Transcritical carbon dioxide underground heat exchange deep-ground long-distance refrigeration system and control method

Abstract

This invention discloses a transcritical underground carbon dioxide heat exchange deep-ground long-distance refrigeration system and its control method. This invention effectively utilizes the natural pressure potential energy at underground depths, reducing dependence on external energy sources, while minimizing energy loss through a multi-functional coupler unit. A circulation loop is formed by connecting the vertical shaft pipeline to the multi-functional coupler unit, completing the refrigeration and heat recovery of carbon dioxide, overcoming the challenges of ultra-deep transportation. This invention employs an underground direct-connection scheme for the phase change heat exchanger, eliminating the vertical section of the refrigerant from the surface to hundreds of meters underground. Liquid carbon dioxide absorbs heat from the refrigerant loop of the terminal air cooler through isothermal liquid-gas phase change within the underground carbon dioxide phase change heat exchanger, retaining cooling capacity using its high latent heat value, achieving low-dissipation energy transfer. The carbon dioxide phase change heat exchanger is located at the same depth as the cooling load center, shortening the low-temperature refrigerant transportation distance and significantly reducing pipeline cooling losses.

Description

Technical Field

[0001] This invention relates to deep-ground long-distance refrigeration technology, specifically to a transcritical carbon dioxide underground heat exchange deep-ground long-distance refrigeration system and its control method. Background Technology

[0002] As development deepens, the high-temperature heat hazards in deep underground spaces become increasingly severe. For example, controlling humidity in underground environments is a global challenge, and the future maximum mining depth will, to some extent, depend on the level of heat hazard management in mines. In subway stations, ventilation and air conditioning systems account for approximately 70% of the total energy consumption of all electromechanical equipment. Theories and technologies related to high-temperature heat hazard management with low energy consumption, low pollution, and high thermodynamic performance have become one of the key technologies for deep underground space development.

[0003] Currently, there are two main types of underground space cooling technologies used worldwide: non-refrigeration cooling and refrigeration cooling. Non-refrigeration cooling is ineffective for high-temperature underground spaces and is no longer used alone. Underground space cooling and air conditioning systems typically use Freon-based refrigerants as their working fluid. Because the use of these refrigerants contributes to ozone layer depletion and the greenhouse effect, it has caused long-term and far-reaching negative environmental impacts. Therefore, phasing out these substances and replacing them with more environmentally friendly alternatives is a key action promoted by international environmental organizations globally.

[0004] Carbon dioxide has a global warming potential (GWP) of 1 and an ozone depletion potential (ODP) of 0. As a naturally occurring substance, its negative impact on climate change as a refrigerant is minimal. Furthermore, transcritical carbon dioxide systems exhibit high energy efficiency due to their superior thermodynamic properties, enabling refrigeration with relatively low energy input. Compared to traditional refrigerants, carbon dioxide systems operate at relatively lower pressures, reducing equipment wear and extending system lifespan. They also demonstrate high heat exchange efficiency in the transcritical range, particularly in the condenser and evaporator processes. Moreover, they maintain good refrigeration performance at high temperatures, making them an important future application direction for refrigeration technology.

[0005] In existing deep underground space cooling technologies, the extreme depth leads to severely excessive fluid pressure, which underground unit equipment and transport pipelines cannot withstand. The extremely long transport distance results in significant heat loss along the way, making it impossible to deliver cooling capacity to the end of the underground space. Due to the extreme depth and long transport distance, the transmission and effect of refrigerant are affected by pressure and heat loss, thus reducing the performance of the cooling system, increasing energy consumption, and increasing technical difficulty. Traditional deep-earth cooling systems often fail to fully utilize the heat transported from deep underground to the surface, leading to significant heat dissipation and environmental pollution. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention proposes a transcritical carbon dioxide underground heat exchange deep-ground long-distance refrigeration system and its control method. This invention can supply heat to heat users and water treatment systems while simultaneously providing refrigeration, effectively recovering heat energy. It has good economic potential, environmental potential, and thermodynamic performance, and can solve the defects of existing technologies, such as fluid dynamics and thermodynamic performance loss, energy waste, and environmental pollution caused by ultra-deep transport depths of over 600 meters and ultra-long transport distances of over 4000 meters.

[0007] One object of the present invention is to propose a transcritical carbon dioxide underground heat exchange deep-ground long-distance refrigeration system.

