Note: Descriptions are shown in the official language in which they were submitted.
CA 02293898 2000-O1-04
54763
PROCESS FOR FLUSHING THE GAS CIRCUIT OF A FUEL CELL, AND
APPARATUS FOR PRACTICING THE FROCESS
The invention relates to a process for purging the
gas circuits of fuel cells, adapted to evacuate from these
circuits, the water in liquid phase which may be there, as
well as if desired the unusable gases or the like that the
circuit is adapted to channel, for example nitrogen in
circuits adapted for the ~~irculation of hydrogen or air-
hydrogen cells; it also relates to a device for practicing
this process.
In fuel cells at the anode of which hydrogen is
~C consumed, a portion of the water produced on the cathode side
of the cell passes through the membrane and accumulates in the
compartment for hydrogen. So as to get rid of this water, it
is necessary to carry out regularly purges of the hydrogen
. circuit. The frequency of the necessary purges largely
15 depends on the cell, and is of the order of several seconds to
several minutes. In the absence of purging, there will be
seen a progressive degradation of the performance of the cell.
Purges permit returning approximately to the initial perfor
mance; they moreover permit eliminating nitrogen which
20 concentrates in the hydrogen compartment by diffusing through
the membrane, as well as possible impurities present in the
hydrogen.
CA 02293898 2000-O1-04
At present, purging the hydrogen circuit of fuel
cells is carried out toward the outside, and there results a
loss of hydrogen; to limit this loss, it is the practice to
limit the flow rate at the outlet of the cell; however, it is
not necessary to greatly delimit this flow, because the purges
lose their effectiveness if the hydrogen flow at the outlet is
insufficient; a total hydrogen loss being unable to be reduced
below several percent, this correspondingly decreases the
yield of combustible. It has been proposed to operate the
0 cell in a "closed circuit", and to re-inject the hydrogen at
the input of the latter with the air of a circulator; it is
thus possible to cause the cell to operate with purges that
are much greater spaced, serving to eliminate the nitrogen
from the cathode portion in the case of cells in which the
combustible is oxygen supplied by a circulation of air;
another advantage of recirculation is that the mixing of the
gases permits better distribution of hydrogen when there is a
great deal of nitrogen, and improves the operation with a
hydrogen-nitrogen mixer, by avoiding a stratification phenome
non.
The practice of recirculation, however, suffers from
drawbacks: one is that a recirculator is a complex rotary
machine, subjected to delicate conditions of operation (the
possible presence of water in liquid state resulting from
condensation); another drawback is that a continuous circula-
tion does not always permit eliminating completely water in
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CA 02293898 2000-O1-04
liquid phase from the cells, because the movement of the gas
created is not always sufficiently abrupt.
The invention has for its object to create abrupt
purges (which is to say with a relatively large and rapid
pressure drop), whilst limiting to the extent possible the
losses of purged gas from the circuit, for example the loss of
hydrogen from the hydrogen circuit, and the loss of oxygen
from the oxygen circuits.
To this end, the invention relates to a purge
0 process for the gas circuit of a fuel cell, in which there is
introduced at least one gas from a supply of gas to an input
of the gas circuit of the cell and residual products are
removed at an evacuation outlet or purge of the gas circuit of
the cell, according to which process at least the water in
liquid state located in the gas circuit, is evacuated at
intervals that are regular or not, by the outlet; character
ized in that the cell being connected to a storage b~T at least
connection means connecting the outlet of the cell to an inlet
of the storage, and the gas pressures in the respective
20 reference regions of the cell and of the capacity being
momentarily approximately equal, the pressure of the gas in
the cell and in the storage is decreased, then there is
established, from gas from the supply, a rapidly increasing
gas flow passing through the connection means, of the battery
25 to the storage, so as to transport simultaneously water in
liquid phase contained in the battery, into the storage.
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CA 02293898 2000-O1-04
The process can moreover have one or several of the
following characteristics:
- to approximately equalize the pressures in the
cell and in the storage, the arrival of gas from the feed to
the inlet of the cell is interrupted, then the pressure in the
storage and in the cell is decreased, typically to about
atmospheric pressure, by letting the cell consume gas from the
cell and from the storage without resupplying the cell with
gas, and the cell is resupplied to increase the pressure in
it, and finally the cell and the storage are again placed in
communication by opening the connection means;
The invention also relates to a device which
comprises a fuel cell having at least two gas circuits of
which at least one comprises a circuit gas inlet connected by
an inlet circuit to a gas supply and an outlet for evacuating
or purging the gas circuit, and a storage comprising an inlet
connected to the outlet of the cell by connection means.
Other characteristics and advantages of the inven
tion will become apparent from the following description, of
embodiments of method and construction of the invention, given
by way of non-limiting example, and illustrated by the
accompanying drawings, in which:
- Figures 1 to 7 are block diagrams respectively of
seven devices for practicing the process according to the
invention, in which the same reference numerals indicate the
corresponding elements from one figure to another, and
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CA 02293898 2000-O1-04
- Figure 8 is a curve representative of the voltage
delivered by the fuel cell as a function of time elapsed from
its being placed in service, respectively for a cell in which
there is no purge, for a cell periodically purged according to
the prior art, and for a cell periodically purged according to
the invention.
The drawings show a fuel cell 1, comprising an inlet
11 for a gas circuit by which a gas permits operation of the
cell, from a gas supply (not shown), which is introduced into
the latter, and an outlet 12 for evacuation or purging of gas,
by which a desired residual portion of gas which has not been
used, as well as the liquid phase water which has passed
thromgh the mer;;~rane of the cell , are evacuated, this nossibl a
residual gas and this water constituting the obstacles to the
free circulation of new gas. In the examples which follow,
the gas circuit is a hydrogen circuit, and there can be, as
residual gas, hydrogen, and as will be seen, nitrogen.
According to tile invention, the outlet 12 of the
cell 1 is connected to an inlet of a storage 2 whose volume is
the order of that of the anode compartment of the cell, by
connection means 3 comprising a conduit 30. At regular
intervals, or not, the pressure in at least one reference
region of the cell located between the inlet 11 and the outlet
12 is approximately equalized, as well as that of the storage,
at a value P; then the gas pressure is decreased at least in
the storage 2, then there is established, with gas from the
feed, a rapidly increasing gas current, passing through the
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CA 02293898 2000-O1-04
connection means, from the cell to the storage, so as to
convey simultaneously liquid phase water contained in the
cell, to the storage; after this, the water in liquid phase
is evacuated from the storage.
In many cases, it is practical that, the value P
being greater than atmospheric pressure, the pressura value at
which the gas flow is established will be approximately ea_ual
to atmospheric pressure.
The values which follow, given by way of example,
correspond to a fuel cell of 30kW and whose internal volume is
24 liters.
In Figure 1, the connection means 3 comprise, in
addition to the outlet conduit 30 from the cell, an outlet
valve 31 interposed in this conduit, and yielding pressure
drops as small as possible; in normal operation (between
purges) , this valve 31 can be open or closed; in the inlet
circuit 4 of the cell 1 connecting the cell to a gas supply
(not shown), a valve 41 is connected to the inlet 11 of the
cell; in normal operation (between purges), this valve 41 is
open. So as to proceed to purge, the outlet valve 31 is
opened if it is initially closed, then successively:
- the arrival of gas from the supply is interrupted,
at the inlet 11 of the cell, and to do this the inlet valve 41
is closed; following consumption of the gas present in the
cell, which is not resupplied, the pressure which initially as
P in the cell 1 and in the storage 2, decreased to P-0P; in
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CA 02293898 2000-O1-04
the selected example, the pressure drop DP is of the order of
0.1 bar every two seconds;
- when 0P reaches a predetermined value of the order
of 0.1 bar to 0.6 bar, the storage is shut off from the cell,
and to do this the outlet valve 3l is closed, and the cell is
resupplied; and to do this, the inlet valve 41 is immediately
opened, which gives rise to a pressure increase in the cell 1;
- finally, when the pressure in the cell 1 has
returned to its initial value P, the cell and the storage are
1C again placed in communication; and to do this, the outlet
valve 31 is reopened, which gives rise to a rapidly increased
gas current to the cell and, passing through the connection
means 3, from the cell to the storage, carrying the liquid
phase water contained in tl~e cell, to the storage 2; it then
suffices to purge the storage to evacuate the water, by means
of a purge member 21 which can also be a valve.
In Figure 2, the connection means 3 comprise an
outlet conduit 30 but no outlet valve, because the pressure
drops of the inlet circuit 4 comprising the inlet valve 41 are
supposedly sufficiently low so as not to impose any limitation
on the gas flow. In this case, for purging, the arriving gas
at the entry of the cell is interrupted; and to do this, the
inlet valve 41 is closed, the pressure falls by DP in the cel l
1 and in the storage 2, as in the case of Figure 1, and the
cell is resupplied; and to do this, the inlet valve 41 is
opened, which gives rise to a rapidly increasing gas flow
toward the cell, into the cell, and from the cell to the
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CA 02293898 2000-O1-04
storage, carrying along the water contained in the cell. If
the pressure drops in the inlet circuit 4 are too great, a
buffer storage 42 can as a modification be disposed in this
circuit, upstream of the inlet valve 41 (in broken lines in
Figure 2) , to store new gas in a predetermined quantity and at
a predetermined pressure.
In the two examples which have been described, the
frequency of the purges of the cell 1 is limited only by the
rapidity of consumption of gas in the cell. Moreover, the
turbulence created by the abrupt movements of the gas can
improve the operation in the presence of nitrogen. However,
if too great a quantity of nitrogen is present, it is neces-
nary to carry out a purge of the excess nitrogen. The
presence of an excess of nitrogen is detected by the ohserva-
tion of a decrease in performance of the cell at the time of
pressure drop. In the first example, the presence of the
storage 2 permits limiting the loss of hydrogen, because to
purge nitrogen, the valve 31 separating the :,vtorage 2 and the
cell 1 is closed, and the storage 2 is opened to the outside,
which evacuates a portion of the mixture impoverished in
hydrogen, which is located there (one-third for an operation
at 1.5 bars); during return to the conditions of normal
operation, the quantity of hydrogen in the anode portion is
increased, and a portion of the mixture is replaced which is
impoverished by the new hydrogen (1/6th in this case).
In Figure 3, the connection means 3 again comprise
an outlet conduit 30 and an outlet valve 31 disposed in this
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CA 02293898 2000-O1-04
conduit; moreover, the storage 2 comprises an outlet 22
connected by a return circuit 5 to the cell 1, so as to re-
inject thereinto the hydrogen contained in the storage,
instead of evacuating from the storage by means of a purge
member 21 which then serves solely to evacuate water and
nitrogen; to this end, the return circuit 5 comprises a pump
50 interposed in a conduit 51 of the return circuit connecting
the outlet of the storage in this case to the inlet circuit 4;
a purge member 52 is also connected to the outlet of the pump
50.
The device shown in this Figure 3 permits carrying
out abrupt and regular purges whilst returning the hydrogen to
c~_rcula'.=ion because the pressure in the storage 2 is decreased
bar means of the pump 50 by which the hydrogen cor_tai n ed i n the
storage is re-injected, for example upstream of the cell 1
(immediately adjacent the inlet 2 or farther upstream), or
even downstream of the cell but upstream of the outlet valve
31 (shoum in broken line in Figure 3 ) , the hydrogen W ing them
reintroduced into the cell though the outlet 12 of the latter.
With this device, 'the outlet valve 31 is closed, and the
pressure in the storage is decreased to the value P-DP; when
this value is reached, the outlet valve 31 is opened, and thus
there is established a rapidly increasing gas current from the
cell to the storage, carrying the water in liquid phase
contained in the cell, into the storage, for its evacuation.
