WO2010066463A1

WO2010066463A1 - Fuel cell system with circular stack

Info

Publication number: WO2010066463A1
Application number: PCT/EP2009/008950
Authority: WO
Inventors: Sascha Kuehn
Original assignee: Ezelleron Gmbh
Priority date: 2008-12-12
Filing date: 2009-12-14
Publication date: 2010-06-17
Also published as: DE112009003663T5

Abstract

A fuel cell system with several tubular fuel cells, especially with microtubular fuel cells with a diameter in the range of 0.1 mm to 30 mm and/or a lenght in the range of 1 cm to 40 cm and/or solid oxide fuel cells (SOFCs), having at least one electrically conductive carrier (1) or carrier section, which is partitioned into several inherently electrically conductive segments (1a, 1b) which are concentrically arranged into each other, wherein different segments are electrically isolated from each other, wherein at least two of the segments comprise at least one, preferably several fuel cells (2), which is/are, at least in sections, arranged on and/or integrated in the respective segment (1a, 1b) in such a way that one electrode of each of said fuel cells is in electrical contact with the respective segment.

Description

Fuel cell system with circular stack

The invention relates to fuel cell systems, especially high- temperature solid oxide fuel cell sys- terns .

Concepts for high- temperature solid oxide fuel cells already exist and are documented in the technical literature (Fuel Cell Handbook 7th edition, EG&G Ser- vices, Inc. U.S. Department of Energy, Office of Fossil Energy, National Energy Technology Laboratory, Morgantown, West Virginia, November 2004; Handbook of Fuel Cells Fundamentals, Technology and Application, Wolf Vielstich, Hubert A. Gasteiger, Arnold Lamm, 2003 John Wiley & Sons, Ltd.) and among them there are tubular, planar and monolithic cell designs. A special case of the tubular design is the so-called microtubular cell, which distinguishes itself by extraordinary stability against high temperature gradi- ents and, associated with that, fast thermal cy- clability. Beyond that, they have an improved volumetric power density due to their higher surface-to- volume ratio (High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications; S. C. Singhal et al . ; ISBN 1856173879; Elsevier Ltd., 2003, Oxford, V. Lawlor, S. Griesser, G. Buchinger, A. Olabi, S. Cordiner, D. Meissner; Journal of Power Sources, 2009, pp. 387-399) .

A particular problem exists for fuel cell systems built with tubular cells, when cells are connected in series (in comparison to planar systems) . Above a certain current density, low- loss parallel circuits are not easy to realize. During the assembling of in- dividual tubular SOFCs, the low- loss power tap of the cells and the minimization of ohmic drops (for an improved degree of efficiency of the stack) present a rather huge challenge. In addition, it must be observed that the voltage of an individually connected cell is thermodynamically limited to approx. 1 V; in order to achieve higher voltages costly voltage converters must be used or cells must be connected in series. In comparison to planar cells, the latter cannot so easily be realized especially for tubular cells. Finally, especially for smaller systems, gas channelling is of decisive importance for the efficient use of introduced reactants and, in many cases, only realizable with great difficulties. In addition, a preferably low weight associated with a preferably compact design plays an important role for the usability of stack-structures.

The problem to be solved by the present invention is to provide a fuel cell system design enabling a low- loss power tap of the cells and a minimization of ohmic drops at a small system weight and volume, con- sidering good gas supply of the fuel cells in the stack and good gas impermeability.

This problem is solved by a fuel cell system accord- ing to claim 1. Other advantageous design forms of a fuel cell system according to the present invention can be found in the depending claims.

The individual characteristics of the design exam- pies, which are described in the following, can be realized independently from each other within the framework of the present invention, thus do not have to be exactly implemented in the characteristic combinations presented in the examples .

The basic idea of the present invention is as follows: Instead of integrating fuel cells in one continuous, e.g. metallic stack with very long current paths and thus high ohmic drops , and only a few or complicated connection possibilities, the fuel cells are integrated into smaller segments, which at first electrically isolated, generate the complete stack. This construction results in a multiplicity of possibilities to electrically connect the cells and seg- ments with each other (as is realized in the following examples and claims) , which allows to adjust a number of parallel and/or serial connected elements in such a way, that requested power and voltages can be obtained, while keeping the current densities at the taps low enough, so that the conductors can be implemented as weight- and space-saving as possible. At the same time, the failure of a single cell or of a single contact will not cause a complete system failure (due to a parallel connection of several cells in the segments or of several segments in the stack) . A concentric arrangement of the segments leads to an optimal space utilisation of the fuel cell stack, especially, if in addition to the fuel cells, also fur- ther components, such as e.g. a burner, a reformer or a heat exchanger are integrated into the construction. An improved thermal management of the fuel cells, especially of high- temperature fuel cells, can be more easily achieved using the concentric design.

Based on the partition in segments, the gas supply to the individual segments can on the one hand be done in parallel, on the other hand (controlled by e.g. valves) it can, if necessary, be individually ad- justed for all segments or the fuel cells of different segments can also relatively simply be connected in series with respect to the gas supply. This serial gas interconnection enables increased fuel utilisation and the use of isolating constructed elements allows the flexible adjustment to different operating conditions, such as operating voltage and/or current density depending on the connection variant.

The basic construction of the invention can be de- scribed as follows: A fuel cell system with several tubular, especially microtubular fuel cells, especially solid oxide fuel cells (SOFCs) , having at least one conductive carrier or carrier section, which is partitioned into several concentrically ar- ranged, interleaving, electrically conductive segments, which are electrically isolated from each other, where at least two of the segments contain at least one, preferably several, fuel cells, which at least in sections are arranged/integrated on and/or in the segment, so that respectively one of their electrodes is in electrical contact with the segment. According to the present invention, this results in the design of a fuel cell system with tubular (preferably microtubular, due to the high stability against temperature variations) SOFCs, where the stack is constructed from individual rings with fuel cells.

Preferred design examples of the invention are pre- sented in figures 1 to 7 , and will now be explained in detail.

Example 1 :

Figure 1 shows in the cross section vertical to the longitudinal axes of the individual fuel cells, an exemplary design of a first fuel cell stack according to the present invention. Preferably several, but at least one fuel cell(s) (2) each is/are integrated in several concentrically interleaving and by insulation layers electrically isolated, electrically conducting rings (1) . A respective electrical isolator (3) is located between adjacent rings (1) . The fuel cells (2) are microtubular SOFCs, which consist of at least one inner electrode and one electrolyte, which mainly encloses the inner electrode and serves as layer between the inner electrode and an outer electrode, which encloses the electrolyte.

Hereby, the electrically conducting rings (1) are electrically connected with one of the electrodes, in this case here with the inner electrode. This connection can be preferably implemented using an electrically conducting layer/coating (e.g. metal solders, especially preferred based on silver) , which serves as seal for the partitioning of different gas com- partments as well. Based on the electrical isolators (3) , a serial connection of fuel cells (2) of the individual rings (1) is now possible and realized. Hereby, the rings (1) can have different widths and/or thicknesses and/or a different number of fuel cells .

Figure 2 shows the longitudinal cross section of the system in figure 1. For clarity purposes, only the first, innermost ring (Ia) is shown here and the ring (Ib) closest to it, enclosing the outer concentric peripheral side of (Ia) , as well as an also circular electrical isolator (3) , which is located between both rings (Ia, Ib) . Each of these elements (Ia) , (Ib) and (3) contains two single, in the longitudinal direction of the configuration (hence vertical to the cross section plane shown in figure 1) distanced from each other, thin, planar ring sections, in-between those elements (4) , (5) and (6) , described below, are constructed, or which are spaced apart of each other by these elements (4) , (5) and (6) .

The fuel cells (2) are on both sides via the inner electrode, which is covered everywhere by the elec- trolyte (5) except in those locations, where the contact with the ring sections (Ia) , (Ib) (which are in the following named as circles as well for simplification) takes place, in electrical contact with the electrically conducting rings (Ia, Ib) .

Between electrolyte (5) , inner electrode and electrical carrier or rings (Ia, Ib) a seal is located at the transition from cell to carrier (Ia, Ib) , which prevents uncontrolled mixing of the atmospheres of the inner and outer electrodes (6) . This seal can for example be a ceramic seal (adhesive, compression seal) and/or a glass solder and/or a metal solder. Latter preferably additionally increases the electrical contact between the inner electrode and the carriers (Ia, Ib) .

The outer electrodes (6) of the tubular fuel cells of the first ring (Ia) are each connected via an electrical contact (4) with the second, electrically conducting ring (Ib) , which results in in a electrical serial connection of the fuel cells of the first electrically conducting ring (Ia) and those of the second electrically conducting ring (Ib) , while the cells of the respective rings (Ia, Ib) are interconnected in parallel.

The in figures 3 to 7 described, additional design examples are basically designed the same way as the design example in figures 1 and 2. Therefore, only the differences will be described below:

Example 2 :

Figure 3 shows the cross section of a system, which is designed comparable to the system in figure 1, whereby here centred, hence within the innermost ring or conductive carrier (1) an additional heat generating or heat absorbing component (7) is integrated. This component (7) may be a heat exchanger, a burner, especially preferred a porous burner, or a component with reforming effect. A possible variant is the integration of a heat exchanger, through which the gas supplied to the system is channelled and thereby heated. An alternative configuration can also use a cell -internal burner as component 7, through which e.g. the entire fuel for the system is channelled, whereby the not burned fuel is warmed. Example 3 :

Figure 4 shows a fuel cell system corresponding to the one in figure 3, where according to the present invention additionally an outer heat-generating or heat-absorbing component (8) exists, which here concentrically encloses the outermost ring (1) . This component (8) may be a heat exchanger, a burner, es- pecially preferred a porous burner, or a component with reforming effect. A possible variant can here be the integration of a heat exchanger (8), through which the gas supplied to the system is channelled and thereby heated. An alternative configuration can also use a cell -internal burner as component 8, through which e.g. the entire fuel for the system is channelled, whereby the not burned fuel is warmed.

Example 4 :

Figure 5 shows a fuel cell system corresponding to the one shown in figure 1, whereas the forms of the electrically conducting rings (1) and the electrical isolators (3) according to the present invention are squares.

Example 5 :

Figure 6 shows a fuel cell system corresponding to figure 2, where the shown conducting rings (Ia) and

(Ib) are connected in series regarding the gas supply of the inner electrode and where the individual fuel cells (2) within the segments are electrically connected in parallel, while in the electrical sum a se- rial connection of the fuel cells of the first electrically conductive ring (Ia) and that of the second electrically conductive ring (Ib) is generated. The gas supply (9) of the inner electrodes of the cells in ring (Ia) is spatially isolated by the exhaust pipes (10) from the exhaust gas of the fuel cells of the inner ring (Ia) , whereby this design helps heating of the supplied gas and cooling of the exhaust. The gas pipes (11) connect the cells of different segments or rings (1) in series. This fluid-related serial connection of the fuel cells can ensure opti- mum gas utilization. Additionally, it is possible to install a burner in the exhaust pipe (10) . The gas in the gas supply (9) is supplied through the inlet pipe (16) . The supply of the outer electrodes is done via lateral gas flow (17) .

Example 6 :

Figure 7 also shows a fuel cell system corresponding to figure 2. Here the gas supply for both segments (Ia) and (Ib) is done in parallel, however in the respective gas supply pipes (12) and (14) for segments (Ia) and (Ib) valves (13) and (15) are located, which can be used to individually control the gas supply, so that for the respective segment the gas supply can be optimally adjusted according to e.g. cell material and/or required temperature and/or power production.

Claims

1. A fuel cell system with several tubular fuel cells, especially with microtubular fuel cells with a diameter in the range of 0.1 mm to 30 mm and/or a length in the range of 1 cm to 40 cm and/or solid oxide fuel cells (SOFCs) , having at least one electrically conductive carrier (1) or carrier section, which is partitioned into sev- eral inherently electrically conductive segments

(Ia, Ib) which are concentrically arranged into each other, wherein different segments are electrically isolated from each other, wherein at least two of the segments comprise at least one, preferably several fuel cells, which is/are, at least in sections, arranged on and/or integrated in the respective segment (Ia, Ib) in such a way that one electrode of each of said fuel cells is in electrical contact with the respective seg- ment.

2. Fuel cell system according to the preceding claim, with two or more circular segments (Ia, Ib) electrically isolated from each other, which are, seen in a cross section perpendicular to the longitudinal axis of the fuel cells arranged in parallel, designed as circular ring segments.

3. Fuel cell system according to one of the preceding claims, wherein at least one electrically conductive segment (1) is, at least in sections, but preferably completely, made from gas-tight material .

4. Fuel cell system according to one of the preceding claims, wherein, seen in a cross section perpendicular to the longitudinal axes of the fuel cells, at least one, preferable all segment (s) has/have the form of a circular ring, an elliptical ring or a polygon.

5. Fuel cell system according to one of the preced- ing claims, wherein, seen from the centre to the outside, at least two, preferably all of the concentric segments are respectively isolated pairwise from each other by electrical isolators, especially ceramic materials, ceramic ad- hesives, glass solders, ceramic layers on metallic materials, ceramic coatings on metallic materials and/or gas compartments .

6. Fuel cell system according to the preceding claim, wherein at least one of the isolators is designed gas-tight and/or mounted reversibly detachable between the segments in such a way that the adjacent segments with their integrated fuel cells are adapted to be removed from the fuel cell system based on a removal of the respective isolator.

7. Fuel cell system according to one of the two preceding claims, wherein at least one of the isolators comprises or consists of a ceramic fibre material, especially aluminium oxide, magne- sium oxide, calcium oxide or zirconium oxide, wherein preferably a seal effect can be generated based on a compressive force compressing the fibre material.

