Normally, this shrinkage occurs in all directions within the material.
However, the material can be stretched, causing its polymer strands to line-up, and directionalizing the shrinkage.
The solar thermal motor operates by using bands of stretched black plastic bag to continually pull a flywheel off-center as it rotates on an axle. The strips are heated by sunlight on one side of a drum/flywheel assembly, pulling the flywheel toward the sun-side. As the strips rotate to the back, they cool in the shadow of the drum and relax. This makes the flywheel continuously off-center on the sun side, causing it to rotate.