Abstract:
An air conditioning system for a vehicle is provided with a heat pump circulation path through which a refrigerant body is circulated through a compressor. The heat pump circulation path is provided with a condenser unit, an expansion valve, a first evaporator, a heater, and a second evaporator. The heat pump circulation path is also provided with a bypass means which, in heating operation, bypasses a main condenser for constituting the condenser unit and connects the heater and a gas-liquid separation-type refrigerant body containing section. The configuration provides effects which minimize an increase in the number of dedicated parts required only in a heating mode and which, by effectively utilizing the system in a cooling mode, prevents a reduction in the air conditioning performance caused by the shortage of the refrigerant body.

Description:
TECHNICAL FIELD  
       [0001]    The present invention relates to a vehicular air conditioning system incorporated in a vehicle for air-conditioning a passenger cabin of the vehicle. 
       BACKGROUND ART  
       [0002]    Vehicles, e.g., engine automobiles having an internal combustion engine, hybrid automobiles having an engine and a secondary battery (or a secondary battery and a fuel cell or the like) in combination, electric automobiles, and fuel cell automobiles, incorporate various types of vehicular air conditioning systems. 
         [0003]    For example, as shown in  FIG. 19 , a vehicular air conditioning apparatus, as disclosed in Japanese Laid-Open Patent Publication No. 2009-023564, includes a compressor  1  for drawing in and discharging a refrigerant, a condenser  3  disposed in an air conditioning unit case  2  for heating air through heat exchange between the air and the refrigerant that is discharged from the compressor  1  in a heating mode, a receiver  4  for receiving the refrigerant flowing in from the condenser  3  and performing gas-liquid separation in the heating mode, a supercooler  5  for supercooling the liquid refrigerant that flows in from the receiver  4  through heat exchange between the liquid refrigerant and ambient air in the heating mode, a depressurizer  6  for depressurizing the refrigerant that has been supercooled by the supercooler  5  in the heating mode, and an outdoor heat exchanger  7  for evaporating the refrigerant depressurized by the depressurizer  6  in the heating mode. 
         [0004]    With the above vehicular air conditioning apparatus, subcooling (i.e., a degree of subcooling) is achieved by the receiver  4 , and further is additionally achieved reliably by the supercooler  5 , which is disposed downstream of the receiver  4 , using ambient air in the heating mode. The vehicular air conditioning apparatus is rendered highly efficient and excellent in heating performance through a relatively simple cyclic arrangement. 
         [0005]    According to Japanese Laid-Open Patent Publication No. 2009-023564, the receiver  4  and the supercooler  5  are used only in the heating mode, and thus, the receiver  4  and the supercooler  5  are not required in a cooling mode. Therefore, the number of components dedicated to the heating mode is increased, which makes the vehicular air conditioning apparatus uneconomical. 
         [0006]    According to Japanese Laid-Open Patent Publication No. 2009-023564, furthermore, the vehicular air conditioning apparatus does not include a buffer in order to make up for a refrigerant shortage in the case that the cooled liquid refrigerant remains trapped in the outdoor heat exchanger  7 , and the amount of refrigerant used for air-conditioning the vehicle is reduced in the cooling mode. Consequently, air-conditioning performance is lowered due to the refrigerant shortage, resulting in the need for a power increase caused by a capability shortage of the compressor  1 , and also resulting in poor mileage on account of the power increase. 
       SUMMARY OF INVENTION  
       [0007]    The present invention has been made in order to solve the aforementioned problems. It is an object of the present invention to provide a vehicular air conditioning system, which is capable of increasing heat exchange efficiency and maintaining good air-conditioning performance as a result of stably circulating a refrigerant by means of a simple and economical arrangement. 
         [0008]    According to the present invention, there is provided a vehicular air conditioning system of the heat pump type comprising a condenser for performing heat exchange between a refrigerant and ambient air, the condenser being connected to a heat pump circulation path for circulating the refrigerant with a compressor, an evaporator connected to the heat pump circulation path for performing heat exchange between the refrigerant and air-conditioning air, and a heater connected to the heat pump circulation path for performing heat exchange between the refrigerant which has been delivered from the compressor and the air-conditioning air that has passed through the evaporator. 
         [0009]    The air conditioning system further comprises a gas-liquid separation refrigerant storage unit, a supercooling heat exchanger, and a bypass means connecting the gas-liquid separation refrigerant storage unit and the supercooling heat exchanger downstream of the heater in bypassing relation to the condenser in a heating mode. 
         [0010]    According to the present invention, the gas-liquid separation refrigerant storage unit functions as a buffer for making up or compensating for a refrigerant shortage in a cooling mode. Therefore, when the air conditioning system operates in the heating mode at an increased ambient air temperature, as well as when the air conditioning system operates in a transient mode such as a dehumidifying heating mode, no refrigerant shortage occurs, thereby allowing the air conditioning system to perform air-conditioning in a stable manner. 
