What is the best approach to storing large quantities of gaseous hydrogen in areas where storage underground or in salt dome formations is not an option? Recently, a large, multinational utility in Europe received five above-ground tanks capable of storing 2.7 metric tons of hydrogen. This calls into question the assumption that in areas without underground storage fields or salt dome formations, the hydrogen will be stored in the pipeline itself. If storing large quantities of gaseous hydrogen in above-ground pressurized storage tanks is desirable, would safety issues make such a scheme untenable? Could big tanks be placed near each other by the dozen, or is that inviting disaster? Is it logical to assume that the storage should be underground? The large tanks seem like they might be a less expensive, higher density (kg/acre) option than large diameter pipes underground. But if they have to be spaced a great distance apart with blast walls between them, then maybe not.

It is possible to store large quantities of gaseous hydrogen above ground, but it will likely require a large footprint due to its relatively low density even at high pressure. Also, if the quantity equals or exceeds 10,000 lb., the facility will need to comply with OSHA 1910.119 process safety management requirements if located in the US. Similar regulations exist in Europe and Asia that increase the regulatory requirements as storage exceeds about 5 tons. Codes such as NFPA 2 aren’t intended to provide full guidance for large facilities and  systematic hazard and risk analysis should be completed to ensure safety. A large storage facility would have to be sited and permitted using methods and risk analysis as typically would be done for a large plant. The technology for gaseous storage is fairly well established and well proven from previous and smaller installations. The same types of tanks, from fully metallic to fully composite construction, would be selected and used based on the operating parameters and economics. Individual tanks may be larger, but large amounts of non-cavern storage are likely to consist of large arrays of multiple vessels due to manufacturing and transportation limitations. The key aspects for large storage systems are as follows: 

1. Materials of construction:  Material science for hydrogen pressure vessels is well established and would mirror smaller storage systems. 
2. Fatigue:  Gaseous storage systems are likely to cycle deeply to increase utilization of the high capital cost of vessels. Deep cycles will typically lead to relatively short intervals between inspections based on fracture mechanics. 
3. A Mechanical Integrity program:  Given the large amount of stored energy, a mechanical integrity program will be very important. The cost of inspections could be high based on quantity and size of vessels. 
4. Minimization of impingement: Piping should be designed such that leaks don’t impinge on a neighboring tank. Impingement fires offer high risk of failure. Fire barriers or intumescent paint could be used at the base for protection. 
5. Relief systems:  Design of relief systems from a fire could be challenging, as well as designing a safe vent system and stack for what are likely to be very large relief devices. 
6. Permitting:  Large quantities of hydrogen need to meet additional regulatory requirements and height of vessels can lead to additional review depending on location. 
7. Foundations:  These will be large and expensive due to weight, height, and seismic considerations. 
8. Pressure: Higher pressure means lower volume required, smaller and/or fewer vessels, and smaller plot space. Lower pressure requires less compression equipment and less compression energy requires less compression.
9. Installation: One overlooked aspect of storage is the cost to transport and then install the vessels including freight, concrete, rigging, and piping. These costs must be added to the capital cost of the vessels. Storage vessels are expensive to ship   due to size and weight.