Method for operating an electrochemical system, and control device

Abstract

The invention relates to a method for operating an electrochemical system for gas separation, in particular for carbon capture, comprising at least one stack (1) which is connected to an external power supply (3) via an intermediate circuit (2), wherein the at least one stack (1) is operated in a charging mode in which electrical energy is received by the stack (1), and the stack is then operated in a discharging mode in which previously received electrical energy is released from the stack (1) and returned to the intermediate circuit (2). The invention also relates to a control device which is suitable for carrying out the method according to the invention or individual steps of a method.

Description

[0001] R.414337

[0002] - 1 -

[0003] Description

[0004] Method for operating an electrochemical plant and control unit

[0005] The present invention relates to a method for operating an electrochemical plant for gas separation, in particular for carbon dioxide separation. Furthermore, the invention relates to a control device suitable for carrying out the method or individual steps of the method.

[0006] State of the art

[0007] Electrochemical cells used for carbon dioxide capture typically have two coated electrodes that are electrically connected and separated by a separator. The separator is permeable to ions and serves to balance the charge between the cells. At a first electrode on the cathode side, a gas, for example, carbon dioxide, is captured from a gas stream through an electrochemical process. The gas capture is typically initiated by a change in the electrical voltage. The process is reversible, meaning that removing or reversing the voltage leads to a controlled release of the previously captured gas. A second electrode on the anode side acts as an electron donor and provides the electrons necessary for the capture reaction.

[0008] To increase efficiency and the amount of bound gas, a large number of electrochemical cells are stacked and electrically connected in practical applications. Within such a cell stack, there are supply channels through which exhaust gas is supplied to the individual cells, and in particular to the first electrode on the cathode side, as input gas, and the processed exhaust gas is removed as output gas. R.414337

[0009] - 2 -

[0010] The supplied gas stream typically originates from combustion processes or other production processes that generate gas mixtures as exhaust gases or byproducts. These gas mixtures often contain carbon dioxide.

[0011] The gas, specifically carbon dioxide (CO2), can be captured from the gas stream using such electrochemical cells. The gas capture reaction requires electrical energy. This energy is partially released again during the controlled release of the captured gas by reversing the electrical voltage. This released electrical energy typically dissipates within the system, resulting in energy losses and thus reduced efficiency of the electrochemical cell or stack.

[0012] The present invention is concerned with the objective of solving the described disadvantages of the prior art and increasing the efficiency in the operation of at least one electrochemical stack.

[0013] The method of claim 1 is proposed to solve the problem. Advantageous embodiments of the invention are described in the dependent claims. Furthermore, a control unit for executing the method or individual steps of the method is specified.

[0014] Disclosure of the invention

[0015] Method for operating an electrochemical plant for gas separation, in particular for carbon dioxide separation, comprising at least one stack connected via an intermediate circuit to an external power supply, wherein the at least one stack is operated in a charging mode in which electrical energy is absorbed by the stack, and the stack is subsequently operated in a discharging mode in which previously absorbed electrical energy is released by the stack and fed back into the intermediate circuit. R.414337

[0016] - 3 -

[0017] The proposed method allows the electrical energy released during discharge to be fed back into an intermediate circuit, thereby enabling its utilization. By feeding back and utilizing this electrical energy, energy losses are minimized and the overall energy requirement of the system is reduced. This results in an increase in the efficiency of the electrochemical plant.

[0018] Preferably, the electrical energy fed back into the intermediate circuit is used to operate at least one additional stack in charging mode. Utilizing the fed-back energy in another stack is particularly advantageous due to the similarity of the components with respect to required voltage, current, and electrical power. This allows for particularly efficient use of the fed-back energy and minimizes energy losses. In particular, simultaneous operation of at least one stack in charging mode and at least one stack in discharging mode within the same system is advantageous, since, due to their proximity, the fed-back energy from the discharging mode can be used directly for the at least one electrochemical stack in charging mode.

[0019] Furthermore, an energetically balanced ratio between the electrical energy released by at least one stack and the energy required by at least one other stack is advantageous. For example, if the system has more than two stacks with the same electrical capacity, ideally the number of stacks in discharge mode should equal the number of stacks in charging mode. This ensures that the electrical energy released by the stacks in discharge mode can be fully utilized.

[0020] Preferably, the electrical energy fed back into the intermediate circuit is temporarily stored in an intermediate storage device. This temporary storage allows for staggered use, i.e., use at a later time. This prevents dissipation of the released energy and thus energy losses. Preferably, the electrical energy is temporarily stored in a capacitor. A capacitor has the advantage of being a simple and inexpensive electrical component, R.414337

[0021] - 4 - is easy to integrate, with the capacitor preferably being integrated into the intermediate circuit. Direct use of the electrical energy fed back into the intermediate circuit is preferred to storage in the intermediate storage, since higher energy losses occur during storage than during direct use.

[0022] Furthermore, the electrical energy fed back into the DC link is preferably fed into an external power supply network. By feeding the released electrical energy back into the external power supply network, less electrical energy is drawn from the network overall. This energy is then available for other uses, for example, for another electrical consumer. This could be, in particular, an internal combustion engine that produces exhaust gas containing carbon dioxide. Feeding electrical energy back from the DC link into the external power supply network is especially advantageous when more electrical energy is available in the DC link than is needed. In this case, the surplus electrical energy can be used elsewhere.

[0023] Surpluses of recovered energy can occur particularly when there are fluctuations in the amount and / or concentration of gas produced, for example, due to a combustion engine being throttled back or shut down. In this case, less exhaust gas would be produced, and a surplus of electrical energy would be released during the discharge processes. For this reason, the recovered electrical energy can be fed into the external power grid.

[0024] Feeding electrical energy back into the external power supply network can also compensate for temporary power peaks, thus ensuring high process stability. For example, this method can be used to bridge the start-up time of an emergency generator and to cushion electrical power drops in the external power supply.

[0025] Since at least one stack of the electrochemical plant is usually operated with direct current, but in the external power supply network R.414337

[0026] - 5 - Since alternating current is usually present, an AC / DC converter is preferably connected between the external power supply network and the intermediate circuit. Preferably, when electrical energy is fed back into the external power supply network, direct current is converted into alternating current using this AC / DC converter. By using an existing AC / DC converter, the process can be made even more efficient.

[0027] In a preferred embodiment of the invention, a constant voltage is applied to the at least one stack in charging mode via an inverter connected between the at least one stack and the intermediate circuit, and a constant current is fed back from the at least one stack to the intermediate circuit in discharging mode. Since the charging and discharging rates of the at least one stack depend on the voltage and current, respectively, a constant voltage and current result in a more homogeneous and thus improved charging and discharging of the stack. This, in turn, increases process accuracy. Furthermore, a constant voltage and current enable better control of the charging and discharging processes. For example, in the case of a constant voltage, the resulting current can be measured, and conversely, in the case of a constant current, the resulting voltage can be measured.Both values ​​represent an important control parameter for adjusting and / or optimizing the processes.

[0028] Furthermore, the inverter connected between the DC link and the at least one stack converts the electrical energy into a voltage and current range advantageous for the stack. This increases the stack's efficiency, as the electrical energy can be absorbed optimally. Moreover, this prevents damage to the stack caused by, for example, excessive current or voltage.

[0029] Preferably, the voltage is measured using the inverter during charging and / or discharging. The voltage applied to the inverter is the same as that applied to the respective stack. Furthermore, the voltage applied to the inverter can be measured very easily. Therefore, R.414337

[0030] - 6 - the electrical voltage state on the stack can be determined very easily and accurately.

[0031] Furthermore, it is preferably preferred that the current flow be measured using a current sensor during charging and / or discharging. Measuring the current flow with a current sensor is both simple and accurate. For the present method, a current sensor that is not integrated into a converter or is located outside of a converter is preferably used. This allows the current flow to be measured with high accuracy.

[0032] Preferably, at least one state variable, in particular an individual charge state and / or an individual aging state, of the at least one stack is determined from the measured voltage and / or current flow. This at least one state variable enables improved monitoring and thus control of the proposed methods compared to simply measuring voltage and / or current flow. For example, the replacement of a stack can be monitored and planned in advance using the aging state. In this way, unplanned failures, downtime, and efficiency losses can be minimized. The charging and discharging process can be monitored using the charge state. For example, the electrical power can be switched off in the case of complete charging or complete discharging.This not only saves electrical energy and thus increases the efficiency of the stack, but also prevents damage to the stacks.

[0033] Advantageously, depending on the measured voltage and / or current flow, preferably depending on the at least one state variable determined from the measured voltage and / or current flow, the at least one stack is temporarily switched off, with a constant maintenance voltage being applied to the temporarily switched-off stack. The temporary switch-off of the at least one stack helps to reduce energy consumption and thus the operating costs of the electrochemical plant. Since the amount of gas supplied to the electrochemical plant can fluctuate during operation, such process fluctuations can be mitigated by temporarily switching off at least one stack.

[0034] - 7 - react. Gas separation can then be achieved with a smaller number of stacks.

[0035] The at least one state variable that can lead to the temporary shutdown of a stack is, in particular, its gas loading state. Since the efficiency of a stack decreases with increasing gas loading, it is advantageous to temporarily shut down the stack with the highest gas loading. The gas loading state of a stack can be individually determined from the measured voltage and / or current flow, thus identifying the stack with the lowest gas phase deposition efficiency.

[0036] Applying a constant maintenance voltage to the temporarily switched-off stack serves to prevent an unwanted reversal of the gas separation reaction.

