Ambient pressure fuel cell system employing partial air humidification

Abstract

A fuel cell system is provided that is capable of operating at high temperatures and near-ambient pressure with partial humidification of air supplied to the fuel cell stack. The fuel cells of the stack incorporate gas diffusion barrier layers at the cathode side thereof. The system includes a cooling loop for circulating a liquid coolant through the stack. In some embodiments, an incoming air stream is partially humidified with water vapor transferred from a cathode exhaust stream in a gas-exchange humidifier or enthalpy wheel. In other embodiments, a cathode recycle is employed to partially humidify the incoming air. The humidity of the air and cathode exhaust streams is maintained below a stack saturation point. Methods of operating the fuel cell system are also provided.

1 . A fuel cell system comprising: 
 a fuel cell stack comprising a plurality of fuel cells, the fuel cells having a cathode gas diffusion barrier layer;    a fuel system for supplying a fuel to the stack;    a blower for supplying air to the stack at near-ambient pressure;    a humidification device in fluid communication with an air stream supplied to the stack and a cathode exhaust stream exiting the stack for transferring water vapor from the cathode exhaust stream to the air stream; and    a coolant loop for circulating a liquid coolant through the stack.    
     
     
         2 . The fuel cell system of  claim 1  wherein the blower is a variable-speed blower.  
     
     
         3 . The fuel cell system of  claim 1 , further comprising an air filter located upstream of the humidification device and in fluid communication therewith.  
     
     
         4 . The fuel cell system of  claim 1  wherein the humidification device comprises a gas-exchange humidifier.  
     
     
         5 . The fuel cell system of  claim 1  wherein the humidification device comprises an enthalpy wheel.  
     
     
         6 . The fuel cell system of  claim 1  wherein the blower is located downstream of the humidification device.  
     
     
         7 . The fuel cell system of  claim 1 , further comprising a cathode recycle loop for returning at least a portion of the cathode exhaust stream to the fuel cell stack.  
     
     
         8 . The fuel cell system of  claim 7 , further comprising a damper disposed in the cathode recycle loop.  
     
     
         9 . The fuel cell system of  claim 7 , further comprising a recycle blower disposed in the cathode recycle loop.  
     
     
         10 . The fuel cell system of  claim 7  wherein the cathode recycle loop is fluidly connected to the humidification device for supplying a remainder of the cathode exhaust stream thereto.  
     
     
         11 . The fuel cell system of  claim 1  wherein the fuel is substantially pure hydrogen.  
     
     
         12 . The fuel cell system of  claim 11  wherein the fuel supply system is dead-ended.  
     
     
         13 . The fuel cell system of  claim 11  wherein the fuel supply system comprises a hydrogen recycle loop.  
     
     
         14 . The fuel cell system of  claim 1  wherein the coolant loop further comprises a heat exchanger.  
     
     
         15 . The fuel cell system of  claim 14  wherein the heat exchanger comprises a radiator.  
     
     
         16 . The fuel cell system of  claim 1  wherein the coolant is selected from the group consisting of deionized water, ethylene glycol and mixtures thereof.  
     
     
         17 . The fuel cell system of  claim 1  wherein the gas diffusion barrier layer comprises an expanded graphite sheet material.  
     
     
         18 . The fuel cell system of  claim 1  wherein the gas diffusion barrier layer comprises a porous, electrically conductive material having a region filled with a solid, thereby reducing the porosity of the region.  
     
     
         19 . The fuel cell system of  claim 1  wherein the gas diffusion barrier layer comprises a laminate having a first layer interposed between a second layer and a fuel cell membrane, the first layer having a lower permeability to water vapor relative to the second layer.  
     
     
         20 . The fuel cell system of  claim 1  wherein the gas diffusion barrier layer comprises a microporous membrane.  
     
     
         21 . A method of operating a fuel cell system, the system including a fuel cell stack and the fuel cell stack including a plurality of fuel cells having a cathode gas diffusion barrier layer, the method comprising: 
 supplying air to the stack at near-ambient pressure and a stoichiometry greater than  1 ;    supplying a cathode exhaust stream to a humidification device;    maintaining the relative humidity of the air below a stack inlet saturation point;    maintaining the relative humidity of the cathode exhaust stream below a stack outlet saturation point; and    operating the stack at a temperature greater than about 75° C.    
     
     
         22 . The method of  claim 21  wherein the air is supplied to the stack at a pressure of about 20 mbar to about 50 mbar.  
     
     
         23 . The method of  claim 21  wherein the air is supplied to the stack at a stoichiometry of about 1.2 to about 3.0.  
     
     
         24 . The method of  claim 21 , further comprising increasing the air stoichiometry as a power output of the fuel cell system to an external load decreases.  
     
     
         25 . The method of  claim 21 , further comprising decreasing the air stoichiometry as the stack temperature increases.  
     
     
         26 . The method of  claim 21 , further comprising circulating a liquid coolant through the stack.  
     
     
         27 . The method of  claim 26 , further comprising circulating the coolant through a heat exchanger.  
     
     
         28 . The method of  claim 21  wherein the air is supplied to the stack by a blower, the method further comprising: 
 monitoring an operating parameter of the stack indicative of the stack temperature; and  
 varying the speed of the blower in response to the monitored parameter.  
 
     
     
         29 . The method of  claim 21 , further comprising returning at least a portion of the cathode exhaust stream to the stack.  
     
     
         30 . The method of  claim 29 , further comprising: 
 monitoring an operating parameter of the stack indicative of the stack temperature; and    varying the portion of the cathode exhaust stream returned to the stack in response to the monitored parameter.    
     
     
         31 . A fuel cell system comprising: 
 a fuel cell stack comprising a plurality of fuel cells, the fuel cells having a cathode gas diffusion barrier means;    a fuel system for supplying a fuel to the stack;    supply means for supplying air to the stack at near-ambient pressure;    humidification means for transferring water vapor from a cathode exhaust stream exiting the stack with an air stream supplied to the stack; and    a coolant loop for circulating a liquid coolant through the stack.    
     
     
         32 . The fuel cell system of  claim 31 , further comprising: 
 a sensor for measuring an operating parameter indicative of an operating temperature of the stack; and    control means adapted to receive an input from the sensor and control a stoichiometry of the air supplied to the stack by the supply means in response to the input.    
     
     
         33 . The fuel cell system of  claim 31 , further comprising recycling means for returning at least a portion of the cathode exhaust stream to the stack.  
     
     
         34 . The fuel cell system of  claim 33 , further comprising: 
 a sensor for measuring an operating parameter indicative of an operating temperature of the stack; and    control means adapted to receive an input from the sensor and, in response to the input, control at least one of a stoichiometry of the air supplied to the stack by the supply means and the portion of the cathode exhaust stream returned to the stack by the recycling means.