Fuel cell system

Abstract

Fuel cell system (1) with a fuel cell (2) and an actuator (3) controlled by a control unit (4) for removing residual gas from a fuel flow of the fuel cell (2), characterized in that a detection means (20) detects the power consumption, the rotational speed and / or stroke frequency of a recirculation device and the control unit (4) determines a fuel concentration in the fuel flow by appropriately converting the provided signal, and depending on this, removes residual gas from the fuel flow by actuating the actuator (3).

Description

[0001] The present invention relates to a fuel cell system according to the preamble of claim 1. State of the art

[0002] Fuel cells, especially those with a proton-conducting polymer film as a partition between the anode and cathode (PEM fuel cells), are operated on the anode side with a fuel gas such as gaseous hydrogen or hydrogen-containing gas mixtures such as reformate gases.

[0003] To improve the efficiency of such fuel cells, it is known to recirculate the residual gas exiting the anode side of the fuel cell back to the anode input side. However, since the fuel cell only consumes pure fuel during operation, usually hydrogen, such recirculation of the fuel flow causes the concentration of pure fuel in the anode region of the fuel cell to decrease over the course of operation.

[0004] This is due to the accumulation of residual gases in this circulation loop, which either originate from the fuel gas supply or diffuse from the cathode side (the side of the fuel cell supplied with air) to the anode side. These gases are primarily nitrogen and water vapor. Because of the opposing behavior—an increase in the residual gas content and a decrease in the pure fuel content in the fuel cell—the electrical voltage of the fuel cell also decreases. With high fuel gas consumption, the dynamics of gas transport to the proton exchange membrane (PEM membrane) also slow down increasingly.

[0005] To prevent excessive accumulation of residual gases in the anode area of ​​the fuel cell, it is known to either continuously release the gas escaping from the anode area of ​​the fuel cell to the environment via a throttle, or, in another embodiment, to release it at regular intervals via a correspondingly controlled valve. Such valves are known, for example, as "purge valves".

[0006] Document US 2004 / 0214059 A1 discloses a fuel cell system in which no gases need to be removed from the anode system. The fuel cell system comprises a fuel cell, an anode system with a recirculation line, a pump, a fuel concentration sensor, and a pump controller.

[0007] Document US 2005 / 0214603 A1 discloses a fuel cell system in which impurities in the gas composition in the anode system are detected using an acoustic sensor.

[0008] Document US 2004 / 0110048 A1 discloses a fuel cell system in which the nitrogen concentration of the exhaust gas recirculated in the anode system is determined and the increase in impurities contained in the circulating exhaust gas is determined in advance based on the amount of hydrogen in the main stream and the amount of circulating exhaust gas.

[0009] Document US 2006 / 0003204 A1 discloses a fuel cell system in which a purge valve is controlled in response to a parameter of a recirculation blower and a pulse width modulation control signal is provided to control the purge valve.

[0010] Document WO 2004 / 105165 A2 discloses a fuel cell system in which a load detector monitors the load on a hydrogen circulation device. When the load exceeds a certain value, a purge valve controller detects that the amount of nitrogen accumulated in the hydrogen circulation path has exceeded a certain value and opens the purge valve. When it is determined that the load on the hydrogen circulation device falls below the specified value, the purge valve closes, as the nitrogen purge is considered complete. Purpose and advantages of the invention

[0011] The present invention is therefore based on the objective of improving a fuel cell system of the type described above.

[0012] This problem is solved by the features of claim 1. Advantageous and expedient embodiments of the invention are specified in the dependent claims.

[0013] Accordingly, the present invention relates to a fuel cell system comprising a fuel cell and an actuator controlled by a control unit for the discharge of residual gas from a fuel cell operating fluid stream. According to the invention, it is characterized in that the control unit includes a control and / or regulation system that takes into account the operating fluid concentration in the operating fluid stream.

[0014] This approach is based on the understanding that conventional methods for removing residual gases—either continuously via a throttle or by opening a valve at regular intervals—not only remove the unwanted portion of residual gases from the fuel cell's operating or fuel stream at the anode side, but also, inevitably, fuel that has not yet been converted by the anode. The lower the concentration of the residual gas, and thus the higher the concentration of the fuel (preferably hydrogen), the greater the losses due to the removal of this unused fuel. This finding applies to both previously known methods for removing residual gases.

[0015] Building on this, the present invention proposes that residual gas be discharged from the operating fluid stream via the actuator only when the concentration of the operating fluid or fuel falls below a certain, predefinable value or when certain undesired concentrations of other substances in the residual gas exceed an impermissible value. This avoids fuel losses due to the unnecessary discharge or venting of residual gas from the operating fluid stream into the fuel cell, thereby increasing the efficiency of the fuel cell.

