Coat one side of each vane in your average window blinds with aluminium foil, and paint the other side black. Twist them one way to absorb heat into the room from the sun, twist them the other way to reflect it all outside, providing you with extra cool shade.
An optional automation system features a series of prisms guiding daylight to one side of a hamster wheel, which in turn is linked via several pulleys and excessive gearing systems to twist the blinds. Hopefully the hamster should run away from the sunlight when it's too bright, and towards it when it's too dark.
Assuming you already have a hamster the whole thing should be completely automatic with no maintenance or power whatsoever.