Aquifer Thermal Energy Storage (ATES) is an underground thermal energy storage technology that provides large capacity (of order MWth to 10s MWth), low carbon heating and cooling to large buildings or complexes of buildings, or district heating/cooling networks.  The technology operates through seasonal capture, storage and re-use of thermal energy in shallow aquifers, reducing carbon emissions and electricity demand for heating and cooling compared to direct ground- or air-sourced heat pump systems.

We demonstrate that ATES could make a significant contribution to decarbonising UK heating and cooling, but uptake is currently very low.  We identify eleven low temperature (LT-ATES) systems operating in the UK, with the first having been installed in 2006. These systems currently meet <0.01% of the UK’s heating and <0.5% of cooling demand.  Despite the current low uptake, the UK has large potential for widespread deployment of LT-ATES, due to its seasonal climate and the wide availability of suitable aquifers which are co-located with urban centres of high heating and cooling demand.  We use a probabilistic approach to estimate that ATES could supply approximately 64.5 % of current UK heating demand, and 80 % of cooling demand.  

A key barrier to increasing UK uptake is lack of awareness of the technology.  We analyse the performance of a successful UK installation, and also report installations in which problems with design and operation have caused sub-optimal performance.  The UK can benefit from experience of both successful and unsuccessful deployments but these need to be more widely reported.

How to cite: Jackson, M., Regnier, G., and Staffell, I.: Prospects for Aquifer Thermal Energy Storage in the UK, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7451, https://doi.org/10.5194/egusphere-egu24-7451, 2024.

Aquifer thermal energy storage (ATES) is a promising technology for sustainable and climate-friendly space heating and cooling. Compared to conventional heating and cooling techniques, ATES-based systems offer several benefits such as lower greenhouse gas emissions and reduced primary energy consumption. Despite these benefits and the availability of suitable aquifers in many places around the world, ATES has yet to see a widespread global utilization. Currently the vast majority of installed systems is located in the Netherlands, Belgium, Sweden and Denmark. Besides technical and hydrogeological feasibility, appropriate national policies driving ATES deployment are therefore of high importance. Hence, this study provides an international comparison of ATES policies, highlighting best practice examples and revealing where appropriate policy measures are missing. To this end, multi-disciplinary views from experts in geothermal energy and ATES from academia, companies, government authorities, national geological surveys and industrial associations in 30 countries were obtained through an online survey. Subsequent semi-structured interviews with a smaller selection of experts revealed further insights. The online survey results show significant differences regarding the existence and the strength of supporting policy elements between countries of different ATES market maturity. Going beyond these descriptive findings, the interviews provided more country-specific details on how favorable conditions came into effect and what obstacles have still to be overcome for an increased ATES deployment. Based on the lessons learned from the online survey and the expert interviews, recommendations for sophisticated ATES policies are derived which address the following areas: legislative and regulatory issues, raising awareness and expertise, the role of ATES in local energy transitions, and social engagement. This work aims at steering energy policy towards a wider international ATES deployment and better harnessing the potential of ATES to decarbonize buildings.

How to cite: Stemmle, R., Hanna, R., Menberg, K., Østergaard, P. A., Jackson, M., Staffell, I., and Blum, P.: Policies for Aquifer Thermal Energy Storage (ATES), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9196, https://doi.org/10.5194/egusphere-egu24-9196, 2024.

In the context where the member states of the European Union are asked to reach the goal of net zero emission of greenhouse gases before 2050, many cities all over Europe are adopting sustainable energy systems. One of the primary sources of CO₂ emission are building heating and cooling systems. As aquifers are commonly present in many urban areas, ground coupled heat pumps are a renewable solution that many countries are adopting to replace fossil based heating and cooling techniques.

Aquifer thermal energy storage (ATES) is attained by storing thermal energy in groundwater aquifers. Hence, a key aquifer characteristic for ATES is the natural groundwater flow velocity. For velocities less than 25 m/y, bi-directional ATES is usually possible. For groundwater flow velocities greater than 25 m/y, ATES is still possible, but requires multiple doublets with a specific well placement to be able to compensate for the groundwater flow via upstream injection and downstream extraction. Alternatively, “pump and dump” systems, which generate thermal plumes that affect the downstream groundwater temperature and are not desirable for other users, are adopted.

A possible way to tackle the issue of aquifers with high ambient groundwater flow velocity is to combine the two solutions described above by applying a uni-directional pumping scheme allowing to compensate for the groundwater flow by constantly injecting in the upstream well and extracting from the downstream well. Spacing of the wells should be adjusted to the storage cycle length and natural groundwater flow velocity to ensure re-capture the energy injected within the aquifer in the previous season from the upstream well and transported to the down-gradient extraction well by the groundwater flow. This concept would also mitigate the downstream effect of a thermal plume. In this research the results of a sensitivity and feasibility analysis of this concept are presented. The results show that optimal inter-well distance not only depends on storage cycle length and groundwater flow velocity, but also on storage volume. Downstream thermal pollution can be avoided and recovery of the heat/cold stored can reach values between 50-60% depending on the conditions.

How to cite: Silvestri, V., Bloemendal, M., Crosta, G., and Previati, A.: Exploring the feasibility of uni-directional ATES in high ambient groundwater flow aquifers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2196, https://doi.org/10.5194/egusphere-egu24-2196, 2024.

The Chalk comprises a highly heterogeneous dual porosity aquifer, characterized by intervals of high permeability formed by fracturing and/or karstification of a low-permeability matrix. Many boreholes in London show evidence of a high-permeability flow zone at the top of the Chalk. Despite this, models used to predict ATES system operation in the Chalk aquifer in London have typically assumed a homogeneous aquifer, so that simulated warm and cold plumes have a simple cylindrical geometry around the wells. In this study, we investigate the impact of aquifer heterogeneity on system operation.

We examine an operating LT-ATES installation. The system employs 4 cold wells and 4 warm wells and is sized to deliver peak heating of 1.8 MW and peak cooling of 2.75 MW. Analysis of flowrate and temperature data shows that the system has a well-balanced energy ratio of 0.09 and exhibits a low but increasing thermal recovery which is currently ca. 40% for warm storage and 25% for cold storage.

We use a Surface-Based Modelling (SBM) approach to represent geological heterogeneity, which allows us to accurately and realistically capture geometrically complex subsurface features. We develop a range of parametrised 3D models of different geological scenarios, to capture uncertainty in geological heterogeneity between the wells. Flow and heat transport during ATES operation are simulated using the Imperial College Finite Element Reservoir Simulator (IC-FERST). The models are calibrated using Nelder-Mead methods to match pressure transient data and well inflow logs obtained from the boreholes prior to commissioning. Temperature and flowrate data collected during operation are subsequently employed in thermal simulations using the calibrated models.

Our findings suggest that aquifer heterogeneity has a significant impact on the formation of the warm and cold plumes. Heterogeneity has resulted in a thermal recovery bias toward warm water, despite ambient aquifer temperature being closer to the injected cool water temperature. Two high-permeability intervals play a pivotal role in the development of pancake-shaped plumes, as opposed to simple cylindrical plumes. Greater conductive heat losses to the overlying and underlying rock is observed with pancake-shaped plumes, resulting in lower thermal recovery. Recovery is predicted to increase as the temperature of the surrounding rock gradually changes through time. Heating and cooling demand on the ATES system is generally low, so the system predominantly utilizes just a single well doublet, with the choice of operational doublets varying through time. Thermal interference between the warm and cold wells in a given operating doublet may result from the laterally extensive plume geometry.

Our results indicate the necessity of recognising and modelling subsurface heterogeneity prior to ATES operation in areas with fractured and/or karstified aquifers. Designing effective operational plans entails incorporating considerations of local heterogeneity. Our insights have broad implications for future planning and design of ATES systems in the UK and globally.

How to cite: Firth, H., Jacquemyn, C., Hampson, G., and Jackson, M.: Impact of Aquifer Heterogeneity on LT-ATES Performance: A Case Study from the Chalk Aquifer, London, UK, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9285, https://doi.org/10.5194/egusphere-egu24-9285, 2024.

Space heating and cooling is responsible for roughly 25% of our final energy use, therefore it is a necessity to decarbonize our heating and cooling systems. Many technologies for space heating and cooling use a heat pump, making space heating and cooling more dependent on electricity. Since other sectors are similarly becoming more dependent on electricity, a main challenge in the energy transition is electricity grid congestion. This is even more a challenging issue given the season variation within heating and cooling demands.

                Hence, there is a need for a technology that disconnects the heating and cooling demand from electricity use. The Aquifer Thermal Energy Storage (ATES) Triplet is a technology that does just that. The ATES Triplet uses local sources for heating and cooling, such as solar collectors and dry coolers, and disconnects the temporal supply and demand asynchrony with subsurface storage. It has a hot well, for when the heat source either has too much or too little supply to match demand, a cold well for the time when a cold source has either too much or too little supply for direct cooling. Furthermore, it has a third well that prevents thermal pollution of the hot and cold well. Because of this integration, the ATES Triplet system can supply heating and cooling without a heat pump.

                Parametric simulations are presented to give insight into operational behaviour. A systematic variation of heating and cooling demands, injection temperatures, minimum operation temperatures and return temperatures gives insight into requirements to design and operate an efficient and reliable system. These parameters mostly influence either the heat recovery from the hot well, or the temperature in the third well. A lower heat recovery in the hot well results in a higher need for heat generation, and is mainly influenced by the heating demand, the injection temperature, and the cutoff temperature. A higher temperature in the third well results in a smaller need for heat generation, but a higher need for cold generation, and is influenced by the heating and cooling demands, and the return temperatures.

How to cite: van Esch, T., Bloemendal, M., Hartog, N., and Vardon, P.: Effect of supply and demand conditions on the storage utilization for the ATES Triplet, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1849, https://doi.org/10.5194/egusphere-egu24-1849, 2024.

