Compound frequency conversion air source heat pump hot water system and operation control method

Abstract

The present invention relates to a cascade variable-frequency air-source heat pump hot water system and an operation control method. The feature is that the cascade variable-frequency air-source heat pump hot water system includes a low-pressure stage compressor, a heat storage heat exchanger, a first three-way valve, an evaporative condenser, a high-pressure stage gas-liquid separator, a high-pressure stage compressor, a three-tube heat exchanger, a three-way proportional regulating valve, a circulating water return temperature sensor, a circulating water outlet temperature sensor, a high-pressure stage electronic expansion valve, a low-pressure stage electronic expansion valve, a bypass electronic expansion valve, an outdoor ambient temperature sensor, an outdoor unit fan, an outdoor unit heat exchanger, an outdoor unit heat exchanger pipe temperature sensor, a second three-way valve, and a low-pressure stage gas-liquid separator. It realizes that the low-pressure stage system of the cascade heat pump still operates in the forward heating mode during defrosting operation and can continue to provide heat for the heat-using end, with stable heat supply.

Description

Technical Field

[0001] The present invention relates to a cascade variable-frequency air-source heat pump hot water system and an operation control method thereof. Background Art

[0002] Compared with a single-stage compression air-source heat pump, a cascade air-source heat pump hot water system can be used at a lower outdoor ambient temperature and can also provide a higher hot water temperature. However, there is also an important problem that when the outdoor ambient temperature is low, frost will form on the surface of the outdoor unit heat exchanger. As the frost layer thickens, the performance of the heat pump system will gradually decline. Therefore, it is necessary to defrost the outdoor unit heat exchanger.

[0003] Currently, the most commonly used defrosting method for a cascade heat pump is that the low-pressure stage heat pump system operates in reverse, the high-pressure stage heat pump system stops operating, and the low-pressure stage heat pump system obtains heat from the original heat-using end. The main problem with this defrosting method is that not only can it not provide heat for the heat-using end during the defrosting process, but it also needs to absorb heat from it. In addition, when the defrosting is over, the compressor of the low-pressure stage heat pump system needs to stop for a while before it can be switched to heating operation and then restarted to ensure the reliable operation of the heat pump system. During the switching process between defrosting operation and heating operation, it not only affects the operation stability of the heat pump system, but also affects system performance such as energy consumption and heating capacity, which is more obvious in the cascade heat pump system. To solve the above problems, the first is to solve the heat source problem of the low-pressure stage heat pump system during defrosting, and the second is to solve the problem of how the low-pressure stage heat pump system always operates in the forward heating direction. Summary of the Invention

[0004] The purpose of the present invention is to overcome the deficiencies of the prior art and provide a cascade variable-frequency air-source heat pump hot water system and an operation control method thereof, so as to realize that the low-pressure stage system of the cascade heat pump still operates in the forward heating direction during defrosting operation and can continue to provide heat for the heat-using end, with stable heating.

[0005] To achieve the above purpose, the technical solution of the cascade variable-frequency air-source heat pump hot water system of the present invention is realized as follows. It is characterized by including a low-pressure stage compressor, a heat storage heat exchanger, a first three-way valve, an evaporative condenser, a high-pressure stage gas-liquid separator, a high-pressure stage compressor, a three-tube heat exchanger, a three-way proportional regulating valve, a circulating water return temperature sensor, a circulating water outlet temperature sensor, a high-pressure stage electronic expansion valve, a low-pressure stage electronic expansion valve, a bypass electronic expansion valve, an outdoor ambient temperature sensor, an outdoor unit fan, an outdoor unit heat exchanger, an outdoor unit heat exchanger pipe temperature sensor, a second three-way valve, and a low-pressure stage gas-liquid separator;

[0006] The exhaust outlet of the low-pressure stage compressor is communicated with the inlet of the heat release heat exchange tube of the heat storage heat exchanger. The outlet of the heat release heat exchange tube in the heat storage heat exchanger is communicated with the A pipe of the first three-way valve. The inlet of the low-pressure refrigerant pipeline of the three-tube heat exchanger is communicated with the B pipe of the first three-way valve. The outlet of the low-pressure refrigerant pipeline of the three-tube heat exchanger is communicated with the inlet of the low-pressure stage electronic expansion valve. The outlet of the low-pressure stage electronic expansion valve is communicated with the inlet of the outdoor unit heat exchanger. The outlet of the outdoor unit heat exchanger is communicated with the A pipe of the second three-way valve. The inlet of the low-pressure stage gas-liquid separator is respectively communicated with the B pipe of the second three-way valve and the outlet of the heat absorption heat exchange tube of the heat storage heat exchanger. The outlet of the low-pressure stage gas-liquid separator is communicated with the suction port of the low-pressure stage compressor. The C pipe of the second three-way valve is communicated with the inlet of the bypass electronic expansion valve. The outlet of the bypass electronic expansion valve is communicated with the inlet of the heat absorption heat exchange tube of the heat storage heat exchanger. The inlet of the condensation pipeline of the evaporation-condenser is communicated with the C pipe of the first three-way valve. The outlet of the condensation pipeline of the evaporation-condenser is communicated with the inlet of the low-pressure stage electronic expansion valve. The inlet of the evaporation pipeline of the evaporation-condenser is communicated with the outlet of the high-pressure stage electronic expansion valve. The outlet of the evaporation pipeline of the evaporation-condenser is communicated with the inlet of the high-pressure stage gas-liquid separator. The suction port of the high-pressure stage compressor is communicated with the outlet of the high-pressure stage gas-liquid separator. The exhaust port of the high-pressure stage gas-liquid separator is communicated with the inlet of the high-pressure refrigerant pipeline of the three-tube heat exchanger. The outlet of the high-pressure refrigerant pipeline of the three-tube heat exchanger is communicated with the inlet of the high-pressure stage electronic expansion valve. The E pipe of the three-way proportional regulating valve is communicated with the circulating water return pipe. The F pipe of the three-way proportional regulating valve is communicated with the inlet of the water pipeline of the three-tube heat exchanger. The G pipe of the three-way proportional regulating valve is respectively communicated with the circulating water outlet pipe and the outlet of the water pipeline of the three-tube heat exchanger. The circulating water return temperature sensed by the circulating water return temperature sensor is T1. The circulating water outlet temperature sensed by the circulating water outlet temperature sensor is T2. The outdoor ambient temperature sensed by the outdoor ambient temperature sensor is T3. The temperature of the outdoor unit heat exchanger tube sensed by the outdoor unit heat exchanger tube temperature sensor is T4.

