WO2025040568A1

WO2025040568A1 - Method and system for operating a solid oxide cell unit

Info

Publication number: WO2025040568A1
Application number: PCT/EP2024/073043
Authority: WO
Inventors: Christian Wix; Catalin losif CIONTEA; Thomas Heiredal-Clausen
Original assignee: Haldor Topsoe AS
Current assignee: Topsoe AS
Priority date: 2023-08-18
Filing date: 2024-08-16
Publication date: 2025-02-27
Anticipated expiration: 2026-02-18

Abstract

A method and system for operating a solid oxide cell (SOC) unit, the method comprising the steps of: i) providing a power supply unit (PSU) comprising a rectifier and converting an AC-current to a DC-current; ii) providing a solid oxide cell (SOC) unit comprising one or more SOC stacks, supplying steam to the one or more SOC stacks and serially connecting said one or more SOC stacks to said rectifier by providing said DC- current to the one or more SOC stacks, thereby powering the one or more SOC stacks for operation in electrolysis cell mode; and outputting hydrogen from said one or more SOC stacks; iii) interrupting said DC-current to the one or more of the SOC stacks, optionally interrupting said supply of steam to the one or more SOC stacks, and supplying a fuel source to the one or more SOC stacks, thereby switching the one or more SOC stacks from operation in electrolysis cell mode to operation in fuel cell mode; and outputting a DC-current from said one or more SOC stacks; iv) providing an electric device such as an electric heater and connecting said electric heater to said one or more SOC stacks operating in fuel cell mode, by directly providing said DC-current from said one or more SOC stacks to the electric heater. The invention provides also a system for carrying out the method.

Description

Title: Method and system for operating a solid oxide cell unit

The present invention relates to an improved method and system for operating a solid oxide cell (SOC) unit by switching between electrolysis cell mode (SOEC) and fuel cell mode (SOFC).

Electrical direct current (DC) power supply units are conventionally used in industrial applications for providing DC current and voltage for e.g. electrolysis units, such as solid oxide electrolysis cell stacks (SOEC stacks) of a SOEC unit. Thus, conventionally, electrical DC power from alternating current (AC) mains is provided by first converting the mains AC voltage into a quasi - DC voltage, and then adapting this voltage to the desired DC-load voltage with a DC-DC converter. A low-pass filter such as an inductor or capacitor may be provided in between both steps to stabilize the DC bus voltage, smoothen the AC mains current and attenuate any EMI (electromagnetic interference) noise, i.e. attenuate electromagnetic noise.

It is also known that it is advantageous for the electrolysis cell unit lifetime to switch between electrolysis cell mode (SOEC mode) and fuel cell mode (SOFC mode). The excess power from the SOFC mode is sent back to the grid via a DC-AC converter. Thus, a DC-AC converter is provided for this purpose, which is typically associated with a power loss of about 5%.

Applicant’s WO 2016000957 discloses (Fig. 6) a system for a situation where the SOEC cell voltage is below 1 .29 V and thus below the thermoneutral potential for splitting steam. The system comprises a rectifier for converting AC current to DC current, a plurality of SOEC stacks arranged in series, an electrical heater and a converter in which its output power is used in the electrical heater. A fast switch connects the electrical heater or converter with the electrolysis stack loads. The excess power from the converter is conventionally sent to the grid.

It would be desirable to provide an improved method and system for powering of a solid oxide electrolysis cell (SOEC) stack. It would also be desirable to provide an improved method of operating a solid oxide cell (SOC) unit in fuel cell mode for very short time to extend the lifetime of the cells.

Accordingly, in a first aspect of the invention there is provided a method for operating a solid oxide cell (SOC) unit, comprising the steps of: i) providing a power supply unit (PSU) comprising a rectifier and converting an AC-current to a DC-current; ii) providing a solid oxide cell (SOC) unit comprising one or more SOC stacks, supplying steam to the one or more SOC stacks and serially connecting said one or more SOC stacks to said rectifier by providing said DC-current to the one or more SOC stacks, thereby powering the one or more SOC stacks for operation in electrolysis cell mode; and outputting hydrogen from said one or more SOC stacks; iii) interrupting said DC-current to the one or more SOC stacks by: iii-1) interrupting said supply of steam to the one or more SOC stacks, and supplying a fuel source to the one or more SOC stacks; or iii-2) maintaining the supply of steam, together with a fuel source, to the one or more SOC stacks; thereby switching the one or more SOC stacks from operation in electrolysis cell mode to operation in fuel cell mode; and outputting a DC-current from said one or more SOC stacks; iv) providing an electrical device and connecting said electrical device to said one or more SOC stacks operating in fuel cell mode, by directly providing said DC-current from said one or more SOC stacks to said electrical device.

