Solid oxide steam electrolysis system and method

Abstract

An object of the invention is a solid oxide steam electrolysis system comprising a steam feed (1), a gas recycle device (10) that supplies hydrogen from feed-in line (51) to the steam feed (1), and flow rate of the hydrogen from the gas recycle device (10) is being configured to control the partial pressure of hydrogen in the inlet of the cathode compartment from fuel gas supply structure (22) of the solid oxide electrolysis stack structure (30). A first heat management system (20) is being configured to heat the steam-hydrogen gas mixture in line (21) to 400 – 900 °C and is being configured to supply the gas from fuel gas supply structure (22) to the cathode compartment of the solid oxide electrolysis stack structure (30) to reduce steam into hydrogen and oxygen ions by a first controlled current from a power source (70). In the system the hydrogen-steam mixture in product gas line (23) being fed to the first heat management system (20) transferring energy to the inlet gas mixture from line (21), and the hydrogen-steam mixture from the first heat management system (20) in fluid line (24) being fed through a second heat management system (40) where the gas mixture is partly condensing and producing two-phase hydrogen-water-steam mixture to line (41). The steam flow rate in fuel gas supply structure (22) to the cathode compartment of the solid oxide electrolysis stack structure (30) is being controlled based on the first controlled current of a power supply (70). The steam flow rate in fuel gas supply structure (22) to the cathode compartment is being determined based on at least one of a sum of the steam feed in line (1), the steam feed from the gas recycle device in line (51), and a flow and composition measurements in the lines (1, 21, 22, 51).

Description

[0001] Solid oxide steam electrolysis system and method

[0002] The field of the invention

[0003] Most of the energy of the world is produced by means of oil, coal, natural gas or nuclear power. All these production methods have their specific problems as far as, for example, availability and friendliness to environment are concerned. As far as the environment is concerned, especially oil and coal cause pollution when they are combusted. The problem with nuclear power is, at least, storage of used fuel.

[0004] Especially because of the environmental problems, new energy sources, more environmentally friendly and, for example, having a better efficiency than the above- mentioned energy sources, have been developed. Fuel cell’s, by means of which energy of fuel, for example biogas, is directly converted to electricity via a chemical reaction in an environmentally friendly process and electrolysers, in which electricity is converted to a fuel, are promising future energy solution devices.

[0005] Renewable energy production methods such as photovoltaic and wind power faces problems in seasonal production variations as their electricity production is limited by environmental effects. In the case of over production, hydrogen production through water electrolysis is suggested to be one of the future energy storing options.

[0006] Furthermore, an electrolysis cell can also be utilized to produce high quality methane gas from renewably biogas stores.

[0007] State of the art

[0008] Fuel cell comprises an anode side and a cathode side and an electrolyte material between them. Here the structure is called as the unit cell. In solid oxide fuel cells (SOFCs) oxygen is fed to the cathode side and it is reduced to a negative oxygen ion by receiving electrons from the cathode. The input stream is depleted from oxygen at the outlet side. The negative oxygen ion transfers through the electrolyte material to the anode side where it reacts with fuel producing electrons, water, and also typically carbon monoxide (CO) and carbon dioxide (CO2), i.e. fuel exhaust gas. Anode and cathode are connected through an external electric circuit comprising a load for the fuel cell withdrawing electrical energy alongside heat out of the system. The fuel cell reactions in the case of methane, carbon monoxide and hydrogen fuel are shown below: Anode: CH4+ H2O = CO + 3H2

[0009] CO + H2O = CO2 + H2

[0010] H2+ O2- = H2O + 2e-

[0011] Cathode: O2 + 4e_= 2O2’

[0012] Net reactions: CH4+ 2O2 = CO2 + 2H2O

[0013] CO + 1 / 202 = CO2

[0014] H2 + 1 / 202 = H2O

[0015] In electrolysis operating mode (solid oxide electrolysis cells (SOEC)) the reaction is reversed, i.e. heat, as well as electrical energy from a source 70, are supplied to the cell where water and often also carbon dioxide are reduced in the cathode compartment of a solid oxide electrolysis stack structure 31 forming oxygen ions, which move through the electrolyte material to the anode compartment of a solid oxide electrolysis stack structure 32 where oxidation reaction takes place. It is possible to use the same solid electrolyte cell in both SOFC and SOEC modes. In such a case and in the context of this description the electrodes are typically named anode and cathode based on the fuel cell operating mode, whereas in purely SOEC applications the oxygen electrode may be named the anode, and the reactant electrode as the cathode.

[0016] Solid oxide electrolyser cells operate at temperatures which allow high temperature electrolysis reaction to take place, said temperatures being typically between 400 - 1000 °C, but temperatures differing the said limits may be useful. These operating temperatures are similar to those conditions of the SOFCs. The net cell reaction produces hydrogen and oxygen gases. The reactions for one mole of water are shown below, with reduction of water occurring at the anode:

[0017] Cathode: H2O + 2e- = 2 H2+ O2’

[0018] Anode: O2’ = 1 / 2O2+ 2e-

[0019] Net Reaction: H2O = H2 + 1 / 202.

