countries thermal comfort in low rise low income houses in high solar radiation Cool roofs: High tech low cost solution for energy efficiency and Energy & Buildings

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Energy & Buildings

journalhomepage:www.elsevier.com/locate/enbuild

Cool roofs: High tech low cost solution for energy efficiency and

thermal comfort in low rise low income houses in high solar radiation countries

Maria Kolokotroni^a,∗, Emmanuel Shittu^a,Thiago Santos^a,b,Lukasz Ramowski^a, Adeline Mollard^a, Kirkland Rowe^c,Earle Wilson^c, João Pereira deBrito Filho^d, Divine Novieto^e

aBrunel University London, Kingston Lane, Uxbridge UB8 3PH, UK

bFederal Institute of Pernambuco, Av . Prof Luiz Freire, 500 Recife, PE, Brazil

cUniversity of Technology, Kingston, Jamaica

dFederal University of Pernambuco, Recife, PE, Brazil

eHo Technical University, Ho. Volta Region, Ghana

a rt i c l e i nf o

Article history:

Received 14 November 2017 Revised 16 May 2018 Accepted 3 July 2018 Available online 17 July 2018 Keywords:

Cool roof paint Quality of life Low cost Thermal comfort Energy efficiency High solar radiation

a b s t ra c t

Coolroofsaremosteffectiveinreducingcoolingloadsandalleviatingoverheatinginlocationswithhigh solarradiationandexternalairtemperature.Thispaperpresentsresultsofanexperimentalstudyofa lowincomehouseinJamaicaandacomputationalstudyinthreecountriesaroundtheequator:Jamaica, NortheastBrazil(Recife)andGhana.Acase-studytypicalofsinglestoreyhousesinJamaicawasmoni- toredbeforeandaftertheinstallationofacoolpaintontheroof;ondayswithaveragesolarradiation intensityof∼420W/m² andambientairtemperatureof∼28°C,internalceilingsurfacetemperatureis reducedbyanaverageof6.8°Candinternalairtemperatureby2.3°C.Monitoringresultswereusedto calibratesuccessfullyanEnergyPlusmodel;similarmodelsweredevelopedforGhanaandBrazildiffering insizeand/orconstructiontoreflect countryspecific practices.Annualsimulationsindicatethatinter- nalceilingsurfacetemperaturesarereducedonaverageby3.2–5.5^oCandinternalairtemperaturesby 0.75–1.2°C.Coolingdemandsimulations(setpoint24°C)indicatesimilarannualpotentialsavingsinthe threelocations(∼190kWh/m²/year)althoughestimatedCO₂emissionsreductiondifferreflectingelectric- itygenerationfuels.Agingofthecoolroofhasanimpactreducingloadsavingsby22–26kWh/m²/year.

© 2018TheAuthors.PublishedbyElsevierB.V.

ThisisanopenaccessarticleundertheCCBYlicense.(http://creativecommons.org/licenses/by/4.0/)

1. Introduction

Many factors influence the energy demand of a building in- cludingits purpose,intendeduseandlocation.Thethermalprop- erties of the materials used for the external walls and roof can haveamajorinfluenceonthesurfacetemperatureandinturnthe amountofheat conductedthroughthe surfaceofthe building.A coolbuildingsurface(roofand/or walls)usesa coatingwithhigh thermalemissivityandsolarreflectancepropertiestodecreasethe solarthermalloadofa buildingthusreducing itsenergyrequire- mentsforcooling.Manyexperimentalandmodellingstudieshave beenpublishedthatcomparebuildingenergyefficiencybenefitsof

∗ Corresponding author.

E-mail address: maria.kolokotroni@brunel.ac.uk (M. Kolokotroni).

coolroofingtechniques[1–3]withcomputationalandexperimen- talstudiesreportingpositiveresultsforresidentialbuildings[4–6]. Moststudiesfocusontheimprovementofresidentialbuildings inwell developedeconomies driven by legislation toreduce CO₂ emissions by buildings [7,8]. There are few studies focussing on areaswhere poorly insulatedbuildings combinedwithhighsolar radiation levels create uncomfortable internal conditions for the most vulnerable populations [9,10]. When these populations can afford it, they install add-on air conditioning systems which in- creaseelectricity demand.Therefore additionalevidence fromin- tervention studies andclimaterelated designguidance isneeded todemonstratetheeffectivenessofcoolroofsastheirperformance dependsbothonclimaticconditionsandthecharacteristicsofthe buildingstock.

