A bionic fractal flow channel reinforced heat exchange system for a medium-deep geothermal well

Abstract

The utility model belongs to the geothermal energy heating technology field relates to a kind of bionic fractal flow passage reinforced heat exchange system for middle-deep geothermal well, comprising: bionic fractal coaxial sleeve type downhole heat exchanger, plate heat exchanger, heat pump unit, the outlet of downhole heat exchanger is connected with tee regulating control valve;Tee regulating control valve connects the hot side import of plate heat exchanger, the hot side export of plate heat exchanger is connected back to the import of downhole heat exchanger, and the cold side of plate heat exchanger is connected with user side circulation;Tee regulating control valve connects the import of evaporator of heat pump unit, and the outlet of evaporator is connected back to the import of downhole heat exchanger, and the condenser of heat pump unit is connected to user side in circulation;The utility model effectively reduces system energy consumption while guaranteeing heating demand, improves overall energy efficiency ratio, adapts the middle-deep geothermal development demand of different temperature gradient, has stronger applicability and practicality.

Description

Technical Field

[0001] This utility model belongs to the field of geothermal energy heating technology, specifically relating to a biomimetic fractal flow channel enhanced heat exchange system for medium-deep geothermal wells. Background Technology

[0002] With the increasing demand for clean energy, medium-deep geothermal energy is widely used for heating due to its stability and environmental friendliness. Currently, the most commonly used downhole heat exchangers are coaxial sleeve type and U-tube type.

[0003] However, existing technologies have the following shortcomings: 1. Limited heat exchange area in downhole heat exchangers. Existing coaxial casing heat exchangers only exchange heat between the annular channel wall and the rock / soil mass, with the heat exchange area limited by the wellbore diameter. Since the casing diameter is typically only 200–300 mm, the limited effective heat exchange area results in insufficient residence time of the circulating working fluid downhole, failing to fully absorb heat from the deep, high-temperature rock mass. U-tube heat exchangers also have limited heat exchange area. This drawback leads to insufficient heat extraction capacity.

[0004] 2. The stratified temperature gradient of the formation was not considered. The temperature of underground rock strata increases with depth, and the thermal conductivity also varies gradient. Existing heat exchangers use a single material casing from the wellhead to the bottom of the well, without differentiated design for different depths. In shallow sections, heat is lost due to heat conduction through the casing wall, while in deeper sections, insufficient heat exchange area prevents full utilization of high-temperature resources. This shortcoming results in significant heat loss in shallow layers and inadequate heat extraction in deeper layers.

[0005] 3. Uneven flow within downhole heat exchangers can easily lead to flow short-circuiting. Existing heat exchangers use a single annular flow channel or a single U-shaped tube for the fluid channel. When the cold fluid flows from top to bottom, due to uneven distribution of flow resistance, some fluid may pass quickly along paths with lower resistance, failing to fully enter the deep, high-temperature region for heat exchange, resulting in insufficient localized heat transfer. Simultaneously, the traditional U-shaped tube arrangement with two branches running side-by-side makes it prone to thermal short-circuiting between the high-temperature and low-temperature working fluids, causing heat loss. This drawback leads to a decrease in heat exchange efficiency.

[0006] 4. The above-ground heating system operates in a single mode, leading to a decrease in energy efficiency in the later stages of the heating season. Under the existing series-cascade utilization mode, geothermal well water must first pass through a plate heat exchanger for cooling before entering the heat pump unit's evaporator. When the geothermal well water temperature drops during the middle of the heating season, the heat pump becomes the primary heat source. However, due to the series structure, the heat pump cannot directly utilize the higher-temperature geothermal water, resulting in a significant decrease in the system's coefficient of performance (COP). This drawback leads to increased energy consumption in the later stages of the heating season.

[0007] Therefore, a device or method with higher heat exchange efficiency is needed to solve the above-mentioned technical problems. Utility Model Content

[0008] This utility model provides the following technical solution: a biomimetic fractal flow channel enhanced heat exchange system for medium-deep geothermal wells, comprising: a biomimetic fractal coaxial shell-and-tube downhole heat exchanger, a plate heat exchanger, and a heat pump unit; the outlet of the biomimetic fractal coaxial shell-and-tube downhole heat exchanger is connected to the inlet of a three-way regulating control valve; one outlet of the three-way regulating control valve is connected to the hot-side inlet of the plate heat exchanger, and the hot-side outlet of the plate heat exchanger is connected back to the inlet of the biomimetic fractal coaxial shell-and-tube downhole heat exchanger; the cold side of the plate heat exchanger is cyclically connected to the user side; the other outlet of the three-way regulating control valve is connected to the inlet of the evaporator of the heat pump unit, and the outlet of the evaporator is connected back to the inlet of the biomimetic fractal coaxial shell-and-tube downhole heat exchanger; the condenser of the heat pump unit is cyclically connected to the user side.

[0009] The biomimetic fractal coaxial sleeve downhole heat exchanger is in the shape of a coaxial sleeve. The heat-insulating inner sleeve of the biomimetic fractal coaxial sleeve downhole heat exchanger is located in the inner cavity of the root-shaped biomimetic fractal outer sleeve. The annular cylindrical flow channel formed between the outer wall of the heat-insulating inner sleeve and the inner wall of the root-shaped biomimetic fractal outer sleeve constitutes the low-temperature working fluid downward pipeline. The low-temperature working fluid downward pipeline is used for the downward flow of low-temperature working fluid, and the inner cavity of the heat-insulating inner sleeve is used for the upward flow of high-temperature working fluid.

[0010] Multiple root-shaped bionic tubes are provided on the outer wall of the root-shaped bionic outer casing. The root-shaped bionic tubes extend from top to bottom. The lower end of the root-shaped bionic tubes is connected to the lower opening of the heat-insulating inner casing through the bottom circulation working fluid collection and communication cavity. A control valve is provided at the connection between the upper end of the root-shaped bionic tube and the root-shaped bionic fractal outer casing. The control valve is used to control the switching and flow rate of the low-temperature working fluid diverted to the lower section of the root-shaped bionic fractal outer casing and / or the root-shaped bionic tubes when it flows downward.

[0011] Preferably, the root-shaped bionic fractal outer jacket is provided with a first-level bionic fractal flow channel main pipe, a second-level bionic fractal flow channel main pipe, and a third-level bionic fractal flow channel main pipe in sequence from top to bottom. Each level of bionic fractal flow channel main pipe is evenly arranged around the root-shaped bionic fractal outer jacket, and the number of each level is two to six.

[0012] Even better, each stage of the biomimetic fractal flow channel main pipe is equipped with two biomimetic fractal flow channel branch pipes at the lower end. The lower end of the biomimetic fractal flow channel branch pipes is connected to the heat insulation inner sleeve. The biomimetic fractal flow channel main pipe is set in the medium-deep heat exchange section, and the biomimetic fractal flow channel branch pipes are set in the deep high-temperature section.

[0013] More preferably, the upper part of the medium-deep heat exchange section is a shallow insulation section, and the lower part of the medium-deep heat exchange section is a deep high-temperature section. The depth range of the shallow insulation section is 0-300m, the depth range of the medium-deep heat exchange section is 300-1200m, and the depth range of the deep high-temperature section is 1200-1500m. The shallow insulation section uses low thermal conductivity insulation cement as cementing material, while the medium-deep heat exchange section and the deep high-temperature section both use high thermal conductivity cement as cementing material.

[0014] Preferably, the control valve is a bellows-type three-way mechanical control valve, which includes a bellows, a baffle, and a circulating working fluid channel. The circulating working fluid channel is located inside the bellows. The baffle can move up and down under the action of the bellows to block or open the circulating working fluid channel. When the bellows is in a stretched state, the baffle opens the circulating working fluid channel or the biomimetic fractal flow channel main pipe. When the control valve is in a compressed state, the baffle closes the circulating working fluid channel or the biomimetic fractal flow channel main pipe.

[0015] Preferably, pumps and valves are respectively provided on the circulation pipeline of the biomimetic fractal coaxial sleeve-type downhole heat exchanger connected to the plate heat exchanger, the circulation pipeline of the biomimetic fractal coaxial sleeve-type downhole heat exchanger connected to the heat pump unit, the circulation pipeline of the plate heat exchanger connected to the user side, and the circulation pipeline of the heat pump unit connected to the user side.

[0016] The beneficial effects of this utility model are: 1. This utility model expands the heat exchange area from a single pipe wall to a networked branch structure by setting up a three-level biomimetic fractal flow channel, thereby increasing the heat exchange area; at the same time, through the layered branch density differential design, it achieves adaptation to the formation temperature gradient, avoiding the problems of shallow heat loss and insufficient deep heat extraction.

[0017] 2. The coaxial sleeve structure of this utility model, combined with the inner tube upward channel, reduces thermal short circuits between the inlet and outlet working fluids; the bellows-type three-way mechanical control valve's step-by-step opening mechanism solves the defects of uneven fluid flow and insufficient heat exchange in some areas.

[0018] 3. This utility model adopts a parallel three-mode switching design, which enables the heat pump to directly utilize the higher temperature of the geothermal energy, avoiding the energy waste of cooling down first and then heating up, and improving the energy efficiency ratio (COP) in the later stage of heating. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the overall structure of a biomimetic fractal flow channel enhanced heat exchange system for medium-deep geothermal wells according to this utility model. Figure 2 This is an enlarged schematic diagram and a working state diagram of the bellows-type three-way mechanical control valve of this utility model; Figure 3This is a schematic diagram showing the structural details of the bellows-type three-way mechanical control valve of this utility model. Figure 4 This is a top view of the biomimetic fractal coaxial sleeve downhole heat exchanger of this utility model.

[0020] The components include: 1. Downhole geological stratification; 2. Bionic fractal coaxial casing downhole heat exchanger; 3. Control valve; 4. Cementing material; 5. Downhole circulating working fluid return pipeline; 6. Ground source side circulation pump; 7. Three-way regulating control valve; 8. Plate heat exchanger side downhole circulating working fluid return pipeline; 9. Plate heat exchanger; 10. Plate heat exchanger side downhole circulating working fluid injection pipeline; 11. First electric control valve; 12. Injection pump; 3. Second electric control valve; 14. Plate heat exchanger side inlet pipe; 15. User side; 16. User side first circulation pump; 17. Plate heat exchanger side outlet pipe; 18. Third electric control valve; 19. Evaporator side downhole circulating working fluid return pipe; 20. Heat pump unit; 21. Evaporator side downhole circulating working fluid injection pipe; 22. Fourth electric control valve; 23. Condenser side inlet pipe; 24. User side second Circulating pump; 25. Condenser side outlet pipe; 26. Fifth electric control valve; 101. Shallow insulation section; 102. Medium-deep heat exchange section; 103. Deep high-temperature section; 201. Insulating inner sleeve; 202. Root-shaped biomimetic fractal outer sleeve; 203. Low-temperature working fluid downflow pipe; 204. First-stage biomimetic fractal flow channel main pipe; 205. First-stage biomimetic fractal flow channel branch pipe; 206. Second-stage biomimetic fractal flow channel Main pipe; 207, Second-stage biomimetic fractal flow channel branch pipe; 208, Third-stage biomimetic fractal flow channel main pipe; 209, Third-stage biomimetic fractal flow channel branch pipe; 210, Bottom-of-well circulating working fluid collection and connecting cavity; 211, High-temperature working fluid upward pipeline; 301, Corrugated pipe; 302, Baffle; 303, Circulating working fluid channel; 2001, Evaporator; 2002, Compressor; 2003, Condenser; 2004, Throttling mechanism. Detailed Implementation

[0021] The relevant technologies of this utility model will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only some embodiments of this utility model, and not all embodiments. Based on the embodiments of this utility model, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of this utility model.

[0022] like Figures 1-4As shown in this embodiment, a biomimetic fractal flow channel enhanced heat exchange system for medium-deep geothermal wells consists of two parts: a surface heating subsystem and a downhole heat exchange subsystem. The downhole subsystem performs heat exchange between the low-temperature circulating working fluid and the formation, while the surface subsystem performs the cascade utilization and distribution of heat from the high-temperature circulating working fluid after heat exchange.

[0023] The overall system structure is as follows Figure 1 As shown, the downhole heat exchange subsystem includes a biomimetic fractal coaxial sleeve downhole heat exchanger 2, which is coaxially fitted with an insulated inner sleeve 201 and a root-shaped biomimetic fractal outer sleeve 202. The internal space of the insulated inner sleeve 201 forms a high-temperature working fluid upward pipeline 211, and the annular space between the insulated inner sleeve 201 and the root-shaped biomimetic fractal outer sleeve 202 forms a low-temperature working fluid downward pipeline 203. The outer wall of the root-shaped biomimetic fractal outer sleeve 202 is filled with cementing material 4 between it and the well wall, and the well depth is divided into three functional sections according to the formation temperature gradient: a shallow insulation section 101, with a depth of 0–300 m; a medium-deep heat exchange section 102, with a depth of 300–1200 m; and a deep high-temperature section 103, with a depth of 1200–1500 m. On the outside of the root-shaped biomimetic fractal outer casing 202 in the shallow insulation section 101, the cementing material 4 uses low thermal conductivity insulation cement; on the outside of the root-shaped biomimetic fractal outer casing 202 in the medium-deep heat exchange section 102 and the deep high-temperature section 103, the cementing material 4 uses high thermal conductivity cement mixed with high thermal conductivity fillers such as graphite.

[0024] Along the cryogenic working fluid downflow pipe 203, from top to bottom, are sequentially arranged a first-stage biomimetic fractal flow channel main pipe 204, a second-stage biomimetic fractal flow channel main pipe 206, and a third-stage biomimetic fractal flow channel main pipe 208. Four of each stage of the biomimetic fractal flow channel main pipe are evenly arranged circumferentially, such as... Figure 4 As shown in the top view, each main pipe is spaced 90° apart. The shallow insulation section 101 does not have biomimetic fractal flow channels; the biomimetic fractal flow channel main pipes are arranged in the medium-deep heat exchange section 102, and the biomimetic fractal flow channel branch pipes are arranged in the medium-deep heat exchange section 102 and the high-temperature heat exchange section 103, forming a denser heat exchange network. Furthermore, each main pipe is connected to two branch pipes, forming a root-like distribution. The circulating working fluid at the end outlet of all biomimetic fractal flow channel branch pipes is collected in the bottom-hole circulating working fluid collection and connecting cavity 210. Finally, it returns to the surface via the high-temperature working fluid upward pipeline 211.

[0025] At each connection point between the biomimetic fractal flow channel main pipe and the cryogenic working fluid downflow pipe 203, a bellows-type three-way mechanical control valve 3 is installed. For example... Figure 2 and Figure 3As shown, the control valve includes a bellows 301, a baffle 302, and a circulating working fluid channel 303. The bellows 301 is mechanically controlled to be in a stretched or compressed state: when stretched, the baffle 302 separates, forming a passage; when compressed, the baffle 302 presses down, eliminating the passage. Furthermore, by controlling the working state of the bellows 301, the circulating working fluid channel 303 can be switched between three working states: In working state one, the cryogenic circulating working fluid only enters the biomimetic fractal flow channel main pipe and does not enter the downstream section of the cryogenic working fluid downflow pipe 203; at this time, the bellows 301 in control valve 3 that communicates with the upstream section of the cryogenic working fluid downflow pipe 203 and the biomimetic fractal flow channel main pipe is in a stretched state, while the bellows 301 that communicates with the downstream section of the cryogenic working fluid downflow pipe 203 is in a compressed state. In operating state two, the cryogenic circulating working fluid simultaneously enters the downstream section of both the biomimetic fractal flow channel main pipe and the cryogenic working fluid downflow pipe 203; at this time, all three bellows 301 in control valve 3 are in a stretched state. In operating state three, the cryogenic circulating working fluid only enters the downstream section of the cryogenic working fluid downflow pipe 203, and does not enter the biomimetic fractal flow channel branch pipe; at this time, the bellows 301 in control valve 3 connected to the upstream and downstream sections of the cryogenic working fluid downflow pipe 203 are in a stretched state, while the bellows 301 connected to the biomimetic fractal flow channel main pipe is in a compressed state. This valve achieves flow distribution through three operating states: operating state one forces all flow to preferentially flow into the deep branches, eliminating flow short-circuiting; operating state two allows continuous adjustment of the flow split ratio to achieve graded quantitative flow distribution; operating state three allows temporary closure of branches that have completed heat exchange or have lower temperatures (such as closing shallow branches in the later stages of heating), forcibly distributing all flow to other branches that still have heat exchange potential. The three elements work together to solve the inherent defects of traditional coaxial tube heat exchangers, namely "shallow overheating and deep underflow".

[0026] The surface heating subsystem includes: 5. Downhole circulating working fluid return pipeline, 6. Ground source side circulating pump, 7. Three-way regulating control valve, 8. Plate heat exchanger side downhole circulating working fluid return pipeline, 9. Plate heat exchanger, 10. Plate heat exchanger side downhole circulating working fluid injection pipeline, 11. First electric control valve, 12. Injection pump, 13. Second electric control valve, 14. Plate heat exchanger side inlet pipeline, 15. User side, 16. User side first circulating pump, 17. Plate heat exchanger side outlet pipeline, 18. Third electric control valve, 19. Evaporator side downhole circulating working fluid return pipeline, 20. Heat pump unit, 2001. Evaporator, 2002. Compressor, 2003. Condenser, 2004. Throttling mechanism, 21. Evaporator side downhole circulating working fluid injection pipeline, 22. Fourth electric control valve, 23. Condenser side inlet pipeline, 24. User side second circulating pump, 25. Condenser side outlet pipeline, 26. Fifth electric control valve. The heat pump unit 20 is a vapor compression heat pump, which consists of an evaporator 2001, a compressor 2002, a condenser 2003, and a throttling mechanism 2004 connected in a closed loop.

