Experimental Analysis of the Effect of Geometry and Façade Materials on Urban District’S Equivalent Albedo

sustainability

Article Experimental Analysis of the Effect of Geometry and Façade Materials on Urban District’s Equivalent Albedo

Elena Morini ID , Beatrice Castellani * ID , Andrea Presciutti ID , Elisabetta Anderini, Mirko Filipponi ID , Andrea Nicolini ID and Federico Rossi

Engineering Department, Interuniversity Research Centre on Pollution and Environment “Mauro Felli” (CIRIAF), University of Perugia, Via G. Duranti 67, 06125 Perugia, Italy; [email protected] (E.M.); [email protected] (A.P.); [email protected] (E.A.); ﬁ[email protected] (M.F.); [email protected] (A.N.); [email protected] (F.R.) * Correspondence: [email protected]; Tel.: +39-0755853853

Received: 26 May 2017; Accepted: 12 July 2017; Published: 16 July 2017

Abstract: Urban Heat Island (UHI) is influenced by urban form, geometry, and the properties of surfaces. Retroreflective (RR) materials have been proposed as a countermeasure to UHI, thanks to their optical property of reflecting most of the incident solar energy back towards the same direction. In this paper, the effect of RR materials on urban districts was investigated. They were applied on building façades of urban districts with different urban forms and orientations. To this aim, an experimental model resembling urban districts with different geometries was built and RR materials on vertical surfaces were tested and compared to conventional construction materials with similar global reflectance. The trend of the instantaneous albedo was monitored during the day and a new parameter called “equivalent albedo” was used to demonstrate the effectiveness of the RR materials. The comparative analysis shows that the RR façades lead to an increase of the equivalent albedo for all of the investigated urban patterns. For a block pattern, the equivalent albedo increase is equal to 3%, while for canyon patterns it is equal to 7%. Results of energy evaluations show that the energy savings obtainable with the use of RR materials is comparable to the values of anthropogenic heat emissions in residential areas.

Keywords: urban heat island; urban district; retroreﬂective materials; equivalent albedo

1. Introduction The Urban Heat Island (UHI) effect is a phenomenon that occurs in urban areas. It is due to build environment and human activities that make cities several degrees warmer than rural and suburban areas [1]. The UHI intensity is mainly due to human modification of the atmospheric environment [2–4]. UHI is influenced by several factors, including lower evaporation, increasing anthropogenic heat, lower air circulation in urban canyons, more pollutants in the atmosphere, topography, city size, low albedo, high heat capacity, and decrease in evapotranspiration [5–7]. Several studies found that the increased urban temperatures heavily impact the energy consumption of buildings during the summer period, affect human health [8], deteriorate indoor and outdoor thermal comfort [9,10], raise the concentration of harmful pollutants [11,12] and increase the carbon footprint of urban facilities and utilities [13]. Strategies for UHI mitigation have been widely investigated, and cool roofs, cool pavements [14– 17], and urban vegetation including roof gardens and wet roofs [18–20] have emerged as new materials for thermal energy storage [21]. Urban vegetation allows the modification of the heat balance of the city by evapotranspiration and by intercepting solar radiation. In particular, this aspect has been

Sustainability 2017, 9, 1245; doi:10.3390/su9071245 www.mdpi.com/journal/sustainability Sustainability 2017, 9, 1245 2 of 12 studied for Cairo’s urban developmentsas a function of two parameters: leaf area index, LAI and leaf area density, LAD [19]. The energy saving potentials of cool roofs have been recognized; they can reduce cooling costs with a possible small increase in heating costs [22,23], they have a positive effect on the quality of life in urban areas [24,25], they can lead to an offsetting of CO2 emissions [26–28], and they can be an effective strategy in cold regions as well [29–31]. Strong efforts have been made to investigate innovative cool materials such as retroreflective materials (RR) [32–34], thermo chromic pigments [35], and directional reflective materials [36]. As regards RR materials, previous experimental studies have focused on: (i) the analytical modelling of RR materials’ behaviour for perpendicular incident radiation [37]; (ii) the evaluation of the optic properties of RR samples for several angles of incidence [38]; and (iii) the evaluation of RR cooling effect at the urban canyon level in terms of reduction of the energy circulating inside the canyon [39]. In particular, results show that RR materials reflect the incident radiation mainly back toward the incoming direction for low incidence angles, while they lose this property for high incidence angles [40]. The distribution of reflected radiation by a diffusive material follows the Lambert law:

W ·r W = i cos α (1) r,α π Measurement results of previous studies [40] demonstrated that the angular distribution of the reflected radiation by a retroreflective surfaces depends on cosn, where “n”, which depends on the material, represents a “concentration factor” of the reflected radiation around the direction of the incident radiation. The previous experimental campaigns have demonstrated the mitigation effects of RR materials in urban canyon scenarios [39,41]. Studies have analysed different strategies to introduce in urban planning tools in order to address climate change and subsequent risk disaster [42]. Generally, such instruments are based on three pillars: energy efficiency (EE), renewable energy sources (RES), and greenhouse gas emission reduction. In this context, solar energy technologies have already been presented as a good opportunity. Due to their demonstrated ability to reduce urban albedo and their cooling potential in terms of energy reflected and sent beyond the urban canyon [38], RR materials could also be considered a mitigation and adaptation strategy of climate change to add to the kit of urban planning tools. The present paper aims to assesses the albedo change in an urban district physical model after the implementation of RR materials and as a function of urban geometry. To this end, RR materials were tested and compared to conventional construction materials with similar global reflectance. The effect of both materials and urban patterns on the energy kept inside the urban canopy and thus on the heat island phenomenon was evaluated by introducing and calculating a new parameter, called “equivalent albedo” of an urban area. The work is organized according to the following scheme: Section2 introduces the materials and methods used for the experimental analysis; Section3 shows the results and the discussion; conclusions are given in Section4. The results show that RR materials increase the equivalent albedo for all of the investigated urban patterns with a major effect on canyon schemes.

2. Materials and Methods

2.1. The Test Field The experimental facility was located in Terni, on the roof of a building of the Applied Physics Department, University of Perugia. The considered urban geometries, orientations, and dimensions were taken from a previous study [41]. Concrete cubes of 15 cm sides were employed to reproduce Sustainability 2017, 9, 1245 3 of 12 urban scenarios with different geometries. The black membrane of the rooﬁng material represents the bitumenSustainability of the street. 2017, 9, The1245 reproduced urban patterns are represented in Figure1. 3 of 12

Sustainability 2017, 9, 1245 3 of 12

Figure 1. Schemes of the urban structure: (a) Blocks; (b) W-E canyons; (c) N-S canyons. Figure 1. Schemes of the urban structure: (a) Blocks; (b) W-E canyons; (c) N-S canyons. The urban districts analyzed could represent the scale model of real urban cities such as Athens

