Radiative Cooling

Passive radiative cooling relies on the high transparency of earth's atmosphere in mid-infrared (mid-IR) wavelengths to cool terrestrial objects by radiating heat to the cold outer space - a concept that is key to maintaining a life-sustaining temperature on earth. While this phenomenon is well known in atmospheric sciences, only recently has it been proposed as a passive cooling solution. However, achieving passive radiative cooling below ambient temperatures during the day has proven challenging as things typically tend to heat up due to the intense solar radiation. Fortunately, some recent studies have demonstrated that it is in fact possible to achieve cooling during the day by rejecting nearly all radiation in the solar spectrum while still emitting strongly in mid-IR. This is possible because solar radiation is confined to shorter wavelengths (higher energy) and the radiation emitted by terrestrial objects as well as earth's high-transparency atmospheric 'window' are at much longer wavelengths (lower energy) and do not overlap. However, designing such cooling solutions are non-trivial and require specialized spectrally engineered photonic structures primarily due to the intense solar radiation.

At the Device Research Laboratory, we are working on a different approach to reduce the influence of solar radiation by relying on the fact that solar radiation is highly directional in the sky - something so well known that we intuitively know where to point the sunshade to protect from the sun. We realized that since almost all the solar radiation is confined around the solar disk, we still have access to the cold upper atmosphere around the sun where we can emit radiation and achieve cooling. This concept allows us to decouple the solar reflection and mid-IR emission, which was the main obstacle for previous work on daytime radiative cooling using photonic structures. As of now, we have demonstrated [1] passive cooling 6 C below ambient temperature using simple materials like aluminum foil, thermal insulation and polyethylene sheet. We envision that cooling as low as 20 C below ambient might be achieved using this approach - enough to meet modern refrigeration needs, without the need for electricity.

We believe this work can really take passive radiative cooling to the masses. In particular, we envision applications in safe storage of food and medicines in rural areas with limited access to electricity. And probably the best thing is anyone can build this cooler with simple guidelines and everyday materials. All you need is a sunshade and you are ready to cool!

Read more about our work in Nature Communications and MIT News.

This work is supported by the Solid State Solar Thermal Energy Conversion Center.

  1. B. Bhatia, A. Leroy, Y. Shen, L. Zhao, M. Gianello, D. Li, T. Gu, J.Hu, M. Sojacic, E.N. Wang, "Passive directional sub-ambient daytime radiative cooling", Nature Communications, 9(5001), 2018.