[brianmcconnell]

Pilot here. I agree with other posts about energy density being an important factor here. I don't expect to see all-solar aircraft anytime soon. The best way to get aviation to become carbon neutral will be via biofuels, which several airlines are already experimenting with.

That said, I can see a couple of areas where solar will contribute.

While it will be very difficult to generate all of the power needed for an airliner via solar, there's no reason not to cover the aircraft in high efficiency (triple junction) solar cells. A large airliner will have a usable sky facing surface area of 1,000 square meters or so (rough estimate). Enough to generate a few hundred kilowatts of power. If the aircraft has hybrid gas/electric engines, that power can reduce the fuel burn a bit (even a small reduction is a big deal).

I can also see where a serial hybrid design could be workable (e.g. lots of fuel cells generating electric power that in turn drives electric fans). This would be attractive, especially in smaller planes, due to the potential to reduce the risk of mechanical failure.