Recovering heat from hot drain water—Experimental evaluation, parametric analysis and new calculation procedure

(1)

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

Mohamad Ramadan

^a,∗

, Thierry lemenand

^b

, Mahmoud Khaled

^a,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.

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

(2)

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.

(3)

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 T_h,i and T_h,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−T_c,i

(2) where ˙mcandCp,carerespectivelythemassﬂowrateandspeciﬁc heatofthecoldwater(waterfromthesupplytank).

Theefﬁciencyoftherecoverysystem,whichrepresentsthe abilityofthesystemtoavoidwastingenergy,isdeﬁnedastheratio oftheheatrecoveredbythecoldwaterovertheheatlostbythe hotwater:

= Q˙R

m˙_hC_p,h

T_h,i−T_h,o

⁽³⁾

where ˙m_handC_p,harerespectivelythemassﬂowrateandspeciﬁc heatofthehotwater(waterfromthedrain).

Theeffectivenessεofthesystemwhichrepresentsameasure- mentoftheperformance oftheheatexchangerwhenusingthe NTUmethod[34]isdefinedastheratiooftheheatrecoveredby thecoldwateroverthemaximumpossibleheattransferthatcan behypotheticallyachievedin acounter-flowheat exchangerof infinitelength,calculatedfromthefollowingrelation:

ε= Q˙R

m˙_hC_p,h

T_h,i−T_c,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

(4)

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 rat e (kg/ s)

Cold flow rat e (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 rat e (%)

Cold flow r ate (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,cT_b,o−m˙_hC_p,hT_h,i (5) whereT_b,oisthetemperatureofwaterattheboileroutlet.

Theratioofthelostheatcanbecalculatedas:

Q˙L

Q˙_b,o =m˙cCp,cTb,o−m˙hCp,hTh,i

m˙cCp,cT_b,o (6)

with ˙Q_b,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

(5)

(a)

(b)

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0

40 50 60 70 80

Losses QL(kW)

Boiler outlet temperature (C)

Flow rate = 0. 058 kg/s Flow rate = 0. 097 kg/s Flow rate = 0. 146 kg/s

0.0 5.0 10.0 15.0 20.0

40 50 60 70 80

Losses QL/Qb,o(%)

Boiler outlet temperature (C)

Flow rate = 0. 058 kg/s Flow rate = 0. 097 kg/s Flow rate = 0. 146 kg/s .

. .

Fig.8.Variationof(a)lossesand(b)percentageoflossesinfunctionoftheboiler outlettemperaturefordifferentcoldﬂowrates.

from32.9kWto37.8kW,fortheboileroutlettemperaturesvarying from40^◦Cto80^◦C.

Thepercentageofheatlost(Fig.8b)increaseswhentheflow rateincreasesfrom0.058kg/sto0.097kg/s,thenitdecreasesfor higherflowrates.Asillustration,theaveragepercentagescorre- spondingtoflowratesof0.058kg/s,0.097kg/s,and0.146kg/sare respectively14.7%,20.2%and17.4%.

Fig.9showsthevariationoftherecoveredheatinfunctionof thecoldﬂowrateandboileroutlettemperature.

Itcanbeshownthattherecoveredheatincreaseswiththeboiler outlettemperature.Asillustrationforacoldﬂowrateof0.058kg/s, therecoveredheatincreasesfrom0.05kWto2.42kWastheboiler outlettemperatureincreasesfrom40^◦Cto80^◦C.Forﬂowratesof 0.097kg/sand0.146kg/s,therecoveredheatincreasesrespectively from0.36kWto4.77kWandfrom0.55kWto5.31kW.

Foraboilertemperatureof50^◦C,therecoveredheatincreases from0.24kWto1.46kWasthecoldwaterﬂowrateincreasesfrom 0.058kg/sto0.146kg/s.Foraboileroutlettemperatureof70^◦C, therecoveredheat increasesfrom1.58kWto3.91kW.Whatis strikingfromFig.9bisthefactthattherecoveredheatincreases highlywhentheﬂowrateincreasesfrom0.058kg/sto0.097kg/s, butseemstoreachaplateaubeyondthisvalue.

Theeffectsofthecoldﬂowrateandboileroutlettemperature ontheeffectivenessoftheexchangeroftherecoverysystemare showninFig.10.

