WO2003001620A2

WO2003001620A2 - Fuel cell stack with higher efficiency density

Info

Publication number: WO2003001620A2
Application number: PCT/DE2002/002283
Authority: WO
Inventors: Norbert Alleborn; Max Brand; Ulrich Gebhardt; Bernd Genenger; Joachim Grosse; Martin Ise; Manfred Poppinger
Original assignee: Siemens Aktiengesellschaft
Priority date: 2001-06-22
Filing date: 2002-06-21
Publication date: 2003-01-03
Also published as: WO2003001620A3; DE10130164A1

Abstract

It is known per se that individual fuel cell units can be mechanically stacked on top of each other according to a predetermined stack length, whereupon they are electrically and fluidically connected in a successive manner. According to the invention, fluidic inertial forces are used to evenly distribute all media, especially the reactants, in the individual fuel cell units and are transferred in precise matching geometrical dimensions of distributing and/or reaction channels.

Description

description

Fuel cell stack with high efficiency density

The invention relates to a fuel cell stack with a high efficiency density, wherein individual fuel cell units are mechanically stacked on one another over a predeterminable stack length, but are electrically and fluidically connected in series.

Constructions of fuel cell stacks previously used in practice are characterized by a comparatively high pressure loss in the operating media. The operating media are the reaction gases (reactants), such as hydrogen / hydrogen-rich gases and oxygen / air, on the one hand, and coolants, such as oil or other cooling liquids, on the other. A pressure drop occurs and is required to distribute gases and liquids evenly over the individual cells of a stack and over the surface of the individual cells, as well as to discharge the product water on the cathode side. Since the operating temperature is lower than the evaporation temperature of the water at the operating pressure of the fuel cell stack, the reaction product of hydrogen (H ₂ ) and oxygen (0 ₂ ) is product water in liquid form. In the prior art, the main flow direction of the reaction gases always runs diagonally over the area of the individual cells. Examples of the design of conventional systems are given in DE 197 43 067 C2.

Current technologies, such as the use of graphite for the bipolar plates required in fuel cells, require a relatively large material thickness due to the comparatively low mechanical stability of the graphite. This has an adverse effect on the overall height of a single cell and especially on the total length of a stack. When assembling and operating a fuel cell stack, mechanical and thermal loads occur on the membranes, which can damage the membranes. The membranes must therefore be handled with particular care during the stacking process and must be packed individually for storage in order to protect them from environmental influences. Special measures are required during operation to avoid damage due to thermal and mechanical loads, such as those caused by moistening the operating gas or mechanical supports.

Sealing concepts that are currently being used use elastomers and adhesives as seals. B. in screen printing or in one-off production by vulcanization on components of the cells. The individual components are then put together in a stack. Since the membrane is not currently protected in conventional designs, damage during assembly and operation is difficult to avoid.

Another problem with the ready-made fuel cell stacks is high contact resistance between the individual, electrically connected cells. Part of the electrical resistance in conventional fuel cell stacks is caused by uneven pressing of the bipolar plates and the electrodes. In the prior art, the contact pressure is applied mechanically via the end plates, sometimes in combination with pressure pads.

Starting from the prior art, it is an object of the invention to improve the constructive implementation of fuel cell stacks which have polymer electrolyte membranes and are suitable for use at temperatures above 100 ° C. For this purpose, a fuel cell stack is to be created which, with a simple structure, has an optimized degree of efficiency. The object is achieved by the entirety of the features of claim 1. Developments are specified in the dependent claims.

The invention results in the use of complex fluid-mechanical interrelationships, such as in particular momentum and mass balances for gas distributors and gas collectors, cell flow, coolant flow, for the constructive implementation of a cell and a stack with a high power density and high system efficiency. Especially compared to common ones

Constructions with sheets as bipolar plates reduce the overall length of the stack by virtue of the fact that the cross-sectional area of the distribution channels can be dimensioned on the basis of flow calculations so that the largest possible active area is achieved with the smallest possible pressure loss for the flow of reaction gases and coolant. This advantageously means that fewer cells are required to build up the stack than in the prior art in order to provide a predetermined electrical output.

