Metal hydrides are one of several ways to store hydrogen. How do metal hydride storage systems work? How effective are metal hydride storage systems? This, and for which technologies it plays a role, is explained in the following article.
What is a metal hydride storage? Hydrogen storage in metal hydrides is possible because some metals and metal alloys are capable of storing gaseous hydrogen. In this process, the H atoms – i.e. hydrogen in dissolved form – are deposited in so-called "interstitial sites.[1] Metal and gas form a compound, the result is a metal hydride. While heat is released during hydrogen absorption, heat must be added for release and, in addition, the pressure must be reduced. Chemical compounds such as lithium hydride and lithium aluminum hydride as well as sodium borohydride and ammine borane are currently considered to be of interest.[2]
Such hydride storage systems are considered promising candidates for hydrogen storage, which could play an important role in fuel cell technology. Thus, hydrogen fuel cells or direct methanol fuel cells in mobile applications such as laptops or in automobiles could become more significant as energy carriers in the future.[2]
The metal hydride storage function is explained quite simply: hydrogen can be stored in the tanks due to the chemical reaction between metal and gas. The hydrogen is chemically bonded, or absorbed by the metal, without the need for compression. Instead, the solid metal hydride functions like a sponge that absorbs and releases the gas. If you want to recover the hydrogen from the metal hydride, you have to add heat.[5]
Due to various properties, the storage of hydrogen proves to be comparatively difficult; conventional gas tanks, for example, are unsuitable. Hydrogen can only be liquefied with appropriate cooling, and it also reacts violently with oxygen and chlorine.[4]
Other reasons are the low enthalpy of combustion (compared to other hydrocarbons) and the associated low volume-related energy density. Since this is only about 30 percent of the energy density of natural gas, hydrogen storage of the same amounts of energy requires a tank three times as large or a pressure three times as high as for the storage of natural gas.[4] The storage of hydrogen in metal hydrides shows a comparatively high efficiency and is therefore treated as an interesting technology and alternative for the introduction of fuel cell vehicles.[7] Thus, metal hydride storage systems are characterized by a high storage density per volume and a low storage and filling pressure. In addition, the waste heat can be used for system cooling.[3]
What does metal hydride storage cost? Metal hydride accumulators bring numerous advantages when it comes to handling and safety. In addition to working at normal pressure, for example, there are no evaporation losses. In addition, the hydrogen is only released when heat is added, which means that it remains bound even if the tank is damaged. Metal hydride storage tanks are already being used commercially, for example in German submarines.[6]
In terms of volume, the storage capacity of metal hydride storage systems is therefore very good. While the stationary use of metal hydrides therefore holds a lot of potential according to experts, they are less suitable for mobile applications. This is mainly due to the fact that they are filled with the metal or alloy, which makes the containers weigh significantly more than when empty. This makes the storage units comparatively heavy and also very expensive due to the high material costs. Anyone who wants to buy a metal hydride storage unit for industrial applications has to dig deep into their pockets.[5] Prices can of course vary depending on the metal hydride storage manufacturer.
How long do metal hydride storage systems last? Metal hydrides for hydrogen storage are considered extremely safe and durable. Since the hydrogen is only released when heat is added, it remains bound even if the container is damaged. Another advantage over compressed gas and liquid hydrogen storage systems is that the hydrogen is chemically bound in the metal hydride and cannot escape explosively. In addition, the storage of hydrogen in metal hydrides allows mostly loss-free storage and very long-lasting storage.[8]
Storage technology using metal hydrides is a promising alternative to conventional storage methods and is already being used on a trial basis in some applications. In some cases, however, there is still a considerable need for development of the storage medium. One of the main problems for vehicle construction is the fact that the metal hydride storage systems currently available are very heavy because they have a low mass-specific storage density. For example, a metal hydride tank designed to replace a normal automotive tank would weigh more than 250 kilograms.[8]
Metal hydride storage systems represent a promising hydrogen storage option for industry. The biggest disadvantages of this storage option are still its comparatively heavy weight and high price. The extent to which metal hydride storage systems for hydrogen will also be used for demanding applications such as cars in the future depends on further research and development of the storage medium.
[1] https://www.physik.uni-augsburg.de/exp1/lehre/Mawi-Praktikum/Anleitungen/zusfass_wasserstoff.pdf
[2] https://www.chemie-schule.de/KnowHow/Metallhydridspeicher
[3] http://www.h2hh.de/downloads/HERA.pdf
[4] https://en.wikipedia.org/wiki/Hydrogen_storage
[5] https://www.produktion.de/technik/wasserstoff-im-metallspeicher-232.html
[7] https://www.hzwei.info/blog/2017/08/09/metallhydridspeicher-und-reversible-hochtemperaturzelle/
[8] https://www.hereon.de/institutes/hydrogen_technology/materials_design/glossary/index.php.de
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