Control unit of fuel cell generating system

The disclosed control unit for a hybrid power generating system including a fuel cell and a storage battery backup includes an auxiliary controller for the fuel cell itself, and an output current controller for controlling the output current drawn from the fuel cell, whereby the storage battery can be charged for recovery within the shortest possible time to reach a target remaining charge capacity under charging conditions that do not cause deterioration of performance of the battery. The system is useful as the power supply in applications subject is sudden load fluctuations such as are encountered in a fork lift truck, for example.

The present invention relates to a control unit of a fuel cell generating 
system employed, for example, for a power supply for an electric car such 
as a forklift truck. 
BACKGROUND OF THE INVENTION 
In general, in a fuel cell generating system formed by combining a fuel 
cell and a fuel reformer, it is difficult to have the output of the fuel 
cell respond quickly with the variation in load when the load is varied 
suddenly under operating conditions of traveling or cargo loading since 
the response speed of the reformer is slow. Such being the case, a fuel 
cell generating hybrid system, in which a storage battery is connected to 
the output side of the fuel cell as a backup for the fuel cell, has been 
used, in which needed electric power not capable of being supplied by the 
fuel cell is discharged from the storage battery under heavy load 
operating conditions, and in which the storage battery is charged for 
recovery with surplus electric power of the fuel cell under light load or 
no-load operation, as proposed by the present applicant under Japanese 
Patent Application No. 121147/87. 
On the other hand, when the storage battery continues to be charged from 
the fuel cell under light load or no-load operation for a long time, the 
storage battery is over-charged. Conversely, when the shortage in electric 
power of the fuel cell is discharged from the storage battery for a long 
time under heavy load operation, the storage battery is over-discharged. 
If such overcharged and overdischarged states are repeated, the life of 
the storage battery is extremely shortened. A control unit of a fuel cell 
generating system, in which the charged quantity of the storage battery is 
monitored, the storage battery is held under a highly charged state so as 
to cope with a heavy load by controlling the output of the fuel cell in 
correspondence with the residual capacity of the storage battery, and the 
stabilization of electric power fed to the load is devised at the same 
time, has been proposed as a countermeasure by the present applicant under 
Japanese Patent Application No. 110634/87. 
Thereupon, it is required to always maintain the storage battery of the 
generating system always under a state of high residual capacity so as to 
cope with a heavy load in case that the load is fluctuated suddenly 
because of cargo handling or travel motion such as in the above-mentioned 
case of a forklift truck. To this end, it is required to finish charging 
the storage battery in shortest possible time within the window of an 
allowable charging condition so as not to shorten the life of the storage 
battery. 
The above-mentioned proposed control systems utilize a basic control system 
that controls and varies only the output of the fuel cell in 
correspondence with the charged quantity of the storage battery. They are 
insufficient to finely control the output of the fuel cell based on the 
charging characteristics of the storage battery to finish charging for 
recovery of the storage battery in the shortest possible time under 
allowable charging conditions, in such a manner that does not cause 
deterioration of performance of the storage battery as described above. 
With reference to FIG. 3, there is shown charging and discharging 
characteristics at a regular time factor of the storage battery with a 
lead storage battery as an object. In regions A and B of said 
characteristics, the battery reactions of charging and discharging are 
performed reversibly, whereas the stable reversible reaction is collapsed 
in the overcharged or over-discharged state shown in region C and an 
irreversible state is produced. It has been known that, if the storage 
battery is used repeatedly in this region, the battery characteristics are 
not only deteriorated, but also the life is shortened sharply. Also, FIG. 
4 shows current to voltage characteristics in the charging of a lead 
storage battery in terms of respective discharged quantities (%), and a 
characteristic line D in the Figure represents a boundary line of the 
allowable charging region. That is, safe charging operation is performed 
with a small charging current below the intersection of characteristic 
lines of respective charging quantities (%) and the boundary line D of the 
charging allowable regions, but deterioration of performance of the 
storage battery progresses rapidly if rapid, which applies a large 
charging current, is repeated in the broken line region which exceeds the 
intersection of said characteristic line. In other words, when a storage 
battery is charged for recovery, the allowable charging current, thus the 
allowable charging voltage, is different in accordance with the state of 
residual capacity from the characteristic aspect of the storage battery. 
In particular, when the light load operating time in the intervals of 
heavy load operation is utilized, as mentioned above and it is required to 
charge the storage battery for recovery in a short time without causing 
deterioration of battery characteristics, fine control of performing 
charging for recovery under charging conditions closest to the very limit 
of the allowable region shown in FIG. 4 is required. 
