Patent Publication Number: US-2018031282-A1

Title: Supercritical refrigeration cycle apparatus and method for controlling supercritical refrigeration cycle apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     Pursuant to 35 U.S.C. §119(a), this application claims the benefit of earlier filing date and right of priority to Korean Application No. 10-2016-0094970, filed in Korea on Jul. 26, 2016, the contents of which is incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Field 
     A supercritical refrigeration cycle apparatus and a method for controlling a supercritical refrigeration cycle apparatus are disclosed herein. 
     2. Background 
     As is known, a refrigeration cycle apparatus may be configured to have a so-called vapor-compression refrigeration cycle including a compressor configured to compress refrigerant, a condenser in which refrigerant is radiated and condensed, an expansion apparatus configured to decompress and expand refrigerant, and an evaporator in which refrigerant is evaporated by absorbing ambient latent heat. On the other hand, there is a supercritical refrigeration cycle apparatus (hereinafter, referred to as a “supercritical refrigeration cycle apparatus”) using carbon dioxide (CO 2 ) as refrigerant. 
     The supercritical refrigeration cycle apparatus may include a compressor configured to compress refrigerant in a supercritical state, a gas cooler configured to radiate heat from the compressed refrigerant, an expansion apparatus configured to decompress and expand refrigerant which has passed through the gas cooler, and an evaporator configured to allow refrigerant to absorb and evaporate ambient latent heat. According to the supercritical refrigeration cycle apparatus, in contrast to a condenser in the refrigeration cycle apparatus in which a phase change typically occurs, a refrigerating capacity and power consumption may vary according to a pressure change due to a change in an outlet temperature of the gas cooler. 
     That is, as illustrated in  FIG. 1 , when comparing a first temperature, a second temperature, and a third temperature with different ambient temperatures, it is seen that the outlet temperature of the gas cooler relatively increases at the second and the third temperature (t 2 , t 3 ) with relatively higher ambient temperatures compared to the first temperature (t 1 ) with the lowest ambient temperature, and thus, their refrigerating capacity respectively decreases. According to the supercritical refrigeration cycle apparatus, there may exist “an appropriate operation high pressure” capable of maximizing a ratio of a power consumption change rate to a change rate of the refrigerating capacity. However, according to the related art supercritical refrigeration cycle apparatus, liquid refrigerant may be suctioned into the compressor, and due to this, an opening degree of the expansion apparatus should be controlled in consideration of a suction superheat degree of refrigerant suctioned into the compressor and the appropriate operation high pressure at the same time to suppress the occurrence of damage to the compressor, thereby causing a relatively large number of constraints in controlling the appropriate high pressure capable of enhancing the operation efficiency of the apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will be described in detail with reference to the following drawings in which like reference numerals refer to like elements, and wherein: 
         FIG. 1  is a graph showing a change in refrigerating capacity according to a gas cooler outlet temperature of a supercritical refrigeration cycle apparatus in the related art; 
         FIG. 2  is a schematic diagram of a supercritical refrigeration cycle apparatus according to an embodiment; 
         FIG. 3  is a pressure-enthalpy diagram for the supercritical refrigeration cycle apparatus of  FIG. 2 ; 
         FIG. 4  is a control block diagram of the supercritical refrigeration cycle apparatus of  FIG. 2 . 
         FIG. 5  is a schematic diagram of a supercritical refrigeration cycle apparatus according to another embodiment; 
         FIG. 6  is a control block diagram of the supercritical refrigeration cycle apparatus of  FIG. 5 ; 
         FIG. 7  is a flow chart of a method for controlling a supercritical refrigeration cycle apparatus according to an embodiment; and 
         FIG. 8  is a flow chart of a method for controlling a supercritical refrigeration cycle apparatus according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. Where possible, like reference numerals have been used to indicate like elements, and repetitive disclosure has been omitted. In describing the embodiments, detailed description will be omitted when a specific description for publicly known technologies to which the embodiments pertain is judged to obscure the gist. 
     Further, the accompanying drawings are used to help easily understand the technical idea of the embodiments and it should be understood that the idea of the present disclosure is not limited by the accompanying drawings. The idea of the present disclosure should be construed to extend to any alterations, equivalents, and substitutes besides the accompanying drawings. 
     As illustrated in  FIGS. 2 through 4 , a supercritical refrigeration cycle apparatus according to an embodiment may include a compressor  110  configured to compress refrigerant in a supercritical state; a gas cooler  140  configured to cool the compressed refrigerant in a supercritical state; a pressure control electronic expansion valve  170  connected to the gas cooler  140  to control a pressure of refrigerant; a receiver  180  configured to temporarily store refrigerant which has passed through the pressure control electronic expansion valve; a flow control electronic expansion valve  190  connected to an outlet side of the receiver  180  to control a flow rate of refrigerant; and a controller  250  configured to control the flow control electronic expansion valve  190  based on a suction superheat degree of refrigerant suctioned into the compressor  110  and a target suction superheat degree, and control the pressure control electronic expansion valve  170  based on a target operation high pressure and a current operation high pressure. The refrigerant may be carbon dioxide (CO 2 ), for example. 
