Subzero ambient shutdown purge operating strategy for PEM fuel cell system

A method for purging water from a fuel cell stack at fuel cell system shut down, said method comprising:

• determining a stack water generation request that defines an amount of water generation that is needed from the stack to control the rate of drying of membranes in the stack;
• determining a cathode catalytic heating water generation request that defines an amount and a duration of water generation that is needed from the stack using cathode catalytic heating to purge water from components in an anode balance of plant;
• determining a maximum charge a battery in the fuel cell system can accept;
• determining an ancillary power request for powering components of the fuel cell system during shutdown that defines an amount of ancillary power that is needed to operate components of the fuel cell system during a shut down purge;
• and
• allocating how much of the water generation request is fulfilled by operating the fuel cell stack to charge the battery and to provide the power needed for the ancillary power request such that by-product water is created and further allocating how much of the water generation request is fulfilled by cathode catalytic heating that produces water and heat in a cathode side of the fuel cell stack.

A system for purging water from a fuel cell stack at fuel cell system shut down, said system comprising:

• a compressor for providing cathode air flow to a cathode side of the fuel cell stack;
• a hydrogen source for providing hydrogen to an anode side of the fuel cell stack;
• an anode recirculation loop for recirculating anode exhaust gas to the fuel cell stack;
• a battery for providing current to the fuel cell system, wherein the battery may be charged by the fuel cell stack;
• a bleed valve in the anode recirculation loop for bleeding the anode exhaust gas to a cathode inlet line so that hydrogen is provided to the cathode side of the stack when cathode catalytic heating is desired;
• and
• a controller configured to determine a stack water generation request that defines an amount of water generation that is needed from the stack and a cathode catalytic heating water generation request that defines an amount and a duration of water generation that is needed from the stack using cathode catalytic heating to purge water from components in an anode balance of plant, said controller also configured to determine a maximum charge that the battery can accept and an ancillary power request for powering components of the fuel cell system during shutdown that defines an amount of ancillary power that is needed to operate components of the fuel cell system during a shut down purge, said controller further configured to allocate how much of the water generation request is fulfilled by operating the fuel cell system to charge the battery and to provide the power needed for the ancillary power request such that by-product water is created, and how much of the water generation request will be fulfilled by cathode catalytic heating that produces water and heat in a cathode side of the fuel cell stack, said controller setting a cathode air flow for the purge, monitoring membrane humidity using a water buffer model and/or high frequency resistance measurement during the purge and adapting the amount of water generated and/or the cathode air flow if the membrane humidity is outside of a predetermined range.