A three-stage cascade heat pump system and control method capable of providing dual-temperature zone cooling and heating.

Abstract

This invention relates to a three-stage cascade heat pump system and control method capable of providing dual-temperature zone cooling and heating. The system includes three loops: high-temperature, medium-temperature, and low-temperature, which are cascaded and connected via condenser-evaporators. The high-temperature loop distributes refrigerant to a first condenser and a second condenser via a first three-way valve. A first vortex tube is installed at the inlet of the second condenser to subcool the refrigerant entering the first condenser-evaporator. A second three-way valve controls whether hot air enters the second condenser. The medium-temperature loop distributes refrigerant to a medium-temperature evaporator and a second condenser-evaporator via a third three-way valve, and before distribution, the refrigerant is mixed with and subcooled by the cold air at the cold end of the second vortex tube. The low-temperature loop distributes refrigerant to a low-temperature evaporator and an internal evaporator via a fourth three-way valve, and before distribution, the refrigerant is mixed with and subcooled by the cold air at the cold end of the third vortex tube. This system, through the medium-temperature evaporator, the low-temperature evaporator, and the two condensers, can meet multi-temperature requirements, improve energy efficiency, and suppress flash evaporation through vortex tube subcooling.

Description

Technical Field

[0001] This invention relates to the field of heat pump technology, specifically to a three-stage cascade heat pump system and control method capable of providing cooling and heating in dual temperature zones. Background Technology

[0002] With the continuous development of refrigeration technology, cascade refrigeration systems have been widely used due to their ability to achieve cooling in lower temperature ranges. However, existing cascade refrigeration systems still have the following significant shortcomings: 1) They have a single cooling temperature range, making it difficult to simultaneously meet the cooling needs of dual temperature ranges and above, such as medium temperature and ultra-low temperature, resulting in limited system adaptability; 2) The refrigerant lacks effective subcooling treatment before throttling, which easily leads to flash evaporation, resulting in increased throttling losses, decreased cooling effect, and thus increased energy consumption; 3) A large amount of energy is directly dissipated during the throttling process, causing energy waste and poor economic efficiency; 4) The cooling mode is single, making it impossible to flexibly adjust according to changes in ambient temperature to achieve efficient and energy-saving operation, resulting in high overall system energy consumption. Summary of the Invention

[0003] To address the technical problems existing in the background art, the present invention provides a three-stage cascade heat pump system and control method capable of providing dual-temperature zone cooling and heating.

[0004] The technical solution of this invention is as follows: A three-stage cascade heat pump system capable of providing cooling and heating in two temperature zones, comprising three loops: high temperature, medium temperature, and low temperature; The high-temperature circuit includes a first compressor, a condensing unit, a first throttle valve, and a first condenser-evaporator connected in series. The condensing unit includes a first condenser and a second condenser connected in parallel. The two are connected to the outlet of the first compressor through a first three-way valve. A first vortex tube is provided on the inlet side of the second condenser. The inlet of the first vortex tube is connected to the first three-way valve. The cold end outlet is connected to the inlet pipe of the first throttle valve. The hot end outlet is connected to the inlet of the second condenser and the inlet of the first compressor through a second three-way valve. The medium-temperature circuit includes a second compressor and a medium-temperature refrigeration unit connected in series. The medium-temperature refrigeration unit includes a medium-temperature evaporator and a second condenser-evaporator connected in parallel. The two are connected to the outlet of the second compressor through a third three-way valve, and both are equipped with a second throttle valve on their inlet side. The outlet of the second compressor is connected to the inlet of the third three-way valve through the first condenser-evaporator. A second vortex tube is provided on the outlet side of the second compressor. The inlet, cold end outlet, and hot end outlet of the second vortex tube are connected to the outlet of the second compressor, the inlet pipeline of the third three-way valve, and the inlet of the second compressor, respectively. The low-temperature circuit includes a third compressor and a low-temperature refrigeration unit connected in series. The low-temperature refrigeration unit includes a low-temperature evaporator and an internal evaporator connected in parallel. The two are connected to the outlet of the third compressor through a fourth three-way valve, and both are equipped with a third throttle valve on their inlet side. The outlet of the third compressor is connected to the inlet of the fourth three-way valve through a second condenser-evaporator. A third vortex tube is provided on the outlet side of the third compressor. The inlet, cold end outlet, and hot end outlet of the third vortex tube are connected to the outlet of the third compressor, the inlet pipeline of the fourth three-way valve, and the inlet of the third compressor, respectively.

[0005] Furthermore, the first ejector and the second ejector are connected in series on the branches of the second condenser-evaporator and the low-temperature evaporator, respectively; The working flow inlet of the first ejector is connected to the inlet pipe of the third three-way valve and is located downstream of the outlet connection position of the cold end of the second vortex tube. The jet flow inlet of the first ejector is connected to the second condenser evaporator, and the outlet is connected to the inlet of the second compressor. The working flow inlet of the second injector is connected to the inlet pipe of the fourth three-way valve and is located downstream of the cold end outlet connection of the third vortex tube. The jet inlet of the second injector is connected to the low-temperature evaporator, and the outlet is connected to the inlet of the third compressor.

[0006] Furthermore, the inlet pipes of the first throttle valve, the third three-way valve, and the fourth three-way valve are respectively connected in series with the first liquid reservoir, the second liquid reservoir, and the third liquid reservoir. The cold end outlet connection positions of the three vortex tubes are respectively located downstream of the corresponding liquid reservoirs.

[0007] Furthermore, a first gas-liquid separator, a second gas-liquid separator, and a third gas-liquid separator are connected in series on the inlet pipes of the first compressor, the second compressor, and the third compressor, respectively. The hot end outlets of the three vortex tubes, the first condenser-evaporator, the medium-temperature refrigeration unit, and the low-temperature refrigeration unit are all connected to the corresponding gas-liquid separators, and the liquid outlets of the three gas-liquid separators are connected to the corresponding liquid receivers.

[0008] Furthermore, oil separators are connected in series on the outlet pipes of the first compressor and the second compressor, respectively.

[0009] Furthermore, the refrigerants for the high-temperature, medium-temperature, and low-temperature circuits are NH3, CO2, and R23, respectively.

