Patent Description:
Propulsion for transport or passenger aircraft is typically provided by jet or combustion engines using hydrocarbon based fuel, which lead to a certain carbon footprint, NOx emissions as well as noise. As an alternative, hydrogen produced by renewable energy for future aircraft is a zero-carbon fuel burned with oxygen from the atmosphere. However, the direct usage of hydrogen in a jet engine also results in the generation of NOx. Additionally, this process results in low efficiency of the engine.

Solid oxide fuel cells (SOFC) are energy conversion devices, which use hydrogen as fuel and converts its chemical energy into electrical and thermal energy. The power is generated by an electrochemical reaction, which is a safe non-combustion reaction. SOFCs are widely used for stationary application and are able to produce power with high fuel utilization of up to <NUM>%. An SOFC consists of an anode, a cathode, electrolyte, and current collecting components/interconnectors. The oxygen reduction reaction occurs in the porous cathode by accepting electrons and producing oxide ions, which travel through the gas tight electrolyte to the anode interface. The hydrogen fuel gets oxidized at the anode interface by accepting oxide ions and producing electrons, which pass to the cathode side through an external electrical circuit. The operating temperature of SOFC is usually in a range of <NUM> to <NUM>, which depends on the choice of SOFC materials.

Concepts exist, in which SOFCs are integrated into drive units for propulsion of aircraft. For example, <CIT> discloses a drive unit, comprising a combustion chamber for combusting a fuel/air mixture, and a solid oxide fuel cell device, wherein the fuel cell device comprises at least one fuel cell which in each case comprises an anode that is couplable to a fuel line, a cathode that is couplable to an air source, and a fluid outlet, and is arranged upstream of the combustion chamber. The combustion chamber comprises a combustion chamber inlet for supplying the fuel/air mixture, and a combustion chamber outlet for discharging exhaust gas, and the combustion chamber inlet is connected to the fluid outlet of the fuel cell device.

SOFC for stationary application are often chosen in the form of planar type SOFC, which have a low gravimetric power density and may experience high thermal stress to the cell at fast heating and cooling rates. The low gravimetric power density of the stack is mainly due to a heavy bipolar plate used in the design. Alternatively, tubular design SOFCs offer a huge advantage on high gravimetric power density, high surface to volume ratio and low thermal stresses. Moreover, an integration of tubular SOFC to a stack is flexible depending on the application. Above-mentioned drive unit uses a ring of tubular SOFC.

Published patent applications <CIT>, <CIT> and <CIT> disclose ring-shaped fuel cell assemblies of tubular solid oxide fuel cells.

Stacks with a large number of tubular SOFCs staggered in two dimensions may experience a disadvantageously wide temperature profile inside the stack and difficulties in extracting the heat from the stack. This may result in the degradation of cell materials and result in degraded performance, and the integration of a cooling system to harmonize the wide temperature profile would be technically challenging.

It is thus an object of the invention to propose an alternative solid oxide fuel cell stack for an aircraft engine having tubular SOFCs staggered in one or two dimensions, which allows to deliver power at a high efficiency and with a clearly reduced risk of degradation.

This object is met by the solid oxide fuel cell stack for an aircraft engine having the features of independent claim <NUM>. Advantageous embodiments and further improvements may be gathered from the sub claims and the following description.

A solid oxide fuel cell stack for an aircraft engine is proposed, comprising a plurality of ring-shaped fuel cell assemblies of a plurality of tubular solid oxide fuel cells each, the fuel cells being arranged parallel to each other and distributed circumferentially around a central axis, at least one first stacking manifold for each fuel cell assembly in contact with a first side of the individual fuel cell assembly, at least one second stacking manifold for each fuel cell assembly in contact with a second side of the individual fuel cell assembly, and a central recess for leading an engine shaft through, wherein the fuel cells each comprise a anode and a cathode, wherein the fuel cell assemblies are stacked in an axial direction through pairs of first and second stacking manifolds contacting each other, wherein each first stacking manifold comprises a hydrogen inlet and is connected to first ends of the anodes of the respective fuel cells, and wherein each second stacking manifold comprises a hydrogen and steam outlet and is connected to second ends of the anodes of the respective fuel cells.

In the following, the term "solid oxide fuel cell" is referred to as "SOFC" for an improved readability. Also, in the context of this invention, the term "fuel cell" refers to a solid oxide fuel cell if another type of fuel cell is not explicitly mentioned.

