The most common type of geothermal plants exploits probes of the vertical type which penetrate into the ground through geothermal wells of limited depth, the order of 100 m.
For a single plant are required more wells the number of which depends on the thermal requirement to be met and the thermal output of the subsoil from which the thermal energy must be extracted. The construction of the wells is preceded by a series of investigations aimed at the determination of:
- thermal conductivity;
- the presence of moisture;
- stratification of the subsoil.
The thermal conductivity of the materials is critical for the design of a geothermal field: in the literature there are numerous tables showing the conductivity of different types of soil and rock.
As you can see in the table below the rocks are the materials with higher conductivity
Type of soil
Yield W / m
|Poor subsurface sediments and dry||20|
|Underground rock and sediment-poor water||50|
|Consolidated rock with high conductivity||70|
|Gravel and sand shoals||<20|
|Gravel and sand saturated with water||55 65|
|Gravel and sand with considerable flow of ground water||80 100|
|Siliceous igneous rocks (granite)||55 70|
|Basic igneous rocks (basalt)||35 55|
These values may vary depending on the structure (eg presence of cracks and alterations).
The probes with vertical development require the perforations with depths varying between 80 and 150m: The operating modes with which realize these perforations are variable according to the type of subsoil and available technologies.
At depths above the ground maintains a temperature that is around 15 ° C without affected by changes in external weather.
The probes are filled by small diameter tubes (20-50 mm) in polyethylene, For civil applications with standard operating conditions (temperature 15 ° C, pressure 16 bar), and cross-linked polyethylene for applications with higher temperatures and pressures: the correct choice of the type of pipe according to the operating conditions, guarantees a useful life of the probes also superior to 100 years.
Among the possibilities for installation of the pipe there is the U-shaped configuration and double U.
La configuration double U presents the dual advantage of allowing a better heat exchange with the ground and at the same time in case of malfunction of a pipe, the other being independent can still power the heat pump.
As can be seen in the figures, in each well, once placed the pipes, is introduced a filling material between the tubes and the walls of the well in order to ensure a good thermal contact with the surrounding soil. In case the probe passes through a flap, the filler material also prevents vertical circulations of water.
The filler material in addition to the thermal conductivity must also ensure a poor solubilityTo avoid interactions with groundwater and at the same time a good elasticity to withstand the rigors of the field: the most widely used material is a grout cementobentonitica that is able to perform all these functions.
Concerning instead the fluid, It consists of water with added antifreeze in a variable percentage between 10% and 30%: the addition of liquid antifreeze lowers the freezing point of water until almost-15 ° C and is particularly important in heating regime.
The following table shows values for length, content and diameter of the probes in the case of tube double U. Soils with poor stability may also require a greater diameter of the hole, to be assessed case by case.
fluid xm (l)
|32||2,12||112 115||approx. 150|
|40||3,34||127 135||approx. 300|
If you want to do the insights on heating with heat pumps for geothermal, You can find valuable information at the following links: