Here is a unique concept called the "Gravity Plane" that theoretically can take off, climb to altitude, cruise, descend, and land using only stored helium and built-in air compressors. It purports to be entirely self-powered, but in reality we know that conservation of energy requires that 100% efficiency be realized in order for that to be so. Still, it is an interesting idea. There are currently automated, unmanned submarines that employ a similar principle for bobbing up and down through the oceans to collect research data.
While there are lots of technical hurdles to overcome, it is an approach to "clean" flight that should be explored further, if for no other reason than to rule it out as a possibility. Maybe this would make a good radio controlled modeling project for someone. I'd do it, but just don't have the time. Come to think of it, there might be some stimulus money out there for funding such a green concept.