WO1998011616A1

WO1998011616A1 - Plate- or rod-like fuel cell cooling element and fuel cell stack with one or more fuel cell cooling elements

Info

Publication number: WO1998011616A1
Application number: PCT/DE1997/001833
Authority: WO
Inventors: Volker Peinecke
Original assignee: Forschungszentrum Jülich GmbH
Priority date: 1996-09-11
Filing date: 1997-08-23
Publication date: 1998-03-19
Also published as: DE19636904C1

Abstract

Plate-like or tubular cooling devices for fuel cells are known through which flows a cooling medium, for example water or air. These elements have a costly design and a temperature gradient in the direction of flow of the cooling medium. The temperature in a heat transfer medium (1) may be strongly evened out by cooling elements with a heat evacuation section (4) and a heat absorbing section (6) in thermoconductive contact with a heat transfer medium (1). The cross-section of the heat absorbing section (6) increases in the direction of the heat evacuation section (4). This solution is suitable for all types of ordinary fuel cells.

Description

PLATE OR BAR FUEL CELL COOLING ELEMENT AND FUEL CELL STACK WITH ONE OR MORE FUEL CELL COOLING ELEMENTS

Description

The invention relates to a plate or rod-shaped fuel cell cooling element with high thermal conductivity. It also relates to a fuel cell stack, consisting of stacked individual cells with one or more fuel cell cooling elements arranged between the individual cells.

Plate or tubular cooling devices for fuel cells are known which are powered by a cooling medium, e.g. Water or air. The cow! cautions have the advantage of being able to dissipate a relatively high amount of heat per unit of time. However, supply and discharge lines for the cooling medium, pumps and heat exchangers must be provided, which make such devices technologically complex and expensive.

Such a cow! Devices are used particularly in the case of fuel cells with a solid electrolyte, such as Membrane fuel cells, which consist of a stack of several individual cells, are used. In terms of fuel technology, the individual cells are arranged in parallel and electrically connected in series. The same current flows through each individual cell. Bipolar plates are placed between the individual cells, each connecting the anode side of one individual cell to the cathode side of the next cell.

These bipolar plates are now partially designed as cooling plates to dissipate the waste heat generated in the individual cells, such as. B. from DE 42 34 093 A1, DE 39 07 819 A1 or from EP 0 295 629 A1 is known. The cooling plates are hollow and are penetrated by a cooling medium. flows. Mostly water is used, less often air. The waste heat is then released from the cooling medium to a heat exchanger.

There are e.g. B. from DE 39 03 261 A1 or EP 0 128 023 A1, further working on the same principle, pl attenformi ge cooling elements are known which are not provided with a cavity. Rather, there are pipes that are trapped by the coolant and are embedded in a plate-shaped body for heat absorption.

Fuel cells cooled in this way are very sensitive to overheating. On the one hand, a sufficiently high temperature must be maintained in order to achieve optimum efficiency in the generation of electrical energy with the fuel cell. On the other hand, the local temperature inside the fuel cell must not exceed a certain level, since even minor overheats can severely shorten the life of an individual cell. The known liquid cooling systems operate with a control system, such as EP 0 295 629 A1, which, however, does not exclude a temperature gradient in the individual cell in the flow direction of the cooling medium and thus local overheats.

A solution is also known from EP 0 473 540 A2, in which the air supplying the oxygen for the process also serves as a cooling agent. On the one hand, a considerable excess of air must be used; on the other hand, special temperature compensation bodies are required, through which the air can absorb the heat. A temperature gradient along the chemically active zones cannot be avoided with this solution.

The invention is therefore based on the object, a Kuhl ^¬ element specify the initially stated type which is of simple construction and the maßigten to a comparable and constant operating temperature of a Warmetra- gers contributes and is therefore suitable for use in fuel cells.

Invention, the object is achieved in that since the cooling element has a heat discharge section and a heat receiver that is in heat-conducting contact with the heat carrier, the cross-section of which increases in the direction of the heat discharge section.

