SAMPLING!OF!BUILDING!SURFACES!TOWARDS!AN!EARLY!ASSESSMENT! OF!BIPV!POTENTIAL!IN!URBAN!CONTEXTS!

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1 SAMPLINGOFBUILDINGSURFACESTOWARDSANEARLYASSESSMENT OFBIPVPOTENTIALINURBANCONTEXTS GiuseppePeronato Ecolepolytechniquefédérale delausanne(epfl) LIPID EmmanuelRey EPFL LAST MarilyneAndersen EPFL LIPID Fig1:BIPVRsuitablesurfacesasdefinedbytheproposedalgorithm (conservativeapproach,threshold=975kwh/m 2 ) WHICHAREYOURARCHITECTURAL(R)SOLUTIONSTOTHESOCIAL,ENVIRONMENTALANDECONOMIC CHALLENGESOFTODAY? Research(summary( AlthoughtheintegrationofPVsystemsinthebuildingenvelope(BIPV)isanimportantfactorforthe acceptabilityofsuchinstallations,currenturbanrscalesolarpotentialmetricsonlypartiallyconsiderthis aspect.aspartofthedefinitionofbipvrsuitablesurfaces,wearguethatageometricrregularitycriterion can help predict the possibledisposition of solar panels already in the early assessment of BIPV potentialinurbancontexts. Toaddressthisneed,wedevelopedanalgorithmforthegeometricsamplingofthepartsofthebuilding envelopeachievingaminimumirradiationthreshold,withtheaimofdefininguniformlyrcoveredactive solarsurfaces.theproposedmethodologyisimplementedinaflexibleparametricdesignplatformand testedinacasestudyinneuchâtel(switzerland).weshowthatintegratinggeometricregularityinthe assessment of BIPV potential can have a significant influence in the calculation of the solar energy productionanddiscussthevalueofsuchinformationinurbanplanningpractices. Keywords:buildingRintegratedphotovoltaics(BIPV),solarpotential,sustainableurbanplanning

2 1.(Introduction( Withthe EnergyStrategy2050 (SFOE,2014), Switzerland is planning to gradually withdraw from the use of nuclear energy and consequently integrate a larger share of renewableenergysources.onthebasisofthe estimated photovoltaic (PV) potential on building surfaces (IEA, 2002), it can be calculated that the solar power production could cover 29.6% of the current electricity consumption of Switzerland (SFOE, 2015). However, despite the continuous advancementsinsolarcelltechnologiesandthe reduction of fabrication costs, the largerscale implementation of photovoltaics as an integratedpartofthebuildingenvelope(bipv) encountersseveralobstacles,suchasthelackof a holistic approach starting from the planning phase(heinstein,ballif,&perretraebi,2013). Different methods exist for calculating the activesolarpotentialoftheurbanfabriconboth roofs and facades, using either CADR (Compagnon, 2004; Kanters, Wall, & Dubois, 2014) or LiDARRbased 3D models (Jakubiec & Reinhart,2012;Redweik,Catita,&Brito,2013). In all of them, the PVRsuitability of building surfacesiscalculatedbytheachievementofa minimum solar irradiation threshold, usually defined by the payback time (Jakubiec & Reinhart,2012;Kantersetal.,2014).However, little attention is given to the effective integration of solar systems in the urban envelope in terms of visual appearance, although this is an important factor for the urban acceptability of such installations and for the case of Switzerland to get public subsidies(munariprobst&roecker,2011). Amongallintegrationcriteria,thearchitectural quality of the intervention is particularly difficult to evaluate before the system being actually designed, e.g. during an urbanrscale assessment.however,wearguethataregular geometric disposition of PVRsuitable surfaces, whichisgenerallynotconsideredinirradiation maps,isaprerequisiteforthecoherenceofthe PVinstallationwiththe globalbuildingdesign logic (MunariProbst&Roecker,2011)andfor itsalignmenttotheexistingbuildingelements (SMS,2013). Thispaperthuspresentsasamplingalgorithm with the goal of introducing a geometricr regularity criterion since the early assessment of BIPV potential in urban contexts. Two approaches towards geometric regularity are evaluated and compared with a standard calculation method based only on a solar irradiation threshold. This algorithm is implementedintoaparametricdesignplatform, coupled with a solar radiation simulation tool, and tested in a CADRbased urban 3D model automaticallyrreconstructed from LiDAR data. We show that the proposed approaches can haveasignificantinfluenceinthecalculationof thesolarenergyproduction.wearguethatsuch difference should be taken into account by plannersanddecisionrmakerswhilecalculating thesolarenergypotentialattheurbanscale. 2.(Methodology( The proposed methodology integrates a geometryrsampling algorithm and an evaluation of the BIPVRsuitability of building surfaces in order to predict the possible installationofbipvsystems. 2.1$Geometry$sampling$ ThesamplingalgorithmisbasedonageometricR regularitycriterion.itsdefinitionisinspiredbya selectionofdesignguidelinesprovidedbysms (2013),suchas regroupingthe[pv]elements, adoptingpreferablyarectangularshape and respecting building contours and the parallelism of lines (translation ours). Two approaches, both aiming at geometric

3 regularityinthedispositionofsolarpanels,but opposed in terms of the use of the solar irradiationthresholdasadecisioncriterion,are proposedhere.intheformer,whichwereferto astheconservativeapproach,thepartsofthe facade that, despite achieving the irradiation threshold,preventaregulardispositionofsolar panels on that surface are discarded. Conversely,theaggressiveapproachtakesinto account also those parts of the facade that, despitenotachievingtheirradiationthreshold, allowaregulardispositionofsolarcellsonthat surface. This concept is implemented in an algorithm thatanalyzesthegridofpointscreatedoneach building surface to be used as sensors in the simulation of solar radiation. These points are organizedinparallelhorizontallines,asshown infig2,sothattheirbelongingtoaparticular line can be used in the sampling algorithm to determineifthegeometricrregularityobjective is fulfilled. In the conservative approach, a sensor p b,s,l is considered suitable for BIPV installationonlyifallthesensorsonthelinel b,s achievetheirradiationthreshold.conversely,in the aggressive approach, if a certain number (n min )ofsensorpointsp b,s,l ofthelinel b,s achieve the irradiation threshold, all the sensor points onthelinel b,s areconsideredsuitableforabipv installation.ascanbeseeninfig2,byusingthe aggressive approach, two points not achieving the threshold are considered acceptable. The minimumnumberofsensorpoints(n min )fora linetobeconsideredsuitablecanbefreelyfixed bytheuser.thesmalleritsvalueis,thegreater ispossiblythenumberofsensorpointsthatwill be considered suitable despite not achieving theirradiationthreshold.inthispaper,themost extreme scenario, i.e. n min = 1, was chosen in ordertoshowthemaximumpossibleincrement in energy production that can be obtained by this approach. However, ifthe choice of the irradiation threshold is determined by an economicorenvironmentalminimumpayback time,thedecisionrmakershouldaccepttherisk of the nonrviability of the installation. If such viabilityisdeterminantfortheprojectandthe geometric regularity is still considered an important factor, the conservative approach shouldbechoseninstead. Fig 2: Definition of sensor points and of the geometricracceptabilityapproaches 2.2$PV$systems$and$building$surfaces$ Allsensorpointsp b,s,l arecoupledwithavector representing the normal n b,s to the surface s theybelongto(fig2).thisvectorcanbeused toidentifytheinclinationofthesurfacesover the horizontal plane. Since the tilt angle influences the direct radiation arriving at an inclined surface(häberlin, 2012, para ) as well as the possible PVRmounting systems, wearguethatsuchinformationcanhelpassign to each building surface the most appropriate PV panel (e.g. tilted panels on flat roofs, enveloperintegrated panels on sloped and vertical opaque surfaces) and solar cell technology (e.g. crystallinersilicon modules on roofs,thinrfilmsonfacades).thischoicecanbe finally supported by additional analyses (not includedinthiswork),suchastheexpositionor thenumberofsunlighthours.

4 3.(Case<study(application( The proposed algorithm is implemented in a casestudyinthecityofneuchâtel(switzerland) followingtheworkflowdescribedinfig6andin the following paragraphs. The three buildings selected for this study (Fig 3) are considered representative of some common building typologies in Swiss urban areas and present relevantcharacteristicsforthepurposeofthis study. Fig 3: Definition of lines on building surfaces; in color,thefootprintofthethreesimulatedbuildings Buildings A and B are two gablerroofed row buildings, whereas building C is a hiprroofed detachedbuilding.theinclinationoftheroofs rangesfrom10r20 forbuildingatoabout30 for buildings B and C (Fig 5). They present different orientations, as can be seen in Fig 4. The envelope of buildings A and B are mostly orientedtowardseastandwest,whilebuilding ChasmainlyaNorthRSouthorientation.Building C thus presents both the best orientation and roofinclination,reachingthehighestirradiation valuesof1294kwh/m 2 (Fig4andFig5).Finally, one should also note that all buildings are prevalently residential and do not have any photovoltaic panel or solar collector installed yet. Irradiation[kWh/m 2 ] A B C Orientation[ N] Fig 4: Distribution of sensor points over the orientationandirradiationforbuildingsa,bandc Irradiation[kWh/m 2 ] A B C Inclination[ ] Fig 5: Distribution of sensor points over the inclinationandirradiationforbuildingsa,bandc 3.1$Modeling$ The 3D model is created using the software BuildingReconstruction (virtualcitysystems, 2014) on the basis of highrdefinition GIS data, accordingtothelevelofdetail2(lod2)ofthe CityGML standard (Kolbe, Gröger, & Plümer, 2005).Thismodelingworkflowisimplemented in Grasshopper (McNeel, 2014) through its standardcomponents. The distance between points, in both u and v directions,wassetto1m,asitisconsidereda standardvalueforurbanrscalesolarsimulations (Compagnon, 2004). However, it is possible to useother values in order to reflect the actual dimensionofsolarpanels.

5 Fig 6 R Flowchart of the modeling, simulation and assessmentphases 3.2$Simulation$ A weather file for the city of Neuchâtel was createdinmeteonorm(remund,2014)onthe basis of the latest available recorded data (1991R2010). The simulation was run through DivaRforRGrasshopper (Solemma, 2014), a graphical interface to Radiance (Ward, 1994), using the GenCumulativeSky algorithm (Robinson& Stone, 2004), which provides the annual cumulative solar irradiation for each sensorpoint,expressedinkwh/m $Assessment$ Thesimulationresultswereprocessedthrough the geometricrregularity algorithm and transformedintoenergyproduction,asshown in Fig 6. We used fixed module efficiency coefficients of 15% for roof and of 8% for facades surfaces, considered among the standardvaluesfor,respectively,polycrystalline siliconr and cadmium telluriderbased modules (Häberlin, 2012, para ). The latter is considered a more suitable technology for facadeinstallation,duetothehigherprobability of having partial shading on some cells, which would more highly affect the performance of polycrystallinemodules(khaing,liang,htay,& Fan,2014),aswellasbecauseofthewiderange of possible installations as thinrfilm (e.g. on shadingdevicesorintegratedinglazing). Theirradiationresults,expressedinkWh/m 2 of exposedsurface,weretransformedinkwhby multiplying the value of each point by the sensorpointmeanarea,calculatedbydividing the area of each surface s b by the number of sensor points p b,s,l. The obtained values were thenconvertedtoenergyproductionusingthe abovermentionedpvrefficiencycoefficientsand finally normalized in kwh/m 2 of footprint surface in order to compare buildings of differentsizeswiththesamereferencescale. We conducted thus a sensitivity analysis in order to check the influence of different irradiationthresholdsontheenergyproduction calculated with the geometricrregularity algorithm. Fig 7 and Fig 8 show that the use of the conservative and aggressive approaches determine, respectively, a reduction and an increment in the energy production with respect to the normal calculation method(i.e. the one without geometry sampling) for all selectedirradiationthresholds,exceptforthose from1175to1250kwh/m 2 asnosensorpoint iswithinthisrange(fig4andfig5).moreover,

6 the difference in energy production can vary significantlydependingonthechosenthreshold andthegeometricrregularityapproach. Energyproduction [kwh/m 2 footprint surface] Fig7:Energyproductionassociatedtotheproposed approaches Differenceofenergyproduction[%] % 75% 25% R25% R75% 50 0 Conservative Normal Aggressive Irradiationthreshold [kwh/m 2 exposed surface] Conservative Aggressive Irradiationthreshold [kwh/m 2 exposedsurface] Fig 8: Difference between the normal and the geometryrsamplingapproaches Ingeneral,thelossinenergyproductiondueto the conservative approach is greater than the possible increment due to the aggressive approach. However, this is not the case for a thresholdof1150kwh/m 2,whichbringsabout a134%incrementfortheaggressiveapproach andonlya71%reductionfortheconservative approach.inthiscase,onlyscatteredroofparts of buildings A and B achieve the irradiation threshold and are hence discarded using the conservative approach, while a much larger surface gets considered if the normal and aggressiveapproachesareimplemented,ascan beobservedinthefourthcolumnoffig9. 4.(Limitations(and(future(work( In this work we used a geometryrsampling method to evaluate the BIPVRsuitability of building surfaces. However, other criteria shouldbeincluded such as, for example, the context sensitivity and the system visibility proposedby Munari Probst& Roecker(2011), as well as a more extensive analysis on the architecturalfeatures,intermsforexampleof materials, colors, surface textures and joints (ibid.). Moreover, the proposed workflow to selecttheappropriatebipvsystemsshouldbe further developed to identify, for instance, partiallyshadedzoneswheretheinstallationof crystalline siliconrbased solar cells should be avoided, unless microrinverters are used in ordertopreventthelossinenergyproduction. The efficiency of the cells should then be adjusted according to the actual installation conditions of the solar panels, for example in termsoftemperature. Regardingthesoftwareplatform,thealgorithm reliesondivarforrgrasshopperastheinterface tothesimulationengine.weaimatportingit intoaplatformrindependentcodesothatitcan be integrated into othersimulationtools(such ashoneybee)orcoupleddirectlywithsimulation enginessuchasradianceorcitysim.finally,the effectiveness of the proposed method in identifying BIPVRsuitable surfaces has to be validated.

7 Fig9:BIPVRsuitablesurfacesasdefinedbytheproposedapproachesfordifferentirradiationthresholds A first step towards validation will be the comparisonoftheresultsobtainedwiththose calculatedonanarchitecturalrscalemodelwith a higher level of detail (LOD3) and carefullyr designedpvintegration. 5.(Conclusions( Thispaperpresentedtheearlydevelopmentof a methodology to assess the BIPV potential in urbancontextsinordertoinformtheplanning and the design processes. We proposed an algorithm integrating geometricrregularity criteria in the definition of BIPVRsuitable surfaces.wedemonstrateditsapplicabilitytoa LOD2 model produced by highrdefinition GIS data, which makes possible its use for large urban areas. Although the results cannot be generalized, we showed that the proposed conservative and aggressive approaches can determine energy production values that greatly differ from those calculated through a methodbasedonlyonanirradiationthreshold and that this difference can vary significantly dependingontheselectedthreshold. Thetwoproposedapproachesbothrespondto the geometricrregularity requirement, even if theyrepresentopposedstrategiestodealwith suchcriterion.althoughitisdifficulttopredict whichstrategywillbechosenintheactualbipv installation, the proposed methodology is expected to provide helpful guidance when evaluating the solar energy potential in urban environments, as it provides an estimation of the error rate that irradiation thresholdrbased methodscanhaveifgeometricregularityisnot considered.

