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Urban water networks as an alternative source fordistrict heating and emergency heat-wave cooling

Xiaofeng Guo, Martin Hendel

To cite this version:Xiaofeng Guo, Martin Hendel. Urban water networks as an alternative source for district heating andemergency heat-wave cooling. Energy, Elsevier, 2017, �10.1016/j.energy.2017.12.108�. �hal-01763551�

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Urbanwaternetworksasanalternativesourcefordistrictheatingandemergencyheat-wavecoolingXiaofengGuo1,2*,MartinHendel1,2

1ESIEEParis,UniversitéParisEst,2boulevardBlaisePascal-CitéDescartes,F-93162,NoisyLeGrand,France2UnivParisDiderot,SorbonneParisCité,LIED,UMR8236,CNRS,F-75013,Paris,France

AbstractUrban water networks can contribute to the energy transition of cities by serving asalternatives sources for heating and cooling. Indeed, the thermal energy potential of theurbanwatercycleisconsiderable.Parisistakenasanexampletopresentanassessmentofthe fieldperformanceofadistrict-scalewastewaterheat recovery systemand toexplorepotential techniques for emergency cold recovery from drinking or non-potable waternetworks in responsetoheat-waves.Thecaseheat recoverysystemwas foundtoprovidesignificant greenhouse gas emission reductions (up to 75%) and limited primary energysavings(around30%).Theselimitedsavingsarefoundtobemainlyduetotheperformanceof the heat pump system. Three emergency cold recovery techniques are presented as aresponse toheat-waves: subwaystationcooling, iceproduction for individual cooling,and“heat-wave shelter” cooling in association with pavement-watering. The cold generationpotential of each approach is assessed with a special consideration for mains watertemperaturesanitarylimitations.Finally,technicalobstaclesandperspectivesarediscussed.

KeywordsUrbanwatercycle;Thermalenergyrecovery;Urbanheatisland;Heat-wave;Centralheatingsupply.

1 IntroductionConcentrating60%to80%oftheworld’senergyconsumption[1],citiesareattheheartoftheenergytransitionchallengefacinghumanityoverthe21stCentury.Thischallengeismademoredifficult by the changes in climate expectedover the courseof the current century,whichwillgraduallyandinevitablyaffectthewayenergyisusedtoheatandcoolbuildings.

Asclimatechangecontinues,citieswillwitnessadecreaseintheirheatingdemandandanincrease in their cooling demand. While the decrease in Heating Degree Days (HDD)forebodesenergysavings,thesemaylikelybecompensatedbythesharpincreaseincoolingdemand[2].Thistrendcanbeobservedinmanymajorcitiesacrosstheglobeandpresentamajorchallengefortheworld’ssuccessfulenergytransition[3]. InParis,ascanbeseen inFigure 1a), building energy demand is clearly heating-dominated. This is reflected by itsaverage2352˚C.dayofHDD,whilecoolingdemandremainssmallwithatotal17°C.dayofcooling degree days (CDD) (the threshold values used are 18°C for heating and 24°C forcooling)[4].Attheendofthiscentury,climatechangeisexpectedtodecreaseHDDby30%

*Corresponding author: Xiaofeng Guo, xiaofeng.guo@esiee.fr, Tel: +33 14592 6058, Fax: +33 14592 6699address:ESIEEParis,departmentSEN,2bdBlaisePascalBP99,93162,NoisyLeGrandCedex,FRANCE

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to1622°C.day,whileCDDshouldincreaseseven-foldto127°C.day[4].Thisshiftisalreadyvisibleoverthelastfewdecades[5].

a)MonthlytemperaturesforParisb)Maximumandminimumdaily

temperaturesduringtheJune2017heat-wave

Figure1.AirtemperaturedataattheMontsourisweatherstationinParisTheyellowbandshownintheleftgivesthehumancomfortzonebetween18°Cand24°C;

TheverticalbandontherightoutlinesthepeakofJune2017heat-wave.

In addition to the local climate, which is the main determinant for building heating andcoolingdemand,citiesarealsosubject totheurbanheat island(UHI)effect.This localizedwarmingphenomenon is the resultof a combinationof radiative trapping, increasedheatstorage, wind obstruction, low vegetation presence, low surface permeability and highconcentrationsofhumanactivityalongwithcorrespondingheatrelease[6].Oneshouldalsomentiontheincreaseofindividual,airsourceair-conditioningsystemsthatintensifytheUHI.Thesemechanismscausehigherairandsurfacetemperaturesincitycentresrelativetothesurrounding rural areas, in the order of 1° to 3°C [7]. In terms of its impact on energyconsumption,UHItendstoincreasecoolingdemandandreducethatofheating.

ParalleltotheglobalclimateshiftandUHIeffect,thefrequencyofextremeweatherevents,in particular heat-waves, is expected to increase [8]. In Paris, heat-waves are expected toincreasefrom1heat-wavedayperyeartoasmanyas26daysperyear[4].CombinedwiththeUHIeffect, theseeventsposeaseriouspublichealthconcern,aswitnessedduringthe2003 heat-wave [9]. Although infrequent, such events are characterized by hightemperaturesduringmorethan3consecutivedays,asshowninFigure1b),andmeritactivecooling techniques. How to deal with short but intense emergency cooling needs intraditionally heating dominated regions ismore of a public security issue than an energyefficiencyconcern.

Inrecentyears,energyrecoveryfromurbanwaternetworkshasgainedincreasingattentionfrom urban planners as well as water utility companies. To date, urban water networks,especially sewer systems, have been seen as potential sources for heat recovery [10,11],while cold recovery has been considered more recently [12,13]. In Paris, industrialapplicationsofbothheatandcoldrecoveryhavebeenbuiltrecently[14–16].However,thefieldperformanceofactualrecoverysystemshasonlyrarelybeenevaluated.Furthermore,coldrecoveryhasneverbeenconsideredasameansofrespondingtoheat-waves.

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In this paper, the Parismetropolitan area is used as a case study for evaluating differentpossibilitiesofusingwaterasheatingandcoolingalternatives.Fortheheatingsupplyfromsewagewaterheat recovery, the fieldperformanceof anexistingheat recovery system isassessed. Regarding cold supply from water mains during extreme heat, a potentialassessment isconducted for threecold recoveryconfigurationsdesignedasanemergencyresponsetoheat-waves.

Therestofthepaperisorganizedasfollows:first,theglobalurbanwatercycleisdescribedwithspecialattentiontotemperaturelevelandthermalenergyrecoverypotentialsineachstep. Then, annual running data from a waste water heat recovery project in Paris isanalyzed. Greenhouse Gas (GHG) emissions and primary energy savings are used asevaluation criteria. The third part gives innovative concept descriptions of three coolingproductions from potable or non-potable water mains. These concepts are expected toproviderealactivesolutionsduringheat-wavesinhighdensityurbancenters.

2 ThermalenergyrecoveryinwaternetworksTheoverallwatercycleinanurbanareabeginsatariverorundergroundwatersourceandends at the outlet ofwastewater treatment plants (WWTP). As shown in Figure 2, afterbeingpumpedfromthesource,treatedwateristransportedtoitsend-usersthroughurbanwatermains.Afterbeingused,sewageiscarriedtoaWWTPviathesewernetwork.Certaincities,suchasParis,areequippedwithsecondarywaternetworksdedicatedtonon-potableuses such as green space irrigation or street cleaning. This watermay also come from asimilarwatersourcewithlessintensivetreatmentormayalsobetreatedwastewater,thesourcebeingtheWWTPoutlet.Regardlessofthespecifics, itscycleremainssimilartothatdepictedinFigure2.

Considering thewholeurbanwatercycle,domestichotwater (DHW)preparation isby farthehighestenergyconsumer,representingapproximately85%oftotalenergyneeds[17].The other twomain energy uses are found at the supply and sewer disposal ends of thecycle.Asameansofcomparison,raisingwatertemperatureby1°Cisalreadyequivalenttotheenergyneedsof thosetwoprocesses.Generally,DHWisheatedto60-65˚Ctocombatbacterialhazards,particularlyLegionellaspp.Giventhatthewaterinletisbetween10°and15°C[18],thetemperaturemustberaisedby45-55°Conaveragethroughouttheyear,notaccountingforseasonalvariations.

Temperaturelevelsinthewholewatercyclerangefrom1°Cto65°C,asshowninFigure2.Inthecycle,twothermalenergyrecoverypotentialscanbepossible:coldrecoveryinthewatermainswhere temperaturesarebelow25°C,aswellasheat recovery in thesewersystemswheretemperaturesarebetween13and35°C.

Forheatrecovery,thesewerwatermustremainabove13°CtomeettheoperationalneedsofWWTPprocesses.Forcoldrecovery,watermainstemperaturemustremainbelow25°Ctoensure thatbacterialgrowthremains limited [19].Therefore, in thecaseofclosed loopsystems(sewageorpotablewater),wherewaterremainsinthewaternetwork,amaximumtemperature difference is permitted. However, in the case of an open loop system(introducedinsection4.3),highertemperaturechangesareallowed.

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Figure2.Temperaturedistributioninwatercycle,fromdrinkingwatertosewagewater

Whiletheflowratefluctuationfeatureofwaternetworkscouldbeadifficultyforrecoveryprojects, its application at the district level is less intermittent. As long as the connectingpopulationissufficientlydenseanddiverse,acontinuousflowrateismaintainedalmostallthroughtheday.Particularlyinthecaseofwastewaterheatrecovery,sewernetworkscantemporallyholdhigheffluent inletssincetheirvolumesare ingenerallyoverdimensioned.Consequently, they can serve asbuffers to stablewastewater flowrate. In this paper,wefocusourattentiontothecollectiveutilisationofwaterthermalresource,i.e.,bysupposingstableflowratesduringheatrecoveryprocesses.

