METHOD FOR OPERATING A FUEL CELL SYSTEM AND ADJUSTING A RELATIVE HUMIDITY OF A CATHODE OPERATING GAS DURING A HEATING PHASE

1 . A method for operating a fuel cell system during a transient operating phase, wherein the fuel cell system comprises a fuel cell stack having anode chambers and cathode chambers separated by polymer electrolyte membranes, a cathode supply for supplying a cathode operating gas into the cathode chambers and discharging a cathode exhaust gas from the cathode chambers, and a cooling system for controlling a temperature of the fuel cell stack, wherein the method comprises:

determining an inlet temperature of the cathode operating gas at an inlet of the fuel cell stack,

setting a coolant setpoint temperature at the inlet of the fuel cell stack to a value which is equal to or less by a predetermined amount than the inlet temperature of the cathode operating gas, and

controlling the cooling system so that a coolant temperature at the inlet of the fuel cell stack is substantially the same as the coolant setpoint temperature.

2 . The method according to claim 1 , wherein the predetermined amount is less than or equal to 10 K.

3 . The method according to claim 1 , wherein the coolant temperature at the inlet of the fuel cell stack is adjusted by changing a cooling capacity of a cooler of the cooling system, changing a bypass opening of a cooler bypass line of the cooler, or changing a power of a conveyor of the cooling system.

4 . A method for adjusting a relative humidity of a cathode operating gas of a fuel cell system during a transient operating phase, the fuel cell system comprising a fuel cell stack having anode chambers and cathode chambers separated by polymer electrolyte membranes, a cathode supply for supplying a cathode operating gas into the cathode chambers and discharging a cathode exhaust gas from the cathode chambers, and a cooling system for controlling a temperature of the fuel cell stack, wherein the method comprises:

determining an inlet temperature of the cathode operating gas at an inlet of the fuel cell stack,

setting a coolant setpoint temperature at the inlet of the fuel cell stack to a value which is equal to or less by a predetermined amount than the inlet temperature of the cathode operating gas,

controlling the cooling system so that a coolant temperature at the inlet of the fuel cell stack is substantially the same as the coolant setpoint temperature;

setting a setpoint value for a relative humidity of the cathode operating gas at the inlet of the fuel cell stack as a function of the inlet temperature of the cathode operating gas at the inlet of the fuel cell stack, and

controlling the cathode supply so that a relative humidity of the cathode operating gas at the inlet of the fuel cell stack is substantially the same as the setpoint value for the relative humidity.

5 . The method according to claim 4 , wherein the relative humidity of the cathode operating gas at the inlet of the fuel cell stack is adjusted by changing a cathode pressure of the cathode operating gas.

6 . The method according to claim 4 , wherein the relative humidity of the cathode operating gas at the inlet of the fuel cell stack is adjusted by changing an opening of a humidifier bypass line.

7 . The method according to claim 4 , wherein the relative humidity of the cathode operating gas at the inlet of the fuel cell stack is adjusted by changing the inlet temperature of the cathode operating gas at the inlet of the fuel cell stack.

8 . A fuel cell system, comprising:

a fuel cell stack with anode chambers and cathode chambers separated by polymer electrolyte membranes;

a cathode supply for supplying a cathode operating gas into the cathode chambers and discharging a cathode exhaust gas from the cathode chambers;

a cooling system for controlling a temperature of the fuel cell stack to a setpoint temperature; and

a control unit configured to control the fuel cell system, during a transient operating phase of the fuel cell system, to:

determine an inlet temperature of the cathode operating gas at an inlet of the fuel cell stack,

set a coolant setpoint temperature at the inlet of the fuel cell stack to a value which is equal to or less by a predetermined amount than the inlet temperature of the cathode operating gas, and

control the cooling system so that a coolant temperature at the inlet of the fuel cell stack is substantially the same as the coolant setpoint temperature.

9 . The fuel cell system according to claim 8 , wherein the control unit is further configured to control the fuel cell system to:

set a setpoint value for a relative humidity of the cathode operating gas at the inlet of the fuel cell stack as a function of the inlet temperature of the cathode operating gas at the inlet of the fuel cell stack, and

control the cathode supply so that a relative humidity of the cathode operating gas at the inlet of the fuel cell stack is substantially the same as the setpoint value for the relative humidity.

10 . The fuel cell system according to claim 8 , wherein the cathode supply includes a humidifier configured to be flowed through by the cathode operating gas and the cathode exhaust gas such that a water vapor transfer occurs from the cathode exhaust gas to the cathode operating gas.

11 . The fuel cell system according to claim 8 , wherein the fuel cell system is a component of a wheeled vehicle.