Method for controlling fuel cells at very low partial pressures down to zero

Abstract

A method (1000) for controlling a fuel cell (1) having an anode (11) and a cathode (12), the method comprising at least the following steps: determining a pressure value (p ) for each of the gas components (2a, 2b, 2c, 2d) present in the anode chamber (110) and / or the cathode chamber (120) that are involved in the operation of the fuel cell; i ) is received (100), - a current (I) for controlling the operation of the fuel cell (1) is set (200), - the received pressure value (p i ) is used to convert the set current (I) to the target voltage (U S ) is calculated (300), and this calculation is S The method relies on a numerical inversion of a predetermined relationship that converts a current (I) from the fuel cell (1) to a set current (I), the numerical inversion relying on addition, subtraction, multiplication, division, and exponent formation, but not including a numerical calculation of logarithms, and includes the steps of: - controlling (400) the operation of a fuel cell (1) by a set current (I); - measuring (500) a voltage (U) generated at the fuel cell (1); - converting the measured voltage (U) to a calculated target voltage (U S ) (600).

Description

[Technical field]

[0001] The invention relates to a method for controlling a fuel cell.The invention also relates to a computer program implementing said method, to a machine-readable data medium comprising such a computer program, as well as to one or more computers comprising said computer program. [Background technology]

[0002] Control and calculation methods for fuel cell operation are based on the positive absolute pressure (partial pressure) of the gases in the fuel cell. For mobile automotive fuel cells, this is usually the hydrogen pressure at the anode and the oxygen pressure at the cathode. In the control methods, the source and no-load voltages are typically calculated using the Nernst equation, where the partial pressure of one gas component in the fuel cell appears as the logarithmic independent variable. Summary of the Invention

[0003] During the initial start-up process, when the gas chambers are not yet filled with the fuel cell working gas, as well as during the stop-down process, when the working gas is decomposed, partial pressures of the working gas can occur that can even reach zero pressure. When the fuel cell stack is shut down, the anode and / or cathode chambers can be closed via valves, so that no further gas can flow in or out. The fuel cell stack can then completely consume the amount of gas remaining in the anode and / or cathode by current draining. The corresponding partial pressures can then drop to values ​​very close to and including zero.

[0004] It has been found that the currently prevalent method of controlling fuel cells can have difficulties in cases where no gas or only very little gas is supplied to at least one electrode. If the partial pressure drops close to or to zero, the Nernst equation for the source and no-load voltage calculation is no longer applicable, and therefore the control does not have information about the current operating state of the fuel cell. This can be the case during the starting or stopping process, as explained above. Similarly, after a relatively long stopping time, the fuel (e.g. hydrogen) and / or the oxidant (e.g. oxygen) can be completely reacted out, so that only nitrogen remains at the anode and / or cathode. Furthermore, in many cases at present, the fact that not only gas is generally present in the anode or cathode chamber, especially due to diffusion processes, is not taken into account or is not taken into sufficient account.

[0005] The method according to the present invention relates to the control of a fuel cell having an anode and a cathode and includes at least the steps described below.

[0006] For each gas component that may be present in the anode or cathode chamber and that is involved in the operation of the fuel cell, a pressure value is received, which may for example come from a sensor or for example from a model calculation.

[0007] Furthermore, a current for operating the fuel cell is set. A target voltage is then determined from the set current using the received pressure value. A known relationship between the set current or the corresponding current density and the target voltage, which takes into account all pressure values ​​involved, is used (Butler-Volmer formula, taking into account mixed potentials). This relationship expresses the current as a function of the target voltage and the pressure values ​​of all gas components involved, which is numerically inverted in the course of the method; an analytical inverse function does not exist for the general case. Within the framework of the numerical inversion, the calculations rely on addition, subtraction, multiplication, division and exponential formation, but do not involve numerical calculations of logarithms. In a next method step, the fuel cell is operated with the set current and the voltage occurring at the fuel cell is measured. The measured voltage value is then compared with the calculated target voltage.

[0008] The gas components involved during the operation of the fuel cell may be at least the fuel supplied at the anode and the oxidant supplied at the cathode. Due to, for example, diffusion processes through the electrolyte used in the fuel cell, in some circumstances a non-negligible amount of fuel may also be present in the cathode chamber and vice versa a non-negligible amount of oxidant in the anode chamber. Reaction products, such as, for example, water, also play a role. Furthermore, in the course of chemical processes taking place inside the fuel cell, further gas components may be generated which may reach the electrode chambers due to diffusion. All gas components which are in principle present and which cannot be ignored should be taken into account when calculating the target voltage and / or when calculating the other quantities involved in the fuel cell, respectively.

[0009] Equations involving logarithmic calculations present numerical difficulties when the independent variable of the logarithm becomes very small and the logarithm therefore takes on a very large (negative) expression. Calculations involving logarithms with small independent variables can be particularly numerically unstable. Numerical inaccuracies can grow into large errors. The latter is also relevant when calculations for controlling a fuel cell are developed and tested on a particular computer type and subsequently implemented in, for example, a control device for an automobile. Such types of control devices are generally relatively insular in terms of computing power and make use of their own libraries and their own logarithm tables, where the software may behave differently there than on the computer for which it was developed. The software may likewise behave differently when implemented in a new control device having a different chipset in which the numerical calculations of the logarithms are performed by a different algorithm. It would therefore be beneficial and advantageous to avoid the above-mentioned sources of error associated with logarithmic calculations and provide a robust calculation that is less susceptible to numerical problems. The invention described herein achieves this.

[0010] The reason why the method does not involve the numerical calculation of logarithms is that it relies on the Butler-Volmer equation, which is obtained by transformation and includes the mixed potential, where ln(p i / p i0 In the above formula, p i represents the partial pressure of the i-th gas component, and p i0 represents the (positive, fixed) reference pressure of the i-th gas component. The ratio p i / p i0 is used as the independent variable of the logarithm, but in this method, i / p i0 ) γ i ±α i / n iappears in each case as the base of , where the exponent is determined by the chemical and physical properties of the i-th gas component and is always greater than 0 in the case of typically applied chemicals. For example, for the reaction of oxygen to water at the cathode, the (effective) reaction order γ can be chosen in the range 0.5..1, the transfer coefficient α can be chosen in the range 0.9..1.5, and the number of electrons involved in the reaction is n=4. At the cathode, γ+α / n is chosen, which in the most unfavourable case gives 0.5+0.9 / 4=0.725.

[0011] In this way, the problem of very large logarithms at small values ​​of the independent variable does not appear. i The case where p = 0 is also fine, because i This is because division by does not occur.

[0012] Very small values ​​of partial pressure of the gas components involved during the operation of the fuel cell are real. They can appear, for example, if the gas supply is (no longer) available under the closed anode and / or cathode chambers, but the current still flows through the resistor, which can occur especially during the parking process. Gas components still present in the respective electrode chambers can still be consumed in the course of chemical reactions inside the fuel cell, which results in a very small contribution to the pressure. Another similar case can occur if the water produced during the operation of the fuel cell freezes. In one embodiment, the operational control of the fuel cell is altered to reduce the discrepancy between the determined target voltage and the voltage measured at the fuel cell, such that the method can be used for error diagnosis and / or compensation when a fuel cell malfunction occurs and is recognized.

[0013] In one embodiment, malfunction and / or degradation of the fuel cell is deduced and assessed, particularly from the discrepancy between the target voltage and the measured voltage.

[0014] The method can thereby ensure error-free operation of fuel cells in motor vehicles and contribute to extending the possible service life of fuel cells.

[0015] In one embodiment, the current set in the above relationship is selected to operate the fuel cell at a working point where degradation of the fuel cell is mitigated.

[0016] Accordingly, the service life of the fuel cell can be extended, and the manufacturing and preparation costs of the fuel cell can be correspondingly reduced.

[0017] In one embodiment, the operation control of the fuel cell is altered so that the power delivered by the fuel cell at the same set current remains constant regardless of degradation.

[0018] In another embodiment, the fuel cell is controlled to operate at a set current in an operating mode in which no fuel is supplied to the anode and / or no oxidant is supplied to the cathode, and the pressure of at least one gas component is significantly reduced at at least one of both electrodes.

[0019] Such a situation may occur, for example, during the parking process, when gas supply is not possible under closed anode or cathode chambers, but current still flows via a resistor. In such a case, the gas still present in the corresponding electrode chambers continues to be consumed or converted in the course of the chemical reactions taking place in the fuel cell, so that the partial pressure of the gas component of interest continues to decrease. The method for controlling a fuel cell described here provides a robust value for the setpoint voltage even under the above-mentioned circumstances, which can be finally incorporated into the control of the fuel cell. This is achieved by the fact that the method does not include a logarithm, which may be a very small pressure (partial pressure) value, as the independent variable.

[0020] In one embodiment, the fuel utilized in the fuel cell may be hydrogen and the oxidant utilized in the fuel cell may be oxygen or air.

[0021] By way of example, the methods described herein may be applied in the control of a PEM fuel cell.

[0022] In one embodiment, there may be a mixture of gases on both the anode and cathode sides.

[0023] By way of example, the anode side gas mixture and the cathode side gas mixture may each consist of at least a fuel supplied to the fuel cell on the anode side and an oxidant supplied to the fuel cell on the cathode side.

[0024] In many cases, it is desirable to prevent the fuel or oxidant, respectively, from diffusing through the electrolyte to the other electrode.

