Patent Application
20160190615
The present invention relates to a fuel cell system, to a motor vehicle containing a fuel cell system of said type, and to a method for operating a fuel cell system.
Fuel cell systems are known in a variety of embodiments. All fuel cell systems have in common the fact that they exhibit only limited dynamics, said limitation normally arising owing to restricted controllability of the oxidation fluid delivery unit contained in the fuel cell system. In the case of a fuel cell system being used in a motor vehicle, therefore, a high level of hybridization of fuel cell and high-voltage accumulator, and thus a high-voltage accumulator (battery) of high power, are necessary specifically in order to provide sufficient energy in an acceleration process (positive step change in load) or else in order to recuperate energy in the event of a negative step change in load. These are, however, susceptible to degradation. Batteries with high power capability are furthermore characterized by a high weight and a large structural volume, which is a disadvantage in particular for use in lightweight constructions. Furthermore, power deficits nevertheless arise, in particular during the acceleration process of a motor vehicle, owing to slow start-up times and reaction times of the oxidation fluid delivery unit and associated poor fuel cell system dynamics.
Taking this prior art as a starting point, it is therefore an object of the present invention to provide a fuel cell system which exhibits good dynamics, which is very powerful and which is designed to store or release energy quickly when required. It is also an object of the invention to provide a motor vehicle operated using a fuel cell system, which motor vehicle is characterized by good driving dynamics and very good driving comfort. It is a further object of the present invention to provide a method for operating a fuel cell system, which method makes it possible for the fuel cell system to be controlled easily and with a high level of variability and thus with dynamic power adaptation.
In the case of a fuel cell system, the object is achieved according to the invention in that the fuel cell system includes a plurality of fuel cells combined to form a fuel cell stack, wherein
As a further constituent of the invention, the fuel cell system includes a control device which is designed to adapt an available electrical and/or thermal power of the fuel cell system by changing a redox state of the electrochemical storage system.
The redox flow fuel cell differs from “normal” fuel cells in that the water-forming reaction, that is to say the formation of water from protons, electrons and oxygen, is spatially relocated, and thus takes place not in the cathode region, which is adjacent to the separator and which is situated opposite the anode region, but in a so-called regenerator which is spatially separate from said cathode region but which is connected to the other components of the fuel cell system by way of a corresponding transport system. Via a transport circuit, the regenerator is supplied with the protons which have been produced in the anode region and which have passed through the proton-permeable separator into the cathode region, and with the electrons which have been produced and which commonly flow via an external consumer. The circuit for the transport of the protons may be identical to the pump circuit for conducting the electrochemical storage system through the cathode region of the redox flow fuel cell, though may also constitute a separate circuit. The oxidation fluid required for the water-forming reaction, that is to say generally an oxidant, for example air or an oxidation gas such as oxygen, or a corresponding liquid (referred to herein by the expression “oxidation fluid”), is supplied to the regenerator via at least one oxidation fluid delivery unit, for example a compressor.
Within the meaning of the invention, an electrochemical storage system includes chemical, redox-active molecules or active redox molecules, which may be present both in reduced form and in oxidized form, wherein both forms form a redox pair, and wherein the electrochemical storage system can receive and release one and/or multiple electrons per redox-active molecule. The electrochemical storage system is preferably provided in the form of a solution of the redox-active molecules, and serves for the storage and the transport of electrons.
It is preferably the case that the active redox molecule itself, or a solvent contained in the electrochemical storage system, transports protons. Furthermore, it is preferably the case that the electrochemical storage system exhibits low electrical conductivity. It is also preferably the case that the electrochemical storage system itself does not discharge, or discharges only very slowly.
The fuel cell system according to the invention may include one or more control devices. Here, a control device is designed such that it can initiate a change of the redox state of the electrochemical storage system and thus adapt the electrical and/or thermal power of the fuel cell system. The information regarding the electrical state of the electrochemical storage system and further parameters, such as liquid level, temperatures, pressures, pH value, conductivity etc., are provided to the control unit by way of sensors and/or model calculations.
If electrical power is to be drawn from the fuel cell system (positive load situation), the electrochemical storage system is changed from the oxidized state to the reduced state. This is performed by promoting the anode reaction of the redox flow fuel cell. The electrons thus released are received by the electrochemical storage system in the cathode region after passing through a load. In other words, a ratio of the reduced form of the electrochemical storage system and of the oxidized form of the electrochemical storage system is adapted in favor of the reduced form. If, for example, the ratio of the reduced form and of the oxidized form tends toward infinity, then from this point in time, only as many electrons can be received as can be released again in the regenerator. This corresponds to a maximum continuous power of the fuel cell system.
In the recuperation situation (negative load situation), it is possible for electrical power either to be supplied to the oxidation fluid delivery unit for the activation and/or operation thereof, and/or, if the fuel cell system has a high-voltage accumulator, to charge the high-voltage accumulator. Here, a ratio of the reduced form of the electrochemical storage system and of the oxidized form of the electrochemical storage system is adapted in favor of the oxidized form.
The control device is thus designed such that, by changing the redox state of the electrochemical storage system, said control device controls the anode reaction (release of electrons) independently of the water-forming reaction (consumption of electrons) and thus adapts the redox state of the electrochemical storage system to the power demands on the fuel cell system. This is possible by virtue of the electrochemical storage system serving as a so-called “buffer” for electrons. Furthermore, the control unit can regulate the concentration of the redox-active molecules or active redox molecules, the solvent content (for example water) and a fill level of the electrochemical storage molecule in the pump circuit, for example by way of temperatures and/or an efficiency of an optionally provided solvent recovery installation (condenser).
Whereas it is the case in a conventional fuel cell that the water-forming cathode reaction necessitates the anode reaction (and vice versa), and thus the power of the fuel cell is restricted substantially by the rate of supply of combustion fluids and oxidation fluids to the respective reaction region, it is possible in the case of the redox flow fuel cell for the anode reaction to be decoupled from the cathode reaction, and for the electrochemical storage system to be adjusted into the intended redox state, by virtue of electrons being received and stored by the electrochemical storage system. In the event of a positive load situation, that is when power is drawn by an external consumer or a load, it is now possible, in addition to the “normal” fuel cell reaction with the conventional production of water through the combination of the cathode reaction and anode reaction, and thus production of energy, for electrons to be received or temporarily stored by the electrochemical storage system, until said electrons are discharged by way of the water-forming reaction in the presence of relatively low loads. Here, the electrochemical storage system changes from the oxidized state into the reduced state. The power of the redox flow fuel cell is thus temporarily increased in relation to a conventional fuel cell.
Owing to the characteristic of the control device of changing the redox state of the electrochemical storage system and adapting the power demands to the fuel cell system, a fuel cell system with dynamic power adaptation is thus obtained, which can deal with very high power demands even within a short period of time. It is thus also possible for energy to be drawn significantly more quickly upon the start-up of the fuel cell system.
In one advantageous refinement of the fuel cell system, the control device is designed to adapt the electrical power of the fuel cell system by way of a change of the redox state of at least 10% of the redox-active molecules (or active redox molecules) of the electrochemical storage system. This improves the dynamic power adaptation of the fuel cell system.
Furthermore, the control device is advantageously designed to increase the electrical power of the fuel cell system beyond the maximum power predefined by the oxidation fluid delivery unit by initiating a reduction of the electrochemical storage system. A particularly high level of electrical power can be drawn in this way.
It is likewise advantageously the case that the control device is designed to provide the electrical power of the fuel cell system without activation of the oxidation fluid delivery unit by initiating a reduction of the electrochemical storage system. Specifically in the event of short positive step changes in load, a release of energy is possible without a time delay. Furthermore, in this way, the inert oxidation fluid delivery units are conserved.
A further advantageous refinement provides that the control device is designed to effect a regeneration of the electrochemical storage system by feeding in recuperation energy. This is realized, for example, by activation of the oxidation fluid delivery unit or by electrochemical charging of the electrochemical storage system.
The regeneration of the electrochemical storage system is advantageously performed by feeding recuperation energy into the oxidation fluid delivery unit.
It is furthermore advantageously the case that the control device is designed to regulate the pump in stepped and/or continuously variable fashion in a manner dependent on a substance amount of the active redox molecules of the electrochemical storage system. This permits operation adapted to the fuel cell and with the best possible efficiency. The substance amount of the active redox molecules of the electrochemical storage system is large if the concentration of the electrochemical storage system in a given constant volume is high.
It is furthermore advantageously the case that the control device is designed such that, if the substance amount of the active redox molecules of the electrochemical storage system is low, that is to say for example in the case of a volume of the electrochemical storage system of less than 8 L/100 kW fuel cell system power, in the event of a positive step change in load, the control device immediately activates the oxidation fluid delivery unit and provides electrical power by initiating a reduction of the electrochemical storage system. In this way, power deficits during the start-up of the oxidation fluid delivery unit are minimized, and faster response behavior of the fuel cell system is promoted.
If the substance amount of the electrochemical storage system is high, for example in the case of a volume of the electrochemical storage system of more than 8 L/100 kW fuel cell system power, the control device is advantageously designed such that in the event of a positive step change in load, the control device provides electrical power by initiating a reduction of the electrochemical storage system and activates the oxidation fluid delivery unit after a delay of several seconds, in particular of 0 to 20 seconds, preferably of 1 to 10 seconds, and particular preferably of 2 to 4 seconds. It is thus possible for a sufficiently high level of power to be drawn from the fuel cell system when required and, at the same time, for the inert oxidation fluid delivery unit to be conserved, wherein the energy-consuming oxidation fluid delivery unit can be activated at a later point in time and thus the full power of the fuel cell system (power of the fuel cell stack plus power from the electrochemical storage system of the redox flow fuel cell) is available immediately upon the starting of the fuel cell system. Alternatively or in addition, a device or a circuit for the smooth start-up of the oxidation fluid delivery unit may be provided (cf.
In an advantageous refinement, the control device is designed such that, in the event of a negative step change in load, the control device supplies recuperation energy that is obtained to the oxidation fluid delivery unit in order to activate or operate the latter. This saves energy upon the restarting of the oxidation fluid delivery unit in the event of a subsequent positive step change in load, without the dynamics of the fuel cell system being adversely impaired.
To improve the dynamic power adaptation of the fuel cell system, the fuel cell system according to the invention has at least one battery. The battery and the storage system preferably provide the required power. Since the electrochemical storage system is likewise suitable for storing energy, it is possible in this case for the battery to have a relatively low capacity or power. Furthermore, the battery is conserved by the buffer action of the electrochemical storage system specifically in the event of intense step changes in power, which lengthens the service life of the battery.
It is furthermore advantageous for the control device to be designed such that, in the event of a negative step change in load, the control device supplies the recuperation energy that is obtained to the oxidation fluid delivery unit and/or to the battery.
For faster provision of the power of the fuel cell system, the control device is preferably designed such that, during the start-up of the fuel cell system or during a cold start or frost start, the control device reduces a pump power of the pump apparatus in order to bring the fuel cell system to operating temperature.
The present invention also relates to motor vehicle which includes a fuel cell system as described above. The fuel cell system according to the invention is, owing to its good dynamics, particularly well-suited for use in a motor vehicle, and thus provides a high level of driving dynamics and a high level of driving comfort.
The refinements, advantages and effects described for the fuel cell system according to the invention also apply to the motor vehicle according to the invention.
The invention likewise also relates to a method for operating a fuel cell system having multiple fuel cells combined to form a fuel cell stack, wherein
Here, the method according to the invention includes the step of adapting an available electrical and/or thermal power of the fuel cell system by changing a redox state of the electrochemical storage system. As discussed above, this step is initiated by way of a control device. For the reasons stated above, and incorporating the effects and advantages already described, it is possible by way of the method according to the invention for a fuel cell system to be controlled easily and with good power dynamics in accordance with the power demands on the fuel cell system.
