GB2510256A

GB2510256A - Fuel cell system and method for operating a fuel cell system

Info

Publication number: GB2510256A
Application number: GB1322582.6A
Authority: GB
Inventors: Richard Fellows; Gustav Bohm; Michael Bader
Original assignee: Daimler AG
Current assignee: Mercedes Benz Group AG
Priority date: 2013-12-19
Filing date: 2013-12-19
Publication date: 2014-07-30
Also published as: DE102014018321A1; GB201322582D0

Abstract

A fuel cell system 10 is disclosed, in particular for a vehicle, with a fuel cell stack 12, a supply pipe 14 for conveying an oxidizing agent to a cathode side of the fuel cell stack 12, an exhaust pipe 16 connected to the cathode side of the fuel cell stack. The supply pipe and the exhaust pipe are connected to each other via a bypass 20. The fuel cell system also comprises a shutoff element 22 configured to open or close the bypass. The fuel cell system further comprises a control unit 26 designed to open the shutoff element upon a shutdown of the fuel cell system. The shutoff element is located downstream of a junction 24 at which the bypass branches off from the supply pipe. Preferably the shutoff element is a flap, which is de-energized in the open position. The shutoff element may be positioned closer to the supply than the exhaust pipe, and the shutoff element may be thermally coupled to a component of the fuel cell system with a larger heat capacity, e.g. a compressor. There may be a further shutoff element 32, 34.

Description

Fuel Cell System and Method for Operating a Fuel Cell System The invention relates to a fuel cell system, in particular for a vehicle, with a fuel cell stack, a supply pipe for conveying an oxidizing agent to a cathode side of the fuel cell stack and an exhaust pipe connected to the cathode side of the fuel cell stack. The supply pipe and the exhaust pipe are connected to each other via a bypass. A shutoff element is configured to open or close the bypass. The fuel cell system further comprises a control unit designed to open the shutoff element upon a shutdown of the fuel cell system. The invention further relates to a method for operating such a fuel cell system.

Fuel cell stacks, in particular fuel cell stacks for vehicles, often utilize hydrogen as fuel.

They comprise a multitude of fuel cells with a cathode and an anode each. The hydrogen is supplied to the anode side of the fuel cell stack. In general compressed air is utilized as an oxidizing agent conveyed to the cathode side of the fuel cell stacks. The anode and the cathode of each cell are separated from each other by a membrane which usually supports catalysts for the electrochemical reaction of the hydrogen with the oxygen.

If a vehicle equipped with a fuel cell system is turned off, for example when parking the vehicle, there is still hydrogen present on the anode side of the fuel cell stack. The air present on the cathode side or air side of the fuel cell stack is preferably depleted from oxygen by a continuing reaction of the oxygen with the hydrogen present in the fuel cell stack. A low concentration of oxygen on the cathode side of the fuel cell stack is advantageous for a restart of the fuel cell system. The longer such a low concentration can be maintained on the cathode side of the fuel cell stack, the better it is for preventing early aging of the catalysts, i.e. a premature degradation of the catalysts.

If the vehicle is turned off for a longer period of time and thus the fuel cell system is also turned off, hydrogen diffuses via the membrane from the anode side to the cathode side of each fuel cell. Simultaneously hydrogen can also diffuse outside the fuel cell stack through gaskets. The diffusion through the membrane is particularly quick, if fresh air with a normal oxygen content infiltrates into the cathode chambers of the fuel cells. Then the hydrogen that has diffused trough the membrane reacts to water at the catalyst. Thus, a high concentration gradient is maintained and the depletion of hydrogen on the anode side of the fuel cell stack continues. A high oxygen content on the cathode side of the fuel cell stack can also lead to a diffusion of oxygen to the anode side of each fuel cell and to a direct oxidation of hydrogen on the anode side.

After all the hydrogen on the anode side of the fuel cell stack is consumed, oxygen accumulates on the anode side of the fuel cell stack until the gas composition on the anode side is equivalent to that of ambient air, i.e. about 21 o/© oxygen and 79 %of nitrogen. If in such a situation the vehicle and the fuel cell system is re-started, a status is reached in which the anode side of the fuel cell stack, which is filled with the oxygen/nitrogen mixture is supplied with hydrogen. This is an undesirable process, as it leads to early aging of the catalysts supported on the electrodes of the fuel cells.

It is therefore desirable to prevent fresh air from quickly infiltrating into the cathode side of the fuel cell stack after the vehicle and the fuel cell system has been shut down.

Document US 2005/0112424 Al describes a fuel cell system with an air flowing passage being connected to a fuel cell stack of the fuel cell system. An exhaust passage is connected to an exhaust side of the fuel cell stack. An air pump is utilized to compress the air conveyed to the fuel cell stack via a supply pipe of the air flowing passage. A bypass branches off from the supply pipe, which allows detouring the fuel cell stack. At a point at which the bypass branches off from the supply pipe a three-way valve is located.

When the operation of the fuel cell system is stopped, a control unit selects the bypass by adjusting the three-way valve such that the airflow through the fuel cell stack is cut off.

When the control unit detects a freezing of the three-way valve, the air pump conveys air to the bypass. By an adiabatic compression of the air a high-pressure heating gas is produced upstream of the three-way valve. Thus, the three-way valve is heated and thawed accordingly.

Such a fuel cell system is rather costly and complex.

A fuel cell system with a bypass connecting a supply pipe and an exhaust pipe of a fuel cell stack is also disclosed in document JR 2009 151 989 A. It is an object of the present invention to provide a fuel cell system and a method of the initially mentioned kind, which is particularly simple and economic.

This object is solved by a fuel cell system having the features of claim 1 and by a method having the features of claim 8. Advantageous configurations with convenient further developments of the invention are specified in the dependent claims.

According to the invention the shutoff element is located downstream of a junction at which the bypass branches off from the supply pipe. Thus, a very simple shutoff element can be utilized to open or close the bypass, and no complex and costly three-way valve is needed. Thus, the fuel cell system is particularly simple and economic.

The invention is based on the finding that without a bypass air can flow trough the fuel cell stack after shutdown of the fuel cell system, i.e. during non driving situations if a vehicle is equipped with the fuel cell system. Such an unintended air flow can be caused by the pressure of wind to which the vehicle and the fuel cell stack are exposed. Also the ingress of air into the fuel cell stack can be caused by thermal convection, i.e. due to the heat distribution in the fuel cell system when it is cooling off after shutdown. Such a thermal convection can lead to an aspiration of fresh air from the outside into the fuel cell stack.

As opening the shutoff element and thus the bypass efficiently prevents the entering of fresh air into the fuel cell stack, the negative effects related to the presence of oxygen in the fuel cell stack can be avoided or at least reduced. As the bypass directly connects the supply pipe with the exhaust pipe fresh air which is entering the supply pipe due to the pressure of wind or due to thermal effects circumvents the fuel cell stack. This is due to the fact that the backpressure of the bypass is by far lower than the backpressure of the fuel cell stack. The convection driven or wind driven air flow through the fuel cell system is mainly directed through the bypass and only to a negligible extend -if at all -through the fuel cell stack. Thus, the negative effects associated with oxygen entering the stack are considerably reduced.

In a vehicle equipped with the fuel cell system the fuel cell stack can be protected from unintended air flow through it over a considerably long period of time by opening the shutoff element located within the bypass compared to a fuel cell system without the bypass. For example, the period of time can be five times longer than the period observed for a fuel cell system without the bypass under the same external wind pressure conditions or internal thermal convection conditions. For example if strong wind leads to an accumulation of oxygen on the hydrogen side of the fuel cell stack within two minutes if no bypass with opened shutoff element is provided, this time can be prolonged to about ten minutes by providing the bypass with the shutoff element moved into its open position.

This is in particular important if the fuel cell system is installed in a vehicle which is utilized for short distance delivery of goods and thus stopped or parked for several minutes in short intervals.

The shutoff element located within the bypass is also advantageous during a start up of the fuel cell system. If hydrogen has migrated to the cathode side of the fuel cell stack after the fuel cell system has been shut down, this hydrogen is blown out of the fuel cell stack during start up. In such a situation operating the shutoff element can help diluting hydrogen in the exhaust pipe. Thus, a concentration of hydrogen in the exhaust pipe can be maintained below an ignition limit, i.e. below 4 %.

Also under certain operation conditions of a compressor utilized to increase the pressure of the oxidizing agent upstream of the fuel cell stack the shutoff element can be operated as a pressure control valve or waste gate. Thus, damage to the fuel cell stack can be avoided by operating the shutoff element.

