A transcritical co2 refrigeration system and method

Abstract

The application belongs to the technical field of air conditioning and refrigeration, and relates to a transcritical CO2 refrigeration system and method, which provides two schemes, scheme one including a main refrigeration cycle and a waste heat recovery driven organic Rankine power cycle, and scheme two further including a mechanical supercooling cycle; the main refrigeration cycle includes an evaporator, a low-pressure stage compressor, a high-pressure stage compressor and a gas waste heat recovery heat exchanger connected in sequence, and the gas waste heat recovery heat exchanger and a first expander; the waste heat recovery driven organic Rankine power cycle includes a pump, a first condenser, a second expander and the gas waste heat recovery heat exchanger connected in sequence; the mechanical supercooling cycle includes a throttle valve, a mechanical supercooling heat exchanger, a compressor and a second condenser connected in sequence, and the mechanical supercooling heat exchanger is arranged between the gas waste heat recovery heat exchanger and the first expander. The application reduces the power consumption of the CO2 refrigeration system, simultaneously reduces the irreversible loss in conventional CO2 gas cooling, and greatly improves the energy efficiency of the CO2 refrigeration system.

Description

Technical Field

[0001] This invention belongs to the field of air conditioning and refrigeration technology, specifically relating to a transcritical CO2 refrigeration system and method. Background Technology

[0002] Currently, due to energy constraints and environmental issues, various sectors are facing increasingly stringent requirements for energy conservation and emission reduction. In the refrigeration and air conditioning industry, the integration of system components and the application of natural refrigerants have attracted widespread attention. CO2 has garnered attention due to its unique properties (non-toxic, non-flammable, and large volumetric refrigeration capacity) and wide availability. However, CO2's relatively low critical temperature and relatively high critical pressure mean that the outlet temperature of the gas cooler has a significant impact on the system's refrigeration efficiency and power consumption. Compared to conventional refrigerants, CO2 refrigeration systems have lower energy efficiency, limiting their application and development. To address this limitation, some researchers have implemented measures such as two-stage compression with intercooling, mechanical subcooling, expanders, and ejectors to improve the energy efficiency of CO2 refrigeration systems.

[0003] Chinese patent CN211041462U discloses a transcritical CO2 refrigeration system for recovering waste heat from a gas cooler. This system also adds an expander component to the system cycle. However, the added expander is for recovering waste heat from the gas cooler and converting it into mechanical work to power the compression process of the mechanical subcooling cycle. The expander is coaxial with the compressor in the mechanical subcooling cycle. The underlying principle is to recover waste heat and pressure difference and convert it into mechanical work by setting up a conventional two-stage compression cycle and an expander in the waste heat recovery cycle. However, the degree of subcooling in this system depends entirely on the amount of heat recovered, and the subcooling amount is not adjustable.

[0004] Invention patent CN113513854A discloses a transcritical CO2 mechanical subcooling refrigeration system with a high-pressure ejector. The system mainly focuses on the recovery and utilization of waste heat from CO2 gas coolers and the recovery and utilization of waste heat from the condenser of the jet subcooling system. The subcooling cycle adopts the jet subcooling cycle, but the ejector has poor performance under variable operating conditions, while the recovered waste heat is used for heating.

[0005] Invention patent CN110513902A discloses a multi-stage evaporative condensation mechanical subcooling transcritical CO2 high-temperature heat pump system. It incorporates two subcooling heat exchangers within a basic transcritical CO2 cycle, further increasing the system's subcooling and reducing the heat exchange temperature difference, thereby improving the overall system energy efficiency. The system features a multi-stage compression subcooling system outside the main cycle, adding three mechanical subcooling compressors and two subcoolers, making the system complex. Waste heat recovery from the CO2 gas cooler in the main cycle loop is primarily used for heating.

