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Albedo effect of external surfaces on the energy loads and thermal comfort in buildings
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International Conference On Materials And Energy 2015, ICOME 15, 19-22 May 2015, Tetouan, Morocco, and the International Conference On Materials And Energy 2016, ICOME 16, 17-20 May 2016, La Rochelle, France The 15th International Symposium on District Heating and Cooling
Albedo effect of external surfaces on the energy loads and thermal Assessing the feasibility ofin using the heat demand-outdoor comfort buildings temperature function for a,b,c, a long-term district heat demand forecast b c Ouarda.Mansouri *, Rafik.Belarbi , Fatiha.Bourbia a a b c Département de Génie-Civil, Faculté de, Technologie, Université Skikda, 21000, Algeria I. Andrića,b,c *, A. Pina , P. Ferrão J. Fournier ., B.de Lacarrière , O. Le Correc a
b
a
Laboratoire des Sciences de l’Ingénieur pour l’Environnement “LaSIE”. Université La Rochelle, 17042. France IN+ Center forc Innovation, and Bioclimatique Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 LaboratoireTechnology d’Architecture et Environnement “ABE”. Université Constantine3. Algeria Lisbon, Portugal b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France
Abstract Abstract Due to climate changes which are growing through time, in addition to the building consumption, considered as the largest sector in energy consumption in the world, the external texture and color of materials covering the urban areas (pavements, facades and District heating networksrole are in commonly addressed inand theaffecting literaturetheasinside one of most effective solutions depending for decreasing the roofs), plays an important energy consumption andtheoutside thermal ambiance, on the greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat climate variations. This study to examine and analyze the and effectbuilding and performance reflective coatings of in thethe building in the sales. Dueattempts to the changed climate conditions renovationofpolicies, heat demand futureenvelope could decrease, context of indoor thermal comfort energy efficiency. prolonging the investment return and period. The applied a housing building in aofMediterranean of Algeria, in particular the for cityheat of Skikda. Thestudy mainwas scope of thisforpaper is to assess the situated feasibility using the heatclimate demandzone – outdoor temperature function demand The study was carried out considering the variation of the building envelope parameters seeing, orientation, outside reflectivity, forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 thermal mass and insulation the externalperiod walls of edifice. The results obtained from (low, modeling simulation, usingthree TRNSYS buildings that vary in bothofconstruction andthetypology. Three weather scenarios medium, high) and district software, shown that the thermal insulation combined with orientation high solar reflectance, has heat influenced a positive renovation scenarios were developed (shallow, intermediate, deep). Toand estimate the error, obtained demandinvalues were way the indoor thermal levels anddemand energymodel, efficiency towardsdeveloped a better environment forby thethe inhabitants. compared withair, results fromcomfort a dynamic heat previously and validated authors. The results showed that when only weather change is considered, the margin of error could be acceptable for some applications ©(the 2017 TheinAuthors. by Elsevier Ltd.20% for all weather scenarios considered). However, after introducing renovation error annual Published demand was lower than Peer-review under responsibility of theupscientific committee of ICOME 2015 and ICOME 2016.scenarios combination considered). scenarios, the error value increased to 59.5% (depending on the weather and renovation The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the Keywords: reflectance, energy loads,hours orientation, thermalduring mass, insulation, thermal comfort decrease Solar in the number of heating of 22-139h the heating season (depending on the combination of weather and renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations. © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. * Corresponding author. Tel.: +213-3874-0736; Keywords: Heat demand; Forecast; Climate change E-mail address: [email protected] (O.Mansouri). 1876-6102 © 2017 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the scientific committee of ICOME 2015 and ICOME 2016. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of ICOME 2015 and ICOME 2016 10.1016/j.egypro.2017.11.255
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1. Introduction Mineralization of urban spaces and the reduction of green areas which were replaced by roads, roofs and facades whose materials absorb heat and increase the discharges of long wavelength, cause the warming of these spaces, and generate what is called the urban heat island. To remedy this problem and try to mitigate its intensity, great interest was focused on surface materials, especially those that have a high reflectivity "high albedo" to minimize the absorption of solar radiation and therefore, reduce its restitution in long wavelength at night. Much research has proven the positive effect of reflective materials on the urban microclimate moderation, on mitigating the outside temperatures and the increasing degrees of comfort for pedestrians and users of outdoor urban spaces [1]. What about its effect on the indoor environment and energy consumption? Solar radiation incident on building envelope can be absorbed, reflected or transmitted. It influences exterior and interior surface temperature. Heat flux enters the building in several different modes, namely by conduction and convection through four walls, roof and floor, by convection in the form of ventilation and infiltration, and by direct gain through glazed area of windows. A multitude of research, old and new, experimental and numerical were conducted in this direction; showed the positive effect of reflective coatings on the mitigation of indoor temperatures and energy consumption of buildings. These solar reflective coatings can be applied to the horizontal wall "roof" by doing what is called "cool roof", or the vertical wall (facade), their application allows a reflect of maximum sunlight instead of absorbing it. This application reduces the outer surface temperature, decreases heat flow entering the building and therefore decreases the inner room temperature. Citing some work in this direction: Givoni and Hoffman [2] performed early experiments on small buildings with different exterior colors in Israel, They compared the resulting indoor temperature for unventilated buildings. They found that buildings with white-colored walls were approximately 3°C cooler in summer than when the same buildings were painted gray. Reagan and Acklam [3] done a study that shows that while changing the roof color from dark (α=0.75) to light (α=0.35), it does greatly reduce