For the purposes of the present application:

The term “comprising” includes also “comprising only”, i.e. “consisting of”.

The term “power supply unit (PSU)” means a unit that converts mains AC to low-volt- age regulated DC power for electrolysis. For instance, the PSU may comprise a rectifier, a low frequency filter and a DC/DC converter.

Suitably, the PSU comprises only a rectifier. The term “suitably” means “optional”, i.e. an optional embodiment.

The term “outputting” means “producing”. For instance, “outputting hydrogen from said one or more SOC stacks” means “producing hydrogen from said one or more SOC stacks”. For instance, “outputting a DC-current from said one or more SOC stacks” means “producing a DC-current from said one or more SOC stacks”. The term “an electrical device” means a device arranged to receive the DC-current from said one or more SOC stacks, such as an electrical device of a plant or process comprising the SOC unit or comprising a SOC section, the SOC section (herein also re referred to as “system”) comprising a plurality of SOC units.

For instance, the term “an electric heater” means any electric heater, such as an electric heater of a plant or process comprising the SOC unit or comprising a SOC section, the SOC section comprising a plurality of SOC units, for instance an electric heater for heating up a reactor or a boiler of said process or plant; or an electric heater directly associated with the one more SOC stacks of the SOC unit. The process or plant is for instance a process or plant for producing ammonia, or methanol, or hydrogen. As used herein, the term “directly associated” means that the heat generated in the electric heater is integrated into any of the SOC stacks or the SOC unit or the SOC section. For instance, the heat generated in step iv) is transferred to the at least one SOC stack operating in electrolysis cell mode. For instance, the hydrogen recycle blower is directly associated with the one more SOC stacks of the SOC unit, and thereby with the SOC unit, by the hydrogen being at least a portion of said hydrogen from step ii). For instance, the hydrogen compressor is directly associated with the one more SOC stacks of the SOC unit, and thereby with the SOC unit, by the hydrogen being at least a portion of said hydrogen from step ii).

The term “directly providing said DC-current from said one or more SOC stacks to the electrical device” means that the nature of the DC-current is maintained, i.e. not changed, prior to being provided to the electrical device. For instance, the term “directly providing said DC-current from said one or more SOC stacks to the electric heater” means that the nature of the DC-current is maintained, i.e. not changed, prior to being provided to the electric heater. For instance, the DC-current is not converted into AC current prior to being provided to the electric heater. For instance, a switch may be provided, as this does not change the nature of the DC-current. Accordingly, the method of the invention does not comprise providing said DC-current to a converter, such as a DC-AC converter, prior to e.g. the electric heater, i.e. prior to supplying to e.g. the electric heater.

The term “present invention” or simply “invention” may be used interchangeably with the term “present application” or simply “application”.

The term “first aspect of the invention” refers to a method for operating a SOC unit. The term “second aspect of the invention” refers a system for carrying out the method.

The term “system” in accordance with the second aspect of the invention means a SOC section.

The term solid oxide cell (SOC) unit encompasses a solid oxide electrolysis (SOEC) unit and a solid oxide fuel cell (SOFC) unit. The SOC unit may thus be regarded as a reversible SOC unit. It is understood that a SOC unit comprises one or more SOC stacks. A SOC stack comprises a plurality of cells. For instance, a SOC unit comprises an array of SOC stacks, and a SOC stack comprises a plurality of cells arranged on top of each other. A SOC section comprises an assembly of SOC units, for instance a sequence of SOC units.

The term “and/or” means in connection with a given embodiment any of three options. The term “and/or” may be used interchangeably with the term “at least one of” the three options.

The use of the article “a” in connection with an item, means “at least one”. For instance, a SOC unit means one or more SOC units. For instance, a SOC stack means one or more SOC stacks. The use of the term “SOC stack(s)” means one or more SOC stacks. The use of the term “SOC unit(s)” means one or more SOC units.

