Glass-hybrid plastic offers passive cooling

If the temperature of buildings could be controlled by simply covering them with a plastic wrap that dissipates daytime heat without consuming electricity, owners could save a bundle on energy costs. Researchers at the University of Colorado, Boulder, have taken a key step in that direction by developing an inexpensive glass-polymer hybrid material that excels in this type of passive cooling (Science 2017, DOI: 10.1126/science.aai7899). Scientists have identified a number of materials for efficient nighttime radiative cooling. But daytime cooling remains a challenge because solar absorbance of just a few percent exceeds many materials’ cooling capacities, causing buildings and other surfaces to become hot. So Colorado mechanical engineers Ronggui Yang, Xiaobo Yin, and coworkers devised a material that strongly emits thermal energy in the form of infrared light and hardly absorbs sunlight. The material, which the team makes via low-cost roll-to-roll manufacturing methods, consists of randomly distributed micrometer-sized SiO₂ spheres embedded in a poly(methylpentene) matrix. A 50-μm-thick film of the transparent material containing 6% microspheres by volume emits intensely throughout the IR region and reflects approximately 96% of solar radiation when backed with a 200-nm-thick silver coating. Having conducted initial lab tests, the team now plans to evaluate the technology this summer in the form of a 20 m² roof cover on Colorado’s engineering building.