Atmospheric-moisture-induced polyacrylate hydrogel system and method for passive cooling

Abstract

A photonic hydrogel film (300) includes a substrate (602) and a hybrid cooling layer (304) attached to the substrate (602) and including a sodium polyacrylate (PAAS) material (308) forming a continuous film structure. The PAAS material (308) is hydrated by atmospheric water molecules so that chains of the PAAS material (308) are uncoiled for maintaining the continuous film structure. The PAAS material simultaneously reflects sunlight due to a porous structure and emits infrared radiation due to molecular vibrations of the chains of the PAAS material.

Description

ATMOSPHERIC-MOISTURE-INDUCED POLYACRYLATE HYDROGEL SYSTEMAND METHOD FOR PASSIVE COOLINGCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63 / 524,948, filed on July 5, 2023, entitled “ATMOSPHERIC-MOISTURE- INDUCED POLYACRYLATE HYDROGELS FOR HYBRID PASSIVE COOLING,” the disclosure of which is incorporated herein by reference in its entirety.BACKGROUND OF THE INVENTIONTECHNICAL FIELD

[0002] Embodiments of the subject matter disclosed herein generally relate to a system and method for passive cooling, and more particularly, to a polyacrylate- based material that exhibits high solar reflectance, thus reducing solar heating, and high mid-infrared emittance, thus maximizing thermal emission.DISCUSSION OF THE BACKGROUND

[0003] Climate change has brought high temperatures to many regions of the globe for longer periods. As a result, rapidly increasing energy demands, in part from air conditioning systems, are overloading the existing energy grids. Energy consumption and associated environmental issues due to compressor-based cooling techniques amplify the need to explore energy-free passive cooling approaches. The natural process of evaporation, condensation, precipitation, and percolation, knownas the “water cycle,” delineates the continuous water movement between the Earth and the atmosphere in three phases. Mainly, a large amount of energy is absorbed during water evaporation (e.g., 2,430 kJ kg'1at 30°C). Therefore, it is possible to use this process for cooling, i.e., evaporating water to absorb ambient heat passively. However, continuous evaporative cooling requires an external water supply, limiting its widespread use in areas with limited water sources. Global atmospheric vapor is estimated at about 12,900 km3and is considered a ubiquitous water resource. Moreover, rising ambient temperatures cause an increase in atmospheric moisture, amplifying the warming effect of greenhouse gases such as moisture (moisture is the most significant component of greenhouse gases and even more challenging to control than carbon dioxide). Therefore, utilizing atmospheric moisture effectively could create a solution to address the challenge of global warming.

[0004] One approach to passive cooling is the use of desiccant materials.These materials, including silica gel, zeolites, hygroscopic hydrogel [1], metal- organic-frameworks, and lithium chloride [2], have been extensively studied for their ability to absorb vapor from humid atmospheres during the nighttime and evaporate it during the daytime, meeting water harvesting needs [3]. The hygroscopic hydrogel, in particular, has been developed for various applications, including atmospheric water harvesting [4], evaporative cooling [5], and interfacial solar evaporation [6, 7], Its high affinity for water absorption and relatively low temperature-activated desorption process (~ 30°C) make it a promising material. Additionally, the inherent molecular vibrations of chemical bonds over infrared wavelengths (5 - 25 pm) givethe hygroscopic hydrogels high thermal emittance, enabling effective heat dissipation through radiative cooling.

[0005] Recently, bilayer structures with polymer fibrous networks atop a hygroscopic hydrogel underlayer have been evaluated to integrate radiative and evaporative cooling [8]. Currently, used hygroscopic hydrogels include a hygroscopic matrix and a moisture absorption enhancer. The hygroscopic matrix includes poly(N- isopropyl acrylamide) (pNIPAM), Polyvinyl alcohol (PVA), or polyacrylamide (PAM), while the moisture absorption enhancer includes LiCI and LiBr salts for enhanced cooling performance. However, these hydrogel layers require a complex synthesis process, which increases the fabrication cost and hinders its extensive engineering implementations. Specifically, fabricating poly (vinyl alcohol)-CaCl2 hydrogel involves time-sensitive and labor-intensive processes such as gelation, freeze-drying, and moisture-absorbent loading. These processes contribute to positive carbon emissions and exacerbate global warming.

[0006] Thus, there is a need for a new material and a “green” fabrication process of this new material, with no carbon emissions, to further promote and advance the passive cooling technique.SUMMARY OF THE INVENTION

[0007] According to an embodiment, there is a photonic hydrogel film that includes a substrate and a hybrid cooling layer attached to the substrate and including a sodium polyacrylate (PAAS) material forming a continuous film structure. The PAAS material is hydrated by atmospheric water molecules so that chains of the PAAS material are uncoiled for maintaining the continuous film structure. The PAAS material simultaneously reflects sunlight due to a porous structure and emits infrared radiation due to molecular vibrations of the chains of the PAAS material.

[0008] According to another embodiment, there is a method for cooling a structure and the method includes placing a photonic hydrogel film on a structure, wherein the photonic hydrogel film includes a sodium polyacrylate (PAAS) material forming a continuous film structure, hydrating the photonic hydrogel film with atmospheric water molecules during night, exposing to sunlight the photonic hydrogel film during day so that the PAAS material is hydrated by atmospheric water molecules so that chains of the PAAS material are uncoiled for maintaining the continuous film structure, and the PAAS material simultaneously reflects the sunlight due to a porous structure and emits infrared radiation due to molecular vibrations of the chains of the PAAS material.

[0009] According to yet another embodiment, there is a construction material to be attached to a structure for passively lowering a temperature of the structure, and the construction material includes a substrate and a hybrid cooling layer attached to the substrate and including a sodium polyacrylate (PAAS) materialforming a continuous film structure. The PAAS material is hydrated by atmospheric water molecules so that chains of the PAAS material are uncoiled for maintaining the continuous film structure. The PAAS material simultaneously reflects sunlight due to a porous structure and emits infrared radiation due to molecular vibrations of the chains of the PAAS material.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings in which:

[0011] FIG. 1 illustrates the atmospheric transmittance versus the wavelength;

[0012] FIGs. 2A and 2B illustrate a hybrid strategy for passive cooling of building envelopes;

[0013] FIG. 3 illustrates a sodium polyacry late-based film having passive cooling properties according to an embodiment;

[0014] FIG. 4 illustrates a method for forming the sodium polyacrylate-based film of FIG. 3;

[0015] FIG. 5 schematically illustrates the transformation of powder sodium polyacrylate into the sodium polyacrylate-based film of FIG. 3;

[0016] FIG. 6 shows a cross-section of a sodium polyacrylate-based film according to an embodiment;

[0017] FIGs. 7 and 8 illustrate the water uptake of various materials and the sodium polyacrylate-based film of FIG. 3;

[0018] FIG. 9 compares the water uptake and water release of the various materials of FIGs. 7 and 8;

[0019] FIG. 10 illustrates the temperature of the various materials of FIGs. 7 and 8 when exposed to the sun;

[0020] FIG. 11 illustrates the ambient temperature and temperature around sodium polyacrylate-based films;

[0021] FIG. 12 illustrates the reflectance and emittance spectra of the sodium polyacrylate-based film compared to a cooling wood, hierarchical polymers, and multilayer photonic structures, displayed against a standard solar irradiance spectrum and a transmittance spectrum of the atmospheric window;

[0022] FIG. 13 schematically shows the sodium polyacrylate-based film formation mechanism by hydrogel bond crosslink;

[0023] FIG. 14 plots temperature variations of roof shingle, polydimethylsiloxane (PDMS), sodium polyacrylate-based photonic film (wet), and sodium polyacrylate-based (dry) film samples; and

[0024] FIG. 15 is a flow chart of a method for using the sodium polyacrylate- based film as an envelope of a construction for lowering a temperature inside the building with no electricity consumption.DETAILED DESCRIPTION OF THE INVENTION

[0025] The following description of the embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, about a sodium polyacrylate (PAAS)-based material that possesses both radiative cooling and thermal emission and can be manufactured at a low cost with effective and scalable processes. However, the embodiments to be discussed next are not limited to only a PAAS- based material but may be applied to variations of this material, as discussed later.