Where there is more than one item, for instance more than one SOC stack, the term “plurality” or “several” may be used interchangeably. For instance, a SOC unit comprises a plurality of SOC stacks, i.e. several SOC stacks, such as an array of SOC stacks. For instance, a SOC section comprises a plurality of SOC units, i.e. several SOC units, such as an assembly of SOC units, for instance a sequence of SOC units. The term “conduit” means a process line, such as a pipe, carrying a given process stream. The term “conduit” may also signify an electrical conduit.

Other definitions are provided in connection with one or more of the above or below embodiments. According to the present invention, as recited above, instead of sending power back to the grid via a DC-AC converter provided after the SOC unit, i.e. downstream the SOC unit, the power generated during fuel cell mode operation is directly consumed in an electrical device connected to the stack, such as an electric heater connected to the stack. The provision of said DC-AC converter which may include an inverter and associated electrical equipment is thus omitted. This is highly advantageous, since the conversion of DC to AC is more complex than converting AC to DC, the former requiring more complicated electronic equipment. It is understood that present invention does not exclude the provision of a DC-DC converter, e.g. to adapt the output DC-current from said one or more SOC stacks to the current required by the heater, since the nature of the DC-current is maintained, i.e. it is still a DC-current. Further, from AC mains, the PSU being a rectifier, optionally also a capacitor, may suffice for converting the AC into DC current, without requiring other typically associated units such as low-pass filter to eliminate fluctuations and/or DC-DC converter for finally adapting to the desired level.

The invention thus enables an improved way, i.e. a simpler way, of operating a SOC electrolysis unit (SOEC unit) in fuel cell mode for very short time at a constant frequency to extend the lifetime of the electrolysis cells by minimizing nickel-migration therein, while at the same time enabling e.g. heat integration where the electrical device is an electric heater.

In an embodiment, the electrical device is at least one of:

- an electric heater;

- the one or more stacks operating in electrolysis cell mode of step ii);

- a blower, such as a hydrogen recycle blower in which the hydrogen is at least a portion of said hydrogen from step ii);

- a compressor, such as a hydrogen compressor in in which the hydrogen is at least a portion of said hydrogen from step ii).

The electrical device, such as the electric heater may be integrated in the system, the system being a SOC section comprising a SOC unit, i.e. one or more SOC units. The electrical device, such as the electric heater, may be external to the system and arranged within a process or plant comprising the system, such as a plant for producing any of ammonia, methanol, or hydrogen. For instance, heat generated in the electric heater is transferred to a process line and/or to a process unit of the plant; for instance, as mentioned above, for heating up a reactor or a boiler of said process or plant.

In an embodiment, in step iv) said DC-current is consumed within the same SOC-unit where the DC-current is output, i.e. produced. This enables a much simpler configuration of the SOC-unit.

In an embodiment, step iv) comprises serially connecting, said electrical device to said one or more SOC stacks operating in fuel cell mode. In particular, the one or more SOC stacks operating in fuel cell mode are electrically connected in series providing the DC-current to the electrical device, thereby powering the electrical device, the electrical device being fluidly connected in series to said SOE stacks. In other words, the one or more SOC stacks operating in fuel cell mode are electrically connected in series delivering power to an electrical device that is fluidly connected in series to said SOE stacks.

This excludes powering (delivering DC-current) other SOE stacks as these are fluidly connected in parallel.

It is understood that the SOC stack(s) are electrically connected to the electric device, such as the electric heater. Suitably, where a plurality of SOC stacks is provided, i.e. where several stacks are provided, these can be connected in series. Accordingly, in an embodiment, the one or more SOC stacks are connected in series to said electric device, such as to said electric heater.

In an embodiment, in step iii-2), said fuel source is hydrogen (H2), and said supply of steam (H2O), together with a fuel source, is a H2/H2O gas mixture comprising 1-20 vol.% H₂.

This provides a simpler method of operating the SOC unit and thereby the SOC section, as there is no need to interrupt the steam supply. In an embodiment, in step iii-2), said fuel source is hydrogen (H2), and said supply of steam (H2O), together with a fuel source, is a H2/H2O gas mixture comprising 1-10 vol.% H2, e.g. 2, 3, 4, 5, 6, 7, 8, 9 vol.% H2. This ^-concentration, albeit low, for instance 3-7 vol.% H2, such as 4 vol.% H2, is sufficient as fuel source. This enables to run a small SOFC-current in a supply of fuel and steam, such as an existing supply of a mixture of fuel and steam, thereby also providing a simpler method of operating the SOC unit, as there is no need to interrupt the steam supply.

In an embodiment, in any of step iii-1) or iii-2), said fuel source is a portion of said hydrogen from step ii). Thereby the hydrogen required for the quick operation in fuel cell mode is advantageously sourced internally. In a particular embodiment, said portion is the entire portion.

The present invention, as already recited, enables utilization of SOFC-generated power, i.e. the DC-current from said one or more SOC stacks operating in fuel cell mode, directly internally in the SOC unit or SOC section, instead of converting it to AC- current, exporting it to the grid, importing it again, and converting it back to DC-current.

Electrical device(s) capable of operating with the generated DC-current from fuel cell operation, are provided, namely: the electric heater; or the one or more stacks operating in electrolysis cell mode of step ii); or the blower, such as a hydrogen recycle blower in which the hydrogen is at least a portion of said hydrogen from step ii); or the compressor, such as a hydrogen compressor in in which the hydrogen is at least a portion of said hydrogen from step ii).

Hence, the invention enables also the powering of SOC stacks running in SOEC mode, or to the powering of a blower such as the hydrogen recycle power, or to the powering of a compressor. For instance, the hydrogen compressor may be directly powered by DC-current, in which at least part of the DC-current is supplied by the SOC stacks running in SOEC mode. This can be achieved by connecting the compressor motor to a DC power source, such as a DC power supply or a DC bus in the system (SOC section) or the SOE unit. This approach eliminates the need for an AC-to-DC conversion. It is understood that hydrogen produced may be recycled via the hydrogen recycle blower and combined with the steam, thereby e.g. providing the H2/H2O gas mixture comprising 1-20 vol.% H2.

It is understood that hydrogen produced may be withdrawn as hydrogen product, suitably the portion of hydrogen that is not recycled via the hydrogen recycle blower, by compressing the hydrogen in the hydrogen compressor.

In an embodiment, the method comprises: in step ii) all SOC stacks in the SOC unit operating in electrolysis cell mode, and in step iii) all SOC stacks switching to operation in fuel cell mode. Hence, all stacks in the SOC unit are first operated in electrolysis cell mode, thus the SOC unit is operated as a SOEC unit, and after switching to fuel cell mode, all stacks in the SOC unit are then operated in fuel cell mode, thus the SOC unit is operated as a SOFC unit. Hence, in connection with step iii), the PSU interrupts the current and optionally a switch, i.e. one or more switches, route the fuel cell power to the electric heater.

Hence, in a SOC assembly comprising a plurality of SOC units, a SOC unit operates in fuel cell mode and a SOC unit operates in electrolysis cell mode.

In an embodiment, the method comprises at least one SOC stack operating in fuel cell mode and at least one SOC stack operating in electrolysis cell mode. Accordingly, in an embodiment, as recited above, a SOC unit operates in fuel cell mode and a SOC unit operates in electrolysis cell mode; or in another embodiment, for instance one SOC stack of a SOC unit operates in fuel cell mode, while the other SOC stacks operate in electrolysis cell mode. Hence, the PSU delivers constant current, yet optionally a switch, i.e. one or more switches, may control the stacks so that at least one of the stacks is always in fuel cell mode. A switch is herein also referred to as “electrical switching device” or “electrical switch” (“El switching”). Suitably, a switch is directly associated to each stack. Hence, stack 1 has a 1^st switch directly associated thereto, stack 2 has a 2^nd switch directly associated thereto, and so forth until the last stack, stack n, having a n-switch directly associated thereto. For instance, an operation mode is that a SOC stack is alternately operated in fuel cell mode and electrolysis cell mode. For instance, now more specifically, as shown in appended Fig. 3, a SOE stack alternates between operation in electrolysis mode and fuel cell mode. The states described follow an order where: state (a) corresponds to Stack 1 of the SOC unit operating in fuel cell mode and the other stacks in electrolysis mode, state (b) corresponds to Stack 2 of the SOC unit operating in fuel cell mode and the other stacks in electrolysis mode, and state (c) corresponds to Stack n of the SOC unit operating in fuel cell mode and the other stacks in electrolysis mode; thus alternating the stacks operating in electrolysis mode and fuel cell mode. The DC-current from Stack 1 is routed directly to the heater. The DC-current from Stack 2 is routed directly to the heater. The DC-current from Stack n is routed directly to the heater.

In an embodiment, the electrical device is an electric heater and the heat generated in step iv) is transferred to the at least one SOC stack operating in electrolysis cell mode, e.g. to at least one SOC unit operating in electrolysis cell mode. It is hereby understood that the heat generated is transferred to the at least one SOC stack operating in electrolysis cell mode or to the SOC unit. It is also understood that the term “heat” comprises latent heat, such as that associated with the evaporation of steam. This is particularly advantageous where the SOC unit operates in electrolysis cell mode, i.e. as a SOEC unit, and the cell voltage is below the thermoneutral potential for splitting steam (1.29 V). The SOEC unit is cooled during operation and thus requires heating, which typically is provided by external sources. Contrary to the prior art, as in above-mentioned applicant’s WO 2016/000957 (Fig. 6 therein), in which the SOE stacks operate in electrolysis cell mode and output power of a converter is used in the electric heater, the method of the present invention is much simpler as at least one of the SOC stacks is switched to operation in fuel cell mode, thus directly driving the electric heater instead of indirectly via a converter, for instance a DC-power converter (e.g. DC-DC converter) which is coupled in series with the electrolysis cell stacks and used in current control mode to control the current through the stacks. While the above approach is envisaged, yet again it is understood that present invention does not exclude the provision of a DC-DC converter to adapt the output DC-current from said one or more SOC stacks to the current required by the heater, since the nature of the DC-current is maintained, i.e. it is still a DC-current. At the same time, the power converter removes the excess power, defined as the power provided by the rectifier but not used by the electrolysis cell stacks. The converter may also be a combination of a DC-DC converter and a DC/AC converter, to enable the excess power being fed back to the grid. Again, the present invention enables i.a. omitting the provision of this converter (DC/AC converter).

In an embodiment, the time of operation in said electrolysis cell mode is at least two times, such as at least three times, or at least five times, or at least ten times, longer that the time of operation in said fuel cell mode. In connection thereto, in an embodiment, the time of operation in electrolysis cell mode is 1 min - 24 hours, i.e. 1 min to 1 day, and the time of operation in fuel cell mode is 0.10-60 seconds, i.e. 0.10 seconds to 1 min. In a particular embodiment, the time of operation in electrolysis cell mode is 1-30 minutes and the time of operation in fuel cell mode is 10-30 seconds.

Hence, the switching into fuel cell mode, is suitably conducted in the order of seconds or fractions of a second, while the operation in electrolysis cell mode is in the order of minutes or hours. Hence, the fuel cell operation mode is conducted for a very short time compared to operation in electrolysis cell mode, for thereby at the same time extending the lifetime of the cell. Heat integration, where the electrical device is an electric heater, while at the same time extending the lifetime of the cells is thus achieved.

Steps i) and ii) are conducted under normal operation. The term “normal operation” means that the normal stack operation temperature, herein also referred to as normal operating temperature, is suitably 600-1000°C, such as 700, 750°C or 800°C, which is the temperature used during production of hydrogen in a stack with a continuously applied electrolysis current. It would also be understood, that step iii) in which there is the switch into fuel cell mode, corresponds to a transient operation, which means non-con- tinuous operation of the corresponding SOC stack(s), thereby also of the associated SOC unit(s), such as where the stack has not reached a steady state corresponding to normal operation of the SOEC, including the supply of a continuous current.

Accordingly, in an embodiment, the one or more SOC stacks of the SOC unit during normal operation is one or more solid oxide electrolysis (SOEC) stacks. Hence, a SOC stack, suitably the SOC unit and thus all SOC stacks, during normal operation, i.e. continuous operation, operates as a SOEC stack or SOEC unit, and is only briefly switched to operation in fuel cell mode.

In a second aspect of the invention there is also provided a system for carrying out the method according to any of the above embodiments of the first aspect (method) of the invention; wherein the system is a SOC section comprising a plurality of SOC units, in which a SOC unit comprises one or more SOC stacks; the SOC section further comprising:

- a power supply unit (PSU) comprising a rectifier arranged to convert an AC-current to a DC-current;

- a conduit arranged to supply steam to the one or more SOC stacks;

- an electrically conductive element, such as an electrical cable, arranged to provide said DC-current to the one or more SOC stacks;

- a conduit arranged to output hydrogen from said one or more SOC stacks, the one or more SOC stacks operating in electrolysis cell mode;

- an interrupting device, such as a valve, arranged to interrupt said supply of steam to the one or more SOC stacks; and a conduit arranged to supply a fuel source to the one or more SOC stacks; or

- a flow regulating device, such as a valve, to maintain the supply of steam, together with a fuel source, to the one or more SOC stacks;

- a conduit, i.e an electrical conduit, arranged to output a DC-current from said one or more SOC stacks, the one or more SOC stacks operating in fuel cell mode;

- an electrical device arranged to directly receive said DC-current from said one or more SOC stacks operating in fuel cell mode.

