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 Ca3(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 μm phonon resonance enable Ca3(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 °C below ambient temperatures on average, and a net cooling power of 95 W/m2 was obtained during daytime. At noon, with a summit solar radiation intensity of 900 W/m2, it remains at ∼38 °C below ambient temperature. Furthermore, during winter, the temperature of the Ca3(PO4)2 paint drops to as low as −9 °C, which is 16 °C lower than the ambient temperature, and an average cooling power of 60 W/m2 was obtained. The paint shows a standard figure of merit of 0.72, which is among the highest of radiative cooling solutions.