A passive radiation refrigeration progressive freezing desalination apparatus and method

Abstract

This invention provides a passive radiative cooling progressive freezing desalination device and method, belonging to the field of seawater desalination and water treatment technology. The device includes a radiative cooling unit, a high thermal conductivity substrate, a crystallization chamber, an insulation layer, and a concentrated brine discharge structure. The radiative cooling unit radiates heat into outer space through an atmospheric infrared window. The cooled energy is laterally homogenized by a metal extension body and then conducted to the brine crystallization surface by the high thermal conductivity substrate. With the assistance of an ice crystal nucleating agent, ice crystals gradually grow downwards from the crystallization surface, and salt is repelled to the lower liquid phase to form concentrated brine. The insulation layer blocks the intrusion of ambient heat from the sides and bottom, and the concentrated brine discharge structure achieves in-situ separation of ice crystals and concentrated brine. Using the device and method provided by this invention, the problem of low freshwater separation efficiency in existing technologies can be solved.

Description

Technical Field

[0001] This invention relates to the field of seawater desalination and water treatment technology, and in particular to a passive radiation cooling progressive freezing desalination device and method. Background Technology

[0002] Freshwater scarcity has become a major challenge restricting the sustainable socio-economic development of the world. Seawater desalination, as a core approach to converting seawater or brackish water into usable freshwater, has received widespread attention. The rigid demand for freshwater is particularly urgent in islands, remote coastal areas, post-disaster emergency response, and areas without power grid coverage. However, these scenarios often lack stable power supply and centralized infrastructure, making it difficult to support large-scale desalination equipment. Therefore, there is an urgent need to develop passive seawater desalination technologies that require no external energy input, have a simple structure, and can be deployed in a distributed manner.

[0003] In existing technologies, radiative cooling technology can utilize an 8μm–13μm atmospheric infrared window to dissipate heat from an object to a cold source in outer space (approximately 3K) in the form of infrared radiation. Under suitable meteorological conditions, this cools the radiating surface below the ambient temperature, achieving passive cooling without any external energy input. Current technologies utilize radiative cooling to lower the temperature of the still's condensing surface to increase the mass transfer driving force, or combine radiative cooling films with solar thermal heating to improve evaporation-condensation efficiency, or construct a photovoltaic-sky radiative cooling coupled humidification-dehumidification system. The technological focus remains on the evaporation path or systems requiring active circulation components such as pumps and fans. Regarding radiative cooling-driven cryogenic desalination, existing research has verified the feasibility of using radiative cooling plates to freeze brine and obtain fresh water through subsequent filtration, washing, and melting of ice crystals. However, in this route, the brine forms an ice crystal suspension or ice-slurry mixture within the cold plate pipes. The ice-brine mixture needs to be removed before filtration and washing to separate the salt, making the process complex. Furthermore, the brine entrained by the ice crystals limits the separation efficiency.

[0004] Existing seawater desalination technologies either rely on electricity and high-temperature heat sources, are limited by solar radiation conditions and evaporation interface failures, or have complex system structures that require active equipment to drive them. Existing radiation-assisted desalination schemes are still concentrated on evaporation-condensation routes or ice-slurry suspension separation methods, which require complex steps such as filtration and washing to separate salts. This results in the limited cooling capacity of passive radiation refrigeration not being used efficiently, leading to low freshwater separation efficiency. Summary of the Invention

[0005] This invention provides a passive radiative cooling progressive freezing desalination device and method, which can solve the problem of low freshwater separation efficiency in the prior art. The technical solution is as follows: In a first aspect, a passive radiative cooling progressive freezing desalination device includes: a radiative cooling unit, a high thermal conductivity substrate, a crystallization chamber, an insulation layer, and a concentrated brine discharge structure. The radiation cooling unit includes a radiation cooling layer and a metal extension body. The radiation cooling layer is arranged facing upwards and has a high infrared emissivity in the 8μm to 13μm atmospheric window band. The metal extension body is attached to the bottom of the radiation cooling layer and is made of a metal plate with high thermal conductivity. The high thermal conductivity substrate is disposed below the metal extension body and is thermally and tightly connected to the metal extension body, and the lower surface of the high thermal conductivity substrate forms a brine crystallization surface; The crystallization cavity is disposed below the high thermal conductivity substrate, the top of the crystallization cavity is in contact with the lower surface of the high thermal conductivity substrate, the crystallization cavity is used to contain the brine to be desalinated, and the crystallization cavity contains an ice crystal nucleating agent; The heat insulation layer covers the sides and bottom of the crystallization cavity; The concentrated brine discharge structure connects the bottom of the crystallization chamber to the outside.

[0006] Optionally, an insulating air layer and an infrared transparent cover layer are sequentially provided above the radiative cooling unit; the infrared transparent cover layer is made of a thin film material with high infrared transmittance in the 8μm to 13μm band; the insulating air layer is enclosed by the heat insulation frame between the infrared transparent cover layer and the radiative cooling unit to form a sealed space, which is filled with dry and still air.

[0007] Optionally, the thickness of the heat-insulating air layer is 10mm to 50mm; the infrared transparent cover layer is made of polyethylene film or fluorinated ethylene propylene film.

[0008] Optionally, the infrared emissivity of the radiation cooling layer in the 8μm to 13μm band is greater than 0.9; the radiation cooling layer is any one of inorganic particle-polymer composite film, porous polymer film or multilayer optical film.

[0009] Optionally, the thermal conductivity of the high thermal conductivity substrate is 200 W / (m·K) to 420 W / (m·K).

[0010] Optionally, the lower surface of the high thermal conductivity substrate is provided with a salt corrosion resistant layer, which is any one of a polytetrafluoroethylene coating, a nickel plating layer, a gold plating layer, a ceramic coating, or a surface passivation film.

[0011] Optionally, the ice crystal nucleating agent is silver iodide particles dispersed in the brine to be desalinated; Alternatively, the ice crystal nucleating agent may be a nucleation auxiliary layer disposed on the lower surface of the high thermal conductivity substrate, wherein the nucleation auxiliary layer may be any one of a micro-nano rough structure layer, an ice-loving coating, a mineral particle coating, an immobilized silver iodide particle layer, a metal oxide coating, or a mechanical scratch structure layer.

[0012] Optionally, the insulation layer is made of any one of extruded polystyrene foam, expanded polystyrene foam, polyurethane foam, aerogel felt, or vacuum insulation board; the thickness of the insulation layer is 10mm to 50mm.

[0013] Optionally, the crystallization chamber is made of a transparent or translucent material; the concentrated brine discharge structure is any one of a corrosion-resistant ball valve, a quick-connect fitting, a removable bottom cover, or a side bottom guide port.

[0014] Secondly, a passive radiation cooling progressive freezing desalination method, based on the aforementioned passive radiation cooling progressive freezing desalination device, includes the following steps: S1: Inject the brine containing the ice crystal nucleating agent into the crystallization cavity, so that the surface of the brine is in contact with the lower surface of the high thermal conductivity substrate; S2: Place the passive radiation cooling progressive freezing desalination device in an open outdoor area, so that the radiation cooling layer faces the sky, and operate at night or under low solar radiation conditions; S3: The radiation cooling layer radiates heat to outer space through the atmospheric infrared window. The generated cold energy is transferred to the high thermal conductivity substrate after being uniformly heated laterally by the metal extension body. Then, the high thermal conductivity substrate conducts the cold energy vertically to the brine crystal surface. The temperature of the brine to be desalinated in contact with the brine crystal surface gradually decreases. With the assistance of the ice crystal nucleating agent, nucleation occurs preferentially at the brine crystal surface. The ice crystal layer grows gradually downward from the brine crystal surface. Salt is repelled and enriched in the lower liquid phase to form concentrated brine. S4: When the thickness of the ice crystal layer reaches a preset value or the running time reaches a preset duration, the concentrated brine discharge structure is activated to discharge the concentrated brine from the bottom of the crystallization chamber. S5: Remove the ice crystal layer and melt it to collect the desalinated water; if it is necessary to further reduce the salinity, re-inject the desalinated water into the crystallization cavity and repeat S1 to S5.

