Seasonal heat storage in partially flooded mines - In-situ investigations at the Freiberg, Saxony, site

Currently, about half of the energy consumption of Germany is spend on heating and cooling, and only about 19% of this energy is generated from renewable sources. In order to decarbonise the overall energy supply, it is essential to integrate seasonal heat storage systems. The aim is, for example, to store excess heat during summer (e.g., from solar thermal energy) for use in winter. One possibility can be the use of partially flooded mines as so-called thermal energy storage (TES) systems, in that specific case known as Mine TES (MTES). As part of the BMBF-funded R&D project 'MineATES', an in-situ laboratory was set up at the Reiche Zeche (Himmelfahrt Fundgrube mine in Freiberg, Saxony). In the underground mine, thermal energy was injected into (charging process) and extracted from (discharging process) a test storage reservoir of approx. 21 m³ volume using a mobile heat pump test rig. The effects on the surrounding rock (Freiberg gneiss), the water chemistry and the technical infrastructure (e.g. heat exchanger plates) were analysed using more than ninety temperature sensors, numerous water analyses and continuous monitoring of control parameters. The first test series show that temporary heat storage is possible, with both the test reservoir’s water body and the surrounding rock acting as a combined storage medium. With regard to the latter, a measurable temperature change at a distance of more than two metres from the test reservoir was demonstrated within a few days. However, considerable heat losses occur during the storage process. The main causes are thought to be losses due to flow through the basin and advective dispersive heat transport within the rock matrix (mainly along the geologically determined fracture structure), and evaporation and heat transfer losses across the large (relative to storage volume) water surface. In parallel to the in-situ investigations, the necessary interplay between mine storage and surface heat supply was analysed. Using GIS-based modelling, potential sources for waste heat from industry, active cooling of buildings, or solar thermal energy were systematically recorded and their possible coupling with heat storage in the mine was investigated. For the exemplary analysis of the city of Freiberg (Saxony), four preferred areas including the old town were identified and analysed. The results show that the share of renewable heat supply can be increased by about 20% through the implementation of MTES.