[0007] In order to achieve the above object, the technical solution of the operation control method of the cascade variable-frequency air source heat pump water heater system of the present invention is realized as follows, and it is characterized in that:

[0008] ① When the heat pump system is operating normally for heating, the A pipe and the C pipe of the first three-way valve are connected. At this time, the A pipe and the B pipe of the first three-way valve are not connected. The heat release heat exchange pipe for the exhaust of the low-pressure stage compressor to the heat storage heat exchanger exchanges heat with the heat storage material in the heat storage heat exchanger through the heat release heat exchange pipe of the heat storage heat exchanger, releasing part of the heat, flowing into the condensation pipeline of the evaporation-condenser from the A pipe of the first three-way valve and flowing out from the C pipe of the first three-way valve. The refrigerant in the condensation pipeline of the evaporation-condenser exchanges heat with the refrigerant in the evaporation pipeline of the evaporation-condenser, releasing heat and condensing into refrigerant liquid. The liquid refrigerant is throttled by the low-pressure stage electronic expansion valve and then enters the outdoor unit heat exchanger for evaporation and absorbs heat from the external environment. The gaseous refrigerant flows into the B pipe of the second three-way valve from the A pipe of the second three-way valve and returns to the low-pressure stage gas-liquid separator, and then returns to the low-pressure stage compressor; the exhaust of the high-pressure stage compressor is discharged to the high-pressure refrigerant pipeline of the three-pipe heat exchanger to exchange heat with the circulating water, releasing heat and condensing into refrigerant liquid. The liquid refrigerant is throttled by the high-pressure stage electronic expansion valve and then enters the evaporation pipeline of the evaporation-condenser for endothermic evaporation. The gaseous refrigerant returns to the high-pressure stage compressor after passing through the high-pressure stage gas-liquid separator;

[0009] ② When the heat pump system is in the defrosting operation, the A pipe and the C pipe of the first three-way valve are connected, and the A pipe and the C pipe of the second three-way valve are connected. The exhaust of the low-pressure stage compressor releases part of the heat through the heat release heat exchange pipe of the heat storage heat exchanger, and then releases part of the heat to the evaporation pipeline of the evaporation-condenser through the condensation pipeline of the evaporation-condenser. At this time, the refrigerant becomes high-dryness saturated wet vapor, and undergoes a primary pressure reduction throttling through the low-pressure stage electronic expansion valve. The refrigerant pressure and temperature decrease, and then it enters the outdoor unit heat exchanger to continue releasing heat. The high-dryness saturated wet vapor is completely condensed into refrigerant liquid. The frost layer on the surface of the outdoor unit heat exchanger absorbs heat and defrosts. The refrigerant liquid flows into the C pipe of the second three-way valve from the A pipe of the second three-way valve and reaches the bypass electronic expansion valve for secondary throttling. The throttled refrigerant enters the heat absorption heat exchange pipe of the heat storage heat exchanger, absorbs heat from the heat storage material through the heat absorption heat exchange pipe of the heat storage heat exchanger and becomes refrigerant vapor. The gaseous refrigerant returns to the low-pressure stage compressor after passing through the low-pressure stage gas-liquid separator; at this time, the high-pressure stage heat pump system still operates as in the normal heating operation of the above-mentioned heat pump system; during the defrosting process, a certain amount of heat is still continuously provided to the circulating water system through the high-pressure stage heat pump system;

[0010] ③ After the defrosting is completed, the A pipe and the B pipe of the second three-way valve are connected, and the heat pump system switches to the normal heating operation. During the defrosting - heating conversion process, both the low-pressure stage compressor and the high-pressure stage compressor do not stop;

[0011] ④ During the defrosting operation of the heat pump system, it is necessary to control the frequency of the low-pressure stage compressor, the frequency of the high-pressure stage compressor, the ratio of the three-way proportional regulating valve, the opening of the high-pressure stage electronic expansion valve, the opening of the low-pressure stage electronic expansion valve, and the opening of the bypass electronic expansion valve. The control method is as follows:

[0012] (a) The low-pressure stage compressor increases its operating frequency to increase the heating capacity of the heat pump system. The defrosting operating frequency range is 80 - 120 Hz. If the operating frequency of the low-pressure stage compressor before defrosting is already greater than 100 Hz, then maintain the original operating frequency; according to the frequency of the low-pressure stage compressor, determine the corresponding frequency of the high-pressure stage compressor during defrosting operation, and this frequency range is 20 - 60 Hz;

[0013] (b) According to the frequency of the low-pressure stage compressor, the frequency of the high-pressure stage compressor, and the return water temperature T1 of the circulating water during defrosting operation, obtain the preset values of the ratio of the three-way proportional regulating valve and the opening degrees of the low-pressure stage electronic expansion valve, the high-pressure stage electronic expansion valve, and the bypass electronic expansion valve through experimental methods;

[0014] (c) Perform secondary adjustment on the frequency of the high-pressure stage compressor according to the temperature difference ΔT = T2 - T1 between the inlet and outlet of the circulating water; when 3°C ≤ ΔT ≤ 7°C, the frequency of the high-pressure stage compressor remains unchanged; when ΔT < 3°C, increase the frequency of the high-pressure stage compressor to increase the heat supply to the circulating water system; when ΔT > 7°C, decrease the frequency of the high-pressure stage compressor to increase the defrosting heat supply to the outdoor unit heat exchanger;