The fuel cell stack according to the invention thus comprises several ring-shaped fuel cell assemblies that are delimited by a first and a second stacking manifold each, wherein the individual assemblies are aligned in an axial arrangement and are electrically connected to each other to form the fuel cell stack. By providing the central recess, an engine shaft can reach through the fuel cell stack. The overall design of the fuel cell stack is hollow-cylindrical and each of the assemblies comprises a ring shape. The ring shape may include at least one ring of tubular SOFCs arranged in a circumferential direction on the same diameter. If desired, several concentric rings may be formed in each assembly, in which the SOFCs are also packed in the radial direction.

The first stacking manifold and the second stacking manifold axially enclose the respective assembly and provide for their electrical interconnection. The stacking manifolds may comprise a single part that electrically connects at least a part of the SOFCs in a parallel connection. However, also segmented stacking manifolds are conceivable that provide a plurality of individual groups of electrically parallel connected SOFCs, which may then be connected to each other in a serial connection, upon desire. The first stacking manifolds are used for delivering hydrogen into the SOFCs. The second stacking manifolds are used for receiving residual hydrogen that flows out of the SOFCs. Thus, the stacking manifolds may provide the function of electrical connection, i.e. a current collection, and the connection to a hydrogen source and hydrogen sink at the same time. Combining these functions into a single component allows to minimize the material usage. The stacking manifolds may be designed as hollow ring-shaped devices that are capable of being flown-through in order to distribute and/or collect fluids. They can be understood as flat chambers with a shape adapted to conform the respective SOFCs. At a side facing the SOFCs, the stacking manifolds may have a flat, platelike shape, which allows to electrically connect the respective stacking manifold to the respective SOFCs.

The individual tubular SOFCs are open on two sides and may comprise one of a variety of different material combinations. When choosing or designing the SOFCs, the internal diameter and the external diameter, the spacing distance, and the surface available for the fuel cell process can be influenced. The tubular shape of the SOFCs ensures that a high mass flow of particularly hydrogen is achieved.

The electrolyte of these SOFCs preferably comprises a ceramic material that is conductive to oxygen ions while having an insulating effect on electrons. Doped zirconium dioxide, for example doped with yttrium, in particular <NUM> mol % yttrium doped zirconium may be a suitable material. The cathode of the SOFC may also be made from a ceramic material, which is conductive both to oxide or oxygen ions and to electrons, for example based on doped lanthanum manganate, with such doping taking place with the use of strontium or the like. Mixed ionic and electronic conductive material may be used, e.g. (La0.60Sr0. <NUM>)<NUM>. 95Co0.20Fe0. An anode of this fuel cell may be made from a cermet material, which comprises ceramic and metallic material. For example, a compound comprising a mixed conducting material, e.g. Ni-8YSZ nickel-doped and yttrium-doped zirconium oxide, in order to conduct ions and electrons. With the use of tubular SOFCs a multitude of self-contained fuel cells can be arranged in a ring-shaped manner around a shaft between a turbine and a compressor so that particularly high electrical power can be provided with integration that is as compact as possible, and at the same time the through-flow of hydrogen and air is improved.

The design of the tubular SOFCs may comprise circular, elliptical, rectangular, polygonal, or other regularly shaped or irregularly shaped cross sections, to which shapes the subject of the invention is not limited. This also relates to the course of a cross section along the length of a tubular SOFC, which course is either constant, resulting for example in a cylindrical shape of the ceramic tube, or is not constant, resulting in a conical shape. Furthermore, the tubular SOFCs may also be designed with integrated form-fitting elements to simplify holding the tubular SOFCs in receiving openings.

The SOFCs may comprise an anode formed by an inner surface, and a cathode formed by an outer surface. The design of the tubular SOFCs is thus particularly simple and robust, and conducting the electrical power provided can be implemented by a mechanical mount of the SOFCs in the stacking manifolds.

As an alternative to this, each SOFC may comprise a separate first electrode, which is connected to the outside of the SOFC, and a separate second electrode, which is connected to the inside of the SOFC. In this way, conduction of the electrical power to the stacking manifolds and thus, to an external circuit, is conducted.

The voltage and power delivered by the fuel cell stack depends on the number of the SOFCs, their electrical connection, as well as of the number of assemblies arranged in the stack. The number of SOFCs in turn depends on the outer diameters, which also influence the packing density given limited dimensions of the stack inside the engine. Generally, SOFCs with the lowest outer diameter may produce a high power stack due to its occupancy of the large number of cells in the assemblies.

In an advantageous embodiment, the plurality of tubular solid oxide fuel cells of each ring-shaped assembly are staggered radially to form at least two rings of fuel cells. Thus, the packing density of the stack is increased and the maximum power to be delivered by the stack can be greatly raised. The SOFCs may form a square or a hexagonal packing structure, wherein the hexagonal packing structure leads to a higher packing density.

However, it is conceivable that the SOFCs are spaced apart from each other, both in the radial and the circumferential direction, in order to let air flow into porous cathodes.

In an advantageous embodiment, at least one of the fuel cell assemblies comprises at least two groups of fuel cells separated in the radial direction, wherein the at least two groups comprise different types of solid oxide fuel cells with different operating temperature ranges. Thus, the stack has the capability to operate at a wide range of operating temperatures, such that an uneven temperature distribution in the stack does not influence the integrity of the stack. Consequently, bigger, and wider stack designs can be used.

In an advantageous embodiment, the operating temperature range of a radially outer group is higher than the operating temperature range of a radial further inward group. Thus, by arranging SOFCs with a higher operating temperature in radially outer regions is preferred.

In an advantageous embodiment, at least one of the fuel cell assemblies comprises three groups of fuel cells separated in the radial direction with different operating temperature ranges. The power of the stack can thus be clearly increased due to a clearly increased volume used by the SOFCs thanks to three consecutively increased operating temperature regions in the radial direction.

In an advantageous embodiment, a radially outermost group comprises electrolyte supported solid oxide fuel cells, wherein a radially central group comprises anode supported solid oxide fuel cells, and wherein a radially innermost group comprises metal supported solid oxide fuel cells. Electrolyte supported SOFCs comprise anodes and cathodes being realized as a coating on opposite surfaces of a ceramic electrolyte. These comprise the highest operating temperature range. A typical anode supported SOFC may comprise an anode made from a cermet mixture of nickel and yttria stabilized zirconia (YSZ). An anode functional layer, an electrolyte, and a cathode layer are sequentially formed on an anode supporter. The operating temperature range may be slightly below the range of the electrolyte supported SOFCs. Still further, metal-supported SOFCs conventionally comprise an anode electrode layer, which is formed on a porous metal support, which may exemplarily be obtained by sintering Fe-Cr-based alloy powder, and an electrolyte layer may be formed thereon to create the SOFC. Here, the operating temperature range is lower than the other two mentioned types of SOFC.

In an advantageous embodiment, the radially outermost group comprises an operating temperature range of <NUM> to <NUM>, wherein the radially central group comprises an operating temperature range of <NUM> to <NUM>, and wherein a radially innermost group comprises an operating temperature range of <NUM> to <NUM>. The operating temperature may thus spread from <NUM> to <NUM> over the radial extension of the stack, which allows to safely operate the stack without requiring a complicated cooling system to cool the stack or harmonize the stack temperature distribution. The design allows to tolerate a large deviation of operating temperatures.

In an advantageous embodiment, the first stacking manifold and the second stacking manifold are configured to alternately flow hydrogen from the hydrogen inlet of the first stacking manifold through circumferentially successive fuel cells in an alternating axial flow direction in a zigzag manner. A laminar flow of hydrogen through the fuel cells can be achieved also at high flow rates.

In an advantageous embodiment, the first stacking manifold is configured to flow hydrogen from the hydrogen inlet of the first stacking manifold through circumferentially successive fuel cells in the same axial flow direction towards the second stacking manifold. This simplifies the hydrogen supply to the individual fuel cells. Appropriately choosing flow resistances along the flow path of the hydrogen to the individual fuel cells allows to let the hydrogen flow laminarly.

In an advantageous embodiment, the fuel cells are spaced apart from each other in a radial direction, wherein the fuel cell stack comprises a housing enclosing the fuel cells, and wherein an air inlet and an air outlet are arranged at the housing to let air flow through the housing to flush the fuel cells with air. Thus, the fuel cells are enclosed by a housing and are placed in an interior space of the housing. Air enters the housing and flows from the first stacking manifold to the second stacking manifold. In doing so, the fuel cells are flushed by the air, which is then able to enter the cathodes, particularly when they are porous and are at the outermost layer of the fuel cells.

It is conceivable to feed colder air to the metal supported SOFCs through air inlets at radially inward positions, since this type of SOFCs requires colder air than the other types. Air inlets at radially outward positions may supply hotter air primarily to the electrolyte supported SOFCs. Once the air enters into the stack, the air travels through the stack in axial direction to maintain an optimal temperature of the stack. Furthermore, air may also be supplied from the side of the stack.

In an advantageous embodiment, the stack further comprises baffle plates inside the housing, wherein the baffle plates are spaced apart from each other in an axial direction and extend in a radial direction, wherein axially consecutive baffle plates have different radial dimensions, and wherein the baffle plates have openings to let the fuel cells pass through. By using the baffle plates, the air flow between the first stacking manifold and the second will repeatedly be deflected. This leads to a greater vorticity, an improved distribution and a more harmonic oxygen density for all fuel cells arranged inside the housing. Additionally, this will provide mechanical support for the tubular SOFCs. When an axial and lateral air flow is provided, the baffle plates may comprise an additional perforation for letting air penetrate in axial direction. This supports maintaining the stack temperature in optimal condition.

In an advantageous embodiment, the stacking manifolds comprise stainless steel and an aluminium oxide coating. This allows to expose the stainless steel only to the preferred locations of the fuel cells, where the current has to be collected. The coated Al2O3 layer prevents high temperature corrosion and chromium evaporation from the stainless steel material. The use of stainless steel is beneficial for the durability, mechanical stability, and corrosion resistance. The shape, size, and locations of uncoated parts of the stacking manifolds depend mainly on the electrical connection circuit, which is explained further below.

In an advantageous embodiment, the stacking manifolds comprise a hole for each of the fuel cells. Preferably, an interior surface of the respective hole is at least partially uncoated to provide an electrical contact to the respective fuel cell. The holes also serve for mechanically supporting the fuel cells.

The invention further relates to an aircraft engine, comprising a solid oxide fuel cell stack according to the above description, a combustion chamber downstream the fuel cell stack, a turbine unit downstream the combustion chamber, and a compressor unit arranged upstream the fuel cell stack and connected to the turbine through an engine shaft extending through the central recess. The engine is able to provide a thrust force and electrical power at the same time by only consuming hydrogen and air, wherein the NOx emission is clearly reduced compared to known hydrogen consuming combustion engines.

In an advantageous embodiment, the combustion chamber is in fluid communication with the hydrogen and steam outlets of the fuel cell assemblies and comprises an air inlet. Thus, residual hydrogen flowing out of the anodes of the fuel cells is combusted inside the combustion chamber. In addition, the combustion chamber may also comprise a separate hydrogen inlet to allow a better mass flow control of hydrogen and to operate the combustion chamber at least partially independent from the operation of the fuel cell stack.

In the following, the attached drawings are used to illustrate exemplary embodiments in more detail. The illustrations are schematic and not to scale. Identical reference numerals refer to identical or similar elements. They show:.

<FIG> shows an aircraft engine <NUM> in a schematic sectional drawing. A front side <NUM> is arranged on the left-hand side of <FIG>, while a rear side <NUM> is arranged on the righthand side of <FIG>. At the front side <NUM>, a compressor unit <NUM> is present, which comprises exemplarily three compressor wheels <NUM> arranged one behind each other on an engine shaft <NUM>. They receive air <NUM> from the surrounding of the engine <NUM> and compress it. A part of the compressor wheels <NUM> may be realized in the form of a fan.

Downstream of the compressor unit <NUM>, a solid oxide fuel cell stack <NUM> is provided, which will be explained in further detail below. It comprises air inlets <NUM>, which are in fluid communication with the compressor unit <NUM>. Air that enters the fuel cell stack <NUM> flows out through air outlets <NUM>. Furthermore, hydrogen inlets <NUM> are provided, which are coupled with a hydrogen source <NUM> for flowing hydrogen into the fuel cell stack <NUM>. Hydrogen and steam outlets <NUM> are provided to let residual hydrogen as well as steam exit the fuel cell stack <NUM>. The fuel cell stack <NUM> is provided for conducting a fuel-cell process by consuming hydrogen from the hydrogen source <NUM> and oxygen from air <NUM> to produce electricity.