[0015] The beneficial effects of the technical solutions provided in the embodiments of the present invention include at least the following: This invention provides a passive radiative cooling progressive freezing desalination device and method. By setting up a radiative cooling unit, wherein the radiative cooling layer is arranged upwards and has high infrared emissivity in the 8μm–13μm atmospheric window band, it can dissipate heat in the form of infrared radiation to a cold source in outer space with a temperature of approximately 3K using the atmospheric infrared window. This achieves completely passive cooling without the need for compressors, pumps, fans, or any external power input, making it suitable for applications lacking a stable power supply, such as islands, remote coastal areas, post-disaster emergencies, and areas without grid coverage. A metal extension body, made of a high thermal conductivity metal plate, is attached to the bottom of the radiative cooling layer, enabling horizontal and uniform heat diffusion from below to the radiative cooling layer, avoiding… Localized heat concentration leads to excessive temperature differences across regions of the radiative cooling layer. To ensure uniform heat dissipation throughout the entire radiative cooling layer, a high thermal conductivity substrate is positioned below and thermally bonded to the metal extension. Its lower surface forms the brine crystallization surface. This creates a continuous heat flow path—sky radiative heat dissipation, lateral temperature equalization, vertical heat conduction, and interface crystallization—through the radiative cooling layer, the metal extension, and the high thermal conductivity substrate. This concentrates the limited cooling capacity generated by radiative cooling onto the crystallization interface at the top of the brine. Ice crystals gradually grow downwards from the brine crystallization surface. Salt, based on the solute segregation effect of water, is excluded from the ice crystal lattice and diffuses downwards under gravity, enriching to form concentrated brine. This establishes a spatial distribution pattern within the crystallization cavity where ice forms at the top and salt is enriched at the bottom, achieving a harmonious interaction between ice crystals and brine. Directional and gradual separation; the crystallization chamber is located below a high thermal conductivity substrate, with its top in contact with the lower surface of the substrate, allowing direct heat exchange between the brine to be desalinated and the brine crystallization surface. The crystallization chamber contains an ice crystal nucleating agent, which reduces the supercooling required for brine solidification. This allows ice crystal formation to begin with the aid of the nucleating agent when the brine temperature drops below freezing, avoiding the problem of insufficient cooling provided by radiative refrigeration to trigger ice crystal nucleation due to excessive supercooling, thus improving the utilization efficiency of the limited cooling capacity of radiative refrigeration. An insulation layer covers the sides and bottom of the crystallization chamber, forming an enclosed insulation structure that effectively blocks ambient heat from entering the crystallization chamber from the sides and bottom, ensuring that the cooling capacity generated by radiative refrigeration is mainly transferred vertically from top to bottom in a unidirectional direction. The cooling brine and driving ice crystal growth, rather than being offset by environmental heat intrusion, ensure the stability of the heat flow direction within the crystallization chamber, further improving the efficiency of cold energy utilization. The concentrated brine discharge structure is located at the bottom of the crystallization chamber, which can directly discharge the unsolidified concentrated brine from the bottom of the crystallization chamber after the ice crystal layer has formed. This achieves in-situ separation of the ice crystal layer and the concentrated brine inside the device. This differs from the complex operation route in traditional freezing methods, where the ice-brine mixture needs to be removed after the ice crystals form an ice slurry suspension in the cold plate pipes, and then filtered and washed to separate the salt. This avoids the problem of salt re-dissolution caused by the remixing of ice crystals and concentrated brine, significantly simplifies the operation process and improves the freshwater separation efficiency, effectively solving the problem of low freshwater separation efficiency in existing technologies. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a schematic cross-sectional view of an overall structure provided in an embodiment of the present invention; Figure 2 This is another overall structural cross-sectional schematic diagram of the initial state provided in the embodiment of the present invention; Figure 3 This is a schematic cross-sectional view of another overall structure in the form of an ice crystal layer provided by an embodiment of the present invention; Figure 4 This is a schematic diagram of the temperature change curves of each part during the operation of the first group of experiments (initial salinity 3.5%) provided in the embodiments of the present invention; Figure 5 This is a schematic diagram of the temperature change curves of each part during the operation of the second set of experiments (initial salinity 1.75%) provided in the embodiments of the present invention; Figure 6 This is a flowchart of the method provided in an embodiment of the present invention.

[0018] In the diagram: 1-Radiative cooling unit; 11-Radiative cooling layer; 12-Metal extension body; 2-High thermal conductivity substrate; 21-Brine crystallization surface; 3-Crystallization cavity; 4-Insulation layer; 5-Concentrated brine discharge structure; 6-Brine to be desalinated; 61-Ice crystal layer; 62-Concentrated brine; 7-Insulating air layer; 8-Infrared transparent cover layer. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0020] Figure 1 This is a schematic cross-sectional view of an overall structure provided in an embodiment of the present invention; Figure 2 This is another overall structural cross-sectional schematic diagram of the initial state provided in the embodiment of the present invention; Figure 3 This is a schematic cross-sectional view of another overall structure in the form of an ice crystal layer provided by an embodiment of the present invention; Figure 4 This is a schematic diagram of the temperature change curves of each part during the operation of the first group of experiments (initial salinity 3.5%) provided in the embodiments of the present invention; Figure 5This is a schematic diagram of the temperature change curves of each part during the operation of the second set of experiments (initial salinity 1.75%) provided in the embodiments of the present invention; Figure 6 This is a flowchart of a method provided in an embodiment of the present invention. Figures 1 to 6A passive radiative cooling progressive cryogenic desalination device is shown, comprising: a radiative cooling unit 1, a high thermal conductivity substrate 2, a crystallization chamber 3, an insulation layer 4, and a concentrated brine discharge structure 5. The radiative cooling unit 1 includes a radiative cooling layer 11 and a metal extension 12. The radiative cooling layer 11 is arranged upwards and has high infrared emissivity in the 8μm–13μm atmospheric window band, used to dissipate heat in the form of infrared radiation through the atmospheric window to a cold source in outer space with a temperature of approximately 3K. The metal extension 12 is attached to the bottom of the radiative cooling layer 11. The metal extension 12 is made of a high thermal conductivity metal plate, forming a tight thermal coupling interface with the radiative cooling layer 11. This interface is used to uniformly transfer heat from below to the radiative cooling layer 11 after horizontal temperature diffusion, thereby avoiding localized heat concentration that could lead to excessive temperature differences in different areas of the radiative cooling layer 11 and ensuring uniform heat dissipation throughout the radiative cooling layer 11. The thermal conductivity of the metal extension body 12 is not less than 150 W / (m·K), preferably a metal material with a thermal conductivity of 200 W / (m·K) to 420 W / (m·K), specifically, any one of pure aluminum plate, aluminum alloy plate, copper plate, or copper-aluminum clad plate can be used. The thickness of the metal extension body 12 is 1 mm to 10 mm, preferably 3 mm to 5 mm; when the thickness is too small, the lateral temperature diffusion capacity of the metal extension body 12 is insufficient, and the temperature difference between different areas of the radiative cooling layer 11 is large; when the thickness is too large, the thermal resistance along the thickness direction increases and the overall weight of the device increases. The planar dimensions of the metal extension body 12 are not less than the upper surface dimensions of the high thermal conductivity substrate 2 to ensure that the cooling capacity can cover the entire brine crystallization surface 21. The high thermal conductivity substrate 2 is disposed below the metal extension 12 and is thermally tightly connected to the metal extension 12. The two are tightly thermally coupled through a low thermal resistance connection, forming a continuous cooling channel from the high thermal conductivity substrate 2 through the metal extension 12 to the radiative cooling layer 11. The specific connection method can be any of the following: the lower surface of the metal extension 12 and the upper surface of the high thermal conductivity substrate 2 are mechanically ground flat and then directly contacted, and a clamping force is applied by bolts or clamps; or flexible thermally conductive gap-filling materials such as thermally conductive silicone grease, thermally conductive phase change pads, or indium foil are filled between them, wherein the coating thickness of the thermally conductive silicone grease is 0.05mm to 0.3mm, the thickness of the thermally conductive phase change pad is 0.2mm to 1mm, and the thickness of the indium foil is 0.05mm to 0.2mm, and the thermal conductivity of the aforementioned gap-filling materials is not less than 1W / (m·K); or the metal extension 12 and the high thermal conductivity substrate 2 are metallurgically bonded together using low-temperature welding or diffusion welding processes. The bonding connection between the radiation cooling layer 11 and the metal extension body 12 is achieved through interface thermal coupling treatment.Specifically, one of the following methods can be used: the radiation cooling layer 11 can be directly coated or pressed onto the upper surface of the metal extension body 12, utilizing the adhesion of the radiation cooling film itself to form a tight contact with the metal plate; or a thin layer of thermally conductive adhesive can be applied between the radiation cooling layer 11 and the metal extension body 12, with a thickness of 0.05mm to 0.5mm and a thermal conductivity of not less than 1W / (m·K), ensuring low interfacial thermal resistance while achieving bonding and fixation; or the radiation cooling layer 11 can be pressed tightly onto the upper surface of the metal extension body 12 by a mechanical clamping frame, and thermally conductive silicone grease can be filled between the contact surfaces to eliminate interfacial air gaps. The lower surface of the high thermal conductivity substrate 2 forms a brine crystallization surface 21, which is an interface for preferential nucleation and directional growth of ice crystals. The crystallization cavity 3 is located below the high thermal conductivity substrate 2, and the top of the crystallization cavity 3 is in contact with the lower surface of the high thermal conductivity substrate 2, so that the brine to be desalinated 6 in the crystallization cavity 3 can directly exchange heat with the brine crystallization surface 21. The crystallization chamber 3 is used to contain the brine 6 to be desalinated. An ice crystal nucleating agent is provided inside the crystallization chamber 3 to reduce the supercooling required for brine solidification and promote preferential nucleation of ice crystals at the brine crystallization surface 21. An insulation layer 4 covers the sides and bottom of the crystallization chamber 3, forming an enclosed insulation structure to block ambient heat from entering the crystallization chamber 3 from the sides and bottom. This ensures that the limited cooling capacity generated by the radiant cooling unit 1 is mainly transferred vertically from top to bottom, without being offset by ambient heat intrusion from the sides and bottom, thus guaranteeing that the heat flow within the crystallization chamber 3 is unidirectional, from top to bottom. A concentrated brine discharge structure 5 is located at the bottom of the crystallization chamber 3 to discharge unsolidified concentrated brine 62 after the formation of the ice crystal layer 61.

