EP2548248A1

EP2548248A1 - Fuel cell system and method for operating a fuel cell system

Info

Publication number: EP2548248A1
Application number: EP10795221A
Authority: EP
Inventors: Meenakshi Sundaresan; Steffen Dehn
Original assignee: Daimler AG
Current assignee: Mercedes Benz Group AG
Priority date: 2010-03-16
Filing date: 2010-12-08
Publication date: 2013-01-23
Also published as: DE102010011559A1; JP2013522828A; CN103109406A; US20130036749A1; WO2011113460A1

Abstract

A fuel cell system (1) has the following components: - at least one fuel cell (2) which has an anode region (3) and a cathode region (4); a burner (10) for burning exhaust gases from the fuel cell (2) and additional fuel which can be optionally supplied; a storage volume (17) for temporarily storing exhaust gases which flow continuously or discontinuously out of the anode region (3) of the fuel cell (2) via a valve device (14), wherein the storage volume (17) is arranged between the anode region (3) and the burner (10). According to the invention, the hot exhaust gases of the burner (1) are expanded in an expansion apparatus (12). A solution according to the method makes provision for operation of a fuel cell system of this kind to be operated with an additional valve device (18) behind the storage volume (17).

Description

Fuel cell system and method for operating a fuel cell system

The invention relates to a fuel cell system according to the closer defined in the preamble of claim 1 and a method for operating a fuel cell system according to the closer defined in the preamble of claim 5 Art.

Fuel cell systems are for the production of electrical energy from the

known in the general art. The following statements are intended to refer to a fuel cell system with a stack of individual fuel cells, which are exemplified as PEM fuel cells. In principle, however, the ideas can also be applied analogously to other types of fuel cells. The fuel cell or the fuel cell stack typically always has a cathode region, which is supplied with oxygen, for example in supplied air. Furthermore, the fuel cell has an anode region, which is supplied with a fuel, typically a hydrogen-containing gas or hydrogen, in gaseous form.

For the anode region, it is now the case that it is either flowed through by the fuel, so that an excess amount of fuel as exhaust gas comes from the anode region. Depending on the embodiment, the design can be chosen such that only a minimal amount of fuel exits the anode region, while the greater part of the fuel in the anode region is used up. This is referred to as a "near-dead-end stack." The alternative would be a fuel cell without an exit in the anode area, a so-called "dead-end stack," in which all the fuel supplied is used up. As another very common alternative in the construction of an anode region, it may also be provided to apply a large excess of fuel to the anode region. Then, a comparatively large amount of fuel as exhaust gas from the Outflow anode area. In order not to waste this fuel, this is then in the circulation, in a so-called "anode loop", back to the entrance of the

Anode area out and mixed there with the fresh fuel flowing to the anode area.

Over time, nitrogen accumulates in the anode region, which diffuses through the membranes of the fuel cells from the cathode region or the air in the cathode region into the anode region. In addition, a part of the product water, which is produced during power generation with the fuel cell, forms in the anode region. In the typically preferred anodes of either anode loop or near-dead-end stack design, these undesirable substances may be removed from the anode region with the exhaust gas or, in the case of an anode loop, are typically exhausted from time to time via a drain valve , All these exhaust gases, whether from an anode loop or from the

Anode area directly, in addition to water and inert gases always have a remainder of the fuel or hydrogen. It is therefore known from the prior art to post-combust these substances via a burner or the like in order to avoid fuel emissions to the environment.

From DE 11 2004 001 483 B4, a construction is known in which the exhaust gas from the anode region of the fuel cell is temporarily stored in a chamber or a storage volume in order then to be supplied to a burner, for example continuously.

A similar structure is also known from US 2005/0214617 A1. Here too, a collecting tank or storage volume for the exhaust gas from the anode area is used. The delivery to the environment is also carried out continuously and comparatively slowly, so that a corresponding mixture with the exhaust gas from the cathode region provides for a total exhaust gas, which is at all times below a critical fuel-oxygen mixture and can be delivered unburned to the environment ,

From the further prior art it is also known that, as it

For example, in DE 103 06 234 B4, the exhaust gases of a

Fuel cell can be burned in a burner. The afterburned Exhaust gases or the hot exhaust gas of this afterburning can then be used in an expansion device, such as a turbine. The cited patent describes the construction of a turbocharger, in which this turbine drives a compressor or compressor for the supply air to the cathode region. In addition, an electric machine may be provided which provides additional drive power for the compressor when needed, and which at a

Excess energy at the turbine can also be operated as a generator. The electrical energy thus generated can then be stored or otherwise used. This structure is also called electric turbocharger or ETC.