Downstream the fuel cell stack <NUM>, a combustion chamber <NUM> is provided, which receives air from the fuel cell stack <NUM> through the air outlet <NUM> as well as residual hydrogen and steam from the hydrogen and steam outlet <NUM>. In addition, a separate hydrogen inlet <NUM> and an additional air inlet <NUM> are provided, through which additional hydrogen from the hydrogen source <NUM> and air from the compressor unit <NUM> is fed into the combustion chamber <NUM>. Resultantly, oxygen depleted air and steam exit the combustion chamber <NUM> and are fed into a turbine unit <NUM> downstream the combustion chamber <NUM>.

The turbine unit <NUM> is coupled with the compressor unit <NUM> through the engine shaft <NUM>. It is impinged by oxygen depleted air and steam and will thus be driven to rotate. The rotation is transferred to the compressor unit <NUM> through the engine shaft <NUM> and leads to providing mechanical power for the compressor unit <NUM>. The engine <NUM> thus produces thrust and electrical power.

<FIG> shows the fuel cell stack <NUM> in a more detailed illustration. Here, a plurality of ring-shaped fuel cell assemblies 36a, 36b, 36c, 36d, 36e, and 36f are shown. They are arranged one after another in an axial direction along the engine shaft <NUM> and form the fuel cell stack <NUM>. To route the engine shaft <NUM> through the fuel cell stack <NUM>, a central recess <NUM> is provided that extends through all ring-shaped fuel cell assemblies 36a to 36f.

Each of the fuel cell assemblies 36a to 36f comprise a first stacking manifold <NUM> and a second stacking manifold <NUM>. The first stacking manifolds <NUM> each comprise a plurality of air inlets <NUM>, which are distributed in a circumferential direction. In this exemplary embodiment, air inlets <NUM> are provided at radial outer regions and radial inner regions at the same time. Exemplarily, an air manifold <NUM> is provided, which delivers air to several circular pipes <NUM>, which each surround one of the first stacking manifolds <NUM> leading to the radial outer air inlets <NUM>. An inner circular pipe <NUM>, which receives air as well, surrounds the engine shaft <NUM> and is connected to the radial inner air inlets <NUM>. Each of the second stacking manifolds <NUM> comprises air outlets <NUM>, e.g. in a radial inner region of the second stacking manifolds <NUM>, which outlets <NUM> are directly connected to the combustion chamber <NUM>.

Still further, each of the first stacking manifolds <NUM> comprises the hydrogen inlet <NUM>, which is connected to a hydrogen supply pipe <NUM>. In analogy to this, each of the second stacking manifolds <NUM> comprises the hydrogen and steam outlet <NUM> connected to a hydrogen and steam pipe <NUM>. Exemplarily, the fuel cell assemblies 36a to 36f may constitute an electrical serial connection by directly contacting them through their opposed stacking manifolds. By providing such a serial arrangement , the delivered voltages of each fuel-cell assembly 36a to 36f are added to a total voltage. Still further, each of the fuel cell assemblies 36a to 36f comprises a plurality of fuel cells, which are electrically interconnected depending on desired electrical parameters and will be explained in the following. However, it may also be possible to provide gaps <NUM> between the individual fuel cell assemblies 36a to 36f and choose a different connection scheme through a suitable wiring.

As apparent from both <FIG>, the combustion chamber <NUM> may comprise a hollow-cylindrical shape having an outer diameter that at least substantially corresponds to the outer diameter of the fuel cell assemblies <NUM>. The detail design of the combustion chamber <NUM> is not crucial for the gist of the invention and is thus not explained in detail herein. A skilled person will be able to consider a suitable design for the combustion chamber <NUM>.

<FIG> shows a part of a first stacking manifold <NUM>, which comprises a plurality of holes <NUM> for receiving a tubular fuel cell <NUM> each. Inside each of the holes <NUM>, an interface connector <NUM> is provided in the form of uncoated stainless steel. The first stacking manifold <NUM> is exemplarily made from stainless steel and comprises a thin aluminium oxide coating <NUM> except in places, where an electrical contact is required, e.g. at said interface connectors <NUM>. This allows to provide a design solution for compiling the stacking manifolds <NUM> and <NUM> and the interface connectors <NUM> as a single component, which minimizes the stack weight, improves the electron transport, and also minimizes chromium release from the stacking manifolds <NUM> and <NUM>.