[0021] Exemplarily, in this embodiment of the invention, when the device is in operation, the radiative cooling layer 11 faces the sky and continuously radiates heat into outer space through an 8μm to 13μm atmospheric infrared window, causing the temperature of the radiative cooling layer 11 to drop below the ambient temperature, thus generating a cooling effect. The cooling energy generated by the radiative cooling layer 11 is first transferred to the metal extension 12 attached to its bottom. The metal extension 12 utilizes its high thermal conductivity to diffuse the cooling energy laterally and uniformly in the horizontal plane, eliminating the temperature gradient caused by uneven heat dissipation in different areas of the radiative cooling layer 11, making the temperature field more uniform. After being uniformly cooled laterally, the cooling energy is transferred downward from the metal extension 12 to the high thermal conductivity substrate 2, which is thermally tightly connected to it. The high thermal conductivity substrate 2 further introduces the cooling energy into the brine crystallization surface 21 formed by its lower surface in the vertical direction. The brine 6 to be desalinated, which is in direct contact with the brine crystallization surface 21, is first cooled, and the temperature gradually decreases below the freezing point. With the assistance of the ice crystal nucleating agent, ice crystals preferentially nucleate at the brine crystallization surface 21. As the cold energy is continuously transferred downwards from the top, the ice crystal layer 61 gradually grows downwards from the brine crystallization surface 21. During the ice crystal solidification process, based on the solute segregation effect of water, salt ions are mainly repelled outside the ice crystal lattice. The repelled salt diffuses downwards under the action of gravity and accumulates in the liquid phase region at the bottom of the crystallization chamber 3, forming concentrated brine 62 with a salinity higher than the initial brine. This establishes a spatial distribution pattern of ice formation at the top and salt enrichment at the bottom within the crystallization chamber 3. During this process, the insulation layer 4 continuously blocks the intrusion of ambient heat from the sides and bottom, ensuring the stable growth direction of the ice crystal layer 61 and high cold energy utilization efficiency. When the thickness of the ice crystal layer 61 reaches a preset value or the operating time reaches a preset duration, the concentrated brine discharge structure 5 is activated to discharge the concentrated brine 62 located at the bottom of the crystallization chamber 3, achieving in-situ separation of the ice crystal layer 61 and the concentrated brine 62 inside the device. After discharging the concentrated brine 62, the ice crystal layer 61 is removed and melted, and the resulting desalinated water can be collected. A continuous heat flow path—from sky radiation heat dissipation, lateral temperature equalization, vertical cooling, to interface crystallization—is constructed using a radiative cooling layer 11, a metal extension body 12, and a high thermal conductivity substrate 2. This concentrates the limited cooling capacity generated by radiative cooling onto the crystallization interface at the top of the brine, preventing the cooling capacity from dispersing throughout the device or the surrounding environment. The insulation layer 4 effectively blocks the intrusion of environmental heat by covering the sides and bottom of the crystallization chamber 3, ensuring a stable and controllable growth position for the ice crystal layer 61. The concentrated brine discharge structure 5 discharges the concentrated brine 62 before the ice crystals melt, preventing the ice crystals from remixing with the concentrated brine and causing salt re-dissolution. This differs from the separation route where ice slurry suspensions are removed and then filtered, washed, and melted, simplifying the operation and improving freshwater separation efficiency. The entire device requires no compressor, pump, fan, or external power input, relying entirely on radiative cooling for passive cooling-driven cryogenic desalination. It is suitable for distributed freshwater supply in islands, remote coastal areas, field emergencies, and areas without power grid coverage.

[0022] Optionally, an insulating air layer 7 and an infrared transparent cover layer 8 are sequentially provided above the radiative cooling unit 1; the infrared transparent cover layer 8 is made of a thin film material with high infrared transmittance in the 8μm to 13μm band; the insulating air layer 7 is enclosed by the heat insulation frame between the infrared transparent cover layer 8 and the radiative cooling unit 1 to form a sealed space, which is filled with dry and still air.

[0023] Exemplarily, in this embodiment of the invention, the infrared transparent cover layer 8 is disposed at the top of the device, above the heat-insulating air layer 7, forming a physical isolation interface between the device and the external environment. The heat-insulating frame is disposed along the perimeter between the infrared transparent cover layer 8 and the radiative cooling unit 1, enclosing the space between the infrared transparent cover layer 8 and the radiative cooling layer 11 into a closed cavity, preventing outside air from entering. The infrared transparent cover layer 8, while ensuring efficient transmission of infrared radiation in the 8μm–13μm band, reduces the impact of convective disturbances of outside air on the surface temperature of the radiative cooling layer 11, preventing dust, frost, and precipitation from directly depositing on the surface of the radiative cooling layer 11 and reducing its infrared emission performance. The dry, still air in the heat-insulating air layer 7 has low thermal conductivity, forming an effective thermal barrier layer between the infrared transparent cover layer 8 and the radiative cooling unit 1, inhibiting ambient heat from intruding into the radiative cooling layer 11 from above through convection and heat conduction, further reducing the surface temperature of the radiative cooling layer 11 and increasing the cooling effect of radiative cooling. Dry air also reduces the risk of water vapor condensation or frost formation inside the insulating air layer 7, preventing frost from blocking the infrared radiation channel and causing radiation cooling failure. During operation, the infrared radiation emitted by the radiation cooling layer 11 passes sequentially through the dry, still air in the insulating air layer 7 and the infrared transparent capping layer 8 before entering the atmosphere and finally radiating to the outer space cold source through the atmospheric infrared window. Due to the synergistic protective effect of the insulating air layer 7 and the infrared transparent capping layer 8, the environmental convection heat transfer and heat conduction interference experienced by the radiation cooling layer 11 are significantly reduced, allowing its surface temperature to drop to a lower level. This increases the temperature difference between the radiation cooling layer 11 and the brine crystallization surface 21, accelerates the rate of cold energy transfer downwards along the metal extension body 12 and the high thermal conductivity substrate 2, and improves the growth rate of the ice crystal layer 61 and the overall cryogenic desalination efficiency.

[0024] Optionally, the thickness of the heat insulation air layer 7 is 10mm to 50mm; the infrared transparent cover layer 8 is made of polyethylene film or fluorinated ethylene propylene film.

