DE102013007561A1 - Method for operating polymer electrolyte membrane fuel cell stack for fuel cell system for vehicle, involves cooling flow channel of stack to temperature at which water condenses, during cooling operation when water is present in stack - Google Patents

Abstract

The method involves changing temperature of a fuel cell stack (10) by using a temperature control element designed as a Peltier element (22). A flow channel (16) of the fuel cell stack exhibiting the temperature control element is cooled by the temperature control element to temperature at which water condenses, during a cooling operation when water is present in the fuel cell stack, where a reactant of respective flow fields is supplied by fuel cells of the fuel cell stack. The flow channel is cooled to the temperature of the fuel cell stack during shutdown of the fuel cell stack. An independent claim is also included for a fuel cell stack.

Description

The invention relates to a method for operating a fuel cell stack, wherein a temperature of the fuel cell stack is changed by means of at least one tempering element. Furthermore, the invention relates to a fuel cell stack with at least one tempering element.

For fuel cell stacks, which are intended for mobile applications, polymer electrolyte membrane fuel cells are generally used. The basic structure of a polymer electrolyte membrane fuel cell - PEMFC for short - is as follows. The PEMFC contains a membrane-electrode assembly - MEA short, which is composed of an anode, a cathode and a polymer electrolyte membrane arranged between them (also ionomer membrane) - PEM - constructed. In turn, the MEA is interposed between two separator plates, with one separator plate having fuel distribution channels and the other separator plate having channels for the distribution of oxidant and the channels facing the MEA. The channels form a channel structure, a so-called flow field. The electrodes, anode and cathode, are generally designed as gas diffusion electrodes - in short GDE. These have the function of dissipating the current generated in the electrochemical reaction (eg 2 H ₂ + O ₂ → 2 H ₂ O) and allowing the reactants, starting materials and products to diffuse through. A GDE consists of at least one gas diffusion layer or gas diffusion layer - in short GDL - and a catalyst layer which faces the PEM and at which the electrochemical reaction takes place. The GDE may also have a gas distribution layer, which adjoins the gas diffusion layer and which faces in the PEMFC a Separatorplatte. Gas diffusion layer and gas distribution layer differ mainly in their pore sizes and thus in the nature of the transport mechanism for a reactant (diffusion or distribution).

Such a fuel cell can produce high power electrical power at relatively low operating temperatures. Real fuel cells are usually stacked into so-called fuel cell stacks - stacks - to achieve a high power output, instead of the monopolar separator plates bipolar separator plates, so-called bipolar plates are used and form monopolar separator plates only the two terminal terminations of the stack. They are z. T. end plates called and can be structurally different from the bipolar plates.

The bipolar plates are generally composed of two partial plates. These sub-plates have substantially complementary and with respect to a mirror plane mirror-image forms. However, the partial plates do not necessarily have to be mirror images. It is only important that they have at least one common contact surface to which they can be connected. The partial plates have an uneven topography. As a result, the channel structures already mentioned above arise at the surfaces of the partial plates that point away from each other. At the respectively facing each other surfaces of the sub-panels z. B. in embossed metal part plates to the o. G. Channel structure complementary channel structure. When the two sub-panels are placed on top of each other, a cavity, which consists of a system of several interconnected tunnels, thus arises between the sub-panels, on their mutually facing surfaces. The cavity or the system of the tunnel is surrounded in a liquid-tight manner by a joint which essentially surrounds the partial plates in the edge region, openings being provided for the coolant supply and removal, so that the cavity can be used for the distribution of a coolant.

Thus, one of the tasks of a bipolar plate is: the distribution of oxidizing agent and reducing agent; the distribution of coolant and thus the cooling (better tempering) of the fuel cell; The fluidic separation of the individual cells of a stack from each other; Furthermore, the electrical contacting of the series-connected individual cells of a stack and thus the passage of the electric current generated by the individual cells.

