Operating method of a fuel cell

Abstract

Title: Method for Operating a Fuel Cell The invention relates to a method for operating a fuel cell having a "ping-pong" architecture, in which two groups (10, 20) of electrochemical cells are alternately powered. Advantageously, a purging phase is carried out synchronously with the displacement of non-reactive fluids accumulated in the form of a plug aggregate (B), when said plug (B) is located at least partially in the discharge lines (120, 220) of the fluidic circuit (4). Figure for the abstract: Fig. 5

Description

Title of the invention: Method for operating a fuel cell technical field

[0001] The present invention relates to the field of fuel cells, in particular proton exchange membrane fuel cells (or PEMFCs for Proton Exchange Membrane Fuel Cells) with alternating, or "ping-pong," feeding. It can advantageously be implemented to optimize the performance of a PEMFC with a ping-pong architecture. STATE OF THE ART

[0002] A fuel cell is formed by a stack of "unit" electrochemical cells, each comprising an anode and a cathode electrically separated from each other by an electrolyte. In the case of a hydrogen fuel cell, the fuel (hydrogen) is brought into contact with the anode, and the oxidant (oxygen) is brought into contact with the cathode. Oxidation and reduction reactions take place at the anode and cathode, respectively, producing electricity, water, and heat. The electrolyte may be in the form of a membrane that allows the protons from the hydrogen oxidation reaction to pass through. This is the case for proton exchange membrane fuel cells (PEMFCs).

[0003] The cell stack is merely the site of the reaction: reactants must be supplied to it, and products and non-reactive species must be removed, as well as the heat produced. Generally separate anodic and cathodic fluidic circuits supply the cell stack with fuel and oxidizer respectively, and remove the products from the stack.

[0004] The hydrogen supplying the anodic compartment can advantageously be reinjected into the anodic compartment, by a recirculation pump on the fluidic circuit, so as to distribute the non-reactive species to the anode, typically the water produced by the reaction and the nitrogen from the cathode through the membrane.

[0005] The fluidic circuit can be more complex in the case of particular stack architectures. In the so-called "ping-pong" architecture disclosed by patent document FR2975227, the stack is divided into several groups of cells fluidically linked to each other by their respective outlets.

[0006] In this known architecture illustrated in [Fig. 1], the inlet 11, 21 of the different groups 10, 20 is supplied alternately during certain operating phases of the fuel cell. A first group 10 is supplied at inlet 11 with the fuel fluid (hydrogen) while the supply of a second group 20 is The fuel fluid is cut off at inlet 21. It passes through the first group, which then operates nominally, and exits moistened and slightly depleted at outlet 12 of the first group 10. This fluid then feeds the second group 20 via outlet 22 of the second group 20. This direct feed to the first group and reverse flow to the second group aims to stir up pockets of highly depleted fluid formed within the cells of the first group, dissipating them through mixing. This prevents cells in the first group from operating for extended periods in the presence of stagnant, highly depleted fluid. The second group 20 is then fed at inlet 21 with the fuel fluid while the supply to the first group 10 is cut off at inlet 11. The fuel fluid passes through the second group, which then operates nominally, and exits moistened and slightly depleted at outlet 22 of the second group 20.This fluid then feeds the first group 10 via outlet 12 of the first group 10. This direct supply to the second group, in the opposite direction to the first group, also helps to dissipate pockets of highly depleted fluid formed within the cells of the second group during the previous cycle, by stirring them. This prevents cells in the second group from operating for extended periods in the presence of stagnant, highly depleted fluid.

[0007] The principle of this ping-pong architecture consists of alternately supplying the different groups of cells so as to alternate phases of nominal operation with phases of reverse supply for each of the groups. After a few cycles of alternating supply, groups 10, 20 are supplied simultaneously and a purge valve 40 common to the different groups is opened in order to evacuate the accumulated non-reactive products and gases, in particular water and nitrogen.

[0008] In constant current (imposed) operation, a voltage drop is generally observed during the purging of these accumulated non-reactive products and gases.

[0009] An object of the present invention is to propose a method of operating a ping-pong architecture fuel cell, improving the performance of the fuel cell, and in particular limiting the voltage or power drop during purging.

[0010] The other objects, features and advantages of the present invention will become apparent from an examination of the following description and accompanying drawings. It is understood that other advantages may be incorporated. SUMMARY

[0011] To achieve this objective, according to one embodiment, a method for operating a fuel cell is provided, said fuel cell comprising a plurality of groups of electrochemical cells connected to each other by a fluidic circuit, said fluidic circuit comprising, for each group, a line input supply line of said group, and a switching device on said supply line, and, an outlet discharge line of said group, said fluidic circuit being configured so that the discharge lines communicate fluidly with each other and are connected to a common purge device.

[0012] The operating method comprises at least: • A cycle of alternating feedings including at least: i. A first supply phase in which a first group among the plurality of groups is directly supplied with feed fluid at its inlet, the switching element of said first group being open while the switching elements of the other groups of the plurality of groups are blocked, said other groups being supplied indirectly, during this first supply phase, at their respective outlets, by the feed fluid from the outlet of the first group, ii. A second supply phase in which a second group, different from the first group, is directly supplied with feed fluid at its inlet, the switching element of said second group being open while the switching elements of the other groups in the plurality of groups are blocked, said other groups being supplied indirectly, during this second supply phase, at their respective outlets, by the feed fluid from the outlet of the second group, • A purging phase following the alternating feed cycle, said purging phase including an opening of the purge device so as to evacuate non-reactive fluids originating at least in part from the feed fluid.

[0013] Advantageously, the purging phase is triggered in a synchronized manner with a movement of non-reactive fluids accumulated in the form of an aggregate called a plug, when said plug is located at least partly in the drainage lines.

[0014] Thus synchronized, the purge phase of a ping-pong fuel cell is triggered at the optimal time with respect to the alternating feed cycle. One principle of the method according to the present invention consists of triggering the opening of the purge device in a controlled manner, by determining the optimal time to trigger this opening, and preferably the duration of this opening. In particular, the triggering of the purge phase occurs during a feed phase j of the alternating feed cycle and before the end of feed phase j.

[0015] This approach is fundamentally different from the approach taught in patent document FR2975227, which simply purges the fuel cell groups after a few power cycles, during a "dead end" phase where both groups are first powered simultaneously to push non-reactive species to the vicinity of the outlet of their respective groups, before the purge valve is opened. This dead end phase is carried out immediately at the end of a standard-duration power cycle for one of the groups. This causes a drop in the voltage delivered by the fuel cell and a decrease in performance during the purge phase. This leads to an increase in the purge frequency, in order to limit the voltage drop. As a result, hydrogen loss during the purges, and therefore hydrogen consumption by the system, is increased.

[0016] On the contrary, the process according to the present invention is based on a better understanding of the reaction and fluidic mechanisms occurring within the fuel cell. During the development of the present invention, it was identified that non-reactive products and gases accumulate locally in the fluidic circuit, in the form of an aggregate called a plug. This plug moves according to the injections of feed fluid alternately into each of the fuel cell groups. This movement causes limited mixing of the gas mixture, inducing only a spreading of the plug, without achieving true dilution of the non-reactive species in the flow of reactive species.

[0017] During the development of the present invention, it was observed that the dead-end phase following the end of a feed phase of a group, as disclosed by document FR2975227, does not allow the plug to be moved out of the cell group. According to this known method, the plug cannot be located in the drain lines. Indeed, it has been identified that the plug is pushed by the feed and pulled by the consumption of reagents. Since the latter does not exist in the drain lines, the dead-end phase stabilizes the position of the plug where it is at the end of the feed phase, without moving it into the drain lines. However, it has been determined that the plug is not located in the drain lines at the end of the feed phase. It is therefore necessary to synchronize the triggering of the purge phase with the movement of the plug when the latter is located in the drain lines.According to the invention, the triggering of the purge phase is carried out before the end of the feed phase which precedes the purge phase.

[0018] By comparison, implementing the operating method according to the present invention makes it possible to significantly reduce the purging frequency. This can reduce the amount of hydrogen released during the purging phase, while obtaining a voltage delivered by the battery that is practically constant.

[0019] It has also been observed that the plug tends to oscillate around an equilibrium position in the fluidic circuit.

[0020] By way of example, when the stack comprises two identical groups connected by symmetrical discharge lines, this equilibrium position is typically located at the midpoint between the group outlets, equidistant from the outlets. In a preferred configuration, the drain valve is located at this equilibrium position. At this equilibrium position, the plug exhibits a substantially constant movement speed regardless of its direction of travel. Triggering the opening of the drain valve is thus facilitated, regardless of the feed cycle preceding this triggering.

[0021] Due to the inertia of the plug's movement, the plug reaches its equilibrium position with a delay after one of the switching elements has been opened. An object of the invention is, in particular, to determine this delay so as to open the purge element at the moment the plug passes through its equilibrium position, as close as possible to the purge element. This minimizes the loss of unburned fuel. This reduces the opening time of the purge element. The performance stability of the fuel cell is improved. Furthermore, by triggering the opening of the purge element when the plug passes in its immediate vicinity, it has been observed that the voltage delivered by the fuel cell, at constant current and constant hydrogen pressure, remains practically unchanged. Proper synchronization of the opening of the purge element with the movement of the plug thus advantageously prevents a drop in the voltage delivered by the fuel cell.With a higher overall fuel cell voltage during this purging phase and a lower rate of hydrogen rejection, the overall efficiency and stability of the fuel cell are improved.

[0022] Consequently, and advantageously, the operating method of a fuel cell with a ping-pong architecture according to the present invention makes it possible, in particular, by optimizing the purge timing, to reduce fuel consumption and improve the electrical performance of this fuel cell. BRIEF DESCRIPTION OF FIGURES

[0023] The aims, objects, features and advantages of the invention will become clearer from the detailed description of an embodiment thereof, which is illustrated by the following accompanying drawings in which:

[0024] [Fig-1] Fig. 1 represents a fuel cell exhibiting a ping-pong architecture according to the prior art.

[0025] [Fig.2] Fig.2 represents a simplified diagram of a fuel cell with a ping-pong architecture, according to an embodiment of the present invention.

[0026] [Fig.3] Fig.3 represents alternating supply cycles at the inputs of the stack groups, according to an embodiment of the present invention.

[0027] [Fig.4A] Fig.4A represents the movement of the plug of non-reactive products and gases during a first phase of feeding the fuel cell, according to an embodiment of the present invention.

[0028] [Fig.4B] Fig.4B represents the movement of the plug of non-reactive products and gases during a second phase of feeding the fuel cell, according to an embodiment of the present invention.

[0029] [Fig.5] Fig.5 represents a purging phase when the plug is in an equilibrium position near the purging device, according to an embodiment of the present invention.

[0030] [Fig.6] Fig.6 represents alternating feed cycles at the inputs of the stack groups, with different purge times, according to an embodiment of the present invention.

[0031] [Fig.7A] Fig.7A represents the voltage delivered by the fuel cell during its operation, by performing periodic purges according to a first purge moment, according to an embodiment of the present invention.

[0032] [Fig.7B] Fig.7B represents the voltage delivered by the fuel cell during its operation, by performing periodic purges according to a second purge moment, according to an embodiment of the present invention.

[0033] [Fig.7C] Fig.7C represents the voltage delivered by the fuel cell during its operation, by performing periodic purges according to a third purge moment, according to an embodiment of the present invention.

[0034] [Fig.7D] Fig.7D represents the voltage delivered by the fuel cell during its operation, by performing periodic purges according to a fourth purge moment, according to an embodiment of the present invention.

[0035] [Fig.8] Fig.8 represents a simulation of the nitrogen concentration at the purge device with respect to the alternating feed phases, according to an embodiment of the present invention.

[0036] The drawings are given by way of example and are not limiting of the invention. They constitute schematic representations of principle intended to facilitate understanding of the invention and are not necessarily to scale with practical applications. In particular, the various components and parts of the fluidic circuit are illustrated by diagrams that are not representative of reality. DETAILED DESCRIPTION

[0037] Before proceeding to a detailed review of embodiments of the invention, optional features that may be used in combination or alternatively are listed below:

[0038] According to one example, the alternating feed cycle has a period T and the purge phase is triggered during a feed phase j and before the end of feed phase j, after a delay following the start of feed phase j such that:

[0039] [Math.l]

[0040] with r = TU the half-period, and

[0041] [Math.2] A ~ ~ 2^V(P-Psat)

[0042] With S the active surface area of ​​the cell, i the current density and the Faraday constant, K the ideal gas constant, η the cell temperature, V the volume through which the fluid flows in the cell, P the fluid pressure in the cell, and Psat the saturated vapor pressure at the cell temperature. The plug typically comprises nitrogen and water vapor in addition to hydrogen, which is present in reduced concentration.

[0043] According to one example, the alternating feed cycle has a period T and the purge phase is triggered during a feed phase j and before the end of feed phase j, after a delay of between 0.3T / 2 <dj < 0,7.T / 2-

[0044] According to one example, the alternating feed cycle has a period T, and the purge phase is triggered during a feed phase j and before the end of feed phase j, after a delay of y / 4. In practice, liquid water is also present in the fluid circuit. This liquid water can affect the movement dynamics of the plug. A delay substantially equal to T / 4 allows for the consideration of a delay in the plug's movement due to the liquid water, and / or for the removal of this liquid water.

