Tag: Thermoregulation

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.

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.