Patent Publication Number: US-2010119898-A1

Title: Fuel cell system

Description:
FIELD OF THE INVENTION 
     The present invention relates to a fuel cell system. 
     BACKGROUND OF THE INVENTION 
     In an environment where the outside-air temperature is low, water generated inside a fuel cell system is frozen after the fuel cell system stops and may break pipes or valves. In light of such a problem, a method for draining water accumulated inside a fuel cell to the outside by performing scavenging processing when the fuel cell system is stopped has been proposed. 
     In order to perform such scavenging processing, an energy source other than the fuel cell is required, and a power storage device (e.g., a capacitor and a battery) for assisting the output of the fuel cell is used as the energy source. 
     Such a power storage device is also used as an energy source for starting the fuel cell system, in addition to the use for anode scavenging processing. Thus, in the situation where the outside-air temperature is lowered after the fuel cell system is stopped and the fuel cell system needs to be started at a low temperature (e.g., below zero), the fuel cell system cannot be started at the low temperature (e.g., below zero), since the amount of power remaining in the power storage device has decreased as a result of the anode scavenging processing. 
     In order to solve such a problem, a method has been proposed, where after starting scavenging processing, the amount of power remaining in a power storage device such as a capacitor and a battery is monitored, and when the monitored amount decreases to a threshold value, the anode scavenging processing is ended, thereby ensuring that a fuel cell system can be started next time (for example, patent document 1). 
     Patent Document 1: JP2006-202520 A 
     DISCLOSURE OF THE INVENTION 
     However, if whether the scavenging processing is ended or not is determined based only on the amount of power remaining in the power storage device, as in the configuration above, the scavenging processing will be continued for an unnecessarily long time even when, for example, the amount of water remaining in a fuel cell is at a proper level, which causes energy efficiency to be lowered or causes an electrolyte membrane of the fuel cell to be dried too much. 
     The present invention has been made in light of the circumstances above, and an object of the present invention is to provide a fuel cell system capable of ensuring that the fuel cell system can be started next time and preventing the scavenging processing from being continued for an unnecessarily long time. 
     In order to solve the problem above, a fuel cell system according to an aspect of the present invention, which includes a fuel cell and a power storage device and performs scavenging processing by supplying a certain gas into the system, includes: a first detector for detecting an amount of remaining water in the fuel cell; a second detector for detecting an amount of remaining power in the power-storage device; a first storage for storing a remaining water amount reference value; a second storage for storing a remaining power amount reference value; and a scavenging controller for controlling whether of not the scavenging processing is ended based on the result of a comparison between the amount of remaining water detected after the start of the scavenging processing and the remaining water amount reference value, or based on the result of a comparison between the amount of remaining power detected after the start of the scavenging processing and the remaining power amount reference value. 
     With such a configuration, whether or not the scavenging processing should be ended is determined in consideration of not only the amount of the remaining power in the power storage device but also the amount of the remaining water in the fuel cell, and thus the next start of the fuel cell system can be ensured while the scavenging processing can be prevented from being continued for an unnecessarily long time. 
     In the configuration above, It is preferable that: the remaining water amount reference value is a remaining water amount threshold value which indicates an amount of water required for starting the system next time; the remaining power amount reference value is a remaining power amount threshold value which indicates an amount of electrical power required for starting the system next time; and the scavenging controller ends the scavenging processing when the amount of remaining water falls below the remaining water amount reference value or when the amount of remaining power falls below the remaining power amount reference value. 
     In addition, it is preferable that the remaining power amount threshold value varies depending on environmental conditions in which the system is started next time. 
     The certain gas is a fuel gas supplied to an anode in the fuel cell or an oxidant gas supplied to a cathode in the fuel cell, and the first detector may detect the amount of remaining water by measuring an impedance of the fuel cell. 
     As described above, the present invention ensures that the fuel cell system can be started next time and prevents the scavenging processing from being continued for an unnecessarily long time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing the configuration of a fuel cell system according to an embodiment of the present invention. 
         FIG. 2  is a diagram showing a scavenging control function of a control unit according to the embodiment. 
         FIG. 3  is a diagram showing an example of the relationship between scavenging time periods and measured impedances according to the embodiment. 
         FIG. 4  is a flowchart showing scavenging control processing according to the embodiment. 
         FIG. 5  is a flowchart showing scavenging control processing according to modification 1. 
         FIG. 6  is a flowchart showing an SOC control processing according to modification 2. 
         FIG. 7  is a diagram showing the relationship between the SOC of a battery and a charge/discharge target power according to modification 2. 
         FIG. 8  is a diagram showing the relationship between the SOC of the battery and a discharge power upper limit according to modification 2. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     An embodiment of the present invention will be described below with reference to the attached drawings. 
     A. Embodiment 
     Overall Configuration 
       FIG. 1  schematically shows the configuration of a vehicle equipped with a fuel cell system  100  according to an embodiment. 
     Although the following description assumes a fuel cell hybrid vehicle (FCHV) as an example of vehicles, the fuel cell system may also be applied to electric vehicles and hybrid vehicles. In addition, the fuel cell system may be applied not only to the vehicles but also to various mobile objects (e.g., ships, airplanes and robots), stationary power supplies and mobile fuel cell systems. 
     The vehicle travels using, as a driving force source, a synchronous motor  61  connected to wheels  63 L and  63 R. Power sources for the synchronous motor  61  are a fuel cell  40  and a battery  20 . Electrical power output from the fuel cell  40  and battery  20  is converted to a three-phase alternating current by an inverter  60  and then supplied to the synchronous motor  61 . The synchronous motor  61  also functions as a power generator during a braking operation. 
     The vehicle travels using as a driving force source a synchronous motor  61  connected to wheels  63 L and  63 R. Power sources for the synchronous motor  61  are a fuel cell  40  and a battery  20 . Electrical power output from the fuel cell  40  and battery  20  is converted to a three-phase alternating current by an inverter  60  and then supplied to the synchronous motor  61 . The synchronous motor  61  also functions as a power generator during a braking operation. 
     The fuel cell  40  is a means for generating electrical power from a supplied fuel gas and a supplied oxidant gas, and has a stack structure in which a plurality of unit cells, each provided with an MEA containing an electrolyte membrane, etc., is stacked in series. Specifically, various types of fuel cells such as polymer electrolyte fuel cells, phosphoric acid fuel cells and molten carbonate fuel cells may be used. 
     A cooling mechanism  70  is a device for cooling the fuel cell  40 , and includes: a pump (not shown) for compressing and circulating a coolant; and a heat exchanger (not shown) for radiating the heat of the coolant to the outside. 
     The fuel cell  40  is provided with: a flow rate sensor  41  for detecting flow rates of gasses to be supplied; and a temperature sensor  43  for detecting a temperature (FC outlet temperature) of the coolant on the fuel cell side. 
     The battery (power storage device)  20  is a dischargeable/chargeable secondary battery constituted from, for example, a nickel hydrogen battery. The battery  20  supplements the output of the fuel cell  40  and supplies a stored energy to the synchronous motor  61 , a vehicle auxiliary apparatus  50 , an FC auxiliary apparatus  51 , etc., when, for example, power generation by the fuel cell  40  stops, while the battery  20  is used as an energy source for starting the system next time. The SOC (State Of Charge) of the battery  20  is detected by an SOC sensor  21  and the detected SOC is managed in a control unit  10 . Note that various type of secondary batteries may be used other than the nickel hydrogen battery. Also, a dischargeable/chargeable power storage device other than the secondary battery, e.g., a capacitor, may be used instead of the battery  20 . This battery  20  is arranged in a discharge path of the fuel cell  40  and connected in parallel to the fuel cell  40 . 
     The fuel cell  40  and the battery  20  are connected in parallel to the inverter  60 , and a circuit between the fuel cell  40  and the inverter  60  is provided with a diode  42  for preventing a current from the battery  20  or a current generated in the synchronous motor  61  from flowing backward. 
     In order to realize a suitable output distribution between the power sources, i.e., the fuel cell  40  and battery  20  which are connected in parallel, the relative voltage difference between the power sources need to be controlled. In order to control such voltage difference, a DC/DC converter  30  is provided between the battery  20  and the inverter  60 . The DC/DC converter  30  is a direct-current voltage converter, which has: a function of adjusting a DC voltage input from the battery  20  and outputting the adjusted DC voltage toward the fuel cell  40 ; and a function of adjusting a DC voltage input from the fuel cell  40  or the motor  61  and outputting the adjusted DC voltage toward the battery  20 . 
     The vehicle auxiliary apparatus  50  and the FC auxiliary apparatus  51  are each connected to the battery  20  and the DC/DC converter  30 , and the battery  20  serves as a power source for these auxiliary apparatuses. The vehicle auxiliary apparatus  50  refers to various types of electrical equipment used for the operation of the vehicle, which may include lighting equipment, an air conditioner, a hydraulic pump, etc. The FC auxiliary apparatus  51  refers to various types of electrical equipment used for the operation of the fuel cell  40 , which may include pumps for supplying a fuel gas and a reformed material, a heater for adjusting the temperature of a reformer, etc. 
