Abstract:
A fuel cell system includes: a fuel cell which generates electricity by supplying hydrogen to an anode electrode, and a reaction gas to a cathode electrode, an anode gas supply device which supplies hydrogen to the anode electrode, and a cathode gas supply device which drives a compressor using electric power generated by the fuel cell, and supplies pressurized reaction gas to the cathode electrode. The fuel cell system further includes a target pressure setting device which sets a target value for cathode pressure of the fuel cell, a correction device which corrects the target value in accordance with atmospheric pressure, and a control device which controls the cathode pressure of the fuel cell to the corrected target value. Efficient electric power generation can be performed in accordance with the ambient environment.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     Priority is claimed on Japanese Patent Application No. 2003-398917, filed Nov. 28, 2003, the contents of which are incorporated herein by reference.  
         [0003]     The present invention relates to a fuel cell system having an anode gas supply device which supplies hydrogen to an anode electrode, and a cathode gas supply device which drives a compressor using electric power generated by the fuel cell, and supplies pressurized reaction gas to the cathode electrode. The present invention also relates to a control method for a fuel cell.  
         [0004]     2. Description of Related Art  
         [0005]     Recently, the development of fuel cell vehicles using electricity generated by a fuel cell is proceeding.  
         [0006]     As this type of fuel cell vehicle, there is one where an anode gas supply device which supplies hydrogen to an anode electrode, and a compressor which supplies reaction gas to a cathode electrode are mounted within a vehicle, and hydrogen is supplied to the anode electrode and reaction gas to the cathode electrode to generate electricity. In practice, since electric power is required to drive the compressor, a part of the electric power generated by the fuel cell is consumed by the compressor. Moreover, since the electric power consumption of the compressor increases accompanying an increase in the electric power generated, it is not always efficient to increase the electric power generated to an unlimited extent. From this point of view, in Japanese Patent Application Unexamined Publication No. 8-45525, a technique is proposed to set the cathode pressure in accordance with the target generated current, so that the electric power generation efficiency of the fuel cell (overall efficiency in consideration of electric power consumption of the compressor) becomes a maximum.  
         [0007]     However, the power generation efficiency of the fuel cell is insufficient if only the target generated current is considered, and it varies in accordance with environmental factors such as ambient temperature and pressure and the like. Therefore, there is the problem that in control with the aforementioned conventional technology, generation of electric power at sufficient efficiency is not possible in some cases.  
       SUMMARY OF THE INVENTION  
       [0008]     It is an object of the present invention to provide a fuel cell system whereby electric power can be generated efficiently in accordance with the ambient environment. It is another object of the present invention to provide a control method for a fuel cell whereby electric power can be generated efficiently in accordance with the ambient environment.  
         [0009]     In order to attain the above object, according to an aspect of the present invention, there is provided a fuel cell system including: a fuel cell having an anode electrode and a cathode electrode, the fuel cell generating electricity by supplying hydrogen to the anode electrode, and a reaction gas to the cathode electrode; an anode gas supply device which supplies hydrogen to the anode electrode; and a cathode gas supply device which drives a compressor using electric power generated by the fuel cell, and supplies the reaction gas pressurized to the cathode electrode, comprising: a target pressure setting device (for example, step S 04  in the embodiment) which sets a target value for cathode pressure of the fuel cell; a correction device (for example, step S 06  in the embodiment) which corrects the target value in accordance with atmospheric pressure; and a control device (for example, step S 12  in the embodiment) which controls the cathode pressure of the fuel cell to the corrected target value.  
         [0010]     According to the thus constructed fuel cell system, since the target value can be corrected in accordance with atmospheric pressure being a cause of variation in the electric power consumption of the compressor, then even when the atmospheric pressure varies, the electric power consumption of the compressor can be suppressed, and a decrease in external output can be prevented.  
         [0011]     Preferably, in the fuel cell system as mentioned above, the correction device decreases the target value when a detected atmospheric pressure decreases.  