[0016] To determine the partial pressure or concentration of the fuel in the fuel stream, a fuel or operating fluid concentration sensor can be provided in a first embodiment, for example, which transmits a corresponding signal to the control unit that actuates the actuator. A fuel sensor with particularly high sensitivity, especially for high fuel concentrations, is particularly preferred for this purpose, thus providing a good, high-resolution, and accurate signal in this operating range. For hydrogen-powered fuel cells, this would be a correspondingly sensitive hydrogen sensor.

[0017] In addition to the approach of monitoring the fuel concentration in the operating fluid stream to control the actuator for the discharge of residual gas, a second approach is proposed below, which can be implemented separately or in conjunction with the first. Here, the nitrogen concentration in the fuel stream is monitored by a nitrogen concentration sensor connected to the control unit. If a certain nitrogen concentration threshold is exceeded, the control unit, as described above, initiates the discharge of residual gases from the operating fluid stream.

[0018] In a further embodiment, the concentration of water vapor in the fuel stream can be monitored, either alone or in combination with one or both of the above-mentioned procedures, and if a certain maximum value is exceeded, the control unit can be prompted to release residual gas via the actuator.

[0019] In addition to these direct monitoring methods for concentration values, the present invention also presents an indirect monitoring method in which a measuring device for the quantity of operating fluid conveyed is provided. For this purpose, a relevant parameter of the power consumption of an operating fluid recirculation device can be used, for example. Current, voltage, and / or power measuring devices, such as corresponding converters, are particularly suitable here, as their signals are tapped and evaluated to initiate appropriate measures.

[0020] The recirculation device can be, for example, a recirculation compressor, a recirculation pump, or similar equipment. Since these devices have some kind of rotary or translational actuation component, the fuel concentration in the operating fluid flow can also be measured by evaluating the signal from a suitably positioned sensor, such as a speed or stroke frequency meter. Volume flow measurement is also conceivable, for example, using a flow meter, such as a paddle wheel flow meter.

[0021] In particular, by comparing with known power consumptions or rotational speeds, frequencies or volume flows that can be assigned to specific fuel concentrations, a clear statement about the concentration of pure fuel in the fuel stream is possible, and thus in turn an exact influence on the discharge of residual gas from the operating fluid stream by actuating the control element via the control unit.

[0022] In a simple embodiment of the control unit, the actuator can be operated using, for example, a time-based control mechanism. This allows for a so-called "purge process" to be initiated, i.e., the release of residual gas by opening the actuator, after a certain, predefined time period. This process is then terminated and, if necessary, repeated until the resulting proportion of foreign gas has decreased to a permissible level or until the fuel has a sufficiently high concentration of pure fuel.

[0023] In addition to such a relatively simple control system, a more sophisticated embodiment can also include a control mechanism for valve actuation. In this case, the control unit can, for example, regulate the flow rate of residual gas through the actuator by varying its opening time. In a further improved embodiment, the effective cross-section of the actuator can also be varied to influence the flow rate of residual gas through it. The actuator could, for example, be a switching valve in the first case and a proportionally controlled valve or a regulating valve in the second.Thus, a variety of control methods are possible, for example, a control system in which the setpoint is achieved by simply opening the actuator, or one in which feedforward control is performed depending on the pressure in the anode path, and precise dosing is achieved through a superimposed control system, and so on. Example of implementation

[0024] The invention is explained in more detail with reference to the drawings and the description that follows. Accordingly, the accompanying figure shows a schematic representation of a fuel cell system as a block diagram.

[0025] The fuel cell system 1 according to the accompanying figure therefore comprises a fuel cell 2 and an actuating element 3 for releasing residual gas from an anode-side fuel stream, which is actuated by a control unit 4 equipped with a control and / or regulation 4.1 depending on certain operating parameters. The operating parameters can preferably be the concentration values ​​of the pure fuel or the other residual gas components such as nitrogen or water vapor.

[0026] To supply the fuel cell 2 with fuel, a fuel storage unit 5 is connected, for example, to a fuel supply line 9 via a valve 6 and, downstream in the supply direction, a pressure regulator 7 and a fuel metering unit 8. The fuel gas supplied in this way, preferably hydrogen or a hydrogen-containing gas mixture such as reformate gas, enters the anode side A of the fuel cell 2 through the inlet 10 in the form of a volume flow at a corresponding pressure.

[0027] During operation of fuel cell 2, only a portion of the hydrogen is used to generate electrical energy. The unused portion of the supplied gas exits through the anode-side outlet 11 of fuel cell 2. To prevent significant hydrogen losses, this residual gas is fed back into the supply line 9 via a recirculation device 13 and a recirculation path 12 at a return point 14.

[0028] The two input and output sectors 15 and 16 are symbolically represented in the drawing for coupling the control unit 4 to the various units of the fuel cell system, as well as for supplying energy and, if necessary, outputting further signals to other control and / or regulation units. In particular, these two vector connections encompass the signal transmission from detection units 17, 18, and 19 to the control unit 4, and from the control unit 4 to the actuator 3.

