Patent Publication Number: US-2023158858-A1

Title: Method for Controlling Vehicle HVAC System

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Patent Application No. 10-2021-0161626, filed on Nov. 22, 2021, which application is hereby incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a method for controlling a vehicle heating, ventilation, and air conditioning (HVAC) system. 
     BACKGROUND 
     It is known to provide heating, ventilation, and air conditioning (HVAC) systems in vehicles. These HVAC systems may heat and cool the air within a passenger compartment for the comfort of occupants. In addition, some vehicle HVAC systems may be configured to selectively change the source of air. In one configuration, the HVAC system draws in fresh air from outside the vehicle, conditions the air, and then circulates the conditioned air into the passenger compartment. In another configuration, the HVAC system draws in a mixture of outdoor air and indoor air, conditions the mixed air, and then pumps the conditioned air into the passenger compartment. 
     The vehicle HVAC system includes an evaporator, a heater core (or an interior condenser), and an air mixing door within an HVAC housing. The HVAC housing has an inlet through which the air is allowed to be drawn in, and a plurality of outlets through which the air is directed into the passenger compartment. An exterior condenser may be adjacent to a grille in a front compartment of the vehicle, and a refrigerant passing through an internal passage of the exterior condenser may be cooled and condensed by the air drawn in through the grille. A cooling fan may be disposed behind the exterior condenser. The exterior condenser may exchange heat with the air forcibly blown by the cooling fan, and the refrigerant passing through the internal passage of the exterior condenser may be condensed and subcooled. The heater core may heat the air entering the passenger compartment. The air mixing door may be disposed between the evaporator and the heater core. The evaporator may be located upstream of the air mixing door, and the heater core may be located downstream of the air mixing door. The air mixing door may be configured to adjust the flow rate of air passing through the heater core, thereby adjusting the temperature of the air entering the passenger compartment. 
     The HVAC system may require the subcooling of the refrigerant to prevent the refrigerant in gas phase from flowing into an expansion valve. The HVAC system according to the related art is basically designed to achieve sufficient subcooling of the refrigerant, so an additional control technology for achieving the subcooling of the refrigerant is not applied thereto. Meanwhile, an actual fan duty of the cooling fan may be higher than a required fan duty for the subcooling of the refrigerant, and thus power may be excessively consumed during the operation of the HVAC system. 
     The above information described in this background section is provided to assist in understanding the background of the inventive concept, and may include any technical concept which is not considered as the prior art that is already known to those skilled in the art. 
     SUMMARY 
     The present disclosure relates to a method for controlling a vehicle heating, ventilation, and air conditioning (HVAC) system. Particular embodiments relate to a method for controlling a vehicle HVAC system capable of reducing power consumption and achieving sufficient subcooling of a refrigerant. 
     Embodiments of the present disclosure can solve problems occurring in the prior art while advantages achieved by the prior art are maintained intact. 
     An embodiment of the present disclosure provides a method for controlling a vehicle heating, ventilation, and air conditioning (HVAC) system capable of accurately calculating a required fan duty of a cooling fan which matches subcooling of a refrigerant, thereby achieving sufficient subcooling of the refrigerant and reducing power consumption. 
     According to an embodiment of the present disclosure, a method for controlling a vehicle HVAC system may include determining, by a controller, a target subcooled temperature of a refrigerant based on temperature and pressure of the refrigerant discharged from an outlet of a compressor when the compressor operates, calculating, by the controller, a change in enthalpy of the refrigerant based on the determined target subcooled temperature in a process of condensing and subcooling the refrigerant, calculating, by the controller, a change in enthalpy of air passing over an exterior surface of a condenser based on the calculated refrigerant enthalpy change, and calculating, by the controller, a required fan duty of a cooling fan based on the calculated air enthalpy change. The cooling fan is configured to blow the air to the condenser. The method may be designed to accurately calculate the required fan duty of the cooling fan which matches the subcooling of the refrigerant based on the refrigerant enthalpy change, the air enthalpy change, and the like, thereby achieving sufficient subcooling of the refrigerant and reducing power consumption during the operation of the HVAC system. 
     The air enthalpy change may be calculated based on a temperature difference between the refrigerant and the air, a specific heat of the air, and a flow rate of the refrigerant. 
     The method may further include monitoring a vehicle speed and an opening degree of a grille. The required fan duty of the cooling fan may be calculated based on the vehicle speed, the opening degree of the grille, and the calculated air enthalpy change. 
     The method may further include preliminarily cooling and condensing the refrigerant using a coolant passing through a heat exchanger located on the upstream side of the condenser, and calculating, by the controller, a change in enthalpy of the coolant based on RPM of a pump of a cooling system. The HVAC system may be thermally connected to the cooling system through the heat exchanger, and the required fan duty of the cooling fan may be calculated based on the calculated coolant enthalpy change and the calculated air enthalpy change. 
     The coolant enthalpy change may be calculated based on a higher RPM of a first RPM determined by an external controller and a second RPM determined by the controller. As the coolant enthalpy change is calculated based on the higher RPM of the first RPM and the second RPM and the flow rate of the coolant passing through the heat exchanger increases, the amount of the refrigerant condensed by the heat exchanger may be relatively increased compared to the amount of the refrigerant condensed by the condenser, and thus the required fan duty of the cooling fan for the subcooling of the refrigerant may be relatively reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of embodiments of the present disclosure will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG.  1    illustrates an example of a heating, ventilation, and air conditioning (HVAC) system, which is applicable to an internal combustion engine vehicle; 
         FIG.  2    illustrates an example of an HVAC system, which is applicable to an electric vehicle; 
         FIG.  3    illustrates a flowchart of a method for controlling the HVAC system illustrated in  FIG.  1   ; 
         FIG.  4    illustrates a flowchart of a method for controlling the HVAC system illustrated in  FIG.  2   , in a condition in which a water-cooled heat exchanger is removed from the HVAC system; 
         FIG.  5    illustrates a flowchart of a method for controlling the HVAC system illustrated in  FIG.  2    according to an exemplary embodiment of the present disclosure; and 
         FIG.  6    illustrates a flowchart of a method for controlling the HVAC system illustrated in  FIG.  2    according to another exemplary embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Hereinafter, exemplary embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In the drawings, the same reference numerals will be used throughout to designate the same or equivalent elements. In addition, a detailed description of well-known techniques associated with the present disclosure will be omitted in order not to unnecessarily obscure the gist of the present disclosure. 
     Terms such as first, second, A, B, (a), and (b) may be used to describe the elements in exemplary embodiments of the present disclosure. These terms are only used to distinguish one element from another element, and the intrinsic features, sequence or order, and the like of the corresponding elements are not limited by the terms. Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meanings as those generally understood by those with ordinary knowledge in the field of art to which the present disclosure belongs. Such terms as those defined in a generally used dictionary are to be interpreted as having meanings equal to the contextual meanings in the relevant field of art, and are not to be interpreted as having ideal or excessively formal meanings unless clearly defined as having such in the present application. 
       FIG.  1    illustrates an example of a heating, ventilation, and air conditioning (HVAC) system, which is applicable to an internal combustion engine vehicle. Referring to  FIG.  1   , a vehicle HVAC system according to an exemplary embodiment of the present disclosure may include an HVAC housing  1 . The HVAC housing  1  may be mounted on a dash panel of the vehicle by which a front compartment and a passenger compartment are divided. 
     The HVAC housing  1  may accommodate an evaporator  2 , a heater core  3 , and an air mixing door  4 . The evaporator  2 , the air mixing door  4 , and the heater core  3  may be sequentially arranged within the HVAC housing  1  in an air flow direction from upstream to downstream. 
