My idea is to create a sun-driven motor using Nitinol (nickel-titanium) memory metal. The concept is to orient memory metal bars around the inside of a ring similar to spokes in a bicycle wheel, but pulling all in the same direction. Each “spoke” would be connected to a ratchet assembly tangentially, somewhat like a stepping relay in a pinball machine. If the Nitinol is pre-strained, and the entire wheel is exposed to the sun, the Nitinol will heat-up and shrink once and the center hub will rotate for only a short time. However, if there is a rotating lid connected to the center of the wheel over the ‘spokes” with slots and lenses oriented such that the sun can be concentrated on one “spoke” at a time the “spokes’ will shrink and will pull the center of the wheel rotationally. After the slots are rotated from over the shortened “spokes”, they will cool and stretch to back to their normal length and reconnect to the ratchet assembly. As long as there is sun, the motor will continue to run. The motor can be scaled-up and connected to various devices or connected to a generator to generate electricity. The motor would probably not rotate at very high RPM's but it would have very high torque.
There are probably many ways to design around this idea, but the sketch attached is the simplest way I could convey the idea.