HEAT PUMP SYSTEM |
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| 申请号 | EP14906430.5 | 申请日 | 2014-11-19 | 公开(公告)号 | EP3222932A1 | 公开(公告)日 | 2017-09-27 |
| 申请人 | Mitsubishi Electric Corporation; | 发明人 | TAKAYAMA, Keisuke; | ||||
| 摘要 | A heat pump system is provided, which is capable of preventing decomposition of a substance, included in a refrigerant and having a carbon-to-carbon double bond, and is capable of increasing a heating temperature of a heating medium. A heat pump system uses a refrigerant which includes a substance having a carbon-to-carbon double bond, and sequentially performs a first operation, a second operation, and a third operation after a compressor is activated. In the first operation, specific enthalpy of the refrigerant at an inlet of a decompressor is kept at a higher level than specific enthalpy at a critical point of the substance. In the first operation, high pressure-side pressure rises from a lower level than critical pressure of the substance to a higher level than the critical pressure. In the second operation, high pressure-side pressure is kept at a level higher than the critical pressure In the second operation, the specific enthalpy of the refrigerant at the inlet of the decompressor drops from a higher level than the specific enthalpy at the critical point to a lower level than the specific enthalpy at the critical point. In the third operation, a temperature of the heating medium flowing out from a heater is kept at a level higher than a critical temperature of the substance. | ||||||
| 权利要求 | |||||||
| 说明书全文 | The present invention relates to a heat pump system. PTL 1 described below discloses a heat cycle system which uses a working medium including 1,1,2-trifluoroethylene (HFO-1123). As a measure to prevent proliferation of legionella bacteria, a heat pump water heater needs to be operated such that discharged hot water temperature is equal to or higher than 65°C. HFO-1123 has a critical temperature of 59.2°C. Therefore, when HFO-1123 is used in a refrigerant of a heat pump water heater, a section, where a state of the refrigerant becomes a critical point of HFO-1123, or a vicinity thereof may be created inside a heater which performs heat exchange between water and the refrigerant. [PTL 1] At a critical point and a vicinity thereof of a substance having a carbon-to-carbon double bond such as HFO-1123, a reaction that cleaves the double bond is promoted. As a result, HFO-1123 is decomposed and may lose its ability to function as a refrigerant. In addition, sludge may be created inside a refrigerant circuit by decomposition products and may cause clogging of an expansion valve or the like. The present invention has been made in order to solve problems such as that described above and an object thereof is to provide a heat pump system capable of preventing decomposition of a substance, which is contained in a refrigerant and which has a carbon-to-carbon double bond, and increasing a heating temperature of a heating medium. A heat pump system according to the present invention includes: refrigerant including a substance having a carbon-to-carbon double bond; a compressor configured to compress the refrigerant; a pump configured to pump a heating medium; a heater coupled to the compressor to exchange heat between the heating medium pumped by the pump and the refrigerant compressed by the compressor ;a decompressor configured to decompress the refrigerant, the decompressor having an inlet and an outlet; an evaporator coupled to the decompressor to evaporate the refrigerant decompressed by the decompressor; and a controller coupled to the compressor, the decompressor and the pump. The controller is configured to sequentially perform a first operation, a second operation and a third operation after the compressor is activated. A specific enthalpy of the refrigerant at the inlet of the decompressor is kept at a higher level than a specific enthalpy at a critical point of the substance in the first operation. A pressure of the refrigerant between the compressor and the inlet of the decompressor rises from a lower level than a critical pressure of the substance to a higher level than the critical pressure in the first operation. The pressure of the refrigerant between the compressor and the inlet of the decompressor is kept at a level higher than the critical pressure in the second operation. The specific enthalpy of the refrigerant at the inlet of the decompressor drops from a higher level than the specific enthalpy at the critical point to a lower level than the specific enthalpy at the critical point in the second operation. A temperature of the heating medium flowing out from the heater is kept at a level higher than a critical temperature of the substance in the third operation. With the heat pump system according to the present invention, decomposition of a substance which is included in a refrigerant and which has a carbon-to-carbon double bond can be prevented and a heating temperature of a heating medium can be increased.
Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that common elements in the respective drawings are denoted by same reference signs and overlapping descriptions will be omitted. The heat pump unit 1 further includes a heating medium pipe 9, an air blower 12 which blows air into the evaporator 6, a first refrigerant temperature sensor 14, a refrigerant pressure sensor 15, a second refrigerant temperature sensor 16, and a controller 50. A heating medium which is a fluid heated by the heat pump unit 1 flows through the heating medium pipe 9. The heating medium pipe 9 has an inlet 91 and an outlet 92. In the first embodiment, the heating medium is water. The heating medium according to the present invention may be a liquid other than water such as antifreeze or brine. The heat pump unit 1 according to the first embodiment is used as a water heater. The heat pump system according to the present invention is also applicable to a heat pump system for heating a heating medium used for purposes (such as indoor-heating) other than water heating. The compressor 2 compresses low pressure refrigerant gas having been taken in and discharges high pressure refrigerant gas. The heater 3 is a heat exchanger which performs heat exchange between the heating medium and the high pressure refrigerant compressed by the compressor 2. The heater 3 has a refrigerant flow path 31 and a heating medium flow path 32. The heating medium pipe 9 guides the heating medium having flowed in from the inlet 91 to an inlet of the heating medium flow path 32 of the heater 3, and guides the heating medium having exited the heating medium flow path 32 from an outlet thereof to the outlet 92. In the heater 3, the heating medium and the refrigerant form counterflows to each other. The high/low pressure heat exchanger 8 has the high pressure section 81 and the low pressure section 82. A high pressure refrigerant having passed through the refrigerant flow path 31 of the heater 3 flows into the high pressure section 81 of the high/low pressure heat exchanger 8. An opening of the decompressor 5 is variable. For example, an expansion valve can be used as the decompressor 5. An inlet of the decompressor 5 is connected to an outlet of the high pressure section 81 of the high/low pressure heat exchanger 8. An outlet of the decompressor 5 is connected to an inlet of the evaporator 6. A high pressure refrigerant having passed through the high pressure section 81 of the high/low pressure heat exchanger 8 is expanded and decompressed when passing through the decompressor 5 and becomes a low pressure refrigerant. The low pressure refrigerant flows into the evaporator 6. When the bypass valve 13 is closed, the low pressure refrigerant having passed through the evaporator 6 flows into the low pressure section 82 of the high/low pressure heat exchanger 8. The evaporator 6 is a heat exchanger which performs heat exchange between the refrigerant and air. The evaporator 6 causes the refrigerant to absorb heat of outside air blown by the air blower 12. In the present invention, a fluid which exchanges heat with the refrigerant in the evaporator 6 is not limited to air and may instead be groundwater, solar-heated water, drainage water, or the like. In the first embodiment, the air blower 12 corresponds to a flow rate regulating section which regulates a flow rate of the fluid which exchanges heat with the refrigerant in the evaporator 6. The high/low pressure heat exchanger 8 performs heat exchange between the high pressure refrigerant after exchanging heat with the heating medium and the low pressure refrigerant having passed through the evaporator 6. The low pressure refrigerant having passed through the low pressure section 82 of the high/low pressure heat exchanger 8 flows into the accumulator 7. When the bypass valve 13 is open, the low pressure refrigerant from the evaporator 6 passes through the bypass passage 11 and flows into the accumulator 7. In the refrigerant having flowed into the accumulator 7, a refrigerant liquid is stored in the accumulator 7 and a refrigerant gas is discharged from the accumulator 7 to be sucked into the compressor 2. In this manner, the accumulator 7 stores excess refrigerant liquid in the refrigerant circuit. In the refrigerant circuit described above, a section between the outlet of the compressor 2 and the inlet of the decompressor 5 will be referred to as a "high pressure side" and a section between the outlet of the decompressor 5 and the inlet of the compressor 2 will be referred to as a "low pressure side". In addition, pressure of the refrigerant on the high pressure side will be referred to as "high pressure-side pressure" and pressure of the refrigerant on the low pressure side will be referred to as "low pressure-side pressure". The first refrigerant temperature sensor 14 detects a temperature of the refrigerant discharged from the compressor 2. The temperature of the refrigerant discharged from the compressor 2 or, in other words, the temperature of the refrigerant after being compressed by the compressor 2 will be hereinafter referred to as a "compressor discharge temperature". The refrigerant pressure sensor 15 detects pressure of the refrigerant discharged from the compressor 2. The pressure of the refrigerant discharged from the compressor 2 or, in other words, the pressure of the refrigerant after being compressed by the compressor 2 will be hereinafter referred to as a "compressor discharge pressure". The second refrigerant temperature sensor 16 detects a