Using photocatalytic coating to maintain solar reflectance and lower cooling energy consumption of buildings

Solar reflectance is one of the main parameters that affect the heat transfer through opaque building envelope and cooling energy consumption. Deposition of airborne black carbon (BC) darkens the building surfaces and increases the cooling energy consumption. The objective of this study is to experimentally demonstrate that photocatalytic coating with TiO2 is able to maintain solar reflectance of opaque building envelope and lower the cooling energy consumption. Portland cement mortar specimens were coated with transparent silicate coating (TSC), white silicate coating (WSC), and white silicate coating incorporating photocatalyst (PWSC). The solar reflectance, color, heat gain, and surface temperatures of the specimens exposed to a sun simulator for 9 h were determined at three different time points: 1) before BC deposition; 2) after BC deposition; and 3) after 300 h exposure to simulated solar irradiation in an accelerated weathering chamber. The BC deposition reduces the solar reflectance of the specimens by 51 - 56% and increases the heat transfer through the specimens exposed to the sun simulator by 27% - 123%, depending on the solar reflectance before BC deposition. The photocatalytic TiO2 is able to remove black carbon and restore the solar reflectance and color of the PWSC specimen after 300 h of exposure to simulated solar irradiation in a weathering chamber. With the removal of black carbon, the heat gain and inner surface temperature of the PWSC specimen exposed to the sun simulator for 9 h are comparable to those before the BC deposition but 53% and 7.2 °C lower than those after the BC deposition, respectively. The results indicate that photocatalytic coating with TiO2 is able to maintain the solar reflectance of opaque building envelope and lower cooling energy consumption of buildings.