Measuring solar reflectance Part I: Defining a metric that accurately predicts solar heat gain

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Solar reflectance can vary with the spectral and angular distributions of incident sunlight, which in turn depend on surface orientation, solar position and atmospheric conditions. A widely used solar reflectance metric based on the ASTM Standard E891 beam-normal solar spectral irradiance underestimates the solar heat gain of a spectrally selective 'cool colored' surface because this irradiance contains a greater fraction of near-infrared light than typically found in ordinary (unconcentrated) global sunlight. At mainland U.S. latitudes, this metric RE891BN can underestimate the annual peak solar heat gain of a typical roof or pavement (slope {le} 5:12 [23{sup o}]) by as much ... continued below

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Levinson, Ronnen; Akbari, Hashem & Berdahl, Paul May 14, 2010.

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Solar reflectance can vary with the spectral and angular distributions of incident sunlight, which in turn depend on surface orientation, solar position and atmospheric conditions. A widely used solar reflectance metric based on the ASTM Standard E891 beam-normal solar spectral irradiance underestimates the solar heat gain of a spectrally selective 'cool colored' surface because this irradiance contains a greater fraction of near-infrared light than typically found in ordinary (unconcentrated) global sunlight. At mainland U.S. latitudes, this metric RE891BN can underestimate the annual peak solar heat gain of a typical roof or pavement (slope {le} 5:12 [23{sup o}]) by as much as 89 W m{sup -2}, and underestimate its peak surface temperature by up to 5 K. Using R{sub E891BN} to characterize roofs in a building energy simulation can exaggerate the economic value N of annual cool-roof net energy savings by as much as 23%. We define clear-sky air mass one global horizontal ('AM1GH') solar reflectance R{sub g,0}, a simple and easily measured property that more accurately predicts solar heat gain. R{sub g,0} predicts the annual peak solar heat gain of a roof or pavement to within 2 W m{sup -2}, and overestimates N by no more than 3%. R{sub g,0} is well suited to rating the solar reflectances of roofs, pavements and walls. We show in Part II that R{sub g,0} can be easily and accurately measured with a pyranometer, a solar spectrophotometer or version 6 of the Solar Spectrum Reflectometer.

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  • Journal Name: Solar Energy; Journal Volume: 84; Journal Issue: 9

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  • Report No.: LBNL-3604E-Pt-I
  • Grant Number: DE-AC02-05CH11231
  • DOI: 10.1016/j.solener.2010.04.018 | External Link
  • Office of Scientific & Technical Information Report Number: 984958
  • Archival Resource Key: ark:/67531/metadc1014527

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  • May 14, 2010

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  • Oct. 14, 2017, 8:36 a.m.

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  • Oct. 18, 2017, 10:33 a.m.

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Levinson, Ronnen; Akbari, Hashem & Berdahl, Paul. Measuring solar reflectance Part I: Defining a metric that accurately predicts solar heat gain, article, May 14, 2010; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc1014527/: accessed August 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.