Our planet constantly produces energy in the form of heat; from the deepest areas it spreads towards the surface: this is the so called flow of heat or geothermal flow. The heat of the sun heats the Earth’s surface with a flow that is almost 6,000 times greater than what is produced from inside the Earth. However the geothermal flow, that is constant and continuous, is an important source of energy, with an average of 0.06 watts per square metre; from the entire surface of the Earth, a quantity of heat, equal to approximately 30,000 billion watts is radiated.
The Earth gets hotter as we dig deeper in the ground, and this is a phenomenon that is well known to miners. Some of the deep mines and galleries reach temperatures that are near the boundary of human survival (which is not the case in caves, where the natural circulation of air and water remarkably lower the temperature, so that an increase in temperature related to depth is practically not felt). The Earth’s heat, is mostly due to energy freed in the decay process of the radioactive isotopes of some elements such as potassium, thorium and uranium. Due to the different thicknesses of the Earth’s crust and the different geological situations which can cause the rise of warmer materials from deeper zones, the geothermal gradient (i.e. the increase in temperature, due to greater depth) is not equal all over the World. On average, the temperature increases 2-3°C per 100 m in depth, but the increase can vary from 1° up to 5°C/100 m.
In order to measure the geothermal gradient, wells are dug at least 300 m deep (so that the effect of daily and annual variations in the temperature, due to climatic influences, is not felt). In these wells, special thermometers which record the temperature at the different depths are positioned.
The flow of heat is greater where the thickness of the lithosphere is less, as for example on the ocean ridges or in the continental rifting zones or in volcanic areas where different geological processes lead to rock melting, or in areas where there is slowly cooling magma in the subsoil.
In order to find the most suitable areas to exploit geothermal resources, surveys are conducted both on the surface and underground. The results of the survey are used to understand what the geological, hydro-geological, thermal characteristics and the output capacity of the geothermal system are.
Where it is
With reference to the “plate tectonics” theory (according to which the earth’s crust can be divided into approximately 20 micro-areas called “plates” which every year move between zero and 18 centimetres), the hottest geothermal areas of the globe are generally positioned along the breaking and collision margins of the plates.
Where it is produced
Tuscany and the northern part of the Lazio region are well-known for their production of geothermal energy and host the largest geothermal plants in Italy (and Europe), near Piancastagnaio, at the foot of the Amiata mountain, and in Civitavecchia. The biggest plant is “The Geysers” plant, located 140 km in the north of San Francisco in California (USA), with a total power of 1590 megawatts.
A bit of history
The use of geothermal waters is a very ancient phenomenon probably dating back to the higher Palaeolithic. Nevertheless, its development from a more specifically health viewpoint originated in Japan and in Italy approximately 2000 years ago. However, although in Japan it was limited at national level, Romans disseminated it from Italy to any region of their Empire (Hungary, Germany, France, Spain, Great Britain, Turkey and Arabia).
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.
High temperature geothermal systems
In high temperature geothermal systems, the underground waters are very hot, usually, over 140 ° C. Temperatures can be even higher, such as, for example, in Larderello (Toscana) (260° C), Cerro Prieto (Messico) (388° C) or S. Vito (Campi Flegrei, Campania) (400 °C): the latter area has registered the highest temperature ever observed in a geothermal system. In these systems the heat flux is 3-4 times higher than normal and can generally be found in correspondence with cooling magma intrusions, between 3 and 15 km of depth.
Low temperature geothermal systems
In low temperature geothermal systems, where temperatures are below 140° C, direct production of electricity from vapour is not generally convenient. However, if temperatures are above 90° C it is possible to use warm fluids to vaporise a second fluid that has a lower boiling temperature (such as freon, isobuthane or ethyl cloride), thus obtaining vapour for indirect electricity production, even if the productivity level of this process is rather low.
A "cascade" of applications
After being used for electricity production and domestic heating, geothermal fluids still have a certain quantity of heat that allows their usage in a variety of ways, some of which very peculiar: in Sapporo (Japan) and Klamath Falls (USA), for example, hot waters are used for heating the roads during the winter to avoid the formation of ice.
Our local systems
Italy, because of its geological situation, is rich in both high- and low-temperature geothermal fields. The “symbol” and flagship of the geothermal energy in our country is definitely represented by the Larderello-Travale-Radicondoli thermal field, in Tuscany.
Geothermal energy is universally considered a clean energy. The characteristic that makes this source renewable and preferable to the others is its constant availability. In fact in the case of geothermal energy, electricity is available 24/24h, 365 days a year. In its natural condition, the geothermal fluid is present in the reservoir in the form of steam, as is the case in Italy in Larderello; or in the liquid form as on Mount Amiata.
Technological development and the need to retrieve energy from the highest possible number of sources are contributing to the rediscovery of geothermal energy and an increase of its areas of utilisation: of the “clean and cheap” energy provided by our planet, nothing is wasted!