     The compressor  110  may include an airtight container  111 , a compression unit or device  115  provided within the airtight container  111 , and a motor unit or motor  121  configured to provide a drive force to the compression unit  115 , for example. 
     The compression unit  115  may include a low stage compression unit or device  117  configured to suction and compress the evaporated refrigerant and a high stage compression unit or device  119  configured to compress the refrigerant compressed in the low stage compression unit  117 . The high stage compression unit  119  may be configured to compress the refrigerant at a pressure above a critical pressure of the refrigerant. 
     According to this embodiment, a case in which the compressor  110  is implemented as a two-stage compressor has been described; however, this is merely an example, and embodiments are not limited thereto. 
     An accumulator  130  configured to separate refrigerant into a gas and a liquid may be provided at a suction side of the compressor  110 , for example. The gas cooler  140  may be connected to a discharge-side pipe of the compressor  110  to communicate therewith. The pressure control electronic expansion valve  170 , which may be configured to reduce a pressure of refrigerant cooled in the gas cooler  140 , for example, may be provided at an outlet-side pipe of the gas cooler  140 . 
     The receiver  180  may be provided at an outlet side pipe of the pressure control electronic expansion valve  170 . Refrigerant decompressed and expanded while passing through the pressure control electronic expansion valve  170  may be temporarily stored within the receiver  180 . 
     The flow control electronic expansion valve  190 , which may be configured to control a flow rate of refrigerant which has passed the receiver  180 , for example, may be provided at an outlet-side pipe of the receiver  180 . An evaporator  210 , in which refrigerant may absorb and evaporate ambient latent heat, may be provided at an outlet-side pipe of the flow control electronic expansion valve  190 . 
     An outlet-side pipe of the evaporator  210  may be connected to a suction side of the compressor  110  to communicate therewith. The outlet-side pipe of the evaporator  210  may be connected to the accumulator  130 , for example. 
     Refrigerant which has passed through the evaporator  210  may exchange heat with refrigerant which has passed through the gas cooler  140 . Due to this, a suction superheat degree of refrigerant suctioned into the compressor  110  may increase. 
     An outlet-side pipe of the evaporator  210  may be configured to exchange heat with an outlet-side pipe of the gas cooler  140 . That is, an inter-refrigerant heat exchanger  150  or intermediate heat exchanger (hereinafter, referred to as “intermediate heat exchanger  150 ”) configured to exchange heat between high-pressure refrigerant and low-pressure refrigerant may be provided at an outlet side of the gas cooler  140 . 
     The intermediate heat exchanger  150  may include a first heat exchanger  151  configured to receive refrigerant which has passed through the gas cooler  140  and a second heat exchanger  153  configured to receive refrigerant which has passed the evaporator  210 . The first heat exchanger  151  and second heat exchanger  153  may be configured to exchange heat, for example. 
     A supercritical refrigeration cycle apparatus according to this embodiment may include the controller  250  implemented with a microprocessor provided with a control program, as illustrated in  FIG. 4 , for example. The controller  250  may be configured to control the flow control electronic expansion valve  190  so as to decrease an opening degree of the flow control electronic expansion valve  190  when the suction superheat degree of the refrigerant is less than the target suction superheat degree, and increase an opening degree of the flow control electronic expansion valve  190  when the suction superheat degree of the refrigerant is greater than the target suction superheat degree. 
     The controller  250  may be configured to control the pressure control electronic expansion valve  170  so as to increase an opening degree of the pressure control electronic expansion valve  170  when the current operation high pressure is greater than the target operation high pressure, decrease the opening degree of the pressure control electronic expansion valve  170  when the current operation high pressure is less than the target operation high pressure, and maintain the opening degree of the pressure control electronic expansion valve  170  when the current operation high pressure is the same as the target operation high pressure. 
     The controller  250  may include a operation unit (calculation unit or calculator)  255  configured to calculate a suction superheat degree of refrigerant in the compressor  110 , for example. A compressor suction temperature sensor  270  configured to sense a temperature of refrigerant suctioned into the compressor  110 , an evaporator temperature sensor  265  configured to sense a temperature of refrigerant at an outlet side of the evaporator  210 , a gas cooler temperature sensor  260  configured to sense an outlet temperature of the gas cooler  140 , and a pressure sensor  275  configured to sense a current operation high pressure may all be connected to the controller  250  in a communicable manner. The pressure sensor  275  may be provided at an outlet side of the gas cooler  140 , for example. 
     The operation unit  255  may be configured to compute the target operation high pressure (P CT ) from an outlet temperature (T C ) of the gas cooler  140  and an evaporation temperature (T E ) of the evaporator  210 , for example. The target operation high pressure (P CT ) may be configured such that a ratio of a refrigerating capacity change rate to a power consumption change rate is a maximum (highest). 
     That is, the target operation high pressure (target pressure) may be calculated using Equation 1 below, for example. 
         P   CT   =A*T   C   +B*T   C   *T   E   +C , where  A, B, C  are preset or predetermined constants, respectively, according to a system.  [Equation 1]
 
     The operation unit  255  may subtract an outlet temperature of refrigerant in the evaporator  210  from a suction temperature of refrigerant in the compressor  110  to compute a suction superheat degree (=suction temperature−outlet temperature) of refrigerant in the compressor  110 , for example. The controller  250  may be configured to control an opening degree of the flow control electronic expansion valve  190  to secure a suction superheat degree of the compressor  110  so as to suppress the occurrence of damage to the compressor  110  due to wet compression (liquid compression), and then control an opening degree of the pressure control electronic expansion valve  170  to maintain an appropriate operation high pressure so as to enhance the operation efficiency of the apparatus. 
     When an operation is started, low-pressure refrigerant may be suctioned into the compressor  110  and compressed at a pressure above a critical pressure and discharged in a supercritical state. The discharged refrigerant may be cooled by exchanging heat with air in the gas cooler  140 . 
     Refrigerant which has passed through the gas cooler  140  may be decompressed while passing through the pressure control electronic expansion valve  170  via the intermediate heat exchanger  150 . Refrigerant which has passed through the pressure control electronic expansion valve  170  may be introduced into the receiver  180 , and refrigerant which has passed through the receiver  180  may be decompressed while passing through the flow control electronic expansion valve  190 . Refrigerant which has passed through the flow control electronic expansion valve  190  may be introduced into the compressor  110  and evaporated by absorbing ambient latent heat. 
     Hereinafter, a method for controlling a supercritical refrigeration cycle apparatus according to an embodiment will be described with reference to  FIG. 7 . 
     The operation unit  255  may calculate a suction superheat degree of refrigerant in the compressor  110  from a difference between a temperature sensed by the compressor suction temperature sensor  270  and a temperature sensed by the evaporator temperature sensor  265  (S 110 ). The controller  250  may compare the calculated suction superheat degree and a target suction superheat degree to control an opening degree of the flow control electronic expansion valve  190  (S 120 ). When the calculated suction superheat degree is the same as the target suction superheat degree (S 110 ), the controller  250  may maintain a current opening degree of the flow control electronic expansion valve  190  (S 160 ) and control an opening degree of the pressure control electronic expansion valve  170 . 
     According to this embodiment, the target suction superheat degree may be set to prevent liquid refrigerant from being suctioned into the compressor  110 , and thus, when the calculated suction superheat degree is maintained to be the same as the target suction superheat degree, the wet compression (liquid compression) of the compressor  110  may be prevented, enhancing reliability of the compressor  110 . When the calculated suction superheat degree is greater than the target suction superheat degree (S 130 ), the controller  250  may control the flow control electronic expansion valve  190  to increase an opening degree of the flow control electronic expansion valve  190  (S 140 ). 
     Further, when the calculated suction superheat degree is less than the target suction superheat degree (S 130 ), the controller  250  may control the flow control electronic expansion valve  190  to decrease an opening degree of the flow control electronic expansion valve  190  (S 150 ). When the calculated suction superheat degree is the same as the target suction superheat degree (S 120 ), the controller  250  may control the flow control electronic expansion valve  190  to maintain a current opening degree of the flow control electronic expansion valve  190  (S 160 ), and control an opening degree of the pressure control electronic expansion valve  170 . 
     On the other hand, the controller  250  may sense a gas cooler temperature and an evaporator temperature from the gas cooler temperature sensor  260  and the evaporator temperature sensor  265 , respectively, and sense a current pressure by the pressure sensor  275  (S 170 ). The operation unit  255  may compute a target operation high pressure using the gas cooler temperature and evaporator temperature based on the Equation 1 (S 180 ). 
     When the target operation high pressure is computed, the controller  250  may compare the computed target operation high pressure with the current pressure (S 190 ), and when the current pressure is the same as the target operation high pressure, the controller  250  may control the pressure control electronic expansion valve  170  to maintain a current opening degree of the pressure control electronic expansion valve  170  (S 230 ). When the current pressure is greater than the target operation high pressure (S 200 ), the controller  250  may control the pressure control electronic expansion valve  170  to increase an opening degree of the pressure control electronic expansion valve  170  (S 210 ). When the current pressure is less than the target operation high pressure (S 200 ), the controller  250  may control the pressure control electronic expansion valve  170  to decrease an opening degree of the pressure control electronic expansion valve  170  (S 220 ). 
     According to this embodiment, the target operation high pressure may be set to maximize a ratio of the refrigerating capacity change rate to the power consumption change rate, and then, when the current pressure is maintained to be the same as the target operation high pressure, the operation efficiency may be enhanced. 
     Hereinafter, a supercritical refrigeration cycle apparatus according to another embodiment will be described with reference to  FIGS. 5 and 6 . 
     As illustrated in  FIGS. 5 and 6 , a supercrtical refrigeration cycle apparatus according to this embodiment may include compressor  110  configured to compress refrigerant in a supercritical state; gas cooler  140  configured to cool the compressed refrigerant; a pressure control electronic expansion valve  170  connected to the gas cooler  140  to control a pressure of refrigerant; a phase separator  280  configured to accommodate refrigerant which has passed through the pressure control electronic expansion valve to perform phase separation; a flow control electronic expansion valve  190  connected to an outlet side of the phase separator  280  to control a flow rate of refrigerant; an injection pipe  285 , one or a first end of which may be connected to the phase separator  280 , and the other or a second end of which may be connected to the compressor  110  to provide gaseous refrigerant in the phase separator  280  to the compressor  110 ; a switching valve  286  configured to open or close the injection pipe  285 ; and controller  250  configured to control the flow control electronic expansion valve  190  based on a suction superheat degree of refrigerant suctioned into the compressor  110  and a target suction superheat degree, and control the pressure control electronic expansion valve  170  based on a target operation high pressure and a current operation high pressure, and control the switching valve  286  based on a compression ratio of the refrigerant and a refrigerant discharge temperature of the compressor  110 . The refrigerant may be carbon dioxide (CO 2 ), for example. 
     The compressor  110  may include airtight container  111 , compression unit or device  115  provided within the airtight container  111 , and motor unit or motor  121  configured to provide a drive force to the compression unit  115 , for example. The compression unit  115  may include low stage compression unit or device  117  and high stage compression unit or device  119 . 
     Accumulator  130  may be provided at a suction side of the compressor  110 . According to this embodiment, a case in which the accumulator  130  is provided at a suction side of the compressor  110  is illustrated, but it is merely an illustration, and the accumulator  130  may not be provided. 
     The gas cooler  140  may be connected to the discharge-side pipe of the compressor  110 . Intermediate heat exchanger  150  may be provided at the outlet side of the gas cooler  140 . 
     The pressure control electronic expansion valve  170  may be provided at the outlet side of the intermediate heat exchanger  150 . The phase separator  280  may be connected to the outlet side of the pressure control electronic expansion valve  170 . 
     The flow control electronic expansion valve  190  may be provided at one side of the phase separator  280 . The evaporator  210  may be connected to the discharge side of the flow control electronic expansion valve  190 . The phase separator  280  may be configured to allow refrigerant decompressed while passing through the pressure control electronic expansion valve  170 , for example, to be introduced and separated into gaseous liquid (a gas and a liquid). 
     The phase separator  280  may include a case  281  configured to form an accommodation space therein and a first outlet  282  configured to discharge gaseous refrigerant within the case  281 , for example. The phase separator  280  may further include a second outlet  283  configured to discharge liquid refrigerant within the case  281 , for example. One or a first end of a pipe may be connected to the evaporator  210 , and the other or a second end thereof may be connected the second outlet  283 . 
     One or a first end of the injection pipe  285  may be connected to the compressor  110 , and the other or a second end thereof may be connected to the first outlet  282 , for example. Due to this, gaseous refrigerant within the phase separator  280  may be provided to the compressor  110 . 
     The injection pipe  285  may be configured to provide gaseous refrigerant in the phase separator  280  to a suction side of the high stage compression unit  119  of the compressor  110 . The switching valve  286  may be provided at the injection pipe  285  to open or close a passage of the injection pipe  285 . 
     This embodiment may be configured to include the controller  250  implemented as a microprocessor provided with a control program, for example. The flow control electronic expansion valve  190 , the pressure control electronic expansion valve  170 , and the switching valve  286  may be respectively connected to the controller  250  in a controlled manner, as illustrated in  FIG. 6 . The controller  250  may include the operation unit  255  configured to compute a refrigerant suction superheat degree and a target operation high pressure (target pressure) of the compressor  110 . 
     Gas cooler temperature sensor  260  configured to sense a temperature of the gas cooler  140  evaporator temperature sensor  265  configured to sense the evaporator temperature, compressor suction temperature sensor  270  configured to sense a temperature of refrigerant suctioned into the compressor  110 , a compressor discharge temperature sensor  290  configured to sense a temperature of refrigerant discharged from the compressor  110  may be connected to the controller  250 , and pressure sensor  275  configured to sense a current pressure (high pressure) of the refrigerant may each be connected to the controller  250 . 
     The operation unit  255  may be configured to subtract an outlet temperature of the evaporator  210  from a refrigerant suction temperature of the compressor  110  to calculate a refrigerant suction superheat degree of the compressor  110 . The operation unit  255  may be configured to compute a target operation high pressure using Equation 1 as described above. 
