The OP suggested fuel cells. This would replace IC engines and should result in complete conversion to CO2+H2O (with the whole process being carbon neutral beca

The OP suggested fuel cells. This would replace IC engines and should result in complete conversion to CO2+H2O (with the whole process being carbon neutral because the CO2 came from the air originally). I.e no NOx, fine particles etc. I agree on feeding people however e.g. algae produced bio oil could be very space conservative -- and could be used to e.g. clean waste water. NB use of could and should rather than would throughout this post. Algae produced bio oils are a matter of research at the moment and I don't think that the fuel cells exist yet.