[0010] This application also provides a control method for a three-stage cascade heat pump system, applied to the aforementioned three-stage cascade heat pump system capable of dual-temperature zone cooling and heating. The three-stage cascade heat pump system further includes a control system and a temperature sensor for detecting ambient temperature. The method includes: S1. Real-time acquisition of ambient temperature T; S2. Compare T with preset temperature thresholds t1, t2, t3, and t4, where t1 > t2 > t3 > t4; When T≥t1, execute S3; When t1 > T ≥ t2, execute S4; When t2>T≥t3, execute S5; When t3 > T ≥ t4, execute S6; When T≤t4, execute S7; S3. Only open the high temperature and medium temperature circuits. Open the first three-way valve to distribute the refrigerant to the first condenser and the first vortex tube in a ratio of a1:a2. Open the second three-way valve, but close the outlet connected to the second condenser. Open the third three-way valve, but close the outlet connected to the second condenser-evaporator. Where a1 > a2. S4. Open the three circuits: high temperature, medium temperature, and low temperature. Open the first three-way valve to distribute the refrigerant to the first condenser and the first vortex tube in the ratio of a1:a2. Only open the second three-way valve to connect to the outlet of the second condenser. Open the two outlets of the third three-way valve with the same opening degree. Open the two outlets of the fourth three-way valve with the same opening degree. S5. Open the three circuits: high temperature, medium temperature, and low temperature. Open the first three-way valve to distribute the refrigerant to the first condenser and the first vortex tube in the ratio of b1:b2. Only open the second three-way valve to connect to the outlet of the second condenser. Open the two outlets of the third three-way valve with the same opening degree. Open the two outlets of the fourth three-way valve with the same opening degree. Where a1>b1>b2>a2. S6. Open the three circuits: high temperature, medium temperature, and low temperature. Open the first three-way valve to distribute refrigerant to the first condenser and the first vortex tube in a ratio of a1:a2. Open the second three-way valve, but close the outlet connected to the second condenser-evaporator. Open the third three-way valve to distribute refrigerant to the second condenser-evaporator and the medium temperature evaporator in a ratio of c1:c2, where c1>c2. Open the fourth three-way valve to distribute refrigerant to the low temperature evaporator and the internal evaporator in a ratio of d1:d2, where d1>d2. S7. Only open the high-temperature circuit, wherein only open the outlet of the first three-way valve connected to the first vortex tube, and only open the outlet of the second three-way valve connected to the second condenser.

[0011] Furthermore, before S1, at system startup time t1, the opening of the first throttle valve opens to the preset opening degree D1 at a preset rate. At time t2, the openings of the second and third throttle valves open to the preset opening degrees D2 and D3 at the same preset rate, respectively, where t2 > t1.

[0012] Furthermore, the high-temperature circuit is also equipped with a temperature sensor to detect the refrigerant temperature at the outlet side of the first condenser-evaporator. In any step S3-S7, the refrigerant temperature T_eva1 at the outlet side of the first condenser-evaporator is obtained at a preset time period and compared with a preset target temperature range. When T_eva1 is greater than the target temperature range for a consecutive preset number of cycles, the opening of the first throttle valve is reduced by a preset unit until T_eva1 is within the preset target temperature range. When T_eva1 is less than the preset target temperature range for a consecutive preset number of cycles, the opening of the first throttle valve is increased by a preset unit until T_eva1 is within the preset target temperature range. If the opening of the first throttle valve exceeds the preset limit range and T_eva1 also exceeds the preset target temperature range for a preset time, the opening of the first three-way valve connected to the first condenser will be increased by a preset unit change value until T_eva1 is within the preset target temperature range.

[0013] Furthermore, when executing any of steps S3-S6, if the system has been running continuously and stably for 1 hour, the operating frequency of the corresponding compressor is reduced by 10% to 20%.

[0014] The present invention provides a three-stage cascade heat pump system and control method capable of providing dual-temperature zone cooling and heating, which has the following beneficial effects: 1. Through a three-stage superimposed loop of high, medium and low temperature, and with separate medium-temperature evaporators, low-temperature evaporators and two condensers, it can simultaneously meet the cooling needs of both medium and ultra-low temperature zones, and can switch to heating, realizing high-temperature heat dissipation, waste heat recovery heating and cold and heat coordinated energy supply mode, improving energy utilization, realizing integrated cold and heat supply, and meeting the energy supply needs under multiple temperature conditions.

[0015] 2. A vortex tube is installed before the throttling valve in each circuit. The cold end outlet of the tube is used to subcool the refrigerant in the main circuit, which effectively suppresses the flashing phenomenon before throttling and reduces throttling losses.

[0016] 3. The hot end outlet of the vortex tube draws the high-temperature gas back to the compressor inlet, and at the same time uses the ejector to recover the residual pressure energy after throttling, realizing energy recovery and utilization, reducing compressor power consumption, and improving economy.

[0017] 4. The control system automatically switches the operating mode according to the ambient temperature and dynamically adjusts the distribution ratio of each three-way valve, so that the system can cool more efficiently and operate in energy-saving mode under different climatic conditions. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the system; Figure 2 This is a schematic diagram of the system's control flow.

[0019] The components represented by the various reference numerals in the diagram are: 11. First compressor; 12. First three-way valve; 131. First condenser; 132. Second condenser; 14. First throttle valve; 15. First condensing evaporator; 16. First vortex tube; 17. Second three-way valve; 18. First liquid receiver; 19. First gas-liquid separator; 21. Second compressor; 22. Medium-temperature evaporator; 23. Second condensing evaporator; 24. Third three-way valve; 25. Second throttle valve; 26. Second vortex tube; 27. First ejector; 28. Second liquid receiver; 29. ​​Second gas-liquid separator; 31. Third compressor; 32. Low-temperature evaporator; 33. Internal evaporator; 34. Fourth three-way valve; 35. Third throttle valve; 36. Third vortex tube; 37. Second ejector; 38. Third liquid receiver; 39. Third gas-liquid separator. Detailed Implementation

[0020] Example 1 like Figure 1 As shown, this embodiment provides a three-stage cascade heat pump system capable of providing cooling and heating in dual temperature zones, comprising three independent circulation loops: a high-temperature loop, a medium-temperature loop, and a low-temperature loop. The three loops can operate independently or work in concert to achieve multiple functions, including medium-temperature cooling, ultra-low-temperature cooling, and heating.

[0021] The high-temperature circuit includes a first compressor 11, a condensing unit, a first throttle valve 14, and a first condenser-evaporator 15 connected in series by pipelines.

