Fuel cell system and method for operating a fuel cell system in a high-pressure operation

Abstract

The invention relates to a fuel cell system (100) for converting energy. The fuel cell system (100) comprises: - a fuel cell stack (101) and - a computing unit (103). The computing unit (103) is configured so as to switch the fuel cell system (100) to an intermittent operation, wherein in the intermittent operation, the fuel cell system (100) is configured so as to switch repeatedly between a dry operation (211) of the fuel cell stack (101) and a humid operation (207) of the fuel cell stack (101), and, in the dry operation (211), set a degree of humidity in the fuel cell stack (101) which is reduced in comparison to the humid operation (207) and, in the humid operation (211), set a degree of humidity in the fuel cell stack (101) which is increased in comparison to the dry operation (207). The computing unit (103) is configured so as to only activate the intermittent operation when the pressure of an air mass flow supplied to the fuel cell stack (101) lies above a specified pressure threshold.

Description

[0001] Description

[0002] title

[0003] Fuel cell system and operating method for a fuel cell system in high pressure operation

[0004] The presented invention relates to a fuel cell system, an operating method for operating a fuel cell system and a vehicle according to the appended claims.

[0005] State of the art

[0006] Typical fuel cell systems operate at moderate temperatures of approximately 70°C and moderate pressures of approximately 2.5 bar.

[0007] Since the cooling capacity is very challenging, for example when operating in a vehicle under extreme conditions such as hot climates or when driving uphill, a temporary increase in temperature is essential.

[0008] In order to prevent the respective fuel cells from drying out during operation under extreme conditions, counteracting measures must be taken, such as supplying moisture by means of external humidification, increasing the pressure and / or reducing the power.

[0009] Fuel cell systems are known that operate in high pressure mode at approx.

[0010] 4 bar, utilizing two-stage compression with energy recovery via a turbine in the air system. High-pressure operation not only enables high-temperature operation but also improves the fuel efficiency of the fuel cell system through better humidification and a higher oxygen partial pressure in the fuel cell stack. At the system level, this efficiency gain can be maintained or even increased due to energy recovery.

[0011] Disclosure of the invention

[0012] Within the scope of the invention presented, a fuel cell system, an operating method for operating the fuel cell system, and a vehicle are presented. Further features and details of the invention emerge from the respective subclaims, the description, and the drawings. Features and details described in connection with the operating method according to the invention naturally also apply in connection with the fuel cell system according to the invention, the operating method according to the invention, or the vehicle according to the invention, and vice versa, so that with regard to the disclosure of the individual aspects of the invention, reference is always made to each other.

[0013] The invention presented serves in particular to provide a robust and fuel-efficient fuel cell system.

[0014] Thus, according to a first aspect of the invention presented, a fuel cell system for converting energy is presented.

[0015] The presented fuel cell system comprises a fuel cell stack and a computing unit, wherein the computing unit is configured to switch the fuel cell system into intermittent operation, wherein in the intermittent operation the fuel cell system is configured to repeatedly switch between a dry operation of the fuel cell stack and a humid operation of the fuel cell stack, and to set a reduced humidity in the fuel cell stack in the dry operation compared to the humid operation and to set a higher humidity in the fuel cell stack in the humid operation compared to the dry operation, and wherein the computing unit is configured to activate the intermittent operation only when a pressure of an air mass flow supplied to the fuel cell stack is above a predetermined pressure threshold value.

[0016] In the context of the invention presented, intermittent operation or a so-called “toggle mode” is to be understood as an operating state of a fuel cell system in which there is a repeated change between a dry operation of the fuel cell stack with reduced humidity and a humid operation of the fuel cell stack with increased humidity.

[0017] In the context of the invention presented, a computing unit is understood to mean a computer, a processor, a subprocessor, a control unit or any other programmable circuit.

[0018] To adjust the humidity in the fuel cell stack, a stoichiometry, for example, an air mass flow supplied to the fuel cell stack and / or a pressure of an air mass flow supplied to the fuel cell stack and / or external humidification can be adjusted. For example, in dry operation, the pressure of an air mass flow supplied to the fuel cell stack can be reduced and / or the air mass flow can be increased and / or external humidification, i.e., the introduction of moisture or water by a humidification system, can be reduced.

[0019] The fuel cell system according to the invention is based on the principle that the intermittent operation is only activated when a pressure of an air mass flow supplied to the fuel cell stack is above a predetermined pressure threshold value.

[0020] Since the intermittent operation provided according to the invention reliably prevents flooding of, for example, a cathode of a fuel cell stack, the intermittent operation significantly expands the operating window for high-pressure operation with high pressure or an air mass flow whose pressure lies above a predetermined pressure threshold, so that high-pressure operation can be set as quasi-stationary operation in the fuel cell system presented and is not merely reserved for extreme conditions. Intermittent operation is generally activated when a minimum gas velocity u is reached in humid operation. minThe water flow must be below the specified threshold for liquid water discharge to ensure a specified membrane moisture level. A brief violation of the specified stoichiometry limits is permissible during intermittent operation, as the fuel cells can store a significant amount of water. This means that the specified limits are alternately violated and maintained during intermittent operation.

[0021] It may be provided that the computing unit is configured to activate the intermittent operation only when a temperature of the fuel cell stack is below a predetermined temperature threshold.

[0022] Particularly at moderate temperatures below a predetermined temperature threshold of, for example, 85°C, preferably 75°C, the intermittent operation prevents flooding of the cathode of the presented fuel cell system, so that the intermittent operation can be activated particularly in this temperature range.

[0023] By specifying a pressure threshold specifically for a temperature in the fuel cell stack, permissible reaction conditions in the fuel cell stack can be specified for a given pressure to be provided, thus preventing a condition that could, for example, lead to flooding of the cathode, or preventing the activation of intermittent operation, which could, for example, lead to flooding of the cathode. Accordingly, the proposed fuel cell system can be switched to intermittent operation safely and without the risk of damage to the fuel cell system.

[0024] It can further be provided that the computing unit is configured to set a stoichiometry that is increased in dry operation compared to wet operation and a stoichiometry that is reduced in wet operation compared to dry operation.

[0025] In intermittent operation, the target activity of the fuel cell system can be reduced for a specified period of time to adjust the minimum gas velocity for liquid water discharge by increasing the stoichiometry in the fuel cell stack and / or reducing the pressure within the fuel cell stack (dry operation). After the specified period of time, the target activity can be increased again to ensure sufficient humidification of the membrane (wet operation).

[0026] It can further be provided that the computing unit is configured to activate the humid operation more frequently and / or for a longer period in the intermittent operation than the dry operation.

[0027] By keeping the operating point for dry operation shorter than the operating point for wet operation, flooding of the fuel cells is avoided and fuel cell aging is minimized. Furthermore, excessively dry membrane conditions are avoided and the efficiency of the fuel cell system is maximized. In particular, this enables the fuel cell system to achieve high-pressure capability even at moderate operating temperatures, thus expanding the operating range of the fuel cell system.

[0028] It can further be provided that the computing unit is configured to activate the humidity operation in the intermittent operation only for so long and / or so frequently that a liquid water content in the fuel cell stack is permanently below a maximum water content specified for full-load operation of the fuel cell stack.

[0029] A certain liquid water content, i.e., a certain volume fraction of liquid water, is permissible in the gas paths of a fuel cell system. However, if a maximum water content is exceeded, the supply of reactants to the fuel cell stack is restricted to such an extent that performance losses and even irreversible damage to the fuel cell system can occur.

[0030] It can further be provided that the computing unit is configured to set the fuel cell system in a high-pressure operation such that the fuel cell stack is operated with an air mass flow supplied to the fuel cell stack, the pressure of which is above the pressure threshold value and a temperature is set in the fuel cell stack which is below a predetermined temperature threshold value and to activate the intermittent operation while the high-pressure operation is active.

[0031] High pressure operation with activated intermittent operation allows high pressure operation to be activated quasi-stationary or for a very long time, especially under normal or non-extreme conditions.

[0032] Accordingly, it can be provided that the computing unit is configured to set quasi-stationary operating conditions in the fuel cell system during high-pressure operation.

[0033] Due to quasi-stationary operating conditions in the fuel cell system, the fuel cell system is operated in continuous operation or normal operation, so that the high-pressure operation can be active in normal operation or continuous operation of the fuel cell system.

[0034] It can further be provided that the computing unit is configured to activate the humidity operation in the intermittent operation only for so long and / or so frequently that a liquid water content in the fuel cell stack is permanently below a maximum water content specified for full-load operation of the fuel cell stack.

[0035] A certain liquid water content, i.e. a certain volume fraction of liquid water, is permissible in the gas paths of a fuel cell system. However, if a maximum water content is exceeded, the supply of reactants to the fuel cell stack is restricted to such an extent that power losses or even irreversible damage to the fuel cell system can occur. The maximum water content can, for example, be selected depending on a desired current strength. This means that at low current strengths, an effective area for gas transport by liquid water can be significantly reduced without causing damage, or at high current strengths, the gas paths are largely, possibly even almost completely, free of liquid water. Accordingly, the maximum water content can be selected to be higher for low current strengths than for high current strengths.

[0036] It may further be provided that the pressure threshold value is between 2.5 bar and 6 bar, in particular between 3.5 bar and 4.5 bar.

[0037] In particular, the pressure threshold value is selected in such a way that it requires operation with a two-stage compression device.

[0038] It can further be provided that the fuel cell system comprises a two-stage compression device for supplying air to the fuel cell stack and the computing unit is configured to activate the intermittent operation when a first stage and a second stage of the compression device act together to compress an air mass flow to be supplied to the fuel cell stack.

[0039] Since two-stage compression typically results in high-pressure operation, activation of two-stage compression can be used as an activation signal to activate intermittent operation.

[0040] According to a second aspect, the presented invention relates to an operating method for a fuel cell system, in particular for a fuel cell system according to the first aspect of the invention.

[0041] The proposed operating method comprises switching the fuel cell system to intermittent operation, wherein in the intermittent operation, switching is repeatedly carried out between a dry operation of the fuel cell stack and a humid operation of the fuel cell stack. In the dry operation, a reduced humidity is set in the fuel cell stack compared to the humid operation, and in the humid operation, a higher humidity is set in the fuel cell stack compared to the dry operation. The intermittent operation is only activated when a pressure of an air mass flow supplied to the fuel cell stack is above a predetermined pressure threshold. The same advantages apply to the operating method as have been described in detail above for the fuel cell system according to the first aspect of the invention.

[0042] According to a third aspect, the presented invention relates to a vehicle comprising an embodiment of the presented fuel cell system. The same advantages apply to the vehicle with a fuel cell system according to the first aspect of the invention as previously described in detail for the fuel cell system according to the first aspect of the invention.

[0043] Further advantages, features, and details of the invention will become apparent from the following description, which describes exemplary embodiments of the invention in detail with reference to the drawings. The features mentioned in the claims and in the description may be essential to the invention individually or in any combination.

[0044] They show:

[0045] Figure 1 is a schematic representation of a possible design of the presented fuel cell system,

[0046] Figure 2 shows a possible design of the presented operating procedure,

[0047] Figure 3 shows a possible design of the presented vehicle.

[0048] Figure 1 shows a fuel cell system 100. The fuel cell system 100 comprises a fuel cell stack 101 and a computing unit 103. The computing unit 103 is configured to switch the fuel cell system 100 into intermittent operation and, in the intermittent operation, to repeatedly switch between a dry operation of the fuel cell stack 101 and a wet operation of the fuel cell stack 101.

[0049] Furthermore, the computing unit 103 is configured to set a reduced humidity in the fuel cell stack 101 in the dry mode compared to the wet mode and to set an increased humidity in the fuel cell stack 101 in the wet mode compared to the dry mode and to activate the intermittent mode only when a pressure of an air mass flow supplied to the fuel cell stack 101 is above a predetermined pressure threshold value.

[0050] For example, the computing unit 103 is configured to activate the wet operation more frequently and / or for a longer period in the intermittent operation than the dry operation, so that flooding of fuel cells of the fuel cell stack 101 is avoided and efficiency, ie, energetic efficiency of the fuel cell system, is maximized.

[0051] Furthermore, the fuel cell system 100 comprises a user interface 105 through which the intermittent operation can be activated or deactivated.

[0052] Figure 2 shows the operating method 200 for the fuel cell system 100 according to Figure 1.

[0053] In a first activation step 201, a high-pressure operation is carried out to supply the fuel cell stack 101 with a high-pressure air mass flow through, for example, a two-stage compression device.

[0054] In a calibration step 203, a measured pressure of the air mass flow is compared with a predefined pressure threshold and, optionally, a temperature prevailing in the fuel cell stack 101 is compared with a predefined temperature threshold. If the measured pressure is above the pressure threshold and the temperature prevailing in the fuel cell stack 101 is below the temperature threshold, intermittent operation is activated in a second activation step 205.

[0055] Accordingly, a humidity operation 207 is initially set.

[0056] In a determination step 209, it is determined whether an activation condition for dry operation 211 is met, for example, by comparing a humidity measured in the fuel cell stack with a humidity threshold. If the measured humidity is above the humidity threshold, dry operation 211 is activated. Otherwise, wet operation remains activated.

[0057] In a termination step 213, the operating method 200 is terminated. The termination step 213 can be activated, for example, if the measured pressure falls below the pressure threshold or if a user sends a termination command via the user interface 105.

[0058] Figure 3 shows a vehicle 300. The vehicle 300 includes the fuel cell system 100 shown in Figure 1.

Claims

Claims 1. A fuel cell system (100) for converting energy, the fuel cell system (100) comprising: - a fuel cell stack (101), and - a computing unit (103), wherein the computing unit (103) is configured to switch the fuel cell system (100) into intermittent operation, wherein in the intermittent operation, the fuel cell system (100) is configured to repeatedly switch between a dry operation (211) of the fuel cell stack (101) and a humid operation (207) of the fuel cell stack (101), and to set a reduced humidity in the fuel cell stack (101) in the dry operation (211) compared to the humid operation (207) and to set an increased humidity in the fuel cell stack (101) in the humid operation (211) compared to the dry operation (207), and wherein the computing unit (103) is configured to activate the intermittent operation only when a pressure of an air mass flow supplied to the fuel cell stack (101) is above a predetermined pressure threshold value 2. Fuel cell system (100) according to claim 1, characterized in that the computing unit (103) is configured to activate the intermittent operation only when a temperature of the fuel cell stack (101) is below a predetermined temperature threshold value.

3. Fuel cell system (100) according to claim 1 or 2, characterized in that the computing unit (103) is configured to set an increased stoichiometry in the dry operation (211) compared to the wet operation (207) and a reduced stoichiometry in the wet operation (207) compared to the dry operation (211).

4. Fuel cell system (100) according to one of the preceding claims, characterized in that the computing unit (103) is configured to activate the wet operation (207) more frequently and / or for a longer time in the intermittent operation than the dry operation (211).

5. Fuel cell system (100) according to one of the preceding claims, characterized in that the computing unit (103) is configured to set the fuel cell system (100) in a high-pressure operation such that the fuel cell stack (101) is operated with an air mass flow supplied to the fuel cell stack (101), the pressure of which is above the pressure threshold value and a temperature is set in the fuel cell stack (101) which is below a predetermined temperature threshold value and to activate the intermittent operation while the high-pressure operation is active.

6. Fuel cell system (100) according to one of the preceding claims, characterized in that the computing unit (103) is configured to set quasi-stationary operating conditions in the fuel cell system (100) during high-pressure operation.

7. Fuel cell system (100) according to one of the preceding claims, characterized in that the computing unit (103) is configured to activate the humidification mode (207) in the intermittent operation only for so long and / or so frequently that a liquid water content in the fuel cell stack (101) is permanently below a maximum water content predetermined for full-load operation of the fuel cell stack (101).

8. Fuel cell system (100) according to claim 7, characterized in that the pressure threshold value is between 2.5 bar and 5 bar, in particular between 3.5 bar and 4.5 bar.

9. Fuel cell system (100) according to one of the preceding claims, characterized in that the fuel cell system (100) comprises a two-stage compression device for supplying air to the fuel cell stack (101) and the computing unit (103) is configured to activate the intermittent operation when a first stage and a second stage of the compression device act together to compress an air mass flow to be supplied to the fuel cell stack (101).

10. Operating method (200) for a fuel cell system, the operating method (200) comprising: - Switching the fuel cell system (100) into an intermittent mode of operation, wherein in the intermittent mode of operation, switching is repeatedly carried out between a dry mode (211) of the fuel cell stack (101) and a humid mode (207) of the fuel cell stack (101), wherein in the dry mode (211) a reduced humidity is set in the fuel cell stack (101) compared to the humid mode (207) and in the humid mode (207) a higher humidity is set in the fuel cell stack (101) compared to the dry mode (211), wherein the intermittent operation is activated only when a pressure of an air mass flow supplied to the fuel cell stack (101) is above a predetermined pressure threshold.

11. Vehicle (300), wherein the vehicle comprises a fuel cell system (100) according to one of claims 1 to 9.

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