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Comment by phreeza | Hacker News Reader

One thing I have never quite understood about geothermal, maybe someone can enlighten me: the energy flow from the Earth's core to the surface is not that huge, less than 1 watt per square meter. Doesn't that fundamentally limit the usefulness of geothermal power as a general solution outside of exceptional spots where this gradient is locally much higher, or there is an opportunity to collect from a wide area with a single small borehole? And if I drill a hole and collect 500 watts from it on a 100 sqm plot, am I effectively siphoning the heat from my neighbors plots?

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The cited value is the energy flow through the surface, which is about 0.1 W/m^2. But this is conducted through kilometers of rock and soil, which acts as insulation. Geothermal power works through convection instead of conduction. You inject cold water into a borehole, and hot water (steam) comes back, and spins a turbine.

Convection can extract energy at a much higher rate than conduction through the crust.

If you boil a pot of water, you can still comfortably hold the pot's handle if it's long enough, indefinitely. This is heat conduction. On the other hand, if you try drinking from the pot with a straw, you'll find it very painful. This is convection.

Perhaps a stupid question, but... what are the risks? Wolndn't extracting too much energy from the earth's core cool it down, at least a little bit? Or does it contain so much energy that extracting it to replace all of 'surface generation' won't make even a little difference?

We can't drill anywhere near the mantle, geothermal extracts energy from the upper part of the crust.

Wouldn't drawing energy from the crust cool it down, and wouldn't a cooler crust in turn 'draw' more heat from the lower layers? I guess the earth already radiates out a lot of energy, the process of extracting geothermal energy will presumably lead it to radiate more energy. I don't know by how much though, or if it will make any real difference, or if that's how it even works.

I guess if you could extract enough energy from the core it would reduce convection which would in turn reduce the strength of the earth’s magnetic field.

Look at a cross section of the Earth: https://www.britannica.com/science/oceanic-crust

The deepest bore of all time was 12 km deep. The crust is between 5 km and 100 km and thinnest under the ocean. The numbers involved here are staggering. One might as well hope to stop the Earth's winds with a windmill.

I saw a documentary about some long running geothermal projects and basically the temperature in the well cooled down and made it uneconomical. They said they would have to wait, I think, 30 years for it to heat up again.

Earthquakes! There are couple comments mentioning "fracking" around, that's destabilizing the land by injecting acid to get energy out. The acid is dangerous, and so are breaking up soil deep down.

> Wouldn't extracting too much energy from the earth's core cool it down, at least a little bit?

The earth generates ~50 terawatts of energy through radiation/other processes, while global energy consumption over the last year was 0.003 terawatts. I think we're fine.

Where are you getting 0.003 terawatts? Another user elsewhere in the thread[0] claimed "Global total energy (not just electricity) consumption is currently 180,000TWh/year, or about 20TW."

Google is showing me other figures like 25,000 terawatt hours of electricity consumption annually.

[0] - https://news.ycombinator.com/item?id=43234856

One might also be careful to count energy properly. The fossil fuel industry has been counting "total energy" including losses to make fossil look bigger and harder to replace. But a gas car throws away like 70% of the energy, so going electric, you don't need the same energy to run the car. Not even close.

Oops, I missed the thousands. So 3 TW, which is larger, going by Google stats, and 20 TW by the other users. So that's not negligible.

What stops you from cooling down the area around the borehole to the point where you are conduction-limited again?

On what timescale? Over even just a few second timescale you are cooling rock, but digging deeper gives higher starting temperatures and more volume to remove heat. So, dig far enough and you can get an above some temperature an arbitrary long period say 100 years.

Given your geothermal power plant operates for 100 years and pulls from say 1w/m2. Then you move to a new location for 100 years, and then come back you’re limited to 1w/m2 + 1w/m2 = 2w/m2. Have not 2 locations but many and eventually you’re fully recharged.

You probably do cool down the surrounding area.

But my guess is that the 1w/m² quoted by GP is no where near the energy we get from the sun. Quick Google says sunlight in the order of 1kw/m² (sure that's dependent on where you are, it sun doesn't shine at night, but we're off by 3 orders of magnitude here).

So probably it'll have no effect on surface temperature.

Besides, the heat is mostly released anyways when driving a steam turbine, and the electricity also becomes heat, in your computer or whatever.

Pulling heat from km below the surface isn’t going to reduce surface temperature by 1w/m2, but ~0.001/m2 over thousands of years. Thus the issue is warming the area more so than cooling it.

What matters here is the recharge rate, but all power plants have a finite lifespan. You can simply move somewhere else up to the point you run out of untapped geothermal energy across the surface of the earth which is a rather crazy number.

Enhanced geothermal uses fracking to expose absolutely massive amounts of m^2 through fracking between two parallel, long, horizontally drilled bore holes. Current efforts seem to show a minimum of 30 years before there will be loss of heat quality.

The sorts of drilling talked about for enhanced geothermal are on the scale far far above the needs of a house, IIRC about 5MW per bore hole pair, with many connecting from a single point on the surface. It's also at distances kilometers into the earth.

There is no way to tell yet what the longevity of the resource will be as it's too early. In fracked resources the main issue is "short circuiting" where increased flow rates travel along preferential paths between the doublet wells as the source rock cools and cooling rate of the source rock in general. This causes the MWt of the resource to decline per injection / production well. Fervo is getting around this by drilling extra wells per pad to be turned on in response. Many geothermal resources decline over time as heat is slowly extracted and these declines are somewhat manageable by tuning the injection production well rates and drilling new wells. They are built into the economics of existing plants. Geothermal is kind of extractive and not "renewable" in this way over medium term time scales, you need to continuously keep drilling at a certain rate. Rock is a good insulator and it takes a long, long time for it to heat back up.

> And if I drill a hole and collect 500 watts from it on a 100 sqm plot, am I effectively siphoning the heat from my neighbors plots?

Pretty much. But the Earth's crust has a lot of thermal mass, so there's enough energy stored there to last for a long while.

That has always been my understanding.

It’s useful for HVAC for a home for example, where you aren’t trying to do a conversion to electricity but instead are directly leveraging the consistent temperature to reduce strain on the system.

Yea that is a heat pump scenario where you are also putting energy back in at certain times, that makes sense to me too, it doesn't have much to do with extracting geothermal energy from the core iiuc.

Geothermal is mining heat stored in the crust. It can extract heat at a much higher rate than the average crustal heat flow, at least for a time.

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