High Temperature Aquifer Thermal Energy Storage (HT-ATES) allows the large scale seasonal storage of sustainable heat available in summer, enabling its utilization in winter, hence reducing GHG emissions. The feasibility and potential of HT-ATES systems is largely determined by its recovery efficiency. The stored groundwater, at 40 to 80 °C higher temperature than the ambient groundwater, has a relatively low density and viscosity. The differences in density and viscosity between the stored and ambient groundwater create the potential for buoyancy-driven flow to the top of the aquifer during storage, which may lead to increased heat losses during recovery as commonly observed in earlier HT-ATES studies. Conventionally, these studies assume an uniform in- and outflow distribution of water through the well screen during injection and extraction. However, as density and viscosity changes also affect the pressure state in the well and influence the flow resistance of the aquifer, the flow distribution through the well screen is potentially impacted by these changes of density an viscosity, especially at higher storage temperatures. Therefore, this study addresses the effect of density and viscosity differences on the flow distribution through HT-ATES screens during injection and extraction for relevant storage conditions, and further assesses its impact on heat recovery compared to simulations including uniform well flow distribution. Results show that, due to both density and viscosity changes at high storage temperature, the flow distribution through the well screen may be significantly changed depending on mainly the hydrogeological and well operating conditions. Compared to HT-ATES simulations with uniform flow distribution, increased thermal recovery efficiencies are observed ranging from 0 – 8% in the 5^th year of operation for the varied conditions in this study.

How to cite: Beernink, S., Hartog, N., Vardon, P. J., and Bloemendal, M.: How temperature-induced density and viscosity differences affect the flow distribution through well screens and may influence heat recovery of HT-ATES systems, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10981, https://doi.org/10.5194/egusphere-egu24-10981, 2024.

To mitigate the increasing greenhouse gas emissions, the technology of high-temperature aquifer thermal energy storage system (HT-ATES) is attracting the public’s attention as an alternative to traditional fossil fuels for domestic heating and cooling. Based on a wellbore model and a reservoir model, we did a comprehensive economic assessment of the target HT-ATES planning in Burgwedel near Hannover, Germany. The levelized cost of heat (LCOH), payback time, and CO₂ emission reduction are selected to assess the HT-ATES performance. Results show that the total energy loss during the stages of injection, production and reservoir storage is ca. 9%, of which ca. 2.7% during injection, ca. 2.2% during production, and ca. 4% within the reservoir. Provided that the heat exchange efficiency between the subsurface part and end-use system is 70%, the HT-ATES starts to profit from the 3^rd operation year with 30-year LCOH of nearly 2.3 cent per kWh, similar to the currently running ATESs. The 30-year net CO₂ emission reduction is ca. 58.1 kt with an average of ca. 1937 t/year, which is more considerable than the low-temperature ATES, i.e., ranging from 150 and 1500 t/year. The economic assessment of the HT-ATES indicates that the planning project can provide heating and cooling services for the district of Burgwedel with lower price and CO₂ emission.

How to cite: Zhou, D., Li, K., Gao, H., Tatomir, A., and Sauter, M.: Techno-economic assessment of high-temperature aquifer thermal energy storage system, insights from a study case in Burgwedel, Germany, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13438, https://doi.org/10.5194/egusphere-egu24-13438, 2024.

At present, over half of all primary energy used in Europe is used for heating and cooling. Therefore, decarbonizing the heating supply is essential to achieve climate targets.  Underground thermal energy storage is a key enabling technology for the energy transition to buffer the large seasonal mismatch between thermal energy demand and sustainable thermal energy production capabilities. In Delft, a High-Temperature Aquifer Thermal Energy Storage (HT-ATES) system will be installed at the campus of Delft University of Technology (TU Delft). It will be integrated in the wider heating system on and around the TU Delft campus, which itself is undergoing a transformation to optimally supply sustainable thermal energy. The district heating network will be extended and utilize the thermal energy from a geothermal doublet producing heat at around 75-80°C with a flow rate of ~350m³/hr. Excess energy produced by the geothermal well in summer will be stored in the HT-ATES system, and will be utilised when demand exceeds production throughout the winter. The HT-ATES system will comprise of 7 wells (3 hot wells of 80°C and 4 warm wells of 50°C) to a depth of approximately 200m, with storage in an unconsolidated sedimentary aquifer between 160-200m depth. It is designed so that the instantaneous excess power from the geothermal project can be stored and demand from the district heating network be extracted from the system.  

The HT-ATES system at TU Delft is partially funded by local stakeholders and the European commission within the PUSH-IT project and has two primary goals: (i) to reduce carbon emissions on TU Delft campus , and (ii) to create a unique demonstration, education and research infrastructure. The complexity of a HT-ATES requires innovative solutions during the entire system life cycle.  The scientific programme that is initially planned within the project is therefore focusing on various research fields and includes:
- Characterisation of the subsurface formations including mechanical, hydraulic, thermal, and chemical properties.

- Evaluation and monitoring of the biological conditions and microbial diversity, and potential impact on water quality.

- Innovations in drilling and completion, monitoring and performance.

- Quantification of the system performance and system impact during multiple storage cycles and the full lifecycle of the HT-ATES. This will include extensively monitoring temperature distribution and water quality in the subsurface to characterise behaviour and improve models.

- Demonstrate and develop the implementation of HT-ATES in an urban setting,  including control of the system in the built-environment and transforming the conventional heat network to a future-proof  heat network.

- To allow access to other universities or institutions with active programmes in the field of Geothermal Science and Engineering to jointly carry out research and perform experiments.

