Methods of hydrogen storage for subsequent use span many approaches, including high pressures, cryogenics, and chemical compounds that reversibly release H2 upon heating. Underground hydrogen storage is useful to provide grid energy storage for intermittent energy sources, like wind power, as well as providing fuel for transportation, particularly for ships and airplanes.
Most research into hydrogen storage is focused on storing hydrogen as a lightweight, compact energy carrier for mobile applications.
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 by design for liquid hydrogen tanks is adding costs for this method. Liquid hydrogen has less 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.
Compressed hydrogen, in comparison, is quite different to store. 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 hydrogen tank). Compressed hydrogen will require 2.1% of the energy content to power the compressor. Higher compression without energy recovery will mean more energy lost to the compression step. Compressed hydrogen storage can exhibit very low permeation.
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