school energy&environment

Different types of probes

To “capture” the heat of the subsoil, two types of probes are used, depending on how deep they are positioned.
In order to use geothermal energy, “vertical probes” are set up. These are simply a couple of U shaped pipes with a 10 – 18 cm diameter, that are positioned in wells at variable depths, between 50 and 350 m. The optimum depth for a home of approximately 100 m² is about 70-100 m; if the volume to be heated is greater, the probe can be positioned at depths up to 300 – 350 m, but not deeper, because drilling deeper has decidedly non competitive costs. The pipes are made of polyethylene, a material that is inert respect to the chemical composition of the soil, it does not corrode and can guarantee good thermal conductivity. The empty space between the pipes and the walls of the well is filled in with bentonite, a special clay which guarantees good thermal contact between the probe and the ground. The pipes are then filled with a mixture of water with 15-20% “thermovector” fluid, which is similar to the antifreeze liquid used for cars, which can absorb the heat of the ground to a higher degree than water alone. The pipes connect directly into the heat pump, and the circuit is sealed to guarantee there are no leaks: therefore there is no pollution and there is water saving, as the water is made to circulate constantly without adding more water. For the vector fluid, non-toxic substances are used, that do not damage the ozone in the atmosphere (so called “ozone friendly” compounds, without CFC), so also disposal , once the system closes, does not cause environmental problems.
A geothermal domestic system is installed in 3 – 4 days, the time required to dig the wells for the probes and to connect the system to the home distribution network. The probes last approximately 50 – 100 years and the system practically does not need maintenance. Instead, in order to use energy absorbed by the ground from Sun radiation, so called “horizontal sensors” are used. The principle is the same as for the vertical probes, but instead of digging a well into the ground, a coil of pipes (made of copper or polyethylene) are laid about 60 cm underground (just under the superficial layer of the soil, which could freeze during the winter), or a series of small probes (“heat energy piles”) is positioned a couple of metres deep. For a home of approximately 100 m², 120 – 150 m² of capture surface, in contact with the soil is necessary: it is therefore an economic solution, that is easy to install if there is a small garden where it is possible to lay the sensors underground. The only limitations linked to the use of the garden for a system with horizontal sensors are that the area above the probes must not be covered by paving or tar and tall trees cannot be planted because the roots may damage the sensors: apart from this, the garden or the orchard can be cultivated and used as usual. Also in this case, the circuit that conveys the fluid (usually water with glycol) to the pump is closed, so there are no problems regarding emissions of gas or fumes: also this is a clean and environment respectful method.
In the presence of a water table, the so called “capture wells” can be used. These use the water directly from the water-table, which is extracted and then sent back into the ground: in this case the water is used both as a source of energy and as a the vector fluid. It is a very efficient system but it is more expensive and cannot always be used, in particular in the presence of a water table used a source of drinking water. An analogous system can be realized, by exchanging heat and the water of a lake or of a pond close to the home: this type of system has been set up to supply air conditioning in the Palace of Nations in Geneva, using the water of Lake Geneva (Lac Léman).