[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.