A downhole heat exchanger with variable length inner tube and a control method thereof

Abstract

The application discloses a downhole heat exchanger with variable inner tube length and a control method thereof, which comprises a closed well pipe, an inner tube and a source side circulating pump, and further comprises a variable tube length device, wherein at least one Y-shaped pipe is arranged on the inner tube; the Y-shaped pipe comprises an inlet, an outlet and a Y-shaped branch pipe; the inlet and the outlet are communicated with the inner tube; the variable tube length device comprises an air bag, a capillary tube, a gas storage tank and an air compressor; the air bag is arranged in a pipe opening of the Y-shaped branch pipe; one end of the air bag is connected with the capillary tube; the other end of the capillary tube is connected with the gas storage tank; and the gas storage tank is connected with the air compressor. Through the variable tube length device based on the air bag, the capillary tube and a pressure regulating valve, the length of the inner tube of the downhole heat exchanger is conveniently adjusted; when the building load is low, the length of the heat exchanger actually running can be reduced, the power consumption of the circulating pump is reduced, and the operation cost is saved.

Description

Technical Field

[0001] This invention belongs to the field of medium-deep geothermal utilization, specifically a medium-deep downhole heat exchanger with variable inner tube length and its control method. Background Technology

[0002] Geothermal energy is characterized by its large reserves, wide distribution, cleanliness, environmental friendliness, stability, and reliability, making it a practical and competitive clean energy source.

[0003] Ground source heat pumps with buried pipes are the most common form of ground source heat pumps, extracting energy from shallow soil through buried pipe heat exchangers. However, traditional buried pipes have low single-hole heat exchange capacity, and the land area occupied by buried pipe networks typically reaches 30% to 55% of the building's energy supply area. This large land area limits the development of shallow buried pipe ground source heat pump systems in urban areas and other regions where surface space is scarce. Therefore, it is urgent to improve the single-hole heat exchange capacity of underground heat exchangers and reduce their land area, thus breaking through the bottleneck in the development of ground source heat pumps.

[0004] 2. Advantages and problems of medium-deep well downhole heat exchanger technology

[0005] To address the aforementioned issues, medium-deep ground source heat pumps have seen rapid development in recent years. Medium-deep ground source heat pumps typically employ coaxial medium-deep downhole heat exchangers to extract underground heat energy, as shown in the system diagram below. Figure 1 As shown. The outer tube 1 is generally a metal well pipe with good thermal conductivity, while the inner tube 2 is generally made of a material with poor thermal conductivity. The circulating medium (usually water) flows in from the top of the outer tube of the downhole heat exchanger, flows into the inner tube at the bottom of the well, and then flows out from the top of the inner tube. This flow pattern ensures that the circulating medium heated by geothermal energy can flow out from the well-insulated inner tube, avoiding heat loss. The depth of medium-deep downhole heat exchangers can generally reach 1000-3000m, and the heat exchange per unit depth can generally reach 100-150W / m. Therefore, the heat exchange of a single-hole heat exchanger can be equivalent to dozens of holes of traditional shallow buried pipes, which can effectively reduce the land area occupied.

[0006] However, medium-deep downhole heat exchangers suffer from high friction loss and high power consumption of the circulating pump. For traditional shallow buried pipes, the "Technical Specification for Ground Source Heat Pump System Engineering" (2020 draft for comments) requires that "the number of vertical buried pipe holes connecting each group of supply and return water manifolds should not exceed eight, and shut-off valves should be installed between the manifolds and the distribution / manifolds." Therefore, by rationally setting the number of activated buried pipe zones, the number of operating buried pipes can be matched with the building load, thereby reducing the energy consumption of the circulating pump. For example, at the beginning of the heating season in November, the heat load is relatively low, and only a portion of the buried pipe zones needs to be operated to obtain sufficient heat exchange. However, for conventional medium-deep downhole heat exchangers, their depth must be designed according to the maximum building load. In actual operation, regardless of the building load, the entire heat exchanger must be operated, resulting in wasted pump power and high energy consumption when the building load is low.

[0007] 3. Methods and shortcomings for solving the problem of high pump power in medium-deep well downhole heat exchangers

[0008] To address the aforementioned issues, Zhang Bingbing of Shandong Jianzhu University proposed a variable flow rate operation mode in his 2019 master's thesis, "Energy Efficiency Study of Medium-Deep Casing-Type Buried Pipe Heat Exchanger and Ground Source Water System," using a smaller operating flow rate when the building load is low. However, in this method, the circulating medium still needs to flow through the entire downhole heat exchanger, resulting in relatively high frictional resistance. Furthermore, the dimensions of the heat exchange channels within the downhole heat exchanger and heat pump unit are already fixed; reducing the flow rate would alter the flow state of the circulating medium and reduce the convective heat transfer effect.

[0009] Another approach is to arrange multiple inner tubes of varying lengths within the downhole heat exchanger, using shorter inner tubes when the building load is low. However, because the inner tubes of mid-to-deep downhole heat exchangers require materials with good insulation and high strength, the cost is high, significantly increasing the initial investment for this method. Furthermore, due to the limited space within the well, this method only allows for switching between a few lengths.

[0010] In summary, there is an urgent need for a design and operation control method that allows for easy modification of the inner tube length of medium-deep downhole heat exchangers in order to improve system energy efficiency and economy. Summary of the Invention

[0011] The technical problem to be solved by the present invention is to make it possible to easily change the inner tube length of a medium-deep downhole heat exchanger. In order to solve the above problem, a medium-deep downhole heat exchanger with variable inner tube length and its control method are provided.

[0012] The object of this invention is achieved in the following manner:

[0013] A medium-deep wellbore heat exchanger with variable inner tube length includes a sealed well pipe 1, an inner tube 2, and a source-side circulation pump 12, and also includes a variable tube length device. The inner tube 2 is provided with at least one Y-shaped pipe 3. The Y-shaped pipe 3 includes an inlet 31, an outlet 32, and a Y-shaped branch pipe 33. The inlet 31 and the outlet 32 ​​are connected to the inner tube 2. The variable tube length device includes an air bladder 4, a capillary tube 5, an air storage tank 7, and an air compressor 8. The air bladder 4 is disposed inside the opening of the Y-shaped branch pipe 33. The air bladder 4 is connected to one end of the capillary tube 5, and the other end of the capillary tube 5 is connected to the air storage tank 7. The air storage tank 7 is connected to the air compressor 8.

[0014] The capillary tube 5 is equipped with a pressure regulating valve 6, which is communicatively connected to the control system 11.

[0015] It also includes a monitoring system and a control system 11; the monitoring system includes a flow meter 9 and a temperature sensor 10; the flow meter 9 is installed on the outlet circulation pipeline of the source-side circulation pump 12 and is used to measure the flow rate of the circulating medium in the heat exchanger; the temperature sensor 10 is installed at the inlet and outlet of the heat exchanger respectively and is used to measure the inlet and outlet temperatures of the heat exchanger; the flow meter 9 and the temperature sensor 10 are respectively connected to the control system 11 in communication.

[0016] The Y-shaped branch pipe 33 has an opening that faces upwards at an angle, and a perforated baffle 34 is installed at the opening.

[0017] The number of Y-type pipes 3 is designed according to the following principle: when the total depth of the heat exchanger is 1000-1600m, 1600m-2300m, and 2300-3000m, the number of Y-type pipes is 3-4, 4-5, and 5-6, respectively.

[0018] The depth distribution of the Y-shaped pipe 3 is consistent with the building load distribution.

[0019] The airbag 4 is made of nitrile rubber, and the capillary 5 is made of corrosion-resistant material.

[0020] A control method for a medium-deep wellhead heat exchanger with variable inner tube length includes the following steps:

[0021] Step 1: Based on equipment sample data or actual measurement data, obtain the energy efficiency model of the source-side circulating pump 12 and heat pump unit 13 connected to the medium-deep well downhole heat exchanger by fitting model or neural network algorithm.

[0022] Step 2: Based on the actual parameters of the medium-deep downhole heat exchanger, obtain the heat exchanger pressure drop model;

[0023] Step 3: Based on the sample data of airbag 4, the density of the circulating medium and the depth information of each airbag 4, calculate the pressure of each airbag 4 when it is fully inflated and fully contracted.

[0024] Step 4: Monitor the flow rate and inlet / outlet temperature of the deep well heat exchanger in real time using flow meter 9 and temperature sensor 10, calculate the heat exchanger's heat transfer capacity and store it in real time.

