An oil return system and operation method of a single-cycle middle-deep geothermal heat pump

Abstract

The application provides a single-cycle middle-deep geothermal heat pump oil return system and an operation method. The system comprises a downhole heat exchanger, a regenerator, a compressor, an oil separator, a gas cooler, an auxiliary evaporator and an oil return pipeline. The outlet of the downhole heat exchanger is connected with the first side inlet of the regenerator, and the first side outlet of the regenerator is connected with the inlet of the compressor. The outlet of the compressor is connected with the working medium inlet of the oil separator, and the working medium outlet of the oil separator is connected with the hot side inlet of the gas cooler. The low-pressure channel outlet of the auxiliary evaporator is connected with the second side inlet of the regenerator, and the second side outlet of the regenerator is connected with the inlet of the downhole heat exchanger. One technical effect of the application is that the oil separator is combined with a two-stage evaporation process, so that carbon dioxide and lubricating oil are fully separated in the ground system, and the separated lubricating oil is introduced back to the compressor through the oil return pipeline, thereby ensuring safe and stable operation of the compressor.

Description

Technical Field

[0001] This invention belongs to the field of new energy technology, specifically relating to an oil return system and operation method of a single-cycle deep ground source heat pump. Background Technology

[0002] Currently, the development and utilization of medium-deep geothermal energy are mainly divided into open-loop injection and closed-loop extraction. Closed-loop extraction is more environmentally friendly, but compared to injection, it has a lower outlet temperature and lower heat extraction power. Heating is the primary use of geothermal energy; therefore, given the low outlet temperature of closed-loop extraction, coupled heat pump heating is a common and necessary technology, namely, medium-deep ground source heat pumps.

[0003] However, existing medium-deep ground source heat pump systems mostly employ a dual-cycle (dual-working-fluid) approach for both heat extraction and consumption. The heat extraction cycle has high pumping power consumption, and the heat exchange between the two working fluids occurs within the heat pump evaporator, resulting in intermediate heat exchange losses, indicating significant potential for optimization. Using carbon dioxide as the working fluid for both the downhole heat exchange and surface heat pump systems, and simultaneously utilizing the downhole heat exchanger as the heat pump's "evaporator," can effectively solve these problems by constructing an integrated downhole and surface heat pump heating system. However, this also introduces new challenges: when carbon dioxide leaves the compressor, it carries some lubricating oil, which deposits in the downhole heat exchanger, leading to difficulties in oil return. This deteriorates heat transfer in the downhole heat exchanger and increases the risk of corrosion; furthermore, it can cause compressor oil shortages and malfunctions.

[0004] Therefore, there is an urgent need for a single-cycle deep ground source heat pump oil return system and operation method to solve the above-mentioned technical problems. Summary of the Invention

[0005] The present invention aims to at least solve one of the technical problems existing in the prior art, and to provide a new technical solution for the oil return system and operation method of a single-cycle deep ground source heat pump.

[0006] According to a first aspect of the present invention, a return oil system for a single-cycle deep ground source heat pump is provided, comprising: The downhole heat exchanger, the regenerator, and the compressor are provided, wherein the outlet of the downhole heat exchanger is connected to the first side inlet of the regenerator, and the first side outlet of the regenerator is connected to the inlet of the compressor. An oil separator and a gas cooler are provided, wherein the outlet of the compressor is connected to the working fluid inlet of the oil separator, and the working fluid outlet of the oil separator is connected to the hot side inlet of the gas cooler; An auxiliary evaporator is provided, which has a high-pressure channel and a low-pressure channel. The hot side outlet of the gas cooler is connected to the high-pressure channel inlet of the auxiliary evaporator. The high-pressure channel outlet of the auxiliary evaporator is connected to the low-pressure channel inlet of the auxiliary evaporator. The low-pressure channel outlet of the auxiliary evaporator is connected to the second side inlet of the regenerator. The second side outlet of the regenerator is connected to the inlet of the downhole heat exchanger. The auxiliary evaporator, the gas cooler, and the oil separator are connected in sequence through the oil return line and then connected to the lubricating oil inlet of the compressor.

[0007] Optionally, the downhole heat exchanger includes an outer tube and an inner tube. The outer tube is sleeved outside the inner tube and coaxially arranged with the inner tube. The lower end of the outer tube is connected to the lower end of the inner tube, and an annular channel is formed between the outer tube and the inner tube. The carbon dioxide working fluid flows in from the upper inlet of the outer tube, flows downward through the outer tube to the bottom, then turns into the inner tube and flows upward, exiting from the upper outlet of the inner tube. The upper outlet of the inner tube is connected to the first side inlet of the regenerator.

