Radiative sky cooling: Fundamental principles, materials, and applications

Passive daytime radiative cooling (PDRC) involves spontaneously cooling a surface by reflecting sunlight and radiating heat to the cold outer space. Current PDRC designs are promising alternatives to electrical cooling, but are either inefficient or have limited applicability. We present a simple, inexpensive and scalable phase-inversion-based method for fabricating hierarchically porous poly(vinylidene fluoride-co-hexafluoropropene) (P(VdF-HFP)HP) coatings with excellent PDRC capability. High, substrate-independent hemispherical solar reflectances (0.96 ± 0.03) and long-wave infrared (LWIR) emittances (0.97 ± 0.02) allow for sub-ambient temperature drops of ~6°C and cooling powers of ~96 W m–2 under solar intensities of 890 and 750 W m–2 respectively. The performance equals or surpasses those of state-of-the-art PDRC designs, while the technique offers a paint-like simplicity.