Passive radiation-evaporative cooling composite coating and preparation method thereof

Abstract

The invention discloses a passive radiation-evaporative cooling composite coating and a preparation method thereof. The passive radiation-evaporative cooling composite coating comprises a radiation cooling film on the upper layer and hygroscopic hydrogel on the lower layer. The radiation cooling layer with high sunlight reflectivity and high and medium infrared emissivity provides all-weather radiation cooling and protects the hydrogel on the lower layer from direct solar radiation; and the hydrogel can be evaporated and cooled in the daytime and can absorb moisture at night to realize self-regeneration. The invention has the advantages of low cost of required raw materials, simple preparation and large-scale production. The prepared double-layer cooling material has excellent and adjustable optical properties, the temperature of the double-layer cooling material is reduced by more than 15 DEG C under the direct irradiation of sunlight in the daytime, and meanwhile, the double-layer cooling material has a hydrophobic surface. As a refrigeration technology without power consumption and emission, the method is expected to reduce energy consumption and carbon emission of traditional refrigeration, and can be widely applied to spontaneous cooling of the surfaces of devices and facilities.

Description

Technical Field

[0001] The invention belongs to the field of passive cooling, and in particular relates to a passive radiation-evaporative cooling composite coating and a preparation method thereof. Background Art

[0002] Most current cooling technologies, such as air compressor-based cooling systems, usually consume a lot of energy, directly or indirectly emit a large amount of carbon dioxide, and aggravate global warming. Passive cooling technology that does not require additional power input provides a viable alternative for reducing indoor cooling energy consumption. Among them, radiative cooling technology and evaporative cooling technology do not require additional energy input and can achieve high energy efficiency while achieving a minimal carbon footprint, so they are gradually attracting people's attention. Radiative cooling technology uses the atmospheric transparent window (8-13 μm) to emit thermal radiation into space, thereby achieving a sub-ambient cooling effect. However, the maximum limit of the cooling power of the radiative cooling device is 150 W·m -2 , which seriously limits the practical application of radiative cooling technology; evaporative cooling usually requires additional water supply accessories, which increases the complexity of the system. Therefore, researching and developing passive cooling materials that are simple to prepare, low-cost, efficient, environmentally friendly, and can be mass-produced is currently a major challenge.

[0003] Patent Publication No.: CN110216924A discloses a composite radiation cooling film, the film has holes and additives in the emission layer, which can improve the emissivity of the atmospheric window band, but does not improve the spectral reflectivity of solar radiation. This patent sets a reflection layer below the emission layer to achieve high reflection of the solar energy band, and the reflection layer still uses metal materials. Therefore, the patent has a complex structure and fails to avoid ultraviolet absorption of metal.

[0004] Patent Publication No.: CN114714692A discloses a visible-near-infrared frequency division type radiation cooling film based on a bionic rose petal micro-nano structure. This film is based on a bionic rose petal micro-nano structure, and a transparent bionic micro-nano structure layer and a multi-layer film structure layer are arranged from top to bottom, so that the radiation cooling film can selectively transmit solar radiation energy, realize the visible-near-infrared band frequency division function and the atmospheric window high emission performance, take into account both lighting and cooling performance, and broaden the scope of application of radiation cooling materials. Although the film uses low-cost polymer materials, the processing of the bionic micro-nano structure introduces wet etching technology and template hot embossing method, and the multi-layer film structure layer uses vacuum magnetron sputtering method, electron beam evaporation method, and chemical vapor deposition method. These processing processes are costly and the size is limited by the equipment, so this type of film does not have the advantage of large-scale application.

