Heat exchanger

Abstract

The present invention provides a heat exchanger and a method of making a heat exchanger, the heat exchanger being formed of at least one pair of single repeat units, each single repeat unit comprising a support layer and an interconnect layer, and in a first pair of the single repeat units, a first single repeat unit and a second single repeat unit is configured with their interconnect layers facing each other to define a first fluid passageway between those interconnect layers; and a second fluid passageway is provided on an opposite side of at least one of those interconnect layers so that the at least one interconnect layer is adapted to exchange heat between fluids within the first and second fluid passageways.

Description

[0001] Heat Exchanger

[0002] The present invention relates to a heat exchanger and a method of making a heat exchanger. The method also relates to a method of recycling.

[0003] In a fuel cell and in an electrolyser, and in particular in medium or high temperature fuel cells and electrolysers - particularly those involving temperatures in excess of 400 degrees C, such as solid oxide fuel cells (SOFCs) or solid oxide electrolyser cells (SOECs), it is important that any latent heat in the off gas can be recirculated or recuperated into the stack for improving the efficiency of the system. This is often done by recuperating it into the infeed gases for the stack. For this purpose, heat exchangers are commonly used within these systems, connected between the outflows (e.g. for the off-gases) and the inflows (e.g. for the infeed gases) of the stack via the pipework of the system. For example, SOFCs benefit in their efficiency if they can utilise the off gases to preheat the infeed gas - the fuel (e.g. a hydrocarbon such as methane, or hydrogen) - before it enters the stack, particularly if it can be used to assist with reforming the fuel.

[0004] In some such systems, the efficiency of the heat exchangers is important. In an efficient system, the heat of both of the off gas streams exiting the stack is beneficially recuperated to pre-heat the infeed gases to achieve maximum efficiency. In an electrolyser, perhaps about 2 / 3 of the heat is stored in the produced hydrogen of a wet hydrogen off-gas stream exiting the cathode side of the electrolyser, so the wet hydrogen stream is commonly seen to be the most valuable off-gas to heat exchange with. However, in total there is typically only about 20% surplus of latent heat in the combined off gas streams in an electrolyser, so by additionally using the anode (02) off-gas stream, that 20% surplus can be optimally utilised. For example, by utilising all of that 20%, the heat exchangers can often bring the infeed gases (often steam for a water electrolyser) close enough to stack temperatures without the need for an inline electric heater, especially if the heat exchanger can achieve >90% efficiency. Losing the heat of the anode off gas, or using a less efficient heat exchanger, can tip the balance enough to then require an inline electric heater for the inflow gas. This would increase both the cost of the system (capex) and operating cost (opex). The efficiency of the heat exchanger is thus important for avoiding an inline electric heater (adding an inline electric heater decreases the efficiency of the system).

[0005] In the prior art, the infeed gases on the cathode side of the stack - for both a fuel cell and an electrolyser - may be referred to as the “fuel”, as the steam entering an electrolyser equates to the fuel flowing into the stack for a fuel cell in electricity generation mode. As such, a common name for this electrode in both SOFC and SOEC is a ‘fuel electrode’. The opposing electrode is termed an ‘air’ or ‘oxygen’ electrode.

[0006] Another characteristic of SOFCs and SOECs is that over time the electrochemical efficiency of the electrochemically active layers of the cell can deteriorate. One cause of this can be a “poisoning” of the electrochemically active layers by impurities in the fuel or infeed fluids. Although typically a gradual and slow process, it is desirable for a stack to have a maximum possible service life as commonly the cells are non- serviceable, and thus upon a stack having a sufficiently diminished efficiency, the whole stack will need replacement. It thus would also be desirable to provide a “poison getter” for the system to draw out or diminish the prevalence of such “poisons” in the infeed, or at least to reduce the impact of any such poisoning of cells in the system on the efficiency of the system.

[0007] According to the present invention there is provided a heat exchanger formed of at least one pair of single repeat units, each single repeat unit comprising a support layer and an interconnect layer, and in a first pair of the single repeat units, a first single repeat unit and a second single repeat unit is configured with their interconnect layers facing (or bearing against) each other to define a first fluid passageway between those (facing or connected) interconnect layers; and a second fluid passageway is provided on an opposite side of at least one of those interconnect layers so that the at least one interconnect layer is adapted to exchange heat between fluids within the first and second fluid passageways.

[0008] In some embodiments, the first and second single repeat units each further comprise an electrochemical layer. In some embodiments, the second fluid passageway is in fluid communication with at least one of the electrochemical layers.

[0009] In some embodiments, each pair of single repeat units is configured such that at least one of its two single repeat units comprises an electrochemical layer.

[0010] In some embodiments, the second fluid passageway in each pair of single repeat units is in fluid communication with at least one electrochemical layer.

[0011] In some embodiments, the single repeat layers are configured such that the electrochemical layers in each single repeat unit are on an opposite side of the support layer to its interconnect layer.

[0012] In some embodiments, the single repeat layers are configured such that an electrochemical layer in at least one single repeat layer is between the at least one single repeat layer’s support layer and interconnect layer.

[0013] In some embodiments, the or each support layer is porous so as to provide fluid communication through the support layer with the electrochemical layer. Conversely, the interconnect layers are not porous, thus preventing mixing of the fluids within the fluid passageways across the interconnect layer.

[0014] The interconnect layers that are facing each other may be bearing against each other, for example either directly or indirectly. In other embodiments, they may be spaced apart from one another. For example, a spacer may separate them.

