A layered cascade phase change nano underground heat exchange structure and heat pump system

Abstract

The present invention discloses a layered stepped phase-change nano underground heat exchange structure, which includes an outer surface constraint tube, a first inner tube, a second inner tube and a heat exchange tube arranged from the outside to the inside, wherein the heat exchange tube is arranged in the second inner tube, and a plurality of separators are evenly distributed along the length direction of the first inner tube and the second inner tube, and the separators divide the first inner tube and the second inner tube into a plurality of layers; each layer is filled with two phase-change materials, and the two phase-change materials do not interfere with each other.

Description

Technical Field

[0001] The present invention relates to the technical field of ground source heat pumps, and in particular to a layered and stepped phase-change nano underground heat exchange structure and a heat pump system. Background Art

[0002] With rapid socioeconomic development, energy issues are gaining increasing attention in more and more countries. To meet the ever-increasing demand for energy, my country's traditional energy structure urgently needs to be optimized. In recent years, clean and renewable energy sources such as wind, solar, and geothermal energy have been vigorously developed. Geothermal energy, in particular, has attracted widespread attention due to its large reserves and widespread distribution. Furthermore, centralized heating is not only a rigid demand for civil construction in northern China but has also been widely adopted in southern China in recent years, leading the country to vigorously promote the development of related technologies. Against this backdrop, ground-source heat pump technology emerged.

[0003] The ground source heat pump air conditioning system consists of three parts: underground heat exchange structure, water source heat pump unit system, and indoor heating and air conditioning terminal system. It "uses underground heat exchange structure to extract low-grade energy from soil and groundwater, and upgrades low-grade energy to high-grade energy through a small amount of electricity". It has the advantages of energy saving, high efficiency, economy and environmental protection.

[0004] Traditional ground-source heat pump air conditioning systems require separately buried heat exchangers, which takes up more land resources and incurs expensive drilling costs. Furthermore, considering the thermal imbalance of the soil, during actual use, the soil may experience cold or hot accumulation due to uneven heat release and heat extraction, which will affect soil recovery effectiveness and the long-term operating efficiency of the unit.

[0005] Incorporating the pipe pile foundations commonly used in current buildings, a "hollow sandwich steel tube concrete column geothermal heating system" has been proposed. This technology installs heat exchange pipes within the pile foundation, meaning that the concrete columns serve as the building's vertical load-bearing structure while also containing plumbing pipes for the indoor heating and air-conditioning system within the sandwich concrete. Plate heat exchangers are also installed within the pipes to accelerate the heat exchange efficiency between the vertical heat pump system and the water-source heat pump unit. This design saves building space by laying pipes within the foundation columns. It also eliminates the need for underground heat exchangers, reducing costs. Concrete and steel also have higher thermal conductivity than soil, further improving energy conversion efficiency. However, this technology still fails to effectively address the large and rapid fluctuations in soil temperature, and the accumulation of hot and cold soil temperatures remains a challenge.

[0006] Other experts have proposed designing a prefabricated energy pile, in which the longitudinal reinforcement and spiral stirrups within the pile foundation are hollowed out, allowing heat exchange to occur through a heat-carrying, anti-corrosion fluid flowing through the steel cage. This type of pile, with its densely packed steel, extends the heat exchange path, resulting in more efficient heat exchange and smaller temperature fluctuations. However, this also involves complex manufacturing processes and high costs.

[0007] Energy piles using phase change materials have also been proposed. Because phase change materials are corrosive to concrete, the idea is to store paraffin wax inside steel balls as aggregate. After arranging the steel cage, heat exchange tubes, and several steel balls, concrete is poured. The steel balls containing the phase change material are evenly distributed throughout the pile. This type of phase change material energy pile is complex to manufacture, and the use of stainless steel balls to package the phase change material requires a large number of balls, resulting in high costs. Summary of the Invention

[0008] In response to the shortcomings of the prior art, the present invention aims to provide a layered cascade phase-change nano underground heat exchange structure and heat pump system. The cross-sectional form of the heat exchange structure of the present invention can be a circular tube or a square tube. The heat exchange tube is arranged in the hollow part between the inner tube walls. It is a fixed heat exchange tube. The internal cavity is filled with two layers of PCM (phase change material) in layers along the radial direction. The two layers of phase change material are separated by a second inner tube, and the outer PCM and concrete are separated by a first inner tube. The use of two layers of phase change material in the radial direction and multiple layers of different phase change materials in the depth direction can effectively phase-change and store heat at different temperatures, slow down the accumulation of cold and heat in the soil, and improve the operating energy efficiency of the ground source heat pump. Nanofluid material circulates in the heat exchange tube, and an external water source heat pump unit can be connected to obtain a complete ground source heat pump air conditioning system based on the layered cascade phase-change nano underground heat exchange structure.

[0009] In order to achieve the above object, the present invention adopts the following technical solutions:

[0010] A layered stepped phase-change nano underground heat exchange structure comprises an outer surface constraint tube, a first inner tube, a second inner tube, and a heat exchange tube arranged from outside to inside, wherein the heat exchange tube is arranged in the second inner tube, and a plurality of separators are evenly distributed along the length direction of the first inner tube and the second inner tube, and the separators divide the first inner tube and the second inner tube into a plurality of layers; each layer is filled with two phase-change materials, and the two phase-change materials do not interfere with each other.

[0011] It should be noted that the phase change material includes each inner layer of PCM n1 , outer PCM n2 (n is the nth layer along the depth direction); when the inner PCM n1 When the second inner tube of the nth layer is filled, the outer PCM n2 Filled between the first inner tube and the second inner tube of the n-th layer.

[0012] It should be noted that the filling method adopts radial filling of two different phase change materials and depth filling of n different phase change materials.

[0013] It should be noted that the circulating fluid in the heat exchange tube is a nanofluid with high thermal conductivity.

[0014] It should be noted that concrete is poured between the outer surface constraint tube and the first inner tube.

[0015] The present invention also provides a method for preparing a layered stepped phase-change nano underground heat exchange structure, the method comprising the following steps:

[0016] S1 prefabricates a heat exchange structure consisting of an outer surface tube, a first inner tube, and a second inner tube in sequence;

[0017] S2: tying stress-bearing steel bars, stirrups and filling concrete between the outer surface restraining tube and the first inner tube;

[0018] S3 installs heat exchange tubes in the second inner tube and divides it into several layers using separators, and then places the configured inner PCM layers n1 and outer PCM n2 (n is the nth layer along the depth direction) perform layered filling;

[0019] S4 will phase change material PCM 11 Filled in the second inner tube of the first layer, the phase change material PCM 12 Then fill back between the first inner tube and the second inner tube of the layer; similarly, the phase change material PCM n1 Filled in the second inner tube of the nth layer, the phase change material PCM n2 Then, backfill the space between the first inner tube and the second inner tube of the layer. Fill the inner and outer phase change materials of different layers in the same manner;

[0020] S5 fills the heat exchange tube with nanofluid with high thermal conductivity.

[0021] It should be noted that the heat exchange tube is a U-shaped tube or a spiral tube.

[0022] The present invention further provides a heat pump system with a layered cascade phase change nano underground heat exchange structure, the system includes a summer air conditioning mode and a winter heat pump mode, wherein

[0023] The summer air conditioning mode includes a layered cascade phase change nano underground heat exchange structure, a booster pump, a condenser, a throttle valve, an evaporator and an indoor terminal connected in sequence, wherein a compressor is also provided, and the two ends of the compressor are respectively connected to the condenser and the evaporator;

[0024] The winter heat pump mode includes a layered cascade phase change nano underground heat exchange structure, a booster pump, an evaporator, a throttle valve, a condenser and an indoor terminal connected in sequence, wherein a compressor is also provided, and the two ends of the compressor are respectively connected to the evaporator and the condenser; a three-way valve is provided between the booster pump and the evaporator, and the three-way valve is connected to one end of the water tank, and the other end of the water tank is connected to the domestic hot water terminal.

[0025] The beneficial effects of the present invention are that the layered and stepped phase-change nano underground heat exchange structure not only improves the heat exchange efficiency of geothermal energy extraction through the heat exchange tubes through nanofluids, but also achieves dual-purpose structure and performance, which can simultaneously improve the heat exchange performance of the system in both winter and summer, greatly reducing production and construction costs. At the same time, the backfill phase change material selected according to the material's thermal performance parameters as a buffer can effectively absorb and release the phase change latent heat, greatly reducing the temperature fluctuation range, effectively solving the problem of soil cold and hot accumulation, improving the unit's operating efficiency, and extending the unit's service life.

[0026] In general, the present invention has the following advantages:

[0027] 1. Flexible and convenient construction. The underground heat exchange structure is mainly composed of prefabricated components. The heat exchange tubes in the cavity can also be prefabricated in the factory. The position of the heat exchange tubes can be determined during on-site construction. After positioning is completed, PCM can be backfilled layer by layer.

[0028] 2. Minimal disturbance to the surrounding soil, accelerating soil recovery. Phase-change backfill materials, acting as buffers, can effectively absorb or release heat through phase change, slowing down the rate of soil temperature change, reducing the magnitude of soil temperature fluctuations, and minimizing the radius of soil thermal influence. The greater the latent heat of the material's phase change, the smaller the soil temperature change, effectively resolving the problem of hot and cold soil accumulation.

[0029] 3. Enhanced heat exchange capacity. The circulating fluid in the layered and stepped phase-change nano underground heat exchange structure uses nanofluid with high thermal conductivity, which can further improve the heat exchange capacity of the underground heat exchange structure.

[0030] 4. Save land resources and reduce construction costs. Placing the heat exchange tubes in the tube cavity and filling it with composite phase change materials not only eliminates the need to bury an outdoor underground heat exchanger, thus avoiding additional land occupation, but also helps improve the bearing capacity of the structure while backfilling.

[0031] 5. Superior performance, environmental protection, and economic benefits. The inner and outer tubes increase the thermal conductivity of the structure, significantly improving the heat transfer efficiency of the underground heat exchange structure while also ensuring structural rigidity and meeting load-bearing requirements. Both steel pipes and concrete facilitate more complete heat exchange, providing a large amount of clean energy for the building.

[0032] 6. Greater applicability, achieving cascaded energy utilization. Using two layers of phase change material in the radial direction and multiple layers of different phase change materials in the depth direction can effectively store heat at different temperatures through phase change, slowing down the accumulation of hot and cold soils and improving the operating efficiency of ground-source heat pumps. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a schematic diagram of the layered and stepped phase-change nano-underground heat exchange structure of the present invention;

[0034] Figure 2 Schematic diagram of the cross-sectional structure of the layered and stepped phase-change nano underground heat exchange structure of the present invention after being filled with phase-change material;

[0035] Figure 3 Schematic diagram of the layered step-by-step phase-change nano-underground heat exchange structure in Example 1 of the present invention;

[0036] Figure 4 This is a structural diagram of a plurality of steel piles connected into long piles by steel pile connecting sleeves and bolts in Example 1 of the present invention;

[0037] Figure 5 This is a schematic diagram of the structure of connecting multiple tubes through straight-through joints when the heat exchange tubes are long in Example 1 of the present invention;

[0038] Figure 6 Schematic diagram of the separator installed in the layered stepped phase change nano underground heat exchange structure in Example 1 of the present invention;

[0039] Figure 7 for Figure 6 Schematic diagram of the structure of the middle separator;

[0040] Figure 8 This is a schematic diagram of the summer air-conditioning mode of the ground source heat pump system in Example 2 of the present invention;

[0041] Figure 9 This is a schematic diagram of the winter heat pump mode of the ground source heat pump system in Example 3 of the present invention. DETAILED DESCRIPTION

[0042] The present invention will be further described below. It should be noted that the following examples are based on the present technical solution and provide detailed implementation methods and specific operating processes, but the protection scope of the present invention is not limited to these examples.

[0043] like Figure 1As shown, the present invention is a layered stepped phase change nano underground heat exchange structure, which includes an outer surface constraint tube 1, a first inner tube 2, a second inner tube 3 and a heat exchange tube 4 arranged from the outside to the inside, wherein the heat exchange tube 4 is arranged in the second inner tube 3, and a plurality of separators 5 are evenly distributed along the length direction of the first inner tube 2 and the second inner tube 3. The separators 5 divide the first inner tube 2 and the second inner tube 3 into several layers; each layer is filled with two phase change materials, and the two phase change materials do not interfere with each other.

[0044] Further, such as Figure 1 As shown, the phase change material of the present invention includes configured inner layers of PCM n1 and outer PCM n2 , where n is the nth layer along the depth direction, and the filling is done in layers. 11 Filled in the second inner tube of the first layer, the phase change material PCM 12 Then fill back between the first inner tube and the second inner tube of the layer; similarly, the phase change material PCM n1 Filled in the second inner tube of the nth layer, the phase change material PCM n2 Then backfill between the first inner tube and the second inner tube of the layer; fill the inner and outer phase change materials of different layers according to the filling method. Further, the filling method of the present invention adopts radial filling of two different phase change materials and length direction filling of multiple different phase change materials.

[0045] Furthermore, the circulating fluid in the heat exchange tube of the present invention is a nanofluid with high thermal conductivity.

[0046] Furthermore, in the present invention, concrete is poured between the outer surface constraint tube and the first inner tube.

[0047] The present invention also provides a method for preparing a layered stepped phase-change nano underground heat exchange structure, the method comprising the following steps:

[0048] S1 prefabricates a heat exchange structure consisting of an outer surface constraint tube, a first inner tube, and a second inner tube in sequence;

[0049] S2: tying stress-bearing steel bars, stirrups and filling concrete between the outer surface restraining tube and the first inner tube;

[0050] S3 installs heat exchange tubes in the second inner tube and divides it into several layers using separators, and then places the configured phase change material PCM inside each layer. n1 and external phase change material PCM n2 (n is the nth layer along the depth direction) perform layered filling;

[0051] S4 will phase change material PCM11 Filled in the second inner tube of the first layer, the phase change material PCM 12 Then fill back between the first inner tube and the second inner tube of the layer; similarly, the phase change material PCM n1 Filled in the second inner tube of the nth layer, the phase change material PCM n2 Then, backfill the space between the first inner tube and the second inner tube of the layer. Fill the inner and outer phase change materials of different layers in the same manner;

[0052] S5 fills the heat exchange tube with nanofluid with high thermal conductivity.

[0053] Furthermore, the heat exchange tube of the present invention is a U-shaped tube or a spiral tube.

[0054] The present invention further provides a heat pump system with a layered cascade phase change nano underground heat exchange structure, the system includes a summer air conditioning mode and a winter heat pump mode, wherein

[0055] The summer air-conditioning mode includes a layered cascade phase change nano underground heat exchange structure, a booster pump, a condenser, a throttle valve, an evaporator and an indoor terminal connected in sequence, wherein a compressor is also provided, and the two ends of the compressor are respectively connected to the condenser and the evaporator;

[0056] The winter heat pump mode includes a layered cascade phase change nano underground heat exchange structure, a booster pump, an evaporator, a throttle valve, a condenser and an indoor terminal connected in sequence, wherein a compressor is also provided, and the two ends of the compressor are respectively connected to the evaporator and the condenser; a three-way valve is provided between the booster pump and the evaporator, and the three-way valve is connected to one end of the water tank, and the other end of the water tank is connected to the domestic hot water terminal.

[0057] The manufacturing and installation process of the present invention will be better described through the following examples.

[0058] Example 1

[0059] The layered step phase change nano underground heat exchange structure of the present invention is also a reinforced concrete structure, which can be prefabricated in the factory. The outer surface constraint tube 1 with a closed bottom surface and the first inner tube 2 and the second inner tube 3 are customized in advance, wherein the first inner tube 2 and the second inner tube 3 share a bottom surface through welding. The function of the outer surface constraint tube 1 is to constrain the concrete and improve the thermal conductivity of the pile body; the function of the first inner tube 2 is to prevent the outer layer PCM from leaking and corroding the concrete; the function of the second inner tube 3 is to prevent the two PCMs from mixing and prevent PCM leakage. The first inner steel pipe 2 is placed inside the outer surface constraint tube 1, and the cavities of the two are tied with stress-bearing steel bars 13, stirrups 12 and filled with concrete 4, as shown Figure 3In addition, the total length of the structure of the present invention can be calculated according to the required cooling and heating load of the building environment. If the structure is long, in order to facilitate construction, multiple steel piles can be connected into long piles through steel pile connecting sleeves 17 and bolts 18, as shown in FIG. Figure 4 shown.

[0060] After the prefabricated structure of the present invention is cured, it is transported to the construction site for installation. A heat exchange tube 10 is installed in the cavity of the second inner tube. The heat exchange tube 10 can be a U-shaped tube or a spiral tube according to actual conditions. If the heat exchange tube 10 is long, multiple tubes can be connected through a straight-through joint 19. The heat exchange tube 10 is inserted into the straight-through joint and fixed by a buckle 20. Figure 5 Then, the configured inner layers of the phase change material PCM n1 and external phase change material PCM n2 (n is the nth layer along the depth direction) perform layered filling;

[0061] On the other hand, the phase change material PCM 11 Filled in the second inner tube of the first layer, the phase change material PCM 12 Then fill back between the first inner tube and the second inner tube of the layer; similarly, the phase change material PCM n1 Filled in the second inner tube of the nth layer, the phase change material PCM n2 Then fill back between the first inner tube and the second inner tube of the layer. Fill the inner and outer layers of the different layers of phase change material according to this filling method. The inner and outer layers of PCM in the radial direction are separated by the second inner tube. The N layers of PCM in the depth direction are separated by a heat exchange tube and a circular separator 9 to ensure the sealing of the phase change material. Figure 6 As shown. The circular separator is composed of an annular steel sheet sandwiched between two layers of oil-resistant rubber pads. Specifically, after the backfill of the lower layer of phase change material is completed, a bottom layer of oil-resistant rubber pad 15, annular steel sheet 16 and a top layer of oil-resistant rubber pad 14 are laid on the surface in sequence. Figure 7 shown.

[0062] Example 2

[0063] Capric-lauric acid mixed acid was used as PCM1, oleic acid as PCM2, and CuO-water solid-liquid hybrid nanofluid was used. PCM1 was found to be beneficial in summer conditions, while PCM2 was found to be beneficial in winter conditions.

[0064] It should be pointed out that the capric acid-lauric acid mixed acid phase change material can be prepared according to the following procedure. Capric acid and lauric acid are both solid at below 26°C. First, capric acid and lauric acid are heated separately to a molten state, and then measured according to a quantitative ratio (66:34) and then mixed and stirred until uniform.

[0065] To enhance heat transfer performance, the circulating fluid in the heat exchange tubes is a CuO-water solid-liquid mixed nanofluid with a molar concentration of 1%-10%. The CuO-water nanofluid can be prepared using a two-step process. Nanoparticles are first prepared in a proportional manner. Ultrasonic vibration is then used to disrupt the mutual attraction between the small particles, or a dispersant is used to generate a strong repulsive force between the particles. This allows the CuO nanoparticles to be stably dispersed in water, forming the CuO-water solid-liquid mixed nanofluid.

[0066] When cooling the room in summer, the four-way valve is turned to the air conditioning position according to the cooling condition. The condenser in the heat pump unit is connected to the stratified cascade phase change nano underground heat exchange structure through the ground source side circulating water system. The CuO-water nanofluid circulated from the heat pump unit to the underground heat exchange structure enters the stratified cascade phase change nano underground heat exchange structure. First, the heat exchange efficiency of the underground heat exchange structure is improved by the CuO-water nanofluid in the heat exchange tube. Due to the presence of the inner and outer layers of phase change materials, capric acid-lauric acid mixed acid, a large part of the heat in the heat exchange tube is first used for the phase change of the mixed acid. The solid mixed acid absorbs heat and melts, and the excess heat is transferred to the soil. This realizes the transfer of heat from the high-grade heat source room to the low-grade heat source soil. At the same time, due to the phase change absorption of the mixed acid, the soil temperature changes little and the heating rate is also slowed down, reducing the heat accumulation of the soil. In this way, the mixed acid PCM1 in the stratified cascade phase change nano underground heat exchange structure plays a role. Figure 8 As shown, the layered step phase change nano underground heat exchange structure 1, the booster pump 2, the compressor 3, the condenser 4, the throttle valve 5, the evaporator 6, and the indoor terminal 7.

[0067] Example 3

[0068] When heating in winter, turn the four-way valve to the heat pump position and operate according to the heating condition. The evaporator in the heat pump unit is connected to the stratified cascade phase change nano underground heat exchange structure through the ground source side circulating water system. In winter, the low-temperature CuO-water nanofluid circulated from the heat pump unit to the energy underground structure enters the stratified cascade phase change nano underground heat exchange structure. First, the heat exchange efficiency of the underground heat exchange structure is improved by the CuO-water nanofluid in the heat exchange tube. The heat of the water source unit near the ground comes from the heat of the soil on the one hand and the oleic acid phase change on the other hand. While obtaining heat from the soil, the low-temperature CuO-water nanofluid in winter flows into the heat exchange tube of the soil. First, the inner and outer layers of oleic acid PCM2 are condensed into a solid state and release a large amount of solidification heat, which reduces the heat extracted from the soil, reduces the cooling amplitude of the soil, and reduces the cold accumulation of the soil. In this way, the oleic acid PCM in the stratified cascade phase change nano underground heat exchange structure plays a role. Figure 9As shown, the layered cascade phase change nano underground heat exchange structure 1, the booster pump 2, the compressor 3, the evaporator 4, the throttle valve 5, the condenser 6, the indoor terminal 7, the three-way valve 8, the water tank 9, and the domestic hot water terminal 10.

[0069] Those skilled in the art can make various corresponding changes and modifications based on the above technical solutions and concepts, and all of these changes and modifications should be included in the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a layered and stepped phase-change nano-underground heat exchange structure, characterized in that: The method comprises the following steps: S1 prefabricates a heat exchange structure consisting of an outer surface restraining tube, a first inner tube, and a second inner tube in sequence; S2 Tie the stress-bearing steel bars, stirrups and fill with concrete between the outer surface restraining tube and the first inner tube; S3 Install heat exchange tubes in the second inner tube and use separators to divide the tubes into several layers. Then, place the configured inner PCM layers on the heat exchange tubes. n1 and outer PCM n2 , where n is the nth layer along the depth direction, and layered filling is performed; S4 Phase change material PCM 11 Filled in the second inner tube of the first layer, the phase change material PCM 12 Then fill back between the first inner tube and the second inner tube of the layer; similarly, the phase change material PCM n1 Filled in the second inner tube of the nth layer, the phase change material PCM n2 Then, backfill the space between the first inner tube and the second inner tube of the layer; and fill the inner and outer phase change materials of different layers according to the filling method; S5 fills the heat exchange tube with high thermal conductivity nanofluid.

2. The method for preparing the layered and stepped phase-change nano underground heat exchange structure according to claim 1, characterized in that: The heat exchange tube is a U-shaped tube or a spiral tube.

3. A heat pump system having a layered cascade phase change nano underground heat exchange structure prepared by the preparation method according to any one of claims 1-2, characterized in that: The system includes a summer air conditioning mode and a winter heat pump mode, wherein The summer air-conditioning mode includes a layered cascade phase change nano underground heat exchange structure, a booster pump, a condenser, a throttle valve, an evaporator and an indoor terminal connected in sequence, wherein a compressor is also provided, and the two ends of the compressor are respectively connected to the condenser and the evaporator; The winter heat pump mode includes a layered cascade phase change nano underground heat exchange structure, a booster pump, an evaporator, a throttle valve, a condenser and an indoor terminal connected in sequence, wherein a compressor is also provided, and the two ends of the compressor are respectively connected to the evaporator and the condenser; a three-way valve is provided between the booster pump and the evaporator, and the three-way valve is connected to one end of the water tank, and the other end of the water tank is connected to the domestic hot water terminal.

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