The present application relates to the field of propulsion of underwater vehicles, and,
5 in particular, anaerobic propulsion (Air Independent Propulsion - AIP) with a fuel cell, in
particular with a hydrogen cell.
Anaerobic propulsion makes it possible to significantly increase the immersion
autonomy of underwater vehicles, and to limit regular returns to the surface to run the engines
in order to recharge the batteries used when immersed, the latter representing undesirable
10 periods of indiscretion.
The ability to store as much hydrogen as possible in a limited volume is a fundamental
issue in AIP fuel cell power systems.
Borohydrides or aluminohydrides (for example KBH4, LiBH4, NaAlH4, LiAlH4 or
NaBH4) are a means of storage and production of hydrogen by hydrolysis. These borohydrides
15 or aluminohydrides are denoted XH4 hereinafter.
The usual XH4 AIP systems are based on aqueous storage. Thus, WO 2017145068
describes an AIP system with a Phosphoric Acid Fuel Cell (PAFC), and a hydrogen production
system based on an NaBH4 solution in liquid form. NaBH4 is thus stored in the form of an
aqueous solution.
20 Storing XH4 solutions, however, gives rise to significant technical disadvantages, such
as the weight, size and stability of the solutions.
It is therefore necessary to provide an alternative on-board storage means and a
process allowing the production of hydrogen.
25 The present invention provides an improved propulsion system based on on-board
XH4 in solid form.
According to a first object, the present invention relates to an anaerobic propulsion
process (AIP) of an underwater vehicle comprising the production of hydrogen (H2) by
hydrolysis of a borohydride or of an aluminohydride, of formula XH4 in which X is chosen from
30 KB, LiB, NaB, KAl, LiAl, NaAl, said process comprising:
i) On-board storage of NaBH4 in solid form, distributed in at least one storage
reservoir;
2
ii) Addition of an aqueous dissolution solution and a basic aqueous solution to
one of the storage reservoirs;
iii) Forced circulation of the mixture thus obtained within a dissolution chamber
and said storage reservoir;
5 iv) Extraction of the aqueous NaBH4 solution thus obtained from the dissolution
chamber towards the H2 production reactor;
v) Repetition of steps ii) to iv) in sequence starting with another storage reservoir.
Without being exhaustive, the present invention makes it possible to solve, inter alia,
10 the problems inherent in the aqueous storage of XH4 and to optimize the dissolution of XH4:
• In the absence of water, the solid XH4 does not produce hydrogen, which makes it
possible to control the production of hydrogen during the underwater vehicle’s
mission;
• Suppression of unwanted hydrogen releases when hydrogen production is not
15 required;
• Elimination of XH4 crystallization problems not controlled in the reservoirs;
• Increase in the quantity of on-board XH4 per unit of volume (the mass fraction of
hydrogen in solid XH4 being greater than a concentrated solution of XH4);
• Forced circulation through a storage reservoir and a dissolution chamber makes it
20 possible to optimize the dissolution;
• Continuous production of aqueous XH4 from a set of separate solid XH4 reservoirs;
• Optimization of the XH4 dissolution time through mechanical stirring of XH4 by water
recirculation;
• Elimination of the conveying of solids on board the submarine, as this represents a
25 source of noise and significant technical risk;
• Elimination of solid pump type means for injecting XH4 into the reactor.
In particular, the process according to the invention makes it possible to create an
aqueous solution of XH4 on board, whenever the hydrogen production reactor is supplied.
30 XH4 may be stored in a storage reservoir or distributed in at least two storage
reservoirs. The fractionation of the storage of XH4 within several reservoirs and the successive
sequencing of the delivery of the dissolution solution reservoir by reservoir, makes it possible
to dedicate a limited volume to the dissolution of XH4 and to decrease the concentration thereof
3
to avoid any risk of crystallization of the XH4 in solution, whatever the temperatures in which
the underwater vehicle is operating.
According to one embodiment, XH4 is, in particular, NaBH4 or LiBH4. According to one
embodiment, the solid XH4 is stored in the form of tablets (“caplets”), granules (“pellets”) or
5 powder, more particularly in the form of caplets.
According to one embodiment, the process further comprises the step of introducing
the XH4 aqueous solution thus obtained after extraction in the hydrolysis reactor to produce H2.
The aqueous dissolution solution may result from the recycling of the aqueous
solution at the end of the hydrolysis step. According to one embodiment, the process may
10 comprise recycling the water produced during the production of H2 at the outlet of the hydrolysis
reactor for the supply of the aqueous dissolution solution.
To initiate the process of the invention, it is then necessary to have an aqueous
priming solution, which, added to the first of said storage reservoirs, then makes it possible to
constitute the aqueous dissolution solution. According to one embodiment, the process may
15 comprise the preliminary priming step, said priming step comprising dissolving solid XH4 using
an on-board aqueous priming solution.
According to one embodiment, the basic aqueous solution comprises soda (NAOH)
and/or potash (KOH). Typically, the basic aqueous solution comprises one or more bases with
a base concentration of between 1 and 10% by weight.
20 The process may be carried out under an inert atmosphere, for example under a
nitrogen atmosphere. According to one embodiment, the process comprises the application of
a flow of nitrogen (N2), for example at 0.1 bar, within the storage reservoirs and the dissolution
chamber.
The process according to the invention may also comprise means for stirring solid
25 XH4 and/or the dissolution solution loaded with XH4.
According to one embodiment, the process according to the invention further
comprises the extraction of the hydrogen created during the dissolution towards a fuel cell.
It is important that the dissolution solution has an XH4 concentration below the critical
30 crystallization concentration of XH4. In fact, it is possible to follow the concentration of XH4 in
the dissolution solution and to set a concentration Cmax that is strictly lower than the critical
crystallization concentration of XH4, which it must not exceed. According to one embodiment,
4
the dissolution may be carried out up to a concentration of XH4(max) that is lower than the
critical XH4crystallization concentration.
The concentration may, for example, be measured by voltammetry or electrochemical
impedance spectroscopy...
5 Typically, said critical concentration depends on the temperature of the XH4 solution.
In the above and what follows:
"On-board" means the storage of the XH4 on board an underwater vehicle, which
generally involves a double limitation, of a spatial order (reduced size) and energy (limited
consumption).
10 The terms “reserve” and “storage” are used here interchangeably and denote the
stock of XH4, in one or more containers.
The term “underwater vehicle” means, in particular, a submarine proper, or any other
equipment or underwater vehicle requiring for its propulsion or its operation, a source of
hydrogen supplied by XH4.
15 The term “hydrolysis reactor” denotes the apparatus generally consisting of at least
one container and a stirring means in which the reaction between water and XH4 is carried out,
leading to the formation of hydrogen, according to the reaction:
XH4 + 2 H2O -> 4 H2 + XO2
The term "aqueous dissolution solution" means a water-based solution used to
20 dissolve the solid XH4, so as to form an aqueous solution of XH4. Said aqueous dissolution
solution essentially contains water, but may also contain one or more other dissolved agents,
such as aqueous NaOH or KOH bases, which allow stabilization of the solution.
The term “dissolution chamber” denotes the apparatus generally consisting of at least
one container and a stirring means, in which the mixture of XH4 in water may be produced, the
25 mixture being able to be produced continuously. The dissolution chamber may, therefore, be
filled, at least in part, with the aqueous dissolution solution.
According to one or other of the embodiments, the XH4 may be advantageously
stabilized by one or more bases. Typically, the base(s) may be present in the form of solid XH4,
in the aqueous dissolution solution, and/or in the XH4 aqueous solution.
30 The term "circulation" is understood here to mean circulation and recirculation
between the dissolution chamber and said storage reservoir.
