Thermocouple power/current production efficiency
The fact that thermocouples produce a (relatively low) EMF is key to understanding how to get power out of them.

You can't do anything about the EMF of a single junction (for a given temperature difference, which is limited by your heat source and material melting points) but you can increase the current by reducing the electrical resistance (i.e. using thicker wire).

However this also reduces the thermal resistance, conducting heat from the hot junction directly to the cold junction - and this is where the efficiency limit comes from. You cannot conduct more current from that junction without conducting more precious heat away from the hot junction.

he crucial parameters in a thermocouple are thus:


The potential (EMF)
The ratio of electrical conductivity to thermal conductivity.


It's one of those annoying facts of life that good electrical conductors are normally also good thermal conductors, so (2) tends to be pretty similar (and lower than you want for this purpose) across materials.

Typical metals yield efficiencies of 3% or below : to get 8% you need fairly exotic materials (bismuth and tellurium compounds I think). I don't see any prospect of improving this without some fundamental breakthrough in solid state physics.

Series or parallel combinations of junctions cannot affect this : all those thermal shorts are always in parallel. You can increase power by using more in series (increasing the voltage) or more in parallel (another way of thickening the wire) but the efficiency won't improve : those thermal shorts are still always in parallel.