Energy performance and outdoor thermal comfort of angular-selective radiative cooling walls in urban canyons: a Monte Carlo based study

Radiative cooling coatings with angularly asymmetric emitters have demonstrated strong potential for wall applications by achieving sub-ambient surface temperatures. However, their energy-saving performance in urban street environments with complex radiative interactions remains insufficiently explored. This study introduces a novel approach to evaluate their energy performance within urban canyons by employing Monte Carlo ray tracing method to simulate the radiative behavior under realistic street geometries. The model coupled the method with the urban canopy model and the building energy model and has been validated using against experimental data. And then five emitters with varying spectral characteristics are analyzed across a range of street aspect ratios and climate zones. Results reveal a geometry-dependent performance: in narrow streets (aspect ratio = 0.5, the ratio of street width to building height), emitters with the narrowest angular range (∼30°) achieve up to 55.2 GJ of cooling-load savings in hot-summer warm-winter zones (e.g., Sanya, China). Conversely, in wider streets (aspect ratio = 2.0), emitters with the broadest angular range deliver up to 302.7 GJ of cooling-load savings, outperforming narrow-range emitters by 25.8 %. A nationwide analysis further shows that cities benefiting from narrower-angular-range emitters are more geographically widespread than those favoring broader ranges. Furthermore, wider streets equipped with these emitters also enhance outdoor thermal comfort. This study presents a novel method for modelling angularly asymmetric radiative coatings in realistic urban environments and clarifies how emitter angular range and street geometry influence building energy performance across China. The findings offer critical insights for optimizing vertical radiative cooling technologies in urban applications.