Refrigeration device

Abstract

The present disclosure provides a refrigeration device capable of enhancing efficiency.　The refrigeration device according to the present disclosure comprises a refrigerant circuit in which a compression mechanism, a heat source side heat exchanger, a gas-liquid separator, and a user side heat exchanger are connected. The refrigeration device has: a liquid pump that sends liquid refrigerant from the gas-liquid separator to a cooling side heat exchanger from among the heat source side heat exchanger and the user side heat exchanger; a check valve that is provided in parallel with the liquid pump and prevents backflow of the refrigerant toward the gas-liquid separator; and an expansion mechanism that expands the refrigerant that has flowed through a gas cooler from among the heat source side heat exchanger and the user side heat exchanger and sends it to the gas-liquid separator. The compression mechanism is provided in a gas vent pipe into which gas refrigerant from the gas-liquid separator flows, and operates coaxially with the expansion mechanism.

Description

Refrigeration device 【0001】 The present disclosure relates to a refrigeration device. 【0002】 Patent Document 1 discloses a refrigeration device that uses carbon dioxide as a refrigerant and enables high-efficiency operation. This refrigeration device includes a decompression tank provided on the downstream side of the gas cooler, an auxiliary circuit that sucks the refrigerant in the decompression tank into the intermediate pressure part of the throttle compression means, and a main circuit that heat-exchanges the refrigerant flowing out of the decompression tank with the refrigerant throttled in the auxiliary circuit and flows it to the main throttle means. Patent Document 2 discloses a refrigeration cycle device that aims to improve efficiency by using an ejector. This refrigeration cycle device includes an ejector into which the refrigerant discharged from the compressor and passing through the outdoor heat exchanger flows and discharges the refrigerant to the gas-liquid separator, and an internal heat exchanger that cools the refrigerant flowing from the gas-liquid separator to the indoor heat exchanger by a part of the liquid refrigerant in the gas-liquid separator sucked by the ejector. 【0003】 Patent Document 3 discloses a refrigeration cycle device that improves efficiency by an expansion mechanism and a sub-refrigerant circuit independent of the main refrigerant circuit. This refrigeration cycle device includes a main expansion mechanism that expands the refrigerant flowing toward the main utilization-side heat exchanger and recovers power, and a sub-utilization-side heat exchanger that further cools the refrigerant after power recovery and then flows it to the utilization-side heat exchanger. Patent Document 4 discloses a refrigeration device that uses an ejector and enables the refrigerant to easily circulate through the evaporator while maintaining the efficiency of the refrigerant circuit. This refrigeration device includes a bypass passage branched from a pipe between the downstream side of the radiator and the inlet of the ejector, and an adjustment mechanism that adjusts the amount of refrigerant in the bypass passage. 【0004】 Japanese Patent Publication No. 6292480 【0005】 Japanese Patent Publication No. 5213986 【0006】 Japanese Patent Publication No. 7193706 【0007】 Japanese Patent Publication No. 5786481 【0008】 The present disclosure provides a refrigeration device capable of achieving high efficiency. 【0009】This specification includes all the contents of Japanese Patent Application No. 2024-216168, filed on December 11, 2024. A refrigeration apparatus according to a first aspect of this disclosure comprises a refrigerant circuit connecting a compression mechanism, a heat source side heat exchanger, a gas-liquid separator, and a utilization side heat exchanger, and includes a liquid pump that sends the liquid refrigerant from the gas-liquid separator to the cooling side heat exchanger of the heat source side heat exchanger and the utilization side heat exchanger, a check valve provided in parallel with the liquid pump to prevent backflow of refrigerant toward the gas-liquid separator, and an expansion mechanism that expands the refrigerant that has flowed through the gas cooler of the heat source side heat exchanger and the utilization side heat exchanger and flows it toward the gas-liquid separator, wherein the compression mechanism is provided in a gas venting pipe into which the gas refrigerant from the gas-liquid separator flows and operates coaxially with the expansion mechanism. This specification contains all the contents of Japanese Patent Application No. 2024-220985, filed on December 17, 2024. A refrigeration system according to a second aspect of this disclosure comprises a refrigeration circuit connecting a high-stage compressor, a low-stage compressor, a heat source side heat exchanger, a gas-liquid separator, and a utilization side heat exchanger, and includes a liquid pump that sends the liquid refrigerant from the gas-liquid separator to the cooling side heat exchanger among the heat source side heat exchanger and the utilization side heat exchanger, a check valve provided in parallel with the liquid pump to prevent backflow of refrigerant toward the gas-liquid separator, a gas venting pipe that removes gaseous refrigerant from the gas-liquid separator, and a control unit, wherein the gas venting pipe is connected to the high-stage compressor via a high-stage side valve and to the low-stage compressor via a low-stage side valve, and the control unit determines a target compression ratio based on the ambient temperature, and based on the target compression ratio, switches between a two-stage compression operation that drives the high-stage compressor and the low-stage compressor by opening and closing the low-stage side valve and stopping the low-stage compressor and driving the high-stage compressor. 【0010】The refrigeration system according to the first aspect of this disclosure can reduce the pressure of the refrigerant entering the gas-liquid separator while recovering power using an expansion mechanism, and efficiently utilize the recovered power to operate the compression mechanism. Therefore, energy consumption can be suppressed and the efficiency of the refrigeration system can be improved. The refrigeration system according to the second aspect of this disclosure can easily maintain an appropriate compression ratio and easily suppress the starting and stopping of the compressor by switching between single-stage compression operation and two-stage compression operation based on the target compression ratio. Therefore, the efficiency of the refrigeration system can be improved. 【0011】 Figure 1 shows the refrigerant circuit of the refrigeration system according to Embodiment 1. Figure 2 shows the refrigerant circuit during medium-load cooling operation. Figure 3 shows the refrigerant circuit during air-cooled cooling operation. Figure 4 shows the refrigerant circuit during high-load cooling operation. Figure 5 shows the refrigerant circuit during heating operation. Figure 6 shows the refrigerant circuit of the refrigeration system according to Embodiment 2. Figure 7 is a flowchart of the refrigeration system. Figure 8 shows the refrigerant circuit of the refrigeration system according to Embodiment 3. Figure 9 shows the refrigerant circuit during low-load cooling operation. Figure 10 shows the refrigerant circuit during air-cooled cooling operation. Figure 11 shows the refrigerant circuit during heating operation. Figure 12 shows the refrigeration circuit of the refrigeration system according to Embodiment 4. Figure 13 shows the medium-load Figure 14 shows the refrigeration circuit during cooling operation, Figure 15 shows the refrigeration circuit during air-cooled cooling operation, Figure 16 shows the refrigeration circuit during high-load cooling operation, Figure 17 shows the refrigeration circuit during heating operation, Figure 18 shows the refrigeration circuit of the refrigeration system according to Embodiment 5, Figure 19 shows the refrigeration circuit during medium-load cooling operation, Figure 20 shows the refrigeration circuit during high-load cooling operation, Figure 21 shows the refrigeration circuit during heating operation, Figure 22 shows the refrigeration circuit during air-cooled cooling operation, Figure 23 shows the refrigeration circuit of the refrigeration system according to Embodiment 6, Figure 24 is a flowchart showing the operation of the refrigeration system, Figure 25 is a flowchart showing the operation of the refrigeration system. 【0012】(Knowledge and other information forming the basis of this disclosure) At the time the inventors conceived this disclosure, there was a technology in the field of refrigeration equipment that used carbon dioxide, a type of natural refrigerant, as a refrigerant. Carbon dioxide has a low global warming potential, is non-flammable and non-toxic, and is expected to be used more widely as a refrigerant with a small environmental impact. Although carbon dioxide has been considered to have efficiency issues because its critical temperature is within the range of operating temperatures, in low-temperature equipment of about -45°C to -5°C, the high intake density of the compressor can be utilized, and refrigeration equipment using carbon dioxide as a refrigerant has been put into practical use with performance close to that of equipment using fluorocarbons as a refrigerant. However, when using carbon dioxide as a refrigerant, it is difficult to utilize the high intake density, especially in equipment in the air conditioning temperature range, making it difficult to achieve high efficiency, and the inventors discovered that further technological development for higher efficiency is required, and in order to solve this problem, they came to form the subject of this disclosure. Therefore, this disclosure provides a refrigeration equipment that can achieve high efficiency. 【0013】 The embodiments will be described in detail below with reference to the drawings. However, unnecessary details may be omitted. For example, detailed explanations of already well-known matters or redundant explanations of substantially identical configurations may be omitted. This is to avoid the following explanation becoming unnecessarily verbose and to facilitate understanding for those skilled in the art. The accompanying drawings and the following explanation are provided to enable those skilled in the art to fully understand this disclosure and are not intended to limit the subject matter described in the claims. 【0014】 (Embodiment 1) Embodiment 1 will be described below with reference to the drawings. [1-1. Configuration] [1-1-1. Overall Configuration] Figure 1 is a diagram showing the refrigerant circuit 2 of the refrigeration device 1 according to Embodiment 1. In the figure, the on-off valve and throttle valve in the open state are shown in white, and the on-off valve and throttle valve in the closed state are shown in white. Also, in the figure, the wiring through which the refrigerant flows is shown with thick lines, and the piping through which the refrigerant does not flow is shown with thin lines. In this specification, the refrigerant circuit may be referred to as the refrigeration circuit. 【0015】The refrigeration system 1 is a device having a refrigerant circuit 2 that transfers heat through a refrigeration cycle. The refrigeration system 1 in this embodiment is an air conditioner installed in buildings such as commercial buildings, office buildings, and hotels. The refrigeration system 1 has an outdoor unit 10 and an indoor unit 20. The refrigerant circuit 2 is formed as a circuit through which the refrigerant circulates when the outdoor unit 10 and the indoor unit 20 are connected. In this embodiment, carbon dioxide (R744), a natural refrigerant that is non-flammable, non-toxic, and has little environmental impact, is used as the refrigerant in the refrigerant circuit 2. 【0016】 The outdoor unit 10 is a device that is mainly installed outdoors. In this embodiment, the outdoor unit 10 is installed on the roof of a building. The outdoor unit 10 has a heat source side heat exchanger 14 that exchanges heat between the internal refrigerant and the outside air. 【0017】 The indoor unit 20 is a device installed in a conditioned space, such as the interior of a building. The number of indoor units 20 installed in the refrigeration system 1 is not particularly limited as long as there is one or more, but Figure 1 shows one indoor unit 20 installed on the upper floor and one on the lower floor of the building. Hereafter, when distinguishing between the indoor units 20, the indoor unit 20 installed on the upper floor will be called indoor unit 20H, and the indoor unit 20 installed on the lower floor will be called indoor unit 20L. In other words, in this embodiment, the outdoor unit 10 is located at a higher position than the indoor unit 20H, and the indoor unit 20H is located at a higher position than the indoor unit 20L. 【0018】 Each indoor unit 20 has a user-side heat exchanger 21 and a user-side throttle valve 22. The user-side heat exchanger 21 is a heat exchanger that exchanges heat between the refrigerant inside and the air in the air-conditioned space. The user-side throttle valve 22 is a valve that adjusts the flow rate of the refrigerant flowing into the user-side heat exchanger 21. The indoor unit 20 conditioned the air in the air-conditioned space by heating or cooling the air in the air-conditioned space using the user-side heat exchanger 21. In this embodiment, of the components of the refrigerant circuit 2, all equipment and devices other than the user-side heat exchanger 21, the user-side throttle valve 22, and the piping connecting the outdoor unit 10 and the indoor unit 20 are installed in the outdoor unit 10. 【0019】[1-1-2. Refrigerant Circuit Configuration] The refrigerant circuit 2 includes a low-stage compressor 11 and a high-stage compressor 12. The low-stage compressor 11 and the high-stage compressor 12 correspond to the "compressors" in this disclosure. The low-stage compressor 11 and the high-stage compressor 12 are connected in series to enable two-stage compression. The low-stage compressor 11 compresses the refrigerant and discharges it to the suction side of the high-stage compressor 12. The high-stage compressor 12 compresses the refrigerant on the suction side and discharges it toward the oil separator. The oil separator 13 returns the oil in the refrigerant to each of the compressors 11 and 12. The discharge side of the high-stage compressor 12 is connected to the first switching mechanism 51 via the oil separator 13. 【0020】 The first switching mechanism 51 is a mechanism for switching the flow path of the refrigerant. The first switching mechanism 51 has a first cooling valve 53, a first heating valve 54, a second cooling valve 55, and a heating throttle valve 56, which are connected in an annular manner in order. 【0021】 The discharge side of the high-stage compressor 12 described above is connected via an oil separator 13 between the first cooling valve 53 and the first heating valve 54 of the first switching mechanism 51. In the first switching mechanism 51, the high-temperature side of the heat source side heat exchanger 14 is connected between the first cooling valve 53 and the second heating valve 56. The high-temperature side of the user side heat exchanger 21 is connected between the first heating valve 54 and the second cooling valve 55. 【0022】 The first switching mechanism 51 switches the destination of the refrigerant discharged from the high-stage compressor 12 between the heat source side heat exchanger 14 and the utilization side heat exchanger 21 by opening either the first cooling valve 53 or the first heating valve 54. Of the heat exchangers 14 and 21, the one designated as the destination for the refrigerant discharged from the high-stage compressor 12 functions as a gas cooler. In this specification, a gas cooler means a heat exchanger that dissipates heat to the gaseous refrigerant inside. That is, during cooling operation of the refrigeration system 1, the heat source side heat exchanger 14 becomes a gas cooler, and during heating operation, the utilization side heat exchanger 21 becomes a gas cooler. 【0023】The low-temperature side of the heat source side heat exchanger 14 is connected to the second switching mechanism 52. The low-temperature side of the utilization side heat exchanger 21 is connected to the second switching mechanism 52 via the utilization side throttle valve 22. In other words, regardless of whether the heat exchanger 14 or 21 functions as a gas cooler, the refrigerant that has passed through the gas cooler flows into the second switching mechanism 52. 【0024】 The second switching mechanism 52 is a mechanism for switching the flow path of the refrigerant. The second switching mechanism 52 has a third cooling valve 57, a third heating valve 58, a fourth cooling valve 59, and a heating throttle valve 60, which are connected in an annular manner in order. 【0025】 The low-temperature side of the heat source side heat exchanger 14 is connected between the third cooling valve 57 and the heating throttle valve 60 in the second switching mechanism 52. The low-temperature side of the user side heat exchanger 21 is connected between the third heating valve 58 and the fourth cooling valve 59 via the user side throttle valve 22. In addition, in the second switching mechanism 52, the inlet of the gas-liquid separator 17 is connected between the third cooling valve 57 and the third heating valve 58 via the high-pressure receiver 15 and the throttle valve 16. 【0026】 The second switching mechanism 52 opens either the third cooling valve 57 or the third heating valve 58, whichever is located between the gas cooler and the high-pressure receiver 15. This allows the second switching mechanism 52 to allow the refrigerant that has passed through the gas cooler to flow into the gas-liquid separator 17, regardless of whether the heat exchanger 14 or 21 functions as the gas cooler. In this embodiment, the second switching mechanism 52 also has check valves 61 and 62 to prevent backflow of refrigerant from the high-pressure receiver 15 towards the gas cooler. Check valve 61 is provided between the high-pressure receiver 15 and the third cooling valve 57. Check valve 62 is provided between the high-pressure receiver 15 and the third heating valve. 【0027】 Whether the second switching mechanism 52 opens the third cooling valve 57 or the third heating valve 58 depends on which of the heat exchangers 14 and 21 the first switching mechanism 51 designates as the gas cooler. In other words, the first switching mechanism 51 and the second switching mechanism 52 are controlled to operate in conjunction. Hereafter, the first switching mechanism 51 and the second switching mechanism 52 together will be referred to as the flow path switching mechanism 50. 【0028】As described above, a high-pressure receiver 15 is provided between the second switching mechanism 52 and the inlet of the gas-liquid separator 17. The high-pressure receiver 15 is a so-called receiver tank. The high-pressure receiver 15 temporarily stores the refrigerant cooled by the gas cooler and flows the liquid refrigerant from the stored refrigerant to the throttle valve 16. 【0029】 The throttle valve 16 is installed upstream of the inlet of the gas-liquid separator 17. The throttle valve 16 depressurizes the liquid refrigerant supplied from the high-pressure receiver 15, creating a gas-liquid mixture before allowing it to flow into the gas-liquid separator 17. 【0030】 The gas-liquid separator 17 is a device that separates the incoming refrigerant into gaseous refrigerant and liquid refrigerant. The gas-liquid separator 17 flows the separated gaseous refrigerant into the gas venting pipe 30. The gas venting pipe 30 is a pipe that connects the gas-liquid separator 17 to the suction side of each compressor 11 and 12. The gas venting pipe 30 has a bifurcated low-stage branch pipe 31 and a high-stage branch pipe 32. The low-stage branch pipe 31 is connected to the suction side of the low-stage compressor 11 via a low-stage throttle valve 33. The high-stage branch pipe 32 is connected to the suction side of the high-stage compressor 12 via a high-stage throttle valve 34. 【0031】 Furthermore, a liquid pump 40 is provided at the liquid-side outlet of the gas-liquid separator 17. The liquid pump 40 is driven to send the liquid refrigerant from the gas-liquid separator 17 toward the second switching mechanism 52. The liquid pump 40 is, for example, a centrifugal pump, an axial flow pump, or a mixed flow pump. 【0032】 A main pipe 41 is connected to the downstream side of the liquid pump 40. The main pipe 41 is the pipe that connects the outlet of the liquid pump 40 to the second switching mechanism 52. In other words, the main pipe 41 carries the liquid refrigerant from the gas-liquid separator 17, which has been supplied to the liquid pump 40, to the second switching mechanism 52. 【0033】 Furthermore, the refrigerant circuit 2 is provided with a path 42 in parallel with the liquid pump 40. Path 42 is a pipe that allows the liquid refrigerant from the gas-liquid separator 17 to flow to the main pipe 41 downstream of the liquid pump 40, without passing through the liquid pump 40. Path 42 connects the liquid outlet of the gas-liquid separator 17, which is different from the liquid outlet to which the liquid pump is connected, to the main pipe 41. 【0034】A check valve 43 is provided in the path 42 to prevent backflow of refrigerant toward the gas-liquid separator 17. That is, the check valve 43 is provided in parallel with the liquid pump 40. 【0035】 Cooling pipes 44 branch off from the main pipe 41. Cooling pipes 44 connect the main pipe 41 to the vent pipe 30. More specifically, cooling pipes 44 branch off from the main pipe 41 upstream of the point where the check valve 43 merges with the main pipe 41. Cooling pipes 44 are equipped with a cooling throttle valve 45 for reducing the pressure of the refrigerant. 【0036】 Downstream of the liquid pump 40, a subcooling heat exchanger 46 is provided to exchange heat between the liquid refrigerant in the main pipe 41 and the refrigerant in the cooling pipe 44 after it has passed through the cooling throttle valve 45. Since the refrigerant in the cooling pipe 44 is cooled after passing through the cooling throttle valve 45, in the subcooling heat exchanger 46, the refrigerant in the main pipe 41 is cooled by the refrigerant in the cooling pipe 44. More specifically, the subcooling heat exchanger 46 cools the refrigerant in the main pipe 41 downstream of the junction with the path 42. In other words, the subcooling heat exchanger 46 can cool both the refrigerant supplied to the liquid pump 40 and the refrigerant that has passed through the path 42. 【0037】 Furthermore, the refrigerant downstream of the subcooled heat exchanger 46 in the main piping 41 is cooled by an external cooling device 47. The external cooling device 47 cools the refrigerant in the main piping 41 using a refrigeration cycle that uses refrigerant that has been disconnected from the refrigerant in the refrigerant circuit 2. 【0038】 The external cooling equipment 47 only needs to have the minimum equipment necessary to establish the refrigeration cycle and cool the refrigerant in the main piping 41. Therefore, the configuration of the external cooling equipment 47 can be simpler than that of the refrigerant circuit 2, and refrigerant leakage from the external cooling equipment 47 is less likely to occur. In addition, since the external cooling equipment 47 can be easily configured to operate with a smaller amount of refrigerant than the refrigerant circuit 2, even if refrigerant leaks from the external cooling equipment 47, the amount of leakage can be easily reduced. In other words, the external cooling equipment 47 can be easily configured to have a low risk of refrigerant leakage. 【0039】Therefore, the refrigeration cycle of the external cooling device 47 can use a refrigerant that is more flammable, toxic, and has a greater environmental impact in the event of leakage than carbon dioxide, but is more energy-efficient than carbon dioxide. In this embodiment, the external cooling device 47 can properly manage leakage even when using a refrigerant that is more energy-efficient in the refrigeration cycle than carbon dioxide, such as an HFC (HydrofluoroCarbon) refrigerant, an HFO (HydrofluoroOlefin) refrigerant, or a mixed refrigerant containing these. 【0040】 The main piping 41 is connected downstream of the external cooling equipment 47, between the fourth cooling valve 59 and the heating throttle valve 60 of the second switching mechanism 52. 【0041】 The second switching mechanism 52 switches the destination of the liquid refrigerant inlet of the main piping 41 between the heat source side heat exchanger 14 and the utilization side heat exchanger 21 by opening either the fourth cooling valve 59 or the heating throttle valve 60. At this time, the heat exchanger designated as the destination of the liquid refrigerant inlet of the main piping 41 functions as the cooling side heat exchanger. In this specification, the cooling side heat exchanger means a heat exchanger that causes the refrigerant inside to absorb heat. That is, during cooling operation of the refrigeration system 1, the utilization side heat exchanger 21 becomes the cooling side heat exchanger, and during heating operation, the heat source side heat exchanger 14 becomes the cooling side heat exchanger. In this embodiment, the second switching mechanism 52 has a check valve 63 that prevents backflow from the utilization side throttle valve 22 toward the main piping 41. The check valve 63 is provided in the second switching mechanism 52 between the fourth cooling valve 59 and the utilization side throttle valve 22. 【0042】 As described above, the high-temperature sides of the heat exchangers 14 and 21 are connected to the first switching mechanism 51, so the refrigerant that has passed through the cooling-side heat exchanger flows into the first switching mechanism 51. In the first switching mechanism 51, the suction-side piping 18, which is connected to the suction side of the low-stage compressor 11, is connected between the second cooling valve 55 and the second heating valve 56. The return piping 70 is connected between the second cooling valve 55 and the second heating valve 56. The return piping 70 is connected to the first switching mechanism 51 in parallel with the suction-side piping 18. 【0043】The first switching mechanism 51 opens one of the second cooling valve 55 and the second heating valve 56, which is located between the cooling-side heat exchanger and the suction-side pipe 18. Therefore, the first switching mechanism 51 can flow the refrigerant of the cooling-side heat exchanger into the suction-side pipe 18 and the return pipe 70 regardless of whether the heat exchangers 14 and 21 function as the cooling-side heat exchanger. 【0044】 A suction-side opening / closing valve 19 is provided in the suction-side pipe 18. Specifically, the suction-side opening / closing valve 19 is located upstream of the confluence portion of the low-stage side branch pipe 31 and the suction-side pipe 18 in the suction-side pipe 18. 