Thermoeconomic assessment of a multi-engine, multi-heat-pump CCHP system

ContentslistsavailableatScienceDirect

EnergyandBuildings

journalhomepage:www.elsevier.com/locate/enbuild

EvaluationofaturbinedrivenCCHPsystemforlargeofficebuildingsunderdifferentoperatingstrategies

PedroJ.Mago∗,AnnaK.Hueffed

DepartmentofMechanicalEngineering,MississippiStateUniversity,210CarpenterEngineeringBuilding,P.O.BoxME,MississippiState,MS39762-5925

articleinfoabstract

Combinedcooling,heating,andpower(CCHP)systemsusewasteheatfromon-siteelectricitygenerationtomeetthethermaldemandofthefacility.ThispapermodelsaCCHPsystemforalargeofficebuildingandexaminesitsprimaryenergyconsumption(PEC),operationalcosts,andcarbondioxideemissions(CDE)withrespecttoareferencebuildingusingconventionaltechnologies.Theprimemoverusedinthisinvestigationisaloadshareturbine,andtheCCHPsystemisevaluatedunderthreedifferentoperationstrategies:followingtheelectricdemandofthefacility,followingthethermaldemandofthefacility,andfollowingaseasonalstrategy.Forthevariousstrategies,thepercentagesoftotalcarbondioxideemissionsbysourcearepresented.ThispaperexplorestheuseofcarboncreditstoshowhowthereductionincarbondioxideemissionsthatispossiblefromtheCCHPsystemcouldtranslateintoeconomicbenefits.Inaddition,thecapitalcostsavailablefortheCCHPsystemaredeterminedusingthesimplepaybackperiod.ResultsindicatethatfortheevaluatedofficebuildinglocatedinChicagotheCCHPoperationreducestheoperationalcost,PEC,andCDEfromthereferencebuildingbyanaverageof2.6%,12.1%,and40.6%,respectively,forallthedifferentoperationalstrategies.

©2010ElsevierB.V.Allrightsreserved.

Articlehistory:

Received19February2010

Receivedinrevisedform7April2010Accepted19April2010Keywords:CCHP

PrimaryenergyreductionCarbondioxideemissionsTrigenerationCarboncredits

1.Introduction

Combinedcooling,heatingandpower(CCHP)istheproductionofpower(electricalormechanical)andusableheatfromasinglefuelsource.Oftenidentifiedastrigeneration,CCHPisabroadtermreferringtoasetofintegratedtechnologiessuchasturbines,recip-rocatingengines,microturbines,fuelcells,heatpumps,thermallyactivatedtechnologies,and/orwasteheatrecoverytechnologiesthatcanbeimplementedindifferentconfigurationstosuitdif-ferentneeds.Thermallyactivatedtechnologiestransformthermalenergyintousefulheating,cooling,humiditycontrol,thermalstor-age,andshaft/electricalpower.CCHPsystemshavethepotentialforhigherthermalefficiencyovertheseparateproductionofpowerandheat;therefore,lessfuelisconsumedforthesameoutput,therebyreducinggreenhousegasemissionsandloweringopera-tionalcosts.Theheatgeneratedasaby-productfromtraditional,centralizedpowergenerationistypicallylosttotheatmospherethroughcoolingtowers,fluegas,orothermeans.Overtwo-thirdsofallthefuelusedtogeneratepowerintheU.S.islostasheat.Byplacingthepowerproductionatornearthesiteofconsumption,a

Abbreviations:CHP,combinedheatingandpower;CCHP,combinedcooling,heating,andpower;PGU,powergenerationunit(primemover);REF,referencebuilding(noCCHPsystem).

∗Correspondingauthor.Tel.:+6623256602;fax:+6623257223.E-mailaddress:mago@me.msstate.edu(P.J.Mago).0378-7788/$–seefrontmatter©2010ElsevierB.V.Allrightsreserved.doi:10.1016/j.enbuild.2010.04.005

formofdistributedgeneration,CCHPsystemscanusethewould-bewasteheattosatisfysomeorallofthefacility’sthermaldemand.Distributedgenerationalsoeliminatestransmissionanddistribu-tionlossesassociatedwithdeliveringelectricityfromthepowerplanttotheuser.

ThesizesandapplicationsofCCHPsystemsvarytoaconsid-erabledegree,ranginginsizefromafewkilowattstomegawattsofpowerproduction,withapplicationstoresidential,commercial,industrial,orlarge-scaledistrictenergysystems.Ingeneral,CCHPsystemsareusuallyoperatedusingtwobasicstrategies:follow-ingtheelectricload(FEL)andfollowingthethermalload(FTL).TheCCHPoperationstrategywilldictatetheloadingandfuelcon-sumptionoftheprimemoverandthustheenergyconsumptionprofileoftheCCHPsystem.InthecaseofFELoperationstrategy,theprimemoverisloadedinordertosatisfytheelectricdemandofthefacilitythroughthegenerator.Theprimemoverandgeneratorformthepowergenerationunit.Thewasteheatfromthisloadingisthenrecoveredinordertosatisfythethermalloadofthefacil-ity.Forthisoperationstrategy,iftherecoveredthermalenergyisnotenoughtohandlethethermalload(coolingorheating)ofthefacility,additionalheathastobeprovidedbytheauxiliaryboileroftheCCHPsystem.FortheFTLstrategy,theprimemoverisloadedsuchthattherecoveredwasteheatwillbeadequatetosupplythefacilitywiththenecessarythermalenergytosatisfytheheatingandcoolingrequirements.Forthisoperatingstrategytheamountofelectricityproducedmayormaynotbeenoughtoprovidetheelectricityrequiredbythebuilding.Therefore,iftheelectricitypro-

P.J.Mago,A.K.Hueffed/EnergyandBuildings42(2010)1628–16361629

Nomenclature

VariablesCCcarboncreditvalueCDEecarbondioxideemissionfactorfordeliveredelec-tricity