     The HVAC housing  1  may include an inlet through which the air is allowed to be drawn in, and an outlet through which the air is directed into the passenger compartment. 
     The evaporator  2  may be located upstream in the HVAC housing  1 , and the evaporator  2  may be configured to cool the air. An expansion valve  7  may be connected to an inlet of the evaporator  2 , and a compressor  5  may be connected to an outlet of the evaporator  2 . A condenser  6  may be connected to an outlet of the compressor  5 . A refrigerant loop may connect the compressor  5 , the condenser  6 , the expansion valve  7 , and the evaporator  2 , and a refrigerant may circulate through the refrigerant loop. 
     The condenser  6  may be adjacent to a grille  8  of the vehicle, and a cooling fan  9  may be located behind the condenser  6 . The refrigerant passing through an internal passage of the condenser  6  may be cooled by the air forcibly blown by the cooling fan  9 , and accordingly the refrigerant may be condensed and subcooled while passing through the internal passage of the condenser  6 . 
     According to an exemplary embodiment, an active air flap  8   a  may be configured to adjust the opening degree of the grille  8 , and the active air flap  8   a  may be disposed between the grille  8  and the condenser  6 . Each flap of the active air flap  8   a  may be rotatable to thereby adjust the opening degree of a corresponding opening of the grille  8 . 
     The heater core  3  may be located on the downstream side of the evaporator  2 , and the heater core  3  may be configured to heat the air. According to an exemplary embodiment, the heater core  3  may heat the air using an engine coolant heated by an engine. According to another exemplary embodiment, a coolant may be heated by waste heat generated when power electronics such as a motor, a power converter (an inverter, a converter, etc.), an on-board charger (OBC), and an automated driving controller are operating, and the heater core  3  may heat the air using the coolant heated by the waste heat. According to another exemplary embodiment, the heater core  3  may be configured to heat the air using a refrigerant compressed by a heating operation (heat pump function) of a refrigeration cycle. A positive temperature coefficient (PTC) heater  3   a  may be located on the downstream side of the heater core  3 . 
     A controller  100  may be configured to control the compressor  5 , the cooling fan  9 , and the like. 
       FIG.  2    illustrates an example of an HVAC system, which is applicable to an electric vehicle. Referring to  FIG.  2   , an HVAC system  11  according to an exemplary embodiment of the present disclosure may include a refrigerant loop  21 , and a refrigerant may circulate through the refrigerant loop  21 . 
     A battery cooling system  12  may be thermally connected to the HVAC system  11  through a battery chiller  37  and a water-cooled heat exchanger  70 , and a powertrain cooling system  13  may be thermally connected to the HVAC system  11  through the water-cooled heat exchanger  70 . 
     The HVAC system  11  may be configured to heat or cool the air in the passenger compartment of the vehicle using the refrigerant circulating in the refrigerant loop  21 . The refrigerant loop  21  may be fluidly connected to an evaporator  31 , a compressor  32 , an interior condenser  33 , a heating-side expansion valve  16 , the water-cooled heat exchanger  70 , an exterior heat exchanger  35 , and a cooling-side expansion valve  15 . In  FIG.  2   , the refrigerant may sequentially pass through the compressor  32 , the interior condenser  33 , the heating-side expansion valve  16 , the water-cooled heat exchanger  70 , the exterior heat exchanger  35 , the cooling-side expansion valve  15 , and the evaporator  31  through the refrigerant loop  21 . 
     The evaporator  31  may be configured to evaporate the refrigerant received from the cooling-side expansion valve  15 . That is, the refrigerant expanded by the cooling-side expansion valve  15  may absorb heat from the air and evaporate in the evaporator  31 . During a cooling operation of the HVAC system  11 , the evaporator  31  may cool the air using the refrigerant cooled by the exterior heat exchanger  35  and expanded by the cooling-side expansion valve  15 , and the air cooled by the evaporator  31  may be directed into the passenger compartment. 
     The compressor  32  may be configured to compress the refrigerant received from the evaporator  31  and/or the battery chiller  37 . According to an exemplary embodiment, the compressor  32  may be an electric compressor which is driven by electric energy. 
     The interior condenser  33  may be configured to condense the refrigerant received from the compressor  32 . That is, the refrigerant compressed by the compressor  32  may transfer heat to the air and be condensed in the interior condenser  33 . Accordingly, the interior condenser  33  may heat the air using the refrigerant compressed by the compressor  32 , and the air heated by the interior condenser  33  may be directed into the passenger compartment. 
     The exterior heat exchanger  35  may be adjacent to a grille  14  of the vehicle. Since the exterior heat exchanger  35  is exposed to the outside, heat may be transferred between the exterior heat exchanger  35  and the ambient air. 
     According to an exemplary embodiment, a cooling fan  75  may be located behind the exterior heat exchanger  35 , and the exterior heat exchanger  35  may exchange heat with the ambient air forcibly blown by the cooling fan  75  so that a heat transfer rate between the exterior heat exchanger  35  and the ambient air may be further increased. 
     According to an exemplary embodiment, an active air flap  14   a  may be configured to adjust the opening degree of the grille  14 , and the active air flap  14   a  may be disposed between the grille  14  and the exterior heat exchanger  35 . Each flap of the active air flap  14   a  may be rotatable to thereby adjust the opening degree of a corresponding opening of the grille  14 . 
     During the cooling operation of the HVAC system  11 , the exterior heat exchanger  35  may be configured to condense the refrigerant received from the interior condenser  33 . That is, the exterior heat exchanger  35  may serve as an exterior condenser that condenses the refrigerant by transferring heat to the ambient air during the cooling operation of the HVAC system  11 . In particular, the exterior heat exchanger  35  may serve as a subcooling condenser that cools the refrigerant during the cooling operation of the HVAC system  11 . 
     During a heating operation of the HVAC system  11 , the exterior heat exchanger  35  may be configured to evaporate the refrigerant received from the water-cooled heat exchanger  70 . That is, the exterior heat exchanger  35  may serve as an exterior evaporator that evaporates the refrigerant by absorbing heat from the ambient air during the heating operation of the HVAC system  11 . In particular, the exterior heat exchanger  35  may serve as a superheating evaporator that superheats the refrigerant during the heating operation of the HVAC system  11 . 
     During the cooling operation of the HVAC system  11 , the refrigerant passing through an internal passage of the exterior heat exchanger  35  may be cooled by the air forcibly blown by the cooling fan  75 , and accordingly the refrigerant may be condensed and subcooled when passing through the internal passage of the exterior heat exchanger  35 . 
     The water-cooled heat exchanger  70  may transfer heat among the refrigerant loop  21  of the HVAC system  11 , a battery coolant loop  22  of the battery cooling system  12 , and a powertrain coolant loop  23  of the powertrain cooling system  13 . Specifically, the water-cooled heat exchanger  70  may be disposed between the interior condenser  33  and the exterior heat exchanger  35  in the refrigerant loop  21 . The water-cooled heat exchanger  70  may include a first passage  71  fluidly connected to the powertrain coolant loop  23 , a second passage  72  fluidly connected to the battery coolant loop  22 , and a third passage  73  fluidly connected to the refrigerant loop  21 . 
     During the heating operation of the HVAC system  11 , the water-cooled heat exchanger  70  may be configured to evaporate the refrigerant which is received from the interior condenser  33  using the heat which is received from the powertrain cooling system  13 . That is, during the heating operation of the HVAC system  11 , the water-cooled heat exchanger  70  may serve as an evaporator that evaporates the refrigerant by recovering waste heat from electric motors  51   a  and  51   b  and power electronics  52   a ,  52   b , and  52   c  of the powertrain cooling system  13 . 
