Abstract:
Cooling of a battery pack of an electrified vehicle is performed with an optimized energy usage and with minimal impact on cooling of the passenger cabin. The battery is actively cooled by circulating coolant from the battery to a chiller of an air conditioning system when a battery temperature is above a predetermined power-limiting temperature. The battery is passively cooled by circulating coolant from the battery to a radiator when the battery temperature is between a first threshold and the power-limiting temperature and a difference between a battery coolant temperature and an ambient air temperature is greater than a predetermined difference. The battery is actively cooled using the chiller when the battery temperature is between the first threshold and the power-limiting temperature and the difference between the battery coolant temperature and the ambient air temperature is less than the predetermined difference.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not Applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    Not Applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates in general to battery cooling in electrified vehicles, and, more specifically, to a liquid-cooled battery with active and passive cooling modes. 
         [0004]    When an electrical storage battery (e.g., battery pack) is used to provide power to an electric motor to drive an electrified vehicle (e.g., hybrid electric or full electric), the temperature of the battery can increase when the motor is operating for extended periods of time. The battery pack is usually installed in a relatively small, enclosed space which tends to retain the heat generated. Increases in battery temperature can reduce battery charge or discharge efficiency and impede battery performance. If the battery is not cooled, the power generation, battery life, and fuel economy may suffer. 
         [0005]    Passenger vehicles typically have a passenger air conditioning system to actively cool the passenger compartment, including a compressor, a refrigerant line, and a heat exchanger such as an evaporator. One way that high battery temperatures have been addressed is to use at least a portion of the passenger compartment air conditioning system to cool the battery. Because the air conditioning system is used to cool the passenger compartment, the same compressor can be used to cool the battery, with an additional refrigerant line and evaporator. U.S. Pat. No. 7,658,083 discloses a shared cabin/battery cooling system wherein an evaporator core is provided for cooling the battery via air circulated by a battery fan across the evaporator core and the battery. 
         [0006]    In order to more effectively cool the battery, liquid cooling systems have been introduced because water has a higher thermal conductivity (can move heat faster) and a higher specific heat capacity (can absorb more heat) than air. The liquid coolant can be circulated through a cold plate in contact with the battery cells to remove the heat. The liquid coolant can convey the heat to a battery chiller which shares the refrigerant of the passenger air conditioning system. 
         [0007]    Another trend in passenger air conditioning systems is the use of separately cooled zones (e.g., front seating and rear seating zones) within the passenger cabin. Each zone may have a respective evaporator which is individually coupled to the refrigerant circuit for on-demand cooling of air in the respective zone. In an electrified vehicle with multiple passenger cooling zones, the demand on the shared refrigerant supply subsystem can become large. Increasing the size of shared cooling subsystem components (e.g., compressor, condenser, evaporator) can be undesirable due to losses in efficiency and increases in cost. Thus, it would be desirable to optimize performance of and energy use by the chiller and evaporators to reduce the overall size of the A/C components while balancing cooling system operation to best meet performance targets when the separate cooling sections reach their peak demands. 
       SUMMARY OF THE INVENTION 
       [0008]    In one aspect of the invention, an electrified vehicle comprises an electric drive adapted to selectably move the vehicle wherein a battery pack provides electrical energy to the electric drive. The battery pack includes a cooling conduit for conveying a liquid coolant. A battery sensor senses a battery temperature. A passive radiator is exposed to an ambient air temperature. A liquid pump pumps the coolant through the cooling conduit. A shared cooling subsystem includes a compressor and a condenser circulating a refrigerant. A main evaporator is selectably coupled to the shared cooling subsystem and is adapted to evaporate refrigerant to cool a passenger cabin of the vehicle. A chiller is selectably coupled to the shared cooling subsystem and is adapted to evaporate refrigerant to cool the coolant. A diverting valve has a first configuration connecting the radiator with the pump and cooling conduit and has a second configuration connecting the chiller with the pump and cooling conduit. A controller provides commands to the valve for selecting one of the configurations. When the battery temperature is between a first threshold temperature and a predetermined power-limiting temperature then the controller commands the first configuration provided that a difference between a battery coolant temperature and the ambient temperature is greater than a predetermined difference. Otherwise (i.e., if the difference is less than the predetermined difference), the controller commands the second configuration. When the battery temperature is greater than the power-limiting temperature then the controller commands the second configuration. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a block diagram of a conventional electrified vehicle. 
           [0010]      FIG. 2  is a block diagram of a prior art cooling system for a passenger cabin and a battery pack of an electrified vehicle. 
           [0011]      FIG. 3  is an embodiment of a shared cabin/battery cooling system of the present invention. 
           [0012]      FIG. 4  is a graph showing regimes for active and passive battery cooling according to one embodiment of the invention. 
           [0013]      FIG. 5  is a flowchart showing an embodiment of a method of the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0014]    Referring to  FIG. 1 , an electrified vehicle  10  has a passenger cabin  11 . An electric drive  12  (e.g., an inverter-driven traction motor) receives electrical power from a battery pack  13 . A controller  14  may include a battery control module for monitoring battery performance (including battery temperature) and a system controller for operating the inverter. A battery cooling system  15  provides a cooling fluid (such as a cooled liquid coolant or a cooled air flow) to battery pack  13  under control of controller  14 . Conventional systems have utilized an independent source of cooled air in cooling system  15  and have used a shared cooling system with a passenger A/C system  16  (for either air-cooled or liquid-cooled batteries). 
         [0015]      FIG. 2  shows a prior art shared cooling system  20  including a passenger compartment air conditioning (A/C) system  21  capable of cooling passenger compartment  22 . The passenger compartment A/C system  21  includes an accumulator  23 , a compressor  24 , a condenser  25 , a shutoff valve  26 , an expansion device  27  (such as an electronic expansion valve, temperature expansion valve, or an orifice tube), and an evaporator core  28 . These elements are configured to allow a refrigerant to flow between them and operate in a manner known in the art. The flow of refrigerant is determined in part by shutoff valve  26 . 
         [0016]    Passenger compartment A/C system  21  also includes an air blower  29  operable to facilitate air flow between evaporator core  28  and vehicle compartment  22 . Cooling system  20  also includes a battery A/C subsystem  30  capable of cooling a battery  31 . Battery A/C subsystem  30  includes a shutoff valve  32 , a thermal expansion valve  33 , and an evaporator core  34 . 
         [0017]    Battery A/C subsystem  30  shares accumulator  23 , compressor  24 , and condenser  25  with the passenger compartment A/C system  21 . These elements are configured to allow a refrigerant to flow between them and operate in a manner known in the art. The flow of refrigerant between thermal expansion valve  33  and evaporator core  34  is determined by shutoff valve  32 . Battery A/C subsystem  30  also includes a battery fan  35  operable to facilitate air flow between battery  31  and evaporator core  34 . 
         [0018]      FIG. 3  shows one preferred embodiment of the invention wherein an electrified vehicle having a battery pack  40  for providing electrical energy to an electric drive. Battery  40  includes a cooling conduit  41  for conveying a liquid coolant that absorbs heat from battery  40  and then releases it in one of either an active or passive cooling mode as described below. Conduit  41  may pass through a cold plate which contacts the battery cells, for example. 
         [0019]    A coolant pump  42  circulates the coolant through a coolant circuit including a plurality of coolant lines interconnecting conduit  41 , a passive battery radiator  44 , an active battery chiller  46 , and a three-way diverter valve  43 . Passive radiator  44  may include a battery fan  45  (or a shared engine cooling fan) for increasing heat removal as coolant passes through battery radiator  44 . In the passive cooling mode, diverter valve  43  selectably connects radiator  44  to pump  42  in response to a command signal from a controller circuit  50 . Controller  50  may include dedicated logic circuits, programmable gate arrays, or a programmable general-purpose microcontroller, for example. For the passive cooling mode, controller  50  configures valve  43  to couple its outlet  43   a  to a first inlet  43   b  and activates pump  42  to circulate the coolant through conduit  41  and radiator  44 . Controller  50  may also activate fan  45  while in the passive cooling mode as necessary. 
         [0020]    A battery temperature sensor  47  is incorporated with battery pack  40 , and an ambient air temperature sensor  48  is mounted to the vehicle where it is exposed to outside air. A sensor  49  measures a temperature of the coolant T C  as it exits the battery cold plate. Sensors  47 ,  48 , and  49  are coupled to controller  50  for providing battery temperature and ambient air temperature, respectively, to controller  50  for use in determining when to activate the passive or active cooling modes as described below. 
         [0021]    For operating in the active cooling mode, controller  50  configures diverter valve  43  so that outlet  43   a  is coupled to inlet  43   c , thereby directing the flow from pump  42  through conduit  41  and a battery chiller  46 . Battery chiller  46  is coupled to a shared cooling subsystem  51  for the passenger cabin. 
         [0022]    In shared cooling subsystem  51 , a refrigerant is circulated from a compressor  52  to an outside heat exchanger (OHX)  53  operating as a condenser. Refrigerant is applied selectively through respective valves to a front (main) evaporator  54 , rear (zone) evaporator  55 , and battery chiller  46 . Front evaporator  54  is a main evaporator for serving a main zone such as the front passenger cabin or even the entire passenger cabin when no other zone evaporator is present. Battery chiller  46  is selectively coupled to receive refrigerant in the shared cooling subsystem under control of an electronic expansion valve (EXV)  56  that is wired for receiving a control signal from controller  50 . EXV  56  is able to be completely closed in order to avoid any consumption by battery chiller  46  when not being used. A sensor  57  is incorporated in battery chiller  46  and is coupled to the controller  50  for providing a chiller outlet refrigerant temperature and refrigerant pressure signal. The sensor is only needed when using an EXV. If EXV  56  is replaced with a TXV and a refrigerant shutoff valve, then sensor  57  is not needed. 
         [0023]    For selectively coupling the cabin cooling evaporators to the shared cooling subsystem, either an EXV or a thermostatic expansion valve (TXV) may be used. Thus, TXVs  60  and  61  supply refrigerant to evaporators  54  and  55 , respectively, wherein the flow rates through TXVs  60  and  61  automatically adapt to control the superheat of the evaporators in a manner known in the art. In order to completely shut off refrigerant flow in a branch circuit when not needed by evaporators  54  or  55 , shutoff supply valves  62  and  63  are connected in series with TXVs  60  and  61  which are controlled by appropriate command signals from controller  50 . 
         [0024]    In the embodiment shown, each evaporator is individually controlled to consume the appropriate quantity of refrigerant when in use in order to provide the desired superheat for the evaporator or battery chiller. Since battery chiller  46  uses an EXV, a refrigerant temperature and pressure signal from chiller temperature sensor  57  is used by controller  50  in order to set an appropriate flow rate through valve  56  to control the superheat of the chiller. Temperature sensors  58  and  59  may be provided for evaporators  54  and  55 , especially if EXVs are substituted for the TXVs. In the preferred embodiment, an EXV is used at least for battery chiller  46  in order to achieve a necessary fine level of control for battery chiller  46  so that the cooling load actually used for the battery does not inadvertently exceed the necessary level because any unnecessary loss of cooling capacity could have a negative impact on cabin cooling. 
         [0025]    In operation, the battery cooling system in  FIG. 3  uses a minimum of energy as a result of 1) using passive cooling whenever possible and 2) by imposing strict control of refrigerant used by the battery chiller once active cooling becomes required.  FIG. 4  illustrates some temperature relationships for defining active and passive cooling regimes used by the cooling system. Selection of active or passive cooling modes may be determined by measured battery temperature T Bat  and ambient temperature T Amb  according to various temperature thresholds. Another battery-related temperature which may be used in the control algorithm is a measured temperature of the coolant T C  as it exits the battery cold plate. A first threshold T 1  shown at  65  defines a lowest battery temperature at which cooling of the battery pack is desired (e.g., about 10° C.). A power-limiting threshold T PL  shown at  66  is a function of a lowest battery temperature at which electrical output from the battery pack is negatively impacted to the degree that it becomes worthwhile to expend more energy to reduce the battery temperature (e.g., about 40° C.). For example, threshold T PL  may be set a few degrees less than the actual temperature at which the battery performance is affected. Thus, when battery temperature T Bat  is greater than power-limiting temperature T PL  then the battery cooling system enters the active cooling mode in active regime  70  (i.e., the controller issues command signals to position the diverter valve  43  to circulate liquid coolant from the battery cooling conduit through the battery chiller and to open the expansion valve feeding refrigerant to the battery chiller). 
