Optimizing surface performance for urban heat island mitigation using microsphere-based coatings

Urban Heat Island is a detrimental overheating phenomenon that can be mitigated by using materials with tailored reflective emissive properties. This study develops innovative coatings for radiative cooling containing glass, ceramic, and chromed stainless-steel microspheres ranging from 10 to 50 μm, 150 to 250 μm, and 400 to 600 μm. Microspheres were applied on a pure aluminum layer, as well as on a black- and white-painted aluminum. Spectrophotometry and FTIR spectroscopy were employed to evaluate samples’ solar reflectance and thermal emittance, exploring the interplay between substrates and microspheres, and assessing how granulometry and material influenced the performance. In addition, surface temperature measurements were performed in a climatic chamber simulating summer and winter days to assess the coatings’ thermal behavior. Ceramic microspheres proved to be the most effective, exhibiting higher solar reflectance than the other materials. Although they caused a slight decrease in solar reflectance compared to aluminum and white references, they increased the UV reflectance by 20 % in white samples, and over 30 % in black samples in the entire spectrum analyzed. Moreover, ceramic microspheres also improved thermal emittance within the atmospheric window wavelengths (over 50 % for aluminum-based samples). Finally, climatic chamber simulations demonstrated that ceramic microspheres reduce surface temperatures if compared to samples without microspheres (up to 10 °C for black samples). In conclusion, these coatings represent an effective strategy to mitigate the Urban Heat Island through optimization and customization of surface performance.