Fuel cell system and method for operating a fuel cell system

Abstract

The disclosure relates to a fuel cell system comprising a fuel cell stack for providing an electrical power P.sub.stack depending on a power demand, at least one auxiliary unit for operating the fuel cell stack with an electrical power consumption P.sub.aux, at least one consumer with an electrical power request P.sub.use, and a control unit for regulating the power demand as well as a method for controlling such a fuel cell system. It is provided that the control unit is configured to selectively operate the fuel cell system in a first operating mode or in a second operating mode, whereby the fuel cell stack is turned off depending on the operating mode upon the falling below of an optimal efficiency degree operating point P(η.sub.max) of the fuel cell system or a minimum operating point P.sub.min of the fuel cell stack. In particular, at least one auxiliary unit is also turned off in the first operating mode, when the optimal efficiency degree operating point decreases.

Claims

1. A fuel cell system, comprising: a fuel cell stack for providing an electrical power P.sub.stack depending on a power demand; at least one auxiliary unit for operating the fuel cell stack with electrical power consumption P.sub.aux; at least one consumer with electrical power request P.sub.use; and a control unit for regulating the power demand, wherein the control unit is programmed to: operate the fuel cell system selectively in a high efficiency operating mode and in a high power range operating mode; increase the power request from the at least one consumer in the high efficiency operating mode responsive to the power demand falling below a first threshold power demand corresponding to an optimal efficiency degree operating point P(η.sub.max) of the fuel cell system, and increase the power request from the at least one consumer in the high power range operating mode responsive to the power demand falling below a second threshold power demand corresponding to a minimum operating point P.sub.min of the fuel cell stack, wherein the first threshold power demand is higher than the second threshold power demand.

2. The fuel cell system according to claim 1 wherein the control unit is further programmed to turn off at least one auxiliary unit in the high efficiency operating mode when the power demand falls below the first threshold power demand.

3. The fuel cell system according to claim 1 wherein the control unit is further programmed to determine, in the first operating mode, an electrical power requested by at least one consumer P.sub.use; and to increase the electrical power requested by at least one consumer, if the determined power request P.sub.use is lower than the net power provided by the fuel cell system P.sub.stack−P.sub.aux.

4. The fuel cell system according to claim 1 wherein the control unit is further programmed to require at least a minimum power demand from the fuel cell stack in the first operating mode so that the electrical power provided by the fuel cell stack P.sub.stack exceeds the electrical power consumed by at least one auxiliary unit P.sub.aux; and the fuel cell system is operated above the optimal efficiency operating point P(η.sub.max).

5. A vehicle with a fuel cell system according to claim 1 wherein it indicates at least one consumer of a traction motor.

6. The vehicle according to claim 5 wherein the control unit is further programmed to operate the fuel cell system depending on a manual input of a driver or based on a driver type detection either in the high efficiency operating mode or in the high power range operating mode.

7. A method for operating a fuel cell system, the fuel cell system including: a fuel cell stack for providing an electrical power P.sub.stack depending on a power demand; at least one auxiliary unit for operating the fuel cell stack with an electrical power consumption P.sub.aux; at least one consumer with an electrical power request P.sub.use; and a control unit for regulating the power demand; wherein the method comprises: selectively operating the fuel cell system in a high efficiency operating mode or in a high power range operating mode; increasing the power request from the at least one consumer in the high efficiency operating mode responsive to the power demand falling below a first threshold power demand corresponding to an optimal efficiency degree operating point P(η.sub.max) of the fuel cell system, and increasing the power request from the at least one consumer in the high power range operating mode responsive to the power demand falling below a second threshold power demand corresponding to a minimum operating point P.sub.min of the fuel cell stack, wherein the first threshold power demand is higher than the second threshold power demand.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) The disclosure is explained below in exemplary embodiments with reference to the associated drawings. The figures show:

(2) FIG. 1 is a schematic representation of a fuel cell system according to an embodiment;

(3) FIG. 2 is a schematic representation of a vehicle according to an embodiment; and

(4) FIG. 3 is current-voltage characteristic curve of a fuel cell stack (U.sub.stack) and efficiency degree curve of a fuel cell system (η.sub.system);

DETAILED DESCRIPTION

(5) FIG. 1 shows a fuel cell system, denoted with 100 overall, according to a preferred embodiment of the present disclosure. The fuel cell system 100 is part of a vehicle not shown in further detail, in particular of an electric vehicle, which comprises an electric traction motor, which is supplied with electrical energy by the respective fuel cell system 100.

