DE102021115672A1 - Method for determining and adjusting voltage-current pairs of at least one fuel cell, fuel cell system and motor vehicle - Google Patents

Abstract

The invention relates to a method for determining and setting voltage-current pairs (U/I) of fuel cells in a fuel cell stack (104) in a fuel cell system (100) as a function of the electrical power (Pel) and/or thermal power requested by a control unit Power (Pth) at which the current (I) to be set for the required power is determined using a quotient from the sum of all required powers (Pel, Pth) and the product of the number of cells with a voltage (U), where the voltage ( U) is composed of the upper calorific value voltage (UOHS) and a correction term taking into account the boiling point curve of the water present in the fuel cell, which is dependent on a temperature sensor (120) measured mean coolant temperature between the inlet and the outlet of the fuel cell stack and one of one Pressure sensor (122) measured mean pressure value on the Cathode side of the fuel cell stack, and in which, after determining the current to be set (I), the voltage to be set (U) is set using a quotient of the electrical power (Pel) and the current to be set (I). The invention also relates to a fuel cell system (100) for carrying out the method and a motor vehicle with a fuel cell system (100).

Description

The invention relates to a method for determining and setting voltage-current pairs of fuel cells in a fuel cell stack in a fuel cell system as a function of the electrical power and/or thermal power requested by a control unit. The invention also relates to a fuel cell system for carrying out the method and a motor vehicle with such a fuel cell system.

Fuel cells use the chemical reaction of a fuel with oxygen to form water to generate electrical energy. For this purpose, fuel cells contain the so-called membrane electrode assembly (MEA for membrane electrode assembly) as a core component, which is a composite of a proton-conducting membrane and one electrode (anode and cathode) arranged on both sides of the membrane. In addition, gas diffusion layers (GDL) can be arranged on both sides of the membrane-electrode assembly on the sides of the electrodes facing away from the membrane. As a rule, the fuel cell is formed by a large number of MEAs arranged in a stack, the electrical power of which is added up. By converting chemical energy directly into electrical energy, fuel cells achieve improved efficiency compared to other electricity generators by bypassing the Carnot factor.

In order to increase the efficiency of the fuel cell system, it is common for the fuel cell or a fuel cell stack to request voltage-current pairs (operating points) that lie on an ideal voltage-current characteristic (V/I characteristic). Additional components are often used as power sinks and as heat sources in order to meet the requirements of a high-voltage system and the requirements for heat demand in the vehicle network. These additional components increase system complexity, manufacturing costs, and fuel consumption due to the individual efficiency penalties of each component.

From the DE 11 2010 005 600 T5 discloses a fuel cell system and a method in which a map of operating points previously created by means of a measurement is read out in order to heat to a predetermined temperature in a targeted manner as a function of the water content within the fuel cell stack. In this case, the heating output is regulated until a critical temperature is reached, so that no more frost can occur if such a fuel cell system is shut down prematurely. The map used at the operating points is initially very complex to determine with the help of a measuring device using an almost ideal fuel cell. Only when this map has been recorded and stored in the control unit can the method described in this document be run through in order to prevent the prevailing frost conditions from causing ice to form inside the fuel cell stack.

In the DE 10 2015 109 502 A1 a fuel cell system is described in which the stack is coupled to a current converter, the current converter requesting an increased current from the fuel cell stack, so that the individual fuel cells are operated with a reduced efficiency and are therefore subject to increased heat generation. In this way, a cold start procedure can be carried out in which operating points deviating from an ideal voltage-current characteristic are selected and requested.

From the JP 2010 103 066 A it is known to apply an alternating current to the fuel cell stack when the reaction spaces are blocked by ice. This alternating current can be used to identify whether and which fuel cells in the fuel cell stack are blocked by ice, and these can then be selectively heated, which shortens the frost start procedure. At the same time, the individual fuel cell can be operated with a reduced efficiency in order to promote additional heating.

Also in the U.S. 2016 / 0 372 768 A1 describes a method and a fuel cell system that can be started in freezing conditions. Here, too, the fuel cell is operated at reduced power in order to warm it up more quickly. Here, the solubility of water within the membrane electrode assembly is taken into account based on the ambient temperature and the prevailing water content in the membrane electrode assembly, which are known from a shutdown procedure run through before shutdown.

Proceeding from this, it is the object of the present invention to specify a method for determining and setting voltage-current pairs, in which a less complex construction of the fuel cell system results and which automatically adapts to the current operating situation of the fuel cell stack. Furthermore, it is the object of the present invention to specify a less complex fuel cell system for carrying out this method and a motor vehicle with such a fuel cell system.

These objects are achieved by a method having the features of claim 1, by a fuel cell system having the features of claim 9 and by a motor vehicle having the features of claim 10. Advantageous configurations with expedient developments of the invention are specified in the dependent claims.

The method according to the invention for determining and setting voltage-current pairs of fuel cells of a fuel cell stack in a fuel cell system takes place as a function of the electrical power and/or thermal power requested by a control unit. In the method, the current to be set for the required power is determined using a quotient from the sum of all required powers and the product of the number of cells with a voltage. This voltage is composed of the upper calorific value voltage and a correction term that takes into account the boiling point curve of the water present in the fuel cell, which is dependent on an average coolant temperature measured by a temperature sensor between the inlet and the outlet of the fuel cell stack and an average pressure value measured by a pressure sensor the cathode side of the fuel cell stack. In the method, after the current to be set has been determined, the voltage to be set is determined using a quotient formation of the electrical power and the current to be set, as previously specified or determined.

In this way, it is possible for a voltage-current pair to be determined as required during operation, even under dynamic load changes, without having to create a characteristic diagram that was expensive beforehand. Such a characteristic diagram for the selection of a suitable voltage-current pair (operating point) can therefore be dispensed with.

festgelegt wird, wobei I der elektrische Strom, P_el die angeforderte elektrische Leistung, P_th die angeforderte thermische Leistung, U_OHS die obere Heizwertspannung, T die mittlere Kühlmitteltemperatur, p der mittlere Luftdruck, z die Anzahl der übertragenen Elektronen, F die Faradaykonstante, ΔS die Entropieänderung und Zellanzahl die Anzahl der Zellen im Brennstoffzellenstapel.It has been found that for the modeling of the voltage-current pairs, not only the upper calorific value voltage should be taken into account, since this is based on the ideal case of the presence of only liquid water inside the fuel cell stack. However, because a vaporous portion of the water can also be present within the fuel cell stack - depending on pressure and temperature - it is advantageous to choose a modified approach for the voltage, which takes the Clausius-Clapeyron equation into account. For this reason, it is therefore advantageous if the current to be supplied with the equation $$I=\frac{P_{el}+P_{tH}}{\left(u_{OHS}-\left(-1\right)\cdot \left(\frac{ln\left(p\right)\cdot T\cdot R-T\cdot \Delta S}{\mathrm{e.g}\cdot f}\right)\right)\cdot Zellen\mathrm{e.g}aHl}$$

where I is the electrical current, P _el is the required electrical power, P _th is the required thermal power, U _OHS is the upper heating value voltage, T is the average coolant temperature, p is the average air pressure, z is the number of electrons transferred, F is the Faraday constant, ΔS the entropy change and cell number the number of cells in the fuel cell stack.

festgelegt werden, wobei U die Spannung, P_el die elektrische Leistung und I der festgelegt Strom ist.The total electrical power can then also be used to determine the voltage to be set. The voltage to be set can then preferably be calculated using the equation $$u=\frac{P_{el}}{I}$$

where U is the voltage, P _el is the electrical power and I is the specified current.

In order to carry out the method even more reliably, it is advantageous if a voltage converter is present and causes a current draw from the fuel cell stack that corresponds to the specified current in order to provide the required power, for which a stoichiometry of the air mass flow supplied to the cathode is between 0.9 and 1.1, in particular set or maintained by 1. This ensures that the current actually drawn from the fuel cell stack by the voltage converter corresponds to the calculated current value I.

