How much geothermal energy is there?
There amount of geothermal energy stored deep in the earth is much greater than the amount of energy that humans will use even if we stay on earth until the sun turns into a red giant star, billions of years into the future. But that energy is not easily accessible.
The source of geothermal energy is radioactive decay, occurring all over the vast volume of the earth's interior, from the surface clear down 6730 kilometers (4000 miles) to the center of the earth. Except for certain hot spots where magma lies near the surface, the crust of the earth is relatively cool, but warmer and warmer as you go deeper.
There are some mines that go a few miles into the ground where it is too hot to work unless the region is cooled by a large flow of air from the surface. Imagine having to do hard physical labor when the temperature is 150 degrees F (66 degrees Celsius)!
To get some idea of the forces holding the rock together, the pressure one mile deep in the ocean is about 176 atmospheres; the compressive pressure of rock at that depth is on the order of 500 atmospheres.
But the uncomfortable temperature at that depth is, by standards of heat engines, rather cold. The efficiency of engines that convert heat to work is limited, according to the immutable 2nd law of thermodynamics. An absolutely perfect engine that could take in heat at 66 degrees C and then reject waste heat into the environment at 20 degrees C would have an efficiency of only 13.6%.
The several geothermal power plants in existence use hot spots in the earth, where the temperature is much higher than the 150 (F, 66 C) in the example. On the other hand, their engines are not the theoretical perfect engines, either.
According to the Energy Information Agency, the average efficiency of
American geothermal power plants is about 16%.