Analyses of ancient dog DNA suggest that dogs were likely domesticated from Eurasian wolves around 23,000 years ago, possibly in Siberia.
Image credit: Pixabay/Chiemsee2016.
Edited by John B. Pendry, Imperial College London, London, United Kingdom, and approved August 18, 2015 (received for review May 19, 2015)
Significance
The coldness of the universe is an enormous but strikingly underexploited thermodynamic resource. Its direct utilization on Earth therefore represents an important frontier for renewable energy research. In many applications, including solar cells and outdoor structures, the absorption of sunlight is intrinsic either from operational or aesthetic considerations, but the resulting heating by sunlight is undesirable. Here we experimentally demonstrate a thermal photonic scheme that can cool these structures by thermal radiation to outer space, while preserving the structures’ solar absorption. Our work shows, for the first time to our knowledge, that radiative cooling can be used in combination with the utilization of sunlight, and opens new possibilities for using the coldness of the universe to improve the performance of terrestrial energy systems.
Abstract
A solar absorber, under the sun, is heated up by sunlight. In many applications, including solar cells and outdoor structures, the absorption of sunlight is intrinsic for either operational or aesthetic considerations, but the resulting heating is undesirable. Because a solar absorber by necessity faces the sky, it also naturally has radiative access to the coldness of the universe. Therefore, in these applications it would be very attractive to directly use the sky as a heat sink while preserving solar absorption properties. Here we experimentally demonstrate a visibly transparent thermal blackbody, based on a silica photonic crystal. When placed on a silicon absorber under sunlight, such a blackbody preserves or even slightly enhances sunlight absorption, but reduces the temperature of the underlying silicon absorber by as much as 13 °C due to radiative cooling. Our work shows that the concept of radiative cooling can be used in combination with the utilization of sunlight, enabling new technological capabilities.
Footnotes
↵1L.Z. and A.P.R. contributed equally to this work.
- ↵2To whom correspondence should be addressed. Email: shanhui{at}stanford.edu.
Author contributions: L.Z., A.P.R., and S.F. designed research; L.Z., A.P.R., and S.F. performed research; L.Z., A.P.R., and S.F. analyzed data; S.F. supervised the project; and L.Z., A.P.R., and S.F. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Passive radiative cooling of solar absorbers
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- Applied Physical Sciences
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