Patent Application
20090220828
1. Field of the Invention
Fuel cells may be used to supply power in a wide variety of applications. Exemplary transportation applications include hybrid electric vehicles (HEV), electric vehicles (EV), Heavy Duty Vehicles (HDV) and Vehicles with 42-volt electrical systems. Exemplary stationary applications include backup power for telecommunications systems, uninterruptible power supplies (UPS), and distributed power generation applications.
Electrochemical fuel cells convert reactants, namely a fuel and oxidant, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode.
2. Description of the Related Art
One type of electrochemical fuel cell is the proton exchange membrane (PEM) fuel cell. PEM fuel cells generally employ a membrane electrode assembly (MEA) comprising a solid polymer electrolyte or ion-exchange membrane disposed between two electrodes.
In a fuel cell, an MEA is typically interposed between two electrically conductive separator or fluid flow field plates that are substantially impermeable to the reactant fluid streams. The separator plates act as current collectors and may provide mechanical support for the MEA. In addition, the separator plates have channels, trenches, or the like formed therein which serve as paths to provide access for the fuel and the oxidant fluid streams to the anode and the cathode, respectively. Also, the fluid paths provide for the removal of reaction byproducts and depleted gases formed during operation of the fuel cell.
In a fuel cell stack, a plurality of fuel cells are connected together, typically in series but sometimes in parallel or a combination of series and parallel, to increase the overall output power of the fuel cell system. In such an arrangement, one side of a given separator plate may be referred to as an anode separator plate for one cell and the other side of the plate may be referred to as the cathode separator plate for the adjacent cell.
When a fuel cell has been shut down for a long period of time, the gas composition present at the cathode and anode flow fields of the fuel cell typically consists mainly of air. This may be due for example to air crossover through the membrane as well as air leaks in the seals and valves of the fuel cell system. On starting up such a fuel cell, adding hydrogen fuel to the anode electrode results in a wavefront as the air present at the anode is displaced by the hydrogen. This wavefront causes the cathode potential downstream of the wavefront to rise to a value that may contribute to corrosion of the cathode electrode.
Various solutions have been proposed to mitigate the above described problem. Some solutions have proposed purging the flow fields with inert gasses during the shut down operation of the fuel cell, or drawing an electrical load from the fuel cell during startup of the fuel cell to limit the cathode potential. These approaches to dealing with the described problem often give rise to substantially increased complexity and cost of the fuel cell system which is undesirable. US-2002-0076582-A1 proposes using an extremely rapid purging of the anode flow field upon start up with a hydrogen reducing fluid fuel so that air is purged from the anode flow field in no more than 1 second, or as quickly as no more than 0.05 seconds. This solution may reduce the corrosion effects, but has not proved effective in eliminating them.
U.S. Pat. No. 6,838,199-B2 proposes a method for starting up a fuel cell including the steps of: purging the cathode flow field with the reducing fluid fuel; then, directing the reducing fluid fuel to flow through the anode flow field; next, terminating flow of the fuel through the cathode flow field and directing an oxygen containing oxidant to flow through the cathode flow field; and connecting a primary load to the fuel cell so that electrical current flows from the fuel cell to the electrical load. This method merely shifts the corrosion problem from the cathode of the fuel cell to the anode of the fuel cell.
Solutions to eliminate or further minimize electrode corrosion upon startup of a fuel-cell are therefore desirable.
In one embodiment, a method for starting operation of a fuel cell system comprises supplying a fuel to both the anode electrode and the cathode electrode of the fuel cell system at substantially the same time during a first stage in the startup process, ceasing the supply of the fuel to the cathode electrode during a second stage in the startup process, and supplying an oxidant to the cathode electrode during a third stage in the startup process.
In another embodiment, a fuel cell system comprises an anode electrode with an adjacent anode flow field, a cathode electrode with an adjacent cathode flow field, a fuel supply device coupled to the anode flow field and coupleable to the cathode flow field, and a controller configured to control the fuel supply device to supply both the anode flow field and the cathode flow field with a fuel at substantially the same time during a start up of the fuel cell system.
In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some of these elements are arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn, are not intended to convey any information regarding the actual shape of the particular elements, and have been solely selected for ease of recognition in the drawings.
In the following description and enclosed drawings, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the invention. One skilled in the art will understand, however, that the invention may be practiced without all of these details. In other instances, well-known structures associated with fuel cell systems have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the invention.
Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open sense, that is as “including, but not limited to.”
Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Further more, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
The headings provided herein are for convenience only and do not interpret the scope or meaning of the claimed invention.
The fuel cell system 100 further comprises cathode inlet 116 and anode inlet 118 to enable the introduction of the oxidant and fuel streams into the cathode flow field 112 and the anode flow field 114 respectively. Cathode outlet 120 and anode outlet 122 provide for the removal of reaction byproducts and depleted fluids formed during operation of the fuel cell.
Valves 134, 136 are coupled to the outlets 120, 122 to either regulate the pressure of the fluids within the fuel cell 102, or as purge valves, to expel the reaction byproducts and depleted fluids formed during operation of the fuel cell 102 from the fuel cell 102.
At a stage in the start up operation of the fuel cell system 100, a controller 127 operates valves 128, 130, and 132 in concert to supply fuel from the fuel source 126 to both the cathode electrode 104 and the anode electrode 106 at substantially the same time. For the purposes of this invention, this is defined as the first stage in the start up operation of the fuel cell system. It should however be noted that the controller 127 may take other actions during the start up of the fuel cell system either before, after, in-between, or simultaneously with the stages described in this disclosure. These actions may for example comprise: purging the electrodes 104, 106 with a passivating fluid, connecting an electrical load to the fuel cell, circulating cooling fluid through the fuel cell, and operating heaters, among other actions. While such activities are not described in detail, these and other start up actions are well known and persons of ordinary skill in the art can readily select suitable start up actions for a given application. The controller 127 may employ information (arrows pointing toward controller 127) received from sensors and monitors, and may provide control signals (arrows pointing away from controller 127) to various valves, switches, actuators, solenoids, relays, contactors, motors, pumps, fans, blowers, compressors and other equipment.
The timing of the actuation of the valves 128, 130, and 132 may depend on factors such as the relative volumes of the cathode flow field 112 and the anode flow field 114, as well as the volume of the piping leading to the inlets 116, 118, among other factors. Calculating the actual timing and sequence of the valve operations is well within the abilities of an individual of ordinary skill in the art using well established principles.
For example, assuming the volume of the cathode flow field 112 is equal to the volume of the anode flow field 114, and assuming the valves 130, 132 are placed close enough to the cathode inlet 116 and the anode inlet 118 that the volume of the piping between the valves 130, 132 and the inlets 116, 118 is negligible, operating valve 128 first, and then operating valves 130 and 132 at substantially the same time would supply the fuel to both the cathode electrode 104 and the anode electrode 106 at substantially the same time.
The presence of a fuel on a cathode electrode and an anode electrode at substantially the same time should provide symmetrical conditions at the cathode electrode and the anode electrode, which in turn should avoid the creation of a high potential region, which in turn contributes to the minimization or elimination of the corrosion problem previously mentioned. This is described in more detail below.
At some period after the fuel has been introduced into the cathode flow field 112, the controller 127 halts the supply of the fuel to the cathode flow field 112. This is defined as the second stage of the start up of the fuel cell system 100. This may be accomplished by, for example, closing valve 130. This period may be predefined, or may be calculated or otherwise determined by the controller 127 during operation of the fuel cell system.
In some embodiments, once the hydrogen-air wavefronts have been eliminated from the anode flow field 114 by the passage of the fuel through the anode flow field 114 (i.e., the air has been substantially expelled from the flow field, or has been thoroughly mixed in to the fuel gas so that a wavefront is no longer present), an oxidant may be supplied to the cathode flow field 112. This is defined as the third stage of the start up of the fuel cell system 100. It should be appreciated that the amount of time required to eliminate the hydrogen-air front on the anode electrode 106 may be calculated for a given fuel cell system, and therefore the various components described may be actuated for a pre-determined period of time.
Oxidant is provided to the cathode electrode 104 of the fuel cell 102 by an oxidant source 124. In some embodiments the oxidant source 124 may comprise a storage device such as oxygen tanks. In other embodiments the oxidant source 124 may comprise an active device such as an air compressor or an air blower, among others. In some embodiments the oxidant source 124 may further comprise various other components such as filters, two-way valves and/or check valves. In some embodiments the oxidant source 124 may include means to prevent the fuel from escaping to atmosphere or from contaminating the oxidant source 124 during the first stage of the start up of the fuel cell 102. For example, in embodiments where a compressor is used to supply oxidant to the fuel cell 102, the compressor might be operated at a low speed to inhibit the fuel from traveling towards the oxidant source 124, or to dilute the fuel entering the fuel cell 102.
Once the fuel is present at the anode electrode 106, and an oxidant is present at the cathode electrode 104, the fuel cell 102 may be ready to supply power to an external load (not shown), and the start up procedure is complete. In some embodiments it may be desirable to connect an electrical load (not shown) to the fuel cell 102 during some or all of the above described stages to further minimize the corrosion, or to produce more rapid heating of the fuel cell 102.
In some embodiments it may be desirable to supply the cathode electrode with a known volume of fuel during the start up process. For example, to prevent the exhaust of fuel from the fuel cell 202 to the atmosphere it may be desirable to cease the supply of the fuel to the cathode electrode 204 before the fuel completely fills the cathode flow field 212. Using an accumulator 240 as shown, can therefore be useful to supply a known quantity of fuel to the cathode electrode 204.
In some embodiments valve 244 may be used to isolate the oxidant source 224 from the cathode flow field 212 during some stages of the start up process, in order to prevent the fuel from contaminating the oxidant source 224 during the startup process.
On start up, the controller 127 operates valve 328 to supply fuel from the fuel source 326 to the recirculation pump 352. The controller 127 then operates recirculation pump 352, and valves 354 and 356 to supply the fuel to both the cathode electrode 304 and the anode electrode 306 at substantially the same time. Three-way valve 356 is operated to direct the fluid exhausted from the cathode outlet 320 into the recirculation system 350 to be circulated through both the cathode flow field 312 and the anode flow field 314. In some embodiments it may be desirable to begin circulating the fluid already in the flow fields 312, 314 before operating valve 328 to supply the fuel to the system.
In some embodiments, valve 328 may be operated to only supply a limited amount of the fuel to the recirculation pump 352. For example, valve 328 may be operated to supply an amount of fuel to the recirculation pump 352 such that the concentration of hydrogen in the air present in flow fields 312, 314 remains below a threshold value (for example below a flammable limit of 4% hydrogen in air).
Similar to the examples above, once sufficient fuel has been introduced into the flow fields 312, 314, the valve 354 is operated to fluidly isolate the cathode inlet 316 from the anode inlet 318. Valve 356 is operated to isolate the cathode outlet 320 from the recirculation system 350, and may be further operated to exhaust any fluids from the cathode flow field 312 to atmosphere. Valve 344 is then operated to supply an oxidant from the oxidant source 324 to the cathode inlet 316.
On start up, the controller 127 operates valve 428 to supply a fuel from the fuel source 426 to the valves 432, 454. The controller 127 then operates valves 432, 454, recirculation pump 452, and blower 464 to supply the fuel to both the cathode electrode 404 and the anode electrode 406 at substantially the same time.
In some embodiments it may be desirable to begin circulating the fluid already in the flow fields 412, 414 before operating valves 432, 454 to supply the fuel to the fuel cell 402. In some embodiments it may be desirable to operate the recirculation pump 452 and the blower 464 at different speeds to vary the rate of recirculation of the fluids in recirculation systems 450, 460.
In some embodiments, valves 432, 454 may be operated to only supply a limited amount of the fuel to either or both the anode flow field 414 and the cathode flow field 412. For example, valves 432, 454 may be operated to supply an amount of fuel to the flow fields 412, 414 such that the concentration of hydrogen in the air present in flow fields 412, 414 remains below a threshold value (for example below a flammable limit of 4% hydrogen in air).
Similar to the examples above, once sufficient fuel has been introduced into the flow fields 412, 414, the valve 454 may be operated to fluidly isolate the cathode inlet 416 from the anode inlet 418.
Three-way valve 466 is then operated to supply an oxidant from the oxidant source 424 to the blower 464.
Valves 134, 136 are operated to exhaust any fluids from the flow fields 412, 414 to atmosphere as required. Valves 134, 136 may also be used to regulate the pressures of the fluids in the flow fields 412, 414.
Three-way valve 466 may be operated to vary the proportions of fluid recirculated through the cathode recirculation system 460, and the proportion of fluid introduced into the system from an oxidant source 424.
As can be seen in
In some embodiments, the hydrogen-air wavefronts do not progress through the flow fields 612, 614 at the same rate. In further embodiments there might be a delay between the formation of a wavefront in one flow field, and the formation of a wavefront in the opposite flow field.
Therefore, as used herein and in the appended claims, supplying fuel to both flow fields at substantially the same time is defined as supplying fuel to both flow fields so that at some period of time, hydrogen-air wavefronts exist within both flow fields.
A suitable fuel for the purposes of this invention comprises a hydrogen containing fluid. The fuel could for example comprise a substantially pure hydrogen gas, a hydrogen-rich fluid such as reformate, methanol, or other suitable compounds containing hydrogen.
The foregoing detailed description has set forth various embodiments of the devices and/or processes via the use of block diagrams, schematics, and examples. Insofar as such block diagrams, schematics, and examples contain one or more functions and/or operations, it will be understood by those skilled in the art that each function and/or operation within such block diagrams, flowcharts, or examples can be implemented, individually and/or collectively, by a wide range of hardware, software, firmware, or virtually any combination thereof. In one embodiment, parts of the present subject matter may be implemented via Application Specific Integrated Circuits (ASICs). However, those skilled in the art will recognize that the embodiments disclosed herein, in whole or in part, can be equivalently implemented in standard integrated circuits, as one or more computer programs running on one or more computers (e.g., as one or more programs running on one or more computer systems), as one or more programs running on one or more controllers (e.g., microcontrollers) as one or more programs running on one or more processors (e.g., microprocessors), as firmware, or as virtually any combination thereof, and that designing the circuitry and/or writing the code for the software and or firmware would be well within the skill of one of ordinary skill in the art in light of this disclosure.
In addition, those skilled in the art will appreciate that the methods and control mechanisms taught herein are capable of being distributed as a program product in a variety of forms, and that an illustrative embodiment applies equally regardless of the particular type of signal bearing media used to actually carry out the distribution. Examples of signal bearing media include, but are not limited to, the following: recordable type media such as floppy disks, hard disk drives, CD ROMs, digital tape, and computer memory; and transmission type media such as digital and analog communication links using TDM or IP based communication links (e.g., packet links).
Although specific embodiments of and examples for a fuel cell system and methods are described herein for illustrative purposes, various equivalent modifications can be made without departing from the spirit and scope of the disclosure, as will be recognized by those skilled in the relevant art.
For example, those skilled in the art will recognize that the embodiments disclosed herein, in whole or in part, can be equivalently implemented using a wide variety of standard components and circuits. For example three-way valves may be replaced by two two-way valves. Two-way valves may be replaced by check valves or other devices chosen to fulfill a similar purpose. Designing the circuitry and/or hardware and/or control strategies would be well within the skill of one of ordinary skill in the art in light of this disclosure.
The various embodiments described above can be combined to provide further embodiments.
These and other changes can be made to the invention in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims, but should be construed to include all fuel cell systems. Accordingly; the invention is not limited by the disclosure, but instead its scope is to be determined entirely by the following claims.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US2007/006355 | 3/13/2007 | WO | 00 | 1/7/2009 |
| Number | Date | Country | |
|---|---|---|---|
| 60783100 | Mar 2006 | US |