(6) The fuel cell system 100 comprises as core components a fuel cell stack 10, which comprises a plurality of individual cells 11, which are arranged in the form of a stack and which are formed by alternately stacked membrane electrode assemblies (MEAs) 14 and bipolar plates 15 (see detailed view). Each individual cell 11 thus respectively comprises an MEA 14 with an ion-conductive polymer electrolyte membrane not shown in more detail here and catalytic electrodes arranged thereon on both sides. These electrodes catalyze the respective partial reaction of the fuel conversion. The anode and cathode electrodes are designed as coating on the membrane and comprise a catalytic material, such as platinum, which is provided on an electrically conductive substrate material, with a large specific surface, such as a carbon-based material.

(7) As shown in the detailed view of FIG. 1, an anode chamber 12 is formed between a bipolar plate 15 and the anode and the cathode chamber 13 is formed between the cathode and the next bipolar plate 15. The bipolar plates 15 serve to supply the operating media in the anode and cathode chambers 12, 13 and further establishes the electrical connection between the individual fuel cells 11. Optionally, gas diffusion layers can be arranged between the membrane electrode assemblies 14 and the bipolar plates 15.

(8) To supply the fuel cell stack 10 with the operating media, the 100 fuel cell systems comprise an anode supply 20, on the one hand and a cathode supply 30 on the other hand.

(9) The anode supply 20 of the 100 fuel cell system, shown in FIG. 1, comprises an anode supply path 21, which serves to supply an anode operating medium (the fuel), such as hydrogen, to the anode chambers 12 of the fuel cell stack 10. For this purpose, the anode supply paths 21 connects a fuel storage tank 23 to an anode inlet of the fuel cell stack 10. The anode supply 20 further comprises an anode exhaust gas path 22, which discharges the anode exhaust gas from the anode chambers 12 via an anode outlet of the fuel cell stack 10. The anode operating pressure on the anode sides 12 of the fuel cell stack 10 can be adjusted via an initial agent 24 in the anode supply path 21.

(10) In addition, the anode supply 20 of the fuel cell system shown in FIG. 1 comprises a recirculation line 25, which connects the anode exhaust gas path 22 to the anode supply path 21. The recirculation of fuel is usual, in order to return the fuel, which is in most cases used overstoichiometrically, to the fuel cell stack 10. In the recirculation line 25 a recirculation conveyor 26 is arranged, preferably a recirculation fan.

(11) The cathode supply 30 of the fuel cell system 100, shown in FIG. 1, comprises a cathode supply path 31, which supplies an oxygen-containing cathode operating medium, in particular, air taken in from the environment, to the cathode chambers 13 of the fuel cells stack 10. The cathode supply 30 further comprises a cathode exhaust gas path 32, which discharges the cathode exhaust gas (in particular the exhaust air) from the cathode chambers 13 of the fuel cell stack 10 and if necessary supplies it to an exhaust gas system, not shown. For conveying and compacting the cathode operating medium, a compressor 33 is arranged in the cathode supply path 31. In the exemplary embodiment shown, the compressor 33 is designed as a compressor 33, which is mainly driven by an electric motor 34 equipped with appropriate power electronics 35. The compressor 33 can further be auxiliary driven by a turbine 36 (if necessary with variable turbine geometry) arranged in the cathode exhaust gas path 32 via a common shaft, (not shown).

(12) The fuel cell system 100, shown in FIG. 1 further comprises a humidifier module 39. The humidifier module 39 is arranged in the cathode supply path 31, on the one hand, so that the cathode operating gas can flow through it. On the other hand, the arrangement in the cathode exhaust gas path 32 allows the cathode exhaust gas can flow through it. A humidifier 39 typically comprises a plurality of water vapor permeable membranes, which are designed to be either flat or in the form of hollow fibers. Thereby, the comparatively dry cathode operating gas (air) flows over one side of the membranes and the comparatively moist cathode exhaust gas (exhaust gas) flows over the other side. Driven by the higher partial pressure of the water vapor in the cathode exhaust gas, water vapors pass over the membrane into the cathode operating gas, which is moistened in this way. The cathode supply 30 further comprises a bypass line 37, which connects the cathode supply line 31 to the cathode exhaust gas line 32. An agent 38 arranged in the bypass line 37 serves to control the amount of the cathode operating medium surrounding the fuel cell stack 10.

(13) Different additional details of the anode and cathode supply 20, 30 are not shown in the simplified FIG. 1, for reasons of clarity. For example, a water separator can be installed in the anode and/or cathode exhaust gas path 22, 32 in order to condense and drain product water arising from the fuel cell reaction. Finally, the anode exhaust gas line 22 can merge into the cathode exhaust gas line 32 so that the anode exhaust gas and the cathode exhaust gas are discharged via a common exhaust gas system.

(14) The fuel cell system 100 further comprises a control unit 60, which requires a power demand from the fuel cell stack 10, and at least one consumer 44, 51 with the electrical power request P.sub.use. A detailed description of the function of the control unit 60 in connection with at least one consumer 44, 51 is given in the description of FIG. 2.

(15) FIG. 2 shows a vehicle, which is denoted with 200 overall and which comprises the fuel cell system 100, from FIG. 1, the electronic control unit 60 contained therein, an electrical power system 40, and a vehicle drive system 50. At least one consumer 44, 51 of the fuel cell system is in this case constituted by components of the vehicle.

(16) The electrical power system 40 comprises a voltage sensor 41 for detecting a voltage generated by the fuel cell stack 10, and a current sensor 42 for detecting a current generated by the fuel cell stack 10. The electrical power system 40 further comprises an energy storage unit 44, such as a high-voltage battery or a capacitor. In the power system 40 a converter 45 is further arranged, designed in triport topology (triport converter). The battery 44 is connected to the first side of the double DC/DC converter 45. All traction network components of the drive system 50 are connected to a second side of the converter 45, with a fixed voltage level. In the same or a similar manner, the auxiliary units of the fuel cell system itself, such as the electric motor 34 of the compressor 33 (see FIG. 1), or other electrical consumers of the vehicle, such as a compressor for an air-conditioning unit or the like, can be connected to the power network.

(17) The drive system 50 comprises an electric motor 51, which serves as traction motor of the vehicle 200. To this end, the electric motor 51 drives a drive axle 52 with drive wheels 53 arranged thereon. The traction motor 51 is connected via an inverter 43 to the electronic power system 40 of the fuel cell system 100 and constitutes the main electrical consumer of the system.

(18) The electronic control unit 60 controls the operation of the fuel cell system 100, in particular its anode and cathode supply 20, 30, its electrical power system 40 and the traction motor 51. For this purpose, the control unit 60 receives different input signals, such as the voltage U, detected using the voltage sensor 41, of the fuel cell 10, the current I, detected using the current sensor 42, of the fuel cell stack 10, the power P.sub.stack, resulting from the voltage U and the current I, of the fuel cell 10, information about the temperature T of the fuel cell stack 10, the p pressures in the anode and/or cathode chamber 12, 13, the charge state SOC of the energy storage unit 44, the n rotational speed of the traction motor 51, and other input variables. Alternatively, some of the aforementioned values, such as P.sub.stack, can also be determined in the control unit 60 itself. Further, the electrical power P.sub.use requested by the electrical consumers of the vehicle 200, in particular by the traction motor 51 and/or the energy storage unit 44 and the electrical power consumed by the auxiliary units of the fuel cell stack 10 P.sub.aux are received by the control unit 60. The requested electrical power P.sub.use can contain, as components, a traction power requested by the driver of the vehicle 200 P.sub.W and the power requested by an air-conditioning system. The variable P.sub.W is, in particular, detected via a pedal sensor from the force used to operate an accelerator pedal not shown here. The control unit 60, shown in FIG. 2, can also be provided in a vehicle 200 as a distributed control system, for example comprising a control subunit for the fuel cell system and an additional control subunit for the drive.

