FUEL CELL SYSTEM

Abstract

The invention relates to a fuel cell system comprising a stack of electrochemical cells forming a polymer ion-exchange membrane fuel cell (6), a fuel gas supply circuit and an oxidant gas supply circuit.

Said oxidant gas supply circuit comprises a compressor (3) intended to compress the ambient air before it enters the fuel cell (6), and an outlet exhaust (10) intended to discharge the gases leaving the fuel cell.

Said supply circuit is connected to the fuel cell at a first access point (7) and a second access point (8).

The system additionally comprises a switching element (11) that has two positions: a first position in which the outlet of the compressor (3) is connected to the first access point (7), and the second access point (8) is connected to the outlet exhaust (10), and a second position in which the outlet of the compressor (3) is connected to the second access point (8), and the first access point (7) is connected to the outlet exhaust (10).

The system is characterized in that it contains a moisture reservoir positioned in the oxidant gas supply circuit, upstream of the first access point (7).

Claims

1-10. (canceled)

11: A fuel cell system comprising: a stack of electrochemical cells forming a polymer ion-exchange membrane fuel cell; a fuel gas supply circuit for supplying a fuel gas; an oxidant gas supply circuit for supplying an oxidant gas, the oxidant gas supply circuit having a first access point and a second access point, the oxidant gas supply circuit including: a compressor that compresses ambient air before the ambient air enters the fuel cell, and an outlet exhaust through which gas leaving the fuel cell is discharged; a moisture reservoir positioned in the oxidant gas supply circuit upstream of the first access point; and a switch having a first position and a second position, wherein, when the switch is in the first position, the first access point is connected to an outlet of the compressor, and the second access point is connected to the outlet exhaust, and wherein, when the switch is in second position, the second access point is connected to the outlet of the compressor, and the first access point is connected to the outlet exhaust.

12: The system according to claim 11, wherein the moisture reservoir is formed of a hygroscopic material.

13: The system according to claim 11, wherein the electrochemical cells are separated by bipolar plates, wherein each of the bipolar plates includes faces, and a channel is formed in each of the faces for circulation of the fuel gas and the oxidant gas, and wherein the first and second access points form an inlet and an outlet of the channels.

14: The system according to claim 11, wherein the switch includes a four-way valve.

15: The system according to claim 14, wherein the switch includes a permanent-magnet angular motor coupled to the four-way valve.

16: The system according to claim 11, further comprising first and second pressure sensors installed in the oxidant gas supply circuit, the first pressure sensor being installed between the switch and the first access point, and the second pressure sensor being installed between the switch and the second access point.

17: The system according to claim 11, wherein the switch is installed on an end plate of the fuel cell, and wherein the system further comprises at least one fuel-cell management system.

18: A process for controlling a fuel cell system that includes a stack of electrochemical cells forming a polymer ion-exchange membrane fuel cell; a fuel gas supply circuit for supplying a fuel gas; an oxidant gas supply circuit for supplying an oxidant gas, the oxidant gas supply circuit having a first access point and a second access point, the oxidant gas supply circuit including a compressor that compresses ambient air before the ambient air enters the fuel cell, and an outlet exhaust through which gas leaving the fuel cell is discharged; a moisture reservoir positioned in the oxidant gas supply circuit upstream of the first access point; and a switch having a first position and a second position, wherein, when the switch is in the first position, the first access point is connected to an outlet of the compressor, and the second access point is connected to the outlet exhaust, and wherein, when the switch is in second position, the second access point is connected to the outlet of the compressor, and the first access point is connected to the outlet exhaust, the process comprising a step of: controlling the switch to move from the first position to the second position according to an asymmetric regular cycle.

19: The process according to claim 18, wherein the regular cycle has a duration of 15 seconds.

20: The process according to claim 18, further comprising a step of: measuring a temperature within the fuel cell, wherein the step of controlling the switch is performed only when the temperature of the fuel cell is higher than a predetermined threshold.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0030] Other advantages and features of the invention will appear with the description, given non-limitingly, of various embodiments illustrated by the following figures:

[0031] FIG. 1 shows the cathode circuit of a fuel cell system of the prior art;

[0032] FIG. 2 shows the cathode circuit of a fuel cell system according to the present invention;

[0033] FIGS. 3a and 3b, already described, show the circulation of the gases in the membrane-electrode assembly of a fuel cell, in a “co-flow” situation and a “counter-flow” situation;

[0034] FIGS. 4a and 4b show the operation of a four-way valve coupled to an angular motor according to the invention;

[0035] FIG. 5 shows the angular position of the cylinder of the valve during the switching between two positions of the valve;

[0036] FIG. 6 shows a cross-sectional view of the motor/valve assembly used in a system according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0037] FIG. 1 shows a system comprising a fuel cell 6 of the type having an electrolyte in the form of a polymer membrane (i.e. of the PEFC (polymer electrolyte fuel cell) or PEM (proton exchange membrane) type). The fuel cell 6 is supplied with two gases, namely the fuel (hydrogen stored or generated on board the vehicle) and the oxidant (air or pure oxygen), which gases supply the electrodes of the electrochemical cells. For this purpose, the system comprises two gas circuits: a fuel gas supply circuit, also referred to as an anode circuit, and an oxidant gas supply circuit, also referred to as a cathode circuit. FIG. 1 only represents the elements of the cathode circuit useful for understanding the invention. Of course, the present invention is not limited to these elements, and may include all the devices known to a person skilled in the art and that can be used in the case of such a fuel cell system.

[0038] Thus, the equipment comprises an air supply circuit on the cathode side. This circuit comprises an air inlet filter 1, a flow meter 2 that makes it possible to measure the flow rate of the incoming air, an air compressor 3, and a non-return valve 4 that makes it possible to prevent the gas leaving the fuel cell from coming back in the direction of the compressor 3. As indicated above, at the outlet of the compressor 3, the air is hot and dry, and would therefore run the risk, if it was introduced over too long a duration into the fuel cell, of degrading the polymer membrane. Consequently, in a conventional fuel cell, a humidifier 5, for example of the type of those of the Permapure® brand, is placed upstream of the inlet 7 which enables the entry of oxidant gas into the fuel cell.

[0039] The principle of the humidifier 5 is the following: it is known that the gases leaving the fuel cell are loaded with moisture, due to the chemical half-reaction that takes place at the cathode: O.sub.2+4 H.sup.++4 e−=2 H.sub.2O which produces water. This gas leaving through the outlet 8 of the fuel cell is introduced into the humidifier 5 at the same time as the dry gas leaving the compressor 3. The humidifier 5 comprises a polymer membrane, for example of Nafion® type. Through this membrane, a portion of the moisture present in the gases leaving the fuel cell is transferred to the dry gases before they enter the fuel cell, which makes it possible to guarantee a sufficient level of moisture in order not to damage the polymer membrane of the fuel cell 6. The gases leaving the fuel cell are then sent, after passing into the humidifier 5, to an outlet exhaust 10, via a pressure-regulating valve 9. Such a configuration has various drawbacks linked to the use of such a humidifier. Specifically, this humidifier is very bulky, since it represents a fraction of the not inconsiderable volume of the fuel cell 6 (it should be noted that the dimensions used in FIGS. 1 and 2 are not to scale). However, in the case of a mobile application, such as use in a vehicle, it is useful to be able to reduce the weight and bulkiness of the assembly as much as possible. Furthermore, the polymer membranes used in the humidifier are relatively expensive. Moreover, in the case of a use of such a humidifier, the humidity is not uniform in the channel of the bipolar plate on the cathode side. Indeed, since the gases are loaded with moisture in the course of their journey in the channel, and therefore in the course of the electrochemical reaction, this results in a very high humidity at the end of the channel.

