Geothermal facilities in the North German Basin are frequently affected by corrosion and scaling due to high salinity as well as microbial induced corrosion. To study biofilm formation, corrosion processes and countermeasures, a mobile bypass system was installed at the Neubrandenburg geothermal heat storage plant. The reservoir was located in a depth of 1300m in a saline aquifer (~130g/L NaCl). Operation was conducted in two different seasonal modes. In the warm months, heat was stored, while in the winter months the direction of flow was reversed and heat was recovered. At temperatures lower than 60°C, at the cold side of the aquifer, corrosion was promoted by microbial activity, which led to the formation of biofilms on plant components and pipes. Biofilms consisted mainly of various genera of fermentative, sulfate reducing and hydrogen consuming bacteria. Longer incubation time as well as inoculation with fresh biofilm showed an independency of seasonal mode and enrichment of highly adapted community composition with the dominating sulfate-reducing genus Desulfallas.

As a countermeasure to corrosion, heat shocks were evaluated in the bypass system and tested also two times at the large scale plant. Heat shocks led to significantly reduced biofilm formation on corrosion coupons and correspondingly reduced iron sulfide precipitates and corrosion rates from 0.538mm/a to 0.170mm/a over an observation period of 48 days. The impact of one heat shock lasted for more than four weeks. Overall the use of periodic heat shocks showed its preventative measure against microbial induced corrosion and scaling in geothermal plants.

Column and field tests related to ATES found significantly elevated Mo, V, and other oxo-anions that could not be explained by reductive dissolution of Fe oxyhydroxides. A common hypothesis levied was that the oxo-anion mobilization appears to be related to a thermal desorption process. However, for an accurate prediction of the concentration changes, there is a lack of thermodynamic parameters to prove that hypothesis which was the aim of this study.

Batch equilibrium adsorption experiments with oxo-anions such as molybdate and vanadate were performed using goethite suspensions with different concentrations, ionic strengths, pH values, and at four temperatures between 10 and 75 °C. The results of this large number (>500) of individual batch equilibrium experiments showed that the amount of an oxo-anion adsorbed decreased with increasing temperature. The experimental data were fitted using the CD-MUSIC surface complexation model framework. Temperature variations in the complexation constants were in turn fitted using the two-term van’t Hoff equation to obtain molar enthalpies and entropies. The enthalpies were negative, indicating that the adsorption of the oxo-anions is exothermic and therefore the adsorption affinity decreases with increasing temperature. The entropies could be correlated to the adsorbate molecule volumes together with those previously determined for other oxo-anions, which can be used to extrapolate the adsorption entropies of many other oxo-anions for which EXAFS-based adsorbate structures are available, but for which such data are not yet available. Hydration differences across the bivalent oxo-anion molecule series apparently affect the derived enthalpies and hence the adsorption energies for oxo-anions.

A thorough characterization of aquifer parameters is crucial for long-term predictions of the ATES system's functioning. Single well tests, also known as push-pull tests, have been widely used to identify effective porosity, flow velocity, decay constants, sorption coefficients, and heat storage capacity of the aquifer. For more than fifty years, multiple analytical and numerical approaches have been developed to validate push-pull test data and to identify model sensitivity. Despite the relatively straightforward approach, the main bottleneck of the push-pull test calibration is the non-uniqueness of the inverse problem solution. Especially in a deep ATES system data scarcity induces the parametric uncertainty and thus calls for the stochastic parameter optimization. To address this issue, a sensitivity-acknowledging surrogate modeling-based optimization technique for stochastic parameter optimization has been developed. Based on the analytical solution for heat and conservative tracer, a surrogate modeling-based optimization approach was developed to identify the heat storage from the push-pull test data. The optimization procedure has been validated against a synthetic dataset with parameter ranges from one of the ATES sites in Berlin. Results confirm that doing a push-pull test with heat and conservative tracer together enables uncertainty reduction. At the same time, sensitivity acknowledging optimization results in a much narrower posterior parameter distribution than the instant fusion of all available data. The modeling procedure highlights that objective function selection, as well as measurement accuracy, define the confidence interval and calibration precision.

