[flushandforget]

Exactly it may take years and lots of heat energy extraction, but the Earth could end up cold. I don't think it's a renewable.

On the flip side you can do this the other way round: trap solar in a heat store below ground, piping the heat down below. It becomes a giant battery. The earth insulates the heat store (providing there is no water run through / water table to wick out the heat). You reverse the process during cold spells into a building.

[dpark]

Then nothing is renewable. Eventually the sun runs out of fuel.

Renewable cannot be taken to mean truly limitless or nothing is renewable. Renewable means practically limitless. How much heat do we have to pull from the Earth before it causes a problem? I don't know, but I bet it's a lot more than we will in the next 10K years even with heavy investment in geothermal.

The Earth also doesn't have a fixed amount of heat inside. Heat is constantly being produced by, e.g., tidal forces and radioactive decay.

[masklinn]

> the Earth could end up cold. I don't think it's a renewable.

The earth's mantle and crust contain decaying radioisotope, and it gives off primordial heat, totalling about 45TW to the surface split roughly half and half (that's just the stuff naturally heating you from the ground).

While that's small potatoes compared to incoming solar radiation (let alone solar energy generation) the current electrical budget of humanity is 20000 TWh per year. That corresponds to about 450h of the "thermal waste" of the planet, or a bit under 3 weeks.

And that's just the waste energy, for the planet to end up cold we'd have to tap into the core, we can't even remotely reach there (humanity's deepest boreholes are ~12km deep, which is about half a thin continental crust and rougly 0.2% through to the earth's center)

[Retric]

While there is plenty of heating going on, it's not actually needed for this to be considered 'renewable'.

Currently stored heat is "approximately 100 billion times current (2010) worldwide annual energy consumption". So at 10x current energy extraction the sun will engulf the earth before the earth cools, thus warming it back up.

[masklinn]

Good point, I just went with what the earth gives off as "waste" to the surface anyway, but you're completely right that we could be tapping into the primordial reservoir and not make a dent into it anyway.