A simplified framework to assess the feasibility of zero-energy at the neighbourhood / community scale

ContentslistsavailableatScienceDirect

Energy

and

Buildings

jo u r n al h om ep age :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

A

simplified

framework

to

assess

the

feasibility

of

zero-energy

at

the

neighbourhood/community

scale

Anne-Franc¸

oise

Marique

,

Sigrid

Reiter

UniversityofLiege,LEMA(LocalEnvironment:Management&Analysis),ChemindesChevreuils,1B52/3,4000Liege,Belgium

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received7March2014

Receivedinrevisedform13June2014 Accepted1July2014

Availableonline9July2014 Keywords:

Zero-energybuilding Zero-energycommunity Zero-energyneighbourhood Dailymobility

On-siterenewableenergy Urbanform

Buildingstock Retrofitting Energymutualisation

a

b

s

t

r

a

c

t

Zero-energybuildings(ZEBs)areattractingincreasinginterestinternationallyinpoliciesaimingatamore sustainablybuiltenvironment,thescientificliteratureandpracticalapplications.Although“zeroenergy” canbeconsideredatdifferentscales(e.g.,community,city),themostcommonapproachadoptsonlythe perspectiveoftheindividualbuilding.Moreover,thefeasibilityofthisobjectiveisnotreallyaddressed, especiallyasfarastheretrofittingoftheexistingbuildingstockisconcerned.Therefore,thispaperaims firsttoinvestigatetheopportunitytoextendthe“zero-energybuilding”concepttotheneighbourhood scalebytakingintoaccounttwomainchallenges:(1)theimpactofurbanformonenergyneedsand theon-siteproductionofrenewableenergyand(2)theimpactoflocationontransportationenergy consumption.Itproposesasimplifiedframeworkandacalculationmethodthatisthenappliedtotwo representativecasestudies(oneurbanneighbourhoodandoneruralneighbourhood)toinvestigatethe feasibilityofzero-energyinexistingneighbourhoods.Themainparametersthatactupontheenergy balanceareidentified.Thepotentialof“energymutualisation”attheneighbourhoodscaleishighlighted. Thispapertherebyshowsthepotentialitiesofanintegratedapproachlinkingtransportationandbuilding energyconsumptions.

©2014ElsevierB.V.Allrightsreserved.

1. Introduction

1.1. Zero-energyatthebuildingscale

Thebuildingsectorisamajorconsumerofenergyworldwide [1–3].Forexample,itrepresentsover40%oftheoverallenergy consumedintheEuropeanUnion[1–3].Inthecurrentcontextof growinginterest in environmentalissues, reducing energy con-sumption in the buildingsector is an important policy target. Politicians,stakeholdersandevencitizensarenowawareofthe issueofenergy consumptionin buildings,especially asa result ofthepassageoftheEuropeanEnergyPerformanceofBuildings DirectiveanditsadaptationtotheMemberStates.Itsmainaim wastoestablishminimumstandardsfortheenergyperformance ofnewbuildingsandexistingbuildingslargerthan1000m2_subject tomajorrenovation[4].Anothermajortrendcommonlyproposed toreducetheenergyconsumptionoftheexistingbuildingstockis theimprovementofthethermalperformanceoftheenvelopeof existingbuildings(sometimesincombinationwithmoreefficient heating/ventilationsystems)[1,3].Asaresult,newconstruction

∗ Correspondingauthor.Tel.:+3243669367;fax:+3243662909. E-mailaddress:afmarique@ulg.ac.be(A.-F.Marique).

and renovationstandards ((very)low-energystandards,passive housestandards)[5–7]havebeendevelopedtodrastically min-imisetheenergyconsumptionofnewandretrofittedbuildingsand theassociatedgreenhousegasemissions.Duringthelastfewyears, “zero-energybuildings”havearousedincreasinginterest interna-tionallyinthescientificliterature(e.g.,[8–14]),policiesaimingat amoresustainablebuiltenvironmentandevenconcrete applica-tions.

Intheliterature,the“zero-energy”objectiveismostoften con-sidered onthebuilding scale.Although existing definitions are commonlyarticulatedaroundanannualenergybalanceequalto zero(theenergy demandofthebuildingiscompensatedbyits renewable production) [10,11],numerous differences exist and severaldefinitionscoexist[8,12]dependingonsuchelementsas specificlocalconditions,politicaltargets,connection(ornot)tothe gridandmeasurestoaddressenergyefficiencybeforeusing renew-ableenergysources.The“zero-energybuilding”(ZEB)ispresented asageneralconceptthatalsoincludesautonomousbuildingsnot connectedtoenergygrids. Theterm“netzero-energybuilding” (nZEB)“underlinesthefactthat thereisabalancebetweenenergy takenfromandsuppliedbacktotheenergygridsoveraperiodoftime, nominallyayear”[8,p.220].Theconceptofa“nearlyzero-energy building”ispresentedbytheEuropeanDirectiveontheenergy per-formanceofbuildings[15]asa“buildingthathasaverygoodenergy http://dx.doi.org/10.1016/j.enbuild.2014.07.006

performance.Thenearlyzeroenergyorverylowamountofenergy requiredshouldbesuppliedtoaverysignificantextentbyenergyfrom renewablesources,includingenergyfromrenewablesourcesproduced on-siteornearby”.