     During the cooling operation of the HVAC system  11 , the water-cooled heat exchanger  70  may be configured to condense the refrigerant received from the interior condenser  33 . The water-cooled heat exchanger  70  may serve as a condenser that condenses the refrigerant by cooling the refrigerant using a battery-side coolant circulating in the battery coolant loop  22  of the battery cooling system  12  and a powertrain-side coolant circulating in the powertrain coolant loop  23  of the powertrain cooling system  13 . During the cooling operation of the HVAC system  11 , the exterior heat exchanger  35  and the water-cooled heat exchanger  70  may serve as a condenser. As the water-cooled heat exchanger  70  is located on the upstream side of the exterior heat exchanger  35 , the refrigerant may be preliminarily cooled and condensed by the water-cooled heat exchanger  70 , and then be condensed and subcooled by the exterior heat exchanger  35 . 
     According to another exemplary embodiment of the present disclosure, the water-cooled heat exchanger  70  may be removed from the HVAC system  11 . 
     The heating-side expansion valve  16  may be located on the upstream side of the water-cooled heat exchanger  70  in the refrigerant loop  21 . Specifically, the heating-side expansion valve  16  may be disposed between the interior condenser  33  and the water-cooled heat exchanger  70 . During the heating operation of the HVAC system  11 , the heating-side expansion valve  16  may adjust the flow of the refrigerant or the flow rate of the refrigerant into the water-cooled heat exchanger  70 . The heating-side expansion valve  16  may be configured to expand the refrigerant received from the interior condenser  33  during the heating operation of the HVAC system  11 . 
     According to an exemplary embodiment, the heating-side expansion valve  16  may be an electronic expansion valve (EXV) having a drive motor  16   a . The drive motor  16   a  may have a shaft which is movable to open or close an orifice defined in a valve body of the heating-side expansion valve  16 , and the position of the shaft may be varied depending on the rotation direction, rotation degree, and the like of the drive motor  16   a , and thus the opening degree of the orifice of the heating-side expansion valve  16  may be varied. A controller  1000  may control the operation of the drive motor  16   a . The heating-side expansion valve  16  may be a full open type EXV. When the HVAC system  11  does not operate in a heating mode, the heating-side expansion valve  16  may be fully opened so that the refrigerant may pass through the heating-side expansion valve  16  without a change in pressure of the refrigerant. When the HVAC system  11  operates in a cooling mode, the opening degree of the heating-side expansion valve  16  may be 100% so that the refrigerant may pass through the heating-side expansion valve  16  without expansion of the refrigerant (without any change in the pressure of the refrigerant). 
     The opening degree of the heating-side expansion valve  16  may be varied by the controller  1000 . As the opening degree of the heating-side expansion valve  16  is varied, the flow rate of the refrigerant into the third passage  73  may be varied. The heating-side expansion valve  16  may be controlled by the controller  1000  during the heating operation of the HVAC system  11 . The cooling-side expansion valve  15  may be disposed between the exterior heat exchanger  35  and the evaporator  31  in the refrigerant loop  21 . As the cooling-side expansion valve  15  is located on the upstream side of the evaporator  31 , the cooling-side expansion valve  15  may adjust the flow of the refrigerant or the flow rate of the refrigerant into the evaporator  31 , and the cooling-side expansion valve  15  may be configured to expand the refrigerant received from the exterior heat exchanger  35  during the cooling operation of the HVAC system  11 . 
     According to an exemplary embodiment, the cooling-side expansion valve  15  may be a thermal expansion valve (TXV) which senses the temperature and/or pressure of the refrigerant and adjusts the opening degree of the cooling-side expansion valve  15 . Specifically, the cooling-side expansion valve  15  may be a TXV having a shut-off valve  15   a  selectively blocking the flow of the refrigerant toward an internal passage of the cooling-side expansion valve  15 , and the shut-off valve  15   a  may be a solenoid valve. The shut-off valve  15   a  may be opened or closed by the controller  1000 , thereby unblocking or blocking the flow of the refrigerant toward the cooling-side expansion valve  15 . When the shut-off valve  15   a  is opened, the refrigerant may be allowed to flow into the cooling-side expansion valve  15 , and when the shut-off valve  15   a  is closed, the refrigerant may be blocked from flowing into the cooling-side expansion valve  15 . According to an exemplary embodiment, the shut-off valve  15   a  may be mounted in the inside of a valve body of the cooling-side expansion valve  15 , thereby opening or closing the internal passage of the cooling-side expansion valve  15 . According to another exemplary embodiment, the shut-off valve  15   a  may be located on the upstream side of the cooling-side expansion valve  15 , thereby selectively opening or closing an inlet of the cooling-side expansion valve  15 . 
     When the shut-off valve  15   a  is closed, the flow of the refrigerant into the cooling-side expansion valve  15  may be blocked, and accordingly the refrigerant may only be directed into the battery chiller  37  without flowing into the cooling-side expansion valve  15  and the evaporator  31 . That is, when the shut-off valve  15   a  of the cooling-side expansion valve  15  is closed, the cooling operation of the HVAC system  11  may not be performed, and only the battery chiller  37  may be cooled or the heating operation of the HVAC system  11  may be performed. When the shut-off valve  15   a  is opened, the refrigerant may be directed into the cooling-side expansion valve  15  and the evaporator  31 . That is, when the shut-off valve  15   a  of the cooling-side expansion valve  15  is opened, the cooling operation of the HVAC system  11  may be performed. 
     The HVAC system  11  may include an HVAC housing  30  having an inlet and outlets. The HVAC housing  30  may be mounted on a dash panel of the vehicle while facing front seats of the vehicle. The HVAC housing  30  may be configured to allow the air to be directed into the passenger compartment of the vehicle. The evaporator  31  and the interior condenser  33  may be located in the HVAC housing  30 . An air mixing door  34   a  may be disposed between the evaporator  31  and the interior condenser  33 , and a PTC heater  34   b  may be located on the downstream side of the interior condenser  33 . 
     The HVAC system  11  may further include an accumulator  38  disposed between the evaporator  31  and the compressor  32  in the refrigerant loop  21 , and the accumulator  38  may be located on the downstream side of the evaporator  31 . The accumulator  38  may separate a liquid refrigerant from the refrigerant which is received from the evaporator  31 , thereby preventing the liquid refrigerant from entering the compressor  32 . 
     The HVAC system  11  may further include a branch conduit  36  branching off from the refrigerant loop  21 . The branch conduit  36  may branch off from an upstream point of the cooling-side expansion valve  15  and be connected to the compressor  32  in the refrigerant loop  21 . The battery chiller  37  may be fluidly connected to the branch conduit  36 , and the battery chiller  37  may be configured to transfer heat between the branch conduit  36  and the battery coolant loop  22  to be described below. That is, the battery chiller  37  may be configured to transfer heat between the refrigerant circulating in the refrigerant loop  21  of the HVAC system  11  and the battery-side coolant circulating in the battery coolant loop  22  of the battery cooling system  12 . 
     Specifically, the battery chiller  37  may include a first passage  37   a  fluidly connected to the branch conduit  36 , and a second passage  37   b  fluidly connected to the battery coolant loop  22 . The first passage  37   a  and the second passage  37   b  may be adjacent to or contact each other within the battery chiller  37 , and the first passage  37   a  may be fluidly separated from the second passage  37   b . Accordingly, the battery chiller  37  may transfer heat between the battery-side coolant passing through the second passage  37   b  and the refrigerant passing through the first passage  37   a . The branch conduit  36  may be fluidly connected to the accumulator  38 , and the refrigerant passing through the branch conduit  36  may be received in the accumulator  38 . 