3 Heatrecoveryfromsewagewatersystem

3.1 PrincipleWaste water effluent has a temperature range of 35-27˚C at the outlet of buildings. InFrance,thetemperatureleveldecreasesalongseweragechannelsuntil13˚CbeforeenteringWWTPs. Lower temperatures should be avoided as most treatment processes require awarm environment for efficient nitrogen removal [20], even though recent studies haveproposed low-temperature treatments [21,22]. From a heat recovery point of view, weconsideratemperaturelevelfrom35˚Cto13˚Cinthisstudy.

Thoughtemperaturelevelsarehigherclosertobuildingseweroutlets,flowratesarelowerandintermittent.Therefore,fordistrict-scalesystemsheatrecoveryisonlyfeasibleinsewercollectors where the flow is continuous, despite lower temperature levels. Closer to the

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effluent source where temperatures are higher, individual solutions compatible withintermittentflowscanbeusedforheatrecoveryforDHWsystemsforexample[10,23].

Atthedistrict-scale,manycentralizedheatrecoveryprojectsexistandcanbereferredto.Insuchsystems,aheat transfer fluid (HTF) isused to transport the thermalenergy fromthewastewatertoaheatpump(HP)waterheater.Oneadvantageofdistrict-scalerecoveryisthepossibilityofprovidingDHWinadditiontoheating.Bydoingthis,thepaybackperiodisshorter and the running hours of the installed equipment are greater. An example of acentral heat recovery system is depicted in Figure 3. In such a system, two majorcomponentsshouldbeaddressed:heatexchangersandHP.

Figure3.Districtheatingwithsewagewaterheatrecoveryintegration

3.2 HeatexchangertechnologyAtthedistrictscale,twotypesofheatexchangerscanbeused:thoseincorporatedintothesewagemains (integrated type)or spiralheatexchangers (external type).Both reduce theriskoffoulingandfrequentmaintenance.Heatexchangershavethreemaincharacteristics:capacity, exchange surface and heat transfer coefficients. To minimize the temperaturedifferenceinwastewaterheatrecovery,whereaHTFisused,itispreferabletohavealargeexchangesurface(A)andahightransfercoefficient(U).

Integratedexchangerscanbeaddedtotheexistingsewersystemorincorporatedintonewsewer mains sections. The desired capacity is reached by assembling several modules orsectionsinseries.Becauseoftheircomparativelylowheatexchangecoefficient,i.e.approx.300W.m-2.K-1versustypicalvaluesof2000-5000W.m-2.K-1 [24],reachingthedesiredheatcapacity often requires long segments. Spiral heat exchangers are much more compactthanks to theirhighexchange surfaceandhighperformance [25],but requireadedicatedsewagenetworkderivation.Thechoicebetweenbothtechnologieswilltendtobedictatedby theopportunityofaddingaheatexchanger toanexistingsewersystemasopposedtocreatinga sewagederivation.Moreover,modern spiral heat exchangershold self-cleaninganti-fouling options that reduce the frequency of maintenance. Integrated tube heatexchangers,however,shouldberegularlycleanedifthecarriedwastewaterishighlyorganicloaded.

RecentexamplesofbothtechnologiescanbefoundinFrance[14,26–29].

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3.3 HeatpumpsTwo important characteristics of HP should be considered: heating capacity and COP(CoefficientofPerformance).Today,a large rangeofheatingcapacitiesareavailable,withnominalvaluesfrom2kWto20000kW[30].Theirproductioncanbeadaptedtodemandbutonlywithincertain limits:belowacertainthreshold,e.g.10%ofnominalcapacity, theHP must be stopped and replaced by auxiliary heating devices such as a boiler. Thisvulnerabilitymakesitimportanttoconsiderthetemporalfluctuationsofbothdemand-andsource-sides.

The COP of a HP is expressed as the ratio of its heating capacity and the compressor’selectricconsumption.Intheliterature,theCOPforsewerheatrecoveryprojectsisreportedtorangefrom1.8to10.6[31].

The large performance range is partially due to the temperature differences between thehotandcoldside,whichisgivenbyCarnotHPperformancecoefficient:

𝐶𝑂𝑃$%&'( =𝑇+

𝑇+ − 𝑇- (1)

Inpractice,duetoexergylossesinarealcycle,weputabouthalfoftheCarnotCOPvalueasthatofrealone,inadditiontoapartloadfactorconsidered:

𝐶𝑂𝑃.&'( =𝑄+

𝑊-123= 0.55×𝑃𝐿𝐹×

𝑇+𝑇+ − 𝑇-

(2)

wherePLFstandsforthepart-loadfactor,anditsvalueislowerthan1whentheHPrunsatnon-nominalconditions.𝑄+ and𝑊-123 represent respectivelyheating rateat thehot sideandcompressorelectricpowerconsumption.

