Originally my question was, “How deep could we dig?”- if we threw all of our resources at the problem, how deep a hole could we make, what are the biggest problems encountered and so on. Feel free to talk about that too.
Obviously at some point we find magma. Before things get all melty, presumably it gets hot enough to boil water. So, even without setting an all-time record for the deepest hole ever, surely we could dig deep enough to boil water and run some turbines. Something like this would probably be a pain to build and maintain, but once it was running we could have fuel-free power, potentially on an unlimited scale.
It’s easier to go to areas of the earth where geothermal heat is closer to the surface. Improvements in the technology and high energy prices will extend the suitable areas.
There’s already several geothermal energy plants across the globe, mainly along lines of tectonic plates and thinner crust. It’s either not technically feasible or cost-effective to dig deep enough into earth’s crust, otherwise, to obtain the thermal energy needed for a power plant.
As I understand it the problem isn’t the depth, we can already drill deep enough. The problem is creating a flow of water to use in the heat extraction cycle. Present geothermal plants use water that’s already there, but creating a flow of our own means somehow connecting the bottoms of two shafts that we’ve drilled deep into the Earth and it’s been an intractable problem last I’ve heard.
It’s not so much in connecting two shafts as having a reservoir of sufficient size at the bottom so that heat is extracted efficiently. The temperature has to be above the boiling point for efficiently driving a steam turbine, usually by maintaining the underground water well under pressure well above the boiling point and transferring the heat to a seperate clean water supply to drive a turbine.
Techniques similar to fracking where large underground areas are become saturated with hot water and steam are alleged to be a cause of earthquakes, which could eliminate a simple solution to the heat transfer problem.
Small geothermal wells are being drilled all over the place. The smaller ones need less surface area to conduct heat below the surface. But the cost of drilling places a minimium size on a geothermal plant that would efficiently produce electricity for sale.
I think part of the problem would be cost per watt. Sure, you can probably dig down far enough to get to a temp of 212F, but how many such holes would you have to dig in order to produce x watts of electricity. Even with horizontal drilling I suspect it would still be cost prohibitive.
Now consider that the US is the Saudi Arabia of coal and nat gas (using fraking though) and the fact that nat gas is something less than $3 per MMBTU (thousand thousand BTU’s or million BTU’s), and the cost issue really becomes the predominant one.
It would be pretty expensive, especially considering I am imagining a project on a massive scale e.g. the 3 Gorges Dam of geothermal excavation projects. It’d cost a fortune to build, but OTOH the amount of energy that stands to be gained seems practically limitless. It isn’t like the Earth is going to just cool off…
It makes sense to have a lot of smaller geothermal wells operating. Large enough to be cost efficient for the investment, but small enough to serve a local area without high losses in transmitting electricity over a distance. The cost of drilling for each well is no more than for an oil well, but the reservoir/heat exchange problems have to be resolved.
If the area of excavation were on the scale of square miles, perhaps we could expose a very large bit of hot earth, build what is necessary on/in it and then fill it back in. How deep would that be anyway?
The issue with geothermal, in areas of good resource (see map on third link), is the upfront cost. But otherwise it is comparable to nuclear - low carbon, solid baseline power, low operating expenses. From the second link:
Where water runs down mountains, you can build water-wheels to run mills. Hydroelectric generators, like inside Hoover Dam, are just big hungus water wheels, right?
So how about this: Where molten lava (that’s just magma, right?) runs down the side of a volcano, you can build magma wheels. Falling magma turns wheel, which turns generator.
I’m not sure where to do this. My idea is not traditional geothermal, as I’m suggesting simply digging until it gets boiling hot, which theoretically you could do literally anywhere. Obviously we wouldn’t do this too close to a populated area or someplace like Yellowstone which we want to preserve. It is something like the vast solar arrays people sometimes imagine for the desert southwest, miles and miles of generation in the middle of nowhere connected to the rest of the world via massive DC delivery lines.
The plan is not to find someplace where there is steam already leaking out of the ground, but rather to simply find a few square miles that not too many people will object to basically destroying utterly and digging until you hit lava/it is hot enough for the purpose. One question is: what area of ‘hot Earth’ would need to be uncovered to provide enough steam to power turbines equivalent to say 50 nuclear power plants? (Maybe more; does the space taken up by turbines, pipes and etc. for a nuke plant take up even a football field’s worth of space? Does it keep getting hotter the deeper you go, or does it level off?) The hole would have to take the form of an upside-down pyramid (or maybe not…), so the area at the surface is proportional to the area of hotness required. The turbines might be more conveniently located partway down, closer to the heat, or maybe ringing the surface too, I dunno. The point is to do it as massively as possible to really leverage the effect of economy of scale, and also because the amount of heat in the Earth is effectively unlimited if one tries hard enough.
