ELECTROLYZER SYSTEM INCLUDING BACK PRESSURE CONTROL VALVE AND METHODS OF OPERATING SAME

FIELD

[0001]The present disclosure is directed to electrolyzer cell systems including a back pressure control valve and methods of operating the same.

BACKGROUND

[0002]In a solid oxide electrolyzer cell (SOEC), a cathode electrode is separated from an anode electrode by a solid oxide electrolyte. When a SOEC is used to produce hydrogen through electrolysis, a positive potential is applied to the air side of the SOEC and oxygen ions are transported from the fuel (e.g., steam) side to the air side. Throughout this specification, the SOEC anode will be referred to as the air electrode, and the SOEC cathode will be referred to as the fuel electrode. During SOEC operation, water (e.g., steam) in the fuel stream is reduced (H₂O+2e⁻→O²⁻+H₂) to form H₂ gas and O²⁻ ions, the O²⁻ ions are transported through the solid electrolyte, and then oxidized (e.g., by an air inlet stream) on the air side (O²⁻ to O₂) to produce molecular oxygen (e.g., oxygen enriched air).

SUMMARY

[0003]In various embodiments, an electrolyzer system comprises a stack of electrolyzer cells configured to electrolyze steam into a hydrogen product stream; an air inlet conduit; an air blower configured to provide an air inlet stream to the stack via the air inlet conduit; an exhaust conduit fluidly connected to an oxygen exhaust of the stack; and a variable air valve located on the exhaust conduit and configured to control a pressure of an oxygen exhaust stream flowing through the exhaust conduit from the stack.

[0004]In various embodiments, a method of operating an electrolyzer system comprises providing steam and an air inlet stream to a stack of electrolyzer cells; electrolyzing the steam into a hydrogen product stream and an oxygen exhaust stream; and controlling a pressure of the oxygen exhaust stream to maintain a pressure differential between a fuel side and an air side of each of the electrolyzer cells in the stack to be less than 1 psig.

FIGURES

[0005]FIG. 1A is a perspective view of a solid oxide electrolyzer cell (SOEC) stack, and FIG. 1B is a side cross-sectional view of a portion of the stack of FIG. 1A.

[0006]FIG. 2 is a schematic view of a portion of an electrolyzer system, according to various embodiments of the present disclosure.

[0007]FIG. 3 is a schematic view of an electrolyzer module that may be included in the electrolyzer system of FIG. 2.

[0008]FIG. 4 is a schematic view of an electrolyzer system, according to various embodiments of the present disclosure.

[0009]FIG. 5 is a schematic view of alternative electrolyzer system, according to various embodiments of the present disclosure.

DETAILED DESCRIPTION

[0010]The various embodiments will be described in detail with reference to the accompanying drawings. The drawings are not necessarily to scale and are intended to illustrate various features of the invention. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. References made to particular examples and implementations are for illustrative purposes and are not intended to limit the scope of the invention or the claims.

[0011]FIG. 1A is a perspective view of an electrolyzer cell stack 100, such as a solid oxide electrolyzer cell (SOEC) stack, and FIG. 1B is a side cross-sectional view of a portion of the stack 100 of FIG. 1A. Referring to FIGS. 1A and 1B, the stack 100 includes multiple electrolyzer cells (e.g., SOECs) 1 that are separated by interconnects 10, which may also be referred to as gas flow separator plates or bipolar plates. Each electrolyzer cell 1 includes an air electrode 3, an electrolyte 5, such as a solid oxide electrolyte for a SOEC, and a fuel electrode 7. The stack 100 also includes internal fuel riser channels 22.

[0012]Each interconnect 10 electrically connects adjacent electrolyzer cells 1 in the stack 100. In particular, an interconnect 10 may electrically connect the fuel electrode 7 of one electrolyzer cell 1 to the air electrode 3 of an adjacent electrolyzer cell 1. FIG. 1B shows that the lower electrolyzer cell 1 is located between two interconnects 10.

[0013]Various materials may be used for the air electrode 3, electrolyte 5, and fuel electrode 7. For example, the air electrode 3 may comprise an electrically conductive material, such as an electrically conductive perovskite material, such as lanthanum strontium manganite (LSM). Other conductive perovskites, such as LSCo, etc., or metals, such as Pt, may also be used. The electrolyte 5 may comprise a stabilized zirconia, such as scandia stabilized zirconia (SSZ) or yttria stabilized zirconia (YSZ), yttria-ceria-stabilized zirconia (YCSZ), ytterbia-ceria-scandia-stabilized zirconia (YbCSSZ) or blends thereof. In YbCSSZ, scandia may be present in an amount equal to 9 to 11 mol %, such as 10 mol %, ceria may present in amount greater than 0 and equal to or less than 3 mol %, for example 0.5 mol % to 2.5 mol %, such as 1 mol %, and ytterbia may be present in an amount greater than 0 and equal to or less than 2.5 mol %, for example 0.5 mol % to 2 mol %, such as 1 mol %, as disclosed in U.S. Pat. No. 8,580,456, which is incorporated herein by reference. Alternatively, the electrolyte 5 may comprise another ionically conductive material, such as a doped ceria. The fuel electrode 7 may comprise a cermet comprising a nickel containing phase and a ceramic phase. The nickel containing phase may consist entirely of nickel in a reduced state. The ceramic phase may comprise a stabilized zirconia, such as yttria and/or scandia stabilized zirconia and/or a doped ceria, such as gadolinia, yttria and/or samaria doped ceria. The electrodes and the electrolyte may each comprise one or more sublayers of one or more of the above described materials.

[0014]Each interconnect 10 includes fuel ribs 12A that at least partially define fuel channels 8A, and air ribs 12B that at least partially define air channels 8B. The interconnect 10 may operate as a gas-fuel separator that separates a fuel, such as steam, flowing to the fuel electrode 7 of one electrolyzer cell 1 in the stack 100 from oxidant, such as air, flowing to the air electrode 3 of an adjacent electrolyzer cell 1 in the stack 100. At either end of the stack 100, there may be an air end plate or fuel end plate (not shown) for providing air or fuel, respectively, to the end electrode. Alternatively, the air end plate or fuel end plate may comprise the same interconnect structure used throughout the stack.

[0015]Each interconnect 10 may be made of or may contain electrically conductive material, such as a metal alloy (e.g., chromium-iron alloy) which has a similar coefficient of thermal expansion to that of the solid oxide electrolyte in the cells (e.g., a difference of 0-10%). For example, the interconnects 10 may comprise a metal (e.g., a chromium-iron alloy, such as 4-6 weight percent iron, optionally 1 or less weight percent yttrium and balance chromium alloy). Alternatively, any other suitable conductive interconnect material, such as stainless steel (e.g., ferritic stainless steel, SS446, SS430, etc.) or iron-chromium alloy (e.g., Crofer™ 22 APU alloy which contains 20 to 24 wt. % Cr, less than 1 wt. % Mn, Ti and La, and balance Fe, or ZMG™ 232L alloy which contains 21 to 23 wt. % Cr, 1 wt. % Mn and less than 1 wt. % Si, C, Ni, Al, Zr and La, and balance Fe).

[0016]FIG. 2 is a schematic view of a portion of an electrolyzer system 300, according to various embodiments of the present disclosure. Referring to FIG. 2, the system 300 may include multiple fluidly connected modules 200. The modules 200 may comprise electrolyzer modules (e.g., hydrogen generation modules) which generate a hydrogen product from electrolysis of water (e.g., steam). While three modules 200 are shown, the system 300 may include any suitable number of modules 200. The system 300 may include a steam conduit (e.g., a pipe, header or manifold) 230 configured to fluidly connect each module 200 to a steam source 30, a product conduit (e.g., a pipe, header or manifold) 240 configured to fluidly connect each module 200 to a hydrogen processor 40, a hydrogen conduit (e.g., a pipe, header or manifold) 250 configured to fluidly connect a hydrogen storage vessel (e.g., hydrogen tank) 50 to the steam conduit 230, and an exhaust conduit (e.g., a pipe, header or manifold) 270 configured to receive oxygen exhaust from the modules 200. The steam source 30 may comprise any suitable source of steam, such as a building or factory steam source (e.g., external boiler, etc.), which provides byproduct steam to the steam conduit 230, and/or a dedicated steam generator which is part of the system 300. The hydrogen processor 40 may comprise any component which may compress and/or store the hydrogen product, such as a mechanical compressor, an electrochemical hydrogen separator (e.g., a proton exchange membrane), and/or a hydrogen storage vessel.

[0017]The system 300 may also include a system controller 225, an optional system recycling conduit 244, an optional vent conduit 246, and an optional ejector 245. The ejector 245 may be located on the steam conduit 230, and the system recycling conduit 244 may fluidly connect the product conduit 240 to the ejector 245 located on the steam conduit 230. The vent conduit 246 may fluidly connect the product conduit 240 to an individual module exhaust or to a common system exhaust conduit which is fluidly connected to the vent conduits 246 of all modules. The ejector 245 may operate to pull a portion of the hydrogen product stream from the product conduit 240 through the system recycling conduit 244 and into the steam conduit 230 to recycle a portion of the hydrogen product stream back into the modules 200, while a remaining portion of the hydrogen product stream is provided to the hydrogen processor 40. In some embodiments, the ejector 245 may be replaced with a system recycle blower.

