Ground source heat pump technology using buried pipes is currently one of the cutting-edge research topics in the air conditioning field. Due to its advantages such as high coefficient of performance, good energy efficiency, utilization of renewable energy, environmental friendliness, and system simplicity, buried pipe ground source heat pump systems are widely used in Europe and America. However, in my country, it is still in its initial stage, with only a few buried pipe ground source heat pump units currently in operation. my country has a vast territory and abundant shallow geothermal resources (usually less than 400m). Therefore, effectively utilizing these shallow geothermal resources to overcome the limitations and shortcomings of traditional heat pump air conditioning technology is of great significance and practical value. The reason why buried pipe ground source heat pumps have not developed as rapidly as air source heat pumps is not only due to the high initial investment and requirements for soil and rock conditions, but also the lack of reliable system design and simulation tools. Cane and Forgas calculated that the length of the buried pipe heat exchanger in the current North American buried pipe ground source heat pump project examples is 10% to 30% larger than the actual length[1], which makes it more impossible to recover funds in the short term and is not conducive to the development and promotion of buried pipe ground source heat pumps. The heat transfer between the buried pipe heat exchanger and the rock and soil is unsteady and occurs over an infinitely large area. The process is very complex and has many influencing factors. According to the existing literature, the analysis of the heat transfer mechanism of underground heat exchangers is mainly focused on theoretical research and trial application. The study of the temperature field of rock and soil is based on the principle of heat conduction. There is little research on the heat transfer mechanism and heat exchange mechanism considering the movement of groundwater. However, domestic and foreign literature shows that the transverse seepage of groundwater has a great influence on the heat transfer process of rock and soil[25]. Therefore, in order to understand the impact of the installation of buried pipe heat exchangers on the original underground environment characteristics, it is necessary to study the heat transfer mechanism of buried pipe heat exchangers under the combined action of heat conduction and groundwater movement. 1. Heat Transfer Model of Buried Pipe Heat Exchanger Many factors influence the performance of buried pipe ground source heat pump systems, including groundwater flow, backfill material properties, the possibility of phase change around the heat exchanger, and changes in soil and rock properties along the pipe length. Improving the heat transfer model of the buried pipe heat exchanger to better simulate the actual heat exchange situation and determining the optimal size of the buried pipe heat exchanger is crucial for the development and promotion of buried pipe ground source heat pumps. Buried pipe systems are currently in the research stage and have always been a challenge in buried pipe ground source heat pump technology. Existing design methods for buried pipe ground source heat pumps are mostly based on experimental research on buried pipe heat exchangers. Buried pipe heat exchangers generally come in three forms: vertical buried pipes, horizontal buried pipes, and spiral buried pipes. Horizontal buried pipes are usually shallowly buried, have lower development technology requirements, and often have lower initial investment than vertical buried pipes; however, because the heat exchange capacity of horizontal buried pipes is often lower than that of vertical buried pipes, and the laying area is larger, the excavation work is more extensive, and it is sometimes not economical. Depending on the burial method, vertical buried pipes are usually of two types: U-shaped pipes and sleeve pipes. U-shaped buried pipe heat exchangers are commonly used in ground source heat pump projects both domestically and internationally. Although the heat exchange capacity of sleeve pipe heat exchangers is better than that of U-shaped pipe heat exchangers, they are rarely used in engineering projects due to their large initial investment and are only used for shallow burial methods. Existing buried pipe heat exchanger design software is mainly based on the linear heat source theory, cylindrical heat source theory [68], energy balance theory [915], etc. to establish control equations. When designing buried pipe heat exchangers, it is necessary to consider the temperature rise or fall of the soil and rock mass caused by the imbalance between heat extraction and release after long-term operation. Analytical methods are favored because they can easily and quickly obtain long-term operating results. However, when considering factors such as inlet and outlet water temperatures, water flow rates, different geological layers, and the influence of backfill soil, analytical methods become difficult to apply. Therefore, some simplifications must be made, such as equating the U-shaped pipe to an equivalent single pipe for column heat source theory, or treating it as an infinitely long line heat source for line heat source theory. For long-term operation, these simplifications have little impact on the results, but this is not the case for short-term operation, where numerical methods are more effective. Therefore, some models combine both numerical and analytical methods. The main analytical calculation models are: Ingersol model [1617], IGSHPA model [18], Hart and Couvillion model, Kavanaugh model [19] and Mei model [20], which can be found in reference [21]. The numerical design calculation models are mainly as follows: 1) Mei and Emerson heat transfer model and calculation method [22] Mei and Emerson developed a mathematical model suitable for horizontal pipe sections that takes into account the influence of frozen soil around the pipe. The