【0045】 The return pipe 70 is a pipe that returns the refrigerant from the first switching mechanism 51 to the gas-liquid separator 17. A return-side opening / closing valve 71 is provided in the return pipe 70. Further, a check valve 72 for preventing the backflow of the refrigerant from the gas-liquid separator 17 side toward the first switching mechanism 51 is provided in the return pipe 70. The suction-side opening / closing valve 19 and the return-side opening / closing valve 71 correspond to the "opening / closing valve" in the present disclosure. 【0046】 An oil separator 73 is provided in the return pipe 70. The oil separator 73 separates the oil mixed in the refrigerant of the return pipe 70 and returns it to the suction side of each compressor 11 and 12. In the present embodiment, the oil separator 73 returns the oil in the return pipe 70 to the gas venting pipe 30 located on the suction side of each compressor 11. 【0047】 Further, a heat exchanger 74 is provided in the return pipe 70. The heat exchanger 74 is a heat exchanger that cools the refrigerant in the return pipe 70 by the outside air. That is, the heat exchanger 74 can liquefy the gas component evaporated in the cooling-side heat exchanger among the refrigerant in the return pipe 70. 【0048】 The refrigeration device 1 has a blower device 74a that blows the outside air to the heat exchanger 74. Further, the refrigeration device  1 has a water supply means 74b that supplies water for lowering the temperature of the suction air of the heat exchanger 74 by the latent heat of evaporation. Therefore, the heat exchanger 74 can cool the refrigerant in the return pipe 70 even when the outside air temperature is about 5°C higher than the refrigerant in the return pipe 70. 【0049】Similarly, the refrigeration system 1 has a blower 14a that blows outside air to the heat source side heat exchanger 14. The refrigeration system 1 also has a water supply means 14b that supplies water to lower the temperature of the intake air to the heat source side heat exchanger 14 by latent heat of vaporization. The water supply means 14b supplies water to lower the temperature of the intake air to the heat source side heat exchanger 14 by latent heat of vaporization when the heat source side heat exchanger 14 functions as a gas cooler. As a result, the efficiency of heat exchange when the heat source side heat exchanger 14 functions as a gas cooler is improved. 【0050】 The water supply means 14b and 74b may be, for example, devices that directly spray water onto the heat exchangers 14 and 74. Alternatively, the water supply means 14b and 74b may be so-called indirect water-spraying devices that supply water to a permeable filter provided on the outside air intake side of the heat exchangers 14 and 74, thereby lowering the temperature of the intake air of the heat exchangers 14 and 74 due to the latent heat of vaporization of the water adhering to the filter. Furthermore, the water supply means 14b and 74b may be other devices, mechanisms, etc. that can lower the temperature of the intake air of the heat exchangers 14 and 74 by utilizing the latent heat of vaporization of water. 【0051】 [1-1-3. Configuration of the Control Device] As shown in Figure 1, the refrigeration device 1 is provided with a control unit 90. The control unit 90 is a device that controls each part of the refrigeration device 1. The control unit 90 has a processor such as a CPU (Central Processing Unit) and an MPU (Micro-Processing Unit), and a storage medium such as a hard disk, flash memory, and optical disc. The control unit 90 controls each part of the refrigeration device 1 by having the processor read a program from the storage medium and executing the program using the processor. Alternatively, the control unit 90 may have wired logic such as an ASIC (Application Specific Integrated Circuit) instead of a processor and storage medium. Alternatively, the control unit 90 may have a combination of a processor, storage medium, and wired logic. 【0052】Furthermore, the control unit 90 is equipped with communication hardware such as connectors and communication circuits that conform to wireless or wired communication standards. The control unit 90 communicates with each part of the refrigeration system 1 via this communication hardware. 【0053】 The control unit 90 individually controls the on / off state and rotational speed of each compressor 11, 12 and the liquid pump 40. The control unit 90 switches the open and closed states of each valve 19, 53-60, 71, which are on / off valves. The control unit 90 also switches the open and closed states and adjusts the opening degree of each throttle valve 16, 22, 33, 34, 45, 60, which are throttle valves with adjustable opening degree. The control unit 90 switches the on / off state and controls the airflow rate of each blower 14a, 74a. The control unit 90 controls the presence or absence of water supply and the amount of water supplied by the water supply means 14b, 74b. 【0054】 [1-2. Operation] The operation of the refrigeration system 1, configured as described above, will be explained below. The refrigeration system 1 is designed to easily improve the Annual Performance Factor (APF) by switching the refrigerant flow path according to the cooling load to increase energy efficiency. First, the operation during cooling when the cooling load is low or medium, that is, when the cooling load is medium or lower, will be explained. 【0055】 [1-2-1. Operation during cooling operation at medium load or lower] Figure 2 shows the refrigerant circuit 2 during cooling operation at medium load. Figure 1 shows the refrigerant circuit 2 during cooling operation at low load. As shown in Figures 1 and 2, during cooling operation at medium load or lower, the control unit 90 opens each of the cooling valves 53, 55, 57, and 59. The control unit 90 also closes each of the heating valves 54, 56, and 58 and the heating throttle valve 60. As a result, the heat source side heat exchanger 14 functions as a gas cooler, and the user side heat exchanger 21 functions as a cooling side heat exchanger. 【0056】Furthermore, the control unit 90 closes the intake-side on-off valve 19 and opens the return-side on-off valve 71. As a result, each compressor 11 and 12 no longer directly inhales the refrigerant that has passed through the utilization-side heat exchanger 21, which is the cooling-side heat exchanger, and instead inhales the gaseous refrigerant separated in the gas-liquid separator 17 through the gas venting pipe 30. 【0057】 In detail, when the cooling load is low, the control unit 90 opens the high-stage throttle valve 34 and closes the low-stage throttle valve 33, as shown in Figure 1, and then drives only the high-stage compressor 12 among the compressors 11 and 12. As a result, the gaseous refrigerant separated in the gas-liquid separator 17 is compressed in a single stage by the high-stage compressor 12 and discharged as high-temperature gaseous refrigerant. 【0058】 On the other hand, when the cooling load is at a moderate load, the control unit 90 closes the high-stage throttle valve 34 and opens the low-stage throttle valve 33, as shown in Figure 2, and then drives both compressors 11 and 12. As a result, the gaseous refrigerant separated in the gas-liquid separator 17 is compressed in two stages by the two compressors 11 and 12 and discharged as high-temperature gaseous refrigerant by the high-stage compressor 12. 【0059】 Regardless of whether the cooling load is low or medium, the refrigerant discharged by the high-stage compressor 12 flows into the first switching mechanism 51 via the oil separator 13, passes through the first cooling valve 53, and flows into the heat source side heat exchanger 14. 【0060】 In the heat source side heat exchanger 14, the high-temperature gaseous refrigerant is cooled by releasing heat into the outside air. In addition, the control unit 90 activates the water supply means 14b during cooling operation to promote the cooling of the refrigerant. After releasing heat in the heat source side heat exchanger 14, the refrigerant flows into the second switching mechanism 52 and then into the high-pressure receiver 15 via the third cooling valve 57. 【0061】 Of the refrigerant that flows into the high-pressure receiver 15, the liquid refrigerant is depressurized by the throttle valve 16 to become a medium-temperature gas-liquid mixture and flows into the gas-liquid separator 17. The refrigerant that flows into the gas-liquid separator 17 is separated into gaseous refrigerant and liquid refrigerant, and the separated gaseous refrigerant is drawn back into each compressor 11 and 12 via the venting pipe 30. 【0062】Thus, when the cooling load is moderate or lower, the gaseous refrigerant in the gas-liquid separator 17 is compressed by each compressor 11 and 12, dissipates heat in the heat source side heat exchanger 14, and returns to the gas-liquid separator 17 without passing through the utilization side heat exchanger 21, where it is separated into gaseous and liquid refrigerant. In other words, each compressor 11 and 12 performs the work of liquefying the gaseous refrigerant in the gas-liquid separator 17 and returning it to the gas-liquid separator 17. On the other hand, the compressors 11 and 12 do not perform the work of supplying the liquid refrigerant from the gas-liquid separator 17 to the utilization side heat exchanger 21. 【0063】 A liquid pump 40 is used to supply liquid refrigerant to the heat exchanger 21 on the user side. The control unit 90 drives the liquid pump 40 to supply the liquid refrigerant stored in the gas-liquid separator 17 to the heat exchanger 21 on the user side. When the liquid pump 40 is driven, the liquid refrigerant from the gas-liquid separator 17 flows into the main piping 41. A check valve 43 prevents backflow of the refrigerant supplied to the liquid pump 40 towards the gas-liquid separator 17. 【0064】 The control unit 90 uses the subcooling heat exchanger 46 and the external cooling equipment 47 to cool the refrigerant in the main piping 41 to a predetermined target temperature. In this case, the control unit 90 prioritizes using the external cooling equipment 47 over the subcooling heat exchanger 46 to cool the refrigerant in the main piping 41. That is, when the refrigerant in the main piping 41 can be cooled to the target temperature using only the external cooling equipment 47, the control unit 90 closes the cooling throttle valve 45 and does not perform cooling by the subcooling heat exchanger 46. Also, when the refrigerant in the main piping 41 cannot be subcooled to the target temperature using only the external cooling equipment 47, the control unit 90 opens the cooling throttle valve 45 and performs cooling by the subcooling heat exchanger 46. 【0065】 The liquid refrigerant that has passed through the main piping 41 flows into the second switching mechanism 52, passes through the fourth cooling valve 59 and the check valve 63, flows into each indoor unit 20, and flows into the utilization-side heat exchanger 21 through the utilization-side throttle valve 22. In the utilization-side heat exchanger 21, the liquid refrigerant absorbs heat from the air in the air-conditioned space, and a portion of it becomes gaseous refrigerant, which cools the air in the air-conditioned space. As a result, the air-conditioned space is cooled. 【0066】The refrigerant that has passed through the utilization-side heat exchanger 21 flows into the first switching mechanism 51, and via the second cooling valve 55, reaches the suction-side piping 18 and the return piping 70. As described above, when the cooling load is medium load or less, the suction-side on-off valve 19 is in the closed state and the return-side on-off valve 71 is in the open state, so the refrigerant that has passed through the first switching mechanism 51 is not drawn into the compressors 11 and 12, but returns to the gas-liquid separator 17 via the return piping 70. 【0067】 The refrigerant passing through the return pipe 70 is cooled by the heat exchanger 74. When the cooling load is medium or lower, the control unit 90 activates the water supply means 74b to facilitate the liquefaction of the refrigerant in the return pipe 70. The refrigerant that has passed through the heat exchanger 74 is returned to the gas-liquid separator 17. 【0068】 Thus, when the cooling load is moderate or lower, the liquid refrigerant in the gas-liquid separator 17 is sent by the liquid pump 40 to the user-side heat exchanger 21 to cool the conditioned space, and returns to the gas-liquid separator 17 through a closed cycle without passing through the compressors 11 and 12. In other words, the refrigerant circuit 2 is configured to independently perform the storage of liquid refrigerant in the gas-liquid separator 17 by driving the compressors 11 and 12, and the sending of the liquid refrigerant from the gas-liquid separator 17 to the user-side heat exchanger 21 by driving the liquid pump 40. 【0069】 If, due to insufficient cooling in the heat exchanger 74, refrigerant including gaseous refrigerant returns to the gas-liquid separator 17 from the return pipe 70, and the conditioned space is cooled by the operation of the liquid pump 40, the amount of gaseous refrigerant in the gas-liquid separator 17 increases and the amount of liquid refrigerant decreases. The control unit 90 drives the compressors 11 and 12 to compensate for the decrease in liquid refrigerant in the gas-liquid separator 17. 【0070】To achieve the above operation, when the cooling load is low, the control unit 90 controls each part of the refrigeration system 1 so that the pressure in each part of the refrigerant circuit 2 satisfies the following inequality (A): P1 < P5 ≤ P9 < P8 ≤ P0 < P7 (A) However, as shown in Figure 1, in inequality (A), P0 is the pressure on the outlet side of the liquid pump 40, P1 is the suction pressure of the high-stage compressor 12, P5 is the pressure inside the gas-liquid separator 17, P7 is the pressure of the piping on the inlet side of the indoor unit 20L on the lower floor, P8 is the pressure of the piping on the inlet side of the indoor unit 20H on the upper floor, and P9 is the pressure of the piping on the outlet side of the indoor units 20L and 20H. Note that P7 is the pressure of the piping located at the same height as the indoor unit 20L, and P8 is the pressure of the piping located at the same height as the indoor unit 20H. 【0071】 When the cooling load is moderate or lower, as described above, the liquid refrigerant in the gas-liquid separator 17 returns to the gas-liquid separator 17 through a closed cycle that bypasses the compressors 11 and 12, due to the operation of the liquid pump 40. In this closed cycle, even when the liquid pump 40 has stopped after it has operated, natural circulation of the refrigerant may occur between the gas-liquid separator 17 and the user-side heat exchanger 21. 【0072】 When natural circulation occurs, the liquid refrigerant in the gas-liquid separator 17 flows into the main piping 41 via path 42, rather than into the stopped liquid pump 40, which has high resistance. After flowing into the main piping 41, the liquid refrigerant flows into the utilization-side heat exchanger 21 and return piping 70 through a closed cycle similar to when the liquid pump 40 is running, and returns to the gas-liquid separator 17. By utilizing natural circulation, the control unit 90 can continue cooling operation simply by driving the liquid pump 40 when natural circulation is generated or when natural circulation is attenuated due to flow resistance, making it easier to improve energy efficiency. 【0073】In this embodiment, the refrigeration system 1 is installed under conditions that facilitate natural circulation. Specifically, since the outdoor unit 10 equipped with the gas-liquid separator 17 is located higher than the indoor units 20L and 20H, the difference in height between the gas-liquid separator 17 and the indoor units 20L and 20H makes it easy to circulate the refrigerant between the gas-liquid separator 17 and the indoor units 20L and 20H. Also, as described above, a portion of the liquid refrigerant evaporates due to heat absorption in the heat exchanger 21 on the utilization side. Therefore, the refrigerant with a lower specific gravity that has partially evaporated in the heat exchanger 21 on the utilization side is easily pushed back to the gas-liquid separator 17 by the liquid refrigerant with a higher specific gravity moving from the gas-liquid separator 17 towards the heat exchanger 21 on the utilization side of the indoor units 20L and 20H. 【0074】 Furthermore, since the liquid refrigerant in the gas-liquid separator 17 returns to the gas-liquid separator 17 through a closed cycle that does not pass through the compressors 11 and 12, the oil that flows into the gas-liquid separator 17 cannot return to the compressors 11 and 12 during the closed cycle. In contrast, in this embodiment, the oil that flows into the gas-liquid separator 17 is returned to the compressors 11 and 12 by an oil separator 73 that returns the oil in the refrigerant in the return pipe 70 to the gas vent pipe 30 outside the closed cycle. 【0075】 [1-2-2. Operation during air-cooled cooling operation] Figure 3 shows the refrigerant circuit 2 during air-cooled cooling operation. When the cooling load is medium load or lower, under conditions where the refrigerant evaporated in the user-side heat exchanger 21 is all returned to liquid refrigerant by the heat exchanger 74 of the return pipe 70, the amount of liquid refrigerant in the gas-liquid separator 17 does not decrease. In such cases, it is possible to perform air-cooled cooling operation by using only the heat exchanger 74 to liquefy the refrigerant without operating the compressors 11 and 12. For example, when the refrigeration system 1 is operated in a cooling operation to prevent the room temperature from rising due to exhaust heat from equipment installed in the air-conditioned space in an environment with a low outside temperature, air-cooled cooling operation is easy to perform. 【0076】When performing cooling operation using air cooling, the control unit 90 stops the compressors 11 and 12 and closes the throttle valve 16, the low-stage throttle valve 33, the high-stage throttle valve 34, and the cooling throttle valve 45 when the cooling operation state is at medium load or lower. As a result, the suction and discharge sides of the compressors 11 and 12 are no longer in communication with the gas-liquid separator 17. In addition, the refrigerant in the main piping 41 no longer passes through the cooling piping 44, and the refrigerant in the main piping 41 is not cooled by the subcooled heat exchanger 46. 【0077】 During air-cooled cooling operation, the control unit 90 drives the liquid pump 40 to return the liquid refrigerant from the gas-liquid separator 17 to the user-side heat exchanger 21 and return piping 70 via a closed cycle similar to when the cooling load is medium or lower. In addition, the control unit 90 stops the liquid pump 40 when natural circulation of the refrigerant occurs in the closed cycle, similar to when the cooling load is medium or lower, and drives the liquid pump 40 only when natural circulation occurs or when natural circulation has decreased. 【0078】 Thus, during air-cooled cooling operation, the compressors 11 and 12, which consume a particularly large amount of energy, are not driven, thus significantly reducing the energy required for cooling operation. The more frequently air-cooled cooling operation is possible throughout the year, the easier it is to improve the APF of the refrigeration system 1. 【0079】 [1-2-3. Operation during high-load cooling operation] Figure 4 shows the refrigerant circuit 2 during high-load cooling operation. In operation when the cooling load is medium or lower, as described above, the supercooled liquid refrigerant is flowed to the utilization-side heat exchanger 21, so when the ambient temperature is high and the cooling load is high, heat leakage tends to be large. Since such heat leakage occurs largely in the gas-liquid separator 17, in order to reduce heat leakage, the refrigerant temperature drop in the gas-liquid separator 17 is suppressed, and the liquid refrigerant is cooled by the supercooled heat exchanger 46 and external cooling equipment 47, thereby increasing supercooling and enhancing the refrigeration effect. Furthermore, when the cooling load is high, the refrigeration system 1 switches to superheat control where there is a temperature difference between the inlet and outlet of the utilization-side heat exchanger 21, which is the cooling-side heat exchanger, and cools the air in the conditioned space using the latent heat of vaporization. 【0080】The control unit 90 opens the cooling valves 53, 55, 57, and 59, and closes the heating valves 54, 56, and 58 and the heating throttle valve 60, as it does when the load is moderate or lower. However, unlike when the load is moderate or lower, the control unit 90 opens the intake side on-off valve 19 and closes the return side on-off valve 71. As a result, the refrigerant that has passed through the utilization side heat exchanger 21, which is the cooling side heat exchanger, does not return to the gas-liquid separator 17 via the return pipe 70, but is compressed in two stages by the compressors 11 and 12. 【0081】 The refrigerant compressed in two stages by compressors 11 and 12 flows into the gas-liquid separator 17 through the same path as in the case of medium load or less. The control unit 90 closes the low-stage throttle valve 33 and opens the high-stage throttle valve 34. As a result, the gaseous refrigerant separated in the gas-liquid separator 17 is drawn into the high-stage compressor 12 via the venting pipe 30, lowering the discharge refrigerant temperature of the high-stage compressor 12. 【0082】 When the cooling load is high, the suction-side shut-off valve 19 is open, so the main piping 41 and the path 42 communicate with the suction side of the compressors 11 and 12. For this reason, the control unit 90 stops the liquid pump 40 and allows the liquid refrigerant separated in the gas-liquid separator 17 to flow into the main piping 41 via the path 42 through circulation accompanying the operation of the compressors 11 and 12. The liquid refrigerant in the main piping 41 flows through the same path as in the case of medium load or lower, and flows into each indoor unit 20 while being cooled by the subcooled heat exchanger 46 and the external cooling equipment 47. 【0083】 The refrigerant flowing into each indoor unit 20 is depressurized at the utilization-side throttle valve 22, absorbs heat in the utilization-side heat exchanger 21, and evaporates. The refrigerant that has passed through the utilization-side heat exchanger 21 passes through the second cooling valve 55 and the suction-side on / off valve 19 of the first switching mechanism 51, through the suction-side piping 18, and is compressed in two stages by the compressors 11 and 12. The control unit 90 controls the opening of the utilization-side throttle valve 22 so that the refrigerant flowing into the utilization-side heat exchanger 21 reaches a specified degree of superheating, thereby preventing liquid compression in the compressors 11 and 12. Furthermore, if the cooling load is high, the refrigerant does not pass through the return piping 70, so the control unit 90 stops the water supply means 74b. 【0084】In other words, when the cooling load is high, the compressors 11 and 12 perform not only the work of liquefying the refrigerant and storing it in the gas-liquid separator 17, as in a normal two-stage compression cycle, but also the work of flowing the liquid refrigerant from the gas-liquid separator 17 to the heat exchanger 21 on the utilization side. 【0085】 [1-2-4. Operation during heating] Figure 5 shows the refrigerant circuit 2 during heating operation. The refrigeration system 1 is configured to perform heating operation in addition to cooling operation according to the cooling load. During heating operation, the control unit 90 opens the heating valves 54, 56, 58 and the heating throttle valve 60 of the flow path switching mechanism 50, and closes the cooling valves 53, 55, 57, 59. As a result, the heat source side heat exchanger 14 functions as a cooling side heat exchanger, and the utilization side heat exchanger 21 functions as a gas cooler. 【0086】 Furthermore, the control unit 90 opens the intake-side on-off valve 19 and closes the return-side on-off valve 71. As a result, the refrigerant that has passed through the heat source-side heat exchanger 14, which is the cooling-side heat exchanger, is compressed in two stages by the compressors 11 and 12 and flows into the first switching mechanism 51 via the oil separator 13. 【0087】 The refrigerant that flows into the first switching mechanism 51 flows into each indoor unit 20 via the first heating valve 54, and is cooled by releasing heat into the air of the heated space in the user-side heat exchanger 21. As a result, the heated space is heated. 【0088】 The refrigerant that has passed through the user-side heat exchanger 21 passes through the user-side throttle valve 22, the third heating valve 58 and check valve 62 of the second switching mechanism 52, and flows into the high-pressure receiver 15. Of the refrigerant that has flowed into the high-pressure receiver 15, the liquid refrigerant is depressurized by the throttle valve 16 to become a low-temperature gas-liquid mixture and flows into the gas-liquid separator 17. 【0089】 During heating operation, the control unit 90 closes the low-stage throttle valve 33 and opens the high-stage throttle valve 34. As a result, the gaseous refrigerant separated in the gas-liquid separator 17 is drawn into the high-stage compressor 12 via the gas vent pipe 30, bringing the pressure inside the gas-liquid separator 17 to a specified value and lowering the discharge refrigerant temperature of the high-stage compressor 12. 【0090】Furthermore, during heating operation, the intake side on-off valve 19 is open, so the main piping 41 and the path 42 communicate with the intake side of the compressors 11 and 12. For this reason, the control unit 90 stops the liquid pump 40 and allows the liquid refrigerant separated in the gas-liquid separator 17 to flow to the main piping 41 via the path 42 through circulation accompanying the operation of the compressors 11 and 12. In other words, during heating operation, just like in a normal two-stage compression cycle, the compressors 11 and 12 also perform the work of flowing the liquid refrigerant from the gas-liquid separator 17 to the heat source side heat exchanger 14. The control unit 90 also opens the cooling throttle valve 45 and cools the refrigerant in the main piping 41 with the subcooled heat exchanger 46. The control unit 90 also stops the external cooling equipment 47. 【0091】 The refrigerant that has passed through the main pipe 41 flows into the heat source side heat exchanger 14 through the heating throttle valve 60 of the second switching mechanism 52. The refrigerant absorbs heat and evaporates in the heat source side heat exchanger 14, passes through the second heating valve 56 and the suction side on / off valve 19 of the first switching mechanism 51, and then passes through the suction side pipe 18 and is compressed in two stages by the compressors 11 and 12. The control unit 90 stops the water supply means 14b during heating operation. The control unit 90 also controls the opening degree of the heating throttle valve 60 so that the refrigerant passing through the heat source side heat exchanger 14 reaches a specified degree of superheating, thereby preventing liquid compression in the compressors 11 and 12. 