     The operation unit  255  may be configured to compute a ratio (compression ratio) of an operation high pressure of refrigerant to an operation low pressure, for example. The operation high pressure may denote a pressure of high-pressure refrigerant from a discharge-side pipe of the compressor  110  prior to introduction to the pressure control electronic expansion valve  170 , for example. 
     The operation high pressure may be a pressure value sensed by the pressure sensor  275  provided in the gas cooler  140 , for example. The operation high pressure may be a pressure value converted from a temperature of the gas cooler  140 . 
     The operation low pressure may be a pressure value of low-pressure refrigerant from a discharge side of the flow control electronic expansion valve  190  prior to the suction of the compressor  110 , for example. The operation low pressure may be a pressure value converted from a temperature sensed from the evaporator temperature sensor  265 , for example. The operation low pressure may be a pressure value sensed by a pressure sensor (not shown) configured to sense a pressure of the evaporator  210 , for example. 
     The controller  250  may decrease an opening degree of the flow control electronic expansion valve  190  when the suction superheat degree of the refrigerant is less than the target suction superheat degree, increase an opening degree of the flow control electronic expansion valve  190  when the suction superheat degree of the refrigerant is greater than the target suction superheat degree, and maintain a current opening degree of the flow control electronic expansion valve  190  when the suction superheat degree of the refrigerant is the same as the target suction superheat degree. In this way, a suction superheat degree of refrigerant suctioned into the compressor  110  may be appropriately maintained to suppress the occurrence of damage to the compressor  110  due to wet compression (liquid compression), thereby enhancing reliability of the compressor  110 . 
     The controller  250  may control the pressure control electronic expansion valve  170  to increase an opening degree of the pressure control electronic expansion valve  170  when the current operation high pressure is greater than the target operation high pressure, decrease an opening degree of the pressure control electronic expansion valve  170  when the current operation high pressure is less than the target operation high pressure, and maintain an opening degree of the pressure control electronic expansion valve  170  when the current operation high pressure is the same as the target operation high pressure. In this way, a ratio of a refrigerating capacity change rate to a consumption power change rate may be maximized to enhance an operation efficiency of the supercritical refrigeration cycle apparatus according to this embodiment. 
     On the other hand, the controller  250  may be configured to control the switching valve  286  to open a passage of the injection pipe  285  so as to provide refrigerant in the phase separator  280  to the compressor  110  when a ratio of the operation high pressure of the refrigerant to the operation low pressure, namely, the compression ratio of the refrigerant, is above a set or predetermined value or the discharge temperature of the compressor  110  is above a set or predetermined temperature. Gaseous refrigerant in the phase separator  280  may be provided to the compressor  110  to reduce a temperature of the refrigerant, thereby decreasing a work (load) of the compressor  110  (actually, the high stage compression unit  119 ). 
     When the operation of the supercritical refrigeration cycle apparatus according to this embodiment is started, the compressor  110  may suction and compress low-pressure refrigerant in a supercritical state and discharge it to the gas cooler  140 . Refrigerant cooled in the gas cooler  140  may be decompressed while passing through the flow control electronic expansion valve  190  via the intermediate heat exchanger  150 . 
     The refrigerant which has passed through the flow control electronic expansion valve  190  may be introduced into the phase separator  280 , and phase-separated into a gas and a liquid. The liquid refrigerant of the phase separator  280  may be discharged through the second outlet  283  and decompressed while passing through the flow control electronic expansion valve  190 . The refrigerant which has passed through the flow control electronic expansion valve  190  may be evaporated by absorbing latent heat while passing through the evaporator  210 . 
     Hereinafter, a method of controlling a supercritical refrigeration cycle apparatus according to another embodiment will be described with reference to  FIG. 8 . 
     The controller  250  may control the operation unit  255  to calculate a suction superheat degree of refrigerant in the compressor  110  (S 110 ). The controller  250  may determine whether the suction superheat degree of the refrigerant is the same as a target suction superheat degree (S 120 ). 
     When the suction superheat degree of the refrigerant is greater than the target suction superheat degree (S 130 ), the controller  250  may control the flow control electronic expansion valve  190  to increase an opening degree of the flow control electronic expansion valve  190  (S 140 ). When the suction superheat degree of the refrigerant is less than the target suction superheat degree (S 130 ), the controller  250  may control the flow control electronic expansion valve  190  to decrease an opening degree of the flow control electronic expansion valve  190  (S 150 ). 
     The controller  250  may repeatedly perform an opening degree control process of the flow control electronic expansion valve  190 , and control the flow control electronic expansion valve  190  to maintain a current opening degree of the flow control electronic expansion valve  190  (S 160 ) when the suction superheat degree is the same as the target suction superheat degree (S 120 ). When the current opening degree of the flow control electronic expansion valve  190  is maintained (S 160 ), the controller  250  may perform the control of the pressure control electronic expansion valve  170 . 
