While the Sun has long been understood to be a source of energy, the possibility that one can use the coldness of outer space as a renewable thermodynamic resource here on the surface of the Earth has been largely ignored. By using thermal radiation one can in fact access this coldness by exploiting a transparency window in the atmosphere between 8-13 µm, which overlaps strongly with the blackbody spectrum of room temperature objects. At night, passive cooling below ambient air temperature has been demonstrated using a technique known as radiative cooling, where one uses a thermal emissive surface exposed to the sky to radiatively emit heat to outer space through the transparency window. Peak cooling demand however occurs during the daytime, and is a major driver of peak electricity demand. Air conditioning of buildings, for example, accounts for 15% of the primary energy used to generate electricity in the United States. A passive cooling strategy that cools without any electricity input during the day could therefore have a significant impact on global energy consumption. To achieve cooling one needs to be able to reach and maintain a temperature below the ambient air. Daytime radiative cooling below ambient under direct sunlight has never before been achieved because sky access during the day results in heating of the radiative cooler by the Sun.
In this talk, we show how a thermal nanophotonic approach enables one, for the first time, to achieve passive radiative cooling below ambient air temperature during peak daylight hours. We first highlight the theoretical requirements necessary for daytime radiative cooling and discuss the need for a photonic approach. We next present a nanophotonic design that has the required spectral characteristics to achieve daytime radiative cooling: it is strongly reflective over visible and near-IR wavelengths but strongly emissive between 8 and 13 µm. We then present results of the first experimental demonstration of daytime radiative cooling, where we achieve a temperature of nearly 5°C below the ambient air temperature under direct sunlight. Finally, we discuss how one can use thermal photonic approaches to passively maintain solar cells at lower temperatures, while maintaining their solar absorption, indicating how photonic radiative cooling can improve a range of energy conversion processes here on Earth.
Aaswath Raman is a Research Associate with the Ginzton Laboratory and Dept. of Electrical Engineering at Stanford University where he works with the group of Professor Shanhui Fan. He received his Ph.D. in Applied Physics from Stanford University in 2013, and his A.B. in Physics & Astronomy and M.S. in Computer Science from Harvard University in 2006. His research interests lie at the intersection of fundamental nanophotonics and new applications to energy efficiency and production. He has authored 18 journal articles that have been cited over 700 times. In 2013, he was the recipient of the Stanford Postdoctoral Research Award, and in 2011, the SPIE Green Photonics Award for his work on nanophotonic light trapping for solar cells, and the Sir James Lougheed Award of Distinction from the Government of Alberta, Canada.