mux wrote:The thing is though, and at the risk of sounding apologetic to CNG or H2, you can design and maintain to reduce those kinds of failures to close to zero. Most CNG tank explosions you're going to see are in very old, badly maintained vehicles. Same should be true for hydrogen at a certain point in the future.
The issue I have with hydrogen in this regard isn't that it is inherently dangerous to have an extremely explosive gas, with on top of that a good 10% of its embodied energy just in gas pressure added to that. It's the fact that you NEED to maintain, inspect and replace these tanks. It is an inherently fragile chain of trust to get to high reliability.
And it gets worse, from an engineering perspective at least. Most tanks are made from carbon fiber reinforced plastics, not metals. CFRP is extremely strong but brittle, meaning any failure will actually be catastrophic.
I note that the Mirai tanks are only certified for 15 years, after which they need to be retired. IDK what if any inspection regime is required, but certainly any such regime needs to be (and eventually will be) designed to ensure adequate safety. As with all other such hazmat, safety rules will be written in blood.
<OT, IIRR Al scuba tanks are also fixed lifetime, unlike steel tanks which can be used as long as they pass the required inspections and tests. While Al scuba tanks are the norm for warm-water recreational diving, I don't use them given the colder, deeper diving I do, which requires both more gas and more insulation, i.e. more weight to compensate for the buoyancy of that insulation. Steel tanks are more negative than Al while also being lighter for the same capacity, so you need to add less lead. In short, there are always design tradeoffs.
As for CFRP, I'd be perfectly happy to use them for scuba even though they're god-awful expensive, owing to their greater capacity (due to being fillable to 4,500-5,500k PSI instead of the 2,400-3,500 for steel tanks, 3,000 for Al). But almost no one outside of the military or other well-funded organizations use them, because aside from being more expensive (and more susceptible to damage from careless handling. They require similar annual visual inspections as with steel tanks), virtually all commercial dive ops have compressors that are incapable of filling them to anywhere near max. pressure, so you'd never get the benefit of the extra capacity. end OT>
mux wrote:There are ways to design much more ductile materials in there to reduce this risk and limit 'wear and tear' failures to (quite fast) leaks instead of explosive decompression. But these tanks would be unreasonably heavy to put in a car (rough rule of thumb: about ~2.5x the weight of CFRP tanks, which already weigh about 100kg for the Nexo and Mirai).
Re weight: Note that a battery pack to provide similar range will weigh 500 kg or more. It's more a space than weight issue; The larger of the two tanks in the Clarity basically eliminates 2/3rds of the trunk's capacity. The Nexo went to three uniformly-sized smaller tanks instead of the Tucson's two differently sized ones, both to make packaging easier and (guessing) probably to benefit from economies of scale.
mux wrote:All of this isn't as much of an indictment of hydrogen as vehicle fuel as Oils4AsphaultOnly implies though. Things can, should and certainly will be engineered to be safe. But clearly, right now, it isn't. Worldwide there are about 100 commercial hydrogen vending stations and about 5000 currently running H2 vehicles. So far we've had 2 stations catastrophically fail. This is an unreasonable failure rate. If this were the aerospace industry, EVERYTHING would be shut down. This is an enormous deal. I think the industry is just 1 Mirai exploding away from basically shutting down.
There are considerably more than 5,000 H2-fueled FCEVs running around worldwide [Edit
: Per the IEA report I posted in the H2 and FCEV topic, current world total is 11,200], and the fleet is growing at an increasing pace. China's going big into FCEVs, for commercial vehicles (buses etc.) initially, where BEVs can't meet the operational requirements. BTW, we've only had one station fail; the other explosion was at Air Products H2 production plant in the Bay Area, placed (as it should be) well away from residential areas.