Optimizing boron nitride nano-fibers scatterers arrangement to enhance solar reflectance and selective thermal emissivity towards passive daytime radiative cooling

Spontaneous cooling offers a sustainable and electricity-free strategy to mitigate global warming and reduce urban heat accumulation. Building upon our earlier experiments on nanoporous films and glass bubble enhanced Passive Radiative Cooling (PRC) paints, which demonstrated solar reflectance above 94% and broadband emissivity exceeding 95%, this study advances PRC design through full-wave electromagnetic modeling. Using the Finite Element Method, we optimized multilayer disordered optical coatings incorporating randomly distributed dielectric boron nitride (BN) nano-fibers as light scatterers. Two architectures were investigated: a five-layer stacked (5LS) structure with graded particle sizes (100 - 600 nm), and a single-layer Dense-to-Coarse (DtC) configuration featuring a vertical gradient in scatterer size. Polarization-resolved simulations and diffraction order decomposition were employed to generate accurate, unpolarized reflectance spectra across the full solar range (280 - 2500 nm). Both designs exhibited total and solar-weighted reflectance values exceeding 97%, demonstrating their effectiveness for Passive Daytime Radiative Cooling (PDRC). Such high reflectance and emissivity in the sky transparency window (STW) provides the optical properties needed to be integrated into a tailored thermal model for simulating realistic working conditions. The optimized coatings achieved a temperature drop of up to \(18^\circ\) C compared with the same system covered by a perfect absorber and up to \(4^\circ\) C below ambient, approaching the performance of ideal PDRC materials.