In a preferred manner, the cooling element can be designed in such a way that the cross section of the heat absorption section increases.

In a likewise preferred manner, the cooling element can also be designed in such a way that the cross section of the heat absorption section increases quadratically.

Other possibilities for increasing cross-section are conceivable.

With such a construction it is possible to use e.g. the surface of a single cell in fuel cells to obtain a uniform temperature and a high heat transfer rate over the entire surface. Cooling water or cooling air can be dispensed with up to a certain range of the heat output to be dissipated.

In a preferred manner, the cooling element can also be designed in such a way that the heat absorbing part is surrounded by a shell that complements the cooling element to form a cuboid or cylindrical shape and whose thermal conductivity is lower than that of the cooling element. It can therefore be snug against the warming elements. Individual rod-shaped cooling elements can also be introduced into a plate-shaped shell.

The cooling element can preferably consist of copper or aluminum. The wrapping of the cooling element according to the invention has the particular advantage that it can also be used for fuel cells with aggressive media. The cover, which covers the heat absorption section, in which the cooling element is in heat-conducting contact with the heat carrier, can thus be made of a resistant material, for example titanium or stainless steel.

The cooling element is particularly suitable for use in fuel cell stack, consisting of stacked single cells with one or more fuel cells arranged between the single cells, which connect the individual cells electrically and which are in heat-conducting contact with the individual cells via the heat absorption section stand.

The invention will be explained in more detail below on the basis of an exemplary embodiment. In the associated drawings show:

Fιg.1 the schematic sectional view of a plate-shaped cooling element according to the invention in the assembly with the individual cells of a fuel cell,

Fig. 2 shows the construction of a cooling element according to the invention. 1 in section,

Fig. 3 shows the heat flow in an arrangement. Fig. 2,

4 shows a diagram for the heat release in the heat carrier,

5 shows a diagram for the heat flow in the longitudinal direction of the cooling element, 6 shows a diagram for the temperature profile of a known cooling device with a constant cross section,

7 shows a diagram for the temperature profile with a linearly increasing cross section,

8 shows a diagram for the temperature profile with a square increasing cross section of the heat absorption section,

9 shows a conventional fuel stack 11 in the stack

Schmtt,

FIG. 10 shows the perspective illustration of a fuel cell stack with a cooling element according to FIGS. 2 and

11 shows the perspective illustration of a fuel cell stack with a rod-shaped cooling element according to the invention.

The exemplary embodiment relates to an application of the invention for a fuel stack 11. 1 shows four individual cells 1 of such a stack, a cooling plate 2 being inserted between two of the individual cells. The cooling plate 2 extends beyond the upper level of the individual cells 1 and the cell frame 3 and forms a heat dissipation section 4 there.

As FIG. 2 clearly shows, the cooling plate 2 consists of two parts, an outer cooling plate 5 and the cooling element according to the invention with the heat absorption section 6 and the heat discharge section 4. This protrudes beyond the cell frame 3 and becomes outside the fuel cell cooled, which is primarily air cooling. The air can either be passed by natural convection or blown past by forced convection. the. If water is used as the cooling medium, it is pumped past the hot discharge sections 4 of the individual cooling plates 2. The cooling elements are inserted in a form-locking manner in the cooling plates 1 and 5, which are designed with a cavity. The material of the cooling plate sleeve 5 has a lower thermal conductivity than the cooling elements, which are made of copper or aluminum, for example. Although these materials are not very corrosion-resistant, due to their construction they have no contact with the channels of the fuel cell that are traversed by corrosive media. The Warmnahmnahmeabschm 6 are designed so that their cross section increases outwards. 2, the cross section increases linearly towards the outside.

To compensate for the temperature, the local, outward temperature gradient oil / _> x must be kept low. This depends on the total heat flow Q (x) along the length x of the cooling plate 2 in the direction of the heat discharge section 4, the cross section A (x) of the heat absorption sections 6 and the thermal conductivity, as is illustrated in FIG. 3:

_) τ / 3x = Q (x) / (A (x) 7t)

If one takes a constant release of goods over the length x

q (x) = const. = qo

As is the case with an ideally operated fuel cell (FIG. 4), the total heat flow Q (x) in the heat absorption section 6 increases linearly with the length x towards the outside

Q (x) = Qo x

With a constant cross section A (x) = const. = Ao of the heat absorption section 6 take the temperature gradient) T tx linear and the temperature T quadratically with the length x to:

3 τ / - »x = Q (x) / (A (x) λ,) = Qo / (Ao λ) x

T (x) = To + 0.5 Qo / (Aoλ) x ² = To + Ki χ ²

This variant corresponds to the known cooling plates with a constant cross-section. A diagram of such a temperature profile is shown in FIG. 6. This means that the temperature T rises sharply and the temperature distribution 1 becomes uneven, which is undesirable.

On the other hand, if the cooling elements according to the invention are used with a cross section A (x) ≠ const increasing over the barrel length x. > 0, the temperature gradient t) T / Px is reduced and the temperature T is compared.

For example, the cross section A (x) increases linearly with the length x (FIG. 7)

A (x) = Ai x,

the temperature gradient e> T / -x remains constant and the temperature T has a linear increase:

e> T / 2x = Q (x) / (A (x) λ) = Qo / (Aι λ)

T (x) = To + Qo / (Aι λ) χ = To + K2X

In the case of a square cross section increasing with the length x

A (x) = A2X ²

the temperature gradient ^ T / ^ x becomes smaller with the length x with? T / Px ^ l / x (x ^ 0) and the temperature is logarithmic with the length x according to T (x) _* *! n (): 3T / _) x = Q (x) / (A (x)) = Qo / (A ₂ ) 1 / X

T (x) = To + K ₃ ln (x)

(qualitative, since undefined for x = 0).

The temperature profile for this variant is shown in FIG. 8.

9, on the other hand, shows a conventional fuel cell, the individual cells 1 of which are clamped between two end plates 7 and connected to one another by bipolar plates 8. A cooling medium, usually water, flows through the bipolar plates 8 via supply and discharge lines, not shown here, and act as cooling plates. The waste heat is then released from the cooling medium to a heat exchanger, also not shown here.

FIG. 10 shows again a plate-shaped cooling element according to the invention in assembly with the individual cells of a fuel cell as in FIG. 2, but this time in a perspective view.

Another variant is shown in FIG. The cooling plate 2 here consists of a cooling plate sleeve 1 e 5 and a rod-shaped cooling element, which in turn has a heat absorption section 6, which is conical here, and a cyl inderfor i gene heat discharge section 6.

Claims

Expectations

1. Platten- or stabforrmges Fuel cell len- cooling element with high thermal conductivity, characterized in that the cooling element has a heat discharge section (4) and one with the heat carrier (1) in heat-conducting contact heat absorption section (6), the cross section of which in the direction of the Heat discharge section (4) increases.

2. Cooling element according to claim 1, characterized in that the cross section of the heat absorption abt (6) 1 i near increases.

3. Cooling element according to claim 1, characterized in that the cross section of the heat absorption abt (6) increases quadratically.

4. Cooling element according to one of the preceding claims, characterized in that the heat-absorbing part (6) is surrounded by a sheath which supplements the cooling element to form a cuboid or cylindrical shape and whose thermal conductivity is lower than that of the cooling element.

5. Cooling element according to one of the preceding claims, characterized in that it consists of copper.

6. Cooling element according to one of the preceding claims, characterized in that it consists of aluminum.

7. Fuel cell lenstapel, consisting of stacked individual cells with one or more fuel cell cooling elements arranged between the individual cells according to one or more of the preceding claims, which electrically connect the individual cells and which over the The heat absorption section (6) is in warm contact with the individual cells.