Here is my 5 year old thread about the Russian Thermocluster company products. They had a line of products based on thermocoupling effect. Their web site is down unfortunately. All I've been able to find is their price list in Russian. They had a pot which was able to generate 12W at 12V while boiling water over a campfire. I saw its blueprints on some web-site - it's almost indestructible in a typical camping environment. The price must be cheap too because there is nothing special involved in its thermocouple design (just an array of different metal pairs welded to the bottom plate and covered with an end-cap).

This one is thermo-acoustic. So, some moving parts are involved...