Modeling and labeling heterogeneous directional reflective roofing materials

摘要

Solar reflectance (R) of roofing materials is a function of irradiance magnitude and direction, and surface properties. Current methods of labeling the surface reflectance are applicable to homogeneous materials. However, reflectivity of recently developed directional reflective materials (DRM) has a strong dependence on the heterogeneity of the surface. We have developed a model to calculate the hourly reflectance of DRMs as a function of zenith and azimuth angles. We investigate options to represent the reflectance of DRMs. Two main options are proposed for labeling; (1) Option 1: using one reflectance for the whole year, and (2) Option 2: using seasonal reflectance (a number representing summer reflectance, a number representing winter reflectance, and the mean of the winter and summer reflectances for other months of a year). We propose a simple method based on Option 2 to label the solar reflectance of DRMs. The proposed option assigns the average reflectance of DRMs as the winter reflectance and the reflectance of the DRM according to a 20° zenith toward the reflective side as the summer reflectance. In summer, the selected metric estimates the mean hourly heat absorption and energy saving for building energy models accurately, and peak heat absorption of the surface by 8% (< 40 W/m~2) error. In winter, the potential energy savings of buildings is overestimated by an error of less than 20%. and mean hourly and peak heat absorptions are estimated by an error of 22 kJ/m~2 and 9 W/m~2, respectively.