         [0012]     According to the thus constructed fuel cell system, since the target value is decreased when the atmospheric pressure decreases, then a drive power equivalent to when the atmospheric pressure does not tend to decrease is required of the compressor, and an increase in the electric power consumption of the compressor can be prevented, and a decrease in external output can be prevented.  
         [0013]     Preferably, the fuel cell system as mentioned above further comprises a back pressure valve located downstream of the cathode electrode, whose opening is feedback-controlled by the control device such that the cathode pressure of the fuel cell matches the corrected target value for the cathode pressure.  
         [0014]     Preferably, the fuel cell system as mentioned above further comprises a regulator located between the anode gas supply device and the anode electrode, the regulator being operated in accordance with a pilot pressure input thereto from downstream of the compressor to regulate the hydrogen in pressure before the hydrogen is supplied to the anode electrode.  
         [0015]     According to another aspect of the present invention, there is provided a fuel cell system including: a fuel cell having an anode electrode and a cathode electrode, the fuel cell generating electricity by supplying hydrogen to the anode electrode, and a reaction gas to the cathode electrode; an anode gas supply device which supplies hydrogen to the anode electrode; and a cathode gas supply device which drives a compressor using electric power generated by the fuel cell, and supplies the reaction gas pressurized to the cathode electrode, comprising: a target pressure setting device which sets a target value for cathode pressure of the fuel cell; a correction device (for example, step S 30  in the embodiment) which corrects the target value in accordance with intake air temperature; and a control device which controls the cathode pressure of the fuel cell to the corrected target value.  
         [0016]     According to the thus constructed fuel cell system, since the target value can be corrected in accordance with the intake air temperature being a cause of variation in the electric power consumption of the compressor, then even when the intake air temperature varies, the electric power consumption of the compressor can be suppressed and a decrease in external output can be prevented.  
         [0017]     Preferably, in the fuel cell system as mentioned above, the correction device decreases the target value when a detected intake air temperature increases.  
         [0018]     According to the thus constructed fuel cell system, since the target value is decreased when the intake air temperature increases, then a drive power equivalent to when the intake air temperature does not increase is required of the compressor, and an increase in the electric power consumption of the compressor can be prevented, and a decrease in external output can be prevented.  
         [0019]     Preferably, the fuel cell system as mentioned above further comprises a back pressure valve located downstream of the cathode electrode, whose opening is feedback-controlled by the control device such that the cathode pressure of the fuel cell matches the corrected target value for the cathode pressure.  
         [0020]     Preferably, the fuel cell system as mentioned above further comprises a regulator located between the anode gas supply device and the anode electrode, the regulator being operated in accordance with a pilot pressure input thereto from downstream of the compressor to regulate the hydrogen in pressure before the hydrogen is supplied to the anode electrode.  
         [0021]     According to yet another aspect of the present invention, there is provided a fuel cell system including: a fuel cell having an anode electrode and a cathode electrode which generates electricity by supplying hydrogen to the anode electrode, and a reaction gas to the cathode electrode; an anode gas supply device which supplies hydrogen to the anode electrode; and a cathode gas supply device which drives a compressor using electric power generated by the fuel cell, and supplies the reaction gas to the cathode electrode, comprising: a target flow setting device which sets a target value for flow of the reaction gas to the cathode electrode; an actual flow detecting device that detects an actual flow of the reaction gas to the cathode electrode; and a control device which controls the compressor such that the actual flow of the reaction gas matches the target value for flow of the reaction gas to the cathode electrode.  
         [0022]     Preferably, the fuel cell system as mentioned above further comprises a back pressure valve located downstream of the cathode electrode, which is feedback-controlled by the control device to control cathode pressure of the fuel cell.  
