Thermal energy storage and extraction at the MTES real laboratory site Reiche Zeche: microbial response and implications

Microbial processes such as biofilm formation (biofouling), microbially induced mineral dissolution, mineral precipitation and corrosion can affect the thermal, chemical and physical stability of mine thermal energy storages (MTES). High concentrations of sulfate and metals, typical for mine water, enable microbial iron and sulfur cycling. Oxic conditions promote acidification through iron and sulfur oxidation, while anoxic conditions enable the production of H₂S or methane, that are not only climate but also safety relevant. Processes such as sulfate reduction and iron oxidation can corrode technical components of the heat pump system. Together with biofouling and mineral precipitation on technical equipment as well as in the water-bearing mine galleries, microbial processes can compromise the efficiency of MTES. To evaluate the microbial impact on the performance of MTES, it is crucial to characterize the site-specific hydrogeochemical conditions, the microbiome inherent to the MTES site, and its response to the changing thermal conditions.  

At the MTES real laboratory site Reiche Zeche, Freiberg (Germany), we monitored the microbial community in a mine water filled rock pool, its responses to several cycles of charging and discharging and their implications on different materials used in the heat exchangers. We analyzed the microbial abundance via quantitative PCR and the community composition based on amplicon sequencing in water and biofilm samples as well as the hydrogeochemical conditions after every heating and cooling cycle.

The test site is located at 147.5 m below surface in the first level of the mine. The mine water was characterized by an in-situ temperature of 11 °C, hyperacidic conditions (pH 2.6), concentrations of 1.1 mg L^-1 DOC, ~700 mg L^-1 of sulfate and 13-30 mg L^-1 of iron. The microbial community was dominated by aerobic, acidophilic autotrophs related to iron and sulfur oxidation. During 1.5 years of construction at the test site, the bacterial taxa dominating the mine water shifted from the iron-oxidizing Gallionella and Sideroxydans to Ferrovum, Leptospirillum and Thiomonas, the latter potentially capable of both iron and sulfur oxidation. These taxa and their potential activity involve the risk of corrosion, biofilm formation and iron mineral scaling as well as further acidification. Especially Leptospirillum, a meso- to thermophilic genus, is expected to play a crucial role also during the heating cycles up to 50°C.

Results of this study will help to better understand the microbial response to changing thermal conditions in an oxic, acidic mine environment and its impact on technical equipment of different metal-based materials.