Problems to solve before we can entertain those thoughts:
Contraption: For capturing liquid nitrogen and for regenerating it in the contraption
Alternative: For liquid Nitrogen
Super-conductors: A much better under-standing of how superconductivity really occurs. Holy-grail would be to find and/or create materials that are superconductors under normal conditions. Meaning room temperature and normal pressure. link
Material-Science: This appears to be a very interesting Science. Breakthroughs in material science are going to revolutionize and spawn a ton of other industries. Hopefully these breakthroughs can also help answer some of the questions related to superconducting materials.
Magnets, electro-magnets: We’d probably end up depleting Iron as a basic resource at the very least. Specially if we decide to build a system for wide-scale use and implementation of this technology. Electro-magnets too, need iron. Solution? Mine the asteroids? I guess my thoughts and my ideas really aren’t that crazy. Consider this and this (more recent)