Method for changing a fuel cell system over to a standby mode as well as such a fuel cell system

Abstract

A method for changing a fuel cell system from a normal mode of operation over to a standby mode comprises the following steps: a) reducing the load withdrawalvia the electric circuit of the fuel cell stackdown to a load within the range from 1% to +5% around a load with an optimal system efficiency, b) regulating down the anode pressure down via the anode supply system, c) in the meantime, maintaining and controlling the cathode gas feed via the cathode supply system so that the pressure differential between the anode spaces and the cathode spaces does not exceed a prescribed maximum pressure differential, d) switching off the cathode gas feed if the pressure differential between the anode spaces and of the fuel cell stack and the environment has reached the prescribed maximum pressure differential, and e) switching off the load withdrawal via the external electric circuit at the latest when a prescribed minimum limit voltage of the fuel cell stack has been reached.

Claims

1. A method for changing a fuel cell system from a normal mode of operation over to a standby mode, the fuel cell system having a fuel cell stack comprising cathode spaces and anode spaces, an anode supply system, a cathode gas supply system as well as an external electric circuit, a maximum efficiency of the fuel cell system being exhibited at a first load, the first load defining an optimal system efficiency load, the method comprising the following steps: a) reducing a loadvia the electric circuit of the fuel cell stackdown to a load within a range from 1% to +5% of the optimal system efficiency load, b) regulating or controlling an anode pressure down via the anode supply system so that an anode operating pressure is reduced, c) in the meantime, maintaining and controlling a cathode gas feed via the cathode supply system in such a way that a pressure differential that sets in between the anode spaces and the cathode spaces does not exceed a prescribed maximum pressure differential, d) switching off the cathode gas feed if a further pressure differential between the anode spaces and of the fuel cell stack and an environment has reached the prescribed maximum pressure differential, and e) switching off the load via the external electric circuit at the latest when a prescribed minimum limit voltage of the fuel cell stack has been reached.

2. The method as recited in claim 1 wherein an electric energy of the fuel cell stack generated to switch off the power withdrawal from the stack in step (e) is fed to an electric consumer or to a battery connected to the fuel cell stack via the electric circuit.

3. The method as recited in claim 2 wherein the electric consumer is an auxiliary aggregate of the fuel cell system or an external consumer.

4. The method as recited in claim 1 wherein the switching off the cathode gas feed includes switching off a conveying device for a cathode operating gas.

5. The method as recited in claim 4 wherein the conveying device is a compressor.

6. The method as recited in claim 1 wherein a recirculation of the anode operating gas is maintained while the anode pressure is being controlled or regulated in step (b) and while the cathode gas feed is being maintained in step (c).

7. The method as recited in claim 1 wherein the load set in step (a) is within the range from 0.7% to +3% of the optimal system efficiency load.

8. The method as recited in claim 7 wherein the load set in step (a) is within the range from 0.5% to +1% of the optimal system efficiency load.

9. The method as recited in claim 1 wherein the prescribed maximum pressure differential is within a range from 0.1 bar to 0.3 bar.

10. The method as recited in claim 1 wherein the prescribed maximum pressure differential is within a range from 0.15 bar to 0.25 bar.

11. A fuel cell system with a fuel cell stack comprising: cathode spaces and anode spaces; an anode supply system; a cathode gas supply system; and an external electric circuit the fuel cell system performing the method as recited in claim 1.

12. The method as recited in claim 1 wherein during the regulating or controlling the anode pressure down via the anode supply system, the anode operating pressure is reduced linearly.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be described below in embodiments on the basis of the accompanying drawings. The following is shown:

(2) FIG. 1 a fuel cell system in a first embodiment;

(3) FIG. 2 a curve of the efficiency of a fuel cell stack (.sub.FC) and of a fuel cell system (.sub.Sys) as a function of the output/load;

(4) FIG. 3 a flow chart of the method sequence for changing a fuel cell system from a normal mode of operation over to standby mode of operation according to an embodiment of the invention; and

(5) FIG. 4 a schematic depiction of the anode and cathode pressures of a fuel cell stack during the course of the method according to the invention.

