Hydrogen (H2) is the most abundant chemical element in the universe. That’s great, because hydrogen is the fuel of choice for successfully finalizing the energy transition. As a fuel, it offers the promise of a low-emission and sustainable power supply – from production to application. One kilogram of hydrogen contains almost as much energy as three kilograms of gasoline.1 It is far more efficient as a fuel, and its conversion into electricity comes without environmentally harmful emissions. These are clear advantages over fossil fuels such as gasoline and diesel. Less great: Hydrogen is a colorless and odorless gas, making it highly volatile. Storing hydrogen therefore poses major challenges for industry and consumers. However, hydrogen storage, like the associated fuel cell technology with hydrogen fuel cells and direct methanol fuel cells, is a core element on the road to a hydrogen economy.
The EU wants to reduce greenhouse gas emissions by 55 percent by 2030.2 That is an ambitious target. It can only be achieved if power supply is rethought. New means, above all, climate-neutral. Hydrogen produced with electricity from renewable sources – so-called green hydrogen – such as wind, water and solar energy offers this possibility. The main advantage is its storage capability. It can be transported to where it is needed: at the user’s site. Hydrogen storage includes injection, storage and withdrawal. In its function as an energy carrier of the future, H2 is comparatively easy to store. Various methods and processes are used. Hydrogen can be stored in liquid form, in gaseous form in a pressurized gas storage or pressure storage system, or as a deposit in metals at the molecular level.3 The liquid form of hydrogen storage is widely used, especially for vehicles. H2 has the highest storage density when it is liquefied. This occurs at a temperature of -253°C.4
After being liquefied, the next step in hydrogen storage is for the gas to go into a so-called cryotank. It has the best insulation properties so that the temperature can be kept as constant as possible. If it rises, the gas evaporates. In this context, experts refer to hydrogen storage as evaporation losses. As mentioned, this form of hydrogen storage is popular in the automotive sector. The energy content of liquid hydrogen in relation to its weight has proven to be particularly advantageous. This is why liquid hydrogen is used in space travel as a rocket fuel.5 However, the weight advantage is also offset by a disadvantage. When hydrogen-powered vehicles are stationary for long periods, the aforementioned evaporation losses cannot always be avoided. If liquid H2 is required, the industry resorts to stationary hydrogen storage solutions for cryogenic hydrogen. This is also the case when delivery by truck is preferred for space reasons. At a filling station, the liquid hydrogen can be converted back into gaseous hydrogen. As compressed hydrogen, delivery by truck only makes sense if only short distances have to be covered.
And what is compressed hydrogen? This is another form of hydrogen storage. It is quite simple. We talk about pressurized storage whenever gas is stored at a higher pressure than usual. The hydrogen pressure vessel used for hydrogen storage differs according to the intended use and the pressure level. For example, it makes a difference whether the tank is used stationary or mobile. Installed in a car or truck, it should take up as little space as possible, be lightweight, but still provide full power. In the case of hydrogen storage, this is a particularly big challenge. Automakers prefer pressure tanks made of composite materials such as aluminum or polyethylene for hydrogen storage. The thin material must withstand pressure levels of up to 70 MPa (700 bar)6. Since the late 1990s, scientists have been working on solid hydrogen storage systems that can store even more hydrogen in less space.7
In addition to the common methods of hydrogen storage, there are other alternatives. For example, some metal alloys can be used. The process for hydrogen storage is similar to that of a sponge absorbing water. Metal hydride stores are basically chemical compounds that store hydrogen in a metal lattice. H2 is adsorbed by the metal, forming metal hydrides. These have the property of releasing heat when filled during hydrogen storage. If the hydrogen is now needed, users have to go in the opposite direction. By adding heat, the storage unit releases H2 again.8 The advantages of this type of hydrogen storage are the high storage volume and the low evaporation losses. A disadvantage is the high weight. Metal hydride storage systems are therefore unsuitable for mobile applications in vehicles. In addition, they are comparatively expensive due to the high material costs. Typical applications for this type of hydrogen storage are submarines. In 2002, the first submarine with a diesel-electric drive and fuel cell on board was launched.9
This by no means exhausts the possibilities. Even hydrogen storage in oil is possible. This involves liquid organic hydrogen carriers (LOHC), which enable hydrogen storage. Catalysts are also required to bind the hydrogen to the oil or to release it again. Just as with the other hydrogen storage processes, there are advantages and disadvantages. The storage density of the LOHC-based storage process is about five times higher than that of pressurized tanks. Usually, to store one kilogram of hydrogen, it takes a bottle as big as a person. LOHC technology is more efficient in this area. It only requires a portable 20-liter canister for hydrogen storage.10 However, the high energy consumption is problematic when the stored hydrogen has to be released again. Scientists are currently working on making the technology more efficient. Regardless of which method of hydrogen storage ultimately proves to be the most efficient, in combination with hydrogen fuel cells from SFC Energy as a decentralized and powerful conversion technology, it forms a powerful duo for an environmentally friendly energy supply.
8 Electrochemical storages; Peter Kurzweil, Otto K. Dietlmeier, Springer Vieweg, Springer Fachmedien Wiesbaden GmbH, S. 490.