[0025] For example, in this embodiment of the invention, when the thickness of the insulating air layer 7 is too small, its thermal resistance is insufficient to effectively suppress heat conduction and convective heat transfer between the infrared transparent cover layer 8 and the radiative cooling layer 11. Ambient heat can easily penetrate the insulating air layer 7 and invade the radiative cooling layer 11, reducing the cooling effect of radiative cooling. When the thickness of the insulating air layer 7 is too large, the air in the sealed space may generate natural convection, which will increase the convective heat transfer and weaken the insulation effect. At the same time, it will also increase the overall height and volume of the device, which is not conducive to miniaturization and portability. Within the thickness range of 10mm to 50mm, the insulating air layer 7 can achieve a good balance between suppressing heat conduction and avoiding natural convection, and the thickness is preferably about 20mm. The infrared transparent cover layer 8 is made of polyethylene film or fluorinated ethylene propylene film. Polyethylene film has high infrared transmittance in the 8μm to 13μm band, low cost and easy to process and mold, and is suitable for application scenarios with high economic requirements. Fluorinated ethylene propylene film also exhibits excellent infrared transmittance in the same wavelength band, and its weather resistance, UV aging resistance, and mechanical strength are superior to polyethylene film, making it suitable for stable operation in long-term outdoor exposure environments. The thickness of the infrared transparent cover layer 8 is 10μm to 100μm, preferably 15μm to 50μm. When the film thickness is too small, the mechanical strength is insufficient, and it is prone to cracking or excessive relaxation deformation under outdoor wind loads and its own weight, making it difficult to maintain the structural integrity of the sealed space. When the film thickness is too large, the absorption of infrared radiation in the 8μm to 13μm wavelength band by the film itself increases, the infrared transmittance decreases, and the net radiative heat dissipation capacity of the radiative cooling layer 11 to outer space is weakened. Within the thickness range of 10μm to 100μm, both polyethylene film and fluorinated ethylene propylene film can maintain an infrared transmittance of over 0.7 in the atmospheric window band, while possessing sufficient mechanical strength and weather resistance. Both of these film materials can ensure efficient transmission of infrared radiation emitted by the radiative cooling layer 11, while providing reliable physical protection for the radiative cooling unit 1.

[0026] Optionally, the infrared emissivity of the radiation cooling layer 11 in the 8μm to 13μm band is greater than 0.9; the radiation cooling layer 11 is any one of inorganic particle-polymer composite film, porous polymer film or multilayer optical film.

[0027] For example, in this embodiment of the invention, the 8μm to 13μm band corresponds to the main infrared transparency window of the Earth's atmosphere. Within this band, the atmosphere's absorption rate of infrared radiation is extremely low. The heat radiated by the radiation cooling layer 11 through this window can penetrate the atmosphere to the maximum extent without being absorbed or reflected back, thus achieving net radiative heat dissipation. When the infrared emissivity of the radiation cooling layer 11 in this band is greater than 0.9, the radiation cooling layer 11 can radiate heat into outer space through the atmospheric window with near-blackbody efficiency, maximizing the radiative heat dissipation power per unit area. This allows the surface temperature of the radiation cooling layer 11 to be reduced as low as possible below the ambient temperature, generating greater cooling output and facilitating sufficient cooling driving force for the brine crystallization surface 21. The inorganic particle-polymer composite membrane disperses inorganic particles such as silica and barium sulfate in a polymer matrix. The phonon resonance absorption characteristics of the inorganic particles enhance the infrared emissivity of the membrane in the atmospheric window band. Simultaneously, the polymer matrix provides flexibility and processability, enabling large-area, low-cost fabrication. Porous polymer films, by constructing micro-nano-scale pore structures within the polymer film, utilize the refractive index difference between the pores and the polymer matrix to generate infrared scattering and enhanced emission effects, achieving high emissivity in the atmospheric window band. Multilayer optical films, by alternately stacking thin film layers with different refractive indices, utilize thin film interference effects to achieve selective high emissivity in the target band, and band-selective radiation characteristics can be designed as needed. In this embodiment, the radiation cooling layer 11 serves as the cooling source for the entire device. The radiation cooling layer 11 is positioned towards the sky, continuously emitting infrared radiation in the 8μm–13μm band towards the outer space cold source, dissipating heat from itself and the heat transferred below in the form of electromagnetic waves. Due to the extremely low temperature of the outer space cold source and the high transmittance of the atmospheric window band, the net radiative heat dissipation of the radiation cooling layer 11 is greater than the radiative heat absorbed from the environment, causing the temperature of the radiation cooling layer 11 to continuously drop below the ambient temperature, forming a spontaneous passive cooling effect. This cooling energy is then transferred to the brine crystallization surface 21 along the cooling path of the metal extension 12 and the high thermal conductivity substrate 2, driving the brine to freeze and crystallize.

[0028] Optionally, the thermal conductivity of the high thermal conductivity substrate 2 is 200 W / (m·K) to 420 W / (m·K); the high thermal conductivity substrate 2 is any one of copper plate, aluminum plate, graphite composite plate, graphene composite plate or metal-ceramic composite plate.

[0029] Exemplarily, in this embodiment of the invention, the high thermal conductivity substrate 2 plays a crucial role in the device by vertically conducting the cooling energy generated by the radiative cooling unit 1 from the metal extension body 12 downwards to the brine crystallization surface 21. When the thermal conductivity of the high thermal conductivity substrate 2 is in the range of 200 W / (m·K) to 420 W / (m·K), the cooling energy can be transferred from the upper surface of the high thermal conductivity substrate 2 to the brine crystallization surface 21 on its lower surface with low thermal resistance and small temperature loss, ensuring that the temperature of the brine crystallization surface 21 can be effectively reduced to below the freezing point of the brine, providing sufficient driving force for ice crystal nucleation and gradual growth. If the thermal conductivity is too low, the thermal resistance of the high thermal conductivity substrate 2 itself will consume a large temperature difference, resulting in the temperature of the brine crystallization surface 21 not being able to drop to a sufficiently low level, making it difficult for ice crystals to nucleate and grow smoothly. The thermal conductivity of the copper plate is approximately 398 W / (m·K), which is the best thermal conductivity among the above materials, making it suitable for applications with the highest requirements for cooling efficiency. The aluminum plate has a thermal conductivity of approximately 237 W / (m·K), combining good thermal conductivity with low density, which helps reduce the overall weight of the device and facilitates handling and deployment. Graphite composite plates and graphene composite plates utilize the high in-plane thermal conductivity of graphite or graphene, exhibiting excellent temperature uniformity in the planar direction, making them suitable for scenarios requiring large-area uniform cooling. Metal-ceramic composite plates combine the high thermal conductivity of metals with the corrosion resistance of ceramics, reducing the performance degradation of the high thermal conductivity substrate 2 in long-term contact with brine environments. During operation, the upper surface of the high thermal conductivity substrate 2 is tightly connected to the metal extension 12 to receive cooling energy, which is vertically conducted along the thickness direction of the high thermal conductivity substrate 2 to the brine crystallization surface 21 on its lower surface. Due to the high thermal conductivity of the high thermal conductivity substrate 2, the temperature gradient of this vertical cooling process is small, allowing the brine crystallization surface 21 to achieve a low temperature level close to that of the bottom of the metal extension 12, ensuring that the brine 6 to be desalinated in contact with the brine crystallization surface 21 is effectively cooled and smoothly initiates the solidification process with the assistance of the ice crystal nucleating agent.

[0030] Optionally, the lower surface of the high thermal conductivity substrate 2 is provided with a salt corrosion resistant layer, which is any one of polytetrafluoroethylene coating, nickel plating, gold plating, ceramic coating or surface passivation film.

[0031] Exemplarily, in this embodiment of the invention, the lower surface of the high thermal conductivity substrate 2, namely the brine crystallization surface 21, is in direct contact with the brine during long-term operation of the device. Because brine is highly corrosive, if the lower surface of the high thermal conductivity substrate 2 is not protected, chloride ions and other corrosive ions in the brine will continuously erode the metal surface of the high thermal conductivity substrate 2, causing pitting corrosion, crevice corrosion, or uniform corrosion. This leads to uncontrollable changes in the surface roughness of the high thermal conductivity substrate 2, decreased thermal conductivity, reduced structural strength, and even the generation of corrosion products that pollute the desalinated water, seriously affecting the long-term service life of the device and the quality of the desalinated water. A salt-resistant corrosion-resistant layer covers the lower surface of the high thermal conductivity substrate 2 between it and the brine to be desalinated 6, forming a chemically inert protective barrier. The polytetrafluoroethylene coating has extremely low surface energy and excellent chemical corrosion resistance, effectively resisting brine erosion. The nickel plating layer has good corrosion resistance in a brine environment and high surface hardness, resisting mechanical wear during ice crystal growth and desorption. The gold plating layer exhibits extremely high chemical stability, exhibiting no corrosion in various salt and acid / alkali environments, making it suitable for applications requiring extremely high durability. The ceramic coating combines corrosion resistance and wear resistance with some thermal conductivity, protecting the surface of the high thermal conductivity substrate 2 while minimizing the thermal resistance to cold transfer. The surface passivation film is generated in situ on the metal surface of the high thermal conductivity substrate 2 using chemical or electrochemical methods, resulting in a dense oxide protective film with a simple process and strong adhesion to the substrate. During operation, the presence of the salt corrosion resistant layer ensures that the lower surface of the high thermal conductivity substrate 2 remains stable under long-term repeated contact with high-salinity brine and concentrated brine, allowing for continuous and uniform cold transfer from the high thermal conductivity substrate 2 to the brine crystallization surface 21, guaranteeing the stability and repeatability of the ice crystal nucleation and growth process.

[0032] Optionally, the ice crystal nucleating agent is silver iodide particles dispersed in the brine 6 to be desalinated; or, the ice crystal nucleating agent is a nucleation auxiliary layer disposed on the lower surface of the high thermal conductivity substrate 2, and the nucleation auxiliary layer is any one of the following: micro-nano rough structure layer, ice-loving coating, mineral particle coating, immobilized silver iodide particle layer, metal oxide coating, or mechanical scratch structure layer.

[0033] For example, in this embodiment of the invention, brine contains dissolved salts, its freezing point is lower than that of pure water, and it exhibits significant supercooling during solidification. This means the brine temperature must drop far below its equilibrium freezing point to spontaneously form ice crystal nuclei. In passive cryogenic desalination driven by radiative cooling, the cooling capacity generated by radiative cooling is limited. If the brine is excessively supercooled, the temperature drop provided by radiative cooling may be insufficient to overcome the supercooling barrier and trigger ice crystal nucleation, resulting in the brine remaining in a supercooled liquid state and unable to crystallize, even though it has dropped below the freezing point. The role of the ice crystal nucleating agent is to reduce the supercooling required for ice crystal nucleation, allowing the brine to initiate the ice crystal formation process with the aid of the nucleating agent as soon as its temperature drops below the freezing point, thereby improving the utilization efficiency of the limited cooling capacity of radiative cooling. When the ice crystal nucleating agent is silver iodide particles dispersed in the brine to be desalinated (6), the crystal structure of the silver iodide particles has a high lattice matching degree with the crystal structure of ice, and can serve as a heterogeneous nucleation substrate to reduce the free energy barrier and supercooling required for ice crystal nucleation. Silver iodide particles are uniformly dispersed in brine. When the brine temperature drops to near the freezing point, ice crystal nucleation is preferentially triggered on the surface of the silver iodide particles. The particle size range of silver iodide is 0.1 μm to 50 μm, preferably 1 μm to 10 μm. If the particle size is too small, the particles are prone to agglomeration, making dispersion difficult and reducing nucleation efficiency. If the particle size is too large, the particles have poor suspension stability in the brine and tend to settle quickly to the bottom of the cavity, away from the brine crystallization surface 21, and cannot play an effective nucleation-promoting role at the ice crystal growth front. The mass concentration of silver iodide particles added to the brine to be desalinated 6 is 0.01 g / L to 1 g / L, preferably 0.05 g / L to 0.2 g / L. If the concentration is too low, the number of nucleation sites is insufficient, and it is difficult to initiate the nucleation process even when the brine is highly supercooled. If the concentration is too high, it may have an adverse effect on the quality of the desalinated water and reduce economic efficiency. After adding silver iodide particles, ultrasonic stirring is used for 5 min to 30 min to ensure uniform dispersion of the particles in the brine. Because the temperature is lowest at the brine crystallization surface 21, silver iodide particles in this region first reach the nucleation conditions. Ice crystals preferentially form near the brine crystallization surface 21 and fuse with the already nucleated ice crystal layer 61, ensuring the directional and gradual growth of the ice crystal layer 61. When the ice crystal nucleating agent is a nucleation aid layer placed on the lower surface of the high thermal conductivity substrate 2, the nucleation aid effect occurs directly on the brine crystallization surface 21. The micro-nano rough structure layer increases the effective contact area and provides a large number of edges and defect sites by constructing micro-nano-scale protrusions, pits, or textures on the brine crystallization surface 21, which is conducive to the preferential nucleation of ice crystals at these sites. The ice-loving coating makes the brine crystallization surface 21 ice-loving through surface chemical modification, reduces the interfacial energy between ice crystals and the surface, and promotes the preferential formation of ice crystals on the surface. The mineral particle coating fixes mineral particles with the ability to promote ice crystal nucleation on the brine crystallization surface 21, and uses the lattice matching effect of mineral particles to assist ice crystal nucleation.The immobilized silver iodide particle layer fixes silver iodide particles onto the brine crystallization surface 21 through bonding or deposition, combining the high nucleation-promoting ability of silver iodide with the advantages of reusability and non-contamination of desalinated water after immobilization. The metal oxide coating utilizes the interfacial affinity between the surface of some metal oxides and ice crystals to assist ice crystal nucleation. The mechanically scratched structure layer forms regular or irregular scratch patterns on the brine crystallization surface 21 through mechanical processing, providing physical defect sites for ice crystal nucleation. Typical preparation methods for the above-mentioned nucleation auxiliary layers are as follows: The micro / nano rough structure layer is obtained by sandblasting, acid etching, or laser etching of the lower surface of the high thermal conductivity substrate 2, with a surface roughness Ra controlled within the range of 0.5 μm to 50 μm; the ice-loving coating is formed by spin-coating or spraying a polymer solution containing hydrophilic groups onto the lower surface of the high thermal conductivity substrate 2 and then drying and curing it, with a coating thickness of 1 μm to 50 μm, and any one of polyvinyl alcohol, polydopamine, or hydroxyl-functionalized silane coupling agent can be used as the coating material; the mineral particle coating is formed by mixing mineral microparticles with ice crystal nucleation ability, such as kaolin, feldspar powder, or mica powder, with a binder, coating them onto the lower surface of the high thermal conductivity substrate 2, and then curing them, with the particle size of the mineral particles in the coating being 1 μm to 100 μm. The coating thickness is 10μm to 200μm; the immobilized silver iodide particle layer is formed by dispersing silver iodide particles in binders such as epoxy resin, silicone or polyurethane to form a slurry, which is then coated onto the lower surface of the high thermal conductivity substrate 2 and cured at room temperature or by heating, with a coating thickness of 10μm to 100μm; the metal oxide coating is prepared by sol-gel method, magnetron sputtering or chemical vapor deposition on the lower surface of the high thermal conductivity substrate 2 to form alumina, titanium oxide or zinc oxide film, with a coating thickness of 0.1μm to 10μm; the mechanical scratch structure layer is formed by grinding with sandpaper, diamond scribing pen or CNC milling to form regular or irregular grooves with a spacing of 0.1mm to 2mm and a depth of 0.01mm to 0.5mm on the lower surface of the high thermal conductivity substrate 2. The above-mentioned nucleation assistance method makes the nucleation of ice crystals at the brine crystallization surface 21 more reliable and repeatable, ensuring that ice crystals can be stably formed and gradually grown on the predetermined crystallization interface in each operating cycle, thereby improving the stability and consistency of the device operation.

[0034] Optionally, the insulation layer 4 is made of any one of extruded polystyrene foam, expanded polystyrene foam, polyurethane foam, aerogel felt or vacuum insulation board; the thickness of the insulation layer 4 is 10mm to 50mm.