[0020] In solid oxide fuel cell (SOFC) and solid oxide electrolysis (SOE) stacks, commonly here referred as solid oxide cell stack, where the flow direction of the cathode gas relative to the anode gas internally in each cell as well as the flow directions of the gases between adjacent cells, are combined through different cell layers of the stack. Further, the cathode gas or the anode gas or both can pass through more than one cell before it is exhausted, and a plurality of gas streams can be split or merged after passing a primary cell and before passing a secondary cell. These combinations serve to increase the current density and minimize the thermal gradients across the cells and the whole stack.

[0021] The added value of a solid oxide steam electrolysis compared to low temperature water electrolysis is its considerably higher stack efficiency (90 ... 110 %-LHV compared to 65 ... 85 %-LHV). SOE stack is operated between 500 ... 900 °C and is composed of ceramic unit cells, metallic flow field and current collection structures (interconnect plates) and sealing solutions. A unit cell is composed of at least an electronically conductive porous cathode electrode where water is reduced to hydrogen, an electronically conductive porous anode electrode where oxygen is formed, and an oxygen ion conducting electrolyte that is between the anode and cathode electrodes separating the gases from each other. Due to its high operating temperature, strength of the structures is reduced, and thus low internal stress levels are favorable to reduce the risk of device failures. Operating gas pressure and especially pressure differences between anode and cathode sides can increase the internal stress levels and thus low operating pressures and low differential pressures are favorable.

[0022] In prior art document US 12 043 909 B2 (Bloom Energy Corporation) is presented a solid oxide electrolyzer system that has at least one SOE stack producing hydrogen from steam connected to at least a first hydrogen pump, i.e. an electrochemical compressor, compressing hydrogen to a first pressure level, and the first hydrogen pump is connected to at least a second hydrogen pump compressing hydrogen to another pressure level, and both hydrogen pumps have hydrogen recycle loops. The document US 12 043 909 B2 is lacking a system parts and method for an effective heat integration of the system, an effective water management system parts and method for the electrochemical compressor, an effective hydrogen recycling system parts and method for the whole steam electrolysis system, and an effective control system and method.

[0023] Brief description of the invention

[0024] An object of the invention is to enable cost optimized hydrogen production by maximizing the efficiency and lifetime of a solid oxide steam electrolysis system. This is achieved by a solid oxide steam electrolysis system comprising at least a solid oxide electrolysis stack structure containing at least one solid oxide stack that is composed of at least a unit cell having at least one anode electrode, at least one cathode electrode and electrolyte between the anode and cathode electrodes and at least one inlet port connected to a fuel gas supply structure and at least one inlet port connected to an oxygen rich gas supply structure and at least one outlet port connected to a product gas line and at least one outlet port connected to an oxygen rich gas exhaust structure, The system comprises a steam feed, a gas recycle device that supplies hydrogen from feed-in line to the steam feed, and flow rate of the hydrogen from the gas recycle device being configured to control the partial pressure of hydrogen in the inlet of the cathode compartment from fuel gas supply structure of the solid oxide electrolysis stack structure, a first heat management system being configured to heat the steam-hydrogen gas mixture in line to 400 - 900 °C and being configured to supply the gas from fuel gas supply structure to the cathode compartment of the solid oxide electrolysis stack structure to reduce steam into hydrogen and oxygen ions by a first controlled current from a power source, and to use the current to increase and control the hydrogen flow rate at the outlet of the solid oxide electrolysis stack structure and to feed the gas mixture to product gas line, and in the system the hydrogen-steam mixture in product gas line being fed to the first heat management system transferring energy to the inlet gas mixture from the steam-hydrogen gas mixture line, and the hydrogen-steam mixture from the first heat management system in fluid line being fed through a second heat management system where the gas mixture is partly condensing and producing two-phase hydrogen-water-steam mixture to an electrochemical compressor, the two-phase hydrogen-water-steam mixture being fed to the electrochemical compressor where part of the hydrogen is being transferred through a membrane with a second controlled current from a second power supply producing compressed hydrogen, and the gas pressure of the two-phase hydrogen-water-steam mixture in gas inlet to an electrochemical compressor line being controlled with the second control current from power supply, and the hydrogen gas being fed to the gas recycle device, and the second heat management system being connected from its other side to a working fluid line in which the fluid is in liquid form when being fed to the second heat management system and being vaporized to an outlet line from an evaporator in the second heat management system, and the flow rate of the working fluid in line, and properties of the working fluid in line being selected so that the temperature of two- phase hydrogen-water mixture in line is being optimized based on the properties of the electrochemical compressor for the temperature and for the amount of liquid water, and the first and the second electrical current from power supplies being dynamically controlled, and the steam flow rate in fuel gas supply structure to the cathode compartment of the solid oxide electrolysis stack structure being controlled based on the first controlled current of a power supply, and the steam flow rate in fuel gas supply structure to the cathode compartment being determined based on at least one of a sum of the steam feed, the steam feed from the gas recycle device in feed-in line, and a flow and composition measurements in the lines.

[0025] A focus of the invention is also a solid oxide steam electrolysis method, characterized in that in the method is supplied hydrogen from a feed-in line to the steam feed, and flow rate of the hydrogen from the gas recycling is controlling the partial pressure of hydrogen in an inlet of a cathode compartment of a solid oxide electrolysis stack structure from fuel gas supply structure of a solid oxide electrolysis stack structure, steam-hydrogen gas mixture is heated in line to 400 - 900 °C and gas from fuel gas supply structure is supplied to the cathode compartment of the solid oxide electrolysis stack structure to reduce steam into hydrogen and oxygen ions by a first controlled current from a power source, and to use the current to increase and control the hydrogen flow rate at the outlet of the solid oxide electrolysis stack structure and to feed the gas mixture to product gas line, and in the method the hydrogen-steam mixture in product gas line is fed to a first heat management system transferring energy to the inlet gas mixture from line, and the hydrogen-steam mixture from the first heat management system in fluid line is fed through a second heat management system where the gas mixture is partly condensing and producing two-phase hydrogen-water-steam mixture to an electrochemical compressor, and the two-phase hydrogen -water-steam mixture is fed to an electrochemical compressor where part of the hydrogen is transferred through a membrane with a second controlled current from a second power supply increasing the pressure in pressurized hydrogen line producing compressed hydrogen, and the gas pressure of the two-phase hydrogen-water-steam mixture in electrochemical compressor line is controlled with the second control current from power supply, and the hydrogen gas is fed to the gas recycling, and the second heat management system is connected from its other side to a working fluid line in which the fluid is in liquid form when the fluid is fed to the second heat management system and the fluid is vaporized to line in the second heat management system, and the flow rate of the working fluid in line, and properties of the working fluid in the working fluid line are selected so that the temperature of two-phase hydrogen-water mixture is optimized based on the properties of the electrochemical compressor for the temperature and for the amount of liquid water, and the first and the second electrical current from power supplies are dynamically controlled, and the steam flow rate in fuel gas supply structure to the cathode compartment of the solid oxide electrolysis stack structure is controlled based on the first controlled current of a power supply, and the steam flow rate in fuel gas supply structure to the cathode compartment is determined based on at least one of a sum of the steam feed, the steam feed from the gas recycle device in feed-in line, and a flow and composition measurements in the lines.

[0026] The invention is based on a solid oxide steam electrolysis system comprising a steam feed, a gas recycle device that supplies hydrogen from feed-in line to the steam feed, and flow rate of the hydrogen from the gas recycle device being configured to control the partial pressure of hydrogen in the inlet of the cathode compartment from fuel gas supply line of the solid oxide electrolysis stack structure. The invention is further based on a first heat management system which is configured to heat the steam-hydrogen gas mixture in line and configured to supply the gas from line to the cathode compartment of the solid oxide electrolysis stack structure to reduce steam into hydrogen and oxygen ions by a first controlled current from a power source, and to use the current to increase and control the hydrogen flow rate at the outlet of the solid oxide electrolysis stack structure and to feed the gas mixture to line, and in the system the hydrogen-steam mixture in cathode inlet line being fed to the first heat management system transferring energy to the inlet gas mixture from steam-hydrogen gas mixture line, and the hydrogen-steam mixture from the first heat management system in line is fed through a second heat management system where the gas mixture is partly condensing and producing two- phase hydrogen-water-steam mixture to gas inlet line to an electrochemical compressor. The invention is further based on that properties of the working fluid in working fluid line are selected so that the temperature of two-phase hydrogen-water mixture in line is optimized based on the properties of the electrochemical compressor for the temperature and for the amount of liquid water. The first and the second electrical current from power supplies are dynamically controlled, and the steam flow rate in fuel gas supply line to the cathode compartment of the solid oxide electrolysis stack structure are controlled based on the first controlled current of a power supply, and the steam flow rate in fuel gas supply line to the cathode compartment being determined based on at least one of a sum of the steam feed in steam feed line, the steam feed from the gas recycle device in feed-in line, and a flow and composition measurements in the lines.

[0027] Benefit of the invention is that efficiency of a solid oxide steam electrolysis system can be considerably improved and lifetime lengthened.

[0028] Brief description of the drawings

[0029] Figure 1 presents a first exemplary embodiment according to the present invention.

[0030] Figure 2 presents a second exemplary embodiment according to the present invention.

[0031] Figure 3 presents a third exemplary embodiment according to the present invention.

[0032] Reference signs

[0033] 1 steam feed

[0034] 2 liquid water feed

[0035] 3 second fluid feed compressed hydrogen anode product gas outlet gas cycle device a water cleaning system b water pump or pressure regulator c water evaporator d ejector, pump or compressor e heat exchanger heat management system a heat exchanger b trim heater, radiative heat exchanger, or their combination steam-hydrogen gas mixture line fuel gas supply structure product gas line fluid line solid oxide electrolysis stack structure cathode compartment of a solid oxide electrolysis stack structure anode compartment of a solid oxide electrolysis stack structure second heat management system a evaporator b expander c regenerator d pump e heat exchanger f generator gas inlet to an electrochemical compressor inlet fluid to an evaporator (40) outlet fluid from an evaporator (40) electrochemical compressor 50b valve

[0036] 51 feed-in line

[0037] 52 pressurized hydrogen line

[0038] 60 third heat management system

[0039] 60a heat exchanger

[0040] 60b trim heater, radiative heat exchanger or their combination

[0041] 60c gas recycle device

[0042] 60d gas separation system

[0043] 60e control valve

[0044] 62 oxygen rich gas supply structure

[0045] 63 oxygen rich gas exhaust structure

[0046] 70 power supply

[0047] 80 second power supply

[0048] Detailed description of the invention

[0049] Typical hydrogen applications require pressurized hydrogen. The hydrogen compression can be done most efficiently with an electrochemical compressor which works as an enthalpy pump driven by the external current feed. The average efficiency of mechanical compressors is around 45%, while EHCs tend to be over 60 % [Sdanghi G, Maranzana G, Celzard A, Fierro V. Review of the current technologies and performances of hydrogen compression for stationary and automotive applications. Renew Sustain Energy Rev 2019;102:150e70. https: / / doi.Org / 10.1016 / j.rser.2018.11.028.]. The electrochemical compressor also purifies the compressed gas as it is selective for hydrogen in contrast to mechanical compressors and thus less purification cycles are required. The electrochemical compressor is based on similar structures and materials as a polymer electrolyte fuel cell or a polymer electrolyte electrolysis stack. Its optimal operation requires about 60 - 90 °C operation temperature and two-phase flow conditions for the hydrogen - water mixture. The two-phase flow conditions are determined by hydrogen-to-steam ratio at the cathode outlet of the solid oxide stack and the temperature of this fluid at the electrochemical compressor inlet. The fluid temperature can be effectively and accurately controlled in a heat exchanger that has at the other side phase transfer conditions. This can be accomplished and controlled by a selection of an organic fluid. The organic fluid can be further connected to a turbine to produce electricity for the system and further to typical Rankine cycle. The lifetime of the electrochemical compressor and solid oxide steam electrolysis stack can be extended if their reactant utilizations can be decreased. This means that hydrogen compressor respects high hydrogen flow rate through its structure and solid oxide electrolysis stack high steam flow rate. The cathode materials in solid oxide electrolysis stack can be further stabilized at increased hydrogen concentrations. Thus, it is favorable to increase both hydrogen and steam flow rates throughout the solid oxide stack’s cathode compartment system. This fluid recycling can be accomplished with a pump, compressor or ejector. Both pump and compressor require further electricity and whereas ejector can be operated with pressurized gas including steam making it an attractive choice from the efficiency perspective.

[0050] The objective of the invention is to enable cost optimized hydrogen production by maximizing the efficiency and lifetime of a solid oxide steam electrolysis system. The present invention enables this by combining the solid oxide electrolysis stack with a steam supply, a cathode side heat management system, a (organic) Rankine cycle, electrochemical compressor, hydrogen recycle device, anode side heat management system, and electrical current control system. The presented system, when integrated to a high temperature heat source, can produce hydrogen with less than 37 kWh / kg energy consumption at ambient pressure and for example at 200 bar compression levels below 40 kWh / kg.

[0051] In figures 1-3 are presented exemplary embodiments of a solid oxide steam electrolysis system according to the present invention. The system comprises at least a solid oxide electrolysis stack structure 30 which contains at least one solid oxide stack that is composed of at least a unit cell having at least one anode electrode, at least one cathode electrode and electrolyte between the anode and cathode electrodes. At least one inlet port is connected to a fuel gas supply structure 22 and at least one another is inlet port connected to an oxygen rich gas supply structure 62 and at least one outlet port is connected to a product gas line 23 and at least one another outlet port is connected to an oxygen rich gas exhaust structure 63. A fuel gas supply structure 22 can be for example a pipe, compartment, duct, or a manifold structure providing gas to a single or multiple solid oxide electrolysis stacks that may be connected from their gas inlet side in parallel or serial connection. An oxygen rich gas supply structure 62 can be for example a pipe, compartment, duct, or a manifold structure providing gas to a single or multiple solid oxide electrolysis stacks that may be connected from their oxygen rich inlet side in parallel or serial connection. A product gas line 23 can be for example a pipe, compartment, duct, or a manifold structure feeding out the product gas from a single or multiple solid oxide electrolysis stacks that may be connected from their gas outlet side in parallel or serial connection. An oxygen rich gas exhaust structure 63 can be for example a pipe, compartment, duct, or a manifold structure feeding out oxygen rich gas from a single or multiple solid oxide electrolysis stacks that may be connected from their oxygen rich inlet side in parallel or serial connection. The system comprises a steam feed line 1 , a gas recycle device 10 that supplies hydrogen from feed-in line 51 to the steam feed line 1 . The recycle device 10 can be at least one of a pump, a compressor, and an ejector 10d which motive fluid is the steam at a controlled pressure. The steam pressure can be controlled by the pressure of the liquid water with at least one of a pump 10b and a pressure regulator 10b in water line 2. A flow rate of the hydrogen from the gas recycle device 10 is configured to control the partial pressure of hydrogen in the inlet of the cathode compartment from fuel gas supply structure 22 of the solid oxide electrolysis stack structure 30. The working pressure of the cathode compartment of a solid oxide electrolysis stack structure 31 and the anode compartment of a solid oxide electrolysis stack structure 32 of the solid oxide electrolysis stack can be in the range of 0 to 500 mbar(g) in a preferred implementation but may exceed also 50 bar(a) if the whole system is pressurized.