-Societal engagement and legal evaluation for improving the just energy transition.

Acknowledgements: Funded by the European Union under grant agreement 1011096566 (PUSH-IT project). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or CINEA. Neither the European Union nor CINEA can be held responsible for them.

How to cite: Vardon, P., van der Schans, M., Koulidis, A., Grubben, T., Beernink, S., and Bloemendal, M.: High-Temperature Aquifer Thermal Energy Storage (HT-ATES) system for research development and demonstration on the TU Delft campus, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14989, https://doi.org/10.5194/egusphere-egu24-14989, 2024.

Concepts for achieving sustainable energy provision in urban areas require also an appropriate exploration concept for a succesful planning of a possible site-development. Geophysical exploration data and deep reaching boreholes are commonly rare in such areas. In Berlin, Germany, the German Research Centre for Geosciences drilled a research borehole to investigate the usability of Mesozoic aquifers as underground thermal storage reservoirs, the Gt BtrKoe 1/2021 borehole in Berlin-Adlershof. The information gained from this borehole provides new structural data and encounters several Mesozoic potentially ATES aquifers. The general depth of those aquifers are largely determined by the ascent and location of the underlying Permian salt structure. The research well was completely cored from a depth of 210 m (close to the top of pre-Tertiary) to the final depth of the well at 456 m. Based on an in-depth characterization of the cores that included also geophysical measurements on cores (like gamma-ray, bulk density, thermal properties, and p-XRF) and a detailed geological description, the porosity, permeability, and lithological heterogeneity of the drilled succession was evaluated and integrated in a 3D geological model. In the cored borehole section, several promising sandy reservoir sections were drilled, with a main storage reservoir in the Lower Jurassic Hettangian at a depth of 360–400 m. The sandstone is very fine grained and only weakly consolidated, showing porosities of 25–30% and permeabilities of more than 1 Darcy (> 10-12 m²). A first simulation allows for a successful integration of an ATES using this horizon at this site. The simulation is based on the seasonal provision of 35 GWh of storeable heat provided from a wood-fired combined heat and power plant  for a time period of 4 months with a temperature of 90°C, which is stored in the 24°C warm sandstone reservoir. The simulation shows that it will take about two years of operation to reach the full storage potential of the reservoir. Based on the findings, a real ATES will be established at the site, allowing to study in detail possible rock-fluid interactions and the overall performance of the system. The site activities are funded by the BMWK (FKZ 03EE4007 and FKZ 03EWR022C) and the EU project PUSH-IT (https://www.push-it-thermalstorage.eu/).

How to cite: Norden, B., Kranz, S., Blöcher, G., Regenspurg, S., Virchow, L., and Saadat, A.: The realization of a high-temperature ATES in Berlin: from explorational drilling to implementation in Berlin-Adlershof, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19244, https://doi.org/10.5194/egusphere-egu24-19244, 2024.

High temperature aquifer thermal storage (HT-ATES) in hot deep aquifers is considered to optimize the usage of commonly available energy sources. This study investigates potential near-well mineralogical changes induced by HT-ATES in two different Danish geothermal reservoirs: the mineralogically mature and calcite-free Gassum Formation, and the calcite-containing Bunter Sandstone Formation.

Core flooding experiments at reservoir conditions and temperatures up to 150°C were performed with a core specimen from each of the two geothermal reservoirs and with synthetic formation water as the flooding fluid. Effluent brines for chemical analysis were collected during the experiments and mineralogical changes of the core material identified by mineralogical characterisation of the core material before and after the experiment. A 1D reactive transport model was constructed to simulate the laboratory experiment and the model was calibrated against the experimental data. This calibrated model was then extended to simulate the continuous injection for six months of heated formation water into the two sandstone reservoirs to evaluate potential near-well mineralogical and porosity changes. For the Bunter Sandstone Formation, the formation water was modified to a saturation index for calcite of -0.1 prior to injection to replicate a situation where precautions are made to avoid loss of injectivity due to calcium carbonate scaling at elevated temperatures.

For the Gassum Formation, injecting formation water up to 100°C showed no significant changes in porosity. At temperatures ≥120°C, albite and siderite contents decreased, but the impact on reservoir porosity was minimal. However, calcite precipitation near the injection well at temperatures ≥120°C may reduce injectivity. In the Bunter Sandstone Formation, the use of slightly modified formation water resulted in considerable calcite dissolution in the reservoir at all investigated temperatures and dolomitization at 150°C. Our findings indicate a potential risk of near-well clogging caused by the precipitation of dolomite at this temperature. At lower temperatures, the calcite dissolution led to increased porosity, posing a potential risk to the rock's integrity.

The study suggests that HT-ATES in reservoirs with high calcium carbonate content, like the Bunter Sandstone Formation, is challenging. Precautions to avoid calcium carbonate scaling may lead to dissolution, while neglecting precautions may cause scaling and clogging issues. In contrast, mineralogically mature and calcite-free sandstones, such as the Gassum Formation, may be suitable for excess heat storage, particularly at temperatures <100°C.

How to cite: Holmslykke, H. D. and Kjøller, C.: Laboratory investigations and reactive transport modelling of potential near-well mineralogical changes during seasonal heat storage (HT-ATES) in Danish geothermal reservoirs, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17870, https://doi.org/10.5194/egusphere-egu24-17870, 2024.