[0022] The condensing unit includes a first condenser 131 and a second condenser 132 connected in parallel. They are connected to the outlet of the first compressor 11 via a first three-way valve 12. The first three-way valve 12 regulates the ratio of refrigerant flowing into the branches containing the first condenser 131 and the second condenser 132. The first condenser 131 is mainly used for cooling and heat dissipation, releasing heat into the environment. The second condenser 132 is used to release heat to users in heating mode, such as for supplying process hot water.

[0023] An oil separator is connected in series on the pipeline between the first compressor 11 and the first three-way valve 12 to separate the lubricating oil carried in the exhaust of the first compressor 11, preventing the lubricating oil from entering the two condensers and affecting the heat exchange efficiency. A one-way valve is connected in series on the pipeline between the oil separator and the first three-way valve 12 to prevent high-temperature and high-pressure refrigerant gas from flowing back into the first compressor 11.

[0024] A first vortex tube 16 is connected in series on the branch where the second condenser 132 is located. The first vortex tube 16 is located on the inlet side of the second condenser 132. The inlet of the first vortex tube 16 is connected to one outlet of the first three-way valve 12. The cold end outlet of the first vortex tube 16 is connected to the inlet pipe of the first throttling valve 14. The hot end outlet of the first vortex tube 16 is connected to the inlet of the second condenser 132 and the inlet of the first compressor 11 through the second three-way valve 17. The low-temperature gas at the cold end outlet of the first vortex tube 16 can subcool the refrigerant before throttling, suppress flashing, and reduce throttling losses. The high-temperature gas at the hot end outlet of the first vortex tube 16 can be selected to enter the second condenser 132 for heating or return to the inlet of the first compressor 11, depending on the mode.

[0025] A first liquid receiver 18 is connected in series on the pipeline between the first throttle valve 14 and the condenser unit. It is used to store and regulate the amount of refrigerant required for the cycle and output liquid refrigerant. The cold end outlet of the first vortex tube 16 is connected to the pipeline between the first throttle valve 14 and the first liquid receiver 18. Its low-temperature gaseous refrigerant mixes and subcools with the liquid refrigerant flowing out of the first liquid receiver 18 before entering the first throttle valve 14 for further cooling and depressurization.

[0026] The first condenser-evaporator 15 is preferably a shell-and-tube cascade condenser-evaporator, and the outlet of the first throttle valve 14 is connected to the tube-side inlet of the first condenser-evaporator 15 via a pipeline. The tube-side outlet of the first condenser-evaporator 15 is connected to the inlet of the first compressor 11 via a pipeline.

[0027] Furthermore, a first gas-liquid separator 19 is connected in series on the pipeline between the first condenser-evaporator 15 and the first compressor 11 to prevent unevaporated droplets from entering the first compressor 11 and causing liquid slugging, thus ensuring the safe operation of the first compressor 11. One outlet of the second three-way valve 17 is connected to the first gas-liquid separator 19 through a pipeline, and the hot end outlet of the first vortex tube 16 is connected to the inlet of the first compressor 11 via the second three-way valve 17 and the first gas-liquid separator 19.

[0028] NH3 is preferred as the refrigerant in high-temperature circuits. NH3 has excellent heat exchange performance and a high critical temperature, making it suitable as a high-temperature working fluid. It is also safe and environmentally friendly.

[0029] The intermediate-temperature circuit includes a second compressor 21 connected in series via piping and an intermediate-temperature refrigeration unit. This intermediate-temperature refrigeration unit includes a medium-temperature evaporator 22 and a second condensing evaporator 23 connected in parallel. Both are connected to the outlet of the second compressor 21 via a third three-way valve 24, and both have a second throttling valve 25 on their inlet side. The medium-temperature evaporator 22 provides medium-temperature cooling capacity, for example, -20 to -35°C, while the second condensing evaporator 23 provides a condensing cold source for the low-temperature circuit; it is preferably a shell-and-tube cascade condensing evaporator.

[0030] The outlet of the second compressor 21 is connected to an oil separator via a pipeline to separate the lubricating oil carried in the exhaust of the second compressor 21. The oil separator is connected to the shell-side inlet of the first condenser-evaporator 15 via a pipeline, and the shell-side outlet of the first condenser-evaporator 15 is connected to the inlet of the third three-way valve 24 via a pipeline. The first condenser-evaporator 15 of the high-temperature circuit is used as the condenser of the medium-temperature circuit, so that the high-temperature circuit provides a cold source for the medium-temperature circuit, forming a superposition.

[0031] Furthermore, a second liquid receiver 28 is connected in series in the pipeline between the first condenser evaporator 15 and the third three-way valve 24 to store and regulate the amount of refrigerant required for the cycle and output liquid refrigerant.

[0032] The outlet side of the second compressor 21 is also equipped with a second vortex tube 26. The inlet of the second vortex tube 26 is connected to the pipeline between the oil separator and the first condenser-evaporator 15 via a pipeline. The cold end outlet of the second vortex tube 26 is connected to the pipeline between the third three-way valve 24 and the second liquid receiver 28 via a pipeline. The hot end outlet of the second vortex tube 26 is connected to the inlet of the second compressor 21. The function of the second vortex tube 26 is similar to that of the high-temperature circuit, that is, to subcool the refrigerant before throttling and recover the hot end gas, thereby improving the system energy efficiency.

[0033] Furthermore, a first ejector 27 is also provided in the intermediate temperature circuit. The working flow inlet of the first ejector 27 is connected to the pipeline between the third three-way valve 24 and the second liquid receiver 28 via a pipeline, and the connection position is located downstream of the cold end outlet connection position of the second vortex tube 26. That is, the liquid refrigerant flowing out of the second liquid receiver 28 is first mixed and subcooled with the gaseous refrigerant flowing out of the cold end of the second vortex tube 26, and then part of it flows into the first ejector 27 as the ejector fluid, and the other part is distributed to the intermediate temperature evaporator 22 and / or the second condensing evaporator 23 via the third three-way valve 24. The ejector flow inlet of the first ejector 27 is connected to the second condensing evaporator 23, and the outlet of the first ejector 27 is connected to the inlet of the second compressor 21. The first ejector 27 uses the ejection effect of the high-pressure working fluid to draw in the low-pressure refrigerant of the second condensing evaporator 23, pressurize it, and send it back to the second compressor 21, thereby recovering the residual pressure energy after throttling and reducing the compressor power consumption.

