Patent Publication Number: US-2010124682-A1

Title: Fuel cell system

Description:
FIELD OF THE INVENTION 
     The present invention relates to fuel cell systems which can output, via a pair of output terminals, electric energy produced through reaction between oxygen contained in air and hydrogen contained in fuel gas. 
     BACKGROUND OF THE INVENTION 
     To meet a growing demand for global environment protection, fuel cell systems have been in practical use which do not involve discharge of unwanted carbon dioxide gas. One example of such fuel cell systems is disclosed in Japanese Patent Application Laid-Open Publication No. 2004-235093 (hereinafter referred to as “the patent literature”). 
       FIG. 6  hereof is a view explanatory of the basic principles of the fuel cell system disclosed in the patent literature. In the fuel cell system  100  disclosed in the patent literature, hydrogen H 2  contained in fuel gas is supplied to a fuel electrode  102  located to the left of a power generation cell  101  while oxygen O 2  contained in air is supplied to an air electrode  103  located to the right of a power generation cell  101 , so that electric power is supplied to an external apparatus via output terminals  104  and  105  connected to the power generation cell  101 . 
       FIG. 7  hereof is a graph showing relationship between an output electric current and fuel gas pressure of the fuel cell system  100  disclosed in the patent literature, where the horizontal axis represents the fuel gas pressure P and the vertical axis represents the output electric current I. Since the electric power is proportional to the electric current if the electric voltage is assumed to be contact, the following description will be given, replacing the electric power of the fuel cell system  100  with the output electric current I. Because the fuel gas pressure P and the electric power I are in proportional relationship with each other in the fuel cell system  100 , there would arise a need to increase the fuel gas pressure P while monitoring the fuel gas pressure P with a gas pressure sensor  106  of  FIG. 6 . 
       FIG. 8  is a graph showing variation of the fuel gas pressure in the fuel cell system  100 , where the horizontal axis represents time T and the vertical axis represents the fuel gas pressure P. A solid line in the figure is a plot of settings of the fuel gas pressure P, while a broken line in the figure is a plot of actual measurements of the fuel gas pressure P that would increase with a time delay T L  from the settings for various reasons. Namely, because the actual fuel gas pressure is lower by a level P S  than the setting, the output electric current ( FIG. 7 ) too would become short by an amount corresponding to the pressure shortage P S , which would undesirably invite a temporary electric power shortage. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing prior art problems, it is an object of the present invention to provide an improved fuel cell system which can reliably avoid any undesired temporary electric power shortage. 
     In order to accomplish the above-mentioned object, the present invention provides an improved fuel cell system, which comprises: a fuel cell for outputting, via a pair of output terminals, electric energy produced through reaction between oxygen contained in air supplied to an air electrode and hydrogen contained in fuel gas supplied to a fuel electrode; a subsidiary power supply device for supplying electric power to the output terminals from outside the fuel cell; a gas pressure detection section for monitoring a gas pressure with which the fuel gas is supplied to the fuel electrode; and a control section for, when the gas pressure detected by the gas pressure detection section is lower than required gas pressure, performing control to cause electric power, corresponding to a difference between the detected gas pressure and the required gas pressure, to be supplied from the subsidiary power supply device to the output terminals. 
     According to the present invention, the subsidiary power supply device is provided for supplying (subsidiary) electric power, corresponding to a difference between the detected gas pressure and the required gas pressure, to the output terminals from outside the fuel cell. Thus, even when the electric power output from the fuel cell has run short temporarily, the electric power shortage can be supplemented with the electric power supplied from the subsidiary power supply device. In this way, the present invention can reliably avoid any temporary electric power shortage. 
     Further, in the present invention, the subsidiary power supply device is a secondary cell or a capacitor. Because the secondary cell or capacitor can repetitively store and discharge electric energy, the subsidiary power supply device can be used for a longer time than a primary cell. Preferably, the fuel cell system of the invention further comprises a storage section having prestored therein a gas pressure map and an output electric power map, and the control section determines, with reference to the gas pressure map and the output electric power map, the electric power to be supplied from the subsidiary power supply device to the terminals. 