It is an object of the present invention to provide a control unit of a 
fuel cell generating system which makes it possible to perform fine 
control so as to charge the storage battery for recovery within the 
shortest possible time to reach the target charged quantity under such 
charging conditions that do not cause deterioration of performance of the 
storage battery, and which exhibits excellent effects in a fuel cell 
generating hybrid system used as a power supply in applications subject to 
sudden load fluctuations such as are encountered in a fork lift truck, for 
example. 
SUMMARY OF THE INVENTION 
In order to achieve the above-mentioned object, the control unit of the 
present invention comprises output control means for controlling the 
output of the fuel cell, output current control means for controlling the 
output current drawn out of the fuel cell, means for detecting the storage 
battery voltage, means for detecting the charged quantity of the storage 
battery, and a controller for controlling the output control means and the 
output current control means of the fuel cell by comparing a set value of 
an allowable charging voltage corresponding to each charged state and a 
target value of charged quantity to be always maintained, said set and 
target values being predetermined based on charging characteristics of the 
storage battery with the detected values of the voltage and charged 
quantity of the storage battery obtained from the respective detecting 
means. 
The set value of the allowable charging voltage and the target value of the 
charged quantity are stored in a memory provided in the controller, in 
which the target value of the charged quantity to be maintained is set 
preferably at more than 80%, and the charging control pattern is set 
corresponding to the residual capacity of the storage battery as shown 
with a charging allowable boundary line E in FIG. 2 which is determined on 
the basis of current-voltage characteristics of the storage battery 
described in connection with FIG. 4. Also, the residual capacity of the 
storage battery is obtainable by monitoring the charged and discharged 
quantity of the storage battery through charged quantity detecting means 
and computing with comparison with the rated capacity. 
When electricity is discharged from the storage battery because of heavy 
load operation at the time of operation of the generating system and the 
remaining capacity is lowered to less than 80%, the detected value of the 
charged quantity and the voltage value of the storage battery are compared 
with respective set values stored in the memory of the controller as 
previously described, and the output control means and the output current 
control means are controlled so as to have the output and the output 
current of the fuel cell increase under such a condition that the charging 
voltage of the storage battery, i.e., the output voltage fed to the load 
and the storage battery from the fuel cell, does not exceed the set value 
of the allowable charging voltage of the storage battery, thus charging 
for recovery until the storage battery reaches the target charged 
quantity. Practical control is performed by means of a system that PID 
control is performed after determining the set value at one point over the 
remaining capacity at 80% of the storage battery or 2-position control is 
performed by determining two points over the remaining capacity at 80%. 
In such a manner, by controlling the output of the fuel cell while 
controlling the charging voltage to the set value of the allowable 
charging voltage in accordance with the remaining capacity of the backup 
storage battery, it is possible to charge the storage battery for recovery 
in a short time under such a charging condition that deterioration of 
performance of the storage battery is not caused when the remaining 
capacity of the storage battery is lowered to below the target value and, 
at the same time, to always feed stable electric power to the load 
corresponding to sharp fluctuations of the load.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a block diagram showing a control unit of a fuel cell generating 
system according to an embodiment of the present invention. In FIG. 1, 
reference numeral 1 denotes a raw material tank having reforming raw 
material such as methanol in store, 2 denotes a reformer, 3 denotes a fuel 
cell, 4 denotes a DC--DC convertor as an output current control means of 
the fuel cell 3, 5 denotes a load and 6 denotes a storage battery for 
backup. Also, 7 denotes a raw material transfer pump feeding reforming 
material to the reformer 2 from the raw material tank 1, 8 denotes a 
combustion air blower for supplying combustion air to a burner of the 
reformer 2, 9 denotes a reaction air blower for supplying reaction air 
into the fuel cell 3 and 10 denotes an auxiliary controller as output 
control means which performs output control of the fuel cell by 
controlling the operation of this auxiliary equipment. The fuel cell 
generating system is composed of these equipments. Besides, 
above-mentioned reformer 2 is arranged to obtain reforming reaction heat 
by burning the off-gas discharged from the fuel cell 3, and the electric 
power consumed by the above-mentioned auxiliary equipment and the 
auxiliary controller 10 is fed from the output side of the fuel cell 3. 
