Method for constructing heat transfer model of ground heat exchanger and method for determining hole depth of ground heat exchanger

Abstract

The invention discloses a method for constructing a heat transfer model of a buried pipe heat exchanger and determining the hole depth of the buried pipe heat exchanger, and aims to solve the technical problem that the drilling depth of the current buried pipe heat exchanger is unreasonable. According to the technical scheme, based on the heat transfer model of the ground heat exchanger, the interval approximation method is adopted, the better burial depth range is gradually refined, so that the optimal arrangement hole depth within the specific depth range is found, the method is simple and scientific, the accuracy of predicting the performance of the ground heat exchanger can be improved, and then the efficiency and reliability of a ground source heat pump system are improved.

Description

[0001] This application is a divisional application of the invention patent application No. 202411822020.7, entitled "Construction of Heat Transfer Model of Buried Pipe Heat Exchanger and Method for Determining Hole Depth of Buried Pipe Heat Exchanger", with the parent application date being December 11, 2024. Technical Field

[0002] This invention relates to the field of geothermal energy utilization technology, specifically to the construction of a heat transfer model for a buried pipe heat exchanger and a method for determining the borehole depth of the buried pipe heat exchanger. Background Technology

[0003] Ground source heat pump systems with buried pipes are highly efficient, energy-saving, and environmentally friendly air conditioning systems that utilize shallow geothermal resources for both heating and cooling. The performance of the core component, the buried pipe heat exchanger, directly affects the efficiency and reliability of the entire system. The buried pipe heat exchanger stores and extracts heat by burying pipes underground, utilizing the constant temperature characteristics of the underground soil and the natural flow of groundwater. In practical applications, the performance of the buried pipe heat exchanger is closely related to the selection of the borehole depth. Traditional methods for determining the borehole depth of buried pipe heat exchangers mainly rely on engineering experience and estimation. This method has significant uncertainties and often leads to unreasonable drilling depths (too shallow a borehole may not meet the system's heat exchange requirements, resulting in low system efficiency; too deep a borehole will waste resources and increase the system's construction and operating costs), thus affecting the system's heat exchange efficiency and operating costs. Therefore, developing a scientific and accurate method for determining the borehole depth of buried pipe heat exchangers is of great significance for improving the efficiency and reliability of ground source heat pump systems.

[0004] The information disclosed in this background section is intended only to enhance the understanding of the background technology of this disclosure and should not be construed as an admission or in any way implying that the information constitutes prior art known to those skilled in the art. Summary of the Invention

[0005] In view of at least one of the above technical problems, this disclosure provides a method for constructing a heat transfer model of a buried pipe heat exchanger and determining the hole depth of the buried pipe heat exchanger. The method adopts an interval approximation method to gradually refine the optimal burial depth range, thereby determining the optimal hole depth within a specific depth range, providing a more accurate scientific basis for the design of buried pipe heat exchangers.

[0006] According to one aspect of this disclosure, a method for constructing a heat transfer model of a buried pipe heat exchanger is provided, comprising the following steps: (1) Determine the range of borehole depth in the area to be determined, analyze the hydrogeological conditions, thermal properties of the soil and rock and the geothermal field characteristics within the range, and perform grid subdivision of the area to be determined; (2) Based on the hydrogeological conditions and the thermal properties of the rock and soil, the aquifer is generalized and divided into shallow aquifer temperature change zone, shallow aquifer temperature increase zone, weakly permeable layer and deep aquifer temperature increase zone in the vertical direction. (3) Set the boundary conditions, characterize the source and sink terms of the aquifer in the region to be determined, and on this basis, construct a steady flow model to simulate the initial flow field distribution of the model under given conditions; (4) Based on the aforementioned solution conditions, the buried pipe heat exchanger and the surrounding rock and soil are characterized, and the heat transfer control equation is established to construct the heat transfer model of the buried pipe heat exchanger. The buried pipe heat exchanger is characterized using the one-dimensional finite element method, and its governing equations are as follows: ; ; ; ; ; ; ; ; In the formula: ρ r c r —Heat capacity of the fluid inside the pipe; ε g —Porosity of the backfill material; ρ g c g —Heat capacity of the backfill material; u—Flow velocity of the circulating fluid inside the pipe; ∇—Differential operator heat dissipation flux; Λ r —Heat diffusion flux of fluid inside the pipe; —The convective heat transfer coefficient between the fluid inside the double-U heat exchanger tube and the inlet tube wall; —The convective heat transfer coefficient between the fluid inside the double-U heat exchanger tube and the tube wall outside the tube; —The convective heat transfer coefficient between the two tubes; —The convective heat transfer coefficient between the backfill material and the surrounding soil; The surrounding soil and rock mass was treated using a three-dimensional finite element method, and its heat transfer control equation is as follows: ; ; ; In the formula: s—water storage coefficient, dimensionless; superscript f—represents liquid; superscript s—represents soil; h—water head; Q—flow rate; β—thermal expansion coefficient; q—Darcy velocity; ∇—vector differential operator; T—temperature; ε—porosity; ρ—density; c—specific heat capacity; H—heat source sink term; λ—thermal conductivity; αL—longitudinal heat diffusion; αT—lateral heat diffusion; I—unit characteristic matrix.

[0007] Preferably, in step (2), the water-bearing medium in the shallow aquifer temperature zone includes silt, medium sand and fine sand.

[0008] Preferably, in step (2), the water-bearing medium in the shallow aquifer warming zone includes fine sand, silt and silty sand.

[0009] Preferably, in step (2), the soil layer of the weakly permeable layer includes a thick layer of silty clay and / or a thin layer of silt.

[0010] Preferably, in step (2), the water-bearing medium in the warming zone of the deep aquifer includes fine sand, silt and / or a thin layer of silty clay.

[0011] Preferably, in step (3), the boundary conditions are: the surface temperature is taken as the annual average temperature of the area to be determined, the geothermal constant temperature zone is taken as 17.3℃, and the values ​​of each layer below are taken by multiplying the depth from the constant temperature zone by the geothermal warming rate.

[0012] Preferably, in step (3), the flow boundary can be generalized in the horizontal direction as a recharge boundary, a discharge boundary and a zero flow boundary, and in the vertical direction as a flux boundary surface and a water-blocking boundary surface.

[0013] Preferably, in step (3), the surface and lower boundary of the area to be determined are generalized to a constant temperature boundary.

[0014] Preferably, in step (1), the Triangle algorithm is used to perform triangular meshing on the super mesh surface in the region to be determined.

[0015] According to a second aspect of this disclosure, a method for determining the borehole depth of a buried pipe heat exchanger is provided, comprising the following steps: (1) Based on the simulation cycle of the heat transfer model of the buried pipe heat exchanger established above, the operating power and circulation velocity of the heat exchange hole are set, and the geothermal prediction model is established by simulating the cycle time series. (2) Based on the geothermal prediction model, simulate and compare the outlet temperature of the buried pipe at a specific interval during the heating and cooling periods of the year, and calculate the inlet and outlet temperature difference of the buried pipe body at different burial depths respectively. (3) Draw a “temperature difference-depth” line graph and calculate the slope of different burial depth intervals during the cooling and heating periods of the year; (4) Determine the period in which the slope changes with depth, further refine the lower limit of the burial depth interval where the smallest absolute value of the slope is located in the period, and further reduce the burial depth interval, simulate the outlet temperature of the underground pipe at different burial depths in the period, and then calculate the temperature difference between the inlet and outlet of the underground pipe body at different burial depths, and draw a "temperature difference-depth" line graph. (5) Take the middle value of the two adjacent burial depth intervals with the largest difference in absolute slope of the "temperature difference-depth" line graph as the optimal hole depth.

[0016] One or more technical solutions provided in the embodiments of this application have at least one of the following technical effects or advantages: The method for determining the depth of the heat exchanger borehole in this application is simple, scientific, and yields accurate and reliable results. It can not only improve the heat exchange efficiency of the ground source heat pump system and reduce operating costs, but also optimize drilling parameters, reduce resource waste and environmental pollution during construction, and is of great significance for promoting the widespread application and sustainable development of ground source heat pump technology. Attached Figure Description

[0017] Figure 1 This is a bar chart of the R17 thermal response test wells in one embodiment of this application.

[0018] Figure 2 This is a bar chart of the R18 thermal response test wells in one embodiment of this application.

[0019] Figure 3 This is a bar chart of the R19 thermal response test wells in one embodiment of this application.

[0020] Figure 4 This is a bar chart of the R20 thermal response test wells in one embodiment of this application.

[0021] Figure 5 This is an embodiment of the present application showing the internal structure and thermal resistance relationship of the double U-shaped heat exchange holes.

