Researchers from City University of Hong Kong (CityU) announced a breakthrough in passive radiative cooling (PRC) materials designed to dramatically reduce the energy needs of buildings and cities. This technology offers a path to cleaner cooling that could replace traditional, polluting, and costly air conditioning systems. The study appeared in Science, underscoring the significance of the discovery for climate-friendly building strategies in North America and beyond.
The material, termed cooling ceramics, delivers effective cooling using optics that do not require power or coolant. Its potential for cost efficiency, durability, and broad applicability makes it an attractive option for commercialization, with strong relevance to modern building construction.
Cooling ceramics lessen the thermal load on structures and maintain stable cooling performance across diverse climates. Researchers stated that the technology can boost energy efficiency and help mitigate global warming. PRC is regarded as a leading green cooling approach to meet the rising demand for air conditioning while cutting environmental pollution, according to CityU researchers.
Although similar materials have existed, many rely on nanophotonic structures that raise costs and complicate real-world use. Polymeric photonic options often lack durability against weather and fail to reflect sunlight effectively. Cooling ceramics overcome these hurdles with a simpler, more robust composition.
Improved optical properties and applicability
Researchers note that the quenching ceramic delivers enhanced optical performance and practical versatility. Key attributes include color stability, weather resistance, mechanical strength, and the ability to suppress the Leidenfrost effect, which otherwise hinders heat transfer on hot surfaces. These properties ensure durability and broad applicability of cooling ceramics.
The material features a hierarchically porous bulk ceramic structure that can be produced from accessible inorganic starting materials such as alumina through a straightforward two-step process involving phase inversion and sintering. No specialized equipment or expensive materials are required, enabling scalable production of cooling ceramics.
Optical performance divides into two wavelength ranges: the solar spectrum (0.25–2.5 µm) and the mid-infrared range (8–13 µm). Effective cooling relies on high reflectivity in the solar range to minimize heat gain and high emissivity in the mid-infrared to maximize radiative heat loss. The alumina basis keeps solar absorption low, aiding efficient cooling across daylight hours.
96.5% solar reflectance rate
By imitating the whiteness of specific natural structures and tailoring pore geometry based on Mie scattering principles, the cooling ceramic achieves near-total scattering of sunlight, with solar reflectance approaching 99.6% and mid-infrared emission around 96.5%. These figures exceed those of many newer PRC materials and mark a notable advance in optical design for cooling applications.
The ceramic relies on alumina, which provides strong UV resistance and resists degradation common to polymer-based PRC designs. It also demonstrates excellent fire resistance, withstanding temperatures well over 1,000°C, far surpassing typical polymer or metal-based RPC materials.
Excellent weather resistance
Beyond optics, cooling ceramics exhibit strong weather resistance, chemical stability, and mechanical durability, making them suitable for long-term outdoor use. At extreme temperatures, the material displays superhydrophilicity, promoting rapid moisture interaction and facilitating fast impregnation thanks to its interconnected porosity. This property helps prevent evaporation and mitigates the Leidenfrost effect, a limitation of traditional building materials and cooling approaches. As a result, evaporative cooling becomes more efficient.
The strength of quenching ceramics lies in meeting high-performance PRC standards while staying practical for real-world environments. The material can be colored with a double-layer design to enhance aesthetics according to consumer taste.
Initial experiments indicate that applying cooling ceramics to rooftop areas can yield noticeable energy savings, with reports suggesting more than 20% electricity reductions in some scenarios. For space cooling, the ceramic’s potential is substantial, offering a sustainable route to reduce reliance on traditional active systems and to avoid grid stress, greenhouse gas emissions, and urban heat islands. Researchers emphasize the broader goal of expanding passive thermal management toward wider adoption across sectors such as textiles, energy systems, and transportation.
Further work is planned to refine passive cooling strategies and broaden their applicability to improve energy efficiency and sustainability across industries. The study signals a path toward broader accessibility and integration of PRC technologies in buildings and other applications.
Reference work: DOI: 10.1126/science.adi4725
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Notes on the environmental implications accompany the research, highlighting the added value of materials that contribute to cleaner, more resilient infrastructure.