     A chiller-side expansion valve  17  may be located on the upstream side of the battery chiller  37  in the branch conduit  36 . The chiller-side expansion valve  17  may adjust the flow of the refrigerant or the flow rate of the refrigerant into the battery chiller  37 , and the chiller-side expansion valve  17  may be configured to expand the refrigerant received from the exterior heat exchanger  35 . 
     According to an exemplary embodiment, the chiller-side expansion valve  17  may be an EXV having a drive motor  17   a . The drive motor  17   a  may have a shaft which is movable to open or close an orifice defined in a valve body of the chiller-side expansion valve  17 , and the position of the shaft may be varied depending on the rotation direction, rotation degree, and the like of the drive motor  17   a , and thus the opening degree of the orifice of the chiller-side expansion valve  17  may be varied. The controller  1000  may control the operation of the drive motor  17   a . The chiller-side expansion valve  17  may be a full open type EXV. 
     As the opening degree of the chiller-side expansion valve  17  is varied, the flow rate of the refrigerant into the battery chiller  37  may be varied. For example, when the opening degree of the chiller-side expansion valve  17  is greater than a reference opening degree, the flow rate of the refrigerant into the battery chiller  37  may be relatively increased above a reference flow rate, and when the opening degree of the chiller-side expansion valve  17  is less than the reference opening degree, the flow rate of the refrigerant into the battery chiller  37  may be similar to the reference flow rate or be relatively lowered below the reference flow rate. Here, the reference opening degree refers to an opening degree of the chiller-side expansion valve  17  required for maintaining a target evaporator temperature, and the reference flow rate refers to a flow rate of the refrigerant which is allowed to flow into the battery chiller  37  when the chiller-side expansion valve  17  is opened to the reference opening degree. When the chiller-side expansion valve  17  is opened to the reference opening degree, the refrigerant may be directed into the battery chiller  37  at a corresponding reference flow rate. 
     As the opening degree of the cooling-side expansion valve  15  and the opening degree of the chiller-side expansion valve  17  are adjusted by the controller  1000 , the refrigerant may be distributed to the evaporator  31  and the battery chiller  37  at a predetermined ratio, and thus the cooling of the HVAC system  11  and the cooling of the battery chiller  37  may be performed simultaneously or selectively. 
     The HVAC system  11  may include a first refrigerant bypass conduit  25  connecting a downstream point of the third passage  73  of the water-cooled heat exchanger  70  and the branch conduit  36 . An inlet of the first refrigerant bypass conduit  25  may be connected to the downstream point of the water-cooled heat exchanger  70 , and an outlet of the first refrigerant bypass conduit  25  may be connected to the branch conduit  36 . Specifically, the inlet of the first refrigerant bypass conduit  25  may be connected to a point between the water-cooled heat exchanger  70  and the exterior heat exchanger  35 , and the outlet of the first refrigerant bypass conduit  25  may be connected to a point between the battery chiller  37  and the compressor  32  in the branch conduit  36 . A first three-way valve  61  may be disposed at a junction between the inlet of the first refrigerant bypass conduit  25  and the refrigerant loop  21 . Accordingly, the first three-way valve  61  may be disposed between the exterior heat exchanger  35  and the water-cooled heat exchanger  70  in the refrigerant loop  21 . When the first three-way valve  61  is switched to open the inlet of the first refrigerant bypass conduit  25 , the refrigerant having passed through the third passage  73  of the water-cooled heat exchanger  70  may be directed into the compressor  32  through the first refrigerant bypass conduit  25  and the accumulator  38 . That is, when the inlet of the first refrigerant bypass conduit  25  is opened by the switching of the first three-way valve  61 , the refrigerant may bypass the exterior heat exchanger  35 . When the first three-way valve  61  is switched to close the inlet of the first refrigerant bypass conduit  25 , the refrigerant having passed through the third passage  73  of the water-cooled heat exchanger  70  may be directed into the exterior heat exchanger  35  without passing through the first refrigerant bypass conduit  25 . That is, when the inlet of the first refrigerant bypass conduit  25  is closed by the switching of the first three-way valve  61 , the refrigerant may pass through the exterior heat exchanger  35 . 
     The controller  1000  may be configured to control respective operations of the shut-off valve  15   a  of the cooling-side expansion valve  15 , the heating-side expansion valve  16 , the chiller-side expansion valve  17 , the compressor  32 , and the like, and thus the overall operation of the HVAC system  11  may be controlled by the controller  1000 . According to an exemplary embodiment, the controller  1000  may be a full automatic temperature control (FATC) system. 
     When the HVAC system  11  operates in the cooling mode, the shut-off valve  15   a  of the cooling-side expansion valve  15  may be opened, and accordingly the refrigerant may sequentially circulate through the compressor  32 , the interior condenser  33 , the heating-side expansion valve  16 , the third passage  73  of the water-cooled heat exchanger  70 , the exterior heat exchanger  35 , the cooling-side expansion valve  15 , and the evaporator  31 . 
     When the HVAC system  11  operates in the heating mode, the shut-off valve  15   a  of the cooling-side expansion valve  15  may be closed, and accordingly the refrigerant may sequentially circulate through the compressor  32 , the interior condenser  33 , the heating-side expansion valve  16 , the third passage  73  of the water-cooled heat exchanger  70 , the exterior heat exchanger  35 , the chiller-side expansion valve  17 , the first passage  37   a  of the battery chiller  37 , and the compressor  32 . Meanwhile, during the heating operation of the HVAC system  11 , when the shut-off valve  15   a  of the cooling-side expansion valve  15  is closed and the inlet of the first refrigerant bypass conduit  25  is opened by the switching of the first three-way valve  61 , the refrigerant may sequentially circulate through the compressor  32 , the interior condenser  33 , the heating-side expansion valve  16 , the third passage  73  of the water-cooled heat exchanger  70 , and the compressor  32 . 
     The HVAC system  11  according to an exemplary embodiment of the present disclosure may further include a branch conduit  26  branching off from the refrigerant loop  21 . The branch conduit  26  may branch off from a point between the heating-side expansion valve  16  and the water-cooled heat exchanger  70  in the refrigerant loop  21 , and the branch conduit  26  may extend to a downstream point of the cooling-side expansion valve  15 . A shut-off valve  27  may be provided to selectively block or unblock the flow of the refrigerant in the branch conduit  26 . When dehumidification of the passenger compartment is required during the heating operation of the HVAC system  11 , the shut-off valve  27  may be opened so that a portion of the refrigerant flowing from the heating-side expansion valve  16  to the water-cooled heat exchanger  70  may be directed into the evaporator  31  through the branch conduit  26 . Accordingly, the refrigerant entering the evaporator  31  may absorb heat from the air passing through the evaporator  31 , and thus the heating and dehumidification of the passenger compartment may be simultaneously performed. 
     The battery cooling system  12  may include the battery coolant loop  22 , and the battery-side coolant for cooling a battery  41  may circulate through the battery coolant loop  22 . 
     The battery cooling system  12  may be configured to cool the battery  41  or increase a temperature of the battery  41  using the battery-side coolant circulating in the battery coolant loop  22 . The battery coolant loop  22  may be fluidly connected to a battery radiator  43 , a reservoir tank  48 , a first battery-side pump  44 , the battery chiller  37 , a heater  42 , the battery  41 , a second battery-side pump  45 , and the water-cooled heat exchanger  70 . In  FIG.  2   , the battery-side coolant may sequentially pass through the battery radiator  43 , the reservoir tank  48 , the first battery-side pump  44 , the battery chiller  37 , the heater  42 , the battery  41 , the second battery-side pump  45 , and the second passage  72  of the water-cooled heat exchanger  70  through the battery coolant loop  22 . 