the roof heat gain. The reductions were 6.4% and 4.8% in July day in Tucson, Arizona, for houses with ceiling thermal resistances of 2.50 and 5.88m²k/w respectively. Taha et al. [4] simulated building cooling load reduction of 18.9% for summer days in Sacramento, California, for an albedo increase of both roof and walls from 0.30 to 0.90. Bansal et al. [5] have studied experimentally as well as theoretically the effect of external surface colour on the thermal behavior of a building, they found that the black painted enclosure recorded a maximum of 7°C higher temperature than the corresponding white painted enclosure during hours of maximum solar radiation. Taha at al. [6] found that white electrometric coatings with a reflectivity of over 0.72 could be as high as 45°C cooler than black coatings with a reflectivity of 0.08, in varying reflectivity materials used in urban surfaces. Parker et al. [7] monitored six homes in Florida before and after application of high-albedo coatings on their roofs. Reduction in air-conditioning electricity consumption was measured between 11% and 43% with an average saving of 9.2kwh/day, and reduction in peak power demand (occurs between 5 and 6 pm) was 0.4–1.0kw with an average reduction of 0.7kw. Akbari et al. [8] monitored peak power and cooling energy savings from high-albedo coatings at one house and two school bungalows in Sacramento, California. They found savings of 2.2kwh/d for one house (80% of base case use), and peak demand reductions of 0.6 kw. In the school bungalows, cooling energy was reduced 3.1kwh/d (35% of base case use), and peak demand by 0.6kw. Simpson and McPherson [9], Reductions in total and peak air-conditioning load of approximately 5% were measured for two identical white (SR≈0.75) compared to gray (SR≈0.30) and silver (SR≈0.50) roofed scale model buildings in Tucson Arizona. Shariah et al. [10] carried out a series of simulations for two mild and hot climates in Jordan. They found that, as the reflectance changes from 0 to 1, the total energy load decreases by 32% and 47% for non-insulated buildings and by 26% and 32% for insulated buildings in Amman and Aqaba respectively. Cheng et al. [11] performed investigation with test cells about the effect of envelope colour and thermal mass on indoor temperatures under hot and humid weather condition. They showed that the maximum difference of inside air temperature between a black and a white cell was about 12°C for lightweight construction. Synnefa et al. [12] studied the impact from using cool roof coatings on the cooling and heating loads and estimated the indoor thermal comfort conditions of residential buildings for various climatic conditions. The results show that increasing the roof solar reflectance reduces cooling loads by 18–93% and peak cooling demand in air-conditioned buildings by 11–27%. Zinzi et al. [13] evaluated the solar properties of ecological cool coatings and the benefit achievable for building applications for different Mediterranean localities. They showed that there is an influence of
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cool materials on the energy performance in all zones especially where higher insulation levels coupled with solar control lead to strong energy reductions and have consequent improvement of thermal conditions inside the built environment. Uemoto et al. [14] demonstrated that the cool colored paint formulations produced significantly higher near infrared radiation reflectance than conventional paints of similar colors, and that the surface temperatures were more than 10°C lower than those of conventional paints when exposed to infrared radiation. Shen et al. [15] performed an experimental study on the impact of reflective coatings on indoor environment and building energy consumption. They found that exterior and interior surface temperatures can be reduced by up to 20°C and 4.7°C respectively using different coatings, depending on location, season and orientation. The maximum reduction in globe temperature and mean radiant temperature was 2.3°C and 3.7°C in that order. For the conditioned case, the annual reduction in electricity consumption for electricity reached 116kwh. For their part, Bozonnet et al. [16] studied the impact of cool roofs on building thermal response in French context through experimental study which was completed by dynamic simulations. They found that cool roof decreases the mean outside surface temperature by more than 10°C, with low differences for lower temperatures, but a strong impact on the highest temperatures. In another recent study, Pisello et al [17] applied a cool roof as an innovative technique to reduce building energy requirements for cooling and to improve indoor thermal comfort conditions. Their solution consists of a prototyped cool clay tile on a traditional residential building in central Italy. They found that a maximum effect of decreasing summer peak indoor overheating of the attic by up to 4.7°C. The corresponding winter maximum overcooling reduction is 1.2°C. Lapisa et al. [18] from their part modeled the behavior of a building "reference" in order to study the main parameters that impact on energy demand and comfort. It is shown that the simultaneous use of "cool roof" and the natural ventilation combined with a high thermal inertia of the building may be a sufficient passive cooling solution in summer from commercial buildings. 2. Analysis and modelling To estimate the effect of the reflectivity of materials (albedo) combined with other factors (orientation, Thermal mass and thermal insulation) on the annual energy needs for heating and cooling and thermal comfort, a parametric study was carried out on a building reference situated in Skikda city, in North-east of Algeria (latitude 36.54°N and a longitude of 6.52°E). This building represents the most common type of buildings in the region in a Mediterranean climate context (hot and humid in summer and mild with low amplitudes in winter). The simulations of this study are performed for 02 years by the mean of TRNSYS17 [19] “Transient System Simulation”, with a time step of one hour. The building used in this study which represents a reference case is a parallelepiped of 10m length, 7m width and 3m heigth, it consists of cinderblock walls with flat roof (not accessible). We considered for these simulations that the reflectivity of the roof is fixed to a value of 0.5 but the reflectivity of the walls is changed between 0.1 and 0.9 (knowing that a perfectly reflecting or absorbing material does not exist in reality). The building orientation is varied in the range (90, 270) towards the south with step of 45°. The roof has a fixed u-value of 2.42, but wall facades are varied of 1.57 for cinderblock wall, and 1.69 for concrete wall, the two main materials used in the region. Simulations are performed with and without insulation of an air void layer of 5cm thick. The first simulations are performed for opaque walls in order to deduce their effect, then repeated by introducing a glazed surface of 6m² to the south side of the building to deduct the influence of penetration of solar radiation on the energy needs. The window has a wooden frame with single glazing of 4mm thick, and g-value of 0.87. Set point temperatures are 20°C and 26°C for heating and cooling respectively. Moreover, internal gains of human that define the conditions of comfort such as "clothes" and "metabolic activity" are taken from ISO7730. Infiltration and ventilation are considered null to deduct the net effect of the reflectivity of the opaque walls on actual energy needs and the inner ambience in the absence of air sweeping. The same simulations were repeated under free floating conditions to estimate the effect of change solar reflectance on comfort conditions in the building. The characteristics of the different envelopes and the conditions of dynamic thermal simulation are summarized in the following table:
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Table.1. Summary of the different parameters considered in the simulation Building Element
Characteristics Solar reflectivity Orientation
Concrete wall Cinderblock wall Building surface Building volume Set cooling temperature Set heating temperature
0.1-0.9 0.1-0.9 70m² 210 m3 26°C 20°C
90-270 90-270
U-value (w/m².k) 1.69 1.57
3. Results and discussion The analysis of the results of all these simulations allowed us to highlight the effect of the reflectivity (albedo) of the opaque vertical wall on energy requirements and thermal comfort, as well as modification of this effect once the glazing introduced. 3.1. Energy loads The analysis of the parametric study results is expressed by the following graphs according to various different parameters: Solar reflectance effect: According to the graph, we note that energy needs are inversely proportional to the values of the solar reflectivity. The reference building with a reflectivity of 0.5 presents an annual energy needs for heating of 37.81kwh/m².y, and cooling needs of 56.36kwh/m².y, which will make a total of 94.18kwh/m².y. When changing the coating of outer surfaces of the walls by a reflective coating with a value of 0.9, the heating needs increase by 8.65kwh/m².y with a percentage of 18.61%, the need for cooling decrease by 12.41kwh/m².y, making a percentage of 22%. Hence, the total needs decreased by 3.77kwh/m².y equivalent to a percentage of about 4%. However, the allocation of a reflectivity value of 0.1 (absorbing materials) for facade walls gives opposite results compared with the reference building. In fact, the heating needs decreased by 7.25kwh/m².y for a percentage of 19.17%. The cooling requirements increased by 13.44kwh/m².y, giving a percentage of 19.25%, hence the total requirements increased by 6.17kwh/m².y, where a percentage of 6.14%.
Fig.1. Solar reflectance effect on annual energy loads for cinderblock insulated building oriented South
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We can therefore conclude that the external colour of a building affects the thermal performance of the building although its impact is little in our climate. Orientation effect: The effect of orientation shows very small nuances between the different simulated orientations, either for heating demand, air conditioning or total requirements. The better is the orientation 0 (South) with a minimum total annual demand of 94.18kwh/m².y. The one with the maximum demand is the orientation 45 (South-West) with total needs of 95.96kwh/m².y which is not far from the South-East orientation (315) with a value of 95.94kwh/m².y, where as two remaining orientations 90 and 270 (East and West) have exactly the same needs for heating, cooling and total needs. Fig.2. summarizes these results.
Fig.2. Orientation effect on annual energy loads for building with cinderblock walls and solar reflectivity of 0.5
It can be deduced that for cinderblock walls and solar reflectance value of 0.5, the orientation of the building plays a negligible role in mitigating the energy needs. Type of material and insulation effect: The change in the constitution of the vertical wall of the building between the cinderblock wall and concrete wall gave approximately similar results. The heating requirements for buildings with cinderblock wall are 48.23kwh/m².year while for the concrete walls, they are 51.19kwh/m².y, hence an increase of 2.96kwh/m².y representing a percentage of 5.78%. The cooling needs are 55.84kwh/m².y for cinderblock walls and 54.63kwh/m².y for concrete walls, where the difference is 1.21kwh/m².y and the percentage is 2.16%. The total requirement in turn spend 104.07kwh/m².y for the first case to 105.82kwh for the second case giving a difference of 1.65%. This small difference is due to the thermal mass of the two materials which is almost nearly the same. The effect of the insulation is estimated by comparing the same type of wall with and without insulation. For the cinderblock wall, heating requirements when the insulation is applied are less by 10.42kwh to the needs obtained for a wall without insulation, with a percentage of 21.60%, whereas the cooling needs increased insignificantly. As consequence, the total needs decrease is 9.89kwh/m².y with a percentage of 9.5%. For the concrete wall, the same trend is noticed, the heating needs for the insulated wall are lower than those of the uninsulated wall about 12.08kwh, where a percentage of 23.59%. The change in the field of cooling requirements is negligible, which results in a reduction of the total needs of 11.29kwh with a percentage of 10.66% between the concrete wall with and without insulation. Furthermore, the change in the annual energy needs for heating, cooling and total requirements between the two walls when the insulation is applied show the same trends as the change between the 2 types of walls without insulation, i.e. negligible results. This is due to the almost similarity of thermal mass of the two walls.
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Fig.3. Type materials and insulation effect on annual energy loads for solar reflectivity of 0.5 and orientation 0
After introducing a window in the south wall of the building, the energy needs have changed. For the cinderblock wall facing south, with a reflectivity of 0.5, there was a decrease in the heating needs of 6.14kwh/m².y i.e. with a percentage of 16.23%. While the cooling requirements have increased of 4.79kwh/m².y with a percentage of 7.83% resulting in a decrease in total needs of 1.35kwh/m².y, so a percentage of 1.43%. For the reflectivity of 0.1, the heating needs for the pierced wall decrease of 4.48kwh/m².y at a percentage of 14.65%. The cooling needs are increased by 3.87kwh/m².y, resulting in a percentage of 5.25% .This gives a negligible decrease in the total requirements. For the reflectivity of 0.9, the heating requirements decreased of 8.11kwh/m².y with a percentage of 17.45%. The need for cooling increased of 5.54kwh/m².y where a percentage of 11.19%, and total requirements decreased to a
very low rate of approximately 2.84%, as it is showed in the following figure.
