Method and regulation apparatus for regulating a fuel cell or a fuel cell stack

Abstract

In a method for regulating a fuel cell stack (1), a current-voltage characteristic of the fuel cell stack is detected and evaluated to determine an operating point of the fuel cell stack, wherein a current-voltage characteristic of the fuel cell stack (1) is detected at time intervals in operation whose gradient has a minimum, a characteristic value (R.sub.min) for the minimum of the gradient is respectively determined from the detected current-voltage characteristic and a desired value for the operating point is determined by addition of a predefined offset value (R.sub.offset) to the characteristic value, and wherein the fuel cell stack (1) is regulated by the desired value determined in this manner.

Claims

1. A method for regulating a fuel cell or a fuel cell stack comprising: detecting a current-voltage characteristic of the fuel cell or of the fuel cell stack at time intervals during operation of the fuel cell or of the fuel cell stack, wherein the detected current-voltage characteristic has a gradient with a minimum; determining an operating point of the fuel cell or of the fuel cell stack by determining a first value corresponding to a value for the minimum of the gradient or a resistance value related to the minimum of the gradient, from the detected current-voltage characteristic, and adding a predefined offset value to the first value; and regulating the fuel cell or the fuel cell stack using the determined operation point.

2. A method in accordance with claim 1, wherein the fuel cell or the fuel cell stack are regulated via a regulable consumer or a regulable current sink which are connected to the output of the fuel cell or of the fuel cell stack.

3. A method in accordance with claim 1, wherein the regulable consumer is a voltage converter or an inverter whose outputs are connectable to a power grid.

4. A method in accordance with claim 1, wherein the value for the minimum of the gradient is the value of the internal resistance or of the area specific resistance of the fuel cell or of the fuel cell stack in the minimum of the gradient or wherein the resistance value related to the minimum of the gradient is linked with the value of the internal resistance or of the area specific resistance of the fuel cell or of the fuel cell stack in the minimum of the gradient.

5. A method in accordance with claim 1, wherein the value for the minimum of the gradient or the resistance value related to the minimum of the gradient is mathematically determined from the current-voltage characteristic-numerically or by mathematical derivation from the current-voltage characteristic.

6. A method in accordance with claim 1, wherein the fuel cell or the fuel cell stack is voltage controlled by regulating cell voltage or output voltage, or wherein the fuel cell or the fuel cell stack is current controlled by regulating current or current density.

7. A method in accordance with claim 1, wherein first a sum value is determined by addition of the predefined offset value to the first value and wherein a value corresponding to the sum value is determined with the aid of the current-voltage characteristic for cell voltage or for output voltage or for current or for current density which serves as a desired value for regulation of the fuel cell or of the fuel cell stack.

8. A method in accordance with claim 1, wherein the current-voltage characteristic of the fuel cell or of the fuel cell stack is detected in operation a second time after at least 200 h or at least 500 h or at least 1000 h or every 200 h or every 500 h or every 1000 h and the operating point is determined again from the current-voltage characteristic detected the second time.

9. A regulating apparatus for a fuel cell, a fuel cell stack or a fuel cell system, configured to regulate the fuel cell or the fuel cell stack or the fuel cell system by means of a method in accordance with claim 1.

10. A regulating apparatus in accordance with claim 9, wherein the regulating apparatus is connected to an output of the fuel cell or of the fuel cell stack, and contains a measuring and regulating unit which are configured to detect the current-voltage characteristic of the fuel cell or of the fuel cell stack and to determine the value for the minimum of the gradient or the resistance value related to the minimum of the gradient from the detected current-voltage characteristic in order to determine the operating point.

11. A regulating apparatus in accordance with claim 9, additionally containing a regulable consumer or a regulable current sink which can be connected to the output of the fuel cell or of the fuel cell stack to regulate the fuel cell or the fuel cell stack via the regulable consumer or the regulable current sink.

12. A fuel cell or fuel cell stack or fuel cell system having a regulating apparatus in accordance with claim 9.

Description

(1) The invention will be explained in more detail in the following with reference to the embodiments and to the drawing. There are shown:

(2) FIG. 1 a typical power-current curve of a fuel cell;

(3) FIG. 2 two typical current-voltage characteristics of a fuel cell for two different internal resistances without taking account of the gas consumption;

(4) FIG. 3 two typical current-voltage characteristics of a fuel cell for two different internal resistances while taking account of the gas consumption,

(5) FIG. 4 three typical current-voltage characteristics of a fuel cell for three different fuel gas flows or fuel gas qualities without taking account of the internal resistance;

(6) FIG. 5 three typical current-voltage characteristics of a fuel cell for three different fuel gas flows or fuel gas qualities while taking account of the internal resistance;

(7) FIG. 6 a typical curve of the area specific resistance (ASR) of a fuel cell;

(8) FIG. 7 typical ASR curves of a new and of an aged fuel cell;

(9) FIG. 8 typical ASR curves of a fuel cell with two different fuel gas flow rates or fuel gas qualities; and

(10) FIG. 9 a fuel cell stack having an embodiment of a regulating apparatus in accordance with the present invention.

(11) FIG. 9 shows a fuel cell stack having an embodiment of a regulation apparatus 9 in accordance with the present invention. An arrangement of a fuel cell stack having a regulating apparatus is also called a fuel cell system in the following. The fuel cell stack 1 can, for example, be made up of high-temperature fuel cells of the SOFC (solid oxide fuel cell) type which are typically operated at a temperature of 600 C. to 1000 C. and allows the utilization of energy of a fuel by the energy conversion. In this respect, both electric energy, which is generated on the basis of electrochemical processes, and thermal energy, which arises in the form of hot waste gases, can be utilized. Gaseous flows of two reactants A, B are conducted separately through the cells. The first reactant A, which is in particular environmental air, contains oxidizing components; the second reactant B reducing components.

(12) A gas containing methane (e.g. natural gas) is advantageously used as the second reactant B which is conducted through a reformer before the entry into the cells and is there converted into the reducing components of hydrogen and carbon monoxide by means, for example, of an additional infeed of water W and optionally air and while supplying process heat. The hot waste gas can advantageously be used as the source for the process heat required in the reformer.

(13) The fuel cell stack 1 is as a rule connected to a heat exchanger 2 in which heat can be extracted from the hot waste gases of the fuel cell stack. The heat exchanger 2 is advantageously connected to a heating circuit 2. The waste gases C can subsequently be led into the free air or the residual oxygen in the waste gases can be utilized in an additional burner which is not shown in FIG. 9.

(14) In the embodiment shown, a regulating apparatus 8 is connected to an output 9 of the fuel cell stack 1. The regulating apparatus 8 contains a measuring and regulating unit 6 which is configured to detect a current-voltage characteristic of the fuel cell stack in time intervals whose gradient has a minimum, for example by means of a current sensor 4 and of a voltage sensor 5, and to determine a value for the minimum of the gradient or a value R.sub.min related to the minimum of the gradient from the detected current-voltage characteristic in order to determine a desired value for the operating point by a mathematical linking of the determined values with a predefined offset value R.sub.offset, for example by addition of a predefined offset value to the determined value, and to regulate the fuel cell stack 1 using the desired value determined in this manner.

(15) In an advantageous embodiment, the regulating apparatus 8 additionally contains a regulable consumer or a regulable current sink 3 which is connected to the output 9 of the fuel cell stack to regulate the fuel cell or the fuel cell stack via the regulable consumer or via the regulable current sink. For this purpose, the measuring and regulating unit 6 can be connected to the regulable consumer or to the regulable current sink via a line 7.

(16) The regulable consumer 3 is advantageously a voltage converter or an inverter whose outputs can be connected to a power grid 3 to feed the current generated in the fuel cell stack into the power grid.

(17) The invention furthermore includes a fuel cell or a fuel cell stack 1 or a fuel cell system having a regulating apparatus or an embodiment of the regulating apparatus in accordance with the above description.