[0026] Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification does not necessarily refer to the same embodiment. Further, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0027] According to an embodiment, a PAAS powder is transformed into a white and continuous photonic hydrogel film (also called “PAAS-based material,” or simply “film” in this disclosure) using only atmospheric moisture as a water source, thus creating an environmentally friendly photonic hydrogel film with a carbon-freemanufacturing process. Note that the acrylate (A) in PAAS can be a salt, ester, and / or a conjugate base of acrylic acid. The acrylate ion is the anion CH2=CHCO“2. The term “acrylate” often refers to esters of acrylic acid, the most common member being methyl acrylate. These acrylates contain vinyl groups. These compounds are of interest because they are bifunctional: the vinyl group is susceptible to polymerization, and the carboxylate group carries myriad functionalities.

[0028] The obtained photonic hydrogel film achieves radiative and evaporative cooling. The porous structure of the photonic hydrogel film allows efficient sunlight reflection, which reduces solar heating. Additionally, the intrinsic molecular vibrations of the PAAS polymer chains of the photonic hydrogel film provide a high mid-infrared emittance, which expedites radiative heat dissipation through the atmospheric window. These two features of the photonic hydrogel film are called “hybrid passive cooling.” In addition, the photonic hydrogel film can store water harvested from the atmosphere at night, which can be employed for daytime evaporative cooling to enhance its cooling effect. For example, under a partly cloudy sky with a solar intensity of 800 W nr2, a sub-ambient temperature reduction of 5°C (from 37°C to 32°C) was achieved with the photonic hydrogel film due to the hybrid cooling mechanism. This performance is superior to conventional radiative cooling systems (i.e. , the pure radiative cooling sample climbed 2°C above the ambient temperature under identical environmental conditions).

[0029] In one embodiment, the photonic hydrogel film can be recycled through crushing and re-crosslinking (i.e., self-healing) with minimum cost, as discussed later, which maintains its optical performance, prolongs its lifespan, and hencedecreases the lifespan cost. This PAAS-based photonic film's hybrid passive cooling feature expands its potential applications in conditions of weakened radiative cooling in high-humid weather or in urban areas where high-rise buildings restrict access to the open sky. In one embodiment, this moisture-absorption-induced PAAS-based hydrogel film facilitates a micro-water cycle for passive cooling, representing a prospective usage of atmospheric moisture. Combined with its environmental sustainability during manufacturing, implementing this passive cooling film will result in net harmful carbon emissions in the long-term deployment.

[0030] Passive radiative cooling recently emerged as a transformative cooling technique that potentially provides “carbon-free” space cooling. It is considered a sustainable alternative to conventional cooling technologies by exploiting the balance of radiative heat flow between terrestrial objects and outer space. It achieves cooling by manipulating the balance of radiative heat flow between terrestrial objects and outer space. As illustrated in FIG. 1 , the Earth’s atmosphere has a transparency window 100 for electromagnetic waves between 8-13 pm, corresponding to the peak thermal radiation spectral range of terrestrial structures at typical ambient temperature (e.g., ~20°C to 45°C in tropical areas). This infrared transparent window 100 is a cooling channel through which a thermal body on Earth’s surface can radiate heat into the cold outer space.

[0031] To realize daytime radiative cooling, the photonic hydrogel film meets two criteria: (1) for the solar wavelength range (0.3-2.5 pm), strongly reflects solar irradiance to minimize solar heating, and (2) in the long-wavelength infrared range (LWIR, specifically 8-13 pm), spontaneously emits thermal radiation into theuniverse through a clear sky. The photonic hydrogel film achieves these spectrally selective features, which will be discussed herein.

[0032] The photonic hydrogel film discussed herein is configured to achieve one or more of: high solar reflectance and high infrared thermal emittance for efficient radiative cooling, manufactured by a completely green processing by taking advantage of ambient water vapor, has a super reliable functionality for an extended working lifetime, and has good adhesion onto various building surfaces.

[0033] The two passive cooling strategies discussed above i.e., radiative cooling and evaporative cooling, are integrated into a single-layered system (the photonic hydrogel film) to enable a more robust cooling capability that is less dependent on weather conditions. During the daytime, as illustrated in FIG. 2A, the photonic hydrogel film 200 reflects sunlight 202 and emits thermal radiation 204, enabling strong radiative cooling. In addition, film 200 evaporates the moisture 206 stored in the film for evaporative cooling, which is not dependent on atmospheric transparency. During nighttime, as illustrated in FIG. 2B, the photonic hydrogel film 200 absorbs moisture 208 from the air and restores its evaporative cooling capability for the next day's cooling operation. This dual functionality enables a stronger cooling potential to largely overcome the challenges faced by radiative cooling technologies

[0034] The general principle of radiative cooling is now discussed. A blackbody radiator facing the sky emits a power of PB centered around 10 pm wavelength. It also receives an incoming radiation power of PA from the atmosphere. Since the emissivity of the atmosphere e is zero within the atmospheric window, theincoming radiation power is mainly outside the transparent window. In practice, clouds and water vapors cause a small non-zero emissivity, resulting in a small portion of radiation energy in the atmospheric transparent window. In addition to radiative energy, the blackbody also gains heat from the environment because its temperature is below ambient. This undesirable parasitic heat gain can be described as Pc= h (Ta- Te). Here Teand Taare the tadiator and tambient temperatures respectively, and h is the heat transfer coefficient ranging from 0 to 10 W nr2K’1, depending on thermal insulation. The net cooling power can be obtained as Pn= PB- PA~ Pc- Thus, the net radiative cooling power is a function of the temperature of the radiator, which has a unity emissivity within the transparent window and zero outside. This radiator can create a cooling power of around 100 W nr2when it is at ambient temperature. As it cools down, the net cooling power decreases, i.e., when the net cooling power reaches zero, it reaches equilibrium. In the ideal case when the transparent window has zero emissivity, the radiator can reach the lowest temperature of 262.2 K. While this temperature is quite impressive, in practice, the performance of the radiator degrades rapidly with increasing atmospheric emissivity. However, the incoming heat introduced by solar illumination will change PA and PC significantly for daytime cooling. Therefore, realizing the cooling effect during daytime under high-intensity direct sunlight in desert climates is a challenge.

