Hydrogen FCV Safety Risks
In my opinion, the most significant argument against hydrogen fuel cell vehicles involves the safety risks from hydrogen explosions. This is far beyond the potential consequences of gasoline, diesel, or battery fires in personal vehicles.
I work with industrial high pressure hydrogen processes as a chemical engineer. A significant part of my job is preventing Loss of Primary Containment (LOPC) accidents involving high pressure hydrogen and other flammable gases.
During discussions of hydrogen risks, one often sees comparisons made to the Hindenburg airship disaster. I think that's a poor comparison. The hydrogen in the Hindenburg was not at high pressure.
Fuel cell vehicles such as the Toyota FCV car store hydrogen at very high pressure -- 6 kg of H2 is stored in the car at up to 10,000 psig. This pressure is so high that the decompression force upon catastrophic LOPC could produce a large damage radius even if the compressed gas was merely air. But it's not just air, hydrogen is a flammable gas that has a very wide range between the lower and upper explosive limits when released into air, and has a very low energy for detonation.
The evidence from accidents involving rapid decompression of high pressure hydrogen demonstrates that these events often result in detonation of hydrogen-air mixtures.
Proponents of hydrogen FCVs point to photos showing the controlled release of a vehicle's H2 tank through the tank's pressure relief device (PRD), a very small orifice that is sized to produce the flame shown in the photos. A well-designed PRD in this service will produce a large vertical jet of flame, the least bad direction to aim the flamethrower (assuming the vehicle stays upright). What they don't tell you is that the PRD specified for FCV H2 tanks would not be legal for use to protect industrial vessels used in high pressure H2 service, where we are required to use pressure relief devices to protect against any feasible overpressure scenario.
Hydrogen fuel cell vehicle PRDs and CNG vehicle PRDs protect only against specific fire impingement scenarios -- the PRD has to reach a sufficient temperature to
melt the plug before it will activate. The idea is that the PRD will activate before the high temperature from an external fire increases the vehicle tank pressure to the catastrophic rupture point. Unfortunately these PRDs don't work in all overpressure scenarios, including all fire scenarios. The PRD did not protect a
Honda's CNG tank in this fire. The CNG tank heated up, overpressured, and ruptured before the PRD activated, blowing open the doors and blowing off the roof, and propelling parts of the Honda as much as 100' across a wide radius.
In all likelihood, H2 FCV and CNG PRDs don't provide broader overpressure protection because the consequences of a false-positive activation could be pretty severe -- the vertical flamethrower anywhere the car might be located.
Proponents of hydrogen FCVs also point to the DOT bullet test, which pierces the FCV H2 tank with a rifle bullet. However this is misleading. The bullet-sized hole produces a high speed flame jet. It does not illustrate the much more rapid LOPC and explosion that could result if the tank catastrophically ruptured.
The H2 tank in a FCV is undoubtedly very strong. It has to be strong to contain 10,000 psig. In a bad accident, such as the FCV car being rear ended by a large truck or a fire started by some external source, the FCV H2 tank is probably less likely to rupture than a gasoline tank in an ICE vehicle. But in the event of a FCV H2 tank rupture, the consequences could be far worse than an ICE gasoline fire. High pressure vessels also have other failure scenarios, such a brittle fracture, or the failure of associated valves, fittings, or piping.
There was an industrial accident that demonstrates the magnitude of the
consequences from an explosion involving 3.5 - 7 kg of hydrogen -- which is coincidentally the amount of H2 in one FCV tank. The amount of H2 that exploded in this accident was calculated by the blast damage:
- prefabricated sections of concrete walls weighing over 1 ton each were blown out along a long length of the building during the explosion
- windows in an adjacent building were shattered, and large shards of glass were embedded in the opposite wall
- all ordinary window glass within 100m and some as far as 700m away was shattered
- the 700 kg/m2 roof of the building was lifted 1.5m from the overpressure of the explosion
- large metal structures were bent, twisted, and some were propelled tens of meters
- two workers were killed, and causalities would have been far higher had it not been a Saturday
Try to imagine this blast damage -- one of the worst industrial hydrogen explosions in history -- in a residential neighborhood, a parking garage, a busy highway, or in a dense commercial district at a filling station.
It's one thing to deal with these amounts of high pressure H2 in industrial settings, far away from the public. It's quite another thing to deal with it in thousands or millions of personal vehicles in homes, cities, or on our public roads. This is an unnecessary risk, there are a number of means of personal vehicle transport that do not involve the high consequences of a 6 kg hydrogen explosion.
A fire in a gasoline or diesel ICE car does not cause explosive blast damage of this magnitude. Only the small percentage of the fuel that has vaporized can explode.
A fire in a BEV car cannot cause damage of this magnitude. The stored energy is simply not there.
Authorities at the San Francisco International Airport didn't have any problem imagining the blast damage from H2 FCVs, because
they apparently would not allow a FCV H2 fueling station to be located near their airport.
"A hydrogen explosion in close proximity to active runways and a highway could be catastrophic and result in significant loss of life," [Airport Director] Martin wrote.
This airport handles over 2 million gallons of jet fuel a day, but a nearby H2 filling station for fuel cell vehicles was deemed to be an excessive risk. The
H2 supplier Linde -- who has vast experience producing and handling hydrogen on a huge industrial scale -- refused to assume the liability for the FCV H2 fueling station near the airport. If the vicinity of an airport can be deemed an excessive risk for siting a FCV H2 station, imagine the NIMBY fights over locating H2 fueling stations in many of the other places gas stations get located.