[0006] Based on the problems existing in the current CO2 transcritical refrigeration system, this invention is proposed. Summary of the Invention

[0007] The purpose of this invention is to solve the aforementioned problems in the prior art and to propose a transcritical CO2 refrigeration system and method, applicable to transcritical CO2 multi-stage compression refrigeration and waste heat recovery methods for gas waste heat recovery heat exchangers. This invention uses a waste heat recovery system to drive the output power of an organic Rankine cycle (ORC) to power a high-pressure stage compressor, while simultaneously using a parallel mechanical subcooling system to subcool the transcritical CO2 main cycle and cool the condenser of the ORC, thereby reducing the condensation temperature of the ORC and increasing its output power, thus improving the energy efficiency of the CO2 refrigeration system.

[0008] The technical solution of this invention is:

[0009] A transcritical CO2 refrigeration system includes a main refrigeration cycle and a waste heat recovery driven organic Rankine power cycle. The main refrigeration cycle includes an evaporator. The outlet of the evaporator is sequentially connected to a low-pressure stage compressor and a high-pressure stage compressor. The outlet of the high-pressure stage compressor is connected to a gas waste heat recovery heat exchanger. The outlet of the gas waste heat recovery heat exchanger is connected to a first expander. The first expander is connected to the evaporator.

[0010] The waste heat recovery-driven organic Rankine power cycle includes a first condenser and a second expander connected to the first condenser. The outlet of the gas waste heat recovery heat exchanger is connected to the second expander, the outlet of the second expander is connected to the inlet of the first condenser, and the outlet of the first condenser is connected to a pump. The pump is connected to the gas waste heat recovery heat exchanger.

[0011] The first expander drives the low-pressure stage compressor, and the second expander drives the high-pressure stage compressor.

[0012] Furthermore, the present invention also protects a refrigeration method employing the above-mentioned transcritical CO2 refrigeration system, wherein the refrigeration system includes a main refrigeration cycle and a waste heat recovery-driven organic Rankine power cycle, and the refrigeration method includes:

[0013] In the main refrigeration cycle refrigeration method, the refrigerant evaporates and absorbs heat in the evaporator to form refrigerant gas. The refrigerant gas enters the low-pressure stage compressor and the high-pressure stage compressor for compression. The resulting high-temperature and high-pressure gas releases heat through the gas waste heat recovery heat exchanger and then enters the first expander to do work to drive the low-pressure stage compressor. After exiting the first expander, it enters the evaporator.

[0014] The waste heat recovery-driven organic Rankine cycle refrigeration method involves a working fluid that is pressurized in a pump and then enters a gas waste heat recovery heat exchanger to absorb heat, becoming a high-temperature, high-pressure gas that enters a second expander to do work. The expansion output power drives a high-pressure stage compressor. The low-temperature, low-pressure gas exiting the second expander is cooled by a first condenser and then enters the pump to complete the cycle.

[0015] Furthermore, it also includes a mechanical subcooling cycle, which includes a mechanical subcooling heat exchanger, a compressor, a second condenser, and a throttling valve connected in sequence. The mechanical subcooling heat exchanger is located between the gas waste heat recovery heat exchanger and the first expander. The outlet of the gas waste heat recovery heat exchanger is connected to the mechanical subcooling heat exchanger, and the mechanical subcooling heat exchanger is connected to the first expander.

[0016] Furthermore, the outlet of the throttle valve is connected to a mechanical subcooling heat exchanger and a first condenser, respectively. The outlets of the mechanical subcooling heat exchanger and the first condenser are connected to a compressor, and the outlet of the compressor is connected to a second condenser and the throttle valve in sequence.

