Method for operating a fuel cell system

Abstract

The invention relates to a method for operating a fuel cell system (100), wherein the fuel cell system (100) comprises at least one fuel cell stack (101), an air path (10), an off-gas pipe (12), a fuel pipe (20) with a recirculation circuit (50), and a battery, the battery being capable of storing the electrical energy generated by the fuel cell stack. A maximum charge state SOC_lim of the battery is determined depending on a potential performance gradient.

Description

[0001] Description

[0002] title

[0003] Method for operating a fuel cell system

[0004] The invention relates to a method for operating a fuel cell system having the features of the preamble of patent claim 1.

[0005] State of the art

[0006] Hydrogen-based fuel cell systems are considered the mobility concept of the future because they emit only water as exhaust gas and enable fast refueling times. Fuel cell systems require air and hydrogen for the chemical reaction within the cells. To provide the required amount of energy, the fuel cells arranged within a fuel cell system are interconnected to form so-called fuel cell stacks. The waste heat from the cells is dissipated via a cooling circuit and released into the environment. The hydrogen required to operate fuel cell systems is usually supplied to the systems from high-pressure tanks.

[0007] Document DE 10 2006 013 699 A1 describes a fuel cell system with a fuel cell and an actuator, actuated by a control unit, for removing residual gas from a fuel flow of the fuel cell. It is characterized by the fact that the control unit includes a control and / or regulation system that takes the fuel concentration in the fuel flow into account.

[0008] Disclosure of the Invention The inventive method for operating a fuel cell system with the features of the independent claim has the advantage that, by determining a maximum state of charge SOCJim of the battery as a function of a power gradient, it is ensured that dynamic depletion does not occur. Dynamic depletion can occur if, during a sudden power reduction, the operating pressure is reduced by hydrogen consumption, and predominantly nitrogen remains in the anode circuit. This significantly reduces the hydrogen partial pressure, and a local undersupply (depletion) can occur.

[0009] The object of the method according to the invention is to adjust the battery's state of charge (SOC) in such a way that a sudden reduction in the required electrical power does not result in hydrogen depletion. A hydrogen depletion can lead to degradation of the fuel cell stack, which has a negative impact on the service life of the fuel cell stack.

[0010] By the method according to the invention, the maximum state of charge SOCJim is determined depending on a possible power gradient, so that the battery can continue to store energy, so that the power gradient can be reduced and no dynamic depletion occurs.

[0011] If the battery is only charged to the maximum state of charge (SOCJim) or less, the negative power gradient of the fuel cell system can be delayed so that dynamic depletion does not occur. The excess energy can be stored in the battery.

[0012] Advantageous embodiments and further developments of the fuel cell system according to the invention are specified in the dependent claims.

[0013] It is advantageous if the possible power gradient depends on the current or future driving mode, since, for example, a large power gradient can generally be assumed in the Sport driving mode. It can also be helpful if the future driving mode is determined based on a planned route profile. Lowering the current battery state of charge (SOC) when the current state of charge (SOC) is above the maximum state of charge (SOCJim) is advantageous, as this ensures that the battery can absorb the energy not required by the vehicle.

[0014] A simple way to lower the battery's current SOC is to use electrical power sinks and auxiliary loads in the vehicle, especially the air conditioning system. The current SOC can be lowered by feeding energy into an electric motor in the vehicle.

[0015] Since dynamic depletion can only occur at a high power of the fuel cell system, it is advantageous if the current state of charge SOC is only reduced if, in addition, the current power of the fuel cell stack is greater than a limit value P_lim of the power.

[0016] It is advantageous to derive the SOCJim limit from the possible excess energy E_over and the battery capacity C_Bat, where the excess energy E_over is determined from the maximum possible power gradient at which no depletion occurs. The SOCJim limit can be calculated using the following formula:

[0017] SOC Jim=1 -E_über / C_bat

[0018] The consideration of other performance and energy consumption limiting factors, in particular the battery temperature and / or battery aging, is advantageous, as this allows for more precise control of the purge strategy, which can take into account changes in the battery due to its age.

[0019] The method according to the invention can be used in particular in fuel cell-powered motor vehicles. However, it is also conceivable for use in other fuel cell-powered means of transport, such as cranes, ships, rail vehicles, aircraft, or even stationary fuel cell-powered vehicles. The figures show:

[0020] Fig. 1 is a schematic representation of a fuel cell system according to the invention according to a first embodiment,

[0021] Fig. 2 is a flow chart of the individual steps of a method according to the invention according to an embodiment and

[0022] Fig. 3 two diagrams showing the exemplary target power curve and resulting surplus energy.

[0023] Figure 1 shows a schematic topology of a fuel cell system 100 according to a first embodiment of the invention, comprising at least one fuel cell stack 101. The at least one fuel cell system 100 has an air path 10, an exhaust line 12, and a fuel line 20. The at least one fuel cell stack 101 can be used for mobile applications with high power requirements, for example, in vehicles or trucks, or for stationary applications, for example, in generators.

[0024] The air path 10 serves as an air supply line to supply air from the environment to a cathode 105 of the fuel cell stack 101 via an inlet 16. Components required for the operation of the fuel cell stack 101 are arranged in the air path 10. An air compressor 11 and / or compressor 11 is arranged in the air path 10, which compresses or draws in the air according to the respective operating conditions of the fuel cell stack 101. Downstream of the air compressor 11 and / or compressor 11, a humidifier 15 can be located, which humidifies the air in the air path 10 to a higher relative humidity.

[0025] Further components, such as a filter and / or a heat exchanger and / or valves, may be provided within the air path 10. Oxygen-containing air is supplied to the fuel cell stack 101 via the air path 10. The fuel cell system 100 may further comprise a cooling circuit configured to cool the fuel cell stack 101. The cooling circuit is not shown in Figure 1, as it is not part of the invention.

[0026] A tank 21 and a shut-off valve 22 are located at the inlet of the fuel line 20. Further components can be arranged in the fuel line 20 to supply an anode 103 of the fuel cell stack 101 with fuel as needed.

[0027] In order to always supply the fuel cell stack 101 with sufficient fuel, there is a need for a superstoichiometric metering of fuel via the fuel line 20. The excess fuel, as well as certain amounts of water and nitrogen that diffuse through the cell membranes to the anode side, are returned to a recirculation circuit 50 and mixed with the metered fuel from the fuel line 20.

[0028] To drive the flow in the recirculation circuit 50, various components can be installed, such as a jet pump 51 driven by the metered fuel or a blower 52. A combination of jet pump 51 and blower 52 is also possible.

[0029] Since the amount of water and nitrogen continues to increase over time, the recirculation circuit 50 must be flushed from time to time so that the performance of the fuel cell stack 101 does not decrease due to an excessively high nitrogen concentration in the fuel line 20.

[0030] A purge line 40 is arranged between the recirculation circuit 50 and an exhaust gas line 12 so that the gas mixture can flow from the recirculation circuit 50 into the exhaust gas line 12.

[0031] A purge valve 41 is located in the purge line 40, which can open and close the connection between the recirculation circuit 50 and the exhaust line 12. The purge valve 41 is usually opened for a short time, so that the gas mixture is directed via the purge line 40 into the exhaust line 12. The components and the piping system responsible for supplying the anode with fuel are referred to as the anode side.

[0032] The exhaust line 12 serves to transport exhaust gas into the environment via an outlet 18. The exhaust gas comprises a gas mixture with air components from the air path 10 and water. The exhaust gas from the exhaust line 12 can also contain hydrogen (H2) because portions of the hydrogen from the fuel line 20 can diffuse through the membrane of the fuel cell stack 101. Furthermore, hydrogen and a gas mixture with nitrogen can enter the exhaust line 12 via the purge line 40.

[0033] An H2 sensor 45 can be arranged in the exhaust line 12 to measure the concentration of hydrogen in the exhaust gas, since excessive hydrogen must not be released into the environment via the exhaust line 12. Furthermore, the formation of an explosive mixture must be avoided.

[0034] The components and the piping system responsible for supplying the cathode with air are called the cathode side.

[0035] A significant reduction in the power of the fuel cell system 100 can lead to a so-called dynamic depletion on the anode side. If the fuel cell system is operated at high power, relatively high operating pressures (2.5-3.5 bar) are typically set on the cathode and anode sides. When the load is reduced, the operating pressure is reduced significantly in a short time (for example, to 1-1.5 bar), which is achieved on the anode side by consuming the hydrogen. However, if a significant amount of nitrogen has accumulated due to gas transfer from the cathode side, the majority of the remaining nitrogen remains after the pressure reduction due to hydrogen consumption.

[0036] As a result, the partial pressure of the remaining hydrogen drops noticeably and can lead to severely damaging conditions for the fuel cell stack (local fuel depletion).

[0037] In stationary high-load operation, a certain nitrogen concentration is permissible, since the higher total pressure still provides sufficient hydrogen partial pressure and a reduction in the nitrogen content could only be achieved by more frequent purging and thus higher hydrogen losses.

[0038] The method according to the invention is based on the basic idea of ​​avoiding a strong reduction in the power of the fuel cell stack, so that dynamic depletion on the anode side does not occur.

[0039] If the battery is only slightly or at least not fully charged, the power gradient of the fuel cell system can be delayed so that dynamic depletion does not occur. The excess power can be stored in the battery.

[0040] It is therefore proposed to limit the maximum state of charge (SOCJim) of the battery depending on the expected (negative) power gradient of the fuel cell system. This means that the more negative the power gradient (higher rate of change), the more energy must be absorbed by the battery to avoid dynamic depletion.

[0041] Fig. 2 shows a flow diagram of the individual steps of the method according to the invention for operating a fuel cell system 100.

[0042] In a method step 200, the fuel cell system is started.

[0043] In the subsequent method step 210, the power P of the fuel cell system 100 is monitored during operation of the fuel cell system 100.

[0044] In method step 220, a check is performed to determine whether an expected power gradient is greater than a predefined limit value GW. The expected power gradient can be derived from the driving mode. For example, a higher expected power gradient can be assumed when the vehicle is in sport mode.

[0045] Another factor that favors a higher expected power gradient is the number of active fuel cell stacks in a multi-stack system, as this allows for a very high power drop. It is also possible to infer an expected high power gradient based on a prediction using a planned route profile (GPS), for example, based on the elevation profile or a winding route.

[0046] A very general check to determine whether an expected power gradient is greater can be performed by checking in method step 220 whether the current power of the fuel cell stack is greater than a power limit value P_lim. A power gradient that exceeds the specified limit value GW is only possible if the fuel cell system is operated at a power that exceeds the power limit value P_lim.

[0047] If the expected power gradient is greater than the specified limit value GW, the process proceeds to step 230. In step 230, the current state of charge (SOC) of the battery is reduced to a level below the maximum state of charge (SOC_im). If the current state of charge (SOC) is already below the maximum state of charge (SOC_Lim), the state of charge does not need to be reduced. The process then proceeds again to step 210.

[0048] If the expected power gradient is greater than the specified limit value GW, the process goes directly to step 210.

[0049] The limit value SOCJim is derived from the potential excess energy E_excess and the battery capacity C_Bat. The excess energy E_excess results from the maximum permissible power gradient of the fuel cell system at which no depletion occurs with normal purge rate.

[0050] In particular, the limit value SOCJim can be calculated using the following formula:

[0051] SOC Jim=1 -E_über / C_bat

[0052] In order to achieve increased accuracy in the process of operating a fuel cell system, further performance and energy consumption limiting factors, in particular battery temperature and / or battery aging, can be taken into account.

[0053] Figure 3 shows two diagrams in which the performance of a fuel cell system is plotted over time. The first diagram A shows an example

[0054] The target power curve and the resulting excess energy E_über during a delayed power reduction of the fuel cell system are shown. The excess energy E_über corresponds to the dotted area between the power curve during a delayed power reduction (dashed line) and the power curve during a rapid power reduction (solid line).

[0055] The second diagram B shows an example of a target power curve with rapid power reduction without excess energy.

Claims

Claims 1 . A method for operating a fuel cell system (100), wherein the fuel cell system (100) has at least one fuel cell stack (101), an air path (10), an exhaust line (12), a fuel line (20) with a recirculation circuit (50) and a battery, wherein the battery can store the electrical energy generated by the fuel cell stack, characterized in that a maximum state of charge SOCJim of the battery is determined depending on a possible power gradient.

2. Method according to claim 1, characterized in that the possible power gradient depends on the current or future driving mode 3. Method according to claim 1, characterized in that the future driving mode is determined depending on a planned route profile.

4. The method according to claim 1, characterized in that a current state of charge SOC of the battery is lowered if the current state of charge SOC is above the maximum state of charge SOCJim.

5. Method according to claim 1, characterized in that the current state of charge SOC is reduced by electrical power sinks or additional consumers in the vehicle, in particular the air conditioning system.

6. The method according to claim 5, characterized in that the current state of charge SOC is reduced by feeding energy into an increased operation of an electric motor in the vehicle.

7. The method according to claim 5, characterized in that the current state of charge SOC is only reduced if, in addition, the current power of the fuel cell stack is greater than a limit value P_lim of the power.

8. Method according to claim 1, characterized in that the limit value SOCJim is derived from the possible excess energy E_über and the battery capacity C_Bat, the excess energy E_über resulting from the maximum possible power gradient at which no depletion occurs 9. Method according to claim 8, characterized in that the limit value SOCJim is calculated using the following formula: SOC Jim=1 -E_over / C_bat 10. Method according to one of the preceding claims, characterized in that further power and energy consumption limiting factors, in particular the battery temperature and / or battery aging, are taken into account.

Images