I've often wondered if a balloon-based system might be the way that this is eventually achieved. As solar energy becomes more and more efficient every year (app

I've often wondered if a balloon-based system might be the way that this is eventually achieved. As solar energy becomes more and more efficient every year (apparently it's on an expotential price/performance growth curve), you coat the balloon with a light-weight, flexible solar array, and either heat the air like a hot air balloon, or as it becomes cheaper and more efficient to extract hydrogen from water, fill the balloon with hydrogen, and constantly replace the hydrogen that leaks out with hydrogen extracted from water vapor in the air: http://www.technologyreview.com/view/512996/a-cheaper-way-to-make-hydrogen-from-water/ http://www.technologyreview.com/view/512996/a-cheaper-way-to... You could use low powered propellers (like a blimp), and take advantage of weather patterns (like Google's Loon project) in order to maintain position or to slowly travel.