[0027] The system workflow is as follows: Downhole heat exchange process: The cryogenic circulating working fluid from the primary circulation (geosource side) on the surface enters the inlet of the biomimetic fractal coaxial casing downhole heat exchanger 2 through the injection pump 12, and then enters the cryogenic working fluid downflow pipeline 203. Initially, all control valves 3 are in working state three (i.e., only supplying downstream, not entering the biomimetic fractal flow channel branch pipe).

[0028] The cryogenic circulating working fluid passes through the shallow insulation section 101 along the cryogenic working fluid downflow pipe 203. Since the outer side of the root-shaped biomimetic fractal outer sleeve 202 of this section is filled with insulating cement (low thermal conductivity) and no biomimetic fractal flow channel branch pipe is set in this section, the cryogenic circulating working fluid only flows downward along the downflow pipe.

[0029] When the cryogenic circulating working fluid reaches the node between the first-stage biomimetic fractal flow channel main pipe 204 and the cryogenic working fluid downstream pipe 203 in the mid-deep heat exchange section 102, control valve 3 switches to operating state one. First, switching the valve to operating state one allows all the cryogenic circulating working fluid to enter the first-stage biomimetic fractal flow channel main pipe 204 and the first-stage biomimetic fractal flow channel branch pipe 205, thus temporarily closing the downstream main channel. The cryogenic circulating working fluid flows in a root-like manner in the branch pipe, fully exchanging heat with the surrounding rock strata. After a stable flow is formed within this branch, the valve is then switched to operating state two, allowing some of the cryogenic circulating working fluid to continue flowing downstream, while some enters the branch pipe for heat exchange.

[0030] Repeat the above operation, sequentially opening the control valves at the node between the second-stage biomimetic fractal flow channel main pipe 206 and the cryogenic working fluid downflow pipe 203, and at the node between the third-stage biomimetic fractal flow channel main pipe 208 and the cryogenic working fluid downflow pipe 203. The opening sequence strictly follows the principle of top-down and from sparse to dense, that is, first fully open the second-stage branch, and after it stabilizes, open the third-stage branch, thereby ensuring that the cryogenic circulating working fluid preferentially enters the deep high-temperature region for heat exchange and avoids flow short-circuiting.

[0031] All the high-temperature circulating working fluid in the biomimetic fractal flow channel branch pipes eventually converges into the bottom-hole circulating working fluid collection and connecting cavity 210, and then enters the high-temperature working fluid upward pipeline 211 inside the thermally insulated inner sleeve 201. Because the thermally insulated inner sleeve 201 uses a low thermal conductivity material, the heat loss of the high-temperature circulating working fluid during its ascent is minimal. The high-temperature circulating working fluid returns to the surface via the downhole circulating working fluid return pipeline 5, powered by the ground-source side circulation pump 6.

[0032] Above-ground heating process: The high-temperature circulating working fluid returning to the surface enters the surface heating subsystem. After passing through the three-way regulating control valve 7, it splits into two paths: one path enters the heat source side of the plate heat exchanger 9 via the downhole circulating working fluid return pipeline 8 on the plate heat exchanger side; the other path enters the evaporator 2001 of the heat pump unit 20 via the downhole circulating working fluid return pipeline 19 on the evaporator side. The system switches operating modes according to the geothermal well temperature (i.e., the high-temperature circulating working fluid temperature), as follows: Mode 1: High-Temperature Condition (T≥45℃). The first electric control valve 11 is open, the second electric control valve 13 is open, the third electric control valve 18 is open, the fourth electric control valve 22 is closed, and the fifth electric control valve 26 is closed. All the high-temperature circulating working fluid enters the heat source side of the plate heat exchanger 9, where it exchanges heat with the user-side return water. The user-side working fluid, driven by the user-side first circulation pump 16, enters the plate heat exchanger 9 through the plate heat exchanger-side outlet pipe 17, absorbs heat, and then rises in temperature before being sent to the user-side 15 for heating via the plate heat exchanger-side inlet pipe 14. After heat exchange, the temperature of the downhole circulating working fluid decreases, and it is sent back downhole via the plate heat exchanger-side downhole circulating working fluid injection pipe 10 and injection pump 12 to absorb heat again. The heat pump unit 20 shuts down in this mode and does not consume electricity.

[0033] Mode 2: Medium Temperature Condition (35℃≤T<45℃). The first electric control valve 11 is open, the second electric control valve 13 is open, the third electric control valve 14 is open, the fourth electric control valve 22 is open, and the fifth electric control valve 26 is open. The high-temperature circulating working fluid is simultaneously distributed to the plate heat exchanger 9 and the evaporator 2001 of the heat pump unit 20 via the three-way regulating control valve 7 in a certain proportion. The portion entering the plate heat exchanger 9 directly exchanges heat to produce hot water for supply to the user side 15; the portion entering the evaporator 2001 transfers heat to the refrigerant inside the heat pump. After absorbing heat and evaporating, the refrigerant is compressed into a high-temperature, high-pressure gas by the compressor 2002, then enters the condenser 2003 to release heat, increasing the heat in the circulating working fluid, and is then delivered to the user side 14 via the condenser-side inlet pipe 23. The two stages of heat converge at the user side 14 to jointly meet the heating demand. After heat exchange, the circulating working fluid (from the plate heat exchanger and evaporator) decreases in temperature and is returned to the well via the plate heat exchanger side downhole circulating working fluid injection pipeline 10 and the evaporator side downhole circulating working fluid injection pipeline 21, respectively. In this mode, the heat pump unit 20 operates at partial load.

[0034] Mode 3: Low Temperature Condition (T<35℃). The first electric control valve 11 is closed, the second electric control valve 13 is closed, the third electric control valve 18 is closed, the fourth electric control valve 22 is open, and the fifth electric control valve 26 is open. All the high-temperature circulating refrigerant enters the evaporator 2001 of the heat pump unit 20. The heat pump unit 20 operates at full load. The evaporator 2001 extracts low-grade heat from the circulating refrigerant, which is then boosted by the compressor 2002 and used to generate heat by the condenser 2003. The heat is then delivered to the user side 15 via the condenser-side inlet pipe 23. After heat exchange, the temperature of the circulating refrigerant decreases, and it is returned to the well via the evaporator-side downhole circulating refrigerant injection pipe 21. In this mode, the heat pump unit 20 can still meet heating needs.

[0035] This implementation significantly increases the heat exchange area: by setting up a biomimetic fractal flow channel, especially a denser branching network in the deep section, the heat exchange area is increased compared to traditional coaxial sleeves, significantly enhancing heat extraction capacity. This implementation allows for layered optimization to adapt to formation temperature gradients: the design of shallow insulation, sparse branching in the middle layer, and dense branching in the deep layer ensures that the heat exchanger matches the formation temperature distribution, reducing ineffective heat loss and concentrating the extraction of high-temperature heat from deep layers. This implementation eliminates flow short-circuiting and ensures uniform heat exchange: using a bellows-type three-way mechanical control valve to open each level of branch sequentially and as needed avoids the problem of the low-temperature circulating working fluid preferentially "taking a shortcut" back to the surface from the shallow branches, ensuring sufficient flow to the deep, high-temperature branches. This implementation's ground system is highly energy-efficient and flexible in operation: adopting a parallel plate heat exchanger + heat pump system, it automatically switches between three modes according to the geothermal temperature, avoiding the use of the heat pump at high temperatures (saving electricity) and relying on the heat pump for heating at low temperatures, thus improving the overall coefficient of performance (COP).

[0036] The three-level hierarchical structure and branch density differentiation of the biomimetic fractal flow channel in this embodiment are as follows: A three-stage biomimetic fractal flow channel main pipe is installed from top to bottom within the low-temperature working medium downflow pipeline 203. Each stage of the main pipe has four branches evenly distributed around its circumference, and each stage of the main pipe is connected to branching branches, forming a root-like network with no branches in the shallow layer, sparse branches in the middle layer, and dense branches in the deep layer. This structure increases the heat exchange area compared to traditional coaxial sleeves and matches the formation temperature gradient, achieving tiered heat extraction of "shallow layer insulation, middle layer heat exchange, and deep layer strong heat".

[0037] The bellows-type three-way mechanical control valve of this embodiment and its three working states: This valve is installed at the connection node between the main biomimetic fractal flow channel and the downstream pipeline at each stage. The valve's three states are switched by the stretching or compression of the bellows 301: State 1 (entering only the fractal flow channel), State 2 (entering both the fractal flow channel and the downstream side simultaneously), and State 3 (entering only the downstream side). This valve achieves three major functions: forced priority injection into the deep branch, continuous adjustment of the flow ratio, and temporary closure of the low-temperature branch. It completely solves the flow short-circuit problem of "shallow overheating and deep underflow" in traditional heat exchangers and can reduce pumping energy consumption.

[0038] This embodiment features a parallel three-mode switching design for the above-ground heating subsystem: The high-temperature circulating working fluid is divided into two paths via a three-way regulating control valve 7: one path goes to the plate heat exchanger 9, and the other path goes to the evaporator 2001 of the heat pump unit 20. Based on the geothermal well outlet temperature (T), three operating conditions are automatically switched: high-temperature condition (T≥45℃) with only the plate heat exchanger providing heating; medium-temperature condition (35℃≤T<45℃) with both the plate heat exchanger and the heat pump providing heating; and low-temperature condition (T<35℃) with only the heat pump providing heating. This parallel design allows the heat pump to directly utilize the higher-temperature working fluid, avoiding the energy waste of the series mode and significantly improving the system's energy efficiency in the later stages of heating.

[0039] This implementation method defines the downhole layered cementing materials and functional section divisions: The well depth was divided into a shallow insulation section 101 (0–300 m), a medium-deep heat exchange section 102 (300–1200 m), and a deep high-temperature section 103 (1200–1500 m). Low-thermal-conductivity insulation cement and high-thermal-conductivity cement were used for cementing in these sections, respectively. This design reduces shallow heat loss while enhancing deep heat exchange, and, in conjunction with biomimetic fractal flow channels, optimizes thermal efficiency throughout the well.

[0040] In this embodiment, the structural fit between the high-temperature working fluid upward pipeline and the bottom-of-well connecting cavity is as follows: The downhole heat exchanger employs a coaxial casing structure, with an annular downward channel formed between the outer and inner casings, and an upward channel formed within the inner casing's cavity. All outlets of the biomimetic fractal branch channels converge at the bottom-hole connecting cavity, then enter the inner casing's upward channel to return to the surface. This structure ensures that the high-temperature working fluid after heat exchange can return to the surface independently and with insulation. Simultaneously, the inner tube can be made of low thermal conductivity materials or have an added insulation layer to further reduce heat loss during the upward flow.

[0041] In summary, this invention, by setting up graded and differentiated biomimetic fractal flow channels in the downhole heat exchanger, combined with the flow regulation of the corrugated pipe three-way mechanical control valve, and integrating a plate heat exchanger + heat pump parallel heating subsystem with switchable operating modes on the surface, significantly increases the heat exchange area and heat extraction efficiency of medium-deep geothermal wells. It solves the flow short-circuit problem of "overheating in shallow layers and underflow in deep layers" in traditional coaxial casing heat exchangers, and can flexibly adjust the operating mode according to the geothermal outlet temperature. While ensuring heating demand, it effectively reduces system energy consumption, improves the overall energy efficiency ratio, and adapts to the development needs of medium-deep geothermal resources with different temperature gradients, thus possessing stronger applicability and practicality. It should be emphasized that the above are merely preferred embodiments of the present utility model and are not intended to limit the present utility model in any way. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present utility model shall still fall within the scope of the technical solution of the present utility model.

Claims

1. A biomimetic fractal flow channel enhanced heat transfer system for medium-deep geothermal wells, characterized in that, include: A biomimetic fractal coaxial shell-and-tube downhole heat exchanger (2), a plate heat exchanger (9), and a heat pump unit (20) are provided. The outlet of the biomimetic fractal coaxial shell-and-tube downhole heat exchanger (2) is connected to the inlet of a three-way regulating control valve (7). One outlet of the three-way regulating control valve (7) is connected to the hot side inlet of the plate heat exchanger (9), and the hot side outlet of the plate heat exchanger (9) is connected back to the inlet of the biomimetic fractal coaxial shell-and-tube downhole heat exchanger (2). The cold side of the plate heat exchanger (9) is cyclically connected to the user side (15). The other outlet of the three-way regulating control valve (7) is connected to the inlet of the evaporator (2001) of the heat pump unit (20), and the outlet of the evaporator (2001) is connected back to the inlet of the biomimetic fractal coaxial shell-and-tube downhole heat exchanger (2). The condenser (2003) of the heat pump unit (20) is cyclically connected to the user side (15). The biomimetic fractal coaxial sleeve downhole heat exchanger (2) is in the shape of a coaxial sleeve. The heat-insulating inner sleeve (201) of the biomimetic fractal coaxial sleeve downhole heat exchanger (2) is located in the inner cavity of the root-shaped biomimetic fractal outer sleeve (202). The annular cylindrical flow channel formed between the outer wall of the heat-insulating inner sleeve (201) and the inner wall of the root-shaped biomimetic fractal outer sleeve (202) constitutes a low-temperature working fluid downward pipe (203). The low-temperature working fluid downward pipe (203) is used for the downward flow of low-temperature working fluid, and the inner cavity of the heat-insulating inner sleeve (201) is used for the upward flow of high-temperature working fluid. The outer wall of the root-shaped bionic fractal outer sleeve (202) is provided with multiple root-shaped bionic tubes. The root-shaped bionic tubes extend from top to bottom. The lower end of the root-shaped bionic tubes is connected to the lower opening of the heat-insulating inner sleeve (201) through the bottom circulation working fluid collection and communication cavity (210). A control valve (3) is provided at the connection between the upper end of the root-shaped bionic tube and the root-shaped bionic fractal outer sleeve (202). The control valve (3) is used to control the switching and flow rate of the low-temperature working fluid diverted to the lower section of the root-shaped bionic fractal outer sleeve (202) and / or the root-shaped bionic tube when it flows downward.

2. The biomimetic fractal flow channel enhanced heat transfer system for medium-deep geothermal wells according to claim 1, characterized in that, The root-shaped bionic fractal outer tube (202) is provided with a first-level bionic fractal flow channel main tube (204), a second-level bionic fractal flow channel main tube (206), and a third-level bionic fractal flow channel main tube (208) in sequence from top to bottom. Each level of bionic fractal flow channel main tube is evenly arranged around the root-shaped bionic fractal outer tube (202), and the number of each level is two to six.

3. The biomimetic fractal flow channel enhanced heat transfer system for medium-deep geothermal wells according to claim 2, characterized in that, Each stage of the biomimetic fractal flow channel main pipe is provided with two biomimetic fractal flow channel branch pipes at the lower end. The lower end of the biomimetic fractal flow channel branch pipe is connected to the heat insulation inner sleeve (201). The biomimetic fractal flow channel main pipe is set in the medium-deep heat exchange section (102), and the biomimetic fractal flow channel branch pipe is set in the deep high-temperature section (103).

4. The biomimetic fractal flow channel enhanced heat transfer system for medium-deep geothermal wells according to claim 3, characterized in that, Above the medium-deep heat exchange section (102) is the shallow insulation section (101), and below the medium-deep heat exchange section (102) is the deep high-temperature section (103). The depth range of the shallow insulation section (101) is 0-300m, the depth range of the medium-deep heat exchange section (102) is 300-1200m, and the depth range of the deep high-temperature section (103) is 1200-1500m. The shallow insulation section (101) uses low thermal conductivity insulation cement as cementing material, while the medium-deep heat exchange section (102) and the deep high-temperature section (103) both use high thermal conductivity cement as cementing material.

5. A biomimetic fractal flow channel enhanced heat transfer system for medium-deep geothermal wells according to claim 1, characterized in that, The control valve (3) is a bellows-type three-way mechanical control valve. The control valve (3) includes: bellows (301), baffle (302), and circulating working fluid channel (303). The circulating working fluid channel (303) is located in the inner cavity of the bellows (301). The baffle (302) can move in the up and down direction under the action of the bellows (301) to block or open the circulating working fluid channel (303). When the bellows (301) is in a stretched state, the baffle (302) opens the circulating working medium channel (303) or the biomimetic fractal flow channel main pipe. When the bellows (301) is in a compressed state, the baffle (302) closes the circulating working medium channel (303) or the biomimetic fractal flow channel main pipe.

6. A biomimetic fractal flow channel enhanced heat transfer system for medium-deep geothermal wells according to claim 1, characterized in that, Pumps and valves are respectively provided on the circulation pipeline of the biomimetic fractal coaxial casing downhole heat exchanger (2) circulating to the plate heat exchanger (9), the circulation pipeline of the biomimetic fractal coaxial casing downhole heat exchanger (2) circulating to the heat pump unit (20), the circulation pipeline of the plate heat exchanger (9) circulating to the user side (15), and the circulation pipeline of the heat pump unit (20) circulating to the user side (15).

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