JP2000088364A - Supercritical refrigerating cycle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To operate an air conditioner while maintaining an overall efficiency of a CO2 cycle (air conditioner) high. SOLUTION: At the time of a low load mode, it is controlled that a refrigerant pressure at an outlet side of a radiator 2 becomes lower than an inlet pressure of a target pressure control valve so that a theoretical COP becomes maximum in the state that a rotational speed of a compressor 1 is held at a predetermined rotational speed. Thus, since a discharge pressure is lowered so that a compression ratio of the compressor 1 is lowered, an efficiency η of the compressor 1 is raised. Accordingly, at the time of the low load mode, the COP is lowered, but the efficiency of the compressor 1 is improved. Therefore, the air conditioner can be driven while maintaining the efficiency of an overall CO2 cycle (air conditioner) high.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a supercritical refrigeration cycle in which a refrigerant pressure in a radiator (high pressure side) exceeds a critical pressure of the refrigerant, and is effective when applied to an air conditioner of an electric vehicle. By the way, the critical pressure refers to a pressure at which the refrigerant molecules move like a gas phase state (supercritical state) while the density is substantially equal to the liquid density. Specifically, the critical pressure in the Mollier diagram The pressure corresponding to the maximum value of the saturated liquid line / saturated vapor line.

[0002]

2. Description of the Related Art A supercritical refrigeration cycle using carbon dioxide (CO ₂ ) as a refrigerant (hereinafter, this supercritical refrigeration cycle is called CO 2
Called _two cycles. In the invention described in JP-A-9-264622, the pressure control valve (pressure reducing valve) is set so that the refrigerant pressure at the radiator outlet side becomes the target pressure determined based on the refrigerant temperature at the radiator outlet side. By controlling the opening degree of CO _2, CO ₂ can be maintained while maintaining a high coefficient of performance (COP).
It is described that the cycle is operated.

Incidentally, the coefficient of performance (COP) is the ratio of the specific enthalpy difference Δi between the inlet side and the outlet side of the evaporator to the specific enthalpy difference ΔL between the suction side and the discharge side of the compressor (= Δi
/ ΔL).

[0004]

By the way, the inventors of the present invention carried out a test and study on the CO ₂ cycle described in the above-mentioned publication, and found that the opening degree of the pressure control valve was based on the refrigerant temperature at the radiator outlet side. Just controlling the CO ₂
It has been discovered that the efficiency of the entire cycle (air conditioner) cannot always be kept high.

That is, the opening degree of the pressure control valve is determined by the theoretical CO
The refrigeration capacity of the CO ₂ cycle (the cooling capacity of the air conditioner) is determined based on the refrigerant temperature at the radiator outlet side such that P becomes maximum (maximum). It is controlled (adjusted) by increasing or decreasing the mass flow through the evaporator. The theoretical COP is a ratio of the specific enthalpy difference Δi between the inlet side and the outlet side of the evaporator to the specific enthalpy difference ΔL between the suction side and the discharge side of the compressor, which can be read on a Mollier diagram of the refrigerant ( = Δi / ΔL).

[0006] On the other hand, as is well known, the efficiency of the compressor changes so as to become maximum (maximum) at a predetermined rotation speed and to be upwardly convex with respect to a change in rotation speed. Here, as is well known, the efficiency of the compressor refers to the compression work actually performed by the compressor with respect to the energy given to the compressor (for example, the power consumption of the electric motor when the compressor is driven by the electric motor). (ΔL × refrigerant flow rate) Ratio of Wout (= Wout / Wi)
n).

[0007] For this reason, if the rotational speed of the compressor is simply reduced to reduce the refrigerating capacity, the efficiency of the compressor is reduced. Therefore, even if the pressure control valve is controlled so that the theoretical COP is maximized, There is a problem that the efficiency of the entire CO ₂ cycle (air conditioner) is reduced. In view of the above, an object of the present invention is to operate a supercritical refrigeration cycle while maintaining a high efficiency of the entire supercritical refrigeration cycle.

[0008]

The present invention uses the following technical means to achieve the above object. Claim 1
According to the invention described in 3, the required rotation speed of the compressor (1) capable of exerting the refrigerating capacity required in the evaporator (4) by the evaporator (4) is higher than the predetermined rotation speed by a high load. In this state, the opening of the pressure control valve (3) is adjusted so that the refrigerant pressure at the outlet of the radiator (2) becomes the first target pressure determined based on the refrigerant temperature at the outlet of the radiator (2). On the other hand, in a low load state where the required rotation speed is equal to or lower than the predetermined rotation speed, the refrigerant pressure at the outlet of the radiator (2) is
The opening of the pressure control valve (3) is controlled so that the second target pressure is lower than the first target pressure.