[0005] Patent Publication No.: CN112375418A discloses a method for preparing a multi-level porous radiation cooling thin film coating. This invention uses a simple and low-cost preparation method to generate a multi-level pore structure to enhance the reflectivity of the solar radiation band. However, due to the limited structural regulation function, its cooling effect is not obvious enough. Summary of the invention

[0006] The purpose of the present invention is to provide a passive radiation-evaporative cooling composite coating and a preparation method thereof. Due to the micro-nanoscale pore structure of the upper radiation cooling film, its solar reflectivity is as high as 91.6%, the emissivity in the infrared atmospheric window band (8-13 μm) is as high as 90.0%, and the emissivity in the mid-infrared band (5-25 μm) is as high as 88.6%, which can achieve efficient radiation cooling throughout the day. The lower hydrogel layer can take away heat through the evaporation of water during the day to achieve evaporative cooling; at night, it absorbs water vapor in the air to achieve self-regeneration. Under direct sunlight in summer, the double-layer polymer passive cooling material can achieve a temperature reduction of about 15°C. In addition, the double-layer cooling material also has a series of advantages such as simple preparation, good hydrophobicity, excellent mechanical ductility and low cost, and can be applied on a large scale on various complex surfaces.

[0007] In order to achieve the above object, the present invention adopts the following technical solution: A method for preparing a passive radiation-evaporative cooling composite coating comprises the following steps: (1) adding a high molecular weight polymer or silane and an additive into a first solvent and mixing them evenly to obtain an organic solution or emulsion; (2) pouring the organic solution or emulsion obtained in step (1) into a mold of a desired size or attaching it to the substrate plane by coating to obtain an initial coating; (3) performing solvent replacement of the initial coating obtained in step (2) with a second solvent to obtain a multiphase mixed coating with water phase intrusion replacement; (4) placing the multiphase mixed coating obtained in step (3) together with the mold or substrate plane in a ventilated place indoors and drying naturally to obtain a solidified porous coating, and directly peeling it off from the mold or substrate plane to obtain a porous radiative cooling film; (5) Using chemical crosslinking or ionic crosslinking methods to prepare evaporatively cooled hydrogels; (6) soaking the hydrogel obtained in step (5) in a high concentration salt solution to obtain a hygroscopic hydrogel; (7) Placing the porous radiation cooling film obtained in step (4) on the surface of the hygroscopic hydrogel obtained in step (6), and irradiating the film under sunlight to make the porous radiation cooling film and the hydrogel adhere to each other, thereby obtaining a dual-functional double-layer polymer passive cooling material.

[0008] A further improvement of the present invention is that in step (1), the high molecular polymer is any one or more of ethylene polymer, vinyl fluoride homopolymer, polyurethane, epoxy resin polymer, styrene polymer, polyethylene terephthalate, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, (meth)acrylate polymer and organosiloxane polymer; The silane is any one or more of dimethylsiloxane, methyltriethoxysilane, vinyltriethoxysilane, dimethyldiethoxysilane and tetraethoxysilane; The additive is any one or more of a curing agent, a binder and an initiator for maintaining or adjusting the state of an organic solution or emulsion, and the volume ratio of any additive to the high molecular polymer or silane is 1:8 to 1:10; The first solvent is any one or more of toluene, xylene, octane, cyclohexane, cyclohexanone, chlorobenzene, dichloromethane, methanol, ethanol, isopropanol, propylene oxide, N,N-dimethylformamide, dimethylformamide, dimethylacetamide, dimethyl sulfoxide, methyl acetate, ethyl acetate, propyl acetate, acetone, methyl butyl ketone, methyl isobutyl ketone, N-methylpyrrolidone, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, trichloroethylene, tetrahydrofuran, acetonitrile and triethanolamine; The high molecular weight polymer or silane accounts for 10-60 wt % of the organic solution or emulsion.

[0009] A further improvement of the present invention is that in step (2), the coating method includes: any one or more of spin coating, scraper coating, spray coating and roller coating; The base plane includes: any one or a composite plane of glass, polytetrafluoroethylene, stainless steel, wood or cloth fabric.

[0010] A further improvement of the present invention is that in step (3), the second solvent is any one or more of water, methanol, ethanol, ether and propanol; The method for replacing the solvent between the coating and the second solvent comprises: placing the initial coating in a high humidity environment where the relative humidity of the second solvent is greater than 50%, or immersing the initial coating in the second solvent, or spraying the atomized second solvent on the initial coating; wherein the second solvent is soluble in the first solvent, and the second solvent is insoluble in the high molecular weight polymer or silane in step (1); The duration of the solvent replacement process is 0.5-2h.

[0011] A further improvement of the present invention is that in step (4), natural drying is performed at room temperature of 20°C to 30°C for a drying time of 12-24 hours.

[0012] A further improvement of the present invention is that in step (6), the high concentration salt solution is a hygroscopic salt solution of calcium chloride, lithium chloride, magnesium chloride or calcium nitrate.

[0013] A passive radiation-evaporative cooling composite coating is prepared by the preparation method, and the thickness of the upper porous radiation cooling film is 200-10000 μm, and the thickness of the lower highly hygroscopic hydrogel is 0.5-2 cm.

[0014] A further improvement of the present invention is that the pore size distribution of the porous radiation cooling film layer is as follows: nano-scale pores are distributed at 350±270nm, and micro-scale pores are distributed at 2.40±1.25μm.

[0015] A further improvement of the present invention is that the reflectivity of the porous radiation cooling film layer in the solar radiation band is as high as 91.6%, the emissivity in the infrared atmospheric window band, i.e., 8-13 μm, is as high as 90.0%, and the emissivity in the mid-infrared band, i.e., 5-25 μm, is as high as 88.6%; The water contact angle of the porous radiation cooling film layer is between 90° and 135°, and the contact angle of the hydrogel layer is between 10° and 20°.

[0016] A further improvement of the present invention is that the highly hygroscopic hydrogel has excellent evaporative cooling and hygroscopic properties in environments with different relative humidity, i.e., 40%-80%.

[0017] Compared with the prior art, the present invention has at least the following beneficial technical effects: (1) In the present invention, the upper radiation cooling film is distributed with abundant micro-nano pore structures, and a tiny scattering interface with differentiated refractive index is constructed through pores and nanoparticles. The reflectivity of solar radiation is improved by strengthening multiple scattering. At the same time, the micron-level pores themselves can increase the thermal emissivity. (2) In the present invention, the hygroscopic hydrogel in the lower layer can perform evaporative cooling during the day and absorb moisture at night to achieve self-regeneration. (3) In the present invention, the double-layer passive cooling material has a significant cooling effect and achieves a higher temperature reduction among the same type of double-layer passive cooling materials. (4) In the present invention, the double-layer passive cooling material has a low material selection cost, a simple preparation process, and production does not rely on complex equipment technology, and can be mass-produced; (5) In the present invention, the upper radiation cooling film has excellent hydrophobic properties, and the double-layer passive cooling material has good mechanical ductility and can be used in multiple scenarios.

[0018] (2) The passive radiation-evaporative cooling composite coating and its preparation method provided by the present invention improve the comprehensive passive cooling performance of the material by combining radiation cooling and evaporative cooling. During the day, the radiation cooling film on the upper layer can strongly reflect sunlight and effectively transmit thermal radiation to outer space. In addition, the porous structure of the radiation cooling film allows water molecules to pass through, so that the hydrogel on the lower layer can achieve good evaporative cooling. At night, the radiation cooling effect of the radiation cooling film can keep the hydrogel at a lower temperature, which is conducive to the hydrogel absorbing moisture from the air for evaporative cooling under high temperature on the next day, thereby achieving good self-circulation. In terms of material selection, firstly, by selecting a high molecular polymer or silane with high absorption rate characteristics in the atmospheric window band as the matrix material of the radiation cooling film, the film as a whole has good radiation performance; the hydrogel is prepared by methods such as thermal crosslinking and ionic crosslinking, and the hydrogel is preferably treated with a hygroscopic salt solution with strong hygroscopicity and low price. In terms of preparation process, a porous structure is formed by using phase separation, and numerous solid-gas phase interfaces are introduced inside the radiation cooling film coating. The refractive index difference of the phase interface can cause multiple scattering of light, thereby enhancing the macroscopic reflectivity; hygroscopic salt is loaded into the hydrogel by immersion method to obtain a hydrogel that can absorb moisture at night. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic structural diagram of an embodiment of the passive radiation-evaporative cooling composite coating of the present invention.

[0020] Figure 2 Appearance photos and water contact angles of the passive radiation-evaporative cooling composite coating prepared in the present invention.

[0021] Figure 3 This is a scanning electron microscope image of the upper radiation cooling film.

[0022] Figure 4 Size distribution of (a) nanoscale pores and (b) microscale pores in thin film coatings for radiative cooling.

[0023] Figure 5 The reflection spectrum of the upper radiation cooling film is 0.28-2.5 μm and the emission spectrum of the mid-infrared is 2.5-25 μm.

[0024] Figure 6 Dynamic adsorption-desorption curve of the hygroscopic hydrogel layer.

[0025] Figure 7 This is the temperature variation curve of the passive radiation-evaporative cooling composite coating under isolated convective heat transfer. DETAILED DESCRIPTION

[0026] In order to clearly and completely describe the technical solutions and advantages of the present invention, the present invention is further described below in conjunction with specific embodiments and drawings. The described embodiments are part of the examples of the present invention, but the present invention is not limited to the following embodiments. The methods described are conventional methods unless otherwise specified. The raw materials and processing equipment and instruments can be obtained from public commercial channels unless otherwise specified. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present invention.

[0027] The embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0028] The present invention provides a method for preparing a passive radiation-evaporative cooling composite coating, comprising the following steps: (1) adding a high molecular weight polymer or silane and an additive into a first solvent and mixing them evenly to obtain an organic solution or emulsion; (2) pouring the organic solution or emulsion obtained in step (1) into a mold of a desired size or attaching it to the substrate plane by coating to obtain an initial coating; (3) performing solvent replacement of the initial coating obtained in step (2) with a second solvent to obtain a multiphase mixed coating with water phase intrusion replacement; (4) placing the multiphase mixed coating obtained in step (3) together with the mold or substrate plane in a ventilated place indoors and drying naturally to obtain a solidified porous coating, and directly peeling it off from the mold or substrate plane to obtain a porous radiative cooling film; (5) Using chemical crosslinking or ionic crosslinking methods to prepare evaporatively cooled hydrogels; (6) soaking the hydrogel obtained in step (5) in a high concentration salt solution to obtain a hygroscopic hydrogel; (7) Placing the porous radiation cooling film obtained in step (4) on the surface of the hygroscopic hydrogel obtained in step (6), and irradiating the film under sunlight to make the porous radiation cooling film and the hydrogel adhere to each other, thereby obtaining a dual-functional double-layer polymer passive cooling material.

[0029] Preferably, in step (1), the high molecular polymer is any one or more of ethylene polymer, vinyl fluoride homopolymer, polyurethane, epoxy resin polymer, styrene polymer, polyethylene terephthalate, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, (meth)acrylate polymer and organosiloxane polymer; the silane is any one or more of dimethylsiloxane, methyltriethoxysilane, vinyltriethoxysilane, dimethyldiethoxysilane and tetraethoxysilane; the additive is any one or more of the curing agent, adhesive and initiator for maintaining or adjusting the state of the organic solution or emulsion. Any one or more, any additive and the volume ratio of the high molecular polymer or silane is 1:8 to 1:10; the first solvent is any one or more of toluene, xylene, octane, cyclohexane, cyclohexanone, chlorobenzene, dichloromethane, methanol, ethanol, isopropanol, propylene oxide, N,N-dimethylformamide, dimethylformamide, dimethylacetamide, dimethyl sulfoxide, methyl acetate, ethyl acetate, propyl acetate, acetone, methyl butyl ketone, methyl isobutyl ketone, N-methylpyrrolidone, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, trichloroethylene, tetrahydrofuran, acetonitrile and triethanolamine. The high molecular polymer or silane accounts for 10-60wt% of the organic solution or emulsion.

[0030] Preferably, in step (2), the coating method includes: any one or more of spin coating, scraper coating, spray coating and roll coating; the substrate plane includes: any one or more of glass, polytetrafluoroethylene, stainless steel, wood or cloth fabric.

[0031] Preferably, in step (3), the second solvent is any one or more of water, methanol, ethanol, ether and propanol; the method for solvent replacement between the coating and the second solvent comprises: placing the initial coating in a high humidity environment where the relative humidity of the second solvent is greater than 50%, or immersing the initial coating in the second solvent, or spraying atomized second solvent on the initial coating; wherein the second solvent is soluble in the first solvent, and the second solvent is insoluble in the high molecular polymer or silane in step (1); and the duration of the solvent replacement process is 0.5-2 hours.

