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The Practical Effect of the Top PDMS Emitter on Window Cooling (A and B) Visible (A) and IR (B) images of NIR reflective glazing with (sample 1) and without (sample 2) PDMS show the same appearance and obvious temperature difference. (C) The solar transmittance spectra of bare glazing, samples 1 and 2. (D) The emissivity spectra of samples 1 and 2. (E) Measured ambient temperature, air humidity, and calculated precipitable water (PW) at noon in Wuhan, China (30.62 N, 114.13 E) on October 31, 2018. (F) The real-time temperatures of the 2 samples at noon. The simulated steady-state temperatures of the 2 samples are plotted for reference.
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Solar and thermal management of transparent windows is important for the energy efficiency of human-made structures. Functional layers that partially block solar radiation would also be heated by sunlight, thus accelerating the interior heat exchange and increasing daytime cooling energy consumption. Here, we propose a strategy to improve energy ef...
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... we propose a cooling energy-saving strategy by using a transparent double-layer coating on windows, in which the NIR reflective underlayer blocks undesired NIR in sunlight, and the top emitter as complementary layer enhances the mid-infrared (MIR) emittance to cool windows ( Figure S1). 14,15 The top PDRC emitter as the ''solid refrigerant'' for window cooling needs to meet a series of attributes, especially the optical properties (Table S1), which not only requires highly visible-transparent for observation, lighting, or aesthetic needs Figure S2), but also extremely low solar absorption (a, 0.3-2.5 mm) for minimizing heat gain and high MIR emittance (ε, 2.5-20 mm) for maximizing thermal loss ( Figures S3 and S4). Thus far, the reported PDRC materials that have been successfully demonstrated in roof and wall cooling are not suitable for window applications because of their hazy or opaque appearance. ...
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... experimentally demonstrated the feasibility of daytime window cooling using the transparent double-layer coating (Figure 3). Two glazing with a NIR reflective layer (indium tin oxide [ITO]) pointed to the sky were framed by Cu foil as the windows ( Figure 3A), and the left sample 1 was covered with a PDMS film and the right sample 2 remained the same. ...
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... experimentally demonstrated the feasibility of daytime window cooling using the transparent double-layer coating (Figure 3). Two glazing with a NIR reflective layer (indium tin oxide [ITO]) pointed to the sky were framed by Cu foil as the windows ( Figure 3A), and the left sample 1 was covered with a PDMS film and the right sample 2 remained the same. Both had the same appearance and had significantly reduced NIR transmittance compared with the ordinary bare glazing ( Figure 3C), which can play an important role in interior cooling energy savings. ...
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... glazing with a NIR reflective layer (indium tin oxide [ITO]) pointed to the sky were framed by Cu foil as the windows ( Figure 3A), and the left sample 1 was covered with a PDMS film and the right sample 2 remained the same. Both had the same appearance and had significantly reduced NIR transmittance compared with the ordinary bare glazing ( Figure 3C), which can play an important role in interior cooling energy savings. On the other side, the top PDMS emitter in sample 1 significantly enhances the emissivity in MIR wavelength ( Figure 3D) without creating extra solar absorption (<1%), thus efficiently cooling the heat in the windows that was increased by the sunlight absorption of ITO and oxide components in glazing (e.g., Na 2 O, CaO, MgO). ...
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... had the same appearance and had significantly reduced NIR transmittance compared with the ordinary bare glazing ( Figure 3C), which can play an important role in interior cooling energy savings. On the other side, the top PDMS emitter in sample 1 significantly enhances the emissivity in MIR wavelength ( Figure 3D) without creating extra solar absorption (<1%), thus efficiently cooling the heat in the windows that was increased by the sunlight absorption of ITO and oxide components in glazing (e.g., Na 2 O, CaO, MgO). 21 The thermal imaging captured from the back surfaces with the same emissivity shows a much lower temperature for sample 1 ( Figure 3B). ...
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... the other side, the top PDMS emitter in sample 1 significantly enhances the emissivity in MIR wavelength ( Figure 3D) without creating extra solar absorption (<1%), thus efficiently cooling the heat in the windows that was increased by the sunlight absorption of ITO and oxide components in glazing (e.g., Na 2 O, CaO, MgO). 21 The thermal imaging captured from the back surfaces with the same emissivity shows a much lower temperature for sample 1 ( Figure 3B). The warmer edge of the glazing was due to the non-radiative heat exchange with the heated metal frame. ...