(19) Depending on the input variables, in particular from the power requested by the consumers P.sub.use and the power required for the auxiliary units P.sub.aux, the control unit 60 determines a power to be demanded from the fuel cell system P.sub.system and a power demand to be required, thus, from the fuel cell stack 10. From this, the control unit 60 determines the required mass currents or operating pressures of the anode and cathode operating medium, from calculations or appropriately stored characteristic diagrams and controls the operating medium supply of the fuel cell system, for example, via the electric motor 34 of the compressor 33, as well as the agents 24, 38, etc. of the fuel cell system 100. Further, the control unit 60 controls the inverter 43 in order to supply energy to the traction motor 51 as well as the converter 45 and possibly other converters in order to charge or discharge the energy storage unit 44 and to supply energy to the consumers connected to the power network.

(20) FIG. 3 shows an efficiency curve η.sub.system of a fuel cell system (100) according to the disclosure and a current-voltage characteristic curve U.sub.stack of the fuel cell stack (10) arranged therein.

(21) It is obvious from FIG. 3 that at a low load, i.e., in a low load range, of the fuel cell system, i.e., at a low current draw from the fuel cell system, a low power output of the fuel cell system or a low efficiency of the overall system is caused by a comparatively high consumption of the auxiliary units. An operating point A of the fuel cell stack is unambiguously determined by a point of the current-voltage characteristic curve U.sub.stack via P.sub.A=U.sub.A*I.sub.A. An operating point B of the fuel cell system is unambiguously determined by a point of the efficiency curve η.sub.B and by the voltage and current of the fuel cell stack at that point by P(η.sub.B)=η.sub.B*U.sub.B*I.sub.B. In FIG. 3 an upper limit voltage U.sub.max is further drawn in, whose exceeding of can lead to damage to the fuel cell as a result of a degradation of the catalytic material and, thus, to cell aging. This upper limit voltage is reached by the fuel cell stack at its minimum operating point P.sub.min, at which the fuel cell stack outputs a current I.sub.min=P.sub.min/U.sub.max. Further, an optimal efficiency operating point of the fuel cell system P(η.sub.max)=η.sub.max*I(η.sub.max)*U(η.sub.max) is drawn in, which corresponds to a local maximum η.sub.max of the efficiency degree of the fuel cell system.

(22) According to the disclosure, the fuel cell stack is turned off in a first operating mode of the fuel cell system, when the optimal efficiency operating point P(η.sub.max) decreases so that the shaded area (low load range) is not available to the fuel cell system and the fuel cell stack, in the first operating mode. In a second operating mode of the fuel cell system, according to the disclosure, the fuel cell system is not turned off, until the minimum operating point P.sub.min decreases, to prevent the maximum cell voltage U.sub.max from being exceeded. Upon reaching the minimum operating point or the limit voltage U.sub.max, the fuel cell stack is either completely turned off or alternatively placed into a standby mode. In the standby mode, the auxiliary units of the fuel cell stack continue to operate so that the fuel cell stack can be quickly started up again from the standby mode.

(23) Preferably, the transition into the standby mode takes place by interrupting the air supply into the cathode chambers upon reaching the minimum operating point and by the oxygen present in the cathode chambers reacting with the fuel (hydrogen), which continues to be supplied. In the meantime, an additional electrical power discharge from the stack takes place, until the chemical reaction stops. In doing so, the discharge process is controlled via a voltage-dependent discharge current. As a result of the oxygen under supply, the discharge current is reduced when the voltage is constant.

(24) Based on the efficiency curve, shown in FIG. 3, and the displayed current-voltage characteristic curve, different operating states of a vehicle 200 with a fuel cell system 100, an energy storage unit 44, and an electric traction motor 51 will now be explained. Thus, in particular, it is addressed how the operating states differ depending on whether the fuel cell system is operated in the first operating mode or in the second operating mode. The control unit 60 is configured to perform these operating states in the first operating mode and in the second operating mode. The vehicle 200 has the operating states fuel cell operation, boost operation, battery operation, and recuperation operation, without being limited to them.