Coolmaterialsaremosteffectiveinlocationswithhighsolarra- diationandexternalairtemperature.Thispaperpresentsresultsof an experimental andcomputational studyforlow incomehouses https://doi.org/10.1016/j.enbuild.2018.07.005

0378-7788/© 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license. ( http://creativecommons.org/licenses/by/4.0/ )

Fig. 1. Location of case-studies.

inonecountry,Jamaica,andcomputationalstudiesinanothertwo countrieswithtropical climate:Northeast Brazil(Recife- Pernam- bucoState)andGhana (Fig.1). Thetargetedcountriesarearound theequatorwithhighsolarradiationintensitythroughouttheyear (4–6kWh/m²) andhighexternalairtemperaturesleadingto high cooling demand in buildings oroverheating conditions especially inpoorlyinsulatedbuildingswherecoolmaterialsare mosteffec- tive [11]. InBrazil, voluntarybuildingguidelines [12]recommend aU-valueofupto2.00W/m²·KwhileinGhanaandJamaicaeven highervaluesarederivedduetothelackofinsulationinthecon- crete/brickwallsandconcrete/metalroofing.Thereisevidencethat overheatingisexperiencedinbuildingsinthesecountriesandadd- onair-conditioningisinstalledwhereitisaffordable.However,in some cases(Jamaica)highelectricity rates(48%increase between 2010 and2014)prohibit air-conditioning duetooperational costs even if capitalcosts were affordable [13]while electricity power cuts are commonin some countries. Studies suggeststhat a 25%

cooling energy demand reduction is possible in non- residential buildingslocatedinthetargetedregions.[11].

Thispaperfirstpresentsaninterventionexperimentinahouse in Jamaica (Portmore). The house was monitored before and af- ter the application of the cool roof paint. Data from the experi- mentwere usedtocalibrateamodeldevelopedusingEnergyPlus.

The developedmodel wasthen usedto quantifyimprovementof the internal thermal environment in low rise typical houses in the threecase-studycountriesaswell asquantificationof poten- tial electricity and carbonsavings by avoiding installation ofair- conditioningtoachievethermalcomfortconditions.

2. ExperimentalcasestudyinJamaicaandmonitoringresults 2.1. Descriptionofcase-studyandappliedcoolroof

The experimental case-study building is a typical example of manylow-income singlestorey semi-detachedhouses builtinJa- maica.Thesurrounding areaisan urban contextcomprisinglow- risebuildingswithaminimumcontributiontotheshading ofthe

case-study building.The floor plan is shownin Fig. 2; photos of thehousearepresentedinFig.3.

Acommerciallyavailable cool paintsuitable forflat roofswas appliedtotheroofofthehouseinJamaica.Thispaintisincluded incoolroofratingdatabases[14,15]withthefollowingcharacter- istics:

• Initial solar reflectance: 0.82, reduced to 0.72 after three years.

• Thermalemittance:0.90.

• InitialSolarReflectivityIndex(SRI):160,reducedto90after threeyears.

Afterapriorcleaning,aprimercoatwasappliedontheprecast concreteslabon22ndMarch2017.Duetothebadweathercondi- tion,thethreelayersofthecoolpaintwereappliedfromthe31st March2017to16thofApril2017.Fig.4showstheroofbeforeand aftertheapplicationofcoolroof.

2.2.Experimentalmonitoringprocedure

Thepurpose ofmonitoringwasto acquire datafroman oper- ationalhousepre-andafterintervention.Monitoring focussedon surfacetemperatureoftheceilingandairtemperatureinside the house.Someweather data(airtemperature,relativehumidityand globalsolarirradiance)weremeasuredtofacilitatethecalibration ofthecomputationmodel.Anon-sitesurveywascarriedouttode- terminethegeometry(includingareasofwindowsanddoors)and constructionofthehouseaswellasequipment(includinglighting) assourcesofinternal heatgains.It wasnot possibleto carryout moredetailed monitoringof the occupantsbehaviour anduse of thehouseduetolimitedfundingbuttheoccupanthasprovideda scheduleofnormalactivities.