[0032] Preferably, in step (4), natural drying is performed at room temperature of 20°C to 30°C for 12-24 hours.

[0033] Preferably, in step (6), the high concentration salt solution is a hygroscopic salt solution of calcium chloride, lithium chloride, magnesium chloride or calcium nitrate.

[0034] Example 1: First, 1.5 g of P(VdF-HFP) powder was dissolved in 12 mL of N,N-dimethylformamide and stirred for 2 hours. Then, the uniform P(VdF-HFP) solution was poured into a polytetrafluoroethylene square mold and allowed to stand for 5 minutes. Next, deionized water was sprayed on the surface of the solution ten times, with an interval of three minutes each time, to allow the solvent and non-solvent to be fully exchanged. Finally, the mixed solution above the film was poured off and the film was allowed to dry at room temperature for 24 hours. The prepared film is as shown in FIG. Figure 2 As shown, it has a bright white appearance under daylight.

[0035] 0.5 g of sodium alginate and 5.0 g of acrylamide were completely dissolved in 40 mL of deionized water. Then, 0.016 g of N,N'-methylenebisacrylamide was added as a crosslinker and 20 μL of N,N,N',N'-tetramethylethylenediamine was added as an accelerator. Then, 0.05 g of ammonium persulfate was added as a thermal initiator. Then, the solution was quickly poured into a 10×10 cm mold and baked in an oven at 50 °C for 2 h. Finally, the sample was immersed in 2.5 mol / L CaCl 2 The solution was incubated for 8 hours to crosslink the sodium alginate ions and obtain PAAm / alginate-CaCl 2 Hydrogel. The prepared hygroscopic hydrogel is as follows Figure 1 As shown, it has a translucent appearance under daylight.

[0036] Example 2: The experimental apparatus and operation are the same as in Example 1, except that 1.5 g of P(VdF-HFP) powder is replaced with 1.8 g of P(VdF-HFP) powder, and the other conditions remain unchanged, thereby obtaining a 1.8 g P(VdF-HFP) radiation cooling coating.

[0037] Example 3: The experimental apparatus and operation are the same as in Example 1, except that 1.5 g of P(VdF-HFP) powder is replaced with 2.1 g of P(VdF-HFP) powder, and the other conditions remain unchanged, to obtain 2.1 g of P(VdF-HFP) radiation cooling coating.

[0038] Example 4: The experimental apparatus and operation are the same as in Example 1, except that 1.5 g of P(VdF-HFP) powder is replaced with 2.4 g of P(VdF-HFP) powder, and the other conditions remain unchanged, thereby obtaining 2.4 g of P(VdF-HFP) radiation cooling coating.

[0039] Example 5: The experimental apparatus and operation are the same as in Example 1, except that 1.5 g of P(VdF-HFP) powder is replaced with 1.0 g of thermoplastic polyurethane (TPU) powder, and the other conditions remain unchanged, to obtain a porous TPU radiation cooling film layer.

[0040] Example 6: The experimental device and operation are the same as in Example 1, except that 12 mL of N,N-dimethylformamide is replaced with 12 mL of acetone, the suspension is transferred to a polytetrafluoroethylene square mold with an opening on the top surface instead of being spin-coated on a glass plane, and the drying condition at room temperature for 24 h is changed to drying at room temperature for 8 h. The other conditions remain unchanged.

[0041] Example 7: The experimental apparatus and operation are the same as in Example 1, except that 0.5 g of sodium alginate and 5.0 g of acrylamide are replaced with 5.5 g of acrylamide, and the other conditions remain unchanged to obtain PAAm-CaCl 2 Hydrogel.

[0042] like Figure 2 As shown, the P(VdF-HFP) thin film coating in Example 1 has excellent hydrophobic properties, and its contact angle is greater than 130°. The hydrophobicity enables the radiation cooling layer to have self-cleaning ability, which can protect the underlying hydrophilic hydrogel from damage by dust and dirt, enhance the durability of the hydrogel, and make it more suitable for outdoor application scenarios.

