Refrigeration cycle device

Abstract

When a first compressor (11) and a second compressor (51) are in a stopped state, a control unit (100) executes a first operation in which the first compressor (11) is operated while keeping the second compressor (51) in the stopped state to cause heat dissipation of a first refrigerant in a refrigerant heat exchanger (52). After the first operation, the control unit (100) executes a second operation in which the second compressor (51) is operated.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a refrigeration cycle apparatus.BACKGROUND ART

[0002] Patent Document 1 discloses a refrigeration cycle apparatus that performs a binary refrigeration cycle. In the refrigeration cycle apparatus, a low-stage first refrigerant circuit and a high-stage second refrigerant circuit are connected by a cascade heat exchanger. A refrigeration cycle is performed in each of the first refrigerant circuit and the second refrigerant circuit.CITATION LISTPATENT DOCUMENT

[0003] Patent Document 1: Japanese Unexamined Patent Publication No. 2004-132647SUMMARY OF THE INVENTIONTECHNICAL PROBLEM

[0004] When a refrigeration cycle apparatus, such as one described in Patent Document 1, is stopped, refrigerant may dissolve into refrigerating machine oil inside a compressor of the second refrigerant circuit. This phenomenon is also called "refrigerant accumulation." If refrigerant accumulation occurs, the substantial amount of the refrigerant for use in the refrigeration cycle of the second refrigerant circuit is reduced. If the compressor of the second refrigerant circuit is operated in this state, low pressure drops sharply due to a shortage in the amount of refrigerant, and stable startup of the compressor cannot be achieved.

[0005] It is an object of the present disclosure to reduce a sharp drop of the low pressure at a startup of a compressor of a second refrigerant circuit.SOLUTION TO THE PROBLEM

[0006] A first aspect is directed to a refrigeration cycle apparatus. The refrigeration cycle apparatus includes: a first refrigerant circuit (10) through which a first refrigerant circulates to perform a refrigeration cycle; a second refrigerant circuit (50) through which a second refrigerant circulates to perform a refrigeration cycle; a refrigerant heat exchanger (52) configured to exchange heat between the first refrigerant in the first refrigerant circuit (10) and the second refrigerant in the second refrigerant circuit (50); and a control unit (100) configured to control the first refrigerant circuit (10) and the second refrigerant circuit (50). The first refrigerant circuit (10) includes a first compressor (11), a heat-source-side heat exchanger (12), and a first expansion mechanism (13, 14). The second refrigerant circuit (50) includes a second compressor (51), a second expansion mechanism (53), and a utilization-side heat exchanger (54). The control unit (100) is configured to execute, when the first compressor (11) and the second compressor (51) are in a stopped state, a first operation in which the first compressor (11) is operated while keeping the second compressor (51) in the stopped state to cause heat dissipation of the first refrigerant in the refrigerant heat exchanger (52). The control unit (100) is configured to execute, after the first operation, a second operation in which the second compressor (51) is operated.

[0007] According to the first aspect, in the first operation, the first compressor (11) is operated while the second compressor (51) is in the stopped state. In the first operation, the first refrigerant discharged from the first compressor (11) dissipates heat in the refrigerant heat exchanger (52). Thus, in the refrigerant heat exchanger (52), the second refrigerant in the second refrigerant circuit (50) is heated, and the inside of the second compressor (51) is also heated. As a result, refrigerant accumulation can be eliminated before startup of the second compressor (51).

[0008] When the second operation is executed after the first operation, the second compressor (51) is put into operation. Since refrigerant accumulation has been eliminated in the second compressor (51), a sharp drop in low pressure is reduced, allowing the second compressor (51) to start stably.

[0009] A second aspect is an embodiment of the first aspect. In the second aspect, the control unit (100) is configured to execute the first operation if a stop time of the first compressor (11) and the second compressor (51) is longer than a predetermined time.

[0010] According to the second aspect, if the stop time of the first compressor (11) and the second compressor (51) is longer than a predetermined time, the amount of the refrigerant dissolved into refrigerating machine oil inside the second compressor (51) increases. To address this, if the stop time is longer than the predetermined time, the control unit (100) executes the first operation to eliminate refrigerant accumulation.

[0011] A third aspect is an embodiment of the first or second aspect. In the third aspect, the control unit (100) is configured to control a rotational speed of the second compressor (51) to low speed in the second operation. Strictly speaking, the "low speed" as used herein refers to a rotational speed lower than the intermediate rotational speed within the control range of the rotational speed of the second compressor (51).

[0012] According to the third aspect, in the second operation, the second compressor (51) is controlled to operate at low speed, thereby reducing a sharp drop in low pressure of the second refrigerant circuit (50).

[0013] A fourth aspect is an embodiment of any one of the first to third aspects. In the fourth aspect, the control unit (100) is configured to execute the first operation again if a first condition is met during the second operation, the first condition being that an evaporation temperature, an evaporation pressure, or a low pressure of the second refrigerant circuit (50) is lower than a predetermined value.

[0014] According to the fourth aspect, if the first condition is met during the second operation, there is a possibility that refrigerant accumulation has not been sufficiently eliminated. To address this, if the first condition is met, the control unit (100) executes the first operation again to eliminate refrigerant accumulation.

[0015] A fifth aspect is an embodiment of any one of the first to fourth aspects. In the fifth aspect, during the second operation, the control unit (100) is configured to stop the second compressor (51) if a state in which a temperature difference between the first refrigerant and the second refrigerant in the refrigerant heat exchanger (52) is greater than a predetermined value continues for a predetermined time or longer.

[0016] According to the fifth aspect, when the second refrigerant leaks from the second refrigerant circuit (50), the low pressure of the second refrigerant circuit (50) is maintained at low level, and therefore, the temperature difference between the first refrigerant and the second refrigerant in the refrigerant heat exchanger (52) is maintained to be greater than a predetermined value. If such a condition is met, it is highly likely that the second refrigerant in the second refrigerant circuit leaks. Accordingly, if the state in which a temperature difference between the first refrigerant and the second refrigerant in the refrigerant heat exchanger (52) is greater than a predetermined value continues for a predetermined time or longer, the control unit (100) stops the second compressor (51).

[0017] A sixth aspect is an embodiment of any one of the first to fifth aspects. In the sixth aspect, the second refrigerant includes propane.

[0018] According to the sixth aspect, propane, which is a highly flammable refrigerant, is used as the second refrigerant. Propane is a highly flammable refrigerant and has a risk of ignition. Accordingly, the amount of the refrigerant which fills the second refrigerant circuit (50) is restricted, and the amount of refrigerant for use in the refrigeration cycle tends to be insufficient. Thus, if refrigerant accumulation occurs in the second compressor (51), the problem of a drop in low pressure becomes significant. To address this, the first operation is performed, which can eliminate refrigerant accumulation before startup of the second compressor (51); therefore, a drop in low pressure can be reduced.BRIEF DESCRIPTION OF THE DRAWINGS

[0019] [FIG. 1] FIG. 1 is a piping system diagram of a hot water supply apparatus of an embodiment. [FIG. 2] FIG. 2 is a block diagram of main devices of the hot water supply apparatus. [FIG. 3] FIG. 3 is a piping system diagram of the hot water supply apparatus, and illustrates a flow of a refrigerant in a first heating mode of a binary heating operation and that in a second operation. [FIG. 4] FIG. 4 is a piping system diagram of the hot water supply apparatus, and illustrates a flow of the refrigerant in a second heating mode of the binary heating operation. [FIG. 5] FIG. 5 is a flowchart of startup control. [FIG. 6] FIG. 6 is a piping system diagram of the hot water supply apparatus, and illustrates a flow of the refrigerant in a first operation. [FIG. 7] FIG. 7 is a piping system diagram of the hot water supply apparatus, and illustrates a flow of the refrigerant in a unitary heating operation. DESCRIPTION OF EMBODIMENTS

[0020] Embodiments of the present disclosure will be described in detail below with reference to the drawings. The present disclosure is not limited to the embodiments shown below, and various changes can be made within the scope without departing from the technical concept of the present disclosure. Since each of the drawings is intended to illustrate the present disclosure conceptually, dimensions, ratios, or numbers may be exaggerated or simplified as necessary for ease of understanding.(1-1) General Configuration

[0021] The refrigeration cycle apparatus is applied to a hot water supply apparatus (1). The hot water supply apparatus (1) generates hot water. The generated hot water is stored in a hot water storage tank and supplied to a predetermined target. The hot water supply apparatus (1) of this embodiment has an outdoor unit (OU) installed outdoors and an indoor unit (IU) installed indoors.

[0022] As illustrated in FIG. 1, the hot water supply apparatus (1) includes a first refrigerant circuit (10), a second refrigerant circuit (50), and a water circuit (60). The first refrigerant circuit (10) is filled with a first refrigerant, and the second refrigerant circuit (50) is filled with a second refrigerant different from the first refrigerant. The first refrigerant is carbon dioxide, and the second refrigerant is propane (R290), which is a highly flammable refrigerant. The highly flammable refrigerant has a high burning rate. The highly flammable refrigerant may be, for example, methane (R50), ethane (R170), butane (R600), or isobutane (R600a). The second refrigerant may be a single component refrigerant consisting of one type of highly flammable refrigerant, or may be a refrigerant mixture including the highly flammable refrigerant and one or more other types of refrigerant.

[0023] The hot water supply apparatus (1) performs a binary refrigeration cycle. In other words, in the hot water supply apparatus (1), the low-stage first refrigerant circuit (10) and the high-stage second refrigerant circuit (50) are connected by a refrigerant heat exchanger (52), which is a so-called "cascade heat exchanger." The refrigerant heat exchanger (52) includes a first flow path (52a) through which the first refrigerant in the first refrigerant circuit (10) flows and a second flow path (52b) through which the second refrigerant in the second refrigerant circuit (50) flows. In other words, the first refrigerant circuit (10) includes the first flow path (52a) of the refrigerant heat exchanger (52), and the second refrigerant circuit (50) includes the second flow path (52b) of the refrigerant heat exchanger (52).(1-2) First Refrigerant Circuit

[0024] The first refrigerant circuit (10) performs a refrigeration cycle using the first refrigerant. The first refrigerant circuit (10) includes, as its main components, a first compressor (11), an outdoor heat exchanger (12), a first expansion valve (13), and a second expansion valve (14). The first refrigerant circuit (10) of this embodiment further includes a four-way switching valve (15), a receiver (16), and a bridge circuit (20). These components are provided in the outdoor unit (OU).

[0025] The first compressor (11) compresses a refrigerant sucked therein and discharges the compressed refrigerant. The first compressor (11) is a so-called high-pressure dome compressor. Specifically, the inside of the casing of the first compressor (11) is filled with a high-pressure refrigerant discharged from a compression mechanism. An oil reservoir for refrigerating machine oil is formed at the bottom of the casing. The refrigerating machine oil is supplied to sliding portions of the compression mechanism and bearings by an oil supply pump.

[0026] The outdoor heat exchanger (12) is an example of a heat-source-side heat exchanger. The outdoor heat exchanger (12) is an air heat exchanger that exchanges heat between outdoor air transferred by an outdoor fan (17) and the first refrigerant. The first expansion valve (13) and the second expansion valve (14) are examples of a first decompression mechanism. The first expansion valve (13) and the second expansion valve (14) are electronic expansion valves, for example, and decompress the refrigerant.

[0027] The four-way switching valve (15) has a first port (P1), a second port (P2), a third port (P3), and a fourth port (P4). The four-way switching valve (15) switches between a first state indicated by solid curves in FIG. 1 and a second state indicated by broken curves in FIG. 1. The four-way switching valve (15) in the first state makes the first port (P1) and the second port (P2) communicate with each other, and the third port (P3) and the fourth port (P4) communicate with each other at the same time. The four-way switching valve (15) in the second state makes the first port (P1) and the third port (P3) communicate with each other, and the second port (P2) and the fourth port (P4) communicate with each other at the same time.

