Apparatus and method for electrolytic power conversion

Abstract

One object of the invention is a system for electrolytic power conversion, comprising electrolyzer cells arranged as a controllable series-connected group of cells (103), means (142) for electrolysis operation at a first voltage in the range of 1.0-2.5 V per cell, and means (144) for at least intermittently drawing current from said group of cells at a second voltage in the range of 0.4-1.0 V per cell. The system comprises at least one capacitor bank (150) maintained at the first voltage and at least one capacitor bank (151) maintained at a second voltage, the capacitor banks (150, 151) and the group of cells (103) having one pole in common, and at least one bidirectional non-isolated DC / DC converter (146, 148) connecting the capacitor banks of the first and second voltages. The system further includes means (152) for controlling the first and second voltage levels, and at least one half-bridge switch pair (154) for each controllable cell group (103) for individually alternating between the application of the first and second voltage levels to the cell group (103) to prevent increased imbalance and cell degradation.

Description

[Technical Field]

[0001] Most of the world's energy is produced from oil, coal, natural gas, or nuclear power. All of these production methods have their own unique problems, for example, in terms of availability and environmental considerations. Regarding the environment, oil and coal, in particular, cause pollution when burned. The problems associated with nuclear power include, at the very least, the storage of spent fuel.

[0002] In particular, due to environmental concerns, new energy sources have been developed that are more environmentally friendly and, for example, more efficient than conventional energy sources.

[0003] Solid oxide batteries, operating through chemical reactions in an environmentally friendly process, represent a highly promising future energy conversion device. The intermittency of renewable energy sources presents challenges to the stability of the power grid, necessitating increased demand and supply-side flexibility, as well as new energy storage and conversion technologies. [Background technology]

[0004] Electrochemically active solid oxide cells can be used as fuel cells or electrolytic cells. Fuel cells generate electricity and heat from various fuels, while electrolytic cells generate electricity and heat from chemicals such as hydrogen, methane, ammonia, and carbon monoxide from, for example, water vapor, CO2, and nitrogen. Such batteries that operate in both modes as fuel cells and electrolytic cells are called solid oxide electrochemical cells (SOECs), reversible solid oxide cells (rSOCs), or simply solid oxide cells (SOCs).

[0005] A solid oxide cell (SOC) has a fuel side, an oxygen-rich side, and an electrolyte material between them. In a solid oxide fuel cell (SOFC), oxygen is supplied to the oxygen-rich side and reduced to negative oxygen ions. The negative oxygen ions move through the electrolyte material to the fuel side, where they react with the fuel to produce water and, typically, carbon dioxide (CO2). The fuel side and the oxygen-rich side are connected via an external electrical circuit that has a load for the fuel cell operating mode, which draws electrical energy from the system. The fuel cell also generates heat in the reactant effluent. In the electrolytic (electrolysis) operating mode, the current flow is reversed, and the solid oxide cell acts as a load to which electricity is supplied. Depending on the operating conditions, the cell operation can be endothermic, exothermic, or thermally neutral.

[0006] The fuel cell reactions for methane, carbon monoxide, and hydrogen fuels are shown below: Fuel side: CH4 + H2O = CO + 3H2 CO + H2O = CO2 + H2 H2 + O 2- =H2O+2e - Oxygen-rich side: O2 + 4e - =2O 2- Net reaction: CH4 + 2O2 = CO2 + 2H2O CO + 1 / 2O2 = CO2 H2 + 1 / 2O2 = H2O

[0007] In the electrolytic operation mode (solid oxide electrolytic cell, (SOEC)), the reaction is reversed; that is, electrical energy from the source is supplied to the cell, where water, and often carbon dioxide, are reduced on the fuel side to form oxygen ions, which then move through the electrolyte material to the oxygen-rich side, where the oxidation reaction takes place. It is possible to use the same solid oxide battery in both SOFC mode and SOEC mode.

[0008] Conventional solid oxide electrolytic cells operate at temperatures that allow high-temperature electrolytic reactions to occur, typically between 500 and 1000°C, although temperatures exceeding 1000°C may also be useful. These operating temperatures are similar to those of solid oxide fuel cells (SOFCs). The net cell reaction produces hydrogen and oxygen gases. The reaction of steam electrolysis is shown below: Fuel side: H2O+2e-→2H2+O 2- Oxygen-rich side: O 2- → 1 / 2O2 + 2e - Net reaction: H2O → H2 + 1 / 2O2

[0009] In co-electrolysis, in addition to vapor, carbonaceous species are supplied to the cell in a proportion favorable for the subsequent purification of the resulting gas, typically according to the Fischer-Tropsch process. Carbon dioxide can be directly reduced to carbon monoxide, or it can interact with hydrogen via a water-gas shift reaction to form vapor with carbon monoxide. Also, the net reaction: CO2 → CO + 1 / 2O2 It is also possible to use solid oxide batteries for the electrochemical reduction of carbon dioxide to carbon monoxide, according to the method described.

