Geothermische Methoden
Hverir in Island. © Fotolia/Kichatov

Geothermal energy refers to naturally or artificially stored energy underground. Even if the Earth’s surface is usually perceived to be relatively cold, temperature increases continually with depth and reaches around 4000°C at the core. The continuously radiated heat from the Earth’s interior has been used for centuries primarily in the form of thermal spas, recently also in the energy industry for renewability reasons (for heating and power generation).

General

The temperature inside the Earth continuously increases with depth (geothermal gradient) at a rate of around 30°C per kilometre in the upper crust (lithosphere). This can be expressed through the terrestrial heat flux [mW/m²] which averages around 70 mW/m² on the continental crust. Besides conduction, the regional heat transport in the Earth’s crust can also take place through convection of circulating depth waters (advection, free convection). Heat transfer processes and heat sources are important constituents of this process. In the continental crust, in particular in acidic igneous rocks, radiogenic heat production as a result of radioactive decay of natural isotopes (nuclides 232Th, 235U, 40K) plays an essential role. In the alpine region it can be assumed that almost 70% of the heat reaching the Earth’s surface stems from radioactive sources in acidic crustal rocks.

Situation in Austria

In central Europe, the variable thickness of the lithosphere presents a considerable geothermal parameter, where a thicker crust shows a reduced heat flux density. The lowest terrestrial heat flux in Austria is thus found in regions of the Northern Calcareous Alps and the Karawanken. Here the large crustal thickness together with the convective effect of infiltrating precipitation into the mountains has a negative influence on the thermal regime of the region. The highest heat flux densities and thus the most advantageous thermal underground conditions can be found in the East and South-East of the country (Burgenland, South-Eastern Styria). In this area, the thin crust – equivalent to a shallow-lying mantle in the region of the Pannonian Basin – has a positive effect on the terrestrial heat field. Far-reaching hydraulic circulation systems lead to above-average geothermal conditions in some regions of the western molasses (Innviertel, Hausruckviertel).

Possible Usage

The utilisation of geothermal energy sources occurs in two forms: deep geothermal energy and shallow geothermal energy. Currently Austria uses deep geothermal energy for providing thermal water for supplying hot water spas as well as for generating power and energy for heating. This is predominately used in Eastern Styria and the Inn- and Hausruckviertel (Upper Austria).

In contrast to deep geothermal energy, we have the shallow downhole heat exchangers and collectors which are grouped under the term “near surface geothermal energy”, which use open or closed systems to produce an artificial heat current at maximum depths of several hundreds of meters (though usually max. 150 m). These systems usually rely on heat pumps to use the gained energy, but have the advantage of also allowing indoor rooms to be cooled in summer [Free Cooling].