[0035] Exemplarily, in this embodiment of the invention, the insulation layer 4 covers the sides and bottom of the crystallization cavity 3. Its core function is to construct a thermal barrier, preventing ambient heat from penetrating into the crystallization cavity 3 from the sides and bottom. The cooling capacity generated by radiative cooling is limited. If a large amount of ambient heat penetrates into the crystallization cavity 3 from the sides and bottom, the intruding heat will cancel out the cooling capacity generated by radiative cooling, preventing the brine temperature inside the crystallization cavity 3 from dropping below the freezing point. This prevents ice crystals from nucleating and growing, thus hindering the desalination function. The insulation layer 4, through its low thermal conductivity material forming a thermal barrier, minimizes the intrusion path of ambient heat, allowing the cooling capacity generated by radiative cooling to be primarily used for cooling the brine and driving ice crystal growth, rather than resisting ambient heat intrusion, thereby improving the efficiency of cooling capacity utilization. Extruded polystyrene foam has a closed-cell structure, low water absorption, and low thermal conductivity, making it suitable for working environments where it may come into contact with water vapor or splashing brine. Expanded polystyrene foam is low in cost and easy to process into the required shape, making it suitable for applications with high economic requirements. Polyurethane foam has the lowest thermal conductivity among common foam insulation materials, enabling it to achieve high thermal resistance with a relatively small thickness, making it suitable for designs with high requirements for device compactness. Aerogel felt has extremely low thermal conductivity, providing superior insulation performance compared to the aforementioned foam materials, and its flexibility and bendability facilitate the covering of irregularly shaped crystallizing cavities. Vacuum insulation panels eliminate gas heat conduction and convective heat transfer under vacuum conditions, resulting in optimal insulation performance. They achieve excellent insulation effects with extremely thin thicknesses, making them suitable for applications with strict limitations on device size and weight. The thickness of insulation layer 4 ranges from 10mm to 50mm, determined based on the thermal conductivity of different insulation materials and actual operating conditions. Greater thickness results in better insulation, but increases device size and weight; smaller thickness makes the device more compact and portable, but reduces insulation performance. Within the 10mm to 50mm range, all the aforementioned insulation materials provide sufficient thermal protection for the crystallizing cavity 3, allowing the cooling energy from radiative cooling to effectively drive the growth of the ice crystal layer 61.

[0036] Optionally, the crystallization chamber 3 is made of a transparent or translucent material; the concentrated brine discharge structure 5 is any one of a corrosion-resistant ball valve, a quick-connect fitting, a removable bottom cover, or a side bottom guide port.

[0037] For example, in this embodiment of the invention, the crystallization chamber 3 is made of a transparent or semi-transparent material, allowing the operator to directly observe the growth state, thickness changes, interface position between the ice crystal layer 61 and the concentrated brine 62, and the salt repulsion and enrichment process through the wall of the crystallization chamber 3. The transparent or semi-transparent material can be acrylic, polycarbonate, or glass, etc. This visualization feature facilitates the operator's judgment on whether the ice crystal layer 61 has reached the preset thickness, determines the timing for activating the concentrated brine discharge structure 5, and assesses the operating status of the device. This avoids improper timing of operation due to the inability to observe the internal situation, excessively thin ice crystal layer 61 affecting desalination water production, or excessively thick ice crystal layer 61 preventing the concentrated brine 62 from being discharged smoothly. The concentrated brine discharge structure 5 is located at the bottom of the crystallization chamber 3 and is used to discharge the concentrated brine 62 located at the bottom of the crystallization chamber 3 after the ice crystal layer 61 has formed. The corrosion-resistant ball valve can withstand long-term corrosion from high-salinity brine. It is easy to operate and has excellent sealing properties. It remains closed during ice crystal solidification to prevent brine leakage and opens quickly after solidification to discharge concentrated brine 62. The quick-connect fitting facilitates rapid connection and disconnection of the discharge line, making it suitable for experimental or portable applications requiring frequent disassembly and reassembly. The removable bottom cover allows for rapid discharge of concentrated brine 62 and removal of the ice crystal layer 61 by removing the cover plate at the bottom of the crystallization chamber 3. Its simple structure is suitable for small devices. A side-bottom guide port is located on the side wall of the crystallization chamber 3 near the bottom, allowing concentrated brine 62 to flow out naturally using gravity without additional force. During operation, in the ice crystal solidification stage, the concentrated brine discharge structure 5 remains closed. The crystallization chamber 3 is a sealed container where the brine to be desalinated 6 receives cooling energy from the top and gradually solidifies to form the ice crystal layer 61. Once the operator observes through the transparent wall of the crystallization chamber 3 that the ice crystal layer 61 has reached the predetermined thickness, or that the operating time has reached the preset duration, the concentrated brine discharge structure 5 is activated. The concentrated brine 62 flows out from the bottom of the crystallization chamber 3 under gravity and is collected. After the concentrated brine 62 is discharged, the ice crystal layer 61 remains on the upper part of the crystallization chamber 3, adhering to the lower surface of the high thermal conductivity substrate 2 or the top area of ​​the crystallization chamber 3. The ice crystal layer 61 is then removed and melted, and the resulting desalinated water is collected.

[0038] A passive radiation refrigeration progressive freezing desalination method, based on the aforementioned passive radiation refrigeration progressive freezing desalination device, includes the following steps: S1: Inject the brine 6 containing ice crystal nucleating agent into the crystallization chamber 3, so that the liquid surface of the brine 6 is in contact with the lower surface of the high thermal conductivity substrate 2.

[0039] Specifically, in this embodiment, the brine to be desalinated 6 can be natural seawater, simulated seawater, or other saline bodies. During injection, the liquid level should be controlled to ensure sufficient contact between the brine surface and the lower surface of the high thermal conductivity substrate 2, guaranteeing that cooling energy can be directly transferred from the brine crystallization surface 21 to the brine to be desalinated 6. The ice crystal nucleating agent can be pre-dispersed in the brine to be desalinated 6, for example, by adding silver iodide particles to the brine and then ultrasonically stirring to ensure uniform dispersion; or the ice crystal nucleating agent can be pre-placed on the lower surface of the high thermal conductivity substrate 2 in the form of a nucleation auxiliary layer. During this stage, the concentrated brine discharge structure 5 is in a closed state, ensuring that the crystallization chamber 3 is a sealed container and preventing brine leakage.

[0040] S2: Place the passive radiation cooling progressive freezing desalination device in an open outdoor area, so that the radiation cooling layer 11 faces the sky, and operate at night or under low solar radiation conditions.

[0041] Specifically, in this embodiment, the device should be placed in an area with a wide, unobstructed view of the sky, allowing the radiative cooling layer 11 to face the sky to the maximum extent, increasing the solid angle of heat dissipation radiated into outer space. During nighttime operation, without solar radiation interference, the net radiative heat dissipation of the radiative cooling layer 11 is maximized, resulting in the best cooling effect. Under low solar irradiance conditions, such as cloudy days, early morning, or evening, the radiative cooling layer 11 can also achieve a certain degree of net radiative heat dissipation. Clear skies, low cloud cover, and low humidity are conducive to increasing the transmittance of the atmospheric window, enhancing the radiative cooling effect. The suitable ambient temperature range for device operation is -5℃ to 15℃. Under suitable meteorological conditions, radiative cooling technology can typically achieve a temperature reduction of 5℃ to 15℃ below the ambient temperature on the radiating surface. When the ambient temperature is above 15℃, even if radiative cooling reaches its maximum reduction, the radiating surface temperature may still be above the freezing point of brine, making it difficult for the brine to freeze; when the ambient temperature is below -5℃, the natural low temperature of the environment is sufficient to freeze the brine, and the advantages of radiative cooling are not significant. Within the aforementioned temperature range, the cooling effect of radiative cooling, combined with the low-temperature effect of the environment itself, can effectively lower the temperature of the brine crystallization surface 21 below the freezing point of the brine, driving ice crystal nucleation and gradual growth. Furthermore, the relative humidity of the environment should ideally be below 70%. Excessive humidity will increase the absorption of 8μm–13μm infrared radiation by water vapor in the atmosphere, weakening the radiative cooling effect and increasing the risk of condensation or frost formation on the surface of the radiative cooling layer 11. During operation, the ambient wind speed should ideally be below 3m / s. Excessive wind speed will intensify convective heat transfer on the surface of the radiative cooling layer 11, partially offsetting the cooling effect of radiative cooling. If operating in an environment with high wind speeds, windbreaks can be installed around the device for protection. The height of the windbreaks should not be less than the overall height of the device, and a distance of 50mm–200mm should be maintained between the windbreaks and the device. The top of the windbreaks should remain open to avoid obstructing the view of the radiative cooling layer 11 radiating towards the sky.

[0042] S3: The radiation cooling layer 11 radiates heat to outer space through the atmospheric infrared window. The generated cold energy is transferred to the high thermal conductivity substrate 2 after being uniformly heated laterally by the metal extension body 12. Then, it is vertically cooled by the high thermal conductivity substrate 2 to the brine crystallization surface 21. The temperature of the brine 6 to be desalinated, which is in contact with the brine crystallization surface 21, gradually decreases. With the assistance of the ice crystal nucleating agent, it preferentially nucleates at the brine crystallization surface 21. The ice crystal layer 61 grows gradually downward from the brine crystallization surface 21. The salt is repelled and enriched in the lower liquid phase to form concentrated brine 62.

