I wonder if one could use a couple of smallish soft pads like the ones used to clean aquariums with a solar panel and a small motor, so that they recharge during the day and then move randomly on the window while there is no sun.

They'd need to be cheap enough that one should be able to buy one per window panel, however, and probably also won't work where the night-day cicle is very unbalanced.

Maybe having to put them on the window when they are needed (and then on a recharge pad) would be more practical.

Changing bedsheets sounds more complicated, and cables belong to the Dukes of Chaos and any attempt to arrange them neatly is bound to backfire :)