Passive Daytime Radiative Cooling (PDRC) is an electricity-free method for cooling terrestrial entities. In PDRC, a surface has a solar reflectance of nearly 100% to avoid solar heating, and high emissivity close to 1 in the long wavelength infrared (LWIR) transparent window of the atmosphere to radiate heat to the cold sky (wavelength l = 8- 13 um). Hence, such a surface can achieve sub-ambient cooling without any electricity. It is attractive for building envelope. Here a scalable solution-based method was developed to make porous fluoropolymer coating with excellent PDRC properties. The pores are hierchicially nano and mesoscale pores with a bimodal distribution centered at 200 nm and 5 um, respectively. Such interfaces between the polymer and air cause strong light scattering which leads to high reflectance, as the polymer is not absoptive in the solar spectrum. On the other side, the polymer is absorptive in infrared, and the porous structure leads to ultrahigh thermal emittance. Consequently, high solar reflectance of 0.96 and high thermal emittance of 0.97 are achieved. Sub-ambient cooling of 6 oC is achieved in Phoenix, Arizona under strong solar flux. Similarly, cooling at 2-5 oC is also achieved in New York City and Chittagong, Bangladesh. The corresponding cooling power can achieve \~100 W / m2. This strategy can also be applied to various kinds of polymers. 

 

Moreover, switchable devices are fabrciated by tuning difference in refractive index between the polymer and media in the pores (e.g. air or liquid), and the device can perform well for at least 100 cycles. High switchable contrast of 0.5-0.6 is achieved in solar band and the switching time is less than 1 min. Moreover, colored cooling paints are also fabricated by blending polymer with dyes with limited infrared absorption. A 20-50% higher infrared reflectance is achieved in different colors, including black, blue, red and yellow, which leads to 5-10 oC cooler comapred to normal paint with the same color. 

 

References:

1\. Mandal, J.; Fu, Y.; Overvig, A.; Jia, M.; Sun, K.; Shi, N.; Zhou, H.; Xiao, X.; Yu, N.; Yang, Y.\*; *Hierarchically porous polymer coatings for highly efficient passive daytime radiative cooling. *Science, 362(6412), 315-319 (2018). 

2\. Mandal, J.; Jia, M.; Overvig, A.; Fu, Y.; Che, E.; Yu, N.; Yang, Y.\*; *Porous Polymers with Switchable Optical Transmittance for Optical and Thermal Regulation*. Joule, 3(12), 3088-3099 (2019).

3\. Yijun Liu, Jyotirmoy Mandal, Nanfang Yu and Yuan Yang, Science Advances, accepted. 

 

 

 


Salable Radiative Cooling Paint for Thermal Management in Buildings

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Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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