Geothermal energy comes from the structure of the earth and its interior heat source and circulation.
There is a continual flow of heat energy outwards towards the surface
Surface Manifestations Include:
Conventional Uses of Geothermal Energy:
There are production sites in New Zealand, Iceland and, of course, California
Averaged over the earths surface, the heat energy flow is 0.06 Watts per square meter (500 times less than incoming solar energy flux)
Of course, there are local concentrations of geothermal energy that are quite high (e.g. yellowstone).
On average though, it is clear that to get large scale electrical energy generation from geothermal we will be withdrawing energy more rapidly than it is replenished. Hence we are essentially mining geothermal energy in the same way that we mine fossil fuels.
Existing Production Sites:
The main problem with geothermal, of course, is lack of easily accessible surface sites. Turbines on Old Faithful would probably not be well received.
Geothermal using natural steam sources is quite efficient, almost 100%. Only real source of loss is turbine friction.
Note that Geothermal does pollute the environment with small amounts of sulfur and carbon emission but to date, the tapped sources are natural so this would happen anyway
Categories of Geothermal Resources:
Note: Although the efficiency of geothermally heated steam powered turbines is quite high, only a small fraction of the available thermal heat goes into producing steam
Example -- The Geysers Geothermal Site:
8x1019Joules / 3.15 x 1018 Joules/yr = 26 years
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