[0043] Specifically, in this embodiment, this step is the core cryo-crystallization stage of the device, and the energy transfer process is as follows: The radiative cooling layer 11 continuously emits infrared radiation in the 8μm~13μm band towards the sky, and its surface temperature gradually drops below the ambient temperature, forming a cold source; the cold energy is transferred downward from the radiative cooling layer 11 to the metal extension body 12, and the metal extension body 12 uses its high thermal conductivity to diffuse laterally in a horizontal plane to eliminate temperature non-uniformity; the cold energy after temperature equalization continues to be transferred downward from the metal extension body 12 to the high thermal conductivity substrate 2, and the high thermal conductivity substrate 2 conducts the cold energy vertically to the brine crystallization surface 21 along the thickness direction. The temperature of the brine 6 to be desalinated, which is in contact with the brine crystallization surface 21, gradually decreases under the continuous input of cold energy. When the brine temperature drops below its freezing point and reaches the nucleation supercooling corresponding to the ice crystal nucleating agent, ice crystals preferentially nucleate at the brine crystallization surface 21. After nucleation, the ice crystal layer 61 grows gradually from top to bottom with the brine crystallization surface 21 as the starting interface, driven by continuous cold energy. During ice crystal solidification, water molecules enter the ice crystal lattice to form a solid phase, while salt ions, due to differences in size and charge, cannot embed themselves in the ice crystal lattice structure and are repelled from the ice crystal growth front. The repelled salt diffuses and settles downwards under the combined effects of gravity and concentration gradient, gradually accumulating in the liquid phase region at the bottom of the crystallization chamber 3, forming concentrated brine 62 with a salinity higher than the initial desalinated brine 6. As the ice crystal layer 61 continues to grow downwards, the salinity of the top ice crystal layer 61 gradually decreases and becomes pure, while the salinity of the concentrated brine 62 in the bottom liquid phase region continues to increase, forming a stable top ice formation and a salt-rich spatial concentration distribution in the lower part of the crystallization chamber 3. Throughout this process, the insulation layer 4 continuously acts as a thermal barrier, preventing ambient heat from intruding from the sides and bottom of the crystallization chamber 3, ensuring that the cooling capacity is primarily used to drive the downward growth of the ice crystal layer 61, rather than being offset by ambient heat intrusion.

[0044] S4: When the thickness of the ice crystal layer 61 reaches the preset value or the running time reaches the preset duration, the concentrated brine discharge structure 5 is activated to discharge the concentrated brine 62 from the bottom of the crystallization chamber 3.

[0045] Specifically, in this embodiment, the preset values ​​for the thickness of the ice crystal layer 61 and the preset operating time can be determined in advance based on parameters such as the initial brine salinity, brine volume, environmental meteorological conditions, and target desalination water salinity. Alternatively, they can be determined by the operator through direct observation of the transparent crystallization chamber 3 wall. As a reference guideline, the preset thickness of the ice crystal layer 61 is typically 20% to 60% of the initial brine surface depth within the crystallization chamber 3. Under seawater conditions with an initial salinity of 3.5%, a depth of 30% to 50% is preferred as the preset thickness. If the ice crystal layer 61 is too thin, the desalination water production will be low; if it is too thick, the brine concentration at the ice crystal growth front will significantly increase, leading to more salt inclusions in the ice crystals and a decrease in the desalination rate. The preset operating time is typically 6 hours to 14 hours, matching a nighttime operating cycle. In practice, the appropriate time to stop is when the ice-water interface at the lower edge of the ice crystal layer 61 no longer significantly advances downwards, or when white turbidity appears at the lower edge of the ice crystal layer 61 (indicating increased salt entrainment), as observed through the wall of the transparent crystallization chamber 3. After activating the concentrated brine discharge structure 5, the concentrated brine 62 is discharged and collected from the bottom of the crystallization chamber 3 under gravity. This step should be completed before the ice crystal layer 61 melts to avoid re-mixing with the concentrated brine 62 after the ice crystals melt, which would cause salt re-dissolution and reduce desalination efficiency. The discharge rate of the concentrated brine discharge structure 5 should be moderate to avoid disturbances caused by excessively rapid discharge that could damage the integrity of the ice crystal layer 61.

[0046] S5: Remove the ice crystal layer 61 and melt it to collect the desalinated water; if it is necessary to further reduce the salinity, inject the desalinated water back into the crystallization chamber 3 and repeat S1 to S5.

[0047] Specifically, in this embodiment, after the ice crystal layer 61 is removed from the crystallization chamber 3, its surface can be quickly rinsed with a small amount of low-salt water or fresh water if necessary to remove residual salt and concentrated brine film adhering to the ice crystal surface, further reducing the salinity of the desalinated water after melting. The rinsed ice crystal layer 61 is allowed to melt naturally or accelerated at room temperature, and the melted water is collected as desalinated water. If the salinity of the obtained desalinated water does not meet the usage requirements, the obtained desalinated water can be reinjected into the crystallization chamber 3 as a new source of desalinated water, and the operation process from S1 to S5 can be repeated to achieve multi-stage freezing crystallization to gradually reduce salinity. After each stage of freezing crystallization, the salinity of the ice melt water will be further reduced, and low-salinity desalinated water that meets the usage requirements can be obtained after multi-stage treatment.

[0048] In one specific embodiment, the heat-insulating air layer 7, with a thickness of 20 mm, forms a sealed space with the infrared transparent cover layer 8 through a sealed frame constructed of surrounding insulation material, and is filled with dry, still air. The radiative cooling layer 11 uses a radiative cooling film with an emissivity greater than 0.9 in the 8 μm to 13 μm wavelength band, and a 5 mm thick, 0.25 m × 0.25 m pure aluminum plate is tightly bonded underneath it as a metal extender 12, with an effective radiation area of ​​approximately 0.0625 m². 2 The high thermal conductivity substrate 2 is made of a 5mm thick copper plate, with a dense polytetrafluoroethylene coating sprayed on its lower surface as a salt corrosion resistant layer. The crystallization chamber is made of a 5mm thick transparent acrylic plate, with an effective internal volume of approximately 75mL. The sidewalls and bottom are covered with a 10mm thick extruded polystyrene foam insulation layer. The experiment adopted a parallel control scheme. Simulated seawater with a salinity of 3.5% or 1.75% sodium chloride was prepared in advance. Silver iodide ice crystal nucleating agent was quantitatively added to the simulated seawater used in both the experimental and control groups, and then the nucleating agent was uniformly dispersed in the solution by ultrasonic stirring. 75mL of the pretreated solution with the same ratio was injected into two crystallization chambers 3 with identical dimensions. The sidewalls and bottom of both chambers were covered with 10mm thick extruded polystyrene foam insulation material. The only difference between the two is the top structure: the experimental group chamber is equipped with a complete radiative cooling unit 1 at the top, while the control group chamber has no radiative cooling structure at the top. Both sets of devices were simultaneously placed in a clear, low-cloudy outdoor environment, with the experimental group's radiative cooling layer facing the sky and operating continuously for 8–11 hours. The experimental group achieved cooling by radiating heat into outer space through the radiative cooling membrane. The cold energy was conducted to the brine surface via a high-thermal-conductivity substrate of pure aluminum and copper plates. Under the action of silver iodide nucleating agent, ice crystals grew downwards from the lower surface of the copper plate. The control group had no radiative cooling source and was only affected by the ambient temperature at night. After solidification, the concentrated brine was separated and the ice crystals were melted to obtain desalinated water. The control group showed virtually no ice crystal formation throughout the experiment, while only the experimental group could stably freeze and produce water. This demonstrates that the cold energy required for the brine to freeze in the experimental group was provided by the device's radiative cooling structure, and not by spontaneous freezing due to low ambient temperature. Experimental results show that the first group of experiments with an initial salinity of 3.5% yielded approximately 13 mL of melted ice water with a salinity of approximately 0.95% and a desalination rate of approximately 72.86%; the second group of experiments with an initial salinity of 1.75% yielded approximately 37 mL of melted ice water with a salinity of approximately 0.27% and a desalination rate of approximately 84.57%. These results verify the technical feasibility of this invention, which achieves desalination by driving the gradual freezing and crystallization of brine through passive radiative refrigeration.

