Evaluation of radiation-induced buoyant flow for building facades: a theoretical model for high-angle solar incidence

With the advancement of transparent materials on building facades, solar incidence causes complex heat-flow coupling that impacts energy saving and urban heat evaluations. Environmental factors further compound such effects due to drastic optical variation caused by high-to-extreme solar angles. Yet, little is known about solar angles' impact on building facades' thermal-flow performance when applied in different climate and temporal conditions. This study proposes a new theoretical model for the naturally ventilated double-skin facade (NVDSF), focusing on addressing high to extreme solar angles' impact on passive ventilation and thermal performance. The change in latitude, season and time of the day are integrated with optical properties, energy balance and the fully-mixed model. Moreover, details of buoyancy flow are revealed numerically to deepen the understanding of radiation-convection coupling. New Incidence Angle Modifiers (IAMs) correlations, considering polarization under high angles, are empirically proposed for reflectivity, absorptivity, and transmissivity, yielding mean absolute errors of 8 %, 1 %, and 5 %, respectively. The coefficients are then integrated into a new model to reflect the angular impact on transparent panes. Compared to numerical results, the analytical model shows average errors of 4 % in natural ventilation rate, 8 % and 4 % in glazing temperatures (outer and inner). The model's implementation for a city at L = 31.15 ° during transition seasons promptly suggests the NVDSF's suitability, e.g. during the 150 days, natural ventilation provides only 33 % satisfaction for 1 m 2 NVDSF, but 95 % for 2 m 2 ; temperature differences between the outer surface and air are 39 % under 5 °C and 61 % between 5–11 °C. The new model provides a readily calculable form, supporting a new solution for future integrations with passive ventilation design, building thermal plumes and related urban evaluations.