Freeze under sunshine by radiative cooling of water

Ancient Persians harnessed radiative cooling to freeze thin layers of water in shallow pools on clear nights, even when the ambient temperature remained above the freezing point. However, due to solar heating, daytime ice-making without electricity has yet to be achieved even now. Inspired by passive daytime radiative cooling, sub-ambient cooling during the day can be attained by minimizing solar absorption and maximizing thermal emission. Intriguingly, water exhibits desirable optical properties for that purpose. Here, we demonstrated that liquid water can be utilized to construct a high-performance daytime radiative cooler, i.e., the water cooler. The water cooler's thermal emission and solar absorption can be controlled by adjusting the water’s thickness, as determined by theoretical calculations and experimental measurements. We observed a 6 °C sub-ambient cooling of the water cooler in the field test in Xi’an on 1st Dec. 2023 at noon. Daytime ice-making without electricity was also showcased using the same setup. Based on the results, the water was cooled below the freezing point for most of the day when the ambient temperature was above 0 °C.