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

Technical Field

[0001] The present invention relates to the technical field of geothermal energy utilization, and in particular to the construction of a heat transfer model of a buried pipe heat exchanger and a method for determining the hole depth of the buried pipe heat exchanger. Background Art

[0002] The buried pipe ground source heat pump system is an efficient, energy-saving and environmentally friendly air-conditioning system that uses shallow underground geothermal resources for both heating and cooling. The performance of its 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 and 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 hole depth. The traditional method for determining the hole depth of the buried pipe heat exchanger mainly relies on engineering experience and estimation. This method has great uncertainty and often leads to unreasonable drilling depth (if the drilling is too shallow, it may not meet the heat exchange requirements of the system, resulting in low system efficiency; if the drilling is too deep, it will cause waste of resources and increase the construction and operation costs of the system), which in turn affects the heat exchange efficiency and operation costs of the system. Therefore, developing a scientific and accurate method for determining the hole depth of the buried pipe heat exchanger is of great significance to improving the efficiency and reliability of the ground source heat pump system.

[0003] The information disclosed in this background technology section is only used to deepen the understanding of the background technology of the present disclosure, and should not be regarded as acknowledging or suggesting in any form that the information constitutes the prior art known to those skilled in the art. Summary of the invention

[0004] In view of at least one of the above technical problems, the present 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 interval approximation method is used to gradually refine the optimal burial depth range, so as to optimally arrange the hole depth within a specific depth range, providing a more accurate scientific basis for the design of the buried pipe heat exchanger.

[0005] According to one aspect of the present disclosure, a method for constructing a heat transfer model of a ground pipe heat exchanger is provided, comprising the following steps: (1) Determine the drilling depth range of the area to be determined, analyze the hydrogeological conditions, thermal properties of the rock and soil, and geothermal field characteristics within the range, and perform grid division on the area to be determined; (2) Based on the hydrogeological conditions and the thermal physical properties of the rock and soil, the aquifers are generalized (a method of processing geographic data in relevant software, such as ArcGIS, to simplify the geometric shape of the data, reduce the complexity of the data, and at the same time maintain the main characteristics and distribution patterns of the data as much as possible), and the aquifers are divided vertically into the shallow aquifer variable temperature zone, the shallow aquifer warming zone, the weak permeable layer and the deep aquifer warming zone; (3) Setting the solution conditions, characterizing the source and sink terms of the aquifer in the determined area, and on this basis, constructing a steady flow model to simulate the initial flow field distribution of the model under given conditions; (4) Based on the 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 a heat transfer model of the buried pipe heat exchanger.

[0006] In some embodiments of the present disclosure, in step (2), the water-containing medium in the shallow aquifer temperature change zone includes silt sand, medium sand and fine sand; the water-containing medium in the shallow aquifer warming zone includes fine sand, silt and silt sand; the rock and soil layer of the weak permeable layer includes a thick layer of silty clay and / or a thin layer of silt; the water-containing medium in the deep aquifer warming zone includes fine sand, silt and / or a thin layer of silty clay.

[0007] In some embodiments of the present disclosure, in step (3), the solution conditions are as follows: the surface temperature is the annual average temperature of the area to be determined, the geothermal isothermal zone is 17.3°C, and the values ​​of the layers below are determined according to the depth from the isothermal zone multiplied by the geothermal warming rate; the flow boundary can be generalized into a recharge boundary, a discharge boundary and a zero flow boundary horizontally, and can be generalized into a flux boundary surface and a water-isolating boundary surface vertically; the surface and lower boundaries of the area to be determined are generalized as constant temperature boundaries.

[0008] In some embodiments of the present disclosure, in step (4), the buried pipe heat exchanger is characterized by a one-dimensional finite element method and divided into 8 heat exchange zones, namely, 2 inlet pipe zones, 2 outlet pipe zones, and 4 backfill material zones. The 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 rock and soil. The control equation is as follows: Where: ρ r , c r —heat capacity of the fluid in the tube; ε g —Porosity of backfill material; ρ g 、c g —Heat capacity of backfill; u —Flow rate of the circulating fluid in the tube; ∇—differential operator heat diffusion flux; Λ r —heat diffusion flux of the fluid in the tube; —Convection heat transfer coefficient between the fluid in the double U heat exchange tube and the inlet tube wall; —Convection heat transfer coefficient between the fluid in the double U heat exchange tube and the outlet tube wall; —Convection heat transfer coefficient between two tubes; —Convective heat transfer coefficient between backfill and surrounding soil.

[0009] In some embodiments of the present disclosure, in step (4), the surrounding rock and soil body is processed using a three-dimensional finite element method, and the heat transfer control equation is as follows: In the formula: s is the water storage coefficient, dimensionless; superscript f represents liquid; superscript s represents soil; h is the hydraulic head; Q is the flow rate; β is the thermal expansion coefficient; q is the Darcy velocity; ∇ is the vector differential operator; T is the temperature; ε is the porosity; ρ is the density; c is the specific heat capacity; H is the heat source and sink; λ is the thermal conductivity; αL is the longitudinal thermal dissipation; αT is the transverse thermal dissipation; I is the unit characteristic matrix.

[0010] In some embodiments of the present disclosure, in step (1), the super mesh surface is triangulated using the Triangle algorithm in the area to be determined.

[0011] According to a second aspect of the present disclosure, a method for determining a hole depth of a buried pipe heat exchanger is provided, comprising the following steps: (1) The simulation cycle of the heat transfer model of the buried pipe heat exchanger established above is set, and the operating power and circulation flow rate of the single heat exchange hole are set. The geothermal prediction model is established by simulating the cyclic time series; (2) Based on the geothermal prediction model, simulate and compare the outlet temperatures of the buried pipes at different burial depths at specific intervals during the heating and cooling periods of the year, and calculate the inlet and outlet temperature differences of the buried pipes at different burial depths; (3) Draw a “temperature difference-depth” line graph and calculate the slopes of different burial depth intervals during the cooling and heating periods of the year; (4) Determine the period when the slope changes with depth, further refine the lower limit of the depth interval where the minimum absolute value of the slope is located during the period, and further narrow the depth interval, simulate the outlet temperature of the buried pipe at different burial depths during the period, and then calculate the inlet and outlet temperature differences of the buried pipe body at different burial depths, and draw a "temperature difference-depth" line graph; (5) The middle value of the two adjacent burial depth intervals with the largest difference in absolute slope in the “temperature difference-depth” line graph is taken as the optimal hole depth.

[0012] One or more technical solutions provided in the embodiments of the present application have at least any of the following technical effects or advantages: The method for determining the hole depth of the heat exchanger in this application is simple and scientific, and the results are accurate and reliable. It can not only improve the heat exchange efficiency of the ground source heat pump system and reduce operating costs, but also optimize the drilling construction parameters and reduce resource waste and environmental pollution during the construction process. It is of great significance to promote the widespread application and sustainable development of ground source heat pump technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is a histogram of the R17 thermal response test hole in one embodiment of the present application.

[0014] Figure 2 This is a histogram of the R18 thermal response test hole in one embodiment of the present application.

[0015] Figure 3 This is a histogram of the R19 thermal response test hole in one embodiment of the present application.

[0016] Figure 4 This is a histogram of the R20 thermal response test hole in one embodiment of the present application.

[0017] Figure 5 The internal structure and thermal resistance relationship of the double U heat exchange hole in one embodiment of the present application.

[0018] Figure 6 This is a distribution diagram of four-hole U-shaped buried pipes in one embodiment of the present application.

[0019] Figure 7 This is a grid division diagram in one embodiment of the present application.

[0020] Figure 8 This is the R17 hole heat exchanger setting in one embodiment of the present application.

[0021] Fig. 9 This is the R18 hole heat exchanger setting in one embodiment of the present application.

[0022] Fig.10 This is the R19 hole heat exchanger setting in one embodiment of the present application.

[0023] Fig.11 This is an R20 hole heat exchanger arrangement in one embodiment of the present application.

[0024] Fig.12 This is the fitting effect of the water outlet temperature of the R17 hole drilled heat exchanger in one embodiment of the present application.