(18) An embodiment of a method in accordance with the present invention will be described with reference to FIGS. 6 to 9. In the method for regulating a fuel cell or a fuel cell stack 1, a current-voltage characteristic of the fuel cell or of the fuel cell stack is detected and evaluated to determine an operating point of the fuel cell or of the fuel cell stack. The method is characterized in that a current-voltage characteristic of the fuel cell or of the fuel cell stack is detected at time intervals in operation whose gradient has a minimum; in that a value for the minimum of the gradient or a value R.sub.min related to the minimum of the gradient is respectively determined from the detected current-voltage characteristic; in that a desired value is determined for the operating point by a mathematical linking of the determined value with a predefined offset value R.sub.offset, for example by addition of a predefined offset value to the determined value; and in that the fuel cell or the fuel cell stack is regulated using the desired value thus determined.

(19) The fuel cell or the fuel cell stack 1 are advantageously regulated via a regulable consumer or via a regulable current sink 3 which are connected to the output 9 of the fuel cell or of the fuel cell stack 1, with the regulable consumer 3, for example, being able to be a voltage converter or an inverter, for example, whose outputs 3 can be connected to a power grid to feed the current generated in the fuel cell or in the fuel cell stack into the power grid.

(20) The value for the minimum of the gradient can, for example, be the value of the internal resistance or of the area specific resistance (ASR) of the fuel cell or of the fuel cell stack in the minimum of the gradient or the value R.sub.min related to the minimum of the gradient can from case to case be linked to the value of the internal resistance or of the area specific resistance of the fuel cell or of the fuel cell stack in the minimum of the gradient. The area specific resistance ASR is calculated from the internal resistance of a fuel cell in that the internal resistance is multiplied by the electrochemically active area of the fuel cell or in that the internal resistance of a duplicating unit of a fuel cell stack is multiplied by the electrochemically active area of a duplicating unit.

(21) The value for the minimum of the gradient or the value R.sub.mm related to the minimum of the gradient is mathematically determined from the current-voltage characteristic, for example numerically or by mathematical derivation of the current-voltage characteristic.

(22) In two advantageous embodiment variants, the fuel cell or the fuel cell stack 1 is voltage controlled, i.e. regulated with a desired value U.sub.soll for the cell voltage or the output voltage, or it is current controlled, i.e. regulated with a desired value I.sub.soll for the current or the current density.

(23) In an advantageous embodiment, first a sum value R.sub.Summe=R.sub.min+R.sub.offset is determined by addition of a predefined offset value R.sub.offset to the value for the minimum of the gradient or to the value R.sub.min related to the minimum of the gradient and a value corresponding to the sum value is determined with the aid of the current-voltage characteristic for the cell voltage or for the output voltage U.sub.soll or for the current or for the current density I.sub.soll which serves as a desired value for the regulation of the fuel cell or of the fuel cell stack 1.

(24) It is assumed by way of example in the following that the value R.sub.min for the minimum of the gradient of the current-voltage characteristic of the fuel cell or of the fuel cell stack corresponds to the value of the area specific resistance ASR in the minimum of the gradient.

(25) FIG. 6 shows a typical curve ASR(I) of the area specific resistance ASR of a fuel cell in dependence on the current density I. The curve shown was obtained in that the output voltage of a fuel cell was detected in dependence on the current density I and subsequently the gradient, i.e. the area specific resistance ASR(I) was determined by mathematical derivation. The curve ASR(I) of the area specific resistance has, as shown in FIG. 6, a minimum ASR.sub.min characteristic for fuel cells.

(26) Corresponding to the embodiment of the method described in the preceding paragraph, first a sum value R.sub.Summe=ASR.sub.min+ASR.sub.offset is determined, called ARS.sub.opt in FIG. 6, by addition of a predefined offset value ASR.sub.offset to the value ASR.sub.min for the minimum of the gradient and a value corresponding to the sum value ASR.sub.opt is determined for the output voltage U.sub.soll or for the current density I.sub.soll with the aid of the current-voltage characteristic, said value serving as a desired value for the regulation of the fuel cell or of the fuel cell stack 1. The value I1 for the current density corresponding to the sum value ASR.sub.opt can be seen directly from the curve of ASR(I) in FIG. 6.

