Ehrenfriedersdorf Secondary School

Since 1994, the secondary school in Ehrenfriedersdorf uses mine water available in the „Nordwest-Feld” for supplying heat. Approximately 200,000 m³ of water storage volume is available from decommissioned mine tunnels that have been flooded. When added to the volume of the natural cavities in the underground rock, this yields a total stored water volume of 530.000 m³ [1]. Efforts were made by the Energy Model Saxony association (Energiemodell Sachsen e.V.) to put this available geothermal energy to use.

In order to tap the reservoir, one production well and one injection well were drilled. During operation, the water is discharged back to the underground cavities at a location (Direction N 2020, 510 mNN, 2nd tunnel) distant from the extraction point (Direction N 5000, 415 mNN, 5th tunnel) after energy is recovered from the mine water.  Thus, the temperature of the water is allowed to be regenerated by geothermal energy. Originally, the water needed to be actively pumped to the surface to be used in geothermal applications; however, with the increasing groundwater level, the pump could be taken out of service because the extraction point of the production well was below the water level. This makes pumping only necessary for water circulation, providing prime conditions for energy efficiency.

 

Geothermal mine water system at the secondary school in Ehrenfriedersdorf (Graphic: Freiberg University of Mining and Technology)

The system has been updated on several occasions as experience has been collected over time. In the first variation, the mine water was used directly to feed the heat pump. However, this led to strong fouling of the heat exchanger used to vaporize the heating fluid. Consequently, operation of the system was temporarily halted in 2000 because the pressure drop became too high. Further developments led to a second, dismountable heat exchanger being installed before the heat pump. This not only facilitates easy cleaning, but the high-mineral content mine water no longer runs through the heat pump, preventing fouling. Originally, a combined heat and power unit and two compression heat pumps were used for supplying heat. Today, the heating cycle is driven with three absorption heat pumps, and peak loads are covered with the help of a gas boiler. Furthermore, the originally installed steel pipes have been almost completely substituted with polyethylene pipes, and the original pump for pumping the water to the surface has been removed. In order to achieve this, the water level was decreased, the pump apparatus removed, and the mine tunnels reflooded through groundwater ingress [2].

Literatur

[1] Rottluff F. (1994) Ehrenfriedersdorf: Grubenwärmenutzung in Betrieb genommen. Geothermische Energie - Mitteilungsblatt der Geothermischen Vereinigung e. V. 3(8)
[2] Rottluff F. (2012) Abschlussbericht zur Sanierung der Bohrungen zur Grubenwasserentnahme/-rückführung aus dem Nord-West-Feld (NWF) der ehemaligen Zinngrube Ehrenfriedersdorf, Zschopau