Cheap clean energy is indeed the core problem. With enough energy even inefficient processes like creating synthetic high energy fuels to condense more energy

Cheap clean energy is indeed the core problem. With enough energy even inefficient processes like creating synthetic high energy fuels to condense more energy in to less weight for air travel are possible. Short term: Build MODERN, waste reducing, nuclear (fission) plants. If proliferation is a concern, require the military be hired to protect them and the logs/supervision open to the public with short delay. Long term: I like the 3001 / Gundam OO / generic scifi solution. Solar collectors. Point lots of solar energy at a focal location and do useful things with it.