【0092】[1-3. Effects, etc.] As described above, in this embodiment, the refrigeration system 1 is equipped with a refrigerant circuit 2 connecting compressors 11 and 12, a heat source side heat exchanger 14, a gas-liquid separator 17, and a utilization side heat exchanger 21. A liquid pump 40 is provided to send the liquid refrigerant from the gas-liquid separator 17 to the cooling side heat exchanger among the heat source side heat exchanger 14 and the utilization side heat exchanger 21, and a path 42 is provided to allow the liquid refrigerant from the gas-liquid separator 17 to flow downstream of the liquid pump 40 without going through the liquid pump 40. As a result, the liquid refrigerant in the gas-liquid separator 17 can be sent to the cooling side heat exchanger without relying on the compressors 11 and 12, making it easier to reduce the amount of work that the compressors 11 and 12 have to do to compress the gaseous refrigerant. Furthermore, especially when the gas-liquid separator 17 is installed on the roof of a building and the cooling-side heat exchanger is installed on a lower floor of the building, if the gas-liquid separator 17 is located higher than the cooling-side heat exchanger, the liquid refrigerant can easily circulate between the gas-liquid separator 17 and the cooling-side heat exchanger via the path 42 even after the liquid pump 40 has stopped. This makes it easier to reduce energy consumption and improve the efficiency of the refrigeration system 1. 【0093】 As in this embodiment, in the refrigeration system 1, the compressors 11 and 12 discharge the gaseous refrigerant from the gas-liquid separator 17 to the gas cooler among the heat source side heat exchanger 14 and the utilization side heat exchanger 21, and return it to the gas-liquid separator 17 in a gas-liquid mixed state via the throttle valve 16. The liquid pump 40 sends the liquid refrigerant from the gas-liquid separator 17 to the cooling side heat exchanger, allows it to absorb heat, and then returns it to the gas-liquid separator 17. This configuration allows the operation of the compressors 11 and 12 to be separated from the supply of liquid refrigerant to the cooling side heat exchanger, and to be performed with the objective of liquefying the gaseous refrigerant and returning it to the gas-liquid separator 17, thereby reducing the workload of the compressors 11 and 12. As a result, energy consumption can be easily reduced, and the efficiency of the refrigeration system 1 can be improved. 【0094】 As in this embodiment, the refrigeration system 1 may be configured such that a check valve 43 is provided in the path 42 to prevent backflow of refrigerant toward the gas-liquid separator 17. This prevents the liquid refrigerant supplied to the liquid pump 40 from flowing back into the gas-liquid separator 17 via the path 42. As a result, the liquid pump 40 can more easily supply the liquid refrigerant to the gas-liquid separator 17, thereby improving the efficiency of the refrigeration system 1. 【0095】 As in this embodiment, the refrigeration system 1 may be configured to include a main pipe 41 provided downstream of the liquid pump 40, through which refrigerant flows toward the cooling-side heat exchanger, a cooling pipe 44 branched from the main pipe 41, a cooling throttle valve 45 that reduces the pressure of the refrigerant in the cooling pipe 44, and a subcooled heat exchanger 46 that cools the refrigerant in the main pipe 41 with the refrigerant reduced in pressure by the cooling throttle valve 45. This allows the liquid refrigerant to be cooled after passing through the gas-liquid separator 17, which has a large surface area, thereby reducing heat leakage of the liquid refrigerant. As a result, the refrigeration effect can be increased while suppressing losses, and the efficiency of the refrigeration system 1 can be improved. 【0096】 As in this embodiment, the refrigeration system 1 may be configured to include an external cooling device 47 that cools the refrigerant after it has passed through the subcooled heat exchanger 46 in the main piping 41. This allows for further cooling of the liquid refrigerant flowing into the cooling-side heat exchanger. As a result, it is possible to improve and stabilize the refrigeration capacity, thereby increasing the efficiency of the refrigeration system. 【0097】 As in this embodiment, the refrigerant circuit 2 may use carbon dioxide as the refrigerant, and the external cooling device 47 may use a refrigeration cycle that utilizes a refrigerant with higher energy efficiency than carbon dioxide to cool the refrigerant in the main piping 41. This configuration allows the refrigeration capacity of the refrigeration system 1, which uses carbon dioxide as the refrigerant with low environmental impact, to be improved by the highly energy-efficient external cooling device 47. Furthermore, since the external cooling device 47 can be made simpler in configuration than the refrigerant circuit 2 of the refrigeration system 1, even if a refrigerant such as HFC or HFO, which is highly efficient but has a greater environmental impact than carbon dioxide, is used in the external cooling device 47, the risk of refrigerant leakage from the external cooling device 47 is less likely to increase. Therefore, it is possible to improve the efficiency of the refrigeration system 1 while suppressing environmental impact. 【0098】As in this embodiment, the refrigeration system 1 may be configured to prioritize the use of an external cooling device 47 over a subcooled heat exchanger 46 to cool the refrigerant flowing to the cooling-side heat exchanger to the target temperature. This allows the external cooling device 47, which is easier to configure with high energy efficiency, to be used in priority over the subcooled heat exchanger 46. As a result, the efficiency of the refrigeration system 1 can be improved. 【0099】 As in this embodiment, the refrigeration system 1 may be configured to include a water supply means 14b that supplies water to lower the temperature of the intake air of the heat source side heat exchanger 14 by latent heat of vaporization. This makes it easier to improve the refrigeration capacity of the refrigeration system with low energy consumption. Therefore, it is possible to improve the efficiency of the refrigeration system. 【0100】 As in this embodiment, the compressor may be configured to include a high-stage compressor 12 and a low-stage compressor 11. This makes it easier to increase the efficiency of each compressor 11 and 12, especially when using carbon dioxide or the like as a refrigerant, which has a large pressure difference within the refrigerant circuit 2. As a result, the efficiency of the refrigeration system 1 can be increased. 【0101】As described above, in this embodiment, the refrigeration system 1 is equipped with a refrigerant circuit 2 connecting compressors 11 and 12, a heat source side heat exchanger 14, a gas-liquid separator 17, and a utilization side heat exchanger 21. It also has a liquid pump 40 that sends the liquid refrigerant from the gas-liquid separator 17 to the cooling side heat exchanger among the heat source side heat exchanger 14 and the utilization side heat exchanger 21, and a check valve 43 provided in parallel with the liquid pump 40 to prevent backflow of refrigerant toward the gas-liquid separator 17. The refrigerant that has passed through the cooling side heat exchanger branches and flows into an suction side pipe 18 connected to the suction side of the compressors 11 and 12 and a return pipe 70 connected to the gas-liquid separator 17. The suction side pipe 18 and the return pipe 70 are equipped with an suction side on / off valve 19 and a return side on / off valve 71, respectively. As a result, when the cooling load on the cooling-side heat exchanger is not high, the refrigerant from the cooling-side heat exchanger can be returned to the gas-liquid separator 17 via the return pipe 70 by the operation of the liquid pump 40 or by natural circulation via the check valve 43, thereby reducing the workload of the compressors 11 and 12. Also, when the cooling load on the cooling-side heat exchanger is high, the refrigerant that has flowed into the cooling-side heat exchanger via the check valve 43 can be compressed by the compressors 11 and 12. Therefore, it becomes easier to improve the APF and increase the efficiency of the refrigeration system. 【0102】 As in this embodiment, the refrigeration system 1 may be configured such that the compressor includes a low-stage compressor 11 and a high-stage compressor 12, and the venting pipe 30 for removing gaseous refrigerant from the gas-liquid separator 17 is connected to the low-stage compressor 11 and the high-stage compressor 12 via a low-stage side throttle valve 33 and a high-stage side throttle valve 34, respectively. This allows the low-stage compressor 11 to be stopped and the gaseous refrigerant in the gas-liquid separator 17 to be liquefied by the operation of only the high-stage compressor 12 when the load is small, for example, when the liquid refrigerant is flowed to the cooling-side heat exchanger by the liquid pump 40 or natural circulation. This makes it easier to improve the APF and increase the efficiency of the refrigeration system 1. 【0103】As in this embodiment, the refrigeration system 1 may be configured to include a heat exchanger 74 that exchanges heat between the refrigerant in the return pipe 70 and the outside air. This makes it easier to increase the liquid component of the refrigerant flowing into the gas-liquid separator 17 by releasing heat from the refrigerant in the return pipe 70 to the outside air, thereby reducing the workload of the compressors 11 and 12. As a result, it becomes easier to improve the APF and increase the efficiency of the refrigeration system 1. 【0104】 As in this embodiment, the refrigeration system 1 may be configured to have a water supply means 74b that supplies water to lower the temperature of the intake air of the heat exchanger 74 by latent heat of vaporization. This makes it easier to dissipate heat from the refrigerant in the return pipe 70 to the outside air, so that the liquid refrigerant in the gas-liquid separator 17 is less likely to run out, and the work of the compressors 11 and 12 can be reduced more easily. As a result, it becomes easier to improve the APF and the efficiency of the refrigeration system 1 can be increased. 【0105】 As in this embodiment, the refrigeration system 1 may be configured to include an oil separator 73 that recovers oil from the return pipe 70 and returns it to the suction side of the compressors 11 and 12. This prevents oil from accumulating in the gas-liquid separator 17 when the liquid refrigerant from the gas-liquid separator 17 is circulated to the cooling-side heat exchanger and returned to the gas-liquid separator 17 via the return pipe 70. This makes it easier to ensure reliability while increasing the efficiency of the refrigeration system 1. 【0106】As in this embodiment, the refrigeration system 1 is equipped with a flow path switching mechanism 50 that switches the gas cooler, which dissipates heat from the refrigerant discharged from the compressors 11 and 12, between the user-side heat exchanger 21 and the heat source-side heat exchanger 14. The flow path switching mechanism 50 may be configured to allow the refrigerant that has passed through the gas cooler to flow into the gas-liquid separator 17, regardless of whether the user-side heat exchanger 21 or the heat source-side heat exchanger 14 is functioning as the gas cooler. This allows the heat exchanger functioning as the gas cooler to be switched between the user-side heat exchanger 21 and the heat source-side heat exchanger 14, while the refrigerant liquefied in the gas cooler is stored in the gas-liquid separator 17 and pumped by the liquid pump 40. For this reason, when the refrigeration system 1 is an air conditioner, as in this embodiment, the workload of the compressors 11 and 12 can be easily reduced during both cooling and heating operations. 【0107】 As in this embodiment, the refrigeration system 1 is equipped with a flow path switching mechanism 50 that switches the cooling-side heat exchanger between the utilization-side heat exchanger 21 and the heat source-side heat exchanger 14. The flow path switching mechanism 50 may be configured to ensure that the refrigerant that has passed through the cooling-side heat exchanger reaches the suction-side piping 18 and the return piping 70, regardless of whether the utilization-side heat exchanger 21 or the heat source-side heat exchanger 14 is functioning as the cooling-side heat exchanger. This allows the system to switch the heat exchanger that functions as the cooling-side heat exchanger between the utilization-side heat exchanger 21 and the heat source-side heat exchanger 14, and to select whether to return the refrigerant that has passed through the cooling-side heat exchanger to the gas-liquid separator 17 or compress it in the compressors 11 and 12, depending on the load. For this reason, when the refrigeration system 1 is an air conditioner, as in this embodiment, the work of the compressors 11 and 12 can be easily reduced during both cooling and heating operations. 【0108】 The following describes an embodiment that modifies part of Embodiment 1. In the following, configurations different from Embodiment 1 will be described, and configurations similar to Embodiment 1 will not be described. 【0109】(Embodiment 2) As described above, the refrigeration system 1 of Embodiment 1 operates in a manner similar to a normal two-stage compression cycle when the cooling load is high and during heating operation. In contrast, the refrigeration system 301 of Embodiment 2 is configured to easily improve energy efficiency even under high load conditions. 【0110】 [2-1. Configuration] Figure 6 shows the refrigerant circuit 302 of the refrigeration device 301 according to Embodiment 2. In the refrigerant circuit 302 of Embodiment 2, an expansion mechanism 316 is provided instead of the throttle valve 16 of Embodiment 1. The expansion mechanism 316 is a device that recovers the pressure difference due to the flow of refrigerant as power and reduces the pressure of the refrigerant. The power recovered by the expansion mechanism 316 can be used for power generation by a generator, driving compressors 11 and 12, etc. In this embodiment, carbon dioxide is used as the refrigerant, which has a particularly large pressure difference in the refrigerant circuit 302, making it easier to recover power using the expansion mechanism 316. Note that the refrigeration device 301 of this embodiment is not provided with the external cooling equipment 47 of Embodiment 1. 【0111】 Furthermore, the refrigeration system 301 of the second embodiment is provided with a pressure sensor 381 for measuring the pressure inside the gas-liquid separator 17. The pressure sensor 381 transmits the measured pressure inside the gas-liquid separator 17 to the control unit 90. In addition, the refrigeration system 301 is provided with a temperature sensor 382 for measuring the outlet temperature of the liquid refrigerant in the gas-liquid separator 17. In this embodiment, the temperature sensor 382 measures the temperature of the refrigerant at one of the liquid-side outlets of the gas-liquid separator 17 that is connected to the liquid pump 40. The temperature sensor 382 transmits the measured temperature of the outlet temperature of the liquid refrigerant in the gas-liquid separator 17 to the control unit 90. 【0112】[2-2. Operation] The operation of each part of the refrigerant circuit 302 during cooling and heating operation of the refrigeration unit 301 is the same as the operation of each part of the refrigerant circuit 2 in Embodiment 1, except for the operation of the cooling throttle valve 45 and the expansion mechanism 316. In Embodiment 2 as well, during both heating and cooling operation, the flow path switching mechanism 50 causes the refrigerant that has passed through the gas cooler to flow into the expansion mechanism 316 from the same direction via the high-pressure receiver 15. Therefore, during both cooling and heating operation, the refrigerant pressure reduction and power recovery by the expansion mechanism 316 are possible. 【0113】 During the operation of compressors 11 and 12, as described above, the refrigerant that has flowed through the gas cooler flows into the expansion mechanism 316, is depressurized, and flows into the gas-liquid separator 17. However, the expansion mechanism 316 is more difficult to control the amount of refrigerant depressurization than the throttle valve 16, and it is possible that refrigerant that has not been sufficiently depressurized may flow into the gas-liquid separator 17. In such cases, the pressure of the refrigerant in the gas-liquid separator 17 will rise. Also, since the temperature of the refrigerant in the gas-liquid separator 17 is equivalent to the saturation temperature calculated based on the pressure of the refrigerant in the gas-liquid separator 17, the temperature of the refrigerant in the gas-liquid separator 17 will rise as the pressure in the gas-liquid separator 17 rises. In such cases, the temperature of the liquid refrigerant flowing from the main pipe 41 to the cooling-side heat exchanger rises, inhibiting the heat absorption of the refrigerant in the cooling-side heat exchanger, and reducing the refrigeration effect. 【0114】 In contrast, in this embodiment, when the refrigerant pressure reduction by the expansion mechanism 316 is insufficient, the opening of the cooling throttle valve 45 is increased, thereby strengthening the cooling of the refrigerant in the main pipe 41 by the subcooled heat exchanger 46 and stabilizing the temperature of the refrigerant flowing into the cooling-side heat exchanger. 【0115】 Figure 7 is a flowchart of the refrigeration system 301, showing the operation of the control unit 90 while at least one of the compressors 11 and 12 is in operation. The control unit 90 repeatedly performs the operation shown in Figure 7 while at least one of the compressors 11 and 12 is in operation. 【0116】 First, in step SA1, the control unit 90 acquires the measured values ​​from the pressure sensor 381 or the temperature sensor 382. 【0117】In step SA2, the control unit 90 determines whether the pressure reduction by the expansion mechanism 316 is insufficient based on the measurement value obtained in step SA1. Specifically, if the control unit 90 obtained a measurement value from the pressure sensor 381 in step SA1, it determines whether the obtained measurement value is equal to or greater than the first pressure. If the measurement value from the pressure sensor 381 is equal to or greater than the first pressure, the control unit 90 determines that the pressure reduction by the expansion mechanism 316 is insufficient (step SA2: YES) and proceeds to step SA3. If the measurement value from the pressure sensor 381 is less than the first pressure, the control unit 90 determines that the pressure reduction by the expansion mechanism 316 is not insufficient (step SA2: NO) and terminates the operation shown in Figure 7. 【0118】 Furthermore, in step SA2, if the control unit 90 obtained a measurement value from the temperature sensor 382 in step SA1, it determines whether the obtained measurement value is equal to or greater than the first temperature. If the measurement value from the temperature sensor 382 is equal to or greater than the first pressure, the control unit 90 determines that the pressure reduction by the expansion mechanism 316 is insufficient (step SA2: YES) and proceeds to step SA3. If the measurement value from the temperature sensor 382 is less than the first pressure, the control unit 90 determines that the pressure reduction by the expansion mechanism 316 is not insufficient (step SA2: NO) and proceeds to step SA4. 【0119】 The first pressure or first temperature used for the decision in step SA2 may be stored in the control unit 90 as a predetermined constant value. Alternatively, the first pressure and first temperature may be values ​​determined by the current opening of the cooling throttle valve 45, the target temperature of the refrigerant in the main piping 41 for cooling the subcooled heat exchanger 46, etc. For example, if the current opening of the cooling throttle valve 45 is large, the value of the first pressure or first temperature will be large, and if the target temperature of the refrigerant in the main piping 41 is low, the value of the first pressure or first temperature will be small. 【0120】In step SA3, the control unit 90 increases the opening of the cooling throttle valve 45. This increases the flow rate of refrigerant in the cooling pipe 44, and strengthens the cooling of the refrigerant in the main pipe 41 by the subcooled heat exchanger 46. Therefore, even if the pressure reduction by the expansion mechanism 316 is insufficient, the temperature of the refrigerant flowing into the cooling-side heat exchanger can be stabilized. In step SA3, the control unit 90 may be configured to increase the opening of the cooling throttle valve 45 more as the difference between the first pressure or first temperature and the measured values ​​of sensors 381 and 382 increases. After step SA3 is executed, the operation of the control unit 90 proceeds to step SA4. 【0121】 In step SA4, the control unit 90 determines whether the intermediate pressure, i.e., the suction pressure P1 of the high-stage compressor 12, is below a specified value. For example, if the opening of the cooling throttle valve 45 is increased in step SA3, the flow rate of refrigerant flowing into the venting pipe 30 after heat absorption in the subcooling heat exchanger 46 increases, and the intermediate pressure rises. If the suction pressure P1 of the high-stage compressor 12 is below a specified value (step SA4: YES), the control unit 90 terminates the operation shown in Figure 7. If the suction pressure P1 of the high-stage compressor 12 exceeds a specified value (step SA4: NO), the control unit 90 proceeds to step SA5. 【0122】 In step SA5, the control unit 90 increases the operating speed of the high-stage compressor 12. This allows the intermediate pressure to be reduced until it reaches a pressure that matches the liquid temperature in the gas-liquid separator 17. After step SA5, the control unit 90 terminates the operation shown in Figure 7. 【0123】[2-3. Effects, etc.] As described above, in this embodiment, the refrigeration system 301 is equipped with a refrigerant circuit 302 connecting compressors 11 and 12, a heat source side heat exchanger 14, a gas-liquid separator 17, and a utilization side heat exchanger 21, a liquid pump 40 that sends the liquid refrigerant from the gas-liquid separator 17 to the cooling side heat exchanger of the heat source side heat exchanger 14 and the utilization side heat exchanger 21, a check valve 43 provided in parallel with the liquid pump 40 to prevent backflow of refrigerant toward the gas-liquid separator 17, and the heat source side heat exchanger 14 and the utilization side The refrigeration system 301 includes an expansion mechanism 316 that expands the refrigerant flowing through the gas cooler into the gas-liquid separator 17, a main pipe 41 provided downstream of the liquid pump 40 through which the refrigerant flows toward the cooling-side heat exchanger, a cooling pipe 44 that branches off from the main pipe 41 and returns the refrigerant to the suction side of the compressors 11 and 12, a cooling throttle valve 45 that adjusts the flow rate of the cooling pipe 44, and a subcooled heat exchanger 46 that cools the refrigerant in the main pipe 41 with the reduced-pressure refrigerant from the cooling throttle valve 45. This allows for improved refrigeration capacity by using the expansion mechanism 316 to recover power while reducing the pressure of the refrigerant entering the gas-liquid separator 17, and by subcooling the refrigerant flowing into the cooling-side heat exchanger with the subcooled heat exchanger 46. As a result, refrigeration capacity can be improved while recovering power, and the efficiency of the refrigeration system 301 can be increased. 【0124】 As in this embodiment, the control unit 90 of the refrigeration system 301 may be configured to increase the opening of the cooling throttle valve 45 when it determines that the pressure reduction by the expansion mechanism 316 is insufficient. This allows the refrigerant flowing into the cooling-side heat exchanger to be stably subcooled. As a result, the refrigeration capacity can be improved while recovering power, and the efficiency of the refrigeration system 301 can be increased. In particular, in this embodiment, when the intermediate pressure, i.e., the suction pressure P1 of the high-stage compressor 12, exceeds a specified value as a result of increasing the opening of the cooling throttle valve 45, the control unit 90 increases the operating speed of the high-stage compressor 12. This suppresses the increase in intermediate pressure. As a result, the temperature rise of the liquid refrigerant in the gas-liquid separator 17 can be suppressed. 【0125】As in this embodiment, the control unit 90 may be configured to determine that the depressurization by the expansion mechanism 316 is insufficient when the pressure inside the gas-liquid separator 17 is equal to or greater than a first pressure, or when the outlet temperature of the liquid refrigerant from the gas-liquid separator 17 is equal to or greater than a first temperature. This allows for stable supercooling of the refrigerant flowing into the cooling-side heat exchanger. As a result, the refrigeration capacity can be improved while recovering power, and the efficiency of the refrigeration system can be increased. 【0126】 (Embodiment 3) In Embodiment 2, it was explained that the power recovered by the expansion mechanism 316 can be applied to purposes such as power generation or refrigerant compression. In Embodiment 3, a refrigeration system 401 is described that can efficiently utilize the power recovered by the expansion mechanism 316 to operate the compression mechanism 411 and achieve high efficiency. Below, the differences from the refrigeration system 301 of Embodiment 2 will be explained, and the same components will not be described. 【0127】 [3-1. Configuration] Figure 8 shows the refrigerant circuit 402 of the refrigeration system 401 according to Embodiment 3, and shows the refrigerant circuit 402 during high-load cooling operation. As shown in Figure 8, in Embodiment 3, a compression mechanism 411 is provided in place of the low-stage compressor 11 of Embodiment 2. Furthermore, in Embodiment 3, a compressor 412 is provided in place of the high-stage compressor 12. 【0128】 The compression mechanism 411 is located in the venting pipe 30 that removes the gaseous refrigerant from the gas-liquid separator 17. More specifically, the suction side of the compression mechanism 411 is located downstream of the connection point with the oil separator 73 in the venting pipe 30. In other words, the oil separator 73 recovers oil from the return pipe 70 and returns the recovered oil to the suction side of the compression mechanism 411. 