Geothermal energy, in the broadest sense, is the natural heat of the earth. Immense amounts of thermal energy are generated and stored in the earth's core, mantle, and crust. The heat is transferred from the interior towards the surface mostly by conduction, and this conductive heat flow makes temperatures rise with increasing depth in the crust on average 25 - 30C/km. This is called the geothermal gradient. The recoverable thermal energy theoretically suitable for direct applications has been estimated at 2.9 x 10^24 > Joules, which is about 10,000 times the present annual world consumption of primary energy without regard to grade. Most of the earth's heat is, however, far too deeply buried to be tapped by man, even under the most optimistic assumptions of technological development