This device and this process permitting a recircula
tion of the hydrogen, require the use of a pump. However,
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CA 02293898 2000-O1-04
this solution has its advantage, relative to the prior art
technique using a circulator, that the pump need not circulate
large quantities of hydrogen under a low pressure DP, but
simply need drop the pressure in the storage during the
interval of time which separates two purges. In the same
example as before (cell of 30kW and 24 liters), with a purge
every two minutes for a pressure difference of 0.5 bar, the
flow rate is 6 N1/mn, and can thus be ensured by a miniature
pump of several watts. But the fundamental advantage of this
device and of this process is that the cell is no longer
placed under vacuum in the course of a purge, and there is
accordingly no drop in performance even the slightest, in the
course of thi s under-pressuring, except :when the storage
rec:_i?;es nitrogen, which permits detecting the moment at wr,i ch
the storage must be purged.
The devices of Figures l, 2 and 3 require purging
the storage to eliminate nitrogen and various impuriti es; they
can be provided, so as to re-inject into the cell oily
hydrogen and to purge the impurities with a minimum loss of
this gas, with purification means for example of palladium, an
organic membrane, or a molecular sieve.
In Figure 4, the connection means 3 comprise an
outlet conduit 30 and an outlet valve 31 of the three-passage
type. The storage 2 is provided with a separation apparatus
23, in this case a membrane, to recover the hydrogen present
in the latter. The outlet of this apparatus 23 is connected
by a return circuit 5 comprising a conduit 51 with a second
CA 02293898 2000-O1-04
inlet (third passage) of the valve 31. The device also
comprises an inlet valve 41 in its inlet circuit 4, and a
purge member 21 for the storage, such as a valve. The process
used in this device, which is an improvement on that of Figure
1, is the following: for purging, the inlet valve 41 is
closed, and the inlet of the outlet valve 31 is placed in
communication with the outlet 22 of the separation apparatus
23 such that the cell 1 draws in the content of the storage 2
through the membrane of the separation apparatus 23; when the
pressure in the storage reaches the value P-DP or stabilizes
at a higher value (which then signifies that there is almost
no more nitrogen and impurities in the storage); then at this
point the outlet valve 31 is closed and the inlet vai.ve 41 is
opened; after which the storage is purged if the pressure
L~ stabilizes at a value higher than P-DP (or at a predetermined
value), to eliminate at least a portion of the impurities;
thus there is reached a value P-DP (fcr example adjacent
at~r~ospheric pressure) in the storage; finally, the inlet of
the outlet valve 31 is placed in communication with the inlet
of the storage by the outlet conduit 30, and thus the cell is
purged as was described for the device of Figure 1.
As modifications (not shown):
- there could be connected at the outlet for purging
the storage, a suction pump for the impurities, permitting
reaching a lower pressure in the storage and eliminating more
impurities;
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CA 02293898 2000-O1-04
- the three-passage outlet valve 31 could be
replaced by a conventional valve with two flaps, namely a
return flap for hydrogen pumping (in a direction from the
separation apparatus 23 toward the cell), and a purge flap (in
a direction from the cell toward the storage).
In Figure 5, showing a device which is a development
of that of Figure 2, in which the connection means comprise an
outlet conduit 30 but no outlet valve; thus, the outlet valve
is in this case replaced by a principal flap 32 whose passing
direction is from the cell 1 toward the storage 2, and a
return circuit 5 comprises a return flap 52 for pumping
hydrogen which connects the separation apparatus 23 to
upstream oz the principal valve 32 with a passing direction
from this apparatus 23 upstream. During intake, already
described in relation to the device of Figure 4, the flap 52
for pumping hydrogen opens and permits the transfer of
hydrogen from the storage to the cell, which decreases the
pressure in the storage; when there is no more hydroaer~ in the
storage, the pressure again falls in the cell, but cannot fall
over in the storage; if the storage is purged, the principal
flap 32 does not open to the extent the cell remains at a
lower pressure than the storage; after this, the purge member
21 i.s re-closed, the inlet valve 41 is opened, and thus there
is established a flow of purge gas as in the case of Figure 2.
With the devices of Figures 4 and 5, there can also
be carried out the purge of the cell without closing the purge
member of the storage, by selecting a geometry of the storage
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CA 02293898 2000-O1-04
which promotes the piston effect exerted by the gas leaving
the cell; there is thus eliminated a larger part of the
accumulated impurities.
In Figure 6, which shows a device which is a
development of that of Figure 3, the connection means 3 also
comprise an outlet conduit 30 and an outlet valve 31 disposed
in this conduit; the storage 2 is provided with a hydrogen
separation apparatus 23 , with a membrane; it is this apparatus
23 which is connected by a return circuit 5 to the inlet 11 of
the cell or to the inlet circuit 4, or else upstream of the
outlet valve 31 (shown ir~ broken line in Figure 6) so as to
re-inj ect into the cell the hydrogen contained in the storage,
the return circuit 5 comprising a pump 50 incorporated in the
return conduit 51 as described ice. connection with Figure 3.
As before, the hydrogen is thus drawn from the storage toward
the pump this time by means of the selective membrane, and
then transmitted from the pump to the cell; the purge proce-
dure is with this difference almost the same as that which has
been described with reference to Figure 3. As is the case for
the devices of Figures 4 and 5, the requirement to purge the
storage is detected when the pressure in the latter stabilizes
above the usual pressure.
In Figure 7 is shown a device which is also a
modification of the device of Figure 1; there is therefore
again an outlet valve 31 between the cell and the storage, the
purge member 21 for the storage, and an inlet valve 41, in the
same mounting; simply said, the conventional inlet valve 41 is
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CA 02293898 2000-O1-04
replaced by a three-passage valve whose second inlet (third
passage) is connected by a return circuit 5 comprising a
conduit 51, to the outlet 22 of the storage.
As in the preceding cases, in which there is an
outlet valve 31, the latter can be in normal operation open or
closed; the inlet valve 41 connects the cell to the hydrogen
supply; at an appropriate time, the position of the inlet
valve is reversed and the pressure in the storage is decreased
by transferring gas from the latter to the cell; then the
outlet valve 31 is closed if it was open; when the pressure in
the storage has decreased to the selected value, the position
of the inlet valve 41 is reversed to return the cell into
communication with the gas supply, and the outlet valve 31 is
opened and thus there is established a rapidly increasing gas
flow carrying the water from the cell to the storage, after
which the outlet valve 31 is re-closed to isolate the cell
from the storage, if such are the normal operating conditions.
The same type of modification can be carried out
with respect ,to Figure 2, and thus, in an embodiment (not
shown), Figure 2 is modified by replacing the conventional
inlet valve 41 by a three-passage valve whose second inlet is
connected by a return circuit to the outlet of the storage.
Naturally, the problem of the appearance of water is
not limited to the hydrogen circuit of a fuel cell, but it
also exists in the oxygen circuit.
The examples described above, which do not rely on
techniques specific to hydrogen, namely those of Figures 1 , 2,
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CA 02293898 2000-O1-04
3 and 7, are also applicable to the pure oxygen circuit of
such hydrogen-oxygen cells. Moreover, the greater quantity of
water present on the oxygen side requires more frequent
purges, but the frequency depends, as for hydrogen, on the
cell; on the other hand, there is less trouble from the
accumulation of other gases.
It will be noted that the frequency of the purges of
the storage (of water, of nitrogen, etc.) can be very much
less than that of the purges of the cell.
The efficiency obtained by practice of the invention
can be seen in Figure 8, which shows in several cases the
voltage delivered by a fuel cell such as that described above
by way of example, as a function of time: the curve A
represents the voltage delivered by the cell as a function of
time when this cell is not purged, the voltage whose rapidity
of decrease is characteristic; curve B shows the voltage for
an identical cell in which the hydrogen circuit is purged
every five minutes, according to the prior art process, but
losing hydrogen with each purge; curve C shows the voltage for
an identical cell in which the hydrogen circuit is purged by
means of the process of the device according to the invention
shown in Figure 1. This latter curve shows perfectly the
constancy of performance with time, of a cell thus purged
according to the invention.