Following an increase in demand and extent of renewable energy systems, one of the most urgent challenges in the energy transition is the storage of intermittent renewable energy. Storage would enable continuous energy supply and ease current difficulties in balancing the energy grid. For example, it is estimated that the UK spent over £500 million on wind energy curtailment payments in 2023, wasting thousands of GWh during valuable high output times. This curtailed wind energy could be stored as Underground Thermal Energy (UTES) and utilised when demand increases and outstrips production. Hundreds of abandoned legacy mine shafts across the UK contain a large volume of water and concrete lining surrounded by low permeability rocks, suggesting they can be used as valuable pre-existing structures for low-cost heat storage systems. The locality, availability and volume of flooded mine shafts make them an interesting asset of energy storage for space heating and cooling especially in Scotland.

The STEaM project (Subsurface Thermal Energy storAge: Engineered structures and legacy Mine shafts) is investigating the challenges and safety of heat storage in abandoned coal mine shafts that have refilled with water via a joint experimental/modelling investigation and an upcoming pilot project at a legacy mineshaft in Scotland. While the overall project includes hydrogeochemical analysis, energy systems modelling, as well as mechanical and material investigation, this presentation focuses on simulating the coupled hydro-thermal-mechanical processes impacting the shaft and near-field environment such as e.g., development of natural convection cells. Specifically, this presentation will showcase a modelling sensitivity analysis of some of the common variabilities in mine shafts (such as shaft dimension, geology and groundwater flow) to evaluate heat storage efficiency for cyclic heating and cooling. Basis for the presented model is a 3D finite element analysis with COMSOL to study spatial and temporal developments of heat gain and loss underground.

How to cite: Dassow, J., Molnar, I., Burnside, N., Ewe, W. E., Flett, G., Flude, S., Mukherjee, I., Tuohy, P., Wang, H., Whittington, D., Yang, S., and Shipton, Z.: Heat storage in abandoned coal mine shafts in Scotland, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11654, https://doi.org/10.5194/egusphere-egu24-11654, 2024.

Thermal Energy Storage (TES) has gained prominence as a viable solution for storing surplus heat and cold underground. While traditional TES systems primarily rely on natural aquifers, an emerging approach known as ”Mine Thermal Energy Storage“ (MTES) has garnered attention for repurposing former underground mines.

Despite the promise of TES in both Aquifer Thermal Energy Storage (ATES) and MTES, several operational challenges persist, including clogging, scaling, corrosion, and energy loss across system boundaries. These challenges impact not only the geological matrix but also the integrity of technical infrastructure components such as pipes and heat exchangers.

The “MineATES” research and development project, funded by the German Federal Ministry of Education and Research (BMBF), investigates the feasibility and limitations of employing water-filled cavities in underground mines for TES. The study primarily focuses on the ”Reiche Zeche“ silver mine, designated as a teaching and research site at the TU Bergakademie Freiberg in Freiberg, Germany. An in-situ “real-laboratory” is established there with the purpose of simulating the periodically varying heat exchange between mine water - here stored in an experimental basin and slowly flowed through with acid precipitation water (pH ~ 2 - 3) - and the surrounding Freiberg Gneiss rock. Comprehensive monitoring of hydrochemical parameters and molecular biological analyses complement multiple heating and cooling cycles within the mine.

Furthermore, laboratory-scale column flow experiments and batch reactor tests mimic TES cycles, heating columns up to 60 ⁰C and cooling them down to 10 ⁰C (Figure 1). Comparative investigations include rock materials and mine waters from two other mines in Saxony, namely a former tin ore mine in Ehrenfriedersdorf and a former hard coal mine in Lugau/Oelsnitz.

The overarching goal of all experiments is to ascertain the nature, extent, and location of potential chemical alterations during TES operations.

Figure 1-  Overview of column experiments

How to cite: Arab, A., Binder, M., Engelmann, C., and Scheytt, T.: Revitalizing underground mines: unlocking the potential of thermal energy storage (MineATES), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19543, https://doi.org/10.5194/egusphere-egu24-19543, 2024.

The (over-)seasonal storage of excess heat and cold in the subsurface is considered a promising solution to the manifold challenges of the energy transition. Many underground thermal energy storage (TES) systems are focusing on natural aquifers. In parallel, there has also been increasing attention on using artificial cavities in (partially) flooded underground mines. This special form is known as mine thermal energy storage (MTES).

Like other underground TES systems, MTES faces several challenges. Many former mines are actively dewatered to keep a defined flooding level and, therefore, significant water flows can be present, especially in the main tunnels and shafts. The unintentional transport of stored heat energy out of the original storage area, whether through heat advection or conduction, ultimately leads to reduced recovery rates and suboptimal efficiency. Adoption of localized and hydraulically (more or less) isolated mine sections instead of entire levels may provide a solution to this technical challenge.

