Method for operating a fuel cell system in a sub-zero start-up

Abstract

The invention relates to a method for operating a fuel cell system (1) comprising a fuel cell stack (200), which has a plurality of fuel cells (2a, 2b) and through which cooling channels (201) extend, the channels being fed a coolant via a circuit (25) by means of a coolant pump (26), wherein: at the beginning of a sub-zero start-up (40), in an initial step (41) a constant current target value (Itarget) is first set, according to the principle of air starvation, in order to achieve, by voltage control by means of the air mass flow rate (Mfair), a desired target voltage (Utarget); if the present actual voltage (Uist) falls below (42) at least a specified minimum voltage (Umin), the initial voltage control by means of the air mass flow rate (Mfair) is interrupted and instead a constant air mass flow rate (Mfair const) is set until the actual voltage (Uist) rises above the minimum voltage (Umin) by current control.

Description

[0001] Description

[0002] Title:

[0003] Method for operating a fuel cell system during a freeze-start

[0004] The present invention relates to a method for operating a fuel cell system and a control device which is configured to carry out steps of the method.

[0005] State of the art

[0006] A fuel cell typically converts a fuel, such as hydrogen, and oxygen into electrical energy, heat, and water. To increase performance, a large number of fuel cells are typically connected to form a fuel cell stack and supplied with reaction gases via supply channels running through the fuel cell stack. The heat generated in the fuel cells during the electrochemical process is dissipated using a cooling circuit and released into the environment via a cooler - in mobile applications usually the vehicle radiator. The coolant in the cooling circuit is pumped through coolant supply channels running through the fuel cell stack using a coolant pump integrated into the cooling circuit. A directional control valve is usually integrated into the cooling circuit to bypass the cooler.Bypassing the cooler can be advantageous during start-up, for example, because the fuel cell stack should be heated up as quickly as possible, particularly at ambient temperatures below 0°C, to avoid the accumulation of condensation and / or ice, which could delay or even prevent start-up. However, the risk of icing is only averted when the coolant has been safely warmed above 0°C before entering the fuel cell stack. This prevents freezing conditions from occurring when the coolant is pumped into the fuel cell stack. During freeze-starts, the coolant is generally heated either externally to the stack or by the electrochemical reaction in the fuel cell stack. In both cases, however, this prolongs the start-up process. Furthermore, the ice tolerance of the fuel cells must be increased due to the constant cooling below 0°C.This is usually done by installing ice buffers in the fuel cells, additional heaters in the fuel cell system, and the like.

[0007] During freeze-start with heat generation directly in the fuel cell stack, the coolant flow rate must be high enough to prevent so-called "hot spots" and an excessive temperature difference between the cell inlet and outlet, but low enough to prevent the fuel cell inlet area from icing up due to the temperature drop caused by the cold inflowing coolant. At very low temperatures from approximately -20°C, the water formed on the cathode side freezes almost instantly, thus blocking the active centers. Due to the resulting continuous decrease in the active area, the voltage continues to drop to values ​​close to 0 V. In this case, the reduction and recombination of H+ to gaseous H2 occurs more intensively on the cathode side, so that the H2 concentration in the cathode exhaust gas increases significantly.As soon as preheated coolant flows into the fuel cell stack, the ice melts and the resulting liquid water can be removed.

[0008] As a countermeasure, it is also commonly known to initially set a constant current setpoint at the beginning of a freeze start, based on the so-called air depletion principle, in order to achieve the desired target voltage via voltage control of the air mass flow. However, this countermeasure is often insufficient.

[0009] The object of the present invention is to detect cathode-side icing in fuel cells as early as possible in order to initiate effective countermeasures based on initial current limitation using simple technical means. Disclosure of the Invention

[0010] The object is achieved based on a method according to the preamble of claim 1 in conjunction with its characterizing features. The following dependent claims describe advantageous developments of the invention. Claim 10 is directed to a control unit implementing the method.

[0011] The invention includes the process engineering teaching that at the beginning of a freeze start according to the principle of air depletion, a constant current setpoint is initially set in order to achieve a desired target voltage via a voltage control of the air mass flow, wherein in the event that the current actual voltage falls below at least a predetermined minimum voltage, the initial voltage control of the air mass flow is interrupted and instead a constant air mass flow is set until the actual voltage rises above the minimum voltage again via current control.

