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The Optimal Choice of the Outside Innovative Surface Finishes for Buildings from a Thermal and Energetic Point of View
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 101 (2016) 980 – 987
71st Conference of the Italian Thermal Machines Engineering Association, ATI2016, 14-16 September 2016, Turin, Italy
The optimal choice of the outside innovative surface finishes for buildings from a thermal and energetic point of view Concetta Marinoa* a
Dipartimento di Ingegneria Industriale, Università degli Studi di Napoli Federico II, P.le Tecchio 80, Napoli 80125, IT
Abstract
The surface finishes have an important role on the energy requirements of the buildings. This paper presents a new factor denominated “outside coating factor” (OCF) for an air-conditioned building and for a building without summer thermal control. It synthetically describes the climate of the location considered and can be useful for building planners for choosing the optimal outside surface finishes from thermal and energetic point of view. This factor depends on the cooling/heating degrees-day and solar radiation. For positive values of the OCF (0, +2.5), the optimal choice falls on the high solar reflectance and high infrared emissivity surface finishes, while the opposite occurs for negative values of the OCF. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Scientific Committee of ATI 2016. Peer-review under responsibility of the Scientific Committee of ATI 2016. Keywords: building envelope; existing residential buildings; surface finishes; outside coating factor; solar radiation; energy saving; HVAC systems; optimal choice; adaptive comfort; high solar reflectance
1. Introduction In the European Union (EU), more than 40 % of the global energy consumption depends on buildings [1]. Therefore, a significant part of pollutant gas emissions derives from the building sector [2]. The European Directive 2010/31/EU (EPBD recast) [3] affirms that the building sector is expanding, so it is bound to increase its energy
* Corresponding author. E-mail address: [email protected], [email protected]
1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Scientific Committee of ATI 2016. doi:10.1016/j.egypro.2016.11.124
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requirements. Consequently, the saving energy and the use of renewable energy sources in the buildings constitute important measures needed to reduce the EU energy dependency and greenhouse gas emissions. These actions should allow maintaining the global temperature rise below 2 °C and reducing of 20 %, by 2020, overall greenhouse gas emissions, compared to the 1990 levels. So, the EPBD recast promotes, in the building sector, the energy saving and the use of passive techniques for the building envelope. For example, innovative materials, i.e. phase change material (PCM) are studied in [4, 5]. In temperate or hot climates, the use of air conditioning systems in summer season is widespread also in residential sector, causing the increase of the peak electric demand and blackouts [6]. In Italy, many historic buildings are characterized by low energy performances. For this reason it is important to act on these buildings [7]. The first step to achieve energy saving in the buildings is represented by passive solutions. An important option to reduce the building energy demand is the optimization of the radiative characteristics of the surface finishes. Outside finishes with clear colour absorb about 40 % of the solar radiation, while the percentage rises to 90 % in the case of dark surface finishes [8]. The albedo control allows reflecting the shortwave radiation toward the space. Three effects are obtained with this control: the global warming mitigation, the decrease of urban heat islands and the reduction of the energy demand in the buildings [9]. The heat island phenomenon increases the outside temperature of the cities, worsens the environmental comfort in the urban space and rises the cooling energy requirements. The results show that cool pavements decrease the effects of the summer season, reducing the temperature of the urban environment [10]. Also, the urban heat islands are studied in Santamouris [11], Xu et al. [12]. The cool paints used as external surface finishes (on the roof) can improve the summer comfort conditions and reduce the operative/mean radiant temperature of the rooms [13]. Various passive strategies for decreasing the cooling energy demand by using solar shading devices are studied in Bellia et al. [14, 15]. The influence of innovative surface finishes is evaluated in Marino et al. [16]. This work shows that the use of outside cool paints on the roof and walls decreases the summer energy demand (up to 60 %), but rises the heating requirements (up to 10 %). Also, the effect of the coating of the building envelope is studied in Ascione et al. [17]. This work introduced the Surface Factor (SF, dimensionless) as reported by the Eq. 1: (1) This factor is obtained dividing the monthly average daily solar radiation on the horizontal plane by a fixed reference value of 10000 Wh/m2 and the winter degrees-day with a reference value of 1000 K.day. In Ferrari et al. [18], innovative solutions for the ceramic tiles are planned. This work analyses the use of coolcolors and insulation layer for the tiles. The incidence of innovative surface finishes on the energy primary requirements is considered in Marino et al. [19]. Also, the building envelope insulating is considered. The effects of the cool coatings are preferable with respect to green roofs. The first action has great potential in reducing the incident solar radiation and the energy management costs, as studied in Ascione et al. [20]. Other innovative technologies are studied in Buonomano et al. [21] and in Ascione et al. [22]. The energetic and economic effects of thermal insulation on existing buildings is analyzed in de’ Rossi et al. [23]. Pisello et al. report an experimental study on cool coatings for clay tiles [24]. The results show that these coatings reduce the number of hours (18 % in summer season) when the operative temperature is over 26 °C, while this data is negligible in winter. The American Society for Testing and Materials (ASTM) introduced for roofs the solar reflectance index (SRI). High values of SRI reduce the cooling energy requirements by 10–60% [25]. The surface finishes have an important role on the energy requirements of the buildings. In the cooling season, the heat transfer through the building envelope does not depend only on the thermal transmittance, heat capacity, temperature difference between outside and inside, but also the solar radiation incident on the building is an important factor. The solar-air temperature is often used to evaluate this heat transfer and the energy performance of the opaque building envelope; it takes into account the solar absorption factor, the solar reflectance and the infrared emissivity. In this paper, an analysis is performed on the energy saving obtainable by applying innovative surface finishes
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on the outside envelope of a typical existing residential building. A dynamic building energy simulation software (Energy Plus) [26] is used. In particular, an attic of the historic center of some towns is considered as case study. A new factor denominated “outside coating factor” (OCF) is presented. It synthetically describes the climate of the location considered and can be useful for building planners for choosing the optimal outside surface finishes from thermal and energetic point of view. This factor depends on the cooling and heating degrees-day and solar radiation. The coating factor is evaluated for an air-conditioned building and also for a building without summer thermal control. Besides the energy analysis, an investigation about thermal comfort (adaptive comfort) is also performed. Nomenclature EPBD Energy Performances of Buildings Directive HVAC Heating, Ventilation and Air Conditioning ICF Inside Coating Factor (ND) OCF Outside Coating Factor (ND) PE primary energy (kWh/m2y) SEER seasonal energy efficiency ratio (ND) SPB simple payback time (number of years) TE thermal energy needs of the building envelope (kWh/m2y) U thermal transmittance (W/m2K) WWR window-to-wall ratio (%) α absorptance (ND), Ԑ emissivity (ND), ρ reflectance (ND) ƞgl seasonal global efficiency of the heating system (ND) ƞthermoelectric efficiency of the national electric system (ND) Subscripts: y=yearly; sol= solar 2. Methodology and case study The research activity is carried out by using a dynamic energy simulation code [26]. EnergyPlus was subject to extensive validation procedures. Currently, analytical tests, comparative tests, release and executable tests are conducted. EnergyPlus provides different testing reports for building envelope [27] and HVAC equipment [28]. Buonomano and Palombo [29] compared EnergyPlus to other simulation codes and the deviation was lower than 10%. In Becchio et al. [30], the dynamic energy simulation was also compared to stationary energy simulation and the deviation was equal to 15%. This study is conducted on a reference residential building built in the 1920–1960 period (Fig. 1) characterized by window-to-wall ratio (WWR) equal to 20 %. The main characteristics of the building and systems (Tab. 1) are identified by TABULA project [31] for the European cities. The U-values of the building envelope components are reported in Tab. 2. The considered innovative coating, analyzed also in Marino et al. [16, 19], is a red tile cool paint on pitched roof (ρsolar =0.79; Ԑinfrared = 0.89). This paper evaluates the energy performances and the adaptive comfort of the residential reference building when applying the innovative surface finish on the pitched roof. The heart of the paper is represented by the determination of a new factor named OCF, which describes the climate of the locality analyzed and has the aim of optimizing the choice of outside surface finishes as a function of the climatic characteristics of the site examined. To determinate the climate of the city, different parameters are considered: heating degrees-day; cooling degrees-day; solar radiation. Depending on their use, it is possible to obtain two equations (Eq. 2 and Eq. 3). To choose in the best way the outside surface finishes, also the solar radiation has to be taken into account (Eq. 3). ICF = [(CDD - HDD) / 1000]) (2) OCF = [(CDD - HDD) / 1000] + (RAD / 10000) (3) where: - HDD: heating degrees-day (reference temperature equal to 18.3 ° C) [K.day]; - CDD: cooling degrees-day (reference temperature equal to 18.3 ° C) [K.day]; - RAD: maximum average monthly direct radiation on a horizontal surface [Wh / m 2].
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Fig. 1. The reference building considered for different countries proposed. From (Tabula Project, 2013 [31]; Eurostat, 2013 [32], Marino et al. [20]) Table 1 – Main characteristics of the simulation, case study building and systems. From Marino et al. [19], Eurostat, 2013 [32] Heat Balance Algorithm: Conduction Transfer Function, 6 timestep/hour Weather Data: IWEC Length (north-south direction): 11.0 m Minimum height: 1.5 m Width (east-west direction): 9.0 m Maximum height: 3.5 m Floor area: 99.0 m2 Gross volume: 247.5 m3 Indoor air Tset point (heating/cooling): 20 °C / 26 °C Metabolic index: 1.5 met per person Heating system ƞgl: 0.66 for IT, ES, FR, DE Thermal load due to artificial illumination and computer or kitchen: 20 W·m -2 (10 for lighting, 10 for computer or kitchen) Thermoelectric system efficiency (ƞthermoelectric): 0.466 for IT, 0.465 for ES, 0.547 for FR, 0.475 for DE Cooling system SEER: 2.7 for IT (Palermo) and ES (Seville); 2.8 for IT (Rome); 2.9 for IT (Milan), FR (Paris)and DE (Berlin) Table 2 – Thermal transmittance (U-value, W·m-2K-1) of the building components of the reference case [31] Opaque and transparent building component IT
ES
FR, DE, UK
Walls Roof Window Walls Roof Window Walls Roof Window
Double layer of bricks and air gap Uninsulated pitched roof (boards in wood) Frame of wood and single clear glass Single layer of bricks Uninsulated pitched roof (ceramic pieces layer as support of the tiles) Frame of wood and single clear glass Single layer of bricks Insulated pitched roof Frame of wood and double clear glass
U W·m-2K-1 1.15 1.83 4.90 2.63 4.17 5.70 (glass) – 2.20 (frame) 1.20 0.80 3.50
