Methods for operating a fuel cell system and fuel cell system

Abstract

Method for operating a fuel cell system (100) comprising at least one fuel cell (106) which is integrated into a coolant circuit (102) comprising a cooler (104) in which a coolant pump (108) is provided for circulating a coolant, and to which a bypass line (110) is assigned, into which at least a portion of the coolant flow can be introduced by means of a bypass valve (128), thereby reducing the portion of the coolant flow flowing through the cooler (104) by the portion flowing into the bypass line (110), comprising the steps: - Detecting the coolant pressure before an inlet (200, 204, 208) of at least one of the components (104, 106, 108) integrated into the coolant circuit (102), and - Controlling or regulating the bypass valve (128) depending on the pressure detected at the inlet (200, 204, 208) such that the pressure at the inlet (200, 204, 208) is kept below a predetermined or predeterminable limit pressure.

Description

[0001] The invention relates to a method for operating a fuel cell system, which comprises at least one fuel cell integrated into a coolant circuit including a cooler, in which a coolant pump is provided for circulating a coolant, and to which a bypass line is assigned, into which at least a portion of the coolant flow can be introduced by means of a bypass valve, thereby reducing the portion of the coolant flow flowing through the cooler by the portion flowing into the bypass line. The invention also relates to a fuel cell system.

[0002] Fuel cell systems are used to generate electrical energy through an electrochemical reaction. This reaction produces heat within the fuel cell, in addition to generating electrical energy and product water. The fuel cell operates most efficiently within a specific temperature range, and the resulting heat is dissipated via the coolant circuit through the cooler. Liquid-cooled units are typically cooled using air-to-water heat exchangers. The coolant is not routed through the cooler if the fuel cell or a fuel cell stack has not yet reached its operating temperature. In such cases, the coolant is routed through a bypass. For known systems, the sole criterion for determining whether or not the coolant is routed through the bypass is the temperature of the coolant or the fuel cell.Thermostatic valves were therefore most commonly used as bypass valves, enabling automatic opening or closing and thus diverting the coolant into the coolant bypass line. When controlling the fuel cell, the goal is to minimize the temperature difference between the inlet temperature and the outlet temperature of the fuel cell or fuel cell stack, i.e., to maintain a temperature as constant as possible for identical reaction conditions. This allows for a very high coolant flow rate, for example, 200 liters per minute (l / min). Consequently, the coolant can be subjected to a very high pressure of up to five bar, as is the case, for example, in internal combustion engines.

[0003] At such pressures, the fuel cell or fuel cell stack can leak, as it is less stable than, for example, an internal combustion engine.

[0004] US patent 2017 / 0 352 896 A1 describes a fuel cell system in which a thermostatic valve is used to direct the coolant flow through a bypass line. US patent 2004 / 0 001 985 A1 describes a pressure regulating valve installed upstream of the fuel cell stack to prevent coolant entering the fuel cell stack from being introduced at excessively high pressure.

[0005] It is therefore an object of the present invention to provide a method for operating a fuel cell system in which the control strategy for cooling the fuel cell effectively avoids damage. Furthermore, it is an object of the present invention to provide a corresponding fuel cell system.

[0006] The problem relating to the method is solved by a method having the features of claim 1. The problem relating to the fuel cell system is solved by a fuel cell system having the features of claim 9. Advantageous embodiments with expedient further developments of the invention are specified in the dependent claims.

[0007] The process is characterized in particular by the following steps: - Detecting the coolant pressure before it enters at least one of the components integrated into the coolant circuit, and - Controlling or regulating the bypass valve depending on the pressure detected at the inlet such that the pressure at the inlet is kept below a predetermined or predeterminable limit pressure.

[0008] This means that pressure is used as an input variable for controlling the bypass valve and thus for directing a portion of the coolant flow through the bypass line. Due to the bends or branches in the coolant circuits within the radiator, the greatest pressure drop of the coolant occurs here. This fact is therefore used to reduce the proportion of coolant flowing through the radiator by increasing the inlet pressure at the inlet of the relevant component. Such pressure-controlled regulation prevents pressures from exceeding the maximum permissible operating pressure of the respective component.

[0009] In this context, it has therefore proven advantageous if, upon reaching or exceeding the limit pressure, the bypass valve increases the proportion of coolant flow passing through the cooler, thereby reducing the inlet pressure. With this design, it is possible to achieve a lower pressure at the fuel cell stack inlet without reducing the speed of the coolant pump in the coolant circuit. The pressure reduction is achieved solely by the larger proportion of coolant passing through the cooler.

[0010] When the fuel cell system is used in a motor vehicle, it has proven advantageous to predict the pressure limit and / or to use a performance model. When the pressure limit is expected to be reached, the bypass valve increases the proportion of coolant flow passing through the cooler, thereby reducing the inlet pressure. The bypass valve settings can be determined, for example, using data from the fuel cell system's operating history or, in particular, using forecast data regarding expected temperatures or traffic conditions. Furthermore, it is possible to infer the likelihood of reaching the pressure limit from route data entered into the vehicle's navigation system, for instance, if uphill driving or driving under full load is anticipated.

[0011] To prevent damage to the fuel cell system or the fuel cell itself, it has proven advantageous to reduce the speed of the coolant pump when the pressure at the inlet has not dropped below the limit pressure, even if the coolant flow is completely routed through the cooler.

[0012] In this context, it has proven advantageous to monitor the pressure at the fuel cell inlet. The fuel cell or fuel cell stack is considered a particularly critical component because hydrogen flows through it, which could be the subject of an oxyhydrogen reaction. Therefore, it must be ensured that the fuel cell or fuel cell stack is sufficiently leak-proof and that the pressure at the fuel cell inlet does not exceed the limit pressure.

[0013] For reliable and continuous or pulsed adjustment of the bypass valve, it has proven advantageous to design the bypass valve as a rotary slide valve actuated by a server motor. This design allows for very precise switching positions of the bypass valve, enabling precise metering of the coolant flow through the bypass line and the cooler. The server motor can be controlled, for example, by a control unit.

[0014] It has proven advantageous if the pressure drop of the coolant flowing through the cooler is greater than the pressure drop of the coolant flowing through the fuel cell. This allows the pressure at the fuel cell inlet to be reliably controlled by appropriately adjusting the bypass valve position and thus the proportions of coolant flowing through the bypass line and coolant flowing through the cooler.

[0015] In this context, it has also proven advantageous if the pressure gradient of the coolant flow through a primary circuit of the coolant system bypassing the cooler is lower than the pressure gradient of the coolant flow through a secondary circuit of the coolant system enclosing the cooler. Thus, by partially routing the coolant through the cooler, the pressure can be significantly reduced, resulting in a reduced pressure at the fuel cell inlet as well.

[0016] The advantages of this method are particularly evident in a fuel cell system suitable for carrying out the process described above. This fuel cell system comprises at least one fuel cell or several fuel cells combined into a fuel cell stack. The fuel cell is integrated into a coolant circuit that includes a cooler and a first coolant pump for circulating the coolant. The coolant circuit also includes a bypass line into which at least a portion of the coolant flow can be diverted via a bypass valve, thus reducing the coolant flow through the cooler by the amount flowing into the bypass line. The fuel cell system also includes a control unit.It is characterized in particular by a pressure sensor at the inlet of a component of the fuel cell system for detecting the pressure of the coolant, wherein the control unit is designed to control or regulate the bypass valve in such a way as to control the pressure detected at the inlet, so that the pressure at the inlet is kept below a limit pressure.

[0017] By regulating the pressure of the coolant flow through the cooler and the complementary flow through the bypass line, the inlet pressure can be adjusted and, if necessary, reduced by increasing the amount of coolant flowing through the cooler. The cooler contains deflections in the coolant flow channels, resulting in a pressure drop. Therefore, the pressure at the fuel cell inlet is preferably monitored by a first pressure sensor to keep the measured pressure below the limit pressure.

[0018] The fuel cell system is therefore designed to be particularly reliable, although speed control of the coolant pump is no longer strictly necessary, but still possible. In this context, it has also proven advantageous that only a single coolant pump is integrated into the coolant circuit, thereby reducing the complexity of the overall system.

[0019] To precisely control the proportions of coolant flow through the bypass line and the radiator, it has proven advantageous for the bypass valve to be a rotary spool valve actuated by a servo motor. This servo motor is also controlled or regulated by the control unit depending on the pressure detected by the pressure sensor.

[0020] The features and combinations of features mentioned above in the description, as well as those subsequently mentioned in the figure description and / or shown in the figures alone, can be used not only in the combinations specified, but also in other combinations or on their own, without departing from the scope of the invention. Thus, embodiments that are not explicitly shown or explained in the figures, but which can be derived and generated from the explained embodiments by separate combinations of features, are also to be considered as encompassed and disclosed by the invention.

[0021] Further advantages, features and details of the invention will become apparent from the claims, the following description of preferred embodiments, and the drawings. These show: Fig. 1 a highly schematic representation of a fuel cell system comprising a coolant circuit, and Fig. 2 a position-dependent representation of the pressure (p in bar) distributed across the cooling circuit, which ranges from a minimum pressure (p min ) and a maximum pressure (p max ) lies.

[0022] In Fig. Figure 1 shows the part of a fuel cell system 100 necessary to explain the invention, wherein the fuel cell system 100 comprises a coolant circuit 102 in which a cooler 104 is integrated and which has a first coolant pump 108 for circulating a coolant. A bypass line 110 is associated with the coolant circuit 102, into which a portion of the coolant flow can be introduced by means of a bypass valve 128. A fuel cell 106, in particular a plurality of fuel cells 106 combined to form a fuel cell stack, is also integrated into the coolant circuit 102.

[0023] Each of the fuel cells 106 comprises an anode, a cathode, and a proton-conducting membrane separating the anode from the cathode. The membrane is formed from an ionomer, preferably a sulfonated tetrafluoroethylene polymer (PTFE) or a polymer of perfluorinated sulfonic acid (PFSA). Alternatively, the membrane can be formed as a sulfonated hydrocarbon membrane.

[0024] A catalyst may be added to the anodes and / or the cathodes, wherein the membrane is preferably coated on its first side and / or on its second side with a catalyst layer made of a precious metal or a mixture comprising precious metals such as platinum, palladium, ruthenium or the like, which serve as reaction accelerators in the reaction of the respective fuel cell 106.

[0025] Fuel (for example, hydrogen) can be supplied to the anode via an anode compartment. In a polymer electrolyte membrane fuel cell (PEM fuel cell), the fuel or fuel molecules are split into protons and electrons at the anode. The PEM allows the protons to pass through but is impermeable to the electrons. The reaction that occurs at the anode, for example, is: 2H₂ → 4H₂ + + 4e - (Oxidation / Electron Release). While the protons pass through the PEM to the cathode, the electrons are conducted to the cathode or an energy storage device via an external circuit. Cathode gas (for example, oxygen or oxygen-containing air) can be supplied to the cathode via a cathode compartment, so that the following reaction takes place on the cathode side: O₂ + 4H₂ + + 4e - → 2H2O (reduction / electron uptake).

[0026] To ensure ionic conductivity for hydrogen protons through the PEM, the presence of water molecules within the PEM is required. Therefore, the cathode gas is humidified before being supplied to the fuel cell to induce moisture saturation of the PEM.

[0027] Since several fuel cells 106 are combined in the fuel cell stack, a sufficiently large quantity of cathode gas must be supplied. Therefore, a compressor provides a large mass flow of cathode gas, whereby its temperature increases significantly as a result of compression. The conditioning of the cathode gas, i.e., its adjustment with respect to the parameters desired in the fuel cell stack, takes place in a charge air cooler and a humidifier.

[0028] In the Fig. Figure 1 shows, purely as an example, a single fuel cell 106 into which reactant is supplied so that the electrochemical reaction for the generation of electrical energy can take place in a controlled manner within the fuel cell 106. To regulate the temperature of the fuel cell 106, and in particular to dissipate the heat generated during the electrochemical reaction, the coolant circuit 102 with the cooler 104 is assigned to the fuel cell 106. The cooler 104 ensures that the coolant temperature at the inlet 204 of the fuel cell 106 is at the desired value. The coolant is heated as it passes through the fuel cell 106 or the fuel cell stack, resulting in a temperature increase for the coolant.Downstream of the fuel cell 106, the bypass line 110 branches off from the coolant circuit 102. This bypass line rejoins the coolant circuit 102 downstream of the cooler 104, upstream of a coolant pump 108, and thus upstream of the fuel cell 106. This creates two sub-circuits for the coolant flow: a small primary circuit 112 bypassing the cooler 104 and a large secondary circuit 114 enclosing the cooler 104. These sub-circuits divide the coolant flow into two complementary components. In other words, the portion of the coolant conveyed by the bypass line 110 is routed around the cooler 104 (bypass). By bypassing the cooler 104, the coolant receives no or only significantly reduced cooling.