[0025] Fig.13 This is the fitting effect of the water outlet temperature of the R18 hole drilled heat exchanger in one embodiment of the present application.

[0026] Fig.14 This is the fitting effect of the water outlet temperature of the R19 hole drilled heat exchanger in one embodiment of the present application.

[0027] Fig.15 This is the fitting effect of the water outlet temperature of the R20 hole drilled heat exchanger in one embodiment of the present application.

[0028] Fig.16 This is a temperature field comparison diagram with an operating cycle of 1 year in one embodiment of the present application; wherein 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.

[0029] Fig.17 This is a diagram showing the temperature variation at the outlet of the buried pipe at different burial depths in one embodiment of the present application.

[0030] Fig.18 This is the “temperature difference-burial depth” diagram of the buried pipe in one embodiment of the present application (50m intervals).

[0031] Fig.19 This is the “temperature difference-burial depth” diagram of the buried pipe in one embodiment of the present application (5m intervals). DETAILED DESCRIPTION

[0032] In order to better understand the technical solution of the present application, the above technical solution will be described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0033] Embodiment: This application takes the Liuji test field in Zhongmou, Zhengzhou City as the research area and determines the hole depth of the buried pipe heat exchanger in this area.

[0034] 1. Storage conditions of shallow geothermal energy in the analysis area 1. Hydrogeological conditions: The study area is the Liuji test site in Zhongmou, Zhengzhou City. The landform is the breach fan of the Yellow River alluvial plain. It is the latest breach fan formed after the breach of the Huayuankou levee in 1938. It is a geomorphic relic of the Yellow River breach and flooding. The ground elevation is 77-90m, and the terrain is low-lying and slightly inclined to the southeast. The surface lithology is silt and fine sand formed by the Holocene alluvial, with a thickness of more than 30m. The aquifer is mainly composed of medium and fine sand, the groundwater is buried at a depth of 12m, the top plate is buried at a depth of 8m, the permeability is 15-25m / d, and the water richness is 300-500m 3 / d•m.

[0035] The main source of groundwater recharge is precipitation infiltration, followed by irrigation infiltration, surface water lateral infiltration and lateral runoff. Most of the study area has a relatively flat terrain, with a ground slope of 1 / 3000 to 1 / 5000, slow surface runoff, shallow groundwater depth, and most of the lithology of the aeration zone is silt and silt sand, with a loose structure, which is very conducive to atmospheric precipitation infiltration recharge. The runoff of groundwater is basically the same during the flood season and the dry season. Affected by the lateral infiltration of the Yellow River, the groundwater runoff generally flows from north to south. The discharge of groundwater mainly takes the form of exploitation, evaporation and runoff.

[0036] 2. Thermophysical properties of rock and soil: According to the test results of the thermophysical parameters of rock and soil samples and the collected data, the thermophysical parameters were statistically analyzed according to different rock types, see Table 1.

[0037] Table 1 Statistics of thermal physical parameters of single-hole rock and soil .

[0038] 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℃. The temperate zone is 25m deep, the constant temperature is 17℃, and the geothermal gradient is 2.57℃ / 100m.

[0039] 4. Characteristics of the on-site thermal response test: The on-site thermal response test refers to a test that uses an artificial cold (heat) source to continuously heat (cool) the rock and soil using a buried pipe heat exchange system, and records the temperature change and circulation volume of the heat transfer medium to determine the thermal conductivity of the rock and soil. There are two types of tests based on the way the pipes are buried. One is the horizontal on-site horizontal thermal response test for horizontally buried heat exchange pipes, and the buried pipe depth is generally about 1.5m; the other is the vertical on-site vertical thermal response test for vertically buried heat exchange pipes. Based on whether temperature sensors are installed, it can be divided into on-site layered thermal response tests (temperature sensors are installed at the required spacing) and conventional thermal response tests (temperature sensors are not installed).