(27) The value of ASR.sub.min varies in dependence on the fuel cells used, on the degradation, on the fuel quantity, on the fuel quality and on any leaks. The value of ASR.sub.min typically lies in the range between 0.3 cm.sup.2 and 1.0 cm.sup.2. The offset value ASR.sub.offsset is characteristic for the respective fuel cell concept and can be determined by experiments. The offset value ASR.sub.offset typically lies between 0.3 cm.sup.2 and 1.0 cm.sup.2, preferably between 0.5 cm.sup.2 and 0.8 cm.sup.2.

(28) In practice, the total curve of ASR(I) does not need to be detected. It is rather sufficient first to determine the value ASR.sub.min for the minimum of the gradient from the current-voltage characteristic. The sum value ASR.sub.opt=ASR.sub.min+ASR.sub.offset is determined from the respective determined value ASR.sub.min and the offset value ASR.sub.offset characteristic for the respective fuel cell technology. The current-voltage characteristic is now detected and evaluated until the gradient reaches the value ASR.sub.opt. At this point, the corresponding value of the current density I.sub.soll is determined which corresponds to the value of the current density I1 in FIG. 6 and/or the corresponding value of the voltage U.sub.soll is determined.

(29) The fuel cell stack is advantageously subsequently regulated and/or operated with voltage control with U=U.sub.opt.

(30) In a further advantageous embodiment, a current-voltage characteristic of the fuel cell or of the fuel cell stack is detected in operation after at least 200 h or at least 500 h or at least 1000 h or every 200 h or 500 h or every 1000 h and the desired value is determined again from the detected current-voltage characteristic.

(31) The behavior with an aged fuel cell is explained with reference to FIG. 7. In FIG. 7, the ASR curve of the fuel cell of FIG. 6 (curve ASR.sub.1(I)) is shown and by way of example that of an aged fuel cell (curve ASR.sub.2(I)). It can be recognized that despite differences in the sum values ASR.sub.opt1 and ASR.sub.opt2 determined from the respective ASR curves, an almost equally high value for the desired value of the current density results (I1I2). Despite the aging of the cell, the stack can be operated close to the maximum power, i.e. close to the optimum.

(32) The behavior with a changed fuel gas quality or with a changed fuel gas flow will be explained with reference to FIG. 8. In FIG. 8 the ASR curve of the fuel cell of FIG. 6 (curve ASR.sub.1(I)) and by way of example that of a fuel fell which is operated at a 10% lower fuel gas flow is shown (curve ASR.sub.3(I)). It can be recognized that the sum values ASR.sub.opt1 or ASR.sub.opt2 determined from the respective ASR curves are of almost the same magnitude. However, clearly different desired values for the current density (I1I3) result from the different course of the curves ASR.sub.1(I) and ASR.sub.3(I).

(33) An additional benefit of the above-described method and of the above-described regulation apparatus for regulating a fuel cell or a fuel cell stack results when gas flow sensors are used with which the fuel gas flow and the minimal air requirement (L.sub.min) of the gas used can be determined sufficiently precisely. In this case, the current density can be calculated at the ideal operating point through Faraday's Law. The described method can then be used for monitoring or also for calibrating the sensor during operation. Calibrated sensors are currently not available at prices which allow a use in small fuel cell devices to appear sensible.

(34) The method and the regulating apparatus for regulating a fuel cell or a fuel cell stack as well as the fuel cell and the fuel cell stack in accordance with the present invention have the advantage that the fuel cell or the fuel cell stack can be reliably operated even if the operating conditions vary over time since influence factors such as the age of the fuel cells, the fuel quantity, the fuel quality and any leaks are mapped in the gradient curve of the current-voltage characteristic and are thus taken into account in the regulation of the fuel cell or of the fuel cell stack. This allows a safe operation of the fuel cell or of the fuel cell stack close to the maximum power, i.e. close to the optimum.