Center Researcher develops Glass-Polymer Hybrid Metamaterial for Daytime Radiative Cooling (February 2017)
Center seed investigator Xiaobo Yin and a team of University of Colorado Boulder engineers have developed a scalable manufactured metamaterial – an engineered material with extraordinary properties not found in nature – to act as a kind of air conditioning system for structures. It has the ability to cool objects even under direct sunlight with zero energy and water consumption.
When applied to a surface, the metamaterial film cools the object underneath by efficiently reflecting incoming solar energy back into space while simultaneously allowing the surface to shed its own heat in the form of infrared thermal radiation.
The material takes advantage of passive radiative cooling, the process by which objects naturally shed heat in the form of infrared radiation, without consuming energy. Thermal radiation provides some natural nighttime cooling and is used for residential cooling in some areas, but daytime cooling has historically been more of a challenge. For a structure exposed to sunlight, even a small amount of directly-absorbed solar energy is enough to negate passive radiation.
The challenge for the CU Boulder researchers, then, was to create a material that could provide a one-two punch: reflect any incoming solar rays back into the atmosphere while still providing a means of escape for infrared radiation. To solve this, the researchers embedded visibly-scattering but infrared-radiant glass microspheres into a polymer film. They then added a thin silver coating underneath in order to achieve maximum spectral reflectance.
Glass-polymer hybrid metamaterial. (A) Schematic of the hybrid metamaterial with randomly distributed SiO2 microsphere inclusions and backed with a thin silver film. The silver film diffusively reflects most of the incident solar irradiance while the hybrid material absorbs all incident infrared irradiance and is highly infrared emissive. (B) Three-dimensional confocal microscope image of the hybrid metamaterial. The microspheres are visible due to the autofluorescence of SiO2.
The researchers have demonstrated high-throughput, economical roll-to-roll manufacturing of the metamaterial, vital for promoting radiative cooling as a viable energy technology. "We feel that this low-cost manufacturing process will be transformative for real-world applications of this radiative cooling technology," said Yin. "That makes a big difference at scale."
This work was published in
Y. Zhai, Y. Ma, S. N. David, D. Zhao, R. Lou, G. Tan, R. Yang, and X. Yi, "Scalable-manufactured randomized glass-polymer hybrid metamaterial for daytime radiative cooling," Science (2017).
Read the press release.