JP6996208B2 - Rankine cycle system and its control method - Google Patents

Description

The present invention relates to a Rankine cycle system and a control method thereof.

The Rankine cycle is known, and is applied to, for example, waste heat recovery of an in-vehicle engine from the viewpoint of energy saving. The Rankine cycle system mainly has a closed circuit in which a pump, an evaporator, an expander, and a condenser are arranged in this order and a working fluid circulates. The pump pumps the working fluid towards the evaporator so that it circulates in the closed circuit of the Rankine cycle system. In an example applied to an in-vehicle engine, the evaporator is configured as a heat exchanger to heat and evaporate the working fluid with waste heat from the engine as a heat source, and the expander to draw power from the working fluid through the evaporator. The condenser can be configured as a heat exchanger to condense and liquefy the working fluid that has passed through the expander.

In such a Rankine cycle system, it is proposed in Patent Document 1 to make the flow rate of the working fluid circulating in the closed circuit variable in order to improve the cycle efficiency. More specifically, in the Rankine cycle system of Patent Document 1, a bypass path is provided in which a condenser and a pump are bypassed and a receiver tank is arranged, and a solenoid valve is provided in this bypass path. Then, in the system of Patent Document 1, the degree of supercooling of the working fluid after passing through the condenser is calculated, and the amount of refrigerant circulating in the closed circuit is controlled by controlling the above-mentioned solenoid valve according to the degree of supercooling. To control.

Japanese Unexamined Patent Publication No. 2008-231981

By the way, the heating capacity of the evaporator and the cooling capacity of the condenser in the Rankine cycle system change depending on the state (particularly temperature) of the medium that exchanges heat with the working fluid, respectively. In addition, in the Rankine cycle system, the temperature of the working fluid through the condenser correlates with the pressure of the working fluid. Here, in the Rankine cycle system in which the exhaust gas of the engine is used to heat the working fluid in the evaporator and the engine cooling water is used to cool the working fluid in the condenser, the temperature of the engine cooling water significantly exceeds the assumed value. Imagine when it is below. In this case, if the various settings of the Rankine cycle are kept constant at the assumed values, the pressure of the working fluid drops as a whole, and as a result, the pressure of the working fluid at the outlet of the inflator becomes more than necessary, specifically. There is a risk that the pressure will be lower than the atmospheric pressure. Such a decrease in the pressure of the working fluid at the outlet of the inflator may cause an excessive burden on the closed structure of the pipe joint of the Rankine cycle system, which is not preferable in terms of the life of the pipe joint.

On the other hand, if the degree of supercooling of the working fluid that has passed through the condenser becomes too small due to a decrease in the cooling capacity of the condenser as described above, there may be a problem that cavitation occurs in the pump.

Therefore, an object of the present invention is to prevent cavitation in a pump and prevent the working fluid from becoming lower than atmospheric pressure in the Rankine cycle system.

According to one aspect of the invention, in a Rankine cycle system in which the pumped working fluid flows through an evaporator, an inflator, and a condenser in that order and is pumped again by the pump, the pressure of the working fluid is increased. The pressure variable means configured to be variable, and the working fluid downstream of the inflator and upstream of the pump to keep the pressure above atmospheric pressure and through the condenser. A Rankine cycle system is provided that includes a control means configured to control the pressure variable means so as to keep the degree of cooling above a predetermined degree of overcooling.

Preferably, the pressure variable means is configured in a tank provided between the condenser and the pump so as to achieve gas-liquid separation of the working fluid so that the capacity in the tank is variable. It is equipped with a capacity variable means. Specifically, the control means calculates the degree of supercooling of the working fluid based on the acquired temperature and pressure of the working fluid upstream of the pump and downstream of the condenser, and the calculated supercooling degree is the said. The capacity variable means may be controlled so as to have a degree of supercooling or higher.

According to a further aspect of the present invention, it is a control method in a Rankin cycle system in which a working fluid pumped by a pump flows through an evaporator, an expander, and a condenser in order and is pumped again by the pump. The Rankin cycle system is a capacity variable means configured to make the capacity in the tank variable in a tank provided between the condenser and the pump so as to separate gas and liquid from the working fluid. Based on the temperature and pressure of the working fluid upstream of the pump and downstream of the condenser, the step of calculating the overcooling degree of the working fluid that has passed through the condenser and the calculated overcooling degree are predetermined. Control, including a step of controlling the capacitance variable means to be above cooling and to keep the pressure of the working fluid downstream of the inflator and upstream of the pump above atmospheric pressure. The method is provided.

Since the Rankine cycle system according to the above aspect of the present invention has the above configuration, the degree of supercooling of the working fluid that has passed through the condenser is maintained at a predetermined degree of supercooling or higher, and the working fluid downstream of the expander and upstream of the pump is used. The pressure can be kept higher than the atmospheric pressure. Therefore, according to the above aspect of the present invention, it is possible to suitably prevent the occurrence of cavitation in the pump and the pressure of the working fluid being lower than the atmospheric pressure.

It is a schematic block diagram of the power recovery apparatus provided with the Rankine cycle system which concerns on 1st Embodiment of this invention. It is a flowchart of control in 1st Embodiment. It is a schematic block diagram of a balloon mechanism and its surroundings in the power recovery apparatus provided with the Rankine cycle system which concerns on 2nd Embodiment of this invention.

Hereinafter, embodiments according to the present invention will be described with reference to the drawings. First, the first embodiment according to the present invention will be described.

FIG. 1 is a schematic configuration diagram of a power recovery device 1 provided with the Rankine cycle system 10 of the present embodiment. The power recovery device 1 extracts power from the exhaust gas of an internal combustion engine for a vehicle (hereinafter referred to as an engine) by an expander of the Rankin cycle system 10 described below. Although the power recovery device 1 is configured to drive the generator with the power recovered by the expander, another machine, for example, an engine may be used as the expander load.

The Rankine cycle system 10 includes a pump 12, an evaporator 14, an expander 16 and a condenser 18. The pump 12, the evaporator 14, the expander 16 and the condenser 18 are sequentially arranged in the annular passage 20 through which the working fluid flows, and these form a closed circuit. Here, ethanol is used as the working fluid, but the present invention does not limit the working fluid to ethanol.

The pump 12 pumps the working fluid to the evaporator 14 side to circulate the annular passage 20 of the Rankine cycle system 10. Here, the pump 12 is driven by the motor 12a, but may be driven by the power (driving force) of another power source.

Further, the evaporator 14 is configured as a heat exchanger. The evaporator 14 is configured as a countercurrent type heat exchanger. That is, in the evaporator 14, the flow direction of the exhaust gas and the flow direction of the working fluid are substantially opposite to each other. The evaporator 14 heats the working fluid (of the liquid phase) pumped by the pump 12 by exchanging heat with the exhaust gas flowing in the exhaust pipe 24 of the engine 22 as a heat source. The working fluid is vaporized (evaporated) by being heated by the heat (waste heat) of the exhaust gas in the evaporator 14, and the vaporized working fluid flows into the expander 16.

The expander 16 takes out the expansion energy of the working fluid vaporized by the evaporator 14 as power (mechanical energy) and rotates the drive shaft 16a. Here, the expander 16 is configured as a so-called turbine, and its drive shaft 16a is connected to the generator 26.

The condenser 18 cools the working fluid inflated by the expander 16 to condense and liquefy it. The condenser 18 is configured as a countercurrent type heat exchanger. Then, the cooling water of the engine 22 (engine cooling water) is used as a cooling source of the working fluid of the condenser 18. That is, in the condenser 18, the flow direction of the engine cooling water and the flow direction of the working fluid are substantially opposite to each other. The condenser 18 cools the working fluid (gas phase or gaseous) that has passed through the expander 16 by exchanging heat with the engine cooling water as a cooling source. The working fluid is condensed (liquefied) by being cooled by heat exchange with the engine cooling water in the condenser 18. Here, the engine cooling water flows into the condenser 18 after passing through the radiator 28, but the present invention is not limited to such a configuration, and is cooled by the radiator 28 after passing through the condenser 18. You may. The working fluid condensed by the condenser 18 is sucked into the pump 12 and circulates in the annular passage 20 again.

By the way, in the Rankine cycle system 10, a tank 30 is provided between the condenser 18 and the pump 12. The tank 30 is provided so as to separate the working fluid from gas and liquid. The working fluid flow path (upstream side flow path portion) 32 extending from the condenser 18 side toward the tank 30 is provided so as to communicate with the upper region in the tank 30. Further, the working fluid flow path (downstream side flow path portion) 34 extending from the tank 30 toward the pump 12 side is provided so as to communicate with the lower region in the tank 30. Here, the lower region in the tank 30 is a region in which the working fluid of the liquid phase is accumulated, and thus may be referred to as a liquid phase region. On the other hand, the above-mentioned upper region above the liquid phase region in the tank 30 in the vertical direction can be referred to as a gas phase region because it is a region in which the working fluid of the gas phase accumulates. That is, the upstream side flow path portion 32 has an opening 32a in the gas phase region in the tank 30, and the downstream side flow path portion 34 has an opening 34a in the liquid phase region in the tank 30.

