Both plants are are run by consortia that include DONG Energy as the majority stakeholder. Thisted was built 1981-84 (enlarged in 2000). The Amager plant in Copenhagen was begun in 2001 and its first geothermal energy was produced in November 2004. A third plant is planned for Sønderborg in 2012, with another in Copenhagen around 2015 and more anticipated on the Jutland peninsula.

The principle at work is to use the Earth's naturally percolated and heated water resources to either power turbines or run heating (or cooling) systems directly. Below a surprisingly shallow depth (a few metres), the temperature of the Earth is stable. But drilling deeply gets you into rising temperatures, since the Earth gets hotter as you get nearer to its core. Generally, it rises 20-30° C per km — more if you happen to be near a volcano.

Water trapped underground in aquifers heats up naturally and the hot fluid is extracted by drilling into porous rock. This fluid is used to heat the supply pipes in district heating schemes, or it is flash-heated into steam and used to power electricity generators such as those at the Amager plant (see box at right).

Geothermal energy is clean and sustainable, though some external power — usually about 10% — is used to transfer the heat from the boreholes to district heating supply pipes. Spent (cool) liquid from the process can be returned to the aquifer for reheating through a second borehole, maintaining the pressure underground.

Copenhagen's Amager plant is in the same locality as the city's largest district heating network and combined heat and power (CHP) plant, and produces 14MW from its two wells. It's adjacent to a major geological fault line (through which water percolates to the aquifers). The water from its wells is more saline and has a higher concentration of CO2 than the water from Thisted.

The planned new plant in Copenhagen will have up to six times the capacity of Amager and could cost around one billion Kroner. The major investment of any geothermal scheme is in the drilling of its very deep wells. Seismic surveys are used to identify suitable sites.