[0052] A first heat management system 20 is configured to heat the steam-hydrogen gas mixture in line 21 to 400 - 900 °C and to supply the gas from fuel gas supply structure 22 to the cathode compartment of the solid oxide electrolysis stack structure 30 to reduce steam into hydrogen and oxygen ions by a first controlled current from a power source 70. A power source may also be called a rectifier, AC / DC converter, or DC / DC converter. The current is used to increase and control the hydrogen flow rate at the outlet of the solid oxide electrolysis stack structure 30 and to feed the gas mixture to the product gas line 23. Preferred voltage level of a power source 70 can be for example optimized by the electrical supply network and can be for example in the range of 100 V - 2000 V. A voltage of a solid oxide electrolysis stack may be smaller than the supply voltage of the power source but the stacks can be connected in electrical series to increase the overall voltage of the system. The heat management system 20 can comprise at least one heat exchanger 20a, radiative heat exchanger, and a trim heater 20b between the gas cycle device 10 and the fuel gas supply structure 22, the radiative heat exchanger being placed in a close vicinity with the solid oxide electrolysis stack structure 30, and the trim heater 20b being based on at least one of electrical heating and heat from an external source, which can be at least one of exhaust gas from a combustion process, for example biogas combustion, industrial waste heat, for example heat from steel production or ammonia production, geothermal heat, heat from solar heat process, and heat from nuclear reactions.

[0053] In the system the hydrogen-steam mixture in a product gas line 23 is fed to the first heat management system 20 transferring energy to the inlet gas mixture from steamhydrogen gas mixture line 21 , and the hydrogen-steam mixture from the first heat management system 20 in fluid line 24 is fed through a second heat management system 40 where the gas mixture is partly condensing and producing two-phase hydrogen-water-steam mixture to a gas inlet line 41 to an electrochemical compressor 50. The two-phase hydrogen-water-steam mixture is fed to the electrochemical compressor 50 where part of the hydrogen is transferred through a membrane with a second controlled current from a second power supply 80 increasing the pressure in pressurized hydrogen line 52 to produce compressed hydrogen 5. The second power source may also be called a rectifier, AC / DC converter, or DC / DC converter. Preferred voltage level of a power source 80 can be for example optimized by the electrical supply network and can be for example in the range of 10 V - 2000 V. A voltage of an electrochemical compressor may be smaller than the supply voltage of the power source but multiple electrochemical compressors can be connected in electrical series to increase the overall voltage of the system. The gas pressure of the two-phase hydrogen-water-steam mixture in line 41 is controlled with the second control current from power supply 80. The temperature of the two-phase hydrogen-water-steam stream 41 fed to the electrochemical compressor 50 can be between 50 and 95 °C. The hydrogen gas is fed to a gas recycle device 10, and the second heat management system 40 is connected from its other side to a working fluid line 42 in which the fluid is in liquid form when being fed to the second heat management system 40. The fluid is vaporized to the outlet line 43 from an evaporator 40a in the second heat management system 40. The working fluid in the second heat management system can be at least one of butane, isobutane, isopentane, pentane, neopentane, toluene, benzene, nonane, decane, dodecane, water, R600, R600a, R601 , R601a, R245fa, SES36, R1234yf, R134a, R1234ze, R1233zd, R124, R142b, R227ea, R152a, RC318, R227ea, R123, R236ea, R114, and their mixtures.

[0054] The system according to the present invention can comprise a feed system 2 supplying liquid water, a water cleaning system 10a reducing the amount of solid particles and ions, and a first evaporator 10c producing steam from the cleaned water. Energy for evaporation can be based on at least one of an electrical heater providing heat for the water evaporation process and hot fluid with a heat exchanger structure.

