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
Exploring the meteorological impacts of surface and rooftop heat mitigation strategies over a tropical city
(American Geophysical Union, 2023) Khan, Ansar; Khorat, Samiran; Doan, Quang-Van; Khatun, Rupali; Das, Debashish; Hamdi, Rafiq; Carlosena Remírez, Laura; Santamouris, Mattheos; Georgescu, Matei; Niyogi, Dev; Ingeniería; Ingeniaritza
Different heat mitigation technologies have been developed to improve the thermal environment in cities. However, the regional impacts of such technologies, especially in the context of a tropical city, remain unclear. The deployment of heat mitigation technologies at city-scale can change the radiation balance, advective flow, and energy balance between urban areas and the overlying atmosphere. We used the mesoscale Weather Research and Forecasting model coupled with a physically based single-layer urban canopy model to assess the impacts of five different heat mitigation technologies on surface energy balance, standard surface meteorological fields, and planetary boundary layer (PBL) dynamics for premonsoon typical hot summer days over a tropical coastal city in the month of April in 2018, 2019, and 2020. Results indicate that the regional impacts of cool materials (CMs), super-cool broadband radiative coolers, green roofs (GRs), vegetation fraction change, and a combination of CMs and GRs (i.e., “Cool city (CC)”) on the lower atmosphere are different at diurnal scale. Results showed that super-cool materials have the maximum potential of ambient temperature reduction of 1.6°C during peak hour (14:00 LT) compared to other technologies in the study. During the daytime hours, the PBL height was considerably lower than the reference scenario with no implementation of strategies by 700 m for super-cool materials and 500 m for both CMs and CC cases; however, the green roofing system underwent nominal changes over the urban area. During the nighttime hours, the PBL height increased by CMs and the CC strategies compared to the reference scenario, but minimal changes were evident for super-cool materials. The changes of temperature on the vertical profile of the heat mitigation implemented city reveal a stable PBL over the urban domain and a reduction of the vertical mixing associated with a pollution dome. This would lead to crossover phenomena above the PBL due to the decrease in vertical wind speed. Therefore, assessing the coupled regional impact of urban heat mitigation over the lower atmosphere at city-scale is urgent for sustainable urban planning.
Open Access
Urban cooling potential and cost comparison of heat mitigation techniques for their impact on the lower atmosphere
(Springer, 2023) Khan, Ansar; Carlosena Remírez, Laura; Khorat, Samiran; Khatun, Rupali; Das, Debashish; Doan, Quang-Van; Hamdi, Rafiq; Aziz, Sk Mohammad; Akbari, Hashem; Santamouris, Mattheos; Niyogi, Dev; Ingeniería; Ingeniaritza
Cool materials and rooftop vegetation help achieve urban heating mitigation as they can reduce building cooling demands. This study assesses the cooling potential of different mitigation technologies using Weather Research and Forecasting (WRF)- taking case of a tropical coastal climate in the Kolkata Metropolitan Area. The model was validated using data from six meteorological sites. The cooling potential of eight mitigation scenarios was evaluated for: three cool roofs, four green roofs, and their combination (cool-city). The sensible heat, latent heat, heat storage, 2-m ambient temperature, surface temperature, air temperature, roof temperature, and urban canopy temperature was calculated. The effects on the urban boundary layer were also investigated. The different scenarios reduced the daytime temperature of various urban components, and the effect varied nearly linearly with increasing albedo and green roof fractions. For example, the maximum ambient temperature decreased by 3.6 °C, 0.9 °C, and 1.4 °C for a cool roof with 85% albedo, 100% rooftop vegetation, and their combination. The cost of different mitigation scenarios was assumed to depend on the construction options, location, and market prices. The potential for price per square meter and corresponding temperature decreased was related to one another. Recognizing the complex relationship between scenarios and construction options, the reduction in the maximum and minimum temperature across different cool and green roof cases were used for developing the cost estimates. This estimate thus attempted a summary of the price per degree of cooling for the different potential technologies. Higher green fraction, cool materials, and their combination generally reduced winds and enhanced buoyancy. The surface changes alter the lower atmospheric dynamics such as low-level vertical mixing and a shallower boundary layer and weakened horizontal convective rolls during afternoon hours. Although cool materials offer the highest temperature reductions, the cooling resulting from its combination and a green roof strategy could mitigate or reverse the summertime heat island effect. The results highlight the possibilities for heat mitigation and offer insight into the different strategies and costs for mitigating the urban heating and cooling demands.
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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