WO2017191353A1

WO2017191353A1 - A stack row structure and method of high temperature fuel cell

Info

Publication number: WO2017191353A1
Application number: PCT/FI2016/050280
Authority: WO
Inventors: Kim ÅSTRÖM; Vesa Kyyhkynen; Hulda AMINOFF
Original assignee: Convion Oy
Priority date: 2016-05-02
Filing date: 2016-05-02
Publication date: 2017-11-09
Also published as: EP3452639A1; KR20190003665A; CN109415828A

Abstract

An object of the invention is a stack arrangement of high temperature fuel cell system or electrolysis cell system, each cell in the cell system comprising an anode side (100), a cathode side (102), and an electrolyte (104) between the anode side and the cathode side, the cell system comprising the cells in cell stacks (103). The arrangement comprises the stacks (103) arranged in row arrangement, wherein the stacks are arranged side by side at least in two rows, and the arrangement comprises air feed-in ducting (120) for feeding air to the stacks (103), the ducting having air inlet ends (130), which are conveyed to a sealed air feed space (132), which is formed between the stack (103) rows having at least two sides of the air feed space enclosed by the stacks themselves. The arrangement comprises a fuel feed-in common rail (133), a fuel feed-in ducting (122) and individual feed-in channeling (131) for feeding fuel to the stacks (103), and at least one radiative heat transfer compensation element (134) along at least one of the fuel feed-in common rail (133), the fuel feed-in ducting (122), the individual feed-in channeling (131) and the air feed-in ducting (120), means (135) for performing flow and temperature balanced air feed-in flows for the stacks (103), and means (137) for performing flow balanced fuel flows in the cell system.

Description

A stack row structure and method of high temperature fuel cell system

The field of the invention

Most of the energy of the world is produced by means of oil, coal, natural gas or nuclear power. All these production methods have their specific problems as far as, for example, availability and friendliness to environment are concerned. As far as the environment is concerned, especially oil and coal cause pollution when they are combusted. The problem with nuclear power is, at least, storage of used fuel.

Especially because of the environmental problems, new energy sources, more environmentally friendly and, for example, having a better efficiency than the above-mentioned energy sources, have been developed.

Fuel cell's, by means of which energy of fuel, for example biogas, is directly converted to electricity via a chemical reaction in an environmentally friendly process, are promising future energy conversion devices.

The state of the art

Fuel cell, as presented in fig 1, comprises an anode side 100 and a cathode side 102 and an electrolyte material 104 between them. In solid oxide fuel cells (SOFCs) oxygen 106 is fed to the cathode side 102 and it is reduced to a negative oxygen ion by receiving electrons from the cathode. The negative oxygen ion goes through the electrolyte material 104 to the anode side 100 where it reacts with fuel 108 producing water and also typically carbon dioxide (CO2). Anode 100 and cathode 102 are connected through an external electric circuit 111 comprising a load 110 for the fuel cell withdrawing electrical energy out of the system. The fuel cells also produce heat to the reactant exhaust streams.

In figure 2 is presented a SOFC device as an example of a high temperature fuel cell device. SOFC device can utilize as fuel for example natural gas, bio gas, methanol or other compounds containing hydrocarbons. SOFC device in figure 2 comprises more than one, typically plural of fuel cells in stack formation 103 (SOFC stack). Each fuel cell comprises anode 100 and cathode 102 structure as presented in figure 1. Part of the used fuel can be recirculated in feedback arrangement 109 through each anode. SOFC device in fig 2 also comprises a fuel heat exchanger 501 and a reformer 107.

Typically several heat exchangers are used for controlling thermal conditions at different locations in a fuel cell process. Reformer 107 is a device that converts the fuel such as for example natural gas to a composition suitable for fuel cells, for example to a composition containing hydrogen and methane, carbon dioxide, carbon monoxide and inert gases. Anyway in each SOFC device it is though not necessary to have a reformer.

For example inert gases are purge gases or part of purge gas compounds used in fuel cell technology. For example nitrogen is a typical inert gas used as purge gas in fuel cell technology. Purge gases are not necessarily elemental and they can be also compound gases.

