Hydrogen gas contains a lot of energy per pound, but like all gases it is difficult to compress. In order to get enough hydrogen fuel on a vehicle to operate for several hundred miles or more between fill ups, it must be stored at very high pressures—typically between 5,000 and 10,000 pounds per square inch (psi). Gaseous hydrogen fuel with the same amount of energy as one gallon of diesel fuel would only weigh about one third as much, but would occupy almost seven times the volume if stored at 10,000 psi.8 If stored at only 3,000 psi, the hydrogen would occupy almost seventeen times the volume of the diesel fuel.
The use of high-pressure storage does introduce some potential hazards due to the large amount of mechanical energy in the compressed gas. However, the high-pressure storage tanks used to hold compressed hydrogen are designed with a high margin of safety, and designs are verified with extensive qualification testing. See Figure 14 for photos of the types of tests required for certification of a high-pressure storage tank design. High-pressure storage tanks are also protected from excessive pressure build-up inside the tank using pressure relief devices (PRD) and/or temperature relief devices (TRD). These devices act to vent hydrogen to relieve pressure in the tank if it gets so high that there will be danger of a rupture.
High-pressure storage tanks must be protected from abrasion or damage by road debris, and both the tanks and lines should be adequately protected from vibration to minimize the possibility of leaks.
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