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MiniGeoThermal Generators

Recycling 90% of exploratory oil wells
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They say that in the oil industry, 90% of holes drilled are "dry"; they don't yield oil. So far as I know, this has been true since the early days, and is still true despite improvements in scanning rock formations underground before doing any drilling.

However, just because they might have wasted a bunch of money drilling 9 holes out of 10, that don't produce oil, they still have drilled 9 out of 10 pretty deep holes that could be used for something else. So, here's what I suggest:

It is widely known that the deeper one digs underground, the hotter it gets. Exploratory oil well holes typically go down a mile or more (at one time the cost was something like a million dollars a mile, but that didn't stop them from often going deeper, and it doesn't stop them now, with the cost inflated over the years). Obviously, the rocks at the bottom of a "dry" hole are going to be pretty hot!

In a "standard" geothermal energy plant, there is a natural presence of water, which gets boiled by hot underground rock, and steam comes out at the surface, which we simply trap to drive turbines and generators. Variants on the theme include injecting water underground at one place, and collecting the steam at another place. Not much has been done about extracting heat from the "dry" rock at the bottom of a single deep hole.

The likeliest reason for lack of efforts along that line has to do with the desire for large power plants. The problem is that a large power plant needs a lot of heat to generate a lot of power. There might BE a lot of heat at the bottom of a deep hole, but after it is removed, then what? You have to wait for more heat to reach the hole from the surrounding rock, before you can generate more power!

Now consider a scenario in which a SMALL power plant is built on top of a "dry" exploratory oil well. We know how deep the hole is and we know how hot it is at the bottom, AND we can make a pretty good estimate about how much heat we can withdraw, such that it is STEADILY replaced by heat from the surrounding rock. This is a scenario which can produce power (not a huge amount) for a nice long time.

And there are more than a hundred years' worth of "dry" exploratory oil wells waiting to have these small power plants built on top of them. THAT'S how we can get a lot of power from this idea!

Vernon, Jul 21 2008

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       If this worked, they would be using it to pump fresh air (never mind air-conditioning) to deep mines, a la DRD's or ERPM's. Truth is, the energy is 1,5 - 3,5 km deep and all plus some is used on moving a working fluid. Geothermal is wonderful, when found close to the surface.   

       I appreciate that you mitigate this with "already existing holes of no use". But to pump any working fluid, (even gas), from 1 mile deep, needs more energy than is given. Else the surface of earth would be one big Ol' Faithful matrix.
4whom, Jul 21 2008
  

       [4whom], it's not as bad as you think, if we consider a closed-loop system, say a tube-within-a-tube. The wall of the inner tube is an insulator. Water under pressure (to prevent it from boiling) goes down the hole in the outer tube and hot water comes up the hole in the inner tube. The hot water exchanges heat in a steam generator and a turbine generates power from the steam.   

       Sure, it takes energy to pump the pressurized water around the loop, but, after it gets going, it only takes energy to overcome the frictional losses, because at that point Momentum is working in our favor (that's the factor I think you have neglected). Obviously the tubes need to be coated with a low-friction material, to minimize those frictional losses. At the temperatures involved, probably ordinary Teflon will work.
Vernon, Jul 21 2008
  

       Yes, I have been giving the "Esspresso machine" hexagonal thingy, a bit of thought, at least atmospheric pressure lends a hand. You know... create a vacuum and the stuff gets sucked/pushed up. And that additional power could be fed from solar?
4whom, Jul 21 2008
  

       //Water ... goes down the hole in the outer tube and hot water comes up the hole in the inner tube. The hot water exchanges heat in//   

       The tubes up and down ARE a counter-flow heat exchanger. Without some hellacious insulation, the heat exchange takes place in the tubes, and the whole system dies.
baconbrain, Jul 21 2008
  

       [baconbrain], I'm aware that a most excellent insulator is required. I will suggest vacuum if necessary, although this means the tube-walls, already strong enough to hold pressurized water, need to be even stronger.
Vernon, Jul 21 2008
  

       Halleluah!
A [vernon] idea I can actually understand, and don't celebrate two birthdays reading. [+]
AbsintheWithoutLeave, Jul 21 2008
  

       [Vernon]: do you have any data on the temperatures likely seen at the depths you describe? I ask because I know that there are thermodynamic cycles that can take advantage of temperature differentials of less than 100C.   