Accordingly, the compression ratio of the compressor (1) decreases as described later in conjunction with the decrease in the discharge pressure of the compressor (1), and the efficiency of the compressor (1) increases. Therefore, in a low load state, although the theoretical COP is reduced, the efficiency of the compressor (1) is improved. Therefore, it is possible to operate the supercritical refrigeration cycle while maintaining the efficiency of the entire supercritical refrigeration cycle high. it can.

In a low load state, it is desirable to control the compressor (1) such that the rotation speed of the compressor (1) becomes a predetermined rotation speed. According to a third aspect of the present invention, the predetermined rotation speed is selected so as to increase as the refrigerant pressure on the outlet side of the radiator (2) increases. Thereby, as described later, the supercritical refrigeration cycle can be operated while maintaining the efficiency of the entire supercritical refrigeration cycle even higher.

In the invention according to claim 4, the evaporator (4)
When the required refrigerating capacity required in the above is larger than the predetermined refrigerating capacity, the refrigerant pressure at the outlet of the radiator (2) is determined based on the refrigerant temperature at the outlet of the radiator (2).
The opening of the pressure control valve (3) is controlled so as to reach the target pressure. On the other hand, when the required refrigerating capacity is equal to or lower than the predetermined refrigerating capacity, the refrigerant pressure at the outlet of the radiator (2) is higher than the first target pressure. It is characterized in that the opening of the pressure control valve (3) is controlled so as to have a low second target pressure.

Thus, the supercritical refrigeration cycle can be operated while maintaining the efficiency of the entire supercritical refrigeration cycle high, as in the first aspect of the present invention. Incidentally, the reference numerals in parentheses of the above means are examples showing the correspondence with specific means described in the embodiments described later.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment)
The CO ₂ cycle (supercritical refrigeration cycle) according to the present invention is applied to an air conditioner of an electric vehicle, and FIG. 1 is a schematic diagram of an air conditioner according to the present embodiment.

Reference numeral 1 denotes a compressor for sucking and compressing refrigerant (CO ₂ ). The compressor 1 is a so-called electric compressor integrated with an electric motor (not shown) for driving the compressor 1. Reference numeral 2 denotes a radiator (gas cooler) that cools the refrigerant discharged from the compressor 1 and whose internal pressure exceeds the critical pressure of the refrigerant. Reference numeral 3 denotes an electric pressure control valve (electric expansion valve) for reducing the pressure of the refrigerant flowing out of the radiator 2 and controlling the refrigerant pressure on the outlet side (high pressure side) of the radiator 2. And a proportional solenoid type solenoid valve in which the valve opening is controlled (adjusted) by controlling the applied voltage.

Reference numeral 4 denotes an evaporator (evaporator) for evaporating the refrigerant depressurized by the pressure control valve and exhibiting a refrigerating ability. Reference numeral 5 denotes a refrigerant flowing out of the evaporator 4 into a gaseous refrigerant and a liquid refrigerant. An accumulator (liquid receiver) that separates and allows only the gas-phase refrigerant to flow out to the suction side of the compressor 1 and stores excess refrigerant during the CO ₂ cycle. Reference numeral 6 denotes an electronic control unit (control means) for controlling the pressure control valve 3 and the compressor 1 (electric motor). The electronic control unit (hereinafter, referred to as ECU) 6 includes a temperature sensor (temperature detecting means) 7 for detecting a refrigerant temperature at the outlet side of the radiator 2, a radiator 2
A pressure sensor (pressure detecting means) 8 for detecting the refrigerant pressure at the outlet side, an outside air temperature sensor (outside air temperature detecting means) 9 for detecting the temperature of the air outside the vehicle interior, and an internal air temperature sensor (internal air temperature) for detecting the temperature of the air inside the vehicle interior Detecting means) 10, a solar radiation sensor (infrared radiation detecting means) 11 for detecting the amount of solar radiation poured into the vehicle interior, and a temperature sensor for detecting the temperature of air cooled by the evaporator 4 and blown out toward the vehicle interior. Outlet air temperature detecting means) 12
Temperature setting device (temperature setting means) 13 for setting and inputting the detected value of and the room temperature desired by the occupant by manual operation of the occupant
The set temperature entered in is input.