[0025] Step 5: Set the current operating time as time n, obtain the building load prediction data and historical heat exchange data at time n+1; use algorithms such as resistance-capacitance heat transfer model or finite element model to couple the energy efficiency models of source-side circulating pump 12 and heat pump unit 13, calculate the power consumption of source-side circulating pump 12 and heat pump unit 13 under different inner pipe 2 lengths, and obtain the optimal length of inner pipe 2 based on the principle of minimizing total power consumption.

[0026] Step six: When the system reaches time n+1, based on the optimal length of the inner tube 2, the control system 11 outputs a signal to manually or automatically adjust the pressure regulating valve 6 corresponding to the optimal length, so that the pressure inside the corresponding airbag 4 is slightly lower than the pressure when it is fully contracted, allowing the circulating medium to flow freely into the opening of the corresponding Y-shaped branch pipe 33; for other airbags 4 with shallower depths, the control system 11 outputs a signal to manually or automatically adjust the corresponding pressure regulating valve 6, so that the pressure of the airbag 4 is slightly higher than the pressure when it is fully inflated, and the outlet of the Y-shaped branch pipe 33 is completely closed; for other airbags 4 with deeper depths, their pressure does not need to be adjusted.

[0027] Step 7: After the pressure of airbag 4 is adjusted, return to step 4 to continue data monitoring and inner tube 2 length adjustment.

[0028] The beneficial effects of this invention are as follows: This invention provides a medium-deep downhole heat exchanger with variable inner tube length. Through a variable tube length device based on components such as air bladders, capillary tubes, and pressure regulating valves, the inner tube length of the heat exchanger can be conveniently adjusted. When the building load is low, the actual operating length of the heat exchanger can be reduced, lowering the power consumption of the circulating pump and saving operating costs. Furthermore, the control method of this invention for a medium-deep downhole heat exchanger with variable inner tube length allows for optimal selection of the inner tube length based on the building load, further improving the overall energy efficiency of the system. Attached Figure Description

[0029] Figure 1 This is a system diagram of existing technology.

[0030] Figure 2 This is a schematic diagram of a medium-deep well downhole heat exchanger with variable inner tube length according to the present invention.

[0031] Figure 3 This is a schematic diagram of the Y-shaped pipe in this invention.

[0032] In the diagram: 1-Sealed well casing, 2-Inner pipe, 3-Y-type pipe, 31-Inlet, 32-Outlet, 33-Y-type branch pipe, 34-Perforated baffle, 4-Airbag, 5-Capillary tube, 6-Pressure regulating valve, 7-Air storage tank, 8-Air compressor, 9-Flow meter, 10-Temperature sensor, 11-Control system, 12-Source-side circulation pump, 13-Heat pump unit. Detailed Implementation

[0033] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0034] It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same technical meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0035] like Figure 2 and Figure 3 As shown, a medium-deep downhole heat exchanger with variable inner tube length includes a sealed well pipe 1, an inner tube 2, and a source-side circulation pump 12. It also includes a variable tube length device. The inner tube 2 is provided with at least one Y-shaped pipe 3 that matches the variable tube length device. The Y-shaped pipe 3 includes an inlet 31, an outlet 32, and a Y-shaped branch pipe 33. The inlet 31 and the outlet 32 ​​are connected to the inner tube 2. The variable tube length device includes an air bladder 4, a capillary tube 5, an air storage tank 7, and an air compressor 8. The air bladder 4 is disposed inside the opening of the Y-shaped branch pipe 33. The air bladder 4 is connected to one end of the capillary tube 5, and the other end of the capillary tube 5 is connected to the air storage tank 7. The air storage tank 7 is connected to the air compressor 8.

[0036] The capillary tube 5 is equipped with a pressure regulating valve 6, which is communicatively connected to the control system 11. The control system 11 outputs the optimal length of the inner tube 2 at the next moment with the goal of minimizing energy consumption, and controls the operating parameters of the corresponding pressure regulating valve 6 in the variable tube length device accordingly.

[0037] It also includes a monitoring system and a control system 11; the monitoring system includes a flow meter 9 and a temperature sensor 10; the flow meter 9 is installed on the outlet circulation pipeline of the source-side circulation pump 12 and is used to measure the flow rate of the circulating medium in the heat exchanger; the temperature sensor 10 is installed at the inlet and outlet of the heat exchanger respectively and is used to measure the inlet and outlet temperatures of the heat exchanger; the flow meter 9 and the temperature sensor 10 are respectively connected to the control system 11 in communication.

