FR2788169A1 - Feeding a reactive gas to a fuel cell involves increasing and then reducing gas pressure in at least one reference region between gas inlet and outlet of cell - Google Patents

Abstract

Reactive gas is fed to fuel cell (1) through inlet (11) and residual gas is evacuated through outlet (12). Recurrently and successively, gas pressure in at least one reference region between inlet (11) and outlet (12) is increased and then lowered, to create gas turbulence during at least part of its transit and on at least a part of the gas path between the inlet and outlet. An Independent claim is given for the feed device.

Description

The invention relates to a method for supplying

  reactive gas, from fuel cell, intended to ensure an

  regular carrying of this gas, for example air or hydrogen, in the cell, independently of the fluctuations in operation of the latter; it also concerns a

  device for implementing this method.

  For example in current fuel cells in which the oxidant is oxygen, this is

  supplied either by a stream of pure oxygen or by a flow

  rant of oxidizing gas, generally a stream of mixing

zeux like air.

  Current air supply devices

  carry this air whose oxygen is necessary for the functioning-

  on the cathode side of the cells, by means of a continuous circulation which must be large enough to ensure

  an over-stoichiometry of a factor of at least 1.5 in each

  that cell, and allow the evacuation of the water produced (and sometimes introduced on the air side for cooling, as

  in the technique known as "with porous").

  This circulation continues, unless precautions are taken

  can however be disturbed in different ways.

  negated by fluctuations that are difficult to control in the operation of the battery. In particular: - In fuel cells, the supply of the various cells is carried out in parallel; when circulation in a cell is slightly hampered for example by a water plug at the entrance or at the exit, it

  is insufficiently powered, resulting in deterioration

  performance ration and sometimes damage to the

cell.

  - In the case of the "with porous" technique, the gas supply is carried out through a layer of conductive and porous material; due to the absence of

  gas distribution channels, this one is less well

  new in certain parts of the cell, which can be inerted by the presence of the nitrogen left there after the consumption of oxygen. Again, the performance of

  the cell is degraded. These degradations of per-

  formances must be detected, resulting in the need for

  to monitor the cell voltage individually,

  in order to trigger purges in case the cell voltage

  the becomes below a predetermined threshold.

  - The last part of the gas journey to par-

  coming to the electrolyte is by diffusion in a layer

  porous where the produced water and the incoming gas intersect.

  This diffusion layer is filled with the nitrogen remaining after consumption of the corresponding oxygen, which forces the new oxygen to make its way by diffusing through this nitrogen. As this diffusion zone is not located on the gas path, there is no forced circulation of the gas through the layer; on the contrary, a gas flow of

  water vapor flows in the opposite direction and slows down the diffusion

oxygen.

  Another example of a problem to solve is that

  concerning the hydrogen circuit of such fuel cells-

  tible, circuit in which nitrogen appears from the air circuit by diffusion through the membrane; in this case, the correct functioning of the battery involves avoiding the accumulation of nitrogen, for example in the form of stratum (s)

in certain regions of the circuit.

  The invention aims to remedy these drawbacks, and to this end relates to a process for supplying reactive gas to a fuel cell, according to which this reactive gas is introduced at a gas inlet of the cell and a

  residual gas at a gas outlet from the cell, charac-

  terrified in that, repeatedly, successively, we

  increases gas pressure in at least one pressure region

  ference of the battery located between the inlet and outlet up to

  that at a first value, and we decrease the gas pressure

  in the reference region up to a second in-

  lower than the first, in such a way that gas turbulence is created during at least part of the transit and on at least part of the path of the gas between the inlet and

the battery outlet.

  Thanks to the fact that the pressure in the stack is continuously varied between two extreme values, the

  gas circulation and supply are guaranteed

  dynamic, and the "lung effect" thus created ensures a gas supply at all points of the cells between the two

  access to the battery, and prevents the accumulation of water in the liquid state in certain places. The process according to the invention can moreover present

  ter one or more of the following characteristics: - the gas pressure in the cell is changed between

  the supply pressure at the inlet of the battery and the pressure

discharge ion at its exit;

  - the gas pressure in the stack is increased up to

  that at a value equal to or very close to the supply pressure at the inlet of the stack;

  - the gas pressure in the stack is reduced to

  that at a value equal to or very close to the discharge pressure at the outlet of the cell; - is introduced into the stack, at said entry of

  gas therefrom, an oxidizing gas, and evacuated to said outlet

  gas from the battery, a residual gas in which the pro-

  portion of oxygen is practically nonexistent or noticeable-

  ment reduced compared to the proportion of oxygen in the oxidizing gas at the inlet of the cell; - A gas consisting of at least hydrogen is introduced into the cell, at said gas inlet thereof, and a residual gas is discharged at said gas outlet from the cell

  containing hydrogen and nitrogen.

