Geothermal systems: A topical introduction

The earth's thermal regime differs depending on the depth of a geothermal system. The usable heat in the near-surface zone, that is within the first 20 m below the ground, is determined almost exclusively by solar radiation and leachate. In contrast, the geothermal heat flux from the interior of the earth has a greater impact on plants in deeper areas between 20 and 100 m.

From there, the temperature increases by the geothermal gradient of about 3 K per 100 m to about 20 °C at a depth of 400 m. A significantly higher temperature - which can even be used to generate heat and electricity through geothermal power stations - prevails at depths of several km. In this region, the earth receives its thermal energy mainly through heat conduction and production by means of radioactive decay processes in the Earth's interior. As a result, temperatures remain constant in this area almost all year round.

Working principle of geothermal thermosiphons

Geothermal thermosiphons present an innovative way of producing geothermal energy by means of a perpetual, closed-loop system. In this autonomous process, a cooling medium, that is a liquid with a low boiling point (e.g. propane or carbon dioxide), is vaporized through geothermal absorption, and subsequently condensed in a heat exchanger.

A geothermal thermosiphon consists of a long vertical pipe of 60 to 200 m. It can either be segmentally welded or spirally coiled - resulting in a smooth or corrugated pipe wall, respectively - and is tightly sealed at the bottom. At the upper end, the condensate from the heat exchanger is applied to the inside of the pipe through a pipe head.

Mine water geothermics

Through entering leachate or rising thermal deep waters, mine workings can cause large volumes of mine water that are subject to geothermal influence. Therefore, at times large heat quantities can be extracted from these mine waters using appropriate heat pump technology, which presents an energetically very favorable way of cooling buildings. Due to the distinct structural conditions of the respective mine workings, a wide range of different mine waters and corresponding utilization concepts for their geothermal application exist.

Logo of the European fund for regional development on the left; logo of the cooperation program between the Free State of Saxony and the Czech Republic from 2014 - 2020 on the right
GEOTHERMICS
Energy from the earth

Insights into existing systems

The geothermal system “Reiche Zeche” is located in the Free State of Saxony on the grounds of the TU Bergakademie Freiberg. The plant consists of a total of 8 probes and has been in operation for research and teaching purposes since 2007. In addition to geothermal thermosiphons for heat generation, the system also includes liquid circulation probes which are used for refrigeration. This way, laboratories of the TU Bergakademie Freiberg can be heated and cooled. The heating and cooling cycles are completely separated from each other.

VODAMIN II

The cooperation program “Free State of Saxony - Czech Republic 2014 to 2020” aims to reduce border barriers and improve the quality of life in the frontier area. VODAMIN II is a follow-up of the VODAMIN project (“Voda” = water, “min” = mine), which aims to enhance the protection of surface and groundwater from the effects of mine and tailings water. Furthermore, the geothermal utilization potential of mine water is to be investigated.

Monitoring of operating data

As part of the VODAMIN II project, the TU Bergakademie Freiberg carries out investigations into the performance of 5 mine water geothermal systems in Freiberg (located at the exhibition mine “Reiche Zeche”, the Castle “Freudenstein” and the district hospital) and Ehrenfriedersdorf (situated at the exhibition mine “Zinngrube” and the secondary school “Schule des Friedens”). Since the plant layout depends in large measure on the existing mine structure, different technical system concepts have been realized. For a plant efficiency comparison, the necessary operating data are collected and evaluated.

 

 

Mine water temperature

Due to the geothermal gradient of approx. 3 K per 100 m on average, the temperature of the earth - along with its thermal deep waters - increases with depth. Pits flooded by warmer waters that have risen from greater depths provide a favorable energetic condition to operate heat pumps for the cooling of buildings.

 

 

Mine water flow rate

The flow rate indicates the mine water volume per hour conveyed by a heat pump. Since the usable thermal energy is dependent on the disposable amount of mine water, its year-round availability is essential for a mine water geothermal system.

 

 

Performance factor of a plant

The performance factor serves as an indicator for the efficiency of a geothermal system. It reveals the supplied amount of thermal energy relative to the input energy for a given period of time. For an accurate energy assessment, the cost-benefit ratio of the entire process chain must be taken into account.