         [0011]    In the heating mode, the heater, the gas-liquid separation refrigerant storage unit, and the supercooling heat exchanger are connected in bypassing relation to the condenser. Consequently, the gas-liquid separation refrigerant storage unit functions as a subcooling tank. As a result, liquid refrigerant, which is produced upon separation of gas contained in the refrigerant, flows through the supercooling heat exchanger (subcooling condenser) and is cooled to an ambient air temperature range. Accordingly, there is no need to provide a subcooling tank and a subcooler, which would be used only in the heating mode. 
         [0012]    It is thus possible to increase heat exchange efficiency and to maintain good air-conditioning performance by stably circulating the refrigerant by means of a simple and economical arrangement. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1  is a schematic block diagram of a vehicular air conditioning system according to a first embodiment of the present invention; 
           [0014]      FIG. 2  is a schematic view showing the manner in which the vehicular air conditioning system operates in a heating mode; 
           [0015]      FIG. 3  is a diagram showing a cycle on a Mollier chart plotted when the vehicular air conditioning system operates in the heating mode; 
           [0016]      FIG. 4  is a schematic view showing the manner in which the vehicular air conditioning system operates in a dehumidifying heating mode; 
           [0017]      FIG. 5  is a schematic view showing the manner in which the vehicular air conditioning system operates in a cooling mode; 
           [0018]      FIG. 6  is a schematic block diagram of a vehicular air conditioning system according to a second embodiment of the present invention; 
           [0019]      FIG. 7  is a diagram showing a cycle on a Mollier chart plotted when the vehicular air conditioning system operates in a heating mode; 
           [0020]      FIG. 8  is a schematic block diagram of a vehicular air conditioning system according to a third embodiment of the present invention; 
           [0021]      FIG. 9  is a schematic block diagram of a vehicular air conditioning system according to a fourth embodiment of the present invention; 
           [0022]      FIG. 10  is a schematic view showing the manner in which the vehicular air conditioning system operates in a heating mode; 
           [0023]      FIG. 11  is a schematic view showing the manner in which the vehicular air conditioning system operates in a dehumidifying heating mode; 
           [0024]      FIG. 12  is a schematic view showing the manner in which the vehicular air conditioning system operates in a cooling mode; 
           [0025]      FIG. 13  is a schematic block diagram of a vehicular air conditioning system according to a fifth embodiment of the present invention; 
           [0026]      FIG. 14  is a schematic view showing the manner in which the vehicular air conditioning system operates in a cooling mode; 
           [0027]      FIG. 15  is a schematic block diagram of a vehicular air conditioning system according to a sixth embodiment of the present invention; 
           [0028]      FIG. 16  is a schematic view showing the manner in which the vehicular air conditioning system operates in a cooling mode; 
           [0029]      FIG. 17  is a schematic block diagram of a vehicular air conditioning system according to a seventh embodiment of the present invention; 
           [0030]      FIG. 18  is a schematic block diagram of a vehicular air conditioning system according to an eighth embodiment of the present invention; and 
           [0031]      FIG. 19  is a diagram illustrating the vehicular air conditioning apparatus disclosed in Japanese Laid-Open Patent Publication No. 2009-023564. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0032]    As shown in  FIGS. 1 and 2 , a vehicular air conditioning system  10  according to a first embodiment of the present invention is incorporated in an automobile (vehicle)  12  for air-conditioning a passenger cabin (vehicle compartment)  14  of the automobile  12 . 
         [0033]    The air conditioning system  10  has a heat pump circulation path  18  for circulating a refrigerant via a compressor  16 . The heat pump circulation path  18  includes therein a condenser unit (condenser)  20  for performing heat exchange between the refrigerant and ambient air, an expansion valve  22  for depressurizing the refrigerant delivered from the condenser unit  20 , a first evaporator (evaporator) for performing heat exchange between the refrigerant that has passed through the expansion valve  22  and air-conditioning air, and a heater  26  for performing heat exchange between the refrigerant delivered from the compressor  16  and the air-conditioning air that has passed through the first evaporator  24 . 
         [0034]    The heat pump circulation path  18  branches into a branch path  28 , which includes a second evaporator (rear evaporator)  30  for performing heat exchange between a heat medium discharged from the cabin  14  (waste heat gas from the cabin  14 ) and the refrigerant. 
         [0035]    Since the heating medium used for heat exchange in the second evaporator  30  is an exhaust heat gas from the cabin  14 , heat that is carried in the cabin  14  can effectively be utilized without being abandoned. When the air conditioning system  10  is activated for warming the cabin  14 , heat used to warm the cabin  14  is retrieved and introduced again into the air conditioning system  10 . Therefore, the air conditioning system  10  can be started up quickly. 
