WO2008148486A1

WO2008148486A1 - Fuel circuit for a fuel cell system, and method for its operation

Info

Publication number: WO2008148486A1
Application number: PCT/EP2008/004227
Authority: WO
Inventors: Hans-Jörg Heidrich
Original assignee: Daimler Ag; Ford Global Technologies, Llc
Priority date: 2007-06-04
Filing date: 2008-05-28
Publication date: 2008-12-11
Also published as: DE102007026004A1

Abstract

The invention relates to a fuel circuit for a fuel cell system, comprising a fuel cell unit (4) which produces electrical energy by being fed with fuel and an oxidant; a fuel supply line (5) for supplying fuel to the fuel cell unit (4); an ejector (2) arranged in the fuel supply line (5); an anode feedback line (6) which connects the anode outlet (4B) to the fuel supply line (5) and branches into a first line element (6A), which leads to the ejector (2), and into a second line element (6B), which opens into the fuel supply line (5) between the ejector (2) and the fuel cell unit (4), with the two line elements (6A, 6B) together having at least one first and one second valve (8A, 8B), wherein a control unit is provided and operates the two valves (8A, 8B) such that one and only one of the two valves (8A, 8B) is ever open, and the other is in each case closed.

Description

Daimler AG and

Ford Global Technologies, LLC

rcuit Tor a fuel cell system, and method for its operation

The invention relates to a fuel circuit for a fuel cell system, and to a method for operation of a fuel circuit such as this according to the precharacterizing clauses of Patent Claim 1, Patent Claim 6 and Patent Claim 15.

Fuel cell systems of the type on which this is based are known. Because of their high electrochemical efficiency and their versatile options for use, fuel cell systems have quickly gained importance. In this case, PEM (Polymer Electrolyte Membrane) fuel cell systems or SO (Solid Oxide) fuel cell systems are being used by preference, in which the electrical energy is produced by chemical reactions with the involvement of elementary hydrogen.

Because of their advantageous operating temperatures, low- temperature fuel cell systems are particularly suitable for use in automobiles. Electrical energy can be obtained by supplying hydrogen to the anode side and oxygen or air to the cathode side in a manner known per se for the fuel cell reaction in a fuel cell. Fuel cell stacks comprising a plurality of individual cells are normally used in fuel cell systems. In a fuel cell stack such as this, hydrogen and oxygen are supplied to the individual fuel cells via gas distribution structures. In principle, the fuel supplied to a fuel cell is not consumed completely in the fuel cell but results in an anode off-gas with residual fuel gases such as H2, for example, and inert gases and H2O. The anode off-gas can on the one hand be reburned in a burner, and let out into the surrounding area as off-gas. Another option is to recirculate the anode off-gas into the fuel supplied to the fuel cell. In this case, the anode off-gas is fed back to the anode inlet and, mixed with fresh hydrogen, is supplied to the anode again. However, these systems require so-called "purging" at intervals, in order to force water droplets and inert gases out of the gas distribution structure of the fuel cells. During this process, clean and dry hydrogen gas is blown at an increased flow rate and/or in a pulsed form through the gas distribution structure. However, this has the disadvantage that purging results in some of the hydrogen being lost, thus reducing the fuel-cell efficiency.

Finally, DE 102 51 878 Al discloses a fuel circuit of this generic type for a fuel cell system, as is defined in the precharacterizing clause of the present Patent Claim 1. This document relates to the production of uniform pressure conditions for the recirculation of the anode off-gas. However, the known fuel cell system is less suitable for generation of pressure pulses for subsequent purging.

The object of the present invention is to provide a fuel circuit for a fuel cell system and a method for operation of a fuel circuit such as this, by means of which the water/anode off-gas from a fuel cell can be disposed of more efficiently and safely.

According to the invention, the object is achieved by the characterizing features of Patent Claim 1, of Patent Claim 6 and of Patent Claim 15. The fuel circuit according to the invention for a fuel cell system comprises a fuel cell unit which produces electrical energy by being fed with fuel and an oxidant. Furthermore, it comprises a fuel supply line for supplying fuel to the fuel ceil unit and an ejector arranged in this line, as well as an anode feedback line which connects the anode outlet to the fuel supply line and branches into a first line element, which leads to the ejector, and into a second line element, which opens into the fuel supply line between the ejector and the fuel cell unit. Together, the two line elements have at least one first and one second valve. According to the invention, a control unit is provided and operates the two valves such that one and only one of the two valves is ever open, and the other is in each case closed.

In a development of the invention according to Claim 2, a recirculation pump is arranged in the second line element.