[0038] The Y-shaped branch pipe 33 has an opening that faces upwards at an angle, and a perforated baffle 34 is installed at the opening.

[0039] The number of Y-type pipes 3 is designed according to the following principle: when the total depth of the heat exchanger is 1000-1600m, 1600m-2300m, and 2300-3000m, the number of Y-type pipes is 3-4, 4-5, and 5-6, respectively.

[0040] The depth distribution of the Y-shaped pipe 3 is consistent with the building load distribution.

[0041] The airbag 4 is made of rubber materials with good water resistance, heat resistance and aging resistance, such as nitrile rubber.

[0042] The capillary tube 5 is made of a high-strength, corrosion-resistant material.

[0043] The head of the source-side circulating pump 12 can be changed by connecting multiple pumps in series or by using a frequency converter.

[0044] A control method for a medium-deep wellhead heat exchanger with variable inner tube length includes the following steps:

[0045] Step 1: Based on equipment sample data or actual measurement data, obtain the energy efficiency model of the source-side circulating pump 12 and heat pump unit 13 connected to the medium-deep well downhole heat exchanger by fitting model or neural network algorithm.

[0046] Step 2: Based on the actual parameters of the medium-deep downhole heat exchanger, obtain the heat exchanger pressure drop model;

[0047] Step 3: Based on the sample data of airbag 4, the density of the circulating medium and the depth information of each airbag 4, calculate the pressure of each airbag 4 when it is fully inflated and fully contracted.

[0048] Step 4: Monitor the flow rate and inlet / outlet temperature of the deep well heat exchanger in real time using flow meter 9 and temperature sensor 10, calculate the heat exchanger's heat transfer capacity and store it in real time.

[0049] Step 5: Assume the current operating time is time n, obtain the building load prediction data and historical heat exchange data at time n+1, where the building load prediction data and historical heat exchange data at time n+1 are obtained from the design institute; through algorithms such as the resistance-capacitance heat transfer model or the finite element model, coupled with the energy efficiency models of the source-side circulating pump 12 and the heat pump unit 13, calculate the power consumption of the source-side circulating pump 12 and the heat pump unit 13 under different inner pipe 2 lengths, and obtain the optimal length of the inner pipe 2 based on the principle of minimizing the total power consumption;

[0050] Step six: When the system reaches time n+1, based on the optimal length of the inner tube 2, the control system 11 outputs a signal to manually or automatically adjust the pressure regulating valve 6 corresponding to the optimal length, so that the pressure inside the corresponding airbag 4 is slightly lower than the pressure when it is fully contracted, allowing the circulating medium to flow freely into the opening of the corresponding Y-shaped branch pipe 33; for other airbags 4 with shallower depths, the control system 11 outputs a signal to manually or automatically adjust the corresponding pressure regulating valve 6, so that the pressure of the airbag 4 is slightly higher than the pressure when it is fully inflated, and the outlet of the Y-shaped branch pipe 33 is completely closed; for other airbags 4 with deeper depths, their pressure does not need to be adjusted.

[0051] Step 7: After the pressure of airbag 4 is adjusted, return to step 4 to continue data monitoring and inner tube 2 length adjustment.

[0052] The structure, workflow, and working principle of the present invention are further illustrated below with reference to embodiments thereof:

[0053] A medium-deep downhole heat exchanger with variable inner tube length, such as... Figure 2As shown, the system includes a sealed well casing 1, an inner pipe 2, a variable pipe length device, and a source-side circulation pump 12. The inner pipe 2 is equipped with several Y-shaped pipes 3 that match the variable pipe length device. Each Y-shaped pipe 3 includes an inlet 31, an outlet 32, and Y-shaped branch pipes 33. The inlet 31 and outlet 32 ​​are connected to the inner pipe 2. The variable pipe length device includes an air bladder 4, a capillary tube 5, a pressure regulating valve 6, an air storage tank 7, and an air compressor 8. It also includes a monitoring system and a control system 11. The monitoring system includes a flow meter 9 and a temperature sensor 10. The flow meter 9 measures the flow rate of the circulating medium in the heat exchanger. The temperature sensor 10 measures the inlet and outlet temperatures of the heat exchanger. The control system 11 outputs the optimal length of the inner pipe 2 for the next moment with the goal of minimizing energy consumption, and controls the operating parameters of the corresponding pressure regulating valve 6 in the variable pipe length device accordingly.

