WO2014177207A1

WO2014177207A1 - Heat exchanger in sofc stack

Info

Publication number: WO2014177207A1
Application number: PCT/EP2013/059065
Authority: WO
Inventors: Claus FRIIS PEDERSEN
Original assignee: Topsøe Fuel Cell A/S
Priority date: 2013-05-01
Filing date: 2013-05-01
Publication date: 2014-11-06

Abstract

A Solid oxide fuel cell (SOFC) system has at least one integrated heat exchanger which minimizes the hot surface area of the heat exchanger and the system.

Description

Title: Heat exchanger in SOFC stack

Solid Oxide Cells are used for a wide range of purposes including the generation of electricity from different fuels (fuel cell mode) and the generation of synthesis gas (CO + H2) from water and carbon dioxide (electrolysis mode) .

Regardless of the application, the solid oxide stacks typically operate at temperatures in the range from 600 to 1000 °C . At these temperatures reduction of heat loss is essential to obtain a good efficiency.

The two essential components to reduce heat loss are thermal insulation materials and heat exchangers. Thermal insulation is used to reduce heat loss to the environment and heat exchangers are used to transfer heat from the outlet gas flows to the inlet gas flows. Heat exchangers are therefore critical components in almost any solid oxide stack system (SOSS) and it is essential to optimise these to enhance heat transfer and reduce heat loss.

This invention relates to the design and integration of thermally matched heat exchangers and their integration with solid oxide stacks and systems to enhance the overall efficacy of these systems.

In traditional non-integrated solid oxide stack system configurations a significant amount of heat is lost through hot surfaces and piping between components.

An example of a non-integrated system is shown in Fig. 1. On both the fuel and the oxygen side a solid oxide stack in electrolysis mode is fed via an input/output heat exchanger in series with an electrical heater. To be able to compare heat losses of different set-up a parameter called the ^xhot surface area' (HSA) can be defined as :

Where T (A) is the temperature for a given area, T_Max is the maximum temperature across the entire area and T_ext (A) is the temperature of the adjacent components and surroundings external environment and Τ_Μ±_η is the minimum external

temperature (typically room temperature) . The ^xhot surface area' can be interpreted as the actual surface temperature difference integrated for the entire device area and

normalised with the maximum temperature difference. Considering, the large left heat exchangers in Fig. 1, the active area measures 8 cm x 8 cm x 12 cm giving it a surface area of 500 cm². The temperature of the heat exchanger however varies (linearly) from the maximum temperature to the cold input temperature. If the cold input temperature is similar to the external temperature, then the hot surface area of the heat exchanger is 250 cm². This number does not include the manifolding and the piping which adds additional 250 cm² of hot surface area. For reference the stack (without enclosure and compression system) measures 12 cm x 12 cm x 10 cm and has a (hot) surface area of 750 cm². To reduce the heat loss of solid oxide stack systems it has been proposed to integrate the heat exchangers with the stacks and thereby to reduce the heat losses from hot

manifolding and hot piping.

EP1602141 is an example of such a proposed invention and reveals a Solid Oxide Stack System that is modularly built, wherein the additional components such as the heat exchangers are directly arranged in the high-temperature fuel cell stack to avoid piping. Referring to Fig. 2 (Fig. 4 from EP1602141) cold air (luft) is being preheated by exchanging heat with warm off gas (abgas) coming from a post-combustion

(nachverbrenner) unit.

There are however fundamental limitation to the heat loss improvements which can be obtained with the invention in EP1602141. Fig. 2 shows that the heat exchanger

( luftvorwarmer) in EP1602141 is based on a traditional cross flow heat exchanger. In this case the heat exchanger will have a hot ^xleft' side and a cold ^xright' side as indicated in italics in Fig. 2. The heat exchanger is facing a hot component (nachverbrenner) on top and a cold component

(Vorreformer) at the bottom. This distribution of hot and cold surfaces leads to two issues.

1) In the configuration shown in EP1602141, the heat

exchanger will on the left side have hot surfaces (A) facing the relatively cold prereformer ("vorreformer") and have cold surfaces (B) facing the hot post combustor

("nachverbrenner") . Thermal insulation materials are

therefore necessary between the different elements to avoid high heat loss at the hot/cold interfaces A) and B) . To obtain a good thermal insulation between adjacent hot and cold surfaces, typically at least 0.1 mm/K of insulation materials will have to be used¹.

