I recall a comment on stove fans that sit on top of wood/multifuel stoves
but cannot now trace it. Anyone else remember it or who commented please?

I've been looking at some on-line and they appear to fall into two
categories - stirling engine and 'electric' but I can't figure out how
the electric ones work without mains or batteries.

I'd have thought the simplest format would be a simple heat rise
updraught from the stove to turn the fan blades.

What obvious thing am I missing and does anyone have any recommendations
for *currently available* ones at reasonable price. The "Aldiddles" both
seem to have had some but no longer.