U.S. patent application number 13/634043 was filed with the patent office on 2013-02-07 for high temperature electrolyzer (hte) including a plurality of cells, having improved operation in the event of breakage of at least one cell and during ageing.
This patent application is currently assigned to Commissariat a l'energie atomique et aux energies alternatives. The applicant listed for this patent is Patrick Le Gallo, Christian Perret. Invention is credited to Patrick Le Gallo, Christian Perret.
Application Number | 20130032490 13/634043 |
Document ID | / |
Family ID | 42358982 |
Filed Date | 2013-02-07 |
United States Patent
Application |
20130032490 |
Kind Code |
A1 |
Le Gallo; Patrick ; et
al. |
February 7, 2013 |
HIGH TEMPERATURE ELECTROLYZER (HTE) INCLUDING A PLURALITY OF CELLS,
HAVING IMPROVED OPERATION IN THE EVENT OF BREAKAGE OF AT LEAST ONE
CELL AND DURING AGEING
Abstract
A process of electrolyzing water at high temperatures
implemented by a cell stack reactor, including: a) simultaneously
circulating water vapour at each cathode and at each anode as a
leaching gas, temperatures of the water vapour at an inlet of each
anode and each cathode being lower than high temperatures at which
electrolysis is carried out and the water vapour circulating at the
anode being at an overpressure with respect to the cathode; b) upon
starting the electrolysis, supplying electrical power having a
substantially constant electrical voltage across terminals of the
stack and maintaining same. In event of breakage of one or more
cells, complete destruction of the stack is avoided and high
production efficiency is maintained, and efficiency is maintained
during ageing.
Inventors: |
Le Gallo; Patrick; (Saint
Appolinard, FR) ; Perret; Christian; (Grenoble,
FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Le Gallo; Patrick
Perret; Christian |
Saint Appolinard
Grenoble |
|
FR
FR |
|
|
Assignee: |
Commissariat a l'energie atomique
et aux energies alternatives
Paris
FR
|
Family ID: |
42358982 |
Appl. No.: |
13/634043 |
Filed: |
March 11, 2011 |
PCT Filed: |
March 11, 2011 |
PCT NO: |
PCT/EP11/53725 |
371 Date: |
October 25, 2012 |
Current U.S.
Class: |
205/628 ;
204/270 |
Current CPC
Class: |
Y02E 60/366 20130101;
C25B 1/04 20130101; Y02E 60/36 20130101; C25B 9/18 20130101 |
Class at
Publication: |
205/628 ;
204/270 |
International
Class: |
C25B 1/04 20060101
C25B001/04; C25B 9/18 20060101 C25B009/18; C25B 9/06 20060101
C25B009/06 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 12, 2010 |
FR |
10 51782 |
Claims
1-8. (canceled)
9. A process of electrolyzing water at high temperatures
implemented by an electrochemical reactor including a stack of
elementary electrochemical cells each formed of a cathode, an
anode, and an electrolyte inserted between the cathode and the
anode, at least one interconnecting plate being arranged between
two adjacent elementary cells and in electrical contact with an
electrode of one of the two elementary cells and an electrode of
the other of the two elementary cells, in which at least the water
vapour is made to circulate in contact with the cathode and a
leaching gas is made to circulate in contact with the anode to
evacuate oxygen produced, the method comprising: a) simultaneously
circulating the water vapour containing at most 1% of hydrogen at
each cathode and at each anode as a leaching gas, temperatures of
the water vapour at an inlet of each anode and each cathode being
lower than high temperatures at which electrolysis is carried out,
and the water vapour circulating at the anode being at an
overpressure with respect to the cathode; and b) imposing upon
starting the electrolysis and maintaining a substantially constant
level of electrical voltage across terminals of the stack of
electrolysis cells.
10. A process of electrolyzing water according to claim 9, wherein
the overpressure of the water vapour containing at the most 1% of
hydrogen at the anode compared to that at the cathode is between 5
and 100 mbars, or is 30 mbars.
11. A process of electrolyzing water according to claim 9, wherein
a number of elementary cells and a level of voltage imposed and
maintained constant are such that unitary voltage across the
terminals of each elementary cell is of an order of 1.3 volts.
12. A process of electrolyzing water according to claim 9, wherein
an initial conversion rate into hydrogen is of an order of
100%.