2) Due to thermal expansion and the different temperatures across the heat exchanger in the contact plane, the heat exchanger will move relative to the adjacent components during start up and cool-down of the system. With a 12 x 12 cm stack and a (steel) thermal expansion coefficient of 15 E- 6 (K^-1) , this relatively movement will be of the order of 1.5 mm if the post combustor reaches the stack operating

temperature of 750°C. If the prereformer, heat exchanger and post combustor are free to move relative to each other this is not necessarily an issue. However gas needs to flow between the adjacent components and the three elements therefore have to be fixed to each other as indicated in Fig. 3, where the prereformer, heat exchanger and post combustor are shown together with the necessary connections between these. Fig. 3 also indicate hot areas with ^xblack' colour and cold areas with ^xwhite' colour.

To summarise, the heat exchanger of invention in EP1602141 is therefore characterised by:

^■ At points A and B at least 0.1 mm x ΔΤ of insulation

materials have to be inserted. Here ΔΤ is the

temperature difference across the heat exchanger (hot inlet temperature - cold inlet temperature)

■ At points C and D the heat exchangers have to be fixed to the adjacent components. Since the heat exchanger

¹ Using for example high insulation materials from microthermgroup.com moves relative to the adjacent components during start^¬ up and cool down cycles, this fixation have to be of a solid nature e.g. with steel connections brazed to the heat exchanger and to the adjacent component.

Assuming a ΔΤ of 750°C and a simple realisation of the steel connection along the side of the stack measuring 12 cm (stack length) x 1.5 cm (for air holes and brazing zones) x 7.5 cm (insulation height) then the hot surface area of these steel connections become 135 cm. This is comparable to the surface area of the piping and manifolding of the traditional bulk heat exchanger design of Fig. 1.

Another challenge of the invention of EP1602141 is that Fig. 2 indicates flow transfer also at the hot/cold interfaces A and B, which are therefore not free to move. Consequently, the invention of EP1602141 is only suited to deliver the desired flows and provide the desired reduction in piping and manifolding surface areas for relatively low values of inlet/outlet temperature differences (ΔΤ) .

The maximum operating temperature and efficiencies of the heat exchanger of invention EP1602141 and similar designs are limited by the fact that the heat exchanger thermal gradient (left to right) is perpendicular to the thermal gradient (top to bottom) of the adjacent components in the stack assembly. Alternative system and heat exchanger designs are proposed in this invention which has thermal gradients oriented in the same direction (thermally matched) as those of the adjacent stack components. This allows the heat exchangers to operate efficiently also in systems with relatively high values of inlet/outlet temperature differences (ΔΤ) .

Here we propose to use thermally matched heat exchangers closely integrated with the other components of solid oxide stack systems. By thermally matched is here understood that the thermal gradients of the heat exchanger are oriented in the same direction as those of the adjacent stack components. This can also be expressed as a criterion that the maximum temperature difference δΤ between two adj acent /touching points of the heat exchanger and the adjacent components is substantially smaller than the maximum temperature difference ΔΤ across the heat exchangers, please refer to Fig. 4.

Such heat exchangers have the advantages of being able to practically eliminate the heat loss from manifolding and piping. Furthermore, they can also be used to:

1. Reduce heat loss from the compression system

2. Reduce the overall heat loss of the system as they can reduce the hot surface area of the system

3. Improve the efficiency of the electrical system

4. Reduce the heat exchanger cost as the assembly of these can be integrated with the stack assembly.

One example of an embodiment (Hex A) of thermally matched heat exchanger is shown in Fig. 5. The heat exchanger is in principle a ^xbended' counter flow plate heat exchanger. The flow propagates from one side ( ^xleft' or ^xright' ) to the other side ( ^xright' or ^xleft' ) and at the left and right edges, the flow is directed to another layer in the heat exchanger and the flow direction is reversed. Between each ^xlayer' of the heat exchanger, a thermally insulation layer is inserted. This layer could for example be just a hollow section using still air as insulation material. The number of layers ^λΝ' used in such a heat exchanger is determined by the maximum acceptable temperature difference δΤ . As each layer handles 1/Nth of the overall temperature difference ΔΤ, N can be expressed as N > ΔΤ/δΤ.

