Purge circuit of a fuel cell

Abstract

A purging circuit for purging an anodic compartment of a cell of a fuel cell, this circuit including: a capacity, forming a related volume at least equal to 500 ml, for containing and homogenising a recovery gas, including an inlet and an outlet; a first nonreturn valve to prevent the recovery gas from returning through the outlet and allowing gas to flow from the first outlet to an inlet of the compartment; a second nonreturn valve to prevent gas from being discharged from the capacity through the inlet; a pressure sensor able to measure the pressure of a fluid present in the circuit; a valve controlling the flow of a supply gas to and from the compartment as a function of data of the sensor and allowing gas to flow from the first nonreturn valve to the inlet of the compartment.

Claims

1. A method for operating a purging circuit for purging an anodic compartment of an electrochemical cell of a fuel cell, this circuit including: a volume, having connexity, of at least equal to 300 ml, containing and homogenising a recovery gas comprising hydrogen and an inert gas, said volume comprising a first inlet and a first outlet; a first nonreturn valve connected to the first outlet of said volume containing a recovery gas, said first nonreturn valve preventing a gas from being introduced into said volume through the first outlet and further allowing a part of a gas to be flown, from the first outlet to an inlet of the anodic compartment; a second nonreturn valve connected to the first inlet of said volume for containing a recovery gas, preventing a gas from being discharged, from said volume, through the first inlet; said method including: measuring with a pressure sensor a pressure of a fluid present in said purging circuit; a valve allowing a supply gas to be flown to the inlet of said compartment, or prohibiting it from flowing to said inlet, as a function of pressure data of said pressure sensor.

2. The method according to claim 1, the compartment being supplied with recovery gas when the pressure of the recovery gases is higher than the pressure in said anodic compartment, the first nonreturn valve being thereby open, or said valve allowing a gas to be flown to the inlet of said compartment being open when the pressure in the compartment and in said volume containing the recovery gas comes below a so-called minimum pressure value.

3. The method according to claim 1, a pressure variation caused by the opening of the valve allowing a gas to be flown to the inlet of said compartment causing the first nonreturn valve to be closed or causing the second nonreturn valve to be open or causing water and gas present in the anodic compartment to be discharged.

4. The method according to claim 1, including closing the second nonreturn valve before the pressures are balanced in the means for containing the recovery gas and in the anodic compartment.

5. The method according to claim 1, including closing the valve, prohibiting a gas from being flown to the inlet of said compartment when the pressure, detected by said pressure sensor, is higher than a predetermined pressure threshold, called a maximum pressure VB.

6. The method according to claim 1, said pressure being measured with said pressure sensor upstream of the second nonreturn valve.

7. The method according to claim 1, wherein, in response to changes in pressure within the purging circuit, the first nonreturn valve and the second nonreturn valve alternate between a state in which both the first nonreturn valve and the second nonreturn valve are closed and a state wherein one of the first nonreturn valve and the second nonreturn valve is open.

8. The method circuit according to claim 1, a connector comprising a first branch connected to the first nonreturn valve, a second branch and a third branch, said valve allowing a supply gas to be flown to the inlet of said compartment being connected to the third branch of the connector.

9. The method according to claim 8, the inlet of said anodic compartment being connected to the second branch of said connector, so as to allow a gas outflowing from the anodic compartment to be reintroduced into said compartment through the purging circuit.

10. The method circuit according to claim 1, said valve: allowing a supply gas to be flown to the inlet of said compartment, to increase the pressure therein or when the pressure sensor measures a pressure lower than a minimum threshold, or when a pressure measured by said pressure sensor increases; not allowing a supply gas to be flown to the inlet of said compartment, to decrease the pressure therein or when the pressure sensor measures a pressure higher than a maximum threshold, which is higher than the minimum threshold pressure or when the pressure measured by said pressure sensor decreases.

11. The method circuit according to claim 1, a pressure reducer, connected upstream of the inlet of the valve, allowing a gas to be flown or prohibiting it from being flown to the inlet of said compartment.

12. The method according to claim 1, further comprising discharging at least part of the liquid state water present in said volume for containing a recovery gas through a second outlet.

13. The method according to claim 1, further including measuring the volume of the liquid water present in said volume for containing a recovery gas.

14. The method according to claim 12, a valve or a solenoid valve controlling said second outlet.

15. The method according to claim 14, further comprising measuring the frequency of purging cycles and controlling the operation of said valve controlling said second outlet as a function of this frequency.

