Radiative cooling improvement by retro-reflective materials

Abstract

Cool materials are essential for reducing energy demand in buildings and for mitigating the Urban Heat Island (UHI) phenomenon. Their effectiveness relies on two primary physical properties: the ability to reflect solar energy and the capacity to emit infrared radiation, both of which are especially beneficial on horizontal surfaces like roofs and pavements. However, vertical surfaces, such as façades, also play a significant role in urban thermal balance. Conventional materials often underperform on these surfaces due to non-directional properties. This study measures the emissivity of Retro-Reflective (RR) materials, investigating their behaviour in the thermal infrared range. Results show that emissivity depends just on the superficial temperature and there are no angular variations. Therefore, RR materials have a directional behaviour only in the reflected radiation and not in the emitted one. Since emissivity is one of the parameters used in the calculation of the Cooling Power Potential (CPP), a critical knowledge gap regarding the CPP of RR coatings at varying orientations was found in literature. To address this limitation, an original measurement campaign was conducted, where several kinds of RR materials were realized by varying the size and density of embedded glass beads. RR materials significantly enhance CPP compared to conventional diffusive surfaces. At a typical façade temperature of 55 °C, RR materials increased CPP by an average of 20 %, demonstrating their superior cooling capability. Further research should focus on the long-term durability and environmental impact of RR materials to ensure their effectiveness over time.