Direct expansion heat pump system based on spectral adaptive composite coating, working method and phase change hydrogel layer preparation method

Abstract

This invention relates to the field of solar heat pump and building energy conservation technology, and discloses a direct expansion heat pump system based on a spectrally adaptive composite coating, its operating method, and a method for preparing a phase change hydrogel layer. The system includes a compressor, an indoor heat exchanger, a throttling element, an outdoor flat plate heat exchanger, and a four-way valve. The outdoor flat plate heat exchanger includes a metal thermally conductive substrate, an outdoor working fluid pipe, and a composite coating. The composite coating sequentially includes a solar heat-absorbing coating, a phase change hydrogel layer, and a transparent cover layer. The phase change hydrogel layer is transparent below the phase change temperature and opaque and scattering above the phase change temperature. This allows the outdoor flat plate heat exchanger to receive solar radiation and transfer heat to the refrigerant in heating mode, and to reflect or scatter solar radiation and dissipate heat through long-wave infrared radiation via the transparent cover layer in cooling mode. This allows the same outdoor flat plate heat exchanger to passively switch between heat collection and heat dissipation states depending on the surface temperature, improving the system's heat exchange adaptability under different seasonal operating conditions.

Description

Technical Field

[0001] This invention relates to the fields of solar heat pumps, radiative heat exchange, and building energy conservation, and particularly to a direct expansion heat pump system based on a spectrally adaptive composite coating, its operating method, and a method for preparing a phase change hydrogel layer. Background Technology

[0002] Direct expansion solar heat pumps are a type of heat pump technology that couples a solar collector with a heat pump evaporator. They typically utilize refrigerant to directly evaporate and absorb heat within the solar collector or outdoor heat exchanger, thereby reducing intermediate heat exchange stages and improving evaporation temperature and heating performance in low-temperature environments. Compared to ordinary air-source heat pumps, direct expansion solar heat pumps have certain advantages in winter conditions with good sunshine and low ambient temperatures, and are therefore used in building heating, domestic hot water supply, and some combined cooling and heating (CCHP) scenarios.

[0003] However, most existing direct-expansion solar heat pumps use heat-absorbing coatings with fixed spectral characteristics for their outdoor collector components. These coatings typically have high absorptivity in the solar radiation band. In winter heating mode, this characteristic is beneficial for the outdoor heat exchanger to absorb solar energy, causing the low-temperature, low-pressure refrigerant to evaporate. However, in summer cooling mode, if the same outdoor heat exchanger is used as a condenser, the high-absorptivity coating will continuously absorb solar radiation, causing additional heat accumulation on the condenser surface. This leads to increased condensing temperature, increased compressor discharge pressure, and decreased cooling efficiency. Therefore, direct-expansion solar heat pumps exhibit a significant seasonal contradiction during year-round operation: increasing solar absorption is necessary in winter, while suppressing solar absorption and enhancing heat dissipation are required in summer.

[0004] For example, patent document CN101571329B discloses a direct-expansion multifunctional solar-assisted heat pump system, which achieves functions such as heating, cooling, or domestic hot water supply through direct-expansion solar collector heat exchange components, a compressor, a reversing valve, and multiple heat exchange circuits. This type of system can expand the application scenarios of solar heat pumps to some extent, but its focus remains on the combination of refrigerant flow paths and functional modes; the solar absorption characteristics of the outdoor collector surface are usually relatively fixed. When the system is in summer cooling mode, the solar radiation heat input from the fixed heat-absorbing surface may still affect the outdoor condensation heat dissipation capacity.

[0005] For example, patent document WO2019024061A1 discloses a PVT heat pump system that can utilize solar radiation and cold sky radiation to achieve heating and cooling supply. It achieves multi-energy complementarity through photovoltaic modules, refrigerant circulation, and sky radiation heat exchange. This technology has addressed the utilization of solar radiation and cold sky radiation in the heat pump system, but its core lies more in the coupling between the PVT module and the heat pump cycle. It does not address the opposite heat exchange requirements faced by the same outdoor flat plate heat exchanger in a direct expansion heat pump when it acts as an evaporator in winter and a condenser in summer, and does not propose a structural design based on passive switching of surface spectral states.

[0006] Furthermore, thermochromic materials, phase change materials, and radiative cooling materials have been used in building envelopes, smart windows, and passive thermal management. For example, patent document CN113999585A discloses a thermochromic radiative cooling coating or film that utilizes the optical state changes of temperature-sensitive materials at different temperatures to achieve solar reflection and radiative cooling; patent document CN109945363B discloses a temperature-adaptive radiative cooling system that adjusts the spectral transmittance characteristics of a transmission cover plate through a phase change material. However, these technologies are mostly applied to building walls, windows, radiative cooling panels, or independent films, and their control targets are mainly the building's exterior surface or passive cooling structure. They do not integrate the temperature-sensitive spectral control layer with the refrigerant circulation of a direct expansion heat pump, the reversing of a four-way valve, and the evaporation / condensation dual-mode operation of an outdoor flat plate heat exchanger.

[0007] Therefore, although existing technologies disclose direct-expansion solar heat pumps, sky radiation heat exchange, and thermochromic radiative cooling materials, a system is still lacking that can utilize the same outdoor flat-plate heat exchanger to achieve solar absorption in heating mode and solar reflection or scattering and long-wave infrared radiation heat dissipation in cooling mode. How to match the surface spectral state of the outdoor flat-plate heat exchanger with the heat pump's operating mode without adding complex active control mechanisms is a key technical problem that needs to be solved for direct-expansion solar heat pumps to operate efficiently year-round. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a direct expansion heat pump system that enables the same outdoor flat plate heat exchanger to absorb solar radiation in heating mode and to suppress solar radiation input and perform long-wave infrared radiation heat dissipation in cooling mode.

[0009] To achieve the above objectives, the present invention provides a direct expansion heat pump system, including a compressor, an indoor heat exchanger, a throttling element, an outdoor flat plate heat exchanger, and a four-way valve. The first port of the four-way valve is connected to the exhaust port of the compressor, the second port is connected to the suction port of the compressor, the third port is connected to the indoor heat exchanger, and the fourth port is connected to the outdoor flat plate heat exchanger. The outdoor flat plate heat exchanger includes a metal thermally conductive substrate, an outdoor working fluid pipe, and a composite coating. The outdoor working fluid pipe is located on the back of the metal thermally conductive substrate and connected to the refrigerant circulation loop. The composite coating is located on the front of the metal thermally conductive substrate. The composite coating consists of a solar heat-absorbing coating, a phase change hydrogel layer, and a transparent cover layer, arranged sequentially from the side closest to the metal thermally conductive substrate to the side furthest from the metal thermally conductive substrate. The solar heat-absorbing coating is attached to the front side of the metal thermally conductive substrate, the phase change hydrogel layer is sandwiched between the solar heat-absorbing coating and the transparent cover layer, and the transparent cover layer covers the outside of the phase change hydrogel layer. The phase transition temperature of the phase transition hydrogel layer is 28–32°C. It is transparent when the temperature is below the phase transition temperature and opaque and scattering when the temperature is above the phase transition temperature.

[0010] Furthermore, the metal thermally conductive substrate is an aluminum plate, a copper plate, or a stainless steel plate; The outdoor working fluid pipeline is a copper pipe or a stainless steel pipe. The solar heat-absorbing coating is a blue titanium coating, a black chrome coating, or a black paint coating; The transparent cover layer is a polymethyl methacrylate cover layer or a polyethylene cover layer.

[0011] Furthermore, the outdoor working fluid pipe is arranged in a serpentine pattern and welded to the back of the metal heat-conducting substrate. One end of the outdoor working fluid pipe is connected to the fourth interface of the four-way valve, and the other end is connected to the throttling element.

[0012] Furthermore, a cavity for accommodating the phase change hydrogel layer is formed between the solar heat-absorbing coating and the transparent cover layer, the phase change hydrogel layer being located within the cavity and in contact with the solar heat-absorbing coating.

[0013] Furthermore, the thickness of the solar heat-absorbing coating is 2–5 μm, the thickness of the transparent cover layer is 0.5–2 mm, and the average emissivity of the transparent cover layer in the 8–13 μm band is greater than 0.9.

[0014] Furthermore, the phase change hydrogel layer is a poly(N-isopropylacrylamide) hydrogel film, wherein the phase change hydrogel layer is in a hydrated, light-transmitting state below the phase change temperature and in a dehydrated, scattering state above the phase change temperature.

[0015] Furthermore, the four-way valve has a heating passage and a cooling passage; In the heating circuit, the outdoor flat plate heat exchanger is connected between the throttling element and the compressor's suction port; In the refrigeration circuit, the outdoor flat plate heat exchanger is connected between the compressor's exhaust port and the throttling element.

[0016] The present invention also provides a method for operating the above-mentioned direct expansion heat pump system, comprising the following steps: S11. In building heating mode, switch the four-way valve to the heating path so that the low-temperature, low-pressure refrigerant output by the throttling element enters the outdoor working fluid pipe of the outdoor flat plate heat exchanger. S12. When the surface temperature of the outdoor flat plate heat exchanger is lower than the phase change temperature of the phase change hydrogel layer, the phase change hydrogel layer is in a transparent state. Solar radiation passes through the transparent cover layer and the phase change hydrogel layer in sequence and reaches the solar heat absorption coating. The solar heat absorption coating, the metal heat-conducting substrate and the outdoor working fluid pipe are heat-transferred in sequence. The refrigerant changes from a liquid or gas-liquid two-phase state to a gaseous state in the outdoor working fluid pipe. S13. In building cooling mode, switch the four-way valve to the cooling passage so that the high-temperature and high-pressure refrigerant discharged by the compressor enters the outdoor working fluid pipe of the outdoor flat plate heat exchanger. S14. When the surface temperature of the outdoor flat plate heat exchanger is higher than the phase change temperature of the phase change hydrogel layer, the phase change hydrogel layer is in an opaque scattering state. Solar radiation is reflected or scattered at the phase change hydrogel layer, and the transparent cover plate layer emits long-wave infrared radiation to the outside. The refrigerant changes from a gaseous state to a liquid state in the outdoor working fluid pipe.

[0017] Furthermore, in S11 to S12, the refrigerant passes through the outdoor flat plate heat exchanger and then enters the compressor through a four-way valve. After being compressed by the compressor, it enters the indoor heat exchanger through a four-way valve and then returns to the outdoor flat plate heat exchanger through a throttling element. In S13 to S14, the refrigerant enters the throttling element after passing through the outdoor flat plate heat exchanger. After being throttled by the throttling element, it enters the indoor heat exchanger and then returns to the compressor through the four-way valve.

[0018] The present invention also provides a method for preparing the phase change hydrogel layer, comprising the following steps: S21. Weigh the raw materials according to the ratio of deionized water: NIPAm monomer: N,N-methylenebisacrylamide crosslinking agent: ammonium persulfate initiator = 500mL: 50g: 1g: 1g, and prepare the catalyst according to the dosage of 4μL LTEMED for every 1g of NIPAm monomer. S22. Add NIPAm monomer and N,N-methylenebisacrylamide crosslinking agent to deionized water, stir for 30 minutes, then add ammonium persulfate initiator and continue stirring for 10 minutes; S23. Add TEMED catalyst and stir for 1 minute, then place the resulting precursor solution in an ice bath to cool for at least 1 minute; S24. The cooled precursor solution is injected into the cavity formed between the solar heat-absorbing coating and the transparent cover plate layer, and the mixture is left to stand and crosslink in the cavity for 12 to 24 hours to form a phase change hydrogel layer in situ on the surface of the solar heat-absorbing coating.

[0019] Compared with existing technologies, this invention does not simply add a common solar collector coating to a traditional direct expansion heat pump system, nor does it use thermochromic materials alone as the exterior surface of a building. Instead, it configures the refrigerant cycle formed by the compressor, indoor heat exchanger, throttling element, outdoor flat plate heat exchanger, and four-way valve in a corresponding manner with the composite coating set on the front of the outdoor flat plate heat exchanger, so that the refrigerant thermodynamic state of the outdoor flat plate heat exchanger, the surface temperature of the metal thermally conductive substrate, and the optical state of the phase change hydrogel layer are linked together.

[0020] Specifically, in building heating mode, the outdoor flat plate heat exchanger is located between the throttling element and the compressor's suction port. Low-temperature, low-pressure refrigerant enters the outdoor working fluid pipe. The phase change hydrogel layer is in a light-transmitting state at a low surface temperature. Solar radiation can pass through the transparent cover layer and the phase change hydrogel layer to reach the solar heat-absorbing coating, and then be transferred to the refrigerant in the outdoor working fluid pipe via the metal thermally conductive substrate, so that the outdoor flat plate heat exchanger is adapted to the heat absorption requirements under heating conditions.

[0021] Meanwhile, in building cooling mode, the outdoor flat plate heat exchanger is located between the compressor's exhaust port and the throttling element. High-temperature and high-pressure refrigerant enters the outdoor working fluid pipe. The phase change hydrogel layer is in an opaque scattering state at a high surface temperature. Solar radiation is reflected or scattered at the phase change hydrogel layer, and the transparent cover layer further radiates long-wave infrared radiation to the outside. This makes the outdoor flat plate heat exchanger compatible with the heat dissipation requirements under cooling conditions, and avoids the fixed heat-absorbing coating from continuing to increase solar radiation heat input under summer cooling conditions.

[0022] Furthermore, the phase change hydrogel layer is sandwiched between the solar heat-absorbing coating and the transparent cover layer, and can be formed on the surface of the solar heat-absorbing coating through in-situ cross-linking, resulting in a relatively stable interlayer positional relationship between the phase change hydrogel layer and the heat exchange surface of the outdoor flat plate heat exchanger. This structure reduces reliance on additional active control components, which is beneficial for accommodating both winter heat absorption and summer heat dissipation on the same outdoor flat plate heat exchanger, thereby improving the adaptability of the direct expansion heat pump system to various operating conditions throughout the year. Attached Figure Description

[0023] To more clearly illustrate the technical solution of the present invention, the accompanying drawings used in this specification are briefly described below.

[0024] Figure 1 This is a schematic diagram of the direct expansion heat pump system in an embodiment of the present invention; Figure 2 This is a schematic diagram of the layer structure of the outdoor flat plate heat exchanger in an embodiment of the present invention; Figure 3 This is a schematic diagram illustrating the working principle of the direct expansion heat pump system in building heating mode in an embodiment of the present invention; Figure 4 This is a schematic diagram illustrating the working principle of the direct expansion heat pump system in building cooling mode in an embodiment of the present invention; Figure 5 This is a comparison diagram of the temperature changes of a composite plate with a phase change hydrogel layer and a single heat-absorbing plate under irradiation conditions in an embodiment of the present invention. Figure 6 These are comparative photographs of phase change hydrogel layers at different temperature states in embodiments of the present invention, wherein: Figure 6 (a) is a photograph of the phase change hydrogel layer in a transparent state; Figure 6 (b) is a photograph of the phase change hydrogel layer in an opaque scattering state.

[0025] The attached figures are labeled as follows: 1. Compressor; 2. Indoor heat exchanger; 3. Throttling element; 4. Outdoor flat plate heat exchanger; 5. Four-way valve; 41. Metal thermally conductive substrate; 42. Outdoor working fluid pipe; 43. Composite coating; 431. Solar heat-absorbing coating; 432. Phase change hydrogel layer; 433. Transparent cover layer. Detailed Implementation

[0026] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. It should be understood that the following embodiments are used to illustrate the structural composition, working process, and preparation method of the present invention, and are not intended to limit the scope of protection of the present invention. Equivalent substitutions or conventional modifications made by those skilled in the art based on the content of this specification without departing from the concept of the present invention should all fall within the scope of protection of the present invention.

[0027] Example 1 like Figure 1 As shown, this embodiment provides a direct expansion heat pump system based on a spectrally adaptive composite coating, including a compressor 1, an indoor heat exchanger 2, a throttling element 3, an outdoor flat plate heat exchanger 4, and a four-way valve 5. The compressor 1, indoor heat exchanger 2, throttling element 3, and outdoor flat plate heat exchanger 4 are connected via refrigerant piping to form a refrigerant circulation loop. The four-way valve 5 is located between the compressor 1, indoor heat exchanger 2, and outdoor flat plate heat exchanger 4 to change the flow direction of the refrigerant in the system. The indoor heat exchanger 2 is located on the indoor side, and the outdoor flat plate heat exchanger 4 is located on the outdoor side. The compressor 1 and the four-way valve 5 are located in the refrigerant circulation loop. The throttling element 3 is located on the piping between the indoor heat exchanger 2 and the outdoor flat plate heat exchanger 4, enabling the system to switch between building heating and building cooling modes.

[0028] Specifically, the first port of the four-way valve 5 is connected to the discharge port of the compressor 1, the second port of the four-way valve 5 is connected to the suction port of the compressor 1, the third port of the four-way valve 5 is connected to the indoor heat exchanger 2, and the fourth port of the four-way valve 5 is connected to the outdoor flat plate heat exchanger 4. By switching the pathway of the four-way valve 5, the outdoor flat plate heat exchanger 4 can be connected between the throttling element 3 and the suction port of the compressor 1 in building heating mode, and also between the discharge port of the compressor 1 and the throttling element 3 in building cooling mode. Therefore, the same outdoor flat plate heat exchanger 4 can serve as both a heat exchange component for low-temperature, low-pressure refrigerant absorption and evaporation, and a heat exchange component for high-temperature, high-pressure refrigerant release and condensation.

[0029] In this embodiment, the compressor 1, indoor heat exchanger 2, throttling element 3, and four-way valve 5 can all be commercially available products capable of meeting the corresponding refrigerant cycle conditions. For example, the compressor 1 can be a commercially available scroll compressor, rotary compressor, or other compressor suitable for heat pump systems; the indoor heat exchanger 2 can be a commercially available finned tube air heat exchanger or other heat exchangers suitable for indoor heat exchange; the throttling element 3 can be a commercially available electronic expansion valve, thermostatic expansion valve, or capillary throttling element; and the four-way valve 5 can be a commercially available heat pump reversing four-way valve. The specific models of the above components can be selected according to the type of refrigerant, heating capacity, cooling capacity, and installation space.

[0030] Combination Figure 2 As shown, the outdoor flat plate heat exchanger 4 includes a metal thermally conductive substrate 41, an outdoor working fluid pipe 42, and a composite coating 43. The metal thermally conductive substrate 41 has a front and a back side disposed opposite to each other. The outdoor working fluid pipe 42 is disposed on the back side of the metal thermally conductive substrate 41 and connected to the refrigerant circulation loop. The composite coating 43 is disposed on the front side of the metal thermally conductive substrate 41. Figure 2 The left side shows the overall layered arrangement of the outdoor flat plate heat exchanger 4. The outdoor working fluid pipe 42 is located on the back of the metal heat-conducting substrate 41, and the composite coating 43 is located on the front of the metal heat-conducting substrate 41, thereby forming a heat exchange path between the external radiation environment, the composite coating 43, the metal heat-conducting substrate 41, the outdoor working fluid pipe 42 and the refrigerant.

[0031] Furthermore, the composite coating 43 comprises, sequentially from the side closest to the metal thermally conductive substrate 41 to the side furthest from the metal thermally conductive substrate 41, a solar heat-absorbing coating 431, a phase change hydrogel layer 432, and a transparent cover layer 433. The solar heat-absorbing coating 431 is attached to the front side of the metal thermally conductive substrate 41, the phase change hydrogel layer 432 is sandwiched between the solar heat-absorbing coating 431 and the transparent cover layer 433, and the transparent cover layer 433 covers the outside of the phase change hydrogel layer 432. Figure 2The magnified view on the right shows the layer structure relationship under low-temperature and high-temperature conditions. Under low-temperature conditions, the phase change hydrogel layer 432 is in a transparent state; under high-temperature conditions, the phase change hydrogel layer 432 is in an opaque scattering state. This layered arrangement allows solar radiation to pass sequentially through the transparent cover layer 433 and the phase change hydrogel layer 432 when entering the outdoor flat plate heat exchanger 4, and determines whether to continue to the solar heat-absorbing coating 431 based on the state of the phase change hydrogel layer 432.

[0032] Optionally, the metal thermally conductive substrate 41 can be an aluminum plate, a copper plate, or a stainless steel plate. When using an aluminum plate, the metal thermally conductive substrate 41 offers good thermal conductivity and a lighter structural weight; when using a copper plate, the metal thermally conductive substrate 41 provides higher thermal conductivity; and when using a stainless steel plate, the metal thermally conductive substrate 41 offers good corrosion resistance. The outdoor working fluid conduit 42 can be a copper pipe or a stainless steel pipe. Copper pipes facilitate efficient thermal connection with the metal thermally conductive substrate 41, while stainless steel pipes are suitable for environments requiring high corrosion resistance.

[0033] Optionally, the outdoor working fluid pipe 42 is arranged in a serpentine pattern and can be welded to the back of the metal heat-conducting substrate 41. One end of the outdoor working fluid pipe 42 is connected to the fourth interface of the four-way valve 5, and the other end is connected to the throttling element 3. The serpentine arrangement increases the coverage area of ​​the outdoor working fluid pipe 42 on the back of the metal heat-conducting substrate 41, allowing the refrigerant to flow along the back of the metal heat-conducting substrate 41, thereby improving the heat exchange uniformity between the metal heat-conducting substrate 41 and the outdoor working fluid pipe 42. When welded, a stable thermally conductive contact is formed between the outdoor working fluid pipe 42 and the metal heat-conducting substrate 41, reducing contact thermal resistance and ensuring that the heat on the composite coating 43 side can be transferred to the refrigerant in the outdoor working fluid pipe 42 through the metal heat-conducting substrate 41, or that the heat in the outdoor working fluid pipe 42 can be released to the outside through the metal heat-conducting substrate 41 and the composite coating 43.

[0034] Optionally, the solar heat-absorbing coating 431 is a blue titanium coating, a black chrome coating, or a black paint coating. The solar heat-absorbing coating 431 is attached to the front side of the metal thermally conductive substrate 41. When the phase change hydrogel layer 432 is in a light-transmitting state, solar radiation can reach the solar heat-absorbing coating 431 and be absorbed by it, subsequently being transferred to the outdoor working fluid pipe 42 through the metal thermally conductive substrate 41. The transparent cover layer 433 is a polymethyl methacrylate cover layer or a polyethylene cover layer. The transparent cover layer 433 covers the outside of the phase change hydrogel layer 432, serving both to define and protect the phase change hydrogel layer 432, and to provide an outer long-wave infrared radiation channel.

[0035] Optionally, a cavity for accommodating the phase change hydrogel layer 432 is formed between the solar heat-absorbing coating 431 and the transparent cover layer 433. Specifically, the solar heat-absorbing coating 431 can be formed first on the front side of the metal thermally conductive substrate 41, and then the transparent cover layer 433 can be positioned opposite the solar heat-absorbing coating 431, with the transparent cover layer 433 located outside the solar heat-absorbing coating 431. Subsequently, a sealing layer or sealing frame is provided between the periphery of the solar heat-absorbing coating 431 and the transparent cover layer 433, sealing the periphery of the two together. The sealing layer, sealing ring, or sealing frame can be formed using weather-resistant sealing materials, such as silicone rubber sealing materials, epoxy sealing materials, or other sealing materials that can withstand outdoor temperature changes and humid environments. Thus, a cavity is formed between the solar heat-absorbing coating 431 and the transparent cover layer 433. An injection port can be provided on at least one side of the cavity, and an exhaust port can also be provided if necessary, so that the precursor solution of the phase change hydrogel layer 432 can be injected into the cavity.

[0036] Furthermore, the phase change hydrogel layer 432 is located within the cavity and is in contact with the solar heat-absorbing coating 431. After the precursor solution is injected into the cavity, the phase change hydrogel layer 432 can be formed in situ on the surface of the solar heat-absorbing coating 431 through static cross-linking, and then the injection port and exhaust port are sealed. Through the above structure, the transparent cover layer 433, the peripheral sealing structure, and the solar heat-absorbing coating 431 together define the position of the phase change hydrogel layer 432, making it difficult for the phase change hydrogel layer 432 to detach from the surface of the solar heat-absorbing coating 431, while maintaining it on the path of solar radiation entry and forming a corresponding relationship with the heat exchange area where the metal thermally conductive substrate 41 and the outdoor working fluid pipe 42 are located.

[0037] Alternatively, in addition to using a sealant layer or a sealing frame to form the cavity, a spacer or circumferential support strip can be provided between the solar heat-absorbing coating 431 and the transparent cover layer 433. The cavity thickness can be controlled by using the spacer or circumferential support strip, and then the periphery can be sealed with sealant. Both of these methods can form a cavity to accommodate the phase change hydrogel layer 432 without affecting the switching of the phase change hydrogel layer 432 between a low-temperature transparent state and a high-temperature opaque scattering state.

[0038] Optionally, the thickness of the solar heat-absorbing coating 431 can be 2 μm, 3.5 μm, or 5 μm. When the thickness of the solar heat-absorbing coating 431 is 2 μm, a relatively thin heat-absorbing functional layer can be formed on the front side of the metal thermally conductive substrate 41; when the thickness of the solar heat-absorbing coating 431 is 3.5 μm, both coating continuity and heat conduction path length can be considered; when the thickness of the solar heat-absorbing coating 431 is 5 μm, a relatively complete solar radiation absorption layer can be formed on the front side. Within the above thickness range, the solar heat-absorbing coating 431 is attached to the front side of the metal thermally conductive substrate 41 and is disposed adjacent to the phase change hydrogel layer 432.

[0039] Optionally, the thickness of the transparent cover layer 433 can be 0.5 mm, 1.2 mm, or 2 mm. A thickness of 0.5 mm allows for a thinner outer covering layer; a thickness of 1.2 mm balances coverage and light transmission; and a thickness of 2 mm improves the overall integrity of the outer covering structure. The average emissivity of the transparent cover layer 433 in the 8–13 μm band is greater than 0.9, enabling it to radiate long-wave infrared light outwards during building cooling mode. The 8–13 μm band corresponds to the atmospheric window band, which is beneficial for the outdoor flat plate heat exchanger 4 to release heat to the outside.

[0040] Optionally, the phase change hydrogel layer 432 is a poly(N-isopropylacrylamide) hydrogel film, and the phase change temperature of the phase change hydrogel layer 432 is 28℃~32℃, which can match the seasonal surface temperature changes of common building heat pump outdoor heat exchangers.

[0041] In this embodiment, the surface temperature of the outdoor flat plate heat exchanger 4 can be understood as the temperature of the area where the composite coating 43 is located, or the temperature of the area adjacent to the phase change hydrogel layer 432 and the solar heat absorption coating 431; this temperature can reflect the surface thermal state of the outdoor flat plate heat exchanger 4 under heating evaporation or cooling condensation conditions.

[0042] Example 2 This embodiment describes the operation of a direct expansion heat pump system in building heating mode. The direct expansion heat pump system used in this embodiment is the same as that in Embodiment 1, and the same structural components and connection relationships will not be repeated here.

[0043] like Figure 3 As shown, in building heating mode, the four-way valve 5 switches to the heating path, and the low-temperature, low-pressure refrigerant output by the throttling element 3 enters the outdoor working fluid pipe 42 of the outdoor flat plate heat exchanger 4. At this time, the outdoor flat plate heat exchanger 4 is connected between the throttling element 3 and the suction port of the compressor 1, and the outdoor flat plate heat exchanger 4 serves as a heat exchange component for refrigerant heat absorption and evaporation. Figure 3 In the diagram, solar radiation shines on the front of the outdoor flat plate heat exchanger 4, while the indoor heat exchanger 2 is located on the indoor side and releases heat into the room. The arrows indicate the flow direction of the refrigerant in the heating mode.

[0044] Furthermore, in the building heating mode, when the temperature of the area where the composite coating 43 of the outdoor flat plate heat exchanger 4 is located is lower than the phase change temperature of the phase change hydrogel layer 432, the phase change hydrogel layer 432 is in a light-transmitting state. Solar radiation passes sequentially through the transparent cover layer 433 and the phase change hydrogel layer 432 to reach the solar heat-absorbing coating 431. After absorbing solar radiation, the solar heat-absorbing coating 431 transfers heat to the metal heat-conducting substrate 41, and then from the metal heat-conducting substrate 41 to the outdoor working fluid pipe 42. After absorbing heat from the solar heat-absorbing coating 431 and the outside air, the refrigerant in the outdoor working fluid pipe 42 changes from a liquid or gas-liquid two-phase state to a gaseous state.

[0045] Subsequently, the gaseous refrigerant passes through the outdoor flat plate heat exchanger 4 and then through the four-way valve 5 into the compressor 1. After being compressed by the compressor 1, it forms a high-temperature, high-pressure refrigerant, which then passes through the four-way valve 5 into the indoor heat exchanger 2. In the indoor heat exchanger 2, the refrigerant releases heat to the indoor air or the indoor heat exchange medium, then passes through the throttling element 3 to reduce its pressure and temperature before returning to the outdoor flat plate heat exchanger 4. Thus, in the building heating mode, Figure 3 The refrigerant flow path shown is... Figure 2 Corresponding to the low-temperature light-transmitting layer structure shown, the solar heat-absorbing coating 431 can participate in the low-temperature side heat absorption process, thereby improving the heat exchange conditions in the building heating mode.

[0046] Example 3 This embodiment describes the operation of a direct expansion heat pump system in building cooling mode. The direct expansion heat pump system used in this embodiment is the same as that in Embodiment 1, and the same structural components and connection relationships will not be repeated here.

[0047] like Figure 4 As shown, in building cooling mode, the four-way valve 5 switches to the cooling path, and the high-temperature, high-pressure refrigerant discharged from the compressor 1 enters the outdoor working fluid pipe 42 of the outdoor flat plate heat exchanger 4. At this time, the outdoor flat plate heat exchanger 4 is connected between the exhaust port of the compressor 1 and the throttling element 3, and the outdoor flat plate heat exchanger 4 serves as a heat exchange component for refrigerant heat release and condensation. Figure 4 In the process, solar radiation is reflected or scattered after hitting the front of the outdoor flat plate heat exchanger 4. The transparent cover layer 433 emits long-wave infrared radiation to the outside. The indoor heat exchanger 2 is located on the indoor side and absorbs indoor heat. The arrows indicate the flow direction of the refrigerant in the cooling mode.

[0048] Furthermore, in building cooling mode, after the high-temperature, high-pressure refrigerant discharged from compressor 1 enters the outdoor working fluid pipe 42, the surface temperature of the outdoor flat plate heat exchanger 4 rises. When the surface temperature of the outdoor flat plate heat exchanger 4 is higher than the phase change temperature of the phase change hydrogel layer 432, the phase change hydrogel layer 432 is in an opaque scattering state. After solar radiation reaches the transparent cover layer 433 and enters the composite coating 43, it is reflected or scattered at the phase change hydrogel layer 432, reducing the proportion of solar radiation that continues to be transferred to the solar heat-absorbing coating 431; at the same time, the transparent cover layer 433 radiates long-wave infrared radiation to the outside, and the high-temperature, high-pressure refrigerant in the outdoor working fluid pipe 42 releases heat to the outside through the metal heat-conducting substrate 41 and the composite coating 43, changing from a gaseous state to a liquid state.

[0049] Subsequently, the liquid refrigerant passes through the outdoor flat plate heat exchanger 4 and then enters the throttling element 3. After being throttled by the throttling element 3, it becomes a low-temperature, low-pressure refrigerant, which then enters the indoor heat exchanger 2. After absorbing heat from the indoor environment in the indoor heat exchanger 2, the refrigerant returns to the compressor 1 through the four-way valve 5, starting the next cycle. Thus, in building cooling mode, Figure 4 The refrigerant flow path shown is... Figure 2 Corresponding to the high-temperature opaque scattering layer structure shown, the composite coating 43 can suppress solar radiation heat input under condensation and heat dissipation conditions, and form an outward long-wave infrared radiation heat dissipation path in conjunction with the transparent cover plate layer 433.

[0050] Further integration Figure 5 As shown, the temperature changes of the composite plate with phase change hydrogel layer 432 and the single heat absorber were compared under irradiation conditions. Figure 5 In this study, as the irradiance increases, the temperature of a single absorber plate rises significantly, while the temperature of the composite plate with the phase change hydrogel layer 432 remains relatively low. For example, at an ambient temperature of 35°C and a solar irradiance of 800 W / m², the temperature of the composite plate increases more dramatically. 2 Under the test conditions, using a composite plate with a phase change hydrogel layer 432, the surface temperature of the outdoor plate heat exchanger 4 can be reduced by 30-40°C compared to the traditional heat-absorbing coating, and the system's coefficient of performance (COP) can be improved by 25%-35%. This comparison shows that after the phase change hydrogel layer 432 transforms into an opaque scattering state at high temperatures, it can reduce the proportion of solar radiation that continues to be transferred to the solar heat-absorbing coating 431. Simultaneously, the transparent cover layer 433 provides a long-wave infrared radiation path, allowing the composite plate to maintain a lower surface temperature under strong irradiation conditions. This demonstrates that the composite coating 43 has a good regulatory effect on the surface temperature of the outdoor plate heat exchanger 4 under building cooling modes.

[0051] Further integration Figure 6 (a) and Figure 6 As can be seen from (b) in the figure, the appearance of the phase change hydrogel layer 432 changes with temperature. Figure 6 (a) shows the sample state when the phase change hydrogel layer 432 is in the light-transmitting state, at which point the bottom solar heat-absorbing coating 431 can be observed; Figure 6 (b) shows the sample state when the phase change hydrogel layer 432 is in an opaque scattering state, at which time the surface of the composite plate shows a more obvious light color or milky white state.

[0052] Example 4 This embodiment provides a method for preparing the phase change hydrogel layer 432. In this embodiment, the basic structure of the outdoor flat plate heat exchanger 4 is the same as that in Embodiment 1, and the same structural composition and connection relationships will not be repeated here.