An MTES Geo-Lab has been recently designed and established as part of the R&D project "MineATES", funded by the German Federal Ministry of Education and Research (BMBF). The in-situ Geo-Lab is located in the former silver mine "Reiche Zeche (Himmelfahrt Fundgrube)" at the TU Bergakademie Freiberg in Saxony, Germany, and focuses on the controlled simulation of TES cycles on a manageable scale. Specifically, a cuboid-shaped experimental reservoir (water capacity of approx. 21 cubic meters) in the northern field of the Reiche Zeche’s first level, slowly flown through by acidic precipitation water (pH values between 2 and 3), was chosen. The prevalent geological formation in this area is the Freiberg gneiss. Given the pilot-scale of the study site, heat losses across system boundaries are expected to be of an experimentally manageable magnitude – and are intentionally so.

The immediate vicinity of the experimental reservoir has been equipped with an extensive thermal monitoring system. This includes more than 90 temperature sensors embedded at various distances from the reservoir walls, with some up to two meters deep and distributed across 18 boreholes. First measurements showed a background temperature in the rock of approx. 11.5°C on average. This monitoring system enables continuous tracking of transient temperature distributions in the surrounding rock, facilitating the quantification of heat losses and efficiency reductions during periodic heat/cold injection and extraction experiments, emulating real-world TES cycles. Furthermore, the Geo-Lab is equipped with multiple sampling points to monitor hydrochemical parameters over time.

How to cite: Binder, M., Arab, A., Engelmann, C., Oppelt, L., Chen, C., Grab, T., Nagel, T., and Scheytt, T.: The Reiche Zeche Geo-Lab for in-situ simulation of mine thermal energy storage (MTES): Design and insights., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18502, https://doi.org/10.5194/egusphere-egu24-18502, 2024.

Decarbonizing the industrial and building heating and cooling sectors is a crucial step toward achieving carbon neutrality, necessitating innovative and sustainable solutions for the over-seasonal storage of excess heat energy. With Germany alone having more than 10,000 old mines, repurposing these sites to implement a controlled thermal energy storage strategy, known as mine-based thermal energy storage (TES), has emerged as a potential solution. To effectively utilize such partially flooded artificial cavities, it is crucial to fully understand the heat transport and storage behavior in these systems.

In this work, a three-dimensional hydro-thermo-component (HTC) model was developed using the open-source simulation code OpenGeoSys (OGS). The model was initially verified against analytical solutions for the single fracture flow of heat and solute transport, respectively. Subsequently, stochastic discrete fracture matrix (DFM) geometries and meshes were generated using the computational suite Frackit, based on data from a pilot heat storage site in a water-filled mining cavity in Freiberg, Germany. This test site is geologically characterized as the Freiberg gneiss, a metamorphic fractured rock formation.

The developed setup allows for investigating the thermal energy storage capacity and the energy recovery efficiency based on process simulations in OGS. The study evaluated the thermally affected zone in the fractured formation and quantified the amount of heat stored and recovered during cyclic operation. In addition, the solute transport distance within the surrounding rock can be evaluated under different hydraulic conditions. The general modeling workflow provides a basis for conducting techno-economic feasibility analysis of mine-based TES systems.

How to cite: Chen, C., Binder, M., Oppelt, L., Hu, Y., Engelmann, C., Arab, A., Xu, W., Scheytt, T., and Nagel, T.: Evaluation of the mine thermal energy storage potential with a stochastic discrete fracture matrix model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8027, https://doi.org/10.5194/egusphere-egu24-8027, 2024.

The subsurface offers significant opportunities for geothermal heat extraction and storage. Disused, flooded coal mines, in particular, are capable of providing a major low-enthalpy, renewable and long-term heat resource if carefully managed. To establish a mine water geothermal system requires significant capital investment and a large amount of time. Therefore, a solid investigation of the feasibility of a mine as heat source or storage medium is essential.

We have developed a modelling tool, GEMSToolbox, to investigate the feasibility of abandoned mines for long-term heat extraction and/or storage. This tool allows for a computationally fast and a low-cost study into the viability of mine water heating for given mine workings. We combine numerical and analytical methods with digitised legacy mine plan data to estimate the variations in the abstraction water temperature over the lifetime of a project. We couple the heat transfer approximation method originally proposed by Rodriguez and Diaz (2009) to that of flow in a pipe network as described by Todini and Pilati (1987), and refine the original heat transfer approximation by accounting for a flow regime specific heat transfer coefficient between the rock mass and the water, as prescribed by Loredo et al. (2017). A novel weighting function is also developed to account for the interference between adjacent mine galleries. This tool has been successfully applied to a range of mine system in the North East of England, and the results of this study are used to draw widely applicable conclusions on the feasibility of mine workings for heat extraction or storage more generally.

References:

• Loredo C, Banks D, Roqueñí N. Evaluation of analytical models for heat transfer in mine tunnels. Geothermics 2017; 69; 153-164.
• Rodriguez R and Díaz M. Analysis of the utilization of mine galleries as geothermal heat exchangers by means a semi-empirical prediction method. Renewable Energy 2009; 34(7), 1716-1725.
• Todini E and Pilati S. A gradient method for the analysis of pipe networks. Computer app. In water supply 1987; 1-20, v1.

How to cite: van Hunen, J., Mouli-Castillo, J., Sweeney, A., Tu, J., Wang, Y., and Adams, C.: Modelling the feasibility of using disused mines for heat extraction and storage, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9394, https://doi.org/10.5194/egusphere-egu24-9394, 2024.

