If a solar sail works, a windmill type design like this might work.
http://inhabitat.com/city-windmills-enable-clean-energy-and-unique-advertising-opportunites/city-windmill/?extend=1 The vanes rotate to flatten for maximum surface area when being pushed then rotate to go thin edge first through resisting winds. Given that light will be pushing it, some thermal reacting spring might help the vanes pivot on the dark side of the revolution and then snap back to "full sail" when they hit the sunlight again.
Interesting to think about but there are better ways to get power out there... I would like to try temperature difference using sunlight and shade to vaporize and condense a volatile liquid, passing it through a turbine in the middle, then flip the rig over (sunny -> shady side) to repeat. Wonder what kind of metal fatigue issues would arise transitioning between full sun and shade...