Aquifer thermal energy storage (ATES) is comparatively rarely used in Germany. Since there is a lack of demonstration plants nationally, the goal of our BMBF-funded joint project “DemoSpeicher” (Development and Monitoring of Seasonal Heat and Cold Storage for the Demonstration of Aquifer Storage) is to implement and scientifically accompany a near-surface LT-ATES. Within the scope of the project, the entire construction cycle of an LT-ATES is to be covered, which ranges from design and planning to grid integration and commissioning to thermal energy supply. Legal admission requirements are developed in the process. An urban site in Germanys capital Berlin-Mitte was selected for the implementation of the demonstration plant. An extensive monitoring program is planned for the thermal-hydraulic underground processes. Another focus of the project will be possible changes in groundwater chemistry and temperature-sensitive groundwater ecology because of thermal loading. Monitoring of energy flows is also planned to estimate the thermal energy exchange between the aquifer reservoir and the building's systems engineering. This will include a heating and cooling demand analysis, as well as an assessment of potential synergistic use effects with other technologies that could be used, for example, for thermal loading of aquifer storage. All results will be presented in a coupled thermal-hydraulic modeling. The project and the first results of the implementation of an LT-ATES in a densely populated urban area will be presented and discussed in this presentation.

In progressively decarbonized district heating systems with high shares of renewable energies, seasonal large-scale heat storage systems are a central component for overcoming the seasonal offset between heat generation and demand. In addition to Pit Thermal Energy Storages (PTES), which often encounter high spatial resistance in urban contexts, Aquifer Thermal Energy Storages (ATES) in the three geothermal regions in Germany offer a suitable solution for large-scale and cost-effective thermal energy storage with high surface area efficiency. Currently, however, there is no operating high-temperature ATES in Germany that is integrated into an urban heating network. The focus of this presentation is on aspects of interaction between aquifer storage, large-scale heat pumps and district heating networks in Germany.

The framework conditions for the technical integration of heat from aquifer storage are subject to constant change. On the one hand, district heating systems are usually operated with sliding supply and return temperatures, on the other hand, the temperature continuously decreases during discharging period.

This requires different solutions for the integration of aquifer storage, which are to be systematized and classified. Central integration possibilities are the direct use in supply line, the increase of the return temperature as well as the use of large heat pumps.

The lecture will present the findings from the research project "OptInAquiFer" on reasonable integration possibilities of aquifer storage in German district heating networks. In addition, current research on large-scale HP configurations will be highlighted to identify suitable and efficient HP capacities, refrigerants and HP configurations for ATES applications.

Achieving sustainable and climate-friendly space heating and cooling is essential to the energy transition. Aquifer Thermal Energy Storage (ATES) is a promising technology to significantly reduce greenhouse gas emissions compared to conventional heating and cooling technologies. Therefore, in this study the technical potential of shallow low-temperature ATES systems is quantified for the city of Freiburg im Breisgau, Germany in terms of reclaimable energy. Using 3D heat transport models, heating and cooling power densities are determined accounting for several different ATES configurations in various hydrogeological subsurface conditions. High groundwater flow velocities of up to 13 m d^-1 lead to a significant loss of stored energy limiting power densities to a maximum of 3.2 W m^-2. Nevertheless, comparing these power densities to the existing demands of heating and cooling energy reveals that substantial heating and cooling supply rates are possible with ATES. While heating energy supply rates of larger than 60 % are determined for about 50 % of all residential buildings, the cooling energy demand could be supplied entirely by ATES systems for 92 % of the buildings. For ATES heating alone, this results in greenhouse gas emission savings of up to 70,000 tCO₂eq a^-1. This equals about 40 % of the current greenhouse gas emissions caused by space and water heating in the study area’s residential building stock. In the future, the application of the modeling approach proposed in this study for other regions with similar hydrogeological conditions could obtain estimations of local ATES supply rates supporting city-scale energy planning.

