WELL, FOR STARTERS THEY BURN HYDROGEN: Here’s How Hydrogen Engines Work.

I drove a GM hydrogen car for Popular Mechanics a few years back. I still have the same reservations about the hydrogen fuel cycle. To wit: “The car advertises itself as petroleum-free, which is true. But—and here’s my problem with hydrogen cars—it’s not really fossil fuel free. Most hydrogen is made by ‘steam reformation’ of natural gas, which is still a fossil fuel. You can also make it out of water, via electrolysis, but unless you’ve got a non-fossil source of electricity the hydrogen is really just functioning as an energy-storage medium, rather than a source of energy. Of course, build lots of nice, clean nuclear plants, or orbiting solar power plants, or whatever, and that problem goes away.”

Maybe the abundance of fracked natural gas changes this calculation a bit, but I’m not sure.