Transcritical carbon dioxide hot water system

Abstract

The invention discloses a transcritical carbon dioxide hot water system. The system comprises a first-stage compressor, a first heat exchanger used for exchanging heat with user side cooling water, a second-stage compressor, a second heat exchanger used for exchanging heat with user side cooling water, a third heat exchanger used for exchanging heat with a liquid-phase refrigerant and a gas-phase refrigerant, an expansion valve, a fourth heat exchanger used for exchanging heat with ambient air, a buffer water tank and a first proportional valve. A first proportional valve, a second proportional valve, a third proportional valve, a fourth proportional valve, a defrosting valve, a refrigerant path bypass valve and the like; according to the system, single-stage and double-stage compression switching in high-temperature weather and low-temperature weather can be achieved, it is guaranteed that the unit has good heating capacity and energy efficiency ratio when high-temperature water is discharged in the high-temperature weather and the low-temperature weather, and meanwhile the water outlet temperature control problem and the defrosting problem of the transcritical carbon dioxide single-double-unit compression system are solved.

Description

[0001] This invention is a divisional application of a Chinese invention patent application filed on December 28, 2021, with application number 2021116230412 and titled “A transcritical carbon dioxide single- and double-stage compression hot water system and its control method”. Technical Field

[0002] The invention relates to the technical field of transcritical carbon dioxide heat pumps, and in particular to a transcritical carbon dioxide hot water system. Background Art

[0003] Carbon dioxide has good environmental properties with an ODP value of 0 and a GWP of 1. As a natural working fluid, carbon dioxide also has good physical properties under low temperature conditions. As an environmentally friendly, efficient, stable and reliable comprehensive thermal energy utilization system, the transcritical carbon dioxide heat pump system is often used as a building air conditioner to meet the winter heating and summer cooling needs of large buildings in the commercial and public service fields. Studies have shown that the maximum temperature in the gas cooler of the transcritical carbon dioxide heat pump system can reach 140°C. Therefore, the transcritical carbon dioxide heat pump system can provide hot water at a higher temperature.

[0004] However, the current carbon dioxide heat pump water heater faces a series of problems when producing high-temperature water under low-temperature conditions, such as serious attenuation of the unit's energy efficiency ratio and heating capacity, and increased exhaust temperature. At the same time, the intermediate and high-pressure pressure control and outlet water temperature control of the current transcritical carbon dioxide two-stage compression system are not perfect enough, and it is impossible to switch between single and double-stage compression, resulting in the unit being unable to be used normally in high-temperature weather, and there may also be erroneous defrosting actions. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and provide an improved transcritical carbon dioxide single-stage and double-stage compression hot water system, which can realize single-stage and double-stage compression switching in high temperature weather and low temperature weather, ensuring that the unit has good heating capacity and energy efficiency ratio when outputting high temperature water in high temperature weather and low temperature weather, and at the same time solves the problem of controlling the outlet water temperature and defrosting of the transcritical carbon dioxide single-stage and double-stage compression system.

[0006] To achieve the above-mentioned purpose, the technical solution adopted by the present invention is: a transcritical carbon dioxide single-stage and double-stage compressed hot water system, the transcritical carbon dioxide single-stage and double-stage compressed hot water system comprising: a first-stage compressor, a first heat exchanger for heat exchange with user-side cooling water, a second-stage compressor, a second heat exchanger for heat exchange with user-side cooling water, a third heat exchanger for heat exchange between liquid-phase refrigerant and gas-phase refrigerant, an expansion valve, a fourth heat exchanger for heat exchange with ambient air, a buffer water tank, a first proportional valve, a second proportional valve, a third proportional valve, a fourth proportional valve, a defrost valve, and a refrigerant bypass valve;

[0007] The primary compressor, the first heat exchanger, the secondary compressor, the second heat exchanger, the liquid-phase refrigerant flow side of the third heat exchanger, the expansion valve, the fourth heat exchanger, and the gas-phase refrigerant flow side of the third heat exchanger are circulated and connected in sequence;

[0008] The two ends of the refrigerant bypass valve are respectively connected to the suction port of the first compressor and the suction port of the second compressor, and the two ends of the defrost valve are respectively connected to the exhaust port of the second compressor and the refrigerant inlet of the fourth heat exchanger;

[0009] The buffer water tank, the first proportional valve, the first heat exchanger, the third proportional valve, and the second heat exchanger are connected in sequence, the inlet of the second proportional valve is connected to the buffer water tank, the outlet of the second proportional valve is connected to the inlet of the third proportional valve and the inlet of the fourth proportional valve respectively, the inlet of the fourth proportional valve is also connected to the first heat exchanger, and the outlet of the fourth proportional valve is connected to the buffer water tank.

[0010] The system realizes intelligent switching between single-stage and double-stage transcritical operation under variable operating conditions. It has outstanding heating capacity and energy efficiency ratio under all operating conditions, significantly improving the comprehensive performance of producing high-temperature hot water in low-temperature environments. It also solves the defrosting and oil return problems of the two-stage compression hot water system.

[0011] According to some preferred aspects of the present invention, the transcritical carbon dioxide single-stage and double-stage compression hot water system also includes a compressor oil separator, and the compressor oil separator includes an oil separator refrigerant inlet, an oil separator refrigerant outlet and an oil separator lubricating oil outlet. The oil separator refrigerant inlet is connected to the exhaust port of the secondary compressor, and the oil separator refrigerant outlet is respectively connected to the second heat exchanger and the defrost valve, and the oil separator lubricating oil outlet is respectively connected to the oil return port of the first compressor and the oil return port of the secondary compressor.

[0012] According to some preferred aspects of the present invention, the transcritical carbon dioxide single-stage and double-stage compression hot water system also includes a first oil circuit solenoid valve and a second oil circuit solenoid valve, wherein the two ends of the first oil circuit solenoid valve are respectively connected to the lubricating oil outlet of the oil separator and the oil return port of the secondary compressor, and the two ends of the second oil circuit solenoid valve are respectively connected to the lubricating oil outlet of the oil separator and the oil return port of the primary compressor.

[0013] According to some preferred aspects of the present invention, the transcritical carbon dioxide single-stage and double-stage compressed hot water system also includes a liquid reservoir, which is respectively connected to the refrigerant outlet of the second heat exchanger and the liquid-phase refrigerant flow side of the third heat exchanger; and / or, the transcritical carbon dioxide single-stage and double-stage compressed hot water system also includes a gas-liquid separator, which is respectively connected to the gas-phase refrigerant flow side of the fourth heat exchanger and the third heat exchanger.

[0014] According to some preferred aspects of the present invention, the transcritical carbon dioxide single-stage and double-stage compression hot water system further includes a water pump, which is respectively connected to the buffer water tank, the first proportional valve, and the second proportional valve.

[0015] According to some preferred aspects of the present invention, the transcritical carbon dioxide single-stage and double-stage compression hot water system further comprises a fan, which is used to blow ambient air toward the fourth heat exchanger and faces the fourth heat exchanger.

[0016] According to some specific aspects of the present invention, the fourth heat exchanger is a fin-tube evaporator.

[0017] According to some preferred aspects of the present invention, the primary compressor is a variable frequency compressor, and the secondary compressor is a fixed frequency compressor.

[0018] According to some preferred aspects of the present invention, the transcritical carbon dioxide single-stage and double-stage compressed hot water system further includes an ambient temperature sensor, a buffer water tank outlet water temperature sensor, a first heat exchanger outlet water temperature sensor, a second heat exchanger outlet water temperature sensor, a first-stage compressor exhaust pressure sensor, a first-stage compressor exhaust temperature sensor, a first-stage compressor suction pressure sensor, a first-stage compressor suction temperature sensor, a second-stage compressor exhaust pressure sensor, a second-stage compressor exhaust temperature sensor, a second-stage compressor suction pressure sensor, a second-stage compressor suction temperature sensor, a second-stage compressor refrigerant outlet temperature sensor, a fourth heat exchanger surface temperature sensor, and a fourth heat exchanger refrigerant evaporation pressure sensor;

[0019] The buffer water tank outlet water temperature sensor is arranged at the water outlet of the buffer water tank, the water outlet temperature sensor of the first heat exchanger is arranged at the water outlet of the first heat exchanger, the water outlet temperature sensor of the second heat exchanger is arranged at the water outlet of the second heat exchanger, the exhaust pressure sensor of the first-stage compressor and the exhaust temperature sensor of the first-stage compressor are respectively arranged at the exhaust port of the first-stage compressor, the suction pressure sensor of the first-stage compressor and the suction temperature sensor of the first-stage compressor are respectively arranged at the suction port of the first-stage compressor, the exhaust pressure sensor of the second-stage compressor and the exhaust temperature sensor of the second-stage compressor are respectively arranged at the exhaust port of the second-stage compressor, the suction pressure sensor of the second-stage compressor and the suction temperature sensor of the second-stage compressor are respectively arranged at the suction port of the second-stage compressor, the refrigerant outlet temperature sensor of the second heat exchanger is arranged at the refrigerant outlet of the second heat exchanger, and the surface temperature sensor of the fourth heat exchanger and the refrigerant evaporation pressure sensor of the fourth heat exchanger are arranged on the fourth heat exchanger.

[0020] Another technical solution provided by the present invention is a control method for the above-mentioned transcritical carbon dioxide single-stage and double-stage compressed hot water system, the control method comprising a two-stage compressor operation control step and a single-stage compressor control step, and respectively detecting the evaporation pressure P0 of the transcritical carbon dioxide single-stage and double-stage compressed hot water system, the temperature t of the refrigerant at the outlet of the second heat exchanger gout , the surface temperature of the fourth heat exchanger t e , the optimal exhaust pressure of the first-stage compressor is recorded as P (1,o) The optimal exhaust pressure of the secondary compressor is denoted as P (2,o) ;

[0021] When the transcritical carbon dioxide single-stage and double-stage compressed hot water system is in double-stage compressor operation, the double-stage compressor operation control step includes: the refrigerant bypass valve is in a closed state, according to the formula:

[0022]

[0023] P 2，0 =f1(t gout , P 10 , t e )

[0024] f1(t gout ,P 1,0 , t e )=(3.896-0.0223*t e )×t gout +(0.496*t e -10.55)+1.032×P 1,0 0.13

[0025] Solve the problem together and solve the P (1,o) and P (2,o) As the target value, compare the actual exhaust pressure P1 of the first compressor and the actual exhaust pressure P2 of the second compressor obtained by actual detection. ΔP is the pressure correction value and is between -5 bar and 10 bar. (1,o) The difference between P2 and P (2,o) The difference between the expansion valve opening and the operating frequency of the first-stage compressor is adjusted to make P1 close to P (1,o) , P2 is close to P (2,o) ;

[0026] When the transcritical carbon dioxide single-stage and double-stage compression hot water system is operated in a single-stage operation, the single-stage compressor control step includes:

[0027] The refrigerant bypass valve is open, the first compressor is shut down, the opening of the first proportional valve is 0, and the opening of the second proportional valve is 100%.

