I had planned on using a 3.6V cordless screwdriver motor and gearset to turn the drum. I figured the startup amps for the motor could be determined with a multimeter and then the solar array sized accordingly. I know zeners are typically used in parallel with a voltage source as shunt to regulate voltage but in this case I'd like to try using it in series to only allow current through in full sun. Don't really have any specific numbers yet as it's beeing planned and specific components haven't been sourced yet. Based on your response, the voltage drop across the zener would be 10Volts. I thought when they broke down they were essentially a short, which is why I'd like to wire it in series. Power wasting is not of concern as the the device is to turn the drum a bit once an a while so I don't have to do it manually....because I'm lazy and forgetful. I suppose if it shortened the life of the diode I would be concerned about that.
the best I can do on the numbers is that the array will be series up out of 0.5V cells to get around 3.6 volts and then paralleled up to get the required current. the actual wattage expected wuring operation through the circuit would be on the order of 2 or 3 watts, 5 at the most.