Radiative Cooling Under an Urban Glow and a Narrow View of the Sky*

Passive radiative cooling (PRC) of objects involves radiative heat loss into the cold outer space through the long wavelength infrared (LWIR) atmospheric transmission window. Due to its passive operation and net cooling effect, it is a promising alternative or complement to electrical cooling.^1-2 For efficient PRC of objects, an unimpeded view of the sky is ideal. However, the view of the sky is often limited - for instance, the walls of buildings have > 50% of their field of view subtended by the earth, while low-lying roofs are overshadowed by taller buildings. Moreover, objects on earth become heat sources under sunlight. Therefore, buildings with hot terrestrial objects in view experience reduced cooling or even heating.

We show that by using common materials with selective LWIR emittances, walls and roofs can radiatively cool to considerably lower temperatures than achievable by using traditional broadband emitters like paints. Cooling enhancements (theoretical and demonstrated) achieved by such materials opens opportunities for the appropriate selection and design of materials for building envelopes.

[1] A. Raman et. al., Nature 515, 541 (2014)
[2] J. Mandal et. al., Science 362, 315 (2018)