[0055] The flow rate of the working fluid in line 42, and properties of the working fluid in line 42 are selected so that the temperature of two-phase hydrogen-water mixture in line 41 is optimized based on the properties of the electrochemical compressor 50 for the temperature and for the amount of liquid water. The first and the second electrical current from power supplies 70, 80 are dynamically controlled, and the steam flow rate in fuel gas supply structure 22 to the cathode compartment of the solid oxide electrolysis stack structure 30 is controlled based on the first controlled current of a power supply 70, and the steam flow rate in fuel gas supply structure 22 to the cathode compartment is determined based on at least one of a sum of the steam feed in line 1 , the steam feed from the gas recycle device in feed-in line 51 , and a flow and composition measurements in the lines 1 , 21 , 22, 51 .

[0056] In one preferred embodiment the fluid mixture from the electrochemical compressor 50 can be connected at least to one controlled valve 50b for purging the fluid mixture and valve control which is based on at least one of time sequences and gas measurement techniques. In a preferred embodiment the evaporated working fluid from an evaporator 40a can be fed to an expander 40b in which gas pressure is been reduced. The expander 40b can be at least one of turbine, screw, and scroll, being connected to a generator 40f producing electricity. The reduced pressure working fluid from expander 40b is fed to a regenerator 40c in which the fluid is cooled, the reduced pressure working fluid is fed to the third heat exchanger 40e in which heat from the working fluid circuit is transported away to another fluid and the working fluid is condensed into liquid phase. The liquid working fluid is fed to a pump 40d for increasing the pressure of the fluid circuit, and the fluid from a pump 40d is fed to the regenerator 40c for heating the liquid working fluid. The pressurized liquid working fluid is fed to the evaporator 40a.

[0057] The oxygen rich gas circuiting in the third heat management system 60 can comprise at least one of a second fluid feed 3, a heat exchanger 60a, at least one of a trim heater, radiative heat exchanger and their combination 60b, and an anode product gas outlet 7. The third heat management system 60 can comprise at least a control valve 60e, a gas separation system 60d with at least one anode product gas outlet 7, a gas recycle device 60c, a second fluid feed 3 for thermal control means, and the second fluid is at least one of air, nitrogen, carbon dioxide, argon, helium, neon, krypton, xenon, radon, steam, and their mixtures.

[0058] The system can comprise according to the present invention a liquid water supply means 2 that supplies cooling water to the heat exchanger 40e in order to remove the heat from the working fluid and to condensate the working fluid, and the water being fed to at least one of a water tank of a cleaning system 10a and the water evaporator 10c. The fluid pressure at the outlet of the electrochemical compressor 50 can be supplied to a heat exchanger 10e to evaporate the liquid water from the two-phase mixture, and the gas mixture being supplied to the recycle device 10d.

[0059] The system according to the present invention can have a feed system 2 supplying liquid water, a water cleaning system 10a reducing the amount of solid particles and ions, and a first evaporator 10c producing steam from the cleaned water. Energy for evaporation can be based on at least one of an electrical heater providing heat for the water evaporation process and hot fluid with heat exchanger structure providing heat for the water evaporation process. In the hot fluid can be at least one of an exhaust gas from a combustion process for example biogas combustion, an industrial waste heat stream for example heat from steel production, or ammonia production, a geothermal heat stream, a heat stream from solar heat process, and a heat stream from nuclear reactions.

[0060] In one preferred embodiment the fluid mixture from the electrochemical compressor 50 can be connected at least to one controlled valve 50b that is used to purge the fluid mixture in order to reduce water and second content of the gas mixture, where nitrogen or other dilutant gas is transported from the solid oxide electrolysis stack’s anode side to the cathode side. The valve control is based at least one of time sequences and gas measurement techniques.

[0061] In one preferred embodiment the electrical current fed to the solid oxide electrolysis stack structure 30 can be controlled with the power supply 70 between nominal current and lower current level in faster than one hertz scale to reduce voltage increase in the solid oxide electrolysis stack.

[0062] The electrical control system for a solid oxide steam electrolysis stack and for the electrochemical compressor is preferably a rectifier or a rectifier and a DC / DC converter which voltage is optimized based on the voltage level of a single stack or serial connected stacks. The preferred control method is current control because the hydrogen production rate of both electrochemical devices is a direct function of supplied current. The current control should be made dynamic to enable direct follow-up of the supply network as the electricity may be produced for example by solar or wind energy and the production needs. The current control can also be made dynamic in second, milli-second or micro-second level to further stabilize the electrodes in both electrochemical devices.

[0063] Thus, while there have been shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the invention may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements which perform substantially the same results are within the scope of the invention. Substitutions of the elements from one described embodiment to another are also fully intended and contemplated. It is also to be understood that the drawings are not necessarily drawn to scale but they are merely conceptual in nature. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.

Claims

Claims1 . Solid oxide steam electrolysis system comprising at least a solid oxide electrolysis stack structure (30) containing at least one solid oxide stack that is composed of at least a unit cell having at least one anode electrode, at least one cathode electrode and electrolyte between the anode and cathode electrodes and at least one inlet port connected to a fuel gas supply structure (22) and at least one inlet port connected to an oxygen rich gas supply structure (62) and at least one outlet port connected to a product gas line (23) and at least one outlet port connected to an oxygen rich gas exhaust structure (63), characterized in that the system comprises a steam feed (1 ), a gas recycle device (10) that supplies hydrogen from feed-in line (51 ) to the steam feed (1 ), and flow rate of the hydrogen from the gas recycle device (10) being configured to control the partial pressure of hydrogen in the inlet of the cathode compartment from fuel gas supply structure (22) of the solid oxide electrolysis stack structure (30), a first heat management system (20) being configured to heat the steam-hydrogen gas mixture in line (21 ) to 400 - 900 °C and being configured to supply the gas from fuel gas supply structure (22) to the cathode compartment of the solid oxide electrolysis stack structure (30) to reduce steam into hydrogen and oxygen ions by a first controlled current from a power source (70), and to use the current to increase and control the hydrogen flow rate at the outlet of the solid oxide electrolysis stack structure (30) and to feed the gas mixture to product gas line (23), and in the system the hydrogen-steam mixture in product gas line (23) being fed to the first heat management system (20) transferring energy to the inlet gas mixture from the steam-hydrogen gas mixture line (21 ), and the hydrogen-steam mixture from the first heat management system (20) in fluid line (24) being fed through a second heat management system (40) where the gas mixture is partly condensing and producing two-phase hydrogen -water-steam mixture to an electrochemical compressor (50), the two-phase hydrogen-water-steam mixture being fed to the electrochemical compressor (50) where part of the hydrogen is being transferred through a membrane with a second controlled current from a second power supply (80) producing compressed hydrogen (5), and the gas pressure of the two-phase hydrogen-water-steam mixture in gas inlet to an electrochemical compressor line(41) being controlled with the second control current from power supply (80), and the hydrogen gas being fed to the gas recycle device (10), and the second heat management system (40) being connected from its other side to a working fluid line(42) in which the fluid is in liquid form when being fed to the second heat management system (40) and being vaporized to an outlet line (43) from an evaporator (40a) in the second heat management system (40), and the flow rate of the working fluid in line (42), and properties of the working fluid in line (42) being selected so that the temperature of two-phase hydrogen-water mixture in the line (41) is being optimized based on the properties of the electrochemical compressor(50) for the temperature and for the amount of liquid water, and the first and the second electrical current from power supplies (70, 80) being dynamically controlled, and the steam flow rate in fuel gas supply structure (22) to the cathode compartment of the solid oxide electrolysis stack structure (31 ) being controlled based on the first controlled current of a power supply (70), and the steam flow rate in fuel gas supply structure (22) to the cathode compartment being determined based on at least one of a sum of the steam feed (1 ), the steam feed from the gas recycle device in feed-in line (51 ), and a flow and composition measurements in the lines (1 , 21 , 22, 51 ).

2. Solid oxide steam electrolysis system according to claim 1 , characterized in that the recycle device (10) is at least one of a pump, a compressor, and an ejector (10d) which motive fluid is the steam at a controlled pressure.

3. Solid oxide steam electrolysis system according to claim 1 , characterized in that the fluid mixture from the electrochemical compressor (50) being connected at least to one controlled valve (50b) for purging the fluid mixture and valve control being based on at least one of time sequences and gas measurement techniques.

4. Solid oxide steam electrolysis system according to claim 1 , characterized in that the evaporated working fluid from an evaporator (40a) being fed to an expander (40b) in which gas pressure is being reduced and the expander (40b) being at least one of turbine, screw, and scroll, being connected to a generator (40f) producing electricity, the reduced pressure working fluid from expander (40b) being fed to a regenerator (40c) in which the fluid is being cooled, the reduced pressure working fluid being fed to the third heat exchanger (40e) in which heat from the working fluid circuit being transported away to another fluid and the working fluid being condensed into liquid phase, and the liquid working fluid being fed to a pump (40d) for increasing the pressure of the fluid circuit, and the fluid from a pump (40d) being fed to the regenerator (40c) for heating the liquid working fluid, and the pressurized liquid working fluid being fed to the evaporator (40a).

5. Solid oxide steam electrolysis system according to claim 1 , characterized in that the heat management system (20) comprises at least one heat exchanger (20a), radiative heat exchanger, and a trim heater (20b) between the gas cycle device (10) and the fuel gas supply structure (22), the radiative heat exchanger being placed in a close vicinity with the solid oxide electrolysis stack structure (30), and the trim heater (20b) being based on at least one of electrical heating and heat from an external source.

6. Solid oxide steam electrolysis system according to claim 1 , characterized in that the oxygen rich gas circuiting in the third heat management system (60) comprising at least one of a gas inlet (3), a heat exchanger (60a), at least one of a trim heater, radiative heat exchanger and their combination (60b), and a gas outlet (7).