         [0036]    The condenser unit  20  includes a main condenser (condensing device)  32 , a gas-liquid separation refrigerant storage unit (subcooling tank)  34 , and a subcondenser (supercooling heat exchanger)  36 , which are connected mutually in series downstream of the heater  26 , and through which the refrigerant flows in a cooling mode. A solenoid-operated valve  38   a  is disposed upstream of the main condenser  32 . 
         [0037]    A bypass means  40  is connected to the heat pump circulation path  18  for connecting the heater  26  to the gas-liquid separation refrigerant storage unit  34  and the subcondenser  36  in bypassing relation to the main condenser  32  in a heating mode. The bypass means  40  includes a first bypass path  42   a , which branches from the heat pump circulation path  18  and is connected to the gas-liquid separation refrigerant storage unit  34  of the condenser unit  20 . The first bypass path  42   a  includes a solenoid-operated valve  38   b.    
         [0038]    The expansion valve  22  includes a means (not shown) for detecting the temperature of the refrigerant delivered from the first evaporator  24 , which cools the air-conditioning air. An opening of the expansion valve  22  is variable automatically depending on the temperature of the refrigerant delivered from the first evaporator  24 , for thereby varying the flow rate of the refrigerant. 
         [0039]    The heat pump circulation path  18  also includes a three-way valve  44   a  at a junction between a portion of a path near the expansion valve  22  and an inlet of the branch path  28 . The heat pump circulation path  18  further includes a three-way valve  44   b  at a junction between an outlet of a second bypass path  42   b , which bypasses the first evaporator  24 , and the heat pump circulation path  18 . The second evaporator  30  is disposed in a rear portion of the automobile  12  (see  FIG. 2 ). 
         [0000]    timfyt2139 
         [0040]    Between the first evaporator  24  and the heater  26 , there is disposed an air mixing damper  46  for introducing air-conditioning air, having been cooled by the first evaporator  24 , into the cabin  14  in bypassing relation to the heater  26 . 
         [0041]    The automobile  12  has an ambient air inlet  48  for introducing ambient air as the air-conditioning air. The first evaporator  24  and the heater  26  are successively disposed in this order downstream of the ambient air inlet  48 . The air conditioning system  10  includes a controller (ECU)  50 , which functions as a flow path switching means, for controlling the solenoid-operated valves  38   a ,  38   b  and the three-way valves  44   a ,  44   b  to switch between the heating mode and the cooling mode, and which also controls the air conditioning system  10  in its entirety (see  FIG. 1 ). 
         [0042]    Operations of the air conditioning system  10  will be described below with reference to a cycle diagram shown in  FIG. 3 . 
         [0043]    When the air conditioning system  10  operates in a heating mode, as shown in  FIG. 2 , the compressor  16  is actuated to deliver refrigerant into the heat pump circulation path  18 . The refrigerant is supplied to the heater  26 , which carries out heat exchange between the refrigerant and the air-conditioning air (radiates heat into the air-conditioning air) in order to increase the temperature of the air-conditioning air. 
         [0044]    The solenoid-operated valve  38   a  is closed and the solenoid-operated valve  38   b  is opened, so as to allow the refrigerant, which is discharged from the heater  26 , to pass through the first bypass path  42   a  and directly into the gas-liquid separation refrigerant storage unit  34 , in bypassing relation to the main condenser  32 . The refrigerant flows from the gas-liquid separation refrigerant storage unit  34  through the subcondenser  36 . The subcondenser  36  cools the refrigerant and delivers the cooled refrigerant to the expansion valve  22 . 
         [0045]    The refrigerant is depressurized by the expansion valve  22  and branches through the three-way valve  44   a  into the branch path  28 , from which the refrigerant is introduced into the second evaporator  30 . The second evaporator  30  carries out heat exchange between the refrigerant and a heat source in the cabin  14 . The refrigerant then bypasses the first evaporator  24  and flows back into the compressor through the second bypass path  42   b  and the expansion valve  22 . 
         [0046]    According to the first embodiment, the main condenser  32 , the gas-liquid separation refrigerant storage unit  34 , and the subcondenser  36  are connected mutually in series downstream of the heater  26 . The heat pump circulation path  18  includes the bypass means  40 , which connects the heater  26  to the gas-liquid separation refrigerant storage unit  34  and the subcondenser  36 , in bypassing relation to the main condenser  32  in the heating mode. 
         [0047]    When the air conditioning system  10  operates in the heating mode, as shown in  FIG. 2 , a portion of the heat pump circulation path  18  downstream of the heater  26  is connected to the gas-liquid separation refrigerant storage unit  34  and the subcondenser  36  in bypassing relation to the main condenser  32 . The gas-liquid separation refrigerant storage unit  34  and the subcondenser thus function respectively as subcooling tanks (see the gas-liquid separation refrigerant storage unit  34  and the subcondenser  36  in  FIG. 3 ). 
