Patent Application
20080145718
The invention relates to a fuel cell system and a method for operating a fuel cell system.
When a fuel cell system is operated, for producing electric current a combustion gas, for example hydrogen, is customarily ducted on the anode side to a fuel cell block formed from stacked fuel cells, and air or oxygen is customarily ducted thereto as a further reaction gas on the cathode side. There are now many different types of fuel cell systems that differ with respect to their physical structure and, in particular, the electrolytes employed, as well as in terms of the required operating temperature. In what is termed a PEM (proton exchange membrane) fuel cell, a polymer membrane that is permeable to hydrogen protons is arranged between a gas permeable anode and a gas-permeable cathode. Since a single fuel cell supplies a voltage of only around 0.7 to 0.9 V, a plurality of fuel cells are electrically connected in series to form a stack. The individual fuel cells are therein customarily mutually separated by a bipolar plate. Said bipolar plate therein as a rule has a kind of fluted structure and abuts the anode or, as the case may be, cathode. By means of the fluted structure a gas space through which the reaction gases flow is formed between the bipolar plate and the anode or, as the case may be, cathode.
When a PEM fuel cell is operated, hydrogen protons migrate through the electrolyte to the oxygen side and react with the oxygen, with water being the product of this reaction. Water is additionally introduced into the gas spaces as a result of the customary humidifying of the reaction gases before they enter the fuel cell. Depending on the purity of the reaction gases used, inert gases also arise alongside the water. In the case of a fuel cell system having a plurality of fuel cell stacks arranged one after the other in series in a cascaded manner, the water and inert gases will accumulate in the last stack or fuel cell, where inert gas will consequently be added to the reaction gases. This “reactant diluting” causes a voltage drop at the last fuel cell or, as the case may be, the last fuel cell stack. Said fuel cell stack is therefore purged at specific intervals, which is to say a purge line connected at the gas outlet side to the stack is opened via a purge valve so that the accumulated water and inert gases will be discharged. The last fuel cell or last fuel cell stack is therefore referred to also as a purge cell or, as the case may be, purge cell stack. The voltage drop is customarily employed as the control signal for opening the purge valve. The concentration of inert gases is reduced by said purging so that the voltage level is raised again.
These conditions in the purge cell give rise to a risk of corrosion for the bipolar plates, in particular if the voltage drops to a region of a corrosion potential of the material used for the bipolar plates.
The object of the invention is to enable reliable operation of a fuel cell system with minimal risk of corrosion.
Said object is achieved according to the invention by means of a fuel cell system having a fuel cell stack, consisting of a plurality of fuel cells, to which reaction gases can be ducted on the gas inlet side and which on the gas outlet side has at least one purge valve on a purge cell. The system further contains a control device, in particular a regulating device, that controls actuating of the purge valve as a function of the purge cell's voltage. Voltage tapping for measuring the voltage in the purge cell therein takes place in the vicinity of a gas outlet toward the purge valve. What is understood therein by “in the vicinity of a gas outlet” is the voltage tap's being sited approximately at the same level as the gas outlet. Voltage tapping therein takes place expediently in the—viewed in the direction of flow of the reaction gases—lower or nethermost region of the purge cell.
This embodiment is based on the consideration that the accumulated inert gases are not distributed evenly in the purge cell but rather collect in the lower region thereof in the direction of flow. During operation a concentration gradient of the reaction gases therefore becomes established in the direction of gas flow from a top gas inlet to the bottom gas outlet. Owing to that, the voltage produced in the purge cell is in part significantly higher, depending on the concentration of inert gas, in the vicinity of the top gas inlet than the voltage in the lower region in the vicinity of the gas outlet. With controlling by means of a top voltage tap in the vicinity of the gas inlet there would hence be a risk that only low voltages will be maintained in lower partial regions of the purge cell so that there will be a high risk of corrosion there. By means of the voltage tap in the lower region near the gas outlet, very precise and very sensitive controlling or regulating is achieved for actuating the purge valve, with significant improvements being gained in terms of control. What is particularly avoided thereby is that a predefined minimum voltage value constituting the lower control limit will be undershot in partial regions of the fuel cell. There will be more purging operations, which is to say the purge rate will be increased, compared to when a voltage tap is arranged in the top region. Voltage tapping preferably therein takes place in the vicinity of an edge side of a bipolar plate by which the fuel cell is delimited, with the gas outlet for the reaction gas being provided in the vicinity of said edge side.
To maximize the efficiency with which the reaction gases can be utilized, the fuel cell system has a plurality of fuel cell stacks arranged in a cascaded manner. What is understood thereby is a series of fuel cell stacks through which the reaction gases flow in succession, with the number of fuel cells in the individual stacks arranged one after the other successively reducing in number in the reaction gases' direction of flow. The reduction in the number of fuel cells is therein harmonized with the respective residual amount of gas exiting the preceding fuel cell stack. The last fuel cell stack is embodied as a purge cell stack having one or a plurality of purge cells followed by the purge valve.
