Phase change energy storage type backfill material for ground heat pump and application thereof in ground heat pump

Abstract

The present application relates to a phase change energy storage type backfill material for a buried pipe and its application in a ground source heat pump, and belongs to the field of geothermal energy. The backfill material comprises a phase change composite material and a backfill base material. Graphite powder, polyvinyl alcohol, aluminum hydroxide and aluminum sol are mixed, and a graphite-PVA composite gel is formed through freezing and thawing crosslinking. The graphite-PVA composite gel is soaked in a polyaluminum chloride solution, dried, and then calcined at a temperature of 720-730 DEG C in an inert atmosphere to obtain a graphite-carbon-aluminum oxide composite. The graphite-carbon-aluminum oxide composite is impregnated in a phase change core material to obtain a phase change composite material. The phase change composite material is mixed with the backfill base material and used for backfilling of a buried pipe. The phase change energy storage type backfill material for a buried pipe has high thermal conductivity, high latent heat, low leakage, durability and summer and winter adaptation performance.

Description

Technical Field

[0001] This invention relates to the field of geothermal technology, specifically to phase change energy storage type buried pipe backfill material and its application in ground source heat pumps. Background Technology

[0002] Ground source heat pump systems utilize shallow geothermal energy for heating and cooling, offering advantages such as high efficiency, energy saving, and environmental friendliness. The buried pipe heat exchanger is the core component of a ground source heat pump, and the thermal properties of its backfill material directly affect heat exchange efficiency and the long-term operational stability of the system. Traditional backfill materials for buried pipes often use inorganic materials such as cement, sand, and bentonite. While these materials possess good mechanical and thermal conductivity, their volumetric specific heat capacity is low, resulting in limited energy storage density. This makes it difficult to alleviate the problem of thermal or cold accumulation in the soil around the buried pipes, leading to a decrease in the system's energy efficiency ratio over time.

[0003] In recent years, phase change energy storage materials have been explored for use in backfill materials for buried pipelines due to their high energy density and near-isothermal heat storage and release characteristics. However, existing technologies have the following shortcomings:

[0004] Phase change materials are prone to leakage: conventional phase change materials use porous carriers to adsorb fatty acids or paraffin. Under long-term hot and cold cycles and groundwater pressure, the phase change medium is prone to dissolution, leading to a decrease in energy storage performance.

[0005] Poor thermal conductivity: Most phase change materials have low thermal conductivity. Adding them will reduce the thermal conductivity of the backfill material and affect the heat exchange efficiency.

[0006] In materials design, there is often a certain "competition" or "trade-off" relationship between thermal conductivity and latent heat of phase change. For example, to improve thermal conductivity, it is necessary to add high thermal conductivity fillers (graphite, metals, carbon fibers, etc.). These materials themselves do not participate in phase change, and the higher the proportion of fillers, the lower the proportion of phase change material. On the other hand, high latent heat of phase change requires a high content of phase change media (fatty acids, paraffin, etc.). These media have extremely poor thermal conductivity, and the higher the proportion of phase change material, the lower the proportion of thermally conductive fillers. Therefore, simply mixing high thermal conductivity materials and phase change materials presents a contradiction: increasing the phase change material will reduce thermal conductivity, while increasing the thermally conductive fillers will reduce latent heat.

[0007] Therefore, developing a phase change energy storage type buried pipe backfill material that combines high thermal conductivity, high energy storage, low leakage, pressure and vibration resistance, and can adapt to different operating requirements in summer and winter has important engineering value. Summary of the Invention

[0008] This invention discovered that using a small amount of highly thermally conductive filler (such as graphite) to form a three-dimensional continuous framework can significantly improve the overall thermal conductivity, even with a low filler volume fraction (e.g., 10-20%). In this case, the phase change material can still occupy a relatively large volume fraction (80-90%), resulting in minimal latent heat loss. Furthermore, multi-level porous carrier technology can be employed, such as the graphite-carbon-alumina composite in this invention, which has multi-level pores. This allows for efficient loading of the phase change material (high latent heat) and the formation of thermally conductive pathways through the mutual contact of graphite particles (high thermal conductivity).

[0009] This invention addresses the shortcomings of existing technologies by providing phase change energy storage type buried pipe backfill material and its application in ground source heat pumps. The technical solution is as follows:

[0010] On the one hand, phase change energy storage type underground pipe backfill materials include phase change composite materials and backfill base materials;

[0011] The preparation method of the phase change composite material includes the following steps:

[0012] Graphite powder, polyvinyl alcohol, aluminum hydroxide, and aluminum sol are mixed and then crosslinked through freezing and thawing to form a graphite-PVA composite gel.

[0013] The graphite-PVA composite gel was immersed in a polyaluminum chloride solution, dried, and then calcined at 720-730°C under an inert atmosphere to obtain a graphite-carbon-alumina composite.

[0014] The graphite-carbon-alumina composite is impregnated in a phase change core material to obtain a phase change composite material;

[0015] The phase change composite material is mixed with the backfill base material and then used for backfilling of buried pipes.

[0016] As a further embodiment of the present invention, the phase change core material is paraffin or a mixture of fatty acids;

[0017] The fatty acid mixture is classified as either a summer type or a winter type:

[0018] The summer type is a mixture of decanoic acid and lauric acid in a mass ratio of 60~70:30~40;

[0019] The winter type is a mixture of lauric acid and stearic acid in a mass ratio of 65~70:30~35.

[0020] As a further aspect of the present invention, the mass fraction of the polyaluminum chloride solution is 2.7%.

[0021] As a further embodiment of the present invention, the surface of the phase change composite material is further coated with an epoxy resin encapsulation layer.

[0022] As a further aspect of the present invention, the freezing and thawing crosslinking process is as follows: freezing at a temperature of -15°C or below for at least 10 hours, followed by thawing at room temperature for at least 1.5 hours; the cycle is repeated at least 5 times.

[0023] As a further embodiment of the present invention, the mass ratio of the graphite powder, polyvinyl alcohol, aluminum hydroxide, and aluminum sol solid phase is (5~15):(30~60):(1~5):(1~3).

[0024] As a further aspect of the present invention, the graphite powder and aluminum hydroxide need to be mixed with water to form a corresponding suspension before mixing.

[0025] As a further embodiment of the present invention, the backfill material comprises cement, standard sand, and bentonite;

[0026] The backfill material includes bentonite, standard sand, and kaolin.

[0027] Secondly, the application of the phase change energy storage type buried pipe backfill material in ground source heat pumps includes the following steps:

[0028] The phase change composite material is surface-encapsulated with epoxy resin, and the encapsulated particles are mixed with backfill material.

[0029] The deep well section of the buried pipe is constructed using a layered backfilling method: the phase change energy storage buried pipe backfill material is poured in layers from bottom to top.

[0030] Thirdly, the ground source heat pump system includes a buried pipe, and the buried pipe is surrounded by the phase change energy storage type buried pipe backfill material.

[0031] Compared with the prior art, the beneficial effects of the present invention are:

[0032] 1. By constructing a graphite-carbon-alumina composite, the graphite particles form a continuous thermally conductive network, making the thermal conductivity of the phase change composite material and the backfill material much higher than that of traditional organic phase change materials. At the same time, the high fatty acid content results in high latent heat of phase change and apparent latent heat of backfill material, and the volumetric heat capacity is increased by 20-50% compared to the system without phase change composite material, significantly enhancing the thermal buffering capacity around the buried pipe.

[0033] 2. After calcination, the three aluminum sources (aluminum hydroxide, aluminum sol, and polyaluminum chloride) form a multi-level pore system with interconnected micropores, mesopores, and macropores, achieving both physical anchoring and chemical wetting of fatty acids. Following epoxy resin encapsulation, the multi-component synergistic design of this invention significantly improves leak-proof performance.

[0034] 3. By adjusting the types and ratios of fatty acids, the backfill material can effectively store and release heat under different seasonal working conditions, and can be backfilled in layers and alternately to further optimize heat exchange efficiency.

[0035] 4. The steps of freezing-thawing crosslinking, solution soaking, calcination, and impregnation are well-established, and the optimal framework structure can be obtained when the polyaluminum chloride concentration is within the 2.7% window.

[0036] In summary, the phase change energy storage type buried pipe backfill material provided by this invention and its application in ground source heat pumps effectively solve the problems of easy leakage, low thermal conductivity and poor durability of phase change materials in the prior art, and have significant technological progress and industrial application value. Detailed Implementation

[0037] The present invention will be described in detail below with reference to specific embodiments. These embodiments are merely some, not all, implementations of the present invention. All other implementations obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0038] Example 1

[0039] 1. Add 100g of graphite powder to 5000mL of deionized water and disperse it ultrasonically in an ultrasonic tank while stirring with a stirring paddle for 25 minutes to obtain a graphite suspension.

[0040] 2. Add 30g of aluminum hydroxide to 1200mL of deionized water and disperse it ultrasonically in an ultrasonic tank while stirring with a paddle for 15min to obtain an aluminum hydroxide suspension.

[0041] 3. Add 500g of polyvinyl alcohol (PVA, degree of polymerization 1750±50) powder to the above graphite suspension; heat in a water bath to 90℃ and stir continuously for 2 hours until the PVA is completely dissolved to form a viscous and uniform first mixture; add aluminum hydroxide suspension to the first mixture and stir continuously at 90℃ for 30 minutes to form a viscous and uniform second mixture.

[0042] Add 80g of aluminum sol (25% solid content) of 16% PVA mass to the second mixture and stir continuously for 1 hour to form a viscous and homogeneous third mixture; pour the third mixture into a mold and remove air bubbles.

[0043] 4. Then place the mold in the refrigerator and freeze at -20℃ for 12 hours; remove it and thaw naturally at room temperature (25℃) for 2 hours.

[0044] 5. Repeat the above "freeze-thaw" cycle 5 times to perform freeze-thaw crosslinking and obtain a black gel.

[0045] 6. Remove the obtained black gel from the mold and wash it with deionized water to remove uncrosslinked PVA. The final product is the graphite-PVA composite gel. Graphite particles are uniformly dispersed and fixed in the three-dimensional physical crosslinked network of PVA.

[0046] 7. Prepare a 2.7% (w / w) polyaluminum chloride (PAC) solution using deionized water. Completely immerse the graphite-PVA composite gel in the PAC solution and allow it to stand at room temperature for 12 hours. During this process, PAC slowly diffuses into the gel, neutralizing and coordinating with the PVA molecular chains and the graphite surface to form a composite gel. After immersion, remove the composite gel, rinse with deionized water, and dry it in an oven at 80°C. The dried composite gel is then calcined at 720-730°C under an inert gas atmosphere. At this temperature, PVA decomposes, while aluminum sol and aluminum hydroxide are converted into different forms of alumina (e.g., γ-alumina or α-alumina), ultimately forming a graphite-carbon-alumina composite (GAPC).

[0047] 8. Weigh the mixed acid according to the specified ratio (e.g., decanoic acid-lauric acid mass ratio is 65:35 in summer, lauric acid-stearic acid mass ratio is 70:30 in winter), heat in a water bath to 70~72℃, and stir at a constant temperature until completely melted. To ensure complete impregnation, add the graphite-carbon-alumina composite to the liquid mixed acid at a ratio of 9:1 (mixed acid to graphite-carbon-alumina composite), and keep it at a constant temperature and stir (or ultrasonically vibrate) for more than 2 hours to allow the mixed acid to be fully adsorbed into the graphite-carbon-alumina composite. Centrifuge or filter to remove excess mixed acid, and cool to room temperature. The final phase change composite material is obtained, and its performance indicators are shown in Table 1.

[0048] Table 1

[0049]

[0050] Example 2

[0051] The phase change composite materials obtained in Example 1 (divided into summer phase change composite materials and winter phase change composite materials) were used to make underground pipe backfill materials (divided into summer underground pipe backfill materials and winter underground pipe backfill materials, the only difference between the two being the type of phase change composite material). The specific formula is as follows:

[0052] Phase change composite material of Example 1: 18 to 22 parts, preferably 20 parts (1 part is the basic mass unit, for example, 1 part is 50g).

[0053] Standard sand: 40-50 parts; preferably 50 parts;

[0054] Cement: 15-25 parts, preferably 20 parts;

[0055] Bentonite: 2-3 parts, preferably 3 parts;

[0056] Water: 8 to 12 parts, preferably 11 parts.

[0057] The relevant indicators of the buried pipe backfill material (obtained by taking upper and lower limits after multiple measurements) are shown in Table 2:

[0058] Table 2

[0059]

[0060] In contrast, if the phase change composite material of Example 1 is not added to the backfill material of the buried pipe in this example, the corresponding volumetric specific heat capacity is 2.0~2.4 MJ / (m³). 3 ·K).

[0061] The thermal conductivity of the cement-sand-bentonite system is generally above 2 W / (m·K), which is reduced after adding the buried pipe backfill material.

[0062] In this embodiment, when preparing the backfill material for buried pipes, a retarder can be added to slow down the cement hydration rate, extend the hydration heat release cycle, and reduce the peak heat release; or, low-heat cement can be selected to reduce the total heat release of hydration; or, dry material premixing can be used, followed by the addition of ice water for mixing.

[0063] In deep well sections, a layered (segmented) backfilling method is used, with sufficient time allowed between each layer to allow heat to dissipate before pouring the next layer, thus avoiding heat accumulation. For example, summer-type and winter-type buried pipe backfill materials can be used for alternating backfilling in layers.

[0064] In some other embodiments, the phase change composite material of Example 1 can reduce leaching by surface impregnation with a coupling agent.

[0065] Example 3

[0066] The phase change composite material obtained in Example 1 was used to prepare a backfill material for buried pipes, with the following specific formula:

[0067] Bentonite (sodium-based): 4-6 parts; preferably 5 parts;

[0068] Standard sand: 50-60 parts; preferred: 55 parts;

[0069] Kaolin: 24-36 parts; preferably 30 parts;

[0070] Phase change composite material of Example 1: 20 parts;

[0071] Water: 15-18 parts; preferably 16 parts.

[0072] Construction process: Mix bentonite, standard sand and kaolin thoroughly by dry mixing, then add phase change composite material and mix thoroughly; then add water and mix, and finally pour and cure.

[0073] The relevant indicators of the buried pipe backfill material (obtained by taking upper and lower limits after multiple measurements) are shown in Table 3:

[0074] Table 3

[0075]

[0076] In contrast, if the phase change composite material of Example 1 is not added to the backfill material for the buried pipe in this example, the corresponding volumetric specific heat capacity is 1.5~2.0 MJ / (m³). 3 ·K).

[0077] Example 4

[0078] In practical applications, to ensure a service life of more than 10 years, the phase change composite material of Example 1 usually needs to be encapsulated. The encapsulation process uses conventional epoxy resin encapsulation, such as mixing bisphenol A type epoxy resin and 650 curing agent in a ratio of 100:50 to make epoxy resin glue. Then, a multi-spray process or fluidized bed spraying process is used to coat the surface of the phase change composite material particles with liquid epoxy resin, which is then cured to form a shell.

[0079] In this invention, the three different aluminum components in the phase change composite material described in Example 1 will form Al2O3 with multi-level channels after calcination. For example, the carbon skeleton pores left by PVA carbonization are generally micropores, the structural pores of Al2O3 itself are mesopores, and the gaps between graphite particles, template pores, etc. are generally macropores. In addition, the thermal decomposition difficulty of different components such as polyaluminum chloride, aluminum sol, and aluminum hydroxide is different. Finally, a composite particle with multi-level channels and a complex three-dimensional structure will be constructed. This microstructure can achieve physical anchoring, construct a stress buffer layer, and enhance chemical wetting bonding.

[0080] The following tests were all conducted using the summer phase change composite material from Example 1.

[0081] Control Experiment 1: The difference between this example and Example 1 is that in the preparation of the phase change composite material in this example, polyaluminum chloride is not added, but an equal mass of aluminum hydroxide is used instead; all other aspects are the same; finally, control material 1 is obtained. Control material 1 is encapsulated with epoxy resin and then tested.

[0082] Control Experiment 2: The difference between this example and Example 1 is that in the preparation of the phase change composite material in this example, aluminum sol is not added; instead, an equal mass of aluminum hydroxide is used instead. All other aspects are the same. Control material 2 is then obtained. Control material 2 is encapsulated with epoxy resin and then tested.

[0083] Control Experiment 3: The difference between this example and Example 1 is that in the preparation of the phase change composite material in this example, aluminum hydroxide is not added, but an equal mass of polyaluminum chloride is used instead; all other aspects are the same; finally, control material 3 is obtained. Control material 3 is encapsulated with epoxy resin and then tested.

[0084] Control Experiment 4: The difference between this example and Example 1 is that in the preparation of the phase change composite material in this example, polyaluminum chloride is directly added to the third mixture, and then air bubbles are removed in the mold. A "freeze-thaw" cycle is then performed, unlike Example 1 where the graphite-PVA composite gel is soaked in a polyaluminum chloride solution. Finally, both are calcined at 720~730℃ to obtain Control Material 4. Control Material 4 is encapsulated with epoxy resin and then tested.

[0085] Control Experiment 5: The difference between this example and Example 1 is that in the preparation of the phase change composite material in this example, aluminum chloride hexahydrate was used instead of polyaluminum chloride. An aluminum chloride solution was prepared and used to soak the graphite-PVA composite gel. All other aspects remained the same, resulting in control material 5. Control material 5 was then encapsulated with epoxy resin and tested.

[0086] 1. Hydrostatic pressure test

[0087] The test sample (e.g., the particulate matter obtained after encapsulating the phase change composite material of Example 1 with epoxy resin) was poured into a water tank, immersed in water, pressurized to 1 MPa, then heated to 60°C, and then naturally cooled to room temperature; this cycle was repeated from 60°C to room temperature, and the sample was left to stand for 3 days. The mass loss rate was calculated based on the mass loss of the test sample before and after the test. Additionally, the thermal conductivity of the test sample before and after the test also needed to be tested, and the thermal conductivity decay rate was calculated.

[0088] 2. Dynamic water pressure test

[0089] The test sample (e.g., the particulate matter obtained after encapsulating the phase change composite material of Example 1 with epoxy resin) was poured into a water tank, immersed in water, and pressurized to 1 MPa. An ultrasonic vibrating rod was installed inside the tank. During the test, the ultrasonic frequency was 20 kHz, and the sample was heated to 60°C, then allowed to cool naturally to room temperature. This cycle was repeated from 60°C to room temperature, and the sample was left to stand for 3 days. The mass loss rate was calculated based on the mass loss of the test sample before and after the test. Additionally, the thermal conductivity of the test sample before and after the test was also tested, and the thermal conductivity decay rate was calculated.

[0090] Under high pressure, ultrasonic high-frequency vibration can further cause the epoxy resin shell to break, leading to leakage of the phase change composite material. This simulation is used to model the impact of adverse factors such as high pressure during backfilling, construction vibration, and potential environmental vibration on the encapsulated phase change composite material.

[0091] The results of Example 1 and Control Experiments 1-5 are shown in Table 4:

[0092] Table 4

[0093]

[0094] As is known in the art, if the encapsulation material leaks and causes the mixed acid to overflow, the apparent latent heat will certainly decrease significantly. However, the change in thermal conductivity is not necessarily a decrease, because the thermal conductivity of fatty acids is not as good as that of thermally conductive materials such as graphite. As a phase change filler, its loss will lead to an increase in porosity, and the interfacial thermal resistance may increase. Whether the thermal conduction path is necessarily blocked depends on whether the three-dimensional thermal conduction structure is changed. Therefore, with the overflow of mixed acid, the thermal conductivity may decrease slightly.

[0095] As shown in Comparative Experiments 1-3 and Table 4, the three different aluminum components in the phase change composite material are the basis for the continuous porous skeleton with corresponding multi-level channels and complex three-dimensional structures that are formed subsequently. If any one of them is missing, the porous skeleton may not be as stable as expected due to unreasonable stacking structure. Although the graphite particles can be relatively uniformly distributed in the above porous skeleton (the thermal conductivity attenuation rate is not very large), the compatibility of the porous skeleton with epoxy resin is very poor when it is encapsulated in the subsequent process. Once under high pressure and high vibration environment, irreversible damage occurs between the encapsulated epoxy resin shell and the phase change composite material, resulting in fatty acid leakage.

[0096] In control experiment 2, the high mass loss indicates that a large amount of fatty acid leaked. The reason why the thermal conductivity did not decrease significantly may be because the structure of the thermally conductive skeleton did not change much, and the leaked fatty acid had little thermal conductivity, so it did not have much impact on the overall thermal conductivity.

[0097] Control experiment 5 further confirms that replacing polyaluminum chloride with aluminum chloride hexahydrate disrupts the entire synthesis system, preventing the formation of a stable porous framework. Furthermore, the phenomenon of small mass loss but high thermal conductivity decrease in control experiment 5 may be due to the fact that replacing polyaluminum chloride with aluminum chloride hexahydrate creates a relatively closed alumina layer with poor connectivity during calcination, preventing sufficient impregnation of fatty acids and resulting in a large number of pores being occupied by air. The extremely low thermal conductivity of air makes the initial thermal conductivity of the material heavily dependent on fatty acids. Therefore, even with a small amount of fatty acid leakage, the thermal resistance increases significantly after air enters the pores, leading to a substantial decrease in thermal conductivity.

[0098] As shown in control experiment 4, the addition step of polyaluminum chloride directly affects its distribution in the porous framework and also affects the final porous framework structure.

[0099] Experimental Example 1: The difference between this example and Example 1 is that in the preparation of the phase change composite material, the mass fraction of the polyaluminum chloride solution used in step 7 is 1%, 2%, 3%, and 5%, respectively. The corresponding experimental results are shown in Table 5.

[0100] Table 5

[0101]

[0102] The possible reasons are as follows: if the concentration of the polyaluminum chloride solution is too low, it may cause the skeleton to crack, making the gel brittle. During subsequent drying and calcination, microcracks will form due to internal stress concentration. After a limited number of freeze-thaw cycles, these microcracks will expand, forming channels that cause fatty acid leakage, ultimately leading to thermal cycling failure. Conversely, if the concentration of the polyaluminum chloride solution is too high, it will form uneven agglomerates or excessively cross-linked networks, increasing the material's brittleness. Once calcined, this will easily cause the collapse of the three-dimensional hierarchical porous structure, directly reducing the initial latent heat of phase change.

[0103] Furthermore, it should be understood that those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A phase change energy storage type underground pipe backfill material, characterized in that, This includes phase change composite materials and backfill materials; The preparation method of the phase change composite material includes the following steps: Graphite powder, polyvinyl alcohol, aluminum hydroxide, and aluminum sol are mixed and then crosslinked through freezing and thawing to form a graphite-PVA composite gel. The graphite-PVA composite gel was immersed in a polyaluminum chloride solution, dried, and then calcined at 720-730°C under an inert atmosphere to obtain a graphite-carbon-alumina composite. The graphite-carbon-alumina composite is impregnated in a phase change core material to obtain a phase change composite material; The phase change composite material is mixed with the backfill base material and then used for backfilling of buried pipes.

2. The phase change energy storage type buried pipe backfill material according to claim 1, characterized in that: The phase change core material is paraffin or a mixture of fatty acids; The fatty acid mixture is classified as either a summer type or a winter type: The summer type is a mixture of decanoic acid and lauric acid in a mass ratio of 60~70:30~40; The winter type is a mixture of lauric acid and stearic acid in a mass ratio of 65~70:30~35.

3. The phase change energy storage type buried pipe backfill material according to claim 1, characterized in that: The polyaluminum chloride solution has a mass fraction of 2.7%.

4. The phase change energy storage type buried pipe backfill material according to claim 1, characterized in that: The surface of the phase change composite material is also coated with an epoxy resin encapsulation layer.

5. The phase change energy storage type buried pipe backfill material according to claim 1, characterized in that: The freezing and thawing crosslinking process is as follows: freezing at -15°C or below for at least 10 hours, followed by thawing at room temperature for at least 1.5 hours; the cycle is repeated at least 5 times.

6. The phase change energy storage type buried pipe backfill material according to claim 1, characterized in that: The mass ratio of graphite powder, polyvinyl alcohol, aluminum hydroxide, and aluminum sol solid phase is (5~15):(30~60):(1~5):(1~3).

7. The phase change energy storage type buried pipe backfill material according to claim 1, characterized in that: The graphite powder and aluminum hydroxide need to be mixed with water to form a corresponding suspension before mixing.

8. The phase change energy storage type buried pipe backfill material according to claim 1, characterized in that: The backfill material includes cement, standard sand, and bentonite; or, The backfill material includes bentonite, standard sand, and kaolin.

9. The application of the phase change energy storage type buried pipe backfill material according to any one of claims 1 to 8 in ground source heat pumps, characterized in that, Includes the following steps: The phase change composite material is surface-encapsulated with epoxy resin, and the encapsulated particles are mixed with backfill material. The deep well section of the buried pipe is constructed using a layered backfilling method: the phase change energy storage buried pipe backfill material is poured in layers from bottom to top.

10. A ground source heat pump system, including buried pipes, characterized in that, The buried pipe is surrounded by the phase change energy storage type buried pipe backfill material as described in any one of claims 1 to 8.

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