Method and arrangement for operating an electrolyzer, electrolysis plant and methanization plant

Abstract

Method for operating an electrolyzer (10a , ..., 10f) in which the electrolyzer is supplied with voltage via an alternating voltage rectified by means of a rectifier (70), in particular a three-phase alternating voltage, and in which the voltage applied to the electrolyzer is set by means of a voltage control (1a, ..., 1f), characterized in that the voltage control is at least partially decoupled from the rectifier and that the circuit connecting the electrolyzer and the rectifier is repeatedly interrupted during the voltage control.

Description

[0001] The invention relates to a method for operating an electrolyzer, in which the electrolyzer is supplied with voltage via an alternating voltage rectified by means of a rectifier, in particular a three-phase alternating voltage, and in which the voltage applied to the electrolyzer is adjusted by means of a voltage control.

[0002] Such processes are known. Since electrolysis on an industrial scale requires very high power and current, electrolyzers are typically connected to the low-voltage grid, or, if necessary, via a transformer to the medium-voltage grid. The three-phase grid voltage is rectified by a rectifier, thus providing a power supply for the electrolyzer. The particular advantage of modern rectifier technology using thyristors lies in the fact that the rectifier also allows for voltage control of the voltage applied to the electrolyzer. Phase control of the rectifier's electronic switching components achieves voltage regulation for the electrolyzer.For example, by adjusting the rectifier, the voltage is reduced to a value below that supplied by the rectifier in order to supply the electrolyzer with a desired voltage, and as the electrolyzer degrades, the output voltage at the rectifier is increased by changing the phase control to maintain a desired operating point.

[0003] The control range of such rectifiers, controlled by, for example, thyristor bridges, is approximately 20 to 100%. This allows for voltage control during the operation of an electrolyzer, such as a stacked electrolyzer, or multiple combined electrolyzers, when their internal resistance decreases due to, for example, higher electrolyte temperatures. An example of such a controlled rectifier is one based on a twelve-pulse circuit, whose DC voltage output, when applied to three-phase AC voltage, depends on the control angle α of the phase control and is given by U. DC = 1.35 U AC × cos α, where the control angle α is applied as an electrical pulse from a control electronics to the thyristor gates.

[0004] In US 2010 / 0 089 746 A1, an electrolyzer is disclosed in which a 3-phase alternating voltage is passed through a circuit similar to a frequency converter with a rectifier and six IGBTs for voltage control.

[0005] DE 10 2012 216 090 A1 discloses a combined plant for the production of chemical products which can serve as a methanization plant.

[0006] The invention is based on the objective of further improving a method of the type mentioned above, in particular with regard to a suitable integration of the method into the power supply of the electrolyzer from the mains.

[0007] This problem is solved by the invention through a further development of the method of the type mentioned at the outset, which is essentially characterized in that the voltage control is at least partially decoupled from the rectifier and that the circuit connecting the electrolyzer and the rectifier is repeatedly interrupted during voltage control.

[0008] The solution according to the invention thus enables voltage control of the electrolyzer, which relieves the rectifier and, in particular, eliminates the need for phase control of its components or controllable components. In this way, reactive power that would otherwise be fed back into the grid due to phase control of the rectifier can be reduced or even avoided, while still maintaining the desired voltage control at the electrolyzer. In one possible embodiment, the decoupling can be complete; the rectifier then no longer provides any control options for the electrolyzer voltage control. In this context, it is provided that the circuit connecting the electrolyzer and the rectifier is intermittently interrupted during voltage control.The number of recurring interruptions per second should be on average greater than 400, preferably greater than 1000, and especially greater than 2000.

[0009] In a preferred embodiment of the process, the charge discharge from an electrolyzer end plate is subject to fluctuations on a timescale of less than 0.0025 s due to the voltage control, particularly with a fluctuation amplitude of more than 5%, preferably more than 10%, and especially more than 15%, based on the time-averaged charge discharge, and / or of less than 70%, preferably less than 50%. On average over larger timescales, the fluctuations are therefore hardly noticeable, and the electrolyzer delivers a constant hydrogen flow, but at a lower level than without fluctuations, for example, if the time-averaged charge discharge were constant at its maximum level.

[0010] In one variant of the process, it is further provided that a cycle, beginning with an interruption and ending before the next interruption, occurs periodically at a frequency of less than 40 kHz, preferably less than 25 kHz, and particularly less than 10 kHz. This results in a suitable interaction between the charge state on the electrolyzer end plates and their recharging during the periods of uninterrupted connection between the electrolyzer and the rectifier.

[0011] It is preferably provided that the ratio of time intervals with and without interruption for achieving a desired voltage applied to the electrolyzer is adjusted within the interval of 0.2 to 3, preferably within the interval of 0.3 to 2, and particularly within the interval of 0.4 to 1.5. This ensures a stable recharging dynamic for the electrolyzer end plates, which lose charge during the interrupted connection periods. For example, if the applied voltage without interruption were U = U max , and this time interval ratio is denoted by x, resulting in an effective voltage of U applied to the electrolyzer. eff = U max[x / (1 + x)]. For voltage reduction, values ​​are particularly preferred in this context that result in the ratio of the voltage applied to the electrolyzer on average over time due to the voltage control to the rectified supply voltage being less than 90%, preferably less than 85%, particularly less than 80%, and / or greater than 20%, preferably more than 30%, particularly more than 40%. These values ​​apply particularly when the voltage control is optimized for the use of the available electrical power in a way that minimizes reactive power feed-in. However, it is also conceivable to provide fine-tuning of the voltage control, aiming for a voltage difference of less than 5%, even less than 3%, and particularly less than 2%.

[0012] In a preferred process design, the decoupled voltage control operates near at least one end plate of the electrolyzer. In larger systems, the electrolyzer is typically spatially separated from the rectifier and transformer, with the corresponding transmission lines leading to the electrolyzer via, for example, appropriately designed cables or busbars. Spatially, the voltage control is thus more closely associated with the electrolyzer than with the rectifier; in particular, it is conceivable that the corresponding voltage control circuits are located close to the electrolyzer, for example, directly at the electrolyzer end plates.

[0013] In one embodiment of the process, the temperature gradient existing from the inside to the outside of the end plate is reduced by actuating the voltage control. This means that the waste heat generated by the voltage control provides the electrolyzer end plate with an external heat input that reduces the temperature gradient within the plate, ultimately reducing heat loss in the electrolyte and thus limiting the need for reheating.

[0014] Preferably, the rectified alternating voltage is supplied via a transformation from the medium-voltage network. Medium-voltage systems on the order of approximately 20 kV and correspondingly low currents result in the high currents in the range of several kA required for the electrolyzers due to the transformation.

[0015] Hydrogen is preferably produced by electrolysis, in particular by the electrolysis of water. It is especially preferred that the hydrogen production is followed by catalytic methanation of the hydrogen thus produced with a gas in the form of carbon oxide, in particular carbon dioxide, which is then combined with the hydrogen.

[0016] In terms of the apparatus, the object of the invention is solved by an arrangement for operating an electrolyzer, comprising a rectifier that rectifies a voltage supply provided in the form of an alternating voltage and a voltage control that adjusts the voltage applied to the electrolyzer, which is essentially characterized in that the voltage control is at least partially decoupled from the rectifier and that the circuit connecting the electrolyzer and the rectifier is repeatedly interrupted during the voltage control.

[0017] The advantages of the arrangement according to the invention will become apparent from the above explanations of the method according to the invention.

[0018] In this arrangement, it is particularly provided that the voltage control has a first circuit part spaced apart from the rectifier, which causes fluctuations in the voltage applied to the electrolyzer on a time scale of less than 0.0025 s with fluctuation amplitudes of more than 40%, preferably more than 60%, and in particular more than 80%, based on the time-averaged applied voltage.

[0019] In a particularly preferred embodiment, the first circuit component is a transistor, in particular an IGBT. This is especially suitable for cases in which a recurring circuit interruption is part of the voltage control.

[0020] In a particularly preferred embodiment, the voltage control comprises at least a second circuit section for adjusting the voltage of a second electrolyzer, which is coupled to the same rectifier as the first circuit section. In this arrangement, a central DC power supply is thus provided for the at least two electrolyzers. This allows for a simple design and reliable operation of the arrangement. A multi-pulse, and especially a twelve-pulse, diode rectifier is particularly suitable. This minimizes feedback to the supply network, for example, with regard to reactive power and harmonics. Due to the voltage control via the decoupled circuit sections, particularly exclusively in the DC section, the three-phase system providing the electrical power is thus decoupled from the DC-side controlled loads.

[0021] In particular, it is intended that two, but also more than two, especially more than four, and certainly more than six, and especially more than twelve electrolyzers, can be connected to the same rectifier, taking into account the transformer power and that of the diode rectifier, which must not be exceeded. It is possible, but not required, for all electrolyzers to be connected to have such a voltage-controlling circuit component. It is also conceivable to equip at least one, or even several, especially at least all but two, or all but one electrolyzer without such a circuit component.

[0022] On the other hand, each of the electrolyzers connected to the same rectifier can also be equipped with such a circuit component. At maximum load, which is also the intended operating point, all electrolyzers are switched on. At lower loads, it is specifically designed that the electrolyzers with the least degradation are switched on first, thus ensuring a uniform degradation of all electrolyzers.

