Bill Menke's BLOG Page: Is Geothermal Energy Really Renewable?
A recent MIT-led report, "The Future of Geothermal Energy*", argues for the expansion of US geothermal energy production. A hot cube of rock, just a kilometer or so across, contains enough heat energy to run a big, one-gigawatt power plant for more than a decade, and there are many many such cubes beneath western US. Extensive mining of this heat for electric power could substantially reduce US carbon emissions.
But while there's a lot of heat within the earth, shallow hot areas are comparatively rare. And once cooled, heat takes millions of years to seep back up from depth to recharge them. Thus geothermal heat, like oil, is just another finite, albiet underexploited resource.
Rather than to get caught up in an estimate of how long it might last**, I prefer to ponder the following: How long did people expect oil to last, back a few years after 1859, when Edward Drake struck oil in Titusville, Pennsylvania? Keep in mind that the main use of petroleum at that time was to provide kerosene for lamps.
**But for the record, if the report's estimate of 13,000,000 exajoules of heat energy is correct, then we could run four million 1GW power plants for a century. Given that worldwide electical production in 2004 was equivalent to about 1800 1GW power plants, we would conclude we have nothing to worry about for a very time - 200,000 years! But recovering anything but a very small fraction of this heat is probably unrealistic.