Selective LWIR Emitters – Jyotirmoy Mandal

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.

Takeaways:

* 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.

Publication

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!

Preprint

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.