Hydrogen Storage Methods

 The current methods of storage being developed are:

• Compressed gas: H2 can be compressed in 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 reach 6,000 psi which must be periodically tested and inspected to ensure their safety.

• Liquid Hydrogen: When Hydrogen is liquefied, it is 845 times the density of the gas but at 20 degrees Kelvin (~253 degrees C), is very cold and requires almost thirty percent of its energy to cool and compress it enough to liquefy it. It must be stored in a super insulated tank and even then one to two percent of the Hydrogen must be bled off and evaporated every day to keep the rest cold. (Liquid Hydrogen and liquid Oxygen are the main fuels which power the Space Shuttle). Safety is a function of keeping it cold, and in the liquid state.

• Chemically Stored Hydrogen: A recent development has allowed Hydrogen to be stored as a pelletized sodium, potassium or lithium compounds. When released into water the individual pellets combine with the water to release H2 gas with a resulting recyclable solution of Sodium Hydroxide, Potassium Hydroxide, etc. 

• Metal Hydride: A newer method of storing Hydrogen is to trap it in a metal hydride, which is an inter-metallic compound that soaks up Hydrogen like a sponge. Hydrides require moderate pressure but are currently expensive, need to operate at high temperature to store a lot of Hydrogen, and are typically very heavy. One other drawback is they must have only very pure Hydrogen supplied or they will get contaminated and stop operating properly. From a safety standpoint, Hydrides are intrinsically safe, as the Hydrogen must be released from the Hydride before it can oxidize or burn rapidly.

• Cryogenic Activated Carbon: This technique is being improved rapidly but works best at 77 degrees Kelvin (~196 C). The best theoretical projections that fifty-four percent by weight is the upper limit for carbon storage.

• Glass Micro Spheres: These tiny hollow balls require very high pressure to force the Hydrogen inside the spheres. Once stored though, the pressure in the carrying tank can be low, adding to safety. They are inert, resistant to contamination and only require moderate heat to release the Hydrogen. Experiments are being done with crushing the spheres to release Hydrogen at a faster rate.

• Raw Iron Pellets: While not a true storage method, Hydrogen can be created when needed by using raw iron to separate the Hydrogen from Oxygen in the high tech process known as rust. The iron process is low-cost and up to four and one-half percent by weight effective. The only requirement is to recycle the iron and remove the Oxygen so that the iron can be reused. One drawback is the high weight requirement of the iron.

• Liquid Carrier Storage: Hydrogen can be stored in Methanol, liquid Methane, Ammonia, Toluene or other liquid forms from which it must be extracted before it can be utilized.

Of all these processes, the ones that are currently most practical are compressed gas, liquid, or hydride or chemical. Many of the others are promising for the future.