[0015] (2) When the operating condition range of the heat pump is narrow, that is, the temperature difference between the hot water supply temperature and the outdoor ambient temperature is small, and the system only needs to operate in a single stage with the low-pressure stage heat pump system, the control method flow is as follows:

[0016] ① When the heat pump system is operating in normal heating mode, the A pipe and the B pipe of the first three-way valve are connected, and at this time, the A pipe and the C pipe of the first three-way valve are not connected. The heat release heat exchange pipe for the exhaust of the low-pressure stage compressor to the heat storage heat exchanger exchanges heat with the heat storage material in the heat storage heat exchanger through the heat release heat exchange pipe of the heat storage heat exchanger, releases part of the heat, and then flows into the A pipe of the first three-way valve and flows out from the B pipe of the first three-way valve to the low-pressure refrigerant pipeline of the three-pipe heat exchanger to exchange heat with the circulating water, releases heat and condenses into refrigerant liquid. The liquid refrigerant throttles through the low-pressure stage electronic expansion valve and then enters the outdoor unit heat exchanger for evaporation and absorbs heat from the external environment. The gaseous refrigerant flows into the A pipe of the second three-way valve and flows out from the B pipe of the second three-way valve back to the low-pressure stage gas-liquid separator, and then returns to the low-pressure stage compressor; at this time, the high-pressure stage compressor does not operate;

[0017] ②When the heat pump system is in the defrosting operation, the A pipe and the B pipe of the first three-way valve are connected, and the A pipe and the C pipe of the second three-way valve are connected. Part of the heat is released from the exhaust of the low-pressure stage compressor through the heat release heat exchange pipe of the heat storage heat exchanger, and then flows into the low-pressure refrigerant pipeline of the three-pipe heat exchanger from the A pipe of the first three-way valve and flows out from the B pipe of the first three-way valve to exchange heat with the circulating water, releasing part of the heat. At this time, the refrigerant becomes high-quality saturated wet steam, and is throttled down once by the low-pressure stage electronic expansion valve. The refrigerant pressure and temperature decrease, and then it enters the outdoor unit heat exchanger to continue releasing heat. The high-quality saturated wet steam is completely condensed into refrigerant liquid. The frost layer on the surface of the outdoor unit heat exchanger absorbs heat to defrost. The refrigerant liquid flows into the second three-way valve from the A pipe and flows to the bypass electronic expansion valve through the C pipe of the second three-way valve for secondary throttling. The throttled refrigerant enters the heat absorption heat exchange pipe of the heat storage heat exchanger, absorbs heat from the heat storage material through the heat absorption heat exchange pipe and becomes refrigerant vapor. The gaseous refrigerant returns to the low-pressure stage compressor through the low-pressure stage gas-liquid separator; at this time, the high-pressure stage compressor does not operate; during the defrosting process, a certain amount of heat is still provided to the circulating water system through the low-pressure stage heat pump system;

[0018] ③After the defrosting is completed, the A pipe and the B pipe of the second three-way valve are connected, and the heat pump system switches to the normal heating operation. The low-pressure stage compressor does not stop during the defrosting-heating conversion process;

[0019] ④During the defrosting operation of the heat pump system, it is necessary to control the frequency of the low-pressure stage compressor, the ratio of the three-way proportional regulating valve, the opening degree of the low-pressure stage electronic expansion valve, and the opening degree of the bypass electronic expansion valve. The control methods are as follows:

[0020] (a) The low-pressure stage compressor increases the operating frequency to increase the heat output of the heat pump system. The defrosting operation frequency range is 80 - 120 Hz. If the operating frequency of the low-pressure stage compressor before defrosting is already greater than 100 Hz, the original operating frequency is maintained;

[0021] (b) According to the frequency of the low-pressure stage compressor during the defrosting operation and the return water temperature T1 of the circulating water, the preset values of the ratio of the three-way proportional regulating valve and the opening degrees of the low-pressure stage electronic expansion valve and the bypass electronic expansion valve (13) are obtained through experimental methods;

[0022] (c) The opening degree of the low-pressure stage electronic expansion valve is adjusted secondarily according to the temperature difference ΔT = T2 - T1 between the inlet and outlet of the circulating water; when 3 °C ≤ ΔT ≤ 7 °C, the opening degree of the low-pressure stage electronic expansion valve remains unchanged; when ΔT < 3 °C, the opening degree of the low-pressure stage electronic expansion valve is reduced to increase the heat supply to the circulating water system; when ΔT > 7 °C, the opening degree of the low-pressure stage electronic expansion valve is increased to increase the defrosting heat supply to the outdoor unit heat exchanger;

[0023] In this technical solution, the particularly preferred frequency of the low-pressure stage compressor during defrosting operation is 100 Hz.

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] (1) During the defrost operation, it can continue to provide partial heat for the circulating water system, reduce the temperature fluctuation of the circulating water system, and more importantly, avoid the disadvantages caused by taking heat from the circulating water system;

[0026] (2) During the defrost operation, the refrigerant of the heat pump system still flows forward. During the switching process of heating - defrosting - heating, the low-pressure stage compressor does not need to stop and then start, reducing the compressor energy consumption, improving the operating condition of the compressor, and increasing the total heat output of the heat pump system;

[0027] (3) The overall heat pump system is simplified, making it more suitable for the cascade heat pump system that requires switching between cascade operation and single-stage operation. Brief Description of the Drawings

[0028] Figure 1 It is the schematic diagram of the implementation of the present invention. Detailed Embodiment

[0029] The embodiments of the present invention will be described in detail below. The examples of the embodiments are shown in the drawings. The embodiments described below with reference to the drawings are exemplary and are only used to explain the present invention and should not be construed as a limitation of the present invention.