I suppose someplace where the Earth is rocky/solid/stable all the way down is the best choice for a project like this simply because it’ll hold its shape rather than caving in. Maybe the parts of Nevada where the US did nuclear testing and nobody visits? I don’t know, I am not a geologist. Maybe do it in one of the largest existing open-pit mines since the project is already underway in those places.
If done on a colossal scale it would amount to a major, permanent energy source which, if given a long enough time frame, would necessarily be worth it at some point.
One problem is that with today’s technology, one would have to burn one hell of a lot of fossil fuels to do an excavation like this. To make it more fun, plan the engineering such that as much work as possible is done with manpower/animal power, say in a global-warming panicked future world. The US may not be the best place for this version, but rather someplace really crowded and more desperate, or say China or India simply because they have literally hundreds of millions of extremely poor people.
Clearly being realistic is not the top priority here. I’m more interested in where the limits to an idea like this come into play, what is theoretically possible, that kind of thing.
Geothermal energy can be extracted anywhere in the world. It is easier in natural ‘hot spots’, but you just have to go a little deeper other places.
The massive economy of scale has limits. There is loss in the transmission of electricity, and having it generated all in one place is inefficient.
There is also a heat transfer rate issue. Numerous small wells will be more efficient than a few large ones because of the geometry of the surface are in the well to conduct heat. The wells then have to be spaced sufficiently apart to prevent cooling the earth in a small area.
Take a look at long running geothemal production in California. They are taking advantage of a hot spot. But the same level of production could be maintained anywhere with more extensive drilling.
Why do you want to go any deeper than hot enough to make water boil? That’s how current geothermal for electricity generation works. No need to hit lava. See the links provided.
The maps in those links show that at 6 km down across most of the country you would hit over 100 degrees Celsius, enough to drive turbines, and much closer (cheaper to exploit) in most of Nevada and into Oregon Washington Idaho and Northern California.
You don’t need to expose it all. You need to drill and put down pipes, just like drilling for oil, except that you need to make a complete circuit.
From the linkDSeid provided, This is what I am talking about:
There is simply a staggering amount of potential there. It isn’t clear to me to what extent the Earth’s internal heat regenerates itself if heavily tapped. Does it amount to basically ‘free’ heat because the energy ultimately comes from gravity? I’m not sure. What I’m asking here is if the potential of magmatic systems can really be compared to oil, since oil can be burned and then be gone whereas I assume magmatic systems are constantly re-heated from below.
You guys are insisting that drilling is better than the kind of massive excavation I am imagining. But with drilling 6km, you have all your equipment 3 miles away from the energy source, whereas with an enormous excavation you could place your turbines much closer, on terraces halfway down or whatever. And a huge excavation could be planned for huge expansion potential. Hundreds of pipes coming out of the hot earth at the bottom powering hundreds of turbines on the terraces which pump out DC current to points far and wide. If you want still more, drill another dozen or two wells into the terraces, you’re already 3-5 km down at that point.
I don’t know, maybe it really is ridiculous. How do they do it now? Drill a wide-diameter bore hole, construct the u-link at the surface and lower it into the hole, then attach pipes to that section and lower it in, over and over until it reaches the bottom?
The heat from coalescence of the Earth, which was ultimately from gravity, is long gone. What we have now comes from the decay of long-lived radioactive elements like uranium and thorium. Which is an ongoing process, but that doesn’t necessarily mean that it’s sustainable: The processes that make coal and oil are ongoing, too, but we’re depleting the accumulated reserves of æons. Similarly, we also now have a large supply of “fossil heat” built up, and it could be that we might deplete that reserve faster than it’s replenished.
There are enormous practical problems locating and operating a steam plant far below the surface like that. It’s really hot for one thing. In addition, geometry is a problem. The large the volume of water you need to heat, the less conductive surface area there will be unless you start using complex shapes, which increase your costs. The rock that far under the earth is under enormous stress and you don’t really want to be creating odd shaped chambers that could collapse. And there’s no reason to think this has any greater economy at a massive scale. There’s a reason we don’t make current power plants 50X bigger now. It isn’t practical. That doesn’t mean that our energy needs can’t be supplied by many geothermal wells. As I’ve pointed out before, there is an economy in numbers there. It places the power generation closer to the end user, minimizing transmission losses.