Patent Publication Number: US-2006003204-A1

Title: Controlling fuel cell fuel purge in response to recycle fuel blower operating conditions

Description:
TECHNICAL FIELD  
      This invention relates to monitoring the operating conditions of a fuel cell recycle fuel blower, such as speed or current, to estimate the ratio of hydrogen to non-hydrogen gases in the recycle stream, thereby to control a fuel purge valve.  
     BACKGROUND  
      It is well known that fuel cell power plants cannot be run at 100% fuel utilization, that is, providing the exact amount of fuel which is consumed in producing the desired electrical load, without resulting in fuel starvation at various regions of various fuel cells in the stack. Fuel cell starvation results in corrosion of the carbonaceous catalyst supports, resulting in reduced system power performance. To overcome this problem, recycling a portion of the fuel, which exits the anode fuel flow fields, to the inlets of the anode fuel flow fields provides overall fuel cell stack utilization of nearly 100%, while having a lower fuel cell utilization on a single pass, cell by cell basis.  
      It is also known that recycle fuel contains a depleted amount of hydrogen which is mixed with inerts, such as nitrogen which crosses over from the air in the cathode through the porous membrane electrolyte. To clear the anode of the inerts, and to assure inward flow of fresh hydrogen, purging is accomplished, either in a small, steady amount, or more typically by pulse purging; that is, opening the purge valve on a periodic, duty cycle basis. The rate of purging is typically determined during initial testing of fuel cell models, on the basis of the density of current being produced. Thereafter, purging is performed in a predetermined fashion as a function of fuel cell stack current density.  
      The problem with this method is that it fails to take into account surges and variations in nitrogen crossover due to changes in the membrane, performance losses in the fuel cell, and so forth.  
      To overcome these problems, the purge is purposely set somewhat high, providing a marginal purge to assure that the purge will be sufficient; this naturally reduces the efficiency of the overall system below that which could possibly be attained. If the purge gas is released into a cabinet containing the fuel cell systems, the marginal, extra purge also increases on the cabinet ventilation system which increases the noise level and the parasitic power loss.  
     DISCLOSURE OF INVENTION  
      Objects of the invention include: a simple, effective control over fuel cell anode purge which does not require additional equipment; providing information on hydrogen flow through the anode of a fuel cell without the need of hydrogen sensors; providing improved startup and shutdown of fuel cell power plants; providing purge control in a fuel cell power plant which is responsive to actual conditions in the anode fuel flow; assuring adequate fuel flow in a fuel cell power plant without marginal extra flow; and fuel cell power plant anode purge which is responsive to actual gas composition of the anode gas flow.  
      The term “recycle blower” is used herein for convenience and is to be understood to include any suitable fuel recycle gas mover or impeller which returns at least a portion of gas exiting the fuel flow fields of a fuel cell to an inlet of fuel cell fuel flow fields. This term includes fan-like blowers, pumps, compressors and other suitable impellers.  
      According to the present invention, an operating condition of a fuel cell fuel recycle blower, such as speed or current, is utilized to control a fuel purge valve. According to the invention, pulse purging may be accommodated by controlling the pulse width modulation of a purge valve in response to recycle fuel blower speed, current pressure rise, temperature or current. In accordance with one embodiment of the present invention, density of recycle fuel in a recycle blower nominally operated at constant speed or with a constant drive signal, is estimated from parameters, such as recycle blower conditions including current, speed and pressure rise, recycle gas temperature, and load current. In accordance with one aspect of the invention, a function of fuel recycle blower speed(s) and recycle gas temperature (T) or load current (I), provides a calculated, estimated pressure rise, which is compared with the actual measured pressure across the recycle blower, the error thereof being used to alter or trim the load current signal used to control the fuel purge valve. An example is: aI+bS+cIS. However, other combinations of parameters related to fuel cell stack performance and the recycle blower may be used within the purview of the invention.  
      Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a simplified, stylized schematic illustration of the fuel portion of a fuel cell stack known to the prior art.  
       FIG. 2  is a simplified, stylized schematic illustration of the fuel portion of a fuel cell power plant utilizing a simple embodiment of the present invention.  
       FIG. 3  is a simplified, stylized schematic illustration of the fuel portion of a fuel cell power plant employing another embodiment of the invention.  
       FIG. 4  is a functional, illustrative diagram of an exemplary control methodology for the configuration of  FIG. 3 .  
       FIG. 5  is a fragmentary, simplified schematic illustrating that the recycle blower parameter used to control the purge valve may be blower motor current. 
    