According to another object, the present invention also relates to an anaerobic
propulsion system on board an underwater vehicle comprising:
5
- a system (10) of on-board hydrogen produced from XH4,
- a plurality of reservoirs (1, 1', 1'') for storage of solid XH4 (2, 2', 2 ''), the plurality
of storage reservoirs (1, 1', 1'') being mounted parallel to each other;
- a dissolution chamber (11);
5 - a hydrolysis reactor (10) connected to the dissolution chamber,
- at least one means (3, 3’, 3’’) for the introduction of an aqueous dissolution
solution (7, 7’) into at least one storage reservoir (1, 1’, 1’’)
Characterized in that it comprises at least one means (4') for circulating the aqueous
dissolution solution (7') between the dissolution chamber (11) and at least one of the storage
10 reservoirs (1, 1', 1'').
According to one embodiment, said system may further comprise:
- a reservoir of an aqueous priming solution; and
- at least one means for introducing the aqueous priming solution into the reservoir
15 (1, 1’, 1’’) for storing solid XH4 (2, 2’, 2’’).
According to one embodiment, said system may comprise at least one means for
recycling the water produced during the production of H2 to the dissolution chamber.
According to one embodiment, it may be advantageous to increase the pH of the
dissolution solution in order to prevent crystallization of XH4. Thus, the system may be provided
20 with a reservoir for storing a basic aqueous solution, and at least one means for introducing the
basic aqueous solution into the dissolution solution. Therefore, the aqueous dissolution solution
and the basic aqueous solution may be mixed to obtain a basic aqueous solution, or may
remain as two separate solutions.
The invention will be better understood upon reading the description which follows,
25 given solely by way of example and made with reference to the appended drawings, in which:
Figure 1 schematically represents the system according to the invention in its initial
state, for example for a departure of the submarine on mission.
Figure 2 shows schematically the dissolution of the first XH4 storage reservoir before
30 starting the propulsion system (AIP).
Figure 3 shows schematically the consumption of the first reserve of XH4.
Figure 4 shows schematically the circulation of the dissolution solution and the filling
of the second reservoir.
6
Figure 5 shows schematically the dissolution of XH4 from the second storage reservoir.
Figure 6 shows schematically the consumption of the second reservoir.
Figures 1 to 6, in fact, illustrate the implementation of the process according to the
5 invention for a system comprising 3 storage reservoirs denoted by the general references 1, 1′
and 1″ of solid XH4 denoted respectively by the references general 2, 2' and 2'', as the first
reservoir 1 and then the second storage reservoir 1' dissolve successively. The solid XH4 may,
for example, be in the form of powder, tablets, pellets or the like.
10 As illustrated in Figure 1, the system in its initial state includes storage reservoirs 1,
1' and 1'' filled with solid XH4 2, 2' and 2'' respectively, said reservoirs being arranged parallel
to each other, and also arranged in series with a dissolution chamber denoted by the general
reference 11.
The valves 3, 3', 3'' and 3a, 3a', 3a'' are placed respectively at the inlet and outlet of
15 each of the storage reservoirs 1, 1' and 1'' to control the circulation of the dissolution solution
between the chamber 11 and the various reservoirs.
Said chamber 11 is filled, at least in part, with an aqueous dissolution solution denoted
by the general reference 7, the volume being completed by an inert atmosphere, for example
20 nitrogen N2 under pressure denoted by the general reference 13. A non-return valve denoted
by the general reference 14 closes the chamber 11 at its top to vent gas overpressures. A nonreturn valve denoted by the general reference 14’ also equips the outlet of the solution from the
dissolution chamber 7 towards the circuit supplying the reservoirs, in order to avoid possible
returns to the chamber 11.
25 The chamber 11 is also connected to the hydrolysis reactor denoted by the general
reference 10 allowing the production of hydrogen denoted by the general reference 6, from said
XH4. A pump denoted by the general reference 4 controls the circulation from the chamber 11
to the reactor 10.
The system may comprise a pressure gauge represented by the general reference 5
30 to measure the pressure of the hydrogen formed in the reactor 10. Said pressure gauge may
be connected to the pump 4 to modulate the flow of entry of the dissolution solution into the
reactor inlet 10.
