This review covers the recent progress made in the nanophotonic devices-based daytime passive radiative coolers. The radiative cooling capabilities along with the structural description of various natural species are discussed. The design principle along with key characteristics of the omnidirectional solar reflectors as well as thermal adiators is discussed in detail. Several analogues planner one-dimensional and two-dimensional photonic nanostructures and their current state-of-the-art techniques have been discussed. For each kind of the photonic structure, the novelty, measurement principle, and their respective daytime radiative cooling capability are presented. The reported works and the corresponding results predict the possibility to realize an efficient and commercially viable radiator for passive radiative cooling applications.
Cooling represents a significant sector of energy consumption.1 It is the most significant driver of peak electricity demand as well as considerable end use of energy globally.2 The global energy consumption can be reduced by adopting a cooling strategy that can cool without any electrical input, which is also known as passive cooling technique. The cooling can be achieved if one is able to reach and maintain a temperature below ambient temperature. Radiative cooling, as a passive radiative cooling scheme, does not require any external active devices, such as fans, air conditioners, or thermoelectric.3–5 It works on the principle that the atmosphere is almost transparent between 8 and (also known as ‘‘sky window”). Thus, utilizing outer space as a heat sink by allowing the emission of thermal radiation from surface to the universe.6,7
In the past few decades, the radiative cooling strategy has been widely explored. Its significance along with potential application in cooling during nighttime was practically demonstrated.8–10 A number of different materials showing the capability of intrinsic infrared (IR) emissions for substantial radiative cooling have also been explored previously.11–13 These results well demonstrate the capability of the radiative cooling technique. However, the work was mostly limited for the nighttime radiative cooling only. This is because of the availability of suitable materials that possess good IR emission within the atmospheric window. But the primary cooling power requirement usually occurs during the presence of the sun (daytime as compared to nighttime).14 The presence of Sun also affects the radiator’s efficiency. Therefore, for daytime cooling, both high emissivity (in sky window) and high reflectivity (of the complete solar spectrum) are required. This approach utilizes the universe as a heat sink to release the heat through the atmospheric window and minimize the absorption of incoming atmospheric radiation. Then the surface temperature can be decreased below the ambient. Therefore, certain natural materials, such as polymeric materials,15,16 titanium dioxide,17–19 silicon nitrides,20 and silicon-monoxide (SiO),21 have potential for limited selectivity. But it is difficult to find out a simple bulk material with high emissivity and high reflectivity in the desirable spectrum range simultaneously.22 However, many natural species possess the capability to cool down themselves by exhibiting both the aforementioned properties simultaneously. Inspired from natural species, a lot of work is also carried out to design nanophotonic devices for daytime radiative cooling applications. This facilitates capacity to simultaneously possess a reflectivity of more than 97% for complete solar spectrum and strong IR emission within the sky window.23–26 The development of nanophotonics structures is certainly an important factor that helps in the current resurrection of passive radiative cooling techniques.
Interestingly, one recent article demonstrated the potential of one-dimensional photonic crystal (1D-PhC) structure that can cool down an entity below the ambient temperature using the same technique to that of Sahara silver ants, i.e., highly reflective and emissive for the desirable spectrum range.27 Similarly, another recent report has also been reported for this purpose by mimicking the structural attributes of Morpho butterfly wings and successfully demonstrated the passive radiative cooling strategy.28 These emerging nanophotonic devices can offer an efficient cooling strategy with their strictly selective and achievable properties and have triggered significant research interest in this area.
Within this context, this review provides sufficient details regarding design principle, radiative cooling characteristics, and recent progress made toward the development of daytime passive radiative cooling devices. Section 2 presents an overview of biomimetic analogy of radiative cooling and motivation behind developing these devices. Section 3 presents a detail working principle of radiative cooling. The approach for nighttime and daytime applications is identified. This section also discusses the design strategy for omnidirectional reflector (ODR). In Sec. 4, we review the different material systems along with various 1D and 2D nanophotonic devices developed for daytime passive radiative cooling applications in the earlier studies. Their working principles and performances of the reported designs are thoroughly investigated and possible efficiency improvement strategies are discussed. Section 5 discusses the current challenges and scopes of daytime passive radiative cooling technique and finally Sec. 6 concludes the review.
Radiative Cooling’s Biomimetic Analogy
While the proposed passive daytime radiative cooling concept is potentially transformative for metallic structures, the idea of selective passive radiative cooling is not new to nature. Various insects and animals living in hot environment, such as Morpho butterfly, Pompeii worm (Alvinella pompejana), Sahara Desert ant (Cataglyphis bicolor), and water bear or Tardigrade (Hypsibius dujardini) possess the capability to withstand extreme climate changes. For example, Pompeii worm and Sahara Desert ant can withstand a temperature as high as 176°F (80°C) and 122°F (50°C), respectively.29,30 Water bear is capable of surviving the temperature more than 302°F (150°C).31 This happens only because of their unique body structure that helps them to overcome the heating challenges. Alongside, beetles (Coleoptera) exhibit a number of interesting optical phenomena, i.e., metallic colors and spectral iridescence.32 Recently, the white structure presents on beetles’ elytra has also attracted researchers’ interest to explore this mechanism. Many authors have further explored the optical functionalities of these species to develop various new biological photonic structures.33–36
If an entity keeps on absorbing the significant portion of solar radiation, it will not survive. Therefore, most of the natural substances have their body structures that help them to reflect solar radiation. For example, Morpho butterfly has empty pores within the thin membranes, which helps them to scatter near-IR radiation and avoid excessive heating under sunlight.37 The Morpho butterfly image and corresponding pictorial representation of internal structure are shown in Figs. 1(a) and 1(b), respectively. Furthermore, these species also exhibit strong emissive properties in mid-infrared (MIR) range, thereby using universe as heat sink as explained earlier.28,38 Similarly, the internal structure of Bistonina biston butterfly’s wings also exhibits spectrally selective emissivity values. This helps butterfly to cooldown the wing surface. Because of its unique structure, it exhibits a high emissivity in the MIR (2.5 to ) to dissipate heat and it is highly reflective to the solar spectrum, which helps it to cool the wings.39
Saharan silver ant is another widely studied insect that possesses the capability to stay cool in the Saharan Desert under the direct sunlight. The amazing temperature management ability of Saharan silver ants is also due to their unique structural geometry. The Saharan ants are covered with a dense array of hairs with triangular shapes on both the top and sides of their bodies as shown in Fig. 2.27,40 It has been recently demonstrated that the periodical arrangement of their hair structures makes them reflective to the solar spectrum (0.4 to ) and highly emissive in the MIR range (2.5 to ).27 These uniquely shaped silvery hairs protect and enable the ants to maintain lower body temperatures by two main mechanisms (i.e., reflection of most of the solar spectrum and transferring heat to the universe by improving the emissivity in the MIR region) similar to the Bistonina biston butterfly. The combined effect helps the silver ant to maintain steady-state body temperature.
Another example present in nature is the cocoon made by wild silk moths. The cocoons comprise a number of fibers arranged randomly and scatter the incident light. This attributes to high solar reflectivity along with high MIR emissivity.41,42 This helps moth pupae from overheating under the direct sun. In 2018, Choi et al.43 explored the internal structure of silkworm fibers and found the presence of microcavities. These microcavities are responsible for both reflections of the solar spectrum and suppression of the transmission spectrum via Anderson localization.
The thermoregulatory solutions demonstrated by natural species are the examples that show the capability of an animal to control electromagnetic waves over an extremely broad range of the spectrum (almost complete solar spectrum). These remarkable effects observed in these natural species could be very interesting and have a massive technological impact and inspiration for the development of nanophotonic devices for passive radiative cooling applications.
