There are several threads on this topic, but I'll update this "somewhat-less-old" one. Two interesting videos of applications of this concept.
(1) A 10 grade girl who won second prize in a science fair with a "hollow flashlight" charged by her hand (I think she claimed 50% efficiency):
(2) A guy charging a 12v deep cycle battery with TEGs and a biomass heat source (what's important is that he is doing it DIY):
I could see tackling some version of these with my 11 year old engineering-interested daughter in a couple years. I plan for the "Black & Green" Civic to still be around too, so why not?
-james