FR2865069A1 - Fuel cell system useful in providing continuous power supply to electrical load, comprises electrical energy storage device(s) electrically connectible with and/or disconnectable from fuel cell stack by electronic switching element(s) - Google Patents

Abstract

A fuel cell system comprises a fuel cell stack (2) and electrical energy storage device(s) (7) electrically connectible with and/or disconnectable from the fuel cell stack by electronic switching element(s) (8). The fuel cell stack is cooled using a cooling circuit (3). The switching element(s) is in thermally conductive contact with the cooling circuit.

Description

Description

  A fuel cell system having a fuel cell stack and at least one electrical energy storage device that can be separated and / or electrically connected to the fuel cell stack. via at least one electronic switching element, the stack of fuel cells being cooled.

  A fuel cell system of this type is described in DE 101 25 106 A1. The fuel cell system described therein is provided with an electronic preference switch, by means of which a synchronized current flow can be taken from the stack of fuel cells.

  The setting of the fuel cell stack is such that when a critical limit value is reached, for example a minimum voltage, a critical pressure, a predetermined internal resistance or the like, the current leaving the stack of cells The fuel switch is interrupted by the switch to prevent the voltage of each of the fuel cells or some of them from falling below a damaging limit. Typically, such a damaging limit is 0 volts for a battery voltage. When the flow of current leaving the fuel cell is interrupted, the power supply of the consumer is made from the electric energy accumulator, here a combination of a battery and a capacitor with large capacity. The modulated pulse width switching imposes very high stresses on the switching element itself, in particular for high frequencies, since the currents must generally be switched in load and the internal resistance of the switching element should be kept as low as possible to avoid significant loss of power.

  This level of requirement can be achieved by using semiconductor switches, for example in the form of field effect transistors based on semiconductor oxide metal components, commonly referred to as MOS F.E.T. The extremely short switching times of the MOS F.E.T., and this also in load, make it possible to obtain high pulse width modulation frequencies while recording only moderate power losses. In addition, such MOS F.E.T. generally support extremely large current pulses, such as those created for example when low-resistance intermediate accumulators are used, such as the capacitance capacitor previously mentioned.

  Despite the relatively moderate power loss of such electronic switches, high currents and powers produce a relatively large amount of heat due to power losses. Having to plan specific measures to evacuate this loss of power manifesting itself in the form of thermal energy is a serious drawback of the already very complex fuel cell system.

  The ambient temperature of the electronic switches is relatively high, particularly in fuel cell systems that draw the necessary hydrogen from gas generator systems, which makes convection cooling by means of cooling elements more difficult. or conventional cooling bodies.

  The object of the present invention is therefore to create a structure avoiding the aforementioned drawbacks and optimizing the application of electronic switches in fuel cell systems such as those presented in the preamble.

  This object is achieved with the fuel cell system of the invention wherein at least one of the switching elements is thermally coupled with a cooling circuit for cooling the fuel cell stack.

  The thermal coupling of the switching elements with the cooling system already present in a stack of fuel cells, for example based on PEM fuel cells, makes it possible to use the cooling circuit already present to cool the electronic switching elements. Cooling of the fuel cell stack by the cooling circuit is typically configured as active and tuned cooling. In the case of a stack of PEM fuel cells, the temperatures are for example set to approximately 70 to 90 C, with a tolerance of 1 C to ensure its proper operation. Since this temperature range is very well supported by the generally used electronic switching elements based on semiconductor components, the cooling circuit can take care of both the cooling of the fuel cell stack and the cooling of the components. of power electronics.

  In a particularly advantageous configuration of the invention, provision is made to configure the switching element in the form of a semiconductor component provided with at least one switch. According to a very advantageous improvement, this switch can be configured in the form of a semiconductor oxide metal oxide field effect transistor (MOS F.E.T.), in particular in the form of a MOS F.E.T. with channel n. The advantage of such MOS F.E.T. This is due to the extremely short switching times already mentioned, and this also in load, for moderate power losses at the internal resistance of the MOS F.E.T., as well as in the very high switching frequencies that the MOS F.E.T. allow to reach. The use of MOS F.E.T. With n-channel it furthermore has the known advantage that the n-channel semiconductor components have lower internal resistance than the corresponding p-channel components. The use of MOS F.E.T. with p-channel can however be theoretically considered.

  According to another very advantageous configuration of the fuel cell system according to the invention, it is furthermore provided that at least two of the MOS-based switching elements F.E.T. are connected head-to-tail for separating and / or electrically connecting the fuel cell stack and the energy storage device.

  The particular advantage of this type of assembly lies in the separation of the electrical connection due to the head-to-tail assembly of the MOS FETs, which makes it possible to obtain a true electrical separation between the stack of fuel cells and the less an energy storage device. The head-to-tail arrangement makes it possible to eliminate the disadvantage of using the substrate diode typically present in the MOS F. E. With the head-to-toe arrangement, the current flow is securely blocked in both directions when the MOS F.E.T. are locked. The connection between the fuel cell stack and the at least one energy storage device is bipolar electrically separated, thus bidirectional, so that no current can flow from the fuel cell to the fuel cell. at least one energy storage device and, more importantly, to prevent the fuel cell from being damaged, from the energy storage device to the fuel cell.

  According to a very advantageous improvement of the fuel cell system according to the invention, the at least one switching element is in electrically conductive and thermal conductor contact on at least one end plate of the fuel cell stack. .

  In addition to the direct thermal coupling of the switching element with the stack of cooled fuel cells or its also cooled end plates, the ideal cooling of the switching element as well as its electrical connection with the fuel cell is obtained. or its stack of fuel cells. That relatively large currents circulating here, of an order of magnitude up to a few hundred amps, can represent a considerable advantage in terms of difficulty level, mounting, space and cost by saving an electrical connection pipe which would have been otherwise necessary.

  The transfer, in the zone of the end plates of the stack of fuel cells, of the thermal energy originating from the power loss at the level of the switching elements has the further advantage that the temperature prevailing in this zone of the The fuel cell stack is generally slightly lower than the center of the fuel cell stack. The thermal connection connecting the switching element and the end plate of the fuel cell stack therefore ensures, on the one hand, good cooling of the switching element and, on the other hand, heating of the peripheral zone of the fuel cell. stack of fuel cells. The entire stack of fuel cells thus has a more homogeneous temperature on the different zones so that all the fuel cells of the stack can operate more homogeneously. It can further be expected that the performance of the fuel cell stack will be improved.

  Other advantageous configurations of the invention emerge from the variant embodiments in particular because: l'the switching element based on MOS F.E.T. can measure the voltage drop in the internal resistance (RDs0n) of at least one of the MOS F.E.T. ; and / or E each of the switching elements may be provided with at least one electronic switch (MOS F.E.T.) and an integrated circuit for its control; and / or each of the switching elements may be provided with at least one electronic switch (MOS F.E.T.) and a device for measuring and amplifying the voltage drop at the internal resistance (RDs ON) of the switch; and / or each of the switching elements may be provided with a plurality of electronic switches (MOS F.E.T.) connected in parallel; and / or E at least one of the switching elements can be arranged on the end plate of the fuel cell stack connected to the anode, and at least one other switching element can be arranged on the end plate of the stack of fuel cells connected to the cathode; and / or E the switching element may be in heat-conducting contact with the coolant of the cooling circuit prior to its entry into the fuel cell stack.

  An exemplary embodiment is presented below with the aid of the figures which show: FIG. 1 a fuel cell system; Figure 2 a MOS F.E.T. serving as an electrical switching element; Figure 3 a block diagram of such a switching element; Figure 4 a possibility of mounting head to tail switching elements; and Figure 5 a variant of head-to-tail assembly of switching elements.

  FIG. 1 represents a fuel cell system (1) provided with a stack of fuel cells (2) represented here symbolically, said stack of fuel cells being able to be configured for example as a stack of PEM fuel cells; . The stack of fuel cells (2) consists of several fuel cells which are each mounted in series with the others. The fuel cell stack may further comprise a plurality of sub-stacks which in turn are serially connected to one another. The following embodiments are described with a single stack of fuel cells (2), but it is also possible to use several stacks of fuel cells connected in parallel.

