New Cooling Ceramic for Construction Sector Saves Energy

Researchers at City University of Hong Kong (CityU) have made a significant breakthrough in developing a passive radiative cooling (PRC) material.

The material, known as cooling ceramic, has achieved high-performance optical properties for energy-free and refrigerant-free cooling generation. Its cost-effectiveness, durability, and versatility make it suitable for commercialization in numerous applications, particularly in building construction. 

By reducing the thermal load of buildings and providing stable cooling performance, even in diverse weather conditions in all climates, cooling ceramic enhances energy efficiency and can combat global warming, the research found.

PRC is considered one of the most promising green cooling technologies for curbing soaring demand for space cooling, reducing environmental pollution, and combating global warming, according to Professor Edwin Tso Chi-yan, associate professor in the School of Energy and Environment (SEE) at CityU, one of the authors of the paper.

However, current PRC using nanophotonic structures are limited by its high cost and poor compatibility with existing end uses, while polymeric photonic alternatives lack weather resistance and effective solar reflection.

Enhanced optical properties and applicability 

“But our cooling ceramic achieves advanced optical properties and has robust applicability,” said Professor Tso. “The color, weather resistance, mechanical robustness and ability to depress the Leidenfrost effect — a phenomenon that prevents heat transfer and makes liquid cooling on the hot surface ineffective — are key features ensuring the durable and versatile nature of the cooling ceramic.”  

The cooling ceramic has a hierarchically porous structure as a bulk ceramic material, which is easily fabricated using highly accessible inorganic materials such as alumina through a simple two-step process involving phase inversion and sintering. No delicate equipment or costly materials are required, making scalable cooling ceramics manufacturing highly feasible.

Optical properties determine the cooling performance of PRC materials in two wavelength ranges: solar range (0.25-2.5 µm) and mid-infrared range (8-13 µm). Efficient cooling requires high reflectivity in the former range to minimize the solar heat gain and high emissivity in the latter range to maximize the radiative heat dissipation. Owing to the high bandgap of alumina, the cooling ceramic keeps solar absorption to a minimum.

By mimicking the bio-whiteness of the Cyphochilus beetle and optimizing the porous structure based on Mie scattering, the cooling ceramic efficiently scatters almost all the wavelength of sunlight, resulting in near-ideal solar reflectivity of 99.6% (a recorded high solar reflectivity) and achieves a high mid-infrared thermal emission of 96.5%. These advanced optical properties surpass those of current state-of-the-art materials. 

“The cooling ceramic is made of alumina, which provides the desired UV resistance degradation, which is a concern typical of most polymer-based PRC designs. It also exhibits outstanding fire resistance by withstanding temperatures exceeding 1,000°C, which surpasses the capabilities of most polymer-based or metal-based PRC materials,” said Tso.

The cooling ceramic also exhibits excellent weather resistance, chemical stability, and mechanical strength, making it ideal for long-term outdoor applications. At extremely high temperatures, the cooling ceramic exhibits superhydrophilicity, enabling immediate droplet spreading, and facilitating rapid impregnation of the droplets due to its interconnected porous structure. This superhydrophilic characteristic inhibits the Leidenfrost effect that hinders evaporation, commonly found in traditional building envelope materials, and enables efficient evaporative cooling. 

“The beauty of the cooling ceramic is that it fulfils the requirements for both high-performance PRC and applications in real-life settings,” said Professor Tso, adding that the cooling ceramic can be colored with a dual-layer design, meeting aesthetic requirements as well.

“Our experiment found that applying the cooling ceramic on a house roof can achieve more than 20% electricity for space cooling, which confirms the great potential of cooling ceramic in reducing people’s reliance on traditional active cooling strategies and provides a sustainable solution for avoiding electricity grid overload, greenhouse gas emissions and urban heat islands,” added Tso.

The research team intends to advance further passive thermal management strategies and to explore the application of these strategies to enhance energy efficiency, promote sustainability, and increase the accessibility and applicability of PRC technologies in various sectors, including textiles, energy systems, and transportation.

The findings have just been published in the prestigious scientific journal Science titled “Hierarchically structured passive radiative cooling ceramic with high solar reflectivity.”