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Recovering heat from hot drain water-Experimental evaluation, parametric analysis and new calculation
procedure
Mohamad Ramadan, Thierry Lemenand, Mahmoud Khaled
To cite this version:
Mohamad Ramadan, Thierry Lemenand, Mahmoud Khaled. Recovering heat from hot drain water-
Experimental evaluation, parametric analysis and new calculation procedure. Energy and Buildings,
Elsevier, 2016, 128, pp.575-582. �10.1016/j.enbuild.2016.07.017�. �hal-02525529�
ContentslistsavailableatScienceDirect
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
Recovering heat from hot drain water—Experimental evaluation, parametric analysis and new calculation procedure
MohamadRamadana,∗,Thierrylemenandb,MahmoudKhaleda,c
aEnergyandThermo-FluidGroup,SchoolofEngineering,LebaneseInternationalUniversityLIU,P.O.Box146404,Beirut,Lebanon
bLARISEA7315,ISTIA,UniversityofAngers,Angers,France
cUnivParisDiderot,SorbonneParisCité,InterdisciplinaryEnergyResearchInstitute(PIERI),Paris,France
a r t i c l e i n f o
Articlehistory:
Received8February2016
Receivedinrevisedform26May2016 Accepted6July2016
Availableonline6July2016
Keywords:
Heatrecovery Energymanagement Drainwater Prototype Heattransfer Calculationprocedure
a b s t r a c t
Inthelastdecade,atremendousefforthasbeenmadetofindsolutionspermittingtodecreasethecon- sumptionoffossilenergy.Energyrecoveryisoneoftheemergingsolutions.Itconsistsinrecuperating thewasteenergythatexistsinmanysystemsandreutilizingitinausefulway.Heatrecoveryfromhot waterdrainisoneofenergyrecoverysystemsthatistakingitsreputationnowadaysduetothemajor partoftheelectricbilloccupiedbyheatingdomesticwater.Thispaperreportsacalculationprocedure thatcanbeappliedtomanydrainheatrecoverysystems.Itcanbeutilizedasapre-calculationtoolto evaluateandanalyzedrainheatrecoverysystemsorasanoptimizationtechniquetoenhanceanexisting system.Toproceed,agenericexperimentalsetupisdevisedandaparametricexperimentalanalysisis performedusingthedevelopedsetup.Experimentshaveshownthatthesystemcanconsiderablyincrease thetemperatureofthecoldsupplywaterforsomeconfigurations.Basedonexperimentalresults,many drainsystemscanbepreliminaryevaluatedandanalyzedusinganewsuggestedsystematiccalculation procedurebasedonexperimentallydeterminedparameters.
©2016ElsevierB.V.Allrightsreserved.
1. Introduction
Thehighdemandonenergyistremendouslyincreasingdueto threemainreasonsthataretheexpansionoftheindustrialdomain, thedepletionofthefuelresourceandtherapidpopulationgrowth.
Hugeeffortsarebeingmadetoovercomethisseverecrisis.Solu- tionscanbeclassifiedintothreecategories:
Usingrenewableenergyresourcessuchaswindenergy[1,2], solarenergy[3,4]andgeothermalenergy[5,6].
Improvingenergymanagement[7,8]whichconsistsinorganiz- ingenergyresourcessothatthelossofenergyisminimizedand theuseofenergyisoptimized.
Developingenergyrecovery[9,10]whichconsistsinrecovering thelostheatinenergysystemsandusingitinotherapplications.
Indeedenergyrecoverymaytakeseveralforms.Themostdevel- opedoneisheatrecovery[11–13]wheretherecoveredenergyis heat.Inotherterms,thelostheatofasystemcanbere-usedas heatsourceforanotherapplication.Heatrecoverystudiescover widerangeofenergydomains.Industrialapplicationsaresourcesof
∗Correspondingauthor.
