Method for operating a fuel cell system, control device

Abstract

The invention relates to a method for operating a fuel cell system (100), in particular a mobile fuel cell system, comprising a fuel cell stack (101) with media channels which are dried by the application of air from an air path (10) before the fuel cell system (100) is switched off. Before the fuel cell system is switched off, the temperature T_K of a coolant in a cooling circuit (30) is detected and the drying time dt is adapted as a function of the temperature T_K. The invention also relates to a control device for a fuel cell system (100).

Description

[0001] Description

[0002] Title:

[0003] Procedures for operating a

[0004] The invention relates to a method for operating a fuel cell system, in particular a mobile fuel cell system or fuel cell vehicle. Furthermore, the invention relates to a control unit configured to execute steps of the method.

[0005] State of the art

[0006] Hydrogen-based fuel cells are considered a mobility concept of the future, as they emit only water as exhaust gas and allow for rapid refueling. In the fuel cells, hydrogen is converted together with oxygen into electrical energy, heat, and water. To increase electrical output, a large number of these fuel cells are combined to form a fuel cell stack.

[0007] PEM fuel cells are known for their high power density and fast start-up capability; however, water freezing within the cells at low temperatures poses a significant challenge. This can lead to component damage and impaired functionality. To counteract this, thorough drying of the stack during the shutdown procedure is essential to prevent water freezing and the associated problems.

[0008] Previous approaches to drying PEM fuel cell stacks during the shutdown procedure have primarily focused on passing a large volume of air through the cathode at a low load point to remove water from the stack as water vapor. The effectiveness of this process depends significantly on the temperature of the air entering and exiting the fuel cell stack, which in turn is influenced not only by the ambient temperature but also, and crucially, by the temperature of the coolant.

[0009] The temperature of the coolant, which is regulated via the intercooler and the stack itself, plays a crucial role in determining the amount of water discharged. Despite the importance of this temperature parameter, existing methods have primarily considered only the ambient temperature for determining the drying time, based on the assumption that the coolant temperature in a warm system is typically around 60°C.

[0010] From the application with the German file number 102022211441 .8, a method for controlling the drying process of a fuel cell system, in particular for preparing for a freeze start, is known. It includes initiating the drying process by activating an air compression system, adjusting operating parameters to a constant level, monitoring temperatures, in particular the outlet temperatures from the stack, and determining a completion time for the drying process based on monitoring temperature gradients or differences.

[0011] The present invention is concerned with the objective of taking into account the ambient temperature and the coolant temperature during the drying period in order to enable a more precise and efficient drying of the fuel cell stack.

[0012] To solve the problem, the method with the features of claim 1 is proposed. Advantageous embodiments of the invention are described in the dependent claims. Furthermore, a control unit for executing steps of the method is specified.

[0013] Disclosure of the invention

[0014] A method for operating a fuel cell system, in particular a mobile fuel cell system, is proposed, comprising a fuel cell stack with media channels, wherein the media channels are dried after the fuel cell system is switched off by supplying air from an air path, wherein the temperature T_K of a coolant in a cooling circuit of the fuel cell system is detected before the fuel cell system is switched off and the drying time dt is adjusted depending on the temperature T_K.

[0015] The method according to the invention is more effective than the prior art because it incorporates the coolant temperature T_K as a critical factor in determining the drying time dt. Previous approaches have largely ignored the coolant temperature T_K, which can lead to incomplete drying, particularly under cold ambient conditions when the fuel cell system is switched off shortly after startup and the system may not be fully warmed up. Insufficient drying can manifest as a surge of water during a subsequent startup, indicating the presence of undrained water in the fuel cell stack.

[0016] One advantage of the method according to the invention is therefore increased reliability. By taking the coolant temperature T_K into account, a more thorough drying of the fuel cell stack is achieved, which minimizes the risk of water freezing and the associated damage to the fuel cell stack.

[0017] Furthermore, a complete drying of the fuel cell stack ensures optimal operating conditions for the fuel cell, leading to improved performance.

[0018] The method according to the invention enables flexible adjustment of the drying time dt to different environmental and operating conditions, which expands the application possibilities of the fuel cell system in different climate zones.

[0019] Furthermore, the energy consumption of the drying process can be optimized by precisely controlling the drying time dt based on the actual coolant temperature T_K. The present invention offers an innovative solution to the challenges associated with drying fuel cell stacks under variable climatic conditions. By incorporating the coolant temperature T_K into the drying time dt, more efficient, reliable, and powerful operation of the fuel cell system is enabled.