The refinements, advantages and effects described for the fuel cell system according to the invention and the motor vehicle according to the invention also apply to the method according to the invention for operating a fuel cell system.
In an advantageous refinement of the method according to the invention, the method is characterized by the step of adapting the electrical power of the fuel cell system by way of a change of the redox state of at least 10% of the redox-active molecules of the electrochemical storage system. This improves the dynamics of the power provision of the fuel cell system.
To provide a particularly high level of power that goes beyond the “normal” power of a fuel cell system, the method provides for increasing the electrical power of the fuel cell system beyond the maximum power predefined by the oxidation fluid delivery unit by initiating a reduction of the electrochemical storage system.
By means of the step, provided in accordance with an advantageous refinement, of providing the electrical power of the fuel cell system without activation of the oxidation fluid delivery unit by initiating a reduction of the electrochemical storage system, it is possible, specifically in the case of short positive step changes in load, for energy to be released without a time delay. Furthermore, in this way, the inert oxidation fluid delivery units are conserved.
Furthermore, the method advantageously includes the step of regenerating the electrochemical storage system by feeding in recuperation energy. In this way, an at least partial, preferably complete, oxidation of the electrochemical storage system is realized, such that then, in a subsequent positive load situation, the full electrical power of the fuel cell system can be provided by initiating a reduction of the electrochemical storage system.
By way of regulating the pump apparatus in stepped and/or continuously variable fashion in a manner dependent on a substance amount of the active redox molecules of the electrochemical storage system, it is made possible for the required energy to be provided more precisely and more quickly.
The method according to the invention furthermore advantageously provides that, if the substance amount of the active redox molecules of the electrochemical storage system is low, in the event of a positive step change in load, the oxidation fluid delivery unit is immediately activated and power is provided by initiation of a reduction of the electrochemical storage system. In this way, power deficits during the start-up of the oxidation fluid delivery unit are minimized, and faster response behavior of the fuel cell system is promoted.
If the substance amount of the active redox molecules of the electrochemical storage system is high, the method according to the invention as per one refinement provides that, in the event of a positive step change in load, power is provided by initiation of a reduction of the electrochemical storage system and the oxidation fluid delivery unit is activated after a delay of several seconds, in particular of 0 to 20 seconds, preferably of 1 to 10 seconds and more preferably of 2 to 4 seconds. It is thus possible for a sufficiently high level of electrical power to be drawn from the fuel cell system when required and, at the same time, for the inert oxidation fluid delivery unit to be conserved. Alternatively or in addition, a device or a circuit for the smooth start-up of the oxidation fluid delivery unit may be provided (cf.
To save energy upon a restart of the oxidation fluid delivery unit in the event of a positive load situation following a negative load situation, without the dynamics of the fuel cell system being adversely impaired, it is provided according to one refinement of the method that, in the event of a negative step change in load, the recuperation energy that is obtained is supplied to the oxidation fluid delivery unit in order to activate or operate the latter.
To optimize the dynamic adaptation of the power of the fuel cell system, the fuel cell system has at least one battery, wherein here, the method is refined in that, in the event of a negative step change in load, the recuperation energy that is obtained is supplied to the oxidation fluid delivery unit and/or to the battery. In this way, the battery is conserved in the event of intense step changes in power, which lengthens the service life of the battery.
For faster provision of the power of the fuel cell system, the method is preferably refined in that, during the start-up of the fuel cell system or during a cold start or frost start, a pump power of the pump is reduced in order to bring the fuel cell system to operating temperature.
The solutions according to the invention and the refinements thereof yield the following advantages:
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.
The figures illustrate only those aspects of the present invention that are of interest here; all other aspects have been omitted for the sake of clarity.
