The most important hydrogen storage methods

Hydrogen has a pivotal part to play in the world of renewable energy. The reason? This small molecule has a high energy density, making it the perfect energy source of the future. But what's the best way to store and transport this volatile, highly explosive gas? In this blog post, we'll be taking a look at the most important storage methods and explosion protection challenges.

When it comes to energy density, no other fuel is a match for hydrogen: While one kilo of heating oil contains 9.8 to 11.4 kWh of energy, one kilogram of hydrogen boasts an impressive 33.3 kWh. But at normal temperatures and air pressure, the energy of hydrogen is distributed over quite a large volume. As a result, one litre of hydrogen offers just 3 Wh of energy. This means that there isn't one perfect storage system for hydrogen, but rather a vast array of options, each with its own benefits and drawbacks.

Compressed gas storage is by far the most well known method of storing hydrogen. It involves compressing gas in special tanks under high pressure. This takes place at pressures of up to 700 bar or even higher. The advantage of this method is that the technology is common and relatively straightforward. It is mainly used in the automotive industry for fuel cell vehicles. Underground storage caverns, for example salt domes, are ideal for storing large volumes of this energy source. However, a lot of energy is needed to compress the hydrogen and there is a risk of losing some of the hydrogen through diffusion.

Another approach is liquefied gas storage, where the hydrogen is cooled down to -253 °C and liquefied. This method allows for a much higher energy density, meaning that the hydrogen takes up less space during storage and transportation. Liquefied hydrogen is used in the aerospace industry in particular, as well as in heavy industry. But liquefaction is energy-intensive – it uses up around one third of the stored energy – and well-insulated cryogenic tanks are needed for safe storage. These also have to be continuously cooled in order to reduce the boil-off effect, i.e. the evaporation of hydrogen.

Chemical storage makes it possible to bind hydrogen in another chemical compound and then release it again as required. Important examples include ammonia and methanol, which have a high energy density and are easier to transport and store than pure hydrogen. This method offers greater safety and could be particularly useful for transport over long distances. Other compounds like formic acid and hydrazine are less important. One thing they all have in common is that the conversion process always leads to energy loss.

Liquid organic hydrogen carriers (LOHCs) like dibenzyltoluene are a promising new chemical storage option. This technology makes it possible to store and transport hydrogen safely and efficiently in normal ambient conditions by chemically binding the gas to a liquid. LOHCs use existing infrastructure and enable a high storage density which is five times that of compressed storage. In spite of the benefits, like the use of thermal energy from the hydration and dehydration process, there are certain challenges that have to be considered, such as the overall efficiency and the additional mass due to the carrier substances. LOHCs represent a good option for storing and transporting hydrogen, especially over short distances.

Metal hydride storage uses special alloys which can absorb hydrogen like a sponge. This technology makes it possible to store the gas safely and with a relatively high density without the need for high pressure or extremely low temperatures. This method is already being used on submarines. However, it has certain limitations due to the weight of the metal hydrides and the slow reaction times when releasing the hydrogen.

Adsorption storage is another option, involving binding hydrogen to the surface of materials like activated carbon or zeolites. This method is particularly effective at low temperatures and offers a safe storage option. However, it is still in the very early stages of development and will require further research to improve efficiency and capacity.

All of the aforementioned storage technologies have their own unique benefits and drawbacks. However, one thing they all have in common is the question of how to protect people, systems and the environment from risks presented by hydrogen. Hydrogen stands out from other combustible gases due to its very wide explosion range: Mixtures containing between 4 vol% and 77 vol% are explosive – and in hydrogen explosions, the flames spread very quickly. What's more, minimal energy is needed to ignite a mixture of hydrogen and air: At 0.02 mJ, the ignition energy is lower than that of methane by a factor of 15. This is another reason why hydrogen is classified in the most hazardous ignition group (IIC). The fact that explosions involving hydrogen are relatively rare is in large part due to the considerable experience of using the gas that has been gained in the (petro)chemical industry. However, compliance with the relevant safety standards in explosion protection is an essential prerequisite, including IEC 60079 and IEC 80079, as well as the specific requirements for the construction and operation of electrolysis systems described in ISO 22734.

Alongside the system's leak-tightness (primary explosion protection), preventing ignition sources (secondary explosion protection) plays an important role. The zone classification resulting from the risk assessment is crucial here, with Zone 0 referring to areas in which explosive atmospheres are present for prolonged periods or occur frequently, while in Zone 2 during normal operation they occur only rarely and only for brief periods. Due to hydrogen's explosive characteristics, it is categorised in the highest gas group (IIC) and the maximum permissible surface temperature is only 450 °C. The technology used in hazardous areas therefore must have a safe design.