【0129】 The compression mechanism 411, upon operation, draws in the gas refrigerant gas from the venting pipe 30, compresses it, and discharges it. In this embodiment, the venting pipe 30 does not have branch pipes 31 and 32, and the entire amount of refrigerant passing through the venting pipe 30 is drawn into the compression mechanism 411. As a result, the oil returned by the oil separator 73 is easily drawn into the compression mechanism 411, and the compression mechanism 411 is easily lubricated with oil. 【0130】 Furthermore, in this embodiment, the cooling pipe 44 is connected to the venting pipe 30 between the suction side of the compression mechanism 411 and the gas-liquid separator 17. As a result, the suction force of the compression mechanism 411 makes it easier for the refrigerant in the cooling pipe 44 to flow toward the compression mechanism 411. 【0131】 The compression mechanism 411 is coaxially connected to the expansion mechanism 316 via shaft 411a. The compression mechanism 411 is also operated by the power recovered by the expansion mechanism 316. Specifically, when the expansion mechanism 316 rotates using the power recovered from the refrigerant flow, the compression mechanism 411 rotates and operates coaxially with the expansion mechanism 316. By providing a compression mechanism 411 that operates coaxially with the expansion mechanism 316 in this way, energy loss can be reduced compared to, for example, a case where the power recovered by the expansion mechanism 316 is converted into electricity. 【0132】 A low-stage discharge pipe 413 is connected to the discharge side of the compression mechanism 411. The low-stage discharge pipe 413 branches into a first branch pipe 414 and a second branch pipe 415. The first branch pipe 414 is connected to the suction side of the compressor 412. The first branch pipe 414 is also provided with a first on-off valve 414a for opening and closing the first branch pipe 414. The second branch pipe 415 is connected to the discharge side of the compressor 412. More specifically, the second branch pipe 415 merges with a discharge pipe 416 that connects the discharge side of the compressor 412 to the oil separator 13. The second branch pipe 415 is also provided with a second on-off valve 415a for opening and closing the second branch pipe 415. The first on-off valve 414a and the second on-off valve 415a correspond to examples of "on-off valves" in this disclosure. 【0133】 In other words, the discharge side of the compression mechanism 411 is connected to both the suction side and the discharge side of the compressor 412 via the on-off valves 414a and 415a, respectively. Furthermore, the control unit 90 can switch between single-stage and two-stage compression of the refrigerant in the gas venting pipe 30 by opening and closing the first on-off valve 414a and the second on-off valve 415a. 【0134】In detail, by opening the first on-off valve 414a and closing the second on-off valve 415a, the refrigerant in the venting pipe 30 is compressed in two stages by the compression mechanism 411 and the compressor 412 and flows into the discharge pipe 416. Alternatively, by opening the second on-off valve 415a and closing the first on-off valve 414a, the refrigerant in the venting pipe 30 is compressed in one stage by the compression mechanism 411 and then flows into the discharge pipe 416 without being compressed by the compressor 412. Note that each on-off valve 414a and 415a can be switched between an open state and a closed state by the control unit 90. For this reason, each on-off valve 414a and 415a may be a valve that can only be switched between an open state and a closed state, or it may be a flow control valve that allows adjustment of the opening degree. 【0135】 The compressor 412 is powered by an electric motor or an engine powered by fuel. In this embodiment, the compressor 412 is powered by an electric motor. The compressor 412 compresses the gaseous refrigerant drawn in from the first branch pipe 414 and discharges it into the discharge pipe 416. 【0136】 The discharge pipe 416 is connected to the oil separator 13. The oil separator 13 recovers the oil contained in the refrigerant flowing in from the discharge pipe 416. The oil separator 13 also flows the refrigerant, after the oil has been recovered, to the first switching mechanism 51. In this embodiment, the oil separator 13 has an oil tank 13a for storing the recovered oil. 【0137】 Furthermore, in Embodiment 3, the refrigeration device 401 is provided with an oil return mechanism 420 that returns the oil recovered by the oil separator 13 to the expansion mechanism 316 and the compressor 412. The oil return mechanism 420 supplies the oil stored in the oil tank 13a to the expansion mechanism 316 and the compressor 412. 【0138】 The oil return mechanism 420 is connected to the oil tank 13a and has an oil return pipe 421 through which oil flows. The oil return pipe 421 branches into a first oil return branch pipe 422 and a second oil return branch pipe 423. The first oil return branch pipe 422 is connected to the upstream side of the expansion mechanism 316. The second oil return branch pipe 423 is connected to the compressor 412. 【0139】A first oil return branch pipe 422 is provided with a first oil return volume adjustment valve 424. A second oil return volume adjustment valve 425 is provided with a second oil return branch pipe 423. Each oil return volume adjustment valve 424 and 425 is a valve whose opening and closing and degree of opening can be adjusted by the control unit 90. The first oil return volume adjustment valve 424 adjusts the amount of oil returned from the oil separator 13 to the expansion mechanism 316. The second oil return volume adjustment valve 425 adjusts the flow rate of oil returned from the oil separator 13 to the compressor 412. Each oil return volume adjustment valve 424 and 425 is controlled by the control unit 90 so that the sliding parts of the expansion mechanism 316 and the compressor 412 are sufficiently lubricated, and so that a large amount of oil does not accumulate downstream of the expansion mechanism 316 and the compressor 412. This allows for appropriate management of the oil circulation rate in the refrigerant circuit 402. 【0140】 In this embodiment, the casing of the expansion mechanism 316 and the casing of the compression mechanism 411 are connected by an oil pipe 411b. The oil pipe 411b is a pipe that carries oil between the expansion mechanism 316 and the compression mechanism 411. In this embodiment, the pressure on the suction side of the compression mechanism 411 is lower than the pressure of the expansion mechanism 316. Therefore, the oil returned to the expansion mechanism 316 by the oil return mechanism 420 is distributed to the compression mechanism 411 via the oil pipe 411b. 【0141】 Furthermore, in Embodiment 3, similar to Embodiment 1, an external cooling device 47 is provided to cool the refrigerant in the main piping 41 after it has passed through the subcooled heat exchanger 46. Similar to Embodiment 1, this external cooling device 47 uses a refrigeration cycle that employs a refrigerant with higher energy efficiency than carbon dioxide to cool the refrigerant in the main piping 41. 【0142】 Furthermore, in Embodiment 3, the suction-side piping 18 and suction-side on / off valve 19 connected to the first switching mechanism 51 are not provided. Therefore, of the heat source-side heat exchanger 14 and the utilization-side heat exchanger 21, the refrigerant that has passed through the cooling-side heat exchanger always flows into the return piping 70 after passing through the first switching mechanism 51. 【0143】[3-2. Operation] [3-2-1. Operation during high-load cooling operation] As shown in Figure 8, during high-load cooling operation, the control unit 90 opens each of the cooling valves 53, 55, 57, and 59. The control unit 90 also closes each of the heating valves 54, 56, and 58 and the heating throttle valve 60. As a result, the heat source side heat exchanger 14 functions as a gas cooler, and the user side heat exchanger 21 functions as a cooling side heat exchanger. 【0144】 Furthermore, the control unit 90 opens the first on-off valve 414a and closes the second on-off valve 415a. The control unit 90 also opens the return on-off valve 71. 【0145】 During high-load cooling operation, the control unit 90 switches the opening and closing of each valve as described above, and then drives the compressor 412. As a result, the compressor 412 draws in refrigerant from the low-stage discharge pipe 413 and discharges it into the discharge pipe 416. 【0146】 The refrigerant discharged into the discharge pipe 416 flows into the heat source side heat exchanger 14 via the oil separator 13 and the first switching mechanism 51. The heat source side heat exchanger 14 functions as a gas cooler, cooling the refrigerant by exchanging heat with air. At this time, the control unit 90 operates the water supply means 14b, as in embodiments 1 and 2, to lower the temperature of the intake air to the heat source side heat exchanger 14 by the latent heat of vaporization of water. 【0147】 The refrigerant cooled by the heat source side heat exchanger 14 reaches the expansion mechanism 316 via the second switching mechanism 52 and the high-pressure receiver 15. The refrigerant that reaches the expansion mechanism 316 is depressurized by the expansion mechanism 316 and flows into the gas-liquid separator 17, where it is separated into gaseous refrigerant and liquid refrigerant. 【0148】 As described above, the expansion mechanism 316 and the compression mechanism 411 operate coaxially, so when the expansion mechanism 316 reduces the pressure of the refrigerant, the compression mechanism 411 operates. Due to the operation of the compression mechanism 411, the gaseous refrigerant separated in the gas-liquid separator 17 is drawn into the compression mechanism 411 via the gas venting pipe 30. 【0149】In this embodiment, the expansion mechanism 316 is provided at the inlet of the gas-liquid separator 17, and the compression mechanism 411, which operates coaxially with the expansion mechanism 316, is provided at the gas venting pipe 30, which is the outlet for the gaseous refrigerant in the gas-liquid separator 17. That is, a portion of the refrigerant that flows into the gas-liquid separator 17 through the expansion mechanism 316 becomes gaseous refrigerant and is drawn into the compression mechanism 411. For this reason, the flow rates of the refrigerant passing through the expansion mechanism 316 and the refrigerant drawn into the compression mechanism 411 are correlated, and the ratio of their flow rates does not fluctuate significantly. Therefore, the power recovered by the expansion mechanism 316 can be effectively utilized for compression by the compression mechanism 411. 【0150】 The gaseous refrigerant compressed by the compression mechanism 411 is drawn back into the compressor 412 via the low-stage discharge pipe 413. In other words, during high-load cooling operation, the refrigeration system 401 uses a two-stage compression system, with the compression mechanism 411 on the low-stage side and the compressor 412 on the high-stage side, to liquefy the gaseous refrigerant in the gas-liquid separator 17. 【0151】 The liquid refrigerant separated in the gas-liquid separator 17 flows into the main piping 41 by driving the liquid pump 40. However, as in Embodiment 1, when natural circulation occurs between the user-side heat exchanger 21 and the gas-liquid separator 17, the control unit 90 may stop driving the liquid pump 40 and allow the liquid refrigerant from the gas-liquid separator 17 to flow into the main piping 41 via the path 42 and the check valve 43. 【0152】 The liquid refrigerant flowing into the main pipe 41 branches off into the cooling pipe 44 midway through its flow, is depressurized by the cooling throttle valve 45, and passes through the subcooling heat exchanger 46. In the subcooling heat exchanger 46, the refrigerant in the cooling pipe 44 subcools the refrigerant in the main pipe 41. After passing through the subcooling heat exchanger 46, the refrigerant in the cooling pipe 44 flows into the vent pipe 30 and is drawn into the compression mechanism 411. Therefore, the suction force of the compression mechanism 411 can be used to facilitate the flow of the refrigerant in the cooling pipe 44, and the refrigerant in the main pipe 41 can be easily subcooled. 【0153】The refrigerant in the main pipe 41 passes through the subcooling heat exchanger 46 and is then supercooled by the external cooling device 47. Although the heat exchange by the subcooling heat exchanger 46 is within the refrigerant of the refrigerant circuit 402 and therefore less prone to losses, the amount of supercooling of the refrigerant in the main pipe 41 by the subcooling heat exchanger 46 is limited by the operating conditions of the refrigeration system 401. Furthermore, the external cooling device 47 uses a refrigerant that is more efficient than carbon dioxide, and the supercooling of the refrigerant in the main pipe 41 is easier to adjust, but unlike the subcooling heat exchanger 46, the operation of the external cooling device 47 requires additional power. For this reason, the control unit 90 may select the more efficient of the subcooling heat exchanger 46 and the external cooling device 47 and use them to cool the main pipe 41. 【0154】 The refrigerant cooled by the external cooling device 47 flows into the user-side heat exchanger 21 via the second switching mechanism 52, cooling the air in the conditioned space. The refrigerant that has flowed through the user-side heat exchanger 21 flows into the return pipe 70 via the first switching mechanism 51. In the return pipe 70, the refrigerant passes through the oil separator 73 and reaches the heat exchanger 74, where it is cooled. The control unit 90 activates the water supply means 74b to lower the temperature of the intake air in the heat exchanger 74, making it easier to liquefy the refrigerant. The refrigerant cooled in the heat exchanger 74 flows back into the gas-liquid separator 17. 【0155】 [3-2-2. Operation during low-load cooling operation] Figure 9 shows the refrigerant circuit 402 during low-load cooling operation. During low-load cooling operation, the control unit 90 switches the refrigerant circuit 402 from the state during high-load cooling operation to a state where the compressor 412 is stopped. The control unit 90 also switches the first on-off valve 414a to the closed state and the second on-off valve 415a to the open state. 【0156】As a result, the refrigerant discharged to the lower-stage discharge pipe 413 by the compression mechanism 411 flows into the discharge pipe 416 without being drawn into the compressor 412. The refrigerant that flows into the discharge pipe 416 reaches the expansion mechanism 316 via the oil separator 13, the first switching mechanism 51, and the high-pressure receiver 15. The refrigerant that reaches the expansion mechanism 316 is depressurized by the expansion mechanism 316 and flows into the gas-liquid separator 17, where it is separated into gaseous refrigerant and liquid refrigerant. The gaseous refrigerant separated in the gas-liquid separator 17 is then drawn back into the compression mechanism 411 via the gas vent pipe 30. In other words, during low-load cooling operation, the refrigeration system 401 liquefies the gaseous refrigerant in the gas-liquid separator 17 by single-stage compression by the compression mechanism 411. 【0157】 The liquid refrigerant separated in the gas-liquid separator 17 flows into the utilization-side heat exchanger 21 via the same path as in high-load cooling operation, and cools the air in the air-conditioned space. The refrigerant that has flowed through the utilization-side heat exchanger 21 returns to the gas-liquid separator 17 via the same path as in high-load cooling operation. 【0158】 If low-load cooling operation continues, the pressure difference between the upstream and downstream sides of the expansion mechanism 316 will gradually decrease, making it impossible to recover sufficient power from the expansion mechanism 316 and thus preventing the compression mechanism 411 from operating. For this reason, when the pressure difference between the upstream and downstream sides of the expansion mechanism 316 decreases, the control unit 90 switches the first on-off valve 414a to the open state and the second on-off valve 415a to the closed state, and then temporarily operates the compressor 412. After a sufficient pressure difference is secured between the upstream and downstream sides of the expansion mechanism 316, the control unit 90 stops the compressor 412 again and switches the first on-off valve 414a to the closed state and the second on-off valve 415a to the open state, thereby continuing low-load cooling operation. 【0159】 [3-2-3. Operation during air-cooled cooling operation] The refrigeration device 401 of Embodiment 3 can perform air-cooled cooling operation in the same manner as in Embodiment 1. Figure 10 shows the refrigerant circuit 402 during air-cooled cooling operation. 【0160】During air-cooled cooling operation, the refrigerant circuit 402 is configured to a state where the expansion mechanism 316 and the compression mechanism 411 are also stopped, in addition to the state during low-load cooling operation when the compressor 412 is stopped. Furthermore, during low-load cooling operation, the control unit 90 closes both the on-off valves 414a and 415a. As a result, the gaseous refrigerant separated in the gas-liquid separator 17 is no longer drawn into the compression mechanism 411 and the compressor 412, and refrigerant no longer flows to the heat source side heat exchanger 14, which is a gas cooler. 【0161】 Furthermore, during air-cooled cooling operation, the control unit 90 closes the cooling throttle valve 45. In addition, similar to Embodiment 1, the liquid refrigerant separated in the gas-liquid separator 17 flows into the utilization-side heat exchanger 21 by being pumped by the liquid pump 40 or by natural circulation via the path 42 and check valve 43, and returns to the gas-liquid separator 17 via the return pipe 70. At this time, the control unit 90 operates the external cooling equipment 47 to lower the temperature of the refrigerant flowing into the utilization-side heat exchanger 21 to the temperature necessary for air conditioning in the conditioned space. The control unit 90 also operates the water supply means 74b to lower the temperature of the intake air to the heat exchanger 74, thereby facilitating the liquefaction of the refrigerant returning to the gas-liquid separator 17. 【0162】 [3-2-4. Operation during heating] Figure 11 shows the refrigerant circuit 402 during heating operation. As shown in Figure 11, during heating operation, the control unit 90 opens the heating valves 54, 56, 58 and the heating throttle valve 60 of the flow path switching mechanism 50, and closes the cooling valves 53, 55, 57, 59. As a result, the heat source side heat exchanger 14 functions as a cooling side heat exchanger, and the utilization side heat exchanger 21 functions as a gas cooler. 【0163】 Furthermore, the control unit 90 opens the first on-off valve 414a and closes the second on-off valve 415a. The control unit 90 also opens the return on-off valve 71. 【0164】During heating operation, the control unit 90 switches the opening and closing of each valve as described above, and then drives the compressor 412. As a result, the compressor 412 draws in refrigerant from the low-stage discharge pipe 413 and discharges it into the discharge pipe 416. The refrigerant discharged into the discharge pipe 416 flows into the first switching mechanism 51 via the oil separator 13. 【0165】 The refrigerant that flows into the first switching mechanism 51 flows into each indoor unit 20 via the first heating valve 54, and is cooled by releasing heat into the air of the heated space in the user-side heat exchanger 21. As a result, the heated space is heated. 【0166】 The refrigerant that has passed through the user-side heat exchanger 21 reaches the expansion mechanism 316 via the second switching mechanism 52 and the high-pressure receiver 15, where it is depressurized and becomes a low-temperature gas-liquid mixture. Subsequently, the refrigerant flows into the gas-liquid separator 17, where it is separated into gaseous refrigerant and liquid refrigerant. 【0167】 The gaseous refrigerant separated in the gas-liquid separator 17 is drawn into the compression mechanism 411 via the gas venting pipe 30, compressed, and discharged into the low-stage discharge pipe 413. The refrigerant discharged into the low-stage discharge pipe 413 is drawn back into the compressor 412 via the first branch pipe 414. 【0168】 The liquid refrigerant separated in the gas-liquid separator 17 flows into the main pipe 41. The refrigerant that has passed through the main pipe 41 passes through the second switching mechanism 52 and flows into the heat source side heat exchanger 14. The refrigerant absorbs heat in the heat source side heat exchanger 14 and returns to the gas-liquid separator 17 through the return pipe 70. The control unit 90 stops the water supply means 14b and 74b during heating operation. 【0169】[3-3. Effects, etc.] As described above, in this embodiment, the refrigeration device 401 is equipped with a refrigerant circuit 402 connecting a compression mechanism 411, a heat source side heat exchanger 14, a gas-liquid separator 17, and a utilization side heat exchanger 21. It includes a liquid pump 40 that sends the liquid refrigerant from the gas-liquid separator 17 to the cooling side heat exchanger among the heat source side heat exchanger 14 and the utilization side heat exchanger 21, a check valve 43 provided in parallel with the liquid pump 40 to prevent backflow of refrigerant toward the gas-liquid separator 17, and an expansion mechanism 316 that expands the refrigerant that has flowed through the gas cooler among the heat source side heat exchanger 14 and the utilization side heat exchanger 21 and sends it to the gas-liquid separator 17. The compression mechanism 411 is provided in the gas venting pipe 30 into which the gas refrigerant from the gas-liquid separator 17 flows, and operates coaxially with the expansion mechanism 316. This allows the refrigerant entering the gas-liquid separator 17 to be depressurized while power is recovered using the expansion mechanism 316, and the recovered power can be efficiently used to operate the compression mechanism 411. As a result, energy consumption can be suppressed and the efficiency of the refrigeration system 401 can be increased. 【0170】 As in this embodiment, the refrigeration system 401 may have a compressor 412 operated by a motor, and the discharge side of the compression mechanism 411 may be connected to both the suction side and the discharge side of the compressor 412 via a first on-off valve 414a and a second on-off valve 415a, respectively. This makes it possible to switch between single-stage compression using only the compression mechanism 411 and two-stage compression using both the compression mechanism 411 and the compressor 412, depending on the load. As a result, energy consumption for operating the compressor 412 can be suppressed, and the efficiency of the refrigeration system 401 can be improved. 【0171】As in this embodiment, the refrigeration system 401 is provided downstream of the liquid pump 40 and includes a main pipe 41 through which refrigerant flows toward the cooling-side heat exchanger, a cooling pipe 44 branching from the main pipe 41, a cooling throttle valve 45 for adjusting the flow rate of the cooling pipe 44, and a subcooled heat exchanger 46 that cools the refrigerant in the main pipe 41 with the reduced-pressure refrigerant from the cooling throttle valve 45. The cooling pipe 44 may be connected between the compression mechanism 411 and the gas-liquid separator 17 in the venting pipe 30. This configuration allows the refrigerant flowing into the cooling-side heat exchanger to be subcooled by utilizing the suction force of the compression mechanism 411, which operates coaxially with the expansion mechanism 316. As a result, the refrigeration effect can be increased while suppressing energy consumption, and the efficiency of the refrigeration system 401 can be improved. 【0172】 As in this embodiment, the refrigeration system 401 may be configured to include an external cooling device 47 that cools the refrigerant after it has passed through the subcooled heat exchanger 46 in the main piping 41. This allows the refrigerant flowing toward the cooling-side heat exchanger to be subcooled even when the pressure reduction by the expansion mechanism 316 is insufficient. As a result, the efficiency of the refrigeration system 401 can be improved while stabilizing the refrigeration effect. 【0173】 As in this embodiment, the refrigerant circuit 402 may use carbon dioxide as the refrigerant, and the external cooling device 47 may use a refrigeration cycle that utilizes a refrigerant with higher energy efficiency than carbon dioxide to cool the refrigerant in the main piping 41. This configuration allows the refrigeration capacity of the refrigeration system, which uses carbon dioxide as the refrigerant and has a low environmental impact, to be improved by the highly energy-efficient external cooling device 47. Furthermore, since the external cooling device 47 can be made simpler in configuration than the refrigeration system 401, even if a refrigerant such as HFC or HFO, which is highly efficient but has a greater environmental impact than carbon dioxide, is used in the external cooling device 47, the risk of refrigerant leakage from the external cooling device 47 is less likely to increase. Therefore, it is possible to improve the efficiency of the refrigeration system 401 while suppressing environmental impact. 【0174】As in this embodiment, the system may be configured to include a return pipe 70 through which the refrigerant that has passed through the cooling-side heat exchanger flows and is connected to a gas-liquid separator 17, and a heat exchanger 74 that exchanges heat between the refrigerant in the return pipe 70 and the outside air. This makes it easier to increase the liquid component of the refrigerant flowing into the gas-liquid separator 17 by releasing heat from the refrigerant in the return pipe 70 to the outside air, thereby reducing the workload of the compressor 412. As a result, it becomes easier to improve the APF and increase the efficiency of the refrigeration system 401. 【0175】 As in this embodiment, the refrigeration system 401 may be configured to include a water supply means 74b that supplies water to lower the temperature of the intake air of the heat exchanger 74 by latent heat of vaporization. This makes it easier to dissipate heat from the refrigerant in the return pipe 70 to the outside air, thus reducing the workload of the compressor 412. As a result, it becomes easier to improve the APF and increase the efficiency of the refrigeration system 401. 【0176】 As in this embodiment, the refrigeration system 401 may be configured to include a water supply means 14b that supplies water to lower the temperature of the intake air of the heat source side heat exchanger 14 by latent heat of vaporization. This makes it easier to improve the refrigeration capacity of the refrigeration system 401 with low energy consumption. As a result, the efficiency of the refrigeration system 401 can be increased. 