Utilisation of Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 467

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1882, geothermal power production was invented by Prince P. G. Conti in
Lardarello, Italy in 1904. Geothermal power production in Tuscany has continued
since then and amounted to 128 MW of installed electrical power in 1942 and to
about 790 MW in 2003. In 1958, a small geothermal power plant began operating
in New Zealand, in 1959 another in Mexico, and in 1960 commercial production
of geothermal power began in the USA within the Geysers Field in California.
Today 25 countries use geothermal energy for power production, and the world-
wide installed electrical capacity has increased to about 8,930 MW in the year
2004 with an average annual increase of about 17 % between the year 1995 and
the year 2000 /10-6/, /10-11/, /10-12/. One of the main reasons for this success is
the base load ability of geothermal power generation.
Today, geothermal power production is economic viable only when high tem-
peratures are found at relatively shallow depth. In regions with a normal or a
slightly above normal geothermal gradient of about 3 K per 100 m, one has to drill
more than 5,000 m deep in order to achieve temperatures above 150 °C. Such
deep wells are expensive (generally more than 5 Mio. €) and there is a high risk of
failure. For this reason under economic considerations geothermal power produc-
tion is mainly restricted to geothermal fields with extremely high temperature
gradients and high heat flows. Such fields often show surface manifestations of
geothermal activity like fumaroles or hot springs. A scheme of such a geothermal
field is shown in Fig. 10.13. The heat source are hot magmatic bodies that have
risen up in the earth crust from a greater depth (often several tens of kilometres) to
more shallow regions. The high heat flow in the overlaying crust, resulting from
these magmatic bodies, heats up water of meteoric or marine origin in porous or
fractured rock formations, that are in most cases covered by a cap rock of low
permeability. Due to buoyancy effects the water starts to converge within the host
rock bringing high temperatures closer to the surface. Depending on temperature
and pressure the fluid may start to boil at certain depths and vapour is produced.
The amount of vapour characterises those systems as liquid or vapour dominated
reservoirs. Typical fluid temperatures range from 150 to 300 °C at depths between
a few hundred and 3,000 m. Even higher temperatures are encountered at greater
depths.
Most of the geothermal fields used today are in zones with active volcanism.
Generally, not the volcanoes, which cool down rapidly, but the magma chambers
buried underneath the volcanoes are the heat source for geothermal manifestations
over prolonged time periods and are the indirect sources for geothermal power
production. Magma chambers contain silitic or basaltic magma. Their volume
measures from about 1 to 10 5 km 3 . Their heat content is tremendous (up to 10 23 J)
/10-6/.
Geothermal fields accompanied with volcanism /10-6/ are found along sub-
duction zones at active continental margins where oceanic crust is pushed under-
neath the continental crust. Examples of these areas are aligned along the Pacific
Ring of Fire, such as the Altiplano of the Andes, the Taupo Region in New Zea-