Cascade heat pump system and defrosting control method thereof

Abstract

The present application relates to the technical field of cascade heat pump system, and particularly provides a cascade heat pump system and a defrosting control method thereof, aiming to solve the problem that the defrosting mode of the existing cascade heat pump system easily leads to a larger water temperature fluctuation range of the produced hot water and high energy consumption. To this end, the cascade heat pump system comprises a high-pressure refrigerant circulation loop, a low-pressure refrigerant circulation loop and a defrosting branch, and the defrosting branch can send the refrigerant discharged by the low-pressure stage compressor to the low-pressure stage heat exchanger. The cascade heat pump system can provide another defrosting mode, which can effectively avoid the problem of the water temperature fluctuation range of the produced hot water being too large caused by the refrigerant backflow defrosting, and can also improve the defrosting efficiency, reduce the defrosting energy consumption and cost, and thus effectively improve the user experience.

Description

Technical Field

[0001] This invention relates to the field of cascade heat pump systems, specifically providing a cascade heat pump system and its defrosting control method. Background Technology

[0002] When cascade heat pump systems are used in low-temperature winter environments, their outdoor heat exchangers are prone to frost formation, which significantly impacts the system's heating performance. Therefore, timely defrosting is essential. For cascade heat pump systems operating at high temperatures, which involve two-stage circulation (high-pressure and low-pressure stages), configuring the defrosting scheme for these two stages and coordinating their operation to achieve optimal defrosting performance presents a pressing technical challenge.

[0003] Existing cascade heat pump systems are equipped with reversing valves to reverse the refrigerant circulation in the refrigerant loop to achieve defrosting. However, this defrosting method can easily lead to a larger fluctuation range in the hot water temperature produced by the heat pump system, and can also easily lead to increased energy consumption of the heat pump system, thus increasing the operating cost.

[0004] Accordingly, there is a need in the field for a new cascade heat pump system and its defrosting control method to solve the above-mentioned technical problems. Summary of the Invention

[0005] The present invention aims to solve the above-mentioned technical problems, namely, to solve the problem that the defrosting method of the existing cascade heat pump system easily leads to a large range of water temperature fluctuations and high energy consumption in the production of hot water.

[0006] In a first aspect, the present invention provides a defrosting control method for a cascade heat pump system. The cascade heat pump system includes a high-pressure refrigerant circulation loop, a low-pressure refrigerant circulation loop, and a defrosting branch. The low-pressure refrigerant circulation loop is equipped with a low-pressure compressor and a low-pressure heat exchanger. The high-pressure refrigerant circulation loop and the low-pressure refrigerant circulation loop are configured to exchange heat through an intermediate heat exchanger. The defrosting branch is configured to deliver refrigerant discharged from the low-pressure compressor to the low-pressure heat exchanger. The defrosting control method includes: acquiring the ambient temperature of the cascade heat pump system and the coil temperature of the low-pressure heat exchanger; and selectively controlling the cascade heat pump system to perform a defrosting operation based on the ambient temperature and the coil temperature.

[0007] In the preferred embodiment of the above defrosting control method, the step of "selectively controlling the cascade heat pump system to perform defrosting operation according to the ambient temperature and the coil temperature" specifically includes: if the ambient temperature is greater than or equal to a first preset ambient temperature and the coil temperature is less than or equal to a target defrosting temperature, then the cascade heat pump system is controlled to perform a first defrosting operation; if the ambient temperature is greater than a second preset ambient temperature and less than the first preset ambient temperature, and the coil temperature is less than or equal to the target defrosting temperature, then the cascade heat pump system is controlled to perform a second defrosting operation; if the ambient temperature is less than or equal to the second preset ambient temperature and the coil temperature is less than or equal to the target defrosting temperature, then the cascade heat pump system is controlled to perform a third defrosting operation.

[0008] In the preferred embodiment of the above defrosting control method, the first defrosting operation includes: controlling the operation of the defrosting branch.

[0009] In the preferred embodiment of the above defrosting control method, the cascade heat pump system further includes a bypass branch, which is connected to the low-pressure stage refrigerant circulation loop, and both ends of the bypass branch are connected to the exhaust port and intake port of the low-pressure stage compressor. The second defrosting operation includes controlling the operation of the defrosting branch and controlling the operation of the bypass branch.