temperature of a high pressure refrigerant at an outlet of the refrigerant flow path 31 of the heater 3. The high pressure-side pressure can be considered approximately equal to the compressor discharge pressure. The controller 50 includes a CPU (central processing unit) 50a, a storage section 50b, and an input/output interface 50c. The storage section 50b includes a ROM (read only memory), a RAM (random access memory), a nonvolatile memory, and the like. The storage section 50b stores a control program, data, and the like. The input/output interface 50c is used to input and output external signals to and from the CPU 50a. The various actuators and sensors included in the heat pump unit 1 are electrically connected to the controller 50. The controller 50 controls operations of the heat pump unit 1 by having the CPU 50a execute the control program stored in the storage section 50b. The controller 50 is capable of controlling the opening of the decompressor 5. The controller 50 is capable of controlling a capacity of the compressor 2. The capacity of the compressor 2 corresponds to an amount of the refrigerant which is pumped out by the compressor 2 per unit time. The controller 50 can control the capacity of the compressor 2 by changing a rotating speed of the compressor 2 using, for example, inverter control. The controller 50 can control a flow rate of the heating medium (water) of the heating medium flow path 32 of the heater 3 by controlling an operation of a water pump 22 to be described later. The controller 50 can control the flow rate of water in the heating medium flow path 32 of the heater 3 by changing a rotating speed of the water pump 22 using, for example, inverter control. A temperature of the heating medium (water) flowing out from the heating medium flow path 32 of the heater 3 will be hereinafter referred to as a "heating medium heating temperature". When the water flow rate through the heater 3 increases, the heating medium heating temperature drops. When the water flow rate through the heater 3 decreases, the heating medium heating temperature rises. By adjusting the water flow rate through the heater 3, the controller 50 can control the heating medium heating temperature so as to match a target temperature. The target temperature of the heating medium heating temperature during a steady-state operation (a third operation to be described later) of the heat pump unit 1 is desirably 65°C or higher. Accordingly, since the temperature of hot water stored in the hot water storage tank 21 to be described later can be set to 65°C or higher, proliferation of legionella bacteria can be reliably prevented. The refrigerant of the heat pump unit 1 contains a substance having a carbon-to-carbon double bond. Hereinafter, this substance will be referred to as a "first substance". A critical temperature of the first substance is a temperature lower than the heating medium heating temperature during a steady-state operation (the third operation) of the heat pump unit 1. In the first embodiment, 1,1,2-trifluoroethylene is used as the first substance. In the following description, 1,1,2-trifluoroethylene will be referred to as HFO-1123. HFO-1123 has a critical temperature of 59.2°C. The heat pump system according to the first embodiment contributes toward suppression of global warming by using the first substance which has a carbon-to-carbon double bond and a low global warming potential (GWP) in a refrigerant. The refrigerant of the heat pump unit 1 may be a mixed refrigerant further containing a substance other than the first substance or a single refrigerant solely constituted by the first substance. When the heat pump unit 1 uses a mixed refrigerant, a proportion of concentration of the first substance is favorably 60 mass% or higher. The refrigerant of the heat pump unit 1 may be a mixed refrigerant containing the first substance and a second substance. A critical temperature of the second substance is desirably higher than the heating medium heating temperature during a steady-state operation (the third operation) of the heat pump unit 1. A boiling point of the second substance is desirably higher than a boiling point of the first substance. For example, 2,3,3,3-tetrafluoropropene (HFO-1234yf) is preferably used as the second substance. HFO-1234yf has a critical temperature of 95°C. HFO-1234yf has a carbon-to-carbon double bond. By using a substance having a carbon-to-carbon double bond as the second substance, the impact on global warming can be further prevented when using a mixed refrigerant containing the second substance. A water supply pipe 25 is further connected to the lower part of the hot water storage tank 21 of the tank unit 20. Water supplied from an external water source such as waterworks passes through the water supply pipe 25 and flows into and stored in the hot water storage tank 21. The inside of the hot water storage tank 21 is constantly kept in a fully-filled state due to inflow of water from the water supply pipe 25. A mixing valve 26 is further provided in the tank unit 20. The mixing valve 26 is connected to the upper part of the hot water storage tank 21 via a hot water outflow pipe 27. In addition, a water supply branch pipe 28 which is branched from the water supply pipe 25 is connected to the mixing valve 26. One end of a hot water supply pipe 29 is further connected to the mixing valve 26. Although not illustrated, another end of the hot water supply pipe 29 is connected to a hot water supply terminal such as a tap, a shower, or