Open Access
Optically modulated passive broadband daytime radiative cooling materials can cool cities in summer and heat cities in winter
(MDPI, 2022) Khan, Ansar; Carlosena Remírez, Laura; Feng, Jie; Khorat, Samiran; Khatun, Rupali; Doan, Quang-Van; Santamouris, Mattheos; Ingeniería; Ingeniaritza
Broadband passive daytime radiative cooling (PDRC) materials exhibit sub-ambient surface temperatures and contribute highly to mitigating extreme urban heat during the warm period. However, their application may cause undesired overcooling problems in winter. This study aims to assess, on a city scale, different solutions to overcome the winter overcooling penalty derived from using PDRC materials. Furthermore, a mesoscale urban modeling system assesses the potential of the optical modulation of reflectance (ρ) and emissivity (ε) to reduce, minimize, or reverse the overcooling penalty. The alteration of heat flux components, air temperature modification, ground and roof surface temperature, and the urban canopy temperature are assessed. The maximum decrease of the winter ambient temperature using standard PDRC materials is 1.1 ◦C and 0.8 ◦C for daytime and nighttime, respectively, while the ρ+ε-modulation can increase the ambient temperature up to 0.4 ◦C and 1.4 ◦C, respectively, compared to the use of conventional materials. Compared with the control case, the maximum decrease of net radiation inflow occurred at the peak hour, reducing by 192.7 Wm−2 for the PDRC materials, 5.4 Wm−2 for ρ-modulated PDRC materials, and 173.7 Wm−2 for ε-PDRC materials; nevertheless, the ρ+ε-modulated PDRC materials increased the maximum net radiation inflow by 51.5 Wm−2 , leading to heating of the cities during the winter.
Open Access
On the winter overcooling penalty of super cool photonic materials in cities
(Elsevier, 2021) Khan, Ansar; Carlosena Remírez, Laura; Khorat, Samiran; Khatun, Rupali; Doan, Quang-Van; Feng, Jie; Santamouris, Mattheos; Ingeniería; Ingeniaritza
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.

Carlosena Remírez, Laura

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