  The stack of fuel cells (2) is cooled by a cooling circuit (3) in which a liquid cooling fluid circulates, for example a mixture of water and antifreeze. The cooling circuit (3) comprises, in addition to a heat exchanger (4) located in the stack of fuel cells (2), a circulation device (5) and a heat exchanger (6) for transferring the thermal energy of the coolant to the environment. The stack of fuel cells (2) is actively cooled by this cooling circuit (3) to a predetermined temperature, generally in a range of 70 to 90 C. The active cooling of the fuel cell stack ( 2) is done here with a very high precision since the fuel cells of the stack (2) preferably operate with a very limited temperature variation of a preferred order of magnitude of 1 to 2 K. A sensor ( 5a) located in the cooling circuit (3) is provided for controlling the cooling, said sensor being shown here in the inlet zone of the cooling fluid in the heat exchanger (4). The flow rate of the cooling fluid is set on the basis of the data input by this sensor, as well as possibly depending on the ambient temperature of the heat exchanger (6), for example by means of a fluid circulation device. cooling device (5) controllable or adjustable, as shown here by the dashed arrow, and / or a valve device for influencing the volumetric flow rate of the cooling fluid and / or an aerator (6a). ) located at the heat exchanger (6).

  The fuel cell system (1) further comprises corresponding devices known per se and therefore not shown fuel supply, such as for example hydrogen or a mixture of hydrogen-enriched reforming gas and air. The fuel cell system (1) shown here further comprises an energy storage device (7) which can be connected to and / or electrically separated from the fuel cell by means of at least one switching element. electronic (8). Such an assembly of the fuel cell system (1), as known in the state of the art, serves to supply electrical current to an electrical consumer (9) shown here. The fuel cell system (1) shown here is controlled by pulse width modulated synchronous current pickup from the fuel cell stack (2) via the switching element. (8). The consumer (9) is always connected to the energy storage device (7) and to the stack of fuel cells (2) when the switching element (8) is closed, so that when the switching element (8) is open, the power for the electrical consumer (9) can be taken from the energy storage device (7). The fuel cell system (1) thus allows a continuous power supply of the electric consumer (9) with an ideal use of the fuel cell stack (2) or the feedstock with which it is fed.

  The energy storage device (7) may comprise a battery (7a), it may also, as shown here, include a parallel connection of the battery (7a) and a capacitor (7b) which presents an internal resistance substantially lower than that of the battery (7a). Such an arrangement is described with its various configuration possibilities and advantages in DE 102 25 106 A1 presented previously.

  Fig. 2 again shows the switching element (8) in a possible arrangement. In the representation, the switching element (8) is provided with a switch which is configured as a metal oxide semiconductor field effect transistor, such as a MOS F.E.T. (10). In principle, only one MOS F.E.T. (10) should suffice as a switch in the switching element (8). However, it is possible to use several MOS F.E.T. (10) connected in parallel as a switch in the switching element (8) for mounting with the lowest possible resistance in the on state. This makes it possible, besides obtaining a connection with low resistance in the on state, to insert several MOS F.E.T. (10) of smaller size, which increases the total area required for MOS F.E.T. (10). This increased surface makes it possible to improve the cooling of the MOS F.E.T. (10) since these are cooled by means of the substrate surface, which typically represents at the same time the drain fitting (D). In the exemplary embodiment shown in FIG. 2, the drain (D) of the MOS F.E.T. (10) is connected to the stack of fuel cells (2), for example to the end plate located on the anode side. This link, which is represented by way of example by line (11) in FIG. 2, is ideally mounted directly on the MOS F.E.T. and therefore to its substrate, and therefore to the drain connection (D), on the end plate. This direct editing of the MOS F.E.T. (10) on the end plate of the fuel cell stack (2), which at the same time represents its electrical pole in the generally used fuel cell stacking structures (2), achieves thermal coupling and electrical ideal switching element (8) to the stack of fuel cells (2). This makes it possible to obtain an ideal embodiment of the switching element (8) from the point of view of the electrical and thermal connection, and with minimal investment and without the need for a connection line for currents of a few hundred or so. typically circulating amps in this area.

  The order of MOS F.E.T. (10) is by means of a control voltage applied for example by an electronic control and / or adjustment device in the area of the door (G) of the MOS F.E.T. (10) and thus releases the link connecting the drain (D) and the source (S) for a current flow.

  Instead of the direct connection, from a thermal point of view, of the switching element (8) to the end plate of the fuel cell stack (2), it could also be envisaged to put the switching element (8) with the coolant circulating in the cooling circuit (3), the zone designated by the reference (12) in FIG. 1, situated before the cooling fluid enters the heat exchanger, being adapted for this purpose. The power loss to be cooled of the switching element (8) is so low that it does not heat the cooling fluid, so as to disadvantageously influence the cooling of the fuel cell stack (2) by the heat exchanger (4). On the other hand, the cooling fluid is substantially colder before it enters the stack of fuel cells (2) and / or the heat exchanger (4) than at its outlet, so that it it is possible to guarantee an ideal cooling of the switching element (8) by the cooling fluid (3) in this zone (12) due to the larger temperature difference.