CDEelectricitycarbondioxideemissionsfordeliveredelectric-ity

CDEfuel-onsite-boilerboileron-sitecarbondioxideemissions

resultingfromfuelcombustion

CDEfuel-onsite-pguPGUon-sitecarbondioxideemissionsresult-ingfromfuelcombustion

CDEfuel-pre-combustionpre-combustioncarbondioxideemis-sionsforfueldeliver

CDEng-onsite-boilercarbondioxideemissionfactorforon-site

naturalgascombustioninaboiler

CDEng-onsite-pgucarbondioxideemissionfactorforon-site

naturalgascombustioninPGU

CDEng-pre-combustioncarbondioxideemissionfactorfornatural

gasdelivery

Costoperationoperationalcost(withoutpurchasingcarbon

credits)

Costtotalcosttooperatefacility(includespurchasingcarbon

creditstooffsetemissions)

Eelectricity

Epgu◦nominalPGUloadEreqtotalelectricityrequiredbyofficebuildingFfuelinputLRloadratioQboilerheatproducedbytheboilerQccoolingloadofofficeQchheatneededtooperateabsorptionchiller(=Qc/Ách)

Qh

heatingloadofofficeQhcheatneededtorunheatingcoil(=Qh/Áhc)

Qhw

hotwaterloadofofficeQhwsheatrequiredtorunhotwatersystem(=Qwh/Áhws)

Qrec

heatrecoveredfromthePGUexhaustQreqtotalheatrequiredbybuildingGreekÁthermalefficiency

󰀁cost

differenceincapitalcostsoftheCCHPsystemandreferencebuilding

Subscriptsboilerboilerchabsorptionchillereelectricityequipmentequipmentexportexportfanfanhcheatingcoilhwshotwatersystemgridgridlightslightsngnaturalgaspgupowergenerationunitrecrecoveredrefreferencebuildingreqrequired

ducedisnotenoughtohandletheelectricloadadditionalelectricity

hastobeimportedfromthegrid.SomeresearcherssuchasCardonaandPiacentino[1,2],Jalalzadeh-Azar[3],Magoetal.[4],andMagoandChamra[5]amongothershaveinvestigatedtheoperationofCCHPsystemsunderthesetwooperationstrategies.CardonaandPiacentino[1]refertothesetwostrategiesasElectricDemandMan-agementandThermalDemandManagement.Theyconcludedthattheuseofthetwostrategiesdependsonseveralfactorssuchas:theloadingoftheprimemoverandtheabilitytosellbackelectricitytothegridorstoreitonsiteforlateruse.Inaddition,thepriceoffuelversusthatofelectricitypurchasedfromatraditionalsourcecanaffectthemanagementofaplant[2].Jalalzadeh-Azar[3]performedananalysisofenergycostandprimaryenergyconsumptionofCCHPsystemsoperatingunderFELandFTLstrategieswhileutilizingagasfiredmicroturbineindifferentclimates.Theresultsyieldedan11%reductionintotalenergyconsumptionwhentheCCHPoperatesaccordingtoFTLversusthatofFEL.Magoetal.[4]comparedFELandFTLstrategiesforbothCHPandCCHPsystemsthatusedaninternalcombustionengineastheprimemoverforasmallofficebuilding(140m2)infourdifferentclimateregions.Comparisonsweremadebasedonprimaryenergyconsumption(PEC),cost,andcarbondioxideemissions(CDE).Anationalaverageprimaryenergyconsumptionfactorforelectricitywasusedtodeterminethepri-maryenergyconsumption,costwasfiguredfromasingleflatrateforbothelectricityandnaturalgas,andthecalculatedcarbondiox-ideemissionsdependedontheregionalmixoffuelusedtoproducegridelectricity.Magoetal.[4]foundthat,ingeneral,FTLperformedbetterthanFEL.Inanotherstudy,MagoandChamra[5]optimizedCCHPsystemsthatwereoperatingunderFELandFTLstrategiesbasedonenergy,cost,andemissions.Inaddition,theyevaluatedanoptimizedoperationalstrategyinwhichaCCHPsystemfollowsahybridelectric–thermalloadstrategy(HETS).MagoandChamra[5]reportedthattheHETSisagoodalternativeforoperationofCCHPsystemssinceityieldedgoodreductionsofPEC,cost,andCDE.

Inadditiontothebasicoperationstrategies(FELandFTL),thispaperalsoevaluatesoperationoftheCCHPsystemfollowingasea-sonalstrategy(FSS).Foreachmonthunderthisstrategy,theCCHPsystemwilloperatetoeitherFELorFTLdependingonthemonthlyelectric-to-thermalloadratio.

Ingeneral,mostoftheinvestigationsmentionedaboveusedanaturalgasinternalcombustionengineastheprimemover.How-ever,otherresearchershaveinvestigatedtheuseofaturbineprimemoverforCCHPapplications.SavolaandKeppo[6]modeledfourexistingsteamturbineCHPsystems(1–20MWe)operatingatpartload.Theyfoundthatalthoughthepartloadpowerproductioncanbedescribedquiteaccuratelywithasingleline,thereisasmallnonlinearreductioninthepowerproductionastheheatloaddecreases.Kongetal.[7]presentedasimplelinearprogrammingmodeltodeterminetheoptimalstrategiesthatminimizetheover-allcostofenergyfortheCCHPsystem.Theenergysystemconsistsofagasturbine,anabsorptionchillerandaheatrecoveryboiler.Theydemonstratedthattheoptimalsystemoperationisdependentupontheloadconditionsbeingsatisfied.Theyalsoreportedthatforthecaseofalowelectrictogascostratioitmaynotbeoptimaltooperatetheturbine.Khanetal.[8]presentedanovelcoolingandpowercyclethatcombinesasemi-closedgasturbinecyclewithavaporabsorptionrefrigerationsystemforpower,waterextrac-tion,andrefrigeration.Thecombinedcycleefficiencywasfoundtobe44%.Colomboetal.[9]presentedanddiscussedtheresultsofanexperimentalinvestigationofamicroturbinecogenerationplant.ExperimentaltestswererunonaTurbecT100-CHPmicrotur-bineunitwhilevaryingtheelectricalpoweroutputbetween50and110kWandforwatertemperaturesattherecuperatoroutletrang-ingfrom60to80◦C.Theyreportedthattheperformanceremainsessentiallyconstantintherangeof80–110kWwhileamoderate

