Researchers from City University of Hong Kong, known as CityU, unveiled a groundbreaking discovery. Passive radiative cooling PRC material promises to significantly reduce the cooling load on buildings and cities. It offers a technology capable of fighting global warming while cutting the need for traditional, costly, polluting air conditioning. The findings appeared in the journal Science.
Cooling ceramics deliver high performance optics that enable cooling without power or coolant. Their cost effectiveness, durability and versatility make them strong candidates for commercialization across many sectors, especially building construction.
These ceramics lower the thermal load on buildings and provide stable cooling across diverse climates. The researchers noted that energy efficiency improves and global warming can be mitigated.
Passive radiative cooling, or PRC, is viewed as one of the most promising green cooling approaches to curb rising air conditioning demand, reduce environmental pollution and combat climate change, according to Professor Edwin Tso Chi-yan from CityU, one of the paper’s corresponding authors.
Although similar materials existed before, earlier versions used nanophotonic structures that carried high costs and limited compatibility with current end uses. Polymeric photonic options lacked weather resistance and effective solar reflection.
Improved optical properties and applicability
“But our quenching ceramic demonstrates improved optical properties and strong practical potential”, said Professor Tso. “Color, weather resistance, mechanical strength and the ability to reduce the Leidenfrost effect, which prevents heat transfer on hot surfaces, are key traits that ensure the durable and versatile nature of cooling ceramics.”
The cooling ceramics stand out with a hierarchically porous bulk structure that is easy to fabricate from common inorganic materials such as alumina through a simple two step process of phase inversion and sintering. No precision equipment or expensive materials are required, making scalable production feasible.
Optical properties determine PRC performance in two spectral ranges: the solar range from 0.25 to 2.5 micrometers and the mid infrared range from 8 to 13 micrometers. Efficient cooling relies on high reflectivity in the solar range to minimize heat gain and high emissivity in the mid infrared to maximize heat loss. The high band gap of alumina keeps solar absorption to a minimum.
96.5 percent solar reflectance
By mimicking the white appearance of a certain insect species and tuning the pore structure, the cooling ceramic scatters almost all sunlight, achieving a solar reflectance near 99.6 percent and a mid infrared emission around 96.5 percent. These optical traits exceed today’s leading next generation materials.
The ceramic is based on alumina, offering UV resistance to degradation, a common concern for polymer based PRC designs. It also shows excellent fire resistance, enduring temperatures above 1,000 C, which surpasses the capabilities of most polymer or metal based RPC materials, according to Professor Tso.
Excellent weather resistance
Beyond striking optics, the cooling ceramics demonstrate strong weathering resistance, chemical stability and mechanical durability, ideal for long term outdoor use. At very high temperatures, the quenching ceramic becomes super hydrophilic, enabling rapid droplet spread. Its interconnected porous structure speeds impregnation and helps prevent evaporation. This superhydrophilic property reduces the Leidenfrost effect, a common issue with traditional building envelopes. The result is efficient evaporative cooling.
“The beauty of quenching ceramics is that they meet the needs of high performance PRC and real world applications”, said Professor Tso. The material can be colored with a double layer design to enhance aesthetics according to consumer preference.
“Tests show that applying cooling ceramics to a house roof can save more than 20 percent of electricity use”, he stated.
“For space cooling, the great potential of ceramic cooling is clear, reducing dependence on traditional active systems and offering a sustainable path to prevent grid overload, greenhouse gas emissions and urban heat islands”, said Professor Tso.
Based on these observations, the team plans to explore more passive thermal management solutions. The goal is to broaden energy efficiency, boost sustainability and improve the accessibility and applicability of RPC technologies in sectors such as textiles, energy systems and transportation.
Reference work: DOI: 10.1126/science.adi4725
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