Geothermal energy is a form of usable energy that comes from the heat present in the deeper layers of the earth's crust. In fact, deep into the earth's surface, the temperature becomes gradually higher, increasing by an average of about 30 ° C for km in the crust (30 ° C / km and 80 ° C / 100 km respectively in the mantle and in the nucleus, it is average values, in some areas in fact, you can find lower or higher gradients tens of times).
The deposits of this energy, however, are dispersed and in depth so high as to prevent the exploitation. To extract and use the heat trapped in the Earth, it is necessary to identify the areas with positive thermal anomaly where the earth's heat is concentrated: the tank or geothermal reservoir. To obtain an optimal heating of houses or greenhouses is put in place the action of fluids at low temperature; instead, to get electricity use is made of high temperature fluids. There are several geothermal systems, but currently only the hydrothermal systems are used in industry, constituted by permeable formations in which rainwater and rivers infiltrates and is heated by layers of high temperature rocks. The temperatures reached vary from 50-60 ° C up to a few hundreds of degrees.
The use of this energy has advantages as in the short times inexhaustibility, if exploited in a rational way, and less pollution of the surrounding environment; a certain pollution is not excluded for the possible entry into the toxic elements, such as sulfur, mercury and arsenic present in geothermal fluids, for this reason the geothermal areas are subjected to annual environmental audits. Looking only to a production of thermal energy, it is the low enthalpy geothermal system that exploiting the natural heat of the ground with the aid of a heat pump is able to produce thermal energy for the domestic hot water and for the heating of buildings. In some particular areas may present conditions in which the temperature of the subsoil is higher than the average, a phenomenon caused by tectonic or volcanic phenomena. In these "hot spots" energy it can be easily recovered using geothermal energy.
Geothermal energy is to pipe the steam coming from the water sources of the underground routing to turbines used for the production of electricity and reusing steam for district heating, crops in greenhouses and spas. To power the production of water vapor is often used in of cold water in depth, a technique useful for keeping constant the flow of the steam. This way you can be working at full capacity turbines and produce heat continuously. In this sense it seems to be very promising developments related to geothermal energy, which has a very wide geographical distribution because of the absence of water use but only heat. The natural radioactivity of the earth is the cause of geothermal energy. It is estimated that the total flow of heat to the earth's surface is 16 TW, and since the earth has a mean radius of 6371 km, the average power produced by geothermal street is 32 & n For comparison the average solar radiation is, to European latitudes, of about 200 W / m². The power is significantly greater close to the limits of tectonic fractures where the crust is less thick. Moreover, the circulation of water in depth can further increase the thermal power per unit area.