         [0023]     According to still another aspect of the present invention, there is provided a control method for a fuel cell in which: a fuel cell having an anode electrode and a cathode electrode generates electricity by supplying hydrogen to the anode electrode, and a reaction gas to the cathode electrode; hydrogen is supplied to the anode electrode; and a compressor is driven using electric power generated by the fuel cell to supply the reaction gas pressurized to the cathode electrode, comprising the steps of: setting a target value for cathode pressure of the fuel cell; correcting the target value in accordance with atmospheric pressure; and controlling the cathode pressure of the fuel cell to the corrected target value.  
         [0024]     Preferably, in the control method as mentioned above, the correcting step decreases the target value when a detected atmospheric pressure decreases.  
         [0025]     According to yet another aspect of the present invention, there is provided a control method for a fuel cell in which: a fuel cell having an anode electrode and a cathode electrode generates electricity by supplying hydrogen to the anode electrode, and a reaction gas to the cathode electrode; hydrogen is supplied to the anode electrode; and a compressor is driven using electric power generated by the fuel cell to supply the reaction gas pressurized to the cathode electrode, comprising the steps of: setting a target value for cathode pressure of the fuel cell; correcting the target value in accordance with intake air temperature; and controlling the cathode pressure of the fuel cell to the corrected target value.  
         [0026]     Preferably, in the control method as mentioned above, the correcting step decreases the target value when a detected intake air temperature increases. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]      FIG. 1  is a block diagram of a fuel cell system in a first embodiment of the present invention.  
         [0028]      FIG. 2  is a flowchart for pressure control processing in the fuel cell system of  FIG. 1 .  
         [0029]      FIG. 3  is a flowchart for flow control processing in the fuel cell system of  FIG. 1 .  
         [0030]      FIG. 4  is a flowchart for pressure control processing in the fuel cell system of  FIG. 1 , in accordance with another embodiment of the present invention.  
         [0031]      FIG. 5  is a graph showing a relationship between cathode gas pressure, and electric power generated by a fuel cell, and electric power consumption of a compressor.  
         [0032]      FIG. 6  is a graph showing a relationship between target generated current and target pressure.  
         [0033]      FIG. 7  is a graph showing a relationship between atmospheric pressure and correction coefficient.  
         [0034]      FIG. 8  is a graph showing a relationship between intake air temperature and correction coefficient.  
         [0035]      FIG. 9  is a schematic diagram showing a relationship between height above sea level and generated electric power in the fuel cell system of  FIG. 1 .  
         [0036]      FIG. 10  is a schematic diagram showing a relationship between height above sea level and generated electric power in a conventional fuel cell system. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0037]     Hereunder is a description of a fuel cell system according to the present invention, with reference to the drawings.  FIG. 1  is a block diagram of the fuel cell system in a first embodiment of the present invention.  
         [0038]     A fuel cell (FC)  1  comprises cells of solid polymer electrolyte membrane formed from a solid polymer ion exchange membrane or the like, sandwiched between an anode electrode  2  and a cathode electrode  3 , with a plurality of layers of cells each sandwiched between separators ( FIG. 1  is a simplified diagram and therefore only a single cell is shown). When hydrogen gas is supplied to the anode electrode  2  as fuel gas, and air including oxygen is supplied to the cathode electrode  3  as reaction gas, hydrogen ions generated by the catalytic reaction at the anode electrode  2  pass through the solid polymer electrolyte membrane and migrate to the cathode electrode  3 , giving rise to an electrochemical reaction with oxygen at the cathode electrode  3 , and generating electricity and water.  
         [0039]     Air is pressurized to a predetermined pressure by a compressor  10  such as a supercharger (S/C) or the like, then supplied to the cathode electrode  3  of the fuel cell  1  from an air supply flow path  11 , and discharged as air off-gas from the fuel cell  1  from an air off-gas flow path  12  via a back pressure valve  13 .  
         [0040]     On the other hand, the hydrogen gas supplied from an anode gas supply system  4  having a high-pressure hydrogen tank (H 2 ) is decreased in pressure to a predetermined pressure by a regulator  5  provided midway along a hydrogen gas supply flow path (fuel supply flow path)  6 , and supplied to the anode electrode  2  of the fuel cell  1 . The hydrogen gas supplied to the fuel cell  1  is employed in generation of electricity, and discharged as hydrogen off-gas to a hydrogen off-gas circulation flow path (circulation flow path)  7  from the fuel cell  1 .  
         [0041]     The hydrogen off-gas circulation flow path  7  is connected to the hydrogen gas supply flow path  6  downstream of the regulator  5  via an ejector  8 . Thus, the hydrogen off-gas discharged from the fuel cell  1  is merged with the hydrogen gas supply flow path  6  via the ejector  8 , and thus the hydrogen off-gas is mixed with fresh hydrogen gas supplied from the anode gas supply system  4 , and supplied again to the anode electrode  2  of the fuel cell  1 .  
         [0042]     Here, a branch flow path  16  is provided in the hydrogen off-gas circulation flow path  7 , and a discharge valve  15  is provided in this branch flow path  16 .  
         [0043]     Furthermore, a branch flow path  18  branched from the air supply flow path  11  downstream of the compressor  10  is connected to the regulator  5  via an orifice  19 , and the regulator  5  is operated in accordance with a pilot pressure input from the branch flow path  18 .  
         [0044]     Moreover, the anode gas supply system  4 , the compressor  10 , and the back pressure valve  13  are each connected to a controller (ECU)  14 . This controller  14  computes the electric power required for operation of the load, and sends control signals to the anode gas supply system  4  and the compressor  10  based on the computed electric power. Thus, the amount of reaction gases supplied from the anode gas supply system  4  and the compressor  10  is adjusted, and the amount of electricity generated in the fuel cell  1  controlled.  
         [0045]     Furthermore, the controller  14  is connected to an accelerator pedal opening sensor  21 , an atmospheric pressure sensor  22 , an airflow sensor  23  which detects the amount of airflow supplied to the cathode electrode  3  from the compressor  10 , an air pressure sensor  24  which detects the air pressure, and an intake air temperature sensor  32 , and control is conducted in accordance with the detected values detected by these sensors  21  through  24 , and  32 . Hereunder is a description of this control.  
         [0046]      FIG. 2  is a flowchart for control of the pressure of the reaction gas supplied to the fuel cell  1 , in the fuel cell system. Firstly in step S 02 , a target generated current for the fuel cell  1  is computed in accordance with the accelerator pedal opening detected by the accelerator pedal opening sensor  21 . In step S 04 , the target pressure is set so that maximum efficiency is reached at the target generated current. This target pressure is set using  FIG. 6 .  FIG. 6  is a graph showing the relationship between target generated current and target pressure in the standard state (state wherein the atmospheric pressure value is the reference pressure value, and intake air temperature value is the reference temperature value). Here, the inlet pressure of the cathode electrode  3  is set as the standard pressure, however, the outlet pressure may also be set.  
         [0047]     In step S 06 , the atmospheric pressure PO is detected by the atmospheric pressure sensor  22 , and a correction coefficient is computed in accordance with this atmospheric pressure PO. This correction coefficient is computed using  FIG. 7 .  FIG. 7  is a graph showing the relationship between atmospheric pressure and the correction coefficient. In the present embodiment, the pressure value at a predetermined height above sea level (for example, 0 m above sea level) is set as the reference pressure value, and the correction coefficient at this reference value is set to 1.0. If the atmospheric pressure detected by the atmospheric pressure sensor  22  is greater than the reference pressure value, the correction coefficient is increased, and if the atmospheric pressure detected by the atmospheric pressure sensor  22  is less than the reference pressure value, the correction coefficient is decreased.  
         [0048]     In step S 08 , the target pressure is corrected in accordance with the correction coefficient. This correction is conducted by multiplying the target pressure value set in step S 04  by the correction coefficient.  