[0018]The system 300 may also include various flow control elements to control fluid flow to and/or from the modules 200. For example, the system 300 may include a primary steam valve 232 located on the steam conduit 230 and configured to control steam flow from the steam source 30 through the steam conduit 230, and optional module shutoff valves 234 located between the steam conduit 230 and the respective modules 200 and configured to control the steam flow from the steam conduit 230 into the respective modules 200. The system 300 may also include a product valve 242 located on the product conduit 240 and configured to control hydrogen product flow from the product conduit 240 into the hydrogen processor 40. The system 300 may also include an optional recycling valve 249 located on the optional system recycling conduit 244 and configured to control a flow of a portion of the hydrogen product stream through the system recycling conduit 244. The system 300 may also include a hydrogen valve 252 located on the hydrogen conduit 250 and configured to control hydrogen flow from the hydrogen storage vessel 50 into the hydrogen conduit 250. The hydrogen valve 252 may be opened during system startup and shutdown modes to provide hydrogen from the hydrogen storage vessel 50 to the modules 200, and closed during a steady-state operating mode of the system 300 during which the system 300 generates the hydrogen product. The system 300 may also include a vent valve 248 located on the vent conduit 246 and configured to control hydrogen product flow through the vent conduit 246. For example, the vent valve 248 may be opened during system shutdown to vent the product conduit 240 and depressurize the system 300. In some embodiments, one or more of the valves may comprise gas solenoid valves or other suitable valves.

[0019]The system controller 225 may include a central processing unit and a memory. The system controller 225 may be wired or wirelessly connected to various elements of the system 300, and may be configured to control the same. For example, the system controller 225 may be configured to control the various valves and the operation of the modules 200. In one embodiment, the system controller 225 may be located in a power module which includes a housing separate from the housings of the electrolyzer modules 200. The power module may also include an AC/DC rectifier configured to convert alternating current (AC) power from a power source (e.g., power grid) to direct current (DC) power provided to the electrolyzer modules 200. The remaining components of the system 300 may be located either in a gas distribution module which includes a housing separate from the housings of the electrolyzer modules 200 and the power module, and/or outside the module housings of the system 300. For example, the steam source 30 and/or the hydrogen processor 40 may be located in the gas distribution module or separate from the module housings of the system 300. Likewise, the steam conduit 230 may extend from the gas distribution module to the electrolyzer modules 200 in or over a common base supporting the gas distribution module and the electrolyzer modules 200.

[0020]FIG. 3 is a schematic view of an electrolyzer module 200 that may be included in the electrolyzer system 300 of FIG. 2, according to various embodiments of the present disclosure. Referring to FIGS. 1A, 1B, 2 and 3, each module 200 may include an electrolyzer cell stack 100 including multiple electrolyzer cells, such as solid oxide electrolyzer cells (SOECs), as described with respect to FIGS. 1A and 1B. The stack 100 may be located in an electrolyzer cell column including plural stacks. Alternatively, the column may contain only a single stack 100. The module 200 may also include a steam recuperator heat exchanger 108, one or multiple steam heaters 110, an air recuperator heat exchanger 112, and one or multiple air heaters 114. The module 200 may also include an optional product cooler/air preheater heat exchanger 116, and an optional stack heater (not shown for clarity).

[0021]The module 200 may include a hotbox 202 to house various components, such as the stack 100, the steam recuperator 108, the steam heater 110, the air recuperator 112, and/or the air heater 114. In some embodiments, the hotbox 202 may include multiple stacks 100 and/or columns of stacks. The module 200 may also include a cabinet 204 configured to house the hotbox 202 and other module 200 components located outside of the hotbox 202. Optionally, the module 200 may also include a controller 125, such as a central processing unit, which is configured to control the operation of the module 200. For example, the controller 125 may be wired or wirelessly connected to various elements of the module 200 to control the same. Alternatively, the controller 125 may be located outside the housing of the electrolyzer module 200 (e.g., in the power module of the system 300). The product cooler/air preheater heat exchanger 116 can be located inside the hotbox 202, or it can be located outside of the hotbox 202.

[0022]During operation, the stack 100 may be provided with steam from the steam source 30 and may be provided with electric power (e.g., DC current or voltage) from an external power source, such as a power grid. In particular, the steam may be provided to the fuel electrodes 7 of the electrolyzer cells 1 of the stack 100, and the power source may apply a voltage between the fuel electrodes 7 and the air electrodes 3, in order to electrolyze water molecules at the fuel electrodes 7 to form hydrogen gas and oxygen ions. In SOECs 1, the oxygen ions are transported through the solid electrolyte 5 to the air electrodes 3. Air may optionally be provided to the air electrodes 3 of the stack 100, in order to sweep the oxygen from the air electrodes 3. The stack 100 may output a hydrogen stream (e.g., hydrogen product which may also contain residual steam) into a module product conduit 120, and an oxygen-rich exhaust stream (e.g., an oxygen exhaust stream), such as an oxygen-rich air stream (i.e., oxygen enriched air) into a module exhaust conduit 126.