[0034] The two outlets of the third three-way valve 24 are connected to the two second throttle valves 25 through pipelines. The two second throttle valves 25 are connected to the inlet of the medium-temperature evaporator 22 and the tube-side inlet of the second condensing evaporator 23 through pipelines. The tube-side outlet of the second condensing evaporator 23 is connected to the jet inlet of the first ejector 27 through pipelines.

[0035] A second gas-liquid separator 29 is connected in series on the inlet side pipe of the second compressor 21. The outlet of the medium-temperature evaporator 22, the outlet of the first ejector 27, and the hot end outlet of the second vortex tube 26 are all connected to the second gas-liquid separator 29. The second gas-liquid separator 29 is connected to the inlet of the second compressor 21 to prevent liquid refrigerant from entering the second compressor 21 and causing liquid slugging.

[0036] CO2 is preferred as the refrigerant in the medium-temperature circuit. CO2 has the advantages of being environmentally friendly, inexpensive, and having a large cooling capacity per unit volume, making it suitable as a medium-temperature working fluid.

[0037] The cryogenic circuit includes a third compressor 31 connected in series via pipelines and a cryogenic refrigeration unit. The cryogenic refrigeration unit includes a cryogenic evaporator 32 and an internal evaporator 33 connected in parallel. Both are connected to the outlet of the third compressor 31 via a fourth three-way valve 34, and both are equipped with a third throttle valve 35 on their inlet sides. The cryogenic evaporator 32 is used to provide ultra-low temperature cooling capacity externally, while the internal evaporator 33 is used to maintain the temperature stability of the medium-temperature circuit, especially the temperature stability of the gas drawn into the third compressor 31, ensuring the long-term stable operation of the third compressor 31 under ultra-low temperature conditions.

[0038] The outlet of the third compressor 31 is connected to the inlet of the fourth three-way valve 34 after passing through the second condenser-evaporator 23. Specifically, the outlet of the third compressor 31 is connected to the shell-side inlet of the second condenser-evaporator 23 through a pipeline, and the shell-side outlet of the second condenser-evaporator 23 is connected to the inlet of the fourth three-way valve 34 through a pipeline. The second condenser-evaporator 23 is used as the condenser of the low-temperature circuit, so that the medium-temperature circuit provides a cold source for the low-temperature circuit, forming a superposition.

[0039] Furthermore, a third liquid receiver 38 is connected in series in the pipeline between the second condenser evaporator 23 and the fourth three-way valve 34 to store and regulate the amount of refrigerant required for the cycle and output liquid refrigerant.

[0040] The outlet side of the third compressor 31 is equipped with a third vortex tube 36. The inlet of the third vortex tube 36 is connected to the pipeline between the third compressor 31 and the second condenser-evaporator 23 via a pipeline. The cold end outlet of the third vortex tube 36 is connected to the pipeline between the fourth three-way valve 34 and the third liquid receiver 38 via a pipeline. The hot end outlet of the third vortex tube 36 is connected to the inlet of the third compressor 31. The third vortex tube 36 is also used to achieve subcooling before throttling and energy recovery. Especially for ultra-low temperature conditions, subcooling is particularly important for improving system performance.

[0041] Furthermore, a second ejector 37 is also provided in the cryogenic circuit. The working flow inlet of the second ejector 37 is connected to the pipeline between the fourth three-way valve 34 and the third liquid receiver 38 via a pipeline, and the connection position is located downstream of the cold end outlet connection position of the third vortex tube 36. That is, the liquid refrigerant flowing out of the third liquid receiver 38 is first mixed and subcooled with the gaseous refrigerant flowing out of the cold end of the third vortex tube 36, and then part of it flows into the second ejector 37 as the ejector fluid, and the other part is distributed to the cryogenic evaporator 32 and / or the internal evaporator 33 via the fourth three-way valve 34. The ejector flow inlet of the second ejector 37 is connected to the cryogenic evaporator 32, and the outlet of the second ejector 37 is connected to the inlet of the third compressor 31. The second ejector 37 uses the ejection effect of the high-pressure working fluid to draw in the low-pressure refrigerant of the cryogenic evaporator 32, pressurize it, and send it back to the third compressor 31, thereby recovering the residual pressure energy of the low-pressure refrigerant at the outlet of the cryogenic evaporator 32, increasing the compressor suction pressure, thereby reducing compressor energy consumption, which is especially suitable for ultra-low temperature and large temperature difference conditions.

[0042] Similarly, a third gas-liquid separator 39 is connected in series on the inlet pipe of the third compressor 31. The outlet of the internal evaporator 33, the outlet of the second ejector 37, and the hot end outlet of the third vortex tube 36 are all connected to the third gas-liquid separator 39. The third gas-liquid separator 39 is connected to the inlet of the third compressor 31 to prevent liquid refrigerant from entering the third compressor 31 and causing liquid slugging.

[0043] R23 is preferred as the refrigerant in the low-temperature circuit. R23 has a low boiling point and good low-temperature fluidity, making it suitable as a working fluid for ultra-low temperature stages.

[0044] Temperature and pressure sensors are installed in all three loops, and liquid level sensors are installed in each gas-liquid separator to monitor system operating parameters in real time. When system parameters exceed the safe range, the control system automatically triggers the protection mechanism to close relevant valves and stop the compressor, ensuring the safe and stable operation of the system.

[0045] Example 2 This embodiment provides a control method for the three-stage cascade heat pump system described in Embodiment 1. The three-stage cascade heat pump system is also equipped with a control system and a temperature sensor for detecting ambient temperature. The method includes: S1. Real-time acquisition of ambient temperature T.

[0046] S2. Compare the ambient temperature T with preset thresholds t1, t2, t3, and t4, and automatically switch operating modes to ensure the system always operates within its optimal efficiency range under different environmental conditions, avoiding unnecessary energy consumption under inefficient operating conditions. The preset thresholds satisfy t1 > t2 > t3 > t4.

[0047] In this embodiment, t1 is 25°C, t2 is 20°C, t3 is 10°C, and t4 is -50°C.

[0048] When T≥t1, execute S3; when t1>T≥t2, execute S4; when t2>T≥t3, execute S5; when t3>T≥t4, execute S6; when T≤t4, execute S7.

[0049] S3. When the ambient temperature T≥25℃, such as in summer, the system enters the medium-temperature cooling mode M1.

