Power conversion system for electrolysis stacks

Abstract

The present invention relates to a power conversion system for multiple electrolysis cell stack units, the power conversion system comprising a parallel arrangement of multiple DC / DC conversion modules, each DC / DC conversion module configured to supply power to a single electrolysis cell stack unit, each DC / DC conversion module being capable of supplying a predetermined variation of current, power and / or voltage to the electrolysis cell stack unit such that near-thermoneutral operation at partial load is enabled by matching the integral Joule heat generation with the integral reaction heat consumption in the electrolysis cell stack unit, and / or each DC / DC conversion module being capable of reversing the current supplied to said electrolysis cell stack unit, causing said electrolysis cell stack unit to operate in fuel cell mode. The power conversion system allows for an facilitated and inexpensive power distribution, a long life span and improved thermal management during operation of the electrolysis cell stack. In further aspects, the invention relates to a power distribution system and an electrolysis plant comprising said power conversion system, as well as related methods.

Description

[Technical field]

[0001] The present invention relates to a power conversion system for multiple electrolysis cell stack units that allows for easy and inexpensive power distribution and improved thermal management during operation of the electrolysis cell stack and / or extended lifetime of the electrolysis cell stack.

[0002] In further aspects, the present invention relates to power distribution systems and electrolysis plants including said power conversion systems, and associated methods. [Background technology]

[0003] Due to their inherent ability to convert electrical energy into chemical energy, electrolysis systems are generally considered to be a key technology for the renewable energy economy. However, the efficient operation of large electrolysis plants (e.g., operating at MW-scale power) containing multiple electrolysis cell stacks poses several challenges.

[0004] Effective thermal management represents one of these challenges. For example, high temperature electrolysis cells (such as solid oxide electrolysis cells (SOECs)), which typically operate at temperatures between 600 and 900 °C, require that heat be supplied to the cell to sustain the endothermic electrolysis reaction. The heat supply can be provided by preheating the inlet gases or, alternatively, by heating the electrolysis cell stack to a thermal neutral potential E tn Thermoneutral potential can be achieved by operating at a temperature of 0.1° C. for 10 min at 1000° C. The thermoneutral potential represents the SRU (single repeating unit consisting of electrolysis cells and interconnects) voltage where Joule heat (i.e., heat generated by current flowing through internal resistance in the SRU) is matched to the heat required by the electrolysis reaction, thereby minimizing external input or output energy and enhancing electrolysis efficiency. However, for SOEC systems operating under typical temperatures and gas compositions at atmospheric pressure (e.g., steam and / or CO 2 In the electrolytic cell, E tnOperation of a single repeat unit at E typically results in high electrode overpotentials at the SRU and / or excessive degradation due to adsorption of impurities at electrochemically active sites at the electrode. tn Increasing the current density (and hence the electrolysis current density) beyond this will lead to even faster degradation, lower conversion efficiency, and increased need for thermal control to dissipate excess Joule heat. On the other hand, if a high temperature electrolysis cell stack is used, the OCV and E tn If the electrolysis stack is operated at an SRU voltage between 0.1 V and 0.5 V, the gas in the stack generally cools from the inlet to the outlet. The temperature from the gas inlet to the gas outlet can drop significantly, despite extensive efforts to limit the temperature drop in the stack by sweep gas. The temperature drop increases the internal resistance, which in turn reduces the absolute current density at the stack outlet, resulting in a non-uniform current distribution in the stack. Operating the electrolysis stack by drawing a constant current from the power supply (and thus resulting in constant voltage operation) can therefore result in a relatively large temperature drop across the electrolysis stack. Alternatively, when dynamically changing the SRU voltage and current, the temperature distribution in the stack can also change over time due to changes in the reaction and Joule heat generation. The resulting non-uniform heat distribution creates thermo-mechanical stresses, which can lead to contact losses at the interfaces between the various layers in the stack (typically between the stack and the bipolar interconnect plate).

[0005] C. Graves et al., Nature Materials 2015, 14, 239-244, disclose that in the context of a single steam electrolysis cell, reverse cycling between electrolysis and fuel cell operation by periodically reversing the current leads to a more stable cell voltage and an extended cell life. However, the beneficial effect was observed in tests of a single cell mounted in an externally heated near-isothermal enclosure. Dynamic operation of a larger stack would result in significantly larger temperature changes, which could induce higher decomposition rates due to thermal stresses, obscuring the beneficial effect ("Solid oxide electrolysis for grid balancing", Final Report for Energinet.dk, project no. 2013-1-12013, Fig. 27, Pp. 35).

[0006] Considering the above problems, WO2020 / 201485A1 proposes to operate one or more electrolysis cells by providing one or more voltage changes to the electrolysis cells by at least one power electronics unit, the voltage changes being configured such that near-thermal neutral operation at partial load is enabled by matching the integral Joule heat generation with the integral reaction heat consumption in the cells. The method allows the provision of a low-cost electrolysis system, which at the same time allows for fast response dynamic operation, improved electrolysis efficiency, extended life and high impurity tolerance.

[0007] However, a power supply configuration that allows efficient and stable operation of multiple electrolysis stacks in a MW-scale plant under these conditions has not been disclosed.

[0008] Generally, the supply of the electrical DC power required for electrolysis operation involves two stages: the conversion of the source AC voltage to a (pseudo) DC voltage, and the adaptation of the latter voltage to the desired DC load voltage level by a DC-DC converter, which may be selectively galvanically isolated. Between these two stages, a low-pass filter (typically an LC filter consisting of inductors and capacitors) is typically inserted to reduce ripples in the DC input voltage, smooth the AC source current, and attenuate noise originating from electromagnetic interference. A typical approach for powering a corresponding electrolysis stack is shown in FIG. 1A, where a DC / DC conversion stage is coupled in parallel to the load, i.e. the electrolysis stack. Such a configuration may also include a converter connected to the same load using phase-shift control, such as, for example, the three-phase interleaved buck converter disclosed in B. Yodwong et al., Electronics 2020, 9, 912.

[0009] However, efforts have been made to dispense with the need for capacitors in the filter stage, especially the capacitors used, since they contribute substantially to the cost and complexity of the power supply system and are often prone to failure during long-term operation. For this purpose, EP2963761A1 proposes an AC-DC power conversion unit configuration according to FIG. 1B, in which the electrolytic stack and the power converter are coupled in series, so that the same current flows through the input of the electrolytic stack to the power converter, and the input power to the series of loads and converters is provided by the output of the AC-DC rectifier. In this configuration, the voltage ripple at the load is advantageously removed by the converter acting as an active filter, but is not suitable for effective dynamic near-thermal neutral operation at partial loads by matching the integral Joule heat generation with the integral reaction heat consumption in the electrolytic stack.

[0010] U.S. Patent Application Publication No. 2017 / 0005357 (A1) discloses a grid-tied power distribution system for a reversible solid oxide fuel cell stack, which incorporates a bidirectional AC-DC converter to provide power to or draw power from a fuel cell system, but does not address the above problems.

[0011] In view of the above, there is still a need to provide a simple and inexpensive power source that enables the fast-response dynamic operation of a large-scale electrolysis plant including a number of electrolysis stacks simultaneously and has a long lifespan.

[0012] WO2018 / 033948A1 discloses a hydrogen production system comprising an electrolysis cell stack, a power source for supplying a constant current to the stack, and a temperature control mechanism configured to control the temperature of the cell stack such that the generated voltage achieves a previously set target voltage. However, WO2018 / 033948A1 does not disclose or suggest a parallel arrangement of a plurality of DC / DC converter modules.

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0013] The present invention solves these objectives by the subject matter of the claims defined herein. Further advantages of the present invention are explained in more detail in the following sections.

MEANS FOR SOLVING THE PROBLEMS

[0014] Generally, the present invention relates to a power converter system for multiple electrolysis cell stack units, the power converter system comprising a parallel arrangement of multiple DC / DC conversion modules, each DC / DC conversion module configured to supply power to a single electrolysis cell stack unit, each DC / DC conversion module being capable of supplying a predetermined variation in current, power and / or voltage to the electrolysis cell stack unit, thereby enabling near-thermal neutral operation at partial load by matching integral Joule heat generation with integral reaction heat consumption in the electrolysis cell stack unit, and / or each DC / DC conversion module being capable of reversing the current supplied to the electrolysis cell stack unit, causing the electrolysis cell stack unit to operate in fuel cell mode.

[0015] In a further embodiment, the present invention provides a power distribution system for multiple electrolysis cell stack units comprising a common bus comprising a transformer, one or more rectifiers and optional input filters, and the aforementioned power conversion system connected to the common bus.

[0016] In another embodiment, the present invention relates to an electrolysis power plant comprising a power distribution system as described above and a plurality of electrolysis cell stack units.

[0017] In another embodiment, the invention relates to a method for distributing power to a plurality of electrolysis cell stack units, the method comprising: coupling a common bus comprising a transformer, one or more rectifiers and an input filter between a power grid and a plurality of DC / DC conversion modules arranged in parallel; connecting each DC / DC conversion module to a separate electrolysis cell stack unit; independently supplying a predetermined variation of current, power and / or voltage to one or some of the electrolysis cell stack units via the DC / DC conversion module, whereby near-thermal neutral operation at partial load is enabled by matching the integrated Joule heat generation with the integrated reaction heat consumption in the electrolysis cell stack unit; and / or independently reversing the current supplied to one or some of the plurality of electrolysis cell stacks via the DC / DC converter to cause said electrolysis cell stack units to operate in a fuel cell mode.

[0018] Advantageously, the present invention enables efficient, cheap and effective power management for large scale electrolysis plants by connecting a DC / DC converter to multiple loads (i.e. electrolysis stacks) and enabling load shift coordination between these multiple loads to provide a constant DC link voltage and at the same time near-thermal neutral operation and / or high degradation resistance of the electrolysis stacks.

[0019] Preferred embodiments of the systems and associated methods for operating one or more electrolysis cells, as well as other aspects of the invention, are set forth in the following description and claims. [Brief description of the drawings]

[0020] [Figure 1] 1A illustrates a conventional power distribution system with an AC / DC conversion stage and a DC / DC conversion stage coupled in parallel to a load, i.e., an electrolytic stack, and (B) illustrates a conventional power distribution system in which an electrolytic stack and a power converter are coupled in series. [Diagram 2]FIG. 1 shows a schematic diagram of a system of transformers, rectifiers, input filters, modules and a storage tank for the electrolysis product. [Diagram 3] FIG. 1 is a system diagram showing a single module consisting of N units, each unit consisting of N converters and N electrolytic stacks. [Figure 4] FIG. 1 illustrates an equivalent electrical circuit model for a stack of 75 series-connected cells. [Diagram 5] FIG. 2 shows the characteristics of an electrolytic stack based on the Thevenin model, with maximum current, voltage and power indicated by stars. [Figure 6] FIG. 14 shows the current at 5%, 35%, 65% and 95% rated power for thermal balance, calculations are done for HO electrolysis, i.e., K=−0.3 V per cell. [Figure 7] FIG. 13 shows cell voltages at 5%, 35%, 65% and 95% nominal power for thermal balance, where calculations are done for HO electrolysis, i.e., K=−0.3 V per cell. [Figure 8] FIG. 1 shows Joules and heat of reaction for "a" and "b" intervals, calculations are done for CO2 electrolysis, i.e., K=-0.5V per cell. [Figure 9] The top row shows the mean (solid line) and average (dashed line) module currents for two (far left), three, four, five and six (far right) units respectively, the next row shows the unit currents. The vertical dotted black lines indicate periods. The figures are shown for units operating in fuel cell mode 20% of the time. [Figure 10] FIG. 13 shows the current in the units and modules for five units at different duty cycles for N=5 units, where IEb=-0.5IEa. [Figure 11] FIG. 13 shows the maximum, average and minimum modulus current for N=5 units at different duty cycles. [Figure 12] FIG. 1 is a diagram showing an LTSpice simulation model. [Figure 13] DC link capacitor voltage (grey curve), current through the LC filter inductor (upper black curve) and input current to one converter (lower black curve) for duty cycles of 0.75 (top), 0.80 (middle) and 0.85 (bottom). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] For a more complete understanding of the present invention, reference is now made to the following description of exemplary embodiments thereof.

[0022] Power Conversion Systems In a first embodiment, the present invention relates to a power conversion system for multiple electrolysis cell stack units, comprising a parallel arrangement of multiple DC / DC conversion modules, each DC / DC conversion module configured to supply power to a single electrolysis cell stack unit, each DC / DC conversion module being capable of supplying a predetermined variation in current, power and / or voltage to said electrolysis cell stack unit, whereby near-thermal neutral operation at partial load is enabled by matching the integral Joule heat generation with the integral reaction heat consumption in the electrolysis cell stack unit, and / or each DC / DC conversion module being capable of reversing the current supplied to said electrolysis cell stack unit, causing said electrolysis cell stack unit to operate in fuel cell mode.

[0023] In this context, "plurality" refers to a plurality of DC / DC conversion modules. The specific number of DC / DC conversion modules is not particularly limited and preferably corresponds to the number of electrolysis cell stack units to be powered. Typically, the number of conversion modules ranges from 2 to 100, e.g., from 3 to 50.

[0024] In practice, electrolysis systems typically operate under conditions that are neither completely isothermal nor completely adiabatic. The term "near-thermonutral operation", as used herein, refers to electrolysis operation in which the absolute value of the difference between the integrated Joule heat production and the integrated reaction heat consumption (both integrated over a period longer than 3600 seconds) is lower than the absolute value of the integrated heat consumption or the absolute value of the integrated heat production, or both.

[0025] In a preferred embodiment, "near-thermoneutral operation" is understood as electrothermal balance operation, which can be distinguished from conventional thermal balance operation, where electrothermal (Joule heating) is used to balance the required reaction heat, and the heat capacity of the excess air flow is used to limit the temperature changes in the electrolysis cells and stack. A side effect of the present invention is that the need to blow hot air through the stack is reduced. In addition, the impurity tolerance can be improved, which reduces the need for gas purification. Both side effects substantially reduce the overall cost of the system.

[0026] The expression "part load" refers to a condition where the electrolysis stack operates at less than 100% of its maximum power output, for example 99.9% or less, preferably 0.1% or more.

[0027] The term "fluctuations", as used herein, refers to predetermined variations of the cell current, power and / or voltage, which may be applied in the form of periodic variations recurring at predetermined intervals. With a view to reducing mechanical tensions, the duration of each fluctuation, i.e. the duration of the deviation of the current, power and / or voltage from the normal operating values, is preferably set in the range of 1 μs to 1000 s, more preferably in the range of 1 μs to 100 s. By operating the electrolysis system correspondingly, the duration of each fluctuation is short enough that the fluids (e.g. gases) as well as temperature changes in the cells and stack are negligible. In this way, the accumulation of mechanical tensions at weak interfaces in the stack can be avoided, making it possible to achieve an extended lifespan made possible by the reversible operation. In a further preferred embodiment, the frequency of the fluctuations is in the range of 10 mHz to 100 kHz. Frequencies in the range of 10 mHz to less than 20 kHz are further preferred, while frequencies of 20 mHz to 10 kHz are particularly preferred. In particular, the present invention differs from converters using phase shift control. Because the phase-shift controls are connected to the same load, their time shift is limited by the switching frequency (usually higher than 20 kHz).

[0028] The shape of the variation is not particularly limited. However, variations including sinusoidal and / or square wave-shaped variation profiles are preferred. Although symmetrical and especially asymmetrical square wave-shaped variations are generally the most effective and practical, smooth sinusoidal variations may be preferred to minimize stray inductance and eddy currents in the SRU. A combination of sinusoidal and square wave-shaped variations may be preferred to minimize peak voltages and avoid malfunction conditions associated with induction phenomena.

[0029] In principle, either the current, the power and / or the voltage may be modulated to allow near-thermal neutral operation at partial load. However, from the point of view of practicality, a certain variation of the voltage is particularly preferred. In this context, the range of the voltage variation is generally 0.2V to 2.0V, particularly preferably 0.5V to 1.9V.

[0030] However, in an alternative preferred embodiment, current control may be preferred over voltage control mode due to the increased ability to control the temperature in the stack. For example, if the temperature at the stack outlet increases slightly, the regional resistance in said region is decreased. In voltage control mode, the decreased resistance in said region implies a higher steam conversion rate and thus an increased Nernst voltage. As a result, during stack operation in electrolysis mode, the heat consumption from the electrolysis reaction decreases, but during stack operation in electrolysis mode, the heat generation increases, which leads to a net increase in heat generation. Depending on the electrolysis and fuel cell voltage settings, the Joule heat generation can increase or decrease. However, this means that the heat generation in the stack outlet region is likely to increase, which may create a risk for thermal runaway. In the case of current control, the situation is different, because a slight temperature increase in the stack outlet region causes a decrease in the regional resistance in said region. When operating in current control mode, the net reaction heat is not affected by the decrease in resistance. However, the Joule heat generation decreases, whereby the slight temperature increase is offset by the decrease in heat generation. Therefore, stable operation at a controlled temperature can be easily ensured.

[0031] In other embodiments, power controlled operation is preferred. In yet other preferred embodiments, a hybrid of current, power or voltage controlled operation may be preferred.

[0032] According to the present invention, by connecting each of the multiple DC / DC conversion modules present in a parallel arrangement to a single electrolysis cell stack unit, the dynamic near-thermal neutral operation of each electrolysis cell stack unit can not only be controlled separately, but also effectively coordinated between the electrolysis cell stack units. By coordinating the shift between the multiple loads (i.e., electrolysis cell stack units), it is advantageously possible to obtain a constant and smooth DC link voltage without the need for expensive (large) filter capacitors that potentially limit the life of the power supply system, while still maintaining the near-thermal neutral operation of each electrolysis cell stack and its associated advantages. Thus, all electrolysis cell stack units can be operated at optimal performance.

[0033] Generally, it may be preferable to configure the cell to evaporate, desorb or dissolve side reaction compounds that are adsorbed, precipitated or otherwise formed on the cell electrodes, for example by increasing (oxidation) or decreasing (reduction) the oxidation state of the side reaction compounds, leading to reduced degradation, more stable cell voltage and extended cell life. Without being limited thereto, as long as their formation is reversible, these side reaction compounds may be undesirable intermediates or may originate from impurities in the reactants (e.g., hydroxides formed by alkaline earth metals, hydrocarbons, sulfur-based compounds, formaldehyde, ammonia formate, halogenated compounds) or electrolysis cell materials (e.g., Si-based impurities from glass components).