    
     MODE(S) FOR CARRYING OUT THE INVENTION  
      Referring to  FIG. 1 , a fuel cell stack  9  includes cathode flow fields  1   0 , and includes anode flow fields  11  which are provided with fuel reactant gas in a fuel inlet conduit  12  through a valve  13  from a source  14  of fuel, such as hydrogen. The fuel exhaust in a conduit  18  provides fuel recycle gas in a conduit  19  to a recycle pump  20 , the output of which is connected by a conduit  21  to the fuel inlet conduit  12 , all as is known in the art. The valve  13  is controlled by a signal on a line  1   5  from a controller  16 .  
      In order to control the amount of inert (non-fuel) gases in the anode flow fields  11 , a purge valve  26  may periodically release small amounts of gas exiting the anode flow fields  11  to exhaust  27 , which may be a suitably vented ambient or a burner, as is known. Control over the purge valve  26  may be in response to a pulse width modulation command on a signal line  28  from the controller  16  having a portion  30  that responds to current in the load to determine the amount of purge gas to expel from the system.  
      An indication of load current is provided by a sensor  33  in response to current in the fuel cell stack output lines  34 ,  35  that provide current to the load  36 . In the known system of  FIG. 1 , the ratio of open and closed times is established in response to some function of load current which may be determined as appropriate for each type of fuel cell, or for each fuel cell if desired. However, the system in  FIG. 1  does not take into account changes in the system over time, differences between one system and another, and particularly, the variation in nitrogen crossover which may occur depending upon operating conditions. The system of  FIG. 1  also does not take into account rapid surges in load current and other perturbations in attempting to provide just the right amount of purge so as to achieve almost 100% overall fuel utilization without fuel starvation in any of the cells, and without unnecessary waste.  
      In  FIG. 2 , a simple embodiment of the present invention provides a signal on a line  40  indicative of the rotary speed, S, of the recycle blower  20 , which is used in the function  30   a  of the controller  16  to determine the pulse width modulation signal on the line  28  to control the on/off operation of the purge valve  26 , thereby controlling the average rate of purge of gas exiting from the anode flow fields. The function  30   a  may be an empirically determined monotonic function, either calculated in real time or stored in a look-up table.  
      A more extensive version of the present invention is illustrated in  FIG. 3 . The configuration of  FIG. 3  utilizes not only the current signal from the sensor  33  as in  FIG. 1 , and the speed signal on the line  40  as in  FIG. 2 , but also utilizes the temperature of the recycle gas determined by a temperature sensor  44  which provides a temperature signal on a line  45 .  
      The configuration of  FIG. 3  also responds to the pressure rise across the recycle pump  20  as determined by two pressure sensors  48 ,  49  providing signals on corresponding lines  50 ,  51  to the controller  16 . The portion  30   b  of the controller is illustrated in  FIG. 4 . Therein, the actual, measured pressure rise of the recycle blower is determined by a summer  54  which subtracts the downstream pressure from the upstream pressure to provide the actual delta pressure, dPa. A function generator  55  generates an estimated correct pressure rise for the fuel cycle blower, dPc, in response to blower speed, S, and temperature of the recycle gas, T. The function  55  may, for example, be aI+bS+cIS. The difference between the actual pressure rise and the calculated pressure rise is determined by a summer  57  and the error, dPe, is applied to a proportional/integral amplifier  58 , which has limits applied to the output thereof by a limit circuit  59 , the output of which is fed to a multiplier  60 . The functions of the summers  54 ,  57  may be combined. Although shown as discrete circuit blocks, the foregoing will typically be performed by software.  
      The effect of the multiplier  60  is, when the actual pressure rise is deemed proper for the current speed and temperature, the signal from the limit circuit  59  will be 1.0, causing the signal on the line  33  to be unaltered by the multiplier  60 . If the actual pressure rise is greater than the estimated pressure rise, this indicates that there are more inerts in the fuel recycle stream than there should be, so that the multiplier  60  will increase the current signal by some amount greater than one. If the actual pressure rise is less than the estimated pressure rise, that means there is more hydrogen in the fuel recycle than is normal, so that the signal from the limiter  59  will reduce the current signal by a value which is slightly less than one. The multiplier  60  provides a signal to a conventional pulse width multiplier circuit (or function, in a computerized controller)  62  which provides the purge valve control signal on the line  28 .  
      The foregoing embodiments utilize blower speed, which may typically be provided in terms of frequency (Hertz). However, the blower current may be utilized, instead, as an indication of the work performed by the blower, and therefore the density of the recycle gas, as illustrated by the line  40   a  in  FIG. 5 . The function in that case will be a slight variation of the function that is utilized with speed as the input.  
      Although the embodiments herein employ pulse width modulation of the purge valve, a valve may be continuously metered in response to a signal which is a function of the recycle blower condition indicative of density of the gas being impelled by the blower.  
      Thus, although the invention has been shown and described with respect to exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without departing from the spirit and scope of the invention.