8. Fuel cell system according to one of the preced- ing claims, wherein a heat-generating and/or heat -absorbing component is located adjacent to and/or in at least one of the segments and/or between at least two segments .

9. Fuel cell system according to one of the preceding claims,

wherein a heat-generating and/or heat-absorbing component is located within the innermost segment and/or

wherein a heat-generating and/or heat -absorbing component is located outside of the outermost segment, wherein preferably the heat -generating and/or heat-absorbing component is designed as electrical isolator.

10. Fuel cell system according to one of the two preceding claims,

wherein the heat -generating component is a fuel- oxidizing component, a burner, a reformer, a heat exchanger and/or at least one microtubular

SOFC adapted to be used to burn fuel for heat generation,

and/or

wherein the heat-absorbing component is a reformer and/or a heat exchanger and/or designed to reduce the outer temperature of the system.

11. Fuel cell system according to one of the preceding claims, wherein respectively the cathodes of the fuel cells of at least one segment are electrically connected in series with the anodes of the fuel cells of at least one other segment.

12. Fuel cell system according to one of the preceding claims, wherein, at least in one of the segments, several, preferably all fuel cells of the segment are electrically connected in parallel.

13. Fuel cell system according to one of the preceding claims, wherein in at least one electrical isolation between segments a heat-generating or a heat-absorbing component is integrated.

14. Fuel cell system according to one of the preced- ing claims, wherein the fuel cells are at least partially microtubular SOFCs being adapted to burn fuel for heat generation.

15. Fuel cell system according to one of the preceding claims, wherein the inner electrodes of the fuel cells of at least one segment are adapted to be supplied with the exhaust gas of the inner electrodes of the fuel cells of at least one other segment and/or wherein the outer electrodes of the fuel cells of at least one segment are adapted to be supplied with the exhaust gas of the outer electrodes of the fuel cells of at least one other segment.

16. Fuel cell system according to one of the preceding claims,

wherein the exhaust gas(es) of one electrode or of both electrodes of one fuel cell, of several fuel cells or of all fuel cells of at least one segment is/are adapted to be fed to a burner

and/or

wherein the product gas of a reformer is adapted to be fed to at least one electrode of one fuel cell, of several fuel cells or of all fuel cells of at least one segment.

17. Fuel cell system according to one of the preced- ing claims,

wherein the gas supply of one electrode or of both electrodes of the fuel cells of at least on segment is independently adjustable from the gas supply of the electrodes of the fuel cells of at least one other segment

and/or

wherein the gas supply of the heat-generating and/or heat -absorbing component (s) of at least one segment is independently adjustable from the gas supply of the heat-generating and/or heat- absorbing component (s) of at least one other segment and/or of the fuel cells,

wherein preferably this independent adjustment of the gas supply is adapted to be used to control the generated power and/or to optimize the utilization of the gaseous reactants and/or to adjust the temperature of the system.

18. Fuel cell system according to one of the preceding claims,

wherein for several serial connections of fuel cells of different segments always the same number of fuel cells is consecutively connected

or

wherein for several serial connections of fuel cells of different segments not always the same number of fuel cells is consecutively connected.

19. Fuel cell system according to the preceding claim, wherein the serial connection of not always the same number of fuel cells is designed so as to ensure a maximum current flow by taking into account that the fuel cells of individual segments provide different current densities at one and the same voltage due to different materials and/or different quality and/or different temperatures and/or different concentration of the gaseous reactants and/or different pressures .

20. Fuel cell system according to one of the preceding claims, wherein electrical isolators between conductive segments and such segments are adapted to be plugged into one another in a form-fitting and/or force-fitting manner.

21. Fuel cell system according to one of the preceding claims, wherein tubular fuel cells, especially microtubular SOFCs, are, when seen in the direction of their longitudinal axis, arranged at both of their respective ends in electrically conductive segments in ring form as carrier sections, therein the tubular fuel cells being electrically connected at one end via the inner electrode of the fuel cell with the one electrically conductive ring, in which the respective fuel cell is integrated, and being electrically- connected at the other end via the outer electrode and/or via a preferably not porous conductive layer connected to the outer electrode with the other electrically conductive ring, in which the respective fuel cell is integrated, wherein adjacent rings are electrically connected in an alternating manner with the outer electrode and with the inner electrode of the fuel cells af- fixed in the rings and wherein the electrically conductive rings are connected with each other in such a way as to create a serial connection of the fuel cells, which are respectively integrated into the individual electrically conduc- tive rings.