Efficient wind turbine alternators connected directly to batteries do not have the right speed range to make the best use of the blades. Adding a controller that converts the voltage between the windmill and the batteries can improve performance (rather in the way that grid-connect inverters do), but such controllers aren’t simple. Outback Power Systems ‘Flexmax’ controllers have been used for this, but now the company is in new hands and this experimental application has been dropped.
Here are two companies that offer ‘market ready’ controllers for optimising the blade speed. Finnprop specialise in heating controllers but they can do some battery chargers too. Midnite offer their ‘Classic’ PV controller with an additional ‘Clipper’ box that adapts it to use with wind turbines (at a price).
“Finnprop is a company, which is solely dedicated to produce windmill´s power controllers. Our products make possible to utilisize electicity captured from wind to varied type of usage. FinnProp® windmill power controllers are designed to maximize windmill efficiency and allow propellers to rotate whenever the wind is blowing. We also produce fully tailor-made solutions.”
http://www.finnprop.fi/index6.html
“The Classic MPPT controller allows the wind turbine to operate at a much higher voltage
than has been possible in the past. The Classic is an extremely efficient DC to DC
converter that tracks the wind and transforms the DC voltage down to the battery voltage.
Preliminary testing shows 100% more power from some existing turbines.”
This is not a recommendation as I have no experience of them, but here is another manufacturer, this one of small scale 3-phase DC wind MPPT: http://www.schams-solar.de/windmppt-ENG.html
I’ve given quite a bit of thought to these MMPTs.
1) If their effect is felt at low and medium wind speeds then I think they might be cost effective. Of course, their cost would have to be balanced against building a turbine with a greater swept area. At high wind speeds, my experience suggests that we’d be in the middle of a storm cycle during which the high output coupled with its duration would mean that the batteries would, fairly quickly, reach full charge anyway. Also, adding some PV capacity to the wind system is also a very fine way to bolster the low-end amount of juice trickling into the batteries, when there is little or no wind.
2) MMPTs would work fine with PM alternators because the magnetic flux is constant and independent of the alternator’s output voltage voltage. Unfortunately, with the older DC generators, such as the Jacobs, Wincharger, etc.., MMPTs may not be appropriate. Here, the magnetic flux is somewhat proportional to the generator voltage. Output voltages that are lower than the rated output of the generator would cause a fairly straight-line decrease in magnetic flux and a diminishing of generator output. If the generator output is allowed to rise above the rated voltage then the flux values would remain fairly constant -a near flat line on the graph- BUT allowing the generator to spin faster would still increase the output voltage. This would increase the current through the field coils causing them to heat up and, if things got extreme, they could burn out. My guess is that the DC generator may be allowed to “freewheel” somewhat, to operate at a higher voltage, but some research would need to be done to properly adapt these electronics to particular DC generators. Here, I don’t think it’s a one-size-fits-all kind of thing.