Fuel cell cathode flow field
A fuel cell cathode flow field has multiple channels each with a cross-sectional area that varies along the length of the channel such that oxygen availability at every lengthwise position along the channel is kept substantially constant for a given channel length and air stoichiometry ratio. Each channel comprises a flat floor with substantially constant depth and a pair of side walls extending upwardly from the floor; the side walls each taper inwards from channel inlet to outlet with a convex curve relative to the channel centreline. Achieving substantially uniform oxygen availability throughout the flow field results in substantially uniform current density throughout the flow field, which is desirable for efficient fuel cell operation and improved performance.
1. A fuel cell cathode flow channel comprising:
(a) an inlet;
(b) an outlet;
(c) a length and a floor of substantially constant depth having a substantially straight pathway between said inlet and said outlet; and
(d) a pair of adjacent and opposed convexly curved side walls extending upwards from said floor and defining a channel width between said side walls that decreases exponentially along said length of the channel from the inlet to the outlet.
2. A fuel cell cathode flow channel as claimed in claim 1 wherein the channel width decreases according to a natural exponential function.
3. A fuel cell cathode flow channel as claimed in claim 2 wherein the channel width at a selected lengthwise position of the channel is proportional to a natural exponential function of the selected lengthwise position.
4. A fuel cell cathode flow channel as claimed in claim 3 wherein the natural exponential function is also a function of a constant derived from an air stoichiometry ratio of a fuel cell in which the flow channel is incorporated.
5. A fuel cell cathode flow channel as claimed in claim 4 wherein the constant is a natural logarithm of a function of the air stoichiometry ratio.
6. A fuel cell cathode flow channel as claimed in claim 4 wherein the channel width at the selected lengthwise position is also proportional to the channel width at the channel inlet.
7. A fuel cell cathode flow channel as claimed in claim 1 wherein the channel width at the selected lengthwise position is equal to:
W
(
0
)
ⅇ
-
x
L
ln
(
STOICH
STOICH
-
1
)
wherein x is the selected lengthwise position, W(O) is the channel width at the inlet, L is the channel length, and STOICH is an air stoichiometry ratio of a fuel cell in which the flow channel is incorporated.
8. A fuel cell separator plate comprising at least one flow channel as claimed in claim 1 .
9. A fuel cell cathode flow channel comprising a cross-sectional area that varies along a length of the channel such that oxygen availability at every lengthwise position along the channel is kept substantially constant for a selected air stoichiometry ratio and channel length, and further comprising:
(a) an inlet;
(b) an outlet;
(c) a length and a floor of substantially constant depth having a substantially straight pathway between said inlet and said outlet; and
(d) a pair of adjacent and opposed convexly curved side walls extending upwards from said floor and defining a channel width between said side walls that decreases exponentially along said length of the channel from the inlet to the outlet.
10. A fuel cell cathode flow channel as claimed in claim 9 wherein the channel width at a selected lengthwise position of the channel” is proportional to a base exponentiated to a power that is a function of the selected lengthwise position.
11. A fuel cell cathode flow channel as claimed in claim 10 wherein the base is a natural logarithmic base e.
12. A fuel cell cathode flow channel as claimed in claim 11 wherein the channel width at the selected lengthwise position is also proportional to the channel width at the channel inlet.
13. A fuel cell cathode flow channel as claimed in claim 12 wherein the power is also a function of an air stoichiometry ratio of a fuel cell in which the flow channel is incorporated.
14. A fuel cell cathode flow channel as claimed in claim 13 wherein the channel width at the selected lengthwise position is equal to:
W
(
0
)
ⅇ
-
x
L
ln
(
STOICH
STOICH
-
1
)
wherein “x” is the selected lengthwise position, “W(O)” is the channel width at a channel start position, “L” is the channel length, and “STOICH” is an air stoichiometry ratio of a fuel cell in which the flow channel is incorporated.
15. A fuel cell cathode flow channel comprising a cross-sectional area that varies along a length of the channel such that oxygen availability at every lengthwise position along the channel is kept substantially constant for a selected air stoichiometry ratio and channel length wherein the channel area “A(x)” at the selected lengthwise position is equal to:
A ( x )=(1.7. times. 10−14).rho.air (( I Stoich)− I d .intg. 0× W R ( x ) x )( I Stoich)/AVAIL O 2( x )[kg m/s 2]
wherein “x” is the selected lengthwise position, “I” is an entire channel current load, “stoich” is an air stoichiometry ratio of a fuel cell in which the flow channel is incorporated, “Id” is current density, WR(X) is a width of the oxygen reaction area at the selected lengthwise position “x”, and “AVAIL02(x)” is oxygen availability at the selected lengthwise position “x”.