[0043] A scanning electron microscope image of a cross section of the prepared thin film coating is shown in FIG3 , which shows that the interior contains micrometer and nanometer-level pores, thereby enhancing the reflectivity. Figure 4 The results of pore size measurement show a bimodal distribution, with nanoscale and micron-scale pore sizes concentrated at 350 nm and 2.4 μm, respectively. Based on the Mie scattering principle, the multi-level pore structure can produce a strong scattering effect and enhance the reflectivity of the solar band. At the same time, the porous structure allows water molecules to pass through without hindering the evaporation and moisture absorption of the lower hydrogel.

[0044] Figure 5 The reflection spectra of the radiation cooling films in Examples 1 to 4 at 0.28-2.5 μm and the emission spectra of the mid-infrared at 2.5-25 μm show that the reflectivity in the solar radiation band is as high as 91.6%, and it exhibits broadband reflection characteristics; the emissivity in the infrared atmospheric window band (8-13 μm) is as high as 90.0%, and the emissivity in the mid-infrared band (5-25 μm) is as high as 88.6%, and it exhibits broadband emission characteristics, and can radiate its own heat into the cosmic background to a large extent.

[0045] Figure 6 (a) and Figure 6 (b) The mass changes of the hydrogel in Example 1 at 20°C (adsorption) and 45°C (evaporation) under different humidity conditions (RH=40%, 60% and 80%). It can be seen that the hydrogel has excellent moisture absorption or evaporation ability under different humidity conditions to adapt to various outdoor environments. Figure 6(c) is the desorption curve of the hydrogel covered with a radiation film and the hydrogel not covered with a radiation film in Example 1 at 45°C and a relative humidity of 60%. The evaporation capacity of the lower layer of the hydrogel in the double-layer structure is not greatly affected.

[0046] Figure 7 (a) is the temperature tracking curve (12:00-24:00) of the aluminum plate and the single-layer radiation cooling film, the single-layer hydrogel and the double-layer cooling material in Example 1 under isolated convection heat transfer. Figure 7 (b) is the temperature difference curve of single-layer radiation cooling film, single-layer hydrogel and double-layer material and the control group aluminum plate. It can be seen that the temperature of the double-layer cooling material is always the lowest, the maximum instantaneous temperature difference reaches 18.0℃, and the average daytime (12:00-16:00) temperature difference reaches 15.4℃.

[0047] The above shows and describes the basic principles and main features of the present invention and the advantages of the present invention. It is obvious to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic features of the present invention. Therefore, no matter from which point of view, the embodiments should be regarded as exemplary and non-restrictive. The scope of the present invention is defined by the attached claims rather than the above description, and it is intended that all changes falling within the meaning and scope of the equivalent elements of the claims are included in the present invention. Any figure mark in the claims should not be regarded as limiting the claims involved.

[0048] In addition, it should be understood that although this specification is described in accordance with the implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation modes that can be understood by those skilled in the art. The above content is only to illustrate the technical idea of ​​the present invention, and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution according to the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A method for preparing a passive radiation-evaporative cooling composite coating, characterized in that: The following steps are involved: (1) adding a high molecular weight polymer or silane and an additive into a first solvent and mixing them evenly to obtain an organic solution or emulsion; (2) pouring the organic solution or emulsion obtained in step (1) into a mold of a desired size or attaching it to the substrate plane by coating to obtain an initial coating; (3) performing solvent replacement of the initial coating obtained in step (2) with a second solvent to obtain a multiphase mixed coating with water phase intrusion replacement; (4) placing the multiphase mixed coating obtained in step (3) together with the mold or substrate plane in a ventilated place indoors and drying naturally to obtain a solidified porous coating, and directly peeling it off from the mold or substrate plane to obtain a porous radiative cooling film; (5) Using chemical crosslinking or ionic crosslinking methods to prepare evaporatively cooled hydrogels; (6) soaking the hydrogel obtained in step (5) in a high concentration salt solution to obtain a hygroscopic hydrogel; (7) Placing the porous radiation cooling film obtained in step (4) on the surface of the hygroscopic hydrogel obtained in step (6), and irradiating the film under sunlight to make the porous radiation cooling film and the hydrogel adhere to each other, thereby obtaining a dual-functional double-layer polymer passive cooling material.