【0177】 As in this embodiment, the refrigeration system 401 may be configured to include a return pipe 70 through which the refrigerant that has passed through the cooling-side heat exchanger flows and is connected to the gas-liquid separator 17, and an oil separator 73 that recovers oil from the return pipe 70 and returns it to the suction side of the compression mechanism 411. This prevents oil from accumulating in the gas-liquid separator 17 when the liquid refrigerant from the gas-liquid separator 17 is circulated by flowing it to the cooling-side heat exchanger and returning it to the gas-liquid separator 17 via the return pipe 70. As a result, it becomes easier to ensure reliability while increasing the efficiency of the refrigeration system 401. 【0178】As in this embodiment, the refrigeration system 401 may be configured to include an oil separator 13 that recovers oil from the discharge pipe 416 of the compressor 412, and an oil return mechanism 420 that returns the oil recovered by the oil separator 13 to the compressor 412 and the expansion mechanism 316. This allows for a stable supply of oil to the compressor 412 and the expansion mechanism 316. This makes it easier to ensure reliability while increasing the efficiency of the refrigeration system 401. In particular, in this embodiment, the oil return mechanism 420 has a first oil return amount adjustment valve 424 and a second oil return amount adjustment valve 425 that adjust the amount of oil returned to the expansion mechanism 316 and the compressor 412. This makes it easier to supply an appropriate amount of oil to each of the expansion mechanism 316 and the compressor 412. In addition, in this embodiment, the refrigeration system 401 is provided with an oil pipe 411b that allows oil to flow between the casing of the expansion mechanism 316 and the casing of the compression mechanism 411. Therefore, when oil is returned to the expansion mechanism 316 by the oil return mechanism 420, it becomes easier to ensure lubrication of the compression mechanism 411. 【0179】 (Embodiment 4) As described above, the refrigeration system 1 of Embodiment 1 operates in a manner similar to a normal two-stage compression cycle when the cooling load is high and during heating operation. In contrast, the refrigeration system 101 of Embodiment 4 is configured to easily improve energy efficiency even under high load conditions. 【0180】 [4-1. Configuration] Figure 12 shows the refrigeration circuit 102 of the refrigeration device 101 according to Embodiment 4. In the refrigeration circuit 102 of Embodiment 4, an ejector 116 is provided instead of the throttle valve 16 of Embodiment 1. The ejector has a nozzle and a suction port, etc. The ejector 116 reduces the pressure of the liquid refrigerant flowing in from the high-pressure receiver 15 by spraying it from the nozzle. The ejector 116 also uses the low pressure of the refrigerant sprayed from the nozzle to suck in refrigerant from the suction port, mixes the sucked refrigerant with the sprayed refrigerant, and flows it into the gas-liquid separator 17. In this embodiment, a suction pipe 116a is connected to the suction port of the ejector 116. 【0181】The suction pipe 116a is connected to one port of a flow path switching valve 175 provided in the return pipe 70. The flow path switching valve 175 is a so-called three-way valve. The flow path switching valve 175 is provided in the middle of the return pipe 70 on the downstream side of the heat exchanger 74 and is connected to the suction pipe 116a, the upstream side of the return pipe 70, and the downstream side of the return pipe 70. In other words, the suction pipe 116a is provided on the outlet side of the cooling-side heat exchanger, of the heat source-side heat exchanger 14 and the utilization-side heat exchanger 21, via the first switching mechanism 51, the return pipe 70, and the flow path switching valve 175. The flow path switching valve 175 can be switched between a first state and a second state by the control unit 90. In the first state, the flow path switching valve 175 connects the upstream side of the return piping 70 to the suction pipe 116a and closes the downstream side of the return piping 70 to the flow path switching valve 175. As a result, when the flow path switching valve 175 is in the first state, refrigerant that has passed through the cooling side heat exchanger flows into the suction pipe 116a. In the second state, the flow path switching valve 175 connects the upstream and downstream sides of the return piping 70 to the flow path switching valve 175 and closes the suction pipe 116a. 【0182】 Furthermore, in Embodiment 4, a gas venting throttle valve 135 is provided to adjust the flow rate of the refrigerant in the gas venting pipe 30. The gas venting throttle valve 135 is a valve whose opening degree can be adjusted by the control unit 90. More specifically, the gas venting throttle valve 135 is provided in the gas venting pipe 30 on the upstream side of the connection portion with the cooling pipe 44, that is, on the side closer to the gas-liquid separator 17. 【0183】 Furthermore, in Embodiment 4, unlike the suction-side piping 18 in Embodiment 1, the suction-side piping 118, which is the suction-side piping of the low-stage compressor 11, is not connected to the first switching mechanism 51. Also, the suction-side on / off valve 19 of Embodiment 1 is not provided. The suction-side piping 118 is connected only to the low-stage side branch pipe 31 of the gas venting piping 30. For this reason, in Embodiment 4, the compressors 11 and 12 cannot directly inhale the refrigerant that has passed through the cooling-side heat exchanger, but can inhale the gaseous refrigerant separated by the gas-liquid separator 17. 【0184】 [4-2. Operation] The operation of the refrigeration device 101, configured as described above, will be explained below. 【0185】 [4-2-1. Operation during cooling operation at medium load or less] Figure 13 shows the refrigeration circuit 102 during cooling operation at medium load. Figure 12, mentioned above, shows the refrigeration circuit 102 during cooling operation at low load. As shown in Figures 12 and 13, when the cooling load is medium load or less, the control unit 90 performs the same control as in Embodiment 1 for each part other than the flow path switching valve 175, the gas venting throttle valve 135, and the cooling throttle valve 45. 【0186】 In Embodiment 4, when the cooling load is medium load or lower, the control unit 90 sets the flow path switching valve 175 to the second state and closes the suction pipe 116a. As a result, the flow path of the refrigerant in the refrigeration circuit 102 becomes the same as in Embodiment 1. That is, as in Embodiment 1, the compressors 11 and 12 only perform the work of liquefying the gaseous refrigerant in the gas-liquid separator 17 and returning it to the gas-liquid separator 17. The liquid refrigerant in the gas-liquid separator 17 is then supplied to the user-side heat exchanger 21 by the liquid pump 40 or by natural circulation. Therefore, as in Embodiment 1, the workload of the compressors 11 and 12 can be reduced. 【0187】 Furthermore, in Embodiment 4, the control unit 90 controls the gas venting throttle valve 135 and the cooling throttle valve 45 so that the cooling effect in the user-side heat exchanger 21 is enhanced and the discharge gas temperature of the compressors 11 and 12 is optimized. 【0188】 The temperature of the refrigerant in the gas-liquid separator 17 is equivalent to the saturation temperature corresponding to the pressure in the gas-liquid separator 17. Therefore, when the control unit 90 increases the opening of the gas venting throttle valve 135, the pressure in the gas-liquid separator 17 decreases, and the temperature of the refrigerant in the gas-liquid separator 17 also decreases. When the control unit 90 increases the opening of the cooling throttle valve 45, the cooling by the subcooled heat exchanger 46 increases, the temperature of the liquid refrigerant in the main piping 41 decreases, and the refrigeration effect in the user-side heat exchanger 21 increases. In addition, when the control unit 90 increases the opening of the cooling throttle valve 45, the flow rate of refrigerant flowing from the cooling piping 44 to the gas venting piping 30 increases, so the suction temperature and discharge gas temperature of the compressors 11 and 12 decrease. 【0189】[4-2-2. Operation during air-cooled cooling operation] Figure 14 shows the refrigeration circuit 102 during air-cooled cooling operation. In Embodiment 4, as in Embodiment 1, air-cooled cooling operation of the heat exchanger 74 is possible with the compressors 11 and 12 stopped. In air-cooled cooling operation, the control unit 90 stops the compressors 11 and 12 when the cooling load is medium load or lower, and closes the cooling throttle valve 45, the gas venting throttle valve 135, and the third cooling valve 57. As a result, the discharge side and suction side of the compressors 11 and 12 are no longer in communication with the gas-liquid separator 17. Also, as in Embodiment 1, the control unit 90 cools the conditioned space by using the liquid pump 40 and natural circulation to flow the supercooled liquid refrigerant to the utilization side heat exchanger 21. 【0190】 [4-2-3. Operation during high-load cooling operation] Figure 15 shows the refrigeration circuit 102 during high-load cooling operation. As shown in Figure 15, when the cooling load is high, the control unit 90 switches the flow path switching valve 175 to the first state from the state where the cooling load is medium load. As a result, the refrigerant in the return pipe 70 that has been cooled by the heat exchanger 74 does not return directly to the gas-liquid separator 17, but is instead drawn into the suction port of the ejector 116 via the suction pipe 116a. 【0191】 When the cooling load is high, the gaseous refrigerant in the gas-liquid separator 17 passes through the venting pipe 30, is compressed in two stages by the compressors 11 and 12, and dissipates heat in the heat source side heat exchanger 14. The refrigerant that has dissipated heat in the heat source side heat exchanger 14 flows into the ejector 116 via the high-pressure receiver 15, is depressurized, and returns to the gas-liquid separator 17. As the refrigerant passes through the ejector 116, the ejector 116 draws the refrigerant from the return pipe 70, which has been cooled by the heat exchanger 74, through the suction port and suction pipe 116a. 【0192】Here, the heat exchanger 74 is connected to the liquid-side outlet of the gas-liquid separator 17 via the return pipe 70, the utilization-side heat exchanger 21, the main pipe 41, and the path 42, etc. Therefore, when the ejector 116 sucks the refrigerant from the return pipe 70, the liquid refrigerant from the gas-liquid separator 17 flows towards the utilization-side heat exchanger 21 via the path 42 and the main pipe 41. In other words, the ejector 116 can perform the work of returning the liquid refrigerant from the gas-liquid separator 17 to the gas-liquid separator 17 through the utilization-side heat exchanger 21. 【0193】 Furthermore, when the cooling load is high, the control unit 90 adjusts the opening of the gas venting throttle valve 135 and the cooling throttle valve 45 in the same way as when the cooling load is medium or lower, in order to improve the cooling effect in the user-side heat exchanger 21 and to optimize the discharge gas temperature of the compressors 11 and 12. 【0194】 [4-2-4. Operation during heating] Figure 16 shows the refrigeration circuit 102 during heating operation. During heating operation, the control unit 90 closes the cooling valves 53, 55, 57, and 59 when the cooling load is high, and opens the heating valves 54, 56, 58 and the heating throttle valve 60. As a result, the gaseous refrigerant from the gas-liquid separator 17 is drawn into the compressors 11 and 12 via the gas venting pipe 30, compressed in two stages, and then flows into the utilization-side heat exchanger 21 via the oil separator 13 to dissipate heat. This heats the heated space. 【0195】 Furthermore, the refrigerant that has released heat in the user-side heat exchanger 21 flows into the ejector 116 via the high-pressure receiver 15, just as during cooling operation, and is depressurized before flowing into the gas-liquid separator 17. At this time, the ejector 116 sucks the refrigerant from the return pipe 70 after it has passed through the heat exchanger 74 via the suction pipe 116a. 【0196】Here, the heat exchanger 74 is connected to the liquid-side outlet of the gas-liquid separator 17 via the return pipe 70, the heat source-side heat exchanger 14, the main pipe 41, and the path 42, etc. Therefore, when the ejector 116 sucks the refrigerant from the return pipe 70, the liquid refrigerant from the gas-liquid separator 17 flows towards the heat source-side heat exchanger 14 via the path 42 and the main pipe 41. In other words, the ejector 116 can perform the work of returning the liquid refrigerant from the gas-liquid separator 17 to the gas-liquid separator 17 through the heat source-side heat exchanger 14. 【0197】 During heating operation, the control unit 90 stops the external cooling equipment 47 and the water supply means 14b and 74b. Also, similar to during cooling operation, the control unit 90 adjusts the opening of the gas venting throttle valve 135 and the cooling throttle valve 45 to improve the cooling effect in the user-side heat exchanger 21 and to optimize the discharge gas temperature of the compressors 11 and 12. 【0198】 [4-3. Effects, etc.] As described above, in this embodiment, the refrigeration system 101 is equipped with a refrigeration circuit 102 connecting compressors 11 and 12, a heat source side heat exchanger 14, a gas-liquid separator 17, and a utilization side heat exchanger 21. It includes a liquid pump 40 that sends the liquid refrigerant from the gas-liquid separator 17 to the cooling side heat exchanger among the heat source side heat exchanger 14 and the utilization side heat exchanger 21, a check valve 43 provided in parallel with the liquid pump 40 to prevent backflow of refrigerant toward the gas-liquid separator 17, and an ejector 116 into which the refrigerant that has flowed through the gas cooler among the heat source side heat exchanger 14 and the utilization side heat exchanger 21 flows and directs the incoming refrigerant toward the gas-liquid separator 17. The suction port of the ejector 116 is connected to a suction pipe 116a provided on the outlet side of the cooling side heat exchanger. This makes it easier to return the refrigerant from the cooling-side heat exchanger to the gas-liquid separator by utilizing the suction force of the ejector. As a result, the workload of the liquid pump 40 can be reduced, and the efficiency of the refrigeration system can be improved. In particular, in this embodiment, a flow path switching mechanism 50 is provided that allows the refrigerant that has passed through the gas cooler to flow into the gas-liquid separator 17 via the ejector 116, while the refrigerant that has passed through the cooling-side heat exchanger flows into the return pipe 70, both during cooling and heating operation. As a result, the workload of the liquid pump 40 can be reduced during both cooling and heating operation. 【0199】Furthermore, as in this embodiment, in the refrigeration system 101, a return pipe 70 for returning the refrigerant to the gas-liquid separator 17 is provided on the outlet side of the cooling-side heat exchanger, the suction pipe 116a is branched from the return pipe 70, and the return pipe 70 is provided with a flow path switching valve 175 that directs the refrigerant that has passed through the cooling-side heat exchanger to either the suction pipe 116a or the gas-liquid separator 17. This configuration allows the refrigerant to be returned to the gas-liquid separator 17 without going through the ejector 116, thereby facilitating natural circulation that returns to the gas-liquid separator 17 through the return pipe 70 while avoiding the resistance of the ejector 116. For this reason, the ejector 116 can be effectively utilized under high load conditions, and natural circulation can be utilized under low load conditions, thereby improving the efficiency of the refrigeration system 101. 【0200】 As in this embodiment, the return pipe 70 may be configured to include a heat exchanger 74 located upstream of the flow path switching valve 175, which exchanges heat between the refrigerant in the return pipe 70 and the outside air. This makes it easier to increase the liquid component of the refrigerant returning to the gas-liquid separator 17 when the outside temperature is low, and the liquid refrigerant in the gas-liquid separator 17 is less likely to be depleted, thus making it easier to maintain natural circulation. As a result, the workload of the compressor can be reduced, and the efficiency of the refrigeration system can be improved. 【0201】 As in this embodiment, the refrigeration system 101 may be configured to include a venting pipe 30 that removes gaseous refrigerant from the gas-liquid separator 17 and returns it to the compressors 11 and 12, a venting throttle valve 135 for adjusting the flow rate of the venting pipe 30, a main pipe 41 provided downstream of the liquid pump 40 through which refrigerant flows toward the cooling-side heat exchanger, a cooling pipe 44 that branches off from the main pipe 41 and is connected to the part of the venting pipe 30 downstream of the venting throttle valve 135, and a cooling throttle valve 45 for adjusting the flow rate of refrigerant in the cooling pipe 44. This allows the refrigerant temperature in the gas-liquid separator 17 to be adjusted by adjusting the opening of the venting throttle valve 135, and the suction temperature of the compressors 11 and 12 to be adjusted by adjusting the opening of the cooling throttle valve 45. As a result, the discharge temperature of the compressors 11 and 12 can be adjusted, improving the reliability of the refrigeration system. 【0202】As in this embodiment, a subcooled heat exchanger 46 may be provided that cools the refrigerant in the main pipe 41 with the reduced-pressure refrigerant at the cooling throttle valve 45. This allows the refrigerant heading towards the cooling-side heat exchanger to be subcooled, improving the cooling capacity of the refrigeration system 101. As a result, the efficiency of the refrigeration system can be increased. 【0203】 (Embodiment 5) [5-1. Configuration] Figure 17 is a diagram showing the refrigeration circuit 202 of the refrigeration device 201 according to Embodiment 5. As shown in Figure 17, in the refrigeration circuit 202 of Embodiment 5, an ejector 216 is provided in place of the throttle valve 16 of Embodiment 1. The ejector 216 depressurizes the refrigerant of the high-pressure receiver 15 and discharges it toward the gas-liquid separator 17. 【0204】 Furthermore, the refrigeration circuit 202 does not have the cooling pipe 44, cooling throttle valve 45, and subcooling heat exchanger 46 of Embodiment 1. Instead, the refrigeration circuit 202 is provided with a cooling suction pipe 244, which is a pipe that branches off from the main pipe 41 and is connected to the suction port of the ejector 216, and a cooling throttle valve 245 that reduces the pressure of the refrigerant in the cooling suction pipe 244. Therefore, when the refrigerant from the high-pressure receiver 15 flows into the ejector 216, the refrigerant from the main pipe 41 flows into the cooling suction pipe 244, is reduced in pressure by the cooling throttle valve 245, and is drawn into the ejector 216. 【0205】 Furthermore, the refrigeration circuit 202 is equipped with a heat exchanger 246 that exchanges heat between the refrigerant, which has been depressurized by the cooling throttle valve 245, and the refrigerant that flows through the ejector 216 toward the gas-liquid separator 17. The heat exchanger 246 cools and liquefies the refrigerant that flows through the ejector 216 toward the gas-liquid separator 17. 【0206】 Furthermore, the refrigeration circuit 202 is provided with a bypass pipe 248 that connects the cooling suction pipe 244, specifically the downstream side of the heat exchanger 246, i.e., the suction port side of the ejector 216, to the gas vent pipe 30. The bypass pipe 248 is also provided with a bypass pipe throttle valve 249 that adjusts the flow rate of the refrigerant passing through the bypass pipe 248. 【0207】[5-2. Operation] The operation of the refrigeration circuit 202 according to Embodiment 5 is the same as that of the refrigeration circuit 2 according to Embodiment 1, except for the operation of the cooling throttle valve 245 and the bypass pipe throttle valve 249. Below, only the differences in operation from Embodiment 1 will be described. 【0208】 [5-2-1. Operation of the compressor during operation] Figure 18 shows the refrigeration circuit 202 during medium-load cooling operation. Figure 19 shows the refrigeration circuit 202 during high-load cooling operation. Figure 20 shows the refrigeration circuit 202 during heating operation. Figure 17 shows the refrigeration circuit 202 during low-load cooling operation. 【0209】 During low to high load cooling operation and heating operation as shown in Figures 17 to 20, when the compressors 11 and 12 are driven, the control unit 90 opens the cooling throttle valve 245. As a result, the refrigerant in the main piping 41 is drawn into the ejector 216 via the cooling suction pipe 244. Then, the refrigerant, depressurized by the cooling throttle valve 245, flows into the heat exchanger 246, and the refrigerant that has passed through the gas cooler, high-pressure receiver 15, and ejector 216 is cooled and liquefied. When the control unit 90 can draw in enough refrigerant for cooling by the heat exchanger 246 through the ejector 216, it closes the bypass pipe throttle valve 249. 【0210】 In contrast, for example, when the rotational speed of the compressors 11 and 12 is low, the refrigerant drawn from the main piping 41 through the cooling suction pipe 244 to the suction port of the ejector 216 may not be sufficient to cool the heat exchanger 246. In such cases, the control unit 90 opens the bypass pipe throttle valve 249 and connects the cooling suction pipe 244 and the gas vent pipe 30 via the bypass piping 248. Since the gas vent pipe 30 is connected to the suction side of the compressors 11 and 12, when the bypass pipe throttle valve 249 is open, the liquid refrigerant in the main piping 41 flows into the cooling suction pipe 244 due to the suction of the compressors 11 and 12, and passes through the cooling throttle valve 245 and the heat exchanger 246. As a result, the refrigerant that has passed through the ejector 216 can be cooled and liquefied by the heat exchanger 246. 【0211】Thus, in Embodiment 5, the liquefaction of the refrigerant flowing into the gas-liquid separator 17 can be promoted both when the suction force of the ejector 216 is sufficient and when the suction force is insufficient. 【0212】 [5-2-2. Operation when the compressor is stopped] Figure 21 shows the refrigeration circuit 202 during air-cooled cooling operation. When the compressors 11 and 12 are stopped, such as during air-cooled cooling operation, the control unit 90 closes the cooling throttle valve 245 and the bypass pipe throttle valve 249, blocking the flow of refrigerant in the cooling suction pipe 244 and the bypass piping 248. In addition, during air-cooled cooling operation, the control unit 90 closes the third cooling valve 57, the low-stage throttle valve 33, and the high-stage throttle valve 34, etc., so that the suction and discharge sides of the compressors 11 and 12 are not in communication with the gas-liquid separator 17. As a result, similar to Embodiment 1, the liquid refrigerant from the gas-liquid separator 17 can be sent to the utilization-side heat exchanger 21 for cooling using the liquid pump 40 or natural circulation, and the refrigerant can be liquefied in the heat exchanger 74 before being returned to the gas-liquid separator 17. 【0213】[5-3. Effects, etc.] As described above, in this embodiment, the refrigeration system 201 is equipped with a refrigeration circuit 202 connecting compressors 11 and 12, a heat source side heat exchanger 14, a gas-liquid separator 17, and a utilization side heat exchanger 21, and a liquid pump 40 that sends the liquid refrigerant from the gas-liquid separator 17 to the cooling side heat exchanger of the heat source side heat exchanger 14 and the utilization side heat exchanger 21, a check valve 43 provided in parallel with the liquid pump 40 to prevent backflow of refrigerant toward the gas-liquid separator 17, and the gas-liquid separator 40 of the heat source side heat exchanger 14 and the utilization side heat exchanger 21 The system includes an ejector 216 into which the refrigerant flowing through the air pump flows and which directs the incoming refrigerant to a gas-liquid separator 17; a main pipe 41 located downstream of the liquid pump 40, through which the refrigerant toward the cooling-side heat exchanger flows; a cooling suction pipe 244 branching from the main pipe 41 and connected to the suction port of the ejector 216; a cooling throttle valve 245 that reduces the pressure of the refrigerant in the cooling suction pipe 244; and a heat exchanger 246 that cools the refrigerant flowing from the ejector 216 to the gas-liquid separator 17 with the reduced pressure of the refrigerant in the cooling throttle valve 245. This allows the flow rate of the cooling suction pipe 244 to be increased using the suction force of the ejector 216, enabling cooling by the heat exchanger 246, and thus making it easier to liquefy the refrigerant flowing into the gas-liquid separator 17. As a result, the workload of the compressors 11 and 12 can be reduced, and the efficiency of the refrigeration system 201 can be improved. 【0214】 As in this embodiment, the refrigeration system 201 may be configured to include a venting pipe 30 that removes gaseous refrigerant from the gas-liquid separator 17 and returns it to the compressors 11 and 12, a bypass pipe 248 that connects the venting pipe 30 to the downstream side of the cooling suction pipe 244 of the heat exchanger 246, and a bypass pipe throttle valve 249 provided on the bypass pipe 248. This allows the flow rate of the cooling suction pipe 244 to be increased by utilizing the suction of the compressors 11 and 12 when the suction force of the ejector 216 is insufficient, thereby increasing the supercooling of the refrigerant flowing into the gas-liquid separator 17. As a result, the refrigeration capacity can be improved, and the efficiency of the refrigeration system 201 can be increased. 【0215】As in this embodiment, the refrigeration system 201 may be configured to include an external cooling device 47 for cooling the refrigerant in the main piping 41. This allows for greater supercooling of the refrigerant flowing into the cooling-side heat exchanger. As a result, the refrigeration capacity can be improved, and the efficiency of the refrigeration system 201 can be increased. 【0216】 As in this embodiment, the refrigeration circuit 202 of the refrigeration system 201 may use carbon dioxide as the refrigerant, and the external cooling device 47 may use a refrigeration cycle that utilizes a refrigerant with higher energy efficiency than carbon dioxide to cool the refrigerant in the main piping 41. This configuration allows the refrigeration capacity of the refrigeration system 201, which uses carbon dioxide as the refrigerant with low environmental impact, to be improved by the highly energy-efficient external cooling device 47. Furthermore, since the external cooling device 47 can be made simpler in configuration than the refrigeration system 201, even if a refrigerant such as HFC or HFO, which is highly efficient but has a greater environmental impact than carbon dioxide, is used in the external cooling device, the risk of refrigerant leakage from the external cooling device 47 is less likely to increase. Therefore, it is possible to improve the efficiency of the refrigeration system 201 while suppressing environmental impact. 【0217】 As in this embodiment, the refrigeration device 201 may be configured to include a water supply means 14b that supplies water to lower the temperature of the intake air of the heat source side heat exchanger 14 by latent heat of vaporization. This makes it easier to improve the refrigeration capacity of the refrigeration device 201 with low energy consumption. As a result, the efficiency of the refrigeration device 201 can be increased. 【0218】 (Embodiment 6) Next, the refrigeration apparatus 501 according to Embodiment 6 will be described. 【0219】 [6-1. Configuration] Figure 22 is a diagram showing the refrigeration circuit 502 of the refrigeration device 501 according to Embodiment 6. The refrigeration circuit 502 of the refrigeration device 501 has the same configuration as the refrigeration circuit 102 of Embodiment 4, in addition to having a pressure sensor 581. The pressure sensor 581 measures the pressure P5 inside the gas-liquid separator 17. The pressure sensor 581 also transmits the measured value of the pressure P5 inside the gas-liquid separator 17 to the control unit 90. 【0220】The refrigeration circuit 502 has a first refrigerant temperature sensor 582. The first refrigerant temperature sensor 582 measures the temperature T1 of the liquid refrigerant in the gas-liquid separator 17. More specifically, the first refrigerant temperature sensor 582 measures the temperature T1 as the outlet temperature of the liquid refrigerant in the gas-liquid separator 17. The first refrigerant temperature sensor 582 transmits the measured value of the liquid refrigerant temperature T1 in the gas-liquid separator 17 to the control unit 90. 【0221】 The refrigeration circuit 502 has a second refrigerant temperature sensor 583. The second refrigerant temperature sensor 583 measures the temperature T2 of the refrigerant between the heat source side heat exchanger 14 and the second switching mechanism 52. That is, when the refrigeration system 501 is performing cooling operation, the second refrigerant temperature sensor 583 measures the temperature T2 of the refrigerant on the outlet side of the heat source side heat exchanger 14, which is a gas cooler. The second refrigerant temperature sensor 583 transmits the measured temperature T2 to the control unit 90. 【0222】 Furthermore, the refrigeration unit 501 has an outside air temperature sensor 584. The outside air temperature sensor 584 is installed, for example, on the outdoor unit 10. The outside air temperature sensor 584 measures the outside air temperature T0 at the location where the refrigeration unit 501 is installed. The outside air temperature sensor 584 transmits the measured value of the outside air temperature T0 to the control unit 90. 【0223】 Furthermore, in this embodiment, the ejector 116 is configured to allow adjustment of the throttle opening under the control of the control unit 90. When the throttle opening of the ejector 116 is reduced, the amount of pressure reduction of the liquid refrigerant flowing in from the high-pressure receiver 15 becomes greater than when the throttle opening is large. 【0224】 [6-2. Operation] The operation of the refrigeration system 501 configured as described above will be explained below. The control of the refrigeration system 501 when it performs cooling operation will be explained below. Specifically, in the following operation, the cooling valves 53, 55, 57, and 59 are set to the open state, and the heating valves 54, 56, and 58 and the heating throttle valve 60 are set to the closed state. Also, during the following operation, the return side on / off valve 71 is set to the open state. As a result, during the following operation, the heat source side heat exchanger 14 functions as a gas cooler, and the utilization side heat exchanger 21 functions as a cooling side heat exchanger. 【0225】[6-2-1. Operation to switch between single-stage compression operation and double-stage compression operation] Figure 23 is a flowchart of the refrigeration unit 501, showing the operation to switch between single-stage compression operation and double-stage compression operation when the refrigeration unit 501 is in cooling operation. The operation in Figure 23 may be performed repeatedly at predetermined time intervals (for example, every 5 minutes) while the refrigeration unit 501 is performing cooling operation, or it may be configured to be performed when the refrigeration unit 501 starts cooling operation. In this specification, single-stage compression operation means operation in which only the high-stage compressor 12 of the compressors 11 and 12 is driven, and double-stage compression operation means operation in which both compressors 11 and 12 are driven. 【0226】 At the beginning of the operation shown in Figure 23, in step SD1, the control unit 90 performs a determination regarding the ambient temperature T0. The control unit 90 uses the measurement value from the ambient temperature sensor 584 for the determination. Specifically, the control unit 90 determines whether the ambient temperature T0 is less than the low-temperature reference temperature TL, whether the ambient temperature T0 is greater than or equal to the low-temperature reference temperature TL and less than or equal to the high-temperature reference temperature TH, or whether the ambient temperature T0 exceeds the high-temperature reference temperature TH. 【0227】 The high-temperature reference temperature TH and the low-temperature reference temperature TL are pre-stored in the storage medium of the control unit 90. The high-temperature reference temperature TH is set to a higher temperature than the low-temperature reference temperature TL. In this embodiment, the high-temperature reference temperature TH is 25°C. Also in this embodiment, the low-temperature reference temperature TL is 20°C. 【0228】 In step SD1, if the control unit 90 determines that the outside air temperature T0 is equal to or greater than the low-temperature reference temperature TL and less than or equal to the high-temperature reference temperature TH (step SD1: TL ≤ T0 ≤ TH), the operation of the control unit 90 proceeds to step SD2. 【0229】 In step SD1, if the control unit 90 determines that the outside air temperature T0 is less than the low-temperature reference temperature TL (step SD1: T0 < TL), the operation of the control unit 90 proceeds to step SD3. 【0230】In step SD1, if the control unit 90 determines that the outside air temperature T0 exceeds the high-temperature reference temperature TH (step SD1: TH < T0), the operation of the control unit 90 proceeds to step SD4. 【0231】 In step SD2, the control unit 90 determines whether the temperature T2 of the refrigerant at the outlet side of the heat source side heat exchanger 14, which is a gas cooler, exceeds the sum of the ambient temperature T0 and the reference temperature difference dT. In other words, in step SD2, the control unit 90 determines whether the temperature difference obtained by subtracting the ambient temperature T0 from the temperature T2 of the refrigerant at the outlet side of the heat source side heat exchanger 14 exceeds the reference temperature difference dT. In step SD2, the control unit 90 uses the measurement value of the ambient temperature sensor 584 as the ambient temperature T0 and the measurement value of the second refrigerant temperature sensor 583 as the temperature T2. The reference temperature difference dT is stored in the storage medium of the control unit 90. In this embodiment, the reference temperature difference is 2K. 【0232】 When the temperature T2 exceeds the sum of the ambient temperature T0 and the reference temperature difference dT, that is, when the temperature difference obtained by subtracting the ambient temperature T0 from the temperature T2 exceeds the reference temperature difference dT, it can be estimated that the refrigerant in the heat source side heat exchanger 14, which is a gas cooler, is sufficiently hotter than the ambient temperature T0 and can dissipate heat sufficiently. Conversely, when the temperature T2 does not exceed the sum of the ambient temperature T0 and the reference temperature difference dT, that is, when the temperature difference obtained by subtracting the ambient temperature T0 from the temperature T2 does not exceed the reference temperature difference dT, it can be estimated that the refrigerant in the heat source side heat exchanger 14, which is a gas cooler, is not sufficiently hotter than the ambient temperature T0 and cannot dissipate heat sufficiently. Thus, in step SD2, it can be said that the control unit 90 is determining whether sufficient heat dissipation is occurring in the heat source side heat exchanger 14, which is a gas cooler. 【0233】 In step SD2, if the temperature T2 exceeds the sum of the ambient temperature T0 and the reference temperature difference dT (step SD2: YES), the control unit 90 proceeds to step SD3. In step SD2, if the temperature T2 does not exceed the sum of the ambient temperature T0 and the reference temperature difference dT (step SD2: NO), the control unit 90 proceeds to step SD4. 【0234】In step SD3, the control unit 90 sets a target high pressure according to the ambient temperature T0. The target high pressure is the target value of the high pressure, which is the pressure of the refrigerant in the range from the discharge port of the high-stage compressor 12 to the ejector 116 during the operation of the refrigeration system 501. The target high pressure set by the control unit 90 in step SD3 is the pressure of the first pressure range. The first pressure range is a range of pressures. In this embodiment, the first pressure range is the subcritical pressure of carbon dioxide, which is the refrigerant of the refrigeration circuit 502. 【0235】 In step SD4, the control unit 90 sets a target high pressure according to the ambient temperature T0, similar to step SD3. The target high pressure set by the control unit 90 in step SD4 is the pressure in the second pressure zone. The second pressure zone is a range of pressures higher than the first pressure zone. In this embodiment, the second pressure zone is the supercritical pressure for carbon dioxide, which is the refrigerant in the refrigeration circuit 502. 【0236】 In detail, in step SD3 and step SD4, the control unit 90 sets the target high pressure so that when the gaseous refrigerant in the gas-liquid separator 17 is compressed and raised to the target high pressure, the temperature of the pressurized refrigerant becomes higher than the ambient temperature T0 by a predetermined temperature difference. The predetermined temperature difference is, for example, 10K. This allows the refrigerant, which has been pressurized to the target high pressure, to dissipate heat sufficiently in the heat source side heat exchanger 14, which is a gas cooler. For example, the control unit 90 may store a table or the like that associates the temperature of the pressurized refrigerant with the value of the target high pressure of the ambient temperature T0 for a combination of the refrigerant pressure P5 or temperature T1 in the gas-liquid separator 17 and the ambient temperature T0 during cooling operation. 【0237】 Thus, in each of steps SD3 and SD4, the control unit 90 sets a target high pressure higher than the target high pressure when the ambient temperature T0 is high. After the completion of step SD3 and step SD4, the operation of the control unit 90 proceeds to step SD5. 【0238】In step SD5, the control unit 90 determines whether the target compression ratio is less than a specified value. The target compression ratio is a value calculated based on the target high pressure set in steps SD3 and SD4 and the refrigerant pressure P5 of the gas-liquid separator 17 during cooling operation. The specified value is, for example, a value stored in the storage medium of the control unit 90. 【0239】 In step SD5, if the control unit 90 determines that the target compression ratio is less than a specified value (step SD5: YES), the control unit 90 proceeds to step SD6. In step SD6, the control unit 90 closes the low-stage throttle valve 33 and opens the high-stage throttle valve 34. After step SD6, the control unit 90 proceeds to step SD7. In step SD7, the control unit 90 stops the low-stage compressor 11 and drives the high-stage compressor 12 to start single-stage compression operation. After step SD7, the control unit 90 terminates the operation shown in Figure 23. The high-stage throttle valve 34 is an example of a "high-stage valve" in this disclosure. The low-stage throttle valve 33 is an example of a "low-stage valve" in this disclosure. 【0240】 In step SD5, if the control unit 90 determines that the target compression ratio is not below a specified value (step SD5: NO), the control unit 90 proceeds to step SD8. In step SD8, the control unit 90 opens the low-stage throttle valve 33 and closes the high-stage throttle valve 34. After step SD8, the control unit 90 proceeds to step SD9. In step SD9, the control unit 90 drives the low-stage compressor 11 and the high-stage compressor 12 to start two-stage compression operation. After step SD9, the control unit 90 terminates the operation shown in Figure 23. 【0241】 Thus, when the target compression ratio is below a specified value, the increase in energy consumption can be suppressed by single-stage compression operation, and when the target compression ratio is not below a specified value, the load on each compressor 11 and 12 can be reduced by two-stage compression operation. 【0242】[6-2-2. Control of the flow path switching valve] Figure 24 is a flowchart showing the operation of the refrigeration unit 501, and shows the operation of switching the flow path switching valve 175. The operation in Figure 24 may be repeatedly performed at predetermined time intervals (for example, every 5 minutes) while the refrigeration unit 501 is performing cooling operation, or it may be configured to be performed when the refrigeration unit 501 starts cooling operation. 【0243】 At the start of the operation shown in Figure 24, in step SB1, the control unit 90 determines whether the ambient temperature T0 exceeds a first specified temperature Tc. The first specified temperature Tc is stored in the storage medium of the control unit 90. If the control unit 90 determines in step SB1 that the ambient temperature T0 exceeds the first specified temperature Tc (step SB1: YES), the operation of the control unit 90 proceeds to step SB2. If the control unit determines in step SB1 that the ambient temperature T0 does not exceed the first specified temperature Tc (step SB1: NO), the operation of the control unit 90 proceeds to step SB4. 【0244】 In step SB2, the control unit 90 determines whether the rotational speed N of the operating low-stage compressor 11 or high-stage compressor 12 exceeds the first specified rotational speed Nc. During single-stage compression operation, the control unit 90 uses the rotational speed N of the high-stage compressor 12 to make the determination in step SB2. During two-stage compression operation, the control unit 90 uses the rotational speed N of either the low-stage compressor 11 or the high-stage compressor 12 to make the determination in step SB2. The first specified rotational speed Nc is stored, for example, in the storage medium of the control unit 90. If the control unit 90 determines in step SB2 that the rotational speed N of the operating low-stage compressor 11 or high-stage compressor 12 exceeds the first specified rotational speed Nc (step SB2: YES), the operation of the control unit 90 proceeds to step SB3. In step SB2, if the control unit 90 determines that the rotational speed N of the operating low-stage compressor 11 or high-stage compressor 12 does not exceed the first specified rotational speed Nc (step SB2: NO), the operation of the control unit 90 proceeds to step SB4. 【0245】In step SB3, the control unit 90 switches the state of the flow path switching valve 175 to the first state. That is, in step SB3, the control unit 90 controls the flow path switching valve 175 to direct the refrigerant that has passed through the utilization-side heat exchanger 21, which is the cooling-side heat exchanger, to the suction port of the ejector 116 via the suction pipe 116a. After step SB3, the control unit 90 terminates the operation shown in Figure 24. If the flow path switching valve 175 was originally in the first state in step SB3, the control unit 90 maintains the flow path switching valve 175 in the first state. In this way, when the ambient temperature T0 exceeds the first specified temperature Tc (step SB1: YES) and the rotational speed N exceeds the first specified rotational speed Nc (step SB2: YES), the control unit 90 controls the flow path switching valve 175 to draw in the refrigerant with the ejector 116. As a result, the control unit 90 can cause the ejector 116 to draw in the refrigerant when the refrigerant circulation rate is large and the ejector 116 can draw in enough refrigerant. 【0246】In step SB4, the control unit 90 switches the state of the flow path switching valve 175 to the second state. That is, in step SB4, the control unit 90 controls the flow path switching valve 175 to allow the refrigerant that has passed through the cooling-side heat exchanger, the utilization-side heat exchanger 21, to flow to the gas-liquid separator 17, and not to the suction port of the ejector 116 via the suction pipe 116a. After step SB4, the control unit 90 completes the operation shown in Figure 24. If the flow path switching valve 175 was originally in the second state in step SB4, the control unit 90 maintains the flow path switching valve 175 in the second state. Also in step SB4, the control unit 90 controls the throttle opening of the ejector 116 to be greater than or equal to the first throttle opening. The first throttle opening is stored in the storage medium of the control unit 90. In this embodiment, the first throttle opening is the nearly fully open position of the ejector 116. Thus, when the ambient temperature T0 is below the first specified temperature Tc (step SB1: NO), or when the rotational speed N is below the first specified rotational speed Nc (step SB2: NO), the control unit 90 controls the flow path switching valve 175 to return the refrigerant that has passed through the utilization-side heat exchanger 21, which is the cooling-side heat exchanger, back to the gas-liquid separator 17. This allows the control unit 90 to return the refrigerant that has passed through the utilization-side heat exchanger 21 to the gas-liquid separator 17 without being sucked in by the ejector 116 when the refrigerant circulation rate is small and the ejector 116 cannot adequately suction. Furthermore, by controlling the throttle opening of the ejector 116 to a first throttle opening or higher, the throttling loss in the ejector 116 can be reduced when the refrigerant circulation rate is small. 【0247】 [6-2-3. Operation during Cooling Operation] Figure 25 is a flowchart showing the operation of the refrigeration unit 501, illustrating the operation when the refrigeration unit 501 performs cooling operation. Cooling operation is an operation that starts after a pull-down operation is performed to lower the temperature of the heated space to a set temperature, and is a cooling operation performed to maintain the temperature of the heated space at the set temperature. The operation in Figure 25 may be repeatedly performed at predetermined time intervals (for example, every 5 minutes) while the refrigeration unit 501 is performing cooling operation, or it may be configured to be performed when the refrigeration unit 501 starts cooling operation after a pull-down operation. 【0248】In step SC1, the control unit 90 determines whether the temperature T1 of the liquid refrigerant in the gas-liquid separator 17, or the pressure P5 of the liquid refrigerant in the gas-liquid separator 17, is within a specified range. In step SC1, the control unit 90 may determine only the temperature T1 of the liquid refrigerant in the gas-liquid separator 17, or only the pressure P5 of the liquid refrigerant in the gas-liquid separator 17. The specified range is a range of temperature or a range of pressure, and is stored, for example, in the storage medium of the control unit 90. Furthermore, the control unit 90 uses the measured value of the first refrigerant temperature sensor 582 or the measured value of the pressure sensor 581 for the determination in step SC1. 【0249】 In step SC1, if it is determined that the temperature T1 of the liquid refrigerant in the gas-liquid separator 17, or the pressure P5 of the liquid refrigerant in the gas-liquid separator 17, is within the specified range (step SC1: YES), the control unit 90 proceeds to step SC2. In step SC2, the control unit 90 maintains the throttle opening of the ejector 116 and the rotational speeds of the compressors 11 and 12. This makes it easier to maintain the temperature T1 of the liquid refrigerant in the gas-liquid separator 17 and the pressure P5 of the liquid refrigerant in the gas-liquid separator 17 within the specified range. 【0250】 Furthermore, in step SC1, if it is determined that the temperature T1 of the liquid refrigerant in the gas-liquid separator 17, or the pressure P5 of the liquid refrigerant in the gas-liquid separator 17, is lower than the specified range (step SC1: lower than the specified range or lower than the specified range), the operation of the control unit 90 proceeds to step SC3. In step SC3, the control unit 90 increases the throttle opening of the ejector 116 and decreases the rotational speed of the compressors 11 and 12. As a result, the amount of pressure reduction in the ejector 116 decreases, and the amount of refrigerant drawn from the venting pipe 30 to the compressors 11 and 12 decreases, making it easier for the temperature T1 and pressure P5 of the liquid refrigerant in the gas-liquid separator 17 to rise. Therefore, it becomes easier to adjust the temperature T1 of the liquid refrigerant in the gas-liquid separator 17 and the pressure P5 of the liquid refrigerant in the gas-liquid separator 17 to be within the specified range. 【0251】Furthermore, in step SC1, if it is determined that the temperature T1 of the liquid refrigerant in the gas-liquid separator 17, or the pressure P5 of the liquid refrigerant in the gas-liquid separator 17, is higher than the specified range (step SC1: higher than the specified range or higher than the specified range), the operation of the control unit 90 proceeds to step SC4. In step SC4, the control unit 90 reduces the throttle opening of the ejector 116 and increases the rotational speed of the compressors 11 and 12. As a result, the amount of pressure reduction in the ejector 116 increases, and the amount of refrigerant drawn from the venting pipe 30 to the compressors 11 and 12 increases, making it easier for the temperature T1 and pressure P5 of the liquid refrigerant in the gas-liquid separator 17 to decrease. Therefore, it becomes easier to adjust the temperature T1 of the liquid refrigerant in the gas-liquid separator 17 and the pressure P5 of the liquid refrigerant in the gas-liquid separator 17 to be within the specified range. 【0252】 [6-3. Effects, etc.] As described above, in this embodiment, the refrigeration system 501 is equipped with a refrigeration circuit 502 connecting a high-stage compressor 12, a low-stage compressor 11, a heat source side heat exchanger 14, a gas-liquid separator 17, and a utilization side heat exchanger 21, a liquid pump 40 that sends the liquid refrigerant from the gas-liquid separator 17 to the cooling side heat exchanger among the heat source side heat exchanger 14 and the utilization side heat exchanger 21, a check valve 43 provided in parallel with the liquid pump 40 to prevent backflow of refrigerant toward the gas-liquid separator 17, and a gaseous refrigerant from the gas-liquid separator 17 The system includes a gas venting pipe 30 and a control unit 90. The gas venting pipe 30 is connected to the high-stage compressor 12 via a high-stage throttle valve 34 and to the low-stage compressor 11 via a low-stage throttle valve 33. The control unit 90 determines a target compression ratio based on the ambient temperature T0 and, based on the target compression ratio, opens and closes the low-stage throttle valve to switch between two-stage compression operation, which drives the high-stage compressor 12 and the low-stage compressor 11, and single-stage compression operation, which stops the low-stage compressor 11 and drives the high-stage compressor 12. By switching between single-stage and two-stage compression operation based on the target compression ratio, it becomes easier to maintain an appropriate compression ratio and to suppress the starting and stopping of the compressors 11 and 12. As a result, the efficiency of the refrigeration system 501 can be increased. 【0253】As in this embodiment, the control unit 90 may be configured to determine a target high pressure based on the ambient temperature T0, and then determine a target compression ratio based on the target high pressure. This makes it easier to maintain an appropriate compression ratio by switching between single-stage compression operation and two-stage compression operation based on the target compression ratio, and makes it easier to suppress the starting and stopping of the compressors 11 and 12. As a result, the efficiency of the refrigeration system 501 can be increased. 【0254】 As in this embodiment, the control unit 90 may be configured to close the low-stage throttle valve 33 during single-stage compression operation. This suppresses oil leakage from the low-stage compressor 11, which is stopped during single-stage compression operation. As a result, the efficiency of the refrigeration system 501 can be improved while ensuring the reliability of the compressors 11 and 12. 【0255】 As in this embodiment, the refrigeration system 501 includes an ejector 116 into which refrigerant that has flowed through the gas cooler flows in between the heat source side heat exchanger 14 and the utilization side heat exchanger 21, and which directs the incoming refrigerant to a gas-liquid separator 17, a return pipe 70 provided on the outlet side of the cooling side heat exchanger that returns the refrigerant to the gas-liquid separator 17, and a flow path switching valve 175 provided on the return pipe 70 that directs the refrigerant that has passed through the cooling side heat exchanger to either the suction port of the ejector 116 or the gas-liquid separator 17. The control unit 90 may be configured to switch the flow path switching valve 175 when the outside air temperature T0 is below a first specified temperature Tc, directing the refrigerant that has passed through the cooling side heat exchanger to the gas-liquid separator 17, and setting the throttle opening of the ejector 116 to the first specified opening or greater. This makes it possible to reduce the throttling loss in the ejector 116 when the amount of refrigerant circulating in the refrigeration circuit 502 is small. Therefore, the efficiency of the refrigeration device 501 can be improved. 【0256】As in this embodiment, the control unit 90 may be configured to switch the flow path switching valve 175 based on the rotational speed N of the high-stage compressor 12 or low-stage compressor 11 during operation when the ambient temperature T0 exceeds a first specified temperature Tc. This allows the control unit 90 to switch whether to have the refrigerant in the return pipe 30 drawn into the ejector 116 or returned to the gas-liquid separator 17, depending on whether suction by the ejector 116 is possible. As a result, a highly efficient refrigeration system 501 utilizing the ejector can be realized. In particular, in this embodiment, when the ambient temperature T0 exceeds a first specified temperature Tc and the rotational speed N exceeds a first specified rotational speed Nc, the control unit 90 controls the flow path switching valve 175 to have the ejector 116 draw in the refrigerant. As a result, the control unit 90 can have the ejector 116 draw in the refrigerant when the refrigerant circulation amount is large and sufficient suction by the ejector 116 is possible, thereby improving the efficiency of the refrigeration system 501. 【0257】 As in this embodiment, the control unit 90 may be configured to switch the flow path switching valve 175 when the ambient temperature T0 exceeds a first specified temperature Tc and the rotational speed N of the operating high-stage compressor 12 or low-stage compressor 11 is less than the first specified rotational speed Nc, thereby directing the refrigerant that has passed through the cooling-side heat exchanger to the gas-liquid separator 17. This allows the refrigerant in the return pipe 30 to be returned directly to the gas-liquid separator 17 when suction by the ejector 116 is not possible. Therefore, a highly efficient refrigeration system 501 utilizing the ejector 116 can be realized. 【0258】 As in this embodiment, the refrigeration circuit 502 may be configured to use carbon dioxide as a refrigerant. This reduces the environmental burden caused by the refrigerant. Therefore, it is possible to improve the efficiency of the refrigeration device 501 while reducing the environmental burden caused by the refrigerant. 【0259】(Other Embodiments) As described above, Embodiments 1 to 6 have been explained as examples of the technology disclosed in this application. However, the technology in this disclosure is not limited thereto and can be applied to embodiments that have been modified, replaced, added, or omitted. Furthermore, it is possible to create new embodiments by combining the components described in Embodiments 1 to 6 above. Therefore, other embodiments are described below as examples. 【0260】 In the above embodiment, the refrigeration device 1 was described as an air conditioner, but this is just one example. The refrigeration device 1 may be any device other than an air conditioner that uses a refrigeration cycle to heat or cool an object. For example, the refrigeration device 1 may be a refrigerator, a display case, or other type of refrigeration unit. 【0261】 In the above embodiment, the refrigerant circuit 2 was equipped with a low-stage compressor 11 and a high-stage compressor 12 as compressors, and was configured to enable two-stage compression, but this is just one example. For example, instead of the low-stage compressor 11 and the high-stage compressor 12, a compound compressor capable of two-stage compression may be provided as the compressor of the refrigerant circuit 2. Alternatively, instead of the low-stage compressor 11 and the high-stage compressor 12, a single-stage compressor may be provided as the compressor of the refrigerant circuit 2. Furthermore, the single-stage compressor may be equipped with an injection port that allows for refrigerant injection. In this case, the high-stage side branch pipe 32 may be connected to the injection port. Note that two or more of the low-stage compressor 11, the high-stage compressor 12, or compressors that replace them may be connected in parallel. 【0262】In the above embodiment, the flow path switching mechanism 50 was described as comprising a first switching mechanism 51 having annularly connected valves 53 to 56 and a second switching mechanism 52 having annularly connected valves 57 to 63, but this is just one example. The flow path switching mechanism 50 can switch between the utilization-side heat exchanger 21 and the heat source-side heat exchanger 14 to function as a gas cooler, and it is sufficient that the refrigerant that has passed through the gas cooler can be directed toward the gas-liquid separator 17 regardless of which one is functioning as the gas cooler. In addition, the flow path switching mechanism 50 can switch between the utilization-side heat exchanger 21 and the heat source-side heat exchanger 14 to function as a cooling-side heat exchanger, and it is sufficient that the refrigerant that has passed through the cooling-side heat exchanger can be directed toward the suction-side piping 18 and the return piping 70 regardless of which one is functioning as the cooling-side heat exchanger. In other words, if it has the same function as the flow path switching mechanism 50 of this embodiment, part or all of the flow path switching mechanism 50 may be replaced with an on-off valve, a check valve, a throttle valve, or a four-way valve, etc. 【0263】 In the above embodiment, carbon dioxide was described as being used as the refrigerant in the refrigerant circuit 2, but this is just one example. The type of refrigerant in the refrigerant circuit 2 is not particularly limited and may be a natural refrigerant other than carbon dioxide, or a refrigerant other than a natural refrigerant such as an HFC-based refrigerant or an HFO-based refrigerant. However, if carbon dioxide is used as the refrigerant in the refrigerant circuit 2, the risk of refrigerant leakage, such as environmental impact, can be reduced. 【0264】 In the above embodiment, it was explained that during heating operation, the intake valve 19 is in the open state and the return valve 71 is in the closed state, but this is just one example. The refrigeration system 1 may also be configured to close the intake valve 19 and open the return valve 71 during heating operation, just as during cooling operation, when the heating load is medium load or less. This makes it possible to independently store liquid refrigerant in the gas-liquid separator 17 by driving the compressors 11 and 12, and to independently send the liquid refrigerant from the gas-liquid separator 17 toward the heat source side heat exchanger 14 by driving the liquid pump 40, thereby reducing the energy consumed by driving the compressors 11 and 12. 【0265】In Embodiment 2, it was explained that the refrigeration device 301 is provided with both a pressure sensor 381 and a temperature sensor 382, ​​but this is just one example. The refrigeration device 301 only needs to be provided with either the pressure sensor 381 or the temperature sensor 382. Furthermore, the operation of step SA2 in Figure 7 may be performed using the measurement value of either the pressure sensor 381 or the temperature sensor 382 provided in the refrigeration device 301. 【0266】 The operation shown in Figure 7 in Embodiment 2 is just one example, and the operation of the control unit 90 is not limited thereto. For example, in addition to the operation shown in Figure 7, the control unit 90 may, after step SA3 or the like, compare the measured values ​​of sensors 381 and 382 with a second pressure and a second temperature that are lower than the first pressure and the first temperature, and determine whether the pressure reduction by the expansion mechanism 316 is excessive. Alternatively, the control unit 90 may be configured to reduce the opening of the cooling throttle valve 45 when it determines that the measured values ​​of sensors 381 and 382 are below the second pressure and the second temperature, and that the pressure reduction by the expansion mechanism 316 is excessive. 【0267】 In the above embodiment, a three-way valve was described as being used as the flow path switching valve 175, but this is just one example. The flow path switching valve 175 can be configured to allow the refrigerant from the return pipe 70 to flow through either the suction pipe 116a or the gas-liquid separator 17, and to switch the inflow destination. For example, the flow path switching valve 175 may be composed of multiple on-off valves or throttle valves. 【0268】In the above embodiment, a low-stage throttle valve 33 and a high-stage throttle valve 34 were described as examples of low-stage and high-stage valves provided on the suction side of each compressor 11 and 12, but this is just one example. The valves provided on the suction side of each compressor 11 and 12 only need to be able to switch between single-stage compression operation, in which the high-stage compressor is driven while the low-stage compressor 11 is stopped, and two-stage compression operation, in which both the low-stage compressor 11 and the high-stage compressor 12 are driven. Therefore, the valves provided on the suction side of each compressor 11 and 12 are not limited to a low-stage throttle valve 33 and a high-stage throttle valve 34 that can be switched between an open state and a closed state and the degree of opening can be adjusted. For example, instead of a low-stage throttle valve 33 and a high-stage throttle valve 34, an on-off valve that can only be switched between an open state and a closed state may be used. Alternatively, for example, instead of the low-stage throttle valve 33, an on-off valve that can only be switched between an open state and a closed state may be used, and instead of the high-stage throttle valve 34, a check valve may be provided to block the refrigerant flowing back from the discharge side of the low-stage compressor 11 to the venting pipe 30 via the high-stage branch pipe 32. 【0269】 In Embodiment 6, the refrigeration system 501 was described as having an ambient temperature sensor 584 for measuring the ambient temperature T0, but this is just one example. The refrigeration system 501 only needs to be configured so that the control unit 90 can acquire the ambient temperature T0 at the location where the refrigeration system 501 is installed. For example, the control unit 90 may be configured to be able to connect to an external server or terminal such as a smartphone via a communication network such as the Internet, and to acquire the ambient temperature at the location where the refrigeration system 501 is installed from the external server or terminal. In this case, the ambient temperature sensor 584 may be omitted. 【0270】Furthermore, the operational step units shown in Figure 7 are divided according to the main processing content in order to facilitate understanding of the operation of each part of the refrigeration device 301, and this disclosure is not limited by the way the processing units are divided or the names of those units. Similarly, the operational step units shown in Figures 23 to 25 are divided according to the main processing content in order to facilitate understanding of the operation, and the operation is not limited by the way the processing units are divided or the names of those units. Depending on the processing content, the operation may be further divided into more step units. Alternatively, one step unit may be divided to include even more processing. Furthermore, the order of the steps may be rearranged as appropriate, as long as it does not impede the intent of this disclosure. 【0271】 Since the embodiments described above are for illustrative purposes of the technology described herein, various modifications, substitutions, additions, omissions, etc., can be made within the claims or their equivalents. 【0272】 (Note) The above description of embodiments discloses the following technologies. (Technology 1) A refrigeration system comprising a refrigerant circuit connecting a compression mechanism, a heat source side heat exchanger, a gas-liquid separator, and a utilization side heat exchanger, wherein the system includes a liquid pump that sends the liquid refrigerant from the gas-liquid separator to the cooling side heat exchanger among the heat source side heat exchanger and the utilization side heat exchanger, a check valve provided in parallel with the liquid pump to prevent backflow of refrigerant toward the gas-liquid separator, and an expansion mechanism that expands the refrigerant that has flowed through the gas cooler among the heat source side heat exchanger and the utilization side heat exchanger and flows it to the gas-liquid separator, wherein the compression mechanism is provided in a gas venting pipe into which the gas refrigerant from the gas-liquid separator flows and operates coaxially with the expansion mechanism. 【0273】(Technology 2) The refrigeration apparatus according to Technology 1, characterized in that it has a compressor operated by a motor, and the discharge side of the compression mechanism is connected to both the suction side and the discharge side of the compressor via on-off valves. This makes it possible to switch between single-stage compression using only the compression mechanism and two-stage compression using the compressor in addition to the compression mechanism, depending on the load, etc. Therefore, energy consumption for operating the compressor can be suppressed, and the efficiency of the refrigeration apparatus can be improved. 【0274】 (Technology 3) The refrigeration apparatus according to Technology 1 or 2, characterized in that it has a main pipe provided downstream of the liquid pump through which the refrigerant flows toward the cooling-side heat exchanger, a cooling pipe branching off from the main pipe, a cooling throttle valve for adjusting the flow rate of the cooling pipe, and a subcooling heat exchanger that cools the refrigerant in the main pipe with the refrigerant reduced in pressure by the cooling throttle valve, and the cooling pipe is connected between the compression mechanism and the gas-liquid separator in the gas venting pipe. As a result, the refrigerant flowing into the cooling-side heat exchanger can be subcooled by utilizing the suction force of the compression mechanism which operates coaxially with the expansion mechanism. Therefore, the refrigeration effect can be increased while suppressing energy consumption, and the efficiency of the refrigeration apparatus can be improved. 【0275】 (Technical 4) The refrigeration apparatus according to Technical 3, wherein an external cooling device is provided to cool the refrigerant after it has passed through the subcooled heat exchanger in the main piping. This allows the refrigerant flowing toward the cooling-side heat exchanger to be subcooled even when the pressure reduction by the expansion mechanism is insufficient. Therefore, the efficiency of the refrigeration apparatus can be improved while stabilizing the refrigeration effect. 【0276】(Technical 5) The refrigeration apparatus described in Technical 4, wherein the refrigerant circuit uses carbon dioxide as the refrigerant, and the external cooling equipment uses a refrigeration cycle that utilizes a refrigerant with higher energy efficiency than carbon dioxide to cool the refrigerant in the main piping. This allows the refrigeration capacity of the refrigeration apparatus using carbon dioxide, which has a low environmental impact, to be improved by using an external cooling equipment with high energy efficiency. Furthermore, since the external cooling equipment can be made simpler in configuration than the refrigeration apparatus, even when using refrigerants such as HFCs or HFOs, which are highly efficient but have a greater environmental impact than carbon dioxide, in the external cooling equipment, the risk of refrigerant leakage from the external cooling equipment is less likely to increase. For this reason, it is possible to improve the efficiency of the refrigeration apparatus while suppressing environmental impact. 【0277】 (Technical 6) A refrigeration system according to any one of Technical 1 to 5, comprising a return pipe through which the refrigerant that has passed through the cooling-side heat exchanger flows and which is connected to the gas-liquid separator, and a heat exchanger that exchanges heat between the refrigerant in the return pipe and the outside air. This makes it easier to increase the liquid component of the refrigerant flowing into the gas-liquid separator by releasing heat from the refrigerant in the return pipe to the outside air, and thus easier to reduce the workload of the compressor. As a result, it is easier to improve the APF and make the refrigeration system more efficient. 【0278】 (Technical 7) The refrigeration apparatus according to Technical 6, further comprising a water supply means for supplying water to lower the temperature of the intake air of the heat exchanger by latent heat of vaporization. This makes it easier to dissipate heat from the refrigerant in the return piping to the outside air, thus making it easier to reduce the workload of the compressor. As a result, it becomes easier to improve the APF and increase the efficiency of the refrigeration apparatus. 【0279】 (Technical 8) A refrigeration apparatus according to any one of Technical 1 to 7, comprising a water supply means for supplying water to lower the temperature of the intake air of the heat source side heat exchanger by latent heat of vaporization. This makes it easier to improve the refrigeration capacity of the refrigeration apparatus with low energy consumption. Therefore, it is possible to improve the efficiency of the refrigeration apparatus. 【0280】(Technical 9) A refrigeration apparatus according to any one of Technical 1 to 8, wherein a return pipe is provided through which the refrigerant that has passed through the cooling-side heat exchanger flows and is connected to the gas-liquid separator, and an oil separator is provided to recover oil from the return pipe and return it to the suction side of the compression mechanism. This makes it possible to suppress the accumulation of oil from the compression mechanism in the gas-liquid separator when the liquid refrigerant from the gas-liquid separator is circulated by flowing it to the cooling-side heat exchanger and returning it to the gas-liquid separator via the return pipe. This makes it easier to ensure reliability while increasing the efficiency of the refrigeration apparatus. 【0281】 (Technical 10) The refrigeration apparatus according to Technical 2, comprising an oil separator for recovering oil from the discharge pipe of the compressor, and an oil return mechanism for returning the oil recovered by the oil separator to the compressor and the expansion mechanism. This allows for a stable supply of oil to the compressor and the expansion mechanism. As a result, it becomes easier to ensure reliability while increasing the efficiency of the refrigeration apparatus. 【0282】 (Technical 11) A refrigeration system comprising a refrigeration circuit connecting a high-stage compressor, a low-stage compressor, a heat source side heat exchanger, a gas-liquid separator, and a utilization side heat exchanger, a liquid pump that sends the liquid refrigerant from the gas-liquid separator to the cooling side heat exchanger among the heat source side heat exchanger and the utilization side heat exchanger, a check valve provided in parallel with the liquid pump to prevent backflow of refrigerant toward the gas-liquid separator, a gas venting pipe that removes gaseous refrigerant from the gas-liquid separator, and a control unit, wherein the gas venting pipe is connected to the high-stage compressor via a high-stage side valve and to the low-stage compressor via a low-stage side valve, and the control unit determines a target compression ratio based on the ambient temperature, and based on the target compression ratio, opens and closes the low-stage side valve to switch between a two-stage compression operation that drives the high-stage compressor and the low-stage compressor, and a single-stage compression operation that stops the low-stage compressor and drives the high-stage compressor. This makes it easier to maintain an appropriate compression ratio by switching between single-stage and two-stage compression operation based on the target compression ratio, and also makes it easier to suppress compressor starting and stopping. As a result, the efficiency of the refrigeration system can be improved. 【0283】(Technical 12) The refrigeration apparatus according to Technical 11, wherein the control unit determines a target high pressure based on the ambient temperature and determines the target compression ratio based on the target high pressure. This makes it easier to maintain an appropriate compression ratio and suppress the starting and stopping of the compressor by switching between single-stage compression operation and two-stage compression operation based on the target compression ratio. As a result, the efficiency of the refrigeration apparatus can be increased. 【0284】 (Technical 13) The refrigeration system according to Technical 11 or 12, wherein the control unit closes the low-stage side valve during the single-stage compression operation. This suppresses oil leakage from the low-stage compressor, which is stopped during the single-stage compression operation. Therefore, it is possible to improve the efficiency of the refrigeration system while ensuring the reliability of the compressor. 【0285】 (Technical 14) A refrigeration apparatus according to any one of Technical 11 to 13, comprising: an ejector into which refrigerant that has flowed through a gas cooler flows between the heat source side heat exchanger and the utilization side heat exchanger, and which directs the incoming refrigerant to the gas-liquid separator; a return pipe provided on the outlet side of the cooling side heat exchanger that returns the refrigerant to the gas-liquid separator; and a flow path switching valve provided on the return pipe that directs the refrigerant that has passed through the cooling side heat exchanger to either the suction port of the ejector or the gas-liquid separator, wherein the control unit switches the flow path switching valve when the ambient temperature is below a first specified temperature, directing the refrigerant that has passed through the cooling side heat exchanger to the gas-liquid separator, and setting the throttle opening of the ejector to a first specified opening or greater. This makes it possible to reduce the throttling loss in the ejector when the amount of refrigerant circulating in the refrigeration circuit is small. As a result, the efficiency of the refrigeration apparatus can be increased. 【0286】 (Technical 15) The refrigeration apparatus according to Technical 14, wherein the control unit switches the flow path switching valve based on the rotational speed of the high-stage compressor or the low-stage compressor during operation when the ambient temperature exceeds the first specified temperature. This makes it possible to switch whether to draw the refrigerant in the return pipe into the ejector or return it to the gas-liquid separator, depending on whether suction by the ejector is possible. For this reason, a highly efficient refrigeration apparatus utilizing the ejector can be realized. 【0287】(Technical 16) The refrigeration apparatus according to Technical 15, wherein the control unit switches the flow path switching valve when the ambient temperature exceeds the first specified temperature and the rotational speed of the high-stage compressor or the low-stage compressor in operation is less than the first specified rotational speed, thereby allowing the refrigerant that has passed through the cooling-side heat exchanger to flow to the gas-liquid separator. This allows the refrigerant in the return piping to be returned directly to the gas-liquid separator when suction by the ejector is not possible. Therefore, a highly efficient refrigeration apparatus utilizing the ejector can be realized. 【0288】 (Technical 17) The refrigeration circuit is a refrigeration device according to any one of Technical 11 to 16, which uses carbon dioxide as a refrigerant. This makes it possible to suppress the environmental burden caused by the refrigerant. Therefore, it is possible to improve the efficiency of the refrigeration device while suppressing the environmental burden caused by the refrigerant. 【0289】 This disclosure is applicable to refrigeration equipment. Specifically, this disclosure is applicable to devices such as air conditioners that have a refrigerant circuit. 【0290】1 Refrigeration unit 2 Refrigerant circuit (refrigeration circuit) 10 Outdoor unit 11 Low-stage compressor (compressor) 12 High-stage compressor (compressor) 13 Oil separator 13a Oil tank 14 Heat source side heat exchanger 14a Blower 14b Water supply means 15 High-pressure receiver 16 Throttle valve 17 Gas-liquid separator 18 Suction side piping 19 Suction side on-off valve (on-off valve) 20, 20H, 20L Indoor unit 21 Utilization side heat exchanger 22 Utilization side throttle valve 30 Gas venting piping 31 Low-stage side branch pipe 32 High-stage side branch pipe 33 Low-stage side throttle valve (low-stage valve) 34 High-stage side throttle valve (high-stage valve) 40 Liquid pump 41 Main piping 42 Route 43 Check valve 44 Cooling piping 45 Cooling throttle valve 46 Cooling heat exchanger 47 External cooling equipment 50 Flow path switching mechanism 51 First switching mechanism 52 Second switching mechanism 53 First cooling valve 54 First heating valve 55 Second cooling valve 56 Second heating valve 57 Third cooling valve 58 Third heating valve 59 Fourth cooling valve 60 Heat throttle valve 61-63 Check valve 70 Return piping 71 Return side on / off valve (on / off valve) 72 Check valve 73 Oil separator 74 Heat exchanger 74a Blower 74b Water supply means 90 Control unit 101 Refrigeration device 102 Refrigeration circuit 116 Ejector 116a Suction pipe 118 Suction side piping 135 Gas vent throttle valve 175 Flow path switching valve 201 Refrigeration device 202 Refrigeration circuit 216 Ejector 244 Cooling suction pipe 245 Cooling throttle valve 246 Heat exchanger 248 Bypass piping 249 Bypass pipe throttle valve 301 Refrigeration system 302 Refrigerant circuit 316 Expansion mechanism 381 Pressure sensor 382 Temperature sensor 401 Refrigeration system 402 Refrigerant circuit 411 Compression mechanism 411a Shaft 411b Oil pipe 412 Compressor 413 Stage side discharge piping 414 First branch pipe 414a First on-off valve (on-off valve) 415 Second branch pipe 415a Second on-off valve (on-off valve) 416 Discharge piping 420 Oil return mechanism 421 Oil return piping 422 First oil return branch pipe 423 Second oil return branch pipe 424 First oil return volume adjustment valve 425 Second oil return volume adjustment valve 501 Refrigeration system502 Refrigeration circuit 581 Pressure sensor 582 First refrigerant temperature sensor 583 Second refrigerant temperature sensor 584 Ambient temperature sensor

Claims

1. A refrigeration system comprising a refrigerant circuit connecting a compression mechanism, a heat source side heat exchanger, a gas-liquid separator, and a utilization side heat exchanger, wherein the system includes a liquid pump that sends the liquid refrigerant from the gas-liquid separator to the cooling side heat exchanger among the heat source side heat exchanger and the utilization side heat exchanger, a check valve provided in parallel with the liquid pump to prevent backflow of refrigerant toward the gas-liquid separator, and an expansion mechanism that expands the refrigerant that has flowed through the gas cooler among the heat source side heat exchanger and the utilization side heat exchanger and flows it toward the gas-liquid separator, wherein the compression mechanism is provided in a gas venting pipe into which the gas refrigerant from the gas-liquid separator flows and operates coaxially with the expansion mechanism.

2. The refrigeration apparatus according to claim 1, wherein the compressor is operated by a motor, and the discharge side of the compression mechanism is connected to both the suction side and the discharge side of the compressor via on-off valves.

3. The refrigeration apparatus according to claim 1, comprising: a main pipe provided downstream of the liquid pump and through which refrigerant flows toward the cooling heat exchanger; a cooling pipe branching off from the main pipe; a cooling throttle valve for adjusting the flow rate of the cooling pipe; and a subcooling heat exchanger that cools the refrigerant in the main pipe with refrigerant reduced in pressure by the cooling throttle valve, wherein the cooling pipe is connected between the compression mechanism and the gas-liquid separator in the gas venting pipe.

4. The refrigeration apparatus according to claim 3, wherein an external cooling device is provided to cool the refrigerant after it has passed through the subcooled heat exchanger in the main piping.

5. The refrigeration apparatus according to claim 4, wherein the refrigerant circuit uses carbon dioxide as the refrigerant, and the external cooling equipment uses a refrigeration cycle that utilizes a refrigerant more energy-efficient than carbon dioxide to cool the refrigerant in the main piping.

6. The refrigeration apparatus according to claim 1, comprising: a return pipe through which the refrigerant that has passed through the cooling-side heat exchanger flows and which is connected to the gas-liquid separator; and a heat exchanger that exchanges heat between the refrigerant in the return pipe and the outside air.

7. The refrigeration apparatus according to claim 6, further comprising a water supply means for supplying water to lower the temperature of the intake air of the heat exchanger by latent heat of vaporization.

8. The refrigeration apparatus according to claim 1, further comprising a water supply means for supplying water to lower the temperature of the intake air of the heat source side heat exchanger by latent heat of vaporization.

9. The refrigeration apparatus according to claim 1, further comprising: a return pipe through which the refrigerant that has passed through the cooling-side heat exchanger flows and which is connected to the gas-liquid separator; and an oil separator that recovers oil from the return pipe and returns it to the suction side of the compression mechanism.

10. The refrigeration apparatus according to claim 2, further comprising: an oil separator for recovering oil from the discharge pipe of the compressor; and an oil return mechanism for returning the oil recovered by the oil separator to the compressor and the expansion mechanism.

11. A refrigeration system comprising: a high-stage compressor, a low-stage compressor, a heat source side heat exchanger, a gas-liquid separator, and a utilization side heat exchanger; a liquid pump for supplying liquid refrigerant from the gas-liquid separator to the cooling side heat exchanger among the heat source side heat exchanger and the utilization side heat exchanger; a check valve provided in parallel with the liquid pump to prevent backflow of refrigerant toward the gas-liquid separator; a gas venting pipe for removing gaseous refrigerant from the gas-liquid separator; and a control unit; wherein the gas venting pipe is connected to the high-stage compressor via a high-stage side valve and to the low-stage compressor via a low-stage side valve; and the control unit determines a target compression ratio based on the ambient temperature; and switches between a two-stage compression operation, which drives the high-stage compressor and the low-stage compressor by opening and closing the low-stage side valve based on the target compression ratio, and a single-stage compression operation, which stops the low-stage compressor and drives the high-stage compressor.

12. The refrigeration apparatus according to claim 11, wherein the control unit determines a target high pressure based on the ambient temperature and determines the target compression ratio based on the target high pressure.

13. The refrigeration apparatus according to claim 11 or 12, wherein the control unit closes the low-stage side valve during the single-stage compression operation.

14. The refrigeration apparatus according to claim 11 or 12, comprising: an ejector into which refrigerant that has flowed through a gas cooler flows between the heat source side heat exchanger and the utilization side heat exchanger, and which directs the incoming refrigerant to the gas-liquid separator; a return pipe provided on the outlet side of the cooling side heat exchanger for returning the refrigerant to the gas-liquid separator; and a flow path switching valve provided on the return pipe for directing the refrigerant that has passed through the cooling side heat exchanger to either the suction port of the ejector or the gas-liquid separator, wherein the control unit switches the flow path switching valve when the ambient temperature is below a first specified temperature, directing the refrigerant that has passed through the cooling side heat exchanger to the gas-liquid separator, and setting the throttle opening of the ejector to a first specified opening or greater.

15. The refrigeration apparatus according to claim 14, wherein the control unit switches the flow path switching valve based on the rotational speed of the high-stage compressor or the low-stage compressor in operation when the outside air temperature exceeds the first specified temperature.

16. The refrigeration apparatus according to claim 15, wherein the control unit switches the flow path switching valve when the ambient temperature exceeds the first specified temperature and the rotational speed of the high-stage compressor or the low-stage compressor in operation is less than the first specified rotational speed, thereby allowing the refrigerant that has passed through the cooling-side heat exchanger to flow to the gas-liquid separator.

17. The refrigeration apparatus according to claim 11 or 12, wherein the refrigeration circuit uses carbon dioxide as a refrigerant.

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