In an embodiment according to second aspect of the invention, the electrical device is at least one of: - an electric heater;

- the one or more stacks operating in electrolysis cell mode of step ii);

Any of the embodiments and associated benefits of the first aspect of the invention (method) may be used in connection with the second aspect of the invention (system), or vice versa.

Figure 1 (Fig. 1) shows the general concept of the invention.

Figure 2 (Fig. 2) shows the general concept of the invention with more detail.

Figure 3 (Fig. 3) shows three particular embodiments of the general concept of the invention of Fig. 1 and 2.

Fig. 1 shows a system according to an embodiment of the invention. From AC mains, a PSU, here comprising only a rectifier, converts AC into DC which is then supplied to a solid oxide cell (SOC) unit. The SOC unit under normal operation (continuous operation) is in electrolysis cell mode and comprises stacks 1 , 2, up to n, arranged in series. In the embodiment illustrated in the figure, all stacks 1 to n are thus first operated in electrolysis cell mode. The DC-current to the SOC unit, thus normally operating as a SOEC unit, is then briefly interrupted and switched into operation in fuel cell mode, thereby outputting a DC-current and then directly and serially connecting an electric heater (heater). The heat generated therein is then transferred to the at least one SOC stack operating in electrolysis cell mode, i.e. the at least one SOC stack of a SOC unit operating in electrolysis cell mode, as shown by the stippled line from the heater. The heat generated is transferred to the at least one SOC stack operating in electrolysis cell mode. The stacks are thus suitably switched back into electrolysis cell mode upon said heat transfer. Instead of sending power back to the grid, the power generated during fuel cell mode is consumed in the electric heater connected to the stack and thus to the SOC unit. The electric heater may thus be integrated in the system, the system being a SOC section comprising a SOC unit, i.e. one or more SOC units. The electric heater may be external to the system and arranged within a process or plant comprising the system, such as a plant for producing any of ammonia, methanol, or hydrogen.

Fig. 2 shows in more detail the general concept of the invention. The rectifier of Fig. 1 is here shown as AC/DC converter, thereby producing DC power which via electrical switching devices, herein also referred to as “switches” or in the figures as “electrical switch” (“El. switching”), powers the one or more SOC stacks (Stack 1 , 2 ... n) for operation in electrolysis cell mode. The stacks are part of a SOC unit as shown by the stippled lines encapsulating the stacks. Via the El. Switching, the operation of a SOC stack is switched into operation in fuel cell mode, thereby outputting a DC-current and then directly connecting to an electric heater (heater). The heat generated therein is then transferred to to the at least one SOC stack operating in electrolysis cell mode or to the SOC unit, as shown by the stippled line from the heater.

Fig. 3 shows three states (out of at least n states) in which at least one SOC stack operates in fuel cell mode and at least one SOC stack operates in electrolysis cell mode and the heat generated in the heater is transferred to the at least one SOC stack operating in electrolysis cell mode or to the SOC unit:

(a) Stack 1 operates in SOFC mode and supplies the electric heater (heater); all other stacks operate in SOEC mode. The DC-current from Stack 1 is routed directly to the heater.

(b) Stack 2 operates in SOFC mode and supplies the electric heater (heater); all other stacks operate in SOEC mode. The DC-current from Stack 2 is routed directly to the heater.

(c) Stack n operates in SOFC mode and supplies the heater (heater); all other stacks operate in SOEC mode. The DC-current from Stack n is routed directly to the heater.

The states described above follow the order described, namely (a) - (b) - (c), thus alternating the stack operating in electrolysis mode and fuel cell mode respectively. It is due to the electrical switches (“El switching”) depicted in the figures and the way in which these are controlled that this succession of states is occurring. For instance, in Fig. 3(a), the El switching (or simply “switch”) directly associated to Stack 1 provides for the DC-current being routed directly to the heater, while the switch directly associated to Stack 2 or Stack n, provides for the DC-current being routed to Stack 2 or Stack n, respectively, these being operated in electrolysis mode.