By using measurement means 115 (such as fuel flow meter, current meter and temperature meter) necessary measurements are carried out for the operation of the SOFC device. Part of the gas used at anodes 100 may be recirculated through anodes in feedback arrangement 109 and the other part of the gas is exhausted 114 from the anodes 100. The fuel cell reactions in the case of methane, carbon monoxide and hydrogen fuel are shown below: Anode: CH₄ + H₂0 = CO + 3H₂

CO + H₂O = CO₂ + H₂

H₂ + O^2" = H₂O + 2e

Cathode: O₂ + 4e = 2O^2~

Net reactions: CH₄ + 2O₂ = CO₂ + 2H₂O

CO + l/20₂ = CO₂

In electrolysis operating mode (solid oxide electrolysis cells (SOEC)) the reaction is reversed, i.e. heat, as well as electrical energy from a source 110, are supplied to the cell where water and often also carbon dioxide are reduced in the cathode side forming oxygen ions, which move through the electrolyte material to the anode side where oxidation reaction takes place. It is possible to use the same solid electrolyte cell in both SOFC and SOEC modes. In such a case and in the context of this description the electrodes are typically named anode and cathode based on the fuel cell operating mode, whereas in purely SOEC applications the oxygen electrode may be named the anode, and the reactant electrode as the cathode.

In Solid Oxide Fuel Cell (SOFC) and Solid Oxide Electrolyzer (SOE) stacks, commonly here referred as solid oxide cell stack, where the flow direction of the cathode gas relative to the anode gas internally in each cell as well as the flow directions of the gases between adjacent cells, are combined through different cell layers of the stack. Further, the cathode gas or the anode gas or both can pass through more than one cell before it is

exhausted and a plurality of gas streams can be split or merged after passing a primary cell and before passing a secondary cell. These combinations serve to increase the current density and minimize the thermal gradients across the cells and the whole stack.

A solid oxide fuel cell (SOFC) device is an electrochemical conversion device that produces electricity directly from oxidizing fuel. Advantages of SOFC device include high efficiencies, long term stability, low emissions, and cost. The main disadvantage is the high operating temperature which results in long start up times and both mechanical and chemical compatibility issues. The anode electrode of solid oxide fuel cell (SOFC) typically contains significant amounts of nickel that is vulnerable to form nickel oxide if the atmosphere is not reducing. If nickel oxide formation is severe, the

morphology of electrode is changed irreversibly causing significant loss of electrochemical activity or even break down of cells. Hence, SOFC systems typically require purge gas, i.e. safety gas, containing reductive agents (such as hydrogen diluted with inert such as nitrogen) during the start-up and shut-down in order to prevent the fuel cell's anode electrodes from oxidation. In practical systems the amount of purge gas has to be minimized because an extensive amount of, e.g. pressurized gas containing hydrogen, are expensive and problematic as space-requiring components.

There is a need to minimize the need for purge gases both in system startup and shut-down conditions. According to prior art applications the amount of purge gases during normal start-up or shut-down is minimized by anode recirculation, i.e. circulating the non-used purge gases back to the loop, or by generating reducing gas through steam reforming or catalytic partial oxidation. However, in emergency shut-down (ESD) that may be caused e.g. by gas alarm or black-out, it may not be possible or permissible to operate anode recirculation or active reducing gas generation means, whereby the amount of needed purge gas is increased. In addition, if cathode air flow for cooling the system cannot be supplied during the ESD, the amount of needed purge gas is even more increased as the time to cool the system down to temperatures where nickel oxidation does not happen is even threefold compared to active shut-down situation. As described, current SOFC stacks require reducing purge gas to protect the anode from oxidation during abnormal situations, like emergency shutdowns. However, still the amount of purge gas is considerable for real field application, especially with larger unit sizes. Stacks are vulnerable towards detrimental nickel oxidation above a certain critical temperature, which lies typically somewhere between 300-400 degrees Celsius. Below this

temperature, nickel oxidation reaction is so slow that there is no more need for reductive atmosphere on the anode. In passive emergency shutdown (ESD) situations, the cooling of the unit is extremely slow (even up to 10 hours or more) due to non-existing air flow through the system, high heat capacitance of the components, and good thermal insulation of the system. Even if active air cooling can be utilized, the cooling is slow typically because of high efficiency recuperator bringing most of the heat back to the system.

High temperature fuel cell systems, especially SOFC systems, typically contain several fuel cell stacks within one or more insulation enclosures. With respect to explosion safety the fuel cell stacks themselves as well as any flanges and sealing surfaces used within the fuel supply lines shall be regarded as potential sources of fuel leakage to their surroundings. In case of high temperature fuel cells the temperature of the surroundings is typically above the self-ignition temperatures of gaseous fuels that are used. Therefore, the traditional approach for dealing with the leakages has been to place all potential sources of leakage within an air or exhaust stream taking care that air is always available in excess amounts to combust any leaking fuel. This approach requires a gas tight enclosure suitable for the pressure level of the air/exhaust stream and large enough to contain all fuel manifolding with non-welded connections (i.e. source of leakage). The above described method for dealing with leakages is convenient particularly if leakages are relatively high. In some process and layout configurations, however, the requirement to fit all fuel connectors within a gas tight space can negatively impact system compactness, maintainability and cost. Particularly in the case of open-air manifolded stacks, i.e. stacks are to be sealed against the perimeter of an air feed duct, fitting in all fuel connectors within such a duct is often impractical.

In high temperature fuel cell or electrolysis systems consisting of several stacks, management of operating conditions occurring at different stacks is of key importance for maximizing the system performance and lifetime.

Stacks operating at different temperatures will have different internal resistances and flow resistances, leading to differences in volumetric flow and hence causing differences in fuel utilization, voltage and temperature gradients. Such differences are likely to accelerate the degradation of those stacks operating at the least beneficial conditions and thus limit the lifetime of the system. As a countermeasure, safety margins in operating conditions can be globally increased in order to keep these stacks from experiencing unbeneficial conditions, but the increase of safety margins typically

decreases system output and/or increases cost.

In prior art embodiments, methods to minimize differences in operating conditions include passive means such as use of symmetry, guide vanes and flow restrictors for flow homogenization as well as active means such as means for stack-wise or stack group-wise flow or loading control. A combination of several methods is typically used. The strive to achieve as even operating conditions as possible places constraints on the system layout and typically becomes a decisive factor in the choice of fuel cell or

electrolysis system geometries. In high temperature fuel cell systems significant complexity and cost is typically related to interfaces and boundaries such as gas, electricity and mechanical support feedthroughs between a hot and a cold compartment. Where a connection between a hot and a cold structure needs to be made such as passing a hot pipe through a gas-tight cold wall, management of heat loss and thermomechanical stresses becomes an issue which typically require costly and/or space consuming arrangements. Also, transfer pipes between different hot components may need support from cold structures and thermomechanical compensators. Therefore, minimization of hot-cold feedthroughs and transfer pipes is desirable. By integration of components this can be achieved. In prior art embodiments, various designs integrating burners, reformer(s) and heat exchangers are presented. With proper symmetry or heat management, such arrangements can be brought adjacent to fuel cells. However, such arrangements either do not utilize anode recirculation or the anode recirculation is arranged outside of the stack compartment, requiring costly feedthroughs and/or intermediate cooling of the gas to be recycled. In system comprising multiple stacks it is also beneficial to minimize the amount of feedthroughs and heat losses by integrating all stacks into one compartment. Furthermore, avoiding stack specific or stack group specific insulation reduces system cost and volume. Integrating a high number of stacks in a common environment however typically inherently introduces differences in operating conditions between different stacks. In multiple stack arrangements it is of utmost importance to balance operating conditions within the stack arrangement to minimize environment- induced variations in performance and lifetime and to prevent premature failure of individual stacks. Use of symmetry provides a good approach to inherently minimize differences but with a high number of stacks it may not be possible to achieve perfect symmetry at reasonable cost or within reasonable geometrical dimensions. Furthermore, despite of having a high number of individual stacks it is typically beneficial to use common

components in the balance of plant (BoP), such as reformers, burners, heat exchangers, ejectors, blowers and valves. The interfacing to these typically introduces asymmetry in an otherwise symmetric arrangement.

Instead of a completely symmetric e.g. circular or rectangular configuration of stacks, it may be more cost and space effective to arrange the stacks in a row configuration with feeds from one end, both ends or from the middle of the row. It is beneficial to convey fuel and air inlet structures within a space in which air outlet flows freely, or alternatively convey fuel pipes and air outlet pipes in a space conveying air inlet. In both cases it is beneficial not to insulate said pipes within said space conveying air as harmful particles may peel off from the insulation and migrate with the flow. Furthermore, radiation heat exchange inherently taking place between cooler inlet pipes and hotter outlet pipes is beneficial in reducing the need or duty of separate heat exchangers. However, distribution pipes undergoing heat exchange in a row configuration introduces differences in flow temperatures of different flow branches along the distribution common rail. Pressure losses and dynamic pressures in distribution pipes of reasonable size furthermore introduce pressure differences between flow branches supplying different stacks or groups of stacks. In addition, the temperature change of reactant supply pipes running along the row configuration changes the thermal radiation field experienced by different stacks in the flow configurations. All of these phenomena introduce variations in operation conditions of stacks, which affect their performance and lifetime. To avoid harmful effects of such variations, appropriate safety margins need to be applied on global operating parameters to assure that allowed limits are not locally exceeded.