The fuel cell system is preferably embodied having PEM fuel cells.
The object is furthermore inventively achieved by means of a method for operating a fuel cell system having a fuel cell stack to which reaction gases are ducted on the gas inlet side and which on the gas outlet side has a purge cell having a purge valve, with said purge cell's voltage being measured in the vicinity of a gas outlet toward the purge valve and actuating of the purge valve being controlled as a function of the purge cell's voltage.
The advantages and preferred embodiments cited with reference to the fuel cell system are also to be applied analogously to the method.
An exemplary embodiment of the invention is explained in more detail below with reference to the drawings. The figures listed below are each highly simplified schematics.
According to
The last fuel cell stack is embodied as a purge cell stack 8 having a plurality of purge cells 10. The reaction gas G is ducted in the vicinity of a gas inlet 12 to the purge cell stack 8 and flows downward through the individual purge cells 10 toward a gas outlet 14. Said gas outlet 14 is followed by a purge line 16 that can be closed via a controllable purge valve 18.
The individual purge cells 10 are mutually separated by a bipolar plate 20, shown schematically in
The purge valve 18 is initially closed when the fuel cell system is operating so that the water G forming during the reaction as well as inert gases present in the reaction gases will accumulate in the purge cells 10. The purge cell voltage will drop owing to the accumulation of inert gases. Said voltage is measured and used for controlling a purging operation, which is to say for controlling the purge valve 18. If the voltage falls below a predefined control value, the purge valve 18 will open and the water produced by the reaction as well as the residual gas in the purge cells 10, in particular the inert gases, will be discharged. Both the oxygen side or cathode side and the hydrogen side or anode side of the purge cells 10 are therein expediently preferably each purged, in particular simultaneously, via a separate purge valve 18.
The purge cells will, owing to the accumulation of inert gases, not be supplied adequately with the reaction gases when current is flowing simultaneously. The boundary conditions are thus present for performing an electrolysis of water and the result on the anode side is the partial reaction 4OH−>O2+2H2O+2e−. Oxygen is thus produced which can cause corrosion in the case of the customarily metallic bipolar plate 20. That problem will exist particularly when the voltage in the purge cells 10 drops to the region of the corrosion potential of the material used for the bipolar plates 20.
To prevent corrosion of said type and in order not to allow the voltage of the purge cells 10 to fall below a specific threshold in, where possible, any partial region of the purge cells 10, it is provided for a voltage tap 24 for controlling purging in the lower region of the bipolar plate 20 to be sited approximately at the same level as the gas outlet 14, in particular on the bottom edge side 26 of the bipolar plate 20. The bottom cell voltage UCbottom is measured at said bottom voltage tap 24. A top voltage tap 28 at which a top cell voltage UCtop is tapped is furthermore indicated in
Significantly more sensitive and improved voltage controlling is achieved by means of the bottom voltage tap 24 compared to the top voltage tap 28. What is particularly prevented thereby is that the voltage will fall below a desired threshold of, for example, around 0.5 V in the lower region of the bipolar plate 20. Said threshold is here preferably selected as being above the corrosion potential of the material of the bipolar plate 20. That is because measurements have shown that a significant voltage gradient becomes established between UCtop and UCbottom owing to the inert gases' accumulating primarily in the vicinity of the gas outlet 14. The differences between purge cell controlling performed as a function of UCtop and as a function of UCbottom are apparent from the voltage curves shown in
The individual voltage curves shown by way of example are therein based on a test system having a purge cell stack 8 that has four purge cells 10 through each of which a current of around 560 A flows. The curves of the four purge cells 10 are shown in each of the diagrams.
In
As will be apparent from comparing
In contrast to controlling based on UCtop, controlling based on UCbottom is significantly more sensitive and precise, as is apparent from
As is particularly apparent from comparing
Thus, thanks to more frequently performed purging operations, less water forming as a reaction product and a smaller amount of inert gases will accumulate in the purge cells 10. The purge cell voltage remains significantly higher. The risk of corrosion for the bipolar plates 20 is reduced overall thereby. To minimize the loss of residual reaction gases due to more frequent purging, the purge time or the cross-section of flow of the purge valve 18 is selected as being appropriately small.
| Number | Date | Country | Kind |
|---|---|---|---|
| 04015501.2 | Jul 2004 | EP | regional |
This application is the US National Stage of International Application No. PCT/EP2005/053051, filed Jun. 29, 2005 and claims the benefit thereof. The International Application claims the benefits of European Patent application No. 04015501.2 filed Jul. 1, 2004. All of the applications are incorporated by reference herein in their entirety.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2005/053051 | 6/29/2005 | WO | 00 | 12/28/2006 |