1630P.J.Mago,A.K.Hueffed/EnergyandBuildings42(2010)1628–1636

decreaseisobservedfrom50kWuntilabout60kW.EhyaeiandMozafari[10]studiedtheoptimizationofmicroturbineapplica-tionstomeettheelectrical,heating,andcoolingloadsofabuildingthroughanenergy,economics,andenvironmentalanalysis.Theyevaluatedthreedifferentscenarios:agasmicroturbinetomeettheelectricalpowerdemandofthebuilding;agasmicroturbinetomeettheelectricalpowerdemandofthebuildingaswellasthepowerrequiredbyaheatpumpandamechanicalrefrigerator;andagasmicroturbineforCHPtomeettheelectricalpowerofthebuildingaswellaspartofthepowerrequiredbyheatpumpandmechanicalrefrigerator.Theyconcludedthatthenumberoftur-bineunitsandelectricitycostarehighlydependentonelectricityconsumptionmanagement.

ThispapermodelsaCCHPsystemforalargeofficebuildingandexaminesitsenergyconsumption,operationalcosts,andcarbondioxideemissionswithrespecttoareferencebuildingusingcon-ventionaltechnologies.TheprimemoverusedinthisinvestigationisaloadshareturbineandtheCCHPsystemisevaluatedunderthreedifferentoperationstrategies:followingtheelectricloadofthefacility,followingthethermalloadofthefacility,orfollowingaseasonalstrategy.Anotherobjectiveofthispaperistodeterminethepercentageofemissionsgeneratedfromthedeliveredelectric-ityandtheon-siteelectricityproductionandshowhowreductionsincarbondioxideemissionsthatcouldbeobtainedfromtheopera-tionoftheCCHPsystemcouldbetranslatedintoeconomicbenefitsusingcarboncredits.WhileseveralCCHPandCHPmodelsandopti-mizationschemesavailableintheliteratureuseconstantelectricityandnaturalgaspricesineconomiccalculations[11–16],thecurrentinvestigationconsidersrealelectricityandnaturalgasratesfortheevaluatedcity.ThisisaccomplishedbysimulatingtheCCHPsysteminthecityofChicagowhichincorporatesblockchargesanddemandchargesintheirutilityschedule.ActualmonthlygasratesforIllinoiswerealsoemployedtoaccountforthevariationsingasratesacrosslocationsandfluctuationsinpricesthroughouttheyear.Finally,foragivenpaybackperiodandoperationalcostsavingsoftheCCHPsystem,thecapitalcostavailabletoinvestintheCCHPsystemisdetermined.

2.Carboncredits

Thepurposeofcarboncreditsistocreateeconomicvaluefromdefinedenvironmentalbenefitssuchasthereductionofgreen-housegas(GHG)emissions,whichincludecarbondioxide(CO2),methane(CH4),nitrousoxide(N2O),sulfurhexafluoride(SF6),perfluorocarbons(PFCs),andhydrofluorocarbons(HFCs).Whencomparedtoaconventionalbuilding,theuseofaCCHPsystemcanreducetheamountofemissionsandthereforegainsomeeco-nomicbenefitsusingcarboncredits.OneofthemethodologiessuggestedtoaddressGHGemissionsisamarket-based“capandtrade”system.Basically,forcompaniesandindustriesthatsig-nificantlycontributetoGHGemissions,directemitters,acaporlimitisplacedontheamountofallowableemissions.Thesecom-paniesorindustriesmustreducetheiremissionstoalevelequaltoorbelowthecap.Iftheiremissionsarebelowthetargettheyareissuedcredits.Thosewhocannotmeetthecapmustpur-chasecredits,thusbringingtheiremissionsintocompliancewiththecap.Inadditiontodirectemitters,indirectemitterssuchasoffice-basedbusinessesorindustriesgenerateemissionsindirectlythroughtheconsumptionofelectricityandotherrelatedactivities.Thesetypesofcompaniesmustoffset100%oftheiremissionsbypurchasingcredits.Thisensuresthattheatmosphericburdenofgreenhousegases(GHGs)isnotincreasedbytheentity’sindirectactivities.Forexample,thissystemisusedontheChicagoClimateExchange(CCX),whichiscurrentlyavoluntarybutlegallybind-ingcommitmentforitsmembers[17].Forthelargeofficebuilding

Fig.1.Electric,cooling,heating,andhotwaterloadsfortheevaluatedlargeofficebuildinglocatedinChicago.

inthisinvestigation,onlycarbondioxideemissionsareconsid-ered.Tooffsetcarbondioxideemissions“carboncredits”mustbepurchased,whicharetypicallypricedindollarspermetrictonofCO2-equivalent.3.Referencebuilding

SeveralcommercialbuildingbenchmarkmodelshavebeendevelopedbytheU.S.DepartmentofEnergy.Themainbenefitofthestandardizedbenchmarkmodelsisthattheyformacom-monpointofcomparisonbetweenresearchprojects[18].Sixteentypesofcommercialbuildings(small,medium,andlargeoffices,hotels,hospitals,etc.)insixteendifferentlocationsacrossthreevin-tages(new,pre-1980,andpost-1980construction)weredevelopedandmodeled[19]usingEnergyPlussoftware[20].Thesebuildingsrepresentapproximately70%ofthecommercialbuildingsintheU.S.[21].Thisstudyfocusesonanewlargeofficebuildingwith460,240ft2offloorareaand12floorsplusabasementthatislocatedinChicago,IL.Thereferencebenchmarkbuildingusesanelectricchillerunitforcoolingandaboilerforheating.TheCOPoftheelec-tricchillerunitis5.5andtheoverallheatingefficiencyis78%.Theairdistributionisthroughamulti-zonevariableairvolumesystem.Themonthlyelectric,thermal,andhotwaterloadsforthereferencebuildingarepresentedinFig.1.Inthebenchmarkmodels,elec-tricityimportedfromthegridisusedforlights,equipment,andHVACcomponents.Inaddition,naturalgasissuppliedtoaboilertosatisfytheheatingandhotwaterloadsofthebuildings.Inthisstudy,thelargeofficebenchmarkbuildingwassimulatedinEner-gyPlusandusedasthereferencecase.Fromthesimulationresultsthebuilding’selectric,cooling,heating,andhotwaterloadsweredeterminedandusedfortheCCHPsystemanalysis.4.CCHPsystemmodels

