A SOLAR REFLECTING MATERIAL FOR RADIATIVE COOLING APPLICATIONS - ZNS PIGMENTED POLYETHYLENE

Plastic foils containing nonabsorbing pigments can display a high reflectance of solar radiation combined with a high transmittance in the atmospheric window region in the thermal infrared. Such foils can be applied as selective covers enabling radiative cooling of an underlying material during the night and avoiding heating in direct sunlight during the day. The foils could be used for condensing water or cooling food, buildings, etc. We have prepared ZnS pigmented polyethylene foils with various thicknesses and volume fractions of pigments. The optical properties of the foils were compared with theory, and good agreement was achieved for pigment volume fractions up to 0.1. The optimum solar reflectance of the foil is 0.825 for the available pigment powder; it should be 0.88 if heating were to be avoided at noon with the sun in its zenith. The computed cooling power for a radiator covered with the best sample was 52 W m-2 at night, and the equilibrium temperature of the radiator was predicted to be 12 K below the ambient temperature. Computations showed that heating of the radiator could be avoided 18 hours per day, and the radiator temperature at noon was 8 K above the ambience.