In this context, DE 103 25 452 A1 also describes the possibility of a "Boosf" operation, in which additional fuel is supplied to the burner, which then provides additional energy to the expansion device, if necessary, and so can either improve the air supply to the cathode area or directly generates electrical energy via the electric machine. In the case of application in a vehicle, this boost mode can be used, for example, to quickly provide a large amount of electrical energy in the event of an acceleration request of the vehicle in the short term until its dynamics

comparatively slow fuel cell, the requirement correspondingly implemented and the energy needs in turn completely covers. By means of such a boost operation, the dynamics of the electric power generation by the fuel cell system can thus be improved.

The present invention has now set itself the task of a

Specify fuel cell system, which optimizes the energy use and dynamics in the fuel cell system, and which with minimal space and efficient use of energy used to the fuel cell system directed

Serviced requirements.

According to the invention this object is achieved by the features mentioned in the characterizing part of claim 1. A procedural solution for operating such a fuel cell system results from the features in the characterizing part of claim 5. The solution according to the invention provides that a fuel cell system is constructed in which the exhaust gases from the region of the anode are temporarily stored in a storage volume before they reach the area of a burner from there. In the burner they are then implemented accordingly and the hot exhaust gases of the burner drive an expansion device, in which the hot exhaust gases are relaxed. Thus, with the expansion device, the energy content in the exhaust gases from the region of the anode by combustion, for example, together with the exhaust gases from the cathode, which contain residual oxygen, are used. The

Energy balance of such a system will therefore be better than in a system in which the exhaust gases are only burned to the leakage of

Prevent fuel emissions. In addition, the use of a

Storage volume a very efficient control and a very efficient operation of the expansion device or the burner, since cathode exhaust gas can be collected and selectively fed to the burner, especially if a corresponding energy demand is present.

In corresponding developments of the invention, it is provided for this purpose that the expansion device is designed as a turbine in a turbocharger. If, in addition, a valve device for controlling or regulating the volume flow emerging from the storage volume, according to a very favorable development of the

The fuel cell system according to the invention is provided, then the combustion of the exhaust gases from the anode region can always take place via the turbine as an expansion device in a very targeted manner, when the energy for conveying supply air to the cathode is required in any case.

The method according to the invention for operating a fuel cell system thereby provides a valve device according to the storage volume. The flow of

Anode exhaust gas from the storage volume can be influenced. Particularly preferably, this can be adjusted depending on the degree of filling in the storage volume. Thus, for example, from a discontinuous outflow of the exhaust gas from the anode region, which is particularly advantageous for removing water accumulated in the anode region, a corresponding collection of the discontinuously flowing exhaust gas in the storage volume. From there it can then be continuously or with a corresponding energy requirement via a be continuously supplied to the burner for a certain period of time so as to be able to provide the requested performance in the area of the expansion device.

Further advantageous embodiments of the device according to the invention and the method according to the invention will become apparent from the remaining dependent claims and will be apparent from the embodiment. This will be described in more detail below with reference to the figures.

Showing:

Fig. 1 is a schematic representation of a possible structure of a

fuel cell system according to the invention; and

Fig. 2 is a flowchart for operating the illustrated in Fig. 1

Fuel cell system.

1 shows a fuel cell system 1 by way of example. This consists in the core of a fuel cell 2, which should be constructed as a stack of PEM fuel cells by way of example. This stack 2 or stack of individual fuel cells has an anode region 3 and a cathode region 4. The anode region 3 is supplied with hydrogen from a hydrogen storage device 5, in which case pressure reducers, valves and the like have been dispensed with in the illustration of FIG. Regardless, these are present in the manner known per se. The cathode region 4 of the fuel cell 2 is supplied via a compressor 6, which is designed here as part of an electric turbocharger 7 (ETC) described in more detail later, air. The compressor 6 is preferably formed in the structure shown here as a flow compressor, alternative embodiments and

Constructions of the compressor 6, however, would also be conceivable. The intake via the compressor 6 air then flows to a charge air cooler 8 and from there into the

Cathode region 4 of the fuel cell 2. In the fuel cell 2, the hydrogen in the anode region is reacted in a conventional manner with the oxygen in the cathode region 4 air, wherein water and electrical power is generated. From the cathode region 4 then flows an exhaust gas, which in

Essentially, an oxygen-depleted exhaust air together with a certain proportion of water and water vapor. This comparatively cool exhaust air in turn flows through the charge air cooler 8 and cools there after the compressor. 6 heated supply air on its way to the cathode region 4. After the intercooler 8, the air flows into a mixer 9 and from there into a burner 10, which

for example, as a pore burner, but in particular as a catalytic burner is formed.

