Dropwise condensation reduces selectivity of sky-facing radiative cooling surfaces

Authors: E. Simsek, J. Mandal, A. Raman, L. Pilon

Journal Link: Int. Journal of Heat and Mass Transfer, 198, 123399

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Summary: It is well-known that sky facing surfaces, including radiative coolers, can collect dew at night, but the impact of dew on the optical/spectral properties of radiative coolers, and by extension, their cooling capability, has not really been explored.

We show that dew formation turns sky-facing selective LWIR emitters into broadband emitters, reducing their ability to reach deeper sub-ambient temperatures. This is not so much the case for vertically oriented emitters, which can maintain their optical properties more easily.


* Despite claims in the literature, it is probably not that beneficial to use selective emitters on building roofs for cooling. For vertical facades, that may be a different story.

If you are making radiative coolers that harvest dew, it is best to have dew not form on the sky-facing side as it will reduce cooling capability. If that is not possible, it may be best to make/let it run off.

While the momentum on radiative cooling research has largely been towards materials development, for effective designs, we must also think about the environment in which radiative coolers operate.

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).

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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!

Accurately Quantifying Clear-Sky Radiative Cooling Potentials: A Temperature correction to the Transmittance-based approximation.

Authors: J. Mandal,* X. Huang, A. Raman*

Journal Link: Atmosphere, 12(9), 1195 (2021)

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Summary: Theoretical calculations of the cooling potential of radiative cooling materials are crucial for determining their cooling capability under different meteorological conditions and evaluating their performance. To enable these calculations, accurate models of long-wave infrared downwelling atmospheric irradiance are needed. However, the transmittance-based cosine approximation (ε(θ)=1-τ1/cosθ), which is widely used to determine radiative cooling potentials, does not account for the cooling potential arising from heat loss to the colder reaches of the atmosphere itself.

Here, I show that use of the approximation can lead to > 10% underestimation of the cooling potential relative to MODTRAN 6 outputs. I propose a temperature correction to the transmittance-based approximation which accounts for heat loss to the cold upper atmosphere, and significantly reduces this underestimation, while retaining the advantages of the original model.

In light of the widespread and continued use of the transmittance-based model, this work highlights an important source of potential errors and a means to correct for them. It also indicates that the performances of previously reported radiative cooling designs and radiative cooling potential maps may have to correct for the theoretical underestimation if they used the transmittance-based model. 

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).

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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.

Radiative Cooling and Thermoregulation in the Earth’s Glow

Authors: J. Mandal,* S. Mandal, J. Brewer, A. Ramachandran, A. Raman*.

Preprint: Posted on arXiv (2020-06-21).

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Summary: Passive radiative cooling involves a net radiative heat loss into the cold outer space through the atmospheric transmission windows. Due to its passive nature and net cooling effect, it is a promising alternative or complement to electrical cooling. For efficient radiative cooling of objects, an unimpeded view of the sky is ideal. However, the view of the sky is usually limited – for instance, the walls of buildings have >50% of their field of view subtended by the earth. Moreover, objects on earth become sources of heat under sunlight. Therefore, building walls with hot terrestrial objects in view experience reduced cooling or heating, even with materials optimized for heat loss into the sky.

We show that by using materials with selective long-wavelength infrared (LWIR) emittances, vertical building facades experience higher cooling than achievable by using broadband thermal emitters like typical building envelopes. Intriguingly, this effect is pronounced in the summer and diminishes or even reverses during the winter, indicating a thermoregulation effect. The findings highlight a major opportunity to harness untapped energy savings in buildings.

Colored and Paintable Bilayer Coatings with High Solar-infrared Reflectance for Efficient Cooling.

Authors: J. Mandal,† Y. Chen,† C. Tsai, S. Shrestha, N. Yu, Y. Yang.

Journal Link: Science Advances, 6 (17), eaaz5413 (2020).

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Summary: This work shows that paints can be carefully engineered to absorb visible colors complementary to the one desired (e.g. yellow, if you desire blue), and at the same time, transmit invisible solar heat. When a thin layer of such a paint is painted above a highly reflective coating (say a white paint, or some of the designs below), it can reflect solar heat much better than traditional single layer designs. This enables bi-layers (color paints on white coatings) to stay much cooler (e.g. by 5ºC) than traditional coatings of the same color, while using less colorants.

Note: Bilayer designs have previously been explored by Ronnen Levinson et. al. at the Heat Island Group in Lawrence Berkeley National Laboratory. This work adds to those by providing physical justifications for the bilayer design, and proposing that a near-infrared (NIR) transparent top layer may be generally better than one that scatters NIR.

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)

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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.

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).

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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.

Li4Ti5O12: A Visible-to-Infrared Broadband and Fast Electrochromic Material for Thermal and Optical Management

Authors: J. Mandal, S. Du, M. Dontigny, K. Zaghib, N. Yu, Y. Yang.

Journal Link: Advanced Functional Materials 28 (36), 201802180 (2018).

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Summary: In this work, we looked at Lithium Titanium Oxide (Li4Ti5O12), a promising material for battery anodes which  is known to change from white to black when it is charged. We investigated the optical properties of Li4Ti5O12 across the visible to LWIR wavelengths (i.e. 0.4-14 µm), and found that the transition from white to black actually stretches all the way from the visible to the LWIR. We explore the potential physical mechanisms of the switching, and demonstrate that it can be used for radiative cooling and heating, and thermal camouflage.

Note: the original idea of investigating the Li4Ti5O12 was not mine, but that of my doctoral adviser, Dr. Yuan Yang. He had the hunch that the optical switching of the material can extend into the infrared, and asked me to optical characterize samples he had prepared. After confirming his hypothesis, I explored the physics of the optical transition, and the optical design aspects with the goal of achieving radiative cooling and thermal camouflage.