[0025] However, due to diffusion processes occurring through the electrolyte, it is possible for a gas mixture containing both fuel and oxidant to be present in at least one of the electrode chambers of a fuel cell, and the methods described herein take such cases into account.

[0026] Furthermore, it may occur - and be taken into account in the methods described - that the gas mixture on the cathode and / or anode side contains at least one further component, which is formed, for example, in the course of a chemical reaction taking place inside the fuel cell, and the hydrogen-containing fuel gas, obtained, for example, by reforming a hydrocarbon-containing fuel (for example methanol or natural gas), may contain by-products and impurities originating from the reforming.

[0027] In one embodiment, the fuel cell of interest within the methods described herein may be part of a fuel cell stack.

[0028] Typically, the fuel cells of a fuel cell stack are connected in series and can be used to generate electrical current in a fuel cell vehicle. The method described here allows a reliable and stable control of the situation in which gas is (no longer) supplied to at least one of the electrodes of at least one fuel cell of the stack, but gas continues to be diverted in the electrode chamber by a drain current via a resistor. Numerical problems within the framework of computer-aided fuel cell control or regulation based on very small pressures (partial pressures) are avoided with the method presented here.

[0029] Furthermore, the invention relates to a computer program having machine-readable instructions which, when executed on one or more computers, instruct the one or more computers to carry out the method according to the invention. The invention also relates to a machine-readable data medium on which said computer program is stored, as well as to a computer equipped with said computer program or said machine-readable data medium.

[0030] Other ways of improving the present invention will be described in detail below in conjunction with the description of the preferred embodiments of the present invention with reference to the drawings. [Brief description of the drawings]

[0031] [Figure 1] 1 is an exemplary embodiment of a method for controlling a fuel cell. [Diagram 2] 4 is another example embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] FIG. 1 shows an embodiment of a method 1000 for controlling a fuel cell 1, comprising the steps listed below.

[0033] In step 100, a pressure value p corresponding to the ith gas component that may be present in the anode or cathode chamber of the fuel cell and is relevant during operation of the fuel cell 1 is calculated. iare received, respectively (in FIG. 2, the corresponding gas components are designated 2a, 2b, 2c, 2d and 2e).

[0034] In step 200, the current I for controlling the operation of the fuel cell 1 is set, from which in step 300 the received pressure value p i Then, the target voltage U S The corresponding calculation is then performed to convert the set current I to the target voltage U S (a generalized Butler-Volmer equation including mixed potential calculations).

[0035] In the process of numerical inversion, addition, subtraction, multiplication, division and exponent formation operations are used. However, this procedure does not include the calculation of numerical logarithms. The presented method is intended to cover in particular the cases where at least one of the pressures (partial pressures) in the anode and / or cathode chambers of the fuel cell 1 is very small or even missing. Usually, the Butler-Volmer formula is based on the logarithm ln(p i / p i0 ) appears, where p i0 represents the reference pressure of the i-th gas component.

[0036] This formula is therefore problematic for the pressure contributions, which are small or even missing in the corresponding components. Therefore, in the method presented here, a modified form of the corresponding formula, obtained by rearrangement of the terms, is applied, in which the partial pressure p i no longer appears as a logarithmic independent variable. The numerical inversion based on the modified form allows cases in particular with very small partial pressures to be considered in a numerically stable manner. Situations of this kind can appear, for example, during the stopping process of a fuel cell-powered vehicle, as already explained above.

[0037] In step 400, the fuel cell 1 is operated and controlled by a set current I, and in step 500, the voltage U generated in the fuel cell 1 in response thereto is measured.

[0038] Finally, in step 600, the resulting voltage U is compared to the previously calculated target voltage U S is compared to.

[0039] By this comparison, for example, a deterioration or malfunction of the fuel cell can be diagnosed. In response to the determined deterioration, for example, the fuel cell can be operated in a controlled manner so that the effects of the deterioration are minimized and maximum power can still be extracted from the fuel cell. Alternatively, in response to the diagnosed deterioration or malfunction, it may be intended that the fuel cell be operated in a controlled manner with respect to the current so that the difference between the target voltage and the measured voltage is at least small.

[0040] Alternatively or additionally, the fuel cell can continue to be operated at an operating point where further degradation is mitigated, or the fuel cell can be controlled to operate such that the power delivered remains constant regardless of degradation.

[0041] FIG. 2 shows a fuel cell 1 having an anode 11 and a cathode 12 arranged in an anode chamber 110 or a cathode chamber 120, respectively, in contact with an ion-conducting electrolyte 13. The fuel cell 1 can be supplied with a fuel 31 on the anode side and with an oxidant 32 on the cathode side. The fuel 31 can be, for example, hydrogen, and the oxidant 32 can be oxygen or air. In the anode chamber 110 or the cathode chamber 120, there are gas components 2a, 2b, 2c, 2d, 2e which are involved in the chemical reactions taking place inside the fuel cell 1. The mention of five specific gas components 2a-2e here should be understood as an example only, not as a limitation. The method works in principle for any number of gas components, the computational cost being approximately 100 .mu.m.

[0042] Partial pressure of gas component p i is measured and the target voltage U SIn the course of the calculation, the current I at which the fuel cell 1 is to be subsequently operated is also determined. First, as already explained above, the partial pressure p i Using the set current I, the target voltage U S The target voltage U S After the target voltage U is determined, the fuel cell 1 is operated with the set current I and the voltage U generated in the fuel cell is measured. S The comparison with the target voltage U can then be used for diagnostic purposes, such as to determine a malfunction or deterioration of the fuel cell 1. Subsequently, a new target voltage U is set at a changed current I, for example in order to realize a compensation for deterioration of the fuel cell. S Then, when the fuel cell 1 is operated and controlled with the current I having been changed, it is possible to alleviate, for example, a decrease in the output of the fuel cell 1 due to degradation. [Explanation of symbols]

[0043] 1 fuel cell 2a, 2b, 2c, 2d, 2e Gas components 11 Anode 12 Cathode 31 Fuel 32 Oxidizing Agents 110 Anode chamber 120 Cathode chamber 200 Settings 300 calculations 400 Operation Control 500 measurements 600 comparison 1000 ways I current U Voltage U S Target Voltage p i Pressure Value

Claims

1. A method (1000) for controlling a fuel cell (1) having an anode (11) and a cathode (12), comprising: The method includes at least the following steps: A pressure value (p ｉ ) is received (100), A current (I) for controlling the operation of the fuel cell (1) is set (200); The received pressure value (p ｉ ) and then calculate the target voltage (U Ｓ ) is calculated (300), said calculation including the target voltage (U Ｓ ) to said set current (I), the numerical inversion relying on addition, subtraction, multiplication, division, and exponentiation, but not including the numerical calculation of logarithms; The fuel cell (1) is operated and controlled (400) by the set current (I); The voltage (U) generated by the fuel cell (1) is measured (500); The measured voltage (U) is equal to the calculated target voltage (U Ｓ ) (600).

2. The target voltage (U Ｓ 2. The method (1000) of claim 1, wherein the operating control of the fuel cell (1) is modified with the aim of reducing the discrepancy between the measured voltage (U) and the measured voltage (U).

3. The target voltage (U Ｓ 3. The method (1000) according to any one of claims 1 to 2, wherein malfunction and / or deterioration of the fuel cell (1) is assessed from the difference between the measured voltage (U) and the measured voltage (U).

4. The method (1000) according to any one of claims 1 to 3, wherein the set current (I) is selected such that the fuel cell (1) operates at a working point where degradation of the fuel cell (1) is mitigated.

5. A method (1000) according to any one of claims 1 to 4, wherein the operational control of the fuel cell (1) is changed so that, under the same set current (I), the power emitted from the fuel cell (1) remains constant regardless of degradation.

6. No fuel (31) is supplied at the anode (11) and / or no oxidant (32) is supplied at the cathode (12), and the pressure (p ｉ The method (1000) according to any one of claims 1 to 5, further comprising controlling the fuel cell (1) to operate with the set current (I) under an operating mode in which the current (I) is significantly reduced.

7. The method (1000) according to any one of claims 1 to 6, wherein the fuel (31) used in the fuel cell (1) is hydrogen and the oxidant (32) used in the fuel cell (1) is oxygen or air.

8. The method (1000) according to any one of claims 1 to 7, wherein a gas mixture consisting of a plurality of gas components (2a, 2b; 2c, 2d) is present on the cathode side and on the anode side, respectively.

9. 9. The method (1000) according to claim 8, wherein the anode side gas mixture (2a, 2b) and the cathode side gas mixture (2c, 2d) consist of at least a fuel (31) supplied to the fuel cell (1) on the anode side and an oxidant (32) supplied to the fuel cell (1) on the cathode side.

10. 10. The method (1000) according to claim 8 or 9, wherein the gas mixture (2c, 2d) on the cathode side and / or the gas mixture (2a, 2b) on the anode side further comprises a component (2e) which is formed in the course of a chemical reaction taking place inside the fuel cell (1).

11. The method (1000) of any one of claims 1 to 10, wherein the fuel cell (1) is part of a fuel cell stack.

12. A computer program comprising machine-readable instructions that, when executed on one or more computers, direct the one or more computers to perform the method (1000) of any one of claims 1 to 11.

13. A machine-readable data medium carrying a computer program according to claim 12.

14. A computer equipped with a computer program according to claim 12 and / or a machine-readable data medium according to claim 13.

Images