In its most simplistic form, geothermal energy is produced when steam pressure is moved through a turbine generator which produces electricity. The source of the steam and byproducts of the plant varies depending on the type of geothermal resource the plant is located on.
There are three kinds of geothermal power plants, which depend on the temperature and pressure of the reservoir.
Enhanced Geothermal Systems (EGS) are a newer technology that enables increased production from all types of existing reservoirs for all three types of geothermal plants. EGS pumps waste water into reservoirs in production to increase efficiency or pumps the water into hot rock that are tectonically stressed and fractured to create a new reservoir. The water heats from these hot rocks or existing reservoirs and is pumped back up into add-on plants. This technology is being pushed as it enables the U.S. to reach geothermal production of 100GW, as compared to the 30GW of current total capacity using current technology. See the Jan. ’07 MIT Report for additional information.