[0012] In other words, according to the invention, after detecting cathode icing, the current is initially limited as a countermeasure, while simultaneously or independently adjusting the coolant flow to prevent excessive cooling of the cell inlet. This stabilizes the voltage above the level at which H2 formation begins on the cathode side, i.e., at which the voltage converter can no longer operate. This enables a quick and uninterrupted freeze-start through targeted control measures and increases the service life of the fuel cell stack.

[0013] In other words, the method according to the invention involves setting a constant current setpoint at the beginning of a freeze start. At the same time, an attempt is made to achieve the target voltage by regulating the coolant flow. Up to this point, the countermeasures initiated correspond to a freeze start with air starvation, so-called "air starvation", in order to generate as much heat as possible with the fuel cell stack. However, this initial countermeasure fails when fuel cells increasingly ice up on the cathode side. In this case, voltage regulation via the air mass, i.e. the coolant flow, is no longer possible. Therefore, the method according to the invention provides for monitoring compliance with a voltage corridor or at least a predetermined minimum voltage. The minimum voltage can be the total voltage of the fuel cell stack or an average individual voltage of fuel cells.As soon as one or both of these variables fall below the minimum value or the target corridor, the control of the coolant flow is stopped and a constant value is set instead until the actual voltage rises above the minimum voltage via current control.

[0014] Preferably, the air in the coolant flow is supplied slightly over-stoichiometrically, with a lambda between 1.1 and 1.3, preferably 1.2. At the same time, the current is regulated with the goal of stabilizing the voltage back to the original target voltage. This condition is maintained until the originally set current setpoint is reached again. At the end of the cycle, the air system again takes over control of voltage regulation, and the current is kept constant. A constant current setpoint is therefore set in order to achieve the target voltage again via voltage regulation via the air mass flow.

[0015] Alternatively, the H2 concentration in the exhaust gas can also be used as a trigger for current limitation in the procedure described above. However, in this case the delay is much longer, meaning that a delayed response to icing of the fuel cells is possible. In this case, icing can only be limited. Instead of limiting the current, the coolant flow can also be adjusted. In this case, the coolant flow would be reduced when icing begins, so that less cold coolant is forced into the fuel cell stack. However, this has the consequence of prolonging the freeze-up start, as it takes longer for the entire coolant volume to warm above freezing point and the fuel cells to be thawed.In addition, depending on the fuel cell stack, low coolant flow and high current can lead to the formation of so-called hotspots in the fuel cells, which can ultimately cause irreversible damage to the fuel cell stack.

[0016] According to a measure which further improves the invention, it is proposed that more than one voltage tap is provided per fuel cell to determine the current actual voltage, this preferably taking place by means of a high-precision 1-channel voltage module (CVM). The voltage taps should preferably be located in the area between the air inlet of a fuel cell and the air outlet. This is based on the idea that during operation with air depletion, a locally limited reaction zone forms within which all of the oxygen is consumed. The voltage distribution in the cell is therefore inhomogeneous. It cannot therefore be assumed that the same voltage is measured at both taps. If the reaction zone freezes during the freeze-start, it shifts continuously towards the air outlet, i.e. towards the second voltage tap.In this case, the two voltage values ​​change as the reaction zone moves, i.e., as icing progresses. By comparing the fuel cells, early action can be taken to detect the onset of icing.

[0017] As suggested above, the current is reduced or the air mass flow throttled depending on the stack voltage to prevent further freezing of the fuel cells on the cathode side. A minimum voltage was proposed as a criterion, which can be calculated from both the total voltage of the fuel cell stack and the average voltage of individual fuel cells. However, blockage of individual fuel cells on the anode side can no longer be detected. In this case, individual fuel cells assume strongly negative values, while the average voltage barely changes. In contrast to cathode icing, this highly damaging condition requires a much more aggressive current reduction.

[0018] In order to prevent anode icing in addition to cathode icing, a first variant proposes as a countermeasure to outsource the monitoring and regulation of the minimum voltage to the fuel cell's voltage converter. This is a good idea because voltage converters inherently have a minimum voltage limit below which operation is not possible. To avoid jeopardizing stable operation, the voltage converter will reduce the current to stabilize the voltage. At the same time, the fuel cell system's control unit is signaled that voltage regulation via the air mass flow is being stopped. The control unit then adopts the current setpoint determined by the voltage converter and continuously updates the coolant flow so that the cathode stoichiometry is kept constant at the currently set current setpoint.

[0019] This offers the advantage that individual fuel cells are not increasingly operated at over-stoichiometric levels, which significantly reduces heat production. As the process continues, the control system on the fuel cell's voltage converter attempts to gradually return the current to the original target current value without falling below the specified minimum voltage.

[0020] Preferably, the control unit for regulating the minimum voltage is only activated when a fuel cell is operating within the defined control range. In principle, the total voltage of the fuel cell stack is limited downwards, i.e., to the minimum voltage, via a current limit in the fuel cell's voltage converter and coordination with the fuel cell system's control unit to stop the lambda controller as long as the current limit is active. This counteracts severely negative cell voltages (anode blockage) via an aggressive current setpoint limit on the control unit.

[0021] According to an alternative approach, the implementation and parallel execution of both modes can be carried out on the fuel cell system's control unit, namely current limiting for the overall voltage and aggressive attenuation of negative cell voltages. For this purpose, both controllers—one for regulating the minimum overall voltage of the fuel cell stack and the other for regulating individual fuel cell voltages—are implemented in the voltage module (CVM) on the fuel cell system's control unit, while the fuel cell voltage converter passively adopts the setpoint calculated there.

[0022] Example

[0023] Further measures improving the invention are described in more detail below, together with the description of preferred embodiments of the invention, with reference to the figures. It shows:

[0024] Fig. 1 is a schematic representation of a fuel cell system suitable for carrying out a method according to the invention,

[0025] Fig.2a is a flow chart of the method according to the invention according to a first embodiment,

[0026] Fig. 2b is a flow chart of the method according to the invention according to a second embodiment,

[0027] Fig. 3 is a schematic representation of a fuel cell stack of a fuel cell system,

[0028] Fig. 4 is a diagram illustrating a freeze start with the temporal progression of various parameters of the fuel cell system, and

[0029] Fig. 5 is a diagram showing a comparison of cell voltages near the end plates of a fuel cell stack.

[0030] According to Fig. 1, a fuel cell system 1 for a motor vehicle comprises a plurality of fuel cells 2 (exemplary), each with an anode 3 and a cathode 4. Hydrogen is supplied to the anode 3 via an anode circuit 5. The hydrogen is stored in a tank 15, which is connected to the anode circuit 5 via a valve 13, a heat exchanger 15, a pressure regulator 16, and a suction jet pump 17. Depleted hydrogen escaping from the fuel cell 2 can be passively recirculated with the help of the suction jet pump 17. The suction jet pump 17 is actively assisted by a blower 18. Since the anode gas becomes enriched with nitrogen over time, it is purged from time to time. For this purpose, a purge valve 8 is opened, via which the anode gas is removed from the anode circuit 5. The water produced in the process is separated by means of a water separator 19 and collected in a container 20.By opening a drain valve 9, the container 20 can be emptied.

[0031] The cathode 4 is supplied with air taken from the environment via a cathode supply air path 6. The air passes through an air filter 21 to an air compressor 7, which compresses the air. The air is heated and cooled downstream of the air compressor 7 via a heat exchanger 22 in the cathode supply air path 6. Depleted air exiting the fuel cell 2 is discharged to the outside via a cathode exhaust air path 10. The air supply can be blocked via shut-off valves 11 and 12. Furthermore, a bypass path 23 with an integrated bypass valve 24 to the environment of the at least one fuel cell 2 is provided.

[0032] The fuel cell 2 of the fuel cell stack (not shown) is also connected to a cooling circuit 25 to dissipate the heat generated during the process. The cooling circuit 25 is operated by a coolant pump 26 and includes a fuel cell-side heat exchanger 27 and a cooler 28. A drain valve 29 allows the cooling circuit to be switched between a small cooling circuit for start-up operation and a large cooling circuit including the cooler 28.

[0033] The fuel cell system 1 further includes, as electrical components, an inverter 30 for converting the direct current generated by the fuel cells 2 into a three-phase alternating current for driving an AC motor 31 of the motor vehicle illustrated here. An upstream DC-DC converter 32 converts the direct voltage generated by the fuel cells 2 into a higher DC voltage suitable for the inverter 30. In this exemplary embodiment, at least both components are connected to a higher-level electronic control unit 33 of the fuel cell system 1 or are part of the same.

[0034] According to Fig. 2a, according to a first embodiment of the method according to the invention for operating the fuel cell system described above, at the beginning of a freeze start 40, a constant current setpoint karget is first set in step 41 according to the principle of air depletion in order to control the air mass flow Mf by voltage regulation a tr to achieve a desired target voltage Utar et.

[0035] However, if in a comparison step 42 the current actual voltage Ui St a predetermined minimum voltage Umin, the control of the air mass flow Mf is carried out in a subsequent step 43 a tr is interrupted and instead a constant air mass flow Mfatr const is set until the actual voltage Ui St by current control via the set minimum voltage U m t nincreases, which is determined via a comparison step 44. If the condition is met, the system returns to the initial voltage control via the air mass flow Mfatr.

[0036] The variant shown in Fig. 2b differs from the previous variant in that - as shown on the left - the monitoring and control of the minimum voltage U m t non the voltage converter 32, which is external to at least one fuel cell 2, and the higher-level control unit 33, it is signaled that the voltage regulation via the air mass flow Mfatr has been stopped, after which the voltage converter 32 adopts the current setpoint Itarget determined by the control unit 33 and then continuously updates the air mass flow Mfatr so that the cathode stoichiometry is kept constant at the respectively set current setpoint Itarget. For this purpose, a gradual increase takes place in step 44' until Itarget is reached. The control unit 33 is used to regulate the minimum voltage U minonly active when at least one fuel cell 2 is running within the defined control range. In this case, a current setpoint is determined on the control unit 33 and transmitted to the voltage converter 32. Both values ​​are continuously compared on the voltage converter 32. If the cell voltage regulator on the control unit does not respond, the voltage converter 32 receives its own current setpoint back. As soon as the cell voltage regulator becomes active, the control unit sends a lower setpoint back to the voltage converter 32. Therefore, the voltage converter 32 is always set to the minimum of its own setpoint and the setpoint reported back by the control unit 33. When leaving the cell voltage control range, it is advantageous if the current is gently adjusted to the original setpoint using a ramp as per step 44. Otherwise, there is a risk that critical cells will immediately be pulled back into anode-side depletion.The transition out of the cell voltage control range is configured on the control unit 33. As soon as the originally specified current setpoint is reached again, the voltage converter 32 passively takes over the current setpoint from the control unit 33, which in turn attempts to use the air mass flow Mf. a tr to the desired target voltage karget to regulate.

[0037] In addition to the comparison step 42 to check whether a given minimum voltage l min of the entire system, a parallel comparison step 52 determines whether this also applies to a predetermined minimum cell voltage. If so, current regulation according to the minimum cell voltage U C eii_min. According to the boundary condition 54 applicable to both parallel branches, the air mass flow Mf a tr kept constant.

[0038] According to Fig. 3, an exemplary fuel cell stack 200 consists of several fuel cells 2a, 2b, etc. This stack is traversed by cooling channels 201 of the heat exchanger 27 (not shown in detail) of the cooling circuit 25. The cathode 4 is supplied with air 202. As shown, more than one voltage tap 203 for a voltage module (CVM) is provided per fuel cell 2a and 2b, which is located close to the air inlet into the fuel cells 2a, 2b. The other voltage tap 204 is arranged on the air outlet side. Icing V, which is a threat due to a freezing cold start, originates from the cathode 4.

[0039] The diagram shown in Fig. 4 illustrates the course of the parameters air mass flow Mf a tr, total voltage Ui St stack, the hydrogen concentration xH2, the coolant inlet temperature Ti n and the coolant outlet temperature T out as well as a set of cell voltages UCVM of the individual fuel cells of the fuel cell stack during a freeze start measurement from -20°C.

[0040] Initially, a constant current setpoint of 90 amperes is set according to the principle of air depletion. Subsequently, the air mass flow Mfatr is regulated to achieve a target voltage Utarget of 60 V. The characteristic curve of the fuel cell stack is intentionally degraded to increase heat production. The graph shows that although the air mass flow Mfatr remains approximately constant, the total voltage of the fuel cell stack Ui Ststack continuously drops to a minimum of near 0 V. In this case, it can be assumed that ice growth is progressing on the cathode side, which is causing the active area of ​​the fuel cells to continue to reduce. This means that, at constant current, the current density per active area continues to rise, causing the voltage to drop. At the same time, it is evident that the H2 concentration xH2 directly at the cathode outlet rises sharply to 1.8%, indicating the effect described above. As soon as preheated coolant reaches the fuel cells, which occurs at point X, it is evident that the cell voltages gradually recover.

[0041] According to Fig. 5, a comparison of the cell voltages of fuel cells on the left edge "L" near the media distribution plate with those on the right edge "R" of the fuel cell stack shows that the fuel cells on the left wheel receive oxygen first at start-up, so water production starts there first. However, these fuel cells also receive the preheated coolant first, so they are the first to thaw. The right edge is delayed. These fuel cells start later because the air front must first travel through the stack at start-up. The preheated coolant also reaches these fuel cells later, so the cells remain frozen longer.

Claims

Claims 1. A method for operating a fuel cell system (1), comprising a fuel cell stack (200) having a plurality of fuel cells (2a, 2b), which is traversed by cooling channels (201) which are supplied with a coolant via a cooling circuit (25) using a coolant pump (26), wherein at the beginning of a freeze start (40) according to the principle of air depletion, in an initial step (41), a constant current setpoint (karget) is first set in order to control the air mass flow (Mf) by voltage regulation. a ir) to achieve a desired target voltage (Utar et), characterized in that when the current actual voltage (Ui) is undershot (42) St ) below at least a specified minimum voltage (Umin), the initial voltage regulation via the air mass flow (Mfair) is interrupted and instead a constant air mass flow (Mfair const) is set until the actual voltage (Ui St) by current control via the minimum voltage (U mjn ) increases.

2. Method according to claim 1, characterized in that the air of the air mass flow (Mfair) is supplied over-stoichiometrically with a lambda between 1.1 and 1.3, preferably 1.2, the current being regulated in such a way that the voltage stabilizes again at the target voltage (Utarget).

3. Method according to claim 1, characterized in that the actual voltage (Ui St ) of the total voltage (Ui S t stack) of the fuel cell stack (2) and / or an average single cell voltage (Uist mittei) of fuel cells 0.

4. Method according to claim 1, characterized in that subsequently a constant current Setpoint (karget) is set in order to again use a voltage control via the air mass flow (Mf a ir) to reach the target voltage (Utar et).

5. Method according to claim 1, characterized in that more than one voltage tap is used per fuel cell (2a, 2b) to determine the current actual voltage (Ui St ) is provided.

6. Method according to claim 5, characterized in that the voltage tap is carried out between air inlet 0 and air outlet 0 of the fuel cell (2a; 2b).

7. Method according to claim 1, characterized in that the monitoring and control of the minimum voltage (U min) to a voltage converter (32) which is outsourced to at least one fuel cell (2), and the higher-level control unit (33) is signaled that the voltage regulation via the air mass flow (Mfair) has been stopped, after which the voltage converter (32) adopts the current setpoint (karget) determined by the control unit (33) and then continuously updates the air mass flow (Mfair) so that the cathode stoichiometry is kept constant at the respectively set current setpoint (karget).

8. Method according to claim 7, characterized in that the control device (33) for regulating the minimum voltage (U min ) is only active when at least one fuel cell (2) runs within the defined control range.

9. Method according to claim 1, characterized in that both controls of the minimum total voltage (Ui St stack min) of the fuel cell stack () and the minimum voltage (U min) is carried out on the control unit (33) and the voltage converter (32) passively accepts the setpoint value calculated by the control unit (33).

10. Control unit (33) for a fuel cell system (1) which is configured to carry out steps of a method according to one of the preceding claims.

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