Preparation of a borohydride or aluminhydride solution from solid borohydride
or aluminhydride on-board a sub-marine device
The present application relates to the field of propulsion of underwater 5 vehicles,
and, 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 diving autonomy
of underwater vehicles, and to limit regular returns to the surface to run the engines in order
10 to recharge the batteries used during the immersion, wherein the latter represents
undesirable 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, KAlH4, LiAlH4
15 or NaBH4) are a means of storage and production of hydrogen by hydrolysis. These
borohydrides or aluminohydrides are denoted XH4 hereinafter.
The usual XH4 AIP systems are based on aqueous storage. WO 2017145068 thus
proposes an AIP system with a phosphoric acid fuel cell (PAFC), and a hydrogen production
20 system based on NaBH4 solution in liquid form. NaBH4 is thus stored in the form of an
aqueous solution.
Storing XH4 solutions, however, presents significant technical disadvantages, such
as the weight, size and stability of the solutions.
25 It is therefore necessary to provide alternative on-board storage means.
The present invention provides on-board storage of XH4 in solid form and a process
for preparing an XH4 solution.
According to a first object, the present invention relates to a process for the
30 preparation of an aqueous solution of a borohydride or an aluminohydride denoted XH4,
where X is chosen from KB, LiB, NaAl, KAl, LiAl or NaB and carried in an underwater vehicle,
wherein said method comprises dissolving XH4 in the form of tablets in an aqueous
dissolution solution. Said tablets may also be referred to as "caplets".
2
Without being exhaustive, the present invention makes it possible to solve, inter
alia, the problems inherent in the aqueous storage 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 5 mission;
· Suppression of unwanted hydrogen releases when hydrogen production is not
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
10 hydrogen in solid XH4 being greater than a concentrated XH4 solution).
"On-board" is understood to mean the storage of XH4 on board an underwater
vehicle, generally involving a double limitation, of a spatial order (reduced size) and energy
(limited consumption).
15 The terms “reserve” and “storage” are used here interchangeably to denote the
stock of XH4, in one or more containers.
The term “underwater vehicle” is understood to mean, in particular, an actual
submarine, or any other equipment or underwater vehicle requiring a source of hydrogen
supplied by XH4 for its propulsion or its operation.
20
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 the XH4 is carried
out, leading to the formation of hydrogen, depending on the reaction:
XH4 + 2 H2O -> 4 H2 + XO2
25 The term "aqueous dissolution solution" is understood to mean a water-based
solution used to 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 the stabilization of
the solution. ...
30
The term “dissolution chamber” denotes the apparatus generally consisting of at
least one container and a stirring means, in which the XH4 may be mixed in water, wherein
the mixture may be produced continuously.
3
According to one embodiment, the dissolution is carried out in a dissolution
chamber and/or a reservoir for storing the XH4 tablets.
According to one embodiment, the method comprises the prior step of adding a
basic aqueous solution to water.
According to one embodiment, the basic aqueous solution comprises 5 soda (NAOH)
and/or potash (KOH).
According to one embodiment, the basic aqueous solution comprises one or more
bases at a concentration of between 1 and 10% by weight.
According to one embodiment, the step of dissolving the XH4 in the aqueous
10 dissolution solution is carried out by forced circulation of the aqueous dissolution solution
between said storage reservoir and the dissolution chamber.
According to one embodiment, the dissolution is carried out until the concentration
of XH4 in the aqueous solution reaches a predetermined Cmax value. Cmax is typically
chosen so as to be strictly lower than the critical crystallization concentration of XH4.
15 According to one embodiment, said critical concentration depends on the
temperature of the aqueous dissolution solution.
According to one or other of the embodiments, the XH4 solution may be
advantageously stabilized by one or more bases.
Typically, the base(s) may be present in solid XH4, in the aqueous dissolution
20 solution, and/or in the aqueous solution of XH4.
According to one embodiment, said method comprises the initial priming step by
dissolving solid XH4 with an aqueous priming solution.
According to another object, the present invention also relates to an anaerobic
propulsion process (AIP, Air Independent Propulsion) of an underwater vehicle comprising
25 the production of hydrogen (H2) by hydrolysis of XH4, characterized in that it comprises the
prior step of on-board dissolution of XH4 in solid form according to the invention.
According to one embodiment, said method comprises recycling the water for
dissolving the H2 production phase to supply the aqueous dissolution solution.
30 According to another object, the present invention also relates to an on-board
system within an underwater vehicle comprising:
- A storage reservoir 1 of XH4 in the form of tablets;
- An inlet 12, 12' of a basic aqueous solution in the dissolution solution 7;
4
- A dissolution chamber 11;
Characterized in that there is at least one means of forced circulation of the
aqueous dissolution solution 7 between the dissolution chamber 11 and the storage reservoir
1.
5
According to one embodiment, said system comprises:
- at least one means (15) for measuring the concentration of XH4 in the
dissolution chamber (11) making it possible to control the flow of liquid arriving in
the dissolution chamber (11) by acting on the operation of the pump (4'').
According to one embodiment, said system may also comprise 10 at least one means
for extracting the XH4 solution from the dissolution chamber when the concentration of XH4 in
the dissolution solution reaches a determined value Cmax.
In order to measure the concentration, it is possible, in particular, to use a
voltammeter or an electrochemical impedance spectroscope, for example.
15 According to one embodiment, the system comprises means for recycling the water
produced during the production of H2, to the water inlet.
The invention will be better understood upon reading the description which follows,
given solely by way of example and made with reference to the appended drawing, in which:
20 Figure 1 shows schematically a system according to the invention, allowing the
implementation of the method according to the invention.
Figure 1, in fact, illustrates a system for hydrogen production from on-board solid
XH4 and comprising:
a reservoir denoted by the general reference 1 of solid XH4 denoted by the general
25 reference 2, said solid XH4 2 being conventionally transferred by a conveying system
provided with a valve denoted by the general reference 3, and a pump denoted by the
general reference 4, to a dissolution chamber 11, then to a hydrolysis reactor denoted by the
general reference 10 to allow the production of hydrogen denoted by the general reference 6,
from said solid XH4 2.
30 The hydrogen 6 so formed is then extracted and transferred from the reactor to a
fuel cell, conventionally denoted by the general reference 9, for example by means of a
pump.
5
The system may comprise a pressure gauge represented by the general reference
5 connected to the pump 4, so as to measure the pressure of the hydrogen so formed and to
modulate the inlet flow of the liquid XH4 into the reactor 10.
The system of Figure 1 also comprises means for dissolving the solid 5 XH4 2, prior to
introduction into the reactor 10, according to the invention.
Thus, the system comprises means for the arrival of an aqueous solution denoted
by the general reference 12 to the solid XH4 2, and means for forced circulation of said
aqueous solution 7 between a dissolution chamber denoted by the general reference 11, and
10 the storage reservoir 1 of solid XH4 2, such as the pumps 4' and 4''.
Thus, water is added to the solid XH4 2 and forcibly circulated between the reservoir
1 and the dissolution chamber 11, so as to form an aqueous solution of XH4 denoted by the
general reference 7.
At the outlet of the mixer 11, a pump 4 makes it possible to extract the aqueous
15 solution of XH4 7, and to transfer it to the hydrolysis reactor 10.
The concentration of XH4 in the aqueous dissolution solution is measured via an
indicator 15, using voltammetry or electrochemical impedance spectroscopy, etc. for example
by drawing off said solution from the dissolution chamber. This indicator is connected to a
20 pump 4’’ in order to modulate the flow of the dissolution solution between the dissolution
chamber and the hydrolysis reactor.
The aqueous solution 12, typically water to which is added a basic aqueous solution
12′, may come from a supply such as a storage reservoir and/or may come from the recycling
14 of the water obtained at the end of the hydrolysis reaction.
25 Typically the system operates under a flow of nitrogen, denoted by the general
reference 13.

CLAIMS
1. Process for the preparation of an aqueous solution of a borohydride or an
aluminohydride denoted XH4 where X is chosen from KB, LiB, NaAl, KAl, LiAl or NaB, carried
in an underwater vehicle, said process comprising dissolution of XH4 in the form 5 of tablets (2)
in an aqueous dissolution solution (7).
2. Process according to claim 1 such that the dissolution is carried out in a
dissolution chamber (11) and/or a storage reservoir (1) of XH4 tablets (2).
10
3. Process according to claim 1 or 2 such that it comprises the prior step of adding a
basic aqueous solution to water (12, 12’).
4. Process according to claim 3 such that the basic aqueous solution comprises
15 soda (NAOH) and/or potash (KOH).
5. Process according to any one of claims 3 to 4 such that the basic aqueous
solution comprises one or more bases at a concentration between 1 and 10% by weight.
20 6. Process according to any one of the preceding claims, such that the step of
dissolving XH4 in the aqueous dissolution solution is carried out by forced circulation of the
aqueous dissolution solution (7) in said storage reservoir (1) and the dissolution chamber
(11).
25 7. Process according to any one of the preceding claims, such that the dissolution is
carried out until the concentration of XH4 in the aqueous solution (7) reaches a Cmax value
lower than the critical crystallization concentration of XH4.
8. Process of anaerobic propulsion (AIP, Air Independent Propulsion) of an
30 underwater vehicle comprising the production of hydrogen (H2) by hydrolysis of XH4,
characterized in that it comprises the preliminary step of dissolution of on-board XH4 in solid
form (2) according to any one of claims 1 to 7.
7
9. Process according to claim 8 comprising recycling (14) of the water produced
during the production of H2 in an aqueous dissolution solution.
10. On-board system within an underwater vehicle comprising:
- A storage reservoir (1) of XH4 in the form 5 of tablets (2);
- An inlet (12, 12’) of a basic aqueous solution in the dissolution solution (7);
- A dissolution chamber (11);
Characterized in that it comprises at least one means (4’, 4’’) of forced circulation of
the aqueous dissolution solution (7) between the dissolution chamber (11) and the storage
10 reservoir (1).
11. System according to claim 10 as it comprises:
- at least one means (15) for measuring the concentration of XH4 in the dissolution
chamber (11), and making it possible to control the flow of liquid arriving in the dissolution
15 chamber (11) by acting on the operation of the pump (4'').