Hydrogen is the lightest gas in the entire Universe. One liter of this gas weighs only 90 mg under normal atmospheric pressure, which means that it is 11 times lighter than the air we breathe.
A volume of around 11 m3 (which is the volume of the trunk of a large utility or commercial vehicle) is needed to store just 1 kg of hydrogen, which is the quantity needed to drive 100 km. For this reason, its density must be increased using one of the following techniques:
For easier and more efficient transport, hydrogen is stored in composite tanks or bottles. Air Liquide researchers are working on the mechanical strength of the materials that make up these bottles over time. They perform accelerated fatigue tests by filling and permeability cycles at very high pressure to ensure their perfect tightness. All this research will lay the scientific foundations of the behavior of the materials and allow to determine the criteria of dimensioning of the reservoirs. Thanks to this research, Air Liquide is a decisive player in the definition of safety standards that must be put in place to ensure maximum safety for the user.
Another research area of the Group is the development of bottle control technologies during their use. This step is also essential for the safety of the users and consists in ensuring the absence of defects such as microcracks. For this, researchers appropriate non-destructive testing methods such as acoustic emission to detect this type of anomalies.
The easiest way to decrease the volume of a gas, at constant temperatures, is to increase its pressure.
So, at 700 bar, which is 700 times normal atmospheric pressure, hydrogen has a density of 42 kg/m3, compared with 0.090 kg/m3 under normal pressure and temperature conditions. At this pressure, 5 kg of hydrogen can be stored in a 125-liter tank.
Today, most car manufacturers have opted for the solution that consists in storing hydrogen in the gaseous form, at high pressure. This technology enables us to store enough hydrogen to allow a car that runs on a fuel cell battery to cover between 500 and 600 km between fill-ups.
A state-of-theart technique for storing maximum hydrogen in a restricted volume is to convert hydrogen gas to liquid hydrogen by cooling it to a very low temperature.
Hydrogen turns into a liquid when it is cooled to a temperature below -252,87 °C.
At -252.87°C and 1.013 bar, liquid hydrogen has a density of close to 71 kg/m3. At this pressure, 5 kg of hydrogen can be stored in a 75-liter tank.
In order to maintain liquid hydrogen at this temperature, tanks must be perfectly isolated.
Currently, storing hydrogen in the liquid form is being reserved for certain special applications, in high-tech areas such as space travel. For example, the tanks on the Ariane launcher, designed and manufactured by Air Liquide, contain the 28 tons of liquid hydrogen that will provide fuel to the central engine. These tanks are a genuine example of technological prowess: they weigh only 5.5 tons empty and their casing is not more than 1.3 mm thick.
The storage of hydrogen in solid form, i.e. stored in another material, is also a promising avenue of research.
Methods for storing hydrogen in solid form are techniques involving absorption or adsorption mechanisms of hydrogen by a material.
One example is to form solid metallic hydrides through the reaction of hydrogen with certain metal alloys. This absorption is the result of the reversible chemical combination of hydrogen with the atoms that comprise these materials. The most promising materials are composed of magnesium and alanates.
Only a low mass of hydrogen can be stored in these materials, which is currently the major downside of this technology. In fact, the best materials currently generate a ratio of hydrogen weight to the total weight of the tank of not more than 2 to 3%.
Before considering large-scale applications, it is also important to master certain key parameters such as kinetics (cell performance), and the temperature and pressure of the charge and discharge cycles of hydrogen in these materials.