[0034] A particularly efficient reduction of the degradation due to desorption or dissolution of side reaction compounds can be achieved by reversing the current supplied to the electrolysis cell stack unit for performing the fuel cell operation of said electrolysis cell stack unit. According to the invention, the current reversal can be performed independently of the predetermined variation defined above, i.e. instead of or in addition to the modulation of the current, power and voltage (e.g. either during a part or during the entire predetermined variation of the voltage and / or power). Thus, the "predetermined variation" of the current, including the current reversal, does not necessarily have to be directed towards a near-thermoneutral operation, as long as it performs the dissolution or desorption of undesired intermediates, impurities or other compounds formed by reversible reactions and causing degradation of the electrolysis performance.

[0035] As an example of desorption of undesired intermediates, impurities or other compounds, H 2 O and CO 2 Sulfur and SiO from the Ni surface of Ni / YSZ electrode observed during electrolysis 2 The desorption of CO may be mentioned. 2 In the high temperature electrolysis of , periodic current reversal also allows for desorption of carbon at the reaction sites, thus advantageously resulting in a more rapid CO 2 However, the beneficial effects are not limited to these particular species, but extend in principle to all reversible side reaction mechanisms that may interfere with the performance of the electrolysis cell.

[0036] In particular, the DC / DC conversion module is preferably configured to apply a periodic current reversal at a preferred frequency in the range of 10 mHz to 100 kHz, more preferably in the range of 10 mHz to less than 20 kHz, with a frequency of 20 mHz to 10 kHz being particularly preferred. As with the above-mentioned predetermined variations in current, power and / or voltage, the duration of the current reversal is set in the range of 1 μs to 1000 s, more preferably in the range of 1 μs to 100 s. In this context, the current modulation may for example comprise a sinusoidal and / or square wave shaped current variation profile.

[0037] In an alternative embodiment, it is also possible to apply current reversals at random frequencies or at a range of frequencies, provided that the shifts between multiple loads (i.e. electrolysis cell stack units) are appropriately coordinated to obtain a constant and smooth DC link voltage.

[0038] In a preferred embodiment, each DC / DC conversion module may therefore further include one or more electronic switches configured to reverse the current supplied to the electrolysis cell stack unit. Suitable electronic switches may be selected from electronic or electromechanical switches known to those skilled in the art that are useful in power converters. Advantageously, this process may reduce damage to the electrode microstructure and / or result in desorption or dissolution of side reaction compounds that are adsorbed, precipitated or otherwise formed on the electrodes of the cell. Furthermore, not all products from the integrated electrochemical reactions still present in the cell during the portion of time that the current is reversed are converted back into reactants. Thus, contrary to conventional (DC voltage) operation, there is no need to change the fluid (e.g., gas) composition. By allowing reverse current operation (stack acting as a source) for short periods of time, a constant DC link voltage operation can be ensured. Without load regulation, an energy storage device, e.g., a battery or supercapacitor, would need to be added to handle the stack acting as a source for short periods of time, inevitably resulting in increased cost and complexity.

[0039] The Joule heating due to the required overvoltage and current is positive in both fuel cell and electrolysis modes. In high temperature electrolysis, E tn Near-thermal neutral operation at lower operating voltages is desirable for optimum performance. OCV and E tnNear-thermal neutral operation requires heat addition during the electrolysis process due to the operating voltage between 0.1 V and 0.2 V. In such systems, heat addition is also required to reduce tensile stresses at the interconnect / cell interface, potentially leading to delamination and loss of contact, poor performance and degradation. Conventionally, heat is provided, for example, by the use of heated sweep gas or active heating devices. In contrast, in the present invention, Joule heat is balanced with reaction heat (and heat loss to the surroundings) by supplying one or more fluctuations of current, power and / or voltage to the electrolysis stack via a DC / DC conversion module. Thus, the electrolysis stack unit can be operated near-thermal neutrally without the need for an external heating source.

[0040] It is emphasized that the power conversion system according to the invention may be used to counteract heat losses that are not necessarily caused by reaction heat consumption higher than Joule heat generation during DC operation. For example, heat may be lost to the surroundings through insulating materials or in a heat exchanger. In this case, energy consumption during idle / standby operation may be minimized, since the gas flow (and associated heat loss in the heat exchanger) is limited to maintain the stack at a given temperature while using AC / DC operation instead to counteract heat losses to the surroundings. This allows the maintenance of individual stacks or modules, especially auxiliary equipment, to be simplified and accelerated, while interfering with stack operation to a minimum. Although not limited thereto, such an operation method is particularly advantageous in applications where auxiliary equipment failures tend to be substantially more important than actual fuel cell module failures (e.g. in small modules such as micro-combined heat and power (micro-CHP) systems (see ERNielsen et al., Fuel Cells 2009, 19, 340-345)).

[0041] In a preferred embodiment of the power conversion system, only one or a portion of the DC / DC conversion modules simultaneously supply a given variation in current, power and / or voltage.

[0042] Particularly preferably, the power conversion system further comprises a control unit connected to each of the DC / DC conversion modules and configured to regulate a predetermined variation of the current, power and / or voltage in the DC / DC conversion modules in an alternating manner, whereby the application of the predetermined variation is cycled between one or a subset of the DC / DC conversion modules (hereinafter also referred to as "duty cycle").

[0043] In a further preferred embodiment, the control unit may be configured to regulate predetermined variations in one or more DC / DC conversion modules such that the common DC link voltage remains approximately constant during the predetermined variations. This may be achieved by appropriately adjusting the amplitude, duration and / or frequency of the current, power and / or voltage variations for each individual module or subset of modules, as well as the duty cycle distribution among the modules or subset of modules. The control unit may include a number of electronic switches and synchronization means (e.g. distributed clock signals).

[0044] The means for providing the desired variation in current, power and / or voltage may be selected by one skilled in the art from known components. In a preferred embodiment according to the present invention, the power conversion system includes a pulse width modulation (PWM) circuit, optionally in combination with a motor controller, to provide the predetermined voltage variation. Such components may be integrated into individual DC / DC conversion modules or into a common circuit, provided that independent operation in a parallel arrangement is still possible.