[0008] Optionally, the oil return system of this single-cycle deep ground source heat pump also includes a first-stage throttling valve; The inlet of the primary throttling valve is connected to the high-pressure channel outlet of the auxiliary evaporator, and the outlet of the primary throttling valve is connected to the low-pressure channel inlet of the auxiliary evaporator.

[0009] Optionally, the oil return system of this single-cycle deep ground source heat pump also includes a secondary throttling valve; The inlet of the secondary throttle valve is connected to the second-side outlet of the regenerator, and the outlet of the secondary throttle valve is connected to the inlet of the downhole heat exchanger.

[0010] Optionally, the oil separator is provided with a lubricating oil outlet, which is connected to the lubricating oil inlet of the compressor through the oil return pipeline, for guiding the separated lubricating oil back to the compressor.

[0011] Optionally, the gas cooler is provided with a lubricating oil collection port at the rear end, which is connected to the return oil pipeline to collect the lubricating oil that has been adsorbed onto the pipe wall of the rear end of the gas cooler due to the decrease in temperature and the increase in viscosity.

[0012] Optionally, the low-pressure channel of the auxiliary evaporator is provided with a lubricating oil outlet at the end, which is connected to the oil return pipeline to discharge the lubricating oil that has been precipitated and accumulated at the end of the auxiliary evaporator due to the evaporation of carbon dioxide working fluid.

[0013] According to a second aspect of the present invention, an operation method for the oil return system of a single-cycle medium-deep ground source heat pump is provided, applicable to the oil return system of the single-cycle medium-deep ground source heat pump as described in the first aspect, comprising the following steps: Step S1: After absorbing geothermal energy in the downhole heat exchanger, the carbon dioxide working fluid flows out to the surface through the inner pipe of the downhole heat exchanger. In step S2, the outflowing carbon dioxide working fluid enters the first side of the regenerator to absorb heat and increase its temperature, and then enters the compressor to increase its pressure. In step S3, the pressurized carbon dioxide working fluid undergoes oil-gas separation through the oil separator, separating out most of the lubricating oil carried on it. Step S4: The separated high-temperature and high-pressure carbon dioxide working fluid sequentially enters the high-pressure channels of the gas cooler and the auxiliary evaporator to release heat. In step S5, the exothermic carbon dioxide working fluid is depressurized through a first-stage throttling valve and then enters the low-pressure channel of the auxiliary evaporator to absorb heat and evaporate. Step S6: After the evaporated carbon dioxide working fluid releases heat through the second side of the regenerator, it is further depressurized and cooled by the secondary throttle valve and enters the outer tube of the downhole heat exchanger to complete the circulation. Step S7: During the above cycle, the lubricating oil is separated and the lubricating oil deposited in the auxiliary evaporator, the gas cooler and the oil separator is returned to the compressor through the oil return line.

[0014] Optionally, in step S7, the oil return pipeline is sequentially connected to the lubricating oil outlet of the auxiliary evaporator, the lubricating oil collection port of the gas cooler, the lubricating oil outlet of the oil separator, and the lubricating oil inlet of the compressor, and the lubricating oil is driven to return to the compressor along the oil return pipeline by utilizing the pressure difference between the various devices.

[0015] Optionally, the separation of lubricating oil includes a three-stage separation process: In the first stage of separation, most of the lubricating oil carried by the compressor outlet working fluid is separated by mechanical separation in the oil separator. In the second-stage separation process, at the rear section of the gas cooler, the lubricating oil viscosity increases due to the decrease in temperature, and the lubricating oil is adsorbed onto the rear section pipe wall of the gas cooler and collected. In the third stage of separation, as the carbon dioxide working medium evaporates in the low-pressure channel in the auxiliary evaporator, the lubricating oil dissolved in the carbon dioxide working medium gradually precipitates out and accumulates at the end of the auxiliary evaporator.

[0016] One technical advantage of this invention is that: In this embodiment, the oil return system of the single-cycle deep ground source heat pump simultaneously applies carbon dioxide as the working fluid to both downhole heat exchange and surface heat pump circulation, constructing a single-working-fluid integrated heat pump system. This eliminates the intermediate heat exchange link between the water and organic working fluids in the evaporator, as is common in traditional dual-cycle systems, thus eliminating temperature difference losses during intermediate heat exchange. Furthermore, addressing the problem of compressor lubricating oil being carried to the downhole heat exchanger in single-working-fluid circulation, this invention incorporates an oil separator at the compressor outlet for preliminary oil-gas separation and innovatively includes an auxiliary evaporator with high-pressure and low-pressure channels: the high-pressure channel further cools and releases heat from the high-temperature, high-pressure working fluid at the compressor outlet, while the low-pressure channel allows the depressurized working fluid to absorb heat and evaporate. The auxiliary evaporator, while achieving internal heat exchange to improve circulation efficiency, also utilizes the evaporation of the working fluid to precipitate the lubricating oil dissolved in it, becoming a crucial step in lubricating oil separation. In addition, the oil return pipeline connects the three lubricating oil collection points—the auxiliary evaporator, the gas cooler, and the oil separator—in series to the compressor, forming a complete oil return path and fundamentally solving the oil return problem in a single-working-refrigerant circulation system. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the oil return system of a single-cycle deep ground source heat pump according to an embodiment of the present invention.

[0018] In the diagram: 1. Underground heat exchanger; 11. Outer pipe; 12. Inner pipe; 2. Secondary throttling valve; 3. Regenerator; 4. Compressor; 5. Electric motor; 6. Primary throttling valve; 7. Auxiliary evaporator; 8. Gas cooler; 9. Oil separator.

[0019] The solid line represents the working fluid circulation route, and the dashed line represents the lubricating oil return route. Detailed Implementation

[0020] Various exemplary embodiments of this application will now be described in detail with reference to the accompanying drawings. It should be noted that, unless otherwise specifically stated, the relative arrangement, numerical expressions, and values ​​of the components and steps set forth in these embodiments do not limit the scope of this application.

[0021] The embodiments of this application will now be described in detail. Examples of these embodiments are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

[0022] The terms "first" and "second" in the specification and claims of this application may explicitly or implicitly include one or more of the features. In the description of this application, unless otherwise stated, "multiple" means two or more. Furthermore, "and / or" in the specification and claims indicates at least one of the connected objects, and the character " / " generally indicates that the preceding and following objects are in an "or" relationship.

[0023] In the description of this application, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc., indicating the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application.

[0024] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection between two components. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0025] According to a first aspect of the invention, such as Figure 1 As shown, a return oil system for a single-cycle deep ground source heat pump is provided, comprising: The well includes a downhole heat exchanger 1, a regenerator 3, and a compressor 4. The outlet of the downhole heat exchanger is connected to the first side inlet of the regenerator 3, and the first side outlet of the regenerator 3 is connected to the inlet of the compressor 4. The compressor 4 is driven by an electric motor 5.

[0026] The compressor 4 has an oil separator 9 and a gas cooler 8. The outlet of the compressor 4 is connected to the working fluid inlet of the oil separator 9, and the working fluid outlet of the oil separator 9 is connected to the hot side inlet of the gas cooler 8. An auxiliary evaporator 7 is provided with a high-pressure channel and a low-pressure channel. The hot-side outlet of the gas cooler 8 is connected to the high-pressure channel inlet of the auxiliary evaporator 7. The high-pressure channel outlet of the auxiliary evaporator 7 is connected to the low-pressure channel inlet of the auxiliary evaporator 7. The low-pressure channel outlet of the auxiliary evaporator 7 is connected to the second-side inlet of the regenerator 3. The second-side outlet of the regenerator 3 is connected to the inlet of the downhole heat exchanger. The auxiliary evaporator 7, the gas cooler 8, and the oil separator 9 are connected in sequence through the oil return pipeline and then connected to the lubricating oil inlet of the compressor 4.

[0027] In this embodiment, the oil return system of the single-cycle deep ground source heat pump simultaneously applies carbon dioxide as the working fluid to downhole heat exchange and surface heat pump circulation, constructing a single-working-fluid integrated heat pump system. This eliminates the intermediate heat exchange link between the water and organic working fluids in the evaporator in the traditional dual-cycle system, thus eliminating intermediate heat exchange temperature difference losses. Simultaneously, addressing the problem of compressor lubricating oil being carried to the downhole heat exchanger by the single-working-fluid circulation, this invention sets up an oil separator 9 at the compressor 4 outlet for preliminary oil-gas separation and innovatively sets up an auxiliary evaporator 7 with high-pressure and low-pressure channels: the high-pressure channel further cools and releases heat from the high-temperature, high-pressure working fluid at the compressor 4 outlet, while the low-pressure channel allows the depressurized working fluid to absorb heat and evaporate. The auxiliary evaporator 7, while achieving internal heat exchange to improve circulation efficiency, utilizes the evaporation of the working fluid to precipitate the lubricating oil dissolved in it, becoming a key step in lubricating oil separation. In addition, the oil return pipeline connects the three lubricating oil collection points, namely the auxiliary evaporator 7, the gas cooler 8, and the oil separator 9, in series to the compressor 4, forming a complete oil return path, which fundamentally solves the oil return problem in the single working fluid circulation system.

[0028] Optionally, the downhole heat exchanger includes an outer tube 11 and an inner tube 12. The outer tube 11 is sleeved on the outside of the inner tube 12 and coaxially arranged with the inner tube 12. The lower end of the outer tube 11 is connected to the lower end of the inner tube 12, and an annular channel is formed between the outer tube 11 and the inner tube 12. The carbon dioxide working fluid flows in from the upper inlet of the outer tube 11, flows downward through the outer tube 11 to the bottom, turns into the inner tube 12 and flows upward, and flows out from the upper outlet of the inner tube 12. The upper outlet of the inner tube 12 is connected to the first side inlet of the regenerator.

[0029] In the above embodiment, the coaxial casing structure composed of the outer tube 11 and the inner tube 12 forms a regular annular flow channel. As the carbon dioxide working fluid flows downwards along the annular channel of the outer tube 11, it exchanges heat with the formation, absorbs geothermal energy, and then reaches the bottom before turning upwards into the inner tube 12 to return to the surface. During the downward flow of the working fluid in the outer tube 11, the temperature gradually increases, maintaining a reasonable heat exchange temperature difference with the formation. The working fluid returning upwards in the inner tube 12 experiences minimal heat loss due to its insulating effect, ensuring it still has a high enthalpy value upon reaching the surface. An insulation layer can be provided on the outer wall of the inner tube 12 to further reduce thermal short-circuit losses between the rising working fluid in the inner tube 12 and the descending working fluid in the outer tube 11. This coaxial casing structure exhibits good pressure-bearing capacity and heat transfer performance under high temperature and high pressure conditions in medium-deep wells.

[0030] For example, the heating return water enters the gas cooler 8 through the cold side inlet and flows out through the cold side outlet of the gas cooler 8 to form heating supply water, thereby achieving heating of the heating return water at the gas cooler 8.

[0031] Optionally, the oil return system of this single-cycle deep ground source heat pump also includes a first-stage throttling valve 6; The inlet of the first-stage throttling valve 6 is connected to the high-pressure channel outlet of the auxiliary evaporator 7, and the outlet of the first-stage throttling valve 6 is connected to the low-pressure channel inlet of the auxiliary evaporator 7.

[0032] In the above embodiment, the first-stage throttling valve 6 is located between the high-pressure channel outlet and the low-pressure channel inlet of the auxiliary evaporator 7. It depressurizes the carbon dioxide working fluid, which has released heat and cooled after exiting the high-pressure channel, to an intermediate pressure level, ensuring that it meets the thermal conditions for evaporation at the corresponding temperature when entering the low-pressure channel. The pressure reduction effect of the first-stage throttling valve 6 simultaneously lowers the temperature and pressure of the working fluid, creating the necessary subcooling state for evaporation and heat absorption in the low-pressure channel. At the same time, the first-stage throttling valve 6 maintains a relatively high pressure and temperature for the working fluid in the high-pressure channel, preserving a sufficient heat transfer temperature difference between the high-pressure and low-pressure channels, ensuring effective heat exchange within the auxiliary evaporator 7.

[0033] Optionally, the oil return system of this single-cycle deep ground source heat pump also includes a secondary throttling valve 2; The inlet of the secondary throttle valve 2 is connected to the second side outlet of the regenerator 3, and the outlet of the secondary throttle valve 2 is connected to the inlet of the downhole heat exchanger.

[0034] In the above embodiment, the secondary throttling valve 2 further depressurizes and cools the carbon dioxide working fluid after it has released heat on the second side of the regenerator 3 to the working pressure and temperature level required by the downhole heat exchanger. This ensures that the working fluid enters the outer tube 11 of the downhole heat exchanger in a low-temperature and low-pressure state, creating a sufficient heat transfer temperature difference with the formation, thereby efficiently absorbing medium-deep geothermal energy. The secondary throttling valve 2 and the primary throttling valve 6 together constitute a two-stage throttling and pressure-reducing structure: the primary throttling valve 6 reduces the working fluid to an intermediate pressure to meet the evaporation conditions of the low-pressure channel of the auxiliary evaporator 7, while the secondary throttling valve 2 reduces the working fluid to an evaporation pressure to meet the heat extraction conditions of the downhole heat exchanger. This two-stage throttling allows the thermal state of the working fluid to be matched to the needs of each piece of equipment step by step, optimizing the overall thermal performance of the system.

[0035] Optionally, the oil separator 9 is provided with a lubricating oil outlet, which is connected to the lubricating oil inlet of the compressor 4 through the oil return pipeline, for guiding the separated lubricating oil back to the compressor 4.

[0036] In the above embodiment, the oil separator 9 is located at the outlet of the compressor 4, and uses mechanical separation to immediately separate most of the lubricating oil after the working fluid leaves the compressor 4. This is the first-level barrier for lubricating oil separation, with high separation efficiency, and can remove most of the lubricating oil carried by the working fluid at the outlet of the compressor 4. The separated lubricating oil is directly returned to the lubricating oil inlet of the compressor 4 through the return oil pipeline, ensuring that the compressor 4 maintains a sufficient supply of lubricating oil during continuous operation, and avoiding wear and failure of the compressor 4 due to lubricating oil loss.

[0037] Optionally, the gas cooler 8 is provided with a lubricating oil collection port at the rear end, which is connected to the return oil pipeline and is used to collect the lubricating oil that is adsorbed on the pipe wall of the rear end of the gas cooler 8 due to the decrease in temperature and the increase in viscosity.

[0038] In the above embodiment, the working fluid, after initial separation by the oil separator 9, still carries a small amount of lubricating oil. When the high-temperature working fluid releases heat to the heating return water in the gas cooler 8, the temperature of the working fluid gradually decreases along the flow direction. In the downstream section of the gas cooler 8, the temperature drop causes a significant increase in the viscosity of the lubricating oil. The miscibility between the lubricating oil and the carbon dioxide working fluid decreases with decreasing temperature, causing the lubricating oil to tend to separate from the working fluid and adhere to the pipe wall. By setting a lubricating oil collection port in the downstream section of the gas cooler 8, the second-stage physical separation of the lubricating oil is achieved by utilizing the changes in viscosity and solubility caused by temperature changes. This requires no additional energy consumption and fully utilizes the natural physical phenomena during the cooling process of the gas cooler 8.

[0039] Optionally, the low-pressure channel of the auxiliary evaporator 7 is provided with a lubricating oil outlet, which is connected to the return oil pipeline to discharge the lubricating oil that has been precipitated and accumulated at the end of the auxiliary evaporator 7 due to the evaporation of carbon dioxide working fluid.

[0040] In the above embodiment, after two stages of separation by the oil separator 9 and the gas cooler 8, a small amount of lubricating oil remains dissolved or dispersed in the carbon dioxide working fluid. When the working fluid enters the low-pressure channel of the auxiliary evaporator 7 after being depressurized by the first-stage throttle valve 6, the concentration of lubricating oil in the working fluid gradually increases as the carbon dioxide working fluid continues to evaporate. When the concentration exceeds the solubility limit, the lubricating oil precipitates from the working fluid. Since the density of lubricating oil is greater than that of gaseous carbon dioxide, the precipitated lubricating oil accumulates at the end of the auxiliary evaporator 7 under the action of gravity. By setting a lubricating oil outlet at the end of the low-pressure channel, a third stage of thermodynamic separation of the lubricating oil is achieved. This stage of separation is the most thorough and can remove the residual trace amounts of lubricating oil from the working fluid, ensuring the purity of the carbon dioxide working fluid entering the downhole heat exchanger to the greatest extent.

[0041] According to a second aspect of the present invention, an operation method for the oil return system of a single-cycle medium-deep ground source heat pump is provided, applicable to the oil return system of the single-cycle medium-deep ground source heat pump as described in the first aspect, comprising the following steps: Step S1: After absorbing geothermal energy in the downhole heat exchanger, the carbon dioxide working fluid flows out to the surface through the inner pipe 12 of the downhole heat exchanger. In step S2, the outflowing carbon dioxide working fluid enters the first side of the regenerator 3 to absorb heat and increase its temperature, and then enters the compressor 4 to increase its pressure. In step S3, the pressurized carbon dioxide working fluid undergoes oil-gas separation through the oil separator 9, separating out most of the lubricating oil carried on it. In step S4, the separated high-temperature and high-pressure carbon dioxide working fluid sequentially enters the high-pressure channels of the gas cooler 8 and the auxiliary evaporator 7 to release heat. In step S5, the exothermic carbon dioxide working fluid is depressurized by the first-stage throttling valve 6 and then enters the low-pressure channel of the auxiliary evaporator 7 to absorb heat and evaporate. In step S6, after the evaporated carbon dioxide working fluid releases heat through the second side of the regenerator 3, it is further depressurized and cooled by the secondary throttle valve 2 and enters the outer tube 11 of the downhole heat exchanger to complete the circulation. In step S7, during the above-mentioned cycle, the lubricating oil is separated and the lubricating oil deposited in the auxiliary evaporator 7, the gas cooler 8 and the oil separator 9 is returned to the compressor 4 through the oil return pipeline.

[0042] In the above implementation method, this operation integrates downhole heat extraction with carbon dioxide as the working fluid, surface compression and pressurization, cascaded heat release, two-stage throttling and pressure reduction, and oil-gas separation and return into a continuous closed-loop process. The entire method utilizes a single working fluid, carbon dioxide, to complete the entire thermodynamic process from downhole heat extraction to surface heating, avoiding intermediate heat exchange losses found in dual-working-fluid systems. Simultaneously, multi-stage separation of the lubricating oil is naturally achieved during the working fluid circulation process: from mechanical separation after compression, to physical separation in the gas cooler 8, and then to thermodynamic separation in the auxiliary evaporator 7. Each stage of separation utilizes the temperature and pressure change characteristics at different stages of the working fluid circulation process, eliminating the need for additional energy-consuming equipment for lubricating oil separation. The return oil pipeline continuously returns the separated and collected lubricating oil to the compressor 4 during the circulation process, ensuring that the oil-gas separation and return process are synchronized with the main thermodynamic cycle, guaranteeing continuous, stable, and efficient system operation.

[0043] Optionally, in step S7, the oil return pipeline is sequentially connected to the lubricating oil outlet of the auxiliary evaporator 7, the lubricating oil collection port of the gas cooler 8, the lubricating oil outlet of the oil separator 9, and the lubricating oil inlet of the compressor 4, and the lubricating oil is driven to return to the compressor 4 along the oil return pipeline by utilizing the pressure difference between each device.

[0044] In the above embodiment, the oil return pipeline connects the three lubricating oil collection points in series with the compressor 4 in the order of auxiliary evaporator 7 → gas cooler 8 → oil separator 9, forming a complete oil return path. Utilizing the natural pressure difference between the various devices as the driving force, automatic lubricating oil return can be achieved without the need for additional power equipment such as an oil return pump. The pressure in the low-pressure channel of the auxiliary evaporator 7 is lower than that of the gas cooler 8 and oil separator 9. This pressure difference causes the lubricating oil to flow from the low-pressure side to the high-pressure side, ultimately flowing into the lubricating oil inlet of the compressor 4. This pressure difference-driven oil return method simplifies the system structure, reduces equipment costs and operating energy consumption, and improves system reliability.

[0045] Optionally, the separation of lubricating oil includes a three-stage separation process: In the first stage of separation, most of the lubricating oil carried by the working fluid at the outlet of the compressor 4 is separated by mechanical separation in the oil separator 9. In the second stage of separation, at the rear section of the gas cooler 8, the lubricating oil viscosity increases due to the decrease in temperature, and the lubricating oil is adsorbed onto the rear section pipe wall of the gas cooler 8 and collected. In the third stage of separation, in the auxiliary evaporator 7, as the carbon dioxide working medium evaporates in the low-pressure channel, the lubricating oil dissolved in the carbon dioxide working medium gradually precipitates out and accumulates at the end of the auxiliary evaporator 7.

[0046] In the above implementation, the three-stage separation process is based on the principles of mechanical separation, physical property change, and thermodynamic phase change, respectively. These three separation mechanisms complement each other and progress step by step. The first-stage mechanical separation has the largest processing capacity, separating most of the lubricating oil and reducing the burden on subsequent separations. The second stage utilizes the viscosity and solubility changes caused by temperature variations to naturally separate a moderate amount of lubricating oil in the gas cooler 8, requiring no additional energy consumption. The third stage utilizes the phase change of the working fluid evaporation to completely precipitate the dissolved lubricating oil, achieving the highest processing precision and removing residual trace amounts of lubricating oil. The three-stage separation forms a complete separation system from coarse to fine, resulting in an extremely low oil content in the working fluid that finally enters the downhole heat exchanger. This effectively prevents the deposition of lubricating oil in the downhole heat exchanger, ensuring the heat transfer efficiency and pipe life of the downhole heat exchanger, while simultaneously ensuring that the compressor 4 always has a sufficient supply of lubricating oil.

[0047] It should be noted that the oil return system and operation method of this single-cycle deep ground source heat pump deeply embeds the lubricating oil separation and oil return process into the carbon dioxide thermodynamic cycle. Through the organic coupling of multiple thermodynamic processes, the overall system performance and oil return efficiency are maximized, specifically in the following aspects: Firstly, traditional oil return systems typically only have an oil separator 9 at the compressor 4 outlet for single-stage separation, resulting in limited separation efficiency. This invention embeds the lubricating oil separation point into multiple key stages of the thermodynamic cycle, utilizing the differences in the physicochemical properties of the working fluid under different thermodynamic states to achieve natural and efficient three-stage separation. The first stage utilizes the high-speed swirling flow of the working fluid at the compressor 4 outlet in the oil separator 9 for mechanical separation; the second stage utilizes the viscosity and solubility changes of the working fluid during cooling in the downstream section of the gas cooler 8 for physical separation; and the third stage utilizes the phase change of the working fluid during evaporation in the auxiliary evaporator 7 for thermodynamic separation. These three separation stages correspond to three different state points in the thermodynamic cycle: after compression, after cooling, and after evaporation. They utilize the temperature and pressure changes generated by the working fluid cycle itself, eliminating the need for additional energy input for the separation process and achieving deep coupling between the oil-gas separation process and the thermodynamic cycle.

[0048] Secondly, the auxiliary evaporator 7 is equipped with both a high-pressure channel and a low-pressure channel. The high-temperature, high-pressure carbon dioxide working fluid releases heat (cools) in the high-pressure channel, and after being depressurized by the first-stage throttling valve 6, it absorbs heat and evaporates in the low-pressure channel. The heat released on the high-pressure side is directly absorbed by the low-pressure side, forming an internal heat exchange loop. On the one hand, the high-pressure side working fluid is further cooled (subcooled) before entering the first-stage throttling valve 6, increasing the effective enthalpy difference after throttling and improving the thermodynamic performance of the subsequent regenerator 3 and the second-stage throttling valve 2. On the other hand, the evaporation and heat absorption of the low-pressure side working fluid lowers its temperature to an even lower level, enhancing the heat exchange temperature difference between the second and first sides of the regenerator 3 and improving the heat exchange effect of the regenerator 3. At the same time, the evaporation process of the working fluid in the low-pressure channel provides the phase change conditions for the thermodynamic precipitation of the third-stage lubricating oil, enabling the auxiliary evaporator 7 to simultaneously perform the dual functions of an internal heat exchanger and an oil separator 9.

[0049] Thirdly, the regenerator 3 exchanges heat between the low-temperature working fluid (first side / cold side) at the outlet of the downhole heat exchanger and the higher-temperature working fluid (second side / hot side) at the outlet of the auxiliary evaporator 7. The cold-side working fluid, after being heated, enters the compressor 4, while the hot-side working fluid, after being cooled, enters the secondary throttling valve 2. The heating of the cold side increases the suction enthalpy of the compressor 4, reducing the specific work required to compress the working fluid to the target pressure; the cooling of the hot side increases the subcooling before the secondary throttling valve 2, reducing the dryness of the working fluid after throttling and increasing the amount of heat that the working fluid can absorb in the downhole heat exchanger. The regenerator 3 allows the cycle to operate between a higher average heat absorption temperature and a lower average heat release temperature, effectively improving the thermodynamic perfection of the entire heat pump cycle.

[0050] Fourthly, this invention uses carbon dioxide as the sole working fluid, forming a closed single-working-fluid circulation loop from the downhole heat exchanger to the surface compressor 4, gas cooler 8, auxiliary evaporator 7, and regenerator 3, before returning to the downhole heat exchanger. Compared to the traditional dual-loop mode of "water-based downhole heat extraction + organic-based surface heat pump," this eliminates the evaporator heat exchange link between the two loops, thus eliminating the 5-10°C intermediate heat exchange temperature difference loss. Simultaneously, the downhole heat exchanger directly functions as the evaporator for the heat pump cycle. After absorbing geothermal energy downhole, the working fluid is heated by the regenerator 3 and directly enters the compressor 4 for pressurization, achieving seamless integration of heat extraction and heat pump temperature increase, fundamentally improving the energy utilization efficiency of the medium-deep geothermal heating system. The oil return system solves the lubricating oil management problem caused by the single-working-fluid circulation, making the integrated circulation scheme engineering feasible.

[0051] It is understood that the above embodiments are merely exemplary implementations used to illustrate the principles of the present invention, and the present invention is not limited thereto. For those skilled in the art, various modifications and improvements can be made without departing from the spirit and essence of the present invention, and these modifications and improvements are also considered to be within the scope of protection of the present invention.

Claims

1. A single-cycle intermediate-depth ground-source heat pump oil return system, characterized by, include: The downhole heat exchanger, the regenerator, and the compressor are provided, wherein the outlet of the downhole heat exchanger is connected to the first side inlet of the regenerator, and the first side outlet of the regenerator is connected to the inlet of the compressor. An oil separator and a gas cooler are provided, wherein the outlet of the compressor is connected to the working fluid inlet of the oil separator, and the working fluid outlet of the oil separator is connected to the hot side inlet of the gas cooler; An auxiliary evaporator is provided, which has a high-pressure channel and a low-pressure channel. The hot side outlet of the gas cooler is connected to the high-pressure channel inlet of the auxiliary evaporator. The high-pressure channel outlet of the auxiliary evaporator is connected to the low-pressure channel inlet of the auxiliary evaporator. The low-pressure channel outlet of the auxiliary evaporator is connected to the second side inlet of the regenerator. The second side outlet of the regenerator is connected to the inlet of the downhole heat exchanger. The auxiliary evaporator, the gas cooler, and the oil separator are connected in sequence through the oil return line and then connected to the lubricating oil inlet of the compressor.

2. The oil return system of a single-cycle intermediate-depth ground-source heat pump according to claim 1, characterized in that, The downhole heat exchanger includes an outer tube and an inner tube. The outer tube is sleeved outside the inner tube and coaxially arranged with the inner tube. The lower end of the outer tube is connected to the lower end of the inner tube, and an annular channel is formed between the outer tube and the inner tube. The carbon dioxide working fluid flows in from the upper inlet of the outer tube, flows downward through the outer tube to the bottom, then turns into the inner tube and flows upward, exiting from the upper outlet of the inner tube. The upper outlet of the inner tube is connected to the first side inlet of the regenerator.

3. The oil return system for a single-cycle intermediate-depth ground-source heat pump according to claim 2, characterized in that, It also includes a primary throttle valve; The inlet of the primary throttling valve is connected to the high-pressure channel outlet of the auxiliary evaporator, and the outlet of the primary throttling valve is connected to the low-pressure channel inlet of the auxiliary evaporator.

4. The oil return system of a single-cycle intermediate-depth ground-source heat pump according to claim 3, characterized in that, It also includes a secondary throttle valve; The inlet of the secondary throttle valve is connected to the second-side outlet of the regenerator, and the outlet of the secondary throttle valve is connected to the inlet of the downhole heat exchanger.

5. The oil return system for a single-cycle intermediate-depth ground-source heat pump according to claim 4, characterized in that, The oil separator is provided with a lubricating oil outlet, which is connected to the lubricating oil inlet of the compressor through the oil return pipeline, so as to lead the separated lubricating oil back to the compressor.

6. The oil return system for a single-cycle intermediate-depth ground-source heat pump according to claim 5, wherein The gas cooler is provided with a lubricating oil collection port at the rear end, which is connected to the return oil pipeline and is used to collect the lubricating oil that is adsorbed on the pipe wall of the rear end of the gas cooler due to the decrease in temperature and the increase in viscosity.

7. The oil return system of the single-cycle deep ground source heat pump according to claim 6, characterized in that, The low-pressure channel of the auxiliary evaporator is provided with a lubricating oil outlet, which is connected to the return oil pipeline to discharge the lubricating oil that has been precipitated and accumulated at the end of the auxiliary evaporator due to the evaporation of carbon dioxide working fluid.

8. A method for operating the oil return system of a single-cycle deep ground source heat pump, characterized in that, The oil return system applied to the single-cycle deep ground source heat pump as described in any one of claims 1-7 includes the following steps: Step S1: After absorbing geothermal energy in the downhole heat exchanger, the carbon dioxide working fluid flows out to the surface through the inner pipe of the downhole heat exchanger. In step S2, the outflowing carbon dioxide working fluid enters the first side of the regenerator to absorb heat and increase its temperature, and then enters the compressor to increase its pressure. In step S3, the pressurized carbon dioxide working fluid undergoes oil-gas separation through the oil separator, separating out most of the lubricating oil carried on it. Step S4: The separated high-temperature and high-pressure carbon dioxide working fluid sequentially enters the high-pressure channels of the gas cooler and the auxiliary evaporator to release heat. In step S5, the exothermic carbon dioxide working fluid is depressurized through a first-stage throttling valve and then enters the low-pressure channel of the auxiliary evaporator to absorb heat and evaporate. Step S6: After the evaporated carbon dioxide working fluid releases heat through the second side of the regenerator, it is further depressurized and cooled by the secondary throttle valve and enters the outer tube of the downhole heat exchanger to complete the circulation. Step S7: During the above cycle, the lubricating oil is separated and the lubricating oil deposited in the auxiliary evaporator, the gas cooler and the oil separator is returned to the compressor through the oil return line.

9. The method for operating the oil return system of the single-cycle deep ground source heat pump according to claim 8, characterized in that, In step S7, the oil return pipeline is sequentially connected to the lubricating oil outlet of the auxiliary evaporator, the lubricating oil collection port of the gas cooler, the lubricating oil outlet of the oil separator, and the lubricating oil inlet of the compressor. The pressure difference between the devices drives the lubricating oil to return to the compressor along the oil return pipeline.

10. The method for operating the oil return system of the single-cycle deep ground source heat pump according to claim 9, characterized in that, The separation of lubricating oil involves a three-stage separation process: In the first stage of separation, most of the lubricating oil carried by the compressor outlet working fluid is separated by mechanical separation in the oil separator. In the second-stage separation process, at the rear section of the gas cooler, the lubricating oil viscosity increases due to the decrease in temperature, and the lubricating oil is adsorbed onto the rear section pipe wall of the gas cooler and collected. In the third stage of separation, as the carbon dioxide working medium evaporates in the low-pressure channel in the auxiliary evaporator, the lubricating oil dissolved in the carbon dioxide working medium gradually precipitates out and accumulates at the end of the auxiliary evaporator.

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