     The controller  250  may sense the gas cooler temperature, the evaporator temperature, and a current pressure through the gas cooler temperature sensor  260 , the evaporator temperature sensor  265 , and the pressure sensor (S 170 ). The controller  250  may control the operation unit  255  to calculate a target operation high pressure (S 180 ). 
     The controller  250  may compare the current pressure with the target operation high pressure (S 190 ), and control the pressure control electronic expansion valve  170  to increase an opening degree of the pressure control electronic expansion valve  170  (S 210 ) when the current pressure is greater than the target operation high pressure (S 200 ). When the current pressure is less than the target operation high pressure (S 200 ), the controller  250  may control the pressure control electronic expansion valve  170  to decrease an opening degree of the pressure control electronic expansion valve  170  (S 220 ). 
     The controller  250  may repeat the control process of the pressure control electronic expansion valve  170 , and control the pressure control electronic expansion valve  170  to maintain a current opening degree of the pressure control electronic expansion valve  170  (S 230 ) when the current pressure is the same as the target operation high pressure (S 190 ). 
     On the other hand, the controller  250  may sense an operation high pressure, an operation low pressure of the refrigerant, and a discharge temperature of the compressor  110  (S 240 ), and control the operation unit  255  to compute a compression ratio of the refrigerant (S 250 ). The controller  250  may compare the compression ratio with a set or predetermined value (S 250 ), and control the switching valve  286  to open a passage of the injection pipe  285  when the compression ratio exceeds the set or predetermined value (S 260 ). 
     Further, when a refrigerant discharge temperature of the compressor  110  exceeds a set or predetermined temperature (S 255 ), the controller  250  may control the switching valve  286  to open a passage of the injection pipe  285  (S 260 ). When the passage of the injection pipe  285  is open, gaseous refrigerant within the phase separator  280  may move along a passage of the injection pipe  285  to be provided to the compressor  110 . When the refrigerant discharge temperature of the compressor  110  does not exceed the set or predetermined temperature (S 255 ), the controller  250  may control the switching value  286  to close the passage of the injection pipe  285  (S 265 ). 
     That is, gaseous refrigerant moved along the injection pipe  285  may be introduced to a suction side of the high stage compression unit  119  of the compressor  110 , and mixed with refrigerant discharged from the low stage compression unit  117  and suctioned to a suction side of the high stage compression unit  119 . Gaseous refrigerant in the receiver  180  may have a relatively low temperature, and then when the gaseous refrigerant is mixed with refrigerant compressed in the low stage compression unit  117 , a temperature of refrigerant suctioned into the high stage compression unit  119  may be significantly lower compared to a temperature of refrigerant discharged from the low stage compression unit  117 . In this way, a load of the high stage compression unit  119  of the compressor  110  may be significantly reduced. 
     With a supercritical refrigeration cycle apparatus and a method for controlling a supercritical refrigeration cycle apparatus in accordance with this embodiment, a suction superheat degree of refrigerant suctioned into the compressor  110  through control of the flow control electronic expansion valve  190  may be appropriately secured to suppress the occurrence of wet compression in the compressor  110 , thereby enhancing reliability of the compressor  110 . Further, a ratio of a refrigerating capacity change rate to a power consumption change rate may be appropriately managed (maximized) through control of the pressure control electronic expansion valve  170 , thereby enhancing operation efficiency. Furthermore, when a compression ratio of the refrigerant exceeds a set or predetermined value or a discharge temperature of the compressor  110  is above a set or predetermined temperature, gaseous refrigerant in the phase separator  280  may be provided to the compressor  110  to alleviate a load (work) of the compressor  110 , thereby further enhancing operation efficiency. 
     As described above, according to an embodiment, a pressure control electronic expansion valve connected to a gas cooler to control a pressure of refrigerant, a receiver configured to temporarily store refrigerant which has passed through the pressure control electronic expansion valve, and a flow control electronic expansion valve connected to an outlet side of the receiver to control a flow rate of refrigerant may be provided, thereby implementing flow control and pressure control of the refrigerant in a respectively separate manner. In this way, it may be possible to enhance reliability of the compressor as well as enhance refrigerating capacity. 
     In addition, a pressure control electronic expansion valve, a phase separator configured to accommodate refrigerant which has passed through the pressure control electronic expansion valve to perform phase separation, a flow control electronic expansion valve connected to an outlet side of the phase separator to control a flow rate of refrigerant, an injection pipe, one or a first end of which may be connected to the phase separator, and the other or a second end of which may be connected to the compressor to provide gaseous refrigerant in the phase separator to the compressor, and a switching valve configured to open or close the injection pipe may be provided therein to implement flow control and pressure control of the refrigerant in a respectively separate manner, as well as provide gaseous refrigerant within the phase separator to the compressor so as to reduce a load of the compressor, thereby reducing power consumption. 
