High-Performance Passive Radiative Cooling Material Engineering towards Large-Scale Applications

Abstract

Rising demand for space cooling is imposing enormous implications for electricity grids, greenhouse gas emission, and urban heat islands. At the same time, prevalently adopted cooling strategy using compression-based cooling systems encounter inherent drawbacks of relying on ozone-depleting heat transportation mediums, releasing thermal waste to outdoor environment in city, and causing noise. Therefore, eco-friendly and sustainable cooling strategies are urgently needed. Passive radiative cooling (PRC), an energy-free and refrigerant-free cooling technique, is regarded as a promising solution for reducing the space cooling demand. However, the application of radiative cooling technology is greatly hindered by scalability. Most PRC designs rely on precise parameter control at the nanometer and micrometer levels. These designs not only depend on high-precision fabrication equipment but also involve expensive materials, which makes the designs costly, difficult to maintain, and impractical for large-scale applications.

This study is devoted to the development of practical PRC solutions for large-scale applications. In total, three scalable PRC designs are successfully developed. They are in the form of sheet, coating, and ceramic tile, covering a wide range of potential application scenarios. Firstly, the PRC sheet employed a dual-layer design consisting of a bottom layer of flexible metal reflector sheet and an upper layer of polymeric thermal emitting layer. Without requiring precise optical structures or complex manufacturing processes, this facile design achieved a solar reflection of 92.1% and a mid-infrared emissivity of 94.5%. The PRC sheet, a scalable design, was for the first time demonstrated with continuous sub-ambient cooling in the Hong Kong climate. Moreover, for the first time, the effect of the cooler’s sky view factor on radiative cooling performance was numerically and experimentally investigated in this study, providing a valuable reference for the application of PRC material in real settings. Secondly, the PRC coating was obtained by doping low-solar-absorption inorganic particles with a transparent polymeric binder. With an optimized thickness and particle concentration, the cooling coating obtained a broadband solar reflection of 92.2% and efficient MIR emission of 95.3%. The polymer-based precursor used for PRC coating fabrication offered the convenience of a paint-like application, greatly lowering the barriers for large-scale implementation of radiative cooling technology while maintaining its cooling effectiveness. Finally, the beetle, Cyphochilus, with the whitest appearance in the world, inspired me to develop a PRC ceramic, which delivers nearly perfect reflectivity across the solar spectrum. The all-inorganic material gives the developed PRC ceramic a bulk ceramic-format appearance and tile-like applicability. Targeting large-scale application of radiative cooling technology in above-ambient scenarios, such as building envelopes, I focused my efforts on getting as high solar reflection as possible instead of tailing optical properties in the mid- and far-infrared wavelength range. It demonstrates that the as-obtained PRC ceramic generates stable cooling under various climate conditions. The all-inorganic material empowers the cooling ceramic with great resistance to degradation, which promises stability in longer-term operation and therefore reduced cost of maintenance. The cooling ceramic can withstand high-temperatures and inhibits the Leidenfrost effect for effective evaporative cooling, which is rarely reported in previous research. Besides, the high solar reflection enables the cooling ceramic to achieve balance between a low heat load and a desirable color, fulfilling the need for aesthetics in real applications.

The findings from this study offer evidence regarding the scalability of PRC technology for building applications. Through the successful development of three PRC materials, it has been conclusively demonstrated that PRC technology holds immense promise for enhancing the energy efficiency of buildings. By harnessing the passive cooling properties of PRC materials, buildings can achieve optimal thermal comfort while reducing their reliance on conventional cooling systems that often come with inherent drawbacks. I believe the findings reported in this study can provide crucial insights for researchers, architects, and policymakers alike, who are striving to create more sustainable and resilient cities.

Date of Award	15 Apr 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chi Yan TSO (Supervisor)