[0049] Figure 4 These are the temperature change curves for each part during the first set of experiments. Figure 5The figures show the temperature change curves for each part during the second group of experiments. In the figure, the blue curve represents the ambient temperature, the green curve represents the brine temperature of the control group, the yellow curve represents the brine temperature of the experimental group, the orange curve represents the air temperature of the sealed interlayer, and the gray curve represents the surface temperature of the radiative cooling film. Figure 4 The experiment showed that the ambient temperature fluctuated and decreased throughout the entire test; the surface temperature of the radiative cooling film cooled the fastest and had the lowest overall temperature, efficiently radiating heat outward through the atmospheric infrared window; the temperature of the brine in the experimental group was significantly lower than that in the control group throughout the entire test, and the temperature difference between the two increased continuously with the running time. Ultimately, the temperature of the brine in the experimental group dropped below the freezing point and successfully froze, while the temperature of the brine in the control group remained close to the ambient temperature, above the freezing point throughout the test, and showed no freezing phenomenon. This proves that, under conditions of no frost interference and normal operation of the radiative cooling unit, the additional cooling capacity obtained by the brine in the experimental group comes from the radiative cooling structure of the device, not from the low ambient temperature at night, and that brine freezing and desalination can be achieved by relying on radiative cooling. Figure 5 The experiment showed the temperature change pattern in the early stage and... Figure 4 The results were consistent: the radiative cooling film cooled rapidly, and the temperature of the experimental group was lower than that of the control group. Midway through the experiment, due to high ambient humidity, frost formed on the surface of the infrared transparent cover, blocking the infrared radiation channel and causing radiative cooling to fail. This manifested as a sudden rise in the temperature of the radiative cooling film and the enclosed air. The cooling of the brine in the experimental group was hindered, and its temperature rose synchronously, significantly reducing the temperature difference between the experimental and control groups. In the later stages of the experiment, relying solely on natural ambient cooling, the temperatures of both groups slowly and synchronously decreased, and the experimental group no longer exhibited a significant low-temperature advantage. This demonstrates that frost formation on the infrared cover blocks the radiative heat dissipation path, directly causing the device's cooling performance to fail, thus indirectly confirming that the low temperature in the experimental group was provided by radiative cooling.

[0050] Furthermore, in large-scale applications, an array-based deployment method can be adopted to increase the total water production. Multiple individual units are arranged in an array in an open area, with a spacing of 50mm to 200mm between each unit to avoid mutual obstruction of the radiation view. They are then uniformly connected to the concentrated brine collection pipeline and the desalinated water collection pipeline. The radiation area of ​​the individual units can be scaled up proportionally according to actual needs. When the radiation area increases, the effective volume of the crystallization chamber 3 increases proportionally according to the aforementioned area-volume matching relationship to ensure a balance between the cooling supply and the brine heat load.

[0051] Unless otherwise defined, the technical or scientific terms used herein shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains. The terms “first,” “second,” and similar terms used in this patent application specification and claims do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Similarly, the terms “an” or “a” and similar terms do not indicate a quantity limitation, but rather indicate the presence of at least one. The terms “comprising” or “including” and similar terms mean that the elements or objects preceding “comprising” or “including” encompass the elements or objects listed following “comprising” or “including” and their equivalents, and do not exclude other elements or objects. The terms “connected” or “linked” and similar terms are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. The terms “upper,” “lower,” “left,” and “right” are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0052] The above description is merely an optional embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A passive radiation cooling progressive freezing desalination device, characterized in that, include: Radiation cooling unit (1), high thermal conductivity substrate (2), crystallization cavity (3), insulation layer (4) and concentrated brine discharge structure (5); The radiation cooling unit (1) includes a radiation cooling layer (11) and a metal extension body (12). The radiation cooling layer (11) is arranged facing upward and has a high infrared emissivity in the 8μm to 13μm atmospheric window band. The metal extension body (12) is attached to the bottom of the radiation cooling layer (11). The metal extension body (12) is made of a metal plate with high thermal conductivity. The high thermal conductivity substrate (2) is disposed below the metal extension body (12) and is thermally tightly connected to the metal extension body (12). The lower surface of the high thermal conductivity substrate (2) forms a salt water crystallization surface (21). The crystallization cavity (3) is located below the high thermal conductivity substrate (2), the top of the crystallization cavity (3) is in contact with the lower surface of the high thermal conductivity substrate (2), the crystallization cavity (3) is used to contain the brine to be desalinated (6), and the crystallization cavity (3) is provided with an ice crystal nucleating agent. The heat insulation layer (4) covers the sides and bottom of the crystallization cavity (3); The concentrated brine discharge structure (5) connects the bottom of the crystallization cavity (3) to the outside.

2. The passive radiative refrigeration progressive freeze desalination device of claim 1, wherein, The radiative cooling unit (1) is provided with an insulating air layer (7) and an infrared transparent cover layer (8) in sequence above it. The infrared transparent cover layer (8) is made of a thin film material with high infrared transmittance in the 8μm~13μm band. The insulating air layer (7) is formed by the heat insulation frame between the infrared transparent cover layer (8) and the radiative cooling unit (1) to form a sealed space, which is filled with dry and still air.

3. The passive radiative refrigeration progressive freeze desalination device of claim 2, wherein, The thickness of the heat insulation air layer (7) is 10mm to 50mm; the infrared transparent cover layer (8) is made of polyethylene film or fluorinated ethylene propylene film.

4. The passive radiative refrigeration progressive freeze desalination device of claim 1, wherein, The infrared emissivity of the radiation cooling layer (11) in the 8μm to 13μm band is greater than 0.9; the radiation cooling layer (11) is any one of inorganic particle-polymer composite film, porous polymer film or multilayer optical film.

5. The passive radiative refrigeration progressive freeze desalination device of claim 1, wherein, The thermal conductivity of the high thermal conductivity substrate (2) is 200 W / (m·K) to 420 W / (m·K).

6. The passive radiative refrigeration progressive freeze desalination device of claim 1, wherein, The lower surface of the high thermal conductivity substrate (2) is provided with a salt corrosion resistant layer, which is any one of polytetrafluoroethylene coating, nickel plating, gold plating, ceramic coating or surface passivation film.

7. The passive radiation cooling progressive freezing desalination apparatus according to claim 1, characterized in that, The ice crystal nucleating agent is silver iodide particles dispersed in the brine to be desalinated (6); Alternatively, the ice crystal nucleating agent may be a nucleation auxiliary layer disposed on the lower surface of the high thermal conductivity substrate (2), and the nucleation auxiliary layer may be any one of the following: micro-nano rough structure layer, ice-loving coating, mineral particle coating, immobilized silver iodide particle layer, metal oxide coating or mechanical scratch structure layer.

8. The passive radiative refrigeration progressive freeze desalination device of claim 1, wherein, The insulation layer (4) is made of any one of extruded polystyrene foam, expanded polystyrene foam, polyurethane foam, aerogel felt or vacuum insulation board; the thickness of the insulation layer (4) is 10mm to 50mm.

9. The passive radiative refrigeration progressive freeze desalination device of claim 1, wherein, The crystallization chamber (3) is made of transparent or semi-transparent material; the concentrated brine discharge structure (5) is any one of corrosion-resistant ball valve, quick-connect fitting, removable bottom cover or side bottom guide port.

10. A passive radiation cooling progressive freezing desalination method, based on the passive radiation cooling progressive freezing desalination apparatus according to any one of claims 1-9, characterized in that, Includes the following steps: S1: Inject the brine (6) containing the ice crystal nucleating agent into the crystallization cavity (3) so that the liquid surface of the brine (6) is in contact with the lower surface of the high thermal conductivity substrate (2). S2: Place the passive radiation cooling progressive freezing desalination device in an open outdoor area, so that the radiation cooling layer (11) faces the sky and operates at night or under low solar radiation conditions; S3: The radiation cooling layer (11) radiates heat to outer space through the atmospheric infrared window. The generated cold energy is transferred to the high thermal conductivity substrate (2) after being uniformly heated laterally by the metal extension body (12). Then, the high thermal conductivity substrate (2) conducts the cold energy vertically to the brine crystal surface (21). The temperature of the brine to be desalinated (6) in contact with the brine crystal surface (21) gradually decreases. With the assistance of the ice crystal nucleating agent, it preferentially nucleates at the brine crystal surface (21). The ice crystal layer (61) grows gradually downward from the brine crystal surface (21). The salt is repelled and enriched in the lower liquid phase to form concentrated brine (62). S4: When the thickness of the ice crystal layer (61) reaches a preset value or the running time reaches a preset duration, the concentrated brine discharge structure (5) is opened to discharge the concentrated brine (62) from the bottom of the crystallization chamber (3); S5: Remove the ice crystal layer (61) and melt it to collect the desalinated water; if it is necessary to further reduce the salinity, re-inject the desalinated water into the crystallization cavity (3) and repeat S1 to S5.

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