High-temperature aquifer thermal energy storage (HT-ATES) is a heat storage technology utilising the subsurface, which can reduce greenhouse gas emissions in renewable-dominated heating sectors. Since the temperature difference between the surrounding groundwater and the injected water (> 50 °C) leads to density differences, HT-ATES can induce buoyancy flow. This process results in uneven heat distribution across the aquifer thickness, lower storage efficiency, and increased thermal impacts. The occurrence and intensity of buoyancy flow depends on, among others, vertical and horizontal permeability.

The proposed HT-ATES storage site in Hamburg, Germany, utilizes the Miocene Lower Braunkohlensande (brown coal sands) as the storage aquifer. This geological formation was formed in a coastal transition regime between terrestrial and shallow marine settings and is primarily composed of sands. Layers of brown coal, silt, and clay, embedded in the main storage aquifer, as identified from borehole information, were formed from peat swamps and lagoons and may impede convection due to their low permeability. The influence of these low permeability layers, also considering their lateral extension and position relative to the warm well, on induced convection and on HT-ATES efficiency and thermal impacts is examined in this work by employing a site-specific numerical HT-ATES model representing the coupled thermo-hydraulic processes.

Results show, that including even thin low permeability layers can effectively hinder temperature induced thermal convection, thus increasing thermal efficiency of a HT-ATES. The scenario simulations also show, that convection is already significantly dampened if the layers extend only up to the thermal radius.

Aquifer thermal energy storage (ATES) is a promising technique for the short- to long-term storage of reusable thermal energy in the subsurface. Many ATES projects suffer from operational issues or failures. Main causes are clogging, mineral precipitation and corrosion affecting both the aquifer matrix and technical infrastructure (e.g., pipes, heat exchangers), as well as unfavourable recovery rates due to convective and conductive heat energy losses across natural system boundaries.

While most systems address natural porous aquifers, a range of formerly active underground mines has been considered for ATES as well. The ongoing research project “MineATES” focuses on chances and limitations of such man-made systems.

Specifically, an in-situ real laboratory has been set up at the former silver mine “Reiche Zeche” in Freiberg, Germany, to simulate periodical heat exchange between mine water and surrounding rock. Hydro-/geochemistry changes will be logged simultaneously to the monitoring of heat propagation in both water and rock. In parallel, laboratory-scale flow-through column and batch experiments with multiple combinations of rock types and mine water compositions will be carried out at defined temperatures (~ 5°C to 50°C) to identify scales, types and locations of possible mineralization and further chemical alteration. Reference materials (rocks and mining waters) from the “Reiche Zeche” will be compared to materials from the Saxonian mines “Ehrenfriedersdorf” (former tin ore mine) and “Lugau/Oelsnitz” (former hard coal mine).

The project results are to be compiled into a criteria catalogue, providing guidelines for assessing if and how a mine could be used as an ATES system.

The Winzer project investigates the opportunities and challenges of implementing ATES systems in old groundwater-filled coal mines. For this purpose, a near-surface (<80 m) small coal mine in Bochum, Germany at the Fraunhofer IEG is used as the pilot site. The mine was tapped by three boreholes through which the groundwater is lifted and reinjected. The implementation of geothermal reuse in the old mine building in combination with a concentrated solar power system (CSP) is unique in the world. This allows seasonal storage of fluids with temperatures up to 60°C. In addition, a comprehensive condition monitoring was implemented. The measurement data and findings obtained will be used to make qualitative and quantitative statements on the hydrochemical, microbiological, geo-mechanical and ecological conditions during cyclic operations. The newly developed concepts and technologies will enhance the efficiency of the existing ATES system with regard to scalings and biofoulings at the heat exchangers as well as the safety, i.e. with regard to the mobilization potential of contaminants. Within the project the upscaling from the pilot site at the IEG through the planned development of the site of the former Dannenbaum mine is evaluated. There the hydraulic and geomechanical properties and changes in the ATES system can also be examined during the seasonal heat storage. By including numerical simulations, an optimization of the operation management concepts for the overall system is achieved and potentials for transferability to many other cities and networks in former coal mining regions in Europe will be shown.