Method for operating a fuel cell system, control unit, fuel cell system

Abstract

The invention relates to a method for operating a fuel cell system (1) in which air is supplied to a stack (2) via an air supply path (3) of an air system (4), which is previously moistened by means of water injection, wherein the water is taken from a water tank (6) by means of an electric pump (5) and supplied via a water line (7) to a metering valve (8) arranged on the air supply path (3), the actuation of which is time-coordinated to the operation of the pump (5) in such a way that a pressure wave generated by the pump (5) and propagating via the water line (7) towards the metering valve (8) causes a pressure increase in the metering valve (8) before it is opened. The invention further relates to a control unit (9) for a fuel cell system (1) and a fuel cell system (1) with a corresponding control unit (9).

Description

[0001] The invention relates to a method for operating a fuel cell system having the features of the preamble of claim 1. Furthermore, the invention relates to a control device for a fuel cell system for carrying out steps of the method and to a fuel cell system that can be operated according to the method. State of the art

[0002] Fuel cells are electrochemical energy converters. Hydrogen (H2) and oxygen (O2) can be used as reaction gases. These are converted into electrical energy, water (H2O), and heat using a fuel cell. To increase electrical output, a large number of fuel cells are stacked together to form a fuel cell stack. The fuel cell stack is crisscrossed by channels through which the individual fuel cells are supplied with the required media and through which the depleted media escaping from the fuel cells are transported away.

[0003] To supply a stack with oxygen, ambient air is typically used, which is fed to the stack via an air supply path of an air system. The air system also includes an exhaust air path for removing the air escaping or exhaust air from the stack. Since the electrochemical reaction in the fuel cells requires a certain air mass flow and a certain pressure level, an air compressor is integrated into the supply air path. To cool the air, which heats up considerably during compression, a cooler is arranged downstream of the air compressor in the supply air path. To further condition the air, a humidification device is usually provided in the supply air path. Gas-to-gas membrane humidifiers are frequently used, with the moist exhaust air escaping from the stack usually being used to humidify the air in the supply air path. Another option is humidification by water injection.The water is then injected directly into the supply air path using a suitable dosing valve. In addition to the dosing valve, a water tank is required to store the water and a pumping system to supply the water from the water tank to the dosing valve.

[0004] The present invention is concerned with the task of optimizing the humidification of air in an air supply path of an air system by means of water injection, in particular with regard to dosing pressure and spray quality.

[0005] To achieve this objective, the method with the features of claim 1 is proposed. Advantageous further developments of the invention are set forth in the subclaims. Furthermore, a control unit for executing steps of the method and a fuel cell system with a corresponding control unit are specified. Disclosure of the invention

[0006] In the proposed method for operating a fuel cell system, air is supplied to a stack via an air supply path of an air system, which air is previously humidified by water injection. The water is drawn from a water tank using an electric pump and fed via a water line to a metering valve located on the air supply path. The actuation of the metering valve is timed to coincide with the operation of the pump in such a way that a pressure wave generated by the pump and propagating via the water line toward the metering valve causes a pressure increase in the metering valve before it opens.

[0007] The pressure increase in the dosing valve, which is achieved by the timed operation of the pump and dosing valve, leads to an increased dosing pressure when the dosing valve is opened and thus to an improved spray quality.

[0008] The pressure increase is due to the fact that, upon arrival of the pressure wave, a back pressure builds up in the metering valve. Although the pressure wave is partially reflected, a pressure increase still occurs. If the metering valve is opened shortly thereafter, this pressure increase can be used to meter the water into the supply air path. In this way, a metering pressure can be achieved that is sufficiently high to overcome the injection back pressure in the supply air path.

[0009] The achieved pressure increase enables the use of a smaller pump, thus saving space and costs. Furthermore, a pump can be used that is already known from other applications, for example, in the field of exhaust gas aftertreatment. Here, a pump and a metering valve are used to inject a reducing agent into the exhaust system of an internal combustion engine to reduce the nitrogen oxide content in the exhaust gas. However, the injection backpressure in the exhaust system is generally lower than in the intake air path of a fuel cell system, so the use of such a pump would not be possible without the pressure increase achieved with the aid of the proposed method.

[0010] To implement the proposed method, a diaphragm pump, more preferably a linear-stroke diaphragm pump, is preferably used as the pump. Such pumps are also used, for example, in exhaust gas aftertreatment systems. The advantage of such a pump is that, inherently, pressure waves are generated during pump operation. This is because suction stroke and discharge stroke phases alternate.

[0011] The propagation times of the pressure waves in different operating states of the pump can be determined in advance and stored in a control unit of the fuel cell system.

[0012] Furthermore, it is proposed that the actuation of the metering valve be timed to coincide with the pump's operation using a control unit. It is advantageous if the control unit knows the propagation times of the pressure waves in different operating states of the pump. Ideally, the control unit in which this data is stored is used. This simplifies the required timing.

[0013] To achieve maximum pressure increase, the timing of the metering valve and pump must be as precisely coordinated as possible. If the metering valve opens too early, no back pressure builds up. If the metering valve opens too late, the back pressure may already have dissipated. To prevent this, the optimal time for opening the metering valve is preferably determined in advance and stored in the control unit.

[0014] Advantageously, the air is compressed before humidification by means of water injection using an air compressor integrated into the supply air path. Humidification of the air thus occurs after compression, which has the advantage that humidification simultaneously cools the air, which heats up considerably during compression. This relieves the load on a cooler located downstream of the air compressor in the supply air path.

[0015] In a further development of the invention, it is proposed that the air compressed by the air compressor be used to heat the water line connecting the pump to the metering valve. By heating the water line, it can be kept frost-free at low outside temperatures, or any existing ice can be thawed. For this purpose, the water line can be integrated, at least in sections, into the supply air path. Alternatively, a portion of the compressed air from the supply air path can be diverted to heat the water line.

[0016] Furthermore, a control unit for a fuel cell system is proposed, which is configured to carry out steps of a method according to the invention. Advantageously, data relating to the propagation times of pressure waves in different operating states of the pump are stored in the control unit in order to be able to coordinate the actuation of the metering valve accordingly.

[0017] According to a preferred embodiment of the invention, the pump and the metering valve are each connected to a power output stage integrated into the control unit. In this way, the operation of the pump and the metering valve can be optimally coordinated in time.

[0018] Furthermore, a fuel cell system is proposed. The fuel cell system comprises: - a stack, - an air system with an air supply path through which the stack can be supplied with air, - a device for injecting water into the supply air path, which (i) a pump, (ii) a water tank and (iii) has a dosing valve connected to the pump via a water pipe, and - a control device according to the invention, by means of which the metering valve can be actuated.

[0019] The proposed fuel cell system therefore comprises all the components required to carry out the method according to the invention described above. The fuel cell system is thus suitable for carrying out the method or can be operated according to this method. Consequently, the same advantages can be achieved with the help of the fuel cell system. In particular, increased dosing pressure and high spray quality can be achieved when injecting water into the supply air path.

[0020] According to a preferred embodiment of the invention, the pump is a diaphragm pump, preferably a linear-stroke diaphragm pump. Such pumps are known from the field of exhaust gas aftertreatment, so existing components can be used. Thanks to the achievable pressure increase, a metering pressure sufficiently high to overcome the injection backpressure can be achieved even with a small pump. This is particularly important because the injection backpressure in the intake air path of a fuel cell system is higher than in an exhaust system.

[0021] Furthermore, the pump and the metering valve are preferably each connected via electrical lines to a power output stage integrated into the control unit. This enables optimal timing of the actuation of the metering valve with the operation of the pump.

[0022] Furthermore, the metering valve is preferably arranged downstream of an air compressor integrated into the supply air path. The air in the supply air path is therefore first compressed and only then humidified. Humidifying the air simultaneously achieves a certain degree of cooling of the air, which heats up considerably during compression. This can reduce the load on a cooler downstream of the air compressor.

[0023] Advantageously, the water line connecting the pump to the dosing valve is routed at least partially along the supply air path or is located in the supply air path. The "warm" air present in the supply air path can be used to keep the water line frost-free or thaw it if necessary.

[0024] The invention and its advantages are 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 with an air system and a device for injecting water, as well as Fig. 2 a schematic representation of a device for injecting water for a fuel cell system according to the invention. Detailed description of the drawings

[0025] Fig. 1 shows a fuel cell system 1 with a stack 2 and an air system 4 for supplying the stack 2 with air. The air is supplied to the stack 2 via an air supply path 3 of the air system 4. Since the electrochemical reaction in the fuel cells of the stack 2 requires a certain air mass flow and a certain pressure level, the air supplied to the stack 2 is first compressed using an air compressor 10 integrated into the air supply path 3. To humidify the air, a metering valve 8 of a device 11 for injecting water is integrated into the air supply path 3 downstream of the air compressor 10. The water is drawn from a water tank 6 using a pump 5 and supplied to the metering valve 8 via a water line 7. The metering valve 8 is actuated using a control unit 9.

[0026] The air or exhaust air exiting from the stack 2 is discharged via an exhaust air path 14 into which a throttle valve 15 is integrated.

[0027] A device 11 for injecting water for a fuel cell system 1 according to the invention is shown by way of example in Fig. 2. The pump 5 of the illustrated device 11 is designed as a linear-stroke diaphragm pump and is located outside the water tank 6. During operation, the pump 5 generates pressure waves that propagate via the water line 7 toward the metering valve 8. Upon reaching the metering valve 8, a portion of the pressure wave is reflected. By initially keeping the metering valve 8 closed, a back pressure is simultaneously generated in the metering valve 8, which leads to an increase in pressure. If the metering valve 8 is subsequently opened, an increased metering pressure and thus a high spray quality can be achieved.

[0028] The timing of the actuation of the metering valve 8 with the operation of the pump 5 is achieved with the aid of the control unit 9. For this purpose, the pump 5 and the metering valve 8 are each connected via an electrical line 12 to a power output stage 13 integrated into the control unit 9.

Claims

[1] Method for operating a fuel cell system (1), in which air is supplied to a stack (2) via an air supply path (3) of an air system (4), which air is previously humidified by means of water injection, the water being taken from a water tank (6) with the aid of an electric pump (5) and supplied via a water line (7) to a metering valve (8) arranged on the air supply path (3), the actuation of which is timed to the operation of the pump (5) in such a way that a pressure wave generated by the pump (5) and propagating via the water line (7) in the direction of the metering valve (8) causes a pressure increase in the metering valve (8) before the latter is opened. [2] Method according to claim 1, characterized by that a diaphragm pump, preferably a linear stroke diaphragm pump, is used as pump (5). [3] Method according to claim 1 or 2, characterized bythat the actuation of the dosing valve (8) is timed to match the operation of the pump (5) by means of a control unit (9). [4] Method according to one of the preceding claims, characterized by that the air is compressed before humidification by means of water injection using an air compressor (10) integrated into the supply air path (3). [5] Method according to claim 4, characterized by that the air compressed by the air compressor (10) is used to heat the water pipe (7). [6] Control unit (9) for a fuel cell system (1), wherein the control unit (9) is configured to carry out steps of a method according to one of the preceding claims. [7] Fuel cell system (1), comprising - a stack (2), - an air system (4) with an air supply path (3) through which the stack (2) can be supplied with air, - a device (11) for injecting water into the supply air path (3), which (i) a pump (5), (ii) a water tank (6) and (iii) a dosing valve (8) connected to the pump (5) via a water line (7), and - a control device (9) according to claim 6, by means of which the metering valve (8) can be actuated. [8] Fuel cell system according to claim 7, characterized by that the pump (5) is a diaphragm pump, preferably a linear stroke diaphragm pump. [9] Fuel cell system according to claim 7 or 8, characterized by that the pump (5) and the metering valve (8) are each connected via electrical lines (12) to a power output stage (13) integrated in the control unit (9). [10] Fuel cell system according to one of claims 7 to 9, characterized bythat the metering valve (8) is arranged downstream of an air compressor (10) integrated into the supply air path (3) in the supply air path (3) and / or the water line (7) is guided at least in sections along the supply air path (3) or is arranged in the supply air path (3).

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