Urban scale rooftop super cool broadband radiative coolers in humid conditions

Abstract

The presence of water molecules in the air can impact how super cool broadband radiative coolers behave. Higher humidity in the lower atmosphere traps infrared radiation, reducing heat sent back to outer space. In this study, a mesoscale urban climate model is used to evaluate the newly developed super cool materials with broadband emissivity not selective in atmospheric window as an arsenal for urban heat management of tropical wet and dry cities like Kolkata. The results suggest that the energy balance over urban domain has substantially been altered by the city scale deployment of super cool broadband radiative cooling materials on the building rooftop. Bowen ratio and evaporative fraction values were found decreasing and increasing, respectively with a positive directional polynomial (R² = 0.968) relationship, after the implementation of super cool broadband radiative cooling materials and in comparison, to the unmitigated scenario. At high solar hour (14:00 LT), additional thermal variables of urban domain such as 2 m air temperature, surface skin temperature, urban canopy temperature, and roof surface temperature decrease by 2.3 °C, 5.4 °C, 0.8 °C, and 31.7 °C, respectively. Reflective super cool broadband materials achieve sub-ambient temperatures up to 11.7 °C during peak hours, reduce surface wind speed by 2.5 m s⁻¹, and lower the planetary boundary layer by 1475 m. The average daytime drop is approximately 7.3 °C, and at night, it is close to 2.4 °C. Deployment induces a “regional high” over urban areas, disrupting sea breeze onset and lowering the planetary boundary layer. Finally, an optimal cooling performance for super cool broadband radiative coolers can be achieved in lower humidity conditions, as their efficiency decreases with increased humidity. Though needing further investigation, these findings of nano-science-based super cool broadband materials offer valuable insights for policymakers and urban planners addressing thermal management in densely packed tropical urban environments.