PreliminarymeasurementsstartedinSeptember2016 whileall monitoring sensors were inplace in January 2017.As mentioned beforethecoolpaintwasappliedbetween22Marchand16April 2017 and monitoring continued until July 2017, to include peri- ods before andafter the installation of thecool roof of data ac- quisition. Internal and external roof surface temperatures were

Fig. 2. Floor plan of the case-study house in Jamaica with a floor area of 36 m ²(dimensions in m). Position of measurement points are indicated.

Fig. 3. Case-study house in Jamaica photos. The position of the weather station is indicated. The AC unit is not working.

measured with thermocouples linked with a Campbell Scientific CR10xdatalogger with accuracyof ±0.05% ofthe full-scale input range, at 4 locations outside and 4 inside (see Fig. 2). Internal air temperature was measured at four locations (living room, 2 bedrooms and kitchen) with HOBO UX100-003 dataloggers with accuracy ±0.21°C. External air temperature and relative humid- ity were measured on sitealso using a HOBO UX100-003 data- loggershieldedandventilated(see Fig.3). Datawere recordedat 5min interval andaveraged to one hour forthe analysis.Global solarirradiancewasmeasured onsitewithapyranometerCMP 3 fromKipp & Zonen; for the case-study, the small spectral range from300to2800nm andthemaximumoperationalirradianceof 2000W/m² are sufficient.The output rangeis0 to30mVwitha sensitivityof5to 20μ^{V/W/m2.Bymeasuring globalsolarirradi-}

ance all elementsofsolarradiationare includedin themeasure- mentsasitwasnotpossibletomeasurethecomponentsinmore detail.

2.3. Monitoringresults

Fig.5presentsmeasuredairandinsideceilingsurfacetemper- ature ofthe livingroom andsolar radiationintensity beforeand afterpainting.Itshould benotedthat globalsolarirradiancewas measured and thisis used for the graph of Figs. 5 and6. There is a difference inthe sun’s inclination betweenMarch and April and thismight have affected the incident solar radiation on the roof.Nevertheless,thisdifferenceissmallfortheresultspresented whicharechosentobeascloseintimeaspossible.Averagesolar

Fig. 4. Roof of the case-study house in Jamaica.

Fig. 5. Living room air and internal ceiling surface measured temperature for pre-application (13th March 2017 to 23rd March 2017) and post-application (16th April 2017 to 25th April 2017) of cool roof. Measured solar radiation intensity is indicated.

Fig. 6. Two days measured results of solar radiation intensity, living room air and internal ceiling surface temperature for pre-application (13th March 2017) and post- application (24th April 2017) of cool roof.

Fig. 7. Thermal zones of the house in Jamaica.

radiationintensityduringdaytime andaverageexternal airtem- peraturewere lower inMarch (average of407W/m² and27.4°C) thaninApril (averageof428W/m² and27.9°C).The curvesindi- catethatceilingsurfacetemperatureandinternalairtemperature arelowerinApril(afterpaintperiod)forhighersolarradiationin- tensityandhigheroutsideairtemperature.

Inordertogiveafurtherinsightofthepreandaftercoolroof applicationconditions,Fig.6presentsmeasurementsontwodays.

Thefirst day(13 March) isbefore andthe second day(24 April) afterthe application;both days havesimilar external average air temperature(27.4°Con13Marchand28.2°Con24April)andav- erageglobalsolarradiationintensityduringdaytime(416W/m²on 13Marchand428W/m² on24April).

Internal ceiling surface temperature was higher on the 13 Marchcompared to24Aprilby amaximumof18.6°C andan av- erageof6.8°C. Theinternal airtemperatureshowsthat afterap- plyingcoolpaint,thelivingroomisonanaveragecoolerby2.3°C.

3. Developmentofcomputationalmodelofthehouses 3.1.MethodologyofmodeldevelopmentofthehouseinJamaica

A model of the experimental house was developed using En- ergyPlus[16]and OpenStudio [17]. Thehouse wasmodelled into sixthermalzonesshowninFig.7.Thelocalsurveyprovidedinfor- mationonthe materialsoftheexternal envelopesothat thermal characteristics were calculated (Table 1); the internal heat gains arebasedoninputfromtheoccupantfortheschedule.Maximum internalgainis3kW but2kWare duetothe gasburnerandthe restlighting,occupancyandappliances.