     The following will describe embodiments of the present invention, but it should be appreciated that the present invention is not limited to the described embodiments and various modifications of the invention are possible without departing from the basic principles. The scope of the present invention is therefore to be determined solely by the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain preferred embodiments of the present invention will hereinafter be described in detail, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a block diagram explanatory of the basic principles of a fuel cell system according to an embodiment of the present invention; 
         FIG. 2  is a diagram explanatory of a gas pressure map and an output electric current map employed in the fuel cell system; 
         FIG. 3  is a diagram explanatory of a manner in which an electric current correction amount is calculated on the basis of a difference between a measured gas pressure value and a gas pressure setting; 
         FIG. 4  is a block diagram showing an example detailed hardware setup of the fuel cell system; 
         FIG. 5  is a block diagram showing an example hardware setup of the control section employed in the fuel cell system; 
         FIG. 6  is a view illustrating the basic principles of a conventionally-known fuel cell system; 
         FIG. 7  is a graph showing relationship between an output electric current and fuel gas pressure in the conventionally-known fuel cell system; and 
         FIG. 8  is a graph showing variation of the fuel gas pressure in the conventionally-known fuel cell system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description will be given hereinbelow in relation to a case where a subsidiary power supply device is a capacitor. 
       FIG. 1  is a block diagram explanatory of the basic principles of a fuel cell system  100  according to an embodiment of the present invention. The fuel cell system  10  includes: a fuel cell  12  having a power generation cell  11 ; a fuel gas supply unit  13  connected to the left side of the fuel cell  12  for supplying fuel gas to the fuel cell  12 ; a gas pressure detection section  20  provided in the fuel gas supply unit  13  for detecting or monitoring a supplied pressure of the fuel gas (i.e., a pressure with which the fuel gas is supplied to a fuel electrode  22 ; this pressure will hereinafter be referred to as “fuel gas pressure” or “gas pressure”); and an air supply unit  21  connected to the right side of the fuel cell  12  for supplying air to the fuel cell  12 . 
     The power generation cell  11  includes the fuel electrode  22  which hydrogen H 2  contained in fuel gas supplied via the fuel gas supply unit  13  contacts, and an air electrode  23  which oxygen O 2  contained in air supplied via the air supply unit  21  contacts. By hydrogen  112  contacting the fuel electrode  22  and the oxygen O 2  contacting the air electrode  23 , electric energy can be produced through reaction between the hydrogen H 2  and the oxygen O 2 , and the thus-produced electric energy is provided or output via output terminals  24  and  25  electrically connected to the electrodes  22  and  23 . 
     An unused portion of the supplied fuel gas is returned via a fuel gas returning unit  26 , and an unused portion of the supplied air is returned via an air returning unit  27 . 
     Normally, a combination of a plurality of the power generation cells  11  is used in an actual application; however, it is assumed here that only one such power generation cell  11  is provided, for convenience of description. 
     Further, in the fuel cell system  10 , a capacitor  30 , provided to function as the subsidiary power supply device, is connected to the terminals  24  and  25  via an electric current control  28 . The capacitor  30  is an electric storage device provided for supplying subsidiary power to the terminals  24  and  25  from outside the fuel cell  12 . The capacitor  30  can be used for a longer time than a primary cell because it can repetitively store and discharge electric energy. 
     The capacitor  30  need be replenished with electric energy as necessary. Thus, in the fuel cell system  10 , the capacitor  30  is charged by a power generator  31  that may be a normal power source. 
     The fuel cell system  10  further includes a control section  40  that, when the fuel gas pressure detected by the gas pressure detection section  20  is lower than required fuel gas pressure, controls the electric current control  28  to cause an output electric current, corresponding to a difference between the detected pressure gas and the required gas pressure, to flow from the capacitor  30  to the terminals  24  and  25 , and a storage section  41  connected to the control section  40  and having prestored therein a map indicative of relationship between possible values of the fuel gas pressure and time (i.e., gas pressure map) and a map indicative of relationship between possible values of the output electric current and time (i.e., output electric current map). The fuel cell system  10  further includes a fuel chamber  32 , an air chamber  33 , an electrolyte film  34 , and separators  35  and  36 . 