[0040] A set of thermal response tests was conducted in the R18 hole of the test site. The drilling depth of the test hole was 150 m, the heat exchanger type was double U-type, 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 test was carried out from June 4 to June 14, 2021. The initial average temperature test of the rock and soil body, the high-power constant heat flow test (heating power 10kW), and the low-power constant heat flow test (heating power 5.5kW) were carried out successively, with a total of 60 shifts. After the constant heat flow heating test, the ground temperature was restored. In addition, 3 sets of conventional thermal response test data were collected based on the supporting project, as shown in Table 2.

[0041] Table 2 Thermal response test data and thermal physical parameters of rock and soil .

[0042] 2. Establishment of heat transfer model of buried pipe heat exchanger 1. Hydrogeological conceptual model (1) Aquifer generalization: According to the stratigraphic structure revealed by the boreholes, the vertical range of the simulation area within 200 m depth is composed of Quaternary and Neogene strata, and the lithology is mainly silt, silt sand, silty clay, clay, fine sand, medium-fine sand, etc. Therefore, according to the water content and permeability characteristics of the rock and soil layers, the aquifer is divided into four layers, namely, the burial depth: 0-27 m is the shallow aquifer variable temperature zone, and the water-bearing medium is silt sand, medium sand and fine sand; 27-80 m is the shallow aquifer warming zone, and the water-bearing medium is fine sand, silt soil and silt sand; 80-88 m is the weak permeable layer, and the rock and soil layer is a thick layer of silty clay with a thin layer of silt soil; 88-113 m is the deep aquifer warming zone, and the water-bearing medium is fine sand, silt soil, and a thin layer of silty clay; 113-143 m is the weak permeable layer, and the rock and soil layer is a thick layer of silty clay with a thin layer of silt soil; 143-150 m is the deep aquifer warming zone, and the water-bearing medium is a fine sand layer; 150-155 m is the weak permeable layer, and the rock and soil layer is a thick layer of silty clay; 155-200 m is the weak permeable layer, and the rock and soil layer is a thick layer of silty clay; m is the deep aquifer warming zone, and the water-bearing medium is fine sand with a thin layer of silty clay. In summary, the strata in the simulation area are generalized into 8 layers vertically, as shown in Table 3. The strata structure is detailed in the attached Figures 1 to 4 .

[0043] Table 3 Results of vertical generalization of strata .

[0044] (2) Boundary generalization: Flow boundary generalization: According to 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 is generalized as the discharge boundary, and the eastern and western boundaries are generalized as zero flow boundaries. Vertically, the model considers the rock and soil 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 a flux boundary surface; the lower part, according to the drilling results, is a thick clay layer at a depth of 200 m underground, with weak water conductivity and permeability, and is generalized as a water-blocking boundary surface.

[0045] Generalization of temperature field boundary: Due to the limited area of ​​the simulation area (100 m×100 m), the geothermal temperature difference on the horizontal surface in the area is small. It is assumed that the groundwater temperature at the same elevation in the simulation area is the same. The temperature values ​​of each layer are: ① The surface layer takes the annual average temperature of the study area as 15.0℃; ② The geothermal constant temperature zone takes 17.3℃, and the values ​​of the layers below are multiplied by the geothermal warming rate according to the depth from the constant temperature zone, so the final initial temperature of the bottom plate is taken as 23.65℃. The surface and the lower part are generalized as fixed temperature boundaries.

[0046] 2. Construction of heat transfer model of buried pipe heat exchanger: The buried pipe heat exchanger is characterized by one-dimensional finite element method, and the surrounding rock and soil are processed by three-dimensional finite element method. Heat transfer mainly includes heat transfer in rock and soil and heat transfer in wellbore. The heat transfer control equation in rock and soil is as follows: Where: s—storage coefficient, dimensionless; superscript f—indicates liquid; superscript s—indicates soil; h—water head (m); Q—flow rate (m 3 / s); β—thermal expansion coefficient (℃ -1 ); q—Darcy velocity (m / s); ∇—vector differential operator; T—temperature (℃); ε—porosity; ρ—density (kg / m 3 ); c—specific heat capacity (J / (kg·℃)); H—heat source and sink (W / m); λ—thermal conductivity (W / (m·℃)); αL—longitudinal thermal dissipation (m); αT—transverse thermal dissipation (m); I—unit characteristic matrix.