[0023]The steam output from the steam source 30 may be provided to the multiple modules 200 via the steam conduit 230. The steam entering a module 200 from the steam conduit 230 may be provided to the steam recuperator 108 via a module steam conduit 132. The steam may include small amounts of dissolved air and/or oxygen. As such, the steam may be mixed with hydrogen gas, in order to maintain a reducing environment in the stack 100, and in particular, at the fuel electrodes 7. An optional shutoff valve 134 and an optional non-return valve 136 may be located on module steam conduit 132. The non-return valve 136 is configured to prevent the backflow of steam from the module steam conduit 132 into the steam conduit 230. However, in some embodiments, the non-return valve 136 may be omitted. For example, operation of the shutoff valve 134 may be sufficient to prevent steam backflow.

[0024]Hydrogen may be provided to the steam from the hydrogen storage vessel 50 and/or from a portion of the hydrogen product generated by the stack 100. The hydrogen addition rate may be set to provide an amount of hydrogen that exceeds an amount of hydrogen needed to react with an amount of oxygen dissolved in the steam. The hydrogen addition rate may either be fixed or set to a constant water to hydrogen ratio. However, if the steam is formed using water that is fully deaerated, the hydrogen addition may optionally be omitted.

[0025]In some embodiments, the hydrogen may be provided by the external hydrogen storage vessel 50 during system startup and shutdown. For example, during the system 300 startup and/or shutdown modes, hydrogen may be provided from the hydrogen storage vessel 50 to the steam conduit 230 via the hydrogen conduit 250. In contrast, during the steady-state operation mode, a portion of the hydrogen product (i.e., hydrogen exhaust stream) may be diverted from the product conduit 240 to the steam conduit 230 via the recycling conduit 244, and the hydrogen flow from the hydrogen storage vessel 50 may be stopped by closing the shutoff valve 252 on the hydrogen conduit 250.

[0026]In some embodiments, the module 200 may include a recycle blower 122 configured to selectively divert a portion of the generated hydrogen product to the steam in the module steam conduit 132. For example, the recycle blower 122 may be located on a module recycling conduit 124 which fluidly connects a module product conduit 120 to the module steam conduit 132. Alternatively, a hydrogen pump may be used instead of the recycle blower 122. In some embodiments, a portion of the generated hydrogen product may be diverted from the module product conduit 120 to the module recycling conduit 124 by a splitter and/or valve.

[0027]The steam recuperator 108 may be a heat exchanger configured to recover heat from the hydrogen stream output from the stack 100 into the module product conduit 120. The steam may be heated to at least 600° C., such as 620° C. to 780° C. (depending in part on the stack 100 operating temperature) in the steam recuperator 108.

[0028]The steam output from the steam recuperator 108 may be provided to the steam heater 110 which is located downstream from the steam recuperator 108 on the module steam conduit 132, as shown in FIG. 3. The steam heater 110 may include a heating element, such as a resistive or inductive heating element. The steam heater 110 may be configured to heat the steam to a temperature above the operating temperature of the stack 100. For example, depending on the health of the stack 100, the water utilization rate of the stack 100, and the air flow rate to the stack 100, the steam heater 110 may heat the steam to a temperature ranging from about 700° C. to about 850° C., such as 720° C. to 780° C. Accordingly, the stack 100 may be provided with steam or a steam-hydrogen mixture at a temperature that allows for efficient hydrogen generation. Heat may also be transported directly from the steam heater to the stack by radiation (i.e., by radiant heat transfer). If the stack operating current is sufficiently high to maintain the stack at a desired steady-state operating temperature, then the steam heater and/or the air heater may be turned off. In some embodiments, the steam heater 110 may include multiple steam heater zones with independent power levels (divided vertically, circumferentially, or both), in order to enhance thermal uniformity.

[0029]An air blower 118 may provide an air inlet stream to the air recuperator 112 via a module air inlet conduit 119. The module air inlet conduit 119 fluidly connects the air blower 118 to an air inlet of the stack 100 through the product cooler/air preheater heat exchanger 116. The oxygen exhaust output from the stack 100 may be provided via the module exhaust conduit 126 to the air recuperator 112. The air recuperator 112 may be configured to heat the air inlet stream using heat extracted from the oxygen exhaust.

[0030]Air output from the air recuperator 112 may be provided to the air heater 114 via a portion 119A of the air inlet conduit 119 located in the hotbox 202. The air heater 114 may include a resistive or inductive heating element configured to heat the air to a temperature exceeding the operating temperature of the stack 100. For example, depending on the health of the stack 100, the water utilization rate of the stack 100, and the air flow rate to the stack 100, the air heater 114 may heat the air to a temperature ranging from about 700° C. to about 850° C., such as 720° C. to 880° C. Accordingly, the stack 100 may be provided with air at a temperature that allows for efficient hydrogen generation. Heat may also be transported directly from the air heater to the stack by radiation. In some embodiments, the air heater 114 may include multiple air heater zones with independent power levels (divided vertically or circumferentially or both), in order to enhance thermal uniformity. Air from the air heater 114 is provided to the air electrodes 3 of the stack 100.

[0031]Oxygen exhaust (e.g., oxygen enriched air) output from the stack 100 through the air recuperator 112 may be provided to the exhaust conduit 270 via a portion 126A of the module exhaust conduit 126 located in the hotbox 202, the remaining portion of the module exhaust conduit 126 located outside the hotbox 202 and an exhaust duct 206 of the cabinet 204. A fan 208 or multiple fans 208 may optionally be located in the exhaust duct 206 to improve oxygen exhaust flow through the exhaust conduit 270. The exhaust conduit 270 may be configured to receive oxygen exhaust output from multiple modules 200. In some embodiments, the exhaust conduit 270 may provide the exhaust to a chimney or may provide the air exhaust to the atmosphere. In other embodiments, the oxygen exhaust (e.g., oxygen enriched air) may be provided from the exhaust conduit 270 for purification and/or use. In some embodiments, the cabinet 204 may contain a cabinet ventilation fan that comprises the fan 208 or another fan in addition to the fan 208. The cabinet ventilation stream may be merged with the oxygen exhaust stream to lower the temperature and oxygen concentration of the oxygen exhaust stream before exhausting it to the atmosphere.

[0032]In some embodiments, the module 200 may include an optional product cooler/air preheater heat exchanger 116, which may be located outside (e.g., on top of) of the hotbox 202 or inside of the hotbox 202. The product cooler/air preheater heat exchanger 116 may be fluidly connected to the hydrogen product conduit 240 by the module product conduit 120. The product cooler/air preheater heat exchanger 116 may be configured to preheat the air inlet stream provided to the hotbox 202 via the module air inlet conduit 119 using heat from the hydrogen product in the module product conduit 120, and to cool a hydrogen product output from the stack 100 using the air inlet stream provided from the air blower 118.

[0033]The hydrogen product stream is output from the steam recuperator 108 and the optional product cooler/air preheater heat exchanger 116 via the module product conduit 120 and the product conduit 240 at a temperature of 100° C. to 200° C. The hydrogen product stream may be compressed and/or purified in the hydrogen processor 40, which may include a hydrogen pump (e.g., proton exchange membrane electrochemical pump) that operates at a temperature of from about 40° C. to about 120° C., in order to remove from about 70% to about 90% of the hydrogen from the hydrogen product stream. A remaining water rich stream comprises an unpumped effluent from the hydrogen pump.

[0034]In various embodiments, the hydrogen processor 40 may include at least one electrochemical hydrogen pump, liquid ring compressor, diaphragm compressor or combination thereof. For example, the hydrogen processor may include a series of electrochemical hydrogen pumps, which may be located in series and/or in parallel with respect to a flow direction of the hydrogen stream, in order to compress the hydrogen stream. Electrochemical compression may be more electrically efficient than traditional compression. Traditional compression may occur in multiple stages, with interstage cooling and water knockout. The final product from compression may still contain traces of water. As such, the hydrogen processor 40 may include a dewatering device, such as a temperature swing adsorption reactor or a pressure swing adsorption reactor, to remove this residual water, if necessary.

[0035]In some embodiments, it is desirable to increase the pressure of the hydrogen product stream exiting the electrolyzer modules 200 in order to simplify downstream processing of the hydrogen product stream in the hydrogen processor 40. In order to increase the pressure of the hydrogen product stream, the pressure of the steam from the steam source 30 may be increased. However, increasing the steam pressure may result in a pressure differential inside the stack 100 between the fuel side and the air side of each electrolyzer cell 1. Such pressure differential may deform the glass or glass-ceramic seals which seal the interconnects 10 and/or cells 1 of the stack 100 to each other. A sufficiently large differential may also crack the electrolytes. Therefore, in order to avoid deforming the seals, the pressure differential between the fuel and air sides should be maintained below 1 psig, such as at 0.5 psig or less, for example at 0 to 0.5 psig, including 0.1 to 0.5 psig. The embodiments of the present disclosure provide configurations that distribute steam and/or air in a manner that reduce pressure gradients even when the pressure of the steam and the hydrogen product stream are increased. At least one of the cells 1 may comprise a measurement cell including one or more pressure sensors used to measure the pressure differential across the cell. The pressure differential may be evaluated (e.g., measured) at either end of the cell 1, or at both ends of the cell 1.

[0036]To address the pressure differential, the pressure on the air side of the cells 1 in the stack 100 may be increased to compensate for the higher steam pressure on the fuel side. In some embodiments, the air pressure in the stack may be dynamically controlled (e.g., dynamically increased or decreased) by adding a variable air valve 442 (e.g., oxygen exhaust control valve) shown in FIGS. 4 and 5 on the module exhaust conduit 126 to maintain a pressure differential between the fuel and air sides of the cells 1 below 1 psig. For example, the electrolyzer system may compensate for a sudden loss of steam in the module steam conduit 132 at the steam input side of the stack 100 by a creating a corresponding air pressure reduction. Air pressure reduction can be accomplished by real time adjustment of the variable air valve. Adjusting the air valve can equalize the pressure on the fuel and air sides of each cell 1 in the stack 100 using real time monitoring of steam and air input pressures to the fuel and air sides of the cells 1 of the stack 100. This technique can be used to control the back pressure in the stack 100.

[0037]FIG. 4 illustrates an electrolyzer system 400 that is configured to control the back pressure of the stack 100. The system 400 is similar to system 300. Thus, only the differences between the systems 300 and 400 are described below.

[0038]The electrolyzer system 400 includes valves and pressure sensors that may be coordinated by a controller (e.g., the module controller 125 and/or the system controller 225) so as to reduce pressure differential between the air side and the fuel side of the stack 100, such that the pressure difference is 1 psig or less. On the air side, an air flow meter 410 may monitor the air flow rate from the air inlet conduit 119 to the air blower 118 and may provide the measured air flow rate to the controller 125. In one embodiment, the controller 125 may dynamically control the speed of the air blower 118 based on the measured air flow rate or based on a combination of the measured air flow rate and other process variables described below.

[0039]A pressure of the air stream output from the air blower 118 into the air inlet conduit 119 may be measured by pressure sensor 413 located on the air inlet conduit 119 between the air blower 118 and the hotbox 202 upstream of the stack 100. The pressure sensor 413 may provide the measured air pressure to the controller 125. Alternatively or in addition, the pressure sensor 413 may be located on the portion 119A of the air inlet conduit 119 and/or on the portion 126A of the module exhaust conduit 126 located in the hotbox 202 on the air inlet or the air outlet side of the stack.

[0040]In one embodiment, the stack 100 may be externally manifolded for air. The air stream (shown as dashed arrows in FIG. 4) may pass from one side of the stack 100 through the air channels 8B of the interconnects 10 in the stack 100 and be output from the opposite side of the stack 100 as an oxygen exhaust stream (i.e., oxygen-enriched air stream) into the module exhaust conduit 126. The oxygen exhaust stream in the module exhaust conduit 126 may split into two parallel conduit branches 126A and 126B to pass through two parallel valves located on the respective branches. A variable air valve 442 is located on the first branch 126A and an emergency shutoff/bypass valve 444 is located on the second branch 126B. The variable air valve 442 may be a solenoid valve, a ball valve, or other gas control valve allowing a range of open positions and gas flow rates therethrough. The emergency shutoff/bypass valve 444 may be a pressure sensitive valve which is normally closed, but which opens when the air exhaust pressure exceeds a critical pressure threshold to vent the module exhaust conduit 126 to prevent damage to the stack 100.

[0041]The controller 125 may receive measurements from the pressure sensor 413 via connection line (e.g., Ethernet, inter-integrated circuit (I2C), etc.) 415 and may control an actuator 417 to control an opening or closing of variable air valve 442. After passing through the variable air valve 442, the enriched air stream may continue from the branch 126A to the system exhaust conduit 270. The opening and closing of the variable air valve 442 may be controlled by controller 125 based on the pressure measured by the pressure sensor 413, and optionally on the air blower 118 speed, the air flow meter 410 air flow measurement, the recycle blower 122 speed, a steam pressure from a fuel side pressure sensor 424, and/or other sensors, such as the hydrogen product pressure sensor 432 as will be described below. The opening and closing of the variable air valve 442 may be controlled by the controller 125 to maintain a back pressure of air into the stack 100 so that the air pressure in the stack 100 is within a safe operating range, such that a pressure difference of 1 psig or less, such as 0.5 psig or less is provided across the electrolyzer cells 1 between their air and fuel sides.

[0042]On the fuel side, the steam source 30 may supply steam to the module steam conduit 132. The module steam conduit 132 may include a steam flow rate measurement device 421, a steam pressure sensor 432 and one or more valves 134, 422 thereon. The steam may enter a series of valves 134 and 422. The steam pressure sensor 424 in conduit 132 measures an input steam pressure for the stack 100. If the stack 100 includes fuel inlet and outlet rails which extend vertically adjacent to a side of the stack and provide fuel inside the stack via a splitter plate, then the pressure sensor 424 may also be located at the bottom of either the inlet fuel rail, the outlet fuel rail or both. The steam flow rate measurement device 421 may measure the steam flow rate through the module steam conduit 132 and provide it to the module controller 125. The module controller 125 may adjust an actuator 423 of the steam control valve 422 based on the measured steam flow rate to provide a desired steam flow rate to the stack 100 to match the applied electrical power to the stack 100. In one embodiment, the steam flow rate measurement device 421 may comprise an orifice plate and a differential pressure sensor located on opposite sides of the orifice plate. However, other flow rate measurement devices may also be used.

[0043]The module steam conduit 132 provides the steam through valves 134 and 422 into the inlet steam riser 22 of the stack 100. As described above, the module steam conduit 132 may also receive recycled hydrogen product containing residual steam exhaust from the steam recycle blower 122 and the module recycle conduit 124. The steam pressure at the input to the stack 100 may include the pressure from the recycle blower 122 and the pressure from the steam source 30. After passing through the stack 100, the steam may be output from the steam outlet riser 22 of the stack 100 to the module product conduit 120.

[0044]The module recycle conduit 124 may be joined to the module product conduit 120 at a splitter (e.g., T-junction) 121. A first portion of the hydrogen product stream from the stack 100 may flow to the recycle blower 122 located on the module recycle conduit 124. A second portion of the hydrogen product stream may continue to flow down the module product conduit 120 to the system product conduit 240 located in the above described gas distribution module 450 and then be provided to the hydrogen processor 40. A hydrogen pressure sensor 432 may be located on conduit 240 in the gas distribution module 450 or on the outlet fuel rail, and may measure a pressure of the hydrogen product stream in conduit 240 to determine system hydrogen product pressure, and provide it to the system controller 225 and/or the module controller 125. The variable vent valve 436 serves as a transition valve which may allow the hydrogen product stream to bypass the hydrogen processor 40 when the valve 436 is open. The system controller 225 may receive the pressure value from at least one hydrogen pressure sensor 424 and/or 432 and in response signal an actuator 431 to adjust the variable vent valve 436 located on a first parallel branch 246A of the vent conduit 246. The opening of the variable vent valve 436 may be controlled by the system controller 225 based on the measured hydrogen product pressure by the sensor 424 and/or 432 to avoid providing the hydrogen product stream to the hydrogen processor 40 above the hydrogen processor's maximum pressure rating (e.g., when the pressure of the hydrogen product stream exceeds a threshold value). Thus, the hydrogen product stream is provided to the hydrogen processor 40 if the measured pressure of the hydrogen product stream is below the threshold value, and all or some of the hydrogen product stream bypasses the hydrogen processor 40 if the measured pressure of the hydrogen product stream is above the threshold value. Alternatively, variable vent valve 436 is opened and valve 434 is closed when the hydrogen processor 40 is shut down or not in use.

[0045]The module controller 125, valve 442, and air blower 118 are used to coordinate the target airflow and differential pressure across the cells 1 in the stack 100. The module controller 125 adjusts the valve 442 and/or the air blower 118 speed to maintain a safe differential pressure across the cells 1 in the stack 100 with regard to the steam and hydrogen product backpressure that is imposed on the cells 1 in the stack 100, as measured by pressure sensors 424 and 432. The backpressure can range from atmospheric (0 psig) during venting mode when the fluid streams are vented from the system, and up to 5 psig during the steady-state operating mode, depending on the configuration of the hydrogen processor 40. In one embodiment, the pressure differential in the stacks 100 is maintained by the module controller 125 within a safe operating range (e.g., is less than 1 psig, such as 0.5 psig or less).

[0046]The second vent valve 248 may be located on a second parallel branch 246B of the vent conduit 246. The second vent valve 248 may comprise an emergency shutoff/bypass valve, which may be a pressure sensitive valve which is normally closed, but which opens when the hydrogen product pressure exceeds a critical pressure threshold to vent the hydrogen product in the vent conduit 246 to prevent damage to the stacks 100. The hydrogen product stream in steady-state operation mode passes through the hydrogen product control valve 434 on the conduit 240 to the hydrogen processor 40. The module controller 125 may control the variable air valve 442 and the air blower 118 speed to adjust the back pressure on the air side of the electrolyzer cells 1 to maintain the pressure differential between the fuel side and the air side of the electrolyzer cells to be below a threshold (e.g., below 1 psig). The module controller 125 may also control the speed of the recycle blower 122 to adjust the fuel flow rate to the stack 100.

[0047]FIG. 5 illustrates an electrolyzer system 500 that is configured to control the back pressure into the stack 100. The system 500 is similar to system 400. Thus, only the differences between the systems 500 and 400 are described below.

[0048]In system 500, the steam control valves 422 and 134 are located in the gas distribution module 450 instead of in the electrolyzer module 200. Thus, the opening and closing of the steam control valve 422 may be controlled by the system controller 225 instead of the module controller 125.

