Denmark’s third geothermal plant in Sønderborg is an importatnt part of the ambition of the municipality of Sønderborg (Sønderborg Kommune) to become CO2 neutral.

Danish Geothermal District Heating has assisted Sønderborg Fjernvarme (the local district heating company) with establishing a geothermal plant in the Danish town of Sønderborg. In the Northeastern part of town two geothermal wells with surface installations has been constructed, and next to the local CHP plant Sønderborg Kraftvarmeværk, two new woodchip boilers, delivering driving heat for the four absorption heat pumps of the geothermal plant, have been constructed. The entire geothermal plant was commissioned in the spring of 2013.


About the plant

The subsurface around Sønderborg is interesting with respect to the production of geothermal energy. Sønderborg Fjernvarme and DONG Energy therefore in 2005 began negotiatons concerning the establishment of a geothermal district heating plant. During these negotiations it was confirmed that geothermal energy from an environmental, security of supply and economical point of view is an attractive supplement to the district heating supply of Sønderborg.

The district heating network of Sønderborg is today gets its base load supply from the local CHP plant, while peak load is supplied from natural gas fired boiler units. The district heating production on the local CHP plant is based on waste incineration and a natural gas fired Combined Cycle unit. The geothermal district heating plant will reduce the consumption of natural gas and thereby reduce the CO2 emission of the district heating production.

Sønderborg Fjernvarme and DONG Energy in January 2007 signed a Joint Operating Agreement and was together given a license for exploration and production of geothermal energy in October 2007. In July 2010 both parties agreed that DONG Energy should hand over its share of the project to Sønderborg Fjernvarme, who finished the project on its own.

In the first phase of the project, a new seismic survey was completed in 2007-2008. Next, two geothermal wells were planned and drilled I 2008-2010. In the summer of 2010 pump out tests from the Gassum Formation in a depth of approximately 1.2 km showed, that temperature and reservoir transmissivity of the geothermal reservoir where sufficiently high to continue the project. As a consequence the surface installations with heat exchangers, filters, absorption heat pumps, etc. was constructed in 2011-2012. The entire geothermal plant was commissioned in the spring of 2013.

 Sfjprincip12komma5 Soenderborg
Simple sketch of the principle and design data of the geothermal plant in Sønderborg.

The plant has a design capacity of 12.5 MJ/s from the geothermal reservoir. The plant is split up with absorption heat pumps placed next to the local CHP plant, where the possibilities to feed the produced energy into the district heating network. A new boiler unit with two woodchip fired boilers has been constructed at the same place, delivering heat to power the absorption heat pumps. The geothermal loop with wells, pumps and heat exchangers is placed roughly 4 km to the east, as the geological conditions at the CHP plant are not suited for production of geothermal energy.

 Soenderborg Kort

The location of the absorption heat pumps and the geothermal loop to the east.

Environmental advantages

The geothermal district heating plant will together with the new woodchip fired boiler unit replace natural gas used at the local CHP plant. The shift from natural gas to geothermal energy and wood chip leads to substantial reductions in the climate gas emissions from the district heating production in Sønderborg. Early in the project, cuts in the emission of the climate gas CO2 from 58,000 to 4,800 tons annually (more than 90 %) were estimated. The geothermal plant will therefore contribute significantly to fulfilling the ambitious goal of the municipality of Sønderborg of CO2 neutrality in 2029 (the socalled Project Zero).

The CO2 emissions of Sønderborg Fjernvarmes are significantly reduced.

During operations it is expected, that it will be necessary to dispose of relatively small amounts of geothermal water. The geothermal water is not an environment danger – it largely resembles highly concentrated sea water – but because of the temperature and high salinity it can influence the marine environment just around the point of disposal. A number of measures have therefore been implemented in order to avoid negative environmental effects. For example the water is pumped about 5 km to the waste water treatment plant at Als Sund and not out into the more sensitive Augustenborg Fjord, even though the geothermal loop is situated less than 300 meters from the fjord.