     The operation of each of the elements above is controlled by the control unit  10 . The control unit  10  is constituted as a microcomputer including a CPU, a RAM, a ROM, etc. The control unit  10  controls the operations of the fuel cell  40  and DC/DC converter  30  so that electrical power corresponding to required motive power is supplied. The control unit  10  receives various sensor signals input from an accelerator pedal sensor  11 , the SOC sensor  21 , the flow rate sensor  41 , the temperature sensor  43 , an outside-air temperature sensor  44  for detecting an outside-air temperature, and a vehicle speed sensor  62  for detecting a vehicle speed. The control unit  10  centrally controls the system based on these signals, and constantly captures the SOC of the battery  20 . 
     In addition, the control unit  10  is connected to an ignition switch (IG switch)  45 . The control unit  10  detects an on- or off-operation for the IG switch  45  and controls the power generation to be started or stopped in accordance with the detection result. 
     The fuel cell system  100  having the above-described configuration realizes a scavenging control where a moisture state in the fuel cell  40  (i.e., the amount of remaining water) is detected by measuring an impedance of the fuel cell  40 , while the SOC of the battery  20  is detected by the SOC censor  21 , and the moisture state in the fuel cell  40  is maintained to be at a suitable level based on both of these parameters. A scavenging control function according to this embodiment will be described below. 
     Explanation of Scavenging Control Function 
       FIG. 2  is a diagram explaining the scavenging control unction of the control unit  10 . 
     As shown in  FIG. 2 , the control unit  10  includes an impedance calculator  140 , an impedance comparator  150 , a scavenging control section  160  and an SOC comparator  170 . 
     The scavenging control section (scavenging controller)  160  starts the scavenging processing when the IG switch  45  is turned off and a power generation stop command for the fuel cell  40  is received from the IG switch  45 . 
     Specifically, the scavenging processing is performed in order to reduce the water remaining in the fuel cell  40 , pipes (not shown), etc., by supplying an oxidant gas having a low humidity to a cathode in the fuel cell  40  or by supplying .a fuel gas having a low humidity to an anode in the fuel cell  40 . Note, however, that such scavenging processing is merely an example, and any suitable methods may be employed as long as the water remaining in the system can be reduced. 
     Upon the start of the scavenging processing, the impedance calculator (first detector)  140  intermittently measures impedances, and sequentially stores pairs, each consisting of time elapsed from the start of the scavenging processing (hereinafter referred to as a “scavenging time period”) and a measured impedance (such as (t, in) =(t 1 , in 1 ), (t 2 , in 2 ), etc., as shown in  FIG. 3 ) in a measurement memory  152 . 
     The impedance comparator  150  compares an impedance reference value ins (remaining water amount reference value; see  FIG. 3 ) stored in a memory (first storage)  151  with the measured impedances stored in the measurement memory  152  and determines whether or not the amount of water remaining in the system falls below a threshold value. The impedance reference value ins is a reference value which is set so that the amount of water remaining in the system does not decrease too much (i.e., so that the electrolyte membrane is not dried too much), the impedance reference value indicating a threshold value of the remaining water amount, which represents the amount of water required for starting the system next time. The impedance reference value ins can be obtained in advance through experiments, etc. When determining that the amount of water remaining in the system falls below the threshold value based on the comparison result indicating that a measured impedance exceeds the impedance reference value ins, the impedance comparator  150  provides to the scavenging control section  160  a notification that the scavenging processing should be ended. 
     On the other hand, when determining that the amount of water remaining in the system has not fallen below the threshold value yet based on the comparison result indicating that a measured impedance is below the impedance reference value ins, the impedance comparator  150  provides to the SOC comparator  170  a notification that an SOC comparison should be conducted. 
     Upon the start of the scavenging processing, the SOC sensor (second detector)  21  intermittently detects SOCs of the battery  20 , and sequentially stores the detected SOCs of the battery  20  (hereinafter referred to as a “detected SOC”) in a SOC memory  172 . 
     In accordance with the notification from the impedance comparator  150 , the SOC comparator  170  compares an SOC reference value (remaining power amount reference value) stored in a memory (second storage)  171  with the detected SOCs stored in the SOC memory  172  and determines whether or not each detected SOC is below the SOC reference value. The SOC reference value is a remaining power threshold value representing the amount of electrical power required for, after stopping the system, starting the system again next time, and the SOC reference value can be obtained in advance through experiments. When a detected SOC is below the SOC reference value, the SOC comparator  170  provides to the scavenging control section  160  a notification that the scavenging processing should be ended. Note that the SOC reference value may be a fixed value or may alternatively be a value that varies depending on, for example, an outside-air temperature (environmental condition) detected by the outside-air temperature sensor  44 . 