In order to produce a combustible mixture in the mixer 9, the mixer 9 also flows to an exhaust gas from the anode region 3 of the fuel cell in a manner to be described later. If required, optional hydrogen can also be passed to the mixer 9 via a valve device 1, so that in each case a mixture is produced in the mixer 9, which can be burned in the burner 10. The hot exhaust gases of the burner 10 then flow into an expansion device 12, which in turn is formed here as part of the electric turbocharger 7. The

Expansion device 12 is typically formed as a turbine, which is arranged on a common shaft with the compressor 6. In the embodiment used here as an electric turbocharger 7, an electric machine 13 is also arranged on the common shaft.

In essence, one can distinguish three different operating modes of the electric turbocharger 7. Either the expansion device 12 can provide all the energy needed for the compressor 6, then the electric machine 13 will only idle. With an energy surplus in the range of

Expansion device 2, the electric machine 3 can be operated as a generator. Then, additional electrical energy can be generated via the expansion device 12 and the electric machine 13, which is available as an alternative or in addition to the electrical energy from the fuel cell 2. Thus, for example, when equipping a vehicle with the fuel cell system 1, a sudden increase in the power requirement can be fulfilled within a very short time. If required, optional fuel for the burner 10 is then made available via the valve device 11 so that the electric energy is available at the electric machine 13 via a boost or turbo boost. In the last application, in which the expansion device 12 is not the entire for the

Compressor 6 can provide needed energy, the electric machine 13 can also be operated by a motor, so as to compensate for the required energy difference. In the preferred construction of the invention, the anode region 3 should now be formed as a so-called "near-dead-end" anode region 3. This means that the

Anode region 3 is flowed through by hydrogen and is designed so that only a very small proportion of hydrogen and optionally diffused through the membranes nitrogen and a certain amount of product water is obtained as the exhaust gas. Such near-dead-end anode regions are typically constructed as cascaded anode regions 3, that is to say that from section to section in FIG

Flow direction of the hydrogen, the available active area of the anode region 3 decreases, in particular to a similar extent as the hydrogen in the anode region 3 is consumed. This ensures that approximately the same amount or concentration of hydrogen per active unit area, which is covered by the hydrogen, is available. Such structures allow the abandonment of a complex anode loop, which typically has a

Conveyor, such as a hydrogen circulation fan or the like, is operated to lead unused hydrogen back to the anode inlet.

For example, a near-dead-end anode region 3 can manage with a hydrogen excess of a few percent in a cascaded configuration. This gas is discharged from the fuel cell 2. This can be done with a continuous flow, for example through a diaphragm or the like. However, in particular, it can also be done via a valve device 14, a so-called purge valve, the purge valve 14 in a clocked manner, so that the exhaust gas from the

Anodenbereich 3 is discharged discontinuously or intermittently. This generally allows a better discharge of the product water occurring in the anode region 3, since then there is always a greater pressure difference for blowing off this product water, than with a continuous outflow of the exhaust gases from the anode region 3. The anode exhaust gases then reach the valve device 14 by way of example in a water separator 15, which is designed as a simple water trap. From the water separator 15, the water passes through a valve device 16 and a corresponding line element in the region of the exhaust air after the

Expansion device 12. The exhaust gas freed from the liquid water passes through a check valve in a storage volume 17 and from there via a valve means 18 to the mixer 9, together with the exhaust gas from the cathode region 4 and optionally via the valve means 11 optionally supplied hydrogen from the hydrogen storage device 5 mixed and the burner 10 to be supplied. These streams are shown in the representation of Figure 1 as solid lines.

In the illustration of Figure 1 also various sensors can be seen. One

Pressure sensor 19 is arranged in the region of the storage volume 17. In addition, a hydrogen concentration sensor 20 is located in the region of the flow between the mixer 9 and the burner 10. A flow sensor 21 for hydrogen is also located in the line member which connects the valve means 11 to the mixer 9. The sensors deliver their values, as shown by the dashed lines, to an electronic control unit 22. From this control electronics 22, the existing in the fuel cell system 1 valve devices 11, 14, 16 and 18 are controlled accordingly or the flow rates through these valve devices 11, 14th , 16 and 18 regulated.

The structure according to the invention thus provides a storage volume 17 together with the burner 10, the hot exhaust gases are used in an expansion device 12 in addition to the generation of energy. This structure allows a very efficient operation of the burner 10, as this, the hydrogen from the storage volume 17 can be continuously or continuously supplied as needed. In addition, the storage volume 17 allows a discontinuous discharge of the anode exhaust gases via the valve device 14. This is preferable due to the higher pressure difference compared to conceivable fading over a fixed aperture, as due to the higher pressure difference more water is discharged from the anode region 3. As a result, the system performance of the fuel cell 2 is improved. The construction with the

Storage volume 17 can in particular via the pressure sensor 19 and the

Valve means 18 are controlled so that the outflow of the exhaust gas from the storage volume 17, for example, as a function of the pressure and thus in

Dependence of the degree of filling of the storage volume 17 is variable. In the

Control electronics 22 may also be stored in the discontinuous discharge of exhaust gas from the anode region 3, the frequency of this intermittent delivery via the valve device 14. Depending on the load condition of the

Fuel cell 2 can be selected as a suitable strategy for discharging the exhaust gas from the anode region 3. At the same time, the amount of exhaust gas which flows into the region of the storage volume 17 can be detected via the frequency and the amount of exhaust gases produced corresponding to the load point. That way, without that a pressure sensor 19 is absolutely necessary, also the degree of filling of

Storage volume can be determined and so the continuation of the from the

Storage volume of outflowing gas can be adjusted based on the degree of filling.

In particular, in the boost mode, that is, when the valve device 11 additional hydrogen to the mixer 9 and thus to the burner 10 is promoted, because in the expansion device 12 additional energy is required, this structure with the storage volume 17 can play its special advantages. Via the hydrogen sensor 20, the hydrogen concentration of the gas flowing to the burner 10 can be determined. Thus, an expected temperature during combustion in the burner 10 via the control electronics 22 can be predicted. If this calculation indicates that a permissible maximum temperature threatens to be exceeded, then the flow rate of hydrogen detected via the flow sensor 21 can be correspondingly restricted or regulated to a lower flow rate via the valve device 11. This can ensure that the expected temperature in the burner 10 does not exceed the maximum permissible temperature. Nevertheless, due to the additional hydrogen and the hydrogen stored in the storage volume 17, the requirement for the power to the boost operation up to a system-related upper limit can be met. This is at relatively low demand for additional hydrogen from the

Hydrogen storage device 5 and thus very energy efficient possible.

The size of the storage volume 17 is of crucial importance to the functionality. It may well be appropriate to choose the storage volume comparatively large. However, in particular when using the fuel cell system 1 in a motor vehicle, the size is to be minimized by space limitations and the desire for a low weight of the fuel cell system 1. Taking a fuel cell 2 in a typically used for motor vehicles

Fuel cell system 1, for example, a PEM fuel cell with a power in the order of 50 to 90 kW, so there are per second, depending on the load case of the fuel cell 2 exhaust gas volumes from the anode area 3, if this is operated as a near-dead end stack, which are in the order of 0.2 to about 10 liters. Now it is that, in particular for the operation at low load one

Caching of the anode exhaust gas 3 should be possible over several seconds. At full load, in addition to the anode exhaust gas 3, a comparatively large amount of water is also produced. which must be discharged to maintain the functionality of the anode region 3. In this constellation, the caching of the

Anode exhaust gases 3 therefore only take place for a rather short period. Setting now for the low load a period of a few seconds, for example 4 to 8 seconds, and for the full load a period of less than 1 second, the result is an optimized storage volume in the order of 1 to 3 liters, especially in the Order of about 2 liters for the above system. The structure can thus be optimized with respect to the functionality and the space with a storage volume 7 with a storage capacity of about 2 liters.

An exemplary operation for the fuel cell system 1 shown in FIG. 1 will now be described below with reference to a flowchart in FIG. 2 and will be explained in more detail below.

In FIG. 2, the described control sequence, which will typically be carried out in the control electronics 22, starts in the oval box labeled Start. In step A1, the pressure in the storage volume 17 is detected. In the second

Process step A2 is compared with this pressure, referred to below as P17, with a predetermined reference pressure. The reference pressure typically indicates the pressure value for the full storage volume 17. As soon as the pressure P17 reaches or exceeds this reference pressure, the storage volume 17 is filled. If the pressure P17 detected in the storage volume 17 is above the predetermined reference pressure, the step A3 is triggered, in which the flow through the valve device 18 is increased, the storage volume 17 thus empties or the fill level increases less rapidly. If the pressure P17 in the storage volume 17 is smaller than the predetermined reference pressure, the selection jumps to method step A4 and the valve device 18 of the storage volume 17 is closed. After step A4, the process is completed and can be restarted directly or after a short wait.

In the subsequent method step A5, the operating point of the fuel cell is then detected. Based on the operating point of the fuel cell can then be determined in step A6, whether a discharge of anode exhaust gas is required. If this is not the case, the valve device 14 is closed in step A8. If deflation is required, step A7 is initiated after step A6, in which via the valve device 14th From the anode region 3 exhaust gas is discharged into the storage volume 17. In the following step A9 is then questioned whether the fuel cell system 1 is currently in boost mode. If this is not the case, the system jumps back to the start or to method step A1. Is it against that

Fuel cell system 1 immediately in boost mode, so will continue to

Step A10 jumped and with the hydrogen sensor 20 is the

Concentration of the hydrogen flowing to the burner detected. in the

Process step A11 is then calculated or detected hydrogen flow rate through the valve device 11 to the mixer 9 and influenced accordingly in the method step A12, typically throttled. Then the process ends in the end labeled oval box. The procedure can then be restarted directly or after a short waiting time.

With the described construction and the described method, the fuel efficiency of the fuel cell system 1 can be increased by a storage volume 17 for intermediate storage of the exhaust gas from the anode area 3 and an expansion device 12 downstream of the burner 10, in particular if it is an anode area 3 in near-dead space. End execution is. By the afterburning and the use of the hot exhaust gases in the expansion device 12 so energy can be saved and the efficiency of the overall system can be increased.

Claims

claims

1. fuel cell system with

1.1 at least one fuel cell, which has an anode region and a

Having cathode region;

1.2 a burner for combustion of exhaust gases from the fuel cell and

optional additional feedable fuel;

1.3 a storage volume for temporary storage of exhaust gases, which

continuously or discontinuously via a valve device from the

Anodenbereich the fuel cell flow out; wherein the storage volume is arranged between the anode area and the burner,

characterized in that

1.4 in the flow direction of the hot exhaust gases of the burner (10) after the burner (10) an expansion device (12) is arranged.

2. Fuel cell system according to claim 1,

characterized in that

the expansion device (12) is designed as a turbine in a turbocharger, in particular an electric turbocharger (7).

3. Fuel cell system according to claim 1 or 2,

characterized in that

the fuel cell (2) with an open anode (3), in particular with in

Flow direction cascading decreasing active surface, is formed.

4. Fuel cell system according to claim 1, 2 or 3,

characterized in that

the one valve device (18) for controlling or regulating the from Storage volume (17) exiting flow in the flow direction after the storage volume (17) is arranged.

5. A method of operating a fuel cell system according to claim 4,

characterized in that

the flow of the anode exhaust gas from the storage volume (17) is adjusted in dependence on the degree of filling of the storage volume (17).

6. The method according to claim 5,

characterized in that

the flow of the anode exhaust gas from the storage volume (17) is adjusted in dependence on the pressure (P17) in the storage volume (17).

7. The method according to claim 5 or 6,

characterized in that

the outflow of the exhaust gas from the anode region (3)

takes place intermittently, wherein the flow of the anode exhaust gas from the

Storage volume (17) depending on whether currently exhaust from the

Anodenbereich (3) is discharged or not, is set.

8. The method according to claim 5, 6 or 7,

characterized in that

the flow of the anode exhaust gas from the storage volume (17) is adjusted in dependence on the detected or calculated temperature of the exhaust gases of the burner (10).

9. The method according to any one of claims 5 to 8,

characterized in that

when additional energy is needed at the expansion device (12), optional fuel is supplied, the amount of optional fuel in

Dependence of the detected or predicted temperature in the region of the burner (10) and in dependence on the exhaust gas available from the storage volume (17) is set.

10. The method according to claim 9,

characterized in that

the amount of fuel flowing to the burner (10) with a

Fuel concentration sensor (20) in the flow direction upstream of the burner (10) is monitored.