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.
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).
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. They hope to get down to 15 and expect temperatures of 300 C. Now if we say the surface is 10 C we’re looking at an increase of 20C /km. So we’d need to drill 4.5 km (~3 miles). That’s a long, long way just to pump down some water and get steam. Besides as the steam returns it’ll 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 it’s much easier to let Mother Nature build a geyser.
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.
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.
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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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?
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 
As in, a hole a kilometer on each side, and 135 kilometers deep? I think you need to check your math.
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.
I’m thinking he assumed 1000 cubic metres = 1 cubic kilometre when, in fact, it’s actually 1,000,000,000 cubic metres.
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.
How would you stop the steam coming back up the feed pipe?
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?
Here in Boston, Trinity Church has been using this technology to provide both heat in the Winter and cold in the Summer.
Barry
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.
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?
Thermodynamics wise it would be fine. Actually building, maintaining and operating the thing is cost prohibitive.
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.