Method for operating a fuel cell system, and control device

Abstract

The invention relates to a method for operating a fuel cell system, comprising a fuel cell stack and a cooling circuit via which the stack is cooled during normal operation and is heated in the event of a start under freezing conditions, wherein a coolant is pumped through the stack with the aid of a coolant pump integrated into the cooling circuit. According to the invention, in the event of a start under freezing conditions, a) a superstoichiometric air mass flow is supplied to a cathode region of the stack in a first phase when the coolant pump is switched off or operated intermittently, and b) the coolant pump is operated at a constant pump speed in a second phase and the air mass flow supplied to the cathode region is set substoichiometrically, wherein the cell temperature in the interior of the stack is estimated by evaluating a gradient of the coolant outlet temperature set at the time of the transition from the first phase to the second phase. The invention also relates to a control device for a fuel cell system.

Description

[0001] Description

[0002] Method for operating a fuel cell system, control unit

[0003] The present invention relates to a method for operating a fuel cell system and to a control unit configured to perform steps of the method.

[0004] The preferred application area of ​​the invention is mobile fuel cell systems or fuel cell vehicles.

[0005] State of the art

[0006] Hydrogen-based fuel cells are considered a mobility concept of the future because they emit only water as a byproduct and allow for rapid refueling. They require air and hydrogen as reactant gases, which are converted into electrical energy, heat, and water in an electrochemical reaction within the fuel cells. To increase electrical output, multiple fuel cells are stacked and connected to form a fuel cell stack.

[0007] To dissipate the heat generated by the electrochemical reaction in the fuel cells, the stack is cooled. Cooling is achieved via a cooling circuit that incorporates a radiator, through which the heat is dissipated to the surroundings. A coolant pump and a directional control valve are also typically integrated into the cooling circuit. The coolant pump circulates the coolant through the stack. The directional control valve allows a bypass to be opened, circumventing the radiator so that the coolant mass flow in the cooling circuit is at least partially diverted around the radiator. This is particularly advantageous during a freeze-start of the fuel cell system, as the stack then needs to be heated, not cooled. Rapid heating of the stack reduces water and / or ice formation, thus reducing the risk of water and / or ice hindering or even completely preventing a start.The risk of icing is only eliminated once the coolant has been reliably heated above 0°C. Otherwise, there is a risk that the coolant pumped through the stack will lead to freezing conditions and thus to icing within the stack. This risk is particularly high at the coolant inlet of the stack, as the coolant temperature is lowest there. This means that during a freeze-start, the coolant must also be heated. This occurs through an electrochemical reaction within the stack or externally.

[0008] A fundamental challenge with freeze-start technology is that temperature measurement is performed outside the stack, typically based on the coolant temperature upstream or downstream of the stack. This means there is no direct correlation between the measured temperature and the actual temperature inside the stack or the cell temperature. Optimizing the freeze-start strategy, for example to compensate for aging effects, is therefore difficult.

[0009] The present invention addresses the problem of remedying this situation. In particular, it aims to provide a method that, in the case of a freeze-start, enables a more accurate estimation of the cell temperature inside the stack and thus an optimization of the freeze-start strategy.

[0010] To solve the problem, the method with the features of claim 1 is proposed. Advantageous embodiments of the invention are described in the dependent claims. Furthermore, a control unit for executing steps of the method is specified.

[0011] Disclosure of the invention

[0012] A method for operating a fuel cell system is proposed, comprising a fuel cell stack and a cooling circuit by which the stack is cooled during normal operation and heated in the event of a freeze start. A coolant is pumped through the stack by means of a coolant pump integrated into the cooling circuit. According to the invention, during a freeze start, a) in a first phase, with the coolant pump switched off or operating intermittently, a superstoichiometric air mass flow is supplied to a cathode region of the stack, and b) in a second phase, the coolant pump is operated at a constant pump speed and the air mass flow supplied to the cathode region is set to substoichiometric. The cell temperature inside the stack is estimated by evaluating a gradient in the coolant outlet temperature that develops at the time of the transition from the first phase to the second phase.

[0013] In the proposed method, the freeze-start process occurs in several phases. In the first phase, the coolant pump either remains completely switched off, so that the coolant remains in the cooling circuit, or operates intermittently, meaning it is repeatedly started briefly to supply warmed coolant to a temperature sensor located outside the stack. During the first phase of the freeze-start, the coolant therefore does not contribute to heating the stack, but is itself heated by the waste heat within the stack. It can thus be reasonably assumed that the temperature of the coolant exiting the stack essentially corresponds to the cell temperature inside the stack.

[0014] With the coolant pump activated, coolant flows through each cell. The coolant from all cells flows into a common manifold and is discharged from the stack via this manifold. The coolant temperature at the stack outlet, or the coolant outlet temperature, can therefore be equated with an average cell temperature, representing an average value across all cells. The coolant outlet temperature, particularly during the first 10 seconds of a freeze start, rises with a characteristic gradient that is proportional to the temperature of the coolant flowing out of the cells. Using the proposed analysis of this gradient, the temperature within the cells of the stack can thus be retrospectively determined.

[0015] Knowing the cell temperature at the time of transition from the first to the second phase is particularly important for the stability of the freeze-start process. This is because the transition must occur at a sufficiently high temperature to prevent the temperature from dropping below freezing at the beginning of the second phase. Therefore, it is crucial to verify the temperature reached at the end of the first phase, especially when cell activity decreases over time. If this temperature falls below a predefined target value, for example due to stack aging, the freeze-start strategy can be iteratively adjusted.

[0016] The proposed method thus creates a virtual sensor that enables optimization of the freeze-start strategy, as it provides the information about the cell temperature inside the stack that is necessary for a stable freeze-start.

[0017] In a further development of the procedure, it is proposed that no current be drawn temporarily at the beginning of the second phase. This means that only the coolant pump is started at the beginning of the second phase. This allows the cell temperature to be determined at the end of the first phase without being influenced by the second phase.

[0018] Furthermore, it is proposed that during the first phase, the cell voltages be regulated to values ​​between 0.7 V and 0.8 V via the current. This keeps the cell voltages in the upper range, which promotes rapid and homogeneous heating of the stack.

[0019] The duration of the first phase is preferably determined taking into account the ambient temperature and / or the standby time of the fuel cell system. If the ambient temperature is considered, the duration of the first phase increases as the ambient temperature decreases. This is because the lower the ambient temperature, the longer it takes to heat up the stack. If—alternatively or additionally—the standby time is considered, the duration of the first phase increases with increasing standby time, as the stack will have cooled down more significantly.

[0020] Preferably, during the first phase, the temperature of the air mass flow exiting the stack is determined and taken into account when estimating the cell temperature. This can further increase the accuracy of the estimation. If the temperature of the air mass flow exiting the stack is not measured directly at the stack outlet, it can also be derived from a model or another directly measured quantity.

[0021] Furthermore, the temperature of the air mass flow exiting the stack is preferably compared with a predefined threshold value, and the transition to the second phase only occurs upon reaching this threshold. This further increases the stability of the freeze start.

[0022] As a further development measure, it is proposed that during the first phase, particularly at ambient temperatures below -20°C, the current required to set a target voltage be evaluated and taken into account when estimating the cell temperature. Since strong kinetic inhibition occurs at very low ambient temperatures, an evaluation of the current or current trajectory during target voltage adjustment can also be used to infer the average cell temperature. This allows for an even more precise cell temperature estimate. A further advantage is that the starting temperature does not need to be estimated to determine the duration of the first phase.

[0023] Furthermore, the duration of the first phase is preferably determined based on the evaluation of the current trajectory. During this evaluation, the cell voltages and the respective set currents can be compared with a reference characteristic curve or historical values ​​during normal operation. The first phase can be terminated as soon as the voltage-current pair corresponds to the reference point at normal operating temperature.

[0024] Furthermore, a control unit for a fuel cell system is proposed, wherein the control unit is configured to execute steps of a method according to the invention. In particular, the control unit can be used to evaluate the coolant outlet temperature to estimate the cell temperature. The control unit can also be used to evaluate and analyze other parameters in order to consider them when estimating the cell temperature and / or determining the duration of the first phase of the freeze-start process. The control unit can contain at least one threshold value and / or reference value and / or a reference characteristic curve to perform the respective evaluation or analysis. The control unit can also be used to regulate a target voltage during the first phase. Furthermore, the control unit can be used to control the coolant pump according to the method according to the invention.

[0025] The invention and its advantages are explained in more detail below with reference to the accompanying figures. These show:

[0026] Fig. 1 is a flowchart illustrating the process of a method according to the invention in a preferred embodiment,

[0027] Fig. 2 is a diagram illustrating the curves of cell voltage U_Cell, cell temperature T_Cell and stack current l_Stack during a freeze start according to a method according to the invention and

[0028] Fig. 3 shows a schematic longitudinal section through a stack to illustrate the temperature distribution in a coolant collector during a freeze start.

[0029] Detailed description of the drawings: Figure 1 illustrates the process of a method according to the invention. The method starts with step 1, meaning that step 1 initiates the freezing start, which is divided into a first phase a and a second phase b.

[0030] In the first phase a, in step 2 the coolant pump is operated intermittently or not at all, so that the first phase is carried out with the coolant stationary. In step s, a mass airflow is supplied to a cathode region of the stack, with the air stoichiometry set > 1. Furthermore, a target voltage is regulated via the current, which can, for example, take a value between 0.7 V and 0.8 V. In step 4, the cell temperature is estimated, in particular by evaluating the coolant outlet temperature and / or the current or current trajectory. In step 5, it is checked whether the cell temperature has reached a predefined threshold and can transition from the first phase a to the second phase b. If this is not the case, Steps 3 to 5 are repeated or continued. If the test result is positive ("+"), the second phase b is initiated with step 6. For this, the coolant pump is started or – if it was already operating intermittently – operation is continued at a constant speed. In step 7, the current is then set to a constant value and the voltage is regulated via the air mass flow supplied to the cathode area. In step 8, the gradient of the coolant outlet temperature is determined and the cell temperature is inferred from this. In step 9, the cell temperature is compared with a target value. If this is not reached, the operating strategy in the first phase a can be modified accordingly. In this way, for example, aging effects can be compensated for. In step 10, the freeze start is terminated.

[0031] At very low ambient temperatures, for example -20°C or below, the catalytic activity of the cells in a fuel cell stack is severely limited. If a freeze start is performed under these conditions, the cell voltage U_Cell initially rises with the stack contacts open, as illustrated in the diagram in Figure 2. Once the maximum is reached at time h, the stack contacts are closed, and the current control is adjusted to achieve cell voltages U_Cell between 0.7 V and 0.8 V. The current required for this will initially increase exponentially, but will then transition to a flatter profile as the cell temperature T_Cell rises (in this case, at time t2). Once a minimum threshold of the gradient is reached, or as soon as other thresholds, such as the cathode outlet temperature, are met, the transition to the second phase of the freeze start can occur.The advantage here is that it eliminates the need to pre-control the duration of the first phase.

[0032] Figure 3 shows an example of the temperature distribution in a coolant collector 11 of a fuel cell stack. Since the coolant from all cells 12 flows together in the collector 11, the coolant outlet temperature can be equated with the average cell temperature.

Claims

Claims 1. A method for operating a fuel cell system comprising a fuel cell stack and a cooling circuit by which the stack is cooled during normal operation and heated in the case of a freeze start, wherein a coolant is pumped through the stack by means of a coolant pump integrated into the cooling circuit, characterized in that during a freeze start a) in a first phase with the coolant pump switched off or intermittently operated, a superstoichiometric air mass flow is supplied to a cathode region of the stack, and b) in a second phase the coolant pump is operated at a constant pump speed and the air mass flow supplied to the cathode region is set to substoichiometric, wherein the cell temperature inside the stack is estimated by evaluating a gradient of the coolant outlet temperature that occurs at the time of the transition from the first phase to the second phase.

2. Method according to claim 1, characterized in that no current is drawn temporarily at the beginning of the second phase.

3. Method according to claim 1 or 2, characterized in that during the first phase the cell voltages (U_Cell) are regulated to values ​​between 0.7 V and 0.8 V via the current.

4. Method according to one of the preceding claims, characterized in that the duration of the first phase is determined taking into account the ambient temperature and / or the standby time of the fuel cell system.

5. Method according to one of the preceding claims, characterized in that during the first phase the temperature of the air mass flow exiting the stack is determined and taken into account when estimating the cell temperature.

6. Method according to claim 5, characterized in that the temperature of the air mass flow exiting the stack is compared with a predefined threshold value and the transition to the second phase is only carried out upon reaching the threshold value.

7. Method according to one of the preceding claims, characterized in that during the first phase, particularly at ambient temperatures below -20°C, the current required to set a target voltage is evaluated and taken into account when estimating the cell temperature.

8. Method according to claim 7, characterized in that the duration of the first phase is determined depending on the evaluation of the current trajectory.

9. Control unit for a fuel cell system configured to perform steps of a method according to any of the preceding claims.

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