Regulating Voltage From a Bike Generating Electricity?

I'm trying regulate electricity that is coming from a motor that is being turned by pedals on a bike. The motor is outputting voltage anywhere from 0 volts to 40 volts or more. If you pedal the bike backwards, it will output negative voltage - which will blow up the grid tie inverter we are using. The ideal range of the Grid Tie inverter is 12.3V to 29.3V. I'm trying to make a circuit that regulates the voltage to within this range. I was thinking that I could probably do this with diodes. Any help would be greatly appreciated!