[empraptor]

I don't know what the current state of thermophotovoltaics are but maybe that's one way to go about it. Or maybe the temps are too low for that. Not sure.

https://www.science.org/content/article/thermal-batteries-co...

UPDATE: Papers about geothermal source temperatures refer to tempratures up to 180°C. The thermal battery article above refer to TPV setups that use 1400°C source and efforts are being made to push temps to 2400°C for higher efficiency.

https://www.sciencedirect.com/topics/engineering/geothermal-...