[0050] An ambient temperature of 25°C marks the economic threshold at which the combined COP (Coefficient of Performance) of a three-stage cascade system begins to fall below that of the "intermittent insulation + single-stage refrigeration" combination. When the ambient temperature exceeds 25°C, heat dissipation from the high-temperature circuit to the air becomes difficult, forcing the condensing temperature to rise. This leads to an increase in the compression ratio of the first compressor and a significant drop in COP. If the low-temperature circuit continues to operate at this point, the cumulative efficiency loss of the three-stage compression is amplified. Shutting down the low-temperature circuit can prevent wasted effort in the inefficient range.

[0051] Furthermore, compared to medium-temperature circuits, low-temperature circuits have a larger temperature difference between the working area and the ambient temperature, stronger insulation, and higher cold storage capacity. Therefore, the thermal inertia of the working area corresponding to a low-temperature circuit is significantly greater, resulting in better temperature stability. Experimental calculations show that the temperature rise rate of the working area corresponding to a low-temperature circuit (such as a cold storage facility) is generally about 0.3~0.6℃ / hour, requiring 10~15 hours to rise from -85℃ to -78℃. This high thermal inertia fully allows for temporary shutdowns of the low-temperature circuit under high-temperature environments.

[0052] In high-temperature environments, the system enters a medium-temperature cooling mode. At this time, the most power-consuming low-temperature circuit goes into hibernation, utilizing the thermal inertia of its corresponding operating area to maintain its own temperature. The system primarily maintains cooling for the operating area corresponding to the medium-temperature circuit, ensuring cooling in the relatively less thermally inertial medium-temperature cooling area, reducing overall system energy consumption, and avoiding "operating at a loss." In this embodiment, the preferred medium-temperature cooling temperature is -20 to -35°C.

[0053] Furthermore, the system can also put the low-temperature circuit into hibernation during the day when the temperature is relatively high and the cooling efficiency is relatively low, such as from 6:00 to 19:00. After the temperature drops at night and the cooling efficiency is relatively improved, the low-temperature circuit can be activated to efficiently replenish the cooling and make up for the temperature loss in the corresponding working area during the day, so as to meet the long-term low-temperature demand of the area and thus improve the economic efficiency of the system operation in high-temperature environments.

[0054] The specific control process is as follows: Start the high-temperature and medium-temperature circuits, and shut down the low-temperature circuit.

[0055] In the high-temperature circuit, the first three-way valve 12 is opened to distribute refrigerant to the first condenser 131 and the first vortex tube 16 in a ratio of a1:a2, where a1 > a2, preferably 90% entering the first condenser 131 and 10% entering the first vortex tube 16. Of the two outlets of the second three-way valve 17, the outlet connected to the second condenser 132 is closed, and the other outlet is open. Most of the refrigerant enters the first condenser 131 to dissipate heat to the environment, and a small portion enters the first vortex tube 16 to generate subcooled gas. Since the ambient temperature is high and there is no heating requirement, the gas flowing out of the hot end of the first vortex tube 16 does not enter the second condenser 132 but returns to the first compressor 11. The refrigerant in the first condenser 131 is condensed into a high-pressure saturated liquid by cooling water and flows into the first liquid receiver 18. The high-pressure gas at the cold end outlet of the first vortex tube 16 mixes with the liquid at the outlet of the liquid receiver and is subcooled before being depressurized by the first throttle valve 14 and entering the first condenser-evaporator 15 to provide a cold source for the medium-temperature circuit.

[0056] In the intermediate temperature circuit, the third three-way valve 24 is opened, but only the outlet connected to the intermediate temperature evaporator 22 is opened, while the outlet connected to the second condenser-evaporator 23 is closed. The centralized energy consumption only provides intermediate temperature cooling capacity externally and does not provide a cooling source to the low temperature circuit because the low temperature circuit has been shut down. The second compressor 21 starts, and after condensation by the first condenser-evaporator 15, the liquid mixes and is subcooled with the gas at the cold end outlet of the second vortex tube 26, and then enters the intermediate temperature evaporator 22 for evaporation through the second throttle valve 25, providing intermediate temperature cooling capacity of -20 to -35°C externally.

[0057] The cryogenic circuit was shut down completely.

[0058] S4. When the ambient temperature is 20℃≤T<25℃, such as in the summer or autumn season, the system enters the dual-temperature zone cooling mode M2.

[0059] An ambient temperature of 20-25℃ represents the transitional zone between the COP of cascade systems and single-stage refrigeration, and also marks the onset of heating demand. Within this temperature range, the condensing pressure in the high-temperature loop is moderate, and the COP of the three-stage cascade system is close to that of single-stage refrigeration. Having all three loops fully open can meet potentially simultaneous heating and cooling demands without significant efficiency loss. When the ambient temperature drops below 25℃, domestic hot water demand begins to emerge. A 5℃ buffer zone prevents the system from frequently switching modes near the threshold, reducing compressor start-up and shutdown shocks.

[0060] The specific control process is as follows: The three circuits—high temperature, medium temperature, and low temperature—are activated.

[0061] In the high-temperature circuit, the first three-way valve 12 still distributes heat in the a1:a2 ratio. The second three-way valve 17 switches to open only the outlet connected to the second condenser 132. The high-temperature gas flowing out of the hot end outlet of the first vortex tube 16 flows into the second condenser 132. The second condenser 132 cools the high-temperature gas while supplying heat to the outside for heating or heating domestic water, etc. The preferred heating temperature is 40~55℃. The second condenser 132 recovers the waste heat generated by the first vortex tube 16 to meet the user's heating needs and achieve energy saving.

[0062] In the medium-temperature circuit, the third three-way valve 24 opens two outlets with the same opening degree, that is, it simultaneously supplies refrigerant to the medium-temperature evaporator 22 and the second condenser-evaporator 23, providing medium-temperature refrigeration and providing a cold source for the low-temperature circuit.

[0063] In the low-temperature circuit, the fourth three-way valve 34 opens two outlets with the same degree of opening, that is, simultaneously supplies liquid to the low-temperature evaporator 32 and the internal evaporator 33. The low-temperature evaporator 32 provides a low-temperature cooling capacity of -85 to -105°C to the outside.

[0064] S5. When the ambient temperature is 10℃≤T<20℃, such as in spring or autumn, the system enters the cooling and heating synergy mode M3.

