I've thought about this a fair bit, played a little in the garage (easy when you're on 13 acres), and have bought and used the BioLite wood stove / thermocouple USB charger. A few thoughts:
It's not a thermocouple, but a whole pile of them in series certainly for 5 volts and possibly in parallel for more amperage.
100C ain't a great differential for a thermocouple (or Carnot-cycle power generation either). The more the better, to the limits of materials.
If you can project your hot-side HX into the exhaust gases, there will be a lot of radiant heat transfer (paint it, don't leave it bare metal).
On your cold-side HX, there will be much less radiant transfer going on and more of the heat transfer will be through conduction/convection. Note that the BioLite uses a fan on the cold side (which also fans the flames of the fire).
Chimneys can be too short (VERY common), too wide or too narrow for the size of the fire. Chimneys can not be too tall. Taller is always better (although not as ultralight).
The output of the thermocouples will be limited to some % (5-10%) of the heat passing through them. Don't think of GETTING them hot, so much as the heat FLOWING through that pathway. Therefore, any air gaps between the TCs and HXs will be very performance limiting.
I found with the Biolite, I had to tend it A LOT to keep it putting out power, despite feeding it very dry, split, hardwood bits. Hence, I would want it in the tent, where I'd be sheltered, to keep putting bits of wood in. And, I've recently found, a heated tent is a wonderous thing in cold weather, with clothing to dry, etc.
I've got a thermocouple pile, about 2" x 2" by 0.125" if you want one to play with. Send me your address in a PM.
-David
