Patent Application
20130032490
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.
It also relates to a reactor for implementing said process.
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.
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.
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.
The anode and the cathode are made of porous material into which the gases can flow.
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.
At present, the construction of a factory for producing hydrogen from a large number of high temperature electrolysers is being envisaged.
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.
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.
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.
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.
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.
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.
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.
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:
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,
b/ imposing upon starting the electrolysis and maintaining a substantially constant level of electrical voltage across the terminals of the stack of electrolysis cells.
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.
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.
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.
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.
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.
The solution according to the invention thus consists in a combination of means for carrying out respectively:
an auto-thermal operation of the assembly constituted of the electrolyser and the associated heat exchanger(s),
over-pressurising water vapour at the anode,
a constant electrical voltage across the terminals of the stack.
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.
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).
The following phenomena occur:
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.
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.
Considering a stack of a number of N+1 cells, with the number N very high for example N−1000.
In the absence of breakage, the relations linking the voltage across the terminals of a cell ucel, across the terminals of the stack of N+1 cells with the current may be written as follows:
U
0
=N u
0
+u
cel
0
u
cel
0
=u
0 and I0=i0,
in which U0 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.
Following the breakage of a cell, the voltage across the terminals of the cell concerned is written:
ucel=ucel0−ε
From which:
N u
0
=N u−ε
u being the voltage on the other non-broken cells.
The preceding equation can also be written:
u=u
0
+ε/N
Considering the value R of apparent resistance of a cell, the following relation is obtained:
u0=R i0
and
u=R i
from which the value of the current i in each of the other non-broken cells:
i=i
0
+ε/NR.
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.
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.
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:
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.
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.
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.
The initial conversion rate into hydrogen is preferably of the order of 100%.
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.
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.
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.
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.
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.
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.
The process can operate at temperatures of at least 450° C., typically comprised between 700° C. and 1000° C.
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.
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 U0 across the terminals of two interconnecting plates of the stack the furthest away from each other.
Finally, the invention relates to a hydrogen production assembly comprising a plurality of devices such as that described above.
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:
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° C., typically comprised between 700° C. and 1000° C.
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.
It is pointed out that the representations of the different components are not shown to scale.
Finally, it is pointed out that the representation of the distribution of the current is in the form of straight line segments in
In
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.
The cell C1 comprises a cathode 2.1 and an anode 4.1 between which is arranged an electrolyte 6.1, generally of 100 μm thickness for the cells known as electrolyte support and of several μm thickness for the cells known as cathode support.
The cell C2 comprises a cathode 2.2 and an anode 4.2 between which is arranged an electrolyte 6.2.
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 μm for the cells known as electrolyte support and a thickness of the order of 500 μm for the cathode of the cells known as cathode support and 40 μm for the anode.
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.
An interconnecting plate 8 is interposed between two elementary cells C1, C2.
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.
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.
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.
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.
As represented in
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.
Typically, the temperatures at the inlet of the cathodic compartment 11.1 are of the order of 800° C. for temperatures of operation (during the electrolysis along the cathodic compartment 11) which can reach 820° C.
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.
Finally, the electrical voltage U0 at the terminals of the stack of cells delivered by the power supply source 15 is maintained substantially constant. An electrical voltage U0 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
i.e. substantially equal to 1.3 Volts.
In
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
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.
In
In
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.
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.
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).
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.
The different breakage situations of
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%.
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.
In
The inventors started from the principle according to which this conversion rate α necessarily drops due to the phenomenon of ageing of the electrolysis cells in an electrolyser according to the prior art.
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.
The inventors thus think that with an initially targeted conversion rate α 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.
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.
The advantages of the solution according to the invention are numerous:
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,
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,
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,
the conditions of auto-thermal operation of electrolyser(s) according to the invention imply a high production efficiency,
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).
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 51782 | Mar 2010 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP11/53725 | 3/11/2011 | WO | 00 | 10/25/2012 |
