Method and apparatus for improving the performance of a fuel cell electric power system

Abstract

Current pulsing improves the performance of fuel cells in a fuel cell stack based power system. Voltage clamping limits the voltage peaks that occur after a current pulse. In a hybrid power system, an electric storage device supplies the loads during current pulsing. The electric storage device may sink current to achieve the voltage clamping, and/or power system may employ other the voltage clamping circuits.

1 . A power system, comprising: 
 a power bus;    a fuel cell stack comprising a plurality of serially coupled fuel cells, operable to output a fuel cell stack voltage corresponding to a sum of an output voltage of each of the plurality of serially coupled fuel cells, and electrically coupled in parallel with the power bus;    a pulsing switch operable to selectively provide and remove a first electrical short circuit path across the fuel cells of fuel cell stack;    a controller coupled to control the pulsing switch to current pulse the fuel cell stack from time-to-time; and    a Zener diode having a breakdown voltage, the Zener diode electrically coupled to provide a second electrical short circuit path across the fuel cells of the fuel cell stack at least immediately following the removal of the first electrical short circuit path across the fuel cell stack provided by the pulsing switch when the Zener diode is reversed biased above the breakdown voltage of the Zener diode to limit a positive value of the fuel cell stack voltage below a maximum stack voltage limit of the fuel cell stack.    
   
   
       2 . The power system of  claim 1  wherein the second electrical short circuit path passes through Zener diode.  
   
   
       3 . The power system of  claim 1 , further comprising: 
 a transistor, wherein the Zener diode is electrical coupled to apply a signal to operate the transistor, wherein the second electrical short circuit path passes through transistor.    
   
   
       4 . The power system of  claim 1  wherein the pulsing switch includes at least two metal oxide semiconductor field effect transistors electrically coupled in parallel with each other.  
   
   
       5 . The power system of  claim 1  wherein the controller comprises: 
 an oscillator that operates the pulsing switch to periodically short the fuel cell stack.    
   
   
       6 . The power system of  claim 1  wherein the controller comprises: 
 an oscillator that operates the pulsing switch to periodically electrically couple a load across the fuel cell stack to current pulse the fuel cell stack.    
   
   
       7 . The power system of  claim 1 , further comprising: 
 an electrical storage device electrically coupled in parallel with the fuel cell stack.    
   
   
       8 . The power system of  claim 7  wherein the electrical storage device comprises at least one of a battery and a super-capacitor.  
   
   
       9 . The power system of  claim 7 , further comprising: 
 a stack isolation switch operable to selectively electrically decouple the fuel cell stack from the power bus to discharge the electrical storage device through a load.    
   
   
       10 . The power system of  claim 7  wherein the controller is configured operate the stack isolation switch to electrically decouple the fuel cell stack from the power bus to discharge the electrical storage device through the load before the controller causes the pulsing switch to current pulse the fuel cell stack.  
   
   
       11 . A power system, comprising: 
 a power bus;    a fuel cell stack comprising a plurality of serially coupled fuel cells, operable to output a fuel cell stack voltage corresponding to a sum of an output voltage of each of the plurality of serially coupled fuel cells, and electrically coupled in parallel with the power bus;    a pulsing switch operable to selectively provide and remove a first electrical short circuit path across the fuel cell stack;    a controller coupled to control the pulsing switch to current pulse the fuel cell stack from time-to-time;    an electrical storage device electrically coupled in parallel with the fuel cell stack; and    a Zener diode having a breakdown voltage, the Zener diode electrically coupled in parallel across the fuel cells of the fuel cell stack at least immediately following the removal of the first electrical short circuit path across the fuel cell stack provided by the pulsing switch to provide a second short circuit electrical path through the Zener diode across the fuel cell stack when the Zener diode is reversed biased above the breakdown voltage of the Zener diode to limit a positive value of the fuel cell stack voltage below a maximum stack voltage limit of the fuel cell stack.    
   
   
       12 . The power system of  claim 11  wherein the controller comprises: 
 an oscillator that operates the pulsing switch to periodically short the fuel cell stack.    
   
   
       13 . The power system of  claim 11  wherein the controller comprises: 
 an oscillator that operates the pulsing switch to periodically electrically couple a load across the fuel cell stack to current pulse the fuel cell stack.    
   
   
       14 . The power system of  claim 11  wherein the electrical storage device comprises at least one of a battery and a super-capacitor.  
   
   
       15 . A power system, comprising: 
 a power bus;    a fuel cell stack comprising a plurality of serially coupled fuel cells, operable to output a fuel cell stack voltage corresponding to a sum of an output voltage of each of the plurality of serially coupled fuel cells, and electrically coupled in parallel with the power bus;    a pulsing switch operable to selectively provide and remove a first electrical short circuit path across the fuel cell stack;    a controller coupled to control the pulsing switch to current pulse the fuel cell stack from time-to-time;    an electrical storage device electrically coupled in parallel with the fuel cell stack;    a transistor operable to selectively provide and remove a second electrical short circuit path across the fuel cell stack through the transistor; and    a Zener diode having a breakdown voltage, the Zener diode electrically coupled to provide a signal to the transistor that causes the transistor to provide the second electrical short circuit path across the fuel cells through the transistor at least immediately following the removal of the first electrical short circuit path across the fuel cell stack provided by the pulsing switch when the Zener diode is reversed biased above the breakdown voltage of the Zener diode to limit a positive value of the fuel cell stack voltage below a maximum stack voltage limit of the fuel cell stack.    
   
   
       16 . The power system of  claim 15  wherein the controller comprises: 
 a gate resistor electrically coupled between a cathode of the Zener diode and a gate of the transistor to apply to the signal to the gate of the transistor.    
   
   
       17 . The power system of  claim 15  wherein the controller comprises: 
 an oscillator that operates the pulsing switch to periodically short the fuel cell stack.    
   
   
       18 . The power system of  claim 15  wherein the controller comprises: 
 an oscillator that operates the pulsing switch to periodically electrically couple a load across the fuel cell stack to current pulse the fuel cell stack.    
   
   
       19 . The power system of  claim 15  wherein the electrical storage device comprises at least one of a battery and a super-capacitor.  
   
   
       20 . The power system of  claim 15 , further comprising: 
 a stack isolation switch operable to selectively electrically decouple the fuel cell stack from the power bus to discharge the electrical storage device through a load, wherein the controller is configured operate the stack isolation switch to electrically decouple the fuel cell stack from the power bus to discharge the electrical storage device through the load before the controller causes the pulsing switch to current pulse the fuel cell stack.