         [0048]    The refrigerant can thus be introduced as a perfect liquid medium into the expansion valve  22 , whereby the expansion valve  22  is effectively prevented from trapping gas therein. Therefore, the heat pump circulation path  18  is capable of stably circulating the refrigerant, thereby easily increasing air-conditioning performance and maintaining the air-conditioning performance favorably. 
         [0049]    The gas-liquid separation refrigerant storage unit  34  is used as a subcooling tank. Consequently, the gas-liquid separation refrigerant storage unit  34  can provide a sufficient amount of refrigerant, making it possible to prevent air-conditioning performance from being lowered due to a shortage of refrigerant when the air conditioning system  10  operates in the heating mode at an increased ambient air temperature, as well as when the air conditioning system  10  operates in a transient mode such as a dehumidifying heating mode. 
         [0050]    According to the first embodiment, furthermore, there is no need to provide a subcooling tank and a subcooler for use only in the heating mode, because the gas-liquid separation refrigerant storage unit  34  and the subcondenser  36  of the condenser unit  20 , which as described later serves as a heat radiator in the cooling mode, can be shared with the heating mode. Since there are no devices that are used only in the heating mode, the system installation space in the front portion of the vehicle that incorporates the air conditioning system  10  therein is effectively reduced. 
         [0051]    Consequently, by stably circulating the refrigerant with a simple and economical arrangement, it is possible to increase the heat exchange efficiency and to maintain good air-conditioning performance. 
         [0052]    Operations of the air conditioning system  10  in the dehumidifying heating mode will be described below. 
         [0053]    When the air conditioning system  10  operates in the dehumidifying heating mode, as shown in  FIG. 4 , the three-way valve  44   b  is actuated in order to close the second bypass path  42   b , thereby connecting the first evaporator  24  to the heat pump circulation path  18 . When the compressor  16  is operated, the refrigerant delivered into the heat pump circulation path  18  flows through the heater  26 , which radiates heat from the refrigerant. Thereafter, the refrigerant flows through the gas-liquid separation refrigerant storage unit  34 , the subcondenser  36 , and the expansion valve  22 , whereupon the refrigerant becomes lower in pressure and temperature. The heat of the refrigerant is absorbed by the second evaporator  30  and thereafter the refrigerant is delivered to the first evaporator  24 . 
         [0054]    The first evaporator  24  absorbs heat from the air-conditioning air thereby cooling the air-conditioning air. Thereafter, the temperature of the air-conditioning air is increased by the heater  26  and the air-conditioning air is then introduced into the cabin  14 . Since the air-conditioning air is cooled by the first evaporator  24 , water vapor contained in air that is introduced from outside the automobile  12  is removed, i.e., the introduced air is dehumidified. 
         [0055]    Even if the air-conditioning air that passes through the first evaporator  24  is low in temperature, the second evaporator  30  absorbs a sufficient amount of heat from a heat source that is discharged from the cabin  14 , which is high in temperature and low in humidity, thereby heating the refrigerant that flows into the first evaporator  24 . Therefore, even when the air conditioning system operates in the dehumidifying heating mode, the second evaporator  30  does not freeze and is capable of operating continuously. 
         [0056]    Since the refrigerant is supplied to the gas-liquid separation refrigerant storage unit  34 , the gas-liquid separation refrigerant storage unit  34  also functions as a subcooling tank. As a result, when the refrigerant is distributed at the time that the air conditioning system  10  operates in a transient mode such as the dehumidifying heating mode, no shortage of refrigerant occurs, thus allowing the air conditioning system  10  to operate with stable air-conditioning performance. 
         [0057]      FIG. 5  shows the manner in which the vehicular air conditioning system  10  operates in the cooling mode. 
         [0058]    When the air conditioning system  10  is operated in the cooling mode, the solenoid-operated valve  38   a  is opened and the solenoid-operated valve  38   b  is closed, whereby the condenser unit  20  is connected to the heat pump circulation path  18 . The three-way valves  44   a ,  44   b  are switched in order to disconnect the branch path  28  from the heat pump circulation path  18 , and to connect the first evaporator  24  to the heat pump circulation path  18 . The air mixing damper  46  remains fully closed. 
         [0059]    The compressor  16  is actuated in order to compress the refrigerant to a high temperature. Compressed refrigerant flows through the heater  26  and then the refrigerant is cooled by the condenser unit  20 . Thereafter, the refrigerant is converted by the expansion valve  22  into a refrigerant of low temperature and low pressure, whereupon the refrigerant is supplied to the first evaporator  24 . When the low-temperature refrigerant flows through the first evaporator  24 , the first evaporator  24  carries out heat exchange between the refrigerant and the air-conditioning air. The air-conditioning air is cooled, and the refrigerant flows from the expansion valve  22  back into the compressor  16  after heat from the refrigerant is absorbed by the expansion valve  22 . 
         [0060]    The air-conditioning air, which has been cooled by the first evaporator  24 , is not heated since the air mixing damper  46  is closed, and the air-conditioning air is introduced into the cabin  14 , thereby cooling the cabin  14 . In the cooling mode, the gas-liquid separation refrigerant storage unit  34  performs a dampening action so as to dampen any increase or decrease in the amount of refrigerant. 
         [0061]      FIG. 6  is a schematic block diagram of a vehicular air conditioning system  60  according to a second embodiment of the present invention. Parts of the air conditioning system  60  according to the second embodiment, which are identical to those of the air conditioning system  10  according to the first embodiment, are denoted by identical reference characters, and such features will not be described in detail below. Similarly, parts of air conditioning systems according to later-described third through eighth embodiments of the present invention, which are identical to those of the air conditioning system  10  according to the first embodiment, are denoted by identical reference characters, and such features will not be described in detail below. 
         [0062]    The air conditioning system  60  includes a bypass means  62  connected to the heat pump circulation path  18 , for thereby connecting the heater  26  to the gas-liquid separation refrigerant storage unit  34  in bypassing relation to the main condenser  32  in the heating mode. The bypass means  62  includes the first bypass path  42   a  and a flow rate control valve  64 , for example, a metering valve, a flow rate regulating valve, or the like, which is connected to the first bypass path  42   a  and serves as a pressure loss device for causing the refrigerant to undergo a pressure loss. An opening of the flow rate control valve  64  is adjusted by an actuator such as a motor  66 , for example. 
         [0063]    According to the second embodiment, as shown in a cycle diagram illustrated in  FIG. 7 , since the flow rate control valve  64  is used as a pressure loss device, the subcooling region in the heating mode can be increased. Therefore, when the ambient temperature is very low, the flow rate control valve  64  can be actuated to increase the enthalpy difference at the heater  26 . 
         [0064]    In addition, with such an increased amount of subcooling, the respective inlet temperatures of the gas-liquid separation refrigerant storage unit  34  and the subcondenser  36 , which serves as a supercooling heat exchanger, can be lowered to a temperature equivalent to that of the very low ambient temperature. Therefore, heat radiation from ambient air in the supercooling heat exchanger can be held to a minimum. 
         [0065]    Furthermore, since the amount of subcooling is increased, the capacity of the heater  26  can be increased to a higher temperature and/or higher pressure (from pressure a to pressure al). The heating performance of the heater  26  can thus be effectively increased. 
         [0066]      FIG. 8  is a schematic block diagram of a vehicular air conditioning system  70  according to a third embodiment of the present invention. 
         [0067]    The air conditioning system  70  includes a bypass means  72 , which is connected to the heat pump circulation path  18 , for connecting the heater  26  to the gas-liquid separation refrigerant storage unit  34  in bypassing relation to the main condenser  32  in the heating mode. The bypass means  72  includes the first bypass path  42   a  and a capillary  74 , which is connected to the first bypass path  42   a  and serves as a pressure loss device for causing the refrigerant to undergo a pressure loss. The solenoid-operated valve  38   b  is connected upstream of the capillary  74 . 
         [0068]    According to the third embodiment, since the capillary  74  is used as a pressure loss device, the same advantages as those of the second embodiment are achieved. For example, the subcooling region in the heating mode can be increased. 
         [0069]      FIG. 9  is a schematic block diagram of a vehicular air conditioning system  80  according to a fourth embodiment of the present invention.  FIG. 10  is a schematic diagram of the vehicular air conditioning system  80 . 
         [0070]    The air conditioning system  80  includes a condenser  82  connected to the heat pump circulation path  18  for performing heat exchange between the refrigerant and the ambient air, and a bypass means  84  connected to the heat pump circulation path  18  for connecting the heater  26  to the gas-liquid separation refrigerant storage unit  34  and the subcondenser  36  in bypassing relation to the condenser  82  in the heating mode. 
         [0071]    The condenser  82  comprises a condensing device  86 , a tank  88 , and a supercooler  90 , which are coupled together integrally. The solenoid-operated valve  38   a  is connected between the condenser  82  and the heater  26  and is positioned close to an upstream end of the condenser  82 . 
         [0072]    The bypass means  84  includes the first bypass path  42   a , in which there are included the gas-liquid separation refrigerant storage unit  34 , the subcondenser  36 , and the solenoid-operated valve  38   b.    
         [0073]    The air conditioning system  80  operates in the same manner as shown in the cycle diagram of  FIG. 3 . More specifically, when the air conditioning system  80  is operated in the heating mode, as shown in  FIG. 10 , the compressor  16  is actuated in order to deliver refrigerant into the heat pump circulation path  18 . The refrigerant is supplied to the heater  26 , which carries out heat exchange between the refrigerant and the air-conditioning air (i.e., radiates heat into the air-conditioning air) so as to increase the temperature of the air-conditioning air. 
         [0074]    Then, the solenoid-operated valve  38   a  is closed and the solenoid-operated valve  38   b  is opened in order to allow the refrigerant, having been discharged from the heater  26 , to pass through the first bypass path  42   a  directly into the gas-liquid separation refrigerant storage unit  34  in bypassing relation to the condenser  82 . The refrigerant flows from the gas-liquid separation refrigerant storage unit  34  and through the subcondenser  36 , which cools the refrigerant and delivers the cooled refrigerant to the expansion valve  22 . 
         [0075]    According to the fourth embodiment, the gas-liquid separation refrigerant storage unit  34  and the sub-condenser  36  are connected mutually in series through the first bypass path  42   a  downstream of the heater  26 . The heat pump circulation path  18  includes the bypass means  84 , which connects the heater  26  to the gas-liquid separation refrigerant storage unit  34  and the subcondenser  36 , in bypassing relation to the condenser  82  in the heating mode. 
         [0076]    When the air conditioning system  80  is operated in the heating mode, as shown in  FIG. 10 , a portion of the heat pump circulation path  18  downstream of the heater  26  is connected to the gas-liquid separation refrigerant storage unit  34  and the subcondenser  36  in bypassing relation to the condenser  82 . The gas-liquid separation refrigerant storage unit  34  thus functions as a subcooling tank. When gas contained in the refrigerant supplied to the gas-liquid separation refrigerant storage unit  34  is separated from the refrigerant, a liquid refrigerant is produced through operation of the gas-liquid separation refrigerant storage unit  34 . The liquid refrigerant is cooled to an ambient temperature range when the liquid refrigerant flows through the subcondenser  36 . 
         [0077]    The refrigerant can thus be introduced as a perfect liquid medium into the expansion valve  22 , thereby effectively preventing gas from becoming trapped in the expansion valve  22 . Therefore, the heat pump circulation path  18  is capable of stably circulating the refrigerant, thereby easily increasing air-conditioning performance and maintaining the air-conditioning performance favorably. 
         [0078]    The gas-liquid separation refrigerant storage unit  34  is used as a subcooling tank. Consequently, the gas-liquid separation refrigerant storage unit  34  can provide a sufficient amount of refrigerant, thus making it possible to prevent air-conditioning performance from being lowered due to a shortage of refrigerant when the air conditioning system  80  operates in the heating mode at an increased ambient air temperature, as well as when the air conditioning system  80  operates in a transient mode such as a dehumidifying heating mode. Therefore, the fourth embodiment provides the same advantages as those of the first through third embodiments. 
         [0079]    Since the subcondenser  36  can be located freely, the layout freedom of the air conditioning system  80  is effectively increased. The subcondenser  36  may be located in any position insofar as ambient air can flow through the subcondenser  36 . Thus, the subcondenser  36  can be installed easily and effectively. 
         [0080]    Operations of the air conditioning system  80  in the dehumidifying heating mode will be described below. 
         [0081]    When the air conditioning system  80  is operated in the dehumidifying heating mode, as shown in  FIG. 11 , the three-way valve  44   b  is actuated in order to close the second bypass path  42   b  and to connect the first evaporator  24  to the heat pump circulation path  18 . When the compressor is operated, the refrigerant, which is delivered into the heat pump circulation path  18 , flows through the heater  26 . Thereafter, the refrigerant flows through the gas-liquid separation refrigerant storage unit  34 , the subcondenser  36 , and the expansion valve  22 , whereupon the refrigerant is lowered in pressure and temperature. Heat of the refrigerant is absorbed by the second evaporator  30 , and thereafter, the refrigerant is delivered to the first evaporator  24 . 
         [0082]    The first evaporator  24  absorbs heat from the air-conditioning air in order to cool the air-conditioning air. Thereafter, the temperature of the air-conditioning air is increased by the heater  26 , and then the air-conditioning air is introduced into the cabin  14 . Since the air-conditioning air is cooled by the first evaporator  24 , water vapor contained in air that is introduced from outside the automobile  12  is removed, i.e., the air introduced into the automobile  12  is dehumidified. 
         [0083]      FIG. 12  shows the manner in which the vehicular air conditioning system  80  operates in the cooling mode. 
