Monitoring the thermal potential of low-cost radiative cooling materials under static and dynamic conditions of exposure

Abstract

Reflecting the radiation of the sun while emitting thermal radiation to cold outer space has proven to be an effective solution against urban overheating. The latter severely impact the energy consumption of buildings, outdoor pollution levels, and heat-related morbidity and mortality, which is why recent research has focused on new advanced mitigation technologies to be implemented in cities. Passive radiative cooling (PRC) has the potential to provide a temperature lower than ambient without any energy consumption. While conventional cooling prototypes reject heat to the air, PRCs reject heat to the outer atmosphere emitting radiation mainly in the 8-13 ¿m range, i.e., the so-called atmospheric window. This work investigates the thermal behavior of different radiative cooling materials under various exposure conditions to examine their effective cooling potential. The basic structure of the samples comprehends a highly reflective substrate (aluminum or Vikuiti) and a silica-derived emissive layer. After a preliminary characterization under controlled environmental settings, the samples were exposed outdoors, and their superficial temperature was monitored during the central hours of the day. Comparisons among samples and a benchmark aluminum reference layer were made, also considering the weather data collected during the days of exposure. Although the samples did not reach sub-ambient temperatures during the monitoring, the emissive layer significantly reduced the surface temperature. Furthermore, the effect of a tunable intermediate layer placed between the substrate and the emissive element was demonstrated to positively impact the thermal performance of the sample, thanks to its capability of changing the emissivity spectrum with temperature.