There is a vast amount of heat contained by the earth interior. This internal heat is mainly comprised of the residual heat of planetary accretion and radioactive heat (from radioactive element decay). The hottest part of the earth is the core, a big part of which is in molten state. Heat radiates and gets transferred from the core to the outer layers of the planet by interior fluids and melts. The general geothermal profile of the earth (Figure 8.6) provides an idea of the scale of the thermal resource and the gradual change of the earth temperature at different depths. Because the earth structure is not uniform, heat is more readily transferred in some zones than in others. High heat transfer is usually associated with fracture zones and major faults, which are often located at the boundaries of tectonic plates.
The temperature change rate with depth depends on the density and thermal conductivity of rocks. Subdivision of the earth structure into layers is made according to the rock composition and rheological properties, so we see that the thermal profile within each layer can be quite different. The drastic change in temperature pattern around the boundary between the lower mantle and outer core is apparently related to the transition of the molten state.
The heat flux within the crust (the thin top layer) is highly variable due to the existence of large unified fragments of the crust (plates) divided by plate boundaries, the more mobile zones, where plates collide, spread out, or move relative to one another. Increased mobility of the plate boundary zones may cause creation of faults of various depth, which favor heat transfer to the surface. If you want more background about plate tectonic theory, you will be able to find a lot of resource on the web. For example, An Introduction to Plate Tectonics provides a nice and concise illustrated introduction to this whole idea.
There are a number of technologies that help convert the thermal flux and hydrothermal waters to usable energy. Next, we will refer to the following reading to learn how these technologies work.
Reading Assignment:
“How Geothermal Energy Works”, UCS, 4/1/2014
The following short video (5 min) provides an additional illustration of a utility scale geothermal plant.
Video: How a Geothermal Plant Works (4:45)
While geothermal energy seems to be another unlimited and “free” energy resource, effective conversion of that energy and power distribution incur substantial costs. From economic evaluations, utility scale geothermal and natural gas power plants are comparable in overall cost, but only in the long term. Significant up-front expenditures for construction of the energy facility are much higher for the geothermal plant.