       FIG. 2  is a diagram explanatory of the gas pressure map and the output electric current map prestored in the storage section  41 . Namely, in the storage section  41  of  FIG. 1  are stored in advance the gas pressure map indicative of the relationship between the gas pressure P and the time T (indicated at (a) of  FIG. 2 ) and the output electric current map indicative of the relationship between the output electric current I and the time T (indicated at (b) of  FIG. 2 ). 
     Once a target electric current value Ia is given, this target electric current value Ia is assigned to the vertical axis of (b) of  FIG. 2 , and then a horizontal line  44  is drawn rightward. Then, once the horizontal line  44  intersects an output current vs. time curve  45  at a point  46 , a vertical line  47  is drawn upward from the point  46 . Then, once the vertical line  47  intersects a gas pressure vs. time curve  48 , a gas pressure value Pb at the point where the vertical line  47  has intersected the curve  48  is determined as a gas pressure setting Pb. Intersecting point between the vertical line  47  and the horizontal axis represents a time point T 1 . 
     Namely, if a gas pressure value Pb is given, a target electric current value Ia can be obtained. In other words, the gas pressure value Pb is necessary to generate the target electric current value Ia. Namely, at the time point T 1 , the gas pressure value is Pb, and the output electric current to be generated is Ia. 
     The control section  40  of  FIG. 1  calculates correlationship among the output current I, time T and gas pressure P, using the maps in the aforementioned manner. The following description will be given, assuming that the above-mentioned gas pressure value Pb is defined as a gas pressure setting. 
     The following describe how an electric current correction amount is calculated on the basis of the above-mentioned gas pressure map and output electric current map. 
       FIG. 3  is a diagram explanatory of a manner in which an electric current correction amount is calculated on the basis of a difference between a measured gas pressure value and the gas pressure setting, and (a) and (b) of  FIG. 3  show the same gas pressure map and output electric current map as shown in (a) and (b) of  FIG. 2 . 
     Namely, in the illustrated example, a gas pressure value Pc is detected or measured at the time point T 1 . The measured gas pressure value Pc is smaller than the gas pressure setting Pb; namely, in this case, there is a pressure difference between the gas pressure setting Pb and the measured gas pressure value Pc (Pb−Pc). Thus, a horizontal line is drawn in (a) of  FIG. 3  as indicated by a leftward arrow ( 1 ), and a vertical line is drawn in to (b) of  FIG. 3  as indicated by a downward arrow ( 2 ). An output electric current value corresponding to the measured gas pressure value Pc is determined to be Ic, and thus, this output electric current value Ic is defined as a calculated electric current value. 
     In (b) of  FIG. 3 , a target electric current value at the time point T 1  is Ia. Further, in this case, an actual output electric current value is Ic that is smaller than the target electric current value Ia, and thus, the output electric current correction amount (Ia-Ic) is determined as subsidiary electric applied which has to be applied to the terminals  24  and  25  from outside the fuel cell  12 . 
     When a gas pressure value detected (measured) by the gas pressure detection section  20  is smaller than the gas pressure setting (i.e., required gas pressure), the capacitor  30  in the fuel cell system  10  of  FIG. 1  is controlled to feed an electric current correction amount, corresponding to a difference between the detected gas pressure value and the gas pressure setting, to the terminals  24  and  25  of the fuel cell  12 . Thus, even when the electric power produced or output from the fuel cell  12  has run short temporarily, the electric power shortage can be supplemented with the electric power supplied from the capacitor  30 . In this way, the instant embodiment of the fuel cell system  10  can reliably avoid any temporary electric power shortage. 
     Whereas the fuel cell system  10  has been outlined above, the following describe the fuel cell system  10  in greater detail. 