Further, the annular passage 20 is provided with a pressure variable means PV configured to make the pressure of the working fluid variable. Specifically, as the pressure variable means PV, a piston mechanism 36 configured to make the capacity in the tank 30 variable is provided in the tank 30. The piston mechanism 36 corresponds to the capacity variable means. As simply shown in FIG. 1, the piston mechanism 36 includes a cylinder portion 36a and a piston portion 36b movably provided in the cylinder portion 36a. The piston mechanism 36 has a known closed structure provided with an O-ring or the like so that the airtightness between the piston portions 36b is sufficiently maintained when the piston portions 36b are moved along the inner wall surface of the cylinder portion 36a. (Not shown). Further, a ventilation hole 36c is formed in the cylinder portion 36a so that the piston portion 36b can move in the cylinder portion 36a with good airtightness. The operation of the piston mechanism 36 is controlled by an electronic control unit (hereinafter, ECU) 60, which will be described in detail later. More specifically, the actuator 36d provided for operating the piston portion 36b is controlled by the ECU 60, whereby the piston portion 36b can be moved up and down as shown by an arrow in FIG. By controlling the piston mechanism 36 to the capacity according to the position of the piston portion 36b in the cylinder 36a, the capacity in the tank 32 is variable. Here, the piston mechanism 36 is provided in the tank 30 so that the piston portion 36b moves up and down in the vertical direction, but the installation position and installation direction of the piston mechanism 36, the moving direction of the piston portion 36b, and the like can be changed in design. be. Further, in the present embodiment, the piston mechanism 36 is provided on the gas phase region side in the tank 30, but may be provided on the liquid phase region side.

Now, in the Rankine cycle system, the working fluid flowing into the pump 12 needs to be in a liquid phase so as to prevent cavitation in the pump 12. Therefore, the Rankine cycle system 10 is configured so that the tank 30 is provided as described above and only the working fluid of the liquid phase is supplied to the pump 12.

On the other hand, if the degree of supercooling of the working fluid that has passed through the condenser 18 is too small, cavitation may occur in the pump 12. In addition to changing the amount of waste heat of the engine according to the operating state or operating state of the engine 22, the condensation temperature and the condensation pressure of the working fluid in the condenser 18 change depending on the temperature of the engine cooling water and the like. Therefore, in the present embodiment, the piston mechanism 36 as a capacity variable means is provided in order to keep the supercooling degree of the working fluid that has passed through the condenser 18 to be equal to or higher than the predetermined supercooling degree so as to prevent cavitation in the pump 12. Has been done.

In the present embodiment, by using the piston mechanism 36, the degree of overcooling of the working fluid that has passed through the condenser 18 is maintained at a predetermined degree of overcooling or higher, that is, substantially, a predetermined degree of overcooling at a predetermined pressure. The pressure of the working fluid in the Rankine cycle system 10 is variable so as to adjust the state of the working fluid to the state corresponding to the above working fluid. More specifically, here, the piston mechanism 36 is controlled so that the degree of supercooling of the working fluid that has passed through the condenser 18 is maintained at a predetermined degree of supercooling or higher, and the capacity in the tank 30 is variable. The control by the ECU 60 including the control of the piston mechanism 36 will be described below.

A pressure sensor 62 is provided downstream of the tank 30 and upstream of the pump 12 for controlling the piston mechanism 36. Further, a temperature sensor 64 is provided downstream of the tank 30 and upstream of the pump 12. The pressure sensor 62 and the temperature sensor 64 may be provided at other locations upstream of the pump and downstream of the condenser, respectively. Further, the engine 22 and the power recovery device 1 are provided with various sensors. For example, an engine rotation speed sensor 66 for detecting the engine rotation speed and an engine load sensor 68 for detecting the engine load are provided on the engine 22. The engine load sensor is, for example, an air flow meter (flow rate sensor). Further, a water temperature sensor 70 for detecting the engine cooling water temperature is provided.

The outputs from these sensors are input to the ECU 60. The ECU 60 is configured as a so-called computer so as to perform various controls of the Rankine cycle system 10 and the engine 22, and includes known arithmetic processing units (for example, CPU), storage devices (for example, ROM, RAM), input ports, output ports, and the like. It is configured in preparation. The ECU 60 acquires (detects) various values based on the outputs from these sensors, performs predetermined calculations based on the programs and data stored in advance, and operates the engine 22 (for example, an injector or a throttle valve). An operation signal is output to each of the motor 12a and the actuator 36d for driving the piston of the piston mechanism 36. That is, the ECU 60 has a functional unit corresponding to each of the engine control means, the motor 12a control means, and the control means (that is, the control means of the actuator 36d) of the piston mechanism 36 (of the pressure variable means or the capacitance variable means). , These are associated with each other. The ECU 60 is not limited to one electronic control unit, and may be configured as a complex of a plurality of electronic control units.

The control of the piston mechanism 36 by the ECU 60 will be described in detail with reference to FIG. The flow of FIG. 2 is executed at predetermined time intervals.

First, the ECU 60 calculates the degree of supercooling (step S201). Specifically, the ECU 60 acquires the pressure and temperature of the working fluid based on the outputs of the pressure sensor 62 and the temperature sensor 64. Then, the ECU 60 calculates the degree of supercooling of the working fluid based on a predetermined program or data. For the degree of supercooling, for example, the saturation temperature (condensation temperature) is obtained by searching the pressure of the working fluid obtained from the saturated vapor diagram (data) of the working fluid, and the difference between the obtained saturation temperature and the obtained temperature ( It can be obtained by calculating the absolute value).

Next, the ECU 60 calculates the target pressure of the working fluid for making the supercooling degree of the working fluid a predetermined supercooling degree while keeping the temperature of the working fluid at the acquired temperature (step S203). This target pressure is calculated by performing a predetermined calculation using the saturated vapor diagram (data) of the working fluid described above. The target pressure of the working fluid here is the target value of the pressure downstream of the expander 16 and upstream of the pump 12, and more specifically, the target value of the pressure downstream of the expander 16 and upstream of the condenser 18. be. The ECU 60 is an exhaust gas calculated according to the engine operating state determined based on the temperature of the engine cooling water acquired based on the output of the water temperature sensor 70, the output of the engine rotation speed sensor 66, and the output of the engine load sensor 68. Such target pressures may be corrected based on the temperature of the gas. This is to operate the Rankine cycle system more appropriately.

Here, the target pressure will be described more specifically. In the basic specifications of the Rankine cycle system, the pressure PA (bar) of the working fluid and the temperature TA (° C.) of the working fluid are used as the pressure and temperature of the working fluid acquired based on the outputs of the pressure sensor 62 and the temperature sensor 64. Suppose when is specified. Then, consider the case where the pressure PB (bar) (<PA) and the temperature TB (° C.) (<TA) of the working fluid are acquired based on the outputs of the pressure sensor 62 and the temperature sensor 64. In this case, in order to keep the degree of supercooling of the working fluid that has passed through the condenser 18 at a predetermined degree of supercooling, it is necessary to increase the pressure of the working fluid. The target pressure at this time is calculated as the target pressure.

When the target pressure is calculated, the ECU 60 determines whether or not the calculated target pressure exceeds the atmospheric pressure (step S205). This determination is made to prevent the pressure of the working fluid at the outlet of the inflator 16 from dropping below atmospheric pressure due to this control. Atmospheric pressure may be detected (acquired) by an atmospheric pressure sensor, but here it is set as a default value and stored in a storage device.

Since the calculated target pressure exceeds the atmospheric pressure, if a positive determination is made in step S205, the ECU 60 proceeds to step S207. In step S207, the target control amount for the actuator 36d of the piston mechanism 36 is calculated by a predetermined calculation based on predetermined data or the like so as to reach the calculated target pressure. Then, the actuator 36d is controlled by the controlled amount. The routine ends by going through step S207.

On the other hand, since the calculated target pressure does not exceed the atmospheric pressure, if a negative determination is made in step S205, the ECU 60 proceeds to step S209. In step S209, a predetermined pressure is set as the target pressure instead of the calculated target pressure. This predetermined pressure is a pressure that makes the supercooling degree of the working fluid exceed the above-mentioned predetermined supercooling degree, is a pressure equal to or higher than the atmospheric pressure, and is a value peculiar to the Rankine cycle system 10 based on an experiment in advance. It is good that it is defined. The predetermined pressure in this case may be atmospheric pressure, but it is more preferable to be higher than that. This is to more reliably prevent the pressure of the working fluid at the outlet of the inflator from becoming lower than the atmospheric pressure. By going through step S209, the process proceeds to step S207, and the routine ends. As a result, the control of the piston mechanism 36 as described above is executed.

As described above, according to the present embodiment, the pressure of the working fluid is adjusted so as to keep the degree of supercooling of the working fluid after passing through the condenser 18 (before the pump 12) above the predetermined degree of supercooling. The piston mechanism 36 is controlled. Therefore, it is possible to suitably prevent the occurrence of cavitation in the pump.

The target value of the pressure of the working fluid at this time is also set so as to keep the pressure of the working fluid downstream of the inflator 16 and upstream of the pump 12 at atmospheric pressure or higher. That is, when the pressure for making the overcooling degree of the working fluid a predetermined overcooling degree is lower than the atmospheric pressure, the increase amount or the target value of the pressure of the working fluid is corrected so that the pressure becomes the atmospheric pressure or more, and the actuator The target control amount of 64 is set. Thereby, by the above control, the pressure of the working fluid downstream of the inflator 16 and upstream of the pump 12 is maintained at a pressure equal to or higher than the atmospheric pressure, particularly the pressure of the working fluid downstream of the inflator 16 and upstream of the condenser 18. Can be done. Therefore, it is possible to prevent the pressure of the working fluid from falling below the atmospheric pressure, and it is possible to prevent the inward force (force in the direction in which the pipe member is crushed) due to the atmospheric pressure from acting on the working fluid flow path. Generally, in the Rankine cycle system, the joints such as pipes of the working fluid flow path are designed to ensure airtightness when the internal pressure (of the pipe) is high, and conversely when the internal pressure is less than atmospheric pressure. Not designed with this in mind. However, as described above, according to the above embodiment, since the pressure of the working fluid can be prevented from falling below the atmospheric pressure, the airtightness of the joint portion such as the piping of the working fluid flow path is not impaired.

In the above embodiment, the piston portion 36b of the piston mechanism 36 is continuously controlled according to the calculated supercooling degree. However, when the calculated supercooling degree is equal to or higher than the predetermined supercooling degree and the pressure of the working fluid exceeds the atmospheric pressure, the piston mechanism 36 may not be controlled. This is because there is no problem of cavitation in the pump or problem of airtightness at the joint of the working fluid flow path. This also applies to the next second embodiment.

Next, a second embodiment according to the present invention will be described. The second embodiment differs from the first embodiment only in the configuration of the capacity variable means provided in the tank 30. Therefore, in the following, the overall description of the power recovery device provided with the Rankine cycle system according to the second embodiment will not be given, but the differences will be mainly described with reference to FIG. In the following, the above-mentioned reference numerals will be similarly attached to the components corresponding to the components already described, and the duplicated description will be omitted.

In the second embodiment, the balloon mechanism 80 is provided as the capacity variable means instead of the piston mechanism. The balloon mechanism 80 includes a balloon portion 82 arranged in the tank 30, an air tank (gas supply means) 84 provided for supplying gas into the balloon portion 82, and a control valve 86 which is a three-way valve. Be prepared. The balloon portion 82 is configured as an elastic bag and can be expanded or contracted according to the amount of gas (gas pressure) inside. Further, the ECU 60 (in the control means of the auxiliary pump) is used to maintain the tank internal pressure detected based on the output of a pressure sensor (not shown) at a predetermined pressure or higher so that the air gas having a predetermined pressure or higher is continuously stored in the air tank 84. The corresponding functional unit) is configured to drive the auxiliary pump (connected to the air tank) at a predetermined time. Although the auxiliary pump is electric here, it may be driven by the power of the engine.

Also in the second embodiment, the control described with reference to FIG. 2 is performed in the same manner, and the same arithmetic processing is performed for steps S201 to S205 and S209. In the second embodiment, the ECU 60 (a functional unit corresponding to the control means of the control valve 86 of the balloon mechanism 80) is a target corresponding to the target pressure of the working fluid defined in step S205 or S209 in the above step S207. Calculate the control amount. This target control amount is a control amount for the control valve 86 (actuator). The ECU 60 controls the control valve 86 (actuator) by performing a predetermined calculation based on a predetermined program or data based on the output from the pressure sensor 88 for detecting the pressure in the balloon unit 82. Calculate the amount. Then, as a result, the ECU 60 controls the control valve 86 with the controlled amount.

When it is desired to reduce the capacity (space in which the working fluid can be accumulated) in the tank 30 (to increase the pressure of the working fluid), the ECU 60 in the air tank 84 so as to send the air gas in the air tank 84 to the balloon portion 82. The control valve 86 is controlled so as to open in the balloon portion 82 and close to the external space E (see arrow A1 in FIG. 3A). As a result, the volume occupied by the balloon portion 82 in the tank 30 increases, and the volume in the tank 30 (space on the working fluid side) decreases. On the other hand, when it is desired to increase the capacity in the tank 30 (to reduce the pressure of the working fluid), the ECU 60 does not supply the air gas from the air tank 84, but discharges the air gas in the balloon portion 82 to the external space E. Controls the control valve 86 (see arrow A2 in FIG. 3B). As a result, the volume occupied by the balloon portion 82 in the tank 30 decreases, and the volume in the tank 30 (space on the working fluid side) increases.

The gas supply means is not limited to the air tank 84 as described above. For example, the auxiliary pump may be directly connected to the control valve 86.

Although the typical embodiment of the present invention has been described above, the present invention can be modified in various ways. Various substitutions and modifications are possible without departing from the spirit and scope of the invention as defined by the claims of the present application.

In the Rankine cycle system, engine cooling water is used for cooling in the condenser, but the present invention is not limited to this, and the condenser may be configured to cool the working fluid with the outside air. Further, in the above embodiment, the capacity variable means provided in the tank is used as the pressure variable means, but the present invention is not limited to this, and the pressure variable means having another configuration may be provided in the circulation passage of the working fluid. For example, the balloon mechanism may be provided in a flow path of a working fluid other than the tank. Further, in the above embodiment, the upper limit value of the target pressure of the working fluid is not set, but the upper limit value may be set.

1 Power recovery device 10 Rankine cycle system 12 Pump 12a Motor 12b Drive shaft 14 Evaporator 16 Inflator 18 Condensator 20 Circular passage 22 Engine 24 Exhaust pipe 26 Generator 36 Piston mechanism 60 Electronic control unit (ECU) (Control means)
62 Pressure sensor 64 Temperature sensor 80 Balloon mechanism PV Pressure variable means

Claims (2)

In a Rankine cycle system in which the working fluid pumped by the pump flows through an evaporator, an inflator, and a condenser in that order and is pumped again by the pump.
A pressure variable means configured to make the pressure of the working fluid variable,
The pressure is variable so that the pressure downstream of the expander and upstream of the pump is kept higher than the atmospheric pressure, and the degree of supercooling of the working fluid that has passed through the condenser is kept above a predetermined degree of supercooling. With control means configured to control the means ,
The pressure variable means is a capacity variable means configured to make the capacity in the tank variable in a tank provided between the condenser and the pump so as to achieve gas-liquid separation of the working fluid. Have,
The control means is
The degree of supercooling of the working fluid was calculated based on the acquired temperature and pressure of the working fluid upstream of the pump and downstream of the condenser.
Calculate the target pressure to make the calculated supercooling degree equal to or higher than the predetermined supercooling degree.
When the calculated target pressure is lower than the atmospheric pressure, the target pressure is set to a predetermined pressure equal to or higher than the atmospheric pressure, and the capacity variable means is controlled based on the set predetermined pressure.
Rankine cycle system. A control method in a Rankine cycle system in which the working fluid pumped by a pump flows through an evaporator, an inflator, and a condenser in that order and is pumped again by the pump.
The Rankine cycle system has a capacity-variable means configured to make the capacity in the tank variable in a tank provided between the condenser and the pump so as to achieve gas-liquid separation of the working fluid. Prepare,
A step of calculating the degree of supercooling of the working fluid that has passed through the condenser based on the acquired temperature and pressure of the working fluid upstream of the pump and downstream of the condenser.
A step of calculating a target pressure for making the calculated supercooling degree equal to or higher than the predetermined supercooling degree, and
When the calculated target pressure is lower than the atmospheric pressure, the step of setting the target pressure to a predetermined pressure equal to or higher than the atmospheric pressure and controlling the capacitance variable means based on the set predetermined pressure .
Control methods, including.

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