DE102023204047B4 - High-temperature fuel cell system and methods for its operation - Google Patents

Abstract

Method (100) for controlling the operation of a PEM fuel cell system (200; 300; 400) (PEMFCS), wherein the PEMFCS (200; 300; 400) comprises a set (205) of one or more PEM fuel cells (PEMFC), a compressor with one or more compressor stages (215, 220) for providing a pressurized gaseous oxidizer at a cathode side of the PEMFC (205), and a controller (230) for controlling the operation of the PEMFCS (200; 300; 400), wherein the method (100) comprises:
Control of the PEMFCS (200; 300; 400) by the controller (230) to cause the compressor to compress the gaseous oxidant and supply it to a cathode side of the PEMFC (205) during operation of the PEMFCS (200; 300; 400) depending on a target electricity output O _e of the PEMFCS (200; 300; 400), wherein the control of the PEMFCS (200; 300; 400) includes controlling the following parameters:
(a) a pressure p to which the oxidizing agent is compressed by the compressor,
(b) a flow rate r with which the compressed oxidant is supplied to the cathode side of the PEMFC (205) such that an increase in an operating temperature T of the PEMFC (205) is accompanied by a corresponding increase in the pressure p and a corresponding decrease in a stoichiometric ratio λ of the consumption of the oxidant in its electrochemical reaction with a fuel in the PEMFC.

Description

The present invention relates to the field of proton exchange membrane (PEM) fuel cell systems (PEMFCS) comprising one or more PEM fuel cells (PEMFCs). Typical applications for PEMFCS include mobile applications (e.g., for powering a vehicle such as a truck, bus, or car) and stationary applications (e.g., as a local power plant for one or more buildings, an industrial site, or a server farm). In particular, the invention relates to a method for automatically operating a PEMFCS and to a controller and a PEMFCS configured to operate according to such a method.

Currently known PEMFCS typically require a relatively low operating temperature. PEMFCS are temperature-limited due to two main effects: (i) High temperature reduces the concentration of the oxidizing agent and thus reduces thermal efficiency, and (ii) high temperature reduces membrane moisture, which further decreases thermal efficiency and can cause damage.

Therefore, known PEMFCS lose performance and are at risk of damage when the operating temperature rises above 80°C, especially in the case of PEMFCS for mobile applications. To keep a PEMFCS within such a temperature range, fairly powerful and therefore large cooling devices, such as radiators (e.g., in mobile applications) with sufficient cooling capacity (and thus size), are generally required. However, for a given PEMFCS application, there are typically clear limits to how much the cooling capacity can be increased. In a vehicle, for example, the available frontal area for placing a radiator as a cooling device for the PEMFCS is naturally limited (even in a large vehicle like a truck or bus).

On the other hand, there is a need to keep fuel costs low for operating the PEMFCS, and therefore a reduction in its fuel consumption (e.g., average fuel consumption or fuel consumption per output) is desirable. Such a reduction in fuel consumption could be achieved if the maximum operating temperature of a PEMFCS can be increased without compromising its efficiency and integrity. This is particularly relevant, for example, for high-duty vehicles (HDVs), such as long-haul trucks, which must be able to withstand heavy loads at high temperatures, e.g., in summer and in high-temperature and/or mountainous geographical regions.

US 2003/0072980 A1 This describes a method for operating a fuel cell system and a fuel cell system that can adjust the operating pressure of the fuel cell system to maximize efficiency. The method is based on the fact that, under certain operating conditions, appropriately adjusted operating pressures enable significantly more efficient system operation.

DE 10 2008 010 312 B4 describes the control of the relative humidity of a reactant stream in a fuel cell system and, in particular, the influencing of the operation of cathode components in such a way as to reduce the system-level performance losses associated with such an influencing of the airflow.

Conventional approaches to overcoming thermal limitations involve increasing the size of the fuel cell and reducing the area current density. However, this results in a larger footprint and typically higher costs for the PEMFCS (Power-to-Fuel Fuel Cell System). Particularly in cargo applications, such increased footprint can lead to a reduction in available cargo capacity in commercial vehicles (and thus to increased operating costs).

Accordingly, it is an object of the present invention to provide an improved way to extend the available range of operating temperatures of a PEMFCS, in which it can be operated efficiently and without damage, to higher temperatures.

One solution to this problem is provided by the teachings of the independent claims. Various preferred embodiments of the present solution are provided by the teachings of the dependent claims.

A first aspect of the present solution is directed to a method for controlling the operation of a PEM fuel cell system (PEMFCS), wherein the PEMFCS comprises a set of one or more PEM fuel cells (PEMFC), a compressor with one or more compressor stages for supplying a pressurized gaseous oxidant, such as oxygen ( _O₂ ), at a cathode side of the PEMFC, and a control system for controlling the operation of the PEMFCS. The PEMFC may, in particular, comprise a stack of a plurality of stacked and interconnected PEM fuel cells.

The method involves controlling the PEMFCS by the controller to cause the compressor to compress the gaseous oxidant and supply it to a cathode side of the PEMFC during operation of the PEMFCS depending on a target electricity output O _e of the PEMFCS.

1. (a) ein Drucks p, auf den das Oxidationsmittel durch den Verdichter verdichtet wird,
2. (b) eine Strömungsrate r, mit der das verdichtete Oxidationsmittel der Kathodenseite der PEMFC bereitgestellt wird

sodass eine Erhöhung der Betriebstemperatur T der PEMFC sowohl mit mindestens einer von einer entsprechenden Erhöhung des Drucks p als auch mit einer entsprechenden Verringerung eines stöchiometrischen Verhältnisses λ des Verbrauchs des Oxidationsmittels in seiner chemischen Reaktion mit einem Brennstoff, wie etwa Wasserstoff (H₂), in der PEMFC einhergeht.In particular, controlling the PEMFCS includes controlling the following parameters:

1. (a) a pressure p to which the oxidizing agent is compressed by the compressor,
2. (b) a flow rate r at which the compressed oxidant is supplied to the cathode side of the PEMFC

so that an increase in the operating temperature T of the PEMFC is accompanied by at least one corresponding increase in the pressure p as well as a corresponding decrease in the stoichiometric ratio λ of the consumption of the oxidant in its chemical reaction with a fuel, such as hydrogen (H ₂ ), in the PEMFC.

The term "operating temperature," as used herein, may refer in particular to the temperature of a coolant used for temperature control, especially cooling, of the PEMFC or parts thereof. Alternatively, the operating temperature may be determined, in particular, as a function of the coolant temperature (different from the identity function). The operating temperature may, in particular, be defined as an average, mean, or maximum temperature occurring in the PEMFC or its coolant during a considered operating period of the PEMFC.

The term ‘target electricity output’ (O _e ), as used herein, may refer in particular to an electrical output current or electrical output power that is to be supplied by the PEMFCS during its operation.

The term “stoichiometric ratio,” as used herein, refers to the ratio of the actual mass flow rate of a reagent in a chemical reaction to its mass consumption rate in the chemical reaction. In this case, the term “stoichiometric ratio” refers specifically to the ratio of the actual mass flow rate of the gaseous oxidant to its mass consumption rate in the chemical reaction with the fuel, e.g., hydrogen ( _H₂ ), in the PEMFC.

The terms “first”, “second”, “third”, and the like in the description and in the claims are used to distinguish between similar elements and not necessarily to describe a sequential or chronological order. It is understood that the terms used in this way are interchangeable under reasonable circumstances and that the embodiments of the present solution described herein are capable of operating in sequences other than those described or illustrated herein.

Unless the context otherwise requires, the term “comprehensive” or “including” or any variation thereof such as “includes” or “includes” or “includes” in the present description and claims does not exclude other elements or steps and is to be interpreted in an open, inclusive sense, i.e. as “including but not limited to”.

When an indefinite or definite article is used to refer to a singular noun, e.g., "ein" or "eine", "der/die/das", this includes a plural of that noun unless explicitly stated otherwise.

The use of the phrases "in some embodiments" or "in one embodiment" in the description does not necessarily refer to the same embodiment. Furthermore, the specific features, structures, or characteristics can be combined in any suitable way in one or more embodiments.

Furthermore, unless expressly stated otherwise, "or" refers to an inclusive or and not an exclusive or. For example, a condition A or B is satisfied by any of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present).

According to the method, an increase in the operating temperature T, whether actively controlled or simply a consequence of given circumstances such as the ambient temperature or a past or present electrical load on the PEMFCS, is accompanied by a corresponding increase in the (target) pressure p and a corresponding decrease in the stoichiometric (target) ratio λ. In this way, the thermal efficiency of the PEMFCS can be maintained or even improved despite the increase in the operating temperature T, since the change in pressure p and the stoichiometric ratio λ (and thus the local mole fraction x) of the PEMFC the concentration of the added oxidizing agent is also increased to such an extent that the natural negative effect of a mere temperature increase on the concentration of the oxidizing agent is compensated for or even reversed.

In particular, increasing the pressure p also has a negative effect on water evaporation and thus counteracts at least one of the following: a reduction in membrane moisture, a reduction in thermal efficiency and moisture-dependent damage to the membrane.

While an increase in pressure p naturally requires more compressor power without countermeasures and thus generates more heat, a reduction in the stoichiometric ratio λ allows a reduction in the power requirement of the compressor, so that an increase in pressure p can be achieved while mitigating a significant increase in the operating temperature T associated with it.

Accordingly, the method provides an improved way to extend the available range of operating temperatures of a PEMFCS to higher temperatures without compromising the thermal efficiency and integrity of the PEMFCS, including in particular the integrity of the membrane(s) of the PEMFC.

The following are preferred embodiments of the method of the first aspect, which can be combined arbitrarily with each other or with other aspects of the present solution, unless such a combination is expressly excluded or technically impossible.

In some embodiments, the PEMFCS is controlled by adjusting the parameters as a function of the target electrical output O _{_e} such that the ratio (p·x)/T is increased or at least maintained within a range M = ± 10%, where x is the molar concentration of the compressed oxidant in a returning stream at an outlet on the cathode side of the PEMFC, e.g., x = CO_₂ for air. Accordingly, these embodiments are based on using the ratio (p·x)/T, or an equivalent or corresponding quantity, as a control variable for the control system to regulate the operation of the PEMFCS, thus enabling an effective and efficient implementation of a control scheme to be carried out by the controller.

In some embodiments, controlling the PEMFCS involves controlling the parameters such that the oxidant molar concentration x of the compressed oxidant is reduced to a minimum oxidant molar concentration (x_{_min}) that depends on the target electrical output O_{_e}, where x is the oxidant molar concentration of the compressed oxidant in a returning flow at an outlet on the cathode side of the PEMFC. In this way, the compensation described above for the temperature-increasing effect of a pressure increase can be optimized by simultaneously reducing the stoichiometric ratio λ (and thus the oxidant molar concentration x). In particular, x _{_min} can be defined as dependent on an electrical output current of the PEMFCS or its associated current density. In the specific embodiments discussed further below with reference to the figures, the oxidant is air, and its molar concentration x is instead denoted as CO_₂ .

In some embodiments, at least one of the compressor stages incorporates a variable nozzle turbine (VNT), and controlling the pressure p involves changing the effective aspect ratio of the VNT. Thus, the VNT provides an effective and space-saving means of modifying the pressure p in a controlled manner. The turbine can, in particular, be a turbine on the exhaust side of an electric turbocharger of the at least one compressor stage.

In some embodiments, controlling the flow rate r involves changing the flow rate of at least one compressor stage. In particular, in the case of a centrifugal compressor stage, controlling the flow rate r can include controlling the rotational speed of the compressor stage. Consequently, both the pressure p and the flow rate r can be adjusted by (simply) controlling the operation of the compressor itself.