The heat exchanger proposed here is not only characterised by eliminating the heat loss from manifolding and piping, but it also have a very low effective surface area when integrated into solid oxide stack systems. To understand this equation 1 shows that if T (A) - T_ext (A) is low for a given surface, then the effective hot surface area from this surface is also low. This basically indicates that if two hot surfaces are placed face to face, then the heat loss of both can be eliminated.

To exemplify this, please consider the system in Fig. 6. A 'Hex A' type heat exchanger with a height of 3 cm is

connected to a 12 x 12 cm stack operating at 775°C. The heat exchanger is assumed to have a 25°C inlet from an adjacent manifold δΤ is assumed to be 75°C towards both the manifold and the stack facets. For the stand alone heat exchanger integration according to equation (1) of the 6 sides' yields: cm x 12 cm x (775 -75/2 25) °C 10.3 nr K (TOP) cm x 12 cm x (75/2) 0.5 m K (bottom)

4 x 12 cm x 3 cm x (775 25) 12 °C 5.4 ir K (sides) Leading to a hot surface area of (10.3 + 0.5 + 5.4) m K / (775 - 25) K = 216 cm2

In the stack integrated configuration, the heat loss from the hot side is reduced to 0.5 m² K and the system hot surface are is found as (0.5 + 0.5 + 5.4) m² K / (775 - 25) K = 85 cm2

An interesting aspect of this heat exchanger is that it can become an integrated part of the stack compression system and help to reduce the heat loss from the compression system, which in most solid oxide stack systems are substantial. Due to differences in thermal expansion coefficients between different stack components (e.g. cells and interconnects), external compression systems are typically applied to the two endplates of the stack. One example of heat loss from a compression system is shown in Fig. 7 and used for the system in Fig. 1. Here a steel shield is used to provide the stack compression. The steel shield is bolted to a bottom plate and provides the compression pressure through the stack through a compression mat. As this compression mat has a relatively high thermal conductivity, the steel shield has the same temperature as the stack inside. The surface area of the steel shield is 2000 cm2 or almost 3 times higher than the surface area of the stack. This implies that the hot

compression system will add significantly to the heat loss of the solid oxide stack system.

A very attractive alternative would be to use the proposed embodiment of a stack integrated thermally matched heat exchanger as an integrated part of the compression system. A Solid Oxide Stack System configuration which is very popular when several stacks are need is the so-called 'boxer' configuration. Here two stacks are placed on top of each other with a centre manifold and compression provided from the two ends as shown in Fig. 8a and 8b. One advantages of the 'boxer' configuration is that two hot stack surfaces are facing each other, thereby reducing the system heat loss compared to a configuration with two individual stacks.

Furthermore, a simple compression system providing

compression from the two ends can be used to compress the two stacks simultaneously. This is shown for a standard

configuration in Fig. 8a, indicating that in this case a ^xhot' compression system is needed as the compression will be applied directly at the two hot stack top/bottoms. An alternative and attractive configuration is shown in Fig. 8b. Here, the proposed heat exchanger (Hex a) is inserted between the stacks and the compression system. The heat exchangers have hot end surfaces towards the stacks and cold end surfaces opposite the stacks towards the compression systems. Such heat exchangers could provide cold interfaces for the compression system and thereby in practise eliminate the heat loss from the compression system.

Another source of heat loss in Solid Oxide Stack Systems is the electrical wiring. Typically, the electrical wiring is connected to the hot bottom and top-plates of the stacks as indicated in Fig. 9A. To reduce the ohmic losses of these wires they typically have a relatively broad cross section and hence a considerable thermal conductivity.

By using electrically conductive stack integrated heat exchangers like Hex A and connecting the electrical wires to the cold surfaces, this heat loss would be eliminated as indicated in Fig. 9B .

To give an example of the typical ohmic loss of a heat exchanger, consider again a heat exchanger measuring 12 x 12 x 3 cm. As the thermally matched heat exchangers can be connected directly to the stacks without the use of gaskets, the electrical resistance to the stack is identical to the electrical resistance of the heat exchanger. The heat exchanger plates will be electrically connected in the joining areas which can be assumed to be 12 cm long and a 1 cm width. Using a resistivity of steel of 12E-6 Ohm cm, the Hex A resistance R_Hex to the stack from the cold heat

exchanger surface is found to be:

R_Hex = 12E-6 Ohm cm x 3 cm / (12 cm x 1 cm) = 3 E-6 Ohm

Transmission of even a relative large SOEC current of 100A across the heat exchanger will hence only cause a negligible additional voltage drop.

