I'm curious as to whether any work is being done into stabilizing solar-powered lifting bodies - eg. helium-filled rigid airships where the body itself acts as a wing. The idea would be that they're still too heavy to float on their own, but would require a comparative minimum of forward power to lift off.
That would seem like an ideal type of platform for solar powered flight - if you can solve the issue of wind gusts, which are the main obstacle to widespread use of airships.
With this thing, cool as it is, the efficiency of the engines doesn't sound like the problem, rather the conversion efficiency of the photovoltaics.