Methods and apparatus for improving the cold starting capability of a fuel cell

Abstract

Apparatus and methods of ceasing operation of an electric power generating system improve the cold starting capability of the system. The system comprises a fuel cell stack connectable to an external circuit for supplying electric current to the external circuit. The stack comprises at least one solid polymer fuel cell, and the system further comprises a fuel passage for directing a fuel stream through the stack and an oxidant passage for directing an oxidant stream through the stack, a sensor assembly connected to the stack for monitoring a parameter indicative of stack performance, a controller for controlling at least one operating parameter of the stack, and a control system communicative with the sensor assembly and operating parameter controller. The method comprises adjusting at least one fuel cell stack operating parameter to cause the stack to operate under a drying condition that causes a net outflux of water from the stack, operating the stack under the drying condition until the water content in the stack has been reduced, and interrupting supply of electric current from the stack to the external circuit.

What is claimed is:  
     
         1 . A method of ceasing operation of an electric power generating system comprising a fuel cell stack connectable to an external circuit for supplying electric current to said external circuit, said stack comprising at least one solid polymer fuel cell, said system further comprising a fuel passage for directing a fuel stream through said stack and an oxidant passage for directing an oxidant stream through said stack, said method comprising in sequential order: 
 (a) adjusting at least one fuel cell stack operating parameter to cause said stack to operate under a drying condition that causes a net outflux of water from said stack;    (b) operating said stack under said drying condition until the water content in said stack has been reduced; and    (c) interrupting supply of electric current from said stack to said external circuit.    
     
     
         2 . The method of  claim 1  wherein said at least one operating parameter is selected from the group consisting of stack temperature, oxidant and fuel stream relative humidities, oxidant and fuel stoichiometries, and oxidant and fuel stream pressures.  
     
     
         3 . The method of  claim 2  wherein prior to cessation of system operation at least one of said oxidant and fuel streams directed to said stack is humidified, and wherein in step (a) the degree of humidification of said at least one of said oxidant and fuel streams is reduced.  
     
     
         4 . The method of  claim 3  wherein said humidification reduction is performed by directing at least some of said at least one of said oxidant and fuel streams to said stack without humidification.  
     
     
         5 . The method of  claim 4  wherein said at least one of said oxidant and fuel relative humidities is reduced by directing at least some of said oxidant and fuel streams to said stack in fluid isolation from said humidifier.  
     
     
         6 . The method of  claim 2  wherein prior to cessation of stack operation, a coolant is circulated through said stack to maintain the temperature of said stack within a desired nominal operating range, and wherein in (a), said stack temperature is increased by stopping coolant circulation through said stack.  
     
     
         7 . The method of  claim 2  wherein in step (a), at least one of said oxidant and fuel stream pressures is decreased.  
     
     
         8 . The method of  claim 2 , wherein in step (a), at least one of said oxidant and fuel stoichiometries is increased.  
     
     
         9 . The method of  claim 1  wherein in step (b), a parameter indicative of stack performance is monitored and said operation of said stack under said drying condition is stopped when said stack performance falls to a threshold value.  
     
     
         10 . The method of  claim 9  wherein the resistance of said stack is monitored, and said operation of said stack under said drying condition is stopped when said resistance exceeds a threshold value.  
     
     
         11 . The method of  claim 10 , wherein said resistance threshold value corresponds to or above a critical membrane moisture level.  
     
     
         12 . The method of  claim 9  wherein the voltage of said stack is monitored, and said operation of said stack under said drying condition is stopped when said voltage falls to or below a threshold value.  
     
     
         13 . The method of  claim 12 , wherein said threshold voltage value corresponds to or above a critical membrane moisture level.  
     
     
         14 . A method of ceasing operation of an electric power generating system comprising a fuel cell stack connectable to an external circuit for supplying electric current to said external circuit, and said stack comprising at least one solid polymer fuel cell, said system further comprising a fuel passage for directing a fuel stream through said stack and an oxidant passage for directing an oxidant stream through said stack, said method comprising in sequential order: 
 (a) interrupting the supply of electric current from said fuel cell stack to said external circuit;    (b) adjusting at least one of stack temperature, oxidant or fuel stream flow rate, or oxidant or fuel stream pressure to cause a drying condition with a net outflux of water from said stack; and    (c) flowing at least one of fuel or oxidant streams through said stack under said drying condition until the water content in said stack has been reduced.    
     
     
         15 . The method of  claim 14  wherein prior to cessation of stack operation, a coolant is circulated through said stack to maintain the temperature of said stack within a desired nominal operating range, and wherein in step (b), said coolant circulation through said stack is stopped.  
     
     
         16 . The method of  claim 14  wherein in step (b), at least one of said oxidant and fuel stream pressures is decreased.  
     
     
         17 . The method of  claim 14 , wherein in step (b), at least one of said oxidant and fuel stream flow rates is increased.  
     
     
         18 . The method of  claim 14  wherein in step (c), a parameter indicative of stack performance is monitored and said flow of oxidant or fuel streams or both through said stack under said drying condition is stopped when said stack performance falls to a threshold value.  
     
