Hydrogen poses non trivial challenges for handling and storing: although it has the highest energy content per unit of mass, its very low density means that the resulting energy density per unit of volume is rather low. Therefore, storing energy in the form of hydrogen is a challenge.
In addition, the hydrogen molecule is the smallest in nature, thus making it prone to leaks: hydrogen can in fact escape from tanks and pipes more easily than other fuels.
However, the adoption of hydrogen as a fuel requires ways of storing and transporting it that are both efficient and cost-effective.
Storing hydrogen can be done in three main ways: in compressed form, liquid form and by sorption in solid matrices.
Compressing hydrogen is similar to compressing natural gas, though the compressors need better seals as hydrogen is less dense and more prone to leaking. Hydrogen is normally compressed to between 200 and 250 bar for storage in cylindrical tanks of up to 50 litres. These tanks may be made from aluminium or carbon/graphite compounds and can be used for either small industrial projects or as tanks in vehicles.
If compressed hydrogen is to be used on a larger scale, then pressures of 500-600 bar may be employed, though some of the largest compressed hydrogen tanks in the world (about 15,000 cubic metres) use pressures of only 12-16 bar.
The advantage of liquid hydrogen is its high energy density per unit of mass, three times higher than that of gasoline.
Since hydrogen does not liquefy until it reaches –253°C (20 degrees above absolute zero), the process is both long and energy intensive: up to 40% of the energy content in the hydrogen can be used up in the process.
Also, the low liquefaction point may result in significant losses from hydrogen slowing boiling off the tanks.
Storage in solids
Since long, activated carbons have been known as effective physisorbents as they provide numerous sites onto which gases can easily adsorb and desorb. The interaction between hydrogen molecules and the surface atoms of the material relies on Van der Waals forces, therefore storage capacity depends critically on the specific surface area of the material.
In this respect, good candidates for hydrogen storage are carbon nanostructures, e.g. nanotubes.
Some metals can absorb hydrogen by forming hydrides. This is the safest methods as no hydrogen will be released in the event of an accident.
However, the temperatures and pressure for reversible storage-release in these materials are challenging for many applications (1-10 bar; 0-300°C); in addition, the speed at which these reactions occur poses challenges for their use, in particular for the longer times required in the vehicle refuelling phase.
One of the first elements investigated for storage in solid form was lanthanum; unfortunately, the gravimetric density of hydrogen is low and the cost is high. Higher mass densities can be reached by using lighter metals as Li, Be or Mg.
For further information contact David Hart (email: firstname.lastname@example.org) from E4tech