> small but steady temperature differential twixt down-there and surface ma=
y allow power generation.

I thought someone said that poor conduction of soil was a problem?  You hav=
e a nice steady temperature difference because underground is isolated from=
 heat sources.  Once you start pumping heat down there, via heat pump or th=
ermoelectric devices or whatever, you start heating up the "local" part of =
underground, and if conduction is poor (which is must be, or underground wo=
uld be the same temperature as above-ground, essentially), it rapidly appro=
aches the surface temperature and your scheme stops working very well.  (Pr=
esumably, this is why ground source heat pumps don't work as well as people=
 think.)

A lot of "thermal energy harvesting" seems to have a bigger problem keeping=
 the cool end cool, than providing heat for the hot end.  Stirling engines,=
 too.

BillW


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