7
The hydrogen 2 so formed is then extracted and transferred from the reactor 10 to a
fuel cell, conventionally denoted by the general reference 9, for example by means of a pump.
The dissolution solution 7 may come from a reservoir of an aqueous priming solution
5 (not shown in the figures). In this case, the system may further comprise at least one means
for introducing the aqueous priming solution into the reservoir 1 for storing solid XH4, or into
the chamber 11.
Typically, and although not shown in the figures, the system may also comprise at
least one means for recycling the water produced during the production of H2 towards the
10 dissolution chamber.
In the initial state (Figure 1), the dissolution solution is only present in the chamber
11, and consists essentially of water (to which one or more additives may be added). The
storage reservoirs 1, 1’ and 1’’ are loaded with solid XH4 2, 2’ and 2’’. The valves 3, 3', 3'', 3a,
3a' and 3a'' are closed, and the pump denoted by the general reference 4' is stopped,
15 preventing circulation between the chamber 11 and the reservoirs 1, 1' and 1''.
In operation (Figure 2), if a supply of hydrogen to the cell is required, the dissolution
of the XH4 present in a first chamber 1 is initiated: for this, the circulation of the dissolution
solution 7 between the chamber 11 and the reservoir 1 is triggered by opening valves 3 and 3a
to put the pump 4' into operation. As the solution 7 circulates in the reservoir 1, it dissolves the
20 XH4 and is loaded with XH4. The dissolution solution loaded with XH4 is denoted by the general
reference 7’: solution 7’ differs from solution 7 in that its concentration of XH4 is higher.
If hydrogen is formed during this forced circulation, the hydrogen overpressure may
be vented from the chamber 11 via the non-return valve 14.
The forced circulation of solution 7' is continued until its concentration of XH4 reaches
25 a predetermined value. This concentration may, in particular, be measured with a voltammeter
or an electrochemical impedance spectroscope.
When this value is reached, the circulation through the reservoir 1 is stopped. As
illustrated in Figure 3, it is, for example, possible to purge the reservoir 1 by overpressure, for
example with a nitrogen overpressure. In parallel, the pump 4 is activated so as to initiate the
30 circulation of the loaded solution 7' between the chamber 11 and the reactor 10.
Within the reactor 10, the solution 7' undergoes hydrolysis to form gaseous
dihydrogen 6 which is evacuated towards the fuel cell 9.
8
The flow of the pump is controlled as a function of the hydrogen pressure so formed,
as measured by the pressure gauge 5. When sufficient hydrogen is formed, the feed flow
generated by the pump 4 may thus be reduced or stopped.
The reaction residue is discharged from reactor 8. It is possible to recycle the water
5 at the outlet of the reactor towards the supply of the dissolution solution 7 or 7’.
When the contents of reservoir 1 are drained, water is added to the circulation circuit,
possibly with added base. The water may come from a supply such as a storage reservoir
and/or may come from the recycling of the water obtained at the end of the hydrolysis reaction.
10 As illustrated in Figure 4, sodium hydroxide or potash may be added at a
concentration of 2% by weight to the water thus added, in order to avoid crystallization of XH4
in the dissolution solution 7′. Other bases and/or mixtures of bases may, however, be used, at
concentrations which may typically be between 0.5 and 5% by weight. It is also possible to mix
a base within the solid XH4 2, 2’ and/or 2’’. The system may, therefore, also comprise a storage
15 reservoir for a basic aqueous solution, and at least one means for introducing the basic
aqueous solution into the dissolution solution.
This supply then constitutes a new dissolution solution 7, the circulation of which may
be initiated via the reservoir 1’, as illustrated in Figure 4.
20 To do this, we open valves 3’ and 3a’, while valves 3 and 3a are closed. The solution
7 then circulates within the solid XH4 2’ contained in the reservoir 1’ and via the chamber 11,
by the action of the pump 4’. Along with its passage through the solid XH4 2’, the solution is
loaded with dissolved XH4 so as to reach the concentration of the solution 7’. If hydrogen is
necessary to supply the cell, the solution 7′ thus loaded with XH4 is supplied to the reactor by
25 operation of the pump 4.