16. The purging circuit according to claim 14, said valve controlling said second outlet without releasing in the environment a recovery gas present in said volume containing a recovery gas.

17. The method according to claim 1, a second pressure sensor measuring the pressure in a cathodic compartment belonging to the same electrochemical cell as said anodic compartment.

18. The method according to claim 17, said valve, allowing a supply gas to be flown or prohibiting it from being flown to the inlet of said compartment, as a function of pressure data of said pressure sensor, being controlled as a function of the pressure difference between the anodic compartment and the cathodic compartment.

19. The method according to claim 1, comprising connecting several outlets and several inlets of several anodic compartments belonging to a fuel cell comprising several electrochemical cells.

20. The method according to claim 1, further including supplying a pressurized fuel comprising hydrogen.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further details and characteristics of the invention will appear upon reading the description that follows, made in reference to the following appended figures. Identical, similar or equivalent parts of the different figures bear the same reference numerals so as to facilitate switching from one figure to the other. Different parts represented in the figures are not necessarily drawn at a uniform scale, to make the figures more legible.

(2) FIG. 1 represents a cross-section view of a fuel cell comprising several electrochemical cells.

(3) FIG. 2 represents FIG. 7 of document EP1018774 A1.

(4) FIG. 3 represents an exemplary embodiment of the invention connected with an anodic compartment of a hydrogen cell.

(5) FIG. 4 represents the variation of pressure and flow rate of a fluid flowing in a device represented in FIG. 3 as a function of time (t).

(6) FIG. 5 represents a variant of the exemplary embodiment represented in FIG. 3.

(7) FIG. 6 represents another variant of the exemplary embodiment represented in FIG. 3.

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

(8) The present application aims at providing a purging device enabling an anodic compartment of a hydrogen cell to be purged, in a more efficient, simpler and more reliable manner. An exemplary embodiment of such a device, with several alternatives added thereto, is described below.

(9) The elements making up a hydrogen cell have been described above, the references mentioned herein below correspond to this description.

(10) According to a first exemplary embodiment of the invention, the outlet 14B and the inlet 14A of an anodic compartment 14 of a hydrogen electrochemical cell 2 (or battery), are connected, outside the compartment, through a purging circuit 40 (FIG. 3). The phrase “elements connected” according to the present application means that a fluid can flow between both these elements via the purging circuit.

(11) The purging circuit can for example comprise one or more channels proof to the fluid flowing therein. The purging circuit includes means 20 forming a volume for containing hydrogen and/or a recovery gas (these means are designated by the term “capacitor” herein below) a first inlet 24A of which is intended to be connected to the outlet 14B of the anodic compartment 14, and a first outlet 24B of which is intended to be connected to the inlet 14A of the same compartment. The circuit can be connected to gas supply means, which are not represented in the figures; the gas provided by these supply means is preferably a gas having a very strong concentration of hydrogen, higher than 99%. In the case where hydrogen is mixed with another gas, this other gas can be nitrogen, and/or carbon dioxide, and/or methane.

(12) A first nonreturn valve 42 is disposed downstream of the outlet 24B (in a gas flow direction in the purging circuit) and is intended to be connected to the inlet 14A so as to allow a fluid to be flown only from the capacitor 20 to an input of the anodic compartment 14.

(13) A second nonreturn valve 44 is connected upstream (in a gas flow direction, from the anodic compartment to the capacitor 20) of the first inlet 24A of the capacitor, and is intended to be connected to the outlet 14B of the anodic compartment, so as to allow a fluid to be flown only from this outlet 14B to the capacitor. In other words, the first nonreturn valve 42 does not allow the fluid to be flown into the purging circuit 40, from the anodic compartment 14 to the capacitor 20. And the second nonreturn valve does not allow the fluid to be flown from the capacitor 20 to the anodic compartment 14. The nonreturn valves can be identical and are for example of the bellow, ball or concentric disc type.

(14) The purging circuit includes means 46 that will allow a gas to be inflown into the anodic compartment 14. According to one example, these means include a connector 46 with three branches, a first branch 46A of which is intended to be connected to the first nonreturn valve 42, a second branch 46B of which is intended to be connected to the inlet 14A of the anodic compartment and a third branch 46C of which is intended to be connected to an outlet 48B of a two-way valve 48, for example a solenoid valve. The inlet 48A of this valve can be connected to a pressurised hydrogen supply device (not represented).

