Hydrogen storage is one of the central issues in the transition to a more sustainable energy
system. Indeed, despite its potential as a clean energy carrier, hydrogen presents some
challenges due to its physical and chemical nature.
How hydrogen storage works
In order to analyse how the storage technologies available today work, it is necessary to start
from some fundamental quantities of this gas such as specific energy mass and volumetric
energy, i.e. the density of stored energy considering the mass or volume of the gas.
In fact, hydrogen is the lightest gas present in nature. Its density is so low that, in its gaseous
state, it contains far less energy per unit volume than other fuels.
This makes it essential to adopt solutions that increase energy density so that it can be stored
and transported efficiently.
Hydrogen storage technologies
Today, several technologies exist to increase the density of hydrogen and thus facilitate its
storage. The main ones are:
Let us look at the various processes in more detail.
Hydrogen storage by high-pressure compression
Compression is the most common solution for increasing hydrogen density. It consists of
increasing the pressure of the gas using compressors to very high values, even over 1000 bar.
To achieve these levels, multi-stage compressors are often used, i.e. with several compression
steps to reach the desired value.
In order to contain hydrogen compressed to these pressures, high-performance cylinders have
been developed, such as type 3 and type 4 cylinders, which are mainly made of carbon fibre
and guarantee high strength and lightness.
Liquefaction of hydrogen to increase its density
Another method to increase density is condensation, commonly called liquefaction, i.e. the
cooling of hydrogen to a liquid state. The process requires reaching extremely low
temperatures, i.e. -253°C (20°K), so it is much more complex than compression. Indeed,
liquefaction involves sophisticated equipment, high energy consumption and careful handling of
cryogenic systems.
However, liquid hydrogen has a higher volume density than compressed hydrogen, making this
technology advantageous in some specific applications, e.g. aerospace and long-distance
transport.
Chemical conversion of hydrogen to ammonia for transport
Another way to change the density of hydrogen is to carry out a chemical reaction to transform it
into a new compound. The most common process is the formation of ammonia (NH₃) through a
chemical reaction between nitrogen (N₂) and hydrogen (H₂):
N₂ + 3H₂ → 2NH₃
This transformation, carried out in dedicated plants with high temperatures and catalysts, results
in a compound that is easier to store and transport.
Once generated, ammonia can be easily transported to a liquid state in an aqueous solution at
room temperature, or in pure form at -33°C.
For final use, it is necessary to reconvert the ammonia into hydrogen, a process that in turn
requires energy, heat and catalysts.
Despite the toxicity of ammonia, which requires appropriate precautions, this route is very
promising due to the high density achieved, in some cases higher than that of pure liquid
hydrogen.
Hydrogen absorption in solid metal hydrides
Another innovative technology involves the absorption of hydrogen in a solid material, in
particular metal hydride. In this case, the hydrogen is absorbed into the metal’s crystal lattice,
without an actual chemical reaction. During absorption, the material heats up because the
reaction is exothermic and spontaneous at the right temperature and pressure.
With the discharge, the system cools down (endothermic reaction), thus self-inhibiting the
reaction.
This is a particularly safe procedure because it operates at low pressure and, if the container is
damaged, the reaction slows down on its own. This technology can be used to compress
hydrogen by supplying heat only.
The use of this solution on an industrial scale requires the use of cylinders containing several
small metal hydride disks, designed to facilitate gas extraction and heat exchange.
A current limitation is the overall mass. In fact, as much as 100 kg of hydrides are needed to
store 1 kg of hydrogen. However, achievable volumetric density values can be as high as 150
kg/m³.
Challenges in hydrogen storage
Each of the analysed hydrogen storage technologies has advantages and limitations that need
to be carefully evaluated according to the specific application. The main challenges include:
- Energy efficiency: minimising energy consumption in compression or liquefaction
processes. - Safety: handling hydrogen at high pressures or in liquid form, or in the form of potentially
toxic compounds. - Scalability and cost: make the technologies accessible on a large scale, even in different
industries. - Infrastructure compatibility: integrate storage solutions with hydrogen production,
transport and utilisation systems.
Simplifhy’s support for hydrogen storage
At Simplifhy, as an Italian solution provider specialised in the realisation of strategies and plants
for the decarbonisation of industry through hydrogen, we support companies and designers in
choosing the most suitable storage technology that takes into account all possible variables:
- space and transport requirements
- safety and regulatory constraints
- economic return and sustainability of the system.
Through feasibility studies, tailor-made design and partnerships with leading technology players,
we help companies integrate hydrogen into their processes in a safe, efficient and scalable way.
In particular, our Swiss partner GRZ Technologies offers a pioneering technology using metal
hydrides for hydrogen storage.
Contact us to find out how we can support you in the development of your hydrogen storage
unit.