[0028] P 2，0 =f2(t gout , P0,t e )

[0029] f2(t gout ,P , ,t e )=(3.896-0.0223*t e )×t gout +(0.496*t e -10.55)+1.032×P0 0.13

[0030] The solved P (2,o) As the target value, compare it with the actual exhaust pressure P2 of the secondary compressor obtained by actual detection. ΔP is the pressure correction value and is between -5 bar and 10 bar. (2,o) The difference between the two values ​​is adjusted to adjust the opening of the expansion valve so that P2 is close to P (2,o) .

[0031] According to some preferred aspects of the present invention, the control method further includes a step of controlling the outlet water temperature of the second heat exchanger, and the step of controlling the outlet water temperature of the second heat exchanger includes:

[0032] The suction superheat Δt of the secondary compressor is controlled by adjusting the opening of the first proportional valve and the second proportional valve respectively. s2 , control Δt s2is 5K-10K; and adjust the opening of the third proportional valve and the fourth proportional valve, record the opening of the first proportional valve as EXP1, the opening of the second proportional valve as EXP2, the opening of the third proportional valve as EXP3, the opening of the fourth proportional valve as EXP4, EXP1+EXP2=EXP3+EXP4+ΔEXP, ΔEXP is the opening compensation for hydraulic loss and is 3%-6%.

[0033] According to some preferred aspects of the present invention, the control method further comprises a defrosting control step, and the defrosting control step comprises:

[0034] When the surface temperature of the fourth heat exchanger t e The temperature is lower than the set temperature t1 during the duration T1 and the suction pressure P1 of the secondary compressor is lower than the set pressure P1s during the duration T2. When the transcritical carbon dioxide single-stage and double-stage compression system starts defrosting,

[0035]

[0036] T is 30min to 60min; and when the transcritical carbon dioxide single-stage and double-stage compression system starts defrosting, the expansion valve is closed, the fan is stopped, the first proportional valve is closed, the defrost valve is opened, and the first-stage compressor runs to a frequency of HZ1, HZ1 is the operating frequency of the first-stage compressor, between 50HZ and 65HZ, and the second-stage compressor does not stop.

[0037] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art:

[0038] The hot water system of the transcritical carbon dioxide single-stage and double-stage compression and the control method thereof of the present invention solve the problem of the intermediate pressure control of the transcritical carbon dioxide double-stage compression, ensure that the unit can be in the best operating state under different working conditions, and can also realize the switching of the transcritical single-stage and double-stage compression, taking into account the operating conditions of high ambient temperature and low ambient temperature. In addition, the present invention also solves the defrosting problem of the hot water system of the transcritical carbon dioxide single-stage and double-stage compression, improves the defrosting efficiency, and reduces the erroneous defrosting action.

[0039] In addition, the hot water system of the transcritical carbon dioxide single-stage and double-stage compression of the present invention not only realizes the heat recovery of the first-stage compression, but also realizes the precise control of the outlet water temperature of the unit. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0041] Figure 1 This is a schematic structural diagram of a hot water system for single- and double-stage compression of transcritical carbon dioxide according to an embodiment of the present invention;

[0042] Among them, 1. first-stage compressor; 2. first heat exchanger; 3. second-stage compressor; 4. second heat exchanger; 5. third heat exchanger; 6. expansion valve; 7. fourth heat exchanger; 8. buffer water tank; 9. first proportional valve; 10. second proportional valve; 11. third proportional valve; 12. fourth proportional valve; 13. defrost valve; 14. refrigerant bypass valve; 15. compressor oil separator; 16. first oil circuit solenoid valve; 17. second oil circuit solenoid valve; 18. liquid storage tank; 19. gas-liquid separator; 20. water pump; 21. fan. DETAILED DESCRIPTION

[0043] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention, so the present invention is not limited by the specific embodiments disclosed below.

[0044] In the description of the present invention, “plurality” means at least two, for example, two, three, etc., unless otherwise clearly and specifically defined.

[0045] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise clearly defined. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0046] In the invention, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may mean that the first and second features are in direct contact, or that the first and second features are in indirect contact through an intermediate medium. Moreover, a first feature being "above", "above" or "above" a second feature may mean that the first feature is directly above or obliquely above the second feature, or simply means that the first feature is higher in level than the second feature. A first feature being "below", "below" or "below" a second feature may mean that the first feature is directly below or obliquely below the second feature, or simply means that the first feature is lower in level than the second feature.

[0047] It should be noted that when an element is referred to as being "fixed to" or "disposed on" another element, it may be directly on the other element or there may be an intermediate element. When an element is considered to be "connected to" another element, it may be directly connected to the other element or there may be an intermediate element at the same time.