[0054] In this embodiment, the total depth of the heat exchanger is 1000m, the inner diameter of the sealed well pipe 1 is 180mm, the outer diameter of the inner pipe 2 is 120mm, and the inner diameter of the inner pipe 3 is 100mm. Based on the daily load calculation of the energy-supplying building, the proportions of energy supply days with load rates of 0%–50%, 50%–70%, 70%–85%, and 85%–100% are basically the same. The circulating medium is water with a density of 1000kg / m³. 3 Its specific heat capacity is 4200 J / (kg·K), and its kinematic viscosity is 1×10⁻⁶. -6 m 2 / s; the local gravitational acceleration is 9.8 m / s². 2 The source-side circulating pump 12 is a Southern CDL65-10 (with frequency converter installed), and the heat pump unit 13 is a WaterFurnace NXW60.

[0055] Preferably, there can be 3 Y-shaped pipes.

[0056] Preferably, the depths of the three Y-shaped pipes 3 can be set to 500m, 700m and 850m respectively.

[0057] Preferably, the airbag 4 is disposed in the Y-shaped branch pipe 33. The opening direction of the Y-shaped branch pipe 33 can be obliquely upward. A perforated baffle 34 can be provided at the opening. The baffle can prevent the airbag 4 from falling out and can also ensure that the circulating medium passes through the baffle with a resistance of no more than 100Pa when the airbag 4 is deflated.

[0058] Preferably, the airbag 4 can be made of nitrile rubber.

[0059] Preferably, when the airbag 4 is fully inflated, it can completely seal the outlet portion of the Y-shaped branch pipe 3, and when the airbag 4 is fully contracted, it can allow the circulating medium to pass through the Y-shaped branch pipe 3 with a resistance of no more than 100 Pa.

[0060] Preferably, the capillary tube 5 can be made of 304 stainless steel and has a specification of Φ6×0.8 (mm).

[0061] Preferably, each airbag 4 is connected to a capillary tube 5, which is connected to an air storage tank 7. Each capillary tube is equipped with a pressure regulating valve 6 above the ground. The pressure regulating valve 6 is connected to the control system 11. The pressure regulating valve 6 can be an electric pressure regulating valve to achieve automated control.

[0062] Preferably, the air compressor 8 is connected to the air tank 7, and the rated pressure inside the air tank 7 can be set to 9MPa. When the air pressure is too low, the air compressor 8 will automatically replenish the air.

[0063] Preferably, the source-side circulation pump 12 can be a variable frequency pump.

[0064] A control method for a medium-deep well downhole heat exchanger with variable inner tube length, specifically including the following steps:

[0065] Step 1: Based on the equipment sample data, the energy efficiency models of heat pump unit 13 and source-side circulation pump 12 are fitted. The model for heat pump unit 13 is based on the coefficient of performance (COP) and source-side flow rate (V). S m 3 / h) and return water temperature (T) S,i ,℃), load-side flow rate (V L m 3 / h) and return water temperature (T) L,i The equation for (℃) is shown in equation (1):

[0066]

[0067] Where a1~a 15 These are the model coefficients, obtained through fitting.

[0068] The heat pump power consumption (E) can be calculated using equation (2). hp ,kW):

[0069]

[0070] Where Q represents the heat supply, in kW.

[0071] The source-side circulating pump model 12 includes two models, one of which is the circulating pump head model with respect to head (H). wp m) and flow rate (V) wp m 3 The equation for / h) is shown in equation (3):

[0072]

[0073] Where b1 to b3 are model coefficients, obtained through fitting.

[0074] The power model of the circulating pump is about power (E) wp kW) and flow rate (V) wp m3 The equation for / h) is shown in equation (4):

[0075]

[0076] Where c1 to c3 are model coefficients, obtained through fitting.

[0077] Step 2: Obtain the total pressure drop model (H) of the medium-deep downhole heat exchanger using equation (5). DHE m):

[0078]

[0079] Where L is the length of inner pipe 2, in meters; d is the hydraulic diameter, which is 0.06 meters for the annular region within the sealed well pipe 1 and 0.1 meters for inner pipe 2; S is the flow area, which is 14137.2 mm² for the annular region within the sealed well pipe 1 and 14137.2 mm² for inner pipe 2. 2 7854.0mm 2 ν is the kinematic viscosity of the circulating medium, i.e., 1×10⁻⁶. -6 m 2 / s.

[0080] Step 3: Assume that the pressure of the selected airbag 4 relative to the external environment when it is fully inflated and fully deflated is P1 = 0.6 MPa and P0 = 0 MPa, respectively; calculate the ground surface pressure (P) of the airbag 4 when it is fully inflated and fully deflated using equations (6) and (7), respectively. H,1 P H,0 (MPa):

[0081]

[0082]

[0083] Where ρ is the density of the circulating medium, i.e., 1000 kg / m³ 3 g is the local gravitational acceleration, which is 9.8 m / s². 2 H represents the depth of airbag 4, in meters. The P values ​​for the three airbags 4 are calculated. H,1 The values ​​were 5.50 MPa, 7.46 MPa, and 8.93 MPa, respectively. H,0 The values ​​are 4.90 MPa, 6.86 MPa, and 8.33 MPa, respectively.

[0084] Step 4: Monitor the heat exchanger flow rate (V) in real time using flow meter 9 and temperature sensor 10. S m 3 / h), heat pump unit 13 source-side water supply temperature (T) S,o (℃) and return water temperature (T) S,i The heat exchanger heat transfer capacity (Q, J) can be calculated according to formula (8): (℃)

[0085]

[0086] Where c p τ represents the specific heat capacity of the circulating medium, i.e., 4200 J / (kg·K); τ is the recording time interval, in seconds. Because the underground heat exchange process has strong unsteady-state characteristics and is greatly affected by historical operating conditions, the calculated heat exchange volume needs to be stored in real time and used as the basis for subsequent calculations of the heat exchanger outlet temperature.

[0087] Step 5: Assuming the current operating time is time n, obtain the predicted building load Q and historical heat exchange data for time n+1 (where the predicted building load Q and historical heat exchange data for time n+1 are obtained from the design institute). Equations (3) and (5) both contain the variable Vs. Equating the two equations yields Vs and H for different inner pipe lengths. DHE And E is calculated using equation (4) wp The outlet temperature T of the deep well heat exchanger at that moment was calculated using a resistance-capacitance heat transfer model. S,i Combined with the return water temperature T on the load side L,i and traffic V L The COP of the heat pump unit 13 can be calculated using equation (1), and the E can be calculated using equation (2). hp Total power consumption E on the source side of the power supply system sum =E wp +E hp Different inner tube lengths correspond to different E values. sum When the inner pipe 2 is shorter, the outlet water temperature of the medium-deep well downhole heat exchanger is lower, the COP of the heat pump unit 13 is lower, and E hp Higher, while E wp Lower; while when the inner tube 2 is longer, E hp Lower and E wp The value is relatively high. Therefore, there exists an optimal inner tube length 2 such that E... sum Reach the minimum.

[0088] For this embodiment, assuming Q = 80kW at time n+1, E is calculated when the pipe length is 500m, 700m, 850m, and 1000m. hp The power outputs are 16.52kW, 15.14kW, 14.30kW, and 13.59kW respectively. wp The power outputs are 3.16kW, 4.93kW, 6.48kW, and 8.88kW respectively. sum The corresponding power outputs are 19.68kW, 20.07kW, 20.78kW, and 22.47kW, respectively. Therefore, the optimal length of inner pipe 2 is 500m, which can reduce energy consumption by 12.41% compared with operating the entire pipe length.

[0089] Step 6: Adjust the electric pressure regulating valve 6 corresponding to the 500m depth airbag 4 through the control system 11 to adjust the pressure inside the corresponding airbag 4 to 4.80MPa, so that it is fully contracted and the circulating medium flows freely into the opening of the Y-shaped branch pipe where it is located; for the airbags 4 with depths of 700m and 850m, the pressure does not need to be adjusted.

[0090] Step 7: After the pressure of airbag 4 is adjusted, return to step 4 to continue data monitoring and inner tube 2 length adjustment.

[0091] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several changes and improvements without departing from the overall concept of the present invention, and these should also be considered within the scope of protection of the present invention.

