Patent ID: 7597974

Claim:
A method of operating a fuel cell system to prevent a reaction between a fuel gas and a sweeping gas during a sweeping process, the method comprising the steps of: during a normal power generation period of a fuel cell, passing said fuel gas through a fuel gas flow passage; passing an oxidizer gas through an oxidizer gas flow passage; generating power by a reaction of the fuel gas and the oxidizer gas; supplying the generated power to a load connected to the fuel cell; after receiving a stop request for shutting off said fuel cell system, stopping a supply of the fuel gas to said fuel gas flow passage; keeping the load connected to the fuel cell; continuing an operation of the fuel cell to generate the power after the stop request is received so that the generated power is consumed in the load connected to the fuel cell; determining whether to perform a sweeping for discharging at least water within said fuel gas flow passage, based on an outside temperature measured by a temperature sensor, when receiving said stop request; reducing a pressure within said fuel gas flow passage to a first level lower than a second level when it is determined to perform the sweeping, and to the second level when it is determined not to perform the sweeping; disconnecting the load from the fuel cell if the pressure within the fuel gas flow passage is reduced to the first level when it is determined to perform the sweeping, and to the second level when it is determined not to perform the sweeping; and sweeping said fuel gas flow passage with a sweeping gas after the pressure within said fuel gas flow passage is reduced to the first level to prevent a membrane of the fuel cell from being deteriorated due to a reaction between the fuel gas and the sweeping gas within an anode loop of the fuel cell during the sweeping process so that the fuel cell is protected from the reaction between the fuel gas and the sweeping gas during the sweeping process, wherein the fuel gas is hydrogen and the sweeping gas is the oxidizer gas.