     Accordingly, embodiments disclosed herein provide a supercritical refrigeration cycle apparatus and a method for controlling a supercritical refrigeration cycle apparatus capable of implementing flow control and pressure control of refrigerant in a respectively separate manner. Further, embodiments disclosed herein provide a supercritical refrigeration cycle apparatus and a method for controlling a supercritical refrigeration cycle apparatus thereof capable of controlling flow control and pressure control of refrigerant in a separate manner, as well as reducing a temperature of refrigerant in the compressor to decrease load. 
     Embodiments disclosed herein provide a supercritical refrigeration cycle apparatus that may include a compressor configured to compress refrigerant in a supercritical state; a gas cooler configured to cool the compressed refrigerant in a supercritical state; a pressure control electronic expansion valve connected to the gas cooler to control a pressure of refrigerant; a receiver configured to temporarily store refrigerant which has passed through the pressure control electronic expansion valve; a flow control electronic expansion valve connected to an outlet side of the receiver to control a flow rate of refrigerant; and a controller configured to control the flow control electronic expansion valve based on a suction superheat degree of refrigerant suctioned into the compressor and a target suction superheat degree, and control the pressure control electronic expansion valve based on a target operation high pressure and a current operation high pressure. The controller may decrease an opening degree of the flow control electronic expansion valve when the suction superheat degree of the refrigerant is less than the target suction superheat degree, and increase an opening degree of the flow control electronic expansion valve when the suction superheat degree of the refrigerant is greater than the target suction superheat degree. The controller may increase an opening degree of the pressure control electronic expansion valve when the current operation high pressure is greater than the target operation high pressure, and decrease an opening degree of the pressure control electronic expansion valve when the current operation high pressure is less than the target operation high pressure, and maintain an opening degree of the pressure control electronic expansion valve when they are the same. 
     The supercritical refrigeration cycle apparatus may further include an intermediate heat exchanger in which refrigerant which has passed through the gas cooler and refrigerant which has passed through the evaporator exchange heat with each other. 
     Embodiments disclosed herein further provide a supercritical refrigeration cycle apparatus that may include a compressor configured to compress refrigerant in a supercritical state; a gas cooler configured to cool the compressed refrigerant; a pressure control electronic expansion valve connected to the gas cooler to control a pressure of refrigerant; a phase separator configured to accommodate refrigerant which has passed through the pressure control electronic expansion valve to perform phase separation; a flow control electronic expansion valve connected to an outlet side of the phase separator to control a flow rate of refrigerant; an injection pipe, one or a first end of which may be connected to the phase separator, and the other or a second end of which may be connected to the compressor to provide gaseous refrigerant in the phase separator to the compressor; a switching valve configured to open or close the injection pipe; and a controller configured to control the flow control electronic expansion valve based on a suction superheat degree of refrigerant suctioned into the compressor and a target suction superheat degree, and control the pressure control electronic expansion valve based on a target operation high pressure and a current operation high pressure, and control the switching valve based on a compression ratio of the refrigerant and a refrigerant discharge temperature of the compressor. The controller may decrease an opening degree of the flow control electronic expansion valve when the suction superheat degree of the refrigerant is less than the target suction superheat degree, and increase an opening degree of the flow control electronic expansion valve when the suction superheat degree of the refrigerant is greater than the target suction superheat degree. The controller may increase an opening degree of the pressure control electronic expansion valve when a current operation high pressure is greater than the target operation high pressure, decrease an opening degree of the pressure control electronic expansion valve when the current operation high pressure is less than the target operation high pressure, and maintain an opening degree of the pressure control electronic expansion valve when the current operation high pressure is the same as the target operation high pressure. 
     The supercritical refrigeration cycle apparatus may further include an intermediate heat exchanger in which refrigerant which has passed through the gas cooler and refrigerant which has passed through the evaporator exchange heat with each other. 
     The controller may control the switching valve to open the injection pipe so as to provide refrigerant in the phase separator to the compressor when the compression ratio is above a set or predetermined value, and the discharge temperature of the compressor is above a set or predetermined temperature. 
     Embodiments disclosed herein further provide a method for controlling a supercritical refrigeration cycle apparatus including a compressor configured to compress refrigerant; a gas cooler configured to cool the compressed refrigerant in a supercritical state; a pressure control electronic expansion valve connected to the gas cooler to control a pressure of refrigerant; a receiver configured to temporarily store refrigerant which has passed through the pressure control electronic expansion valve; and a flow control electronic expansion valve connected to an outlet side of the receiver to control a flow rate of refrigerant. The method may include controlling an opening degree of the flow control electronic expansion valve based on a suction superheat degree of refrigerant in the compressor; and controlling an opening degree of the pressure control electronic expansion valve based on an operation high pressure of the refrigerant. 