In some embodiments, parameter control involves reading a setpoint for each parameter to be controlled from one or more predetermined lookup tables, which together define such setpoints as a function of the target electrical output O_{_e} or at least one of these corresponding quantities. This is particularly useful in cases where performance (in terms of processing speed) is important and where one or more predefined setpoint functions can be used without compromising the effectiveness and/or efficiency of the process. These one or more setpoint functions, each containing one or more setpoints as a function of associated parameters, are then used to determine the setpoints. Specific inputs, such as the target electricity output O _{_e} or derived quantities, can thus be defined in one or more lookup tables, making the outputs of the setpoint functions available very quickly and reliably from these tables. Furthermore, lookup tables offer the advantage that even functions that cannot be defined (efficiently or at all) closed by a mathematical formula can still be easily implemented. This is particularly useful when the function to be represented in a lookup table has been derived experimentally, for example, by measuring a discrete set of a limited number of data points, rather than through some kind of mathematical calculation.

In some embodiments, the method further comprises determining the target electricity output _Oe , wherein determining _Oe includes at least one of the following: (i) measuring _Oe ; (ii) measuring at least one quantity corresponding to _Oe and deriving _Oe based on one or more such measured quantities; (iii) receiving information representing at least one quantity corresponding to _Oe from a PEMFCS-external information source and determining _Oe based on the received information. Thus, in cases (i) and (ii), the method, or in particular a PEM fuel cell system implementing the method, becomes largely autonomous, whereas in case (iii) it can utilize information gathered elsewhere, e.g., from a PEM fuel cell system. B. by one or more sensors provided elsewhere and possibly even (also) for other purposes in a system or device (e.g. vehicle) powered by the fuel cell system, so that in such a dual-use situation an increase in efficiency in terms of space and/or cost savings can be achieved.

In some embodiments, the PEMFCS is used to drive a vehicle motor, and the method for controlling the PEMFCS is applied during the operation of the electric vehicle motor, while it is at least partially driven by electricity generated by the PEMFCS. This is particularly useful for covering driving situations in which the power demand of the vehicle, including in particular its electric motor, reaches a very high level and is accompanied by fairly high temperatures. Such a situation could occur, for example, when the vehicle (e.g., a heavy-duty truck with a large load) is driving on a mountainous road at high ambient temperatures, e.g., in a hot desert environment (such as in the southwestern United States) or on a hot summer day.

A second aspect of the present solution relates to a PEM fuel cell system (PEMFCS) comprising: (i) a set of one or more PEM fuel cells (PEMFCs), (ii) a compressor with one or more compressor stages for supplying a pressurized gaseous oxidizer, such as oxygen, to a cathode side of the PEMFC, and (iii) a controller for controlling the operation of the PEMFCS. The controller is configured to control the PEMFCS according to the method of the first aspect, e.g., according to one or more of its embodiments described herein.

The PEM fuel can be supplied, in particular, as components of a fuel cell stack, in which they are arranged in a stacked configuration. A PEM fuel cell stack is a well-known concept for arranging several, often several hundred, PEM fuel cells in a very space-saving manner, e.g., in the field of fuel cells for automotive applications.

The term “controller,” as used herein, refers to any functional unit or set of functional units configured to perform the tasks assigned to the controller, in particular to control all or part of the operation of the PEMFCS. A controller may, in particular, comprise one or more physical devices. For example, one or more of these devices may each be implemented as a hardware circuit comprising custom VLSI circuits or gate arrays, commercially available semiconductors such as logic chips, transistors, or other discrete components. A device may also be implemented in programmable hardware, such as field-programmable gate arrays, programmable array logic, programmable logic devices, or the like. A controller may also be implemented, at least in part, in software for execution by various types of processors. An identified device with executable code may, for example, comprise one or more physical or logical blocks of computer instructions, which may be organized, for example, as an object, procedure, or function. Nevertheless, the executable files of an identified device need not be physically located together, but may comprise different instructions stored in different locations which, when logically connected, represent the device and fulfill its stated purpose. In fact A unit of executable code could be a single instruction or many instructions, and could even be distributed across several different code segments, between different programs, and across multiple storage media. Similarly, operational data in this text can be identified and illustrated within devices and can be embodied in any suitable form and organized within any suitable type of data structure. The operational data can be collected as a single data set or distributed across different locations, including different storage media, and can exist, at least in part, simply as electronic signals within a system or network.

The following are preferred embodiments of the PEMFCS of the second aspect, which can be combined arbitrarily with each other or with other aspects of the present solution, unless such a combination is expressly excluded or technically impossible.

In some embodiments, the PEMFCS further includes one or more temperature control devices (such as one or more heat sinks or other cooling devices, thermal heat pumps, or heaters) for adjusting the temperature of a coolant flowing through the PEMFC. In this way, the temperature of the PEMFC can be proactively influenced, particularly under the control of the controller, to help bring or maintain the PEMFC in an optimal operating state or range of operating states. Accordingly, at least one of the temperature control devices can be controllable by the controller to regulate the operating temperature of the PEMFC.

In some embodiments, at least one of the compressor stages has one or more pressure-changing devices for changing the pressure p under the control of the controller. In this way, the pressure p can be selectively brought to a desired, and in particular optimal, pressure level or maintained there, especially depending on the target electrical output O_{_e}, e.g., target output current.

In some embodiments, at least one of the pressure-changing devices incorporates a variable-geometry turbocharger (VNT) whose effective aspect ratio is controllable by the controller. In particular, the VNT can be used instead of, or in combination with, the pressure control valve discussed above to control the pressure p. A VNT is a highly effective pressure control device that is readily controllable using an appropriate control signal provided by the controller, possibly even via a single actuator.

In some embodiments, at least one of the compressor stages has an electric turbocharger that can be controlled by the control unit. This enables highly efficient (especially energy-efficient) pressurization of the oxidizing agent, e.g., air.

In particular, some of these embodiments of the electric turbocharger incorporate the VNT. This allows for a very compact yet highly efficient pressure changer. For example, the VNT can be part of, or define, a turbine side of the electric turbocharger.

In some embodiments, the PEM fuel cell system has a pressure control valve in a cathode-side outlet channel of the PEMFCS, particularly in an exhaust channel (e.g., for air) of the PEMFCS. Accordingly, the valve can be used to control the pressure p under the control of the controller.

In some embodiments, the PEM fuel cell system further comprises a set of one or more sensors for measuring one or more of the following quantities: (i) the operating temperature T of the PEMFCS (e.g., in the form of a coolant temperature for tempering the PEMFC), (ii) a flow rate Q of the oxidizer before its compression by the compressor; (iii) an inlet pressure P of the oxidizer before its compression by the compressor; (iv) an electrical current I generated by the PEMFC. The controller is configured to control the PEMFC based on at least one of these quantities as measured by the set of sensors. These embodiments are particularly useful for implementing the PEMFCS as a largely autonomous system capable of optimizing its operation under the control of the controller, with little or no need for sensor data to be generated elsewhere.

In some embodiments, the controller is configured to control the rotational speed of an actuator, e.g., a motor that drives at least one of the compressor stages. Since the pressure generated by each compressor stage is thus dependent on the rotational speed, the controller can therefore control the output pressure of each compressor stage.

In some embodiments, the PEMFCS further includes a humidifier configured to humidify the oxidizer after it has passed through at least one, e.g., all, of the compressor stages. Accordingly, the humidifier can be located, in particular, downstream of the compressor and upstream of the PEMFC. The role of the humidifier is, in particular, to maintain the moisture content of the oxidizer within a suitable range, since excessively low moisture can lead to an increase in the ohmic resistance of the PEMFC and thus to a reduced thermal efficiency.

This is particularly relevant at rising and high temperatures when the humidifier incorporates a gas-to-gas moisture exchanger. Such an exchanger directs the cathode process air across a gas-permeable membrane. Water vapor from the cathode outlet gas can diffuse across this membrane to humidify the cathode inlet gas. As the temperature at such a humidifier increases, the vapor concentration in the PEMFC outlet gas is typically reduced.

If the temperature becomes too high and the humidity of the cathode inlet gas consequently approaches a critically low level, the pressure p must be increased to compensate for this in order to maintain the thermal efficiency at a sufficiently high level and to avoid damage to the membrane.

A third aspect of the present solution is directed to a controller for controlling the operation of a PEM fuel cell system (PEMFCS), wherein the PEMFCS comprises a set of one or more PEM fuel cells (PEMFC), a compressor with one or more compressor stages for providing a pressurized gaseous oxidizer at the cathode side of the PEMFC, and wherein the controller is configured to control the PEMFCS according to the method of the first aspect.

A fourth aspect of the present solution is directed to a computer program, in particular a stored computer program or a non-volatile computer-readable storage medium, wherein the computer program contains instructions which, when executed on the controller of the PEM fuel cell system of the second aspect, cause the controller to carry out the method according to the first aspect.

The features and advantages explained with reference to the first aspect of the solution apply accordingly to the other aspects of the solution.

The method according to the first aspect is preferably configured to be executed by the control according to the second aspect or the PEMFCS according to the third aspect (in particular by its control), especially an embodiment thereof described herein.

The computer program (product) can be implemented, in particular, in the form of a data carrier on which one or more programs for carrying out the method are stored. Preferably, this is a data carrier such as a ROM or a flash memory module. This can be advantageous if the computer program product is to be traded as a standalone product, independent of the processor platform on which the one or more programs are to be executed. In another implementation, the computer program product is provided as a file on a data processing unit, in particular on a server, and can be downloaded via a data connection, e.g., the Internet or a dedicated data connection, such as a proprietary or local network.

The control system of the second aspect and/or the PEMFCS of the third aspect may accordingly include a program memory in which the computer program is stored. Alternatively, the system may also be configured to access a computer program that is externally available, for example, in one or more external storage devices or on one or more servers or other data processing units, via a communication link, in particular to exchange data that is used during the execution of the computer program or that represents outputs of the computer program.

BRIEF DESCRIPTION OF THE DRAWINGS

• 1A ein Blockdiagramm zeigt, das eine beispielhafte Ausführungsform des Verfahrens des ersten Aspekts veranschaulicht;
• 1B, 1C und 1D beispielhafte Ausführungsformen von Sollwert-Kennfeldern für drei der in dem Verfahren von 1A definierten Sollwerte zeigen;
• 2 ein Blockdiagramm ist, das eine erste Ausführungsform des PEMFCS veranschaulicht;
• 3 ein Blockdiagramm ist, das eine zweite Ausführungsform des PEMFCS veranschaulicht; und
• 4 ein Blockdiagramm ist, das eine dritte Ausführungsform des PEMFCS veranschaulicht.

Further advantages, features and applications of the present solution are provided in the following detailed description and the accompanying figures, wherein:

• 1A a block diagram illustrating an exemplary embodiment of the method of the first aspect;
• 1B , 1C and 1D Exemplary embodiments of setpoint characteristic maps for three of the methods described in the procedure by 1A show defined target values;
• 2 a block diagram illustrating a first embodiment of the PEMFCS;
• 3 a block diagram illustrating a second embodiment of the PEMFCS; and
• 4 a block diagram illustrating a third embodiment of the PEMFCS.

In the figures, the same reference symbols are used for identical or corresponding elements of the procedure described in this document and of the PEMFCS.

DETAILED DESCRIPTION OF EXECUTION FORMS

1A Figure 1 shows a block diagram illustrating an exemplary embodiment of a method 100 for controlling the operation of a PEM fuel cell system (PEMFCS). Various embodiments of such a PEMFCS are described in the following publications: 2 , 3 and 4 Illustrated. While the following describes procedure 100 with reference to the 1A to 1D in combination with the in 2 The illustrated PEMFCS 200 is discussed, but reference can be made to any of the other embodiments of the PEMFCS provided herein, e.g., any of the embodiments in the 3 and 4 , instead of the one from 2 Reference will be made to this.

