Tag: Vertical Surfaces

Publication

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

Download Paper and Supporting Information

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.

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).

Download Preprint

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.