Remember the Stirling Engine? I have some recollection of it, but not much. Here’s one you can make in your spare time:
Stirling engines are a type of heat engine. They convert heat into mechanical work (you can find out more about how they work here), which means that it is possible to use the sun as a source of heat to general clean power. The video below shows a working stirling engine build out of soda cans that spins up to almost a thousand RPM.