[0010] In the preferred embodiment of the above defrosting control method, a heater is further provided on the low-pressure refrigerant circulation loop, and the third defrosting operation includes: controlling the operation of the defrosting branch, controlling the operation of the bypass branch, and controlling the heater to turn on.

[0011] In a preferred embodiment of the above-mentioned defrosting control method, the cascade heat pump system further includes a hot water exchange circuit, at least a portion of which is disposed in the high-pressure stage heat exchanger on the high-pressure stage refrigerant circulation loop. When the cascade heat pump system performs a defrosting operation, the defrosting control method further includes: obtaining the outlet water temperature of the hot water exchange circuit; obtaining the coil temperature of the low-pressure stage heat exchanger again; and selectively controlling the cascade heat pump system to exit the defrosting operation based on the outlet water temperature and the re-obtained coil temperature.

[0012] In the preferred embodiment of the above defrosting control method, the step of "selectively controlling the cascade heat pump system to exit the defrosting operation based on the outlet water temperature and the re-acquired coil temperature" specifically includes: if the outlet water temperature is less than the preset outlet water temperature and the coil temperature is greater than or equal to the target defrosting temperature, then the cascade heat pump system is controlled to exit the defrosting operation.

[0013] In the preferred embodiment of the above defrosting control method, the method for determining the target defrosting temperature includes: calculating the average value of the ambient temperature of the cascade heat pump system, denoted as the first temperature; obtaining the ambient temperature near the low-pressure stage heat exchanger, denoted as the second temperature; and determining the target defrosting temperature based on the first temperature and the second temperature.

[0014] In the preferred embodiment of the above defrosting control method, the step of "determining the target defrosting temperature based on the first temperature and the second temperature" specifically includes: determining an adjustment coefficient based on the first temperature; establishing a linear relationship between the target defrosting temperature and the first temperature based on the first temperature and the adjustment coefficient; and determining the target defrosting temperature based on the linear relationship between the target defrosting temperature and the first temperature and the second temperature.

[0015] In another aspect, the present invention also provides a cascade heat pump system, the cascade heat pump system including a controller, the controller being capable of executing the defrosting control method described in any of the preferred technical solutions above.

[0016] With the above technical solution adopted, the cascade heat pump system of the present invention includes a high-pressure stage refrigerant circulation loop, a low-pressure stage refrigerant circulation loop, and a defrost branch. The defrost branch can send the refrigerant discharged from the low-pressure stage compressor to the low-pressure stage heat exchanger. The cascade heat pump system of the present invention can provide an alternative defrosting method, which can effectively avoid the problem of excessive water temperature fluctuation range caused by refrigerant backflow defrosting, improve defrosting efficiency, reduce defrosting energy consumption and cost, and thus effectively improve the user experience. Attached Figure Description

[0017] The preferred embodiments of the present invention are described below with reference to the accompanying drawings, in which:

[0018] Figure 1 This is a schematic diagram of the cascade heat pump system of the present invention;

[0019] Figure 2 This is a flowchart of the main steps of the defrosting control method of the present invention;

[0020] Figure 3 This is a flowchart illustrating the specific steps of a preferred embodiment of the defrosting control method of the present invention;

[0021] Figure label:

[0022] 1. High-pressure refrigerant circulation loop;

[0023] 11. High-pressure stage compressor; 12. High-pressure stage heat exchanger; 13. First throttling component; 14. Intermediate heat exchanger;

[0024] 2. Low-pressure refrigerant circulation loop;

[0025] 21. Low-pressure stage compressor; 22. Second throttling component; 23. Low-pressure stage heat exchanger; 24. Heater; 25. Liquid receiver; 26. Check valve;

[0026] 3. Defrosting branch; 31. Defrosting control valve;

[0027] 4. Bypass branch; 41. Bypass control valve;

[0028] 5. Replace the hot water pipes. Detailed Implementation

[0029] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are merely illustrative of the technical principles of the invention and are not intended to limit the scope of protection of the invention. Those skilled in the art can make adjustments as needed to adapt to specific applications. For example, the cascade heat pump system described in this invention can be a residential cascade heat pump system or an industrial cascade heat pump system; these are not limiting. Those skilled in the art can determine the application of the cascade heat pump system of the present invention according to actual usage requirements. Such changes in application do not depart from the basic principles of the present invention and fall within the scope of protection of the present invention.