Claims

1. A medium-deep downhole heat exchanger with variable inner tube length, comprising a sealed well casing (1), an inner tube (2), and a source-side circulation pump (12), characterized in that: It also includes a variable pipe length device, wherein at least one Y-shaped pipe (3) is provided on the inner pipe (2); the Y-shaped pipe (3) includes an inlet (31), an outlet (32) and a Y-shaped branch pipe (33); the inlet (31) and the outlet (32) are connected to the inner pipe (2), and the variable pipe length device includes an air bladder (4), a capillary tube (5), an air tank (7) and an air compressor (8); the air bladder (4) is set in the opening of the Y-shaped branch pipe (33), the air bladder (4) is connected to one end of the capillary tube (5), the other end of the capillary tube (5) is connected to the air tank (7), and the air tank (7) is connected to the air compressor (8).

2. The variable inner tube length downhole heat exchanger according to claim 1, characterized in that: The capillary tube (5) is equipped with a pressure regulating valve (6), which is communicatively connected to the control system (11).

3. The variable inner tube length downhole heat exchanger according to claim 2, characterized in that: It also includes a monitoring system and a control system (11); the monitoring system includes a flow meter (9) and a temperature sensor (10); the flow meter (9) is installed on the outlet circulation pipeline of the source-side circulation pump (12) and is used to measure the flow rate of the circulating medium in the heat exchanger; the temperature sensor (10) is installed at the inlet and outlet of the heat exchanger respectively and is used to measure the inlet and outlet temperatures of the heat exchanger; the flow meter (9) and the temperature sensor (10) are respectively connected to the control system (11) in communication.

4. The variable inner tube length downhole heat exchanger according to claim 1, characterized in that: The Y-shaped branch pipe (33) has an opening direction that is obliquely upward, and a perforated baffle (34) is provided at the opening.

5. The variable inner tube length downhole heat exchanger according to claim 1, characterized in that: The number of Y-type pipes (3) is designed according to the following principles: when the total depth of the heat exchanger is 1000 m to 1600 m, 1600 m to 2300 m, and 2300 m to 3000 m, the number of Y-type pipes is 3 to 4, 4 to 5, and 5 to 6, respectively.

6. The variable inner tube length downhole heat exchanger according to claim 1, characterized in that: The depth distribution of the Y-shaped pipe (3) is consistent with the building load distribution.

7. The variable inner tube length downhole heat exchanger according to claim 1, characterized in that: The airbag (4) is made of nitrile rubber, and the capillary (5) is made of corrosion-resistant material.

8. A control method for a medium-deep wellhead heat exchanger with variable inner tube length, characterized in that: Includes the following steps: Step 1: Based on equipment sample data or actual measurement data, obtain the energy efficiency model of the source-side circulating pump (12) and heat pump unit (13) connected to the medium-deep well downhole heat exchanger by fitting model or neural network algorithm; Step 2: Based on the actual parameters of the medium-deep downhole heat exchanger, obtain the heat exchanger pressure drop model; Step 3: Based on the sample data of airbags (4), the density of the circulating medium and the depth information of each airbag (4), calculate the pressure of each airbag (4) when it is fully inflated and fully contracted; Step 4: The flow rate and inlet / outlet temperature of the deep well heat exchanger are monitored in real time by the flow meter (9) and temperature sensor (10), and the heat exchange capacity of the heat exchanger is calculated and stored in real time. Step 5: Set the current operating time as time n, obtain the building load prediction data and historical heat exchange data at time n+1; use the resistance-capacitance heat transfer model or finite element model algorithm to couple the energy efficiency models of the source-side circulating pump (12) and the heat pump unit (13), calculate the power consumption of the source-side circulating pump (12) and the heat pump unit (13) under different inner pipe (2) lengths, and obtain the optimal length of the inner pipe (2) based on the principle of the lowest total power consumption; Step six: When the system reaches time n+1, based on the optimal length of the inner tube (2), the pressure regulating valve (6) corresponding to the optimal length is manually or automatically adjusted by the output signal of the control system (11) so that the pressure inside the corresponding airbag (4) is slightly lower than the pressure when it is fully contracted, allowing the circulating medium to flow freely into the opening of the corresponding Y-shaped branch pipe (33); for other airbags (4) with shallower depths, the pressure regulating valve (6) is manually or automatically adjusted by the output signal of the control system (11) so that the pressure of the airbag (4) is slightly higher than the pressure when it is fully inflated, and the outlet of the Y-shaped branch pipe (33) is completely closed; for other airbags (4) with deeper depths, their pressure does not need to be adjusted; Step 7: After the airbag (4) pressure is adjusted, return to step 4 to continue data monitoring and inner tube (2) length adjustment.

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