4.7 Article

Angularly selective thermal emitters for deep subfreezing daytime radiative cooling

Journal

NANOPHOTONICS
Volume 11, Issue 16, Pages 3709-3717

Publisher

WALTER DE GRUYTER GMBH
DOI: 10.1515/nanoph-2022-0032

Keywords

angularly selective system; radiative cooling; thermal design; thin film design

Funding

  1. National Natural Science Foundation of China [62134009, 62121005]
  2. Innovation Grant of Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP)
  3. Jilin Provincial Science and Technology Development Project [YDZJ202102CXJD002]
  4. Bill & Melinda Gates Foundation [INV-009181]
  5. Development Program of the Science and Technology of Jilin Province [20200802001GH]

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We theoretically analyze the impact of angular selectivity on the radiative cooling performance of thermal emitters and propose a general scheme to achieve angular and spectral selectivity. We demonstrate that combining these two selectivities is necessary to reach subfreezing temperatures and introduce a thin film stack to realize this scheme.
We theoretically analyze the impact of angular selectivity on the radiative cooling performance of thermal emitters. We investigate the effect of spectral selectivity, environmental conditions, and parasitic heating on the minimum possible equilibrium temperature of the thermal emitter. We show that combining angular and spectral selectivity is necessary to reach deep subfreezing temperatures. We also show that angularly selective thermal emitters increase the cooling performance in humid environments, however, they require management of nonradiative heat transfer processes. We introduce a general scheme to realize angularly and spectrally selective absorption/emission using a thin film stack consisting of an angle dependent transmission filter overlayed on a selective thermal emitter. The thermal emitter total thickness is similar to 16 mu m, an order of magnitude less than previously proposed angular selective thermal emitters/absorbers and operates under s- and p-polarized light without using anisotropic layers. Under realistic conditions and reasonable parasitic heating, the proposed emitter can be cooled down to Delta T = -46 degrees C below ambient temperature. Our work highlights the advantages and drawbacks of angular selective thermal emitters towards practical and efficient radiative cooling devices.

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