Hydrogen serves as a storage and transportation medium for energy. In general there are three different ways of storing hydrogen:
All of them have pros and cons which qualify them for different applications.
- storage in pressure tanks
- storage of liquid hydrogen
- storage via absorption
- Pressurized Hydrogen Storage
We talk of the storage of pressurised gas whenever a gas is stored under higher than normal pressure. The tank constructions for the storage of pressurized gas differ depending on the application and the required pressure levels. Usually stationary tanks have a lower pressure level and therefore are cheaper. The requirements for mobile applications, for example in a motor vehicle, are different, the space for tanks is limited and the storage has to be light. At present most carmakers prefer pressure tanks, the pressure level can be up to 70 MPa (700 bar). Pressure levels like these allow fuel cell vehicles to achieve ranges comparable to petrol cars even today.
Modern pressure tanks are made from composite materials (carbon-fibre or glass-fibre composite materials with a thin internal aluminium or polyethylene liner) and they are much lighter than steel cylinders.
If the storage of large amounts of hydrogen for a future energy economy becomes necessary, hydrogen can be stored within subterranean cavern storages. There it can be stored under a pressure of up to 5 MPa (50 bar). In France and in the USA this method is already in use. In Germany natural gas is stored in such caverns. They could be used for the storage of hydrogen in the future.
- Liquid Hydrogen
Liquified hydrogen storage
Hydrogen has the highest energy density referring to weight when it is liquefied before storing. Hydrogen is liquefied at -235°C.
The quality of the tanks for liquid gases at very low temperatures -the so-called cryo-tanks - is very high today. The losses resulting from the temperature rise of the liquid hydrogen in the tank (waste steam losses) can be kept very low. The storage of liquid hydrogen is especially suited for the use in transport applications, as the energy content of liquid hydrogen in relation to its weight is the highest possible. This is also the reason for using liquid hydrogen as fuel in space crafts. The favourable weight value is opposed to an unfavourable volume value, especially as the tank has to be properly insulated. In practice there are some disadvantages of liquid hydrogen compared to the pressure variant. Waste steam losses at longer times of vehicle standstill can not be completely avoided. This fuel is lost, a fact which doesn´t occur with the pressurised hydrogen.
Stationary liquid storage will only be used when hydrogen is either requested in liquid form, or has to be delivered by trucks because of limited space. It can be steamed into gaseous hydrogen at the filling station. In general the energy required to provide liquid hydrogen is a little bit higher than that required to provide pressurised hydrogen. However, delivery of hydrogen by a truck makes only sense at short distances. Alternatively it is possible to produce the hydrogen directly at the filling station or to supply it by pipeline.
- Other Means of Hydrogen Storage
Metal hydride storage
This storage technology uses certain metal alloys which are storing hydrogen like a sponge becoming saturated with water. The hydrogen is adsorbed by the metal thus building metal hydrides.
If a metal hydride is "filled" with hydrogen it emits heat. To regain the hydrogen heat must be supplied.
Referring to the volume metal hydride storage has a very high storage capacity. Unfortunately these storages are quite heavy and therefore they can actually not be used in mobile applications. In addition they are very expensive because of the high costs of materials.
With regard to handling and safety there are advantages in the use of metal hydride tanks. Almost all of them operate at normal pressures, there are no losses and they effect a cleaning of the hydrogen. Hydrogen is released by the supply of heat and therefore the hydrogen remains bonded in case the tank is damaged.
In submarines this type of storage is in commercial use today.
Beside pressure gas and liquid gas storage there are other methods for the storage of hydrogen as well. Materials with a large inner surface and a suitable pore size have the characteristics of depositing certain gases preferentially or to accelerate the process of chemical reactions. Therefore these materials are often used to clean gas and as a material for catalysers. Porous carbon but also zeolite have been well-known materials with appropriate features.
Only recently a new class of materials has been synthesised by a specific combination of organic and inorganic building blocks, the so-called coordination polymers which specifically allow influencing the inner surface and the porousness. Coordination polymers can achieve inner surfaces up to much more than 3000 m2/g. This is why they are ascribed a good potential for the storage of gas, especially carbon monoxide, methane or hydrogen, a fact which has also been partly observed. A reversible hydrogen storage capacity of about 10 percent by weight and a volume storage capacity of up to 50 g/litre at cryogenic temperatures and pressures up to 90 bars could be measured on the best materials (special so-called metallic-organic frameworks or MOFs) in the laboratory.
However, until today there are no existing working storage systems out of these materials that would be able to match the storage capacities of active porous carbon. The operational dynamic over a wide pressure and temperature range requires a complex storage - management (e.g. waste heat while refuelling) the development of which has only recently started.
By the standards of today the synthesis of new materials provides the opportunity to develop materials which are better adapted to hydrogen and inexpensive at the same time. Certainly it cannot be foreseen whether these efforts lead to a storage system which can meet the requirements of everyday and will outclass conventional possibilities as shown above.