[0028] The bridge circuit (20) includes first to fourth pipes (21, 22, 23, 24), each including a check valve (CV). Each of the check valves (CV) allows the refrigerant to flow in the direction indicated by the arrow shown in FIG. 1, and prevents the refrigerant from flowing in the opposite direction. The outflow end of the first pipe (21) and the outflow end of the second pipe (22) are connected to the inlet side of the receiver (16). The inflow end of the third pipe (23) and the inflow end of the fourth pipe (24) are connected to the outlet side of the receiver (16). The inflow end of the first pipe (21) and the outflow end of the third pipe (23) are connected to the liquid-side end of the outdoor heat exchanger (12). The inflow end of the second pipe (22) and the outflow end of the fourth pipe (24) are connected to the first flow path (52a) of the refrigerant heat exchanger (52).

[0029] The first refrigerant circuit (10) includes a first water heat exchanger (25) and a bypass mechanism (30). These components are provided in the indoor unit (IU).

[0030] The first water heat exchanger (25) is configured as a plate heat exchanger, for example. The first water heat exchanger (25) includes a third flow path (25a) through which the first refrigerant in the first refrigerant circuit (10) flows and a fourth flow path (25b) through which water in the water circuit (60) flows. The first water heat exchanger (25) exchanges heat between the first refrigerant in the third flow path (25a) and water in the fourth flow path (25b). The first water heat exchanger (25) is a counterflow heat exchanger in which the first refrigerant in the third flow path (25a) and water in the fourth flow path (25b) flow in opposite directions.

[0031] The bypass mechanism (30) switches between a first state in which the refrigerant flows through the third flow path (25a) of the first water heat exchanger (25) and a second state in which the refrigerant bypasses the first water heat exchanger (25). The bypass mechanism (30) includes a bypass flow path (31), a first valve (32), and a second valve (33). One end of the bypass flow path (31) is connected to a gas line of the first refrigerant circuit (10). The other end of the bypass flow path (31) is connected to a pipe between the first flow path (52a) and the third flow path (25a) in the first refrigerant circuit (10). The first valve (32) is an on-off valve, for example, and is provided between the one end of the bypass flow path (31) on the gas line and the third flow path (25a) of the first water heat exchanger (25). The second valve (33) is an on-off valve, for example, and is provided in the bypass flow path (31). The bypass mechanism (30) may include a three-way valve instead of the first valve (32) and the second valve (33).(1-3) Second Refrigerant Circuit

[0032] The second refrigerant circuit (50) performs a refrigeration cycle using the second refrigerant. The second refrigerant circuit (50) includes, as its main components, a second compressor (51), the refrigerant heat exchanger (52), a third expansion valve (53), and a second water heat exchanger (54).

[0033] The second compressor (51) compresses a refrigerant sucked therein and discharges the compressed refrigerant. The second compressor (51) is a so-called high-pressure dome compressor. Specifically, the inside of the casing of the second compressor (51) is filled with a high-pressure refrigerant discharged from a compression mechanism. An oil reservoir for refrigerating machine oil is formed at the bottom of the casing. The refrigerating machine oil is supplied to sliding portions of the compression mechanism and bearings by an oil supply pump.

[0034] The refrigerant heat exchanger (52) is configured as a plate heat exchanger, for example. The refrigerant heat exchanger (52) exchanges heat between the first refrigerant in the first flow path (52a) and the second refrigerant in the second flow path (52b). The third expansion valve (53) is an example of a second decompression mechanism. The second water heat exchanger (54) is an example of a utilization-side heat exchanger. The second water heat exchanger (54) is configured as a plate heat exchanger, for example. The second water heat exchanger (54) includes a fifth flow path (54a) through which the second refrigerant in the second refrigerant circuit (50) flows and a sixth flow path (54b) through which water in the water circuit (60) flows. The second water heat exchanger (54) exchanges heat between the second refrigerant in the fifth flow path (54a) and water in the sixth flow path (54b). The second water heat exchanger (54) is a counterflow heat exchanger in which the second refrigerant in the fifth flow path (54a) and water in the sixth flow path (54b) flow in opposite directions. The second water heat exchanger (54) is provided upstream of the first water heat exchanger (25) along the water flow in the water circuit (60).(1-4) Water Circuit

[0035] The water to be supplied to the target circulates in the water circuit (60). The water circuit (60) is provided with a hot water storage tank (not shown) that stores water (hot water) heated by the first water heat exchanger (25) and the second water heat exchanger (54). The water circuit (60) is provided with a pump (61) for circulating water. The hot water storage tank and the pump (61) are provided in the indoor unit (IU).(2) Sensor and Pressure Switch

[0036] The hot water supply apparatus (1) includes a plurality of sensors. As shown in FIG. 1, the sensors include a first discharge pressure sensor (80), a second discharge pressure sensor (81), a discharge temperature sensor (82), a suction pressure sensor (83), and a suction temperature sensor (84). The first discharge pressure sensor (80) detects the high pressure of the first refrigerant in the first refrigerant circuit (10). The first discharge pressure sensor (80) constitutes a low-stage sensor for detecting the temperature of the first refrigerant in the refrigerant heat exchanger (52). The second discharge pressure sensor (81) detects the high pressure in the second refrigerant circuit (50). The discharge temperature sensor (82) detects the temperature of the refrigerant discharged from the second compressor (51). The suction pressure sensor (83) detects the low pressure in the second refrigerant circuit (50). The suction pressure sensor (83) constitutes a high-stage sensor for detecting the temperature of the second refrigerant in the refrigerant heat exchanger (52). The suction temperature sensor (84) detects the temperature of the refrigerant to be sucked into the second compressor (51).

[0037] The sensors include a first air temperature sensor (86) and a second air temperature sensor (87). The first air temperature sensor (86) is disposed around the first compressor (11) and detects the temperature of outdoor air. The second air temperature sensor (87) is disposed around the second compressor (51) and detects the temperature of indoor air.

[0038] The hot water supply apparatus includes a plurality of pressure switches. The pressure switches include a high pressure switch (88) and a low pressure switch (89). The high pressure switch (88) is provided on the discharge side of the second compressor (51) and is activated when the high pressure exceeds a predetermined value. The low pressure switch (89) is provided on the suction side of the second compressor (51) and is activated when the low pressure falls below a predetermined value.(3) Controller

[0039] The controller (100) is an example of a control unit. The controller (100) includes a micro control unit (MCU), an electric circuit, and an electronic circuit. The MCU includes a central processing unit (CPU), a memory, and a communication interface. The memory stores various programs to be executed by the CPU. The controller (100) may be configured as one physically isolated element or may be configured as two or more physically separated elements.

