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.

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. 

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.