Hierarchical Fabric Emitter for Highly Efficient Passive Radiative Heat Release

Intense heat waves pose a serious threat to public health and well-being, especially in outdoor spaces. Outdoor high-temperature environments without air conditioners are major challenges for humanity. However, an achievable approach that can provide outdoor cooling without consuming any energy is lacking. Hence, this work presents a novel hierarchical fabric emitter (HFET) used for sunshade sheds to provide radiative outdoor cooling for humanity, the HFET is composed of polyethylene/silicon dioxide/silicon nitride film, melt-blown polypropylene film, and polydimethylsiloxane film from top to bottom. In addition to reflecting 94% solar irradiance by its top surface, the HFET shows selective emission (0.82 in the atmospheric window and 0.38 outside the atmospheric window) on its top surface to outer space and broadband absorption (0.80 in the longwave infrared band) on its bottom surface from the inside. This bidirectional asymmetric emission enables the simulated skin to avoid overheating by 2-11 ? relative to the reverse HFET and bare cases under direct sunlight. Due to its excellent cooling capability, the HFET will be one of the most considerable solutions for outdoor cooling in hot summer environments.