A so-called C-flow of all media is preferably realized for an optimal distribution with minimal pressure loss and optimum overall efficiency. At the same time, a constructive implementation of components that are easy to manufacture, store and service-friendly is guaranteed.

The sealing concept implemented in the invention also takes the latter aspect into account and realizes assemblies suitable for production and operation at an early stage of manufacture.

Further details and advantages of the invention result from the following description of figures of exemplary embodiments with reference to the drawing in conjunction with the patent claims. Show it 1 shows a section through a fuel cell stack

Clarification of the C flow over the stack length, FIG. 2 shows a graphical representation of the distribution quality of the C flow in FIG. 1 over a standardized stack length,

3 to 6 each show a top view of a fuel cell area with different configurations of the lines for the operating media, FIG. 7 shows a so-called wide-angle diffuser in a sectional view,

8 shows a component for coolant distribution over the surface in a sectional view, FIG. 9 shows a complete fuel cell stack with end plates in a sectional view with reference to a partial section,

10 shows the partial section from FIG. 9, which illustrates internal and external seals of the individual fuel cell units, FIGS. 11 and 12 each show a top view of the bipolar plate according to FIG. 4 with two different concepts of the reactant flow, FIG. 13 shows a graphical representation of the distribution quality as a function of the situation of the barrel length and FIG. 14 shows a graphical representation of the pressure loss over the relative barrel length.

FIG. 1 shows a fuel cell stack 1 which consists of a plurality of fuel cell units 10, 10 ^λ , ... stacked mechanically one on top of the other. Cell 1, cell i and cell z are identified in detail by the fuel cell units 10, ^10λ , ...

The fuel cell stack 1 is operated with hydrogen (H ₂ ) as the fuel gas, which is possibly generated via a reformer not shown in FIG. 1, and with oxygen (0) from the ambient air. It is a line 2 for the air / hydrogen supply and a line 3 for the removal of the residual gas.

FIG. 1 shows that the reaction gases are supplied with a gas stream V. If such a gas flow V is applied to the supply line 2, a gas loading qi results in the cell 1, a gas loading qi in the cell i and a gas loading q ₂ in the last cell. The aim is to achieve a uniform gas distribution on the individual fuel cell units 10, 10 ^Λ , ... in the fuel cell stack 1.

In contrast to the constructions used in the prior art, the uniform gas distribution in the stack and in the individual cells is not ensured by a very high pressure loss between the supply and discharge sides. Rather, a uniform distribution is achieved by utilizing the interaction of inertial forces and viscous forces in the flow and their implementation in coordinated geometric dimensions of the distribution channels on the one hand and the reaction channels on the other.

A combination of geometry and favorable flow guidance in a compact form leads to a particularly low pressure drop with a particularly even distribution of the reaction gases and the coolant. The low pressure drop and the uniform distribution are converted into a particularly high power density and a particularly low parasitic load. Throughputs close to the stoichiometric throughput can be achieved for both the anode- and the cathode-side reaction gas without a drop in performance of the fuel cell stack 1 occurring.

Distributions in the range between 1.05 and 1.2 can be achieved for reformate. For air as the reaction gas, the distribution is between 1.05 and 1.5. A pressure loss of 0.5 bar is assumed. The ratio q / q _{0 of} the gas distribution over the standardized stack length x / L is plotted in FIG. 2, with q indicating the mass flow through a cell and q ₀ indicating the mean value of all mass flows. The distribution quality V _G indicates the maximum deviation of the mass flow through a cell from the mean value and can be defined in the present case as:

VG = 1 + max | (q / q ₀ ) -l | , (Gl 1)

A distribution quality close to 1 is usually sought for the trouble-free operation of the fuel cell stack. Practical experience has shown that the distribution quality VG should have a maximum deviation of 5% from a base value.

It can be seen from curve 21 in FIG. 2 that the distribution quality VG has a value of approximately 1.06 at the start of the stack and a value of approximately 0.97 has been reached at the end of the stack length. In practice, this means that the distribution quality will not drop to an impermissible value.

FIGS. 3 to 6 show advantageous exemplary embodiments for a bipolar plate 11 as part of a fuel cell unit 10. This means that each fuel line unit 10, 10 ... of the stack 1 has its own bipolar plate 11, 11 \ ....

In FIGS. 3 to 6 the middle area of the bipolar plate 11 is the reaction area to the individual fuel cell 10. Here the reaction of the fuel gas with oxygen takes place under proton conduction in the fuel cell membrane and for the generation of the charge carriers. For this purpose there are 11 inputs on the upper side of the bipolar plate for the media required for the reaction. In particular, in FIG. 3 there is an inlet 12 for air and an inlet 13 for reformer gas, both of which are circular. There is still an access 14 between these two entrances for a cooling medium, the access 14 being rectangular. Corresponding outputs 15 to 17 for the reactants and the coolant are provided on the opposite side, with a directly opposite arrangement being realized in each case. In this way, a C-shaped fluid flow is implemented for the reactants both over the stack length and over the area of a fuel cell unit.

Specifically in FIG. 3, the lines for the reactants are circular and the line for the cooling medium is rectangular. Other geometrical designs are also possible, which are illustrated with the aid of FIGS. 4 to 6, with the bipolar plate 11 being of basically the same structure.

In Figure 4, the entrances 12 ^x to 14 ^Λ and the exits 15 ^λ to 17 for the operating media are each rectangular with the same cross-sectional area. Figure 5 has been modified to the extent that the same ^inputs 12 ^λ and 13 ^Vλ or outputs 15 ^X and 16 ^{λX are available} for the reactants, but the coolant is guided laterally with access 14 ^λ and outlet 17 ^λλ . Finally, FIG. 6 shows a modification with accesses 12 ^{λ λ λ} to 14 ^λ and outlets 15 ^λ to 17 ^Λ for the operating media, the inlets and outlets each having a round cross section. Three lines are provided for the coolant.

There are therefore four variants for the bipolar plates 11, which are distinguished by a different use of space and thus a specifically adapted output.

In order for the fuel cells to perform well, the relative usable area of the bipolar plates with the media supply and discharge lines must be optimized in relation to the stack volume. This means that the relative stack power density has to be adjusted. In the following days The relative stack power densities for the four variants of the bipolar plates shown in FIGS. 3 to 6 are given in Bellarian compilation.

table

It can be seen that variant III according to FIG. 5 covers a particularly favorable relative stack power density.

A special design feature of the coolant supply is a flow distributor integrated in one of the end plates, which, according to FIG. 7, is advantageously designed as a so-called wide-angle diffuser. Such a wide-angle diffuser 40 with outer walls 41 and inner walls 42 is shown in FIG. 7, each of which enclose an angle α <15 °. The coolant is evenly distributed from a narrow coolant inlet 43 to a wide coolant outlet 44. The coolant can thus be distributed evenly over the width of a rectangular coolant channel.

As a further special design feature of the devices for coolant distribution, a component 35 with a defined one Pressure loss installed, which ensures the distribution over the area of the individual cells and on the individual cells of the entire stack. Such a component 35 is shown in FIG. 8 and is essentially formed by a perforated plate 36 for coolant distribution. The coolant is distributed over the surface 38 by means of a coolant supply 37 and serves for uniform cooling of the surface 38.

The dimensions of the component 35 are matched to the distance between two cells, which is designed for optimal heat transfer with maximum efficiency of the overall system. Due to the higher heat transfer coefficient, cells with a short distance can already be cooled with smaller coolant mass flows. In order to evenly distribute these small coolant mass flows, the

Pressure loss of the mounting part matched to the pressure loss of the gap flow.

The sealing concept of the fuel cell stack 1 according to FIG. 1 is illustrated in FIG. 9. It is essential here that in addition to the individual fuel cell units 10, 10 ^λ , ... and the feeds 2 and the drains 3, corresponding to FIG. 1, there are end plates 60, 60 ^λ , which in turn have feed connectors 61 to 64 for the operating media. It is important that the fuel cell stack used in operation forms a tight unit in itself, but is equally modular in that individual fuel cell units are sealed in themselves.

The latter is clear from section x of FIG. 9 according to FIG. 10: in the enlarged illustration it is essential that membrane electrode units 90, 90 ^λ (MEA = membrane electrode assembly) of membrane and electrodes are inserted tightly between bipolar plates 70, 70. The gas distribution for the cell takes place in the electrode spaces of the individual cells, the openings 91, 91 ^λ illustrating the gas distribution for the stack. These are both internal in seals in every cell as well as externally on the cell, i.e. on the stack.

In detail, the internal seals for sealing MEA 90 and bipolar plate 70 are designated 95. On the outside, each assembly unit made of a single fuel cell has an outer seal 96, which completes the sealing concept.

The described sealing concept is an integral part of the above-described concept of the fuel cell stack, which assembles preassembled units that can be stored and processed without special protective measures. The seals applied to individual components each connect several functional elements to form storage and assembly units in which the particularly sensitive membrane is protected. Such preassembled units are in particular the two electrodes between which the polymer membrane is inserted, or entire cells including the devices for distributing the reaction gases and the coolant. This enables considerable savings in storage, assembly of the stack and, due to the particularly reliable design, the warranty.

In order to increase the overall efficiency compared to conventional systems, the pressure level of the coolant is raised. This isostatic pressing causes bipolar plates and electrodes to be pressed together evenly over the entire surface of each fuel cell in the stack. This results in a significant reduction in the electrical contact resistance. At the same time, the pressure of the coolant is significantly higher, up to 5 times higher than the pressure of the reactants in the fuel cell.

FIGS. 11 and 12 show, according to FIGS. 3 to 6, the top view of the bipolar plates 11 with the corresponding feeds for air, coolant and reformer gas, such as it is also essentially already shown in FIGS. 3 to 6. In adaptation to FIG. 4, the inlets and outlets for air, coolant and reformer gas are each represented as rectangular ducts of the same cross section in FIGS. 10 and 11.

In FIG. 11, the arrows 101, 101,... Which are carried out in parallel illustrate the flow distribution, for example of the air, from the inlet 12 ^Λ to the outlet 15 ^x . In the prior art, the constructive design of the flow-guiding components means that a total diagonal flow distribution according to arrow 105 is formed along the bipolar plate 11, which is also referred to as the Z flow. The same applies to the reformer gas, which reacts with the air in the fuel cell. From the inlet 13 ^λ to the outlet 16 ^λ , a Z flow with diagonal flow distribution is also realized in FIG. 10.

On the basis of FIG. 12, it is clarified in comparison to FIG. 11 that the reactant, for example the reformer gas in the bipolar plate 11, is led from the inlet 14 along the parallel arrows 102, 102, ... to the opposite outlet 16 ^{λ in} such a way that gives an approximately C-shaped flow according to arrow 110. The same applies in turn to the air as reaction gas from inlet 13 to outlet 16 ^Λ .

Overall, it is achieved with the device described above that in each fuel cell stack there is a C-shaped flow of air and / or fuel gas over the longitudinal direction of the successive cells. In addition, on the other hand, a C-shaped flow is also realized in the surface of the bipolar plates 11, as is exemplified for the reformer gas with reference to FIG. 12. The combination of these measures gives a decisive improvement over the prior art. In FIG. 13, the distribution quality is shown in accordance with FIG. 2 as a function of the standardized run length. Here curve 121 essentially corresponds to curve 21 from FIG. 2. For comparison, a further curve 52 is entered, which results from the Z-flow most commonly used in the prior art. It can be seen that a uniform value for the distribution quality can hardly be achieved with the comparison curve. In order to achieve a distribution quality of 1 in the middle region, the distribution quality is lower at the beginning of the fuel cell stack 1 and the distribution quality is higher at the end of the fuel cell stack 1.

Essentially corresponding results result from FIG. 14, in which the pressure loss is shown as a function of the relative running length. The characteristic mean

131 the pressure loss over a perforated plate and the characteristic

132 the pressure loss across the supply channels. It follows that there is an approximately constant pressure loss over the entire length of the barrel in the designs described. There is therefore no deterioration in the fuel or gas supply.

What is common to the fuel cell stack described using the individual examples is that the flow guidance of all media is C-shaped over the stack length of the stacked fuel cell units. Furthermore, the flow of the reactants over the area of an individual fuel cell unit is C-shaped. With this flow guidance, uniform distributions near the stoichiometric throughput are realized for the anode-side and / or the cathode-side reaction gas.

Claims

claims

1. Fuel cell stack with high efficiency density, wherein individual fuel line units are mechanically stacked on one another over a predeterminable stack length, but electrically and fluidically connected in series, characterized in that for the uniform distribution of all operating media, in particular the reactants (H ₂ ), in the individual fuel line units (10, 10, ...) the interaction of inertial forces and viscous forces in the distribution flow are used and implemented in coordinated geometrical dimensions of distribution and / or reaction channels (2, 3; 12 to 14, 15 to 17).

2. Fuel cell stack according to claim 1, characterized in that the flow of the operating media over the stack length (L) of the stacked fuel cell units (10, 10 ^λ , ...) is C-shaped.

3. Fuel cell stack according to claim 1, characterized in that the flow guidance of the reactants (0 ₂ , H ₂ ) over the surface of a single fuel cell unit (10, 10, ...) is C-shaped.

4. Fuel cell stack according to one of claims 1 to 3, characterized in that for the anode-side and / or the cathode-side reaction gas (H ₂ , 0 ₂ ) uniform distributions are realized close to the stoichiometric throughput.

5. Fuel cell stack according to claim 4, characterized in that for reformate as the reaction gas there is a distribution better than 1.2 and preferably at 1.05 with a pressure loss of 0.2 bar.

6. Fuel cell stack according to claim 4, so that a distribution better than 1.5, and preferably at 1.05, for a pressure loss of 0.2 bar is present for air as the reaction gas.

7. Fuel cell stack according to claim 1, claim 2 or claim 3, d a d u r c h g e k e n n z e i c h n e t that for the coolant supply one in one of the end plates

(40, 40) integrated flow distributor (41) is present,

8. Fuel cell stack according to claim 7, with a rectangular coolant channel, so that the flow distributor has a wide-angle diffuser (40) with which the coolant (KM) is distributed uniformly over the width (b) of the rectangular coolant channel (13).

9. Fuel cell stack according to claim 8, characterized in that a perforated plate is used as a component (35) with a defined pressure loss for the coolant distribution in the surface (30) of a fuel cell unit (10, 10 ^Λ ).

10. The fuel cell stack according to claim 9, so that the dimensions of the component (35) are matched to the distance (d) between two fuel cell units (10, 10 10, ...).

11. Fuel cell stack according to claim 8, characterized in that the pressure loss of the component (35) is matched to the pressure loss of the gap flow between two fuel cell units (10, 10 ^λ , ...) for uniform distribution of the mass flows of the coolant (KM).

12. Fuel cell stack according to one of the preceding claims, characterized in that that pre-assembled fuel cell units (10, 10 ^Λ ) are joined together with seals (95, 96).

13. Fuel cell stack according to claim 12, characterized in that inner seals (95) in the respective fuel cell unit (10 ^Λ ), the membrane electrode unit (90) against bipolar plates (62, 63) and outer seals (96) a complete fuel cell unit (10, 10) seal to the outside.

14. A fuel cell stack according to claim 12, characterized in that the seals (95, 96) each connect a plurality of functional elements (10, 10 ^Λ ) to form storage and assembly modules in which the membrane (91) is arranged in a protected manner.

15. Fuel cell stack according to claim 12, characterized in that a preassembled fuel cell unit (10, 10 ^λ ) from two electrodes (92, 93), between which the membrane (91) is inserted and their

Electrode spaces for gas distribution of bipolar plates (70, 70 ^λ ) are closed.

16. Fuel cell stack according to claim 15, characterized in that the pre-assembled fuel cell unit (10, 10 ^x ) u - all devices for distributing the reaction gases (Hι, 0 ₂ ) and the coolant (KM).

17. Fuel cell stack according to one of the preceding claims, wherein an incompressible medium is used as the coolant, characterized in that by increasing the pressure of the coolant (KM) isostatic pressing of the bipolar plates (11, 11) on the electrodes.

18. Fuel cell stack according to claim 17, so that the pressure of the coolant (KM) is higher than the pressure of the reactants (L, BG) in the fuel cell (10, 10).

19. The fuel cell stack according to claim 18, so that an excess pressure of the coolant with respect to the reactants is between 1 to 8 bar, preferably about 5 bar.