Fig.4. Solar reflectance effect on annual energy loads for cinderblock insulated building oriented South with glazing
We can conclude that introducing window in the south wall can dilute the benefit effect of the high reflectance of a building envelope on the energy demand. 3.2. Thermal comfort To estimate the summer comfort inside the building, we repeated the same simulations without air conditioning (free floating conditions), we took a temperature threshold of 27°C, beyond which we will record the numbers of hours of discomfort. We will present the results obtained after the change in the reflectivity of the façade walls. The number of hours of discomfort for the reference building is from 3088 hours so a percentage of 35%. As expected, increasing the reflectivity of the wall cladding decreases the number of hours of discomfort. For example, for the reflectivity value of 0.9, the number of hours of discomfort will be reduced to 2603 hours equivalent to a
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percentage of 30%. And if we apply a coating with a reflectivity of 0.1, we note that the number of hours of discomfort increases to 3441 hours, resulting in a percentage of 39%. We can conclude that the reflectivity of wall coatings materials plays a role in improving summer comfort conditions inside the building, beyond 0.5, the interior ambience is moderated by reducing the number of hours of discomfort. 4. Conclusion The aim of this study is to estimate the effect of solar reflectivity (albedo) with other parameters on energy consumption and thermal comfort in Mediterranean climatic context. It was found that all the parameters play an important role when they are implemented. The best combination that could be obtained was the one with light colored coatings, applied on the outer walls of an albedo value greater than 0.5, combined with a thermal insulation. This study has as perspective the determination of database , with the right combination of insulation, orientation, thermal mass and of course the high reflectivity coatings of the outer shell of the building, useful for stakeholders (designers, architects, policy makers ... ect) to optimize the energy balance of the building throughout the year and its corollary thermal comfort. References [1] Rosenfeld, A. H, Akbari, H, Bretz, S, Fishman, B. L, Kurn, D. M, Sailor, D, and Taha, H, “Mitigation of urban heat islands: materials,utility programs, updates,” Energy Build., 1995, vol. 22, no. 3, pp. 255–265. [2] Givoni, B,and Hoffiman, M.E, , “Effect of building materials on internal temperatures,” Res. Report, Build. Res. Station. Tech. Haifa, 1968 [3] Reagan, J. A, and Acklam D. M, “Solar reflectivity of common building materials and its influence on the roof heat gain of typical southwestern U.S.A. residences,” Energy Build., 1979, vol. 2, no. 3, pp. 237–248. [4] Taha, H, Akbari, H, Rosenfeld, A, and Huang J, “Residential cooling loads and the urban heat island—the effects of albedo,” Build. Environ., 1988, vol. 23, no. 4, pp. 271–283. [5] Bansal, N. K, Garg, S. N, and Kothari, S, “Effect of exterior surface colour on the thermal performance of buildings,” Build. Environ., 1992, vol. 27, no. 1, pp. 31–37. [6] Taha, H, Sailor, D, Akbari, H, “High albedo materials for reducing cooling energy use,” Lawrence Berkeley Lab. 1992, Rep. 31721, vol. UC530, Berkeley CA. [7] Parker, D.S, Barkaszi, S.F, Sonne, J.K, “Measured cooling energy savings from reflective roof coatings in Florida,”, 1994, Phase II report, Rep. No.FSEC-CR-699-94, Florida Sol. Energy Center, Cape Canaveral, FL. [8] Akbari, H, Bretz, S, Kurn, D. M, and Hanford, J, “Peak power and cooling energy savings of high-albedo roofs,” Energy Build., 1997, vol. 25, no. 2, pp. 117–126. [9] Simpson, J. R, and Mcpherson, E. G, “The effects of roof albedo modification on cooling loads of scale model residences in Tucson , Arizona,” 1997, vol. 25. [10] Shariah, A, Shalabi, B, Rousan, A, and Tashtoush, B, “Effects of absorptance of external surfaces on heating and cooling loads of residential buildings in Jordan,” Energy Convers. Manag., 1998, vol. 39, no. 3–4, pp. 273–284. [11] Cheng, V, Ng, E, and Givoni, B, “Effect of envelope colour and thermal mass on indoor temperatures in hot humid climate,” Sol. Energy, 2005, vol. 78, no. 4 SPEC. ISS., pp. 528–534. [12] Synnefa, A, Santamouris, M, and Akbari, H, “Estimating the effect of using cool coatings on energy loads and thermal comfort in residential buildings in various climatic conditions,” Energy Build., 2007, vol. 39, no. 11, pp. 1167–1174. [13] Zinzi, M, Daneo, A, and Fanchiotti, A, “Paper No 314: Optical properties and influence of reflective coatings on the energy demand and thermal comfort in dwellings at Mediterranean latitudes,” PLEA 2008 – 25th Conf. Passiv. Low Energy Archit. Dublin, 22nd to 24th Oct. 2008, no. 314. [14] Uemoto, K. L, Sato, N. M. N, and John, V. M, “Estimating thermal performance of cool colored paints,” Energy Build., 2010, vol. 42, no. 1, pp. 17–22. [15] Shen, H, Tan, H, and Tzempelikos, A, “The effect of reflective coatings on building surface temperatures , indoor environment and energy consumption — An experimental study,” 2011, vol. 43, pp. 573–580. [16] Bozonnet, E, Doya, M, and Allard, F, “Cool roofs impact on building thermal response: A French case study,” Energy Build., 2011, vol. 43, no. 11, pp. 3006–3012. [17] Pisello, A.L, Cotana,F, " The thermal effect of an innovative cool roof on residential buildings in Italy: Results from two years of continuous monitoring," Energy and Buildings, 2014, vol.69 pp.154–164. [18] Lapisa, R, "Etude du rafraîchissement passif des bâtiments commerciaux ou industriels", Thèse de Doctorat, Université de La Rochelle.2015 [19] “TRNSYS 17”, T. S. S. P. U. Manuel, Sol. Energy Lab. Univ. Wisconsin, Madison, USA.
