Patent Publication Number: US-2022221168-A1

Title: Air conditioner

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of PCT International Application No. PCT/JP2020/029351, filed on Jul. 30, 2020, which claims priority under 35 U.S.C. 119(a) to Patent Application No. 2019-180598, filed in Japan on Sep. 30, 2019 and Patent Application No. 2019-180599, filed in Japan on Sep. 30, 2019, all of which are hereby expressly incorporated by reference into the present application. 
    
    
     TECHNICAL FIELD 
     Air conditioner equipped with ejector 
     BACKGROUND ART 
     For example, as described in PTL 1 (Japanese Patent No. 4069656), a vapor compression refrigerating machine that performs a vapor compression refrigeration cycle using an ejector is known in the related art. The vapor compression refrigerating machine described in PTL 1 is applied to an air conditioner capable of switching between cooling and heating. 
     SUMMARY 
     An air conditioner according to one aspect includes a compression mechanism, a first heat-source-side heat exchanger, a use-side heat exchanger, an ejector that raises a pressure of refrigerant by using energy for refrigerant decompression and expansion, an expansion mechanism, and a switching mechanism. The switching mechanism switches between a refrigerant flow in a first operation and a refrigerant flow in a second operation. The air conditioner is configured such that in the first operation, refrigerant compressed by the compression mechanism radiates heat in the use-side heat exchanger and is decompressed and expanded by the ejector while refrigerant evaporated in the first heat-source-side heat exchanger is raised in pressure by the ejector. The air conditioner is configured such that in the second operation, refrigerant compressed by the compression mechanism radiates heat in the first heat-source-side heat exchanger and is decompressed and expanded by the expansion mechanism before being evaporated in the use-side heat exchanger while refrigerant does not flow through the ejector. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a circuit diagram for explaining a first operation of an air conditioner according to a first embodiment. 
         FIG. 2  is a Mollier diagram illustrating a state of refrigerant in the first operation of the air conditioner in  FIG. 1 . 
         FIG. 3  is a circuit diagram for explaining a second operation of the air conditioner according to the first embodiment. 
         FIG. 4  is a circuit diagram for explaining a third operation of the air conditioner according to the first embodiment. 
         FIG. 5  is a block diagram for explaining a controller of the air conditioner in  FIG. 1 . 
         FIG. 6  is a circuit diagram for explaining a first operation of an air conditioner according to a second embodiment. 
         FIG. 7  is a circuit diagram for explaining a second operation of the air conditioner according to the second embodiment. 
         FIG. 8  is a circuit diagram for explaining a third operation of the air conditioner according to the second embodiment. 
         FIG. 9  is a block diagram for explaining a controller of the air conditioner in  FIG. 6 . 
         FIG. 10  is a circuit diagram for explaining a first operation of an air conditioner according to a third embodiment. 
         FIG. 11  is a Mollier diagram illustrating a state of refrigerant in the first operation of the air conditioner in  FIG. 10 . 
         FIG. 12  is a circuit diagram for explaining a second operation of the air conditioner according to the third embodiment. 
         FIG. 13  is a circuit diagram for explaining a third operation of the air conditioner according to the third embodiment. 
         FIG. 14  is a circuit diagram for explaining an air conditioner according to modification A and modification B. 
         FIG. 15  is a circuit diagram for explaining an air conditioner according to modification C. 
         FIG. 16  is a circuit diagram for explaining an air conditioner according to modification D. 
         FIG. 17  is a circuit diagram for explaining a first operation of an air conditioner according to a fourth embodiment. 
         FIG. 18  is a Mollier diagram illustrating a state of refrigerant in the first operation of the air conditioner in  FIG. 17 . 
         FIG. 19  is a circuit diagram for explaining a second operation of the air conditioner according to the fourth embodiment. 
         FIG. 20  is a block diagram for explaining a controller of the air conditioner in  FIG. 17 . 
         FIG. 21  is a circuit diagram for explaining a first operation of an air conditioner according to a fifth embodiment. 
         FIG. 22  is a circuit diagram for explaining a second operation of the air conditioner according to the fifth embodiment. 
         FIG. 23  is a block diagram for explaining a controller of the air conditioner in  FIG. 21 . 
         FIG. 24  is a circuit diagram for explaining a first operation of an air conditioner according to a sixth embodiment. 
         FIG. 25  is a Mollier diagram illustrating a state of refrigerant in the first operation of the air conditioner in  FIG. 24 . 
         FIG. 26  is a circuit diagram for explaining a second operation of the air conditioner according to the sixth embodiment. 
         FIG. 27  is a block diagram for explaining a controller of the air conditioner in  FIG. 24 . 
         FIG. 28  is a circuit diagram for explaining a first operation of an air conditioner according to a seventh embodiment. 
         FIG. 29  is a Mollier diagram illustrating a state of refrigerant in the first operation of the air conditioner in  FIG. 28 . 
         FIG. 30  is a circuit diagram for explaining a second operation of the air conditioner according to the seventh embodiment. 
         FIG. 31  is a block diagram for explaining a controller of the air conditioner in  FIG. 28 . 
         FIG. 32  is a circuit diagram for explaining an air conditioner according to modification I. 
         FIG. 33  is a circuit diagram for explaining an air conditioner according to modification J. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     (1) Overview of Configuration 
     As illustrated in  FIG. 1 ,  FIG. 3 , and  FIG. 4 , an air conditioner  1  according to a first embodiment includes a compression mechanism  10 , a heat-source-side heat exchanger  31 , a use-side heat exchanger  32 , an ejector  50  that raises the pressure of refrigerant by using energy for refrigerant decompression and expansion, a first expansion valve  41 , and a switching mechanism  20 . The switching mechanism  20  switches between the refrigerant flow in a first operation illustrated in  FIG. 1  and the refrigerant flow in a second operation illustrated in  FIG. 3 . 
     As illustrated in  FIG. 1 , the air conditioner  1  is configured such that, in the first operation, the refrigerant compressed by the compression mechanism  10  radiates heat in the use-side heat exchanger  32  and is decompressed and expanded by the ejector  50  while the refrigerant evaporated in the heat-source-side heat exchanger  31  is raised in pressure by the ejector  50 . 
     As illustrated in  FIG. 3 , the air conditioner  1  is configured such that, in the second operation, the refrigerant compressed by the compression mechanism  10  radiate heat in the heat-source-side heat exchanger  31  and is decompressed and expanded by the first expansion valve  41  before being evaporated in the use-side heat exchanger  32  while refrigerant does not flow through the ejector  50 . 
     The air conditioner  1  having the configuration described above can perform heating in the first operation illustrated in  FIG. 1  by using heat radiated from the refrigerant in the use-side heat exchanger  32 . In the second operation illustrated in  FIG. 3 , the air conditioner  1  can perform cooling by heat absorption due to the evaporation of the refrigerant in the use-side heat exchanger  32 . The air conditioner  1  can improve heating efficiency and cooling efficiency by switching between the heating operation using the ejector  50  and the cooling operation without using the ejector  50 . 
     (2) Detailed Configuration 
     (2-1) Overview of Circuit Configuration of Air Conditioner  1   
     The air conditioner  1  according to the first embodiment includes, in addition to the compression mechanism  10 , the heat-source-side heat exchanger  31 , the use-side heat exchanger  32 , the ejector  50 , the first expansion valve  41 , and the switching mechanism  20  described above, a first flow path F 1 , a second flow path F 2 , a third flow path F 3 , a fourth flow path F 4 , an on-off valve  61 , which is a first valve, and a flow rate control valve  43 , which is a second valve. The flow rate control valve  43  is capable of changing the flow rate of the refrigerant by changing the opening degree thereof. Further, the flow rate control valve  43  is capable of shutting off the refrigerant flow when fully closed. The switching mechanism  20  is constituted by a four-way valve  21 . 
     The first flow path F 1  is a flow path through which the heat-source-side heat exchanger  31  and the use-side heat exchanger  32  communicate with each other. The second flow path F 2  branches off from the first flow path F 1  between the use-side heat exchanger  32  and the on-off valve  61  and communicates with a refrigerant inflow port of the ejector  50 . In the third flow path F 3 , the refrigerant flows from a refrigerant outflow port of the ejector  50  to the heat-source-side heat exchanger  31  during the first operation (see  FIG. 1 ), and no refrigerant flows between the refrigerant outflow port of the ejector  50  and the heat-source-side heat exchanger  31  during the second operation (see  FIG. 3 ). In the fourth flow path F 4 , gas refrigerant flows from the heat-source-side heat exchanger  31  to a refrigerant suction port of the ejector  50  during the first operation (see  FIG. 1 ), and no refrigerant flows between the heat-source-side heat exchanger  31  and the refrigerant suction port of the ejector  50  during the second operation (see  FIG. 3 ). 
     The on-off valve  61  is disposed in the first flow path F 1 . The flow rate control valve  43  is disposed in the second flow path F 2 . During the first operation, as illustrated in  FIG. 1 , the on-off valve  61  closes the first flow path F 1 , and the flow rate control valve  43  opens the second flow path F 2 . During the second operation, as illustrated in  FIG. 3 , the on-off valve  61  opens the first flow path F 1 , and the flow rate control valve  43  closes the second flow path F 2 . 
     In the air conditioner  1  according to the first embodiment, with a simple configuration of the four flow paths, namely, the first flow path F 1  to the fourth flow path F 4 , the on-off valve  61  (first valve), and the flow rate control valve  43  (second valve), the ejector  50  can be bypassed during the second operation. In other words, in the second operation, as illustrated in  FIG. 3 , the refrigerant circulates through a compressor  11 , the four-way valve  21 , the heat-source-side heat exchanger  31 , a second expansion valve  42 , the on-off valve  61 , the first expansion valve  41 , the use-side heat exchanger  32 , the four-way valve  21 , a receiver  91 , and the compressor  11  in this order. However, this circulation path does not include the ejector  50 , and no refrigerant flows through the ejector  50  in the second operation. The compressor  11  is, for example, a compressor whose capacity can be changed, and includes a motor driven by an inverter. 
     (2-2) Details of Circuit Configuration of Air Conditioner  1   
     The air conditioner  1  includes, in addition to the configuration described above, a gas-liquid separator  92 , a check valve  63 , which is a third valve, an on-off valve  64 , which is a fourth valve, a check valve  65 , which is a fifth valve, an on-off valve  66 , which is a sixth valve, a fifth flow path F 5 , and a sixth flow path F 6 . 
     The gas-liquid separator  92  has a refrigerant inlet communicating with the refrigerant outflow port of the ejector  50 , a liquid refrigerant outlet from which separated liquid refrigerant flows out, and a gas refrigerant outlet from which separated gas refrigerant flows out. In the air conditioner  1 , a portion from the refrigerant outflow port of the ejector  50  to the liquid refrigerant outlet of the gas-liquid separator  92  constitutes part of the third flow path F 3 . The liquid refrigerant outlet of the gas-liquid separator  92  communicates with an inlet of the check valve  63 . 
     The check valve  63  is disposed in the third flow path F 3 . As illustrated in  FIG. 1 , the check valve  63  allows the liquid refrigerant to flow from the liquid refrigerant outlet of the gas-liquid separator  92  to the heat-source-side heat exchanger  31  during the first operation. Since the on-off valve  61  is closed during the first operation, the refrigerant that has flowed out of an outlet of the check valve  63  does not flow to the use-side heat exchanger  32 , but flows to the heat-source-side heat exchanger  31  via the second expansion valve  42 . As illustrated in  FIG. 3 , the check valve  63  prevents the flow of liquid refrigerant between the liquid refrigerant outlet of the gas-liquid separator  92  and the heat-source-side heat exchanger  31  during the second operation. During the second operation, since the pressure of the refrigerant at the outlet of the check valve  63  is higher than the pressure of the refrigerant at the inlet of the check valve  63  (the first flow path F 1 ), the refrigerant does not flow through the check valve  63 . 
     The on-off valve  64  is disposed in the fourth flow path F 4 . In the air conditioner  1 , the on-off valve  64  is opened to open the fourth flow path F 4  during the first operation. In the air conditioner  1 , the on-off valve  64  is closed to close the fourth flow path F 4  during the second operation. 
     The fifth flow path F 5  is a flow path through which the gas refrigerant flows from the gas refrigerant outlet of the gas-liquid separator  92  to the suction side of the compressor  11 . The sixth flow path F 6  is a flow path through which the heat-source-side heat exchanger  31  and the compressor  11  communicate with each other. 
     The check valve  65  is disposed in the fifth flow path F 5 . During the first operation, the check valve  65  allows the gas refrigerant to flow from the gas refrigerant outlet of the gas-liquid separator  92  to the suction side of the compressor  11 . During the second operation, the check valve  65  prevents the flow of the gas refrigerant between the gas refrigerant outlet of the gas-liquid separator  92  and the suction side of the compressor  11 . An inlet of the check valve  65  communicates with the gas refrigerant outlet of the gas-liquid separator  92 , and an outlet of the check valve  65  is coupled between the four-way valve  21  and the on-off valve  66 . Thus, during the second operation, since the pressure of the refrigerant at the outlet of the check valve  65  is higher than the pressure of the refrigerant at the inlet of the check valve  65 , the refrigerant does not flow through the check valve  65 . 
     The on-off valve  66  is disposed in the sixth flow path F 6 . The on-off valve  66  prevents the flow of the refrigerant between the heat-source-side heat exchanger  31  and the compressor  11  during the first operation. The on-off valve  66  allows the flow of the refrigerant between the heat-source-side heat exchanger  31  and the compressor  11  during the second operation. 
     In the air conditioner  1 , the gas-liquid separator  92  is used to separate the refrigerant in the gas-liquid two-phase state flowing out of the ejector  50 . The air conditioner  1  can allow the gas refrigerant separated by the gas-liquid separator  92  to flow to the refrigerant suction port of the ejector  50  using the fourth flow path F 4  and the fifth flow path F 5 . When the gas refrigerant separated by the gas-liquid separator  92  flows to the refrigerant suction port of the ejector  50 , the air conditioner  1  can perform air conditioning using the ejector  50 . 
     (3) Overall Operation 
     (3-1) Operation of Air Conditioner  1  during First Operation 
     The operation of the air conditioner  1  during the first operation using carbon dioxide as refrigerant will be described with reference to  FIG. 1  and  FIG. 2 . The refrigerant discharged from a discharge port of the compressor  11  (point a) is in a supercritical state. The refrigerant in the supercritical state discharged from the compressor  11  flows into the use-side heat exchanger  32  via the four-way valve  21 . The refrigerant in the supercritical state radiates heat in the use-side heat exchanger  32 . In the use-side heat exchanger  32 , for example, heat exchange is performed between indoor air and the refrigerant, and the warmed air is used to perform indoor heating. 
     The refrigerant at an outflow point (point b) of the use-side heat exchanger  32  is in a high-pressure state, and the specific enthalpy thereof is smaller than that at the point a. The first expansion valve  41  and the flow rate control valve  43  are open and allow the refrigerant to pass therethrough without substantially decompressing the refrigerant. The refrigerant at an outflow point (point c) of the first expansion valve  41  and the refrigerant at an outflow point (point d) of the flow rate control valve  43  are in substantially the same state as the refrigerant at the point b. 
     The refrigerant that has flowed into the refrigerant inflow port of the ejector  50  from the flow rate control valve  43  is decompressed and expanded by a nozzle (not illustrated) in the ejector  50  into low-pressure refrigerant in a gas-liquid two-phase state at the nozzle (point e). At an outlet of the nozzle (point f), the refrigerant that has flowed in from the refrigerant inflow port and the low-pressure gas refrigerant sucked from the refrigerant suction port of the ejector  50  (point l) are mixed into refrigerant having a specific enthalpy between those of the refrigerant at the point e and the refrigerant at the point l. The refrigerant at the refrigerant outflow port of the ejector  50  (point g) is in a state such that the pressure is raised to be higher than that of the refrigerant at the outlet of the nozzle (point f). The refrigerant that has been raised in pressure and flowed out of the refrigerant outflow port of the ejector  50  is separated by the gas-liquid separator  92 . The refrigerant separated by the gas-liquid separator  92  and flowing out of the liquid refrigerant outlet of the gas-liquid separator  92  (point h) is liquid refrigerant with a low specific enthalpy. The refrigerant passing through the check valve  63  and present between the check valve  63  and the second expansion valve  42  (point i) is in substantially the same state as the refrigerant flowing out of the liquid refrigerant outlet of the gas-liquid separator  92  (point h). In the second expansion valve  42 , the refrigerant present between the check valve  63  and the second expansion valve  42  (point i) is decompressed and expanded. The refrigerant decompressed by the second expansion valve  42  and present between the second expansion valve  42  and the heat-source-side heat exchanger  31  (point j) evaporates into gas refrigerant in the heat-source-side heat exchanger  31 . In the heat-source-side heat exchanger  31 , for example, heat exchange is performed between outdoor air and the refrigerant. The gas refrigerant at an outflow point (point k) of the heat-source-side heat exchanger  31  is gas refrigerant with a high specific enthalpy. Since the on-off valve  64  is open, the refrigerant that has flowed out of the heat-source-side heat exchanger  31  passes through the fourth flow path F 4  and is sucked into the ejector  50  from the refrigerant suction port of the ejector  50  (point l). 
     The refrigerant separated by the gas-liquid separator  92  and flowing out of the gas refrigerant outlet of the gas-liquid separator  92  (point m) is gas refrigerant with a high specific enthalpy. The refrigerant flowing out of the gas refrigerant outlet of the gas-liquid separator  92  (point m) is sucked in from a suction port of the compressor  11  (point o) via the check valve  65 , the four-way valve  21 , and the receiver  91 . The state of the refrigerant present between the closed on-off valve  66  and the four-way valve  21  (point n) and the state of the refrigerant present at the suction port of the compressor  11  (point o) are substantially the same as that of the gas refrigerant at the gas refrigerant outlet of the gas-liquid separator  92  (point m). 
     (3-2) Operation of Air Conditioner  1  during Second Operation 
     The operation of the air conditioner  1  during the second operation using carbon dioxide as refrigerant will be described with reference to  FIG. 3 . The refrigerant discharged from the discharge port of the compressor  11  is in a supercritical state. The refrigerant in the supercritical state discharged from the compressor  11  flows into the heat-source-side heat exchanger  31  via the four-way valve  21  and the on-off valve  66 . In this case, the refrigerant does not flow through the fourth flow path F 4  and the fifth flow path F 5  due to the closed on-off valve  64  and the check valve  65 . The refrigerant in the supercritical state radiates heat in the heat-source-side heat exchanger  31 . In the heat-source-side heat exchanger  31  functioning as a radiator, for example, heat exchange is performed between outdoor air and the refrigerant. 
     The refrigerant flowing out of the heat-source-side heat exchanger  31  is in a high-pressure state, and the specific enthalpy thereof is smaller than that before flowing into the heat-source-side heat exchanger  31 . Since the second expansion valve  42  is open, the on-off valve  61  is open, and the flow rate control valve  43  is closed, all of the refrigerant that has flowed out of the heat-source-side heat exchanger  31  flows to the first expansion valve  41 . The refrigerant that flows from the first expansion valve  41  to the use-side heat exchanger  32  is decompressed and expanded by the first expansion valve  41  before flowing into the use-side heat exchanger  32 . The refrigerant in the gas-liquid two-phase state that has flowed into the use-side heat exchanger  32  evaporates into gas refrigerant in the use-side heat exchanger  32 . In the use-side heat exchanger  32  functioning as an evaporator, for example, heat exchange is performed between indoor air and the refrigerant, and the cooled air is used to perform indoor cooling. The gas refrigerant that has flowed out of the use-side heat exchanger  32  is sucked in from the suction port of the compressor  11  via the four-way valve  21  and the receiver  91 . 
     (3-3) Operation of Air Conditioner  1  during Third Operation 
     As illustrated in  FIG. 4 , during a third operation, the refrigerant discharged from the discharge port of the compressor  11  is sucked in from the suction port of the compressor  11  via the four-way valve  21 , the use-side heat exchanger  32 , the first expansion valve  41 , the on-off valve  61 , the second expansion valve  42 , the heat-source-side heat exchanger  31 , the on-off valve  66 , the four-way valve  21 , and the receiver  91 . During the third operation, the flow rate control valve  43  and the on-off valve  64  are closed, and thus the refrigerant does not flow through the ejector  50 . In the third operation, the refrigerant in the supercritical state discharged from the compressor  11  is cooled in the use-side heat exchanger  32  functioning as a radiator. The first expansion valve  41  remains fully opened and does not decompress the refrigerant. The refrigerant cooled in the use-side heat exchanger  32  passes through the first expansion valve  41  and is decompressed and expanded by the second expansion valve  42  to enter a gas-liquid two-phase state. The refrigerant in the gas-liquid two-phase state is warmed in the heat-source-side heat exchanger  31  functioning as an evaporator and becomes gas refrigerant. The gas refrigerant is sucked into the compressor  11  through the receiver  91 . In the third operation, the air conditioner  1  performs indoor heating by, for example, heat exchange between indoor air and refrigerant in the use-side heat exchanger  32 . 
     (3-4) Control of Air Conditioner  1   
     The air conditioner  1  according to the first embodiment includes a controller  80  illustrated in  FIG. 5  to cause the internal devices to perform the operation described above. The controller  80  is implemented by a computer, for example. The computer includes, for example, a control and arithmetic unit and a memory. The control and arithmetic unit can be implemented using a processor. The controller  80  in  FIG. 5  includes a CPU  81  serving as a processor. The control and arithmetic unit reads, for example, a program stored in the memory and performs predetermined image processing, arithmetic processing, or sequence processing in accordance with the program. Further, for example, the control and arithmetic unit can write an arithmetic result to the memory or read information stored in the memory in accordance with the program. The memory can be used as a database. The controller  80  includes a memory  82  serving as a memory. 
     The controller  80  controls the compressor  11 , the first expansion valve  41 , the second expansion valve  42 , the flow rate control valve  43 , the four-way valve  21 , and the on-off valves  61 ,  64 , and  66 . The three valves, namely, the on-off valves  61 ,  64 , and  66 , can be each implemented using, for example, an electromagnetic valve that switches between an open state and a closed state in accordance with a signal from the controller  80 . The first expansion valve  41 , the second expansion valve  42 , and the flow rate control valve  43  can be each implemented using, for example, an electrically powered valve whose opening degree can be changed in response to a pulse signal. 
     (3-5) Selection between First Operation and Third Operation 
     In the air conditioner  1 , the controller  80  selects to perform the first operation using the ejector  50  or the third operation not using the ejector  50  by determining whether the following conditions are satisfied. At the start time, for example, when a first condition, a second condition, and a third condition are satisfied, the first operation using the ejector  50  is performed. The first condition is a condition that a target value (high-pressure target value) of the pressure of the refrigerant discharged from the compressor  11  is within a first predetermined range. The second condition is a condition that a target value (low-pressure target value) of the pressure of the refrigerant sucked into the compressor  11  is within a second predetermined range. The third condition is a condition that the air conditioning capacity (required capacity) required for the compression mechanism  10  is greater than or equal to a predetermined value. For example, the third condition is set such that the cooling capacity required for cooling is greater than or equal to 2 kW, and the third condition is set such that the heating capacity required for heating is greater than or equal to 3 kW. When the pressure difference between the high-pressure target value and the low-pressure target value is small and it is difficult for the ejector  50  to sufficiently recover energy, efficiency deteriorates due to pressure loss in the ejector  50 . When the high-pressure target value is within the first predetermined range and the low-pressure target value is within the second predetermined range, the pressure difference therebetween is a pressure at which it can be expected that the ejector  50  will improve the operation efficiency of the air conditioner  1 . Accordingly, satisfaction of the first condition and the second condition may be replaced with satisfaction of a condition that the pressure difference between the high-pressure target value and the low-pressure target value is greater than or equal to a predetermined value. 
     The air conditioner  1  may be configured to stop the use of the ejector  50 , for example, if the first condition, the second condition, or the third condition is not satisfied when the air conditioner  1  is in operation. The term “in operation” refers to the situation where a predetermined period of time has elapsed since the start of operation. The operation of the air conditioner  1  is stable after the predetermined period of time has elapsed since the start of operation. Further, the air conditioner  1  may be configured to stop the use of the ejector  50  when a sixth condition that the refrigerant accumulates in the gas-liquid separator  92  is satisfied. The controller  80  determines that the sixth condition is satisfied, for example, when the following three phenomena simultaneously occur: a decrease in the pressure of the refrigerant discharged from the compressor  11 , a decrease in the pressure of the refrigerant sucked into the compressor  11 , and an increase in the degree of superheating of the refrigerant sucked into the compressor  11 . The air conditioner  1  may be configured such that when the air conditioner  1  is in operation, the first condition, the second condition, and the third condition use the ejector  50  that is in stop. 
     Second Embodiment 
     (4) Overview of Configuration 
     As illustrated in  FIG. 6 ,  FIG. 7  and  FIG. 8 , an overview of the configuration of an air conditioner  1  according to a second embodiment is the same as the overview of the configuration according to the first embodiment described in (1) described above. Accordingly, a description of the overview of the configuration of the air conditioner  1  according to the second embodiment will be omitted here.  FIG. 6  illustrates the air conditioner  1  in which the first operation is being performed,  FIG. 7  illustrates the air conditioner  1  in which the second operation is being performed, and  FIG. 8  illustrates the air conditioner  1  in which the third operation is being performed. 
     (5) Detailed Configuration 
     (5-1) Overview of Circuit Configuration of Air Conditioner  1   
     As illustrated in  FIG. 6 ,  FIG. 7 , and  FIG. 8 , an overview of the circuit configuration of the air conditioner  1  according to the second embodiment is the same as the overview of the circuit configuration of the air conditioner  1  described in (2-1) described above. Accordingly, a description of the overview of the circuit configuration of the air conditioner  1  according to the second embodiment will be omitted here. 
     ( 5 - 2 ) Details of Circuit Configuration of Air Conditioner  1   
     The air conditioner  1  according to the second embodiment includes, in addition to the configuration described above, the gas-liquid separator  92 , the check valve  63 , which is a third valve, a check valve  67 , which is a seventh valve, and the fifth flow path F 5 . The switching mechanism  20  is the four-way valve  21  having a first port communicating with the discharge side of the compressor  11 , a second port communicating with the heat-source-side heat exchanger  31 , a third port, and a fourth port communicating with the use-side heat exchanger  32 . In the first operation, the first port and the fourth port of the four-way valve  21  communicate with each other, and the second port and the third port of the four-way valve  21  communicate with each other. In the second operation, the first port and the second port of the four-way valve  21  communicate with each other, and the third port and the fourth port of the four-way valve  21  communicate with each other. 
     The gas-liquid separator  92  has a refrigerant inlet communicating with the refrigerant outflow port of the ejector  50 , a liquid refrigerant outlet from which separated liquid refrigerant flows out, and a gas refrigerant outlet from which separated gas refrigerant flows out. In the air conditioner  1 , a portion from the refrigerant outflow port of the ejector  50  to the liquid refrigerant outlet of the gas-liquid separator  92  constitutes part of the third flow path F 3 . The liquid refrigerant outlet of the gas-liquid separator  92  communicates with the inlet of the check valve  63 . 
     The check valve  63  is disposed in the third flow path F 3 . As illustrated in  FIG. 6 , the check valve  63  allows the liquid refrigerant to flow from the liquid refrigerant outlet of the gas-liquid separator  92  to the heat-source-side heat exchanger  31  during the first operation. Since the on-off valve  61  is closed, the refrigerant that has flowed out of the outlet of the check valve  63  does not flow to the use-side heat exchanger  32 , but flows to the heat-source-side heat exchanger  31  via the second expansion valve  42 . As illustrated in  FIG. 7 , the check valve  63  prevents the liquid refrigerant from flowing between the liquid refrigerant outlet of the gas-liquid separator  92  and the heat-source-side heat exchanger  31  during the second operation. During the second operation, since the pressure of the refrigerant at the outlet of the check valve  63  is higher than the pressure of the refrigerant at the inlet of the check valve  63 , the refrigerant does not flow through the check valve  63 . 
     The fifth flow path F 5  is a flow path through which the gas refrigerant flows from the gas refrigerant outlet of the gas-liquid separator  92  to the suction side of the compressor  11 . 
     The check valve  67 , which is a seventh valve, prevents refrigerant from flowing during the first operation and allows refrigerant to flow during the second operation. The check valve  67  has a first end communicating with the third port of the four-way valve  21  and a second end communicating with the suction side of the compressor  11  through the receiver  91 . The refrigerant suction port of the ejector  50  is coupled between the first end of the check valve  67  and the third port of the four-way valve  21 . The gas refrigerant outlet of the gas-liquid separator  92  is coupled between the second end of the check valve  67  and the suction side of the compressor  11 . More specifically, the gas refrigerant outlet of the gas-liquid separator  92  is coupled between the second end of the check valve  67  and an inflow port of the receiver  91 . 
     In the air conditioner  1 , the gas-liquid separator  92  is used to separate the refrigerant in the gas-liquid two-phase state flowing out of the ejector  50 . During the first operation, the air conditioner  1  can allow the gas refrigerant separated by the gas-liquid separator  92  to flow to the refrigerant suction port of the ejector  50  using the fourth flow path F 4  and the fifth flow path F 5 . When the liquid refrigerant separated by the gas-liquid separator  92  flows to the refrigerant suction port of the ejector  50 , the air conditioner  1  can perform air conditioning using the ejector  50 . Further, the air conditioner  1  can perform air conditioning not using the ejector  50  without causing the gas refrigerant separated by the gas-liquid separator  92  to flow. 
     (6) Overall Operation 
     (6-1) Operation of Air Conditioner  1  during First Operation 
     The operation of the air conditioner  1  according to the second embodiment illustrated in  FIG. 6  during the first operation is different from the operation of the air conditioner  1  according to the first embodiment during the first operation in the operation thereof downstream of the gas refrigerant outlet of the gas-liquid separator  92  (point m) and the operation thereof downstream of the heat-source-side heat exchanger  31 . Accordingly, the operations of the air conditioner  1  according to the second embodiment during the first operation on downstream of the gas refrigerant outlet of the gas-liquid separator  92  (point m) and on downstream of the heat-source-side heat exchanger  31  will be described. A Mollier diagram illustrated in  FIG. 2  is also applicable to the air conditioner  1  according to the second embodiment except the different operations. 
     The refrigerant flowing out of the gas refrigerant outlet of the gas-liquid separator  92  (point m) is sucked in from the suction port of the compressor  11  (point o) via the receiver  91 . The state of the refrigerant present between the check valve  67  and the receiver  91  (point n) and the state of the refrigerant present at the suction port of the compressor  11  (point o) are substantially the same as that of the gas refrigerant at the gas refrigerant outlet of the gas-liquid separator  92  (point m). 
     The refrigerant at the outflow point of the heat-source-side heat exchanger  31  (point k) is gas refrigerant with a high specific enthalpy. The refrigerant that has flowed out of the heat-source-side heat exchanger  31  passes through the four-way valve  21  and the fourth flow path F 4  and is sucked into the ejector  50  from the refrigerant suction port of the ejector  50  (point l). At this time, since the pressure at the inlet of the check valve  67  (point n) is lower than the pressure at the outlet (point k), the check valve  67  does not allow the refrigerant to flow therethrough. 
     (6-2) Operation of Air Conditioner  1  during Second Operation 
     The air conditioner  1  according to the second embodiment illustrated in  FIG. 7  performs the same refrigeration cycle as the vapor compression refrigeration cycle of the air conditioner  1  according to the first embodiment described in ( 3 - 2 ), with refrigerant circulating through the compressor  11 , the heat-source-side heat exchanger  31  functioning as a radiator, the first expansion valve  41 , and the use-side heat exchanger  32  functioning as an evaporator. The operation of the air conditioner  1  according to the second embodiment during the second operation is different from the operation of the air conditioner  1  according to the first embodiment during the second operation in the operation thereof on the downstream side of the four-way valve  21 . 
     In the air conditioner  1  according to the first embodiment illustrated in  FIG. 3 , the refrigerant that has flowed out of the use-side heat exchanger  32  flows into the receiver  91  through the four-way valve  21 . In the air conditioner  1  according to the second embodiment illustrated in  FIG. 7 , in contrast, the refrigerant that has flowed out of the use-side heat exchanger  32  flows into the receiver  91  through the four-way valve  21  and the check valve  67 . The fourth flow path F 4  communicates between the check valve  67  and the four-way valve  21 . The fifth flow path F 5  communicates between the check valve  67  and the receiver  91 . However, the flow rate control valve  43  is fully closed. Further, the pressure in the first flow path F 1  at the outlet of the check valve  63  remains higher than the pressure of the refrigerant in the gas-liquid separator  92 , and the check valve  63  prevents the refrigerant from flowing through the third flow path F 3 . Accordingly, the ejector  50  is not in a state of sucking the refrigerant from the refrigerant suction port, and thus the refrigerant does not flow from between the check valve  67  and the four-way valve  21  toward the refrigerant suction port of the ejector  50 . In addition, the low-pressure refrigerant between the receiver  91  and the check valve  67  does not flow toward the gas-liquid separator  92  through the fifth flow path F 5 . 
     (6-3) Operation of Air Conditioner  1  during Third Operation 
     In the air conditioner  1  according to the second embodiment illustrated in  FIG. 8 , during the third operation, the refrigerant discharged from the discharge port of the compressor  11  is sucked in from the suction port of the compressor  11  via the four-way valve  21 , the use-side heat exchanger  32 , the first expansion valve  41 , the on-off valve  61 , the second expansion valve  42 , the heat-source-side heat exchanger  31 , the four-way valve  21 , the check valve  67 , and the receiver  91 . During the third operation, the flow rate control valve  43  is closed, and thus the refrigerant does not flow through the ejector  50 . The air conditioner  1  according to the second embodiment performs the same refrigeration cycle as the vapor compression refrigeration cycle of the air conditioner  1  described in (3-3), with refrigerant circulating through the compressor  11 , the use-side heat exchanger  32  functioning as a radiator, the second expansion valve  42 , and the heat-source-side heat exchanger  31  functioning as an evaporator. In the third operation, the air conditioner  1  performs indoor heating by, for example, heat exchange between indoor air and refrigerant in the use-side heat exchanger  32 . 
     (6-4) Control of Air Conditioner  1   
     The air conditioner  1  according to the second embodiment includes a controller  80  illustrated in  FIG. 9  to cause the internal devices to perform the operation described above. The controller  80  controls the compressor  11 , the first expansion valve  41 , the second expansion valve  42 , the flow rate control valve  43 , the four-way valve  21 , and the on-off valve  61 . 
     (6-5) Selection between First Operation and Third Operation 
     In the air conditioner  1  according to the second embodiment, the controller  80  selects to perform the first operation using the ejector  50  or the third operation not using the ejector  50 . The selection between the first operation and the third operation of the air conditioner  1  according to the second embodiment can be performed in a way similar to the selection between the first operation and the third operation of the air conditioner  1  according to the first embodiment described in ( 3 - 5 ). Thus, a detailed description of the selection between the first operation and the third operation of the air conditioner  1  according to the second embodiment will be omitted here. 
     Third Embodiment 
     (7) Overview of Configuration 
     As illustrated in  FIG. 10 ,  FIG. 12  and  FIG. 13 , an overview of the configuration of an air conditioner  1  according to a third embodiment is the same as the overview of the configuration according to the first embodiment described in (1) described above. Accordingly, a description of the overview of the configuration of the air conditioner  1  according to the third embodiment will be omitted here.  FIG. 10  illustrates the air conditioner  1  in which the first operation is being performed,  FIG. 12  illustrates the air conditioner  1  in which the second operation is being performed, and  FIG. 13  illustrates the air conditioner  1  in which the third operation is being performed. 
     (8) Detailed Configuration 
     (8-1) Overview of Circuit Configuration of Air Conditioner  1   
     As illustrated in  FIG. 10 ,  FIG. 12  and  FIG. 13 , an overview of the circuit configuration of the air conditioner  1  according to the third embodiment is the same as the overview of the circuit configuration of the air conditioner  1  described in (2-1) described above. Accordingly, a description of the overview of the circuit configuration of the air conditioner  1  according to the third embodiment will be omitted here. 
     (8-2) Details of Circuit Configuration of Air Conditioner  1   
     The air conditioner  1  according to the third embodiment includes, in addition to the configuration described above, an accumulator  93 , the check valve  63 , which is a third valve, and a check valve  68 , which is an eighth valve. The switching mechanism  20  is the four-way valve  21  having a first port communicating with the discharge side of the compressor  11 , a second port communicating with the heat-source-side heat exchanger  31 , a third port, and a fourth port communicating with the use-side heat exchanger  32 . In the first operation, the first port and the fourth port of the four-way valve  21  communicate with each other, and the second port and the third port of the four-way valve  21  communicate with each other. In the second operation, the first port and the second port of the four-way valve  21  communicate with each other, and the third port and the fourth port of the four-way valve  21  communicate with each other. 
     The accumulator  93  has a refrigerant inlet communicating with the refrigerant outflow port of the ejector  50 , a liquid refrigerant outlet from which separated liquid refrigerant flows out, and a gas refrigerant outlet from which separated gas refrigerant flows out. In the air conditioner  1 , a portion from the refrigerant outflow port of the ejector  50  to the liquid refrigerant outlet of the accumulator  93  constitutes part of the third flow path F 3 . The liquid refrigerant outlet of the accumulator  93  communicates with the inlet of the check valve  63 . 
     The check valve  63  is disposed in the third flow path F 3 . As illustrated in  FIG. 10 , the check valve  63  allows the liquid refrigerant to flow from the liquid refrigerant outlet of the accumulator  93  to the heat-source-side heat exchanger  31  during the first operation. Since the on-off valve  61  is closed, the refrigerant that has flowed out of the outlet of the check valve  63  does not flow to the use-side heat exchanger  32 , but flows to the heat-source-side heat exchanger  31  via the second expansion valve  42 . As illustrated in  FIG. 12 , the check valve  63  prevents the liquid refrigerant from flowing between the liquid refrigerant outlet of the accumulator  93  and the heat-source-side heat exchanger  31  during the second operation. During the second operation, since the pressure of the refrigerant at the outlet of the check valve  63  is higher than the pressure of the refrigerant at the inlet of the check valve  63  (the first flow path F 1 ), the refrigerant does not flow through the check valve  63 . 
     The check valve  68 , which is an eighth valve, prevents refrigerant from flowing during the first operation and allows refrigerant to flow during the second operation. The check valve  68  has a first end communicating with the third port of the four-way valve  21  and a second end communicating with the suction side of the compressor  11  through the accumulator  93 . The refrigerant suction port of the ejector  50  is coupled between the first end of the check valve  68  and the third port of the four-way valve  21 . The gas refrigerant outlet of the accumulator  93  communicates with the suction port of the compressor  11 . 
     In the air conditioner  1  according to the third embodiment, the accumulator  93  is used to separate the refrigerant in the gas-liquid two-phase state flowing out of the ejector  50 . When the liquid refrigerant separated by the accumulator  93  flows to the heat-source-side heat exchanger  31  and the gas refrigerant evaporated in the heat-source-side heat exchanger  31  flows to the refrigerant suction port of the ejector  50 , the air conditioner  1  can perform air conditioning using the ejector  50 . 
     (9) Overall Operation 
     (9-1) Operation of Air Conditioner  1  during First Operation 
     The operation of the air conditioner  1  during the first operation using carbon dioxide as refrigerant will be described with reference to  FIG. 10  and  FIG. 11 . The refrigerant discharged from the discharge port of the compressor  11  (point a) is in a supercritical state. The refrigerant in the supercritical state discharged from the compressor  11  flows into the use-side heat exchanger  32  via the four-way valve  21 . The refrigerant in the supercritical state radiates heat in the use-side heat exchanger  32 . In the use-side heat exchanger  32 , for example, heat exchange is performed between indoor air and the refrigerant, and the warmed air is used to perform indoor heating. 
     The refrigerant at the outflow point (point b) of the use-side heat exchanger  32  is in a high-pressure state, and the specific enthalpy thereof is smaller than that at the point a. The first expansion valve  41  and the flow rate control valve  43  are open and allow the refrigerant to pass therethrough without substantially decompressing the refrigerant. The refrigerant at the outflow point (point c) of the first expansion valve  41  and the refrigerant at the outflow point (point d) of the flow rate control valve  43  are in substantially the same state as the refrigerant at the point b. 
     The refrigerant that has flowed into the refrigerant inflow port of the ejector  50  from the flow rate control valve  43  is decompressed and expanded by a nozzle (not illustrated) in the ejector  50  into low-pressure refrigerant in a gas-liquid two-phase state at the nozzle (point e). At the outlet of the nozzle (point f), the refrigerant that has flowed in from the refrigerant inflow port and the low-pressure gas refrigerant sucked from the refrigerant suction port of the ejector  50  (point m) are mixed into refrigerant having a specific enthalpy between those of the refrigerant at the point e and the refrigerant at the point m. The refrigerant at the refrigerant outflow port of the ejector  50  (point g) is in a state such that the pressure is raised to be higher than that of the refrigerant at the outlet of the nozzle (point f). The refrigerant that has been raised in pressure and flowed out of the refrigerant outflow port of the ejector  50  is separated by the accumulator  93 . As illustrated in  FIG. 11 , the state of the refrigerant at the refrigerant outflow port of the ejector  50  (point g) is the same as the state of the refrigerant at the inflow port of the accumulator  93  (point h). The refrigerant separated by the accumulator  93  and flowing out of the liquid refrigerant outlet of the accumulator  93  (point i) is liquid refrigerant with a low specific enthalpy. The refrigerant passing through the check valve  63  and present between the check valve  63  and the second expansion valve  42  (point j) is in substantially the same state as the refrigerant flowing out of the liquid refrigerant outlet of the accumulator  93  (point i). In the second expansion valve  42 , the refrigerant present between the check valve  63  and the second expansion valve  42  (point j) is decompressed and expanded. The refrigerant decompressed by the second expansion valve  42  and present between the second expansion valve  42  and the heat-source-side heat exchanger  31  (point k) evaporates into gas refrigerant in the heat-source-side heat exchanger  31 . In the heat-source-side heat exchanger  31 , for example, heat exchange is performed between outdoor air and the refrigerant. The gas refrigerant at the outflow point of the heat-source-side heat exchanger  31  (point  1 ) is gas refrigerant with a high specific enthalpy. Since the on-off valve  61  is open, the refrigerant that has flowed out of the heat-source-side heat exchanger  31  passes through the fourth flow path F 4  and is sucked into the ejector  50  from the refrigerant suction port of the ejector  50  (point m). 
     The refrigerant separated by the accumulator  93  and flowing out of the gas refrigerant outlet of the accumulator  93  (point n) is gas refrigerant with a high specific enthalpy. The refrigerant flowing out of the gas refrigerant outlet of the accumulator  93  (point n) is sucked in from the suction port of the compressor  11 . 
     (9-2) Operation of Air Conditioner  1  during Second Operation 
     The air conditioner  1  according to the third embodiment illustrated in  FIG. 12  performs the same refrigeration cycle as the vapor compression refrigeration cycle of the air conditioner  1  according to the first embodiment described in (3-2), with refrigerant circulating through the compressor  11 , the heat-source-side heat exchanger  31  functioning as a radiator, the first expansion valve  41 , and the use-side heat exchanger  32  functioning as an evaporator. The operation of the air conditioner  1  according to the third embodiment during the second operation is different from the operation of the air conditioner  1  according to the first embodiment during the second operation in the operation thereof on the downstream side of the four-way valve  21 . 
     In the air conditioner  1  according to the first embodiment illustrated in  FIG. 3 , the refrigerant that has flowed out of the use-side heat exchanger  32  flows into the receiver  91  through the four-way valve  21 . In the air conditioner  1  according to the third embodiment illustrated in  FIG. 12 , in contrast, the refrigerant that has flowed out of the use-side heat exchanger  32  flows into the accumulator  93  through the four-way valve  21  and the check valve  68 . The fourth flow path F 4  communicates between the check valve  68  and the four-way valve  21 . The refrigerant outflow port of the ejector  50  communicates between the check valve  68  and the accumulator  93 . However, the flow rate control valve  43  is fully closed. Further, the pressure in the first flow path F 1  at the outlet of the check valve  63  remains higher than the pressure of the refrigerant in the accumulator  93 , and the check valve  63  prevents the refrigerant from flowing through the third flow path F 3 . Accordingly, the ejector  50  is not in a state of sucking the refrigerant from the refrigerant suction port, and thus the refrigerant does not flow from between the check valve  68  and the four-way valve  21  toward the refrigerant suction port and the refrigerant outflow port of the ejector  50 . 
     (9-3) Operation of Air Conditioner  1  during Third Operation 
     In the air conditioner  1  according to the third embodiment illustrated in  FIG. 13 , during the third operation, the refrigerant discharged from the discharge port of the compressor  11  is sucked in from the suction port of the compressor  11  via the four-way valve  21 , the use-side heat exchanger  32 , the first expansion valve  41 , the on-off valve  61 , the second expansion valve  42 , the heat-source-side heat exchanger  31 , the four-way valve  21 , the check valve  68 , and the accumulator  93 . During the third operation, the flow rate control valve  43  is closed, and thus the refrigerant does not flow through the ejector  50 . The air conditioner  1  according to the third embodiment performs the same refrigeration cycle as the vapor compression refrigeration cycle of the air conditioner  1  described in ( 3 - 3 ), with refrigerant circulating through the compressor  11 , the use-side heat exchanger  32  functioning as a radiator, the second expansion valve  42 , and the heat-source-side heat exchanger  31  functioning as an evaporator. In the third operation, the air conditioner  1  performs indoor heating by, for example, heat exchange between indoor air and refrigerant in the use-side heat exchanger  32 . 
     (9-4) Control of Air Conditioner  1   
     The air conditioner  1  according to the third embodiment includes the controller  80  illustrated in  FIG. 9  to cause the internal devices to perform the operation described above. The controller  80  controls the compressor  11 , the second expansion valve  42 , the flow rate control valve  43 , the first expansion valve  41 , the four-way valve  21 , and the on-off valve  61 . 
     (9-5) Selection between First Operation and Third Operation 
     In the air conditioner  1  according to the third embodiment, the controller  80  selects to perform the first operation using the ejector  50  or the third operation not using the ejector  50 . The selection between the first operation and the third operation of the air conditioner  1  according to the third embodiment can be performed in a way similar to the selection between the first operation and the third operation of the air conditioner  1  according to the first embodiment described in ( 3 - 5 ). Thus, a detailed description of the selection between the first operation and the third operation of the air conditioner  1  according to the third embodiment will be omitted here. 
     (10) Modifications 
     (10-1) Modification A 
     The air conditioner  1  according to the first embodiment, the second embodiment, and the third embodiment has been described in which the compression mechanism  10  is constituted by one compressor  11 . However, the compression mechanism  10  is not limited to one constituted by one compressor  11  as in the air conditioner  1  according to the first embodiment, the second embodiment, and the third embodiment. For example, in the air conditioner  1  according to the third embodiment, as illustrated in  FIG. 14 , the compression mechanism  10  may be constituted by two compressors  12  and  13 . In the compression mechanism  10  illustrated in  FIG. 14 , a discharge port of the compressor  12  communicates with a suction port of the compressor  13 . In other words, the compression mechanism  10  is configured to perform two-stage compression. The compression mechanism  10  may also be configured to perform multi-stage compression in which three or more compressors communicate with each other. When the compression mechanism  10  is configured to perform two-stage compression, for example, one compressor may include a first compression element for low-pressure compression, and a second compression element for high-pressure compression. When the compression mechanism  10  is constituted by a plurality of compressors, the compressors may be coupled in parallel. 
     (10-2) Modification B 
     The air conditioner  1  including a compression mechanism configured to perform multi-stage compression described in modification A may be provided with an economizer circuit  70  illustrated in  FIG. 14 . The economizer circuit  70  includes an economizer heat exchanger  33 , an injection pipe  71 , and an injection valve  72 . The injection pipe  71  branches the refrigerant delivered from the radiator to the expansion valve and returns the branched refrigerant to the suction port of the compressor  13  in the subsequent stage (downstream). The economizer heat exchanger  33  performs heat exchange between the refrigerant delivered from the radiator to the expansion valve and intermediate-pressure refrigerant in the refrigeration cycle flowing through the injection pipe  71 . The injection valve  72  is an expansion valve and decompresses and expands the refrigerant in the injection pipe  71  before the refrigerant enters the economizer heat exchanger  33  along the injection pipe  71 . The refrigerant that has passed through the injection valve  72  is intermediate-pressure refrigerant. In the air conditioner  1 , since intermediate-pressure injection using the economizer heat exchanger  33  and the injection pipe  71  is adopted, the temperature of the refrigerant to be sucked into the compressor  13  in the subsequent stage (downstream) can be kept low with no heat radiate to the outside, and the refrigerant to be delivered to the evaporator can be cooled. For example, in the second operation, the heat-source-side heat exchanger  31  functions as a radiator, the use-side heat exchanger  32  functions as an evaporator, and the first expansion valve  41  functions as the expansion valve described above. 
     (10-3) Modification C 
     The air conditioner  1  including the compression mechanism  10  configured to perform multi-stage compression described in modification A may be provided with an intercooler  34  illustrated in  FIG. 15 . In the first operation, the intercooler  34  functions as an evaporator. In the second operation, the intercooler  34  performs heat exchange to cool the refrigerant discharged from the compressor  12 , which is a first compression element, and causes the cooled refrigerant to be sucked into the compressor  13 , which is a second compression element. The refrigerant to be sucked into the compressor  13  is cooled to decrease the temperature of the refrigerant to be discharged from the compressor  13 , and, as a result, the reliability of the compressor  13  and the efficiency of the refrigeration cycle can be increased. 
     In  FIG. 15 , the compressors  12  and  13  constitute the compression mechanism  10 , and four-way valves  22  and  23  constitute the switching mechanism  20 . The air conditioner  1  in  FIG. 15  includes, in addition to the compression mechanism  10  and the switching mechanism  20 , check valves  73  and  74  and a fourth expansion valve  44 , with which the intercooler  34  communicates. The discharge port of the compressor  12  communicates with a first port of the four-way valve  22 . A second port of the four-way valve  22  communicates with a first inlet/outlet of the intercooler  34 . A second inlet/outlet of the intercooler  34  communicates with an inlet of the check valve  74  and a first end of the fourth expansion valve  44 . A second end of the fourth expansion valve  44  is coupled between the on-off valve  61  and the second expansion valve  42 . An outlet of the check valve  74  communicates with the suction port of the compressor  13 . A third port of the four-way valve  22  communicates with the fourth flow path F 4 . A fourth port of the four-way valve  22  communicates with an inlet of the check valve  73 . An outlet of the check valve  73  communicates with the suction port of the compressor  13 . A discharge port of the compressor  13  communicates with a first port of the four-way valve  23 . A second port of the four-way valve  23  communicates with a first inlet/outlet of the heat-source-side heat exchanger  31 . A second inlet/outlet of the heat-source-side heat exchanger  31  communicates with the second expansion valve  42 . A third port of the four-way valve  23  communicates with the fourth flow path F 4 . A fourth port of the four-way valve  23  communicates with a first inlet/outlet of the use-side heat exchanger  32 . A second inlet/outlet of the use-side heat exchanger  32  communicates with the first expansion valve  41 . 
     The circuit configuration of portions corresponding to the first expansion valve  41 , the on-off valve  61 , the second expansion valve  42 , the flow rate control valve  43 , the ejector  50 , the check valve  68 , the accumulator  93 , and the economizer circuit  70  of the air conditioner  1  illustrated in  FIG. 15  is the same as the circuit configuration of the air conditioner  1  illustrated in  FIG. 14 , and a description thereof will thus be omitted. 
     In the first operation and the third operation, as illustrated in  FIG. 15 , the first port and the fourth port of each of the four-way valves  22  and  23  communicate with each other, and the second port and the third port of each of the four-way valves  22  and  23  communicate with each other. In the second operation, the first port and the second port of each of the four-way valves  22  and  23  communicate with each other, and the third port and the fourth port of each of the four-way valves  22  and  23  communicate with each other. 
     In the first operation and the third operation, the refrigerant discharged from the compressor  12  flows from the four-way valve  22 , the check valve  73 , the compressor  13 , the four-way valve  23 , and the use-side heat exchanger  32  to the first flow path F 1 . In the first operation and the third operation, the difference between the air conditioner  1  in  FIG. 14  and the air conditioner  1  in  FIG. 15  is the travel path of the refrigerant downstream of the third flow path F 3 . In the air conditioner  1  in  FIG. 15 , the refrigerant is divided into refrigerant flowing from the third flow path F 3  to the fourth flow path F 4  via the second expansion valve  42  and the heat-source-side heat exchanger  31  and refrigerant flowing from the third flow path F 3  to the fourth flow path F 4  via the fourth expansion valve  44  and the intercooler  34 . At this time, the refrigerants are decompressed and expanded by the second expansion valve  42  and the fourth expansion valve  44 , and the intercooler  34  functions as an evaporator, like the heat-source-side heat exchanger  31 . 
     In the second operation, the difference between the air conditioner  1  in  FIG. 15  and the air conditioner  1  in  FIG. 14  is the presence or absence of refrigerant flowing through the intercooler  34 . In the air conditioner  1  in  FIG. 15 , the refrigerant discharged from the compressor  12  in the preceding stage flows into the suction port of the compressor  13  in the subsequent stage via the intercooler  34 . The intercooler  34  cools the refrigerant discharged from the compressor  12  in the preceding stage and to be sucked into the compressor  13  in the subsequent stage. 
     (10-4) Modification D 
     The air conditioner  1  described above including one use-side heat exchanger  32  has been described. However, the air conditioner  1  may include a plurality of use-side heat exchangers. When the air conditioner  1  according to the first embodiment includes two use-side heat exchangers  32 , for example, as illustrated in  FIG. 16 , two units each including the use-side heat exchanger  32  and the first expansion valve  41  may be coupled in parallel. 
     (10-5) Modification E 
     While the air conditioner  1  described above including the first expansion valve  41  and the second expansion valve  42  has been described, the first expansion valve  41  and the second expansion valve  42  may be combined into a single expansion valve. For example, the first expansion valve  41  may be omitted, and the second expansion valve  42  may perform decompression and expansion in the second operation. The second expansion valve  42  having the configuration described above serves as a first expansion valve. 
     (10-6) Modification F 
     The air conditioner  1  described above including the check valves  63 ,  65 ,  67 ,  68 ,  73 , and  74  has been described. However, the check valves  63 ,  65 ,  67 ,  68 ,  73 , and  74  may be each replaced with an on-off valve. Further, the air conditioner  1  described above including the flow rate control valve  43  has been described above. However, the flow rate control valve  43  may be replaced with an on-off valve. Alternatively, the flow rate control valve  43  may be replaced with an expansion valve configured to perform decompression and expansion such that refrigerant having an intermediate pressure between a high pressure and a low pressure flows to the refrigerant inflow port of the ejector  50 . 
     (10-7) Modification G 
     The air conditioner  1  described above in which carbon dioxide is used as refrigerant has been described. The refrigerant used in the air conditioner  1  described above is preferably carbon dioxide or a refrigerant mixture containing carbon dioxide in which the refrigerant to be discharged from the compression mechanism  10  has a high pressure. However, the air conditioner  1  described above may use refrigerant other than carbon dioxide or a refrigerant mixture containing carbon dioxide. For example, refrigerant whose saturation pressure is greater than or equal to 4.5 MPa when reaching a saturation temperature of 65° C. may be used. Examples of such refrigerant include R410A refrigerant. Alternatively, chlorofluorocarbon-based refrigerant that reaches a critical state when discharged from the compression mechanism  10  may be used. Examples of such chlorofluorocarbon-based refrigerant include R 23  refrigerant. 
     (11) Features 
     (11-1) 
     In the first operation, the air conditioner  1  described above can perform heating using heat radiated from the refrigerant in the use-side heat exchanger  32 . In the second operation, the air conditioner  1  can perform cooling by heat absorption due to the evaporation of the refrigerant in the use-side heat exchanger  32 . In the air conditioner  1 , the switching mechanism  20  switches between the first operation using the ejector  50  and the second operation without the ejector  50 , thereby providing efficient operation of the air conditioner  1 . The air conditioner  1  described above includes the first expansion valve  41  as an expansion mechanism. 
     (11-2) 
     In the air conditioner  1  described above, with a simple configuration of the on-off valve  61 , which is a first valve, and the flow rate control valve  43 , which is a second valve, in addition to the configuration including the first flow path F 1 , the second flow path F 2 , the third flow path F 3 , and the fourth flow path F 4 , the ejector  50  can be bypassed during the second operation. As a result, the air conditioner  1  capable of switching between the first operation using the ejector  50  and the second operation without the ejector  50  can be constructed at low cost. 
     (11-3) 
     In the air conditioner  1  according to the first embodiment, in the first operation, the gas-liquid separator  92  is used to separate the refrigerant in the gas-liquid two-phase state flowing out of the ejector  50 . In the air conditioner  1 , due to the on-off valve  64 , which is a fourth valve, the check valve  65 , which is a fifth valve, and the on-off valve  66 , which is a sixth valve, the refrigerant does not flow through the fourth flow path F 4  and the fifth flow path F 5  and the refrigerant flows through the sixth flow path F 6  in the second operation, and the refrigerant flows through the fourth flow path F 4  and the fifth flow path F 5  and the refrigerant does not flow through the sixth flow path F 6  in the first operation. Thus, in the first operation, the air conditioner  1  can allow the gas refrigerant separated by the gas-liquid separator  92  to flow to the refrigerant suction port of the ejector  50  along the fourth flow path F 4  and the fifth flow path F 5 . In the air conditioner  1 , due to the check valve  63 , which is a third valve, the refrigerant does not flow through the third flow path F 3  in the second operation, and the refrigerant flows through the third flow path F 3  in the first operation. Thus, in the first operation, the air conditioner  1  can allow the liquid refrigerant separated by the gas-liquid separator  92  to flow to the heat-source-side heat exchanger  31 , which is a first heat-source-side heat exchanger, along the third flow path F 3 . As a result, in the air conditioner  1 , the ejector  50  can efficiently be operated in the first operation. 
     (11-4) 
     In the air conditioner  1  according to the second embodiment, in the first operation, the gas-liquid separator  92  is used to separate the refrigerant in the gas-liquid two-phase state flowing out of the ejector  50 . Due to the check valve  67 , which is a seventh valve, and the four-way valve  21 , the air conditioner  1  can prevent the refrigerant from flowing through the fourth flow path F 4  and the fifth flow path F 5  in the second operation and allow the refrigerant to flow through the fourth flow path F 4  and the fifth flow path F 5  in the first operation. Thus, in the first operation, the air conditioner  1  can allow the separated gas refrigerant to flow to the refrigerant suction port of the ejector  50  along the fourth flow path F 4  and the fifth flow path F 5 . In the air conditioner  1 , due to the check valve  63 , which is a third valve, the refrigerant does not flow through the third flow path F 3  in the second operation, and the refrigerant flow through the third flow path F 3  in the first operation. In the first operation, the air conditioner  1  can allow the separated liquid refrigerant to flow to the heat-source-side heat exchanger  31  along the third flow path F 3 . As a result, in the air conditioner  1 , the ejector  50  can efficiently be operated in the first operation. 
     (11-5) 
     In the air conditioner  1  according to the third embodiment, in the first operation, the accumulator  93  is used to separate the refrigerant in the gas-liquid two-phase state flowing out of the ejector  50 . Due to the check valve  68 , which is an eighth valve, and the four-way valve  21 , the air conditioner  1  can prevent the refrigerant from flowing through the fourth flow path F 4  in the second operation and allow the refrigerant to flow through the fourth flow path F 4  in the first operation. Thus, in the first operation, the air conditioner  1  can allow the separated gas refrigerant to flow to the refrigerant suction port of the ejector  50  along the fourth flow path F 4 . In the air conditioner  1 , due to the check valve  63 , which is a third valve, the refrigerant does not flow through the third flow path F 3  in the second operation, and the refrigerant flows through the third flow path F 3  in the first operation. In the first operation, the air conditioner  1  can allow the separated liquid refrigerant to flow to the heat-source-side heat exchanger  31  along the third flow path F 3 . As a result, in the air conditioner  1 , the ejector  50  can efficiently be operated in the first operation. 
     (11-6) 
     As described in modification A with reference to  FIG. 14 , the compression mechanism  10  is configured such that, for example, the compressor  12 , which is a first compression element, and the compressor  13 , which is a second compression element, perform multi-stage compression. The pressure of the refrigerant is raised to a high pressure by such multi-stage compression of the compression mechanism  10 , which can bring the ejector  50  into efficient operation. 
     (11-7) 
     As described in modification B with reference to  FIG. 14 , the air conditioner  1  including the economizer circuit  70  can increase the efficiency of cooling operation. 
     (11-8) As described in modification C with reference to  FIG. 15 , the intercooler  34  cools the refrigerant to be sucked into the compressor  13 , which is a second compression element. As a result, the reliability of the compressor  13  and the efficiency of the refrigeration cycle can be improved. 
     (11-9) 
     The air conditioner  1  described above includes the second expansion valve  42  that decompresses and expands refrigerant to be caused to flow into the heat-source-side heat exchanger  31 . The switching mechanism  20  is configured to switch to the refrigerant flow in the third operation. Specifically, the switching mechanism  20  switches, for the third operation, to a refrigerant flow similar to that in the first operation. As described with reference to  FIG. 4 ,  FIG. 8 , and  FIG. 13 , the air conditioner  1  is configured such that, in the third operation, the refrigerant compressed by the compression mechanism  10  radiates heat in the use-side heat exchanger  32  and is decompressed and expanded by the second expansion valve  42  before being evaporated in the heat-source-side heat exchanger  31  without passing through the ejector  50 . The air conditioner  1  having the configuration described above switches the operation to the third operation if efficiency is low in the first operation, which makes it possible to suppress a decrease in efficiency. 
     (11-10) 
     The switching mechanism  20  may be configured to switch to the refrigerant flow in the first operation when a condition that a high-pressure target value of the refrigerant to be discharged from the compression mechanism  10  and a low-pressure target value of the refrigerant to be sucked into the compression mechanism  10  are within a predetermined range and that the capacity required for the compression mechanism  10  is greater than or equal to a predetermined value is satisfied, and switch to the refrigerant flow in the third operation when the condition is not satisfied. In the configuration described above, it is possible to appropriately switch between the first operation and the third operation, based on the pressure of the refrigerant and the required capacity. 
     Fourth Embodiment 
     (12) Overview of Configuration 
     As illustrated in  FIG. 17  and  FIG. 19 , an air conditioner  1  according to a fourth embodiment includes a compression mechanism  110 , a first heat-source-side heat exchanger  131 , a second heat-source-side heat exchanger  132 , a use-side heat exchanger  133 , an ejector  150  that raises the pressure of refrigerant by using energy for refrigerant decompression and expansion, an expansion mechanism  140 , and a switching mechanism  120 . The switching mechanism  120  switches between the refrigerant flow in a first operation illustrated in  FIG. 17  and the refrigerant flow in a second operation illustrated in  FIG. 19 . The expansion mechanism  140  includes a first expansion valve  141  and a second expansion valve  142 . 
     As illustrated in  FIG. 17 , in the first operation of the air conditioner  1 , the refrigerant compressed by the compression mechanism  110  radiates heat in the use-side heat exchanger  133 . In the air conditioner  1 , in the first operation, a portion of the refrigerant that has radiated heat in the use-side heat exchanger  133  is decompressed and expanded by the ejector  150 , and the rest of the refrigerant that has radiated heat in the use-side heat exchanger  133  is decompressed and expanded by the first expansion valve  141  (the expansion mechanism  140 ). In the air conditioner  1 , the refrigerant heated by the first heat-source-side heat exchanger  131  after decompressed and expanded by the first expansion valve  141  is raised in pressure by the ejector  150 . In the air conditioner  1 , further, the gas-liquid two-phase refrigerant raised in pressure by the ejector  150  is evaporated in the second heat-source-side heat exchanger  132 . 
     As illustrated in  FIG. 19 , in the second operation of the air conditioner  1 , the refrigerant compressed by the compression mechanism  110  radiates heat in the first heat-source-side heat exchanger  131  and the second heat-source-side heat exchanger  132  and is decompressed and expanded by the second expansion valve  142 . In the air conditioner  1 , after decompressed and expanded by the second expansion valve  142  (the expansion mechanism  140 ), the refrigerant is evaporated in the use-side heat exchanger  133 . As described above, the air conditioner  1  is configured such that no refrigerant flows through the ejector  150  in the second operation. 
     The air conditioner  1  having the configuration described above can perform heating in the first operation by using heat radiated from the refrigerant in the use-side heat exchanger  133 . Further, the air conditioner  1  can perform cooling in the second operation by heat absorption due to the evaporation of the refrigerant in the use-side heat exchanger  133 . As described above, the air conditioner  1  can improve heating efficiency and cooling efficiency by switching between the heating operation using the ejector  150  and the cooling operation without using the ejector  150 . 
     (13) Detailed Configuration 
     (13-1) Overview of Circuit Configuration of Air Conditioner  1   
     The air conditioner  1  according to the fourth embodiment includes, in addition to the compression mechanism  110 , the first heat-source-side heat exchanger  131 , the second heat-source-side heat exchanger  132 , the use-side heat exchanger  133 , the ejector  150 , the expansion mechanism  140  (the first expansion valve  141  and the second expansion valve  142 ), and the switching mechanism  120  described above, a flow rate control valve  143 , an on-off valve  161 , which is a first valve, an on-off valve  162 , which is a second valve, and a check valve  163 , which is a third valve. 
     The first expansion valve  141  has a first end through which refrigerant is allowed to flow between the first expansion valve  141  and the use-side heat exchanger  133 . The ejector  150  has a refrigerant inflow port communicating with the first end of the first expansion valve  141 . Each of the first heat-source-side heat exchanger  131  and the second heat-source-side heat exchanger  132  has a first inlet/outlet into which the refrigerant discharged from the compression mechanism  110  flows in the second operation. 
     The first heat-source-side heat exchanger  131  has a second inlet/outlet communicating with a second end of the first expansion valve  141 . The second heat-source-side heat exchanger  132  has a second inlet/outlet communicating with a refrigerant outflow port of the ejector  150 . The on-off valve  161  is coupled between the first inlet/outlet of the first heat-source-side heat exchanger  131  and the first inlet/outlet of the second heat-source-side heat exchanger  132 . The on-off valve  162  has a first end coupled between the first heat-source-side heat exchanger  131  and the on-off valve  161 , and a second end communicating with a refrigerant suction port of the ejector  150 . The check valve  163  is coupled between the refrigerant inflow port of the ejector  150  and the refrigerant outflow port of the ejector  150 . 
     The on-off valve  161  does not allow the refrigerant to flow during the first operation and allows the refrigerant to flow during the second operation. The on-off valve  162  allows the refrigerant to flow during the first operation and does not allow the refrigerant to flow during the second operation. The check valve  163  does not allow the refrigerant to flow during the first operation and allows the refrigerant to flow during the second operation. The air conditioner  1  is configured such that the refrigerant returns to the compression mechanism  110  from the first inlet/outlet of the second heat-source-side heat exchanger  132  in the first operation, and the refrigerant returns to the compression mechanism  110  from the use-side heat exchanger  133  in the second operation. 
     In the air conditioner  1  according to the fourth embodiment, the ejector  150  can be bypassed during the second operation by using the on-off valve  161 , the on-off valve  162 , and the check valve  163 . In the air conditioner  1 , bypassing the ejector  150  can prevent occurrence of pressure loss in the ejector  150 . In the air conditioner  1 , furthermore, during the first operation, the on-off valve  161  is closed and the on-off valve  162  is opened to allow the refrigerant to flow through the ejector  150 . 
     (13-2) Details of Circuit Configuration of Air Conditioner  1   
     In the air conditioner  1  illustrated in  FIG. 17  and  FIG. 19 , the compression mechanism  110  is constituted by one compressor  111 . The switching mechanism  120  is constituted by a four-way valve  121 . An outflow port of a receiver  191  is coupled to a suction port of the compressor  111 . A discharge port of the compressor  111  communicates with a first port of the four-way valve  121 . A second port of the four-way valve  121  communicates with the first inlet/outlet of the second heat-source-side heat exchanger  132 . A third port of the four-way valve  121  communicates with an inflow port of the receiver  191 . A fourth port of the four-way valve  121  communicates with a first inlet/outlet of the use-side heat exchanger  133 . A second inlet/outlet of the use-side heat exchanger  133  communicates with a first end of the second expansion valve  142 . A second end of the second expansion valve  142  communicates with the first end of the first expansion valve  141  and a first end of the flow rate control valve  143 . A second end of the flow rate control valve  143  communicates with the inflow port at refrigerant of the ejector  150 . Accordingly, the refrigerant inflow port of the ejector  150  communicates with the first end of the first expansion valve  141  through the flow rate control valve  143 . 
     In the first operation, as illustrated in  FIG. 17 , the first port and the fourth port of the four-way valve  121  communicate with each other, and the second port and the third port of the four-way valve  121  communicate with each other. In the second operation, as illustrated in  FIG. 19 , the first port and the second port of the four-way valve  121  communicate with each other, and the third port and the fourth port of the four-way valve  121  communicate with each other. The four-way valve  121  performs switching described above such that, in the first operation, the refrigerant discharged from the discharge port of the compressor  111  flows to the use-side heat exchanger  133  and the refrigerant that has flowed out of the first inlet/outlet of the second heat-source-side heat exchanger  132  returns to the suction port of the compressor  111  through the receiver  191 . In the second operation, the refrigerant discharged from the discharge port of the compressor  111  flows through the first heat-source-side heat exchanger  131  and the second heat-source-side heat exchanger  132  in parallel, and the refrigerant that has flowed out of the first inlet/outlet of the use-side heat exchanger  133  returns to the suction port of the compressor  111  through the receiver  191 . 
     (14) Overall Operation 
     (14-1) Operation of Air Conditioner  1  during First Operation 
     The operation of the air conditioner  1  during the first operation using carbon dioxide as refrigerant will be described with reference to  FIG. 17  and  FIG. 18 . The refrigerant discharged from the discharge port of the compressor  111  (point a) is in a supercritical state. The refrigerant in the supercritical state discharged from the compressor  111  flows into the use-side heat exchanger  133  via the four-way valve  121 . The refrigerant in the supercritical state radiates heat in the use-side heat exchanger  133 . In the use-side heat exchanger  133 , for example, heat exchange is performed between indoor air and the refrigerant, and the warmed air is used to perform indoor heating. 
     The refrigerant at an outflow point (point b) of the use-side heat exchanger  133  is in a high-pressure state, and the specific enthalpy thereof is smaller than that at the point a. The second expansion valve  142  and the flow rate control valve  143  allow the refrigerant to pass therethrough without substantially decompressing the refrigerant. The refrigerant at an outflow point (point c) of the second expansion valve  142 , the refrigerant at an inflow point (point d) and an outflow point (point g) of the flow rate control valve  143 , and the refrigerant at an inflow point (point h) of the ejector  150  are in substantially the same state as the refrigerant at the point b. 
     The refrigerant that has flowed into the refrigerant inflow port of the ejector  150  from the flow rate control valve  143  is decompressed and expanded by a nozzle (not illustrated) in the ejector  150  into low-pressure refrigerant in a gas-liquid two-phase state at the nozzle (point i). At an outlet of the nozzle (point j), the refrigerant that has flowed in from the refrigerant inflow port and low-pressure gas refrigerant sucked from the refrigerant suction port of the ejector  150  (here, the same as that at an outflow point (point f) of the first inlet/outlet of the first heat-source-side heat exchanger  131 ) are mixed into refrigerant having a specific enthalpy between those of the refrigerant at the point i and the refrigerant at the point f. The refrigerant at the refrigerant outflow port of the ejector  150  (point k) is in a state such that the pressure is raised to be higher than that of the refrigerant at the outlet of the nozzle (point j). The refrigerant that has been raised in pressure and flowed out of the refrigerant outflow port of the ejector  150  evaporates into gas refrigerant in the second heat-source-side heat exchanger  132 . The refrigerant flowing out of the first inlet/outlet of the second heat-source-side heat exchanger  132  (point l) is gas refrigerant with a high specific enthalpy. The refrigerant that has flowed out of the second heat-source-side heat exchanger  132  is sucked in from the suction port of the compressor  111  (point m) via the four-way valve  121  and the receiver  191 . The state of the refrigerant present at the suction port of the compressor  111  (point m) is substantially the same as that of the gas refrigerant at the first inlet/outlet of the second heat-source-side heat exchanger  132  (point l). 
     (14-2) Operation of Air Conditioner  1  during Second Operation 
     The operation of the air conditioner  1  during the second operation using carbon dioxide as refrigerant will be described with reference to  FIG. 19 . The refrigerant discharged from the discharge port of the compressor  111  is in a supercritical state. A portion of the refrigerant in the supercritical state discharged from the compressor  111  flows into the second heat-source-side heat exchanger  132  via the four-way valve  121 , and the remaining refrigerant flows into the first heat-source-side heat exchanger  131  via the four-way valve  121  and the on-off valve  161 . In this case, the refrigerant does not flow through the ejector  150  due to the closed on-off valve  162  and the check valve  163 . The refrigerant in the supercritical state radiates heat in either the first heat-source-side heat exchanger  131  or the second heat-source-side heat exchanger  132 . In the first heat-source-side heat exchanger  131  and the second heat-source-side heat exchanger  132  functioning as radiators, for example, heat exchange is performed between outdoor air and the refrigerant. 
     The refrigerant flowing out of the first heat-source-side heat exchanger  131  and the second heat-source-side heat exchanger  132  is in a high-pressure state, and the specific enthalpy thereof is smaller than that before flowing into the first heat-source-side heat exchanger  131  and the second heat-source-side heat exchanger  132 . Since the first expansion valve  141  and the flow rate control valve  143  are open, all of the refrigerants that have exited the first heat-source-side heat exchanger  131  and the second heat-source-side heat exchanger  132  flow to the second expansion valve  142 . The refrigerant that flows from the second expansion valve  142  to the use-side heat exchanger  133  is decompressed and expanded by the second expansion valve  142  before flowing into the use-side heat exchanger  133 . The refrigerant in the gas-liquid two-phase state that has flowed into the use-side heat exchanger  133  evaporates into gas refrigerant in the use-side heat exchanger  133 . In the use-side heat exchanger  133  functioning as an evaporator, for example, heat exchange is performed between indoor air and the refrigerant, and the cooled air is used to perform indoor cooling. The gas refrigerant that has flowed out of the use-side heat exchanger  133  is sucked in from the suction port of the compressor  111  via the four-way valve  121  and the receiver  191 . 
     (14-3) Control of Air Conditioner  1   
     The air conditioner  1  according to the fourth embodiment includes a controller  200  illustrated in  FIG. 20  to cause the internal devices to perform the operation described above. The controller  200  is implemented by a computer, for example. The computer includes, for example, a control and arithmetic unit and a memory. The control and arithmetic unit can be implemented using a processor. The controller  200  in  FIG. 20  includes a CPU  201  serving as a processor. The control and arithmetic unit reads, for example, a program stored in the memory and performs predetermined image processing, arithmetic processing, or sequence processing in accordance with the program. Further, for example, the control and arithmetic unit can write an arithmetic result to the memory or read information stored in the memory in accordance with the program. The memory can be used as a database. The controller  200  includes a memory  202  serving as a memory. 
     The controller  200  controls the compressor  111 , the first expansion valve  141 , the second expansion valve  142 , the flow rate control valve  143 , the four-way valve  121 , and the on-off valves  161  and  162 . The two valves, namely, the on-off valves  161  and  162 , can be each implemented using, for example, an electromagnetic valve that switches between an open state and a closed state in accordance with a signal from the controller  200 . The first expansion valve  141 , the second expansion valve  142 , and the flow rate control valve  143  can be each implemented using, for example, an electrically powered valve whose opening degree can be changed in response to a pulse signal. 
     Fifth Embodiment 
     (15) Overview of Configuration 
     As illustrated in  FIG. 21  and  FIG. 22 , an overview of the configuration of an air conditioner  1  according to a fifth embodiment is the same as the overview of the configuration according to the fourth embodiment described in (12) described above. Accordingly, a description of the overview of the configuration of the air conditioner  1  according to the fifth embodiment will be omitted here.  FIG. 21  illustrates the air conditioner  1  in which the first operation is being performed, and  FIG. 22  illustrates the air conditioner  1  in which the second operation is being performed. 
     (16) Detailed Configuration 
     (16-1) Overview of Circuit Configuration of Air Conditioner  1   
     The air conditioner  1  according to the fifth embodiment includes, in addition to the compression mechanism  110 , the first heat-source-side heat exchanger  131 , the second heat-source-side heat exchanger  132 , the use-side heat exchanger  133 , the ejector  150 , the first expansion valve  141 , the second expansion valve  142 , and the switching mechanism  120  described above, an on-off valve  164 , which is a fourth valve, and a flow rate control valve  144 , which is a fifth valve. 
     The on-off valve  164  is coupled between the refrigerant outflow port of the ejector  150  and the refrigerant suction port of the ejector  150 . Each of the first expansion valve  141  and the flow rate control valve  144  has a first end through which refrigerant is allowed to flow between the corresponding one of the first expansion valve  141  and the flow rate control valve  144  and the use-side heat exchanger  133 . A second end of the flow rate control valve  144  communicates with the refrigerant inflow port of the ejector  150 . The second heat-source-side heat exchanger  132  has a second inlet/outlet communicating with the refrigerant inflow port of the ejector  150 . The first heat-source-side heat exchanger  131  has a first inlet/outlet communicating with the refrigerant suction port of the ejector  150 , and a second inlet/outlet communicating with a second end of the first expansion valve  141 . 
     The on-off valve  164  does not allow the refrigerant to flow during the first operation and allows the refrigerant to flow during the second operation. As illustrated in  FIG. 21 , in the first operation, the refrigerant flow out of a first inlet/outlet of the second heat-source-side heat exchanger  132  to the suction side of the compression mechanism  110 . As illustrated in  FIG. 22 , in the second operation, the refrigerant discharged from the compression mechanism  110  flows into the first inlet/outlet of the second heat-source-side heat exchanger  132 . 
     In the air conditioner  1  according to the fifth embodiment, the ejector  150  can be bypassed during the second operation by closing the flow rate control valve  144  to prevent the refrigerant from flowing to the ejector  150  and opening the on-off valve  164  to allow the refrigerant to flow. In the air conditioner  1 , bypassing the ejector  150  can prevent occurrence of pressure loss in the ejector  150 . In the air conditioner  1 , during the first operation, the on-off valve  164  is closed and the flow rate control valve  144  is opened to allow the refrigerant to flow through the ejector  150 . 
     (16-2) Details of Circuit Configuration of Air Conditioner  1   
     In the air conditioner  1  illustrated in  FIG. 21  and  FIG. 22 , the compression mechanism  110  is constituted by one compressor  111 . The switching mechanism  120  is constituted by a four-way valve  121 . An outflow port of a receiver  191  is coupled to a suction port of the compressor  111 . A discharge port of the compressor  111  communicates with a first port of the four-way valve  121 . A second port of the four-way valve  121  communicates with the first inlet/outlet of the second heat-source-side heat exchanger  132 . A third port of the four-way valve  121  communicates with an inflow port of the receiver  191 . A fourth port of the four-way valve  121  communicates with a first inlet/outlet of the use-side heat exchanger  133 . A second inlet/outlet of the use-side heat exchanger  133  communicates with a first end of the second expansion valve  142 . The second end of the second expansion valve  142  communicates with the first end of the first expansion valve  141  and the first end of the flow rate control valve  144 . 
     In the first operation, as illustrated in  FIG. 21 , the first port and the fourth port of the four-way valve  121  communicate with each other, and the second port and the third port of the four-way valve  121  communicate with each other. In the second operation, as illustrated in  FIG. 22 , the first port and the second port of the four-way valve  121  communicate with each other, and the third port and the fourth port of the four-way valve  121  communicate with each other. The four-way valve  121  performs switching described above such that, in the first operation, the refrigerant discharged from the discharge port of the compressor  111  flows to the use-side heat exchanger  133  and the refrigerant that has flowed out of the first inlet/outlet of the second heat-source-side heat exchanger  132  returns to the suction port of the compressor  111  through the receiver  191 . In the second operation, the refrigerant discharged from the discharge port of the compressor  111  first flows through the second heat-source-side heat exchanger  132  and then flows through the first heat-source-side heat exchanger  131 . In the second operation, furthermore, the refrigerant that has flowed out of the first inlet/outlet of the use-side heat exchanger  133  returns to the suction port of the compressor  111  through the receiver  191 . 
     (17) Overall Operation 
     (17-1) Operation of Air Conditioner  1  during First Operation 
     The operation of the air conditioner  1  during the first operation of the air conditioner  1  according to the fifth embodiment is the same as the operation of the air conditioner  1  according to the fourth embodiment during the first operation described in (14-1) described above. Accordingly, a description of the operation of the air conditioner  1  during the first operation of the air conditioner  1  according to the fifth embodiment will be omitted here. 
     (17-2) Operation of Air Conditioner  1  during Second Operation 
     The operation of the air conditioner  1  according to the fifth embodiment during the second operation using carbon dioxide as refrigerant will be described with reference to  FIG. 22 . The refrigerant discharged from the discharge port of the compressor  111  is in a supercritical state. The refrigerant in the supercritical state discharged from the compressor  111  flows into the second heat-source-side heat exchanger  132  via the four-way valve  121 . The refrigerant that has radiated heat in the second heat-source-side heat exchanger  132  further flows into the first heat-source-side heat exchanger  131  via the on-off valve  164 . In this case, the refrigerant does not flow through the ejector  150  due to the closed flow rate control valve  144  and the opened on-off valve  164 . The refrigerant in the supercritical state radiates heat in both the second heat-source-side heat exchanger  132  and the first heat-source-side heat exchanger  131 . In the first heat-source-side heat exchanger  131  and the second heat-source-side heat exchanger  132  functioning as radiators, for example, heat exchange is performed between outdoor air and the refrigerant. 
     The refrigerant flowing out of the first heat-source-side heat exchanger  131  is in a high-pressure state, and the specific enthalpy thereof is smaller than that before flowing into the second heat-source-side heat exchanger  132 . Since the first expansion valve  141  is open, the refrigerant that has exited the first heat-source-side heat exchanger  131  flows to the second expansion valve  142 . The refrigerant that flows from the second expansion valve  142  to the use-side heat exchanger  133  is decompressed and expanded by the second expansion valve  142  before flowing into the use-side heat exchanger  133 . The refrigerant in the gas-liquid two-phase state that has flowed into the use-side heat exchanger  133  evaporates into gas refrigerant in the use-side heat exchanger  133 . In the use-side heat exchanger  133  functioning as an evaporator, for example, heat exchange is performed between indoor air and the refrigerant, and the cooled air is used to perform indoor cooling. The gas refrigerant that has flowed out of the use-side heat exchanger  133  is sucked in from the suction port of the compressor  111  via the four-way valve  121  and the receiver  191 . 
     (17-3) Control of Air Conditioner  1   
     The air conditioner  1  according to the fifth embodiment includes a controller  200  illustrated in  FIG. 23  to cause the internal devices to perform the operation described above. 
     The controller  200  controls the compressor  111 , the first expansion valve  141 , the second expansion valve  142 , the flow rate control valve  144 , the four-way valve  121 , and the on-off valve  161 . 
     Sixth Embodiment 
     (18) Overview of Configuration 
     As illustrated in  FIG. 24  and  FIG. 26 , an overview of the configuration of an air conditioner  1  according to a sixth embodiment is the same as the overview of the configuration according to the fourth embodiment described in (12) described above. Accordingly, a description of the overview of the configuration of the air conditioner  1  according to the sixth embodiment will be omitted here.  FIG. 24  illustrates the air conditioner  1  in which the first operation is being performed, and  FIG. 26  illustrates the air conditioner  1  in which the second operation is being performed. 
     (19) Detailed Configuration 
     (19-1) Overview of Circuit Configuration of Air Conditioner  1   
     The air conditioner  1  according to the sixth embodiment includes, in addition to the compression mechanism  110 , the first heat-source-side heat exchanger  131 , the second heat-source-side heat exchanger  132 , the use-side heat exchanger  133 , the ejector  150 , the expansion mechanism  140  (the first expansion valve  141  and the second expansion valve  142 ), and the switching mechanism  120  described above, a check valve  171 , which is a sixth valve, a check valve  172 , which is a seventh valve, an on-off valve  173 , which is an eighth valve, an on-off valve  174 , which is a ninth valve, and a flow rate control valve  145 , which is a tenth valve. 
     The compression mechanism  110  includes a compressor  112 , which is a first compression element in the preceding stage, and a compressor  113 , which is a second compression element in the subsequent stage. The switching mechanism  120  includes a four-way valve  122 , which is a first four-way valve, and a four-way valve  123 , which is a second four-way valve. Each of the four-way valves  122  and  123  has a first port, a second port, a third port, and a fourth port. 
     The first port of the four-way valve  122  communicates with a discharge port of the compressor  112 , the second port of the four-way valve  122  communicates with a first inlet/outlet of the second heat-source-side heat exchanger  132 , and the third port of the four-way valve  122  communicates with a suction port of the compressor  112  through the receiver  191 . The first inlet/outlet of the second heat-source-side heat exchanger  132  communicates with the second port of the four-way valve  122 , and a second inlet/outlet of the second heat-source-side heat exchanger  132  communicates with the refrigerant outflow port of the ejector  150 . The first port of the four-way valve  123  communicates with a discharge port of the compressor  113 , the third port of the four-way valve  123  communicates with the third port of the four-way valve  122 , and the fourth port of the four-way valve  123  communicates with a first inlet/outlet of the use-side heat exchanger  133 . 
     The check valve  171  is coupled between the fourth port of the four-way valve  122  and a suction port of the compressor  113 . The check valve  172  is coupled between the second inlet/outlet of the second heat-source-side heat exchanger  132  and the suction port of the compressor  113 . The on-off valve  173  is coupled between the refrigerant suction port of the ejector  150  and a first inlet/outlet of the first heat-source-side heat exchanger  131 . The on-off valve  174  is coupled between the second port of the four-way valve  123  and the first inlet/outlet of the first heat-source-side heat exchanger  131 . 
     As illustrated in  FIG. 24 , during the first operation, the first port and the fourth port of each of the four-way valves  122  and  123  communicate with each other, and the second port and the third port of each of the four-way valves  122  and  123  communicate with each other. As illustrated in  FIG. 24 , during the second operation, the first port and the second port of each of the four-way valves  122  and  123  communicate with each other, and the third port and the fourth port of each of the four-way valves  122  and  123  communicate with each other. The check valve  171  is coupled such that refrigerant is allowed to flow during the first operation and refrigerant is prevented from flowing during the second operation. The check valve  172  is coupled such that refrigerant is prevented from flowing during the first operation and refrigerant is allowed to flow during the second operation. The on-off valve  173  is controlled to allow refrigerant to flow during the first operation and to prevent refrigerant from flowing during the second operation. The on-off valve  174  is controlled to prevent refrigerant from flowing during the first operation and to allow refrigerant to flow during the second operation. 
     (19-2) Details of Circuit Configuration of Air Conditioner  1   
     The air conditioner  1  illustrated in  FIG. 24  and  FIG. 26  further includes a receiver  191 . An inflow port of the receiver  191  communicates with the third ports of both the four-way valves  122  and  123 . An outflow port of the receiver  191  communicates with the suction port of the compressor  112 . 
     (20) Overall Operation 
     (20-1) Operation of Air Conditioner  1  during First Operation 
     The operation of the air conditioner  1  according to the sixth embodiment during the first operation using carbon dioxide as refrigerant will be described with reference to  FIG. 24  and  FIG. 25 . The refrigerant discharged from the discharge port of the compressor  112  (point a) is sucked in from the suction port of the compressor  113  (point b) through the check valve  171 . The refrigerant sucked into the compressor  113  is further compressed by the compressor  113 . The refrigerant discharged from the discharge port of the compressor  113  (point c) is in a supercritical state. The state of the refrigerant at the suction port of the compressor  113  (point b) is the same as the state of the refrigerant at the discharge port of the compressor  112  (point a). 
     The refrigerant in the supercritical state discharged from the compressor  113  flows into the use-side heat exchanger  133  via the four-way valve  123 . The state of the refrigerant at the first inlet/outlet of the use-side heat exchanger  133  (point d) is the same as the state of the refrigerant at the discharge port of the compressor  113  (point c). The refrigerant in the supercritical state radiates heat in the use-side heat exchanger  133 . In the use-side heat exchanger  133 , for example, heat exchange is performed between indoor air and the refrigerant, and the warmed air is used to perform indoor heating. 
     The refrigerant at a second inlet/outlet of the use-side heat exchanger  133  (point e) is in a high-pressure state, and the specific enthalpy thereof is smaller than that at the point d. The second expansion valve  142  and the flow rate control valve  145  allow the refrigerant to pass therethrough without substantially decompressing the refrigerant. The refrigerant at the first end of the second expansion valve  142  (point f), the refrigerant at a first end and a second end (point i) of the flow rate control valve  145  (point f), and the refrigerant at the refrigerant inflow port of the ejector  150  (point i) are in substantially the same state as the refrigerant at the point f. 
     The refrigerant that has flowed into the refrigerant inflow port of the ejector  150  from the flow rate control valve  145  is decompressed and expanded by a nozzle (not illustrated) in the ejector  150  and becomes a low-pressure refrigerant in a gas-liquid two-phase state at the nozzle (point j). At the outlet of the nozzle (point k), the refrigerant that has flowed in from the refrigerant inflow port and low-pressure gas refrigerant sucked from the refrigerant suction port of the ejector  150  (here, the same as that at the first inlet/outlet of the first heat-source-side heat exchanger  131  (point h)) are mixed into refrigerant having a specific enthalpy between those of the refrigerant at the point j and the refrigerant at the point h. The refrigerant at the refrigerant outflow port of the ejector  150  (point  1 ) is in a state such that the pressure is raised to be higher than that of the refrigerant at the outlet of the nozzle (point k). The refrigerant that has been raised in pressure and flowed out of the refrigerant outflow port of the ejector  150  evaporates into gas refrigerant in the second heat-source-side heat exchanger  132 . The refrigerant flowing out of the first inlet/outlet of the second heat-source-side heat exchanger  132  (point m) is gas refrigerant with a high specific enthalpy. The refrigerant that has flowed out of the second heat-source-side heat exchanger  132  is sucked in from the suction port of the compressor  111  (point n) via the four-way valve  122  and the receiver  191 . The state of the refrigerant present at the suction port of the compressor  111  (point n) is substantially the same as that of the gas refrigerant at the first inlet/outlet of the second heat-source-side heat exchanger  132  (point m). 
     (20-2) Operation of Air Conditioner  1  during Second Operation 
     The operation of the air conditioner  1  during the second operation according to the sixth embodiment using carbon dioxide as refrigerant will be described with reference to  FIG. 26 . The refrigerant discharged from the discharge port of the compressor  112  in the preceding stage flows into the second heat-source-side heat exchanger  132  via the four-way valve  122 . The refrigerant cooled in the second heat-source-side heat exchanger  132  is sucked in from the suction port of the compressor  113  in the subsequent stage. During the second operation, the second heat-source-side heat exchanger  132  functions as an intercooler. 
     The refrigerant in a critical state discharged from the compressor  113  in the subsequent stage flows into the first heat-source-side heat exchanger  131  via the on-off valve  174 . The first heat-source-side heat exchanger  131  functions as a radiator and performs heat exchange to take heat from the refrigerant. The refrigerant that has flowed out of the first heat-source-side heat exchanger  131  passes through the first expansion valve  141  and is decompressed and expanded by the second expansion valve  142 . The refrigerant decompressed and expanded by the second expansion valve  142  into a gas-liquid two-phase state flows into the use-side heat exchanger  133 . The use-side heat exchanger  133  functions as an evaporator. For example, heat exchange is performed between indoor air and the refrigerant in the use-side heat exchanger  133 , and the air cooled by the heat exchange is used to perform cooling. The refrigerant that has flowed out of the use-side heat exchanger  133  is sucked into the compressor  112  via the four-way valve  123  and the receiver  191 . 
     (20-3) Control of Air Conditioner  1   
     The air conditioner  1  according to the sixth embodiment includes a controller  200  illustrated in  FIG. 27  to cause the internal devices to perform the operation described above. The controller  200  controls the compressors  112  and  113 , the four-way valves  122  and  123 , the first expansion valve  141 , the second expansion valve  142 , the flow rate control valve  145 , and the on-off valves  173  and  174 . 
     Seventh Embodiment 
     (21) Overview of Configuration 
     As illustrated in  FIG. 28  and  FIG. 30 , an overview of the configuration of an air conditioner  1  according to a seventh embodiment is the same as the overview of the configuration according to the fourth embodiment described in (12) described above. Accordingly, a description of the overview of the configuration of the air conditioner  1  according to the seventh embodiment will be omitted here.  FIG. 28  illustrates the air conditioner  1  in which the first operation is being performed, and  FIG. 30  illustrates the air conditioner  1  in which the second operation is being performed. 
     (22) Detailed Configuration 
     (22-1) Overview of Circuit Configuration of Air Conditioner  1   
     The air conditioner  1  according to the seventh embodiment includes, in addition to the compression mechanism  110 , the first heat-source-side heat exchanger  131 , the second heat-source-side heat exchanger  132 , the use-side heat exchanger  133 , the expansion mechanism  140  (the first expansion valve  141  and the second expansion valve  142 ), and the switching mechanism  120  described above, a check valve  181 , which is an eleventh valve, a check valve  182 , which is a twelfth valve, an on-off valve  183 , which is a thirteenth valve, a check valve  184 , which is a fourteenth valve, and a flow rate control valve  146 , which is a tenth valve. 
     The compression mechanism  110  includes a compressor  112 , which is a first compression element, and a compressor  113 , which is a second compression element. The compressor  112  is arranged in the preceding stage, and the compressor  113  is arranged in the subsequent stage. The compressors  112  and  113  perform two-stage compression such that the refrigerant discharged from the compressor  112  is further compressed by the compressor  113 . Each of the first expansion valve  141  and the flow rate control valve  146  has a first end through which refrigerant is allowed to flow between the corresponding one of the first expansion valve  141  and the flow rate control valve  146  and the use-side heat exchanger  133 . In other words, the first ends of both the first expansion valve  141  and the flow rate control valve  146  communicate with a second inlet/outlet of the use-side heat exchanger  133 . The first expansion valve  141  has a second end communicating with a second inlet/outlet of the first heat-source-side heat exchanger  131 . The flow rate control valve  146  has a second end communicating with the refrigerant inflow port of the ejector  150 . 
     The switching mechanism  120  includes a four-way valve  122 , which is a first four-way valve, and a four-way valve  123 , which is a second four-way valve. Each of the four-way valves  122  and  123  has a first port and a fourth port communicating with each other, and a second port and a third port communicating with each other during the first operation. During the second operation, the first port and the second port of each of the four-way valves  122  and  123  communicate with each other, and the third port and the fourth port of each of the four-way valves  122  and  123  communicate with each other. The first port of the four-way valve  122  communicates with the discharge port of the compressor  112 , the second port of the four-way valve  122  communicates with the first inlet/outlet of the first heat-source-side heat exchanger  131 , and the third port of the four-way valve  122  communicates with the suction side of the compressor  112 . The fourth port of the four-way valve  122  communicates with the suction port of the compressor  112  through the check valve  181 . The first port of the four-way valve  123  communicates with a discharge port of the compressor  113 , the third port of the four-way valve  123  communicates with the third port of the four-way valve  122 , and the fourth port of the four-way valve  123  communicates with a first inlet/outlet of the use-side heat exchanger  133 . The four-way valve  123  allows the refrigerant that flows through the use-side heat exchanger  133  to pass through the fourth port. During the first operation, the refrigerant flows from the fourth port of the four-way valve  123  to the first inlet/outlet of the use-side heat exchanger  133 , and during the second operation, the refrigerant flows from the first inlet/outlet of the use-side heat exchanger  133  to the fourth port of the four-way valve  123 . A first inlet/outlet of the first heat-source-side heat exchanger  131  communicates with the refrigerant suction port of the ejector  150 , and the second inlet/outlet of the first heat-source-side heat exchanger  131  communicates with the suction port of the compressor  113  through the check valve  182 . 
     The check valve  181  has a first end communicating with the fourth port of the four-way valve  122 , and a second end communicating with the suction port of the compressor  113 . In other words, the check valve  181  is coupled between the fourth port of the four-way valve  122  and the suction side of the compressor  113 . The check valve  181  is coupled such that refrigerant is allowed to flow during the first operation and refrigerant is prevented from flowing during the second operation. The check valve  182  has a first end communicating with the second inlet/outlet of the first heat-source-side heat exchanger  131 , and a second end communicating with the suction port of the compressor  113 . In other words, the check valve  182  is coupled between the second inlet/outlet of the first heat-source-side heat exchanger  131  and the suction side of the compressor  113 . The check valve  182  is coupled such that refrigerant is prevented from flowing during the first operation and refrigerant is allowed to flow during the second operation. 
     The on-off valve  183  has a first end communicating with the refrigerant suction port of the ejector  150 , and a second end communicating with the first inlet/outlet of the first heat-source-side heat exchanger  131 . In other words, the on-off valve  183  is coupled between the refrigerant suction port of the ejector  150  and the first inlet/outlet of the first heat-source-side heat exchanger  131 . The on-off valve  183  is controlled to allow refrigerant to flow during the first operation and to prevent refrigerant from flowing during the second operation. The check valve  184  has a first end communicating with a second inlet/outlet of the second heat-source-side heat exchanger  132  and the refrigerant outflow port of the ejector  150 , and a second end communicating with a second end of the flow rate control valve  146 . In other words, the check valve  184  is coupled between the refrigerant outflow port and the refrigerant inflow port of the ejector  150 . The check valve  184  is further coupled between the second end of the flow rate control valve  146  and the second inlet/outlet of the second heat-source-side heat exchanger  132 . The second end of the flow rate control valve  146  communicates with the refrigerant inflow port of the ejector  150 . The refrigerant outflow port of the ejector  150  communicates with the second inlet/outlet of the second heat-source-side heat exchanger  132 . The check valve  184  is coupled such that refrigerant is prevented from flowing during the first operation and refrigerant is allowed to flow during the second operation. 
     (22-2) Details of Circuit Configuration of Air Conditioner 
     The air conditioner  1  illustrated in  FIG. 28  and  FIG. 30  further includes a receiver  191 . An inflow port of the receiver  191  communicates with the third ports of both the four-way valves  122  and  123 . An outflow port of the receiver  191  communicates with the suction port of the compressor  112 . 
     (23) Overall Operation 
     (23-1) Operation of Air Conditioner  1  during First Operation 
     The operation of the air conditioner  1  according to the seventh embodiment during the first operation using carbon dioxide as refrigerant will be described with reference to  FIG. 28  and  FIG. 29 . The refrigerant discharged from the discharge port of the compressor  112  (point a) is sucked in from the suction port of the compressor  113  (point b) through the check valve  181 . The refrigerant sucked into the compressor  113  is further compressed by the compressor  113 . The refrigerant discharged from the discharge port of the compressor  113  (point c) is in a supercritical state. The state of the refrigerant at the suction port of the compressor  113  (point b) is the same as the state of the refrigerant at the discharge port of the compressor  112  (point a). 
     The refrigerant in the supercritical state discharged from the compressor  113  flows into the use-side heat exchanger  133  via the four-way valve  123 . The state of the refrigerant at the first inlet/outlet of the use-side heat exchanger  133  (point d) is the same as the state of the refrigerant at the discharge port of the compressor  113  (point c). The refrigerant in the supercritical state radiates heat in the use-side heat exchanger  133 . In the use-side heat exchanger  133 , for example, heat exchange is performed between indoor air and the refrigerant, and the warmed air is used to perform indoor heating. 
     The refrigerant at a second inlet/outlet of the use-side heat exchanger  133  (point e) is in a high-pressure state, and the specific enthalpy thereof is smaller than that at the point d. The second expansion valve  142  and the flow rate control valve  146  allow the refrigerant to pass therethrough without substantially decompressing the refrigerant. The refrigerant at a first end of the second expansion valve  142  (point f) and the refrigerant at the first end of the flow rate control valve  146  (point f) and at the refrigerant inflow port of the ejector  150  (point j) are in substantially the same state as the refrigerant at the point f. 
     The refrigerant that has flowed into the refrigerant inflow port of the ejector  150  from the flow rate control valve  146  is decompressed and expanded by a nozzle (not illustrated) in the ejector  150  and becomes a low-pressure refrigerant in a gas-liquid two-phase state at the nozzle (point k). At the outlet of the nozzle (point l), the refrigerant that has flowed in from the refrigerant inflow port and low-pressure gas refrigerant sucked from the refrigerant suction port of the ejector  150  (here, the same as that at the first inlet/outlet of the first heat-source-side heat exchanger  131  (point i)) are mixed into refrigerant having a specific enthalpy between those of the refrigerant at the point k and the refrigerant at the point i. The refrigerant at the refrigerant outflow port of the ejector  150  (point m) is in a state such that the pressure is raised to be higher than that of the refrigerant at the outlet of the nozzle (point  1 ). The refrigerant that has been raised in pressure and flowed out of the refrigerant outflow port of the ejector  150  evaporates into gas refrigerant in the second heat-source-side heat exchanger  132 . The refrigerant flowing out of the first inlet/outlet of the second heat-source-side heat exchanger  132  (point n) is gas refrigerant with a high specific enthalpy. The refrigerant that has flowed out of the second heat-source-side heat exchanger  132  is sucked in from the suction port of the compressor  111  (point o) via the four-way valve  123  and the receiver  191 . The state of the refrigerant present at the suction port of the compressor  111  (point o) is substantially the same as that of the gas refrigerant at the first inlet/outlet of the second heat-source-side heat exchanger  132  (point n). 
     (23-2) Operation of Air Conditioner  1  during Second Operation 
     The second operation of the air conditioner  1  during the second operation according to the seventh embodiment using carbon dioxide as refrigerant will be described with reference to  FIG. 30 . The refrigerant discharged from the discharge port of the compressor  112  in the preceding stage flows into the first heat-source-side heat exchanger  131  via the four-way valve  122 . The refrigerant cooled in the first heat-source-side heat exchanger  131  is sucked in from the suction port of the compressor  113  in the subsequent stage. During the second operation, the first heat-source-side heat exchanger  131  functions as an intercooler. 
     The refrigerant in a critical state discharged from the compressor  113  in the subsequent stage flows into the second heat-source-side heat exchanger  132 . The second heat-source-side heat exchanger  132  functions as a radiator and performs heat exchange to take heat from the refrigerant. The refrigerant that has flowed out of the first heat-source-side heat exchanger  131  passes through the first expansion valve  141  and is decompressed and expanded by the second expansion valve  142 . The refrigerant decompressed and expanded by the second expansion valve  142  into a gas-liquid two-phase state flows into the use-side heat exchanger  133 . The use-side heat exchanger  133  functions as an evaporator. For example, heat exchange is performed between indoor air and the refrigerant in the use-side heat exchanger  133 , and the air cooled by the heat exchange is used to perform cooling. The refrigerant that has flowed out of the use-side heat exchanger  133  is sucked into the compressor  112  via the four-way valve  123  and the receiver  191 . 
     (23-3) Control of Air Conditioner  1   
     The air conditioner  1  according to the seventh embodiment includes a controller  200  illustrated in  FIG. 31  to cause the internal devices to perform the operation described above. The controller  200  controls the compressors  112  and  113 , the four-way valves  122  and  123 , the first expansion valve  141 , the second expansion valve  142 , the flow rate control valve  146 , and the on-off valve  183 . 
     (24) Modifications 
     (24-1) Modification H 
     The air conditioner  1  according to the fourth embodiment, the fifth embodiment, the sixth embodiment, and the seventh embodiment in which the compression mechanism  110  is constituted by one compressor  111  or two compressors  112  and  113  has been described. However, the compression mechanism  110  is not limited to one constituted by one compressor  111  or two compressors  112 ,  113 . For example, the compression mechanism  110  may be constituted by three or more compressors. In other words, the compression mechanism  110  may be configured to perform compression in three or more multiple stages. When the compression mechanism  110  is configured to perform two-stage compression, for example, one compressor may include a first compression element for low-pressure compression and a second compression element for high-pressure compression. When the compression mechanism  110  is constituted by a plurality of compressors, the compressors may be coupled in parallel. 
     (24-2) Modification I 
     The air conditioner  1  including a compression mechanism configured to perform multi-stage compression described in the sixth embodiment or the seventh embodiment may be provided with an economizer circuit  210  illustrated in  FIG. 32 . The economizer circuit  210  includes an economizer heat exchanger  211 , an injection pipe  212 , and an injection valve  213 . The injection pipe  212  branches the refrigerant delivered from the radiator to the expansion valve and returns the branched refrigerant to the suction port of the compressor  113  in the subsequent stage (downstream). The economizer heat exchanger  211  performs heat exchange between the refrigerant delivered from the radiator to the expansion valve and intermediate-pressure refrigerant in the refrigeration cycle flowing through the injection pipe  212 . The injection valve  213  is an expansion valve and decompresses and expands the refrigerant in the injection pipe  212  before the refrigerant enters the economizer heat exchanger  211  by the injection pipe  212 . The refrigerant that has passed through the injection valve  213  is intermediate-pressure refrigerant. In the air conditioner  1 , since intermediate-pressure injection using the economizer heat exchanger  211  and the injection pipe  212  is adopted, the temperature of the refrigerant to be sucked into the compressor  113  in the subsequent stage (downstream) can be kept low with no heat radiate to the outside, and the refrigerant to be delivered to the evaporator can be cooled. For example, in the second operation, the first heat-source-side heat exchanger  131  functions as a radiator, the use-side heat exchanger  133  functions as an evaporator, and the second expansion valve  142  performs decompression and expansion. 
     (24-3) Modification J 
     The air conditioner  1  described above including one use-side heat exchanger  133  has been described. However, the air conditioner  1  may include a plurality of use-side heat exchangers. When the air conditioner  1  according to the fourth embodiment includes two use-side heat exchangers  133 , for example, as illustrated in  FIG. 33 , two units each including the use-side heat exchanger  133  and the second expansion valve  142  may be coupled in parallel. 
     (24-4) Modification K 
     While the air conditioner  1  described above including the first expansion valve  141  and the second expansion valve  142  has been described, the first expansion valve  141  and the second expansion valve  142  may be combined into a single expansion valve. For example, the first expansion valve  141  may be omitted, and the second expansion valve  142  may perform decompression and expansion in the second operation. The second expansion valve  142  having the configuration described above also serves as a first expansion valve. 
     (24-5) Modification L 
     The air conditioner  1  described above including the check valves  163 ,  171 ,  172 ,  181 , and  182  has been described. However, the check valves  163 ,  171 ,  172 ,  181 , and  182  may be each replaced with an on-off valve. Further, the air conditioner  1  described above including the flow rate control valves  143 ,  144 ,  145 , and  146  have been described. However, the flow rate control valves  143 ,  144 ,  145 , and  146  may be each replaced with an on-off valve. Alternatively, the flow rate control valves  143 ,  144 ,  145 , and  146  may be each replaced with an expansion valve. The air conditioner  1  may be configured such that, before allowing refrigerant to flow to the ejector  150 , an expansion valve decompresses and expands the refrigerant upstream of the refrigerant inflow port of the ejector  150  and allows refrigerant having an intermediate pressure between a high pressure and a low pressure to flow to the refrigerant inflow port of the ejector  150 . 
     (24-6) Modification M 
     The air conditioner  1  described above in which carbon dioxide is used as refrigerant has been described. The refrigerant used in the air conditioner  1  described above is preferably carbon dioxide or a refrigerant mixture containing carbon dioxide in which the refrigerant to be discharged from the compression mechanism  110  has a high pressure. However, the air conditioner  1  described above may use refrigerant other than carbon dioxide or a refrigerant mixture containing carbon dioxide. For example, refrigerant whose saturation pressure is greater than or equal to 4.5 MPa when reaching a saturation temperature of 65° C. may be used. Examples of such refrigerant include R410A refrigerant. Alternatively, a chlorofluorocarbon-based refrigerant that reaches a critical state when discharged from the compression mechanism  110  may be used. Examples of such chlorofluorocarbon-based refrigerant include R23 refrigerant. 
     (25) Features 
     (25-1) 
     The air conditioner  1  according to the fourth and subsequent embodiments described above can perform, for example, heating in the first operation by using heat radiated from the refrigerant in the use-side heat exchanger  133  and cooling in the second operation by heat absorption due to the evaporation of the refrigerant in the use-side heat exchanger  133 . The air conditioner  1  described above can provide efficient operation by, for example, switching between heating operation using the ejector  150  and cooling operation without the ejector  150 . 
     (25-2) 
     In the air conditioner  1  according to the fourth embodiment illustrated in  FIG. 17  and  FIG. 19 , the ejector  150  can be bypassed during the second operation by using the on-off valve  161 , which is a first valve, the on-off valve  162 , which is a second valve, and the check valve  163 , which is a third valve. As illustrated in  FIG. 17 , in the first operation, in the air conditioner  1  according to the fourth embodiment, the flow rate control valve  143  and the on-off valve  162  are opened and the on-off valve  161  is closed such that no refrigerant flows through the check valve  163 . As a result, refrigerant can be allowed to appropriately flow through the ejector  150 . As illustrated in  FIG. 19 , in the second operation, in the air conditioner  1  according to the fourth embodiment, the on-off valve  162  are closed and the flow rate control valve  143  and the on-off valve  161  is opened such that refrigerant flows through the check valve  163  and no refrigerant flows through the ejector  150 . As a result, in the second operation, the first heat-source-side heat exchanger  131  and the second heat-source-side heat exchanger  132  functioning as radiators, and the use-side heat exchanger  133  functioning as an evaporator can perform cooling, for example. 
     The air conditioner  1  according to the fourth embodiment can allow refrigerant to flow through the first heat-source-side heat exchanger  131  and the second heat-source-side heat exchanger  132  in parallel during the second operation. 
     (25-3) 
     In the air conditioner  1  according to the fifth embodiment illustrated in  FIG. 21  and  FIG. 22 , the ejector  150  can be bypassed during the second operation by using the on-off valve  164 , which is a fifth valve, and the flow rate control valve  144 , which is a fourth valve. As illustrated in  FIG. 21 , in the first operation, in the air conditioner  1  according to the fifth embodiment, the flow rate control valve  144  is opened and the on-off valve  164  is closed. As a result, refrigerant can be allowed to appropriately flow through the ejector  150 . As illustrated in  FIG. 22 , in the second operation, in the air conditioner  1  according to the fourth embodiment, the flow rate control valve  144  is closed and the on-off valve  164  is opened such that no refrigerant flows through the ejector  150 . As a result, in the second operation, the first heat-source-side heat exchanger  131  and the second heat-source-side heat exchanger  132  functioning as radiators, and the use-side heat exchanger  133  functioning as an evaporator can perform cooling, for example. 
     The air conditioner  1  according to the fifth embodiment can allow refrigerant to flow through the first heat-source-side heat exchanger  131  and the second heat-source-side heat exchanger  132  in series during the second operation. 
     (25-4) 
     In the air conditioner  1  according to the sixth embodiment illustrated in  FIG. 24  and  FIG. 26 , the ejector  150  can be bypassed during the second operation by using the check valve  171 , which is a sixth valve, the check valve  172 , which is a seventh valve, the on-off valve  173 , which is an eighth valve, the on-off valve  174 , which is a ninth valve, and the flow rate control valve  145 , which is a tenth valve. As illustrated in  FIG. 24 , in the first operation, in the air conditioner  1  according to the sixth embodiment, the on-off valve  173  and the flow rate control valve  145  are opened and the on-off valve  174  is closed. As a result, refrigerant can be allowed to appropriately flow through the ejector  150 . As illustrated in  FIG. 26 , in the second operation, in the air conditioner  1  according to the sixth embodiment, the on-off valve  173  and the flow rate control valve  145  are closed and the on-off valve  174  is opened such that no refrigerant flows through the ejector  150 . As a result, in the second operation, the first heat-source-side heat exchanger  131  and the second heat-source-side heat exchanger  132  functioning as radiators, and the use-side heat exchanger  133  functioning as an evaporator can perform cooling, for example. 
     The air conditioner  1  according to the sixth embodiment can allow the second heat-source-side heat exchanger  132  to function as an intercooler during the second operation. 
     (25-5) 
     In the air conditioner  1  according to the seventh embodiment illustrated in  FIG. 28  and  FIG. 30 , the ejector  150  can be bypassed during the second operation by using the check valve  181 , which is an eleventh valve, the check valve  182 , which is a twelfth valve, the on-off valve  183 , which is a thirteenth valve, and the check valve  184 , which is a fourteenth valve. As illustrated in  FIG. 28 , in the first operation, in the air conditioner  1  according to the seventh embodiment, the on-off valve  183  is opened. As a result, refrigerant can be allowed to appropriately flow through the ejector  150 . As illustrated in  FIG. 30 , in the second operation, in the air conditioner  1  according to the sixth embodiment, the on-off valve  183  is closed such that no refrigerant flows through the ejector  150 . As a result, in the second operation, the first heat-source-side heat exchanger  131  and the second heat-source-side heat exchanger  132  functioning as radiators, and the use-side heat exchanger  133  functioning as an evaporator can perform cooling, for example. 
     The air conditioner  1  according to the seventh embodiment can allow the first heat-source-side heat exchanger  131  to function as an intercooler during the second operation. 
     (25-6) 
     The compression mechanism  110  of the air conditioner  1  according to the sixth embodiment or the seventh embodiment is configured such that the compressor  112 , which is a first compression element, and the compressor  113 , which is a second compression element, perform multi-stage compression. The pressure of the refrigerant is raised to a high pressure by such multi-stage compression of the compression mechanism  110 , which can bring the ejector  150  into efficient operation. 
     (25-7) 
     The air conditioner  1  according to modification I described with reference to  FIG. 32  can increase the efficiency of cooling operation by using the economizer circuit  210 . 
     While embodiments of the present disclosure have been described, it will be understood that forms and details can be changed in various ways without departing from the spirit and scope of the present disclosure as recited in the claims. 
     EXPLANATION OF REFERENCES 
       1  air conditioner 
       10  compression mechanism 
       11  compressor (example of compression mechanism) 
       12  compressor (example of compression mechanism, first compression element) 
       13  compressor (example of compression mechanism, second compression element) 
       20  switching mechanism 
       21  four-way valve (example of switching mechanism) 
       31  heat-source-side heat exchanger (example of first heat-source-side heat exchanger) 
       32 ,  133  use-side heat exchanger 
       34  intercooler 
       41  first expansion valve (example of expansion mechanism) 
       42  second expansion valve 
       43  flow rate control valve (example of second valve) 
       50  ejector 
       61  on-off valve (example of first valve) 
       63  check valve (example of third valve) 
       64  on-off valve (example of fourth valve) 
       65  check valve (example of fifth valve) 
       66  on-off valve (example of sixth valve) 
       67  check valve (example of seventh valve) 
       68  check valve (example of eighth valve) 
       70  economizer circuit 
       92  gas-liquid separator 
       93  accumulator 
       110  compression mechanism 
       111  compressor 
       112  compressor (example of first compression element) 
       113  compressor (example of second compression element) 
       120  switching mechanism 
       122  four-way valve (example of first four-way valve) 
       123  four-way valve (example of second four-way valve) 
       131  first heat-source-side heat exchanger 
       132  second heat-source-side heat exchanger 
       140  expansion mechanism 
       141  first expansion valve (example of expansion mechanism) 
       144  flow rate control valve (example of fourth valve) 
       145  flow rate control valve (example of tenth valve) 
       146  flow rate control valve (example of fifteenth valve) 
       150  ejector 
       161  on-off valve (example of first valve) 
       162  on-off valve (example of second valve) 
       163  check valve (example of third valve) 
       164  on-off valve (example of fifth valve) 
       171  check valve (example of sixth valve) 
       172  check valve (example of seventh valve) 
       173  on-off valve (example of eighth valve) 
       174  on-off valve (example of ninth valve) 
       181  check valve (example of eleventh valve) 
       182  check valve (example of twelfth valve) 
       183  on-off valve (example of thirteenth valve) 
       184  check valve (example of fourteenth valve) 
     CITATION LIST 
     Patent Literature 
     [PTL 1]Japanese Patent No. 4069656