Fuel cell system for a vehicle, method for monitoring a or the fuel cell system

The invention relates to a fuel cell system for a vehicle having a fuel cell arrangement with a plurality of fuel cells, with the fuel cell arrangement being designed to emit an output voltage, an output current and an output power during operation, and with each fuel cell being designed to emit a cell voltage during operation, having a vehicle drive in the form of a primary load, having a plurality of secondary loads, and having a control apparatus which is designed to control the primary load and the secondary loads, and to a corresponding method.

Fuel cell systems are used as mobile energy sources for vehicles and are a futuristic alternative to the conventional drive concepts using internal combustion engines. However, the implementation of this alternative leads to different requirements for matching to daily use. While conventional internal combustion engines can provide virtually the total maximum output power without delay after being started, the performance data of fuel cell systems is highly dependent on their operating conditions, for example, the pressure, temperature, etc. A further exacerbating factor is that a multiplicity of peripheral components are required to operate a fuel cell system, which have to condition the substances

used and which likewise must be supplied with power during operation. This wide range of requirements and constraints for operation of fuel cell systems means that particular effort must be devoted during development to the control and energy management of the fuel cell systems.

By way of example, the document PAJ 2005190967 (Publication Number) relates to a fuel cell system being controlled as a function of operating parameters. This document proposes a method for starting a fuel cell system and a corresponding fuel cell system, with the output current of the fuel cell system being controlled as a function of temperatures measured at various positions in the fuel cell system.

The invention is based on the object of proposing an apparatus and a method which implement an intelligent monitoring strategy for operation of the fuel cell system, even in particular operating conditions.

This object is achieved by a fuel cell system having the features of Claim 1 and by a method for monitoring a or the fuel cell system having the features of Claim 10. Preferred or advantageous embodiments of the invention are specified in the dependent claims, the following description and the attached figures.

For the purposes of the invention, a fuel cell system is proposed which is suitable and/or designed for integration in a vehicle. The fuel cell system comprises a fuel cell arrangement with a plurality of fuel cells, which are preferably organized to form fuel cell stacks, with the number of fuel cells in a fuel cell stack or in the fuel cell arrangement preferably being more than 100. In one preferred embodiment, the fuel cells have a PEM membrane and are

designed to create an electrochemical reaction between a fuel in the form of hydrogen with an oxidant in the form of environmental air, in order to obtain electrical energy from the chemical energy.

During operation, the electrical energy is emitted from the fuel cell arrangement as an output power at an output voltage and an output current. Each fuel cell emits a cell voltage in accordance with its polarization characteristic, with the totality of cell voltages, with fuel cells preferably connected in series, in total resulting in the output voltage of the fuel cell arrangement or stack.

The fuel cell system has a primary load which is in the form of a vehicle drive.

Furthermore, the fuel cell system has a plurality of secondary loads, which can be subdivided into at least two groups : a first group in this case relates to the peripheral components which are absolutely essential to allow operation of the fuel cell arrangement. By way of example, these peripheral components include an air compressor, fuel recirculation fan, cooling water pump and the like. Peripheral components such as these are also referred to as parasitic components or balance-of-plant (BOP) .

A second group comprises loads which can be switched on optionally, for example DC/DC converters for supplying a high-voltage rechargeable battery or capacitor, a DC/DC converter for supplying low-voltage components, variable loads (electrical heating device) and the like.

The fuel cell system has a control apparatus which is designed to control the primary load and the secondary loads. In particular, the control apparatus is designed for power distribution to the primary load and the secondary loads. In this case, the power can be distributed by switching the primary load and/or the second loads on and off in a stepped form and/or continuously variably.

The invention proposes that the control apparatus have programming and/or circuitry such that a monitoring circuit is provided in a special operating mode of the fuel cell system. For the purposes of the monitoring circuit, the secondary loads are switched on and/or off and controlled as a manipulated variable, to be precise such that the output voltage, as a reference variable, in the monitoring circuit is kept at a low voltage value which is formed by a cell voltage of the fuel cells of less than 0.45 V on average. In particular, the monitoring circuit may be in the form of a closed loop and/or an open loop. In modified embodiments, the monitoring circuit can also be implemented for the purposes of a neural network, fuzzy logic, adaptive regulators, etc.

The invention is in this case based on the idea that, during special operating modes of the fuel cell system, it may be technically important to keep the fuel cells at a low cell voltage. This situation occurs, for example, during so-called cold and/or freezing starting of the fuel cell system, when the operating temperature of the fuel cells is less than the normal operating temperature of about 80 ⁰ C, and in particular is less than 0°C. In this special operating mode, it is worth actively increasing the temperature of the fuel cells. It has been found that the self-heating of the fuel cells by heat losses is approximately inversely proportional to the cell voltage of the fuel cells. It is therefore advantageous for a

rapid heating process for the cell voltages of the fuel cells to be kept as low as possible. Cell voltages of less than 0.45 V, in particular in a range between 0.2 and 0.45 V, have been found to be advantageous in this case. In order to obtain this cell voltage and the low voltage value of the output voltage resulting from it, it is therefore proposed that the secondary loads be switched on in a dynamically stepped or infinitely variable form, and that the output voltage be reduced to the said low voltage value by increasing the output current.

In one preferred embodiment of the invention, the control apparatus is designed to keep the output voltage at a constant value as the low voltage value. This embodiment has the advantage that the operation of the individual fuel cells is not adversely affected by widely fluctuating cell voltages .

The special operating mode is preferably cold starting or freezing starting and/or is arranged for an instantaneous output power from the fuel cell arrangement, in which case, although the output power is greater than the necessary self- supply power for the first group of secondary loads, it is, however, preferably less than the maximum secondary load power.

The maximum secondary load power is characterized by the power value which can be achieved when all the available secondary loads are connected to the fuel cell arrangement and are being operated at maximum load. For an example of a fuel cell system whose normal output voltage is 250 V to 450 V, the output current may extend up to 200 Amperes without moving the vehicle, that is to say without current and power being emitted to the primary load. Analogously, a

maximum secondary load current is characterized by the current value which can be reached when all the available secondary loads are connected to the fuel cell arrangement and are being operated at maximum load at the low voltage value. The special operating mode occurs in this illustration when the output current is greater than the self-supply current for the peripheral components but is less than the maximum secondary load current in each case at the low voltage value.

In one preferred development of the invention, the control apparatus is designed such that the output voltage is greater than the low voltage value in a range between zero power or zero current, that is to say when the fuel cell system is being started, and a or the self-supply power or self-supply current. This development of the monitoring strategy provides for the output current or the power consumption to be increased in steps until the fuel cell arrangement is loaded such that the desired low voltage value occurs.

In a further optional development of the invention, the control apparatus is designed to monitor the stoichiometry of the oxidant-fuel ratio as a further manipulated variable in an output power range in which the output power is greater than the maximum secondary load power and/or is greater than the self-supply power, or a corresponding output-current range, with the stoichiometry preferably being set by an air compressor, fan or the like. The stoichiometry is actually the ratio of the amount of substance supplied and the amount of substance converted in the reaction in accordance with Faraday's Law, for a respective reaction partner. Both the oxidant and the fuel therefore each have their own stoichiometry values. However, in practice, the expression stoichiometry is very often used, as in the following text as

well, for the ratio of these two stoichiometry values. The "stoichiometry" (lambda) in the following text therefore refers to the ratio between the oxidant and the fuel, with stoichiometry of lambda = 1 defining equilibrium between these flows, a value of lambda < 1 indicating a "rich ratio" with excess fuel, and a value of lambda > 1 indicating a "weak ratio" with excess oxidant. The stoichiometry can be used on the one hand to control the output power of the fuel cells and on the other hand likewise to control the cell voltage, with values of lambda < 1 reducing the cell voltage, and values of lambda > 1 increasing the cell voltage.

The aim is therefore preferably for the manipulated variable "secondary loads switched on or off" and the manipulated variable "stoichiometry" to be controlled jointly in order to keep the output voltage value at the low voltage value.

In one possible embodiment of the invention, the control apparatus is designed to emit a driving enable signal as soon as the available output power is greater than the self- supply power plus a variable power margin. This embodiment allows the control apparatus to assign sufficient power to the primary load, with the power output to the secondary loads being reduced at the same time in order to keep the output voltage at or below the low voltage value. The power assigned to the primary load is at least sufficient for careful or emergency operation of the vehicle at reduced power. In this case, the control apparatus is preferably designed to treat the power drawn by the driving operation as a disturbance variable in the monitoring circuit.

A further subject matter of the invention relates to a method for monitoring a or the fuel cell system, in which case, in a special operating mode, for example during cold or freezing

starting, the output voltage of the fuel cell arrangement is kept at a low voltage value by switching secondary loads on or off, which low voltage value is less than the output voltage value which is formed by operation of the fuel cells in the fuel cell arrangement with a cell voltage of less than 0.45 V. This method therefore represents normal use of the fuel cell system just described.

Further features, advantages and effects of the invention will become evident from the following description and from the drawings of preferred exemplary embodiments, in which:

Figure 1 shows a schematic block diagram of a fuel cell system as a first exemplary embodiment of the invention;

Figure 2 shows a graph using the coordinate system representation, which illustrates the relationship between the output current and the output voltage of the fuel cell arrangement in Figure 1 at different temperatures, and

Figure 3 shows a graph using the same coordinate system representation as that in Figure 2, in order to illustrate the method according to the invention.

Figure 1 shows a fuel cell system 1 which has a fuel arrangement 2 with a multiplicity of fuel cells 3. The fuel cell system 1 is preferably integrated in a vehicle. In order to provide intelligent power management, the fuel cell system 1 has a control apparatus 4, which has a monitoring module 5 and a distribution module 6.

On the input side, the control apparatus 4 is connected to the power output 7 of the fuel cell arrangement 2. Furthermore, the control apparatus 4 optionally receives the

signal from a temperature sensor 8 which monitors the temperature T of the fuel cells 3. On the output side, the control apparatus 4 is connected to a primary load in the form of a drive motor 9 for the vehicle. Furthermore, the control apparatus 4 is connected on the output side to secondary loads 10, with a first group of the secondary loads being formed by peripheral components 11, also referred to as parasitic or BOP components, and a second group of secondary loads being formed by switchable loads 12.

The control apparatus 4 optionally has an output 13 which is connected for control purposes to an air compressor 14, with the air compressor 14 being designed to compress or to accelerate the oxidant for the fuel cell arrangement 2 and the fuel cells 3.

In the control apparatus 4, a voltage signal U is tapped off from the power output of the fuel cell arrangement 2 and is passed to the monitoring module 5.

The monitoring module 5 is designed to implement a monitoring strategy, in particular for cold or freezing starting of the fuel cell arrangement 2, and for this purpose receives as input variables the voltage signal U and, optionally, the temperature signal T from the temperature sensor 5. The temperature signal T is used to determine whether cold or freezing starting is occurring. If this is the case or if cold or freezing starting is defined in some other way, then a low voltage value U _nom is defined as the nominal value for the output voltage U, corresponding to an individual cell voltage in the fuel cells of between 0.2 and 0.45 V. A value U _nom = 150 V is assumed for the rest of the explanation in this example.

The monitoring strategy which is implemented by the monitoring module 5 uses as a manipulated variable the power distribution of the output power from the fuel cell arrangement 2 by the distribution module 6. The distribution module 6 is therefore designed to distribute the output power from the fuel cell arrangement 2 between the primary load 9, the secondary loads 10, in particular the peripheral components 11 and the switchable components 12. The power distribution can be implemented on the one hand as illustrated schematically in Figure 1, that is to say with the power being assigned to the loads by the distribution module 6. In alternative embodiments, the individual loads 7, 8, 9, 10 are addressed selectively by a control signal from the control apparatus 4 and increase or decrease their power demand in accordance with the control command.

The fundamental concept of the monitoring strategy is illustrated in Figure 2. Figure 2 shows a plurality of output current-output voltage-characteristics 15a to 15d for different temperatures, with the arrow 16 pointing in the direction of rising temperatures. The monitoring strategy of the monitoring module 6 and of the control apparatus 4 is illustrated in the form of an example by the line profile 17. When the fuel cell arrangement 2 is switched on, loads, in particular peripheral components 11, are initially connected in a ramped form, with the output voltage U initially rising from 0 V to more than 300 V, in order then subsequently to fall to the low voltage value U _nom = 150 V. Beyond this curve point, which is shown in the figure at about 70 Amperes, the distribution module 6 is controlled by the monitoring module 5 such that the power distribution and the process of switching loads on and off are set as a manipulated variable such that the output voltage U from then on corresponds to the low voltage value U _nom . At a current level of about

120 Amperes, an enable signal is emitted, such that the vehicle can be driven in a reduced-power conservation or emergency mode.

Figure 3 once again illustrates the exemplary embodiment of the method according to the invention, in which the peripheral components 11 are switched on successively in a first current range between 0 and 30 Amperes, in order to increase the current drawn, and therefore to reduce the output voltage U and, in the end, to reduce it to the low voltage value U _nom .

Beyond an output current of about 30 Amperes, the peripheral components 11 are supplied completely, so that this current level or power level represents the required self-supply power for the fuel cell arrangement 2.

In a range between 30 Amperes and 150 Amperes, switchable components 12, such as DC/DC converters for low-voltage applications or for an energy storage apparatus (not illustrated) are switched on successively so that the increasing power output from the fuel cell arrangement 2 is compensated for by the loads that are switched on, and the output voltage U is maintained at the low voltage value U _nom .

Beyond a value of about 150 Amperes, further variable loads, for example an electrical heater, must be switched on in order to limit the output voltage U at the low-voltage value ϋ _nom . As an alternative to this, the primary load 9 can also be switched on, in order to tap off power.

In this range, it is impossible or possible only with difficulty to reduce the output voltage U exclusively by switching on secondary loads 10. In order nevertheless to

ensure the low voltage value U _nom , the monitoring module 5 and the control apparatus 4 have the output interface 13 via which the air compressor 14 can be driven.

In this case, the drive for the air compressor 14 corresponds to a manipulated variable, in order to change the stoichiometry in the fuel cells 3. An air stoichiometry of 1.05 and 1.3 is normally chosen for normal operation at an output current of about 170 A. By reducing the air flow, the ratio between the fuel and oxidant is shifted in the direction of more fuel, which leads to a reduction in the maximum output current and the maximum output power and therefore - taking into account the loads that are switched on - to a reduction in the output voltage in the direction of the low voltage value U _nom .

Since the use of the manipulated variable "stoichiometry" for control purposes is considerably more inert than the use of the manipulated variable "secondary loads switched on or off", the manipulated variable "stoichiometry" is used as a macromonitoring variable, and the manipulated variable "secondary loads switched on or off" is used as a micromonitoring variable in the monitoring circuit. This results in an operating point being formed at about 170 Amperes, with one idea of the monitoring strategy being that the macrocontrol of the power generation is carried out by controlling the air compressor 10, and the microcontrol of the power generation and distribution is carried out by means of the distribution module 6.