     The battery  41  may have a coolant passage provided inside or outside thereof, and the battery-side coolant may pass through the coolant passage. The battery coolant loop  22  may be fluidly connected to the coolant passage of the battery  41 . 
     The heater  42  may be disposed between the battery chiller  37  and the battery  41 , and the heater  42  may heat the battery-side coolant circulating in the battery coolant loop  22  to warm up the coolant. According to an exemplary embodiment, the heater  42  may be an electric heater. According to another exemplary embodiment, the heater  42  may heat the battery-side coolant by exchanging heat with a high-temperature fluid. 
     The battery radiator  43  may be adjacent to the front grille of the vehicle, and the battery-side coolant passing through the battery radiator  43  may be cooled using the ambient air forcibly blown by the cooling fan  75 . The battery radiator  43  may be adjacent to the exterior heat exchanger  35 . According to an exemplary embodiment, the battery radiator  43  may be referred to as a low temperature radiator (LTR). 
     The first battery-side pump  44  may be configured to allow the battery-side coolant to circulate through at least a portion of the battery coolant loop  22 , and the second battery-side pump  45  may be configured to allow the battery-side coolant to circulate through at least a portion of the battery coolant loop  22 . 
     The reservoir tank  48  may be located between an outlet of the battery radiator  43  and an inlet of the first battery-side pump  44 . 
     According to an exemplary embodiment, the battery coolant loop  22  may include a first coolant conduit  22   a  and a second coolant conduit  22   b  connected through a first connection conduit  22   c  and a second connection conduit  22   d . The first coolant conduit  22   a  may be fluidly connected to the battery radiator  43 , the reservoir tank  48 , the first battery-side pump  44 , and the second passage  72  of the water-cooled heat exchanger  70 , and the second coolant conduit  22   b  may be fluidly connected to the battery chiller  37 , the heater  42 , the battery  41 , and the second battery-side pump  45 . 
     The first connection conduit  22   c  may connect a downstream point of the first battery-side pump  44  and an upstream point of the second passage  37   b  of the battery chiller  37 . Specifically, an inlet of the first connection conduit  22   c  may be connected to the downstream point of the first battery-side pump  44 , and an outlet of the first connection conduit  22   c  may be connected to the upstream point of the second passage  37   b  of the battery chiller  37 . 
     The second connection conduit  22   d  may connect a downstream point of the second battery-side pump  45  and an upstream point of the second passage  72  of the water-cooled heat exchanger  70 . Specifically, an inlet of the second connection conduit  22   d  may be connected to the downstream point of the second battery-side pump  45 , and an outlet of the second connection conduit  22   d  may be connected to the upstream point of the second passage  72  of the water-cooled heat exchanger  70 . 
     The first battery-side pump  44  may be disposed on a downstream point of the battery radiator  43  in the first coolant conduit  22   a  of the battery coolant loop  22 . 
     The second battery-side pump  45  may be disposed on a downstream point of the battery  41  in the second coolant conduit  22   b  of the battery coolant loop  22 . 
     The first battery-side pump  44  and the second battery-side pump  45  may operate individually and selectively according to the thermal condition and charging condition of the battery  41 , the operating condition of the HVAC system  11 , and the like. 
     The battery cooling system  12  may include a second three-way valve  62  mounted in at least one of the first and second connection conduits  22   c  and  22   d.    
     Referring to  FIG.  2   , the second three-way valve  62  may be disposed at the outlet of the first connection conduit  22   c . That is, the second three-way valve  62  may be disposed at a junction between the first connection conduit  22   c  and the second coolant conduit  22   b.    
     When the second three-way valve  62  is switched to open the outlet of the first connection conduit  22   c , the first coolant conduit  22   a  may be fluidly connected to the second coolant conduit  22   b  through the first connection conduit  22   c  and the second connection conduit  22   d , and accordingly the battery-side coolant may entirely circulate through the first coolant conduit  22   a  and the second coolant conduit  22   b.    
     When the second three-way valve  62  is switched to close the outlet of the first connection conduit  22   c , the first coolant conduit  22   a  may be fluidly separated from the second coolant conduit  22   b , and accordingly the battery-side coolant may circulate in the first coolant conduit  22   a  and the second coolant conduit  22   b  independently of each other. Specifically, in a state in which the second three-way valve  62  is switched to close the outlet of the first connection conduit  22   c , a portion of the battery-side coolant may circulate in the first coolant conduit  22   a  through the first battery-side pump  44  so that it may sequentially pass through the battery radiator  43 , the reservoir tank  48 , and the second passage  72  of the water-cooled heat exchanger  70 , and a remaining portion of the battery-side coolant may circulate in the second coolant conduit  22   b  through the second battery-side pump  45  so that it may sequentially pass through the second passage  37   b  of the battery chiller  37 , the heater  42 , and the battery  41 . 
     The battery cooling system  12  may be controlled by a battery management system  1100 . The battery management system  1100  may monitor the state of the battery  41 , and perform the cooling of the battery  41  when the temperature of the battery  41  is higher than or equal to a threshold temperature. The battery management system  1100  may transmit an instruction for the cooling of the battery  41  to the controller  1000 , and accordingly the controller  1000  may control the compressor  32  to operate and control the chiller-side expansion valve  17  to open. When the operation of the HVAC system  11  is not required during the cooling operation of the battery  41 , the controller  1000  may control the cooling-side expansion valve  15  to close. In addition, the battery management system  1100  may control the operation of the first battery-side pump  44 , the operation of the second battery-side pump  45 , and the switching of the second three-way valve  62  as necessary so that the battery-side coolant may selectively flow through the first coolant conduit  22   a  and the second coolant conduit  22   b.    
     The powertrain cooling system  13  may include the powertrain coolant loop  23 , and the powertrain-side coolant for cooling powertrain components (the electric motors and the power electronics) may circulate through the powertrain coolant loop  23 . 
     The powertrain cooling system  13  may be configured to cool the powertrain components of an electric powertrain using the powertrain-side coolant circulating in the powertrain coolant loop  23 . The powertrain components may include one or more electric motors  51   a  and  51   b  and one or more power electronics  52   a ,  52   b , and  52   c . The powertrain coolant loop  23  may be fluidly connected to a powertrain radiator  53 , a reservoir tank  56 , a powertrain-side pump  54 , the power electronics  52   a ,  52   b , and  52   c , the electric motors  51   a  and  51   b , and the first passage  71  of the water-cooled heat exchanger  70 . Referring to  FIG.  2   , the powertrain-side coolant may sequentially pass through the powertrain radiator  53 , the reservoir tank  56 , the powertrain-side pump  54 , the power electronics  52   a ,  52   b , and  52   c , the electric motors  51   a  and  51   b , and the first passage  71  of the water-cooled heat exchanger  70  through the powertrain coolant loop  23 . 
     The electric motors  51   a  and  51   b  may have a coolant passage provided inside or outside thereof, and the powertrain-side coolant may pass through the coolant passage. The powertrain coolant loop  23  may be fluidly connected to the coolant passage of each of the electric motors  51   a  and  51   b . Referring to  FIG.  2   , the electric motors  51   a  and  51   b  may include a front-wheel-side electric motor  51   a  and a rear-wheel-side electric motor  51   b . The power electronics  52   a ,  52   b , and  52   c  may be one or more power electronics components related to the driving of the electric motors  51   a  and  51   b . The power electronics  52   a ,  52   b , and  52   c  may have a coolant passage provided inside or outside thereof, and the powertrain-side coolant may pass through the coolant passage. The powertrain coolant loop  23  may be fluidly connected to the coolant passage of each of the power electronics  52   a ,  52   b , and  52   c . Referring to  FIG.  2   , the power electronics  52   a ,  52   b , and  52   c  may be a rear-wheel-side inverter  52   a , a front-wheel-side inverter  52   b , and an on-board charger (OBC)/low DC-DC converter (LDC)  52   c.    
     The powertrain radiator  53  may be adjacent to the front grille of the vehicle, and the powertrain-side coolant passing through the powertrain radiator  53  may be cooled using the ambient air forcibly blown by the cooling fan  75 . According to an exemplary embodiment, the powertrain radiator  53  may be referred to as a high temperature radiator (HTR). 
     The exterior heat exchanger  35 , the battery radiator  43 , and the powertrain radiator  53  may be adjacent to each other on the front of the vehicle, and accordingly the exterior heat exchanger  35 , the battery radiator  43 , and the powertrain radiator  53  may come into contact with the ambient air and exchange heat with the ambient air. The cooling fan  75  may be disposed behind the exterior heat exchanger  35 , the battery radiator  43 , and the powertrain radiator  53 . 
     The reservoir tank  56  may be located on the downstream side of the powertrain radiator  53 . In particular, the reservoir tank  56  may be disposed between an outlet of the powertrain radiator  53  and the powertrain-side pump  54  in the powertrain coolant loop  23 . 
     The powertrain-side pump  54  may be located on the upstream side of the electric motors  51   a  and  51   b  and the power electronics  52   a ,  52   b , and  52   c , and the powertrain-side pump  54  may be configured to allow the powertrain-side coolant to circulate through the powertrain coolant loop  23 . 
     The powertrain cooling system  13  may further include a powertrain bypass conduit  55  allowing the powertrain-side coolant to bypass the powertrain radiator  53 . The powertrain bypass conduit  55  may directly connect an upstream point of the powertrain radiator  53  and a downstream point of the powertrain radiator  53  in the powertrain coolant loop  23  so that the powertrain-side coolant may bypass the powertrain radiator  53  through the powertrain bypass conduit  55 . 
     An inlet of the powertrain bypass conduit  55  may be connected to a point between an inlet of the powertrain radiator  53  and the electric motors  51   a  and  51   b  in the powertrain coolant loop  23 . Specifically, the inlet of the powertrain bypass conduit  55  may be connected to a point between the inlet of the powertrain radiator  53  and the first passage  71  of the water-cooled heat exchanger  70  in the powertrain coolant loop  23 . An outlet of the powertrain bypass conduit  55  may be connected to a point between the outlet of the powertrain radiator  53  and the reservoir tank  56  in the powertrain coolant loop  23 . 
     The powertrain cooling system  13  may include a third three-way valve  63  disposed at the inlet of the powertrain bypass conduit  55 . When the third three-way valve  63  is switched to open the inlet of the powertrain bypass conduit  55 , the powertrain-side coolant may pass through the powertrain bypass conduit  55  so that the powertrain-side coolant may bypass the powertrain radiator  53 , and accordingly the powertrain-side coolant may sequentially pass through the first passage  71  of the water-cooled heat exchanger  70 , the powertrain bypass conduit  55 , the reservoir tank  56 , the powertrain-side pump  54 , the power electronics  52   a ,  52   b , and  52   c , and the electric motors  51   a  and  51   b . When the third three-way valve  63  is switched to close the inlet of the powertrain bypass conduit  55 , the powertrain-side coolant may not be directed to the powertrain bypass conduit  55 , and accordingly the powertrain-side coolant may sequentially pass through the first passage  71  of the water-cooled heat exchanger  70 , the powertrain radiator  53 , the reservoir tank  56 , the powertrain-side pump  54 , the power electronics  52   a ,  52   b , and  52   c , and the electric motors  51   a  and  51   b.    
     The powertrain cooling system  13  may be controlled by a powertrain controller  1200 . The powertrain controller  1200  may monitor the states of the powertrain components (the electric motors, the power electronics, and the like), and perform the cooling of the powertrain components when the temperatures of the powertrain components are higher than or equal to a threshold temperature. The switching of the third three-way valve  63  and the operation of the powertrain-side pump  54  may be controlled by the controller  1000 . 
     Referring to  FIG.  2   , the HVAC system  11 , the battery cooling system  12 , and the powertrain cooling system  13  may form a vehicle thermal management system. The vehicle thermal management system may include an interior temperature sensor for measuring the interior temperature of the vehicle, an ambient temperature sensor for measuring the ambient temperature of the vehicle, a battery temperature sensor for measuring the temperature of the battery  41 , a coolant temperature sensor for measuring the temperature of the battery-side coolant, and a refrigerant sensor for measuring the temperature and pressure of the refrigerant. 
       FIG.  3    illustrates a flowchart of a method for controlling the vehicle HVAC system illustrated in  FIG.  1   . 
     Referring to  FIG.  3   , it may be determined whether the compressor  5  of the vehicle HVAC system operates (S 1 ). 
     When the compressor  5  operates, the controller  100  may monitor the state of a refrigerant using the temperature and pressure of the refrigerant measured by a refrigerant sensor (not shown) (S 2 ). When the compressor  5  does not operate, the method according to an exemplary embodiment of the present disclosure may end. 
     The controller  100  may determine a target subcooled temperature of the refrigerant based on the temperature and pressure of the refrigerant discharged from the outlet of the compressor  5  (S 3 ). 
     The controller  100  may calculate a change in enthalpy of the refrigerant based on the determined target subcooled temperature in a process of condensing and subcooling the refrigerant (S 4 ). Here, when the refrigerant flows from the outlet of the compressor  5  to an outlet of the condenser  6 , the enthalpy change of the refrigerant may be calculated by considering a change in a saturated temperature of the refrigerant according to a pressure drop of the refrigerant. 
     The controller  100  may calculate a change in enthalpy of air passing over an exterior surface of the condenser  6  based on the calculated refrigerant enthalpy change (S 1 ). Specifically, the enthalpy change of the air may be calculated based on a temperature difference between the refrigerant and the air, a specific heat of the air, a flow rate of the refrigerant passing through the condenser  6 , and the like. 
     The controller  100  may monitor a vehicle speed and an opening degree of the grille  8  (S 6 ). The vehicle speed may be received from a vehicle controller, and the opening degree of the grille  8  may be adjusted by the active air flap  8   a.    
     The controller  100  may calculate a required fan duty of the cooling fan  9  based on the calculated air enthalpy change, the vehicle speed, and the opening degree of the grille  8  (S 7 ). Here, the required fan duty of the cooling fan  9  refers to a fan duty optimized to meet the target subcooled temperature of the refrigerant. 
     The controller  100  may operate the cooling fan  9  according to the calculated required fan duty (S 8 ). 
     Then, the controller  100  may determine whether the compressor  5  is stopped (S 9 ). When the compressor  5  is not stopped, the method according to this exemplary embodiment may return to S 2 . 
       FIG.  4    illustrates a flowchart of a method for controlling the vehicle HVAC system  11  illustrated in  FIG.  2   . According to an exemplary embodiment illustrated in  FIG.  4   , the method may be performed in a condition in which the water-cooled heat exchanger  70  is removed from the HVAC system  11 . 
     Referring to  FIG.  4   , it may be determined whether the compressor  32  of the vehicle HVAC system  11  operates (S 11 ). 
     During the cooling operation of the vehicle HVAC system, the exterior heat exchanger  35  may serve as a condenser. The controller  1000  may determine whether a refrigerant passes through the exterior heat exchanger  35  serving as the condenser (S 12 ). When the first three-way valve  61  is switched to close the inlet of the first refrigerant bypass conduit  25 , the refrigerant may pass through the internal passage of the exterior heat exchanger  35  without passing through the first refrigerant bypass conduit  25 . Accordingly, the controller  1000  may determine whether the refrigerant passes through the internal passage of the exterior heat exchanger  35  according to the switching operation of the first three-way valve  61 . 