[0010] In solid oxide fuel cells (SOFCs) and solid oxide electrolyzer (SOE) stacks, where the direction of fuel-side gas flow is relative to the oxygen-rich gas within each cell and to the gas flow direction between adjacent cells, multiple stacks are combined through different cell layers of the stack. Furthermore, the fuel-side gas, oxygen-rich gas, or both can pass through two or more cells before being discharged, and multiple gas flows can be split or merged after passing through the primary cell and before passing through the secondary cell. These combinations act to increase current density and minimize the thermal gradient across the cells and the entire stack.

[0011] High operating temperatures in SOC cells and systems present material-related challenges in terms of thermomechanical forces, material properties, chemical stability, and uniformity of operating conditions. These aspects impose practical constraints on feasible SOC cell, stack, and module sizes. Scaling technology for large-scale installations typical of SOEC applications therefore depends primarily on the increase in cells, stacks, and SOC modules. Consequently, minimizing the cost of each increasing unit at all levels is crucial for reducing overall costs.

[0012] A SOC module comprises dozens to hundreds of SOC stacks, support structures, insulation, reactant transport and distribution structures, instrumentation, and electrical and reactive interfaces to applications or other modules. Since high-temperature interfaces are costly, space-consuming, and can constitute ignition sources, it is also beneficial to include heat exchange within the module to lower the temperature of the reactant interface. Furthermore, SOC modules require internal or external means to facilitate safe starting and stopping.

[0013] Industrial-level electrolysis, reaching total power levels ranging from tens of megawatts to gigawatts, is based on a very large number of individual electrolytic cells incorporated into electrolytic modules, including stacks, groups of stacks, and groups of one or more stacks. To drive the electrolytic reaction, a DC or pulsed DC current must be supplied to the cells, although in fuel cell mode, the current is drawn from the cells. Power electronics conversion is typically required to interface the fuel cell with a power source or sink. This could be an AC distribution grid or, for example, an industrial DC distribution system. DC power for electrolysis can be supplied with energy from, for example, an AC distribution network, or directly coupled to renewable sources such as solar, wind, and wave power. Conversion between different voltages and / or frequencies requires power electronics equipment and incurs losses. These play a significant role in both the capital and operating costs of operating both the electrolytic cells and the fuel cell.

[0014] In high-power applications, a series connection of numerous individual cells or groups of cells allows for the attainment of high string voltages. The number of cells can be optimized for a given interface voltage level or a given semiconductor voltage range. However, in the case of reversible operation, the difference in string voltage between the operation of electrolytic cells and fuel cells becomes large. This can mean a low utilization rate of the power electronics circuit in at least one of these modes.

[0015] Typically, the control goal of power electronics is to manage the cell current, because this determines the use of reactants or their respective fuels, which must be controlled in terms of lifespan and gas composition. However, since cells have a fairly large DC series resistance, the current can also be indirectly controlled by controlling the cell voltage. A special characteristic of high-temperature cells is the strong temperature dependence of their internal resistance, or area specific resistance (ASR). The temperature coefficient is negative, meaning that a rise in temperature leads to lower resistance, and consequently, higher current at a given voltage. Therefore, parallel-connected cells or groups of cells exhibit positive feedback behavior, i.e., a tendency to deviate from the initial uniform distribution of current across parallel paths. This can be counteracted by actively controlling the current at each branch, or by active temperature control and / or thermal coupling of the parallel group. In a series connection, assuming there are no short circuits or other unintended current paths across the elements, the current is the same for all elements connected in series. While control can be performed based on the series current and total voltage, it may be advantageous to also measure the voltage in series for operational constraints or protection purposes.

[0016] A simple approach to current management is to have a dedicated power converter for each parallel branch. This could be, for example, a DC / DC converter interfaced with a common DC bus, or an AC / DC converter interfaced with a utility grid. For high-power applications, non-isolated, typically hard-switching converter topologies are preferred for cost and efficiency reasons. Typical topologies for DC / DC conversion are buck, boost, and buck-boost, while for AC / DC, three-phase active full-bridges are used. For cryogenic electrolysis, various passive 6-pulse or 12-pulse rectifiers, as well as thyristor-based bridge topologies, are also used, but these suffer from poor controllability, poor power factor, and / or large grid frequency inductive components. SOFC / SOEC applications require more advanced control, and active topologies are preferred.

[0017] Industrial SOFC / SOEC high-power modules can have dozens of cells in series (strings). Equipping each string with a dedicated power converter allows for maximum control flexibility, but it means a large number of converters, discrete components, and costs compared to a general converter solution. This difference can be reduced by minimizing the dedicated portion or power level. For example, instead of a dedicated DC / DC converter that converts the entire power of a stacked string, a lower-power unidirectional or bidirectional controllable power supply can be placed in series with each string to provide the voltage offset necessary to maintain balance, typically a few percent of the total voltage. All of these series connections can then be connected in parallel to a common power stage. Thus, balancing can be achieved with even lower power levels and losses in the balancing function itself.

