Wavelength-selective thermal nonreciprocity barely improves sky radiative cooling.

Sky radiative cooling has showcased great potential for passive refrigeration without extra energy consumption, while its cooling power and efficiency are 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 a broadband manner, makes nonreciprocal radiative cooling (NRC) possible. However, as there are few reports on NRC either theoretically or experimentally, it is necessary to evaluate the feasibility and worthiness of developing 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, which are the current primary radiative coolers. Counterintuitively, we found that introducing thermal nonreciprocity barely improves sky radiative cooling, and only in the atmospheric window (8-13 µm) even leads to a negative gain. The current findings break the intuition of NRC and offer a negative proof for the development of NRC devices.