Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 96

reservoirs are already in use. Very high temperatures (150 to 250 °C) of the
depth sections of up to 3,000 m only occur in areas with specific tectonic situa-
tions, for example in the fracture zones of the earth's crust, where magma rocks
are uplifted from the deep underground.
− Hydrothermal high-pressure reservoirs. Such deposits contain hot water which
is preloaded and mixed with gas (such as methane) (e.g. in the South of the
USA, in the region of the Gulf Coasts of Texas and Louisiana). They are
formed by porous isolated rock units which are subducted within a short time
into the earth by tectonic processes. The pore waters and gas contents are then
exposed to the prevailing pressure and temperature conditions of those depths.
− Hot dry rocks. Really dry rock layers are the exception at earth's crust in depths
of up to approximately 10 km to be reached with the currently available drilling
techniques. Therefore, also rock formations that do not have enough naturally
available water to enable thermal water loops over a longer period of time
(several years) are called "dry". Thus, a large spectrum of rock layers with dif-
ferent levels of permeability and percentages of water are summed up under the
expression of "hot dry rocks". Such occurrences, however, contain by far the
largest potential of geothermal energy accessible with the currently available
technology.
− Magma deposits. Close to tectonically active zones, melted rocks (so-called
magmas) of temperatures of over 700 °C can be found which often have a
lower density compared to the still solid rocks surrounding them. These par-
tially liquefied materials have been uplifted due to their low density from larger
depths to depths of 3 to 10 km. Fluid systems with high temperatures can gen-
erally be found around such magma chambers. They can be used to supply
high-quality geothermal energy. However, developing such systems is a tech-
nological challenge.
The use of such geothermal reservoirs largely depends on the energy content and
thus on the temperature.
Above 130 °C geothermal reservoirs can be used for electricity generation. Ba-
sically the provision of electrical energy is also possible at temperatures below
this temperature range; the "coldest" geothermal power plant of the world located
in Neustadt-Glewe/Germany operates at a temperature level of approximately
98 °C; but temperatures above 130 °C are necessary to get sufficient efficiency to
convert the earth's heat to electrical power. Geothermal energy of temperatures
above 150 °C is already utilised in several locations (i.e. Italy, New Zealand).
Usually, geothermal power production is independent of the time of the day or the
season and the local weather conditions. The reservoirs can be used economically
for base load energy provision and in most cases in an environmentally friendly
way. However, the appropriate geological requirements have to be met. This is
only the case to a limited extent worldwide.
Even at temperatures below 130 °C geothermal heat can be utilised in many
ways. Typical examples are outlined as follows: