Experimental and theoretical analysis of sub-ambient cooling with longwave radiative coating

With radiative cooling coating it is possible to keep the temperature of a surface around or even below air temperature under solar radiation exposure. This study conducted experimental and theoretical analysis of its effectiveness. Setups with different surface coatings and boundary conditions were placed on a roof of a building in Chongqing, China. The 24-h measurements on a representative summer day showed that different boundary conditions at the bottom of the coating led to varied cooling effects, but the surface temperature of the radiative cooling coating stayed below air temperature most of the day when the incident solar radiation was below 800 W m−2. Theoretical models were also established to investigate the heat transfer process. Analysis showed that the longwave radiation absorption amounted to 89.0%–91.9% of the total daily heat gain of the cooling coating. The longwave radiative emission was the sole and significant heat loss pathway for the cooling coating, making the net longwave radiative heat transfer a pathway for heat dissipation. Sensitivity analysis further demonstrated that on a typical summer day in Chongqing, shortwave reflectivity and incident solar radiation were the dominant factors impacting the diurnal cooling effect, while wind velocity was the most important parameter for nocturnal cooling.