(25) In fuel cell operation, the traction motor 51 is used to drive the vehicle 200 and the required power is provided solely by the fuel cell stack 100. In the first operating mode, the fuel cell stack 100 is operated above the optimal efficiency operating point P(η.sub.max). As long as the power request P.sub.use by the traction motor 51 makes possible an operation of the fuel cell system 10, above the optimal efficiency operating point P(η.sub.max), and as a result of this power demand, an electrical power P.sub.stack is, in particular provided by the fuel cell stack 100, whose electrical power exceeds an electrical power consumed by at least one auxiliary unit P.sub.aux, the energy storage unit 44 is passive and neither outputs nor stores power. As soon as the power request by the traction motor 51 and, thus, the power demand from the fuel cell stack decreases to the extent that the latter is operated below the optimal efficiency operating point P(η.sub.max), the charge state of the energy storage unit 44 is queried and, if it does not exceed a certain limit value, the energy storage unit 44 is charged in addition to the operation of the traction motor 51 and the power request P.sub.use is thereby increased. Only after the charge state of the energy storage unit 44 exceeds a certain limit value and the efficiency gain is, thus, overcompensated for as a result of the increased power request P.sub.use by an efficiency loss in the energy storage unit (transfer losses and storage losses), the fuel cell stack 100 is turned off and the traction motor 51 is supplied only by the energy storage unit 44. In the second operating mode, the fuel cell stack 100 is operated permanently above the minimum operating point P(.sub.min).

(26) In boost operation, the traction motor 51 is used to drive the vehicle 200, wherein the required electrical power is jointly provided by the fuel cell stack 100 and the energy storage unit 44. The boost operation in the first operating mode does not significantly differ from the boost operation in the second operating mode.

(27) In battery operation, the traction motor 51 is used to drive the vehicle 200 and the required power is provided solely by the energy storage unit 44. In the first operating mode, the fuel cell stack 100 and, preferably, also its auxiliary units 24, 26, 33, 34, 38 are deactivated. In the second operating mode, the fuel cell stack 100 and its auxiliary units 24, 26, 33, 34, 38 can also be deactivated but can, in addition, also be present in an activated or a passive state. In battery operation in the second operating mode and with activated fuel cell stack, the energy storage unit 44 is also responsible for providing the drive power. However, the fuel cell stack 100 is activated and permanently provides the minimum power demand P.sub.min. With this power, additional consumers, such as an air-conditioning system, can, for example, be supplied or the energy storage unit 44 can be charged. In addition, a quick switching into boost operation and into fuel cell operation and, thus, a very high driving dynamic is possible. In battery operation in the second operating mode and with passive fuel cell stack 100, the energy storage unit 44 is also responsible for providing the drive power. The energy storage unit further supplies the auxiliary units 24, 26, 33, 34, 38 of the fuel cell stack 100 with energy, while the fuel cell stack 100 is deactivated. This operating state also allows for a quick switching into battery operation, with activated fuel cell or into boost operation and thus a high driving dynamic.

(28) In recuperation operation, the traction motor 51 is used to charge the energy storage unit 44. In this case, in the first operating mode, the fuel cell stack 100 and preferably also its auxiliary units 24, 26, 33, 34, 38 are deactivated, if a power demand by other consumers of the vehicle 200, such as an air-conditioning system (not shown), does not allow for an operation of the fuel cell system 10 above the optimal efficiency degree operating point P(η.sub.max). In addition to the traction motor 51 working as a generator, a charge control of the energy storage unit 44 can possibly also request power from the fuel cell stack 100 and, thus, allow for operating the fuel cell system 10 above the optimal efficiency operating point P(η.sub.max). In the second operating mode, the fuel cell stack 100 and its auxiliary units 24, 26, 33, 34, 38 can also be deactivated but can in addition also be present in an activated or a passive state. The active and the passive state of the fuel cell stack 100 in recuperation operation are in this case equal to these states in battery operation.