Calcium Phosphate Paints for Full-Daytime Subambient Radiative Cooling

Passive radiative cooling is of great significance for energy-saving and global carbon neutrality because of its zero energy consumption, no pollution, and low cost. To achieve radiative cooling, minimum absorption in solar radiation and maximum emission in the sky window are required. In this work, we have developed an all-season full-daytime subambient radiative cooler based on Ca-3(PO4)(2)-acrylic paint, which shows an exceptional solar reflectance of 97.6% and a high sky window emissivity of 0.96. The 5.6 eV bandwidth and 9.3 mu m phonon resonance enable Ca-3(PO4)(2) to efficiently reflect solar flux and dissipate infrared emission through the atmospheric window. The field tests during summer indicate that the surface temperature is more than 18 degrees C below ambient temperatures on average, and a net cooling power of 95 W/m(2) was obtained during daytime. At noon, with a summit solar radiation intensity of 900 W/m(2), it remains at similar to 38 degrees C below ambient temperature. Furthermore, during winter, the temperature of the Ca-3(PO4)(2) paint drops to as low as -9 degrees C, which is 16 degrees C lower than the ambient temperature, and an average cooling power of 60 W/m(2) was obtained. The paint shows a standard figure of merit of 0.72, which is among the highest of radiative cooling solutions.

Automated summary

Passive radiative cooling is significant for energy-saving and carbon neutrality. An all-season full-daytime subambient radiative cooler has been developed, which exhibits high solar reflectance and sky window emissivity. The experimental results show effective cooling performance in both summer and winter.