Fuel cell system and method for operating the fuel cell system

1. A method for operating a fuel cell system having a fuel cell unit with an anode and a cathode, a media flow path leading to the anode, a media flow path leading to the cathode, an anode exhaust-gas flow path, a cathode exhaust-gas flow path, a fan inserted in the media flow path leading to the cathode for supplying air to the cathode, and a catalytic burner arranged in the cathode exhaust-gas flow path, said method comprising:

introducing an anode exhaust gas into, and mixing the anode exhaust gas with a cathode exhaust gas in, at least one of the catalytic burner and the cathode exhaust-gas flow path upstream of the catalytic burner;

converting the anode exhaust gas and the cathode exhaust gas into a combined catalytically converted fuel cell exhaust flow in the catalytic burner,

passing the combined catalytically converted fuel cell exhaust flow from the catalytic burner into an expansion machine; and

feeding the fuel cell unit with a lower excess of hydrogen in part-load operation than in full-load operation.

2. The method according to claim 1 , wherein the fuel cell unit is fed with a hydrogen excess of at most 10% in part-load operation, and is fed with a hydrogen excess of at least 20% in full-load operation.

3. The method according to claim 1 , wherein one of the cathode exhaust gas and the anode exhaust gas, is heated to a first temperature of between 150° C. and 1100° C. in the catalytic burner.

4. The method according to claim 1 , wherein a fuel is metered to the catalytic burner for combustion of the cathode exhaust gas.

5. The method according to claim 4 , wherein metering of the fuel into the catalytic burner is controlled based on data from a characteristic diagram of a control device, which diagram includes a temperature drop of the cathode exhaust gas across the expansion machine as a function of a pressure ratio of the expansion machine.

6. The method according to claim 5 , wherein the cathode exhaust gas is heated in the catalytic burner to a high temperature at which the expansion machine and the fan interact as a generator, whereby the generator increases the useful power of the fuel cell system.

7. The method according to claim 1 , wherein:

the fuel cell system comprises a plurality of the fuel cell unit arranged into a plurality of fuel cell stacks;

at least two fuel cell stacks are connected in parallel in terms of flow; and

at least one fuel cell stack is arranged in series in terms of flow with the at least two fuel cell stacks connected in parallel.