In Fig. 2, the values of Inside Coating Factor (ICF) and OCF are reported for various localities. The introduction of the solar radiation (OCF) varies the position of some cities, i.e. Athens (AT), Izmir (IZ), Naples (NA) and Jerusalem (JE), which present negative values of the coating factor (ICF) only when the solar radiation is not considered; contrariwise, when considering the solar radiation, these cities pass to positive values of the coating factor (OCF). Therefore, the individuation of the parameters in play is important. OCF is obtained dividing the (CDD – HDD) by a fixed value of 1000 K.day and dividing the RAD by a fixed value of 10000 Wh/m2 (Eq. 3). The OCF has been evaluated for building with and without air-conditioned system in summer season (and then this investigation is supported by the energy analysis and also by the adaptive comfort study). In fact, the energy efficiency optimization solutions have not to overlook the main purpose of the buildings, i.e. also providing comfort conditions in the indoor environment. Therefore, a complete analysis can not only consider the energy efficiency and the energy demand reduction in the buildings. In particular, in locations of the Southern Europe, characterized by Mediterranean climates, there is the need to provide comfortable thermal conditions in summer season, too. The standard EN 15251 [33] defines two main types of acceptable comfort: 1. for buildings with summer cooling active systems, the Fanger model (based on the Predicted Mean Vote, PMV); 2. for buildings with only summer cooling passive systems, the Adaptive model, i.e. the ability of the occupants to adapt to the prevailing climate of external spaces. This paper evaluates only the Adaptive model. In this case, the maximum indoor acceptable temperatures are quite high. In fact, the analysis shows that the adaptive algorithm provides the maximum indoor comfort temperatures on summer days equal to 29.4 °C for Palermo. It can be noted that traditional systems in urban residential buildings are considered, i.e. simple autonomous systems for heating season (radiators and hot water boiler) and split systems for cooling, that do not allow the handling of the latent loads. Therefore, the influence of the relative humidity is not investigated in this study. The main characteristics of the considered systems are reported in Tab. 1.
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Hot climates [(CDD - HDD) / 1000] + (RAD / 10000)
[(CDD - HDD) / 1000]) Hot climates Particular attention is necessary for this area
Particular attention is necessary for this area
Negative ICF Positive OCF Cold climates
a)
Cold climates
b)
Fig. 2. Coating Factor for various cities: (a) ICF, if solar radiation is not considered; (b) OCF, if solar radiation is considered. CA Cairo, TE Tenerife, SV Seville, PA Palermo, AT Athens, IZ Izmir, NA Naples, AK Ankara, JE Jerusalem, BA Barcelona, RM Rome, MI Milan, PR Paris, BE Berlin
3. Results Firstly, the values of the parameter OCF are calculated for many localities and reported in Tab. 3. Results and guidelines are provided for the optimal choice of outside surface finishes, as a function of the OCF values assumed, for both air-conditioned buildings and not air-conditioned buildings. For the first ones, an energy analysis (section 3.1) has been performed, while only a thermal analysis for the second ones (section 3.2). Table 3 – HDD, CDD, RAD and OCF values for some localities CA TE SV PA AT OCF
IZ
NA
GE
BA
2.135
1.769
0.979
0.933
0.668
0.33
0.054
0.090
-0.144 -0.287 -1.599 -1.925 -2.361 -2.628
1767
1110
1141
1002
1079
983
742
647
573
555
372
182
227
148
HDD [K day]
393
69
914
802
1164
1409
1368
1364
1389
1523
2654
2643
3299
3287
RAD [Wh/m2]
7614
7285
7518
7328
7529
7572
6797
8069
6695
6805
6827
5355
7113
5109
CDD [K.day] .
RM
MI
PR
AK
BE
3.1. Energy results for air-conditioned building The energy analysis refers to air-conditioned buildings. Fig. 3a shows the optimal choice of the outside innovative surface finishes as a function of the OCF values, for various localities. For positive values of OCF in the range [0, +2.5], i.e. in those cities characterized by hot or very hot climate, the designer should choose outside surface finishes with high ρsol factor and high Ԑinf, because the application of the cool paints leads to relevant reductions of the yearly primary energy for cooling and heating (EP y). Contrariwise, for negative OCF (range [-0.5, -2.5]), the optimal external surface finishes are those characterized by low values of ρsol and Ԑinf. In the interval [0, 0.5] there is a light influence of the spectral characteristics of the coatings of the various opaque building components on the energy performance of the building. In this case, the designer must pay particular attention, since it is not easy determine the most appropriate intervention. In this case, the designer should individuate the main purpose to achieve (privileging the winter energy requirements or the summer ones, or the annual). This occurs, for example, for the city of Naples, characterized by hot summers but also rather cold winters (HDD higher than CDD, Tab. 3). For some localities, Fig. 3a shows also the energy advantage/disadvantage when applying the outside innovative surface finishes, in terms of the percentage primary energy (EPy) variation during the entire year (i.e., energy for both heating and cooling). For example, in Seville (city in positive area of the OCF) the use of cool paints on the pitched roof determines a relevant reduction of the primary energy ('EPy = –11.8%); contrariwise, in Milan (city in negative area of the OCF) the cool paints induce an increase of the primary energy (('EPy = + 4.1 %). Note that an appreciable part of the cooling load related to the solar radiation is due to the transparent components (rather than opaque ones). This part depends mainly on the ratio between glazed and opaque areas in the considered building (WWR, equal to 20% in the case study). To explain how a change in this ratio could affect the presented results (this ratio may change the relative importance of the outside coatings), in Fig. 3b, the primary energy requirements are reported as a function of the WWR (from 10% to 50%), for the city of Palermo.