[0030] It should be noted that, in the description of this preferred embodiment, unless otherwise explicitly specified and limited, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Furthermore, the terms "connected" and "linked" should be interpreted broadly; for example, they can refer to mechanical connections or electrical connections, direct connections or indirect connections via an intermediate medium, or connections within two components. Therefore, they should not be construed as limiting the invention. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0031] Furthermore, it should be noted that although the various steps of the defrosting control method of the present invention are described in a specific order in the description of the present invention, these orders are not restrictive. Without departing from the basic principles of the present invention, those skilled in the art can perform the steps in different orders.

[0032] First refer to Figure 1 , Figure 1 This is a schematic diagram of the cascade heat pump system of the present invention. Figure 1As shown, the cascade heat pump system of the present invention includes a high-pressure refrigerant circulation loop 1, a low-pressure refrigerant circulation loop 2, a defrost branch 3, and a bypass branch 4. The high-pressure refrigerant circulation loop 1 is equipped with a high-pressure compressor 11, a high-pressure heat exchanger 12, a first throttling component 13, and an intermediate heat exchanger 14. The low-pressure refrigerant circulation loop 2 is equipped with a low-pressure compressor 21, a second throttling component 22 of the intermediate heat exchanger 14, and a low-pressure heat exchanger 23. The high-pressure refrigerant circulation loop 1 and the low-pressure refrigerant circulation loop 2 are configured to exchange heat in the intermediate heat exchanger 14.

[0033] It should be noted that the present invention does not impose any restrictions on the specific structure of the high-pressure stage compressor 11 and the low-pressure stage compressor 21, the high-pressure stage heat exchanger 12, the intermediate heat exchanger and the low-pressure stage heat exchanger 23, and the first throttling member 13 and the second throttling member 22. For example, the high-pressure stage compressor 11 and the low-pressure stage compressor 21 can be variable frequency compressors or fixed frequency compressors; the high-pressure stage heat exchanger 12, the intermediate heat exchanger and the low-pressure stage heat exchanger 23 can be plate heat exchangers or shell and tube heat exchangers; and the first throttling member 13 and the second throttling member 22 can be capillary tubes or electronic expansion valves. These are not limiting, and those skilled in the art can set them according to the actual situation.

[0034] Furthermore, it should be noted that the present invention does not impose any restrictions on the specific structures of the high-pressure refrigerant circulation loop 1 and the low-pressure refrigerant circulation loop 2; those skilled in the art can set them according to actual conditions. Preferably, the low-pressure refrigerant circulation loop 2 is further provided with a heater 24 and a liquid storage tank 25. The heater 24 is located between the low-pressure heat exchanger 23 and the suction port of the low-pressure compressor 21, and is used to heat the refrigerant flowing out of the low-pressure heat exchanger 23; the liquid storage tank 25 is located between the intermediate heat exchanger 14 and the second throttling member 22, and is used to buffer the refrigerant pressure of the low-pressure refrigerant circulation loop 2. Of course, the specific structures and specific locations of the heater 24 and the liquid storage tank 25 are not limiting; those skilled in the art can set them according to actual conditions.

[0035] Furthermore, the defrost branch 3 is connected to the low-pressure stage refrigerant circulation loop 2, and the defrost branch 3 is configured to send the refrigerant discharged from the low-pressure stage compressor 21 to the low-pressure stage heat exchanger 23. The bypass branch 4 is connected to the low-pressure stage refrigerant circulation loop 2, and the two ends of the bypass branch 4 are connected to the exhaust port and the suction port of the low-pressure stage compressor 21. The bypass branch 4 can send the refrigerant discharged from the exhaust port of the low-pressure stage compressor 21 to the suction port and back to the low-pressure stage compressor 21.