Claims

1 . A method for operating a solid oxide cell (SOC) unit, comprising the steps of: i) providing a power supply unit (PSU) comprising a rectifier and converting an AC-current to a DC-current; ii) providing a solid oxide cell (SOC) unit comprising one or more SOC stacks, supplying steam to the one or more SOC stacks and serially connecting said one or more SOC stacks to said rectifier by providing said DC-current to the one or more SOC stacks, thereby powering the one or more SOC stacks for operation in electrolysis cell mode; and outputting hydrogen from said one or more SOC stacks; iii) interrupting said DC-current to the one or more SOC stacks by: iii-1) interrupting said supply of steam to the one or more SOC stacks, and supplying a fuel source to the one or more SOC stacks; or iii-2) maintaining the supply of steam, together with a fuel source, to the one or more SOC stacks; thereby switching the one or more SOC stacks from operation in electrolysis cell mode to operation in fuel cell mode; and outputting a DC-current from said one or more SOC stacks; iv) providing an electrical device and connecting said electrical device to said one or more SOC stacks operating in fuel cell mode, by directly providing said DC-current from said one or more SOC stacks to said electrical device.

2. Method according to claim 1 , wherein the electrical device is at least one of:

- an electric heater;

- the one or more stacks operating in electrolysis cell mode of step ii);

3. Method according to any of claims 1-2, wherein: in step iv) said DC-current is consumed within the same SOC-unit where the DC-current is output.

4. Method according to any of claims 1-3, wherein step iv) comprises serially connecting said electric heater to said one or more SOC stacks operating in fuel cell mode.

5. Method according to claim 4, wherein the one or more SOC stacks operating in fuel cell mode are electrically connected in series providing the DC-current to the electrical device, thereby powering the electrical device, the electrical device being fluidly connected in series to said SOE stacks.

6. Method according to any of claims 1-5, wherein: in step iii-2), said fuel source is hydrogen (H2), and said supply of steam (H2O), together with a fuel source, is a H2/H2O gas mixture comprising 1-20 vol.% H2.

7. Method according to claim 6, wherein: in step iii-2), said fuel source is hydrogen (H2), and said supply of steam together with a fuel source is a H2/H2O gas mixture comprising 1-10 vol.% H2, e.g. 2, 3, 4, 5, 6, 7, 8, 9 vol.% H₂.

8. Method according to any of claims 1-7, wherein: in any of step iii-1) or iii-2), said fuel source is a portion of said hydrogen from step ii).

9. Method according to any of claims 1-8, the method comprising: in step ii) all SOC stacks in the SOC unit operating in electrolysis cell mode, and in step iii) all SOC stacks switching to operation in fuel cell mode.

10. Method according to any of claims 1-8, the method comprising at least one SOC stack operating in fuel cell mode and at least one SOC stack operating in electrolysis cell mode.

11. Method according to any of claims 2-10, wherein the electrical device is an electric heater and the heat generated in step iv) is transferred to the at least one SOC stack operating in electrolysis cell mode.

12. Method according to any of claims 1-11, wherein the time of operation in said electrolysis cell mode is at least two times, such as at least three times, or at least five times, or at least ten times, longer that the time of operation in said fuel cell mode.

13. Method according to claim 12, wherein the time of operation in electrolysis cell mode is 1 min - 24 hours and the time of operation in fuel cell mode is 0.10-60 seconds; such as the time of operation in electrolysis cell mode being 1-30 minutes and the time of operation in fuel cell mode being 10-30 seconds.

14. Method according to any of claims 1-13, wherein the one or more SOC stacks of the SOC unit during normal operation is one or more solid oxide electrolysis cell (SO EC) stacks.

15. A system for carrying out the method according to any of claims 1-14; said system being a SOC section comprising a plurality of SOC units, in which a SOC unit comprises one or more SOC stacks; the SOC section further comprising:

- a conduit arranged to supply steam to the one or more SOC stacks;

- an interrupting device, such as a valve, arranged to interrupt said supply of steam to the one or more SOC stacks; and a conduit arranged to supply a fuel source to the one or more SOC stacks; or - a flow regulating device, such as a valve, to maintain the supply of steam, together with a fuel source, to the one or more SOC stacks;

16. System according to claim 15, wherein the electrical device is at least one of:

- an electric heater;

- the one or more stacks operating in electrolysis cell mode of step ii);

- a blower, such as a hydrogen recycle blower in which the hydrogen is at least a portion of said hydrogen from step ii); - a compressor, such as a hydrogen compressor in in which the hydrogen is at least a portion of said hydrogen from step ii).