[0035] Earlier attempts by the inventors to overcome these problems and to generate radiative cooling building envelopes were based on the fact that solar wavelengths from 0.38 pm to 2.5 pm are required for rejecting solar heating effects and high infrared thermal emittance from 5.0 pm to 20 pm to emit thermal energy tothe outer space effectively. The inventors identified that transparent polydimethylsiloxane (PDMS) films can be implemented in a designed architecture to realize high-performance radiative cooling. A 100-pm-thick PDMS layer was coated on an Al mirror and characterized its optical absorption and thermal emission spectra. This thin film is almost transparent in the visible range (with the averaged optical absorption less than 5% in the wavelength range of 400-700 nm). In contrast, its thermal emission in the 8-13 pm range is over 95%. According to experimental validation, this thin film emitter can realize a temperature drop of 11°C below the ambient temperature in the controlled laboratory. The inventors recently improved this radiative cooling architecture and demonstrated a temperature drop over 12°C below the ambient outdoors.

[0036] The fabrication processes for the existing hybrid cooling materials involve hazardous chemicals, which can hinder their ability to be mass-produced. The inventors manufactured the PDMS film discussed above through a sustainable process using microsugar templates. By substituting the chemicals for sugar, the manufacturing procedure produces zero toxic waste and can be endlessly recycled via methods widely used in the sugar industry. The crystal sugar cubic with a 100- 600 pm particle size is employed as the template. The PDMS precursor thoroughly infiltrated the sugar template's pores, then curing and dissolving the sugar with 60°C water. The final porous PDMS sponge exhibits strong visible scattering to reject solar irradiance. Over wavelengths from 0.35 pm to 1 .7 pm, the solar absorptance of porous PDMS sponge is only 0.07, which is much smaller than that of the pristine PDMS film with an absorptance of 0.12 due to the backscattering of microporesintroduced by templating dissolving. The porous PDMS samples show strong, omnidirectional thermal emission in the infrared wavelength range.

[0037] During a 12-hour outdoor experimental measurement, the pristine PDMS sample's temperature was always higher than the ambient due to the direct solar heating effect. In contrast, the porous PDMS sponge realized continuous daytime radiative cooling without needing sunlight shelter. The measurements performed by the inventors (not shown) indicate that the porous PDMS sponge maintains a temperature reduction of 3.2 ± 1 .0 °C below the ambient. The pristine PDMS film can only realize a temperature of 2.0 ± 1 .4 °C higher than the ambient. This experiment demonstrates that the porous PDMS sponge, when used as a roofing material, can reduce the heat leakage to the space under the roof by about 90% compared to a commercial asphalt shingle product. This can efficiently suppress the heat exchange with direct sunlight or the environment, allowing for the development of new energy-efficient building envelope material. This sugar- templating method has also resolved the hazardous inorganic solvent issue other hybrid cooling materials face during the fabrication process.

[0038] However, this method is unsuitable for scalable fabrication due to the limited size of the sugar template. Moreover, the cost of PDMS is still high above the scale-up deployment requirement.

[0039] The method and film now discussed overcome these problems. FIG. 3 shows a film 300 configured to implement the hybrid cooling discussed above, with low cost and a simplified manufacturing process. As illustrated in the figure, an optional top layer 302 possesses high solar reflectivity 310 and mid-infraredemissivity 312 to facilitate radiative cooling. In one application, the top layer 302 can be any white film made of polymer, plastic, cotton, wood, and / or metal wires. The desired feature for this layer is to white in color (or other color that does not reflect light) and it is permeable for moisture. For example, the top layer 302 may be porous PE or PTFE. Meanwhile, a bottom layer 304 (also called “hybrid cooling layer”) serves the purpose of moisture absorption (e.g., due to moisture absorption particles 306 and / or fibers 308) during nighttime and water release for evaporative cooling during daytime. In one embodiment, a single layer (e.g., the bottom layer 304) achieves the functions of both the top and bottom layer, i.e. , the film 300 includes a single layer 304. The bottom layer 304 in one embodiment includes PAAS fibers 308 and CaCl2 particles 306. Note that the CaCl2 particles 306 are optional in this embodiment. This combination retains a large amount of water 309. PAAS 308, also called water lock, is a super-absorbent polymer made of repeated chains of acrylate compounds with many other favorable features, including mechanical durability and excellent thermal resistance.

[0040] Recently, renewed interest in this anionic polyelectrolyte material emerged due to new applications in expansion microscopy for bioimaging and pressure-sensitive adhesives. Generally, it is commercially available in the form of white powders due to the backscattering of ambient light and has been utilized in many daily commercial products, like baby diapers. These white particles can strongly backscatter incident sunlight, which is a desirable property for radiative cooling. However, the powder form in a dry state makes PAAS inconvenient formany practical applications like building envelopes and personal thermal management.

[0041] As illustrated in FIG. 4, the dry PAAS powders 410 are uniformly distributed by a powder feeding device 412 on a plastic film (e.g., polyacrylic substrate) 414 by a blade coating approach (see blade 416 in the figure, which controls the thickness of the PAAS powders). In one embodiment, the height of the feeding device 412 relative to the substrate 414 determines the thickness of the final film 300. The powders and substrate are placed in an open field at midnight to absorb atmospheric moisture 309. Alternatively, steam 418 may be injected by a steam heater (not shown) so that the PAAS becomes a wet film 420. In one embodiment, if the film is made indoors, the steam heater sprays moisture at about 20-25 g min'1for 20 mins, and the temperature is regulated to be between 65 and 70 °C. The PAAS particles 308 are activated by the water vapor molecules 309 (from steam 418 if the film is made indoors, or if the film is made outdoors, the water vapor molecules are naturally absorbed from the ambient) and form a continuous film of uniform density by a hydrogen bond between the water molecules. A drying process follows by applying heat 422 with a heater 424 (if indoors) or at room temperature (if outdoors) to form the PAAS dry film 300. The final film 300 may be rolled, as schematically illustrated in the figure. The process may occur at a relative humidity of 60% or above for about 6 hours if performed during the night in an open environment.

[0042] The same process is schematically illustrated in FIG. 5 for showing an actual film 300 built by the inventors. FIG. 5 displays the transformation process ofthe PAAS photonic film, starting from the raw material of white PAAS dry powder 410 with a diameter of around 100 pm (i.e., a layer of PAAS powder 410 of about 1 mm). By spraying moisture over its top surface and swelling, a wet film 420 is formed, which is dried to obtain a free-standing white film 300 with dimensions of 1 m x 0.4 m x 3 mm.

[0043] The atmospheric moisture-induced process can even fabricate the PAAS photonic film 300 without artificially spraying water. It involves spreading the dry PAAS powders 410 (thickness of about 2 mm) on a flat substrate in an outdoor environment with a high relativity humidity from 65% to 90% at night. A continuous film 300 can be generated within 6 hours during the night. According to an experiment under the ambient temperature of around 22°C, the minimum relative humidity for dry PAAS powders 410 to form a continuous film 300 is about 60%. This “natural” and “green” method further simplifies fabrication, minimizing large-scale deployment labor and energy requirements. Moreover, color pigments can also be introduced in the structure of layer 308 to form a colorful PAAS photonic film 300 without affecting the formation of the continuous film, which increases the versatility of PAAS photonic film for aesthetic purposes. In addition, the PAAS photonic film can also be applied onto diverse substrates with different textures like plastics, wood, and metals, which broadens its application scenarios like roofing, outdoor electronics, automotive, and industrial.