E-mailaddresses:dr.ramadan.mohamad@gmail.com, mahmoudkhaled21@hotmail.com(M.Ramadan).
hugeheatlossthatexplainwhymanyworkshaveinvestigatedheat recoveryfromindustrialmachines[14].Heatrecoveryfrominter- nalcombustionengines[15]hasalsobeenstudied.Otherworks concernheatrecoveryinbuildingapplications[16–18].Heating waterinresidentialbuildingrepresentsthehighercontributionin thetotalamountofenergyconsumption.Ontheotherhand,drain waterisarichsourceofheatlossthatcanberecovered.
Inoneofthefirstworksonheatrecoveryfromdrainwater[19], theconsumptionofenergyforwaterheatingisstudiedintermsof thelife-styleofoccupants.Authorsshowedthatanenergysaving upto10%canbeobtainedinsomeconfigurations.Inanotherwork [20],studyofrecoveringheatfromwastewaterindyeingprocessis presentedandenergysavingisreportedbytheauthors.Heatrecov- eryfromdishwashersisinvestigatedin[21].Theauthorspresented anexperimentalstudyandshowedthatthesystemiseconomically beneficial.Recoveringwasteheatfromsystemsusinghotwater suchassauna,tobeusedinaheatpumpasheatsourceisstud- iedand analyzedin[22].Wasteheatrecoveryfromdrainwater inhighrisebuildingisinvestigatedin[23]:ahorizontalcounter flowheatexchangerisutilizedtoextractheatanduseittoheat coldwater.Authorsshowthatbyinstallingheatrecoverysystem upto15%ofthewastewaterheatcanberecovered.Enhancingheat pumpusedinpublicshowerfacilitiesusingheatrecoverysystem andsolarsystemisstudiedin[24].Beforeheatingwaterbyaheat http://dx.doi.org/10.1016/j.enbuild.2016.07.017
0378-7788/©2016ElsevierB.V.Allrightsreserved.
576 M.Ramadanetal./EnergyandBuildings128(2016)575–582
Fig.1.Drainwaterheatrecoverysystem(DWHRS)workingprinciple.
pump,itisheatedbysolarsystemthenbymeanofheatrecov- erysystem.Itismentionedthatusingsuchasystemreducesthe energyconsumptionaswellasthepollutionlevelandithaslower operatingcost.Heatrecoveryfromhorizontaldrainisinvestigated in[25].Authorsmentionthatusinghorizontalheatrecoverysys- temisapplicableandcanbeefficient; itsefficiencydependson severalfactors.Itisreportedthattheutilizationofsuchasystem reducehighlytheemissionofcarbondioxide.Benntjesetal.[26]
showedthattheeffectivenessofheatrecoverysystemdeceases withtheflowrateandthatthereisacriticalflowratebelowwhich theperformancecannotbeextrapolated.In[27],astudyonverti- calheatrecoverysystemwithheatpumpshowsthatthecontact resistancebetweenthecopperpipesandtheheatresistanceonthe insideofthedrainwaterhasthehighercontributiontotheheat resistance.Ithasbeenshownthatusingsucha heatpump,25%
oftheheatcanberecovered.Afinancialstudyontheutilizationof drainwaterheatrecoverysystemispresentedin[28].Authorspre- sentedamodelallowingestimatingthefinancialefficiencyofthe system.Thestudycoversseveralheatrecoveryconfigurationsand differentinstallationparameters.Utilizationofsewerwaterasheat sourceisstudiedin[29,30]andacasestudyconcerningthecityof Bologna(Italy)isconsidered.Monitoringdataareusedtoobtaina correlationbetweenthewastewaterflowrateanditstemperature.
Parametricstudyondrainwaterheatrecoveryusinginlinevertical heatexchangerforseveralflowscenariosisperformedin[31]:the authorsshowedthattheamountofrecoveredheathighlydepends onthesizingoftheheatrecoverysystem.Heatrecoveryfromwaste wateroftherapysystemsinspaispresentedin[32].Theeffectof theangleoftheheatrecoverysystemwithrespecttothevertical isstudiedin[33]:authorsshowedthattheeffectivenessdecreases whentheanglewithrespecttotheverticalincreases.