[0022] Figure 6 This is a distribution diagram of a four-hole U-shaped underground pipe in one embodiment of this application.

[0023] Figure 7 This is a mesh partitioning diagram in one embodiment of this application.

[0024] Figure 8 An R17-hole heat exchanger is provided in one embodiment of this application.

[0025] Figure 9 An R18-hole heat exchanger is provided in one embodiment of this application.

[0026] Figure 10 An R19-hole heat exchanger is provided in one embodiment of this application.

[0027] Figure 11 An R20-hole heat exchanger is provided in one embodiment of this application.

[0028] Figure 12 This is the fitting effect of the outlet temperature of the R17 hole heat exchanger in one embodiment of this application.

[0029] Figure 13 This is the fitting effect of the outlet temperature of the R18 hole heat exchanger in one embodiment of this application.

[0030] Figure 14 This is the fitting effect of the outlet temperature of the R19 hole heat exchanger in one embodiment of this application.

[0031] Figure 15 This is a fitting effect of the outlet temperature of the R20 hole heat exchanger in one embodiment of this application.

[0032] Figure 16 This is a temperature field comparison diagram with an operating cycle of 1 year in one embodiment of this application; where A: end of the cooling period; B: end of the cooling recovery period; C: end of the heating period; D: end of the heating recovery period.

[0033] Figure 17 This is a graph showing the temperature variation at the outlet of a buried pipe at different burial depths in one embodiment of this application.

[0034] Figure 18 This is a diagram of the "temperature difference-burial depth" of the underground pipe in one embodiment of this application (50m interval).

[0035] Figure 19 This is a diagram of the "temperature difference-burial depth" of the underground pipe in one embodiment of this application (5m interval). Detailed Implementation

[0036] To better understand the technical solution of this application, the above technical solution will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0037] Example: This application takes the Liuji test site in Zhongmu County, Zhengzhou City as the research area to determine the hole depth of the buried pipe heat exchanger in the area.

[0038] I. Analysis of the occurrence conditions of shallow geothermal energy in the analysis area 1. Hydrogeological Conditions: The study area is the Liuji Experimental Site in Zhongmu County, Zhengzhou City. The landform is a breach fan of the Yellow River alluvial plain, the most recent breach fan formed after the breach of the Huayuankou Dike in 1938, representing a geological remnant of the Yellow River's flooding. The ground elevation is 77–90 m, with a low-lying terrain sloping slightly southeast. The surface lithology consists of Holocene alluvial silt and fine sand, with a thickness greater than 30 m. The aquifer is mainly composed of medium to fine sand, with a groundwater depth of 12 m, a top depth of 8 m, a permeability coefficient of 15–25 m / d, and a water-bearing capacity of 300–500 m³.3 / d•m.

[0039] Groundwater recharge is primarily from precipitation infiltration, followed by irrigation backflow, surface water lateral seepage, and lateral runoff. The study area is mostly flat, with a surface slope generally between 1 / 3000 and 1 / 5000, resulting in slow surface runoff, shallow groundwater depth, and a loosely structured vadose zone composed mainly of silt and silty sand, which is highly conducive to precipitation infiltration. Groundwater flow direction is generally consistent between the wet and dry seasons, but influenced by lateral seepage from the Yellow River, the overall flow is from north to south. Groundwater discharge mainly occurs through extraction, evaporation, and runoff.

[0040] 2. Thermal properties of soil and rock: Based on the test results of thermal property parameters of soil and rock samples and the collected data, the thermal property parameters were statistically analyzed according to different lithologies, as shown in Table 1.

[0041] Table 1. Statistical Table of Thermal Properties of Soil and Rock Mass in a Single Hole .

[0042] 3. Characteristics of shallow geothermal field: The shallow geothermal field is mainly controlled by the groundwater flow field. The temperature 2m below the groundwater surface in the study area is 16.8℃. At a depth of 25m, the constant temperature zone is 17℃, and the geothermal gradient is 2.57℃ / 100m.

[0043] 4. Characteristics of Field Thermal Response Tests: Field thermal response tests refer to tests that utilize a buried pipe heat exchange system to continuously heat (cool) soil or rock using an artificial cold (heat) source, and record the temperature changes and circulation volume of the heat transfer medium to determine the thermal conductivity of the soil or rock. Based on the pipe burial method, there are two types: one is the horizontally buried heat exchange pipe test, typically at a depth of about 1.5m; the other is the vertically buried heat exchange pipe test, which can be further divided into the layered field thermal response test (with temperature sensors installed at required intervals) and the conventional thermal response test (without temperature sensors).

