The heat generated in the mantle is a result of decaying radioisotopes. The decay releases heat energy and radiates through soil, rocks and water in the surrounding region. Areas that are geologically younger will have higher amounts of radiating geothermal heat, such as the Pacific Rim’s Ring of Fire shown in the image to the left. Crust on the Ring of Fire is thinner with more active tectonic plates that break up and move water through the ground, heating it up. As the water rises to the surface, natural hot springs and geysers occur, such as Old Faithful at Yellowstone National Park. Most geothermal activity is located on the west coast, as shown in the image on the right.
An image of the Pacific Rim's Ring of Fire
An image of the continental U.S. and the areas with the most geothermal activity.
There are three basic designs. Although different in their approaches, all methods accomplish the same goal: to pull hot water and steam from the ground, use it, and then return the cooled water to the ground. This prolongs the life of the energy source making it more sustainable. Below are images displaying the three main designs used in geothermal energy systems, as described by the U.S. Dept. of Energy:
The simplest design is known as dry steam where the steam goes directly through the turbine, then into a condenser where the steam is condensed into water. In a second approach, very hot water is depressurized or "flashed" into steam which can then be used to drive the turbine.
In the third approach, called a binary cycle system, the hot water is passed through a heat exchanger, where it heats a second liquid—such as isobutane—in a closed loop. Isobutane boils at a lower temperature than water, so it is more easily converted into steam to run the turbine.