(15) A pressure sensor 50 measures the pressure in the purging circuit 40, for example between the outlet 14B of the anodic compartment and the second nonreturn valve 44. The pressure measurement can be performed in any point of the purging circuit. A valve 48, for example a solenoid valve or a driven “on-off” valve is controlled by the pressure measurements thus performed. More precisely, when the pressure sensor detects a pressure, for example at the outlet of the anodic compartment, that is higher than a threshold value, called a maximum pressure threshold VB, the valve 48 is closed. And then, when the pressure measurement becomes lower than a threshold value, called a minimum pressure threshold VA, the valve 48 is open.

(16) The valve 48 can be controlled by means 70, for example a microcomputer or a microprocessor, specifically programmed therefor, or an automatic controller. The value of the minimum pressure threshold VA and the value of the maximum pressure threshold VB depend on the membrane and the pressure at the cathode. Preferably, VA and VB are chosen such that the pressure remains in an admissible differential pressure range between the anode and the cathode, in order to avoid a risk of mechanical degradation of the membrane due to too high a differential pressure with the cathode.

(17) Now, the operation of the purging circuit 40 above will be described. This is connected to the inlet and the outlet of the anodic compartment of a hydrogen cell as mentioned previously.

(18) Upon operating the hydrogen cell, the valve 48 and the 2 valves 42, 44 being in the closed state, the hydrogen amount present in the anodic compartment 14 decreases because of the hydrogen oxidation at the anode and the migration of ions oxidised in the cathodic compartment through the membrane 8.

(19) When the pressure difference between the capacitor 20 and the anodic compartment 14 is higher than the set pressure of the first nonreturn valve 42, this is opened, the valve 44 remaining closed. The capacitor 20 thereby contains a gas, for example pressurised hydrogen, wherein the proportion of inert gas, in particular of nitrogen, can reach 80% therein. This gas is called a recovery gas (see herein below) and will flow to the input of the anodic compartment 14.

(20) The pressure in the capacitor 20 (which, generally, is related, or results from, the change over time in the pressure measured) and in the anodic compartment 14 will decrease because of the hydrogen consumption in the latter. When this pressure reaches the lower value VA, which is detected by the sensor 50, the valve 48 is open. The pressure increases in the anodic compartment 14, which results in the nonreturn valve 42 being closed, the valve 44 being still closed.

(21) Then, the pressure difference between the anodic compartment 14 and the capacitor 20 results in opening the nonreturn valve 44 (because of the higher pressure in the anodic compartment 14), the valve 42 remaining closed. Then, the water and gas present in the anodic compartment 14 can be discharged to the capacitor 20. Indeed, it is then a pressure front which is propagated, for a duration of, for example, a few tenths of a second or a few seconds, for example between 0.1 s and 0.5 s, from the solenoid valve 48 to the capacitor 20, through the anodic compartment 14. This pressure front is very efficient for discharging water and gases (including hydrogen but also possibly at least one inert gas, in particular nitrogen), from the compartment 14 to the capacitor 20.

(22) When the pressure of the anodic compartment reaches the maximum pressure threshold VB, the valve 48 is closed. The hydrogen pressure then begins to decrease in the anodic compartment 14, which results in closing the valve 44, the valve 42 remaining closed. The cycle above can then begin again.

(23) The valves 42, 44 undergo the following cycle: valve 42 closed, valve 44 closed (hydrogen consumption phase in the anodic compartment); valve 42 open, valve 44 closed (recovery gas introduction into the anodic compartment); valve 42 closed, valve 44 closed (hydrogen introduction into the anodic compartment); valve 42 closed, valve 44 open (recovery gas introduction into the capacitor 20); valve 42 closed, valve 44 closed (back to the initial state).

(24) In other words, in the example set forth, each state of the purging circuit in which one of the valves is open is preceded and followed by a state in which both valves are closed.