Hereinafter, the outside air temperature sensor 9, the inside air temperature sensor 10, the solar radiation sensor 11, and the temperature sensor 12 are collectively called an air conditioning sensor. Incidentally, the electric motor of the compressor 1 is D
It is a C brushless motor, and the rotation speed of the compressor 1, that is, the rotation speed of the electric motor, is controlled by the ECU 6 via the inverter 14 that controls the frequency of the alternating current.

A blower 15 blows cooling air to the radiator 2, and a blower 16 blows air toward the evaporator 4. Next, the control operation of the CO ₂ cycle according to the present embodiment will be described based on the flowcharts shown in FIGS. 1. Compressor control mode (see FIG. 2) When a start switch (not shown) of the air conditioner is turned on,
The ECU 6 calculates the compressor 1 based on a control signal to the inverter 14.
The rotation speed of the electric motor (hereinafter, this rotation speed is referred to as the actual rotation speed n) is detected (S100), and the detected values of the air conditioning sensors 9 to 11 (excluding the temperature sensor 12) and the temperature setting device 13 are set. The set temperature input to is read (S11).
0).

Next, the ECU 6 controls the air conditioning sensors 9-11.
Based on the temperature (excluding the temperature sensor 12) and the set temperature, a target temperature of the air blown into the vehicle interior (hereinafter, referred to as a target blown air temperature TAO) is determined (S120), and the blown air is detected from the detected temperature of the temperature sensor 12. (Hereinafter, this temperature is referred to as the actual blown air temperature T) (S1).
30), and compare the target outlet temperature with the actual outlet air temperature (S140).

If the target outlet temperature is lower than the actual outlet air temperature and the required refrigerating capacity required in the evaporator 4 is larger than the actual refrigerating capacity actually exhibited, the necessary refrigerating capacity in the evaporator 4 is used. The rotation speed of the compressor 1 is increased by a predetermined amount (S150) so that
Go to 190. On the other hand, when the target outlet temperature is equal to or higher than the actual outlet air temperature and the required refrigerating capacity is equal to or lower than the actual refrigerating capacity,
Actual speed n of the compressor 1 it is determined whether greater than the predetermined rotational speed n _min (S160), when the actual rotational speed n of the compressor 1 is larger than the predetermined rotational speed n _min, the rotational speed of the compressor 1 Is decreased by a predetermined amount (S170), and the process proceeds to S190. The details of the predetermined rotation speed n _min will be described later.

If the actual rotation speed n of the compressor 1 is equal to or less than the predetermined rotation speed n _{min, the} rotation speed of the compressor 1 is set to the predetermined rotation speed n _min (S180), and the process proceeds to S190. And
At S190, the actual rotational speed n of the compressor 1 is detected again, and it is again determined whether or not the actual rotational speed n of the compressor 1 is greater than the predetermined rotational speed n _min (S200). When the number n is larger than the predetermined number of revolutions n _min , the air conditioner (CO ₂ cycle) increases the required number of revolutions of the compressor capable of exerting the required refrigerating capacity in the evaporator 4 to a value higher than the predetermined number of revolutions n _min. Assuming that it is under load,
The operation shifts to the high load mode in S220 and thereafter.

On the other hand, when the actual rotation speed n of the compressor 1 is equal to or lower than the predetermined rotation speed n _{min, the} rotation speed of the compressor 1 is set to the predetermined rotation speed n _min (S210), and the mode shifts to the low load mode of S300 or lower. Note that the compressor 1 detected in S190
The actual rotation speed n of, which corresponds to the required rotation speed of the compressor 1 can be exhibited required refrigeration capacity in the evaporator 4, the predetermined rotational speed n _min is a predetermined refrigerating capacity of the evaporator 4 Is equivalent to

2. High load mode (refer to FIG. 3) The refrigerant temperature at the outlet side of the radiator 2 is detected from the detection value of the temperature sensor 7 (S220), and based on the map shown in FIG. 5, the target pressure control valve inlet pressure (first target pressure). _PT is determined (S230). Here, the map shown in FIG.
This shows the relationship between the refrigerant pressure and the refrigerant temperature at the outlet side of the radiator 2 so that P becomes maximum (maximum).