     The controller  1000  may determine whether the heating-side expansion valve  16  is fully opened (S 13 ). During the cooling operation of the HVAC system  11 , the opening degree of the heating-side expansion valve  16  may be 100%, and accordingly the refrigerant may pass through the heating-side expansion valve  16  without expansion of the refrigerant. 
     When the compressor  32  operates, the refrigerant passes through the exterior heat exchanger  35  serving as the condenser, and the heating-side expansion valve  16  is fully opened, the controller  1000  may monitor the state of the refrigerant using the temperature and pressure of the refrigerant measured by the refrigerant sensor (not shown) (S 14 ). 
     When the compressor  32  does not operate, the refrigerant does not pass through the exterior heat exchanger  35 , or the heating-side expansion valve  16  is not fully opened, the method according to this exemplary embodiment may end. 
     The controller  1000  may determine a target subcooled temperature of the refrigerant based on the temperature and pressure of the refrigerant discharged from an outlet of the compressor  32  (S 15 ). 
     The controller  1000  may calculate a change in enthalpy of the refrigerant based on the determined target subcooled temperature in a process of condensing and subcooling the refrigerant (S 16 ). Here, when the refrigerant flows from the outlet of the compressor  32  to the internal passage of the exterior heat exchanger  35 , the enthalpy change of the refrigerant may be calculated by considering a change in a saturated temperature of the refrigerant according to a pressure drop of the refrigerant. 
     The controller  1000  may calculate a change in enthalpy of air passing over an exterior surface of the exterior heat exchanger  35  based on the calculated refrigerant enthalpy change (S 17 ). Specifically, the enthalpy change of the air may be calculated based on a temperature difference between the refrigerant and the air, a specific heat of the air, a flow rate of the refrigerant passing through the exterior heat exchanger  35 , and the like. 
     The controller  1000  may monitor a vehicle speed and an opening degree of the grille  14  (S 18 ). The vehicle speed may be received from a vehicle controller  1300 , and the opening degree of the grille  14  may be adjusted by the active air flap  14   a . A flow rate of the air passing over the exterior surface of the exterior heat exchanger  35  may be calculated based on the vehicle speed and the opening degree of the grille  14 . 
     The controller  1000  may calculate a required fan duty of the cooling fan  75  based on the calculated air enthalpy change, the vehicle speed, and the opening degree of the grille  14  (S 19 ). Here, the required fan duty of the cooling fan  75  refers to a fan duty optimized to meet the target subcooled temperature of the refrigerant. 
     The controller  1000  may operate the cooling fan  75  according to the calculated required fan duty (S 20 ). 
     Then, the controller  1000  may determine whether the compressor  32  is stopped (S 21 ). When the compressor  32  is not stopped, the method according to this exemplary embodiment may return to S 14 . 
       FIG.  5    illustrates a flowchart of a method for controlling the vehicle HVAC system  11  illustrated in  FIG.  2   . According to an exemplary embodiment illustrated in  FIG.  5   , the method may be performed in a condition in which a refrigerant is cooled using the water-cooled heat exchanger  70  in the vehicle HVAC system  11 . 
     Referring to  FIG.  5   , it may be determined whether the compressor  32  of the vehicle HVAC system  11  operates (S 31 ). 
     During the cooling operation of the vehicle HVAC system, the exterior heat exchanger  35  may serve as a condenser. The controller  1000  may determine whether a refrigerant passes through the exterior heat exchanger  35  serving as the condenser (S 32 ). When the first three-way valve  61  is switched to close the inlet of the first refrigerant bypass conduit  25 , the refrigerant may pass through the internal passage of the exterior heat exchanger  35  without passing through the first refrigerant bypass conduit  25 . Accordingly, the controller  1000  may determine whether the refrigerant passes through the internal passage of the exterior heat exchanger  35  according to the switching operation of the first three-way valve  61 . 
     The controller  1000  may determine whether the heating-side expansion valve  16  is fully opened (S 33 ). During the cooling operation of the HVAC system  11 , the opening degree of the heating-side expansion valve  16  may be 100%, and accordingly the refrigerant may pass through the heating-side expansion valve  16  without expansion of the refrigerant. 
     When the compressor  32  operates, the refrigerant passes through the exterior heat exchanger  35  serving as the condenser, and the heating-side expansion valve  16  is fully opened (that is, when it is determined in S 33  that the heating-side expansion valve  16  is fully opened), the controller  1000  may monitor the state of the refrigerant using the temperature and pressure of the refrigerant measured by the refrigerant sensor (not shown) (S 34 - 1 ). 
     When the compressor  32  does not operate, the refrigerant does not pass through the exterior heat exchanger  35 , or the heating-side expansion valve  16  is not fully opened, the method according to this exemplary embodiment may end. 
     The controller  1000  may determine a target subcooled temperature of the refrigerant based on the temperature and pressure of the refrigerant discharged from the outlet of the compressor  32  (S 34 - 2 ). 
     The controller  1000  may calculate a change in enthalpy of the refrigerant based on the determined target subcooled temperature in a process of condensing and subcooling the refrigerant (S 34 - 3 ). Here, when the refrigerant flows from the outlet of the compressor  32  to the internal passage of the exterior heat exchanger  35 , the enthalpy change of the refrigerant may be calculated by considering a change in a saturated temperature of the refrigerant according to a pressure drop of the refrigerant. 
     When the compressor  32  operates, the refrigerant passes through the exterior heat exchanger  35  serving as the condenser, and the heating-side expansion valve  16  is fully opened (that is, when it is determined in S 33  that the heating-side expansion valve  16  is fully opened), the controller  1000  may receive a first RPM of each of the first battery-side pump  44  and/or the second battery-side pump  45 , and the powertrain-side pump  54  from the external controllers  1100 ,  1200 , and  1300  (S 35 ). The first RPM may include at least one of the following: the RPM of the first battery-side pump  44  and/or the second battery-side pump  45  determined by the battery management system  1100 ; the RPM of the powertrain-side pump  54  determined by the powertrain controller  1200 ; and the RPM of each of the first battery-side pump  44  and/or the second battery-side pump  45 , and the powertrain-side pump  54  determined by the vehicle controller  1300 . 
     The controller  1000  may calculate a change in enthalpy of a coolant passing through the water-cooled heat exchanger  70  (a change in enthalpy of the battery-side coolant and a change in enthalpy of the powertrain-side coolant) based on the received first RPM (S 36 ). 
     The controller  1000  may calculate a change in enthalpy of air passing over the exterior surface of the exterior heat exchanger  35  based on the calculated refrigerant enthalpy change, the calculated battery-side coolant enthalpy change, and the calculated powertrain-side coolant enthalpy change (S 37 ). Specifically, the enthalpy change of the air may be calculated based on a temperature difference between the refrigerant and the air, a specific heat of the air, a flow rate of the refrigerant passing through the exterior heat exchanger  35 , and the like. 
     The controller  1000  may monitor a vehicle speed and an opening degree of the grille  14  (S 38 ). The vehicle speed may be received from the vehicle controller  1300 , and the opening degree of the grille  14  may be adjusted by the active air flap  14   a . A flow rate of the air passing over the exterior surface of the exterior heat exchanger  35  may be calculated based on the vehicle speed and the opening degree of the grille  14 . 
     The controller  1000  may calculate a required fan duty of the cooling fan  75  based on the calculated air enthalpy change, the vehicle speed, and the opening degree of the grille  14  (S 39 ). Here, the required fan duty of the cooling fan  75  refers to a fan duty optimized to meet the target subcooled temperature of the refrigerant. 
