Building a small wind turbine, then build bigger and bigger.
Put on a generator to get free electricity.
If you want, you can mount the wind-discs above each other on the same axis.
You can also place multiple axes next to each other and couple them together.
At first, I intend to start by using a 1 meter standard 12 M threaded rod, 12 mm nuts and 12 mm ball bearings.
Common woodworks may last so long.
A small problem seems to be what kind of generator I'm going to use, thinking of a regular car generator. The problem is that it starts charging at a quite high speed so I have to change the gear.