Fundamental Principles of Passive Radiative Cooling
Design Principle of Passive Radiative Cooler
In this section, the fundamental principle behind the passive radiative cooling strategy is presented. The environmental radiations comprise both solar radiation and emitted IR radiation from the Earth’s surface, which is further used to maintain the energy balance.44 This energy balance is required to maintain an equilibrium temperature. When a surface under consideration is exposed to the environment, the cooling power is considered to be the resultant out-going radiative energy flux and can be represented byFig. 3), the net radiative cooling power can be obtained by modifying Eq. (1) and represented by following equations:45
Figure 3 represents the pictorial working principle of daytime passive radiative cooling devices, where the mentioned terminologies are already explained earlier. According to Eq. (5), and behave as parasitic cooling losses those attribute to the reduction in the overall cooling efficiency. This further leads to increase in the cooling time to obtain the thermal equilibrium. In addition, the cooling efficiency also depends on the emission characteristic of the radiator, i.e., broadband or selective radiator. The ideal broadband radiator possesses the capability with unity emittance throughout the IR spectrum along with negligible absorption for the complete solar spectrum in daytime radiative cooling applications. Spectrally selective cooler possesses the unity emittance only in the atmospheric window.46
The structural net radiative cooling efficiency or cooling time can be improved by minimizing the absorption of atmospheric and solar radiation. Generally, the radiation property of a perfect absorber (blackbody) is also very strong and research is being carried out to design perfect absorber.47,48 The solar radiations are considered to be equivalent to that of a blackbody radiations for an effective temperature of around 5800 K. Figure 4(a) represents the normalized atmospheric transmission for different wavelengths, where it appears to be highly transparent from 8 to . One more sky window also appears between 16 and . Due to its weak transmittance, it has very less significance for radiative cooling and neglected in general. The Earth’s atmosphere exhibits good emissivity outside this window. Coincidentally, the peak thermal radiation characteristic of a blackbody (at around 300 K) overlaps with the atmospheric window, as shown in Fig. 4(a). The emitted IR rays pass through the Earth’s atmosphere and move into the outer space. Thereby, it uses universe as a heat sink. Figure 4(b) shows the spectral irradiance graph of AM 1.5 solar spectrum (at irradiance of about ). The absorption of this power can drastically affect the cooling performance of the radiator. Apart from these, atmospheric conditions also affect the performance of daytime radiative cooling structures. This has already been discussed in various earlier reports.49–52 To minimize the effects of incoming solar radiation, an IR transparent reflector needs to be integrated on the top side of the surface. Alternatively, a cooling device itself can be designed that possesses the capability to serve as both solar reflector and IR radiator simultaneously.
Design Principle of Dielectric Reflectors
The dielectric reflector is a structure that possesses the capability to reflect the complete solar spectrum at all incident angles. This is also considered as omnidirectional broadband reflector. The structure works on the principle of distributed Bragg reflector (DBR), where optical length optimization is carried out to enhance the reflectivity. Optical length can be varied by changing layer properties, such as height, RI, and porosity. Each layer causes a partial reflection of incident radiation as shown in Fig. 5. In Fig. 5(a), layer “A” corresponds to the material of high RI and layer “B” is of low RI. The structure is designed in such a way that all the reflected waves constructively interfere to give an overall high reflection. This constructive interference condition can easily be governed by Bragg’s law as shown in Fig. 5(b).
The structure can be designed using a quarter-wavelength Bragg reflector. The analysis can be carried out by considering a material system of high and low refractive indices that should also have high emissivity in the atmospheric window. Generally, the structure is designed to reflect a broadband spectrum centered at wavelength (). For normal incident, the layer thicknesses are obtained using Eqs. (6) and (7), while Eq. (8) is used for oblique incident.53,54 simulated annealing,55 jump method,56 and memetic algorithm.57
Recent Progress and Discussions
Achieving subambient radiative cooling during nighttime is straightforward. But, in the daytime, the presence of the sun creates the difference. The cooling scenario can further be divided into four major parts, i.e., daytime above-ambient and subambient, nighttime above-ambient and subambient as shown in Fig. 6. Most of the radiators are designed to exhibit the characteristic similar to these ideal emissivity/absorptivity spectral curves. For nighttime cooling, 2.5- to wavelength range is of interest. For daytime, it is of around 0.3 to (i.e., extra 0.3 to is because of solar spectrum).
