Computational fluid dynamic (CFD) simulations under worst-cast fueling conditions should be utilized to assist in evaluating the risk of short or long term liner or composite damage scenarios.
Starting from the inside of a Type 4 vessel: hot gas at the end of fill is usually transient in nature, where it hits a peak temperature and then immediately starts to cool when the fill stops. The liner bulk material temperature continues to heat up even as the fill has stopped, but it does not reach the peak gas temperature. A heat transfer model should be used to determine the peak bulk liner material temperature under worst-case conditions to compare to the liner softening temperature. Recognizing that the liner material’s degradation will be a result of its “time at temperature,” it can be evaluated whether a lifetime of these sporadic peaks will cause liner degradation, resulting in embrittlement (cracking) and leakage.
Local effects should also be considered when excessive gas temperatures (>85˚C / 185˚F) are prevalent. The liner/end boss interface can be a concern when the tank design utilizes a mechanical interlocking mechanism to attach the liner to the boss. Depending upon the design of the interlock, areas of thinner liner material that the hot gas may have softened may conspire with higher permeability, resulting in the trapping of gas between the layer of plastic and the metal boss. At low tank pressures, these trapped pockets of pressurized hydrogen gas can “pop out” the interlocking plastic material and cause leakage. Tank manufacturers may be consulted to explore if this “trapped gas” scenario has been mitigated (e.g. by design or by using a venting feature in the boss).
Note: the "end boss” in a Type 4 tank is a metallic component that contains the threaded valve opening along with a means of attaching the plastic liner to form a gas tight barrier for the tank.
Moving outwards to the composite resin: the glass transition temperature (Tg) should be comfortably above 105˚C (221˚F) in accordance with industry standard CSA/ANSI HGV 2. A heat transfer model can be used to determine the peak bulk composite material temperature under worst-case fueling conditions when compared to the resin Tg. The composite bulk temperature may neither attain such a high value nor maintain it for any sufficient duration. “Time at temperature” should also be used as a means to gauge whether resin degradation is possible, but the risk may be low if the resin Tg is above 105˚C (221˚F). Tank rupture may occur if the composite properties were to degrade as a result of resin “failure,” but these instances have only been seen to occur in service where the Tg was determined to be significantly lower than 85C.
Generally speaking, the International Fire Code and NFPA 2 apply to non-transportation use of hydrogen. These are maturing quickly, with NFPA 2 currently having issued its most recent edition in 2023. Standards for both on-board LH2 tanks and LH2 tankers for bulk fuel transport are managed by the U.S. Department of Transportation (DOT) and are well established. DOT transport requirements for the U.S. can be found in 49 CFR.
There is growing activity regarding the use of LH2 as a vehicle fuel and there are several prototype trucks in operation. The on-board tanks may lack some reference standards, but vehicle fuel storage systems are typically self-certified by the original equipment manufacturers, particularly at the current state of development. While there is risk of impeding the development of a commercial market due to a lack of approved and common hardware, there is also risk of finalizing a standard prior to completion of development and testing. ISO TC 197 has several working groups including WG 1 and WG 35 to further develop the necessary standards for fuel tanks, fueling connections, and filling protocols. Liquid hydrogen transfer is well proven and established for industrial applications. However, consumer use of a cryogenic product has not yet been proven and will require refinement of hardware and processes.
The vehicle manufacturers can also provide guidance as to their efforts to meet required regulations.
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:
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.
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