[0044] A set of thermal response tests was conducted at borehole R18 in the test site. The borehole depth was 150 m, the heat exchanger type was double U-shaped, the buried pipe depth was 150 m, the buried pipe diameter was DN32 mm, and the wall thickness was 3.0 mm. The thermal response tests were carried out from June 4th to June 14th, 2021, and included initial average temperature tests of the soil and rock mass, high-power constant heat flow tests (heating power 10 kW), and low-power constant heat flow tests (heating power 5.5 kW), totaling 60 shifts. After the constant heat flow heating tests, ground temperature recovery was carried out. In addition, three sets of conventional thermal response test data were collected based on the project, as detailed in Table 2.

[0045] Table 2 Thermal response test data and thermal physical properties of soil and rock mass .

[0046] II. Establishment of the heat transfer model for buried pipe heat exchangers 1. Hydrogeological conceptual model (1) Aquifer generalization: According to the stratigraphic structure revealed by the borehole, the simulated area is composed of Quaternary and Neogene strata within a vertical depth of 200 m, and the lithology is mainly silt, silty sand, silty clay, clay, fine sand, medium and fine sand, etc. Therefore, based on the water-bearing and permeability characteristics of the soil and rock layers, the aquifer is divided into four layers according to the following depths: 0-27 m is the shallow aquifer temperature-changing zone, with the aquifer medium consisting of silt, medium sand, and fine sand; 27-80 m is the shallow aquifer temperature-increasing zone, with the aquifer medium consisting of fine sand, silt, and silt; 80-88 m is a weakly permeable layer, with the soil and rock layer consisting of a thick layer of silty clay and a thin layer of silt; 88-113 m is the deep aquifer temperature-increasing zone, with the aquifer medium consisting of fine sand and silt, and a thin layer of silty clay; 113-143 m is a weakly permeable layer, with the soil and rock layer consisting of a thick layer of silty clay and a thin layer of silt; 143-150 m is the deep aquifer temperature-increasing zone, with the aquifer medium consisting of a fine sand layer; 150-155 m is a weakly permeable layer, with the soil and rock layer consisting of a thick layer of silty clay; 155-20 ... m represents the warming zone of the deep aquifer, with the aquifer medium consisting of fine sand interbedded with thin layers of silty clay. In summary, the vertical stratigraphy of the simulated area is generalized to eight layers, as shown in Table 3. For detailed stratigraphic structure, please refer to the appendix. Figures 1-4 .

[0047] Table 3. Stratigraphic Vertical Generalization Results .

[0048] (2) Boundary generalization: Flow boundary generalization: Based on the relationship between the boundary and the groundwater flow field, the northern part of the simulation area is generalized as the recharge boundary, the southern part as the discharge boundary, and the eastern and western boundaries as zero flow boundaries. Vertically, the model considers the soil and rock layers from the surface to a depth of 200 m. The upper part is the surface, which is recharged by atmospheric precipitation infiltration and is generalized as the flux boundary surface; the lower part, according to the drilling results, is a thick layer of cohesive soil at a depth of 200 m, which has weak water conductivity and permeability and is generalized as the impermeable boundary surface.

[0049] Temperature field boundary generalization: Due to the limited area of ​​the simulation zone (100 m × 100 m), the geothermal temperature difference on the horizontal plane within the zone is relatively small. Therefore, it is assumed that the groundwater temperature at the same elevation within the simulation zone is the same. Temperature values ​​for each layer are as follows: ① The surface layer is taken as the annual average air temperature of the study area, 15.0℃; ② The geothermal isothermal zone is taken as 17.3℃, and for each layer below, the value is determined by multiplying the depth from the isothermal zone by the geothermal warming rate. Thus, the initial temperature of the final base plate is taken as 23.65℃. The surface and subsurface are generalized as constant temperature boundaries.