A PEM can contain several components. The most important component is one or more proton-conductive ionomers. Furthermore, reinforcing components such. As organic fibers (especially PTFE fibers) and / or inorganic fibers (especially glass fibers) may be included, the z. B. may be formed as a woven or knitted fabric. Furthermore, fillers may be included, such as. B. metal oxide particles (especially silica gel, SiO ₂ ), the z. B. assume a function in the moisturization of the PEM. In addition, other components may be included, such as. As phosphoric acid, low molecular weight amphoteric (especially imidazole and / or pyrazole). However, a PEM can also consist of a proton-conductive glass film, in particular of a nanoporous phosphosilicate glass film. If a catalyst layer is applied to one or both of the main surfaces of the PEM, it is generally referred to as a catalyst coated membrane-CCM for short.

The US 2005/0112436 A1 describes a fuel cell stack in which between one of an anode and a cathode facing side of a bipolar plate, a layer of Peltier elements is arranged. The Peltier elements are in this case distributed over the entire surface of the bipolar plates, that a uniform heat distribution over the individual fuel cell and the fuel cell stack can be achieved. By contrast, without the provision of the layer of Peltier elements between the two sides of the bipolar plate, a temperature gradient with increasing temperature would be established from the entry region of a reactant into a flow field towards the exit region of the reactant from the flow field. This can be counteracted by the provision of the Peltier elements. The Peltier elements can be used both for cooling and for heating. By cooling, an occurrence of high temperatures is avoided, which could otherwise lead to damage to the catalysts of the fuel cell. At low temperatures, however, ice may form within the fuel cell stack, which can be avoided by driving the layer of Peltier elements.

A disadvantage here is the fact that it can nevertheless lead to malfunction in such a fuel cell stack.

Object of the present invention is therefore to provide a method and a fuel cell stack of the type mentioned, by means of which can be particularly easy to avoid malfunction.

This object is achieved by a method having the features of patent claim 1 and by a fuel cell stack having the features of patent claim 10. Advantageous embodiments with expedient developments of the invention are specified in the dependent claims.

In the method according to the invention, in a cooling operation by means of the at least one tempering element, at least one region of the fuel cell stack having the at least one tempering element is cooled to a temperature at which water condenses, provided that water is present in the fuel cell stack. This is based on the finding that a fuel cell stack, in particular after switching off or shutdown, can lead to a condensed water accumulation. Namely, in the fuel cell reaction, water is formed, and when the fuel cell stack shuts down, its temperature decreases. In this case, in the liquid phase transferred, so condensed water can then freeze and clog so particular critical points within the fuel cell stack. However, accumulations of unfrozen water can make it difficult to start the fuel cell system and lead to a significant increase in the start time. However, the presence of ice may even cause the startup process to abort.

In the present case, however, the at least one region of the fuel cell stack is purposefully cooled by means of the at least one tempering element to the temperature at which water condenses. As a result, liquid water is formed in the at least one area, and it can not uncontrollably reach other areas of the fuel cell stack, in which the accumulation of condensed water and in particular the formation of ice impairs or even prevents the loading of the fuel cell with the reactant. Thus, the condensation is deliberately shifted in the at least one area, so as to avoid the freezing of water at more vulnerable sites.

The fuel cell stack is thus deliberately withdrawn water by means of the manner of a cold trap operated at least one tempering. As a result, the fuel cell stack is prepared particularly well for a subsequent startup, in particular a freeze start.

In cooling mode, therefore, the at least one tempering element acts as a controlled condensate collector and prevents the uncontrolled condensation of water and as a result a possible later formation of ice at critical points. It is therefore possible to avoid malfunctions in a particularly simple manner. The fact that damage or starting difficulties are prevented as a result of ice formation, there is an increase in the life of the fuel cell stack. Also, a shortening of the time required to start up or start up the fuel cell stack is achievable. Furthermore, this improves the water management within the fuel cell stack

In an advantageous embodiment of the invention, the at least one region of the fuel cell stack is cooled when the fuel cell stack is lowered to the temperature at which any water present condenses. In particular, when shutting down or shutdown of the fuel cell stack, it is of particular importance that does not accumulate water at critical points.

It has also proven to be advantageous if the at least one region of the fuel cell stack is cooled to a temperature at which the condensed water freezes. For then it is particularly largely certain that not Water droplets can get into the critical in terms of a possible blockage areas of the fuel cell stack.