[0030] As Figure 1 shown, it is a cascade variable-frequency air-source heat pump water heater system, which includes a low-pressure stage compressor 1, a heat storage heat exchanger 2, a first three-way valve 3, an evaporative condenser 4, a high-pressure stage gas-liquid separator 5, a high-pressure stage compressor 6, a three-tube heat exchanger 7, a three-way proportional regulating valve 8, a circulating water return temperature sensor 9, a circulating water outlet temperature sensor 10, a high-pressure stage electronic expansion valve 11, a low-pressure stage electronic expansion valve 12, a bypass electronic expansion valve 13, an outdoor ambient temperature sensor 14, an outdoor unit fan 15, an outdoor unit heat exchanger 16, an outdoor unit heat exchanger pipe temperature sensor 17, a second three-way valve 18, and a low-pressure stage gas-liquid separator 19;

[0031] The exhaust outlet of the low-pressure stage compressor 1 is communicated with the inlet of the heat release heat exchange tube in the heat storage heat exchanger 2. The outlet of the heat release heat exchange tube in the heat storage heat exchanger 2 is communicated with port A of the first three-way valve 3. The low-pressure refrigerant pipeline inlet of the three-tube heat exchanger 7 is communicated with port B of the first three-way valve 3. The outlet of the low-pressure refrigerant pipeline of the three-tube heat exchanger 7 is communicated with the inlet of the low-pressure stage electronic expansion valve 12. The outlet of the low-pressure stage electronic expansion valve 12 is communicated with the inlet of the outdoor unit heat exchanger 16. The outlet of the outdoor unit heat exchanger 16 is communicated with the A pipe of the second three-way valve 18. The inlet of the low-pressure stage gas-liquid separator 19 is respectively communicated with the B pipe of the second three-way valve 18 and the outlet of the heat absorption heat exchange tube in the heat storage heat exchanger 2. The outlet of the low-pressure stage gas-liquid separator 19 is communicated with the suction port of the low-pressure stage compressor 1. The C pipe of the second three-way valve 18 is communicated with the inlet of the bypass electronic expansion valve 13. The outlet of the bypass electronic expansion valve 13 is communicated with the inlet of the heat absorption heat exchange tube in the heat storage heat exchanger 2. The condensation pipeline inlet of the evaporative-condenser 4 is communicated with the C pipe of the first three-way valve 3. The condensation pipeline outlet of the evaporative-condenser 4 is communicated with the inlet of the low-pressure stage electronic expansion valve 12. The evaporation pipeline inlet of the evaporative-condenser 4 is communicated with the outlet of the high-pressure stage electronic expansion valve 11. The evaporation pipeline outlet of the evaporative-condenser 4 is communicated with the inlet of the high-pressure stage gas-liquid separator 5. The suction port of the high-pressure stage compressor 6 is communicated with the outlet of the high-pressure stage gas-liquid separator 5. The exhaust port of the high-pressure stage compressor 6 is communicated with the high-pressure refrigerant pipeline inlet of the three-tube heat exchanger 7. The high-pressure refrigerant pipeline outlet of the three-tube heat exchanger 7 is communicated with the inlet of the high-pressure stage electronic expansion valve 11. The E pipe of the three-way proportional regulating valve 8 is communicated with the circulating water return pipe. The F pipe of the three-way proportional regulating valve 8 is communicated with the water pipeline inlet of the three-tube heat exchanger 7. The G pipe of the three-way proportional regulating valve 8 is respectively communicated with the circulating water outlet pipe and the water pipeline outlet of the three-tube heat exchanger 7;

[0032] The circulating water return temperature T1 is sensed by the circulating water return temperature sensor 9. The circulating water outlet temperature T2 is sensed by the circulating water outlet temperature sensor 10. The outdoor ambient temperature T3 is sensed by the outdoor ambient temperature sensor 14. The outdoor unit heat exchanger tube temperature T4 is sensed by the outdoor unit heat exchanger tube temperature sensor 17.

[0033] In this embodiment, the operation control process of the cascade variable-frequency air source heat pump water heating system is as follows:

[0034] (1) When the heat pump operates in a wide range of working conditions, that is, the temperature difference between the hot water supply temperature and the outdoor ambient temperature is large, and the system operates in a cascade mode, the control method process is as follows:

[0035] (1)When the heat pump system is operating in normal heating mode, the A pipe and the C pipe of the first three-way valve 3 are connected, and at this time, the A pipe and the B pipe of the first three-way valve 3 are not connected. The exhaust gas of the low-pressure compressor 1 flows into the heat release heat exchange pipe of the heat storage heat exchanger 2, and exchanges heat with the heat storage material in the heat storage heat exchanger 2 through the heat release heat exchange pipe, releasing part of the heat. It flows from the A pipe of the first three-way valve 3 and flows out from the C pipe of the first three-way valve 3 into the condensation pipeline of the evaporation-condenser 4. The condensation pipeline of the evaporation-condenser 4 exchanges heat with the evaporation pipeline of the evaporation-condenser 4, releasing heat and condensing into a refrigerant liquid. The liquid refrigerant is throttled by the low-pressure electronic expansion valve 12 and then enters the outdoor unit heat exchanger 16 for evaporation and absorbs heat from the external environment. The gaseous refrigerant flows into the B pipe of the second three-way valve 18 from the A pipe of the second three-way valve 18 and returns to the low-pressure gas-liquid separator 19, and then returns to the low-pressure compressor 1. The exhaust gas of the high-pressure compressor 6 is discharged into the high-pressure refrigerant pipeline of the three-pipe heat exchanger 7 to exchange heat with the circulating water, releasing heat and condensing into a refrigerant liquid. The liquid refrigerant is throttled by the high-pressure electronic expansion valve 11 and then enters the evaporation pipeline of the evaporation-condenser 4 for endothermic evaporation. The gaseous refrigerant returns to the high-pressure compressor 6 after passing through the high-pressure gas-liquid separator 5.

