While backpacking in Northern New Mexico, I ran across a solar panel that the Forestry service had up that tracked the sun without using any electric current at all. The frame had Freon or some low boiling point fluid in it and was arranged such that the fluid would boil and re-condense elsewhere in the hollow frame to control the tilt. Very cool. I had to sit and watch it for a couple of hours just to prove to myself that it worked.