[0029] A first detection unit is represented, for example, by the operating fluid concentration sensor 17, which is connected to the recirculation path 12 at output 11. Preferably, in this exemplary embodiment, this is a hydrogen sensor with high sensitivity in the range of high hydrogen concentrations, so that the control unit 4 can be provided with the finest possible resolution and accuracy for the most common operating condition.

[0030] If the concentration of pure fuel in the residual gas falls below a predefined minimum value, the control unit 4 transmits a signal via output vector 16 to actuate the actuator 3 in order to discharge a specific quantity of residual gas from the fuel stream via this so-called "purge device." This results in a comparatively more concentrated fuel flowing into the anode area of ​​the fuel cell, as it is now essentially supplied only via the fuel metering unit provided by the fuel storage unit 5. With this increased fuel partial pressure, the electrical voltage induced by the fuel cell at the electrical connection elements of the fuel cell 2 (not shown here) also increases. The dynamics of the fuel cell also improve significantly, particularly under high fuel gas consumption.

[0031] A second possibility for signaling decreasing fuel concentration in the fuel stream is provided by the arrangement of a nitrogen concentration sensor 18, which is also connected to the control unit 4 via the input vector 15. If a certain nitrogen concentration is exceeded, which can be caused, for example, by diffusion from the cathode side K of the fuel cell 2 to the anode side A, the purge valve 3 can again be actuated by the control unit 4, as described above.

[0032] Another way to actuate this purge valve 3 due to impermissible conditions in the operating fluid flow is, for example, by a signal indicating an excessively high water vapor concentration from a corresponding concentration sensor 19. For clarity, this water vapor concentration sensor 19 is also shown as a nitrogen concentration sensor 18. For implementation, either only one of these sensors can be arranged, or two in separate configurations. A combined embodiment with both sensors, or with the operating fluid concentration sensor 17, is also conceivable. Alternatively, all three can be combined.

[0033] Another possibility for actuating the purge valve 3 is proposed by detecting a relevant parameter of the power consumption of the operating fluid recirculation device 13. In a preferred embodiment, such a detection means is a current-, voltage-, or power-detecting electrical or electronic component, which is preferably housed in the control unit 4 so that no additional signal lines are required. Of course, however, a separate design of these components is also possible.

[0034] Another possibility for controlling or regulating the purge valve, depending on the embodiment of the valve or the control unit 4, is by detecting the rotational speed of a rotating element of the recirculation device 13, for example by means of a tachometer 21. The signal provided to the control unit 4 by a corresponding conversion of the rotational speed can, for example, represent an input vector of a computational algorithm, which, in conjunction with a stored set of parameters, serves to actuate the purge valve 3.

[0035] In particular, by monitoring fuel concentrations and / or concentrations of other unwanted gas components in the residual gas, corresponding to different engine speeds as measured by the tachometer, an exact determination of the residual gas composition is possible. The control of the purge valve 3 by the control unit 4 can vary in complexity. In a simple embodiment, a purge valve, designed as a simple switching valve, can be activated for a specific period of time when a critical value of a residual gas component falls below or exceeds a certain threshold, and then closed again by removing this signal. For more demanding or precise metering, a control system can also be provided in which the control unit 4 switches a correspondingly designed valve 3 either in different stages or, if necessary, regulates it in stages or continuously.With such a design, the duration of the purging process can be optimally adapted to the respective operating state of the fuel cell.

Claims

[1] Fuel cell system (1) comprising a fuel cell (2) and an actuator (3) controlled by a control unit (4) for the discharge of residual gas from a fuel cell (2) operating fluid stream, characterized by , that a detection means (20) detects the power consumption, the rotational speed and / or stroke frequency of a recirculation device and the control unit (4) determines an operating fluid concentration in the operating fluid stream by appropriate conversion of the signal provided, and depending on this, discharges residual gas from the operating fluid stream by actuating the control element (3). [2] Fuel cell system according to claim 1, characterized by , that an operating fluid concentration sensor (17, 18, 19) is provided. [3] Fuel cell system according to claim 2, characterized by, that the preferred operating range of the fuel concentration sensor (17, 18, 19) corresponds to the predominant fuel concentration range of the fuel cell. [4] Fuel cell system according to any of the preceding claims, characterized by , that a hydrogen concentration sensor (17) is provided. [5] Fuel cell system according to any of the preceding claims, characterized by , that a nitrogen concentration sensor (18) is provided. [6] Fuel cell system according to any of the preceding claims, characterized by , that a water vapor concentration sensor (19) is provided. [7] Fuel cell system according to any of the preceding claims, characterized by , that the control unit (4) regulates the flow rate of residual gas by varying the period and / or the injection time for a pulse valve and / or the valve stroke for a proportional valve. [8] Fuel cell system according to any of the preceding claims, characterized by , that the control unit (4) regulates the flow rate of residual gas by varying the effective cross-section or the actuation times of the actuator (3).

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