[0036] (2)When the heat pump system is in the defrosting operation, the A pipe and the C pipe of the first three-way valve 3 are connected, and the A pipe and the C pipe of the second three-way valve 18 are connected. The exhaust gas of the low-pressure compressor 1 releases part of the heat through the heat release heat exchange pipe of the heat storage heat exchanger 2, and then releases part of the heat to the evaporation pipeline of the evaporation-condenser 4 through the condensation pipeline of the evaporation-condenser 4. At this time, the refrigerant becomes a high dryness saturated wet vapor, and undergoes a primary pressure reduction and throttling through the low-pressure electronic expansion valve 12. The refrigerant pressure and temperature decrease, and then it enters the outdoor unit heat exchanger 16 to continue releasing heat. The high dryness saturated wet vapor is completely condensed into a refrigerant liquid. The frost layer on the surface of the outdoor unit heat exchanger 16 absorbs heat and defrosts. The refrigerant liquid flows into the C pipe of the second three-way valve 18 from the A pipe of the second three-way valve 18 and then enters the bypass electronic expansion valve 13 for secondary throttling. The throttled refrigerant enters the heat absorption heat exchange pipe of the heat storage heat exchanger 2, absorbs heat from the heat storage material through the heat absorption heat exchange pipe and becomes a refrigerant vapor. The gaseous refrigerant returns to the low-pressure compressor 1 after passing through the low-pressure gas-liquid separator 19. At this time, the high-pressure heat pump system still operates according to the operation process when the heat pump system is operating in normal heating mode; during the defrosting process, the high-pressure heat pump system still continues to provide a certain amount of heat to the circulating water system.

[0037] (3)After the defrosting is completed, the A pipe and the B pipe of the second three-way valve 18 are connected, and the heat pump system switches to normal heating operation. During the defrosting-heating conversion process, both the low-pressure compressor 1 and the high-pressure compressor 6 do not stop operating.

[0038] During the defrosting operation of the heat pump system, it is necessary to control the frequency of the low-pressure stage compressor 1, the frequency of the high-pressure stage compressor 6, the ratio of the three-way proportional regulating valve 8, the opening degree of the high-pressure stage electronic expansion valve 11, the opening degree of the low-pressure stage electronic expansion valve 12, and the opening degree of the bypass electronic expansion valve 13. The control method is as follows:

[0039] (a)The low-pressure stage compressor 1 increases its operating frequency to increase the heating capacity of the heat pump system. The defrosting operation frequency range is 80 - 120 Hz. For example, the defrosting operation frequencies are 80 Hz, 90 Hz, 100 Hz, 110 Hz, and 120 Hz, etc. However, if the operating frequency of the low-pressure stage compressor 1 before defrosting is already greater than 100 Hz, the original operating frequency is maintained; according to the frequency of the low-pressure stage compressor 1, the corresponding frequency of the high-pressure stage compressor 6 during defrosting operation is determined, and this frequency range is 20 - 60 Hz. For example, 20 Hz, 30 Hz, 40 Hz, 50 Hz, and 60 Hz, etc.;

[0040] (b)Based on the frequency of the low-pressure stage compressor 1, the frequency of the high-pressure stage compressor 6, and the return water temperature T1 of the circulating water during defrosting operation, the preset values of the ratio of the three-way proportional regulating valve 8 and the opening degrees of the low-pressure stage electronic expansion valve 12, high-pressure stage electronic expansion valve 11, and bypass electronic expansion valve 13 are obtained through experimental methods;

[0041] (c)The frequency of the high-pressure stage compressor 6 is adjusted secondly according to the temperature difference ΔT = T2 - T1 between the inlet and outlet water of the circulating water; when 3 °C ≤ ΔT ≤ 7 °C, the frequency of the high-pressure stage compressor 6 remains unchanged; when ΔT < 3 °C, the frequency of the high-pressure stage compressor 6 increases to increase the heating supply to the circulating water system; when ΔT > 7 °C, the frequency of the high-pressure stage compressor 6 decreases to increase the defrosting heating supply to the outdoor unit heat exchanger 16.

[0042] (II)When the operating condition range of the heat pump is narrow, that is, the temperature difference between the hot water supply temperature and the outdoor ambient temperature is small, and the system only needs to operate in a single stage with the low-pressure stage heat pump system, the control method flow is as follows:

[0043] (1) When the heat pump system is operating normally for heating, the A pipe and the B pipe of the first three-way valve 3 are connected, and at this time, the A pipe and the C pipe of the first three-way valve 3 are not connected. The exhaust gas of the low-pressure stage compressor 1 flows into the heat release heat exchange pipe of the heat storage heat exchanger 2, exchanges heat with the heat storage material in the heat storage heat exchanger 2 through the heat release heat exchange pipe, releases part of the heat, and then flows into the low-pressure refrigerant pipeline of the three-pipe heat exchanger 7 from the A pipe of the first three-way valve 3 and flows out from the B pipe of the first three-way valve 3 to exchange heat with the circulating water, releases heat and condenses into refrigerant liquid. The liquid refrigerant throttles through the low-pressure stage electronic expansion valve 12 and then enters the outdoor unit heat exchanger 16 for evaporation and absorbs heat from the external environment. The gaseous refrigerant flows into the B pipe of the second three-way valve 18 from the A pipe of the second three-way valve 18 and returns to the low-pressure stage gas-liquid separator 19, and then returns to the low-pressure stage compressor 1. The high-pressure stage compressor 6 does not operate;

