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Comment by Mordisquitos | Hacker News Reader

Specifically Albania, Bhutan, Nepal, Paraguay, Iceland, Ethiopia and the Democratic Republic of Congo.

Not to downplay the positive steps that are being taken towards using renewable energy worldwide, but one must point out that all those countries except one are almost exclusively using hydroelectric power, whose availability at such scale is a geographical lottery. As for Iceland, which also relies mostly on hydroelectric power but not in such great a proportion, it makes up for it thanks to easy and abundantly available geothermal power (which, though environmentally friendly, is arguably not technically renewable).

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Well yes, hydro and geothermal are the easiest (and earliest perfected) renewable sources to provide consistent base load. It would be odd if the first countries to achieve fully renewable power weren’t making use of those technologies.

Other countries will have to be more reliant on interconnects, diverse renewable mixes and batteries. Luckily this is now almost always cheaper and more secure than fossil fuels and the trend lines point towards that continuing to be more and more true over time.

>at such as scale

Not to downplay the positive steps that are being taken but we are conveniently skipping over the denominator here at least in the case of Ethiopia and DRC who both have a grid that is only serving their full population at a fraction of the level needed to make this story one about geographical lotteries and abundance instead of one about poverty preventing them from access to the traditional carbon power generating routes to server the rest of the population.

Well, when geothermal stops being renewable there will be no humans around to need energy generation.

You are still technically correct, which is the best kind of correct.

But if we follow that rationale, in a long enough timeline, solar and wind is also not renewable.

Also, many of these countries are tropical or subtropical, with optimal conditions for solar energy year round. Nepal and Bhutan are relatively far from equator, but have many days of unobstructed sunshine.

The vast majority of humans live in regions with plentiful sun for solar.

Why geothermal is not renewable? Earth is not going to cool its magma soon enough

“Technically”

Then no power source is "technically" renewable.

Then solar and wind aren't technically renewable either, because the sun is going to eventually consume the earth and explode.

Geothermal is renewable.

However much solar or wind energy we use, the Sun will last exactly as long. This is not a matter of scale. Even if we were to build a photovoltaic Dyson sphere around the Sun, it would have the same lifespan.

That is not the case for geothermal. It could in theory be cooled down if exploited at a massive scale.

Saying geothermal is not renewable is not an indictment nor a criticism. Geothermal is great and we should use it more. It's just technically not renewable, but that doesn't matter.

I guess there's a point where that's arguably true. But it's not correct universally, and it's certainly not true if you look at earth and the sun.

The heat generated within the earth comes from nuclear decay and the energy radiates out from the planet. It would do that no matter what. Humans can tap into it and it would have the same lifespan.

If humans somehow started extracting the heat leftover from the planets formation, then yeah, that is exhaustible. There's about 100,000,000,000 times our total annual energy (not electricity) use in there. Planet only has 5 billion years left, and only about 1 billion livable years.

That time gets considerably shorter if we start pumping that energy to the surface, because we cook the surface with the waste heat.

So even assuming it was technologically possible to access the heat there, you could not draw enough to exhaust it in the time frame you have remaining to do so without killing the biosphere.

And a new star will eventually form from the debris, so "renewable" is a function of time scale.

And after a hundred generations of this there will be no fusible material left. We can extract energy from rotating black holes until they stop, and then the universe is dead.

So solar energy is renewable over a human lifetime, not renewable over a stellar lifetime, renewable over a stellar formation cycle, not renewable over the lifetime of a universe, and renewable if universes turn out to be cyclical. And all but the first are pendantry in the context of renewable energy conversations.

The Earth's heat content is about 1×10^19 TJ. This heat naturally flows to the surface by conduction at a rate of 44.2 TW and is replenished by radioactive decay at a rate of 30 TW. These power rates are more than double humanity's current energy consumption from primary sources, but most of this power is too diffuse (approximately 0.1 W/m^2 on average) to be recoverable.

https://en.wikipedia.org/wiki/Geothermal_power#Resources

In comparison, averaged over the year and the day, the Earth's atmosphere receives 340 W/m^2 from the Sun.

https://en.wikipedia.org/wiki/Solar_irradiance#On_Earth's_su...