The(a) urban and New districts York (b,c). analyzed For these could cities, represent the UHI exce theeds scale 4° [43] model and 8° of [44] real respectively. urban cities An suchin-depth as Athens (a) and Newweather York analysisFigure (b,c). is1. For providedSchemes these of in cities,the Reference urban the structure: UHI[45,46]. exceeds (a ) Blocks; 4 (◦b)[ 43W-E] andcanyons; 8◦ [ (44c) N-S] respectively. canyons. An in-depth In the blocks structure, the cubes were placed at a distance of 15 cm from each other. The weather analysis is provided in Reference [45,46]. outermostThe urban vertexes districts of the analyzed cubes at couldthe corners represent form the a square scale modelof 135 cmof real by 135 urban cm. cities In the such W-E as canyons, Athens In thethe(a) and blockscubes New were structure, York placed (b,c). in the For five cubes these rows cities, were with thea placed distance UHI exce at between aeds distance 4° [43]of 15 ofand cm. 15 8° cmThe [44] fromcubes respectively. eachdisposed other. An in in-depththis The way outermost vertexesformedweather of the four cubesanalysis canyons at is the provided in corners the West-Eastin Reference form a direction. square [45,46]. of The 135 value cm byof the 135 H/D cm. ratio In the that W-E characterizes canyons, the the cubes were placedcanyonsIn in the was five blocks 1. rows The structure, outermost with a distancethe vertexes cubes betweendefined were placed an ofarea 15at of cm.a 135distance Thecm by cubes of 150 15 cm. cm disposed In from the N-Seach in canyons, thisother. way Thethe formed four canyonscubesoutermost were in the vertexes in West-East five rows,of the direction.withcubes 15 at cmthe Thedistancecorners value form between of a thesquare each H/D of other, 135 ratio cm and thatby formed135 characterizes cm. fourIn the canyons W-E the canyons, canyonsin the was the cubes were placed in five rows with a distance between of 15 cm. The cubes disposed in this way 1. The outermostNorth-South vertexes direction. defined The H/D an ratio area was of 1. 135 The cmoccupied by 150 area cm. was In 150 the cm N-S by canyons,135 cm. The the facility cubes were wasformed exposed four canyonsto the sun in fromthe West-East sunrise to direction.sunset. Th Thee three value configurations of the H/D ratiowere thatdesigned characterizes to have the in five rows,samecanyons exposed with was 15 1. vertical The cm outermost distance surface inside betweenvertexes the definedperimeter. each other,an arToea pursue andof 135 formed this cm condition,by 150 four cm. canyons referring In the N-S to in canyons,the the schemes North-South the direction.depictedcubes The were H/D in Figurein ratiofive 1,rows, was25 blocks with 1. The 15we cm occupiedre installeddistance area inbetween the was urban each 150 structure other, cm by and (a) 135 (blockformed cm. shape) The four facilitycanyons and 50 was blocksin the exposed to the sunwereNorth-South from installed sunrise direction. in the to urban sunset. The structures H/D The ratio three (b,c)was configurations(W-E1. The and occupied N-S canyons). area were was designed 150 cm by to 135 have cm. the The same facility exposed verticalwas surface exposedFor each inside schemeto the the sun shown perimeter. from in sunrise Figure To to1, pursue measurementssunset. thisThe three condition, were configurations carried referring out with were two to designed the different schemes to materials: have depicted the in same exposed vertical surface inside the perimeter. To pursue this condition, referring to the schemes Figure1,traditional 25 blocks concrete were installed and retroreflective in the urban (RR) membrane structure [47]. (a) (block Figure 2 shape) shows andthe test 50 fields blocks with were lateral installed surfacesdepicted covered in Figure with 1, 25 RR blocks materials. were installed in the urban structure (a) (block shape) and 50 blocks in the urbanwere installed structures in the (b,c) urban (W-E structures and N-S (b,c) canyons). (W-E and N-S canyons). For eachFor scheme each scheme shown shown in Figure in Figure1, measurements1, measurements were carried carried out out with with two twodifferent different materials: materials: traditionaltraditional concrete concrete and retroreflective and retroreflective (RR) (RR) membrane membrane [47]. [47]. Figure Figure 2 shows2 shows the thetest testfields fields with lateral with lateral surfacessurfaces covered covered with RRwith materials. RR materials.

Figure 2. Schemes of the urban structure with RR lateral surfaces: (a) Blocks; (b) W-E canyons; (c) N- S canyons.