Subambient daytime cooling applications require the emissivity equivalent to blackbody emissivity in the atmospheric window and emissivity close to zero in the solar spectrum. This radiator has to be selective as explained in Sec. 3. The major research works for nighttime subambient radiative cooling devices are based on developing structures for selective IR emission in the sky window that is represented by a solid blue line in Fig. 6.58–61 However, for daytime cooling (either above-ambient or subambient) as shown by black solid and red dashed lines in Fig. 6, the major challenge is to achieve both high solar reflectance and high emissivity over the atmospheric window. A lot of research studies have been carried out to design structures that can reflect complete solar spectrum as well as are highly emissive in the atmospheric transparent window. The following section describes various strategies to design a broadband solar reflector followed by their applications in realizing daytime radiative cooling devices.
Dielectric Omnidirectional Reflector
The recent progress on developing nanophotonic devices based on radiators for passive radiative cooling applications has provided a new direction toward achieving highly efficient devices. Thus, improving the ability to effectively work directly under the sun by maintaining their temperatures below the ambient. Photonic crystal (PhC)-based structures are extensively investigated because of their light controlling and bandgap properties.62–67 These properties are further used to design various one-dimensional (1D) and two-dimensional (2D) PhC-based structures for passive radiative cooling applications.68–71 The PhC structure shows a complete photonic bandgap (PBG), when no electromagnetic mode is allowed to propagate through the structure and behaves as a perfect reflector.72,73 The width of reflection spectrum (or PBG) is important while designing an ODR specifically, for applications such as daytime radiative cooling.74 To achieve daytime radiative cooling, an ODR is required that should have the capability to reflect complete solar spectrum for both TE and TM polarization.75–77 The metallic reflectors are widely accepted for reflection purposes but their widespread uses are limited because of their lossy properties at optical wavelengths.78 Therefore, dielectric-based structures are considered to be good alternatives for this purpose as they also provide design flexibility in terms of reflection wavelength.79 Different types of 1D-PhC structures have already been investigated to design ODRs.
Fink et al.80 and Winn et al.81 presented a 1D-PhC-based dielectric structure with nine layers of an alternating stack of polystyrene/tellurium materials and reported an ODR over the wavelength range from 10 to . The ODR bandgap can also be enlarged by considering heterojunction photonic multilayer structures.82–84 Joseph et al.85 proposed a dispersive 1D-PhC structure made of (as low-index medium) and GaSb (as high index medium) and reported an ODR in 1480- to 1680-nm wavelength range. The combination of dielectric and birefringent material has also been explored for designing of ODR structures. Upadhyay et al.86 used potassium titanyl phosphate (KTP)/lead sulfide (PBS) as birefringent materials and as a dielectric material and reported an ODR over the wavelength range of 3.155 to . The ODR bandgap was further enhanced by considering graded structures of same material and reported an ODR over a wavelength range of 3.188 to .87
Various natural species (i.e., Papilio palinurus butterfly, silvery fish, etc.) have also been studied to understand their ODR phenomena. Jordan et al.88 and later Zhao et al.89 studied and reported a nonpolarizing optical mechanism found in the broadband guanine-cytoplasm “silver” multilayer reflectors of three species of fish. The silvery reflections from the fish are because of the multilayer stacks of guanine crystals and index cytoplasm.90,91 These work as high and low refractive index media, respectively. Similarly, Han et al.23 studied the Papilio palinurus butterfly structure and theoretically and experimentally reported the findings of novel omnidirectional reflective self-stable properties. Furthermore, based on this study, the authors successfully designed and fabricated a -material-based bioinspired color reflector using a simple biotemplate method. Although, the reported papers clearly demonstrate capability of dielectric-based 1D-PhC structure to design ODR but width of reflection spectrum is still a tricky task to cover complete solar spectrum. Table 1 summarizes the recent work done on designing ODRs.
