On the winter overcooling penalty of super cool photonic materials in cities

Abstract

Daytime radiative coolers appear to be the most triumphant and promising technology for urban thermal management, as they could improve the thermal field of the cities, especially during the summertime. However, during the colder months, it can lead to an overcooling penalty, a widely overlooked phenomenon. This study aims to determine the cooling penalty derived from using super-cool materials (SCMs) at a city scale. We used a mesoscale urban modeling system to assess the overcooling of three broadband SCM emitters with different reflectivity and emissivity values. A significant change was found in radiation and energy balance compared to the control case (CTRL) during the daytime and nighttime. Under the most reflective and emissive SCM scenario, the maximum decrease of net radiation at peak hour was 354.9 Wm−2, therefore choosing a scenario with lower albedo values for walls and ground would be more beneficial. The mean decrease of ambient temperature, surface temperature, roof temperature and canopy were 2.8 °C, 4.7 °C, 12.9 °C and 6 °C, respectively. This SCMs assessment is a first stride to understand better the unexplored behavior of the boundary layer meteorology and its depiction in the mesoscale climate model for winter seasons. The implementation of SCMs during winter could create an inversion layer near the surface, leading to a buildup of stagnant air over the urban environment, resulting in heating during the night in the winter seasons as usual with SCMs as with the CTRL. Further research is needed on material development to modulate materials’ spectral configuration to address overcooling during the winter and improve SCMs’ year-round performance at city scale.