Refrigerant circuit system

Abstract

[Problem] To propose a refrigerant circuit system that is capable of switching a freezing cycle in accordance with necessary heating capacity. [Solution] This two-stage refrigerant circuit system is formed by connecting a high-stage side circuit (40) and a low-stage side circuit (30) via a cascade heat exchanger (67) and uses two types of refrigerants. The high-stage side circuit (40) comprises a high-stage side compressor (69), a high-stage side utilization heat exchanger (70), and a high-stage side expansion mechanism (71). The low-stage side circuit (30) comprises a low-stage side compressor (51), a low-stage side utilization heat exchanger (68), low-stage side expansion mechanisms (54, 55), and a low-stage side heat source heat exchanger (52), and is provided with a switching circuit (80) for a first cycle in which a refrigerant flows from the low-stage side compressor (51) to the cascade heat exchanger (67) and a second cycle in which the refrigerant which has flowed from the low-stage side compressor (51) to the low-stage side utilization heat exchanger (68) flows to the cascade heat exchanger (67).

Description

Refrigerant circuit system

[0001] The present disclosure relates to a refrigerant circuit system.

[0002] Patent Document 1 discloses a refrigeration cycle device that includes a high-side refrigerant circuit using propane, a low-side refrigerant circuit using carbon dioxide, and a heat medium circuit, and that is capable of achieving high performance in both heating and cooling.

[0003] Patent No. 7146117

[0004] For example, in order to further improve the cooling or heating capacity of the refrigeration cycle device disclosed in Patent Document 1, it is necessary to increase the amount of refrigerant charged. However, due to their flammability, restrictions are placed on the use of flammable natural refrigerants, such as propane. Therefore, in a refrigeration cycle using a flammable natural refrigerant, for example, the refrigerant cannot be charged to more than a certain amount, limiting the capacity. This disclosure proposes a refrigerant circuit system capable of switching the refrigeration cycle according to the required heating capacity.

[0005] The refrigerant circuit system of the present disclosure is a two-stage refrigerant circuit system in which a high-stage side circuit (40) using a first refrigerant and a low-stage side circuit (30) using a second refrigerant are connected via a cascade heat exchanger (67), and the high-stage side circuit (40) is provided with a high-stage side compressor (69), a high-stage side utilization heat exchanger (70), and a high-stage side expansion mechanism (71), and the low-stage side circuit (30) is provided with a low-stage side compressor (5 The refrigeration cycle can be switched depending on the required heating capacity. The refrigerant circuit system of the present disclosure further includes a first cycle in which a refrigerant flows from the low-stage side compressor (51) to the cascade heat exchanger (67), a low-stage side utilization heat exchanger (68), a low-stage side expansion mechanism (54, 55), and a low-stage side heat source heat exchanger (52), and includes a switching circuit (80) between a first cycle in which a refrigerant flows from the low-stage side compressor (51) to the low-stage side utilization heat exchanger (68) and a second cycle in which the refrigerant, having flowed from the low-stage side compressor (51) to the low-stage side utilization heat exchanger (68), flows to the cascade heat exchanger (67). In this case, the refrigeration cycle can be switched depending on the required heating capacity. The refrigerant circuit system of the present disclosure further includes a heat medium circuit (50) in which the high-stage side utilization heat exchanger (70) and the low-stage side utilization heat exchanger (68) are water heat exchangers, and includes heat medium piping (85, 86, 87, 89), a pump (84), and a tank (82). In the refrigerant circuit system of the present disclosure, the low-temperature side circuit (30) further includes a bypass flow path (811), a first cock (65), and a second cock (66). The refrigerant circuit system of the present disclosure switches to the second cycle when it determines that the required heating capacity will not be reached during heating operation using the first cycle. In this case, the refrigeration cycle can be switched by comparing the current heating capacity with the required heating capacity. In the refrigerant circuit system of the present disclosure, the required heating capacity is determined based on three factors: a target hot water outlet temperature set by a user, a current inlet water temperature, and a water flow rate. The refrigerant circuit system of the present disclosure can switch to the second cycle when the rotation speed of the high-temperature side compressor (69) is equal to or greater than an upper limit value or a certain threshold, and the pump command value controlling the heat medium flow rate is equal to or less than a certain threshold, but the first heat medium temperature from the heat medium pipe (87) has not reached the target value.The refrigerant circuit system of the present disclosure switches to the second cycle when the rotation speed of the high-stage side compressor (69) is equal to or greater than an upper limit or a certain threshold, the heating capacity estimated from the inlet water temperature, the outlet hot water temperature, and the pump command value has not reached the target capacity, and it is determined that the heating capacity has not improved for a certain period of time. In this case, it is possible to prevent switching to the refrigeration cycle before the heating capacity has fully increased. The refrigerant circuit system of the present disclosure changes the pump command value used to determine whether to switch from the first cycle to the second cycle depending on the heat medium pipe length setting. In this case, the same water flow rate can be maintained even when the heat medium pipe length is different. The refrigerant circuit system of the present disclosure also performs heating operation using only the low-stage side circuit (30) when an abnormality occurs in the high-stage side circuit (40). In this case, even if the high-stage side circuit is unavailable, a certain level of heating capacity can be ensured using only the low-stage side circuit. The refrigerant circuit system of the present disclosure also issues an abnormality alert when an abnormality occurs in the high-stage side circuit (40). In this case, the occurrence of an abnormality can be confirmed without being on-site, and the user can also determine whether to perform heating operation using only the low-stage side circuit. In the refrigerant circuit system of the present disclosure, the high-stage side circuit (40) is installed indoors. In this case, even when a flammable refrigerant is used in the high-stage side circuit, the flammable refrigerant filling amount regulation can be satisfied, and the system can be installed indoors even where only a certain amount of ventilation is possible. In addition, the refrigerant circuit system of the present disclosure uses natural refrigerants as both of the refrigerants. In addition, the refrigerant circuit system of the present disclosure uses propane in the high-stage side circuit (40) and carbon dioxide in the low-stage side circuit (30). In addition, in the refrigerant circuit system of the present disclosure, the heat medium supplied from the pump (84) in the heat medium circuit (50) flows sequentially through the low-stage side utilization heat exchanger (68) and the high-stage side utilization heat exchanger (70). In this case, the refrigeration cycle can be switched according to the required heating capacity by switching the switching circuit (80) without switching the flow path on the heat medium circuit (50) side. The refrigerant circuit system of the present disclosure further includes a four-way selector valve (53), and switches between heating operation and cooling operation by switching the four-way selector valve (53).

[0006] 1 is a refrigerant circuit diagram showing an example of the configuration of a refrigerant circuit system according to the present embodiment. FIG. 2 is a heat medium circuit diagram showing an example of the configuration of a heat medium circuit according to the present embodiment. FIG. 3 is a refrigerant circuit diagram showing the flow of refrigerant during heating operation in a first cycle according to the present embodiment. FIG. 4 is a pressure-specific enthalpy diagram during heating operation in the first cycle, where (a) is the pressure-specific enthalpy diagram in the high-stage side circuit and (b) is the pressure-specific enthalpy diagram in the low-stage side circuit. FIG. 5 is a refrigerant circuit diagram showing the flow of refrigerant during heating operation in a second cycle according to the present embodiment. FIG. 6 is a pressure-specific enthalpy diagram during heating operation in the second cycle, where (a) is the pressure-specific enthalpy diagram in the high-stage side circuit and (b) is the pressure-specific enthalpy diagram in the low-stage side circuit. FIG. 7 is a block diagram showing an example of the configuration of major devices according to the present embodiment. FIG. 8 is a flowchart of switching from the first cycle to the second cycle during heating operation, where (a) shows an example when an upper limit value of the rotation speed of the high-stage side compressor is used and (b) shows an example when a threshold value of the rotation speed of the high-stage side compressor is used. 1 is a flowchart of switching from the second cycle to the first cycle during heating operation, where (a) shows an example when a required heating capacity is used and (b) shows an example when a target hot water outlet temperature is used. FIG. 2 is a flowchart of starting up the refrigerant circuit system according to the present embodiment. FIG. 3 is a schematic refrigerant circuit diagram of a switching circuit in a refrigerant circuit system according to Modification 1. FIG. 4 is a schematic refrigerant circuit diagram of a switching circuit in a refrigerant circuit system according to Modification 2. FIG. 5 is a schematic refrigerant circuit diagram of a switching circuit in a refrigerant circuit system according to Modification 3. FIG. 6 is a schematic heat medium circuit diagram in a refrigerant circuit system according to Modification 4. FIG. 7 is a schematic heat medium circuit diagram in a refrigerant circuit system according to Modification 5.

[0007] An embodiment will be described in detail below with reference to the accompanying drawings. <Configuration of Refrigerant Circuit System> FIG. 1 is a refrigerant circuit diagram showing an example configuration of a refrigerant circuit system according to this embodiment. The refrigerant circuit system 1 according to this embodiment is, for example, a hot water supply system. The hot water supply system supplies hot water to an object. In this embodiment, the object includes a hot water supply object such as a faucet, a shower, or a bath, and a heating device using hot water. The refrigerant circuit system 1 according to this embodiment includes, for example, an outdoor unit 10 installed outdoors and an indoor unit 20 installed indoors. The outdoor unit 10 includes a low-temperature side circuit 30, a portion of which is connected to the indoor unit 20. The indoor unit 20 includes a high-temperature side circuit 40 and a heat medium circuit 50. The refrigerant circuit system 1 according to this embodiment may also include a control unit 100 (see FIG. 7A). A natural refrigerant, for example, is used as the first refrigerant charged in the high-temperature side circuit 40 and the second refrigerant charged in the low-temperature side circuit 30. The low-side circuit 30 is filled with, for example, carbon dioxide, and the high-side circuit 40 is filled with, for example, propane.

[0008] The low-temperature side circuit 30 includes a low-temperature side compressor 51, a low-temperature side heat source heat exchanger 52, a four-way switching valve 53, a first expansion valve 54, shut-off valves 61, 62, 63, and 64, a first cock 65, a second cock 66, a cascade heat exchanger 67, and a low-temperature side utilization heat exchanger 68. The low-temperature side circuit 30 also includes a switching circuit 80 that directs refrigerant to the cascade heat exchanger 67 without directing refrigerant to the low-temperature side utilization heat exchanger 68. The low-temperature side circuit 30 may further include a second expansion valve 55, a receiver tank 56, and check valves 57, 58, 59, and 60. The low-temperature side circuit 30 is not limited to the above-described configuration. For example, the low-temperature side circuit 30 may include a filter, a heat sink, an oil separator, and the like. It may also include a pressure sensor and a high-pressure switch serving as a protective detector.

[0009] The low-temperature side compressor 51 draws in refrigerant from its suction side and discharges the compressed refrigerant from its discharge side. The low-temperature side compressor 51 may be equipped with an accumulator on its suction side that separates the refrigerant into gas and liquid. The low-temperature side compressor 51 has its discharge side connected to a first port (P1) of a four-way switching valve 53, and its suction side connected to a third port (P3) of the four-way switching valve 53. The low-temperature side heat source heat exchanger 52 exchanges heat between the refrigerant and outside air. The low-temperature side heat source heat exchanger 52 functions as an evaporator during heating operation. The low-temperature side heat source heat exchanger 52 may be equipped with an outdoor fan.

[0010] The four-way selector valve 53 has a first port (P1), a second port (P2), a third port (P3), and a fourth port (P4), and can switch between a state in which the first port (P1) and the second port (P2) are connected and the third port (P3) and the fourth port (P4) are connected, and a state in which the first port (P1) and the fourth port (P4) are connected and the second port (P2) and the third port (P3) are connected. During heating operation, the four-way selector valve 53 is in a state in which the first port (P1) and the second port (P2) are connected and the third port (P3) and the fourth port (P4) are connected.

[0011] The first expansion valve 54 and the second expansion valve 55 are examples of low-temperature side expansion mechanisms. They are configured to have variable openings and function to reduce the pressure of the refrigerant circulating in the low-temperature side circuit 30 and expand the refrigerant. The receiver tank 56 is a tank that stores the liquid refrigerant condensed in the cascade heat exchanger 67. The low-temperature side circuit 30 may be configured to include piping connecting the receiver tank 56 and the low-temperature side compressor 51.

[0012] The check valves 57, 58, 59, and 60 are valves for preventing backflow of the refrigerant and determine the direction of the refrigerant flow. The refrigerant flow will be described in detail later. The shut-off valves 61, 62, 63, and 64 function to connect the piping of the outdoor unit 10 and the piping of the indoor unit 20.

[0013] The cascade heat exchanger 67 exchanges heat between the low-temperature side circuit 30 and the high-temperature side circuit 40. The cascade heat exchanger 67 has a flow path connected to the low-temperature side circuit 30 and a flow path connected to the high-temperature side circuit 40. The cascade heat exchanger 67 is, for example, a double-pipe heat exchanger in which two pipes of different diameters are combined in an inner and outer double configuration. The cascade heat exchanger 67 may also be another type of heat exchanger, such as a plate heat exchanger. During heating operation, the cascade heat exchanger 67 functions as a condenser in the low-temperature side circuit 30 and as an evaporator in the high-temperature side circuit 40.

[0014] The low-temperature side utilization heat exchanger 68 exchanges heat between the low-temperature side circuit 30 and the heat medium circuit 50. The low-temperature side utilization heat exchanger 68 has a flow path connected to the low-temperature side circuit 30 and a flow path connected to the heat medium circuit 50. During heating operation, the low-temperature side utilization heat exchanger 68 functions as a condenser. The heat medium flowing through the heat medium circuit 50 is warmed by removing heat from the second refrigerant flowing through the low-temperature side circuit 30, thereby performing a heating function. The low-temperature side utilization heat exchanger 68 is a water heat exchanger, but is not limited to this and may be configured as an air heat exchanger, for example.

[0015] The low-temperature side circuit 30 has a bypass flow path 811. The bypass flow path 811 connects a flow path connecting the low-temperature side utilization heat exchanger 68 and the cascade heat exchanger 67 with a flow path connecting the low-temperature side utilization heat exchanger 68 and the low-temperature side compressor 51. The first and second cocks 65 and 66 have the function of switching whether or not the second refrigerant flows through the low-temperature side utilization heat exchanger 68 by opening and closing them. The first cock 65 is disposed on the flow path connecting the low-temperature side utilization heat exchanger 68 and the low-temperature side compressor 51, and the second cock 66 is disposed on the bypass flow path 811. When refrigerant is to flow through the low-temperature side utilization heat exchanger 68, the first cock 65 is opened and the second cock 66 is closed. On the other hand, when the refrigerant is not flowing through the low-temperature side utilization heat exchanger 68 but is flowing through the bypass passage 811 to the cascade heat exchanger 67, the first cock 65 is closed and the second cock 66 is opened. The opening and closing of the first cock 65 and the second cock 66 is controlled by, for example, the control unit 100 (see FIG. 7A ) according to a switching condition described later.

[0016] The high-temperature side circuit 40 includes a cascade heat exchanger 67, a high-temperature side compressor 69, a high-temperature side utilization heat exchanger 70, and a high-temperature side expansion mechanism 71. The high-temperature side circuit 40 is not limited to the above-described configuration. For example, the high-temperature side circuit 40 may include a filter, a heat sink, an oil separator, etc. Furthermore, the high-temperature side circuit 40 may include a pressure sensor, a high-pressure switch serving as a protective detector, etc.

[0017] The high-stage compressor 69 draws in refrigerant from its suction side and discharges the compressed refrigerant from its discharge side. The high-stage compressor 69 may be provided with an accumulator on its suction side that separates the refrigerant into gas and liquid. The high-stage compressor 69 has its discharge side connected to the high-stage utilization heat exchanger 70 and its suction side connected to the cascade heat exchanger 67.

[0018] The high-temperature side utilization heat exchanger 70 exchanges heat between the high-temperature side circuit 40 and the heat medium circuit 50. During heating operation, the high-temperature side utilization heat exchanger 70 functions as a condenser. The heat medium flowing through the heat medium circuit 50 is warmed by removing heat from the refrigerant flowing through the high-temperature side circuit 40, thereby performing a heating function. The high-temperature side utilization heat exchanger 70 is a water heat exchanger, but is not limited to this and may be configured as an air heat exchanger, for example. The high-temperature side expansion mechanism 71 has the function of reducing the pressure of the refrigerant circulating within the high-temperature side circuit 40 and expanding the refrigerant. The high-temperature side expansion mechanism 71 is, for example, a capillary tube. Because the capillary tube has a narrow flow path, resistance to the refrigerant flow occurs, causing a pressure drop. The high-temperature side circuit 40 may also be configured to have an expansion valve instead of a capillary tube.

[0019] FIG. 7A is a block diagram showing an example of the configuration of the main devices according to this embodiment. The control unit 100 is, for example, a controller 100. The controller 100 controls the operation of the hot water supply system. The controller 100 includes an MCU (Micro Control Unit), an electric circuit, and an electronic circuit. The MCU includes a CPU (Central Processing Unit) 101, a memory 102, and a communication interface (I / F) 103. The memory 102 stores various programs executed by the CPU 101. The controller 100 may be composed of a single physically independent element or two or more physically separated elements. The controller 100 controls the low-side circuit 30 and the high-side circuit 40. Specifically, the controller 100 controls the low-side compressor 51, the high-side compressor 69, the four-way valve 53, the first expansion valve 54, and the second expansion valve 55.

[0020] The heat medium circuit 50 exchanges heat with the low-temperature side circuit 30 and the high-temperature side circuit 40 via a low-temperature side heat exchanger 68 and a high-temperature side heat exchanger 70, providing functions utilizing heat exchange. The heat medium circuit 50 is filled with, for example, water, seawater, lake water, or brine such as antifreeze as the heat medium. Alternatively, antifreeze such as tetra-n-butylammonium bromide hydrate (TBAB), silicone oil, or ethylene glycol may be filled as the heat medium. An example configuration of the heat medium circuit 50 will be described with reference to FIG. 2 . FIG. 2 is a heat medium circuit diagram showing an example configuration of the heat medium circuit 50 according to this embodiment. In addition to the low-temperature side heat exchanger 68 and the high-temperature side heat exchanger 70, the heat medium circuit 50 includes, for example, a three-way cock 81, a tank 82, a heat exchanger 83, a pump 84, and heat medium pipes 85 to 89. The three-way cock 81 may be replaced with, for example, a T-pipe.

[0021] The heat medium flowing through the heat medium pipe 85 of the heat medium circuit 50 is driven by a pump 84 and circulates within the heat medium pipe 85. The heat medium flowing through the heat medium pipe 85 passes through the low-temperature side heat utilization heat exchanger 68 and the high-temperature side heat exchanger 70, and is warmed by exchanging heat with the refrigerant flowing through the low-temperature side circuit 30 and the high-temperature side circuit 40. Here, the heat medium passes through the low-temperature side heat utilization heat exchanger 68 and then flows into the high-temperature side heat utilization heat exchanger 70. The hot water discharged from the high-temperature side heat utilization heat exchanger 70 is sent by a three-way cock 81 to a tank 82 or heat medium pipe 86, depending on its intended use.

[0022] When heating is used, the three-way cock 81 sends hot water to the heat medium pipe 86. The heat medium pipe 86 is connected to each room that uses heating, and performs the heating function in each room. The water that has been used for heating and cooled returns through the heat medium pipe 89. The water that returns from the heat medium pipe 89 enters the low-temperature side heat exchanger 68 and the high-temperature side heat exchanger 70, where it is warmed by heat exchange, and sent back to each room.

[0023] When hot water is used, the three-way cock 81 sends hot water to the tank 82. Water is stored in the tank 82, and heat exchange occurs in the heat exchanger 83 between the water flowing through the heat medium pipe 85 and the water stored in the tank 82. The water stored in the tank 82 is heated by the heat exchanger 83 and sent to the heat medium pipe 87, where it is used to supply hot water for showers, etc. Meanwhile, the water flowing through the heat medium pipe 85 is cooled by the heat exchanger 83, enters the low-temperature side utilization heat exchanger 68 and the high-temperature side utilization heat exchanger 70, is heated by heat exchange, and is sent back to the tank 82. When the hot water stored in the tank 82 is used, city water or the like is replenished through the heat medium pipe 88.

[0024] The refrigerant circuit system 1 according to this embodiment can switch between the first cycle and the second cycle by using a switching circuit 80. The switching circuit 80 switches between the first cycle and the second cycle by using a first cock 65, a second cock 66, and a bypass flow path 811. When the first cycle is in use, the low-temperature side circuit 30 closes the first cock 65 and opens the second cock 66, thereby causing the second refrigerant to flow through the cascade heat exchanger 67 without flowing refrigerant through the low-temperature side utilization heat exchanger 68. When the second cycle is in use, the first cock 65 is closed and the second cock 66 is opened, causing the second refrigerant to flow through the low-temperature side utilization heat exchanger 68, and heat exchange occurs in the low-temperature side utilization heat exchanger 68. The first cycle, the second cycle, and the switching conditions therefor are described below.

[0025] <First Cycle> Next, the operation of the refrigerant circuit system 1 during heating operation in the first cycle according to this embodiment will be described with reference to Figures 3 and 4. Figure 3 is a refrigerant circuit diagram showing the flow of refrigerant during heating operation in the first cycle according to this embodiment. Figure 4 is a pressure-specific enthalpy diagram during heating operation in the first cycle, where (a) is the pressure-specific enthalpy diagram in the high-temperature side circuit 40 and (b) is the pressure-specific enthalpy diagram in the low-temperature side circuit 30.

[0026] When the first cycle is in use, the first cock 65 is closed and the second cock 66 is open. During heating operation, the four-way valve 53 of the low-temperature side circuit 30 is in a state where the first port (P1) and the second port (P2) are connected and the third port (P3) and the fourth port (P4) are connected.

[0027] When the first cycle is in operation, the second refrigerant circulates along the arrows shown in FIG. 3 . More specifically, in the low-temperature side circuit 30, the refrigerant is first compressed from a low-pressure state (see point 91 in FIGS. 3 and 4B) by the low-temperature side compressor 51. The compressed refrigerant passes through the four-way valve 53, the shut-off valve 63, the shut-off valve 64, and the second cock 66 before entering the cascade heat exchanger 67. When the first cycle is in operation, no refrigerant flows through the low-temperature side utilization heat exchanger 68, and no heat exchange occurs. Therefore, the state of the refrigerant does not change between points 92 and 93 in FIG. 3 (see points 92 and 93 in FIG. 4B). The cascade heat exchanger 67 functions as a condenser in the low-temperature side circuit 30, and the refrigerant is cooled by heat exchange with the high-temperature side circuit 40 (see point 94 in FIGS. 3 and 4B).

[0028] The refrigerant leaving the cascade heat exchanger 67 passes through the shutoff valve 62, the shutoff valve 61, the check valve 60, and the second expansion valve 55 before entering the receiver tank 56. The refrigerant leaving the receiver tank 56 passes through the first expansion valve 54 and the check valve 57 before entering the low-temperature side heat source heat exchanger 52. The refrigerant is decompressed as it passes through the second expansion valve 55 and the first expansion valve 54 (see point 95 in Figures 3 and 4(b)). The low-temperature side heat source heat exchanger 52 functions as an evaporator, and the refrigerant is heated by heat exchange with outside air (see point 91 in Figures 3 and 4(b)). The refrigerant leaving the low-temperature side heat source heat exchanger 52 passes through the four-way switching valve 53 and enters the low-temperature side compressor 51 again. In this manner, the refrigerant is circulated in the low-temperature side circuit 30.

[0029] In the high-temperature side circuit 40, the first refrigerant circulates along the arrows shown in Fig. 3. The refrigerant is first compressed from a low-pressure state (see point 96 in Fig. 3 and Fig. 4A) by the high-temperature side compressor 69 (see point 97 in Fig. 3 and Fig. 4A). The compressed refrigerant enters the high-temperature side utilization heat exchanger 70. The high-temperature side utilization heat exchanger 70 functions as a condenser, and the refrigerant flowing through the high-temperature side circuit 40 loses heat to the heat medium flowing through the heat medium circuit 50, thereby being cooled (see point 98 in Fig. 3 and Fig. 4A).

[0030] Next, the refrigerant passes through the high-stage expansion mechanism 71 and is decompressed in the process (see point 99 in FIGS. 3 and 4A). The decompressed refrigerant enters the cascade heat exchanger 67. The cascade heat exchanger 67 functions as an evaporator in the high-stage circuit 40, and the refrigerant is heated by heat exchange with the low-stage circuit 30 (see point 96 in FIGS. 3 and 4A). The refrigerant leaving the cascade heat exchanger 67 enters the high-stage compressor 69 again. In this way, the refrigerant is circulated in the high-stage circuit 40.

[0031] When the first cycle is in use, the heat medium flowing through the heat medium circuit 50 is warmed by heat exchange with the refrigerant flowing through the high-temperature side circuit 40 via the high-temperature side utilization heat exchanger 70. On the other hand, when the first cycle is in use, heat exchange in the low-temperature side utilization heat exchanger 68 is not performed.

[0032] <Second Cycle> Next, the operation of the refrigerant circuit system 1 during heating operation in the second cycle according to this embodiment will be described with reference to FIGS. 5 and 6. FIG. 5 is a refrigerant circuit diagram showing the flow of refrigerant during heating operation in the second cycle according to this embodiment. FIG. 6 is a pressure-specific enthalpy diagram during heating operation in the second cycle, where (a) is the pressure-specific enthalpy diagram in the high-temperature side circuit 40 and (b) is the pressure-specific enthalpy diagram in the low-temperature side circuit 30. When the second cycle is in use, the first cock 65 is open and the second cock 66 is closed. Furthermore, during heating operation, the four-way switching valve 53 is in a state in which the first port (P1) and the second port (P2) are connected and the third port (P3) and the fourth port (P4) are connected.

[0033] When the second cycle is in use, the refrigerant circulates along the arrows shown in Figure 5. More specifically, in the low-temperature side circuit 30, the refrigerant is first compressed from a low-pressure state (see point 91 in Figures 5 and 6(b)) by the low-temperature side compressor 51 (see point 92 in Figures 5 and 6(b)). The compressed refrigerant passes through the four-way valve 53, the shut-off valve 63, the shut-off valve 64, and the first cock 65, and enters the low-temperature side utilization heat exchanger 68. The low-temperature side utilization heat exchanger 68 functions as a condenser, and the refrigerant flowing through the low-temperature side circuit 30 is cooled by the heat medium flowing through the heat medium circuit 50, removing heat (see point 93 in Figures 5 and 6(b)). The refrigerant exiting the low-temperature side utilization heat exchanger 68 enters the cascade heat exchanger 67. The cascade heat exchanger 67 functions as a condenser in the low-temperature side circuit 30, and the refrigerant is cooled by heat exchange with the high-temperature side circuit 40 (see point 94 in FIGS. 5 and 6(b)).

[0034] The refrigerant leaving the cascade heat exchanger 67 passes through the shutoff valve 62, the shutoff valve 61, the check valve 60, and the second expansion valve 55 before entering the receiver tank 56. The refrigerant leaving the receiver tank 56 passes through the first expansion valve 54 and the check valve 57 before entering the low-temperature side heat source heat exchanger 52. The refrigerant is decompressed as it passes through the second expansion valve 55 and the first expansion valve 54 (see point 95 in Figures 5 and 6(b)). The low-temperature side heat source heat exchanger 52 functions as an evaporator, and the refrigerant is heated by heat exchange with outside air (see point 91 in Figures 5 and 6(b)). The refrigerant leaving the low-temperature side heat source heat exchanger 52 passes through the four-way switching valve 53 and enters the low-temperature side compressor 51 again. In this manner, the refrigerant is circulated in the low-temperature side circuit 30.

[0035] In the high-temperature side circuit 40 when the second cycle is used, the refrigerant flow and the pressure-specific enthalpy diagram during heating operation are the same as those when the first cycle is used. When the second cycle is used, the heat medium flowing through the heat medium circuit 50 is heated by heat exchange with the refrigerant flowing through the low-temperature side circuit 30 and the high-temperature side circuit 40, respectively, via the low-temperature side utilization heat exchanger 68 and the high-temperature side utilization heat exchanger 70.

[0036] <Switching Conditions> For example, when a flammable refrigerant such as propane is used in the high-temperature side circuit 40, its filling amount is limited. Therefore, in the first cycle, the heating capacity is limited by heat exchange only through the high-temperature side heat exchanger 70. On the other hand, in the second cycle, both the low-temperature side heat exchanger 68 and the high-temperature side heat exchanger 70 are used for heat exchange, and the heat medium flowing through the heat medium circuit 50 is warmed. When the second cycle is used, heat exchange also occurs in the low-temperature side heat exchanger 68, so it is possible to provide a higher heating capacity than when the first cycle is used.

[0037] In the second cycle, the refrigerant flowing through the low-temperature side circuit 30 is cooled in the low-temperature side utilization heat exchanger 68 and further cooled in the cascade heat exchanger 67. This allows for a higher specific enthalpy than when using the first cycle, thereby improving the refrigeration effect. On the other hand, in the second cycle, the high pressure must be increased to ensure the hot water outlet temperature (first heat medium temperature), and the compressor speed must be increased. In contrast, in the first cycle, the high pressure is not required as much as when using the second cycle, allowing for efficient operation.

[0038] For the above reasons, it is desirable to perform highly efficient operation using the first cycle when the required heating capacity is small, and to switch to the second cycle when the required heating capacity is large to ensure sufficient heating capacity. Switching between the first cycle and the second cycle in the refrigerant circuit system 1 according to this embodiment will be described below.

[0039] <<Switching from the First Cycle to the Second Cycle During Heating Operation>> In the refrigerant circuit system 1 according to the present embodiment, switching from heating operation using the first cycle to heating operation using the second cycle is performed, for example, when the control unit 100 determines that the required heating capacity will not be reached. When switching from the first cycle to the second cycle, the first cock 65 is opened and the second cock 66 is closed. The required heating capacity is determined by the control unit 100 based on three factors: the outlet hot water temperature (target outlet hot water temperature) set by the user, the current inlet water temperature, and the water flow rate.

[0040] An example of a case where it is determined that the required heating capacity will not be reached is when the rotation speed of the high-temperature side compressor 69 is equal to or greater than the upper limit or a certain threshold, the pump command value controlling the water flow rate is equal to or less than the certain threshold, but the outlet hot water temperature has not reached the target value. The outlet hot water temperature is the temperature of the water supplied from the tank 82 to the heat medium pipe 87. An example of the flow of switching from the first cycle to the second cycle during heating operation will be described with reference to FIG. 7B. FIG. 7B is a flowchart of switching from the first cycle to the second cycle during heating operation, where (a) shows an example when the upper limit of the rotation speed of the high-temperature side compressor 69 is used, and (b) shows an example when the threshold value of the rotation speed of the high-temperature side compressor 69 is used.

[0041] 7B(a) will be described. In FIG. 7B(a), first, operation using the first cycle is performed (step 1001). The control unit 100 determines whether the rotation speed of the high-side compressor 69 is equal to or greater than the upper limit (step 1002). If the rotation speed is equal to or less than the upper limit (NO in step 1002), the process returns to step 1001, and operation using the first cycle is performed. If the rotation speed is equal to or greater than the upper limit (YES in step 1002), the control unit 100 determines whether the pump command value that controls the water flow rate is equal to or less than the threshold (step 1003). If the pump command value is equal to or greater than the threshold (NO in step 1003), the process returns to step 1001, and operation using the first cycle is performed.

[0042] If the pump command value is equal to or less than the threshold value (YES in step 1003), the control unit 100 determines whether the outlet hot water temperature has reached the target value (step 1004). If the outlet hot water temperature has not reached the target value (NO in step 1004), the process returns to step 1001 and operation using the first cycle is performed. If the outlet hot water temperature has reached the target value (YES in step 1004), the control unit 100 switches to operation using the second cycle (step 1005).

[0043] Next, FIG. 7B(b) will be described. In FIG. 7B(b), first, operation using the first cycle is performed (step 1011). The control unit 100 determines whether the rotation speed of the high-stage side compressor 69 is equal to or greater than a threshold value (step 1012). If the rotation speed is equal to or less than the threshold value (NO in step 1012), the process returns to step 1011, and operation using the first cycle is performed. If the rotation speed is equal to or greater than the threshold value (YES in step 1012), the control unit 100 determines whether the pump command value that controls the water flow rate is equal to or less than a threshold value (step 1013). If the pump command value is equal to or greater than the threshold value (NO in step 1013), the process returns to step 1011, and operation using the first cycle is performed.

[0044] If the pump command value is equal to or less than the threshold value (YES in step 1013), the control unit 100 determines whether the outlet hot water temperature has reached the target value (step 1014). If the outlet hot water temperature has not reached the target value (NO in step 1014), the process returns to step 1011 and operation using the first cycle is performed. If the outlet hot water temperature has reached the target value (YES in step 1014), the control unit 100 switches to operation using the second cycle (step 1015).

[0045] The rotation speed of the high-temperature side compressor 69 being at its upper limit means that the compressor is rotating at a speed equal to or higher than the upper limit rotation speed of the compressor that can be continuously operated and that is set for each compressor. The rotation speed of the high-temperature side compressor 69 being at or above a certain threshold means, for example, that the high-temperature side compressor 69 is rotating at a speed equal to or higher than 75% of its rotational capacity. The pump command value controlling the water flow rate being below a certain threshold means, for example, that the output value is 50% or lower. The outlet hot water temperature not reaching the target value means, for example, that the target value is 55°C, but the outlet temperature only reaches 50°C. In such a case, it is determined that the use of the first cycle will not achieve the heating capacity required to set the outlet hot water temperature to the target value, and the first cycle is switched to the second cycle.

[0046] The above-described threshold values ​​for the rotation speed of the high-temperature side compressor 69 and the pump command value are merely examples and are not limiting. For example, each threshold value used as a basis for switching the refrigeration cycle may be configured to be changeable. By switching between the first cycle and the second cycle according to the above-described switching conditions, the refrigeration cycle can be switched based on the rotation speed of the high-temperature side compressor 69, the pump command value, and the outlet hot water temperature.

[0047] Furthermore, the determination that the required heating capacity will not be reached refers to, for example, the case where the rotational speed of the high-temperature side compressor 69 is equal to or greater than the upper limit or a certain threshold, the heating capacity estimated from the inlet water temperature, the outlet hot water temperature, and the pump command value has not reached the target capacity, and the heating capacity has not improved for a certain period of time. When using the first cycle, if the high-temperature side compressor 69 is operating at a rotational speed equal to or greater than the upper limit or a certain threshold, but the heating capacity has not reached the target capacity and the heating capacity has not improved for a certain period of time, it is determined that the required heating capacity will not be reached in the first cycle. For example, if the high-temperature side compressor 69 is rotating at 100% of its rotational capacity and the heating capacity estimated from the inlet water temperature, the outlet hot water temperature, and the pump command value has not reached the target capacity for five minutes, the system switches from the first cycle to the second cycle.

[0048] The rotation speed and duration of the high-temperature side compressor 69 described above are merely examples and are not limited to these. By switching between the first cycle and the second cycle according to the above-described switching conditions, it is possible to switch to the refrigeration cycle when it is determined that the required heating capacity cannot be achieved in the first cycle. Furthermore, by switching when it is determined that the heating capacity has not improved for a certain period of time, it is possible to prevent switching to the refrigeration cycle before the heating capacity has reached its full potential.

[0049] Furthermore, the control unit 100 may switch from heating operation using the first cycle to heating operation using the second cycle, for example, when the current rotational speed of the high-temperature side compressor 69 is at its upper limit and the current heating capacity does not reach the required heating capacity, or when the current outlet hot water temperature does not reach the target outlet hot water temperature. The control unit 100 determines the current heating capacity based on the current outlet hot water temperature, the current inlet water temperature, and the water flow rate. Note that the switching may also be controlled based on the temperature of another part, for example, the temperature of the heat medium supplied from the heat medium pipe 86. The temperature is measured, for example, by a thermometer (not shown).

[0050] The pump command value used to determine whether to switch from the first cycle to the second cycle may be changed by setting the heat medium pipe length. The heat medium pipe length refers to the length of the heat medium pipes 86, 87 (see FIG. 2) that send the heat medium flowing through the heat medium circuit 50 to each room that uses heating or hot water. Even if the pump command value is the same, if the heat medium pipe lengths are different, the water flow rate cannot be made uniform. Even when the pump 84 (see FIG. 2) is used with the same pump command value, the amount of water that can be flowed will differ depending on the heat medium pipe length.

[0051] When estimating heating capacity from the pump command value, if the amount of water that can be flowed varies depending on the heat medium pipe length, it is impossible to accurately estimate the heating capacity. Therefore, to reduce errors in estimating heating capacity, for example, the control unit 100 accepts input of the heat medium pipe length and changes the pump command value according to the heat medium pipe length setting. For example, if the pump command value is 40% when the heat medium pipe length is 15 m, the pump command value can be set to 70% when the heat medium pipe length is 30 m, thereby correcting for differences in water volume due to differences in heat medium pipe length. With this configuration, it is possible to flow water at the same flow rate even when the heat medium pipe length is different.

[0052] <<Switching from the Second Cycle to the First Cycle During Heating Operation>> In the refrigerant circuit system 1 according to the present embodiment, for example, the control unit can switch from heating operation using the second cycle to heating operation using the first cycle when the current heating capacity reaches the required heating capacity, or the current hot water outlet temperature reaches the target hot water outlet temperature, and the current rotation speed of the high-temperature side compressor 69 has a margin above the upper limit of the rotation speed of the high-temperature side compressor 69. When switching from the second cycle to the first cycle, the first cock 65 is closed and the second cock 66 is opened. When the rotation speed of the high-temperature side compressor 69 has a margin above the upper limit, for example, the current rotation speed of the high-temperature side compressor 69 is less than 90% of the upper limit of the rotation speed of the high-temperature side compressor.

[0053] An example of the flow of switching from the second cycle to the first cycle during heating operation will be described with reference to Figure 7C. Figure 7C is a flowchart of switching from the second cycle to the first cycle during heating operation, where (a) shows an example when the required heating capacity is used, and (b) shows an example when the target hot water outlet temperature is used.

[0054] 7C(a) will be described. In FIG. 7C(a), first, operation using the second cycle is performed (step 2001). The control unit 100 determines whether the current heating capacity has reached the required heating capacity (step 2002). If the current heating capacity has not reached the required heating capacity (NO in step 2002), the process returns to step 2001 and operation using the second cycle is performed.

[0055] If the current heating capacity has reached the required heating capacity (YES in step 2002), the control unit 100 determines whether the current rotation speed of the high-stage side compressor 69 has a margin with respect to the upper limit of the rotation speed of the high-stage side compressor 69 (step 2003). If there is no margin with respect to the upper limit (NO in step 2003), the control unit 100 returns to step 2001 and performs operation using the second cycle. If there is a margin with respect to the upper limit (YES in step 2003), the control unit 100 switches to operation using the first cycle (step 2004).

[0056] Next, Fig. 7C(b) will be described. In Fig. 7C(b), first, operation using the second cycle is performed (step 2011). The control unit 100 determines whether the current outlet heated water temperature has reached the target outlet heated water temperature (step 2012). If the current outlet heated water temperature has not reached the target outlet heated water temperature (NO in step 2012), the process returns to step 2011 and operation using the second cycle is performed.

[0057] If the current outlet hot water temperature has reached the target outlet hot water temperature (YES in step 2012), the control unit 100 determines whether the current rotation speed of the high-temperature side compressor 69 has a margin relative to the upper limit of the rotation speed of the high-temperature side compressor 69 (step 2013). If there is no margin relative to the upper limit (NO in step 2013), the process returns to step 2011 and operation using the second cycle is performed. If there is a margin relative to the upper limit (YES in step 2013), the control unit 100 switches to operation using the first cycle (step 2014).

[0058] <<Startup of Refrigerant Circuit System 1>> FIG. 7D is a flowchart illustrating the startup of the refrigerant circuit system according to this embodiment. When starting up the refrigerant circuit system 1 according to this embodiment, the control unit determines whether to perform heating operation using the first cycle or the second cycle based on the switching determination capability. The switching determination capability is an index for determining whether to use the first cycle or the second cycle during heating operation, and is determined by the control unit 100 based on the target hot water outlet temperature and the outdoor air temperature. For example, as shown in FIG. 7D , if the switching determination capability exceeds the required heating capacity (YES in step 3001), the control unit 100 starts operation of the refrigerant circuit system 1 using the first cycle (step 3002). On the other hand, if the switching determination capability is lower than the required heating capacity (NO in step 3001), the control unit 100 starts operation of the refrigerant circuit system 1 using the second cycle (step 3003).

[0059] <<Regarding Control Steps of Refrigerant Circuit System 1>> The control flow of the refrigerant circuit system 1 according to this embodiment includes, for example, a first step in which a user sets a hot water outlet temperature, a second step in which the control unit 100 calculates a required heating capacity and a switching determination capacity, and a third step in which the control unit 100 determines whether to perform heating operation using the first cycle or the second cycle based on the calculated switching determination capacity. If heating operation using the first cycle is performed in the third step, a fourth step is performed in which heating operation using the second cycle is performed if (1) the current rotation speed of the high-temperature side compressor 69 is at its upper limit and (2) the current heating capacity does not reach the required heating capacity, or (3) the current hot water outlet temperature does not reach the target hot water outlet temperature. If heating operation using the second cycle is performed in the third step, a fifth step is performed in which heating operation using the first cycle is performed if (1) the current rotation speed of the high-temperature side compressor 69 has a margin relative to the upper limit of the rotation speed of the high-temperature side compressor 69 and (2) the current heating capacity reaches the required heating capacity, or (3) the current hot water outlet temperature reaches the target hot water outlet temperature. For example, when heating operation using the first cycle is currently being performed, the control unit 100 may periodically determine whether any of the following conditions (1) to (3) applies: (1) whether the current rotation speed of the high-temperature side compressor 69 is at its upper limit, (2) whether the current heating capacity does not reach the required heating capacity, or (3) whether the current outlet hot water temperature does not reach the target hot water outlet temperature. Furthermore, when heating operation using the second cycle is currently being performed, the control unit 100 may periodically determine whether any of the following conditions (1) to (3) applies: (1) whether the current rotation speed of the high-temperature side compressor 69 has a margin relative to the upper limit of the rotation speed of the high-temperature side compressor 69, (2) whether the current heating capacity reaches the required heating capacity, or (3) whether the current outlet hot water temperature reaches the target hot water outlet temperature.

[0060] Furthermore, the configuration may be such that heating operation is performed only using the low-source side circuit 30 when an abnormality occurs in the high-source side circuit 40. An abnormality is an event that makes it impossible to operate using the high-source side circuit 40, such as a failure of the high-source side compressor 69. Also, the configuration may be such that heating operation is performed only using the low-source side circuit 30 even in the case of a minor failure that is not severe enough to prevent operation. With this configuration, even when the high-source side circuit 40 is unavailable, a certain degree of heating capacity can be ensured using only the low-source side circuit 30.

[0061] Furthermore, a configuration may be adopted in which an abnormality alert is issued when an abnormality occurs in the high-temperature side circuit 40. The user who receives the abnormality alert will understand that the high-temperature side circuit 40 is unavailable and can take action quickly. This configuration allows the occurrence of an abnormality to be confirmed even when not on-site, and also allows the user to make the decision to perform heating operation using only the low-temperature side circuit.

[0062] In this embodiment, the high-temperature side circuit 40 is installed indoors. When propane is used as the refrigerant filled in the high-temperature side circuit 40 and the circuit is installed indoors, according to IEC 60335-2-40 Ed. 7, if the refrigerant amount is 152 g or less, there is no restriction on the floor area of ​​the room where the equipment is installed. However, if the refrigerant amount exceeds 152 g, the floor area of ​​the room where the equipment is installed is restricted, and safety measures must be taken in case of leakage. In this invention, by suppressing the maximum heating capacity in the first cycle, the heat exchanger volume can be reduced, thereby suppressing the refrigerant amount. If the heating capacity is insufficient, the system switches to the second cycle, ensuring heating capacity on the low-temperature side circuit 30 side, thereby expanding the maximum heating capacity. By suppressing the refrigerant amount in this way, the high-temperature side circuit 40 filled with propane can be installed indoors, even in areas where only limited ventilation is possible.

[0063] <Effects> The refrigerant circuit system of the present disclosure is a two-stage refrigerant circuit system in which a high-stage side circuit (40) using a first refrigerant and a low-stage side circuit (30) using a second refrigerant are connected via a cascade heat exchanger (67), the high-stage side circuit (40) includes a high-stage side compressor 69, a high-stage side utilization heat exchanger 70, and a high-stage side expansion mechanism 71, the low-stage side circuit (30) includes a low-stage side compressor 51, a low-stage side utilization heat exchanger 68, a first expansion valve 54, a second expansion valve 55, and a low-stage side heat source heat exchanger 52, and has a switching circuit 80 between a first cycle in which refrigerant flows from the low-stage side compressor 51 to the cascade heat exchanger 67 and a second cycle in which refrigerant flowed from the low-stage side compressor 51 to the low-stage side utilization heat exchanger 68 flows to the cascade heat exchanger 67. In this case, the refrigeration cycle can be switched depending on the required heating capacity. In the refrigerant circuit system of the present disclosure, the high-temperature side heat exchanger 70 and the low-temperature side heat exchanger 68 are water heat exchangers, and the system further includes a heat medium circuit 50 including heat medium pipes 85, 86, 87, and 89, a pump 84, and a tank 82. In the refrigerant circuit system of the present disclosure, the low-temperature side circuit 30 further includes a bypass flow path 811, a first cock 65, and a second cock 66. The refrigerant circuit system 1 of the present disclosure switches to the second cycle when it determines that the required heating capacity will not be reached during heating operation using the first cycle. In this case, the refrigeration cycle can be switched by comparing the current heating capacity with the required heating capacity. In the refrigerant circuit system of the present disclosure, the required heating capacity is determined based on three factors: the outlet hot water temperature (target outlet hot water temperature) set by the user, the current inlet water temperature, and the water flow rate. Here, the refrigerant circuit system 1 of the present disclosure switches to the second cycle when the rotation speed of the high-temperature side compressor 69 is equal to or greater than an upper limit value or a certain threshold value, and the pump command value that controls the heat medium flow rate is equal to or less than a certain threshold value, but the first heat medium temperature from the heat medium pipe 87 has not reached a target value. In this case, the refrigeration cycle can be switched based on the rotation speed of the high-temperature side compressor 69, the pump command value, and the outlet hot water temperature.The refrigerant circuit system 1 of the present disclosure switches to the second cycle when the rotation speed of the high-temperature side compressor 69 is equal to or greater than an upper limit or a certain threshold, the heating capacity estimated from the inlet water temperature, the outlet hot water temperature, and the pump command value has not reached the target capacity, and it is determined that the heating capacity has not improved for a certain period of time. In this case, it is possible to prevent switching to the refrigeration cycle before the heating capacity has fully increased. The refrigerant circuit system 1 of the present disclosure also changes the pump command value used to determine whether to switch from the first cycle to the second cycle depending on the heat transfer medium pipe length setting. In this case, the same water flow rate can be maintained even when the heat transfer medium pipe length is different. The refrigerant circuit system 1 of the present disclosure also performs heating operation using only the low-temperature side circuit 30 when an abnormality occurs in the high-temperature side circuit 40. In this case, even if the high-temperature side circuit 40 is unavailable, a certain level of heating capacity can be maintained using only the low-temperature side circuit 30. The refrigerant circuit system 1 of the present disclosure also issues an abnormality alert when an abnormality occurs in the high-temperature side circuit 40. In this case, the occurrence of an abnormality can be confirmed without being on-site, and the user can also make the decision to perform heating operation using only the low-stage side circuit 30. Furthermore, in the refrigerant circuit system 1 disclosed herein, the high-stage side circuit 40 is installed indoors. In this case, even if a flammable refrigerant is used in the high-stage side circuit, the flammable refrigerant fill amount regulation can be satisfied, and the system can be installed indoors even if only a certain amount of ventilation is possible. Furthermore, the refrigerant circuit system 1 disclosed herein uses natural refrigerants for both of the two refrigerants. Furthermore, the refrigerant circuit system 1 disclosed herein uses propane in the high-stage side circuit 40 and carbon dioxide in the low-stage side circuit 30. Furthermore, in the refrigerant circuit system 1 disclosed herein, in the heat medium circuit 50, the heat medium supplied from the pump 84 flows sequentially through the low-stage side utilization heat exchanger 68 and the high-stage side utilization heat exchanger 70. In this case, the refrigeration cycle can be switched according to the required heating capacity by switching the switching circuit 80 without switching the flow path on the heat medium circuit 50 side. The refrigerant circuit system of the present disclosure further includes a four-way selector valve (53), and switches between heating operation and cooling operation by switching the four-way selector valve (53).

[0064] <Modifications> Modifications of the refrigerant circuit system 1 of the present disclosure will be described. The following modifications can be combined as appropriate.

[0065] <Modification 1> In the refrigerant circuit system 1 of the present disclosure, the first cock 65 is arranged on the flow path connecting the low-temperature side utilization heat exchanger 68 and the low-temperature side compressor 51 in the bypass flow path 811 of the low-temperature side circuit 30, but the present invention is not limited to this. Fig. 8A is a schematic refrigerant circuit diagram of a switching circuit in a refrigerant circuit system according to Modification 1. As shown in Fig. 8A, the first cock 65 is arranged on the flow path connecting the low-temperature side utilization heat exchanger 68 and the cascade heat exchanger 67 (see Fig. 1).

[0066] <Modification 2> Fig. 8B is a schematic refrigerant circuit diagram of a switching circuit in a refrigerant circuit system according to Modification 2. In the refrigerant circuit system 1 according to the present disclosure, the first cock 65 and the second cock 66 are used in the bypass flow path 811 of the low-stage side circuit 30 to switch whether or not the second refrigerant is to flow to the low-stage side utilization heat exchanger 68. In contrast, as shown in Fig. 8B , in the refrigerant circuit system 1 according to Modification 2, a three-way valve 812 is disposed at a point where one end of the bypass flow path 811 is connected to a flow path that connects the low-stage side utilization heat exchanger 68 and the low-stage side compressor 51 (see Fig. 1 ).

[0067] <Modification 3> Fig. 8C is a schematic refrigerant circuit diagram of a switching circuit in a refrigerant circuit system according to Modification 3. As shown in Fig. 8C, in the refrigerant circuit system 1 of Modification 3, a three-way valve 813 is disposed at a location where one end of a bypass flow path 811 is connected to a flow path that connects the low-temperature side utilization heat exchanger 68 and the cascade heat exchanger 67 (see Fig. 1 ).

[0068] <Modification 4> Fig. 9A is a schematic heat medium circuit diagram of a refrigerant circuit system according to Modification 4. In the heat medium circuit 50 of the refrigerant circuit system 1 according to the present disclosure, the heat medium passes through the low-temperature side heat utilization heat exchanger 68 and then flows to the high-temperature side heat utilization heat exchanger 70, but is not limited to this. As shown in Fig. 9A, in the heat medium circuit 50 according to Modification 4, the heat medium passes through the high-temperature side heat utilization heat exchanger 70 and then flows to the low-temperature side heat utilization heat exchanger 68.

[0069] <Modification 5> Fig. 9B is a schematic heat medium circuit diagram of a refrigerant circuit system according to Modification 5. As shown in Fig. 9B, in a heat medium circuit 50 according to Modification 5, the heat medium circulates independently through a low-temperature side heat utilization heat exchanger 68 and a high-temperature side heat utilization heat exchanger 70.

[0070] Although the embodiments have been described above, the technical scope of the present disclosure is not limited to the scope of the above-described embodiments. It is clear from the claims that combinations of two or more of the above-described embodiments, and various modifications or improvements to the above-described embodiments, are also included in the technical scope of the present disclosure.

[0071] 1... Refrigerant circuit system, 10... outdoor unit, 20... indoor unit, 30... low-stage side circuit, 40... high-stage side circuit, 50... heat medium circuit, 51... low-stage side compressor, 52... low-stage side heat source heat exchanger, 53... four-way switching valve, 54... first expansion valve, 55... second expansion valve, 67... cascade heat exchanger, 68... low-stage side utilization heat exchanger, 69... high-stage side compressor, 70... high-stage side utilization heat exchanger, 71... high-stage side expansion mechanism, 80... switching circuit

Claims

1. A two-stage refrigerant circuit system in which a high-stage side circuit (40) using a first refrigerant and a low-stage side circuit (30) using a second refrigerant are connected via a cascade heat exchanger (67), the high-stage side circuit (40) is provided with a high-stage side compressor (69), a high-stage side utilization heat exchanger (70), and a high-stage side expansion mechanism (71), the low-stage side circuit (30) is provided with a low-stage side compressor (51), a low-stage side utilization heat exchanger (68), a low-stage side expansion mechanism (54, 55), and a low-stage side heat source heat exchanger (52), the first cycle being for flowing a refrigerant from the low-stage side compressor (51) to the cascade heat exchanger (67), and a second cycle being for flowing the refrigerant flowed from the low-stage side compressor (51) to the low-stage side utilization heat exchanger (68) to the cascade heat exchanger (67). A refrigerant circuit system comprising a switching circuit (80) for switching between a refrigerant 2. The refrigerant circuit system according to claim 1, wherein the high-temperature side utilization heat exchanger (70) and the low-temperature side utilization heat exchanger (68) are water heat exchangers, and further comprising a heat medium circuit (50) including heat medium piping (85, 86, 87, 89), a pump (84), and a tank (82).

3. The refrigerant circuit system according to claim 2, wherein the low-temperature side circuit (30) further comprises a bypass flow path (811), a first cock (65), and a second cock (66).

4. A refrigerant circuit system according to claim 1 or 2, which switches to the second cycle when it is determined that a required heating capacity is not reached during heating operation using the first cycle.

5. A refrigerant circuit system according to claim 4, wherein the required heating capacity is determined based on three factors: a target hot water outlet temperature set by a user, a current inlet water temperature, and a water flow rate.

6. A refrigerant circuit system as described in claim 4, which switches to the second cycle when the rotation speed of the high-temperature side compressor (69) is equal to or higher than an upper limit value or a certain threshold value, and the pump command value controlling the heat medium flow rate is also equal to or lower than a certain threshold value, but the first heat medium temperature from the heat medium piping (87) has not reached a target value.

7. A refrigerant circuit system as described in claim 4, which switches to the second cycle when it is determined that the rotation speed of the high-temperature side compressor (69) is above an upper limit value or a certain threshold value, and the heating capacity estimated from the inlet water temperature, the outlet water temperature, and the pump command value has not reached the target capacity, and the heating capacity has not improved for a certain period of time.

8. A refrigerant circuit system according to claim 4, wherein a pump command value used to determine whether to switch from the first cycle to the second cycle is changed by setting a length of heat medium piping.

9. The refrigerant circuit system according to claim 2, wherein when an abnormality occurs in the high-temperature side circuit (40), heating operation is performed only in the low-temperature side circuit (30).

10. A refrigerant circuit system according to any one of claims 2 to 9, wherein an abnormality alert is issued when an abnormality occurs in the high temperature side circuit (40).

11. The refrigerant circuit system according to any one of claims 2 to 9, wherein the high temperature side circuit (40) is provided indoors.

12. A refrigerant circuit system according to any one of claims 2, 4 and 9, wherein both of the two types of refrigerants are natural refrigerants.

13. A refrigerant circuit system according to claim 12, wherein propane is used in the high-temperature side circuit (40) and carbon dioxide is used in the low-temperature side circuit (30).

14. A refrigerant circuit system as described in claim 2, wherein in the heat medium circuit (50), the heat medium supplied from the pump (84) flows through the low-temperature side utilization heat exchanger (68) and then through the high-temperature side utilization heat exchanger (70) in that order.

15. The refrigerant circuit system according to claim 2, further comprising a four-way switching valve (53), wherein the heating operation and the cooling operation are switched by switching the four-way switching valve (53).

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