In a broader context, we invite contributions that show how to enhance the societal perceptions and engagement in UTES developments and how to integrate UTES technologies in wider energy system. Both in research and in practice, accurate characterization of subsurface flow and heat transport based on observations of induced or natural variations of the thermal regime is essential. Thus, we invite contributions that reveal new insight into advances in experimental design, report novel field observations, as well as demonstrate new sequential or coupled modelling concepts. The seasonal and long-term development of thermal and mechanical conditions in aquifers and heat transfer across aquifer boundaries are focus points. This also includes the role of groundwater and geothermal energy in the context of UTES for predicting the long-term performance of storage and production of thermal energy (heating and cooling), as well as integration into urban planning and policy making. We also invite hydrogeological studies that examine heat as a natural or anthropogenic tracer with the aim of enhancing thermal response testing or improving our understanding of relevant transport processes in the underground.
Accurate prediction of heat transport is fundamental to the design and performance assessment of underground thermal energy storage (UTES) systems. Most groundwater heat-transport models assume local thermal equilibrium (LTE) between the solid matrix and pore water, implying instantaneous interfacial heat exchange and a single temperature field. However, recent experimental and modelling studies show that this assumption can break down under conditions commonly encountered in permeable and heterogeneous aquifers.
Here we present a synthesis of a multi-scale research programme that identifies when and why local thermal non-equilibrium (LTNE) becomes relevant for subsurface heat transport, spanning grain-scale laboratory experiments and field-scale numerical modelling. At the grain scale, laboratory experiments resolving solid and fluid temperatures independently demonstrate that rate-limited interfacial heat exchange results in persistent solid–fluid temperature differences for coarse grains and elevated Darcy velocities representative of UTES operation. These effects are governed primarily by grain size and solid thermal properties and cannot be captured by LTE formulations.
At the aquifer scale, three-dimensional stochastic simulations show that LTNE-like behaviour can emerge even when pore-scale LTE holds, solely due to hydraulic conductivity heterogeneity. Preferential advection along high-permeability pathways accelerates thermal fronts, while delayed heat diffusion into low-permeability domains leads to effective thermal retardation that deviates fundamentally from predictions based on volumetric heat capacity. This field-scale LTNE depends systematically on the variance and correlation length of hydraulic conductivity and the thermal Péclet number.
Together, these results reveal a continuum of LTNE behaviour across scales: grain size controls interfacial heat exchange at the pore scale, while hydraulic conductivity heterogeneity governs delayed heat uptake at the aquifer scale. Ignoring either mechanism can potentially bias predictions of thermal plume migration, retardation, and heat recovery efficiency, with direct implications for UTES modelling, performance assessment, and design reliability.
How to cite: Rau, G. C., Bayer, P., Blum, P., Lee, H., Gabhardt, H., and Zech, A.: From grains to aquifers: Under what conditions does Local Thermal Non-Equilibrium heat transport matter?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5976, https://doi.org/10.5194/egusphere-egu26-5976, 2026.
Most operational low-temperature aquifer thermal energy storage (LT-ATES) systems are implemented in productive sandy aquifers, the traditional storage target. In many regions, such resources are scarce or absent, and we have to resort to more complex subsurface settings. In Belgium, 14 LT-ATES systems already operate in the fractured basement rock. The heterogeneous distribution of fractures leads to uncertainty on preferential flow paths and thereby complicates predictions of thermal recovery efficiency. Although many studies focused on characterizing the fractured basement aquifer, their practical use to build models remains limited because spatial correlation scales are smaller than average ATES well spacing. To enable larger-scale adoption, we propose an improved modelling approach that captures the well connectivity to the surrounding rock and the connectivity between both wells. The IC-FERST simulator is used to model groundwater flow and (heat) transport, as it efficiently handles highly complex models with a (simplified) explicit fracture representation. Fracture zones were conceptualized to only represent distinct connectivity features. Several alternative conceptual models were semi-automatically generated, including uncertainty on the number and size of fracture zones and fracture orientation. As such all concepts have a different degree of connectivity. A short-term fluorescence tracer test was performed and the simulated breakthrough curve for each scenario was compared to field data. Several concepts led to inconsistent tracer behaviour and were quickly falsified, while others captured the first arrival, peak or tail of the breakthrough curve. Recirculation of the tracer showed to be essential to explain the trailing breakthrough behaviour. The concepts that best explained field data were kept for further uncertainty analysis using Monte Carlo simulations. Variability was included on fracture and matrix porous properties, natural groundwater flow, diffusivity and dispersivity. The resulting prior distribution was not falsified by the observed field data. Finally, long-term ATES simulations were performed for the selected concepts. The (simplified) explicit fracture representation produced highly complex storage volume geometries. Concepts that fit field data well, still showed large variability in the prediction of the outflow temperature of the ATES system, highlighting that uncertainty quantification is indispensable for ATES feasibility studies in fractured reservoirs. In conclusion, the well connectivity modelling strategy to simulate an ATES doublet successfully predicts field data, outperforming previous equivalent porous media modelling efforts in the fractured reservoir. This study also highlights that feasibility studies and design standards for ATES in complex subsurface settings should differ from those developed for the traditional setting. In the future, we aim to apply the Bayesian Evidential Learning (BEL) framework to predict the probability of reaching certain ATES recovery efficiency and to optimize ATES design. Since BEL maps a direct relationship between data and prediction without any model calibration, it is well suited to systems with complex heterogeneity.
How to cite: Tas, L., Jacquemyn, C., Bahlali, M. L., Jackson, M. D., and Hermans, T.: Improved Modelling Approach for LT-ATES Systems under Uncertainty in Fractured Reservoirs: A Case Study in Brussels, Belgium, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14202, https://doi.org/10.5194/egusphere-egu26-14202, 2026.
Frost damage not only affects the normal functioning of tunnels but also jeopardizes their structural stability, possibly leading to tunnel abandonment. Passive methods (e.g., insulation layers, thermal insulation doors, and anti-snow sheds) cannot completely eliminate frost damage due to cumulative freezing effects, while active methods (e.g., electric heat tracing, air curtain insulation, and ground source heat pump) suffer from high installation and maintenance costs and significant energy consumption. To effectively eliminate frost damage while reducing costs and energy consumption, a novel technology utilizing underground thermal energy storage technology for cross-seasonal frost protection in tunnels is proposed. This technology converts solar energy into heat energy via solar collectors before the cold season, then the heat energy is induced and stored into the surrounding rock around areas prone to frost damage by heat pipes. The stored heat energy automatically heats the tunnel during the cold season, driven by temperature differentials. To evaluate the feasibility of this technology in various cold regions, a coupled heat transfer model of solar-geothermal exchanger-heat pipe is developed, and a model test is conducted to validate the accuracy of this coupled model. The effects of groundwater seepage, heat storage locations, and tunnel ventilation on the frost protection performance of this technology are investigated through the validated model. Meanwhile, the influence mechanisms of these parameters on underground thermal energy storage and cross-seasonal frost protection in tunnels are determined by analyzing computational results. The optimal thermal storage locations and timing under different groundwater seepage and tunnel ventilation conditions are also identified. Specifically, this underground thermal energy storage technology can raise the average temperature of frozen areas in tunnels above 0 degrees Celsius. When groundwater seepage exists near the tunnel, the heat storage location should be situated upstream of the groundwater, and the distance between the heat storage location and the tunnel should gradually increase as the groundwater seepage velocity increases. The length of the energy storage area gradually increases as the wind speed at the tunnel entrance rises. The appropriate location for heat storage can significantly reduce heat storage time and enhance the antifreeze effectiveness of tunnels. This technology utilizes underground thermal energy storage to precisely eliminate tunnel frost damage while offering the advantages of low energy consumption and low cost, providing a green and sustainable frost prevention solution for tunnels in cold regions.
How to cite: Yu, Z. and Zhang, G.: Application of Underground Thermal Energy Storage in Cross-Seasonal Freeze Prevention of Tunnels in Cold Regions , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1914, https://doi.org/10.5194/egusphere-egu26-1914, 2026.
Understanding sector coupling of thermal and electrical power is increasingly important as the energy sector transitions towards widespread electrification. In scenarios with high-levels of intermittent renewable energy sources in the national energy generation mix, demand-side management offers opportunities to maximise the use of this generation while producing operational cost benefits to time-shifting grid loads. Included within this dynamic response to grid state are energy system components with power-to-heat and thermal energy storage functionality, as in the case of aquifer thermal energy storage (ATES). The control of these integrated energy systems requires simulation through reliable modelling frameworks.
The work herein explores the impact of electricity price signals on an energy system with low-temperature ATES in the United Kingdom. Using a commercial-scale greenhouse simulation as the basis of heating and cooling loads, a fixed-order control approach was applied to the energy system, using price signals as a key variable for operational decision-making. This used a receding horizon approach to energy system scheduling and applied machine learning models to forecast day-ahead wholesale electricity prices. A Python-FEFLOW co-simulation model was developed to investigate the impact of demand-side response on the energy system components, using key indicators of technical and economic performance.
How to cite: Nibbs, W. and Falcone, G.: Investigating demand-side response with aquifer thermal energy storage (ATES) in the U.K. , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16009, https://doi.org/10.5194/egusphere-egu26-16009, 2026.
Aquifer Thermal Energy Storage (ATES) systems offer a promising way to decarbonise heating and cooling in dense urban environments, but their performance depends on reliable operation within geologically heterogeneous aquifers and under time-varying building demand. Beneath central London, the Chalk aquifer is highly heterogeneous, which leads to complex thermal plume behaviour and potential for significant thermal interference when multiple boreholes are deployed on a confined urban site. This study presents the design of a large ATES system proposed for Imperial College London’s South Kensington campus, a small site (dimensions) with high heating demand.
The simulated system configuration and operating limits are based on previously developed probabilistic scoping scenarios (Jackson et al., 2026, EGU26). Monte Carlo sampling of uncertain design and operational parameters was used to define feasible ranges of ATES performance. In the present work, we choose to take forward selected probabilistic scenarios for detailed numerical simulation to investigate their behaviour under realistic geological and operational constraints.
A bespoke dynamic flow-rate controller is implemented in the simulator through explicit time-dependent borehole boundary conditions. The controller continuously adjusts individual doublet flow rates to meet heating and cooling demand derived from campus load profiles, while also accounting for evolving production temperatures and heat-pump performance. Flow is allocated across a ranked subset of doublets ordered by inter-borehole distance: the most widely spaced doublets are activated first, and progressively closer doublets are only activated as demand increases. Partial loading of marginal boreholes is allowed, and automated temperature-based shut-downs are applied to borehole doublets whenever production temperatures fall below (warm boreholes) or exceed (cool boreholes) the ambient groundwater temperature, to manage thermal breakthrough.
The base-case simulation represents a P50 probabilistic scenario and includes five active ATES doublets. Lateral plume spreading through high permeability karst intervals in the Chalk aquifer leads to thermal interference and, in some boreholes, thermal breakthrough, which is managed using the borehole flow-rate control system. Despite the complex plume development in the aquifer, the system can deliver the target heating and cooling demand. This would not be possible if the boreholes had been spaced based on multiples of the thermal radius, as is commonly done. Here, allowing thermal interference reduced thermal recovery but supplied higher heating and cooling that could be achieved by a system designed to avoid interference. Thermal breakthrough was managed by monitoring borehole temperature and controlling production in response to this. Our results suggest that high borehole density coupled with active borehole monitoring and control may be preferable in dense urban environments to maximize energy supplied.
How to cite: Bahlali, M., Jacquemyn, C., and Jackson, M.: Design of an ATES system in a heterogeneous aquifer to supply a small urban site with high demand: Imperial College London’s South Kensington campus, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12378, https://doi.org/10.5194/egusphere-egu26-12378, 2026.
Underground Thermal Energy Storage (UTES) offers the potential to manage seasonal demand for heat, thereby reducing reliance on fossil fuels. The Horizon EU project PUSH-IT (Piloting Underground Storage of Heat In geoThermal reservoirs) is testing three ways to store heat up to 90°C, in mines, boreholes and aquifers at six pilot sites. The project also explores governance, policies, business models, and societal engagement.
Societal engagement is essential to ensure that such technologies are acceptable, fair and legitimate (Soutar et al., 2024). A key to effective engagement lies in strategic listening (Pidgeon and Fischhoff, 2011), which involves understanding existing public perceptions; a critical area that remains underexplored in the context of underground thermal energy storage (Environment Agency, 2025).
To address this gap, we conducted a 15-minute online questionnaire (N=5,800) with nationally representative samples (n=1,000) across each of four countries (Germany, the Netherlands, the Czech Republic, and the United Kingdom) and locally representative samples (n=300) in each of six communities within 20 km of each site.
We analyse public perceptions and understandings of UTES, including its perceived advantages and disadvantages, emotional responses to the technology, trust in stakeholders, and overall levels of support nationally and close to each site. We then outline the implications of our findings for designing effective societal engagement strategies for UTES and other underground energy technologies, thereby contributing to sustainable energy goals.
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: Thomas, M., Soutar, I., Rohse, M., Kechagia, M. (., and Devine-Wright, P.: Public Perceptions of Underground Thermal Energy Storage in local and national contexts , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3870, https://doi.org/10.5194/egusphere-egu26-3870, 2026.
Geothermal energy offers a growing opportunity to meet the future energy demand. High-temperature reservoir thermal energy storage (HT-RTES) can decrease seasonal mismatches between heat generation and usage while supporting the stability of the power network. In Upper Rhine Graben, HT-RTES is favored from suitable geological conditions, permeable reservoirs and formation temperatures which makes these reservoirs appropriate for injection temperatures above 100 °C.
The study incorporates data from wells within the Leopoldshafen depleted field as well as two additional wells Eggenstein 1 and Stutensee 1 (EG 1 and ST 1) located approximately 2 and 4 km from the main Leopoldshafen study area. These wells target two Oligocene stratigraphic formations: the Niederrödern formation, deposited in a meandering system, and the Karlsruhe subformation, characterized by marine depositional environments.
Through integration of the existing borehole logging data, we developed an approach to identify potential sandstone reservoir horizons and to evaluate regional heat storage capacity. This methodology relies primarily on the interpretation of self-potential (SP) logs, which enables the identification of sandstone bodies, their internal architecture, and lateral continuity. Based on the newly acquired core- and plug porosity and permeability measurements, the observed reservoir heterogeneity reflects significant variability associated with the distribution of different sedimentary facies.
As a result, a three-dimensional facies-based geological model was constructed to identify the most suitable storage horizons and their associated channel geometries, which also enabled estimation of the potentially storable heat volumes.
How to cite: Garipi, X., Bauer, F., Hinderer, M., David, C., and Schill, E.: High-temperature heat storage capacity in the depleted hydrocarbon fields in the Upper Rhine Graben , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12533, https://doi.org/10.5194/egusphere-egu26-12533, 2026.
Reducing the carbon footprint of building heating and cooling is essential for reaching climate change mitigation goals. Seasonal High-Temperature Aquifer Thermal Energy Storage (HT-ATES) is a promising method to achieve these goals. However, the injection of high-temperature water may significantly alter aquifer geochemistry and microbiology by influencing redox equilibria and mineral solubility, and by strongly impacting the structure and activity of microbial communities, favouring shifts toward thermophilic microorganisms and changes in metabolisms. Although microbial life is ubiquitous in aquifers, biogeochemical processes occurring under HT-ATES conditions remain poorly understood, raising concerns regarding environmental impacts, system performances and long-term sustainability.
We investigated the effects of heating and cooling cycles on the structure and functions of aquifer microbial communities. HT-ATES conditions were simulated in the laboratory using a pressurised (13 bar) flow-through column with the BRGM’s BioREP platform. Groundwater from a monitoring HT-ATES well in TU Delft campus (Netherlands) was injected through the aquifer sediments while the temperature of the experimental device varied cyclically within a range typical of HT-ATES warm wells (30°C to 50°C), before a last phase returning to the natural aquifer temperature (12°C). Geochemical parameters, such as pH, redox potential, conductivity, redox-sensitive elements were monitored in circulating water at the inlet and outlet points of the column. Changes in microbial community composition of sediments and circulating water were assessed through 16S rRNA genes Illumina sequencing.
Preliminary results indicate that the aquifer sediments are quartz-rich, with presence of carbonates and clay minerals, and that the groundwater is of reduced brackish Na-Cl type. While intermediate and final sediment mineralogy analysis are still ongoing, groundwater analysis show that the chemistry remained stable throughout the experiment. A gradual clogging of the column (increase of the inlet pressure) was observed. Upon opening the experimental setup, precipitates were observed at the outlet of the column. Their origin, whether chemical or biological, are under investigation.
Initial microbial analysis of the groundwater revealed a dominance of bacteria (89%), with major phyla including Firmicutes, Bacteroidota, Proteobacteria, Desulfobacterota, and Spirochaetota along with Halobacterota. Many of these taxa are associated with anaerobic and slightly saline environments and include fermentative microorganisms. Several dominant groups are mesophilic and/or non-spore forming taxa. Higher and fluctuating temperatures may promote alternative, more thermotolerant microbial assemblage. Ongoing metagenomic analyses aim to determine how temperature perturbations under HT-ATES conditions influence microbial communities’ composition and possible functions and assess the implications for associated biogeochemical processes.
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: Watkinson, M., Stephant, S., Battaglia-Brunet, F., van der Schans, M., Regenspurg, S., and Rad, S.: Effects of temperature variations on aquifer biogeochemistry during high-temperature thermal energy storage operation: flow-through laboratory experiment insights, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13156, https://doi.org/10.5194/egusphere-egu26-13156, 2026.
Abandoned, flooded mine workings present a promising opportunity for seasonal underground thermal energy storage (UTES), offering large subsurface water volumes that can hold waste heat from high-performance computing (HPC) systems or other low-grade heat sources and release it during periods of high heat demand. The Immersion Cooling and Heat Storage (ICHS) project at Durham University combines prototype immersion cooling for HPC infrastructure with a feasibility study of using disused mine networks for inter-seasonal heat storage and reuse. ICHS aims to position itself as a living lab that integrates practical technology testing with fundamental research into subsurface thermal storage processes, with the aim of advancing low-carbon heat solutions and facilitating heat reuse within campus systems and beyond.
The potential of flooded mine networks to function as effective thermal stores depends critically on mine water circulation and its interaction with ambient groundwater flow. Advective movement of water between mine workings and surrounding aquifers can lead to significant heat loss or redistribution, thereby influencing storage efficiency, recovery rates, and long-term sustainability of a mine-based UTES system. Accurately quantifying and modelling these coupled flow processes is therefore vital to assess the practical capacity of mine water thermal storage and to develop predictive tools for design and optimisation of real systems.
In seasonal storage schemes, heat is typically introduced into the subsurface during summer months when excess thermal energy is available, and withdrawn in winter to supply space heating via heat pump systems. Such systems can also replenish heat over the summer that was depleted in winter. To model this transient behaviour, we have extended the GEMSToolbox framework (Mouli-Castillo et al., 2024) to support time-dependent injection temperatures and flow rates, enabling simulation of seasonal injection–withdrawal cycles under varying operational conditions.
In this presentation, we (i) introduce the ICHS project, describe (ii) the implementation of transient heat and flow boundary conditions in GEMSToolbox, and (iii) preliminary results that illustrate how mine water–groundwater interactions influence heat dispersion and recovery. These results highlight the importance of capturing coupled flow-heat dynamics in assessments of mine water thermal energy storage (MTES) performance and provide insights into how operational strategies and site characteristics can be tuned to maximise storage efficiency in post-industrial subsurface environments.
How to cite: van Hunen, J., Shi, D., Basden, A., Gluyas, J., and Thomas, C.: Seasonal Thermal Energy Storage in Abandoned Mines: Transient Numerical Modelling for the ICHS Project, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7792, https://doi.org/10.5194/egusphere-egu26-7792, 2026.
Flooded and partially flooded mine workings are a promising but still under-quantified option for Underground Thermal Energy Storage (UTES), offering accessible volumes and well-constrained geometry for field-scale experimentation. We report a long-term Mine Thermal Energy Storage (MTES) demonstration in a fully instrumented test basin (≈20 m³) installed at ~147 m depth in the Reiche Zeche underground Geo-Lab (Freiberg, Germany). Three controlled heating–cooling cycles were operated over 504 days, combining dense thermometry in the surrounding gneiss, NaCl point-dilution tracer testing, hydrochemical monitoring, and in-situ heat exchanger fouling and material-performance assessment.
Across the three cycles, 38.0 MWh of heat was supplied. Basin temperatures reached ~26 °C in Cycles 1–2 and ~39 °C in Cycle 3. Wall-rock sensors recorded a delayed but persistent response, with the gneiss warming by 10.1 K at 1.8 m depth after the hottest cycle, consistent with a conduction-dominated regime and long-lived thermal memory. Energy-balance partitioning indicates that the surrounding rock mass stored ~90% of the injected energy, whereas the basin water primarily acted as a rapid heat carrier and exchanger interface.
Hydraulic exchange was quantified by conservative tracer decay, yielding a steady throughflow of ~79 L h⁻¹ (mean residence time ~10.5 days) and an advective heat-loss coefficient of 0.092 kW K⁻¹. This persistent throughflow represents the dominant loss pathway and explains the strong sensitivity of recoverability to hydraulic boundary control. Exchanger-based recovery metrics show a pair recovery fraction of ~0.53 for the actively discharged Cycle 2, while Cycle 3 exhibits multi-cycle conductive “memory” effects, with incremental recovery fractions reaching ~0.7 and a cumulative storage efficiency of ~0.56 over the full experiment.
Thermal cycling also induced pronounced hydrochemical and operational constraints. Warm phases triggered rapid Fe(II) oxidation and precipitation of Fe(III) oxyhydroxides, driving exchanger fouling; uncoated AISI 316L lost ~45% of initial conductance, whereas a hydrophobic coating limited losses to ~18% and a Fe-resistant alloy provided intermediate mitigation.
Overall, the dataset demonstrates reproducible MTES operation under mine conditions and identifies hydraulic isolation/throughflow reduction and oxygen control as the primary levers for improving MTES performance. The derived field metrics (advective-loss coefficient, conduction-driven storage depth response, and fouling resistance under acidic mine-water conditions) provide transferable guidance for designing and benchmarking MTES in post-mining UTES applications.
How to cite: Scheytt, T., Arab, A., Wiedener, R., Oppelt, L., Chen, C., Späker, C., Schenker, F., Lotter, T., Schneider, T., Wunderlich, T., Grab, T., and Nagel, T.: Coupled thermal, hydraulic and geochemical processes in mine thermal energy storage at the Reiche Zeche underground mine (Freiberg, Germany), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-23096, https://doi.org/10.5194/egusphere-egu26-23096, 2026.