7. Solid oxide steam electrolysis system according to claim 1 , characterized in that the system comprises a liquid water supply mean (2) that supplies cooling water to the heat exchanger (40e) in order to remove the heat from the working fluid and to condensate the working fluid, and the water being fed to at least one of a water tank of a cleaning system (10a) and the water evaporator (10c).

8. Solid oxide steam electrolysis system according to claim 1 , characterized in that the fluid pressure at the outlet of the electrochemical compressor (50) being supplied to a heat exchanger (10e) to evaporate the liquid water from the two-phase mixture, and the gas mixture being supplied to the recycle device (10d).

9. Solid oxide steam electrolysis method, characterized in that in the method is supplied hydrogen from a feed-in line (51 ) to the steam feed (1 ), and flow rate of the hydrogen from the gas recycling (10) is controlling the partial pressure of hydrogen in an inlet of a cathode compartment of a solid oxide electrolysis stack structure (31 ) from fuel gas supply structure (22) of a solid oxide electrolysis stack structure (30), steam-hydrogen gas mixture is heated in line (21 ) to 400 - 900 °C and gas from fuel gas supply structure (22) is supplied to the cathode compartment of the solid oxide electrolysis stack structure (31 ) to reduce steam into hydrogen and oxygen ions by a first controlled current from a power source (70), and to use the current to increase and control the hydrogen flow rate at the outlet of the solid oxide electrolysis stack structure (30) and to feed the gas mixture to product gas line (23), and in the method the hydrogen-steam mixture in product gas line (23) is fed to a first heat management system (20) transferring energy to the inlet gas mixture from line (21), and the hydrogen-steam mixture from the first heat management system (20) in fluid line (24) is fed through a second heat management system (40) where the gas mixture is partly condensing and producing two-phase hydrogen-water-steam mixture to an electrochemical compressor (50), and the two-phase hydrogen-water- steam mixture is fed to an electrochemical compressor (50) where part of the hydrogen is transferred through a membrane with a second controlled current from a second power supply (80) increasing the pressure in pressurized hydrogen line (52) producing compressed hydrogen (5), and the gas pressure of the two-phase hydrogen-water-steam mixture in electrochemical compressor line (41 ) is controlled with the second control current from power supply (80), and the hydrogen gas is fed to the gas recycling (10), and the second heat management system (40) isconnected from its other side to a working fluid line (42) in which the fluid is in liquid form when the fluid is fed to the second heat management system (40) and the fluid is vaporized to line (43) in the second heat management system (40), and the flow rate of the working fluid in line (42), and properties of the working fluid in line (42) are selected so that the temperature of two-phase hydrogen-water mixture is optimized based on the properties of the electrochemical compressor (50) for the temperature and for the amount of liquid water, and the first and the second electrical current from power supplies (70, 80) are dynamically controlled, and the steam flow rate in fuel gas supply structure (22) to the cathode compartment of the solid oxide electrolysis stack structure (31) is controlled based on the first controlled current of a power supply (70), and the steam flow rate in fuel gas supply structure (22) to the cathode compartment is determined based on at least one of a sum of the steam feed (1 ), the steam feed from the gas recycle device in feed-in line (51 ), and a flow and composition measurements in the lines (1 , 21 , 22, 51 ).

10. Solid oxide steam electrolysis method according to claim 9, characterized in that the recycling (10) is performed by at least one of a pump, a compressor, and an ejector (10d) which motive fluid is the steam at a controlled pressure.11 . Solid oxide steam electrolysis method according to claim 9, characterized in that the fluid mixture from the electrochemical compressor (50) is connected at least to one controlled valve (50b) for purging the fluid mixture and valve control is based on at least one of time sequences and gas measurement techniques.

12. Solid oxide steam electrolysis method according to claim 9, characterized in that the evaporated working fluid is fed to an expander (40b) in which gas pressure is reduced, and the reduced pressure working fluid from expander (40b) is fed to a regenerator (40c) in which the fluid is cooled, the reduced pressure working fluid is fed to a third heat exchanger (40e) in which heat is transported away to another fluid and the working fluid is condensed into liquid phase, and the liquid working fluid is fed to a pump (40d) for increasing the pressure of a circuiting fluid, and the fluid from a pump (40d) is fed to the regenerator (40c) for heating the liquid working fluid, and the pressurized liquid working fluid is fed to an evaporator (40a).

13. Solid oxide steam electrolysis method according to claim 9, characterized in that cooling water is supplied to the heat exchanger (40e) in order to remove the heat from the working fluid and to condensate the working fluid, and the water is fed to at least one of a water tank of a cleaning system (10a) and the water evaporator (10c).

14. Solid oxide steam electrolysis method according to claim 1 , characterized in that the pressurized fluid at the outlet of the electrochemical compressor (50) is supplied to a heat exchanger (10e) to evaporate the liquid water from the two-phase mixture, and the gas mixture being is supplied to the recycle device (10d).

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