A further and quite significant loss mechanism of Solid Oxide Stack Systems is the DC/AC conversion part of the system, where the power loss can easily be of the order of 5%. Here it is well-known that for both DC/AC and AC/DC conversion, the highest conversion efficiencies for a given electrical power is obtained for the highest possible (switching) voltages. This is similar to the well-known phenomena that the lowest transmission loss of electrical power is achieved for high voltage transmission lines. To reduce switching loss it is therefore advantageous to connect several stacks in series electrically to obtain a high voltage for the DC/AC converter, as indicated for an extended boxer configuration in Fig. 9b. Typically, the compression system has to be electrically grounded, which leads to relatively high voltages between the end stack facets and the compression system, point I) and II) in Fig. 9b. This set limits to the maximum voltage in traditional configurations as there are practical limitations to finding electrical insulation materials which can operate

simultaneously at high voltages, high temperatures and at high pressure during temperature cycles.

By applying an electrically conductive stack integrated heat exchanger with a hot side against the stack and a cold surface towards the compression system, it is however

possible to have the electrical interfaces of the stack at a cold surface and hence to provide the insulation between the stacks and the compression system at much colder

temperatures.

A significant advantage of the Hex A heat exchanger is that it can be assembled in the same assembly processes used for assembling the stacks. As an example Hex A can be

manufactured by stacking of metal plates and spacers with the same foot print and from the same materials as those used for interconnects and spacers in the stack. The heat exchanger can be assembled by the same process (e.g. robot controlled stacking) as the fuel cell stack and the heat exchanger elements can be joined by the same methods used for the stack, e.g. glass soldering, Ni-brazing or diffusion bonding. By using metal components for the heat exchangers it is assured that they as generally desired are electrically conductive .

Another preferred embodiment of thermally matched, stack integrated heat exchangers is shown in Fig. 10. This

thermally matched stack integrated heat exchanger is placed at one of the sides of the stack and uses a counter flow plate heat exchanger configuration. In Fig. 10, the cold inlet is connected to the top of the heat exchanger at the ^xcold' right side and the hot ^xleft' side of the heat exchanger faces the stack. The hot flow between the stack and the heat exchanger goes through a manifolding plate at the bottom of the stack. Hex B can be separate from the stack and for example an external heater can be inserted between Hex B and the stack. Hex B can also be an integrated part of the stack and for example share components such as spacers and interconnects. This will require electrically insulating parts to be included in the heat exchanger design as described in the following.

Fig. 11 shows an embodiment of this invention, where the heat exchanger is realised from stack components by extending the length of interconnects and spacers. One set of heat

exchanger plates can then be based on extensions of the interconnects. To avoid electrical short circuiting between the cells in the stack, every other set of heat exchanger plates (corresponding to the extensions of the cells) have to be based on a material which is electrically nonconductive in the vertical direction. This could for example be a ceramic plate or a ceramically coated metal sheet. The two sets of heat exchanger plates can be separated by extended spacers, which are also used to lead the desired flows across the different heat exchangers plates.

As for Hex A, the heat exchanger plates of Hex B can be assembled in exactly the same process as the stack and similar methods can be used for joining the heat exchanger and stack components. Such joining methods could for example be glass sealing/soldering, brazing or diffusion bonding.

As for Hex A, Hex B also can also have a low hot surface area.

To exemplify this, please consider the system in Fig. 10. A 'Hex B' type heat exchanger with a length of 6 cm is

connected to a 12 x 12 cm stack with a height of 10 cm operating at 775°C. The heat exchanger is assumed to have inlet and outlet flow temperatures of 25 and 50°C,

respectively .

For the stand alone heat exchanger, integral of the hot surface area according to equation (1) can be found as:

12 cm x 10 cm x (775 - 25) °C = 9 m² K (hot surface) 12 cm x 10 cm x (50 - 25) °C = 0.3 m² K (cold surface) 4 x 10 cm x 5 cm x (775 - 25) /2 °C = 7.5 m² K (sides)

This gives a hot surface area of:

(9 + 0.3 + 7.5) m² K/ (775 - 25) = 225 cm²

In the stack integrated configuration, the heat loss from the hot surface is eliminated and the system hot surface are is found as 105 cm² The Hex B heat exchanger can also be realised with low electrical resistance. Every other heat exchanger plate can be realised as a metal plate and thereby be electrically conductive. This makes it possible for example to have cold electrical connections to the top or bottom plates of the stack from the top or bottom plate on the heat exchanger. To give an example of the typical ohmic loss, consider again a heat exchanger measuring 12 x 10 x 5 cm. The heat exchanger plates are assumed to be 0.3 mm thick. In this case the Hex B resistance R_Hex to the stack from the cold heat exchanger surface is found to be:

R_Hex = 12E-6 Ω cm x 5 cm / (12 cm x 0.03 cm) = 1.7 E-4 Ω

A very interesting feature of this heat exchanger embodiment is that it is possible to have cold electrical connections to each interconnect in the stack. This can be used to provide a cold environment for electrical components used to control the current through individual cell or groups of cells. Such current control can for example be used to adjust the current over individual cells or cell groups in order to obtain very high fuel utilisations for all cells or cell groups in a stack. Another application could be to switch off the current in SOEC mode for cell or cell groups with defect (leaking) cells. This would avoid the excessive heating of defect cells to destroy adjacent cells.

The proposed heat exchanger embodiments can be used in many configurations to realise different SOFC, SOEC and even combined SOFC/SOEC configurations. One example is shown in Fig. 12. In Fig. 12a four stacks are placed close to each other each sharing two hot sides with neighbouring stacks. Each stack is also connected to one Hex A and one Hex B, providing cold external interfaces for all in-plane sides of this configuration. It is furthermore possible to cascade several similar configurations in the out-off-plane direction thereby realising a system with very few hot external

surfaces. For SOEC purposes it can be advantageous to include stack integrated heaters between the stacks, these are also indicated in Fig. 12.

Solid oxide stack configurations with two heat exchangers are relevant for both SOEC and SOFC systems. This is exemplified in Fig . 13 and 14.

Fig. 13 shows a simple SOEC configuration which can be realised with two (integrated) heat exchangers and a heater. Fuel (e.g. H20) is fed to the stack through two inlets, where the flows can be adjusted independently. The cold flow (e.g. 101°C steam) from fuel inlet 1 is heated in Hexla by the hot fuel outlet flow coming from the stack. In the same manner the cold flow from fuel inlet 2 is heated in Hex2 by the hot oxygen outlet flow from the stack. No inlet flushing of the oxygen side is used and the output from the oxygen side is therefore 100% 02.

The two fuel inlets are combined and the temperature of the combined flow is increased in a heater before the fuel fed to the stack. The stack is assumed to operate close to the thermo-neutral point and the temperatures of the output flows will therefore be close to the stack operating temperature. The heat exchangers are not ideal and some heat will be lost to the environment, and these heat losses are compensated by the heater. The main motivation for this configuration is that it makes it possible to balance the inlet thermal mass to the outlet thermal mass for all current levels. This is probably best demonstrated by two examples. Assuming that a 75 cell stack, with a 100 cm2 active area per cell is used. The stack can operate at currents up to 70A with a fuel utilisation up to 70%. In this case the fuel input flow to the stack should be 3.4 Nm3/h (steam) .

When the system is operating at maximum load, then 2.4 Nm3/h H20 is converted to H2 and at the same time 1.2 Nm3/h 02 is produced. To achieve the most efficient heat recovery in the two heat exchangers, the thermal mass of the inlet flows should be adjusted to match the thermal mass of the outlet flows. With the specific heat of steam being 37.5 J/ (mol K) and the specific heat of oxygen being 29.4 J/ (mol K) , then the thermal mass balancing flow of inlet 2 should be 1.2 Nm3/h x 29.4/37.5 = 0.94 Nm3/h. The balanced flow of inlet 1 then becomes 3.4 Nm3/h - 0.94 Nm3/h = 2.46 Nm3/h

If no electrolysis is applied (the system is idle) then the most efficient heat recovery is obtained if all the fuel is fed through fuel inlet 1, where it in Hexl can interact with a similar hot outlet flow.

