Pigmented foils for radiative cooling and condensation irrigation

摘要

This paper reports on the development of pigmented polyethylene foils for radiative cooling. The optical properties of the foils were optimized for applications in day-time radiative cooling and water condensation. We first study highly scattering foils used as convection shields. These cover foils combine a high solar reflectance and a high transmittance in the atmospheric window region in the infrared. Different pigment materials were studied and ZnS was the only one that could prevent heating of an underlying blackbody at noon, with the sun in its zenith. A 400 $mu@m thick ZnS pigmented polyethylene foil with a pigment volume fraction of 0.15 was tested in Tanzania. At noon the observed temperature of the covered blackbody was only 1.5 K above the ambient. Secondly, we study the potential for condensation of water in an arid region. Pigmented foils for this purpose should combine a high solar reflectance and a high infrared emittance, in order to promote condensation by the radiative cooling effect. Titanium dioxide is a fairly good infrared emitter, but the emittance can be improved by using a mixture of TiO$-2$/ and BaSO$-4$/ pigments or only employing a composite SiO$-2$//TiO$-2$/. Field tests with a 390 $mu@m thick polyethylene foil with TiO$-2$/ and BaSO$-4$/ pigments gave encouraging results.