ScienceDirect Availableonline onlineatatwww.sciencedirect.com www.sciencedirect.com Available Energy Procedia 00 (2017) 000–000
ScienceDirect ScienceDirect
www.elsevier.com/locate/procedia
Energy (2017) 000–000 571–577 EnergyProcedia Procedia139 00 (2017) www.elsevier.com/locate/procedia
International Conference On Materials And Energy 2015, ICOME 15, 19-22 May 2015, Tetouan, Morocco, and the International Conference On Materials And Energy 2016, ICOME 16, 17-20 May 2016, La Rochelle, France The 15th International Symposium on District Heating and Cooling
Albedo effect of external surfaces on the energy loads and thermal Assessing the feasibility ofin using the heat demand-outdoor comfort buildings temperature function for a,b,c, a long-term district heat demand forecast b c Ouarda.Mansouri *, Rafik.Belarbi , Fatiha.Bourbia a a b c Département de Génie-Civil, Faculté de, Technologie, Université Skikda, 21000, Algeria I. Andrića,b,c *, A. Pina , P. Ferrão J. Fournier ., B.de Lacarrière , O. Le Correc a
b
a
Laboratoire des Sciences de l’Ingénieur pour l’Environnement “LaSIE”. Université La Rochelle, 17042. France IN+ Center forc Innovation, and Bioclimatique Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 LaboratoireTechnology d’Architecture et Environnement “ABE”. Université Constantine3. Algeria Lisbon, Portugal b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France
Abstract Abstract Due to climate changes which are growing through time, in addition to the building consumption, considered as the largest sector in energy consumption in the world, the external texture and color of materials covering the urban areas (pavements, facades and District heating networksrole are in commonly addressed inand theaffecting literaturetheasinside one of most effective solutions depending for decreasing the roofs), plays an important energy consumption andtheoutside thermal ambiance, on the greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat climate variations. This study to examine and analyze the and effectbuilding and performance reflective coatings of in thethe building in the sales. Dueattempts to the changed climate conditions renovationofpolicies, heat demand futureenvelope could decrease, context of indoor thermal comfort energy efficiency. prolonging the investment return and period. The applied a housing building in aofMediterranean of Algeria, in particular the for cityheat of Skikda. Thestudy mainwas scope of thisforpaper is to assess the situated feasibility using the heatclimate demandzone – outdoor temperature function demand The study was carried out considering the variation of the building envelope parameters seeing, orientation, outside reflectivity, forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 thermal mass and insulation the externalperiod walls of edifice. The results obtained from (low, modeling simulation, usingthree TRNSYS buildings that vary in bothofconstruction andthetypology. Three weather scenarios medium, high) and district software, shown that the thermal insulation combined with orientation high solar reflectance, has heat influenced a positive renovation scenarios were developed (shallow, intermediate, deep). Toand estimate the error, obtained demandinvalues were way the indoor thermal levels anddemand energymodel, efficiency towardsdeveloped a better environment forby thethe inhabitants. compared withair, results fromcomfort a dynamic heat previously and validated authors. The results showed that when only weather change is considered, the margin of error could be acceptable for some applications ©(the 2017 TheinAuthors. by Elsevier Ltd.20% for all weather scenarios considered). However, after introducing renovation error annual Published demand was lower than Peer-review under responsibility of theupscientific committee of ICOME 2015 and ICOME 2016.scenarios combination considered). scenarios, the error value increased to 59.5% (depending on the weather and renovation The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the Keywords: reflectance, energy loads,hours orientation, thermalduring mass, insulation, thermal comfort decrease Solar in the number of heating of 22-139h the heating season (depending on the combination of weather and renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations. © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. * Corresponding author. Tel.: +213-3874-0736; Keywords: Heat demand; Forecast; Climate change E-mail address: [email protected] (O.Mansouri). 1876-6102 © 2017 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the scientific committee of ICOME 2015 and ICOME 2016. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of ICOME 2015 and ICOME 2016 10.1016/j.egypro.2017.11.255
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Ouarda Mansouri et al.Procedia / Energy 00 Procedia (2017) 571–577 W. Mansouri et al. / Energy (2017)139 000–000
1. Introduction Mineralization of urban spaces and the reduction of green areas which were replaced by roads, roofs and facades whose materials absorb heat and increase the discharges of long wavelength, cause the warming of these spaces, and generate what is called the urban heat island. To remedy this problem and try to mitigate its intensity, great interest was focused on surface materials, especially those that have a high reflectivity "high albedo" to minimize the absorption of solar radiation and therefore, reduce its restitution in long wavelength at night. Much research has proven the positive effect of reflective materials on the urban microclimate moderation, on mitigating the outside temperatures and the increasing degrees of comfort for pedestrians and users of outdoor urban spaces [1]. What about its effect on the indoor environment and energy consumption? Solar radiation incident on building envelope can be absorbed, reflected or transmitted. It influences exterior and interior surface temperature. Heat flux enters the building in several different modes, namely by conduction and convection through four walls, roof and floor, by convection in the form of ventilation and infiltration, and by direct gain through glazed area of windows. A multitude of research, old and new, experimental and numerical were conducted in this direction; showed the positive effect of reflective coatings on the mitigation of indoor temperatures and energy consumption of buildings. These solar reflective coatings can be applied to the horizontal wall "roof" by doing what is called "cool roof", or the vertical wall (facade), their application allows a reflect of maximum sunlight instead of absorbing it. This application reduces the outer surface temperature, decreases heat flow entering the building and therefore decreases the inner room temperature. Citing some work in this direction: Givoni and Hoffman [2] performed early experiments on small buildings with different exterior colors in Israel, They compared the resulting indoor temperature for unventilated buildings. They found that buildings with white-colored walls were approximately 3°C cooler in summer than when the same buildings were painted gray. Reagan