Fuel cell system and method for humidifying an air path of a fuel cell system

Abstract

Fuel cell system (1) having at least one fuel cell stack (100) which has a cathode (110) and an anode (115), wherein air is supplied to the cathode (110) via an air path (111) and exhaust air emerging from the fuel cell stack (100) is discharged via an exhaust gas path (112), and wherein the anode (120) is supplied with hydrogen via an anode system. A Venturi nozzle is arranged in the air path (111), the Venturi nozzle being connected to a humidification line.

Description

[0001] The invention relates to a fuel cell system having the features of the preamble of patent claim 1.

[0002] Furthermore, the invention relates to a method for humidifying an air path of a fuel cell system having the features of the preamble of patent claim 9. State of the art

[0003] Fuel cells convert a fuel, such as hydrogen, and oxygen into electrical energy, heat, and water. Air, especially ambient air, can serve as the oxygen source. The air is fed to a fuel cell cathode via an air supply path. Since the energy conversion process requires a certain air mass flow and a certain pressure level, the air supplied to the cathode side is first compressed using an air compressor located in the air supply path. Before entering the fuel cells, the air is also humidified to prevent the fuel cell membranes from drying out. Otherwise, there is a risk of damage to the fuel cells.

[0004] A gas-to-gas membrane humidifier can be used for humidification, which transports water, particularly product water generated during fuel cell operation, from the outlet side to the inlet side of the fuel cells. Since the exchange surfaces must be large to transport the required water mass flow, the installation space required for such a humidifier is quite large. In addition, water can only be transferred if there is sufficient water on the outlet side. Since water always remains in the exchange regions, this can lead to damage due to ice pressure in the event of frost. Furthermore, there is no way to intervene in such humidifiers to directly control the water transfer. Typically, a bypass channel is provided on one of the two sides for this purpose, which allows the humidity at the fuel cell inlet to be adjusted using a bypass valve.Precisely regulating this concept is comparatively difficult and therefore time-consuming.

[0005] The present invention is therefore based on the object of providing a concept for humidifying the air in an air supply path of a fuel cell system, which allows a precise adjustment of the inlet humidity of the air and is also easy to implement. Disclosure of the invention

[0006] The fuel cell system according to the invention and the method for humidifying an air path of a fuel cell system with the features according to the independent claims have the advantage of preventing drying out of the cathode inlet area and thus membrane damage. The Venturi nozzle arranged in the air path, which is connected to a humidification line, prevents dry membrane conditions in the fuel cell stack, thus achieving a longer service life of the fuel cell stack.

[0007] Furthermore, the following advantages arise over existing humidification devices and methods. Compared to a membrane humidifier, the proposed solution results in a reduction in costs and a smaller installation space requirement.

[0008] The humidification of the air path or the membrane of the fuel cell stack is carried out with less energy than is known with previous solutions, since the Venturi nozzle is a passive component that does not consume any additional energy compared to a pump or other electrical consumers, so that the system efficiency and complexity of the fuel cell system is not further increased.

[0009] In contrast to other humidification methods, such as the membrane humidifier, the Venturi nozzle is not subject to any significant aging processes, thus ensuring robust operation with a long service life.

[0010] The dependent claims contain advantageous embodiments and further developments of the fuel cell system and of the method according to the invention for humidifying an air path of a fuel cell system.

[0011] It is advantageous if the humidification line is connected to the exhaust gas path, in particular to a water separator in the exhaust gas path, as this provides a source of liquid water.

[0012] A valve, particularly a switching or proportional valve, located in the humidification line is advantageous because it can be used to activate or deactivate humidification. Furthermore, a proportional valve allows the amount of humidity that enters the air path via the humidification line to be adjusted.

[0013] It is advantageous if the humidification line is connected to a water tank, as this ensures a reliable supply of water to the humidification line, especially when the water tank is sufficiently filled with water. This is ensured by the method according to the invention. In addition to filling the water tank with liquid water from the water separator, external filling or filling from other sources is also possible.

[0014] A Venturi nozzle with an adjustable cross-section is advantageous because it allows for optimal adaptation to the existing primary mass flow, especially to the air mass flow in the air path. This allows a desired pressure gradient from the exhaust gas to the supply air to be maintained in all operating conditions. If water injection is not desired, the Venturi nozzle can be fully opened. This minimizes the pressure loss in the supply air path through the Venturi nozzle.

[0015] A further advantage arises when a bypass line is arranged parallel to the Venturi nozzle, as this allows the Venturi nozzle to be partially bypassed. This allows the Venturi nozzle to be designed for a narrower operating range, ensuring that the pressure gradient between exhaust gas and supply air is maintained. The strong overall mass flow variation in the air path is dampened by appropriate control of the bypass valve. An adjustable bypass valve arranged in the bypass line is particularly advantageous here, as it allows the mass flow through the Venturi nozzle or the bypass line to be adjusted.

[0016] It is advantageous to increase the water separation rate in the exhaust path when the water tank is empty or insufficiently filled, or to change the operating point of the fuel cell stack so that a larger amount of water is supplied to the water tank from the water separator. This ensures that a sufficient amount of water is always available for humidification at as many operating points of the fuel cell stack as possible. The separation rate in the water separator can be effectively increased by cooling the water separator. Preferred embodiments:

[0017] The device and fuel cell system according to the invention are explained in more detail below with reference to the drawings. They show schematically: Fig. 1 shows a schematic topology of a fuel cell system according to a first embodiment of the invention, Fig. 2 shows a schematic topology of a fuel cell system according to a second embodiment of the invention and Fig. 3 a block diagram illustrating the sequence of a method according to the invention.

[0018] In the Fig. 1 shows a schematic topology of a fuel cell system 100 according to a first embodiment, comprising at least one fuel cell stack 101. The at least one fuel cell stack 101 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.

[0019] A high-pressure 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 the fuel cell stack 101 with fuel as required.

[0020] In order to always supply the fuel cell stack 101 with sufficient fuel, there is a need for a superstoichiometric dosage 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 in a recirculation line 50 and mixed with the metered fuel from the fuel line 20.

[0021] Various components, such as a jet pump 51 driven by the metered fuel or a blower 52, can be installed to drive the flow in the recirculation line 50. A combination of jet pump 51 and blower 52 is also possible.

[0022] The air path 10 serves as an air supply line to supply air from the environment to the fuel cell stack 101 via an inlet 16. 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.

[0023] Additional components such as a humidifier, 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.

[0024] Furthermore, the fuel cell system 100 has an exhaust line 12, in which water and other air components from the air path 10 are transported to the environment via an outlet 18 after passing through the fuel cell stack 101. The exhaust gas from the exhaust line 12 can also contain hydrogen (H2), because portions of the hydrogen can diffuse through the membrane of the fuel cell stack 101.

[0025] Downstream of the air compressor 11 is a Venturi nozzle 30, which is connected to a humidification line 32. The humidification line 32 is arranged between the air path 10 and the exhaust line 12. The humidification line 32 is connected, in particular, to a water separator 36 arranged in the exhaust line 12.

[0026] The Venturi nozzle 30 draws liquid water through the humidification line 32, allowing it to humidify the air in the air path 10. The local narrowing of the air path 10 in the Venturi nozzle 30 increases the flow velocity of the drawn-in supply air, reducing the static pressure in the supply air (Bernoulli effect), and creating a pressure gradient between the air path 10 and the humidification line 32 or the exhaust line 12. Due to the pressure gradient, liquid water is pumped from the water separator 36 into the air path 10.

[0027] A valve 34, in particular a switching or proportional valve, can be arranged within the humidification line. If the valve is designed as a switching valve, it can be opened to allow the inflow of liquid water. Alternatively, the switching valve can be closed so that no liquid water can flow into the air path 10. If the valve 34 is designed as a proportional valve or a continuous valve, the amount of water supplied to the air line via the Venturi effect can be adjusted.

[0028] In an alternative embodiment, a water tank 37 may be arranged in the humidification line 32, or the humidification line 32 may be connected to a water tank 37. The water tank 37 serves to collect liquid water so that it can be used in situations where only a small amount of liquid water is available in the exhaust gas.

[0029] In a further alternative embodiment, the Venturi nozzle 30 can have an adjustable cross-section, allowing adjustment of the mass flow of supply air within the Venturi nozzle 30 to maintain a desired pressure gradient from the exhaust gas to the supply air in all operating conditions. If water injection is not desired, the Venturi nozzle 30 can be fully opened and the switching valve 34 in the humidification line 32 can be closed. In this way, the pressure loss in the supply air path 10 through the Venturi nozzle 30 can be minimized.