[0045] In another preferred embodiment, the power conversion system according to the present invention further comprises one or more sensors configured to acquire physical data related to the electrolytic cell unit, and a PID (Proportional-Integral-Derivative) controller configured to control fluctuations in current, power and / or voltage based on the measured values of the acquired sensor data. The PID controller may be configured to continuously calculate an error value as the difference between a desired data set point and the measured sensor data, enabling the power conversion module to apply corrections to fluctuations in current, power and / or voltage based on the proportional, integral and derivative terms. Target parameters for the sensor data may include, for example, temperature (e.g., inlet and outlet temperatures of a fluid (gas or liquid) sent to or from the electrolytic cell or stack, or direct temperature measurements in the cell compartments), gas pressure, gas concentration, impedance, resistance and current.

[0046] In a further preferred embodiment, the PID controller may be configured to control fluctuations in current, power and / or voltage based on the dynamic current / voltage response of the electrolytic cell stack or unit.

[0047] In a particularly preferred embodiment, the power conversion system includes, as a sensor, a Laplace transform impedance spectrometer configured to measure the frequency domain impedance spectrum of the electrolytic cell, stack or unit (e.g., by means of a current pulse method or a voltage pulse method) to provide information about the health, temperature and performance of individual electrolytic cells, stacks or units, which may then be selectively sent to a PID device to control fluctuations in current (including current reversal), power and / or voltage.

[0048] To achieve a certain modular current, the power conversion system preferably satisfies the following mathematical formula (Formula 1).

[0049]

Number

[0050]

number

[0051] The expression "electrolysis cell stack unit" as used herein refers to an electrolysis cell stack or an assembly of multiple electrolysis cell stacks, the latter of which may be connected in series and / or in parallel depending on the desired performance, as known to those skilled in the art.

[0052] The electrolysis cells forming each cell stack are not particularly limited, but the invention is most useful for high temperature electrolysis cells, such as those configured to operate above 120° C., for example, between 200° C. and 1100° C., or between 650° C. and 1000° C. Preferred examples thereof include, but are not limited to, solid oxide electrolysis / fuel cells (SOEC / SOFC), molten carbonate electrolysis / fuel cells (MCEC / MCFC), high temperature and pressure alkaline electrolysis / fuel cells, and ceramic electrolyte proton conducting electrolysis / fuel cells (PCEC / PCFC).

[0053] The reactant materials are not particularly limited. In a preferred embodiment, the electrolytic cells forming the stack and stack unit are 2 O, CO 2 Electrolysis of, or H 2 O and CO 2 Co-electrolysis is carried out.

[0054] Details relating to materials and construction techniques for electrolysis cells are well known to those skilled in the art and will not be described herein.

[0055] It will be understood that the term "electrolysis cell", as used herein, also includes reversible fuel cells, such as, for example, reversible solid oxide fuel cells (RSOFCs). In such embodiments, the power conversion stage is preferably bidirectional.

[0056] Power Distribution System In a second embodiment, the present invention relates to a power distribution system for a plurality of electrolysis cell stack units, the power distribution system comprising a common bus comprising a transformer, one or more rectifiers and optional input filters, and a power conversion system according to the first embodiment connected to the common bus. The power distribution system thus allows the distribution of voltage from a single AC grid source to the power conversion systems and thus to the electrolysis stack units.

[0057] The term "connected," as used herein, means electrically connected.

[0058] Since the electrolysis units need to be supplied with a DC voltage, the main purpose of the common bus is to convert the AC voltage from the power grid into a (pseudo) DC voltage.

[0059] In particular, the voltage is scaled down by a transformer before being rectified by one or more rectifiers, which for this purpose may be suitably adopted by those skilled in the art depending on the distribution grid voltage and the voltage supplied to the rectification stage.

[0060] Rectification may be performed by active or passive rectifiers. Examples of passive rectifiers include, but are not limited to, thyristor-based and diode-based rectifiers, and examples of active rectifiers include actively controlled switching elements with diode function, such as bipolar junction transistors (BJTs), metal oxide semiconductor field effect transistors (MOSFETs), insulated gate bipolar transistors (IGBTs) and silicon controlled rectifiers (SCRs). Active rectifiers generally tolerate smaller losses, but are also more expensive. By allowing bidirectional current, active rectifiers also have the ability to send energy back to the grid, although this function is not necessarily required for electrolysis plants. Under these circumstances, it may be preferable to select one or more rectifiers from inexpensive passive rectifiers. The rectifiers may be, for example, one-phase or three-phase rectifiers (e.g., 6-pulse or 12-pulse diode bridge rectifiers).

[0061] An optional input filter, typically consisting of an inductor and one or more capacitors (i.e., an LC filter), allows smoothing of the voltage ripple, which is usually important for optimizing the specific energy consumption and reliability of the electrolysis stack. Advantageously, the invention does not require any large capacitors or capacitors as input filters, since the ripple effects are minimized by regulating the switching between the loads to ensure a constant DC link voltage. If implemented, the total capacitance inserted between the rectification stage and the power conversion system is preferably less than 5000 μF, more preferably less than 1000 μF, and particularly preferably less than 500 μF.

[0062] The power distribution system may include further subsystems including circuit breaker sensors and a master controller device (including a microprocessor having a processor and system memory) that may be coupled to a computer terminal. These subsystems may be suitably implemented on a common bus or in a connected DC / DC power conversion system.

[0063] Electrolysis Power Plant In a third embodiment, the present invention relates to an electrolysis power plant comprising a power distribution system according to the second embodiment and a plurality of electrolysis cell stack units.

[0064] As mentioned above, each electrolysis cell stack unit preferably comprises one or more stacks of a solid oxide electrolysis / fuel cell (SOEC / SOFC), a molten carbonate electrolysis / fuel cell (MCEC / MCFC), a high temperature and pressure alkaline electrolysis / fuel cell, and a ceramic electrolyte proton conducting electrolysis / fuel cell (PCEC / PCFC).

[0065] In a preferred embodiment, the electrolysis power plant has a total electrical input power of 1 MW or more. Of course, it must be ensured that the AC grid voltage selected is appropriate for the load. For example, a 10 kV network is sufficient for a 1 MW load.

[0066] Methods for power distribution In a fourth embodiment, the present invention relates to a method for distributing power to a plurality of electrolysis cell stack units, the method comprising: coupling a common bus comprising a transformer, one or more rectifiers and an input filter between a power grid and a plurality of DC / DC conversion modules arranged in parallel; connecting each DC / DC conversion module to a separate electrolysis cell stack unit; independently supplying a predetermined variation of current, power and / or voltage to one or a portion of the plurality of electrolysis cell stack units via the DC / DC conversion modules such that near-thermal neutral operation at partial load is enabled by matching the integral Joule heat generation with the integral reaction heat consumption in the electrolysis cell stack unit; and / or independently reversing the current supplied to one or a portion of the plurality of electrolysis cell stack units via the DC / DC conversion modules to operate said electrolysis cell stack units in a fuel cell mode.

[0067] Preferably, only one or a portion of the DC / DC conversion modules apply a given variation in current, power and / or voltage at a time.

[0068] The term "part" as used herein refers to a number greater than 1 and less than the total number of units in the plurality of electrolysis stack units. In a preferred embodiment, the ratio of electrolysis stack units to which current reversal is applied (i.e., stack units in fuel cell mode) or the ratio of electrolysis stack units to which a predetermined variation in current, power and / or voltage is applied, relative to the total number of electrolysis stack units, is in the range of 1-45%, more preferably in the range of 1-40%, respectively.

[0069] In a preferred embodiment, one or some of the electrolysis cell stack units are independently supplied with a predetermined variation of current via a DC / DC conversion module or the current supplied to one or some of the electrolysis cell stack units is reversed. This mode of operation allows improved control of the stack temperature. For example, in the case of a slight temperature increase at the stack outlet region, the net reaction heat is not affected by the resulting reduction in resistance. However, the Joule heat generation is reduced, whereby the slight temperature increase is offset by the reduced heat generation. The current control mode therefore allows a particularly stable operation at a controlled temperature.

[0070] In an alternative embodiment, it is also possible to apply current reversals at random or a predetermined range of frequencies in combination with shifting between multiple loads (i.e. electrolysis cell stack units) to obtain a constant and smooth DC link voltage.

[0071] In a preferred embodiment, the method according to the invention comprises acquiring physical data related to the electrolysis cell unit and controlling current reversal and / or predetermined variations in current, power and / or voltage based on measurements of the acquired physical data, for example by using one or more sensors (temperature sensors, lambda sensors, etc.) in combination with a PID (Proportional-Integral-Derivative) controller.

[0072] In a further preferred embodiment, which advantageously allows simple integration into the method and enables effective control based on dynamic current / voltage response, the method includes measuring the frequency domain impedance spectrum of the electrolytic cells, stacks or units via Laplace transform impedance spectroscopy (e.g., by current or voltage pulse methods) to provide information about the health, temperature and performance of the individual electrolytic cells, stacks or units, which can then be selectively fed to a PID device to control current reversal or current, power and / or voltage fluctuations. Suitable methods for determining frequency domain impedance are disclosed, for example, in US Patent Application Publication No. 2003 / 0065461 (A1). It will be understood that the preferred features of the first to fourth embodiments may be freely combined in any combination, except combinations in which at least some of the features are mutually exclusive.

[0073] example Exemplary embodiments of the invention and related considerations are described in further detail below, however it will be understood that the invention is not limited thereto.

[0074] Stacking Properties An exemplary electrolysis plant according to the invention is shown in Fig. 2, which shows the connection of a common bus including a transformer, rectifier and input filter to a number of modules (number of modules = M) in a parallel arrangement, each module consisting of a single unit or a combination of units, each unit consisting of a power conversion system and an electrolysis stack unit connected to the power conversion system. The total number of modules M depends on whether two series-connected electrolysis stacks are used and on the total desired power. The total module current is given by the sum of the individual modules according to the following formula:

[0075]

number

[0076] Each unit in each module is preferably regulated so that the current to each module is constant, even though the current in each electrolytic stack may vary with certain fluctuations in voltage and / or power. The number of modules can simply be added to achieve the desired total power.

[0077] By intelligently controlling each stack, the use of small filter capacitors may be made possible. In this regard, it is particularly preferred to reverse the current in one or part of the electrolysis stack units so that each fuel cell stack can operate in fuel cell mode and partially supply the stack operating in electrolysis mode. In combination with this embodiment, it is preferred that the converter is bidirectional. A suitable converter for such purposes may be of the common buck-boost type. By ensuring that the unidirectional voltage is always higher than the voltage at the electrolysis stack, these converter types can be used just as shown in FIG. 3. In the figure, an exemplary converter is comprised of two switches Q1 and Q2, two diodes D 1 and D. 2 , input capacitor C inand inductor L. The electrolysis mode (buck mode) is switched by Q1, and the fuel cell mode (boost mode) is switched by Q2.

[0078] The diagram in FIG. 3 should be understood as one of the modules included in FIG. 2. A module consists of N units, which themselves consist of N converters and N electrolytic stacks. The current i of a given module Mm is given by the sum of the individual units, i.e.

[0079]

number

[0080] For the following discussion, an electrolysis stack consisting of 75 series-connected cells is considered as a starting point.

[0081] For simplicity, the stack is shown in Figure 4 with an internal voltage source E stack and the internal impedance Z stack The current (I stack ) does not change under constant load, the impedance can be thought of as a single resistor equivalent to the sum of the three resistors, i.e., R stack =R S +R 1 +R 2 =0.7Ω (Equation 4) It is.

[0082] Stack voltage V stack and power P stack In this case, V stack =E stack +R stack I stack (Formula 5) P stack =V stack I stack (Formula 6)

[0083] Using equations 5 and 6 above, the voltage and power characteristics of the stack can be calculated, as shown in Figure 5. It can be seen that the current can be negative. In that situation, the stack operates in fuel cell mode. In this example, the maximum power P stack,nom = 7.8kW is V stack =120V and I stack =65A.

[0084] thermal equilibrium Since the stack consists of cells connected in series, the cell current I is equal to the stack current, i.e. I=I stack It is.

[0085] The heat evolution in the stack is Joule heat P J and reaction heat P R This consists of:

[0086]

number

[0087]

number

[0088] The specific thermal response in coulombs per cell is CO 2 In the case of electrolysis, K=-0.5V, and H 2 In the case of O electrolysis, K=-0.3V. The stack is n cells = 75 series-connected cells, so K stack =-37.5V and K stack =-22.5V for the two states. For the system to be in thermal equilibrium, the sum of the Joule heat and the heat of reaction must equal zero. The current that satisfies this condition is I tlv Therefore, the following calculation applies:

[0089]

number

[0090] Cell voltage for thermal balance V tlv teeth,

[0091]

number

[0092] The thermal equilibrium power level at the cellular level is thus

[0093]

number

[0094] When scaled up to the stack level, the stack power for thermal balance is

[0095]

number

[0096] The stack effect for thermal balance is therefore 59.82% (K=-0.3) and 22.7% (K=-0.5) less than the maximum stack power.

[0097] Maximum power transfer fuel cell conditions When the stack acts as a fuel cell, the power I for maximum power transfer is meo is calculated as follows:

[0098]

number

[0099] Cell voltage V meo Next,

[0100]

number

[0101] Electrical Input Power To control the temperature in the stack, the system a It is operated in electrolytic mode for only T b Assuming that the battery can be operated in electrolysis / fuel cell mode for only 100 s, the following formula applies:

[0102]

number

[0103] where T [s] is the period and f [Hz] is the frequency of the square-wave signal. The duty cycles Da and Db for the two modes are given by:

[0104]

number

[0105] The electrical average power of a single stack is then:

[0106]

number

[0107] Here, V stack_a and I a represent the stack voltage and current, respectively, during time interval “a”, and V stack_b and I b represents the stack voltage and current during time interval "b".

[0108] power balance The thermal evolution in the stack is, as mentioned before, the Joule heat P J and reaction heat P RBy partitioning into "a" and "b" intervals, we arrive at the following representation: P J =P J,a +P J,b (Formula 18) P R =P R,a +P R,b (Formula 19)

[0109] The individual contributions are

[0110]

number

[0111]

number

[0112]

number

[0113]

number

[0114] For the system to be in thermal equilibrium, the sum of the Joule heat and the heat of reaction must again equal zero. Therefore,

[0115]

number

[0116] Solution of a system of equations The two equations for stack effects and thermal balance provide, in principle, two possible solutions. However, for a positive I a Since only a solution for current is desired, one solution option remains below.

[0117]

number

[0118]

number

[0119] Simulation results For a given duty cycle, equations 19 and 20 above can be solved. Figure 6 shows the current in the two modes as a function of duty cycle for 5%, 35%, 65% and 95% of rated power for thermal balance, respectively. The maximum absolute values ​​of the two currents as well as the nominal current values ​​for thermal balance and the current for maximum power transfer are also provided in the figure. For a 5% load, I b It can be seen that I is negative. The larger the duty cycle, the greater the I a and I b Both become smaller. a and I b For the maximum absolute value of D a For the remaining power levels (35%, 55% and 95%), I b is positive for low duty cycles and negative for higher duty cycles.

[0120] Figure 7 shows the cell voltage for the same effect. From both Figures 6 and 7, it can be seen that the higher the power, the less room there is for the choice of duty cycle if the nominal current and voltage values ​​for thermal balance are not exceeded. It can also be seen that for very large duty cycles, about 90%-95% "break" the current and voltage in the "b" interval. The reason for this is that the time interval is short, so the current must necessarily be very large (and therefore the voltage very small) to maintain power balance.

[0121] Figure 8 shows the Joule heat and heat of reaction for two intervals at 35% of nominal power for thermal balance. It can be seen that the higher the duty cycle, the more heat of reaction the system absorbs in the "a" interval. Therefore, the heat of reaction in the "b" interval must be D to be able to supply enough heat so that power balance is observed. a It is necessary for the value to go from negative to positive around 40%.

[0122] Number of units As noted in the specification, each electrolysis stack must be in an electrolysis state for some time and in a fuel cell or electrolysis state for the rest of the time, at a frequency between 10 Hz and 100 Hz. To make the overall system relatively simple and scalable, it is desirable for each module to be independent of the other modules, so that each module must draw a constant current from the DC link for short periods of time (i.e., di Mm / dt=0). This means that the negative current (fuel cell state) must be managed internally in each module among the N units.

[0123] Figure 8 shows a principle diagram of how the flow of a module looks like depending on the number of units that make up the module. In the diagram, the current from each unit has the same numerical value I regardless of whether the current is positive or negative. E This does not necessarily occur in practice, but is just to illustrate the principle. The actual current levels depend on the duty cycle and the efficiency of the converter in the two modes. The duty cycle is also 80%, which means that the stack operates in fuel cell mode 20% of the time. For N=2 units, the current has two levels, namely 2I E9. It can be seen that the stack has a maximum current of 1000 mA and a maximum current of 1000 mA. If the number of units is increased to N=3, the minimum level of the module current can be raised to a value greater than zero. For N=4 units, the minimum level is raised further and the maximum level is raised correspondingly. For N=5 units, there is a perfect balance between the number of units and the proportion of time the stack has to operate in fuel cell mode, and therefore the module flow ideally has only one level, namely the average current. If the number of units is increased to N=6, the same two levels arise as for N=4. However, it should be mentioned that the time at which each unit switches from / to fuel cell mode can be selected individually, which is why a larger difference can be achieved between the maximum and minimum levels than shown in FIG. 9. If all units are operating in fuel cell mode, for example, at the same time, the resulting module current is negative. It is therefore important that there is an appropriate time lag between the devices.

[0124] Optimal number of units N opt is given by Equation 1. For example, if the stack operates in fuel cell mode 20% of the time, the optimal number of units N opt is 5 or all integer multiples thereof, which is also shown in FIG.

[0125] Module Current Fluctuation At the optimum number of units (see Equation 1), there is no current variation for each module. It is now tested what the module current levels would be if the optimum number of units could not be achieved. Each stack is a In part, value V a and I a and the remaining time TT a In V a and I b The converter has efficiencies η el and η fc The DC link voltage V inis fixed. This results in each unit having the following currents in the two intervals:

[0126]

number

[0127]

number

[0128] The system consists of N units, which are uniformly time-shifted with ΔT=T / N seconds between units. Figure 9 shows the currents in the units and modules for N=5 units at different duty cycles.

[0129] It is now possible to determine how many units there will be at maximum and which will operate simultaneously in electrolysis and fuel cell modes respectively.

[0130]

number

[0131]

number

[0132] The module current is the maximum value (I M,max ) and the minimum value (I M,min ) I M,max =N a,max I Ea +(NN a,max )I Eb (Formula 25) I M,min =(NN b,max )I Ea +N b,max I Eb (Equation 26)

[0133] Fluctuation in module current ΔI M is the difference between the maximum and minimum values. ΔI M =I M,max -I M,min =(N a,max -(NN b,max ))I Ea +(NN a,max -N b,max )I Eb (Formula 27)

[0134] N a,max and N b,max The time T when the stack operates in electrolysis and fuel cell modes, respectively. a,max and T b,max is given by:

[0135]

number

[0136]

number

[0137] These times are also shown in Figure 10. The average value of the module current is

[0138]

number

[0139] Figure 11 shows how the maximum and minimum module current depends on the duty cycle for a module with N=5 stacks, each consuming 2kW of power. Note that the average current is independent of the duty cycle, as expected.

