- BACKGROUND ART
This invention relates to cascaded control of fuel flow and voltage of cascaded fuel cell stacks, such as during startup and shutdown.
To achieve very high fuel utilizations, about 98% or 99%, when operating a fuel cell stack 9 on pure hydrogen, a cascade fuel flow field, illustrated in FIG. 1, comprises a plurality of groups 10-12 of fuel cells 13 arranged in flow-series relationship so that fuel from a source (not shown) passing through a fuel inlet valve 16 enters a fuel inlet manifold 17, flows through a first group 10 of fuel cells 13, then enters a first turn-around manifold 19, then flows through the second group 11 of fuel cells 13, thence through a second turn-around manifold 20 and through the third group 12 of fuel cells 13, to an exit manifold 22.
For a typical 40 kilowatt fuel cell stack, the first group 10 has a large number of cells 13, which may be on the order of about 200 cells, the second group 11 has a lesser number of cells 13, which may be on the order of about 70 cells, and the third group 12 may have on the order of about 25 cells. As is known, this assures that all of the cells get adequate hydrogen even with high hydrogen utilization, provided that the last group of cells 12 get adequate hydrogen.
Referring to FIG. 1, during the production of electricity in normal fuel cell operation mode, a microcontroller 25 provides a signal on a line 26 to cause a fuel inlet valve 16 to be open, to provide fuel to the inlet manifold 17. The processor 25 also provides a signal on a line 27 to cause a normal fuel outlet valve 23 to be open. Under this condition, the fuel enters the inlet manifold 17, passes through the group 10 of cells 13, into the first turn-around manifold 19, through the group 11 of cells 13, through the second turn-around manifold 20, through the group 12 of cells 13, through the exit manifold 22, through the outlet valve 23, and to the exhaust 30.
The fuel cell stack may include a recycle loop 38 driven by a pump 39, all in a conventional fashion; however, the use of a recycle loop is optional.
In commonly owned, copending U.S. patent application Ser. No. 09/742,481, filed Dec. 20, 2000, it is shown that the more rapidly the fresh hydrogen-containing fuel flows through the anode flow field upon start-up, to displace the air therein, the quicker the hydrogen/air interface moves through the anode flow field, and the less time there is for the occurrence of corrosion of the platinum catalyst and catalyst support.
In copending U.S. patent application Ser. No. 09/921,809, filed Aug. 3, 2001, a cascade reactant flow field of a fuel cell stack has additional fuel inlet valves to provide inlet fuel directly to each cascade of the stack and at least one additional exhaust valve to remove fuel directly from each cascade of the stack. This may be used for rapid deployment of fuel into the fuel flow field during start-up.
Although rapid purging reduces startup problems referred to hereinbefore, performance decay of cascaded fuel cell stacks is still unacceptable.
It is also known in the art, as illustrated in copending U.S. patent application Ser. No. 10/305,301, filed Nov. 26, 2002 to connect a voltage limiting device 45 across the main electrical output terminals 46, 47 as soon as average voltage per cell of about 0.2 volts is detected by a voltage sensing device 48, which causes the microcontroller 25 to close a switch 50. The voltage limiting device, in the aforementioned application, is simply an auxiliary load resistor.
When it is determined, either by the passage of time or by sensing parameters of the fuel cell stack, that normal operation can be achieved, the micro controller 25 will close a switch 52 to connect the main load 53 across the fuel cell electrical output terminals 46, 47, and open the switch 50.
If the voltage limiting device is not connected across the stack while the groups of cells 10 are being fed hydrogen, then these cells will have excessive voltage, and resulting carbon corrosion and ultimate performance decay. On the other hand, if the voltage limiting device 45 is connected across the stack prior to hydrogen reaching the second and third groups 11, 12 of cells, the anodes in the cells in the second and third groups are driven to an elevated potential that results in corrosion of the carbonaceous catalyst support and other components of the cells.
- DISCLOSURE OF INVENTION
The foregoing problems have resulted in the conclusion that fuel cell stacks with cascade fuel feed are impractical, due to the certainty of early performance decay.
Objects of the invention include: a fuel cell stack having the high-fuel-utilization advantage of a cascade fuel flow field without the startup problems, such as reverse cell voltage, carbon corrosion and performance decay, heretofore associated therewith; and an improved fuel cell stack utilizing cascade flow fields which has no unusual degradation of catalysts and other parts as a consequence of frequent shut-down and start-ups.
