Method for operating a fuel cell system, computer program product, control unit, and fuel cell system

Abstract

The invention relates to a method for operating a fuel cell system (FCS) in different operating modes, in which method different limit values (k1, k2, k3, k4) are used for controlled variables (RG), namely: - an outlet activity (aout), - an outlet oxygen partial pressure (pO2out), and / or - a flow velocity in a cathode path, wherein various manipulated variables (SG), namely: - a pressure (p_air) of a supply air stream or an inlet oxygen partial pressure (p_O2in), - an oxygen excess ratio (λO2), - a stack current (i_S), and - a stack temperature (TS) are adjusted, using an optimisation-based approach, on the basis of limit values (k1, k2, k3, k4) which are taken into account in particular in cost functions (fGrenz,Aktiv, fGrenz,Partial) for the controlled variables (RG).

Description

[0001] R. 415966

[0002] - 1 -

[0003] Description

[0004] title

[0005] Method for operating a fuel cell system, computer program product, control unit and fuel cell system

[0006] The invention relates to a method for operating a fuel cell system. Furthermore, the invention relates to a corresponding computer program product, a corresponding control unit, a corresponding fuel cell system, and a corresponding fuel cell vehicle for carrying out a corresponding method.

[0007] State of the art

[0008] In vehicles (Fuel Cell Vehicle or FCV for short) where propulsion energy is supplied (among other things) by one (or more) fuel cell system(s) (Fuel Cell System or FCS for short), oxygen from the ambient air is usually used as the oxidizing agent and hydrogen as the reducing agent or fuel to react in a fuel cell stack to form water (or water vapor) and thus deliver electrical power through electrochemical conversion.

[0009] In the mobile operation of a fuel cell system, various goals are pursued, such as performance, lifespan, consumption optimization, start-stop capability, partial load capability, etc. Furthermore, the operation of a fuel cell system must cover different operating conditions, such as freezing starts, hot climates, mountain driving, long-distance travel, city driving, etc.

[0010] For optimized operation of a fuel cell system and the interacting subsystems (especially three media systems and the electrical system), R. 415966

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[0012] Water management is a central and essential component of an operating strategy. It interacts with all subsystems and is crucial for the proper functioning of the overall system. The target values ​​for the control parameters are usually determined in advance and provided in tabular form. Water management typically sets strict limits for discharge activity and, if applicable, oxygen partial pressure, and then selects the appropriate control parameters for the operation of the fuel cell system. These strict limits for discharge activity yield comparatively good results at steady-state operating points. However, in dynamic situations (dynamic load changes, start / stop cycles, etc.), the temporal trajectories of discharge activity are often difficult to determine precisely. In such cases, strict limits can be very restrictive for operation.

[0013] The models for determining outflow activity are often inaccurate. Storage and transfer effects of water in or through the membrane are difficult to model.

[0014] Due to the uncertainties of the models, the hard limits may be a. set too narrowly (adverse and significant restriction for the operating range) or

[0015] b. be pulled too far (risk of degradation or damage to the stack and other components).

[0016] The hard limits are effective in both steady-state and dynamic operating conditions. In dynamic operating conditions, these hard limits lead either to drastic restrictions in dynamics or to shutdowns, error messages, implausibilities, or non-robust dynamic operation. Reaching hard limits triggers controller switching. Frequent controller switching can cause the system to oscillate and / or toggling.

[0017] In the context of optimization-based operating strategies, hard boundary conditions often pose significant numerical challenges. For example, the optimization algorithm can become trapped at local optimization points if there are too many hard constraints. R. 415966

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[0019] Disclosure of the invention

[0020] The present invention provides a method for operating a fuel cell system with the features of the independent method claim. Furthermore, the invention provides a corresponding computer program, a corresponding control unit, a corresponding fuel cell system, and a corresponding fuel cell vehicle with the features of the dependent claims. Features and details described in connection with the different embodiments and / or aspects of the invention naturally also apply in connection with the other embodiments and / or aspects, and vice versa, so that the disclosure relating to the individual embodiments and / or aspects always includes, or can include, reciprocal references.

[0021] According to the first aspect, the present invention provides:

[0022] a method for operating a fuel cell system (FCS) in different operating modes (or using different operating strategies) in which different limit values ​​for control variables:

[0023] an exit activity,

[0024] an exit oxygen partial pressure and / or

[0025] a flow velocity in a cathode path is used, with various control variables:

[0026] a pressure of supply air or an inlet oxygen partial pressure, an excess of oxygen,

[0027] a stack current and / or

[0028] a stack temperature

[0029] The parameters are set based on optimization using limit values, which are preferably considered in cost functions (or efficiency functions or limit value functions) for the controlled variables. R. 415966

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[0031] The aforementioned control variables address important parameters. However, the method can also use other control variables and / or derived quantities, e.g., similarity parameters (e.g., the Reynolds number for flow in the cathode path) or substance concentrations.

[0032] The fuel cell system can be used to provide electrical energy for a vehicle (Fuel Cell Vehicle or FCV for short).

[0033] The fuel cell system (or system for short) can have several fuel cell stacks (or stacks for short), each with several stacked fuel cells and the associated functional systems, including: media systems (air or cathode system, fuel or anode system, cooling system) and an electrical system.

[0034] Preferably, the fuel cell system can comprise several modules in the form of individual stacks and the associated functional systems.

[0035] In other words, the invention provides:

[0036] a method which controls variables such as air pressure (alternatively also inlet oxygen partial pressure), oxygen excess or O2 stoichiometry, stack flow and stack temperature

[0037] based on limits for the reference variables, such as outlet activity and, if applicable, oxygen partial pressure, which can be taken into account in cost functions or efficiency functions or limit functions, it is set in an optimization-based manner.

[0038] This somewhat softens or relaxes the requirements regarding the limits. Advantageously, this allows for short-term injuries (at least in situations where it is possible, especially to a certain extent).

[0039] The invention utilizes the following insights:

[0040] A short-term breach of a limit value does not necessarily lead to dry-out, flooding (especially with liquid water), or R. 415966

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[0042] Starvation / depletion of reactants. Long-term steady-state operation should, however, advantageously take place within the specified limits. Short-term operation, e.g., during dynamic load changes, may now violate these limits, albeit to a degree determined by the cost functions or efficiency functions.

[0043] This allows for more robust control and / or regulation with "soft" limits.

[0044] This can reduce the accuracy requirements of system models (e.g., for the stack and / or subcomponents, such as the membrane electrode assembly (MEA), gas diffusion layer (GDL), etc.), and also of BoP models (the balance of plant (BoP) includes all components of a fuel cell system except the stack itself), e.g., for the air system, anode system, etc.

[0045] Optimization-based operating strategies with fewer hard limitations (at least in situations where possible) are computationally easier to handle and more robust to control.

[0046] The computational advantage can also be beneficial during hardware implementation. Furthermore, relaxation can improve the computational speed of optimization problems and thus contribute to their feasibility on electronic control units (ECUs).

[0047] This allows the control system to move more effectively from local optimization points or to overcome local minima more easily.

[0048] The "soft" boundaries allow:

[0049] a. excessively narrow limits are dynamically widened (expansion of operating ranges, improvement of dynamics, avoidance of shutdowns, error messages and controller switching as well as oscillation and / or toggling, ensuring robust dynamic operation) and

[0050] b. excessively wide limits should be avoided (reducing the risk of degradation or damage to the stack and other components). R. 415966

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[0052] By using boundary functions in cost functions, a trade-off can be made between primary optimization goals (such as consumption, aging, etc.) and short-term violations of operating limits. In the long run, this can lead to advantages in terms of system lifespan or consumption.

[0053] The idea according to the invention can be used in all optimization-based operating strategies (meaning in the control and / or regulation of the system) that use cost functions (or efficiency functions), such as MPC (model predictive control), LQR (linear quadratic regulator), RL (reinforcement learning), PMP (Pontryagin's maximum principle), ECMS (equivalent consumption minimization strategy).

[0054] Furthermore, it is conceivable that the method (referring to the control and / or regulation of the system) optimizes cost functions for consumption and / or aging, as well as cost functions for the controlled variables, in order to adjust the various manipulated variables. In this way, improved control and / or regulation of the system can be provided.

[0055] Furthermore, it is conceivable that the various control variables may include further control parameters:

[0056] - Humidity of the supply air,

[0057] - Mass flow rate of the supply air,

[0058] - Fuel surplus,

[0059] - a coolant temperature at an input to a stack and / or at an output from a stack and / or

[0060] - a coolant temperature difference between an input to a stack and an output from a stack.

[0061] This allows for improved control and / or regulation. R. 415966

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[0063] Furthermore, it is conceivable that the various control variables could be taken into account, for example, using a model-based approach, when determining a suitable water activity. Advantageously, environmental parameters such as ambient temperature, ambient humidity, and / or ambient pressure can be considered when determining a suitable water activity. This allows suitable water activities to be selected flexibly for different operating modes and dynamically adapted to varying load changes.

[0064] Depending on the current operating conditions, a corresponding operating mode (or corresponding operating strategy) can be selected to operate the fuel cell system, for which a suitable water activity within permissible limits can be set.

[0065] In this way, improved control and / or regulation of suitable water activity and, if necessary, a suitable outlet oxygen partial pressure can be achieved.

[0066] To adjust the appropriate water activity, the control variables can be used, e.g., control variables of a cathode system and, if applicable, an anode system, such as humidity of an incoming air, pressure of an incoming air, excess oxygen in the cathode system, mass flow of an incoming air, excess fuel in an anode system, etc.

[0067] According to another aspect, the invention provides a computer program product comprising instructions which, when executed by a computer, such as the processing unit of the control unit, cause the computer to carry out the method, which can proceed as described above. The same advantages described above in connection with the method according to the invention can be achieved with the computer program product. These advantages are fully referenced herein. R. 415966

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[0069] A corresponding control unit provides a further aspect of the invention. A computer program in the form of code can be stored in a memory unit of the control unit. When the code is executed by a processing unit of the control unit, this program performs a procedure that can proceed as described above. The same advantages described above in connection with the method according to the invention can be achieved with the help of the control unit. These advantages are fully referenced herein.

[0070] A corresponding fuel cell system (FCS) also constitutes an aspect of the invention. The fuel cell system can serve to provide electrical energy for a vehicle. The same advantages described above in connection with the method according to the invention can be achieved with the fuel cell system. These advantages are fully referenced herein.

[0071] A corresponding fuel cell vehicle (FCV) also represents an aspect of the invention. The same advantages described above in connection with the method according to the invention can be achieved with the fuel cell vehicle. These advantages are fully referenced herein.

[0072] Preferred embodiments:

[0073] The invention, its further developments, and its advantages are explained in more detail below with reference to the drawings. Each drawing schematically shows:

[0074] Figure 1 shows an exemplary visualization of a limit function for an outlet activity and an outlet oxygen partial pressure, and

[0075] Figure 2 shows an exemplary visualization of a limit function for an outlet activity and an outlet oxygen partial pressure at a different temperature level than in Figure 1. R. 415966

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[0077] Figures 1 and 2 serve to explain a method in accordance with the invention, which was developed for operating a fuel cell system (FCS) in which different limit values ​​k1, k2, k3, k4 are used for control variables RG in different operating modes of the fuel cell system.

[0078] The controlled variables RG can include:

[0079] - an exit activity a ou t,

[0080] - an exit oxygen partial pressure po2out and / or

[0081] - a flow velocity in a cathode path.

[0082] The controlled variables RG represent target variables that are to be achieved using various manipulated variables SG within the framework of a control and / or regulation system.

[0083] As further shown in Figures 1 and 2, the various control variables SG can include:

[0084] - a pressure p a with an inlet air or alternatively an inlet oxygen partial pressure PO2 in,

[0085] - an excess of oxygen ÄO2,

[0086] - a stack current is and / or

[0087] - a stack temperature Ts.

[0088] The manipulated variables SG are advantageously set in an optimization-based manner using limit values ​​ki, k2, ks, k4, which can be used in cost functions fcrenz, Aktiv, fcrenz, Partial or efficiency functions or limit value functions for the controlled variables RG.

[0089] In other words, the manipulated variables SG are adjusted based on "soft" limits for the controlled variables RG, which can be enabled by considering the limit values ​​ki, k2, ks, k4 in cost functions fcrenz, active, fcrenz, partial, or efficiency functions or limit value functions for the controlled variables RG. R. 415966

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[0091] In this way, the requirements for the limit values ​​k1, k2, k4 can be relaxed. Advantageously, this allows short-term violations, at least in situations where, after weighing robust dynamic operation against other optimization goals such as aging and consumption, it is permissible, at least to certain extents, which can be taken into account using cost functions fcrenz, Active, and fcrenz, Partial.

[0092] The invention recognizes that a short-term violation of limit values ​​ki, k2, ka, k4 does not necessarily lead to negative effects such as dry-out, flooding (especially with liquid water), or starvation of reactants. Long-term steady-state operation can still take place within limit values ​​ki, k2, k4. Short-term operation, e.g., during dynamic load changes, may now exceed these limit values ​​ki, k2, k4, albeit to a degree determined by the cost functions or efficiency functions.

[0093] The control and / or regulation of the system can be more robust with "soft" limits.

[0094] This can reduce the accuracy requirements of the path models (e.g., for the stack and / or subcomponents, such as membrane electrode assembly (MEA), gas diffusion layer (GDL), etc., and also BoP models, e.g., for air system, anode system, etc.).

[0095] Optimization-based operating strategies with fewer hard limitations (at least in situations where possible) can be handled more computationally and controlled more robustly.

[0096] The reduced computational effort can also be advantageous during hardware implementation. Furthermore, relaxation can improve the computational speed of optimization problems and thus contribute to their feasibility on electronic control units (ECUs). R. 415966

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[0098] This allows the control system to move more effectively from local optimization points or to overcome local minima more easily.

[0099] The "soft" boundaries allow:

[0100] a. excessively narrow limits are dynamically widened (expansion of operating ranges, improvement of dynamics, avoidance of shutdowns, error messages and controller switching as well as oscillation and / or toggling, ensuring robust dynamic operation) and

[0101] b. excessively wide limits should be avoided (reduction of the risk of degradation or damage to the stack and other components).

[0102] By using "soft" boundary functions, a trade-off can be made between primary optimization goals (such as consumption, aging, etc.) and short-term violations of operating limits. In the long run, this can lead to advantages in terms of system lifespan or consumption.

[0103] The idea according to the invention can be used in all optimization-based operating strategies that use cost functions (or efficiency functions), such as MPC (model predictive control), LQR (linear quadratic regulator), RL (reinforcement learning), PMP (Pontryagin's maximum principle), ECMS (equivalent consumption minimization strategy).

[0104] From the control variables SG air pressure p air , O2 stoichiometry λ O2 , Stack current i s and stack temperature T s , which set an operating strategy for a specific operating mode, result in an exit activity a out and possibly an exit oxygen partial pressure p O2out .

[0105] Operating modes can include the following modes:

[0106] - a normal start of the fuel cell system,

[0107] - a cold start of the fuel cell system, especially at temperatures between +3 °C and 40 °C,

[0108] - a freeze start of the fuel cell system, especially at temperatures below +3 °C,

[0109] - normal load operation of the fuel cell system, R. 415966

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[0111] - partial load operation of the fuel cell system,

[0112] - an increase in load, in particular a positive load jump, of the fuel cell system,

[0113] - a load decrease, in particular a negative load jump, of the fuel cell system,

[0114] - a start / stop operation, etc.

[0115] Instead of now specifying the permissible range of values ​​for air pressure p air , O2 stoichiometry λ O2 , Stack current i s and stack temperature T s on values ​​within rigid limits for exit activity a outand, if applicable, the exit oxygen partial pressure p O2out To limit costs, the global cost function f is adjusted.

[0116] The global cost function f could previously include consumption and aging terms:

[0117] f = f verbrauch + f alterung .

[0118] Now, according to the invention, the global cost function f is supplemented by two further limit functions:

[0119] f = f verbrauch + f alterung + fGrenz, Aktiv + fGrenz, Partial.

[0120] The equation can also be extended with further limit functions for other controlled variables, such as a flow velocity in a cathode path.

[0121] For example, an exponential function can be used:

[0122] f > Ä k1 (k2 - aout)

[0123] T-border, active — 6

[0124] In this case, k2 can define a lower limit for the exit activity a outSpecify the value k1. The slope of the cost function fmargin can be used to scale the asset. R. 415966

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[0126] For exit activities a out Below the limit k2, the values ​​of the cost function fLimit increase significantly compared to assets (see left flanks in Figs. 1 and 2).

[0127] The left of the two rising flanks can indicate moisture or the outflow activity a ou The right of the two rising flanks can represent the exit oxygen partial pressure p02out. Outside a pronounced plateau, the values ​​of the cost functions (or limit functions) flimit,active and flimit,partial increase sharply. The cost functions (or limit functions) are determined to be negligible over a wide range and to rise rapidly in the unfavorable range.

[0128] Figures 1 and 2 illustrate how the permissible plateau range can change for different temperatures (Ts = 330 K and Ts = 350 K).

[0129] For the exit oxygen partial pressure po2out, the limit function can be implemented as follows, for example:

[0130] fr i — pk3(k4 - pO2out)

[0131]

[0132] I Border, Partial c

[0133] Alternative cost functions or limit functions can be represented using logarithms.

[0134] Furthermore, many cost functions for lower and upper limits of different sizes can be combined.

[0135] In addition to the discharge activity and the oxygen partial pressure, further cost functions or limit functions can also be formulated for other target variables, such as:

[0136] - Inlet air temperature at the entrance to a cathode,

[0137] - Coolant temperature at an input to a stack and / or at an output from a stack,

[0138] - Coolant temperature gradients,

[0139] - Fuel stoichiometry,

[0140] - Fuel (partial) pressure, R. 415966

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[0142] - Pressure difference between an anode and a cathode,

[0143] - Current gradients, etc.

[0144] A corresponding computer program product, a corresponding control unit, a corresponding fuel cell system (FCS) and a corresponding fuel cell vehicle (FCV) represent further aspects of the invention.

[0145] The preceding description of the figures describes the present invention solely by way of examples. Naturally, individual features of the embodiments can be freely combined with one another, provided it is technically feasible, without departing from the scope of the invention.

Claims

R. 415966 - 15 - Claims 1. Method for operating a fuel cell system (FCS) in different operating modes, in which different limit values ​​(k1, k2, k3, k4) for controlled variables (RC): - an exit activity (a ou t), - an exit oxygen partial pressure (po2out) and / or - a flow velocity in a cathode path be used, where different control variables (CV): - a pressure (pair) of an inlet air or an inlet oxygen partial pressure (pO2in), - an excess of oxygen (AO2), - a stack stream (iS) and / or - a stack temperature (Ts) The parameters are set based on optimization using limit values ​​(k1, k2, k3, k4), which are taken into account in particular in cost functions (fLimit, Active, fLimit, Partial) for the controlled variables (RV).

2. Method according to claim 1, where, for dynamic situations such as dynamic load changes and / or start / stop, a short-term violation of the limit values ​​(ki, k2,, ki) of the controlled variables (RG), in particular to certain extents which can be set, for example, by cost functions (fcrenz, Aktiv, fcrenz, Partial), is permitted. and / or where the limit values ​​(ki, k2,, ki) of the controlled variables (RG) are adhered to for steady-state operation.

3. Method according to any one of the preceding claims, R. 415966 - 16 - where dynamic boundary functions and / or soft boundaries for the controlled variables (RC) are provided using cost functions (fcrenz, Aktiv, fcrenz, Partial), and / or where, using cost functions (fcrenz, Active, fcrenz, Partial), robust control and / or regulation is provided in the operation of the fuel cell system (FCS), which in particular can move more easily from local optimization points and / or overcome local minimization points, and / or where cost functions (fcrenz, Active, fcrenz, Partial) reduce accuracy requirements for path models for a stack and its subcomponents, such as a membrane electrode assembly and / or gas diffusion layers.

4. Method according to any one of the preceding claims, where the method is used for optimization-based operating strategies that use cost functions.

5. Method according to any one of the preceding claims, where cost functions (fcrenz, Aktiv, fcrenz, Partial) are used to weigh primary optimization goals, such as consumption and / or aging, against short-term violations of the limit values ​​(ki, k2, kg, ki) for controlled variables (RG), in particular, cost functions (fcrenz, Aktiv, fcrenz, Partial) improve the computing speed for optimization problems.

6. Method according to any one of the preceding claims, where the procedure involves an optimization of cost functions (f verbrauch , f alterung ) for consumption and / or aging and of cost functions (fLimit, Active, fLimit, Partial) for the controlled variables (RC), in particular, the procedure takes into account further cost functions for the following target variables: - Inlet air temperature at the entrance to a cathode, - Coolant temperature at an input to a stack and / or at an output from a stack, - Coolant temperature gradients, R. 415966 - 17 - - Fuel surplus (ÄH2), - Fuel (partial) pressure, - Pressure difference between an anode and a cathode and / or - current gradients.

7. Method according to any of the preceding claims, where the various control variables (CVs) include further control parameters: - Humidity of the supply air, - Mass flow rate of the supply air, - Fuel surplus (ÄH2), - a coolant temperature at an input to a stack and / or at an output from a stack and / or - a coolant temperature difference between an input to a stack and an output from a stack.

8. Method according to any one of the preceding claims, wherein when determining a suitable water activity (a) the various control variables (CV), e.g. model-based, are taken into account, and / or wherein when determining a suitable water activity (a) environmental parameters, such as ambient temperature, ambient humidity and / or ambient pressure, are taken into account, and / or wherein the method performs a control and / or regulation of a suitable water activity (a).

9. Computer program product comprising instructions which, when the computer program product is executed by a computer, cause the computer to perform the method according to any of the preceding claims.

10. Control unit comprising a storage unit in which a code is stored and a computing unit, wherein, when the code is executed by the computing unit, the method according to one of the preceding claims 1 to 8 is carried out. R. 415966 - 18 - 11. Fuel cell system (FCS) comprising a control unit according to the preceding claim.

12. Fuel cell vehicle (FCV) comprising a fuel cell system (FCS) according to the preceding claim.

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