mathematical model uses the finite difference method to solve three one-dimensional partial differential equations to describe the radioactive heat conduction process around the pipe, in the frozen soil area and in the far end area. In addition, a one-dimensional heat transfer equation of the fluid in the pipe along the pipe length direction is added on this basis, which becomes a pseudo-two-dimensional model. The model uses different time steps for the pipe wall and the frozen soil area, and a much larger time step for the fluid in the pipe and the non-frozen soil area. Mei and Emerson gave a comparison of the simulated and experimental values ​​for 48 days. 2) Eskilsson heat transfer model and calculation method Eskilsson uses a dimensionless temperature response factor g function to simulate the temperature field of the buried pipe heat exchanger tube group. The heat flow of the buried pipe changes with time as a function of single step, and then these functions are superimposed to obtain the temperature field of the entire rock and soil area. 3) Nwwa (national water well association) model and calculation method This model establishes the temperature field of the rock and soil layer based on the analytical solution of the Kelvin linear heat source equation, and then determines the size of the heat exchanger. This method is also a commonly used calculation method for buried pipe heat exchangers. It can directly give the average fluid temperature in the heat exchanger and use the superposition method to simulate the intermittent operation of the heat pump [24]. 4) Gluepro and Gchipcalc models The Gluepro model is based on the heat transfer model of Lund University in Sweden and can analyze the situation for one year or more to design the length of vertical buried pipe heat exchangers. This heat transfer analysis is only suitable for situations where there is no groundwater movement and no unbalanced heat absorption or release. The Gchipcalc model calculates the length of the heat exchanger based on the heat absorbed or released by the soil and rock under the design conditions [25]. 5) Muraya model and calculation method [25] Muraya uses a dynamic, two-dimensional finite element model to analyze the thermal interference problem between the two legs of the U-shaped pipe. The model attempts to quantify this interference problem by defining the heat exchanger efficiency, based on the soil and rock composition and backfill characteristics, the distance between the two legs, the temperature at the far end and inside the pipe, and the thermal diffusivity. The model has been verified by the column heat source theory analytical method under the two conditions of constant heat flow and constant wall temperature. Using this model, the overall heat transfer efficiency and the influence of backfill soil depending on the pipe geometry can be calculated. 6) Rottmayer, Beckman, and Mitchel model [26] Rottmayer, Beckman, and Mitchel proposed a two-dimensional numerical model of a U-shaped buried pipe heat exchanger in 1997. In this model, polar coordinate grids are used to simulate lateral heat transfer for each 3m long pipe segment, ignoring the vertical heat conduction process, and considering the temperature change of the fluid in the pipe along the pipe length. Therefore, this is actually a three-dimensional heat transfer model. In the model verification, it was found that the result differed from the analytical method by 5%, which was attributed to the non-circular pipe geometry model in the numerical simulation process. Therefore, in order to solve this problem, a geometric factor (between 0.3 and 0.5) was proposed to correct the thermal resistance of the soil and backfill soil. The results showed that it was in good agreement with the analytical method. 7) The Shorter and Beck model [27] In 1999, Shorter and Beck proposed a one-dimensional heat transfer model for U-shaped buried pipe heat exchangers. In this model, the U-shaped pipe is equivalent to a single pipe. It is assumed that there is a thin layer around the pipe to simulate the heat capacity of the U-shaped pipe and the fluid inside the pipe. It is also assumed that there is one-dimensional dynamic heat conduction in the thin layer, backfill soil and surrounding rock and soil. The inner and outer edges are the variable heat flow conditions inside the thin layer and the isothermal boundary conditions at the far end, respectively. The finite difference method and the Crank-Nicholson method are used to solve the problem [24]. Domestic research on the heat transfer theory of buried pipe heat exchangers is significantly lagging behind experimental research. The main achievements are as follows: Chongqing University, in combination with the law of conservation of energy, based on the Mei three-dimensional transient far boundary heat transfer model, established a heat transfer model of buried pipe heat exchangers and simulated the operation period and transition period. The calculation results are in good agreement with the measured values ​​[28]. Qingdao University of Technology established a two-dimensional unsteady-state heat transfer model of the temperature field of the rock and soil around the U-shaped vertical buried pipe. The calculation results are in good agreement with the measured values, and the thermal radius of the U-shaped buried pipe heat exchanger was calculated [29]. Tongji University established a one-dimensional unsteady-state heat transfer model [30]. Shandong University of Architecture and Engineering also conducted in-depth research on the buried pipe heat exchanger model and proposed a mathematical model of the axial temperature of the medium in the U-shaped buried pipe heat exchanger [31]. Harbin Institute of Technology proposed a quasi-three-dimensional unsteady U-shaped buried pipe heat exchanger heat transfer model to study the cold storage of soil and rock and the ground source heat pump system of buried pipe. The simulation values ​​and experimental results are in good agreement [32]. In summary, there are many methods and commercial design software