[0008] The transcritical carbon dioxide underground heat exchange deep-ground long-distance refrigeration system of the present invention includes: a surface unit located on the ground and an underground unit located underground; the surface unit includes a compressor, a subcooling tower, first and second gas-liquid separators, a regenerator and a throttling device; the underground unit includes a multi-functional coupler unit and a carbon dioxide phase change heat exchanger; the surface unit and the underground unit are connected by an ultra-deep vertical shaft pipeline;

[0009] The gaseous outlet of the second gas-liquid separator is connected to the low-pressure side of the compressor, and the high-pressure side of the compressor is connected to the inlet of the subcooling tower. The outlet of the subcooling tower is divided into two paths: one path connects to the downward inlet of the regenerator, and the other path connects to the downward outlet of the regenerator and is located before the throttling device. The upward outlet and upward inlet of the regenerator are connected internally through an upward pipe, and the downward inlet and downward outlet of the regenerator are connected internally through a downward pipe. The upward outlet is connected to the gas-liquid two-phase inlet of the second gas-liquid separator. The downward outlet of the regenerator is connected to the gas-liquid two-phase inlet of the first gas-liquid separator via the throttling device. The gaseous outlet of the first gas-liquid separator is connected to the gas-liquid two-phase inlet of the second gas-liquid separator. The liquid outlet of the second gas-liquid separator is connected to the gas-liquid two-phase inlet of the first gas-liquid separator.

[0010] The liquid outlet of the first gas-liquid separator is connected to the downflow inlet of the underground multi-functional coupler unit via a vertical shaft pipeline. The downflow outlet of the multi-functional coupler unit is connected to the refrigerant inlet of the carbon dioxide phase change heat exchanger via a pipeline. The refrigerant inlet and outlet of the carbon dioxide phase change heat exchanger are connected via a refrigerant connecting pipe. The refrigerant outlet of the carbon dioxide phase change heat exchanger is connected to the upflow inlet of the multi-functional coupler unit via a pipeline. The upflow outlet of the multi-functional coupler unit is connected to the upflow inlet of the regenerator via a vertical shaft pipeline. The refrigerant connecting pipe of the carbon dioxide phase change heat exchanger is connected to the various instruments of the surface unit via pipelines to form a refrigerant circulation system. Carbon dioxide is used as the refrigerant in the refrigerant circulation system.

[0011] The outlet of the cooling medium in the carbon dioxide phase change heat exchanger is connected to the inlet of the terminal air cooler, and the outlet of the terminal air cooler is connected to the inlet of the cooling medium in the carbon dioxide phase change heat exchanger. The cooling medium inlet and outlet are connected through a cooling medium connecting pipe. The underground cooling medium connecting pipe is connected to the terminal air cooler to form a cooling medium loop. The cooling medium loop uses a liquid cooling medium that does not undergo phase change to circulate. The cooling medium connecting pipe and the refrigerant connecting pipe are not connected.

[0012] The refrigerant is carbon dioxide; the cooling medium is a liquid medium, such as water, salt solution or organic solvent, which remains in a liquid state and does not undergo phase change within the operating temperature range.

[0013] Furthermore, the present invention also includes an oil separator, which is installed between the high-pressure side of the compressor and the subcooling tower. After passing through the compressor, the supercritical carbon dioxide, which becomes high temperature and high pressure, passes through the oil separator to separate the lubricating oil in the compressor from the carbon dioxide, so as to avoid affecting the subsequent fluid flow and heat exchange process.

[0014] It also includes heat-using unit heat exchangers, located after the compressor and before the subcooling tower. After passing through the compressor, the supercritical carbon dioxide, which is converted into high-temperature, high-pressure carbon dioxide, transfers heat to the heat-using unit heat exchanger for further utilization. Heat-using unit heat exchangers are used by heat users such as manufacturing workshops, steam production, industrial applications, domestic heating, or water treatment units.

[0015] The oil separator outlet is divided into two paths: one path connects to the heat exchanger of the heat-using unit, and the other path connects to the inlet of the subcooling tower. The fluid ratio of the two paths is flexibly allocated according to the heat demand of the heat-using unit. If the heat demand of the heat-using unit is high, more flow is allocated to the heat-using unit; if the heat demand is low, the flow allocation is reduced.

[0016] This invention also includes a circulating pump, which is optionally installed on pipelines with insufficient fluid pressure to increase power when flow is obstructed; it also includes a filter device to remove impurities that appear in the working fluid after the refrigeration system has been in use for a period of time, generally installed in underground pipelines; it also includes a transformer, which is the power supply equipment for the underground circulating pump, solenoid valves, and control panels, generally installed in an underground equipment room; and it includes a surface carbon dioxide storage tank, which is installed on the ground surface. The outlet of the surface carbon dioxide storage tank is connected after the throttling device and located before the gas-liquid two-phase inlet of the first gas-liquid separator, to provide a buffer when the refrigeration load changes and to replenish the loss of the working fluid. A carbon dioxide-water heat exchange device is added after the multi-functional coupler unit in the underground chamber; and an additional cooling equipment chamber is set up in the underground space to house the underground secondary chilled water pump and filter equipment, while an electrical chamber is set up to house electrical equipment such as transformers and secondary water pump frequency converters.

[0017] The multi-functional coupler unit includes a pressure-reducing module, a gas-liquid separation module, and an upward circulation pump. The surface unit is connected to the pressure-reducing module of the multi-functional coupler unit through an ultra-deep vertical shaft pipeline. The pressure-reducing module depressurizes the liquid refrigerant from the ground and then transfers it to the carbon dioxide phase change heat exchanger. After heat exchange with the carbon dioxide phase change heat exchanger to complete the cooling process, the liquid refrigerant is transferred to the gas-liquid separation module to separate any air bubbles that may have entered during transport. If the separated air bubbles are harmless to air quality, they are directly released into the air; if they are harmful to air quality, they are collected in a special container or treated to render them harmless before being released. Then, the liquid refrigerant in the gas-liquid separation module is transferred to the surface unit through the ultra-deep vertical shaft pipeline. An upward circulation pump is installed on the upward vertical shaft pipeline after the gas outlet of the gas-liquid separation module. The upward circulation pump provides the upward power for the refrigerant to return to the surface and complete the circulation. By reducing the excessive pressure generated during the refrigerant's transport at deep underground depths, the system utilizes the pressure potential energy from the underground environment to convert it into kinetic energy, facilitating the smooth transport of the refrigerant to its target location within the underground transport channel. It features dynamic flow and pressure regulation, allowing the system to adjust the refrigerant's flow and pressure in real time to ensure stable operation under various working conditions. In addition to regulating the flow and pressure of the refrigerant, it also incorporates gas-liquid separation to prevent air bubbles generated during long-distance underground transport from mixing into the refrigerant, ensuring the stable operation of the entire system. The refrigerant flows through a multi-functional coupler unit, which not only transports the refrigerant underground but also overcomes the challenges of transporting refrigerants in deep-earth environments.

[0018] The throttling device can be a fixed throttling device, an adjustable flow device, an electronic throttling device, an intelligent throttling device, a multi-hole plug throttling device, or a labyrinth throttling device.

[0019] Another objective of this invention is to propose a control method for a transcritical carbon dioxide underground heat exchange deep-ground long-distance refrigeration system.

[0020] The control method for a transcritical carbon dioxide underground heat exchange deep-ground long-distance refrigeration system of the present invention includes the following steps:

[0021] 1) Low-temperature, low-pressure carbon dioxide gas from the second gas-liquid separator enters the compressor and is compressed into high-temperature, high-pressure supercritical carbon dioxide. It is pressurized and heated in the compressor, resulting in higher temperature and pressure on the high-pressure side of the compressor.

[0022] 2) Carbon dioxide is transferred to the subcooling tower, where its temperature is reduced through heat exchange. The carbon dioxide temperature decreases, but it still remains in a supercritical state.

[0023] 3) After being cooled by the subcooling tower, the low-temperature, high-pressure supercritical carbon dioxide selects its path based on temperature. If the temperature is too high to meet the low-temperature requirement of the throttling device, it is transferred to the downflow inlet of the regenerator and then to the downflow pipe. In the downflow pipe of the regenerator, the low-temperature, high-pressure supercritical carbon dioxide exchanges heat with the carbon dioxide from the underground located in the upflow pipe, further reducing the temperature to meet the temperature requirement of the throttling device, and then flows from the downflow outlet to the throttling device. If the temperature is low enough to meet the low-temperature requirement of the throttling device, it is directly transferred to the downflow outlet of the regenerator and before the throttling device.

[0024] 4) The supercritical carbon dioxide suddenly expands through the throttling device, its temperature drops and it partially vaporizes, entering a low-temperature, low-pressure gas-liquid two-phase state.

[0025] 5) After passing through the throttling device, the gas-liquid two-phase carbon dioxide enters the first gas-liquid separator. The gaseous carbon dioxide rises and is transferred to the second gas-liquid separator, where the liquid and gaseous carbon dioxide are separated. The low-temperature and low-pressure liquid carbon dioxide is separated and settled, and then transported underground through the vertical shaft pipeline.

[0026] 6) After being depressurized by the multi-functional coupler unit, the liquid carbon dioxide is transferred to the refrigerant connection pipe of the carbon dioxide phase change heat exchanger located underground. The liquid carbon dioxide in the refrigerant connection pipe exchanges heat with the refrigerant in the refrigerant connection pipe. The refrigerant in the refrigerant connection pipe is cooled, and the carbon dioxide in the refrigerant connection pipe is heated.

[0027] 7) After the carbon dioxide in the refrigerant connecting pipe is heated, it undergoes gas-liquid separation by the multi-functional coupler unit to remove the incompletely vaporized liquid carbon dioxide. The gaseous carbon dioxide is then transported upward through the vertical shaft pipeline to the upward inlet of the regenerator located on the ground and into the upward pipeline. The gaseous carbon dioxide in the upward pipeline of the regenerator exchanges heat with the low-temperature, high-pressure supercritical carbon dioxide in the downward pipeline, cooling the carbon dioxide in the downward pipeline and increasing the temperature of the gaseous carbon dioxide in the upward pipeline.

[0028] 8) The gaseous carbon dioxide in the upper pipe of the regenerator is transferred to the second gas-liquid separator, and the gaseous carbon dioxide re-enters the compressor for compression to form supercritical carbon dioxide, thereby completing the combined cooling and heating supply.

[0029] 9) The liquid cooling medium in the cooling medium connecting pipe of the cooled carbon dioxide phase change heat exchanger is transported to the terminal air cooler through the pipeline; the liquid cooling medium exchanges heat with the air and absorbs heat from the air to complete the refrigeration.

[0030] 10) After the heat exchange is completed, the cooling working fluid is transported back to the underground carbon dioxide phase change heat exchanger through the cooling working fluid connection pipe. The cooling working fluid in the cooling working fluid connection pipe exchanges heat with the carbon dioxide in the refrigerant connection pipe.

[0031] The carbon dioxide phase change heat exchanger completes the heat exchange between the refrigerant and the cooling medium. The phase change process maintains a stable low-temperature heat exchange interface, avoiding the terminal temperature difference loss in sensible heat exchange and significantly improving heat transfer efficiency. Furthermore, the carbon dioxide phase change heat exchanger is located underground, and liquid carbon dioxide with low latent heat loss is directly transported underground. In the underground carbon dioxide phase change heat exchanger, heat from the cooling medium of the terminal air cooler is absorbed through isothermal liquid-gas phase change, maximizing the utilization of the high latent heat value of the cooling medium to retain cold energy and achieving low-dissipation energy transfer.

[0032] In step 1), the temperature on the high-pressure side of the compressor is 65–120°C and the pressure is 7.38–12 MPa.

[0033] In step 6), the lower limit of the cooling medium temperature at the outlet of the carbon dioxide phase change heat exchanger is the selected cooling medium freezing point, and the upper limit is 15°C.

[0034] The liquid supply of the carbon dioxide phase change heat exchanger is controlled by a throttling device to match the heat load of the terminal air cooler; the flow rate of the compressor is adjusted to stabilize the exhaust pressure, and the cooling capacity of the carbon dioxide phase change heat exchanger located on the ground is used to jointly regulate the carbon dioxide saturation pressure.

[0035] In step 9), the temperature of the cooling medium at the inlet of the terminal air cooler is less than 20°C.

[0036] Advantages of this invention:

[0037] This invention effectively utilizes the natural pressure potential energy at underground depths, reducing dependence on external energy sources, while minimizing energy loss through a multi-functional coupler unit. The multi-functional coupler unit features dynamic flow and pressure regulation, allowing the system to adjust the working fluid's flow and pressure in real time, ensuring stable operation under various working conditions. A circulation loop is formed by connecting the vertical shaft pipeline to the multi-functional coupler unit, completing carbon dioxide refrigeration and heat recovery, overcoming the challenges of ultra-deep transportation. Equipped with multiple gas-liquid separation devices and a pressure-reducing module of the multi-functional coupler unit, the working fluid maintains stable operation during long-distance underground transportation, improving system reliability and service life. It can solve the fluid dynamics and thermodynamic performance losses caused by ultra-deep transportation depths exceeding 600 meters and ultra-long transportation distances exceeding 4000 meters, which are difficult to overcome in ultra-deep underground transportation hubs, underground building spaces, or underground energy extraction. By adjusting the carbon dioxide saturation pressure, the phase change temperature within the carbon dioxide phase change heat exchanger can flexibly match the requirements of the selected cooling working fluid loop, making it particularly suitable for deep-earth variable heat flux density scenarios. Furthermore, most refrigerants are toxic or asphyxiating to humans. The narrow installation space and complex piping arrangements in ultra-deep underground spaces mean that leaks can cause serious consequences, including but not limited to respiratory poisoning and frostbite. This invention places the carbon dioxide phase change heat exchanger underground, using a low-hazard or non-hazardous refrigerant to transport cooling capacity to the terminal air cooler. The refrigerant is responsible for reaching the required depth of the cooling area, while transportation within the underground horizontal tunnels is handled by the refrigerant loop. This minimizes cooling loss while ensuring a low risk of leakage. Moreover, since the rock mass itself is an excellent natural insulator, installing the carbon dioxide phase change heat exchanger underground requires no additional insulation layer. Compared to placing the carbon dioxide phase change heat exchanger on the surface, which consumes a large amount of energy to combat environmental heat intrusion, this method is more conducive to reducing operating energy consumption.

[0038] This invention employs a direct underground phase change heat exchanger scheme, eliminating the need for a vertical section of the refrigerant extending hundreds of meters underground from the surface. In this vertical section, the latent heat loss of liquid carbon dioxide is far less than the sensible heat loss of the refrigerant. Within the underground carbon dioxide phase change heat exchanger, liquid carbon dioxide absorbs heat from the refrigerant circuit of the terminal air cooler through isothermal liquid-gas phase change, maximizing the utilization of its high latent heat value to retain cooling capacity and achieving low-dissipation energy transfer. The carbon dioxide phase change heat exchanger is located at a depth close to or consistent with the center of the cooling load demand, which shortens the low-temperature refrigerant transport distance and eliminates the need for long-distance refrigerant pipelines, significantly reducing pipeline cooling losses. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of an embodiment of the transcritical carbon dioxide underground heat exchange deep-ground long-distance refrigeration system of the present invention. Detailed Implementation

[0040] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0041] like Figure 1 As shown, the transcritical carbon dioxide underground heat exchange deep-ground long-distance refrigeration system of the present invention includes: a surface unit located on the ground and an underground unit located underground; the surface unit includes a compressor, an oil separator, a heat exchanger for heat-using units, a subcooling tower, first and second gas-liquid separators, a regenerator, a throttling device, and a carbon dioxide phase change heat exchanger; the underground unit includes a multi-functional coupler unit and a terminal air cooler; the surface unit and the underground unit are connected by an ultra-deep vertical shaft pipeline;

[0042] The gaseous outlet on the side of the second gas-liquid separator is connected to the low-pressure side of the compressor, and the high-pressure side of the compressor is connected to the inlet of the oil separator. The outlet of the oil separator is divided into two paths: one path connects to the heat exchanger of the heat-using unit; the other path connects to the inlet of the subcooling tower. The outlet of the subcooling tower is divided into two paths: one path connects to the downward inlet of the regenerator, and the other path connects to the downward outlet of the regenerator and is located before the throttling device. The upward outlet and the upward inlet of the regenerator are connected internally through an upward pipe, and the downward inlet and the downward outlet of the regenerator are connected internally through a downward pipe. The upward pipe and the downward pipe are not connected. The upward outlet is connected to the gas-liquid two-phase inlet of the second gas-liquid separator. The downward outlet of the regenerator is connected to the gas-liquid two-phase inlet of the first gas-liquid separator via the throttling device. The gaseous outlet on the side of the first gas-liquid separator is connected to the gas-liquid two-phase inlet of the second gas-liquid separator. The liquid outlet at the bottom of the second gas-liquid separator is connected to the gas-liquid two-phase inlet of the first gas-liquid separator.

[0043] The liquid outlet at the bottom of the first gas-liquid separator is connected to the downflow inlet of the underground multi-functional coupler unit via a vertical shaft pipeline. The downflow outlet of the multi-functional coupler unit is connected to the refrigerant inlet of the carbon dioxide phase change heat exchanger via a pipeline. The refrigerant inlet and outlet of the carbon dioxide phase change heat exchanger are connected via a refrigerant connecting pipe. The refrigerant outlet of the carbon dioxide phase change heat exchanger is connected to the upflow inlet of the multi-functional coupler unit via a pipeline. The upflow outlet of the multi-functional coupler unit is connected to the upflow inlet of the regenerator via a vertical shaft pipeline. The refrigerant connecting pipe of the carbon dioxide phase change heat exchanger is connected to the various instruments of the surface unit via pipelines to form a refrigerant circulation system. Carbon dioxide is used as the refrigerant in the refrigerant circulation system.

[0044] The outlet of the cooling medium in the carbon dioxide phase change heat exchanger is connected to the inlet of the terminal air cooler, and the outlet of the terminal air cooler is connected to the inlet of the cooling medium in the carbon dioxide phase change heat exchanger. The cooling medium inlet and outlet are connected through a cooling medium connecting pipe. The underground cooling medium connecting pipe is connected to the terminal air cooler to form a cooling medium loop. The cooling medium loop uses a liquid cooling medium that does not undergo phase change to circulate. The cooling medium connecting pipe and the refrigerant connecting pipe are not interconnected.

[0045] The control method for the transcritical carbon dioxide underground heat exchange deep-ground long-distance refrigeration system of this embodiment includes the following steps:

[0046] 1) Low-temperature, low-pressure carbon dioxide gas from the second gas-liquid separator enters the compressor and is compressed into high-temperature, high-pressure supercritical carbon dioxide. It is pressurized and heated in the compressor, resulting in higher temperature and pressure on the high-pressure side of the compressor.

[0047] 2) Carbon dioxide is transferred to a subcooling tower, where its temperature is further reduced through heat exchange. The temperature of the carbon dioxide is reduced to 30°C, but it still remains in a supercritical state.

[0048] 3) After being cooled by the subcooling tower, the low-temperature, high-pressure supercritical carbon dioxide selects its path based on temperature. If the temperature is too high to meet the low-temperature requirement of the throttling device, it is transferred to the downflow inlet of the regenerator and then to the downflow pipe. In the downflow pipe of the regenerator, the low-temperature, high-pressure supercritical carbon dioxide exchanges heat with the carbon dioxide from the underground located in the upflow pipe, further reducing the temperature to meet the temperature requirement of the throttling device, and then flows from the downflow outlet to the throttling device. If the temperature is low enough to meet the low-temperature requirement of the throttling device, it is directly transferred to the downflow outlet of the regenerator and before the throttling device.

[0049] 4) The supercritical carbon dioxide suddenly expands through the throttling device, its temperature drops and it partially vaporizes, entering a low-temperature, low-pressure gas-liquid two-phase state.

[0050] 5) After passing through the throttling device, the gaseous and liquid carbon dioxide enters the first gas-liquid separator. The gaseous carbon dioxide rises and is transferred to the second gas-liquid separator, where the liquid and gaseous carbon dioxide are separated. The liquid carbon dioxide settles and is transported underground.

[0051] 6) Liquid carbon dioxide is transported through the multi-functional coupler unit to the refrigerant connection pipe of the carbon dioxide phase change heat exchanger. The liquid carbon dioxide in the refrigerant connection pipe exchanges heat with the refrigerant in the refrigerant connection pipe. The refrigerant in the refrigerant connection pipe is cooled, and the carbon dioxide in the refrigerant connection pipe is heated.

[0052] 7) After the carbon dioxide in the refrigerant connecting pipe is heated, it undergoes gas-liquid separation by the multi-functional coupler unit to remove the incompletely vaporized liquid carbon dioxide. The gaseous carbon dioxide is then transported upward through the vertical shaft pipeline to the upward inlet of the regenerator located on the ground and into the upward pipeline. The gaseous carbon dioxide in the upward pipeline of the regenerator exchanges heat with the low-temperature, high-pressure supercritical carbon dioxide in the downward pipeline, cooling the carbon dioxide in the downward pipeline and increasing the temperature of the gaseous carbon dioxide in the upward pipeline.

[0053] 8) The gaseous carbon dioxide in the upper pipe of the regenerator is transferred from the upper outlet of the regenerator to the second gas-liquid separator. The gaseous carbon dioxide re-enters the compressor for compression to form supercritical carbon dioxide, thereby completing the combined cooling and heating supply.

[0054] 9) The liquid cooling medium in the cooling medium connection pipe of the cooled carbon dioxide phase change heat exchanger is transported to the terminal air cooler through a horizontal (or nearly horizontal) pipeline; the liquid cooling medium exchanges heat with the air and absorbs heat from the air to complete the refrigeration.

[0055] 10) After the heat exchange is completed, the cooling working fluid is transported back to the underground carbon dioxide phase change heat exchanger cooling working fluid connection pipe through a horizontal (or nearly horizontal) pipeline. The cooling working fluid in the cooling working fluid connection pipe exchanges heat with the carbon dioxide in the refrigerant connection pipe.

[0056] The carbon dioxide phase change heat exchanger facilitates heat exchange between the refrigerant and the cooling medium at the Earth's surface. Liquid carbon dioxide absorbs heat from the cooling medium at the Earth's surface through isothermal liquid-gas phase change within the heat exchanger, utilizing its high latent heat value to achieve efficient energy transfer. The phase change process maintains a stable low-temperature heat exchange interface, avoiding end-to-end losses in sensible heat transfer and significantly improving heat transfer efficiency. The surface-mounted carbon dioxide phase change radiator can precisely control the heat exchange boundary conditions using the atmosphere or auxiliary cooling system, preventing the inability to precisely control the CO2 evaporation temperature when it deviates from the design value due to the influence of the geothermal gradient. Furthermore, the surface unit and the underground unit are connected through ultra-deep vertical shaft pipelines to complete the transportation of the cooling medium.

[0057] In this embodiment, water is used as the cooling medium.

[0058] Finally, it should be noted that the purpose of disclosing the embodiments is to help further understand the present invention. However, those skilled in the art will understand that various substitutions and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the present invention should not be limited to the content disclosed in the embodiments, and the scope of protection of the present invention is defined by the claims.

Claims

1. A transcritical carbon dioxide underground heat exchange deep-ground long-distance refrigeration system, characterized in that, The refrigeration system includes: a surface unit located on the ground and an underground unit located underground; the surface unit includes a compressor, a subcooling tower, first and second gas-liquid separators, a regenerator, and a throttling device; the underground unit includes a multi-functional coupler unit and a carbon dioxide phase change heat exchanger; the surface unit and the underground unit are connected by an ultra-deep vertical shaft pipeline. The gaseous outlet of the second gas-liquid separator is connected to the low-pressure side of the compressor, and the high-pressure side of the compressor is connected to the inlet of the subcooling tower. The outlet of the subcooling tower is divided into two paths: one path connects to the downward inlet of the regenerator, and the other path connects to the downward outlet of the regenerator and is located before the throttling device. The upward outlet and upward inlet of the regenerator are connected internally through an upward pipe, and the downward inlet and downward outlet of the regenerator are connected internally through a downward pipe. The upward outlet is connected to the gas-liquid two-phase inlet of the second gas-liquid separator. The downward outlet of the regenerator is connected to the gas-liquid two-phase inlet of the first gas-liquid separator via the throttling device. The gaseous outlet of the first gas-liquid separator is connected to the gas-liquid two-phase inlet of the second gas-liquid separator. The liquid outlet of the second gas-liquid separator is connected to the gas-liquid two-phase inlet of the first gas-liquid separator. The liquid outlet of the first gas-liquid separator is connected to the downflow inlet of the underground multi-functional coupler unit via a vertical shaft pipeline. The downflow outlet of the multi-functional coupler unit is connected to the refrigerant inlet of the carbon dioxide phase change heat exchanger via a pipeline. The refrigerant inlet and outlet of the carbon dioxide phase change heat exchanger are connected via a refrigerant connecting pipe. The refrigerant outlet of the carbon dioxide phase change heat exchanger is connected to the upflow inlet of the multi-functional coupler unit via a pipeline. The upflow outlet of the multi-functional coupler unit is connected to the upflow inlet of the regenerator via a vertical shaft pipeline. The refrigerant connecting pipe of the carbon dioxide phase change heat exchanger is connected to the various instruments of the surface unit via pipelines to form a refrigerant circulation system. Carbon dioxide is used as the refrigerant in the refrigerant circulation system. The outlet of the cooling medium in the carbon dioxide phase change heat exchanger is connected to the inlet of the terminal air cooler, and the outlet of the terminal air cooler is connected to the inlet of the cooling medium in the carbon dioxide phase change heat exchanger. The cooling medium inlet and outlet are connected through a cooling medium connecting pipe. The underground cooling medium connecting pipe is connected to the terminal air cooler to form a cooling medium loop. The cooling medium loop uses a liquid cooling medium that does not undergo phase change to circulate. The cooling medium connecting pipe and the refrigerant connecting pipe are not connected. The multi-functional coupler unit includes a pressure-reducing module, a gas-liquid separation module, and an upward circulation pump. The surface unit is connected to the pressure-reducing module of the multi-functional coupler unit through an ultra-deep vertical shaft pipeline. The pressure-reducing module depressurizes the liquid refrigerant from the ground and then transfers it to the carbon dioxide phase change heat exchanger. After heat exchange with the carbon dioxide phase change heat exchanger to complete the cooling process, the liquid refrigerant is transferred to the gas-liquid separation module to separate any air bubbles that may have entered during transport. If the separated air bubbles are harmless to air quality, they are directly released into the air. If they are harmful to air quality, they are collected in a special container or treated to render them harmless before being released. Then, the liquid refrigerant in the gas-liquid separation module is transferred to the surface unit through the ultra-deep vertical shaft pipeline. An upward circulation pump is installed on the upward vertical shaft pipeline after the gas outlet of the gas-liquid separation module. The upward circulation pump provides the upward power for the refrigerant to return to the surface and complete the circulation.

2. The refrigeration system as described in claim 1, characterized in that, The cooling medium is a liquid medium that remains liquid and does not undergo phase change within the operating temperature range.

3. The refrigeration system as described in claim 1, characterized in that, It also includes an oil separator, which is installed between the high-pressure side of the compressor and the subcooling tower. After passing through the compressor, the supercritical carbon dioxide, which becomes high temperature and high pressure, passes through the oil separator to separate the lubricating oil in the compressor from the carbon dioxide, so as to avoid affecting the subsequent fluid flow and heat exchange process.

4. The refrigeration system as described in claim 1, characterized in that, It also includes a heat-using unit heat exchanger, which is located after the compressor and before the subcooling tower. After passing through the compressor, it becomes high-temperature and high-pressure supercritical carbon dioxide, which transfers heat to the heat-using unit heat exchanger for further utilization of the heat.

5. The refrigeration system as described in claim 1, characterized in that, It also includes a carbon dioxide surface storage tank, which is installed on the ground. The outlet of the carbon dioxide surface storage tank is connected after the throttling device and located before the gas-liquid two-phase inlet of the first gas-liquid separator, so as to provide a buffer when the refrigeration load condition changes and to replenish the loss of working fluid.

6. The refrigeration system as described in claim 1, characterized in that, The throttling device can be a fixed throttling device, an adjustable throttling device, an electronic throttling device, an intelligent throttling device, a multi-hole plug-type throttling device, or a labyrinth-type throttling device.

7. A control method for a deep-ground long-distance refrigeration system with transcritical carbon dioxide underground heat exchange as described in claim 1, characterized in that, The control method includes the following steps: 1) Low-temperature, low-pressure carbon dioxide gas from the second gas-liquid separator enters the compressor and is compressed into high-temperature, high-pressure supercritical carbon dioxide. It is pressurized and heated in the compressor, resulting in higher temperature and pressure on the high-pressure side of the compressor. 2) Carbon dioxide is transferred to the subcooling tower, where its temperature is reduced through heat exchange. The carbon dioxide temperature decreases, but it still remains in a supercritical state. 3) After being cooled by the subcooling tower, the low-temperature, high-pressure supercritical carbon dioxide selects its path based on temperature. If the temperature is too high to meet the low-temperature requirement of the throttling device, it is transferred to the downflow inlet of the regenerator and then to the downflow pipe. In the downflow pipe of the regenerator, the low-temperature, high-pressure supercritical carbon dioxide exchanges heat with the carbon dioxide from the underground located in the upflow pipe, further reducing the temperature to meet the temperature requirement of the throttling device, and then flows from the downflow outlet to the throttling device. If the temperature is low enough to meet the low-temperature requirement of the throttling device, it is directly transferred to the downflow outlet of the regenerator and before the throttling device. 4) The supercritical carbon dioxide suddenly expands through the throttling device, its temperature drops and it partially vaporizes, entering a low-temperature, low-pressure gas-liquid two-phase state. 5) After passing through the throttling device, the gas-liquid two-phase carbon dioxide enters the first gas-liquid separator. The gaseous carbon dioxide rises and is transferred to the second gas-liquid separator, where the liquid and gaseous carbon dioxide are separated. The low-temperature and low-pressure liquid carbon dioxide is separated and settled, and then transported underground through the vertical shaft pipeline. 6) After being depressurized by the multi-functional coupler unit, the liquid carbon dioxide is transferred to the refrigerant connection pipe of the carbon dioxide phase change heat exchanger located underground. The liquid carbon dioxide in the refrigerant connection pipe exchanges heat with the refrigerant in the refrigerant connection pipe. The refrigerant in the refrigerant connection pipe is cooled, and the carbon dioxide in the refrigerant connection pipe is heated. 7) After the carbon dioxide in the refrigerant connecting pipe is heated, it undergoes gas-liquid separation by the multi-functional coupler unit to remove the incompletely vaporized liquid carbon dioxide. The gaseous carbon dioxide is then transported upward through the vertical shaft pipeline to the upward inlet of the regenerator located on the ground and into the upward pipeline. The gaseous carbon dioxide in the upward pipeline of the regenerator exchanges heat with the low-temperature, high-pressure supercritical carbon dioxide in the downward pipeline, cooling the carbon dioxide in the downward pipeline and increasing the temperature of the gaseous carbon dioxide in the upward pipeline. 8) The gaseous carbon dioxide in the upper pipe of the regenerator is transferred to the second gas-liquid separator, and the gaseous carbon dioxide re-enters the compressor for compression to form supercritical carbon dioxide, thereby completing the combined cooling and heating supply. 9) The liquid cooling medium in the cooling medium connecting pipe of the cooled carbon dioxide phase change heat exchanger is transported to the terminal air cooler through the pipeline; the liquid cooling medium exchanges heat with the air and absorbs heat from the air to complete the refrigeration. 10) After the heat exchange is completed, the cooling working fluid is transported back to the underground carbon dioxide phase change heat exchanger through the cooling working fluid connection pipe. The cooling working fluid in the cooling working fluid connection pipe exchanges heat with the carbon dioxide in the refrigerant connection pipe. The carbon dioxide phase change heat exchanger completes the heat exchange between the refrigerant and the cooling medium. The phase change process maintains a stable low-temperature heat exchange interface, avoiding the terminal temperature difference loss in sensible heat exchange and significantly improving heat transfer efficiency. Furthermore, the carbon dioxide phase change heat exchanger is located underground, and liquid carbon dioxide with low latent heat loss is directly transported underground. In the underground carbon dioxide phase change heat exchanger, heat from the cooling medium of the terminal air cooler is absorbed through isothermal liquid-gas phase change, maximizing the utilization of the high latent heat value of the cooling medium to retain cold energy and achieving low-dissipation energy transfer.

8. The control method as described in claim 7, characterized in that, The liquid supply of the carbon dioxide phase change heat exchanger is controlled by a throttling device to match the heat load of the terminal air cooler; the flow rate of the compressor is adjusted to stabilize the exhaust pressure, and the cooling capacity of the carbon dioxide phase change heat exchanger located on the ground is used to jointly regulate the carbon dioxide saturation pressure.

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