The fuel cell <NUM> comprises a cathode <NUM> as an outer layer, an anode <NUM> as an inner layer and an electrolyte <NUM> between both. For their operation, hydrogen is routed into the anodes <NUM> and flows through the tubular fuel cells <NUM>, while air enters the fuel cells <NUM> through pores inside the cathode <NUM>. Hence, the first stacking manifold <NUM> is designed to let the hydrogen flow through holes <NUM> into the anodes <NUM>. The holes <NUM> are distributed on the stacking manifolds <NUM> and <NUM> to space apart the fuel cells <NUM> from each other, such that the cathodes <NUM> can be flushed by air.

In the exemplary embodiment, the first stacking manifold <NUM> and the second stacking manifold <NUM> comprise a plurality of individual segments <NUM>, which provide a parallel connection to a small group of fuel cells <NUM>. Several of these groups can be connected in a serial connection to each other through interface connectors <NUM>, which are also realized in the form of uncoated surfaces of the first stacking manifold <NUM>. The segments and <NUM> are spaced apart in the radial direction, such that a gap <NUM> is created, through which air can enter the respective fuel cell assembly <NUM> to flush the fuel cells <NUM> with air.

<FIG> shows a schematic sectional view of a first stacking manifold <NUM>. Here, exemplarily one of the hydrogen inlets <NUM> is shown, through which hydrogen enters the first stacking manifold <NUM> and is distributed to all associated SOFCs <NUM>. Furthermore, an air inlet <NUM> of the first stacking manifold <NUM> is shown, through which air enters the first stacking manifold <NUM> and is distributed to a plurality of air supply openings <NUM>. These may be separate openings in the respective first stacking manifold <NUM> or may be realized through gaps between segments <NUM> in a radial and/or circumferential direction. In analogy, the second stacking manifold <NUM> may comprise several air exit openings, through which unused air from the air supply openings <NUM> can enter the second stacking manifold <NUM>.

In <FIG>, an exemplary interconnection of several segments <NUM> is shown. For the sake of better understanding, positive terminals in the form of segments 66a are hatched differently than negative terminals 66b. Positive terminals 66a and negative terminals 66b follow on each other in an alternating manner in a circumferential direction to form a ring shape. They are connected through interface connectors <NUM> arranged at the sides of the segments 66a and 66b. A plurality of concentrically ring-shaped arrangements of segments 66a and 66b are created, which are only indicated partially in <FIG>. A radial connection between one of the segments 66a or 66b of one ring is connected to one of the segments 66a or 66b of an adjacent ring through an additional connector <NUM>. Here, a segment 66b constituting a negative terminal is connected to a radially further inwardly positioned segment 66a forming a positive terminal. The interconnections can be tailored to the individual needs.

<FIG> shows a schematic view of a fuel cell assembly <NUM>. Here, a plurality of individual fuel cells <NUM> are shown in a dense packing. Here, several concentrical rings of fuel cells <NUM> are provided that enclose each other and form a hollow-cylindrical shape. Thus, the fuel cell assembly <NUM> a has a plurality of fuel cells <NUM> staggered radially and circumferentially. Exemplarily, three groups 72a, 72b and 72c of fuel cells <NUM> are created, which comprise different types of tubular SOFCs <NUM>. For example, the radially innermost group 72a comprises electrolyte supported SOFC 's, while the radially central group comprises anode supported SOFC 's and while the radially outermost group 72c comprises metal supported SOFC 's. These comprise different operating temperature ranges of substantially <NUM> to <NUM> for the first group 72a, <NUM> to <NUM> for the second group 72b and <NUM> to <NUM> for the third group 72c. Thus, a cumbersome and technically challenging harmonization of operating temperatures inside the fuel cell stack <NUM>, i.e. inside each of the fuel-cell assemblies 36a to 36f, is not required.

<FIG> shows the hydrogen inlet <NUM> arranged in the first stacking manifold <NUM> and the distribution of hydrogen to the individual fuel cells <NUM>. Here, the first stacking manifold <NUM> distributes the hydrogen to individual rings of fuel cells <NUM> one after another from the radial outermost position to a radial innermost position. Here, a flow path 74a directly follows on the hydrogen inlet <NUM> and runs in a circumferential direction about <NUM>°. In doing so, all fuel cells <NUM> connected to this flow path 74a are supplied with hydrogen. Afterwards, hydrogen passes through an interconnection opening 76a and reaches into a second circumferential flow path 74b. Here, the hydrogen flows in a circumferential direction in an opposite direction about <NUM>° and all connected fuel cells <NUM> are supplied with hydrogen as well. Afterwards, a further interconnection opening 76b is passed and hydrogen flows into a further flow path 74c, which again runs in a circumferential direction about <NUM>° in an opposite direction and so on. Hence, all of the fuel cells <NUM> will be supplied with hydrogen flowing into the hydrogen inlet <NUM> one after another.