Wallonia (southern Belgium) experienced intense coal mining in the 19th and 20th centuries. The depths of these abandoned coal mines range from the ground surface to more than 1000 m. These abandoned mines are now being studied as potential reservoirs for inter-seasonal geothermal storage operations as part of the operation of 5th generation heating networks. In the scope of feasibility studies, a major challenge is to reconstruct the geometry of former structures and works, a preliminary but essential step in modelling flows and heat transport underground.

As these mines have long since closed and are no longer accessible, this work is based solely on archive documents such as mining maps from the former collieries and cross-sections held by the mining administration. The information available in these documents is invaluable. However, this information, some of which dates back more than a century, is sometimes difficult to interpret and cannot be considered exhaustive. Moreover, it is not homogeneous and the accuracy of the topographical information varies depending on the source and the time period.

In order to reconstruct a coherent model of the mine workings, galleries and shafts, a geological model constrained by information from archive documents is constructed. Next, the elevation of the boundaries of the mine workings and galleries in layers is adjusted on the layers derived from the geological model in order to preserve the topological links between objects. Finally, a discretised model representing the zones of increased permeability around the workings and galleries is extracted. To carry out all these operations and the associated quality controls, a workflow based on developments in Python and relying on open source libraries has been developed and tested.

How to cite: N'depo, Y., Dupont, N., Martin, T., and Kaufmann, O.: Modelling the geometry of abandoned coal mines for inter-seasonal heat and cold underground storage, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18740, https://doi.org/10.5194/egusphere-egu24-18740, 2024.

Thermal energy storage (TES) is a technology that stocks thermal energy by heating or cooling a storage medium so that the stored energy can be used at a later time for heating and cooling applications and power generation.  TES systems can be divided into three main categories: sensible heat storage (SHS), thermochemical storage (TCS) and latent heat storage (LHS) associated with phase-change materials (PCMs). PCMs are able to release or absorb thermal energy changing their physical state at a quite constant temperature.
This work aims to analyse the properties of different PCMs and supporting materials to be used coupled in closed-loop underground thermal energy storage (UTES) system. The physical and thermal properties of different PCMs (from lower to medium – high temperature) and their supporting materials, in particular diatoms, are analysed separately through laboratory analysis and numerical modelling, in order to find out the best material providing high thermal conductivity and capacity, efficiency, durability and low cost. However, research is also open to other types of materials such as graphene, zeolites, carbon nanotube and biochar.
Then, the best performances PCM is tested to define the best mix between PCMs (the capacitive part) and diatoms (part that will prevent the leakage during the phase change) under different boundary conditions. With the best mix, combined with cement, cylindrical samples 30 cm in diameter and 1 m in length will be constructed with the patented mixtures with a metallic finned tube inserted within it, to represent a thermal storage mass with a tube inside.
This sample will be tested, for low-temperature experiments (below 100 °C), using water as carrier fluid, in a physical model hosted at the UNIPD laboratory constituted by a box (1 m3 volume) that can be filled with dry or saturated loose materials with a metallic tube inserted in the middle to represent the heat exchange between the water flowing in the tube at a certain temperature, and the surrounding material, under different boundary conditions. The experimental device, allows to assess, under controlled conditions, the evolution in time and space of the energetic processes that occur between an heat exchanger and the surrounding ground. A second sample with similar characteristics, on the other hand, will be tested with high temperatures (between 200 °C and 300 °C) using a diathermic oil as heat transfer fluid and will be tested in a different device with a high-temperature heating system. The experimental data obtained will be also used in the construction and calibration process of numerical models by using commercial software (FEFLOW).
Finally, the simulation results are expected to identify the best conditions to apply the new conceived mix in a real test site.

How to cite: Franzè, S., Martelletto, F., Dalla Santa, G., Isania, F., Scotton, P., Doretti, L., and Galgaro, A.: Enhanced closed-loop underground thermal energy storage systems, using experimental materials via numerical modelling and laboratory analysis, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15006, https://doi.org/10.5194/egusphere-egu24-15006, 2024.

A series of combined laboratory scale experiments and numerical simulations of a high-temperature borehole heat exchanger (BHE) in a geological porous medium was carried out under dynamic cyclic thermal loading. The main objective of this work was to investigate the influence of the transient charging/discharging cycle time scales on thermal convection and heat transfer characteristics. The experimental unit was constructed with a vertical, grouted coaxial BHE installed in the center of a water saturated coarse sand within a 1.4 m³ cylindrical barrel. Five cyclic experiments at 70°C charging temperature and cycle durations of 6, 12 and 24 hours after a pre-charging phase, and 6 and 12 hours without pre-charging were conducted under well controlled conditions. A dense sensor grid of 129 thermocouples was deployed to record the evolution of the temperature distribution inside the unit and across the boundary. A 2D-axisymmetric numerical model of the experimental unit was developed in OpenGeoSys and validated for the coupled thermo-hydraulic processes in the porous medium by comparison to previously performed short-term static charging/discharging experiments. The model then was applied to simulate the experiments presented here. The model-to-data comparison indicated a very good agreement, and thus was further used to analyse the impact of the cyclic operation on the contribution of thermal convection to overall BHE heat transfer.

