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

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°]) by as much as 89 W m⁻², and underestimate its peak surface temperature by up to 5 K. Using R_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_g,0, a simple and easily measured property that more accurately predicts solar heat gain. R_g,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,0 is well suited to rating the solar reflectances of roofs, pavements and walls. We show in Part II that R_g,0 can be easily and accurately measured with a pyranometer, a solar spectrophotometer or version 6 of the Solar Spectrum Reflectometer.