A porous PE film, a method for preparing the same and a radiation-evaporative refrigeration device comprising the porous PE film

Abstract

The application discloses a porous PE film, a preparation method thereof and a radiation-evaporation refrigeration device comprising the porous PE film, the porous PE film has high reflectivity (>0.9) in a sunlight wave band (0.3-2.5 microns), high transmittance (>0.9) in an infrared atmospheric window (8-13 microns), and good water vapor transmission performance (water vapor mass transfer resistance is about 580 sm ‑1 -1 under the thickness of 110 microns), the radiation-evaporation refrigeration device prepared from the porous PE film realizes high-efficiency refrigeration below room temperature by introducing an air layer between a PVA-LiBr water gel and the porous PE film and utilizing the low thermal conductivity of static air.

Description

Technical Field

[0001] This invention belongs to the field of passive refrigeration technology, specifically relating to a porous PE membrane, its preparation method, and a radiation-evaporation refrigeration device containing the porous PE membrane. Background Technology

[0002] With population growth and rising living standards, the demand for cooling is constantly increasing. Studies indicate that active cooling technologies, such as compression refrigeration, consume approximately 15% of global electricity and emit about 10% of greenhouse gases annually, placing a heavy burden on global energy supply and environmental protection. Active cooling technology drives heat transfer from a low-temperature heat source to a high-temperature heat source by consuming high-grade energy, while passive cooling technology does not alter the direction of heat transfer and does not require additional electrical energy consumption, thus becoming a hot research topic in recent years.

[0003] Passive cooling technologies include radiative cooling and evaporative cooling. Radiative cooling uses outer space (approximately 3K) as a cold source, achieving efficient heat exchange between the coating and outer space while minimizing heat loss to the surrounding environment by controlling the surface spectrum of the coating. Radiative cooling requires high reflectivity (>0.9) in the solar radiation band (0.3-2.5μm) and high emissivity in the mid-infrared band (2.5-20μm) or the infrared atmospheric window (8-13μm) to achieve daytime cooling. Evaporative cooling utilizes the heat absorption of gas-liquid phase change, and liquid water, due to its high enthalpy of vaporization, low cost, availability, non-toxicity, and wide applicability, has become a primary research subject in evaporative cooling. [1] .

[0004] However, passive refrigeration also faces some problems that need to be solved. Limited by environmental radiation and atmospheric transmittance, the cooling power of radiative refrigeration at room temperature is only about 100 W / m². -2 Therefore, achieving high cooling power using radiative refrigeration requires a large amount of materials and occupies a larger cooling area. Furthermore, radiative refrigeration is significantly affected by environmental factors; high precipitable water (PW), cloud cover, and air pollution all reduce its cooling efficiency. Traditional evaporative refrigeration systems achieve cooling by directly heating water to promote evaporation, requiring a large amount of water and a corresponding water supply system. Interfacial evaporation concentrates heat at the evaporation interface, avoiding heating the water and improving cooling efficiency, but still requires artificial water replenishment.

[0005] Recently, hybrid systems combining radiative and evaporative cooling based on atmospheric water harvesting technology have become a research hotspot. (Yilan Sun) [1] Other researchers have proposed a composite fabric in which the top layer is a porous polyvinylidene fluoride copolymer (P(VdF-HFP)). HPThe bottom layer is made of cotton-polyester fiber, and atmospheric water collection is achieved by evenly distributing CaCl2 ions, a water-absorbing agent, within the fibers. Similarly, Jinlei Li [2] et al. proposed a bilayer structure based on cellulose acetate fibers and polyvinyl alcohol-calcium chloride (PVA-CaCl2) hydrogel; Chunzao Feng [3] et al. proposed a bilayer structure based on porous P(VdF-HFP) and polyacrylamide-lithium bromide (PAAM-LiBr) hydrogels. The top layer of these structures serves as a daytime radiative cooling material, exhibiting high reflectivity in the solar spectrum and high emissivity in the mid-infrared spectrum. The bottom layer is an evaporative cooling material, capable of evaporating water under high-temperature, low-humidity daytime conditions and absorbing water vapor from the air under low-temperature, high-humidity nighttime conditions, thus achieving water regeneration. However, due to the extremely high infrared emissivity of the water molecules used for evaporation in the bottom layer, the infrared-opaque top layer introduces additional radiative thermal resistance. Besides the bilayer structure, Liang Xu et al. also proposed a polyacrylamide / polyvinyl alcohol (NPs / NADES@PAAm / PVA) hydrogel doped with nanoparticles, which has a reflectivity of approximately 0.89 in the solar spectrum and an emissivity of approximately 0.90 in the infrared atmospheric window. It is evident that although monolayer hydrogel structures are simple, the absorption of near-infrared sunlight by water molecules reduces the reflectivity of the hydrogel, affecting its daytime performance. [4] .

[0006] [1]Sun Y,Ji Y,Javed M,et al.Preparation of passive daytime coolingfabric with the synergistic effect of radiative cooling and evaporativecooling[J].Advanced Materials Technologies,2022,7(3):2100803.

[0007] [2]Li J, Wang

[0008] [3]Feng C, Yang P, Liu H, et al. Bilayer porous polymer for efficient passive building cooling [J]. Nano Energy, 2021,85:105971.

[0009] [4]Xu L, Sun DW, TianY, et al. Combined effects of radiative and evaporative cooling on fruitpreservation under solar radiation: sunburn resistance and temperature stabilization [J]. ACSApplied Materials&Interfaces, 2022: acsami.2c11349. Summary of the Invention

[0010] To address the above problems, the primary objective of this invention is to provide a porous PE membrane and its preparation method. This membrane exhibits high reflectivity (>0.9) in the solar radiation band (0.3-2.5 μm), high transmittance (>0.9) in the infrared atmospheric window (8-13 μm), and good water vapor transmission performance (water vapor mass transfer resistance of approximately 580 s m at a thickness of 110 μm). -1 ).

[0011] The second objective of this invention is to provide a radiation-evaporation cooling device comprising a porous PE membrane, which achieves efficient cooling below room temperature by introducing an air layer between the PVA-LiBr hydrogel and the porous PE membrane and utilizing the low thermal conductivity of still air.

[0012] To achieve the above-mentioned technical objectives, the present invention adopts the following technical solution:

[0013] A method for preparing a porous PE membrane involves dissolving high-density polyethylene (HDPE) powder and ultra-high molecular weight polyethylene (UHMWPE) powder in paraffin oil at 120-130℃ to obtain a homogeneous mixture. The mixture is then cooled to 45-55℃, sliced ​​to a thickness of 0.3-0.6 mm, and hot-pressed at 120-130℃ for 6-10 min to obtain a gel membrane. Finally, the gel membrane is sequentially immersed in n-hexane and ethanol, and dried to obtain the porous PE membrane.

[0014] Those skilled in the art will know that polyethylene is classified according to density and the number of branches, with high-density polyethylene (HDPE) being one of the most commonly used types, with a density reaching 0.941 g / cm³.-3 It has a very low degree of branching and high tensile strength. Ultra-high molecular weight polyethylene (UHMWPE) refers to polyethylene with a relative molecular weight reaching one million amu (typically 3.5-7.5 million amu). It has a rigid texture but a relatively low density (0.930-0.935 g / cm³). -3 ).

[0015] Preferably, the mass ratio of the high-density polyethylene (HDPE) powder, ultra-high molecular weight polyethylene (UHMWPE) powder, and paraffin oil is 7:3:40.

[0016] In this invention, high-density polyethylene (HDPE) powder and ultra-high molecular weight polyethylene (UHMWPE) powder are first dissolved in paraffin oil at 120-130℃ and thoroughly mixed to avoid solid-liquid separation after mixing. Then, before hot pressing, the mixture is cooled to 45-55℃, and the hot pressing temperature is controlled at 120-130℃. After cooling, the mixture forms a micro-nano-scale porous structure, and the porous structure will not be completely destroyed at 120-130℃. At the same time, the framework of the porous structure can prevent the membrane from rebounding after hot pressing. Finally, the membrane is soaked and washed in hexane and ethanol in sequence to obtain a porous PE membrane.

[0017] The present invention also provides a porous PE membrane prepared by the above preparation method, the thickness of which is 50-150 μm.

[0018] The present invention also provides a radiation-evaporation cooling device, comprising an upper porous PE membrane, an intermediate air layer and a lower PVA-LiBr hydrogel.

[0019] Preferably, the thickness of the intermediate air layer is designed as follows: two parallel and opposite flat plates are used, and the Rayleigh number Ra = g(T) h -T c )βδ 3 / (va), where g is the acceleration due to gravity, T h T represents the temperature of the upper plate (hot end temperature). c ν is the temperature of the lower plate (cold end temperature), β is the air volume expansion coefficient, δ is the thickness between plates (i.e. the thickness of the intermediate air layer), ν is the air kinematic viscosity, and α is the air thermal diffusivity.

[0020] When Ra < 1200 and ΔT = 10-30K, δ is the thickness of the intermediate air layer.

[0021] Preferably, the preparation process of the PVA-LiBr hydrogel is as follows: PVA solution, acrylamide (AAM), N,N'-methylenebisacrylamide, ammonium persulfate (APS) and lithium bromide (LiBr) powder are thoroughly stirred to obtain a hydrogel mother liquor; then the hydrogel mother liquor is gelled, dried and water-absorbed to obtain the PVA-LiBr hydrogel.

[0022] The advantages of this invention are:

[0023] (1) The porous PE membrane (~110μm) of the present invention has high reflectivity (0.91) in the solar radiation band (0.3-2.5μm), high transmittance (0.92) in the infrared atmospheric window (8-13μm), and good water vapor transmission performance (water vapor mass transfer resistance is about 580s m). -1 The existing commercial porous PE membrane (~25μm) has a solar reflectance of approximately 0.585 and a water vapor mass transfer resistance of approximately 1240 s m. -1 Existing porous PE fibers (~450μm) have an opacity of 0.9 in visible light and a transmittance of less than 0.8 in the infrared atmospheric window.

[0024] (2) The radiation-evaporation cooling device of the present invention introduces an air layer between the porous PE membrane and the PVA-LiBr hydrogel, utilizing the low thermal conductivity of still air to achieve efficient cooling below room temperature, while using the additional water vapor mass transfer resistance generated by the air layer to extend the cooling time of the system. The PVA-LiBr hydrogel uses LiBr as an adsorbent, which can meet the requirements of daytime evaporation and nighttime water collection, and the hydrogel has an emissivity in the mid-infrared that is close to that of human skin. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of a radiation-evaporation cooling device, where 1 is a porous PE membrane, 2 is an air layer, and 3 is a PVA-LiBr hydrogel.

[0026] Figure 2 Visible-near-infrared reflectance and mid-infrared transmittance spectra of porous PE membranes and hydrogels.

[0027] Figure 3 Optical images and SEM images of porous PE films;

[0028] Figure 4 The spectral characteristics of the boron nitride radiation-cooled coating are shown.

[0029] Figure 5 This is a comparative experiment on the temperature drop of the radiation-evaporation refrigeration device of the present invention and the boron nitride radiation refrigeration coating; Figure 5 The three curves in section a, from top to bottom, represent the temperature curves of the environment, the boron nitride radiation cooling coating, and the radiation-evaporation cooling device, respectively. Figure 5 b is a schematic diagram of boron nitride radiation cooling coating (PDRC), whose metal substrate is aluminum alloy; Figure 5 c is a schematic diagram of a radiation-evaporation refrigeration device.

[0030] Figure 6This study compares the temperature drop of porous PE films and infrared high-emissivity radiation-cooling coatings. To control reflectivity in the solar light band, both groups of samples used porous PE films of the same thickness (110 μm). In the first group, the porous PE film was placed directly on a glass dish, referred to as the infrared transmission system. In the second group, the porous PE film was first fixed on an acrylic sheet (PMMA), and the high emissivity of the acrylic sheet and the high transmittance of the porous PE film were used to simulate an infrared radiation-cooling coating. Then, the PMMA was placed on a glass dish, referred to as the infrared emission system. Figure 6 The three curves in section a, from top to bottom, represent the temperature curves of the environment, the infrared emitting system, and the infrared transmitting system, respectively. Figure 6 b is a schematic diagram of the infrared emission system; Figure 6 c is a schematic diagram of an infrared transmission system.