         [0049]     In step S 10 , the inlet pressure (actual pressure) of the cathode electrode  3  is detected by the atmospheric pressure sensor  24 . In step S 12 , the opening of the back pressure valve  13  is feedback-controlled so that the target pressure corrected in step S 08  matches the actual pressure, and the processing for this flowchart is completed.  
         [0050]      FIG. 3  is a flowchart for control of the flow of reaction gas supplied to the fuel cell  1 .  
         [0051]     In step S 22 , the target generated current for the fuel cell  1  is computed in accordance with the accelerator opening detected by the accelerator pedal opening sensor  21 . In step S 04  the target airflow is set in accordance with the target generated current. Here, the inlet flow of the cathode electrode  3  is set as the target airflow, however the outlet flow may also be set. In step S 26 , the inlet flow (actual flow) of the cathode electrode  3  is detected by the airflow sensor  23 . In step S 28 , the rotating speed of the compressor  10  is feedback-controlled so that the actual flow matches the target airflow, and the processing for this flowchart is completed.  
         [0052]     In this way, controlling the airflow for the cathode electrode  3  by the rotating speed of the compressor  10 , and on the other hand independently controlling the air pressure of the cathode electrode  3  by the back pressure valve  13 , is desirable from the point of enabling accurate control of the respective amounts. The compressor  10  and the back pressure valve  13  can be used together for control of the airflow and the air pressure.  
         [0053]      FIG. 5  is a graph showing the relationship between the cathode gas pressure and electric power generated by the fuel cell  1 , and the electric power consumption of the compressor. In this figure, line L 1  represents the electric power generated by the fuel cell  1 , line L 2  represents the electric power consumption of the compressor  10  at the target pressure without the correction shown in steps S 06  and S 08 , and line L 3  represents the electric power consumption of the compressor  10  at the corrected target pressure. Moreover, PA_C is a pressure value approximately the same as the reference pressure value, and PA_A (equal to PA_B) is a pressure value a certain amount less than the reference pressure value. As shown in the same figure, when the atmospheric pressure is comparatively close to the reference pressure value (PA_C), the electric power consumption of the compressor  10  is approximately equal irrespective of whether or not correction is used, and the external output becomes approximately equal. However, when the atmospheric pressure is less than the reference pressure value (PA_A, PA_B), unless the target pressure is corrected, the electric power consumption of the compressor  10  increases, and the electric power extracted from the fuel cell  1  minus the compressor  10  consumption decreases significantly. Conversely, when the target pressure is corrected, the increase in electric power consumption of the compressor  10  can be suppressed, and the decrease in external output can be suppressed.  
         [0054]     Furthermore, the difference between effects of the fuel cell system of the present embodiment and the conventional fuel cell system is explained using  FIG. 9  and  FIG. 10 . As shown in  FIG. 10 , in the conventional fuel cell system, since the electric power consumption of the compressor  10  is low at a low height above sea level (height above sea level: low), an external output (load output) comparatively similar to that required can be obtained. However, even when height above sea level increases to a certain extent (height above sea level: medium), the fuel cell system attempts to maintain the inlet pressure of the cathode electrode  3  at a high state similar to that at a low height above sea level, and the generated electric power output therefore remains approximately unchanged. However, since the amount of compressor  10  work increases due to the decrease in pressure, the electric power consumption of the compressor  10  increases. Therefore the external output decreases significantly as a result. When the height above sea level increases further (height above sea level: high), the performance limit of the compressor  10  is reached and the generated electric power decreases, while on the other hand, the electric power consumption of the compressor  10  increases further, so that the external output decreases further.  