[0045] In one example, the purging phase includes a "dead end" delay, and the opening of the purging device occurs after the dead end delay. In an alternative example, the purging phase does not include a "dead end" delay, and the opening of the purging device occurs when the purging phase is triggered.

[0046] According to one example, the stack comprises only two groups, the outputs of these two groups are located equidistant from the purge device, and the first and second power-up phases of the alternating power cycle have the same duration T / 2. This "symmetrical" construction of the stack and the alternating power cycle This allows for better control of the plug's oscillations within the fluid circuit. This facilitates determining the optimal moment for triggering the opening of the purge valve.

[0047] According to one example, the purge phase includes, before or during the opening of the purge device, an input supply to all groups, each switching device being conducting.

[0048] According to one example, the opening of the purge valve occurs after the inlet supply to all groups, following a so-called "dead-end" delay. This allows the plug to stabilize in its equilibrium position. If the purge phase is triggered at the correct time, as determined by the invention, the dead-end phase prevents the plug from moving between the groups in the discharge lines.

[0049] According to one example, the input supply to all groups is carried out simultaneously with the opening of the purge device.

[0050] According to one example, the purge phase does not include supplying input to all groups before the purge valve is opened. The opening of the purge valve thus directly follows a phase of partial supply to one group. This shortens the purge phase. The stack's performance is improved or more consistent.

[0051] According to one example, a constant electric current is imposed on the fuel cell.

[0052] According to one example, the feed fluid injected during the alternating feed cycle has a constant pressure.

[0053] According to one example, the opening of the purge device is triggered when at least 70%, and preferably at least 80%, of the length LB of the plug is located in the drain lines, the length LB of the plug being measured according to the direction in which the drain line extends at the point considered.

[0054] According to one example, the opening of the purge valve is triggered when the blockage is fully located in the drain lines. The removal of the blockage is optimized. The amount of unused feed fluid discharged is reduced.

[0055] According to one example, the opening of the purge device is triggered when the blockage completely occupies the discharge lines. The blockage may eventually overflow into the active areas of the stack.

[0056] According to one example, the opening of the purge device is maintained so that at least part of the blockage is expelled from the fluid circuit via the purge device, preferably at least 70%, preferably all of it. The duration of the purge device's opening is typically determined based on the volume of the blockage or the quantity of non-reactive products and gases to be evacuated. The opening duration of the purge device is minimized when the blockage is at or as close as possible to the purge device. The quantity expelled from Unconsumed power fluid is reduced. Voltage drop is limited or eliminated. Battery performance is optimized.

[0057] According to one example, the fuel cell comprises: • a first group of electrochemical cells exhibiting a first input and a first output, • a second group of electrochemical cells having a second input and a second output, • a fluidic circuit intended to supply the said first and second groups with a fluid, and to evacuate said fluid from the said first and second groups.

[0058] The fluidic circuit typically includes an upstream part comprising: • a main supply line configured to be connected to a tank storing the fluid at a pressure PI, known as high pressure, • a first power supply line connected to the main power supply conduit and connected to the first input of the first group, • a second power line connected to the main power supply conduit and connected to the second input of the second group.

[0059] The fluidic circuit typically includes a so-called downstream part comprising: • a first evacuation line connected to the first exit of the first group, • a second evacuation line connected to the second outlet of the second group, • the said first and second evacuation lines being connected to each other, so as to allow the first group to be supplied with fluid via the first outlet, by the fluid having passed through the second group, and conversely so as to allow the second group to be supplied with fluid via the second outlet, by the fluid having passed through the first group, • a main exhaust duct connected to the first and second exhaust lines, and configured to be connected to an exhaust.

[0060] The stack typically also includes: • at least one pressure-reducing device located in the upstream part of the fluid circuit, configured to decrease the pressure of the fluid coming from the reservoir, from high pressure to low pressure, • a first switching device on the first supply line, configured to allow or block fluid flow to the first inlet, • a second switching device on the second supply line, configured to allow or block fluid flow to the second inlet, • a purge device on the main discharge pipe, configured to allow or block fluid flow to the exhaust.

[0061] According to one example, at least one expansion member comprises a first expansion member configured to reduce the pressure of the fluid from the reservoir, from high pressure to medium pressure, and a second expansion member configured to reduce the pressure of the fluid from medium pressure to low pressure.

[0062] Except where incompatibility exists, technical features described in detail for a given embodiment may be combined with technical features described in the context of other embodiments described by way of example and without limitation. In particular, elements described or illustrated for certain embodiments of the fuel cell and its operating method may be combined to form another embodiment that is not necessarily illustrated or described. Such an embodiment is obviously not excluded from the invention. A fuel cell according to the present invention comprises at least two groups of electrochemical cells. A person skilled in the art will have no difficulty implementing an embodiment comprising more than two groups of electrochemical cells.

[0063] In the context of the present invention, a "ping-pong architecture fuel cell system" is understood to mean a system comprising at least two groups of electrochemical cells distributed in one or more stacks. Each group thus consists of a series of electrochemical cells interconnected electrically and fluidically. The cells are typically in the form of a membrane-electrode assembly commonly referred to as an AME. The fuel cell comprises at least two stacked cells, and preferably at least ten.

[0064] The ping-pong architecture fuel cell system typically has several operating phases, in particular, a first phase during which the first group is fed directly into the inlet of the first group, the second group being fed only into the outlet of the second group by the gas exiting the first group; a second phase during which the second group is fed directly into the inlet of the second group, the first group being fed only into the outlet of the first group by the gas exiting the second group; and a purge phase. The purge phase optionally includes a third phase of simultaneous feeding of the first and second groups into the inlets of the first and second groups, and opening of the purge valve. The first and second phases are carried out alternately and are typically of equal duration, with changes in operating conditions near the fuel cell. The purge phase follows the first and second phases. One principle of the invention consists of triggering the purge phase after a partial feeding phase, that is, a shortened feeding phase, so that the plug is positioned correctly to open the purge valve. Thus, the feeding phase preceding the purge phase is shorter than the duration of each of the first and second phases of an alternating feeding cycle. Determining this shorter duration is an object of the present invention.

[0065] The terms "high pressure," "medium pressure," and "low pressure" are perfectly clear to those skilled in the art. High pressure is strictly greater than medium pressure. Medium pressure is strictly greater than low pressure. High pressure is generally greater than 50 bar, and often greater than 100 bar. High pressure can reach up to 700 bar, or even more depending on the conditions. Medium pressure is generally between 5 and 40 bar, typically between 5 and 20 bar. Low pressure is generally less than 4 bar.