Flow deviations in flow between branches can be reduced by designing sufficient pressure drop in flow branches such that pressure drops within branches dominate over pressure drops within the distribution arrangement. However, if such a distribution arrangement involves differences in heat-up or cooldown of reactants diverting to different flow branches, such

temperature differences give rise to variations in density and viscosity of fluids which again introduce deviations in flow. Moreover, differences in inlet temperature of reactants to the stacks significantly affect the thermal balance of the stack both through the heat content of fluids themselves and through contained endotherm and/or exotherm of e.g. steam reforming. This can give rise to positive feedback phenomena which amplifies differences between the stacks. In other words, the coolest stack receives the most of fuel (due to highest density) and most of air, whereby it has both the highest cooling by the flows and the highest amount of endothermic steam

reforming, which cools it further. Respectively, the hottest stack receives least cooling reactants and thus has lowest amount of cooling by flows. It also has internal reforming which tends to raise its temperature further.

Flow restrictor elements placed in the outlet streams of stacks are most effective in balancing flows as the temperatures at the outlet tend to have smaller differences than at inlets in case of asymmetric feeds. However, large pressure drops as a means for compensating of otherwise uneven flow distribution increases system parasitic losses and particularly increase the requirements for reactant circulation arrangements. Especially with ejector driven anode or cathode recirculation, additional pressure drops can severely reduce the performance of the recirculation(s). Flow restriction elements are also sensitive to manufacturing tolerances.

Short description of the invention

The object of the present invention is to achieve a cell system with improved flow and temperature balance conditions and with improved conditions. This is achieved by a stack arrangement of high temperature fuel cell system or electrolysis cell system, each cell in the cell system comprising an anode side, a cathode side, and an electrolyte between the anode side and the cathode side, the cell system comprising the cells in cell stacks. The arrangement comprises the stacks arranged in row arrangement, wherein the stacks are arranged side by side at least in two rows, and the

arrangement comprises air feed-in ducting for feeding air to the stacks, the ducting having air inlet ends, which are conveyed to a sealed air feed space, which is formed between the stack rows having at least two sides of the air feed space enclosed by the stacks themselves, and the arrangement comprises a fuel feed-in common rail, a fuel feed-in ducting and individual feed-in channeling for feeding fuel to the stacks, and at least one radiative heat transfer compensation element along at least one of the fuel feed-in common rail, the fuel feed-in ducting, the individual feed-in channeling and the air feed-in ducting, means for performing flow and temperature balanced air feed-in flows for the stacks, and means for performing flow balanced fuel flows in the cell system.

The focus of the invention is also a method of high temperature fuel cell system or electrolysis cell system. In the method is arranged cell stacks in row arrangement so that the stacks are arranged side by side at least in two rows, is fed air to the stacks by an air feed-in ducting having air inlet ends, which are conveyed to a sealed air feed space, which is formed between the stack rows having at least two sides of the air feed space enclosed by the stacks themselves, and in the method is fed fuel to the stacks by a feed-in common rail, a fuel feed-in ducting and individual feed-in channeling, is performed radiative heat transfer compensation along at least one of the fuel feed-in common rail, the fuel feed-in ducting, the individual feed-in

channeling and the air feed-in ducting, is performed flow and temperature balanced air feed-in flows for the stacks, and is performed flow balanced fuel flows in the cell system. The invention is based on an air feed-in ducting having air inlet ends, which are conveyed to a sealed air feed space, for feeding air to the stacks, which are arranged side by side at least in two rows. The sealed air feed space is formed between the stack rows having at least two sides of the air feed space enclosed by the stacks themselves. The invention is further based on radiative heat transfer compensation along at least one of the fuel feed-in common rail, the fuel feed-in ducting, the individual feed-in channeling and the air feed-in ducting, and performing flow and temperature balanced air feed-in flows for the stacks, and performing flow balanced fuel flows in the cell system.

The benefit of the invention is that a compact and economically beneficial fuel cell or electrolysis cell system can be built. Also functionality of the cell system can be improved, because sealing problems can be at least partly eliminated.

Short description of figures

Figure 1 presents a single fuel cell structure.

Figure 2 presents an example of a SOFC device.

Figure 3 presents an exemplary stack arrangement of high temperature cell system according to the present invention.

Figure 4 presents the stack row formation according to the present invention. Detailed description of the invention

Solid oxide fuel cells (SOFCs) can have multiple geometries. The planar geometry (Fig 1) is the typical sandwich type geometry employed by most types of fuel cells, where the electrolyte 104 is sandwiched in between the electrodes, anode 100 and cathode 102. SOFCs can also be made in tubular geometries where for example either air or fuel is passed through the inside of the tube and the other gas is passed along the outside of the tube. This can be also arranged so that the gas used as fuel is passed through the inside of the tube and air is passed along the outside of the tube. Other geometries of SOFCs include modified planar cells (MPC or MPSOFC), where a wave-like structure replaces the traditional flat configuration of the planar cell. Such designs are promising, because they share the advantages of both planar cells (low resistance) and tubular cells.

In large solid oxide fuel cell systems typical fuels are natural gas (mainly methane), different biogases (mainly nitrogen and/or carbon dioxide diluted methane), and other higher hydrocarbon containing fuels or alcohols.

Methane and higher hydrocarbons need to be reformed either in the reformer 107 (Fig 2) before entering the fuel cell stacks 103 or (partially) internally within the stacks 103. The reforming reactions require certain amount of water, and additional water is also needed to prevent possible carbon formation (coking) caused by higher hydrocarbons. This water can be provided internally by circulating the anode gas exhaust flow, because water is produced in excess amounts in fuel cell reactions, and/or said water can be provided with an auxiliary water feed (e.g. direct fresh water feed or circulation of exhaust condensate). By anode recirculation arrangement also part of the unused fuel and diluents in anode gas are fed back to the process, whereas in auxiliary water feed arrangement only additive to the process is water. In the arrangement according to the present invention, stacks are arranged in at least one row configuration with respect to feeds such that deviations in operating conditions of individual stacks are essentially zero despite of the asymmetry in feeds and despite lack of isolation on distribution pipes. Close to zero deviations are achieved by feeding air to the stack rows in a back and forth formation with flow and temperature balanced outlets for stacks, and by having branch-specific pressure drop elements in fuel feed branches and by using at least one heat transfer enhancement element along fuel feed-in common rails. The means for performing flow and temperature balanced air feed-in flows for the stacks 103 can also be accomplished e.g. by perforated pipes. Stacks are arranged in towers and towers further side by side in rows such that between every pair of two rows is formed an essentially cuboid shaped air inlet space in which stack air inlets (and intermediate seals) constitute the at least two largest faces of said cuboid space and air feed structures constitute two of the remaining faces. To prevent air bypass flow of the stacks through clearances between adjacent stack towers, said clearances can be filled with e.g. ceramic blocks or fibers, sealing gaskets or a combination thereof. Within said cuboid space, perforated curved, e.g. U- shaped tubes or straight tubes with a common header in both ends can be used to facilitate a zig-zag air feed pattern in a back and forth arrangement. Tubes facilitating an even flow and temperature profile to the stack air inlets can also be arranged coaxially. In a preferable embodiment stack air inlet faces constitute over 50% of the total area of the cuboid boundaries. In the arrangement according to the present invention, the stack rows can be contained in an isolated gas tight enclosure whose inner volume at least partially constitutes a cathode outlet passage. An afterburner can be located internally or externally to said enclosure. Alternatively, post-oxidation of unspent fuel leaving the fuel system can be arranged at multiple locations within said enclosure. A part or all of the unspent fuel leaving the fuel system can also be extracted for e.g. hydrogen generation. Fuel supply pipes are located inside said cathode outlet passage section and along said rows of adjacent stack towers. Said supply pipings include at least one heat transfer enhancement element and at least one pressure drop structure within each stack row. The at least one heat transfer enhancement elements are placed within fuel feed pipes asymmetrically with respect to stack rows to

compensate for gradients in pipe surfaces along the rows. Heat transfer enhancement elements can be e.g. plates or corrugated structures inside pipes, fins or deformations on the pipe surface. A heat transfer enhancement element can also function as a pressure drop element and different geometries can be used at different locations to achieve to optimize heat transfer vs. pressure drop. Pressure drop elements can consist of various pipe inserts, orifices and holes along reactant supply structures.

In a large geometry undergoing large temperature differences, differences in thermal expansion of different structures need to be taken into account. Typically, compensating means such as bellows are needed along at least a part of distribution pipes. In one preferable embodiment according to the present invention, flow branches to individual stack groups along a row configuration include multiple bends, providing inherent flexibility and thus allowing for minimization of need of bellows.

In an alternative embodiment according to the present invention, the space between each two stack tower rows can be the air outlet space and air inlet can be located on the outside, inside the insulated gas tight vessel. Fuel piping can be arranged within the air inlet or outlet space or between said spaces e.g. underneath or above the stack row. Fuel common railing can be arranged as part of the structures supporting and feeding fuel to the stacks (manifolds). Manifolds can be tower specific, common for two adjacent or for two opposite facing stack towers, or common for four or more towers.

Manifolds can be placed at the middle of a tower, at one end or at both ends with separate manifolds for inlet and outlet fuel. In a high tower configuration it is possible to have multiple manifolds at different heights of the stack tower to facilitate even fuel flow.

Despite of the system being designed to provide equal operating conditions to all stacks, there can arise variations in different part of the system due to various reasons. The air feed arrangement according to the present invention can include means to mix controllable amounts of cold air to the heated air inlet streams of different stack groups allowing for compensating differences in temperatures, currents or flows. Similarly, means to inject small amounts of surplus fuel to different flow branches can be provided as compensation means.

Stacks typically require external compression to be applied at least during operation. Compression can be arranged by springs by applying pressure or in some cases by applying weight on stack towers. The compression counterforce can be transferred through cold structures such as through an air sealing enclosure or using rods or plates running partially through a hot environment. For stacks requiring constant compression there can be a need for special transfer compression arrangements to be applied during system assembly and disassembly. In an embodiment according to the present invention manifold can serve as a force distribution plate for single or multiple stack towers for the runtime compression and/or transfer

compression. In order to prevent short-circuiting of stacks and to avoid the risk of electrical shock it is often required to electrically isolate at least part of fuel cell stacks from the system chassis and from any structures in galvanic contact thereto, including gas feed pipelines. Isolation strength requirements depend on the connection topology, the power conversion topology and on relevant regulations. In embodiments according to the present invention isolation can be located in various and multiple parts of the stack-system interface such as between the stack and manifold, along gas feed pipes and support structures, in relation to seals, integral to the stack or integrated with thermal isolation. Isolation elements can be various seals of low electrical conductivity, discrete ceramic parts, various fibers or a combination thereof. The system can include means to monitor the isolation strength during runtime.

In the embodiments according to the present invention individual

compensation elements can comprise flow restrictions (including holes in a pipe or wall), heat transfer enhancing elements etc. The arrangement according to the present invention can be e.g. an arrangement comprising air and fuel feed-in pipings, i.e. ductings, which both have flows in horizontal flows in opposing directions in any cross section with air channel in the middle.

In the exemplary stack arrangement of high temperature cell system according to the present invention presented in figures 3 and 4 the cell system can be a fuel cell system or an electrolysis cell system. Each cell in the cell system comprises an anode side 100, a cathode side 102, and an electrolyte 104 between the anode side and the cathode side. The cell system comprises the cells in cell stacks 103. The stack arrangement comprises the stacks 103 arranged in row arrangement, wherein the stacks are arranged side by side at least in two rows, and the arrangement comprises air feed-in ducting 120 for feeding air to the stacks 103. The ducting has air inlet ends 130, which are conveyed to a sealed air feed space 132, which is formed between the stack 103 rows having at least two sides of the air feed space enclosed by the stacks themselves. The sides, i.e. faces of the air feed space 132, form together with the stacks preferably a space with rectangular cross-section. Flexible material can be used to seal the space between adjacent stacks 103 and to compensate for dimensional tolerances. Said flexible material can also be electrically isolating material. Also a material combination can be used, in which one material is flexible and the other material is electrically isolating. Flexibility is not necessary, if tolerances for material(s) are large enough. The stack arrangement according to the present invention comprises a fuel feed-in common rail 133, a fuel feed-in ducting 122 and individual feed-in channeling 131 for feeding fuel to the stacks 103, and at least one radiative heat transfer compensation element 134 along at least one of the fuel feed-in common rail 133, the fuel feed-in ducting 122 and the individual feed-in channeling 131. The radiative heat transfer compensation element 134 can be for example an insert element for reduction of hydraulic diameter, for improvement of convective heat transfer or for enhancement of turbulence. Similar heat transfer compensation element 134 can also be applicable for air feed-in ductings 120 according to the present invention. In one embodiment the radiative heat transfer compensation element 134 can be a branch -specific compensation element in order to provide uniform inlet temperatures between the reactant ducting branches.