ThissectionpresentstheequationsusedtomodeltheCCHPsystem.TheschematicofthemodeledCCHPsystemispresentedinFig.2.Fromthisfigureitcanbeseenthatfuelissuppliedtothepowergenerationunit(PGU)toproduceelectricityneededbythebuilding(lights,equipments,andHVACfans).Thewasteheatisrecoveredandusedtoproducecoolingthroughanabsorptionchiller,heatingthroughaheatingcoil,orhotwater.ThissectionfirstdiscussesthebasicoperationalstrategiesoftheCCHPsystem,suchasfollowingtheelectricload(FEL)andfollowingthethermalload(FTL).Inaddition,aseasonaloperationstrategy(FSS)ispre-sentedinwhichthesystemfollowseithertheelectricloadorthethermalloaddependingonthemonthlyelectric-to-thermalloadratio.

Foralltheoperationalstrategies,theelectricitythatmustbesuppliedtothebuildingbytheCCHPsystemand/orthegridfor

P.J.Mago,A.K.Hueffed/EnergyandBuildings42(2010)1628–16361631

Fig.2.SchematicoftheCCHPsystemusedinthisinvestigation.

eachhouris

Ereq=Elights+Eequipment+Efans

(1)

Fig.3.Performancecurves:(a)efficiencyversuspowerforashareloadturbine(adaptedfrom[22])and(b)boilerefficiencyasfunctionofairandfuelinput(adaptedfrom[23]).

whereElights,EequipmentandEfansaretheelectricityrequiredbythelights,equipment,andfans,respectively.Similarly,theheatthatmustbeprovidedbytheCCHPsystem(recoveredfromthePGUexhaust)and/ortheboilerforeachhourisQreq=Qhc+Qch+Qhws

(2)

whereQhc,Qc,andQhwsaretheheatrequiredbytheheatingcoil,absorptionchiller,andhotwatersystem,respectively.TheheatrequiredbytheheatingcoiltohandletheheatingloadisestimatedasQhc

Q=h

Áhc

(3)

ofthePGU,Epgu◦,islessthanthatrequiredbytheofficebuilding,electricitymustbeimportedfromthegrid;Therefore,

󰀁

Egrid=

Ereq−Epgu◦0ifEreq>Epgu◦otherwise

󰀂

(6)

ThePGUfuelenergyconsumption,FpgucanbeestimatedasFpgu=

EpguÁpgu

(7)

whereQhisthebuildingheatingloadandÁhcistheheatingcoilefficiency.TheheatrequiredbytheabsorptionchillertohandlethecoolingloadisestimatedasQch=

QcCOPc

(4)

whereÁpguisthePGUthermalefficiency.Asmentionedbefore,thePGUselectedforthispaperisaloadshareturbine.ThethermalefficiencyofthisturbineispresentedinFig.3(a)andmodeledusingthefollowingcurvefitdata

Ápgu=

⎧4

3+cE2+dEaEpgu+bEpgupgu+eif02+b󰀅󰀅E2+c󰀅󰀅⎪a󰀅󰀅Epgu⎪pgu⎪⎪⎩󰀅󰀅󰀅2󰀅󰀅󰀅2󰀅󰀅󰀅

if400kW(8)

aEpgu+bEpgu+c

whereQcisthebuildingthermalcoolingloadandCOPcisthe

absorptionchillercoefficientofperformance.Similarly,theheatrequiredbythehotwatersystemisQhws=

QhwÁhws

(5)

Therecoveredwasteheatfromtheprimemover,Qrec,canbeestimatedasthedifferencebetweenthePGUfuelenergyconsump-tionandthePGUelectricenergytimestheheatrecoverysystemefficiency,Árec,asfollowsQrec=Árec(Fpgu−Epgu)

(9)

whereQhwisthehotwaterloadandÁhwsisthehotwatersystemefficiency.

4.1.CCHPsystemmodelfollowingtheelectricload(CCHP-FEL)FortheCCHPsystemfollowingtheelectricload,thePGUelec-tricoutput,Epgu,willtrytomatchtherequiredelectricdemandofthebuilding,suchthatEpgu=Ereq.Ifthenominalelectricoutput

TheCCHPsystemhastomeetthebuilding’sthermaldemandatanyspecifichourduringitsoperation.Therefore,iftherecoveredthermalenergyisnotenoughtohandlethethermalload(cooling,heating,orhotwater)additionalheathastobeprovidedbytheauxiliaryboileroftheCCHPsystem.Therefore,

󰀁

Qboiler=

Qreq−Qrec0ifQreq>Qrecotherwise

(10)

1632P.J.Mago,A.K.Hueffed/EnergyandBuildings42(2010)1628–1636

TheboilerfuelenergyconsumptioniscomputedasFboiler=

QboilerÁboiler

(11)

4.3.CCHPsystemfollowingtheseasonaloperationstrategy(CCHP-FSS)

TheseasonaloperationstrategyconsistsofoperatingtheCCHPbasedonaparametercalledtheloadratio(LR)thatisdefinedas:LR=

monthlyelectricloadmonthlythermalload

(20)

whereÁboileristheboilerthermalefficiency,whichisrepresentedbyFig.3(b)andmodeledusingthefollowingcurvefitdataÁboiler=Ax2+Bx+C

(12)

wherexisthepercentinputoffuelandair,andA,B,andCareconstants.ThefuelenergyconsumptionregisteredatthemeterisestimatedasFm=Fpgu+Fboiler

4.2.CCHPsystemmodelfollowingthethermalload(CCHP-FTL)

(13)

Then,ifLR>1theCCHPsystemoperatesfollowingtheelectricload(FEL)duringthatmonthandifLR<1thesystemoperatesfol-lowingthethermalload(FTL)duringthatmonth.Accordingly,theequationspresentedinSections4.2and4.3areusedtomodeltheCCHPsystem’soperation.

