I think I agree the RPM is too high. Consider if the voltage is linear down to 14 volts to charge a 12V battery, then:
102/14=7.3
5120/7.3=702 RPM - that's still pretty fast.
Since VAWT's don't turn very fast, that leaves you with the gearing, but then the VAWT won't start turning on its own... kinda chasing your tail...
Try a search on "Electronically Commutated Motor" or ECM for short.
They are used often in newer HVAC systems and there may be one driving the fan in your house's furnace. No don't use that one!
But I mean you can look at it and see if something similar might be suitable for your project. Several members of Fieldlines have put them to good use in small wind turbines and this might be ideal for yours. The next problem, of course, is that there are a bazillion on e-bay and finding one that's right is possible but takes a bit of research, first.