[0053] First, weigh the raw materials according to the ratio of deionized water: NIPAm monomer: N,N-methylenebisacrylamide crosslinking agent: ammonium persulfate initiator = 500 mL: 50 g: 1 g: 1 g, and prepare the catalyst according to the dosage of 4 μL TEMED per 1 g of NIPAm monomer. Taking 50 g of NIPAm monomer as an example, the corresponding dosage of TEMED is 200 μL. In the above raw materials, NIPAm monomer is used to form the poly-N-isopropylacrylamide hydrogel matrix, N,N-methylenebisacrylamide crosslinking agent is used to form the crosslinking network, and ammonium persulfate initiator and TEMED catalyst are used to initiate and promote the polymerization reaction.

[0054] Then, NIPAm monomer and N,N-methylenebisacrylamide crosslinking agent were added to deionized water and stirred for 30 minutes to fully disperse and dissolve the monomer and crosslinking agent. Ammonium persulfate initiator was then added, and stirring continued for 10 minutes. TEMED catalyst was added and stirred for 1 minute. The resulting precursor solution was then placed in an ice bath to cool for at least 1 minute to lower the precursor solution temperature and inhibit excessively rapid prepolymerization. Finally, the cooled precursor solution was injected through a pre-reserved injection port into the cavity formed between the solar heat-absorbing coating 431 and the transparent cover layer 433. During injection, the precursor solution gradually filled the cavity from one side, and air was expelled through a pre-reserved vent. After the precursor solution filled the cavity, the injection port and vent were sealed, keeping the precursor solution between the solar heat-absorbing coating 431 and the transparent cover layer 433. Subsequently, the cavity was left to crosslink for 12–24 hours, forming a phase change hydrogel layer 432 in situ on the surface of the solar heat-absorbing coating 431.

[0055] Through the above preparation method, the phase change hydrogel layer 432 is not simply prepared separately and then attached to the surface of the outdoor flat plate heat exchanger 4, but is formed in situ through cross-linking within the cavity between the solar heat-absorbing coating 431 and the transparent cover layer 433. The solar heat-absorbing coating 431, the peripheral sealing structure, and the transparent cover layer 433 together limit and protect the phase change hydrogel layer 432, enabling the phase change hydrogel layer 432 to be stably sandwiched between the solar heat-absorbing coating 431 and the transparent cover layer 433, and to switch between a transparent state and an opaque scattering state as the surface temperature of the outdoor flat plate heat exchanger 4 changes.

[0056] Optionally, the static crosslinking time of 12 to 24 hours can be flexibly selected according to the film thickness of the phase change hydrogel layer 432. Through in-situ crosslinking within the cavity between the solar heat-absorbing coating 431 and the transparent cover layer 433, the phase change hydrogel layer 432 can be directly formed on the surface of the solar heat-absorbing coating 431 and is constrained in a fixed position by the transparent cover layer 433, thereby forming a stable layered relationship between the phase change hydrogel layer 432, the solar heat-absorbing coating 431, and the transparent cover layer 433.

[0057] Furthermore, the formed phase change hydrogel layer 432 maintains a hydrated, transparent state below the phase change temperature and transforms into a dehydrated, scattering state above the phase change temperature. Figure 6 (a) and Figure 6 As can be seen in (b), at low temperatures, the phase change hydrogel layer 432 allows the bottom solar heat-absorbing coating 431 to be observed; at high temperatures, the phase change hydrogel layer 432 exhibits a light-colored or milky-white opaque scattering state. This state change indicates that the phase change hydrogel layer 432 formed by the in-situ crosslinking method of this embodiment can meet the spectral state switching requirements of the composite coating 43 in heating and cooling modes.

[0058] In summary, this invention constructs a direct expansion heat pump cycle using a compressor 1, an indoor heat exchanger 2, a throttling element 3, an outdoor flat plate heat exchanger 4, and a four-way valve 5. A composite coating 43, consisting of a solar heat-absorbing coating 431, a phase change hydrogel layer 432, and a transparent cover plate layer 433, is applied to the front of the metal thermally conductive substrate 41 of the outdoor flat plate heat exchanger 4. This allows the outdoor flat plate heat exchanger 4 to correspond to the solar transmission and heat absorption paths in building heating mode, and to correspond to the solar reflection or scattering and long-wave infrared radiation heat dissipation paths in building cooling mode. Through this structure and method, the same outdoor flat plate heat exchanger 4 can form different surface heat exchange states under different operating modes, thereby adapting to the opposite heat exchange directions required by building heating and cooling systems on the outdoor side.

Claims

1. A direct expansion heat pump system based on a spectrally adaptive composite coating, characterized in that, It includes a compressor (1), an indoor heat exchanger (2), a throttling element (3), an outdoor flat plate heat exchanger (4), and a four-way valve (5); The first port of the four-way valve (5) is connected to the exhaust port of the compressor (1), the second port is connected to the suction port of the compressor (1), the third port is connected to the indoor heat exchanger (2), and the fourth port is connected to the outdoor flat plate heat exchanger (4). The outdoor flat plate heat exchanger (4) includes a metal heat-conducting substrate (41), an outdoor working fluid pipe (42), and a composite coating (43). The outdoor working fluid pipe (42) is disposed on the back of the metal heat-conducting substrate (41) and connected to the refrigerant circulation loop. The composite coating (43) is disposed on the front of the metal heat-conducting substrate (41). The composite coating (43) consists of a solar heat-absorbing coating (431), a phase change hydrogel layer (432), and a transparent cover layer (433) from the side closest to the metal heat-conducting substrate (41) to the side furthest from the metal heat-conducting substrate (41). The solar heat-absorbing coating (431) is attached to the front side of the metal thermally conductive substrate (41), the phase change hydrogel layer (432) is sandwiched between the solar heat-absorbing coating (431) and the transparent cover layer (433), and the transparent cover layer (433) covers the outside of the phase change hydrogel layer (432). The phase change hydrogel layer (432) has a phase change temperature of 28-32°C. It is transparent when the temperature is below the phase change temperature and opaque and scattering when the temperature is above the phase change temperature.

