1. Field of the Invention
This invention relates to a cell generating system from which electric power of cells generated is derived with a high efficiency.
2. Description of Prior Art
Heretofore a cell generating system as shown in FIG. 1 has been proposed as such type of system as described above. In such a conventional generating system, however, there have been remarkable unbalances among load in respective fuel cells. As a consequence, there has been such a disadvantage in that only such fuel cells possessing similar output characteristics to each other are selected and they must be utilized in parallel connection in order to obtain a required output voltage.
More specifically, FIG. 1 is a connection diagram showing a conventional cell generating system in which natural gas is supplied from a tank 1 for storing the natural gas being a fuel to a hydrogen gas extractor 3 through a pipe 2. On the other hand, water is supplied from a tank 4 for storing the water to the hydrogen gas extractor 3 through a pipe 5. The hydrogen gas extractor 3 extracts hydrogen gas from the natural gas and feeds the resulting hydrogen gas to stacks 6 and 7 of a fuel cell through a valve 111. Besides the hydrogen extractor 3 feeds also the hydrogen gas extracted to stacks 8 and 9 of another fuel cell through a valve 112.
Furthermore air is supplied from an air feeder 12 to the stacks 6-9, inclusive, of these fuel cells through a pipe 13, respectively. Positive and negative wires 14 and 15 are connected with the electrodes of these stacks 6-9, respectively, and a static chopper means 161 for controlling dc voltage is connected to the wire 14.
The chopper means 161 comprises a thyristor 17 the main electrode of which is connected with the wire 14, a capacitor 18, a reactor 19, and a thyristor 20 for controlling commutation being connected in parallel to the thyristor 17 through the capacitor 18 and the reactor 19.
In addition, a smoothing reactor 211 is connected with the wire 14 through the thyristor 17. A converter 22 involving a plurality of thyristors and effecting dc-ac conversion is connected across the reactor 211 and the wire 15. Besides a transformer 24 is connected to the converter 22.
FIG. 2 is a waveform diagram for explaining operation of the generating system shown in FIG. 1. As illustrated in FIG. 1, the stacks 6-9 generate dc electric power by reacting the hydrogen gas supplied from the hydrogen gas extractor 3 with the air fed from the air feeder 12. The resulting dc electric power is supplied to the converter 22 through the chopper means 161 and the reactor 211 thereby converting into ac electric power, and the ac electric power thus converted is supplied to a load (not shown) through the transformer 24.
FIG. 2(a) is a waveform of the ignition signal supplied to a gate of thyristor 17 in the chopper means 161 in which the ignition signal involves pulses each having a period T.
Next, FIG. 2(b) is a waveform of ignition signal in the thyristor 20, and this ignition signal has different phase from that of the thyristor 17, but has the same period T.
When the thyristors 17 and 20 are alternately turned on in accordance with such ignition signals (a) and (b) in FIG. 2, the chopper means 161 generates such output voltage as shown in FIG. 2(c), whilst the reactor 211 feeds direct current as shown in FIG. 2(d) to the converter 22. From the operation as mentioned above, it is apparent that the voltage at the output terminal of the reactor 211 can be controlled to a prescribed value by changing periods T of the ignition signals from the thyristors 17 and 20, respectively.
Meanwhile the curves in which the voltage is plotted against the current density (mA/cm.sup.2) in a fuel cell indicates that the values in the initial state (represented by means of solid line) fall to 90% (dotted line) the original values, in other words, become inferior 90% thereto after the lapse of 5,000 hours as shown in FIG. 3.
Furthermore FIG. 4 is a graphical representation showing output characteristics of the stacks in FIG. 1 in which line A indicates the output characteristics of the stacks 6 and 7, while line B indicates the output characteristics of the stacks 8 and 9. When line A is compared with line B, it is clear that the stacks 8 and 9 have a more remarkable deterioration due to time aging than that of the stacks 6 and 7, and the curve in which the direct current is plotted against the dc voltage in the stacks 8 and 9 is lower than that of the stacks 6 and 7.
If these stacks 6-9 having different output characteristics as described above are connected in series-parallel to each other, i.e., the parallel operation of them is effected whereby the output of dc voltage to be produced is made to be the same V.sub.0, current I.sub.2 flowing through the stacks 6 and 7 becomes at least 150% larger than current I.sub.1 flowing through the stacks 8 and 9. Thus, the generated energy among the stacks 6-9 becomes remarkably unbalanced, and accordingly amounts of the hydrogen gas and air to be supplied to these stacks must be different from each other in response to the above result, so that whose operating efficiency deteriorates remarkably in this case.