Method for operating an electrolysis plant, process control system

Abstract

The invention relates to a method for operating an electrolysis plant with at least one stack, in which a maximum stack voltage (Umax) is specified, which defines the beginning of a first phase (I) at the end of the stack's lifetime, and upon reaching the maximum stack voltage (Umax), the operating parameter temperature (T) at constant current density (iDens) is increased to such an extent that the maximum stack voltage (Umax) is maintained at least over the first phase (I) of the end of the lifetime. The invention further relates to a process control system for executing steps of the method according to the invention.

Description

[0001] The present invention relates to a method for operating an electrolysis plant with at least one stack. Furthermore, the invention relates to a process control system for an electrolysis plant, which is configured to execute steps of the method.

[0002] A preferred application of the invention is the production of hydrogen by electrolysis. State of the art

[0003] Against the backdrop of decarbonization, the production of "green" hydrogen, that is, hydrogen produced using renewable energy, is becoming increasingly important. This energy is required to split water into its components, hydrogen and oxygen, in an electrochemical process called electrolysis.

[0004] The industrial production of hydrogen takes place in an electrolysis plant with at least one stack, which comprises a large number of electrolysis cells in a stacked arrangement. Furthermore, an electrolysis plant includes components and lines for supplying and conditioning the media, as well as for removing and conditioning the product gases. Power electronics and means for controlling the plant are also required.

[0005] A key criterion in the operation of an electrolysis plant is the levelized cost of hydrogen (LCOH), as this determines the plant's economic viability. The LCOH is calculated by dividing all costs, such as acquisition, construction, operation, and maintenance costs of the plant, by the amount of hydrogen produced, considering the entire lifespan of the plant. Electricity costs constitute a significant portion of the LCOH.

[0006] During the operation of an electrolysis plant, at least one of the stacks ages or degrades over time. This leads to an increase in the cell voltage and thus also the stack voltage, resulting in a decrease in stack efficiency. The higher the temperature and / or current density, the faster the aging process progresses. Simultaneously, a higher temperature has a positive effect on stack efficiency because it reduces the cell voltage, and a higher current density increases the amount of hydrogen produced. However, with increasing current density, the voltage also increases, thus reducing efficiency.

[0007] These relationships are exemplified in the Fig. 1 and Fig. 2 shown. Fig. Figure 1 shows the dependence of efficiency A and lifetime B on temperature T, where an increase in temperature (T > nom.) leads to an increase in efficiency A and a decrease in lifetime B. A decrease in temperature (T < nom.) leads to an increase in lifetime B, but simultaneously to a decrease in efficiency A.

[0008] Again Fig. As can be seen in section 2, an increase in current density (iDens > nom.) leads to a decrease in efficiency A and an increase in hydrogen yield C. A reduction in current density (iDens < nom.) does increase efficiency A, but at the same time reduces hydrogen yield C.

[0009] The present invention is concerned with the objective of providing an operating strategy for an electrolysis plant which not only enables the longest possible and most efficient operation, but also leads to an improvement in LCOH.

[0010] To achieve this objective, the method is provided with the features of claim 1. Advantageous developments of the invention are set forth in the subclaims. Furthermore, a process control system for an electrolysis plant is provided. Disclosure of the invention

[0011] In the proposed method for operating an electrolysis plant with at least one stack, a maximum stack voltage is specified, which defines the beginning of a first phase at the end of the stack's service life. Upon reaching the maximum stack voltage, the operating parameter temperature is increased at a constant current density to such an extent that the maximum stack voltage is maintained at least throughout the first phase of the end of the stack's service life.

[0012] According to the proposed method, the stack voltage is kept constant towards the end of the lifespan of at least one stack. Since the efficiency is also constant at a constant stack voltage, a minimum stack efficiency can be maintained at a constant current density, despite aging. This has a positive effect on the economic viability of the electrolysis plant. To achieve this, the operating temperature is increased at maximum stack voltage to such an extent that the current density remains constant and thus as high as possible. The temperature increase can be carried out continuously and / or incrementally.

[0013] Limiting the stack voltage to a predetermined maximum also has the advantage of ensuring the continued functionality of the interface between the stack and the power electronics connected to the stack (which supply power to the stack) as the stack nears the end of its service life. If the stack voltage could increase indefinitely, it would overload the power electronics or necessitate oversizing them. Depending on the design, the latter would increase system costs and / or compromise the performance of the power electronics.

[0014] In a further development of the invention, it is proposed that during the first phase, the temperature is increased at a constant current density until a predetermined maximum temperature is reached, which defines the beginning of a second phase at the end of the stack's lifetime. Upon reaching the maximum temperature, the current density is then reduced to such an extent that the maximum stack voltage is maintained.

[0015] At the beginning of the second phase, the operating strategy is changed. Instead of varying the temperature, the current density is now varied. This allows a minimum efficiency to be maintained during stack operation even in the second phase. The change in operating strategy is triggered by a threshold value, which at the end of the first phase and at the beginning of the second phase is a maximum operating temperature.

[0016] Reducing the current density during the second phase may be at the expense of the hydrogen yield, but this is acceptable depending on the operating phase.

[0017] Preferably, during the second phase, the current density is reduced until a predetermined minimum current density is reached, defining the start of a third phase at the end of the stack's lifetime or the end of the stack's lifetime itself. In the third phase, the stack can therefore continue operating while maintaining the minimum current density, or operation can be discontinued. This is because, while maintaining the minimum current density, the stack voltage will increase over time, and efficiency will decrease. If the minimum efficiency is no longer achieved, continued operation of the stack is no longer economically viable.

[0018] The proposed method therefore preferably comprises two or three phases, with the first phase marking the beginning of the stack's end-of-life. This first phase is preceded by at least one further phase, which begins with the initial commissioning of the stack and preferably ends at a point in time when the stack's aging has progressed to the point where the stack voltage has reached the predetermined maximum. The proposed method thus focuses on operating the stack at the end of its life. For the LCOH, it is then crucial that a minimum quantity of hydrogen is still produced while maintaining a minimum efficiency.

[0019] Preferably, the maximum stack voltage, which defines the beginning of the first phase of the stack's end-of-life, is determined by power electronics connected to the stack and / or a predefined minimum efficiency requirement. If the power electronics determine the maximum stack voltage, this depends on the maximum voltage that the power electronics can provide. If a minimum efficiency requirement determines the maximum stack voltage, this preferably corresponds to a maximum value for the LCOH (Liquid Life Overhead).

[0020] Furthermore, the stack voltage and temperature are preferably measured using sensors. The measured data then form the basis for implementing the various operating strategies during the phases at the end of the stack's service life. Preferably, the sensors are arranged on the stack so that the data is measured locally. This is particularly advantageous when the electrolysis plant comprises more than one stack. Additionally, suitable sensors can be provided to measure the amount of hydrogen produced or the hydrogen yield.

[0021] Furthermore, the aging state of the stack and / or the relationship between temperature and aging is / are preferably determined by comparing it with historical operating data. The historical data can be taken from an internal data storage device or provided via a communication interface from an external data storage device, such as a cloud. When comparing and communicating with a cloud or similar device, corresponding determinations can also be made for multiple stacks of a production batch.

[0022] Furthermore, it is proposed that the hydrogen production costs be determined taking into account the current electricity price or an electricity price averaged over a specific period. This period could, for example, be a calendar year. Determining the LCOH (Levelized Cost of Hydrogen Production) reveals whether adjustments to the temperature and / or current density are necessary to maintain the minimum efficiency and thus the economic viability of operating the stack.

[0023] The levelized cost of hydrogen (LCOH) is calculated using the following formula: LCOH=l0+∑t=1nlt+Et+Mt(1+r)t∑t=inHt(1+r)t with I0 investment costs in year 0, I t Investment costs per year t (e.g. stack replacement), Mt other operating costs per year t (e.g. maintenance costs), Annual electricity costs (t) including generation costs (all costs e.g. in EUR), Hydrogen yield per year (e.g., in kg) r interest rate factor in %, Lifespan in years

[0024] To optimize LCOH towards the end of the stack's lifespan, assuming the same investment and maintenance costs and hydrogen yield over that lifetime, the operating strategy must be adjusted. Assuming the aforementioned parameters remain constant after the change, this can be achieved by maintaining a minimum efficiency, which reduces the Et share, especially during periods of high electricity costs.