The house is a naturallyventilated buildingcontrolled by the occupantswhoprovided dataofthe openingschedules.Tosimu- latethis,theairflownetworkmodelofEnergyPluswasusedwhich offerstheabilitytosimulatemultizoneairflows.Fig.8showsthe airflownetworkused inthemultizone airflow calculation.Wind pressurecoefficientsapplicableto thiscase-study(low-risebuild- ings)wereobtainedfromCIBSEGuideA[18].Theinfiltrationvaries duringthedayduetoairpressuredifferenceswhicharecalculated hourlyfromtheairtemperatureandwinddataoftheweatherfile.

Areferencecondition should be implementedto initiate the cal- culationof both models. Due to the lack of blower door test in theexperimental case-study house,the initial infiltration ofboth buildingsisbasedontheJamaicanregulation.AccordingtotheNa- tionalBuildingCode[19]theinfiltrationrateforbuildingsshallbe assumedtobe0.0017L/sperm²ofthegrossexteriorwall.Forthe simulation ofcool roof, the simpler wayis to neglect the thick- nessandthermalpropertiesofthecoolpaintandredefinetheso- larabsorbanceoftheexterior roofsurface. Thisapproachiscom-

monly used [20] when modellingpaints andother surfacetreat- mentsasthreecoatingsofconventionalcoolpaintadduptoonly about1mmofthicknesswhendry.Althoughthesolarreflectance oftheconventionalroofcouldnotbemeasuredasnosamplesare available, the value is takenas 0.15due to the colour and com- position.Asolarabsorbanceof0.85correspondstoagreyprecast concrete surface. Initial solarreflectance on the cool paintspeci- fied as0.82, soafter paintingthe solarabsorbancevalue is fixed at0.18.Theonlydifferenceofthebeforeandafterpaintinginthe modelsistheroof’ssolarabsorbance.

3.2. Calibrationofthemodel

AMeteonorm[21]weatherfileforKingston,Jamaicawasmodi- fiedusingtheon-sitemeasuredweatherparameterswhichinclude airtemperature,relativehumidityandglobalhorizontalradiation.

In addition to the Global Horizontal Irradiance (GHI) measured, weather filesto be used withEnergyPlus requiretheDirect Nor- mal Irradiance(DNI) plus theDiffuse Horizontal Irradiance (DHI) linkedwiththefollowingformula(whereθ ^{isthesolarzenithan-}

gle):GHI=DHI+DNI×cosθ^{.Forasunnyday,itiscommontoas-}

sume20%ofthevaluemeasuredbythepyranometercomesfrom the diffusecomponent, and80% fromthe direct[22].In Jamaica, the climate is globally sunny all over the year; the assumption isreasonableforthe periodofsimulationforthe calibration.The modifiedweather filecoverstheperiodfrommidJanuary2017to midJune 2017tocoverbothpre-andafter-coolroofperiods.The modifiedwhetherfilewasusedonlyforcalibrationpurposeswhile the typical whether file for Jamaica was used for the modelling presented inSection 4.Calibration of the modelis performedby comparing the experimental observations and simulations of the internal ceiling surfacetemperature and the indoor air tempera- ture,statistically.

Withthemodifiedweather filewithactual climaticdatafrom mid-Januarytomid-June,the simulationresultscanbe compared totheon-sitemeasureddatafairly.Calibrationofthemodelisper- formed by comparing the experimental data and simulation re- sultsof theceiling temperature(internalsurface) andthe indoor airtemperature. Forexample, theupper graphof Fig. 9presents theresultsofthree days’simulation ofaconventional roofingfor theindoortemperatureandtheinsideceilingsurfacetemperature ofthebedroom1.Theon-site-monitoredtemperaturesareinsolid lines, and their corresponding EnergyPlus simulation values (la- belled EP inthe graphlegend) are indashed lines. Atfirst sight, thesimulationresultsseemtoprovideagoodrepresentationofthe measureddata.Thecomputedairtemperaturesandceilingsurface temperatureareclosetothemeasuredtemperatures.

Thesamegraphisdrawnforafterpainting,Fig.9lowergraph, whichexemplifiesthecomparisonbetweenthemeasuredandsim- ulation temperature values with the cool roof. The only differ- enceofthebeforeandafterpaintingmodelistheroof’ssolarab- sorbance. The roof solarabsorptivities are set to0.85 beforeand 0.18aftertheapplicationofthepaint.

Theaccuracyoftheseresultsisessentialtoreviewtheeffectsof coolroofsonthermalcomfortandcoolingenergydemands.Simu- lations are run andoperationaldetails of themodel are changed until the minimum accepted error for a building of this nature isachieved.Trialanderrorapproachisadopteduntil thesimula- tionresultsarewithinareasonablemargin(10%).Theresultsshow goodagreementsfortheoverallperiod,especiallyafterroofrefur- bishment (Fig. 10). Before painting, simulations under-predict in- ternalairtemperatureswhilethepredictionofinternalceilingsur- facetemperatureissplitbetweenoverandunder-prediction.From mid-January to mid-March(before painting), 75.1% of the hourly pointsare inthe 10% margin. Beforethe useof thecool coating, there isa meandifference of livingroom indoor airtemperature

Table 1

External fabric and thermal data for the three case-study houses.

Jamaica Ghana Recife-Brazil

Orientation Exposed facades with windows: East-West Exposed facades : East-West Exposed facades with windows : NE and SW

Blind exposed facade : North Blind exposed facade : North

Floor/Roof area m ² 36 36 91

Volume m ³ 88 88 236

Exposed to ambient ext wall area m ²

54.5 54.5 65

three walls, facing north/east/west three walls, facing north/east/west two walls, facing north-east and south west

Window area m ² 6 single glazed windows – (5 = 0.7 ×0.98 m ², 1 = 0.42 ×0.43 m ²) 3.6 m ²

6 single glazed windows –

(5 = 0.7 ×0.98 m ², 1 = 0.42 ×0.43 m ²) 3.6 m ²

6 single glazed windows – (1.5 ×2 m ²) = 18 m ² Window opening

schedule Manually controlled according to occupancy Manually controlled according to

occupancy Manually controlled accroding to

occupancy

Occupants 1 1 4

Occupancy Schedule

Working occupant: at home at night and weekends Working occupant: at home at night and weekends

Working family: : at home at night and weekends

Internal heat gains Lighting: 70 W Lighting: 70 W Lighting: 145 W

Equipment: 1960 W (kitchen: 1800 W) Equipment: 1960 W (kitchen: 1800 W) Equipment: 2700 W (kitchen: 2400 W) Material Thickness [cm] U-Value [W/m ²K] Material Thickness

[cm]

U-Value [W/m ²K]

Material Thickness [cm]

U-Value [W/m ²K]

Walls Precast concrete 4 5.91 Mud brick 19 2.48 Clay brick

with plaster

14 2.48

Roof Precast concrete 8 5.68 Metal sheet

with plaster

4 5.68 Terracotta

tile without ceiling

2 4.55

Floor Concrete with tiles 10 4.19 Concrete 10 4.19 Concrete

with tiles

10 4.19

Fig. 8 . Plan view of the airflow network showing possible airflow pattern.

of2.3°C. Forthesecond period(mid-Aprilto mid-June),99.3%of thehourlypointsareinthe10%margin.Afterthepainting,thereis ameandifferenceoflivingroomindoorairtemperatureof1.1°C.

Asmentionedbefore,theresultsshowgoodagreementsforthe overallperiod,especiallyafterroofrefurbishment.However,many factors can createsome uncertainties. First,the measured datais expectedto bemorerandom asitisprone tosudden changesin weather conditionswhich can often occur multiple times within anhour,whilethesimulationonlyusestheweatherdatarecorded athourlyintervals. Moreover,theassumptionof80%oftheglobal

horizontalradiationis direct isnot all the time accurate.Cloudi- ness level is not measured and difficult to evaluate. Inclement weather andlackof directsolar radiationscan explain some dif- ferences between the measurement and the simulation. Besides, theuncertaintiesabouttheoccupancyscheduleandoccupantbe- haviourcouldeasilyexplainsomeoftheobserveddifferenceswith themeasuredtemperatureevolution.

The accuracy of the simulation was also checked using the MeanBiasError(MBE)andtheCoefficientofVariationoftheRoof MeanSquareError(CVRMSE).Theseindicesaredefinedasfollows: