In more than 99% of the volume of the earth, the temperature is over 1,000 ° C. The energy content of the Earth's hot interior is thus several hundred million times the energy content of the Earth's total coal, oil and gas reserves.
Heat from the subsurface flows constantly out of the ground surface in Denmark with a modest level at 0,067 W / m2. This heats up the aquifers underground, where geothermal energy can be extracted in the form of steam or hot water. The quantity of heat continuously flowing out from the subsurface below Denmark corresponds theoretically to about 2/3 of Denmark's total current district heating needs.
In the majority of Denmark the bedrock is covered by deposited (sedimented) layers with thicknesses of up to 7 km. The possibilities of using geothermal energy are dependent on the existence of hot water-bearing sandstone layers in the subsoil from which water can be pumped up. Two main factors affect the possibility of utilizing geothermal energy, the temperature and the water conductivity, both of which change with depth.
The Geological Survey of Denmark and Greenland (GEUS) has identified and mapped five Formations (Frederikshavn, Haldager, Gassum, Skagerrak og Bunter) in Denmark in the depth interval 1-3 kilometers, which is believed to be the depth interval with the best potential as geothermal reservoir.
Mapping of the potential geothermal reservoirs in Denmark.
The temperature increases with the depth, which is attractive even though the drilling cost also increases with depth.
In areas with volcanic activity the thermal gradient is very different and higher. In such areas, such as Iceland, the geothermal energy can be exploited for power production via steam turbines.
Globally the most areas have a moderately thermal gradient, like the one in Denmark at 25-30 °C per kilometer. In such areas the geothermal energy can be used for heating, from shallow depths for individual heating and from deep depths for district heating.
At depths more shallow than 1 km the temperature is usually too low for the larger geothermal plants for district heating.
The water conductivity - or permeability - decreases with increasing depth, due to chemical conversion processes and compaction from the load of overlying sediment layers.
This means that the pumping costs increases dramatically with increasing depth and under Danish conditions it is assessed that sandstone layers below 3 km are too tight to obtain a sustainable company economy.
Local subsurface conditions are crucial for a economical sustainable exploitation of the geothermal energy.
Suitable subsurface is not sufficient. For a viable geothermal heat production also a heat market or heat consumption is needed, e.g. via a district heating network.
A viable geothermal heat production for a district heating network is also dependent upon the existing heat supply, and the anticipated development in price of other fuels like coal, gas and biomass. The trend seems to be in favor of geothermal energy, since technical improvements makes geothermal production less expensive while other fuels seem to become more expensive.
Approximately 85 % of the present district heat consumption is attached to district heat networks with annual heat use above 250 TJ, which is estimated to be the lower limit for establishing a viable geothermal plant. On the long term it seems that an annual production of 25 - 40 PJ seems achievable for Denmark, corresponding to 20 - 30 % of the Danish district heat use. Approximately 1 - 2 % of this market potential is covered today.