[pjc50]

Heat harvesting is tricky because it depends on temperature difference, so you need to keep the hot side and the cold side sufficiently far apart.

[ansible]

And you need some kind of heat engine to extract useful work from the difference. These typically have a significant size/mass associated with them.

I think solar cells are the way to go for now, though vibration harvesting can work for some applications.

[pharke]

No engine required https://en.wikipedia.org/wiki/Thermoelectric_effect