METHOD FOR OPERATING A FUEL CELL SYSTEM

Abstract

A method comprising feeding a fuel and an oxidant to individual cells in a fuel cell stack, each having two electrode layers and an electrolyte layer arranged between the electrode layers. The method further includes compressing the cell stack with a clamping device, and detecting a compression pressure upon the cell stack with at least one pressure sensor. The method also includes determining a moisture content of the two electrolyte layers based on the detected compression pressure.

Claims

1. A method comprising: feeding a fuel and an oxidant to individual cells in a fuel cell stack, each having two electrode layers and an electrolyte layer arranged between the electrode layers; compressing the cell stack with a clamping device; detecting a compression pressure upon the cell stack with at least one pressure sensor; and determining a moisture content of the two electrolyte layers based on the detected compression pressure.

2. The method of claim 1, wherein the at least one pressure sensor is situated between an end of the cell stack and the clamping device.

3. The method of claim 1, wherein the at least one pressure sensor is an at least one piezo element.

4. The method of claim 3, wherein the at least one piezo element is configured to create the compression pressure upon the cell stack.

5. The method of claim 4, further comprising detecting a compression pressure based on an electric voltage which is used by the at least one piezo element for creating the compression pressure.

6. The method of claim 1, further comprising detecting a temperature of the cell stack with a temperature sensor.

7. The method of claim 6, wherein the determining step includes determining the moisture content of the two electrolyte layers based on the detected compression pressure and the detected temperature.

8. The method of claim 1, further comprising adjusting one or more operating parameters of the cell stack with a control device based on the moisture content.

9. The method of claim 1, wherein the one or more operating parameters include a cell stack flow rate, a cell stack temperature, a cell stack moisture content, and/or a cell stack pressure.

10. A fuel cell system comprising: a cell stack including individual cells of two electrode layers and an electrolyte layer between the electrode layers; a clamping device configured to compress the cell stack; a pressure sensor configured to detect a pressure upon the cell stack; and a control device configured to determine a moisture content of one or more of the electrolyte layers based on the pressure.

11. The fuel cell system of claim 10, wherein the individual cells are arranged next to each other.

12. The fuel cell system of claim 10, wherein the pressure sensor is a piezo element.

13. The fuel cell system of claim 10, wherein the pressure sensor is situated between an end of the cell stack and the clamping device.

14. The fuel cell system of claim 10, further comprising a temperature sensor configured to detect a temperature of the cell stack.

15. A fuel cell system comprising: a cell stack including individual cells of two electrode layers and an electrolyte layer; a clamping device configured to compress the cell stack; a pressure sensor configured to detect a cell stack pressure; a temperature sensor configured to detect a cell stack temperature; and a control device configured to determine a moisture content of one or more of the electrolyte layers based on the cell stack pressure and temperature.

16. The fuel cell system of claim 15, wherein the individual cells are arranged next to each other.

17. The fuel cell system of claim 15, wherein pressure sensor is a piezo element.

18. The fuel cell system of claim 15, wherein the pressure sensor is situated between an end of the cell stack and the clamping device.

19. The fuel cell system of claim 15, wherein the control device is further configured to adjust one or more operating parameters of the cell stack based on the moisture content.

20. The fuel cell system of claim 19, wherein the one or more operating parameters include a cell stack flow rate, the cell stack temperature, a cell stack moisture content, and/or the cell stack pressure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 shows an exemplary embodiment of a fuel cell system per one embodiment in a schematic side view,

[0029] FIG. 2 shows a schematic top view of one end of the fuel cell system per FIG. 1, with the clamping plate not shown, and

[0030] FIG. 3 shows a schematic diagram of an exemplary embodiment of the method according to the invention for operating a fuel cell system.

DETAILED DESCRIPTION

[0031] As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention.

[0032] In FIG. 1, a schematic side view of a fuel cell system 10 is shown. The fuel cell system 10 comprises a cell stack 12 with a multiplicity of individual cells 14 which are arranged next to each other. The individual cells 14 are arranged in a sandwich-like manner one on top of the other by their large oppositely disposed lateral surfaces so that an electrical series connection of the individual cells 14 is realized. An electrical parallel connection of a plurality of individual cells 14 of the cell stack 12 in each case is also possible. Each individual cell 14 is supplied via passages or lines (not shown) of the fuel system 10 with fuel, for example hydrogen, methane or another gaseous hydrocarbon, and with oxidant, for example oxygen or air. Correspondingly provided for each fuel cell 14 is a discharge line (not shown in FIG. 1) for reaction products and unconsumed oxidant. An electric voltage or energy which is generated by the cell stack 12 is provided at both ends of the cell stack 12 by electrical contacts (similarly not shown). The fuel cell system 10 in this exemplary embodiment is designed for a mobile application, for example in a vehicle, and is configured in the easiest and most space-saving manner possible for this purpose.

[0033] For generating electric energy, each individual cell 14 contains in each case two electrode layers 16 and an electrolyte layer 18 arranged between them. In addition, each individual cell 14 can contain additional laminations, layers or plates, for example gas diffusion layers (GDL) arranged on the electrode layers 16 for uniform distribution of fuel and oxidant over the entire surfaces of the electrode layers 16, and separating plates for separation of the individual cells 14. In this case, an individual separating plate, a so-called bipolar plate, can be provided for two adjacent individual cells 14. Moreover, passages for the feed of fuel and oxidant and for the discharge of reaction products and unconsumed oxidant can be contained in the separating plates. Furthermore, for each individual cell 14 seals are provided on the outer edge of the cell stack 12 or as additional plates or layers to prevent escape of fuel, oxidant, reaction products or an electrolytic fluid from the cell stack 12. The individual cells 14 are designed for example as a proton exchange membrane fuel cells with a proton exchange membrane (PEM) as the electrolyte layer, to name only one of many individual cell types, which can be used in the fuel cell system 10.

[0034] For the compression and holding together of the cell stack 12, the fuel cell system 10 also contains a clamping device 20. The clamping device 20 has four clamping elements 22, designed as clamping bands, which extend in each case from a first end 24 of the fuel cell system 10 to a second end 26. The clamping elements 22 are arranged in pairs in oppositely disposed sides of the cell stack 12 and extend parallel to each other and to the longitudinal axis of the cell stack 12. Shown in FIG. 1 are two clamping elements 22 with sections 28 removed at the two ends 26 of the fuel cell system 10 to therefore expose elements and structures which lie behind. The four clamping elements 22 hold in each a first clamping plate 30 on the first end 24 and a second clamping plate 32 on the second end 26 of the fuel cell system 10 at a fixed maximum distance apart. In alternative embodiments, more or less than four clamping bands can be used, instead of two clamping bands one clamping band which can be guided in a loop-like manner around both ends 24, 26 and along two oppositely disposed sides can be used, or instead of clamping bands clamping bolts can be used. A rigid frame with integrated clamping elements and clamping plates is also possible. It is only important that the clamping plates 30, 32 have a fixed distance apart to thereby constitute an abutment for creating pressure upon the cell stack 12.

[0035] Four levers 36, of which only two are visible in FIG. 1, are pivotably fastened via joints 38 on the second clamping plates 32. By their free end 40, the levers 36 butt against an end plate 42 which uniformly transmits force from the levers 36 onto one end of the cell stack 12 and vice versa. For this, the end plate 42 which is provided on the second end 26 of the cell stack 12 butts against the cell stack over the entire area of the end of said cell stack 12 and has a base area which corresponds to or is like the cross section of the cell stack 12. Arranged in each lever 36, close to the joint 38, between a bearing surface 58 (see FIG. 2) of the lever 36 and the second clamping plate 32, is a piezo element 44. The piezo elements 44 serve both for detecting and for creating compression pressure upon the cell stack 12. Depending on the variable expansion of a piezo element 44, the corresponding lever 36 is pressed by a greater or lesser degree of force against the end plate 42. The levers 36 therefore constitute a one-sided lever which converts a small expansion of the piezo elements 44 into a larger deflection at the free ends 40 of the levers 36. Conversely, the levers 36 transmit the compression pressure which acts upon the cell stack 12 onto the piezo elements 44. The second clamping plate 32, together with the levers 36 and the joints 38, constitutes a lever mechanism 46 for the piezo elements 44. In an alternative exemplary embodiment, piezo elements and levers are also provided in the first clamping plate 30. Therefore, detection and creation of compression pressure is possible at both ends of the cell stack 12. In further alternative exemplary embodiments, more or less than four piezo elements 44 or levers 36 can be provided on one end 24, 26 or even a two-sided lever instead of a one-side lever.

[0036] Each of the four piezo elements 44 in this exemplary embodiment contains a piezo crystal, a piezo-electric ceramic, or a stack of individual elements made from these materials. Depending on the applied electric voltage, piezo-electric materials assume a different volume. By the same token, piezo-electric materials under pressure generate a corresponding electric voltage. Each piezo element 44 can be individually operated by a control device 48 of the fuel cell system 10 by adjustment of a corresponding electric voltage. For this purpose, the piezo elements 44 are connected via electric leads 50 to the control device 48. In this way, the pressure upon the cell stack 12 can be separately adjusted in the region of each corner of the cell stack 12. Furthermore, the piezo elements 44 and the control device 48 are also provided for measuring a pressure. Therefore, in each corner of the cell stack 12 detection of the pressure which acts via the end plate 42 and the lever 36 upon the piezo element 44 can also be carried out instead of creating pressure. Furthermore, in this exemplary embodiment spring elements 52 are arranged at the second end 26 between the clamping plate 32 and the end plate 42 for additional pressing of the end plate 42 against the cell stack 12. The spring elements 52 are designed for example as disk springs or coil springs.

[0037] The control device 48 is designed for determining a current moisture content of electrolyte layers 18 and to this end also use a current temperature or temperature change at one or more locations of the cell stack 12 in addition to the compression pressure currently acting upon the cell stack 12 or a time change of the this pressure. For this, the control device 48 is connected via an electrical connection 54 to at least one temperature sensor 56. The pressure in each piezo element 44 is determined by the control device 48 directly from the voltage which is used for creating pressure. Alternatively, a voltage which is generated by the piezo elements 44 can also be used for determining the pressure. Furthermore, a determination of the moisture content by a separate calculation device which is separate from the control device 48 is also possible.

[0038] For operating the piezo elements 44 with a corresponding electric voltage, the control device 48 takes into consideration for example a current energy extraction, an ambient temperature, the temperature which is determined by the temperature sensor 56, a previously determined moisture content, a pressure inside the fuel cell system 10, a pressure in a region of the end plate 42, a flow rate, temperature or a moisture content of fuel or oxidant and so forth. To this end, the control device 48 can also be designed for processing values of additional sensors, such as temperature sensors, pressure sensors, strain sensors, current sensors or voltage sensors, and contains an electronic processor for processing data and also a memory for storing data. By processing values which are made available, the control device 48 first of all determines current moisture contents of individual cells 14 and then, depending on the operating state, adjusts operating parameters of the fuel cell system 10, for example the compression pressure in each piezo element 44 or the flow rate, the temperature, the moisture content or the pressure of fuel and oxidant so that an optimum moisture content and operation of the fuel cell system 10 is achieved and maintained.

[0039] FIG. 2 shows a schematic top view of the second end 26 of the fuel cell system 10 according to FIG. 1 without the second end plate 32. Each lever 36, at one end in the region of a corner of the end plate 42, is connected via the joint 38 to the second end plate 32, which is not shown. As a joint 38, provision is made for example for a pin which is fastened on the clamping plate 32 and extends in a hole in the lever 36. The pivot axes of the lever 36 are therefore parallel to the dashed lines in the joints 38. Furthermore, each lever 36 extends along a side edge of the end plate 42 up to the opposite corner of the end plate 42 where the free end 40 of the lever 36 acts upon the end plate 42 by a lever head. In this case, two levers 36 are arranged in each case in a crosswise manner and are designed so that they are not mutually limited in their freedom of movement.

[0040] Adjacent to the joint 38, each lever 36 has a contact face 58 against which butts the respective piezo element 44. The piezo elements 44 are held in position by the second clamping plate 32 which in turn is fixed by the clamping elements 22. Because of such an arrangement of the levers 36 and piezo elements 44, a very compact and space-saving clamping device 20 is realized and at the same time is suitable for detecting a compression pressure. In this case, because of the levers 36 being designed if possible piezo elements 44 with small travel ranges and therefore small dimensions are indicated. Furthermore, three spring elements 52, designed for example as disk springs or coil springs, are arranged in the middle of the end plate 42. Alternatively, more or less than three spring elements 52 can also be provided. The spring elements 52 exert an additional pressure upon the end plate, especially in the middle region of this end plate 42.

[0041] FIG. 3 shows a schematic diagram of a method for operating the fuel cell system 10. First, a determination 100 of a relationship between a compression pressure, a temperature and a moisture content of a determined cell stack 12 in use, consisting of individual cells 14, is carried out during a test operation. In this case, a dependency of the compression pressure or of its change on a moisture content and a temperature of the cell stack 12 can be determined. The relationship can be stored as an algorithm or table in a memory of the control device 48 and enables a determination of a current moisture content of electrolyte layers 18 or of individual cells 14 of the cell stack 12 during an operation.

[0042] For this, a periodic or continuous detection 102 of the current compression pressure P by the piezo elements 44 is carried out by the control device 48 during operation of the fuel cell system by feeding fuel and oxidant. A periodic or continuous detection 104 of at least a current temperature T of the cell stack 12 is also carried out with the aid of the temperature sensor 56. The control device 48, using the detected compression pressure P, or its change rate, and the detected temperature T, or its change, carries out a determination of a current moisture content RH of electrolyte layers 18 or of individual cells 14 based on the stored relationship.

[0043] The determined moisture content RH is compared with a predetermined, optimum value range of the moisture content for the current operating state of the fuel cell system, 108. If the determined moisture content RH lies within the predetermined value range, the method is continued with a new detection 102 of the compression pressure. If the determined moisture content RH is outside the predetermined value range, therefore above an upper threshold value RH.sub.max or below a lower threshold value RH.sub.min, an adjustment 110 of operating parameters, such as flow rate, temperature, moisture content or pressure of the oxidant or of the fuel, is carried out with the aid of the control device 48. The method is then continued with a new detection 102 of the compression pressure. In this way, a control for the moisture content during an operation is realized. The fuel cell system 10 is constantly operated with an optimum moisture content of the cell stack 12. In addition, a suitable compression pressure upon the cell stack 12 can be established by the clamping device 20 at any time.

[0044] While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.