[0065] An ambient temperature of 10-20℃ represents the range with the highest overlap between cooling and heating demands and the best waste heat recovery quality. At this temperature, the condensing pressure is moderate, and cooling demand is relatively lower while heating demand is relatively higher, making it the optimal temperature window for coordinated cooling and heating. Within this temperature range, the condensing temperature of the high-temperature circuit is typically around 30-40℃, and the heat recovered by the second condenser is just enough to produce hot water at 40-55℃, eliminating the need for additional temperature increases and maximizing waste heat utilization. Furthermore, by simultaneously cooling and recovering waste heat, one unit of electricity produces both cooling and heating, improving overall energy efficiency.

[0066] Within this temperature range, the condensing pressure is moderate, and the cooling demand is relatively reduced while the heating demand is relatively increased. In the high-temperature circuit, the first three-way valve 12 distributes 60% to 70% of the refrigerant to the first condenser 131 and 30% to 40% to the second condenser 132. While cooling, waste heat is recovered. One unit of electricity can simultaneously produce both cooling and heating, thus improving overall energy efficiency.

[0067] The specific control process is as follows: The three circuits—high temperature, medium temperature, and low temperature—are activated.

[0068] In the high-temperature circuit, the first three-way valve 12 distributes gas in a b1:b2 ratio, where a1 > b1 > b2 > a2, preferably 60%–70% entering the first condenser 131 and 30%–40% entering the first vortex tube 16. Compared to the medium-temperature refrigeration mode, increasing the proportion of flow entering the first vortex tube 16 generates more hot-end gas for heating. The second three-way valve 17 only opens the outlet connected to the second condenser 132. The hot-end gas separated from the first vortex tube 16 enters the second condenser 132 to release heat and achieve heating, with a heating temperature reaching 40–55°C.

[0069] The activation methods for the medium-temperature and low-temperature circuits are the same as those for the M2 mode, and will not be repeated here.

[0070] S6. When the ambient temperature is -50℃≤T<10℃, such as during the beginning of winter, spring, or winter in some areas, the system enters the low-temperature forced cooling mode M4.

[0071] When the ambient temperature is below 10℃, the high-temperature circuit's heat dissipation capacity to the air increases, the condensing pressure decreases, and the compression ratio decreases significantly, resulting in the lowest power consumption for producing ultra-low temperatures. This is the optimal period for producing ultra-low temperature cooling capacity. An ambient temperature of -50℃ is the lowest temperature at which a conventional heat pump compressor can safely operate its refrigeration cycle. Therefore, in the temperature range of -50℃ ≤ T < 10℃, the "natural cold source" should be fully utilized to complete the ultra-low temperature cooling task with minimal power consumption, reserving sufficient cooling capacity for potential shutdowns and insulation during extremely cold weather. Meanwhile, the demand for medium-temperature cooling decreases as the ambient temperature decreases. The system prioritizes low-temperature cooling; as a trade-off, all heating functions are shut down, focusing on ensuring stable output at ultra-low temperatures of -85℃ to -105℃, with all three circuits operating at full load.

[0072] The specific control process is as follows: The three circuits—high temperature, medium temperature, and low temperature—are activated.

[0073] The high-temperature circuit is activated in the same way as the M1 mode.

[0074] In the medium-temperature circuit, the third three-way valve 24 distributes the refrigerant into the second condenser-evaporator 23 and the medium-temperature evaporator 22 in a ratio of c1:c2, where c1>c2. Preferably, 90% of the refrigerant enters the second condenser-evaporator 23 and 10% enters the medium-temperature evaporator 22. That is, most of the refrigerant enters the second condenser-evaporator 23 to prioritize the condensation cold source of the low-temperature circuit and ensure that the ultra-low temperature circuit receives sufficient cold source input. A small amount of refrigerant enters the medium-temperature evaporator 22 to maintain the refrigeration temperature of the corresponding working area of ​​the medium-temperature circuit.

[0075] In the low-temperature circuit, the fourth three-way valve 34 distributes the refrigerant into the low-temperature evaporator 32 and the internal evaporator 33 in a ratio of d1:d2, where d1>d2. Preferably, 90% of the refrigerant enters the low-temperature evaporator 32 and 10% enters the internal evaporator 33, meaning that most of the refrigerant enters the low-temperature evaporator 32 which supplies cooling to the outside.

[0076] The three circuits operate at full load to ensure stable output at ultra-low temperatures of -85 to -105℃. The system is forced to run at high load for no less than 90 minutes to quickly and deeply cool the corresponding working area of ​​the low-temperature circuit, and to reserve sufficient cooling capacity for possible subsequent shutdown insulation.

[0077] S7. When the ambient temperature T < -50℃, such as in winter in some extremely cold regions, the system enters the extreme cold pure heating mode M5.

[0078] When the ambient temperature is below -50℃, even with a cryogenic refrigerant like R23, its evaporation pressure is close to or below atmospheric pressure, allowing air to easily seep into the system. Simultaneously, the viscosity of the lubricating oil increases dramatically, making compressor starting difficult and causing accelerated wear. Forcing the refrigeration equipment to operate carries an extremely high risk of damage. In extremely cold environments of -50℃, survival is paramount, and building heating is a survival-level necessity. The operating area corresponding to the medium-temperature circuit can rely on ambient temperature in extremely cold environments and does not require refrigeration. However, the operating area corresponding to the low-temperature circuit will experience slower heat loss in extremely cold environments, allowing it to maintain ultra-low temperatures for extended periods. The system then enters full-power heating mode.

[0079] The specific control process is as follows: Turn on the high-temperature circuit and turn off the medium-temperature and low-temperature circuits.

[0080] In the high-temperature circuit, the first three-way valve 12 only opens the outlet connected to the first vortex tube 16 and closes the outlet connected to the first condenser 131. The second three-way valve 17 only opens the outlet connected to the second condenser 132. All the exhaust gas from the first compressor 11 enters the first vortex tube 16, and the separated hot-end gas enters the second condenser 132 to release heat to the heating cycle, achieving pure heating operation.

[0081] Example 3 The difference between this embodiment and Embodiment 2 is that, before S1, during the initial stage of system startup, at time t1, the first throttle valve 14 opens to a preset opening degree D1 at a preset rate. At time t2, the second throttle valve 25 and the third throttle valve 35 open to preset opening degrees D2 and D3 at the same preset rate, respectively, where t2 > t1. This ensures that the throttle valves open gradually according to a preset sequence, avoiding premature opening of the throttle valves when the system pressure has not yet been established and the refrigerant distribution is uneven at the moment the compressor starts, thus preventing liquid slugging risk and evaporator dry-burning problems.

