Prior to the commissioning of the plant, an exploration well was drilled in 1981 to a depth of approximately 3 km, showing that the permeability at this depth was too low. Instead, the well was completed for production from the Gassum Formation in approximately 1.25 km depth, where the permeability is fine. Another well was drilled in 1983, and a demonstration plant with an electrically driven compression heat pump was constructed in 1984. In 1988, the electrically driven heat pump was exchanged for a heat driven absorption heat pump due to changes to the taxation of electricity.
In 2000, the plant was expanded with another absorption heat pump after runnig very well since commissioning in 1984. The plant can produce heat equal to the annual heat consumption of roughly 2,000 regular, but due to the limited heat demand of the town it is only in operation between October and April.
From the 1.25 km deep production well, up to 200 m3 per hour is pumped. The water is after filtering send through the absorption heat pumps, filtered once more before being pumped back into the reservoir through the injection well. Both well are vertical, and the horizontal distance between the wells is about 1.5 km.
Simple diagram showing the principle of the geothermal plant in Thisted.
Thisted Varmeforsyning (the local district heating company) takes care of the day to day operations and maintenance of the plant including the two large absorption heat pumps that transfer the heat from the geothermal water to the district heating network. Only during well operations like change-out of the production pump and soft acidizing the injection well, external assistance is needed.
The plant can produce up to 7 MW of heat from the underground. On top of this comes 10 MW of heat from the driving heat for the absorption heat pumps. The driving heat primarily comes from the nearby straw fired boiler unit, alternatively from the waste incineration plant of Thisted. When the driving heat comes from the straw fired boiler unit, the absorption heat pumps are in reality free to run, as the heat is just passing through the heat pumps on its way to the district heating network. The boiler therefore does not use any more fuel to power the heat pumps than it would otherwise have used to produce district heating directly.
Because of the high permeability of the reservoir, the plant produces 20 times as much heat as the electricity consumed.
The produced heat is transferred to the district heating network of the town, which thereby gets most of is it heat demand covered by stable, secure, affordable and climate friendly energy.
|The geothermal district heating plant of Thisted, Denmark.|