The low temperature of the night sky, typically 8 'to 20' C below that of the ambient air, potentially can be an important resource for the passive cooling of residential and commercial buildings. Until now this resource has not been effectively exploited, largely because durable, low-cost materials do not currently exist for making infrared-transparent glazings. Without such a glazing, the radiator surface cools when exposed to the sky but regains much of the heat convectively by cooling the outdoor air. This reduces the amount of "cooling power" available to displace the building's thermal load. This project is to develop a polymer film about 1 mm thick with high infrared transparency between 8 and 13 /,tm. High- and low-density polyethylene films exhibit the highest infrared transmittance of all polymers within this region but fall short of the desired goal because of the presence of absorbing nonethylene groups incorporated in the chain. Pure polyethylene resins (without additives) will be obtained from major manufacturers and subjected to a variety of chemical and physical treatments to reduce the detrimental infrared absorbance caused by short-branching side chains and the presence of unsaturated groups in the polymer chain. Samples of the most transparent films and thermal insulating foams will be made and evaluated in an outdoor radiative cooling test assembly.Anticipated Results/Potential Commercial Applications as described by the awardee:A polyethylene-based glazing film or foamed slab having high infrared transparency in the 8 to 13 itm region will enable the manufacture of effective radiative cooling assemblies for rooftop mounting on buildings without the use of movable insulation. A related application is the development of a glazing material for desert greenhouse agriculture to reduce the need for evaporative cooling under hot summer conditions. Additional commercial applications also may be found in the fabrication of low-cost infrared optical elements such as fresnel lenses and detector windows for instrumentation and research applications.
