Hierarchically Porous Polymer Coatings for Highly Efficient Passive Daytime Radiative Cooling

Authors: J. Mandal, Y. Fu, A. Overvig, M. Jia, N. Shi, K. Sun, H. Zhou, X. Xiang, N. Yu, Y. Yang.

Journal Link: Science 362, 315–319 (2018).

Download Paper and Supporting Information (Accepted versions)

Summary: Passive Daytime Radiative Cooling is a process where an object under the sky reflects sunlight and radiates heat through the atmosphere into outer space. If an object has a sufficiently high solar reflectance and thermal emittance, solar heating is minimized and radiative heat emission into outer space is maximized. As a result, the object can achieve a net heat loss even under sunlight, and passively cool down to sub-ambient temperatures. Because this process is “zero-energy, zero-carbon”, it is a sustainable alternative to active cooling methods such as air conditioners, or an affordable cooling method in low-resource settings.

Most materials around us are excellent radiators of heat (much more so than what scientists and engineers often claim for their designs), however, they lack the other requirement, a high solar reflectance. One can take thermal emitters such as plastic sheets, or dielectric materials and back them with silver or aluminum to get daytime radiative coolers – this has been done since the 1970s, but it is difficult to apply such designs on buildings, where cooling is needed the most. Cool-roof paints with thermal emittances > 0.90 and solar reflectances ~ 0.85 come the closest to being viable designs, but even a solar reflectance of 0.85 is not high enough to prevent some heating under strong sunlight.

In this paper, we aim to achieve a radiative cooling design with a paint-like convenience using a solution-based phase-inversion method. Using a polymer-solvent-nonsolvent precursors (e.g. poly(vinylidene fluoride-co-hexafluoropropene)-acetone-water) and painting films of those on substrates, we create porous polymer coatings that have solar reflectances that can exceed 0.98 and long-wavelength infrared emittance of 0.97 – near perfect values for radiative cooling. During experiments under noontime spring sunlight (890 W m-2), these coatings are found to be cooler than the ambient air by 6°C (and potentially more). The cooling power is measured to be ~96 W m-2 in the same experiment. These performances, which exceed those of known (to me) designs, is obtained with a paint like convenience.

We further show that our technique is compatible with a wide range of polymers, can be used to coat a variety of substrates, and can also be used to create colored coatings that look the same but stay cooler than traditional designs. Collectively these findings represent a major advancement for practical daytime radiative cooling.

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