My grandson, 6 years old, quite the science kid, decided that we should build a windmill, and not just a wind toy or whirlygig (which we had done) but one that would turn a generator and develop electricity
I started off with a trip to American Science and Surplus and got a
couple of different small DC motors (to use as a generator) and a fan on
a hub. I had a supply of small bearings, 80-20 aluminum shape, and
miscellaneous hardware. I worked up a design with three basic elements
1. Offset Shaft. I followed a rather conventional "old" style design with a directional vane, and with the rotor shaft
able to swing to the side in a high-force (ie, high wind) condition.
This was for over-speed protection.
2. Belt Drive. A typical problem on windmill design is to gear up the rotor hub speed to a more
efficient speed for the generator. I did this with small pulleys and a
small belt, with a 4:1 speed increase.
3. Blade Design. I used the hub of a small fan to start with, drilled out the rivets and removed
the small blades, and then attached 12" blades cut from thin (I think
0.025 inch) aluminum sheet stock. I formed the blades into the
approximate shape I wanted on a bench press, setting up blocks so I
could give the same bend to each of the four blades. When finished, I
pop-riveted them onto the hub again, similar to the original blades. I
then put the hub on the shaft and statically balanced the unit (by
trimming metal off of the end of blades) until, after spinning, it was
just as likely to stop in any position. (If unbalanced, it would tend
to stop with the heaviest side down).
When ready, we set it up with guy wires and a wooden tower framework (which was mostly for looks). This was in Estes Park, Colorado.
What we learned:
Picking a good site is extremely important. Their house is somewhat down in a
valley, protected from the really fierce winds that can blow on the
windward side of mountains. Even when the wind is blowing hard there,
the turbulence from all of the land features causes the wind to
constantly change speed and direction. It will spin the generator, but
only for brief periods. Windmills work best when they're on the top of a
mountain, or much higher than the ground items that can affect it, like
trees and buildings. It's hard to do this in your own back yard.
Making something like this yourself is pretty difficult. I have a lathe, so could turn the fan hub, a press, so could bend the blades,
and a milling machine, so I could cut the slots for the bearing mount.
If you're not ready to deal with things like this, you should probably
buy a "kit" of all of the component parts and basically just assemble
it. It's still very educational.
As a group, perhaps we could make up a bunch of parts and do some engineering, so we could identify
how much power they could develop in specific wind conditions, the
threshold wind to develop power, etc.