High-temperature aquifer thermal energy storage (HT-ATES) in the geological subsurface can help bridge the temporal mismatch between production and demand of energy from renewable sources. Despite great importance for energy system transformation in the heat supply sector, HT-ATES faces some challenges and risks such as regulatory challenges. The heat input experiments at the TestUM –Aquifer test site provide a basis for characterization and verification of hydraulic, thermal, geophysical, microbiological, and geochemical process understanding. A HT-ATES system was experimentally simulated at the field site, representing three phases with varying loading and unloading cycles at injection temperatures of 80°C. More than 500 thermocouples were used to record temperature data over a 579-day period between July 2021 and February 2023. A total of eleven operating cycles divided into three phases were performed, representing a total heat input of 155 MWh. The temperature records are highly resolved spatially, especially in the vicinity of the injection well, with intervals as low as 0.5 m in the vertical and horizontal directions, and a temporal resolution of 10 min. Thus, the temperature distribution in the subsurface and the position of the heat plume is well characterized at any time. Results show, that the temperature distribution is affected by density driven convection, caused by the temperature differences, as well as heat loss to the confining unit. The storage efficiency was determined by measuring return flow rates and temperatures, showing that storage efficiency decreases with cycle length and with downtimes between charging and discharging.

Seasonal ATES systems enable the efficient integration of climate-neutral heat sources into urban heat-supply systems. However, secure and efficient operation presupposes the detection as well as realistic evaluation and prediction of induced hydraulic, thermal, geochemical and microbial effects and their impacts on operation and environment.

To provide the database for extending field-scale process understanding and deriving suitable monitoring strategies, a cyclic HT-ATES field test was conducted. In six fortnight-long charging periods ~300 m³ of water were infiltrated (~15 L/min; ~80 °C) into the storage aquifer (6-15 bgs) and recovered directly or after 21 storage days. Induced hydrogeochemical effects and their reversibility were tracked with a temporal and spatially high-resolute monitoring of ~90 measurement points.

Within ~7 m around the „hot well“, superimposition of formerly stratified calcium and sulphate concentrations in combination with the spatial spreading-patterns of elevated silica concentrations point towards the build-up of a density-driven convection cell, which was also predicted by accompanying numerical thermo-hydraulic simulations. In storage periods, but more so in post operation, decreases in temperature go along with a decline of previously elevated concentrations of e.g. silica, potassium, selenium and vanadium. Moreover, potassium and selenium concentration-peaks drop after the first cycles, indicating depletion of their releasable pools. Although simulated tracers indicate passage of infiltrated water, no induced temperature or concentration changes were monitored 30 m downstream so far.

Overall, highly dynamic flow conditions dominate the hot well’s vicinity and despite scale dependent low heat recovery rates, reversibility of induced effects keeps the wider surrounding geochemically unaffected.

Throughout the past two decades, aquifer thermal energy storage (ATES) has grown increasingly into focus as a suitable geo-energy storage method. In this context, many carbonate aquifers are useable for storing and later retrieval of thermal energy due to their potential natural porosity and adequate permeability. However, numerous ATES projects suffer from operational and maintenance issues or failures. For instance, a reduction in reservoir permeability and clogging (caused by scaling, sintering, flocculation, and microbial growth) belong to the main threats to sustainable and reliable functioning.

In the BMBF-funded research project “UnClog-ATES”, both the aforementioned threats and their practical countermeasures (e.g., adding scaling inhibitors, acids, CO₂) are thoroughly investigated using a combination of flow-through column and batch experiments. These experiments are temperature-controlled to simulate realistic ATES cycles of alternating heating and cooling while monitoring them continuously.

A critical point when assessing ATES systems specifically for carbonate aquifers is that significant inconsistencies exist regarding the kinetics and intensities of mineral dissolution/precipitation processes observed in the laboratory (using pure/synthetic minerals) and those observed in reality. This is because factors such as specific surface, dislodgement from equilibrium, presence of inhibitors as well as the rock’s chemical purity play important roles. For that reason, i.e., to represent natural ATES materials and to gain realistic reaction data, limestone from the Malm (Upper Jurassic), Germany (“Treuchtlinger Marmor”), is used for our experiments.

The overall project results aim towards gaining a variety of insights that are essential for planning, effective implementation, and sustainable operation of ATES in carbonate aquifers.