Next, based on the value detected by the pressure sensor 8, the radiator 2
The outlet side refrigerant pressure P is detected (S240), and the target pressure control is performed.
Valve inlet pressure P_TAnd the refrigerant pressure P (S25
0). Then, the target pressure control valve inlet pressure P_TAnd refrigerant pressure
If P is equal, the process returns to S100, and the target pressure control
Valve inlet pressure P_TIs smaller than the refrigerant pressure P (P _T
−P <0), and the refrigerant pressure P is equal to the target pressure control valve inlet pressure P_T
So that the opening degree of the pressure control valve 3 is reduced (S2
60), after increasing the refrigerant pressure P, the process returns to S100.

On the other hand, when the target pressure control valve inlet pressure _PT is higher than the refrigerant pressure P ( _PT- P> 0), the pressure control valve 3 is controlled so that the refrigerant pressure P becomes the target pressure control valve inlet pressure _PT. Is increased (S270), the refrigerant pressure P is decreased, and the process returns to S100. 3. Low load mode (refer to FIG. 4) The detection values of the air conditioning sensors 9 to 11 (excluding the temperature sensor 12) and the set temperature input to the temperature setting device 13 are read (S300), and the air conditioning sensors 9 to 11 (the temperature sensor 12 Excluding) and the target outlet air temperature TAO based on the set temperature.
Is determined (S310).

Then, the actual blown air temperature T is detected (S
320), the target outlet temperature is compared with the actual outlet air temperature (S330), and when the target outlet temperature is higher than the actual outlet air temperature and the required refrigerating capacity is smaller than the actual refrigerating capacity, the opening of the pressure control valve 3 is increased. Then, after the refrigerant pressure P is decreased (S340), the process returns to S320. For this reason, in the low load mode, the refrigerant pressure on the outlet side of the radiator 2 is controlled to be lower (second target pressure) than the target pressure control valve inlet pressure (first target pressure) _PT .

On the other hand, when the refrigerating capacity decreases as the pressure on the outlet side of the radiator 2 decreases, and the target blowing temperature becomes lower than the actual blowing air temperature and the required refrigerating capacity becomes higher than the actual refrigerating capacity, the pressure control valve 3 Is reduced (S350),
The refrigeration capacity is increased by increasing the refrigerant pressure P. Then, the temperature of the refrigerant at the outlet of the radiator 2 is detected (S360),
Based on the map shown in FIG. 5, the target pressure control valve inlet pressure P
_T is determined (S370), and the refrigerant pressure P on the outlet side of the radiator 2 is determined.
Is greater than the target pressure control valve inlet pressure _PT (S380, S390).

When the refrigerant pressure P becomes larger than the target pressure control valve inlet pressure _PT , it is considered that the state can deviate from the low load mode, and the process returns to S100. If the pressure is equal to or lower than the pressure control valve inlet pressure _PT , it is determined that the vehicle is still in the low load mode, and the process returns to S320. Next, the predetermined rotation speed n _min will be described.

Generally, the efficiency η of a compressor depends on its type.
However, as described above, the maximum (maximum) is reached at a predetermined rotation speed.
As shown in FIG.
Become Therefore, in the present embodiment, as shown in FIG.
The rotational speed at which the efficiency η begins to drop significantly_min
And Incidentally, in the present embodiment, the predetermined rotational speed n
_{m in}Is the maximum efficiency η_maxEfficiency η which is about 45% of_minWhen
Rotation speed.

FIG. 6 is a graph showing the relationship between the number of revolutions of the compressor and the efficiency η when the discharge pressure and the suction pressure are kept constant. /
It is generally known that, when the suction pressure is changed, the efficiency η decreases as the compression ratio increases, as shown in FIG. Incidentally, the graphs shown in FIGS. 6 and 7 are values including the efficiency of the electric motor, but the efficiency η of the compressor is similar to the graphs shown in FIGS. It is also generally known to show a tendency.

Next, the features of this embodiment will be described. According to the present embodiment, in the low load mode, the refrigerant pressure on the outlet side of the radiator 2 is set to a pressure (second target pressure) lower than the target pressure control valve inlet pressure (first target pressure) _PT. , The discharge pressure decreases and the compression ratio of the compressor 1 decreases, so that the efficiency η of the compressor 1 increases.

Therefore, in the low load mode, although the theoretical COP is reduced, the efficiency of the compressor 1 is improved. Therefore, the air conditioner is operated while keeping the efficiency of the entire CO ₂ cycle (air conditioner) high. can do. Also,
Necessary rotation speed of the compressor 1 capable of exhibiting the required refrigeration capacity (the rotation speed of the compressor 1 detected in S190)
But when less than the predetermined rotational speed n _min, so to hold the rotational speed of the compressor 1 to a predetermined rotational speed n _min, the compressor 1 becomes to rotate at a higher than necessary number of revolutions rpm. Therefore, the CO ₂ cycle (air conditioner) can be operated with the efficiency of the compressor improved.

