Introduction
Heat is a form of energy and geothermal energy is, literally, the heat contained within the Earth that generates geological phenomena on a planetary scale. `Geothermal energy` is often used nowadays, however, to indicate that part of the Earth`s heat that can, or could, be recovered and exploited by man, and it is in this sense that we will use the term from now on.

Brief geothermal history
The presence of volcanoes, hot springs, and other thermal phenomena must have led our ancestors to surmise that parts of the interior of the Earth were hot. However, it was not until a period between the sixteenth and seventeenth century, when the first mines were excavated to a few hundred metres below ground level, that man deduced, from simple physical sensations, that the Earth`s temperature increased with depth.

The first measurements by thermometer were probably performed in 1740 by De Gensanne, in a mine near Belfort, in France (Buffon, 1778). By 1870, modern scientific methods were being used to study the thermal regime of the Earth (Bullard, 1965), but it was not until the twentieth century, and the discovery of the role played by radiogenic heat, that we could fully comprehend such phenomena as heat balance and the Earth`s thermal history. All modern thermal models of the Earth, in fact, must take into account the heat continually generated by the decay of the long-lived radioactive isotopes of uranium (U238, U235), thorium (Th232) and potassium (K40), which are present in the Earth (Lubimova, 1968). Added to radiogenic heat, in uncertain proportions, are other potential sources of heat such as the primordial energy of planetary accretion. Realistic theories on these models were not available until the 1980s, when it was demonstrated that there was no equilibrium between the radiogenic heat generated in the Earth`s interior and the heat dissipated into space from the Earth, and that our planet is slowly cooling down. To give some idea of the phenomenon involved and its scale, we will cite a heat balance from Stacey and Loper (1988), in which the total flow of heat from the Earth is estimated at 42 x 1012 W (conduction, convection and radiation). Of this figure, 8 x 1012 W come from the crust, which represents only 2% of the total volume of the Earth but is rich in radioactive isotopes, 32.3 x 1012 W come from the mantle, which represents 82% of the total volume of the Earth, and 1.7 x 1012 W come from the core, which accounts for 16% of the total volume and contains no radioactive isotopes. (See Figure 1 for a sketch of the inner structure of the Earth). Since the radiogenic heat of the mantle is estimated at 22 x 1012 W, the cooling rate of this part of the Earth is 10.3 x 1012 W.

In more recent estimates, based on a greater number of data, the total flow of heat from the Earth is about 6 percent higher than the figure utilized by Stacey and Loper in 1988. Even so, the cooling process is still very slow. The temperature of the mantle has decreased no more than 300 to 350 °C in three billion years, remaining at about 4000 °C at its base. It has been estimated that the total heat content of the Earth, reckoned above an assumed average surface temperature of 15 °C, is of the order of 12.6 x 1024 MJ, and that of the crust is of the order of 5.4 x 1021 MJ (Armstead, 1983). The thermal energy of the Earth is therefore immense, but only a fraction could be utilized by mankind. So far our utilization of this energy has been limited to areas in which geological conditions permit a carrier (water in the liquid phase or steam) to `transfer` the heat from deep hot zones to or near the surface, thus giving rise to geothermal resources; innovative techniques in the near future, however, may offer new perspectives in this sector.

In many areas of life, practical applications precede scientific research and technological developments, and the geothermal sector is a good example of this. In the early part of the nineteenth century the geothermal fluids were already being exploited for their energy content. A chemical industry was set up in that period in Italy (in the zone now known as Larderello), to extract boric acid from the boric hot waters emerging naturally or from specially drilled shallow boreholes. The boric acid was obtained by evaporating the boric waters in iron boilers, using the wood from nearby forests as fuel. In 1827 Francesco Larderel, founder of this industry, developed a system for utilising the heat of the boric fluids in the evaporation process, rather than burning wood from the rapidly depleting forests (Figure 2).

Exploitation of the natural steam for its mechanical energy began at much the same time. The geothermal steam was used to raise liquids in primitive gas lifts and later in reciprocating and centrifugal pumps and winches, all of which were used in drilling or the local boric acid industry. Between 1850 and 1875 the factory at Larderello held the monopoly in Europe for boric acid production. Between 1910 and 1940 the low-pressure steam in this part of Tuscany was brought into use to heat the industrial and residential buildings and greenhouses. Other countries also began developing their geothermal resources on an industrial scale. In 1892 the first geothermal district heating system began operations in Boise, Idaho (USA). In 1928 Iceland, another pioneer in the utilization of geothermal energy, also began exploiting its geothermal fluids (mainly hot waters) for domestic heating purposes.

By 1904 the first attempt was being made at generating electricity from geothermal steam; again, it was to take place at Larderello (Figure 3).

The success of this experiment was a clear indication of the industrial value of geothermal energy and marked the beginning of a form of exploitation that was to develop significantly from then on. Electricity generation at Larderello was a commercial success. By 1942 the installed geothermoelectric capacity had reached 127,650 kWe. Several countries were soon to follow the example set by Italy. In 1919 the first geothermal wells in Japan were drilled at Beppu, followed in 1921 by wells drilled at The Geysers, California, USA. In 1958 a small geothermal power plant began operating in New Zealand, in 1959 another began in Mexico, in 1960 in the USA, followed by many other countries in the years to come.
Present status of geothermal utilization

After the Second World War many countries were attracted by geothermal energy, considering it to be economically competitive with other forms of energy. It did not have to be imported, and, in some cases, it was the only energy source available locally.

The countries that utilise geothermal energy to generate electricity are listed in Table1, which also gives the installed geothermal electric capacity in 1995 (6833 MWe), in 2000 (7972 MWe) and the increase between 1995 and the year 2000 (Huttrer, 2001). The same Table also reports the total installed capacity at the end of 2003 (8402 MWe). The geothermal power installed in the developing countries in 1995 and 2000 represents 38 and 47% of the world total, respectively.

The utilization of geothermal energy in developing countries has exhibited an interesting trend over the years. In the five years between 1975 and 1979 the geothermal electric capacity installed in these countries increased from 75 to 462 MWe; by the end of the next five-year period (1984) this figure had reached 1495 MWe, showing a rate of increase during these two periods of 500% and 223%, respectively (Dickson and Fanelli, 1988). In the next sixteen years, from 1984 to 2000, there was a further increase of almost 150%. Geothermal power plays a fairly significant role in the energy balance of some areas; for example, in 2001 the electric energy produced from geothermal resources represented 27% of the total electricity generated in the Philippines, 12.4% in Kenya, 11.4% in Costa Rica, and 4.3% in El Salvador.

As regards non-electric applications of geothermal energy, Table 2 gives the installed capacity (15,145 MWt) and energy use (190,699 TJ/yr) world-wide for the year 2000. During that year 58 countries reported direct uses, compared to 28 in 1995 and 24 in 1985. The number of countries with direct uses has very likely increased since then, as well as the total installed capacity and energy use.

The most common non-electric use world-wide (in terms of installed capacity) is heat pumps (34.80%), followed by bathing (26.20%), space-heating (21.62%), greenhouses (8.22%), aquaculture (3.93%), and industrial processes (3.13%) (Lund and Freeston, 2001).

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