Hydrogen Storage


Once hydrogen is generated, the question becomes: How do we store the hydrogen? Hydrogen can be stored in a variety of ways, each with specific advantages and disadvantages. The overall criteria for choosing a storage method should be safety and ease of use. Listed below are the different storage methods available today in addition to some techniques that are still in the research and development stage.


Metal Hydride Tanks
Metal hydrides are specific combinations of metallic alloys that act similar to a sponge soaking up water. Metal hydrides posses the unique ability to absorb hydrogen and release it later, either at room temperature or through heating of the tank. The total amount of hydrogen absorbed is generally 1% - 2% of the total weight of the tank. Some metal hydrides are capable of storing 5% - 7% of their own weight, but only when heated to temperatures of 2500 C or higher. The percentage of gas absorbed to volume of the metal is still relatively low, but hydrides offer a valuable solution to hydrogen storage.

Metal hydrides offer the advantages of safely delivering hydrogen at a constant pressure. The life of a metal hydride storage tank is directly related to the purity of the hydrogen it is storing. The alloys act as a sponge, which absorbs hydrogen, but it also absorbs any impurities introduced into the tank by the hydrogen. The result is the hydrogen released from the tank is extremely pure, but the tank's lifetime and ability to store hydrogen is reduced as the impurities are left behind and fill the spaces in the metal that the hydrogen once occupied.

Compressed Hydrogen
Hydrogen can be compressed into high-pressure tanks. This process requires energy to accomplish and the space that the compressed gas occupies is usually quite large resulting in a lower energy density when compared to a traditional gasoline tank. A hydrogen gas tank that contained a store of energy equivalent to a gasoline tank would be more than 3,000 times bigger than the gasoline tank.

Compressing or liquefying the gas is expensive. Hydrogen can be compressed into high-pressure tanks where each additional cubic foot compressed into the same space requires another atmosphere of pressure of 14.7 psi. High-pressure tanks achieve 6,000 psi, and therefore must be periodically tested and inspected to ensure their safety.

Liquid Hydrogen
Hydrogen does exist in a liquid state, but only at extremely cold temperatures. Liquid hydrogen typically has to be stored at 20o Kelvin or -253o C. The temperature requirements for liquid hydrogen storage necessitate expending energy to compress and chill the hydrogen into its liquid state. The cooling and compressing process requires energy, resulting in a net loss of about 30% of the energy that the liquid hydrogen is storing. The storage tanks are insulated, to preserve temperature, and reinforced to store the liquid hydrogen under pressure.

The margin of safety concerning liquid hydrogen storage is a function of maintaining tank integrity and preserving the Kelvin temperatures that liquid hydrogen requires. Combine the energy required for the process to get hydrogen into its liquid state and the tanks required to sustain the storage pressure and temperature and liquid hydrogen storage becomes very expensive comparative to other methods. Research in the field of liquid hydrogen storage centers around the development of composite tank materials, resulting in lighter, stronger tanks, and improved methods for liquefying hydrogen.

Chemically Stored Hydrogen
As the most abundant element in the universe, hydrogen is often found in numerous chemical compounds. Many of these compounds are utilized as a hydrogen storage method. The hydrogen is combined in a chemical reaction that creates a stable compound containing the hydrogen. A second reaction occurs that releases the hydrogen, which is collected and utilized by a fuel cell. The exact reaction employed varies from storage compound to storage compound.

Some examples of various techniques include ammonia cracking, partial oxidation, methanol cracking, etc? These methods eliminate the need for a storage unit for the hydrogen produced, where the hydrogen is produced on demand.

Carbon nanotubes
Carbon nanotubes are microscopic tubes of carbon, two nanometers (billionths of a meter) across, that store hydrogen in microscopic pores on the tubes and within the tube structures. Similar to metal hydrides in their mechanism for storing and releasing hydrogen, the advantage of carbon nanotubes is the amount of hydrogen they are able to store. Carbon nanotubes are capable of storing anywhere from 4.2% - to 65% of their own weight in hydrogen.

The US Department of Energy has stated that carbon materials need to have a storage capacity of 6.5% of their own body weight to be practical for transportation uses. Carbon nanotubes and their hydrogen storage capacity are still in the research and development stage. Research on this promising technology has focused on the areas of improving manufacturing techniques and reducing costs as carbon nanotubes move towards commercialization.

Glass Microspheres
Tiny hollow glass spheres can be used to safely store hydrogen. The glass spheres are warmed, increasing the permeability of their walls, and filled by being immersed in high-pressure hydrogen gas. The spheres are then cooled, locking the hydrogen inside of the glass balls. A subsequent increase in temperature will release the hydrogen trapped in the spheres.

Microspheres have the potential to be very safe, resist contamination, and contain hydrogen at a low pressure increasing the margin of safety.

Liquid Carrier Storage
This is the technical term for the hydrogen being stored in the fossil fuels that are common in today's society. Whenever gasoline, natural gas methanol, etc.. is utilized as the source for hydrogen, the fossil fuel requires reforming. The reforming process removes the hydrogen from the original fossil fuel. The reformed hydrogen is then cleaned of excess carbon monoxide, which can poison certain types of fuel cells, and utilized by the fuel cell.

Reformers are currently in the beta stage of their testing with many companies having operating prototypes in the field.