Figure 2.FigureSchemes 2. Schemes of theof the urban urban structurestructure with with RRRR lateral lateral surfaces: surfaces: (a) Blocks; (a )(b Blocks;) W-E canyons; (b) W-E (c) N- canyons; (c) N-S canyons.S canyons. Sustainability 2017, 9, 1245 4 of 12 Sustainability 2017, 9, 1245 4 of 12

An albedometer, supplied by Delta Ohm Srl (model LP PYRA 05) and constituted by two An albedometer, supplied by Delta Ohm Srl (model LP PYRA 05) and constituted by two pyranometers (Class I), was used to measure the albedo during the whole day. The albedo is pyranometers (Class I), was used to measure the albedo during the whole day. The albedo is calculated calculated as the ratio between the reflected solar radiation and the diffuse radiation incident from as the ratio between the reﬂected solar radiation and the diffuse radiation incident from the bottom the bottom hemisphere. The technical data of the sensors are provided in Reference [48]. hemisphere. The technical data of the sensors are provided in Reference [48]. In accordance with the procedure published in other works [41], it was positioned in a central In accordance with the procedure published in other works [41], it was positioned in a central position of each scheme, at a height of 30 cm above the blocks. It is close enough to the blocks to position of each scheme, at a height of 30 cm above the blocks. It is close enough to the blocks to assess assess that the measured albedo is representative of the characteristic of the scheme, and albedo that the measured albedo is representative of the characteristic of the scheme, and albedo variations variations are due to the variations of the characteristics of the scheme (geometry and properties of are due to the variations of the characteristics of the scheme (geometry and properties of materials). materials). Measurements were carried out from 1 September to 10 September in accordance with the Measurements were carried out from 1 September to 10 September in accordance with the weather weather conditions required, which were clear sky conditions. conditions required, which were clear sky conditions.

2.2. Optic-Energy CharacterizationCharacterization of Materials The solar solar reflectance reflectance of the of blocks the and blocks the RR and film the was RRmeasured film through was measured a spectrophotometric through a spectrophotometricanalysis. The hemispherical analysis. The spectral hemispherical reflectance spectral of reflectancethe samples of thewas samples measured was measuredusing a usingUltraviolet/Visible/Near a Ultraviolet/Visible/Near Infrared (UV/VIS/NIR) Infrared (UV/VIS/NIR) by by Shimadzu by Solid by Spec Shimadzu 3700 spectrophotometer Solid Spec 3700 spectrophotometer[49] equipped with 60 [49 mm] equipped integrating with sphere. 60 mm The wavelength integrating range sphere. of measurements The wavelength was range280–2500 of measurementsnm, which includes was 99% 280–2500 of the nm,solar which energy. includes The sola 99%r reflectance of the solar of the energy. samples The was solar then reflectance calculated ofusing the the samples appropriate was then standards calculated (ASTM using Standard the appropriate G 173 [50,51]). standards The (ASTMinvestigated Standard samples G 173 are [ 50shown,51]). Thein Figure investigated 3. samples are shown in Figure3.

Figure 3. Investigated samples: (a) concrete sample; (b)) RRRR sample.sample.

Spectrophotometric analysis was carried out on one RR membrane sample and on fivefive concrete samples (C1–C5), to take into account the effect of surface irregularity. The reflectancereflectance spectraspectra of thethe samples areare reportedreported inin FigureFigure4 4.. Concrete samplessamples showshow very similar behavior. TheThe RR sample shows higher values of reflectance reflectance between 400 nm and 600 nm, 900 nm and 1650 nm, and 1900 nm and 2100 nm, as well as lower reflectancereflectance valuesvalues forfor thethe remainingremaining intervals.intervals. According toto the calculations, they show a comparable average totaltotal reflectancereflectance equalequal toto 55%55% (55.048%(55.048% forfor thethe CC samplessamples andand 54.98%54.98% forfor thethe RRRR sample).sample). Sustainability 20172017,, 99,, 12451245 5 of 12 Sustainability 2017, 9, 1245 5 of 12

Figure 4. Reflectance spectra of the concrete samples C1, C2, C3, C4 and the RR film. Figure 4. ReflectanceReflectance spectra of the concrete samples C1, C2, C3, C4 and the RR film.film. In accordance with procedure described in Reference [37], diffusive and RR samples were In accordanceaccordance with procedureprocedure describeddescribed in ReferenceReference [[37],37], diffusive and RR samplessamples werewere characterized in terms of angular distribution of the reflected energy (from −90° to 90°). The samples’ characterized inin termsterms ofof angularangular distributiondistribution ofof thethe reflectedreflected energyenergy (from(from− −90°90◦ to 90°). 90◦). The The samples’ samples’ behavior is graphed in Figure 5. behavior is graphed in Figure5 5..

Figure 5. Angular distribution of reflected light: diffusive sample and RR sample [30]. Figure 5. Angular distribution of reflected light: diffusive sample and RR sample [30]. Figure 5. Angular distribution of reflected light: diffusive sample and RR sample [30]. With respectrespect to to the the diffusive diffusive sample, sample, the RR the material RR material has a predominant has a predominant retroreflected retroreflected component With respect to the diffusive sample, the RR material has a predominant retroreflected backwardcomponent to backward the incoming to the direction. incoming direction. component backward to the incoming direction. 3. Results and Discussion 3. Results and Discussion 3. Results and Discussion A monitoring campaign during the first 10 days of September 2015 was carried out in the test A monitoring campaign during the first 10 days of September 2015 was carried out in the test facilityA monitoring for each scheme, campaign with during diffusive the and first RR 10 materials.days of September Daily albedo 2015 trends was carried recorded out duringin the test the facility for each scheme, with diffusive and RR materials. Daily albedo trends recorded during the dayfacility are for shown each inscheme, Figures with6–8. diffusive Each figure and reports RR ma theterials. monitored Daily albedo daily albedotrends valuesrecorded for during the block the day are shown in Figures 6–8. Each figure reports the monitored daily albedo values for the block day are shown in Figures 6–8. Each figure reports the monitored daily albedo values for the block scheme, W-E canyons, and N-S canyons, respectively. The considered time interval is the one that scheme, W-E canyons, and N-S canyons, respectively. The considered time interval is the one that has the minimum cloud cover for all the schemes and conditions (from 7:20 to 16:40). has the minimum cloud cover for all the schemes and conditions (from 7:20 to 16:40). Sustainability 2017, 9, 1245 6 of 12

As regards the block scheme in Figure 6, the daily trend shows that the albedo in the presence Sustainabilityof RR materials2017, 9 is, 1245 higher with respect to the diffusive material during most of the day. Some slightly6 of 12 lower values can be found from 10:20 to 12:00 and at 15:45. scheme,Sustainability W-E 2017 canyons,, 9, 1245 and N-S canyons, respectively. The considered time interval is the one that6 of has 12 the minimum cloud cover for all the schemes and conditions (from 7:20 to 16:40). As regards the block scheme in Figure6 6,, thethe dailydaily trendtrend showsshows thatthat thethe albedoalbedo inin thethe presencepresence of RR materials is higher with respect to the diffus diffusiveive material during most of the day. Some slightly lower valuesvalues cancan bebe foundfound fromfrom 10:2010:20 toto 12:0012:00 andand atat 15:45.15:45.

Figure 6. Daily albedo of the block shape scheme.

Figure 7 shows the daily albedo results for the W-E canyons scheme. The measured albedo in the RR canyons is clearly higher than in the concrete canyons throughout the whole day. The maximum difference can be found in the morning, before 10:00. Then, in the central hours, a quite constant value of albedo forFigure both the 6. Daily materials albedo can of thebe blockobserved. shape scheme.

Figure 7. Daily albedo of the W-E canyons scheme.

FigureFinally,7 showsFigure the8 depicts daily albedo the trend results of albedo for the for W-E the canyons N-S canyons scheme. scheme. The measured In this configuration, albedo in the the albedo in the presence of RR materials is clearly higher than the albedo with diffusive concrete. RR canyons is clearly higher than in the concrete canyons throughout the whole day. The maximum difference can be found in the morning, before 10:00. Then, in the central hours, a quite constant value of albedo for both the materialsFigure can 7. Daily be observed. albedo of the W-E canyons scheme.

