Method for controlling an h2 concentration of a fuel cell

Abstract

In order to provide a method for optimally determining a fuel concentration of a fuel cell, the invention relates to a method for controlling the operation of a fuel cell (100), comprising the following steps: - recording (B1) at least one measured value of an operating parameter of the fuel cell or creating (B2) a model for the time curve of the operating parameter; - ascertaining a target value for an H2 concentration for the fuel gas of the fuel cell on the basis of the recorded measured value (C1) or the model of the operating parameter (C2); and - supplying (D1, D2) fuel gas having the ascertained target value of the H2 concentration.

Description

[0001] R.415380

[0002] - 1 -

[0003] Description

[0004] title

[0005] Method for controlling the H2 concentration of a fuel cell

[0006] The invention relates to a method for controlling the operation of a fuel cell, a fuel cell stack, comprising a control device which is configured to carry out such a method.

[0007] State of the art

[0008] The control of the fuel gas supply in the high-temperature fuel cell system (solid oxide fuel cell - SOFC) aims on the one hand at high fuel gas utilization (FU = Fuel Utilisation) in order to achieve high efficiency, and on the other hand at compliance with the limit values ​​for the minimum fuel concentration (iA hydrogen concentration) in the cells or in the fuel cell stack, i.e. c(H2stackout) > min H2.

[0009] In fuel cell systems with anode exhaust gas recirculation (AOGR), the interplay with the minimum oxygen-to-carbon ratio (min O:C) limit to prevent coking means that at low frequency (FU), a large amount of anode gas must be recirculated (high recirculation rate r). This limits the power and efficiency of the fuel cell.

[0010] The goal of fuel cell optimization is therefore to achieve high tolerances for maximum fuel utilization and low tolerances for minimum hydrogen concentration.

[0011] The lower limit for the minimum fuel concentration at the anode outlet of the fuel cell stack is required to prevent a fuel undersupply to a nickel anode of the fuel cell and thus R.415380

[0012] - 2 - to prevent associated nickel oxidation. This can lead to irreversible cell damage.

[0013] The invention is therefore based on the objective of providing a method for optimizing the determination of a fuel concentration in a fuel cell.

[0014] Disclosure of the invention

[0015] The problem according to the invention is solved by a method for controlling the operation of a fuel cell, comprising the following steps:

[0016] Recording at least one measured value of an operating parameter of the fuel cell or creating a model for the time course of the operating parameter,

[0017] Determining a target value for an H2 concentration for the fuel cell fuel gas based on the recorded measurement or the model of the operating parameter, and

[0018] Supply of fuel gas with the determined target value of the H2 concentration.

[0019] The system according to the invention advantageously enables the H2 concentration during fuel cell operation to be dynamically adjusted based on operating parameters relevant to the minimum H2 concentration threshold. This eliminates the need to set aging allowances or a higher minimum H2 concentration threshold at the start of operation to compensate for decreases in H2 concentration over the fuel cell's operating life. Such a reduction in fuel cell efficiency decreases over its lifetime. Instead, according to the invention, the minimum H2 concentration threshold is continuously adjusted to the current state of the fuel cell.

[0020] Methane is the preferred fuel gas. Natural gas can also be used as fuel, particularly with various compositions of methane and longer-chain hydrocarbons, which may be relevant to the anode control concept described herein. R.415380

[0021] - 3 -

[0022] Basically, there are a number of factors that can lead to a local reduction in fuel concentration or hydrogen concentration in a fuel cell:

[0023] 1. Fuel distribution in the fuel cells and fuel cell stack: Flow and diffusion resistances create different local fuel supplies and concentration differences.

[0024] 2. Manufacturing variation (flow resistances, uniform distribution between fuel cell stacks, between fuel cells and locally in porous electrodes)

[0025] 3. a) the aging of cell resistance due to external electrode poisoning b) the aging of the internal vapor reforming catalyst

[0026] Contributions 1 and 2 occur even when the fuel cell or fuel cell stack is new and are generally not directly measurable. Contributions 3, however, only lead to a reduced H2 concentration and a corresponding risk of cell damage over time. Since both aging mechanisms 3a and 3b can be measured via signals during operation, their impact on the fuel cell budget can be continuously adjusted using a suitable calculation method in the control system or control software.

[0027] This approach preferably utilizes the finding that the H2 concentration of the fuel cell or fuel cell stack is a unique function of the fuel utilization FU, the temperature T and the electric current / through the fuel cell:

[0028] H2,stkout = f(l, T, FU)

[0029] Based on this relationship, a calculation rule can be created which is implemented by a control unit of the fuel cell or fuel cell stack to calculate the required minimum H2 concentration. For this purpose, either real measurement data or values ​​from a model of the fuel cell can be used for the respective operating parameters R.415380.

[0030] - 4 - can be used to calculate the H2 concentration based on this and the calculation rule.

[0031] The current dependency occurs indirectly via the aging effect. After the cell resistance ages, the current density concentrates at the trailing edge of the fuel cell. The higher the current, the more the H2 concentration at the cell outlet is reduced.

[0032] This is one of the lifetime effects that needs to be corrected. The correction factor can be derived as an adjustment factor from simulation models and implemented in a software model for the minimum H2 concentration correction (H2,stkout correction) over the lifetime.

[0033] In case 3 a), this occurs in particular via the known correlation between cell voltage degradation and the reduction of the local minimum H2 concentration.

[0034] Accordingly, the preferred operating parameter is a voltage generated by the fuel cell. The fuel cell stack or the individual fuel cell acts as a voltage source. The generated voltage is typically measured by the fuel cell system for diagnostic and control purposes.

[0035] The generated voltage depends on (a) (e.g. via the internal resistance or ASR) the operating conditions such as temperatures, gas composition, current density (e.g. via the internal resistance or total cell resistance (ASR)) and (b) the aging state of the cells.

[0036] The voltage of the fuel cell is related to its current density. Due to the known relationship between cell voltage degradation or current density and the associated decrease in the local minimum H2 concentration, an additional H2 budget can be advantageously determined, which is necessary to compensate for the decrease in the local minimum H2 concentration.

[0037] Accordingly, a value for the minimum H2 concentration in the fuel can then be determined. R.415380

[0038] - 5 -

[0039] Preferably, an increase in voltage over time leads to an increase in H2 concentration. The relationship between the increase in voltage and the increase in H2 concentration is essentially linear.

[0040] Preferably, the operating parameter can also be the open-circuit voltage (OCV) of the fuel cell. By measuring the open-circuit voltage during operation, the condition of a catalyst, especially an I²R (ilidium) catalyst, can be determined or calculated. For this purpose, the open-circuit voltage is preferably measured directly after the current step, and the H₂ concentration is calculated from the measured cell voltage using the Nernst equation, where Un represents the Nernst voltage minus leakage currents.

[0041] OCV = U N = f(T, H2)

[0042] Preferably, when the cell voltage of the fuel cell decreases over time in the unloaded state, the H2 concentration increases. The relationship between the decrease in cell voltage and the increase in H2 concentration is essentially linear.

[0043] Preferably, the operating parameter can also be the temperature of the fuel cell. In the reaction of natural gas and methane, the residual methane concentration depends on the temperature and has a significant influence on the H₂ concentration. The equilibrium concentration can be calculated using the corresponding chemical equilibria of steam reforming (SR) and the water-gas shift reaction (WGS):

[0044] SR: CH4 + H2O --> CO + 3 H2

[0045] WGS: CO + H2O --> CO2 + H2

[0046] The equilibrium concentrations can then preferably be determined by solving the nonlinear system of equations R.415380

[0047] - 6 - be determined.

[0048] The operating parameters described above can be recorded for the respective operating times either by measurement or determined based on a model of the operating parameters. The model is created specifically based on the relationships described above.

[0049] Preferably, a first measurement is taken for a first operating parameter and a second measurement is taken for a second operating parameter, or models are created for the time course of the operating parameters, where the first operating parameter is the cell voltage of the fuel cell in the unloaded state and the second operating parameter is the current density of the fuel cell. Advantageously, two operating parameters are thus determined when calculating the H₂ concentration for the fuel gas of the fuel cell.

[0050] Preferably, the H2 concentration of the fuel gas is continuously recalculated during fuel cell operation, and the fuel supply is adjusted accordingly. This advantageously ensures that an H2 budget is always maintained for fuel cell operation, based on its actual state, which is recorded by measurement and determined using the underlying model.

[0051] These two adjustments to the control limits for the minimum H2 concentration during the fuel cell's operating time increase its overall efficiency because unnecessary aging allowances for the minimum H2 concentration do not need to be considered when the fuel cell is new. Furthermore, the fuel cell is better protected because the cell protection limits are set according to the actual condition of the fuel cell and / or catalyst. R.415380

[0052] - 7 -

[0053] The problem according to the invention is further solved by a fuel cell stack comprising a control unit configured to carry out a method according to one of the aforementioned embodiments. A fuel cell stack is, in particular, a stack of several fuel cells.

[0054] The preferred fuel cells are high-temperature fuel cells (SO₄FC) of the fuel cell stack.

[0055] Furthermore, the problem according to the invention is solved by a fuel cell designed for use in a fuel cell stack according to one of the aforementioned embodiments.

[0056] Preferably, the fuel cell or fuel cell stack includes sensors for recording the aforementioned operating parameters. In particular, such sensors are already present in known fuel cells, so that, advantageously, no additional sensors need to be provided.

[0057] The sensor readings are preferably provided to the control unit to determine the H2 concentration for the fuel gas.

[0058] Specific embodiments are explained below with reference to the figures. They show:

[0059] Fig. 1 shows a top view of a cathode of a fuel cell,

[0060] Fig. 2 shows a schematic diagram of a fuel cell stack,

[0061] Fig. 3 is a diagram showing the relationship between the methane content at the cell outlet and the Fh content.

[0062] Fig. 4 is a diagram showing the relationship between cell voltage degradation at the cell output and an additional Fh budget required, R.415380

[0063] - 8 -

[0064] Fig. 5a) a flowchart for controlling the operation of the fuel cell based on recorded measured values ​​of at least one operating parameter of the fuel cell, and

[0065] Fig. 5 b) a flowchart for controlling the operation of the fuel cell based on model-based values ​​of the operating parameter of the fuel cell.

[0066] Figure 1 shows a top view of a cathode 10 of a fuel cell 100. It shows how the fuel gas 11 strikes a leading edge 12 of the cathode.

[0067] Due to external impurities in the cathode material, which occur over the lifetime of the cathode 10, a deactivated region 14 forms in this area, which is not available for the reaction. Downstream of this, an active region follows.

[0068] This results in locally higher current densities, which in turn lead to high H2 concentration gradients. Consequently, lower minimum H2 concentrations are present, leading to the disadvantages described in the preceding section. Therefore, these disadvantages must be compensated for in the operation of the fuel cell 100 by increasing the minimum H2 concentration.

[0069] Figure 2 shows a fuel cell stack system 200 with several fuel cells 100 and anode exhaust gas recirculation (r). Methane is supplied as fuel gas 11, whereby a limit value for the minimum oxygen-to-carbon ratio (min O:C) must be maintained at an inlet 17 of the fuel cell 100 to prevent coking.

[0070] At an outlet 19 of the fuel cell 100, however, limit values ​​for the minimum fuel concentration (iA hydrogen concentration) in the cells or in the fuel cell stack, i.e. c(H2StackOut) > min H2, must be observed.

[0071] Therefore, according to the procedure, a minimum H2 concentration for operation is determined based on operating parameters such as the voltage of fuel cell 100 and / or a cell voltage of fuel cell 100 in the unloaded state. R.415380

[0072] - 9 - determined Via a control unit 19 of the fuel cell stack 200, an H2 concentration can be set depending on measured values ​​or model-based values ​​to the operating parameters at an operating time of the fuel cell 100, so that an optimal minimum H2 concentration is always present.

[0073] Figure 3 shows a diagram illustrating the fuel gas fraction, in this case the methane fraction, as a function of the hydrogen content at the cell outlet, expressed as a percentage. It shows that a high hydrogen fraction is associated with a low methane fraction when there is low degradation of the catalyst in fuel cell 100. Over the operating time of fuel cell 100 and the catalyst, the methane concentration at the cell outlet increases, which is due to progressive catalyst degradation. Correspondingly, the H₂ concentration also decreases. Using the method described herein, the reduction of the H₂ concentration can be achieved by means of an additional Fh budget, which is determined based on the operating parameters.

[0074] Figure 4 shows a diagram depicting the cell voltage as a function of the rate of decrease in H2 concentration. It can be seen that the rate of decrease in H2 concentration increases, meaning there is a larger delta between the initial H2 concentration and the lower H2 concentration that develops over the operating time of fuel cell 100, which is reflected in a reduction of the fuel cell 100's voltage. The cell voltage decreases due to degradation, and the internal resistance increases. The progressive decrease in H2 concentration can then be advantageously compensated for by an additional Fh budget, which is determined based on the operating parameters.

[0075] Figure 5 a) shows a flowchart of the procedure for controlling the operation of fuel cell 100 or fuel cell stack 200. The procedure is based on the creation of a calculation rule (A) according to which the required H2 concentration at any given time during the operation of fuel cell 100 or fuel cell stack 200 can be determined. R.415380

[0076] - 10 -

[0077] Based on this calculation method and at least one or more operating parameters, for example, the voltage of fuel cell 100 and / or the cell voltage of fuel cell 100 in the unloaded state, which are detected by means of appropriate sensors and determined by means of a suitable calculation method (B2), a minimum H2 concentration can then be determined based on the measured values ​​of the operating parameters (B2). The minimum H2 concentration thus determined can then be implemented by the control unit 19 and the fuel gas with the corresponding minimum H2 concentration can be supplied to the fuel cell 100.

[0078] Figure 5 b) shows a flowchart of the procedure for controlling the operation of the fuel cell 100 or the fuel cell stack 200, whereby model-based values ​​are used instead of measured values ​​recorded during the operation of the fuel cell 100 to determine the minimum H2 concentration.

[0079] For this purpose, the underlying calculation formula is created and used (A). Subsequently, the model-based values ​​are determined (B2) and used together with the calculation formula to determine the minimum H2 concentration for the fuel gas (C2). The minimum H2 concentration thus determined can then be implemented by the control unit 19 and the fuel gas with the corresponding minimum H2 concentration can be supplied to the fuel cell 100.

Claims

R.415380 - 11 - Claims 1. Method for controlling the operation of a fuel cell (100), comprising the following steps: Acquiring (B1) at least one measured value of an operating parameter of the fuel cell or creating (B2) a model for the time course of the operating parameter, Determining a target value for an H2 concentration for the fuel cell fuel gas based on the acquired measurement value (C1) or the model of the operating parameter (C2), and Supply (D1 , D2) of fuel gas with the determined target value of H2 concentration.

2. Method according to claim 1, wherein the operating parameter is a voltage generated by the fuel cell (100).

3. The method of claim 2, wherein the H2 concentration is increased when the voltage decreases over time.

4. Method according to claim 1, wherein the operating parameter is a cell voltage of the fuel cell (100) in the unloaded state.

5. The method of claim 4, wherein the H2 concentration is increased when the cell voltage decreases over time.

6. Method according to claim 1, wherein a first measured value for a first operating parameter and a second measured value for a second operating parameter are acquired or models for the time course of the operating parameters are created, wherein the first operating parameter is a cell voltage of the fuel cell (100) in the unloaded state and the second operating parameter is a current density of the fuel cell (100). R.415380 - 12 - 7. Method according to one of the aforementioned claims, wherein the H2 concentration for the fuel gas is continuously re-determined during the operation of the fuel cell (100) and the fuel supply is adjusted to the newly determined H2 concentration.

8. Fuel cell stack (200) comprising a control device (19) configured to carry out a method according to one of claims 1 to 7.

9. Fuel cell stack (200) according to claim 9, wherein the fuel cells (100) of the fuel cell stack (200) are high-temperature solid oxide fuel cells (SOFC).

10. Fuel cell (100) configured for use in a fuel cell stack (200) according to claim 8 or 9.

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