Oct 01 2020

Hydrogen-Fueled Trains

Predicting the future is perilous, but I think it is a good bet at this point that, for the transportation industry, battery technology has essentially won over hydrogen fuel-cell technology. The last 20 years were critical, during which time battery technology slowly but steadily improved and now is practical for powering electric cars. The next milestone is for all-electric cars to become cheap enough to be at parity with gasoline cars, and we are getting close to that point. Some in the industry predict this point will occur in 2025. Electric vehicles are already cheaper to run (lower cost per mile for the energy) and have much less maintenance than combustion engine vehicles.

Over this same time the hydrogen fuel cell ran into some roadblocks, specifically a cost-effective and safe way to store large amounts of hydrogen in a moving vehicle. I remember back in the 2000’s when there was a lot of hype surrounding the coming hydrogen economy, there was always a tiny footnote that – all they have to do is figure out the best way to store hydrogen, but they are working on it. Those tiny footnotes are sometime deal-breakers, and that is exactly what allowed battery powered cars to win the race.

But hydrogen fuel cells are not dead as a technology. The recent test run of a hydrogen fuel train in the UK indicates where the industry might go. But before we get to that, let’s discuss cars. There are hydrogen cars on the road, but not many. They store their hydrogen as compressed gas, just bypassing the idea of storing the hydrogen in some kind of matrix. The Toyota Mirai, for example, has two tanks that store compressed hydrogen gas at up to 10,000 psi. This provides a range of 312 miles on a full tank, and they claim high safety via extensive crash testing. This is pretty good, and in the range of the best battery powered cars. It also has the advantage of filling up in about 5 minutes. A major disadvantage is the availability of filling stations. Also, there remains concerns about having highly pressurized tanks in fast moving vehicles with human occupants. Further, how will designs increase this range or power larger vehicles? They will need more or larger tanks or higher pressure.

There is still ongoing research looking for an optimal material to store hydrogen in large amounts without the need for high pressure. Perhaps the most promising is so-called MOF’s (metal organic frameworks). These materials have massive internal surface area, and so can soak-up large amounts of gas or liquid. A recent study, for example, did a computer analysis of 500,000 different MOFs and then did testing and the most promising three materials to find the best MOFs for storing hydrogen. Such materials need to meet certain benchmarks in terms of both weight and volume, and the researchers found that no material tested could to that. Those that exceeded in one, failed in the other. Further they found that the usable hydrogen was much less than the total stored hydrogen – usable being what can be extracted at operating conditions. So even this promising technology is not there yet. The hope is that what was learned in this study will help design an MOF that will meet all targets.

Another option is metallic glass. This class of materials has superior physical properties, and also has high effective surface area to absorb hydrogen for later release. But the research is still in early days for this application. We are basically where we were 20 years ago with ceramics, and so far they have not yet panned out. There may be other technologies out there I am not aware of, but I doubt anyone is close to a practical solution. So for now, we have pressurized tanks.

But let’s get back to that hydrogen train – perhaps cars should not be the focus of the hydrogen fuel-cell industry. Trains have several advantages for hydrogen over cars. First, they are huge, which allows for more and larger tanks. Also, the tanks can be kept at safe distances from any passengers, or widely distributed to that no one tank failure will be catastrophic. Further, there are many fewer train accidents than car accidents. In the US there are about 5,800 train accidents per year, most involving trains hitting cars at railroad crossings. Most such accidents would not be a significant risk for damaging the train.

Another advantage to trains is that they are literally on rails, which means the hydrogen infrastructure would be much simpler. You would only need hydrogen filling stations at train stations, and not even all of them. Infrastructure is probably the most significant factor in determining which of competing technologies wins. It is credited as the key factor that allowed gasoline vehicles to win over steam-powered and electric vehicles over a century ago. It may be the biggest challenge for hydrogen fuel-cell cars. But this could be a trivial issue for trains by comparison.

Trains generally have a much greater efficiency than cars and most other forms of transportation. Switching to hydrogen may make them more efficient still. The quick refueling time compared to recharging batteries makes them much more practical than battery technology.

In the US the debate right now is whether or not we should replace our diesel engine fleet with an electrified rail system. I won’t get into the weeds of this debate, except to say that diesel engines should be replaced as part of our effort to decarbonize our economy. But perhaps we should consider hydrogen vs electrifying the entire rail system. I don’t know which system would be better, but would like to see an expert analysis of all the advantages and disadvantages. At the very least, if we are considering a major change to our train infrastructure, all options should be on the table.

It may come down to infrastructure again, this time which one do we want to ride on the backs of trains. Some argue that making an electrical grid to support electric trains could do double duty as an improved grid to support renewable energy sources. However, a hydrogen infrastructure for trains could be a backbone on which a hydrogen infrastructure is built. So the decision we make for trains may have implications beyond this mode of transport.

Again – predicting what will happen is difficult. But in cases like this, we literally can choose our own future.

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