Finally, Figure 8 depicts the trend of albedo for the N-S canyons scheme. In this configuration, the albedo in the presence of RR materials is clearly higher than the albedo with diffusive concrete. Sustainability 2017, 9, 1245 7 of 12 Sustainability 2017, 9, 1245 7 of 12

Figure 8. Daily albedo of the N-S canyons scheme.

Finally,This means Figure that8 depicts RR materials the trend have of albedoa beneficial for the ef N-Sfect canyonsat each time scheme. of the In thisday configuration,and allow energy the albedosaving within in the presence the urban of form RR materials with a canyon is clearly structur highere. During than the the albedo central with hours diffusive of the concrete. day, in all the investigatedThis means patterns, that RR the materials albedo is have constant. a beneficial In effect, effect at this at each time time the sun of the is dayhigh and and allow the incident energy savingradiation within arrives the perpendicular urban form with to th ae canyon horizontal structure. surfaces During of the the urba centraln district. hours In ofthe the morning day, in and all the in investigatedthe afternoon, patterns, when solar the albedoradiation is constant.hits the surfac In effect,es with at this different time the inclinations, sun is high the and instantaneous the incident radiationalbedo profiles arrives in perpendicularall cases show sharp to the fluctuations, horizontal surfaces suggesting of the that urban urban district. albedo Inis a the time-dependent morning and inparameter. the afternoon, It is whenin fact solar mainly radiation influenced hits the by surfaces the interaction with different between inclinations, urban pattern the instantaneous and solar albedoirradiation, profiles which in all changes cases show during sharp the day. fluctuations, To take into suggesting account that this urban aspect, albedo which is becomes a time-dependent crucial at parameter.an urban district It is in level, fact mainly a new influenced parameter by is thehere interaction introduced between and called urban “urban pattern equivalent and solar irradiation,albedo”. It whichis defined changes as the during ratio between the day. the To reflected take into radiatio accountn thisand aspect,the solar which incident becomes radiation crucial both at integrated an urban districton the hemisphere level, a new and parameter during isthe here day, introduced as shown andin Equation called “urban (2). equivalent albedo”. It is defined as the ratio between the reflected radiation and the solar incident radiation both integrated on the Ω hemisphere and during the day, as shown in= Equation (2). (2) Ω R 24 R 2π WrdΩdt The equivalent albedo was calculated,α = in 0accord0 ance with Equation (2), for the three schemes(2) EQ R 24 R 2π analyzed in the time interval from 07:20 to 16:40.0 Results0 Wid Ωaredt shown in Table 1. The daily albedo of the ground without blocks is 0.17. The presence of blocks (whose reflectance is about 55%, as stated The equivalent albedo was calculated, in accordance with Equation (2), for the three schemes before) causes an increase of albedo. analyzed in the time interval from 07:20 to 16:40. Results are shown in Table1. The daily albedo of the ground without blocksTable is 0.17. 1. Equivalent The presence albedos of blocks and related (whose % increase. reflectance is about 55%, as stated before) causes an increase of albedo. Scheme Equivalentalbedo(Diffusive Concrete) Equivalent Albedo(RR) ΔAlbedo (%) Table 1. Equivalent albedos and related % increase. Blocks 0.21 0.24 +3%