Theworkscitedabovecoverawiderangeofapplicationsrelated todrainheatrecovery.Eachstudypresentsananalysisfocusedona specificparameter.However,noneofthesestudiespresentsagen- eralapproachthatcanbeusedasareferencefordrainheatrecovery calculation.Thatiswhyitisessentialtoproposeageneralizedpro- cedureofstudyfordrainheatrecoverysystemthatcanbeadopted independentlyfromtheconfiguration,theapplicationandthesys- temparameters.Inthiscontext,thispaperpresentsacalculation procedurethatmaybeappliedtogenerallymanydrainheatrecov- erysystemsprovidingthatsomepreliminarytestsaredoneforthe drainsystemconsideredandforcorrespondingrealscenarios.Itcan beutilizedasapre-calculationtooltoevaluatedrainheatrecov- erysystemsorasanoptimizationtechniquetoenhanceanexisting system.
Theoriginalityofthepresentworkresidesinthegenericexper- imentalsetupdevelopedandtheanalysisperformedbasedonthe obtainedresults. Moreover,theexperimentalanalysis wasper- formedinsuchawaythathaspermittosuggestanewsystematic calculationprocedureinordertoevaluateandanalyzedrainheat recoverysystemsbasedonpreliminaryexperimentaltests.
Theremainingofthispaperiscomposedoffourparts.Parttwo isdedicatedtopresenttheexperimentalapproach.Inpartthree,
resultsarediscussed.Newcalculationprocedureispresentedin partfourandfinallypartfiveisreservedtotheconclusions.
2. Materialsandmethods
In this section, the principle of the heat recovery concept (Section2.1), theprototypeimplemented(Section2.2), andthe experimentalsetup(Section2.3)arepresented.
2.1. Principle
Theprincipleofthedrainwaterheatrecoverysystem(DWHRS) consistsin heating/preheatingthesupplywater beforeitenters thewaterheater,bytheheatcontainedinthedrainwater.Indeed, thewatertemperatureafteranydrainagetypeisrelativelyhigh.In othertermsthedrainwatertemperatureisclearlyhigherthanthe watersupplytemperaturewhichmaybebelow5◦Cincoldregions.
Froma heattransfer pointof view,thistemperature difference betweenthedrainwaterandthesupplywatermaybetransformed toaheatratethatcanheatthesupplywater.TheDWHRS(Fig.1) canbeviewedasaheatexchanger,inwhichthehotfluidisthe drainwaterandthecoldfluidisthesupplywater.Severaltypes of(DWHRS)canbeobtainedaccordingtotheheatexchangertype whichistheheartofthesystem.Themostcommonlyusedheat exchangersarethecoiledandconcentrictubeheatexchangersthat areusedinthisstudy.
2.2. Prototype
Tostudytheheatrecoveryfromdrainagesystemandanalyzeits performance,aprototypeisconstructed,asshownschematicallyin Fig.2.
Theprototypeiscomposedoffivemainparts:watersupplytank, waterpump,drainbox,electricheater,andcoiledheatexchanger.
Thewatersupplytankisaplastictankof60Lvolume,utilizedto supplythesystem.Thewaterpumpinsuresthenecessarywater pressuresothatwaterflowsinthesystem.Acontrolvalveisused
Fig.2.Schematicoftheconstructedprototype.
Fig.3.Glassboxsimulatingthedrainagetyperegion.
Fig.4.Coiledheatexchangerusedintherecoverysystem.
directlydownstreamofthepumpinordertocontrolaprescribed flowrateinthecircuit.
Thedrainboxrepresentstheregionofagiventypeofdrainage carryingthehotwaterbeforeitsdrainage.Asillustration,thisbox canbeashowerchamber,asink,adishwasher,awashingmachine, etc.Itconsistsofaglassbox(Fig.3)of3mmthicknessfixedby aluminumrodsatitscorners.Ithasalengthandwidthof40cm,a heightof60cmandatotalsurfaceareaof1.28m2.
Aninsulatedboilerof50Lsizeisusedtoheatthewaterafter passingthroughtheheatexchangertube.Thecoiledheatexchanger isconstructedusingasinglecoppercoilof12.7mminnerdiameter and0.6mmthicknesswrappedaroundacircularcopperpipeof 41.3mminnerdiameter,1.2mmthicknessanda70cmlength,as showninFig.4.Thecircularcopperpipeservesasdrainagepipeand isconnecteddirectlytothedrainageboxbase.Fiberglassinsulation of37.5mmthicknessisutilizedtoreduceheatlosses.
2.3. Experimentalsetup
Inordertotestthethermalperformanceofthesystem,mea- surementsoftemperaturesandflowratesarerequired.FourK-type thermocouplesareusedtomeasuretemperaturesTc,iandTc,oat respectivelytheinletandoutletofthecoiledsupplypipe(coldside oftheexchanger)and Th,i and Th,o atrespectivelytheinletand outletofthedrainagepipe(hotsideoftheexchanger).
Waterflowrates aremeasuredusing thestopwatchmethod tofillagiventankvolume.Themassflowrateisthencalculated accordingtothefollowingrelation:
m˙ =V
t (1)
whereisthewaterdensity,V thefilledvolumeandtthetime requiredtofillthevolumeV.Whenthedifferenttemperaturesand flowratesaremeasuredforagiventestedconfiguration,different performanceparametersoftherecoverysystemcanbecalculated.
Therecoveredheatcanbecalculatedfromthefollowingrelation:
Q˙R=m˙cCp,c
Tc,o−Tc,i
(2) where ˙mcandCp,carerespectivelythemassflowrateandspecific heatofthecoldwater(waterfromthesupplytank).
Theefficiencyoftherecoverysystem,whichrepresentsthe abilityofthesystemtoavoidwastingenergy,isdefinedastheratio oftheheatrecoveredbythecoldwaterovertheheatlostbythe hotwater:
= Q˙R
m˙hCp,h
Th,i−Th,o (3)
where ˙mhandCp,harerespectivelythemassflowrateandspecific heatofthehotwater(waterfromthedrain).
Theeffectivenessεofthesystemwhichrepresentsameasure- mentoftheperformance oftheheatexchangerwhenusingthe NTUmethod[34]isdefinedastheratiooftheheatrecoveredby thecoldwateroverthemaximumpossibleheattransferthatcan behypotheticallyachievedin acounter-flowheat exchangerof infinitelength,calculatedfromthefollowingrelation:
ε= Q˙R
m˙hCp,h
Th,i−Tc,i (4)
InEq.(4),thehotsideisconsideredintheidealcasesincethe massflowrateinthehotsideisexpectedtobelowerthaninthe coldsideduetowaterlossesduetowalladhesioninthedrainage box.
Threesetsofexperimentsarecarriedout.Thefirstsetofexper- imentscorresponds to differentinlet cold temperatures(water supplytank)varyingfrom3.2◦Cto22.4◦Cwithafixedhotinlet temperatureof70◦Candacoldflowrateof0.146kg/s(thehotflow rateisspecifiedfurtherinthepaper).Thedifferentcoldtempera- turesareobtainedbyinitiatingatemperatureof3.2◦Cbyadding icestothecoldsupplytankandperformingsuccessivemeasure- mentswhenicesmelt.
Thesecondsetofexperimentsisperformedfordifferentcold waterflowrates(whichimpliesdifferentvaluesofhotflowrates) varyingfrom0.058kg/sto0.146kg/s.
Thethirdsetiscarriedoutfordifferenthotinlettemperatures varyingfrom40◦Cto80◦C.Thecoldinlettemperatureisrecorded foreachconfigurationofsettwoandthreeandrangesfrom25◦C to40◦C.
3. Resultsandanalysis
Inthissection,thedifferentresultsobtainedaswellasthecorre- spondingobservationsandanalysiswillbepresented.Fig.5shows thevariationofthehotwaterflowrate(hotwatertofallinthe drainage)infunctionofthecoldwaterflowrate(waterfromthe watersupplytank).