[0036] It should be noted that the present invention does not impose any restrictions on the specific connection position between the defrost branch 3 and the low-pressure refrigerant circulation loop 2, as long as the defrost branch 3 can deliver the refrigerant discharged from the low-pressure compressor 21 to the low-pressure heat exchanger 23. As a preferred configuration, the first end of the defrost branch 3 is connected between the exhaust port of the low-pressure compressor 21 and the intermediate heat exchanger 14, and the second end of the defrost branch 3 is connected between the second throttling member 22 and the low-pressure heat exchanger 23, so as to improve the defrosting efficiency of the cascade heat pump system.

[0037] Preferably, a defrost control valve 31 is provided on the defrost branch 3, and the defrost control valve 31 is configured to control the connection state of the defrost branch 3; a bypass control valve 41 is provided on the bypass branch 4, and the bypass control valve 41 is configured to control the connection state of the bypass branch 4. Of course, the specific type and structure of the defrost control valve 31 and the bypass control valve 41 are not limiting, and those skilled in the art can set them themselves. For example, the defrost control valve 31 and the bypass control valve 41 can be solenoid valves.

[0038] More preferably, a one-way valve 26 is also provided on the low-pressure stage refrigerant circulation loop 2. The one-way valve 26 is located between the first end of the defrost branch 3 and the exhaust port of the low-pressure stage compressor 21, and the one-way valve 26 is configured to allow refrigerant to be discharged from the exhaust port of the low-pressure stage compressor 21 to the intermediate heat exchanger 14, the defrost branch 3, or the bypass branch 4.

[0039] In addition, the cascade heat pump system also includes a hot water exchange circuit 5, at least a portion of which is located in the high-pressure stage heat exchanger 12.

[0040] Furthermore, the cascade heat pump system also includes a temperature sensor and a controller. The temperature sensor can detect the ambient temperature and the coil temperature of the low-pressure stage heat exchanger 23 of the cascade heat pump system. The controller can acquire the detection data from the temperature sensor and can also control the operating status of the cascade heat pump system, such as controlling the operating mode and defrosting operation of the cascade heat pump system. These are not limiting factors. Those skilled in the art will understand that the present invention does not impose any limitations on the specific structure and model of the temperature sensor and the controller. The controller can be either the original controller of the cascade heat pump system or a controller separately configured to implement the defrosting control method of the present invention. Those skilled in the art can customize the structure and model of the controller according to actual usage requirements.

[0041] See Figure 2 , Figure 2 This is a flowchart illustrating the main steps of the defrosting control method of the present invention. Figure 2As shown, based on the cascade heat pump system described in the above embodiments, the defrosting control method of the present invention mainly includes the following steps:

[0042] S1: Obtain the ambient temperature of the cascade heat pump system and the coil temperature of the low-pressure stage heat exchanger;

[0043] S2: Selectively control the cascade heat pump system to perform defrosting operation based on ambient temperature and coil temperature.

[0044] First, in step S1, the controller acquires the ambient temperature and the coil temperature of the low-pressure stage heat exchanger 23 of the cascade heat pump system. Of course, the specific timing and method of acquiring the coil temperature of the low-pressure stage heat exchanger 23 and the ambient temperature are not limiting. For example, the controller can acquire them in real time or at intervals. It can acquire the average value of the coil temperature of the low-pressure stage heat exchanger 23 and the average value of the ambient temperature, or it can acquire the lowest value of the coil temperature of the low-pressure stage heat exchanger 23 and the highest value of the ambient temperature within a certain time period, etc. These are not limiting, and those skilled in the art can set them according to the actual situation.

[0045] Next, in step S2, the controller selectively controls the cascade heat pump system to perform a defrost operation based on the ambient temperature and the coil temperature. It should be noted that the present invention does not impose any restrictions on the specific execution method of the above steps. For example, when the average coil temperature of the low-pressure stage heat exchanger 23 is lower than a preset coil temperature and the ambient temperature is lower than a preset ambient temperature, the controller starts controlling the heat pump system to perform a defrost operation. Alternatively, the defrost operation can be achieved by reversing the refrigerant circulation or by turning on the heater 24; those skilled in the art can set the operation according to the actual situation.