[0066] In the context of the present invention, an inlet or outlet of a fuel cell group typically each has a structural and a functional aspect. Structurally, the inlet and outlet correspond to the first and second passage orifices for the fuel fluid. Functionally, the inlet and outlet designate the intake and exhaust of the fuel fluid, respectively. Since the direction of fluid flow within the group is alternately reversed, the inlet and outlet can be functionally interchanged. Thus, the first passage orifice can function as either the inlet or the outlet, and conversely, the second passage orifice can function as either the outlet or the inlet.

[0067] To facilitate understanding with regard to the circulation of the fluid in the cells of the pile, we retain only the terms inlet / outlet and their corresponding references on the accompanying drawings, respectively XI (X= 1...2) for the inlet and X2 (X= 1.. .2) for the outlet, regardless of their functional assignment.

[0068] In the context of the present invention, the outlets of the groups are in fluidic communication with each other, via the drainage lines specific to each group. Each drainage line extends between the outlet of the group in question and a common main drainage duct. Each drainage line may share sections of other lines, for example, other drainage lines. Thus, sections of a given drainage line may be common with sections of other drainage lines. Branching points or junctions may be physically present along the drainage line(s). Components may Control devices, such as flow control devices, may also be present along these discharge lines. The entire set of discharge lines can form a stack outlet manifold connected to the stack's main discharge pipe. The drain device is typically located on the stack's main discharge pipe, preferably near an intersection of the discharge lines.

[0069] The operation of the battery according to the invention is based on a succession of different supply and discharge phases. Unless explicitly stated, the terms "successive" or "successive" do not necessarily imply, although this is generally preferred, that the phases follow each other immediately; intermediate phases or stages may separate them.

[0070] In the accompanying figures, the direction of fluid flow in the fluidic circuit is indicated by an arrow. For clarity, only one stack comprising two groups is shown in the accompanying figures. It is understood that the description of this stack applies to all variants in the number of stacks, groups, etc.

[0071] In the following examples, the fuel cell is described and illustrated for a ping-pong architecture comprising two substantially identical groups of cells. It is nevertheless perfectly feasible to implement more than two groups and / or groups of different sizes, without departing from the general principle of synchronized purging explained below. The groups can also be distributed in different fuel cells connected fluidly to each other.

[0072] The original idea implemented in the development of the present invention consists, in particular, of triggering the opening of the purge valve at the optimal time, when the plug formed by the non-reactive products and gases passes in the immediate vicinity of the purge valve. The existence and movement of a plug have indeed been observed through numerical simulations in the context of ping-pong feeding, whereas this operation could have suggested homogenization due to mixing. Triggering the purge phase at the optimal time limits the release of combustible gas. This reduces the opening time of the purge valve. This stabilizes the voltage delivered by the fuel cell. The overall performance of the fuel cell is thus improved, particularly in terms of consumption and power output.

[0073] The term "plug" refers to a volume of gas comprising a significant fraction of non-reactive species and / or reaction products accumulated and localized or localizable in the fluid circuit. Such a plug does not necessarily mean that the fluid circuit is completely obstructed. The plug may, but is not required to, form a partial obstruction of the fluid circuit. The plug typically resembles a pocket of gas not participating in the reaction. It is primarily composed of nitrogen and water vapor, and may also contain Other non-reactive species present in the air or resulting from side reactions, such as argon or carbon dioxide, can also be involved. This pocket of gas becomes an obstacle to the reaction and forms a plug when the proportion of hydrogen in the gas mixture falls below 70% (excluding humidity). The formation of the plug and its movement can be demonstrated by numerical simulation.

[0074] In the following, the operating method is implemented and described for the anodic compartment of a hydrogen fuel cell, i.e., for the fluidic circuit supplying the cells on the anode side with hydrogen (fuel). The method can also be implemented in the cathodic compartment supplied with oxidant, i.e., for the fluidic circuit supplying the cells on the cathode side, typically when the oxidant consists mainly of oxygen (for example, oxygen content greater than 50% in the dry gas).

[0075] As mentioned above, [Fig. 1] illustrates a fuel cell 1 with a typical ping-pong architecture. It is connected to a high-pressure hydrogen tank 2 and comprises a fluidic circuit consisting of a part 3 upstream of the groups 10, 20 of electrochemical cells, and a part 4 downstream of the groups 10, 20 of electrochemical cells. The groups 10, 20 can be nested in a single stack, with alternating cells from the first group and the second group, as described in patent document FR2975227.

[0076] The upstream section 3 typically comprises a shut-off valve 30 at the outlet of the reservoir 2, a main supply line 300 on which a first pressure regulator 31 and a second pressure regulator 32 are successively mounted. The first pressure regulator 31 reduces the pressure of the fluid from the reservoir 2 from a high pressure of several hundred bar to a medium pressure of several tens of bar. The second pressure regulator 32 reduces the pressure of the fluid from the first pressure regulator 31 from the medium pressure to a low pressure of a few bar.

[0077] The upstream part 3 of the fluidic circuit also includes a first supply line 100 connecting the main supply conduit 300 to the inlet 11 of the first group 10 of electrochemical cells, and a second supply line 200 connecting the main supply conduit 300 to the inlet 21 of the second group 20 of electrochemical cells. The first and second supply lines 100, 200 each include at least one switching element, typically a first switching valve 10v and a second switching valve 20v, ensuring either the passage or the blocking of the fluid to each of the groups 10, 20. Other supply configurations, for example with injectors, are also possible. The operating method described herein focuses on more specifically during the stack purging phase, regardless of the power supply configuration allowing for alternating power supply to the different groups of the stack.

[0078] The downstream part 4 of the fluidic circuit typically comprises a first discharge line 120 connecting the outlet 12 of the first group 10 of electrochemical cells to the main discharge conduit 400, and a second discharge line 220 connecting the outlet 22 of the second group 20 of electrochemical cells to the main discharge conduit 400. The first and second discharge lines 120, 220 typically form an outlet manifold of the stack 1. The main discharge conduit 400 is connected to an exhaust 41, and is provided with a purge valve 40.

[0079] Figure 2 illustrates in a simplified way certain elements of the fuel cell illustrated in [Fig. 1]. In particular, the groups 10, 20 of electrochemical cells and the discharge lines 120, 220 of the fluidic circuit are schematically aligned here along a single x-axis. This makes it easier to visualize the position of the products and non-reactive gases accumulated along the x-axis, as a function of the alternating feed phases. Point M here corresponds to the intersection of the evacuation lines 120, 220 and the main evacuation duct 400. Point M is located equidistant from outlets 12, 22. The stack exhibits a symmetrical construction that facilitates understanding the mechanisms of movement of the "plug" of non-reactive products and gases in groups 10, 20 and in the evacuation lines 120, 220. Although this symmetrical construction is preferred in practice, other stack dimensions are possible, for example with evacuation lines of different lengths connected to groups of different sizes.A professional will be able to adjust this sizing according to their needs.