[0049]Furthermore, the vent conduit 246 may include a single branch containing the two vent valves 436 and 248 arranged fluidically in series in system 500, instead fluidically in parallel on each of two parallel branches 246A and 246B, as in system 400.

[0050]The system 500 may optionally include an auxiliary air blower 518 located on an auxiliary air conduit 530. The auxiliary air conduit 530 may include one or more auxiliary air control valves 532. The system may also optionally include an auxiliary nitrogen conduit 534 which is fluidly connected to a site nitrogen source 540. The site nitrogen source 540 may comprise nitrogen storage vessel which stores nitrogen gas at an elevated pressure (e.g., 45 to 60 psig). The auxiliary nitrogen conduit 534 may include one or more nitrogen control valves 536.

[0051]The auxiliary air conduit 530 and/or the auxiliary nitrogen conduit 534 may be fluidly connected to the above described hydrogen conduit 250. In the system 500, the hydrogen conduit 250 may optionally be fluidly connected to the module steam conduit 132 inside the electrolyzer module 200. The valves 532 and 536 may be normally closed valves which are opened to purge the steam conduit 132 and the stack 100 using air from the auxiliary air blower 518 and/or nitrogen from the site nitrogen source 540 in case of an emergency shutdown of the system 500. In one embodiment, a method of operating the system 500 includes stopping providing steam to fuel electrodes 7 of the electrolyzer cells 1 in the stack 100 and providing a nitrogen purge stream to the fuel electrodes 7 of the electrolyzer cells 1 in the stack 100.

[0052]The module exhaust conduit 126 may include a single branch containing the variable air valve 442 in system 500, instead of the two parallel branches 126A and 126A of system 400. In the system 500, the emergency shutoff/bypass valve 444 may be omitted. The variable air valve 442 is controlled by the controller 125 in the system 500 to equalize the air and fuel pressure on the air and fuel sides of the electrolyzer cells 1 in a similar manner as described above with respect to system 400.

[0053]In summary, referring to FIGS. 4 and 5, an electrolyzer system 400 or 500 includes a stack 100 of electrolyzer cells 1 configured to electrolyze steam into a hydrogen product stream; an air inlet conduit 119; an air blower 118 configured to provide an air inlet stream to the stack 100 via the air inlet conduit 119; an exhaust conduit 126 fluidly connected to an oxygen exhaust of the stack 100; and a variable air valve 442 located on the exhaust conduit 126 and configured to control a pressure of an oxygen exhaust stream flowing through the exhaust conduit 126 from the stack 100.

[0054]In one embodiment, the system 400 or 500 also includes a controller (e.g., module controller 125 and/or system controller 225) configured to control the variable air valve 442 to maintain a pressure differential between a fuel side and an air side of each of the electrolyzer cells 1 in the stack 100 to be less than 1 psig, such as 0.5 psig or less.

[0055]In one embodiment, a pressure sensor 413 is located on the air inlet conduit 119 and is configured to measure a pressure of the air inlet stream in the air inlet conduit 119, and to provide the measured pressure of the air inlet stream to the controller. The controller is configured to control (e.g., partially and/or fully open and/or close) the variable air valve 442 to maintain the pressure differential between the fuel side and the air side of each of the electrolyzer cells 1 in the stack 100 to be less than 1 psig based on the measured pressure of the air inlet stream.

[0056]In one embodiment, a hydrogen product conduit (120, 240) is configured to provide the hydrogen product stream from the stack 100 to a hydrogen processor 40. A hydrogen pressure sensor 432 is located on the hydrogen product conduit (120, 240) and configured to measure a pressure of the hydrogen product stream in the hydrogen product conduit (120, 240) and to provide the measured pressure of the hydrogen product stream to the controller. A vent conduit 246 is fluidly connected to the hydrogen product conduit (120, 240), and a variable vent valve 436 is located on the vent conduit 246. The controller is configured to control the variable vent 436 valve to bypass the hydrogen processor 40 when the pressure of the hydrogen product stream exceeds a threshold value or when the hydrogen processor 40 is shut down or not in use.

[0057]In one embodiment, a recycle conduit 124 fluidly connects the hydrogen product conduit (120, 240) to the steam conduit 132, and a recycle blower 122 is located on the recycle conduit 124. In one embodiment shown in FIG. 5, the system 500 also includes at least one of an auxiliary air conduit 530 or an auxiliary nitrogen conduit 534 fluidly connected to the steam conduit 132. In one embodiment, the electrolyzer cells 1 comprise solid oxide electrolyzer cells.

[0058]The electrolyzer systems of various embodiments of the present disclosure provide a benefit to the climate by reducing greenhouse gas emissions.

[0059]The preceding description of the disclosed aspects is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects without departing from the scope of the invention. Thus, the present invention is not intended to be limited to the aspects shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.