and Acklam [3] done a study that shows that while changing the roof color from dark (α=0.75) to light (α=0.35), it does greatly reduce the roof heat gain. The reductions were 6.4% and 4.8% in July day in Tucson, Arizona, for houses with ceiling thermal resistances of 2.50 and 5.88m²k/w respectively. Taha et al. [4] simulated building cooling load reduction of 18.9% for summer days in Sacramento, California, for an albedo increase of both roof and walls from 0.30 to 0.90. Bansal et al. [5] have studied experimentally as well as theoretically the effect of external surface colour on the thermal behavior of a building, they found that the black painted enclosure recorded a maximum of 7°C higher temperature than the corresponding white painted enclosure during hours of maximum solar radiation. Taha at al. [6] found that white electrometric coatings with a reflectivity of over 0.72 could be as high as 45°C cooler than black coatings with a reflectivity of 0.08, in varying reflectivity materials used in urban surfaces. Parker et al. [7] monitored six homes in Florida before and after application of high-albedo coatings on their roofs. Reduction in air-conditioning electricity consumption was measured between 11% and 43% with an average saving of 9.2kwh/day, and reduction in peak power demand (occurs between 5 and 6 pm) was 0.4–1.0kw with an average reduction of 0.7kw. Akbari et al. [8] monitored peak power and cooling energy savings from high-albedo coatings at one house and two school bungalows in Sacramento, California. They found savings of 2.2kwh/d for one house (80% of base case use), and peak demand reductions of 0.6 kw. In the school bungalows, cooling energy was reduced 3.1kwh/d (35% of base case use), and peak demand by 0.6kw. Simpson and McPherson [9], Reductions in total and peak air-conditioning load of approximately 5% were measured for two identical white (SR≈0.75) compared to gray (SR≈0.30) and silver (SR≈0.50) roofed scale model buildings in Tucson Arizona. Shariah et al. [10] carried out a series of simulations for two mild and hot climates in Jordan. They found that, as the reflectance changes from 0 to 1, the total energy load decreases by 32% and 47% for non-insulated buildings and by 26% and 32% for insulated buildings in Amman and Aqaba respectively. Cheng et al. [11] performed investigation with test cells about the effect of envelope colour and thermal mass on indoor temperatures under hot and humid weather condition. They showed that the maximum difference of inside air temperature between a black and a white cell was about 12°C for lightweight construction. Synnefa et al. [12] studied the impact from using cool roof coatings on the cooling and heating loads and estimated the indoor thermal comfort conditions of residential buildings for various climatic conditions. The results show that increasing the roof solar reflectance reduces cooling loads by 18–93% and peak cooling demand in air-conditioned buildings by 11–27%. Zinzi et al. [13] evaluated the solar properties of ecological cool coatings and the benefit achievable for building applications for different Mediterranean localities. They showed that there is an influence of
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cool materials on the energy performance in all zones especially where higher insulation levels coupled with solar control lead to strong energy reductions and have consequent improvement of thermal conditions inside the built environment. Uemoto et al. [14] demonstrated that the cool colored paint formulations produced significantly higher near infrared radiation reflectance than conventional paints of similar colors, and that the surface temperatures were more than 10°C lower than those of conventional paints when exposed to infrared radiation. Shen et al. [15] performed an experimental study on the impact of reflective coatings on indoor environment and building energy consumption. They found that exterior and interior surface temperatures can be reduced by up to 20°C and 4.7°C respectively using different coatings, depending on location, season and orientation. The maximum reduction in globe temperature and mean radiant temperature was 2.3°C and 3.7°C in that order. For the conditioned case, the annual reduction in electricity consumption for electricity reached 116kwh. For their part, Bozonnet et al. [16] studied the impact of cool roofs on building thermal response in French context through experimental study which was completed by dynamic simulations. They found that cool roof decreases the mean outside surface temperature by more than 10°C, with low differences for lower temperatures, but a strong impact on the highest temperatures. In another recent study, Pisello et al [17] applied a cool roof as an innovative technique to reduce building energy requirements for cooling and to improve indoor thermal comfort conditions. Their solution consists of a prototyped cool clay tile on a traditional residential building in central Italy. They found that a maximum effect of decreasing summer peak indoor overheating of the attic by up to 4.7°C. The corresponding winter maximum overcooling reduction is 1.2°C. Lapisa et al. [18] from their part modeled the behavior of a building "reference" in order to study the main parameters that impact on energy demand and comfort. It is shown that the simultaneous use of "cool roof" and the natural ventilation combined with a high thermal inertia of the building may be a sufficient passive cooling solution in summer from commercial buildings. 2. Analysis and modelling To estimate the effect of the reflectivity of materials (albedo) combined with other factors (orientation, Thermal mass and thermal insulation) on the annual energy needs for heating and cooling and thermal comfort, a parametric study was carried out on a building reference situated in Skikda city, in North-east of Algeria (latitude 36.54°N and a longitude of 6.52°E). This building represents the most common type of buildings in the region in a Mediterranean climate context (hot and humid in summer and mild with low amplitudes in winter). The simulations of this study are performed for 02 years by the mean of TRNSYS17 [19] “Transient System Simulation”, with a time step of one hour. The building used in this study which represents a reference case is a parallelepiped of 10m length, 7m width and 3m heigth, it consists of cinderblock walls with flat roof (not accessible). We considered for these simulations that the reflectivity of the roof is fixed to a value of 0.5 but the reflectivity of the walls is changed between 0.1 and 0.9 (knowing that a perfectly reflecting or absorbing material does not exist in reality). The building orientation is varied in the range (90, 270) towards the south with step of 45°. The roof has a fixed u-value of 2.42, but wall facades are varied of 1.57 for cinderblock wall, and 1.69 for concrete wall, the two main materials used in the region. Simulations are performed with and without insulation of an air void layer of 5cm thick. The first simulations are performed for opaque walls in order to deduce their effect, then repeated by introducing a glazed surface of 6m² to the south side of the building to deduct the influence of penetration of solar radiation on the energy needs. The window has a wooden frame with single glazing of 4mm thick, and g-value of 0.87. Set point temperatures are 20°C and 26°C for heating and cooling respectively. Moreover, internal gains of human that define the conditions of comfort such as "clothes" and "metabolic activity" are taken from ISO7730. Infiltration and ventilation are considered null to deduct the net effect of the reflectivity of the opaque walls on actual energy needs and the inner ambience in the absence of air sweeping. The same simulations were repeated under free floating conditions to estimate the effect of change solar reflectance on comfort conditions in the building. The characteristics of the different envelopes and the conditions of dynamic thermal simulation are summarized in the following table:
