On the effective spectral emissivity of clear skies and the radiative cooling potential of selectively designed materials

Mengying Li, Carlos F.M. Coimbra

Research output: Journal article publicationJournal articleAcademic researchpeer-review

12 Citations (Scopus)

Abstract

Thermophotonic devices are optically designed to be spectrally selective in order to reject heat to outer space through atmospheric windows of low thermal absorption. The determination of thermal equilibrium temperatures for thermophotonic devices requires the knowledge of the effective spectral emissivity of the sky. In this work, individual contributions of participating gases and aerosols to the spectral values of the sky emissivity are analyzed in the entire infrared spectrum as well as in seven distinct bands for which water vapor either dominates or is virtually transparent to infrared radiation. We also propose high-fidelity correlations for the effective sky emissivity as functions of the normalized ambient partial pressure of water vapor (pw) for both broadband and for the seven spectral bands. The correlations are derived using a combination of ground experimental data, high resolution spectral data for the main atmospheric constituents and spectral models to reconstruct the spectral distribution of infrared thermal radiation from the atmosphere to the ground. These results enable direct calculation of the equilibrium temperature and cooling efficiency of radiative cooling devices in terms of meteorological conditions observed at the surface level. For hot and dry conditions, the passive radiative coolers have a cooling potential of 150.8 W m−2 while for humid conditions, the coolers are mostly ineffective.

Original languageEnglish
Pages (from-to)1053-1062
Number of pages10
JournalInternational Journal of Heat and Mass Transfer
Volume135
DOIs
Publication statusPublished - Jun 2019

Keywords

  • Longwave atmospheric radiation
  • Passive radiative cooling
  • Spectral sky emissivity

ASJC Scopus subject areas

  • Condensed Matter Physics
  • Mechanical Engineering
  • Fluid Flow and Transfer Processes

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