FIG. 8 shows the case where the pressure control valve 3 is controlled in the low heat load mode so that the theoretical COP is maximized as described in the above-mentioned publication (hereinafter, this control is referred to as conventional control). Cycle diagram (A-B-C-D)
When the pressure control valve 3 is controlled at a pressure (second target pressure) lower than the target pressure control valve inlet pressure (first target pressure) _{PT at} which the theoretical COP is maximum (hereinafter, referred to as the present embodiment) This control is referred to as a new control.)
EFG) is a Mollier diagram, and a thick line η is
FIG. 6 is a plot of the line (optimal control line) shown in FIG. 5 on a Mollier diagram.

Note that the suction side of the compressor and the discharge
Specific enthalpy difference (L₁, L _Two) (A)
-B, AE) are based on Mollier considering the efficiency η of the compressor.
Since they are plotted on a diagram, they are not necessarily
No change along the pi line. In addition, conventional control requires
In order to demonstrate the required refrigeration capacity Q [J], the refrigeration capacity
(The specific enthalpy change in the evaporator 4) is q [J / kg]
And the rotation speed of the compressor 1 is Nrpm (<n_min)age,
In the new control, the required refrigeration capacity is used to demonstrate Q [J].
Or the refrigerating capacity (specific enthalpy change in the evaporator 4)
q / 2 [J / kg] and the rotation speed of the compressor 1 is 2 Nrp
m (≧ n_min).

As is apparent from FIG. 8, the compression work (L ₂ ) of the conventional control is larger than the compression work (L ₁ ) of the new control, and the new control reduces the theoretical COP. However, since the efficiency of the compressor 1 is improved,
_The air conditioner can be operated while maintaining the efficiency of the entire _two cycles (air conditioner) high. Incidentally, according to trial calculations by the inventors, the new control can improve the efficiency of the entire CO ₂ cycle (air conditioner) in the low heat load mode by about 10 to 40% compared to the conventional control. It has gained.

(Second Embodiment) In the above-described embodiment,
Constant rotation speed n_minWas constant (fixed value).
As shown in FIG. 9, the state of the radiator 2 outlet side (high pressure side)
Focus on the fact that the efficiency of the compressor changes with the refrigerant pressure.
It was done. That is, it is clear from FIG.
U, efficiency η_minRotation speed n required to obtain _{m in}Is
The higher the refrigerant pressure on the outlet side of the radiator 2, the larger
In the present embodiment, as shown in FIG.
In the flowchart of the compressor control mode according to the state,
Between S130 and S140, the value detected by the pressure sensor 8
The refrigerant pressure P on the outlet side of the radiator 2 is detected (S400).
Based on the detected refrigerant pressure P, the map shown in FIG.
Predetermined rotation speed n_minIs determined (S410)
These two control steps are added.

Therefore, in this embodiment, while maintaining the efficiency of the entire CO ₂ cycle (air conditioner) higher,
The air conditioner can be operated. Note that S400, S
Except that the 410 is inserted between S130 and S140, it is the same as the first embodiment, and the description of the operation of the entire CO ₂ cycle (air conditioner) is omitted.

In the above embodiment, the present invention is applied to the CO ₂ cycle using carbon dioxide as a refrigerant.
The present invention is not limited to this, and can be applied to, for example, a supercritical refrigeration cycle using ethylene, ethane, nitrogen oxide, or the like as a refrigerant. Further, the application of the present invention is not limited to an air conditioner for an electric vehicle, but can be applied to other refrigeration devices.

The drive source for driving the compressor 1 is not limited to the electric motor, but may be any drive source dedicated to the compressor 1, for example, a gas heat pump cycle gas engine or a diesel heat pump cycle. It may be a diesel engine. Further, in the above-described embodiment, in the low load mode, the rotation speed of the compressor 1 is fixed to the predetermined rotation speed n _min and only the pressure control valve 3 is controlled, but the rotation speed of the compressor 1 and the pressure control valve 3 are controlled. May be controlled simultaneously. According to this, even in the low load mode, the supercritical refrigeration cycle can be operated in a state where the rotation speed of the compressor 1 is equal to or higher than the predetermined rotation speed n _min .
Further, the efficiency of the supercritical refrigeration cycle can be improved.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a CO ₂ cycle (air conditioner).

FIG. 2 is a flowchart illustrating control of a CO ₂ cycle (air conditioner) according to the first embodiment.

FIG. 3 is a flowchart illustrating control of a CO ₂ cycle (air conditioner) according to the first embodiment.

FIG. 4 is a flowchart illustrating control of a CO ₂ cycle (air conditioner) according to the first embodiment.

FIG. 5 is a graph showing a relationship between a target pressure control valve inlet pressure and a refrigerant temperature at a radiator outlet side.

FIG. 6 is a graph showing the relationship between the efficiency of the compressor and the number of revolutions.

FIG. 7 is a graph showing a relationship between a compressor efficiency and a compression ratio.

FIG. 8 is a Mollier diagram.

FIG. 9 is a graph showing the relationship between the efficiency and the number of revolutions of the compressor.

FIG. 10 is a flowchart illustrating control of a CO ₂ cycle (air conditioner) according to the second embodiment.

FIG. 11 is a graph showing a relationship between a predetermined rotational speed of the compressor and a high-pressure side pressure.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Compressor, 2 ... Radiator, 3 ... Pressure control valve, 4 ... Evaporator, 5 ... Accumulator, 6 ... Electronic control device (control means).

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Yukatsu Ozaki 14 Iwatani, Shimowakaku-cho, Nishio-shi, Aichi Prefecture Inside the Japan Automobile Parts Research Institute (72) Inventor Shin Nishida 1-1-1, Showa-cho, Kariya-shi, Aichi Inside DENSO Corporation

Claims (5)

[Claims] 1. A compressor (1) for compressing a refrigerant, and a radiator (2) for cooling the refrigerant discharged from the compressor (1) and having an internal pressure exceeding a critical pressure of the refrigerant.
And a pressure control valve (3) for controlling the refrigerant pressure at the outlet side of the radiator (2) while reducing the pressure of the refrigerant flowing out of the radiator (2); An evaporator (4) for evaporating the refrigerant, and a control means (6) for controlling the pressure control valve (3) and the compressor (1). The control means (6) comprises: In a high load state in which the required rotation speed of the compressor (1), which can exhibit the refrigerating capacity required in (4) by the evaporator (4), is higher than a predetermined rotation speed, the radiator (2)
Controlling the degree of opening of the pressure control valve (3) such that the refrigerant pressure on the outlet side becomes a first target pressure determined based on the refrigerant temperature on the outlet side of the radiator (2); The means (6) is configured such that, in a low load state in which the required rotation speed is equal to or less than the predetermined rotation speed, the refrigerant pressure at the outlet of the radiator (2) has a second target pressure lower than the first target pressure. Controlling the opening of the pressure control valve (3) as described above. 2. The control means (6) controls the compressor (1) such that the rotation speed of the compressor (1) becomes the predetermined rotation speed in the low load state. The supercritical refrigeration cycle according to claim 1, wherein 3. The supercritical refrigeration cycle according to claim 1, wherein the predetermined rotation speed is selected so as to increase as the refrigerant pressure at the outlet of the radiator (2) increases. 4. A compressor (1) for compressing a refrigerant, and a radiator (2) for cooling the refrigerant discharged from the compressor (1) and having an internal pressure exceeding a critical pressure of the refrigerant.
And a pressure control valve (3) for controlling the refrigerant pressure at the outlet side of the radiator (2) while reducing the pressure of the refrigerant flowing out of the radiator (2); An evaporator (4) for evaporating the refrigerant, and a control means (6) for controlling the pressure control valve (3) and the compressor (1). The control means (6) comprises: When the required refrigerating capacity required in (4) is larger than a predetermined refrigerating capacity, the refrigerant pressure at the outlet of the radiator (2) is determined based on the refrigerant temperature at the outlet of the radiator (2). The control means (6) controls the opening degree of the pressure control valve (3) so as to be one target pressure. On the other hand, when the required refrigerating capacity is equal to or less than the predetermined refrigerating capacity, the control means (6) controls the radiator (2). A) a second target pressure at which the refrigerant pressure at the outlet side is lower than the first target pressure; Supercritical refrigeration cycle, wherein the controller controls the opening degree of the pressure control valve (3) so that the. 5. The supercritical refrigeration cycle according to claim 1, wherein carbon dioxide is used as a refrigerant.