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Experimental in-lab and in-field analysis of waterproof membranes for cool roof application and urban heat island mitigation
Energy and Buildings 114 (2016) 180–190
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Experimental in-lab and in-field analysis of waterproof membranes for cool roof application and urban heat island mitigation A.L. Pisello a,b , V.L. Castaldo a , G. Pignatta a,∗ , F. Cotana a,b , M. Santamouris c a b c
CIRIAF—Interuniversity Research Center on Pollution and Environment “M. Felli”, University of Perugia, via G. Duranti 63, 06125 Perugia, Italy University of Perugia, Department of engineering, via G. Duranti 93, 06125 Perugia, Italy National and Kapodistrian University of Athens, Physics Department Group Building Environmental Research, Panepistimioupolis, 15784 Athens, Greece
a r t i c l e
i n f o
Article history: Received 26 February 2015 Received in revised form 14 May 2015 Accepted 16 May 2015 Available online 22 May 2015 Keywords: Cool roof Albedo Urban heat island Energy efficiency in buildings Weathering Reflective roof Passive cooling
a b s t r a c t Buildings are responsible for about the 40% of the global annual energy consumption, therefore, innovative strategies for buildings’ energy efficiency are under development. Strategies of re-roofing with “cool” materials have a non-negligible cooling energy saving potential, as they contribute to the reduction of the peak ambient temperatures during summer and, moreover, they contribute to the improvement of the urban microclimate by decreasing the intensity of heat island phenomena. In this paper, the experimental characterization and optimization of a new membrane for buildings’ roof is carried out. To this aim, laboratory measurements were performed to determine its optic-energy properties and, therefore, to optimize its “cool roof” behavior. A full scale field test was also setup in order to measure the global solar radiation reflected by each membrane, before and after optimization, with varying boundary conditions, e.g. time during the day, seasonal period, and weather conditions. The in-field experimental campaign allowed to characterize the optic-energy behavior of the cool membranes in real boundary conditions, showing non-negligible variation of measured solar reflection capability with varying environmental constraints in winter conditions. The research showed interesting results from the in-lab optimization campaign, and non-negligible unreliability due to environmental agents affecting in-field albedo measurement. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Buildings’ energy consumption in developed countries has increased up to 40% [1], and has exceeded other major sectors such as industry and transportation. In particular, a considerable increase in the energy requirement for cooling has been detected [2,3], given the increasing urban temperatures due to important climate change phenomena such as urban heat island [4]. In fact, urban heat island and global warming are able to increase the near surface ambient temperature in cities, by consequently modifying buildings’ energy balance [5]. While urban heat islands all around the world are widely investigated [6], the research about the impact of global warming on urban climate still consists of a few works proposing several methodologies to quantify such effect [7] in terms of CO2eq offset. The urban heat island phenomenon is imputable to several factors: urban dense constructions’ configuration [8], solar absorptive materials exposed to solar radiation used for buildings’ envelope [9], impervious surfaces limiting
∗ Corresponding author. Tel.: +39 075 585 3796; fax: +39 075 5153321. E-mail address: [email protected] (G. Pignatta). http://dx.doi.org/10.1016/j.enbuild.2015.05.026 0378-7788/© 2015 Elsevier B.V. All rights reserved.
evapotranspiration [10], released anthropogenic heat, and, in general, lower urban albedo than the extra-urban areas [11]. In this panorama, several experimental and numerical studies have been performed in order to develop mitigation solutions and passive cooling strategies within different climatological boundaries [12,13]. The mitigation technique we are focused on in this work consists of the increase in the albedo of buildings’ envelopes and urban paving [14,15], with a particular attention dedicated to horizontal roofs exposed to the solar radiation [16]. In fact, roofs cover at least 25% of urban surfaces, and increasing their reflectivity would have a significant effect on city total energy balance [17,18]. Additionally, the use of infrared reflective materials for repaving urban surfaces [19,20] is encouraged in [21] for countering the negative effects of global warming such as the increase of cooling energy demand in conditioned buildings. In this panorama, cool colored ceramic tiles, acrylic paints, and bituminous membranes for building envelope applications were developed and experimentally tested [22], by taking into account both cool roof and facade applications. Many studies have been carried out [23–27] in order to assess the effectiveness of such “cool solutions” for the energy saving both at building scale and at inter-building level. Laboratory and
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Nomenclature SW: standard white roof membrane, sample I SG: standard grey roof membrane, sample II W + P: white +30% white paste roof membrane, sample III W + P-opt: white +30% optimized white paste roof membrane, sample IV W + PCM: white +10% paraffin roof membrane, sample V Tout : average daily outdoor dry bulb temperature [◦ C] outmost (maximum or minimum) value of albedo ˛out : registered in each considered day and time interval of measurement [–] ˛ ¯ ref : average albedo measured in summer clear days in the 12:00–2:00 p.m. time interval [–] average albedo measured in summer clear days in ˛ ¯ 2–3 : the 2:00–3:00 p.m. time interval [–] ˛ ¯ w: average albedo measured in winter clear days in the 12:00–2:00 p.m. time interval [–] ˛ ¯ cl : average albedo measured in summer cloudy days in the 12:00–2:00 p.m. time interval [–] ˛ ¯ 3–5 : average albedo measured in summer clear days in the 3:00–5:00 p.m. time interval [–]
in-field measurements of the solar reflectance of light-colored roofing membranes were performed by [28] with varying weather conditions, in order to investigate how the aging and weathering forcing can reduce membranes’ thermal-optic performance. Replacing a dark roof with a solar-reflective white cover could decrease the annual conditioning use by almost 20% [29,30], but these benefits due to cool roofs application could be decreased by soiling and weathering processes [31,32]. Therefore, several studies have been performed [33] in order to investigate the variation of the solar reflectance of cool sheet membrane roofs over time and how the building energy use is affected. Additionally, the variation of the solar irradiance and solar reflectance with (i) surface orientation, (ii) solar position, and (iii) atmospheric conditions has been studied [34,35]. In this scenario, several methodologies have been proposed for achieving reliable measurement of the solar reflectivity of both flat and sloped surfaces, in order to properly predict the solar heat gain absorbed by such surfaces. In fact, the angular and spectral distribution of solar radiation changes with solar position and sky conditions. Consequently, the measured value of solar reflectance varies during the course of the day and of the year, given its dependence to the incident angle and wavelength of the solar radiation. The present research concerns the experimental investigation of the optic-thermal properties of several types of roofing membranes in terms of solar reflectance, thermal-emittance, and in-field albedo. In particular, the possibility to measure the solar reflectivity at large solar zenith angle typical in winter and spring/fall conditions is here questioned. Additionally, the influence of weather agents on the measured albedo is investigated. Several quantitative observations are carried out, with the aim to analyze the albedo variability when field measurements can be taken according or not to the reference international standard procedure [36]. Finally, the variability of the in-field albedo due to varying environmental agents is quantified by taking into account the results collected by the present experimental campaign on polyurethane based membranes for cool roofs. 2. Purpose of the work The present research concerns the experimental analysis and optimization of a waterproof membrane for cool roof application, considered as an effective urban heat island mitigation technique
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[37,38]. This technology consists of a polyurethane-based liquid almost white membrane, which is frequently used for roof covering in Italy in both residential and industrial buildings with flat and low-sloped roof. The experimental campaign aimed at characterizing the cool membrane with varying environmental conditions was carried out through coupled in-lab and in-field analyses. In particular, the analysis of five different prototype membranes was carried out, i.e. (i) standard white, (ii) standard grey, (iii) standard white with additional 30% white paste, (iv) standard white with additional 30% optimized white paste, and (v) standard white with 10% integrated paraffin as phase change material. The optical-thermal properties of such innovative membrane were investigated through a dedicated in-lab measurement campaign, in terms of solar reflectance and thermal emittance. During the in-lab process, the optimization of the cooling potential was performed by increasing specific components, i.e. titanium dioxide (TiO2 ) and hollow ceramic microspheres. Additionally, the superficial finishing of the membrane, named “white paste”, was improved in sample (iv) with the purpose to minimize its sticky effect, typical of polyurethane base products, affecting their self-cleaning capability. An in-field monitoring campaign was therefore carried out in order to measure the albedo variation of the membranes with varying several weather boundary conditions, both before and after optimization. The main objective was to study the measured albedo variation due to different environmental factors, such as the daily time of measurement, the seasonal variation during the course of the year, and the cloudiness. In fact, important variations of in-field measured albedo are expected with varying realistic boundary conditions affecting the reliability of the acknowledged [36] measurement procedure. A final quantitative evaluation with respect to the standardized measurement method was carried out in realistic weather conditions, in a temperate climate zone at 43◦ N Latitude, since precision and bias statement has not been established yet and the field measured solar reflectance, according to [36], is expected to vary both from one location to another and with time. 3. Materials and methods The main steps of the research are described as follows: - selection of the cool roof membranes for cool roofs; - development of the prototype samples for laboratory measurements; - development of the field test setup for measuring albedo in the field; - in-lab experimental analysis for the characterization and optimization of the thermal-optical properties of the prototype membranes; - in-field monitoring campaign for the measurement of the albedo of the membranes with varying environmental boundary conditions in winter months and comparison with respect to summer observations; - data analysis and discussion of the main findings. 3.1. Laboratory analysis The purpose of the preliminary in-lab analysis was the definition of the main optic-thermal properties of the polyurethane based membranes, i.e. solar reflectance and thermal emittance. The measurement of the radiative properties of 10 × 10 cm prototype samples was carried out by means of spectrophotometer and portable emissometer, according to standard methods reported in ASTM E903-6 (2010) [39] and ASTM C1371-04A (2010) [40],
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Fig. 1. In-field experimental campaign: (a) field tests of the exposed membranes, (b) albedometer installed over the membranes, and (c) pyranometer.
respectively. In particular, the solar reflectance of the square samples was measured by means of the Solid Spec 3700 UV–vis-NIR spectrophotometer equipped with an integrating sphere. Spectral reflectance was measured from 300 nm to 2500 nm. The thermal emittance of each sample was therefore measured by means of a portable “D&S” thermal emissometer. The emissometer is able to return the hemispherical IR emittance as a value between 0 and 1 with a two-digit precision. Before the measurement, the instrument was calibrated using a high-emissivity (i.e. 0.88) and low-emissivity (i.e. 0.06) reference samples.
Table 1 Main technical details of pyranometers. Manufacturer
LSI LASTEM srl
Model Principle Housing Spectral range Maximum solar irradiance Sensitivity Uncertainty Response time Cable Data logger compatibility
DPA053 Thermopile anodized aluminum 305–2800 nm 2000 W/m2 10.46 ± 0.13 V/W/m2 (1.24%) 0.0014 mV (0.007% FS) with range 20 mV 30 s 5 m length M-Log (ELO007–008)
3.2. In-field monitoring campaign The in-field monitoring campaign consisted of the measurement of the albedos of the different roofing membranes over time in order to determine the measured albedo variation with different environmental boundary conditions. In particular, both the diurnal and seasonal albedo variations were assessed. The in-field albedo measurements were performed on square field tests of about 10 m2 situated over the flat roof of a university building in Perugia, Italy (Fig. 1a). Each field test was left exposed without being cleaned during the whole monitoring period. An albedometer (or doublepyranometer), able to measure simultaneously the incoming and reflected solar radiation, was positioned over one membrane at a time all day long for comparing the albedo of the five membranes (Fig. 1b). More in details, the two pyranometers (Fig. 1c) were installed horizontally on a South-oriented 1.5 m tilt arm, over the center of each field test area at a height of 0.5 m, as suggested in ASTM standard E1918-06. One pyranometer was faced upward to measure incoming horizontal global solar radiation, while the other one was downward faced to measure the reflected solar radiation. The main technical characteristics of the pyranometers are listed in Table 1. In this paper, the methodological approach proposed in ASTM E1918-06 [36] was applied. The measurement campaign was carried out over the period December-July. Additionally, the in-field measurements were performed also in different conditions with respect to the ASTM E1918-06 standard prescription, both in terms
of weather forcings and time interval of measurement, in order to take into account different environmental agents affecting the measured albedo, one at a time. 3.3. In-field data analysis The in-field monitoring campaign allowed the quantitative analysis of the variability of measured albedo with varying one parameter at a time. In particular, time during the course of the day and of the year and weather conditions (clouds’ presence) were selected as key environmental forcings affecting such in-field measured values of albedo. To this aim, as previously mentioned, the in-field campaign was setup both according and with varying selected boundaries with respect to the reference ASTM E1918-06 standard prescription [36]. Therefore, the role of each one of these boundaries was investigated. Additionally, a comparison between measurements performed (i) according to the Standard ASTM E1918-06, (ii) with varying one parameter at a time always according to [36], and (iii) in different conditions that [36] was carried out. The main purpose was to determine the range of albedo variability related to each different environmental factor. Days in winter and summer were analyzed to perform a comparative analysis of the considered white roof membranes. In particular, the albedometer was moved every day from a membrane
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Fig. 2. Solar reflectance profiles of the membranes according to the spectrophotometer.
to another one depending of the weather conditions. The goal was to obtain at least five reliable measurement days per month, characterized by similar weather conditions for each membrane. The average albedo measured according to the ASTM E1918-06 in a summer clear sunny day between 12:00–2:00 p.m. was selected as “reference albedo” ˛ ¯ ref for the quantification of the albedo variability due to each environmental agent calculated as expressed in the following equation: 1 ˛i N N
˛ ¯ ref =
(1)
i=1
where ˛i is each measurement of albedo taken every 10 min in the considered daily time interval (12:00–2:00 p.m.) in a sunny clearsky day in summer; N is the number of albedo measured values (N = 11) collected every 10 min in the considered two hour long time interval. The albedo standard deviation represents the dispersion of the measured albedo data in each considered time interval when i measurements of albedo were carried out. In particular, it was calculated for each selected day in winter and summer, by considering specific time intervals, as follows:
N 1 ˛ = ¯ 2 (˛i − ˛) N
(2)
i=1
¯ is the average value of albedo calculated for each specific where ˛ time interval and day of measurement, and ˛i is each value of albedo
calculated every 10 min as the ratio between incident and reflected global radiation at each ith measurement. The ˛out represents the outmost value of calculated albedo, i.e. maximum or minimum, in each day and time interval of measurement, as reported in the following equation: ˛out = ˛j
such that maxi
˛i − ˛¯ ref = ˛j − ˛¯ ref
(3)
The measured albedo variation was therefore calculated by considering the maximum absolute values of albedo measured with varying one selected environmental boundary at a time, with ¯ ref , as previously specified. respect to the reference albedo, i.e. ˛ In particular, that variability ˛ was calculated as reported in Eq. (4), in order to compare the albedo sensitivity with respect to each varying environmental parameter. ˛ = ˛out − ˛ ¯ ref
(4)
4. Results and discussion 4.1. Characterization and optimization of the prototype membranes Results of the in-lab spectrophotometer measurements of solar reflectance of the five membranes’ samples by means of spectrophotometer are reported in Table 2. Additionally, Fig. 2 shows the trends of the solar reflectance of the membranes measured by spectrophotometer, with reference to the solar spectrum reported in the standard ASTM G173-03 (2012) [41] over the wavelength interval [300–2500 nm].
Fig. 3. Incident and reflected solar radiation profiles measured by the double-pyranometer on membrane I, in a (a) sunny and (b) cloudy day of March and February, respectively.
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Table 2 Solar reflectance of the five membranes’ samples measured by spectrophotometer. Number of the membrane
Name of the membrane
Solar reflectance [%] (300–2500 nm)
I II III IV
SW standard white SG standard grey W + P white +30% white paste W + P-opt white +30% optimized white paste W + PCM white +10% paraffin
81.6 26.2 83.3 85.4
V
80.7
These in-lab measured values showed a consistent trend of the solar reflectance for all the white membranes, both in terms of profile shape and final calculated values of reflectance. Additionally, Fig. 2 shows that the grey membrane (II) presents a significantly lower profile of solar reflectance, over all the spectral wavelengths. Therefore, it is possible to assess that the “standard grey” membrane is characterized by the lowest solar reflectance, i.e. 26%, not only in the visible region of the spectrum. Moreover, the white membrane with additional 30% optimized white paste (IV) presents the highest values of solar reflectance, i.e. 85%. The thermal emittance values measured at ambient temperature of 20 ◦ C by portable
Fig. 4. Albedo profiles in selected days between December and April, from 9:00 a.m. to 6:00 p.m. The focus on the central time interval is highlighted in red and reported in Fig. 5.
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Fig. 5. Albedo profiles in selected days between December and April, from 12:00 p.m. to 2:00 p.m.
emissometer showed a constant value of about 0.9 for all the evaluated prototype membranes, as expected.
irregular and ragged, and it was almost impossible to define a consistent parabolic trend due to the random cloudy and/or rainy conditions.
4.2. Global radiation with varying weather conditions 4.3. Diurnal variation of measured albedo The albedo measurement campaign allowed the analysis of the global radiation profile described by incoming global radiation and by reflected radiation, with varying weather conditions in typical sunny and cloudy days (Fig. 3) of the monitored months. In sunny days, as expected, the global solar radiation was detected to be relatively low in the morning (9:00 a.m.) and late afternoon (6:00 p.m.), while its peak was clearly registered during the central hours of the day, i.e. 12:00–2:00 p.m. On the contrary, during cloudy and/or rainy days, when the amount of sunlight incident to the surface is low, the profile of the solar radiation was
In order to estimate the diurnal profile of in-field measured albedo, i.e. the ratio between global solar radiation measured by the upward oriented pyranometer divided for the radiation measured by the downward oriented pyranometer, both clear sunny and cloudy/rainy days for each monitored month were selected with reference to each evaluated membrane. The diurnal variation of solar reflectivity of all the evaluated membranes in selected winter-spring days is plotted in Figs. 4–5. The same trend was found in all the five tested membranes. In
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Fig. 6. Monthly average mid-day in-field albedo of the five membranes.
particular, the in-field monitoring showed that typically, during sunny clear days in winter and spring, the albedo is constant during the central hours of the day, especially between 12:00 and 2:00 p.m., and it increases in the afternoon, i.e. 4:00–6:00 p.m., when the sun incidence angle gradually decreases. Therefore, by neglecting the early morning and late afternoon shadowing effect due to the instability of the measurement and the presence of shading infrastructures near the field tests, the measured albedo is almost constant during the central hours of the day, when the sun beam is more perpendicular to the site and the sun angle to the normal from the membrane surface is less than 45◦ , therefore when the conditions of measurement are according to the international standard, as expected [36]. Additionally, the measured albedo varies when the incident angle of the solar irradiance is low, i.e. early morning and late afternoon, given the presence of shading or obstacles impacting the incident sunlight during sunrise and sunset time intervals. 4.4. Seasonal variation of measured albedo The five different membranes were continuously monitored over time without being cleaned, with the purpose of investigating the measured albedo variation over different seasons, i.e. winter and spring conditions. Therefore, both the initial dusting and weathering phenomena resulting in a change of the surface color of the membranes and, consequently, in a change of the measured albedos were analyzed. These considerations were integrated with the observations about the diurnal variation of such property. Non consistent decreasing trend of measured albedo was detected from January to April, showing a non-crucial dusting and soiling effect observed in the exposition site, even if non negligible monthly variations were measured attributable to other causes. Nevertheless, the optimized finishing membranes (III and IV) were the ones presenting a slightly growing trend of measured albedo from January to April, showing that the optimized finishing (white paste) was able to minimize the dusting and soiling effect. The maximum seasonal albedo non-monotonic variation was found for the optimized membrane (white +30% optimized white paste − W + Popt) to be about 6%. Additionally, the results showed similar albedos of the white roofing membranes (I, III, IV, and V), with the exception of the grey membrane (II), which presented lower values during the course of the whole monitored period, as expected from in-lab characterization. In particular, the grey membrane presented almost half of the albedo with respect to the standard white membrane (I), i.e. about 0.3 vs. 0.6. Fig. 6 shows for each membrane the monthly average albedo from January to April, with the aim to investigate the effect of seasonal changes on the mid-day (12:00–2:00 p.m.) measured albedo of the membranes.
All the evaluated membranes except for the optimized one W + P-opt showed a stable and constant trend of the albedos over the monitored months, i.e. with maximum variation of 0.04. Therefore, non-significant change of the mid-day measured albedos was found over the winter/spring monitored seasons. Additionally, the measured albedo was slightly lower during the last monitored spring months due also to the preliminary dusting, soiling, and weathering processes occurring over the monitored period. Fig. 6 shows that the in-field approach [36] applied to estimate each membrane albedo does not represent a reliable procedure to detect relatively small improvements of the performance in terms of solar reflectance capability, as observed by mean of in-lab campaign. In fact, the only visible and consistent result concerned the difference in terms of measured albedo between the standard grey membrane (II) and the group of the white membranes (samples I, III, IV, and V), according to the spectrophotometer measured findings. The variability of environmental conditions, even when according to the application boundaries described in [36], does not allow to perceive such a meticulous variation. In particular, the in-lab results were detected to be significantly higher than the infield measured albedo values. This discrepancy is mainly due to the different in-field measurement conditions. In fact, the spectrophotometer is able to measure solar reflectance in a small area of a surface over the wavelength range 300–2500 nm using integrating spheres within controlled conditions and standard solar spectrum. On the contrary, the albedometer is able to measure the ratio between incoming and reflected global solar radiation over a horizontal plane, under variable boundary conditions and with real solar forcing (and spectrum). Therefore, the in-field measurement is affected by all the realistic varying boundary conditions, i.e. weather parameters, presence of rain water over the membrane samples, aging, dusting, etc. For this reason, the in-lab measurement procedure is more appropriate for characterizing the solar reflectance of materials and for comparison purposes, while the in-field albedo measurement approach is suitable for monitoring the optical properties of materials when applied in real roofs and surfaces. The two methodologies of measurement are therefore complementary. In order to avoid redundant information, the albedo of selected days in winter and spring characterized by random boundaries conditions is displayed in Fig. 7, for all the evaluated membranes. All the graphs in Fig. 7 show that, irrespective of weather conditions and period of the year, the albedo was substantially constant during the course of the day and the measurements presented a gradually increasing trend in the afternoon. Additionally, in winter days the increasing trend of the albedo due to measurement perturbation was detected to be earlier, i.e. 4:00 p.m. if compared to the spring time, when the albedo begins to increase later in the day, i.e. 5:00–6:00 p.m. This was true for all the membranes. 4.5. Effect of environmental boundaries on measured albedo This section concerns the quantification of the variability of the in-field albedo measurement due to several environmental conditions. To this aim, a comparison between measurements performed (i) according to the Standard ASTM E1918-06 and (ii) with varying one parameter at a time was carried out, in order to determine the range of albedo variability related to each different environmental factor. Table 3 reports these calculated values of albedo for the reference membrane IV (white +30% optimized white paste −W + P-opt). Additionally, by considering one varying parameter at a time, several values of albedo were calculated while considering different daily and yearly time intervals and cloudy conditions of the sky. In general, the albedo variation ˛ was calculated with respect to the reference albedo, i.e. ˛ ¯ ref = 0.60, calculated according to the ASTM [36] international standard in the very central ranges
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Fig. 7. Albedo profiles within two selected days in winter and spring for each membrane.
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Table 3 In-field albedo of the tested membrane IV, measured with varying measurement conditions. Environmental boundaries I—season of measurement II—sky conditions III—daily time interval Measured data Average daily outdoor dry bulb temperature Tout [◦ C] ¯ [–] Average albedo ˛ Maximum albedo ˛max [–] Minimum albedo ˛min [–] Outmost albedo ˛out [–] Standard deviation ␣ [–] Albedo variation ˛ [–]
According to the ASTM E1918-06 procedure “2–3” “ref”
“w”
Out of the ASTM E1918-06 procedure “cl” “3–5”
Summer July, 03rd Sunny clear sky day 12:00–2:00 p.m.
Summer July, 03rd Sunny clear sky day 2:00–3:00 p.m.
Winter Dec, 23rd Sunny clear sky day 12:00–2:00 p.m.
Cloudy sky day 12:00–2:00 p.m.
Summer July, 03rd Sunny clear sky day 3:00–5:00 p.m.
27
27
6
25
27
0.60 0.62 0.57 0.57 0.014 −0.03
0.57 0.57 0.56 0.56 0.003 −0.04
0.68 0.68 0.68 0.68 0.003 +0.08
0.61 0.62 0.57 0.57 0.024 −0.03
0.57 0.57 0.54 0.54 0.009 −0.06
of measurements indicated within the standard, i.e. in clear sky conditions, in summer time, and close to noon. The results of the present experimental campaign showed that, if the measurements are performed according to the Standard guidelines, the period of the year is the most impacting factor devi¯w > ˛ ¯ ref by 13%) with respect to ating the albedo measurement (˛ the reference value of 0.60. Additionally, the time interval of measurement was registered to be responsible for a 0.04 points of variability when the measurements are carried out over 2:00–3:00 p.m. (˛ ¯ 2–3 < ˛ ¯ ref ), which represents the boarder of the time interval suggested by the Standard [36]. Therefore, Table 3 shows that, even if the measurements are performed according to the procedure indicated by the Standard, a non-negligible probability to have a range of albedo variability >0.01 in terms of ˛ is experimentally detected, at least in the present monitoring campaign and in the considered temperate climate region and latitude. Furthermore, when the measurements were performed without considering the Standard guidelines, the time of measurement was the main factor affecting the albedo in-field measurement (0.06 points of variation detected over the 3:00–5:00 p.m. time interval, i.e. ˛ ¯ 3–5 < ˛ ¯ ref ), while, the cloudiness generated a relatively lower albedo variability of 5% (i.e. ˛ ¯ cl < ˛ ¯ ref by 0.03). Consistently to these results, such albedo variability was detected also for all the other membranes. A similar trend was found for the membranes in terms of overestimation and underestimation of the in-field measured albedo if compared to the albedo measured according to the Standard procedure in the only time interval 12:00–2:00 p.m. In particular, the measurement taken in winter always led to an overestimation of the measured albedo (i.e. ˛ ¯w > ˛ ¯ ref by +13%). The same result was found for the measurements carried out with varying time interval. In fact, an underestimation of 0.04 and 0.06, with reference to 2:00–3:00 p.m. and 3:00–5:00 p.m. measurement periods, respectively, was detected. Therefore, even if the albedo measurements are carried out according to the procedure described in the ASTM E1918-06, it is extremely difficult to limit the variability of the infield measured albedo lower than 0.01 in realistic conditions, as observed in this experimental campaign. In general, the procedure described in the ASTM E1918-06 could produce more stable measured albedo values in higher latitude areas characterized by clearer sky conditions.
5. Conclusions This paper presents the experimental analysis of a combined albedo measurement campaign for the characterization of an innovative waterproof polyurethane-based membrane for cool
Summer July, 02nd
roof application, before and after optimization. Both in-lab and in-field analyses of five different roofing membranes were carried out in order to evaluate their thermal-optic properties, i.e. solarreflectance, thermal-emittance, and in-field measured albedo, determining their passive cooling potential. The in-lab experimental measurement campaign was carried out by using a spectrophotometer and portable emissometer, in order to characterize the optic-thermal properties of the membranes’ prototype samples. The in-field albedo long-term monitoring was performed by using a dedicated albedometer and automatic data collection/acquisition system. In particular, the standard approach defined in ASTM E1918-06 was applied to measure the albedo during winter-spring season, with the purpose of investigating the variation of measured albedo due to (i) the solar position and (ii) weather conditions. A precision and bias statement has not been established yet for this international reference standard and the measured albedo of most surfaces is expected to vary both from one location to another and with time, as showed in this experimental research. Therefore, the scope of the work was to investigate such expected variability in order to study the reliability of such in-field measurement method. To this aim, the effect of one environmental boundary at a time on the in-field measured albedo was evaluated. The in-lab results showed that the white membranes (samples I, III, IV, and V) present the highest values of solar reflectance, varying within 80.7% and 85.4%, and the standard grey membrane(II) presents the lowest solar reflectance, i.e. 26%. Additionally, all the evaluated membranes presented a constant and high value of thermal emittance, i.e. 0.9, as expected. Therefore, all the membranes with the exception of the grey membrane (II) could be considered as suitable for cool roof applications and urban heat island mitigation if applied in dense urban areas. The in-field experimental campaign showed that the measured albedo is relatively stable during the day, especially in the middle hours, i.e. 12:00–2.00 p.m., and it increases in the afternoon with the decrease of the angle of the sun to the normal from the membrane. Non consistent decreasing trend of measured albedo was detected from January to April, even if non negligible monthly variations were measured. In particular, a maximum seasonal albedo variation of 6% was found for the optimized membrane (IV sample, “white +30% optimized white paste”). The analysis of the in-field albedo variability attributable to environmental agents, such as (i) the period of the year, (ii) the cloudiness, and (iii) the time interval of measurement, showed that it is extremely difficult to measure the in-field albedo with a variability <0.01, at least in the boundary conditions of this
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experimental campaign and in the considered temperate climate region and latitude. In fact, such environmental agents were always responsible for overestimation or underestimation of the measured albedo with respect to the reference albedo value measured according to the ASTM E1918-06 in clear sunny days, during the 12:00–2:00 p.m. time interval. Therefore, such in-field measurement method showed to be not suitable to detect relatively small performance differences, such as the overall 5% improvement achieved by mean of the optimized membrane IV with respect to the membrane V, as it was measured in-lab by mean of spectrophotometer. Nevertheless, more stable in-field measured albedo values are expected in those areas presenting more constant weather conditions, i.e. in higher latitude areas characterized by clearer sky conditions. 6. Future developments Future research will investigate the effect of the aging, soiling, and weathering on the cool sheet membrane’s thermal-radiative properties, i.e. solar reflectance and thermal emittance, through a dedicated time performance analysis. Additionally, starting from the experimental results obtained from this study, an innovative standard procedure for measuring in-field albedo will be developed. Further analyses of the evaluated membranes will therefore be carried out when applied in real-scale buildings. Acknowledgements The authors would like to thank the CVR S.R.L. for providing the roofing membranes prototype samples for the experimental campaign. The authors also acknowledge Gabriele Franceschetti for assisting the experimental setup. The first author acknowledgments are due to the “CIRIAF program for UNESCO” in the framework of the UNESCO Chair “Water Resources Management and Culture”, for supporting her research. References [1] L. Perez-Lombard, J. Ortiz, C. Pout, A review on buildings energy consumption information, Energy Build. 40 (2008) 394–398, http://dx.doi.org/10.1016/ j.enbuild.2007.03.007 [2] D.A. Asimakopoulos, M. Santamouris, I. Farrou, M. Laskari, M. Saliari, S.C. Zerefos, T. Antonakaki, C. Giannakopoulos, Modelling the energy demand projection of the building sector in Greece in the 21th century, Energy Build. 49 (2012) 488–498, http://dx.doi.org/10.1016/j.enbuild.2012.02.043 [3] S. Hassid, M. Santamouris, N. Papanikolau, A. Linardi, N. Klitsikas, C. Georgakis, D.N. Assimakopoulos, Effect of the Athens heat island on air conditioning load, Energy Build. 32 (2) (2000) 131–141, http://dx.doi.org/10.1016/S03787788(99)00045-6 [4] A. Synnefa, M. Santamouris, Advances on technical, policy and market aspects of cool roof technology in Europe: the cool roofs project, Energy Build. 55 (2012) 35–41, http://dx.doi.org/10.1016/j.enbuild.2011.11.051 [5] M. Santamouris, On the energy impact of urban heat island and global warming on buildings, Energy Build. 82 (2014) 100–113, http://dx.doi.org/10.1016/ j.enbuild.2014.07.022 [6] M. Santamouris, Heat island research in Europe—the state of the art, Adv. Build. Energy Res. 1 (2007) 123–150, http://dx.doi.org/10.1080/17512549. 2007.9687272 [7] M.P. McCarthy, M.J. Best, R.A. Betts, Climate change in cities due to global warming and urban effects, Geophys. Res. Lett. 37 (2010) 9, http://dx.doi.org/10. 1029/2010GL042845 [8] H.E. Landsberg, The Urban Climate International Geographic Series, vol. 28, Academic Press, New York,NY, 1981. [9] J.P. Montavez, A. Rodriguez, J.I. Jimenez, A study of the urban heat island of Granada, Int. J. Climatol. 20 (2000) 899–911, http://dx.doi.org/10.1002/10970088(20000630)20:8<899::AID-JOC433>3.0.CO;2-I [10] M.L. Imhoff, P. Zhang, R.E. Wolfe, L. Bounoua, Remote sensing of the urban heat island effect across biomes in the continental USA, Remote Sens. Environ. 114 (2010) 504–513, http://dx.doi.org/10.1016/j.rse.2009.10.008 [11] H. Akbari, S. Konopacki, Calculating energy-saving potentials of heat-island reduction strategies, Energy Policy 33 (2005) 721–756, http://dx.doi.org/10. 1016/j.enpol.2003.10.001
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Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Experimental in-lab and in-field analysis of waterproof membranes for cool roof application and urban heat island mitigation A.L. Pisello a,b , V.L. Castaldo a , G. Pignatta a,∗ , F. Cotana a,b , M. Santamouris c a b c
CIRIAF—Interuniversity Research Center on Pollution and Environment “M. Felli”, University of Perugia, via G. Duranti 63, 06125 Perugia, Italy University of Perugia, Department of engineering, via G. Duranti 93, 06125 Perugia, Italy National and Kapodistrian University of Athens, Physics Department Group Building Environmental Research, Panepistimioupolis, 15784 Athens, Greece
a r t i c l e
i n f o
Article history: Received 26 February 2015 Received in revised form 14 May 2015 Accepted 16 May 2015 Available online 22 May 2015 Keywords: Cool roof Albedo Urban heat island Energy efficiency in buildings Weathering Reflective roof Passive cooling
a b s t r a c t Buildings are responsible for about the 40% of the global annual energy consumption, therefore, innovative strategies for buildings’ energy efficiency are under development. Strategies of re-roofing with “cool” materials have a non-negligible cooling energy saving potential, as they contribute to the reduction of the peak ambient temperatures during summer and, moreover, they contribute to the improvement of the urban microclimate by decreasing the intensity of heat island phenomena. In this paper, the experimental characterization and optimization of a new membrane for buildings’ roof is carried out. To this aim, laboratory measurements were performed to determine its optic-energy properties and, therefore, to optimize its “cool roof” behavior. A full scale field test was also setup in order to measure the global solar radiation reflected by each membrane, before and after optimization, with varying boundary conditions, e.g. time during the day, seasonal period, and weather conditions. The in-field experimental campaign allowed to characterize the optic-energy behavior of the cool membranes in real boundary conditions, showing non-negligible variation of measured solar reflection capability with varying environmental constraints in winter conditions. The research showed interesting results from the in-lab optimization campaign, and non-negligible unreliability due to environmental agents affecting in-field albedo measurement. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Buildings’ energy consumption in developed countries has increased up to 40% [1], and has exceeded other major sectors such as industry and transportation. In particular, a considerable increase in the energy requirement for cooling has been detected [2,3], given the increasing urban temperatures due to important climate change phenomena such as urban heat island [4]. In fact, urban heat island and global warming are able to increase the near surface ambient temperature in cities, by consequently modifying buildings’ energy balance [5]. While urban heat islands all around the world are widely investigated [6], the research about the impact of global warming on urban climate still consists of a few works proposing several methodologies to quantify such effect [7] in terms of CO2eq offset. The urban heat island phenomenon is imputable to several factors: urban dense constructions’ configuration [8], solar absorptive materials exposed to solar radiation used for buildings’ envelope [9], impervious surfaces limiting
∗ Corresponding author. Tel.: +39 075 585 3796; fax: +39 075 5153321. E-mail address: [email protected] (G. Pignatta). http://dx.doi.org/10.1016/j.enbuild.2015.05.026 0378-7788/© 2015 Elsevier B.V. All rights reserved.
evapotranspiration [10], released anthropogenic heat, and, in general, lower urban albedo than the extra-urban areas [11]. In this panorama, several experimental and numerical studies have been performed in order to develop mitigation solutions and passive cooling strategies within different climatological boundaries [12,13]. The mitigation technique we are focused on in this work consists of the increase in the albedo of buildings’ envelopes and urban paving [14,15], with a particular attention dedicated to horizontal roofs exposed to the solar radiation [16]. In fact, roofs cover at least 25% of urban surfaces, and increasing their reflectivity would have a significant effect on city total energy balance [17,18]. Additionally, the use of infrared reflective materials for repaving urban surfaces [19,20] is encouraged in [21] for countering the negative effects of global warming such as the increase of cooling energy demand in conditioned buildings. In this panorama, cool colored ceramic tiles, acrylic paints, and bituminous membranes for building envelope applications were developed and experimentally tested [22], by taking into account both cool roof and facade applications. Many studies have been carried out [23–27] in order to assess the effectiveness of such “cool solutions” for the energy saving both at building scale and at inter-building level. Laboratory and
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Nomenclature SW: standard white roof membrane, sample I SG: standard grey roof membrane, sample II W + P: white +30% white paste roof membrane, sample III W + P-opt: white +30% optimized white paste roof membrane, sample IV W + PCM: white +10% paraffin roof membrane, sample V Tout : average daily outdoor dry bulb temperature [◦ C] outmost (maximum or minimum) value of albedo ˛out : registered in each considered day and time interval of measurement [–] ˛ ¯ ref : average albedo measured in summer clear days in the 12:00–2:00 p.m. time interval [–] average albedo measured in summer clear days in ˛ ¯ 2–3 : the 2:00–3:00 p.m. time interval [–] ˛ ¯ w: average albedo measured in winter clear days in the 12:00–2:00 p.m. time interval [–] ˛ ¯ cl : average albedo measured in summer cloudy days in the 12:00–2:00 p.m. time interval [–] ˛ ¯ 3–5 : average albedo measured in summer clear days in the 3:00–5:00 p.m. time interval [–]
in-field measurements of the solar reflectance of light-colored roofing membranes were performed by [28] with varying weather conditions, in order to investigate how the aging and weathering forcing can reduce membranes’ thermal-optic performance. Replacing a dark roof with a solar-reflective white cover could decrease the annual conditioning use by almost 20% [29,30], but these benefits due to cool roofs application could be decreased by soiling and weathering processes [31,32]. Therefore, several studies have been performed [33] in order to investigate the variation of the solar reflectance of cool sheet membrane roofs over time and how the building energy use is affected. Additionally, the variation of the solar irradiance and solar reflectance with (i) surface orientation, (ii) solar position, and (iii) atmospheric conditions has been studied [34,35]. In this scenario, several methodologies have been proposed for achieving reliable measurement of the solar reflectivity of both flat and sloped surfaces, in order to properly predict the solar heat gain absorbed by such surfaces. In fact, the angular and spectral distribution of solar radiation changes with solar position and sky conditions. Consequently, the measured value of solar reflectance varies during the course of the day and of the year, given its dependence to the incident angle and wavelength of the solar radiation. The present research concerns the experimental investigation of the optic-thermal properties of several types of roofing membranes in terms of solar reflectance, thermal-emittance, and in-field albedo. In particular, the possibility to measure the solar reflectivity at large solar zenith angle typical in winter and spring/fall conditions is here questioned. Additionally, the influence of weather agents on the measured albedo is investigated. Several quantitative observations are carried out, with the aim to analyze the albedo variability when field measurements can be taken according or not to the reference international standard procedure [36]. Finally, the variability of the in-field albedo due to varying environmental agents is quantified by taking into account the results collected by the present experimental campaign on polyurethane based membranes for cool roofs. 2. Purpose of the work The present research concerns the experimental analysis and optimization of a waterproof membrane for cool roof application, considered as an effective urban heat island mitigation technique
181
[37,38]. This technology consists of a polyurethane-based liquid almost white membrane, which is frequently used for roof covering in Italy in both residential and industrial buildings with flat and low-sloped roof. The experimental campaign aimed at characterizing the cool membrane with varying environmental conditions was carried out through coupled in-lab and in-field analyses. In particular, the analysis of five different prototype membranes was carried out, i.e. (i) standard white, (ii) standard grey, (iii) standard white with additional 30% white paste, (iv) standard white with additional 30% optimized white paste, and (v) standard white with 10% integrated paraffin as phase change material. The optical-thermal properties of such innovative membrane were investigated through a dedicated in-lab measurement campaign, in terms of solar reflectance and thermal emittance. During the in-lab process, the optimization of the cooling potential was performed by increasing specific components, i.e. titanium dioxide (TiO2 ) and hollow ceramic microspheres. Additionally, the superficial finishing of the membrane, named “white paste”, was improved in sample (iv) with the purpose to minimize its sticky effect, typical of polyurethane base products, affecting their self-cleaning capability. An in-field monitoring campaign was therefore carried out in order to measure the albedo variation of the membranes with varying several weather boundary conditions, both before and after optimization. The main objective was to study the measured albedo variation due to different environmental factors, such as the daily time of measurement, the seasonal variation during the course of the year, and the cloudiness. In fact, important variations of in-field measured albedo are expected with varying realistic boundary conditions affecting the reliability of the acknowledged [36] measurement procedure. A final quantitative evaluation with respect to the standardized measurement method was carried out in realistic weather conditions, in a temperate climate zone at 43◦ N Latitude, since precision and bias statement has not been established yet and the field measured solar reflectance, according to [36], is expected to vary both from one location to another and with time. 3. Materials and methods The main steps of the research are described as follows: - selection of the cool roof membranes for cool roofs; - development of the prototype samples for laboratory measurements; - development of the field test setup for measuring albedo in the field; - in-lab experimental analysis for the characterization and optimization of the thermal-optical properties of the prototype membranes; - in-field monitoring campaign for the measurement of the albedo of the membranes with varying environmental boundary conditions in winter months and comparison with respect to summer observations; - data analysis and discussion of the main findings. 3.1. Laboratory analysis The purpose of the preliminary in-lab analysis was the definition of the main optic-thermal properties of the polyurethane based membranes, i.e. solar reflectance and thermal emittance. The measurement of the radiative properties of 10 × 10 cm prototype samples was carried out by means of spectrophotometer and portable emissometer, according to standard methods reported in ASTM E903-6 (2010) [39] and ASTM C1371-04A (2010) [40],
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Fig. 1. In-field experimental campaign: (a) field tests of the exposed membranes, (b) albedometer installed over the membranes, and (c) pyranometer.
respectively. In particular, the solar reflectance of the square samples was measured by means of the Solid Spec 3700 UV–vis-NIR spectrophotometer equipped with an integrating sphere. Spectral reflectance was measured from 300 nm to 2500 nm. The thermal emittance of each sample was therefore measured by means of a portable “D&S” thermal emissometer. The emissometer is able to return the hemispherical IR emittance as a value between 0 and 1 with a two-digit precision. Before the measurement, the instrument was calibrated using a high-emissivity (i.e. 0.88) and low-emissivity (i.e. 0.06) reference samples.
Table 1 Main technical details of pyranometers. Manufacturer
LSI LASTEM srl
Model Principle Housing Spectral range Maximum solar irradiance Sensitivity Uncertainty Response time Cable Data logger compatibility
DPA053 Thermopile anodized aluminum 305–2800 nm 2000 W/m2 10.46 ± 0.13 V/W/m2 (1.24%) 0.0014 mV (0.007% FS) with range 20 mV 30 s 5 m length M-Log (ELO007–008)
3.2. In-field monitoring campaign The in-field monitoring campaign consisted of the measurement of the albedos of the different roofing membranes over time in order to determine the measured albedo variation with different environmental boundary conditions. In particular, both the diurnal and seasonal albedo variations were assessed. The in-field albedo measurements were performed on square field tests of about 10 m2 situated over the flat roof of a university building in Perugia, Italy (Fig. 1a). Each field test was left exposed without being cleaned during the whole monitoring period. An albedometer (or doublepyranometer), able to measure simultaneously the incoming and reflected solar radiation, was positioned over one membrane at a time all day long for comparing the albedo of the five membranes (Fig. 1b). More in details, the two pyranometers (Fig. 1c) were installed horizontally on a South-oriented 1.5 m tilt arm, over the center of each field test area at a height of 0.5 m, as suggested in ASTM standard E1918-06. One pyranometer was faced upward to measure incoming horizontal global solar radiation, while the other one was downward faced to measure the reflected solar radiation. The main technical characteristics of the pyranometers are listed in Table 1. In this paper, the methodological approach proposed in ASTM E1918-06 [36] was applied. The measurement campaign was carried out over the period December-July. Additionally, the in-field measurements were performed also in different conditions with respect to the ASTM E1918-06 standard prescription, both in terms
of weather forcings and time interval of measurement, in order to take into account different environmental agents affecting the measured albedo, one at a time. 3.3. In-field data analysis The in-field monitoring campaign allowed the quantitative analysis of the variability of measured albedo with varying one parameter at a time. In particular, time during the course of the day and of the year and weather conditions (clouds’ presence) were selected as key environmental forcings affecting such in-field measured values of albedo. To this aim, as previously mentioned, the in-field campaign was setup both according and with varying selected boundaries with respect to the reference ASTM E1918-06 standard prescription [36]. Therefore, the role of each one of these boundaries was investigated. Additionally, a comparison between measurements performed (i) according to the Standard ASTM E1918-06, (ii) with varying one parameter at a time always according to [36], and (iii) in different conditions that [36] was carried out. The main purpose was to determine the range of albedo variability related to each different environmental factor. Days in winter and summer were analyzed to perform a comparative analysis of the considered white roof membranes. In particular, the albedometer was moved every day from a membrane
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Fig. 2. Solar reflectance profiles of the membranes according to the spectrophotometer.
to another one depending of the weather conditions. The goal was to obtain at least five reliable measurement days per month, characterized by similar weather conditions for each membrane. The average albedo measured according to the ASTM E1918-06 in a summer clear sunny day between 12:00–2:00 p.m. was selected as “reference albedo” ˛ ¯ ref for the quantification of the albedo variability due to each environmental agent calculated as expressed in the following equation: 1 ˛i N N
˛ ¯ ref =
(1)
i=1
where ˛i is each measurement of albedo taken every 10 min in the considered daily time interval (12:00–2:00 p.m.) in a sunny clearsky day in summer; N is the number of albedo measured values (N = 11) collected every 10 min in the considered two hour long time interval. The albedo standard deviation represents the dispersion of the measured albedo data in each considered time interval when i measurements of albedo were carried out. In particular, it was calculated for each selected day in winter and summer, by considering specific time intervals, as follows:
N 1 ˛ = ¯ 2 (˛i − ˛) N
(2)
i=1
¯ is the average value of albedo calculated for each specific where ˛ time interval and day of measurement, and ˛i is each value of albedo
calculated every 10 min as the ratio between incident and reflected global radiation at each ith measurement. The ˛out represents the outmost value of calculated albedo, i.e. maximum or minimum, in each day and time interval of measurement, as reported in the following equation: ˛out = ˛j
such that maxi
˛i − ˛¯ ref = ˛j − ˛¯ ref
(3)
The measured albedo variation was therefore calculated by considering the maximum absolute values of albedo measured with varying one selected environmental boundary at a time, with ¯ ref , as previously specified. respect to the reference albedo, i.e. ˛ In particular, that variability ˛ was calculated as reported in Eq. (4), in order to compare the albedo sensitivity with respect to each varying environmental parameter. ˛ = ˛out − ˛ ¯ ref
(4)
4. Results and discussion 4.1. Characterization and optimization of the prototype membranes Results of the in-lab spectrophotometer measurements of solar reflectance of the five membranes’ samples by means of spectrophotometer are reported in Table 2. Additionally, Fig. 2 shows the trends of the solar reflectance of the membranes measured by spectrophotometer, with reference to the solar spectrum reported in the standard ASTM G173-03 (2012) [41] over the wavelength interval [300–2500 nm].
Fig. 3. Incident and reflected solar radiation profiles measured by the double-pyranometer on membrane I, in a (a) sunny and (b) cloudy day of March and February, respectively.
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Table 2 Solar reflectance of the five membranes’ samples measured by spectrophotometer. Number of the membrane
Name of the membrane
Solar reflectance [%] (300–2500 nm)
I II III IV
SW standard white SG standard grey W + P white +30% white paste W + P-opt white +30% optimized white paste W + PCM white +10% paraffin
81.6 26.2 83.3 85.4
V
80.7
These in-lab measured values showed a consistent trend of the solar reflectance for all the white membranes, both in terms of profile shape and final calculated values of reflectance. Additionally, Fig. 2 shows that the grey membrane (II) presents a significantly lower profile of solar reflectance, over all the spectral wavelengths. Therefore, it is possible to assess that the “standard grey” membrane is characterized by the lowest solar reflectance, i.e. 26%, not only in the visible region of the spectrum. Moreover, the white membrane with additional 30% optimized white paste (IV) presents the highest values of solar reflectance, i.e. 85%. The thermal emittance values measured at ambient temperature of 20 ◦ C by portable
Fig. 4. Albedo profiles in selected days between December and April, from 9:00 a.m. to 6:00 p.m. The focus on the central time interval is highlighted in red and reported in Fig. 5.
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Fig. 5. Albedo profiles in selected days between December and April, from 12:00 p.m. to 2:00 p.m.
emissometer showed a constant value of about 0.9 for all the evaluated prototype membranes, as expected.
irregular and ragged, and it was almost impossible to define a consistent parabolic trend due to the random cloudy and/or rainy conditions.
4.2. Global radiation with varying weather conditions 4.3. Diurnal variation of measured albedo The albedo measurement campaign allowed the analysis of the global radiation profile described by incoming global radiation and by reflected radiation, with varying weather conditions in typical sunny and cloudy days (Fig. 3) of the monitored months. In sunny days, as expected, the global solar radiation was detected to be relatively low in the morning (9:00 a.m.) and late afternoon (6:00 p.m.), while its peak was clearly registered during the central hours of the day, i.e. 12:00–2:00 p.m. On the contrary, during cloudy and/or rainy days, when the amount of sunlight incident to the surface is low, the profile of the solar radiation was
In order to estimate the diurnal profile of in-field measured albedo, i.e. the ratio between global solar radiation measured by the upward oriented pyranometer divided for the radiation measured by the downward oriented pyranometer, both clear sunny and cloudy/rainy days for each monitored month were selected with reference to each evaluated membrane. The diurnal variation of solar reflectivity of all the evaluated membranes in selected winter-spring days is plotted in Figs. 4–5. The same trend was found in all the five tested membranes. In
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Fig. 6. Monthly average mid-day in-field albedo of the five membranes.
particular, the in-field monitoring showed that typically, during sunny clear days in winter and spring, the albedo is constant during the central hours of the day, especially between 12:00 and 2:00 p.m., and it increases in the afternoon, i.e. 4:00–6:00 p.m., when the sun incidence angle gradually decreases. Therefore, by neglecting the early morning and late afternoon shadowing effect due to the instability of the measurement and the presence of shading infrastructures near the field tests, the measured albedo is almost constant during the central hours of the day, when the sun beam is more perpendicular to the site and the sun angle to the normal from the membrane surface is less than 45◦ , therefore when the conditions of measurement are according to the international standard, as expected [36]. Additionally, the measured albedo varies when the incident angle of the solar irradiance is low, i.e. early morning and late afternoon, given the presence of shading or obstacles impacting the incident sunlight during sunrise and sunset time intervals. 4.4. Seasonal variation of measured albedo The five different membranes were continuously monitored over time without being cleaned, with the purpose of investigating the measured albedo variation over different seasons, i.e. winter and spring conditions. Therefore, both the initial dusting and weathering phenomena resulting in a change of the surface color of the membranes and, consequently, in a change of the measured albedos were analyzed. These considerations were integrated with the observations about the diurnal variation of such property. Non consistent decreasing trend of measured albedo was detected from January to April, showing a non-crucial dusting and soiling effect observed in the exposition site, even if non negligible monthly variations were measured attributable to other causes. Nevertheless, the optimized finishing membranes (III and IV) were the ones presenting a slightly growing trend of measured albedo from January to April, showing that the optimized finishing (white paste) was able to minimize the dusting and soiling effect. The maximum seasonal albedo non-monotonic variation was found for the optimized membrane (white +30% optimized white paste − W + Popt) to be about 6%. Additionally, the results showed similar albedos of the white roofing membranes (I, III, IV, and V), with the exception of the grey membrane (II), which presented lower values during the course of the whole monitored period, as expected from in-lab characterization. In particular, the grey membrane presented almost half of the albedo with respect to the standard white membrane (I), i.e. about 0.3 vs. 0.6. Fig. 6 shows for each membrane the monthly average albedo from January to April, with the aim to investigate the effect of seasonal changes on the mid-day (12:00–2:00 p.m.) measured albedo of the membranes.
All the evaluated membranes except for the optimized one W + P-opt showed a stable and constant trend of the albedos over the monitored months, i.e. with maximum variation of 0.04. Therefore, non-significant change of the mid-day measured albedos was found over the winter/spring monitored seasons. Additionally, the measured albedo was slightly lower during the last monitored spring months due also to the preliminary dusting, soiling, and weathering processes occurring over the monitored period. Fig. 6 shows that the in-field approach [36] applied to estimate each membrane albedo does not represent a reliable procedure to detect relatively small improvements of the performance in terms of solar reflectance capability, as observed by mean of in-lab campaign. In fact, the only visible and consistent result concerned the difference in terms of measured albedo between the standard grey membrane (II) and the group of the white membranes (samples I, III, IV, and V), according to the spectrophotometer measured findings. The variability of environmental conditions, even when according to the application boundaries described in [36], does not allow to perceive such a meticulous variation. In particular, the in-lab results were detected to be significantly higher than the infield measured albedo values. This discrepancy is mainly due to the different in-field measurement conditions. In fact, the spectrophotometer is able to measure solar reflectance in a small area of a surface over the wavelength range 300–2500 nm using integrating spheres within controlled conditions and standard solar spectrum. On the contrary, the albedometer is able to measure the ratio between incoming and reflected global solar radiation over a horizontal plane, under variable boundary conditions and with real solar forcing (and spectrum). Therefore, the in-field measurement is affected by all the realistic varying boundary conditions, i.e. weather parameters, presence of rain water over the membrane samples, aging, dusting, etc. For this reason, the in-lab measurement procedure is more appropriate for characterizing the solar reflectance of materials and for comparison purposes, while the in-field albedo measurement approach is suitable for monitoring the optical properties of materials when applied in real roofs and surfaces. The two methodologies of measurement are therefore complementary. In order to avoid redundant information, the albedo of selected days in winter and spring characterized by random boundaries conditions is displayed in Fig. 7, for all the evaluated membranes. All the graphs in Fig. 7 show that, irrespective of weather conditions and period of the year, the albedo was substantially constant during the course of the day and the measurements presented a gradually increasing trend in the afternoon. Additionally, in winter days the increasing trend of the albedo due to measurement perturbation was detected to be earlier, i.e. 4:00 p.m. if compared to the spring time, when the albedo begins to increase later in the day, i.e. 5:00–6:00 p.m. This was true for all the membranes. 4.5. Effect of environmental boundaries on measured albedo This section concerns the quantification of the variability of the in-field albedo measurement due to several environmental conditions. To this aim, a comparison between measurements performed (i) according to the Standard ASTM E1918-06 and (ii) with varying one parameter at a time was carried out, in order to determine the range of albedo variability related to each different environmental factor. Table 3 reports these calculated values of albedo for the reference membrane IV (white +30% optimized white paste −W + P-opt). Additionally, by considering one varying parameter at a time, several values of albedo were calculated while considering different daily and yearly time intervals and cloudy conditions of the sky. In general, the albedo variation ˛ was calculated with respect to the reference albedo, i.e. ˛ ¯ ref = 0.60, calculated according to the ASTM [36] international standard in the very central ranges
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Fig. 7. Albedo profiles within two selected days in winter and spring for each membrane.
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Table 3 In-field albedo of the tested membrane IV, measured with varying measurement conditions. Environmental boundaries I—season of measurement II—sky conditions III—daily time interval Measured data Average daily outdoor dry bulb temperature Tout [◦ C] ¯ [–] Average albedo ˛ Maximum albedo ˛max [–] Minimum albedo ˛min [–] Outmost albedo ˛out [–] Standard deviation ␣ [–] Albedo variation ˛ [–]
According to the ASTM E1918-06 procedure “2–3” “ref”
“w”
Out of the ASTM E1918-06 procedure “cl” “3–5”
Summer July, 03rd Sunny clear sky day 12:00–2:00 p.m.
Summer July, 03rd Sunny clear sky day 2:00–3:00 p.m.
Winter Dec, 23rd Sunny clear sky day 12:00–2:00 p.m.
Cloudy sky day 12:00–2:00 p.m.
Summer July, 03rd Sunny clear sky day 3:00–5:00 p.m.
27
27
6
25
27
0.60 0.62 0.57 0.57 0.014 −0.03
0.57 0.57 0.56 0.56 0.003 −0.04
0.68 0.68 0.68 0.68 0.003 +0.08
0.61 0.62 0.57 0.57 0.024 −0.03
0.57 0.57 0.54 0.54 0.009 −0.06
of measurements indicated within the standard, i.e. in clear sky conditions, in summer time, and close to noon. The results of the present experimental campaign showed that, if the measurements are performed according to the Standard guidelines, the period of the year is the most impacting factor devi¯w > ˛ ¯ ref by 13%) with respect to ating the albedo measurement (˛ the reference value of 0.60. Additionally, the time interval of measurement was registered to be responsible for a 0.04 points of variability when the measurements are carried out over 2:00–3:00 p.m. (˛ ¯ 2–3 < ˛ ¯ ref ), which represents the boarder of the time interval suggested by the Standard [36]. Therefore, Table 3 shows that, even if the measurements are performed according to the procedure indicated by the Standard, a non-negligible probability to have a range of albedo variability >0.01 in terms of ˛ is experimentally detected, at least in the present monitoring campaign and in the considered temperate climate region and latitude. Furthermore, when the measurements were performed without considering the Standard guidelines, the time of measurement was the main factor affecting the albedo in-field measurement (0.06 points of variation detected over the 3:00–5:00 p.m. time interval, i.e. ˛ ¯ 3–5 < ˛ ¯ ref ), while, the cloudiness generated a relatively lower albedo variability of 5% (i.e. ˛ ¯ cl < ˛ ¯ ref by 0.03). Consistently to these results, such albedo variability was detected also for all the other membranes. A similar trend was found for the membranes in terms of overestimation and underestimation of the in-field measured albedo if compared to the albedo measured according to the Standard procedure in the only time interval 12:00–2:00 p.m. In particular, the measurement taken in winter always led to an overestimation of the measured albedo (i.e. ˛ ¯w > ˛ ¯ ref by +13%). The same result was found for the measurements carried out with varying time interval. In fact, an underestimation of 0.04 and 0.06, with reference to 2:00–3:00 p.m. and 3:00–5:00 p.m. measurement periods, respectively, was detected. Therefore, even if the albedo measurements are carried out according to the procedure described in the ASTM E1918-06, it is extremely difficult to limit the variability of the infield measured albedo lower than 0.01 in realistic conditions, as observed in this experimental campaign. In general, the procedure described in the ASTM E1918-06 could produce more stable measured albedo values in higher latitude areas characterized by clearer sky conditions.
5. Conclusions This paper presents the experimental analysis of a combined albedo measurement campaign for the characterization of an innovative waterproof polyurethane-based membrane for cool
Summer July, 02nd
roof application, before and after optimization. Both in-lab and in-field analyses of five different roofing membranes were carried out in order to evaluate their thermal-optic properties, i.e. solarreflectance, thermal-emittance, and in-field measured albedo, determining their passive cooling potential. The in-lab experimental measurement campaign was carried out by using a spectrophotometer and portable emissometer, in order to characterize the optic-thermal properties of the membranes’ prototype samples. The in-field albedo long-term monitoring was performed by using a dedicated albedometer and automatic data collection/acquisition system. In particular, the standard approach defined in ASTM E1918-06 was applied to measure the albedo during winter-spring season, with the purpose of investigating the variation of measured albedo due to (i) the solar position and (ii) weather conditions. A precision and bias statement has not been established yet for this international reference standard and the measured albedo of most surfaces is expected to vary both from one location to another and with time, as showed in this experimental research. Therefore, the scope of the work was to investigate such expected variability in order to study the reliability of such in-field measurement method. To this aim, the effect of one environmental boundary at a time on the in-field measured albedo was evaluated. The in-lab results showed that the white membranes (samples I, III, IV, and V) present the highest values of solar reflectance, varying within 80.7% and 85.4%, and the standard grey membrane(II) presents the lowest solar reflectance, i.e. 26%. Additionally, all the evaluated membranes presented a constant and high value of thermal emittance, i.e. 0.9, as expected. Therefore, all the membranes with the exception of the grey membrane (II) could be considered as suitable for cool roof applications and urban heat island mitigation if applied in dense urban areas. The in-field experimental campaign showed that the measured albedo is relatively stable during the day, especially in the middle hours, i.e. 12:00–2.00 p.m., and it increases in the afternoon with the decrease of the angle of the sun to the normal from the membrane. Non consistent decreasing trend of measured albedo was detected from January to April, even if non negligible monthly variations were measured. In particular, a maximum seasonal albedo variation of 6% was found for the optimized membrane (IV sample, “white +30% optimized white paste”). The analysis of the in-field albedo variability attributable to environmental agents, such as (i) the period of the year, (ii) the cloudiness, and (iii) the time interval of measurement, showed that it is extremely difficult to measure the in-field albedo with a variability <0.01, at least in the boundary conditions of this
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experimental campaign and in the considered temperate climate region and latitude. In fact, such environmental agents were always responsible for overestimation or underestimation of the measured albedo with respect to the reference albedo value measured according to the ASTM E1918-06 in clear sunny days, during the 12:00–2:00 p.m. time interval. Therefore, such in-field measurement method showed to be not suitable to detect relatively small performance differences, such as the overall 5% improvement achieved by mean of the optimized membrane IV with respect to the membrane V, as it was measured in-lab by mean of spectrophotometer. Nevertheless, more stable in-field measured albedo values are expected in those areas presenting more constant weather conditions, i.e. in higher latitude areas characterized by clearer sky conditions. 6. Future developments Future research will investigate the effect of the aging, soiling, and weathering on the cool sheet membrane’s thermal-radiative properties, i.e. solar reflectance and thermal emittance, through a dedicated time performance analysis. Additionally, starting from the experimental results obtained from this study, an innovative standard procedure for measuring in-field albedo will be developed. Further analyses of the evaluated membranes will therefore be carried out when applied in real-scale buildings. Acknowledgements The authors would like to thank the CVR S.R.L. for providing the roofing membranes prototype samples for the experimental campaign. The authors also acknowledge Gabriele Franceschetti for assisting the experimental setup. The first author acknowledgments are due to the “CIRIAF program for UNESCO” in the framework of the UNESCO Chair “Water Resources Management and Culture”, for supporting her research. References [1] L. Perez-Lombard, J. Ortiz, C. Pout, A review on buildings energy consumption information, Energy Build. 40 (2008) 394–398, http://dx.doi.org/10.1016/ j.enbuild.2007.03.007 [2] D.A. Asimakopoulos, M. Santamouris, I. Farrou, M. Laskari, M. Saliari, S.C. Zerefos, T. Antonakaki, C. Giannakopoulos, Modelling the energy demand projection of the building sector in Greece in the 21th century, Energy Build. 49 (2012) 488–498, http://dx.doi.org/10.1016/j.enbuild.2012.02.043 [3] S. Hassid, M. Santamouris, N. Papanikolau, A. Linardi, N. Klitsikas, C. Georgakis, D.N. Assimakopoulos, Effect of the Athens heat island on air conditioning load, Energy Build. 32 (2) (2000) 131–141, http://dx.doi.org/10.1016/S03787788(99)00045-6 [4] A. Synnefa, M. Santamouris, Advances on technical, policy and market aspects of cool roof technology in Europe: the cool roofs project, Energy Build. 55 (2012) 35–41, http://dx.doi.org/10.1016/j.enbuild.2011.11.051 [5] M. Santamouris, On the energy impact of urban heat island and global warming on buildings, Energy Build. 82 (2014) 100–113, http://dx.doi.org/10.1016/ j.enbuild.2014.07.022 [6] M. Santamouris, Heat island research in Europe—the state of the art, Adv. Build. Energy Res. 1 (2007) 123–150, http://dx.doi.org/10.1080/17512549. 2007.9687272 [7] M.P. McCarthy, M.J. Best, R.A. Betts, Climate change in cities due to global warming and urban effects, Geophys. Res. Lett. 37 (2010) 9, http://dx.doi.org/10. 1029/2010GL042845 [8] H.E. Landsberg, The Urban Climate International Geographic Series, vol. 28, Academic Press, New York,NY, 1981. [9] J.P. Montavez, A. Rodriguez, J.I. Jimenez, A study of the urban heat island of Granada, Int. J. Climatol. 20 (2000) 899–911, http://dx.doi.org/10.1002/10970088(20000630)20:8<899::AID-JOC433>3.0.CO;2-I [10] M.L. Imhoff, P. Zhang, R.E. Wolfe, L. Bounoua, Remote sensing of the urban heat island effect across biomes in the continental USA, Remote Sens. Environ. 114 (2010) 504–513, http://dx.doi.org/10.1016/j.rse.2009.10.008 [11] H. Akbari, S. Konopacki, Calculating energy-saving potentials of heat-island reduction strategies, Energy Policy 33 (2005) 721–756, http://dx.doi.org/10. 1016/j.enpol.2003.10.001
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