What are the considerations for the potential for hydrogen to leak from hydrogen fueled vehicles when they are being transported by truck, rail, or ship? What is the probability and quantity that might leak?
FCEVs usually contain only a minimal amount of hydrogen fuel pressure (several Mpa) to support getting the car on and off car carriers. Panel members are not aware of any hydrogen release incidents during vehicle loading and don’t know what the probability of such a leak is considering the loading operations. A risk assessment accounting for the probability of collisions as well as leaks from new cars off the assembly line would be needed. It may be best to connect with the car manufacturers when considering such an assessment.
Regarding the questions on the source of leaks and leak rates, Section 5.2.2.2.2 of SAE J2579 (which is harmonized with the GTR) provides the following: The maximum allowable discharge (non-accident) from the compressed hydrogen storage system for passenger vehicles is A*150 Ncc/min where A = (Vwidth+1)*(Vheight+0.5)*(Vlength+1)/30.4 and Vwidth, Vheight, Vlength are the vehicle width, height, length (m), respectively. Simply speaking, the allowable leakage from the compressed hydrogen storage system of a passenger vehicle that fits in a standard garage in the USA is 150 Ncc/min. Note that the 150 Ncc/min limit for a typical passenger vehicle drops to an expected value of 7.5 Ncc/min (maximum) at a partial fill condition of 3 MPa. Potential sources for leaks during vehicle operation could include joints/fittings in the piping or at the fuel cell stack. This piping is typically at lower pressures than the storage vessel as a result of a system regulator.
Additionally, a solenoid valve is located in the tank. The solenoid closes when the key is turned off or the electrical system deenergized. The result is that only a very minimal amount of hydrogen would be in the piping system unless there is a failure of the solenoid. A useful diagram is included here, showing the hydrogen line connections in FCEV with Type 4 tank (source: Update to the 700 bar Compressed Hydrogen Storage System Cost Projection Webinar). This also shows a thermally activated pressure relief device integrated into the in-tank valve/regulator assembly as typical.