13. A process of electrolyzing water according to claim 9, wherein
a flow rate of water vapour at each cathode is increased, when a
conversion rate into hydrogen initially determined at an outlet of
each cathode drops.
14. A process of electrolyzing water at high temperature according
to claim 9, at temperatures of at least 450.degree. C., or between
700.degree. C. and 1000.degree. C.
15. A device for electrolyzing water at high temperatures,
comprising: an electrical voltage source; a reactor comprising a
stack of elementary electrochemical cells each formed of a cathode,
an anode, and an electrolyte inserted between the cathode and the
anode; at least one interconnecting plate being arranged between
two adjacent elementary cells and in electrical contact with an
electrode of one of the two elementary cells and an electrode of
the other of the two elementary cells, the interconnecting plate
comprising at least one cathodic compartment and at least one
anodic compartment for circulation of gases respectively at the
cathode and at the anode; wherein one of ends of the cathodic
compartments is connected to a supply configured to deliver water
vapour containing at most 1% of hydrogen and one of ends of the
anodic compartments is also connected to a supply configured to
deliver water vapour containing at most 1% of hydrogen at an
overpressure compared to those of the cathode, the supplies are
configured to deliver the water vapour at temperatures below those
at which the electrolysis is carried out; and further comprising
means connected to the electrical voltage source to deliver a
substantially constant voltage across terminals of the two
interconnecting plates of the stack furthest away from each
other.
16. An assembly for producing hydrogen comprising a plurality of
devices according to claim 15.
Description
TECHNICAL FIELD
[0001] The invention relates to a process of electrolysing water at
high temperatures (HTE), also known as high temperature vapour
electrolysis (HTVE), with a view to producing hydrogen.
[0002] It also relates to a reactor for implementing said
process.
[0003] More particularly, it relates to an improvement in the
reliability of operation and the efficiency of high temperature
electrolysers (HTE), in the event of potential breakages of one or
more cells.
[0004] In addition, it relates to an improvement in the efficiency
of said HTE electrolysers, during ageing or in other words after a
considerable time of use.
PRIOR ART
[0005] An electrolyser comprises a plurality of elementary cells
formed of a cathode and an anode separated by an electrolyte, the
elementary cells being electrically connected in series by means of
interconnecting plates interposed, in general, between an anode of
an elementary cell and a cathode of the following elementary cell.
An anode-anode connection followed by a cathode-cathode connection
is also possible. The interconnecting plates are electronic
conductive components formed of a metal plate. Said plates moreover
ensure the separation between the cathodic fluid circulating at an
elementary cell from the anodic fluid circulating in a following
elementary cell.
[0006] The anode and the cathode are made of porous material into
which the gases can flow.
[0007] In the case of the high temperature electrolysis of water to
produce hydrogen, the water vapour circulates at the cathode where
hydrogen is generated in gaseous form, and a leaching gas can
circulate at the anode and thereby participate in the evacuation of
the oxygen generated in gaseous form at the anode. Most high
temperature electrolysers use air as leaching gas at the anode.
[0008] At present, the construction of a factory for producing
hydrogen from a large number of high temperature electrolysers is
being envisaged.
[0009] The current dimensioning of such a factory implies in fact
the simultaneous operation of a large number, typically several
million, of electrolysis cells. These cells are on the one hand
fragile and may thus break at any moment and, on the other hand,
they age and thus produce less hydrogen locally. These two
phenomena run counter to the industrial requirement of large volume
hydrogen production. In fact, it is necessary in this context to
have a constant production in quantity and over time and it must do
so with great reliability.
[0010] In other words, the conception of a factory implies firstly
that it is necessary to envisage the breakage of one or more cells,
which leads to either the stoppage of the electrolyser concerned,
or its operation in degraded mode.
[0011] Until now, it has been envisaged to compensate for the loss
of production stemming from this (these) breakage(s) either by
starting up a new high temperature electrolyser or by increasing
the power injected into each electrolyser not concerned by the
breakage(s), in other words with all of their cells non-broken.
[0012] The drawbacks of these solutions are that this requires an
active management with the use of costly electrical cabinets and
that, above all, the energy efficiency of the HTE electrolyser
concerned by the breakage(s) is reduced.
[0013] Secondly, as stated previously, the design of a factory
implies that it is necessary to take into account the ageing of all
the cells, in other words that, in the same conditions
(temperature, pressure, current) as the initial conditions during
the start of the electrolysis, their reactive performances
drop.
