Search

Many materials have been explored for the purpose of creating structures with high radiative cooling potential, such as nanocellulose-based structures and nanoparticle-based coatings, which have been reported with environmentally friendly attributes and high solar reflectance in current literature. They each have their own advantages and disadvantages in practice. It is worth noting that nanocellulose-based structures have an absorption peak in the UV wavelengths, which results in a lower total solar reflectance and, consequently, reduce radiative cooling capabilities. However, the interwoven-fiber structure of cellulose gives high mechanical strength, which promotes its application in different scenarios. The application of nanoplatelet-based coatings is limited due to the need for high volume of nanoparticles to reach their signature high solar reflectance. This requirement weakens the polymer matrix and results in more brittle structures. This work proposes a dual-layer system, comprising of a cellulose-based substrate as the bottom layer and a thin nanoparticle-based radiative cooling paint as the top layer, where both radiative cooling potential and mechanical strength can be maximized. Experimental and theoretical studies are conducted to investigate the relationship between thickness and reflectance in the top coating layer with a consistent thickness of the bottom layer. The saturation point is identified in this relationship and used to determine the optimal thickness for the top-layer to maximize material use efficiency. With the use of cotton paper painted with a 125 μm BaSO4-based layer, the cooling performance is enhanced to be 149.6 W/m2 achieved by the improved total solar reflectance from 80 % to 93 %. [ABSTRACT FROM AUTHOR]

Many materials have been explored for the purpose of creating structures with high radiative cooling potential, such as nanocellulose-based structures and nanoparticle-based coatings, which have been reported with environmentally friendly attributes and high solar reflectance in current literature. They each have their own advantages and disadvantages in practice. It is worth noting that nanocellulose-based structures have an absorption peak in the UV wavelengths, which results in a lower total solar reflectance and, consequently, reduce radiative cooling capabilities. However, the interwoven-fiber structure of cellulose gives high mechanical strength, which promotes its application in different scenarios. The application of nanoplatelet-based coatings is limited due to the need for high volume of nanoparticles to reach their signature high solar reflectance. This requirement weakens the polymer matrix and results in more brittle structures. This work proposes a dual-layer system, comprising of a cellulose-based substrate as the bottom layer and a thin nanoparticle-based radiative cooling paint as the top layer, where both radiative cooling potential and mechanical strength can be maximized. Experimental and theoretical studies are conducted to investigate the relationship between thickness and reflectance in the top coating layer with a consistent thickness of the bottom layer. The saturation point is identified in this relationship and used to determine the optimal thickness for the top-layer to maximize material use efficiency. With the use of cotton paper painted with a 125 μm BaSO4-based layer, the cooling performance is enhanced to be 149.6 W/m2 achieved by the improved total solar reflectance from 80 % to 93 %.

Transparency is an important characteristic in practical applications of radiative cooling, but the transmitted sunlight trapped in an inner space is generally the main cause of the increasing temperature. A transparent radiative cooler that can lower a temperature during the daytime by transmitting visible light, reflecting near‐infrared (NIR) light, and radiating thermal energy through the atmospheric window is proposed. In contrast to transparent selective emitters that transmit most of the incoming solar irradiance under direct sunlight and opaque radiative coolers that reflect all solar energy, the proposed cooler achieves transparency and the cooling effect simultaneously by selectively blocking solar absorption in the NIR regime. Outdoor rooftop measurements confirm that the cooler can reduce i) the inner temperature of an absorbing system and ii) its own temperature when combined with commercially available paints. During daytime, the cooler provides a temperature reduction of a maximum 14.4 °C and 10.1 °C for the inner and own temperature, respectively, and of an average 5.2 °C and 4.3 °C, respectively, in comparison to the transparent selective emitter. The proposed cooler can reduce the temperature during the daytime while maintaining transparency, confirming its possibilities in practical applications such as passive diurnal cooling of vehicles or buildings and compatibility with current paint technologies for aesthetic use. [ABSTRACT FROM AUTHOR]

Radiative cooling is an appealing heat exchange form based on thermal radiation from terrestrial objects to outer space, which can be potentially applied to numerous cooling applications for system performance improvement without energy input. Due to the poor solar reflective ability of previous cooling materials, radiative cooling technology has been largely limited to nocturnal cooling for several decades. Thanks to the recent successful development of highly efficient selective and broadband thermal emitters either backed with high solar reflective metal films or intrinsically equipped with excellent solar reflectance properties, the daytime radiative cooling has been practically fulfilled, which is arousing worldwide research interests. Notably, the emerging nanophotonics or metamaterials-based fabrication approaches are widely reported with the prominent ability to tailor spectral properties for sub-ambient cooling enhancement. Porous polymer-based scalable paints with micro-and nano-pores are also developed with substantial daytime cooling capacities by considerably backscattering sunlight and emitting thermal radiation. This work comprehensively reviews the latest progress on radiative cooling regarding its theoretical fundamentals, material designs and a variety of novel applications with a special focus on building-integrated cooling performance improvement. Considering the preceding extensive research on cool roofs with remarkable potential for real-world implementations, radiative roof cooling has been discussed in terms of cool roof-based heat transfer models, thermal and energy performance, and economic and environmental benefits. Lastly, research prospects of the radiative cooling technology are envisaged to provide insight for further investigation. [ABSTRACT FROM AUTHOR]

Radiative sky coolers (RSCs) can reduce building roof heat gains by radiating heat to outer space. However, their performance during daytime is limited, with substantial roof heat gains still occurring due to high ambient temperatures. Additionally, much of the cooling produced at night is wasted since air conditioners in non-residential buildings are often not operating. To address these limitations, we propose a novel thermal storage-heat pipe-integrated radiative sky cooler system (TS-HP-RSC). It utilizes water thermal storage to capture nighttime sky cooling for use during the day. A gravity-assisted heat pipe unidirectionally transports this stored cooling to the indoor space, preventing losses to the environment. An experimental platform is established integrating the proposed system, a baseline case, and measurement instrumentation. Compared to the baseline, the TS-HP-RSC system not only eliminated daytime cumulative heat gains (0.55–1.27 kJ) but also provided supplemental cooling (1.57–2.75 kJ). This yielded substantial daytime heat gain reductions of 223.62 %–600 % versus the baseline. Similar reductions occurred in peak heat gains. By enhancing daytime cooling, the TS-HP-RSC system can substantially curb roof heat gains in subtropical climates, enabling significant energy savings. • RSC shows poor daytime cooling performance due to unfavorable ambient environment. • RSC nighttime cooling is wasted in non-residential buildings. • TS-HP-RSC system is proposed and experimentally proven to enhance daytime cooling. • It enhances thermal storage to capture nighttime cooling for daytime use. • Unidirectional heat transfer of heat pipe prevents cooling loss to the environment. [ABSTRACT FROM AUTHOR]

Summer air temperatures will continue to rise in metropolitan regions due to climate change and urbanization, intensifying daytime and nighttime air temperatures and result in greater thermal discomfort for city dwellers. Urban heat may be reduced by trees which provide shade, decreasing air and surface temperatures underneath their canopies. We asked whether tree height and canopy density can help to identify species that provide greater microclimate benefits during day and night. We also asked if increased canopy cover of street trees provides similar microclimate benefits. We used continuous measurements of near-surface air temperatures under 36 park trees and from two urban streets to assess these questions. In the park, trees were grouped according to their height (20 m) and canopy density (low, high), while the effect of canopy cover was tested using streets with high (31%) and low (11%) cover. Daytime near-surface air temperature declined with increasing height and canopy density providing significant cooling benefits. However, this trend was reversed at night when tall trees with dense canopies restricted longwave radiative cooling and trapped warm air beneath their crowns. High canopy cover of street trees reduced daytime air temperatures more, resulting in a lower number of days with hot (>35 °C) and extreme (>40 °C) air temperatures compared to the street that had low canopy cover. These findings suggest that tree species and streetscapes with dense canopy cover improve local thermal conditions during the day but do not seem ideal to allow for nighttime cooling, creating potential discomfort for residents during hot summer nights. Our results indicate that classifying trees using a simple metric can assist in selecting tree species that can alleviate the local negative effect of urban heat during the day, but at the same time, their effect in preventing optimal longwave radiative cooling during the night must be factored into planting strategies.

