Patent Application
20120251901
This application claims the benefit of priority under 35 U.S.C. ยง119 of German Patent Application DE 10 2011 006 469.9 filed Mar. 31, 2011, the entire contents of which are incorporated herein by reference.
The present invention pertains to a fuel cell system, especially of a motor vehicle.
The present invention pertains, furthermore, to a process for operating such a fuel cell system.
A fuel cell system usually has at least one fuel cell, which comprises at least two electrodes and an electrolyte. The two electrodes are called anode and cathode according to their functions and are separated by the electrolyte. The significance of fuel cells is that they convert chemical energy released during the chemical reaction of hydrogen and oxygen into electric energy. This electric energy can then be used by a user in the form of electric current for energy supply or stored. Mainly water is generated as the waste product by the chemical reactions that lead to a function of the fuel cell. This fact makes fuel cells an environmentally friendly type of energy generation. The educts for supplying the fuel cell are called cathode gas and anode gas according to the respective electrodes, to which they are fed. Air or a gas containing oxygen is usually used as cathode gas. As a rule, hydrogen or a gas containing hydrogen, which can be generated, for example, by means of a reformer from hydrocarbons, before it is fed as anode gas to the anode in the form of a reformate gas, is used as anode gas. High-temperature fuel cells, such as solid oxide fuel cells (SOFC from the English Solid Oxide Fuel Cell), usually have operating temperatures of a few hundred degrees Celsius. The fuel cell must therefore be brought to a corresponding temperature until the above chemical reactions start and the fuel cell delivers electric energy.
An object of the present invention is to provide an improved or at least alternative embodiment, which is characterized especially by simplified handling, for a fuel cell system.
The present invention is based on the general idea of providing in a fuel cell system of the type mentioned in the introduction a temperature-measuring device, which measures an electrode temperature of at least one of the electrodes, and of using a control such that it sets a quantity of fuel fed to the reformer and/or a quantity of oxidant gas fed to the reformer depending on the measured electrode temperature. The control consequently sets a quantity of fuel or additionally a quantity of oxidant gas that is fed to the reformer especially depending on the electrode temperature.
The reformate gas has a carbon formation limit temperature, below which carbon is formed from the reformate gas. If the reformate gas reaches a surface that has a surface temperature that is lower than the carbon formation limit temperature, this leads especially to the formation of carbon on that surface. In case of fuel cells, the reformate gas is fed to an anode. If an anode temperature is lower than the carbon formation limit temperature, this leads to the formation of carbon on the anode surface. The consequence is especially a reduction in the performance of the anode, which may increase to the extent that the anode will become entirely unfit for use. The present invention utilizes the discovery that the carbon formation limit temperature can be reduced over broad temperature ranges, especially by varying the ratio of a fuel-oxidant gas mixture fed to the reformer. If it is possible to consistently maintain the carbon formation limit temperature below the anode temperature, the formation of carbon on the anode is interrupted or at least reduced. The variation of the quantity of fuel fed to the reformer and/or of the quantity of oxidant gas, which depends especially on the anode temperature, is therefore a useful and simple way of preventing carbon formation, especially on the anode.
Corresponding to an advantageous embodiment, the control can thus be designed and programmed such that it sets the quantity of fuel fed to the reformer and/or the quantity of oxidant gas fed to the reformer depending on the measured electrode temperature such that a fuel-to-oxidant ratio and hence a reformate gas is obtained, whose carbon formation limit temperature is below the electrode temperature, whereby especially the formation of carbon on the corresponding electrode is prevented or at least reduced. This can be embodied especially by the control setting the quantity of fuel and/or the quantity of oxidant gas corresponding to characteristics or characteristic diagrams assigned to the measured electrode temperature.
Corresponding to a possible embodiment of the solution according to the present invention, the control is coupled by a connection with the temperature-measuring device. The control has, furthermore, a connection with a fuel feed means and/or a connection with an oxidant gas feed means. The quantity of fuel fed by the fuel feed means to the reformer and/or the quantity of oxidant gas fed by the oxidant gas feed means to the reformer is varied by the control depending on the measured electrode temperature. This can be achieved especially by varying a capacity of the corresponding feed means, for example, the corresponding delivery means. For example, a pump, whose capacity is set by the control, may be used as a delivery means. The variation of the quantity of fuel fed to the reformate gas and/or the quantity of oxidant gas serves especially the purpose of avoiding or at least reducing the formation of carbon on the anode due to the reduction of the carbon formation limit temperature.
In another embodiment, the fuel cell system additionally has a recirculating means for returning anode waste gas to the reformer. The above-mentioned control or another control is connected to the recirculating means and designed such that it varies the quantity of anode waste gas returned to the reformer depending on the measured electrode temperature. This can be achieved especially by means of corresponding characteristics or characteristic diagrams or by adding to the existing characteristics or characteristic diagrams. The returning of the anode waste gas to the reformer may serve, for example, the purpose of maintaining the carbon formation limit temperature of the reformate gas below the measured electrode temperature, especially by varying the fuel-to-oxidant ratio. In addition or as an option, the quantity of anode waste gas returned to the reformer may depend on a reformate gas volume flow. The reformate gas volume flow can be taken into account especially by adapting the corresponding characteristics and characteristic diagrams, which are available to the control for varying the quantity of anode waste gas returned.
It should be pointed out that the connections between the control and the fuel feed means and/or the oxidant gas feed means or the temperature-measuring device as well as to the feed means mentioned below and the respective delivery means thereof do not necessarily consist of an electric conductor. In particular, wireless connections for transmitting the corresponding signals are conceivable as well. This also applies to connections between the controls, if a plurality of controls are present. It should, furthermore, be mentioned that the individual connections may also have a return channel, especially for polling the values of the individual components of the fuel cell system and for balancing same.
In another embodiment of the solution according to the present invention, the above-mentioned changes in the quantity of fuel returned to the reformer and/or in the quantity of oxidant gas fed to the reformer and/or in the quantity of anode waste gas returned to the reformer may additionally depend, each individually or together, on the conversion (conversion rate of anode gas and cathode gas) of at least one of the fuel cells. This can be embodied especially by means of corresponding characteristics or characteristic diagrams or by adapting the existing characteristics or characteristic diagrams. The taking into account of the conversion may serve especially the purpose of taking into account the quantity of fuel and/or the quantity of oxidant gas of the anode waste gas, which is/are returned to the reformer.
In an advantageous embodiment of the solution according to the present invention, the quantity of fuel fed to the reformer and/or the quantity of oxidant gas and/or the quantity of anode waste gas returned to the reformer are sent depending on the measured electrode temperature such that the resulting carbon formation limit temperature of the reformate gas is below the measured electrode temperature. This can be embodied especially by means of the above-mentioned characteristics or characteristic diagrams, or additional characteristics and characteristic diagrams, which are based on the measured electrode temperature in relationship to a ratio of the fuel-to-oxidant gas mixture, which is taken into account when feeding the quantity of fuel and/or the quantity of oxidant gas to the reformer and/or the quantity of anode waste gas fed to the reformer as well as the changes therein.
In another embodiment, water is fed to the reformate gas depending on the measured electrode temperature. Consequently, a quantity of water, which is added to the reformate gas, is varied, in particular, depending on the measured electrode temperature. This addition of water or the change in the quantity of water fed to the reformate gas serves especially the purpose of varying the carbon formation limit temperature and especially of maintaining it below the measured electrode temperature. The quantity of water fed to the reformate gas may be varied, in addition or as an alternative, depending on the reformate gas volume flow. This can serve especially the purpose of guaranteeing a percentage of water in the reformate gas for any desired reformate gas volume flows. In addition, the quantity of water fed to the reformate gas may depend on the conversion of at least one of the fuel cells.
In another embodiment, a quantity of anode waste gas returned to the reformate gas is changed depending on the measured electrode temperature. In addition or optionally, the quantity of anode waste gas returned to the reformate gas may be varied depending on the conversion (degree or rate of conversion of anode gas and cathode gas) of the respective fuel cell and/or the reformate gas volume flow. These changes serve especially the purpose of varying a carbon formation limit temperature of the reformate gas, preferably such that the carbon formation limit temperature is below the measured electrode temperature.
It should be noted that the water fed to the reformate gas may be in any state of aggregation. Consequently, it may be especially steam or liquid water. Furthermore, other liquids or gases containing water can lead to the same result.
The above-mentioned changes may take place in the respective embodiments each individually or together or in any desired combination continuously or in a stepped manner. In case of a stepped change, the respective step may be preset especially by the corresponding characteristics or characteristic diagrams. The changes may take place, furthermore, independently from each other or depending on each other. It is apparent that the individual changes may affect the carbon formation limit temperature and hence correspondingly the other variable parameters, which is taken correspondingly into account.
The above-mentioned changes may optionally take place only when the measured electrode temperature is above a preset minimum electrode temperature. As an alternative or in addition, the changes may take place only when the measured electrode temperature is below a preset maximum electrode temperature. Furthermore, corresponding minimum electrode temperatures and/or maximum electrode temperatures may be preset, each individually or together or in any desired combination, for the feeding of the quantity of fuel and/or of the quantity of oxidant gas to the reformer and for the feeding of water of reformate gas as well as returning anode waste gas to the reformer and/or to the reformate gas.
It should be pointed out that the determination of the electrode temperature by the temperature-measuring device does not necessarily have to take place directly at the respective electrode. Temperature determinations at any other desired points are also conceivable, if they make it possible to infer the corresponding electrode temperature. In particular, the temperature measurement of the electrode may take place in a contactless manner.
It is apparent that the above-mentioned features, which will also be explained below, can be used not only in the particular combination indicated, but in other combinations or alone as well, without going beyond the scope of the present invention.
Preferred exemplary embodiments of the present invention are shown in the drawings and will be explained in more detail in the following description, where the same reference numbers designate identical or similar or functionally identical components. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.