[0031] Figure 7 This experiment compares the temperature drop of radiation-evaporation cooling devices with and without an air layer. The experiment is divided into three groups. The first group consists of a radiation-evaporation system with a porous PE film and an air layer, referred to as the isolation system. Figure 7 e); The second group consists of radiation-evaporation systems without an air layer, also known simply as contact systems. Figure 7 d); The third group consists of exposed PVA-LiBr hydrogels, referred to simply as the exposed system ( Figure 7 c); Figure 7 a represents the temperature curves for the environment and the three systems; Figure 7 b represents the changes in relative humidity (solid line) and solar radiation intensity (dashed line) during the experiment. Detailed Implementation

[0032] The foregoing and other technical contents, features and effects of the present invention shall be referenced in conjunction with the following. Figures 1 to 7 The detailed description will clearly illustrate this. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the scope of the invention.

[0033] Example 1

[0034] like Figure 1 As shown, the radiation-evaporation cooling device of the present invention includes an upper porous PE membrane, an intermediate air layer and a lower PVA-LiBr hydrogel.

[0035] (1) Preparation of porous PE membrane:

[0036] The first step involves controlling the oil bath temperature at 130℃ and mixing high-density polyethylene (HDPE) powder, ultra-high molecular weight polyethylene (UHMWPE) powder, and paraffin oil at a mass ratio of 7:3:40, stirring to obtain a homogeneous mixture. The second step involves cooling the mixture to approximately 50℃ and then cutting it into slices approximately 0.5mm thick. The third step involves setting the hot-pressing temperature to 130℃ and the hot-pressing time to 500s, placing the slices into a hot press to form a film. The fourth step involves immersing the film in hexane and ethanol for 1 hour each to completely displace the paraffin within the film's pores, and finally drying it at room temperature. The resulting film has a thickness of approximately 110μm.

[0037] like Figure 3 As shown, strong Mie scattering occurs when the diameter of the scattering gap is comparable to the wavelength of the electromagnetic wave. The diameter of the gap formed by the PE skeleton in the porous PE film is about 0.1-1μm, while the diameter in the solar light band is 0.3-2.5μm. Therefore, the porous PE film has a high solar reflectivity, and the optical image is white.

[0038] Table 1. Water vapor diffusion coefficient and mass transfer resistance of porous PE membranes and other PE materials.

[0039]

[0040] [5]Peng Y, Chen J, Song AY, et al. Nanoporous polyethylene microfibres for large-scale radiative cooling fabric[J]. Nature Sustainability, 2018, 1(2):105–112.

[0041] [6]Hsu PC, Song AY, Catrysse PB, et al. Radiative human body cooling by nanoporous polyethylene textile[J].Science,2016,353(6303):1019–1023.

[0042] (2) Air layer thickness design:

[0043] Set up two parallel and opposite flat plates, and let the Rayleigh number Ra = g(T) h -T c )βδ 3 / (va), where g is the acceleration due to gravity, T h T represents the temperature of the upper plate (hot end temperature). cν is the temperature of the lower plate (cold end temperature), β is the air volume expansion coefficient, δ is the thickness between plates (i.e. the thickness of the intermediate air layer), ν is the air kinematic viscosity, and α is the air thermal diffusivity.

[0044] Take Ra = 235, ΔT (i.e., T) h -T c =20K, and calculation shows that δ = 5mm meets the working requirements.

[0045] (3) Preparation of PVA-LiBr hydrogel:

[0046] Water and PVA powder were mixed at a mass ratio of 9:1 and stirred at 90°C until completely dissolved, then allowed to stand to obtain a PVA solution. 10g of the 10wt% PVA solution, 2g of acrylamide (AAM), 0.024g of N,N'-methylenebisacrylamide (MBA), 0.05g of ammonium persulfate (APS), and 1.33g of lithium bromide (LiBr) powder were stirred evenly at room temperature to obtain a hydrogel stock solution. The stock solution was then poured into a glass mold and gelled at 50°C for 30 minutes. APS served as the initiator for the crosslinking reaction, and MBA served as the crosslinking agent. AAM and MBA crosslinked to form polyacrylamide (PAAm), and the hydroxyl groups in the PVA chains bonded to the amide groups in the PAAm chains, forming hydrogen bonds and thus a PVA / PAAm network. The hydrogel was then placed in a drying oven at 60°C and dried further until the mass fraction of lithium bromide in the hydrogel reached 20wt%.

[0047] (4) Temperature drop comparison experiment between radiation-evaporation refrigeration device and commonly used boron nitride radiation refrigeration coating:

[0048] like Figure 4 As shown, the boron nitride radiation cooling coating has a thickness of approximately 60 μm, a reflectivity of 0.92 in the solar band, and an emissivity of 0.90 in the mid-infrared atmospheric window band.