Passive daytime radiative cooling materials significantly alleviate the current energy shortage and global warming without consuming any energy. However, conventional inorganic nanoparticles can hardly achieve excellent cooling effect during daytime due to their low reflectivity. Broadband super white nanoparticle-based radiators demonstrate excellent solar reflectivity under direct sunlight and low absorption. Accordingly, we introduced TiO 2 nanoparticles with the advantages of high refractive index, high infrared emissivity and low solar absorption into the porous cellulose-based film via a facile phase separation strategy. The cavities were distributed around 5 μm in size due to the limited evaporation of solvents, while the introduction of TiO 2 nanoparticles effectively enhanced the solar reflectance to 0.97. Moreover, the scattering model demonstrated that the high solar reflectivity was mainly produced by the randomly agglomerate effect of nanoparticles and these nano-clusters also contribute to a high mid-infrared emissivity. As a result, the photonic film presented about 10 °C cooling capacity even under the high-density solar radiation (~897 W/m2) according to the out-door measurement. The random nano-clusters filling strategy avoids the complicate process of specific size particles and has a significant potential for propel the polymer-based cooling technology to satisfy the need of large-scale thermal management. [Display omitted] • The three-dimensional porous structure increased the solar reflectivity to 97%. Significantly more than previous polymers. • Flexible 3DPCA/TiO 2 photonic film prepared using a phase separation method, suitable for most substrates. • The 3DPCA/TiO 2 photonic film still exhibited a cooling effect of almost 10 °C at a solar power of 897 W/m2. [ABSTRACT FROM AUTHOR]

Urban landscapes, made of unique buildings, pavements, and vegetation, create complex influences on the atmosphere. These influences induce increased nighttime warming and daytime cooling, especially in arid regions. This study compared three types of surfaces, including rooftop, turfgrass, and canyons, in terms of surface temperature and surface energy fluxes. A 68 sq.km parcel in Phoenix, AZ was considered for comparison. The study found that the canyons' land surface temperatures (LST) were 5 °C lower than the rooftop surfaces. The turfgrass surfaces were 4 °C cooler than the canyon surfaces. Moreover, north and south (N-S) oriented canyons were 2 °C cooler than east and west (E-W) oriented canyons. No significant changes were observed in net radiation for rooftop, turfgrass, or canyon surfaces. However, conductive fluxes, warranting nighttime warming, showed higher absorption of 45 W/m2 on the rooftop surfaces, than in the canyons. The turfgrass showed nighttime cooling, as the heat absorption was 210 W/m2 lower than the rooftop surfaces and 165 W/m2 lower than the canyons. Additionally, a 50 W/m2 difference of heat absorption was observed between N-S oriented canyons and E-W oriented canyons. The study concludes that canyons are major causes of daytime cooling and nighttime warming. • The land surface temperature of canyons was 5 °C lower than the rooftop surfaces. • The canyons demonstrated lower soil heat flux density (45 W/m2). • Turfgrass surface was 4 °C cooler than the canyon surfaces. • Soil heat flux was 210 W/m2 lower in the turfgrass than the rooftop surfaces and 165 W/m2 lower than the canyons. • N-S oriented canyons were 2 °C cooler than E-W oriented canyons. [ABSTRACT FROM AUTHOR]

Summer air temperatures will continue to rise in metropolitan regions due to climate change and urbanization, intensifying daytime and nighttime air temperatures and result in greater thermal discomfort for city dwellers. Urban heat may be reduced by trees which provide shade, decreasing air and surface temperatures underneath their canopies. We asked whether tree height and canopy density can help to identify species that provide greater microclimate benefits during day and night. We also asked if increased canopy cover of street trees provides similar microclimate benefits. We used continuous measurements of near-surface air temperatures under 36 park trees and from two urban streets to assess these questions. In the park, trees were grouped according to their height (<10 m, 10–20 m, >20 m) and canopy density (low, high), while the effect of canopy cover was tested using streets with high (31%) and low (11%) cover. Daytime near-surface air temperature declined with increasing height and canopy density providing significant cooling benefits. However, this trend was reversed at night when tall trees with dense canopies restricted longwave radiative cooling and trapped warm air beneath their crowns. High canopy cover of street trees reduced daytime air temperatures more, resulting in a lower number of days with hot (>35 °C) and extreme (>40 °C) air temperatures compared to the street that had low canopy cover. These findings suggest that tree species and streetscapes with dense canopy cover improve local thermal conditions during the day but do not seem ideal to allow for nighttime cooling, creating potential discomfort for residents during hot summer nights. Our results indicate that classifying trees using a simple metric can assist in selecting tree species that can alleviate the local negative effect of urban heat during the day, but at the same time, their effect in preventing optimal longwave radiative cooling during the night must be factored into planting strategies. [ABSTRACT FROM AUTHOR]