[0044] It is noted that film 300 has enough strength to be suspended from its ends without needing a substrate. The optional top layer 302 may be added to the film 300 before being rolled. Due to their surface chemical features, these two filmsmay be easily pressed and adhere to each other. An industrial mechanical rolling machine can be used to press them together. In another embodiment, it is possible to directly add the PAAS or PAM powder on the layer 302, during the manufacturing. They may naturally stick to each other. In addition, considering the natural rain that may degrade the PAAS-based layer, the layer 302 can introduce hydrophobicity so then the liquid water cannot go through, only the moisture can penetrate through.

[0045] A corrugated cover (not shown) with holes on the sides may be employed to protect the film from wind or rain and to keep regular air circulation through those holes simultaneously. While the process shown in FIG. 4 uses steam 418 and heat 422 produced by a heater 424, in one embodiment, steam 418 is replaced by the natural moisture in the air, and heat 422 is replaced by the sun heat. In this way, the process is completely green, i.e. , it does not use any electricity or other resources and does not generate any harmful chemicals. The initial PAAS dry power 410 can be applied on different substrates, such as plastic, wood, or metal 602, as schematically illustrated in FIG. 6. The figure shows the entire film 300 and its substrate 602 being placed / attached to a structure 603, for example, a building. The film 300 and its substrate 602 (which form a construction material 600) may be used as a roofing material or as the walls of the structure 603. After absorbing moisture, these PAAS dry powders 410 turn into the continuous film 304 and form a bond with the underneath substrate 602. In one embodiment, an adhesive 604 is first applied to substrate 602. Then, the PAAS dry powder is processed to form the film 300, which is adhered with adhesive 604 to substrate 602 to enhance the adhesion of the PAAS-based layer 300 to substrate 602. Thus, the final film 300, together withits substrate 602, may be used in various applications, for example, wooden roofs of buildings, metal cases of outdoor electronics, plastics in automotive and other industries, etc.

[0046] In one embodiment, a functional layer is added to the photonic hydrogel film 300. FIG. 6 shows a cross-section through the photonic hydrogel film 300, which includes the optional substrate 602, the optional adhesive 604, the bottom layer 304, the optional top layer 302, and an optional functional layer 610, located over the top layer 302 (if present), otherwise directly located over the bottom layer 304. The functional layer 610 may be a melamine layer coated with PDMS. This type of layer protects the bottom layer 304 and top layer 302 from rain (e.g., excessive water intake) while maintaining high solar reflectivity and thermal emissivity. Other types of functional layers may be used. If the bottom layer 304 is fixed to the substrate 602, for example, wood, plastic, metal, or other construction material. Such a modified film 300 (i.e., including the substrate 602 and the PAAS or a PAAS-AN-based layer 304, with optional reflecting layer 302 and / or functional layer 610) may be used in constructions, to provide an envelope for a building. The film 300 may also be used in other industrial fields.

[0047] In another embodiment, functional layer 610 is selected to extend the durability of film 300 in harsh environments for long-term spectral selectivity. In this regard, the real-world aging of cool roofing materials mainly includes soiling and weathering. Soiling mostly results from the deposition of airborne carbon black, dust, particulate organic matter, and microbial growth. Weathering is primarily from exposure to UV irradiation of natural sunlight, accompanied by moisture andtemperature variations of day and night. To achieve self-cleaning functionality, a super-hydrophobic surface may be used. Thus, choosing the functional layer to achieve super-hydrophobicity and UV resistance enhances the film's performance in terms of soil resistance and durability for passive cooling buildings.

[0048] The PAAS-based material 410 was mixed with one or more materials, in one embodiment, before forming the film 300. Various films were made of pure PAAS, pure LiCI, pure CaCI2, pure AN, and mixtures of PAAS+LiCI, PAAS+CaCI2, and PAAS+AN. In one application, the ratio of PAAS particles to AN particles is about one-to-one. Other ratios may be used. These films were studied as dry powders, in a moisturized state, and a dry state. Only the PAAS and PAAS+AN films maintained the stable solid film structure in all states. The other films formed solutions during the moisturized state. Thus, the bottom layer 304 of film 300 may be made of PAAS or PAAS+AN but not from the other materials. FIGs. 7 and 8 show the water uptake (i.e. , the amount of water absorbed by the bottom layer 304, if the top layer 302 is not present, during the night for film 300) of the films noted above. It is noted that the water uptake of the PAAS+AN film improves over the pure PAAS film. As the moisture absorption of AN is an endothermic process, this results in an improved cooling effect. The cooling effect lowers the temperature of the mixture, which facilitates moisture absorption.

[0049] Further, the inventors have observed that the water release of the PAAS-based film increases with the addition of AN. Note that it is possible to mix AN powder with PAAS powder in the initial phase of the manufacturing process and then mix them uniformly. In one application, it is also possible to add AN later when thePAAS foam is formed. The addition of AN improves the water uptake capability, which in turn increases the water release rate. FIG. 9 shows both the water sorption and release capabilities for the considered films. It is noted that although desiccant materials like LiCI and CaCl2 have a higher water uptake ability, they become liquid during the manufacturing process, and thus, they cannot be used for generating the envelope of a building. As the moisture absorption of PAAS is an exothermic process that generates heat, and the moisture absorption of AN is an endothermic process, mixing PAAS and AN lower the solution temperature, which is desired. FIG. 10 shows the temperature profile of the various materials during moisture absorption at 80% relative humidity (RH). Further, PAAS + AN film shows better evaporative cooling performance compared to PAAS-baesd film, as illustrated in FIG. 11 .

[0050] In one embodiment, the amount of water required to form a stable bottom layer 304 (PAAS or PAAS + AN) is between 0.5 to 0.8 g / g for a thickness of 2 to 3 mm. The thickness of the bottom layer 304 (i.e. , PAAS or PAAS + AN) may be about 2 to 3 mm. The temperature at which film 300 is made may be between 5 and 65 C, the relative humidity at which film 300 is made may be between 60 and 99%, and the time necessary for making film 300 is between 0.5 and 6h.

[0051] As discussed above, film 300 (mainly due to the bottom layer 304) harvests water at night, and the harvested water evaporates during the day and carries the heat away from the film upon heating, for example, under the illumination of sunlight during the daytime period. In this situation, the porous structure of film 300 (due to either the bottom layer 304 or the top layer 302, or the functional layer 610) backscatters sunlight to reduce the solar heating effect. Simultaneously,thermal radiation dissipates heat to outer space through the atmospheric window (discussed above in FIG. 1 ). As a result, film 300 can achieve hybrid passive radiative cooling due to its highly scattering feature and high thermal emittance from the molecular vibrations of the PAAS polymer chains.

[0052] Advantageously, the PAAS-based hydrogel film 300 can regenerate itself by absorbing moisture when the ambient humidity increases (e.g., at nighttime). The evaporation and self-adsorption processes form a perfect daily water cycle that synchronously facilitates the heat absorption and release process. In addition, the entire manufacturing is amenable to industrial roll-to-roll processes (including PAAS dry powder feeding, moisture-induced crosslinking, and natural air drying, as illustrated in FIG. 4).

[0053] Apart from its scalable and facial fabrication technique, the freestanding PAAS-based photonic film 300 also shows excellent whiteness with a solar reflectance of 0.93 in its dry state, as shown in FIG. 12 (solid curve 1200). The strong solar reflectance results from the efficient backscattering feature of porous structures. The PAAS-based photonic film 300 displays a thermal emittance of 0.99 over the atmospheric window (see curve 1200 over the 8 - 13 pm region in FIG. 12), which is higher than multilayered photonic structures 1202 and cooling wood 1204 and is comparable to the hierarchical polymers 1206. All these results are displaced in FIG. 12 against the standard solar irradiance spectrum 1210 and the transmittance spectrum of the atmospheric window 1212.

[0054] It should be noted that the general photonic structures require complicated nanofabrication equipment. For instance, cooling wood relies on thebleaching process to remove the lignin from the pristine wood and the heat-pressing process to form a densified form of microstructures. The bleaching process requires the use of chemicals, such as sodium hydroxide, sodium sulfite, or hydrogen peroxide. Meanwhile, the subsequent overnight heat-pressing process also requires extra energy consumption and an extended processing time (e.g., overnight). Moreover, the phase inversion process for obtaining the hierarchical polymer films involves utilizing many volatile and harmful organic solvents (e.g., acetone).

[0055] In contrast, the environment-friendly fabrication method for producing PAAS-based photonic films illustrated in FIG. 4 uses atmospheric moisture and cost- effective raw materials, demonstrating the advantage of an industrial-level “green” manufacturing process. The PAAS (or PAAS-AN) photonic film 300 with a thickness of 2 mm manufactured using this “green” process exhibits excellent flexibility, as demonstrated by its ability to recover after being severely folded, rolled quickly, or stretched. In addition, the PAAS (or PAAS-AN) photonic film 300 can be stretched to approximately 1 .8 times its original length before a tearing occurs. Advantageously, after such a tearing occurs, the PAAS photonic film 300 could be reconnected due to the healing process by simply applying 90%-95% relative humidity for 6 hours (overnight). The “healed” PAAS-based photonic film 300 can be stretched up to 1 .5 times without experiencing a mechanical failure. The mechanical flexibility and self- healing ability of the PAAS (or PAAS-AN) photonic films 300 potentially reduce the maintenance cost of numerous outdoor applications, particularly in radiative or evaporative cooling systems.

[0056] In addition to the mechanical and self-healing advantages, PAAS- based photonic film 300 can be easily recycled using the usual steps of freezing the used film with liquid nitrogen, crushing it into powders, and forming the recycled film by moisturizing the mixture of crushed powders with pristine PAAS powders. The recycled PAAS photonic film has a reduction of 0.02 in solar reflectance while keeping its original mid-infrared thermal emittance, extending its lifetime without significantly sacrificing the optical performance. Overall, the cost-effective material (~ $1.2 nr2), high-yield manufacturing process, mechanical robustness, and self-healing capability of the PAAS photonic film render it a promising candidate for large-scale passive cooling applications.

[0057] The physical mechanism of the unique optothermal properties of the PAAS photonic film is next discussed. FIG. 13 depicts the mechanism behind forming a mechanically robust continuous film from friable PAAS powder. The long chains of PAAS polymer are initially coiled up within the dry powder. When the dried chains absorb water, they stretch out due to the presence of a negative charge within each repeating unit of PAAS, i.e., as water is added to the polymer. These areas form negatively charged ions that repel one another and cause the polymer to stretch out. Observation under an optical microscope reveals that, in the presence of moisture, the PAAS powders 410 swell, come into contact with each other, and ultimately form a continuous film 300. When adjacent powders connect during swelling, hydrogel bonds form between different PAAS molecules, forming a crosslinked network.

[0058] The Raman spectra explain the formation of hydrogen bonds between adjacent PAAS powders with different water contents. The peaks at 868 c r1and 898 cmr1(not shown) are attributable to the C-COO- stretching vibrations and the out-of-plane bending in the -OH of C-COOH, respectively. With the increasing water content, the peak at 868 cmr1weakens while the peak at 898 cmr1increases because the degree of ionization of PAAS increases. The Fourier transform infrared spectrum (FTIR) (not shown) elucidates the corresponding molecular vibrations of chemical bonds for the moisturized PAAS overlapping with the atmospheric window. The absorption peak at 3,520 cmr1denotes the H-O-H bond stretching vibration of interlayer water molecules of PAAS after absorbing water. The peak at 1 ,410 cmr1is ascribed to the -(CH2)n“ plane rocking vibrations and carbonates. The peak at 1 ,560 cmr1corresponds to the carboxylate (COO-) radical vibration and C=H asymmetric stretching vibration. The -OH of PAAS gives it a unique feature of moisture adsorption in high-humidity conditions which is essential for evaporative cooling, while chemical groups of -(CH2)n“ and (COO-) offer high thermal emittance over the atmospheric window.

[0059] To investigate the water absorption capability of the PAAS-based photonic film 300, a piece of PAAS sample is placed into a humidity- and temperature-adjustable chamber to record its mass change over the relative humidity. The humidity inside the chamber is adjusted by intermittent work of a humidifier and dehumidifier controlled by a proportional-integral-derivative (P ID) controller. The water absorption rate of PAAS photonic film ranges from 0.05 g g-1(relative humidity, 50%) to 0.27 g g~1(relative humidity, 90%). Thermogravimetry(TGA) characterization manifests that the evaporation water in PAAS-based photonic film 300 is around 41% of its total mass, making it possible for evaporative cooling. Heating the PAAS-based photonic film 300 above 200°C would cause a significant color change that is followed by burning into char, demonstrating its excellent optical and thermal tolerance. Differential scanning calorimetric (DSC) (not shown) explains the evaporation enthalpy of water in the PAAS hydrogel network. The DSC thermogram indicates a sharp peak at 100°C, which is followed by a rapid decrease in heat flux, implying quick water evaporation. Notably, the heat flux signal peak for the PAAS-based photonic film is lower than that of pure water, demonstrating that the film provides a larger surface area for water evaporation, thus enhancing its evaporative cooling capability.

[0060] Adding to the great potential towards heat, the developed PAAS-based photonic films 300 also show advantageous mechanical characteristics in the superseding of the day-and-night cycle. During such a cycle, where the solar intensity and relative humidity continuously change, the mechanical properties of PAAS-based films are affected by varying the water content. Heating and cooling cycles are mimicked in the PAAS photonic films to simulate the process. The heating step is performed by employing a solar intensity of 1 ,000 W nr2, a temperature of 31.4°C, and relative humidity of 17%. The nocturnal cooling step is realized by keeping the PAAS-based photonic film in a dark chamber (zero solar intensity) at a temperature of 23.2°C and relative humidity of 86%. The hardness of the photonic film is then measured at each stage using a durometer with scale A. It was found that the hardness of the PAAS-based photonic film increased and peaked after 6hours of heating, indicating the considerably slow evaporation of water from the film. Upon cooling, the hardness of PAAS-based photonic film decreased as it absorbed water from the environment, indicating the ability of the film to recover its flexibility.

[0061] Furthermore, to quantitively characterize the flexibility and self-healing capability of the film 300, the inventors performed tensile tests on “pristine” and “cutting and healing” PAAS specimens. Here, “pristine” means that the specimens are subjected to a relativity humidity of 90-95% at 25°C, while “cut and rehealing” means that the specimens are severed into two pieces, reconnected, and subjected to relativity humidity of 90-95% at 25 °C for 6 hours. The obtained stress-strain curves (not shown) indicate that the “pristine” PAAS photonic films exhibit a strength of 25 kPa and failure strain of 72%. The rehealing process using ambient temperature with relativity humidity of 90-95% can completely recover the strength of the PAAS photonic film and most of its ductility (failure strain of 48%). It is expected that rehealing the severed specimens for a longer period (> 24 hours) may be able to recover the stretchability completely. Nonetheless, it was demonstrated that there is a great potential for the hydrogel film 300 to heal itself after being severed or cut completely. The implication of “drying” on the mechanical response of PAAS photonic film was studied by leaving the film at an ambient temperature of 25°C and relativity humidity of 55% for 60 days. It was found that the drying process can improve the strength and ductility of up to 2 times. In addition, the potential of PAAS hydrogels for shape- and volume-changing materials was found due to their compressibility.

