I often use my gym time to catch up on interesting podcasts, and NPR's excellent Science Friday series is one of my favorite sources. I just caught up with a recent segment on the development of hydrogen-powered trains, which seem like a particularly clever application of a promising technology that must still overcome serious obstacles in its automotive applications. But while I give the host, Ira Flatow, credit for pursuing the question of where the hydrogen for trains would come from, his guests' answers left something to be desired. That's not just because they tended to downplay the emissions associated with producing hydrogen, but because this omission might result in ignoring what could be an even better, more efficient fuel-cell configuration for trains and other large vehicles.

The basic idea of powering trains with fuel cells offers several important advantages--and one very serious disadvantage--for rail companies and their stakeholders. It also represents a less revolutionary change for rail than for automobiles, since trains are already partially or wholly-electrified, and a fuel cell is just another way to generate that electricity. Even the diesel locomotives that fuel-cell locos would be intended to replace are really diesel-electric hybrids. The key benefits of using fuel cells instead of big diesels for this application include substantial reductions in local pollutants, including soot, along with much quieter operation. Unfortunately, even if fuel cell trains could circumvent many of the infrastructure hurdles that have impeded automotive fuel cells, they still look prohibitively expensive. Diesels are pretty cheap on the basis of $ per kilowatt of generating capacity, while fuel cells are still much pricier, by at least a factor of 10.

Ignoring cost, fuel cell trains would face fewer obstacles to wide-scale deployment than fuel cell cars. As one of the program's guest pointed out, hydrogen storage, the Achilles heel of fuel cell cars, is not a problem in this situation. If necessary, a fuel cell train could carry an entire tank-car of compressed hydrogen behind the locomotive, and it wouldn't alter the train's performance or cost appreciably. That would also reduce the need for a widely-dispersed refueling infrastructure. For that matter, a train could carry along its own refueling set-up, in the form of an electrolyzer and compressor. It would require only fresh water--reminiscent of the coal-burning locos of yore--and a place to plug in. However, when you follow that plug back to its ultimate source, you find that the CO2 emissions of a hydrogen train could be quite a bit higher than zero, and possibly even higher than those of the diesel train it would replace, because our power generating mix is still dominated by fossil fuels.

So whether the H2 for a fuel cell train would be produced from natural gas, as most of the substantial quantity of industrial H2 in the US is, or from grid electricity, it results in CO2 emissions somewhere. In fact, because electrolysis of water into H2 is only about 80% efficient, the associated emissions of electrolytic H2 used to fuel a train would be 25% higher than the average of the grid power used to produce it. And although it's theoretically possible to generate H2 solely from off-peak renewable electricity when the latter is not being used to back out higher-emitting power sources, the capital cost of that route is much higher, because it would only operate a small fraction of the time. At least for the near-to-medium term, most H2 will likely be generated from natural gas, and that argues for a very different configuration for the fuel cell train than the one considered in this episode of Science Friday. Instead of using low-temperature automotive-design fuel cells, which require a source of pure H2, a high-temperature fuel cell of the type used for stationary power generation might make more sense. Not only do these operate more efficiently, resulting in lower overall emissions, but they can also run directly on natural gas and other light hydrocarbons, producing the H2 they require internally, rather than externally. In that case, the fuel tank for a fuel cell locomotive might just be an ordinary propane tank car, for which the entire supply chain is already well-developed.

If you've ever waited for a train in an underground or partially-enclosed station with several diesel locomotives idling away, you'll probably join me in wishing the hydrogen train test project team good luck with this initiative. The benefits of converting trains to fuel cells seem obvious, assuming this can ever be done at a competitive cost. At the same time, I hope the developers will take a broader view of hydrogen as not just another fuel, but as part of our overall energy ecology. That might lead them to an even more viable, beneficial result, with a better chance of showing up in real train yards, and eventually even passenger trains.

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