In figure 3 and 4 is illustrated the stack row formation according to the present invention. The stack arrangement further comprises means 135 for performing flow and temperature balanced air feed-in flows for the stacks 103. The means 135 can be accomplished e.g. by a back and forth arrangement 135a (figure 3). The means 135 for performing flow and temperature balanced air feed-in flows for the stacks 103 can also be accomplished e.g. by perforated pipes. In a preferable embodiment, flow balancing elements providing pressure drop are placed in all parallel branches except the at least one with the lowest flow in an unbalanced situation. Thus, the overall pressure loss increment due to the balancing is essentially zero. The stack arrangement according to the present invention presented in figures 3 and 4 further comprises means 137 for performing flow and temperature balanced fuel flows in the cell system. The means 137 can comprise at least one heat transfer element 142 for each stack 103 row being placed asymmetrically with respect to flow direction and the row. The means 137 for performing flow and temperature balanced reactant flows can also comprise a stack group specific radiative heat transfer compensation element 134 in which pipe length, hydraulic diameter and/or pressure drop characteristic is stack specific to compensate for pressure and temperature differences in the fuel feed-in common rail 133. In other words at least one of pipe length, hydraulic diameter and pressure drop characteristic can be chosen according to each stack to compensate for pressure and temperature differences in the fuel feed-in common rail 133.

In one embodiment the radiative heat transfer compensation element 134 arrangement according to the present invention can comprise curved pipes structures 140 in fuel feed-in side or in air feed-in side or in both of them. In addition to providing uniform inlet temperatures between the reactant ducting branches, radiation heat transfer compensation elements can through different kinds of wall temperature management arrangements be utilized to provide a uniform heat radiation environment for all fuel cell stacks.

The common rail structures according to the present invention make the stack arrangement robust and minimize the maldistribution of flows due to leakages and bypassing flows since excess local flows are removed evenly from several parallel branches.

The stack arrangement according to the present invention can comprise an afterburner for performing burning of residual gas from the anode side 100. In one embodiment the arrangement can comprise an air heater, which is accommodated to the air feed space 132. The air heater can preferably be placed in conjunction with the air distribution between the stacks 103, providing heat by both convection and radiation. The heater can be e.g. an electrical heater or a burner arrangement or a combination of them.

In the embodiments according to the present invention can be utilized insert element(s) 134 to accomplish non-uniform thermal isolation. For example stable external pipe surface temperature can be achieved despite of a thermal gradient in the conveyed fluid in order to form gas flows having desired temperatures. Pipe inserts 134 are preferably designed to have as high a view factor as possible with respect to the pipe or duct surface whose wall is to be thermally balanced. In one embodiment pipe inserts can preferably be coated with engineered emissivity characteristic or catalytic properties to act e.g. as reformer or trap for impurities. In one further embodiment can be performed selection of the material combination of pipe and insert elements 134 to have different coefficients of thermal expansion, and thus the stack arrangement can be designed to compensate for thermal movements in structures.

In one embodiment balancing of relative proportions of radiative and convective heat transfer can improve non-uniform thermal isolation over an extended operating window. Similarly for flow balancing, linear and quadratic pressure drop characteristic can be balanced in pressure loss elements to significantly extend the operating window. Simplification of radiative heat transfer structures through reduced symmetry requirements with respect to sources of radiation also enables non-uniform thermal isolation in the embodiments according to the present invention. Different kind of heat radiation transfer methods can be performed simultaneously inside the piping.

Thus, it will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.

Claims

1. A stack arrangement of high temperature fuel cell system or

electrolysis cell system, each cell in the cell system comprising an anode side (100), a cathode side (102), and an electrolyte (104) between the anode side and the cathode side, the cell system comprising the cells in cell stacks (103), characterized by, that the arrangement comprises the stacks (103) arranged in row

arrangement, wherein the stacks are arranged side by side in at least two rows, and the arrangement comprises air feed-in ducting (120) for feeding air to the stacks (103), the ducting having air inlet ends (130), which are conveyed to a sealed air feed space (132), which is formed between the stack (103) rows having at least two sides of the air feed space enclosed by the stacks themselves, and the

arrangement comprises a fuel feed-in common rail (133), a fuel feed- in ducting (122) and individual feed-in channeling (131) for feeding fuel to the stacks (103), and at least one radiative heat transfer compensation element (134) along at least one of the fuel feed-in common rail (133), the fuel feed-in ducting (122), the individual feed- in channeling (131) and the air feed-in ducting (120), means (135) for performing flow and temperature balanced air feed-in flows for the stacks (103), and means (137) for performing flow balanced fuel flows in the cell system.