[0046] See next Figure 3 , Figure 3 This is a flowchart illustrating the specific steps of a preferred embodiment of the defrosting control method of the present invention. Figure 3 As shown, based on the cascade heat pump system described in the above embodiments, the defrosting control method of the preferred embodiment of the present invention includes the following steps:

[0047] S101: Obtain the ambient temperature of the cascade heat pump system and the coil temperature of the low-pressure stage heat exchanger;

[0048] S102: If the ambient temperature is greater than or equal to the first preset ambient temperature, and the coil temperature is less than or equal to the target defrost temperature, then control the defrost branch to operate.

[0049] S103: If the ambient temperature is greater than the second preset ambient temperature and less than the first preset ambient temperature, and the coil temperature is less than or equal to the target defrost temperature, then control the defrost branch to operate and control the bypass branch to operate.

[0050] S104: If the ambient temperature is less than or equal to the second preset ambient temperature, and the coil temperature is less than or equal to the target defrost temperature, then control the defrost branch to operate, control the bypass branch to operate, and control the heater to turn on.

[0051] S105: Obtain the outlet water temperature of the hot water exchange circuit;

[0052] S106: Obtain the coil temperature of the low-pressure stage heat exchanger again;

[0053] S107: If the outlet water temperature is lower than the preset outlet water temperature and the coil temperature is greater than or equal to the target defrost temperature, control the cascade heat pump system to exit the defrost operation.

[0054] First, in step S101, the controller acquires the ambient temperature and the coil temperature of the low-pressure stage heat exchanger 23 of the cascade heat pump system. Of course, the specific timing and method of acquiring the coil temperature of the low-pressure stage heat exchanger 23 and the ambient temperature are not limiting. For example, the controller can acquire them in real time or at certain intervals. It can acquire the average value of the coil temperature of the low-pressure stage heat exchanger 23 and the average value of the ambient temperature, or it can acquire the lowest value of the coil temperature of the low-pressure stage heat exchanger 23 and the highest value of the ambient temperature within a certain time period, etc. These are not limiting, and those skilled in the art can set them according to the actual situation.

[0055] Next, the controller selectively controls the cascade heat pump system to perform a defrosting operation based on the ambient temperature and the coil temperature. It should be noted that this invention does not impose any limitations on the specific execution method of the above steps; those skilled in the art can set their own methods according to actual conditions.

[0056] Preferably, if the ambient temperature is greater than or equal to a first preset ambient temperature, and the coil temperature is less than or equal to a target defrost temperature, the controller controls the cascade heat pump system to perform a first defrost operation; if the ambient temperature is greater than a second preset ambient temperature and less than the first preset ambient temperature, and the coil temperature is less than or equal to the target defrost temperature, the controller controls the cascade heat pump system to perform a second defrost operation; if the ambient temperature is less than or equal to the second preset ambient temperature, and the coil temperature is less than or equal to the target defrost temperature, the controller controls the cascade heat pump system to perform a third defrost operation. Based on the above methods, the cascade heat pump system of the present invention can selectively choose multiple defrost operations to maximize defrost efficiency, thereby effectively ensuring the normal operating time of the cascade heat pump system.

[0057] Specifically, in steps S102 to S104, if the ambient temperature is greater than or equal to the first preset ambient temperature, and the coil temperature is less than or equal to the target defrost temperature, it indicates that although the ambient temperature of the cascade heat pump system is low, it is not very low. In this case, the controller controls the defrost branch 3 to operate, which can both defrost and reduce defrost energy consumption. If the ambient temperature is greater than the second preset ambient temperature and less than the first preset ambient temperature, and the coil temperature is less than or equal to the target defrost temperature, the controller controls the defrost branch 3 and the bypass branch 4 to operate simultaneously. By bypassing part of the high-temperature refrigerant of the low-pressure stage compressor 21 directly from the exhaust port to the suction port through the bypass branch 4, the evaporation temperature can be effectively increased, the suction state of the low-pressure stage compressor 21 can be improved, and the shortcomings of hot gas bypass defrosting in removing the frost layer under low-temperature conditions can be effectively solved, greatly shortening the defrost time and effectively improving the defrost efficiency. If the ambient temperature is less than or equal to the second preset ambient temperature, and the coil temperature is less than or equal to the target defrosting temperature, it indicates that the frosting situation is relatively severe. In this case, the controller controls the operation of the defrosting branch 3 and the bypass branch 4, and controls the heater 24 to turn on, so as to further improve the defrosting efficiency.