         [0026]    When battery temperature T Bat  is greater than first threshold T 1  and less than power-limiting temperature T PL  then the selection of the cooling mode depends on a difference between battery coolant temperature T C  and ambient air temperature T Amb . This difference is a measure of the ability of the passive radiator to transfer heat to the ambient environment. A difference threshold T Diff  shown at  67  represents the temperature difference that is needed for successful cooling. If the actual difference is greater than T Diff  then the battery cooling system enters the passive cooling mode in passive regime  71  (i.e., the controller issues command signals to position the diverter valve  43  to circulate liquid coolant from the battery cooling conduit through the radiator and to close the expansion valve feeding refrigerant to the battery chiller). In addition, the controller may activate the battery fan (e.g., based on another temperature threshold). If the actual difference is less than T Diff  then the battery cooling system enters the active cooling mode in active regime  72  (i.e., the controller issues command signals to position the diverter valve to circulate liquid coolant from the battery cooling conduit through the battery chiller and to open the expansion valve feeding refrigerant to the battery chiller). 
         [0027]      FIG. 5  shows a preferred method of the invention wherein battery temperature T Bat  is compared to the first threshold T 1  in step  80 . If battery temperature is not greater than the first threshold T 1  then no battery cooling is needed, so a No Cooling mode is entered in step  82  and a return is made to step  80  for continuously monitoring the battery temperature. If battery temperature is greater than first threshold T 1  then battery temperature is compared to the power-limiting threshold T PL  in step  83 . If battery temperature T Bat  is greater than T PL  then the active cooling mode is entered at step  84  wherein the diverter valve set to circulate battery coolant to the battery chiller, the EXV valve is opened, and the passive radiator fan is turned off. Then a return is made to step  80  for continuing to monitor battery temperature. 
         [0028]    If battery temperature T Bat  is not greater than T PL  in step  83 , then a difference between the battery coolant temperature T C  and ambient temperature T Amb  is compared to the difference threshold T Diff  in step  85 . If the actual difference is not greater than the difference threshold then the active cooling mode is entered in step  84 . Otherwise, the passive cooling mode can be adopted in step  86  wherein the diverter valve is set to circulate liquid coolant to the battery radiator, the EXV for the battery chiller is closed, and the blower fan for the battery radiator is turned on. 
         [0029]    A typical air-conditioning system may utilize a variable speed compressor wherein the compressor speed is set according to the cooling load (which is usually determined by a temperature measured at the evaporator output). In the present invention, the existence of multiple evaporators together with a battery chiller wherein these elements may or may not all operate simultaneously, creates complexity for determining a compressor speed. In order to maintain acceptable cabin cooling performance without adding excess complexity to the control system, the present invention employs a priority scheme for selecting an evaporator temperature to use in determining compressor speed and adding feedforward speed bumps when the chiller is turned on. Thus, the controller sets the compressor speed according to a temperature of the main evaporator at all times when the main evaporator is cooling the passenger cabin (i.e., is actively evaporating a share of the refrigerant). As used herein, “main” evaporator refers to a front zone evaporator or a sole evaporator when there is only one zone. During times that the battery chiller is the only element actively being used to evaporate refrigerant, then the compressor speed is set by the controller according to a temperature of the battery chiller output (or the temperature of the coolant at the inlet to the battery cooling conduit). When a zone evaporator such as the rear evaporator is present for evaporating refrigerant to cool a corresponding zone in the passenger cabin, then the compressor speed is set by the controller according to a temperature of the zone evaporator whenever the zone evaporator is cooling its zone and the main evaporator is not cooling the main zone of the passenger cabin. Furthermore, the zone evaporator is given a higher priority than the battery chiller in the event that only the zone evaporator and the battery chiller are actively evaporating refrigerant. 
         [0030]    The foregoing invention has the advantage that all three of the cooling heat exchangers have direct access to the refrigerant so that there are no losses due to intermediate heat exchangers. Furthermore, refrigerant use can be balanced between the three cooling heat exchangers to balance the necessary capacity, thereby providing advantageous energy management.