GEOTHERMAL Geothermal energy (from the ancient Greek geo - earth, and thermos - heat) is a form of energy produced by the earth’s endogenous heat.

Volcanoes, thermal springs and geysers indicate the presence of heat from the earth’s crust, which arrives at the surface through fluid "vectors" such as water and steam.

The temperature within the increases with depth, according to a geothermal gradient of 3°C every 100 meters, although areas exist with abnormal geothermal gradient where the flow of heat is greater (9-12°C every 100 meters).

Two types of geothermal power exist:
Classic geothermal power: exploits geological or volcanological abnormalities to produce electricity.

This energy is produced using three different technologies:
  • "Dry steam" systems, which use geothermal water steam directly at an average temperature of 150C to move the turbines
  • "Flash steam" systems, which use intermediate accumulations of steam before bringing them to the turbines; these systems require a minimum temperature of 180C
  • Binary cycle systems, generally low to medium power: in these systems, water at a high temperature is used to heat a secondary fluid at a low boiling point, which is then vaporized to drive the turbines on a generator and subsequently re-condensed in the cycle

The boreholes, primarily on binary type systems, can reach a maximum depth of 4-5 km, where temperatures exceeding 200C can be obtained.

So-called “hot dry rock” geothermal power (also known as “enhanced” in the USA) is derived from hot dry ground in which a liquid (generally water) is injected through an initial deep borehole, to reach or generate areas of fractured rock; the water is thus heated to return to the surface as steam, through a second borehole.

"Low enthalpy" geothermal power exploits the fact that the temperature of the ground, even at just a few meters in depth, has only limited fluctuations during the year, or is almost always constant: this characteristic is common to any location on the planet.

The constancy of the ground’s temperature has a dual beneficial effect: during the winter, the ground temperature is relatively warmer than that of the surface air, whereas during the summer the temperature is lower than the surface air.