FromFig.5a,itcanbenoticedthatthehotflowrateislowerthan thecoldflowrate.Asillustrationforacoldflowrateof0.146kg/s, thehotflowrateis0.122kg/s.Thismeansthat0.024kg/sarelostin thedrainboxduetoadhesionofwaterontheinnerwalls.Theper- centageofhotwatertothecoldwaterflowrate(Fig.5b)isalmost around81%–85%.Itshouldbenoticedthatgiventhepercentage ofhotflowratetocoldflowrate,aconsiderabletimewastaken betweentwoconsecutivetestsinordertohavesufficientdryingof theinnerwallofthedrainbox.
Fig.6showsthevariationoftheheatrecoveredinthesystemin functionofthecoldinlettemperature(Eq.(2)).Itisshownthatthe
578 M.Ramadanetal./EnergyandBuildings128(2016)575–582
0.04 0.06 0.08 0.10 0.12 0.14
0.05 0.07 0.09 0.11 0.13 0.15
Hot flow rate (kg/s)
Cold flow rate (kg/s) (a)
0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 100.00
0.05 0.07 0.09 0.11 0.13 0.15
Hot flow rate (%)
Cold flow rate (kg/s) (b)
Fig.5.Variationof(a)hotwaterflowrateand(b)percentageofhotwaterflowrate intermsofthecoldwaterflowrate.
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00
2 4 6 8 10 12 14 16 18 20 22 24
Recovered heat (kW)
Cold inlet temperature (0C)
Fig.6.Heatrecoveredinfunctionofthecoldinlettemperature.
heatrecovereddecreasesquasi-linearlyasthecoldinlettemper- atureincreases.Asillustration,theheatrecovereddecreasesfrom 8.24kWto2.62kWwhenthecoldinlettemperatureincreasesfrom 3.2◦Cto22.4◦C.
Toinvestigatedeepertheperformanceofthesysteminrelation withthecoldtemperature,theefficiencyandeffectiveness(Eqs.
(2)–(4))areplottedinFig.7.Itcanbeshownthatwhenthecold inlettemperatureincreases,theefficiencyandtheeffectiveness decreasealmostlinearly.Asillustrationwhenthecoldinlettemper- atureincreasesfrom3.2◦Cto22.4◦C,theefficiencydecreasesfrom around100%to47%andtheeffectivenessdecreasesfromaround
Fig.7. Variationof(a)efficiencyand(b)effectivenessoftherecoverysystemin functionofthecoldinlettemperature.
26%to12%.Itisstrikingtonotethateveniftheefficiencyandthe effectivenessdonotrepresentthesameresult,indeedthequanti- tativevaluesaretotallydifferent,theevolutionofthecurvesof andεareverysimilarand,aboveall,ineachcase,thefinalvalueis around46–47%oftheinitialvalue.
Waterflowingfromtheboilertotheinletofthedrain(inletof theheatexchangerhotside)losesheatduetolossofflowrateand coolingduetocontactwiththeinnerwallsofthedrainbox.The lostheatcanbecalculatedasthedifferenceofenergyratebetween theoutletoftheboilerandtheinletofthedrain:
Q˙L=m˙cCp,cTb,o−m˙hCp,hTh,i (5) whereTb,oisthetemperatureofwaterattheboileroutlet.
Theratioofthelostheatcanbecalculatedas:
Q˙L
Q˙b,o =m˙cCp,cTb,o−m˙hCp,hTh,i
m˙cCp,cTb,o (6)
with ˙Qb,otheheatattheoutletoftheboiler.
Fig.8showsthevariationofthelostheat ˙QLandthepercentage oflostheat ˙QL/Q˙b,oinfunctionofthehotinlettemperaturefor differentcoldwaterflowrates.
Itcanbeshownthatlossesincreasewiththeboileroutlettem- perature.Asillustration foracoldwaterflowrateof0.058kg/s, lossesincreasefrom11kWto12.8kWastheboiler outlettem- peratureincreasesfrom40◦Cto80◦C.Forflowratesof0.097kg/s and0.146kg/s,lossesarerespectivelyfrom25.1kWto29.3kWand