[0062] Next, the optothermal features of PAAS-based photonic film 300 were investigated to reveal its potential for radiative cooling further. The PAAS photonic film appears as a diffused white, as seen by the naked eye. Its solar reflectance is over 0.93, resulting in minimal solar heating under direct sunlight. Compared with a mirror-like radiative cooling surface, the high solar reflectance is mostly attributed to its hierarchical architected structures. The micro-sized PAAS structures coexist with the nanostructured surface textures of hierarchical -architected porous structures, as found from the scanning electron microscope (SEM) images (not shown). As the water content in the PAAS-based photonic film increases, the visible reflectance diminishes due to the decrease in refractive index between PAAS and ambient air. Simultaneously, higher water content leads to a noticeable disparity across infrared wavelengths. Most microstructures range from 10 pm to 50 pm and 100 nm to 600 nm, respectively. As described by the refractive index extracted by the fitting of transmittance spectra (not shown), the PAAS-based film shows a negligible extinction coefficient, k. As a result, its micro- and nanostructures efficiently backscatter longer (> 500 nm) and shorter wavelengths (300 ~ 500 nm) of the sunlight, respectively, as validated by the scattering efficiency analysis. The abundant micro- or nanostructures can efficiently backscatter sunlight of solar wavelengths, where nanostructures reflect visible wavelengths and microstructures reflect visible and near-infrared wavelengths of sunlight.

[0063] To further reveal the photon scattering phenomenon, finite-difference time-domain (FDTD) simulations were conducted for four characteristic wavelengths (300 nm, 500 nm, 1 ,000 nm, and 1 ,700 nm) within the solar spectrum. It wasobserved that the propagation of long wavelengths (i.e. , > = 1 ,000 and 1 ,700 nm) penetrates much deeper into the surface compared to that at shorter wavelengths (i.e., > = 300 and 500 nm). Therefore, the PAAS porous structure is more efficient in scattering the photons at shorter wavelengths, which is also correlated with the solar reflectance reduction in FIG. 12 (see the reduced solar reflectance over near-infrared wavelengths from 1 .0 pm to 2.5 pm). When the PAAS photonic film is moisturized, it exhibits a reduced scattering effect. This can be attributed to the smaller refractive index difference between the moisturized PAAS and air, compared to the difference between dry PAAS and air. Specifically, the absorption of water by the PAAS photonic film results in a decrease in its refractive index, as the refractive indices of PAAS, water, and air are 1 .49, 1 .33, and 1 .0, respectively, at a wavelength of 500 nm. The reduced scattering is also exemplified by the reduced solar reflectance of moisturized PAAS photonic film, where the moisturized PAAS photonic film displays a reduced solar reflectance of 0.89. Compared with the dry PAAS surface, the O-H stretching vibration of absorbed water in the moisturized PAAS photonic film induces a higher thermal emittance for more efficient radiative heat dissipation.

[0064] Quantifying the water evaporation behavior of the moisturized PAAS photonic film under varying solar intensities and different humilities helps understand its cooling potential. The evaporation rate of moisturized PAAS photonic film was measured as a function of solar intensity and humidity. The indoor simulated evaporation tests reveal that water evaporation is promoted when the solar irradiance and wind speed are increased, or the ambient humidity is reduced. The evaporation rate reaches 1 .75 kg nr2h~1at 23 ± 0.5°C, relativity humidity of 20 ± 2%,and a wind speed of 1 m s-1. Under windy conditions, convection is the dominant factor for water evaporation, as the evaporation rate shows a slow increase with a change in solar irradiance intensity from 0 kW nr2to 1 .0 kW nr2. While there is no wind and the relativity humidity is high (e.g., relativity humidity is 60% and wind speed is 0 m s-1), the solar heating-enhanced evaporation is dominant since the evaporation rate increases quickly from 0.12 kg nr2h-1to 0.43 kg nr2h-1with the increase of solar intensity from 0 kW nr2to 1 .0 kW nr2.

[0065] To further reveal the physical mechanism of evaporative cooling potential under various convection scenarios, the inventors simulated the temperature reduction of the PAAS-based photonic film under various heat transfer coefficients and evaporation rates. Due to the high solar reflectance of PAAS-based photonic film, the simulated temperature reduction (AT = Tfilm- Tambient) retains prominence even when the non-radiative heat coefficient is above 20 W rm2K-1due to its high potential for evaporative cooling. The efficient solar scattering effect enables the PAAS-based photonic film to reflect sunlight, thereby reducing the heating effect. Additionally, its evaporative capability facilitates evaporative cooling, resulting in enhanced hybrid cooling performance. The unique combination of scattering and evaporation features introduces additional cooling channels besides radiative cooling. This results in a hybrid passive cooling performance that is less dependent on weather conditions.

[0066] The radiative cooling and evaporative cooling performance of the film 300 was also studied. The spectral selectivity of PAAS-based photonic film over solar and thermal wavelengths makes it achieve hybrid sub-ambient cooling (i.e. ,radiative cooling and evaporative cooling). To decouple the combined effect of these two cooling mechanisms, the inventors separately measured the temperature reduction of PAAS-based photonic film by controlling the experimental settings.

[0067] For the radiative cooling measurement under a clear sky, a polyethylene (PE) film (not shown) was placed over the experimental chamber at a location of 1 cm above the PAAS-based photonic film 300. The PE film is broadly transparent over both solar and infrared wavelengths (0.3 - 25 pm), and it does not block the sunlight and the infrared thermal dissipation. This PE film also prevents the water absorption process of the dry PAAS-based photonic film. Therefore, the temperature drop is mainly introduced by the radiative cooling effect. An outdoor experiment was performed with a solar intensity of approximately 400 W nr2from 3:00 PM to 4:30 PM and an ambient temperature ranging from 16°C to 20°C during the experiment. The temperature reduction of the PAAS-based photonic film stabilizes around 3.7°C below the ambient. The theoretical thermal balance analysis shows that the net radiative cooling power is 76 W nr2under this relatively cool weather condition.

[0068] To evaluate the evaporative cooling performance, the inventors first monitored the temperature reduction and evaporative cooling power in a temperature and humidity-controlled chamber (not shown). In this chamber, the PAAS-based sample adheres to a copper (Cu) plate (which acts as a heat spreader and supports the film), while a Kapton thin film heater is attached to the back of the Cu plate to provide heating power. The PAAS-based sample sides are covered with foam to exclusively characterize evaporative cooling from the top surface. A maximumtemperature drop of approximately 6°C is recorded 20 minutes after initiating the experiment, and the temperature stabilizes at 2.5°C below ambient after roughly 130 minutes. The mass change continually increased while its weight loss rate gradually decreased as evaporation continued from the top surface of the PAAS-based photonic film. This occurs because the time required for water from the bottom portion to diffuse to the top surface increases, decreasing the weight loss rate. As the water absorbed on the film’s top surface evaporates, it takes longer for water from the bottom part of the film to migrate to the top surface, extending the diffusion path to the top evaporative cooling surface. Consequently, the temperature reduction induced by evaporative cooling gradually diminishes.

