦Cforthisday(seeFig.8).

Operationmode4:SystemSwitchedOff−21:00to00:00(sem- inarroom)18:00-00:00(openplanofﬁce)-At21:00or18:00the conditionedperiodinthespaceends,thesystemswitchesoffand alltemperaturesconvergetothesamevalue.Thesystemremains switchedoffuntilmidnightwheretheprocessstartsagain.

3. Operationalperformanceduringtheyear 3.1. Method

Asmentionedbefore,datafromtheunit’scontrolsystemwere availableaswellasadditionalspacemonitoringinsidetheseminar room.Theseareusedfortheperformanceevaluation.Monitoring ofroomtemperatureandrelativehumiditywascarriedoutwithin theseminarroomusing8HOBOloggersmeasuringforoneyear at5mininterval.ThepositionofthesensorsareshowninFig.3.

SensorsH1,H2,H3andH4wereinstalledat0.70mfromtheﬂoor, H5 andH6at 1.80m,H7 atthesamelevelasthesystem’swall mountedusercontrol(1.5m)andH8wasplacedclosetoexhaust grille(locatedontheceiling).Inaddition,fouributtonloggerswere placedintheinletdiffusermeasuringairtemperatureat5mins interval. CO2 measurements weretaken usingtwo Telaire sen- sorsontwodays(25–26/11/2015)toanalyseCO₂distributionand comparewithsystemdata.Sensors’speciﬁcationsarepresentedin Table3.

3.2. Thermalcomfortanalysis–summer

Adaptivethermalcomfortapproachisusedfortheevaluation during cooling season [11], as these are current guidelines for schoolbuildings in theUK[12] and followed for non-ACofﬁce buildings[13].Theupperandlowerlimitsofadaptivethermalcom-

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Fig.5.OperationofLTESunitintheseminarroomonedayinAugust.(1=coolingmode,2=purgemode,3=chargingmode,4=off).

Fig.6.OperationofLTESunitintheopenplanofﬁceonedayinJuly.(1=coolingmode,2=purgemode,3=chargingmode,4=off).

fortarebasedoncategoryII(T_min=T−3andT_max=T+3)withT_min andTmaxcalculatedby:T=0.33Trm +18.8 whereTrm istherun- ningmeanexternaltemperature.Fig.7showstheresultsduring weekdays(8:00to21:00)fortheseminarroomfortwosummers;

temperatureandrelativehumidityistheaverageofallsensorsat 0.7m.Relativehumidityiswithinagoodrangebetween30(apart form3hononedayinMay)and70%(apartfromonedayinAugust).

Temperaturedidnotexceedtheupperthermallimit.Ingeneral,the lowsolargainsbecauseofthesmallareaofwindowsandground ﬂoorpositionoftheroomallowthesystemtomaintaincomfortable conditionsduringsummerperiodswithfulloccupancy;forexam- pleduringexaminationtimesinmidMayandmidAugust2016.

Howeversomeovercoolingoccursforsomehoursindicatingthat nightpurgingmightneedmoredetailedcontrolthanrelyingona

timer;thisisdiscussedinmoredetailinSection4.Comparisonof temperaturesatdifferentheightshaveshownnodeviationfrom thermalcomfortranges[14].

Fig.8 showsthesystemspacetemperatureintheopenplan ofﬁce for one summerseason.There is a gapdue to lostdata.

The systemshows very good performance withregards tothe calculatedthermalcomfort,withveryfewthermalcomfortlim- itsexceedance,duringoneweekinJuly(upperlimit)andinMay (lowerlimit).TheexceedanceinMayisduetolownighttemper- aturesand duringthechargingatnight the18^◦C nightcooling limitisreachedveryquicklysowhenthesystemisturnedoffat 7:00theincreaseintemperatureisnotenoughtoreachthelower limitofthermalcomfortwhentheconditionedperiodbegins.This behaviourofthecontrolsystemisrepresentativeofthetrade-off