Geothermal energy
Origin
Geothermal energy comes from the gradual temperature increase as one penetrates deeper into the earth's crust, either because of the natural gradient (3°C/100 m, with an average flux of 60 mW/m2), or because of geophysical singularities (high temperature natural geothermal reservoirs of porous rock).
It is customary to distinguish three broad categories of reservoirs, according to their temperature levels:
high temperature (>220 °C);
intermediate temperature (100–200 °C);
low temperature (50–100 °C).
In the first case, the geothermal fluid can be essentially composed of water or steam, in the other two it is water, optionally under pressure. A special feature of geothermal fluid is that it is never pure water: it also includes many impurities, corrosive salts (the concentration limit for an operation to be possible is equal to 1.5 mol/kg) and non-condensable gas (NCG) in varying amounts (0.1-10%). We shall see that this feature imposes constraints on thermodynamic cycles that can be used.
For environmental reasons, the geothermal fluid should generally be reinjected into the reservoir after use, but it is not always the case.
Thermodynamic conversion
The thermodynamic conversion of geothermal energy uses four main techniques:
plants called “direct-steam” can be used if the geothermal fluid is superheated steam that can be directly expanded in a turbine. Historically, this type of plant was first implemented in Larderello in Italy since 1904;
flash vaporization power plants can exploit sites where geothermal fluid is in the form of pressurized liquid or liquid-vapor mixture. Today it is the type of plant most used. Geothermal fluid begins by being expanded in a chamber at pressure lower than that of the well, thereby vaporizing a portion, which is then expanded in a turbine;
systems known as binary use a secondary working fluid, which follows a closed Hirn or Rankine cycle, the boiler being a heat exchanger with the geothermal fluid;
fluid mixture systems, such as Kalina cycle, a variant of binary systems where the working fluid is no longer pure but consists of two fluids to achieve a temperature glide during vaporization.
Mixed or combined cycles can use both a direct or flash system and a binary system.
Additional information
A thematic page presents these various types of cycles.