[0047] The double U-tube (2U) heat exchanger is divided into 8 heat exchange zones ( Figure 5 ): 2 inlet areas (i1 and i2), 2 outlet areas (o1 and o2), and 4 backfill material areas (g1, g2, g3, and 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 rock and soil.

[0048] The heat transfer of drilling includes the heat exchange between the inlet and outlet fluids and the backfill material, and the heat exchange between the pipe wall, backfill material and the surrounding rock and soil. The control equations are as follows: Where: The third type of Cauchy boundary is used on the wall; ρ r , c r —Heat capacity of fluid in tube (J / ℃); ε g —Porosity of backfill material; ρ g 、c g —Heat capacity of backfill material (J / ℃); u —Flow rate of the circulating fluid in the tube (m / s); ▽—Differential operator heat diffusion flux (W / m 2 );Λ r —Heat diffusion flux of fluid in the tube (W / m 2 ); —Convection heat transfer coefficient between the fluid in the double U heat exchange tube and the inlet tube wall (W / (m 2 ℃)); —Convection heat transfer coefficient between the fluid in the double U heat exchange tube and the outlet tube wall (W / (m 2 ℃)); —Convection heat transfer coefficient between two tubes (W / (m 2 ℃)); —Convective heat transfer coefficient between backfill and surrounding soil (W / (m 2 ℃)).

[0049] 3. Simulation area and mesh generation 1. Simulation range: Thermal response test holes R17~R20 are located in the southeast of Liuji. The relative position distribution of R17~R20 test holes is shown in Figure 6 , the groundwater runoff direction is from north to south, of which the northern boundary is the recharge boundary, the southern boundary is the discharge boundary, and the east-west boundary is the zero flow boundary. The four-hole U-shaped buried pipes are distributed in a rectangular shape at intervals of 6 m. Based on the actual geological and hydrogeological conditions and the purpose of this simulation study, a 100 m×100 m rectangular block is selected as the simulation range.

[0050] 2. Mesh generation: In terms of spatial discretization of 3D modeling, the Triangle algorithm is used to perform triangular mesh generation on the super mesh surface, and the U-shaped tube, which is the focus of the model, is locally meshed. The model plane is divided into 2936 nodes and 5738 unit grids. The vertical division is 8 layers, resulting in 26424 nodes and 45904 unit grids. Figure 7 shown.

[0051] 4. Solution conditions and parameter assignment 1. Solution conditions: The source and sink items of the aquifer in the simulation area mainly include precipitation infiltration recharge and evaporation discharge, artificial well mining, lateral recharge and discharge, which are specifically characterized as follows: the artificial well mining item uses the MultilayerWell module to characterize the mining well in the FEFLOW model, and is supplemented by time series for dynamic mining control; in the model, the lateral recharge and discharge are assigned flux values ​​using a given flow boundary, and then the flow is calculated using the software to characterize the lateral recharge and discharge of the aquifer at the southern and northern boundaries; the groundwater level in the area is greater than the limit evaporation depth (refer to the experience value of Zhengzhou City, set to 4m), the evaporation value is 0, and the 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 layer of clay soil at a depth of 200m underground, with weak water conductivity and permeability, which is generalized as a water-blocking boundary surface.

[0052] Based on the above parameters, a steady flow model is constructed to simulate the initial flow field distribution of the model under given conditions. According to the water level burial depth of the two boreholes in the area, the initial water level burial depth of the steady flow model is set to 11.2m, that is, 69.3m. The northern and southern boundaries of the simulation area are the recharge and discharge boundaries, respectively. The hydraulic gradient is used to characterize them. According to the given data, the hydraulic gradient values ​​of the two boundaries are both 0.0032. After running the model, the steady flow results of the simulation area are obtained, which is used as the initial flow field of the subsequent transient flow model.

[0053] The upper and lower boundaries are set as fixed temperature boundaries. The temperature values ​​for each layer are: ① The surface layer takes the annual average temperature of the study area as 15.0℃; ② The geothermal constant temperature zone takes 17.3℃, and the values ​​of the layers below are taken according to the depth from the constant temperature zone multiplied by the ground temperature increase rate, so the final initial temperature of the bottom plate is taken as 23.65℃.

