Methods for controlling a fuel cell system and fuel cell system

Abstract

The invention presented relates to a method (100) for controlling a fuel cell system (200). The presented procedure (100) includes: - Determining (101) a risk of frost for the fuel cell system (200) based on an ambient temperature of the fuel cell system (200), and in the event that a risk of frost exists: - Activating (103) an exhaust gas recirculation valve (225) in a cathode exhaust gas recirculation path (205) of the fuel cell system (200) to direct a mass flow through the cathode exhaust gas recirculation path (205), wherein the mass flow consists at least partially of a cathode mass flow compressed by an air compressor (203, 227) of the fuel cell system (200).

Description

[0001] The presented invention relates to a method for controlling a fuel cell system and a fuel cell system according to the appended claims. State of the art

[0002] In fuel cell systems, the oxidizing agent oxygen from ambient air is usually used to react with hydrogen in the respective fuel cells to form water or water vapor and thus to deliver electrical power through electrochemical conversion.

[0003] For this purpose, exhaust gas recirculation systems in the cathode subsystem are known that redirect a portion of the cathode exhaust gas back into a cathode supply air path to achieve, for example, humidification of the fuel cells and / or oxygen depletion in the fuel cell stack and / or inerting of the cathode subsystem and / or homogenization of fluids and conditions of the membrane and gas diffusion layers in the fuel cell stack. Accordingly, an exhaust gas recirculation valve that controls a mass flow through a cathode exhaust gas recirculation path is activated only as needed, i.e., not continuously.

[0004] Due to its connection to an exhaust path, a cathode exhaust gas recirculation path is exposed to water ingress, for example, from condensate or entrained liquid water. Condensation and / or water accumulation can also occur. This can lead to ice formation in the cathode exhaust gas recirculation path at low ambient temperatures, which can, for example, result in restricted cross-sections or inoperable exhaust gas recirculation valves. Furthermore, damage to a seal on the exhaust gas recirculation valve or damage to an air compressor impeller due to flaking ice particles can occur. This affects various operating situations, such as start-up, stoppage, standby, or normal operation.

[0005] For example, when driving downhill, the fuel cell stack goes into standby mode, meaning it is shut down because there is no power demand. The air system usually continues to operate in the cathode bypass. The cathode exhaust gas recirculation path is not flowing through and can easily cool down. Disclosure of the invention

[0006] Within the scope of the invention presented, methods for controlling a fuel cell system and a fuel cell system 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 method according to the invention naturally also apply in connection with the fuel cell system 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.

[0007] The invention presented serves in particular to provide a robust fuel cell system.

[0008] Thus, according to a first aspect of the invention presented, a method for controlling a fuel cell system is presented.

[0009] The presented method comprises determining a risk of frost for the fuel cell system based on an ambient temperature of the fuel cell system, and in the event that a risk of frost exists, activating an exhaust gas recirculation valve in a cathode exhaust gas recirculation path of the fuel cell system to direct a mass flow through the cathode exhaust gas recirculation path, wherein the mass flow consists at least partially of a cathode mass flow compressed by an air compressor of the fuel cell system.

[0010] The presented method is based on the activation of an exhaust gas recirculation valve in order to avoid damage to the exhaust gas recirculation valve and / or other components of the fuel cell system and / or functional limitations of the exhaust gas recirculation.

[0011] To do this, the first step is to check whether ice-forming conditions are expected in the near future, for example by comparing an ambient temperature with a threshold value.

[0012] The ambient temperature can be determined, for example, using a temperature sensor for directly measuring the ambient temperature and / or another temperature sensor for measuring a reference temperature that indirectly depends on the ambient temperature. Of course, multiple values ​​or pieces of information, such as the time and / or date, can be combined to predict an expected ambient temperature, for example.

[0013] Alternatively or additionally, weather data from, for example, an external data source can be evaluated to determine the ambient temperature.

[0014] In addition, it can be checked whether humid conditions exist or whether condensation or water accumulation may have occurred previously, for example, by evaluating values ​​​​determined by a humidity sensor. If it is ensured that purely dry conditions exist, i.e., all surfaces are dry or there is no water accumulation, then the measure cannot be triggered in this case either, and the procedure cannot be executed.

