Method for controlling the operating strategy of a fuel cell system, fuel cell system

Abstract

The invention relates to a method for controlling the stoichiometry λ on an anode side of a fuel cell system (100), in which hydrogen is supplied to an anode (103) of a fuel cell stack (101) via a hydrogen path (20) and recirculated via an anode circuit (50), the recirculation being effected by means of a recirculation pump (52) integrated into the anode circuit (50). The stoichiometry λ is adjusted depending on the nitrogen content in the anode gas. The invention further relates to a fuel cell system (100) for carrying out the method.

Description

[0001] The invention relates to a method for controlling the operating strategy of a fuel cell system, in particular the stoichiometry λ on an anode side of a fuel cell system, with the features of the preamble of claim 1. Furthermore, the invention relates to a fuel cell system that is suitable for carrying out the method or is operable according to the method.

[0002] The invention can be used in particular in a fuel cell vehicle. State of the art

[0003] Hydrogen-based fuel cell systems are considered a mobility concept of the future, as they emit only water as exhaust and allow for rapid refueling. In addition to hydrogen, fuel cells require oxygen to convert the hydrogen into electrical energy, heat, and water. The hydrogen is supplied to an anode, and the oxygen to a cathode. To increase electrical power, multiple fuel cells are typically combined into a fuel cell stack.

[0004] The fuel cells of a fuel cell system are typically supplied with hydrogen at a superstoichiometric concentration. To utilize the excess hydrogen, depleted hydrogen exiting the fuel cells is recirculated via an anode circuit.

[0005] Since recirculated anode gas becomes enriched with nitrogen over time, the anode circuit is purged periodically. For this purpose, a valve, the so-called purge valve, is opened and anode gas is discharged from the anode circuit. The discharged amount is then replaced with fresh hydrogen from the high-pressure tank.

[0006] The present invention is concerned with the objective of controlling the stoichiometry λ on the anode side of the fuel cell system.

[0007] To solve the problem, the method with the features of claim 1 and the fuel cell system with the features of claim 10 are proposed. Advantageous embodiments can be found in the respective dependent claims. Disclosure of the invention

[0008] In the proposed method for controlling the operating strategy of a fuel cell system, in particular for controlling the stoichiometry λ on an anode side of a fuel cell system, in which hydrogen is supplied to an anode of a fuel cell stack via a hydrogen path and recirculated via an anode circuit, the recirculation being effected by means of a recirculation pump integrated into the anode circuit, the stoichiometry λ is adjusted depending on the nitrogen content in the anode gas.

[0009] In this way, the amount of gas to be pumped can be reduced, which lowers the power requirement of the recirculation pump. Another positive effect is that the lower gas flow rate reduces the drying behavior of the membrane. The lower H₂ content in the recirculated gas reduces its waste during the purge process. It is particularly advantageous if the stoichiometry λ is adjusted depending on the nitrogen content in the anode gas, because this allows for more efficient fuel utilization and prevents the membrane from being undersupplied. Furthermore, fuel cell degradation can be avoided.

[0010] The dependent claims specify advantageous embodiments and further developments of the inventive method for controlling the stoichiometry λ on an anode side of a fuel cell system and of the inventive fuel cell system.

[0011] It is advantageous that the stoichiometry λ decreases with increasing nitrogen content, as this allows for precise adjustment of the fuel supply to ensure optimal performance and efficiency of the fuel cell system.

[0012] Lowering the stoichiometry by reducing the hydrogen supply to the anode circuit via the hydrogen path is advantageous because this is a simple way to adjust the stoichiometry and prevents overcompensation of the nitrogen content in the anode gas.

[0013] A further advantage results from lowering the stoichiometry λ by reducing the power of the recirculation pump, as this allows for a dynamic and flexible adjustment of the stoichiometry depending on the nitrogen content.

[0014] It is advantageous if the stoichiometry λ is lowered so that a ratio ∝, which corresponds to the quotient of the stoichiometry λ and the cube of the hydrogen fraction χ_H2, remains constant ∝= λ / (χ_H2) 3 This allows for particularly efficient operation without the membrane drying out. The curve described by λ / χ(H2) represents a physical state where the dynamic pressure remains constant. If the H2 concentration decreases because the N2 concentration increases, the gas becomes heavier. The dynamic pressure rises, and consequently, so does the ability to carry away water droplets. To prevent this from causing the membrane to dry out, the stoichiometry (the numerator of the equation) is simply lowered.

[0015] The application of this method is advantageous in low-load operation of the fuel cell system, as particularly large energy savings are possible here due to the typically high values ​​of the stoichiometry lambda in low-load operation.

[0016] Determining the nitrogen content using a hydrogen sensor allows for a simple and precise measurement of the nitrogen content.

[0017] It is advantageous if the nitrogen content is calculated via the nitrogen transfer at the membrane of the fuel cell stack, as additional components, such as a sensor, can be saved in the fuel cell system.

[0018] Taking humidity and temperature into account allows for greater accuracy in determining the nitrogen content, leading to efficient adjustment of the stoichiometry.

[0019] The invention is explained in more detail below with reference to the accompanying drawings. These show: Fig. 1 a schematic representation of a fuel cell system according to the invention. Detailed description of the drawings

[0020] The one in Fig. Figure 1 of the fuel cell system 100 according to the invention has an anode circuit 50 which serves to supply an anode 103 of a fuel cell stack 101 with hydrogen. The hydrogen is converted in the fuel cells of the fuel cell stack 101 together with oxygen, which is supplied to a cathode 105 of the fuel cell stack 101 for this purpose, into electrical energy, heat and water.

[0021] The hydrogen is stored in a tank 21 and metered into the anode circuit 50 via a hydrogen path 20 using a metering valve 22. The anode circuit 50 supplies the anode 103 of the fuel cell stack 101 not only with fresh fuel as anode gas, but also with recirculated anode gas. A jet pump 51 is integrated into the anode circuit 50 for passive recirculation. Furthermore, a recirculation pump 52 is provided, which allows the anode gas to be actively recirculated. Since the anode gas exiting the fuel cell stack 103 may contain liquid water, it is first fed into a water separator 15. When this is full, a drain valve 17 can be opened to empty it.

[0022] In a fuel cell system 100, it is common to operate with an excess of fuel, particularly hydrogen, on the anode side. If only the exact amount of hydrogen corresponding to the desired electricity production were supplied, the downstream part of a membrane in the fuel cell stack 101 could become depleted, meaning it could no longer react with oxygen. Corrosion or degradation would result.

[0023] For this reason, the hydrogen from the anode 103 is fed into a recirculation circuit 50. The recirculation is driven by the recirculation pump 52 or the jet pump 52. Due to the permeability of the membrane of the fuel cell stack 101, nitrogen also accumulates in the anode circuit 52 during operation. As a result, the proportion of hydrogen in the anode circuit 50 decreases, while the proportion of nitrogen increases.

[0024] By opening a purge valve 41, the anode circuit 50 can be purged via a purge line 40 to transport nitrogen out of the anode circuit 50.

[0025] Depending on the system design, the drain and purge process can also be carried out using only a combined drain / purge valve.

[0026] Air supplied to the fuel cell stack 101 is drawn from the environment via an inlet 16 and fed to an air compressor 11 via an air path 10. The air compressor 11 compresses the supplied air, as the electrochemical process in the fuel cells requires a certain air mass flow rate and a certain pressure level. The air compressor 21 can be located downstream of an air filter in the air path 10. Exiting air is discharged from the fuel cell stack 101 via a cathode exhaust path 12.

[0027] The inventive method for controlling the stoichiometry λ on the anode side of the fuel cell system 100 adjusts the stoichiometry λ depending on the nitrogen content in the anode gas. In particular, the nitrogen content in the anode gas is taken into account.

[0028] Water is produced in the membrane of fuel cell 101, which tends to condense and reduce the accessibility of the fuel gases, especially hydrogen and air, to the membrane. To remove the water from the membrane, a flow of anode gas is maintained to ensure a dynamic pressure capable of carrying liquid water droplets out of the membrane.

[0029] The dynamic pressure is calculated as ρ dyn =1 / 2 ρ v 2, where ρ is the density of the gas and v is the velocity in a channel of the fuel cell stack 101. The same dynamic pressure can be generated by a fast (v large) light gas (p small), but also by a slow (v small) heavy gas (p large).

[0030] Since nitrogen is approximately 14 times heavier than hydrogen, maintaining a constant gas velocity in the anode system is not efficient, as the higher density of nitrogen means that the dynamic pressure can be maintained at a lower velocity. Therefore, the gradually increasing nitrogen content should be taken into account, and the gas velocity (which affects the stoichiometry) should decrease accordingly.

[0031] As the nitrogen content increases, the stoichiometry λ is therefore correspondingly lowered. This can be achieved by reducing the hydrogen supply to the anode circuit 50 via the hydrogen path 20.

[0032] In an alternative embodiment, it is possible to lower the stoichiometry λ by reducing the power of the recirculation pump 52.

[0033] The following example illustrates the adjustment of the stoichiometry lambda λ: Time T= 0 min:

[0034] After the purge process, in which the entire anode circuit was emptied of nitrogen, the hydrogen gas content in the anode circuit is approximately 100%, and the dynamic pressure required to blow off a droplet is, for example, 1.3 Pa. Assuming a current density requirement of 0.15 A / cm² 2With a relative humidity of 100% and an H2 content of 100%, and a stoichiometry of lambda λ=6.5, a speed of v=2.6m / s is required to achieve the dynamic pressure of 1.3Pa to blow away a droplet. Time T = 2 min:

[0035] Due to diffusion processes across the membrane of fuel cell stack 101, nitrogen has accumulated in anode circuit 50. The hydrogen content has decreased to 70%, and the nitrogen content has increased accordingly. Because of nitrogen's higher density, a lambda λ = 3 is sufficient to achieve a dynamic pressure of 1.3 Pa. The gas velocity would be 1.8 m / s.

[0036] Through analytical calculations, it can be shown that the stoichiometry lambda λ is proportional to the hydrogen fraction χ. H2 to the third power always maintains a dynamic pressure that is useful for liquid water discharge, even with decreasing H2 dry content.

[0037] For this reason, according to one embodiment of the invention, the stoichiometry λ is lowered so that a ratio ∝, which corresponds to the quotient of the stoichiometry λ and the third power of the hydrogen content χ, is obtained. H2 corresponds, remains constant ∝=λ / (χH2)3

[0038] The nitrogen content can be determined directly via a hydrogen sensor 45 in the anode circuit 50. Alternatively, a sensor at the output of the cathode 105 is possible, in which case the H₂ transfer is determined by a proportionality analysis. The nitrogen content can also be calculated from the nitrogen transfer at the membrane of the fuel cell stack 101. For the calculation of the nitrogen content, the humidity and temperature of the fuel cell stack 101 can be taken into account via a virtual sensor.

[0039] However, it is also conceivable that the power requirement of the recirculation pump 52 or the pressure loss in the anode path is used or included in the determination of the hydrogen content.

[0040] The method can be preferably used at low loads, especially during low-load operation, since the efficiency improvements are particularly large due to the large lambda values ​​in low-load operation.

Claims

[1] Method for controlling the operating strategy of a fuel cell system (100), in particular the stoichiometry λ on an anode side of a fuel cell system (100), in which hydrogen is supplied to an anode (103) of a fuel cell stack (101) via a hydrogen path (20) and recirculated via an anode circuit (50), wherein the recirculation is effected by means of a recirculation pump (52) integrated into the anode circuit (50), characterized by , that the stoichiometry λ is adjusted depending on the nitrogen content in the anode gas. [2] Method according to claim 1, characterized by , that the stoichiometry λ decreases with increasing nitrogen content. [3] Method according to claim 2, characterized by , that the stoichiometry is lowered by reducing the hydrogen supply to the anode circuit (50) via the hydrogen path (20). [4] Method according to claim 2, characterized by, that the stoichiometry λ is lowered by reducing the power of the recirculation pump (52). [5] Method according to any one of the preceding claims, characterized by , that the stoichiometry λ is lowered so that a ratio ∝, which corresponds to the quotient of the stoichiometry λ and the third power of the hydrogen fraction χ H2 corresponds, remains constant ∝=λ / (χH2)3 [6] Method according to any one of the preceding claims, characterized by , that the procedure is applied during low-load operation of the fuel cell system (100). [7] Method according to any of the preceding claims, characterized by , that the nitrogen content is determined via a hydrogen sensor in the anode circuit (50) or in an exhaust gas path (12). [8] Method according to any one of the preceding claims, characterized by , that the nitrogen content is calculated via the nitrogen transfer at the membrane of the fuel cell stack (101). [9] Method according to claim 8, characterized by , that the humidity and temperature of the fuel cell stack (101) are taken into account for the calculation of the nitrogen content. [10] Fuel cell system (100) comprising a fuel cell stack (101) with an anode (103) which can be supplied with hydrogen from a high-pressure hydrogen tank (21) via a hydrogen path (20) and with recirculated anode gas via an anode circuit (50), wherein the recirculation is effected by means of a recirculation pump (52) integrated into the anode circuit (50), characterized by , that the fuel cell system (100) is configured to carry out a method according to one of claims 1 to 9.

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