Wavelength-selective thermal nonreciprocity barely improves sky radiative cooling

Radiative cooling has showcased great potential for passive refrigeration without extra energy consumption, while its cooling power and efficiency is confined by Kirchhoff's law, that is, the emissivity is equal to the absorptivity. The recent development of thermal nonreciprocity that breaks the limitations of Kirchhoff's law, especially in broadband manner, makes nonreciprocal radiative cooling (NRC) possible. Since there lacks of reports of NRC theoretically or experimentally, it is time to evaluate the feasibility and worthiness of develop NRC. Here, we discussed the effects of NRC at around room temperature (298.15 K) from three perspectives: ideal selective radiators, non-selective radiators, and colored radiators. Contrary to intuition, the introduction of thermal nonreciprocity in the atmospheric window (8-13 μm) only leads to a negative gain. Additionally, it should be noted that the radiators discussed in this work are horizontally placed without the influence of asymmetric external heat sources. The current findings shatter the inherent notion of NRC and offer some theoretical support for the practical realization and application of nonreciprocal radiative refrigeration devices.