Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
1998-11-20
2002-08-13
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C324S434000
Reexamination Certificate
active
06432569
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a method and an apparatus for monitoring a selected group of fuel cells of a fuel cell stack.
It is known that in the electrolysis of water, water molecules are decomposed by electrical current into hydrogen (H
2
) and oxygen (O
2
). In a fuel cell, that process takes place in reverse order. Electrochemically combining hydrogen (H
2
) and oxygen (O
2
) to form water creates electrical current at high efficiency and, if pure hydrogen (H
2
) is used as a combustion gas, without emitting pollutants and carbon dioxide (CO
2
). Even with an industrial combustion gas, such as natural gas or coal gas, and air (which may additionally be enriched with oxygen (O
2
)), a fuel cell produces markedly less pollution and less carbon dioxide (CO
2
) than other energy generators that use fossil fuels. The technical application of the principle of the fuel cell has lead to various devices, specifically with various kinds of electrolytes and with operating temperatures between 80° C. and 1000° C.
The fuel cells are classified depending on their operating temperature as low, medium and high-temperature fuel cells, which in turn differ as a result of various technical embodiments.
A high-temperature fuel cell stack (a fuel cell stack is also simply called a “stack” in the professional literature) composed of many high-temperature fuel cells, includes at least one protection layer, one contact layer, one electrolyte-electrode unit, a further contact layer, a further composite printed circuit board, and so forth, which are disposed in that order below an upper composite printed circuit board that covers the high-temperature fuel cell stack.
The electrolyte-electrode unit includes two electrodes and one solid electrolyte, disposed between the two electrodes and constructed as a diaphragm. Each electrolyte-electrode unit located between adjacent composite printed circuit boards, together with the contact layers immediately contacting both sides of the electrolyte-electrode unit, forms a high-temperature fuel cell, to which the sides of each of the two composite printed circuit boards contacting the contact layers also belong. That type of fuel cell and others are known, for instance, from the Fuel Cell Handbook by A. J. Appleby and F. R. Foulkes, 1989, pp. 440-454.
A stack of fuel cells as a rule has at least 50 cells. The stack must be monitored for the entire period of operation in order to detect whether and where a cell in the stack is defective. If a leak occurs, major damage to the system can in fact occur, and/or hydrogen gas can escape uncontrolled into the environment (leading to a risk of explosion). As a rule a fuel cell reacts immediately to a leak by reversing the polarization of its voltage so that such a state must therefore be detected quickly so that the affected cell can be turned off. It is not possible to monitor each cell individually due to the major technological effort and expense. For the sake of practical functional monitoring, a plurality of cells are each combined into one group, and individual groups are then compared with the other groups in the stack.
A method for functional or power monitoring of a plurality of cells electrically connected in series is known from European Patent Disclosure 0 486 654. In that method, the cells of the stack are first subdivided into groups. At a certain time, the electrical voltages of the groups are detected, and the standardized measured voltage of each group is compared with a first electrical reference voltage, which is equal to a predetermined minimum voltage. In addition, the voltages of the individual groups are compared with one another in various ways. In a first exemplary embodiment, the voltage of a selected group is compared with the voltages of each of the other groups. In a second exemplary embodiment, two adjacent groups are selected in succession and compared with one another. In a third exemplary embodiment, the total of the voltages of the individual fuel cell groups is formed and divided by the number of fuel cell groups. That value is then compared with each individual group. In the monitoring method, the voltages of the individual groups of cells are compared with one another in the most various ways within a predetermined monitoring interval. The method is thus based on many comparisons.
In a further known method, a threshold value for the electrical voltage is determined and is output as a function of the electrical current. The electrical voltages of the individual groups are then compared with the threshold value. Since the threshold value is ascertained from a current measurement value measured at a different time from the electrical voltages of the groups (the threshold value must first be calculated before the comparison), an error can occur if the threshold value and the electrical voltages of the groups pertain to different operating states of the stack.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method and an apparatus for monitoring a selected group of fuel cells of a high-temperature fuel cell stack, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and apparatuses of this general type and which detect a failure of one fuel cell of a stack reliably and with simple measures.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for monitoring a selected group of fuel cells of a fuel cell stack, which comprises ascertaining a change over time in an averaged electrical voltage of the fuel cells of the selected group and comparing the change over time with a reference value encompassing at least a change over time in a voltage of other fuel cells.
Thus, the method is highly dynamic since changes in voltage over time are detected and compared, as compared with load changes in the stack. It is possible by using simple measures according to the method to reliably detect whether a cell of the selected group has failed or at least is not furnishing the proper voltage.
In accordance with another mode of the invention, all of the cells of the stack are divided into a plurality of groups of directly series-connected cells, and only the input voltage of the first cell of a group and the output voltage of the last cell of that one group are measured. The groups each have a number cells, for instance ten, the voltages of which are thus detected in groups, namely through the use of the voltage difference at the input and output of each group. There is no need to detect the electrical voltage of each individual cell in this group. The detection probability of the method suffices for groups with a relatively large number of cells.
In accordance with a further mode of the invention, the relative change in the difference between the input voltage of the first cell and the output voltage of the last cell can be ascertained as the change over time in the voltage of a group. The difference in the electrical voltage of this group can be detected at two different times, with the interval between these two times being 0.5 s, for instance.
In accordance with an added mode of the invention, the sum of the voltages of the cells is measured and divided by the number of cells, yielding the averaged voltage of the cells of the selected group. Independence from the group size (that is, the number of cells in the group) is attained by dividing the added-up voltage by the number of cells. The groups then need no longer all have the same number of cells.
In accordance with an additional mode of the invention, the reference value can encompass the voltages and their changes regarding all of the cells of the stack that do not belong to the selected group. This assures very reliable monitoring of the stack, since a defect in one cell of the selected group does not affect the reference value, and a defect in another cell only slightly adulterates the reference value.
In accordance with yet another mode of the invention, the averaged voltage o
Keim Martin
Stühler Walter
Zeilinger Reinhold
Greenberg Laurence A.
Kalafut Stephen
Mayback Gregory L.
Siemens Aktiengesellschaft
Stemer Werner H.
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