[0017] Furthermore, the present invention also protects a refrigeration method employing the above-mentioned transcritical CO2 refrigeration system, wherein the refrigeration system includes a main refrigeration cycle, a waste heat recovery-driven organic Rankine power cycle, and a mechanical subcooling cycle, and the refrigeration method includes:

[0018] In the main refrigeration cycle refrigeration method, the refrigerant evaporates and absorbs heat in the evaporator to form refrigerant gas. The refrigerant gas enters the low-pressure stage compressor and the high-pressure stage compressor for compression. The resulting high-temperature and high-pressure gas releases heat through the gas waste heat recovery heat exchanger and then enters the mechanical subcooling heat exchanger to further release heat. After becoming subcooled gas, it enters the first expander to drive the low-pressure stage compressor. After exiting the first expander, it enters the evaporator.

[0019] In the mechanical subcooling cycle refrigeration method, the refrigerant after being throttled by the expansion valve is divided into two branches. The refrigerant in one branch enters the compressor through the mechanical subcooling heat exchanger, while the refrigerant in the other branch exchanges heat through the first condenser and then enters the compressor together with the refrigerant gas exiting the mechanical subcooling heat exchanger for compression. After releasing heat through the second condenser, it enters the expansion valve.

[0020] In the waste heat recovery-driven organic Rankine cycle refrigeration method, the working fluid is pressurized in the pump and then enters the gas waste heat recovery heat exchanger to absorb heat. It then enters the second expander to do work to drive the high-pressure stage compressor. The gas coming out of the second expander is cooled by the first condenser and then enters the pump to complete the cycle.

[0021] Furthermore, the mechanical subcooling cycle subcools the refrigerant of the main refrigeration cycle through a mechanical subcooling heat exchanger and cools the first condenser.

[0022] The beneficial effects of this invention are:

[0023] (1) The transcritical CO2 refrigeration system provided by the present invention has a waste heat recovery device installed at the exhaust outlet of the high-pressure stage compressor to recover the waste heat of the CO2 high-pressure stage compressor exhaust and drive the organic Rankine power cycle to generate power to drive the high-pressure stage compressor, thereby reducing the power consumption of the CO2 refrigeration system and reducing the irreversible losses in conventional CO2 gas cooling; at the same time, another expander is used to recover the energy of the high-pressure throttling process to drive the low-pressure stage compressor, thus significantly improving the system energy efficiency.

[0024] (2) The present invention also adds a mechanical subcooling system to improve the power generation efficiency of the organic Rankine power cycle system and the energy efficiency of the main cycle CO2 refrigeration cycle; the mechanical subcooling system adopts a parallel method to subcool the CO2 transcritical main cycle, improve the subcooling degree of CO2 before expansion, and significantly improve the system energy efficiency; at the same time, the mechanical subcooling system cools the condenser of the organic Rankine power cycle driven by the exhaust waste heat of the CO2 high-pressure stage compressor, reduces the condensation temperature, improves the efficiency and output power of the organic Rankine power cycle, and further improves the energy efficiency of the transcritical CO2 refrigeration system.

[0025] (3) Based on the fact that the high heat of the gas waste heat recovery heat exchanger in the existing scheme is directly discharged into the environment, resulting in irreversible loss of graded heat energy, the present invention recovers the heat of the gas waste heat recovery heat exchanger in the CO2 refrigeration cycle, and uses the high temperature exhaust waste heat of CO2 to drive the organic Rankine power cycle. At the same time, an expander is used to recover the loss of the throttling process. By reducing the temperature of the gas waste heat recovery heat exchanger outlet and the refrigerant before throttling, the irreversible loss between the gas waste heat recovery heat exchanger and the outside world is reduced, and the overall energy efficiency of the system is increased.