[0015] In some embodiments, a space between the interconnect layer and the support layer in each of the single repeat units of at least the first pair defines the second fluid passageway and a third fluid passageway, both the second and the third fluid passageways being separated from the first fluid passageway by the respective interconnect layers.

[0016] In some embodiments, the support layer of each single repeat unit in the pair is a metal support layer - for example made of stainless steel. Typically it is a generally flat support layer. It may be planar, for example. In some embodiments, the electrochemical layer of each single repeat unit in the pair comprises multiple layers - for example at least one electrolyte layer, at least one anode layer and at least one cathode layer. Each single repeat unit of the pair may thus be a cell for, or a cell from, a fuel cell or an electrolyser cell (or both) - for example an SOEC or an SOFC - some cells are operational in either manner - i.e. they are useable both as a fuel cell or in reverse mode as an electrolyser cell, dependent upon the system and / or stack configuration into which they are configured.

[0017] In some embodiments, the electrochemical layer of each single repeat unit in the pair defines an electrochemically active layer of an electrochemically active electrolyser cell, or of an electrochemically active fuel cell, or both. The cell may be a fully functional cell for a fuel cell or an electrolyser cell (or both), or a fuel cell / electrolyser cell (or both) in a damaged condition, or having a diminished efficiency due to prior use within a stack. For example, it is possible for one or more of the multiple layers of the electrochemical layer to be absent or non-functional, particularly if the single repeat unit is a reject of a fuel cell or electrolyser cell production line, or a used or broken fuel cell or electrolyser cell.

[0018] Due to the flexibility of the form of the cell, the heat exchanger of the present invention provides a useful product for recycling used cells (i.e. used single repeat units), as the condition of the electrochemical layer is not important when the product is being used just as a heat exchanger.

[0019] In some embodiments, the interconnect layer is a metal interconnect layer - for example stainless steel.

[0020] In some embodiments, the interconnect layer of each single repeat unit in the pair comprises a plurality of protrusions and recesses - for example a series or an array or an arrangement of bumps, lumps or ridges and troughs, recesses or valleys, so as to provide an increased surface area on both sides thereof - for increasing the surface area of both fluid passageways on either side thereof (i.e. the first and second fluid passageways for one of the single repeat units in the pair and the first and the third fluid passageways for the other of the single repeat units in the pair). The increased surface area increases the efficiency of the heat exchanger versus a correspondingly sized one having instead an interconnect layer defined by a flat and planar sheet. As electrolyser cells and fuel cells typically have a correspondingly shaped interconnect layer, rejected or discarded fuel cells or electrolyser cells are particularly suitable as the single repeat units of the present invention, even despite any failure of or reduction in the efficiency of their electrochemical layer, and even if they are rejects from the manufacturing process, as the electrochemical layer is typically not being used actively for its original purpose within the heat exchanger.

[0021] In some embodiments, the electrochemical layer is bonded or baked onto the support layer.

[0022] In some embodiments, the support layer of each single repeat unit of the pair is provided between the interconnect layer and the electrochemical layer of each single repeat unit of each pair - i.e. the electrochemical layer is configured such that the single repeat units have an externally configured electrochemical layer. It is possible, however, for the electrochemical layer to be between the support layer and the interconnect layer, whereby the single repeat units have an internally configured electrochemical layer.

[0023] In some embodiments, the electrochemical layer of each single repeat unit in the pair is configured as a poison getter, with the second and third fluid passageways being in fluid communication with the respective electrochemical layer of its single repeat units. For an internally configured electrochemical layer, this is achieved by virtue of the electrochemical layer being located in direct communication of the fluid passageway (it defines one of the wall surfaces of the fluid passageway). For an externally configured electrochemical layer, the support layer is instead perforated or porous to allow fluid within the fluid passageway to interact with the respective electrochemical layer of its single support unit. In some embodiments, the support layer comprises an array of through holes to the electrochemical layer of its single support unit for enabling that interaction.

[0024] With this configuration, the fluid within the second and third fluid passageways - which fluid may be an infeed fluid for a fuel cell (or electrolyser) system (i.e. for a stack within such a fuel cell or electrolyser system), will encounter or flow across that non-active electrochemical layer. Thus any impurities therein may “poison” that “non-active” electrochemical layer in the heat exchanger rather than an “active” electrochemical layer within the cathode side of the stack. The heat exchanger thus acts as a poison getter, whereby poisoning of the active stack of the system can be reduced, eliminated or slowed down. Typical examples of poisons include silica, sulphur, phosphorous, sodium, potassium etc. These tend to deposit on fuel electrode surfaces, or react with the material, and can deactivate or reduce its kinetic functionality.

[0025] In some embodiments, the single repeat units in each pair are matched - and are arranged in a mirrored configuration. This can provide a mirror image architecture for the stack.

[0026] In some embodiments, a spacer is fitted between the two single repeat units in each pair. The single repeat units in each pair may be joined to that spacer - for example by gluing or welding. In some embodiments the spacer is joined to and located between, the two interconnect layers of the pair. For example it can be welded to both interconnect layers in the pair. In some embodiments the welds extend through the interconnect layers across to the support layers of the single repeat units of the pair to form a single stackable component, for example with the electrochemical layers (or the support layers for single repeat unit with internally positioned electrochemical layers) providing an outside boundary of the single stackable component upon which another such single stackable component can be stacked.

