Underground storage tanks can be either installed in a vault or directly buried. Both offer additional
protection from external impact and fire, but each has unique challenges. Vaults must be properly
ventilated and designed to not create an explosion or asphyxiation risk. Direct burial vessels should not
have any underground leak points and must be protected from corrosion. Both types of installation must
have hydrogen vents routed to a safe location above grade.

By definition, liquid hydrogen can BLEVE, but this is highly unlikely. Liquid hydrogen is stored in a double wall tank with vacuum insulation. This protects the primary pressure vessel from direct impingement and the very cold liquid provides self-cooling of the vessel walls. Tanks are also equipped with redundant pressure relief systems that are sized for fire exposure.

Liquid hydrogen is much less likely to pool than liquified natural gas (LNG) due to its low heat of vaporization. Very large facilities are often equipped with methods to enhance vaporization, such as crushed stone under tanks, as well as diversion systems to allow liquid hydrogen to spill and boil off in a safe area. Care needs to be taken that diversion systems do not create a hazardous situation by reducing ventilation.

Composite cylinders can be manufactured to standards written by CSA, ASME, and ISO depending on the application and local requirements. Several ISO standards can serve as the basis for composite cylinder approvals within North America.

Leakage/loss depends on the vessel design. Metallic or metallic lined vessels have extremely low permeability and losses through the vessel walls are typically imperceptible. Conversely, Type IV composite vessels which have non-metallic liners are subject to permeation. They are required to meet maximum permeation rates as part of their certification. Fugitive emissions from piping systems can also be considered and is dependent on the specific design and level of maintenance.

Store flammable gas cylinders such as hydrogen, separated from oxidizing (e.g. oxygen), toxic, pyrophoric, corrosive, and reactive Class 2, 3, or 4 gases. Non-reactive gases, such as helium, may be co-located. See codes and standards such as NFPA 2 [7.2.1.1 Incompatible Materials] for further guidance.

Regarding the concept of introducing hydrogen gas into natural gas pipelines, this is indeed a hot topic and there are recent quantitative treatments of fatigue crack growth driven by pressure cycling and potentially accelerated by hydrogen.  Some analysis has shown that it can be acceptable to operate natural gas pipelines with a hydrogen blend.  However, this is highly dependent upon the pressure and wall stress.  Whereas low pressure distribution lines with low wall stress are more amenable, the higher pressures and wall stresses of major transportation pipelines may not be.  Deep cyclic stresses that might occur from using pipelines as storage  may also create additional issues since hydrogen can accelerate fatigue crack growth, especially for systems where both hydrogen and deep cycles were not anticipated in the original design.  Individual pipelines likely need to be evaluated based on their design and there is likely to be no single answer for this question of pipeline storage. 

    
For industrial propane tanks, the Panel needs more information about structural materials and their properties (including welds). All propane tanks are not built from the same materials or with the same construction techniques, so these tanks likely need to be evaluated on a case-by-case basis. Propane tanks are often built with techniques that leave potential internal features that are susceptible to the initiation of crack growth. In addition, and depending on the application, it’s likely that the cyclic pressure service will be significantly different for a tank in hydrogen service, particularly one that might be cycled deeply on a daily (or more frequent) basis. From the perspective of the low-pressure storage options, while it is tempting to repurpose old LPG vessels for hydrogen service, the Panel cautions against it due to the potential for hydrogen embrittlement in the steel/weld material. Also, a challenge with newer propane tank designs is that they are moving to thinner walls and higher strength steel, which is notably less resistant to embrittlement than older vessels, so counterintuitively, newer tanks might be less amenable to hydrogen than older tanks.   

Another concern for vessels in hydrogen service is the amount of non-circularity, or “peaking” of the longitudinal welds.  This is not as much of a concern for propane so is likely not to be part of a standard design specification. This manufacturing issue can have a severe effect on cyclic life and is well understood when designing and building high cyclic hydrogen vessels, but not propane. Besides the issue described above for the tank itself, there are similar potential embrittlement issues for PRVs, instrumentation, and other accessories on these tanks. Many incidents have been due to component failures in these accessories. 
In conclusion, propane vessels were not designed for H2 cyclic service. Almost by definition, the reason people want to use them is for storage. and that is likely to be over a wide pressure range. Fracture mechanics should be applied to all cyclic H2 service vessels, especially if they start from another service. Propane tank designs vary and unless a tank is specifically intended for H2, there is no guarantee of what might be used terms of materials and construction techniques.
 

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.
    

Hydrogen cylinders contain pure hydrogen unless they are specifically manufactured for and marked as a mixture. The purity grade is usually between 99.5% and 99.9999%. The balance is typically inert gases (such as nitrogen) with just ppm levels of other contaminants, but this can vary depending upon the production source. When emptied, the residual is still the same purity of hydrogen, just at lower pressure. If emptied to atmospheric pressure, there would still be one atmosphere of hydrogen within the container unless evacuated or purged to another gas. (See Properties section for related FAQs). Note that when a cylinder is reduced close to or at atmospheric pressure, it is very susceptible to air migration into the cylinder, especially if ambient temperature changes can create a slight negative pressure in the cylinder. Air should never be allowed into a hydrogen cylinder until it’s been properly purged with an inert gas such as nitrogen, helium or CO2.

This production rate of hydrogen of about 96 kg/h is quite significant, which depending upon the application might require a significant amount of storage. There will be a need to determine how many kg the project wants to store from this production rate in order to determine how much hydrogen ground storage is needed. Since the project is in Europe, look for pressure vessel manufacturers that offer PED approval for their vessels. 

There are many types of vessel construction other than Type 4 vessels for ground storage.  General vessel types also include Type 1 which are more traditional metallic vessels, Type 2 which are hoop wrapped, Type 3 which are fully wrapped with metallic liners, and Type 4 which are fully wrapped with non-metallic liners.
Selection between these types of vessels is highly dependent upon the design attributes (e.g., weight, service conditions, longevity, etc.), the nature of the process (pressure, temperature, cycles, etc.), and economics. The Panel is not in a position to endorse a particular type, brand or vendor. There are many vendors that can supply a variety of these types of vessels. It is important to develop a process specification for a vendor to quote since the application will have a significant effect on the economics of one type compared to another.

It is important to be aware of the local pressure vessel Codes within your jurisdiction. Virtually any manufacturer can help with that determination if requested but it’s also prudent to confirm with the local authority having jurisdiction. The manufacturer can also assist with the User Defined Specification for your service as well recommend   the storage pressure and number of vessels that might be optimized to store the quantity of hydrogen needed. Storage vessel cost may range from $500 up to $2000 per kg stored depending on the requirements selected. There are also many indirect costs related to storage such as relief devices, piping, freight, foundations, and installation that also must be considered as part of the overall assessment.