Session: Panel-2 Textbooks and Pedagogy in Micro/Nanoscale Heat and Mass Transfer / 05-04 Micro/Nanoscale Thermal Radiation

Paper Number: 130664

130664 - A Hierarchically Designed Metafilm for Efficient Daytime Radiative Cooling 

Abstract: 

As global warming is becoming a serious problem for our planet, pursuing carbon neutrality has become a global consensus. However, traditional active cooling methods (like air conditioning) are still the main solution for providing cooling space in hot conditions. It not only consumes ~10% of global electricity but also emits over 10% greenhouse gas, which further aggravates the global warming effect. A sustainable cooling method is needed to release the cooling load of air conditioners. Daytime radiative cooling (DRC), which provides energy-free cooling for objects through surface spectrum modification, has attracted wide attention. It cools down the surface by emitting thermal heat to the outer space through atmospheric windows and reflecting solar heat to suppress the photothermal effect under direct sunlight simultaneously. Various DRC surfaces have been reported, including multilayer photonic film, hierarchical porous structure, particle scattering paint, and nano/micro-fabricated textiles. Although they meet the spectral selectivity for radiative cooling, it is still challenging for efficient space cooling in real applications.

Since the net cooling power of DRC is lower than 100 W/m² at most daytime in hot areas, sufficient surface area is needed to provide effective space cooling. The strict demand for spectral selectivity of the DRC surface leads to high fabrication costs, as its nano-/micro-structure should be controlled accurately. It limits its large-scale applications. Although DRC paint has been reported for low application cost, it faces many challenges in the painting process. For example, the painting quality is highly dependent on the workers, and hard to guarantee a constant surface spectrum as the same as design. Besides, the paint structure involved several painting steps, which take a long time of working hours with the high cost of manpower. It increases the total cost of DRC applications. Thus, a prefabricated DRC surface with low manufacturing cost and scalability for mass production is in high demand to push the DRC technology for space cooling.

In this work, we demonstrate a novel hierarchical metafilm structure, which comprises a reflective layer and an enhanced layer. The reflective layer contains polyethylene terephthalate (PET) fibers and barium sulfate (BaSO₄) particles for efficient sunlight scattering, leading to an ultra-strong solar reflectance (>98%). The PET fibers attached by BaSO₄ MPs are self-assembly and formed inside the PET film through a fast-stretching process. It contributes to low fabrication costs and mass production rates. Enhanced by a silica (SiO₂) particle-embedded Poly (vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) layer, the metafilm shows high emittance (>95%) in the atmospheric window (8-13 µm), caused by the coupling of strong vibration of polymer molecular and particle-enhanced surface roughness. Besides, the proposed structure is highly scalable, and a 20-meter-long metafilm was fabricated via a mass manufacturing process. The summer field tests in Chongqing, one of the hottest cities in China, demonstrated 4.5 ℃ surface sub-ambient cooling at noontime and 60% energy saving in a continuous 6-week space cooling for meter-scale model houses. The self-cleaning function and outdoor durability guarantee sustainable cooling performance in long-term usage.

Presenting Author: Chongjia Lin The Hong Kong University of Science and Technology

Presenting Author Biography: Dr. Lin is a research assistant professor at HKUST. His works focus on the thermal radiation technique via micro/nano-engineering for radiative cooling, heating, and self-adaptive response for smart thermal regulation. Although he is a young researcher in radiative heat transfer, some of his works have been highly recognized and published in good journals, like Science Advances and Advanced Materials. One of his designed metafilms has been industrialized and commercialized in China for large-scale applications. He aims to push the micro/nano thermal radiation techniques for real sustainable applications.

Authors:

Chongjia Lin The Hong Kong University of Science and Technology