Passive radiative cooling below ambient air temperature under direct sunlight

Aaswath P. Raman, Marc Abou Anoma, Linxiao Zhu, Eden Rephaeli, Shanhui Fan

Research output: Contribution to journalArticlepeer-review

971 Scopus citations

Abstract

Cooling is a significant end-use of energy globally and a major driver of peak electricity demand. Air conditioning, for example, accounts for nearly fifteen per cent of the primary energy used by buildings in the United States1. A passive cooling strategy that cools without any electricity input 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 that of the ambient air. At night, passive cooling below ambient air temperature has been demonstrated using a technique known as radiative cooling, in which a device exposed to the sky is used to radiate heat to outer space through a transparency window in the atmosphere between 8 and 13 micrometres2-11. Peak cooling demand, however, occurs during the daytime. Daytime radiative cooling to a temperature below ambient of a surface under direct sunlight has not been achieved3,4,12,13 because sky access during the day results in heating of the radiative cooler by the Sun. Here, we experimentally demonstrate radiative cooling to nearly 5 degrees Celsius below the ambient air temperature under direct sunlight. Using a thermal photonic approach14-25, we introduce an integrated photonic solar reflector and thermal emitter consisting of seven layers of HfO2 and SiO2 that reflects 97 per cent of incident sunlight while emitting strongly and selectively in the atmospheric transparency window. When exposed to direct sunlight exceeding 850 watts per square metre on a rooftop, the photonic radiative cooler cools to 4.9 degrees Celsius below ambient air temperature, and has a cooling power of 40.1 watts per square metre at ambient air temperature. These results demonstrate that a tailored, photonic approach can fundamentally enable new technological possibilities for energy efficiency. Further, the cold darkness of the Universe can be used as a renewable thermodynamic resource, even during the hottest hours of the day.

Original languageEnglish (US)
Pages (from-to)540-544
Number of pages5
JournalNature
Volume515
Issue number7528
DOIs
StatePublished - Nov 27 2014

All Science Journal Classification (ASJC) codes

  • General

Fingerprint

Dive into the research topics of 'Passive radiative cooling below ambient air temperature under direct sunlight'. Together they form a unique fingerprint.

Cite this