[0080] Figure 3 illustrates alternating feed cycles of the two groups 10 and 20. A feed cycle here has a period T and comprises a first phase where only one of the groups is fed for a duration between t0 and t1, for example, the first group 10, followed by a second phase where only the other group is fed for a duration between t1 and t2, for example, the second group 20. According to one possibility, particularly when groups 10 and 20 are sized identically, the duration of the first phase is equal to the duration of the second phase: t0 = t2 - t1 = T / 2. This simplifies the dynamics of plug movement within the groups. The plug movement typically exhibits periodic or pseudo-periodic oscillations in this case. Other alternating feed cycles are nevertheless possible.

[0081] Figures 4A and 4B schematically illustrate an alternating feed cycle of period T with a first feed phase of duration T / 2 and a second feed phase of duration T / 2. The period T is typically between 0.1 s and 10 s. For example, the period T, i.e., the duration of one feed cycle alternating, is on the order of 1 s. According to an example, the half period T / 2, that is to say the duration of a feeding phase, is between 0.3 s and 0.5 s approximately.

[0082] Figure 4A schematically illustrates the first phase of the alternating feed cycle at time t1, i.e., at the end of the first feed phase, just before the switchover to the second phase. The switching valve 101v is open, the switching valve 201v is closed, and the purge valve 40 is closed. The feed fluid, typically hydrogen in the anodic compartment, is injected into the inlet 11 of group 10. It is partially consumed while passing through group 10, then flows along the discharge lines 120, 220 to enter group 20 through the outlet 22 of group 20. It is partially or totally consumed within group 20. During the reaction of hydrogen from the anodic compartment with oxygen from the cathodic compartment, reaction products, typically water, possibly carbon dioxide, are generated within groups 10, 20.The majority of reactions take place in the cathode compartment, but reaction products are typically transported by diffusion or permeation to the anode compartment through the proton exchange membrane. Other non-reactive species, typically nitrogen, can also pass from the cathode compartment to the anode compartment by permeation through the proton exchange membrane. These reaction products and non-reactive species typically form a plug B, in which the hydrogen content is reduced, which moves here "along the x-axis" due to hydrogen injection and hydrogen consumption.

[0083] Plug B is indeed both pushed by the hydrogen injection into group 10 towards the end of group 20, which is not supplied, and pulled or drawn towards this end by vacuum, due to the disappearance of the reactive gases present in this area. This phenomenon of the plug being drawn towards the second group, linked to the consumption of reagents not compensated by a reagent injection into the second group, significantly influences its movement. Conversely, supplying reagents to the first group compensates for this reagent consumption on the side of the plug corresponding to the injection.

[0084] The displacement kinetics of plug B are therefore linked to the supply of fresh reactive gases at the inlet of the fed group and to the disappearance of reactive gases in the areas where the reactions take place. This displacement kinetics is thus directly proportional to the current supplied to the fuel cell, which controls the reaction kinetics. At time t1, at the end of the first feeding phase, plug B is located on the side of the inlet 21 of group 20 that is not being fed, at a position P2 as illustrated in [Fig. 4A].

[0085] Figure 4B schematically illustrates the second phase of the alternating feed cycle at time t2, i.e., at the end of the second feed phase and just before a new switchover. The switching valve 201v is open, the switching valve 10v is closed, and the purge valve 40 is closed. Hydrogen is injected at the inlet 21 of group 20. It is partially consumed as it passes through group 20, then flows along the discharge lines 220, 120 to enter group 10 through the outlet 12 of group 10. It is partially or totally consumed within group 10. As before, the plug B moves under the influence of the hydrogen injection and consumption. At time t2, at the end of the second power phase, the plug B is on the side of the inlet 11 of group 10 which is not powered, in a position PI as illustrated in [Fig.4B].During alternating feed cycles, the plug oscillates between two positions P1 and P2 located on the side of inlets 11 and 21 of groups 10 and 20. This oscillation occurs around an equilibrium position corresponding here approximately to the midpoint M. It is this particular oscillation dynamic that promotes the grouping of non-reactive products and species into a moving agglomerate, which is referred to here as the plug. Indeed, a non-reactive gas particle will be drawn / displaced towards the side opposite the injection point all the more quickly the further it is from that point, due to the significant reactive surface area between the particle and this end. Similarly, a non-reactive particle located near the end without injection will experience a slight draw / displacement towards it, tending towards zero velocity at the inlet that is not being fed.This kinetic gradient favors the concentration of non-reactive species to form the plug, and its homogeneous movement around point M, rather than a mixing linked to the mixing of these species with reactive species.

[0086] The speed of movement of plug B decreases substantially linearly between the inlet of the hydrogen-fed group and the inlet of the unfed group. The maximum speed of movement of plug B is obtained when it is closest to the open feed valve, at the beginning of a feeding phase, for example at t0, t1, t2, etc. The closer plug B gets to the unfed end (closed feed valve), the more its speed of movement decreases, since the flow rate of hydrogen, which carries the non-reactive gases by suction, decreases. At the midpoint M, the speed of plug B is on the order of the arithmetic mean between the maximum and minimum speeds of the plug. The oscillation frequency of plug B follows the frequency 1 / T of the alternating feed cycles. As the frequency 1 / T of the alternating feed cycles increases, the amplitude A of the oscillations of plug B decreases, and vice versa.

[0087] The volume of plug B and / or the length LB of plug B increase with each alternating feeding phase. It is therefore necessary to remove plug B after a few alternating feeding cycles, before the volume of plug B reaches 20% or 30% of the total volume of the anodic compartment, for example after about ten alternating feeding cycles.

[0088] Figure 5 schematically illustrates a purging phase according to one embodiment of the operating process. The purge valve 40 is open. The switching valves 10v and 201v are also open. This facilitates the evacuation of plug B by pushing symmetrically on each side at the inlets 11 and 21. The opening of the purge valve 40 is triggered when plug B is located approximately at the midpoint position M, closest to the purge valve 40. This optimized triggering advantageously minimizes the opening time, since the plug is directly in the correct position, i.e., at the evacuation point. Plug B then passes through the smallest active surface area before reaching the exhaust. This also minimizes the loss of unreacted hydrogen, since the latter is located primarily in groups 10 and 20 and is not trapped between plug B and the purge valve 40.The opening time of the purge valve 40 is typically between 0.1 s and 10 s.