     The scavenging control section (scavenging controller)  160  starts the scavenging processing by receiving the power generation stop command for the fuel cell  40  from the IG switch  45  as described above, while it ends the scavenging processing in accordance with the notifications received from the impedance comparator  150  or from the SOC comparator  170 . Specifically, the control of the scavenging processing is realized by adjusting amount of the oxidant gas or fuel gas to be supplied to the fuel cell  40 , the degree of opening of a bypass valve (not shown), etc. The configuration described above can realize the scavenging control which can maintain a suitable amount of water remaining in the fuel cell system  100 . The scavenging control processing according to this embodiment will be described below. 
     Explanation of Operation 
       FIG. 4  is a flow chart showing the scavenging control processing performed by the control unit  10 . 
     When receiving a power generation stop command for the fuel cell  40  (i.e., an “off” command by the IG switch  45 ) from the IG switch  45 , the scavenging control section  160  starts the scavenging processing with the power generation stop command serving as a trigger (step S 100  to step S 200 ). When the scavenging processing is started by the scavenging control section  160 , the impedance calculator  140  intermittently measures impedances (step S 300 ) and sequentially stores pairs, each consisting of a scavenging time period and a measured impedance ((t, in) =(t 1 , in 1 ), (t 2 , in 2 ), etc., as shown in  FIG. 3 ) in the measurement memory  152 . 
     The impedance comparator  150  compares the impedance reference value ins stored in the memory  151  (see  FIG. 3 ) with the measured impedances stored in the measurement memory  152  and determines whether or not the amount of water remaining in the fuel cell  40  falls below the threshold value (step S 400 ). As described above, the impedance reference value ins indicates the threshold value of the amount of water remaining in the system. When determining that the amount of water remaining in the system falls below the threshold value based on the comparison result indicating that a measured impedance exceeds the impedance reference value ins (step S 400 ; YES), the impedance comparator  150  provides to the scavenging control section  160  a notification that the scavenging processing should be ended (step S 600 ). The scavenging control section  160  ends the scavenging processing by stopping the supply of the oxidant gas and fuel gas, based on the notification from the impedance comparator  150 . 
     On the other hand, when determining that the amount of water remaining in the system has not fallen below the threshold value yet based on the comparison result indicating that a measured impedance is below the impedance reference value ins (step S 400 ; NO), the impedance comparator  150  provides to the SOC comparator  170  a notification that the SOC comparison should be conducted. 
     The SOC comparator  170  compares the SOC reference value stored in the memory  171  with the detected SOCs stored in the SOC memory  172  based on the notification from the impedance comparator  150 , and determines whether or not each detected SOC is below the reference value (step S 500 ). As described above, the SOC reference value indicates a threshold value for reserving the amount of power required, after the system is stopped, for starting the fuel cell  40  next time. When a detected SOC is not below the reference value (step S 500 ; NO), the processing returns to step S 300 , and the SOC comparator  170  provides to the impedance comparator  150  a notification that the impedance comparison should be conducted. 
     On the other hand, when a detected SOC is below the SOC reference value (step  5500 ; YES), the SOC comparator  170  provides to the scavenging control section  160  a notification that the scavenging processing should be ended (step S 600 ). The scavenging control section  160  ends the scavenging processing by stopping the supply of the fuel gas and oxidant gas based on the notification from the impedance comparator  150 . 
     As described above, in this embodiment, whether or not the scavenging processing should be ended is determined based on: the amount of water remaining in the system, which is detected through the measurement of impedances; and the SOC of the battery, which is detected by the SOC sensor, thereby ensuring that the fuel cell system can be started next time and preventing the scavenging processing from being performed for an unnecessarily long time. 
     B. Modifications 
     Modification 1 
     Although the above-described embodiment has not mentioned the operation status of the fuel cell  40  before the IG switch  45  is turned off, the scavenging control may be changed in accordance with the operation status (operation mode) of the fuel cell  40  before the IG switch  45  is turned off. 
       FIG. 5  is a flowchart showing scavenging control processing according to modification 1. Note that the scavenging control processing shown in  FIG. 5  includes steps S 100   a  and S 100   b  in addition to the scavenging control processing shown in  FIG. 4 . Since the other steps are the same as those in  FIG. 4 , a detailed description for those steps will be omitted with corresponding reference numerals being assigned to corresponding steps. 
     When receiving the power generation stop command for the fuel cell  40  (i.e., an “off” command by the IG switch  45 ) from the IG switch  45 , the scavenging control section  160  stops the power generation of the fuel cell  40  based on the command, and checks the operation mode of the fuel cell  40  before the power generation was stopped (step S 100  to step S 100   a ). The fuel cell  40  has two operation modes—a normal operation mode and a low-temperature operation mode. The low-temperature operation mode refers to an operation mode in which a control is performed for the purpose of improving a starting performance in a low-temperature environment (e.g., a control for water content and a control for dryness of the electrolyte membrane), while the normal operation mode refers to the other operation mode different from the low-temperature operation mode. 
     These two operation modes are switched based on an outside-air temperature detected by the outside-air temperature sensor  44 . More specifically, the control unit  10  operates the fuel cell  40  in the normal operation mode when the detected outside-air temperature exceeds a threshold value, while it operates the fuel .cell  40  in the low-temperature operation mode when the outside-air temperature is below the threshold value. Note that the threshold value to be set may be obtained in advance through experiments. Alternatively (or additionally), the operation modes may be switched based on a user&#39;s manipulation of a low-temperature switch (not shown). 
     When determining that the operation mode was set to the normal operation mode in step S 100   a,  the scavenging control section  160  performs normal scavenging processing according to normal starting, rather than starting at a low temperature (step S 100   b ), and then ends the scavenging processing. Note that the normal scavenging processing refers to processing for conducting scavenging for a set time period without taking the SOC of the battery  20  into account. 
     On the other hand, when determining that the operation mode was set to the low-temperature operation mode in step S 100   a,  the scavenging control section  160  performs low-temperature scavenging processing according to starting at a low temperature (step S 200  to step S 500 ), and then ends the scavenging processing (step S 600 ). Note that since the low-temperature scavenging processing has been described in detail in the embodiment above, a further description thereof will be omitted. As described above, the configuration according to modification 1 can realize an optimum scavenging control suitable for each operation mode of the fuel cell  40 . 
     Modification 2 
     Although modification  1  above has not mentioned the SOC of the battery  20 , a control for the SOC of the battery  20  may be changed in accordance with the operation modes of the fuel cell  40 . 
       FIG. 6  is a flowchart showing SOC control processing according to modification 2. The SOC control processing is intermittently performed by the control unit  10  while the fuel cell  40  is being operated. 
     The control unit  10  first checks the operation mode of the fuel cell  40  at the present moment (step S 200 ). When determining that the operation mode is being set to the normal operation mode, the control unit  10  performs a normal-operation SOC control for the battery  20  (step S 220 ), while when determining that the operation mode is being set to the low-temperature operation mode, the control unit  10  performs a low-temperature-operation SOC control for the battery  20  (step S 210 ). 
       FIG. 7  is a diagram showing the relationship between the SOC of the battery and a charge/discharge target power of the battery in each operation mode.  FIG. 8  is a diagram showing an example of a relationship between the SOC of the battery and a discharge power upper limit of the battery in each operation mode. Note that, in  FIGS. 7 and 8 , the graphs showing the normal operation mode are indicated by alternate long and short dash lines, while the graphs showing the low-temperature operation mode are indicated by solid lines. 
     As described in modification  1 , the operation in the low-temperature operation mode is premised on the system being started next time in a low-temperature environment. Accordingly, when the fuel cell  40  is operated in the low-temperature operation mode, the SOC of the battery required for starting the system next time in the low-temperature environment needs to be reserved, and the control value for the SOC of the battery  20  in the low-temperature operation mode is higher than the control value for the SOC of the battery  20  in the normal operation mode as shown in  FIG. 7  (see SOC 1  and SOC 2  in  FIG. 7 ). On the other hand, regarding the discharge power upper limit of the battery  20 , the discharge power upper limit of the battery  20  in the low-temperature operation mode is lower than the discharge power upper limit of the battery  20  in the normal operation mode as shown in  FIG. 8  (see Pb 1  and Pb 2  in  FIG. 8 ). By performing the SOC control for the battery  20  as described above, the fuel cell system can be securely started even in a low-temperature environment. 
     Modification 3 
     In the embodiment above, although the oxidant gas and the fuel gas have been exemplified as gasses to be supplied to the fuel cell during the scavenging processing in the embodiment above, any suitable gasses, which allow for the measurement of impedance (e.g., nitrogen gas), may be employed.