Wavelength-selective thermal nonreciprocity barely improves sky radiative cooling.

Area of Science:

• Thermodynamics
• Materials Science
• Optics

Background:

• Sky radiative cooling offers energy-free passive refrigeration but is limited by Kirchhoff's law.
• Thermal nonreciprocity breaks Kirchhoff's law, enabling nonreciprocal radiative cooling (NRC).
• Limited research exists on NRC, necessitating feasibility and worthiness evaluations.

Purpose of the Study:

• To evaluate the effects of NRC on sky radiative cooling at room temperature.
• To assess NRC's potential across different types of radiative coolers.
• To determine the practical benefits and limitations of implementing NRC.

Main Methods:

• Theoretical analysis of NRC effects on ideal selective, non-selective, and colored radiators.
• Simulations conducted at room temperature (298.15 K).
• Evaluation of cooling performance considering emissivity and absorptivity.

Main Results:

• Introducing thermal nonreciprocity provides negligible improvement in sky radiative cooling.
• NRC can lead to a negative cooling gain within the atmospheric window (8-13 µm).
• The study analyzed three primary radiative cooler types: ideal selective, non-selective, and colored radiators.

Conclusions:

• NRC offers limited advantages for current sky radiative cooling technologies.
• The findings challenge the anticipated benefits of NRC, particularly in the atmospheric window.
• This study provides a negative proof, questioning the viability of developing NRC devices.