Thanks for that context.

This leads naturally to "artificial geothermal", where solar energy is used to heat rocks or soil, and the heat is later extracted. It doesn't have to be anywhere near as deep as ordinary geothermal, which had to accumulate that heat over many thousands of years. Just ~10 meters is about enough.

That's not where natural geothermal energy is from. It's residual heat from planetary formation and some natural radioactivity.

This form of storage also unfortunately only yields heat (via heat pumps or directly), not electricity, as the temperature difference is much too low in comparison to meaningfully run any heat engine from it.

Great if you need to heat houses; not so great if you were hoping to store the solar energy for a rainy, or rather cloudy, day (or night).

No, that is how natural geothermal energy works. Perhaps you mistakenly thought I was saying the heat comes from sunlight? I didn't. The heat comes from below (or, in some cases, from internal radioactive decay). And this delivery of heat from below (or from decay) is a slow process, taking a very long time, which is why geothermal resources have to be buried deeply (otherwise, that heat just leaks out and the temperature of the geothermal resource is too low).

Yeah, "accumulate the heat over thousands of years" indeed sounds a bit misleading to me. The heat is largely already there (or is generated pretty uniformly through radioactive processes), it's just slowly transmitted outwards down a gradient.

No, the heat is not already there. The heat comes in and goes out; the heat energy initially in the crust decays away exponentially with time and has no effect on the steady stage temperature gradient.

What do you mean? It's already in the core and gradually reaches us through the crust. What's your point/distinction here, exactly?

It was not initially in the rocks that we are tapping for geothermal energy, which would be a few kilometers. I wasn't talking about the Earth as a whole. Remember, this is about why so much more thickness is needed for the rocks for ordinary geothermal energy systems, vs. artificial geothermal.

Heat is extracted at geothermal wells much faster than it is being replenished by the average rate of heat flow from the deeper Earth. It's effectively "heat mining". Granted, there's a lot of heat to be mined.

Geothermal is powered by fission Uranium and other heavy atoms deep in the Earth.

Solar is powered by fusion of Hydrogen in the Sun.

I'd use the same classification for both.

About 20% of this is residual heat from planetary accretion; the remainder is attributed to past and current radioactive decay of naturally occurring isotopes.

Most of the radiogenic heating in the Earth results from the decay of the daughter nuclei in the decay chains of uranium-238 and thorium-232, and potassium-40.

https://en.wikipedia.org/wiki/Radiogenic_heating

Potassium is more or less distributed in the body (especially in soft tissues) following intake of foods. A 70-kg man contains about 126 g of potassium (0.18%), most of that is located in muscles. The daily consumption of potassium is approximately 2.5 grams. Hence the concentration of potassium-40 is nearly stable in all persons at a level of about 55 Bq/kg (3850 Bq in total), which corresponds to the annual effective dose of 0.2 mSv.

https://www.nuclear-power.com/nuclear-engineering/radiation-...

No, not quite. Geothermal is powered by the accumulated heat stored in rocks from fission Uranium and other heavy atoms deep in the Earth (and other phenomena).

Geothermal hotspots do not reheat by fission or otherwise at the same speed that we extract their energy (if they did we'd be in trouble if we weren't extracting it!).

As I mentioned in another comment, build a Dyson sphere of solar panels around the Sun and it will last just as long. Build an all-Earth geothermal plant and the heat will be depleted.

By that definition, hydroelectric dams are not a renewable energy source for most of the year.

How long would it take for the heat to be depleted? Humans have only managed to drill something like 12km into the earth because it gets too hot to go further.

If it were possible to access all of the Earth's stored geothermal energy, probably a very, very, very long time.

But if we're open to applying a quantitative timescale threshold to the thought experiment, at which we can argue geothermal is renewable, that raises the question for nuclear. If we could access all fissile uranium and thorium on Earth, how long would it take for us to deplete its stored energy? Does that mean nuclear energy is renewable?

Almost none of it is from fission. Fission is a very rare natural decay mode of uranium and thorium. Most of their radioactive energy output is from ordinary non-fission radioactive decay.

Can’t speak for large scale sites with abundant volcanic activity… But for residential geothermal the bore hole has a lifetime based on how much ground water there is and how active usage it sees.

This is because using it cools the hole slowly and after a few decades (depending on how quickly ground water can dissipate heat gradient) a new hole need to be drilled a distance away.

Can we cycle the holes? Use one while the other one is warming back up.

There are some modern heat pumps which can put heat energy back into the hole during the warmer season when the pump acts as a (cooler). For my own Daikin pump this would be possible if I had a different flooring than wood.

"Unfortunately" I choose wooden flooring on my heated floors. Cooling this means that the I risk condensation due to moisture build-up and therefore a long-term risk of mould. But I have only need of cooling 1-2 months of the year, so it is not a big deal, I can solve it by clever use of insulation instead.

But for those able to use a geothermal pump like this it also has the short-term benefit of increasing the effectiveness of the hole as well.

Only as a technicality. If you find a geothermal hotspot and start to extract energy from it, the hotspot will eventually cool down faster than if you hadn't (which of course depends on the size of the hotspot and how much heat you're pulling out).

However, given that there's no downsides to cooling down a hotspot other than, well, no longer being able to extract energy from it, geothermal is a bit of an honorary "renewable".

Actual renewables ultimately all come down to recent[0] solar energy, which will never deplete their source however much they are used. All the energy in wind, hydroelectric and biofuels has recently originated in the Sun.

[0] I say "recently" because fossil fuels are all also derived from the Sun, but their rate of regeneration is a bit too slow compared to the speed at which we use them.

If it goes down, what happens to all the buildings using geo/earth heat with these probe heads to collect the energy?

Does this effect occur in lets say 10-20 years or is this longterm like 50y+?

At a certain point there won’t be enough heat recovered from the geothermal side of the loop to generate steam on the process side of the loop and power generation will cease. I’m not smart enough to calculate how long that will take, however. I think you could still use the geothermal energy at a lower temperature for district heating and cooling, but a mechanical engineer would be more qualified to answer that.

A lot of hydroelectric depends on snow pack and glacier runoff that is being adversely affected by global warming. Solar and wind are the only robust hedges against a warm up that might ultimately severely curtail river flow.

We have a lot of uranium and nuclear is fairly renewable at least in the span of a few centuries. The waste issue is a problem.

Nuclear waste is not a huge problem. It's mass is tiny, and it can be contained safely in very small areas

We can make oil from biomass by heating it up in a kiln, and we can do it quickly. Oil is totally renewable. It's weird to me that we are fixating on renewable when the problem is carbon gas, isn't it?

Contrary to a popular belief, most high temperature Geothermal plants have a predicted death date.

This is due to the physics reality of the ground itself: Power of a Geothermal well will decay over time to a point where the well become unusable and need to be closed.

It is due to the fact underground water is rich in minerals and raw elements. This soup will slowly but surely cement the well itself and its associated underground.

There are techniques (similar to 'fraking') to extend the lifetime of a well but only to some extent.

If the topic interests you (and you can bear artificially translated English), a French content creator did a pretty good video on the topic:

https://m.youtube.com/watch?v=q4xZArgOIWc

Additionally, Geothermal plants can emit CO2 (even a lot of CO2) in some geological configuration.

All of that makes Geothermal (for electricity) a bit controversial as "Renewable".

I precise that there is absolutely nothing wrong about low temperature Geothermal energy for residential heating and we should do more.

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