(25) FIG. 4 represents several purging cycles as described above, made from a purging circuit 40 connected to a hydrogen cell comprising 70 anodic compartments. More precisely, the hydrogen cell considered herein includes an active surface area of 200 cm.sup.2, the internal volume of the capacitor 20 is 12l, the total volume of the anodic compartments is 0.25l, the measurements are performed at a temperature of 343 Kelvin and an altitude of 150 m. The hydrogen cell produces an intensity of 60 A, that is a ratio of 0.30 A/cm.sup.2 of active surface area; the stcechiometry at the cathode is λc=3 (there are three times more oxygen than necessary). The measurements performed relate to pressures PA measured by the pressure sensor 50, pressure variations PC measured in the capacitor 20 and flow rate variations PE measured at the outlet 48B of the solenoid valve 48 (FIG. 4).

(26) FIG. 4 shows that upon opening the solenoid valve 48 (at 0 second), a hydrogen flow rate PE, at a flow rate of 4.5 Nm.sup.3/h, is injected at the inlet of the anodic compartments. The hydrogen pressure in these compartments increases, which enables said compartments and the capacitor to be filled with hydrogen (PA and PC, 0<t<8 seconds). It is worth noting that the pressure in the capacitor is lower than the pressure in the anodic compartments, because of the presence of the second nonreturn valve 44 between the inlet 24A of the capacitor and the outlets of the anodic compartments. It is observed that the valve 48 is closed when the pressure in the anodic compartments is higher than 1.42 bar (t=8 seconds). According to this exemplary embodiment, this pressure corresponds to the maximum pressure threshold VB defined above. The pressure in the anodic compartments PA then drops because of the hydrogen consumption by the cell (8<t<13 seconds). It is worth noting that when the pressure of the anodic compartments comes below the pressure PE measured in the capacitor, both these pressures linearly lower, because the first nonreturn valve 42 is open. The pressure then lowers in both these enclosures down to a minimum threshold value (1.15 bar) measured by the pressure sensor 50, again triggering opening the valve 48 (at t=13 s). The hydrogen flow rate abruptly increases, creating a pressure front, closing the first nonreturn valve 42 and opening the second nonreturn valve 44. It can be noticed in FIG. 4 that the pressure increase is instantaneous in the anodic compartments and in the capacitor when the solenoid valve is open. This clearly shows that the pressure front is propagated extremely quickly through the anodic compartments to reach the capacitor 20. The pressure front enables water and gases present in the anodic compartments to be homogeneously discharged.

(27) In comparison with a depressurisation technique as disclosed in document EP1018774 A1, the use of a pressure front enables water and inert gases which are in the compartment 14 to be more readily driven. Indeed, this pressure front enables a denser atmosphere to be made in the compartment, because the pressure therein is consequently higher.

(28) Again, the anodic compartments and the capacitor are filled with hydrogen (13<t<20 seconds). The time interval between two successive openings of the valve 48 can define a purging cycle according to the invention, it is for example in the order of 13 seconds according to the present example.

(29) More generally, in a circuit according to the invention, the time interval between two successive openings of the inlet valve 48 can be between 5 s and 20 s.

(30) Now, several alternatives of the purging circuit will be described below.

(31) The alternatives can be combined together to form other exemplary embodiments of the invention.

(32) According to a first alternative, a pressure reducer 52 is connected between the inlet 48A of the solenoid valve 48 and the pressurised hydrogen supply device (FIG. 5).

(33) Advantageously, the pressure reducer enables the pressure in the anodic compartment to be limited while affording high gas flow rates.

(34) According to a second alternative, the capacitor 20 includes a second outlet 24C, enabling liquid state water 54 present in said capacitor (FIG. 6) to be discharged. The capacitor can have an internal volume that can be for example between 500 ml and 22 l. It is worth noting that the time of a purging cycle varies in particular as a function of the available internal volume 56 in the capacitor. The terms “available internal volume” define a volume likely to be filled with a gas, preferably a pressurised gas. The higher the available internal volume, the longer the purging cycle. The capacitor 20 can include means for measuring the water volume 54 present in the internal volume, as for example a floater and/or means for detecting the presence of water controlling a second valve 58, for example a solenoid valve, connected to the second outlet 24C of the capacitor, so as to discharge at least part of the water 54 to outside said capacitor.