  It can furthermore be envisaged to integrate other electronic components in the region of the switching element (8), in addition to those of the embodiment of the simplest switching element (8) shown in FIG. 2. FIG. 3 further represents a switching element (8) of this type for which the representation of an equivalent diagram of the MOS FET has been chosen. (10). The switching element (8) is provided, in the configuration according to FIG. 3, in addition to the MOS F.E.T. (10) which, as previously described, may consist of a parallel connection of a plurality of MOS F.E.T. (10), a device for determining and amplifying the voltage drop at the internal resistance R05 on the MOS F.E.T. (10) connected. The MOS F.E.T. (10) typically have an internal resistance Ros on, known when switched. This internal resistance is only a few mS) but the switched currents very important in the case presented here cause a measurable voltage drop that can be amplified by means of a corresponding amplification device (13), for example an amplifier operational and can serve as a measurement signal for the current IBz taken from the stack of fuel cells (2). The measurement of the fuel cell current by means of the measured voltage drop at the internal resistance RDs on of the MOS F.E.T. (10) can be performed on 5 to 10% of the current of the fuel cell IBz itself so that this value is amply sufficient for many settings, for example safety disconnections or others. Additional sensory analysis which would lead to additional difficulties and additional costs is therefore superfluous.

  An integrated circuit (14) is furthermore represented in the representation of the switching element (8) according to FIG. 3, said integrated circuit supplying the gate (G) of the MOS F.E.T. (10) in control voltage. The integration of such a circuit (14), which can in turn be controlled by an electronic control and adjustment device, as shown by the dashed arrow, offers the advantage on the one hand that these components are also cooled and secondly that the control of the MOS FET (10) is facilitated by integration into the switching element (8).

  Another peculiarity of the MOS F.E.T. (10) appears from the equivalent diagram of the MOS F.E.T. shown in Figure 3. The MOS F.E.T. (10) contains, due to the semiconductor structure of the MOS F.E.T. (10) in the form of a semiconductor oxide metal component, in addition to an additional diode effect (15) acting as an independent diode. Due to this independent diode, it can create a flow of current from the source (S) to the drain (D) through the MOS F.E.T. (10), here for example from the energy storage device (7) to the stack of fuel cells (2), even when there is no control voltage at the door ( G) MOS FET (10), this situation may influence the MOS F.E.T. disadvantageously and / or destroy it. The following figures therefore show arrangements for real separation or bipolar separation between the stack of fuel cells (2) and the energy storage device (7).

  FIG. 4 shows a mounting in which two of the switching elements (8) are mounted head-to-tail with each other in the manner previously described. The two switching elements (8) are on one side of the stack of fuel cells (2) and are provided with an electrical connection therewith or with one of its terminal plates. The two switching elements (8) are connected for this purpose so that the source connections (S) of their MOS F.E.T. are interconnected. The diodes (15) contained in the MOS F.E.T. (10) are thus mounted folded, so that a current flow can pass through the series of two switching elements (8) only when a control voltage is applied to the gate (G). Two arrangements are possible here for cooling. One possibility is that the drain (D) and therefore typically the substrate of one of the MOS F.E.T. (10) or one of the switching elements (8) can be connected directly to the end plate. The other MOS F.E.T. (10) or the other switching element (8) must however be mounted electrically insulated. It can be cooled by means of the cooling circuit (3), for example in the previously described zone (12) thereof. It is also possible to cool together the two MOS F.E.T. (10) or the switching elements (8) by means of the cooling circuit (3). The assembly allows a very simple control, because common, doors (G) of the various switching elements (8).

  The mounting can naturally be achieved in principle by means of a connection of the two drain connections (D) of the switching elements (8). The direct connection of the drain connections (D) and therefore typically substrates of the MOS F.E.T. (10) ensures a simple cooling preferably to be performed in the zone (12) of the cooling circuit (3) as previously presented.

  FIG. 5 represents another embodiment of the assembly, two of the switching elements (8) being used here as well, mounted head-to-tail with respect to each other.