W-EScheme canyons Equivalentalbedo(Diffusive 0.29 Concrete) Equivalent 0.36 Albedo(RR) ∆Albedo +7% (%) N-S Blockscanyons 0.28 0.21 0.240.35 +3% +7% W-E canyons 0.29 0.36 +7% ThereN-S canyonsis an improvement in the 0.28equivalent albedo values due 0.35to the use of RR materials +7% for all the schemes investigated; this is due to the directional properties of RR façades. In particular, for the blockThere scheme, is an the improvement daily equivalent in the albedo equivalent is 0.21 albedo for diffusive values dueconcrete to the and use 0.24 of RR for materials RR materials, for all with the schemesan increase investigated; equal to 3%. this For is duethe W-E to the canyons directional scheme, properties the equivalent of RR façades. albedo In passes particular, from for 0.29 the to block 0.36 scheme,with RR thematerials. daily equivalent The equivalent albedo albedo is 0.21 forin diffusiveN-S canyons concrete scheme and passes 0.24 for from RR materials,0.28 to 0.35. with The an increase equalreaches to 7% 3%. for For both the W-Ethe canyon canyons configurations. scheme, the equivalent albedo passes from 0.29 to 0.36 with RR materials.Another consideration The equivalent can albedo be made in N-Son the canyons basis of scheme the results passes in fromTable 0.28 1. The to 0.35.equivalent The increase albedo reachesis affected 7% by for the both urban the canyon pattern, conﬁgurations. with higher values in presence of canyon urban structures with respect to the block pattern. This is clearly shown in Figure 9, in which the instantaneous albedo Sustainability 2017, 9, 1245 8 of 12

Another consideration can be made on the basis of the results in Table1. The equivalent albedo is affected by the urban pattern, with higher values in presence of canyon urban structures with respect Sustainability 2017, 9, 1245 8 of 12 to the block pattern. This is clearly shown in Figure9, in which the instantaneous albedo values of the valuesthree schemes of the three are compared schemes are (the compared ﬁrst graph (the a) in first presence graph ofa) diffusivein presence façades of diffusive and the façades second graphand the b) withsecond RR graph surfaces). b) with RR surfaces).

(a)

(b)

Figure 9. Daily instantaneous albedo: (a) Diffusive Materials, (b) RR Materials. Figure 9. Daily instantaneous albedo: (a) Diffusive Materials, (b) RR Materials.

Canyon structures ensure higher albedo during the day than the block structure, in the presence of bothCanyon diffusive structures and RR ensure materials. higher Such albedo a differenc duringe theis mainly day than due the to block the number structure, of concrete in the presence blocks thatof both arediffusive positioned and on RR the materials. ground. SuchIn fact, a difference to have the is mainlysame exposed due to the vertical number surface of concrete on the blocks three urbanthat are structures, positioned 50 on concrete the ground. blocks In fact,were to needed have the for same the exposedblock shape vertical and surface 25 concrete on the blocks three urbanwere insteadstructures, used 50 for concrete the canyon blocks structures. were needed for the block shape and 25 concrete blocks were instead usedThis for the difference canyon structures.in the horizontal surface area, which represents the roofs of the urban district, causesThis different difference albedo in thevalues. horizontal In the presence surface area,of diffusive which concrete represents façades, the roofs with of respect the urban to the district, block structure,causes different the albedo albedo increases values. Inby the7% presencewith N-S of canyons diffusive and concrete 8% with façades, W-E canyons. with respect The todirectional the block property,structure, i.e., the albedothe property increases to refl byect 7% the with radiation N-S canyons back in and the 8%same with direction W-E canyons. of incidence, The directional of the RR property,surface intensifies i.e., the property this effect. to reflect the radiation back in the same direction of incidence, of the RR surfaceThe intensifies results show this effect. that, with an equal amount of exposed vertical surface area, the urban irregularityThe results influences show that, the withequivale an equalnt albedo; amount if increasing of exposed the vertical irregularity surface area,of the the urban urban scheme, irregularity it is necessaryinfluences to the install equivalent more reflective albedo; ifmaterials increasing on thethe irregularityvertical surfaces of the in urban order scheme,to have the it is same necessary effect as that achieved in a more regular scheme. The benefits obtained with the use of RR façades in terms of equivalent albedo of the considered urban district models lead to an improvement of the total amount of energy that is reflected outward from the urban canopy, thus reducing the energy circulating inside the urban volume. Sustainability 2017, 9, 1245 9 of 12 to install more reflective materials on the vertical surfaces in order to have the same effect as that achieved in a more regular scheme. The benefits obtained with the use of RR façades in terms of equivalent albedo of the considered urban district models lead to an improvement of the total amount of energy that is reflected outward from the urban canopy, thus reducing the energy circulating inside the urban volume. According to the value of the annual average solar irradiation in a tropical city, Lao PDR [50] of about 16 MJ/m2 day, the amount of energy saved with RR façades is indeed equal to: (i) 0.48 MJ/m2 day, which is almost 50,000 MJ/day for a 100,000 m2 district with the block structure; (ii) 1.12 MJ/m2 day, which is more than 100,000 MJ/day for a 100,000 m2 district with the canyon structure. Results of energy calculations are summarized in Table2.