[0054] 2. Parameter assignment: The spatial range of the simulation in the study area is 100 m×100 m×200 m. The strata in the simulation area are divided into 8 layers vertically, and the simulation target layer is the second layer. According to the stratigraphic structure revealed by the drilling, 0-27 m is the shallow aquifer temperature change zone, and the water-bearing medium is silt sand, medium sand and fine sand; 27-80 m is the shallow aquifer temperature increase zone, and the water-bearing medium is fine sand, silt and silt sand; 80-88 m is the weak permeable layer, and the rock and soil layer is a thick layer of silty clay with a thin layer of silt; 88-113 m is the deep aquifer temperature increase zone, and the water-bearing medium is fine sand and silt with a thin layer of silty clay; 113-143 m is the weak permeable layer, and the rock and soil layer is a thick layer of silty clay with a thin layer of silt; 143-150 m is the deep aquifer temperature increase zone, and the water-bearing medium is a fine sand layer; 150-155 m is the weak permeable layer, and the rock and soil layer is a thick layer of silty clay; 155-200 m is the deep aquifer temperature increase zone, and the water-bearing medium is fine sand with a thin layer of silty clay. The values ​​of hydrogeological parameters and thermophysical parameters are shown in Tables 4 and 5.

[0055] Table 4 Hydrogeological parameter assignment table .

[0056] Table 5 Physical and thermal parameter assignment table .

[0057] 5. Simulation and depiction of buried pipes According to the design of the on-site thermal response test, the depth of the R17~R20 test holes is 120-200 m, the R17~R19 heat exchanger type is a double U-shaped vertical buried pipe, and the R20 heat exchanger type is a single U-shaped vertical buried pipe. The buried pipe depth of each hole is 20-200 m, the buried pipe diameter is DN32 mm, and the wall thickness is 3.0-3.6 mm. In the thermal simulation of FEFLOW, the BHEBorehole HeatExchanger) model is used to characterize the buried pipe heat exchangers in various places, and the inlet temperature and fluid circulation rate are controlled in a time series manner, such as Figures 8 to 11 shown.

[0058] 6. Simulation calibration test According to the needs of the simulation study, the low-power constant heat flow test data set is used as the basis for the temperature fitting of the geothermal model BHE (borehole heat exchanger). The R17, R18, R19, and R20 holes are fitted respectively, and the fitting period is selected as the low-power constant heat flow test time period of each hole, namely: 2021 / 10 / 27 10:48~10 / 29 10:48 (R17 hole), 2021 / 11 / 15 10:10~6 / 14 10:30 (R18 hole), 2021 / 11 / 3 14:47~11 / 5 14:47 (R19 hole), 2021 / 10 / 27 10:41~10 / 29 10:41 (R20 hole). After the parameter cycle optimization process of "control-adjustment", the fitting effect of the outlet temperature of each hole is finally obtained as follows Figures 12 to 15 As shown in Table 6, it can be seen that the simulated value of the heat exchanger outlet temperature is in good agreement with the observed value. The average absolute error and average relative error of the outlet temperature fitting of each hole are shown in the following table, indicating that the constructed buried pipe heat exchanger heat transfer model is stable and reliable and can be used for subsequent simulation research.

[0059] Table 6 Water temperature fitting error analysis table .

[0060] VII. Simulation cycle and threshold of prediction model 1. Simulation period: Based on the heat transfer model of the underground heat exchanger, June 1, 2022 is taken as the initial time point of the simulation prediction, and the simulation period is set to 10 years, until May 31, 2032. During this period, the operating conditions of the heat exchange holes in the cooling period, cooling intermittent period, heating period, and heating intermittent period are considered (Table 7). The operating power of the heat exchange hole is set to 6 kw in summer and 4.5 kw in winter. The small flow rate of the single hole circulation flow rate is 0.7 m / s and the large flow rate is 1.2 m / s. The dynamic characteristics of the model are characterized in the form of a cyclic time series, thereby constructing a geothermal prediction model for the simulation area, and simulating and predicting the temperature field changes near the underground pipe in the next 10 years. The above data sources are determined in combination with this thermal response test and engineering experience.