  Unlike the embodiment shown above, the switching elements (8) are not directly connected to each other; instead, one of the switching elements (8) is arranged on one side of the fuel cell stack (2) and the other is arranged on the other side of the fuel cell stack ( 2). This assembly allows the drains (D) of each of the MOS F.E.T. (10) of the two switching elements (8), and therefore its large area substrates, can be applied without electrical and thermal insulation on the two end plates of the fuel cell stack (2). One of these terminal plates, that representing the cathode, is in direct electrical and thermoconductive connection with one of the switching elements (8) or the drain (D) of its MOS F.E.T. (10). The other, the end plate of the fuel cell stack (2) connected to the anode, is also in direct thermal and electrical contact, this time with the drain connection (D) of the MOS FET (10). of the other switching element (8).

  The assembly here also constitutes a head-to-tail assembly with respect to the diodes (15) 35 immanent MOS F.E.T. (10) here shown, so that here also an electrical connection and the current flow are possible that when a control signal is applied to the doors (G) of the MOS F.E.T. (10) of the two switching elements (8).

  As shown by the two arrows (16, 17), the control of the doors (G) is no longer jointly but separately, the integrated circuit (14) in each of the switching elements (8), of the type explained in the context of FIG. FIG. 3, serving to emit for this purpose an in situ control signal, for example triggered by an external control (for example a pulse) by an electronic control and / or adjustment device. The advantage over the system previously shown lies here in the direct thermal and electrical coupling with the end plates of the stack of fuel cells (2), said coupling further ensuring the transfer of thermal energy in a zone of the stack of fuel cells (2) in which this thermal energy is very favorable for balancing and harmonizing the temperature curve of the entire stack of fuel cells. This advantage certainly outweighs the disadvantage of controlling the MOS F.E.T. (10) switching elements (8) a little more complex than in the embodiment according to Figure 4.

  All of the switching elements (8) shown here may naturally have the structure shown by way of example in FIG. 3 and have, in addition to possibly a larger number of MOS F.E.T. (10) connected in parallel, corresponding devices for determining the fuel cell current IBZ and corresponding circuits (14) for transmitting the gate control signal (G) of the MOS F.E.T. (10). They can however, without impairing the basic operation of the invention, be configured as a single MOS F.E.T. or as a different electronic switch, the mounting head-bêche can naturally be implemented satisfactorily only in the case of MOS F.E.T.

Claims (11)

claims   A fuel cell system (1) provided with a stack of fuel cells (2) and at least one electric energy storage device (7) which can be separated and / or electrically connected to the fuel cell stacking fuel cells (2) via at least one electronic switching element (8), the stack of fuel cells (2) being cooled, characterized in that at least one of the switching elements (8) is thermally coupled with a cooling circuit (3) for cooling the stack of fuel cells (2).   Fuel cell system according to Claim 1, characterized in that the switching element (8) is designed as a semiconductor switching element with at least one switch.   Fuel cell system according to Claim 2, characterized in that the switching element is designed as field effect transistors on the basis of a semiconductor oxide metal element (MOS FET). including a MOS FET with N. channel Fuel cell system according to claim 3, characterized in that the switching element (8) based on MOS F.E.T. measures the voltage drop in the internal resistance (RDson) of at least one of the MOS F.E.T. (10).   5. fuel cell system according to claim 3 or 4, characterized in that two of the switching elements (8) at least based on MOS F.E.T. (10) are connected head-to-tail for separating and / or electrically connecting the fuel cell stack (2) and the energy storage device (7).   Fuel cell system according to one of the preceding claims, characterized in that each of the switching elements (8) is provided with at least one electronic switch (MOS FET) (10) and a circuit integrated (14) for its order.   Fuel cell system according to one of Claims 3 to 6, characterized in that each of the switching elements (8) is provided with at least one electronic switch (MOS FET) (10) and a device (13) for measuring and amplifying the voltage drop at the internal resistance (RDsON) of the switch.   Fuel cell system according to one of the preceding claims, characterized in that each of the switching elements (8) is provided with a plurality of electronic switches (MOS F.E.T.) (10) connected in parallel.   Fuel cell system according to one of the preceding claims, characterized in that at least one switching element (8) is in electrically conductive and thermally conductive contact on at least one terminal plate of the battery stack. fuel cells (2).   Fuel cell system according to claim 9, characterized in that at least one of the switching elements (8) is arranged on the end plate of the fuel cell stack (2) connected to the anode, and at least one other switching element (8) is arranged on the end plate of the fuel cell stack (2) connected to the cathode.   Fuel cell system according to one of Claims 1 to 8, characterized in that the switching element (8) is in heat-conducting contact with the coolant of the cooling circuit (3) prior to its entry. (zone 12) in the stack of fuel cells (2).