The Naked Scientists

The Naked Scientists Forum

Author Topic: Geothermal energy in 22nd Century (or maybe 21st)  (Read 7065 times)

Offline Atomic-S

  • Hero Member
  • *****
  • Posts: 928
  • Thanked: 18 times
    • View Profile
Virtually unlimited geothermal energy resides under all portions of the Earth's

surface. Obtaining it however has thus far been hindered by mainly 3 factors: 

Pressure, temperature, and volume. The temperatures in geothermally good places

are, of course, by definition high, which can do nasty things to equipment. The

depth at which such meaningfully high temperatures typically exist is a matter of

roughly 10 kilometers (depending on how you define "meaningful"), and the pressures

at that depth are very considerable, resulting in a tendency of any borehole to

collapse, especially if the rock has been softened by heat.  All of this greatly

complicates removal of the last portion of the considerable volume of material that

must be removed to make a hole of that depth. Existing technology is inadequate to

deal with these problems.

But what are the chances for non-existing technology? To answer that question, we

need to examine what the problems are. As noted, temperature, pressure, and volume

are the obstacles.  Today there exists limited means for dealing with temperature

and pressure.  Deep holes typically are drilled with the aid of drilling mud, the

weight of which in a deep well helps to counterbalance the ambient pressure. Such a

fluid also can remove heat while being pumped in and out of the well. It seems

clear that the first line of attack, therefore, in dealing with temperature and

pressure is to look at the drilling medium. 

While existing muds do the job for existing types of wells, trying to use them at

much greater depths is more difficult by reason of the fact that the density of the

mud and of the surrounding rock typically are not equal, because the mud perforce

contains a significant percentage of water, a relatively lightweight material. The

deeper you go, the more the dissimilarity in pressure between the mud and the rock

will be, limiting how far you can go before bore cave-in starts becoming a problem

again. Also, the deeper you go, the more difficulty there is in pumping this fluid,

which will become increasingly necessary if the mud is to play the role of coolant

during drilling into hotter and hotter strata. Also, because the density of mud and

of steel are not equal, the deeper you go the less effective the mud is in

counterbalancing the weight of the drill shaft, so that eventually its own

uncompensated weight becomes a problem in terms of its structural integrity.

And then of course, after drilling through several kilometers of overburden, how

much power can be extracted from one well? The cost analysis here may not look very

good compared to the same drilling costs expended in a petroleum field.

New technology, however, might change this picture. We might start by looking at

the way that very long bores have been made horizontally, as under mountains for

railroads etc. This is not done with a drill shaft, but with some kind of

travelling boring machine, and a separate debris transport system. Applied to

geothermal boring, this would suggest getting rid of the drill shaft and using some

kind of roving mole. Of course such a mole has to be supported in a vertical shaft,

but that could be accomplished by all-terrain wheels on the machine, with the

cuttings being evacuated by some suitable system, such as a suction line, which

could be of a material whose density closely matches that of the drilling medium,

and therfore floats neutrally regardless of depth.

But there is still the issue of uncompensated pressure, due to insufficient density

of the drilling medium. The first thing that comes to mind is to try to find a

fluid that does not have this problem. It may prove, however, difficult to find a

fluid having the density of the surrounding rock, which is affordable in the

volumes required, and also is ecologically acceptable for injection into aquifers,

etc.  And if we could find it, what would it be like to actually function with in

it (in terms of its corrosiveness, murkiness, other detrimental properties)? 

Many problems could be simplified if the drilling medium was just plain water.

Water is environmentally clean, minimally detrimental to machinery (with

appropriate design), easily pumped, cheap, and also optically clear, a property

which as we will see could be a considerable asset.  Its only real problem is its

inability to fully balance the weight of surrounding rock strata at great depth.

But if that problem could be overcome, working in water would have many advantages.

We need some way to adjust the water pressure as we go down. Water pressure is

already adjusted in navigation by the use of locks (Panama Canal). Why not pressure

locks, installed down the borehole every so many hundred feet, to maintain a

pressure difference between segments of the borehole greater than the water alone

would create? In this way, the water could be kept at near-ambient pressure no

matter what the depth was.

This probably means that boring would not be accomplished by means of a drill stem

running all the way to the surface. Instead, a roving mole would go down, served by

a water supply line and a discharge line. Both of these lines would pass through

the pressure locks where turbines or some other equipment would maintain the

pressure difference. Power would presumably be electrical, because electric cables

are unaffected by pressure differences, and can be run to any arbitrary depth so

long as they are fastened to the side of the hole. To permit removal of the mole or

portions of it for servicing (as replacing cutter heads), the pressure locks would

also have to have hatchways permitting this. Pressure adjustment pumps at each lock

would be involve too, possibly incorporated into the water supply and waste

discharge systems.

Such a system solves the pressure problem, and also the temperature problem,

inasmuch as the water would cool the immediate region of drilling.

The volume problem, namely the cost of drilling so deep a hole compared to the

energy to be obtained, argues for another level of advancement in technology.

Because of the high cost of drilling through the relatively cool overburden, as few

such holes as possible are desired. But we may want quite a few holes in the hot

lower regions to extract maximum energy. This argues for a two-step approach: A few

holes would be drilled down to the point where temperatures start getting good and

uncomfortable, and then at that depth, a staging area for more elaborate works

would be constructed. The model is that  of underground mining, where a few shafts

from the surface access numerous galleries undergound. Among the possible layouts

are a system of horizontal tunnels down from which protrude numerous access wells

into the hottest zones, or, a system of angled holes branching out from the end of

each main hole to the surface, like bristles of a brush from the handle.

whatever the design of the subsurface network of passageways, these extreme depths,

and the water filling them, pretty well rule out the presence of human miners.

Enter robots.  Robots could do the deepest work, submerged in the water, and thanks

to water's clarity, easily see what they are doing.  Thanks to the counterbalancing

pressure of the water, large caverns could be carved out, permitting large

equipment to function. All wastes would be pumped to the surface, as noted before.

As the work progressed, the heat removed would increase, and at some point would

approach the operating yield of the installation. Even before the work was

completed, the heat to be exhausted would reach such an amount that boiling would

take place well below surface level, as superheated water from the lowest depths

ascended to depths of less pressure.  Thus, the pressure locks would be ideally

positioned to exploit this phenomenon, by having turbines that would generate

electricity even before the water (steam) had ascended to the surface.

This could happen in the 22nd century, although much of the basic science to do it exists today.


 

another_someone

  • Guest
Geothermal energy in 22nd Century (or maybe 21st)
« Reply #1 on: 05/11/2007 06:57:11 »
Would the rocks at those depths be impermeable to water, or else, how do you contain the water (particularly at such high pressures that it may be supercritical, and possibly even chemically reacting with the surrounding rocks)?

How much energy can you hope to extract without effecting the density and temperature of the underlying magma sufficiently to begin to cause a local collapse of the magma?
 

Offline JimBob

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 6564
  • Thanked: 7 times
  • Moderator
    • View Profile
Geothermal energy in 22nd Century (or maybe 21st)
« Reply #2 on: 06/11/2007 02:35:20 »
The technology for extracting electricity from the geothermal gradient has already been developed and has been in use for quite a few years now. It isn't necessary to drill deep to do this. All that is needed is a temperature gradient and then capitalize on the temperature differential.

It is done with water here in the US. The system is simple a pump a system of many bore holes 100 feet deep, lined with PCV pipe and with rubber hose tubing circulated through the numerous holes drilled. It can be use to both heat and cool a building. Just a few feet below ground level, there is a constant temperature of 72. The water is cooled or heated to this temperature, depending on the season, and is then circulated through the building. This simple technology is being utilized all over the US.

Also see a list of patents here - http://www.freepatentsonline.com/CCL-60-641.4.html - many of which will work.

Lastly, when presented with the opportunity to invest in this, I was broke, having just had some very expensive cancer surgery and treatments. If I had had $50,000 5 years ago, I would be a multi-millionaire now. I could have gotten in on the third round of venture funding. (http://www.powertubeinc.com/)

 

another_someone

  • Guest
Geothermal energy in 22nd Century (or maybe 21st)
« Reply #3 on: 06/11/2007 03:48:18 »
The technology for extracting electricity from the geothermal gradient has already been developed and has been in use for quite a few years now. It isn't necessary to drill deep to do this. All that is needed is a temperature gradient and then capitalize on the temperature differential.

It is done with water here in the US. The system is simple a pump a system of many bore holes 100 feet deep, lined with PCV pipe and with rubber hose tubing circulated through the numerous holes drilled. It can be use to both heat and cool a building. Just a few feet below ground level, there is a constant temperature of 72. The water is cooled or heated to this temperature, depending on the season, and is then circulated through the building. This simple technology is being utilized all over the US.


What you seem to be describing is not so much geothermal energy as using the soil as a heat storage medium for diurnal heat differences (perfectly valid, but somewhat different).  One also has to ask how much land is needed for each building - maybe be fine in rural areas, but would it really work in a metropolitan context?
 

Offline JimBob

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 6564
  • Thanked: 7 times
  • Moderator
    • View Profile
Geothermal energy in 22nd Century (or maybe 21st)
« Reply #4 on: 06/11/2007 20:27:28 »
Geo - the earth
thermal - temperature

It is by definition geothermal energy. Diurnal heating has nothing to do with the process described. Below a given depth in any specific area, temperature is constant. This depth is not very deep in the majority of the world. These systems have been used since the '40's.

http://en.wikipedia.org/wiki/Heat_pump  This is the theory behind the drilling and pumping. It works and IS geothermal. It is applicable to urban settings as well as rural settings.

http://www.eere.energy.gov/consumer/your_home/space_heating_cooling/index.cfm/mytopic=12640

See this links for the UK
http://www.bgs.ac.uk/products/gshp/doc/gshp_jet.pdf
« Last Edit: 06/11/2007 20:40:20 by JimBob »
 

another_someone

  • Guest
Geothermal energy in 22nd Century (or maybe 21st)
« Reply #5 on: 07/11/2007 00:30:41 »
Geo - the earth
thermal - temperature

Insofar as it uses the Earth as a medium for heat storage; but the question that is still not clear is whether the Earth is the heat source (i.e. is heat coming up from the magma, or is heat coming down from the surface and being stored by virtue of the high heat capacity of the soil).

Some of the sites seem still to be ambiguous (not least the reference to using pond-water as an alternative to deep soil, and one would expect the pond-water to be heated more by the atmosphere than the soil).

But it still does not answer my question as to how much ground is required for each residence, and is it plausible in areas of high density housing (you say it is applicable - but more specifics about how much ground area is needed for a given surface volume of building seems not to be apparent)?

I am also concerned about long term maintenance of such systems (particularly if they are using vertical loops, and especially where high density housing might require that the system be built beneath the surface buildings).

I did once work in a purpose built office building where they had ponds outside as a heat sink for their aircon, and it did work well for them, but it was a low rise office block with a fair amount of free land surrounding it.  The building was purpose built, and allowed for such special features; but the problem becomes more acute when you are trying to retro-fit, or mass produce cheap housing using standard designs and standard components.

The problem is that many technologies, as you scale up production, you can begin to reduce the cost of the technology; but in this case, it seems that each solution has to be site specific, so you cannot easily scale up production to reduce cost.

It may make more sense on a communal basis rather than an individual building basis, but then the question remains about the amount of land needed for given volume of surface building (including allowing for the fact that if you have lots of these around, all closely packed, they will begin to interfere with each other).

Ofcourse, one possibility, rather than digging holes to fit these pipes, is to use existing holes (such as underground railway tunnels, or sewage pipes).
« Last Edit: 07/11/2007 00:38:37 by another_someone »
 

Offline JimBob

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 6564
  • Thanked: 7 times
  • Moderator
    • View Profile
Geothermal energy in 22nd Century (or maybe 21st)
« Reply #6 on: 07/11/2007 01:12:29 »
Any source of earth energy is geothermal - see the British article. Th average English Row house back garden has enough room for a heat pump. Long term is answered in the US link. the rest of the questions are answered in the links.

"The problem is that many technologies, as you scale up production, you can begin to reduce the cost of the technology; but in this case, it seems that each solution has to be site specific, so you cannot easily scale up production to reduce cost."

The cost issue is addressed in the US site.

In summation, you are using the heat of the earth to do work. Either colling or heating, it is still work. If I remember my physics, work requires energy. Thus, BY DEFINITION, this is geothermal energy.
« Last Edit: 07/11/2007 01:26:54 by JimBob »
 

Offline Atomic-S

  • Hero Member
  • *****
  • Posts: 928
  • Thanked: 18 times
    • View Profile
Geothermal energy in 22nd Century (or maybe 21st)
« Reply #7 on: 07/11/2007 05:52:43 »
Quote
Would the rocks at those depths be impermeable to water, or else, how do you contain the water (particularly at such high pressures that it may be supercritical, and possibly even chemically reacting with the surrounding rocks)?
Any porosity present that is not already filled with groundwater, would be an avenue of escape for injected water; this would simply have to be made up. At very great depths, the rock pressure is so great that porosity would not appear to be a problem. If leakage at shallower depths were a problem, it would be possible to put in a casing (liner) in the hole. If perchance porosity at quite low depths is significant, that would present another way to collect the heat, even as is done already in some geothermal systems in which water is pumped from one well through rock fissures to an extraction well. Chemical reactions with surrounding rocks? I would assume this would apply, if it does, only at very high temperatures, or in certain types of rock. Both could be avoided. Or exploited. If the water dissolves away rock, that provides a new drilling mode. Supercritical temperatures? At the kind of pressures where that would be encountered, I don't think it is a problem. On heading upward, the heated water undergoing expansion could go through turbines. But if the water is too hot to allow this, then the system has been installed too low, and should be relocated to a somewhat higher, and somewhat cooler, location.

Quote
How much energy can you hope to extract without effecting the density and temperature of the underlying magma sufficiently to begin to cause a local collapse of the magma?
I don't know. That would have to be studied, to see, among other things, to what extent it actually is a problem -- considering also that layers below could be sufficiently molten to fill in with minimal geological disturbance.

Quote
It is done with water here in the US. The system is simple a pump a system of many bore holes 100 feet deep, lined with PCV pipe and with rubber hose tubing circulated through the numerous holes drilled. It can be use to both heat and cool a building. Just a few feet below ground level, there is a constant temperature of 72. The water is cooled or heated to this temperature, depending on the season, and is then circulated through the building. This simple technology is being utilized all over the US.
Electricity? This sounds more like a scheme to stabilize the temperature of a building against the weather, thereby lowering its consumption of energy from utilities.

Quote
The technology for extracting electricity from the geothermal gradient has already been developed and has been in use for quite a few years now. It isn't necessary to drill deep to do this. All that is needed is a temperature gradient and then capitalize on the temperature differential.
I am aware that this has been done to generate power, and works, but my scheme was oriented more towards large-scale power production.

Quote
But it still does not answer my question as to how much ground is required for each residence, and is it plausible in areas of high density housing (you say it is applicable - but more specifics about how much ground area is needed for a given surface volume of building seems not to be apparent)?
Not much ground area should be required at all. It is just a matter of how deep the tubes go. You could easily, within the footprint of a building having only a few floors, put in enough tubes to give it excellent thermal ballast, without descending to anywhere near the point where the natural earth temperature would be significantly hotter than the surface.

Quote
The problem is that many technologies, as you scale up production, you can begin to reduce the cost of the technology; but in this case, it seems that each solution has to be site specific, so you cannot easily scale up production to reduce cost.
Spoken, I assume, of a thermal-ballast system. I would tend to agree with you here. The alternative is to try to do thermal ballasting on a utiliti-scale basis, which would involve installing one very large thermal bank, and then piping water or whatever to and from it from a number of buildings. Then you have to worry about things like thermal losses in transit. Ideas of this kind might work in certain situations, however. One is when there is a nearby body of water than can be used as the thermal reservoir; or perhaps a municipal reservoir could be harnessed for this purpose, it having the advantage that it might be covered and less exposed to the weather.

Quote
I am also concerned about long term maintenance of such systems (particularly if they are using vertical loops, and especially where high density housing might require that the system be built beneath the surface buildings).
Might be a problem. Especially if it is built the way sewer lines often are -- which attract roots which clog them up.

Quote
I did once work in a purpose built office building where they had ponds outside as a heat sink for their aircon, and it did work well for them, but it was a low rise office block with a fair amount of free land surrounding it.
Why not put the lake under the building in a tank?

Quote
Ofcourse, one possibility, rather than digging holes to fit these pipes, is to use existing holes (such as underground railway tunnels, or sewage pipes).
A sewer plant might serve as a good thermal ballast for many buildings.

Quote
In summation, you are using the heat of the earth to do work. Either colling or heating, it is still work. If I remember my physics, work requires energy. Thus, BY DEFINITION, this is geothermal energy.
Undeniably, and this form of geothermal energy demonstrably works in some situations. What I was proposing, much more ambitions and expensive, is geared to a different sort of mission:  primary electricity generation on a large scale, replacing a substantial amount of existing plants -- the kind of thing that was once expected of nuclear fusion.













 

Offline JimBob

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 6564
  • Thanked: 7 times
  • Moderator
    • View Profile
Geothermal energy in 22nd Century (or maybe 21st)
« Reply #8 on: 07/11/2007 19:20:06 »
This scheme posted just would not work for many reasons:

1.) THE MAIN REASON - It just cannot be done. Your idea is not technically feasible. First, the pressure at depth would collapse any known pipe used as a liner. Before you get to the depths needed in your scheme, pressures of over a million pounds per square inch would be encountered. Secondly, the temperature would melt any know substance or combinations of substances - At the depths you plan to go, temperatures of 3-5,000C would be encountered. Just not any known substance that would not melt.

2.) The second reason - there are many, many more cost effective geothermal energy producing techniques already in place that can be efficiently exploited. See the Power Tube link in a previous post, go to their site and read about the technology. It is completely scalable and can be used in a large urban area as well as a smaller setting. Used as wind turbine electric generation is presently used it can do the job you are suggesting.





 
 
 

The Naked Scientists Forum

Geothermal energy in 22nd Century (or maybe 21st)
« Reply #8 on: 07/11/2007 19:20:06 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums