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Hashem Akbari - One of the best experts on this subject based on the ideXlab platform.
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procedure for measuring the solar Reflectance of flat or curved roofing assemblies
Solar Energy, 2008Co-Authors: Hashem Akbari, Ronnen Levinson, Stephanie SternAbstract:In Press, Solar Energy January 10, 2008 Procedure for measuring the solar Reflectance of flat or curved roofing assemblies Hashem Akbari * and Ronnen Levinson Heat Island Group Lawrence Berkeley National Laboratory Berkeley, CA 94720 and Stephanie Stern Cool Roof Rating Council Oakland, CA 94612 Abstract The widely used methods to measure the solar Reflectance of roofing materials include ASTM standards E903 (spectrometer), C1549 (reflectometer), and E1918 (pyranometer). Standard E903 uses a spectrometer with an integrating sphere to measure the solar spectral Reflectance of an area approximately 0.1 cm 2 . The solar spectral Reflectance is then weighted with a solar spectral irradiance to calculate the solar Reflectance. Standard C1549 uses a reflectometer to measure the solar Reflectance of an area approximately 5 cm 2 . Both E903 and C1549 are best suited to measurement of the solar Reflectance of flat, homogeneous surfaces. Standard E1918 uses a pyranometer to measure the solar Reflectance of an area approximately 10 m 2 , and is best applied to large surfaces that may also be rough and/or non- uniform. We describe a technique that uses a pyranometer to measure the solar Reflectance of a uniform or variegated sample with an area of approximately 1 m 2 , and use this technique (referred to as E1918A) to measure the solar Reflectance of low- and high-profile tile assemblies. For 10 large (10 m 2 ) tile assemblies whose E1918 solar Reflectances ranged from 0.10 to 0.50, the magnitude of the difference between the E1918A and E1918 measurements did not exceed 0.02 for unicolor assemblies, and did not exceed 0.03 for multicolor assemblies. Keywords: cool roofs, solar Reflectance, roof tiles, pyranometer, albedometer, solar spectrum reflectometer, spectrometer, albedo, E1918 Corresponding author. Email: H_Akbari@LBL.gov. Tel: +1-510-486-4287
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surface roughness effects on the solar Reflectance of cool asphalt shingles
Solar Energy Materials and Solar Cells, 2008Co-Authors: Paul Berdahl, Hashem Akbari, Jeffry L Jacobs, Frank KlinkAbstract:We analyze the solar Reflectance of asphalt roofing shingles that are covered with pigmented mineral roofing granules. The reflecting surface is rough, with a total area approximately twice the nominal area. We introduce a simple analytical model that relates the 'micro-Reflectance' of a small surface region to the 'macro-Reflectance' of the shingle. This model uses a mean field approximation to account for multiple scattering effects. The model is then used to compute the Reflectance of shingles with a mixture of different colored granules, when the Reflectances of the corresponding mono-color shingles are known. Simple linear averaging works well, with small corrections to linear averaging derived for highly reflective materials. Reflective base granules and reflective surface coatings aid achievement of high solar Reflectance. Other factors that influence the solar Reflectance are the size distribution of the granules, coverage of the asphalt substrate, and orientation of the granules as affected by rollers during fabrication.
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solar spectral optical properties of pigments part i model for deriving scattering and absorption coefficients from transmittance and Reflectance measurements
Solar Energy Materials and Solar Cells, 2005Co-Authors: Ronnen Levinson, Paul Berdahl, Hashem AkbariAbstract:Abstract The suitability of a pigment for inclusion in “cool” colored coatings with high solar Reflectance can be determined from its solar spectral backscattering and absorption coefficients. Pigment characterization is performed by dispersing the pigment into a transparent film, then measuring spectral transmittance and Reflectance. Measurements of the Reflectance of film samples on black and white substrates are also used. A model for extracting the spectral backscattering coefficient S and absorption coefficient K from spectrometer measurements is presented. Interface Reflectances complicate the model. The film's diffuse Reflectance and transmittance measurements are used to determine S and K as functions of a wavelength-independent model parameter σ that represents the ratio of forward to total scattering. σ is used to estimate the rate at which incident collimated light becomes diffuse, and is determined by fitting the measured film Reflectance backed by black. A typical value is σ = 0.8 . Then, the measured film Reflectance backed by white is compared with a computed value as a self-consistency check. Measurements on several common pigments are used to illustrate the method.
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effects of soiling and cleaning on the Reflectance and solar heat gain of a light colored roofing membrane
Atmospheric Environment, 2005Co-Authors: Ronnen Levinson, Paul Berdahl, Asmeret Asefaw Berhe, Hashem AkbariAbstract:A roof with high solar Reflectance and high thermal emittance (e.g., a white roof) stays cool in the sun, reducing cooling power demand in a conditioned building and increasing summertime comfort in an unconditioned building. The high initial solar Reflectance of a white membrane roof (circa 0.8) can be lowered by deposition of soot, dust, and/or biomass (e.g., fungi or algae) to about 0.6; degraded solar Reflectances range from 0.3 to 0.8, depending on exposure. We investigate the effects of soiling and cleaning on the solar spectral Reflectances and solar absorptances of 15 initially white or light-gray polyvinyl chloride membrane samples taken from roofs across the United States. Black carbon and organic carbon were the two identifiable strongly absorbing contaminants on the membranes. Wiping was effective at removing black carbon, and less so at removing organic carbon. Rinsing and/or washing removed nearly all of the remaining soil layer, with the exception of (a) thin layers of organic carbon and (b) isolated dark spots of biomass. Bleach was required to clear these last two features. At the most soiled location on each membrane, the ratio of solar Reflectance to unsoiled solar Reflectance (a measure of cleanliness) ranged from 0.41 to 0.89 for the soiled samples; 0.53 to 0.95 for the wiped samples; 0.74 to 0.98 for the rinsed samples; 0.79 to 1.00 for the washed samples; and 0.94 to 1.02 for the bleached samples. However, the influences of membrane soiling and cleaning on roof heat gain are better gauged by fractional variations in solar absorptance. Solar absorptance ratios (indicating solar heat gain relative to that of an unsoiled membrane) ranged from 1.4 to 3.5 for the soiled samples; 1.1 to 3.1 for the wiped samples; 1.0 to 2.0 for the rinsed samples; 1.0 to 1.9 for the washed samples; and 0.9 to 1.3 for the bleached samples.
Ronnen Levinson - One of the best experts on this subject based on the ideXlab platform.
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procedure for measuring the solar Reflectance of flat or curved roofing assemblies
Solar Energy, 2008Co-Authors: Hashem Akbari, Ronnen Levinson, Stephanie SternAbstract:In Press, Solar Energy January 10, 2008 Procedure for measuring the solar Reflectance of flat or curved roofing assemblies Hashem Akbari * and Ronnen Levinson Heat Island Group Lawrence Berkeley National Laboratory Berkeley, CA 94720 and Stephanie Stern Cool Roof Rating Council Oakland, CA 94612 Abstract The widely used methods to measure the solar Reflectance of roofing materials include ASTM standards E903 (spectrometer), C1549 (reflectometer), and E1918 (pyranometer). Standard E903 uses a spectrometer with an integrating sphere to measure the solar spectral Reflectance of an area approximately 0.1 cm 2 . The solar spectral Reflectance is then weighted with a solar spectral irradiance to calculate the solar Reflectance. Standard C1549 uses a reflectometer to measure the solar Reflectance of an area approximately 5 cm 2 . Both E903 and C1549 are best suited to measurement of the solar Reflectance of flat, homogeneous surfaces. Standard E1918 uses a pyranometer to measure the solar Reflectance of an area approximately 10 m 2 , and is best applied to large surfaces that may also be rough and/or non- uniform. We describe a technique that uses a pyranometer to measure the solar Reflectance of a uniform or variegated sample with an area of approximately 1 m 2 , and use this technique (referred to as E1918A) to measure the solar Reflectance of low- and high-profile tile assemblies. For 10 large (10 m 2 ) tile assemblies whose E1918 solar Reflectances ranged from 0.10 to 0.50, the magnitude of the difference between the E1918A and E1918 measurements did not exceed 0.02 for unicolor assemblies, and did not exceed 0.03 for multicolor assemblies. Keywords: cool roofs, solar Reflectance, roof tiles, pyranometer, albedometer, solar spectrum reflectometer, spectrometer, albedo, E1918 Corresponding author. Email: H_Akbari@LBL.gov. Tel: +1-510-486-4287
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solar spectral optical properties of pigments part i model for deriving scattering and absorption coefficients from transmittance and Reflectance measurements
Solar Energy Materials and Solar Cells, 2005Co-Authors: Ronnen Levinson, Paul Berdahl, Hashem AkbariAbstract:Abstract The suitability of a pigment for inclusion in “cool” colored coatings with high solar Reflectance can be determined from its solar spectral backscattering and absorption coefficients. Pigment characterization is performed by dispersing the pigment into a transparent film, then measuring spectral transmittance and Reflectance. Measurements of the Reflectance of film samples on black and white substrates are also used. A model for extracting the spectral backscattering coefficient S and absorption coefficient K from spectrometer measurements is presented. Interface Reflectances complicate the model. The film's diffuse Reflectance and transmittance measurements are used to determine S and K as functions of a wavelength-independent model parameter σ that represents the ratio of forward to total scattering. σ is used to estimate the rate at which incident collimated light becomes diffuse, and is determined by fitting the measured film Reflectance backed by black. A typical value is σ = 0.8 . Then, the measured film Reflectance backed by white is compared with a computed value as a self-consistency check. Measurements on several common pigments are used to illustrate the method.
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effects of soiling and cleaning on the Reflectance and solar heat gain of a light colored roofing membrane
Atmospheric Environment, 2005Co-Authors: Ronnen Levinson, Paul Berdahl, Asmeret Asefaw Berhe, Hashem AkbariAbstract:A roof with high solar Reflectance and high thermal emittance (e.g., a white roof) stays cool in the sun, reducing cooling power demand in a conditioned building and increasing summertime comfort in an unconditioned building. The high initial solar Reflectance of a white membrane roof (circa 0.8) can be lowered by deposition of soot, dust, and/or biomass (e.g., fungi or algae) to about 0.6; degraded solar Reflectances range from 0.3 to 0.8, depending on exposure. We investigate the effects of soiling and cleaning on the solar spectral Reflectances and solar absorptances of 15 initially white or light-gray polyvinyl chloride membrane samples taken from roofs across the United States. Black carbon and organic carbon were the two identifiable strongly absorbing contaminants on the membranes. Wiping was effective at removing black carbon, and less so at removing organic carbon. Rinsing and/or washing removed nearly all of the remaining soil layer, with the exception of (a) thin layers of organic carbon and (b) isolated dark spots of biomass. Bleach was required to clear these last two features. At the most soiled location on each membrane, the ratio of solar Reflectance to unsoiled solar Reflectance (a measure of cleanliness) ranged from 0.41 to 0.89 for the soiled samples; 0.53 to 0.95 for the wiped samples; 0.74 to 0.98 for the rinsed samples; 0.79 to 1.00 for the washed samples; and 0.94 to 1.02 for the bleached samples. However, the influences of membrane soiling and cleaning on roof heat gain are better gauged by fractional variations in solar absorptance. Solar absorptance ratios (indicating solar heat gain relative to that of an unsoiled membrane) ranged from 1.4 to 3.5 for the soiled samples; 1.1 to 3.1 for the wiped samples; 1.0 to 2.0 for the rinsed samples; 1.0 to 1.9 for the washed samples; and 0.9 to 1.3 for the bleached samples.
Paul Berdahl - One of the best experts on this subject based on the ideXlab platform.
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surface roughness effects on the solar Reflectance of cool asphalt shingles
Solar Energy Materials and Solar Cells, 2008Co-Authors: Paul Berdahl, Hashem Akbari, Jeffry L Jacobs, Frank KlinkAbstract:We analyze the solar Reflectance of asphalt roofing shingles that are covered with pigmented mineral roofing granules. The reflecting surface is rough, with a total area approximately twice the nominal area. We introduce a simple analytical model that relates the 'micro-Reflectance' of a small surface region to the 'macro-Reflectance' of the shingle. This model uses a mean field approximation to account for multiple scattering effects. The model is then used to compute the Reflectance of shingles with a mixture of different colored granules, when the Reflectances of the corresponding mono-color shingles are known. Simple linear averaging works well, with small corrections to linear averaging derived for highly reflective materials. Reflective base granules and reflective surface coatings aid achievement of high solar Reflectance. Other factors that influence the solar Reflectance are the size distribution of the granules, coverage of the asphalt substrate, and orientation of the granules as affected by rollers during fabrication.
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solar spectral optical properties of pigments part i model for deriving scattering and absorption coefficients from transmittance and Reflectance measurements
Solar Energy Materials and Solar Cells, 2005Co-Authors: Ronnen Levinson, Paul Berdahl, Hashem AkbariAbstract:Abstract The suitability of a pigment for inclusion in “cool” colored coatings with high solar Reflectance can be determined from its solar spectral backscattering and absorption coefficients. Pigment characterization is performed by dispersing the pigment into a transparent film, then measuring spectral transmittance and Reflectance. Measurements of the Reflectance of film samples on black and white substrates are also used. A model for extracting the spectral backscattering coefficient S and absorption coefficient K from spectrometer measurements is presented. Interface Reflectances complicate the model. The film's diffuse Reflectance and transmittance measurements are used to determine S and K as functions of a wavelength-independent model parameter σ that represents the ratio of forward to total scattering. σ is used to estimate the rate at which incident collimated light becomes diffuse, and is determined by fitting the measured film Reflectance backed by black. A typical value is σ = 0.8 . Then, the measured film Reflectance backed by white is compared with a computed value as a self-consistency check. Measurements on several common pigments are used to illustrate the method.
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effects of soiling and cleaning on the Reflectance and solar heat gain of a light colored roofing membrane
Atmospheric Environment, 2005Co-Authors: Ronnen Levinson, Paul Berdahl, Asmeret Asefaw Berhe, Hashem AkbariAbstract:A roof with high solar Reflectance and high thermal emittance (e.g., a white roof) stays cool in the sun, reducing cooling power demand in a conditioned building and increasing summertime comfort in an unconditioned building. The high initial solar Reflectance of a white membrane roof (circa 0.8) can be lowered by deposition of soot, dust, and/or biomass (e.g., fungi or algae) to about 0.6; degraded solar Reflectances range from 0.3 to 0.8, depending on exposure. We investigate the effects of soiling and cleaning on the solar spectral Reflectances and solar absorptances of 15 initially white or light-gray polyvinyl chloride membrane samples taken from roofs across the United States. Black carbon and organic carbon were the two identifiable strongly absorbing contaminants on the membranes. Wiping was effective at removing black carbon, and less so at removing organic carbon. Rinsing and/or washing removed nearly all of the remaining soil layer, with the exception of (a) thin layers of organic carbon and (b) isolated dark spots of biomass. Bleach was required to clear these last two features. At the most soiled location on each membrane, the ratio of solar Reflectance to unsoiled solar Reflectance (a measure of cleanliness) ranged from 0.41 to 0.89 for the soiled samples; 0.53 to 0.95 for the wiped samples; 0.74 to 0.98 for the rinsed samples; 0.79 to 1.00 for the washed samples; and 0.94 to 1.02 for the bleached samples. However, the influences of membrane soiling and cleaning on roof heat gain are better gauged by fractional variations in solar absorptance. Solar absorptance ratios (indicating solar heat gain relative to that of an unsoiled membrane) ranged from 1.4 to 3.5 for the soiled samples; 1.1 to 3.1 for the wiped samples; 1.0 to 2.0 for the rinsed samples; 1.0 to 1.9 for the washed samples; and 0.9 to 1.3 for the bleached samples.
W J Collins - One of the best experts on this subject based on the ideXlab platform.
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interannual variability of the earth s spectral solar Reflectance from measurements and simulations
Journal of Geophysical Research, 2014Co-Authors: Constantin Lukachin, Y Roberts, Bruce A Wielicki, Daniel E Feldman, W J CollinsAbstract:The mean solar spectral Reflectance averaged over large spatiotemporal scales is an important climate benchmark data product proposed for the Climate Absolute Radiance and Refractivity Observatory mission. The interannual variability of these Reflectances over the ocean is examined through satellite-measured hyperspectral data and through satellite instrument emulation based on model simulation. Such large domain-averaged Reflectances show small interannual variation, usually under few percent, depending on the latitude region and spatiotemporal scale used for averaging. Although the interannual variation is usually less than the absolute accuracy of model calculation, the model simulated interannual variations are consistent with the measurements because most of the modeling errors in the Reflectance averaged in large climate domains are systematic and are canceled out in the interannual difference spectra. The interannual variability is also shown to decrease as the temporal and spatial scales increase. Both the observational data and the model simulations show that the natural variability in the annual mean Reflectance is about 50% lower than that in the monthly mean over all spectra. The interannual variability determined from observations in large climate domains also compares favorably with that from the climate Observing System Simulation Experiment based on climate model simulations; both show a standard deviation of less than 1% of the mean Reflectance across all spectra for global and annual average over the ocean.
Riccardo Paolini - One of the best experts on this subject based on the ideXlab platform.
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effect of ageing on solar spectral Reflectance of roofing membranes natural exposure in roma and milano and the impact on the energy needs of commercial buildings
Energy and Buildings, 2014Co-Authors: Tiziana Poli, Michele Zinzi, Emiliano Carnielo, Riccardo Paolini, Andrea Giovanni MaininiAbstract:Abstract Highly reflective roofs, widely known as cool roofs, can reduce peak surface temperatures and the energy required to cool buildings, mitigate urban microclimates, and offset CO 2 . However, weathering, soiling, and biological growth affect their solar Reflectance. In this study, the solar spectral Reflectances of 12 roofing membranes were measured before the exposure and after 3, 6, 12, 18, and 24 months of natural ageing in Roma and Milano, Italy. The membranes with an initial solar Reflectance greater than 0.80, for example, decreased in Reflectance by 0.14 in Roma and 0.22 in Milano after two years. Then, for a typical highly insulated commercial building, the annual cooling load savings were calculated to be reduced by 4.1–7.1 MJ m −2 y −1 per 0.1 loss in Reflectance. When the buildings are non-insulated, the savings reduction is 58–71 MJ m −2 y −1 in Milano and 70–84 MJ m −2 y −1 in Roma. Ageing yielded a reduction of the cooling load savings that could be achieved with a new white membrane of 14–23% in Roma and of 20–34% in Milano. Moreover, in Milano, an aged, white, highly insulated roof, which has a solar Reflectance of 0.56, may reach a surface temperature 16 °C higher than a new roof, which has a solar Reflectance of 0.80.
