So there are already rechargeable batteries, and there are already solar-powered devices to charge batteries, but what if you could make a rechargeable battery that was completely self-contained, that all you had to do was put it in the window and let it charge?
Knut Karlsen asked this very question, and actually did something about it. Currently just a proof of concept, Knut was able to wrap a flexible solar cell around a battery, and wire it in such a way that it would charge itself. Right now, all it gets is a trickle, but with the proper redesign (incorporating circuitry and the like) it could be incredibly useful. I know for a fact that would be a product that I would want to buy.