Otherderivedconceptsarealsofoundintheliteraturebasedon variousbalancemetrics[9].Forexample,TorcelliniandCrawley [9]definedfournetzero-energybuildingbalances(netzero pri-maryenergy,netzerositeenergy,netzeroenergycostandnet zeroemissions)andMohamedetal.[16]investigatedthemfora single-familyhousewithdifferentheatingalternatives.ForVoss etal.[10,17],acleardefinition,standardisedbalancingmethodand internationalagreementsonthemeaningof“zero-energybuilding” (ZEB)arelacking.

To address this issue,Marszal et al. [12] recently proposed a reviewof theexisting ZEBdefinitions andvarious calculation methodologies.Theyhighlightedsevenmainissuestobeaddressed in furtherdefinitions: the metric of the balance,the balancing period,thetypeofenergyuseincludedinthebalance,thetypeof energybalance,theacceptablerenewableenergysupplyoptions, theconnectiontotheenergyinfrastructureandfinallythe require-mentsfortheenergyefficiency,theindoorclimateand,inthecase ofgrid-connectedZEB,thebuilding-gridinteraction.Amongstthe morecompleteexistingapproaches,Sartorietal.[8]developeda systematic,comprehensiveand consistentdefinitionframework for“netzero-energybuildings”.Theyconsidered“alltherelevant aspectscharacteringnetZEBandaimsatallowingeachcountryto defineaconsistent(andcomparablewithothers)netZEBdefinition inaccordancewiththecountry’spoliticaltargetsandspecific condi-tions”[8,p.221].Thisframeworkisarticulatedaroundtwotypes ofannualbalances:theimport/exportbalance(balancebetween deliveredand exporterenergy)andtheload/generationbalance (balancebetweenloadandgeneration).Themonthlynetbalance canalsobedeterminedaccordingtothesamephilosophy[8].Voss etal.also[10]proposedaharmonisedterminologyandbalancing procedurethattakesintoaccounttheenergybalanceaswellas theenergyefficiencyandloadmatchingandhighlightedthat“itis theoptimisationandnotthemaximisationofelectricityexportedto thegridthatisanessentialplanninggoalfornetzero-energy build-ings,inadditiontothereductionofenergyconsumption”[10,p.55]. Theseauthorsproposedanewlabel(ZEBx)allowingthe distinc-tionbetweentheneedforseasonalcompensation(thelowerthe “x”value,thelowerthisneedforcompensation).Inthesamevein, Srinivasanetal.[18]introduceda“renewableemergybalance”asa tooltoensurethatbuildingsareoptimisedforthereduced con-sumptionofresources andthat theuseof renewableresources andmaterialsisoptimisedovertheentirelifecycleofthe build-ing.PlessandTorcellini[11]rankedtherenewableenergysources usedinabuildingtoproposeaclassificationgradingsystemforZEB, basedonrenewableenergysupplyoptions.Thegoalofthiswork istoencourage,first,theutilisationofallpossibleenergy-efficient strategiesand,then,theuseofrenewableenergysourcesand tech-nologieslocatedonthebuilding[19].Attiaetal.[20]developed oneoftheonlydecisionsupportbuildingsimulationtoolsthatcan beusedasaproactiveguideintheearlydesignstagesof residen-tialnetzero-energybuildingdesign.Thistoolisdesignedforahot climate(Egypt)andallowsforthesensitivityanalysisofpossible variationsofnZEBdesignparametersandelementstoinformthe decision-makingprocessbyillustratinghowthesevariationscan affectcomfortandenergyperformance.

Asfaraspoliciesareconcerned,theZEBiscurrentlyreceiving anincreasingamountof attentioninseveralcountries[12–14]. In Europe,therecasting of theEuropeanPerformance of Build-ingsDirective(EPBD)requiresallnewbuildings,builtinMember States, to be “nearly zero-energy” buildings (nZEB) by 2020. As a consequence, Member States are currently implementing thisobjectiveintotheirownnationalregulations[14].Thezero

objectivewillthenbeextendedtoexistingbuildingsundergoing majorretrofittingworks[15].IntheUnitedStatesofAmerica,the EnergyIndependenceandSecurityAct(2007)[21],whichconcerns theenergypolicyoftheentirecountry,aimstocreateanationwide netzero-energyinitiativeforhousesbuiltafter2020and commer-cialbuildingsbuiltafter2025.TheAsia-PacificPartnershiponClean DevelopmentandClimate,apublic-privatepartnershipofseven countries(Australia,Canada,China,India,Japan,SouthKoreaand theUnitedStatesofAmerica),aimsalsoatpromotingthe develop-mentofnetzeroenergyhomes[13].

Inpractice,severalbuildingshaverecentlybeenbuiltthatprove that“zeroenergy”,atthebuildingscale,isfeasible.Mostofthese existingzero-energybuildingsare(smallorlarge)residential build-ingsandofficebuildings[17,22].FongandLee[23]showedthatthe netzero-energytargetseemsnottobepossibleforhigh-rise build-ingsinHongKong.However,theynotethatitisfeasibleforlow-rise residentialbuildingsinthissubtropicalclimate.

1.2. Zeroenergyattheneighbourhood/communityscale

Generallyspeaking,mostpapersinvestigatingenergyissuesat theneighbourhood/communityscalefocusoneithertheimpactof urbanformonenergyconsumptioninbuildings[e.g.,24–26]or thepotentialofsolarenergyutilisationforactiveandpassivesolar heatingaswellasphotovoltaicelectricityproduction,lightingand relatedenergysupplyanddemand[e.g.,27–30].Hachemetal.[31] studiedandcomparedtheelectricitygenerationpotentialof neigh-bourhoodsandtheirenergyperformanceintermsofheatingand coolingandfoundoutthatasignificantincreaseintotalelectricity generationcanbeachievedbythebuildingintegratedphotovoltaic systemsofhousingunitsofcertainshape-siteconfigurations,as comparedtotheirreferencecase.Theyalsohighlightedthatthe energyloadofabuildingisaffectedbyitsorientationandshape. Theimpactofurbanformontransportationenergyconsumption hasalsobewidelyhighlightedintheliterature,butitisconsidered eitheralone[e.g.,32–36]or,inafewstudies,incomparisonwith buildingenergyconsumption[e.g.,37–39].

Studies and reports dealing with zero energy at the neigh-bourhood/communityscalesarefewinnumber.Theframework proposedbySartorietal.[8]canalsobeappliedtoaclusterof build-ings.KennedyandSgouridis[40]addressedthequestionofhowto defineazero-carbon,low-carbonorcarbon-neutralurban devel-opmentbyproposinghierarchicalemissionscategories.Todorovic [41]investigatedtheroleofsimulationtoolsintheframeworkof zero-energyurbanplanning.TheNationalRenewableEnergy Lab-oratory[19,p.4]defined,inatechnicalreport,a“zeronetenergy” community (ZEC) as“one that hasgreatly reduced energyneeds throughefficiencygainsuchthatthebalanceofenergyforvehicles, thermal,andelectricalenergywithinthecommunityismetby renew-ableenergy”.Theyhighlighted[19,p.1]that“communityscenarios couldlinktransportation,homeandtheelectricgridaswellasenable largequantitiesofrenewablepowerontothegrid”.Theyalsoapplied theZEBhierarchicalrenewableclassificationproposedbyPlessand Torcellini[11]totheconceptofcommunitytofocusonthemode andlocationofproductionofrenewables.Acommunitythatmet thezeroenergydefinitionthankstorenewableenergiesproduced withinitsbuiltenvironment(orinbrownfields)isatthetopofthe classification(rankA)whereasacommunitythatmetthedefinition throughthepurchaseofrenewableenergycertificatesisrankedC. Althoughnotspecificallydedicatedtothe“Zero-energy” objec-tives,severalneighbourhoodsustainabilityassessmenttoolshave recentlybeendeveloped [42].Examples oftheseNSAtoolsare, amongst others,theSTAR CommunityRatingSystem (Sustaina-bilityToolsforAssessingandRatingCommunities)[43] andthe USGreenBuildingCouncil’sLEED-ND(LeadershipinEnergyand EnvironmentalDesign—NeighbourhoodDevelopment)[44]inthe

UnitedStates,BREEAMCommunities(BREEnvironmental Assess-mentMethod)intheUnitedKingdom[45],HQE2R(HauteQualité EnvironnementaleetÉconomiquedanslaRéhabilitationdes bâti-ments et le Renouvellement des quartiers) in France [46] and CASBEE-UD(ComprehensiveAssessmentSystemforBuilt Environ-mentEfficiency–Urbanenvironment)inJapan[47].Thesetoolsaim toassessandratecommunitiesandneighbourhoodsagainstaset ofdefinedcriteriaandthemes.Theyproposeachecklistofcriteria (mainlyoptional)andarangeofvariousguidelinestohelplocal stakeholders,designersandcitizensmovetowardsmore sustaina-bility.Althoughtheyallincludealargethemededicatedtoenergy, theyneitherallowaquantitativeassessmentofenergy consump-tionorGHGemissionsnortheevaluationoftheenergyefficiency ofretrofittingscenarios.

Asfar as concrete projects are concerned,the West Village isa netzero-energycommunity, including662apartmentsand 343single-familyhomes,underconstructioninDavis,California [48,49].Anotherinteresting developmentattheneighbourhood scaleistheBeddingtonZero(fossil)EnergyDevelopment(BedZED) sustainableneighbourhood,which wasintendedtobethe UK’s largest mixed-use zero-carbon community. However, the zero objectivewasnotachieved.Othersexamplesofverylowenergy neighbourhoods include Hammarby Sjosjad, Augustenborg and BO01inSweden,VaubanandKronsberginGermany,Eva-Lanxmeer intheNetherlandsandVesterbroinDenmark[50].Finally,IEA-EBC (International Energy Agency’s Energy in Buildings and Com-munitiesProgramme) hascurrently a few Annexes/projects on zero-energycommunities[22].

1.3. Aimofthepaper

Thispaperaimstocompletetheexistingapproaches relating to“zeroenergy”bydevelopingandinvestigatingtheopportunities linkedtoanewsimplifiedframeworkdedicatedto“zero-energy neighbourhoods”andarticulatedaroundthefollowingthreemain challenges:

(1)Themajorchallengeoftheadaptationandretrofittingofthe existingbuildingstock(especiallyinthelargepartofEuropein whichtherenewalrateoftheexistingbuildingstockisquite low),incomplementaritywiththenumerousstudiesdealing withtheproductionofnewoptimisedbuildingsand commu-nities,andtheconcretefeasibilityofzero-energyinretrofitting. (2)Theimpacts ofparameters linkedtotheurbanformonthe energyefficiencyofsinglebuildingsaswellasonthechoice andefficiencyof on-siterenewableenergysources (e.g.,the possiblemutualisationofenergysupplyanddemandbetween individualbuildings).

(3)The impact of the location of residences, work places and servicesondailymobility patternsand theirrelated energy consumption, which are considered together with building energyconsumptionbecausebuildingorretrofittingvery effi-cientbuildingscouldbecounterproductiveifitslocationdoes notallowalternativestoprivatecarsfordailymobility(travel towork,school,shops,etc.)andimposeslongtraveldistances. Asarchitectsandurbanplanners,weareparticularlyinterested ininvestigatingthepossibilitiestoadapttheexistingbuildingstock inordertoreachanannualzero-energybalance,atthe neighbour-hoodscale,andinhighlightingthemainurbanandarchitectural parametersthatactupontheenergybalanceofaneighbourhood. Thus,inthefollowing,wewilltakeintoaccountthreemainthemes directlyrelated tothe urbanform of existing neighbourhoods: buildingenergyconsumption,theon-siteproductionofrenewable energiesandtransportationenergyconsumptionofinhabitants(in

ordertotakeintoaccountthelocationofactivitiesontheterritory inthebalance).

1.4. Contentofthepaper

Tothisextent,Section2proposesasimplifiedframeworkand acalculationmethodtoassesszero-energyneighbourhoods.An applicationoftheproposedframeworkisthendevelopedinSection 3totestitsapplicability,investigatethefeasibilityofzero-energy in retrofitting neighbourhoods and highlightkey parameters in theannualenergybalanceoftworepresentativeneighbourhoods (inBelgium).Section4discusseskeychallengestobeaddressed andperspectivestobeinvestigatedinfutureresearch.Finally,the researchfindingsandstrengthsandweaknessesoftheproposed frameworkaresummarisedinSection5.

2. Asimplifiednet“zero-energyneighbourhood” framework:methodandassumptions

Thenet“zero-energyneighbourhood”framework(nZEN) pro-posedinthescopeofthispaperaimstoarticulatethethreemain energyuses(buildingenergyconsumption,theproductionof on-siterenewableenergyandtransportationenergyconsumptionfor dailymobility),attheneighbourhoodscale.Aneighbourhoodis understoodhereasan“urbanblock”(thatistosaythesmallestarea ofacitythatissurroundedbystreets)oragroupofseveral“urban blocks”.Weonlyconsiderresidentialneighbourhoodsalthoughthe generalmethodologycouldbeextendedtoindustries,shops,etc. Alsonotethatpublicservicesenergyusesinaneighbourhood(e.g., streetlighting,trafficlights)arenotassessed.Apreviousresearch [38]namelyshowedthat streetlighting energyconsumption is minimalincomparisonwithbuildingandtransportationenergy consumptions.

ThenZENisheredescribedasaneighbourhoodinwhichthe annual energyconsumption for buildings and transportationof inhabitantsis balanced bythe production ofon-site renewable energy.Themainbalanceisannual,butmonthly,dailyorhourly balances could also be studied according to the same defini-tions tobetter capture the gaps between energy consumption andproductionbyrenewablesources.Asfarasthemetricofthe systemisconcerned,thebalancesareproposedintermsof pri-maryenergy.Theconversionfactorsusedtoconvertgrossenergy into primary energy are 1 for natural gas and petrol and 2.5 forelectricity,asstatedintheWalloonregulationontheenergy performanceofbuildings[51].Onlytheusephase ofthe neigh-bourhoodistakenintoaccountinthesebalances(constructionand deconstructionphasesarenotassessed).Notealsothatanet zero-energyneighbourhoodimpliesinteractionsamongthebuildingsin theneighbourhoodandbetweenthebuildingandtransportation energyconsumptions.Thezero-energybalanceisthusconsidered asawhole,andeachbuildingisnotnecessarilyazero-energy build-ing.Finally,weassumethattheneighbourhoodhasanelectricgrid thatcanprovideenergytotheneighbourhoodwhenon-site gener-ationfromrenewablesislowerthantheload.Ifgreater,theon-site productioncanbesenttothegrid.

2.1. Energyconsumptioninbuildings

Themethodologyusedtoassessbuildingenergyconsumption takesintoaccounttheannualenergyconsumptionforspace heat-ing (ESH), space cooling (ECO), ventilation (EV), appliances(EA), cooking(EC)anddomestichotwater(EHW).Theneighbourhood’s annualenergyconsumptionforbuildings(EB)iscalculatedusing Eq.(1).

2.1.1. Energyconsumptionforspaceheating,coolingand ventilation

Themethoddevelopedtoassessenergyconsumptionforspace heating,coolingandventilationwasextensivelypresentedina pre-viouspaper[38].Thismethodcombinesatypologicalclassification of buildingsand neighbourhoods and thermal dynamic simula-tions.Thistypologicalapproachclassifiedtheresidentialbuilding stockofBelgiumandwasbasedonthefollowingfactors:common ownership (detached,semi-detached orterraced houses, apart-ments),theheatedareaofthedwellinginsquaremeters(m2_),the heatingandventilationsystems,thedateofconstructionandthe levelofinsulation,includingretrofittingworksperformedbythe owners(e.g.,insulationoftheroofand/orreplacementofthe glaz-ing,changeoftheheatingand/orventilation systems).Thermal simulationswereperformedforallofthedwellingtypesofthis typologicalclassificationofbuildings.Theresultsoftheseenergy simulations(ESH and EV)are storedin a databasecomprised of theenergyconsumption ofapproximately250,000buildings.In thesethermalsimulations,Brusselsmeteorologicaldata (temper-ateclimate)areused.Theminimumtemperatureinthedwellings is18◦C,andinternalgainsaredefinedaccordingtothesurfacearea ofthedwelling.A correctionfactor isappliedtoavailablesolar gain accordingtotheneighbourhood type totakeintoaccount thereductionofsolargainswithincreasedbuiltdensity.Thenet andgrossenergyconsumptionandprimaryenergyconsumption forspaceheating,coolingandventilationat theneighbourhood scalearefinallycalculatedbyaddingtheresultsfromtheenergy consumption analysisforeach type ofhouseaccording totheir distributionintheneighbourhoodandtheneighbourhoodtype. Cooling(ECO)isnottakenintoaccountinthecasestudiespresented inSection3,inaccordancewithregionalyearbooks[52].Moreover, theoverheatingindicatordefinedintheEuropeanEnergy Perfor-manceofBuildingDirective[4]astheratiobetweenthesolarand internalgainsofabuildingtotransmissionandventilationlosses wascalculatedby[38]forWalloonresidentialbuildings.Itremains underthethresholdvalueproposedintheDirective(29.8%under thethresholdvaluefortheworstcases),whichindicatethatthe overheatingisnotunacceptableanddoesnotrequirethe installa-tionofcoolingsystem[4].

2.1.2. Energyconsumptionforappliances,cookinganddomestic hotwater

The annualenergy consumptionsrelated to appliances(EA), cooking(EC)anddomestichotwater(EHW)areassumedtodepend onthe number of inhabitantsin thebuilding. In the following application,regionalmeanvalues,gatheredbyaregionalinstitute incharge ofenvironment (the “CelluleEtatde l’Environnement Wallon”[53]), areused; however,in situsurveys couldalsobe implementedinthemodel.Theenergyconsumptionsrelatedto appliancesand cooking are1048kWh per person peryear and 170kWhperpersonperyear,respectively[53].Theenergy con-sumptionforheatingwaterisobtainedbymultiplyingthevolume ofhotwaterneededannuallyattheneighbourhoodscale(m3_)by thedifferenceintemperaturebetweencoldandhotwateranda conversionfactor,usedtoconvertkilocalorieintowatt-hour.This factorisworth1.163kWh/m3◦_{C[54].Weconsidereachinhabitant} toneed100lofcoldwater(10◦C)and40lofhotwater(60◦C)per day,inaccordancewiththeregionaltrends[53].

2.2. Energyconsumptionfordailymobility

The annual energy consumption for daily mobility (EDM) is assessedusingaperformanceindexintroducedbyBoussauwand Witlox[55]andadaptedbyMariqueand Reiter[56].Thisindex isexpressedinkWh/travelperpersonand represents,fora ter-ritorialunit,themeanenergyconsumptionfortravellingforone

personlivingwithinaparticularneighbourhood.Thisindextakes intoaccountthedistancestravelled,themeansoftransportation usedandtheirrelativeconsumptionrates,asexpressedbyEq.(2).In theequation,irepresentstheterritorialunit;mthemeansof trans-portationused(dieselcar,gasolinecar,train,bus,bike,walking); Dmithetotaldistancetravelledbythemeansoftransportationmin territorialuniti;fmtheconsumptionfactorattributedtothemeans oftransportationm;andTithenumberofpersonsintheterritorial uniti.Theconsumptionfactorsdependupontheconsumptionof thevehicles(litresoffuelperkilometre)andtheiroccupationrate. IntheBelgiancontext[56],thesevaluesare0.56kWh/personper kmforadieselcar,0.61kWh/personperkmforanon-dieselcar, 0.45kWh/personperkmforabus,0.15kWh/personperkmfora trainand0fornon-motorisedmeansoftransportation,asthelatter donotconsumeanyenergy[56].

Energy performance index (i)=



Dmifm Ti

(2) Theenergyconsumptionfor dailymobility(EDM)isobtained usingEq.(3)bymultiplyingtheenergyperformanceindexbythe numberofpeople(N)andthenumberoftrips(T)inthe neighbour-hood.

EDM=Energy performance index×NT (3)

InournZENframework,weattribute alltravelstoand from workandtoandfromschooltotheneighbourhood(ratherthanthe portionthatisreallyconsumedwithintheneighborhood)tofocus ontheimpactofresidentiallocationsontransportationenergy con-sumption.

Datausedinthefollowingcasestudiescomefromanational censuscarriedoutinBelgium(theGeneralSocio-EconomicSurvey 2001[57]).Notethatthesedataonlyconcernhome-to-workand home-to-schooltravel;however,wecouldusethesame methodol-ogywithdatafromaninsitusurveyaccountforalltravelpurpose. 2.3. On-siteenergyproductionbyrenewablesources

On-siteenergyproductionviaphotovoltaicpanels(EPV), ther-malpanels (ETH)andsmall windturbines(EWT)are considered whenaccountingforrenewableenergysources.Theannual renew-able energy produced in the neighbourhood(ERP)is calculated usingequation4.

ERP=EPV+ETH+EWT (4)

2.3.1. Photovoltaicpanels(electricity)

Thepotentialofneighbourhoodsforactivesolarheatingand photovoltaic electricity production is obtainedusing numerical simulationsperformedwithTownscopesoftware[58].Only pho-tovoltaicpanelsonroofsareconsideredbecausethoseonfacades arelesseffectiveinBelgium[59].Townscopeallowsthe calcula-tionofthedirect,diffuseandreflectingsolarradiationreachinga pointandtheradiationdistributiononasurface.Ascalculations areperformedunderclear-skyconditions,thesoftwareisusedto determineafirstcorrectionfactorM(thedifferencebetweenthe values calculatedfortheassessedneighbourhoodandtheclean site)toapplytothemeansolarradiationMSRfortheconsidered lat-itude(MSR=1000kWh/m2_{·yearforBelgium[60]).Asecondfactor} Fisappliedtotakeintoaccounttherooforientationandinclination (Table1)[61].

Thesolarenergyreceivedbytheconsideredsurfaceisobtained usingEq.(5).Thepotentialofroofsforphotovoltaicelectricity pro-duction(EPV,inkWhperyear)isobtainedbyEq.(6).InEq.(6),S representsthesurfaceareaoftheconsideredroofs,Cthepercentage oftheroofscoveredbypanels(maximum0.80),ŋpvtheefficiency ofthephotovoltaicpanels,ŋinvtheefficiencyoftheinverterand

Table1

ValuesofthecorrectionfactorF,whichaccountsforrooforientationandinclination [61]. Inclination 0◦ 15◦ 25◦ 35◦ 50◦ Orientation East 0.88 0.87 0.85 0.83 0.77 Southeast 0.88 0.93 0.95 0.95 0.92 South 0.88 0.96 0.99 1 0.98 Southwest 0.88 0.93 0.95 0.95 0.92 West 0.88 0.87 0.85 0.82 0.76

acorrectionfactortakingintoaccountelectricitylosses.Inthe followingcasestudies,theefficiencyofthephotovoltaicpanelsis fixedat0.145,theefficiencyoftheinverterat0.96andthe elec-tricitylosscorrectionfactorat0.2;inaccordancetothetechnical characteristicsofthemostusedtypeofpanelsinWallonia[62].

Esol=MSR×FM(in kWh/m2year) (5)

EPV=EsolSCpv(1−) (6)

2.3.2. Thermalpanels(hotwater)

Thesolarenergyreceivedannuallybytheroofisobtainedusing Eq.(5),wherecorrectionfactorMiscalculatedwithTownscope and correctionfactor F is defined according toTable 1. Eq. (7) allowsthedeterminationofwhethertheroofsofthehousesof theneighbourhoodsareadaptedtotheproductionofhotwater. InEq.(7),Esol representsthesolarenergyreceivedbytheroofs, Sthesurfaceareaofthepanelandŋththeefficiencyofthe ther-malpanels.Weconsiderthat55%oftheproductionofhotwaterof eachhouseholdmustbecoveredthroughthermalpanels(froma technical-economicviewpoint,theoptimumisoftenconsideredto bebetween50%and60%).Undertheseconditions,theefficiencyof thesystemis0.35.

ETH=EsolSth (7)

2.3.3. Windturbines

Theapproximationusedtoevaluatetheannualelectricity pro-ductionofwindturbines(EWT)consistsofmultiplyingtherated powerofthewindturbinebythenumberofoperatinghoursatthis ratedpower,asinEq.(8),inwhichPistheratedpowerofthewind turbineandOHthenumberofoperatinghours.Thisvalueisfixed at1000hforasmallwindturbine[63].

EWT=P·OH (8)

2.4. Annualbalanceattheneighbourhoodscale

Theannualenergyconsumptionoftheneighbourhood(EN)is calculatedbyadding thebuildingenergy consumption(EB)and transportationenergyconsumption(EDM)andsubtractingthe on-siterenewableenergyproduction(ERP),asshowninEq.(9).

EN=EB+EDM−ERP (9)

Themonthlybalancescanalsobestudiedaccordingtothesame typeofequationbyreplacingtheannualenergyconsumptionand productionbythecorrespondingmonthlyvalues.

3. Results

3.1. Presentationofthecasestudies

Thecasestudieschosenaretwocommonarchetypesof neigh-bourhoods(understoodasurbanblocks,thatistosaythesmallest areaofacitythatissurroundedbystreets)andare representa-tiveofthebuildingstockinBelgium[64].Thetwoneighbourhoods

Table2

Maincharacteristicsofthetwocasestudies.

Case1 Case2

Type Urban Suburban

Surfacearea 0.97ha 12.02ha

Population 180inhabitants 150inhabitants

Buildings 57 55

Detachedhouses 7% 75%

Semi-detachedhouses 17.5% 19.6%

Terracedhouses 75.5% 3.6%

Apartments 0% 1.8%

Density 60dw/ha 5dw/ha

%ofthesurfacearea

occupiedbybuildings

29% 5%

containessentiallythesamenumberofbuildingsbutinavery dif-ferenturbanform.Eachurbanformpresentsitsownspecificities andcharacteristics,especiallyasfarasthebuiltdensityandthe typesofbuildingsareconcerned,ashighlightedonTable2,and requirespersonalisedsolutionsregardingtheenergyefficiencyin thebuildingandtransportationsectorsandtheon-siteproduction ofrenewableenergy.

The first case study (Fig. 1) is a dense neighbourhood (60 dwellingsperhectare) representativeof anold compact indus-trialurban fabric. This neighbourhood is located closeto good transportationnetworks(trainsandbuses),workplaces,schools, shopsandservices.Buildingsareverypoorlyinsulated,becausethe neighbourhoodwasbuiltinthe19thcentury.

Thesecondcasestudy(Fig.2)isalow-densitysuburban neigh-bourhood(5dwellingsperhectare)locatedinthesuburbs(18km) of thecitycentre. It is representativeof theurban sprawlthat beganinBelgiuminthe1960s.Publictransportationisminimal, andcardependencyishigh.Theneighbourhoodiscomprisedof detachedhousesbuiltbetween1930and2010.Someretrofitting works(changingoftheglazing,roofinsulation)hasbeenperformed bytheowners.

3.2. Annualenergybalance

Inthecurrentsituation,theenergyconsumptionforspace heat-ingisquitelargeinbothneighbourhoods(184kWh/m2 _peryear in case 1and 235kWh/m2 _{peryear in case 2),and theannual} zero-energybalancecannotbeachieved(Table3).Aclear differ-enceisobservedbetweentheheatingenergyrequirementsofthe twoneighbourhoodsbecausethefirstismadeupterracedhouses, whichconsumeapproximately25%lessenergyforheatingthan thelesscompacturbanform.Similarly tothebuildingscale,to

Table3

Resultsoftheapplicationofthe“zero-energyneighbourhood”frameworktothetwocasestudies.

Casestudy1 Casestudy2

Consumption(kWh) Spaceheatingandventilation:ESH+Ev 1,421,694 2,754,341

(ESH+Ev—low-energyretrofitting) (463,595) (703,235)

(ESH+Ev—passiveretrofitting) (143,156) (214,074)

Appliances:EA 161,139 155,485

Cooking:EC 26,277 25,355

Hotwater:EHW 152,950 127,458

Dailymobility:EDM 339,696 441,072

Production(kWh) Photovoltaicelec.:EPV 139,945 314,669

Hotwaterheating:ETH 80,417 67,170

Windturbine:EWT 0 50,000

Fig. 2. Case study 2—a representative suburban residential neighbourhood

(Belgium).

achieveanetzero-energybalanceattheneighbourhoodscale,the energy demand(heating in thesecase studiesherein) mustbe reducedusingenergyefficiencymeasures(amajorretrofittingof theenvelopeofthebuilding).Theresultmustsatisfythe(very)low, passiveornetzero-energystandards.Moreover,theresultsshow thatthezero-energyneighbourhoodobjectivealsoneedsto min-imisetheenergyneedsforappliances,cookingandhotwater.It isimportanttoemphasisetheinfluenceoflessenergy-consuming devicesaswellasadapteduserbehavioursandlifestyles, param-etersthathavealreadybeenstudiedindetailinotherreferences (e.g.,[65–67]).

Energyconsumptionfordailymobilityisalsohigher (approx-imately 30%) in the suburban neighbourhood, which is highly dependentonprivatecarsandforwhichtravelforworkandschool is acrosslargedistances. Taking intoaccount energy consump-tionfromdailymobility, ashighlightedin ourassumptions,the impactofthelocationoftheneighbourhoodcanbeincludedinthe annualbalance.Thisiscrucialtoavoidsimplyproposingbuilding orretrofittingzero-energybuildingsandneighbourhoods asthe optimalsolutiontocreateamoresustainablebuiltenvironment, regardlessoftheirlocationandtheimpactofthislocationon trans-portationenergyconsumption.Moreover,theresultsshowthatthe zero-energyneighbourhoodobjectivealsorequiresthe minimisa-tionoftheenergyneedsfordailymobility,eveninurbanareas.

Incontrast,asfarason-site renewableenergyproduction is concerned,the photovoltaicproduction is higher in the subur-banneighbourhood(case2)becausesimulationsperformedwith

Townscopetocalculatesolarradiationonroofs(seeSection2.3.1, above)haveshownthat theshadowingeffectismuch lowerin thisareathaninthedenseneighbourhood(case1).Quite inter-estingly,parametricvariationsshowthatifphotovoltaicpanelsare onlylocatedonroofsthatreceiveover90%ofthemaximumsolar energyandiftheelectricityproductionismutualisedatthe neigh-bourhoodscale,theefficiency(kWhproduced perm2 _ofpanel) increasessignificantly(+10.7%incase1and+5.0%incase2).The sameamountofphotovoltaicelectricitycanthusbeproducedby installingfewerpanelsthanreportedinTable3,wherephotovoltaic panelswereinstalledoneachbuilding.Thermalenergyproduction ishigherincase1thankstothesurfaceareasoftheroofsbut sim-ulationsperformedtoassesssolarradiationonroofs(seeSection 2.3.1,above)haveshownthattheshadowingeffectismuchlower inthesuburbanarea.

Theuseofwindturbinesinthefirstcasestudywasnotassessed becauseofthedensecontextinwhichitislocated.Inthe subur-bancase,asmallwindturbineproducesapproximately50,000kWh annually.Thiswindturbinecouldbelocatedinthecentreofthe neighbourhoodbecausethislocationissufficientlyfarfrom exist-inghouses(basedonthenoiseproducedbytheturbineandthe existing regulations);however,this solutionwouldprohibit the futuredensificationoftheneighbourhood,whichisapossible solu-tiontoincreasingthesustainabilityofexistingsuburbanblocks.

In comparisonwithbuildingand transportationenergy con-sumption,andfortheconsideredclimateandcontext,theon-site generationofrenewableenergyisoftenlimited,especiallyinthe densecase study.The zero-energy balancecannotbe achieved, evenifbuildingsareretrofittedtothepassivestandard.Therein, intermediatelevelsofperformance(the[very]low,passiveornet zero-energyneighbourhoods)couldalsobepromoted,especially asfarasinterventionsinexistingneighbourhoodsareconcerned. Ratherthantherespectofazero-energyannualbalance,itseems important to promote, above all, the minimisation of building andtransportationenergyconsumptionsandthemaximisationof renewableenergyproduction.

3.3. Monthlyenergybalances

Anannualbalancewasusedfirstinthispaper.Yearlybalances accountforthesuccessionofthefourseasonsandtheir particular-ities.However,itisalsointerestingtoinvestigateshorterperiods oftime.Monthlyproductionandconsumptioncurveshighlightthe shiftbetweentheproductionandconsumptionpeaksandbetween supplyanddemand,particularlyforsolarenergy(highestin sum-mer)andheatingconsumption(highestinwinter),ashighlighted inFig.3.

4. Discussionandperspectivesforfurtherresearch

Achievinganetzero-energybalanceinthetwoexisting neigh-bourhoods is very difficult, namely because the buildingstock

Fig.3.Monthlyproductionandconsumptioncurvesforthetwocasestudies(casestudy1ontheleftandcasestudy2ontheright).

ispoorlyinsulated.Intermediatemilestoneandtargetscouldbe proposedtohelplocalcommunitiestomovetowardsmore sus-tainability:

1.Improve the energy efficiency of the building stock (e.g., retrofittingtothepassivehousestandard).

2.Minimizeenergydemandforbuildingsandfortransportation throughoccupantbehaviour(e.g.,adaptingtheinner tempera-tureofthedwellings,promotingcarsharing).

3.Maximiseon-siterenewableenergyproduction(e.g.,installing PVpanels).

4.Useoff-site renewable energyproduction (e.g., usingdistrict heatingandimportedrenewableenergy).

Aswehighlighted significantdifferencesbetweentheurban and the suburban case studies, these milestones should be differentiated,accordingtothelocalopportunitiesineach neigh-bourhood.

In this course to reachsustainability in existing neighbour-hoods,oneofthemainadvantagesoftheneighbourhoodscaleis thepotentialforan“energymutualisation”forbothenergy pro-ductionand energy consumption. We have namelyhighlighted theinterest of producing photovoltaic electricity at the neigh-bourhood,ratherthan attheindividual scale.Anotherexample wasthepoolingofthebuiltenvelopeindenseurban neighbour-hoodsthatallowstoreduceenergyneedsofterracedhouses,in comparisonwithdetachedhouses.Thisconceptof“pooling”,at theneighbourhoodscale,providesnumerousavenuestoincrease energyefficiencyin ourbuiltenvironment.In thesamevein, it shouldbenotedthatthis“energymutualisation”or“pooling”at theneighbourhood scale offersinteresting perspectivesfor the compensation of the monthly peaks (aswell as hourlypeaks) betweensupplyanddemandbetweenindividualbuildings, espe-ciallyin neighbourhoodspresenting awide varietyoffunctions withdifferentandshiftedenergyneeds(offices,schools,etc.,versus residences).

Asfarasperspectivesforfurtherresearchareconcerned,the connectionto thegrid, theuse of smart gridsand the storage ofenergyshouldbeinvestigatedinthefuture.Costoptimisation shouldalsobeconsideredbasedoncurrentdiscussionsrelatedto theEuropeanDirectiveontheEnergyPerformanceofBuildings. Finally,werecommendextendingthebalancetotheentirelifecycle ofaneighbourhoodbyincludingtheenergyandCO2 embodied

inmaterialsandtechnicalinstallations(includingtransportation infrastructure).

5. Conclusions

Thegoalofthispaperwastocontributetotheexistingliterature onthe“zero-energy”objectiveinthebuildingsectorby investigat-ingthefeasibilityofthisobjectiveattheneighbourhoodscale.The paperpresentedasimplifiedframeworkandacalculationmethod relatedtothe“netzero-energyneighbourhood”,whichincluded buildingenergyconsumptionandtheon-siterenewableenergyat theneighbourhoodscaleaswellastheimpactofurbanformandthe locationoftheneighbourhoodontransportationenergy consump-tionfordailymobility.Thesedevelopmentswereappliedtotwo casestudies(oneurbanneighbourhoodandonesuburban neigh-bourhoodinBelgium)tohighlightthemainparametersthatact upontheannualenergybalanceofaneighbourhoodandtopropose concretestepstoimprovethesustainabilityofexisting neighbour-hoods.Thisworkhighlightedtheopportunitiesforand interest inextendingtheboundariesoftheexistingframeworksfromthe buildingtotheneighbourhood,whichmainlyconcerntheimpact ofurbanformanddailymobility.TheproposednZENframework allowstoconsiderbuildingenergyconsumption,renewable pro-ductionandtransportationenergyconsumptionasanintegrated system,ratherthanseparatedtopics.

Inamoregeneralperspective,thisworkscallsforabetter inte-grationoftheindividualbuildingintoitscontextinpoliciesdealing withenergyefficiency.Promotingthebuildingandretrofittingof energy-efficientbuildingsisagoodsteptowardsincreasedenergy efficiencyinourbuiltenvironment(i.e.,byimposingmandatory minimumrequirementsontheenergyefficiencyofbuildingsthat arecrucialtoreachanetzeroenergybalance);however,itisnot sufficient.Itisalsocrucialtoconsiderparametersandinteractions linkedtoalargerscale,theurbanplanningscale,tomore effec-tivelyachievetheaimsofthesepolicies.Tothisend,thelocationof newbuildingsanddevelopmentsappearstobecrucialinthetotal balance,whichincludesbothbuildingandtransportationenergy consumption.

Acknowledgements

Thisresearchwasfunded bytheWalloon regionof Belgium underthe SOLEN(“Solutions forLow EnergyNeighbourhoods”) project,Grantnumber1151037.

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