[0082] In this embodiment, t1 is 30 seconds, t2 is 60 seconds, the preset opening rate is 1% / second, D1 is 20%, D2 is 25%, and D3 is 20%.

[0083] Example 4 The difference between this embodiment and Embodiment 2 is that the high-temperature circuit is further equipped with a temperature sensor to detect the refrigerant temperature at the outlet side of the first condenser-evaporator 15. In any step S3-S7, the system acquires the refrigerant temperature T_eva1 at the outlet side of the first condenser-evaporator 15 at a preset time period, compares it with a preset target temperature range, and performs PID negative feedback adjustment based on the comparison result, wherein: When T_eva1 is greater than the target temperature range for a consecutive preset number of cycles, the opening of the first throttle valve 14 is reduced by a preset unit until T_eva1 is within the preset target temperature range. When T_eva1 is less than the preset target temperature range for a consecutive preset number of cycles, the opening of the first throttle valve 14 is increased by a preset unit until T_eva1 is within the preset target temperature range. In this embodiment, the preset time period for acquiring temperature T_eva1 is 10 seconds, and the preset target temperature range is -10 to -15℃. When T_eva1 exceeds the preset target temperature range for three consecutive periods, the opening of the first throttle valve 14 is adjusted by 1% of the preset unit opening.

[0084] In addition, the high-temperature circuit is also equipped with a temperature sensor to detect the refrigerant temperature at the outlet side of the first liquid receiver 18. In any step S3-S7, the system acquires the refrigerant temperature T_sub at the outlet side of the first liquid receiver 18 at a preset time period, compares it with the preset standard temperature range, and performs feedforward compensation adjustment based on the comparison result.

[0085] When T_sub > preset standard temperature range, the first throttle valve 14 is opened wider according to the temperature difference between T_sub and the maximum value of the preset standard temperature range by a preset opening adjustment ratio. When T_sub < preset standard temperature range, the first throttle valve 14 is opened smaller according to the temperature difference between T_sub and the minimum value of the preset standard temperature range by a preset opening adjustment ratio.

[0086] In this embodiment, the preset time period for obtaining T_sub is 2 seconds, and the preset standard temperature range is 30~40℃. When T_sub exceeds the preset standard temperature range, for every 1℃ deviation of T_sub from the preset standard temperature range, the first throttle valve 14 is opened or closed by 1.5%~2%.

[0087] If the opening of the first throttle valve 14 exceeds the preset limit range and T_eva1 also exceeds the preset target temperature range for a preset time, the opening of the first three-way valve 12 connected to the first condenser 131 is increased by a preset unit change value until T_eva1 is within the preset target temperature range.

[0088] In this embodiment, when the opening of the first throttle valve 14 exceeds the preset limit range of 5% to 95% and fails to stabilize the temperature for 50 seconds, the opening ratio of the two openings of the first three-way valve 12 is adjusted to increase the opening of the first three-way valve 12 connected to the first condenser 131 by a preset opening unit change value of 5%.

[0089] Feedforward compensation is used to make adjustments in advance before temperature disturbances affect the outlet temperature of the first condenser-evaporator 15. T_sub is monitored in real time, and adjustments are made immediately once T_sub changes.

[0090] PID negative feedback regulation, based on coarse adjustment using feedforward compensation, performs closed-loop correction according to the deviation between the actual outlet temperature of the first condenser-evaporator 15 and the target temperature range. The adjustment speed of PID negative feedback regulation is slower than that of feedforward compensation regulation, and the adjustment range is also smaller. The rapid response of feedforward compensation regulation avoids drastic fluctuations in T_eva1, allowing PID negative feedback regulation to perform fine adjustment within a relatively stable range.

[0091] Example 5 The difference between this embodiment and embodiment 2 is that when any of steps S3-S6 is executed, if the system has been running continuously and stably for 1 hour, the operating frequency of the compressor that is started is reduced by 10% to 20%. After the system enters a steady state, the compressor speed is reduced by utilizing the system's thermal inertia and load fluctuation tolerance, while meeting the basic needs of the terminal, thereby reducing motor energy consumption under partial load.

[0092] In step S6, under M4 mode, high-load operation is forcibly locked for no less than 90 minutes to ensure that the ultra-low temperature is quickly reached before switching to energy-saving operation as needed. This ensures both rapid cooling of the cold storage and energy saving in subsequent stages.

Claims

1. A three-stage cascade heat pump system capable of providing cooling and heating in dual temperature zones, characterized in that, It includes three circuits: high temperature, medium temperature, and low temperature; The high-temperature circuit includes a first compressor, a condensing unit, a first throttle valve, and a first condenser-evaporator connected in series. The condensing unit includes a first condenser and a second condenser connected in parallel. The two are connected to the outlet of the first compressor through a first three-way valve. A first vortex tube is provided on the inlet side of the second condenser. The inlet of the first vortex tube is connected to the first three-way valve. The cold end outlet is connected to the inlet pipe of the first throttle valve. The hot end outlet is connected to the inlet of the second condenser and the inlet of the first compressor through a second three-way valve. The medium-temperature circuit includes a second compressor and a medium-temperature refrigeration unit connected in series. The medium-temperature refrigeration unit includes a medium-temperature evaporator and a second condenser-evaporator connected in parallel. The two are connected to the outlet of the second compressor through a third three-way valve, and both are equipped with a second throttle valve on their inlet side. The outlet of the second compressor is connected to the inlet of the third three-way valve through the first condenser-evaporator. A second vortex tube is provided on the outlet side of the second compressor. The inlet, cold end outlet, and hot end outlet of the second vortex tube are connected to the outlet of the second compressor, the inlet pipeline of the third three-way valve, and the inlet of the second compressor, respectively. The low-temperature circuit includes a third compressor and a low-temperature refrigeration unit connected in series. The low-temperature refrigeration unit includes a low-temperature evaporator and an internal evaporator connected in parallel. The two are connected to the outlet of the third compressor through a fourth three-way valve, and both are equipped with a third throttle valve on their inlet side. The outlet of the third compressor is connected to the inlet of the fourth three-way valve through a second condenser-evaporator. A third vortex tube is provided on the outlet side of the third compressor. The inlet, cold end outlet, and hot end outlet of the third vortex tube are connected to the outlet of the third compressor, the inlet pipeline of the fourth three-way valve, and the inlet of the third compressor, respectively.