[0014] An aim of the invention is to propose a solution that makes
it possible for a hydrogen production factory comprising a large
number of high temperature electrolysers to have high reliability
and to conserve a constant production efficiency without the
drawbacks of the solutions of the prior art.
[0015] An aim of the invention is thus to propose a solution that
makes it possible, at lower cost, not to suffer losses of
efficiency of a high temperature electrolyser (HTE) due to
potential breakages of cells and moreover to the ageing
thereof.
DESCRIPTION OF THE INVENTION
[0016] To do this, the invention relates to a process of
electrolyzing water at high temperatures implemented by an
electrochemical reactor comprising a stack of N elementary
electrochemical cells each formed of a cathode, an anode and an
electrolyte inserted between the cathode and the anode, at least
one interconnecting plate being arranged between two adjacent
elementary cells and in electrical contact with an electrode of one
of the two elementary cells and an electrode of the other of the
two elementary cells, in which at least water vapour is made to
circulate in contact with the cathode and a leaching gas is made to
circulate in contact with the anode to evacuate the oxygen
produced, characterised in that the following steps are carried
out:
[0017] a/ simultaneously circulating the water vapour containing at
the most 1% of hydrogen at each cathode and at each anode as a
leaching gas, the temperatures of the water vapour at the inlet of
each anode and each cathode being lower than the high temperatures
at which the electrolysis is carried out and the water vapour
circulating at the anode being at an overpressure compared to the
cathode,
[0018] b/ imposing upon starting the electrolysis and maintaining a
substantially constant level of electrical voltage across the
terminals of the stack of electrolysis cells.
[0019] The expression "the temperatures of the water vapour at the
inlet of each anode and each cathode being lower than the high
temperatures at which the electrolysis is carried out" is taken to
mean, within the scope of the invention, that a slightly exothermic
operation of the electrolyser is sought to target a stable, in
other words auto-thermal, operation of the assembly constituted of
the electrolyser (electrochemical reactor) and the associated heat
exchange system. Upstream and downstream of an electrolyser is
installed a heat exchanger system, the function of which is to use
the heat of the outgoing gases to heat the incoming gases (here the
non-hydrogenated water vapour). They ensure the thermal stability
of the assembly, thus a slightly exothermic operation of the
electrolyser is aimed at here.
[0020] Thus, the water vapour containing at the most 1% of hydrogen
enters into the electrolyser at temperatures below (heat at low
temperatures) the high operating temperatures and is reheated
thanks to the energy dissipated by Joule effect (thus of electrical
origin) in the core of the electrolyser, in other words within each
cell.
[0021] The cell breakage configurations envisaged within the scope
of the invention are those that do not lead to an interruption of
the electrical connection at the cell but only create a hydraulic
"short-circuit" between anode and cathode. The inventors have thus
been able to observe that these breakage configurations were those
typically observed in practice, in other words with the
architectures and dimensioning of electrolysers already known to
date. It goes without saying that those skilled in the art will
take care, within the scope of the invention, to ensure that the
architecture and the dimensioning of an electrolyser do not lead to
breakage of electrical connection at each cell.
[0022] Thus, the solution according to the invention makes it
possible to operate a high temperature electrolyser without, or
with little, efficiency losses due to potential breakages of one or
more cells, and to do so without it being necessary to involve
active compensation management.
[0023] In other words, the electrolyser reacts itself and in a
reliable manner to the phenomena of breakage of cells by reducing
any risk of serious damage.
[0024] The solution according to the invention thus consists in a
combination of means for carrying out respectively:
[0025] an auto-thermal operation of the assembly constituted of the
electrolyser and the associated heat exchanger(s),
[0026] over-pressurising water vapour at the anode,
[0027] a constant electrical voltage across the terminals of the
stack.
[0028] Thus, in the event of breakage of a cell, thanks to the
overpressure of non-hydrogenated water vapour circulating at the
anode, the leak caused by this breakage is directed from the anode
to the cathode. In other words, a flow of water vapour more or less
loaded with oxygen arrives at the cathode. The oxygen present then
reacts with the hydrogen produced, which again generates additional
water vapour with a release of heat. The presence of the flow of
water vapour from the anode moderates the rise in temperature.
