Optically Modulated Passive Broadband Daytime Radiative Cooling Materials Can Cool Cities in Summer and Heat Cities in Winter

Abstract

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.

Heat Mitigation in Cities: A Catalyst for Building Energy Saving

“…The recently developed super cool or daytime radiative cooling materials present a very high solar re ectance value combined with high emittance at the atmospheric window, while they have not yet been implemented to mitigate urban overheating. Mesoscale simulations for the city of Kolkata, India, have shown that super cool materials may contribute to decreasing the peak temperature of cities up to 4.5°C, imposing, however, a considerable heating penalty during the winter period [70]. Modulation of their optical properties, mainly re ectance and emittance, to reduce the strength of cooling during the cold period could minimize the problem [71,72].…”

Exploring the Meteorological Impacts of Surface and Rooftop Heat Mitigation Strategies Over a Tropical City

“…Optically modulated SCMs can be an efficient urban heat mitigation technology for use during the summer period without overcooling potential. Khan, Carlosena, et al (2022) concluded that the modulation of both solar reflectivity and infrared emissivity could increase the ambient temperature up to 0.4°C and 1.4°C during the day and night, respectively. Decreasing the burden of heat for urban environments has considerable benefits across society, including reduction of energy consumption, improvement in thermal comfort and health (Akbari et al, 2004;Ali-Toudert et al, 2005; H. Chen et al, 2009;Santamouris, 2020;Santamouris et al, 2015).…”

“…As mentioned before, CMs and SCMs are very reflective and can lead to an overcooling penalty at times. Recent research addressed this knowledge gap, calculating different solutions with optically modulated SCMs (Khan, Carlosena, et al., 2022). Optically modulated SCMs can be an efficient urban heat mitigation technology for use during the summer period without overcooling potential.…”

Toward the Scaling up of Daytime Radiative Coolers: A Review

“…[ 32 ] In the years since, DRCs have gained tremendous popularity and are being investigated for a wide range of applications, including buildings, solar cells, the clothing industry, electricity generation, water harvesting, and UHI mitigation. [ 24,38 ]…”