For the above-mentioned fuel cell generating system, a control system is 
constructed by output control means of the above-mentioned fuel cell, a 
current detector 11 of the storage battery 6 shown with a symbol 11 
including an output current control means, an ampere hour meter 12 for 
calculating charging and discharging currents of the storage battery 
detected by said current detector 11, a voltage detector 13 of the storage 
battery and a controller 15 which outputs a control command signal I.sub.F 
c to the auxiliary controller 10 and the DC--DC convertor 4 according to 
the result of comparison of signals from the ampere hour meter 12 and the 
voltage detector 13 and a charged quantity target value (a value more than 
the remaining capacity at 80%) of the storage battery stored in a memory 
14 with a set value of the allowable charging voltage given with a control 
pattern shown in FIG. 2. A reference numeral 16 denotes a controller of 
the DC--DC convertor 4, 17 denotes an output current detector of the fuel 
cell 3 for detecting the current on the input side of the DC--DC convertor 
4 and 18 denotes a comparator for comparing the detected signal of said 
current detector 17 with the command signal I.sub.F c from the controller 
15. 
Under such an operating condition that the fuel cell generating system is 
operated and power is fed to the load 5, the detected values of charged 
and discharged quantities and the voltage of the storage battery 6 are 
input to the controller 15 through the current detector 11, the ampere 
hour meter 12 and the voltage detector 13. The controller computes the 
residual capacity of the storage battery 6 and also compares the charged 
target value (a value more than the remaining capacity at 80%) of the 
storage battery stored in the memory 14 with the charging voltage value 
set corresponding to the residual capacity of the storage battery shown 
with the control pattern of the allowable charging boundary line E in FIG. 
2, and the command signal I.sub.F c of the fuel cell output is output from 
the controller 15 based on the results of this comparison. This command 
signal is given on the one side to the auxiliary controller 10, and in the 
auxiliary controller 10, the raw material transfer pump 7, the combustion 
air blower 8 and the reaction blower 9 are controlled in accordance with 
the command signal I.sub.F c, thereby to control the output of the fuel 
cell 3. At the same time, said command signal I.sub.F c is compared with 
the output signal from the fuel cell current detector 17 in the comparator 
18, the DC--DC convertor 4 is controlled through the controller 16 with 
the result of comparison as the control signal and the output current 
thereof is varied so as to vary the output current of the fuel cell fed to 
the side of the load 5 and the storage battery 6 corresponding to the 
command signal I.sub.F c. 
Thus an operating state in which electricity is discharged to replenish 
power to the load 5 from the storage battery 6 corresponding to an 
increase of load is produced. As a result, when the residual capacity is 
lowered below the charging target value, the voltage of the storage 
battery 6 also falls into the range of the allowable charging voltage. 
Accordingly, the controller 15 outputs the command signal I.sub.F c so as 
to increase the output current of the DC--DC convertor 4 under such a 
condition that the set value of the allowable charging voltage 
corresponding to the allowable charging boundary line E in the control 
pattern shown in FIG. 2 is not exceeded. The same command signal I.sub.F c 
is applied to the auxiliary controller 10 before the current of the DC--DC 
convertor is increased so that the output of the fuel cell is increased. 
With this, the output currents of the DC--DC converter is increased with a 
delay in time following the output of the fuel cell. This controlled state 
is continued until the charged quantity of the storage battery 6 reaches 
the target value (a certain value more than the remaining capacity at 
80%), and during this charging process, charging for recovery is performed 
while having the command signal I.sub.F c which is output from the 
controller 15, vary momentarily so that the allowable charging voltage set 
value changes along the allowable charging boundary line E given by the 
control pattern in FIG. 2 with the increase in the charged quantity of the 
storage battery 6. This control operation is performed by a control system 
such as PID control or 2-position control as previously described. When 
the charged quantity of the storage battery 5 is recovered to the target 
value, the controller 15 lowers the output of the fuel cell to the 
original operating state. In the case of no external load, such as at the 
time of interruption of operation of a forklift, the fuel cell is 
controlled to stop after the storage battery is charged. 
As described above, according to the present invention, there is provided a 
control unit which displays excellent results for a fuel cell generating 
system particularly one used for a power supply with sharp load variation, 
such as a fork lift, in such a manner that it is possible to complete 
charging of the storage battery in a short time under charging conditions 
that cause no deterioration of performance of the storage battery, thus 
always holding the storage battery at a high charged quantity enabling it 
to cope with a heavy load without shortening the life of the storage 
battery while also maintaining stable power to the load.