[0048] The preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0049] like Figure 1 As shown, this example provides a transcritical carbon dioxide single-stage and double-stage compressed hot water system, which includes: a first-stage compressor 1, a first heat exchanger 2 for heat exchange with user-side cooling water, a second-stage compressor 3, a second heat exchanger 4 for heat exchange with user-side cooling water, a third heat exchanger 5 for heat exchange between liquid-phase refrigerant and gas-phase refrigerant, an expansion valve 6, a fourth heat exchanger 7 for heat exchange with ambient air, a buffer water tank 8, a first proportional valve 9, a second proportional valve 10, a third proportional valve 11, a fourth proportional valve 12, a defrost valve 13, and a refrigerant bypass valve 14;

[0050] The primary compressor 1, the first heat exchanger 2, the secondary compressor 3, the second heat exchanger 4, the liquid phase refrigerant flow side of the third heat exchanger 5, the expansion valve 6, the fourth heat exchanger 7, and the gas phase refrigerant flow side of the third heat exchanger 5 are circulated and connected in sequence;

[0051] The two ends of the refrigerant bypass valve 14 are respectively connected to the suction port of the first compressor 1 and the suction port of the second compressor 3, and the two ends of the defrost valve 13 are respectively connected to the exhaust port of the second compressor 3 and the refrigerant inlet of the fourth heat exchanger 7;

[0052] The buffer water tank 8, the first proportional valve 9, the first heat exchanger 2, the third proportional valve 11, and the second heat exchanger 4 are connected in sequence. The inlet of the second proportional valve 10 is connected to the buffer water tank 8, and the outlet of the second proportional valve 10 is connected to the inlet of the third proportional valve 11 and the inlet of the fourth proportional valve 12 respectively. The inlet of the fourth proportional valve 12 is also connected to the first heat exchanger 2, and the outlet of the fourth proportional valve 12 is connected to the buffer water tank 8.

[0053] In this example, the transcritical carbon dioxide single-stage and double-stage compressed hot water system also includes a compressor oil separator 15, which includes an oil separator refrigerant inlet, an oil separator refrigerant outlet and an oil separator lubricating oil outlet. The oil separator refrigerant inlet is connected to the exhaust port of the secondary compressor 3, and the oil separator refrigerant outlet is respectively connected to the second heat exchanger 4 and the defrost valve 13, and the oil separator lubricating oil outlet is respectively connected to the oil return port of the first compressor 1 and the oil return port of the secondary compressor 3.

[0054] In this example, the transcritical carbon dioxide single-stage and double-stage compression hot water system also includes a first oil circuit solenoid valve 16 and a second oil circuit solenoid valve 17. The two ends of the first oil circuit solenoid valve 16 are respectively connected to the lubricating oil outlet of the oil separator and the return oil port of the secondary compressor 3, and the two ends of the second oil circuit solenoid valve 17 are respectively connected to the lubricating oil outlet of the oil separator and the return oil port of the primary compressor 1.

[0055] In this example, the transcritical carbon dioxide single-stage and double-stage compressed hot water system also includes a liquid reservoir 18 and a gas-liquid separator 19. The liquid reservoir 18 is respectively connected to the refrigerant outlet of the second heat exchanger 4 and the liquid-phase refrigerant flow side of the third heat exchanger 5, and the gas-liquid separator 19 is respectively connected to the gas-phase refrigerant flow side of the fourth heat exchanger 7 and the third heat exchanger 5.

[0056] In this example, the transcritical carbon dioxide single-stage and double-stage compressed hot water system further includes a water pump 20 and a fan 21. The water pump 20 is respectively connected to the buffer water tank 8, the first proportional valve 9, and the second proportional valve 10. The fan 21 is used to blow ambient air to the fourth heat exchanger 7 and faces the fourth heat exchanger 7. Further, the water pump 20 and the fan 21 can be a variable frequency water pump and a variable frequency fan, respectively.

[0057] In this example, the first-stage compressor 1 is a variable frequency compressor, the second-stage compressor 3 is a fixed frequency compressor, and the fourth heat exchanger 7 is a fin-tube evaporator.

[0058] In this example, the transcritical carbon dioxide single-stage and double-stage compressed hot water system also includes an ambient temperature sensor, a buffer water tank outlet water temperature sensor, a first heat exchanger outlet water temperature sensor, a second heat exchanger outlet water temperature sensor, a first-stage compressor exhaust pressure sensor, a first-stage compressor exhaust temperature sensor, a first-stage compressor suction pressure sensor, a first-stage compressor suction temperature sensor, a second-stage compressor exhaust pressure sensor, a second-stage compressor exhaust temperature sensor, a second-stage compressor suction pressure sensor, a second-stage compressor suction temperature sensor, a second-stage compressor refrigerant outlet temperature sensor, a fourth heat exchanger surface temperature sensor, and a fourth heat exchanger refrigerant evaporation pressure sensor;