         [0055]     Conversely, with the fuel cell system of the present embodiment, since the electric power consumption of the compressor  10  is low at a low height above sea level (height above sea level: low), a high external output can be obtained. Moreover, even when the height above sea level increases to a certain extent (height above sea level: medium), correction is applied to decrease the cathode electrode  3  inlet pressure, and the generated electric power decreases, while the increase in the amount of work of the compressor  10  can be suppressed. Therefore, the overall electricity generation efficiency can be maintained in a high state similar to that wherein the height above sea level is low. Furthermore, when the height above sea level increases further (height above sea level: high), even if the performance limit of the compressor  10  is reached, since correction is performed to further decrease the inlet pressure of the cathode electrode  3 , the increase in the amount of work of the compressor  10  can be suppressed, and electricity generation efficiency can be increased beyond the conventional case.  
         [0056]     Next, the fuel cell system in a second embodiment of the present invention is described using  FIG. 1 . The present embodiment differs from the first embodiment in the use of an intake air temperature sensor  32  in place of the atmospheric pressure sensor  22 , and control is in accordance with the intake air temperature TA. This is described using  FIG. 4 .  FIG. 4  is a flowchart for pressure control processing in the fuel cell system of  FIG. 1 . As shown in this figure, in this flowchart, in place of the processing in step S 06  in  FIG. 2 , the intake air temperature TA is detected by the intake air temperature sensor  32  and the correction coefficient is computed in accordance with the intake air temperature TA. This correction coefficient is computed using  FIG. 8 .  FIG. 8  is a graph showing the relationship between intake air temperature and correction coefficient. In the present embodiment, the intake air temperature value at a predetermined height above sea level (for example, 0 m above sea level) is set as the reference temperature value, and the correction coefficient at this reference temperature value is set to 1.0. Then when the intake air temperature detected by the intake air temperature sensor  32  is greater than the reference temperature value, the correction coefficient is decreased, and when the intake air temperature detected by the intake air temperature sensor  32  is less than the reference temperature value, the correction coefficient is increased. Since the volume of air in the compressor  10  also increases when the intake air temperature is greater than the reference temperature value, then when the fuel cell system attempts to maintain the pressure of the cathode electrode  3  to that for when the intake pressure is the same as the reference value, the electric power consumption of the compressor  10  increases. Therefore, by correcting as in step S 32 , the increase in electric power consumption of the compressor  10  can be suppressed.  
         [0057]     In this manner, in the present embodiment, even if the intake air temperature varies, the electric power consumption of the compressor  10  can be suppressed, and a decrease in the external output can be prevented, and electric power can therefore be generated efficiently even if the ambient temperature environment varies.  
         [0058]     According to the present invention, even when the atmospheric pressure varies, the electric power consumption of the compressor can be suppressed, and a decrease in external output can be prevented. Therefore efficient electric power generation can be performed even if the ambient pressure environment varies.  
         [0059]     According to the present invention, efficient electric power generation can be performed even if the atmospheric pressure decreases.  
         [0060]     According to the present invention, even when the intake air temperature varies, the electric power consumption of the compressor can be suppressed, and a decrease in external output can be prevented. Therefore efficient electric power generation can be performed even if the ambient temperature environment varies.  
         [0061]     According to the present invention, efficient electric power generation can be performed even if the intake air temperature increases.  
         [0062]     The details of the present invention are naturally not limited to the embodiments. For example, in the embodiments, a fuel cell system as mounted in a vehicle has been explained, however, this is not limited to a vehicle. Moreover, in the embodiments, the reference pressure value and the reference temperature value are set, and if the detected atmospheric pressure is less than the reference pressure value, or the detected intake air temperature is greater than the reference temperature value, the cathode pressure target value is controlled to decrease. However, the reference pressure value and the reference temperature value need not be set. That is to say, when the detected atmospheric pressure target value tends to decrease, or when the detected intake air temperature tends to increase, the cathode pressure target value may be controlled to decrease. Furthermore, the control in the first embodiment and the control in the second embodiment may be used together.  
         [0063]     While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, omissions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description, and is only limited by the scope of the appended claims.