  The invention also relates to a device for implementing the method defined above, characterized in that it comprises at least one means for modulating the

  pressure in the battery, connected to an access thereof.

  The device according to the invention can moreover pre-

  feel one or more of the following characteristics: - the pressure modulation means is a valve; - The pressure modulation means is a compressor whose output is connected to the input of the battery and which operates in pulsed mode; - The device comprises a pressure drop means connected to one of the accesses of the battery; - The device comprises upstream of the stack at least one means for keeping the supply pressure of the stack approximately constant; - The device comprises downstream of the stack at least one means for keeping the discharge pressure of the stack approximately constant;

  - the means to maintain the approximate pressure

constant is a capacity;

  - the means to maintain approximately

  l0 aunt the battery supply pressure is a detent

deur;

  - the means to maintain approximately

  aunt the battery discharge pressure is an overload;

  - the modulation of the pressure in the pile cor-

  responds to a variation in pressure AP at least equal to

about 0.1 bar.

  Other features and benefits of

  the invention will emerge from the following description of

  embodiments and embodiments of the invention given by way of nonlimiting examples and illustrated by the accompanying drawings in which: - Figure 1 is a diagram showing a part

  a fuel cell supply circuit allows

  both the implementation of the method according to the invention, FIG. 2 is a diagram showing a variant of part of the supply circuit allowing the implementation of the method according to the invention, and

  - Figure 3 is a diagram showing another va-

  laughing part of the power circuit allowing the

  implementation of the method according to the invention.

  The process according to the invention for supplying gas

  reagent, for example in air, from a fuel cell, is

  tiné in a conventional manner to introduce the gas at a gas inlet 11 of a battery 1, and to evacuate at a gas outlet 12 of the battery a residual gas here in which the proportion

  of oxygen is practically non-existent or significantly

  pick with respect to the proportion of oxygen in the gas

  coming to entrance 11, as a result of the use of the ma-

  part of the oxygen in the general chemical reaction

  electricity and water.

  According to the invention, the reactive gas is introduced at the inlet 11 and the residual gas is discharged at the outlet 12 so that the gas pressure in the stack, successive-

  ment, increases to a first value, and decreases until

  has a second value lower than the first, and we re-

  repeatedly fars the cycle of increase and di-

  decrease in pressure in the stack, like a breath

pulmonary ration.

  More precisely, the pressure in the pile, which it is possible to measure in one (or more) region (s) of

  this taken as reference (s), is modified in perma-

  nence between and preferably practically until the pres-

  supply pressure at inlet 11 of the battery and discharge pressure at outlet 12, supply and discharge pressures being kept approximately constant, for example by means of upstream and downstream buffer capacities 13

  14, or regulator and overflow devices, respectively.

  Thus, the first extreme value in the stack can be made equal to or close to the supply pressure at

  entry 11, and the second extreme value equal to or see

  sine of discharge pressure at outlet 12.

  In a first embodiment (FIG. 1), the timing of the pressure increase and decrease cycle in the reference region is achieved by means

  a valve 15, for example a solenoid valve which is

  electrically controlled, the input of which is connected to the output

  of the battery and whose output is connected to the input of the

  downstream capacity 14 or of the spill device. When the valve 15 is closed, the pressure in the stack 1 is caused to increase, preferably until it reaches the practically constant inlet supply pressure.

