In situ switchable nanofiber films based on photoselective asymmetric assembly towards year-round energy saving.

Thermal management of buildings consumes 51% of the world's energy use. Optimization of the energy use can be potentially achieved via daylight harvesting and radiative cooling approaches, yet their simultaneous utilization under static conditions is challenging due to opposite operation principles. Here, an in situ switchable photoselective polymer (PSP) material was prepared by sequential electrospinning of light-reflecting and light-absorbing layers made of contrasting polymer nanofibers. The as-prepared PSP material exhibited a high solar light reflectance of 97.7% and a high broadband emissivity of 94.9% resulting in a radiative cooling power of 111.1 W m⁻². Such "cooling" state of the PSP film can be easily switched to a "heating" one via impregnation of an index matching liquid that suppresses scattering at the film–air interface and reduces the solar band reflectivity of the film. Thanks to the highly porous structure of the designed PSP film, its switching takes less than 5 min and allows an integrated solar absorbance of ∼95.6% to be achieved, resulting in an estimated heating power of 781.6 W m⁻². Performed numerical calculations further supported the high potential of the developed PSP film for thermal management of buildings located at high latitudes with energy savings up to 89.74 GJ m⁻² per year and reduced CO₂ emissions down to 21.69 t. [ABSTRACT FROM AUTHOR]