5.CCHPsystemperformancemetrics

Forthisoperationstrategythetotalheatthatmustberecov-eredfromthePGUwilltrytomatchthethermalenergyrequiredtohandlethecoolingorheatingload.Therefore,Qrec=Qreq=Qhc+Qch+Qhws

(14)

ForagivenoperationalstrategytheperformanceoftheCCHPsystemwasevaluatedbycomparingtheannualoperationalcost,primaryenergyconsumption(PEC),andcarbondioxideemissions(CDE)tothereferencecase.5.1.Operationalcost

Todeterminethetheoperationalcostsactualpricedatawasused.Fornaturalgas,historicalmonthlyaveragesforthestateofIllinoiswereused.Thisaccountsformonth-to-monthpricefluc-tuationsalongwithgeographiclocation.TheutilityratesforthecityofChicagoweretakenfromtheEnergyPlusinputfileforthebenchmarkmodel.Informationaboutsellingelectricitybacktothegridwasunavailable,soassumptionsweremadeastohowtopriceexportedelectricity.Theseutilityratesareforamonthlybillingcycleandincludebaseratesandblockcharges.Therearetwosetsofblockcharges,oneisbasedontotalenergyconsumptionandtheotherincorporatesdemandusage.Energychargesmakeupthebaseofeachmonthlyutilitybill,wheretheenergychargerate($/kWh)ismultipliedbytheelectricityconsumedfromthegridforthatmonth.Forexportedelectricity,thecustomerwouldreceivehalftherateofpurchasedelectricity,sotheenergychargesarecomputedasfollows

EnergyCharges=Egridre−Eexport0.5re

(21)

Sincetherecoveredwasteheatfromtheprimemoverisknown,thefuelenergycanbeestimatedasFpgu=

Qrec

Árec(1−Ápgu)

(15)

wherethevalueforQreccannotexceedthemaximumamountofrecoverableheat(theheatrecoveredwhileoperatingatthenomi-nalload).ThetotalelectricenergysuppliedbythePGUisEpgu=FpguÁpgu

(16)

whereÁpguisdefinedbyEq.(8).Sincethesystemisfollowingthethermalload,theamountofelectricityproducedmayormaynotbeenoughtoprovidetheelectricityrequiredbythebuildingortheremightbemorethanenoughproducedelectricity.Therefore,

󰀁

Egrid=Eexport=

Ereq−Epgu0

Epgu−Ereq0

ifEreq>Epguotherwise

󰀂

(17)

󰀁

ifEpgu>Ereqotherwise

󰀂

(18)

whereEgridistheamountofelectricityrequiredfromthegridandEexportistheamountofexcesselectricitythatcanbeexportedorstoredforfutureuse.Ifthemaximumwasteheatthatcanberecov-eredfromthePGUislessthanthatrequiredbytheofficebuilding,theboilerhastosupplytheadditionalheat.Therefore,

󰀁

Qboiler=

Qreq−Qrec0ifQreq>Qrecotherwise

󰀂

(19)

ThefuelenergyconsumedbytheboilerandthemeteredfuelcanbedeterminedusingEqs.(11)and(13).Forthisoperationalmode,thesystemmayhaveexcesselectricitythatcouldbestoredorsoldbacktothegrid.However,theseoptionsarenotavailableinalllocations.

Table1

Chicagocostdata[24].

A

Size(kWh)

Block1Block2Block3Block4

Energycharges,re($/kWh)Monthlycharge($)Taxes,t(%)

3007001500

Remaining0.004359.48

wherereistheenergychargerateforelectricityin($/kWh),listedinTable1forChicago.UsingthepricingdatainTable1tocalcu-latetheBlockAchargesforChicagoa“net”gridconsumptionofEgrid−0.5×Eexportisfirstdeterminedforthemonth.Fromthis,thefirst300kWhusedisbilledatBlock1’scost,thenext700kWhatBlock2’scost,thenext1500kWhatBlock3’scost,andtheremainingnetgridconsumptionisbilledatBlock4’scost.ThesameprocedureisfollowedforBlockBchargesexceptthesizeofeachblockismultipliedbythemonth’sdemand.Themonthlysubtotalcanthenbecomputedaccordingto

Subtotal=EnergyCharges+BlockChargesA+BlockChargesB

(22)

B

Cost($/kWh)0.0824090.0728730.0616960.041179

Sizeperdemand190110

Remaining

Cost($/kW)0

0.0517730.046965

P.J.Mago,A.K.Hueffed/EnergyandBuildings42(2010)1628–1636

Table2

PECandCDEfactorsforthecityofChicago[24].

PECfactor(kWh/kWh)

CDEfactor(g/MJ)

CDEfactorpre-comb(g/MJ)

CDEFactoron-site(g/MJ)PGU–smallturbine

ElectricityNaturalGas

3.61.092

341.7–

–4.85

–

50.97

1633

Commercialboiler–50.97

andthetotalmonthlycostforelectricityisCoste=(Subtotal+MonthlyCharge)(1+t)

(23)

wheretisthetaxratewhichwastakentobe2%abovetheloca-tion’ssalestaxrate[19]andthemonthlychargeisafixedpriceforconnectiontothegrid.

ThemonthlycostofnaturalgasisCostng=(Fpgu+Fboiler)(rng)(1+t)

(24)

whererngisthemonthlypriceofnaturalgas($/kWh)andthefuelconsumption,Fpgu+Fboiler,isforthegivenmonth.Offsettingcar-bondioxideemissionsthroughthepurchaseofcreditsaddstotheyearlyoperationalcosts,Costoperation,whichisthesummationofthemonthlyelectricitycostandthemonthlynaturalgascostforeachmonthoftheyear.Therefore,thetotalcostisCosttotal=Costoperation+CDE×CC

(25)

whereCCisthecarboncreditvaluein$/metrictonofCO2-equivalentandCDEaretheannualcarbondioxideemissions.IfcarbondioxideemissionsareoffsetbypurchasingcarboncreditsthedirectsavingsduetotheoperationoftheCCHPsystemreducingemissionsare

SavingsCCHP-carbon=(CDEref−CDECCHP)CC(26)

5.2.PECandCDE

Primaryenergy(sometimescalledsourceenergy)allowscom-parisonamongbuildingsthatusedifferentsourcesofenergyandbetterreflectsthebuilding’sresourceconsumption.Itincludesenergyconsumedonsite,theamountofenergymeasuredbythemeter,pluslossesfromproduction,transmission,anddelivery.Forthelargeofficebuilding,therearetwometeredenergysources:gridelectricityandfuel.Todetermineprimaryenergyconsumption,PECconversionfactorsaremultipliedbytheamountofenergyused.Forexample,theelectricityconversionfactorconvertsthesiteelectric-ityfromthegridtotherawfuelusedbythepowerplant.Giventhis,thePECofthebuildingoperatingtheCCHPsystemiscalculatedinthefollowingmanner

PEC=(Egrid−Eexport)PECe+(Fpgu+Fboiler)PECng

(27)wherePECeandPECngaretheprimaryenergyconversionfactorsforelectricityandnaturalgas,respectivley.

Thetotalcarbondioxideemissionscanbebrokendownaccord-ingly

CDE=CDEelectricity+CDEfuel-pre-combstion+CDEfuel-onsite-pgu

+CDEfuel-onsite-boiler

(28)

whereCDEelectricityarethecarbondioxideemissionsfordeliveredelectricity,CDEfuel-pre-combustionarethepre-combustioncarbondiox-ideemissionsforfueldeliverytothebuilding,andCDEfuel-onsite-pguandCDEfuel-onsite-boilerarerespectively,thePGUandboileron-sitecarbondioxideemissionsresultingfromfuelcombustion.Simi-larlytodetermingprimaryenergyconsumption,carbondioxideemissionsarecomputedusingCDEconversionfactors.ThecarbondioxideemissionsfordeliveredelectricitycanbeexpressedasCDEelectricity=(Egrid−Eexport)CDEe

(29)

whereCDEeistheemissionfactorfordeliveredelectricity.Thepre-combustioncarbondioxideemissionsforfueldeliverytobuildingsandtheon-sitecombustioncarbondioxideemissionsinthePGUandboilercanbedetermined,respectively,asCDEfuel-pre-combstion=(Fpgu+Fboiler)CDEng-pre-combstion(30)CDEfuel-onsite-pgu=(Fpgu+Fboiler)CDEng-onsite-pgu(31)CDEfuel-onsite-boiler=(Fpgu+Fboiler)CDEng-onsite-boiler

(32)

whereCDEng-pre-combustion,CDEng-onsite-pgu,andCDEng-onsite-boileraretheemissionfactorsforpre-combustion(extraction,processing,anddeliveryofnaturalgas),PGUon-sitecombustion,andboileron-sitecombustion,respectively.IncomputingPECandCDE,thenetelectricityimportedfromthegridisused.Thisisbecauseexportedelectricitydisplaceselectricitythatmustotherwisebegeneratedbythepowerplanttoserveothercustomers,therebyreducingtheamountofrawfuelconsumed.ThePECandCDEemissionsfactorsforelectricityandnaturalgasinthecityofChicagoarepresentedinTable2.

5.3.Overallcostandpaybackperiod

Whencomparedtothereferencecase,thetotalsavingsfromtheCCHPsystemare

SavingsCCHP=Costref−Costtotal

(33)

WhileinstallingaCCHPsystemcanprovidesavingsintermsofreducedoperationalcosts,therearenormallyhighercapitalcostsassociatedwiththeimplementationofsuchasystem.Whendecid-ingwhetherornottoinvestinaCCHPsystemthistradeoffmustbeweighed,and,onesuchmethodistoevaluatethesimplepaybackperiod.Thesimplepaybackperiodestimatesthenumberofyearsofoperationneededbeforetheinitialinvestmentcanberecouped.Formanybusinesses,themaximumpaybackperiodforanyinvest-mentisset.Inthissituation,themaximumallowableincreaseincapitalcostoverthereferencecasecanbedeterminedfromthesetpaybackperiod.Thiscanbeexpressedas

󰀁cost=CapitalCostCCHP−CapitalCostref=Payback×SavingsCCHP

(34)

Therefore,toguaranteethispaybackperiod,theaboveequationcanbeusedtoestimatethetotalinvestmenttoupgradetoaCCHPsystemortheextrainvestmentoverthereferencecase’scapitalcoststhatcanbemadetoinstallaCCHPsysteminanewbuilding.6.Results

ThissectionpresentstheresultswhichwereobtainedusingtheCCHPoperationalstrategiesdescribedinSection4.Forthesestrate-gies,thevaluesofthevariablesusedtomodeltheCCHPsystemandthePGUcharacteristicsarepresentedinTable3.

Table4presentstheannualtotaloperationalcost(withoutincludingcarboncredits),PEC,andCDEforthedifferentopera-tionstrategiesevaluatedinthispaperandFig.4presentsthesesresultsasthepercentdeviationfromthereferencecase.Whenthepercentdeviationisnegativethissignifiesareductionfromtheref-erencecase.Fig.4illustratesthatalltheoperationstrategiesreduce

1634

Table3

CCHPparameters.ParameterBoilerefficiencyaCoeficient,ACoeficient,BCoeficient,C

HeatrecoverysystemefficiencyHeatingcoilefficiencyChillerCOP

MaxPGUload,L,(kW)

MaxPGUefficiency=L/FpguPGUefficiencybCoefficient,aCoefficient,bCoefficient,cCoefficient,dCoefficient,eCoefficient,a󰀅Coefficient,b󰀅Coefficient,c󰀅Coefficient,a󰀅󰀅Coefficient,b󰀅󰀅Coefficient,c󰀅󰀅

ab

P.J.Mago,A.K.Hueffed/EnergyandBuildings42(2010)1628–1636

Value−448×10−493×10−40.85550.80.80.78000.33

−813.33×10−10427.99×10−07−839.67×10−05783.00×10−03−433.72×10−13−119.99×10−6971.93×10−413.2915

−326.50×10−7477.36×10−416.1135

Fig.5.Comparisonoftheprimaryenergyconsumption(PEC)forthethreeevaluatedoperationalstrategies:FEL,FTL,andFSS.

ASHRAEhandbook[23].Capstone[22].

Fig.4.PercentvariationoftheCCHPbuildingfromthereferencebuildingforcost,PEC,andCDEunderthedifferentoperationstrategies.

theoperationalcost,PEC,andCDE.Thereductionsinoperatingcostsrangefrom1.8%to2.7%.ThecostreductionforCCHP-FTLwithexportelectricity(2.7%)isjustslightlybetterthatthecostreductionforCCHP-FTLwithoutexport(2.5%).RegardingPEC,themaximumPECreductionwasobtainedforCCHP-FSS(15.9%)whilethemin-imumreductionwasCCHP-FTLwithoutexportelectricity(6.3%).CCHP-FTLwithexportandCCHP-FELyieldedareductionof14.1%and14.4%,respectively.TheCDEreductionsforCCHP-FEL,CCHP-FSS,CCHP-FTLwithexport,andCCHP-withoutexportwere49.2%,48.3%,37.7%,and29.5%,respectively.Thisfigureillustratesthatfortheevaluatedlargeofficebuilding,oneofthebiggestadvantagesofutilizingCCHPsystemsisthesignificantreductionofCDE.ThetwostrategiesprovidingthebestCCHPperformancefortheevaluatedlargeofficebuildinginChicagoareCCHP-FELandCCHP-FSS.

Table4

Cost,PEC,andCDEresultsforthedifferentoperationalstrategies.

Cost($/year)

PEC(kWh/year)

Asmentionedbefore,themaximumPECreductionwasobtainedforCCHP-FSS.ThiscanbeexplainedusingFig.5thatgivesthemonthlyPECforthethreedifferentevaluatedoperationalstrate-gies.ThisfigureillustrateshowtheCCHP-FSSfollowstheelectricloadforsomemonthsandfollowsthethermalloadforothers.ForthemonthsofFebruary,March,April,May,September,October,andNovember,LF>1;therefore,theCCHPsystemfollowstheelectricload.Fortheremainingmonths,LF<1andthesystemfollowsthethermalload.TheFSSstrategybasicallyfollowsthestrategythatconsumeslessprimaryenergyineachmonthwhichevidentlywillreducetheoverallPECduringthewholeyear.

Fig.6displaysthedistributionofCDEbysourceforthedifferentCCHPsystem’soperationstrategies.ForCCHP-FELthemajorityoftheemissionsarefromtheon-sitecombustioninthePGU(68.9%).Ontheotherhand,fortheCCHP-FTLwithexportandtheCCHP-FTLwithoutexportmostoftheemissionscomefromthedeliveredelectricity(48.3%and.4%,respectively)followedbythePGUon-sitecombustionemissions(40.5%and35.8%,respectively).Forthelastoperationstrategy,CCHP-FSS,65.5%oftheemissionsarepro-ducedfromtheon-sitePGUcombustionand18.6%arearesultofthedeliveredelectricity.Therefore,forCCHP-FTLthedeliveredelectricitydominatestheCDEwhileforCCHP-FELandCCHP-FSS,thePGUon-sitecombustionisthedominantcontributortoemis-sions.Forallthecases,theemissionsfromtheon-sitecombustionoftheboilerarelow,between6.0%and12%.Forthereferencebuild-ing,93.2%oftheCDEareproducedfromthedeliveredelectricitywhileonly6.8%comesfromfuelconsumption.TheseproportionschangewhenCCHPsystemsareused.ForCCHP-FELandCCHP-FSS88.2%and81.4%,respectively,comesfromfuelutilization.Ontheotherhand,forCCHP-FTLwithandwithoutexport,51.7%and45.6%,respectively,comesfromfuelutilization.

Fig.7illustratestheeffectofcarboncreditsbygivingthetotalcostasapercentvariationfromthereferencecaseforvaryingval-uesofcarboncredit.Thehigherthecarboncreditvalue(in$/metrictonofCO2-equivalent)thelargerthecostreductionduetotheCCHPsystemoperation.HavingthelargestCDEreductionfromtheref-erencecaseamongtheoperationstrategies,theCCHP-FELstrategystandstobenefitthemostfromcarboncredits.Forthisstrategy,theoperationalcostcanbereducedfrom3.0%belowthereference

CDE(kg/year)Electricity

Pre-combustion34,492227,774163,824163,824213,708

On-sitepgu–

1,938,7101,399,1141,399,1141,876,415

On-siteboiler362,482343,236241,826241,826263,349

Total5,829,4722,858,9903,557,7444,040,2382,914,876

ReferenceCCHP-FEL

CCHP-FTL(withexport)CCHP-FTL(withoutexport)FSS448,530440,613436,579437,407440,90217,817,17515,252,49715,299,25216,690,11014,984,2725,432,499349,2701,752,9802,235,474561,404