Nevertheless, this moderate rise in temperature improves the
electrical conductivity at the part of the cathode downstream of
the breakage, which consequently reduces the production of heat by
Joule effect of the initial operation, in other words before the
breakage.
[0029] All of the additional flows of water vapour at the cathode
due on the one hand to the recombination of oxygen coming from the
anode via the broken area with the hydrogen already present at the
cathode and at the leak (water vapour already present at the
anode), leads to a redistribution of the current in the circulation
channel in contact with the cathode (Nernst potential,
Butler-Volmer equation).
[0030] The following phenomena occur:
At the Broken Elementary Cell:
[0031] The electrical voltage across the terminals of the broken
elementary cell drops: in fact, the quantity of water vapour is
greater, the broken elementary cell is hotter. The electrical
voltage across the terminals of the cell being lower, the operation
of the broken elementary cell may be considered exothermic, in
other words that the local electrolysis downstream as upstream of
the breakage consumes part of the excess heat.
[0032] The conditions of gas, temperature, downstream of the
breakage favour an electrolysis downstream rather than upstream of
the breakage. In fact, as mentioned previously, these conditions
lead to an electrical conductivity in the downstream part of the
cathode. Yet, the total current per cell is imposed by the constant
voltage across the terminals of the stack of cells. Thus, due to
this greater electrical conductivity downstream of the breakage and
the total current imposed at the broken elementary cell, there are
less electrochemical reactions upstream of the breakage.
[0033] Considering a stack of a number of N+1 cells, with the
number N very high for example N-1000.
[0034] In the absence of breakage, the relations linking the
voltage across the terminals of a cell u.sup.cel, across the
terminals of the stack of N+1 cells with the current may be written
as follows:
U.sub.0=N u.sub.0+u.sup.cel.sub.0 u.sup.cel.sub.0=u.sub.0 and
I.sub.0=i.sub.0,
[0035] in which U.sub.0 is the voltage maintained constant across
the terminals of the stack according to the invention. In these
equations and in the following equations, by convention, upper case
letters are used for what takes place at the terminals of the
stack, and lower case letters are used for what takes place on a
cell concerned by the breakage.
[0036] Following the breakage of a cell, the voltage across the
terminals of the cell concerned is written:
ucel=ucel.sub.0-.epsilon.
[0037] From which:
N u.sub.0=N u-.epsilon.
[0038] u being the voltage on the other non-broken cells.
[0039] The preceding equation can also be written:
u=u.sub.0+.epsilon./N
[0040] Considering the value R of apparent resistance of a cell,
the following relation is obtained:
u.sub.0=R i.sub.0
[0041] and
u=R i
[0042] from which the value of the current i in each of the other
non-broken cells:
i=i.sub.0+.epsilon./NR.
[0043] Thus, the inventor has arrived at the conclusion that the
variations induced by a breakage of a cell on the other unbroken
cells are smaller the higher the value of N. Yet, in practice, in
the stacks of cells envisaged within the scope of the invention, it
is the case.
[0044] There are thus fewer losses by recombination of products of
the upstream electrochemistry (hydrogen produced upstream of the
breakage and oxygen coming from the anode via the leakage at the
broken area). The electrolysis occurring locally downstream of the
breakage participates in the overall production of hydrogen by the
stack of cells.
[0045] It may thus be considered that the complete electrolyser has
in a way itself reacted to reduce the risks of serious damage.
At the other Non-Broken Elementary Cells:
[0046] Due to the fact that the electrical voltage on either side
of the broken cell has dropped, and that the complete stack of
cells is under a constant imposed electrical voltage, the other
non-broken elementary cells are under a slightly increased
elementary voltage. The elementary current thus consequently
increases slightly, which ensures an excess of overall production
of hydrogen by the stack of cells and compensates the shortfall due
to the breakage.
[0047] The overpressure of the water vapour containing at the most
1% of hydrogen at the anode compared to that at the cathode may be
comprised between 5 and 100 mbars, preferably 30 mbars.
[0048] The number N of elementary cells and the level of voltage
imposed and maintained constant are such that the unitary voltage
level across the terminals of each elementary cell is of the order
of 1.3 volts. This value corresponds to the voltage that enables
the assembly constituted of the electrolyser associated with the
heat system to have a stable operation, in other words auto-thermal
operation, with, if necessary, some heat losses. It goes without
saying that this value is determined for the water vapour
containing at the most 1% of hydrogen.
