[CliffordK (OP)]

Many of the oil wells are over a mile deep, and have a substantial amount of geothermal energy.  Yet, over time, the oil production decreases and they are eventually abandoned.  Although, with new oil extraction techniques and rising oil prices, some are being reopened for additional oil extraction.

Drilling new geothermal wells can often be prohibitively expensive.

Should we be putting heat exchangers into the abandoned oil wells for geothermal energy recovery, either on a large or small scale? 

Ok, so I think I have a great idea, and someone else has already thunk it up   [xx(]
http://peswiki.com/index.php/Directory:Geothermal_Oil_Wells

Although, it is unclear how widespread of a practice this is.

[MikeS]

Would the ground at that depth be sufficiently hot and will that geothermal heat energy be replaced fast enough, can it be done economically and can we accomplish it without doing untold damage.  Maybe but probably not.

[MikeS]

If we could come up with a solar powered (inc. wave, tides etc) method of extracting heat from the sea then might this not be a better and more sustainable alternative.  Or we could use windmills to directly drive heat pumps and pump the heat ashore much the same as we pump electricity ashore.  This would represent a gain of (I think) about 400-500% over generating electricity but of course there are other problems.

[Geezer]

Getting vast amounts of heat from the sea is comparatively easy. What's not so easy is extracting work from the heat.

[CliffordK (OP)]

I was just thinking about something small for the wells that already exist.

Home Heating
Domestic Hot Water Heating
Heating for Small Campuses
etc.

I suppose that most of the wells aren't exactly in Urban areas.  And, it would be expensive to, say put in, say 2 miles of heavy gauge pipe into each well.  But, if the wells have already been drilled, and are essentially abandoned, then why not?

I was envisioning a closed circulatory system, but if the wells are truly mostly empty, perhaps one could circulate water between wells for a better heat exchange system.

[peppercorn]

How about using Phase-change materials lowered down on a lift! []
Then a little railway track to move the 'hot-stuff' to the local town!

[]

[Airthumbs]

I know this is not exactly on the topic of wells but as you said cliffordK someone else has thought of it  

What is the smallest heat exchanger?  If I heated up the coils in my electric fan would it produce electricity!  Like spinning an electric motor in the wind...     

It probably would not but is there not a way of doing this?  More specifically I mean a small device/material that converts heat into electricity? 

[CliffordK (OP)]

You may look up:
Piezoelectric, electricity generated with change in pressure, or bending.
Pyroelectric, electricity generated with a change in temperature
Thermoelectric, electricity generated with a temperature difference applied to two sides of a device.

Unfortunately, none of these work with a static heating, but rather with some kind of a temperature change, or temperature gradient.

A heat pump can create a temperature change or gradient, but requires energy to do so.
--------------------
If you stuck a partly insulated "U" shaped pipe down in an abandoned oil well.
Since cold water is denser than warm water, you should be able to generate circular flow of water through your system with no added energy, and thus bringing warm water to the surface where one could generate a gradient of warm/cold water for power generation. 

[Airthumbs]

That's it, that's what I am thinking of CliffordK, the third one, Thermo....(It's gone I can't see it anymore as I am re-posting)  But anyway, a bigger version of that little picture top right in wiki with a big heat sink the cool side.  Would that do something?  Or how about putting one in between your cold water and hot water tank?  What about making house walls with them so that the difference in temp outside and inside would generate electricity...??     I heard on the moon it is pretty hot in daylight and pretty cold in the shade.  Would these thermo things not work really well in space also?

[CliffordK (OP)]

That's it, that's what I am thinking of CliffordK, the third one, Thermo....
Or how about putting one in between your cold water and hot water tank?  What about making house walls with them so that the difference in temp outside and inside would generate electricity...??

Undoubtedly it will cause a movement of heat.  So, if you had a hot water tank and a cold water tank, after some time, you would end up with two luke-warm tanks.

Likewise, if you are heating the inside of the house, you would loose that heat.  But, you could use a similar device with the diurnal change of temperature, or perhaps with solar heating.  The average solar panel is less than 20% efficient, although the advanced triple junction panels are about 40% efficient.  Anyway, perhaps you could utilize thermal solar heating (vs shade/earth/water temps) for additional power generation.  Or, just glue a thermal-electric device to the back of an ordinary solar cell, to get additional power generation.

I heard on the moon it is pretty hot in daylight and pretty cold in the shade.  Would these thermo things not work really well in space also?

On the moon.

In the polar regions, it is awfully cold in the bottom of the craters where the sun doesn't shine, and awfully hot in the direct sunshine.  And, without the atmosphere, one could have a rotating panel getting the full sunshine all the time also near the poles (is there enough tilt to get a summer and winter?)

In space, many ships are given a roll to even out the sun exposure on all sides.  But, perhaps you've described a new type of solar panel.  Black side directed towards the sun, getting hot.  Back side with a heat sink to radiate the heat away.\

The question is whether the thermal only panels would rival the weight to energy production of the triple-junction panels in use today.  Or, perhaps the would have good longevity. 

Obviously one issue with deep space probes is the further away one gets from the sun, the less energy that received from the sun (and thus the need for some kind of a compact RTG).