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 method of operating a power system comprising a stack of fuel cells, the method comprising:
 from time-to-time,
 current pulsing the stack of fuel cells; and 
 clamping a fuel cell voltage below a maximum fuel cell voltage level during at least a period after current pulsing the stack of fuel cells. 
   
   
   
       2 . The method of  claim 1  wherein current pulsing the stack of fuel cells comprises:
 providing a short circuit path across the stack of fuel cells; and   removing the short circuit path across the stack of fuel cells.   
   
   
       3 . The method of  claim 1  wherein current pulsing the stack of fuel cells comprises:
 electrically coupling a load across the stack of fuel cells; and   electrically uncoupling the load from across the stack of fuel cells.   
   
   
       4 . The method of  claim 1  wherein current pulsing the stack of fuel cells comprises operating at least one switch electrically coupled across a power bus of the stack of fuel cells to short the stack of fuel cells on a periodic basis. 
   
   
       5 . The method of  claim 1  wherein current pulsing the stack of fuel cells comprises operating at least one switch electrically coupled across a power bus of the stack of fuel cells to provide a short circuit path across the stack of fuel cells approximately once every minute. 
   
   
       6 . The method of  claim 1  wherein current pulsing the stack of fuel cells comprises operating at least one switch electrically coupled across a power bus of the stack of fuel cells to provide a short circuit path across the stack of fuel cells approximately once every minute, and to remove the short circuit path approximately 500 milliseconds after providing the short circuit path across the stack of fuel cells. 
   
   
       7 . The method of  claim 1 , further comprising:
 from time-to-time, determining a voltage across the stack of fuel cells, and wherein current pulsing the stack of fuel cells comprises operating at least one switch electrically coupled across a power bus of the stack of fuel cells to provide a short circuit path across the stack of fuel cells when the determined voltage across the stack of fuel cells exceeds a threshold voltage.   
   
   
       8 . The method of  claim 1 , further comprising:
 from time-to-time, determining an actual charge state of an electrical storage device electrically coupled to the stack of fuel cells; and   determining a frequency of operation of at least one switch electrically coupled across a power bus of the stack of fuel cells to maintain a desired charge state of the electrical storage device based on the determined charge state; and wherein current pulsing the stack of fuel cells comprises operating the at least one switch electrically coupled across the output of the bus of the stack of fuel cells at the determined frequency to provide and to remove a short circuit path across the stack of fuel cells at the determined frequency.   
   
   
       9 . The method of  claim 1 , further comprising:
 determining a load condition; and   determining a frequency of operation of at least one switch electrically coupled across a power bus of the stack of fuel cells based on the determined load condition, and wherein current pulsing the stack of fuel cells comprises operating the at least one switch electrically coupled across the output of the bus of the stack of fuel cells at the determined frequency to provide and to remove a short circuit path across the stack of fuel cells at the determined frequency.   
   
   
       10 . The method of  claim 1  wherein clamping a fuel cell voltage below a maximum fuel cell voltage level comprises supplying power to the load from the electrical storage device before current pulsing the stack of fuel cells, and recharging the electrical storage device from the fuel cell stack for a period after current pulsing the stack of fuel cells. 
   
   
       11 . The method of  claim 1 , further comprising:
 clamping the fuel cell voltage above a minimum fuel cell voltage level.   
   
   
       12 . A method of operating a power system to power at least one load, the power system comprising a fuel cell stack and at least one electrical storage device electrically coupled in parallel with the fuel cell stack, the method comprising:
 temporarily shorting the fuel cell stack from time-to-time; and   supplying power from the electrical storage device to the load at least while shorting the fuel cell stack.   
   
   
       13 . The method of  claim 12 , further comprising:
 clamping a voltage on the fuel cells below a maximum fuel cell stack voltage level.   
   
   
       14 . The method of  claim 12 , further comprising:
 supplying current from the electrical storage device to the load for a period prior to shorting the fuel cell stack; and   supplying current from the fuel cell stack to the electrical storage device after the shorting of the fuel cell stack to clamp a fuel cell stack voltage below a maximum fuel cell stack voltage level.   
   