Seasonal thermal energy storage (sTES) using artificial basins filled with water or water-gravel mixtures and embedded in the ground has been investigated for several decades, primarily in the context of district-scale solar thermal applications. Over time, innovation steps have led to the classification of sTES technologies into distinct generations. Building on this evolution, a fifth sTES generation has been introduced by Bott et al. (2019). It is based on the integration of multiple, interacting storage units forming combined multi-storage systems. This concept responds to the fact that, ideally, heating and cooling systems are not static but evolve over time (Müller et al. 2025). In modern mixed-use districts, storage capacities, system configurations and operational requirements may change, including temperature levels, charging/storage/discharging periods and control strategies for heat and cold. As a result, sTES must be designed for adaptability rather than optimised for a single, fixed operation mode.
Our contribution presents numerical simulation results for water-gravel thermal energy storage (WGTES) configured as multi-cascaded, multi-purpose and multi-functional systems. The analysis focuses on key operational parameters, such as temperature ranges, fluid circulation concepts and cascading versus parallel operation. System performance is evaluated in terms of storage efficiency and thermal losses (to the surrounding ground and to the other units). A set of generalised scenario analyses based on representative cases is used to identify robust storage characteristics and to derive conclusions that are transferable beyond specific case studies.
Bott, C., Dressel, I., & Bayer, P. (2019). State-of-technology review of water-based closed seasonal thermal energy storage systems. Renewable and Sustainable Energy Reviews, 113, 109241.
Müller, S., Bott, C., Schmitt, D., Faigl, M., Göttl, K., Strobel, R., Bayer, P., Schrag, T. (2025). Implementation of an Expanding Thermal Source Network as a Step Towards CO₂-Neutral Industry. Energy, 330, 136766.
How to cite: Bayer, P., Hoffmann, D., and Bott, C.: Model-based comparison of operation modes for water-gravel-filled multi-storage basins operated for seasonal thermal energy storage, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18370, https://doi.org/10.5194/egusphere-egu26-18370, 2026.
Seasonal thermal energy storage (sTES) is an enabler for matching a temporal imbalance between thermal supply and demand in energy systems. This increases, among others, system efficiency and resilience. By reducing peak loads and fossil supply, sTES further supports decarbonization strategies, making it a cornerstone of long-term, climate-neutral thermal supply strategies. sTES can be realized through artificial systems (i.e., tank (TTES), water-gravel (WGTES), and pit thermal energy storage (PTES)) (Bott et al. 2019). In contrast to geological storage technologies, they store heat or cold in closed volumes separated from the ground. While they can achieve high efficiencies, new installations are often associated with high costs due to excavation, construction, sealing, and insulation. They amount to emissions (Weise et al. 2025), and their performance can be sensitive to thermal losses, groundwater interactions (Bott et al. 2024), and spatial constraints, limiting scalability in dense urban environments.
The Reno-sTES (renovated sTES) concept addresses these challenges by reusing idle/decommissioned infrastructure. By integrating thermal storage into existing basins, Reno-sTES significantly reduces construction effort, material use, and economic risks, while improving environmental performance and accelerating time-to-operation. Further advantages include no further land use, soil sealing, and increased public acceptance: Instead of installing new, visually intrusive infrastructure, desolate installations are brought back to life. Typical candidates for Reno-sTES include basins for former (waste-) water treatment, swimming pools, gravel pits, stormwater retention, industrial cooling, or fire-water reservoirs, and abandoned industrial tanks.
This study presents the first-time R&D-accompanied implementation of a Reno-sTES system at the incampus, Ingolstadt (Müller et al. 2025). Former water treatment basins of a refinery are being repurposed into a combined, multi-unit WGTES. Our contribution focuses on challenges and solutions during the design and construction/renovation phase, including questions related to handling complex geometries, choosing materials that balance thermal performance and durability, and tailored heat-exchanger designs for effective charging and discharging. Environmental constraints and the need for innovative, new planning/design and construction methods are addressed as well (Dahash et al. 2025). The study summarizes the scientific assessment of these aspects before commissioning, expected in Spring 2026, and includes comprehensive monitoring concepts (e.g. via active distributed temperature sensing) within the basins and surrounding ground. Also on this basis, a successful preparation and construction of this Reno-sTES represents an important contribution to the energy transition and forms the basis for further analyses within the Horizon Europe project INTERSTORES (INTERSTORES 2026).
Literature
Bott et al. (2019). State-of-technology review of water-based closed seasonal thermal energy storage systems. Renewable and Sustainable Energy Reviews, 113, 109241.
Bott et al. (2024). Influence of thermal energy storage basins on the subsurface and shallow groundwater. Journal of Energy Storage, 92, 112222.
Dahash et al. (2025). Simulation-based planning for cost-effective and energy-efficient large-scale seasonal thermal energy storage systems. Renewable Energy, 258, 124813.
INTERSTORES 2026. Available online: https://interstores.eu/.
Müller et al. (2025). Implementation of an Expanding Thermal Source Network as a Step Towards CO₂-Neutral Industry. Energy, 330, 136766.
Weise et al. (2025). Comprehensive life cycle assessment of selected seasonal thermal energy storage systems. Renewable Energy, 124232.
How to cite: Bott, C., Bas, E., Dahash, A., Giordano, F., Trauner, J., Lager, D., Faigl, M., Schneider, C., Hoffmann, D., Weise, J., Müller, S., Akbar, S., Schmitt, D., Trinkl, C., Schrag, T., Bayer, P., and Strobel, R.: Design, construction, and scientific monitoring of a Reno-sTES, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18586, https://doi.org/10.5194/egusphere-egu26-18586, 2026.
Field-scale simulations of heat transport in sedimentary aquifers are commonly based on a single-phase temperature approach that assumes local thermal equilibrium (LTE) between the solid matrix and the pore fluid. However, the validity of this assumption under strong hydraulic heterogeneity and fast flow regimes has been questioned. In such settings, delayed interphase heat exchange may lead to local thermal non-equilibrium (LTNE) effects, resulting in temperature differences between the solid matrix and the fluid phase that cannot be captured by standard modeling approaches.