Referring first to 2 The PEMFCS 200 illustrated therein comprises a set (PEMFC) 205, e.g., a fuel cell stack comprising one or more PEM fuel cells, a compressor with a (typically electric) first compressor stage 215, and a second compressor stage 220. The compressor is configured to receive a supply A _of a gaseous oxidant, such as air, at its inlet and to provide a pressurized flow AH of the oxidant _to a cathode side of the PEMFC 205 at its outlet. In the present example, the second compressor stage 220 comprises an electric turbocharger with a compressor wheel 220a and a turbine 220b. The turbocharger is a variable-geometry turbocharger (VNT) type, so that the effective aspect ratio of its turbine 220b can be varied. Accordingly, stages 215 and 220 in combination form a two-stage electric turbocharger (ETC) of the VNT type. The PEMFCS 200 further comprises a control unit 230 for controlling the operation of the PEMFCS 200. Such control includes, in particular, controlling the effective aspect ratio of the VNT turbine 220b and a rotational speed of at least the first stage 215 (and optionally also the second stage 220) via one or more actuators. Specifically, a control signal X can be used to control the effective aspect ratio of the VNT turbine 220b, and another control signal M can be used to control the respective rotational speed(s) of the compressor stage(s). Additionally, a flow rate sensor can be provided on the supply side of the PEMFCS 200 for measuring a flow rate Q of the incoming current A _of gaseous oxidizer.

Furthermore, the PEMFCS 200 also includes a temperature control device 210 (or is alternatively connected to a PEMFCS-external temperature control device). For example, particularly in the case of a PEMFCS for an automotive application (such as powering a truck), the temperature control device 210 may include a heat sink (e.g., the truck's main radiator). A coolant is circulated through the PEMFC 205, particularly through each of its fuel cells, in suitable channels and through the temperature control device. In particular, during operation of the PEMFCS 200, the flow C _in of cool coolant flows from the temperature control device 210 (in this case: cooling device, e.g., heat sink) to the PEMFC 205, where it absorbs heat generated in the fuel cells, and then returns in a reverse flow C _out to the temperature control device 210, thereby transferring heat from the PEMFC 205 to the temperature control device 210 for cooling.

A temperature sensor for measuring a temperature T of the PEMFC 205 is provided in thermal contact with the coolant, for example in thermal contact with the current C _in . In the latter case, the temperature T corresponds to a temperature at an outlet side of the PEMFC 205.

The PEMFC 205 features a fuel inlet (not shown) for receiving a fuel, such as molecular hydrogen ( _H₂ ), and delivering it to the anode side of the fuel cells. Additionally, the PEMFC 205 has another inlet for receiving a stream _of gaseous oxidant, such as air or a gas containing a higher percentage of oxygen ( _O₂ ). Inside the fuel cells, the fuel reacts chemically with the oxidant to generate electrical energy. When the PEMFC 205 is connected to an electrical circuit, such as an electric motor, an electric current I generated by the PEMFC 205 can flow and be measured by a suitable sensor, such as an ammeter.

At an outlet of the cathode-side PEMFC 205, a backflow AFC _{out of} the (unused) gaseous oxidizing agent can leave the PEMFC 205, and a suitable pressure A sensor is provided to _measure the pressure P of the AFC return flow.

The PEMFCS 200 can further include a humidifier 225. In particular, this humidifier can be arranged, as illustrated, such that both gas streams AFC _in and AFC _out pass through it. While the humidifier 225 humidifies the incoming gas stream AH _in from the compressor to ensure a sufficiently high humidity level in the membranes of the PEM fuel cells via the humidified gas stream AFC _in , it dehumidifies the outgoing gas stream AFC _out to capture moisture that is to be reused at the inlet side to humidify the gas stream AFC _in . In particular, the relative humidity (RH) of the gas stream AFC _out at the cathode-side outlet of the PEMFC 205 can be targeted to be close to 100%. The range can vary, in particular, from RH 80% to 120%. For the dehumidified outgoing gas stream AH _out , a much lower RH is targeted in order to have high efficiency of the humidifier 225 and to retain the moisture of the gas stream AH _out when it reaches and drives the turbine 220b and is finally expelled as exhaust gas stream A _out , low enough to reduce condensation.

Optionally, the temperature control device 210 can have a variable temperature control capability, such as a variable cooling capacity, which can be controlled by the controller 230 via a control signal R. In this way, the temperature T can be influenced via the control signal R. For example, if the measured value of T increases and approaches or even reaches a level that defines an upper temperature limit of an operating range of the PEMFC 205, the control signal R can be used to increase the cooling capacity of the temperature control device 210 in order to keep T below the temperature limit.

All measurements taken by the various sensors are provided to the controller 230 in the form of respective sensor data, and the controller 230 is configured, e.g., by means of one or more suitable computer programs, to process the sensor data in order to derive control signals for controlling the operation of the PEMFCS 200. In particular, such control signals include the control signal X and the control signal M to control the operation of the two-stage VNT turbocharger.

With renewed reference to 1A Procedure 100 for controlling the PEMFCS 200 is explained in more detail. The procedure involves receiving various inputs, including in particular the measured current or at least most recent values of the temperature T, the pressure P, and the flow rate Q.

Another input is a target electrical output O_{_e} . In this example, the target electrical output O_{_e} is a target electrical output current I to be provided by the PEMFCS 200, such as the current requested via the throttle control by a driver of a vehicle powered by the PEMFCS 200. Alternatively, the target electrical output O_{_e} can be set equal to a measured current that the PEMFC 205 can currently provide at the given flow Q. The controller 230 processes all these inputs to derive control signals for controlling the PEMFCS 200, causing it to approach and (if possible) reach the target electrical output O_{_e}. In particular, the fuel feed rate on the anode side could be controlled for this purpose. In addition, the controller 230 also controls, via the control signals X and M, a substance concentration and a feed rate (flow rate) of the oxidant at the cathode side of the PEMFC 205 for this purpose, as will be explained in more detail below.

The processing to be carried out by the control unit 230 includes the calculation of several setpoint values.

In particular, a first process 105 comprises determining a target pressure p of the gaseous oxidant as a function p(I,T) of the input variables I and T. The determined target pressure p defines a first setpoint S_{_p} and can, in particular, refer to the pressure in the outgoing stream AFC_{_out}, whereby the actual pressure P is also measured. A comparison 110 (e.g., by a comparator device in the control 230) between the setpoint S_{_p} and the measured value P is carried out in order to derive a further setpoint, namely a setpoint S_{_x} for influencing the actual pressure, in the present example by controlling the effective aspect ratio of the turbine 220b of the VNT-type ETC. The control signal X can thus be defined such that it is equal to the specified setpoint S _x or otherwise corresponds to it, so that applying the control signal X to the actuator(s) that influence the effective aspect ratio of the turbine 220b leads to achieving an actual effective aspect ratio as determined by the setpoint S _x .

1B This shows an exemplary pressure setpoint characteristic curve for determining the target pressure p as a function of temperature T and for three different values of current I for an exemplary fuel cell design. Accordingly, Cathode outlet pressure values p (and similar P) can exhibit the following properties: The cathode outlet pressure is higher than 100 kPa and lower than 350 kPa. The cathode outlet pressure is constant or increases with temperature T and current I. The variation of the cathode pressure with temperature T is not directly proportional to changes in gas density.

With renewed reference to 1A A further process, 115, involves determining a target molar concentration C _O2 of the gaseous oxidizing agent (in the present example, a concentration of O ₂ in air) as a function C _O2 (j) of the current density j = I/A, based on the input quantity I and the relevant area A of the current path for I. The determined target molar concentration C _O2 defines a further setpoint S _c and can, in particular, refer to the molar concentration C _O2 in the exiting stream AFC _out at the cathode outlet.

1C Figure 1 shows an exemplary target value map of the molar concentration for determining the target molar concentration C _O2 (j) for an exemplary fuel cell design. C _O2 (j) can, in particular, exhibit the following properties: C _O2 (j) is at least substantially proportional to the current density j for regions of the operating range where the Reynolds number Re of the cathode channel is greater than 80 (Re > 80) and/or the current density j is > 0.5 A/ ^cm² . The slope can, in particular, be in the range of 2 to 4 mol ^{m⁻³ cm² A⁻¹} .

With renewed reference to 1A Another process, 120, involves determining a stoichiometric target ratio λ for the consumption of the oxidant in its electrochemical reaction with a fuel in the PEMFC 205. The stoichiometric target ratio λ is determined as a function λ(p, T, _CO2 ), i.e., as a function of the target pressure p, the temperature T, and the molar concentration _CO2 (j) determined by process 115. In particular, λ can be calculated using the molar gas constant R and a fitting parameter K for an MEA-water transition as follows: $$\lambda =\frac{\left(2K-1\right)C_{O2}RT+P}{\left(P-C{}_{O2}RT\left(\mathrm{4,76}+\frac{K{\omega }_{in}}{\mathrm{0,21}}\right)\right)}$$ $${\omega }_{in}=\frac{{\eta }_{Befeuchter}RH\cdot P_{S\ddot{\text{a}}t.}\left(T\right)}{P-{\eta }_{Befeuchter}RH\cdot P_{S\ddot{\text{a}}t.}\left(T\right)}$$

The output of process 120 thus defines a further setpoint S _λ = λ and can in particular refer to the stoichiometric ratio in the incoming currents AH _in or AFC _in at the inlet of the cathode side.

1D This shows an exemplary setpoint characteristic map for the stoichiometric ratio for determining the target stoichiometric ratio S _λ = λ as a function of T and for three different values of the current I. Cathode stoichiometric values λ can, in particular, exhibit the following properties: λ is greater than 1 and decreases with increasing pressure and increasing temperature. It increases with the current I for a given temperature T.

With renewed reference to 1A Another process, 125, involves determining a target flow rate. $$r=\dot{m}=\frac{dm}{dt}=\frac{I}{4F}\frac{M_{Luft}}{\mathrm{0,21}}{N}_{Zellen}\lambda ,$$ where F is the Faraday constant, M _air is the molar mass of air, and N _cells is the number of cells in the fuel cell stack.

The specified target flow rate r, here specifically defined as mass flow rate ṁ, defines a further setpoint S _m and can in particular refer to a flow rate in the incoming stream AH _at the outlet of the compressor stage 220.

With renewed reference to 1A A further process 130 involves determining another setpoint S_{_M} , namely a target flow rate of the gaseous oxidant at the compressor outlet as a function S _{_M} (Q, S_{_m}) of the measured flow rate Q and the determined setpoint S_{_m} . Process 130 includes a comparison (e.g., by a comparator device in the controller 230) between the setpoint S_{_m} and the measured value Q in order to derive the setpoint S_{_M}. The control signal M can thus be defined as equal to the determined setpoint S_{_M} or otherwise corresponding to it, such that applying the control signal M to the actuator(s) (e.g., the compressor's electric drive(s)) results in achieving an actual flow rate AH _at the compressor outlet, as determined by the setpoint S_M.

1. (a) dem Druck p, auf den das Oxidationsmittel durch den Verdichter 215/220 verdichtet wird,
2. (b) einer Strömungsrate r (z. B. Massenströmungsrate, mit der das verdichtete Oxidationsmittel der Kathodenseite der PEMFC 205 zugeführt wird), und
3. (c) einer Betriebstemperatur T der PEMFC 205,

sodass eine Erhöhung der Betriebstemperatur T mit mindestens einer von einer entsprechenden Erhöhung des Drucks p und einer entsprechenden Verringerung eines stöchiometrischen Verhältnisses λ des Verbrauchs des Oxidationsmittels in seiner elektrochemischen Reaktion mit einem Brennstoff, wie etwa Wasserstoff (H₂), in der PEMFC 205 einhergeht.Accordingly, controlling the PEMFCS 200 using procedure 100 includes controlling at least two of the following parameters:

1. (a) the pressure p to which the oxidizing agent is compressed by the compressor 215/220,
2. (b) a flow rate r (e.g. mass flow rate) at which the compressed oxidizing (supplied via the cathode side of the PEMFC 205), and
3. (c) an operating temperature T of the PEMFC 205,

so that an increase in the operating temperature T is accompanied by at least one corresponding increase in the pressure p and a corresponding decrease in the stoichiometric ratio λ of the consumption of the oxidant in its electrochemical reaction with a fuel, such as hydrogen (H ₂ ), in the PEMFC 205.

3 A block diagram illustrates a second embodiment 300 of the PEMFCS. While it bears a strong resemblance to the first embodiment 200 (which is therefore not discussed again here), the following differences exist: The compressor 220 has an electric turbocharger (ETC) configured such that its compressor wheel 220a acts as a first compressor stage, and downstream of it, the turbine 220b acts as a second compressor stage (instead of its normal role as a turboturbine in the exhaust duct). The rotational speed of the compressor 220 is controlled by the controller via the control signal M. Furthermore, a pressure control valve 235 is arranged in the outgoing stream (exhaust duct) and configured to be controlled by the controller 230 via the control signal X to vary the actual pressure P towards the target pressure p.

4 Figure 1 is a block diagram illustrating a third embodiment 400 of the PEMFCS. While it also bears a strong resemblance to the first embodiment 200 (which is therefore not discussed again here), the following differences exist: The compressor 220 has an electric turbocharger (ETC) configured such that its compressor wheel 220a acts as a first compressor stage and the turbine 220b acts in its normal role as a turboturbine in the exhaust duct. The rotational speed of the compressor 220 is controlled by the controller via the control signal M. Furthermore, a pressure control valve 235 is arranged in the outgoing stream (exhaust duct) between the humidifier 225 and the turbine 220b and is configured to be controlled by the controller 230 via the control signal X to vary the actual pressure P towards the target pressure p.

While at least one exemplary embodiment of the present solution has been described above, it should be noted that a large number of variations exist. Furthermore, it is understood that the described exemplary embodiments merely illustrate non-limiting examples of how the present solution can be implemented, and that it is not intended to restrict the scope, application, or configuration of the devices and methods described herein. Rather, the foregoing description provides the person skilled in the art with designs for implementing at least one exemplary embodiment of the present solution, and it is understood that various modifications to the functionality and arrangement of the elements of the exemplary embodiment can be made without deviating from the subject matter defined by the appended claims.

REFERENCE MARK LIST

100

Procedures for controlling the operation of a PEMFCS

105

Process for determining a target pressure setpoint

110

Process for determining a setpoint for an actuator that influences the actual pressure

115

Process for determining a target substance concentration setpoint

120

Process for determining a stoichiometric target ratio

125

Process for determining a target flow rate

130

Process for determining a setpoint for an actuator that influences the actual flow rate

200

first embodiment of the PEMFCS

205

Set of PEM fuel cells (PEMFC)

210

Temperature control device, e.g., heat sink

215

first compressor stage

220

electric turbocharger (ETC), especially as a second compressor stage

220a

compressor wheel

220b

Turbine, e.g. VNT

225

humidifier

230

steering

235

Pressure control valve

300

second embodiment of the PEMFCS

400

third embodiment of the PEMFCS

A

Effective cross-sectional area for determining the current density j from I

Ain

Oxidizing medium flow from its inlet to the compressor

Out

Oxidizing agent stream (in the exhaust duct) from the PEMFCS

AFCin

Oxidizing agent flow from the humidifier to the PEMFC

AFCout

Oxidizing agent flow from the PEMFC to the humidifier

AHin

Oxidizing agent flow from the compressor to the humidifier

AHout

Oxidizing agent flow from the humidifier towards the exhaust duct

C

Constant in formula for stoichiometric ratio λ

Cin

Coolant flow from temperature control device to the PEMFC

Cout

Coolant flow from the PEMFC to the temperature control device

CO2

Molar concentration of the oxidizing agent

Humidifier

Humidifier vapor exchange efficiency

F

Faraday constant, F = 96495 C/mol

I

Target current (or measured current currently possible current)

I1,...,I3

Current values

j

Current density corresponding to current I

K

Adjustment parameters for an MEA water transition

λ

stoichiometric ratio

ṁ

Mass flow rate

M

Control signal for an actuator with an influence on the actual flow rate

N cells

Number of cells in the fuel cell stack

P

measured pressure

p

Target pressure

PSat.

Vapor saturation pressure

Q

measured flow rate of the oxidizing agent upstream of the compressor

r

Flow rate

Sp

Target value for target pressure p

Sx

Setpoint for actuator with influence on actual pressure

Sc

Target value for substance concentration

Sλ

Target value for stoichiometric ratio

Sm

Target value for mass flow rate

SM

Setpoint for actuator with influence on the actual flow rate

T

measured temperature

ωin

Humidity level at the fuel cell inlet

X

Control signal for an actuator with influence on the actual pressure

Claims (21)

