If you use a voltage doubler and by doing so you achieved the same match to the prop as if you had a generator of the correct voltage then you would be in the same position except for the inevitable loss of the boost circuit.
Using voltage doublers that work on the peak of the waveform and generally present a high impedance will almost certainly fail except for very small schemes.
A simple boost converter with fixed pwm chosen as a good compromise may be good enough to get what you want and could just use a shunt mosfet across the winding and a series schottky diode in the line. A 555 followed by a mosfet driver may be good enough. You would need to chop at a modest frequency of about 5kHz . Alternatively you could add a capacitor across the machine brushes and include a ferrite inductor and chop at a high frequency ( 30khz or similar)
To make best use of the scheme you would need to alter the pwm to match the generator speed to the prop's power characteristic. You need to phase the pwm back quickly beyond cut in but slow down the phase back as the wind speed picks up.
The small size of this set up makes me doubt that it is worth a lot of effort needed to make a decent attempt at mppt. The fixed pwm circuit would probably be near enough in this case, there is precious little energy available in low winds to a 4ft prop so there may be no point in going to a lot of effort for a few % improvement. If you had more consistent higher winds then the gain would be considerable.
I suggest that if you are familiar with electronics you just try a simple boost converter. Connect a mosfet across your generator terminals with drain positive, Include a schottky diode in the positive line where you have your conventional blocking diode now( may be good enough to keep it instead of a schottky).Use a decent capacitor across the output of the series diode to common and keep leads short within that loop.
Make sure that the gate drive to the fet can't exceed about 50% duty cycle and make it variable between 50% and zero pwm.
Drive it with a 555 or more likely a microprocessor if that is your line of business, but make sure you use a decent mosfet driver ( chip or discreet) after the drive circuit. Microprocessors will give you all the problems of needing logic level fets and everything else but that is your choice. See how it works, it may work wonders as long as you have wind, otherwise it will just slow your blades as soon as you boost above cut in speed.
Flux
