Uncertainty Study of Reflectance Measurements for Concentrating Solar Reflectors

The solar reflector is one of the main components of concentrated solar thermal systems. Therefore, accurate knowledge of its solar-weighted, near-specular reflectance is highly important. Currently, this parameter cannot be properly measured with a single commercial instrument. There is a great interest in having a suitable procedure that can guarantee the accuracy of reflector quality analysis, which already led to the publication of an international measurement guideline (title “Parameters and method to evaluate reflectance properties of reflector materials for concentrating solar power technology”). Still, more research work is needed to improve the state of the art. At present, both the specular reflectance and the spectral hemispherical reflectance are measured by using commercial portable reflectometers and spectrophotometers, respectively, to gain enough information. This article concentrates on the evaluation and calculation of the type-B (nonstatistical) uncertainties associated with these employed instruments and, therefore, leads to a more accurate definition of the measurement uncertainty. Considering type-B uncertainty, the expanded uncertainties of measurements for most of the reflector types are $U_{\mathrm {B,ref}} = 0.006$ for monochromatic specular reflectance and $U_{\mathrm {B,spec}} = 0.016$ for solar-weighted hemispherical reflectance.