An Energy-free and Low-cost Daytime Passive Radiative Cooling Paint for Energy Saving in Buildings and Decarbonization

Air-conditioning consumes over 30% of the energy used in buildings in Hong Kong, driving peak electricity demand. Compressors of traditional air-conditioning systems consume enormous amounts of energy and the usage of refrigerants is a major cause of ozone layer depletion, contributing to global warming. Therefore, an energy-saving and environmentally friendly cooling technology is urgently needed to reduce the energy for space cooling, and in the long term, mitigate climate change. In this study, an energy-free and low-cost polymer-based daytime passive radiative cooling paint (DPRCP) with a promising cooling performance and wide applicability has been developed, that effectively reflects the sunlight and dissipates the heat to the cold universe (^~ -270 °C). The DPRCP, consisting of a unique porous structure with 48% porosity, has been fabricated using a novel wet phase separation method. Excellent optical performance with 96% solar reflectivity (0.25-2.5 μm) and 95% mid-infrared (8-13 μm) thermal emissivity are achieved. A small-scale prototype field test revealed that the DPRCP can reduce the ambient air temperature by about 4 °C and 6 °C during the daytime and at night, respectively, corresponding to a cooling power of about 81 W/m² and 84 W/m². A scale-up demonstration was also conducted on a rooftop of a real building in Tung Chung, showing a remarkable reduction in the roof surface temperature of about 15 oC under direct sunlight has been achieved compared to the roof without the DPRCP coating. Last, an EnergyPlus simulation further proved that the DPRCP could save around 8% of cooling energy when applied on the one-story building roof. These results strongly demonstrate the potential and capability to perform as an effective and promising passive cooling strategy for buildings and broader applications in various fields.