Referring to the drawings in particular, according to
Control 24 is equipped and programmed such that depending on the anode temperature of anode 4, which is measured by means of temperature-measuring device 8, it varies a quantity of fuel fed to reformer 9 and/or a quantity of oxidant gas fed to reformer 9. This can be implemented especially by varying the delivery capacity of the corresponding delivery means 22 of the fuel feed means 13 and of the oxidant gas feed means 15. Due to a corresponding programming and equipping, control 24 is, moreover, capable of varying a quantity of anode waste gas fed to reformer 9 depending on the anode temperature of anode 4, which is determined by temperature-measuring device 8. This change may be embodied especially by varying the capacity of the delivery means 22 of recirculating means 21. Control 24 is additionally programmed and equipped such that it is capable of varying a quantity of water, which is fed to the reformate gas before the latter enters fuel cell 2. This can be embodied especially by varying the capacity of delivery means 22 of water feed means 11. The individual changes and variations in the corresponding capacities of the delivery means 22 and hence the respective quantity of fuel fed, oxidant gas fed, quantity of anode waste gas and quantity of water or quantity of anode waste gas returned may take place independently or depending on one another. The delivery means 22 may, furthermore, be actuated individually or together or in any desired combination.
Control 24 may now be programmed, corresponding to an advantageous embodiment, such that it can embody the operating process described below on the basis of
Starting from a starting point 25, the process checks the anode temperature of anode 4, which was measured by the temperature-measuring device 8 in a comparison section 26. If a reduction is detected compared to the anode temperature measured last, a quantity of fuel fed to the reformer 9 is reduced and/or a quantity of oxidant gas fed to reformer 9 is increased during an operation 27. The process then returns to starting point 25 and the process is repeated. However, if an increase in the anode temperature of anode 4 compared to the anode temperature measured last is detected in comparison section 26, the quantity of fuel fed to reformer 9 is increased and/or the quantity of oxidant gas fed to reformer 9 is reduced during an operation 28, and the process subsequently returns to starting point 25, after which the process is repeated. If the anode temperature of anode 4 is unchanged in comparison section 26, the process returns to the starting point 25 and the process is repeated. The change of the quantity of fuel fed to reformer 9 and/or of the quantity of oxidant gas fed to reformer 9 can now serve especially the purpose of lowering a carbon formation limit temperature of the reformate gas, below which carbon is formed from the reformate gas, to the extent that it is below the anode temperature. For example, a corresponding fuel-to-oxidant gas ratio can be assigned, for example, to an anode temperature of anode 4, especially in the form of characteristics and characteristic diagrams, and such a ratio is set in the corresponding operations.
Corresponding to the process, the return of anode waste gas to the reformer 9 can be additionally or alternatively varied, especially in operations 27 and 28. This step can be optionally carried out during operations following the operations 27 and 28. Control 24 now changes a quantity of anode waste gas fed to the reformer depending on the measured anode temperature of anode 4. This can be used to maintain the carbon formation limit temperature below the measured anode temperature, especially by fuel and/or oxidant gas possibly present in the anode waste gas.
In an alternative form of the process, control 24 additionally changes a quantity of water fed to the reformate gas, which quantity may additionally depend on a reformate gas volume flow. It is preferred here to increase the quantity of water fed to the reformate gas with decreasing anode temperature and/or with increasing reformate gas volume flow and to reduce it with rising anode temperature and/or decreasing reformate gas volume flow. This serves especially the purpose of maintaining the carbon formation limit temperature of the reformate gas below the anode temperature.
The process can, furthermore, take into account a minimum anode temperature of anode 4, with water being fed to the reformate gas only when the measured anode temperature of the anode is above the minimum anode temperature. This can serve especially the purpose of taking into account a minimum carbon formation limit temperature, below which a further reduction of the carbon formation limit temperature by feed water is not possible. As an alternative or in addition, the process can take into account a maximum anode temperature of anode 4, with water being fed to the reformate gas only when the measured anode temperature is below the maximum anode temperature. This can serve especially the purpose of taking into account anode temperatures that are above the carbon formation limit temperature of the reformate gas without the feed of water.
As an alternative to the above-mentioned process for changing the quantity of water fed to the reformate gas depending on the anode temperature and/or the reformate gas volume flow, a process is also advantageous in which a proportionate quantity of water to the quantity of reformate gas is assigned to each anode temperature or each anode temperature range. This can be embodied especially by characteristics or characteristic diagrams stored in control 24. Control 24 now changes the quantity of water fed to the reformate gas corresponding to the values stored in the characteristics or characteristic diagrams. These stored values may serve especially the purpose of maintaining the carbon formation limit temperature of the reformate gas below the measured anode temperature. The values may depend, furthermore, on the anode temperature and/or the reformate gas volume flow individually or together.
It should be pointed out that the process variants being described here as examples may alternatively or additionally depend on a cathode temperature of a cathode 5 without going beyond the scope of the present invention.
While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2011 006 469.9 | Mar 2011 | DE | national |
