Hydrogen storage describes the methodologies for storing H2 for subsequent use. The methodologies span many approaches, including high pressures and cryogenics, but usually focus on chemical compounds that reversibly release H2 upon heating. Hydrogen storage is a topical goal in the development of a hydrogen economy. Most research into hydrogen storage is focused on storing hydrogen in a lightweight, compact manner for mobile applications.
Some attention has been given to the role of underground hydrogen storage to provide grid energy storage for unpredictable energy sources, like wind power.
Hydrocarbons are stored extensively at the point of use, be it in the gasoline tanks of automobiles or propane tanks hung on the side of barbecue grills. Hydrogen, in comparison, is quite difficult to store or transport with current technology. Hydrogen gas has good energy density by weight, but poor energy density by volume versus hydrocarbons, hence it requires a larger tank to store. A large hydrogen tank will be heavier than the small hydrocarbon tank used to store the same amount of energy, all other factors remaining equal. Increasing gas pressure would improve the energy density by volume, making for smaller, but not lighter container tanks (see pressure vessel). Compressed hydrogen will require energy to power the compressor. Higher compression will mean more energy lost to the compression step.
Alternatively, higher volumetric energy density liquid hydrogen or slush hydrogen may be used (as in the Space Shuttle). However liquid hydrogen requires cryogenic storage and boils around 20.268 K (–252.882 °C or -423.188 °F). Hence, its liquefaction imposes a large energy loss (as energy is needed to cool it down to that temperature). The tanks must also be well insulated to prevent boil off. Insulation for liquid hydrogen tanks is usually expensive and delicate. Assuming all of that is solvable, the density problem remains. Liquid hydrogen has worse energy density by volume than hydrocarbon fuels such as gasoline by approximately a factor of four. This highlights the density problem for pure hydrogen: there is actually about 64% more hydrogen in a liter of gasoline (116 grams hydrogen) than there is in a liter of pure liquid hydrogen (71 grams hydrogen). The carbon in the gasoline also contributes to the energy of combustion.