Hopefully not your every day solar blimp.
1. The temperature at an altitude of 20 kilometres is around minus 60° celsius (-76°Fahrenheit). This is chilly.
2. Stirling engines work on temperature differences.
3. Solar stirling engines are heated on one end by a dish which concentrates the sunlight
and focuses it on one end, while the other end is cooled off by ambient temperatures [see link].
4. The temperature difference between high altitude and sealevel is around 80° celsius. Quite a bit which the stirling can take advantage of.
5. Build huge solar dish stirling from lightweight material. > put dish into a lightweight transparent plastic bulb, which is filled with helium, and acts as a lifting medium. > lift fuselage with electric engine powered by solar dish. > at night, store excess energy in hydrogen, or in a thermal battery or something like that.
6. At our altitude there are no clouds; you capture the sun from dawn to dusk.
7. It looks very strange. I made a drawing [see link].
Could be used for telecom-blimps and such.
Could be quite efficient maybe!