Cellular Inorganic Ceramic for Highly Efficient Daytime Passive Radiative Cooling

Abstract Traditional compression-based cooling systems consume great amounts of energy, causing adverse impacts on the environment by releasing heat and ozone-depleting pollutants. Daytime passive radiative cooling is regarded as an eco-friendly alternative to generate cooling without electricity and coolants. In order to achieve continuous and effective cooling, the cooler requires high solar reflection as well as strong mid infrared emission. However, it remains a great challenge to realize cooling effect in the daytime. Sunlight is an intensive heat source in the daytime, especially during noontime. Under sunlight, even absorption of a few percent of solar irradiation can easily compensate for the thermal heat dissipated by thermal radiation, and therefore fail to achieve any cooling effect. Here we develop a cellular inorganic radiative cooler in the format of ceramic, which possesses advanced optical properties for radiation cooling while exhibiting great outdoor applicability. The cellular ceramic possesses a hierarchically porous structure, which results in efficient light scattering across the whole solar spectrum. The average solar reflection of the cellular ceramic reached a recorded-high of 99.6% with porosity of 70% and thickness of around 600 μm. With the near-ideal solar reflection and high thermal emission (96%), the cellular ceramic can theoretically obtain cooling power in excess of 140 W/m2 under 1000 W/m2 solar intensity in favorable application conditions. Moreover, the high bond strength and chemical inertness of alumina protect the cellular ceramic from the adverse effect of UV exposure, which is desired for long-term outdoor applications. The outstanding performance of cellular ceramic surpasses that of current daytime radiative coolers. The facile fabrication and promising applicability of cellular ceramic exhibits great potential for large-scale applications, such as buildings, to saving the energy for indoor thermal regulation.