[0027] The generally thin nature of the single repeat unit (and in particular the thin nature of the height thereof within the stack or pair thereof versus the width of the fluid passageways within the stack or pair thereof) also serves to improve the efficiency of these heat exchangers as the layers within the resulting heat exchanger will then be even thinner than the thickness of the single repeat units. For example, it is generally preferred that the thickness of each of the two single repeat units in a pair thereof is no more than 5mm (i.e. a pair thickness of 10mm), and preferably each single repeat unit has a thickness of no more than 4mm, whereby the second and third fluid passageways, and the interconnect layers and the support layers are all thinner than that dimension (i.e. they have a lesser height than that).

[0028] Providing a thin interconnect layer is very useful for improving the heat transfer efficiency between the fluid passageways. In some embodiments, the material of the interconnect layer in each single repeat unit is between 0.05 and 0.5mm thick, albeit with the interconnect layer itself - due to the usually-present plurality of protrusions and recesses, will be thicker than that.

[0029] Preferably the material is less than 0.3mm thick, and in some embodiments it is 0.2mm thick, or less than 0.21mm thick.

[0030] The first fluid passageway’s height (thickness) will generally be dictated by the height and depth of the protrusions and recesses of the respective interconnect layers - typically being at least the sum thereof x2, with any spacing between facing (rather than connected) interconnect layers increasing that height.

[0031] In some embodiments, the first fluid passageway’s height does not exceed 6mm, and more preferably it does not exceed 3mm.

[0032] In some embodiments, the width of the single repeat units is no less than 6cm.

[0033] In some embodiments, the length of the single repeat units is no less than 10cm.

[0034] In some embodiments, the heat exchanger comprises multiple pairs of single repeat units configured in a stack. For example, the stack may comprise 15 or more pairs of single repeat units. In some embodiments, a stack comprises up to 50 pairs - i.e. up to 100 single repeat units. Larger stacks - with more pairs - are also possible.

[0035] In some embodiments, the stack of pairs of single repeat units comprises fluidic connections, such as fluid passageways or pipework or manifolding, for connecting the first fluid passageways of each pair, and for connecting the second and third fluid passageways of each pair. These connections may be series or parallel connections or combinations of the two, and can enable two separated fluids passing therethrough to exchange heat between one another. The surface area of each fluid passageway is also relatively high due to the protrusions and recesses on the interconnect layers, and thus the efficiency of the heat exchange can be relatively high. The present invention also provides a method of making a heat exchanger, comprising forming a pair of single repeat units, each single repeat unit of the pair comprising a support layer and an interconnect layer, wherein: a first of the single repeat units of the pair is configured with its interconnect layer facing (or bearing against) the interconnect layer of a second of the pair of single repeat units to define a first fluid passageway between the single repeat units of the pair; and a second fluid passageway is provided on an opposite side of at least one of those interconnect layers so that the at least one interconnect layer is adapted to exchange heat between fluids within the first and second fluid passageways.

[0036] In some embodiments a space between the interconnect layer and the metal support layer in each of the single repeat units of the pair defines the second fluid passageway and a third fluid passageway, both the second and the third fluid passageways being separated from the first fluid passageway by the respective interconnect layers. In some embodiments, the second and third fluid passageways are interconnected to present a common fluid passageway for a second fluid.

[0037] The heat exchanger so produced may be as defined above.

[0038] The present invention also provides a method of recycling single repeat units from a fuel cell stack or from an electrolyser stack or from a discard pile from a manufacturing plant or production line for such single repeat units, the method comprising using the single repeat units to make a heat exchanger as defined above.

[0039] The present invention also provides a method of operating a heat exchanger as defined or made above, comprising passing a first fluid through the first fluid passageway and a second fluid, different to the first fluid, through the second fluid passageway, wherein heat exchanges between the two fluids across the interconnect layer between the first and second fluid passageways.

[0040] Typically the second and third fluid passageways will be fluidically connected, whereby the second fluid will also be fed to the third fluid passageway (as a common fluid passageway) such that heat exchanges between the two fluids across the interconnect layer between the first and third fluid passageways as well. It is possible, however, instead for a third fluid to pass through the third fluid passageway if the second and third fluid passageways are not fluidically connected.

[0041] In some embodiments the heat exchanger comprises multiple pairs of single repeat units configured in a stack and the stack of pairs of single repeat units further comprises fluidic connections, such as via fluid passageways or pipework or manifolding, for connecting the first fluid passageways of each pair, and for connecting the second (and third) fluid passageways of each pair. The method then comprises passing the first fluid through the first fluid passageways of each pair, and the second fluid through the second fluid passageways of each pair. Preferably the second fluid is likewise passed through the third fluid passageways of each pair.

[0042] The fluidic connections between pairs may form series or parallel connections, or combinations of the two, between the fluid passageways in each pair. They enable two separated fluids passing therethrough to exchange heat between one another across the interconnect layers. As indicated above, they can even allow a third separated fluid, and possibly more separated fluids if some pairs of single repeat units are separately connected to other fluid flow paths instead, to exchange heat between one another.

[0043] In some embodiments, the single repeat units of the first pair of repeat units are arranged in a mirrored configuration. This can provide a mirror image architecture for the stack.

[0044] In some embodiments, the single repeat units of the first pair of repeat units are each also paired with third and fourth single repeat units. In some embodiments, the single repeat units of the first pair of repeat units are each arranged in a mirrored configuration with a respective one of the third and fourth single repeat units. This can then provide, or maintain, a mirror image architecture for the stack.

