Bulk material based selective infrared emitter for sub-ambient daytime radiative cooling

Through passively emitting excess heat to the outer space, radiative cooling has been demonstrated as an efficient way for energy saving applications. Instead of well-designed photonic structures, we rediscovered the potential of a bulk material as a selective emitter for sub-ambient daytime radiative cooling. The proposed cooler, consisting of a 1-mm-thick lithium fluoride crystal coated with silver backing, exhibits a low solar absorptance of 4.7% and nearly ideal infrared selectivity with high emission exactly within the atmospheric transmission band (i.e., 8–13 μm). Potentially, the cooler could achieve an ultra-low stagnation temperature up to 57 °C below the ambient. Sub-ambient daytime cooling performance was also demonstrated in an outdoor test with cooler stagnation temperature below the ambient one by 5 °C under solar irradiance above 900 W/m2 and a net cooling power of about 60 W/m2 in thermal equilibrium with the ambient. The proposed simple selective infrared emitter could efficiently and passively cool optical systems due to the wide applications of lithium fluoride crystal as optical windows and mirrors. •The bulk LiF emitter occupies nearly ideal selective emission for radiative cooling.•The emitter could potentially achieve a temperature of 57 K below the ambient.•Sub-ambient daytime cooling effect is demonstrated with the emitter.•This research could provide guidance to radiatively cool optical systems.