[0089] The movement of the stopper to a substantially median position is achieved by applying a partial feeding phase, that is, by following a feeding phase from one group with a feeding phase from the other group of substantially shorter duration. The optimal triggering delay—for which the stopper B is at the median position M—relative to the last feeding phase can be determined from a stopper movement model such as the one described above. In particular, it appears that the stopper B passes through the median position M in the middle of each feeding phase, approximately T / 4 after the start of a feeding phase. The accuracy of the triggering delay is preferably on the order of 10 ms.

[0090] Fig. 6 illustrates four delays dH d25, d50, d75 corresponding respectively to a triggering at 1%, 25%, 50%, 75% of the duration of a power phase, i.e. at times t = 0.01.172, t = 0.25.172, t = 0.50.172, t = 0.75.172.

[0091] Figures 7A to 7D illustrate the cell voltage V as a function of the operating phases Cl, Dl, El, C2, E2, C3 of the stack, for the different triggering delays db d25, d50, d75.

[0092] Phases Cl, C2, C3 each correspond to the operation of the battery during several cycles of alternating power supply, typically about ten consecutive cycles.

[0093] The DI phase corresponds to an optional "dead-end" phase during which all the cells in the stack are simultaneously supplied with power and the drain valve is closed. During this phase, the blockage is forced to stabilize in its position within the fluid circuit. If the blockage is not already located in the drain lines, it should be noted that the dead-end phase does not allow the blockage to be moved out of the cell group, and therefore, in particular, into the drain lines. This is because the blockage is pushed by the supply and pulled by the consumption. However, this consumption does not exist in the drain line.

[0094] This optional phase is therefore preferably carried out at the end of the triggering delay. In this case, the purge valve remains closed and all switching valves are open. Thus, when the triggering delay corresponds to the optimal position of the plug in the fluid circuit, near the purge valve, the dead-end phase can produce a synergistic effect with the synchronized triggering to improve and stabilize the positioning of the plug as close as possible to the purge valve.

[0095] The DI phase can therefore be associated with the purge phase El, typically by being interposed between the triggering delay and the effective opening of the purge valve.

[0096] Phases E1, E2 correspond to purging phases during which the purge valve is open. When the purge valve is opened, the switching valves can be opened, simultaneously or beforehand.

[0097] Phase E2 corresponds to a purge phase without a prior dead-end. The purge phase is thus advantageously shortened. The fuel cell performance is more consistent. The average voltage delivered is higher than in the case of a purge phase with a dead-end. According to one embodiment, the operating method of the invention only provides for purge phases without a dead-end.

[0098] Several triggering delays for the purge phases are shown in Figures 7A to 7D to illustrate the importance of choosing a correct start-up delay for the purge step, which is the subject of the invention. Figure 7A illustrates the triggering of DI + E1 (purge phase with dead end) and E2 (purge phase without dead end) at 1% of a feed phase. In this case, the voltage V drops at the beginning of the purge phase, typically when the plug is not yet brought close to the drain. The hydrogen supply to the group of cells containing the plug is disadvantaged compared to the other group where the pressure drop is lower, which induces this drop in the voltage V delivered by the cells.

[0099] Figure 7B illustrates the triggering of DI + E1 (purge phase with dead end) and E2 (purge phase without dead end) at 25% of a supply phase. In this case, the voltage drop V, always at the beginning of the purge phase, is more moderate than In the previous case, the blockage was still not predominantly positioned in the drainage pipes.

[0100] Figure 7C illustrates the triggering of DI + E1 (purge phase with dead end) and E2 (purge phase without dead end) at 50% of a power supply phase. In this case, the voltage drop V is minimized or even eliminated, particularly when the triggering is not preceded by a dead end. The plug is in the ideal position as close as possible to the purge valve, and the hydrogen participating in the reaction is not evacuated during the purge. It continues to participate in the reaction. This contributes to maintaining the voltage V delivered by the cells.

[0101] Figure 7D illustrates a DI + El (purge phase with dead end) triggering at 75% of a feed phase. In this case, the voltage drop V remains more moderate than in the cases of 1% and 25% triggering. This may be the result of a slight delay in the movement dynamics of the plug, for example, due to substances exhibiting greater movement inertia, such as liquid water. Furthermore, since the water is retained in the drain line when the plug passes, the obstruction caused by the plug to the flow of reagent in the group containing it is reduced. Typically, during a purge before T / 4, the plug is associated with liquid water. This causes a greater pressure drop. The feed to the two groups is unbalanced. The effect of the plug on the voltage is accentuated.During a purge after T / 4, there is less liquid water present with the plug, the supply imbalance is less pronounced, and the effect of the plug on the voltage is reduced.

[0102] According to a preferred practical option, for a two-group assembly, the drain valve is triggered directly in the middle of a feed phase, without a dead end, after a delay corresponding to approximately 50% of the feed phase duration, or approximately T / 4. A certain tolerance with respect to T / 4 remains acceptable for triggering the drain valve opening. This triggering can, for example, occur after a delay of between 30% and 70% of the feed phase duration.

[0103] Other models can be implemented to determine the triggering delay of the purge phase, with or without a dead end, after the start of a feeding cycle, in particular to take into account other parameters and / or variable conditions, for example with regard to the sizing of the units, the durations of the different feeding phases, a variation in electrical current and / or hydrogen pressure, an increase in the size of the plug, etc. Simulations can be carried out on the basis of these models to refine, where necessary, the determination of the triggering delay. Nomographs can be established for a wider use of different ping-pong architecture stacks. An example model is described below.

[0104] Regardless of the number of cell groups, during a given feeding phase, there are always two sets of groups: • The assembly E{ in which no group(s) receive the feed fluid directly at the inlet, • The E2 assembly in which n2 group(s) receive the feed fluid having passed indirectly through the E^ assembly.

[0105] In accordance with the principle of the "ping-pong" architecture, a manifold connected to the purge element is in fluidic connection with all the cell groups, therefore with both sets, regardless of their number of groups.

[0106] The ping-pong process is divided into successive feeding phases of these two fluidly connected series assemblies. The flow in the stack is modeled here as a unidirectional flow along a dimensionless scale graduated between -1 and +1, with position 0 corresponding to the optimal position relative to the purge device. The fluid enters alternately at xf = -1 and xe = +1. The first set E1 of cell groups is traversed in the interval [-1, 0[, and the second set E2 is traversed in the interval ]0, 1]. F, Qv, and v are defined respectively as a molar flux, the total volumetric flow rate, and the velocity of the gas mixture, regardless of its nature, along its position x, which is between -1 and 1. Their signs are positive if xe = -1, and negative if xe = +1.

