ASHRAE.AC-02-15-1-2002 Performance Analysis of U-Tube Concentric Tube

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AC-02-15-1PerformanceAnalysisofU-Tube,ConcentricTube,andStandingColumnWellGroundHeatExchangersUsingaSystemSimulationApproachCenkYavuzturk,Ph.D.AndrewD.Chiasson,P.E.MemberASHRAEABSTRACTtheirconventionalalternatives.Despitetheseadvantages,commercialgrowthofthetechnologyhasbeenhinderedbyThedesignlengthofgroundheatexchangersforgeother-highcapitalcostsofthesesystems,ofwhichasignificantmalheatpumpsystemsisstronglydependentontheboreholeportionisattributedtotheground-loopheatexchanger.thermalresistance.TheboreholethermalresistanceisdefinedbythethermalpropertiesofthematerialsofconstructionandIndesignanddimensioningofground-sourceheatpumpthearrangementofflowchannelsofthegroundheatexchangersystems,theboreholethermalresistanceisanimportantandstronglyinfluencestheperformanceofageothermalheatparameterthataffectsbothshort-andlong-termtemperaturepumpsystem.Simulationstudiesarepresentedthatexamineresponsesoftheheatpumpsystemand,consequently,hasaandcomparethethermalperformanceofsomeofthemorestronginfluenceonthedesignlengthofthegroundheatcommontypesofgroundheatexchangers,suchassingleU-exchanger.Theboreholethermalresistanceisdefinedbyatube,doubleU-tube,concentrictube,andstandingcolumnnumberofdesignvariables,includingthecompositionandwell(withnogroundwaterbleed),usingamathematicalflowrateoftheheattransferfluid,boreholediameter,heatdescriptionforthethermalresistanceofgroundheatexchang-exchangerpipematerial,arrangementoftheflowchannels,ers.Thesystemsimulationsarebasedonashorttimestepandgroutmaterial.Thegreaterthethermalresistanceofthegroundheatexchangermodelandareconductedunderanindividualboreholeelements,thelesswillbetheheattransferequivalentsetofconditionsconsideringasmallofficebuild-ratebetweentheheatcarrierfluidandtheearth,thusrequiringing.ResultsshowthatthesingleU-tuberequiresthegreatestanincreasedlengthofthegroundheatexchanger.Giventhatlengthofborefortheconditionssimulated.Theoreticalreduc-thegoalofthedesignengineeristominimizecapitalcostsoftionsintotalborelengthofapproximately22%,33%,and36%thegroundheatexchanger,theboreholethermalresistanceareseenforthedoubleU-tube,theconcentrictube,andtheshouldbekepttoaminimum.standingcolumnwellwithnobleed,respectively.Acompar-Theboreholeconfigurationthatresultsinminimumbore-ativelife-cyclecostanalysisisalsoconductedconsideringaholethermalresistanceisobviouslythatwheretheheattrans-20-yearsystemoperation.ferfluidisindirectcontactwiththeearth.However,inmanysituations,itisnotpossibleordesirabletoselectthisconfig-INTRODUCTIONurationbecauseofprohibitivegeologicconditionsorpreven-tivegovernmentalregulations.Consequently,acompromiseGeothermalorground-sourceheatpumpsystemshaveintheheatexchangerconstructionmustbemade.beengainingincreasingpopularityforspaceconditioningincommercialandresidentialbuildingsduetotheirreducedSincethepioneeringstudiesinthe1970sregardingenergyandmaintenancecostsascomparedtoconventionalground-loopheatexchangersoftheverticalground-coupledequipment.Ontheaverage,geothermalheatpumpapplica-type,itisclearthattheso-called“U-tube”configurationhastionshavebeenshowntohavealongerlifeexpectancy,oper-emergedasthepreferredmethodofheatexchangerconstruc-atemorequietly,andcauselesscarbondioxideemissionsthantion.Inthistypeofsystem,thegroundheatexchangerCenkYavuzturkisanassistantprofessorintheDepartmentofCivilandArchitecturalEngineering,UniversityofWyoming,Laramie,Wyo.AndrewChiassonisanengineeratHardinGeotechnologies,Warren,Mich.THISPREPRINTISFORDISCUSSIONPURPOSESONLY,FORINCLUSIONINASHRAETRANSACTIONS2002,V.108,Pt.1.NottobereprintedinwholeorinpartwithoutwrittenpermissionoftheAmericanSocietyofHeating,RefrigeratingandAir-ConditioningEngineers,Inc.,1791TullieCircle,NE,Atlanta,GA30329.Opinions,findings,conclusions,orrecommendationsexpressedinthispaperarethoseoftheauthor(s)anddonotnecessarilyreflecttheviewsofASHRAE.WrittenquestionsandcommentsregardingthispapershouldbereceivedatASHRAEnolaterthanJanuary25,2002. commonlyconsistsofafieldofverticalboreholes,eachitisevidentthatthecomponentsofU-tubesystemsprovidecontainingaU-shapedpipethroughwhichaheatexchangesomewhatofaninsulatingeffecttothegroundheatexchanger.fluidiscirculated.Insuchclosed-loopU-tubesystems,heatThesingleU-tubeisthemostcommontypeofgroundtransferbetweentheheatexchangefluidandtheearthisheatexchangerinstalledintheUnitedStates.Publicationsinstronglyinfluencedbythethermalpropertiesoftheheattrans-thescientificliteratureregardingsingleU-tubeheatexchang-ferfluid,theU-tubematerial,andthebackfill(grout)materialersaretoonumeroustolisthere,butmuchoftheresearchbetweentheU-tubeandtheboreholewall.Thepastdecadehaseffortshavebeenincorporatedintoengineeringdesignmanu-seenaconsiderableamountofresearchgearedtowardals,suchasIGSHPA(1988),Caneta(1995),andKavanaughgeothermalheatpumpsystems,andmanyoftheseeffortshaveandRafferty(1997).OtherresearchstudiesthatfocusonU-beenaimedatimprovingtheperformanceofthesingleU-tubetubemodelinghavebeenreportedbyYavuzturketal.