Thanks Peter very interesting, I haven't come across this system before.
Sounds like free energy (to run the motor), but I must be missing something
You don't get free energy it must come for somewhere it must be using some of the heat from the wood burner to run the fan therefore the wood burner will give out less heat
Absolutely right, or almost. The thermo-electric generator does absorb energy and turn it into electricity to turn the fan, but the electric energy all gets dissipated as heat again, so there's no nett loss. The whole point of the fan is to blow air across the stove, and therefore put more heat into the room for a given stove skin temperature, which means you get a bit more useful energy per unit fuel consumed.
The word "thermopile" is usually reserved for arrays of metal thermocouples, most likely chromel-alumel types which will give about 10 millivolts with the cold side at 0C and the hot side at 50C. You used to be able to buy cylindrical mounted stacks of these things in the US, designed for running a small electric motor to power a boat for fishing. I think they were mostly gas fired, but I suppose in principle you could have used a small fire of sticks or something. You'd certainly need a thousand or more separate thermocouples.
The things on the ECO fan are slightly different in that they employ a semiconductor junction made of something horrible (to a semiconductor engineer such as myself) like bismuth telluride rather than two metals. The details of the physics are just a little different, but you get something over 100 millivolts for a 50C drop, so a lot less junctions are needed. Identical devices are used in cold boxes for the car and suchlike.
Are they worth while? Quite possibly for a wood burning stove without water jacket, although if the latter does apply and radiators are run, I doubt it.
Just in case anyone considered it, NEVER eat bismuth telluride: it'll give you the most dreadful indigestion, possibly fatal.