Preferably, as the at least one region of the fuel cell stack, a flow channel is cooled, via which a reactant is supplied to respective flow fields of a plurality of fuel cells of the fuel cell stack. Such a flow channel or collection channel, which branches off to the individual flow fields or flow fields of the fuel cell stack, is also referred to as header. Since this has a particularly large volume that can be flowed through, a targeted accumulation of condensed water is particularly uncritical here.

Additionally or alternatively, as the at least one region of the fuel cell stack, a flow channel into which a reactant flows from respective flow fields of a plurality of fuel cells of the fuel cell stack can be cooled. Also, the flow channel or header, in which the reactant enters after flowing through the flow fields or flow fields, namely, has a comparatively large flow-through cross-section and is thus suitable for targeted accumulation of condensate.

In this case, it has proven to be advantageous if the at least one tempering element in the at least one flow channel flows around the outer peripheral side of the reactant. For then the at least one tempering element provides a particularly large surface on which the condensed water can accumulate. In addition, by means of such a tempering element arranged centrally in the stream of the reactant, the gas stream of the reactant can be cooled particularly uniformly.

It can be cooled to the individual fuel cell inflowing or entering from the flow fields in the at least one flow channel stream of fuel to achieve a targeted dehumidification of this reactant.

Preferably, however, at least the at least one flow channel is preferably cooled by means of the at least one tempering element, which is acted upon by an oxidizing agent as the reactant. Namely, on the side of the oxidant-charged cathode of each fuel cell, comparatively much water exists due to the humidification of the oxidant supplied to the fuel cell stack and the formation of product water of the fuel cell reaction. Therefore, the targeted dehumidification of freezing gas openings is particularly favorable here.

In an operating mode different from the cooling mode, the at least one tempering element can advantageously be used to heat the at least one region of the fuel cell stack to a temperature at which frozen water thaws. This ensures that there is no permanent presence of ice within the fuel cell stack. In addition, the reactants or operating gases can thus be preheated, so that there are favorable conditions for the electrochemical reactions particularly quickly. Existing ice blocks can be defrosted very quickly and the resulting water can be discharged. By heating the reactants or operating gases and the formation of ice can be prevented.

Also for this purpose of the tempering, namely the introduction of heat into the fuel cell stack, it is useful to provide the at least one tempering in at least one of the flow channels through which the fuel or the oxidant supplied to the fuel cells or discharged from them. Because of the comparatively large volume present here, the formation of liquid water due to the thawing of ice is particularly disturbing.

In particular, it has proven to be advantageous if the at least one region is heated to the temperature at the startup of the fuel cell stack, at which frozen water thaws. During commissioning, namely, the rapid removal of any existing ice blocks is particularly advantageous. This applies in particular to a freeze start, that is to say when the fuel cell stack is put into operation or started up when ambient temperatures of less than 0 ° C. are present.

Finally, it has proven to be advantageous if at least one Peltier element is used for introducing heat into the fuel cell stack or for cooling the same as the at least one tempering element. Peltier elements are particularly easy to use both for cooling and for heating, they only need to be charged with electrical energy to produce the desired heating effect or cooling effect. A provision of complex fluid channels can thus be omitted. In particular, a stack of Peltier elements can be used whose geometry is adapted to the fuel cell stack.

Peltier elements also provide a Lastsenke, so a waste heat-generating consumer ready. This can be the self-heating of the Support fuel cell stack, especially during the freeze start of the fuel cell stack.

The fuel cell stack according to the invention comprises at least one tempering element, by means of which a temperature of the fuel cell stack is variable. In this case, the at least one tempering element is arranged in at least one region of the fuel cell stack, which can be flowed through by a reactant during operation of the fuel cell stack. By means of the tempering the range is coolable to a temperature at which water condenses when water is present in the fuel cell stack. As a result of the arrangement of the tempering element in the region through which the reactant can flow, the tempering element comes into direct contact with the reactant. It can thus withdraw moisture from the reactants in cooling mode, thus acting as a condensate collector.

By providing such a tempering element malfunction of the fuel cell stack can be particularly easy to avoid. In a heating operation, the at least one tempering element is preferably used as a particularly rapidly acting gas heater.

The advantages and preferred embodiments described for the method according to the invention also apply to the fuel cell stack according to the invention 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 figures and / or in the figures alone can be used not only in the respectively indicated combination but also in other combinations or in isolation, without the scope of To leave invention.

Further advantages, features and details of the invention will become apparent from the claims, the following description of preferred embodiments and from the drawings. Showing:

1 a partial view of a bipolar plate of a fuel cell stack, which can illustrate the arrangement of in terms of blockage by ice critical areas thereof;

2 in sections, the bipolar plate according to 1 in a view on their flow field; and

3 a fragmentary fuel cell stack, in which schematically the arrangement of a Peltier element is illustrated in a possible for this arrangement range.

From one in 3 shown fuel cell stack 10 is in 1 a bipolar plate 12 shown. The fuel cell stack 10 can be used in particular in a fuel cell system provided for a vehicle.

The bipolar plate 12 has a comparatively large breakthrough in a border area 14 which is in the fuel cell stack 10 with the corresponding breakthroughs 14 the adjacent bipolar plates 12 flees. Through the aligned openings 14 is thus a flow channel 16 formed (cf. 3 ), which is also called a header. Through this flow channel 16 flows during operation of the fuel cell stack 10 a reactant, for example the oxidizing agent in the form of air or oxygen.

At the in 1 shown bipolar plate 12 the oxidizing agent flows out of the area of the breakthrough 14 over a transitional area 18 which is also referred to as a backfeed tunnel, towards another, smaller breakthrough 20 in the bipolar plate 12 ,

About the smaller breakthrough 20 then the oxidizing agent reaches the opposite side of the bipolar plate 12 , In other words, the backfeed tunnel and the (backfeed) breakthrough serve 20 ie the transfer of the oxidizing agent towards the back of the bipolar plate 12 ,

It has been shown that, in particular, this transitional area 18 between the large, in cooperation with the other bipolar plates 12 the flow channel 16 forming breakthrough 14 and the little breakthrough 20 is particularly critical in terms of accumulation of water and concomitant formation of ice in cold outside temperatures.

In the present case, therefore, means are provided which make it possible upstream of this transition region 18 to collect condensing water when the fuel cell stack 10 shut down.

These means comprise, for example, at least one tempering element, which is preferably in the form of a Peltier element 22 is trained. The Peltier element 22 For example, it may be tubular or rod-shaped and thus centrally in the flow channel 16 be arranged so that it can be flowed around by the oxidizing agent (see. 3 ).

When shutting down the fuel cell stack 10 becomes the Peltier element 22 for cooling used. It becomes the Peltier element 22 in the flow channel 16 cooled to a temperature at which it comes to a condensation of water. The Peltier element 22 acts as a condensate collector, preventing water droplets from entering the transition area 18 and, optionally later, ice is formed by freezing the water.

Especially in conjunction with 2 will be particularly apparent that with the through the breakthroughs 14 formed flow channel 16 a particularly large flow-through cross-section is available, in which the accumulation of water is relatively uncritical. In contrast, the transition area 18 particularly critical in terms of ice blockage. If in fact in the transition area 18 Ice forms, so the oxidizer can no longer, as with the help of flow arrows 24 in 2 illustrated on the other side of the bipolar plate 12 and thus into a flow field 26 or Flow Field.

The targeted dehumidification of freeze-endangered gas passages, ie in particular the transition areas 18 and the breakthroughs 20 can, as shown here by way of example, take place in the inflow region, ie before the oxidant has passed from the flow channel 16 into the flow field 26 , However, it can also be in the discharge area, ie in the flow channel, in which the of the respective flow fields 26 the individual fuel cells of the fuel cell stack 10 collect incoming reactants, a Peltier element 22 be provided as a capacitor collector. Otherwise, in the analogue transition region from the respective flow field into the flow channel or header arranged downstream thereof, ice blockage due to freezing of water droplets may otherwise occur.

As in particular from 3 shows are by further openings or recesses in the respective bipolar plates 12 of the fuel cell stack 10 another flow channel 28 for a fuel 30 formed, and optionally an additional flow channel 32 in which the oxidizing agent can flow. Also in the area of the fuel 30 provided flow channel 28 it is possible to provide a tempering element, in particular in the form of the Peltier element shown schematically 22 , Thus, a targeted dehumidification can therefore also be carried out on the anode side.

Out 3 is further evident that via another flow channel 34 a coolant 36 in the fuel cell stack 10 can be introduced to remove heat during operation of this.

That in at least one or more of the flow channels 16 . 28 . 32 arranged Peltier element 22 is present also when starting up the fuel cell stack 10 used, especially at a freeze start, but then not for cooling, but for targeted heating of the fuel 30 or of the oxidizing agent. Then any existing ice blocks, especially in the flow fields 26 , Removed particularly quickly by defrosting and the resulting water from the fuel cell stack 10 be discharged.

Instead of one through the entire fuel cell stack 10 passed Peltier element 22 can also be one in its geometry to the fuel cell stack 10 be provided adapted stack of Peltier elements.

In this case, a respective Peltier element can be assigned to a respective fuel cell, or in each case one Peltier element can heat or cool power for a plurality of fuel cells of the fuel cell stack 10 provide.

LIST OF REFERENCE NUMBERS

10

fuel cell stack

12

bipolar

14

breakthrough

16

flow channel

18

Transition area

20

breakthrough

22

Peltier element

24

flow arrow

26

flow field

28

flow channel

30

fuel

32

flow channel

34

flow channel

36

coolant

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 2005/0112436 A1 [0007]

Claims (10)

Method for operating a fuel cell stack ( 10 ), in which by means of at least one tempering element ( 22 ) a temperature of the fuel cell stack ( 10 ) is changed, characterized in that in a cooling operation by means of the at least one tempering ( 22 ) at least one the tempering ( 22 ) ( 16 ) of the fuel cell stack ( 10 ) is cooled to a temperature at which water condenses when in the fuel cell stack ( 10 ) Water is present. Method according to claim 1, characterized in that the at least one area ( 16 ) when shutting down the fuel cell stack ( 10 ) is cooled to the temperature. Method according to claim 1 or 2, characterized in that the at least one region ( 16 ) of the fuel cell stack ( 10 ) is cooled to a temperature at which the condensed water freezes. Method according to one of claims 1 to 3, characterized in that as the at least a portion of the fuel cell stack ( 10 ) a flow channel ( 16 ) is cooled over which a reactant of respective flow fields ( 26 ) a plurality of fuel cells of the fuel cell stack ( 10 ) and / or - in which a reactant of respective flow fields ( 26 ) a plurality of fuel cells of the fuel cell stack ( 10 ) flows in here. Method according to claim 4, characterized in that the at least one tempering element ( 22 ) in the at least one flow channel ( 16 ) is flowed around by the reactant Außenumfangsseitg. Method according to claim 4 or 5, characterized in that the at least one flow channel ( 16 ) is acted upon with an oxidizing agent as the reactant. Method according to one of claims 1 to 6, characterized in that in a different operating mode of the cooling operation by means of the at least one tempering ( 22 ) the at least one area ( 16 ) of the fuel cell stack ( 10 ) is heated to a temperature at which frozen water thaws. Method according to claim 7, characterized in that the at least one area ( 16 ) during commissioning, in particular during a freeze start, of the fuel cell stack ( 10 ) is heated to the temperature at which frozen water thaws. Method according to one of claims 1 to 8, characterized in that as the at least one tempering at least one Peltier element ( 22 ) for introducing heat into the fuel cell stack ( 10 ) or used for cooling the same. Fuel cell stack with at least one tempering element ( 22 ), by means of which a temperature of the fuel cell stack ( 10 ) is variable, characterized in that the at least one tempering ( 22 ) in at least one area ( 16 ) of the fuel cell stack ( 10 ), which during operation of the fuel cell stack ( 10 ) can be flowed through by a reactant, wherein by means of the tempering ( 22 ) the area ( 16 ) is coolable to a temperature at which water condenses when in the fuel cell stack ( 10 ) Water is present.

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