[0020] Furthermore, a control unit for a fuel cell system is proposed, which is configured to execute steps of a method according to the invention. For example, the drying time dt can be controlled with the aid of the control unit as a function of the temperature T_K of the coolant. For this purpose, the control unit can receive the measurement data from a temperature sensor, with which the current temperature T_K of the coolant can be determined. The method can thus be largely automated.

[0021] The dependent claims specify advantageous embodiments and further developments of the fuel cell system according to the invention and of the method for operating a fuel cell system.

[0022] It is advantageous to measure the temperature T_K only when the operating time falls below a predefined limit, as this simplifies efficiency by reducing the number of steps required. For most ambient temperatures, it can be assumed that the coolant temperature T_K remains approximately constant after the fuel cell system has operated for a period exceeding the predefined limit. In a particular embodiment, the coolant temperature after operating for a period exceeding the predefined limit is between 50 and 70 degrees Celsius, specifically around 60 degrees Celsius.

[0023] The drying time dt is advantageously extended if the coolant temperature T_K is below a predetermined minimum temperature T_min. If the coolant temperature T_K is below a predetermined minimum temperature T_min, the air temperature is significantly lower than under normal conditions and therefore can carry away considerably less water. The drying time dt must be extended accordingly. The drying time dt remains the same as before if the coolant temperature T_K is above a predetermined minimum temperature T_min.

[0024] It is advantageous if the current drying time is a constant, as this greatly simplifies the process. The constant drying time, which is applied under normal circumstances, particularly when the coolant temperature T_K is above a predetermined minimum temperature, can be determined experimentally. Since the drying time also depends on the temperature and humidity of the ambient air, the current drying time can also take these factors into account and, for example, be stored in a two-dimensional table considering the humidity and temperature of the ambient air.

[0025] Since the temperature T_K of the coolant is crucial for the temperature of the air that dries the fuel cell stack, the drying time can be advantageously extended by a factor f, which is determined using the saturation vapor curve p_(T_K) of the air at the temperature of the coolant T_K.

[0026] It is advantageous to extend the drying time by a factor f, where f is the quotient of the saturation vapor pressure p_(T_D) of the medium at an average cooling temperature T_D and the saturation vapor pressure p_(T_K) of the medium at the temperature of the coolant T_K f=p_(T_D) / p_(T_K)

[0027] It is advantageous to determine the saturation vapor pressure p_(T) for air at a temperature T using the Magnus formula p_(T)= 6.112 hPa + exp ( [17.62*T] / [243.12°C+T] ) for -45°C < T < 60°C, as this is a simple way of implementation.

[0028] The invention and its advantages are described in more detail below with reference to the accompanying figure. This figure shows a schematic topology of a fuel cell system according to an embodiment of the invention. Detailed description of the drawing

[0029] The figure shows a schematic topology of a fuel cell system 100 with at least one fuel cell stack 101. The at least one fuel cell stack 101 has an air path 10, an exhaust gas channel 12, and a fuel line 20. The at least one fuel cell stack 101 can be used for mobile applications with high power requirements, e.g., in trucks, or for stationary applications, e.g., in generators.

[0030] The air path 10 serves as an air supply line to provide the fuel cell stack 101 with ambient air 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 a compressor 11 is located in the air path 10, which compresses or draws in the air according to the respective operating conditions of the fuel cell stack 101. A humidifier 15 may be located downstream of the air compressor 11 and / or compressor 11, which increases the concentration of liquid in the air in the air path 10.

[0031] Within air path 10, further components such as a filter and / or a heat exchanger and / or valves may be provided. Oxygen-rich air is supplied to the fuel cell stack 101 via air path 10.

[0032] Furthermore, the fuel cell system 100 has an exhaust gas path 12 in which water, as well as other components of the air from the air path 10, are transported into the environment via an outlet 18 after passing through the fuel cell stack 101.

[0033] At the inlet of the fuel line 20 are a high-pressure tank 21 and a shut-off valve 22. Further components, such as a jet pump 51, a recirculation line 50, or a recirculation pump 52, can be arranged in the fuel supply to supply the fuel cell stack 101 with fuel as needed. Furthermore, a purge and / or drain valve can be present in the recirculation line. However, since these valves are not essential to the invention, they are not shown in the figure.

[0034] The fuel cell system 100 further comprises a cooling circuit 30 which is designed to cool the fuel cell stack 101. The cooling circuit 30 is a closed circuit in which a coolant circulates. A heat exchanger 32 and a coolant pump 34 are arranged in the cooling circuit 30. Additional components may also be arranged in the cooling circuit 30.

[0035] According to the inventive method, the drying time is adjusted depending on the temperature T_K of the coolant in the cooling circuit 30. Before the fuel cell system 100 is switched off, the temperature T_K of the coolant is detected by means of a temperature sensor 36, which is arranged in the cooling circuit 30. Based on the temperature T_K, the drying time dt is adjusted. This adjustment enables more efficient drying of the fuel cell stack 101 by taking into account the physical properties of the coolant and its influence on the drying process.

[0036] In a further embodiment of the invention, the temperature T_K of the coolant is only measured if the operating time of the fuel cell system is below a predetermined limit. This takes into account that during short operating times the coolant, and consequently the system, may not be fully warmed up, which can affect the drying efficiency.

[0037] The drying time dt is extended if the temperature T_K of the coolant falls below a predetermined minimum temperature T_min. This takes into account the fact that at lower temperatures, the capacity of the air to absorb and carry away water is reduced, necessitating a longer drying time dt to ensure complete drying.

[0038] The drying time dt corresponds to a previous drying time when the temperature T_K of the coolant is above the specified minimum temperature T_min. This allows for the standardization of the drying process under conditions considered optimal for efficient drying.

[0039] The drying time can be a constant duration or determined from a stored table based on the ambient temperature and / or humidity. This offers flexibility in adapting the drying process to the ambient conditions to achieve optimal drying.

[0040] If the temperature T_K of the coolant is below the specified minimum temperature T_min, the drying time can be extended by a factor f, which is determined via the saturation vapor curve of the medium at the temperature T_K of the coolant.

[0041] This factor f is the quotient of the saturation vapor pressure p_(T_D) of the air at an average cooling temperature T_D, specifically 60°C, and the saturation vapor pressure p_(T_K) of the air at the temperature T_K of the refrigerant. f = p_(T_K) / p_(T_D)

[0042] This adjustment of the drying time dt by multiplication with the factor f enables precise control of the drying time dt based on the physical properties of the drying medium and the specific temperature T_K of the coolant, which ensures efficient and complete drying of the fuel cell stack 101.

[0043] The saturation vapor pressure can be determined using the Magnus formula p_(t)= 6.112 hPa + exp ( [17.62*t] / [243.12°C+t] ) for -45°C < t < 60°C.

[0044] A control unit 200 is arranged in the fuel cell system 100, which is designed to carry out the method according to the invention.

Claims

Claims 1. Method for operating a fuel cell system (100), in particular a mobile fuel cell system, comprising a fuel cell stack (101) with media channels which are dried by supplying air from an air path (10) before the fuel cell system (100) is switched off, characterized in that the temperature T_K of a coolant in a cooling circuit (30) is detected before the system is switched off and the drying time dt is adjusted as a function of the temperature.

2. Method according to claim 1, characterized in that the temperature T_K is only recorded if the operating time is below a predetermined limit value.

3. Method according to claim 1 or 2, characterized in that the drying time dt is extended if the temperature of the coolant T_K is below a predetermined minimum temperature T_min.

4. Method according to claim 1 or 2, characterized in that the drying time dt corresponds to a previous drying time if the temperature of the coolant T_K is above a predetermined minimum temperature T_min.

5. Method according to claim 4, characterized in that the drying time to date is a constant drying time or is determined depending on the temperature and / or humidity of the ambient air.

6. Method according to claim 4, characterized in that the drying time dt is extended by a factor f, which is determined via the saturation vapor curve dp_(T_K) of the air at the temperature T_K of the coolant.

7. Method according to claim 6, characterized in that the drying time dt is extended by a factor of f, wherein f is the quotient of the saturation vapor pressure p_(T_D) of the medium at an average cooling temperature T_D and the saturation vapor pressure p_(T_K) of the medium at the temperature of the coolant T_K f=p_(T_D) / p_(T_K) 8. Method according to claim 7, characterized in that the saturation vapor pressure for a temperature T is determined using the Magnus formula p_(T) = 6.112 hPa + exp ( [17.62*7] / [243.12°C+7] ) for -45°C < T < 60°C 9. Control unit for a fuel cell system (100), comprising at least one fuel cell stack (101), an air path (10) wherein air from the environment is supplied to the fuel cell via the air path (10), an exhaust gas path (12), a fuel line (20) wherein fuel is transported to the fuel cell stack (101) via the fuel line (20) and a cooling circuit (300) for cooling the fuel cell stack (101), which is configured to perform steps of a method according to one of the preceding claims.

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