A they already built a solar aircraft and B they say it can scale to 50x the size and carry 50 people. IMO, that would be more useful for lifting cargo vs people.

Second wing area and lift is related to velocity (not power) right before you stall outside of that it's a looser connection. Velocity is indirectly related to power in steady level flight based on drag. Drag in level flight is a function of weight, shape, altitude, and velocity.

Economicly I don't think this is a good idea. Physically it seems possible with cheaper and more efficemt solar cells it might make this more reasonable up to a point. Especially if they can boost altitude and thus speed.

Yes they've built an airplane with the wing span of a 747 and a lower lift than a Cessna. Please show me where they stated that they can scale it 50x the size, that would mean it will have a wing span of 10000 feet, that's 10 Eiffel towers....

The article says electric aircraft holding 50 people in 50 years. Granted, not pure solar aircraft but electric.

50x the power takes 50x the surface area and 7.1x the length / width. Or less than that with a lifting body approach. Or significantly less than that if it's daytime only.

I've seen 2 things there 1. electric and 2. 50 people nothing to do with 50x time size. The Solar Impulse can lift up about 5 people as it is (if there was room), scaling it to 50 might might be possible but cargo, and at the weight you were talking about no sorry not going to happen.

Solar Impulse is a marvel of engineering, and the fact that during on of the legs of the flight a single person flew the damn thing for 6 days straight is an attestment to human ingenuity and courage but there is a very very big difference between flying 50 people using electric aircraft (again electric not specifically solar) in 50 years to flying 50,000 of lbs on solar power.

We already have electric aircraft capable of 1.5-2h of flight scaling those up on electric power alone without solar might actually be easier as you do not inherit the weight of the solar panels. Efficient fuel cells and ultra light batteries / nano super caps could potentially make electric flight possible within 50 years but again relying on solar to transport cargo long haul no sorry not happening.

Li-ion battery's are 0.265 kWh per kg. Over a 2 hour flight the lightest thin film solar cells produce 12kWh. Battery's might be improving but not 50x as good. See 6w per gram: http://news.mit.edu/2016/ultrathin-flexible-solar-cells-0226

You would probably want a thicker coating to protect the cell, but in 50 years solar can add very minimal weight.