This invention is predicated in part on our discovery that fuel distribution in the diverse groups of fuel cells within a cascaded fuel cell stack is operationally ineffective, and in part on the fact that voltage control during startup and shutdown of a fuel cell stack must be accomplished in conjunction with each group of fuel cells receiving fuel.
According to the present invention, a cascade of groups of fuel cells arranged in serial fuel flow arrangement includes an inlet fuel distributing fuel inlet manifold for each group of the series, thereby providing operationally adequate fuel distribution in each group of fuel cells. According further to the invention, voltage control during startup of a fuel cell stack in a serial fuel flow, cascaded fuel cell stack, is accomplished one group of fuel cells at a time, thereby responding directly to the introduction of fuel to each group.
The invention allows startup of each group of a cascaded group of fuel cells in a stack to be started up, one group at a time, each group having advantageous startup conditions as would be the case of individual, non-cascaded fuel cell stacks known to the prior art.
BRIEF DESCRIPTION OF THE DRAWINGS
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.
FIG. 1 is a schematic diagram of a three-group cascade fuel cell stack known to the prior art.
FIG. 2 is a schematic diagram of an exemplary three-group cascade fuel cell stack, in accordance with the invention.
MODE(S) FOR CARRYING OUT THE INVENTION
FIGS. 3-5 are partial, simplified schematic diagrams of various voltage limiting devices.
Referring to FIG. 2, a cascaded fuel cell stack 9 a according to the invention includes an inlet fuel distributing fuel inlet manifold 17 a at the inlet to the fuel flow fields of the fuel cells 13 in the first group 10. The fuel distributing manifold 17 a may be a cascade fuel inlet manifold as described in copending U.S. patent application Ser. No. 10/269,654, filed Oct. 10, 2002. The device disclosed therein divides the fuel in half a number of times, such as four times, so that it is evenly distributed across the entire stack of fuel cells, whereby each fuel cell fuel flow field receives a uniform amount of fuel, simultaneously with the fuel flow fields of the other fuel cells. The fuel distributing manifold 17 a may also take the form of a permeable baffle inlet fuel gas distributor, as disclosed in copending U.S. patent application Serial No. (Docket No. C-2950), filed Dec. 15, 2003, entitled “Permeable Inlet Fuel Gas Distributor for Fuel Cells”, in which fuel is evenly distributed by being forced through a permeable baffle, which may be porous, have orifices, be in the form of screening, mesh or other materials. Or, other inlet fuel distributing, composite fuel inlet manifolds may be used.
The fuel passing through the fuel flow fields of the first group 10 of fuel cells 13 will reach a fuel exit manifold 19 a, which is not itself in direct communication with the fuel cells 13 of the group 11. Instead, the fuel exit manifold 19 a feeds a fuel conduit 19 b which in turn feeds another fuel distributing fuel inlet manifold 19 c. Although the manifolds 17 a, 20 a, and 20 c and fuel conduit 20 b are shown as separate elements, they may be integrated into a single structure; similarly, the manifolds 19 a, 19 c, and 22 may be integrated into a single structure along with fuel conduit 19 b. This manifold, similar to the manifold 17 a, will distribute the fuel evenly throughout the group 11 of fuel cells 13. This is in contrast to the manner in which fuel is distributed, in the prior art shown in FIG. 1, in which there is an inherent tendency for the fuel cells 13 of the group 11 which are closest to the group 10 to receive the fuel first, and to receive more fuel than those fuel cells 13 which are disposed closer to the group 12. This is an important aspect of the present invention.
Instead of a plain turnaround manifold between the group 11 and the group 12, the group 11 has a fuel exit manifold 20 a which feeds a fuel conduit 20 b which carries the fuel to another fuel distributing fuel inlet manifold 20 c, similar to the other manifolds 17 a, 19 c. This in turn causes a uniform, simultaneous flow of fuel entering the fuel flow fields of the fuel cells 13 in the group 12. This is in contrast to the tendency, shown in the prior art of FIG. 1, for the fuel to enter those fuel cells 13 of the group 12 which are closer to the group 11, causing less fuel and a delay in a fuel arrival for the fuel cells 13 which are farther away from the group 11. The group 12 of fuel cells 13 all communicate with a fuel exit manifold 22, which in turn may feed the fuel recycle apparatus 38, 39 illustrated in FIG. 1, if desired.
Thus, a first aspect of the present invention includes distributing the fuel evenly to each group in turn so that all of the fuel cells in a given group receive a uniform amount of fuel simultaneously with the other fuel cells of that same group.
A second aspect of the present invention includes substack voltage monitors 48 a-48 c which in turn cause the microcontroller 25 to operate separate voltage limiting device (VLD) switches 50 a-50 c. The substack voltage monitors 48 a-48 c are connected either to two conductive separator plates 58, 59 or to one end plate 56, 57 and one separator plate 58, 59.
When the fuel cell is started up, the microcontroller will open the valve 16 by means of a signal on the line 26 and hydrogen will begin to flow through the fuel distributing fuel inlet manifold 17 a. The fuel will enter the fuel cells simultaneously and this will cause voltage to develop between the end plate 57 and the separator plate 59. When the sub stack voltage monitor 48 a determines a suitable average cell voltage, which might be on the order of 0.2 volts per cell, the microcontroller 25 will close the switch 50 a connecting the voltage limiting device across the group 10 of fuel cells 13. The two other groups of fuel cells have not yet received fuel and are not yet connected to the voltage limiting device. Once fuel begins entering the group 11 of fuel cells 13, uniformly dispersed by the fuel distributing fuel inlet manifold 19 c, the cell voltage will begin to build up between the separator plate 58 and the separator plate 59. When the sub stack voltage monitor 48 b senses an adequate voltage, which may be on the order of 0.2 volts per cell, the microcontroller 25 will cause the switch 50 b to close and the switch 50 a to thereafter open, thereby connecting the group 11 in series with the group 10 across the voltage limiting device 45 through the switch 50 b. Thus, voltage is limited at the appropriate time when fuel is building up in the fuel flow fields of the fuel cells 13.
Eventually fuel will reach the fuel distributing fuel inlet manifold 20 c and voltage will begin to build up between the end plate 56 and the separator plate 58. When the sub stack voltage monitor 48 c indicates an average voltage, which might be on the order of 0.2 volts/cell, the microcontroller 25 will close the switch 50 c and thereafter open the switch 50 b, thereby causing all three groups 10-12 to be connected across the voltage limiting device 45 through the switch 50 c. When the microcontroller determines that all three groups are operating normally, it will cause the switch 52 to close thereby connecting the main load 53 across the stack 9 a by connecting it to the pressure plates 56, 57, and the microcontroller will immediately open the switch 50 c.
Upon shutdown, as is known, the air to the cathode is turned off after which the microcontroller will close the switch 50 c, so that the voltage limiting device 45 consumes the energy as the oxygen in the cathode oxidant channels of all three groups 10-12 is depleted. Thereafter, the microcontroller 25 will shut off the fuel by means of the valve 16; the switch 50 c typically will remain closed until the next time that the fuel cell stack is to be operated.
The voltage limiting device 45 may be a simple resistive auxiliary load 45 a (FIG. 4) as in the aforementioned application Ser. No. 10/305,301, or it may be a resistive load 45 b (FIG. 5) which is connected and disconnected, repetitively, in a pulse width modulation fashion by a control 45 c and a switch 45 d as disclosed in copending U.S. patent application Ser. No. 10/681,493 filed Oct. 7, 2003. The voltage limiting device 45 may be an energy recovery and storage apparatus as disclosed in copending U.S. patent application Ser. No. 10/669,273 filed Sep. 23, 2003, which includes batteries 45 e (FIG. 6) and capacitors as well as buck and boost controls 45 f.
FIG. 3 illustrates that instead of a single voltage limiting device 45 for the entire stack, each group may have its own voltage limiting device 45 a-45 c; in such case, each switch 50 a-50 c, once closed, will remain closed until the microcontroller closes switch 52.
Although disclosed in a three group configuration herein, the invention may be used in cascaded fuel cell stacks having only two groups, or having three, four or more groups. The voltage limiting aspects of the invention are preferably used with the inlet fuel distributing aspects of the invention, but the various aspects of the invention may be used separately.
The aforementioned patent applications are incorporated herein by reference.
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.