[CRConrad]

Actually, calling geothermal energy "renewable" is a bit of a misnomer, isn't it? At least if the heat energy in the Earth's crust, which is what "geothermal energy" harvests, comes from the inside. The Earth's core may not be cooling down very fast, but we know for sure it's not getting any warmer (not before the Sun in its death throes swells up into a red giant and swallows the inner planets, anyway).

Yeah, I know, super-nitpicky — but, hey, it's the Best Kind Of Correct™. (Unless the crust is actually heated more by the Sun than from below, but I doubt that.)

[kragen]

Most geothermal energy comes from the decay of radioactive elements in the Earth's crust, although heat from Earth's formation is a non-negligible fraction of it. If you check out the web site of Iceland's geothermal energy agency, I believe they do have a calculation there of the sustainable power level that could be extracted (without cooling down the crust), but I don't remember if they're currently above or below it.

If I recall correctly, however, the fossil heat trapped in the crust under Iceland is several billion years of the sustainable extraction rate.

And, on the third hand, even if you only extract energy at the rate that radioactive decay produces it, in only a few tens of billions of years, most of the radioisotopes will have decayed away if you don't replenish them.

You are correct that the crust is heated more from below than by the Sun. That's why the bottom of the crust, where it contacts the mantle, is hotter than the surface.