Fig. 14 shows a simple SOFC configuration which can be realised with two (integrated) heat exchangers. Fuel (e.g. H2) is fed to the stack through fuel inlet 1 and the cold inlet flow is heated in Hexl by the hot fuel outlet from the stack. In the same manner the cold air inlet flow is heated in Hex2 by the hot air outlet flow from the stack. In contrast to the situation in SOEC mode, then it is

desirable that the two inlet flows are colder than the stack operating temperature as the colder inlet flows provide the necessary cooling of the stack. To give an example of typical operating parameters then a 75 cell stack, with an electrical output of 1.5 kW, an electrical efficiency of 50% and a fuel utilisation of 70% can be considered. With a lower heating value of hydrogen of 3.5 kWh/Nm3, then the SOFC needs a hydrogen input of 1.5 kW / 3.5 kWh/Nm3 / 50%/ 70% = 1.8 Nm3/h H2

Assuming a stack inlet temperature of 650°C, a outlet

temperature of 850°C, and a specific heat of air of 29 J(mol K) then it is found that roughly 21 Nm3/h of air is needed to provide cooling for the 1.5 kW of heat produced by the stack. In practise this will be somewhat lower as some cooling typically also will be provided by the fuel inlet and there will be some heat lost to the environment.

It is also possible to realise solid oxide stack system with more than two heat exchangers per stack using these

integrated heat exchangers. One possible embodiment is shown in Fig. 12b. Here there are 12 stacks and 14 heat exchangers arranged in a very compact configuration. Using manifolding plates for fuel, air and oxygen distribution, it is possible for several stacks to share one heat exchanger and thereby realise for example the combined SOEC and SOFC system shown in Fig. 15. This configuration is the common denominator of the SOEC and SOFC configurations shown in Fig. 13 and 14. When operating in SOEC mode the air inlet and outlets are shut off (on the cold side of Hex3) and the system is operating exactly as the SOEC system in Fig. 13.

When operating in SOFC mode the Oxy outlet and the fuel inlet 2 are shut off (on the cold side of Hex 2), the heater is shut off and the system is operating in a way very similar to the configuration of Fig. 14. The only difference is that in most cases Hexla is larger than Hexlb, implying that the temperature of the fuel fed to the stack is higher in Fig. 15 than in the typical SOFC configuration. This means that a marginally higher air flow is needed to cool the stack.

However, as the fuel flow is much smaller (more than one order of magnitude) than the airflow this increased airflow will only have a negligible influence on the SOFC system efficiency .

To implement the system configuration of Fig. 15 with the integrated stacks shown in Fig. 12b, then it is possible to use hex B type heat exchangers for the air flow (Hex 3) as Hex B type heat exchangers can easily be designed for low pressure drop, which is less simple with the Hex A type design .

On the other hand in might be advisable to use Hex A type heat exchangers for the main fuel heat exchangers (hex lb) as these are used in both SOEC and SOFC mode and therefore can provide cold interfaces for the compression system for both operating modes. Features of the invention

1. A solid oxide fuel cell (SOFC) system comprising one or more SOFC stack (s) and one or more stack-integrated, temperature-matched heat exchanger (s) in direct physical contact on at least 1/12, preferably at least 1/6, preferably at least 1/3 of the heat exchanger surface area with the SOFC stack (s), wherein the maximum

temperature difference δΤ between two adjacent and contacting points of the heat exchanger and the adjacent SOFC stack (s) is smaller than the maximum temperature difference ΔΤ across the heat exchanger ( s ) , wherein δΤ/ΔΤ < 50%, preferably δΤ/ΔΤ < 20% preferably δΤ/ΔΤ < > o ·

A solid oxide fuel cell system according to feature 1, wherein the hot surface area of the system (HSA_system) relative to the hot surface area of the heat

exchanger (s) in a not integrated, stand-alone (HSA_sa) situation is HSA_system/HSA_sa< 1, preferably HSA_system/HSA_sa 0.3, preferably HSA_system/HSA_sa < 0.1.

A solid oxide fuel cell system according to feature 1, wherein the heat exchanger (s) have a temperature

difference, ΔΤ, between the maximum and minimum inlet temperatures of ΔΤ > 300°C, preferably ΔΤ > 450°C, preferably ΔΤ > 600°C.