         [0084]    When the air conditioning system  80  is operated in the cooling mode, the solenoid-operated valve  38   a  is opened and the solenoid-operated valve  38   b  is closed, thereby connecting the condenser  82  to the heat pump circulation path  18 . The three-way valves  44   a ,  44   b  are switched in order to disconnect the branch path  28  from the heat pump circulation path  18  and to connect the first evaporator  24  to the heat pump circulation path  18 . The air mixing damper  46  remains fully closed. 
         [0085]    The compressor  16  is actuated in order to compress the refrigerant to a high temperature. Compressed refrigerant flows through the heater  26  and then the refrigerant is cooled by the condenser  82 . Thereafter, the refrigerant is converted by the expansion valve  22  into a refrigerant of low temperature and low pressure, whereupon the refrigerant is supplied to the first evaporator  24 . When the low-temperature refrigerant flows through the first evaporator  24 , the first evaporator  24  carries out heat exchange between the refrigerant and the air-conditioning air. The air-conditioning air is cooled, and the refrigerant flows from the expansion valve  22  back into the compressor  16  after heat from the refrigerant is absorbed by the expansion valve  22 . 
         [0086]    The air-conditioning air, which has been cooled by the first evaporator  24 , is not heated since the air mixing damper  46  is closed. The air-conditioning air is introduced into the cabin  14 , thereby cooling the cabin  14 . In the cooling mode, the gas-liquid separation refrigerant storage unit  34  performs a dampening action so as to dampen any increase or decrease in the amount of refrigerant. 
         [0087]      FIG. 13  is a schematic block diagram of a vehicular air conditioning system  100  according to a fifth embodiment of the present invention. 
         [0088]    In the air conditioning system  100 , a bypass means  102  is connected to the heat pump circulation path  18  for connecting the heater  26  to the gas-liquid separation refrigerant storage unit  34  and to the subcondenser  36  in bypassing relation to the condenser  82  in the heating mode. The bypass means  102  includes the first bypass path  42   a , which is connected parallel to the condenser  82  and includes a solenoid-operated valve  38   b.    
         [0089]    The heat pump circulation path  18  includes the gas-liquid separation refrigerant storage unit  34  and the subcondenser  36 , which are positioned between the outlet of the condenser  82  and the inlet of the expansion valve  22 . 
         [0090]    According to the fifth embodiment, when the air conditioning system  100  is operated in the heating mode, as shown in  FIG. 13 , the solenoid-operated valve  38   a  is closed and the solenoid-operated valve  38   b  is opened. When the compressor  16  is actuated, refrigerant is discharged from the heater  26  and flows through the first bypass path  42   a  directly into the gas-liquid separation refrigerant storage unit  34  in bypassing relation to the condenser  82 . The refrigerant flows from the gas-liquid separation refrigerant storage unit  34  and through the subcondenser  36 , which cools the refrigerant and delivers cooled refrigerant to the expansion valve  22 . The gas-liquid separation refrigerant storage unit  34  thus functions as a subcooling tank. Liquid refrigerant, which is produced when gas contained in the refrigerant is separated, flows through the subcondenser  36 , thereby producing a perfect liquid medium. Therefore, the heat pump circulation path  18  is capable of stably circulating the refrigerant therethrough, thereby easily increasing and maintaining good air-conditioning performance. Therefore, the fifth embodiment provides the same advantages as those of the first through fourth embodiments. 
         [0091]    When the air conditioning system  100  is operated in the cooling mode, as shown in  FIG. 14 , the solenoid-operated valve  38   a  is opened and the solenoid-operated valve  38   b  is closed while the three-way valves  44   a ,  44   b  are switched. When the compressor  16  is actuated to compress the refrigerant to a high temperature, compressed refrigerant flows through the heater  26 , and thereafter, the refrigerant is cooled by the condenser  82 . Then, the refrigerant is converted by the subcondenser  36  and the expansion valve  22  into a refrigerant of low temperature and low pressure, whereupon the refrigerant is supplied to the first evaporator  24 . In the cooling mode, therefore, similar to the heating mode, the gas-liquid separation refrigerant storage unit  34  performs a dampening action so as to dampen any increase or decrease in the amount of refrigerant. 
         [0092]      FIG. 15  is a schematic block diagram of a vehicular air conditioning system  110  according to a third embodiment of the present invention. 
         [0093]    The air conditioning system  110  includes a bypass means  112  connected to the heat pump circulation path  18  for connecting the heater  26  to the gas-liquid separation refrigerant storage unit  34  in bypassing relation to the condenser  82  in the heating mode. The bypass means  112  includes the first bypass path  42   a , which includes the solenoid-operated valve  38   b , the gas-liquid separation refrigerant storage unit  34 , and the subcondenser  36 . The gas-liquid separation refrigerant storage unit  34  and the subcondenser  36  are disposed between the first evaporator  24  and the heater  26 . 