[0037] Furthermore, a control unit for an electrochemical plant for gas separation, in particular for carbon dioxide separation, is proposed. The control unit is configured to execute a method according to the invention or individual steps of a method according to the invention. The control unit can be used, in particular, for controlling and monitoring the operation of the electrochemical plant.

[0038] The invention and its advantages are explained in more detail below with reference to a figure. This figure shows a schematic representation of an electrochemical plant for gas separation, which is suitable for carrying out a process according to the invention or can be operated according to such a process.

[0039] Detailed description of the drawing

[0040] The figure shows an electrochemical plant for gas separation, in particular carbon dioxide separation. It comprises several stacks 1, each connected via a converter 5 to a DC link 2. The DC link 2, in turn, is connected via an AC / DC converter 4 to a network of an external power supply 3. R.414337

[0041] - 8 -

[0042] During operation of the electrochemical plant, a gas is supplied to at least one of the stacks 1 for gas separation. Gas separation requires electrical energy, which is supplied to the respective stack 1 via the external power supply 3. The AC / DC converter 4 converts alternating current into the direct current required by the stacks 1. In the charging mode of a stack 1, a constant voltage is applied to it. In a subsequent discharging mode of the stack 1, electrical energy in the form of a constant current flow is fed back from the stack 1 via the inverter 5 into the intermediate circuit 2. The inverter 5 serves to homogenize the voltage and / or current flow in both the charging and discharging modes. This optimizes the operation of the stack(s) 1 and prevents damage, for example, from excessive voltage and / or current flow.In charging and discharging mode, the voltage at the inverter 5 is measured. The current flow is measured via a current sensor 6.

[0043] The electrical energy fed back into the DC link 2 during the discharge mode of a stack 1 can be temporarily stored in an intermediate storage device 7, for example, a capacitor, and used at a later time. For instance, it can be used to operate a stack 1 that is in charging mode. Direct use of the fed-back electrical energy from a stack 1 in discharge mode to operate another stack 1 in charging mode is also possible and particularly energy-efficient, as this minimizes losses caused by intermediate storage of the electrical energy. Furthermore, the released and fed-back electrical energy can be fed into the external power supply network 3. This is particularly advantageous in the case of large surpluses of fed-back electrical energy in the DC link 2 and / or temporary power peaks in the external power supply network 3.In the latter case, power dips in the external power supply 3 can be mitigated. When the electrical energy is fed back into the network of the external power supply 3, it is converted from direct current (DC) back into alternating current (AC) using the AC / DC converter 4.

Claims

R.414337 - 9 - Claims 1. Method for operating an electrochemical plant for gas separation, in particular for carbon dioxide separation, comprising at least one stack (1) which is connected to an external power supply (3) via an intermediate circuit (2), wherein the at least one stack (1) is operated in a charging mode in which electrical energy is absorbed by the stack (1), and the stack (1) is subsequently operated in a discharging mode in which previously absorbed electrical energy is released by the stack (1) and fed back into the intermediate circuit (2).

2. Method according to claim 1, characterized in that the electrical energy fed back into the intermediate circuit (2) is used to operate at least one further stack (1) in charging mode.

3. Method according to claim 1 or 2, characterized in that the electrical energy fed back into the intermediate circuit (2) is temporarily stored in an intermediate storage device (7), preferably in a capacitor.

4. Method according to one of the preceding claims, characterized in that the electrical energy fed back into the intermediate circuit (2) is supplied to a network of the external power supply (3), wherein direct current is preferably converted into alternating current by means of an AC / DC converter (4) which is connected between the intermediate circuit (2) and the external power supply (3).

5. Method according to one of the preceding claims, characterized in that a converter (5) connected between the at least one stack (1) and the intermediate circuit (2) is used in the R.414337 - 10 - In charging mode a constant voltage is applied to at least one stack (1) and in discharging mode a constant current flow is fed back from at least one stack (1) to the intermediate circuit (2).

6. Method according to claim 5, characterized in that the voltage is measured in the charging and / or discharging mode using the inverter (5).

7. Method according to one of the preceding claims, characterized in that the current flow is measured in the charging and / or discharging mode using a current sensor (6).

8. Method according to claim 7, characterized in that at least one state variable, in particular an individual charging state and / or an individual aging state, of the at least one stack (1) is determined from the measured voltage and / or the measured current flow.

9. Method according to claim 7 or 8, characterized in that, depending on the measured voltage and / or the measured current flow, preferably depending on at least one state variable determined from the measured voltage and / or the measured current flow, at least one stack (1) is temporarily switched off, wherein a constant maintenance voltage is applied to the temporarily switched-off stack (1).

10. Control unit for an electrochemical plant for gas separation, in particular for carbon dioxide separation, wherein the control unit is configured to execute a method or individual steps of a method according to one of the preceding claims.

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