@atyh @PawelK LiFePO4 batteries still need PMUs and they aren't easy for me to find at least for my requirements. Using 4 in series would be a 12.8V bundle but the individual cells need to be managed too. I've looked into engineering lifepo4 replacement batteries for laptops and it readily becomes apparent that they would rather die.

Can you construct a Stirling Engine? It's very simple and only requires a temperature differential to operate. It can either be operated with heat or cold on the respective sides. Geothermal heat from 20m down and cold from the environment would likely work. Also solar panel cooling lines could run through it before being used for hot water.