Method for optimizing the purge strategy of a fuel cell system

Abstract

The invention relates to a method for optimizing the purge strategy of a fuel cell system (100), wherein the fuel cell system (100) comprises a fuel cell stack (101), an air path (10), an exhaust 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 purge rate is adjusted according to a state of charge SOC of the battery.

Description

[0001] Description

[0002] title

[0003] Procedure for the one

[0004] The invention relates to a method for optimizing the purge strategy of a fuel cell system with 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] The purge strategy of a fuel cell system is usually time-based or model-based, e.g. by integrating the drawn current as an indication of the air flowing through the cathode, or the amount of nitrogen flowing through the cathode and diffused into the anode.

[0008] Document DE 10 2006 013 699 A1 describes a fuel cell system with a fuel cell and an actuator operated 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.

[0009] Disclosure of the invention

[0010] The inventive method for optimizing the purge strategy of a fuel cell system with the features of the independent claim has the advantage that, by adapting the purge strategy to the battery's state of charge (SOC), harmful conditions for the fuel cell stack caused by dynamic depletion are avoided. Dynamic depletion can occur when, during a sudden power reduction, the operating pressure is reduced by hydrogen consumption, leaving mostly nitrogen in the anode circuit. This significantly reduces the hydrogen partial pressure, and a local undersupply (depletion) can occur.

[0011] The object of the method according to the invention is to adapt the purge strategy, and thus in particular the nitrogen content (e.g., partial pressure N2), depending on 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. The method according to the invention aligns the purge strategy with the required demand, and only in certain situations does a purge strategy be applied that leads to sub-energy-optimal operation. Overall, this leads to efficient operation of the fuel cell system while simultaneously avoiding damaging conditions.

[0012] If the battery is fully or almost fully charged (high SOC), the purge rate is increased to reduce the nitrogen content in the recirculation circuit. Even if this results in efficiency losses due to increased hydrogen loss, this is the only way to prevent a harmful operating condition caused by dynamic depletion with a rapid reduction in the target power. 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 energy can be stored in the battery. Advantageous embodiments and further developments of the fuel cell system according to the invention are specified in the dependent claims.

[0013] It is advantageous to adjust the purge rate depending on the current power of the fuel cell stack, especially when the current power is greater than a power limit P_lim, since a significant reduction in power of the fuel cell system can only occur at high power levels, resulting in dynamic depletion on the anode side. When the system is operated at high power levels, high operating pressures are typically set on the cathode and anode sides (2.5-3.5 bar). When the load is reduced, the pressure is reduced significantly in a short time (to 1-1.5 bar), which is achieved on the anode side by the fuel cell consuming the hydrogen.However, if a significant amount of nitrogen has accumulated due to gas transfer from the cathode side to the anode side, the majority of nitrogen remains in the recirculation circuit after the pressure reduction through H2 consumption. This significantly reduces the partial pressure of the remaining hydrogen, and can lead to severely damaging conditions of local fuel depletion (dynamic depletion).

[0014] A purge rate in an energy-optimal range is advantageous when the current power of the fuel cell stack is less than a power limit P_lim, as this ensures high efficiency of the fuel cell system. Energy-optimized purge rate operation minimizes hydrogen loss.

[0015] It is advantageous to increase the purge rate above an energy-optimal range if the battery's state of charge (SOC) is above a limit value (SOCJim). The increased purge rate can reduce the nitrogen content in the recirculation circuit. Even if this results in efficiency losses due to increased hydrogen loss, it is the only way to prevent a harmful operating condition caused by dynamic depletion when the target power is quickly reduced. Maintaining the purge rate, particularly in energy-optimal operation when the battery's state of charge is below a limit value (SOCJim), is advantageous because this prevents unwanted hydrogen losses. If the battery's state of charge is below the limit value (SOCJim), the power gradient of the fuel cell system can be delayed so that there is no risk of depletion even without adjusting the purge rate. The excess energy can be stored in the battery.

[0016] It is advantageous if the limit value SOCJim is derived from the possible excess energy E_over and the battery capacity C_Bat, where the excess energy E_over results from the maximum possible power gradient at which no depletion occurs. The limit value SOCJim 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.

[0020] They show:

[0021] Fig. 1 is a schematic representation of a fuel cell system according to the invention according to a first embodiment,

[0022] Fig. 2 is a flow chart of the individual steps of a method according to the invention according to an embodiment, and Fig. 3 is two diagrams illustrating the exemplary target power curve and resulting surplus energy.

[0023] Figure 1 shows a schematic topology of a fuel cell system 100 according to an 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 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.

[0026] The fuel cell system 100 may further include a cooling circuit configured to cool the fuel cell stack 101. The cooling circuit is not shown in Figure 1 because it is not part of the invention.

[0027] At the inlet of the fuel line 20 are a tank 21 and a shut-off valve 22. Additional components can be arranged in the fuel line 20 to supply an anode 103 of the fuel cell stack 101 with fuel as needed. To ensure that the fuel cell stack 101 is always sufficiently supplied with fuel, a superstoichiometric metering of fuel via the fuel line 20 is necessary. 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 arranged 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 into the exhaust line 12 via the purge line 40.

[0032] The components and piping system responsible for supplying the anode with fuel are called the anode side.

[0033] 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.

[0034] 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.

[0035] The components and the piping system responsible for supplying the cathode with air are called the cathode side.

[0036] The method according to the invention is based on the fundamental idea that a purge rate, and thus in particular the nitrogen content in the recirculation circuit 50, is adjusted depending on the battery's state of charge (SOC). This prevents a sudden reduction in the required electrical power from leading to a hydrogen deficiency, which would impact the service life of the fuel cell stack.

[0037] 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.

[0038] 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).

[0039] 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.

[0040] If the battery is fully or nearly fully charged (high battery state of charge SOC), the purge rate is increased to reduce the nitrogen content in the recirculation circuit 50. Even if this results in efficiency losses due to increased hydrogen loss, this is the only way to prevent a harmful operating condition caused by dynamic depletion with a rapid reduction in the target power. 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.

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

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

[0043] In method step 220, the power P of the fuel cell system 100 is compared with a limit value of the power P_lim.

[0044] If the power P of the fuel cell system 100 is less than the limit value of the power P_lim, the process proceeds to step 250 and the previous purge rate is maintained or returned to energy-optimal operation.

[0045] If the power P of the fuel cell system 100 is greater than the power limit value P_lim, the state of charge SOC (state of charge) of the battery is monitored in method step 230.

[0046] In method step 240, the battery's state of charge (SOC) is compared with a battery limit value (SOCJim). If the battery's state of charge (SOC) is above the limit value (SOCJim), the method proceeds to method step 260.

[0047] In process step 260, the purge rate is increased above an energy-optimal range to reduce the nitrogen content in the recirculation circuit 50. A purge rate above the energy-optimal range can result in efficiency losses due to increased hydrogen loss from the recirculation circuit 50. A purge rate within the energy-optimal range avoids increased hydrogen loss.

[0048] After process step 260, process step 210 is repeated.

[0049] If the state of charge SOC of the battery is below a limit value SOCJim, the process proceeds from step 240 to step 250 and the previous purge rate is maintained or reset to energy-optimal operation.

[0050] After process step 250, process step 210 is repeated.

[0051] To increase the purge rate, the purge duration is increased and / or the purge interval is reduced. The purge duration is the opening time of the purge valve 41. The purge interval is the time interval between the initiation of two purge processes.

[0052] 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.

[0053] In particular, the limit value SOCJim can be calculated using the following formula:

[0054] SOC Jim=1 -E_über / C_bat In order to achieve increased accuracy of the process for optimizing the purge strategy, further performance and energy consumption limiting factors, in particular the battery temperature and / or battery aging, can be taken into account.

[0055] Figure 3 shows two diagrams plotting the performance of a fuel cell system over time. The first diagram, A, shows an example of the target power curve and the resulting excess energy E_über during a delayed power reduction of the fuel cell system. The excess energy E_über corresponds to the dashed area between the power curve during a delayed power reduction (dashed line) and the power curve during a rapid power reduction (solid line).

[0056] The second diagram B shows an example of a target power curve with rapid power reduction without excess energy.

Claims

Claims 1 . Method for optimizing the purge strategy of a fuel cell system (100), wherein the fuel cell system (100) comprises a 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 purge rate is adjusted depending on a state of charge SOC of the battery.

2. Method according to claim 1, characterized in that the purge rate is adjusted if, in addition, the current power of the fuel cell stack is greater than a limit value P_lim of the power.

3. Method according to claim 1 or 2, characterized in that the purge rate is operated in an energy-optimal range when the current power of the fuel cell stack is less than a limit value P_lim of the power.

4. Method according to claim 1, characterized in that the purge rate is increased over an energy-optimal range when the state of charge SOC of the battery is above a limit value SOCJim.

5. The method according to claim 1, characterized in that the purge rate is maintained, in particular in an energy-optimal range, when the state of charge of the battery is below a limit value SOCJim.

6. Method according to claim 3, characterized in that in order to increase the purge rate, the purge duration is increased and / or the purge interval is reduced.

7. Method according to one of claims 3 or 4, characterized in that the limit value SOCJim is calculated from the possible excess energy E_über and the battery capacity C_Bat, wherein the excess energy E_über results from the maximum possible power gradient at which no depletion occurs.

8. Method according to claim 6, characterized in that the limit value SOCJim is calculated using the following formula: SOC_lim=1 -E_über / C_bat 9. 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.

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