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... temperatures of the two samples were also directly measured under the open conditions without any windshield to investigate the practical cooling effect of PDMS during daylight hours in Wuhan, China (located in a humid subtropical climate). The solar irradiation (I solar ) was $630 W m À2 and mild wind speed was <2 m s À1 , and according to the measured ambient temperature (T amb ) and relative humidity (RH), the precipitable water (PW) can be estimated using the empirical equation, 13 which was $12 mm during the test ( Figure 3E). Real-time measurements revealed that sample 1 had a significant temperature decrease of $7 C compared to sample 2. The steady-state temperature simulations based on actual measurement parameters (constant Figure 3F), and the notable temperature fluctuation for the hotter sample 2 was due to the more significant thermal convection. ...
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... solar irradiation (I solar ) was $630 W m À2 and mild wind speed was <2 m s À1 , and according to the measured ambient temperature (T amb ) and relative humidity (RH), the precipitable water (PW) can be estimated using the empirical equation, 13 which was $12 mm during the test ( Figure 3E). Real-time measurements revealed that sample 1 had a significant temperature decrease of $7 C compared to sample 2. The steady-state temperature simulations based on actual measurement parameters (constant Figure 3F), and the notable temperature fluctuation for the hotter sample 2 was due to the more significant thermal convection. Replacing the ITO with other reflectors with higher NIR reflectivity, which always exhibit lower MIR emissivity, would lead to more obvious temperature decreases according to the cooling mechanism. ...
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Passive daytime radiative cooling (PDRC) has attracted intensive attention due to its great potential in reduced energy consumption. However, the practical application for PDRC is greatly affected by geographical location and the intrinsic cooling power. Herein, a composite flexible fabric with the synergistic effect of evaporative cooling and radi...
Citations
... Owing to their low-cost nature, polymer materials show promise in producing cost-effective and scalable PDRC films. 11 Studies on polymer films, 12 glass−polymer metamaterials, 13 hierarchically porous polymers, 14 and polymer nanofiber films 15 have demonstrated satisfactory cooling effects. A selection of polymer materials, including polyvinyl fluoride (PVF), polyvinyl chloride (PVC), and polymethylpentene (TPX), has been previously scrutinized and utilized for their high emissivity in the ATW. ...
... In this study, the windows at various positions of the vehicle are composed of EC with a transparent radiative , the T sol of EC under full bleaching, 50% coloring and full coloring are 72%, 41% and 10%, respectively, as shown in Figure 4. The transparent radiative cooling film has a visible light transmittance of up to 94% and a thermal emissivity of more than 90%, so it can be used as a good transparent emitter on the outer surface of windows without affecting the daylighting of windows (Zhou et al. 2020b). According to the atmospheric absorption spectrum in Figure 4, in the atmospheric window band (8-13 μm), the atmosphere absorbs less electromagnetic radiation in a specific wavelength range, while the ground receives energy from the sun (mainly in the visible and near infrared regions), and then radiates it back to outer space in the form of long-wave infrared thermal radiation. ...
In hot climates, the large amount of cooling load in electric vehicle (EV) results in a lot of battery energy consumption, leading the decrease of driving range. With the widespread application of windows in EV, the electrochromic glass (EC) shows great prospect in lowering the cooling load. However, researches on the application of EC in EV lack the consideration of both passive cooling measures and passenger comfort, which limits the further application of EC. In this paper, we proposed an idea combining the novel techniques of both electrochromism and radiative cooling. Computational fluid dynamics (CFD) is modeled to simulate the application of electrochromic and radiative cooling coupled smart windows in hot parking conditions, exploring the improvement effect of the window on the thermal environment, comfort and energy saving of the EV. The results indicate that, under the intense sunlight with an outdoor temperature of 33 °C, activating the air conditioning to maintain an average interior temperature of 26 °C, the coupled windows reduced the cooling capacity of the air conditioning by 762 W compared to regular windows, which can further increase the range of EV. Meanwhile, compared to simple electrochromic fully colored glass, the integration of radiative cooling technology can lower the window surface temperature by up to 10.7 °C. Moreover, compared to regular windows, the coupled windows lowered the standard effective temperature (SET*) for passengers by approximately 7 °C, significantly improving comfort. These research findings are expected to provide guidance for optimizing window design and enhancing the performance of EV.
... Another widely noticed PDRC bulk appears in cooling windows [240][241][242]. Zhou et al. [243] prepared a transparent glass by depositing silver nanowires onto one side of an epoxy-infiltrated delignified bamboo slice, as shown in Fig. 14a. ...
Passive daytime radiative cooling (PDRC) is an emerging cooling technique of a sunshine-exposed terrestrial surface by dissipating excessive heat into the deep-cold outer space. It is a passive
technique without fuel consumption and paves a promising way to overcome the issues of energy shortage and environmental pollution at the global scale. In the contemporary era, highly developed nanophotonic engineering allows spectrally precise emissivity/reflectivity of a thermal surface, significantly improving a PDRC structure’s cooling power. Furthermore, scalable manufacturing techniques have also been successfully developed for PDRC material preparation at affordable costs, promoting their practical implementations. However, a comprehensive review of PDRC materials for real-world applications is still lacking. This work begins with the fundamentals of PDRC, continues with the power enhancement and scaling up process of PDRC materials, boosts with the advances of three typical types of scalable PDRC materials, including films, textiles, and bulk materials, and ends with an outlook that addresses the limitations and challenges on PDRC materials for extensive real-world applications. This review can help scientists
and engineers carry forward the design and implementation of PDRC materials, promote the mitigation of global issues such as scorching and water shortage, and make the planet healthier and more comfortable.
... The PDRC materials reflect sunlight (0.3 ~ 2.5 μm) to minimize solar heating and simultaneously emit thermal radiation through the atmosphere's longwave infrared (LWIR) transparency window (8 ~ 13 μm), enabling the transmission of energy to the outer space to realize surface radiation cooling [3][4][5]. Recently, daytime radiative cooling technology has made significant advancements in various applications, such as building cooling [6][7][8], photovoltaic cooling [9,10], cryogenic cooling [11], atmospheric water harvesting [12][13][14], personal thermal management, and wearable devices [15][16][17], et al. The reported PDRC materials include white paints [8,18], photonic structure coatings [19,20], metamaterials [21], bulk materials (cooling wood [22], aerogels [23,24], nanoporous textiles [15,16], and hierarchical metafabrics [17], etc. ...
... To reduce global warming, in recent years, the attention of scientists and environmental activists has been drawn to methods that are green and use no external energy. Some of these methods are based on exchanging energy by thermal emission through transparent atmosphere windows (AWs) with the cosmic background (with the 3 • K temperature) as the ultimate natural heat sink [3,[5][6][7][8][9]. The most important AW is in the 8-13 µm mid-IR band, because in ordinary temperatures, based on Wien's law, the peak of thermal emission is in this band. ...
... In general, we can categorize the radiative coolers (RCs) in two main categories based on reflecting or transmitting the visible spectrum. If the structure is transparent for the visible band, it is called transparent radiative cooler (TRC) [3,[7][8][9]. TRCs have been presented more recently, and they are very useful for situations where transparency is an important property, such as roof glazing. ...
... By using the directional spectral emissivity and thermal emission theories [3,[5][6][7][8][9], the net radiated power in AW vs. δ for different ground surface temperatures can be calculated. λ 0 Λ in the above equation should be determined for the middle wavelength of AW. ...
Photonic crystals are known for their band-gap structures. Due to their band-gaps, they can act as filters in both temporal and spatial domains. However, in most cases, due to their physical symmetry, their angular responses are symmetrical. Here, a structure based on a 1D photonic crystal is introduced and analyzed, which has an asymmetric angular selectivity. The structure is analyzed using the plane wave expansion method. The properties of the structure are expressed and verified by a commercial full-wave simulator software. Based on the analysis and its results, some simple design rules are derived. By using the extracted rules and some approximations, the potential of the structure to be used in radiative coolers, which are not completely toward the sky, is introduced. It is shown that if the structure is used as windows in buildings, it can save up to tens of watts per square meter in energy consumption for air conditioning. Finally, the whole structure including the radiative cooler is simulated, and the results support the calculations and approximations.