[0026] (4) This system is mainly used for refrigeration. The organic Rankine cycle saves compressor power consumption. The organic Rankine cycle does not have a boiler. It uses the waste heat of the gas in the refrigeration cycle to recover the waste heat of the heat exchanger to heat the medium of the organic Rankine cycle ORC cycle. It uses organic working fluid and is suitable for organic working fluid Rankine power generation cycle at lower temperatures (around 100℃). Attached Figure Description

[0027] Figure 1 This is a schematic diagram provided for Embodiment 1 of the present invention;

[0028] Figure 2 This is a schematic diagram provided for Embodiment 2 of the present invention;

[0029] In the above diagram, 1 is the evaporator; 2 is the low-pressure stage compressor; 3 is the high-pressure stage compressor; 4 is the gas waste heat recovery heat exchanger; 5 is the first expander; 6 is the second expander; 7 is the first condenser; 8 is the pump; 9 is the mechanical subcooling heat exchanger; 10 is the compressor; 11 is the second condenser; and 12 is the throttle valve. Detailed Implementation

[0030] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] To further understand the present invention, it will be further described in conjunction with the accompanying drawings and embodiments.

[0032] Example 1

[0033] like Figure 1 As shown, the transcritical CO2 refrigeration system provided in this embodiment is a two-stage compression CO2 refrigeration system using two expanders. The system includes two cycles: the main refrigeration cycle, namely the transcritical CO2 cycle, and the waste heat recovery-driven organic Rankine power cycle.

[0034] In the main refrigeration cycle, the evaporator 1, the low-pressure stage compressor 2, the high-pressure stage compressor 3, the gas waste heat recovery heat exchanger 4, and the first expander 5 are connected in sequence to form a closed loop.

[0035] The outlet of evaporator 1 is connected in sequence to low-pressure stage compressor 2 and high-pressure stage compressor 3. The outlet of high-pressure stage compressor 3 is connected to gas waste heat recovery heat exchanger 4. The outlet of gas waste heat recovery heat exchanger 4 is connected to first expander 5. First expander 5 is connected to evaporator 1.

[0036] In the organic Rankine power cycle driven by waste heat recovery, the first condenser 7, the second expander 6, the gas waste heat recovery heat exchanger 4 and the pump 8 are connected in sequence to form a closed loop.

[0037] In this loop, the outlet of pump 8 is connected to gas waste heat recovery heat exchanger 4, the outlet of gas waste heat recovery heat exchanger 4 is connected to the inlet of second expander 6, the outlet of second expander 6 is connected to the inlet of first condenser 7, and the outlet of first condenser 7 is connected to pump 8.

[0038] The refrigeration method of the above-mentioned transcritical CO2 refrigeration system is as follows:

[0039] The main refrigeration cycle works as follows: The refrigerant evaporates and absorbs heat in the evaporator 1 to cool the outside. The refrigerant gas exiting the evaporator 1 enters the low-pressure stage compressor 2 and the high-pressure stage compressor 3 for compression. The high-temperature and high-pressure gas exiting the high-pressure stage compressor 3 releases heat through the gas waste heat recovery heat exchanger 4 and then enters the first expander 5. After exiting the first expander 5, it enters the evaporator 1. The expansion work recovered by the first expander 5 is used to drive the low-pressure stage compressor 2.

[0040] The working process of the organic Rankine cycle driven by waste heat recovery is as follows: After being pressurized by pump 8, the working fluid enters the gas waste heat recovery heat exchanger 4 to absorb heat, becoming a high-temperature, high-pressure gas. This gas then enters the second expander 6 to expand and perform work, and the expanded output power drives the high-pressure stage compressor 3. The low-temperature, low-pressure gas at the outlet of the second expander 6 is cooled by the condenser and then pressurized by pump 8 to complete this power cycle.

[0041] The first expander 5 and the second expander 6 recover the expansion work of the high-pressure refrigerant. By coaxially connecting or motor connecting them to the low-pressure compressor 2 and the high-pressure compressor 3 respectively, the power consumption of the low-pressure compressor 2 and the high-pressure compressor 3 in the system is reduced, thereby improving the system energy efficiency.

[0042] Example 2

[0043] like Figure 2 As shown, the transcritical CO2 refrigeration system provided in this embodiment is based on Embodiment 1 ( Figure 1 Based on the previous scheme, a mechanical subcooling cycle and a cooling branch for waste heat recovery to drive the organic Rankine power cycle were added.