[0044] (2) When the heat pump system is defrosting, the A pipe and the B pipe of the first three-way valve 3 are connected, and the A pipe and the C pipe of the second three-way valve 18 are connected. The exhaust gas of the low-pressure stage compressor 1 releases part of the heat through the heat release heat exchange pipe of the heat storage heat exchanger 2, and then flows into the low-pressure refrigerant pipeline of the three-pipe heat exchanger 7 from the A pipe of the first three-way valve 3 and flows out from the B pipe of the first three-way valve 3 to exchange heat with the circulating water, releasing part of the heat. At this time, the refrigerant becomes high-dryness saturated wet steam, undergoes a primary pressure reduction throttling through the low-pressure stage electronic expansion valve 12, the refrigerant pressure and temperature decrease, and then enters the outdoor unit heat exchanger 16 to continue releasing heat. The high-dryness saturated wet steam is completely condensed into refrigerant liquid, and the frost layer on the surface of the outdoor unit heat exchanger 16 absorbs heat and defrosts. The refrigerant liquid flows into the C pipe of the second three-way valve 18 from the A pipe of the second three-way valve 18 and reaches the bypass electronic expansion valve 13 for secondary throttling. The throttled refrigerant enters the heat absorption heat exchange pipe of the heat storage heat exchanger 2, absorbs heat from the heat storage material through the heat absorption heat exchange pipe and becomes refrigerant vapor, and the gaseous refrigerant returns to the low-pressure stage compressor 1 through the low-pressure stage gas-liquid separator 19; at this time, the high-pressure stage compressor 6 does not operate; during the defrosting process, the low-pressure stage heat pump system still continues to provide a certain amount of heat for the circulating water system;

[0045] (3) After the defrosting is completed, the A pipe and the B pipe of the second three-way valve 18 are connected, and the heat pump system switches to normal heating operation. The low-pressure stage compressor 1 does not stop during the defrosting-heating conversion process;

[0046] (4) During the defrosting operation of the heat pump system, it is necessary to control the frequency of the low-pressure stage compressor 1, the ratio of the three-way proportional regulating valve 8, the opening degree of the low-pressure stage electronic expansion valve 12, and the opening degree of the bypass electronic expansion valve 13. The control method is as follows:

[0047] (a)The low-pressure stage compressor 1 increases its operating frequency to increase the heating capacity of the heat pump system. The defrosting operating frequency range is 80 - 120 Hz. For example, the defrosting operating frequencies are 80 Hz, 90 Hz, 100 Hz, 110 Hz, 120 Hz, etc. However, if the operating frequency of the low-pressure stage compressor 1 before defrosting is already greater than 100 Hz, the original operating frequency is maintained.

[0048] (b)Based on the frequency of the low-pressure stage compressor 1 during defrosting operation and the return water temperature T1 of the circulating water, the preset values of the ratio of the three-way proportional regulating valve 8 and the opening degrees of the low-pressure stage electronic expansion valve 12 and the bypass electronic expansion valve 13 are obtained through experimental methods.

[0049] (c)The opening degree of the low-pressure stage electronic expansion valve 12 is adjusted secondly according to the temperature difference ΔT = T2 - T1 between the inlet and outlet of the circulating water. When 3 °C ≤ ΔT ≤ 7 °C, the opening degree of the low-pressure stage electronic expansion valve 12 remains unchanged. When ΔT < 3 °C, the opening degree of the low-pressure stage electronic expansion valve 12 is reduced to increase the heating supply to the circulating water system. When ΔT > 7 °C, the opening degree of the low-pressure stage electronic expansion valve 12 is increased to increase the defrosting heating supply to the outdoor unit heat exchanger 16.

[0050] In this embodiment, the particularly preferred defrosting operating frequency of the low-pressure stage compressor 1 is 100 Hz.

[0051] Although the embodiments of the present invention have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and purposes of the present invention. The scope of the present invention is defined by the claims and their equivalents.

Claims

1. A cascade variable-frequency air-source heat pump water heating system, characterized in that it includes a low-pressure stage compressor (1), a heat storage heat exchanger (2), a first three-way valve (3), an evaporative condenser (4), a high-pressure stage gas-liquid separator (5), a high-pressure stage compressor (6), a three-tube heat exchanger (7), a three-way proportional regulating valve (8), a circulating water return temperature sensor (9), a circulating water outlet temperature sensor (10), a high-pressure stage electronic expansion valve (11), a low-pressure stage electronic expansion valve (12), a bypass electronic expansion valve (13), an outdoor ambient temperature sensor (14), an outdoor unit fan (15), an outdoor unit heat exchanger (16), an outdoor unit heat exchanger pipe temperature sensor (17), a second three-way valve (18) and a low-pressure stage gas-liquid separator (19); The exhaust outlet of the low-pressure stage compressor (1) is communicated with the inlet of the heat-releasing heat exchange tube of the heat storage heat exchanger (2). The outlet of the heat-releasing heat exchange tube in the heat storage heat exchanger (2) is communicated with the A pipe of the first three-way valve (3). The inlet of the low-pressure refrigerant pipeline of the three-tube heat exchanger (7) is communicated with the B pipe of the first three-way valve (3). The outlet of the low-pressure refrigerant pipeline of the three-tube heat exchanger (7) is communicated with the inlet of the low-pressure stage electronic expansion valve (12). The outlet of the low-pressure stage electronic expansion valve (12) is communicated with the inlet of the outdoor unit heat exchanger (16). The outlet of the outdoor unit heat exchanger (16) is communicated with the A pipe of the second three-way valve (18). The inlet of the low-pressure stage gas-liquid separator (19) is respectively communicated with the B pipe of the second three-way valve (18) and the outlet of the heat-absorbing heat exchange tube of the heat storage heat exchanger (2). The outlet of the low-pressure stage gas-liquid separator (19) is communicated with the suction port of the low-pressure stage compressor (1). The C pipe of the second three-way valve (18) is communicated with the inlet of the bypass electronic expansion valve (13). The outlet of the bypass electronic expansion valve (13) is communicated with the inlet of the heat-absorbing heat exchange tube of the heat storage heat exchanger (2). The inlet of the condensing pipeline of the evaporative condenser (4) is communicated with the C pipe of the first three-way valve (3). The outlet of the condensing pipeline of the evaporative condenser (4) is communicated with the inlet of the low-pressure stage electronic expansion valve (12). The inlet of the evaporating pipeline of the evaporative condenser (4) is communicated with the outlet of the high-pressure stage electronic expansion valve (11). The outlet of the evaporating pipeline of the evaporative condenser (4) is communicated with the inlet of the high-pressure stage gas-liquid separator (5). The suction port of the high-pressure stage compressor (6) is communicated with the outlet of the high-pressure stage gas-liquid separator (5). The exhaust port of the high-pressure stage gas-liquid separator (5) is communicated with the inlet of the high-pressure refrigerant pipeline of the three-tube heat exchanger (7). The outlet of the high-pressure refrigerant pipeline of the three-tube heat exchanger (7) is communicated with the inlet of the high-pressure stage electronic expansion valve (11). The E pipe of the three-way proportional regulating valve (8) is communicated with the circulating water return pipe. The F pipe of the three-way proportional regulating valve (8) is communicated with the inlet of the water pipeline of the three-tube heat exchanger (7). The G pipe of the three-way proportional regulating valve (8) is respectively communicated with the circulating water outlet pipe and the outlet of the water pipeline of the three-tube heat exchanger (7). The circulating water return temperature sensed by the circulating water return temperature sensor (9) is T1. The circulating water outlet temperature sensed by the circulating water outlet temperature sensor (10) is T2. The outdoor ambient temperature sensed by the outdoor ambient temperature sensor (14) is T3. The outdoor unit heat exchanger tube temperature sensed by the outdoor unit heat exchanger tube temperature sensor (17) is T4.

