Geothermal energy and steam

One of the most promising applications of geothermal energy can be found in its ability to generate steam.

The minimum temperature to achieve this is 120 degrees Celsius with current technology. This means that the geothermal fluid brought to the surface must have this temperature. This means that in Western European conditions, where the average geothermal gradient is 3 degrees Celsius per 100 meters, geothermal heat must be generated at a depth of at least 4,000 meters. If the gradient is much more favorable, such as in parts of Spain, Greece and Italy, then drilling to lesser depths may be sufficient.

Once the steam has been brought to the surface, various systems are available for further processing.

One of the applications for steam production via geothermal energy is the production of electricity.

There are three possible methods for this

Direct

Steam power plants use hydrothermal fluids, primarily steam. The steam goes directly to a turbine, which drives a generator that produces electricity. Steam eliminates the need to burn fossil fuels to run the turbine. (No more transportation and storage of fuels is needed either!)

This is the oldest type of geothermal power plant. It was first used at Lardarello in Italy in 1904.

Flash

Hydrothermal fluids above 360°F (182°C) can be used in flash plants to make electricity.

The liquid is injected into a tank that is at a much lower pressure than the liquid, causing some of the liquid to quickly vaporize, or "flash." The vapor then drives a turbine, which drives a generator.

If any liquid remains in the tank, it can be "flashed" again in a second tank (double flash) to gain even more energy.

Binairy

Most geothermal areas contain water of moderate temperature (less than 400°F). Energy is extracted from these fluids in binary cycle power plants.

Hot geothermal fluid and a secondary (hence "binary") fluid with a much lower boiling point than water pass through a heat exchanger. Heat from the geothermal fluid vaporizes the secondary fluid, which then drives the turbines.

Because this is a closed-loop system, virtually nothing is emitted into the atmosphere. Temperate temperature water is by far the most common geothermal source, and most geothermal power plants in the future will be binary cycle plants.