[0044] Among them, 1-5&9 are transcritical CO2 basic cycles (i.e., main refrigeration cycles), 4-6-7-8 are waste heat recovery driven organic Rankine power cycles, and 7&9-10-11-12 are mechanical subcooling cycles.

[0045] In the closed loop of the main refrigeration cycle, a mechanical subcooling heat exchanger 9 is added between the gas waste heat recovery heat exchanger 4 and the first expander 5. The outlet of the gas waste heat recovery heat exchanger 4 is connected to the mechanical subcooling heat exchanger 9, and the mechanical subcooling heat exchanger 9 is connected to the first expander 5. That is, the evaporator 1, the low-pressure stage compressor 2, the high-pressure stage compressor 3, the gas waste heat recovery heat exchanger 4, the mechanical subcooling heat exchanger 9, and the first expander 5 are connected in sequence to form a closed loop.

[0046] The waste heat recovery-driven organic Rankine power cycle is also a closed loop consisting of the first condenser 7, the second expander 6, the gas waste heat recovery heat exchanger 4, and the pump 8 connected in sequence.

[0047] In the mechanical subcooling cycle, the expansion valve 12, the first condenser 7, the mechanical subcooling heat exchanger 9, the compressor 10, and the second condenser 11 are connected in sequence to form a closed loop. The outlet of the expansion valve 12 is connected to the first condenser 7 and the mechanical subcooling heat exchanger 9, respectively. The outlets of the first condenser 7 and the mechanical subcooling heat exchanger 9 are connected to the inlet of the compressor 10. The outlet of the compressor 10 is connected to the second condenser 11. The outlet of the second condenser 11 is connected to the expansion valve 12.

[0048] The refrigeration method of the above-mentioned transcritical CO2 refrigeration system is as follows:

[0049] The main refrigeration cycle workflow is as follows: Unlike the scheme in Example 1, the gaseous refrigerant from the high-pressure stage compressor 3 does not directly enter the first expander 5 after releasing heat through the gas waste heat recovery heat exchanger 4. Instead, it further releases heat through the mechanical subcooling heat exchanger 9. The high-temperature and high-pressure gas is cooled into subcooled gas and then enters the first expander 5. After exiting the first expander 5, it enters the evaporator 1. The expansion work recovered by the first expander 5 is used to drive the low-pressure stage compressor 2.

[0050] The working process of the mechanical subcooling cycle is as follows: After the refrigerant is throttled by the expansion valve 12, it is divided into two branches. One branch subcools the refrigerant at the outlet of the waste heat recovery heat exchanger 4 of the transcritical CO2 cycle, which is the main refrigeration cycle. The low-temperature and low-pressure refrigerant in the other branch cools and exchanges heat with the first condenser 7 of the waste heat recovery driving organic Rankine power cycle. The refrigerant that has absorbed heat enters the compressor 10 together with the refrigerant gas exiting the mechanical subcooling heat exchanger 9 for compression. The high-temperature and high-pressure gas from the compressor 10 releases heat through the second condenser 11 and then enters the expansion valve 12, thus completing the cycle process.

[0051] The working process of the organic Rankine power cycle driven by waste heat recovery is as follows: After the working fluid is pressurized by pump 8, it enters the gas waste heat recovery heat exchanger 4 to absorb heat, and then enters the second expander 6 to expand and do work. The expansion output power is used to drive the high-pressure stage compressor 3. The gas coming out of the second expander 6 is cooled by the cooling branch of the mechanical subcooling cycle through the first condenser 7, and then the pressure is increased by pump 8 to complete this power cycle.

