Patent Application
20090029210
The invention relates to a low-temperature fuel-cell stack and method of operating the same.
Prior-art fuel-cell stacks normally have a central operating-media manifold. The individual fuel cells are supplied from this feed through supply lines. The operating medium is then as a rule subsequently distributed as uniformly as possible across the surface of a membrane electrode unit (MEA) via passages in the bipolar plate or through network structures.
The prior-art devices, however, have the disadvantage that the desired uniform distribution of the operating agents across the individual cells frequently cannot be implemented satisfactorily. In particular, in water droplets formed within the cells, gas bubbles, and imprecisions in fabrication regularly result in an undersupply of the individual fuel cells. However, specifically in the case of direct-methanol fuel cells a precise and rapid control of the methanol concentration in the individual cells is required in order to maximize the efficiency and the power output.
The object to be attained by the invention is to provide a method by which the adjustment of defined concentrations of methanol for individual fuel cells is possible in one stack. In addition, the object to be attained by the invention is to provide a corresponding device to implement the method.
The objects of the invention are attained by a low-temperature fuel-cell stack comprising the totality of features specified in the main claim, as well as by a method specified in the corresponding claim. Advantageous embodiments of the method and of the low-temperature fuel-cell stack are found in the respective claims related thereto.
The invention relates to a method by which it is possible to adjust or to provide a defined concentration of fuel or water for adding humidity at certain supply points to the individual fuel cells in a low-temperature fuel-cell stack.
To this end, sensors are located a various points within the fuel-cell stack that are able to provide information on the concentration of the fuel, on the humidity of the oxidation gas, or on the cell voltage. This information functions as a controlling element for one or a plurality of dosing devices that are located at certain points of the fuel-cell stack. Suitable dosing devices include, in particular, controllable valves, controllable piezoelectric pumps, or thermally controllable feeds.
The usual means can be utilized for the dosing devices. The method of dosing can be effected, for example, in a manner analogous to a bubble-jet method, i.e. as in ink-jet printers. There are essentially two different methods here for generating an ink droplet: 1) piezoelectric pumps, or 2) pumps in which part of the ink is vaporized by a brief high application of energy, thereby driving the ink out of the nozzle head. A dosing device functioning in this manner comprising a thermally controllable feed is what is meant within the scope of this invention.
Each of the dosing devices is connected to an additional supply device or storage device that is filled with an operating medium, fuel, or oxidation medium. This supply device or storage device can be, in particular, an additional pressurized line that, for example, leads into a reservoir.
During the operation of a low-temperature fuel-cell stack, the fuel is passed through the usual central supply line to the individual cells. Sensors installed at suitable points advantageously respectively determine the specified control information, such as, i.e. the local concentrations of operating media, the humidity content of the oxidation medium, or also appropriate cell voltages for the individual cells. Whenever a sensor detects a deviation from a previously defined set-point value, this results in a controlling action by a dosing unit that is associated with this sensor and is advantageously located within the spatial proximity of the sensor. An undersupply of an operating medium, in particular fuel, which undersupply is detected at a sensor by a lower concentration relative to the set-point value, results in an activation of the corresponding dosing unit that then feeds additional fuel to this point. In the event of an excessively low humidity of the oxidation medium, water, for example, can be supplied through the dosing units.
The actual dosing can be implemented based on conventional control algorithms. What is conceivable is both a pulsed metered dosing, or, on the other hand, also a continuous metered dosing, until the sensor detects a signal that corresponds to the specified set-point value. This then stops the dosing unit. Also conceivable is control system that operates using multiple limit values instead of one specified set-point value. The dosing for fuel here is advantageously effected with concentrated fuel, i.e. in particular for a methanol-water fuel cell with highly concentrated or even pure methanol.
In addition to the usual central supply of operating media, the low-temperature fuel-cell stack according to the invention has at least one additional supply or feed device for a liquid operating medium, in particular, a liquid fuel or water. This device can, for example, be an additional supply line that is fed in particular with a more highly concentrated fuel.
The new concept for a direct fuel-cell stack provides the following technical features:
the fuel-cell stack has at least one additional interior chamber to store a liquid operating agent, in particular, an additional supply or storage device for a liquid operating medium;
the liquid operating media are, in particular, methanol, a highly concentrated methanol solution, or also water;
the interior chamber is connected at multiple points to the dosing devices;
the dosing devices lead to a supply line that feeds an operating medium to the individual cells;
the dosing devices are separately controllable;
the dosing devices are, in particular, piezoelectric pumps, or thermally controlled feed, or analogously controlled valves;
the valves or pumps are controlled by a controller that collects information on the load or operating states of the individual cells.
These fundamental technical features advantageously enable the concentrations of operating media to be locally adjusted for the supply to the individual fuel cells.
The afore-mentioned design of the low-temperature fuel-cell stack can be employed both on the anode side for dosing the fuel and also advantageously on the cathode side, where water is then advantageously used as the liquid medium for humidifying purposes.
The following discussion describes the subject matter of the invention in more detail based on figures, without the subject matter of the invention's being limited thereby.
In contrast thereto,
Suitably installed sensors enable a highly precise control or regulation of the supply of an operating medium to be effected as long as the sensor information is provided to the individually controllable dosing devices.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2005 015 660.6 | Apr 2005 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/DE2006/000481 | 3/18/2006 | WO | 00 | 10/16/2008 |