Forannualanalyses,theEnergyFactor(EF)isoftenused,whichconsidersbothcompressorand auxiliary equipment consumptions as well as their seasonal variation. Whiletemperaturelevelsarerelativelystableinourcase(comparedwithair-sourceHPswithhighseasonalvariation),PLFcanplaysamajorroleinthefinalannualperformance.TheannualEFisgivenby:

𝐸𝐹';;<'( =𝑄+

𝑊-123 +𝑊'<> (3)

3.4 CasestudyDuringtheconstructionofalowcarbondistrictinNanterre(suburbofParis),a800mlongdistrictheatingnetworkwasbuiltandissuppliedbysewageheatrecoverywithanauxiliarygasboiler.Thedistrictisinadenseurbanareaandistransformingfromashut-downfactorysite to an eco-district. The transformation started in 2005 and the buildings arecommissioned progressively between 2011 and 2017. In 2015, the mini-district heatingnetwork delivers heat and DHW to 650 high energy performance residential flats and anewlyconstructedschool,totallingaheatingsurfaceofaround54000m2.

ThedistrictheatingsystemconsistsoftwosuppliesprovidingrespectivelyDHWandcentralheatingandonecommonreturn.Fortherecoveryside,anoverall lengthof200mofheatexchangerisintegratedintothesewernetwork,representingatotalheatexchangesurfaceof112m2.Thesewernetwork transportseffluentsata flowrateof115m3/h, rejectedby

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around15000equivalenthabitants. Thedesignedheatexchange capacity is 370kW for anominaltemperaturedifferenceof11°C.TheglobalheattransfercoefficientbetweenHTFandwastewateris300W.m−2.K−1.Toraisethetemperatureleveltothedeliveringvalue,i.e.,65°CforDHWand72°Cfordistrictheatingsupply,twoHPsareused.Eachofthemhasaheating rateof400kWth.Oneof them isoperatedonly for summerDHWsupplyand theother forheatingandDHWinwinter.Theaboveparametersabout thiscasestudycanbesummarizedinTable1.

Table1.Technicaldetailsofsewageheatrecoverycasestudy for theheatingsupplyofaneco-districtinParissuburb

Thermalenergydemands Totalheatingground(m2) 54000

AnnualHDDin2015(siterecorded,˚C) 1670Annualheatingdemand(MWh) 2548DHWenergydemand(MWh) 1207Heatdeliverytemperature(°C) 72

Heatrecoverybyheatpump Heatpumpheatingcapacity(kWth) 400

AnnualHPenergyfactor(-) 2.7Electricityinputtocompressionandauxiliary(kWM) 1417

Sewagewater HTFoutlettemperature(˚C) 10Heatexchangercapacity(kW) 370

Totalheatexchangersurface(m2) 112

AyearlyassessmenthasbeenconductedandmonthlydetailsareshowninFigure4a/b/c.The total annual energy demand in 2015 (January to December) was 3889MWh with2548MWh dedicated to space heating (October to May) and 1 207MWh for DHW.134MWhwerelostduringthecalorictransport.Itisworthnotingthatthefirstsemesterofheating season in2015wasparticularlyharsh,withmonthlyHDDofup to349°C.day.ThetotalannualHDDvalueoftheyearis1670°C.day.Themaindemandiscentralheatingwithanannualprimaryfractionof66%.

Total HP production supplies 84%of the annual energy demand,with the remaining 16%providedbygasboilers.Theannualshareofrecoveredenergy(evaporatorsideofHP,shownin orange in Figure 4b) is 58%,whichmakes the project eligible as a low carbon energysystemforcertainsubsidies[32].EFrangesfrom2.6to3.0,exceptinJulyandAugustwhentheHPisonlypartiallyrunduetolowenergydemand.Atotalof1165MWhofelectricityisconsumed by the district heating through HP compressors, representing 26% of the totalthermal energy demand. Another 252MWh of electric energy is consumed by auxiliaries,i.e.,pumpsandflowdistribution.

OperationmaintenancesarecarriedoutduringthefirstweekofMay,aswellasallthroughthe summerperiodbetween late June and the endof September.During this period, theheatexchangerisentirelycleanedtoavoidfoulingeffecttothesystemperformance.

Figure4cshowsmonthlyHPproductionasashareoftotalprimaryenergyconsumption.InFrance,electricenergymustbemultipliedby2.58forconversiontoprimaryenergy,while

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recoveredheatisalreadyconsideredtobeprimaryenergy.Figure4cexposesthe“darkside”ofHP thermal energy recovery projects:whileHPproduction satisfies at least 75%of thetotal demand duringmostmonths (shown as ○ in the figure), the recovered energy onlyrepresents 30-40% of monthly primary energy consumption (shown as green bars). Theremainder,i.e.gasandelectricity,represents68%ofprimaryenergyconsumptionovertheyear. This issue isparticularlyembarrassing forHP recoveryprojects sincebuildingenergyregulations generally consider primary energy consumption, not final energy. Inconsequence,comparedwithagasboiler,aHPsystemwhoseEFisunder2.58isnotabletobringprimaryenergysavings,evenwithwasteheatrecovery.

