Managing Cities with Broadband Passive Daytime Radiative Cooling

Among the proposed technological solutions to this problem, broadband PDRCs (daytime passive radiative cooling materials) have emerged as candidates for keeping cities cool during the warmer months, although there are cooling problems. excessive during the winter months. An article published in Durability assessed city-wide strategies to overcome this key issue.

To study: Optically modulated passive broadband daytime radiative cooling materials can cool cities in summer and warm them in winter. Image Credit: TWStock/Shutterstock.com

Increase in urban temperature

Extreme heat is a pressing problem in many urban areas, especially during the summer months.

Climate change is an urgent existential problem facing human society in the 21st century. It has been driven by human activity, urban growth and increasing industrialization. In addition to the damage it causes to fragile ecosystems, climate change is having a profound effect on urban areas, causing high temperatures during the hottest months that lead to health problems, deaths, high energy consumption and an increased concentration of pollutants.

The configuration of the WRF model showing three domains.  The parent domain, D01, has a horizontal grid spacing of 18 km, D02 with a grid spacing of 6 km, and D03 indicates the inner domain of the area of ​​interest that encompasses the high-density urban area of ​​the metropolitan area of Kolkata (KMA) and has a grid spacing of 2 km.

The configuration of the WRF model showing three domains. The parent domain, D01, has a horizontal grid spacing of 18 km, D02 with a grid spacing of 6 km, and D03 indicates the inner domain of the area of ​​interest that encompasses the high-density urban area of ​​the metropolitan area of Kolkata (KMA) and has a grid spacing of 2 km. Image Credit: Khan, A et al., Sustainability

Nearly 25% of the world’s population is exposed to deadly temperature rises in urban areas. Materials commonly used in cities such as concrete and asphalt, as well as the scarcity of vegetation, contribute to extreme temperatures. In recent years, deadly heat waves have multiplied. Since the 1980s, the number of days per year when people are exposed to extreme heat and humidity has tripled.

Extreme urban heat is a phenomenon that has been widely documented in 450 cities around the world, and the magnitude of the phenomenon could reach 10ohC. Extreme temperatures in urban areas and their impact on residents are a major concern for many governments around the world.

Extreme Urban Heat Mitigation

To mitigate the effect of this phenomenon, several strategies have been put in place by urban planners and governments. Urban design is a strategy that plays a key role in this regard. Street orientation and geometry affect access to sunlight and channel airflow through urban canyons, affecting the thermal comfort of residents. Tall buildings can provide much-needed shade when temperatures are at their peak. The orientation of buildings and spaces is important.

Temperature box plots of urban environment variables (hourly) over a high density urban area where PDRC materials are deployed for D03.  Results of 24 h × 169 scenario points (unmodulated and modulated PDRC materials) minus the control case (?T), for Tambient, Troof, Tsurface and Tcanopy.  These temperatures were calculated using the energy balance on the canyon surface of each facet (roof, wall and floor) and their influence on heat fluxes.

Temperature box plots of urban environment variables (hourly) over a high density urban area where PDRC materials are deployed for D03. Results of 24 h × 169 scenario points (unmodulated and modulated PDRC materials) minus the control case (ΔT), for TambientTroofTareaand Tcanopy. These temperatures were calculated using the energy balance on the canyon surface of each facet (roof, wall and floor) and their influence on heat fluxes. Image Credit: Khan, A et al., Sustainability

Technological strategies to mitigate extreme urban warming include evaporative systems, biophilic design and green infrastructure, and advanced materials. Advanced materials include chromic, fluorescent, reflective, and photonic coatings. Studies have shown that these significantly reduce the impact of heating in urban areas.

CRDP materials

PDRC materials have advantageous properties that make them suitable for this purpose. These properties (high reflectance in the solar spectrum and enhanced emissivity values ​​in the atmospheric window) give PDRCs sub-ambient surface temperatures when exposed to sunlight. Installing these materials on buildings helps meet cooling load requirements during peak hours. Over the past decade, research into these advanced materials has grown exponentially.