Itcanbeshownthattheexchangereffectivenessincreaseswith theboileroutlettemperature.Asillustrationforacoldﬂowrateof

(a)

(b) .

0.00 1.00 2.00 3.00 4.00 5.00 6.00

40 50 60 70 80

Heat recovered QR(kW)

Boiler outlet temperature (⁰C) Flow rate = 0.058 kg/s

Flow rate = 0.097 kg/s

Flow rate = 0.146 kg/s

0.00 1.00 2.00 3.00 4.00 5.00 6.00

0.058 0.078 0.098 0.118 0.138

Heat recovered QR(kW)

Cold flow rate (kg/s) Tb,o = 40 °C Tb,o = 50 °C Tb,o = 60 °C Tb,o = 70 °C

Tb,o = 80 °C

.

Fig.9.Variationofrecoveredheat(a)infunctionoftheboileroutlettemperature fordifferentcoldﬂowratesand(b)infunctionofthecoldﬂowratefordifferent boileroutlettemperatures.

0.058kg/s,theeffectivenessincreasesfrom2%to23%astheboiler outlettemperatureincreasesfrom40^◦Cto80^◦C.Forﬂowratesof 0.097kg/sand0.146kg/s,theeffectivenessincreasesrespectively from9%to30%andfrom10%to21%.

FromFig.10b,itcanbeshownthattheeffectivenesscurvesreach maximaintheirvariationswithrespecttothecoldmassﬂowrate.

Toexemplifyforaboileroutlettemperatureof60^◦C,theeffective- nessincreasesfrom16%to22%whenthewaterﬂowrateincreases from0.058kg/sto0.097kg/sandthendecreasesto16%whenthe coldﬂowrateincreasesfrom0.097kg/sto0.146kg/s.

4. Newcalculationprocedure

Basedonmanyfeaturesoftheresultsshownintheprevioussec- tion,anewprocedureofcalculationsthatpermitstoestimatethe performanceofarecoverysystemappliedtoanysystemofdrains and usinganyheatexchangertype willbepresentedhereafter.

Thisprocedurereliesonsomescenariosrelatedtotheoperational modeofagiventypeofdrain.Fig.11showstheﬂowchartofthe calculationprocedure.

Asshown inFig.11, theinputs tothecalculationprocedure arethewaterﬂowratefromthesupplytankthatthedrainsys- tem(showerchamber,sink,dishwasher,washingmachine,etc.) requires,thewatertemperaturein thesupply tankandthehot temperaturerelatedtothesystemmode(forexamplewaterheater temperatureinshowerapplication).Thecalculationprocedurewill

(6)

(b)

0 5 10 15 20 25 30 35

40 50 60 70 80

Effectiveness (%)

Boiler outlet temperature (⁰C)

Flow rate = 0. 058 kg/s

Flow rate = 0. 097 kg/s Flow rate = 0. 146 kg/s

(a)

0 5 10 15 20 25 30

0.058 0.078 0.098 0.118 0.138

Effectiveness (%)

Cold flow rate (kg/s)

Tb,o = 40 °C Tb,o = 50 °C Tb,o= 60 °C Tb,o = 70 °C Tb,o = 80 °C

Fig.10.Variationoftheexchangereffectiveness(a)infunctionoftheboileroutlet temperaturefordifferentcoldﬂowratesand(b)infunctionofthecoldﬂowratefor differentboileroutlettemperatures.

estimateseveralparameters,mainlytherecoveredheatandthe powerconsumptionreduction.

Step1:Fromthecoldmassﬂowrate ˙mc,thehotmassﬂowrate m˙_hcanbecalculatedfromthefollowingrelation:

m˙h=R1m˙c (7)

whereR1 isaratiothatcanbeobtainedfrompreliminarymea- surementsforagivendrainsystemandwhichisfunctionofthe operationalmodescenario.AsillustrationfromFig.4b,thisratiois around81%to86%.

Step2:Thetemperatureofhotwaterattheheatexchangerinlet canbecalculatedfromEq.(6)rearrangedinadifferentmanneras follows:

Th,i=Tb,oRC

R₁(1−R2) (8)

whereR_C=Cp,c/C_p,h istheratioof thespeciﬁcheatofthecold watertothatofhotwaterandR₂=Q˙L/Q˙_b,otheratiooftherate ofenergylostinthedrainsystemtotherateofabsoluteenergy containedinthewateratthehotwatersystemofthedrainbefore enteringthedrainitself.