Finally, from a carbon emissions point of view, the waste heat recovery project is verypromising. In France, GHG emission factors for electricity are estimated to be 0.055t-eqCO2/MWh and 0.206t-eqCO2/MWh for natural gas [33]. In addition, the gas boiler isconsideredtohaveanenergyefficiencyof1.02thankstocondensationgeneration.Resultsshowthatthecaseprojecthasanannualcarbonfootprintof204t-eqCO2in2015,including127t-eqCO2fromthegasboiler,64t-eqCO2fromtheHPcompressorsand14t-eqCO2fromauxiliaries. With a gas-only heating supply, the GHG emissions would have been 799t-eqCO2, i.e. almost 4 timesmore. This is particularly interesting in the light of the recentClimatePlanbeingdevelopedbytheFrenchgovernmentwhichhasrecentlysetthegoalofcarbonneutralityby2050[34].

a)Energydemandfromcentralheating,DHWandheatlosses

b)Productionfromwastewaterheatrecovery,gasboilerandHPelectricity

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4 Coldrecoveryfromdrinkingorirrigationwaternetworks

4.1 UndergroundrailwaystationcoolingIntheParisMetrosystem,stationsareequippedwithmechanicalventilation.Thesystemisdesignedmainlyagainstairpollutionor forsmokeextraction,butstationsarenotactivelycooled.Duringheat-waves,highambient temperaturesanddensecrowdscanmake thesestationsveryuncomfortableandthermallystressfulforpassengers.

Onewayofcontainingtemperatureextremesistousewatermainscoldrecoverytoprovidecooling tounderground railway stations. InFigure5,wepropose to transformexistingairsupply systems (2) into complete Air Handling Units (AHU) with an integrated heatexchanger.Thanks toaHTF,coolingproducedbya refrigerationsystem(3) isdelivered tothe AHU heat exchangers. On the cold recovery side, another HTF provides condensercoolingandinjectsheatintothewatermainsthroughaheatexchanger(1).

c)TotalHPproductionandrecoveryshareofheatingdemand,convertedtoprimary

energyFigure4.Districtheatingwithsewagewaterheatrecoveryperformancedatafromon-site

monitoring

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ThefollowingassessmentisconductedfortheParismetrostationPortedeClignancourt,atypical small-sized metro station with only one passenger line (Line 4). While no activecooling is operated in themetro station, the configuration shown in Figure 5 could helpreduce air temperaturebyblowing cooled air (26°C) into the station insteadof untreatedoutsideair(35˚C).

Cooling the supply air in thismannerwould require 261kWof refrigerating power. For arefrigeration unitwith an energy efficiency ratio (EER) of 6.9 (e.g. Cooling unitMultistack[35],technicaldetailsinTable2),38kWofelectricpowerwouldberequired.Atotal299kWof heatwould then need to be dissipated into thewatermains. For a 1.2˚C temperatureincrease,amainsflowrateof216m3/hwouldberequired.Supposingthattheflowrateiscontinuousover24h,thisflowratecorrespondstothedailyconsumptionof25920people(~200Lperpersonperday).InParis,theobservedaveragewaterconsumptionpercapitaisapproximately230L/day.

Figure5.MetrostationcoolingbyAHUsuppliedwithcoldrecoveryfromwatermains

(1)heatexchanger,(2)circulator,(3)water-to-waterrefrigerator,(4)AHU,(5)undergroundMetrostation

(1)

(5)

(2)(3)

(4) (2)

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Table2.Estimationdetailsofwatermainscoldrecoverytoprovidemetrostationcooling

Metrostationloadestimation Airblowingflowrate(m3/h) 86400Outsideairtemperature(˚C) 35Blowingtemperature(˚C) 26

Coolingload(kW) 261Coolingunit

Manufacturer/Model/Refrigerant Multistack/MS050XN/R410aEvaporatorHTFoutlet(˚C) 16CondenserHTFinlet(˚C) 24

EER(-) 6.9Electricityinput(kW) 38

Mainswater Coldrecoveryheatexchangertemperaturedifference(˚C) 4

Source-sideheatexchangercapacity(kW) 299Mainswaterinlettemperature(°C) 20

Potablewatertemperatureincrease(˚C) 1.2

Approximatively300similarstationsaredistributedoverParis.Giventhetotalflowrateofpotablewater, the describedmethod could be deployed to 99 such stations located nearpotablewatermains, provided that the initialwater temperaturepermits it. This capacitycouldalsobe increased to141 stationsbyalsousing thenon-potablewatermains,whichsupplyanaverage220000m3/day.

Itisworthmentioningthat,duetofiresafetyregulations,mostmetrostationsareequippedwithairventilationsystems.Addinganair-waterheatexchangerbeforeexistingventilatorscanbedoneeasily.Thisfacilitatestheimplementationofmetrostationcooling.

4.2 Movablesolutionfornightcoolingduringheat-wavesAttheindividualscale,coolingisgenerallyprovidedbyanair-conditioning(AC)system.Fewsuch units are installed in Paris, namely due to how short their use period would be, inaddition to their aesthetic implicationswhichmay seem ill-suited toParis’ historic center.Moreover, the heat released by AC units outdoors intensifies the UHI effect and thusworsens the impact of heat-waves, particularly for pedestrians and persons not equippedwith AC units. While there is significant improvement to be expected from behaviouraladaptationtoheat-waves[36],onealternativeforactivecooling,alsobasedonclosedloopmainswatercoldrecovery,istoproduceicewithachillerunitandletpeoplecollecttheicetocooltheirbedroomsduringparticularlyhotnights.Ourfollowingestimationsarebasedonparameters of typical Paris buildings and a special attention is given to the ice quantityneeded.

ThismobilecoolingsolutionisillustratedinFigure6.Anicemaker(3)isusedtoproduceandstore low temperature ice blocks (4) at -9˚C. People in the neighbourhood, in particularthoseidentifiedbeforehandasbeingvulnerabletoheat-waves,e.g.thosealreadysigneduptotheCHALEXlist[37]usedinParis,collectoraredeliveredthenecessaryamountoficeintheevening.Theiceisthenallowedtomelt(5)intheirbedroomsduringthenight.Ablower

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canbe added to intensify themeltingprocessby forced ventilation. The cold recoverybyheatexchanger(1)issimilartothatdescribedinsection4.1.

Figure6.Nightcoolingbyiceblocksproducedbycoldrecoveryfrommainswater

(1)heatexchanger,(2)circulator,(3)water-to-waterglacemaker,(4)modularicestorage,(5)individualnightcoolingbymovableice

Load calculation and sizing are detailed in Table 3. On the load side, a standard 12m2bedroom is occupiedby twopersons for 8h at night. Theheat loss coefficient, i.e. theU-value of the 4 lateral walls, is 0.75W/(m²·˚C). Cooling loads from floor and ceiling areconsiderednegligiblesincetheyarenexttoadjacentbedrooms.Thehourlyairreplacementrateisassumedtobe1ACH(AirChangeperHour),withanoutsidetemperatureof30˚Canda setpoint temperature of 26˚C in the bedroom. An internal load of 100W is consideredfrom the blower and other electronics, in addition to 120 W representing two sleepingoccupants.Intotal,thecoolingloadis394W/bedroom,i.e.3.15kWh/bedroom/night.

Consideringbothsensibleandlatentenergy,melting1kgoficefrom-9˚Cto26˚Crequiresatotal enthalpy of 0.128kWh. Therefore, 24.7kg of ice are required for one night, perbedroom.Whilenotnegligible,thismassisacceptableandcouldbedeliveredeverydayoverseveraldaysunderaheat-waveemergencysituation.

Running under such conditions, a typical chiller runs at an EER of 1.4 [38]. This value isestimatedbytaking-13˚Castheevaporationtemperatureand25˚Casthecondensationonethroughequations(4)-(5).TheyproviderespectivelytheidealCarnotefficiencyandpracticalvaluereducedby4.

𝐸𝐸𝑅$%&'( =𝑇+

𝑇+ − 𝑇- (4)

𝐸𝐸𝑅.&'( = 0.25×𝑇-

𝑇+ − 𝑇- (5)

(1)

(5)

(2)(3)

(4)

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Ifweconsideratotalof1200bedrooms,andthetotalicedemandis29597kg,equivalentto 3786 kWhof cooling. If the chiller is operated continuously during 24h, the electricityconsumptionandcondenserheatgainsare115kWhand273kWh, respectively.Assumingthesameflowrateasinsection4.1,thewatertemperaturewouldincreaseby1.1°C.

Scaledtothewholecity’spotablewaternetwork,nearly120000bedroomscouldbecooledthisway,orupto170000 ifthenon-potablewaternetworkwasalso included.Giventhattherecurrentlyare8400peopleon theCHALEX list inParis [37], thepotential forcoolingtheirbedroomsgreatlyexceedsdemand.

Table3.Estimationdetailsofwatermainscoldrecoveryforiceproduction

Bedroomcoolingloadestimation Roomdimensions(m) w*l*h:3*4*3

HeatlosscoefficientU,(W/(m2·˚C)) 0.75Airrenewalrate(cycle/h) 1

Internalload,2personssleeping,(W) 2x60Internalload,blowerandotherelectronics,(W) 100

Outsideairtemperature(˚C) 30Roomtemperature(˚C) 26

Coolingload(W) 394Nightduration(h) 8

Numberofbedrooms(-) 1200Totalchillingdemand(kWh) 3786

Icequantity Initialtemperature(˚C) -9

Finaltemperatureaftermelting(˚C) 26Totalenthalpyinitial-final(kWh/kg) 0.128

Quantityoficeforabedroomduring8h(kg) 24.7Quantityoficeforallbedrooms(kg) 29597

Chiller–icemaker Manufacturer/Model/Refrigerant Teknotherm/F-45/R404a

EvaporatorHTFoutlet(˚C) -13CondenserHTFinlet(˚C) 25

EERCarnot(-) 6.8EER(estimated,-) 1.4

Dailyelectricityconsumption(kWh) 2767Mainswater Coldrecoveryheatexchangertemperaturedifference(˚C) 4Source-sideheatexchangercapacityaveragedby24h(kW) 273

Potablewaterflowrate(m3/h) 216Equivalentnumberofinhabitants(-) 25920Mainswaterinlettemperature(°C) 20

Potablewatertemperatureincrease(˚C) 1.1

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4.3 Emergencyurbancoolingduringheat-wavesOne response to heat-waves currently under consideration for cities such as Paris ispavement-watering [39,40]. By depositing a water film on street surfaces, the methodprovides evaporative cooling able to positively affect air temperature and pedestrianthermalstress[41].FieldmeasurementsconductedinParishavereportedcoolingofupto0.8°Cand3.7°Cinairandmeanradianttemperatureswithamaximumincreaseofrelativehumidity of 4%. These combined microclimatic impacts resulted in a positive impact onpedestrianthermalstresswithamaximumreductionoftheUniversalThermalClimateIndex(UTCI)equivalenttemperatureof1.5°C[42].

InJapan,pavement-wateringsystemsarealreadyinstalledinthestreetsofNagaokaCityformeltingsnowfallandhavebeenusedtostudypavement-wateringforsummertimecooling[39]. InParis,asimilarpavement-watering infrastructurecouldbebasedonthecity’snon-potablewaternetwork,alreadypresentinmostofitsstreets.Figure7illustrateshowsuchasystemcouldbeinstalledandconnectedtotheexistingwaternetwork.Aheatexchangerforcoldrecoverycouldbeplacedbetweenthesewerandpavementsprinkler.Sincepavement-watering would only be activated during a heat-wave, the associated water use wouldcoincidewiththeproposedemergencycoolingsystemsdiscussedpreviously.

(7)

(1) (3)

(2)

(4)(5)(6)

(8)

Figure7.Crosssectionofstreetstructurewithsewerandwatermains.Thepavement-

wateringsystemcouldbeconnectedtothenon-potablewaternetwork.(1)non-potablewatersupply,(2)wastewater,(3)pavementsprinkler,(4)stormdrain,

(5)heatexchanger,(6)coolingunit,(7)AHU,(8)emergencyheat-waveshelter

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In their study, Hendel et al. analysed the thermal effects of pavement-watering andproposedanoptimisedwateringstrategyfortheirsite[43].Underdirectinsolationandwitha watering rate of 0.41 mm/h, surface cooling of up to 265W/m² was found, i.e.approximately 7W/m² of sensible cooling and 257W/m² of evaporative cooling. Sensiblecoolingthereforeonlyaccountsfor3%oftotalpavement-wateringcooling.

Givenitslimitedcontribution,thesensiblecomponentcouldbeusedinamoreconcentratedform,e.g.forspacecoolingofnearbycafés,restaurantsorpubliclibraries,withonlyaminorimpact on total pavement-watering cooling. To achieve this, a heat exchanger could beplacedbetweenthewatermainandthesprinklingnozzleinthecenterofthestreetinFigure7.Thewaterbeingusedforpavement-wateringwouldsimplybepreheatedfromits initialtemperature (20-25°C) to thewater film temperature (35-40°C depending on the street’sinsolationconditions).

Asopposed to thepreviousclosed-loopapproaches,wherewater remainsor is reinjectedinto the network, this method is open-loop. While a closed loop requires low watertemperature changes for there to beno impact on its quality andusability, anopen loopallowsamuchhighertemperaturechangetobeused.Therefore,evensmallflowratescanprovidelargeamountsofpower.

As a case study, let us consider a 200m long portion of street 20mwide, i.e. an area of4000m²,watered at the rate of 0.41mm/h reported in Hendel et al., i.e. approximately20m3/day [43]. During daytime watering, the average delivered flow is 0.46L/s. With atemperaturegradientof15°C,thisflowcanabsorb28.6kW.ConsideringaheatpumpCOPof3,approximately21.4kWofcoolingpowercanbecontinuouslyproducedinassociationwithpavement-watering.

Attheurbanscale,pavement-wateringwouldbeconductedduringaheat-waveinonlythemostintensivelywalkedstreetsduringdirectinsolation,locatedoutsideofthecoolingreachof parks or rivers and where no shading is available to pedestrians. The cold recoveryproposedherewouldthereforecoincidespatiallyandtemporallywiththeareasandperiodsidentifiedashavingthehighestsolargain.However,whiletheyarewellsuitedforreachingthefullpotentialofpavement-watering,theseperiodsmaynotoccurwhentemperaturesorpedestrian activity are highest. A priori, cold recovery would target peak indoortemperatures which are reached around 8 pm or 9 pm (UTC+2) for Paris [36], i.e. afterscheduledwatering.Tomakeupforthispotentialshortfall,thermalstoragecouldbeusedtooffset the recovered cooling energy to periods with higher temperatures and pedestriantraffic.Awatertankprefilledthedayprevioustopavement-wateringcouldbeusedforthispurpose.Thisreservoirwouldallowforcoldrecoveryindependentlyofscheduledpavement-wateringproportionallytoitsvolume.

Furthermore, the proposed cooling could be used to create “heat-wave shelters” forpedestriansor residents living inparticularlywarmhousing. These spaces could simplybepartofanexistingcafé,restaurant,publiclibraryorschoolortheycouldevenbepartofthestreetfurniture,forexampleacooledbusstop,telephoneboothorsomethingelsedesignedforthispurposealone.

5 Conclusionsandperspectives

Themain contributionof this study is to demonstrate the effectiveness of usingwater asheatingandcoolingsourcesattheurbanscale.Totheauthors’knowledge,veryfewstudies

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intheliteratureassessedthispossibilityinaglobalpointofviewandinparticularthecoolingrecovery potential. A heat recovery case study was conducted and three cold recoverysolutionsthatmightbeusedaspartofanemergencyheat-waveresponsestrategyforPariswerepresentedanddiscussed.

Heatrecoveryfromwastewater isconsideredasagreenalternativeforheatingandDHWproduction.TheanalysesconductedonourcasestudyindicateGHGemissionreductionsofapproximately 75%, from 799t-eqCO2 to 204t-eqCO2with a HP system covering 84% ofyearly demand. An annual primary energy saving of 32% is reached, limited by the HPsystem’sEFwhichrangesfrom2.6to3.0overtheyear.

Threeemergencyheat-wavecoolingstrategiesareexplored,basedonclosedoropenloopsystemsforpotableornon-potablewatercoldrecovery.Ofthe300metrostationspresentinParis,99to141ofthemcouldbecooledbymainswatercoldrecovery.Alternatively,upto 170000 bedrooms could be cooled with ice blocks produced from water mains coldrecovery. Both solutions result in a water temperature increase of around 1˚C, which isacceptable fromawaterquality standpoint. The last solution (open-loop), couplinghighertemperaturelifting(upto20˚C)coldrecoveryfromnon-potablewaterusedforpavement-watering, provides a spatially-distributed emergency cooling solution. For a 200 m longportion of road, the potential cooling production can reach 21.4kW, able to cool a localheat-waveshelter.

Othersourcesofwaterarealsoavailable forheatorcoldrecovery inParisorothercities.Theseincludeexistingusesofpotableornon-potablewater,e.g.forstreetcleaningorgreenspaceirrigationwhichmaybeusedsimilarlytotheopenloopsystem.Furthermore,groundwater which seeps into underground structures such as metro stations or parking lots.Currentlythiswaterismostlypumpeddirectlyintothesewernetworkwithoutservinganythermalenergysupply,despitestabletemperaturesthroughouttheyear.

Heat pumps and heat exchangers are key elements for both heat and cold recovery. Theperformance of heat pump or refrigeration units depends highly on the temperaturedifference,andisthuscloselyrelatedtoheatexchangeperformance.Theheatrecoverycasestudy is based on a temperature difference of 11˚C between waste water and the HTF.Efficientheatexchangerswithhighheatexchangecoefficientscanallowbetterheatpumpperformance and thus better primary energy savings. The spiral heat exchanger solutionappears to be promising given its compact character. Otherwise, the integrated heatexchanger requiresminimalmaintenancebut a largeheat transfer surface (112m2 in thecasestudy).

Forcoldrecoveryfromdrinkingwater,anadaptedheatexchangerhasyettobedeveloped.Themainchallengestheyfaceincludesanitarycompatibility,compactnessandlowpressuredrop. The possibility of pipeline integrated heat exchanger without direct potable waterpassagetoheatexchangerispromising,buttheheatexchangercoefficientshouldbehigherthan300W·°C-1·m-2inorderfortheoccupiedlengthtobeacceptable.

In the current estimation, the heat pump and chillers are simplified with manufacturerperformances,andtheheatexchangerconfigurationisnotexplored.Ourfutureworkswillbe concentrated on compact heat exchanger development as well as rigorous dynamicsimulationsconsideringdemand-andsource-sidefluctuations.

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AcknowledgementThe authorswould like to acknowledge the contribution of Laurent Royon for his fruitfulsuggestionsanddiscussions.ThisworkispartiallysupportedbyEFFICACITY,ajointresearchinstituteforurbanenergytransition.

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