Due, in particular, to a reduction in reserves of petroleum, energies which are alternatives to petroleum are being sought. One of the promising directions for these sources of energy is hydrogen, which can be used in fuel cells to produce electricity.
Hydrogen is an element which is very widespread in the universe and on earth, it can be produced from coal, natural gas or other hydrocarbons but also by simple electrolysis of water by using, for example, electricity produced by solar or wind energy.
Hydrogen cells are already used in certain applications, for example in automobiles, but are not yet very widespread, in particular due to the precautions which have to be taken and the difficulties of storing hydrogen.
Hydrogen may be stored in the form of compressed hydrogen between 350 and 700 bars, which raises safety problems. So, tanks capable of withstanding these pressures must be provided, knowing, moreover, that when these tanks are installed in vehicles they may be subject to impacts.
Hydrogen can be stored in liquid form, but such storage only ensures a low storage yield and does not allow long-term storage. The passage of a volume of hydrogen from liquid state to gaseous state in normal conditions of pressure and temperature produces an increase in its volume by a factor of about 800. Tanks for hydrogen in liquid form are not, in general, very resistant to mechanical impacts, which raises serious safety problems.
There also exists storage of what is called “solid” hydrogen in the form of hydride. Such storage allows a considerable volumetric density of storage and makes use of moderate hydrogen pressure while minimizing the energy impact of storage on the overall yield of the hydrogen chain, i.e. from its production to its conversion into a different energy.
The principle of solid storage of hydrogen in the form of hydride is the following: certain materials and, in particular, certain metals possess the capacity of absorbing hydrogen to form a hydride—this reaction is called absorption. The hydride formed may again give hydrogen gas and a metal. This reaction is called desorption. Absorption or desorption take place depending on the partial pressure of hydrogen and on temperature.
Absorption and desorption of hydrogen on a metallic powder or matrix M take place according to the following reaction:M+x/2 H2←MHx+ΔH(Heat)                M being the metallic powder or matrix.        MHx being the metallic hydride.        
For example, a metallic powder is used which is put in contact with hydrogen, a phenomenon of absorption appears and a metallic hydride is formed. Release of the hydrogen takes place according to a mechanism of desorption.
The storage of hydrogen is an exothermic reaction, i.e. one which releases heat, while the release of hydrogen is an endothermic reaction, i.e. one which absorbs heat.
In particular, rapid charging of the metallic powder with hydrogen is sought. To obtain such rapid charging, the heat produced during this charging must be evacuated to avoid delaying the absorption of hydrogen on the metal powder or matrix. When hydrogen is released, heat is taken in.
The tank is therefore equipped with a heat exchanger including a circuit in which a heat transfer medium circulates, and this circuit is connected to a circuit on the outside of the tank and means of introducing hydrogen into the tank for its absorption and means of collection of hydrogen in the desorption phase.
A phase of absorption followed by a phase of desorption of hydrogen is called a “hydration cycle”.
The tank must also be able to withstand the pressure of hydrogen. In present applications, an interesting pressure interval is situated within the 1-30 bar absolute Interval, knowing that, depending on the type of hydride used, this interval is likely to become wider or narrower. “Hybridisation” of this type of storage using hydrides with storage of hydrogen under pressure can also be considered. Currently, the pressure interval is from 1 to 300 bars or even 1-700 bars.
This requirement is even more difficult to fulfill easily, as reduction in the mass of tanks is being sought, in particular for tanks installed in vehicles.
Document US-2005/0188847 describes a tank for hydrogen stored in the form of hydride including a shell composed of a central tubular part and two longitudinal hemispherical ends. One of the hemispherical ends and the tubular part are made in one piece from stainless steel and the second hemispherical end, also made of stainless steel, is attached by screwing after the installation of the heat exchanger. The shell is then coated with a composition made of carbon fibre to increase the tank's ability to withstand hydrogen pressure. The hydride is stored in the shell inside spaces bordered by the blades of the heat exchanger.
The introduction of the heat exchanger is complicated, as is the making of the shell, which must be done in two stages.
Document W0-2007/011476, describes a hydrogen tank comprised of a shell which is also composed of a central tubular part and two longitudinal hemispherical ends, the whole of which is made of steel. Each of the ends has an opening, one to allow hydrogen supply and hydrogen collection, and the other the circulation of the heat transfer medium. This tank is not easy to dismantle, which makes any work inside the tank difficult. In addition, if the shell is made by welding, as hydride is very sensitive to air, this operation must be performed using a glove box, which makes its fabrication complicated.
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