[0049] The initial conversion rate into hydrogen is preferably of
the order of 100%.
[0050] According to an advantageous embodiment, the flow rate of
water vapour at each cathode is increased when the conversion rate
into hydrogen initially determined at the outlet of each cathode
drops. A given production rate that does not drop is thereby
guaranteed.
[0051] The expression "conversion rate into hydrogen at the outlet
of the cathode" is taken to mean the proportion of water vapour at
the inlet of the cathode, which is transformed by electrolysis into
hydrogen at the outlet of the cathode. Thus, if at the inlet of the
cathode, non-hydrogenated water vapour is made to circulate, and
that the conversion rate initially determined is 100%, one
collects, apart from any breakage, uniquely the hydrogen at the
outlet of the cathode. Those skilled in the art will take care to
determine the necessary surface of each elementary cell and the
initial flow rate of water vapour at each cathode to arrive at the
desired initial conversion rate. They will then take care through
design to over-dimension the necessary cell surface.
[0052] In this way, it is ensured that the ageing of the cells does
not adversely affect the hydrogen production efficiency. In fact,
the reserve of cell surface by over-dimensioning thereof that does
not serve the electrolysis before ageing and which is situated
downstream, come to be used when the cell ages. Thus, the
conversion rate of a cell and the efficiency thereof remain
correct.
[0053] Obviously, if the initial conversion rate determined is of
the order of 100%, those skilled in the art will take care to
install a condensation stage to condense the water vapour not
converted during ageing.
[0054] As of considerable ageing, when all of the surface of the
cell is already used, the conversion rate is going to decrease and
thus the flow rate of hydrogen also. This behaviour is the normal
behaviour of electrolysers.
[0055] If it is wished to maintain this hydrogen flow rate and
conserve good thermodynamic efficiency, it is necessary to increase
the flow rate of water vapour to the detriment of the utilisation
rates.
[0056] The process can operate at temperatures of at least
450.degree. C., typically comprised between 700.degree. C. and
1000.degree. C.
[0057] The invention also relates to a device for electrolysing
water at high temperatures, comprising an electrical voltage source
and a reactor comprising a stack of elementary electrochemical
cells each formed of a cathode, an anode and an electrolyte
inserted between the cathode and the anode, at least one
interconnecting plate being arranged between two adjacent
elementary cells and in electrical contact with an electrode of one
of the two elementary cells and an electrode of the other of the
two elementary cells, the interconnecting plate comprising at least
one cathodic compartment and at least one anodic compartment for
the circulation of gases respectively at the cathode and the
anode.
[0058] According to the invention, one of the ends of the cathodic
compartments is connected to a supply adapted to deliver water
vapour containing at the most 1% of hydrogen and one of the ends of
the anodic compartments is also connected to a supply adapted to
deliver water vapour containing at the most 1% of hydrogen at an
overpressure compared to those of the cathode, the supplies being
adapted to deliver the water vapour at temperatures below those at
which the electrolysis is carried out, the device comprises means
connected to the electrical voltage source to deliver a
substantially constant voltage U.sub.0 across the terminals of two
interconnecting plates of the stack the furthest away from each
other.
[0059] Finally, the invention relates to a hydrogen production
assembly comprising a plurality of devices such as that described
above.
BRIEF DESCRIPTION OF DRAWINGS
[0060] Other advantages and characteristics will become clearer on
reading the detailed description given for illustration purposes
and non-limiting, and by referring to the following drawings, among
which:
[0061] FIG. 1 is a side view of an embodiment of a reactor for
electrolysis at high temperatures according to the present
invention,
[0062] FIG. 1A is a sectional view of the reactor of FIG. 1 along
the plane A-A in electrolysis operation without breakage of
cells,
[0063] FIG. 1B is a sectional view of the reactor of FIG. 1 along
the plane B-B also in electrolysis operation without breakage of
cells,
[0064] FIG. 2 is a view analogous to FIG. 1B but schematically
showing a breakage of a cell,
[0065] FIGS. 3A, 3B, and 3C show schematically a distribution of
the current along a channel in an electrolysis reactor according to
the invention respectively in operation without breakage of
electrolysis cells, in operation with a breakage localised in a
first cell area and, in operation with a breakage localised in a
second cell area distinct from the first area,
[0066] FIG. 4 schematically shows the evolution of the conversion
rate into hydrogen along a cell according to the invention and not
having undergone ageing.
DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS
[0067] The invention is described in relation to a type of high
temperature water electrolyser architecture for producing hydrogen.
It goes without saying that the invention can apply to other
architectures. The high temperatures at which the represented
electrolyser operates are at least equal to 450.degree. C.,
typically comprised between 700.degree. C. and 1000.degree. C.
[0068] It is pointed out that the terms "upstream" and "downstream"
are used with reference to the direction of circulation of the
water vapour and the hydrogen produced at the cathode.
[0069] It is pointed out that the representations of the different
components are not shown to scale.
[0070] Finally, it is pointed out that the representation of the
distribution of the current is in the form of straight line
segments in FIGS. 3A to 3C for simplification: it goes without
saying that in reality, these current distributions are decreasing
curve portions.
[0071] In FIG. 1 is represented an HTE electrolyser according to
the present invention comprising a plurality of stacked elementary
cells C1, C2 . . . .
[0072] Each elementary cell comprises an electrolyte arranged
between a cathode and an anode. In the remainder of the
description, the cells C1 and C2 and their interface will be
described in detail.
[0073] The cell C1 comprises a cathode 2.1 and an anode 4.1 between
which is arranged an electrolyte 6.1, generally of 100 .mu.m
thickness for the cells known as electrolyte support and of several
.mu.m thickness for the cells known as cathode support.
[0074] The cell C2 comprises a cathode 2.2 and an anode 4.2 between
which is arranged an electrolyte 6.2.
[0075] The cathodes 2.1, 2.2 and the anodes 4.1, 4.2 are made of
porous material and have for example a thickness of 40 .mu.m for
the cells known as electrolyte support and a thickness of the order
of 500 .mu.m for the cathode of the cells known as cathode support
and 40 .mu.m for the anode.
[0076] The anode 4.1 of the cell C1 is electrically connected to
the cathode 2.2 of the cell C2 by an interconnecting plate 8 coming
into contact with the anode 4.1 and the cathode 2.2. Furthermore,
it enables the electrical power supply of the anode 4.1 and the
cathode 2.2.
[0077] An interconnecting plate 8 is interposed between two
elementary cells C1, C2.
[0078] In the example represented, it is interposed between an
anode of an elementary cell and the cathode of the adjacent cell.
But, it could be provided that it is interposed between two anodes
or two cathodes.
[0079] The interconnecting plate 8 defines with the adjacent anode
and the cathode channels for the circulation of fluids. More
precisely, they define anodic compartments 9 dedicated to the
circulation of gases at the anode 4 and cathodic compartments 11
dedicated to the circulation of gases at the cathode 2.
[0080] In the example represented, an anodic compartment 9 is
separated from a cathodic compartment by a wall 9.11. In the
example represented, the interconnecting plate 8 comprises in
addition at least one conduit 10 delimiting, with the wall 9.11,
the anodic compartments 9 and the cathodic compartments 11.
[0081] In the example represented, the interconnecting plate 8
comprises a plurality of conduits 10 and a plurality of anodic 9
and cathodic 11 compartments. In an advantageous manner, the
conduit 10 and the compartments have hexagonal, honeycomb sections
which makes it possible to increase the density of compartments 9,
11 and conduits 10. Other sections can also be suitable for the
sections of the compartments.
[0082] As represented in FIG. 1A, water vapour containing at the
most 1% of hydrogen is made to circulate at each cathode 2.1, 2.2
and at each anode 4.1, 4.2 as a leaching gas. The arrows 12 and 13
of FIG. 1A thus clearly represent the simultaneous path in the
anodic 9 and cathodic 11 compartments. It goes without saying that
within the scope of the invention the flow symbolised may just as
easily be in the other direction (arrows 12 and 13 in the opposite
direction). As represented in FIG. 1B, the architecture of the
electrolyser makes it possible in addition to connect the first end
10.1 of the conduit 10 to a supply of water vapour containing at
the most 1% of hydrogen via another conduit and to connect the
second end 10.2 of the conduit 10 to the cathodic compartment 11.
The arrow 14 thus symbolises the return flow of the water vapour
from its flow in the conduit 10 (arrow 16) to the cathodic
compartment 11. It is pointed out here that the initial circulation
in the conduit 10 of the water vapour makes it possible to
homogenise the temperatures and thus avoid heat gradients capable
of damaging the cells.
[0083] According to the invention, upon starting a water
electrolysis cycle, care is firstly taken initially to ensure a
slightly exothermic operation of the electrolyser at high
temperatures: thus the water vapour circulating at the inlet 11.1
of each cathodic compartment 11 is at lower operating temperatures,
in other words those at which the electrolysis of the water along
each cathodic compartment is carried out by heating the vapour
using the energy dissipated by Joule effect.
[0084] Typically, the temperatures at the inlet of the cathodic
compartment 11.1 are of the order of 800.degree. C. for
temperatures of operation (during the electrolysis along the
cathodic compartment 11) which can reach 820.degree. C.
[0085] According to the invention, the water vapour circulating in
the channel or anodic compartment 9 is also over-pressurised
(arrows 12) compared to that circulating in the channel or cathodic
compartment 11 (arrows 13). Typically, the overpressure is
comprised between 15 and 100 mbars, preferably of the order of 30
mbars.
[0086] Finally, the electrical voltage U.sub.0 at the terminals of
the stack of cells delivered by the power supply source 15 is
maintained substantially constant. An electrical voltage U.sub.0 is
advantageously chosen such that for a stack of N electrolysis cells
C1, C2 . . . Cn, the average unitary voltage across the terminals
of each cell
U 1 = U 0 N , ##EQU00001##
i.e. substantially equal to 1.3 Volts.
[0087] In FIG. 2 is represented a situation of breakage of cell C1
typically observed in already tested electrolysers: the electrolyte
6.1 is broken but the electrical connection is still ensured by the
interconnecting plate 8.
[0088] Due to the overpressure of the water vapour in the anodic
compartment 9, a hydraulic short-circuit is in a way created and
the water vapour loaded with oxygen already collected flows via the
broken part 17 of the anodic compartment 9 to the cathodic
compartment (arrow 13.1). The part represented broken 17 is
voluntarily exaggerated in FIG. 2 and can consist in reality in a
fissure sufficient to allow gases to pass. The oxygen having passed
through the broken area 17 recombines with the hydrogen already
present upstream in the cathodic compartment 11 to form water with
a release of heat.
[0089] All of the additional flows of water vapour at the cathode
2.1 due on the one hand to the recombination of the oxygen coming
from the anode 4.1 via the broken area 17 with the hydrogen already
present at the cathode and to the leak (water vapour already
present at the anode), leads to a redistribution of the current in
the circulation compartment 11 in contact with the cathode.
Different models, such as the Nernst potential and the
Butler-Volmer law, exist to take this redistribution of the current
into account.
[0090] In FIG. 3A is represented the distribution line of the
current along a cathode 2 of an electrolysis cell according to the
invention not having undergone breakage: the surface of the hatched
area represents the total current flowing through the elementary
cell. This total current serves integrally for the local
electrolysis at the cell cathode 2.
[0091] In FIG. 3B, 3C is represented respectively the segments of
distribution line of the current along this same cathode but in a
breakage situation, the localisation of the broken area 17 in FIG.
3B being distinct from that of FIG. 3C. The surface of the hatched
areas here also represents the total current still applied to the
elementary cell. But, here the current represented by the hatched
area in solid lines, in other word corresponding to the part of the
cell downstream of the breakage 17, contributes mainly to the
electrolysis at the cell. In fact, the current represented by the
hatched area in broken lines, in other words upstream of the
breakage 17, contributes to a minor extent to the electrolysis.
[0092] In fact, at the broken elementary cell, the electrical
voltage across the terminals of the broken elementary cell drops.
The electrical voltage across the terminals of the cell being
lower, the operation of the broken elementary cell may be
considered endothermic, in other words that local electrolysis
downstream as upstream of the breakage consumes part of the excess
heat.
[0093] The conditions of gas, temperature, downstream of the
breakage favour an electrolysis downstream rather than upstream of
the breakage. In fact, as mentioned beforehand, these conditions
lead to an electrical conductivity in the part of the cathode
downstream of the breakage 17. Yet, the total current per cell is
imposed by the constant voltage across the terminals of the stack
of cells. Thus, due to this greater electrical conductivity
downstream of the breakage 17 and the total current imposed at the
broken elementary cell, there are less electrochemical reactions
upstream of the breakage 17.