With such an operating strategy, the efficiency of the fuel cell stack can be varied as desired (within the physical limits) in order to provide greater thermal output as required and thus enable rapid heating. This heating can take place in particular in different circles; Each circuit can (but does not have to) have its own control unit assigned to request electrical power and/or thermal power.

In this context, it is advantageous if there is a first circuit for providing power to an interior, a second circuit for providing power to a high-voltage battery, and if there is at least one further third circuit for providing power to a constituent of the fuel cell system. Each circuit can be assigned its own control unit, with this control unit then being different proportions of electrical power and thermal power can be provided.

Due to the operating strategy selected here, high-voltage heating devices that would be required, for example, for heating the fuel cells and/or the vehicle interior, can be omitted. For this purpose, for example, electrical power is required from the drive control unit and thermal power is required from the temperature control unit. For this purpose, the desired voltage-current pair is then determined, specifically depending on the respective proportions of the electrical power and the thermal power.

In this context, it is advantageous in favor of a frost start of the fuel cell system if the interior is provided with a proportion of electrical power and a proportion of thermal power, and if these proportions are reduced in favor of heating the fuel cell stack if it turns out that the ambient temperature or the stack temperature has dropped below a limit.

Analogously, there is the possibility that the high-voltage battery is provided with a proportion of electrical power and a proportion of thermal power, and that these proportions are reduced in favor of heating the fuel cell stack if the ambient temperature or the stack temperature falls below a limit value has sunk.

A proportion of electrical power and a proportion of thermal power can also be provided for those constituents that are present in at least the third circle, with these proportions being reduced in favor of heating the fuel cell stack if it turns out that the ambient temperature or the stack temperature is below has fallen to a limit.

The advantages, effects and advantageous configurations explained in connection with the method according to the invention apply in the same way to the fuel cell system according to the invention. This can also be operated with an optimized operating strategy and is less complex due to the fact that no additional HV heaters are used.

The advantages, advantageous effects and refinements described in connection with the fuel cell system according to the invention apply in the same way to a motor vehicle with such a fuel cell system, which participates in the low complexity of the fuel cell system.

The features and combinations of features mentioned above in the description and the features and combinations of features mentioned below in the description of the figures and/or shown alone in the figure can be used not only in the combination specified in each case, but also in other combinations or on their own, without going beyond the scope of the leave invention. Therefore, the invention also includes and discloses embodiments that are not explicitly shown or explained in the figure, but result from the explained embodiments and can be generated by separate combinations of features.

• 1 ein schematisch dargestelltes Brennstoffzellensystem.

Further advantages, features and details of the invention result from the claims, the following description of preferred embodiments and with reference to the drawings. It shows:

• 1 a schematically illustrated fuel cell system.

In 1 1 shows a fuel cell system 100 which comprises a fuel cell stack 104 having cathode compartments and anode compartments. The fuel cell stack 104 has a plurality of fuel cells connected in series.

Each of the fuel cells includes an anode and a cathode, and a proton conductive 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.

A catalyst can also be added to the anodes and/or the cathodes, the membranes preferably being coated on their first side and/or on their second side with a catalyst layer made of a noble metal or mixtures comprising noble metals such as platinum, palladium, ruthenium or the like , which serve as a reaction accelerator in the reaction of the respective fuel cell.

Fuel (for example hydrogen) is supplied to the anodes via the anode chambers within the fuel cell stack 104 . In a polymer electrolyte membrane fuel cell (PEM fuel cell), fuel or fuel molecules are split into protons and electrons at the anode. The membrane lets the protons (e.g. H ⁺ ) through, but is impermeable to the electrons (e). The following reaction takes place at the anode: 2H ₂ → 4H ⁺ + 4e ^- (oxidation/elec ron delivery). While the protons pass through the membrane 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 cathodes via the cathode chambers within the fuel cell stack 104, so that the following reaction takes place on the cathode side: O ₂ +4H ⁺ +4e- → 2H ₂ O (reduction/electron uptake).

The fuel cell stack 104 is assigned a cathode gas supply on the cathode side, which has a cathode supply line 103 fluidically connected to the cathode chambers on the stack entry side for supplying the cathode gas to the cathodes of the fuel cells of the fuel cell stack 104 . In the present case, a humidifier 102 is integrated into the cathode supply line 103 for preconditioning the cathode gas. The humidifier 102 is connected to the cathode chambers of the fuel cell stack 104 by its cathode-side outlet. Upstream of the humidifier 102 there is a compressor 112 which draws in the cathode gas in order to then make it available in compressed form via the cathode supply line 103 at a fresh gas inlet of the humidifier 102 . Since the cathode gas heats up very strongly during compression, it is already cooled down by means of a charge air cooler 105 before it enters the humidifier 102 in order to avoid damage. The cathode gas supply also includes a cathode exhaust gas line 106 for discharging cathode exhaust gas from the fuel cell stack 104 , the cathode exhaust gas line 106 also being fluidically connected to the cathode chambers of the fuel cell stack 104 . In the present case, the humidifier 102 is also integrated into the cathode exhaust gas line 106, so that in other words its cathode-side inlet is also connected to the cathode chambers via the cathode exhaust gas line 106, via which unreacted cathode gas or moist cathode exhaust gas is returned to the humidifier 102. The humidifier 102 is formed from a plurality of water vapor permeable membranes configured to extract moisture from the cathode exhaust gas and add it to the fresh cathode gas. The cathode off-gas line 6 continues after the humidifier 102, so that the (partially) dried cathode off-gas can be fed to further utilization or discharged to the environment.

The fuel cell system 100 is assigned a fuel supply on the anode side in order to supply the fuel to the anode chambers and thus to the anodes of the fuel cells present in the fuel cell stack 104 . The anode chambers are connected via an anode supply line 107 to a fuel reservoir 108 that provides the fuel. Fuel that has not reacted at the anodes can be fed back to the anode chambers via an anode recirculation line 109 . The anode recirculation line 109 thus opens out again into the anode feed line 107 upstream of the fuel cell stack 104 and forms an anode circuit with the part of the anode feed line 107 formed downstream of the outlet. In order to be able to circulate the anode waste gas by means of an adjustable supply of fresh fuel, an ejector 114 is integrated at the point of the mouth. A fuel actuator 110 can be arranged upstream of the ejector 114 for further regulation of the supply of fuel in the anode supply line 107 . This fuel actuator 110 is preferably formed as a pressure control valve. A heat exchanger 111 in the form of a recuperator for heating the fuel is provided upstream of the pressure control valve. The fuel cell system 100 also includes a control unit that is not shown in detail.

The fuel cell system is typically operated at an ideal voltage-current characteristic, situations can arise in which too much power is provided by the fuel cells from a primary drive, in particular an electric drive unit with an electric motor and / or the constituents, i.e a coolant pump, a cooler, the compressor 112, the humidifier 102 or the like. In addition, situations can arise in which the individual parts or components are already very hot and therefore cannot absorb any further thermal power, so that it is necessary to adjust the power both thermally and electrically.

festgelegt, wobei I der elektrische Strom, P_el die angeforderte elektrische Leistung, P_th die angeforderte thermische Leistung, U_OHS die obere Heizwertspannung, T die mittlere Kühlmitteltemperatur, p der mittlere Luftdruck, z die Anzahl der übertragenen Elektronen, F die Faradaykonstante, ΔS die Entropieänderung und Zellanzahl die Anzahl der Zellen im Brennstoffzellenstapel.The method according to the invention is explained in more detail below. Method for determining and adjusting voltage-current pairs (U/I) of fuel cells of a fuel cell stack 104 in a fuel cell system 100 as a function of the electrical power (P _el ) and/or thermal power (P _th ) requested by a control unit which the current (I) to be set for the required power (P _el , P _th ) is determined using a quotient formation from the sum of all required powers (P _el , P _th ) and the product of the number of cells with a voltage (U), where the voltage (U) is composed of the upper calorific value voltage (U _OHS ) and a correction term that takes into account the boiling point curve of the water present in the fuel cell, which is dependent on an average coolant temperature measured by a temperature sensor 120 between the inlet and the outlet of the fuel cell stack 104 and an average pressure value measured by a pressure sensor 122 on the cathode side of the fuel cell stack 104, and in which, after the determination of the current (I) to be set, the voltage (U) to be set is determined using a quotient formation of the electrical power (P _el ) and the current to be set ( i). The current to be supplied is calculated using the equation $$I=\frac{P_{el}+P_{tH}}{\left(u_{OHS}-\left(-1\right)\cdot \left(\frac{ln\left(p\right)\cdot T\cdot R-T\cdot \Delta S}{\mathrm{e.g}\cdot f}\right)\right)\cdot Zellen\mathrm{e.g}aHl}$$

where I is the electrical current, P _el is the required electrical power, P _th is the required thermal power, U _OHS is the upper heating value voltage, T is the average coolant temperature, p is the average air pressure, z is the number of electrons transferred, F is the Faraday constant, ΔS the entropy change and cell number the number of cells in the fuel cell stack.

Only the pressure p and the temperature T are recorded (continuously: continuously or clocked) in order to determine the voltage-current pair. All other variables result from information known approximately for fuel cell systems.

festgelegt, wobei U die Spannung, P_el die elektrische Leistung und 1 der festgelegt Strom ist.The voltage to be set is then calculated using the equation $$u=\frac{P_{el}}{I}$$

where U is the voltage, P _el is the electrical power and 1 is the specified current.

With this operating strategy, the efficiency of the fuel cell stack 104 can be varied as required (at least within the physical limits) in order to provide greater thermal power as required and thus rapid heating not only of the fuel cell stack 104, but also of circuits to be controlled via other control units (see below) to heat up. With this operating strategy, high-voltage heaters that would be required, for example, for heating up fuel cell system 100 and/or a vehicle interior, can also be omitted. For example, electrical power is required from the drive control unit and thermal power is required from the temperature control unit, so that the desired heating occurs. The equations already described above are used for this.

A voltage converter (DC/DC converter) is present in the fuel cell system 100, which causes a current draw from the fuel cell stack 104 which corresponds to the specified current (I) in order to provide the required power (P _el , P _th ), for which a stoichiometry of the air mass flow supplied to the cathode of between 0.9 and 1.1, in particular of 1, is set or maintained. In this way, the calculated value of the current essentially corresponds to the current actually drawn from the fuel cell stack 104 .

There are typically a number of circuits which require power, with the circuits being able to be assigned their own control devices which communicate with one another via a bus system which is known per se. In this case, for example, a first circuit can be present for providing power to a (vehicle) interior, with a second circuit also being present for providing power to a high-voltage battery, and with at least one further third circuit being present for providing power to one of the constituents 102, 105, 111, 112 , 114, 118 of the fuel cell system 100.

A proportion of electrical power (P _el ) and a proportion of thermal power (P _th ) are made available to the interior, and these proportions can be reduced in favor of heating the fuel cell stack 104 if the ambient temperature or the stack temperature falls below one limit has fallen.

Alternatively or additionally, the high-voltage battery is also provided with a proportion of electrical power (P _el ) and a proportion of thermal power (P _th ), these proportions can be reduced in favor of heating the fuel cell stack 104 if it turns out that the ambient temperature or the Stack temperature has dropped below a limit.

Alternatively or additionally, the third circuit with the at least one constituent 102, 105, 111, 112, 114, 118 is provided with a proportion of electrical power (P _el ) and a proportion of thermal power (P _th ), these proportions in favor of the Heating of the fuel cell stack 104 can be reduced if it turns out that the ambient temperature or the stack temperature has fallen below a limit.

The present invention reduces system complexity and fuel consumption, with heat being provided where it is needed when needed. A particularly rapid heating of the relevant constituents 102, 105, 111, 112, 114, 118 and/or the high-voltage battery (not shown in detail) and/or the fuel cell stack 104 is possible as a result. Due to the possibility of doing without additional heaters, the fuel cell sys System 100 and the motor vehicle according to the invention are also characterized by a reduction in their complexity.

Reference List

100

fuel cell system

102

humidifier (constituent)

103

cathode supply line

104

fuel cell stack

105

Intercooler (constituent)

106

cathode exhaust line

107

anode supply line

108

fuel storage

109

anode recirculation line

110

fuel actuator

111

heat exchanger (constituent)

112

compressor (constituent)

114

jet pump (constituent)

116

cooling circuit

118

Constituent of the cooling circuit (e.g. KM pump, cooler or the like)

120

temperature sensor

122

pressure sensor

QUOTES INCLUDED IN DESCRIPTION

This list of the documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• DE 112010005600 T5 [0004]
• DE 102015109502 A1 [0005]
• JP 2010103066 A [0006]
• US20160372768A1 [0007]

Claims (10)

Method for determining and setting voltage-current pairs (U/I) of fuel cells of a fuel cell stack (104) in a fuel cell system (100) as a function of the electrical power (P _el ) and/or thermal power (P _th ) for which the current (I) to be set for the required power (P _el , P _th ) is determined using a quotient formation from the sum of all required powers (P _el , P _th ) and the product of the number of cells with a voltage ( U), where the voltage (U) is made up of the upper calorific value voltage (U _OHS ) and a correction term which takes into account the boiling point curve of the water present in the fuel cell and which is dependent on an average coolant temperature measured by a temperature sensor (120) between the inlet and the outlet of the fuel cell stack and a mean pressure value measured by a pressure sensor (122) on the cat test side of the fuel cell stack, and in which, after the determination of the current (I) to be set, the voltage (U) to be set is determined using a quotient formation of the electrical power (P _el ) and the current (I) to be set. procedure after claim 1 , characterized in that the current to be supplied with the equation $$I=\frac{P_{el}+P_{tH}}{\left(u_{OHS}-\left(-1\right)\cdot \left(\frac{ln\left(p\right)\cdot T\cdot R-T\cdot \Delta S}{\mathrm{e.g}\cdot f}\right)\right)\cdot Zellen\mathrm{e.g}aHl}$$

is determined, where / is the electric current, P _el is the required electric power, P _th is the required thermal power, U _OHS is the upper heating value voltage, T is the average coolant temperature, p is the average air pressure, z is the number of electrons transferred, F is the Faraday constant, ΔS the entropy change and cell number the number of cells in the fuel cell stack. procedure after claim 1 or 2 , characterized in that the voltage to be set with the equation $$u=\frac{P_{el}}{I}$$

is specified, where U is the voltage, P _el is the electrical power and I is the specified current. Procedure according to one of Claims 1 until 3 , characterized in that a voltage converter is present and causes a current draw from the fuel cell stack (104) which corresponds to the specified current (I) in order to provide the required power (P _el , P _th ) for which a stoichiometry of the air mass flow supplied to the cathode of between 0.9 and 1.1, in particular of 1, is set or maintained. Procedure according to one of Claims 1 until 4 , characterized in that a first circuit is present for providing power to an interior, that a second circuit is present for providing power to a high-voltage battery, and that at least one further third circuit is present for providing power to a constituent (102, 105, 111, 112, 114, 118) of the fuel cell system (100). procedure after claim 5 , characterized in that the interior is provided with a proportion of electrical power (P _el ) and a proportion of thermal power (P _th ), and that these proportions are reduced in favor of heating the fuel cell stack (104) if it turns out that the Ambient temperature or the stack temperature has dropped below a limit. procedure after claim 5 or 6 , characterized in that the high-voltage battery is provided with a proportion of electrical power (P _el ) and a proportion of thermal power (P _th ), and that these proportions are reduced in favor of heating the fuel cell stack (104) if it turns out that the Ambient temperature or the stack temperature has dropped below a limit. Procedure according to one of Claims 5 until 7 That the third circuit with the at least one constituent (102, 105, 111, 112, 114, 118) is provided a proportion of electrical power (P _el ) and a proportion of thermal power (P _th ), and that these shares in favor the heating of the fuel cell stack (104) can be reduced if it turns out that the ambient temperature or the stack temperature has dropped below a limit value. Fuel cell system (100) with a for carrying out a method according to one of Claims 1 until 8th configured control unit. Motor vehicle with a fuel cell system (100). claim 9 .

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