Table 2. Energy saving with RR façades in different urban patterns.

Urban Pattern Solar Irradiation (MJ/m2day) ∆Equivalent Albedo Energy Saving (MJ/m2·day) Blocks 16 0.03 −0.48 Canyons 16 0.07 −1.12

The energy savings, i.e., cooling potential of RR materials in terms of energy reflected and sent beyond the urban canyon (measured in this case in MJ/m2 day) have been roughly calculated by considering an ∆equivalent albedo equal to 0.03 and 0.07 as in Table2, with a typical solar irradiation of 16 MJ/m2·day and a 100,000 m2 district. The estimated energy saving obtainable with the use of RR materials is comparable to the values of anthropogenic heat emissions expressed in MJ/m2·day for residential areas in tropical cities, as estimated [52]. Adhikari et al. [52] estimates the temporal variability of the anthropogenic heat flux density by separately considering the major sources of waste heat in urban environments, which include heat release from vehicular traffic, buildings, and human metabolism, respectively. The calculated anthropogenic heat flux density varies as a function of weekdays and study area, and ranges from 1.1 MJ/m2·day to 7.5 MJ/m2 day, which has a comparable order of magnitude of the hypothesized benefit above.

4. Conclusions This paper investigates the influence of retroreflective materials on the albedo of experimental models resembling urban districts with different geometries. Three different geometries were investigated: a block structure, a West-East canyon structure, and a North-South canyon structure. For the three urban structures, the albedo was monitored during the day both in the presence of conventional diffusive building material and RR material. In addition, a new parameter called “urban equivalent albedo” is introduced to take into account the interaction between solar irradiation and urban pattern, which changes during the day. The comparative analysis shows that the RR façades lead to an increase of the equivalent albedo for all of the investigated urban patterns. This is due to the reflective properties of the RR membrane, which reflects the incident solar energy outward towards the same direction of incidence and thus does not diffuse the energy within the urban canopy. For the block structure, the equivalent albedo increase is equal to 3%, while for the canyon schemes the equivalent albedo increase is equal to 7%. It follows that a more irregular urban structure leads to the need for a more reflective material to gain the same albedo improvement as in a more regular urban scheme. Energy evaluation shows that the estimated energy saving obtainable with the use of RR materials is comparable to the values of anthropogenic heat emissions in residential areas. The present work focused on the analysis of the optic-energy properties of materials. As for future developments of the research, it will investigate how such properties can interact in the real scale, and the fluid dynamics of the setup will be studied to check the similitude with real-scale applications. Sustainability 2017, 9, 1245 10 of 12

In effect, the obtained results in the small scale needed to be veriﬁed in real-scale applications in order to also consider the convective phenomena. Further future studies will be focused on the relation between albedo and outdoor comfort by considering the connection between urban degree of compactness and equivalent albedo. A future goal will also be the assessment of albedo-improvement costs in current cities.

Author Contributions: E.M., A.P., E.A. developed the experimental setup. E.M., B.C., A.P. and E.A. carried out the experimental campaign and analyzed data. E.M., B.C., E.A. wrote the paper. A.P., M.F. and A.N. contributed analysis tools. F.R. supervised the research activities. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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