The present invention relates to a fuel cell device. More particularly, it relates to a fuel cell device suitable for a fuel cell such as a solid polymer electrolyte fuel cell in which hydrogen is used as the fuel and the air is employed as the oxidant.
Stacking unit cells, in each of which an electrolyte layer is held by being sandwiched between a fuel flow field and an air flow field, forms the main body of the fuel cell. The fuel flow field and the air flow field are supplied with fuel gas and air, respectively. Then, an electrical chemical reaction is caused to occur, thereby generating the electric power. Moreover, the main body of the fuel cell has a characteristic that, if a load current density is increased, activated polarization of an electrode catalyst, the ohmic loss and the concentration polarization bring about a drop in an output voltage from the fuel cell. On account of this, when power output is performed to an external load that consumes the power with the fuel cell as the power supply, the main body of the fuel cell is used as the following system: The use of a DC-DC converter or a DC-AC converter makes it possible to output, as a constant voltage, a direct current power outputted from the fuel cell. Also, the main body of the fuel cell is slow in the response of the output voltage to a variation in the load current density. Because of this, when a sudden change occurs in the external load, the output voltage is temporarily lowered exceedingly, becoming an output voltage smaller than the minimum operation voltage that the external load side requires. This has resulted in a fear that the system itself may come to a halt.
In order to solve this problem, in publications such as JP-A-50-116925, the following system has been proposed: A secondary battery is located in parallel to the fuel cell and, at the time of the sudden change of the external load, the power is supplied from the secondary battery to the external load so that the external load variation on the fuel cell side is reduced.
However, in the system where, as illustrated in FIG.2, the power is supplied from the secondary battery to the external load at the time of the sudden change of the external load, a voltage needed for charging the secondary battery differs from a voltage needed for the load. This has required that the DC-DC converter or the DC-AC converter be equipped with 2 lines of outputs, i.e., an output for the load and an output for charging the secondary battery, thereby bringing about complexities and cost-up of the appliances.
Also, as illustrated in FIG. 3, in the case of a system where the output for charging the secondary battery is branched from the output line for the load, it turns out that the output line for the secondary battery is inputted into the DC-DC converter or the DC-AC converter. At that time, the power to be supplied from the secondary battery to the load is outputted to the load by way of the DC-DC converter or the DC-AC converter. As the result, the power from the secondary battery is multiplied by conversion efficiency of the DC-DC converter or that of the DC-AC converter. This has caused the power loss to occur.