What would be the theoretical problems of mass geothermal power?

[Cikomyr2]

Basically the question in the title. I know geothermal power uses heat from the magma of the earth, and AFAIK nothing is taken out of the ground.

However, I understand heat transfer. Geothermal power takes heat OUT of the earth. Does it mean there is more heat in the atmosphere? Would there be consequences in mass extracting heat from the magma mantle? I know current plants are basically a drop in the ocean, but would there be consequences doing in on a Super Large Scale?

[NichG]

I would guess that mass geothermal would actually produce less waste heat than the same power generation provided through nuclear power, coal, oil, etc, because it's intercepting a portion of heat that would have entered the environment anyhow, and extracting some usable work from that rather than just letting it slop down into equilibrium; whereas fuel-driven power generation takes energy that was otherwise bound up and releases it. You're still accelerating the transfer of energy from mantle to surface, so you'd still be increasing surface temperatures, but less than producing the same energy from a purely fuel-based method.

Aside from dumping waste heat into the environment, I guess really mass-scale geothermal power generation would decrease natural events in which that heat transfer is performing work. So you might see undersea geothermal vents drop in temperature, or even have a reduction in volcanic activity. If you did enough to change mantle convection patterns, you might affect the composition of the atmosphere over very long timescales. Basically, everything on the surface periodically circulates through the mantle on geological timescales, keeping minerals and oxidation/reduction state refreshed. If you extracted things in a way such that it was actually enough to kill off continental drift (even if you still had deeper down mantle convection), you'd start to deplete things on the surface. I'm not entirely sure which way it'd end up going, but I'd guess you'd tend to see more acidic conditions as carbonate reservoirs in the oceans wouldn't get recycled. But at that scale of activity, you're probably boiling the surface with waste heat anyhow so...

[Sermil]

Not seriously, but if you wanted a cool story...

Extracting enough heat from the mantle would cool it enough to shrink it. Which means the outer shell now needs to contract, which probably involves a lot of new mountains forming and earthquakes. You might actually get more volcanoes for a while as you cracked the outer crust more often.

[Mastikator]

If you cool the mantle all the way down to the core so much that earth becomes entirely solid then the magnetic field would dissipate. Which would leave us vulnerable to cosmic and solar radiation. (but don't worry, we couldn't get there even if we tried, climate change will stop us)

[Anymage]

Using existing technologies? I don't think we'll be able to have any significant effect either way. The second issue that world governments would face if they all decided to start tapping geothermal is that there are only so many hot spots available, and they can't be scaled up well. (Plus you might deplete some of them with use, but that won't have any large scale effects beyond using them up.)

Your primary problem would be the simple fact that a lot of geothermal hot spots are already in use, particularly for tourism purposes. Compounding on that, many of the surrounding areas would get upset if you tried to build a power plant there. You could get a lot of power out of Yellowstone if you tried, but people would get very mad if you built your station on top of Old Faithful.

Future technologies that let us access the mantle more directly would have all the environmental issues that massive geoengieering and strip mining bring. Energetically it wouldn't be that significant (think about all the things people have said about the comparative thickness of the crust vs. the mantle), but accessing the mantle would bring its own set of problems.

[Tvtyrant]

Basically the question in the title. I know geothermal power uses heat from the magma of the earth, and AFAIK nothing is taken out of the ground.

However, I understand heat transfer. Geothermal power takes heat OUT of the earth. Does it mean there is more heat in the atmosphere? Would there be consequences in mass extracting heat from the magma mantle? I know current plants are basically a drop in the ocean, but would there be consequences doing in on a Super Large Scale?

They would break a lot. The same thing that makes geothermal good makes it very unreliable. Imagine sticking the US' infrastructure on Yellowstone and having it break from one of the multiple earthquakes it gets a year (much less the volcano blasting it off the planet.) Of course that is true of anywhere to a certain extent, geology is very unstable from any sort of longer than one human time view.

[Altair_the_Vexed]

There's far more hot Earth than there is energy need.

*begins scribbling on the back of an envelope*

Collectively, humans use about 600 terajoules of energy each year (i.e. 6 x 10^14 J).

Looking at the heat capacity of a couple of key elements - iron, silicon - let's say that 1 kg of Earth mantle cools 1 degree C for every 20 joules we take out. It's very approximate, but we're in the right ball park.
(BTW, to keep in SI units, I'll use kelvin, "K", for temperature from now on).

We're not going to take heat evenly out of the whole of the Earth at once, so let's look at the hot bit just under the crust only - it's reckoned to extend about 700 km deep below the crust, so about 10% of the Earth's total mass. Let's imagine that you're only going to cool this bit down. It weighs about 6 x 10^23 kg.

So to cool all of that mantle by 1 K (which is a tiny fraction of its temperature), we need to extract 3 x 10^22 J of energy. That's about 50 million times the amount of energy that humans use each year.

Even accounting for poor efficiencies in extraction and conversion, and any order of magnitude errors I've made in my sums, we're looking at a very long time. It's unlikely that we'd still exist as a recognisable species over that sort of period - 50 million years ago, the continents weren't even quite in the recognisable positions they're in now.

---

Far more urgent issues are that there are some difficulties in the heat extraction methods - it's kind of like fracking, and it might even release greenhouse gases from underground.
https://en.wikipedia.org/wiki/Geothe...nmental_impact

That's not to say it's not a good energy source - just we need to be careful. I think we need to consider all the non-hydrocarbon energy sources we can.