[0040] The controller (100) controls the first refrigerant circuit (10) and the second refrigerant circuit (50). Specifically, the controller (100) controls the startup and stop of the first compressor (11), the rotational speed of the first compressor (11), the opening degree of the first expansion valve (13), the opening degree of the second expansion valve (14), the open / closed state of the first valve (32), the open / closed state of the second valve (33), the startup and stop of the outdoor fan (17), the rotational speed of the outdoor fan (17), the startup and stop of the second compressor (51), the rotational speed of the second compressor (51), and the opening degree of the third expansion valve (53). The controller (100) receives signals detected by the various sensors described above.(4) Heating Operation

[0041] The hot water supply apparatus (1) performs a heating operation for generating hot water in the water circuit (60). The heating operation includes a binary heating operation and a unitary heating operation. In the binary heating operation, a binary refrigeration cycle is performed, in which the outdoor heat exchanger (12) of the first refrigerant circuit (10) functions as an evaporator, while at the same time the second water heat exchanger (54) functions as a radiator. The binary heating operation includes a first heating mode and a second heating mode. The first heating mode is an operating mode in which the first refrigerant flows through the first water heat exchanger (25). The second heating mode is an operating mode in which the first refrigerant bypasses the first water heat exchanger (25).(4-1) First Heating Mode

[0042] In the first heating mode shown in FIG. 3, the control unit (100) operates the first compressor (11), the second compressor (51), the outdoor fan (17), and the pump (61), sets the four-way switching valve (15) to the first state, sets the first valve (32) to the open state, sets the second valve (33) to the closed state, and adjusts the opening degrees of the first to third expansion valves (13, 14, 53) appropriately.

[0043] In the first refrigerant circuit (10), the first refrigerant compressed by the first compressor (11) dissipates heat in the first water heat exchanger (25) to water in the water circuit (60). The first refrigerant that has dissipated heat dissipates heat in the refrigerant heat exchanger (52) to the second refrigerant in the second refrigerant circuit (50). As a result, the degree of subcooling of the first refrigerant increases. The first refrigerant that has passed through the refrigerant heat exchanger (52) is decompressed by the first expansion valve (13), and then passes through the receiver (16) and is further decompressed by the second expansion valve (14). Thereafter, the first refrigerant absorbs heat from outdoor air in the outdoor heat exchanger (12) and evaporates, and is sucked into the first compressor (11).

[0044] In the second refrigerant circuit (50), the second refrigerant compressed by the second compressor (51) dissipates heat in the second water heat exchanger (54) to water in the water circuit (60). The second refrigerant that has dissipated heat is decompressed by the third expansion valve (53). Thereafter, the second refrigerant absorbs heat from the first refrigerant in the refrigerant heat exchanger (52) and evaporates, and is sucked into the second compressor (51).

[0045] In the water circuit (60), water transferred by the pump (61) is heated by the second water heat exchanger (54) and the first water heat exchanger (25), and is used to generate hot water in the hot water storage tank.(4-2) Second Heating Mode

[0046] In the second heating mode shown in FIG. 4, the control unit (100) operates the first compressor (11), the second compressor (51), the outdoor fan (17), and the pump (61), sets the four-way switching valve (15) to the first state, sets the first valve (32) to the closed state, sets the second valve (33) to the open state, and adjusts the opening degrees of the first to third expansion valves (13, 14, 53) appropriately.

[0047] In the first refrigerant circuit (10), the first refrigerant compressed by the first compressor (11) dissipates heat in the refrigerant heat exchanger (52) to the second refrigerant in the second refrigerant circuit (50) after bypassing the first water heat exchanger (25). The first refrigerant that has passed through the refrigerant heat exchanger (52) is decompressed by the first expansion valve (13), and then passes through the receiver (16) and is further decompressed by the second expansion valve (14). Thereafter, the first refrigerant absorbs heat from outdoor air in the outdoor heat exchanger (12) and evaporates, and is sucked into the first compressor (11).

[0048] In the second refrigerant circuit (50), the second refrigerant compressed by the second compressor (51) dissipates heat in the second water heat exchanger (54) to water in the water circuit (60). The second refrigerant that has dissipated heat is decompressed by the third expansion valve (53). Thereafter, the second refrigerant absorbs heat from the first refrigerant in the refrigerant heat exchanger (52) and evaporates, and is sucked into the second compressor (51). In the water circuit (60), water transferred by the pump (61) is heated by the second water heat exchanger (54), and is used to generate hot water in the hot water storage tank.(5-1) Refrigerant and Refrigerating Machine Oil

[0049] The first refrigerant in the first refrigerant circuit (10) is carbon dioxide. The first refrigerant circuit (10) contains a first refrigerating machine oil for lubricating sliding portions of the first compressor (11). The first refrigerating machine oil contains, for example, polyalkylene glycol (PAG) as a main component. The second refrigerant in the second refrigerant circuit (50) is a highly flammable refrigerant, specifically, propane. The second refrigerant circuit (50) contains a second refrigerating machine oil for lubricating sliding portions of the second compressor (51). The second refrigerating machine oil in the second refrigerant circuit (50) contains, for example, polyalkylene glycol (PAG) as a main component. The second refrigerating machine oil may contain polyol ester (POE) or polyvinyl ether (PVE) as a main component.

[0050] In the second refrigerant circuit (50), a highly flammable refrigerant is used as the refrigerant. A highly flammable refrigerant may ignite. For example, when the second refrigerant circuit (50) is filled with propane as a refrigerant and installed indoors, a restriction is imposed on the floor area of a room where the device is installed and a safety measure against leakage needs to be taken if the refrigerant amount exceeds 152 g according to the standard IEC 60335-2-40 Ed.7. In this embodiment, the second refrigerant circuit (50) is installed in the indoor unit (IU). Thus, the amount of the refrigerant which fills the second refrigerant circuit (50) is restricted. For this reason, the amount of the refrigerant which fills the second refrigerant circuit (50) is smaller than the amount of the refrigerant which fills the first refrigerant circuit (10). For example, the amount of the refrigerant which fills the first refrigerant circuit (10) is 15 times or more and 25 times or less the amount of the refrigerant which fills the second refrigerant circuit (50). For example, if the amount of the refrigerant which fills the second refrigerant circuit (50) is 152 g or less, the amount of the refrigerant which fills the first refrigerant circuit (10) is 2500 g or more and 3500 g or less.