(1999),groundheatexchanger.AlthoughsignificantpracticalandShonderandBeck(1999),Rottmayeretal.(1997),andtheoreticaladvanceshavebeenmade,littletonoattentionhasMurayaetal.(1996).beengiventoothergroundheatexchangerconfigurations.AsignificantportionofrecentresearcheffortsregardingAnumberofgroundheatexchangerconfigurationsareU-tubegroundheatexchangershasdealtwiththedevelop-possible,andmanyhavebeensuccessfullyconstructedandmentofthermallyenhancedgrouts(groutsofhigherthermaloperated.Inthisstudy,weareconcernedonlywithsomeoftheconductivities)forthepurposeofimprovingoverallheatmorecommontypes,asshowninFigure1.Theseground-exchangerperformance,andtherebyreducingnecessarybore-couplingconfigurationsarethe(a)singleU-tube,(b)doubleholelengths.RemundandLund(1993)reportedonimprovingU-tube,(c)closedconcentrictube,and(d)standingcolumnthethermalconductivityofbentonitegroutswiththeuseofwellwithoutgroundwaterbleed.quartzitesand.AllanandKavanaugh(1999)usedsilicasand,Theobjectiveofthiswork,therefore,hasbeentotheoret-aluminagrit,steelgrit,andsiliconcarbideasfillerstoincreaseicallyexamineandtocomparetheheattransferperformancethethermalconductivityofbentonitegroutstovaluesof0.98ofthemostcommontypesofgroundheatexchangerconfig-to1.90Btu/h⋅ft⋅ºF(1.70to3.29W/m⋅K)andachievedatheo-urations.Thiscomparisonismadebymeansofasystemsimu-reticalreductioninrequiredborelengthof22%to37%forthelationapproachusingashorttimestepgroundheatexchangeruseofcement-sandgrouts.AnanalysisofthecostsavingsofmodeldevelopedbyYavuzturkandSpitler(1999).Themainboreholefieldswiththermallyenhancedgrouts(Universityofadvantageofusingasimulationapproachforthisstudyisthateachtypeofgroundheatexchangercanbeevaluatedunderanidealized,equalsetofoperatingconditions,ataskthatisinher-entlydifficulttoaccomplishbymeansofanoutdoorexperi-mentalinvestigation.BACKGROUNDThefirsttypeofgroundheatexchangerexaminedinthisstudyisthesingleU-tube(Figure1a).Dependingupongeologicconditions,boreholesforsingleU-tubeheatexchangersaretypicallydrilledtodepthsrangingfrom50ft(15.2m)to400ft(121.9m),withdiametersrangingfrom3in.(76mm)upto7in.(178mm).Theboreholeannulusisgenerallybackfilledwithabentonite-basedgrouttofacilitatethesealingoftheU-tubeandtoimprovethecontactareaforheattransfer.TypicalU-tubesaremadeofhigh-densitypoly-ethylene(HDPE)andhaveadiameterintherangeof0.75in.(19mm)to1.5in.(38mm).Theheattransferfluidiscommonlypurewaterorasolutionofwaterandpropyleneglycol.TypicalmaterialsofconstructionofsingleU-tubegroundheatexchangersresultinarelativelyhighboreholethermalresistance.Forexample,HDPEplastictypicallyhasathermalconductivityof0.23Btu/h⋅ft⋅ºF(0.40W/m⋅K)andbentonite-basedgroutshaveathermalconductivityofabout0.40Btu/Figure1Schematicofgroundheatexchangerh⋅ft⋅ºF(0.69W/m⋅K).Sincethegeneralrangeofthermalconfigurationsexaminedinthisstudy:(a)singleconductivityvaluesforgeologicmaterialsis0.3to2.2Btu/U-tube,(b)doubleU-tube,(c)concentricclosedh⋅ft⋅ºF(0.5to3.8W/m⋅K)forsoilsand0.6to4.0Btu/h⋅ft⋅ºFloop,and(d)standingcolumnwellwithout(1.0to6.9W/m⋅K)forrocks(KavanaughandRafferty1997),groundwater“bleed.”2AC-02-15-1 Alabama2000a)suggeststhatanupperpracticallimitofgroutsystemperformance.Theanalysesshowedthatusingacasingthermalconductivityisapproachedatvaluesofabout0.85ofhigherthermalconductivitythanthatofPVCsignificantlyBtu/h⋅ft⋅ºF(1.47W/m⋅K).Thisupperlimitisattributedtoanimprovedtheamountofheatexchangedbetweenthefluidandoffsetofthereductionindrillingcostsbyincreasedlaborandtheearth,buttheimprovementbecamenegligiblewhenthehandlingcostsofadditivesthatmustbemixedintothermallycasing’sthermalconductivityapproachedorexceededthatofenhancedgrouts.thesurroundingearth.ItwasalsoshownthatincreasingtheThesecondtypeofgroundheatexchangerexaminedincasingdiameterincreasedtheheatexchanger’scapacity,butthisstudyisthedoubleU-tube(Figure1b).Itsbasicconstruc-notinproportiontotheincreaseinsurfacearea.tionconsistsoftwosingleU-tubesinstalledinoneboreholeThefourthtypeofgroundheatexchangerexaminedinandpipedineitheraseriesorparallelflowcircuit.Theadvan-thisstudyisthestandingcolumnwell(Figure1d).Thisconfig-tageinthermalperformanceofthedoubleU-tubegroundheaturationissimilartotheclosedconcentricloopheatexchangerexchangeroverthesingleU-tubeisthatasecondchannelfordescribedabove,exceptthattheboreholeisuncasedandthetheflowoftheheattransferfluidreplacesaregionofthebore-standingcolumnofgroundwaterintheholeiscirculatedandholethatwouldotherwisebefilledwithgrout.usedastheheattransferfluid.Assuch,thestandingcolumnThedoubleU-tubegroundheatexchangerhasnotwellissometimesreferredtoassemi-openloopsystemsand,receivednearlyasmuchattentioninNorthAmericaasthetherefore,theirapplicationsarelimitedtogeologicregionssingleU-tube,asrevealedbythesparsenessofrelatedpubli-withcompetentrockandgoodgroundwaterquality.cationsinthescientificliterature.TheInternationalGroundThebenefitofthestandingcolumnwelloverothertypesSourceHeatPumpAssociationpresentsabriefdiscussiononofverticalgroundheatexchangersisthattheheatexchangethedoubleU-tubeIGSHPA(1988),andHellstrom(1991)fluidisindirectcontactwiththeearth,thuseliminatingtheprovidesmathematicaldetailsandanalyticalsolutionsforthethermalresistanceduetoboreholecasingandgrout.MoredoubleU-tubeboreholethermalresistance.WetterandHuberimportantly,theuncasedboreholeallowstheabilityto“bleed”(1997)developedadetailedverticalboreholeheatexchangerwaterfromthesystemduringpeaktimes.ThispracticeallowsmodelwithdoubleU-pipes.ASHRAE(1998)describesonefreshgroundwatertobedrawnintothewellbore,therebycasestudyinCanadawheredoubleU-tubeswereinstalledatmoderatingthetemperatureofwaterintheboreandsignifi-acorrectionalfacility.RybachandSanner(2000)reportthatcantlyincreasingthesystemthermalperformance.WedonotthedoubleU-tubeheatexchangeristhemostcommontypeaddresstheprocessofgroundwaterbleedisthispaper,butinstalledinSwitzerland,whereanestimated20,000geother-reserveitforfutureresearch.Orio(1999)hassummarizedmalheatpumpsystemsexistedasof1998.somecommercialcasestudies,notingsignificantreductionsThethirdtypeofgroundheatexchangerexaminedinthisintotalborelengthpertonofinstalledheatpumpcapacity.studyistheclosedconcentrictube(Figure1c).Inthisconfig-Typicalboreholesintheseapplicationshaveadiameterof6in.uration,aclosedcasingwithaconcentricinnerpipeisback-anddepthrangingfrom500to1500feet.Asubmersiblepumpfilledinaborehole.Theinnerpipeisusedtoeitherdeliveraisgenerallyplacedineachwell,butin-linepumpsinacentralheatexchangefluidtoorfromneartheboreholebottomviaamechanicalroomcanalsobeused.perforatedsection,suchthattheusefulheatexchangewiththeOnlyafewpublicationsexistinthescientificliteraturesurroundingearthoccursintheannularflowregion.Someregardingthestandingcolumnwell.YuillandMikler(1995)thermal“short-circuiting”occursbetweentheinnerandouterconductedanexperimentalandtheoreticalstudytoanalyzeflowchannels,butthiscanbereducedwiththeuseofalowcoupledthermalandhydraulicenergytransferaroundawell.thermalconductivityinnerpipe.Thedifferentialequationdescribingthecoupledenergytrans-Theclosedconcentricloopgroundheatexchangerwasferwassimplifiedbytheintroductionofaso-called“ground-firststudiedbyBose(1981).Braudetal.(1983)notedgreaterwaterfactor”torepresenttheratiobetweenconvectionandconductancevaluesoftheconcentricarrangementovertheconduction.ThesolutionwasusedtocalculateaneffectivesingleU-tubearrangementwhentheconcentricheatthermalconductivitytoaccountforinducedgroundwaterexchangerwasconstructedwithasteeloutercasing(ratherflow.thanPVC)andPVCinnerpipe.MeiandFisher(1983)HellstromandKjellsson(2000)conductedlaboratoryconductedanexperimentalstudyofaconcentricgroundheatmeasurementsondifferenttypesofboreholeheatexchangersexchangerwithbothPVCinnerandouterpipes.todeterminethethermalresistancetotheflowofheatbetweenThedesignofanefficientclosedconcentricloopgroundtheheattransferfluidandtheboreholewall.Theinvestigatedheatexchangerisanoptimizationproblem,dependentuponinstallationsincludedasingleU-tube,adoubleU-tube,aeconomics.Thematerialsofconstructionandtheboreholecoaxialtube,andanewprototypeboreholeconfiguration.Thediameteralsocontrolthedesignheatexchangerlength.MeiU-tubeswereplacedinwater-filledboreholes.TheresultsandFisher(1983)usedanumericalmodeltoconductapara-showedthatthenaturalconvectioninthewaterhasasignifi-metricanalysisoftheeffectofsomedesignvariablesoncantimpactontheeffectiveboreholethermalresistance.AC-02-15-13 