     The controlling the opening degree of the flow control electronic expansion valve may include checking a suction superheat degree of refrigerant in the compressor; comparing the suction superheat degree of the compressor with a target suction superheat degree, and maintaining a current opening degree of the flow control electronic expansion valve when the suction superheat degree of the compressor is the same as the target suction superheat degree. The method may further include increasing an opening degree of the flow control electronic expansion valve when the suction superheat degree of the compressor is greater than the target suction superheat degree, and decreasing an opening degree of the flow control electronic expansion valve when the suction superheat degree is less than the target suction superheat degree. 
     When the suction superheat degree of the compressor is the same as the target suction superheat degree, maintaining a current opening degree of the flow control electronic expansion valve, and then controlling an opening degree of the pressure control electronic expansion valve may be carried out. The controlling the opening degree of the pressure control electronic expansion valve may include checking an outlet temperature of the gas cooler and an evaporation temperature of the evaporator, respectively; calculating a target operation high pressure using the outlet temperature of the gas cooler and the evaporation temperature of the evaporator, and calculating a current operation high pressure corresponding to the outlet temperature of the gas cooler; comparing the target operation high pressure with the current operation high pressure; and maintaining a current opening degree of the pressure control electronic expansion valve when the current operation high pressure is the same as the target operation high pressure. The method may further include increasing an opening degree of the pressure control electronic expansion valve when the current operation high pressure is greater than the target operation high pressure, and decreasing an opening degree of the pressure control electronic expansion valve when the current operation high pressure is less than the target operation high pressure. 
     Embodiments disclosed herein also provide a method for controlling a supercritical refrigeration cycle apparatus including a compressor configured to compress refrigerant; a gas cooler configured to cool the compressed refrigerant in a supercritical state; a pressure control electronic expansion valve connected to the gas cooler to control a pressure of refrigerant; a phase separator configured to accommodate refrigerant which has passed through the pressure control electronic expansion valve to perform phase separation; a flow control electronic expansion valve connected to an outlet side of the phase separator to control a flow rate of refrigerant; an injection pipe, one or a first end of which may be connected to the phase separator, and the other or a second end of which may be connected to the compressor to provide gaseous refrigerant in the phase separator to the compressor; and a switching valve configured to open or close the injection pipe. The method may include controlling an opening degree of the flow control electronic expansion valve based on a suction superheat degree of the compressor; controlling an opening degree of the pressure control electronic expansion valve based on an operation high pressure of the refrigerant; and controlling the switching valve based on a compression ratio of the refrigerant and a refrigerant discharge temperature of the compressor. 
     The controlling the switching valve may include controlling the switching valve to open the injection pipe so as to provide refrigerant in the phase separator to the compressor when the compression ratio is above a set or predetermined value, and a discharge temperature of the compressor is above a set or predetermined temperature. When the suction superheat degree of the compressor is the same as the target suction superheat degree, maintaining a current opening degree of the flow control electronic expansion valve, and controlling an opening degree of the pressure control electronic expansion valve may be carried out. 
     The controlling an opening degree of the flow control electronic expansion valve may include checking a suction superheat degree of refrigerant in the compressor; comparing the suction superheat degree of the compressor with a target suction superheat degree; maintaining a current opening degree of the flow control electronic expansion valve when the suction superheat degree of the compressor is the same as the target suction superheat degree, and increasing an opening degree of the flow control electronic expansion valve when the suction superheat degree of the compressor is greater than the target suction superheat degree, and decreasing an opening degree of the flow control electronic expansion valve when the suction superheat degree of the compressor is less than the target suction superheat degree. The controlling the opening degree of the pressure control electronic expansion valve may include checking an outlet temperature of the gas cooler and an evaporation temperature of the evaporator, respectively; calculating a target operation high pressure using the outlet temperature of the gas cooler and the evaporation temperature of the evaporator, and calculating a current operation high pressure corresponding to the outlet temperature of the gas cooler, comparing the target operation high pressure with the current operation high pressure; and maintaining a current opening degree of the pressure control electronic expansion valve when the current operation high pressure is the same as the target operation high pressure, and increasing an opening degree of the pressure control electronic expansion valve when the current operation high pressure is greater than the target operation high pressure, and decreasing an opening degree of the pressure control electronic expansion valve when the current operation high pressure is less than the target operation high pressure. 
     It is obvious to those skilled in the art that embodiments may be embodied in other specific forms without departing from the concept and essential characteristics thereof. The detailed description is, therefore, not to be construed as illustrative in all respects but considered as restrictive. The scope of the invention should be determined by reasonable interpretation of the appended claims and all changes that come within the equivalent scope are included in the scope. 
     Any reference in this specification to “one embodiment,” “an embodiment,” “example embodiment,” etc., means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. The appearances of such phrases in various places in the specification are not necessarily all referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with any embodiment, it is submitted that it is within the purview of one skilled in the art to effect such feature, structure, or characteristic in connection with other ones of the embodiments. 
     Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.