... The measured emissivity spectrum of the optimized hierarchical-morphology coating in Fig. 1e indicates strong emittance in the MIR region, with a high emissivity of 95.1% in the atmospheric window. The Si-O-Si asymmetric stretching vibrations in SiO 2 are typical Lorentz oscillators [54] that exhibit a reflection band near the resonance frequency of 9 μm. However, PDMS (which contains specific functional groups such as the methyl group and vinyl-terminated cross-linkers bonded to chains comprising alternating Si and O atoms) exhibits reduced reflection in the range of 7-14 μm, and counteracts the reflection band of SiO 2 . ...
... All the samples exhibit high emissivity over the entire IR region, which decreases slightly on decreasing the proportion of PDMS in the sample. Pristine PDMS exhibits a high broadband IR emissivity, and the Si-O-Si asymmetric stretching vibrations in SiO 2 exhibit a reflection band at ~ 9 μm [54]; therefore, the replacement of SiO 2 nanoparticles in the PDMS planar structure reduces the atmosphericwindow emissivity of the system slightly. The hierarchicalmorphology coating fabricated with a PDMS/PFPE mixing ratio of 1:1 and nanobead mass fraction of 15 wt% exhibits optimal values of IR emissivity and solar reflectivity. ...
Passive daytime radiative-cooling materials (characterized by a high solar reflectance and thermal emittance) exhibit a cooling effect under direct sunlight with zero energy consumption, thereby decreasing the demand for air conditioning. Although various well-designed radiative-cooling materials have been reported to date, their syntheses are environmentally harmful and unsuitable for large-scale operation (as they involve complicated, high-cost, or solution-processed methods). In this study, a hierarchical-morphology coating for large-scale radiative-cooling applications was constructed by a one-step, inexpensive, solution-free, and environmentally friendly strategy. The hierarchical morphology (comprising nanospheres and micropores randomly dispersed throughout a polymer matrix) was fabricated through simple mechanical stirring (without the use of templates); no solvents or by-products were produced during the manufacturing process. The optimal coating showed high emissivity (95.1%) in the atmospheric-window band, strong solar reflectivity (94.0%), and a cooling power of 62.94 W m⁻² (according to field tests). Moreover, covering the roof of a model with the as-prepared hierarchical-morphology coating reduced the average roof temperature by 11.5 ℃ (according to outdoor tests). According to simulations, the coating enabled annual cooling-energy-consumption savings in the range of 14.5–41.2% for typical buildings located in different climatic regions, indicating high potential as an energy-saving building-envelope material.
Graphical Abstract
A one-step, scalable and sustainable strategy has been developed to fabricate hierarchical-morphology coatings for passive daytime radiative cooling.
... Various other types of heat shield materials have been studied, such as indium-tin-oxide-based coatings [4], dielectric coatings [5], and electrochromic [6] and thermochromic materials [7]. For instance, because environmental factors can change the characteristics of electrochromic and thermochromic window reflection, transmission, and absorption, these windows can be used in both warm and cold climates. ...
Windows with passive multilayer coatings can allow less energy to be used when maintaining comfortable indoor temperatures. As a type of effective solar energy management, these coatings can prevent the generation of excessive heat inside buildings or vehicles by reflecting near-infrared solar radiation (750–2000 nm) while retaining visible light transmission (400–750 nm) over a large range of viewing angles. To prevent overheating, they must also reflect rather than absorb near-infrared radiation. A transparent heat-shielding window is numerically and experimentally demonstrated in this study. High visual transparency (77.2%), near-infrared reflectance (86.1%), and low infrared absorption ( ${\lt}{20}\%$ < 20 % ) over a wide range of oblique incident angles were achieved using nanometer-scale cross-shaped metamaterials manufactured by electron beam lithography. Furthermore, high terahertz transmittance (up to 82%) was also achieved for 6G communication system applications.