[0107] For each element dx of the flow, the mass balance for hydrogen can be written:

[0108] [Math.3] FH2(x + dx) = FH2(x) + SH,(x)

[0109] Where Shi is a source term corresponding to hydrogen consumption.

[0110] By integrating [Math 3], we obtain the molar flow rate at abscissa x: [YES] [Math.4] FH2(x) = FH2(xe)-\xsH,(x)dx

[0112] With

[0113] [Math.5] ri Fn2(xe) = xe] x)dx

[0114] The term source means:

[0115] [Math.6] , . i(x)S(x) Sh,(x) = --2$~

[0116] With S(x) the active surface of the cell where the reaction takes place, i(x) the current density, and the Faraday constant. If the two sets E1 and E2 have a different number of cells, S(x) will take a different value depending on whether x is positive or negative. This formulation therefore allows the problem to be generalized to any number of cell groups.

[0117] Thus, for xe — 1 with * and S constants, we obtain:

[0118] [Math.7] r- / xr / 1\ / x.3. 0-^(-WS

[0119] [Math.8] and Fh2(-1)=0

[0120] For xe = -1 with z and S constants, we obtain:

[0121] [Math.9] c / X c / 1 x r1 / x J 2iS , 0^X5 Fh2(x) = FhJ-1) -]xSH2(x)dx= 25 + ~2$~ = ~25~

[0122] [Math. 10] FhA-I)^ and F^2(l)=0

[0123] The total gas flux Fgaz(x) contains water vapor:

[0124] [Math. 11] Fgalx) = FH ^x)+F ^xj

[0125] The total volumetric flow rate is given by:

[0126] [Math. 12]

[0127] The stack has a gas-accessible volume V(X), with a possible distinction between negative and positive abscissas, as for S(x). The flow length in a group set F2 or F2 being 1, the cross-sectional area perpendicular to the flow is V(X).

[0128] We can deduce the velocity as a function of the abscissa:

[0129] [Math. 13] , , 1 ( f1 , , , P z , , \ Fx) R^x) V(X) = ^[xei1sH2(x)dx-]xSH2(x)dx

[0130] y(x) ~ y(x) 2^(P(x)-Pva^x)) ri A \ -xJ j(x)dx + ]xi(x)dx )

[0131] In order to determine the best time for purging, from the velocity equation, the position of a molecule in the flow can be calculated using the equation:

[0132] [Math. 14] x(7) = x(0) + .|^v(x( / ) )dt

[0133] This differential equation can be solved numerically in the general case using standard simulation tools (for example, via Matlab Simulink software). The simulation makes it possible to predict the time 1 for which x(t) = 0, corresponding to the passage of the molecule between the two sets E1 and E2 of cell groups. If the molecule is detrimental to the operation of the fuel cell, this is when the purge should be opened. For controlling the fuel cell, a purge management algorithm can therefore be implemented, either directly incorporating this model or a table of simulation results under the operating conditions attainable by the fuel cell.

[0134] More complete models can be developed, also taking into account the passage of gas through inactive areas (in particular the purge manifold), or even the flow of liquid water. The latter case is more complex, but also more precise, because the purge serves to remove not only inert gases, but also liquid water, the presence of which interferes with the distribution of the feed fluid, also called the reactive fluid (fuel or oxidizer).

[0135] Without going towards a complexification of the model, it is possible to adapt the version presented here so that it takes into account on the one hand additional volumes by increasing for example the value of y(x), and on the other hand a flow of liquid water by considering that it circulates with the gas at a speed lower than the latter, making it appear later in the collector.

[0136] According to an example of an implementation of the model, the two sets and E2 are considered to each comprise a group of cells. They are identical, and are traversed by the same electric current.

[0137] The following approximations are also made:

[0138] P(x)=P

[0139] [°14°i pmp(x) = pjp)

[0141] i(x)=i

[0142] S(x)=S

[0143] V(x) = V = eS, with e the thickness of the flow volume with frontal surface 5'.

[0144] He comes:

[0145] [Math. 15] v(x(*) ) (-2xe(t)i + i(x£t)-x(i)) )

[0146] v(x(t)) (-x(t) -x^t})

[0147] x^t) is a piecewise constant function, which alternately takes the values ​​-1 and +1. The position as a function of time x( / ) must first be defined for a molecule starting from xe. We denote by tj the time of hydrogen injection at the beginning of a feeding phase of index j. We can write the following relation for ' greater than tj and without change of hydrogen entry point:

[0148] [Math. 16] dxtt-tA -dT = v{x{t-tj) ) = A(x(t-tj) ) + B

[0149] With

[0150] [Math. 17] A SR0i 4 - - l^VjP-P^

[0151] B = xe(tj)A- -xe(tj) 2<&V(P-psat}

[0152] This is a first-order linear differential equation whose solution is of the form:

[0153] [Math. 18] x(t-tj) =€€^^-1

[0154] Since A is negative, we have for 1 tending towards infinity:

[0155] [Math. 19] Ümx(t-tj) = -% = -xe(tj)

[0156] Furthermore, at t = tj:

[0157] [Math.20] x(0) = C-^ = C-xe(tj) = xjjj)

[0158] Let c =

[0159] In the end, he comes:

[0160] [Math.21] x(t-tj) = 2xe(tj)exp(- (t-tj) )-xe(tj)

[0161] During an alternation between the two hydrogen inputs, the molecule does not have time to travel through both groups of cells (it converges to this point at infinity). The starting point for phase j + 1 is denoted x ( -1)+[ ), with ^+1 The instant of opening of the hydrogen inlet for phase j + 1, and ^+1 the instant when this same valve should have opened for the molecule to reach this location from the inlet. We can then write that the starting point for phase j + 1 is the same as the stopping point for phase j:

[0162] [Math.22] ( tj+[ - tj+\) — x(tj+i-1j )

[0163] 2xe(i}+1)exp(A(^+1 -t / / )-x / t / / = 2xe(t / )exp(A(fj+xt}))

[0164] We note that xe () = - xe (tj+1), hence:

[0165]

[0166]

[0167]

[0168]

[0169]

[0170]

[0171]

[0172]

[0173]

[0174]

[0175]

[0176]

[0177]

[0178]

[0179]

[0180]

[0181]

[0182]

[0183]

[0184]

[0185]

[0186]

[0187]

[0188] [Math.23] 2xe(^i)exp(A(^+i-<;+i) ) = -2xe(Z%i)exp() + ^(^+0 exp(A(^+H}+i) ) = l-exp(A(^+1- / }) ) The final result is: [Math.24] ^+i = -Aln[ l-exp( A( / ?+|-4) ) ] +^+l The theoretical purge time is obtained for x = 0. The purge time of the supply phase j is denoted tj. It satisfies: [Math.25] x(tj-tj-) = 2xe(r7)exp(A(^-^-) ) -xe(tj)=0 exp(A(^-^))=| ~ Â^n2 Furthermore, in steady state, the difference between and fj is constant and equal to d. We also have the relation ^+1 = tj + t, with t - T ! 2 the half-period of alternation between the feeding phases. We can write: [Math.26] - 4+ï = a ln [ 1 - exp( A( t%x - 4+i + r ) ) ] By setting 5 = ty+) we obtain: [Math.27] exp(A5) = 1 -exp(A(ô + r) ) = 1 -exp(A<5)exp(Ar) expAù) " expCAr) Â^nl+exp(Ar) Finally, the purge trigger delay ,P j is written: j [Math.28] toP_tP fl-fi in___1___ “JO “VOA \m2 lnl+exp(Ar) ] - Am 2 Digital application: p = 2 105Pa θ = 353k P-= 5 x 104 Pa i = 1 A / cm2 S = 350 cm2 e = 0.3 mm (this value typically takes into account inactive volumes in the flow) T = 0.4 s (period T = 0.8 s)

[0189] A= -3.38s1

[0190]

[0191]

[0192]

[0193] The triggering delay obtained for these numerical values ​​is: dP. — g J 4 s, which corresponds to 34.3% of the half-period T. Figure 8 illustrates the simulated evolution of the nitrogen concentration in the collector at the purge valve (curve N) as a function of the feed phase changes. This Matlab Simulink model allows visualization of the spread of the blockage, which is mainly composed of nitrogen. In this particular case, the point at which the nitrogen concentration is at its maximum in front of the purge valve (local maximum) occurs well before the halfway point of the half-period, as in the previous analytical calculation. The optimal synchronized opening around 50% of the half-period observed in practice is probably attributable to liquid water. Therefore, we will ensure that: [Math. 29] H 1+exfXA-r) dj  ln“"~“

[0194] Through the examples described above, it is clear that the operating method according to the invention makes it possible to improve the consumption and performance of a ping-pong architecture fuel cell. This method can be advantageously implemented for automotive or heavy transport applications (land, sea, air, etc.).

[0195] The invention is not limited to the embodiments described above. Several stacks, with groups of nested cells, can also be envisaged within the framework of the present invention, by implementing the principle of synchronized triggering of the purge phase.

Claims

Demands

1. A method for operating a fuel cell (1), said fuel cell (1) comprising a plurality of groups (10, 20) of electrochemical cells connected to each other by a fluidic circuit (3, 4), said fluidic circuit (3, 4) comprising, for each group, a supply line (100, 200) at the inlet of said group, and a switching element (101v, 201v) on said supply line, and a discharge line (120, 220) at the outlet of said group, said fluidic circuit being configured such that the discharge lines (120, 220) communicate fluidly with each other and are connected to a common purge element (40), said method comprising at least: A cycle (Cl, C2) of alternating feeds comprising at least: • A first supply phase in which a first group (10) among the plurality of groups is supplied directly with feed fluid, at its inlet (11), the switching element (10v) of said first group (10) being open while the switching elements (20v) of the other groups (20) of the plurality of groups are blocked, said other groups (20) being supplied indirectly, during this first supply phase, at their respective outlets (22), by the feed fluid from the outlet (12) of the first group (10), • A second supply phase in which a second group (20), different from the first group (10), is directly supplied with feed fluid at its inlet (21), the switching element (20Iv) of said second group (20) being conducting while the switching elements (10Iv) of the other groups (10) of the plurality of groups are blocked, said other groups (10) being supplied indirectly, during this second supply phase, at their outlets (12). espective, by the feed fluid from the outlet (22) of the second group (20), • A purge phase (E1, E2) following the alternating feed cycle, said purge phase comprising an opening of the purge device (40) so as to evacuate non-reactive fluids originating at least in part from the feed fluid, Said process being characterized in that the purge phase is triggered synchronously with a displacement of non-reactive fluids accumulated in the form of an aggregate called plug (B), when said plug (B) is located at least in part in the discharge lines (120, 220), the alternating feed cycle having a period T and the purge phase being triggered during a feed phase j and before the end of the feed phase j, after a delay d? such that: 1 l+exp(Ar) 2 with r = T / 2 the half-period, and AA - - 2*V(P-Psei) With $ the active area of ​​the cell,* the current density and d? the Faraday constant, R the ideal gas constant, 0 the temperature of the cell, V the volume through which the fluid flows in the cell, P the pressure of the fluid in the cell and Psat the saturated vapor pressure at the cell temperature.

2. A method according to the preceding claim, wherein the alternating feed cycle has a period T and the purge phase is triggered during a feed phase j and before the end of feed phase j, after a delay between 0.3T / 2 < dj < 0.7T / 2.

3. A method according to any one of the preceding claims, wherein the alternating feed cycle has a period T and the purge phase is triggered during a feed phase j and before the end of feed phase j, after a delay dJ = T / 4-

4. A method according to any one of the preceding claims wherein the stack comprises only two groups (10, 20), the outlets (12, 22) of these two groups (10, 20) are located at an equal distance from the purge device (40), and the first and second feeding phases of the alternating feeding cycle have the same duration 172.

5. A method according to any one of the preceding claims wherein the purging phase comprises, before or during the opening of the purging member (40), an input supply (11, 21) to all the groups (10, 20), each switching member (10v, 201v) being conducting.

6. Method according to the preceding claim wherein the inlet supply to all groups is carried out simultaneously with the opening of the purge device (40).

7. Method according to claim 5 wherein the opening of the purge element (40) is carried out after the inlet supply of all groups, after a so-called "dead-end" delay.

8. A method according to any one of claims 1 to 4 wherein the purging phase does not include supplying inlet to all groups before opening the purging device (40).

9. A method according to any one of the preceding claims wherein a constant electric current is imposed on the fuel cell.

10. A method according to any one of the preceding claims wherein the feed fluid injected during the alternating feed cycle has a constant pressure.

11. A method according to any one of the preceding claims wherein the opening of the purge member (40) is triggered when at least 70%, and preferably at least 80%, of the length LB of the plug (B) is located in the discharge lines (120, 220), the length LB of the plug (B) being measured along the direction in which the discharge line extends at the point considered.

12. A method according to any one of the preceding claims wherein the opening of the purge member (40) is triggered when the plug (B) is entirely located in the discharge lines (120, 220).

13. A method according to any one of the preceding claims wherein the opening of the purge member (40) is triggered when the plug (B) completely occupies the discharge lines (120, 220).

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