   
       15 . The method of  claim 12 , further comprising:
 reducing an amount of current supplied from the fuel cell stack to the load for a period prior to shorting the fuel cell stack;   supplying an amount of current from the electrical storage device to the load for a period prior to shorting the fuel cell stack; and   supplying current from the fuel cell stack to the electrical storage device after the shorting of the fuel cell stack to clamp a fuel cell stack voltage below a maximum fuel cell stack voltage level.   
   
   
       16 . The method of  claim 12 , further comprising:
 determining a load condition from time-to-time; and   ceasing the temporarily shorting of the fuel cell stack until the determined load condition exceeds a threshold load condition.   
   
   
       17 . The method of  claim 12 , further comprising:
 from time-to-time, determining when a stack voltage falls below a preset voltage limit; and wherein temporarily shorting the fuel cell stack from time-to-time comprises temporarily shorting the fuel cell stack in response to determining that the stack voltage has fallen below the preset voltage limit.   
   
   
       18 . The method of  claim 12 , further comprising:
 from time-to-time, determining when a stack voltage falls below a float voltage of the electrical storage device; and wherein temporarily shorting the fuel cell stack from time-to-time comprises temporarily shorting the fuel cell stack in response to determining that the stack voltage has fallen below the float voltage of the electrical storage device.   
   
   
       19 . The method of  claim 12 , further comprising:
 from time-to-time, determining when a stack voltage falls below a preset voltage limit; and wherein temporarily shorting the fuel cell stack from time-to-time comprises temporarily shorting the fuel cell stack in response to determining that the stack voltage has fallen below the preset voltage limit; and   determining a measure of pollutants based on a frequency of the shorting of the fuel cell stack.   
   
   
       20 . A method of operating a power system comprising a stack of fuel cells and an electrical storage device electrically couplable to supply power to a load, the method comprising:
 from time-to-time,
 operating at least one switch to provide an electrical short circuit across the stack of fuel cells; 
 operating the at least one switch to remove the electrical short circuit across the stack of fuel cells; 
 supplying power from the electrical storage device to the load at least while shorting the fuel cell stack; and 
 clamping a fuel cell voltage below a maximum fuel cell voltage level during at least a period after removing the short circuit path across the stack of fuel cells. 
   
   
   
       21 . The method of  claim 20  wherein from time-to-time is a periodic basis. 
   
   
       22 . The method of  claim 20  wherein operating the at least one switch to remove the electrical short circuit across the stack of fuel cells occurs approximately 500 milliseconds after operating the at least one switch to provide the electrical short circuit across the stack of fuel cells. 
   
   
       23 . The method of  claim 20 , further comprising:
 from time-to-time, determining a voltage across the stack of fuel cells, and wherein from time-to-time, operating at least one switch to provide an electrical short circuit across the stack of fuel cells comprises operating the at least one switch to provide the electrical short circuit when the determined voltage across the stack of fuel cells exceeds a threshold voltage.   
   
   
       24 . The method of  claim 20  wherein clamping a fuel cell voltage below a maximum fuel cell voltage level comprises supplying power to the load from the electrical storage device before providing the short circuit path across the stack of fuel cells, and recharging the electrical storage device from the fuel cell stack for a period after removing the short circuit path across the stack of fuel cells. 
   
   
       25 . A method of operating a power system having a fuel cell stack and an energy storage device electrically coupled to the fuel cell stack, to power a load and a balance of system, the method comprising:
 measuring a stack current being supplied by the fuel cell stack to the load and the balance of system;   determining an amount of energy required by the load and the balance of system during a short circuit of the fuel cell stack;   determining an amount of energy to be clipped off after the short circuit;   determining an amount of energy to be pre-removed from the energy storage device based on the determined amount of energy required by the load and the balance of system during the short circuit and based on the determined amount of energy to be clipped off after the short circuit;   determining a time required to pre-remove the determined amount of energy from the electrical storage device at a present load;   disconnecting the load and the balance of system from the fuel cell stack;   the determined time required to pre-remove energy from the electrical storage device after disconnecting the load and the balance of system, shorting the fuel cell stack;   reconnecting the load and the balance of system to the fuel cell stack; and   stopping the shorting of the fuel cell stack after shorting the fuel cell stack for a shorting duration.