Control of nitrogen fraction in a flow shifting fuel cell system
A device and method to predict and regulate nitrogen concentration in a flow shifting system. In one aspect of the system, a bleed valve fluidly coupled to multiple fuel cell stacks is used to reduce the presence of nitrogen in an anode flowpath. One or more sensors can be used to measure voltage within one or both of the fuel cell stacks. By assessing fuel cell voltage changes within the anode flowpath and equating such changes with nitrogen fraction buildup, the system can manipulate the bleed valve at appropriate times to improve system operability. In one form of equating the sensed voltage changes with the nitrogen fraction buildup, a predictive algorithm can be used by a logic device in a controller to compare the sensed voltage so that the controller instructs the bleed valve when to open and close. In a variation, the controller can compare the sensed voltages against stored data rather than rely on a formula or related algorithm.
1. A method of operating a fuel cell system, said method comprising:
configuring said system to comprise:
a first fuel cell stack and a second fuel cell stack, each comprising a plurality of individual fuel cells within, each of said individual fuel cells comprising an anode, a cathode and a membrane disposed between said anode and cathode;
a cathode flowpath configured to couple said cathodes of said individual fuel cells within each of said first and second stacks to an oxygen source;
an anode flowpath configured to couple said anodes of said individual fuel cells within each of said first and second stacks to a fuel source, said anode flowpath configured to be connected between said first and second stacks such that fuel present within said anode flowpath cycles back and forth between said first and second stacks;
at least one bleed valve fluidly coupled to said anode flowpath; and
at least one sensor configured to measure a voltage;
introducing fuel into said anode flowpath such that said fuel flows through said stacks in a first direction;
introducing oxygen into said cathode flowpath;
repeatedly reversing said flow of fuel through said anode flowpath such that said fuel flows through stacks in a second direction from the second fuel cell stack to the first fuel cell stack;
sensing a voltage difference between the fuel cell stacks associated with operation of said system;
correlating said sensed voltage difference with a fuel concentration in said anode flowpath; and
upon attainment of a threshold value of said voltage difference based on said correlating, manipulating said at least one bleed valve to allow at least a portion of said fluid present in said anode flowpath to escape.
2. The method of claim 1 , wherein said system further comprises a controller cooperative with said at least one sensor and said at least one bleed valve such that said controller performs said correlating between said sensed voltage difference and said fuel concentration in said anode flowpath to determine if said attainment of said threshold has been reached.
3. The method of claim 2 , wherein said fuel concentration in said anode flowpath is represented in said controller by values in a lookup table.
4. The method of claim 2 , wherein said fuel concentration in said anode flowpath is represented in said controller by a formula according to the following equation:
U
0
=
U
0
+
R
T
2
F
(
ln
p
H
2
cat
p
0
+
1
2
ln
p
O
2
cat
p
0
)
+
Δ
R
S
2
F
(
T
-
T
0
)
.
5. The method of claim 1 , wherein said sensing a voltage difference associated with operation of said system comprises sensing a voltage of said first and second stacks substantially simultaneously.
6. The method of claim 5 , wherein said sensing a voltage difference of said first and second stacks substantially simultaneously further comprises determining an average voltage within each of said first and second stacks such that said correlating comprises correlating said average voltages with said fuel concentration in said anode flowpath.
7. The method of claim 1 , wherein said sensing a voltage difference associated with operation of said system comprises sensing a plurality of voltage differences within one of said first and second stacks such that an average value within said one of said first and second stacks can be determined.
8. The method of claim 1 , wherein said sensing a voltage difference associated with operation of said system comprises sensing a plurality of voltage differences within a single fuel cell within at least one of said first and second stacks.
9. The method of claim 1 , wherein said manipulating said at least one bleed valve further comprises closing said at least one bleed valve after a passage of time that corresponds to no more than one complete anode flowpath operating cycle.
10. The method of claim 1 , wherein said manipulating said at least one bleed valve further comprises closing said at least one bleed valve after a passage of time that is proportional to the magnitude of voltage difference between said first stack and said second stack.
11. The method of claim 1 , wherein said at least one bleed valve comprises at least one three-way valve such that said at least one bleed valve selectively places said fuel source in fluid communication with said anode flowpath.
12. The method of claim 1 , wherein said at least one bleed valve is fluidly disposed between said first and second stacks.