a bathtub. When performing a heat accumulating operation for increasing an amount of heat stored in the hot water storage tank 21, the controller 50 operates the compressor 2 and the water pump 22. During the heat accumulating operation, water stored in the hot water storage tank 21 is sent to the heat pump unit 1 through the water channel 23 by the water pump 22, heated by the heater 3 of the heat pump unit 1, and becomes high temperature hot water. The high temperature hot water passes through the water channel 24 and returns to the tank unit 20, and flows into the hot water storage tank 21 from above. As a result of the heat accumulating operation described above, hot water is stored in the hot water storage tank 21 so as to form a temperature stratification where an upper side has a high temperature and a lower side has a lower temperature. During the heat accumulating operation, the controller 50 can control the heating medium heating temperature so as to match a target temperature by adjusting the water flow rate through the heater 3. A plurality of temperature sensors are attached at intervals in a vertical direction on a surface of the hot water storage tank 21. By detecting a temperature distribution inside the hot water storage tank 21 in the vertical direction with the temperature sensors, the controller 50 can calculate an amount of stored hot water, an amount of stored heat, an amount of remaining hot water, and the like in the hot water storage tank 21. The controller 50 controls start and stop timings of heat accumulating operations and the like based on the amount of stored hot water, the amount of stored heat, or the amount of remaining hot water in the hot water storage tank 21. When supplying hot water from the hot water supply pipe 29 to the hot water supply terminal, the high temperature hot water in the hot water storage tank 21 is supplied to the mixing valve 26 through the hot water outflow pipe 27 and, at the same time, low temperature water is supplied to the mixing valve 26 through the water supply branch pipe 28. The high temperature hot water and the low temperature water are mixed at the mixing valve 26 and supplied to the hot water supply terminal through the hot water supply pipe 29. The controller 50 can control a hot water supply temperature to the hot water supply pipe 29 by adjusting a mixing ratio of the high temperature hot water and the low temperature water with the mixing valve 26.
A flow channel passing through the low pressure section 82 of the high/low pressure heat exchanger 8 is longer than the bypass passage 11, and as a result a fluid passing through the former receives larger resistance than that through the latter. Therefore, when the bypass valve 13 is fully open, most of the refrigerant having exited the evaporator 6 passes through the bypass passage 11 without passing through the low pressure section 82 of the high/low pressure heat exchanger 8. In the following description, for convenience's sake, the entire refrigerant having exited the evaporator 6 is assumed to pass through the bypass passage 11 during the first operation. A to G in As the refrigerant is compressed by the compressor 2, the state of the refrigerant changes from point G to point A. As the refrigerant is cooled by the heater 3, the state of the refrigerant changes from point A to point B. Since the water flow rate through the heater 3 is at zero or at a low level, a difference in specific enthalpy between point A and point B is smaller than that during the third operation. Since the refrigerant having exited the evaporator 6 does not pass through the low pressure section 82 of the high/low pressure heat exchanger 8, heat exchange does not take place in the high/low pressure heat exchanger 8. Therefore, the state of the refrigerant (point C) having passed through the high pressure section 81 of the high/low pressure heat exchanger 8 is substantially the same as that at point B. By passing through the decompressor 5 and expanding, the state of the refrigerant changes from point C to point D. Therefore, the state (point D) of the refrigerant having passed through the decompressor 5 is a gas-liquid two-phase state. As the refrigerant is heated by outside air in the evaporator 6, the state of the refrigerant changes from point D to point E. The state (point E) of the refrigerant at the outlet of the evaporator 6 is a gas-liquid two-phase state or a saturated gas state. The refrigerant having exited the evaporator 6 passes through the bypass passage 11 and directly flows into the accumulator 7 without passing through the low pressure section 82 of the high/low pressure heat exchanger 8. Therefore, the states of the refrigerant at point E and point F are substantially the same as that at point G. In the accumulator 7, gas and liquid of the refrigerant are separated from each other and mainly refrigerant gas is sucked into the compressor 2 (point G). During the first operation, since the opening of the decompressor 5 is larger than the opening of the decompressor 5 during the third operation, the refrigerant at the outlet of the evaporator 6 is likely to assume a gas-liquid two-phase state. Therefore, the refrigerant liquid readily accumulates in the accumulator 7. A density of the refrigerant at the inlet of the decompressor 5 during the first operation is lower than the density of the refrigerant at the inlet of the decompressor 5 during the third operation. By expanding the opening of the decompressor 5, the refrigerant can flow more readily when the density of the refrigerant is low. During the first operation, in accordance with an amount of the refrigerant stored in the accumulator 7 (a height of a liquid surface inside the accumulator 7), the state of the refrigerant sucked into the compressor 2 assumes a state of slightly wet vapor instead of a saturated gas state. Therefore, when the amount of the refrigerant liquid accumulated in the accumulator 7 increases, the compressor discharge temperature drops. During the first operation, the controller 50 desirably controls the opening of the decompressor 5 so that the compressor discharge temperature as detected by the first refrigerant temperature sensor 14 is slightly lower than the compressor discharge temperature during the third operation. Accordingly, the refrigerant liquid can be prevented from overflowing from the accumulator 7 and the height of the liquid surface of the accumulator 7 can be properly controlled. During the first operation, since the water flow rate through the heater 3 is set to zero or to a low level, a cooling amount of the refrigerant by the heater 3 is smaller than that during the third operation. Therefore, the specific enthalpy of the refrigerant at the inlet of the decompressor 5 is higher than the specific enthalpy at the critical point of the first substance. By maintaining the specific enthalpy of the refrigerant at the inlet of the decompressor 5 at a higher level than the specific enthalpy at the critical point of the first substance during the first operation, the refrigerant can be reliably prevented from passing through the operation-prohibited region which is the critical point of the first substance and a vicinity thereof. As a result, decomposition of the first substance can be prevented. During the first operation, by increasing the amount of the refrigerant stored in the accumulator 7 as compared to that during the third operation, operation is enabled on a side (gas side) where specific enthalpy is higher than the critical point of the first substance. During the first operation, the controller 50 desirably controls the air blower 12 so that a flow rate of air which passes through the evaporator 6 is lower than a flow rate of air which passes through the evaporator 6 during the third operation. Accordingly, the refrigerant at the outlet of the evaporator 6 more readily assumes a gas-liquid two-phase state and refrigerant liquid more readily accumulates in the accumulator 7. The controller 50 is capable of calculating specific enthalpy of the refrigerant at the outlet of the refrigerant flow path 31 of the heater 3 based on the temperature of the refrigerant at the outlet of the refrigerant flow path 31 of the heater 3 as detected by the second refrigerant temperature sensor 16 and on compressor discharge pressure (high pressure-side pressure) as detected by the refrigerant pressure sensor 15. During the first operation, the controller 50 may control the operation of the water pump 22 and adjust the water flow rate through the heater 3 so that a calculation value of the specific enthalpy of the refrigerant at the outlet of the refrigerant flow path 31 of the heater 3 is higher than the specific enthalpy at the critical point of the first substance. Accordingly, the specific enthalpy of the refrigerant at the inlet of the decompressor 5 can be reliably maintained at a higher level than the specific enthalpy at the critical point of the first substance. The high pressure-side pressure (points A1, B1, and C1) at the start of the first operation is at a lower level than the critical pressure of the first substance. The high pressure-side pressure (points A2, B2, and C2) at the end of the first operation is at a higher level than the critical pressure of the first substance. During the first operation, the high pressure-side pressure rises from a lower level (points A1, B1 and C1) than the critical pressure of the first substance to a higher level (points A2, B2 and C2) than the critical pressure of the first substance. When the compressor discharge pressure detected by the refrigerant pressure sensor 15 becomes sufficiently higher than the critical pressure of the first substance, the controller 50 ends the first operation and makes a transition to the second operation. During the second operation, the controller 50 performs control as follows.
During the second operation, the refrigerant accumulated in the accumulator 7 moves to the high pressure side. During the second operation, by setting the opening of the decompressor 5 smaller than the opening during the first operation, the refrigerant at the outlet of the evaporator 6 assumes a state of superheated steam. During the second operation, by setting the capacity of the compressor 2 higher than that during the first operation, movement of the refrigerant accumulated in the accumulator 7 to the high pressure side is promoted. During the second operation, an excessive rise of high pressure can be prevented by performing a heat exchange between the refrigerant flowing through the high pressure section 81 and the refrigerant flowing through the low pressure section 82 in the high/low pressure heat exchanger 8. During the second operation, the refrigerant from the outlet of the refrigerant flow path 31 of the heater 3 is cooled by the high/low pressure heat exchanger 8. From the start to the end of the second operation, the state of the refrigerant circuit changes as follows.
During the third operation, the controller 50 performs control as follows.
