What is geothermal energy

Freely adapted from the original text by Mary H. Dickson and Mario Fanelli, CNR Istituto di Geoscienze e Georisorse, Pisa, Italy.

Heat is a form of energy and, strictly speaking, geothermal energy is the heat contained in the Earth's interior. It is the source of many geological phenomena on a planetary scale. However, the term 'geothermal energy' is generally used to refer to that part of the Earth's heat that can, or could, be extracted from underground and used by humans. All modern thermal models of the Earth have to take into account the heat produced continuously by the decay of the long-lived radioactive isotopes of uranium (U²³⁸, U²³⁵), thorium (Th²³²) and potassium (K⁴⁰), which are present in the interior of the globe. In addition to radiogenic heat, other sources of heat, such as the planet's primordial heat, are added in not exactly defined proportions.

It has been estimated that the total heat contained in the Earth, assuming an average surface temperature of 15°C, is of the order of 12.6x10²⁴ MJ and that the heat contained in the crust is of the order of 5.4x10²¹ MJ (Armstead, 1983). The Earth's thermal energy is therefore huge, but only a part of it can be used by man. Until now, the use of this energy has been essentially limited to those areas where geological conditions allow a carrier (water in the liquid phase or steam) to 'transport' heat from deep hot formations to or near the surface, forming what we call hydrothermal resources. However, new avenues could be opened up in the near future by innovative methods and cutting-edge technologies, some of which are already being demonstrated, to use other kinds of geothermal resources.

In many areas, especially in the past, practice has often preceded theory. Many resources, including geothermal energy, were first used without knowing exactly what they were, and only later were they scientifically studied and the technology developed.

Geothermal fluids were already being used for their energy content in the early part of the nineteenth century. At that time, in the area that later took the name of Larderello (Tuscany), a small chemical industry had been built to extract boric acid from the hot waters that flowed naturally from the ground or were extracted from shallow wells. Boric acid was obtained by evaporating hot boron-rich fluids in metal kettles, using wood from nearby forests as fuel. In 1827, Francis Larderel, the owner of this industry, devised a way to use the heat of the boric fluids themselves in the evaporation process, instead of burning wood from the rapidly depleting forests. At the same time, the mechanical energy of natural steam was also used. This was used to lift water in simple "gas lift" systems and, later, to operate pumps and winches used in drilling operations or in the boric acid industry. Between 1850 and 1875, the Larderello chemical industry held a monopoly on boric acid production in Europe. In the same geothermal area, between 1910 and 1940, the use of low-pressure steam for heating residential and industrial buildings and greenhouses began and gradually expanded. In 1928, Iceland, another pioneer in the use of geothermal energy in Europe, also began to use geothermal fluids, mainly hot water, for heating buildings.

The first attempt to produce electricity from the energy contained in geothermal steam was made at Larderello in 1904. The success of this experiment demonstrated the industrial value of geothermal energy and marked the beginning of a form of utilisation that is now widespread in many countries. The production of electricity at Larderello was a commercial success, as well as a technical one, so much so that, by 1942, installed geothermal power had reached 127,650 kW. The Italian example was followed by many other countries. In 1919 the first geothermal well was drilled in Japan, at Beppu, and, in 1921, in the United States, at The Geysers in California. In 1958 the first geothermal power plant went into operation in New Zealand, in 1959 in Mexico, in 1960 in the United States, and in the following years in many other countries.
ARMSTEAD, H.C.H., Geothermal Energy, E. & F.N. Spon, London, 1983, 404 pp.