[0061] Table 7 Summary of operating conditions during the year .

[0062] 2. Selection of discrimination threshold: According to the project operation requirements, when the temperature difference between the inlet and outlet of the underground pipe is less than or equal to 5°C during the operation period, it is considered that the thermal breakthrough / penetration effect has significantly reduced the heat exchange efficiency of the underground pipe and is unacceptable. For cooling conditions, an inlet water temperature of 36°C (the ideal design value in actual engineering) and an outlet water temperature greater than 31°C are unacceptable; the same is true for heating, an inlet water temperature of 5°C (the ideal design value in actual engineering) and an outlet water temperature lower than 10°C are unacceptable.

[0063] 8. Hole Depth Simulation Based on the geothermal prediction model constructed in Example 7, the interval approximation method is used to gradually refine the optimal burial depth range, so as to find the optimal hole depth within the depth range of 200 m.

[0064] First, with an interval of 50 m, the outlet temperature of the buried pipe at a depth of 50 m, 100 m, 150 m, and 200 m was simulated and compared to find the optimal large interval of burial depth. The simulation results are as follows: Fig.17 As shown in the figure, it can be seen that the outlet temperature of the buried pipe is affected by the burial depth of the pipe body. When the inlet liquid temperature changes alternately during the heating period and the cooling period of the year, as the buried depth of the buried pipe increases, the temperature change at the outlet of the pipe body becomes more and more stable, and the temperature difference with the inlet liquid during the same period becomes larger. Among them, during the heating period, the temperature differences under each burial depth scenario are: 0.15℃ (burial depth 50 m), 0.4℃ (burial depth 100 m), 0.64℃ (burial depth 150 m), 0.91℃ (burial depth 200 m); during the cooling period, the temperature differences under each burial depth scenario are: -0.76℃ (burial depth 50 m), -1.2℃ (burial depth 100 m), -1.42℃ (burial depth 150 m), -1.5℃ (burial depth 200 m).

[0065] Draw the above results into a "temperature difference-depth" line graph, such as Fig.18 As shown in the figure, it can be seen that the temperature difference during the heating period basically increases linearly with the buried depth of the underground pipe, and the overall slope of the straight line is 0.005℃ / m; while in the cooling period, the absolute value of the slope of the buried pipe temperature difference-buried depth straight line gradually decreases with the increase of depth, and the slope in the 100-150 m buried depth section is -0.0044℃ / m, and the slope in the 150-200 m buried depth section becomes -0.0016℃ / m. Therefore, it can be considered that there may be an optimal buried depth value near the 150 m buried depth, so that the buried pipe effect achieves the best cost-effectiveness.

[0066] On this basis, the burial depth range of 140 m to 170 m was further refined with an interval of 5 m, and the optimal hole depth was obtained by comparing the slope of the line segment of "inlet and outlet temperature difference-burial depth" during the cooling period. The simulation results are shown in the figure below. Fig.19 As shown in the figure, it can be seen that the absolute value of the slope after 150 m is relatively small, much slower than the line segment before 150 m. Therefore, 150 m is the best hole depth.

[0067] Although some preferred embodiments of the present invention have been described, those skilled in the art may make other changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0068] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the inventive concept. Thus, if these modifications and variations of the present application fall within the scope of the claims of the present application and their equivalents, the present application is also intended to include these modifications and variations.

Claims

1. A method for constructing a heat transfer model of a buried pipe heat exchanger, comprising the following steps: (1) Determine the drilling depth range of the area to be determined, analyze the hydrogeological conditions, thermal properties of the rock and soil, and geothermal field characteristics within the range, and perform grid division on the area to be determined; (2) Based on the hydrogeological conditions and the thermal physical properties of the rock and soil, the aquifers are generalized and vertically divided into the shallow aquifer variable temperature zone, the shallow aquifer warming zone, the weak permeable layer and the deep aquifer warming zone; (3) Setting the solution conditions, characterizing the source and sink terms of the aquifer in the determined area, and on this basis, constructing a steady flow model to simulate the initial flow field distribution of the model under given conditions; (4) Based on the 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 a heat transfer model of the buried pipe heat exchanger.

