Hierarchically porous polymer coatings for highly efficient passive daytime radiative cooling

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Science  19 Oct 2018:
Vol. 362, Issue 6412, pp. 315-319
DOI: 10.1126/science.aat9513

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  • RE:
    • Jyotirmoy Mandal, Gradaute Student, Columbia University, Department of Applied Physics and Applied Mathematics

    My colleagues and I would like to first thank Dr. Chiu and his colleagues for their scrutiny of our work, and also apologize for the delayed response. The e-letter by Chiu et. al. came to our attention only recently, and I would address the points raised in it.
    To ensure that the temperatures measured were not affected by sunlight, we kept the thermocouple tips (and at least 1-2 inches of the thermocouple wire leading from them) shaded from direct sunlight. the thermocouples were placed near the shaded southern wall within the box, which had a roughly east-west orientation. The thermocouples were close to the sample, so their exposure to diffuse sunlight was also limited. We did not cover our thermocouples with any white coatings, as it might radiatively cool the thermocouple. To my knowledge, our setup is similar to what has been devised in previous studies [1-2]. Additionally, in previous trials, we tried attaching infrared transparent poly(ethene) to our ‘air’ thermocouples to increase their thermal inertia and minimize rapid temperature fluctuations, but it did not have any noticeable impact.
    We also took additional care to do our experiments in wide-open areas, either on large rooftops (New York and Chittagong) or a field (Phoenix), to prevent “microclimates” that might be set up by convection shields. Furthermore, our use of the setup shown in Figure 3 might be conservative, as the white paper used is itself a good radiative cooler, and lowers the tempera...

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    Competing Interests: None declared.
  • RE: Hierarchically porous polymer coatings for highly efficient passive daytime radiative cooling
    • Ka-lok Chiu, Research Fellow, Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hong Kong
    • Other Contributors:
      • Songmin Shang, Associate Professor, Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hong Kong
      • Yuxiang Wang, Student, Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hong Kong

    Mandal et al. have synthesized a promising radiative cooling material with record-breaking cooling powers of 96 W m−2 under the direct solar light. They provide a detailed description of sample preparation, radiative cooling power measurement, sample characterization in the article. However, they have not clearly described how to precisely measure ambient temperature during radiative cooling power measurement.

    In Figure 3A, they simply measured ambient temperature by placing thermocouple exposed in air. No detailed information about what thermocouple was used or how to protect thermocouple during the measurement. The experiment is under strong sunlight (over 650W/m2). The thermocouple may absorb sunlight and the measured ambient temperature may be higher than the actual ambient temperature.

    The standard measurement of ambient temperature use thermometer enclosures like Stevenson screen to provide protection from shortwave (solar) and longwave (terrestrial) radiation, and precipitation [1]. Meanwhile, the thermometer enclosure needs to be examined if they are uniform in temperature. New models of radiation shield are also available specially designed for the measurement of ambient temperature [1, 2].

    According to Mandal et al., the cooling power of material is characterized via two ways: measure 1) the sub-ambient temperature drop and 2) how much heat energy is needed to maintain the material at ambient temperature. The cooling ability of the materials...

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    Competing Interests: None declared.

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