(6) FIG. 1 shows a fuel cell system designated in its entirety by the reference numeral 100, according to an advantageous embodiment of the invention.

DETAILED DESCRIPTION

(7) As its core component, the fuel cell system 100 comprises a fuel cell stack 10. The fuel cell stack 10 has a plurality of stacked individual cells, of which only one is indicated here by way of an example. Each individual cell comprises an anode space 11 as well as a cathode space 12 that are separated from each other by an ion-conductive polymer electrolyte membrane 13. The anode and cathode spaces 11, 12 each have a catalytic electrode, namely, the anode or the cathode, which catalyze the appertaining partial reaction of the fuel cell reaction. Between two such membrane electrode units, there is also a bipolar plate that serves to feed the operating media into the anode and cathode spaces 11, 12 and that also establishes the electric connection between the individual fuel cells.

(8) In order for the fuel cell stack 10 to be supplied with the operating gases, the fuel cell system 100 has an anode supply system 20 on the one hand, and a cathode supply system 30 on the other hand.

(9) The anode supply system 20 comprises an anode supply path 21 that serves to feed an anode operating gas, for example, hydrogen, into the anode spaces 11. For this purpose, the anode supply path 21 connects a fuel reservoir 23 to the fuel cell stack 10. A regulating means 24 arranged in the anode supply path 21 serves to regulate the mass flow of the fuel. The regulating means 24 is configured, for instance, as a control valve. The anode supply system 20 also comprises an anode exhaust gas path 22 that discharges the anode exhaust gas out of the anode spaces 11 of the fuel cell stack 10. Moreover, the anode supply system 20 has a recirculation line 26 with another regulating means 27 that connects the anode exhaust gas path 22 to the anode supply path 21. The recirculation of fuel is a conventional process to return and to utilize the fuel, which is usually employed more than stoichiometrically. FIG. 1 does not show an optional conveying device which effectuates the circulation of the recirculated anode gas.

(10) The cathode supply system 30 comprises a cathode supply path 31 that feeds a cathode operating gas containing oxygen into the cathode spaces 12 of the stack 10. The cathode operating gas is preferably air. In order to convey and compress the air, there is a compressor 33 in the cathode supply path 31. A cathode exhaust gas path 32 conveys the cathode exhaust gas (exhaust air) out of the cathode spaces 12 and, if applicable, conveys it to an exhaust gas system (not shown here). The compressor is driven by an electric motor 35. Optionally, as shown here, the compressor 33 can be driven with the assistance of a turbine 34 that is arranged in the cathode exhaust gas path 32. In this context, the compressor 33 and the turbine 34 are connected to each other via a shared shaft.

(11) A wastegate line 36 that branches off from the cathode supply path 31 connects the cathode supply path 31 to the cathode exhaust gas path 32. The wastegate line 36 serves to bypass the fuel cell stack 10 when the compressed cathode operating gas is not needed, for example, during low-load phases in the fuel cell stack 10, but when the compressor 33 nevertheless is not supposed to be ramped down. Optionally, a regulating means 37 that is configured, for instance, as a flap or as a control valve can be arranged in the wastegate line 36. The mass flow passing through the wastegate line 36 is regulated by the regulating means 37, thereby regulating the output of the fuel cell stack 10.

(12) Another regulating means 38 can be arranged in the cathode supply path 31, preferably downstream from the branch-off site of the wastegate line 36. Yet another regulating means 39 can be present in the cathode exhaust gas path 32, preferably upstream from an entry site of the wastegate line 36. The regulating means 38, 39 are likewise configured as flaps or as valves and they allow the separation of the cathode spaces 12 of the fuel cell stack 10 from the environment, for example, when the stack 10 is supposed to be switched off.

(13) In a variation of the embodiment shown, the anode exhaust gas path 22 can open up into the cathode exhaust gas path 32 so that anode and cathode exhaust gas can be transported away for a shared exhaust gas after-treatment.

(14) Several additional individual details of the anode and cathode supply systems 20, 30 are not depicted in FIG. 1 for the sake of clarity. In particular, the cathode supply system 30 can have a humidifier in which the cathode operating gas that is to be humidified is humidified by the cathode exhaust gas via a water vapor-permeable membrane. Moreover, the cathode supply system 30 can have a heat exchanger that serves to pre-heat the air that has been compressed by the compressor 33. The warm exhaust air stemming from the cathode spaces 12 normally flows as a heat carrier through the heat exchanger. In this process, the heat exchanger can be bypassed by an appropriate bypass line on the side of the cathode supply path 31 as well as on side of the cathode exhaust gas path 32. There can also be a turbine bypass line that bypasses the turbine 34 on the side of the cathode exhaust gas path 32. Furthermore, systems without a turbine 34 are also known. Moreover, a water separator can be installed in the anode and/or cathode exhaust gas path 32 in order to condense and drain the product water generated by the fuel cell reaction.

(15) The fuel cell system 100 also comprises an external electric circuit 40 that connects the fuel cell stack 10 to an external power system (not shown here) such as, for instance, the electrical system of a vehicle. A number of electric consumers can be connected to the electrical system, especially an electric traction motor for an electric vehicle and/or a battery. By the same token, the electrical peripheral components of the fuel cell system 100, especially the compressor 33, can be supplied via the system. Preferably, the electric circuit 40 is connected to the external power system via a switch with which it can be disconnected.

(16) The fuel cell system 100 shown in FIG. 1 has the following function:

(17) During normal operation of the fuel cell stack 10, that is to say, as long as electric power in being demanded and tapped from the fuel cell stack 10 via the electric circuit 40, the anode spaces 11 of the stack are supplied with the anode operating gas, especially hydrogen, via the anode supply path 21, and the anode gas is discharged via the anode exhaust gas path 22 and at least partially recirculated via the line 26. In this context, the regulating means 25 is only opened occasionally in order to prevent the anode operating gas from becoming enriched with nitrogen, water vapor, etc. At the same time, the cathode operating gas (air) is conveyed out of the environment via the compressor 33 and compressed, after which it is fed into the cathode spaces 12 of the fuel cell stack 10. The cathode gas is discharged via the cathode exhaust gas path. The wastegate regulating means 37 is closed or else partially or completely opened, depending on the load point of the system. During normal operation, the operating pressures in the anode and cathode spaces 11, 12 are kept within the range of, for example, 1.1 bar to 3 bar, depending on the load or output. In this process, the anode pressure is typically set at a slightly higher value than the cathode pressure, for instance, approximately 0.2 bar higher.

(18) FIG. 2 shows the load-dependent curves of a fuel cell stack (.sub.FC) as well as of a fuel cell system (.sub.Sys) which comprises electric peripheral components that consume electric energy (parasitic energy) for their own operation, thus reducing the total efficiency of the system in comparison to the efficiency of the fuel cell over the entire operating range. The current-voltage curve of the fuel cell (not shown here) is similar to the curve .sub.FC of the fuel cell stack in FIG. 2. It can be seen that, at a low load of the system, the efficiency of the stack (as well as its voltage) rises somewhat, but the higher efficiency of the stack is overcompensated by a relatively high consumption on the part of the peripheral components, so that the efficiency of the entire system at low loads drops considerably.

(19) FIG. 2 also shows an upper limit voltage U.sub.max which, if it is exceeded, can cause damage to the fuel cell due to degradation of the catalytic material, and this can lead to cell ageing. For this reason, the value during the operation of fuel cells is not allowed to fall below a lower output limit (indicated here by the operating point (C)). Rather, when the lower output limit or the upper limit voltage U.sub.max is reached, the fuel cell stack is changed over to a standby mode from which the fuel cell can be quickly ramped up again (start-stop mode).

(20) In the state of the art, the transition to the standby mode takes place in that, when the lower output limit (C in FIG. 2) is reached, the air feed into the cathode spaces is interrupted and the oxygen present in the cathode spaces finishes reacting with the fuel (hydrogen) that has been additionally added. In the meantime, there is an additional discharge of electric power out of the stack until the chemical reaction has come to an end. In this process, the discharging process is controlled via a voltage-dependent discharge current. At a constant voltage, the discharge current decreases due to an inadequate supply of oxygen. In the state of the art, the change-over to the standby mode takes place exclusively from points at a low system load and thus at low operating pressures in the fuel cell stack so that the pressure differential over the membrane cannot rise above an overcritical value when the air supply is switched off.

(21) The sequence of the method according to the invention for changing a fuel cell system over to a standby mode according to a preferred embodiment is shown in the flowchart of FIG. 3.

(22) The method starts in step S1, in which the presence of a standby condition is ascertained. This condition can be, for instance, the standstill of the vehicle without the ignition having been switched off, for example, stopping at a traffic light or the overrun mode of the vehicle.

(23) If such a standby condition is present, the method proceeds to step S2, in which the changing of the fuel cell over to the standby mode begins. Starting from a load point L.sub.A during normal operation, which is shown by way of an example with the operating point (A) in FIG. 2, the load withdrawn via the electric circuit is reduced to a load L.sub.B (see operating point (B) in FIG. 2). Here, the load L.sub.B is a load that corresponds essentially to a load with an optimal system efficiency (L.sub.opt) of the fuel cell system 100 (lower curve in FIG. 2). In particular, the withdrawn load is set as precisely as possible (L.sub.B=L.sub.opt) to the load with an optimal system efficiency (L.sub.opt).

(24) Then, in step S3, a systematic ramping down of the anode pressure p.sub.A begins, so that, at a reduced load withdrawal, the anode operating pressure is gradually reduced to such an extent that sufficient fuel is present for the electrochemical reaction. This is done especially in accordance with a prescribed target-pressure curve, which is shown by way of an example with the curve p.sub.A in FIG. 4. The ramping down of the anode pressure p.sub.A can be controlled or regulated, preferably regulated by means of the regulating means 24 configured as a control valve of the anode supply system 20 (see FIG. 1). During the ramping down of the anode pressure p.sub.A, the regulating means 25 arranged in the anode exhaust gas line 22 is preferably closed by appropriately actuating the regulating means 24, while the regulating means 27 in the recirculation line 26 remains open. In this manner, the anode recirculation is kept active. The cathode gas supply is maintained at the same time.

(25) In the subsequent step S4, the anode pressure p.sub.A that is present or that is established in the anode spaces 11, the cathode pressure p.sub.K as well as the ambient pressure p.sub.U are all read in.

(26) In step S5, the pressure differential p.sub.AK between the momentary anode pressure p.sub.A and the momentary cathode p.sub.K is calculated, and so is the calculated pressure differential p.sub.AU between the anode pressure and the ambient pressure.

(27) In the subsequent query S6, it is checked whether the previously ascertained pressure differential p.sub.AU between the anode pressure and the ambient pressure is equal to or smaller than a prescribed maximum pressure differential p.sub.max of, for instance, 0.2 bar. At the beginning of the method, in which the anode pressure p.sub.A is normally still at a relatively high level, the response to the query is no, so that the method proceeds to the next query in step S7.

(28) In step S7, it is checked whether the pressure differential p.sub.AK between the anode pressure p.sub.A and the cathode pressure p.sub.k is equal to or smaller than the maximum pressure differential p.sub.max. If the response to this query is no, the anode pressure p.sub.A is consequently still too high, and the method returns to step S3, where the anode pressure is regulated further down and subsequently, in step S4, the differing pressures are once again ascertained and read in. If, in contrast, the answer to the query S7 is yes, that is to say, if the pressure differential between the anode and cathode spaces of the fuel cell is sufficiently small, then the method proceeds to step S8.

(29) In step S8, the cathode target pressure is reduced by a prescribed predetermined increment dp. This can be done in that the output of the compressor 33 is correspondingly reduced or, in the case of a constant operation of the compressor, in that the wastegate line 36 is opened by partially opening the valve 37. Subsequently, the method returns to S4, where the differing pressures are once again read in.

(30) During the cyclical execution of steps S3 through S8, the hydrogen that is fed in to the system in a diminished quantity is converted by the fuel cell reaction. In this process, the anode pressure p.sub.A is gradually reduced and the cathode pressure p.sub.K is adapted at a constant distance from it which essentially corresponds to the maximum pressure differential p.sub.max. This can be seen in FIG. 4, in which the anode and cathode pressures p.sub.A and p.sub.K are depicted as a function of the time t. During normal operation, up to a point in time t.sub.1, while the fuel cell system is being operated at a correspondingly high output, the pressures p.sub.A and p.sub.K are at a correspondingly high level. In this context, the anode is always operated at a certain low excess pressure with respect to the cathode. At the point in time t.sub.1, the method starts to change the fuel cell over to the standby mode in that the load point is shifted from L.sub.A over to the efficiency-optimized load L.sub.B=L.sub.opt and the anode gas feed is switched off. Starting at the point in time t.sub.1, the anode pressure p.sub.A is continuously ramped down and the fed-in fuel continues to react chemically. The cathode pressure p.sub.K is continuously adapted to the anode pressure p.sub.A in such a way that the pressure differential p.sub.AK between the anode spaces and the cathode spaces of the fuel cell stack 10 does not exceed the maximum pressure differential p.sub.max.

(31) The cycles S3 through S8 in FIG. 3 are carried out until the anode pressure p.sub.A has sunk to such an extent that the pressure differential p.sub.AU with respect to the ambient pressure is equal to or smaller than the maximum pressure differential p.sub.max. As soon as this is the case, the answer to the query in step S6 is yes and the method proceeds to step S9, where the cathode gas feed to the fuel cell stack 10 is switched off. This is done by switching off the compressor 33.

(32) Subsequently, in step S10, the momentary voltage U of the fuel cell stack 10 (alternatively the output or load) is read in. In step S11, it is checked whether the momentary voltage U has already reached a prescribed minimum limit voltage U.sub.min. As long as this is not the case and thus the answer to the query in step A11 is yes, the method returns to step S10, whereby the appertaining parameters are once again read in.

(33) As soon as the cell voltage U has reached the minimum limit voltage U.sub.min, the method proceeds to step S12, whereby the load withdrawal from the fuel cell stack 10 via the electric circuit 40 is terminated. This ends the method and the fuel cell system 100 is in the standby mode.

(34) The time when the maximum pressure differential between the anode spaces 11 of the stack and the environment has been reached is indicated by the point in time t.sub.2 in FIG. 4. At this point in time, according to the explained method, the air feed is switched off. As a result, the cathode pressure p.sub.K is established at the value of the ambient pressure p.sub.U. The anode pressure p.sub.A, in contrast, continues to drop since hydrogen continues to be reacted.

(35) If the system is to be shut down completely, for instance, because the vehicle is to be parked (the ignition is switched off), all of the peripheral and control devices of the system are switched off. Moreover, additional measures can be taken in order to generate and maintain an inert atmosphere in the anode and cathode spaces of the stack 10. In particular, the anode and cathode spaces 11 and 12 are segregated from the environment in that the regulating means 24, 25, 38 and 39 are closed.

LIST OF REFERENCE NUMERALS

(36) 100 fuel cell system 10 fuel cell stack 11 anode space 12 cathode space 13 polymer electrolyte membrane 20 anode supply system 21 anode supply path 22 anode exhaust gas path 23 fuel tank 24 regulating means/valve 25 regulating means 26 recirculation line 27 regulating means/valve 30 cathode supply system 31 cathode supply path 32 cathode exhaust gas path 33 conveying device/compressor 34 turbine 35 electric motor 36 wastegate line 37 wastegate regulating means 38 regulating means/valve 39 regulating means/valve 40 external power circuit p.sub.A pressure in the anode space, anode pressure p.sub.K pressure in the cathode space, cathode pressure p.sub.U ambient pressure p.sub.AK pressure differential between the anode pressure and the cathode pressure p.sub.AU pressure differential between the anode pressure and the ambient pressure U voltage L load or output .sub.FC efficiency of the fuel cell stack .sub.Sys efficiency of the fuel cell system