METHODOLOGYInthisstudy,thecomparisonofthegroundheatexchangerperformanceisaccomplishedbymeansofasystemsimulationapproachusingtheTRNSYS(SEL1997)model-ingenvironment.Thecorecomponentmodelsareaverticalgroundheatexchangermodelandasimplewater-to-airheatpumpmodel(YavuzturkandSpitler1999).Thesystemsimu-lationsareperformedusingasmallofficebuildinglocatedinTulsa,Oklahoma,astheexamplebuilding.Theheatingandcoolingloadsoftheexamplebuildingaredeterminedonanhourlybasisusingabuildingenergysimulationprogram(BLAST1986)andarereadintotheTRNSYSenvironment.TheGroundHeatExchangerModelThegroundheatexchangermodelusedinthisstudyisthatdevelopedbyYavuzturkandSpitler(1999),whichisanextensionoftheworkofEskilson(1987).Eskilson(1987)solvesforthetemperaturedistributionaroundaverticalbore-Figure2Shorttime-stepcomponentmodelconfiguration.holebymakinguseofacombinationofanalyticalandnumer-icalsolutiontechniques.Thethermalcapacitanceoftheindividualgroundheatexchangercomponents,suchaspipei=indextodenotetheendofatimestepandgrout,areneglected.ThetemperaturefieldsfromasingleEskilson(1987)approximatedthevalidityoftheg-func-boreholearesuperimposedinspacetoobtainthetemperature25rresponseofvariousboreholefieldconfigurations.Thesearetionstotimescalesgreaterthan------------------------borehole-.Eskilson’slongthenconvertedtoanondimensionalsetofresponsefactors,αwhichEskilson(1987)identifiedas“g-functions.”timestepg-functionsarenotapplicableformodelingboreholeTheg-functionallowsthecalculationofthetemperaturetemperatureresponsescorrespondingtotypicalshorttimechangeattheboreholewallinterfaceinresponsetoastepheatstep(hourly)loadfluctuationsofbuildings.Yavuzturketal.injectionorextractionpulse.Oncetheresponseofthebore-(1999)usedatwo-dimensionalfinitevolumeapproachtoholefieldtoasinglestepheatpulseisrepresentedwithag-modeltheheattransferaroundasingleboreholeandextendfunction,theresponsetoanyarbitraryheatrejection/extrac-theg-functionsdowntotimescalesoflessthanonehour.Itistionfunctioncanbedeterminedbydevolvingtheheatrejec-theseshorttimestepg-functionsthathavebeenusedbytion/extractionintoaseriesofstepfunctionsandYavuzturkandSpitler(1999)todevelopagroundheatsuperimposingtheresponsetoeachstepfunction.Thebore-exchangercomponentmodelforsystemsimulationprogramsholewalltemperatureattheendoftheithtimeperiodcanbesuchasTRNSYS(SEL1997).Themodelincludesaflexibledeterminedbysummingtheresponsesoftheprevioussteploadaggregationalgorithmthatsignificantlyreducescomput-functions.ingtime.ThecomponentmodelconfigurationisshowninFigure2.n()-----------------------------Qi–Qi–1g-------------------tn–ti–1-,---r-Tborehole=Tground+∑2πktH(1)MathematicalRepresentationofsi=1BoreholeThermalResistancewhereTheshorttimestepg-functions,asdevelopedbyYavuz-t=time,sturkandSpitler(1999),representthetime-varyingthermalts=timescale,sresistanceofthegroundformationaroundaU-tubeground=H2/9α(whereα=groundthermaldiffusivity,ft2/sheatexchangerboreholeandaccountforthethermalresis-[m2/s])tanceandcapacitanceoftheindividualboreholecomponents.H=activeboreholedepth,ft(m)Theshorttimestepg-functionsarethenusedtocalculatethetemperatureattheboreholewall.Thetemperatureoftheheatk=groundthermalconductivity,Btu/h·ft·°Fexchangefluidiscorrectlypredictedwiththeuseofthe(W/m·°C)steady-stateboreholethermalresistance(YavuzturkandTborehole=averageboreholewalltemperature,°F(°C)Spitler2001).Inthiscomparativestudy,theshorttimestepg-Tground=undisturbedearthtemperature,°F(°C)functionsoftheU-tubeconfigurationareusedtoapproximateQ=stepheatinjection/extractionpulseperunitlengththetemperatureresponseofotherboreholeconfigurations.ofbore,Btu/h·ft(W/m)Theboreholethermalresistanceorfluid-to-groundther-rb=boreholeradius,ft(m)malresistance(Hellstrom1991)isthesumofconvectiveplus4AC-02-15-1 conductiveresistancesbetweentheheatcarrierfluidandtheBtu/h·ft·°F(W/m·°C)boreholewallinterface.Itisrelatedtotheheatrejection/r1=boreholeradius,ft(m)extractionrateofthegroundheatexchangerbyrb=piperadius,ft(m)1q=R-----------------------()Tfluid–Tborehole(2)b=eccentricityparameter,borehole=U-tubeshankspacingboreholediameter÷whereRp=convectiveplusconductiveresistanceofasingleq=heatinjection/extractionrate,Btu/h⋅ft(W/m);pipe,h·ft·°F/Btu(m·°C/W)Rborehole=boreholethermalresistance,h⋅ft⋅ºF/Btu(m⋅ºC/W);=thesumofthefirsttwotermsofEquation3Tfluid=averagetemperatureoftheheattransferfluid,ºF(ºC);Theboreholethermalresistancefortheconcentricpipearrangementconsistsoftwoparts:(1)theresistancebetweenTborehole=averagetemperatureoftheboreholewallinterface,theinnerandouterflowchannelsand(2)theresistanceºF(ºC).betweentheouterflowchannelandtheboreholewall.SinceTheboreholethermalresistanceforthesingleU-tubetheheattransferbetweentheinnerandouterflowchannelsis(Rborehole,singleU-tube)isdefinedperlengthofboreasmuchlessthanthatbetweentheouterflowchannelandtheboreholewall,theresistancebetweentheinnerandouterflow1ln()DoutDinchannelsmaybeneglected.TheboreholethermalresistanceR=-----------------------+----------------------------------Borehole,SingleUTube–2πDh4πkininPipefortheconcentricpipearrangement(Rconcentricpipe)isthere-(3)1foredefinedperlengthofboreas+------------------------------------------------------------------------β1kβ()DDgrout0boreholePipe1ln()DDln()DD=-------------------++-----------------------------oin-----------------------------------boreoR(6)whereConcentricPipeπDh2πk2πkininPipeGroutk=thermalconductivity,Btu/h⋅ft⋅ºF(W/m⋅ºC);whereDinandDoutrefertotheinnerandouterdiametersoftheD=diameter,ft(m);outercasingpipeandDboreistheboreholediameter.Thetermhin=convectioncoefficientbasedontheinsidepipehinrepresentstheconvectioncoefficientfortheannularspacediameter,Btu/h⋅ft2⋅ºF(W/m2/ºC);betweentheinnerandouterflowchannels.β0,β1=resistanceshapefactorcoefficients(Paul1996)TheboreholethermalresistanceforastandingcolumnbasedonU-tubeshankspacing.wellapplicationisidenticaltotheconcentricpipearrange-Theconvectionresistance,thepipeconductionresis-mentwithnowellcasing.Therefore,itisdefinedperlengthofboreastance,andthegroutresistancearerepresentedbythefirst,second,andthirdterms,respectively,inEquation3.The1convectioncoefficient(hin)isdeterminedbytheDittus-RSdtaningColumnWell=π------------------------Dh-.(7)boreinBoeltercorrelation,0.8nBuildingDescriptionandLoadsCalculation0.023RePrk≅-----------------------------------------------fluid-,h(4)inDAsmallofficebuildingnearTulsa,Okla.,waschosenforinsimulatingtheperformanceofthevariousgroundheatwheren=0.4forheatingand0.3forcooling;ameanvalueofexchangertypes.Thetotalfloorareaofthebuildingisapprox-0.35isused.imately14,205ft2(1320m2).TheannualbuildingloadsareTheboreholethermalresistanceforthedoubleU-tubedeterminedusingBuildingLoadsAnalysisandSystemTher-(RBorehole,DoubleU-tube)isdefinedperlengthofboreafterHell-modynamics(BLAST1986)simulationsoftware.Thefollow-strom(1991).ingassumptionshavebeenusedtodeterminetheannualbuildingloads:1R=---------Borehole,()DoubleU-tube2πk1.Thebuildingisdividedintoeightdifferentthermalzones.ln----r1–3---+b2–1---ln()1–b8–1---ln---------------2br1–1---ln2-----------br1+R-----p-2.Foreachzone,asingle-zonedraw-throughfansystemisrb442rb4rb4specified.Thetotalcoilloadsobtainedfromsystemsimu-lationareequaltotheloadstobemetwiththeground-(5)sourceheatpumpsystem.where3.Theofficeoccupancyistakenas1personper100ft2(9.3k=thermalconductivityofregionsurroundingpipes,m2)witha70%radiantheatgainof450Btu/h.AC-02-15-15 Figure3AnnualbuildingloadsforsmallofficebuildinglocatedinTulsa,Okla.(coolingloadsarenegativeandheatingloadsarepositive).4.A1.1W/ft2(11.8W/m2)officeequipmentplugloadassuggestedbyKomor(1997)isused.22Figure4SchematicconfigurationoftheTRNSYSsystem5.Thelightingloadsareassumedtobe1W/ft(10.8W/m).simulation.6.Athermostatsetpointof68.0ºF(20.0ºC)duringtheday(8a.m.to6p.m.)and58.0ºF(14.4ºC)duringthenightisusedforallzonesinthebuilding.Onlyheatingisenteringfluidtemperature,andfluidmassflowrate.Theprovidedduringthenight,dependingontherequirement.modelusesaquadraticcurve-fitapproachtomanufacturer’scatalogdatatocomputetheheatofrejectionincoolingmode,7.Schedulesforofficeoccupancy,lighting,equipment,andheatofabsorptioninheatingmode,andtheheatpumpenergythermostatcontrolsarespecified.consumption.OutputsprovidedbythemodelincludeexitingThebuildingwassimulatedusingTypicalMeteorologicalfluidtemperature,energyconsumption,andfluidmassflowYear(TMY)weatherdataforTulsa,Okla.Theannualbuildingrate.loadsdeterminedonanhourlybasisareshowninFigure3.Thepeakenteringfluidtemperature(EFT)totheheatAnalysisProcedurepumpisthecriticalparameterinsizingthegroundheatexchangersystemsforthisstudy.AhypotheticalsingleU-tubeThecomponentinterconnectdiagramofthegeothermalground-loopheatexchangerisdesignedfortheexamplebuild-heatpumpsystemisshowninFigure4.Thesystemhasbeeningandsimulatedfora20-yearperiod.TheheatpumpEFTsimplementedintheTRNSYSmodelingenvironmentusingandpowerconsumptionoftheU-tubeconfigurationareusedstandardandnonstandardcomponentmodels.ThestandardasabaselineforcomparisontotheothergroundheatTRNSYScomponentmodelsforcomponentssuchaspumps,exchangertypes.ThemaximumpermissibleenteringheatT-pieces,flowdiverters,andthedifferentialcontrollerarepumpfluidtemperatureissettobenohigherthan90ºFandnodescribedbySEL(1997).lowerthan40ºF.SystemsimulationsareperformedfortheThegroundheatexchangerdesignparametersusedinthisremaininggroundheatexchangerconfigurationssuchthatthestudy,includingtheboreholethermalresistancevaluesforpeakheatpumpenteringfluidtemperatureismatchedtotheeachcase,aresummarizedinTable1.AnequivalentsetofsingleU-tubecase.Anattemptismadetocontainboreholeoperatingconditionsissimulatedforeachcaseandidenticaldepthstoapracticalrangeof240ft(73.2m)to310ft(94.5m).heatpumpunitsarestipulatedalongwithaconstantheattrans-Acomparisonofgroundheatexchangerperformanceisthenferfluidflowrate.Limestonerockischosenasthegeologicmadebyconductingasimplelife-cyclecostanalysisofeachmediumforthisstudysinceitiscommonlyoccurringandconfiguration.quitesuitableforinstallationofanyoneofthegroundheatexchangersexaminedinthisstudy.Ahighgroundwaterqual-Thesimulationofthestandingcolumnwellneglectsityisstipulatedfortheconcentrictubeaswellasthestandingadvectionofheatfromtheboreholeduetogroundwaterflow.columnwellboreholeconfigurationssothatnoisolationheatThisassumptionisbasedonthestudyofChiassonetal.exchangersareneeded.(1999),whoshowedthatheatconductionthroughthesolidThebuildingisnotmodeledexplicitlyinthisapplication.rackmatrixisamuchmorerapidprocessthanheatadvectionThehourlybuildingthermalloadsdescribedpreviouslyarethroughtheporespaceduetonaturalgroundwaterflow.Thereadfromafileandpassedontotheheatpumpsubroutine,processofadvection,however,isquiteimportantinthewhichisasimplewater-to-airheatpumpmodel.Inputstothemodelingofstandingcolumnwellswithgroundwaterbleed.heatpumpmodelincludesensibleandlatentbuildingloads,Thebleedprocessinvolvescoupledfluidflowandheattrans-6AC-02-15-1 TABLE1SummaryofGroundHeatExchangerDesignParametersCase3Case4Case1Case2ConcentricStandingcolumnwellDesignparameterSingleU-tubeDoubleU-tubeclosed-loop(nobleed)GeologicconditionsRocktypeLimestoneThermalconductivity,Btu/h⋅ft⋅ºF(W/m⋅ºC)1.4(2.4)Vol.heatcapacity,Btu/ft3⋅ºF(kJ/m3⋅ºC)35(2347)Undisturbedearthtemperature,ºF(ºC)63(17.2)Borehole:Diameter,in.(mm)5(127.0)Heattransferfluid:CompositionPurewaterTotalsystemflowrate,gpm,(L/s)48(3.03)U-tubeorinnerpipe:Diameter,in.(mm)1(25.4)1.5(38.1)MaterialHDPEPVCWallthickness,in.(mm)0.12(3.0)0.15(3.8)Thermalconductivity,Btu/h/ft⋅ºF(W/m⋅ºC)0.23(0.4)0.08(0.14)Outercasing:Diameter,in.(mm)N/AN/A3(76.2)N/AMaterialN/AN/ASteelN/AWallthickness,in.(mm)N/AN/A0.216(5.5)N/AThermalconductivity,Btu/h/ft⋅ºF(W/m⋅ºC)N/AN/A26(44.9)N/AGrout:TypeThermallyenhanced--Thermalconductivity,Btu/h/ft⋅ºF(W/m⋅ºC)0.85(1.47)--Boreholethermalresistance:Rborehole,ºF/Btu/h⋅ft(ºC/W/m)0.2431(0.1405)0.1259(0.0728)0.0706(0.0408)0.0038(0.0022)AC-02-15-17 portduetopumpingandisbeyondthelimitationsofcurrentheatpumpEFToverthesimulationperiodisshowninFiguremodelingtoolsandthereforeisnotaddressedinthispaperbut5.AmaximumEFTof88.7°F(31.5°C)isreachedattheisthesubjectoffuturework.4816thhourofthe20thyear.TheminimumEFTtotheheatpumpispredictedtobe50.7°F(10.4°C),occurringattheSYSTEMSIMULATIONRESULTSANDDISCUSSION202ndhouroftheveryfirstyearofthesimulations.TheannualaverageheatpumpenergyconsumptionforthesinglePerformanceComparisonsoftheU-tubeconfigurationis18,315kW(Table3).AnincreaseofGroundHeatExchangerTypesonly7.7%ispredictedbetweenthefirstandtwentiethyearinTheresultsofthedetailedsystemsimulationsaretheheatpumpenergyconsumption,mostofwhichisexperi-providedinTables2and3andFigures5,6,7,and8,andencedinthefirsttenyearsofsystemoperation.furtherdiscussionispresentedbelow.TemperatureversustimecurvesforthedoubleU-tube,Initialsizingoftheground-loopheatexchangerwasconcentrictube,andstandingcolumnwellgroundheatperformedfortheU-tubeconfiguration,andthiscaseisusedexchangersareshowninFigures6,7,and8,respectively.asthebasecaseforthesubsequentcomparativeanalyses.TheThesecurvesareessentiallyidenticaltothesingleU-tuberesultingborefielddesignforthesingleU-tubecaseis16boreholesin4×4configuration,wherethedepthofeachcase,withdifferencesinthepeakminimumtemperaturesofboreholeis240ft(73.2m)(Table2),yieldingatotalloopabout1°F(0.55°C).Thisobservationmaynotseemtoolengthof3840ft(1171m).Theshorttimestepbehaviorofthesurprising,giventhateachboreholefieldwasdesignedtoTABLE2SummaryofGroundHeatExchangerDesignandSimulationResultsGroundHeatExchangerBoreholeGeometryBoreholeLengthEFTMaxEFTMinTypeft(m)°F(°C)°F(°C)Single4×4240(73.2)88.7(31.5)50.7(10.4)U-TubeDouble3×4248(75.6)88.7(31.5)51.3(10.7)U-TubeConcentric3×3285(86.9)88.7(31.5)51.4(10.8)TubeStandingColumnWell2×4258(78.7)88.7(31.5)51.7(10.9)(NoBleed)Figure5Hourlymaximumheatpumpenteringfluidtemperaturesvs.time(U-tube).8AC-02-15-1 