. A solid oxide fuel cell system according to feature 1, wherein at least one of the heat exchangers is used for feeding current to the stack (s) or to and from the stack (s) and the at least one of the heat exchangers has an electrical resistance R_Hex between an cold surface and a stack element of R_Hex < 1 mOhm, preferably R_Hex < 0.1 mOhm, preferably R_Hex < 0.01 mOhm.

A solid oxide fuel cell system according to feature 4, wherein the stack element is the top or the bottom plates of the stack.

A solid oxide fuel cell system according to feature 4, wherein the stack element is a heater.

A solid oxide fuel cell system according to feature 4, wherein the stack element is cells or cell groups and current control of these is performed at cold interfaces of the at least one heat exchanger.

A solid oxide fuel cell system according to feature 1, wherein stack compression is applied to the cold surface of at least one of the heat exchangers.

A solid oxide fuel cell system according to feature 8, wherein the stack compression is at least 200 mBar.

A solid oxide fuel system according to feature 1, wherein the assembly of the integrated heat exchangers is integrated with the stack assembly, at least one of the heat exchangers share at least an interconnect or a spacer .

A solid oxide fuel system according to feature wherein the hot surfaces of the heat exchanger (s) the stack operating temperature or at maximum 50 °C colder .

12. A solid oxide stack system, applying at least on heat exchanger according to feature 1, where the thermal management system is designed in a way where the system can operate with high efficiency in both SOEC and SOFC mode . 13. A solid oxide stack system according to feature 12, wherein the thermal management system can operate with as few as three heat exchangers .

A solid oxide stack system according to feature 12, where the thermal management system can operate without any hot valves.

Claims

A solid oxide fuel cell (SOFC) system comprising one or more SOFC stack (s) and one or more stack- integrated, temperature-matched heat exchanger (s) in direct physical contact on at least 1/12, preferably at least 1/6, preferably at least 1/3 of the heat exchanger surface area with the SOFC stack (s), wherein the maximum temperature difference δΤ between two adjacent and contacting points of the heat exchanger and the adjacent SOFC stack (s) is smaller than the maximum temperature difference ΔΤ across the heat exchanger ( s ) , wherein δΤ/ΔΤ < 50%, preferably δΤ/ΔΤ < 20% preferably δΤ/ΔΤ < 5%.

A solid oxide fuel cell system according to claim 1, wherein the hot surface area of the system (HSA_system) relative to the hot surface area of the heat exchanger (s) in a not integrated, stand-alone (HSA_sa) situation is HSA_system/HSA_sa< 1, preferably

HSA_system/HSA_sa < 0.3, preferably HSA_system/HSA_sa < 0.1.

A solid oxide fuel cell system according to claim 1, wherein the heat exchanger (s) have a temperature difference, ΔΤ, between the maximum and minimum inlet temperatures of ΔΤ > 300°C, preferably ΔΤ > 450°C, preferably ΔΤ > 600°C. A solid oxide fuel cell system according to claim 1, wherein at least one of the heat exchangers is used for feeding current to the stack (s) or to and from the stack (s) and the at least one of the heat exchangers has an electrical resistance R_Hex between an cold surface and a stack element of R_Hex < 1 mOhm, preferably R_Hex < 0.1 mOhm, preferably R_Hex < 0.01 mOhm.

A solid oxide fuel cell system according to claim 4, wherein the stack element is the top or the bottom plates of the stack.

A solid oxide fuel cell system according to claim 4, wherein the stack element is a heater.

A solid oxide fuel cell system according to claim 4, wherein the stack element is cells or cell groups and current control of these is performed at cold

interfaces of the at least one heat exchanger.

A solid oxide fuel cell system according to claim 1, wherein stack compression is applied to the cold surface of at least one of the heat exchangers.

A solid oxide fuel cell system according to claim wherein the stack compression is at least 200 mBar

10. A solid oxide fuel system according to claim 1,

wherein the assembly of the integrated heat

exchangers is integrated with the stack assembly, at least one of the heat exchangers share at least an interconnect or a spacer. A solid oxide fuel system according to claim 1, wherein the hot surfaces of the heat exchanger (s) are at the stack operating temperature or at maximum 50 °C colder .