Tag: PVdF

Perspective/Opinion, Publication

Paints as a scalable and effective radiative cooling technology for buildings

Authors: J. Mandal,* N. Yu, Y. Yang, A. Raman*.

Journal Link: Joule, 4, 1-7 (2020).

Download Paper (Accepted Version)

Summary: In recent years, the field of radiative cooling (see below) has seen a fair bit of interest and different designs for various applications. However, cooling roofs and walls of buildings remain its greatest application, and white paints, owing to their convenience and modest radiative cooling capability, remain the benchmark for radiative coolers. Curiously, they are seldom mentioned in prominent works that have come out of late, and the paint industry, in turn, has been somewhat distant from advances made in the field.

This article aims to draw research interest into paints as highly efficient radiative coolers. Specifically, simple material and morphological alterations that can greatly enhance the cooler performances of paints are shown, and interdisciplinary challenges associated with their usage, such as the effect of dust or the need for durability, are discussed.

Publication

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.