A flexible and scalable solution for daytime passive radiative cooling using polymer sheets

Daytime passive radiative cooling shows great potential in thermal management by highly reflecting the solar irradiation and intensively releasing mid-infrared thermal emission to the cold universe (~3 K). Although the desired optical property for radiative cooling can be achieved by optical-scale structures, a solution for mass production and cost reduction is still being sought. A flexible and scalable daytime passive radiative cooler in the form of a sheet that consists of a reflector layer and a polydimethylsiloxane (PDMS) layer is demonstrated to achieve promising sub-ambient cooling in Hong Kong's hot and humid climate. The proposed cooling sheet exhibits an average solar reflection of 92.1% and mid-infrared emissivity of 94.5%. A 48-hour continuous sub-ambient cooling, with an average ambient temperature drop of 2.4 °C, is experimentally demonstrated without any windshields or sun shields. Under a clear sky condition, cooling powers of 52.4 W/m² and 84.7 W/m² are measured at noontime and night, respectively. The impact of the sky view factor on cooling performance has also been numerically investigated for the first time. In the scenario of obstruction by a wall, a cooling power degradation of 29 W/m² is assessed during the daytime when the wall temperature is 15 K above ambient air with a view factor of 0.2. To assess the feasibility for cooling as building exterior materials, a field test using a scaled-down model house has been conducted. The cooler-applied model house produced a better indoor thermal environment with lower indoor air temperature and inner wall temperature compared with one coated with commercial white paint. Last but not least, to investigate the heat exchange of the cooler with ventilated air, a passive radiative air-cooling system has been built and tested, and sub-ambient cooling with 1 °C air temperature reduction was achieved through a 24-hour operation. This work leads to a new solution for daytime passive radiative cooling, which can potentially be applied in various scenarios, such as buildings and vehicles, with its great flexibility and scalability.