[0069] To characterize the evaporation cooling power, the sample is attached to a Cu plate on top of a heater, which a PID controller controls to ensure that the PAAS-based sample surface temperature is in equilibrium with the ambient temperature. The heating power can be considered the cooling power because the back of the heater is thermally insulated. When the thin film heater and the PID controller are activated, the temperature of the PAAS-based sample is adjusted to match the ambient after 43 minutes. Afterward, the evaporation cooling power fluctuates around 190 W nr2and gradually decreases due to the water loss during continuous evaporation, which can be observed from the decreasing slope of the fitted curve (not shown).

[0070] The inventors also tested dry and moisturized PAAS-based photonic films alongside two other sample-roof shingles, a common building material for roofs, and transparent PDMS coated on a polished aluminum plate (PDMS (T)) underidentical outdoor conditions to demonstrate the overall hybrid cooling performance of moisturized PAAS-based photonic film. According to experiment data, the PAAS- based photonic film 300 thickness is optimized to be 2 mm. This thickness balances enhanced mechanical robustness and increased moisture absorption, facilitating prolonged and continuous evaporative cooling.

[0071] During the experiment, the average solar intensity was 800 W nr2’ and the average relativity humidity was 60%. At noontime, as illustrated in FIG. 14, the commercial shingle without PE cover film reaches 40°C above ambient, further validating the need for photonic cooling materials to save energy. The dry PAAS sample reaches 2°C above the ambient without evaporative cooling enhancement, while the PDMS (T) sample temperature is 4°C above the ambient due to direct solar illumination (with a peak solar intensity of 920 W nr2). In contrast, the PAAS-based photonic 300 film exhibited a sub-ambient temperature drop of 5°C, demonstrating the superior performance of hybrid cooling in partly cloudy weather. During the nighttime, the PDMS (T) and shingle temperatures are both below that of the moisturized PAAS-based sample, even though they had similar infrared emittance spectra. Due to the moisture adsorption into the PAAS-based film at night, the regeneration process generated heat, resulting in a higher temperature than the PDMS (T) and shingle samples.

[0072] As day and night alternated, the moisturized PAAS-based photonic film evaporates and absorbs moisture from the air, resulting in a hybrid cooling performance. The cooling performance of the moisturized PAAS-based photonic film 300 surpassed that of dry PAAS and PDMS (T) with only radiative cooling capabilityand commercial shingles with high solar absorption. Furthermore, under a clear sky, the temperature of the moisturized PAAS-based photonic film (with hybrid passive cooling) is 7.2°C below the dry PAAS sample (with radiative cooling only), confirming the advantage of hybrid passive cooling over radiative cooling under a clear sky condition.

[0073] An additional 3-day experiment involving moisturized PAAS-based photonic film 300 under diverse weather conditions - including a sunlit day with smoky air, a partially cloudy and warm day, and a predominantly cloudy day - underscores the superior efficacy of the hybrid mechanism relative to pure radiative cooling. It was found that the continuous water evaporation from the PAAS-based photonic film remains viable for an impressive span exceeding 9 hours. This endurance reaffirms the capability of hybrid cooling to operate effectively throughout a significant portion of the daytime. The PAAS photonic film 300's cooling performance was also investigated under a steady heat source within an enclosure, similar to real-life situations. The results demonstrate the hybrid cooling of PAAS photonic film outperforms pure radiative cooling, elucidating the marked advantage of our enhanced heat dissipation capability compared to the conventional pure radiative cooling sample.

[0074] The above-discussed embodiments disclose a spectrally selective film for scalable hybrid passive cooling by employing atmospheric moisture-induced polyacrylate hydrogels. The moisture absorption facilitates the transfiguration of PAAS hydrogel from loose powders to continuous and flexible films. A dynamic water cycle is formed by moisture absorption at nighttime and evaporation cooling inthe daytime. The strong hydrogel link between neighboring PAAS particles guarantees mechanical robustness for long-term engineering applications. Furthermore, the dry PAAS photonic film exhibits a high reflectance of 0.93 over the broadband solar wavelengths resulting from the efficient backscattering of these randomized photonic pores. It also shows a high infrared thermal emittance of 0.99 over the atmospheric transparent window due to its intrinsic molecular vibrations. This spectral selectivity enables efficient passive cooling, featured by a temperature reduction of 5°C under a partly cloudy sky under a solar intensity of 800 W nr2. This PAAS-based photonic film (optionally modified with AN, or enhanced with a top reflecting layer and / or a functional layer) simultaneously brought evaporative cooling that is independent of access to the clear sky assisted by its hygroscopic feature. This hybrid passive cooling strategy is expected to reduce global carbon emissions compared to current electricity-driven air-conditioning facilities. The scalable and economical hygroscopic hydrogel has the potential to drive further development of passive cooling applications, including infrastructure cooling of electronic devices and vehicles, and outdoor personal thermal management. Moreover, the novel PAAS-based photonic film's passive cooling properties could benefit oil tankers, refrigerated ships, and trucks that maintain relatively low temperatures to preserve food and temperature-sensitive goods. Importantly, the entire atmospheric moisture process utilizes no additional chemicals, making it a completely environmentally friendly and high-yield production with zero carbon emissions and even harmful emissions from saved energy consumption in cooling.

[0075] The film 300 and / or construction material 600 including the film 300 may be used to cool a structure, e.g., a building, as now discussed. A method for cooling the structure includes a step 1500 of placing the photonic hydrogel film on the structure, where the photonic hydrogel film includes a sodium polyacrylate (PAAS) material forming a continuous film structure, a step 1502 of hydrating the photonic hydrogel film with atmospheric water molecules during night, and a step 1504 of exposing to sunlight the photonic hydrogel film during day so that the PAAS material is hydrated by atmospheric water molecules so that chains of the PAAS material are uncoiled for maintaining the continuous film structure, and the PAAS material simultaneously reflects the sunlight due to a porous structure and emits infrared radiation due to molecular vibrations of the chains of the PAAS material.

[0076] The method may have the photonic hydrogel film include a substrate and a hybrid cooling layer attached to the substrate, the hybrid cooling layer including the PAAS material. In one application, a mass of the atmospheric water molecules relative to a mass of the PAAS material is between 0.5 to 0.8 g / g for a thickness of 2 to 3 mm. The hybrid cooling layer further includes AN. A mass ratio between the PAAS and AN is about 1 :1. The substrate includes one of wood, metal or plastic.

[0077] The term “about” is used in this application to mean a variation of up to 20% of the parameter characterized by this term.

[0078] It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. Forexample, a first object or step could be termed a second object or step, and, similarly, a second object or step could be termed a first object or step, without departing from the scope of the present disclosure. The first object or step, and the second object or step, are both, objects or steps, respectively, but they are not to be considered the same object or step.

[0079] The terminology used in the description herein is for the purpose of describing particular embodiments and is not intended to be limiting. As used in this description and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term "and / or" as used herein refers to and encompasses any possible combinations of one or more of the associated listed items. It will be further understood that the terms "includes," "including," "comprises" and / or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. Further, as used herein, the term "if" may be construed to mean "when" or "upon" or "in response to determining" or "in response to detecting," depending on the context.