[0051] Propane is a hydrocarbon refrigerant and has high miscibility with the refrigerating machine oil. Thus, the miscibility of the second refrigerant with the second refrigerating machine oil (e.g., PAG) is higher than the miscibility of the first refrigerant with the first refrigerating machine oil (e.g., PAG).(5-2) Problem of Low Pressure Caused By Refrigerant Accumulation

[0052] When the hot water supply apparatus (1) is stopped, the temperatures of the first compressor (11) and the second compressor (51) decline. Thus, so-called refrigerant accumulation, a phenomenon in which the refrigerant in a compressor dissolves into the refrigerating machine oil, is likely to occur in the first compressor (11) and the second compressor (51).

[0053] As described above, a highly flammable refrigerant is used in the second refrigerant circuit (50), and the amount of the refrigerant is small. In addition, propane has high miscibility with the refrigerating machine oil, as described above; therefore, in the second compressor (51), a larger amount of refrigerant dissolves into the refrigerating machine oil. For example, in the second refrigerant circuit (50), the charge amount of refrigerant is 0.5 or more and 0.8 or less relative to the charge amount of refrigerating machine oil. The necessary amount of refrigerating machine oil in the second refrigerant circuit (50) is determined based on the second compressor (51). Thus, even when the charge amount of refrigerant is small, it is difficult to reduce the charge amount of refrigerating machine oil in accordance with the charge amount of the refrigerant. Thus, if refrigerant accumulation occurs in the second compressor (51), the substantial absolute amount of the second refrigerant in the second refrigerant circuit (50) becomes smaller because the amount of refrigerant relative to the refrigerating machine oil is small. If the second compressor (51) is started in this state, the low pressure of the second refrigerant circuit (50) decreases rapidly. As a result, the second compressor (51) may not be able to start stably. Specifically, the low pressure of the second refrigerant circuit (50) may fall below a threshold value, which may lead to the execution of the protection control of the second compressor (51) or activation of the low pressure switch (89). In this embodiment, startup control is performed to solve such a problem.(5-3) Startup Control

[0054] The startup control will be described in detail. The startup control is executed when the hot water supply apparatus (1) transitions from a stopped state to a binary heating operation. When the hot water supply apparatus (1) is not in operation, the first compressor (11) and the second compressor (51) are stopped. FIG. 5 is a flowchart of the startup control.

[0055] For example, a user operates a remote controller to enter a command to perform the binary heating operation into the controller (100) ("YES" in Step ST11). Then, in Step ST12, the controller (100) determines whether or not the stop time of the hot water supply apparatus (1) is a first time ΔT1 or longer. The stop time as used herein refers to the time from the point at which the hot water supply apparatus (1) was stopped most recently to the present time. The first time ΔT1 of this embodiment is a fixed value and is set to be three hours, for example. If the stop time is shorter than the first time ΔT1 in Step ST12, refrigerant accumulation is less likely to occur in the second compressor (51). Thus, if the stop time is shorter than the first time ΔT1 in Step ST12, the controller (100) allows execution of the binary heating operation as usual in Step ST23. In Step ST23, the controller (100) causes the first compressor (11) and the second compressor (51) to operate at the same timing.

[0056] If the stop time is the first time ΔT1 or longer in Step ST12, refrigerant accumulation is likely to occur in the second compressor (51). Thus, if the stop time is the first time ΔT1 or longer in Step ST12, the controller (100) executes a first operation in Step ST13.

[0057] In the first operation, the controller (100) operates the first compressor (11) while keeping the second compressor (51) in the stopped state. The controller (100) controls the rotational speed of the first compressor (11) to the first rotational speed R1. Specifically, in the first operation, the controller (100) operates the first compressor (11), sets the four-way switching valve (15) to the first state, adjusts the opening degrees of the first expansion valve (13) and the second expansion valve (14), and operates the outdoor fan (17). As a result, as shown in FIG. 6, the first refrigerant circuit (10) performs a refrigeration cycle, in which the first refrigerant dissipates heat in the refrigerant heat exchanger (52) and evaporates in the outdoor heat exchanger (12). In other words, in the first operation, a refrigeration cycle is performed in which the first flow path (52a) of the refrigerant heat exchanger (52) functions as a radiator and the outdoor heat exchanger (12) functions as an evaporator. The controller (100) of this embodiment sets the bypass mechanism (30) to the second state in the first operation. Thus, the first refrigerant bypasses the first water heat exchanger (25).

[0058] In the refrigerant heat exchanger (52) during the first operation, the first refrigerant in the first flow path (52a) dissipates heat, and this heat is transferred to the second compressor (51) through the refrigerant heat exchanger (52) (see the broken arrow in FIG. 6). Thus, the second compressor (51) is heated, and the refrigerant dissolved in the second refrigerating machine oil evaporates. Refrigerant accumulation in the second compressor (51) can thus be eliminated. In the first operation, the first refrigerant bypasses the first water heat exchanger (25), thereby reducing heat dissipation from the first refrigerant to the water circuit (60). Accordingly, the heating capacity of the second compressor (51) can be improved.

[0059] In the first operation in Step ST13, the controller (100) of this embodiment determines the first rotational speed R1 based on the ambient temperature of the first compressor (11), specifically, the outdoor air temperature detected by the first air temperature sensor (86). When the outdoor air temperature is relatively low, it takes time for the first refrigerant to reach a temperature sufficient to heat the second refrigerant. In addition, when the outdoor air temperature is low, the density of a low-pressure gas refrigerant of the first refrigerant decreases; therefore, the amount of the refrigerant circulating through the first refrigerant circuit (10) in the first operation decreases. Thus, the lower the outdoor air temperature, the more the controller (100) increases the first rotational speed R1. It is therefore possible to heat the second refrigerant quickly under low outdoor air temperature conditions.

[0060] If a second time ΔT2 has elapsed from the start of the first operation in Step ST14, the controller (100) executes a second operation in Step ST15, in which the second compressor (51) is operated. Specifically, in the second operation, the controller (100) maintains operation of the first compressor (11) and operates the second compressor (51). In the second operation, the controller (100) adjusts the opening degree of the third expansion valve (53). As a result, the second refrigerant circuit (50) performs a refrigeration cycle in which the second refrigerant evaporates in the refrigerant heat exchanger (52) and dissipates heat in the second water heat exchanger (54). In other words, in the second operation, a refrigeration cycle is performed in which the second flow path (52b) of the refrigerant heat exchanger (52) functions as an evaporator and the fifth flow path (54a) of the second water heat exchanger (54) functions as a radiator. Since the refrigerant accumulation is reduced in the first operation before the second operation, a sharp drop in low pressure of the second refrigerant circuit (50) at the startup of the second compressor (51) is reduced.

[0061] In the second operation, the controller (100) controls the rotational speed of the second compressor (51) to the second rotational speed R2. In this embodiment, the controller (100) controls the second compressor (51) to operate at low speed. The "low speed" as used herein refers to the rotational speed lower than the intermediate rotational speed within the control range of the rotational speed of the second compressor (51). In this embodiment, the second rotational speed R2 is a fixed value within the range of from 25 rps to 58 rps, for example. The second rotational speed R2 is preferably the lowest rotational speed of the second compressor (51) (e.g., 25 rps). In the second operation, the second compressor (51) is controlled to operate at low speed or the lowest rotational speed, thereby reducing a sharp drop in low pressure of the second refrigerant circuit (50).

[0062] Next, in Step ST16, the controller (100) determines whether or not a first condition indicating that the low pressure is low is met. The first condition of this embodiment is that the evaporation temperature Te of the second refrigerant circuit (50) is lower than a predetermined temperature (e.g., -15°C). The controller (100) obtains, as the evaporation temperature Te, the gas saturation temperature corresponding to the low pressure detected by the suction pressure sensor (83), for example. If the first condition is met, there is a possibility that the refrigerant accumulation has not been sufficiently eliminated. To address this, if the first condition in Step ST16 is met, the controller (100) stops the second compressor (51) in Step ST19. The first condition may be that the low pressure or the evaporation pressure of the second refrigerant circuit (50) is lower than a predetermined value.

[0063] Next, the controller (100) adds 1 to a flag N in Step ST20. The flag N refers to the number of times the first condition is met after the first operation under the startup control. If the flag N is less than 3 in Step ST21, the process returns to Step ST14. Subsequently, the controller (100) executes the second operation again after the second time ΔT2 has elapsed. If the first condition is met ("YES" in Step ST16) and the first operation is repeatedly performed thereafter, and the flag N becomes greater than 3 ("YES" in Step ST21), there is a possibility that the second refrigerant is leaking from the second refrigerant circuit (50), because normally, refrigerant accumulation of the second compressor (51) would be eliminated by repeatedly executing the first operation. Thus, if the condition in Step ST21 is met, the controller (100) determines in Step S22 that leakage of the second refrigerant in the second refrigerant circuit (50) is suspected. In this case, the controller (100) executes the unitary heating operation in Step ST24. The unitary heating operation will be described in detail later.

[0064] If the first condition is not met in Step ST16, the second operation is continuously executed during a third time ΔT3 in Step ST17. The third time ΔT3 is 15 minutes, for example. Next, in Step ST18, the controller (100) determines whether or not the state in which α is greater than 5°C has continued for a fourth time ΔT4 or longer. Here, α is the difference (Tr1 - Tr2) between the temperature Tr1 of the first refrigerant in the refrigerant heat exchanger (52) and the temperature Tr2 of the second refrigerant in the refrigerant heat exchanger (52). The saturation temperature based on the high pressure of the first refrigerant detected by the first discharge pressure sensor (80) is used as the temperature Tr1 of the first refrigerant. The saturation temperature based on the low pressure of the second refrigerant detected by the suction pressure sensor (83) is used as the temperature Tr2 of the second refrigerant.

[0065] Pressure sensors may be provided in the vicinity of the first flow path (52a) and the second flow path (52b) of the refrigerant heat exchanger (52), and the temperature Tr1 of the first refrigerant and the temperature Tr2 of the second refrigerant may be obtained as the saturation temperatures corresponding to the pressures detected by these pressure sensors. Alternatively, the refrigerant heat exchanger (52) may be provided with temperature sensors that directly detect the temperature Tr1 of the first refrigerant and the temperature of the second refrigerant. When the second refrigerant in the second refrigerant circuit (50) leaks, the low pressure of the second refrigerant circuit (50) does not increase easily. Accordingly, the temperature Tr2 of the second refrigerant remains low, and the state in which α (= T1 - T2) is relatively great continues. To address this, if the state in which α is greater than 5°C has continued for the fourth time ΔT4 or longer in Step ST18, the controller (100) determines in Step ST22 that leakage of the second refrigerant in the second refrigerant circuit (50) is suspected. The fourth time ΔT4 is four minutes, for example.

[0066] If the absolute value of the temperature Tr2 of the second refrigerant is used to determine refrigerant leakage in Step ST18, the accuracy of refrigerant leakage determination decreases due to the influence of the ambient temperature of the refrigerant heat exchanger (52). However, by using the temperature difference α between the first refrigerant and the second refrigerant in the refrigerant heat exchanger (52), it is possible to reduce the possibility of a decrease in the accuracy of refrigerant leakage determination due to the influence of the ambient temperature.

[0067] If the condition in Step ST18 is not met, and refrigerant leakage is not suspected, the first compressor (11) and the second compressor (51) continue to operate, and the operation transitions to a normal binary heating operation in Step ST23.(5-4) Unitary Heating Operation

[0068] The unitary heating operation in Step ST24 will be described. In the unitary heating operation shown in FIG. 7, the controller (100) operates the first compressor (11), operates the outdoor fan (17), sets the four-way switching valve (15) to the first state, sets the bypass mechanism (30) to the first state, and adjusts the opening degrees of the first expansion valve (13) and the second expansion valve (14). Since the controller (100) stops the second compressor (51), the second refrigerant circuit (50) does not perform a refrigeration cycle.

[0069] The first refrigerant circuit (10) performs a refrigeration cycle, in which the first refrigerant dissipates heat in the first water heat exchanger (25) and evaporates in the outdoor heat exchanger (12). The second compressor (51) is in the stopped state in the second refrigerant circuit (50). The risk of refrigerant leakage can thus be reduced. In the first water heat exchanger (25), the first refrigerant dissipates heat to water in the water circuit (60). It is possible to generate hot water in the water circuit (60) while keeping the second compressor (51) in the stopped state.(6) Advantages of Embodiment

[0070] (6-1) When the first compressor (11) and the second compressor (51) are in the stopped state, the controller (100) executes the first operation in which the first compressor (11) is operated while keeping the second compressor (51) in the stopped state to cause heat dissipation of the first refrigerant in the refrigerant heat exchanger (52). After the first operation, the controller (100) executes the second operation in which the second compressor (51) is operated.

[0071] According to this configuration, the first refrigerant discharged from the first compressor (11) by the first operation dissipates heat in the refrigerant heat exchanger (52). Thus, in the refrigerant heat exchanger (52), the second refrigerant in the second refrigerant circuit (50) is heated, and the inside of the second compressor (51) is also heated. As a result, refrigerant accumulation can be eliminated before startup of the second compressor (51).

[0072] When the second operation is executed after the first operation, the second compressor (51) is put into operation. Since refrigerant accumulation has been eliminated in the second compressor (51), a sharp drop in low pressure is reduced, allowing the second compressor (51) to start stably.

[0073] (6-2) If the stop time of the first compressor (11) and the second compressor (51) is longer than the first time ΔT1, the controller (100) executes the first operation. Accordingly, under the conditions in which the stop time of the first compressor (11) and the second compressor (51) is so long that refrigerant accumulation easily occurs in the second compressor (51), such refrigerant accumulation can be eliminated. If the stop time of the first compressor (11) and the second compressor (51) is shorter than the first time ΔT1, the controller (100) does not execute the first operation and performs the binary heating operation in which the first compressor (11) and the second compressor (51) are operated. Accordingly, under the conditions in which refrigerant accumulation is less likely to occur in the second compressor (51), the binary heating operation can be started quickly.

[0074] (6-3) The controller (100) controls the rotational speed of the second compressor (51) to low speed in the second operation. Accordingly, a sharp drop in low pressure of the second refrigerant circuit (50) can be reduced in the second operation. In particular, by setting the rotational speed of the second compressor (51) to the lowest rotational speed, a sharp drop in low pressure of the second refrigerant circuit (50) can be effectively reduced.

[0075] (6-4) The controller (100) executes the first operation again if the first condition is met during the second operation, the first condition being that the evaporation temperature, evaporation pressure, or low pressure of the second refrigerant circuit (50) is lower than a predetermined value. Accordingly, under the conditions in which the low pressure of the second refrigerant circuit (50) is low and there is a possibility that refrigerant accumulation has not yet been eliminated, refrigerant accumulation can be eliminated. As a result, an excessive drop in low pressure of the second refrigerant circuit (50) can be reduced.

[0076] (6-5) During the second operation, the controller (100) stops the second compressor (51) if the state in which the temperature difference α between the first refrigerant and the second refrigerant in the refrigerant heat exchanger (52) is greater than a predetermined value continues for the fourth time ΔT4 or longer. If this condition is met, there is a possibility that refrigerant leaks from the second refrigerant circuit (50). At this time, stopping the second compressor (51) can reduce facilitation of the refrigerant leakage. False determination due to the influence of the ambient temperature of the refrigerant heat exchanger (52) can be reduced by using the temperature difference α also as a criterion value.

[0077] (6-6) Since propane, which is a highly flammable refrigerant, is used as the second refrigerant, the amount of the refrigerant which fills the second refrigerant circuit (50) is reduced, in particular. This makes the problem of refrigerant accumulation in the second compressor (51) significant. To address this, the first operation is performed, which can eliminate refrigerant accumulation before startup of the second compressor (51).

[0078] Since carbon dioxide as the first refrigerant is not a highly flammable refrigerant, the amount of the refrigerant which fills the first refrigerant circuit (10) is larger than the amount of the second refrigerant which fills the second refrigerant circuit (50). In addition, compared to propane, carbon dioxide exhibits low miscibility with the refrigerating machine oil. Thus, it is less likely that the substantial amount of refrigerant in the first refrigerant circuit (10) is insufficient at the startup of the first compressor (11). Thus, in the first operation, the low pressure in the first refrigerant circuit (10) is less likely to drop sharply even when the first compressor (11) is started. Thus, by performing the first operation and the second operation of this embodiment, a sharp drop in low pressure in both the first refrigerant circuit (10) and the second refrigerant circuit (50) can be reduced.(7) Variations

[0079] The above embodiment may be modified into the following variations. In the following description, the differences from the above embodiment will be described.(7-1) First Variation

[0080] When the first compressor (11) and the second compressor (51) are in the stopped state, the controller (100) may determine whether to execute the first operation, based on another condition according to which it is estimated that refrigerant accumulation has occurred. For example, when the ambient temperature of the second compressor (51) is lower than a predetermined temperature, the controller (100) may execute the first operation, and when the ambient temperature of the second compressor (51) is higher than the predetermined temperature, the controller (100) may execute a binary heating operation without executing the first operation.(7-2) Second Variation

[0081] In the first operation in Step ST13 in FIG. 5, the controller (100) may control the first rotational speed R1 of the first compressor (11) to a predetermined fixed value. The fixed value may be a predetermined rotational speed higher than the lowest rotational speed of the first compressor (11), or may be the highest rotational speed of the first compressor (11).(7-3) Third Variation

[0082] The controller (100) may determine the time during which the first operation is executed (the second time ΔT2 in Step SST14), based on the stop time of the first compressor (11) and the second compressor (51) before the first operation. If the stop time is long, the amount of the second refrigerant dissolved into the refrigerating machine oil increases. Thus, the controller (100) increases the second time ΔT2 if the stop time is long. Accordingly, refrigerant accumulation can be sufficiently eliminated in the first operation. If the stop time is short, the amount of the second refrigerant dissolved into the refrigerating machine oil decreases. Thus, the controller (100) shortens the second time ΔT2 if the stop time is short. Accordingly, it is possible to reduce the possibility that the first operation becomes excessively long and to shorten the time necessary to transition to a normal binary heating operation.(7-4) Fourth Variation

[0083] In the second operation in Step ST15 in FIG. 5, the controller (100) may control the second rotational speed R2 such that, as an index (the low pressure, the evaporation temperature, or the evaporation pressure) indicating the low pressure of the second refrigerant circuit (50) becomes smaller, the second rotational speed R2 becomes lower. As a result, in the second operation, an excessive drop in low pressure of the second refrigerant circuit (50) can be reduced.

[0084] In the second operation, the controller (100) may operate the second compressor (51) while keeping the first compressor (11) in the stopped state.(7-5) Fifth Variation

[0085] If the first condition in Step ST16 is met, the controller (100) may execute the second operation in Step ST15 immediately after the second compressor (51) is stopped. In other words, the controller (100) may execute the second operation before a lapse of the second time ΔT2, if the first condition is met.(7-6) Sixth Variation

[0086] The hot water supply apparatus (1) may include an alarm, a ventilator, and a shut-off valve as refrigerant leakage countermeasure devices. The alarm notifies a user of information on the refrigerant leakage through sound, light, or an indication. The ventilator ventilates an indoor space. The shut-off valve closes a predetermined flow path of a refrigerant circuit. For example, if it is determined in Step ST22 in FIG. 5 that refrigerant leakage is suspected, the controller (100) activates these countermeasure devices. If it is determined in Step ST22 in FIG. 5 that refrigerant leakage is suspected, the controller (100) may stop the hot water supply apparatus (1) without executing a unitary operation.(7-7) Seventh Variation

[0087] The controller (100) may set the bypass mechanism (30) to the first state in the first operation. In this case, in the first operation, water in the water circuit (60) can be heated before the binary heating operation. In the first operation, the controller (100) may operate the first compressor (11) with the first expansion valve (13) and the second expansion valve (14) fully open.(7-8) Eighth Variation

[0088] In the binary heating operation, the controller (100) may perform the above-described startup control when a thermo-on command is issued after the indoor unit (IU) has entered a thermo-off state. Refrigerant accumulation in the second compressor (51) is more likely to occur when the indoor unit (IU) enters a thermo-off state. To address this, the controller (100) performs startup control, similarly to the embodiment, when a thermo-on command is issued during the thermo-off state. It is preferable that the controller (100) perform the first operation when the stop time of the first compressor (11) and the second compressor (51) becomes longer than a predetermined time due to the thermo-off state of indoor unit (IU).(8) Other Embodiments

[0089] The refrigeration cycle apparatus does not have to be a hot water supply apparatus. The refrigeration cycle apparatus may be an air conditioner configured to adjust the temperature of indoor air or a cooling apparatus configured to cool inside air.

[0090] The first refrigerant circuit (10) may have a configuration without the first water heat exchanger (25).

[0091] A first expansion mechanism and a second expansion mechanism do not have to be expansion valves, and may be capillary tubes or expanders. The first expansion mechanism includes two expansion valves (13, 14), but may be configured as a single expansion valve configured to drop the pressure of the high-pressure refrigerant to low pressure.

[0092] While the embodiments and variations thereof have been described above, it will be understood that various changes in form and details may be made without departing from the spirit and scope of the claims. The embodiment, the variations thereof, and the other embodiments may be combined appropriately and replaced with each other without deteriorating intended functions of the present disclosure.

[0093] The expressions "first," "second," "third," ... , described above are used to distinguish the terms to which these expressions are given, and do not limit the number and order of the terms.INDUSTRIAL APPLICABILITY

[0094] As can be seen from the foregoing description, the present disclosure is useful for a refrigeration cycle apparatus.DESCRIPTION OF REFERENCE CHARACTERS

[0095] 1Hot Water Supply Apparatus (Refrigeration Cycle Apparatus) 10First Refrigerant Circuit 11First Compressor 12Outdoor Heat Exchanger (Heat-Source-Side Heat Exchanger) 50Second Refrigerant Circuit 51Second Compressor 52Refrigerant Heat Exchanger 53Third Expansion Valve (Second Expansion Mechanism) 54Second Water Heat Exchanger (Utilization-Side Heat Exchanger) 100Controller (Control Unit)

Claims

1. A refrigeration cycle apparatus comprising: a first refrigerant circuit (10) through which a first refrigerant circulates to perform a refrigeration cycle; a second refrigerant circuit (50) through which a second refrigerant circulates to perform a refrigeration cycle; a refrigerant heat exchanger (52) configured to exchange heat between the first refrigerant in the first refrigerant circuit (10) and the second refrigerant in the second refrigerant circuit (50); and a control unit (100) configured to control the first refrigerant circuit (10) and the second refrigerant circuit (50), the first refrigerant circuit (10) including a first compressor (11), a heat-source-side heat exchanger (12), and a first expansion mechanism (13, 14), the second refrigerant circuit (50) including a second compressor (51), a second expansion mechanism (53), and a utilization-side heat exchanger (54), the control unit (100) being configured to execute when the first compressor (11) and the second compressor (51) are in a stopped state, a first operation in which the first compressor (11) is operated while keeping the second compressor (51) in the stopped state to cause heat dissipation of the first refrigerant in the refrigerant heat exchanger (52), and after the first operation, a second operation in which the second compressor (51) is operated.

2. The refrigeration cycle apparatus of claim 1, wherein the control unit (100) is configured to execute the first operation if a stop time of the first compressor (11) and the second compressor (51) is longer than a predetermined time.

3. The refrigeration cycle apparatus of claim 1 or 2, wherein the control unit (100) is configured to control a rotational speed of the second compressor (51) to low speed in the second operation.

4. The refrigeration cycle apparatus of any one of claims 1 to 3, wherein the control unit (100) is configured to execute the first operation again if a first condition is met during the second operation, the first condition being that an evaporation temperature, an evaporation pressure, or a low pressure of the second refrigerant circuit (50) is lower than a predetermined value.

5. The refrigeration cycle apparatus of any one of claims 1 to 4, wherein during the second operation, the control unit (100) is configured to stop the second compressor (51) if a state in which a temperature difference between the first refrigerant and the second refrigerant in the refrigerant heat exchanger (52) is greater than a predetermined value continues for a predetermined time or longer.

6. The refrigeration cycle apparatus of any one of claims 1 to 5, wherein the second refrigerant includes propane.

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