2. The method for preparing a passive radiation-evaporative cooling composite coating according to claim 1, characterized in that: In step (1), the high molecular polymer is any one or more of ethylene polymer, vinyl fluoride homopolymer, polyurethane, epoxy resin polymer, styrene polymer, polyethylene terephthalate, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, (meth)acrylate polymer and organosiloxane polymer; The silane is any one or more of dimethylsiloxane, methyltriethoxysilane, vinyltriethoxysilane, dimethyldiethoxysilane and tetraethoxysilane; The additive is any one or more of a curing agent, a binder and an initiator for maintaining or adjusting the state of an organic solution or emulsion, and the volume ratio of any additive to the high molecular polymer or silane is 1:8 to 1:10; The first solvent is any one or more of toluene, xylene, octane, cyclohexane, cyclohexanone, chlorobenzene, dichloromethane, methanol, ethanol, isopropanol, propylene oxide, N,N-dimethylformamide, dimethylformamide, dimethylacetamide, dimethyl sulfoxide, methyl acetate, ethyl acetate, propyl acetate, acetone, methyl butyl ketone, methyl isobutyl ketone, N-methylpyrrolidone, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, trichloroethylene, tetrahydrofuran, acetonitrile and triethanolamine; The high molecular weight polymer or silane accounts for 10-60 wt % of the organic solution or emulsion.

3. The method for preparing a passive radiation-evaporative cooling composite coating according to claim 1, characterized in that: In step (2), the coating method includes any one or more of spin coating, scraper coating, spray coating and roller coating; The base plane includes: any one or a composite plane of glass, polytetrafluoroethylene, stainless steel, wood or cloth fabric.

4. The method for preparing a passive radiation-evaporative cooling composite coating according to claim 1, characterized in that: In step (3), the second solvent is any one or more of water, methanol, ethanol, ether and propanol; The method for replacing the solvent between the coating and the second solvent comprises: placing the initial coating in a high humidity environment where the relative humidity of the second solvent is greater than 50%, or immersing the initial coating in the second solvent, or spraying the atomized second solvent on the initial coating; wherein the second solvent is soluble in the first solvent, and the second solvent is insoluble in the high molecular weight polymer or silane in step (1); The duration of the solvent replacement process is 0.5-2h.

5. The method for preparing a passive radiation-evaporative cooling composite coating according to claim 1, characterized in that: In step (4), the mixture is naturally dried at room temperature of 20°C to 30°C for 12-24 hours.

6. The method for preparing a passive radiation-evaporative cooling composite coating according to claim 1, characterized in that: In step (6), the high concentration salt solution is a hygroscopic salt solution of calcium chloride, lithium chloride, magnesium chloride or calcium nitrate.

7. A passive radiation-evaporative cooling composite coating, characterized in that: The passive radiation-evaporative cooling composite coating is prepared by the preparation method described in any one of claims 1 to 6, and the thickness of the upper porous radiation cooling film is 200-10000 μm, and the thickness of the lower highly hygroscopic hydrogel is 0.5-2 cm.

8. The passive radiation-evaporative cooling composite coating according to claim 7, characterized in that: The pore size distribution of the porous radiation cooling film layer is as follows: nano-scale pores are distributed at 350±270 nm, and micro-scale pores are distributed at 2.40±1.25 μm.

9. The passive radiation-evaporative cooling composite coating according to claim 7, characterized in that: The reflectivity of the porous radiation cooling film layer in the solar radiation band is as high as 91.6%, the emissivity in the infrared atmospheric window band, i.e., 8-13 μm, is as high as 90.0%, and the emissivity in the mid-infrared band, i.e., 5-25 μm, is as high as 88.6%; The water contact angle of the porous radiation cooling film layer is between 90° and 135°, and the contact angle of the hydrogel layer is between 10° and 20°.

10. The passive radiation-evaporative cooling composite coating according to claim 7, characterized in that: The highly hygroscopic hydrogel has excellent evaporative cooling and moisture absorption properties in environments with different relative humidity, i.e., 40%-80%.

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