[0029] In this case, pressure sensors are connected upstream and downstream of the components, with a first pressure sensor 116 integrated at the inlet 204 of the fuel cell 106 into the coolant circuit 102. Furthermore, a second pressure sensor 118 is connected downstream of the fuel cell 106 at the outlet 206 of the fuel cell 106 into the coolant circuit 102. Preferably, a third pressure sensor 120 is also connected at the inlet 208 of the cooler 104 into the coolant circuit 102, in particular into the secondary circuit 114. Downstream of the cooler 104, the secondary circuit 114, i.e., at the outlet 210 of the cooler 104, has a fourth pressure sensor 122. In addition, a fifth pressure sensor 124 is connected upstream of the coolant pump 108, which is positioned in particular at the inlet 200 of the coolant pump 108. Downstream of the coolant pump 108, i.e. at its outlet 202, a sixth pressure sensor 126 is integrated into the coolant circuit 102.The coolant pressure values ​​detected by the pressure sensors 116, 118, 120, 122, 124, 126 can be evaluated in a control unit of the fuel cell system 100 (not shown in detail). This control unit is designed to control or regulate the bypass valve 128, depending on the pressure detected at one of the inlets, such that the pressure present at the respective inlet is kept below a limit pressure, which is preferably no more than five bar. A different limit pressure can also be selected depending on the design of the individual components of the fuel cell system 100.

[0030] In order to be able to actuate the bypass valve 128 by controlling the control unit, it has proven advantageous if the bypass valve 128 is designed as a rotary slide valve that can be actuated by means of a server motor.

[0031] The following example explains how the bypass valve 128 is controlled or regulated to keep the pressure measured at the inlet 204 of the fuel cell 106 below a predetermined or predefinable limit pressure. For this purpose, reference is made to Fig. 2 referred to the pressure conditions within the coolant circuit 102 at the individual in Fig. The positions shown in Figure 1 are graphically represented. The lowest pressure (p) is usually found at the inlet 200 of the coolant pump 108. min ) of the coolant circuit 102. The coolant pump 108 builds up a delivery pressure such that downstream of the coolant pump 108, i.e. at its outlet 202, the highest pressure (p max ) of coolant circuit 102 is present. The highest pressure (p max ) is dependent on the lowest pressure (p min) is set, because the delivery pressure exerted by the coolant pump 108 is (mostly) constant. In other words: If there is a very low pressure at the inlet 200, then there is also a lower pressure at the outlet 202. If the pressure at the inlet 200 is higher, then the pressure at the outlet 202 also increases to the same extent. The highest pressure (p max ) is therefore the relative pressure with respect to the lowest pressure (p min ) to understand.

[0032] In the coolant lines, the coolant is subject to friction against the line walls, and various bends can also lead to pressure losses that occur in the individual components of the fuel cell system 100. Between the outlet 202 of the coolant pump 108 and the inlet 204 of the fuel cell 106, there is only a small pressure loss, as can be seen from Fig.This results in 2. Since the coolant is generally guided through channels in the bipolar plates of the fuel cell 106 and frequently undergoes deflections, a significant pressure drop of the coolant is observed, so that the pressure at the outlet 206 of the fuel cell 106 is considerably reduced. If the coolant flow is guided completely through the cooler 104, it can be seen that there is a slight pressure loss between the outlet 206 of the fuel cell 106 and the inlet 208 of the cooler 104 due to wall friction. Since the coolant within the cooler 104 is generally guided in a meandering pattern and undergoes many deflections, a very significant pressure loss is also observed between the inlet 208 and the outlet 210 of the cooler 104. The graph shows that the pressure loss of the coolant flowing through the cooler 104 is greater than the pressure loss of the coolant flowing through the fuel cell 106.This results in a lower pressure gradient of the coolant flow through the primary circuit 112 of the coolant circuit 102, which bypasses the cooler 104, than the pressure gradient of the coolant flow through the secondary circuit 114 of the coolant circuit 102, which includes the cooler 104. The present invention utilizes and incorporates this finding, enabling the pressure at the inlet 204 of the fuel cell 106 to be reduced by directing a larger proportion of the coolant flow through the secondary circuit 114, i.e., through the cooler 104. By means of suitable control or regulation, the pressure at the inlet 204 of the fuel cell 106 can thus be kept below the limit pressure.However, if the coolant flow is completely routed through the cooler 104 and the pressure at the inlet 204 of the fuel cell 106 is still at or above the limit pressure, the speed of the coolant pump 108 can also be reduced to lower the maximum pressure (p. max ), therefore to lower it due to the lower delivery pressure in order to avoid component damage.

[0033] The inventive method and the inventive fuel cell system 100 are characterized by a simplified design, reliably preventing leaks or damage to the components. Overall, the fuel cell system 100 is designed with less complexity. REFERENCE MARK LIST: 100 fuel cell systems 102 Coolant circuit 104 Radiators (Main Water Radiator) 106 Fuel cell 108 Coolant pump 110 Bypass line 112 Primary circuit (small circuit) 114 Secondary circulation (large circulation) 116 first pressure sensor 118 second pressure sensor 120 third pressure sensor 122 fourth pressure sensor 124 fifth pressure sensor 126 sixth pressure sensor 128 Bypass valve 200 Position (Pressure) before coolant pump 202 Position (pressure) after coolant pump 204 Position (pressure) at the fuel cell inlet 206 Position (pressure) at the fuel cell outlet 208 Position (pressure) at the cooler inlet 210 Position (pressure) at the cooler outlet 212 Position (pressure) before coolant pump

Claims

[1] Method for operating a fuel cell system (100) comprising at least one fuel cell (106) which is integrated into a coolant circuit (102) comprising a cooler (104) in which a coolant pump (108) is provided for circulating a coolant, and to which a bypass line (110) is assigned, into which at least a portion of the coolant flow can be introduced by means of a bypass valve (128), thereby reducing the portion of the coolant flow flowing through the cooler (104) by the portion flowing into the bypass line (110), comprising the steps: - Detecting the coolant pressure before an inlet (200, 204, 208) of at least one of the components (104, 106, 108) integrated into the coolant circuit (102), and - Controlling or regulating the bypass valve (128) depending on the pressure detected at the inlet (200, 204, 208) such that the pressure at the inlet (200, 204, 208) is kept below a predetermined or predeterminable limit pressure. [2] Method according to claim 1, characterized by , that when the limit pressure is reached or exceeded, the bypass valve (128) increases the proportion of the coolant flow passing through the cooler (104), thereby reducing the pressure at the inlet (200, 204, 208). [3] Method according to claim 1, characterized by , that when the limit pressure is expected to be reached, the bypass valve (128) increases the proportion of the coolant flow passing through the cooler (104), thereby reducing the pressure at the inlet (200, 204, 208). [4] Method according to any one of claims 1 to 3, characterized by, that the speed of the coolant pump (108) is reduced if the pressure at the inlet (200, 204, 208) has not dropped below the limit pressure, even if the coolant flow is completely directed through the radiator (104). [5] Method according to any one of claims 1 to 4, characterized by , that the pressure at the inlet (200) of the fuel cell (106) is measured. [6] Method according to any one of claims 1 to 5, characterized by , that the bypass valve (128) is designed as a rotary slide valve that can be actuated by means of a servo motor. [7] Method according to any one of claims 1 to 6, characterized by , that the pressure loss of the coolant flowing through the cooler (104) is greater than the pressure loss of the coolant flowing through the fuel cell (106). [8] Method according to any one of claims 1 to 7, characterized by, that a gradient of the coolant flow pressure via a primary circuit (112) of the coolant circuit (102) bypassing the cooler (104) is less than the gradient of the coolant flow pressure via a secondary circuit (114) of the coolant circuit (102) enclosing the cooler (104). [9] Fuel cell system (100) comprising at least one fuel cell (106) which is integrated into a coolant circuit (102) comprising a cooler (104) in which a coolant pump (108) is provided for circulating a coolant, and to which a bypass line (110) is assigned, into which at least a portion of the coolant flow can be introduced by means of a bypass valve (128), thereby reducing the portion of the coolant flow flowing through the cooler (104) by the portion flowing into the bypass line (110), and comprising a control unit, characterized by, that a pressure sensor (116, 120, 124) is provided at the inlet (204, 208, 200) of a component of the fuel cell system (100) for detecting the coolant pressure, and that the control unit is designed to control or regulate the bypass valve (128) as a function of the pressure detected at the inlet (204, 208, 200) such that the pressure at the inlet (204, 208, 200) is kept below a limit pressure. [10] Fuel cell system (100) according to claim 9, characterized by , that the bypass valve (128) is designed as a rotary slide valve that can be actuated by means of a servo motor.

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