[0030] Fig. Figure 2 shows a schematic topology of a fuel cell system 1 according to a second embodiment of the invention. The fuel cell system 1 of the second embodiment corresponds to the fuel cell system of the first embodiment except for the components described below.

[0031] A bypass line 38 is arranged parallel to the Venturi nozzle 30 so that the Venturi nozzle 30 can be partially or completely bypassed. If the Venturi nozzle 30 is partially bypassed, only a portion of the supply air flows through the Venturi nozzle 30. If the Venturi nozzle 30 is completely bypassed, the entire supply air flows through the bypass line and past the Venturi nozzle 30.

[0032] To appropriately regulate the amount of supply air flowing through the bypass line 38, a bypass valve 39 is arranged in the bypass line 38. This allows the Venturi nozzle 30 to be designed for a narrower operating range, ensuring that the pressure gradient between the exhaust line 12 and the supply air path 10 is maintained. The strong overall mass flow variation is dampened by appropriate control of the bypass valve 39.

[0033] The method according to the invention for humidifying an air path 10 of a fuel cell system 1 according to one of the described embodiments, in particular with a water tank 37, is described below with reference to Fig. 3 is explained.

[0034] In a method step 100, a condition monitoring system for the fuel cell stack 101 is activated with the aid of a control unit. In a method step 110, it is checked whether the water tank 37 is sufficiently filled. If the water tank 37 is empty or insufficiently filled, the method proceeds to step 140.

[0035] In method step 140, the water separation rate in the exhaust gas path 12 is increased. Alternatively or additionally, the operating point of the fuel cell stack 101 can be changed so that more water can be separated in the water separator 36 during humid operation. Adequate filling of the water tank 37 is ensured if enough water is available for injection at each operating point of the fuel cell stack 101.

[0036] To increase the separation rate, the water separator 36, for example, can be cooled.

[0037] If the water tank 37 is sufficiently filled, process step 120 is used to check whether water injection is required. This may be the case if, without water injection, excessively dry inlet conditions would prevail at the stack. Excessively dry inlet conditions lead to dry membrane conditions, which can reduce service life.

[0038] If water injection is required, process step 130 is initiated, and water is injected via the Venturi nozzle 30. This can be accomplished by opening the valve 34 in the humidification line 32. Furthermore, with a Venturi nozzle 30 with an adjustable cross-section, the cross-section can be adjusted to the required amount of liquid water. Changing the cross-section changes the pressure gradient between the supply air path 10 and the exhaust line 12 and thus the amount of liquid water supplied. After the required amount of liquid water has been injected in process step 130, process step 100 is initiated again.

[0039] If no water injection is required, process step 120 goes directly to process step 100.

Claims

[1] Fuel cell system (1) with at least one fuel cell stack (101) having a cathode and an anode, wherein air is supplied to the cathode via an air path (10) and exhaust air emerging from the fuel cell stack (101) is discharged via an exhaust path (12), and wherein the anode is supplied with hydrogen via an anode system, characterized by that a Venturi nozzle (30) is arranged in the air path (10), wherein the Venturi nozzle (30) is connected to a humidification line (32). [2] Fuel cell system (1) according to claim 1, wherein the humidification line (32) is connected to the exhaust gas path (12), in particular a water separator (36) in the exhaust gas path (12). [3] Fuel cell system (1) according to claim 1, wherein a valve (34), in particular a switching or proportional valve, is arranged in the humidification line (32). [4] Fuel cell system (1) according to one of the preceding claims, wherein the humidification line (32) is connected to a water tank (37). [5] Fuel cell system (1) according to one of the preceding claims, wherein the Venturi nozzle (30) has an adjustable cross-section. [6] Fuel cell system (1) according to claim 1, wherein the Venturi nozzle (30) is arranged downstream of an air compressor (11). [7] Fuel cell system (1) according to claim 1, wherein a bypass line (38) is arranged parallel to the Venturi nozzle (30). [8] Fuel cell system (1) according to claim 7, wherein a bypass valve (39) is arranged in the bypass line (38). [9] Method for humidifying an air path (10) of a fuel cell system (1) according to one of the preceding claims, wherein the fuel cell system (1) has a water tank (37), characterized bythat when the water tank (37) is empty or not sufficiently filled, the separation rate of water in the exhaust gas path (12) is increased or the operating point of the fuel cell stack (101) is changed. [10] Method according to claim 9, characterized by that the water separator (36) is cooled to increase the separation rate.

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