polymers – Jyotirmoy Mandal

Do-it-yourself radiative cooler as a radiative cooling standard and cooling component for device design

Authors: Xin Huang, Jyotirmoy Mandal,* Aaswath Raman*

Journal Link: Journal of Photonics for Energy, 12(1), 012112 (2021).

Download Paper and Supporting Information

Datasets: Link

Summary: If you are interested in radiative cooling technologies, and want to try out a design for yourself, this fun little paper reports a convenient way to make radiative coolers at home. Made from scotch tape (with acrylic adhesive) and aluminum foil, our radiative coolers can reach sub-ambient temperatures (by 11 C or more under the right conditions). Under clear skies, one could use it to harvest dew, couple it to thermoelectric modules to generate electricity, or just cool things.

Although it was a fun DIY project, my motivation for designing this was its potential as a standard and experimental control. The radiative cooling field has seen a lot of work over the past few years, but few ways to verify experimental results or compare designs. I hope that the simplicity and reproducibility of this design will make it useful as a reference. Towards that end, detailed optical parameters of the scotch tape is provided on dataset link above. Have fun making!

Publication

Porous Polymers with Switchable Optical Transmittance for Optical and Thermal Regulation

Authors: J. Mandal, M. Jia, A. Overvig, Y. Fu, E. Che, N. Yu, Y. Yang.

Journal Link: Joule 3, 1-12 (2019)

Download Paper + Supporting Information (accepted version)

Summary: This work shows that porous polymers, which normally scatter light and appear white due to the air voids in them, can be turned transparent or translucent by wetting them with suitable liquids. The idea itself has been known for as long as humans have noticed paper or cloth turn more translucent when wetted. What I try is to push it to a limit and use it for switchable cooling/heating applications.

The key to achieving a white to near-transparent switching is choosing the right liquid. In its porous form, the polymer contains air voids which have a different refractive index (n~1) from that of the polymer (n~1.4-1.5), causing light to scatter off the pores and yield the white colour. But if the pores are filled with a liquid that has the same refractive index as the polymer, then to light the whole system just behaves like one uniform material, so light transmits, as it would through glass.

We achieve this behaviour by using common materials and liquids, and also extend the switching behavior to thermal infrared wavelengths. Promisingly, the switching in the thermal is opposite to that in the solar wavelengths, meaning that porous polymers can switch from icehouse to greenhouse states – something that has not been observed with electrochromic or other switchable designs as far as I know.

With regard to applications, we show that this behavior can be used for switchable heating and cooling of buildings depending on the season, controlling daylight in buildings, thermal camouflage and other uses. Given the low cost and simplicity of the designs we use, they could potentially see large scale uses.

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.