Journal
SOLAR ENERGY
Volume 84, Issue 9, Pages 1717-1744Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.solener.2010.04.018
Keywords
Solar reflectance; Solar heat gain; Pyranometer; Solar spectrophotometer; Solar Spectrum Reflectometer; Spectrally selective cool colored surface
Categories
Funding
- California Energy Commission (CEC)
- Assistant Secretary for Renewable Energy [DE-AC02-05CH11231]
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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 US latitudes, this metric R-E891BN can underestimate the annual peak solar heat gain of a typical roof or pavement (slope <= 5:12 [23 degrees]) by as much as 89W m(-2), and underestimate its peak surface temperature by up to 5 K. Using RE891BN 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 (AMIGH) solar reflectance R-g,R-0, a simple and easily measured property that more accurately predicts solar heat gain. R-g,R-0 predicts the annual peak solar heat gain of a roof or pavement to within 2 W m(-2), and overestimates N by no more than 3%. R-g,R-0 is well suited to rating the solar reflectances of roofs, pavements and walls. We show in Part II that R-g,R-0 can be easily and accurately measured with a pyranometer, a solar spectrophotometer or version 6 of the Solar Spectrum Reflectometer. (C) 2010 Elsevier Ltd. All rights reserved.
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