Preferred embodiments of the invention are shown below.
A hydrogen line A for supplying and discharging hydrogen is connected to the fuel cell stack 1 provided with a temperature sensor 7 and a hydrogen inlet valve 2, a hydrogen exit valve 3, a hydrogen pressure sensor 4, a hydrogen pump 10, a hydrogen purge valve 11, an air introduction valve 12 are provided in the hydrogen line A. In the same manner, an air line B for supplying and discharging air is connected to the fuel cell stack 1 and an air inlet valve 5 and an air pressure sensor 6 are provided in the air line B. A load 8 such as a motor or a battery is connected to the power line C of the fuel cell stack 1 and, further, a small controlling load 9 is connected to the power line C. While heat is generated upon power generation of the fuel cell and a cooling line for cooling the fuel cell is usually mounted, this is not illustrated in this embodiment. Information from the sensors are transmitted to a control apparatus 20 and instructions judged in the control apparatus 20 are transmitted to each of auxiliary equipment to control the operation thereof.
A system starting method by using this embodiment of the fuel cell system is to be described. The switch for the small controlling load 9 connected with the fuel cell stack 1 is turned ON. This is for reducing the voltage in the OCV state by flowing a current to the small controlling load 9 in a state where the reaction gas is supplied to the fuel cell, thereby protecting the cell constituent materials such as the catalyst or the electrolyte.
Then, for supplying the reaction gas to the fuel cell, the hydrogen inlet valve 2 and the hydrogen exit valve 3 are put to an open state and a predetermined amount of hydrogen is supplied to the fuel cell stack 1 from a hydrogen reservoir connected with the hydrogen line A. Substantially at the same time, the air inlet valve 5 on the air line B is opened and a predetermined amount of air is supplied to the fuel cell stack 1 from an air supply blower connected with the air line B. For the hydrogen gas discharged from the fuel cell, a hydrogen gas at a high concentration is discharged. Therefore, the discharged hydrogen gas is boosted by the hydrogen pump 10 and supplied into the fuel cell inlet portion for supplying the discharged hydrogen gas again to the fuel cell stack 1.
In the hydrogen circulating line A, in a case where air is present upon starting, even when hydrogen is supplied, the hydrogen concentration in the hydrogen circulating line A does not reach 100%. Then, the hydrogen purge valve 11 is put to an open state for a short time in a state of supplying hydrogen to drive off air with hydrogen. According to this operation, the hydrogen concentration in the hydrogen line A reaches substantially 100% to complete the preparation for the power generation by the fuel cell.
Then, by operating the load 8, a current is taken out of the fuel cell. In this case, since the current flows also to the small controlling load 9, it is necessary to control such that the current value for the total flowing to the load 8 and the small controlling load 9 is below the maximum setting current for the fuel cell. In a case where the flow of the current to the load 8 can be confirmed and there is no abnormality in the state of the cell and the operation of the auxiliary equipment, the switch for the small controlling load 9 is turned OFF to conduct electric disconnection. Afterward, for conducting power generation from the fuel cell stack 1 in accordance with the amount of power required for the load, the amount of hydrogen supplied, the amount of air supplied, the amount of hydrogen pump operation, the amount of heat dissipation in the cooling line and the operation of each auxiliary equipment is controlled based on the instruction from the control apparatus 20.
In the same manner, a system stopping method by using this embodiment of the fuel cell system is shown below. The small controlling load 9 is turned ON. In this case, since the current flows also to the load 8, the load 8 is previously adjusted such that the maximum setting current of the fuel cell stack 1 is not exceeded. Then, the load 8 is disconnected electrically. The air blower is stopped and the air inlet valve 5 is closed. When it is confirmed that the cell voltage is decreased to 6 V or lower, supply of hydrogen is stopped, the hydrogen inlet valve 2 and the hydrogen exit valve 3 are closed, and the hydrogen pump 10 is stopped. After confirming that the cell voltage is at about 0 V, the small controlling load 9 is turned OFF. The operation of the cooling line is continued or stopped as required. This stopping state is the first stage stopping state. The small controlling load 9 may be left in the ON state as it is. According to this topping method, the stack voltage can be decreased without depressurizing the hydrogen line A and the air line B.
While the details for the principle of voltage decreasing are now under examination, since there is no change in the hydrogen electrode potential, it has been found so far that the potential of the air electrode is decreased. It is considered that since the electrode reaction proceeds in a state not supplying air, water as a reaction product functions as a material for suppressing the reaction between the electrode catalyst and air to decrease the potential of the air electrode.
Then, the starting method from the first stage stopping state is shown below. In a case where the small controlling load 9 is in the OFF state, it is turned to the ON state. In the first stage stopping state, since hydrogen in the hydrogen line is kept at a pressure equal with that in the power generation state and the hydrogen concentration is substantially 100% excluding water, a purging operation for increasing the hydrogen concentration is not necessary. Accordingly, hydrogen is circulated by putting the hydrogen inlet valve 2 and the hydrogen exit valve 3 into the open state, supplying hydrogen to the fuel cell stack 1 and operating the hydrogen pump 10. Substantially at the same time, the air inlet valve 5 is put to an open state and the air blower is operated to supply air to the fuel cell stack 1. Subsequent procedures are identical with those in the usual starting method. Upon initiation of starting, the control apparatus 20 can unify the information such as present stage of the stopping state, cell voltage, elapse of time after stopping, pressure in the cell, and the cell temperature, and judge whether the purge for the gas substitution in the hydrogen line A is conducted or not.
Since this starting method conducts starting from the state where hydrogen remains in the hydrogen line A, operation of increasing the hydrogen concentration in the hydrogen line A is not necessary and it is expected to be advantageous in view of the shortening of the starting time, insurance for safety, improvement of the power generation efficiency, etc.
However, when long time stopping is conducted in the first stage stopping state, the cell may possibly become instable. Since it is considered for this stopping method that water of formation suppresses the electrochemical reaction between the catalyst and the air on the air electrode, it may be a possibility that water is localized with lapse of a long time or decreased due to evaporation to result in decrease of the reaction suppressing effect and abruptly increase the cell voltage. In a case where the cell is left in a state near the OCV voltage, deterioration of the electrode catalyst or the electrolyte material is promoted to damage the cell. Even when the small controlling load 9, etc. is in the ON state, when current flows suddenly, it may be considered, for example, abnormal heat generation in a case where a heater is assumed, or overcharging in a case where a cell is assumed as the small controlling load 9. Accordingly, it is previously programmed such that the state of the cell is recognized by various kinds of information and, in a case where abnormalities are detected, the state is transferred from the first stage stopping state to the second stage stopping state capable of coping with long time stopping.
Operation accompanying the transition from the first stage stopping state to the second stage stopping state is as described below. From the first stage stopping state, all the hydrogen purge valve 11, the air introduction valve 12, the hydrogen inlet valve 2, and the hydrogen exit valve 3 are put to the open state, and the hydrogen pump 10 is operated. Since the hydrogen purge valve 11 is a three-way valve, the gas in the line A boosted by the hydrogen pump 10 is discharged without backflow. On the other hand, for compensating the negative pressure due to operation of the hydrogen pump 10, air out of the system is introduced passing through the air introduction valve 12 into the hydrogen line A, and the hydrogen concentration is decreased. With these operations described above, the potential on the hydrogen electrode is substantially at the same level as the potential on the air electrode and the cell voltage becomes 0 V. Further, in this state, since the atmosphere in the cell does not change even when it is left for a long time, it goes stably.
The transfer of the stopping state is judged by the control apparatus 20. The control apparatus 20 monitors information such as the temperature of the cell, cell voltage, and the gas pressure in the cell and, in a case where the value detected by each of the sensors exceeds a predetermined value, the control apparatus 20 judges that the cell is in an unstable state and conducts a transfer from the first stage stopping state to the second stage stopping state. Further, the control apparatus 20 counts the lapse of time after the first stage stopping state and in a case where it exceeds a predetermined time, it may also conduct the transfer of the stopping state.
In this embodiment, in a case where the voltage of the cell increases to 0.2V or higher per 1 cell, in a case where the cell temperature changes from 50° C. or lower to 50° C. or higher, in a case where the gage pressure in the hydrogen line A goes to 0 kPa or lower, or in a case where one hour or more has been lapsed after transfer to the first stage stopping state in the first stage stopping state, it automatically transfers to the second stage stopping state. This is due to the respective reasons for suppressing degradation of cell members due to high potential, avoiding oxidation reaction of the hydrogen gas due to abnormal heat generation thereby ensuring safety, suppressing degradation of the seal material and electrolyte membrane due to increase in the differential pressure, ensuring the system stopping time, etc.
Then, the starting and stopping method in a case of using a comparative example of fuel cell system in the present invention is shown below. The system constitution is shown in
The starting method in the comparative example is shown below. For supplying a reaction gas to a fuel cell, a predetermined amount of hydrogen is supplied from a hydrogen reservoir connected to a hydrogen line A. Substantially at the same time, a predetermined amount of air is supplied from an air supply blower connected with an air line B to a fuel cell stack 1. For the hydrogen gas discharged from the fuel cell, a hydrogen gas at a high concentration is discharged. Therefore, the discharged hydrogen gas is boosted by the hydrogen pump 10 and supplied into the fuel cell inlet portion for supplying the discharged hydrogen gas again to the fuel cell stack 1.
In the hydrogen circulating line A, in a case where air is present upon starting, even when hydrogen is supplied, the hydrogen concentration in the hydrogen circulating line A does not reach 100%. Then, the hydrogen purge valve 11 is put to an open state for a short time in a state of supplying hydrogen to drive off air with hydrogen. According to this operation, the hydrogen concentration in the hydrogen line A reaches substantially 100% to complete the preparation for the power generation by the fuel cell. Then, by operating the load 8, a current is taken out of the fuel cell. Afterward, for conducting power generation of the fuel cell stack 1 in accordance with the amount of power required for the load, the hydrogen supply amount, the air supply amount, the hydrogen operation amount, the cooling line heat dissipation amount, and the operation of each auxiliary equipment are controlled based on the judging instruction of the control apparatus 20.
Then, the stopping method in a case of using the comparative example of fuel cell system to be described below. The load 8 connected so far with the fuel cell stack 1 is electrically disconnected and the supply of air and hydrogen are stopped. Then, the hydrogen purge valve 11 and the air introduction valve 12 are put to the open state. Then, air outside of the system is introduced into the hydrogen line A to decrease the hydrogen concentration. With the operations described above, the potential on the hydrogen electrode is substantially at the same level as the potential on the air electrode and the cell voltage becomes 0 V. Afterward, the operation of auxiliary equipment is stopped. The operation of the cooling line is continued or stopped as required.
The content of a starting and stopping continuous test conducted in the present embodiment and the comparative example are shown below. Within 2 min after starting from the stopping state, the operation of the fuel cell system is transferred to a rated power generation state and the rated power generation was conducted as it was for 5 min. Afterward, the operation of the fuel cell system was transferred to the stopping state within 2 min, and the stopping state was maintained as it was for 10 min. This was defined as one cycle of starting and stopping operation. The starting and stopping operation was conducted for 300 cycles in total and the amount of hydrogen consumed during the starting and stopping operation was compared. The amount of hydrogen consumed during the rated power generation was calculated based on the amount of power generation and a corresponding amount was decreased previously.
The result of the starting and stopping continuous test is shown in
As described above, In the fuel cell system of the present invention, the stopping method is divided into two stages and, further, a method of stopping while remaining hydrogen as it is in the hydrogen line is adopted in the first stage stopping state, as a result, the amount of discharged hydrogen from the hydrogen line can be decreased greatly upon re-starting. Accordingly, it is possible to improve the power generation efficiency, improve the safety, shorten the starting time, and shorten the stopping time for the fuel cell power generation system.
Number | Date | Country | Kind |
---|---|---|---|
2006-263724 | Sep 2006 | JP | national |