The results indicated a cumulative increase of sand temperature levels during the first cycles in the experiments without pre-charging and a decrease when cyclic operation was started after a stationary pre-charging stage, until a quasi stable behavior was reached between consecutive cycles. In general, the difference in obtained results between both operating modes for a given cycle duration was only observed within the first three to four cycles. An increase of the magnitude of convective flow characterised by increases of heat transfer rates, Rayleigh numbers, buoyant flow velocities, and temperature gradients with the cycle duration from 6, 12 to 24 hours was found. The estimated contribution of thermal convection to heat transfer at the last cycle decreased from 23% at 24 hours cycle duration to 20% and 18% at 12 and 6 hours with pre-charging, and 21% and 16 % without pre-charging, respectively. These results demonstrate that for the given experimental setup and scale, convective flow can no longer fully develop within the shorter dynamic charging cycle durations, and thus is less effective for increasing the BHE-to-sand heat transfer during heat charging.

How to cite: Djotsa Nguimeya Ngninjio, V., Beyer, C., and Bauer, S.: Impact of cyclic thermal loading on thermal convection along a high-temperature borehole heat exchanger: Laboratory-scale experimental and numerical study, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20748, https://doi.org/10.5194/egusphere-egu24-20748, 2024.

Modelling heat transport in porous aquifers is generally based on the assumption of an instantaneous local thermal equilibrium (LTE) between the solid and the fluid phase. Previous studies have revealed that this assumption can be violated, e.g. in the presence of fast or preferential flow causing delayed heat diffusion into the grain structure matrix, referred to as local thermal non-equilibrium (LTNE). However, conditions and scales at which LTNE effects should be taken into account in natural heterogeneous sediments are almost unexplored. We study the relation between macro-scale heterogeneity, thermal dispersion and LTNE through numerical simulations of heat transport in three-dimensional heterogeneous hydraulic conductivity fields. The advection-diffusion equation is solved using the Multiphysics Object-Oriented Simulation Environment (MOOSE), an open-source, parallel finite element framework. The spatial and temporal evolution of the heat plume generated by a line source under steady-state flow conditions is examined. For understanding the propagation of heat plumes, the role of delayed diffusion caused by LTNE effects needs to be distinguished from hydro-mechanical dispersion. Therefore, we estimate the thermal dispersion and the effective thermal retardation for each time step using a stochastic approach. LTNE effects are present, when the effective thermal retardation deviates from the predicted, apparent thermal retardation. Simulations show good agreement between the effective and the apparent thermal retardation for homogeneous hydraulic conductivity. With increasing heterogeneity, characterized by a higher variance of the log-conductivity, the effective retardation becomes lower than the apparent retardation at early times. Furthermore, we estimate the effective thermal retardation for a homogeneous flow field with added thermal dispersion based on the dispersion coefficients resulting from the heterogeneous simulations. We find that there is a significant difference in the evolution of effective retardation between the homogeneous and the heterogeneous case both of the same thermal dispersion, which we associate to LTNE effects. Our modelling approach thus allows to quantify LTNE induced by field-scale heterogeneity.

How to cite: Gebhardt, H., Zech, A., Rau, G., and Bayer, P.: Local thermal non-equilibrium in heterogeneous porous media identified through 3D heat transport modelling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5318, https://doi.org/10.5194/egusphere-egu24-5318, 2024.

Sand, an inexpensive and abundantly available natural geomaterial, holds promise as a thermal energy storage (TES) material in diverse solar thermal systems such as concentrated solar power, solar heating, and solar gasification. Sand possesses the ability to capture solar radiation during daylight hours, preserving it as heat on both a daily and seasonal basis, and then releasing it when demand is high. Although sand exhibits lower thermal conductivity and specific heat capacity when compared to molten salt and engineered TES materials, it compensates for these drawbacks by withstanding high temperatures and being economically advantageous.

For sand to effectively function as a TES medium, it needs to acquire a high energy density. This entails a considerable specific heat capacity and resilience to substantial temperature variations. Optimal for this purpose, pure quartz sand stands out due to its elevated specific heat capacity, high thermal conductivity, and its ability to maintain integrity without agglomerating or degrading even at temperatures exceeding 1000°C. Impurities adversely affect the energy density of sand. The presence of clays, carbonates, and feldspars has been identified as leading to premature agglomeration, early degradation, and/or reduced specific heat capacity in sands. At a temperature of 600°C, clays were observed to enhance the tendency of sand to agglomerate, with agglomeration increasing with rising temperature and pressure. Below 800°C, carbonate minerals undergo decarbonization, resulting in mass loss and alterations in grain-size distribution. For instance, calcite grains transform into lime powder, which exhibits an extremely low specific heat capacity. Below 1200°C, feldspars undergo vitrification, leading to agglomeration that hinders fluidization and the flow of sand through the system. To avoid these complications, it is imperative to limit impurities to less than 2%.

How to cite: Radwan, O. and Humphrey, J.: Sand as a thermal energy storage material for solar thermal technologies, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2421, https://doi.org/10.5194/egusphere-egu24-2421, 2024.