... Lastly, it provides shading in conjunction with the soil to achieve passive indoor temperature control for the building (Fig. 6e). Moreover, PDRC imparts the ability to further diminish plant and soil temperatures through radiative cooling mechanisms, whereby PDRC can be positioned as a mulching film, thereby enhancing thermal dissipation [46,47]. This arrangement is expected to lower the temperature of the microclimate by 11.00 • C, effectively alleviating heat stress in high-temperature climates [48]. ...
... PDMS is a chemically stable and inexpensive material, in which methyl groups and the vinyl cross-linkers are bonded on the chains of alternating silicon and oxygen atoms, bringing relatively low solar radiation absorption and high atmospheric window emission [23]. Zhao et al. covered 200µm-PDMS film on encapsulated commercial silicon solar cell as radiative cooler for the first time. ...
... The transmissivity of PDMS layer in silicon solar cell absorption band decreases with the thickness increasing as shown in Fig. 2(a). A part of incident light is absorbed by PDMS layer due to the C-H band bring stretching vibrations as shown in Fig. 2(c) left part [23]. The PDMS is a good radiative cooler candidate with high emissivity in first atmospheric window band. ...
Passive daytime radiative cooling (PDRC) as a zero-energy consumption cooling method has broad application potential. Common commercial crystalline silicon (c-Si) solar cell arrays suffer working efficiency loss due to the incident light loss and overheating. In this work, a radiative cooler with PDMS (polydimethylsiloxane) film and embedded SiO2 microparticles was proposed to use in silicon solar cells. Both anti-reflection and radiative cooling performance can be improved through numerical parametric study. For the best performing of PDMS/SiO2 radiative cooler, the thickness of PDMS layer, volume fraction and radius of the embedded SiO2 particles have been determined as 55 µm, 8% and 500 nm, respectively. 94% of emissivity in first atmospheric window band (8–13 µm) for radiative cooling and 93.4% of solar transmittance at the crystalline silicon absorption band (0.3–1.1 µm) were achieved. We estimated that the PDMS/SiO2 radiative cooler can lower the temperature of a bare c-Si solar cell by 9.5°C, which can avoid 4.28% of efficiency loss. More incident light can enter and be utilized by silicon layer to enhance the efficiency of the solar cells. The proposed difunctional radiative cooling coating may become guidance for next generation encapsulation of crystalline silicon solar cells.
... Therefore, an advanced radiative cooling design for transparent functions with simultaneous thermal regulation is needed. Some researches of transparent radiative cooling technologies such as nanospheres 159,160 , nanowire [160][161][162] , and photonic crystal 163,164 have been demonstrated, and this section introduces multilayer [165][166][167] and micro spherical photonic 168,169 used in transparent radiative cooling. ...
... Outdoor rooftop experiments demonstrated that this colored cooler reduced the system interior temperature by an average 5.2°C. Zhou et al. 165 introduced a simple double-layered structure of a transparent radiative cooling window transmitting visible light and reflecting the NIR while emitting thermal radiation (Fig. 9b (left)). This transparent cooler uses two materials of indium tin oxide (ITO) and PDMS. ...
... b Schematic structure of a transparent radiative cooler (left) optical and IR image of NIR reflection with and without PDMS (middle) measured emissivity spectra of #1 and #2 in mid-IR range (right). Reproduced with permission 165 . Copyright 2020, Elsevier. ...
Radiative cooling is a passive cooling technology without any energy consumption, compared to conventional cooling technologies that require power sources and dump waste heat into the surroundings. For decades, many radiative cooling studies have been introduced but its applications are mostly restricted to nighttime use only. Recently, the emergence of photonic technologies to achieves daytime radiative cooling overcome the performance limitations. For example, broadband and selective emissions in mid-IR and high reflectance in the solar spectral range have already been demonstrated. This review article discusses the fundamentals of thermodynamic heat transfer that motivates radiative cooling. Several photonic structures such as multilayer, periodical, random; derived from nature, and associated design procedures were thoroughly discussed. Photonic integration with new functionality significantly enhances the efficiency of radiative cooling technologies such as colored, transparent, and switchable radiative cooling applications has been developed. The commercial applications such as reducing cooling loads in vehicles, increasing the power generation of solar cells, generating electricity, saving water, and personal thermal regulation are also summarized. Lastly, perspectives on radiative cooling and emerging issues with potential solution strategies are discussed.