In other words, as a result of the electrochemical reaction in the anode region 1, electrons are released which, after passing through a load, are received by the electrochemical storage system in the cathode region 2, which electrochemical storage system thus changes into a reduced state. The electrochemical storage system is then transported by the pump circuit 3 to the regenerator R. Here, if oxidation fluid and protons are also supplied to the regenerator R, the electrochemical storage system changes into the oxidized state, with a release of electrons. Based on the targeted control of the change of the redox state of the electrochemical storage system, it is possible according to the invention for the electrical power of a fuel cell system that contains the redox flow fuel cell 10 to be adapted.
Here, the fuel cell system has multiple fuel cells that have been stacked to form a fuel cell stack, wherein at least one fuel cell is a redox flow fuel cell. The fuel cell system need not include a high-voltage accumulator. Furthermore, the fuel cell system has a large substance amount of active redox molecules.
Owing to the large substance amount of active redox molecules of the electrochemical storage system, an oxidation fluid delivery unit can be activated with a time delay (for example 0 to 20 seconds, preferably 1 to 10 seconds and more preferably 2 to 4 seconds), such that, directly upon the start-up of the fuel cell system, no energy has to be expended for setting the oxidation fluid delivery unit in operation, which would lessen the overall power of the fuel cell system. The power absorbed by the oxidation fluid delivery unit and absent from the overall power of the system is illustrated in curve C. It can be clearly seen that, here, a smooth start-up of the oxidation fluid delivery unit is provided.
Curve A shows the electrical power of the overall fuel cell stack composed, for example, of “normal” fuel cells and redox flow fuel cells. After a minimum start-up time, which, in relation to a start-up time of a conventional fuel cell system comprising exclusively “normal” fuel cells, a constant power is delivered which originates from the electrode reactions. The very short start-up time is realized in that, during the start-up of the fuel cell system, electrochemical energy stored in the electrochemical storage system is additionally released. A relatively long time delay of the power rise of the fuel cell stack would otherwise be expected in this case too. For constancy of the power of the fuel cell stack, it is necessary, inter alia, for the oxidation fluid delivery unit to be activated in order that the regenerator is supplied with oxidation fluid. As an electrical consumer, the oxidation fluid delivery unit extracts power from the overall system (see curve C), which is evident in the fall in the power curve B of the fuel cell system after passing through a maximum. The resulting hatched region D is the energy available to a consumer, for example to a motor vehicle, owing to delayed activation of the oxidation fluid delivery unit.
By contrast to the fuel cell system from
Curve E shows the contribution made by the high-voltage accumulator to the power. It can be seen that the high-voltage accumulator, like conventional fuel cells, is not capable of providing power without a time delay upon the start-up of the fuel cell system. This is manifest in a slow rise of the curve E, the power curve of the high-voltage accumulator. The power deficit is in turn compensated by way of the electrochemical storage system, which yields an immediate rise of the overall power (curve F) composed of power of the fuel cell system (curve B) and power of the high-voltage accumulator (curve E) to a maximum. The maximum overall power (curve F) that is attained is greater than that from
The fuel cell system includes, similarly to that from
Owing to the small substance amount of active redox molecules of the electrochemical storage system, the oxidation fluid delivery unit is activated without a time delay, such that adequate power of the fuel cell system can be quickly provided immediately upon the start-up of the fuel cell system. A time delay of the initiation of the oxidation fluid delivery unit would be a disadvantage here because, owing to the small substance amount of active redox molecules, electrochemical power can be drawn from the electrochemical storage system only for a short period of time.
As in
The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2013 217 858.1 | Sep 2013 | DE | national |
This application is a continuation of PCT International Application No. PCT/EP2014/068885, filed Sep. 4, 2014, which claims priority under 35 U.S.C. §119 from German Patent Application No. 10 2013 217 858.1, filed Sep. 6, 2013, the entire disclosures of which are herein expressly incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2014/068885 | Sep 2014 | US |
| Child | 15060872 | US |
