Constant Voltage of a Peltier TEG Tile
Peltier effect delivers a voltage according to the temperatures (which must be
different to create nonzero output) of the junctions.   This can be
made constant by putting one end in an icewater bath, and the other in boiling
water.  Assuming the ice melting temperature and water-at-a-boil temperature
are nearly constant, so will the output voltage be.

This is a common issue with ALL heat engines, the temperature difference
is essential to get any energy out, so energy becomes unavailable when
thermal equilibrium occurs.   The removal of energy from a Peltier device
causes heating of the cold plate, and cooling of the hot plate, in proportion
to the current drawn, so any electrical energy removed will cause the ice to melt faster, and the boiling water to require more fuel.