Hereinafter, changes in pressure and specific enthalpy of the refrigerant during the third operation will be described. The refrigerant from the outlet of the refrigerant flow path 31 of the heater 3 is cooled by the high/low pressure heat exchanger 8 and changes from point B3 to point C3. By passing through the decompressor 5 and expanding, the state of the refrigerant changes from point C3 to point D3. The state of the refrigerant having passed through the decompressor 5 is a low-pressure, gas-liquid two-phase state. As the refrigerant is heated by outside air in the evaporator 6, the state of the refrigerant changes from point D3 to point E. The state (point E) of the refrigerant at the outlet of the evaporator 6 is a gas-liquid two-phase state, a saturated gas state, or a superheated steam state. The refrigerant having exited the evaporator 6 is further heated by the high/low pressure heat exchanger 8 and changes from the state of point E to the state of point F3. The state of point F3 is a state of superheated steam. The refrigerant in the state of superheated steam is sucked into the compressor 2 via the accumulator 7. Therefore, refrigerant liquid is not accumulated in the accumulator 7. The controller 50 can estimate the state of the refrigerant to be sucked into the compressor 2 based on the compressor discharge temperature detected by the first refrigerant temperature sensor 14. The controller 50 may adjust the opening of the decompressor 5 so that the state of the refrigerant to be sucked into the compressor 2 which is estimated in this manner assumes a state of superheated steam. When the refrigerant circuit of the heat pump unit 1 is stopped in a state where the third operation is being performed, the controller 50 performs the fourth operation. During the fourth operation, the controller 50 performs control as follows.
From the start to the end of the fourth operation, the specific enthalpy of the refrigerant at the inlet of the decompressor 5 rises. Due to a decrease in the water flow rate through the heater 3 and a decrease in a cooling amount of the refrigerant by the heater 3, the specific enthalpy of the refrigerant at the inlet of the decompressor 5 rises. At the end of the fourth operation, the specific enthalpy of the refrigerant at the inlet of the decompressor 5 is higher than the specific enthalpy at the critical point of the first substance. During the fourth operation, the high pressure-side pressure is maintained at a higher level than the critical pressure of the first substance. During the fourth operation, refrigerant liquid gradually accumulates in the accumulator 7. When the specific enthalpy of the refrigerant at the outlet of the refrigerant flow path 31 of the heater 3 reaches a higher level than the specific enthalpy at the critical point of the first substance, the controller 50 stops the compressor 2. At the end of the fourth operation, the amount of the refrigerant stored in the accumulator 7 is sufficiently large. By performing the fourth operation described above, when stopping the refrigerant circuit, the refrigerant can be prevented from passing through the operation-prohibited region which is the critical point of the first substance and a vicinity thereof and, consequently, decomposition of the first substance can be prevented. In the case of a refrigerant of which a hot water outflow temperature and a critical temperature are close to each other, the condensation temperature is in the vicinity of the critical temperature. In particular, HFO-1123 has a critical temperature of 59.2°C, and when the hot water outflow temperature is set to 65°C in order to prevent the proliferation of legionella bacteria, the condensation temperature is in the vicinity of the critical temperature. Therefore, the control according to the present invention is particularly effective when using HFO-1123 as the first substance of the refrigerant. The critical pressure of HFO-1123 is 4.60 MPa (absolute pressure). CO2 which is widely used as a refrigerant of a conventional heat pump water heater has a critical temperature of 31°C and critical pressure of 7.38 MPa (absolute pressure). Since the refrigerant in a supercritical state undergoes a continuous temperature change, the hot water outflow temperature (heating medium heating temperature) is readily increased. With the heat pump system according to the present first embodiment, the heating medium heating temperature is readily increased as the first substance assumes a supercritical state. When using HFO-1123 as the first substance of the refrigerant, since supercritical is reached at lower pressure as compared to a CO2 refrigerant, design pressure of the refrigerant circuit can be set lower. Therefore, a heat pump system can be constructed with lower cost. With the heat pump system according to the first embodiment, since decomposition of the first substance having a carbon-to-carbon double bond can be reliably prevented, the possibility that the first substance loses its ability to function as a refrigerant can be reduced. In addition, since the creation of sludge in the refrigerant circuit by decomposition products is prevented, the occurrence of clogging of the decompressor 5 and the like can be prevented. With the heat pump system according to the first embodiment, by providing a liquid reservoir (the accumulator 7) on a low pressure side of the refrigerant circuit, refrigerant liquid in the liquid reservoir does not assume a supercritical state and decomposition of the refrigerant can be reliably prevented.
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