[0045] In some embodiments, the third and fourth single repeat units are welded to the adjacent one of the single repeat units of the first pair of single repeat units, whereas in other embodiments they are not welded to each other.

[0046] In some embodiments, the single repeat units of the first pair of repeat units are welded to each other, but in other embodiments, they are not welded to each other. In some embodiments, each single repeat unit additionally comprises a cell spacer for separating a part of its support layer from a part of its interconnect layer. There may be at least two such cell spacers in each single repeat unit - for example one at each end of the single repeat unit.

[0047] In some embodiments, a pair spacer is provided to separate at least parts of the two facing interconnect layers of the first pair of single repeat units. This pair spacer may be to maintain a separation across the full extent of the two facing interconnect layers, thus widening the first fluid passageway.

[0048] In each aspect of the present invention, the surface area of each fluid passageway is provided to be relatively high (versus the surface area of a planar sheet of a similar size) due to the protrusions and recesses on the interconnect layers, and thus the efficiency of the heat exchange between the fluids can be relatively high - and typically greater than 90%.

[0049] The present invention therefore enables a heat exchanger in which heat from one fluid (e.g. the off gas), which is contained between two interconnect layers of a pair of single repeat units, and another fluid (infeed gas, such as fuel or water), which is free to pass in the other passageway(s) (i.e. on both sides of a or each support layer - commonly a porous metal support layer - for example made porous by an array of holes therethrough (which holes may have been drilled therethrough, e.g. with a laser or water cutter) metal support), can be efficiently exchanged.

[0050] In typical embodiments, the second (other) fluid can access both sides of the support layer due to the porosity. The second fluid may even pass beyond any electrochemical layer on that support layer - for example when the electrochemical layer (which is typically a plurality of separately formed ceramic layers) is damaged or cracked, and which thus might not be leak-proof, even if its initial purpose was to include it being leak-proof.

[0051] The present invention also enables an optional ‘poison getter’ aspect of the invention. The present invention also enables the formation of the heat exchanger with single repeat units which were rejected in a fuel cell or electrolyser production line prior to any electrochemical layer being applied, as the electrochemical layer is optional.

[0052] In some embodiments the operation of the heat exchanger is within an electrolyser system - for example an electrolyser system for generating hydrogen and oxygen from water (steam), or for generating carbon monoxide and oxygen from carbon dioxide. In some embodiments the system may be for achieving both. The fuel or infeed fluid is thus typically water or carbon dioxide (or air), or a mixture of these

[0053] Alternatively, the operation of the heat exchanger may be in a fuel cell system, the fuel cell system being for generating electricity across a stack of fuel cells.

[0054] In some embodiments, the method (i.e. the electrolyser system or the fuel cell system) is operated at a temperature in excess of 400 degrees C - for example between 450 and 650 degrees C.

[0055] In some embodiments, the electrolyser system (or the fuel cell system) comprises a separate stack of similar single repeat units, albeit configured within the separate stack differently to the pairs of single repeat units of the heat exchanger. For example, the separate stack is configured to operate as an electrolyser stack (or a fuel cell stack) - by having the single repeat units configured such that their interconnect layers are not touching or facing interconnect layers of neighbouring single repeat units.

[0056] It is to be noted that the separate stack may additionally operate as a heat exchanger - for the fluids passing through separate flow paths within that separate stack.

[0057] In some embodiments, the or each single repeat unit in the pair(s) thereof is a solid oxide electrolyser cell or a solid oxide fuel cell - preferably a metal supported solid oxide electrolyser cell or a solid oxide fuel cell (or both in some embodiments). In other words, the electrochemically active region of the electrolyser cell is a solid oxide. Such cells typically operate in the 400-900 degrees C range. However, they can include intermediate and high temperature cells. There are many possible forms of solid oxide fuel cell, or solid oxide electrolyser cell, each using different electrochemically active electrolyte chemistries. For example, three well known electrolyte materials are yttria-stabilized zirconia (YSZ), scandia stabilized zirconia (ScSZ) and gadolinium doped ceria (GDC or CGO). Anode and cathode materials can also vary. The present invention can be used with any of these, along with many other forms of electrolyser cell or fuel cell.

[0058] For some chemistries, the cells are just operating between 400 and 700 degrees C, or more particularly in the 450-650 degrees C temperature range. Such electrolyser (or fuel) cells may be referred to as intermediate temperature solid oxide electrolyser (or fuel) cells, or IT-SOECs / IT-SOFCs. When operating above 700 degrees C, they are instead commonly referred to as high temperature solid oxide electrolyser (or fuel) cells, or HT-SOECs / HT-SOFCs

[0059] Typically the present invention is configured to electrolyse water in the form of steam, rather than liquid water. An advantage of steam-based electrolysers is that steam electrolysis - particularly intermediate and high temperature steam electrolysis at temperatures above 400 degrees C and 700 degrees, respectively, efficiently produces hydrogen as the high temperature environment can reduce the electric power requirements for the electrolysis of the water molecules from steam compared to electrolysis of liquid water. Additionally, the higher temperature can relatively increase the reaction activity with the electrolyser versus that of liquid water. The use of the heat exchanger in the system can also effectively recirculate off-gas heat back to the infeed. Electrolysis of other fluids, such as carbon dioxide, is also possible, and useful. In such a case, the heat-exchanging gases will be carbon monoxide as an off-gas exchanging heat with carbon dioxide as the feed gas.

