This was built as a lark, a whimsical attempt to recapture some of the hundreds of dollars I spend on air conditioning each year. It's a wind turbine powered by the compressor fan on my air conditioning unit.
But, It is a ponderable notion. And you could reverse the rotor, add a tail and yaw bearing and make a regular wind turbine.

Step 1: The Alternator

I started off by messing around with a stepper motor I had, it was rated 6 volts 0.4 Amps per phase and was a two phase motor, meaning max input would be 4.8 watts. So running it backwards could I get that much out?

I'm not really an electronics guy, so I stuck my meter in AC mode across the terminals, spun it with my fingers, and got a reading of 50 volts! On current it read .2 amps. Umm okay, never mind the meter, would it light LEDs?

So wanting to see if it would light LEDs I wired 5 red LEDs together, four as a full wave bridge rectifier and the fifth as the load. As you can see in the video and pics, it had no problem lighting those LEDs, nor the five super bright white LEDs arranged in the same manner.

Step 2: The Rotor

In the vein of keeping the design simple I wanted to have a simple rotor. My solution was to pull the blade assembly off of a cheap box fan that had reached the end of it's life. The rotor had been attached to the fan on a 1/2 inch shaft. To accommodate the 5mm shaft on my stepper motor, I press fitted a half inch dowel into the fan and then bored a 7/32nds hole through the dowel, and split the end of the dowel with a razor saw to allow the use of a hose clamp to secure the fan to the motor shaft.

Step 3: Housing and Mount

The housing and mount are made from PVC, the housing is made from two 2" end caps, a 2"ring to align the caps and hold the motor in place and a 1/2" elbow cut down to attach the housing to the mount. The two end caps are held together with aluminum tape.

Step 4: The Load

The load is a group of five LEDs per phase arranged as a full wave bridge with the fifth as the load. Instead of poorly drawn schematics, I suggest you read this wikipedia article. Here are some pix.

Step 5: All That's Left

Is to place the motor mount onto the stand and place it on top of the compressor. adjust for best RPM and you are good to go. You might note that contrary to expectations, ie, that you could just set it on top in line with the fan, the best RPM comes from an offset 45 degree placement.

Addendum, I constructed a diode bridge with diodes salvaged with from a CFL and replaced one of the LED loads and recorded an open circuit voltage of 10 volts. Now here's a question for smarter people. Could I put the two phases in series for an OCV of 20 and charge a 12 volt battery?

Step 6: Acknowledgements

These persons are directly or indirectly responsible for this project.

William Kwamba for his inspiration

Alan Parekh who provided the LEDs.

and

The Reuk website for the info on steppers.

Step 7: Addendum

Not as neat as I'd like, but I bodgered up the turbine in conventional form for those who might be interested, alas Code Enforcement would be all over me if I actually installed a wind turbine.

PVC fittings
broom stick boom
road spam tail
and
bamboo stake for yaw bearing.

All held together with screws.