       Efficiency is irrelevant with a virtually infinite supply of power.
TIB, Jul 22 2008
  

       [+] Why do you need pump for water circulation? Make a wide trough at the mouth of the well to collect rain water & allow it to go in the well. If there is sufficient heat, steam will come to surface of earth as in case of natural geyser. And now there are plenty of saturated steam turbine suppliers with wide capacity range. During dry season, keep the plant idle or inject water from nearby source, if available.
vedarshi, Jul 22 2008
  

       Of course as well as supplying power, this will cause the interior of the Earth to cool, eventually causing it to solidify, which will in turn cause the end of tectonic plate movement, earthquakes and volcanoes, and the deterioration of the Earth's magnetic field and the protection this gives us from the lethal solar wind.
hippo, Jul 22 2008
  

       [TIB], yes, I know about OTEC systems, which use a temperature differential of some tens of degrees. I'm sure this will be rather more than that, but the main problem, as I said, has to do with how rapidly heat withdrawn from the hole can be replaced by heat migrating through the rocks down there. A deeper hole will of course allow our system to interact with more (and hotter) rocks, and thus more heat can be withdrawn, but in general I wouldn't expect to be able to get more than a megawatt from any example of this type of power plant.   

       [vedarshi], an "open" system such as you describe is an already well-known idea in the geothermal energy field, and Ideas posted here are suppose to be different. Also, you have to deal with minerals transported by the heated water, and the effects of those minerals upon the first stages of your power plant equipment. A closed loop such as I described here can't gather as much heat, but maintenance costs will be lots less.   

       [hippo], when the Earth is cool enough for plate tectonics and volcanoes to stop, the core will still be molten enough to support the planetary magnetic field, but a different problem than what you describe will be more important and sooner than in your scenario. Active geology is the only thing the ground surface of the Earth has, to keep it above sea level. About 100 million years after most surface activity ends, the continents will have eroded into the seas.
Vernon, Jul 22 2008
  

       //[vedarshi], an "open" system such as you describe is an already well-known idea in the geothermal energy field, and Ideas posted here are suppose to be different.//   

       [Vernon] to be honest, i had not fully read your initial annotation. now i have observed, the 4th para of your initial annotation & my first annotation are almost similar.   

       having worked for many years in thermal power plant, i'm very well aware of the criticality of boiler feed water demineralization. but will you explain, what do they do to demineralize the steam obtained from geothermal source before input to steam turbine?   

       and in case of the well, if it is operated only during rainy season, during dry season, it will gradually get emptied due to escape of steam & finally you will get solidified minerals as by-product; some of these may be valuable ones.   

       and if you want to do away with water pumping, consider connecting the well with underground water source by drilling a trench.
vedarshi, Jul 26 2008
  

       [vedarshi], what they do in ordinary geothermal-steam power plants is irrelevant to this Idea, since I'm talking about a closed loop for the water that is sent down the hole inside a pipe, and comes back up the same hole inside a second (nested) pipe. There is no need for me to specify any other water than will fit in the closed piping system. Also, this Idea would work best if the entire diameter of the hole was occupied by the outer pipe, so rain can be ignored. Ordinary groundwater can also be ignored; it's not going to flood the already-filled-with-pipe "dry" exploratory oil borehole.
Vernon, Jul 28 2008
  

       Try no water movement?   

       1. Can the plant be at the bottom of the hole, like wells where the pump is underground? Then just wires to traverse the distance up, instead of insulation and thermal losses.   

       2. Semiconductor or other non-turbine heat-to-electricity technologies, with smaller footprints at the bottom? Assume the bottom is always hot; then just need to get outside cold down there via aluminum, oil (but just cold- not hot), etc. heat transfer medium.
Bcrosby, Jul 28 2008
  

       [Bcrosby], well, if something goes wrong at the bottom, you have a problem accessing your heat-conversion hardware and fixing it. Simpler to use the aluminum or whatever to carry heat from down there to up here, where we then do the conversion.   

       One of the most efficient heat-carrying systems is known as a "heat pipe". I don't know if it would work across a vertical distance of a couple of kilometers. Perhaps we could "stack" multiple individual heat pipes in the bore hole. Internally, a heat pipe uses convection. The solid aluminum notion would use conduction as the heat-transfer metod, which is rather less efficient than convection. When we pump water through a hot spot, we are EMULATING convection, and of course we know this can work across long distances, with suitable insulation.   

       It MAY be possible for conduction to compete with convection, provided the right substance is used, although I don't know that for certain. The very best solid heat conductor is diamond. It's so hard that it treats heat-vibrations as sound waves, and the sound waves are then conducted throughout the diamond at the speed of sound. Of course, filling a kilometers-long borehole with solid diamond, just to conduct heat out of it, would be an expensive proposition!
Vernon, Jul 28 2008
  
      
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