[0059] The buffer water tank outlet water temperature sensor is arranged at the water outlet of the buffer water tank 8, the water outlet temperature sensor of the first heat exchanger is arranged at the water outlet of the first heat exchanger 2, the water outlet temperature sensor of the second heat exchanger is arranged at the water outlet of the second heat exchanger 4, the exhaust pressure sensor of the first-stage compressor and the exhaust temperature sensor of the first-stage compressor are respectively arranged at the exhaust port of the first-stage compressor 1, the suction pressure sensor of the first-stage compressor and the suction temperature sensor of the first-stage compressor are respectively arranged at the suction port of the first-stage compressor 1, the exhaust pressure sensor of the second-stage compressor and the exhaust temperature sensor of the second-stage compressor are respectively arranged at the exhaust port of the second-stage compressor 3, the suction pressure sensor of the second-stage compressor and the suction temperature sensor of the second-stage compressor are respectively arranged at the suction port of the second-stage compressor 3, the refrigerant outlet temperature sensor of the second heat exchanger is arranged at the refrigerant outlet of the second heat exchanger 4, and the surface temperature sensor of the fourth heat exchanger and the refrigerant evaporation pressure sensor of the fourth heat exchanger are arranged on the fourth heat exchanger 7.

[0060] Furthermore, in this example, the first heat exchanger 2 is a condenser, and the second heat exchanger 4 is a gas cooler.

[0061] Furthermore, in this example, the first-stage compressor 1 can be one or multiple compressors connected in series. The second-stage compressor 3 can be one or multiple compressors connected in series.

[0062] Furthermore, in this example, the transcritical carbon dioxide single-stage and double-stage compressed hot water system also includes a control system, which is respectively communicated with the ambient temperature sensor, the buffer water tank outlet water temperature sensor, the first heat exchanger outlet water temperature sensor, the second heat exchanger outlet water temperature sensor, the exhaust pressure sensor of the first compressor, the exhaust temperature sensor of the first compressor, the suction pressure sensor of the first compressor, the suction temperature sensor of the first compressor, the exhaust pressure sensor of the second compressor, the exhaust temperature sensor of the second compressor, the suction pressure sensor of the second compressor, the suction temperature sensor of the second compressor, the refrigerant outlet temperature sensor of the second heat exchanger, the surface temperature sensor of the fourth heat exchanger, the refrigerant evaporation pressure sensor of the fourth heat exchanger, the variable frequency first compressor, the variable frequency fan, the variable frequency water pump, and the like.

[0063] This example also provides a control method for the above-mentioned transcritical carbon dioxide single-stage and double-stage compressed hot water system, the control method includes a two-stage compressor operation control step and a single-stage compressor control step, and respectively detecting the evaporation pressure P0 of the transcritical carbon dioxide single-stage and double-stage compressed hot water system, the temperature t of the refrigerant at the outlet of the second heat exchanger 4, and the temperature of the refrigerant at the outlet of the second heat exchanger 4. gout , the surface temperature t of the fourth heat exchanger 7 e , the optimal exhaust pressure of the first stage compressor is recorded as P (1,o) The optimal exhaust pressure of the secondary compressor 3 is denoted as P (2,o) ;

[0064] When the hot water system of transcritical carbon dioxide single-stage and double-stage compression is in double-stage compressor operation, the double-stage compressor operation control steps include: the refrigerant bypass valve 14 is in a closed state, according to the formula:

[0065]

[0066] P 2，0 =f1(t gout , P 1,0 , t e )

[0067] f1(t gout ,P 1,0 ,t e )=(3.896-0.0223*t e )×t gout +(0.496*t e -10.55)+1.032×P 1,0 0.13

[0068] Solve the problem together and solve the P (1,o) and P (2,o) As the target value, compare the actual exhaust pressure P1 of the first compressor 1 and the actual exhaust pressure P2 of the second compressor 3 obtained by actual detection, ΔP is the pressure correction value to ensure that the calculated P (1,o) The critical pressure of carbon dioxide gas is not exceeded. The specific value is obtained through experiments, for example, it can be -5 bar to 10 bar. (1,o) The difference between P2 and P (2,o) The difference between the expansion valve opening and the operating frequency of the first-stage compressor is adjusted adaptively to make P1 close to P (1,o) , P2 is close to P (2,o) ;

[0069] When P1≥P (1,o) +[pressure deviation 1], the expansion valve tends to open wider. (1,o) The size of the difference determines the adjustment speed of the expansion valve. The larger the difference, the faster the speed, and the smaller the difference, the slower the speed.

[0070] When P1≤P (1,o) -[Pressure deviation 2], the expansion valve tends to open small, according to P1 and P (1,o) The size of the difference determines the adjustment speed of the expansion valve. The larger the difference, the faster the speed, and the smaller the difference, the slower the speed.

[0071] When P (1,o) -[Pressure deviation 2]≤P1≤P(1,o) +[Pressure deviation 1], the expansion valve maintains its original opening.

[0072] [Pressure deviation 1] and [Pressure deviation 2] can be between 2-7bar, depending on the actual operating conditions of the unit.

[0073] When the hot water system of transcritical carbon dioxide single-stage and double-stage compression is operated in single-stage operation, the single-stage compressor control steps include:

[0074] The refrigerant bypass valve 14 is in an open state, the first-stage compressor 1 is shut down, the opening of the first proportional valve 9 is 0%, and the opening of the second proportional valve 10 is 100%.