[0140] verification To demonstrate the calculations made for the module currents, an LTSpice model of the system shown in Figure 12 was established. The model includes a 150V input voltage source to accommodate a (simplified) unidirectional voltage from the transformer, followed by an LC filter and an N=5 converter and an electrolytic stack. To reduce the simulation time, the converters are designed as average value models, which means that they do not consider transition events during each switch period. Figure 13 shows the current in the filter inductor, the capacitor voltage and the input current to one of the converters for three different duty cycles (0.75, 0.80 and 0.85). It can be seen that the maximum and minimum values ​​for the module currents correspond to the values ​​in Figure 11. However, the capacitor voltages are D a = 0.75 and D a = 0.85, there is a deviation, which is not taken into account in the theoretical calculation. a = 0.8, the number of units in this situation (N = 5) is the optimal number (N opt =1 / Db=1 / 0.2=5), so in practice there is no change in either the filter inductor current or voltage.

[0141] Thus, the above results show that the present invention enables load shifting coordination among these multiple loads to provide a constant DC link voltage, while dynamically operating the electrolysis stack in the near-thermal neutral regime, thereby enabling cheap and effective power management for large-scale electrolysis plants.

[0142] Given the above disclosure, many other features, modifications, and improvements will be apparent to those of ordinary skill in the art.

Claims

1. 1. A power conversion system for a plurality of electrolysis cell stack units, comprising: A parallel arrangement of a plurality of DC / DC conversion modules is provided, Each DC / DC conversion module is configured to power a single electrolysis cell stack unit; each DC / DC conversion module is capable of supplying a predetermined variation of current, power and / or voltage to said electrolysis cell stack unit such that near-thermoneutral operation at partial load is enabled by matching the integral Joule heat generation with the integral reaction heat consumption in said electrolysis cell stack unit; and / or A power conversion system, wherein each DC / DC conversion module is capable of reversing the current supplied to the electrolysis cell stack unit, causing the electrolysis cell stack unit to operate in a fuel cell mode.

2. The power conversion system of claim 1 , wherein only one or a portion of the DC / DC conversion modules apply the predetermined variation or reversal of current, power and / or voltage at a time.

3. 3. The power conversion system of claim 1 or 2, wherein each DC / DC conversion module further comprises one or more electronic switches configured to reverse the current supplied to the electrolysis cell stack unit either for a portion or during the entirety of the predetermined variation in current, power and / or voltage, resulting in fuel cell operation of the electrolysis cell stack unit.

4. 4. The power conversion system of claim 1, wherein the predetermined variation or the reversal of current, power and / or voltage is periodic.

5. 5. The power conversion system of claim 1, further comprising a control unit connected to each of the DC / DC conversion modules and configured to regulate the inversion or the predetermined variation of current, power and / or voltage in an alternating manner in the DC / DC conversion modules.

6. 6. The power conversion system of claim 5, wherein the control unit is configured to regulate the reversal or the predetermined variation in current, power and / or voltage in one or more DC / DC conversion modules such that a common DC link current, power and / or voltage remains approximately constant during the reversal or the predetermined variation.

7. 7. The power conversion system of claim 1, further comprising a sensor configured to acquire physical information related to the electrolysis cell stack unit, and a PID (Proportional-Integral-Derivative) controller configured to control the reversal or the variation of current, power and / or voltage based on measurements of the acquired sensor data.

8. 8. The power conversion system according to any one of claims 1 to 7, wherein the duration of each current reversal or each variation of current, power and / or voltage applied to a single electrolysis cell stack unit is in the range of 1 μs to 1000 s.

9. 【Number 1】 where n ｃ represents the total number of DC / DC conversion modules, and n ｅ represents the total number of electrolysis cell stack units, and T ｐ represents the power-on time of the electrolysis cell stack unit, x is an integer equal to or greater than 1, and T ｂ 9. The power conversion system of claim 1, wherein x represents the duration of the reversal or predetermined variation of current, power and / or voltage applied by one of the DC / DC conversion modules.

10. 1. A power distribution system for a plurality of electrolysis cell stack units, comprising: Transformers, one or more rectifiers; and Input Filter a common bus having A power distribution system comprising: a power conversion system according to any one of claims 1 to 9 connected to the common bus.

11. An electrolysis power plant comprising the power distribution system of claim 10 and a plurality of electrolysis cell stack units.

12. 12. The electrolysis power plant of claim 11, wherein each of the one or more electrolysis cell stack units comprises one or more stacks of a solid oxide electrolysis / fuel cell (SOEC / SOFC), a molten carbonate electrolysis / fuel cell (MCEC / MCFC), a high temperature and pressure alkaline electrolysis / fuel cell, and a ceramic electrolyte proton conducting electrolysis / fuel cell (PCEC / PCFC).

13. 13. The electrolysis power plant of claim 11 or 12, wherein the electrolysis power plant has a total electrical input power of 1 MW or greater.

14. 1. A method for distributing power to a plurality of electrolysis cell stack units, comprising: coupling a common bus, comprising a transformer, one or more rectifiers and an input filter, between a power grid and a plurality of DC / DC conversion modules arranged in parallel; connecting each DC / DC conversion module to a separate electrolysis cell stack unit; Independently supplying voltage to one or a portion of the electrolysis cell stack units via a DC / DC conversion module with predetermined variations in current, power and / or voltage to enable near-thermal neutral operation at partial load by matching the integral Joule heat generation with the integral reaction heat consumption in the electrolysis cell stack units; and / or and independently reversing the current supplied to one or some of the plurality of electrolysis cell stack units via a DC / DC conversion module to effect fuel cell operation of the electrolysis cell stack units.

15. A method for distributing power to a plurality of electrolysis cell stack units as claimed in claim 14, comprising: The method further includes the steps of acquiring physical data associated with one or more electrolysis cell units and controlling current reversal and / or predetermined variations in current, power and / or voltage based on measurements of the acquired physical data.

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