To investigate the potential influence of pore-scale LTNE effects at the field scale, a two-phase heat transport model was applied, resolving separate temperature fields within the fluid and solid phases and coupling them through an interphase heat transfer coefficient at the scale of a sedimentary aquifer. The model was implemented within the Multiphysics Object-Oriented Simulation Environment (MOOSE) and it numerically solves the coupled groundwater flow and heat transport equations. Simulations focused on the evolution of the thermal plume generated by a borehole heat exchanger. A systematic parameter study was carried out to assess the impact of homogeneous and heterogeneous hydraulic conductivity distributions, grain sizes, mean groundwater flow velocity, porosity, and injection temperature on the transient temperature differences between the fluid and solid phases.
For homogeneously distributed hydraulic conductivities, simulation results indicate that temperature differences between the fluid and solid phases remain below 10−3 K for most of the investigated cases and parameter combinations. Given the typical measurement accuracy of temperature sensors, differences of this magnitude are negligible. Preliminary results for heterogeneous hydraulic conductivity fields show that local temperature differences can exceed those observed in the corresponding homogeneous case. However, when averaged over an ensemble of 100 heterogeneous realizations with identical log-conductivity statistics, the mean temperature difference between the solid and fluid phases remains generally very close to that of the homogeneous case. Upstream of the borehole heat exchanger, ensemble-averaged temperature differences in the heterogeneous case are higher than those in the homogeneous case, whereas downstream the opposite trend is observed. Overall, the study quantifies the magnitude and spatiotemporal variability of LTNE effects under field-scale conditions, providing a basis for assessing the relevance of two-phase LTNE modeling for underground thermal energy storage.
How to cite: Gebhardt, H., Zech, A., Rau, G. C., and Bayer, P.: Field-scale analysis of local thermal non-equilibrium in sedimentary aquifers using two-phase heat transport modeling, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4823, https://doi.org/10.5194/egusphere-egu26-4823, 2026.
Ground Source Heating and Cooling (GSHC) and Aquifer Thermal Energy Storage (ATES) systems offer low carbon heating and cooling. Selecting the best technology or combination of technologies for a given installation requires estimates of system performance early in the design process when detailed, site-specific data are not available. It is common for early performance estimates to test only a few selected values of key input parameters. This approach fails to capture the range of potential performance, or the probability of a given performance, and does not allow identification of key uncertain parameters that impact predicted behaviour.
He we present a simple methodology for rapid, probabilistic assessments of GSHC and ATES system performance from uncertain data using a Monte-Carlo approach. The method does not require complex numerical simulations; rather, it allows the use of data from analogue systems to guide the range of input parameters that impact performance. It is assumed that the system under analysis is energy balanced and approximately volume balanced, and that system parameters and predictions can be represented using average values over a given heating or cooling cycle. The methodology is implemented in an open-source software tool. The method and software are demonstrated using a case study of Imperial’s South Kensington campus in London.
How to cite: Jackson, M., Yan, Y., and Bahlali, M.: A probabilistic approach for assessing the potential capacity of open-loop Ground Source Heating and Cooling (GSHC) and Aquifer Thermal Energy Storage (ATES) deployments implemented in open-source software, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11336, https://doi.org/10.5194/egusphere-egu26-11336, 2026.
Low-temperature (LT-) and high-temperature underground thermal energy storage (HT-UTES) are used to provide low carbon heating and cooling. Here, we investigate the potential for ultra-high temperature underground thermal energy storage (UHT-UTES) to balance seasonal fluctuations in electricity supply and demand. During periods of excess supply, groundwater is pumped from naturally porous, permeable underground reservoirs at depths > 500 m, heated to high temperature (>150°C) and pumped back underground where it is stored. When demand exceeds renewable supply, the high temperature water is pumped back to surface and used to generate electricity.
UHT-UTES offers several potential advantages over comparable underground storage technologies such as hydrogen storage: the surface facilities are conventional, comprising boilers and turbines, and expertise can be shared from hydrothermal electricity production. The technology can utilise deep saline aquifers and end-of-life hydrocarbon reservoirs which are geographically widespread and offer large storage and flow capacity.
Despite the potential for UHT-UTES, significant uncertainties remain concerning storage efficiency when the stored water temperature is significantly higher than the ambient reservoir temperature. There are also possible risks to sustainable operation, such as scaling and associated loss of storage and flow capacity, and/or the potential for rock weakening and seismicity.
This study reports the results of numerical modelling to determine key controls on the storage efficiency of UHT-UTES considering a range of operational storage temperatures, reservoir geological settings and heterogeneities, and real-life patterns of intermittent electricity generation and demand. Results show that the underground storage efficiency of UHT-UTES for thermal energy is high and losses are due to both thermal conduction and the convection of hot, buoyant groundwater, and varies depending on how heterogeneity impacts on thermal plume migration.
Despite the high underground storage efficiency, the round-trip efficiency for electricity generation from UHT-UTES is constrained by the conversion efficiency of the turbines. Round-trip efficiency can be maximized by choosing a storage temperature that yields maximum turbine efficiency and minimal thermal storage losses in the reservoir. Current work is focused on addressing the potential for chemical reaction to impact sustainable operation.
How to cite: Liu, R., Jackson, M. D., Hampson, G., Jacquemyn, C., and Bahlali, M.: Ultra-High-Temperature Underground Thermal Energy Storage (UHT-UTES) for Large-scale, Inter-seasonal Electricity Storage, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8721, https://doi.org/10.5194/egusphere-egu26-8721, 2026.