[0023] This aspect is also considered independently worthy of protection by the invention and is disclosed accordingly. The invention thus also relates to a method for operating an arrangement with at least two electrolyzers, which further comprises a rectifying voltage supply provided in the form of an alternating voltage and at least two circuit sections assigned to each electrolyzer, which are connected to the rectifier and can disconnect and connect the electrolyzer assigned to them from the rectifier, wherein the connection and disconnection is carried out depending on the states relating to the wear of the assigned electrolyzers in such a way that the sum of the (suitably detected) wear differences is reduced.

[0024] The arrangement(s) according to the invention are expediently equipped with a control unit containing a control program, with which the arrangement is controlled for carrying out a method according to one of the method aspects explained above.

[0025] Furthermore, the invention provides for an electrolysis plant comprising such an arrangement, i.e. an electrolysis plant with at least one, preferably at least two electrolyzers, in particular of the stack type, which is characterized by an arrangement according to one of the aspects mentioned above.

[0026] In such an electrolysis plant, it is particularly provided that one or more circuit components are arranged on an end plate of the respective electrolyzer. Preferably, the power output of the electrolysis plant is in the range between 200 kW and 8 MW. It is preferably provided that the total power output of the electrolyzers (sub-units) coupled to the same rectifier is at least 1 MW, and in particular at least 1.5 MW. The aforementioned maximum load need not necessarily be fully utilized; expediently, even in comparatively large plants, a maximum power output of 4 MW or less, in particular 2.4 MW or less, can be designed.

[0027] For a single stack-type electrolyzer, i.e., a subunit of such a plant, it is preferred that its power output be at least 100 kW, in particular at least 200 kW, but preferably not exceed 500 kW, in particular not exceed 400 kW.

[0028] Such a system allows for an advantageous power supply to the individual subunits and is comparatively uncomplicated in its construction and proves to be reliable in operation and handling. Finally, the invention also relates to a methanization system with a methanizer for the catalytic methanization of hydrogen produced by electrolysis after its combination with carbon in the form of carbon oxide, in particular carbon dioxide-containing gas, which is characterized by an electrolysis system producing the hydrogen according to one of the aspects mentioned above.

[0029] Further details, features and advantages of the invention will become apparent from the description with reference to the accompanying figures, of which Fig. 1 shows an overview of an electrolysis plant, Fig. Figure 2 shows a rectifier arrangement and its coupling to electrolysis units, Fig. 3 shows an equivalent circuit diagram for an electrolyzer coupled to a DC power source with circuit section, Fig. 4 Current and voltage waveforms for a first circuit control are shown, and Fig. 5 shows current and voltage waveforms for a second circuit control.

[0030] The in Fig. The electrolysis plant 100, shown in a system overview, is connected to the medium-voltage grid via a circuit 90. The medium-voltage level (primary voltage) of approximately 20 kV is transformed down to a secondary voltage level of 220 V AC in this embodiment by transformer 80. A central diode rectifier 70 is coupled to the secondary side of the transformer, which converts the input primary voltage of 220 V AC into 300 V direct current (DC) in this embodiment. The rectifier 70 is a twelve-pulse diode rectifier designed for a power output of 2 MW in this embodiment.

[0031] In this embodiment, six stack-type electrolyzers 10a to 10f are connected in parallel via distribution 60. An IGBT 1i is arranged on an end plate 5i of each electrolyzer 10i, via which voltage control of the associated electrolyzer 10i is carried out by means of a central control unit 99. A more detailed description of the coupling of the diode rectifier with the star and delta connections of the transformer is given in Fig. 2 shown.

[0032] Fig. Figure 3 shows an equivalent circuit diagram of an electrolyzer, which itself is constructed from a number of electrode plates, as is known to those skilled in the art. In the equivalent circuit diagram of Fig. 3. These individual plates can be represented in the form of parallel and series circuits of capacitors and resistors. An IGBT 1i is caused by the controller 99 to undergo chopper control, i.e., a recurring interruption of the connection between the electrolyzer 10i and the rectifier 70, see also the explanations below. Fig. 4 and Fig. 5. During the switch-on time t e The capacities are recharged and during the shutdown time t a The system is discharged accordingly. Due to the inertia of the physical electrolysis system on the one hand and the high switching frequencies between the on-time and off-time of 4 kHz in this embodiment, a continuous current flow in the electrolyzer results despite the circuitry provided by the IGBT 1i. This current flow averages over time and is calculated as the quotient U eff is determined for the internal resistance of the electrolyzer.

[0033] The voltage U applied to electrolyzer 10i over timeeff This in turn results from the characteristics of the chopper-like circuitry of the IGBTs 1i. For example, in Fig. 4 a duty cycle of t e / (t a + t e ) set to 1:2, the effective voltage U eff This is therefore only half of the maximum voltage U. max . Also shown is in Fig. 4. The current curve, which represents the charging and discharging of the capacitors. In the case of voltage control according to... Fig. In contrast, in 5 the on-time is twice as long as the off-time, resulting in a correspondingly higher effective voltage U. eff results.

[0034] The voltage control of the individual electrolyzers 10a,..., 10f is thus completely decoupled from the twelve-pulse diode rectifier 70, which, when uncontrolled, causes negligible reactive power feed-in to the grid. Furthermore, the individual electrolyzers 10i can be controlled without affecting the rectifier 70, for example, to achieve uniform degradation.

[0035] The invention is not limited to the details shown in the description of the figures. Rather, the individual features of the following claims and of the above description may be essential, individually or in combination, for the realization of the invention in its various embodiments.

Claims

[1] Method for operating an electrolyzer (10a , ..., 10f) in which the electrolyzer is supplied with voltage via an alternating voltage rectified by means of a rectifier (70), in particular a three-phase alternating voltage, and in which the voltage applied to the electrolyzer is set by means of a voltage control (1a, ..., 1f), characterized by that the voltage control is at least partially decoupled from the rectifier and that the circuit connecting the electrolyzer and the rectifier is repeatedly interrupted during voltage control. [2] Method according to claim 1, wherein the charge output of an electrolyzer end plate is subject to fluctuations due to voltage control on a time scale of less than 0.0025 seconds, in particular with a fluctuation amplitude of more than 5%, preferably more than 10%, in particular more than 15%, based on the charge output on average over time, and / or of less than 70%, preferably less than 50%. [3] Method according to any of the preceding claims, wherein the number of recurring interruptions per second is on average greater than 400, preferably greater than 1000, in particular greater than 2000. [4] Method according to one of the preceding claims, wherein a cycle beginning with an interruption and ending before the next interruption is periodic at a frequency of less than 40 kHz, preferably less than 25 kHz, in particular less than 10 kHz. [5] Method according to one of the preceding claims, wherein a ratio of the time intervals with and without interruption lies in the interval of 0.2 to 3, preferably in the interval of 0.3 to 2, in particular in the interval of 0.4 to 1.

5. [6] Method according to one of the preceding claims, wherein the ratio of the voltage applied to the electrolyzer on average over time due to the voltage control to the rectified supply voltage is less than 90%, preferably less than 85%, in particular less than 80% and / or greater than 20%, preferably less than 30%, in particular less than 40%. [7] Method according to any of the preceding claims, wherein the decoupled voltage control acts near at least one end plate (5a) of the electrolyzer. [8] Method according to claim 7, wherein a temperature gradient existing from inside to outside in the end plate (5a) is reduced by actuating the voltage control. [9] Method according to one of the preceding claims, wherein the rectified alternating voltage is provided via a transformation from the medium voltage network. [10] A method according to one of the preceding claims, wherein the method serves to produce hydrogen and in particular an electrolysis of water takes place, wherein in particular the hydrogen produced is subsequently combined with a carbon in the form of a carbon oxide, in particular containing carbon dioxide, and is catalytically methanized. [11] Arrangement for operating an electrolyzer (10a), comprising a rectifier (70) that rectifies a voltage supply provided in the form of an alternating voltage and a voltage control that adjusts the voltage applied to the electrolyzer, characterized bythat the voltage control is at least partially decoupled from the rectifier and that the voltage control is designed to repeatedly interrupt the circuit connecting the electrolyzer and the rectifier. [12] Arrangement according to claim 11, wherein the voltage control comprises a first circuit part (1a) spaced apart from the rectifier, which causes fluctuations of the voltage applied to the electrolyzer on a time scale of less than 0.0025 seconds with fluctuation amplitudes of more than 40%, preferably more than 60%, in particular more than 80%, based on the time-averaged voltage applied. [13] Arrangement according to claim 12, wherein the first circuit part is a transistor, in particular an IGBT (1a , ..., 1f). [14] Arrangement according to one of claims 11 to 13, wherein the voltage control comprises at least a second circuit part (1b , ..., 1f) for adjusting the voltage of a second electrolyzer (10b , ..., 10f) which is coupled to the same rectifier as the first circuit part. [15] Arrangement according to one of claims 11 to 14, wherein the rectifier (70) is an uncontrolled rectifier. [16] Arrangement according to one of claims 11 to 15, comprising a control unit (99) containing a control program, with which the arrangement is controlled for carrying out a method according to one of claims 1 to 10. [17] Electrolysis plant (100) with at least one, preferably at least two electrolyzers (10a , ..., 10f), in particular of the stack type, characterized by an arrangement according to any one of claims 11 to 16. [18] Electrolysis system according to claim 17, in which one or more circuit parts (1a) are arranged on an end plate (5a) of the respective associated electrolyzer (10a). [19] Methanization plant, comprising a methanizer for the catalytic methanization of hydrogen produced by electrolysis after its combination with a carbon in the form of a carbon oxide, in particular carbon dioxide-containing gas, characterized by an electrolysis plant producing hydrogen according to claim 17 or 18.

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