2. A stack arrangement of high temperature cell system in accordance with claim 1, characterized by, that the means (135) for performing flow and temperature balanced air feed-in flows for the stacks (103) comprise a back and forth arrangement (135a).

3. A stack arrangement of high temperature cell system in accordance with claim 1, characterized by, that the means (135) for performing flow and temperature balanced air feed-in flows for the stacks (103) comprise perforated pipes.

4. A stack arrangement of high temperature cell system in accordance with claim 1, characterized by, that the arrangement comprises at least one radiative heat transfer compensation element (134) for each stack (103) row being placed asymmetrically with respect to flow direction and the row.

5. A stack arrangement of high temperature cell system in accordance with claim 1, characterized by, that the means (137) for performing flow and temperature balanced fuel flows comprise a stack group specific radiative heat transfer compensation element (134) in at least one of pipe length, diameter and pressure drop characteristics to compensate for pressure and temperature differences in the fuel feed- in common rail (133).

6. A stack arrangement of high temperature cell system in accordance with claim 1, characterized by, that the arrangement comprises an air heater, which is accommodated in the air feed space (132).

7. A stack arrangement of high temperature cell system in accordance with claim 1, characterized by, that the radiative heat transfer compensation element (134) comprises curved pipe structures (140).

8. A stack arrangement of high temperature cell system in accordance with claim 1, characterized by, that the arrangement comprises flexible material to seal the space between adjacent stacks (103) and to compensate for dimensional tolerances.

9. A method of high temperature fuel cell system or electrolysis cell system, characterized by, that in the method is arranged cell stacks (103) in row arrangement so that the stacks are arranged side by side at least in two rows, is fed air to the stacks (103) by an air feed-in ducting (120) having air inlet ends (130), which are conveyed to a sealed air feed space (132), which is formed between the stack (103) rows having at least two sides of the air feed space (120) enclosed by the stacks (103) themselves, and in the method is fed fuel to the stacks (103) by a feed-in common rail (133), a fuel feed-in ducting (122) and individual feed-in channeling (131), is performed radiative heat transfer compensation along at least one of the fuel feed-in common rail (133), the fuel feed-in ducting (122), the individual feed- in channeling (131) and the air feed-in ducting (120), is performed flow and temperature balanced air feed-in flows for the stacks (103), and is performed flow balanced fuel flows in the cell system.

10. A method of high temperature fuel cell system or electrolysis cell system in accordance with claim 9, characterized by, that in the method is performed flow and temperature balanced air feed-in flows for the stacks (103) by a back and forth arrangement (135a).

11. A method of high temperature fuel cell system or electrolysis cell system in accordance with claim 9, characterized by, that in the method is performed flow and temperature balanced air feed-in flows for the stacks (103) by utilizing perforated pipes.

12. A method of high temperature fuel cell system or electrolysis cell system in accordance with claim 9, characterized by, that in the method is performed radiative heat transfer compensation

asymmetrically with respect to flow direction and the row, the radiative heat transfer compensation being performed for each stack (103) row.

13. A method of high temperature fuel cell system or electrolysis cell system in accordance with claim 9, characterized by, that in the method is performed flow and temperature balanced fuel flows by a stack group specific radiative heat transfer compensation in at least one of pipe length, diameter and pressure drop characteristics to compensate for pressure and temperature differences in the fuel feed- in.

14. A method of high temperature fuel cell system or electrolysis cell system in accordance with claim 9, characterized by, that in the method is heated air in the air feed space (132).

15. A method of high temperature fuel cell system or electrolysis cell system in accordance with claim 9, characterized by, that in the method is performed radiative heat transfer compensation by using curved pipe structures (140).

16. A method of high temperature fuel cell system or electrolysis cell system in accordance with claim 9, characterized by, that in the method is used flexible material to seal the space between adjacent stacks (103) and to compensate for dimensional tolerances.