Geothermal power plants use the heat from the earth beneath our feet to produce steam.  That steam is used to turn a turbine attached to a generator to produce electricity.  The type of conversion used depends on the nature of the underground fluid (whether steam or water) and its temperature.  Three geothermal power plant technologies are currently being used to convert hydrothermal fluids to electricity, and a fourth (EGS) is being developed:

Dry Steam methods were used for the first geothermal power generation plants.  The steam from the geothermal reservoir is used directly from the wells.  The steam is routed directly through turbine/generator units to produce electricity.

Flash steam plants are the most common type of geothermal power generation plants in operation today.  They use water at temperatures greater than 360°F (182°C) that is pumped under high pressure to the generation equipment at the surface.

Binary Cycle plants differ from Dry Steam and Flash Steam systems because water or steam from the geothermal reservoir never comes in contact with the turbine/generator units.

Enhanced Geothermal System is a new type of geothermal power technology that does not require natural convective hydrothermal resources (naturally occurring heat, water, and rock permeability).

Dry steam plants use hydrothermal fluids that are primarily steam. The steam goes directly to a turbine, which drives a generator that produces electricity.  The steam eliminates the need to burn fossil fuels to run the turbine.  (Also eliminating the need to transport and store fuels!) This is the oldest type of geothermal power plant.  It was first used at Lardarello in Italy in 1904, and is still very effective.  Steam technology is used today at The Geysers in northern California, the world's largest single source of geothermal power.  These plants emit only excess steam and very minor amounts of gases.

Hydrothermal fluids above 360°F (182°C) can be used in flash plants to make electricity.  Fluid is sprayed into a tank held at a much lower pressure than the fluid, causing some of the fluid to rapidly vaporize, or "flash".  The vapor then turns a turbine, which drives a generator.  If any liquid remains in the tank, it can be flashed again in a second tank to extract even more energy.

Source for images: US Department of Energy

Most geothermal areas contain moderate-temperature water (below 400°F).  Energy is extracted from these fluids in binary-cycle power plants.  A hot geothermal fluid and a secondary fluid (hence, "binary") with a very low boiling point pass through a heat exchanger.  Heat from the geothermal fluid causes the secondary fluid to flash to vapor, which then drives the turbines.  Because this is a closed-loop system, virtually nothing is emitted to the atmosphere.  Moderate-temperature water is by far the more common geothermal resource, and most geothermal power plants in the future will be binary-cycle plants.

EGS is a new type of geothermal power technology that does not require natural convective hydrothermal resources. Until recently, geothermal power systems have exploited only resources where naturally occurring heat, water, and rock permeability are sufficient to allow energy extraction. However, by far the most geothermal energy within reach of conventional techniques is in dry and impermeable rock. EGS technologies enhance and/or create geothermal resources in this hot dry rock (HDR) through 'hydraulic stimulation' with non-toxic methods.

(Not to be confused with "fracking" which uses different methods and chemicals.)

1 Reservoir
2 Pump house
3 Heat exchanger
4 Turbine hall
5 Production well
6 Injection well
7 Hot water to district heating
8 Porous sediments
9 Observation well
10 Crystalline bedrock