The supply of one ring of tubular SOFCs <NUM> with hydrogen is shown in a spatial illustration in <FIG> for further clarification. Here, hydrogen enters the hydrogen inlet <NUM>, is distributed in a circumferential direction of the first stacking manifold <NUM>, to feed hydrogen into all individual fuel cells <NUM>. After flowing through the fuel cells <NUM>, hydrogen reaches the second stacking manifold <NUM> and exits through the hydrogen and steam outlet <NUM>. It is clear that the hydrogen supply to the hydrogen inlet <NUM> is sufficient to feed further rings of tubular SOFCs by appropriately balancing all flow resistances in the first stacking manifold <NUM> and the tubular fuel cells <NUM> and choosing a suitable supply pressure at the hydrogen inlet <NUM>.

<FIG> shows a modified approach for distributing hydrogen. Here, the first stacking manifold <NUM> and the second stacking manifold <NUM> are configured to alternately flow hydrogen from the hydrogen inlet <NUM> of the first stacking manifold <NUM> through circumferentially successive fuel cells <NUM> in an alternating axial flow direction in a zigzag manner.

In <FIG>, a sectional view of a fuel cell assembly <NUM> is shown. Here, a central axis <NUM> is indicated, along which the central recess <NUM> extends. All tubular fuel cells <NUM> are arranged parallel to the central axis <NUM> and are packed between the first stacking manifold <NUM> and the second stacking manifold <NUM>. As shown before, several air inlets <NUM> are provided, which let air flow in a radial direction into the first stacking manifold <NUM>, from which it will be distributed into an interior space <NUM> of a housing <NUM> of the fuel cell assembly <NUM>. The fuel cell assembly <NUM> exemplarily comprises a closed housing <NUM> having an outer housing skin <NUM> and an inner housing skin <NUM>, which are provided extending between the first stacking manifold <NUM> and the second stacking manifold <NUM>. The inner housing skin <NUM> has a hollow cylindrical shape with an inner diameter corresponding to the outer diameter of the central recess <NUM>. The outer housing skin <NUM> has an outer diameter that corresponds to the outer diameter of the stacking manifolds <NUM> and <NUM>. Air that enters the air inlets <NUM> are flowing through the interior space <NUM> and thereby all fuel cells <NUM> are flushed with air. After passing the fuel cells <NUM>, the air exits the air outlets <NUM>. In the exemplary embodiment of <FIG>, the air flows in a substantially axial direction.

Claim 1:
Solid oxide fuel cell stack (<NUM>) for an aircraft engine (<NUM>), comprising:
a plurality of ring-shaped fuel cell assemblies (<NUM>) of a plurality of tubular solid oxide fuel cells (<NUM>) each, the fuel cells (<NUM>) being arranged parallel to each other and distributed circumferentially around a central axis (<NUM>),
at least one first stacking manifold (<NUM>) for each fuel cell assembly (<NUM>) in contact with a first side of the individual fuel cell assembly (<NUM>),
at least one second stacking manifold (<NUM>) for each fuel cell assembly (<NUM>) in contact with a second side of the individual fuel cell assembly (<NUM>), and
a central recess (<NUM>) for leading an engine shaft (<NUM>) through,
wherein the fuel cells (<NUM>) each comprise an anode (<NUM>) and a cathode (<NUM>),
wherein the fuel cell assemblies (<NUM>) are stacked in an axial direction through pairs of first stacking manifolds (<NUM>) and second stacking manifolds (<NUM>) contacting each other,
wherein each first stacking manifold (<NUM>) comprises a hydrogen inlet (<NUM>) and is connected to first ends of the anodes (<NUM>) of the respective fuel cells (<NUM>), and
wherein each second stacking manifold (<NUM>) comprises a hydrogen and steam outlet (<NUM>) and is connected to second ends of the anodes (<NUM>) of the respective fuel cells (<NUM>).