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b)
a)
PA: EPy: - 7 % EPc: - 20 % EPh: + 5 % PALERMO Fig. 3. (a) Energy advantage/disadvantage by applying the cool paints (high ρsol and Ԑinf) on the pitched roof for air-conditioned buildings, as a function of the OCF value of the considered locality (WWR= 20 %); (b) Primary energy for heating and cooling as a function of the WWR for Palermo.
3.2. Thermal comfort results for not air-conditioned building
Toperative
Tmax Toptimal Tmin
a)
Operative temperature [°C]
Operative temperature [°C]
This analysis of the adaptive comfort conditions refers to not air-conditioned buildings. It is carried out for different cities, but this paper shows mainly the results for Palermo (Southern Italy), characterized by hot climate. Three cases are evaluated: 1) traditional building with not insulated envelope; 2) building block with insulation (5 cm thick wood fiber); 3) not insulated building with outside cool paint on the pitched roof. The indoor operative temperatures and adaptive comfort ranges obtained in summer for Palermo are shown in Fig. 4, for the cases 1 and 3. The study shows that the worst case (the greatest number of hours outside of the range of the adaptive comfort temperatures) is that related to the traditional insulation (case2): the number of hours in which the summer overheating occurs is equal to 2065 hours (71% of summer total hours). In traditional building without insulation, the number of summer discomfort hours is equal to 1461 hours (about 50% of the summer total hours) (Fig. 4a). The best case is the traditional building without insulation and with cool paint on pitched roof, which allows to minimize the number of discomfort hours (31% of the summer total hours) (Fig. 4b). Fig. 5 shows the percentage summer discomfort hours when applying the cool paints on the pitched roof for not air-conditioned buildings, as a function of the OCF value of the considered locality. It can be noted that in the buildings without thermal control in summer, the convenience area of the cool paints based on the OCF values becomes larger [+2.5, -0.5], but could even more extend. From Fig. 5, it can be noted that the use of cool paints, from thermal comfort point of view: - is convenient for hot climates (for example, for Palermo, the summer discomfort hours are from 50% to 31% of the total hours); could be convenient even for negative OCF values. For example, in Rome the number of hours of overheating in summer is 10 % (acceptable according to the EN 15251 [33]), although the yearly primary energy increases by 7 % (due to worse performance in winter conditions).
Toperative
Tmax Toptimal Tmin
b)
Fig. 4. Indoor operative temperature and adaptive comfort ranges in summer for Palermo. (a) Not insulated building envelope; (b) Cool paints on not insulated building envelope.
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Fig. 5. Percentage summer discomfort hours when applying the cool paints (high ρsol and Ԑinf) on the pitched roof for not air-conditioned buildings, as a function of the OCF value of the considered locality
4. Conclusions The surface finishes have an important role on the energy requirements and thermal comfort of the buildings. In the cooling season, the heat transfer through the building envelope does not depend only on the thermal transmittance, heat capacity and temperature difference between outside and inside, but also the solar radiation incident on the building is an important factor. The solar-air temperature is often used to evaluate this heat transfer and the energy performance of the opaque building envelope; it takes into account the solar absorption factor, the solar reflectance and the infrared emissivity, which depend on the choice of the surface finishes of the building envelope. In this paper, an analysis is performed on the energy saving obtainable by applying innovative surface finishes on the outside envelope of a typical existing residential building, by means of a dynamic building energy simulation software (Energy Plus). In particular, an attic of the historic center of some towns is considered as case study. A new factor denominated “outside coating factor” is presented. It synthetically describes the climate of the location considered and can be useful for building planners for choosing the optimal surface finishes from thermal and energetic point of view. This factor depends on the cooling and heating degrees-day and solar radiation. For positive values of the outside coating factor (0, +2.5), the optimal choice falls on the high solar reflectance and high infrared emissivity surface finishes (high ρsol and Ԑinf), i.e. cool paints; instead, for negative values of OCF (-0.5, 2.5), the outside finishes should have low solar reflectance and low emissivity (low ρ sol and Ԑinf). In the range [0, 0.5] greater attention is required, and a more specific analysis should be carried out. The coating factor is evaluated for an air-conditioned building and also for a building without summer thermal control. For example, in Seville (positive OCF value equal to about +1), the use of cool paints on the pitched roof determines a relevant reduction of the primary energy (EPy=–11.8 %). Contrariwise, in Milan (negative OCF value, about -1.5), the cool paints induce an increase of the primary energy (EP y=+4.1 %). In the building without air-conditioning in summer, the OCF convenience area for the cool paints becomes larger [+2.5, -0.5], but could even more extend. For not air-conditioned buildings, this convenience is in terms of reduced number of summer discomfort hours rather than in terms of energy saving. Besides the energy analysis, an investigation about thermal comfort (adaptive comfort) is also performed. The best case refers to the traditional building (without insulation) with cool paint on pitched roof, which allows to minimize the number of discomfort hours (31 % of the summer total hours) for Palermo, while this value is 71% in the same case but without the cool paints. So a relevant reduction of the indoor summer overheating is obtained. Acknowledgements This work was developed in the framework of the European project "Italian Training qualificatiOn Workforce in buildiNg" (Acronym: BUILD UP Skills I-TOWN), Intelligent Energy - Europe (IEE), Call for proposals CIP-IEE2013 - BUILD UP Skills Initiative. Contract N°: IEE/13/BWI/721/SI2.680178.
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ScienceDirect Energy Procedia 101 (2016) 980 – 987
71st Conference of the Italian Thermal Machines Engineering Association, ATI2016, 14-16 September 2016, Turin, Italy
The optimal choice of the outside innovative surface finishes for buildings from a thermal and energetic point of view Concetta Marinoa* a
Dipartimento di Ingegneria Industriale, Università degli Studi di Napoli Federico II, P.le Tecchio 80, Napoli 80125, IT
Abstract
The surface finishes have an important role on the energy requirements of the buildings. This paper presents a new factor denominated “outside coating factor” (OCF) for an air-conditioned building and for a building without summer thermal control. It synthetically describes the climate of the location considered and can be useful for building planners for choosing the optimal outside surface finishes from thermal and energetic point of view. This factor depends on the cooling/heating degrees-day and solar radiation. For positive values of the OCF (0, +2.5), the optimal choice falls on the high solar reflectance and high infrared emissivity surface finishes, while the opposite occurs for negative values of the OCF. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Scientific Committee of ATI 2016. Peer-review under responsibility of the Scientific Committee of ATI 2016. Keywords: building envelope; existing residential buildings; surface finishes; outside coating factor; solar radiation; energy saving; HVAC systems; optimal choice; adaptive comfort; high solar reflectance
1. Introduction In the European Union (EU), more than 40 % of the global energy consumption depends on buildings [1]. Therefore, a significant part of pollutant gas emissions derives from the building sector [2]. The European Directive 2010/31/EU (EPBD recast) [3] affirms that the building sector is expanding, so it is bound to increase its energy
* Corresponding author. E-mail address: [email protected], [email protected]
1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Scientific Committee of ATI 2016. doi:10.1016/j.egypro.2016.11.124
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requirements. Consequently, the saving energy and the use of renewable energy sources in the buildings constitute important measures needed to reduce the EU energy dependency and greenhouse gas emissions. These actions should allow maintaining the global temperature rise below 2 °C and reducing of 20 %, by 2020, overall greenhouse gas emissions, compared to the 1990 levels. So, the EPBD recast promotes, in the building sector, the energy saving and the use of passive techniques for the building envelope. For example, innovative materials, i.e. phase change material (PCM) are studied in [4, 5]. In temperate or hot climates, the use of air conditioning systems in summer season is widespread also in residential sector, causing the increase of the peak electric demand and blackouts [6]. In Italy, many historic buildings are characterized by low energy performances. For this reason it is important to act on these buildings [7]. The first step to achieve energy saving in the buildings is represented by passive solutions. An important option to reduce the building energy demand is the optimization of the radiative characteristics of the surface finishes. Outside finishes with clear colour absorb about 40 % of the solar radiation, while the percentage rises to 90 % in the case of dark surface finishes [8]. The albedo control allows reflecting the shortwave radiation toward the space. Three effects are obtained with this control: the global warming mitigation, the decrease of urban heat islands and the reduction of the energy demand in the buildings [9]. The heat island phenomenon increases the outside temperature of the cities, worsens the environmental comfort in the urban space and rises the cooling energy requirements. The results show that cool pavements decrease the effects of the summer season, reducing the temperature of the urban environment [10]. Also, the urban heat islands are studied in Santamouris [11], Xu et al. [12]. The cool paints used as external surface finishes (on the roof) can improve the summer comfort conditions and reduce the operative/mean radiant temperature of the rooms [13]. Various passive strategies for decreasing the cooling energy demand by using solar shading devices are studied in Bellia et al. [14, 15]. The influence of innovative surface finishes is evaluated in Marino et al. [16]. This work shows that the use of outside cool paints on the roof and walls decreases the summer energy demand (up to 60 %), but rises the heating requirements (up to 10 %). Also, the effect of the coating of the building envelope is studied in Ascione et al. [17]. This work introduced the Surface Factor (SF, dimensionless) as reported by the Eq. 1: (1) This factor is obtained dividing the monthly average daily solar radiation on the horizontal plane by a fixed reference value of 10000 Wh/m2 and the winter degrees-day with a reference value of 1000 K.day. In Ferrari et al. [18], innovative solutions for the ceramic tiles are planned. This work analyses the use of coolcolors and insulation layer for the tiles. The incidence of innovative surface finishes on the energy primary requirements is considered in Marino et al. [19]. Also, the building envelope insulating is considered. The effects of the cool coatings are preferable with respect to green roofs. The first action has great potential in reducing the incident solar radiation and the energy management costs, as studied in Ascione et al. [20]. Other innovative technologies are studied in Buonomano et al. [21] and in Ascione et al. [22]. The energetic and economic effects of thermal insulation on existing buildings is analyzed in de’ Rossi et al. [23]. Pisello et al. report an experimental study on cool coatings for clay tiles [24]. The results show that these coatings reduce the number of hours (18 % in summer season) when the operative temperature is over 26 °C, while this data is negligible in winter. The American Society for Testing and Materials (ASTM) introduced for roofs the solar reflectance index (SRI). High values of SRI reduce the cooling energy requirements by 10–60% [25]. The surface finishes have an important role on the energy requirements of the buildings. In the cooling season, the heat transfer through the building envelope does not depend only on the thermal transmittance, heat capacity, temperature difference between outside and inside, but also the solar radiation incident on the building is an important factor. The solar-air temperature is often used to evaluate this heat transfer and the energy performance of the opaque building envelope; it takes into account the solar absorption factor, the solar reflectance and the infrared emissivity. In this paper, an analysis is performed on the energy saving obtainable by applying innovative surface finishes
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on the outside envelope of a typical existing residential building. A dynamic building energy simulation software (Energy Plus) [26] is used. In particular, an attic of the historic center of some towns is considered as case study. A new factor denominated “outside coating factor” (OCF) is presented. It synthetically describes the climate of the location considered and can be useful for building planners for choosing the optimal outside surface finishes from thermal and energetic point of view. This factor depends on the cooling and heating degrees-day and solar radiation. The coating factor is evaluated for an air-conditioned building and also for a building without summer thermal control. Besides the energy analysis, an investigation about thermal comfort (adaptive comfort) is also performed. Nomenclature EPBD Energy Performances of Buildings Directive HVAC Heating, Ventilation and Air Conditioning ICF Inside Coating Factor (ND) OCF Outside Coating Factor (ND) PE primary energy (kWh/m2y) SEER seasonal energy efficiency ratio (ND) SPB simple payback time (number of years) TE thermal energy needs of the building envelope (kWh/m2y) U thermal transmittance (W/m2K) WWR window-to-wall ratio (%) α absorptance (ND), Ԑ emissivity (ND), ρ reflectance (ND) ƞgl seasonal global efficiency of the heating system (ND) ƞthermoelectric efficiency of the national electric system (ND) Subscripts: y=yearly; sol= solar 2. Methodology and case study The research activity is carried out by using a dynamic energy simulation code [26]. EnergyPlus was subject to extensive validation procedures. Currently, analytical tests, comparative tests, release and executable tests are conducted. EnergyPlus provides different testing reports for building envelope [27] and HVAC equipment [28]. Buonomano and Palombo [29] compared EnergyPlus to other simulation codes and the deviation was lower than 10%. In Becchio et al. [30], the dynamic energy simulation was also compared to stationary energy simulation and the deviation was equal to 15%. This study is conducted on a reference residential building built in the 1920–1960 period (Fig. 1) characterized by window-to-wall ratio (WWR) equal to 20 %. The main characteristics of the building and systems (Tab. 1) are identified by TABULA project [31] for the European cities. The U-values of the building envelope components are reported in Tab. 2. The considered innovative coating, analyzed also in Marino et al. [16, 19], is a red tile cool paint on pitched roof (ρsolar =0.79; Ԑinfrared = 0.89). This paper evaluates the energy performances and the adaptive comfort of the residential reference building when applying the innovative surface finish on the pitched roof. The heart of the paper is represented by the determination of a new factor named OCF, which describes the climate of the locality analyzed and has the aim of optimizing the choice of outside surface finishes as a function of the climatic characteristics of the site examined. To determinate the climate of the city, different parameters are considered: heating degrees-day; cooling degrees-day; solar radiation. Depending on their use, it is possible to obtain two equations (Eq. 2 and Eq. 3). To choose in the best way the outside surface finishes, also the solar radiation has to be taken into account (Eq. 3). ICF = [(CDD - HDD) / 1000]) (2) OCF = [(CDD - HDD) / 1000] + (RAD / 10000) (3) where: - HDD: heating degrees-day (reference temperature equal to 18.3 ° C) [K.day]; - CDD: cooling degrees-day (reference temperature equal to 18.3 ° C) [K.day]; - RAD: maximum average monthly direct radiation on a horizontal surface [Wh / m 2].
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Fig. 1. The reference building considered for different countries proposed. From (Tabula Project, 2013 [31]; Eurostat, 2013 [32], Marino et al. [20]) Table 1 – Main characteristics of the simulation, case study building and systems. From Marino et al. [19], Eurostat, 2013 [32] Heat Balance Algorithm: Conduction Transfer Function, 6 timestep/hour Weather Data: IWEC Length (north-south direction): 11.0 m Minimum height: 1.5 m Width (east-west direction): 9.0 m Maximum height: 3.5 m Floor area: 99.0 m2 Gross volume: 247.5 m3 Indoor air Tset point (heating/cooling): 20 °C / 26 °C Metabolic index: 1.5 met per person Heating system ƞgl: 0.66 for IT, ES, FR, DE Thermal load due to artificial illumination and computer or kitchen: 20 W·m -2 (10 for lighting, 10 for computer or kitchen) Thermoelectric system efficiency (ƞthermoelectric): 0.466 for IT, 0.465 for ES, 0.547 for FR, 0.475 for DE Cooling system SEER: 2.7 for IT (Palermo) and ES (Seville); 2.8 for IT (Rome); 2.9 for IT (Milan), FR (Paris)and DE (Berlin) Table 2 – Thermal transmittance (U-value, W·m-2K-1) of the building components of the reference case [31] Opaque and transparent building component IT
ES
FR, DE, UK
Walls Roof Window Walls Roof Window Walls Roof Window
Double layer of bricks and air gap Uninsulated pitched roof (boards in wood) Frame of wood and single clear glass Single layer of bricks Uninsulated pitched roof (ceramic pieces layer as support of the tiles) Frame of wood and single clear glass Single layer of bricks Insulated pitched roof Frame of wood and double clear glass
U W·m-2K-1 1.15 1.83 4.90 2.63 4.17 5.70 (glass) – 2.20 (frame) 1.20 0.80 3.50
In Fig. 2, the values of Inside Coating Factor (ICF) and OCF are reported for various localities. The introduction of the solar radiation (OCF) varies the position of some cities, i.e. Athens (AT), Izmir (IZ), Naples (NA) and Jerusalem (JE), which present negative values of the coating factor (ICF) only when the solar radiation is not considered; contrariwise, when considering the solar radiation, these cities pass to positive values of the coating factor (OCF). Therefore, the individuation of the parameters in play is important. OCF is obtained dividing the (CDD – HDD) by a fixed value of 1000 K.day and dividing the RAD by a fixed value of 10000 Wh/m2 (Eq. 3). The OCF has been evaluated for building with and without air-conditioned system in summer season (and then this investigation is supported by the energy analysis and also by the adaptive comfort study). In fact, the energy efficiency optimization solutions have not to overlook the main purpose of the buildings, i.e. also providing comfort conditions in the indoor environment. Therefore, a complete analysis can not only consider the energy efficiency and the energy demand reduction in the buildings. In particular, in locations of the Southern Europe, characterized by Mediterranean climates, there is the need to provide comfortable thermal conditions in summer season, too. The standard EN 15251 [33] defines two main types of acceptable comfort: 1. for buildings with summer cooling active systems, the Fanger model (based on the Predicted Mean Vote, PMV); 2. for buildings with only summer cooling passive systems, the Adaptive model, i.e. the ability of the occupants to adapt to the prevailing climate of external spaces. This paper evaluates only the Adaptive model. In this case, the maximum indoor acceptable temperatures are quite high. In fact, the analysis shows that the adaptive algorithm provides the maximum indoor comfort temperatures on summer days equal to 29.4 °C for Palermo. It can be noted that traditional systems in urban residential buildings are considered, i.e. simple autonomous systems for heating season (radiators and hot water boiler) and split systems for cooling, that do not allow the handling of the latent loads. Therefore, the influence of the relative humidity is not investigated in this study. The main characteristics of the considered systems are reported in Tab. 1.
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Hot climates [(CDD - HDD) / 1000] + (RAD / 10000)
[(CDD - HDD) / 1000]) Hot climates Particular attention is necessary for this area
Particular attention is necessary for this area
Negative ICF Positive OCF Cold climates
a)
Cold climates
b)
Fig. 2. Coating Factor for various cities: (a) ICF, if solar radiation is not considered; (b) OCF, if solar radiation is considered. CA Cairo, TE Tenerife, SV Seville, PA Palermo, AT Athens, IZ Izmir, NA Naples, AK Ankara, JE Jerusalem, BA Barcelona, RM Rome, MI Milan, PR Paris, BE Berlin
3. Results Firstly, the values of the parameter OCF are calculated for many localities and reported in Tab. 3. Results and guidelines are provided for the optimal choice of outside surface finishes, as a function of the OCF values assumed, for both air-conditioned buildings and not air-conditioned buildings. For the first ones, an energy analysis (section 3.1) has been performed, while only a thermal analysis for the second ones (section 3.2). Table 3 – HDD, CDD, RAD and OCF values for some localities CA TE SV PA AT OCF
IZ
NA
GE
BA
2.135
1.769
0.979
0.933
0.668
0.33
0.054
0.090
-0.144 -0.287 -1.599 -1.925 -2.361 -2.628
1767
1110
1141
1002
1079
983
742
647
573
555
372
182
227
148
HDD [K day]
393
69
914
802
1164
1409
1368
1364
1389
1523
2654
2643
3299
3287
RAD [Wh/m2]
7614
7285
7518
7328
7529
7572
6797
8069
6695
6805
6827
5355
7113
5109
CDD [K.day] .
RM
MI
PR
AK
BE
3.1. Energy results for air-conditioned building The energy analysis refers to air-conditioned buildings. Fig. 3a shows the optimal choice of the outside innovative surface finishes as a function of the OCF values, for various localities. For positive values of OCF in the range [0, +2.5], i.e. in those cities characterized by hot or very hot climate, the designer should choose outside surface finishes with high ρsol factor and high Ԑinf, because the application of the cool paints leads to relevant reductions of the yearly primary energy for cooling and heating (EP y). Contrariwise, for negative OCF (range [-0.5, -2.5]), the optimal external surface finishes are those characterized by low values of ρsol and Ԑinf. In the interval [0, 0.5] there is a light influence of the spectral characteristics of the coatings of the various opaque building components on the energy performance of the building. In this case, the designer must pay particular attention, since it is not easy determine the most appropriate intervention. In this case, the designer should individuate the main purpose to achieve (privileging the winter energy requirements or the summer ones, or the annual). This occurs, for example, for the city of Naples, characterized by hot summers but also rather cold winters (HDD higher than CDD, Tab. 3). For some localities, Fig. 3a shows also the energy advantage/disadvantage when applying the outside innovative surface finishes, in terms of the percentage primary energy (EPy) variation during the entire year (i.e., energy for both heating and cooling). For example, in Seville (city in positive area of the OCF) the use of cool paints on the pitched roof determines a relevant reduction of the primary energy ('EPy = –11.8%); contrariwise, in Milan (city in negative area of the OCF) the cool paints induce an increase of the primary energy (('EPy = + 4.1 %). Note that an appreciable part of the cooling load related to the solar radiation is due to the transparent components (rather than opaque ones). This part depends mainly on the ratio between glazed and opaque areas in the considered building (WWR, equal to 20% in the case study). To explain how a change in this ratio could affect the presented results (this ratio may change the relative importance of the outside coatings), in Fig. 3b, the primary energy requirements are reported as a function of the WWR (from 10% to 50%), for the city of Palermo.
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b)
a)
PA: EPy: - 7 % EPc: - 20 % EPh: + 5 % PALERMO Fig. 3. (a) Energy advantage/disadvantage by applying the cool paints (high ρsol and Ԑinf) on the pitched roof for air-conditioned buildings, as a function of the OCF value of the considered locality (WWR= 20 %); (b) Primary energy for heating and cooling as a function of the WWR for Palermo.
3.2. Thermal comfort results for not air-conditioned building
Toperative
Tmax Toptimal Tmin
a)
Operative temperature [°C]
Operative temperature [°C]
This analysis of the adaptive comfort conditions refers to not air-conditioned buildings. It is carried out for different cities, but this paper shows mainly the results for Palermo (Southern Italy), characterized by hot climate. Three cases are evaluated: 1) traditional building with not insulated envelope; 2) building block with insulation (5 cm thick wood fiber); 3) not insulated building with outside cool paint on the pitched roof. The indoor operative temperatures and adaptive comfort ranges obtained in summer for Palermo are shown in Fig. 4, for the cases 1 and 3. The study shows that the worst case (the greatest number of hours outside of the range of the adaptive comfort temperatures) is that related to the traditional insulation (case2): the number of hours in which the summer overheating occurs is equal to 2065 hours (71% of summer total hours). In traditional building without insulation, the number of summer discomfort hours is equal to 1461 hours (about 50% of the summer total hours) (Fig. 4a). The best case is the traditional building without insulation and with cool paint on pitched roof, which allows to minimize the number of discomfort hours (31% of the summer total hours) (Fig. 4b). Fig. 5 shows the percentage summer discomfort hours when applying the cool paints on the pitched roof for not air-conditioned buildings, as a function of the OCF value of the considered locality. It can be noted that in the buildings without thermal control in summer, the convenience area of the cool paints based on the OCF values becomes larger [+2.5, -0.5], but could even more extend. From Fig. 5, it can be noted that the use of cool paints, from thermal comfort point of view: - is convenient for hot climates (for example, for Palermo, the summer discomfort hours are from 50% to 31% of the total hours); could be convenient even for negative OCF values. For example, in Rome the number of hours of overheating in summer is 10 % (acceptable according to the EN 15251 [33]), although the yearly primary energy increases by 7 % (due to worse performance in winter conditions).
Toperative
Tmax Toptimal Tmin
b)
Fig. 4. Indoor operative temperature and adaptive comfort ranges in summer for Palermo. (a) Not insulated building envelope; (b) Cool paints on not insulated building envelope.
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Fig. 5. Percentage summer discomfort hours when applying the cool paints (high ρsol and Ԑinf) on the pitched roof for not air-conditioned buildings, as a function of the OCF value of the considered locality
4. Conclusions The surface finishes have an important role on the energy requirements and thermal comfort of the buildings. In the cooling season, the heat transfer through the building envelope does not depend only on the thermal transmittance, heat capacity and temperature difference between outside and inside, but also the solar radiation incident on the building is an important factor. The solar-air temperature is often used to evaluate this heat transfer and the energy performance of the opaque building envelope; it takes into account the solar absorption factor, the solar reflectance and the infrared emissivity, which depend on the choice of the surface finishes of the building envelope. In this paper, an analysis is performed on the energy saving obtainable by applying innovative surface finishes on the outside envelope of a typical existing residential building, by means of a dynamic building energy simulation software (Energy Plus). In particular, an attic of the historic center of some towns is considered as case study. A new factor denominated “outside coating factor” is presented. It synthetically describes the climate of the location considered and can be useful for building planners for choosing the optimal surface finishes from thermal and energetic point of view. This factor depends on the cooling and heating degrees-day and solar radiation. For positive values of the outside coating factor (0, +2.5), the optimal choice falls on the high solar reflectance and high infrared emissivity surface finishes (high ρsol and Ԑinf), i.e. cool paints; instead, for negative values of OCF (-0.5, 2.5), the outside finishes should have low solar reflectance and low emissivity (low ρ sol and Ԑinf). In the range [0, 0.5] greater attention is required, and a more specific analysis should be carried out. The coating factor is evaluated for an air-conditioned building and also for a building without summer thermal control. For example, in Seville (positive OCF value equal to about +1), the use of cool paints on the pitched roof determines a relevant reduction of the primary energy (EPy=–11.8 %). Contrariwise, in Milan (negative OCF value, about -1.5), the cool paints induce an increase of the primary energy (EP y=+4.1 %). In the building without air-conditioning in summer, the OCF convenience area for the cool paints becomes larger [+2.5, -0.5], but could even more extend. For not air-conditioned buildings, this convenience is in terms of reduced number of summer discomfort hours rather than in terms of energy saving. Besides the energy analysis, an investigation about thermal comfort (adaptive comfort) is also performed. The best case refers to the traditional building (without insulation) with cool paint on pitched roof, which allows to minimize the number of discomfort hours (31 % of the summer total hours) for Palermo, while this value is 71% in the same case but without the cool paints. So a relevant reduction of the indoor summer overheating is obtained. Acknowledgements This work was developed in the framework of the European project "Italian Training qualificatiOn Workforce in buildiNg" (Acronym: BUILD UP Skills I-TOWN), Intelligent Energy - Europe (IEE), Call for proposals CIP-IEE2013 - BUILD UP Skills Initiative. Contract N°: IEE/13/BWI/721/SI2.680178.
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