Forwarded from Anthony (thermal energy)

loupaulsen at loupaulsen at
Wed Dec 4 09:06:39 MST 2002

> It seems to me the potential of thermal energy is
> being underrated in the discussion about the
> developing fossil fuel energy crisis. Thermal energy
> currently available is pretty much limited to
> naturally occuring geysers and hot springs.
> But what is the deep drilling techniques developed by
> the oil industry were to be extended to the
> artificicial production of thermal energy?
> Such deep drililng could be relatively shallow
> anywhere along the junctures of tectonic plates all
> over the planet. The introduction of water - salt or
> fresh - would produce the thermal power.
> It seems to me that the energy available in the
> earth's core could be accessed more easily in the
> short and medium run, with less serious environmental
> side effects, and less capital investment, than solar
> power.

I actually had the same idea once and discussed it with my brother, who is an
engineer.  He wasn't as optimistic.  What he said was this:  What you're
talking about is a 'heat engine'.  You will have some kind of medium that you
pump into the hole and evaporate, and then you bring it 'up top' to do work,
and you transfer the heat off to some cooling medium and let the heat transfer
medium condense and pump it back down the hole again.  The efficiency of this
is limited by the difference in temperature between the hole and the cooling
medium.  You need a source of cold water for the cooling medium, and this is
going to turn into warm water, so you already have a heat pollution problem.
Some of the energy that you get is going to have to be used to pump the medium
up and down the hole.  There are going to be problems with corrosion and
mineral deposits everywhere in the system.  But all this isn't the big problem.

The big problem is that if you drill this hole, you now have a cylinder lined
with rock.  Well, rock isn't a very good conductor of heat.  In fact it's an
insulator.  If you pump your transfer medium down into the hole, it will be
absorbing the heat from the rock in a layer around the hole a couple inches
thick, and cooling it off, and you will have to wait for a long while for the
earth's heat to filter back in and warm that layer back up again.  Of course
you could drill a LOT of holes and use a LOT of pipes, but it gets to the
point where you are using more energy to produce pipes and drills than you are
getting out of the whole system.

The reason hot springs and geysers work is that there are natural veins and
crevices in the rock down below that have a lot of surface area whereby the
earth's heat can be transferred to the water.  And the reason you don't have
them all over the place is that this kind of natural situation isn't that
common.  It is not a trivial task to try and reproduce this set-up
artificially, or at least not so far.  Maybe there is some way you can drill a
hole and drop a nuke down the shaft and shatter the rock the way you want it,
but it's not off-the-shelf technology exactly.

Probably it's still worth thinking about, and it would probably be good to put
a research institute onto it.

Lou Paulsen

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