Hans Erhorn and Johann Reiss 12.10.1 Concept
Geothermal heat can be used to generate electricity and heat; but the geology of Central Europe limits its application for heating purposes. In spite of its huge energy potential, this environmentally friendly technology is currently only rarely used. In comparison to solar energy that reaches the Earth (5.4 x 1015 MJ/a), the terrestrial heat flux through the Earth's crust is nearly 6000 times smaller (1012 MJ/a). This is, however, still three times greater than the total worldwide energy consumption.
How it is used
In Central Europe, geothermal heat can primarily be used in the following ways (depending on the respective geological situation and appearance):
• petrophysical systems (use of the heat embedded in rocks - for example, magna or hot dry rock);
• geothermal systems with a high enthalpy of > 600 J/kg (high pressure water lenses, steam systems);
• hydro-geothermal systems with a low enthalpy of < 600 J/kg (aquifer or thermal springs);
• surface-near geothermal systems with temperatures below 25°C (geothermal heat probes/ground loops, absorber piles, geothermal heat collectors, groundwater wells); and
• deep geothermal ground loops (below a depth of 400 m).
The different heat densities and temperature levels lead to the following applications:
• electrical power generation (petrophysical and geothermal systems with high entropy);
• direct use (hydro-geothermal systems with low enthalpy and near surface geothermal systems for preheating of air); and
• near-surface direct use enhanced by using a heat pump to extract additional heat before the water is returned (see Figure 12.10.1).
Source: H. Erhorn and J. Reiss, Fraunhofer Institut für Bauphysik, Stuttgart
Figure 12.10.1 Scheme of a heating distribution net using geothermal heat for a large community
There are also systems that simply store heat (surplus heat or cool air from solar or heat pump systems), such as aquifers or caverns.
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