Today, hydrogen can be stored in a variety of forms. Table 1. shows the hydrogen storage methods and hydrogen capacity, energy capacity and application areas of these methods. The data in the table are experimentally calculated maximum values.
One of the most important storage methods of hydrogen is the hydrogen storage in compressed gas, storage in liquid form and storage in metal hydrides. Underground hydrogen storage is another form of compressed gas storage. Each storage method has its own advantages and disadvantages. For example, in the case of hydrogen storage in the liquid state, hydrogen has the highest storage density in liquid form compared to other storage methods, but also requires isolated storage containers and a liquefaction process which requires energy.
Storage of compressed gas hydrogen
The simplest and most widely used method for the present time is a compressor and a pressure tank, the only equipment required to store hydrogen in a compressed gas state (Amos, 1998).
This method is the most cost-effective, most convenient and suitable for short-term applications among all the above hydrogen storage methods (Carpetis, 1985). The main problem in the storage of compressed gaseous hydrogen is the low storage density which is related to storage pressure.
High storage pressures require high investment and operating costs. The storage of gas hydrogen in small quantities and pressurized tanks can be very easy and cost- effective, but as the amount of hydrogen to be stored increases, so does the cost.
Thus this method is not economical for large amounts of hydrogen.
Underground storage of gas hydrogen
Underground storage of hydrogen is another form of compressed gas storage. Underground storage of hydrogen is the lowest cost storage method for the large quantities of hydrogen. Hydrogen can be stored in natural or unnatural caves. Underground gas depots require minimum investment costs.
The disadvantage of this method is that the stored pressurized hydrogen is lost by 5% by volume. One of the cost increasing reasons in underground storage is the gas that occurs when the storage system is at the end of the discharge cycle. Disposal of this gas requires an additional cost. There are 3 different formations to store the pressurized hydrogen gas in the underground.
- Emptied oil / gas wells,
- Pit rock caves,
- Great salt caves.
The features that should be available in any underground storage area are:
– Water permeable structure below the surface (150-900 m) porous layer, usually sand or sandstone,
– Airtight rock head with sufficient thickness,
– Suitable dome-shaped geological structure.
Before the storage of hydrogen gases in underground, cavities are created on layers and surfaces are covered with cement or similar chemical materials. Then the hydrogen is injected into the cavities by the compressor.
Storage of hydrogen in metal hydride
Metal hydrides are formed by absorption capacity of hydrogen with metals. Absorption and release of hydrogen by a metal depends on a number of parameters.
The main parameters are:
- Pressure of hydrogen,
- The temperature of the metal,
- Flow rate of hydrogen.
By this method, hydrogen can be stored with chemically bonding to metals, metalloid elements or alloys. Metal hydrides are composed of metal atoms that have lattice structure and hydrogen atoms held in intermediate places within this lattice structure. Metal and hydrogen can generally combine two different forms. One of these forms is suitable for hydrogen storage and the other is a fully filled form;
During the filling phase, hydrogen is spread through the full surface to create a suitable form for storage.
During the unloading process, hydrogen spreads out to create a storage state and forms H molecules.
Metal hydride storage systems are the safest one for hydrogen storage that can be stored under pressure of 3 to 6 MPa. Suitable metal alloys have empty spaces in their cages where hydrogen atoms can settle. Most of metal alloys can store hydrogen very safely that can be chemically converted while forming metal hydride. Hydrides store both of hydrogen and heat with the combination of chemical bonding, reaction heat of hydrogen and metals which can be used in both stationary and mobile technical process by means of heat-hydrogen combination.
Metal hydrides store hydrogen as a decomposable chemical compound. There are many elements, metals and alloys that can react with gas hydrogen to form metal hydride. Such reactions can be seen in Figure 1 as below;
During storage, the heat is released that must be removed to maintain the reaction continuity. During the hydrogen release process, the heat must be supplied to the storage tank.
Advantage of the storage of hydrogen in the hydrating agents is the safety aspect. Serious damage to a hydride tank (such as a collision) does not pose a fire hazard as the hydrogen remains in the metal structure.
Liquid hydrogen storage
Favorable characteristics of liquid hydrogen include high heating value per unit mass and large cooling capacity due to its high specific heat.
The liquefaction temperature of hydrogen at atmospheric pressure is a relatively low temperature, such as -253 °C.
Therefore, the liquefaction process would require approximately 30% of the ignition energy of hydrogen that must be in the form of electrical energy.
Evaporation losses are the main problem in the production and storage of liquid hydrogen. Total losses from liquefaction unit to final use are between 30-70%.
Liquid hydrogen can be stored in both large and small capacities. If it is stored in the larger tank, losses will be at a lower level. However, liquid hydrogen cannot be stored in cylindrical tanks where natural gas is stored.
This is due to high evaporation losses. Heat leakage losses are generally proportional to the ratio of the surface area to the volume of storage vessel. Therefore, the most suitable shape is spherical because the surface / volume ratio is the lowest.
The storage of liquid hydrogen is the most economical method for large amounts of gas and long-term storage.
The tanks where liquid hydrogen is stored are vacuum insulated between the inner and outer walls, as it can be seen in Figure 2.