GB2552975A

GB2552975A - Fuel cell stack

Info

Publication number: GB2552975A
Application number: GB1614042.8A
Authority: GB
Inventors: Hm Prasanna; Kumar Saurabh
Original assignee: Daimler AG
Current assignee: Mercedes Benz Group AG
Priority date: 2016-08-17
Filing date: 2016-08-17
Publication date: 2018-02-21
Also published as: GB201614042D0

Abstract

A fuel cell stack 1 with a plurality of single cells separated by metal bipolar plates 5, comprising cooling channels 6 and a heating system 8 positioned on at least one side of the stack parallel to the stacking direction S and in thermal contact to at least some of the metal bipolar plates. The heating system may comprise at least one resistance wire coil 10, e.g. of Nickel-chrome, and there may be an electrically isolating material 12, e.g. mica, between the heating system and the plates. The isolating material may be shaped as a flat plate with ribs 13 orthogonal to the stacking direction S, with the plates positioned between the ribs, and the single cells beneath the ribs on their side(s) facing away from the flat plate. There may be thermal insulators between cells and ribs, the heating system may have a channel 9 for a heating fluid at about 100 degrees Celsius, e.g. also with the coil in the channel, and there may be thermal insulation away from the stack opposite the heating system. The invention is designed to allow an improved distribution of temperature in PEM fuel cell freeze starts to start-up vehicles.

Description

(54) Title of the Invention: Fuel cell stack

Abstract Title: Fuel cell stack with side heating system for improving temperature distribution in freeze startups (57) A fuel cell stack 1 with a plurality of single cells separated by metal bipolar plates 5, comprising cooling channels 6 and a heating system 8 positioned on at least one side of the stack parallel to the stacking direction S and in thermal contact to at least some of the metal bipolar plates. The heating system may comprise at least one resistance wire coil 10, e.g. of Nickel-chrome, and there may be an electrically isolating material 12, e.g. mica, between the heating system and the plates. The isolating material may be shaped as a flat plate with ribs 13 orthogonal to the stacking direction S, with the plates positioned between the ribs, and the single cells beneath the ribs on their side(s) facing away from the flat plate. There may be thermal insulators between cells and ribs, the heating system may have a channel 9 for a heating fluid at about 100 degrees Celsius, e.g. also with the coil in the channel, and there may be thermal insulation away from the stack opposite the heating system. The invention is designed to allow an improved distribution of temperature in PEM fuel cell freeze starts to start-up vehicles.

Fig-1

1/1

Fig.2

Fuel Cell Stack

The invention is related to a fuel cell stack with a plurality of stacked single cells, as defined in more detail in the preamble of claim 1.

To build fuel cells in the form of a stack or a pile of single cells is well known in the art. Typically the single cells of such a fuel cell stack are separated by a so called separator. It is also well known in the art to build this separator as a so called metal bipolar plate. Such a metal bipolar plate has the advantage that the needed stiffness can be reached with a relatively thin separator. Additionally such a metal bipolar plate comprises cooling channels for a cooling medium. Due to the effect that metallic materials do allow a very quick and good heat transfer the use of such metal bipolar plates is very advantageous for the cooling of the a fuel cell stack during operation.

It is also well known from the state of the art that the freeze start of a fuel cell stack, especially with PEM fuel cells, might be time intensive. Especially for the use in a vehicle to generate the electric power for driving the vehicle this is a relevant disadvantage.

Therefore during the start-up of a fuel cell system heating of the fuel cell stack is required. It is known in general from the state of the art to heat the cooling medium during such a start-up operation. Such a heating of the fuel cell stack via the heated cooling medium is also relative time intensive because the heat had to be transferred into the cooling medium for example a cooling fluid first and then from the cooling fluid into the bipolar plates and from the bipolar plates to the single cells itself. The time needed e.g. to remove ice from the single cells of the fuel cell stack is therefore relatively long.

To avoid this short coming it is known from the state of the art and is for example described in the US 2004/0247967 A1 that the so called end plates of the fuel cell stack can be heated electrically. Such a heating of the end plates allows a quicker heating of the fuel cell stack. But typically the warmth has to be transferred from the end cell through the separators and the single cells to heat the neighborhood cell. Therefore, the single cells and separators in the middle of the pile will be heated much slower than those in the neighborhood of the end plates. The result is an inhomogeneous distribution of temperature in the stack.

In the Japanese patent abstract JP 2004-220946 A a heater for warming a separator itself is known. Therefore, the separator, which is here a separator made of a non-metallic material is heated by a resistance wire. This can be used for heating the separators and via the separators for the whole fuel cell stack. This allows a homogenous heating. The short coming of this solution is the very complex design of the separator. Typically the separator comprises the cooling channels for a cooling medium during the normal operation. According to the above mentioned Japanese patent abstract the separator had additionally to carry an electric heating system. This makes the separator very broad in the direction of stacking. As this is a commonly known problem of separators made of non-metallic materials this problem will be increased by the very complex design of such a separator including an electric heating system additionally.

The object of the invention is to provide a fuel cell stack which allows a good heating on the one hand and a simple and compact design on the other hand.

This objection of the invention is solved by a fuel cell stack with the features in the characterizing portion of claim 1. Further advantages and developments are shown with the features of the depending claims.

The fuel cell stack according to the invention has a plurality of single cells separated by metal bipolar plates. The metal bipolar plates comprise cooling channels for a cooling medium such as a cooling fluid. Additionally the fuel cell stack is equipped with a heating system. In the fuel cell stack according to the invention the heating system is positioned at least at one side of the stack in parallel to the stacking direction. The heating system is in thermal contact to at least some, preferably to all, of the metal bipolar plates. The metal bipolar plates are able to transfer heat. Therefore they can be brought into contact on their narrow side with the heating system such as a body which is in thermal contact to preferably all of the metal bipolar plates. Therefore, the fuel cell stack can be heated very homogenously. Preferably all of the metal bipolar plates are heated at the same time for example on one of their narrow sides or according to a further development of the invention in a preferred way on two opposite sides of the fuel cell stack. The heat can then be carried very directly into the metal bipolar plates. The heat can easily be transferred in a very homogenous way to all of the single cells in between those metal bipolar plates.

The fuel cell stack can therefore be heated very efficiently which allows a quick start of the fuel cell stack even under freezing conditions.

According to a further advantageous development of the idea the heat system comprises at least one coil of resistance wire which according to a further preferred development can be a Ni-chrome wire.

To avoid an electric contact of the at least one coil of resistance wire according to a further preferred development of the fuel cell stack electric isolators are positioned between the heating system and the metal bipolar plates. Such electric isolators should be have bad insulating properties for the transfer of heat. Otherwise they will delay the heat transfer.

According to a further preferred development such electric isolators can be made of mica or comprise mica. Isolators on the basis of mica are for example known from building generators with several 100 MVA for power plants. The solid mineral material can be used directly or can be used as a powder fixed on a carrier.

According to a further preferred development of the idea the fuel stack according to the invention is built in such a way that the heating system comprises channels for a heating fluid. For example instead of the at least one coil of resistance wire a heating fluid can be transported through channels of the heating system. Such a heating fluid had to be relatively hot, for example its temperature has to be in the range of about 100° C which is more than the standard operation temperature of a typical PEM-fuel cell. This guaranties a quick heating of the fuel cell stack during start-up.

According to a very preferred further development of the idea the fluid and the coil of the resistance wire are in the same channel. This preferred embodiment of the heating system allows a heating with the coil of resistance wire on the one hand and additionally a heating with a heating fluid flowing through the capillaries of the coil on the other hand. In such an embodiment the heating can be realized as an electric heating and/or a heating by a heating fluid whichever is more efficient according to the actual state of the system with the fuel cell stack. For example such an embodiment allows an electric heating if there is enough electric energy in a start-up battery. If not, the fuel cell can be heated by a heating fluid for example with the heat generated by burning a portion of hydrogen which is used for the fuel cell as a fuel. On the other hand both techniques can be combined for example if the temperatures are very low and the user of the fuel stack prefers a very quick start-up of the fuel cell system.

According to a further preferred embodiment of the idea a thermal insulation on at least one side of the heating system facing away from the stack is used. Such a thermal insulation positioned on the backside of the heating system prevents the heating of the surrounding of the fuel cell stack by the heating system. Almost all the heat available can then be used to heat the fuel cell stack. This makes the heating of the fuel cell stack e.g. during a freeze start-up more efficient.

Further preferred developments of the idea are claimed in the further depending claims and are described in possible embodiments which are shown in the drawings and will be described with reference to the drawings thereafter.

Brief description of the drawings:

Figure 1 shows a principal cross section through a central part of a fuel cell stack; Figure 2 shows an alternative embodiment of the heating system; and Figure 3 shows a further alternative embodiment of the heating system.

In figure 1 a central part of a fuel cell stack 1 is shown in a principal cross section drawing. The fuel cell stack is made of a plurality of single fuel cells 2. Two of them are shown in figure 1. In figure 1 these single cells 2 comprise a membrane 3 made of a polymer. This membrane 3 is also referred to as proton exchange membrane. Beneath the membrane 3 on both sides in the stacking direction S of the fuel cell stack 1 gas diffusing electrodes 4 are positioned. In between those single cells 2 there are separators in the form of metal bipolar plates 5. Three of them are shown in figure 1. The metal bipolar plates 5 comprise almost in the center of the metal bipolar plate cooling channels 6 for a cooling medium for example a liquid cooling fluid. Typically those cooling channels 6 are designed as a meander. Furthermore, on the surfaces of the metal bipolar plates 5 in the stacking direction S there are gas distributing channels 7. They might also be designed in the form of a meander. Through those gas distributing channels 7 on the one side of the metal bipolar plate 5 air or oxygen is distributed to the gas diffusing electrode 4. On the other side of the metal bipolar plate 5 hydrogen is transported to the gas diffusing electrode 4.

As far as described the fuel cell stack 1 according to the exemplary embodiment of the figure 1 is well known in the state of the art. The first embodiment according to the invention is shown on the upper and the lower side in parallel to the stacking direction S. There are heating systems 8 provided on those two sides of the fuel cell stack 1. Every heating system 8 comprises a channel 9 with a coil 10 of Ni-chrome wire as a resistance wire for electric heating therein. Furthermore, a heating medium can flow through the channel 9 as an additional or alternative method to heat the heating system 8. By transferring heat to the heating system 8 the metal bipolar plates 5 which are in thermal contact to the heating system 8 are heated. The heated metal bipolar plates 5 will then transfer the heat to the single cells 12 which leads to a very homogenous distribution of heat in the fuel cell stack 1. To prevent the heat to escape into the surrounding a thermal insulation 11 is placed on the upper and lower end of the fuel cell stack 1 shown in the figure 1.

To avoid an electric flow from the resistance wire 10 to the metal bipolar plates 5 or vice versa an electrically isolating but thermally conducting material 12 in form of a mica sheet is positioned in between them. The preferred embodiment of the electrically isolating material 12 is also shown in the figure. The electrically isolating material 12 is shaped as a flat plate with ribs 13 orthogonal to the stacking direction S. In the assemble state of the fuel cell stack 1 as shown in figure 1 the metal bipolar plates 5 are oppositional in between those ribs 13. Therefore, the ribs 13 also made from electrically isolating material which is a good thermal conductor such as mica allows a mechanical support which can reduce the mechanical stress on the single cells when the fuel cell stack is tightened for example with ties or tension straps. To avoid a direct thermal contact of the heating system 8 and the single cells 2, especially with the membrane 3 which can lead to a drying out of the membrane 3 thermal insulators 14 are positioned in between the electrically isolating material 12 especially it’s ribs 13 and the single cells 2 itself.

The use of the channels 9 in the heating system 8 allows an additional function. Instead of a heating fluid a cooling fluid can flow through the channels 9 to allow an additional cooling of the fuel cell stack 1 if such an extra cooling is needed for some operational states of the fuel cell stack 1.

In figure 2 an alternative embodiment of the heating system 8 is shown. Instead of the implementation of the coil 10 of a Ni-chrome wire this coil 10 of a Ni-chrome wire is directly brought into the heating system 8 e.g. the electrically isolating material 12. There is no channel 9 and therefore with a heating system 8 according to the embodiment shown in figure 2 only an electric heating can be provided.

In figure 3 a further alternative embodiment of the heating system 8 is shown. The heating system 8 is shown together with the thermal insulation 14 analog to figure 2. In figure 3 there is no coil 10 of resistance wire. Instead there is only the channel 9 which can be heated by passing a heating fluid through the cannel 9. In this embodiment heating or cooling with a heating or cooling fluid is possible but no electrical heating can be performed. In this embodiment according to figure 3 the electrically isolating material 12 might not be needed. In fact it is helpful to have such an electrically isolating material 12 in this embodiment as well to avoid an electric conduction by the heating or cooling fluid which in the case of electric conduction can decrease the electric isolating resistance for the whole system which might become a safety risk.

Claims

1. A fuel cell stack (1) with a plurality of single cells (2) separated by metal bipolar plates (5), comprising cooling channels (6) for a cooling medium, and with an additional heating system (8), wherein the heating system (8) is positioned at at least one side of the stack (1) in parallel to the stacking direction (S), the heating system (8) is in thermal contact to at least some of the metal bipolar plates (5).

2. The fuel cell stack (1) according to claim 1, wherein the heating system (8) comprises at least one coil (10) of resistance wire.

3. The fuel cell stack (1) according to claim 2, wherein the coil of resistance wire is made of Ni-chrome wire.

4. The fuel cell stack (1) according to claim 1,2 or 3, wherein an electrically isolating material (12) is positioned between the heating system (8) and the metal bipolar plates (5).

5. The fuel cell stack (1) according to claim 4, wherein the electrically isolating material (12) is shaped as a flat plate with ribs (13) orthogonal to the stacking direction (S), whereas in the assembled stack (1) the metal bipolar plates (5) are positioned between the ribs (13) and the narrow side of the single cells (2) are positioned beneath the ribs (13) on their side facing away from the flat plate.

6. The fuel cell stack (1) according to claim 5, wherein thermal insulators (14) are positioned between the single cells (2) and the rips (13) of the electrically isolating material (12).

7. The fuel stack (1) according to one of claims 3 to 6, wherein the electrically isolating material (12) comprises mica or is made of mica.

8. The fuel cell stack (1) according to one of claims 1 to 7, wherein the heating system (8) comprises channels (9) for a heating fluid.

9. The fuel cell stack (1) according to claim 8, wherein the coil (10) of the resistance wire is positioned in the same channel which can be used for the heating fluid.

10. The fuel cell stack (1) according to one of claims 1 to 9, wherein a thermal insulation (11) is positioned on at least one side of the heating system (8) facing away from the stack (1).

Intellectual

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Office

Application No: GB1614042.8 Examiner: Mr Robin Newman