2. The operation control method of the cascade variable-frequency air source heat pump water heating system according to claim 1, characterized in that: (I) When the heat pump operating condition range is wide, that is, the temperature difference between the hot water supply temperature and the outdoor ambient temperature is large, and the system operates in a cascade mode, the control method flow is as follows: ①When the heat pump system is operating normally for heating, the A pipe and the C pipe of the first three-way valve (3) are connected. At this time, the A pipe and the B pipe of the first three-way valve (3) are not connected. The exhaust gas of the low-pressure stage compressor (1) enters the heat release heat exchange pipe of the heat storage heat exchanger (2), and exchanges heat with the heat storage material in the heat storage heat exchanger (2) through the heat release heat exchange pipe of the heat storage heat exchanger (2), releasing part of the heat. It flows into the condensation pipeline of the evaporation-condenser (4) from the A pipe of the first three-way valve (3) and out from the C pipe of the first three-way valve (3). The refrigerant in the condensation pipeline of the evaporation-condenser (4) exchanges heat with the refrigerant in the evaporation pipeline of the evaporation-condenser (4), releasing heat and condensing into refrigerant liquid. The liquid refrigerant throttles through the low-pressure stage electronic expansion valve (12) and then enters the outdoor unit heat exchanger (16) for evaporation and absorbs heat from the external environment. The gaseous refrigerant flows into the B pipe of the second three-way valve (18) from the A pipe of the second three-way valve (18) and returns to the low-pressure stage gas-liquid separator (19), and then returns to the low-pressure stage compressor (1); the exhaust gas of the high-pressure stage compressor (6) is discharged into the high-pressure refrigerant pipeline of the three-pipe heat exchanger (7) to exchange heat with the circulating water, releasing heat and condensing into refrigerant liquid. The liquid refrigerant throttles through the high-pressure stage electronic expansion valve (11) and then enters the evaporation pipeline of the evaporation-condenser (4) for endothermic evaporation. The gaseous refrigerant returns to the high-pressure stage compressor (6) after passing through the high-pressure stage gas-liquid separator (5); ②When the heat pump system is defrosting, the A pipe and the C pipe of the first three-way valve (3) are connected, and the A pipe and the C pipe of the second three-way valve (18) are connected. The exhaust gas of the low-pressure stage compressor (1) releases part of the heat through the heat release heat exchange pipe of the heat storage heat exchanger (2), and then releases part of the heat to the evaporation pipeline of the evaporation-condenser (4) through the condensation pipeline of the evaporation-condenser (4). At this time, the refrigerant becomes high-quality saturated wet steam, and undergoes a primary pressure reduction throttling through the low-pressure stage electronic expansion valve (12). The refrigerant pressure and temperature decrease, and then it enters the outdoor unit heat exchanger (16) to continue releasing heat. The high-quality saturated wet steam is completely condensed into refrigerant liquid. The frost layer on the surface of the outdoor unit heat exchanger (16) absorbs heat and defrosts. The refrigerant liquid flows into the C pipe of the second three-way valve (18) from the A pipe of the second three-way valve (18) and reaches the bypass electronic expansion valve (13) for secondary throttling. The throttled refrigerant enters the heat absorption heat exchange pipe of the heat storage heat exchanger (2), absorbs heat from the heat storage material through the heat absorption heat exchange pipe of the heat storage heat exchanger (2) and becomes refrigerant vapor. The gaseous refrigerant returns to the low-pressure stage compressor (1) through the low-pressure stage gas-liquid separator (19); at this time, the high-pressure stage heat pump system still operates as in the normal heating operation of the above heat pump system; during the defrosting process, the high-pressure stage heat pump system still continues to provide a certain amount of heat to the circulating water system; ③After the defrosting is completed, the A pipe and the B pipe of the second three-way valve (18) are connected, and the heat pump system switches to normal heating operation. During the defrosting-heating conversion process, both the low-pressure stage compressor (1) and the high-pressure stage compressor (6) do not stop running; ④ During the defrosting operation of the heat pump system, it is necessary to control the frequency of the low-pressure stage compressor (1), the frequency of the high-pressure stage compressor (6), the ratio of the three-way proportional regulating valve (8), the opening degree of the high-pressure stage electronic expansion valve (11), the opening degree of the low-pressure stage electronic expansion valve (12), and the opening degree of the bypass electronic expansion valve (13). The control method is as follows: (a) The low-pressure stage compressor (1) increases its operating frequency to increase the heating capacity of the heat pump system. The defrosting operation frequency range is 80 - 120 Hz. If the operating frequency of the low-pressure stage compressor (1) before defrosting is greater than 100 Hz, the original operating frequency is maintained; according to the frequency of the low-pressure stage compressor (1), the corresponding frequency of the high-pressure stage compressor (6) during defrosting operation is determined, and this frequency range is 20 - 60 Hz; (b) According to the frequency of the low-pressure stage compressor (1), the frequency of the high-pressure stage compressor (6), and the return water temperature T1 of the circulating water during defrosting operation, the preset values of the ratio of the three-way proportional regulating valve (8) and the opening degrees of the low-pressure stage electronic expansion valve (12), high-pressure stage electronic expansion valve (11), and bypass electronic expansion valve (13) are obtained through experimental methods; (c) The frequency of the high-pressure stage compressor (6) is adjusted secondarily according to the temperature difference ΔT = T2 - T1 between the inlet and outlet water