         [0094]    According to the sixth embodiment, when the air conditioning system  110  is operated in the heating mode, as shown in  FIG. 15 , the solenoid-operated valve  38   a  is closed and the solenoid-operated valve  38   b  is opened. When the compressor  16  is actuated, refrigerant is discharged from the heater  26  and flows through the first bypass path  42   a  directly into the gas-liquid separation refrigerant storage unit  34  in bypassing relation to the condenser  82 . The refrigerant flows from the gas-liquid separation refrigerant storage unit  34  and through the subcondenser  36 , which cools the refrigerant and delivers cooled refrigerant to the expansion valve  22 . 
         [0095]    When the air conditioning system  110  is operated in the cooling mode, the solenoid-operated valve  38   a  is opened and the solenoid-operated valve  38   b  is closed while the three-way valves  44   a ,  44   b  are switched. Upon actuation of the compressor  16  to compress the refrigerant to a high temperature, the compressed refrigerant flows through the heater  26  and then is cooled by the condenser  82 . 
         [0096]    According to the sixth embodiment, therefore, the heat pump circulation path  18  is capable of stably circulating the refrigerant, thereby easily increasing and maintaining good air-conditioning performance. Thus, the sixth embodiment provides the same advantages as those of the first through fifth embodiments. 
         [0097]      FIG. 17  is a schematic block diagram of a vehicular air conditioning system  120  according to a seventh embodiment of the present invention. 
         [0098]    The air conditioning system  120  includes a bypass means  122  connected to the heat pump circulation path  18  for connecting the heater  26  to the gas-liquid separation refrigerant storage unit  34  and the subcondenser  36  in bypassing relation to the condenser  82  in the heating mode. 
         [0099]    The bypass means  122  includes a flow rate control valve  124 , for example, a metering valve, a flow rate regulating valve, or the like, which is connected to the first bypass path  42   a  and serves as a pressure loss device for causing the refrigerant to undergo a pressure loss. The opening of the flow rate control valve  124  is adjusted by an actuator such as a motor  126 , for example. 
         [0100]    According to the sixth embodiment, since the flow rate control valve  124  is used as a pressure loss device, in the same manner as the cycle diagram shown in  FIG. 7 , a subcooling region can be increased in the heating mode. Therefore, when the ambient temperature is very low, the flow rate control valve  64  can be actuated in order to increase the enthalpy difference at the heater  26 . 
         [0101]    In addition, due to the increased amount of subcooling, the inlet temperature of the gas-liquid separation refrigerant storage unit  34  and the subcondenser  36 , which serves as a supercooling heat exchanger, can be lowered to a temperature that is equivalent to that of the very low ambient temperature. Therefore, heat radiation from ambient air in the supercooling heat exchanger can be held to a minimum. 
         [0102]    Furthermore, since the amount of subcooling is increased, the capacity of the heater  26  can be increased to a higher temperature and/or higher pressure. The heating performance of the heater  26  can thus be effectively increased. 
         [0103]    In essential features thereof, the seventh embodiment is based on the fourth embodiment. However, the seventh embodiment is not strictly limited to the fourth embodiment, but may be based on the fifth embodiment or the sixth embodiment. The same also applies to the eighth embodiment, as described below. 
         [0104]      FIG. 18  is a schematic block diagram of a vehicular air conditioning system  130  according to an eighth embodiment of the present invention. 
         [0105]    The air conditioning system  130  includes a bypass means  132  connected to the heat pump circulation path  18  for connecting the heater  26  to the gas-liquid separation refrigerant storage unit  34  and the subcondenser  36  in bypassing relation to the condenser  82  in the heating mode. The bypass means  72  includes a capillary  134 , which is connected to the first bypass path  42   a  and serves as a pressure loss device for causing the refrigerant to undergo a pressure loss. The solenoid-operated valve  38   b  is disposed upstream of the capillary  134 , whereas the gas-liquid separation refrigerant storage unit  34  and the subcondenser  36  are disposed downstream of the capillary  134 . 
         [0106]    According to the eighth embodiment, since the capillary  134  is used as a pressure loss device, the same advantages as those of the seventh embodiment are achieved. For example, the subcooling region in the heating mode can be increased. 
         [0107]    In each of the above embodiments, the heat medium, which undergoes heat exchange with the refrigerant in the second evaporator  30 , may be, aside from the waste heat gas from the cabin  14 , any medium that is higher in temperature than the refrigerant flowing into the second evaporator  30 , e.g., a medium carrying heat from the motor, heat from the battery, heat from an internal combustion engine assuming the vehicle has an internal combustion engine, heat from the controller  50 , or heat from ambient air, etc. 
         [0108]    The three-way valves  44   a ,  44   b  for switching between flow paths may be formed from a combination of a branch block and a solenoid-operated valve for switching between flow paths, instead of an integral assembly of a three-way branch and a valve mechanism.