school energy&environment

Special zones: geothermal fields

Zones that are characterised by a high and anomalous heat flux are those where the release of energy from the ground is greater, however to be able to use this source a fundamental ingredient, together with hot rocks, is water.
Water heats up thanks to the contact with hot rocks below the surface and, if the temperature and pressure conditions allow it, it can even turn into steam. In order to understand the phenomena in these anomalously hot zones, we must recall that the temperature at which water turns into steam depends on the pressure: when pressure measures 1 atm, vaporisation temperature is, as we know, 100 degrees Celsius, but at 10 atm (equal to the pressure of a 100 m water column, o about 30 m of rock), it goes up to 180° C. In this way, therefore, high pressures keep water at the liquid state even at much higher temperatures than those 100° C that we associate with water boiling in a pan!
The areas where a high heat flux warms the subterranean waters are called geothermal fields and are generally distinguished in high and low temperature geothermal systems (also called high and low enthalpy systems). In these areas it is possible, with the right technologies, to exploit the Earth’s natural energy to produce electricity, for domestic heating and for many other industrial uses: an available for free and renewable energy source. Unfortunately, geothermal fields that are able to produce a good quantity of energy are not many, in the world.
How does a geothermal field look like?
All geothermal systems’ structures look a bit like hydrocarbon traps and the techniques to individuate them, which use geophysical prospecting, are also very similar to the ones used in petroliferous research.
A geothermal system is constituted by:

• a source of heat (for example, magma undergoing a cooling process);
• an aquifer, that is a permeable geological formation, where water can infiltrate and circulate freely through pores or fractures;
• an impermeable cover rock that acts as a “trap” for the hot waters, preventing their dispersion on the surface and keeping them under pressure.

Finally, in order to allow a lasting exploitation of this energy source it is necessary to have a constant refill of water coming from the surface, generally meteoric waters, that can “refill” the aquifer, integrating the water drawn by man: where this is not so, it is necessary to input  fluids artificially.