Investigating performance prediction and optimization of spectral solar reflectance of cool painted layers

The air temperatures are generally found to be higher in the urban areas in comparison with the suburban or rural areas. One of the countermeasures to mitigate this urban heat island is to coat the surfaces of buildings with paint that has higher solar reflectance; the method is commonly referred to as "cool painting". It is important to evaluate the solar reflectance of the paints used for such paintings. Though solar reflectance can be measured by a spectrophotometer with an integrating sphere, accurate numerical prediction of reflective properties of the painted layer including arbitrary pigments can contribute to further development of the cool paint material. The effect of diffuse reflection in the painted layer is more significant than that of specular reflection on the painted surface. It is also important to evaluate the scattering properties of pigments, which are related to the reflection performance. For evaluating the scattering properties of a pigment in a medium, anisotropic scattering should be considered. In this study, we applied the radiation element method for predicting the solar reflectance of a painted layer. The hemispherical spectral reflectance of a painted layer, including TiO₂, ZnO or Al₂O₃, was evaluated. Our results showed that spectral reflectance of a painted layer with arbitrary values of particle sizes, volume fraction of pigment, and coating thickness can be calculated by this method. Further, the validity of this numerical method was evaluated by comparison with the reflective property of the actual painted layer by means of spectrophotometry.