[0075] P 2,0 =f2(t gout , P0,t e )

[0076] f2(t gout ,P0,t e )=(3.896-0.0223*t e )×t gout +(0.496*t e -10.55)+1.032×P0 0.13

[0077] The solved P (2,o) As the target value, compare the actual exhaust pressure P2 of the secondary compressor 3 obtained by actual detection, ΔP is the pressure correction value and is between -5 bar and 10 bar. (2,o) The difference between the two values ​​is adjusted to adjust the opening of the expansion valve 6 so that P2 is close to P (2,o) .

[0078] In this example, the working mode of the transcritical carbon dioxide single-stage and double-stage compression hot water system is as follows: after receiving the start-up command, the current ambient temperature of the unit is detected. If the detected ambient temperature t a1 ≤t a +Δt, the system is in transcritical two-stage compression operation mode; if the detected ambient temperature t a1 ≥t a The system is in transcritical single-stage compression operation mode.

[0079] Δt can be between 1°C and 10°C, t a It can be between -7℃ and -15℃, depending on the specific situation.

[0080] Furthermore, in actual operation, when the hot water system is in the transcritical two-stage compression operation mode, the refrigerant bypass valve 14 is in a closed state, and the refrigerant carbon dioxide gas enters the first heat exchanger 2 (condenser) from the exhaust port driven by the first-stage compressor 1, is cooled to a certain temperature by low-temperature water, and then enters the second-stage compressor 3, is further compressed in the second-stage compressor 3, and then enters the compressor oil separator 15, and the lubricating oil and carbon dioxide gas are separated in the compressor oil separator 15, and then enters the second heat exchanger 4 (gas cooler) and is cooled, and then passes through the liquid storage tank 18 and the third heat exchanger 5, and becomes low-temperature and low-pressure carbon dioxide gas under the action of the expansion valve 6, and then absorbs heat from the air in the fourth heat exchanger 7, i.e., the fin-tube evaporator, and then passes through the third heat exchanger 5, and returns to the first-stage compressor 1;

[0081] When the hot water system is in the transcritical single-stage compression operation mode, the refrigerant bypass valve 14 is in the open state, and the primary compressor 1 is in the shutdown state. Driven by the secondary compressor 3, the carbon dioxide gas enters the compressor oil separator 15 from the exhaust port, and after the lubricating oil and carbon dioxide gas are separated in the compressor oil separator 15, it enters the second heat exchanger 4 (gas cooler) and is cooled. After passing through the liquid storage tank 18 and the third heat exchanger 5, it becomes low-temperature and low-pressure carbon dioxide gas under the action of the expansion valve 6, absorbs heat from the air in the fourth heat exchanger 7, i.e., the fin-tube evaporator, passes through the third heat exchanger 5, and returns to the secondary compressor 3.

[0082] Furthermore, the control method further includes a step of controlling the outlet water temperature of the second heat exchanger 4, and the step of controlling the outlet water temperature of the second heat exchanger 4 includes:

[0083] The suction superheat Δt of the secondary compressor 3 is controlled by adjusting the opening of the first proportional valve 9 and the second proportional valve 10 respectively. s2 , control Δt s2 It can be 5K-10K; and adjust the opening of the third proportional valve 11 and the fourth proportional valve 12, record the opening of the first proportional valve 9 as EXP1, the opening of the second proportional valve 10 as EXP2, the opening of the third proportional valve 11 as EXP3, the opening of the fourth proportional valve 12 as EXP4, EXP1+EXP2=EXP3+EXP4+ΔEXP, ΔEXP is the opening compensation of hydraulic loss, which can be positive or negative, and is calculated according to the actual flow loss of the unit pipeline, generally 3%-6%.

[0084] Furthermore, the control method further includes a defrosting control step, and the defrosting control step includes:

[0085] When the surface temperature t of the fourth heat exchanger 7 e The suction pressure P1 of the secondary compressor 3 is lower than the set pressure P1 during the duration T1 and the suction pressure P2 of the secondary compressor 3 is lower than the set pressure P2 during the duration T2. 1s , When the transcritical carbon dioxide single-stage and double-stage compression system starts to defrost,

[0086]

[0087] Ambient temperature a The higher the value of a is, the larger the value of a is; the ambient temperature t a The lower it is, the smaller the value of a; T is 30min to 60min; and when the transcritical carbon dioxide single-stage and double-stage compression system starts defrosting, the expansion valve 6 is closed, the fan 21 is stopped, the first proportional valve 9 is closed, the defrost valve 13 is opened, and the first-stage compressor 1 runs to the frequency HZ1, HZ1 is the operating frequency of the first-stage compressor 1, between 50HZ and 65HZ, and the second-stage compressor 2 does not stop.

[0088] In summary, the hot water system of transcritical carbon dioxide single-stage and double-stage compression and the control method thereof of the present invention solve the problem of intermediate pressure control of transcritical carbon dioxide double-stage compression, ensure that the unit can be in the best operating state under different working conditions, and can also realize the switching of transcritical single-stage and double-stage compression, taking into account the operating conditions of high ambient temperature and low ambient temperature. In addition, the present invention also solves the defrosting problem of the hot water system of transcritical carbon dioxide single-stage and double-stage compression, improves the defrosting efficiency, and reduces the erroneous defrosting action.

[0089] In addition, the hot water system of the transcritical carbon dioxide single-stage and double-stage compression of the present invention not only realizes the heat recovery of the first-stage compression, but also realizes the precise control of the outlet water temperature of the unit.