[0060] The present invention is thus highly suitable for use with solid oxide electrolyser (or fuel) cells operating at temperatures above 400 degrees C.

[0061] Sometimes the cut-off between intermediate temperature and high temperature SOECs (or SOFCs) is 750 degrees C, rather than 700 degrees C. In some embodiments, the heat exchanger operated with fluids passing therethrough at a pressure greater than 1 bar. Typically the pressure will be at about 2 bar. In some embodiments the pressure may be between 2 bar and 10 bar.

[0062] These and other features of the present invention will now be described in further detail, purely by way of example, by reference to the accompanying drawings in which:

[0063] Figure 1 schematically shows a pair of single repeat units arranged in a mirrored configuration to form at least a part of a heat exchanger that is defined with a first fluid passageway, a second fluid passageway and a third fluid passageway, configured for heat exchange between fluids in the first, second and third fluid passageways;

[0064] Figure 2 schematically shows a stack comprising a first pair of single repeat layers units arranged in a mirrored configuration with electrochemical layers on the outsides of the support layer, and with a pair spacer between the first pair of single repeat layers units, each single repeat layer of the first pair being also paired with a further single repeat layer, i.e. third and fourth single repeat units;

[0065] Figure 3 schematically shows an alternative configuration for the stack of Figure 2 in which the electrochemical layer is instead between the interconnect layer and the support layer of each single repeat unit;

[0066] Figure 4 schematically shows a further alternative configuration for the stack of Figure 2, in which the single repeat units have no electrochemical layers; and .

[0067] Figure 5 shows a series of Nano-SIMs analysis showing silica concentration in a fuel electrode at the inlet or outlet.

[0068] Referring to Figure 1 , there is schematically shown a heat exchanger 10 comprising a first pair 14 of single repeat units 12. The first pair 14 of single repeat units 12 comprises a first single repeat unit 22 and a second repeat unit 24 stacked upon one another such that interconnect layers 18 of the two single repeat layers 12 are facing and contacting one another - they bear against one another in their pair 14. This arrangement defines a first fluid passageway 30 through the centre of the first pair 14 of single repeat units 12 - between the interconnect layers 18. Each single repeat unit 12 comprises the respective interconnect layers, plus also a support layer 16 - which in this embodiment is a metal support layer - for example formed of stainless steel, although other materials can be used.

[0069] Each support layer is coated with an electrochemical layer 36, which in this embodiment is a series of ceramic layers baked onto the metal support layer. Each single repeat layer could thus potentially function as a fuel cell, or an electrolyser cell, due to the electrochemical layer thereon. However, this electrochemical layer may be cracked or broken either through use or via a manufacturing defect, thus preventing or making less efficient, such an operational use. The single repeat units are thus suitable for redeployment as a component of the heat exchanger 10, rather than in a fuel cell (or electrolyser) stack. Fully functional or new fuel cells and electrolyser cells can also be utilised in the heat exchanger of the present invention, although due to their high cost of production, their use is better utilised in a fuel cell (or electrolyser) stack.

[0070] The interconnect layers 18 form a barrier within each single repeat unit 12, such that there is also a second and third fluid passageway 32, 34 within the pair of single repeat units. A first supply of fluid can thus be passed through the first fluid passageway 30 and a second supply of fluid can be passed through the second and third fluid passageways 32, 34 for exchanging heat between the first and second fluids across the interconnect layers 18.

[0071] In this example, the first fluid 60 is off-gas from an electrolyser stack and the second fluid 62 is sourced from a supply of water, and will be water or steam for being preheated or vaporised / evaporated by the heat within the off-gas prior to downstream distribution into the electrolyser stack. The heat exchanger 10 can thus provide the function of preheating the steam for the electrolyser stack.

[0072] As shown in Figure 1, the first and second single repeat unit each has a first weld line 56 connecting the support layer to the interconnect layer. This first weld line creates a seal around the single repeat unit, enclosing the edges thereof. This is because in certain forms of fuel cell unit, or electrolyser unit, the fluid passing through the second and third fluid passageways must be kept separate from the fluid passing to the other side of the interconnect layer. That weld line can be retained in this repurposing of that single repeat unit as such single repeat units also usually have fluid access ports to enable passage of fluid into and out of the single repeat unit.

[0073] In this repurposed use within a heat exchanger, it is likely that the electrochemical layer 36 will be cracked or otherwise broken, porous or absent, due to the single repeat unit being a reject from the production line, or a reject following use thereof in a stack, and it is thus being repurposed. Access into the cell from outside of the electrochemical layer is thus often also possible through both the electrochemical layer (through the cracks, for example) and the support layer. Regarding that support layer, it is typically porous in such single repeat layers to allow the fuel or water / steam to access the cathode layer of the electrochemical layer.

[0074] Such flow through the electrochemical layer 36 and the support layer 16 is in addition to using any pre-existing fluid access ports.

[0075] A pair spacer 38 is also provided between the two interconnect layers 18 within the pair 14. This pair spacer closes the edges of the first fluid passageway, and may be made of a common material to the interconnect layers - e.g. stainless steel - although other materials are also possible, so that it can be welded to the two interconnect layers to seal those edges. Other seals can instead be used, such as gaskets and compression through the stack.