Summary of the recent development in ODR design.
|Structure||Materials||ODR range (μm)||ODR width (μm)||Year||References|
|Planner||Polystyrene/tellurium||10 to 15||5||1998||Fink et al.80|
|Planner||and GaSb||1.48 to 1.68||0.20||2014||Joseph et al85|
|Planner||KTP/PBS as birefringent and as dielectric||3.155 to 4.202||1.047||2012||Upadhyay et al.86|
|Graded||3.188 to 4.655||1.467||2018||Kumar et al.87|
|Decreasing width multilayer||CsBr/Te||12 to 20||08||2017||del Barco et al.137|
|Multistacking||and||0.3 to 2.4||2.1 (TE only)||2019||Ratra et al.138|
|Multistacking||and||0.3 to 2.3||2.0||2020||Ratra et al.139|
|Planner||Porous silicon||1.0 to 2.0||2.0||2020||Castillo et al.140|
Radiative Coolers Based on 1D Nanophotonic Structure
Rephaeli et al.92 proposed a metal–dielectric-based multilayer structure with three sets of five bilayers of and materials as shown in Fig. 7(a). The proposed structure is embedded between two layers of 2D photonic crystal (2D-PhC) made of quartz and SiC and shows the omnidirectional reflectivity for complete solar spectrum. The proposed structure shows its capability of achieving a net cooling power of around at environmental temperature. Similarly, Raman et al.93 designed a structure with planar photonic device consisting of seven alternating layers of hafnium oxide () and (having different layer thicknesses) on a silver-coated silicon substrate as shown in Fig. 7(b). The bottom -based thinner structure is responsible for solar reflection, while the top thicker structure is designed for thermal radiation. The authors experimentally reported a 97% reflection of the complete solar spectrum and an average emissivity of about 0.65 in the transparency window. They experimentally demonstrated a temperature reduction of 4.9°C below the ambient under direct sunlight. This structure has been widely studied by many researchers. Different materials are considered for further improvement in the net cooling power. Kecebas et al.94 replaced with material along with a top layer. Jeong et al.95 further optimized this -based daytime radiative cooler structure with an average emissivity of 0.84 in the sky window along with 94% reflectivity of incident solar energy as shown in Fig. 7(c). The authors estimated the net cooling power around . This shows improvement in cooling power compared to that of the material-based photonic radiative cooler structure. The authors successfully demonstrated a reduction of 7.2°C in temperature along with a net cooling power of under direct sunlight using solar shading.
Fu et al.96 demonstrated a daytime passive radiative cooler composed of porous anodic aluminum oxide (AAO) membranes as shown in Fig. 7(d). The porosity (or air doping) and AAO thickness were optimized to get the desired results. The designed structure is capable of providing a power density of at environmental temperature (at humidity of ) under direct sunlight. The authors experimentally showed the structure cooling by reducing temperature by 2.6°C below the ambient air under direct sunlight. In another report, Chen et al.97 used silicon nitride (), silicon (Si), and aluminum (Al) to design a multilayer structure [as shown in Fig. 7(e)] that works as an ideal selective thermal emitter for sky window. The authors reported an average 42°C temperature reduction below ambient.
Furthermore, calcium fluoride () and germanium (Ge)-based material systems are also explored and used by Huang et al.98 Authors designed an “invisible” radiative cooling structure by considering a nichrome metal film on which seven alternating layers of were deposited. The structural parameters were optimized to get the desired results. To reduce the cost, polymer materials are also explored for this application. Polyvinyl fluoride, polyvinyl chloride, poly-dimethyl-siloxane (PDMS), polyethylene terephthalate, and polymethylpentene (TPX) are commonly used polymer materials, which are widely used for radiative cooling applications.99–102 These materials are beneficial as they offer very strong IR emission and possess the capability of large-scale production along with the low cost.
