Geothermal Energy

Geothermal System

A geothermal system can be described schematically as convecting water in the upper crust of Earth, which, in a confined space, transfers heat from a heat source to a heat sink, usually the free surface. A geothermal system is made up of three main elements: a heat source, a reservoir, and a fluid, which is the carrier that transfers the heat (Figure 1). The heat source can be either a very high-temperature (>600°C) magmatic intrusion that has reached relatively shallow depths (5–10 km [kilometers]) or, as in certain low-temperature systems, Earth's normal temperature. The reservoir is a volume of hot, permeable rocks from which the circulating water is generally overlain by a cover of impermeable rocks and connected

Figure 1 Schematic representation of a typical geothermal system

Source: Dickson, M., & Fanelli, M. (1995). Geothermal energy (UNESCO Energy Engineering Series). Chichester, UK: Wiley.

to a superficial recharge area through which the meteoric waters can replace or partly replace the fluids that escape from the reservoir.

Geothermal History

In the early part of the 19th century, 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 hot waters issuing naturally or from specially drilled shallow boreholes. In 1827, Francesco de Larderel developed a system for using the heat of the geothermal fluids in the evaporation process, rather than burning wood from the rapidly depleting forests. Between 1910 and 1940, the low-pressure steam in this area of Tuscany was brought into use to heat the industrial and residential buildings and greenhouses. In 1928, Iceland, another pioneer in the use of geothermal energy, also began exploiting its geothermal fluids for domestic heating.

The first attempt at generating electricity from geothermal steam was made at Larderello in 1904. The success of this experiment indicated the industrial value of geothermal energy. By 1942, the installed capacity had reached 128 MWe (megawatts electrical). The example set by Italy was followed by several countries. The first geothermal wells in Japan were drilled at Beppu in 1919 and in the United States at The Geysers, California, in 1921. In 1958, a small geothermal power plant began operating in New Zealand, in 1959 in Mexico, in 1960 in the United States, and in many other countries in the years to follow.

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