[0018] Electrical impedance spectroscopy is a widely used method for characterizing fuel cells. Typical solid oxide cell spectra show that frequencies in the 1–10 Hz range influence diffusion and concentration phenomena within the cell, while higher frequencies are dominated by capacitance characteristics associated with different cell functional layers. Based on impedance spectroscopy, an equivalent circuit representation of a solid oxide cell can be constructed. This equivalent circuit representation typically consists of a global series resistance element in series with numerous additional resistors having parallel capacitances representing different functional layers. Series inductance may also be included, especially if cabling to the cell is involved. In pure DC current, only the resistive elements remain, and their sum represents the total DC resistance of the cell. For both fuel cell operation and electrolysis, it is the DC component of the current that produces the net conversion. Any AC component above it causes increased losses in the resistive elements without contributing to the net reaction rate.

[0019] Figure 1 shows an example of a prior art configuration in which each stack group 103 has its own non-isolated AC / DC converter. For simplicity, only one stack group is shown in Figure 1. According to the prior art, the same configuration 115 is increased for all stack groups. Switch-mode power converters inherently produce ripple currents at their switching frequency, and in the case of AC / DC conversion, they often also produce ripple currents at twice the grid frequency. Numerous studies have been conducted to understand the effects of ripple at various frequencies in solid oxide batteries. While the results are inconclusive as to whether the ripple current itself can have a life-degrading effect, it is clear to those skilled in the art that the effect of increasing resistive losses is evident from the equivalent circuit representation. Since fuel cell reactions are exothermic in themselves, i.e., require heat removal, it is obvious that additional heat generation is undesirable. In this regard, fuel cells, compared to batteries, for example, impose more stringent requirements on ripple reduction as a means of improving efficiency and, in some cases, lifespan, due to their higher internal resistance. Numerous studies in the field of power electronics have been published, particularly focusing on topologies and strategies for ripple mitigation in low-power fuel cell applications. Ripple mitigation also serves the purpose of minimizing electromagnetic interference, which is a limiting factor, especially in residential applications.

[0020] In high-temperature electrolysis, the reaction is endothermic, requiring extra heat to maintain cell temperature. Operating at a sufficiently high current density can provide this heat through overvoltage (a resistive loss element in the equivalent circuit). The thermal neutral voltage, i.e., the operating voltage equal to the required heat input through resistive loss, is approximately 1.3V. The current density required to achieve this voltage depends on the stack characteristics, temperature, and other operating conditions. However, operating at such high current densities is not always possible. Operating in an endothermic regime means the cell will cool unless external heat is supplied. The possibility of supplying heat, for example, through the reactants or from the environment, is limited and constitutes an additional cost to the system. Therefore, utilizing the ability to increase heat generation within the cell through ripple injection can be a cost-effective way to maintain thermal neutrality at low current densities. It is most beneficial to be able to control the amount of ripple on the supplied DC current to avoid applying unnecessary ripple when additional heating is not desired. Therefore, pulse control (pulsing) at a frequency lower than the switching frequency is beneficial. As pulse frequency increases, more switching losses are generated in power electronics, while their heating effect in the cell decreases due to capacitive elements in the equivalent circuit. Therefore, intentional heat generation by pulse control is most efficient at lower frequencies, with the lower limit being when pulse control causes undesirable concentrated overvoltages, i.e., below approximately 10 Hz. Hence, the optimal pulse frequency is likely to be found in the range between 10 Hz and 100 Hz, and possibly up to 1 kHz. Such frequencies are about two orders of magnitude lower than typical switching frequencies, i.e., not complex to achieve controllly. Alternating between thermal neutral voltage and open-circuit voltage makes the overall operation thermal neutral, but the average current is proportional to the duty cycle. Frequency can also be a function of operating current or temperature.

[0021] Thermal control and balancing of multiple groups are performed based on information on the operating temperature in the (one or more) stacks. The operating temperature of cells, stacks, and stack groups can be obtained, for example, by thermocouple measurements from inside or outside the cells. However, it is not practical to deploy a large number of physical measurements, and high-temperature instrumentation also has reliability concerns. Temperature information can also be obtained by indirect means, for example, based on current, voltage, and the flow and temperature information of reactants. Preferably, real-time dynamic thermodynamic modeling can be used as part of model-based system control of system conditions. The model can estimate the temperature profile across the stack or cell group. Control code that can run the thermodynamic model in parallel with real-time system control can be implemented, for example, on an industrial PC. Current and voltage information can be easily obtained from the power converter without cumbersome measurements in the stack environment. Flow information can also be based on thermodynamic modeling with a minimal amount of physical sensors in the stack environment in general.

[0022] Regarding solid oxide electrolysis, it has been shown that alternating between the electrolysis cell mode and the fuel cell mode can have a life improvement effect, for example, through the accumulation of oxygen pressure and the suppression of microstructural damage. Such regeneration can be applied in several ways. If the system is reversibly operable, it can be periodically switched to the fuel cell mode as needed. It has been shown that an alternating interval in the range of several hours may be sufficient to achieve the regeneration effect. However, alternating between operating modes at a pace determined by the need for internal regeneration may not match the preferences of the application's operating mode. In applications with multiple independent electrolyzer modules, this drawback can be compensated by operating one module at a time in the fuel cell mode while maintaining the other modules for electrolysis. However, the ability for bidirectional operation adds cost and complexity at the level of each module.

[0023] In a system or module that includes a plurality of stack groups having a dedicated controllable power converter arranged with a common fuel-side recirculation loop, such that, for example, while other groups operate in electrolysis, one group at a time can be brought into fuel cell mode operation. Thus, the majority of the current is driven in the electrolysis direction and the system has an overall net fuel production. The produced fuel from the groups operating in electrolysis is supplied as fuel to the groups in fuel cell mode through recirculation. Thus, the module or system as a whole, apart from the bidirectional power electronics, does not require additional complexity of reverse operation. The switching between the operating modes at the stack group level can be arbitrarily slow. However, the module-produced gas, which is a mixture of the electrolysis-mode produced gas and the fuel-cell-mode produced gas, reduces the overall reactant utilization, i.e., requires more feedstock (e.g., steam) and subsequent drying of the outlet gas. In co-electrolysis, the mixture of the fuel-cell-mode produced gas and the electrolysis-mode produced gas will also affect the product gas equilibrium.

[0024] If the alternation between the operating modes is carried out at a switching speed similar to that associated with the aforementioned pulsed thermal injection, similar benefits are achieved. Alternating at a frequency that exceeds the threshold for forming the reverse concentration gradient but is still low enough to achieve the regeneration effect makes the alternation invisible to the flow control part of the system. Thus, mode alternation can be achieved without a system-level reversibility capability and without sacrificing the reactant utilization or the product gas equilibrium composition of the system.

[0025] Clearly, mode alternation, regardless of the approach employed, has the effect of reducing overall electrolytic production at a given electrolytic current density. When semi-simultaneous electrolytic and fuel cell operation occurs, the portion of operation in fuel cell mode consumes the fuel produced in electrolytic mode. Typically, the fuel cell mode current density is half that of electrolytic current density. Therefore, for example, if 20% of operation is in fuel cell mode and 80% is electrolytic, and it operates with a rectangular waveform, the average production density is 80% - 0.5 * 20% = 70% compared to continuous electrolytic operation at a given current density. If it is sufficient to operate 10% in fuel cell mode, the average production is 85%. Even in applications intended for continuous electrolytic operation, this capacity loss can be justified if it fundamentally prevents degradation phenomena. This then allows for an increase in current density, respectively. As research shows, even infrequent alternation between operating modes on the order of hours may be sufficient to achieve beneficial degradation offsetting effects, and the operating strategy may involve abandoning mode change pulse control during peak demand times and compensating, in some cases, with a higher degree of regeneration (fuel cell) mode during off-peak times.

[0026] The requirement for frequent transitions between electrolytic and fuel cell operation imposes constraints on power conversion. Switching between modes means frequent voltage cycling between approximately 50% and 100% of the electrolytic voltage, which is problematic, especially for large capacitors. Individual buck or boost converters for each stack group can be configured to handle such transients, but this comes at the cost of requiring a pair of switch semiconductors and a dedicated full-current inductor, as well as potentially additional high-frequency ripple filter elements, for each controllable group. Furthermore, all switches must be sized to match the full electrolytic voltage. [Overview of the project]

[0027] The object of the present invention is to achieve an advanced system for electrolytic power conversion in which size and conduction loss are reduced and life is extended. This is achieved by a system for electrolytic power conversion, which comprises an electrolytic cell configured as a controllable series-connected cell group, means for electrolytic operation at a first voltage in the range of 1.0–2.5V per cell, and means for drawing current at least intermittently from the cell group at a second voltage in the range of 0.4–1.0V per cell. The system comprises at least one capacitor bank maintained at the first voltage and at least one other capacitor bank maintained at the second voltage, wherein these capacitor banks and the cell group have one pole in common, at least one bidirectional non-isolated DC / DC converter connecting the capacitor banks of the first voltage and the capacitor banks of the second voltage, means for controlling the first voltage level and the second voltage level, and at least one controllable half-bridge switch pair per cell group for individually alternating between applying the first voltage level and applying the second voltage level to the cell group to prevent increased imbalance and cell degradation.

[0028] The focus of the present invention is also a method of electrolytic power conversion, wherein an electrolytic cell is configured as a controllable series-connected cell group, electrolytic operation is performed at a first voltage in the range of 1.0–2.5V per cell, and current is drawn at least intermittently from the cell group at a second voltage in the range of 0.4–1.0V per cell. In the method, at least one capacitor bank is maintained at the first voltage, at least one capacitor bank is maintained at the second voltage, these capacitor banks and the cell group have one pole in common, and at least one bidirectional non-isolated DC / DC converter is connected to the capacitor banks of the first and second voltages, and in the method, the first and second voltage levels are controlled to alternate individually between applying the first voltage level and applying the second voltage level to the cell group in order to prevent increased imbalance and cell degradation.

[0029] The present invention is based on the use of at least one capacitor bank maintained at a first voltage and at least one other capacitor bank maintained at a second voltage, wherein these capacitor banks have one pole common to the cell group. The present invention further comprises at least one bidirectional non-isolated DC / DC converter connecting the capacitor banks of the first and second voltages, means for controlling the first and second voltage levels, and at least one controllable half-bridge switch pair per cell group for individually alternating between applying the first voltage level and applying the second voltage level to the cell group.

[0030] The advantages of this invention are that a single DC / DC converter can serve multiple groups, thus reducing system size and cost. Furthermore, conduction losses and electromagnetic interference can be minimized. [Brief explanation of the drawing]

[0031] [Figure 1] This shows an example of a conventional technology configuration in which each stack group has its own non-isolated AC / DC converter. [Figure 2] This shows an exemplary system for electrolytic power conversion according to the present invention. [Figure 3] An exemplary circuit according to the present invention is shown. [Figure 4] This shows a control means according to the present invention. [Figure 5] This diagram illustrates the phase shift of pulse control between cell groups for a fluctuating waveform. [Figure 6] This shows an example voltage waveform illustrating the phase shift of pulse control between cell groups against a fluctuating waveform. [Modes for carrying out the invention]

[0032] A system according to the present invention has at least two capacitor banks for alternating between two different voltage levels. A capacitor bank may consist of a single high-voltage discrete capacitor or multiple capacitors in parallel and / or series. These two capacitor banks share one common pole and are connected by a bidirectional non-isolated DC / DC converter. These high-voltage and low-voltage capacitor banks are common to all cell groups. These common poles are also common to all individual fuel cell groups. In the following description and reference diagrams, the negative pole is chosen to be common, but the topology can be reversed to have a common rail on the positive side.

[0033] Individual control of each group is performed by a half-bridge switch pair between the high-voltage and low-voltage capacitor voltages. The high-side switch connects the cell group to the high-voltage capacitor, and the low-side switch connects the cell group to the low-voltage capacitor. The high-voltage capacitor bank is controlled to the electrolysis voltage, and the low-voltage capacitor is controlled to the fuel cell mode voltage. In a preferred embodiment, the fuel cell group consists of approximately 750 cells in series, so that operation at a voltage of 1.3–1.4V per cell in electrolysis mode produces a DC link voltage of 975–1100V, while fuel cell operation within the range of 0.7–0.85V produces a low-side capacitor voltage of 525–640V. This high-side voltage is optimal for active rectification from a 690V AC source. The number of cells or the rectification source voltage can be optimized for a given topology in the power supply stage of the high-side capacitor.

[0034] By changing the half-bridge switch state specific to each fuel cell group, the group can be switched between electrolysis, open-circuit, and fuel cell operation without the need for dedicated inductors. Switching can be performed at low frequencies, such as 10-100 Hz, minimizing switching losses. A further advantage of this topology is that the cell group-specific half-bridge experiences only the voltage difference between the high-side and low-side capacitors. Using the voltage example above, this results in a maximum voltage difference of 575V. This allows for the use of lower-voltage gears in the group-specific switch, further reducing size, cost, and conduction losses.

[0035] A DC / DC converter that interfaces a low-voltage capacitor with a high-voltage capacitor is responsible for recirculating the power drawn during fuel cell mode pulses to the high-side capacitor bank. Its power level, and consequently the size of the switch and inductor, are significantly lower compared to electrolytic power delivery. For example, with a 20% fuel cell mode ratio having half the current density and about half the voltage of electrolysis, the average current is about 10% of the electrolytic power, and the average power is only 5% of the electrolytic power. Also, a duty cycle of nearly 50% for DC / DC conversion is advantageous for the size of the inductor. By interleaving pulses from different cell groups, the DC / DC converter can serve all groups simultaneously while still maintaining its very low power dimensions. This DC / DC converter can be a discrete converter or one leg of, for example, a four-leg inverter. The low-voltage capacitor may be individual capacitances or a subset of high-voltage capacitors, as will be described later.

[0036] In all operating modes involved in rapid pulse control between electrolysis, open-circuit, and / or fuel cell operating modes, the thermal equilibrium of individual cell groups can be controlled using the duty cycle of each mode. Electrolysis operation is thermally neutral, negative, or positive, depending on the voltage. Open-circuit is thermally neutral, while fuel cell mode is inherently always thermally positive. The flow of reactants also affects thermal equilibrium, typically causing net heat removal as well as heat loss to the environment. For example, cell groups can be kept thermally balanced by slightly adjusting the duty cycle of the fuel cell modes of individual groups. Additional mechanisms can be used. Intentional fluctuations of the high-side and / or low-side capacitor voltages can be introduced at the switching frequency. These fluctuations may be, for example, 1-10% of the average voltage. The fluctuation waveform may be sinusoidal, triangular, or square. Advantageously, the low-side voltage is varied in phase with the high-side voltage. When cell groups alternate between electrolysis mode and fuel cell mode (or open circuit) in an interleaved manner, the timing of pulses in response to voltage fluctuations results in different average voltages for different groups. Cell groups that are hotter than average are set to have their fuel cell mode pulses between the peaks of the fluctuations, thereby having the lowest average voltage in electrolysis and the highest average voltage in fuel cell mode, minimizing current flow in both modes. The coldest or least performing group is set to the opposite phase, i.e., connected to the low-side voltage at the bottom of the waveform, and therefore maximizing current. Thus, an average voltage difference of the order of several percent can be achieved for different groups, which is typically sufficient to cancel out the imbalance.

[0037] The amount of voltage fluctuation can be adjusted according to the need for balancing. By dynamically adjusting the interleaving of different cell groups, it is possible to alternate which group receives the highest or lowest voltage according to the need for balancing. Synchronization between switching devices can be achieved via an external synchronization signal, internal phase, or external phase-locked loop. The fluctuation (and thus pulse control) frequency can be equal to the grid AC frequency or twice that frequency. Such fluctuations in capacitor voltage can be easily obtained by controlling the three-phase current with a small phase imbalance. In multiple parallel systems where the imbalance is applied to different phases, the overall imbalance will cancel each other out. Alternatively, the frequency does not have to be an even multiple of the grid frequency; for example, it could be 36Hz for a 50Hz grid, so that the fluctuation is out of phase with the grid and does not appear as a harmonic. Multiple parallel systems may use slightly offset frequencies so that they cancel each other out at the grid level.

[0038] In short, this topology enables customizable switching between fuel cell mode and electrolysis mode with reduced switching losses and minimized inductive components of amount and size. The prerequisite for eliminating fuel cell group-specific inductors is that the high-side capacitor voltage can be adjusted to the operating needs. In such cases, electrolysis can be performed at the desired voltage, and consequently, at the desired current, without the need for inductors and high-frequency switching. Further fine-tuning of group-specific voltages and currents is possible using the method described above. The inductance inherent in the cell group and associated cabling limits the inrush current during switching. If the flexibility of the high-side voltage is more limited, or if it is desirable to minimize the above-mentioned inrush current, the amount of high-frequency switches and group-specific inductors or LC filters can be reduced. This two-level capacitor configuration still offers benefits. The lower voltage difference across the half-bridge switch reduces ripple on the inductor, allowing its size to be reduced by more than half.

[0039] If the high-side voltage is higher than the electrolysis voltage, the buck operation of the half-bridge will draw power from the low-side capacitance during electrolysis. For example, if the high-side capacitor voltage is equivalent to 1.4V per cell, the electrolysis operating voltage is 1.3V, and the low-side voltage is 0.7V per cell, current will be drawn from the high-side and low-side inversely proportional to the voltage difference between the low-side and high-side, i.e., 0.1V:0.6V. In these voltage examples, 14% of the electrolysis current is drawn from the low-side capacitance. By selecting the voltage, this portion can be adjusted between 0% and, for example, 20%. The alternation between electrolysis and fuel cell operation causes the current flow to the low-side capacitance in fuel cell mode to cancel out the power draw during electrolysis mode. In the example shown above, this average current was within 10% of the average electrolysis current. With appropriate voltage selection, the opposing current flows in electrolysis and fuel cell modes can cancel each other out, eliminating the need for power flow through DC / DC converters interfaced with the low-voltage and high-voltage capacitors. With the appropriate control strategy, this separate DC / DC converter can be completely eliminated. The low-side capacitor's initial charge can be performed in parallel with the high-side voltage charge via a half-bridge switch, and then maintained at the desired level via a combination of active and passive means.

[0040] In a preferred embodiment, the high-side capacitance and low-side capacitance are partially combined such that the high-side capacitance consists of at least two series-connected capacitors or capacitor banks, and the low-side capacitance is a subset thereof. In the example presented above, the low-side voltage was preferably exactly half of the high-side voltage, i.e., the midpoint between two equal capacitances in series. When the current, and therefore the voltage, at this midpoint is controlled, the voltage can be offset from the midpoint within the allowable range of the capacitor voltage. High-voltage capacitor banks consisting of multiple series-connected banks can be readily found in standard inverter devices.

[0041] In systems configured for truly bidirectional operation, i.e., continuous operation as opposed to the intermittent pulsed operation in fuel cell mode, the capacitor bank interfaced with the DC / DC converter must be sized to match the continuous fuel cell current, unless the high-side voltage can be reduced to the fuel cell level. However, the power level of this DC / DC converter and associated inductor is only about 25% of the electrolytic power, due to the lower current density and approximately half the voltage. The benefit of this topology is that a single DC / DC can serve multiple groups, while preventing the groups from becoming unbalanced in current sharing is achieved by intermittently turning off groups that would otherwise have too high a current share. Since the differences between groups are small, an off-pulse with a few percent duty cycle for the highest-performing group should be sufficient to prevent increasing the imbalance. The optimal pulse frequency, again in this case, is in the range of 10 Hz–100 Hz, thereby minimizing switching losses and electromagnetic interference.