[0080] The disclosed embodiments provide a PAAS-based hydrogel film that lowers a substrate temperature without electricity consumption, thus, cooling down its substrate relative to the ambient. It should be understood that this description is not intended to limit the invention. On the contrary, the embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit andscope of the invention as defined by the appended claims. Further, in the detailed description of the embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.

[0081] Although the features and elements of the present embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.

[0082] This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.ReferencesThe entire content of all the publications listed herein is incorporated by reference in this patent application.

[0001] D. K. Nandakumar, Y. Zhang, S. K. Ravi, N. Guo, C. Zhang, and S. C. Tan, “Solar energy triggered clean water harvesting from humid air existing above sea surface enabled by a hydrogel with ultrahigh hygroscopicity,” Advanced Materials, vol. 31 , no.10, p. 1806730, 2019.[2] S. Pu, J. Fu, Y. Liao, L. Ge, Y. Zhou, S. Zhang, S. Zhao, X. Liu, X. Hu, K. Liu, and J. Chen, “Promoting energy efficiency via a self-adaptive evaporative cooling hydrogel,” Advanced Materials, vol. 32, no. 17, p. 1907307, 2020.[3] C. Feng, P. Yang, H. Liu, M. Mao, Y. Liu, T. Xue, J. Fu, T. Cheng, X. Hu, H. J. Fan, and K. Liu, “Bilayer porous polymer for efficient passive building cooling,” Nano Energy, vol. 85, p. 105971 , 2021.[4] X. Chang, S. Li, N. Li, S. Wang, J. Li, C. Guo, L. Yu, P. Murto, and X. Xu, “Marine biomass-derived, hygroscopic and temperature-responsive hydrogel beads for atmospheric water harvesting and solar-powered irrigation,” Journal of Materials Chemistry A, 2022.[5] F. Ni, P. Xiao, C. Zhang, W. Zhou, D. Liu, S.-W. Kuo, and T. Chen, “Atmospheric hygroscopic ionogels with dynamically stable cooling interfaces enable a durable thermoelectric performance enhancement,” Advanced Materials, vol. 33, no. 49, p. 2103937, 2021.[6] Y. Tian, X. Liu, S. Xu, J. Li, A. Caratenuto, Y. Mu, Z. Wang, F. Chen, R. Yang, J.Liu, M. Minus, and Y. Zheng, “Recyclable and efficient ocean biomass-derivedhydrogel photothermal evaporator for thermally-localized solar desalination,” Desalination, vol. 523, p. 115449, 2022.[7] P. Liu, Y.-b. Hu, X.-Y. Li, L. Xu, C. Chen, B. Yuan, and M.-L. Fu, “Enhanced solar evaporation using a scalable mos2-based hydrogel for highly efficient solar desalination,” Angewandte Chemie, p. e202208587, 2022.[8] J. Li, X. Wang, D. Liang, N. Xu, B. Zhu, W. Li, P. Yao, Y. Jiang, X. Min, Z. Huang, S. Zhu, and J. Zhu, “A tandem radiative / evaporative cooler for weather-insensitive and high-performance daytime passive cooling,” Science Advances, vol. 8, no. 31 , p. eabq0411 , 2022.[9] T. F. Stocker and C. C. Raible, “Water cycle shifts gear,” Nature, vol. 434, no. 7035, pp. 830-833, 2005.

Claims

WHAT IS CLAIMED IS:1 . A photonic hydrogel film (300) comprising: a substrate (602); and a hybrid cooling layer (304) attached to the substrate (602) and including a sodium polyacrylate (PAAS) material (308) forming a continuous film structure, wherein the PAAS material (308) is hydrated by atmospheric water molecules so that chains of the PAAS material (308) are uncoiled for maintaining the continuous film structure, and wherein the PAAS material simultaneously reflects sunlight due to a porous structure and emits infrared radiation due to molecular vibrations of the chains of the PAAS material.

2. The film of Claim 1 , wherein a mass of the atmospheric water molecules relative to a mass of the PAAS material is between 0.5 to 0.8 g / g for a thickness of 2 to 3 mm.

3. The film of Claim 1 , wherein the hybrid cooling layer further includes AN.

4. The film of Claim 3, wherein a mass ratio between the PAAS and AN is about 1 :1 .

5. The film of Claim 1 , further comprising: a functional layer covering the hybrid cooling layer, wherein the functional layer protects the hybrid cooling layer from excessive water.

6. The film of Claim 5, wherein the functional layer includes a melamine layer coated with polydimethylsiloxane (PDMS).

7. The film of Claim 1 , wherein the substrate includes one of wood, metal or plastic.

8. The film of Claim 1 , further comprising: an adhesive layer located between the substrate and the hybrid cooling layer.

9. The film of Claim 1 , wherein the hybrid cooling layer is configured to lower a temperature behind the film, relative to an ambient temperature, with no electricity consumption.

10. A method for cooling a structure, the method comprising: placing (1500) a photonic hydrogel film (300) on a structure (603), wherein the photonic hydrogel film (300) includes a sodium polyacrylate (PAAS) material (308) forming a continuous film structure;hydrating (1502) the photonic hydrogel film (300) with atmospheric water molecules during night; and exposing (1504) to sunlight the photonic hydrogel film (300) during day so that the PAAS material (308) is hydrated by atmospheric water molecules so that chains of the PAAS material (308) are uncoiled for maintaining the continuous film structure, and the PAAS material simultaneously reflects the sunlight due to a porous structure and emits infrared radiation due to molecular vibrations of the chains of the PAAS material.

11. The method of Claim 10, wherein the photonic hydrogel film includes a substrate (602) and a hybrid cooling layer (304) attached to the substrate (602), the hybrid cooling layer including the PAAS material.

12. The method of Claim 10, wherein a mass of the atmospheric water molecules relative to a mass of the PAAS material is between 0.5 to 0.8 g / g for a thickness of 2 to 3 mm.

13. The method of Claim 11 , wherein the hybrid cooling layer further includes AN.

14. The method of Claim 13, wherein a mass ratio between the PAAS and AN is about 1 :1 .

15. The method of Claim 11 , wherein the substrate includes one of wood, metal or plastic.

16. A construction material (600) to be attached to a structure (603) for passively lowering a temperature of the structure (603), the construction material (600) comprising: a substrate (602); and a hybrid cooling layer (304) attached to the substrate (602) and including a sodium polyacrylate (PAAS) material (308) forming a continuous film structure, wherein the PAAS material (308) is hydrated by atmospheric water molecules so that chains of the PAAS material (308) are uncoiled for maintaining the continuous film structure, and wherein the PAAS material simultaneously reflects sunlight due to a porous structure and emits infrared radiation due to molecular vibrations of the chains of the PAAS material.

17. The construction material of Claim 16, wherein a mass of the atmospheric water molecules relative to a mass of the PAAS material is between 0.5 to 0.8 g / g for a thickness of 2 to 3 mm.

18. The construction material of Claim 16, wherein the hybrid cooling layer further includes AN.

19. The construction material of Claim 18, wherein a mass ratio between the PAAS and AN is about 1 :1.

20. The construction material of Claim 16, further comprising: a functional layer covering the hybrid cooling layer, wherein the functional layer protects the hybrid cooling layer from excessive water, wherein the functional layer includes a melamine layer coated with polydimethylsiloxane (PDMS), and wherein the substrate includes one of wood, metal or plastic.

Images