of the circulating water; when 3°C ≤ ΔT ≤ 7°C, the frequency of the high-pressure stage compressor (6) remains unchanged; when ΔT < 3°C, the frequency of the high-pressure stage compressor (6) increases to increase the heat supply to the circulating water system; when ΔT > 7°C, the frequency of the high-pressure stage compressor (6) decreases to increase the defrosting heat supply to the outdoor unit heat exchanger (16); (2) When the operating condition range of the heat pump is narrow, that is, the temperature difference between the hot water supply temperature and the outdoor ambient temperature is small, and the system only needs to operate in a single stage with the low-pressure stage heat pump system, the control method flow is as follows: ① During the normal heating operation of the heat pump system, the A pipe and the B pipe of the first three-way valve (3) are connected, and at this time, the A pipe and the C pipe of the first three-way valve (3) are not connected. The heat release heat exchange pipe through which the low-pressure stage compressor (1) discharges gas to the heat storage heat exchanger (2) exchanges heat with the heat storage material in the heat storage heat exchanger (2) through the heat release heat exchange pipe of the heat storage heat exchanger (2), releases part of the heat, and then flows into the low-pressure refrigerant pipeline from the A pipe of the first three-way valve (3) and flows to the three-pipe heat exchanger (7) from the B pipe of the first three-way valve (3) to exchange heat with the circulating water, releases heat and condenses into a refrigerant liquid. The liquid refrigerant throttles through the low-pressure stage electronic expansion valve (12) and then enters the outdoor unit heat exchanger (16) for evaporation and absorbs heat from the external environment. The gaseous refrigerant flows into the B pipe of the second three-way valve (18) from the A pipe of the second three-way valve (18) and returns to the low-pressure stage gas-liquid separator (19), and then returns to the low-pressure stage compressor (1); at this time, the high-pressure stage compressor (6) does not operate; ②When the heat pump system defrosts, the A pipe and the B pipe of the first three-way valve (3) are communicated, the A pipe and the C pipe of the second three-way valve (18) are communicated. The exhaust gas of the low-pressure stage compressor (1) releases part of the heat through the heat release heat exchange pipe of the heat storage heat exchanger (2), and then flows into the A pipe of the first three-way valve (3) and flows out from the B pipe of the first three-way valve (3) to the low-pressure refrigerant pipeline of the three-pipe heat exchanger (7) to exchange heat with the circulating water and release part of the heat. At this time, the refrigerant becomes high-dryness saturated wet steam, and undergoes a primary pressure reduction and throttling through the low-pressure stage electronic expansion valve (12). The pressure and temperature of the refrigerant decrease, and then it enters the outdoor unit heat exchanger (16) to continue releasing heat. The high-dryness saturated wet steam is completely condensed into refrigerant liquid. The frost layer on the surface of the outdoor unit heat exchanger (16) absorbs heat to defrost. The refrigerant liquid flows into the A pipe of the second three-way valve (18) and flows out from the C pipe of the second three-way valve (18) to the bypass electronic expansion valve (13) for secondary throttling. The throttled refrigerant enters the heat absorption heat exchange pipe of the heat storage heat exchanger (2), absorbs heat from the heat storage material through the heat absorption heat exchange pipe and becomes refrigerant vapor. The gaseous refrigerant returns to the low-pressure stage compressor (1) through the low-pressure stage gas-liquid separator (19); at this time, the high-pressure stage compressor (6) does not operate; during the defrosting process, the low-pressure stage heat pump system still continues to provide a certain amount of heat for the circulating water system; ③After the defrosting is completed, the A pipe and the B pipe of the second three-way valve (18) are communicated, and the heat pump system switches to normal heating operation. During the defrosting-heating conversion process, the low-pressure stage compressor (1) does not stop; ④During the defrosting operation of the heat pump system, it is necessary to control the frequency of the low-pressure stage compressor (1), the ratio of the three-way proportional regulating valve (8), the opening of the low-pressure stage electronic expansion valve (12), and the opening of the bypass electronic expansion valve (13). The control methods are as follows: (a) The low-pressure stage compressor (1) increases the operating frequency to increase the heat output of the heat pump system. The defrosting operating frequency range is 80 - 120 Hz. If the operating frequency of the low-pressure stage compressor (1) before defrosting is already greater than 100 Hz, the original operating frequency is maintained; (b) According to the frequency of the low-pressure stage compressor (1) during defrosting operation and the return water temperature T1 of the circulating water, the preset values of the ratio of the three-way proportional regulating valve (8) and the openings of the low-pressure stage electronic expansion valve (12) and the bypass electronic expansion valve (13) are obtained through experimental methods; (c) The opening of the low-pressure stage electronic expansion valve (12) is adjusted secondarily according to the temperature difference ΔT = T2 - T1 between the inlet and outlet of the circulating water; when 3 °C ≤ ΔT ≤ 7 °C, the opening of the low-pressure stage electronic expansion valve (12) remains unchanged; when ΔT < 3 °C, the opening of the low-pressure stage electronic expansion valve (12) is reduced to increase the heat supply to the circulating water system; when ΔT > 7 °C, the opening of the low-pressure stage electronic expansion valve (12) is increased to increase the defrosting heat supply to the outdoor unit heat exchanger (16).

3. The operation control method of the cascade variable-frequency air-source heat pump water heater system according to claim 2, characterized in that The particularly preferred frequency of the low-pressure stage compressor (1) during defrosting operation is 100 Hz.

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