[0058] Based on the above settings, when the ambient temperature is slightly high, the first defrosting operation is performed, which consumes the least energy while still ensuring timely and thorough defrosting. When the ambient temperature is slightly low, the second defrosting operation is performed, which effectively increases the evaporation temperature and improves the suction state of the low-pressure stage compressor 21, not only removing the frost layer completely but also significantly shortening the defrosting time. When the ambient temperature is very low, the third defrosting operation is performed, effectively solving the problem of difficulty in removing the frost layer at extremely low temperatures.

[0059] It should be noted that the present invention does not impose any restrictions on the specific setting values ​​of the first preset ambient temperature and the second preset ambient temperature. Preferably, the first preset ambient temperature and the second preset ambient temperature are 5°C and 0°C, respectively. Of course, this is not restrictive, and those skilled in the art can set them according to the actual operation of the cascade heat pump system.

[0060] Furthermore, it should be noted that the present invention does not impose any restrictions on the specific setting method of the target defrost temperature. As a preferred setting method, the controller first calculates the average value of the ambient temperature of the cascade heat pump system, which is recorded as the first temperature; then it obtains the ambient temperature near the low-pressure stage heat exchanger 23, which is recorded as the second temperature; then, based on the first temperature and the second temperature, it determines the target defrost temperature so as to set a reasonable target defrost temperature, which can effectively ensure that the cascade heat pump system can be defrosted in a timely manner, and can also avoid the cascade heat pump system repeatedly entering the defrost operation due to the setting temperature being too high.

[0061] In one specific implementation, the controller determines an adjustment coefficient based on the first temperature. The degree of frost formation in the cascade heat pump system varies with different ambient temperatures. Determining the adjustment coefficient based on the first temperature helps to determine a more suitable target defrost temperature. Next, the controller establishes a linear relationship between the target defrost temperature and the first temperature based on the first temperature and the adjustment coefficient. Finally, the controller determines the target defrost temperature based on the linear relationship between the target defrost temperature and the first temperature, and the second temperature. Based on this setting method, the specific set value of the target defrost temperature changes constantly with the overall ambient temperature of the cascade heat pump system and the ambient temperature of the low-pressure stage heat exchanger 23. This effectively ensures that the cascade heat pump system can operate more rationally and with lower defrost energy consumption, thereby improving defrost efficiency.

[0062] Furthermore, in steps S105 and S106, when the cascade heat pump system is performing a defrost operation, the controller again acquires the coil temperature of the low-pressure stage heat exchanger 23 and the outlet water temperature of the hot water exchange circuit 5. Then, based on the outlet water temperature and the reacquired coil temperature, the controller selectively controls the cascade heat pump system to exit the defrost operation.

[0063] It should be noted that the present invention does not impose any restrictions on the specific timing and method of exiting the defrosting operation; those skilled in the art can set these themselves. Preferably, in step S107, if the outlet water temperature is lower than the preset outlet water temperature and the coil temperature is greater than or equal to the target defrosting temperature, it indicates that the outlet water temperature of the hot water exchange circuit 5 is too low, significantly affecting normal use by the user. Furthermore, at this time, the low-pressure stage heat exchanger 23 can ensure that there is no risk of frost formation. In this case, the controller controls the cascade heat pump system to exit the defrosting operation, which ensures thorough defrosting and maximizes the satisfaction of user needs.

[0064] It should be noted that the present invention does not impose any restrictions on the specific setting value of the preset outlet water temperature. Those skilled in the art can set it themselves according to the actual usage needs of the user and the actual operation of the cascade heat pump system.

[0065] The technical solution of the present invention has been described above with reference to the preferred embodiments shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of the present invention is obviously not limited to these specific embodiments. Without departing from the principles of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after such changes or substitutions will all fall within the scope of protection of the present invention.

Claims

1. A defrosting control method for a cascade heat pump system, characterized in that, The cascade heat pump system includes a high-pressure refrigerant circulation loop, a low-pressure refrigerant circulation loop, and a defrost branch. The low-pressure refrigerant circulation loop is equipped with a low-pressure compressor and a low-pressure heat exchanger. The high-pressure and low-pressure refrigerant circulation loops are configured to exchange heat through an intermediate heat exchanger. The defrost branch is configured to deliver the refrigerant discharged from the low-pressure compressor to the low-pressure heat exchanger. The cascade heat pump system also includes a bypass branch connected to the low-pressure refrigerant circulation loop, with both ends of the bypass branch connected to the exhaust port and intake port of the low-pressure compressor. The defrosting control method includes: Obtain the ambient temperature of the cascade heat pump system and the coil temperature of the low-pressure stage heat exchanger; Based on the ambient temperature and the coil temperature, the cascade heat pump system is selectively controlled to perform a defrost operation. The steps include: if the ambient temperature is greater than a second preset ambient temperature and less than a first preset ambient temperature, and the coil temperature is less than or equal to a target defrost temperature, then the cascade heat pump system is controlled to perform a second defrost operation; the second defrost operation includes: Control the operation of the defrosting branch and the bypass branch.

2. The defrosting control method according to claim 1, characterized in that, The step of "selectively controlling the cascade heat pump system to perform defrosting operation based on the ambient temperature and the coil temperature" specifically includes: If the ambient temperature is greater than or equal to the first preset ambient temperature, and the coil temperature is less than or equal to the target defrost temperature, then the cascade heat pump system is controlled to perform the first defrost operation. If the ambient temperature is less than or equal to the second preset ambient temperature, and the coil temperature is less than or equal to the target defrost temperature, then the cascade heat pump system is controlled to perform a third defrost operation.

3. The defrosting control method according to claim 2, characterized in that, The first defrosting operation includes: Control the operation of the defrosting branch.

4. The defrosting control method according to claim 2, characterized in that, A heater is also installed in the low-pressure refrigerant circulation loop, and the third defrosting operation includes: The defrosting branch is controlled to operate, the bypass branch is controlled to operate, and the heater is controlled to turn on.

5. The defrosting control method according to any one of claims 2 to 4, characterized in that, The cascade heat pump system further includes a hot water exchange circuit, at least a portion of which is located in the high-pressure stage heat exchanger on the high-pressure stage refrigerant circulation loop. When the cascade heat pump system performs a defrost operation, the defrost control method further includes: Obtain the outlet water temperature of the hot water exchange circuit; The coil temperature of the low-pressure stage heat exchanger is obtained again; Based on the outlet water temperature and the re-acquired coil temperature, the cascade heat pump system is selectively controlled to exit the defrosting operation.

6. The defrosting control method according to claim 5, characterized in that, The step of "selectively controlling the cascade heat pump system to exit defrost operation based on the outlet water temperature and the re-acquired coil temperature" specifically includes: If the outlet water temperature is lower than the preset outlet water temperature and the coil temperature is greater than or equal to the target defrost temperature, then the cascade heat pump system is controlled to exit the defrost operation.

7. The defrosting control method according to claim 5, characterized in that, The method for determining the target defrosting temperature includes: Calculate the average ambient temperature of the cascade heat pump system, and denote it as the first temperature; The ambient temperature near the low-pressure stage heat exchanger is obtained and denoted as the second temperature. The target defrosting temperature is determined based on the first temperature and the second temperature.

8. The defrosting control method according to claim 7, characterized in that, The step of "determining the target defrost temperature based on the first temperature and the second temperature" specifically includes: Based on the first temperature, determine the adjustment coefficient; Based on the first temperature and the adjustment coefficient, a linear relationship is established between the target defrosting temperature and the first temperature; The target defrost temperature is determined based on the linear relationship between the target defrost temperature and the first temperature and the second temperature.

9. A cascade heat pump system, characterized in that, The cascade heat pump system includes a controller capable of executing the defrosting control method according to any one of claims 1 to 8.

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