Method (100) for controlling the operation of a PEM fuel cell system (200; 300; 400) (PEMFCS), wherein the PEMFCS (200; 300; 400) comprises a set (205) of one or more PEM fuel cells (PEMFC), a compressor with one or more compressor stages (215, 220) for supplying a pressurized gaseous oxidant to a cathode side of the PEMFC (205), and a controller (230) for controlling the operation of the PEMFCS (200; 300; 400), wherein the method (100) comprises: controlling the PEMFCS (200; 300; 400) by means of the controller (230) to cause the compressor to compress the gaseous oxidant and supply it to a cathode side of the PEMFC (205) during operation of the PEMFCS (200; 300; 400) provides power as a function of a target electrical output O_{_e} of the PEMFCS (200; 300; 400), wherein controlling the PEMFCS (200; 300; 400) includes controlling the following parameters: (a) a pressure p to which the oxidant is compressed by the compressor, (b) a flow rate r at which the compressed oxidant is supplied to the cathode side of the PEMFC (205), such that an increase in an operating temperature T of the PEMFC (205) is accompanied by a corresponding increase in the This is accompanied by a reduction in pressure p as well as a corresponding reduction in the stoichiometric ratio λ of the consumption of the oxidant in its electrochemical reaction with a fuel in the PEMFC. Procedure (100) according to Claim 1 , wherein the control of the PEMFCS (200; 300; 400) is the control of the parameters depending on the target electricity output O _e such that the ratio (p·x)/T is increased or kept at least within a range M = ± 10 %, where x is the oxidant molar concentration of the compressed oxidant in an outgoing backflow (AFCout) at an outlet of the cathode side of the PEMFC (205). Method (100) according to any of the preceding claims, wherein controlling the PEMFCS (200; 300; 400) comprises controlling the parameters such that the oxidant molar concentration x of the compressed oxidant is reduced to a minimum oxidant molar concentration which depends on the target electricity output O _e , wherein x is the oxidant molar concentration of the compressed oxidant in an outgoing backflow (AFCout) at an outlet of the cathode side of the PEMFC (205). Method (100) according to one of the preceding claims, wherein at least one of the compressor stages (220) comprises a variable geometry turbocharger (220b) (VNT) and controlling the pressure p comprises effecting a change in an effective aspect ratio of the VNT (220b). Method (100) according to one of the preceding claims, wherein controlling the flow rate r comprises effecting a change in the flow rate of at least one stage of the compressor. Method (100) according to any of the preceding claims, wherein controlling the parameters comprises reading a respective setpoint of each parameter to be controlled from one or more predetermined lookup tables which together define such setpoints depending on the target electricity output O _e or at least one of these corresponding quantities. Method (100) according to any of the preceding claims, further comprising determining the target electricity output _Oe , wherein the determination of _Oe comprises at least one of the following: - measuring _Oe ; - measuring at least one quantity corresponding to _Oe and deriving _Oe based on one or more such measured quantities; - receiving information representing at least one quantity corresponding to _Oe from a PEMFCS-external information source and determining _Oe based on the received information. Method (100) according to one of the preceding claims, wherein the PEMFCS (200; 300; 400) is used to drive an electric vehicle motor and the method (100) is used to control the PEMFCS (200; 300; 400) during the operation of the electric vehicle motor, while it is at least partially driven by electricity generated by the PEMFCS (200; 300; 400). Control unit (230) for controlling the operation of a PEM fuel cell system (200; 300; 400) (PEMFCS), wherein the PEMFCS (200; 300; 400) comprises a set (205) of one or more PEM fuel cells (PEMFC), a compressor with one or more compressor stages (215, 220) for providing a pressurized gaseous oxidant at a cathode side of the PEMFC (205), wherein the control unit (230) is configured to control the PEMFCS (200; 300; 400) according to the method (100) according to one of the preceding claims. PEM fuel cell system (200; 300; 400) (PEMFCS), comprising: a set (205) of one or more PEM fuel cells (PEMFC), a compressor with one or more compressor stages (215, 220) for supplying a pressurized gaseous oxidizer at a cathode side of the PEMFC (205), and a controller (230) for controlling operation of the PEMFCS (200; 300; 400); wherein the controller (230) is configured to operate the PEMFCS (200; 300; 400) according to the method (100) according to one of the Claims 1 until 8 to control. PEM fuel cell system (200; 300; 400) according to Claim 10 , further comprising one or more temperature control devices (210) for adjusting the temperature of a coolant flowing through the PEMFC (205). PEM fuel cell system (200; 300; 400) according to Claim 11 , wherein at least one of the temperature control devices (210) can be controlled by the controller (230) to control the operating temperature of the PEMFC (205). PEM fuel cell system (200) according to one of the Claims 10 until 12 , wherein one or more of the compressor stages (215, 220) have at least one pressure changing device for changing of the pressure p under control of the control (230). PEM fuel cell system (200) according to Claim 13 , wherein at least one of the pressure changing devices has variable geometry turbochargers (220b, VNT) whose effective aspect ratio can be controlled by the control (230). PEM fuel cell system (200; 300; 400) according to one of the Claims 10 until 14 , wherein at least one of the compressor stages (215, 220) has an electric turbocharger which is controllable by the control unit (230). PEM fuel cell system (200) according to Claim 14 and 15 , wherein the electric turbocharger has the VNT (220b). PEM fuel cell system (300; 400) according to Claim 10 , further comprising a pressure control valve (235) in a cathode-side outlet channel of the PEMFCS. PEM fuel cell system (200; 300; 400) according to one of the Claims 10 until 17 , further comprising a set of one or more sensors for measuring one or more of the following quantities: - the operating temperature T of the PEMFCS (200; 300; 400), - a flow rate Q of the oxidizing agent before its compression by the compressor; - an inlet pressure P of the oxidizing agent before its compression by the compressor, - an electric current generated by the PEMFC (205); wherein the controller (230) is configured to control the PEMFC (205) on the basis of at least one of these quantities as measured by the set of sensors. PEM fuel cell system (200; 300; 400) according to one of the Claims 10 until 18 , wherein the controller (230) is configured to control a speed of an actuator that controls at least one of the compressor stages (215, 220). PEM fuel cell system (200; 300; 400) according to one of the Claims 10 until 19 , further comprising a humidifier (225) configured to humidify the oxidizing agent after its compression by at least one of the compression stages (215, 220). Computer program, in particular a stored computer program or a non-volatile computer-readable storage medium, wherein the computer program contains instructions which, when executed on the controller (230) of the PEM fuel cell system (200; 300; 400) according to one of the Claims 10 until 20 cause the control (230) to execute the procedure (100) according to one of the Claims 1 until 8 to carry out.