[0052] The mechanical subcooling cycle uses the mechanical subcooling heat exchanger 9 to subcool the refrigerant in the main refrigeration cycle and cool the first condenser 7. On the one hand, the waste heat recovery improves the operating conditions of the organic Rankine power cycle, increasing output power and improving cycle performance. On the other hand, the subcooling of the refrigerant in the main cycle loop increases the system's unit cooling capacity. The first expander 5 and the second expander 6 recover the expansion work of the high-pressure refrigerant, reducing the overall power consumption of the corresponding two compressors in the system. Specifically, the power recovery of the first expander 5 reduces the power consumption of the low-pressure stage compressor 2, and the output power of the second expander 6 reduces the power consumption of the high-pressure stage compressor 3, thereby improving system performance.

[0053] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, alterations, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A transcritical CO2 refrigeration system, characterized in that, It includes a main refrigeration cycle and a waste heat recovery driven organic Rankine power cycle. The main refrigeration cycle includes an evaporator. The outlet of the evaporator is connected in sequence to a low-pressure stage compressor and a high-pressure stage compressor. The outlet of the high-pressure stage compressor is connected to a gas waste heat recovery heat exchanger. The outlet of the gas waste heat recovery heat exchanger is connected to a first expander. The first expander is connected to the evaporator. The waste heat recovery-driven organic Rankine power cycle includes a first condenser and a second expander connected to the first condenser. The outlet of the gas waste heat recovery heat exchanger is connected to the second expander, the outlet of the second expander is connected to the inlet of the first condenser, and the outlet of the first condenser is connected to a pump. The pump is connected to the gas waste heat recovery heat exchanger. The refrigeration system also includes a mechanical subcooling cycle, which includes a mechanical subcooling heat exchanger, a compressor, a second condenser and a throttle valve connected in sequence. The mechanical subcooling heat exchanger is located between the gas waste heat recovery heat exchanger and the first expander. The outlet of the gas waste heat recovery heat exchanger is connected to the mechanical subcooling heat exchanger, and the mechanical subcooling heat exchanger is connected to the first expander. The outlet of the throttle valve is connected to the mechanical subcooling heat exchanger and the first condenser, respectively. The outlets of the mechanical subcooling heat exchanger and the first condenser are connected to the compressor. The outlet of the compressor is connected to the second condenser and the throttle valve in sequence.

2. The refrigeration method using the transcritical CO2 refrigeration system as described in claim 1, characterized in that, The refrigeration system includes a main refrigeration cycle, a waste heat recovery-driven organic Rankine power cycle, and a mechanical subcooling cycle; the refrigeration method includes: In the main refrigeration cycle refrigeration method, the refrigerant evaporates and absorbs heat in the evaporator to form refrigerant gas. The refrigerant gas enters the low-pressure stage compressor and the high-pressure stage compressor for compression. The resulting high-temperature and high-pressure gas releases heat through the gas waste heat recovery heat exchanger and then enters the mechanical subcooling heat exchanger to further release heat. After becoming subcooled gas, it enters the first expander to drive the low-pressure stage compressor. After exiting the first expander, it enters the evaporator. In the mechanical subcooling cycle refrigeration method, the refrigerant after being throttled by the expansion valve is divided into two branches. The refrigerant in one branch enters the compressor through the mechanical subcooling heat exchanger, while the refrigerant in the other branch exchanges heat through the first condenser and then enters the compressor together with the refrigerant gas exiting the mechanical subcooling heat exchanger for compression. After releasing heat through the second condenser, it enters the expansion valve. In the waste heat recovery-driven organic Rankine cycle refrigeration method, the working fluid is pressurized in the pump and then enters the gas waste heat recovery heat exchanger to absorb heat. It then enters the second expander to do work to drive the high-pressure stage compressor. The gas coming out of the second expander is cooled by the first condenser and then enters the pump to complete the cycle.

3. The refrigeration method of the transcritical CO2 refrigeration system according to claim 2, characterized in that, The mechanical subcooling cycle subcools the refrigerant of the main refrigeration cycle through a mechanical subcooling heat exchanger and cools the first condenser.

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