[0090] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with this technology to understand the contents of the present invention and implement them accordingly. They cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A transcritical carbon dioxide hot water system, characterized in that: The transcritical carbon dioxide hot water system includes a primary compressor, a first heat exchanger for exchanging heat with user-side cooling water, a secondary compressor, a second heat exchanger for exchanging heat with user-side cooling water, a third heat exchanger for exchanging heat between liquid-phase refrigerant and gas-phase refrigerant, an expansion valve, a fourth heat exchanger for exchanging heat with ambient air, a buffer water tank, a first proportional valve, a second proportional valve, a third proportional valve, a fourth proportional valve, a defrost valve, a refrigerant bypass valve, a gas-liquid separator, and a water pump; The primary compressor, the first heat exchanger, the secondary compressor, the second heat exchanger, the liquid-phase refrigerant flow side of the third heat exchanger, the expansion valve, the fourth heat exchanger, and the gas-phase refrigerant flow side of the third heat exchanger are circulated and connected in sequence; The two ends of the refrigerant bypass valve are respectively connected to the suction port of the first compressor and the suction port of the second compressor, and the two ends of the defrost valve are respectively connected to the exhaust port of the second compressor and the refrigerant inlet of the fourth heat exchanger; The buffer water tank, the first proportional valve, the first heat exchanger, the third proportional valve, and the second heat exchanger are connected in sequence, the inlet of the second proportional valve is connected to the buffer water tank, the outlet of the second proportional valve is connected to the inlet of the third proportional valve and the inlet of the fourth proportional valve respectively, the inlet of the fourth proportional valve is also connected to the first heat exchanger, and the outlet of the fourth proportional valve is connected to the buffer water tank; The gas-liquid separator is communicated with the gas-phase refrigerant flow sides of the fourth heat exchanger and the third heat exchanger respectively; the water pump is communicated with the buffer water tank, the first proportional valve and the second proportional valve respectively.

2. The transcritical carbon dioxide hot water system according to claim 1, characterized in that: The first heat exchanger is a condenser, and the second heat exchanger is a gas cooler.

3. The transcritical carbon dioxide hot water system according to claim 1, characterized in that: The first-stage compressor is composed of one or more compressors connected in series, and the second-stage compressor is composed of one or more compressors connected in series.

4. The transcritical carbon dioxide hot water system according to claim 1, characterized in that: The transcritical carbon dioxide hot water system further comprises a liquid reservoir, which is respectively connected to the refrigerant outlet of the second heat exchanger and the liquid phase refrigerant flow side of the third heat exchanger; The first-stage compressor is a variable frequency compressor, the second-stage compressor is a fixed frequency compressor, and the fourth heat exchanger is a fin-tube evaporator.

5. The transcritical carbon dioxide hot water system according to claim 1, characterized in that: The transcritical carbon dioxide hot water system also includes an ambient temperature sensor, a buffer water tank outlet water temperature sensor, a first heat exchanger outlet water temperature sensor, a second heat exchanger outlet water temperature sensor, a first compressor exhaust pressure sensor, a first compressor exhaust temperature sensor, a first compressor suction pressure sensor, a first compressor suction temperature sensor, a second compressor exhaust pressure sensor, a second compressor exhaust temperature sensor, a second compressor suction pressure sensor, a second compressor suction temperature sensor, a second compressor refrigerant outlet temperature sensor, a fourth heat exchanger surface temperature sensor, a fourth heat exchanger refrigerant evaporation pressure sensor, and a control system; The buffer water tank outlet water temperature sensor is arranged at the water outlet of the buffer water tank, the first heat exchanger outlet water temperature sensor is arranged at the water outlet of the first heat exchanger, the second heat exchanger outlet water temperature sensor is arranged at the water outlet of the second heat exchanger, the exhaust pressure sensor of the first-stage compressor and the exhaust temperature sensor of the first-stage compressor are respectively arranged at the exhaust port of the first-stage compressor, the suction pressure sensor of the first-stage compressor and the suction temperature sensor of the first-stage compressor are respectively arranged at the suction port of the first-stage compressor, the exhaust pressure sensor of the second-stage compressor and the exhaust temperature sensor of the second-stage compressor are respectively arranged at the exhaust port of the second-stage compressor, the suction pressure sensor of the second-stage compressor and the suction temperature sensor of the second-stage compressor are respectively arranged at the suction port of the second-stage compressor, the refrigerant outlet temperature sensor of the second heat exchanger is arranged at the refrigerant outlet of the second heat exchanger, and the surface temperature sensor of the fourth heat exchanger and the refrigerant evaporation pressure sensor of the fourth heat exchanger are arranged on the fourth heat exchanger; The control system is respectively communicated with the ambient temperature sensor, the buffer water tank outlet water temperature sensor, the first heat exchanger outlet water temperature sensor, the second heat exchanger outlet water temperature sensor, the exhaust pressure sensor of the first compressor, the exhaust temperature sensor of the first compressor, the suction pressure sensor of the first compressor, the suction temperature sensor of the first compressor, the exhaust pressure sensor of the second compressor, the exhaust temperature sensor of the second compressor, the suction pressure sensor of the second compressor, the suction temperature sensor of the second compressor, the refrigerant outlet temperature sensor of the second heat exchanger, the surface temperature sensor of the fourth heat exchanger, and the refrigerant evaporation pressure sensor of the fourth heat exchanger.

