Just for fun, I once built a flyback boost inverter to convert around 10mV to six volts. It used a SINGLE thermocouple junction and it worked, provided the drive electronics was externally powered.
The thermocouple was a pilot flame detector salvaged from an old gas hot water heater. The thermocouple generated around 10mV at 50mA (from memory) when attached to a soldering iron tip !
I did get my six volts, but even my multimeter loaded the output significantly. Flea power indeed, only half a milliwatt input power, but it was an interesting thing to try.
It is what happens during retirement, too much spare time.