[0050] 2. Construction of the heat transfer model for the buried pipe heat exchanger: The buried pipe heat exchanger was characterized using the one-dimensional finite element method, while the surrounding soil and rock mass was treated using the three-dimensional finite element method. Heat transfer mainly includes heat transfer within the soil and rock mass and heat transfer through the wellbore. The governing equations for heat transfer within the soil and rock mass are as follows: ; ; ; In the formula: s—water storage coefficient, dimensionless; superscript f—represents liquid; superscript s—represents soil; h—water head (m); Q—flow rate (m³ / h). 3 / s); β—coefficient of thermal expansion (°C) -1 ); q—Darcy velocity (m / s); ∇—vector differential operator; T—temperature (°C); ε—porosity; ρ—density (kg / m³) 3 ); c—specific heat capacity (J / (kg·℃)); H—heat source / sink term (W / m); λ—thermal conductivity (W / (m·℃)); αL—longitudinal heat dispersion (m); αT—lateral heat dispersion (m); I—unit characteristic matrix.

[0051] The double U-tube (2U) heat exchanger is divided into 8 heat exchange zones. Figure 5 ): 2 inlet pipe zones (i1 and i2), 2 outlet pipe zones (o1 and o2), and 4 backfill material zones (g1, g2, g3, g4). Rfig is the thermal resistance between the fluid and the inlet pipe wall, Rfog is the thermal resistance between the fluid and the outlet pipe wall, Rgg is the thermal resistance between the two pipes, and Rgs is the thermal resistance between the backfill material and the soil / rock mass.

[0052] Borehole heat transfer includes heat exchange between the inlet and outlet fluids and the backfill material, and heat exchange between the pipe wall, the backfill material, and the surrounding soil and rock. The governing equations are as follows: ; ; ; ; ; ; ; ; In the formula: the wall surface uses a Cauchy boundary of type 3; ρ r c r —Heat capacity of the fluid inside the pipe (J / ℃); ε g—Porosity of the backfill material; ρ g c g —Recycled packing heat capacity (J / ℃); u—Circulating fluid velocity in the pipe (m / s); ▽—Differential operator heat dissipation flux (W / m³) 2 ); Λ r —Heat diffusion flux of fluid inside the pipe (W / m) 2 ); —The convective heat transfer coefficient between the fluid inside the double-U heat exchanger tube and the inlet tube wall (W / (m²)) 2 · ℃) —The convective heat transfer coefficient between the fluid inside the double-U heat exchanger tube and the tube wall outside the tube (W / (m²)) 2 · ℃) — Convective heat transfer coefficient between the two pipes (W / (m²)) 2 · ℃) — Convective heat transfer coefficient between backfill material and surrounding soil (W / (m²)) 2 · ℃).

[0053] III. Simulation Region and Mesh Generation 1. Simulation range: Thermal response test wells R17~R20 are located southeast of Liuji. The relative positions of test wells R17~R20 are shown in the figure. Figure 6 The groundwater flow direction is from north to south, with the northern boundary being the recharge boundary, the southern boundary the discharge boundary, and the east and west boundaries the zero-flow boundaries. The four U-shaped underground pipes are distributed in a rectangular pattern at 6-meter intervals. Based on the actual geological and hydrogeological conditions and the purpose of this simulation study, a rectangular block of 100 m × 100 m was selected as the simulation area.

[0054] 2. Mesh Generation: In the spatial discretization of the 3D model, the Triangle algorithm was used to generate triangular meshes on the supermesh surface. Local mesh refinement was applied to the U-shaped tubes, a key feature of the model. This resulted in 2936 planar mesh nodes and 5738 element meshes. Vertical meshing was performed in 8 layers, resulting in 26424 nodes and 45904 element meshes. Figure 7 As shown.

[0055] IV. Boundary conditions and parameter assignment 1. Boundary Conditions: The main source and sink terms of the aquifer in the simulation area include precipitation infiltration recharge and evaporation discharge, artificial well extraction, lateral recharge and discharge, which are specifically characterized as follows: The artificial well extraction term is characterized using the MultilayerWell module in the FEFLOW model, supplemented by time series for dynamic control of extraction; In the model, the flux is assigned using a given flow boundary, and then the flow is calculated using software to characterize the lateral recharge and discharge of the aquifer at the southern and northern boundaries; The groundwater level depth in the area is greater than the limit evaporation depth (referring to the experience value of Zhengzhou City, set to 4m), the evaporation value is 0, and evaporation discharge is negligible. The upper part is the surface, which is recharged by atmospheric precipitation infiltration, and is generalized as a flux boundary surface. The lower part, according to the drilling results, is a thick cohesive soil layer at a depth of 200m, with weak water conductivity and permeability, and is generalized as an impermeable boundary surface.

[0056] Based on the above parameters, a steady-state flow model was constructed to simulate the initial flow field distribution under given conditions. According to the water level depths of the two boreholes in the area, the initial water level depth for the steady-state flow model was set to 11.2m, equivalent to 69.3m. The northern and southern boundaries of the simulation area were designated as the recharge and discharge boundaries, respectively, and hydraulic gradients were used to characterize them. Based on the given data, the hydraulic gradient values ​​for both boundaries were found to be 0.0032. After running the model, the steady-state flow results for the simulation area were obtained, which were then used as the initial flow field for the subsequent instantaneous flow model.

[0057] The upper and lower boundaries are defined as constant temperature boundaries. The temperature values ​​for each layer are as follows: ① The surface layer is taken as the annual average temperature of the study area, which is 15.0℃; ② The geothermal constant temperature zone is taken as 17.3℃. For each layer below, the temperature is determined by multiplying the depth from the constant temperature zone by the geothermal warming rate. Thus, the initial temperature of the final base plate is taken as 23.65℃.

[0058] 2. Parameter assignment: The simulated spatial range of the study area is 100 m × 100 m × 200 m. Vertically, the strata of the simulated area are generalized to 8 layers, and the target layer for simulation is the second layer. According to the stratigraphic structure revealed by the boreholes, the following layers are identified: 0-27 m is the shallow aquifer temperature-changing zone, with the aquifer medium consisting of silt, medium sand, and fine sand; 27-80 m is the shallow aquifer temperature-increasing zone, with the aquifer medium consisting of fine sand, silt, and silt; 80-88 m is a weakly permeable layer, with the soil layer consisting of a thick layer of silty clay and a thin layer of silt; 88-113 m is the deep aquifer temperature-increasing zone, with the aquifer medium consisting of fine sand and silt, and a thin layer of silty clay; 113-143 m is a weakly permeable layer, with the soil layer consisting of a thick layer of silty clay and a thin layer of silt; 143-150 m is the deep aquifer temperature-increasing zone, with the aquifer medium consisting of a fine sand layer; 150-155 m is a weakly permeable layer, with the soil layer consisting of a thick layer of silty clay; 155-200 m is the deep aquifer temperature-increasing zone, with the aquifer medium consisting of fine sand and a thin layer of silty clay. The values ​​of hydrogeological parameters and thermal properties are shown in Tables 4 and 5.

[0059] Table 4 Hydrogeological Parameter Assignment Table .

[0060] Table 5. Assignment Table of Thermal Properties .

[0061] V. Simulation and depiction of buried pipes Based on the on-site thermal response test design, the test hole depth for R17~R20 is 120-200 m, the heat exchanger type for R17~R19 is double U-shaped vertical buried tube, and the heat exchanger type for R20 is single U-shaped vertical buried tube. The buried tube depth for each hole is 20-200 m, the buried tube diameter is DN32 mm, and the wall thickness is 3.0-3.6 mm. In FEFLOW's thermal simulation, the BHEBorehole Heat Exchanger model is used to characterize the buried tube heat exchangers in each location, and the inlet temperature and fluid circulation rate are controlled in a time series manner, such as... Figures 8-11 As shown.

[0062] VI. Simulation Calibration Verification Based on the needs of the simulation study, the low-power constant heat flow test dataset was used as the temperature fitting basis for the geothermal model BHE (borehole heat exchanger). Fitting was performed separately for boreholes R17, R18, R19, and R20. The fitting period was selected as the test period of the low-power constant heat flow test for each borehole, namely: 2021 / 10 / 27 10:48~10 / 29 10:48 (Borehole R17), 2021 / 11 / 15 10:10~6 / 14 10:30 (Borehole R18), 2021 / 11 / 3 14:47~11 / 5 14:47 (Borehole R19), 2021 / 10 / 27 10:41~10 / 29 10:41 (Borehole R20). After a parameter cyclic optimization process of "comparison-parameter adjustment", the final fitting effect of the outlet temperature for each borehole was obtained as follows: Figures 12-15 As shown in Table 6, the simulated and observed values ​​of the heat exchanger outlet temperature are in good agreement. The average absolute error and average relative error of the fitting of the outlet temperature of each hole are shown in the table below, indicating that the established heat transfer model of the buried pipe heat exchanger is stable and reliable and can be used for subsequent simulation studies.

[0063] Table 6. Water Temperature Fitting Error Analysis Table .

[0064] VII. Simulation Period and Threshold of the Prediction Model 1. Simulation Period: Based on the established heat transfer model of the buried pipe heat exchanger, June 1, 2022, was set as the initial time point for simulation prediction, and the simulation period was set to 10 years, ending on May 31, 2032. During this period, the operating conditions of the heat exchange holes during the annual cooling period, cooling intermittent period, heating period, and heating intermittent period were considered (Table 7). The operating power of a single heat exchange hole was set to 6 kW in summer and 4.5 kW in winter, with a small circulation velocity of 0.7 m / s and a large circulation velocity of 1.2 m / s. The dynamic characteristics of the model were characterized in the form of a cyclic time series, thereby constructing a geothermal prediction model for the simulation area to simulate and predict the temperature field changes near the buried pipe over the next 10 years. The above data are based on this thermal response test and engineering experience.

[0065] Table 7 Overview of Operating Conditions During the Year .

[0066] 2. Threshold Selection: Based on the engineering operation requirements, if the temperature difference between the inlet and outlet of the buried pipe is less than or equal to 5℃ during operation, the thermal breakthrough / through effect is considered to have significantly reduced the heat exchange efficiency of the buried pipe, which is unacceptable. For cooling operation, an inlet water temperature of 36℃ (the ideal design value in actual engineering) and an outlet water temperature greater than 31℃ are unacceptable; the same applies to heating, an inlet water temperature of 5℃ (the ideal design value in actual engineering) and an outlet water temperature lower than 10℃ are unacceptable.

[0067] 8. Simulation of Hole Depth Layout Based on the geothermal prediction model established in Example 7, the interval approximation method is used to gradually refine the optimal burial depth range, thereby finding the optimal borehole depth within a depth range of 200 m.

[0068] First, at 50 m intervals, the outlet temperature of the buried pipe at burial depths of 50 m, 100 m, 150 m, and 200 m was simulated and compared to find the optimal burial depth range. The simulation results are as follows. Figure 17 As shown, the outlet temperature of the buried pipe is affected by the burial depth. During the alternating heating and cooling periods of the year, the temperature at the pipe outlet becomes increasingly stable with increasing burial depth, while the temperature difference between the outlet and the inlet liquid increases. Specifically, during the heating period, the temperature differences at different burial depths are: 0.15℃ (50 m), 0.4℃ (100 m), 0.64℃ (150 m), and 0.91℃ (200 m); during the cooling period, the temperature differences at different burial depths are: -0.76℃ (50 m), -1.2℃ (100 m), -1.42℃ (150 m), and -1.5℃ (200 m).

[0069] Plot the above results as a "temperature difference-depth" line graph, such as... Figure 18 As shown, during the heating period, the temperature difference increases almost linearly with the burial depth of the underground pipe, with an overall slope of 0.005℃ / m. However, during the cooling period, the absolute value of the slope of the linear relationship between temperature difference and burial depth gradually decreases with increasing depth, reaching -0.0044℃ / m in the 100-150 m burial depth range and -0.0016℃ / m in the 150-200 m burial depth range. Therefore, it can be considered that an optimal burial depth may exist around 150 m, resulting in the best cost-effectiveness of the underground pipe.

[0070] Based on this, the burial depth range of 140 m to 170 m was further refined at 5 m intervals. The optimal borehole depth was determined by comparing the slope of the "inlet / outlet temperature difference - burial depth" line segment during the cooling period. The simulation results are as follows: Figure 19 As shown, the absolute value of the slope is smaller after 150m, much gentler than the line segment before 150m. Therefore, 150m is the optimal borehole depth.

[0071] Although some preferred embodiments of this invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this invention.

[0072] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from the spirit and scope of its inventive concept. Therefore, if these modifications and variations fall within the scope of the claims of this application and their equivalents, this application also intends to include these modifications and variations.

