I think that an interesting way to look at this proposal might be to consider '
Heat Pipes'.
This principle is used in many evacuated tube solar collectors. The idea is to find an appropriate working fluid that would vaporize at a temp found at the bottom of the heat pipe, and change state into a liquid (condense) at the temperature found near the top of the heat pipe. The vaporization/ condensation point can be adjusted by changing the pressure of the working fluid.
The heat absorbed by vaporization and then released by condensation is considerably larger than the delta-T would suggest.
This would not supply enough heat to run an engine, but it could provide a way to bring low level heat energy to the surface, with no moving parts.
Quote:
if the below ground temp is always ~55*
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...the average temp depends on the latitude. It's around 55 degrees near the around here in Oregon, for instance, much warmer in the southern states, colder in the northern regions.
-AC_Hacker