[0042] A further advantage of this topology is that the capacitor bank interfaced with the DC / DC converter can perform electronic oxidation protection of the fuel cell during process abnormalities and / or flow interruptions. An inherent feature of the cell group-specific half-bridge is that their diodes allow current to flow from the low-side capacitor to the fuel cell group when the fuel cell voltage drops below the low-side capacitor voltage. To prevent oxidation, the cell voltage should be kept within the range of 0.8-1.0V per cell. Therefore, to perform protection, it is sufficient to ensure that the capacitor bank maintains this voltage during process abnormalities. This can be done by the DC / DC converter itself, given that the high-side voltage remains available. Furthermore, there can be redundant feeds supplying energy from, for example, a battery bank. The battery bank can be directly connected to the capacitor or power supply, or it can be rectified from a protected AC source. Because the power level is low, this redundant supply can be configured in multiple cost-effective ways. The ability to provide the above protection with the main converter in a passive state provides robustness against failures in the power stage. To prevent simultaneous energy supply and unwanted current flow to areas other than the fuel cell, a means of isolating the fuel cell circuit from the high-side voltage circuit may be necessary.

[0043] Figure 2 shows an exemplary system for electrolytic power conversion according to the present invention. A power unit 140 supplies electricity to the stack 103. A gas (e.g., air, oxygen O2, carbon dioxide CO2, nitrogen N2) is supplied from the gas control unit 126 through the temperature control 128 to the oxygen side 109. A reactant (i.e., water H2O, carbon dioxide CO2, synthesis gas) supply control 132 receives water or a mixture of water and carbon dioxide from the reactant washing unit 134 and supplies it to the steam generator 136 to generate steam. The generated steam is supplied to the fuel side 107 through the temperature control unit 138. The electrolyte side 104 is located between the fuel side 107 and the oxygen side 109. There may also be a direct route from the reactant supply control unit 132 to the temperature control unit 138 due to the carbon content of the co-electrolysis.

[0044] From the fuel side, steam is circulated to the product gas outlet 122 through the temperature control unit 138 and optionally through the pressure control unit 120. The product gases are, for example, hydrogen (H2), ammonia, methane, and / or carbon monoxide. In one embodiment, the steam may also be recirculated to the reactant supply control unit 132 or to the steam generator 136. The steam may be discharged from the steam discharge unit 130. There may also be a path from the steam discharge unit 130 to the temperature control unit 138 for ejector recirculation functionality. From the oxygen side 109, oxygen is sent to the oxygen outlet 124 through the temperature control unit 128 and optionally through the pressure control unit 120.

[0045] Figure 3 shows an exemplary circuit according to the system of the present invention. The system has an electrolytic cell configured as a controllable series-connected cell group 103, and means 142 for electrolytic operation at a first voltage in the range of 1.0–2.5V per cell. Means 144 draw current at least intermittently from the cell group at a second voltage in the range of 0.4–1.0V per cell. The system has at least one capacitor bank 150 maintained at the first voltage, and at least one other capacitor bank 151 maintained at the second voltage. These capacitor banks and cell group share one pole. In one embodiment, the system may have a high-voltage capacitance bank and a low-voltage capacitance bank that are partially combined such that the high-voltage capacitance bank has at least two series-connected capacitor banks, and the low-side capacitance bank is a subset of thereof. At least one bidirectional non-isolated DC / DC converter 146–148 connects the first and second voltage capacitor banks. The system further includes means for controlling first and second voltage levels, and at least one pair of controllable half-bridge switches per cell group for individually alternating between applying the first voltage level and the second voltage level to the cell group to prevent increased imbalance and cell degradation. In one embodiment, the system may include means for generating a synchronization signal and at least one phase-locked loop for performing synchronization between the half-bridge switches. The system may also include means 156 (Figure 4) for generating a synchronization signal and at least one phase-locked loop for performing synchronization between the half-bridge switches.

[0046] The first and second voltage ranges that are prompted are determined so as to cover low-temperature electrolysis as well. The first voltage range may be extended up to 2.5 V per cell in PEM and alkaline electrolysis applications, and the second voltage range may be extended up to 0.4 - 1.0 V. In high-temperature applications, these voltage ranges can be made narrower. For example, the first voltage can be within the range of 1.2 - 1.5 V, and the second voltage can be within the range of 0.6 - 0.9 V.

[0047] Figure 4 shows a schematic diagram of control means 152 and 156 according to the present invention. The control means is based on a microprocessor and is controlled based on measurement results (such as flow velocity, flow rate, temperature, voltage, current, etc.) to instruct the operation of the exemplary circuit 160 shown in Figure 3.

[0048] In a preferred embodiment, the control means 152 is configured to alternate cell groups between a first voltage level and a second voltage level within a frequency range of 10 Hz - 100 Hz in order to minimize switching losses and electromagnetic interference. The control means 152 can be configured to pulse-control cell groups between the electrolysis cell voltage, the fuel cell voltage, and the open circuit. The half-bridge switch 154 (Figure 3) can be controlled to operate as non-insulated DC / DC converters 146 and 148 at a switching frequency that is at least 10 times higher than the frequency of alternation between the first voltage level and the second voltage level. One definition for the first voltage can be that the first voltage is higher than 800 V and the second voltage is lower than 800 V.

[0049] In a preferred embodiment, the means for controlling the voltage level is configured to provide voltage fluctuations controlled around an average for at least one of the capacitor banks. The fluctuation frequency can be equal to the pulse control frequency of the cell groups, whereby the phase shift (t seoec , t sofc , t ocvThe open-cell voltage provides a different average voltage for each cell group. An illustrative current diagram is shown in Figure 5, and an illustrative Figure 6 shows the voltage waveform (UL, UH).

[0050] In one embodiment, the control means may be configured to alternate in a capacitor bank configured for a second voltage level by supplying a voltage to eliminate the flow of current in the opposite direction during the electrolysis mode and fuel cell mode in the cell group.

Claims

1. A system for electrolytic power conversion, comprising an electrolytic cell configured as a controllable series-connected cell group (103), means (142) for electrolytic operation at a first voltage in the range of 1.0-2.5V per cell, and means (144) for drawing current at least intermittently from the cell group at a second voltage in the range of 0.4-1.0V per cell. The system in question is A capacitor bank (150) maintained at the first voltage and a capacitor bank (151) maintained at the second voltage, wherein the capacitor banks (150, 151) and the cell group (103) share one common pole. At least one bidirectional non-isolated DC / DC converter (146, 148) connecting the capacitor banks of the first voltage and the second voltage, Means (152) for controlling the first voltage level and the second voltage level, To prevent increased imbalance and cell degradation, a pair of at least one controllable half-bridge switches (154) for each cell group (103) is provided to individually alternate between applying the first voltage level and the second voltage level to the cell group (103), A system for electrolytic power conversion characterized by having the following features.

2. The system for electrolytic power conversion according to claim 1, characterized in that the controlling means (152) is configured to alternate the cell group between the first voltage level and the second voltage level within a frequency range of 10 Hz to 100 Hz in order to minimize switching losses and electromagnetic interference.

3. The system for electrolytic power conversion according to claim 1, characterized in that the controlling means (152) is configured to pulse control the cell group between the electrolytic cell voltage, the fuel cell voltage, and the open circuit.

4. The system for electrolytic power conversion according to claim 3, characterized in that the controlling means (152) is configured to alternate in the capacitor bank (151) configured for the second voltage level by supplying a voltage to eliminate the flow of current in the opposite direction during the electrolytic mode and fuel cell mode in the cell group (103).

5. The system for electrolytic power conversion according to claim 1, characterized in that the system comprises means (156) for generating a synchronization signal and at least one phase-locked loop for performing synchronization between the half-bridge switches (154).

6. The system for electrolytic power conversion according to claim 1, characterized in that the system includes a high-voltage capacitance bank (150) and a low-voltage capacitance bank (151), such that the high-voltage capacitance bank has at least two capacitor banks connected in series, a subset of which constitute the low-voltage capacitance bank, in a partial combination.

7. A method for electrolytic power conversion, wherein an electrolytic cell is configured as a controllable series-connected cell group (103), electrolytic operation is performed at a first voltage in the range of 1.0-2.5V per cell, and current is drawn at least intermittently from the cell group at a second voltage in the range of 0.4-1.0V per cell. In this method, at least one capacitor bank (150) is maintained at the first voltage, at least one capacitor bank (151) is maintained at the second voltage, the capacitor banks (150, 151) and the cell group (103) share one pole, and at least one bidirectional non-isolated DC / DC converter (146, 148) is connected to the capacitor banks of the first and second voltages. In this method, the first voltage level and the second voltage level are controlled to alternate individually between applying the first voltage level and the second voltage level to the cell group (103) in order to prevent increased imbalance and cell degradation. A method for electrolytic power conversion characterized by the following.

8. The electrolytic power conversion method according to claim 7, characterized in that, in order to minimize switching losses and electromagnetic interference, the cell groups are alternated between the first voltage level and the second voltage level within a frequency range of 10 Hz to 100 Hz.

9. The electrolytic power conversion method according to claim 7 or 8, characterized in that the half-bridge switch (154) is controlled to operate as a non-isolated DC / DC converter at a switching frequency at least 10 times higher than the alternation frequency between the first voltage level and the second voltage level.

10. The electrolytic power conversion method according to claim 7, characterized in that the cell group is pulse-controlled between the electrolytic cell voltage, the fuel cell voltage, and the open circuit.

11. The electrolytic power conversion method according to claim 10, characterized in that, in the capacitor bank (151) configured for the second voltage level, alternation is performed by supplying a voltage to eliminate the flow of current in the opposite direction during the electrolytic mode and fuel cell mode in the cell group (103).

12. The electrolytic power conversion method according to claim 7, characterized in that a synchronization signal is generated and at least one phase-locked loop performs synchronization between half-bridge switches (154) configured to alternate between applying the first voltage level and applying the second voltage level to the cell group (103).

13. The electrolytic power conversion method according to claim 7, characterized in that the high-voltage capacitance bank (150) and the low-voltage capacitance bank (151) are partially combined such that the high-voltage capacitance bank has at least two capacitor banks connected in series, a subset of which constitutes the low-voltage capacitance bank.

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