P.J.Mago,A.K.Hueffed/EnergyandBuildings42(2010)1628–16361635

Fig.6.DistributionofCDEbysourcefor(a)CCHP-FEL,(b)CCHP-FTLwithexport,(c)CCHP-FTLwithoutexport,and(d)CCHP-FSS.

caseto7.3%belowthereferenceforcarboncreditvaluesof2$/met-rictonofCO2-equivalentand$10/metrictonofCO2-equivalent.Inaddition,withoutcarboncreditsCCHP-FEListhemostexpen-sivestrategy,butatabout$6.00/metrictonofCO2-equivalentitbecomesoneoftheleastexpensivestrategy.TheoperationstrategythatwouldbenefittheleastfromthecarboncreditsistheCCHP-FTLwithoutexport,withcostreductionsfrom3.2%belowthereferencecaseto5.6%belowthereferenceforcarboncreditvaluesof$2/met-rictonofCO2-equivalentand$10/metrictonofCO2-equivalent.TheoperationalcostfortheCCHP-FSScanbereducedfrom3.3%belowthereferencecaseto7.4%belowthereferenceforcarboncreditvaluesof$2/metrictonofCO2-equivalentand$10/metrictonofCO2-equivalent,respectively.Whileoffsettingemissionsbypurchasingcarboncreditsisnotcurrentlyrequired,thepotentialfinancialbenefitsfromloweringemissionsthroughtheoperationofaCCHPsystemareimportanttoestimate.

Fig.8showstheextracost(overthereferencebuildingcost)thatcanbeinvestedtoupgradetoaCCHPsystemfordifferentpay-backperiods.ThisfigureillustratesthattheCCHP-FSSallowsthelargestamountofextrainvestmentwhiletheCCHP-FTLwithout

exportprovidestheleastamountofaddedinvestment.Foratypi-calpaybackperiodof3years,anadditional$79,311canbeinvestedtoupgradetoaCCHP-FSSsystemfromthereferencecase.Forthesamepaybackperiod,anadditional$,280canbeinvestedforasystemoperatingunderCCHP-FTLwithoutexport.

Finally,Fig.9illustratestheeffectoftheexportelectricitypriceontheannualcostoftheCCHP-FTLwithexportfordifferentcarboncreditvalues.Thevaluesonthex-axisrepresenttheratiooftheexportpricetotheelectricitypriceforimport,whilethey-axisgivestheoperationalcostasapercentvariationformthereferencecase.Astheratioincreases,theresultingcostreductionsincrease.Also,thisfigureconfirmsthathighervaluesofthecarboncredityieldhigherreductionsinthecostofoperation.Fortheparticularcaseofzerocarboncreditvalue,theoperationalcostis2.8%lowerthanthereferencecaseiftheratioisequaltozero(basicallynoexport)anddecreasesto3.2%belowthereferencecaseiftheratioisequalto1.However,forcarboncreditof$6/metrictonofCO2-equivalenttheoperationalcostis5.4%lowerthanthereferencecaseiftheratioisequaltozeroanddecreasesto5.7%belowthereferencecaseif

Fig.7.EffectofthecarboncreditvalueontheoverallCCHPsystemoperationalcost.

Fig.8.ExtrainvestmentsontheCCHPprojectoverconventionaltechnologiesfordifferentpaybackperiodsusingacarboncreditvalueof$6/metrictonofCO2-equivalent.

1636P.J.Mago,A.K.Hueffed/EnergyandBuildings42(2010)1628–1636

Fig.9.Effectoftheexportelectricitypriceonthepercentvariationfromtherefer-encecaseoftheoverallCCHPoperationalcost.

theratioisequalto1.ThiscanbemoresignificantforothercasesandlocationsifthereisproportionatelymoreelectricitytoexportduringtheCCHPsystemoperation.7.Conclusion

ThispaperpresentedtheperformanceofaturbinedrivenCCHPsystemforalargeofficebuilding.Thecost,energy,andemissionresultsthatwereobtainedfromoperatingthesystemundertheoperationstrategiesoffollowingtheelectricdemandofthefacil-ity,followingthethermaldemandofthefacility,orfollowingaseasonalstrategywerecomparedtoareferencebuildingoperat-ingunderconventionaltechnologiestodeterminetheadvantagesordisadvantagesoftheCCHPsystem’soperation.ThispaperalsodeterminedthedistributionofcarbondioxideemissionsbysourceandhowreducingcarbondioxideemissionsthroughtheuseofaCCHPsystemcouldtranslateintoeconomicbenefitsusingcarboncredits.ResultsindicatethatfortheevaluatelargeofficebuildinglocatedinChicago,IL,theCCHPoperationunderallthestrategiesreducestheoperationalcost,primaryenergyconsumption,andcarbondioxideemissionsbyanaverageof2.6%,12.1%,and40.6%,respectively,fromthereferencecaseforazerocarboncreditvalue.TheproposedFSSstrategyyieldsthelargestPECreductionfromthereferencecase(15.9%).ThetwostrategiesprovidingthebestCCHPperformanceareCCHP-FELandCCHP-FSS.

WiththeuseofaCCHPsystemthepercentofcarbondioxideemissionsfromdeliveredelectricityisreduced.ThemajorityofCDEfortheFELandFSSstrategiesisaresultofthePGUoperation.Ontheotherhand,themajorityoftheemissionsfortheFTLstrategiesisduetodeliveredelectricity.

Carboncreditscansuccessfullyyieldfinancialrewardforreduc-ingcarbonemissions.Thehigherthecarboncreditvalue(in$/metrictonofCO2-equivalent)thelargerthecostreductionoftheCCHPsystemoperation.HavingthelargestCDEreductionfromthereferencecaseamongtheoperationstrategies,CCHP-FELstandstobenefitthemostfromcarboncredits.Forthisstrategy,theoperationalcostcanbereducedfrom3.0%belowthereferencebuildingto7.3%belowthereferencebuildingforcarboncreditvaluesof$2/metrictonofCO2-equivalentand$10/metrictonofCO2-equivalent.

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