Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 101

in the large sediment structures existing at many locations worldwide. Fig. 2.63
shows exemplarily the existing sediment basins. The most interesting area, in
terms of its hydrothermal conditions, is the Molasse Basin between the Danube
and the Alps, where the water of the Malm (i.e. a geological layer with limestone
rocks in the Upper Jurassic), can be used.
Fig. 2.62 Temperature distribution at a depth below NN of 2,000 m within Germany (see
/2-34/, /2-35/, /2-36/)
The geothermal heat flow is independent of daily or seasonal influences. Thus,
there is no variation in the energy supply in human time dimensions. Only in geo-
logical timeframes the geothermal heat flow might change. A slow cooling of the
heat stored in the interior of the earth is certainly the most lengthy process. Within
shorter periods - still periods of maybe several million of years - the migration of
locally confined heat centres within the earth's crust can, for example, lead to a
change in the location of the reservoir through tectonic processes. Also, the proc-
esses associated with the rising salts in the sediment basin (i.e. the growing of salt
domes) also play a certain role. However, all these slow time-related changes in
the heat content have no impact on energetic utilisation.