However, the process of developing these materials is complex. Recent studies have focused on concerns about climate suitability, active versus passive applications, cost, tunability, scalability, materials development, and overcooling penalty. Recently, polymer-based materials have gained interest due to their cost competitiveness, ability to be mass-produced, and applicability to large systems. Paints and films have been evaluated in the field, with varying results.

There are two types of PDRC materials: broadband and wavelength-selective emitters. While it is generally accepted that wavelength-selective transmitters have the best cooling performance in a variety of climates, broadband transmitters perform better under certain conditions. It is when the material is warmer than the surrounding air and when the surface temperature is within a specific temperature range, depending on the climate. For large-scale applications, the development of broadband PDRC materials is advantageous.

The study

Current studies of PDRC materials have been conducted at locations with high sky window factors and are unaffected by solar radiation from surrounding buildings. Thus, they do not reflect urban conditions and overestimate the cooling effect. The study in Durability investigated city-scale simulations of broadband PDRC material cooling performance and supercooling potential.

Potential temperature changes over time in a city.  The potential temperature during a diurnal cycle makes it possible to evaluate the effects of the cooling of CRDP materials on the stability of the boundary layer.  The potential temperature remains constant when a parcel experiences a change in adiabatic pressure.  The dry static stability of the atmosphere is determined by the vertical gradient of potential temperature.  During the peak hour, the convective boundary layer rose fastest and gradually equilibrated in the lower atmosphere with the (a) CTRL versus the (b) unmodulated, (c) modulated in ?, (d ) e-modulated and the (e) ?+e-modulated PDRC materials.  The modulated PDRC materials do not significantly affect the PBL height (white line) and the vertical wind speeds during the winter season.  Due to this phenomenon, normal vertical convective mixing and typical PBL height were possible.  In other words, the PBL decreases rapidly after sunset due to a decrease in thermals rising from the surface.

Potential temperature changes over time in a city. The potential temperature during a diurnal cycle makes it possible to evaluate the effects of the cooling of CRDP materials on the stability of the boundary layer. The potential temperature remains constant when a parcel experiences a change in adiabatic pressure. The dry static stability of the atmosphere is determined by the vertical gradient of potential temperature. During the peak hour, the convective boundary layer rose fastest and gradually equilibrated in the lower atmosphere with the (a) CTRL versus the (b) unmodulated, (c) ρ-modulated, (d ) ε-modulated and the (e) ρ + ε modulated PDRC materials. The modulated PDRC materials do not significantly affect the PBL height (white line) and the vertical wind speeds during the winter season. Due to this phenomenon, normal vertical convective mixing and typical PBL height were possible. In other words, the PBL decreases rapidly after sunset due to a decrease in thermals rising from the surface. Image Credit: Khan, A et al., Sustainability

The standard PDRCs were evaluated alongside three other PDRC materials. These additional materials had modulated emissivity, reflectivity, and both emissivity and reflectivity. The scenarios were run using a mesoscale climate model.

Four main conclusions were drawn from the study. First, modulation of solar emissivity or reflectance has an insignificant potential to attenuate the material’s overcooling potential day or night. Second, the daily maximum latent heat of materials and the maximum daily heat storage of materials were increased by modulating emissivity and reflectivity.

Third, the modulation of PDRC materials caused a positive energy balance during the winter months. Finally, by optically modulating these materials, excellent heat attenuation during the summer months without the associated over cooling potential during the winter is provided.

The authors identified future work opportunities in the development and integration of modulation technologies into PDRC materials. Additionally, they identified cost and scale challenges, as well as durability and regularity requirements for large-scale implementation of these materials in urban areas. However, they predicted that these materials will soon hit the market, offering significant benefits in reducing extreme urban heat with little penalty from overcooling in colder months.

Further reading

Khan, A et al. (2022) Optically modulated passive broadband daytime radiative cooling materials can cool cities in summer and warm them in winter [online] Durability 14(3) 1110 | mdpi.com. Available at: https://www.mdpi.com/2071-1050/14/3/1110

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