(35) Alternatively, the operation of the second solenoid valve 58 can be controlled through a device (not represented) measuring the frequency of the purging cycles. The frequency of the purging cycles increases in proportion to the water volume 54 contained in the capacitor. Thus, beyond a threshold frequency of the purging cycles, predetermined as a function of the internal volume of the capacitor, opening the second valve 58 can be controlled by the device measuring said frequencies. For example, a tachometer measures the frequency of the opening and closing cycle of the valve 48. When this frequency comes to a high threshold level (which means that the volume 56 has reached an admissible minimum threshold, below which it is not desired that it decreases), the control, which can be made by an automatic controller, or a microcontroller or a computer opens the valve 58 which purges part or all of the water present in the recuperator 20. This discharge increases the volume 56 which has the effect to lower the frequency of the opening and closing cycles of the valve 48. When this frequency, measured by the tachometer, comes under a low threshold, this means that the water volume 54 has reached an admissible target minimum value. The valve 58 is then closed.

(36) The second valve 58 enables the frequency of the purging cycles to be modified at will by controlling the water volume 54 present in the capacitor. The devices above also enable only the water present in the liquid state of the capacitor to be discharged, without releasing the hydrogen or recovery gas present in the available internal volume 56 into the environment.

(37) According to a third alternative, the purging circuit 40 can include a second pressure sensor measuring the pressure in the cathodic compartment, or upstream or downstream thereof, so as to know the pressure difference between the anodic compartment 14 and the cathodic compartment 16. The solenoid valve 48 can be open or closed also as a function of the pressure difference between both these compartments, so as to avoid too high mechanical biases of the membrane likely to damage it. Using both these sensors, a differential measurement between the anodic compartment and the cathodic compartment is made. The solenoid valve 48 is open in order to change over time the pressure measured by the sensor 50 such that the differential pressure between the anode and the cathode reaches a first threshold, called a high saturation threshold. When this first threshold is reached, opening the means 48 supplying the anodic compartment is commanded. The differential pressure between the anode and the cathode drops until a second threshold, called a low saturation threshold, is reached, controlling reopening the means 48.

(38) According to a fourth alternative, the set pressures of the nonreturn valves 42 and 44 are as low as possible, so as to be able to use at best the pressure variations afforded by the membrane 8. Each of these set pressures is for example between 5 mbar and 50 mbar.

(39) In the embodiments and examples described above, the pressure sensor is disposed so as to measure the pressure between the outlet of the anodic compartment and the second nonreturn valve 44. However, alternatively, the pressure measurement can be made at any place of the gas line. For example, it can be made between the nonreturn valves, or even in the tank 20. The operation described above remains the same. Further, all the alternatives described above are applicable.

(40) In conclusion, the present application is concerned with a reflow circuit enabling hydrogen, outflowing from an anodic compartment, to be reintroduced into this compartment.

(41) The outlet of an anodic compartment is connected to the inlet on the purging circuit, the outlet of which joins the inlet of the anodic compartment. The reflow circuit advantageously includes a capacitor 20, the available internal volume 56 of which can be controlled by purging liquid water 54 present in said capacitor. In this way, the frequency of the purging cycles can be controlled.

(42) The durations of the different steps of a purging cycle can be set by a relationship between the cell current, the internal volume of the capacitor and the volume of the anodic compartment, the pressure thresholds for opening and closing the valve and the set pressure of the nonreturn valves. In particular: the higher the current and the more hydrogen is consumed, and thus the quicker the pressure drops and the longer the pressure to be increased again; the higher the volumes, the longer their filling and emptying durations; the closer both thresholds, and the more quickly they are reached, and the shorter the cycle; the greater the set pressure, and less often the valves will be open; this will shorten the cycle time.

(43) On the other hand, the volume of liquid water 54 contained in the capacitor 20 can be known as a function of the frequency of the purging cycles: hence, the purging of water from the capacitor 20 can be made without releasing hydrogen in to the environment. Opening and closing the second solenoid valve 58 can be controlled so as to discharge an amount of liquid water equal to or lower than the liquid water present in the capacitor. Thereby, it is possible to limit hydrogen losses upon purging the capacitor.

(44) The invention also enables the mechanical deformations exerted on the membrane 8 to be controlled during purging cycles: the frequency of these cycles can be advantageously adapted so as not to degrade irreversibly the mechanical properties of the membrane.

(45) Another advantage related to the invention is that it is not necessary to utilise compressor or pump type devices to flow hydrogen in the purging circuit 40. Therefore, the invention offers the advantage not to cause an energy extra cost since the purging circuit 40 does not have a pump. Hence, the cell performance is less affected by the pressure variations in the anodic compartment due to the purging circuit, the cell weight is decreased and its overall space is diminished in comparison with a present circuit comprising a compressor.