Patent ID: 7858258

Claim:
A cascaded fuel cell system comprising: a stack comprising a plurality of individual fuel cells, each of said individual fuel cells comprising a membrane electrode assembly, an anode, an anode flowpath fluidly coupled to said anode, a cathode, and a cathode flowpath fluidly coupled to said said plurality of individual fuel cells defining: a first group comprising a portion of said individual fuel cells, said first group arranged such that: said anode flowpath and said cathode flowpath are in counterflow relationship relative to one another, a concentration of reactants delivered to each of said individual fuel cells therein is substantially equal, a first group anode inlet is fluidly coupled to a fuel source, and a first group cathode inlet is fluidly coupled to a second group cathode outlet; and a second group comprising the remainder of said individual fuel cells, said second group arranged such that: said anode flowpath and said cathode flowpath are in counterflow relationship relative to one another, a concentration of reactant delivered to each of said individual fuel cells is substantially equal, a second group anode inlet is fluidly coupled to a first group anode outlet, and a second group cathode inlet is fluidly coupled to an oxygen source; a fresh reactant supply comprising a supplemental oxygen source fluidly coupled to said cathode flowpath between said first group and said second group; a cooling loop thermally coupled to said stack between a coolant inlet to said stack and a coolant outlet from said stack; a first temperature sensor for measuring a coolant-inlet temperature; a second temperature sensor for measuring a coolant-outlet temperature; a recirculation loop fluidly coupled to said anode flowpath; a recirculation pump fluidly coupled to said recirculation loop; and a controller cooperative with said recirculation pump, said first temperature sensor, and said second temperature sensor, such that said controller determines a relative humidity of a cathode exhaust as a function of said coolant-inlet temperature and said coolant-outlet temperature and, based on a predetermined set-point value for said relative humidity, directs said recirculation pump to cycle at an anode recirculation rate an amount of water-rich anode effluent from the second group anode outlet toward fuel flowing into said first group anode inlet, so as to promote a predetermined amount of membrane electrode assembly hydration.