Figure 5 illustrates the operation of the system during forced circulation via the
chamber 13, the reservoir 1’ and the supply to the reactor 10: in stationary mode, the water
supply may be stopped.
30
When the concentration of solution 7’ in XH4 reaches a predetermined value,
circulation through the reservoir 1’ is stopped. As illustrated in Figure 6, we may, for example,
purge reservoir 1’ by overpressure, for example using nitrogen overpressure. At the end of this

purging, it is then possible to close the valves 3' and 3a' and to open the valves 3'' and 3a'' in
order to initiate circulation through the reservoir 1'' and to dissolve the XH4 2''. This may be
done as explained above.

CLAIMS
1. Process of anaerobic propulsion (AIP) of an underwater vehicle comprising the
production of hydrogen (H2) by hydrolysis of borohydride or aluminohydride, of formula XH4 in
5 which X is chosen from KB, LiB, NaB, KAl, LiAl, NaAl, said process comprising:
i) On-board storage of XH4 in solid form, distributed in at least one storage reservoir;
ii) Addition of an aqueous dissolution solution and a basic aqueous solution to one of
the storage reservoirs;
iii) Forced circulation of the mixture thus obtained within a dissolution chamber and
10 said storage reservoir;
iv) Extraction of the aqueous solution of XH4 thus obtained from the dissolution
chamber towards the H2 production reactor;
v) Repetition of steps ii) to iv) in sequence starting with another storage reservoir.
15 2. Process according to claim 1 comprising the introduction of the aqueous solution
of XH4 thus obtained after extraction in the hydrolysis reactor to produce H2.
3. Process according to any one of the preceding claims, such that the solid XH4 is
stored in the form of tablets ("caplets"), granules ("pellets") or powder.
20
4. Process according to any one of the preceding claims comprising the priming step,
comprising the dissolution of solid XBH4 by an on-board aqueous priming solution.
5. Process according to any one of the preceding claims comprising recycling the
25 water produced during the production of H2 at the outlet of the hydrolysis reactor as an aqueous
dissolution solution.
6. Process according to any one of the preceding claims, such that the basic aqueous
solution comprises soda (NAOH) and/or potash (KOH).
30
7. Process according to any one of the preceding claims, such that the basic aqueous
solution comprises one or more bases with a base concentration of between 1 and 10% by
weight.
11
8. Process according to any one of the preceding claims, such that it comprises a flow
of nitrogen (N2) within the storage reservoirs and the dissolution chamber.
9. Process according to any one of the preceding claims, such that it comprises the
5 extraction of the hydrogen created during the dissolution towards a fuel cell.
10. Process according to any one of the preceding claims, such that the dissolution
is carried out up to a concentration of XH4(max) that is less than or equal to the critical
concentration of crystallization of XH4.
10
11. Anaerobic propulsion system on board an underwater vehicle comprising
- a system (10) for on-board hydrogen production from XH4,
- a plurality of reservoirs (1, 1', 1'') for storage of solid XH4 (2, 2', 2''), the plurality of
storage reservoirs (1, 1', 1'') being mounted parallel to each other;
- a dissolution chamber (11); 15
- a hydrolysis reactor (10) connected to the dissolution chamber,
- at least one means (3, 3’, 3’’) for introducing an aqueous dissolution solution (7, 7’)
into at least one storage reservoir (1, 1’, 1’’)
Characterized in that it comprises at least one means (4') for circulating the aqueous
20 dissolution solution (7') between the dissolution chamber (11) and at least one of the storage
reservoirs (1, 1', 1'').
12. System according to claim 11, further comprising:
- a reservoir for an aqueous priming solution; and
25 - at least one means for introducing the aqueous priming solution into the reservoir
(1, 1’, 1’’) for storing solid XH4 (2, 2’, 2’’).
13. System according to any one of claims 11 or 12 comprising at least one means
for recycling the water produced during the production of H2 to the dissolution chamber.