[0076] To create a stack of these pairs of single repeat units, multiple such pairs can be stacked one upon the other, electrochemical layers of one pair being stacked upon electrochemical layers of adjacent pairs. In this manner, a multi-layered stack can be produced, with a first gas (usually the hotter off-gas) being directed through the first fluid passageways 30 of all the pairs 14 and a second gas (usually water in the case of an electrolyser and a fuel such as hydrogen or methane in the case of a fuel cell) can be passed through all of the second and third fluid passageways. Pipework and manifolding can be utilised for guiding these first and second fluids through the respective fluid passageways, as well known in the art. Referring next to Figure 2, there is schematically shown an alternative form of stack 20 of single repeat units 12. The single repeat units are configured with a first pair 14 of single repeat units 12 in the middle of the stack 20, that first pair being formed of a first single repeat unit 22 and a second single repeat unit 24 spaced apart by a pair spacer 38. In this embodiment the pair spacer provides a wider spacing between the interconnect layers, to make the first fluid passageway wider than that of the first embodiment. This is optional, but can enable larger fluid flow rates.

[0077] In addition to the first pair 14, a third single repeat unit 26 is provided below the second single repeat unit and a fourth single repeat unit 28 is provided above the first single repeat unit 22. A top plate 52 then caps the top of the stack 20, and a bottom plate 54 caps the bottom of the stack 20.

[0078] In this embodiment, rather than multiple pairs, there is only a single pair, with single repeat units above and below that pair. It is possible, however, to incorporate the stacked pairs described above between the third and fourth single repeat units, such that the third single repeat unit is below those pairs, with the bottom plate below it, and the fourth single repeat unit above those pairs, with the top plate above it.

[0079] In this embodiment, the single repeat units of the first pair of repeat units are arranged in a mirrored configuration. This provides a mirror image architecture for the pair within the stack, much like the mirror image architecture for the pair in Figure 1, albeit with a wider spacing between the interconnect layers.

[0080] With the positioning of the third and fourth single repeat units below and above the first pair, the single repeat units of the first pair of repeat units are each also paired with the third and fourth single repeat units, albeit instead with the electrochemical layers facing or against each other. The single repeat units of the first pair of repeat units in this example are thus each arranged in a mirrored configuration with a respective one of the third and fourth single repeat units. This maintains a mirror image architecture for the stack.

[0081] In this example, the third and fourth single repeat units are welded to the adjacent ones of the single repeat units of the first pair of single repeat units. This can be, as shown, via a welded internal skirt 48 as there is spacing between the ends of the interconnect layers 18 and the support layers 16, unlike in Figure 1 , and since there is spacing between the adjacent support layers 16 due to the electrochemical layers 36 thereon / therebetween.

[0082] Due to these spacings in this embodiment, in addition to the pair spacer 38, there is a cell spacer 64 at each end of each single repeat unit for providing compression resistance to the single repeat units in the stacking direction, and for closing the sides of the second fluid passageways (along with the welded internal skirt 48). The cell spacer in each single repeat unit also maintains the separation between a part of its support layer from a part of its interconnect layer. As this is commonly at the ends, as shown, there may be at least two such cell spacers in each single repeat unit - for example one at each end of the single repeat unit.

[0083] Compression bolts may be used to hold the stack together - extending between the top and bottom plates. However, in this example, an external welded skirt 50 is instead used to hold the stack together.

[0084] Having an external welded skirt can also provide, along with the internal skirts and the interconnect layers 18, two separate fluid chambers - a first containing the first fluid passageway(s) 30 and extending between the external welded skirt 50, the welded internal skirts 48 and one side of the interconnect layers 18 of the first pair (or each stacked pair when more than one such pair is provided), and potentially also including the fluid passageway between the lower and upper sides of the interconnect layers 18 of the third and fourth single repeat units 26, 28 and the top and bottom plates 52, 54, if fluidically connected thereto, and a second containing the second and third fluid passageways 32, 34 (which are fluidically connected in this embodiment, and thus just annotated as reference sign 32), and extending between the welded internal skirts 48 and the other sides of the interconnect layers 18.

[0085] For feeding the first and second fluids through these chambers, in this example inlets and outlets are formed at either end of the single repeat units via ports 66, 68 in the bottom plate 54 for an infeed gas and via ports 70, 72 for an off-gas in the top plate 52.

[0086] In this example, there is an inlet feed gas 40 (e.g. steam for an electrolyser, or fuel for a fuel cell) entering the second chamber via a first port 66 from an infeed gas supply, and exiting as an outlet feed gas 42 from the second chamber via second port 68. Between these ports 66, 68, the fluid will pass through the support layer (as it is porous), and also across the electrochemical layers via ports or holes or gaps provided therebetween, with manifolded holes provided through the interconnect layers and the support layers to allow that flow, but without any flow through the first fluid passageway. There is also an off gas entering the first chamber as an inlet off gas 44 via a third port 70 from the fuel cell or electrolyser stack (such as wet hydrogen for an electrolyser, or carbon dioxide and water, plus remnant fuel, for a fuel cell), and exiting the first chamber as an outlet feed gas 46 via fourth port 72. Between these ports 70, 72, the fluid will pass through the first fluid passageways 30 to exchange heat with the inlet feed gas as the latter passes through the second fluid passageways 32.

[0087] Referring next to Figure 3, a modification to the configuration of Figure 2 is shown. In this modification, a pair of single repeat units is again arranged in a mirrored configuration to form at least a part of a heat exchanger that is defined with a first fluid passageway, a second fluid passageway and a third fluid passageway, fluidically connected to the second fluid passageway, and configured for heat exchange between fluids in the first, second and third fluid passageways. However, in this embodiment the electrochemical layers are positioned between the support layer and the interconnect layer in each single repeat unit. Nevertheless, the principle of operation is the same. An inlet feed gas 40 (e.g. steam for an electrolyser, or fuel for a fuel cell) enters the second chamber via a first port 66 from an infeed gas supply, and exits as an outlet feed gas 42 from the second chamber via a second port 68. Between these ports 66, 68, the fluid will pass through the support layer (as it is porous), and also across the electrochemical layers, via ports or holes or gaps provided therebetween, with manifolded holes provided through the interconnect layers and the support layers to allow that flow, but without any flow through the first fluid passageway 30. There is also an off gas entering the first chamber as an inlet off gas 44 via a third port 70 from the fuel cell or electrolyser stack (such as wet hydrogen for an electrolyser, or carbon dioxide and water, plus remnant fuel, for a fuel cell), and exiting the first chamber as an outlet feed gas 46 via fourth port 72, both the third and fourth ports in this example being in the top plate. Between these ports 70, 72, the fluid will again similarly pass through the first fluid passageways 30 to exchange heat with the inlet feed gas as the latter passes through the second fluid passageways 32. Similar internal and external skirts 48, 50 are again provided, with manifolded passageways being provided through the pair spacer for the off gas.

[0088] In these examples, the cell spacer 64 can be a pre-existing spacer that was already welded inbetween the support layer and the interconnect layer of each single repeat unit.

[0089] As for the pair spacers 38, they can provide fluid passageway between upper and lower chambers, while also providing fluid passageway for the off gas through the first chamber via the first fluid passageway, without the fluids mixing. It could be a compression gasket similar in form and function to those used in a fuel cell or electrolyser cell stack, or another sealant, rather than a welded interface, or a combination of the two.

[0090] The welded external skirt 52 could also serve the function of holding compression between the layers of the heat exchanger 10.

[0091] It is important that the two fluids are always separated by an interconnect, which won’t be porous. The support layer, after all, generally is porous and the electrochemical layers are not expected to be gas tight, as they could be cracked when utilised in the heat exchanger - either before that use or due to the stacking of the cells and the compression thereof in the heat exchanger. It is only the interconnect layer which operates as the predominant heat exchange interface.

[0092] Referring next to Figure 4 there is schematically shown a third configuration for the stack, where this time the electrochemical layers are absent. For simplicity the heights of the outer pairs are kept the same as in Figures 2 and 3, but in practice in this embodiment the gap between the support layers may be much smaller than that shown, thus resulting in a more compact stack. As the electrochemical layers serve little function in the heat exchange, they are non-essential so even single repeat units that were rejected during their production prior to applying the electrochemical layers can be utilised within the heat exchanger of the present invention.

[0093] The difference between this embodiment and the previous embodiments is that this embodiment, due to the absence of the electrochemical layers, will not additionally function as a poison getter. That is because it is the electrochemical layers in the single repeat units that allow the heat exchanger of the present invention to additionally serve as a poison getter when being used as a heat exchanger for infeed gas for an electrolyser or a fuel cell. As discussed earlier in this specification, the infeed gas, prior to entering the stack, will be exposed to the electrochemical components of the electrochemical layers, and thus any poisons in the infeed gas would react with those electrochemical layers in the heat exchanger, rather than with the corresponding layers in the stack, thus creating damage to the heat exchanger rather than the stack, which heat exchanger damage does not degrade the heat exchange functionality of the heat exchanger. On the other hand, had the infeed gas instead reacted with the electrochemical layers within the electrolyser stack, it would have degraded the electrolyser’s efficiency.

[0094] Figure 5 shows a Nano-SIMS (secondary ion mass spectrometry) analysis of cell potions nearer an inlet (Figures 5a on a silica tested cell and 5b on an untested cell) and a further analysis nearer an outlet of a tested cell (Figure 5c). Each image shows the substrate 16, and the ceramic layers 36 (fuel electrode 82, electrolyte 84, and air electrode 86).

[0095] Figures 5a and 5b show a portion of a cell nearer its inlet, the difference being in Figure 5a a poison (in this case silica) is intentionally introduced into the fluid flow, whereas in Figure 5b the poison was not introduced (i.e. it is untested for silica poisoning). A high concentration of silica (the speckled shading) can be seen in the fuel electrode 82 in the tested analysis (Figure 5a) compared to the ‘untested’ cell (Figure 5b), as significant silica is absorbed by the fuel electrode 82. It should be noted that in Figure 5b, which is also showing the silica concentration, it is not possible to distinguish the ceramic layers 16 from one-another. That is because in Figure 5b the concentration of silica across the three layers is uniform (and essentially zero). The substrate 16 has a non-zero silica concentration in all three images in this example.

[0096] Figure 5c instead shows a cell portion nearer the outlet of the tested cell (i.e. one in which a poison has intentionally been introduced). In this analysis, essentially no silica is again present in the fuel electrode 82. This demonstrates that the silica has already been absorbed by the fuel electrode before it can reach the outlet, i.e. this occurs at or near the fluidic inlet of the cell. Silica poisons the stack by reacting to or with the electrode material, in this case, doped Ceria. A chemical reaction between doped ceria and silica occurs, producing a stable compound of silica ceriate. Formation of this compound reduces the efficacy of the fuel electrode when acting in its intended purpose (i.e. a reduction of steam in electrolysis mode). A similar reaction, and thus a similarly deleterious result, happens with other elements which form ceriate compounds (such as chlorine, arsenic, phosphor, sulphur), i.e. some of the other potential absorbable poisons.