[0025] As an alternative or supplement to determining the LCOH, it is proposed that the quotient of stack efficiency and hydrogen yield, or the quotient of the product of stack efficiency and electricity price and a fictitious maximum hydrogen price, be determined as a subordinate control variable. In the latter case, this is based on current or assumed electricity costs C.el (in EUR / kWh) and a fictitious maximum hydrogen price P H2,max (in EUR / kg) a maximum energy expenditure Q H2,max (in kWh / kg) for the production of hydrogen according to the following formula: QH2,max=PH2,maxCel

[0026] Together with the energy content of hydrogen (H H2 =39.4 kWh / kg) results in a minimum efficiency η according to the following equation. min : ηmin=HH2QH2,max=HH2∗CelPH2,max Assuming electricity costs of 4 ct / kWh and a maximum hydrogen price of 3 EUR / kg, the minimum efficiency is 53%. With a maximum hydrogen price of 2 EUR / kg, the minimum efficiency is 79%.

[0027] In the proposed method, the end of a phase is preferably detected using a central or decentralized process control system, and the corresponding change in the operating strategy is initiated. The method therefore requires the presence of a suitable process control system. This can be a control unit or a programmable logic controller (PLC). In an electrolysis plant with multiple stacks, the process control system can be arranged centrally or decentrally. In the latter case, each stack is preferably equipped with its own corresponding process control system.

[0028] As a further development measure, it is proposed that the recorded and / or determined data be stored in a data storage system for documentation purposes using the centralized or decentralized process control system. This can be an internal data storage system or an external data storage system, such as a cloud. Using the stored data, the improvement in LCOH achieved using the proposed method can be quantitatively demonstrated.

[0029] Furthermore, a process control system, in particular a control unit or a programmable logic controller (PLC), is proposed for an electrolysis plant. This system is configured to execute steps of a process according to the invention. Preferably, the process control system uses data acquired by sensors to detect when a predetermined limit value has been reached or a phase has ended, thus requiring a change in the operating strategy. The corresponding change can then be implemented using the process control system. Additionally, all processes can be stored in a data memory of the process control system for documentation purposes or forwarded via a communication interface to an external data storage device, such as a cloud.

[0030] The invention and its advantages are explained in more detail below with reference to the accompanying drawings. These show: Fig. 1. A diagram illustrating the relationships between temperature T, efficiency A, and lifetime B. Fig. 2 a diagram illustrating the relationships between the current density iDens, the efficiency A and the hydrogen yield C, as well as Fig. 3 a) to c) Diagrams illustrating the curves of the operating temperature T, the current density iDens and the stack voltage U over the lifetime of a stack operated according to a method according to the invention. Detailed description of the drawings

[0031] Regarding the Fig. 1 and Fig. 2. To avoid repetition, reference is made to the introductory description. The information contained in the Fig. 1 and Fig. The relationships described in section 2 form the basis for the method according to the invention, which is explained below using the following examples. Fig. 3 is explained.

[0032] In the Fig. Figure 3 shows the lifetime of a stack plotted on the x-axis, which in this case is divided into four phases, starting with phase 0 and ending with phase III. During phase 0, the stack ages, which results in an increase in the stack voltage U (see Fig. 3c)). At time t1, the stack voltage U reaches a maximum stack voltage (Umax) due to stack aging. This can be predetermined, for example, by power electronics connected to the stack. Time t1 marks the beginning of the end of the stack's service life and thus the beginning of phase I of the method according to the invention.

[0033] The aim of the process according to the invention is to produce a minimum amount of hydrogen at a minimum efficiency towards the end of the stack's lifetime. According to the process according to the invention, this is achieved in phase I by increasing the temperature T (see Fig. 3a)) at constant current density iDens (see Fig. 3b)) and constant voltage U (see Fig. 3c)) achieved. In this way, it is possible to keep the hydrogen yield constant despite aging and thus potentially increase it compared to maintaining the operating strategy from phase 0, while adhering to a predetermined limit for the hydrogen yield and / or the LCOH.

[0034] In the Fig. 3a) shows not only the temperature T (solid line) but also a temperature T' (dashed line), which is required to maintain a specified minimum efficiency. Similarly, in the Fig. 3b) The current density iDens and the current density iDens' are shown. The curves demonstrate that the minimum efficiency can be maintained by increasing the temperature T while keeping the current density iDens constant.

[0035] At time t2, the temperature T reaches a given maximum Tmax (see Fig. 3a)). The temperature T cannot be increased any further, so the operating strategy must be changed. This can be the case, for example, because the design of the lines or subsystems supplying the stack has a temperature limitation and / or the stack itself undergoes irreversible changes at higher temperatures. Instead of the temperature T, the current density iDens is then reduced during a phase II following phase I (see Fig. 3b)). The voltage U—and thus the efficiency—can be kept constant in this way, but the hydrogen yield decreases.

[0036] At time t3, the current density iDens reaches a predetermined minimum iDens,min. Maintaining the minimum efficiency would require a further reduction in the current density iDens (see the dashed line in the Fig.3b)). Since this does not occur, the stack voltage U increases in Phase III, which follows Phase II. In Phase III, continued operation is only possible with a minimum current density iDens. The efficiency drops below the minimum efficiency. Continued operation of the stack is therefore likely no longer economically viable or technically no longer feasible due to plant restrictions.

Claims

[1] Method for operating an electrolysis plant with at least one stack, in which a maximum stack voltage (Umax) is specified, which defines the beginning of a first phase (I) at the end of the service life of the stack, and when the maximum stack voltage (Umax) is reached, the operating parameter temperature (T) at constant current density (iDens) is increased to such an extent that the maximum stack voltage (Umax) is maintained at least over the first phase (I) of the end of the service life. [2] Method according to claim 1, characterized by that during the first phase (I) the temperature (T) is increased at a constant current density (iDens) until a predetermined maximum temperature (Tmax) is reached, which defines the beginning of a second phase (II) at the end of the life of the bag, and when the maximum temperature (Tmax) is reached the current density (iDens) is reduced to such an extent that the maximum stack voltage (Umax) is maintained. [3] Method according to claim 2, characterized by that during the second phase (II) the current density (iDens) is reduced until a predetermined minimum current density (iDens,min) is reached, which defines the beginning of a third phase (III) at the end of the lifetime or the end of the lifetime of the stack. [4] Method according to one of the preceding claims, characterized by that the maximum stack voltage (Umax) is specified by power electronics connected to the stack and / or a predefined minimum efficiency to be maintained. [5] Method according to one of the preceding claims, characterized by that the stack voltage (U) and the temperature (T) are detected by means of a sensor which is preferably arranged on the stack. [6] Method according to one of the preceding claims, characterized by that the aging state of the stack and / or the relationship between temperature (T) and aging is / are determined by comparison with historical operating data. [7] Method according to one of the preceding claims, characterized by that the hydrogen production costs (LCOH) are determined taking into account the current electricity price or an electricity price averaged over a period of observation. [8] Method according to one of the preceding claims, characterized by that the quotient of stack efficiency and hydrogen yield or the quotient of the product of stack efficiency times electricity price and a fictitious maximum hydrogen price is determined as a subordinate control variable. [9] Method according to one of the preceding claims, characterized by that with the help of a central or decentralized process control system, for example a control unit or a programmable logic controller, the end of a phase (I, II, III) is detected and the associated change in the operating strategy is initiated. [10] Method according to claim 9, characterized bythat with the help of the central or decentralized process control system, the recorded and / or determined data are stored in a data storage device for documentation purposes. [11] Process control system, in particular control device or programmable logic controller, for an electrolysis plant, which is designed to carry out steps of a method according to one of the preceding claims.

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