[0040] In order to resolve these drawbacks, the present invention proposes a solution, one exemplary embodiment of which is shown in FIG. 2. This equipment comprises a valve 11, connected on one side to the non-return valve 4 and to the pressure-regulating valve 9, and on the other side to the fuel cell, at the inlet 7 and outlet 8. The valve 11 is a four-way valve, which may be monostable or bistable. The choice between the two will be made in particular in view of the energy constraints of the system, since in one case it is necessary to maintain an electric current in order to keep the valve in the second position, whereas in the other case a simple pulse makes possible to move the valve from one to the other of the positions, which proves to be advantageous in terms of energy consumption.

[0041] Owing to the control of the valve 11, the outlet of the air compressor 3 is connected alternately to the inlet 7 of the fuel cell, and the outlet 8 of the fuel cell. The terms “inlet” and “outlet” are used here in similarity with FIG. 1, but in the configuration of FIG. 2, these access points 7 and 8 are alternately inlets and outlets of the fuel cell. Thus, in a first position, the gas from the compressor 3 enters the fuel cell through the inlet 7, it travels through the channel located on the bipolar plate, in the course of which journey the electrochemical reaction takes place. The gases resulting from this reaction emerge from the fuel cell through the outlet 8 and are then sent to the outlet control valve 9. In a second position, the gas from the air compressor 3 is sent to the outlet 8, it travels through the channel located on the bipolar plate to the inlet 7, and the outgoing gases are then sent, via the valve 11, to the outlet control valve 9.

[0042] Thus, the gas circulates alternately in one direction and in the other direction in the channel. However, as explained above, during its journey in the channel, the gas is loaded with water due to the electrochemical reaction that takes place. Thus, the portion of the channel that is found at the end of the journey has a very high degree of humidity. By alternating the inlet of the gas, it is thus possible for the dry gas from the compressor 3 to enter the fuel cell through a portion of channel that has a high humidity, and thus to be loaded with water, in order not to degrade the polymer membrane. Thus, a system is provided that makes it possible to guarantee a correct humidification of the gases in contact with the membrane without it being necessary to humidifier them before their entry into the fuel cell 6. This is very advantageous in terms of cost and bulkiness, since the four-way valve is a common device, available at low cost and not being very bulky.

[0043] Moreover, alternating the gas inlet between the access points 7 and 8 makes it possible to alternate the direction of travel of the gas in the channel, and thus to make the humidity uniform throughout this channel. Thus, the humidity varies parabolically along the channel, with a high point reached at the middle of the channel, without reaching the very high levels reached at the end of the channel in a conventional system.

[0044] FIGS. 4a and 4b show respectively a first and second position of the four-way valve, actuated by a permanent-magnet angular motor, operating as an electromagnet. FIG. 4a corresponds to a “co-flow” situation and FIG. 4b corresponds to a “counter-flow” situation.

[0045] Preferably, the control of this valve is asymmetric. Specifically, the “co-flow” situation has a tendency to dry out the membrane more quickly than the “counter-flow” situation, and it is therefore advantageous to have a cycle in which the “co-flow” situation lasts between 5 and 15 seconds, and the “counter-flow” situation between 10 and 25 seconds.

[0046] FIG. 5 shows, on the G1 curve, the angular position of the cylinder of the valve during the switching between two positions of the valve. It is thus observed that the displacement time of the cylinder Td is less than 40 milliseconds, which makes it possible not to observe a break in power at the outlet of the fuel cell, since the capacitive effect of the fuel cell is sufficient to maintain the power during the brief switching.

[0047] Preferably, and as shown in FIG. 6, the axis of the motor 100 is coupled with the axis of the four-way valve. Moreover, elastic stops 102 are advantageously installed to absorb the energy stored by the cylinder 101 during its displacement, and to limit the rebound phenomenon, that appears in the box C1 in FIG. 5.

[0048] Thus, the present invention makes it possible to provide a fuel cell system such that the humidification of the gases is preserved, without increasing the cost and bulkiness of the system excessively.