The promise of German Aerospace Center's HY4 aircraft is tremendous: short, emissions-free flights in Europe's crowded and environmentally conscience skies. The plane, which is set to have its first flight in the summer of 2016 out of Stuttgart Airport, is an all-electric craft, powered by a proton exchange membrane fuel cell. It's a lofty goal, but one unlikely to be realized by next summer.

Proton exchange membrane fuel cells are an old idea, and were used on early space exploration missions, though they were later pulled because of weak power generation. The fuel cell runs on hydrogen and oxygen, and it produces water as a byproduct. Fuel cells have been powering experimental airplanes for a little while now, but the technique that would make them economical always seems to be five years away.