An advantageous embodiment according to Claim 3 is obtained by arranging a valve in each of the two line elements.

A further advantageous embodiment according to Claim 4 is obtained by both valves being arranged in the first line element which leads to the ejector.

In a developed embodiment according to Claim 5, a gas reservoir is arranged between the two valves in the first line element.

In the method according to the invention for operation of a fuel circuit for a fuel cell system according to Claim 6, the two valves are operated such that one and only one of the two valves is ever open, and the other is in each case closed. In one advantageous embodiment of the method according to Claim 7, the power of the recirculation fan is controlled as a function of the respective valve state.

According to a development of the method in Claim 8, the recirculation fan is operated during normal operation such that a volume flow which is as constant as possible is applied to the anode inlet of the fuel cell unit.

Furthermore, Claim 9 results in the advantage that the recirculation fan is operated in the purge mode such that, at the time at which the second valve is opened, the recirculation fan also produces a high feed power in order to briefly produce a major reduced pressure at the anode outlet of the fuel cell unit.

A further advantageous refinement option of the method according to Claim 10 results in the purge mode being used in the medium and high load ranges of the fuel cell unit.

Furthermore, according to Claim 11, it is a major advantage for the recirculation fan to be operated such that it provides all or at least the majority of the recirculation of the anode off-gas during low-load operation of the fuel cell unit .

In one preferred embodiment of the method according to Claim 12, the hydrogen is metered from the supply container into the ejector as a function of the position of the first valve and/or of the second valve, and/or as a function of the operation of the recirculation fan such that, at least at times, the metered amount of hydrogen from the supply container into the ejector is increased, thus resulting in pulsed metering and in a boost to the operation of the ejector.

According to one development of the method in Claim 13, the increase in the metered amount of hydrogen from the supply container into the ejector at times takes place when the first valve is open.

According to one particularly preferred development of the method in Claim 14, the pulsed metering of hydrogen into the ejector takes place when the fuel cell unit is in a low load range .

A further embodiment of the method according to the invention for operation of the fuel circuit for a fuel cell system according to Claim 15 comprises the following steps:

production of a reduced pr essure by means of an ejector in specific parts of the fuel circuit on the induction side of the ejector, temporary storage of this reduced pressure in these specific parts, while the pressure in other parts of the circuit is higher, pressure equalization between these two parts of the fuel circuit, with the increase in the flow rate which results from this in the anode circuit being used particularly in the purge mode or during starting as a drive for recirculation of the anode off-gas.

Further advantageous refinements of the invention will become evident from the description and the drawings. The invention will be explained by way of example in the following text with reference to schematic drawings, in which:

Figure 1 shows a fuel circuit for a fuel cell system with a gas reservoir between two valves in a first line element;

Figure 2 shows a fuel circuit for a fuel cell system without a gas reservoir, but with two valves in a first line element, and

Figure 3 shows a fuel circuit for a fuel cell system without a gas reservoir, but with one valve in each case in a first and a second line element.

In order to illustrate the invention, Figure 1 shows a first preferred refinement of a schematically illustrated fuel circuit for a fuel cell system with an arrangement for recirculation of anode off-gas (anode circuit) . The fuel cell system comprises a fuel cell unit 4 for production of electrical energy, which unit has a plurality of individual fuel cells which in this exemplary embodiment are in the form of PEM fuel cells. On the anode side, the fuel cell stack has an anode inlet 4A via which hydrogen is supplied to the fuel cell unit 4 via a fuel supply line 5 and via an ejector 2, which is located in this line 5, from a supply container 1, preferably a high-pressure hydrogen tank, and has an anode outlet 4B, which is connected to an anode feedback line 6 for feeding the anode off-gas back into the fuel supply line 5. The anode feedback line 6 contains an apparatus 7 for separation of liquid water, and this is referred to in the following text as a separator. Downstream from the separator 7, the anode feedback line 6 branches into a first line element 6A, which is connected to the fuel supply channel 5 via the induction opening 2A of the ejector 2, and a second line element 6B, which opens into the fuel supply channel 5 in an area downstream from the ejector 2, with the second line element 6B having a recirculation fan 3 or a recirculation pump which is advantageously designed to be proof against explosions. Downstream from the branch, the first line element 6A has a second valve 8B, a downstream container 9, preferably a gas container or a gas reservoir, followed by a first valve 8A which is arranged at a distance from the induction opening 2A of the ejector 2, with the anode off-gas emitted from the fuel cell unit 4 being sucked in through the induction opening 2A by use of a reduced pressure which is created when hydrogen flows in the ejector 2 and is passed from the supply container 1 into the fuel cell unit 4.

In addition, a gas purging function for removal of inert gases may be integrated in the liquid water separator 7 so that, in addition to water, gas components, in particular inert gases, can be removed from the anode feedback line 6 in order to avoid adversely affecting the performance of the fuel cells by the presence of these inert gases. This gas purging function may, however, also be carried out by separate gas purging valves (not illustrated) , which are preferably arranged in a separate purging line downstream from the liquid water separator 7. These purging valves may be synchronized to the operation of the ejector 2 and/or the cycling of the valves 8A and/or 8B. A purging valve such as this is preferably opened when the hydrogen concentration at the anode outlet 4B of the fuel cell unit 4 is particularly low. This is the situation whenever the proportion of the recirculated gas in the fuel supply line 5 is particularly low, that is to say when the recirculation fan 3 is being operated at low power and/or the second valve 8B is closed, or less hydrogen is being supplied via the ejector 2. Furthermore, the gas purging function can be carried out by means of a gas purging line (not illustrated) which opens into an air supply line, which is not illustrated, upstream of a cathode inlet, which is not shown. The air supply (which is not illustrated separately here) for the fuel cell unit 4 is provided by means of a compressor, intercooler and gas/gas moisturizing unit.

Identical components or those having the same effect are provided with corresponding reference symbols in the following drawings to those in Figure 1. In order to avoid repetition, reference is also made to the description relating to Figure 1 for corresponding components.

Figure 2 shows a second embodiment according to the invention on the basis of a schematically illustrated fuel circuit in which the gas container or reservoir 9 shown in Figure 1 and arranged in the first line element 6A has been dispensed with. In this case, the pipeline volume located between the two valves 8A and 8B in the line element section 6A¹ of the first line element 6A is used as a gas reservoir.

The third refinement of a fuel circuit according to the invention, as illustrated in Figure 3, which represents a modification of the second embodiment, likewise dispenses with a gas reservoir 9. In contrast to Figure 2, the two valves 8A and 8B are arranged in different line elements: the first valve 8A is located in the first line element 6A while, in contrast, the second valve 8B is arranged in the second line element 6B, downstream or else upstream of the recirculation fan 3 or recirculation pump. In the exemplary embodiments shown in Figures 1 to 3, electromagnetic, electrical, hydraulic, mechanical or pneumatically operated shut-off valves may be used as the valves .

In order to avoid enrichment of inert gases in the fuel cell anode, to smooth out the fuel concentration and to remove product water from the anode, on the one hand the off-gas, which is rich in hydrogen, from the anode outlet 4B is fed back to the anode inlet 4A of the fuel cell unit 4 and, on the other hand, parts of the anode off-gas such as inert gases and water are removed from the anode circuit by purging via so-called purging valves. However, one particular disadvantage in this case is that some of the hydrogen is lost during purging, thus reducing the efficiency of the fuel cell unit 4. Since product water accumulates in the fuel cell unit 4 particularly when there is no load on it, this phenomenon resulting from a greater loss of anode pressure has been counteracted until now by increasing the fuel stoichiometry when there is no load. However, this has the further disadvantage that more energy is consumed to operate the recirculation pump 3. The fuel circuit according to the invention in conjunction with the method according to the invention for operation of the fuel circuit considerably reduces the loss of hydrogen during removal of inert gases and product water from the anode circuit, with the power of the recirculation fan 3 or the recirculation pump being controlled as a function of the position of the valves 8A and 8B such that the specified fuel volume flow is always achieved in the fuel cell unit 4.

For this purpose, the method according to the invention comprises two different operating phases, an operating phase 1 which is used to smooth out the volume flow applied to the anode inlet 4A, and a purging mode (operating phase 2) for actually removing water from the fuel cell unit 4. Both operating phases result in a considerable improvement in efficiency for water removal from the fuel cell unit 4.

In the operating phase 1, the first valve 8A is open and the second valve 8B is closed at the same time, while in contrast, in the purging mode, the first valve 8A is closed and the second valve 8B is open at the same time.

The first valve 8A is open in the operating phase 1. Once a certain amount of time has elapsed after opening of the first valve 8A, the volume flow at the ejector inlet 2A decreases when the pressure in the gas container 9, as shown in Figure 1 or in the line element section 6A¹ , which is illustrated in Figure 2 and is located between the valves 8A and 8B in the line element 6A, has decreased to close to a final value, as a result of the effect of the ejector 2. In this case, the ejector 2 produces a vacuum in the gas container 9 and in the line element section 6A¹. As the volume flow at the ejector inlet 2A decreases ever more, the power of the recirculation fan 3 is increased successively in order to produce a volume flow which is as constant as possible at the anode inlet 4A of the fuel cell unit 4.

During the subsequent purging mode, the first valve 8A is closed, and the second valve 8B is opened immediately after this. In this case, the reduced pressure stored in the gas reservoir 9 or the line element section 6A¹ results in brief, intensive purging in the anode feedback line 6 and in the fuel cell unit 4, so that, according to the invention, liquid water is removed extremely efficiently from the fuel cell unit 4 (purging effect) . At the same time, the feed power for the recirculation fan 3 can be briefly increased in order to enhance the reduced pressure at the anode outlet 4B and thus to boost the purging effect.

The power of the recirculation fan 3 is then reduced again, when the volume flow at the inlet 2A of the ejector 2 is increased again by closing the second valve 8B and opening the first valve 8A. In principle, the recirculation fan 3 is operated in the purging mode such that, at the time at which there is a high volume flow at the inlet 2A of the ejector 2, the recirculation fan 3 produces a reduced feed power in order to produce a constant volume flow at the anode inlet 4A of the fuel cell unit 4.

The power of the recirculation fan 3 can be increased or reduced periodically depending on the clock cycle of the valves 8A and 8B. In this case, the opening or closing times may differ so that the operating phase 1 (first valve 8A open, second valve 8B closed, higher pump power of the recirculation fan 3) lasts for longer than the purging mode (first valve 8A closed, second valve 8B open, lower pump power of the recirculation fan 3). Cycling of the valves 8A, 8B in the purging mode results in a pulsed flow in the anode feedback line 6 and in the fuel cell unit 4. The water removal process, which is considerably more efficient than that in the prior art, advantageously counteracts excessive water accumulation within the fuel cell unit 4 and in the same way allows fuel to be transported better in the fuel cell unit 4, therefore increasing the efficiency of the fuel cell. The valve control dynamics therefore also improve the purging effect in the fuel cell unit 4.

This control strategy is preferably used in the medium and high load ranges since this is where the ejector 2 produces the greatest power. In the low load range, the feedback of anode off-gas into the fuel cell operation is provided mainly or completely by the recirculation fan 3.

A further advantageous effect of the embodiments described above is that a vacuum pressure or reduced pressure can be maintained in the gas reservoir 9 and in the line element section 6A' during a phase in which the fuel cell system is shut down, so that the anode circuit can be passively purged or recirculated during the shutdown time or during the next starting process without having to operate the hydrogen supply container and the fuel cell system. This has advantages in terms of safety aspects relating to hydrogen- powered drive systems.

The method according to the invention as described above is also used in Figure 3. In contrast to Figure 1 and Figure 2, only one first valve 8A is, however, arranged in the line element 6A in Figure 3. The valve 8A is in this case advantageously arranged as far from the ejector 2 as possible in the line element 6A in order to ensure that the volume of the line element section 6A" between the ejector 2 and the valve 8A, and therefore also the purging effect, are as great as possible. The second valve 8B in the line element 6B is closed and opened again cyclically and therefore likewise making it possible to achieve the effects described above on the feed power in the anode circuit. If the second valve 8B is arranged downstream from or upstream of the recirculation fan 3, purging and/or a pulsed flow in the line element 6B is likewise achieved by cyclic opening and closing of the valve 8B. This advantageously results in an additional purging effect. In this case, the valves 8A and 8B are alternately opened and closed, so that the two valves are never open at the same time, that is to say the valves 8A and 8B are opened and closed on opposite cycles: the valve 8A is opened when the valve 8B is closed, and vice versa. In this case, the power of the recirculation pump 3 is thus synchronized to the position of the valve 8B such that the power of the pump 3 is reduced or restricted to zero when the valve 8B is closed and the valve 8A in the line element 6A is opened. This control strategy is used in particular in the medium and high load ranges since, in this case, the ejector 2 has the necessary power to maintain the feedback of the anode off-gas even when the valve 8B is closed, that is to say when the recirculation pump 3 is more or less inactive.

When the valve 8A is closed and the valve 8B is open, the power of the recirculation pump 3 can be increased such that it takes over or compensates for the restricted feed power of the ejector. This is preferably done in the low load range since in this case the recirculation of the anode off-gas in the anode circuit is provided mainly or completely by the recirculation pump 3.

In order to boost the effect of the ejector in particular in the low load range, the operation of the valves 8A and/or 8B and/or of the recirculation pump 3 can additionally be synchronized to the metering of fresh hydrogen from the hydrogen supply container 1. The metering apparatus can in this case preferably be integrated in the ejector 2, by arrangement of the metering valve, or else may be fitted in the line between the hydrogen supply container 1 and the ejector 2. The metering of hydrogen can be increased when the valve 8A is open, and can be reduced when the valve 8A is closed. This results in advantageous pulsed metering of fresh hydrogen. During the high-pulse phase, the amount of hydrogen supplied is increased, thus boosting the effect of the ejector 2, particularly in the low load range. In principle, in the described method according to the invention for operation of the fuel circuit for a fuel cell system, a reduced pressure is produced in specific parts of the induction side of the fuel circuit by means of an ejector, which reduced pressure is temporarily stored in these parts of the fuel circuit, while the pressure in other parts of the circuit is higher. The pressure is equalized between these two parts of the fuel circuit, and the increase in the flow rate in the anode circuit resulting from this is used in particular as the drive for recirculation of the anode off-gas during the purging mode or during starting.

Claims

Daimler AG andFord Global Technologies, LLCPatent Claims

1. Fuel circuit for a fuel cell system, comprising a fuel cell unit (4) which produces electrical energy by being fed with fuel and an oxidant; a fuel supply line (5) for supplying fuel to the fuel cell unit (4) ; an ejector (2) arranged in the fuel supply line (5) ; an anode feedback line (6) which connects the anode outlet (4B) to the fuel supply line (5) and branches into a first line element (6A), which leads to the ejector (2), and into a second line element (6B), which opens into the fuel supply line (5) between the ejector

(2) and the fuel cell unit (4), with the two line elements (6A, 6B) together having at least one first and one second valve (8A, 8B), characterized in that a control unit is provided and operates the two valves (8A, 8B) such that one and only one of the two valves (8A, 8B) is ever open, and the other is in each case closed.

2. Fuel circuit according to Claim 1, characterized in that a recirculation pump (3) is arranged in the second line element (6B) .

3. Fuel circuit according to one of Claims 1 or 2, characterized in that a valve (8A, 8B) is arranged in each of the two respective line elements (6A, 6B) .

4. Fuel circuit according to one of Claims 1 or 2, characterized in that both valves (8A, 8B) are arranged in the first line element (6A) to the ejector (2) .

5. Fuel circuit according to Claim 4, characterized in that a gas reservoir (9) is arranged between the two valves (8A, 8B) in the first line element (6A) .

6. Method for operation of a fuel circuit for a fuel cell system according to one of Claims 1 to 5, with the two valves (8A, 8B) being operated such that one and only one of the two valves (8A, 8B) is ever open, and the other is in each case closed.

7. Method according to Claim 6, with the power of the recirculation fan (3) being controlled as a function of the respective valve state.

8. Method according to Claim 6 or 7, characterized in that the recirculation fan (3) is operated during normal operation such that a volume flow which is as constant as possible is applied to the anode inlet (4A) of the fuel cell unit (4) .

9. Method according to one of Claims 6 to 8, characterized in that the recirculation fan (3) is operated in the purge mode such that, at the time at which the second valve (8B) is opened, the recirculation fan (3) also produces a high feed power in order to briefly produce a major reduced pressure at the anode outlet (4B) of the fuel cell unit (4) .

10. Method according to Claim 9, characterized in that the purge mode is used in the medium and high load ranges of the fuel cell unit (4) .

11. Method according to one of Claims 6 to 10, characterized in that the recirculation fan (3) is operated such that it provides all or at least the majority of the recirculation of the anode off-gas during low-load operation of the fuel cell unit (4) .

12. Method according to one of Claims 6 to 11, characterized in that the hydrogen is metered from the supply container (1) into the ejector (2) as a function of the position of the first valve (8A) and/or of the second valve (8B), and/or as a function of the operation of the recirculation fan (3) such that, at least at times, the metered amount of hydrogen from the supply container (1) into the ejector (2) is increased, thus resulting in pulsed metering and in a boost to the operation of the ejector (2) .

13. Method according to Claim 12, characterized in that the increase in the metered amount of hydrogen from the supply container (1) into the ejector (2) at times takes place when the first valve (8A) is open.

14. Method according to one of Claims 12 or 13, characterized in that the pulsed metering of hydrogen into the ejector (2) takes place when the fuel cell unit (4) is in a low load range.

15. Method for operation of the fuel circuit for a fuel cell system, having the following steps: production of a reduced pressure by means of an ejector (2) in specific parts of the fuel circuit on the induction side of the ejector (2), temporary storage of this reduced pressure in these specific parts, while the pressure in other parts of the circuit is higher, pressure equalization between these two parts of the fuel circuit, with the increase in the flow rate which results from this in the anode circuit being used particularly in the purge mode or during starting as a drive for recirculation of the anode off-gas.