https://en.wikipedia.org/wiki/Solar_irradiance#On_Earth's_su...
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).
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.
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.
Does this effect occur in lets say 10-20 years or is this longterm like 50y+?
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.
Solar is powered by fusion of Hydrogen in the Sun.
I'd use the same classification for both.
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-...
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.
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?
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.
Geothermal is renewable.
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.