for the design of buried pipe heat exchangers. All of these design software are based on the principle of heat conduction and the determination of the thermal conductivity and volumetric specific heat capacity of soil and rock. However, soil and rock are porous media with solid, liquid and gas phases mixed. For underground vertical buried pipe heat exchangers, due to the passage through various geological layers of different properties, the performance of each geological layer will greatly affect its heat transfer process. In particular, most of the coil pipe section is located in the saturated zone of soil and rock below the groundwater level. The influence of groundwater flow is particularly important. For aquifers with high porosity and high permeability, the effect is more obvious. Existing domestic and foreign data have also confirmed this. Therefore, it is necessary to consider the influence of different geological layers, groundwater flow and other factors. 2 The impact of groundwater flow on buried pipe heat exchangers In the current field test analysis method of thermal conductivity of soil and rock and the design of buried pipe heat exchangers, only the heat conduction process is considered. Therefore, groundwater flow will affect the design process in the following two ways: 1) the thermal conductivity value of soil and rock tested in the field is often too high; 2) the capacity of the heat exchanger designed with a higher thermal conductivity may be too large. In Minnesota, the thermal conductivity of soil and rock tested in the field was extremely high, which was analyzed to be caused by groundwater flow [2]. The Croydon building in the UK is a three-story office building. In the autumn of 2000, the British Engineering Council funded the construction of the largest ground source heat pump project in the UK to date. During the heating season, the average temperature test results of the fluid inside the pipe were very close to the simulation results (simulated using Earth Energy Designer software). However, in the summer, the average temperature test value of the fluid inside the pipe was much lower than the simulation value, reaching only 3°C. Analysis showed that this was caused by the temperature around the pipe being lowered due to the flow of groundwater [3]. In addition, similar phenomena were found in several field tests and simulation experiments using artificial groundwater flow [4]. Dr. Witte of the Netherlands proposed a geothermal reaction test method. Using the geothermal reaction device he developed (as shown in Figure 1), the heat transfer process of the closed-loop buried pipe heat exchanger under the input of heat or cold can be tested on site, thereby obtaining some important parameters, such as thermal conductivity, far-end temperature, pipe resistance, etc. To analyze the impact of groundwater flow on coil heat transfer, experiments were conducted on the same buried pipe with and without groundwater flow. Water was pumped from another well 5 m away from the experimental well to simulate groundwater flow. The experimental results are shown in Figure 2. In the first 18 hours of the experiment, groundwater flow had little impact on the results. This was mainly due to the large temperature difference in the early stage, where heat conduction played a major role. From the 20th hour onwards, the temperature of the soil and rock without groundwater flow was significantly higher than that with groundwater flow at the same time, and the temperature rise rate of the former was also significantly higher than that of the latter. Furthermore, the thermal conductivity values ​​of the soil and rock obtained from the two experiments also differed greatly. The result of the experiment without groundwater flow was (2.34±0.007) W/(m·K), while the result of the experiment with groundwater flow was (3.22±0018) W/(m·K). Moreover, as the groundwater flow velocity increased, the estimated value of the thermal conductivity of the soil and rock also increased significantly, and the effect was already significant even at low flow velocities (when the Darcy velocity was less than 3.5 m/a, the estimated value of thermal conductivity had already increased by 6%) [4]. It can be seen that the groundwater flow greatly affects the heat exchange process of the buried pipe heat exchanger. Based on the above, the author established a heat transfer model for a buried pipe heat exchanger considering the combined effects of heat conduction and groundwater flow, and conducted a preliminary analysis of a single-well buried pipe. The results showed that groundwater seepage can enhance the heat exchange capacity of the coil. The temperature field of the seeping soil and rock has been deformed compared with the near-centrally symmetrical temperature field of the non-seeping soil and rock. Therefore, if the coil is buried in the seeping soil and rock and the influence of seepage is not considered in the design calculation, the design capacity will be too large, resulting in economic and resource waste [34]. Therefore, the urgent problem to be solved is to further improve the heat transfer model of the buried pipe heat exchanger and the soil and rock, fully consider the influence of different geological layers it passes through, and describe the heat transfer process under the combined effects of heat conduction and groundwater flow. 3 Conclusion The perfection of the buried pipe heat exchanger model is the main influencing factor on whether the buried pipe ground source heat pump system can be promoted and applied. The geological layers that the vertical buried pipe passes through and the groundwater flow have a great influence on its heat transfer performance. Therefore, further in-depth research on heat transfer mechanisms, improvement of heat transfer models for buried pipe heat exchangers, and exploration of the heat transfer process of buried pipe heat exchangers under the combined influence of heat conduction and groundwater flow are essential for the development and promotion of buried pipe ground source heat pump systems.