Hydrogen Storage Systems

Hydrogen can be stored as a gas or a liquid, or even as a solid in the form of a "hydride" – a metal or metal alloy that has the capacity to absorb hydrogen at a certain pressure and temperature.

For hydrogen vehicle applications, hydrogen is usually stored as a gas, because this is the technologically simplest storage method.  The most prevalent forms of hydrogen production – steam reforming of methane (natural gas) and electrolysis – both release hydrogen in gaseous form.  The hydrogen gas can then be stored at room temperature by pumping it into a pressurized tank.  However, gaseous hydrogen storage has one key disadvantage – the tanks required are very large and heavy.  As the lightest of all elements, hydrogen has a density of only 0.07 grams per cubic centimeter, less than 1/10th the density of gasoline.  Therefore, for a hydrogen-fueled vehicle to achieve the same operating range as a gasoline-fueled vehicle, with the same size fuel tanks, the hydrogen must be compressed to at least 5,000 psi.  Storage of hydrogen at this pressure requires either metal tanks, which are very heavy, or composite tanks, which are very expensive.  Specialized transfer devices and plumbing are also required to transfer high-pressure hydrogen from stationary tanks to vehicles, and from vehicle tanks to the hydrogen-burning engine or fuel cell.  Future advances in storage tank technology will enable hydrogen storage at pressures of 10,000 psi or above.  These advances will enable increases in vehicle operating range and reduce the size of tanks required to store a given amount of hydrogen at a refueling facility.  However, the weight and cost of high-pressure tanks will continue to be an issue for the foreseeable future.

Storage of hydrogen as a liquid avoids the issues of high-pressure gas storage, but requires the hydrogen to be super-cooled to below its boiling point – minus 423 degrees F (-252.8° C).  Liquefaction systems for cooling gaseous hydrogen to liquid form are expensive and energy intensive.  Energy equaling 30-40% of that in the fuel is presently required for liquefaction.  Storage of the liquid hydrogen then requires specially insulated tanks to prevent the hydrogen from boiling off.  Even in well-insulated tanks, hydrogen can be expected to boil off at a rate of 1.7% per day.  The combination of the energy losses from liquefaction and the storage losses from boil-off make liquid hydrogen impractical for most vehicle applications today.  However, future improvements in the efficiency of liquefaction and in tank insulation technology may make liquid hydrogen a more attractive option in a few years.  The main advantage of liquid hydrogen storage is that much larger quantities of hydrogen can be stored in tanks of a given size, as compared with storage of hydrogen as a compressed gas.

Metal hydrides show promise for eliminating many of the issues relating to gaseous and liquid hydrogen storage, but the technology is in its early stages and has not yet been demonstrated in a form that is practical for widespread vehicle application.  A metal hydride is formed when gaseous H2 molecules dissociate into individual hydrogen atoms and bond with metal atoms in the storage alloy.  Removing heat drives this absorption process, while adding heat reverses the chemical reaction, causing the hydrogen atoms to reform as H2 molecules inside the storage vessel.  Two key issues with hydrides are the high weight of the metal hydride compared to the amount of hydrogen stored, and the high temperatures required to release usable hydrogen gas from the hydride.  For example, NaAlH4 – even though it is one of the most promising of the hydrides – contains only 4% hydrogen by weight and has a release temperature of 150° C. 

TransPower offers expertise and access to the leading suppliers of hydrogen storage solutions, including manufacturers of compressed gas and liquid hydrogen storage systems and leading developers of metal hydride storage technology.  TransPower can help hydrogen vehicle operators determine which storage technology best meets their needs, and can assist hydrogen storage suppliers in identifying potential vehicle customers.