Radiative cooling exploits the part of the infrared spectrum where the Earth’s atmosphere absorbs little thermal radiation to expel heat to the outer Universe. It has emerged as a promising technology for passive space cooling in buildings. However, while encouraging cooling performance has been demonstrated, the viability of the technology remains a challenge. For instance, existing radiative coolers based on nanophotonic structures are generally expensive, whereas those based on polymers struggle to remain stable under real-world atmospheric conditions. Now, Chi Yan Tso, Zuankai Wang and colleagues from City University of Hong Kong, The Hong Kong Polytechnic University and Hong Kong University of Science and Technology have developed a ceramic for radiative cooling based on hierarchically porous alumina, a relatively low-cost and resistant material with easy manufacturing.
The ceramic has a densely-packed outer layer and an inner layer with numerous internal voids that affords high solar reflectivity of 99.6% and high emissivity in the atmospheric window. These properties result in a cooling power over 130 W m–2 at noon. Beyond the cooling ability, the researchers demonstrate the practical potential of the ceramic: the device is made through a two-step process — a phase inversion and a sintering step — that is in principle scalable and allows the fabrication of curved shapes and coloured ceramics. Furthermore, the stability of the device is tested for real-world stress factors: Tso and team show that the ceramic withstands one-year exposure to outdoor conditions, bending tests, ultraviolet irradiation and fire. The demonstration of both high cooling performance and features of practical relevance for real-world applications is crucial to advance the development and deployment of the technology in practical settings.
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