Geothermal completely slipped my mind! facepalm Obviously the consensus view is that geothermal is sustainable. I have some mild concerns about how many years a installation in a non-geologically/thermally active area can run before it changes the temperature of its lithic heat sink enough to significantly reduce power output, but many installations are proving the success of the technology overall, and that's more an economic challenge (lifetime/ROI of installation) than a question of sustainability.