       FIG. 4  is a block diagram showing an example detailed hardware setup of the fuel cell system  10 , where similar elements to those in  FIG. 1  are indicated by the same reference numerals and characters as used in  FIG. 1 . The fuel gas supply unit  13  includes: a fuel gas container  52  containing the fuel gas and connected to the fuel cell  12  via a fuel gas supply pipe  51 ; a supplied gas adjusting valve  53  provided in a portion of the fuel gas supply pipe  51  located rightwardly or downstream of the fuel gas container  52  (i.e., closer to the fuel cell  12  than the fuel gas container  52 ) for adjusting a flow rate of the fuel gas; and an ejector  54  provided in a portion of the fuel gas supply pipe  51  located downstream of the supplied gas adjusting valve  53  for supplying the fuel gas to the fuel electrode  22 . 
     The fuel gas returning unit  26  includes a fuel gas returning pipe  42  provided for returning an unused portion of the supplied fuel gas from the fuel chamber  32  to the ejector  54 . 
     The air supply unit  21  includes an air blower  56  connected to the fuel cell  12  via an air supply pipe  55  for supplying air to the air electrode  23 , and an air pressure detection section  57  provided in a portion of the air supply pipe  55  between the air blower  56  and the air chamber  33  for monitoring a supplied pressure of the air (i.e., a pressure with which the air is supplied to the air electrode  23 ). 
     Further, the air returning unit  27  includes an air returning pipe  58 , and an air adjusting valve  59  provided in the air returning pipe  58 . 
     Furthermore, an electric motor  43  is connected to the terminals  24  and  25  via an inverter  61  so that electric power produced by the fuel cell system  10  can be supplied to the electric motor  43 . Reference numeral  62  indicates a diode, and  63  is a target electric current value input section. 
       FIG. 5  is a block diagram showing an example hardware setup of the control section  40  employed in the embodiment of the fuel cell system  10 . The control section  40  includes: a gas pressure setting section  64  connected not only to the target electric current value input section  63  but also to the storage section  41 ; a gas pressure calculation section  65  connected to the gas pressure setting section  64 ; an electric current calculation section  66  connected to the gas pressure calculation section  65 ; and an electric current correction amount calculation section  67  connected to the electric current calculation section  66 . 
     The target electric current value input section  63  is a section via which a human operator inputs a target electric current value Ia (see  FIG. 2 ). The storage section  41  is a section having prestored therein the gas pressure map ((a) of  FIG. 2 ) and the output electric current map ((b) of  FIG. 2 ). 
     The gas pressure setting section  64  is a section that determines a gas pressure setting Pb in accordance with a target electric current value Ia input via the target electric current value input section  63  and on the basis of the gas pressure map, as set forth above in relation to  FIG. 2 . 
     The gas pressure calculation section  65  is a section that determines a difference between the gas pressure setting Pb and a measured gas pressure value Pc (i.e., Pb−Pc) in accordance with the input target electric current value Ia and on the basis of the gas pressure map, as set forth above in relation to (a) of  FIG. 3 . 
     The electric current calculation section  66  is a section that determines a calculated electric current value Ic corresponding to the measured gas pressure value Pc on the basis of the output electric current map, as set forth above in relation to (b) of  FIG. 3 . 
     Further, the electric current correction amount calculation section  67  determines an output electric current correction amount (Ia-IC) on the basis of the output electric current map. 
     Whereas the embodiment of the fuel cell system  100  has been described as determining an electric current correction amount in accordance with a difference between the gas pressure setting and the measured (detected) gas pressure value, an electric voltage correction amount or electric power correction amount may be determined in accordance with a difference between the gas pressure setting and the measured (detected) gas pressure value; in such a case, not only the gas pressure map but also a map indicative of relationship between possible values of the output electric voltage or output electric power are prestored in the storage section  41 . 
     Further, whereas the fuel cell system  100  of the present invention has been described as using a capacitor as the subsidiary power supply device, the subsidiary power supply device may be any other suitable device than a capacitor, such as an ordinary power supply device like a secondary cell, as long as it can supply electric power to the output terminals  24  and  25 . 
     The fuel cell system of the present invention is well suited for application to power generation apparatus.