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View Full Version : If temperatures are higher deep underground why can't we generate energy this way?


astro
07-31-2003, 09:35 PM
I was watching Modern Marvels on the History Channel and they were discussing the challenges of mining the Comstock silver mine and how temperatures at 2000 feet below the earth are 130+ F degrees all the time. I know we can't generate energy using this differential between the surface and the depths or we'd be doing it, but I've forgotten precisely why in my dotage.

Remind me why this is impossible.

scr4
07-31-2003, 09:46 PM
We are doig it. (http://www.eere.energy.gov/geothermal/geopowerplants.html) Though I believe it's only economically feasible in volcanic areas.

Whack-a-Mole
07-31-2003, 09:54 PM
Geothermal energy is used in many places around the world including in the US. Usually though it is only in places where thermal energy is relatively close to the surface (IIRC Iceland is a big user of geothermal energy). Realize they aren't looking for any old hotspot but for reservoirs of hot water already existing underground (so this narrows where you can build one).

I think mostly the issue is cost. Unless a suitable site with (relatively) easily accesible energy is found it is cheaper to build and run a conventional power plant (e.g. coal).

Grey
07-31-2003, 10:23 PM
Here's my take. Note to actual geologist semi-wild assumption coming up.

Deepest drilling I know of was in Siberia and they only got down 12 km (http://hypertextbook.com/facts/2003/AdamCassino.shtml). They hope to get down to 15 and expect temperatures of 300 C. Now if we say the surface is 10 C were looking at an increase of 20C /km. So wed need to drill 4.5 km (~3 miles). Thats a long, long way just to pump down some water and get steam. Besides as the steam returns itll loose heat to the cooler surrounding rock and your efficiency tanks.

Yeah you could use a lower boiling point fluid, but the cooling problems remain along with all the issues of relative stability of the surrounding rock etc. As others have mentioned its much easier to let Mother Nature build a geyser.

Thaumaturge
07-31-2003, 11:04 PM
Well.....you could build the generater at depth. Then you wouldn't have to pump the water up, only the power, which is far easier to pump. Of course, building the generator that far down has it's own challenges such that it would probably not be worth it.

Grey
07-31-2003, 11:26 PM
Actually if it was economically feasible for more people a heat pump in the basement fed into the ground for about 20/30 feet would provide a heat sink in the summer and heat source in the winter. Some people do this already but it does cost.

eburacum45
07-31-2003, 11:33 PM
Another problem is that rock is a good insulator, so that eventually you would cool the rock down around the generator, and have to drill a new hole; this problem is not so apparent in active volcanic areas where the throughput of energy is a lot greater.
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I Love Me, Vol. I
07-31-2003, 11:41 PM
Could you let gravity take the water down into the deep hole, and then let the steam rise out by itself-- thus using no pumping? And would an insulated pipe for the returning steam keep the temperature hot enough to still be cost effective?

Grey
07-31-2003, 11:48 PM
Perhaps, if it was cost effective in the first place to dig a 3 mile hole in the ground. I didn't mention this but the drilling in Russian began in 1970 and hit 12 km around 1994. This was a "drill small diameter hole way down deep" kind of experiment.

Given that you'd likely want to be able to perform maintenance in the piping and you'd want a substantial amount of working fluid in eth first place lets say the diameter if 5m across on both sides. We now need to drill/dig out 135 cubic km of earth. For that price we could buy Cape Code and build the wind farm :D

Chronos
08-01-2003, 02:10 AM
We now need to drill/dig out 135 cubic km of earth.As in, a hole a kilometer on each side, and 135 kilometers deep? I think you need to check your math.

scr4
08-01-2003, 02:43 AM
Originally posted by Thaumaturge
Well.....you could build the generater at depth. Then you wouldn't have to pump the water up, only the power, which is far easier to pump.
I'm afraid that won't work. It's not the high temperature that does the work, it's the large temperature difference. You need some way to transport heat so something hot (e.g. hot steam) meets something cold (e.g. cold water or radiator) at the generator.

Gest
08-01-2003, 06:20 AM
Originally posted by Chronos
As in, a hole a kilometer on each side, and 135 kilometers deep? I think you need to check your math. I'm thinking he assumed 1000 cubic metres = 1 cubic kilometre when, in fact, it's actually 1,000,000,000 cubic metres.

Grey
08-01-2003, 07:25 AM
Nope you right. Looks like I multiplied 30m with 4.5km. As Gest pointed out it should be 135,000 m^3not 135 km^3.

Mangetout
08-01-2003, 07:45 AM
Originally posted by I Love Me, Vol. I
Could you let gravity take the water down into the deep hole, and then let the steam rise out by itself-- thus using no pumping? And would an insulated pipe for the returning steam keep the temperature hot enough to still be cost effective? How would you stop the steam coming back up the feed pipe?