  11; when the valve 15 is opened, the reduction is caused

  tion of the pressure in the pile preferably until

  that it reaches the practically constant pressure of de-

  charge; then we start the flow modulation cycle again

  of the valve therefore of the pressure in the pile in a

  common; when the pressure in the cell decreases, the pressure of inert gas in poorly ventilated areas decreases; the pressure rise which follows takes place with "new" gas which then irrigates all the parts of the stack; as the exchanges are all the better as the flow takes place in turbulent regime, the closings and openings of the valve are carried out relatively suddenly. It will be noted that here, the renewal of the gas in the poorly ventilated regions is no longer ensured by diffusion, but by global movements of this one: the pressure in the pile is dropped to evacuate the used gases and one

  fills the battery with fresh gas like a breath

pulmonary tion.

  Another advantage of abrupt switching is the elimination of liquid water from the places where it

  is overabundant, and its ejection out of the pile.

  Alternatively (Figure 2), the valve can be replaced

  outlet 15 by an inlet valve 16, for example also

  a solenoid valve, the input of which is connected to the output of the upstream capacitor 13 and the output of which is connected to the input of the battery. In this case, when the valve is opened, the pressure in the valve increases.

  battery, and when you close the valve, it causes the decrease

  the pressure in the pile; the same type of pressure modulation is thus carried out as with the

Figure 1.

  It is also possible to implement, by coordinating their operation, an inlet valve 16 and a

  exit 15; this configuration (not shown) which

  results from the combination of the configurations of Figures 1 and

  2 adjusts the speed of increase and decrease

pressure in the pile.

  It is also possible to use a single valve 15 or 16 at the outlet or at the inlet, and to slow down the increase or decrease in pressure as the case may be, by inserting a pressure drop means known per se slightly upstream of the 'entry 11 or slightly downstream of the exit

12, respectively.

  In another embodiment (FIG. 3), buffer capacities upstream and downstream are not used respectively as in FIGS. 1 and 2, but there is a compressor 17 upstream of the stack, and the output of the compressor at input 11 of battery 1. To obtain "breathing", the compressor is controlled to operate it in pulsed mode at the selected pressures and at the

  desired cadence, and if necessary we adjust the speed

  braids for increasing and decreasing pressure, for example as previously by means of a valve 15 or of a pressure drop means, this time downstream of the outlet 12 of the stack insofar as the compressor 17 of modulation

is upstream of the entrance.

  The choice of breathing characteristics

  depends on a number of parameters of the battery to be

  bustible. A low rhythm and low amplitude is enough to somewhat improve the performance of the battery. But the

  process is only very effective if you make sure

  renew the gas at a rate higher than its consumption.

  If the difference in pressures in a region of the stack taken as the reference region, between the two extreme values of pressure, is AP, and if P is the pressure at the inlet 11 of the stack, R the constant of ideal gases, T the absolute temperature, F the Faraday constant, I the intensity of the electric current, x0 the molar fraction in contact with the diffusion zone of the battery, and e the thickness

  of the zone comprising the porous layer and the diffusion zone

  ion of the cell, a fraction AP / P of the gas is renewed, and the respiration frequency f must be at least of the order

from (P / AP) (R.T / P) (I / 4F) (1 / xo.e).

  For AP / P = 1/3 (for example AP = 0.33 bar and P = 1 bar), we find, for I = 3A / cm2 and a thickness of 2 mm,

an optimal frequency of 1.5 Hz.

  However, as the gas renewal rate

  in the poorly ventilated parts of the stack is not complete

  zero, a frequency quite significantly lower than the frequency calculated by the above formula (about 20% of

  this) gives still acceptable results.

  On the other hand, the frequencies even lower

  act mainly by elimination of water in the form li-

what.

  It is therefore necessary that the amplitude of the res-

  piration is sufficient to not only ensure the elimination of water in the liquid state, but also create gas turbulence in the stack during at least part of the transit and on at least part of the path of the gas between the inlet 11 and outlet 12, thus ensuring that the AP / P fraction of the gas from the poorly ventilated parts

  reach well ventilated areas to be well ventilated

  without returning to its starting point when

  the next pressure increase.

  In practice, a pressure difference of at least

0.1 bar is required.

  The embodiment already mentioned comprising a valve near each access 11, 12 of the stack, namely an inlet valve 16 and an outlet valve 15, makes it possible to precisely control the flow rate passing through the stack in

  opening the two valves successively at an appropriate rate.

  It is then easy to pass through the

  pile a molar flow rate equal to f (AP / R.T) V o f is the

  quence and V the internal volume of the battery air circuit.

