The subterranean structure of Denmark contains geological structures which contain hot water. This can be utilised for district heating production, either directly or via absorption heat pumps or electric heat pumps.
Geothermal heat from the interior of the Earth continually flows towards the Earth’s surface. In Denmark, where the temperature in the subsoil layers typically increases by 25–30°C for every 1,000 m of depth, it is possible to utilize this heat for heating purposes in the form of district heating. The hot water that is found in porous and permeable sandstone layers is pumped up to the surface via wells. Here, the heat is extracted via heat exchangers, and the cooled water is then pumped back into the subsoil via another well.
In Denmark, there is considerable potential for extracting geothermal heat. Throughout much of Denmark, there are porous and permeable sandstone layers from which geothermal heat can be produced for district heating purposes. However, the sandstone layers become less porous and permeable with depth, so although the layers, and therefore the water contained in the layers, become hotter with depth, there is a maximum depth at which it ceases to be cost-effective to extract geothermal heat. In Denmark, experience has shown that this limit is normally around a depth of 2,500 m.
The Metropolitan Geothermal Alliance (abbreviated HGS, from its Danish name), an alliance consisting of Centralkommunernes Transmissionsselskab I/S (CTR) (18 per cent), DONG VE A/S (28 per cent), KE Varme P/S (18 per cent), DONG Energy Power A/S (18 per cent), and Vestegnens Kraftvarmeselskab I/S (18 per cent), made an assessment in 2008 of the geothermal reserves in the metropolitan region. The conclusion is that there are geothermal reserves of about 60,000 PJ in the entire licence area. The reserves are assessed to cover 30-50 per cent of the district heating requirements in the metropolitan region for thousands of years, and can thus contribute to increasing the share of renewable energy in Denmark if the necessary production installations are established.