     The controller  1000  may operate the cooling fan  75  according to the calculated required fan duty (S 40 ). 
     Then, the controller  1000  may determine whether the compressor  32  is stopped (S 41 ). When the compressor  32  is not stopped, the method according to this exemplary embodiment may return to S 34 - 1 . 
       FIG.  6    illustrates a flowchart of a method for controlling the vehicle HVAC system  11  illustrated in  FIG.  2   . According to an exemplary embodiment illustrated in  FIG.  6   , the method may be performed in a condition in which a refrigerant is cooled using the water-cooled heat exchanger  70  in the vehicle HVAC system  11 . 
     Referring to  FIG.  6   , it may be determined whether the compressor  32  of the vehicle HVAC system  11  operates (S 51 ). 
     During the cooling operation of the vehicle HVAC system, the exterior heat exchanger  35  may serve as a condenser. The controller  1000  may determine whether a refrigerant passes through the exterior heat exchanger  35  serving as the condenser (S 52 ). When the first three-way valve  61  is switched to close the inlet of the first refrigerant bypass conduit  25 , the refrigerant may pass through the internal passage of the exterior heat exchanger  35  without passing through the first refrigerant bypass conduit  25 . Accordingly, the controller  1000  may determine whether the refrigerant passes through the internal passage of the exterior heat exchanger  35  according to the switching operation of the first three-way valve  61 . 
     The controller  1000  may determine whether the heating-side expansion valve  16  is fully opened (S 53 ). During the cooling operation of the HVAC system  11 , the opening degree of the heating-side expansion valve  16  may be 100%, and accordingly the refrigerant may pass through the heating-side expansion valve  16  without expansion of the refrigerant. 
     When the compressor  32  operates, the refrigerant passes through the exterior heat exchanger  35  serving as the condenser, and the heating-side expansion valve  16  is fully opened (that is, when it is determined in S 53  that the heating-side expansion valve  16  is fully opened), the controller  1000  may monitor the state of the refrigerant using the temperature and pressure of the refrigerant measured by the refrigerant sensor (not shown) (S 54 - 1 ). 
     When the compressor  32  does not operate, the refrigerant does not pass through the exterior heat exchanger  35 , or the heating-side expansion valve  16  is not fully opened, the method according to this exemplary embodiment may end. 
     The controller  1000  may determine a target subcooled temperature of the refrigerant based on the temperature and pressure of the refrigerant discharged from the outlet of the compressor  32  (S 54 - 2 ). 
     The controller  1000  may calculate a change in enthalpy of the refrigerant based on the determined target subcooled temperature in a process of condensing and subcooling the refrigerant (S 54 - 3 ). Here, when the refrigerant flows from the outlet of the compressor  32  to the internal passage of the exterior heat exchanger  35 , the enthalpy change of the refrigerant may be calculated by considering a change in a saturated temperature of the refrigerant according to a pressure drop of the refrigerant. 
     When the compressor  32  operates, the refrigerant passes through the exterior heat exchanger  35  serving as the condenser, and the heating-side expansion valve  16  is fully opened (that is, when it is determined in S 53  that the heating-side expansion valve  16  is fully opened), the controller  1000  may receive a first RPM of each of the first battery-side pump  44  and/or the second battery-side pump  45 , and the powertrain-side pump  54  from the external controllers  1100 ,  1200 , and  1300  (S 55 - 1 ). The first RPM may include at least one of the following: the RPM of the first battery-side pump  44  and/or the second battery-side pump  45  determined by the battery management system  1100 ; the RPM of the powertrain-side pump  54  determined by the powertrain controller  1200 ; and the RPM of each of the first battery-side pump  44  and/or the second battery-side pump  45 , and the powertrain-side pump  54  determined by the vehicle controller  1300 . 
     In addition, the controller  1000  may determine a second RPM of the first battery-side pump  44  and/or the second battery-side pump  45 , and a second RPM of the powertrain-side pump  54  (S 55 - 1 ). The second RPM may be determined by considering a flow rate of a battery-side coolant, a flow rate of a powertrain-side coolant, overload of each of the pumps  44 ,  45 , and  54 , and the like. 
     The controller  1000  may determine whether the first RPM is less than or equal to the second RPM (S 55 - 2 ). 
     When it is determined in S 55 - 2  that the first RPM is less than or equal to the second RPM, the controller  1000  may calculate a change in enthalpy of the coolant passing through the water-cooled heat exchanger  70  (a change in enthalpy of the battery-side coolant and a change in enthalpy of the powertrain-side coolant) based on the second RPM (S 55 - 3  and S 56 ). 
     When it is determined in S 55 - 2  that the first RPM exceeds the second RPM, the controller  1000  may calculate a change in enthalpy of the coolant passing through the water-cooled heat exchanger  70  (a change in enthalpy of the battery-side coolant and a change in enthalpy of the powertrain-side coolant) based on the first RPM (S 55 - 4  and S 56 ). 
     In other words, the controller  1000  may calculate the enthalpy change of the battery-side coolant and the enthalpy change of the powertrain-side coolant based on the higher RPM of the first RPM determined by the external controllers  1100 ,  1200 , and  1300  and the second RPM determined by the controller  1000 . As the flow rate of the coolant passing through the water-cooled heat exchanger  70  increases, the amount of the refrigerant condensed by the water-cooled heat exchanger  70  may be relatively increased compared to the amount of the refrigerant condensed by the exterior heat exchanger  35 , and thus a required fan duty of the cooling fan  75  for achieving the subcooling of the refrigerant by the exterior heat exchanger  35  may be relatively reduced. 
     The controller  1000  may calculate a change in enthalpy of air passing over the exterior surface of the exterior heat exchanger  35  based on the calculated refrigerant enthalpy change, the calculated battery-side coolant enthalpy change, and the calculated powertrain-side coolant enthalpy change (S 57 ). Specifically, the enthalpy change of the air may be calculated based on a temperature difference between the refrigerant and the air, a specific heat of the air, a flow rate of the refrigerant passing through the exterior heat exchanger  35 , and the like. 
     The controller  1000  may monitor a vehicle speed and an opening degree of the grille  14  (S 58 ). The vehicle speed may be received from the vehicle controller  1300 , and the opening degree of the grille  14  may be adjusted by the active air flap  14   a . A flow rate of the air passing over the exterior surface of the exterior heat exchanger  35  may be calculated based on the vehicle speed and the opening degree of the grille  14 . 
     The controller  1000  may calculate a required fan duty of the cooling fan  75  based on the calculated air enthalpy change, the vehicle speed, and the opening degree of the grille  14  (S 59 ). Here, the required fan duty of the cooling fan  75  refers to a fan duty optimized to meet the target subcooled temperature of the refrigerant. 
     The controller  1000  may operate the cooling fan  75  according to the calculated required fan duty (S 60 ). 
     Then, the controller  1000  may determine whether the compressor  32  is stopped (S 61 ). When the compressor  32  is not stopped, the method according to this exemplary embodiment may return to S 54 - 1 . 
     As set forth above, the method for controlling the vehicle HVAC system according to exemplary embodiments of the present disclosure may accurately calculate the required fan duty of the cooling fan which matches the subcooling of the refrigerant based on the refrigerant enthalpy change, the air enthalpy change, and the like, thereby achieving sufficient subcooling of the refrigerant and reducing power consumption during the operation of the HVAC system. 
     Hereinabove, although the present disclosure has been described with reference to exemplary embodiments and the accompanying drawings, the present disclosure is not limited thereto, but may be variously modified and altered by those skilled in the art to which the present disclosure pertains without departing from the spirit and scope of the present disclosure claimed in the following claims.