Alessan
08-01-2003, 08:35 AM
Hmm. What if we developed a metal alloy that conducted heat extremely well, wrapped all of it - except the top and bottom - with an insulator, stuck it 5 km into the ground and built a generator on top of it? Sort of like sticking a poker without a handle into a fire. Would that work?

godzillatemple
08-01-2003, 08:40 AM
Here in Boston, Trinity Church (http://www.popsci.com/popsci/hometech/article/0,12543,220187,00.html) has been using this technology to provide both heat in the Winter and cold in the Summer.

Barry

Alereon
08-01-2003, 09:43 AM
scr4: Expose 300C water to normal pressure air. Use resulting high-pressure steam to turn turbine. Granted, condensing more of the steam back to water is going to be difficult, but if you can get some ventillator shafts going, it may be doable.

Alessan: No need to use a solid tube of metal, we have vapor-phase heat pipe technology.

astro
08-01-2003, 11:28 AM
Eveyone has been very kind about responding to this thread, but I guess my OP question is still (for me) still floating out there specifically in terms of why the heat exchange relationship won't work except in near surface geothermal zones.

You dig a deep hole and it's 200C - 300C or so at the bottom. With this kind of heat you can boil water, run a Stirling engine of some kind etc. etc. Why can't we just throw some kind of rugged, modified heat exchange turbine down the hole and get near limitless energy? Where specifically does this cunning plan break down thermodynamics wise?

Grey
08-01-2003, 11:42 AM
Thermodynamics wise it would be fine. Actually building, maintaining and operating the thing is cost prohibitive.

RM Mentock
08-01-2003, 11:45 AM
You still have to dig the whole, and heat doesn't conduct through the rock all that well, so you'd use a lot of energy getting there, then suck out the available heat quickly--and there wouldn't be that much of it to compensate.

A lot of the geothermal applications depend upon convection rather than conduction. Advances in technology may change the picture.

Grey
08-01-2003, 12:01 PM
What if we took existing mines as our hole in the ground? They are serviceable, accessible and some of the South African ones require massive cooling equipment to prevent their workers from being cooked, so they may be hot enough.

You could use the ambient temperature and not the rock itself to draw heat from. The issue then is getting the working fluid to a place where it can be condensed and cycled back through without loosing too much efficiency. Hmmmm.

astro
08-01-2003, 12:29 PM
Originally posted by RM Mentock
You still have to dig the whole, and heat doesn't conduct through the rock all that well, so you'd use a lot of energy getting there, then suck out the available heat quickly--and there wouldn't be that much of it to compensate.

A lot of the geothermal applications depend upon convection rather than conduction. Advances in technology may change the picture.

I see. So beyond the construction cost issues, if we pull the heat out of the surrounding soil or air to do work and essentially cool the area down, the rate at which it will re-heat would be quite slow in getting back up to useful temperatures that we can generate energy with. Is this it?

Sofa King
08-01-2003, 12:45 PM
Can't you generate some electricity by utilizing the Seebeck effect (http://www.xyroth-enterprises.co.uk/seebeck.htm)? Seems like you could do it without moving parts.

prisoner6655321
08-01-2003, 12:46 PM
Yes. Geothermal energy, though exciting, is not a viable alternative energy resource. Even locations that utilize naturally occuring geothermal resources, like The Geysers in California, have witnessed a significant decrease in energy output, due to man's involvement. I wrote a paper on this in College. Imagine in 50 years old faithful not being so faithful. There was a church that wanted to tap geothermal resources just outside Yellowstone. That would have devastated the geysers in the Park.

What the OP is talking about, using the ambient temperature of the rock, could generate electricity. Enough for a reasonable sized home. But the energy output would be short lived.

MEBuckner
08-01-2003, 12:51 PM
A Japanese company which specializes in Ocean Thermal Energy Conversion (http://www.xenesys.com/english/otec.html) systems is apparently trying to make something like the idea in the OP work (http://www.xenesys.com/english/stec.html).

prisoner6655321
08-01-2003, 05:55 PM
Whoops. That "Yes" was in response to this:[i]Originally posted by astro
I see. So beyond the construction cost issues, if we pull the heat out of the surrounding soil or air to do work and essentially cool the area down, the rate at which it will re-heat would be quite slow in getting back up to useful temperatures that we can generate energy with. Is this it?

Rhum Runner
08-01-2003, 10:04 PM
Also, don't forget there would be a massive energy investment in digging the hole to begin with. Think of all the energy required simply to move the drill bits up and down as you dig, the man power to constantly be adding/taking off pipes from the drill etc.

eburacum45
08-01-2003, 10:41 PM
Like I said earlier, you end up creating a cool spot underground... pity that it is too dangerous to tap the phenomenal energy of an active volcano

unless anybody knows a safe method?

I noticed a link to a Japanese OTEC company earlier on . Now that is a technology that seems promising- heavy on initial investment tho'.