Fortherangesoftemperaturesofcoldandhotwaterforthemost commontypesofdrain,theratioRCisalmostequalto1.TheratioR2

isalsoaratiothatcanbeobtainedfrompreliminarymeasurements foragivendrainsystemandisfunctionoftheoperationalmode scenario.AsillustrationfromFig.7b,thisratioisaround14%–21%.

Step3:Fromcurvesofeffectivenessandefficiencyobtainedon theheatexchangerseparatelyfromthesystemandfordifferent coldandhottemperaturesanddifferentwaterflowrates,theeffec- tivenessandefficiencyoftheheatexchangerunderconsideration intherecoverysystemcanbeobtained.Itshouldbenoticedfrom theexperimentalanalysisperformedintheprevioussectionthat theeffectivenessoftheexchangerincreasesquasi-linearlywiththe differenceintemperaturebetweentheinlethotandcoldtempera- turesandincreasesexponentiallywiththeflowrateuptoacertain valuefromwhichitstartstodecrease.

Step4:Therateofheatexchangedinthesystemwhichcorre- spondstotherecoveredheatcanthenbecalculatedasfollows:

Q˙R=εm˙_hC_p,h

T_h,i−T_c,i

(9) Step5:Thecoldwateroutlettemperaturewhichcorresponds tothetemperatureattheinletoftheheatingsystemofthedrain typeunderconsiderationcannowbecalculated:

Tc,o=T_c,i+ Q˙R

m˙cCp,c (10)

Step6:Therateofenergyconsumedbytheheatingsystemof thedraintypecannowbecalculated:

Q˙heating=m˙cCp,c

Tb,o−Tc,o

(11) Step7:Thepowerconsumptionreduction(PCR)obtainedwith theinstallationoftherecoverysystemcanbeobtainedfrom:

PCR= Q˙R

Q˙R+Q˙_Heating (12)

Step8:Thehotwateroutlettemperaturewhichcorrespondsto thetemperatureofwaterthatwillbedischargedintheﬁnaldrain canalsobecalculatedfrom:

Th,o=Th.i− Q˙_R

m˙hCp,h (13)

Theprocedurepresentedabovecanbeappliedtoanytypeof heatrecoverysystemfromanytypeofhotdrainsystemtoestimate parametersrelatedtotheperformanceoftherecoveryconcept.This procedurenecessitates preliminarymeasurements ona generic experimentalsetupsuchasthatpresentedaboveinthismanuscript toobtainsomekeyparametersandratiosthatareusedinthedif- ferentstepsofcalculations.

5. Conclusions

Recoveringthewasteheatfromsystemsutilizedinthedailylife isasolutiontodecreasethedomesticenergyconsumption.Inthis paper,recoveringheatfromhotdrainwaterisinvestigated.Particu- larly,agenericexperimentalsetupisdevelopedandanappropriate experimentalparametricanalysisiscarriedout.

Intheparametricanalysis,itwasparticularlyfoundthat:

(1)Around15%ofthewaterﬂowratecanbelost,i.e.notavailable inthesystemdrainagebox;

(2)The effectiveness and efﬁciency of the recovery system decreasewiththecold inlettemperature.Thisdecreasecan reach50%for20^◦Cdifference;

(3)Around15–20%oftheheatofwaterattheboileroutletarelost inthesystemdrainagebox;

(4)Theexchanger effectivenessincreaseswiththeboileroutlet temperature.Asillustrationforacoldﬂowrateof0.058kg/s, theeffectivenessincreasesfrom2%to23%astheboileroutlet temperatureincreasesfrom40^◦Cto80^◦C.

(5)Theeffectivenesscurvesreachmaximaintheirvariationswith respecttothecoldmassﬂowrate.Toexemplifyforaboiler

(7)

Cold water Flow rate

Cold water (supply) inlet temperature

m

c

T

_c_,_i

Calculation procedure

Ratio of flow rates R1 Ratio of energy lost R2 Effectiveness Efficiency Drain system type

Operational mode scenario

Hot temperature of system mode

Preliminar y tests

o

T

b_,

Hot water flow rate

m

h Eq. 7

Eq. 8 Hot water inlet temperature

Effectiveness Exp.

Curves

Recovered heat

Cold water outlet temperature

Energy rate of heating system

Power consumption reduction

Cold water outlet temperature i

T

h_,

Q

R

o

T

c_,

heating

Q

PCR

o

T

h_, Eq. 9

Eq. 10

Eq. 11

Eq. 12

Eq. 13

1

2

3

4

5

6

7

8 Fig.11.Flowchartofthecalculationprocedure.

outlettemperatureof60^◦C,theeffectivenessincreasesfrom 16%to22%whenthewaterﬂowrateincreasesfrom0.058kg/s to0.097kg/sandthendecreasesto16%whenthecoldﬂowrate increasesfrom0.097kg/sto0.146kg/s.

Finally and based onthe parametric analysisshown, a new designapproachconsistingofasystematiccalculationprocedure waspresented.Itcanbeutilizedasapre-calculationtooltosize drainheatrecoverysystemsorasanoptimizationtechniqueto enhanceanexistingsystem.Itwasshownthatanydrainsystemcan bepreliminarysizedusingthenewsuggestedsystematicapproach basedonexperimentallydeterminedparameters.

References

[1]B.Wang,L.D.Cot,L.Adolphe,S.Geoffroy,J.Morchain,Estimationofwind energyoverroofoftwoperpendicularbuildings,EnergyBuild.88(2015) 57–67.

[2]L.Lu,K.Sun,Windpowerevaluationandutilizationoverareferencehigh-rise buildinginurbanarea,EnergyBuild.68(2014)339–350.

[3]E.Shojaeizadeh,F.Veysi,A.Kamandi,Exergyefficiencyinvestigationand optimizationofanAl2O3—waternanofluidbasedflat-platesolarcollector, EnergyBuild.101(2015)12–23.

[4]S.Tamvakidis,V.K.Firﬁris,A.Martzopoulou,V.P.Fragos,T.A.Kotsopoulos, Performanceevaluationofahybridsolarheatingsystemforfarrowing houses,EnergyBuild.97(2015)162–174.

[5]N.Naili,M.Hazami,I.Attar,A.Farhat,Assessmentofsurfacegeothermal energyforairconditioninginnorthernTunisia:directtestanddeploymentof groundsourceheatpumpsystem,EnergyBuild.111(2016)207–217.

[6]A.Kecebas,C.Coskun,Z.Oktay,A.Hepbasli,Comparingadvancedexergetic assessmentsoftwogeothermaldistrictheatingsystemsforresidential buildings,EnergyBuild.81(2014)141–151.

[7]C.R.Touretzky,M.Baldea,Ahierarchicalschedulingandcontrolstrategyfor thermalenergystoragesystems,EnergyBuild.110(2016)94–107.

[8]M.Ameri,Z.Besharati,Optimaldesignandoperationofdistrictheatingand coolingnetworkswithCCHPsystemsinaresidentialcomplex,EnergyBuild.

110(2016)94–107.

[9]S.Delfani,H.Pasdarshahri,M.Karami,Experimentalinvestigationofheat recoverysystemforbuildingairconditioninginhotandhumidareas,Energy Build.49(2012)62–68.

[10]Y.ElFouih,P.Stabat,P.Rivière,P.Hoang,V.Archambault,Adequacyof air-to-airheatrecoveryventilationsystemappliedinlowenergybuildings, EnergyBuild.54(2012)29–39.

[11]M.S.Todorovic,J.T.Kim,Insearchforsustainablegloballycost-effective energyefﬁcientbuildingsolarsystem—heatrecoveryassistedbuilding integratedPVpoweredheatpumpforair-conditioning,waterheatingand watersaving,EnergyBuild.85(2014)346–355.

[12]J.Jia,W.L.Lee,Applyingstorage-enhancedheatrecoveryroom

air-conditioner(SEHRAC)fordomesticwaterheatinginresidentialbuildings inHongKong,EnergyBuild.78(2014)132–142.

(8)

[13]S.Gendebien,S.Bertagnolio,V.Lemort,Investigationonaventilationheat recoveryexchanger:modelingandexperimentalvalidationindryand partiallywetconditions,EnergyBuild.54(2012)29–39.

[14]S.Garimella,Low-gradewasteheatrecoveryforsimultaneouschilledandhot watergeneration,Appl.Therm.Eng.42(2012)191–198.

[15]B.Peris,J.Navarro-Esbri,F.Molès,BottomingorganicRankinecycle conﬁgurationstoincreaseinternalcombustionenginespoweroutputfrom coolingwaterwasteheatrecovery,Appl.Therm.Eng.61(2013)364–371.

[16]J.White,M.C.Gillott,C.J.Wood,D.L.Loveday,K.Vadodaria,Performance evaluationofamechanicallyventilatedheatrecovery(MVHR)systemaspart ofaseriesofUKresidentialenergyretroﬁtmeasures,EnergyBuild.110 (2016)220–228.

[17]M.Kassai,M.R.Nasr,C.J.Simonson,Adevelopedproceduretopredictannual heatingenergybyheat-andenergy-recoverytechnologiesindifferent climateEuropeancountries,EnergyBuild.109(2015)267–273.

[18]J.Zhang,A.S.Fung,S.Jhingan,Analysisandfeasibilitystudyofresidential integratedheatandenergyrecoveryventilatorwithbuilt-ineconomizer usinganexcelspreadsheetprogram,EnergyBuild.75(2014)430–438.

[19]J.E.Smith,Recoveryandutilisationofheatfromdomesticwastewater,Appl.

Energy1(1975)205–214.

[20]S.Kannoh,Heatrecoveryfromwarmwastewateratdyeingprocessby absorptionheatpump,J.HeatRecoverySyst.2(1982)443–451.

[21]M.DePaepe,E.Theuns,S.Lenaers,J.VanLoon,Heatrecoverysystemfor dishwashers,Appl.Therm.Eng.23(2003)743–756.

[22]N.C.Baek,U.C.Shin,J.H.Yoon,Astudyonthedesignandanalysisofaheat pumpheatingsystemusingwastewaterasaheatsource,Sol.Energy78 (2005)427–440.

[23]L.T.Wong,K.W.Mui,Y.Guan,Showerwaterheatrecoveryinhigh-rise residentialbuildingsofHongKong,Appl.Energy87(2010)703–709.

[24]L.Liu,L.Fu,Y.Jiang,Applicationofanexhaustheatrecoverysystemfor domestichotwater,Energy35(2010)1476–1481.

[25]A.McNabola,K.Shields,Efﬁcientdrainwaterheatrecoveryinhorizontal domesticshowerdrains,EnergyBuild.59(2013)44–49.

[26]I.Beentjes,R.Manouchehri,M.R.Collins,Aninvestigationofdrain-side wettingontheperformanceoffallingﬁlmdrainwaterheatrecoverysystems, EnergyBuild.82(2014)660–667.

[27]J.Wallin,J.Claesson,Investigatingtheefﬁciencyofaverticalinlinedrain waterheatrecoveryheatexchangerinasystemboostedwithaheatpump, EnergyBuild.80(2014)7–16.

[28]D.Slys,S.Kordana,Financialanalysisoftheimplementationofadrainwater heatrecoveryunitinresidentialhousing,EnergyBuild.71(2014)1–11.

[29]S.S.Cipolla,M.Maglionico,Heatrecoveryfromurbanwastewater:analysisof thevariabilityofﬂowrateandtemperature,EnergyBuild.69(2014)122–130.

[30]S.S.Cipolla,M.Maglionico,Heatrecoveryfromurbanwastewater:analysisof thevariabilityofﬂowrateandtemperatureinthesewerofbolognaItaly, EnergyProcedia82(2014)660–667.

[31]J.Wallin,J.Claesson,Analyzingtheefﬁciencyofaheatpumpassisteddrain waterheatrecoverysystemthatusesaverticalinlineheatexchanger,Sustain.

EnergyTechnol.Assess.8(2014)109–119.

[32]R.Manouchechri,C.J.Banister,M.R.Collins,Impactofsmalltiltanglesonthe performanceoffallingﬁlmdrainwaterheatrecoverysystems,EnergyBuild.

102(2015)181–186.

[33]R.Amon,M.Maulhardt,T.Wong,D.Kazama,C.W.Simmons,Wasteheatand waterrecoveryopportunitiesinCaliforniatomatopasteprocessing,Appl.

Therm.Eng.78(2015)525–532.

[34]F.P.Incropera,D.P.DeWitt,FundamentalsofHeatandMassTransfer,John WileyandSons,2011.