Mine Thermal Energy Storage (MTES) systems represent a promising solution for seasonal heat storage to balance the seasonal regenerative energy supply-demand mismatch in former mining regions. MTES may induce hydrochemical changes due to repeated thermal cycling. This study investigates the hydrochemical evolution of mine water during the operation of a pilot-scale MTES system in the Research and Teaching Mine Reiche Zeche of the TU Bergakademie Freiberg (Germany) and in laboratory experiments in the BMFTR-funded project 'MineATES'.
The MTES pilot system consisted of a 20 m3 mine water filled basin located in the unsaturated zone of the mine. The Mine Water in the basin and the inflows showed typical acid mine drainage characteristics with original pH at 2.7, redox potential at 840 mV and sulphate values around 500 mg/l, up to 30 mg/l zinc and up to 26 mg/l dissolved iron. In total three heating and three cooling cycles were conducted at the test site from original 11.6 °C to water temperatures reaching 26 °C in the first two heating cycles and 39 °C in the last heating cycle. The basin was sampled weekly and the inlets into the storage basin were also monitored.
Results indicate that the mine water chemistry was mostly controlled by temperature, mine water influx, and evaporation. We observed iron precipitation during heating after high inflow periods. After the inflow was significantly reduced iron precipitation did so, too. Iron concentration decreased from 23.7 mg/l to 2.5 mg/l during the first cycle with high inflow conditions. After the inflow was reduced a decrease from 1.5 mg/l to 0.2 mg/l iron was observed in the third heating cycle. The third heating cycle reached 39 °C and induced evaporation through the gaps of the basin cover leading to an enrichment in components by the factor 2 and to gypsum formation above the water line. For future optimisation of MTES systems, results suggest a reduction of new mine water inflow to prevent repeated iron precipitation and full contact with the surrounding rock to minimize evaporation effects.
How to cite: Wiedener, R., Arab, A., Schenker, F., Späker, C., and Scheytt, T.: Hydrochemical Evolution during Thermal Cycling in a Mine Thermal Energy Storage System: Insights from the Reiche Zeche MTES site, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12664, https://doi.org/10.5194/egusphere-egu26-12664, 2026.
Integrating high-temperature underground thermal energy storage (HT-UTES) into dense urban areas requires a precise understanding of how heat plumes evolve in groundwater, including potential implications for drinking-water resources and compliance with regulatory temperature criteria. Here, laboratory experiments at mock-up scale are combined with numerical heat-transport modelling using MODFLOW 6–GWE to assess the thermal footprint of HT-UTES on groundwater. The modelling accounts for temperature-dependent density effects to capture buoyancy-driven flow that can influence plume geometry, which is particularly pronounced at high temperatures.
The numerical model is calibrated against a controlled setup using distributed thermal fibre-optic sensing. Observations of transient temperature fields and plume geometry are used to constrain key transport processes and parameters. The laboratory-constrained thermal transport parameters are subsequently applied in an urban-scale model to simulate HT-UTES operation under representative hydrogeological conditions.
We analyse the thermal plume behaviour across a range of hydraulic gradients and compare two operational strategies: (i) cyclic heating/cooling operation and (ii) active plume management using downstream abstraction wells to limit plume migration. The proposed upscaling workflow provides an experimentally constrained basis to evaluate UTES-induced temperature anomalies and thermal interference in groundwater, supporting impact assessment and permitting in urban settings.
How to cite: Vremec, M., Kainz, F., Rebhan, M. J., Marte, R., Monsberger, C., Wendl, S., Seelig, S., Birk, S., and Winkler, G.: Upscaling experimental data to model the impact of high-temperature underground thermal energy storage, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20323, https://doi.org/10.5194/egusphere-egu26-20323, 2026.
As part of the energy transition, aquifer thermal energy storage (ATES) systems are increasingly used to store thermal energy for heating or cooling purposes in the built environment. In the Netherlands, most ATES systems have a legally defined maximum temperature of 25°C with as justification that this temperature has a limited effect on groundwater. The water quality effects that do occur in this temperature range are mainly caused by the mixing of different types of water (e.g., fresh and brackish or salt water). Currently, there is increasing interest in the application of medium and high-temperature ATES (MT/HT-ATES) with storage temperatures between 25 °C and 90 °C. The advantage of this is that more energy can be stored in the same volume, enabling larger-scale seasonal storage.
While various studies have modeled these effects and investigated them experimentally in laboratories (e.g. Luders et al, 2020; Bonte et al., 2013), the actual impact and potential risk on longer time-scales has not yet been investigated much in practice and needs to be better understood. In the Netherlands, only a few MT-ATES (25-60 °C) systems and one HT-ATES (60-90 °C) system are currently operational. In this study, field monitoring results, including geochemical and microbiological analysis of groundwater at two MT/HT-ATES are discussed. At the HT-ATES location, there is particular focus on the effects to the shallower overlying aquifer, as storing heat in the subsurface will not only increase the temperature of the groundwater at the injection depth, but can also affect shallower groundwater in overlying layers by conduction from the hot wells and the hot reservoir. Results for the shallow aquifer above the HT-ATES sites show that the observed effects are actually not directly temperature induced but result from upwards buoyancy flow caused by heat conduction from the hot well. In the storage aquifer at the MT-ATES site, mixing of water types due to pumping is shown to be the main driver of changes in groundwater composition.
How to cite: Hoving, A., Schout, G., Koenen, M., and Griffioen, J.: Biogeochemical effects of high temperature storage on groundwater quality – field monitoring and modelling results, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19331, https://doi.org/10.5194/egusphere-egu26-19331, 2026.
The efficiency of underground thermal energy storage (UTES) and ground source heat pump (GSHP) systems depends strongly on subsurface thermal properties and groundwater dynamics. Groundwater flow introduces an advective component to heat transport, which can significantly affect thermal conductivity measurements and system performance. Enhanced Thermal Response Testing (ETRT) provides high-resolution depth profiles of effective thermal conductivity (λeff), enabling detection of geological heterogeneities and zones influenced by groundwater flow.
In this study, ETRT data from boreholes up to 100 m depth were analyzed using the Infinite Line Source (ILS) method, while the Péclet number (Pe) approach was used to estimate Darcy fluxes and assess the relative contribution of advection versus conduction. Calculated Darcy fluxes were compared with regional groundwater flow data for three Slovenian sites representing distinct hydrogeological settings: Ljubljana (a Quaternary aquifer), Veliko Črnelo (fractured dolomites), and Brod v Podbočju (a silty aquitard). Results reveal a clear gradation of regimes: strong advective influence in Ljubljana, mixed conditions with localized advective zones in Veliko Črnelo, and predominantly conductive transport in Brod v Podbočju.
The Péclet-based analysis proved robust for differentiating these regimes, provided that conductive thermal conductivity (λcond) is accurately determined from conductive segments or laboratory measurements. These findings highlight the importance of incorporating advective processes into geothermal system design and modelling, particularly in high-flow areas.
Acknowledgements
The research borehole was drilled within project V1-2213 GeoCOOL FOOD – Cold storage of food using shallow geothermal energy, funded by the Slovenian Research and Innovation Agency and the Ministry of Agriculture, Forestry and Food under the Targeted Research Program "Our Food, Rural Areas and Natural Resources." Research was a combination of work at ARIS projects within the framework of the Young Researcher Program of K. Borko, research core funding No. P1-0020 Groundwater and Geochemistry, CRP GeoCOOL FOOD, DP Geo-OPT and UNESCO IGCP project 636. The study focused on the analysis of Enhanced Thermal Response Test (ETRT) data from boreholes up to 100 m depth, interpreted using the Infinite Line Source (ILS) model.
How to cite: Borko, K., Chapman, F., Raymond, J., Vidmar, A., and Rman, N.: Evaluating Darcy Flux Through Enhanced Thermal Response Testing, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16819, https://doi.org/10.5194/egusphere-egu26-16819, 2026.
For the successful subsurface thermal energy storage, accurate characterization of a target aquifer is essential. The active-distributed temperature sensing (DTS) thermal test, which often utilizes a fiber-optic cable both to monitor temperature and to serve as a heat source, has emerged as a promising tool for high-resolution estimation of Darcy flux. For the interpretation, most studies have assumed thermal dispersion to be negligible, yet thermal dispersion is expected to become significant under high flow velocity and heterogeneous hydraulic conductivity field. Despite its importance, estimating in-situ dispersivity remains highly challenging. To evaluate the possibility and sensitivity of thermal dispersivity estimates obtained from active-DTS tests, we incorporate thermal dispersion into the moving infinite line source model and validate it through the numerical model. After that, a sensitivity analysis was performed with two-dimensional numerical simulations under various Darcy fluxes (q, 1 – 10 m/d) and thermal longitudinal dispersivity conditions (α, 0 – 0.01 m). Our results demonstrate that increasing thermal dispersivities systematically reduced the magnitude of temperature increase and delayed the time to reach the plateau, both effects intensified as q increased. Based on these results, we jointly estimate Darcy flux and thermal dispersivity from single or multiple active-DTS tests and evaluate their uncertainties quantitatively. We expect that these findings will extend the applicability of active-DTS thermal tests as a versatile tool for aquifer characterizations and provide a chance for in-situ thermal dispersivity estimation.
Keywords: Thermal dispersion; Active-DTS; Aquifer characterization; Moving infinite line source
How to cite: Baek, J.-Y., Bour, O., le Borgne, T., and Klepikova, M.: Quantifying Thermal Dispersivity and Darcy Fluxes with Active-DTS thermal tests, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19224, https://doi.org/10.5194/egusphere-egu26-19224, 2026.
Aquifer Thermal Energy Storage (ATES) systems are among the technologies needed to decarbonize the heating and cooling of buildings. This study assesses the suitability of ATES systems of Spanish aquifers that are prone to groundwater level decline. The focus of this study is on aquifers that are affected by recurrent droughts and where groundwater availability and stability are critical constraints. These aquifers are identified and selected for detailed analysis within the framework of this study.
Aquifer suitability is evaluated using thermal recovery efficiency as the primary performance indicator for ATES feasibility. The assessment is conducted under both present and future energy demand and groundwater level scenarios, accounting for projected changes in groundwater (phreatic) levels and external temperature conditions associated with climate change. Subsurface conditions are first characterized, including aquifer and aquitard properties and the degree of aquifer saturation.
Future groundwater level scenarios are estimated using an established conceptual relationship between reductions in aquifer recharge and corresponding declines in groundwater levels, applied here using aquifer recharge projections for 2050. In parallel, future thermal demand scenarios for 2050 are derived for residential and tertiary buildings based on projected scenarios with changes in external air temperatures. These projections result in spatially maps of groundwater level change and thermal demand, which are subsequently used as inputs for ATES performance simulations.
Coupled groundwater flow and heat transport modeling is then applied to simulate current and future thermal recovery efficiencies, enabling a direct comparison of ATES performance under evolving hydro-climatic conditions. Preliminary results include national-scale prediction maps of groundwater level decline and future thermal demand, highlighting regions where ATES suitability may decrease or remain viable under future groundwater level decline conditions.
This work represents an extended integrated assessment of ATES suitability in Spain, explicitly linking groundwater availability, climate-driven changes, and thermal recovery efficiency. The proposed framework is expected to provide a decision-support tool that reduces uncertainty, helping private stakeholders and decision-makers to better understand where ATES systems may remain viable or become constrained under declining groundwater level conditions, and why, thereby potentially strengthening confidence in the responsible deployment of ATES in water-stressed regions.
How to cite: Ramos Escudero, A. and Bloemendal, M.: Impacts of Groundwater Decline on Aquifer Thermal Energy Storage Suitability in Spain, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19070, https://doi.org/10.5194/egusphere-egu26-19070, 2026.