     
         19 . The method of  claim 18  wherein the resistance of said stack is monitored, and said flow of oxidant or fuel streams or both through said stack under said drying condition is stopped when said resistance exceeds a threshold value.  
     
     
         20 . The method of  claim 18 , wherein said threshold resistance corresponds to or above a critical membrane moisture level.  
     
     
         21 . The method of  claim 18  wherein the voltage of said stack is monitored, and said flow of oxidant or fuel streams or both through said stack under said drying condition is stopped when said voltage falls below a threshold value.  
     
     
         22 . The method of  claim 21 , wherein said threshold voltage corresponds to or above a critical membrane moisture level.  
     
     
         23 . An electric power generation system comprising: 
 (a) a fuel cell stack connectable to an external circuit for supplying electric current to said external circuit, said stack comprising at least one solid polymer fuel cell and fluid flow passages through said stack;    (b) a sensor assembly connected to said stack for monitoring at least one parameter indicative of stack performance;    (c) a controller for controlling at least one operating parameter of said stack; and    (d) a control system communicative with said sensor assembly and said operating parameter controller, such that upon receipt of a shut down instruction by said control system, said operating parameter controller is operable to adjust at least one stack operating parameter such that said stack operates in a drying condition that causes a net outflux of water from said stack, until the water content in said stack has been reduced.    
     
     
         24 . The electric power generation system of  claim 23  wherein said sensor assembly comprises a sensor for monitoring stack resistance.  
     
     
         25 . The electric power generation system of  claim 23  wherein said sensor assembly comprises a sensor for monitoring stack voltage.  
     
     
         26 . The electric power generation system of  claim 23  wherein said sensor assembly comprises at least one sensor for monitoring said at least one operating parameter.  
     
     
         27 . The electric power generation system of  claim 26  wherein said sensor assembly comprises a sensor for monitoring fuel stream pressure.  
     
     
         28 . The electric power generation system of  claim 26  wherein said sensor assembly comprises a sensor for monitoring oxidant stream pressure.  
     
     
         29 . The electric power generation system of  claim 26  wherein said sensor assembly comprises a sensor for monitoring fuel stream relative humidity.  
     
     
         30 . The electric power generation system of  claim 26  wherein said sensor assembly comprises a sensor for monitoring oxidant stream relative humidity.  
     
     
         31 . The electric power generation system of  claim 26  wherein said sensor assembly comprises a sensor for monitoring stack temperature.  
     
     
         32 . The electric power generation system of  claim 31  wherein said sensor assembly comprises temperature sensors for monitoring coolant inlet and outlet temperatures.  
     
     
         33 . The electric power generation system of  claim 23  wherein said operating parameter controller comprises apparatus to control fuel and oxidant relative humidities, stoichiometries, pressures, and stack temperature.  
     
     
         34 . The electric power generation system of  claim 33  wherein said control system comprises a microcontroller.  
     
     
         35 . The electric power generation system of  claim 33  further comprising a humidifier for humidifying at least one of a fuel or oxidant stream supplied to said stack during normal operation, and wherein said relative humidity control apparatus comprises a humidifier bypass system having at least one bypass conduit for directing one of fuel or oxidant to said stack in fluid isolation from said humidifier.  
     
     
         36 . The electric power generation system of  claim 35  wherein upon receipt of a shut down instruction by said control system, said humidifier bypass system directs at least some of said oxidant or fuel streams through an associated said bypass conduit during a system shut down procedure, and to discontinue transmission through said bypass conduit when the water content in said stack has been reduced.  
     
     
         37 . The electric power generation system of  claim 36  wherein said bypass conduit comprises an inlet end connected to one of said reactant stream passages at a location upstream of said humidifier, and an outlet end connected to the same reactant stream passage at a location downstream of said humidifier.  
     
     
         38 . The electric power generation system of  claim 36  wherein said bypass conduit comprises an inlet end connectable to a reactant supply, and an outlet end connected to one of said reactant stream inlet passages at a location downstream of said humidifier.  
     
     
         39 . The electric power generation system of  claim 36  wherein said humidifier bypass system comprises a bypass inlet valve connected to one of said reactant passages at a location upstream of said humidifier, and a bypass outlet valve connected to the same reactant passage at a location downstream of said humidifier, and wherein said bypass conduit is connected to said bypass inlet and outlet valves.  
     
     
         40 . The electric power generation system of  claim 33  wherein said oxidant stoichiometry control apparatus comprises a compressor connected to said oxidant inlet passage.  
     
     
         41 . The electric power generation system of  claim 33  wherein said pressure control apparatus is a pressure regulator on at least one of an oxidant and fuel passage.  
     
     
         42 . The electric power generation system of  claim 33  wherein said stack temperature control apparatus is a coolant system having a coolant passage through said stack and a coolant pump communicative with said control system.  
     
     
         43 . The electric power generation system of  claim 23  wherein said control system is operable such that said drying operation is discontinued after said operating parameter indicative of stack performance measured by said sensor assembly reaches a threshold value.