[0015] In the event that ice-forming conditions are likely to occur soon or an ambient temperature is or is expected to be below a predetermined threshold, the exhaust gas recirculation valve is activated, ie, independent of a control or normal operation of the fuel cell stack, ie, independent of a demand-based activation.

[0016] Activating the exhaust gas recirculation valve causes liquid water to be removed from surfaces such as the exhaust gas recirculation valve, lines, and sensors, and / or from water accumulation, as well as reducing the humidity of the fluid in the recirculation path. This prevents the exhaust gas recirculation valve from being blocked by freezing water, and the exhaust gas recirculation valve seal is protected.

[0017] Secondly, activating the exhaust gas recirculation valve heats the cathode exhaust gas recirculation path by introducing a mass flow that consists at least partially of a cathode mass flow compressed and thus heated by an air compressor of the fuel cell system. Accordingly, the compressed cathode mass flow increases or maximizes the enthalpy of the mass flow flowing into the cathode exhaust gas recirculation path, so that it is heated or kept warm to prevent water from freezing in the cathode exhaust gas recirculation path.

[0018] Accordingly, the presented method is particularly suitable for operating points in which a fuel cell stack of the fuel cell system is deactivated or switched to standby mode and is reactivated promptly, such as when driving downhill without any power demand on the fuel cell system.

[0019] It can be provided that the mass flow is adjusted by means of a cathode bypass valve, which directs air compressed by an air compressor via the cathode bypass path into the exhaust gas recirculation path.

[0020] By introducing air compressed by an air compressor via the cathode bypass path into the exhaust gas recirculation path, the humidity of the fluid in the exhaust gas recirculation path is reduced because the drier air introduced can absorb water.

[0021] A cathode bypass valve, which directs a mass flow of compressed air by an air compressor past a fuel cell stack, can be used to direct a portion of the compressed air bypassing the fuel cell stack into the exhaust gas recirculation path. By using the portion of the compressed air bypassing the fuel cell stack to heat the exhaust gas recirculation path, the fuel cell stack can be operated independently of the heating process of the exhaust gas recirculation path.

[0022] It may further be provided that a speed of the air compressor is increased compared to a target speed specified by a current operating point of the fuel cell system in order to maximize the mass flow through the cathode exhaust gas recirculation path.

[0023] Increasing the mass flow through the cathode exhaust gas recirculation path results in increased water entrainment, so that the cathode exhaust gas recirculation path is dried by mechanical (i.e., fluidic or thermodynamic) water removal due to a two-phase flow. Furthermore, the enthalpy of the mass flow is increased, so that the cathode exhaust gas recirculation path is additionally heated.

[0024] It can further be provided that a rotational speed of the air compressor is increased compared to a target rotational speed predetermined by a current operating point of the fuel cell system in such a way that a predetermined mass flow flows through the cathode exhaust gas recirculation path and a fuel cell stack of the fuel cell system is supplied with an air quantity predetermined for the current operating point.

[0025] It may further be provided that the exhaust gas recirculation valve is activated continuously for a predetermined period of time or repeatedly with a predetermined frequency.

[0026] Continuous activation of the exhaust gas recirculation valve changes the flow conditions, allowing water to be discharged onto different surfaces and preventing the valve from freezing due to movement.

[0027] By repeatedly activating the exhaust gas recirculation valve at a predetermined frequency, a shaking or mechanical abrasive movement is achieved, which destroys or loosens any ice adhering to the exhaust gas recirculation valve or prevents ice formation at critical points.

[0028] It can further be provided that inlet and / or outlet valves of a fuel cell stack of the fuel cell system are opened or closed.

[0029] By closing the inlet and / or outlet valves of a fuel cell stack, the stack can be switched to standby mode and a mass flow through the cathode exhaust gas recirculation path can be maximized or the humidity in the cathode exhaust gas recirculation path can be reduced or minimized.

[0030] It may further be provided that a mass flow directed into the cathode exhaust gas recirculation path is heated.

[0031] Heating of a mass flow conducted into the cathode exhaust gas recirculation path by, for example, a heating element or reducing the cooling of intake air causes a particularly rapid or strong heating of the cathode exhaust gas recirculation path.

[0032] It can further be provided that the cathode exhaust gas recirculation path is flowed through by a cathode supply path in the direction of a cathode outlet.

[0033] By directing the mass flow counter to the direction intended during normal operation to heat the cathode exhaust gas recirculation path, moisture, water, and / or ice particles, in particular, can be effectively discharged to the environment via the exhaust gas path. For this purpose, for example, a mass flow provided by a first air compressor can be introduced into the cathode exhaust gas recirculation path, so that it is discharged into the exhaust gas path.

[0034] Accordingly, it can be provided that the fuel cell system comprises a first air compressor and a second air compressor arranged downstream of the first air compressor in the intake direction, and the mass flow flowing through the cathode exhaust gas recirculation path is provided past the second air compressor by the first air compressor and discharged into an exhaust gas path.

[0035] To introduce the mass flow into the cathode exhaust gas recirculation path against a flow direction intended during normal operation, for example, a cathode bypass valve, a cathode inlet valve and a cathode outlet valve of the fuel cell stack can be closed while the first air compressor is activated and the second air compressor is deactivated.

[0036] A key advantage of flow reversal is that water, moisture, and ice discharged from the cathode exhaust gas recirculation path are not discharged into the second air compressor and the respective lines downstream of the second air compressor, but instead flow directly into the exhaust gas path. This also provides component protection for the second air compressor and downstream components.

[0037] It can further be provided that the fuel cell system comprises a first fuel cell stack and a second fuel cell stack, wherein a cathode exhaust gas path of the first fuel cell stack is fluidly connected to a cathode inlet path of the second fuel cell stack, and a cathode exhaust gas path of the second fuel cell stack is fluidly connected to a cathode inlet path of the first fuel cell stack, and a portion of an exhaust gas of the first fuel cell stack is directed into the cathode inlet path of the second fuel cell stack and / or a portion of an exhaust gas of the second fuel cell stack is directed into the cathode inlet path of the first fuel cell stack.

[0038] In a system topology with multiple fuel cell stacks, fluid from a first fuel cell stack can be used to heat or dehumidify / dewater or dry the cathode exhaust gas recirculation path of another fuel cell stack that is, for example, switched to a standby mode.

[0039] It can further be provided that an air bearing path of the fuel cell system is flowed through with a mass flow, wherein the mass flow consists at least partially of the cathode mass flow compressed by the air compressor of the fuel cell system.

[0040] By flowing through an air bearing path of the fuel cell system with a mass flow that partially consists of the cathode mass flow compressed by the air compressor of the fuel cell system, the air bearing path is heated and the components exposed to the flow, especially the bearings, are protected from damage caused by icing. For this purpose, the mass flow can optionally be further heated and / or accelerated particularly strongly using additional elements, such as a heater, in order to mechanically (i.e., fluidically or thermodynamically) expel any water present in the air bearing path or to dry the air bearing path.

[0041] According to a second aspect, the presented invention relates to a fuel cell system for converting energy.

[0042] The presented fuel cell system comprises a number of fuel cell stacks, a number of air compressors, a number of cathode exhaust gas recirculation paths and a computing unit, wherein the computing unit is configured to carry out a possible embodiment of the presented method.

[0043] Advantages described in detail in the method for controlling a fuel cell system according to the first aspect of the invention equally apply to the fuel cell system for converting energy according to the second aspect of the invention.

[0044] 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.

[0045] They show: Fig. 1 a possible design of the presented procedure, Fig. 2 a possible design of the presented fuel cell system, Fig. 3 another possible design of the presented fuel cell system, Fig. 4 another possible design of the presented fuel cell system, Fig. 5 another possible design of the presented fuel cell system.

[0046] In Fig. 1 shows a method 100 for controlling a fuel cell system.

[0047] The presented method 100 comprises a determination step 101 in which a risk of frost for the fuel cell system is determined based on an ambient temperature of the fuel cell system.

[0048] For this purpose, for example, an outside temperature or an ambient temperature of the fuel cell system can be compared with a threshold value, such as 4°C, so that if the ambient temperature is lower than the threshold value, it can be assumed that a risk of frost is expected or is beginning to occur in the near future.

[0049] In the event that it is determined in the determination step 101 that a risk of frost will occur, the method 100 comprises an activation step 103 in which an exhaust gas recirculation valve in a cathode exhaust gas recirculation path of the fuel cell system is activated to direct a mass flow through the cathode exhaust gas recirculation path, wherein the mass flow consists at least partially of a cathode mass flow compressed by an air compressor of the fuel cell system.

[0050] In the event that the exhaust gas recirculation valve is occupied by a control operation of a fuel cell stack of the fuel cell system, i.e., for example, is open to humidify fuel cells of the fuel cell stack, the fuel cell stack can optionally be adjusted such that the exhaust gas recirculation valve is released and can be used to provide the mass flow into the cathode exhaust gas recirculation path.

[0051] In Fig. 2 shows a fuel cell system 200. The fuel cell system 200 includes a fuel cell stack 201, an air compressor 203, a cathode exhaust recirculation path 205, and a computing unit 207.

[0052] Furthermore, the fuel cell system 200 comprises a cathode supply air path 209 with a cathode inlet valve 211, a cathode exhaust air path 213 with a cathode outlet valve 215 and a cathode bypass path 217 with a cathode bypass valve 219.

[0053] The computing unit 207 is configured to carry out the method 100 according to Fig. 1 and to activate an exhaust gas recirculation valve 225 arranged in the cathode exhaust gas recirculation path 205 accordingly by controlling it continuously or at a predetermined frequency.

[0054] The mass flow through the cathode exhaust gas recirculation path 205 can be adjusted, for example, by means of the cathode bypass valve 219, possibly in combination with a speed or power of the air compressor 203.

[0055] A heat exchanger 221 can be used to adjust the temperature of an air flow flowing through the fuel cell system 200, in particular to cool it. Accordingly, by reducing the power of the heat exchanger 221, for example by throttling a coolant flow conducted through the heat exchanger 221, the temperature of a mass flow directed to the cathode exhaust gas recirculation path 205 can be increased.

[0056] Optionally, the fuel cell system 200 includes additional temperature control elements, such as heaters, which can be used to heat the mass flow to the cathode exhaust gas recirculation path 205.

[0057] In Fig. Figure 3 shows the fuel cell system 200 with an additional air compressor 227 and an additional heat exchanger 229. Additional temperature control elements can be provided optionally.

[0058] The further air compressor 227 can be used to flow through the cathode exhaust gas recirculation path 205 in the direction of the outlet valve 215 of the fuel cell stack 201, as indicated by arrow 235, so that the cathode supply air path 209 flows through it in the direction of the cathode exhaust air path 213, ie, opposite to a direction provided during normal operation of the fuel cell system 200.

[0059] In Fig. 4, the air compressor 203 is according to Fig. 2 and Fig. 3 in detail. Here, it can be seen that the air compressor 203 includes an air bearing path 237 through which an air flow is directed to cool moving parts of the air compressor 203, in particular to the bearings. The air flow is tempered, ie, cooled during normal operation, by means of a temperature control element 239.

[0060] By introducing a compressed mass flow from the air compressor 203 into the air bearing path 237, the air bearing path 237 is dehumidified and dewatered or dried.

[0061] In Fig. 5 shows the fuel cell system 200 with another fuel cell stack 241.

[0062] The cathode exhaust air path 213 of the fuel cell stack 201 is fluidly coupled to a cathode supply air path 243 of the further fuel cell stack 241 via a further cathode exhaust gas recirculation path 245.

[0063] A cathode exhaust air path 247 of the further fuel cell stack 241 is fluidly coupled to a cathode supply air path 209 of the fuel cell stack 201 via a cathode exhaust gas recirculation path 205.

[0064] Accordingly, depending on the pressure gradient, either the cathode exhaust gas recirculation path 205 or the cathode exhaust gas recirculation path 245 can be heated via exhaust gas discharged from the fuel cell stack 201 and / or dried with air via the cathode bypass and / or water / moisture can be removed from the respective cathode exhaust gas recirculation path.

Claims

[1] A method (100) for controlling a fuel cell system (200), the method (100) comprising: - determining (101) a risk of frost for the fuel cell system (200) based on an ambient temperature of the fuel cell system (200), and in the event that a risk of frost exists: - activating (103) an exhaust gas recirculation valve (225) in a cathode exhaust gas recirculation path (205) of the fuel cell system (200) in order to direct a mass flow through the cathode exhaust gas recirculation path (205), wherein the mass flow consists at least partially of a cathode mass flow compressed by an air compressor (203, 227) of the fuel cell system (200). [2] Method (100) according to claim 1, characterized by that the mass flow is adjusted by means of a cathode bypass valve (219) which directs air compressed by the air compressor (203, 227) via the cathode bypass path into the cathode exhaust gas recirculation path (205). [3] Method (100) according to claim 2, characterized by that a speed of the air compressor (203, 227) is increased compared to a target speed predetermined by a current operating point of the fuel cell system (200) in order to increase the mass flow through the cathode exhaust gas recirculation path (205). [4] Method (100) according to claim 2 or 3, characterized by that a speed of the air compressor (203, 227) is increased compared to a target speed predetermined by a current operating point of the fuel cell system (200) in such a way that a predetermined mass flow flows through the cathode exhaust gas recirculation path (205) and a fuel cell stack (201) of the fuel cell system (200) is supplied with an air quantity predetermined for the current operating point. [5] Method (100) according to one of the preceding claims, characterized bythat the exhaust gas recirculation valve (225) is activated continuously for a predetermined period of time or repeatedly at a predetermined frequency. [6] Method (100) according to one of the preceding claims, characterized by that inlet and / or outlet valves (211, 215) of a fuel cell stack (201) of the fuel cell system (200) are opened or closed. [7] Method (100) according to one of the preceding claims, characterized by that a mass flow directed into the cathode exhaust gas recirculation path (205) is heated. [8] Method (100) according to one of the preceding claims, characterized by that the cathode exhaust gas recirculation path (205) is flowed through by a cathode feed path (209) in the direction of a cathode outlet (213). [9] Method (100) according to one of the preceding claims, characterized bythat the fuel cell system (200) comprises a first air compressor (227) and a second air compressor (203) arranged downstream of the first air compressor (227) in the intake direction, and the mass flow flowing through the cathode exhaust gas recirculation path (205) is provided past the second air compressor (203) by the first air compressor (227) and discharged into an exhaust gas path. [10] Method (100) according to one of the preceding claims, characterized byin that the fuel cell system (200) comprises a first fuel cell stack (201) and a second fuel cell stack (241), wherein a cathode exhaust gas path (213) of the first fuel cell stack (201) is fluidly connected to a cathode inlet path (243) of the second fuel cell stack (241), and a cathode exhaust gas path (243) of the second fuel cell stack (241) is fluidly connected to a cathode inlet path (209) of the first fuel cell stack (201), and a portion of an exhaust gas of the first fuel cell stack (201) is guided into the cathode inlet path (243) of the second fuel cell stack (241) and / or a portion of an exhaust gas of the second fuel cell stack (241) is guided into the cathode inlet path (209) of the first fuel cell stack (201). [11] Method (100) according to one of the preceding claims, characterized bythat an air bearing path (237) of the fuel cell system (200) is flowed through with a mass flow, wherein the mass flow consists at least partially of the cathode mass flow compressed by the air compressor (203) of the fuel cell system (200). [12] Fuel cell system (200) for converting energy, the fuel cell system (200) comprising: - a number of fuel cell stacks (201, 241), - a number of air compressors (203, 227), - a number of cathode exhaust gas recirculation paths (205, 245), - a computing unit (207), wherein the computing unit (207) is configured to carry out a method (100) according to one of claims 1 to 11.

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