However, polymer material-based radiators have various concerns that are discussed in detail in later sections. Gentle et al.103 used polymer materials to demonstrate reduction in temperature below ambient. The structure was designed by considering birefringent polymer materials arranged in stacked-like structure. Thus, creating a polymer-based reflector for the visible spectrum. To cover the complete solar spectrum, a silver metallic layer was deposited at the bottom side, which is used to reflect the IR wavelength range of the spectrum. Based on the structures, the authors successfully demonstrated cooling of 2°C below the environmental temperature under the direct sunlight with the irradiation of about . In 2017, a polymer-coated fused silica mirror was also proposed. The designed structure exhibits the characteristic of a near-ideal blackbody as well as near-ideal reflector in the MIR and in the solar spectrum, respectively. The authors demonstrated the radiative cooling below ambient air temperature during daytime (8.2°C) and during nighttime (8.4°C).26 Recently, Yang et al.104 used polytetrafluoroethylene (PTFE) and Ag film on Si substrate [as shown in Fig. 7(f)] and successfully demonstrated radiative cooling to 11°C below the environmental temperature.
The reported results demonstrated well the possibilities of 1D nanophotonic structures to control light in the multiple photonic bands (sky window and solar spectrum) that are extremely desirable for daytime passive radiative cooling applications. However, designing an ODR that can cover complete solar spectrum [ultraviolet (UV) to IR] using simple 1D photonic structures is difficult. Furthermore, the application of metal as back-reflector also affects the reflection of UV spectrum. This is because of the metallic absorption coefficient in the UV wavelength region.105 Working toward this goal, Yao et al.106 proposed a dual-band selective emitter with multinanolayers. Authors used a tandem PhC-based structure with different PBGs (PhC1 and PhC2), which are stacked over a silver metallic reflector. The proposed design (as shown in Fig. 8) shows its capability of radiative cooling to 11°C below the environmental temperature under direct sunlight.
The average emissivity of the design can also be improved using colored radiative coolers. Compared to traditional radiative cooling structures, this technique leads to increase in overall emissivity within the sky window and leads to reduction in the cooling loss. Recently, Sheng et al.107 designed a colored radiative cooler structure based on optical Tamm resonance. The proposed structure possesses the capabilities to produce high-performance cooling and can retain high-purity subtractive primary colors (CMY). The authors used the Tamm structure, composed of a DBR () on top of a silver film, and successfully presented a temperature reduction of 5°C to 6°C. Table 2 summarizes the recent work done on designing radiative cooler based on 1D nanophotonic structure.
Summary of the recent development in 1D nanophotonic structures.
|Materials||Cooling power (W/m2)||Temperature reduction (°C)||Year||References|
|-based structure||7.2||2020||Jeong et al. 95|
|Porous AAO membranes on aluminum substrate||2.6||2019||Fu et al.96|
|PDMS film on a reflective metal substrate||—||3.2||2019||Zho et al.101|
|A 150-nm-thick silver (Ag) and 50-nm-thick on a -thick substrate||—||5.9||2019||Zhao et al.141|
|Tandem||—||11||2019||Zho et al.101|
|multilayer on top of a Ag film||—||5 to 6||2019||Sheng et al.107|
|Structure made by ultrawhite glass-plated Ag||—||2.5||2019||Ao et al.142|
|Zinc phosphate sodium () particles on aluminum (Al) substrate||—||1.5||2019|
|(PTFE) and Ag film on Si substrate||—||11||2017||Yang et al.104|
|Two layers of acrylic resin embedded with and carbon black particles||100||6||2017||Huang et al.76|
|Stack of PDMS//Ag||127||8.2||2017||Kou et al.26|
Radiative Coolers Based on 2D Nanophotonic Structures
Furthermore, 2D structures are also explored for designing daytime radiative coolers. Nanophotonic structures with structural dimensions around wavelength exhibit distinct radiation properties.108,109 Microresonator structures are excellent candidates to reflect or absorb a given wavelength.110–112 A number of nanophotonic structures, such as conical pillar arrays, dielectric resonator metasurfaces, metal–dielectric–metal resonators, and multilayer pyramidal nanostructures, are explored to design daytime passive radiative coolers. These structures possess the capability to tailor the spectral selectivity of the surfaces. Initially, Rephaeli et al.92 explored the possibility of daytime radiative cooling using a nanophotonic structure and successfully demonstrated a temperature reduction of almost 4.9°C below the ambient.