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Table.1. Summary of the different parameters considered in the simulation Building Element
Characteristics Solar reflectivity Orientation
Concrete wall Cinderblock wall Building surface Building volume Set cooling temperature Set heating temperature
0.1-0.9 0.1-0.9 70m² 210 m3 26°C 20°C
90-270 90-270
U-value (w/m².k) 1.69 1.57
3. Results and discussion The analysis of the results of all these simulations allowed us to highlight the effect of the reflectivity (albedo) of the opaque vertical wall on energy requirements and thermal comfort, as well as modification of this effect once the glazing introduced. 3.1. Energy loads The analysis of the parametric study results is expressed by the following graphs according to various different parameters: Solar reflectance effect: According to the graph, we note that energy needs are inversely proportional to the values of the solar reflectivity. The reference building with a reflectivity of 0.5 presents an annual energy needs for heating of 37.81kwh/m².y, and cooling needs of 56.36kwh/m².y, which will make a total of 94.18kwh/m².y. When changing the coating of outer surfaces of the walls by a reflective coating with a value of 0.9, the heating needs increase by 8.65kwh/m².y with a percentage of 18.61%, the need for cooling decrease by 12.41kwh/m².y, making a percentage of 22%. Hence, the total needs decreased by 3.77kwh/m².y equivalent to a percentage of about 4%. However, the allocation of a reflectivity value of 0.1 (absorbing materials) for facade walls gives opposite results compared with the reference building. In fact, the heating needs decreased by 7.25kwh/m².y for a percentage of 19.17%. The cooling requirements increased by 13.44kwh/m².y, giving a percentage of 19.25%, hence the total requirements increased by 6.17kwh/m².y, where a percentage of 6.14%.
Fig.1. Solar reflectance effect on annual energy loads for cinderblock insulated building oriented South
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We can therefore conclude that the external colour of a building affects the thermal performance of the building although its impact is little in our climate. Orientation effect: The effect of orientation shows very small nuances between the different simulated orientations, either for heating demand, air conditioning or total requirements. The better is the orientation 0 (South) with a minimum total annual demand of 94.18kwh/m².y. The one with the maximum demand is the orientation 45 (South-West) with total needs of 95.96kwh/m².y which is not far from the South-East orientation (315) with a value of 95.94kwh/m².y, where as two remaining orientations 90 and 270 (East and West) have exactly the same needs for heating, cooling and total needs. Fig.2. summarizes these results.
Fig.2. Orientation effect on annual energy loads for building with cinderblock walls and solar reflectivity of 0.5
It can be deduced that for cinderblock walls and solar reflectance value of 0.5, the orientation of the building plays a negligible role in mitigating the energy needs. Type of material and insulation effect: The change in the constitution of the vertical wall of the building between the cinderblock wall and concrete wall gave approximately similar results. The heating requirements for buildings with cinderblock wall are 48.23kwh/m².year while for the concrete walls, they are 51.19kwh/m².y, hence an increase of 2.96kwh/m².y representing a percentage of 5.78%. The cooling needs are 55.84kwh/m².y for cinderblock walls and 54.63kwh/m².y for concrete walls, where the difference is 1.21kwh/m².y and the percentage is 2.16%. The total requirement in turn spend 104.07kwh/m².y for the first case to 105.82kwh for the second case giving a difference of 1.65%. This small difference is due to the thermal mass of the two materials which is almost nearly the same. The effect of the insulation is estimated by comparing the same type of wall with and without insulation. For the cinderblock wall, heating requirements when the insulation is applied are less by 10.42kwh to the needs obtained for a wall without insulation, with a percentage of 21.60%, whereas the cooling needs increased insignificantly. As consequence, the total needs decrease is 9.89kwh/m².y with a percentage of 9.5%. For the concrete wall, the same trend is noticed, the heating needs for the insulated wall are lower than those of the uninsulated wall about 12.08kwh, where a percentage of 23.59%. The change in the field of cooling requirements is negligible, which results in a reduction of the total needs of 11.29kwh with a percentage of 10.66% between the concrete wall with and without insulation. Furthermore, the change in the annual energy needs for heating, cooling and total requirements between the two walls when the insulation is applied show the same trends as the change between the 2 types of walls without insulation, i.e. negligible results. This is due to the almost similarity of thermal mass of the two walls.
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Fig.3. Type materials and insulation effect on annual energy loads for solar reflectivity of 0.5 and orientation 0
After introducing a window in the south wall of the building, the energy needs have changed. For the cinderblock wall facing south, with a reflectivity of 0.5, there was a decrease in the heating needs of 6.14kwh/m².y i.e. with a percentage of 16.23%. While the cooling requirements have increased of 4.79kwh/m².y with a percentage of 7.83% resulting in a decrease in total needs of 1.35kwh/m².y, so a percentage of 1.43%. For the reflectivity of 0.1, the heating needs for the pierced wall decrease of 4.48kwh/m².y at a percentage of 14.65%. The cooling needs are increased by 3.87kwh/m².y, resulting in a percentage of 5.25% .This gives a negligible decrease in the total requirements. For the reflectivity of 0.9, the heating requirements decreased of 8.11kwh/m².y with a percentage of 17.45%. The need for cooling increased of 5.54kwh/m².y where a percentage of 11.19%, and total requirements decreased to a
very low rate of approximately 2.84%, as it is showed in the following figure.
Fig.4. Solar reflectance effect on annual energy loads for cinderblock insulated building oriented South with glazing
We can conclude that introducing window in the south wall can dilute the benefit effect of the high reflectance of a building envelope on the energy demand. 3.2. Thermal comfort To estimate the summer comfort inside the building, we repeated the same simulations without air conditioning (free floating conditions), we took a temperature threshold of 27°C, beyond which we will record the numbers of hours of discomfort. We will present the results obtained after the change in the reflectivity of the façade walls. The number of hours of discomfort for the reference building is from 3088 hours so a percentage of 35%. As expected, increasing the reflectivity of the wall cladding decreases the number of hours of discomfort. For example, for the reflectivity value of 0.9, the number of hours of discomfort will be reduced to 2603 hours equivalent to a
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percentage of 30%. And if we apply a coating with a reflectivity of 0.1, we note that the number of hours of discomfort increases to 3441 hours, resulting in a percentage of 39%. We can conclude that the reflectivity of wall coatings materials plays a role in improving summer comfort conditions inside the building, beyond 0.5, the interior ambience is moderated by reducing the number of hours of discomfort. 4. Conclusion The aim of this study is to estimate the effect of solar reflectivity (albedo) with other parameters on energy consumption and thermal comfort in Mediterranean climatic context. It was found that all the parameters play an important role when they are implemented. The best combination that could be obtained was the one with light colored coatings, applied on the outer walls of an albedo value greater than 0.5, combined with a thermal insulation. This study has as perspective the determination of database , with the right combination of insulation, orientation, thermal mass and of course the high reflectivity coatings of the outer shell of the building, useful for stakeholders (designers, architects, policy makers ... ect) to optimize the energy balance of the building throughout the year and its corollary thermal comfort. References [1] Rosenfeld, A. H, Akbari, H, Bretz, S, Fishman, B. L, Kurn, D. M, Sailor, D, and Taha, H, “Mitigation of urban heat islands: materials,utility programs, updates,” Energy Build., 1995, vol. 22, no. 3, pp. 255–265. [2] Givoni, B,and Hoffiman, M.E, , “Effect of building materials on internal temperatures,” Res. Report, Build. Res. Station. Tech. Haifa, 1968 [3] Reagan, J. A, and Acklam D. M, “Solar reflectivity of common building materials and its influence on the roof heat gain of typical southwestern U.S.A. residences,” Energy Build., 1979, vol. 2, no. 3, pp. 237–248. [4] Taha, H, Akbari, H, Rosenfeld, A, and Huang J, “Residential cooling loads and the urban heat island—the effects of albedo,” Build. Environ., 1988, vol. 23, no. 4, pp. 271–283. [5] Bansal, N. K, Garg, S. N, and Kothari, S, “Effect of exterior surface colour on the thermal performance of buildings,” Build. Environ., 1992, vol. 27, no. 1, pp. 31–37. [6] Taha, H, Sailor, D, Akbari, H, “High albedo materials for reducing cooling energy use,” Lawrence Berkeley Lab. 1992, Rep. 31721, vol. UC530, Berkeley CA. [7] Parker, D.S, Barkaszi, S.F, Sonne, J.K, “Measured cooling energy savings from reflective roof coatings in Florida,”, 1994, Phase II report, Rep. No.FSEC-CR-699-94, Florida Sol. Energy Center, Cape Canaveral, FL. [8] Akbari, H, Bretz, S, Kurn, D. M, and Hanford, J, “Peak power and cooling energy savings of high-albedo roofs,” Energy Build., 1997, vol. 25, no. 2, pp. 117–126. [9] Simpson, J. R, and Mcpherson, E. G, “The effects of roof albedo modification on cooling loads of scale model residences in Tucson , Arizona,” 1997, vol. 25. [10] Shariah, A, Shalabi, B, Rousan, A, and Tashtoush, B, “Effects of absorptance of external surfaces on heating and cooling loads of residential buildings in Jordan,” Energy Convers. Manag., 1998, vol. 39, no. 3–4, pp. 273–284. [11] Cheng, V, Ng, E, and Givoni, B, “Effect of envelope colour and thermal mass on indoor temperatures in hot humid climate,” Sol. Energy, 2005, vol. 78, no. 4 SPEC. ISS., pp. 528–534. [12] Synnefa, A, Santamouris, M, and Akbari, H, “Estimating the effect of using cool coatings on energy loads and thermal comfort in residential buildings in various climatic conditions,” Energy Build., 2007, vol. 39, no. 11, pp. 1167–1174. [13] Zinzi, M, Daneo, A, and Fanchiotti, A, “Paper No 314: Optical properties and influence of reflective coatings on the energy demand and thermal comfort in dwellings at Mediterranean latitudes,” PLEA 2008 – 25th Conf. Passiv. Low Energy Archit. Dublin, 22nd to 24th Oct. 2008, no. 314. [14] Uemoto, K. L, Sato, N. M. N, and John, V. M, “Estimating thermal performance of cool colored paints,” Energy Build., 2010, vol. 42, no. 1, pp. 17–22. [15] Shen, H, Tan, H, and Tzempelikos, A, “The effect of reflective coatings on building surface temperatures , indoor environment and energy consumption — An experimental study,” 2011, vol. 43, pp. 573–580. [16] Bozonnet, E, Doya, M, and Allard, F, “Cool roofs impact on building thermal response: A French case study,” Energy Build., 2011, vol. 43, no. 11, pp. 3006–3012. [17] Pisello, A.L, Cotana,F, " The thermal effect of an innovative cool roof on residential buildings in Italy: Results from two years of continuous monitoring," Energy and Buildings, 2014, vol.69 pp.154–164. [18] Lapisa, R, "Etude du rafraîchissement passif des bâtiments commerciaux ou industriels", Thèse de Doctorat, Université de La Rochelle.2015 [19] “TRNSYS 17”, T. S. S. P. U. Manuel, Sol. Energy Lab. Univ. Wisconsin, Madison, USA.