8 6.(Acknowledgements( This work has been conducted as part of the ACTIVE INTERFACES research project (Lufkin, Rey,Wuestenhagen,Wittkopf,&Bacher,2015), whichreceivedfinancialsupportfromtheswiss NationalScienceFoundation(SNSF)withinthe framework of the National Research Program NRP 70 Energy Turnaround. Additional support has been granted by the Ecole polytechnique fédérale de Lausanne (EPFL). The authors would like to thank Dr. Sophie Lufkin, Dr. Luisa Pastore and Emilie Nault for theirconstructivefeedback. 7.(References( Compagnon,R.(2004).Solaranddaylightavailability intheurbanfabric.energy$and$buildings,36(4), Häberlin, H. (2012). Photovoltaics$ System$ Design$ and$practice.chichester,uk:johnwiley&sons. Heinstein,P.,Ballif,C.,&PerretRAebi,L.RE.(2013). Building Integrated Photovoltaics (BIPV): Review,Potentials,BarriersandMyths.Green, 3(2). IEA. (2002). Potential$ for$ Building$ Integrated$ Photovoltaics$E$Full$Technical$Report(PVPST7R 4). Jakubiec, J. A., & Reinhart, C. F. (2012). Towards validatedurbanphotovoltaicpotentialandsolar radiation maps based on lidar measurements, GIS data, and hourly daysim simulations. In Proceedings$of$SimBuild$2012.Madison Kanters, J., Wall, M., & Dubois, M.RC. (2014). Development of a Façade Assessment and Design Tool for Solar Energy (FASSADES). Buildings,4(1), Khaing,H.H.,Liang,Y.J.,Htay,N.N.M.,&Fan,J. (2014). Characteristics of Different Solar PV Modules under Partial Shading. International$ Journal$of$Electrical,$Computer,$Electronics$and$ Communication$Engineering,8(9), Kolbe, T. H., Gröger, G., & Plümer, L. (2005). CityGML: Interoperable Access to 3D City Models. In P. D. P. van Oosterom, D. S. Zlatanova, & E. M. Fendel (Eds.), GeoE information$for$disaster$management(pp ).SpringerBerlinHeidelberg. Lufkin,S.,Rey,E.,Wuestenhagen,R.,Wittkopf,S.,& Bacher,P.(2015,April24).Building$integrated$ photovoltaics$ $ ACTIVE$ INTERFACES. Poster presented at the KickRoff meeting of the NationalResearchProgrammesNRP70"Energy Turnaround" and NRP 71 "Managing Energy Consumption",Luzern. McNeel. (2014). Grasshopper (Version ) [Windows].Seattle,WA,USA:RobertMcNeel& Associates. MunariProbst,M.C.,&Roecker,C.(2011).Urban acceptability of building integrated solar systems:lesoqsvapproach.inproceedings$of$ ISES$2011.Kassel. Redweik, P., Catita, C., & Brito, M. (2013). Solar energy potential on roofs and facades in an urbanlandscape.solar$energy,97, Remund,J.(2014).Meteonorm:Irradiationdatafor every place on Earth (Version 7). Bern: Meteotest. Robinson, D., & Stone, A. (2004). Irradiation modelling made simple: the cumulative sky approachanditsapplications.inproceedings$of$ PLEA$2004.Eindhoven. SFOE. (2014). Energy Strategy Swiss Federal OfficeofEnergy(SFOE). SFOE.(2015).ProductionetConsommationTotales d Énergie Électrique en Suisse Swiss FderalOfficeofEnergy(SFOE). SMS. (2013). Panneaux solaires: fiche des recommandations. Canton de Genève, Service desmonumentsetdessites(sms). Solemma. (2014). DIVARforRGrasshopper (Version )[Windows].Cambridge:Solemma. virtualcitysystems. (2014). BuildingReconstruction (Version ) [Windows]. Berlin: virtualcitysystemsgmbh. Ward,G.J.(1994).TheRADIANCElightingsimulation andrenderingsystem.inproceedings$of$the$21st$ annual$ conference$ on$ Computer$ graphics$ and$ interactive$techniques(pp ).NewYork: ACM.