TABLE3meetthesamemaximumheatpumpEFT.ThesimilaritiesinHeatPumpEnergyConsumptionforVariousGroundthetemperatureresponsecurvestranslateoverintotheHeatExchangerTypesobservedsimilaritiesinheatpumppowerconsumptioninallcases(Table3).Thereisaslightdecrease(about1.2%)intheGroundHeatHeatPumpEnergyConsumption(kW)averageannualenergyconsumptionbetweenthesingleU-ExchangerType1stYear20thYear20-YearAveragetubeandthestandingcolumnwellwithoutbleedcase,whichisduetothefactthesystemruns1οF(0.6οC)warmerintheSingle17,37518,71618,315U-Tubelatterconfigurationduringtheheatingseason.Double17,22618,69318,264Thetotalboreholelengthforeachcase(normalizedtotheU-TubesingleU-tubecase)isshowninFigure9,plottedagainsttheConcentric17,16218,66618,229correspondingboreholethermalresistance.Asexpected,theTuberequiredtotalboreholelengthisastrongfunctionofthebore-StandingColumnWell16,96218,54518,095holethermalresistance.ThesingleU-tubecaserequiresthe(NoBleed)greatestlengthofbore,andreductionsfromthislengthofFigure6Hourlymaximumheatpumpenteringfluidtemperaturesvs.time(doubleU-tube).Figure7Hourlymaximumheatpumpenteringfluidtemperaturesvs.time(concentrictube).AC-02-15-19 Figure8Hourlymaximumheatpumpenteringfluidtemperaturesvs.time(standingcolumnwellwithnobleed).Figure9Totalboreholelength(normalizedtothesingleU-Figure10Volumeofwaterinthegroundloopforthevarioustubecase)versusboreholethermalresistance.groundheatexchangercases.approximately22%,33%,and36%areseenforthedoubleU-Itshouldbenotedthatthetotalborelengthsdeterminedtube,theconcentrictube,andthestandingcolumnwellwithforthedoubleU-tube,theconcentrictube,andthestandingnobleed,respectively.However,therelationshipbetweencolumnwellarelikelytobeoverconservative.EvenwiththeboreholethermalresistanceandrequiredtotalboreholelengthconsiderabledecreaseintotalborelengthsforthesegroundheatexchangersascomparedtothesingleU-tubeconfigura-isnotanentirelylinearone.Fortheconditionssimulatedhere,tion,thereisasignificantlygreatervolumeofwaterinthesealowerlimitoftheboreholethermalresistanceisapproachedsystemsthaninthesingle-U-tubesystem(Figure10).Anearavalueequivalenttothatoftheconcentrictubearrange-reviewofFigure10showsthatwithrespecttothesingleU-ment.Belowthis“criticalvalue,”decreasingtheboreholetubecase,thereisanincreaseinthevolumeofwaterbyathermalresistancebyafactorof2resultsinadecreaseinthefactorof1.5,2.7,and5.7forthedoubleU-tube,theconcentrictotalboreholelengthonlyontheorderof1%.Abovethiscrit-tube,andthestandingcolumnwell,respectively.Thisincreaseicalvalue,halvingtheboreholethermalresistanceresultsinainthermalmassofthesystemwillhavetheeffectofdampingdecreaseinthetotalboreholelengthontheorderof30%.peakhourlyresponsesofthefluid,therebyincreasingheat10AC-02-15-1 pumpoperatingefficiencies.Thedegreetowhichthisis•A6%annualpercentagerateisusedforthepresentimportantisthesubjectoffutureresearch;currentmodelingvalueanalysis.toolsuseasteady-stateresistanceterm,whichisgenerallyvalidforthesingleU-tubeconfiguration(Hellstrom1991)butTheresultsofthecostanalysisarelistedinTable4andmustremainquestionableforconfigurationswithsignifi-showngraphicallyinFigure11.Thisanalysisshowsthatcantlygreaterfluidthermalmass.significantcapitalcostsavingsoverthesingleU-tubecasecanberealizedifothergroundheatexchangertypesareutilized.EconomicAnalysisHowever,whenthetotalnetpresentvalueofthesystemcostThethermalperformanceofeachgroundheatexchangerbasedona20-yearlife-cycleanalysisisconsidered,someofhasbeendiscussedabove.However,itisultimatelyeconom-thesavingsinfirstcostareoffsetbythefactthatthereappearsicsthatdictatethepracticalityofthegroundheatexchanger.tobenosignificantdifferencesinoperatingcostsbetweentheAlthoughadetailedeconomicanalysisisbeyondthescopeofvariouscases.Thisisespeciallydemonstratedbythestandingthispaper,asimpleeconomicanalysismaygivesomeinsightcolumnwellcase.Aspreviouslystated,thesimulatedheatintojustifyingonegroundheatexchangerconfigurationoverpumppowerconsumptionmayhavebeensignificantlylowertheotherunderanequalsetofconditions.Infact,adetailedinthedoubleU-tube,concentrictube,andstandingcolumneconomicanalysismaynotbepossiblegiventhelargewellconfigurationsiftheheatcapacityofthewaterwasdiscrepancyofloopdrillingcostsacrosstheUnitedStates,asconsidered.shownbyarecentanalysisconductedbytheUniversityofAlabama(2000b).CONCLUDINGREMARKSANDInordertocomparethecostofeachgroundheatFURTHERRECOMMENDATIONSexchangertype,thenetpresentvalueiscalculatedforthefirstSimulationstudiesareconductedtoexamineandcostand20-yearoperatingcostforeachsystem.Thenetcomparetheheattransferefficiencyofsomeofthemorepresentvalueisbasedonaseriesofassumptions.commontypesofgroundheatexchangers,suchasU-tube,doubleU-tube,concentrictube,andstandingcolumnwell•TheinstallationcostperfootofboreforasingleU-tubeusingamathematicaldescriptionfortheboreholethermalgroundheatexchangerisusedasthebasecostandisresistanceofthegroundheatexchangers.Thesystemsimula-assumedtobe$6.00perfootofbore.Thisamountalsotionsarebasedonashorttimestepgroundheatexchangerincludestrenchingandheadering.modelandareconductedunderanequivalentsetofoperating•Thecostperfootofboreisbrokendownintodrillingconditionsconsideringasmall,cooling-dominatedofficecosts(includingpipeinstallation,trenching,andheader-building.Thefollowingconclusionsaredrawnfromtheanal-ing),pipecosts(materialonly),andgroutingcostsysespresentedinthisstudy:(laborandmaterials).Forthesingleone-inchdiameterU-tubebasecomparison,thesecostsareestimatedtobe1.Thetotalboreholelengthisstronglydependentuponthe$4.80,$0.60,and$0.60,respectively.overallthermalresistanceoftheborehole.Basedonthefour•Theinstallationcostsfortheothergroundheatgroundheatexchangerconfigurationsexaminedhere,aexchangertypesareadjustedfromthesingleU-tube“criticalvalue”oftheboreholethermalresistanceiscasebasedonconversationswithvendorsandcontrac-observed,belowwhichhalvingitsvalueresultsinators.Fortheconcentrictubegroundheatexchanger,decreaseinthetotalboreholelengthontheorderof1%.increaseddrillingcostsreflectincreasedlaborinhan-Abovethiscriticalvalue,halvingtheboreholethermaldlingandassemblingsteelpipe.Forthestandingcol-resistanceresultsinadecreaseinthetotalboreholelengthumnwellinstallation,higherdrillingcostsareattributedontheorderof30%.totheassumptionthattheupper50feetoftheboreare2.ThedoubleU-tubeboreholeismoreadvantageousthanthecasedwithsteel.CostbreakdownsareshowninTable4.singleU-tube.Areductionofabout22%isachievedinthe•Astaticwaterlevelforthestandingcolumnwellappli-totalrequiredboreholelength.Althoughthecostforthecationof50feetisassumed.pipematerialisdoubledforthiscase,the20-yearlife-cycle•Aconstantpumpingrateforeachcaseisassumed.Thecostisapproximately10%lowerthanthesingleU-tubeoperatingcostsduetopumpingenergyconsumptionareconfiguration.determinedbasedonapipingnetworkdesignconsid-eringthegeometryofeachgroundheatexchangercon-3.Althoughtheconcentrictubeboreholeconfigurationfiguration.Theground-loopnetworkissizedsoasnotresultsinareductionofthetotalrequiredboreholelengthbytoexceedtheASHRAE-recommendedpressuredropofabout30%whencomparedtothesingleU-tubeandabout4ftofwaterper100ftofpipe.Thisresultedinpump10%whencomparedtothedoubleU-tube,thehighcostofcapacitiesrangingfrom0.62hp(0.47kW)to0.72hpconcentrictubematerials(primarilysteelpiping)offsets(0.54kW).Theelectricalenergyconsumptionisdeter-someofthesavingsachievedthroughreduceddrillingminedassumingan80%motorefficiency.costs.Asaresult,the20-yearlife-cyclecostanalysisshows•Thecostofelectricityisassumedtobe$0.07perkWh.(Table4)acostsavingsintheconcentrictubeovertheAC-02-15-111 TABLE4Summaryof20-YearLife-CycleCostAnalysisStandingSingleDoubleConcentricColumnWellU-TubeU-TubeTube(NoBleed)GroundHeatExchangerSystemSummaryNumberofboreholes4×44×33×32×4Boreholedepth(ft)240248285258Additionalboredepthtoreachwatertable(ft)---50Totalborelengthrequired(ft)3,8402,9762,5652,464Maximumflowrate(gpm)48484848Max.heatpumpenteringfluidtemperature(oF)88.788.788.788.7Min.heatpumpenteringfluidtemperature(oF)50.751.351.451.7GroundHeatExchangerSystemCapitalCostUnitinstallationcosts($/ftbore):Drilling,trenching,pipehandling,andinstallation$4.80$4.80$5.00$5.25Pipematerials$0.60$1.20$2.75$0.20Groutmaterialsandplacement$0.60$0.50$0.35$-Totalunitinstallationcost$6.00$6.50$8.10$5.45Totalgroundheatexchangerloopcost$23,040$19,355$20,777$13,429GroundHeatExchangerSystemOperatingCostNetPresentValue-Heatpumppluscirculatingpumpelectrical$19,321$18,780$18,611$18,428powerconsumptionPresentValueofTotalCost$42,361$38,135$39,388$31,85712AC-02-15-1 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