2. The three-stage cascade heat pump system capable of dual-temperature zone cooling and heating as described in claim 1, characterized in that, The first ejector and the second ejector are connected in series on the branches of the second condenser-evaporator and the low-temperature evaporator, respectively; The working flow inlet of the first ejector is connected to the inlet pipe of the third three-way valve and is located downstream of the outlet connection position of the cold end of the second vortex tube. The jet flow inlet of the first ejector is connected to the second condenser evaporator, and the outlet is connected to the inlet of the second compressor. The working flow inlet of the second injector is connected to the inlet pipe of the fourth three-way valve and is located downstream of the cold end outlet connection of the third vortex tube. The jet inlet of the second injector is connected to the low-temperature evaporator, and the outlet is connected to the inlet of the third compressor.

3. A three-stage cascade heat pump system capable of providing dual-temperature zone cooling and heating as described in claim 2, characterized in that, The first throttle valve, the third three-way valve, and the fourth three-way valve are connected in series with the first liquid reservoir, the second liquid reservoir, and the third liquid reservoir, respectively. The cold end outlets of the three vortex tubes are located downstream of their respective liquid reservoirs.

4. A three-stage cascade heat pump system capable of providing dual-temperature zone cooling and heating as described in claim 3, characterized in that, The first, second, and third compressors have a first gas-liquid separator, a second gas-liquid separator, and a third gas-liquid separator connected in series on their inlet pipes, respectively. The hot end outlets of the three vortex tubes, the first condenser-evaporator, the medium-temperature refrigeration unit, and the low-temperature refrigeration unit are all connected to the corresponding gas-liquid separators. The liquid outlets of the three gas-liquid separators are connected to the corresponding liquid receivers.

5. A three-stage cascade heat pump system capable of providing dual-temperature zone cooling and heating as described in claim 4, characterized in that, Oil separators are connected in series on the outlet pipes of the first compressor and the second compressor, respectively.

6. A three-stage cascade heat pump system capable of providing dual-temperature zone cooling and heating as described in claim 5, characterized in that, The refrigerants used in the high-temperature, medium-temperature, and low-temperature circuits are NH3, CO2, and R23, respectively.

7. A control method for a three-stage cascade heat pump system, characterized in that, Applied to the three-stage cascade heat pump system capable of dual-temperature zone cooling and heating as described in any one of claims 1-6, the three-stage cascade heat pump system further includes a control system and a temperature sensor for detecting ambient temperature, the method comprising: S1. Real-time acquisition of ambient temperature T; S2. Compare T with preset temperature thresholds t1, t2, t3, and t4, where t1 > t2 > t3 > t4; When T≥t1, execute S3; When t1 > T ≥ t2, execute S4; When t2>T≥t3, execute S5; When t3 > T ≥ t4, execute S6; When T≤t4, execute S7; S3. Only open the high temperature and medium temperature circuits. Open the first three-way valve to distribute the refrigerant to the first condenser and the first vortex tube in a ratio of a1:a2. Open the second three-way valve, but close the outlet connected to the second condenser. Open the third three-way valve, but close the outlet connected to the second condenser-evaporator. Where a1 > a2. S4. Open the three circuits: high temperature, medium temperature, and low temperature. Open the first three-way valve to distribute the refrigerant to the first condenser and the first vortex tube in the ratio of a1:a2. Only open the second three-way valve to connect to the outlet of the second condenser. Open the two outlets of the third three-way valve with the same opening degree. Open the two outlets of the fourth three-way valve with the same opening degree. S5. Open the three circuits: high temperature, medium temperature, and low temperature. Open the first three-way valve to distribute the refrigerant to the first condenser and the first vortex tube in the ratio of b1:b2. Only open the second three-way valve to connect to the outlet of the second condenser. Open the two outlets of the third three-way valve with the same opening degree. Open the two outlets of the fourth three-way valve with the same opening degree. Where a1>b1>b2>a2. S6. Open the three circuits: high temperature, medium temperature, and low temperature. Open the first three-way valve to distribute refrigerant to the first condenser and the first vortex tube in a ratio of a1:a2. Open the second three-way valve, but close the outlet connected to the second condenser-evaporator. Open the third three-way valve to distribute refrigerant to the second condenser-evaporator and the medium temperature evaporator in a ratio of c1:c2, where c1>c2. Open the fourth three-way valve to distribute refrigerant to the low temperature evaporator and the internal evaporator in a ratio of d1:d2, where d1>d2. S7. Only open the high-temperature circuit, wherein only open the outlet of the first three-way valve connected to the first vortex tube, and only open the outlet of the second three-way valve connected to the second condenser.

8. The control method for a three-stage cascade heat pump system as described in claim 7, characterized in that, Before S1, at time t1 when the system starts, the opening of the first throttle valve opens to the preset opening D1 at a preset rate. At time t2, the openings of the second and third throttle valves open to the preset openings D2 and D3 at the same preset rate, respectively, where t2 > t1.

9. The control method for a three-stage cascade heat pump system as described in claim 7, characterized in that, The high-temperature circuit is also equipped with a temperature sensor to detect the refrigerant temperature at the outlet side of the first condenser-evaporator. In any step S3-S7, the refrigerant temperature T_eva1 at the outlet side of the first condenser-evaporator is obtained at a preset time period and compared with a preset target temperature range. When T_eva1 is greater than the target temperature range for a consecutive preset number of cycles, the opening of the first throttle valve is reduced by a preset unit until T_eva1 is within the preset target temperature range. When T_eva1 is less than the preset target temperature range for a consecutive preset number of cycles, the opening of the first throttle valve is increased by a preset unit until T_eva1 is within the preset target temperature range. If the opening of the first throttle valve exceeds the preset limit range and T_eva1 also exceeds the preset target temperature range for a preset time, the opening of the first three-way valve connected to the first condenser will be increased by a preset unit change value until T_eva1 is within the preset target temperature range.

10. The control method for a three-stage cascade heat pump system as described in claim 7, characterized in that, When executing any step S3-S6, if the system has been running stably for 1 hour, the operating frequency of the corresponding compressor will be reduced by 10% to 20%.

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