Ok, so running 5 TEC's in series would give you 17.5 volts at no current. As you draw more current, the voltage drops. You have to be above the battery voltage to produce any current at all.
Most TECs are the type to used cool things. These don't allow very high operating temperatures, and their efficiency as power generators is very low. You would have to find a company that manufactured modules specifically for power generation. I think what i would do would be to put the hot side on the exhuast, and cool the cold side with engine coolant. You'd probably want to insulate the exhaust system ahead of the power generation pack to ensure temps were higher. It would also work well in combination with a warm air intake, as it would boost exhaust temperatures.
You wouldn't need much coolant to keep the cold side at more or less coolant temperature, as it take a phenomenal amount of heat to heat any significant flow of engine coolant.
I think 50w would be totally doable. That's just a little over 4 amp at 12v.