Claims

1. A method for constructing a heat transfer model for a buried pipe heat exchanger, comprising the following steps: (1) Determine the range of borehole depth in the area to be determined, analyze the hydrogeological conditions, thermal properties of the soil and rock and the geothermal field characteristics within the range, and perform grid subdivision of the area to be determined; (2) Based on the hydrogeological conditions and the thermal properties of the rock and soil, the aquifer is generalized and divided into shallow aquifer temperature change zone, shallow aquifer temperature increase zone, weakly permeable layer and deep aquifer temperature increase zone in the vertical direction. (3) Set the boundary conditions, characterize the source and sink terms of the aquifer in the region to be determined, and on this basis, construct a steady flow model to simulate the initial flow field distribution of the model under given conditions; (4) Based on the aforementioned solution conditions, the buried pipe heat exchanger and the surrounding rock and soil are characterized, and the heat transfer control equation is established to construct the heat transfer model of the buried pipe heat exchanger. The buried pipe heat exchanger is characterized using the one-dimensional finite element method, and its governing equations are as follows: ； ； ； ； ； ； ； ； In the formula: ρ r c r —Heat capacity of the fluid inside the pipe; ε g —Porosity of the backfill material; ρ g c g —Heat capacity of the backfill material; u—Flow velocity of the circulating fluid inside the pipe; ∇—Differential operator heat dissipation flux; Λ r —Heat diffusion flux of fluid inside the pipe; —The convective heat transfer coefficient between the fluid inside the double-U heat exchanger tube and the inlet tube wall; —The convective heat transfer coefficient between the fluid inside the double-U heat exchanger tube and the tube wall outside the tube; —The convective heat transfer coefficient between the two tubes; —The convective heat transfer coefficient between the backfill material and the surrounding soil; The surrounding soil and rock mass was treated using a three-dimensional finite element method, and its heat transfer control equation is as follows: ； ； ； In the formula: s—water storage coefficient, dimensionless; superscript f—represents liquid; superscript s—represents soil; h—water head; Q—flow rate; β—thermal expansion coefficient; q—Darcy velocity; ∇—vector differential operator; T—temperature; ε—porosity; ρ—density; c—specific heat capacity; H—heat source sink term; λ—thermal conductivity; αL—longitudinal heat diffusion; αT—lateral heat diffusion; I—unit characteristic matrix.

2. The construction method according to claim 1, characterized in that, In step (2), the water-bearing medium in the shallow aquifer temperature zone includes silt, medium sand and fine sand.

3. The construction method according to claim 1, characterized in that, In step (2), the water-bearing medium in the shallow aquifer warming zone includes fine sand, silt and silt.

4. The construction method according to claim 1, characterized in that, In step (2), the soil layer of the weakly permeable layer includes a thick layer of silty clay and / or a thin layer of silt.

5. The construction method according to claim 1, characterized in that, In step (2), the water-bearing medium in the warming zone of the deep aquifer includes fine sand, silt and / or thin layers of silty clay.

6. The construction method according to claim 1, characterized in that, In step (3), the boundary conditions are: the surface temperature is taken as the annual average temperature of the area to be determined, the geothermal constant temperature zone is taken as 17.3℃, and the values ​​of each layer below are taken by multiplying the depth from the constant temperature zone by the geothermal warming rate.

7. The construction method according to claim 1, characterized in that, In step (3), the flow boundary can be generalized horizontally as a recharge boundary, a discharge boundary and a zero flow boundary, and vertically as a flux boundary surface and a water-blocking boundary surface.

8. The construction method according to claim 1, characterized in that, In step (3), the surface and lower boundary of the area to be determined are generalized to a constant temperature boundary.

9. The construction method according to claim 1, characterized in that, In step (1), the Triangle algorithm is used to divide the super mesh surface into triangular meshes in the region to be determined.

10. A method for determining the borehole depth of a buried pipe heat exchanger, comprising the following steps: (1) Based on the simulation cycle of the heat transfer model of the buried pipe heat exchanger established in claim 1, the operating power and circulation velocity of the heat exchange hole are set, and the geothermal prediction model is established by simulation through the circulation time series. (2) Based on the geothermal prediction model, simulate and compare the outlet temperature of the buried pipe at a specific interval during the heating and cooling periods of the year, and calculate the inlet and outlet temperature difference of the buried pipe body at different burial depths respectively. (3) Draw a "temperature difference-depth" line graph and calculate the slope of different burial depth intervals during the cooling and heating periods of the year; (4) Determine the period in which the slope changes with depth, further refine the lower limit of the burial depth interval where the smallest absolute value of the slope is located in the period, and further reduce the burial depth interval, simulate the outlet temperature of the underground pipe at different burial depths in the period, and then calculate the temperature difference between the inlet and outlet of the underground pipe body at different burial depths, and draw a "temperature difference-depth" line graph. (5) Take the middle value of the two adjacent burial depth intervals with the largest difference in absolute slope in the "temperature difference-depth" line graph as the optimal hole depth.

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