Systems and methods are provided for using fuel cell staging to reduce or minimize variations in current density when operating molten carbonate fuel cells with elevated CO.sub.2 utilization. The fuel cell staging can mitigate the amount of alternative ion transport that occurs when operating molten carbonate fuel cells under conditions for elevated CO.sub.2 utilization.

A fuel cell system includes a fuel cell stack of a plurality of power generation cells and an impedance measuring device for measuring the impedance in the fuel cell stack. When stopping the operation of the fuel cell system, a method for stopping the operation of the fuel cell system operates the plurality of power generation cells to generate electric power, until the impedance value becomes equal to or greater than an objective impedance value. After the impedance value has become equal to or greater than the objective impedance value, the operation stopping method still continues the power generation of the multiple power generation cells for a given period of time.

A method for accurately measuring the impedance of the fuel cell stack in the vehicle during operation of the vehicle includes determining whether measuring the impedance of the fuel cell stack is requested during driving of the vehicle driven by using a power of a fuel cell stack, switching a DC-DC converter connecting the fuel cell stack and a battery to each other to a buck mode when measuring the impedance is requested, thereby blocking output current of the fuel cell stack from flowing to the battery through the DC-DC converter, determining a first current value of the fuel cell stack for measuring the impedance, controlling a resistance value of a COD variable resistor consuming the output current of the fuel cell stack according to the first current value, and measuring the impedance of the fuel cell stack while the output current of the fuel cell stack is maintained at the first current value.

A fuel cell system includes: a fuel cell stack; a first cooling medium circuit through which a cooling medium for cooling the fuel cell stack flows; an ion exchanger that removes ions in the cooling medium; a second cooling medium circuit in which the average ion concentration of the cooling medium is lower than that of the cooling medium in the first cooling medium circuit; a switching valve that switches between a flow state and a low flow state; a pump configured to cause the cooling medium in the second cooling medium circuit to flow into the first cooling medium circuit; and a control unit that, when a stop period of the fuel cell system is longer than a reference period, drives the pump with the switching valve switched to the flow state after the instruction to start the fuel cell system is input.

In one or more embodiments of the novel aircraft fuel cell system without the use of a buffer battery, the fuel cell and compressor would be sized sufficiently larger for the intended application, allowing the compressor to change speeds much faster. This in turn would allow power outputs to change much quicker. If power outputs can change as quickly as the application dictates, then a buffer battery is not necessary. In one or more embodiments, because the system is mostly electronically controlled, software can be written to protect the fuel cell from instantaneous power pikes. If a large power output is suddenly requested of the fuel cell, the software can smooth out the demand curve to provide an easier load profile to follow.

A method for accurately measuring the impedance of the fuel cell stack in the vehicle during operation of the vehicle includes determining whether measuring the impedance of the fuel cell stack is requested during driving of the vehicle driven by using a power of a fuel cell stack, switching a DC-DC converter connecting the fuel cell stack and a battery to each other to a buck mode when measuring the impedance is requested, thereby blocking output current of the fuel cell stack from flowing to the battery through the DC-DC converter, determining a first current value of the fuel cell stack for measuring the impedance, controlling a resistance value of a COD variable resistor consuming the output current of the fuel cell stack according to the first current value, and measuring the impedance of the fuel cell stack while the output current of the fuel cell stack is maintained at the first current value.

Systems and methods are provided for using fuel cell staging to reduce or minimize variations in current density when operating molten carbonate fuel cells with elevated CO.sub.2 utilization. The fuel cell staging can mitigate the amount of alternative ion transport that occurs when operating molten carbonate fuel cells under conditions for elevated CO.sub.2 utilization.

In a method for regulating a fuel cell stack (1), a current-voltage characteristic of the fuel cell stack is detected and evaluated to determine an operating point of the fuel cell stack, wherein a current-voltage characteristic of the fuel cell stack (1) is detected at time intervals in operation whose gradient has a minimum, a characteristic value (R.sub.min) for the minimum of the gradient is respectively determined from the detected current-voltage characteristic and a desired value for the operating point is determined by addition of a predefined offset value (R.sub.offset) to the characteristic value, and wherein the fuel cell stack (1) is regulated by the desired value determined in this manner.

In one or more embodiments of the novel aircraft fuel cell system without the use of a buffer battery, the fuel cell and compressor would be sized sufficiently larger for the intended application, allowing the compressor to change speeds much faster. This in turn would allow power outputs to change much quicker. If power outputs can change as quickly as the application dictates, then a buffer battery is not necessary. In one or more embodiments, because the system is mostly electronically controlled, software can be written to protect the fuel cell from instantaneous power spikes. If a large power output is suddenly requested of the fuel cell, the software can smooth out the demand curve to provide an easier load profile to follow.

A method of controlling a fuel cell device includes: a step of determining whether impedance between a fuel electrode and an oxidant electrode is greater than a predetermined threshold during a steady operation of the fuel cell device; a step of decreasing a flow rate of a gas circulating via a circulation passage connecting a gas introduction passage of the gas supply unit to a gas discharge passage when the impedance is greater than the predetermined threshold; a step of measuring the voltage between the fuel electrode and the oxidant electrode when the impedance is equal to or less than the predetermined threshold, and determining whether the voltage is equal to or less than a first predetermined threshold; and a step of increasing the flow rate of the gas circulating via the circulation passage when the voltage is equal to or less than the first predetermined threshold.