A novel selective emissivity spectrum for radiative sky cooling

Radiative sky cooling is a passive cooling method to obtain the sub-ambient cooling phenomenon by dissipating waste heat into the cold outer universe, relying on the transparent atmospheric window (i.e., 8–13 μm). In recent years, radiative sky cooling has drawn much attention from the fields of engineering to materials due to its potential for energy-saving and low-carbon applications. Generally, an emitter that has high solar reflectivity and emits selectively within the whole atmospheric window is the best candidate for sub-ambient radiative sky cooling that can achieve a low cooling temperature. Here, we proposed a novel selective emissivity spectrum for radiative sky cooling based on the finding that there exists a large transmittance drop within the atmospheric window and this drop will harm radiative sky cooling process, breaking the inherent cognition of the optimal spectrum of the emitter. A theoretical analysis was conducted to predict the radiative sky cooling performance of the emitter with the novel spectrum, and the performance comparison to traditional ideal selective emitter has been demonstrated in terms of sub-ambient cooling temperature and maximum cooling power. Moreover, the underlying reasons and effective application conditions were discussed in detail. In addition, the effect of the parasitic cooling loss process and content of O3 on the cooling performance of our proposed spectrum was investigated as well. In summary, this study provides a new thinking for designing spectrally selective emitters for obtaining remarkable lower cooling temperatures.