Advantageously the shutoff element can be designed as a flap. A flap as shutoff element is particularly reliable in operation and also very economic. Also, the utilization of a flap allows to open a passage within the bypass, which has a comparably large diameter. In particular the diameter of the bypass cleared by the flap can be about the same diameter as the supply pipe and the exhaust pipe respectively. Thus, by opening the flap the advantage of a very low backpressure of the bypass can be fully exploited.

Preferably the shutoff element is in an open position if the shutoff element is de-energized. With such a configuration no energy is needed to maintain the shutoff element in the open position, i.e. when the fuel cell system is shut down.

In a further advantageous embodiment within the bypass the shutoff element is closer to the supply pipe than to the exhaust pipe. Such an arrangement assures that the shutoff element is sufficiently far away from the exhaust pipe and therefore not affected by the water contained in the exhaust of the fuel cell stack. This is important to avoid freezing of the shutoff element when the temperature of the fuel cell system drops below zero degrees Celcius.

In a further advantageous embodiment the shutoff element is thermally coupled to a component of the fuel cell system which has a larger heat capacity than the shutoff element. Due to the large heat capacity this component cools off more slowly than other components of the fuel cell system. This also slows down the cooling off of the shutoff element. In this way a freezing of the shutoff element can be delayed.

This is in particularly true if the shutoff element is thermally coupled to a compressor of the fuel cell system, wherein the compressor is configured to increase the pressure of the oxidizing agent upstream of the fuel cell stack. If the shutoff element is connected to the compressor, for example by a highly heat conductive element, the heat stored in the compressor efficiently delays a potential freezing of the shutoff element. Also, in this way the shutoff element is located very close to the supply pipe and thus not influenced by the humidity of the exhaust gas.

Additionally or alternatively the shutoff element can be thermally coupled to the fuel cell stack and/or to an electric machine. If the heat capacity of the fuel cell stack itself is utilized to delay the freezing of the shutoff element, it is advantageous to locate the shutoff element far away from components of the fuel cell stack which transport water, i.e. a product of the electrochemical reaction within the fuel cell stack.

In particular if the fuel cell system is utilized within a vehicle, an electric machine configured for propelling the vehicle can be utilized as the component with a large heat capacity.

Still further, the shutoff element can be, in particular temporarily, heated to avoid freezing of the shutoff element. Such a heating of the shutoff element can be performed if the temperature outside the fuel cell stack drops below zero and/or if a restart of the fuel cell system can be expected within a certain period of time. It can thus be assured that during or before start up of the fuel cell system the shutoff element can be readily moved into the position in which it closes the bypass at least partially.

It has further proven to be advantageous if the fuel cell system comprises at least one further shutoff element. Such a further shutoff element can be located within the supply pipe and can be configured to open or close an inlet of the fuel cell stack. The further or supplementary shutoff element arranged in the supply pipe efficiently prevents ambient air to enter the fuel cell stack both due to the pressure exerted by wind and due to thermal convection.

Alternatively or additionally the further shutoff element can be located within an exhaust pipe, and it can be configured to open or close an outlet of the fuel cell stack. Such a shutoff element also reliably prevents any air flow through the fuel cell stack after shutdown of the fuel cell system.

The at least one further shutoff element can in particular be designed to be in an open position if it is de-energized.

According to the inventive method for operating a fuel cell system comprising a fuel cell stack, a supply pipe for conveying an oxidizing agent to a cathode side of the fuel cell stack and an exhaust pipe connected to the cathode side of the fuel cell stack, wherein the supply pipe and the exhaust pipe are connected to each other via a bypass, upon a shutdown of the fuel cell system a control unit effects the opening of the bypass. This is done by operating a shutoff element configured to open or close the bypass, wherein the shutoff element is located downstream of a junction at which the bypass branches off from the supply pipe. Such an operating method is particularly simple and economic in preventing an unintended airflow through the fuel cell stack after shutdown of the fuel cell system.

It is advantageous, if during the shutdown of the fuel cell system the amount of the oxidizing agent which is conveyed to the cathode side of the fuel cell stack is decreased, while electric energy is drawn from the fuel cell stack. Preferably, the bypass is opened subsequently, i.e. after a at least partial depletion of oxygen on the cathode side of the fuel cell stack. The electric energy produced by the fuel cell stack during the shutdown can in particular be stored in a battery. Utilizing the electric energy produced by the fuel cell stack during shutdown causes the electrochemical reaction to continue. Thus the oxygen content within the fuel cell stack can be lowered to particularly small values. If in such a situation the shutoff element is closed, the low oxygen content can be maintained over a particularly long period of time.

The advantages and preferred embodiments described with respect to the fuel cell system also apply to the method for operating the fuel cell system and vice versa.

The features and feature combinations mentioned above in the description as well as the features and feature combinations mentioned below in the description of the figure and/or shown in the figure alone are usable not only in the respectively specified combination but also in other combinations or alone without departing from the scope of the invention.

Thus, implementations not explicitly shown in the figure or explained, but which result and can be generated by separated feature combinations of the explained implementations are also to be considered encompassed and disclosed by the invention.

Further advantages, features and details of the invention are apparent from the claims, the following description of preferred embodiments as well as based on the drawing.

Therein shows the Fig. components of a fuel cell system for a vehicle, wherein a bypass connects a supply pipe to an exhaust pipe of a fuel cell stack, wherein a flap is arranged within the bypass.

The Fig. shows elements of a fuel cell system 10 comprising a fuel cell stack 12. The fuel cell stack 12 comprises anode chambers to which hydrogen is supplied and cathode chambers to which an oxidizing agent such as air or oxygen is supplied. In the fuel cell system 10 of the Fig. air is supplied to the cathode side of the fuel cell stack via a supply pipe 14. An exhaust pipe 16 is also connected to the cathode side of the fuel cell stack, wherein the exhaust pipe 16 diverts air or oxygen together with the product of the electrochemical reaction taking place within the fuel cell stack 12, in particular in the form of water. When the fuel cell system 10 is shut down, a compressor 18 arranged in the supply pipe 14 is turned off. The compressor 18 can in particular be designed as an electric turbo charger. Even though the compressor 18 is turned off, the electrochemical reaction of oxygen and hydrogen within the fuel cell stack 12 continues as electric energy is drawn from the fuel cell stack 12, for example in order to charge a battery of the vehicle. Thus, the oxygen content within the fuel cell stack 12 decreases.

It is desirable to maintain a low oxygen content within the fuel cell stack 12 after shutdown of the fuel cell system 10. However, in conventional fuel cell system outside air can enter the fuel cell stack 12 through the supply pipe 14. A reason for this can be wind which pushes the outside air through the supply pipe 14 and then through the fuel cell stack 12.

Also effects of thermal convection may lead to an aspiration of outside air through the fuel cell stack 12.

In particular during a restart of the fuel cell system 10, i.e. during startup after a longer period of the fuel cell system's 10 shutdown, the presence of oxygen can lead to an early aging or degradation of the catalysts utilized for the electrodes of each fuel cell in the fuel cell stack 12. This is in particular due to a diffusion of oxygen to the anode side of each fuel cell within in the fuel cell stack 12, wherein the oxygen passes through the membrane separating the anode electrode and the cathode electrode of each fuel cell from each other.

In order to maintain a low concentration of oxygen within the fuel cell stack 12 over a prolonged period of time the supply pipe 14 and the exhaust pipe 16 are fluidly coupled to each other via a bypass 20. Within the bypass 20 a shutoff element is arranged, which can be designed as a flap 22. The flap 22 can be moved into an open position in which air can flow through the bypass 20. The bypass 20 branches off from the supply pipe 14 at a junction 24.

Preferably the flap 22 is located directly adjacent to the junction 24. Thus, humid air flowing through the exhaust pipe 16 does not easily come into contact with the flap 22.

The flap 22 is operated by a controller 26, which also executes the shutdown of all other components of the fuel cell system 10. After the oxygen within the fuel cell stack 12 has been depleted through the continuing reaction of remaining oxygen with remaining hydrogen in the fuel cell stack 12, the flap 22 is moved into an open position. Thus, the bypass 20 is opened. Consequently air which is being pushed or aspired into the supply pipe 14 circumvents the fuel cell stack 12 and flows through the bypass 20. This is due to the fact that the bypass 20 has a much lower backpressure than the fuel cell stack 12.

This main airflow through the bypass 20 is visualized in the Fig. by an arrow 28.

Another arrow 30 illustrates a minor airflow through fuel cell stack 12 which may occur. To prevent even this airflow the fuel cell system 10 preferably comprises supplementary shutoff elements such as a first shutoff valve 32 and a second shutoff valve 34. The first shutoff valve 32 is located upstream of the fuel cell stack 12, i.e. in the supply pipe 14 but downstream of the junction 24. The first shutoff valve 32 is thus configured to close an air inlet 35 of the fuel cell stack 12.

The second shutoff valve 34 is located downstream of the cathode side of the fuel cell stack 12, i.e. within the exhaust pipe 16. By operating this shutoff valve 34 an air outlet 38 of the fuel cell stack 12 can be closed.

To prevent freezing of the flap 22 the flap 22 can be thermally coupled to a component of the fuel cell system 10 which has a large heat capacity. For example, the flap 22 can be thermally coupled to the compressor 18 or a part of the fuel cell stack 12. If the flap 22 is connected, for example via a heat conductive element, to the fuel cell stack 12 care should be taken that no exhaust gas conies into contact with the flap 22, as the exhaust gas contains a considerable amount of water.

Instead of a flap 22 a two-way-valve can be arranged in the bypass 20 and opened upon shutdown of the fuel cell system 10. This also allows ambient air which enters the supply pipe 14 of the fuel cell system 10 to bypass the fuel cell stack 12 and possibly other components located in the cathode branch of the fuel cell system 10.

However, utilizing the flap 22 is a particularly simple and economic way to reduce unintended air flow through the fuel cell stack 12 and thus to reduce degradation of the catalysts within the fuel cells of the fuel cell stack 12.

List of reference signs fuel cell system 12 fuel cell stack 14 supply pipe 16 exhaust pipe 16 compressor bypass 22 flap 24 junction 26 controller 26 arrow arrow 32 shutoff valve 34 shutoff valve 36 air inlet 38 air outlet

Claims

Claims Fuel cell system (10), in particular for a vehicle, with a fuel cell stack (12), a supply pipe (14) for conveying an oxidizing agent to a cathode side of the fuel cell stack (12), an exhaust pipe (16) connected to the cathode side of the fuel cell stack (12), wherein the supply pipe (14) and the exhaust pipe (16) are connected to each other via a bypass (20), and with a shutoff element (22) configured to open or close the bypass (20), wherein the fuel cell system (10) further comprises a control unit (26) designed to open the shutoff element (22) upon a shutdown of the fuel cell system (10), characterized in that the shutoff element (22) is located downstream of a junction (24) at which the bypass (20) branches off from the supply pipe (14).
2. Fuel cell system (10) according to claim 1, characterized in that the shutoff element (22) is designed as a flap.
3. Fuel cell system (10) according to claim 1 or 2, characterized in that the shutoff element (22) is in an open position if the shutoff element (22) is de-energized.
4. Fuel cell system (10) according to any one of claims ito 3, characterized in that within the bypass (20) the shutoff element (22) is closer to supply pipe (14) than to the exhaust pipe (16).
5. Fuel cell system (10) according to any one of claims ito 4, characterized in that the shutoff element (22) is thermally coupled to a component of the fuel cell system (10), which has a larger heat capacity than the shutoff element (22).
6. Fuel cell system (10) according to claimS, characterized in that the shutoff element (22) is thermally coupled -to a compressor (18) configured to increase the pressure of the oxidizing agent upstream of the fuel cell stack (12) and/or -to the fuel cell stack (12) and/or -to an electric machine.
7. Fuel cell system (10) according to any one of claims ito 6, characterized in that the fuel cell system comprises at least one further shutoff element (32, 34) -located within the supply pipe (14) and configured to open or close an inlet (36) of the fuel cell stack (12) and/or -located within the exhaust pipe (16) and configured to open or close an outlet (38) of the fuel cell stack (12).
8. Method for operating a fuel cell system (10), in particular for a vehicle, with a fuel cell stack (12), a supply pipe (14) for conveying an oxidizing agent to a cathode side ofthefuel cell stack (12), and an exhaust pipe (16) connected to the cathode side of the fuel cell stack (12), wherein the supply pipe (14) and the exhaust pipe (16) are connected to each other via a bypass (20), wherein upon a shutdown of the fuel cell system 00) a control unit (26) effects the opening of the bypass (20), characterized in that the control unit (26) effects the opening of the bypass (20) by operating a shutoff element (22) configured to open or close the bypass (20) and located downstream of a junction (24) at which the bypass (20) branches off from the supply pipe (14).
9. Method cell system according to claim 8, characterized in that during the shutdown of the fuel cell system (10) the amount of the oxidizing agent being conveyed to the cathode side of the fuel cell stack (12) is decreased while electric energy is drawn from the fuel cell stack (12), wherein the bypass (20) is opened subsequently.