The study in the last three years provides new insight and new designs of radiative cooling materials for a sustainable future, which has applications in building envelopes, transportation, cold chain and other scenarios where radiative cooling is a part of thermal management. Specifically we have made fundamental contributions in following aspects.

1. Modeling and theoretical framework. Porous polymers are attractive for passive daytime radiative cooling (PDRC) since they have excellent performance and scalability. A fundamental question remaining is how PDRC performance depends on pore properties (e.g., radius, porosity), which is critical to guiding future structure designs. We developed a 2D full electromagnetic wave model to answer this question which balanced accuracy and computational cost. Effects of pore size, porosity, and thickness are studied. We find that mixed nanopores (e.g., radii of 100 and 200 nm) have a much higher solar reflectance R_solar (0.951) than single-sized pores (0.811) at a thickness of 300 μm. With an Al substrate underneath, solar reflectance, thermal emittance in the long wavelength infrared window, and net cooling power P_cool reach 0.980, 0.984, and 72 W/m², respectively, under a semihumid atmospheric condition. These simulation results provide a guide for designing high-performance porous coating for PDRC applications.

2. Aqueous processing of porous PVdF-HFP radiative cooling paint. The traditional approach to make porous PVdF-HFP radiative cooling paint requires 70-80 wt% acetone, which is too much for onsite deployment. We developed a new aqueous processing with only 71 g/L volatile organic compound (VOC). Moreover, we identified that a key to realize high-performance aqueous radiative cooling coating is to use adequate force to break agglomeration and reassemble the dispersed particles into secondary microparticles. High solar reflectance of 0.94 and high emittance of 0.97 are achieved by this aqueous process and the film is stable in water for over two months. 

3. Radiative cooling walls. Current research on passive daytime radiative cooling mainly focuses on roofs, but limited attention has been paid to the walls, which typically occupy more surface area of building envelope than the roof. We designed a new zigzag structure to control the angular asymmetry of thermal emittance of a wall, and thus a wall has a high emittance towards the cold sky and low emittance towards the hot land surface. At a ground surface temperature > 50 ?C, such asymmetry leads to a relative cooling power up to 67 W m^-2 compared to conventional walls coated with PDRC materials, and temperature drops of 3.1 ?C (peak) / 2.3 ?C (daily average) compared to the control wall. The zigzag wall can provide up to 37 GJ (28.2 MJ m^-2, ~15%) annual energy saving for a typical midrise apartment, and benefit 42% population in the U.S., particularly in the southern warm areas.

The project results in 6 publications in Nano Letters, Advanced Functional Materials, Next Energy, and two more in preparation or submitted. 

In terms of broader impact, the patent on aqueous processing has been licensed to a startup. The project has trained 1 postdoc, 2 Ph.D., 4 master students, 4 undergraduates and 1 high school students. The team gave >10 presentations in other universities and top conferences in the last three years.

Last Modified: 10/18/2023
Modified by: Yuan Yang