Vertical Radiative Cooling Enabled by Reflective Mirror Geometry and Spectrally Selective Physical Modeling

Abstract

Radiative cooling is a passive cooling method that can reduce electricity use by emitting heat into outer space through the atmospheric transparency window (8–13?µm). Most current designs use horizontal emitters, which are limited by rooftop space in cities. In contrast, vertical surfaces such as building façades are more abundant, but have received little attention due to fundamental physical limitations: they suffer from restricted sky exposure and increased absorption of thermal radiation from the ground and surrounding structures, which significantly reduce net cooling performance when analyzed using conventional models. In this study, we propose a new physical model for vertical radiative cooling, incorporating angular and spectral selectivity as well as geometrical view factor constraints. Based on this model, we design a vertical radiative cooling system that uses angularly and spectrally selective emitters, combined with a reflective mirror placed below the emitter. This design improves the effective emission angle and blocks thermal radiation from the ground. Simulation results show that under typical urban conditions like Phoenix, Arizona, USA (30 °C ambient, atmospheric transmittance ? = 0.8, parasitic heat coefficient h = 0.8?W/m²·K), the system can achieve a net cooling power of 16.13 W/m². We also show that with four vertical panels and one horizontal emitter, the system can deliver over 1 kW of total cooling power. Even without rooftop components, a vertical surface area of 62 m² is enough to reach kilowatt-level cooling. These results suggest that vertical radiative cooling is a practical and scalable solution for reducing cooling energy use in dense urban areas, enabled by a physics-based framework tailored to vertical emitter geometry.