WO2015150307A1

WO2015150307A1 - Adaptive insulation for soc stack system

Info

Publication number: WO2015150307A1
Application number: PCT/EP2015/056856
Authority: WO
Inventors: Henrik Franz Weineisen; Thomas Rostrup-Nielsen
Original assignee: Haldor Topsøe A/S
Priority date: 2014-04-04
Filing date: 2015-03-30
Publication date: 2015-10-08

Abstract

A Solid Oxide Cell stack system has adaptive insulation which insulates the stack system and also enables regulation of the insulation effect and thereby also regulation of the stack system temperature.

Description

Title: Adaptive Insulation for SOC Stack System

FIELD OF THE INVENTION The invention relates to an adaptive insulation for a solid oxide cell (SOC) stack system, in particular a solid oxide fuel cell (SOFC) stack system or a solid oxide electrolysis cell (SOEC) stack system. BACKGROUND OF THE INVENTION

In the following, the structure of a solid oxide cell stack is explained in relation to fuel cells. The fuel cells may, however, also run in "reverse mode" and thus operate as electrolysis cells.

A Solid Oxide Fuel Cell (SOFC) comprises a solid electro^¬ lyte that enables the conduction of oxygen ions, a cathode where oxygen is reduced to oxygen ions and an anode where hydrogen is oxidised. The overall reaction in a SOFC is that hydrogen and oxygen electrochemically react to produce electricity, heat and water. In order to produce the re^¬ quired hydrogen, the anode normally possesses catalytic ac^¬ tivity for the steam reforming of hydrocarbons, particular- ly natural gas, whereby hydrogen, carbon dioxide and carbon monoxide are generated. Steam reforming of methane, the main component of natural gas, can be described by the fol^¬ lowing equations:

CO + 3H₂

2CO + 2H₂

C0₂ + H₂ During operation an oxidant such as air is supplied to the solid oxide fuel cell in the cathode region. Fuel such as hydrogen is supplied in the anode region of the fuel cell. Alternatively, a hydrocarbon fuel such as methane is sup^¬ plied in the anode region, where it is converted to hydro^¬ gen and carbon oxides by the above reactions. Hydrogen passes through the porous anode and reacts at the anode/- electrolyte interface with oxygen ions generated on the cathode side that have diffused through the electrolyte.

Oxygen ions are created in the cathode side with an input of electrons from the external electrical circuit of the cell . To increase voltage, several cell units are assembled to form a stack and are linked together by interconnects. In^¬ terconnects serve as a gas barrier to separate the anode (fuel) and cathode (air/oxygen) sides of adjacent cell units, and at the same time they enable current conduction between the adjacent cells, i.e. between an anode of one cell with a surplus of electrons and a cathode of a neigh^¬ bouring cell needing electrons for the reduction process. Further, interconnects are normally provided with a plural^¬ ity of flow paths for the passage of fuel gas on one side of the interconnect and oxidant gas on the opposite side. To optimize the performance of a SOFC stack, a range of positive values should be maximized without unacceptable consequence on another range of related negative values which should be minimized. Some of these values are: VALUES TO BE MAXIMIZED VALUES TO BE MINIMIZED

- Fuel utilization - Price

- electrical efficiency - Dimensions

- life time - (temperature, to a point)

- production time

- fail rate

- number of components

- Parasitic loss (heating, cooling, blowers..)

Almost all the above listed values are interrelated, which means that altering one value will impact other values. Some relations between the characteristics of gas flow in the fuel cells and the above values are mentioned here:

Fuel utilization:

The flow paths on the fuel side of the interconnect should be designed to seek an equal amount of fuel to each cell in a stack, i.e. there should be no flow- "short-cuts" through the fuel side of the stack.

Parasitic loss:

Design of the process gas flow paths in the SOFC stack and its fuel cell units should seek to achieve a low pressure loss per flow volume at least on the air side and poten^¬ tially on the fuel side of the interconnect, which will re^¬ duce the parasitic loss to blowers. Electric efficiency:

The interconnect leads current between the anode and the cathode layer of neighbouring cells. Hence, to reduce in- ternal resistance, the electrically conducting contact points (hereafter merely called "contact points") of the interconnect should be designed to establish good electri^¬ cally contact to the electrodes (anode and cathode) and the contact points should no where be far apart, which would force the current to run through a longer distance of the electrode with resulting higher internal resistance.

Lifetime : Depends in relation to the interconnect on even flow dis^¬ tribution on both fuel and air side of the interconnect, few components and even protective coating on the materials among others . Price :

The interconnect price contribution can be reduced by not using noble materials, by reducing the production time of the interconnect and minimizing the material loss.

Dimensions :

The overall dimensions of a fuel stack is reduced, when the interconnect design ensures a high utilization of the ac- tive cell area. Dead-areas with low fuel- or air flow should be reduced and inactive zones for sealing surfaces should be minimized. Temperature :

The temperature should be high enough to ensure catalytic reaction in the cell, yet low enough to avoid accelerated degradation of the cell components. The interconnect should therefore contribute to an even temperature distribution giving a high average temperature without exceeding the maximum temperature.

Production time.

Production time of the interconnect itself should be mini^¬ mized and the interconnect design should also contribute to a fast assembling of the entire stack. In general, for eve^¬ ry component the interconnect design renders unnecessary, there is a gain in production time.

Fail rate.

The interconnect production methods and materials should permit a low interconnect fail rate (such as unwanted holes in the interconnect gas barrier, uneven material thickness or characteristics) . Further the fail-rate of the assembled cell stack can be reduced when the interconnect design re^¬ duces the total number of components to be assembled and reduces the length of seal surfaces.

Number of components. Apart from minimizing errors and assembling time as already mentioned, a reduction of the number of components leads to a reduced price. The way the anode and cathode gas flows are distributed in an SOFC stack is by having a common manifold for each of the two process gasses. The manifolds can either be inter^¬ nal or external. The manifolds supply process gasses to the individual layers in the SOFC stack by the means of chan- nels to each layer. The channels are normally situated in one layer of the repeating elements which are comprised in the SOFC stack, i.e. in the spacers or in the interconnect.

The cooling requirement of an SOFC stack or stack module varies with varying load (current) and also with time due to increasing ASR as a result of degradation. The insula^¬ tion of the stack or stack modules in a system is usually optimized for the nominal operating point at beginning of life. The design trade-off is between more insulation/more air for cooling and less insulation/less air for cooling. If a system is operated in another operating point, e.g. higher or lower current, than the nominal operating point, the balance between insulation and air cooling is no longer optimal .

W09829917 discloses A mono-container fuel cell generator contains a layer of interior insulation, a layer of exterior insulation and a single housing between the insulation layers, where fuel cells, containing electrodes and elec- trolyte, are surrounded by the interior insulation in the interior of the generator, and the generator is capable of operating at temperatures over about 650 DEG C, where the combination of interior and exterior insulation layers have the ability to control the temperature in the housing below the degradation temperature of the housing material. The housing can also contain integral cooling ducts, and a plu- rality of these generators can be positioned next to each other to provide a power block array with interior cooling.

The above described known art does not provide a simple, efficient and fail-safe solution to the above described problems. But these and other objects are achieved by the invention as described below and in the claims.

SUMMARY OF THE INVENTION At operating points moderately higher or lower than the nominal operating point, the air utilization can be adjusted (more or less air) in order to keep the stack at the de^¬ sired temperature. When moving further away from the nominal operating point the air flow may no longer be a practi- cal solution for modulating the stack temperature.

At currents higher than the nominal design point or when operating a degraded stack, the net AC efficiency will at some point suffer dramatically from the increased air flow rate required to cool the stack. In addition to this, the off-gas burner would need a bypass solution on the air side. Otherwise the air flow will get so high that stable burner operation will not be possible (high excess air factor, lambda) .

At low currents, the air utilization will, at some point, reach its upper limit and from there on the system fuel utilization, and as a result also the system electrical ef^¬ ficiency, needs to be lowered in order maintain the stack temperature. Lowering the system fuel utilization means that more fuel is combusted in the off-gas burner or sup- port burner, thus generating more heat for air pre-heating. As an example, operating the Power Core at currents below about 12-15 A requires decreasing the system fuel utiliza^¬ tion and therefore the overall electrical efficiency of the system by sending more of the system input fuel to the off- gas burner, where it heats the inlet air.

Invention: The problem described above could be addressed by a system that results in an adaptable insulation of the stack or stack module. Such a system could for example con- sist of two (or more) insulating walls (shells) of equal or different thicknesses with and air gap between them. When high insulation capability is required, the air in the gap(s) is not allowed to leave the gap(s) and therefore a steady state temperature will be reached in each gap and also the next insulation layer (next from the hot source) will also act as efficient insulation layer. In order to prevent free convection, the gaps could be filled with a highly porous material or constructed as such directly, e.g. by creating air channels directly in an impermeable insulation material.

If less insulation is required, the air in the gap or high^¬ ly porous part of the insulation is continuously replaced by new and colder air. This will act as to lower the over- all insulating effect of the multi shell insulation system.

The inlet airflow temperature and rate would then determine the apparent heat conductivity of the insulation system. The air flow could be achieved by a small fan or pump or by free convection (requires a valve to open in order to let the air out) . If based on free convection, the system could possibly be made self-regulating because the air exchange rate would increase as a result of stack temperature in^¬ crease and vice versa.