2. The direct expansion heat pump system based on spectrally adaptive composite coating according to claim 1, characterized in that, The metal thermally conductive substrate (41) is an aluminum plate, a copper plate, or a stainless steel plate; The outdoor working fluid pipeline (42) is a copper pipe or a stainless steel pipe; The solar heat-absorbing coating (431) is a blue titanium coating, a black chrome coating, or a black paint coating; The transparent cover layer (433) is a polymethyl methacrylate cover layer or a polyethylene cover layer.

3. The direct expansion heat pump system based on spectrally adaptive composite coating according to claim 1, characterized in that, The outdoor working fluid pipe (42) is arranged in a serpentine pattern and welded to the back of the metal heat-conducting substrate (41). One end of the outdoor working fluid pipe (42) is connected to the fourth interface of the four-way valve (5), and the other end is connected to the throttling element (3).

4. The direct expansion heat pump system based on spectrally adaptive composite coating according to claim 1, characterized in that, A cavity for accommodating a phase change hydrogel layer (432) is formed between the solar heat-absorbing coating (431) and the transparent cover layer (433), the phase change hydrogel layer (432) being located within the cavity and in contact with the solar heat-absorbing coating (431).

5. The direct expansion heat pump system based on spectrally adaptive composite coating according to claim 1, characterized in that, The thickness of the solar heat-absorbing coating (431) is 2-5 μm, the thickness of the transparent cover layer (433) is 0.5-2 mm, and the average emissivity of the transparent cover layer (433) in the 8-13 μm band is greater than 0.

9.

6. The direct expansion heat pump system based on spectrally adaptive composite coating according to claim 1, characterized in that, The phase change hydrogel layer (432) is a poly(N-isopropylacrylamide) hydrogel film. The phase change hydrogel layer (432) is in a hydrated and transparent state below the phase change temperature and in a dehydrated and scattering state above the phase change temperature.

7. The direct expansion heat pump system based on spectrally adaptive composite coating according to claim 1, characterized in that, The four-way valve (5) has a heating passage and a cooling passage; In the heating circuit, the outdoor flat plate heat exchanger (4) is connected between the throttling element (3) and the suction port of the compressor (1); In the refrigeration path, the outdoor flat plate heat exchanger (4) is connected between the exhaust port of the compressor (1) and the throttling element (3).

8. The operating method of the direct expansion heat pump system based on spectrally adaptive composite coating as described in any one of claims 1 to 7, characterized in that, Includes the following steps: S11. In the building heating mode, switch the four-way valve (5) to the heating path so that the low-temperature and low-pressure refrigerant output by the throttling element (3) enters the outdoor working fluid pipe (42) of the outdoor flat plate heat exchanger (4). S12. When the surface temperature of the outdoor flat plate heat exchanger (4) is lower than the phase change temperature of the phase change hydrogel layer (432), the phase change hydrogel layer (432) is in a light-transmitting state. Solar radiation passes through the transparent cover layer (433) and the phase change hydrogel layer (432) in sequence and reaches the solar heat-absorbing coating (431). The solar heat-absorbing coating (431), the metal heat-conducting substrate (41) and the outdoor working fluid pipe (42) are heat-transferred in sequence. The refrigerant changes from a liquid state or a gas-liquid two-phase state to a gas state in the outdoor working fluid pipe (42). S13. In the building cooling mode, switch the four-way valve (5) to the cooling passage so that the high-temperature and high-pressure refrigerant discharged by the compressor (1) enters the outdoor working fluid pipeline (42) of the outdoor flat plate heat exchanger (4). S14. When the surface temperature of the outdoor flat plate heat exchanger (4) is higher than the phase change temperature of the phase change hydrogel layer (432), the phase change hydrogel layer (432) is in an opaque scattering state. Solar radiation is reflected or scattered at the phase change hydrogel layer (432), and the transparent cover layer (433) radiates long-wave infrared radiation to the outside. The refrigerant changes from a gaseous state to a liquid state in the outdoor working fluid pipe (42).

9. The working method according to claim 8, characterized in that, In S11 to S12, the refrigerant passes through the outdoor flat plate heat exchanger (4) and then enters the compressor (1) through the four-way valve (5). After being compressed by the compressor (1), it enters the indoor heat exchanger (2) through the four-way valve (5) and then returns to the outdoor flat plate heat exchanger (4) through the throttling element (3). In S13 to S14, the refrigerant enters the throttling element (3) after passing through the outdoor flat plate heat exchanger (4), and enters the indoor heat exchanger (2) after being throttled by the throttling element (3), and then returns to the compressor (1) through the four-way valve (5).

10. A method for preparing a phase change hydrogel layer in a direct expansion heat pump system based on a spectrally adaptive composite coating as described in any one of claims 1 to 7, characterized in that, Includes the following steps: S21. Weigh the raw materials according to the ratio of deionized water: NIPAm monomer: N,N-methylenebisacrylamide crosslinking agent: ammonium persulfate initiator = 500mL: 50g: 1g: 1g, and prepare the catalyst according to the dosage of 4μL LTEMED for every 1g of NIPAm monomer. S22. Add NIPAm monomer and N,N-methylenebisacrylamide crosslinking agent to deionized water, stir for 30 minutes, then add ammonium persulfate initiator and continue stirring for 10 minutes; S23. Add TEMED catalyst and stir for 1 minute, then place the resulting precursor solution in an ice bath to cool for at least 1 minute; S24. The cooled precursor solution is injected into the cavity formed between the solar heat-absorbing coating (431) and the transparent cover layer (433), and left to stand and crosslink in the cavity for 12 to 24 hours to form a phase change hydrogel layer (432) on the surface of the solar heat-absorbing coating (431).

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