A general strategy for achieving color-preserving radiative cooling using an -quartz nanostructure has also been proposed.113 The structure comprises an array of -quartz bars placed on top of another arrayed structure made of silicon bars [as shown in Fig. 9(a)]. The color-preserving radiative cooling strategy can also be implemented using a multilayer photonic radiator.114 Zhu et al.115 used a -based PhC absorber for radiative cooling applications. The oxide material is considered because of its strong phonon–polariton resonance properties in the sky window. The structure is designed on a double-sided polished fused silica substrate, in which air holes of depth are etched. The air holes are arranged in a square lattice of periodicity (lattice constant) . The structure shows its capability by reducing the temperature by 13°C from ambient under the direct sunlight.
Hossain et al.116 designed a thermal emitter by introducing a conical metamaterial array made of Al and Ge material and arranged in a pillar-type structure as shown in Fig. 9(b). This is the first research carried out on antenna-type structures. The aluminum and germanium possess the thickness of around 30 and 110 nm, respectively. The proposed structure exhibits a radiative cooling power of at ambient temperature. Working further on the structure, Wu et al.117 reported a dielectric material-based micropyramid structure as shown in Fig. 9(c). The proposed design theoretically retains the best capability of radiative cooling effect of 47°C below the ambient but wide acceptability is still a concern due to its complex fabrication. Recently, Cho et al.118 reported an antenna design using tungsten (W) material. The authors designed a cone-type structure and successfully fabricated the structure using laser-interferometric lithography. They demonstrated that emission improves along with the increase of aspect ratio of cone. A simpler metal-loaded dielectric resonator structure is designed by Zou et al.119 The designed surface consists of phosphorus-doped n-type silicon and silver layer as shown in Fig. 9(d). The proposed structure is advantageous in terms of integration possibility with various silicon-based platforms.
Inspired by Morpho didius butterfly wings’ structure, Didari et al.120 presented a designer metamaterial system for radiative cooling applications. This nanophotonic structure consists of SiC material-based tree structures as shown in Fig. 10(b). The actual internal structure is schematically represented in Fig. 10(a). To enhance the spectral emissivity, the authors further optimized the structural dimensions and complete structure placed in close proximity of a thin film in a vacuum separated by nanoscale gaps as shown in Figs. 10(c) and 10(d). This modification in design improves the near-field radiative transfer within the sky window. Another studied on white beetles Goliathus has also been carried out recently.121 It possesses the unique structure comprising an exterior shell and a unique interior of packed hollow cylinders as shown in Fig. 10(e). This structure not only contributes to a lower equilibrium temperature under direct sunlight but also enhances the broadband omnidirectional reflection of solar spectrum. This is because of the thin-film interference, Mie resonance, and total internal reflection. Furthermore, the structure is also highly emissive in the MIR range. Combining all these effects, it possesses the capability to reduce their temperature approximated around in air. Table 3 summarizes the recent work done on designing radiative cooler based on 2D nanophotonic structure.
Summary of the recent development in 2D nanophotonic structure.
|References||Cooling power (W/m2)||Temperature reduction (°C)||Year||Structure|
|Zhang et al.143||90.8||5.1||2020||Photonic film comprising a micropyramid-arrayed polymer matrix with random ceramic particles|
|Yang et al.144||60||5||2019||1-mm-thick lithium fluoride crystal coated with silver backing|
|Wu et al.117||122||47°C below the ambient temperature||2018||Two-dimensional pyramidal nanostructure of the radiative cooler is composed of alternating aluminum oxide () and silica () multilayer thin films and a bottom silver layer|
|Atiganyanun et al.145||—||4||2018||Randomly packed low-index microspheres|
|Mandal et al.146||96||6||2018||Porous P(VdF-HFP) HP coating|
|Bao et al.147||—||5||2017||Two layers of , , and SiC nanoparticles|
|Zhai et al.148||93||—||2017||Micrometer-sized spheres randomly distributed in a matrix material of TPX|
Future Perspective and Challenges
Development of nanophotonic devices-based passive radiator structures is a hot research topic for radiative cooling, where incorporation of photonic technology is an added advantage due to its unique capability to tailor the spectral properties of the radiator for designing an efficient daytime passive radiative cooling device. This has promoted the development of devices for subambient radiative cooling in recent years. Early research demonstrated that photonic technology-based structures can be used for these applications by thoroughly optimizing their properties to achieve better spectral emissivity for sky window while using a back-reflector for solar spectrum. Most of the reported work focuses on efficiency optimization of the device. However, this can also be designed to work selectively depending on environmental variation. For example, passive radiative cooling may not be required during winter and nighttime. Thereby, self-adapting radiators have desired those work according to different ambient temperatures.122–124 Thus, it becomes desirable to adopt a material whose properties change according to environmental conditions. Phase change materials seem to be best materials for these types of designs.125,126 The phase change materials convert from one phase to other by heating them up to their transition temperature.127,128 Various phase change materials, such as , , and modified graphene oxide, are explored in literature.122,129,130 Working toward this direction, Ono et al.122 designed a photonic structure to realize self-adaptive radiative cooling. The authors designed a spectrally selective filter made of 11 layers of on the top of a three-layer structure as shown in Fig. 11(a). Here, works as the phase change material. Similarly, Wu et al.123 also proposed a structure with [as phase change material shown in Fig. 11(d)] stacked with and successfully demonstrated the change in cooling power by phase changing of .
Furthermore, there are a number of additional challenges that need to be overcome to design an efficient radiator. The fabrication procedure of 2D and 3D photonic radiator structures is challenging that constraint them to become commercial application.131,132 Laser-interferometric lithography and femtosecond laser cross-linking techniques are explored to fabricate these complex structure.118,133 The techniques utilize a laser beam that is incident of a precursor material. The incident laser beam needs an effective control and guiding mechanism to generate such complex geometries. The femtosecond laser technique can also be used to create nanoholes array in aluminum film.134 Similarly, the 1D nanophotonic structures seem to be better as their fabrication is quite simpler than alternate 2D and 3D nanophotonic structures. Additionally, large-scale production is also difficult to achieve at present. Therefore, photonic devices-based passive radiators are still in the early development stage and restricted to laboratory research and exploration. Polymer-based radiative cooling structures are likely to be the future devices due to their significant advantage in terms of ease of manufacturability and cost.135 However, reliability and long-term stability are the main critical challenges that will always be there. Polymer degradation with time affects their long-term high solar reflectivity. The same is true for the metal, where moisture may lead to formation of metal oxide. Thereby, exploration of a new and effective material system is always required for functionality and efficiency enhancement of the radiative cooler structures.
In this review, a thorough study in the field of daytime passive radiative cooling has been carried out, which focuses on recent progress made in the field. Reported works and their corresponding results demonstrate that photonic nanostructures play a significant role in the efficiency improvement of these devices. Furthermore, different nanophotonic structures, their radiative properties, and corresponding design mechanisms are discussed in details. Nature-inspired nanophotonic structures seem to have an edge for radiative cooling. However, their practical implementation is still in process. Furthermore, in the future, the key technologies of biomimetic daytime radiator will be the development, device integration, and explorations of new designs and methods, which will open a new way to design highly efficient passive daytime radiative coolers.
Authors acknowledge all members of ECE Department at JIIT for their help and support. Authors declare no conflicts of interest.