2. The construction method according to claim 1, characterized in that: In step (2), the water-containing medium in the shallow aquifer temperature change zone includes silt sand, medium sand and fine sand; the water-containing medium in the shallow aquifer temperature increase zone includes fine sand, silt and silt sand; the rock and soil layer of the weak permeable layer includes a thick layer of silty clay and / or a thin layer of silt; the water-containing medium in the deep aquifer temperature increase zone includes fine sand, silt and / or a thin layer of silty clay.

3. The construction method according to claim 1, characterized in that: In step (3), the solution conditions are as follows: the surface temperature is the annual average temperature of the area to be determined, the geothermal constant temperature zone is 17.3°C, and the values ​​of the layers below are determined according to the depth from the constant temperature zone multiplied by the geothermal warming rate; the flow boundary can be generalized into a supply boundary, a discharge boundary and a zero flow boundary in the horizontal direction, and can be generalized into a flux boundary surface and a water-isolating boundary surface in the vertical direction; the surface and lower boundaries of the area to be determined are generalized as constant temperature boundaries.

4. The construction method according to claim 1, characterized in that: In step (4), the ground heat exchanger is characterized by a one-dimensional finite element method, and its control equation is as follows: Where: ρ r , c r —heat capacity of the fluid in the tube; ε g —Porosity of backfill material; ρ g 、c g —Heat capacity of backfill; u —Flow rate of the circulating fluid in the tube; ∇—differential operator heat diffusion flux; Λ r —heat diffusion flux of the fluid in the tube; —Convection heat transfer coefficient between the fluid in the double U heat exchange tube and the inlet tube wall; —Convection heat transfer coefficient between the fluid in the double U heat exchange tube and the outlet tube wall; —Convection heat transfer coefficient between two tubes; —Convective heat transfer coefficient between backfill and surrounding soil.

5. The construction method according to claim 1, characterized in that: In step (4), the surrounding rock and soil body is processed by a three-dimensional finite element method, and the heat transfer control equation is as follows: In the formula: s is the water storage coefficient, dimensionless; superscript f represents liquid; superscript s represents soil; h is the hydraulic head; Q is the flow rate; β is the thermal expansion coefficient; q is the Darcy velocity; ∇ is the vector differential operator; T is the temperature; ε is the porosity; ρ is the density; c is the specific heat capacity; H is the heat source and sink; λ is the thermal conductivity; αL is the longitudinal thermal dissipation; αT is the transverse thermal dissipation; I is the unit characteristic matrix.

6. The construction method according to claim 1, characterized in that: In the step (1), the super mesh surface is triangulated using the Triangle algorithm in the area to be determined.

7. A method for determining the hole depth of a buried pipe heat exchanger, comprising the following steps: (1) Setting the simulation cycle of the heat transfer model of the buried pipe heat exchanger established in claim 1, setting the operating power and circulation flow rate of the single heat exchange hole, simulating through the cyclic time series, and establishing a geothermal prediction model; (2) Based on the geothermal prediction model, simulate and compare the outlet temperatures of the buried pipes at different burial depths at specific intervals during the heating and cooling periods of the year, and calculate the inlet and outlet temperature differences of the buried pipes at different burial depths; (3) Draw a "temperature difference-depth" line graph and calculate the slopes of different burial depth intervals during the cooling and heating periods of the year; (4) Determine the period when the slope changes with depth, further refine the lower limit of the depth interval where the minimum absolute value of the slope is located during the period, and further narrow the depth interval, simulate the outlet temperature of the buried pipe at different burial depths during the period, and then calculate the inlet and outlet temperature differences of the buried pipe body at different burial depths, and draw a "temperature difference-depth" line graph; (5) The middle value of the two adjacent burial depth intervals with the largest difference in absolute slope in the "temperature difference-depth" line graph is taken as the optimal hole depth.

Images