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Introduction

Conclusions

Pushing the limits of current technology

The most traditional method of hydrogen storage, high pressure tanks remain the most studied, and until revolutionary advances are made in some of the newer methods of hydrogen storage, high pressure storage also remains the most practical option for storage. A key drawback of hydrogen gas is that it has an extremely low density. At ambient temperatures and pressures, hydrogen has an energy density of only 10.7kJ/L, compared to 31.6MJ/L for gasoline.

Thus, in order to improve the energy density of hydrogen to useful levels, pressurization to as high as 5,000 or 10,000psi is necessary. In contrast, natural gas is generally not stored at pressures above 3,600psi. Thus, traditional tanks for fossil fuel storage will not be sufficient for hydrogen storage. Particularly when the hydrogen needs to be stored in a vehicle, the material needed for the tank needs to be light and strong.

Carbon composite such as wound carbon fibers, with a yield strength of 1,900MPa, offer the lightest and strongest material properties, but they are often quite expensive. Steel, which offers fairly high yield strength (690MPa) become subject to hydrogen embrittlement, which significantly increases the risk of burst failure. Titanium and aluminum are much lighter but cannot offer the strength of carbon fiber tanks. One precaution that must be taken is the installation of a liner to prevent hydrogen diffusion through the carbon tank.

Even at 10,000psi, however, the energy density of hydrogen gas is only 4.4MJ/L, which means that a hydrogen fuel tank will need to be several times larger than a conventional gas tank, once greater efficiencies of hydrogen fuel cells are factored in. Thus there is a great deal of research into materials that can provide hydrogen energy densities higher than 4.4MJ/L. These methods include cryogenic storage, storage in hydrides, storage in nanostructures and storage in hydrocarbons.