[0049] like Figure 5 As shown, the experiment was conducted from 10:30 AM to 12:30 PM on April 26, 2023. The relative humidity was 29±5% and the solar radiation was approximately 975 Wm² over the two hours. -2 Under suitable environmental conditions, the internal temperature of the hydrogel in the radiation-evaporation cooling device and the surface temperature of the boron nitride radiation-cooling coating were measured. During the experiment, the average temperature of the radiation-evaporation cooling device was approximately 26.6℃, the average temperature of the boron nitride radiation-cooling coating was approximately 29.9℃, and the average ambient temperature was approximately 35.4℃. This means the radiation-evaporation cooling device was on average 8.8℃ lower than the ambient temperature and 3.3℃ lower than the boron nitride radiation-cooling coating. This demonstrates that the radiation-evaporation cooling device exhibits better cooling performance than the boron nitride radiation-cooling coating.

[0050] In addition, the ambient temperature fluctuated significantly during the experiment, with a range of 15.1℃ and a variance of 10.7; the temperature range of the boron nitride radiation cooling coating was 5.6℃ and the variance was 1.6; while the temperature range of the radiation-evaporation cooling device was 2.2℃ and the variance was 0.2. This indicates that the introduction of an air layer effectively reduced the impact of ambient temperature fluctuations on the internal temperature of the device.

[0051] (5) Comparative experiment on temperature drop between porous PE film and infrared high-emission radiation cooling coating:

[0052] To further investigate the influence of the infrared spectral characteristics of porous PE films on the temperature drop of radiative cooling, this invention also designed a comparative experiment for two radiative cooling coatings. For example... Figure 6 As shown, the experiment used a glass dish as the substrate, whose infrared emissivity is approximately 1. To control the reflectivity in the solar light band, both groups of samples used porous PE films of the same thickness (110 μm). In the first group, the porous PE film was placed directly on the glass dish, referred to as the infrared transmission system. In the second group, the porous PE film was first fixed on an acrylic sheet (PMMA), utilizing the high emissivity of the acrylic sheet and the high transmittance of the porous PE to simulate an infrared emission radiation cooling coating, and then the PMMA was placed on the glass dish, referred to as the infrared emission system.

[0053] The experiment was conducted from 13:10 to 15:10 on April 26, 2023, with a relative humidity of 30±5% and solar radiation of approximately 900 W / m². -2 In this environment, the average temperature of the infrared transparent system is approximately 29.4℃, the average temperature of the infrared emitting system is approximately 32.7℃, and the average ambient temperature is approximately 40.2℃. It is evident that the bottom temperature of the infrared transparent system is lower than that of the infrared emitting system (an average of 3.3℃ lower).

[0054] In addition, the temperature fluctuations of both systems relative to the environment decreased. Between 14:10 and 15:10, the temperature ranges of the infrared transparent system, the infrared emitting system, and the environment were 1.3℃, 1.8℃, and 9.2℃, respectively; the variances were 0.1, 0.2, and 3.84, respectively.

[0055] (6) Temperature drop comparison experiment of radiation-evaporative refrigeration device with and without air layer:

[0056] like Figure 7 As shown, to illustrate the effect of the air layer on the radiation-evaporation cooling device, the following experimental groups were set up. The first group was a radiation-evaporation cooling device with a porous PE membrane and an air layer (hereinafter referred to as the isolation system), the second group was a radiation-evaporation cooling device without an air layer (hereinafter referred to as the contact system), and the third group was an exposed PVA-LiBr hydrogel (hereinafter referred to as the exposed system).

[0057] This experiment was conducted from 11:20 to 15:20 on April 30, 2023. The experiment can be analyzed in three time periods.

[0058] The first phase was from 11:20 to 12:20. At this time, the average temperatures of the three systems were relatively similar, with the contact system having the lowest temperature at approximately 27.4℃, the isolation system at approximately 28.2℃, and the exposed system at approximately 28.5℃, all about 11℃ lower than the ambient temperature. At this point, the hydrogel concentration was consistent across all systems. The exposed hydrogel had the lowest water vapor mass transfer resistance, resulting in the highest evaporative cooling power. Even though the hydrogel absorbed some solar radiation, its temperature remained lower than the ambient temperature, even approaching that of the other two systems with porous PE membranes. In the contact system, the hydrogel was in direct contact with the porous PE membrane. Besides the lack of an air layer, the wetted surface of the hydrogel penetrated into the porous structure of the membrane, eventually adhering to it. This resulted in very low water vapor mass transfer resistance and a high evaporation rate in the contact system, leading to the largest initial temperature drop under the combined effects of radiation and evaporative cooling.

[0059] The second phase, from 12:20 to 13:20, saw a slow temperature increase in all three systems as the hydrogel lost more water. The isolation system exhibited the lowest heating rate (~0.4℃ / h). -1 ), followed by contact systems (~0.7℃h) -1 The exposed system experiences the highest temperature rise (~1.4℃ / h). -1 Around 13:20, the temperature of the isolation system began to consistently fall below that of the contact and exposed systems. This was because their hydrogels lost more water, leading to an increase in LiBr concentration and inhibiting the evaporative cooling effect. In contrast, the first group of samples, with its air layer and porous PE membrane, experienced less water loss from the hydrogel and maintained a stable evaporation rate.

[0060] The third stage, from 13:20 to 15:20, saw a continuous decrease in solar radiation intensity. During this period, the temperatures of the contact and exposed systems slowly increased, while the temperature of the isolation system continued to decrease. From 14:20 to 15:20, the average temperatures of the isolation, contact, and exposed systems were 26.7℃, 30.6℃, and 34.4℃, respectively. The average ambient temperature was 39.5℃, indicating that the isolation system maintained good cooling performance even after prolonged operation. The temperature of the exposed system actually increased as solar radiation decreased, suggesting that the reduction in light absorption by the hydrogel was insufficient to compensate for the decrease in total cooling power caused by the decreased evaporation rate. The temperature of the isolation system decreased with decreasing light intensity, indicating that its evaporation rate remained relatively stable.

[0061] In the experiment, the average evaporation rates of the hydrogel in the isolated, contact, and exposed systems were 110, 226, and 268 g h, respectively. -1 m -2This is consistent with the experimental results. The experimental results show that the air layer plays a key role in improving the cooling performance of the radiation-evaporation refrigeration device. It can not only act as a heat insulation material to improve the system temperature drop, but also, when combined with a porous PE membrane, regulate the evaporation rate of the hydrogel, enabling the system to achieve a cooling effect below the ambient temperature for a long time under high temperature, low humidity, and strong sunlight conditions.

Claims

1. A radiation-evaporation refrigeration device, characterized in that: It consists of an upper porous PE membrane, an intermediate air layer, and a lower PVA-LiBr hydrogel. The porous PE membrane has a thickness of 50-150 μm, and its specific preparation process is as follows: High-density polyethylene powder and ultra-high molecular weight polyethylene powder are dissolved in paraffin oil at 120-130℃ to obtain a homogeneous mixture. The mixture is then cooled to 45-55℃, sliced ​​to a thickness of 0.3-0.6 mm, and hot-pressed at 120-130℃ for 6-10 min to obtain a gel membrane. Finally, the gel membrane is sequentially immersed in n-hexane and ethanol, and dried to obtain a porous PE membrane. The mass ratio of high-density polyethylene powder, ultra-high molecular weight polyethylene powder, and paraffin oil is 7:3:

40. The thickness of the intermediate air layer is designed as follows: two parallel and opposite flat plates are used, with a Rayleigh number of = ,in g It is the acceleration due to gravity. T h This refers to the temperature of the upper plate, i.e., the hot end temperature. T c This refers to the temperature of the lower plate, i.e., the cold end temperature. β The coefficient of air volume expansion. δ This refers to the thickness between the plates, i.e., the thickness of the intermediate air layer. ν The viscosity of air motion. α The thermal diffusivity of air; Pick Ra <1200, Δ T =10~30 K δ That is, the thickness of the intermediate air layer, where Δ T That is T h - T c .

2. The radiation-evaporation refrigeration device according to claim 1, characterized in that: The preparation process of the PVA-LiBr hydrogel is as follows: PVA solution, acrylamide, N,N'-methylenebisacrylamide, ammonium persulfate and lithium bromide powder are thoroughly stirred to obtain a hydrogel mother liquor; then the hydrogel mother liquor is gelled, dried and water-absorbed to obtain the PVA-LiBr hydrogel.

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