When running two or more stacks (or stack modules) in parallel with regards to air flow, it would be beneficial to have a means by which the heat loss from each stack (or module) could be individually controlled in order to com^¬ pensate for small differences in heat generation resulting from differences in ohmic resistance. Fig. 1 shows an embodiment of the invention where tempera^¬ ture 1 and temperature 2 are the temperature of the inside of the insulation adjacent to the Solid Oxide Cell stack system and temperature of the surroundings. The two insula^¬ tion walls have a gap in-between which allows for a gas flow to control the insulation efficiency. The gap is in this embodiment filled with a porous bypass layer.

FEATURES OF THE INVENTION 1. Solid oxide cell stack system comprising a plurality of stacked cell units and thermal insulation covering at least one of the stack system sides, wherein said insulation is adaptive, thereby allowing variation in the insulation capability. 2. Solid oxide cell stack system according to feature 1, wherein the insulation comprises at least two walls with a gap in-between allowing fluid flow between the walls. 3. Solid oxide cell stack system according to any of the preceding features, wherein the stack system comprises a single solid oxide cell stack.

4. Solid oxide cell stack system according to any of the preceding features, wherein the stack system comprises a plurality of cell stacks arranged in a module.

5. Solid oxide cell stack system according to feature 2, wherein the insulation walls have equal thickness.

6. Solid oxide cell stack system according to any of the features 2 - 5, wherein the gap is at least partly filled with a porous material allowing fluid flow through the gap. 7. Solid oxide cell stack system according to any of the features 2 - 5, wherein the gap is at least partly filled with an impermeable material comprising channels which al^¬ lows fluid flow. 8. Solid oxide cell stack system according to any of the preceding features, further comprising a fan or a pump adapted to provide fluid flow within the insulation, in channels within the insulation or between the walls of the insulation .

9. Solid oxide cell stack system according to any of the features 1 - 7, further comprising at least one regulation valve, which allows regulation of a free convection fluid flow within the insulation, in channels within the insulation or between the walls of the insulation. 10. Solid oxide cell stack system according to any of the preceding features, wherein the insulation is made of mica.

11. Solid oxide cell stack system according to any of the preceding features, wherein the insulation is adapted to allow for an air flow or a cooling liquid flow within the insulation, in channels within the insulation or between the walls of the insulation.

12. Process for regulation the temperature of a solid oxide cell stack system according to any of the features 1 - 11, the process comprising the steps of -

• providing a fluid flow within the insulation, in channels within the insulation or between the walls of the insulation when the insulation capability needs to be lowered.

• shutting of the fluid flow within the insulation, in channels within the insulation or between the walls of the insulation, when the insulation capability needs to be raised.

Claims

CLAIMS .

1. Solid oxide cell stack system comprising a plurality of stacked cell units and thermal insulation covering at least one of the stack system sides, wherein said insulation is adaptive, thereby allowing variation in the insulation capability.

2. Solid oxide cell stack system according to claim 1, wherein the insulation comprises at least two walls with a gap in-between allowing fluid flow between the walls.

3. Solid oxide cell stack system according to any of the preceding claims, wherein the stack system comprises a sin- gle solid oxide cell stack.

4. Solid oxide cell stack system according to any of the preceding claims, wherein the stack system comprises a plurality of cell stacks arranged in a module.

5. Solid oxide cell stack system according to claim 2, wherein the insulation walls have equal thickness.

6. Solid oxide cell stack system according to any of the claims 2 - 5, wherein the gap is at least partly filled with a porous material allowing fluid flow through the gap.

7. Solid oxide cell stack system according to any of the claims 2 - 5, wherein the gap is at least partly filled with an impermeable material comprising channels which al^¬ lows fluid flow.

8. Solid oxide cell stack system according to any of the preceding claims, further comprising a fan or a pump adapted to provide fluid flow within the insulation, in channels within the insulation or between the walls of the insulation.

9. Solid oxide cell stack system according to any of the claims 1 - 7, further comprising at least one regulation valve, which allows regulation of a free convection fluid flow within the insulation, in channels within the insulation or between the walls of the insulation.

10. Solid oxide cell stack system according to any of the preceding claims, wherein the insulation is made of mica.

11. Solid oxide cell stack system according to any of the preceding claims, wherein the insulation is adapted to allow for an air flow or a cooling liquid flow within the insulation, in channels within the insulation or between the walls of the insulation.

12. Process for regulation the temperature of a solid oxide cell stack system according to any of the claims 1 - 11, the process comprising the steps of - · providing a fluid flow within the insulation, in channels within the insulation or between the walls of the insulation when the insulation capability needs to be lowered .