[0097] The example of silica is relevant because is a relatively common contaminant.

[0098] More generally, the invention works on the principle that any potential foreign element entering in the supply streams that would normally react with the electrode materials will do so within the upstream heat exchanger since this effectively contains the same cell materials.

[0099] It should be noted that the interconnect may also contain a ceria-containing material, for example in a backbone material that supports an internal reforming catalyst.

[0100] The present invention has therefore been described above purely by way of example with reference to the accompanying drawings. Modifications in detail may be made to the invention within the scope of the claims as appended hereto.

Claims

CLAIMS:

1. A heat exchanger formed of at least one pair of single repeat units, each single repeat unit comprising a support layer and an interconnect layer, and in a first pair of the single repeat units, a first single repeat unit and a second single repeat unit is configured with their interconnect layers facing each other to define a first fluid passageway between those interconnect layers; and a second fluid passageway is provided on an opposite side of at least one of those interconnect layers so that the at least one interconnect layer is adapted to exchange heat between fluids within the first and second fluid passageways.

2. The heat exchanger of claim 1, wherein the first and second single repeat units each further comprise an electrochemical layer.

3. The heat exchanger of claim 1 or claim 2, wherein the second fluid passageway is in fluid communication with at least one of the electrochemical layers.

4. The heat exchanger of any one of the preceding claims, wherein each pair of single repeat units is configured such that at least one of its two single repeat units comprises an electrochemical layer.

5. The heat exchanger of any one of the preceding claims, wherein the second fluid passageway in each pair of single repeat units is in fluid communication with at least one electrochemical layer.

6. The heat exchanger of any one of the preceding claims, wherein the single repeat layers are configured such that the electrochemical layers in each single repeat unit are on an opposite side of the support layer to its interconnect layer.

7. The heat exchanger of any one of the preceding claims, wherein the single repeat layers are configured such that an electrochemical layer in at least one single repeat layer is between the at least one single repeat layer’s support layer and interconnect layer.

8. The heat exchanger of any one of the preceding claims, wherein the or each support layer is porous so as to provide fluid communication through the support layer with the electrochemical layer.

9. The heat exchanger of any one of the preceding claims, wherein the electrochemical layer of each single repeat unit in the pair defines an electrochemically active layer of an electrochemically active electrolyser cell, or of an electrochemically active fuel cell, or both.

10. The heat exchanger of any one of the preceding claims, wherein the interconnect layer of each single repeat unit in the pair comprises a plurality of protrusions and recesses.

11. The heat exchanger of any one of the preceding claims, wherein the electrochemical layer of each single repeat unit in the pair is configured as a poison getter.

12. The heat exchanger of any one of the preceding claims, wherein the single repeat units in each pair are matched and are arranged in a mirrored configuration.

13. The heat exchanger of any one of the preceding claims, wherein a spacer is fitted between the two single repeat units in each pair.

14. The heat exchanger of claim 13, wherein the spacer is joined to and located between, the two interconnect layers of the pair.

15. The heat exchanger of any one of the preceding claims, wherein the thickness of each of the two single repeat units in a pair thereof is no more than 5mm.

16. The heat exchanger of any one of the preceding claims, wherein the material of the interconnect layer in each single repeat unit is between 0.05 and 0.5mm thick.

17. The heat exchanger of any one of the preceding claims, wherein the heat exchanger comprises multiple pairs of single repeat units configured in a stack.

18. The heat exchanger of claim 17, wherein the stack comprises 15 or more pairs of single repeat units.

19. A method of making a heat exchanger, comprising forming a pair of single repeat units, each single repeat unit of the pair comprising a support layer and an interconnect layer, wherein: a first of the single repeat units of the pair is configured with its interconnect layer facing the interconnect layer of a second of the pair of single repeat units to define a first fluid passageway between the single repeat units of the pair; and a second fluid passageway is provided on an opposite side of at least one of those interconnect layers so that the at least one interconnect layer is adapted to exchange heat between fluids within the first and second fluid passageways.

20. The method of claim 19, wherein the heat exchanger is as defined in any one of claims 1 to 18.

21. A method of recycling single repeat units from a fuel cell stack or from an electrolyser stack or from a discard pile from a manufacturing plant or production line for such single repeat units, the method comprising using the single repeat units to make a heat exchanger as defined in any one of claims 1 to 18.

22. A method of operating a heat exchanger as defined in any one of claim 1 to 18, or as made by the method of any one of claims 19 to 21, comprising passing a first fluid through the first fluid passageway and a second fluid, different to the first fluid, through the second fluid passageway, wherein heat exchanges between the two fluids across the interconnect layer between the first and second fluid passageways.

23. The method of claim 22, wherein the heat exchanger comprises multiple pairs of single repeat units configured in a stack and the stack of pairs of single repeat units further comprises fluidic connections for connecting the first fluid passageways of each pair, and for connecting the second fluid passageways of each pair.

24. The method of claim 22 or claim 23, wherein the operation of the heat exchanger is within an electrolyser system.

25. The method of claim 22 or claim 23, wherein the operation of the heat exchanger is in a fuel cell system.

26. The method of any one of claims 22 to 25, wherein the method is operated at a temperature in excess of 400 degrees C.

27. The method of any one of claims 22 to 26, wherein the or each single repeat unit in the pair(s) thereof is a solid oxide electrolyser cell or a solid oxide fuel cell.

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