Schloss Freudenstein Freiberg

Schloss Freudenstein Freiberg (Graphic: Freiberg University of Mining and Technology)

The geothermal system at the Freudenstein Castle in Freiberg, Saxony, went into operation in 2009 and has since supplied both heating and cooling to sections of the castle, including the mineral museum “terra mineralia” and the mining archives. Providing for the castle is the “Alte Tiefe Fürstenstolln” or translated, the Old Deep Prince’s Gallery, which lies approximately 50 m under the castle and has 10,3°C mine water dammed up to a length of 250 m. This water is pumped to an above-ground heat exchanger, where the heat is transferred to (or from) an intermediate circuit that feeds a heat pump in the castle’s utility room. In the case of cooling, the heat pump can be bypassed, and the intermediate circuit used directly for cooling. Thanks to close cooperation with the Freiberg University of Mining and Technology, many student projects have been carried out to improve and further develop the system.

Geothermal mine water system at the Freudenstein Castle in Freiberg (Graphic: Freiberg University of Mining and Technology)

In order to pump the warm mine water from a depth of 50 m to the surface, two feed pumps each with a flow rate of 3 L/s are employed [1].  After the heat is transferred to an above-ground heat exchanger, a separate circuit brings the absorbed heat to the utility room of the castle. Finally, the water in the intermediate circuit is used for two different purposes:

  • In the case of heating, the water in the circuit is heated to approximately 8-10°C by the mine water, after which the heat pump provides the heat source, heating the water to approximately 42°C. This hot water is stored in a buffer tank, which supplies the castle with heat. The mine water, which has cooled by approximately 5 K, is in returned to the heat exchanger above the mine water pumps, where it is reheated. The heat pump has a maximum heat duty of approximately 126 kW, while using approximately 29 kW of electricity. This fulfills the base-load heating requirements of the castle’s underfloor heating system [2,3].
  • When being used for cooling, the heat pump is disconnected and the water in the intermediate circuit is directed to a second heat exchanger. The water that is used for cooling the castle is directed through the pipes of the underfloor heating system, transfering the absorbed heat back to the intermediate heating loop. This heating circuit then transfers the heat back to the mine water through the heat exchanger above the mine water pumps. The system provides a total cooling output of 120 kW [2].
Measuring weir for the flow rate measurement (Grafic: Freiberg University of Mining and Technology)

To facilitate the control and monitoring of the geothermal system, sensors for measuring the temperature of the mine water are incorporated at different points. Additionally, the temperature of the water is measured at three separate points in the mine to exclude the possibility of temperature stratification. Another significant measuring device in the mine is the measuring weir (see Figure above), whose ultrasonic distance sensor measures the water flowrate by reading the level of the triangular weir. Above ground, there are also heat-flow, volumetric-flow, and electricity counters in addition to the temperature transmitters, which aid in evaluating the performance of the system [2,3].


[1] Dillenardt J., Kranz K. (2010) Diplomkartierung: Geologisch-Hydrogeologische Kartierung im Alten Tiefen Fürstenstolln zur geothermischen Grubenwassernutzung im Schloss Freudenstein, Technische Universität Bergakademie Freiberg
[2] Büttner R., Kupka M., Werner S. (2010) Wärmepumpentechnik bei Einsatz von Grubenwasser am Beispiel "Schloss Freudenstein", Studienarbeit, Technische Universität Bergakademie Freiberg
[3] Grötzsch S., Kleutges J. (2011) Geothermieanlagen zur Grubenwassernutzung für Heizung und Kühlung "Reiche Zeche" & "Schloss Freudenstein", Projektarbeit, Technische Universität Bergakademie Freiberg