6. The transcritical carbon dioxide hot water system according to claim 1, characterized in that: The control method of the transcritical carbon dioxide hot water system includes a two-stage compressor operation control step and a single-stage compressor control step, and respectively detecting the evaporation pressure P0 of the transcritical carbon dioxide hot water system, the temperature tgout of the refrigerant at the outlet of the second heat exchanger, and the surface temperature t e , the optimal exhaust pressure of the first-stage compressor is recorded as P (1,o) The optimal exhaust pressure of the secondary compressor is denoted as P (2,o) ; When the transcritical carbon dioxide hot water system is in two-stage compressor operation, the two-stage compressor operation control step includes: the refrigerant bypass valve is in a closed state, according to the formula: P 2，0 =f1(t gout P 1，0 ,t e ) f1(t gout ,P 1,0 ,t e )=(3.896-0.0223*t e )×t gout +(0.496*t e -10.55)+1.032×P 1,0 0.13 Solve the problem together and solve the P (1,o) and P (2,o) As the target value, compare the actual exhaust pressure P1 of the first compressor and the actual exhaust pressure P2 of the second compressor obtained by actual detection. ΔP is the pressure correction value and is between -5 bar and 10 bar. (1,o) The difference between P2 and P (2,o) The difference between the expansion valve opening and the operating frequency of the first-stage compressor is adjusted to make P1 close to P (1,o) , P2 is close to P (2,o) ; When the transcritical carbon dioxide hot water system is in single-stage operation, the single-stage compressor control step includes: The refrigerant bypass valve is open, the first compressor is shut down, the opening of the first proportional valve is 0, and the opening of the second proportional valve is 100%. P 2，0 =f2(t gout ,P0t e ) f2(t gout ,P0,t e )=(3.896-0.0223*t e )×t gout +(0.496*t e -10.55)+1.032×P0 0.13 The solved P (2,o) As the target value, compare it with the actual exhaust pressure P2 of the secondary compressor obtained by actual detection. ΔP is the pressure correction value and is between -5 bar and 10 bar. (2,o) The difference between the two values ​​is adjusted to adjust the opening of the expansion valve so that P2 is close to P (2,o) .

7. The transcritical carbon dioxide hot water system according to claim 6, characterized in that: The control method further includes a step of controlling the outlet water temperature of the second heat exchanger, and the step of controlling the outlet water temperature of the second heat exchanger includes: The suction superheat Δt of the secondary compressor is controlled by adjusting the opening of the first proportional valve and the second proportional valve respectively. s2 , control Δt s2 is 5K-10K; and adjust the opening of the third proportional valve and the fourth proportional valve, record the opening of the first proportional valve as EXP1, the opening of the second proportional valve as EXP2, the opening of the third proportional valve as EXP3, the opening of the fourth proportional valve as EXP4, EXP1+EXP2=EXP3+EXP4+ΔEXP, ΔEXP is the opening compensation for hydraulic loss and is 3%-6%.

8. The transcritical carbon dioxide hot water system according to claim 6, characterized in that: The control method further includes a defrosting control step, wherein the defrosting control step includes: When the surface temperature of the fourth heat exchanger t e The suction pressure P1 of the secondary compressor is lower than the set pressure P1 during the duration T1 and the suction pressure P2 of the secondary compressor is lower than the set pressure P2 during the duration T2. 1s , When the transcritical carbon dioxide single-stage and double-stage compression system starts defrosting, ， T is 30min to 60min; and when the transcritical carbon dioxide single-stage and double-stage compression system starts defrosting, the expansion valve is closed, the fan is stopped, the first proportional valve is closed, the defrost valve is opened, and the first-stage compressor runs to a frequency of HZ1, HZ1 is the operating frequency of the first-stage compressor, between 50HZ and 65HZ, and the second-stage compressor does not stop.

9. The transcritical carbon dioxide hot water system according to claim 6, characterized in that: When P1≥P (1,o) +When the pressure deviation is 1, the expansion valve tends to open wider. (1,o) The difference in value determines the adjustment speed of the expansion valve. The larger the difference, the faster the speed; the smaller the difference, the slower the speed. When P1≤P (1,o) -When the pressure deviation is 2, the expansion valve tends to open slightly. (1,o) The difference in value determines the adjustment speed of the expansion valve. The larger the difference, the faster the speed; the smaller the difference, the slower the speed. When P (1,o) -Pressure deviation 2≤P1≤P (1,o) +When the pressure deviation is 1, the expansion valve maintains the original opening; The pressure deviation 1 and pressure deviation 2 are 2-7 bar respectively.

10. The transcritical carbon dioxide hot water system according to claim 6, characterized in that: The working mode of the transcritical carbon dioxide hot water system is as follows: after receiving the start-up command, the current ambient temperature of the unit is detected. If the detected ambient temperature t a1 ≤t a +Δt, the system is in transcritical two-stage compression operation mode; if the detected ambient temperature t a1 ≥t a When , the system is in transcritical single-stage compression operation mode; Δt is 1℃ to 10℃, t a -7℃ to -15℃.

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