[fsloth]

As near future sci-fi options go from 100ft up I don't see anything too problematic with hydrogen economy. Except for transport infrastructure.

Or, we remain with carbon based fuels, except they are synthesized from atmospheric carbon in regions with plenty of solar energy.

These approaches surely have challenges, but I would like to know what are the biggest ones?

[Zigurd]

Ammonia is more practical than hydrogen. Just not as sexy. Should be rebranded as the "hydrogen-nitrogen economy."

[petre]

Ammonia has issues like being toxic and higlhy volatile.

I think DME has some potential as an alternative fuel because it can be produced through CO2-to-DME hydrogenation or by using syngas/manure and burned in cleaner diesels (no particulate emissions).