[0094] There are thus fewer losses through recombination of
products of the upstream electrochemistry (hydrogen produced
upstream of the breakage 17 and oxygen coming from the anode via
the leak 13.1 at the broken area 17).
[0095] Thus, despite the breakage 17 of a cell, the electrolysis
taking place locally downstream thereof participates in the overall
production of hydrogen by the stack of cells.
[0096] The different breakage situations of FIG. 3B and 3C are
distinguished by the fact that, in the configuration of FIG. 3C,
the current is not zero at the outlet of the cathodic compartment
11.2: it may thus be considered that, in comparison to the
configuration of FIG. 3B, the local production of hydrogen is
less.
[0097] It may thus also be deduced from these examples that the
lower the initial conversion rate (apart from any breakage), the
less efficient the auto-regulation targeted by the invention. It is
thus necessary to target a conversion rate as high as possible, at
the best of the order of 100%.
[0098] Different experiments have made it possible to validate the
solution according to the invention, namely an overpressure of
water vapour at the anode compared to at the cathode combined with
an auto-thermal operation and a constant electrical voltage across
the terminals of the stack of cells. Thus, overall the inventors
think that such a solution makes it possible not to affect the
overall production of hydrogen of a series of high temperature
electrolysers, even in the event of breakage of one or more
electrolysis cells.
[0099] In FIG. 4 is represented the evolution of the conversion
rate of hydrogen a which takes place through the electrolysis
reaction along a channel (cathodic compartment 11) circulating
along a cathode 2.i of an electrolysis cell Ci. As in all the
electrolysers, this conversion rate a increases as the gases
progress. In the optimal conditions of the invention, one initially
targets, in other words at the design of the electrolyser according
to the invention, a conversion rate a of the order of 0 at the
inlet 11.1 of the compartment, corresponding to a non-hydrogenated
water vapour, and of the order of 100% at the outlet of the
cathodic compartment 11.2, corresponding uniquely to hydrogen.
[0100] The inventors started from the principle according to which
this conversion rate .alpha. necessarily drops due to the
phenomenon of ageing of the electrolysis cells in an electrolyser
according to the prior art.
[0101] They then reached the conclusion that over-dimensioning the
cells, in other words providing for an additional surface for the
electrolysis, compared to the initially expected electrolysis
reaction could again lead to increasing this conversion rate or in
other words the hydrogen production efficiency. In fact, by
combining an increase in the available electrolysis surface with an
increase in the flow rate of water vapour at the inlet if
necessary, one shifts in a way more downstream of the cell the
electrolysis during ageing. Thus, the decreasing conversion rate of
hydrogen during ageing is again increased by an electrolysis
shifted downstream in the cell.
[0102] The inventors thus think that with an initially targeted
conversion rate .alpha. of 100% at the cell outlet, an increase of
10 to 20% of the available surface of the cell and an increase in
the flow rate of water vapour of this same order of magnitude if
necessary, it is possible despite ageing to conserve a conversion
rate of the order of 100%, the reduction of the rate then taking
place much later.
[0103] This may be envisaged even more so given that presently the
design of HTE electrolyser production factories necessarily
provides for the use of condensers that could condense the low
percentage of vapour not converted into hydrogen and that, in the
near future, an industrial objective is to produce ceramic
electrolytes of dimensions greater than 200*200 mm.
[0104] The advantages of the solution according to the invention
are numerous:
[0105] it is simple to implement, reliable and consists in a way in
a passive and instantaneous reactivity of the stack of electrolysis
cells, which does not require restrictive measures as in the prior
art,
[0106] compared to the solutions of the prior art, which imply the
use of a greater number of electrolysers in the event of breakage
of one or more cells, the process according to the invention is
less costly; at the most it is necessary to over-dimension the
electrolysis cells compared to the targeted overall hydrogen
production efficiency,
[0107] imposing and maintaining a substantially constant electrical
voltage across the terminals of an electrolyser at high
temperatures according to the invention is simpler than managing a
current as in the prior art,
[0108] the conditions of auto-thermal operation of electrolyser(s)
according to the invention imply a high production efficiency,
[0109] the regulation according to the invention is only carried
out within the electrolyser and it naturally locally adjusts
itself, only the increase in the flow rate of non-hydrogenated
water vapour needs to be adjusted by the user of the electrolyser
depending on his needs (targeted conversion rate).
* * * * *