  For example, for a 30kw battery that has an internal volume

  of 24 liters, a frequency of 2.2Hz is sufficient to ensure the de-

  bit corresponding to a stoichiometric ratio equal to 2 (i.e. approximately 1.1 moles / s) with an amplitude of AP of 0.5

  bar. The frequency can be reduced by adding vo-

  additional lume; it must also be adapted to take into account the maximum amplitude authorized for AP between

the air and hydrogen circuits.

  In addition, it is possible to vary the pressure in the stack as a function of time between a value

  maximum below the pressure at inlet 11 and a

  their minimum greater than the pressure at the outlet 12 of the stack. Of course, the invention is not limited to

  modes and embodiments described above and

  presented, and we can plan others without going beyond its scope; in particular, the invention can be applied to any other gas and for example to hydrogen in the hydrogen circuit of a fuel cell, with minor modifications5 to the process and the device described; in this case, a stack 1 is introduced into the stack 1, at the inlet 11 thereof.

  gas consisting of at least hydrogen and which may be a mixture also containing nitrogen, and a residual gas containing little or less hydrogen is discharged at outlet 12 and, as a result of the use of the invention, more nitrogen.

Claims (16)

  1. Method for supplying reactive gas to a fuel cell (1), according to which this reactive gas is introduced at a gas inlet (11) from the cell and a residual gas is discharged at a gas outlet (12) of the cell, process characterized in that, repeatedly, successively, the gas pressure is increased in at least one reference region of the   battery (1) located between the inlet (11) and the outlet (12) up to   that at a first value, and we decrease the gas pressure   in the reference region up to a second in-   lower than the first, in such a way that gas turbulence is created during at least part of the transit and on at least part of the path of the gas between the inlet and the battery outlet.   2. Method according to claim 1, characterized in that the gas pressure in the cell is modified between the supply pressure at the inlet (11) of the cell and the   discharge pressure at its outlet (12).   3. Method according to claim 1, characterized in that the gas pressure in the cell is increased to a value equal to or very close to the pressure   supply to the battery inlet (11).   4. Method according to claim 1, characterized in that the gas pressure in the cell is reduced to a value equal to or very close to the discharge pressure at the battery outlet (12).   5. Method according to claim 1, characterized in that one introduces into the stack (1), at said inlet (11) of   gas therefrom, an oxidizing gas, and evacuated to said outlet   tie (12) of stack gas, a residual gas in which the   proportion of oxygen is practically nonexistent or nota-   significantly reduced compared to the proportion of oxygen in   the oxidizing gas at the inlet of the cell.   6. Method according to claim 1, characterized in that one introduces into the cell (1), at said gas inlet (11) thereof, a gas consisting at least of hydrogen, and   evacuated at said outlet (12) of gas from the cell, a gas   sidual containing hydrogen and nitrogen.   7. Device for supplying oxidizing gas, to fuel cell, for implementing the method according to one   any one of claims 1 to 6, characterized in that it comprises at least one means (15; 16; 17) for modulating the pressure in the stack (1), connected to an access (11, 12) thereof .   8. Device according to claim 7, characterized in that the pressure modulation means is a valve (15, 16).   9. Device according to claim 7, characterized in   what the means of pressure modulation is a compress   sor (17) whose outlet is connected to the inlet (11) of the   battery (1) and which operates in pulsed mode.   10. Device according to claim 7, characterized in that it comprises a pressure drop means connected to one accesses (11, 12) of the stack (1).   11. Device according to claim 7, characterized in that it comprises upstream of the stack (1) at least one means (13) for keeping the pressure approximately constant power supply.   12. Device according to claim 7, characterized in that it comprises downstream of the stack (1) at least one means (14) for keeping the pressure approximately constant of the battery.   13. Device according to claim 11 or the claim   cation 10, characterized in that the means (13, 14) for maintaining the pressure approximately constant is a capacity. 14. Device according to claim 11, characterized   in what way to maintain approximately   aunt the battery supply pressure (1) is a   tensioner. 15. Device according to claim 12, characterized   in that the means for maintaining approximately   aunt the battery discharge pressure (1) is a discharge   sister. 16. Device according to claim 7, characterized in   what the modulation of the pressure in the pile (1) corresponds to   lays at a variation in pressure AP at least equal to approximately 0.1 bar.