Aquifer Thermal Energy Storage Research Articles

Developing meshing workflows in Gmsh v4.11 for the geologic uncertainty assessment of high-temperature aquifer thermal energy storage

Abstract. Evaluating uncertainties of geological features on fluid temperature and pressure changes in a reservoir plays a crucial role in the safe and sustainable operation of high-temperature aquifer thermal energy storage (HT-ATES). This study introduces a new automated surface fitting function in the Python API (application programming interface) of Gmsh (v4.11) to simulate the impacts of structural barriers and variations of the reservoir geometries on thermohydraulic behaviour in heat storage applications. These structural features cannot always be detected by geophysical exploration but can be present due to geological complexities. A Python workflow is developed to implement an automated mesh generation routine for various geological scenarios. This way, complex geological models and their inherent uncertainties are transferred into reservoir simulations. The developed meshing workflow is applied to two case studies: (1) Greater Geneva Basin with the Upper Jurassic (“Malm”) limestone reservoir and (2) the 5° eastward-tilted DeepStor sandstone reservoir in the Upper Rhine Graben with a uniform thickness of 10 m. In the Greater Geneva Basin example, the top and bottom surfaces of the reservoir are randomly varied by ± 10 and ± 15 m, generating a total variation of up to 25 % from the initially assumed 100 m reservoir thickness. The injected heat plume in this limestone reservoir is independent of the reservoir geometry variation, indicating the limited propagation of the induced thermal signal. In the DeepStor reservoir, a vertical sub-seismic fault juxtaposing the permeable sandstone layers against low permeable clay-marl units is added to the base case model. The fault is located in distances varying from 4 to 118 m to the well to quantify the possible thermohydraulic response within the model. The variation in the distance between the fault and the well resulted in an insignificant change in the thermal recovery (∼ 1.5 %) but up to a ∼ 10.0 % pressure increase for the (shortest) distance of 4 m from the injection well. Modelling the pressure and temperature distribution in the 5° tilted reservoir, with a well placed in the centre of the model, reveals that heat tends to accumulate in the updip direction, while pressure increases in the downdip direction.

Life Cycle Assessment of an Aquifer Thermal Energy Storage System: Influence of design parameters and comparison with conventional systems

This paper presents a Life Cycle Assessment (LCA) of a low-power capacity Aquifer Thermal Energy Storage (ATES) system supplying a building on Bordeaux INP’s university campus, powered by the French low-carbon electricity mix. It compares environmental impacts with alternative thermal energy supply solutions, analyzing various scenarios including design choices, construction methods, equipment lifespan, and failures.Results show that the operational phase contributes 60% to the total environmental impact, mainly from electricity consumption by the heat pump and pumping wells. The ATES operational phase has a climate change impact of 37 gCO2eq kWhth−1, lower than reported in existing literature, but borehole construction, particularly chromium steel production for the casing, poses environmental burdens. The comparative assessment of the different scenarios underlines the pivotal influence of energy delivery optimization, borehole casing material choice, and heat pump performance coefficient on LCA outcomes.Comparative analyses demonstrate that ATES outperforms conventional natural gas or biomass heating and vapor compressor chillers for cooling, but slightly trails air or ground-source heat pumps. Despite high construction phase impacts, mitigation occurs relatively quickly, except for human health concerns.The study underscores the potential of low-power ATES systems with low-carbon electricity to replace fossil fuel or biomass-based heating, reducing environmental footprints. The choice between ATES and other heat pumps depends on the electricity mix, performance factors, and delivered energy.

Thermal performance of an aquifer thermal energy storage system: Insights from novel multilateral wells

Aquifer thermal energy storage (ATES) is an effective time-shifting thermal energy storage technology. Considering the enormous technical and economic input of the well pattern layout, the storage volume of a single well needs to be improved. This study proposes a novel aquifer thermal energy storage system in which several multilateral wells are side-tracked from the vertical well in the aquifer. Radial branches can enhance the connectivity of the wellbore to the aquifer. The research creates an unsteady-state 3D model to analyze the novel ATES system's fluid flow and heat transfer performance. The model is verified using experimental data. The temperature and pressure fields of the multilateral-well ATES are analyzed. Additionally, the effects of injection rate, radial branch length, number of radial branches, aquifer temperature, and aquifer thickness on system performance are studied. According to the research results, compared with the vertical wells, the performance coefficients of the multilateral hot well and cold well increase by 113.06 and 39.25, respectively, and the economic efficiency increases by 14.19% and 23.87%, respectively. The findings can provide a crucial reference for scientific research and the application of aquifer thermal energy storage systems with multilateral wells.

Experimental study on the effect of micro-carbon additives in vegetable oils for aquifer thermal energy storage

Abstract Aquifer thermal energy storage is a versatile method for regulating building temperatures, utilizing groundwater as a medium for both summer cooling and winter heating. Water has high thermal conductivity and specific heat but is corrosive, creating a mineral build-up that causes scaling. Additionally, its high freezing point presents operational challenges. Vegetable oils emerge as a promising alternative, owing to their lower freezing points. In light of environmental concerns, researchers are exploring vegetable oils as substitutes for petroleum-derived mineral oils. This paper is intended as an initial study using vegetable oils, i.e. coconut and sunflower oil, as the heat-transfer medium in aquifer thermal energy storage. The experiments assess the heat-transfer coefficient of coconut, sunflower, mineral, and synthetic oils when exposed to the same heat source. The study also evaluates the impact of introducing micro-carbon (graphite and charcoal) to the oils. Results indicate that sunflower oil has the highest heat-transfer coefficient of 374.4 W/m2 K among the oils, making it suitable for aquifer thermal energy storage applications. Furthermore, augmenting sunflower oil with charcoal powder enhances its performance by increasing the heat-transfer coefficient to 474.9 W/m2 K, or a 27% increase. In contrast, coconut oil proves unsuitable for aquifer thermal energy storage deployment because of its low heat-transfer coefficient of 293.7 W/m2 K. The heat-transfer coefficient of synthetic oil increases with graphite powder but decreases with charcoal powder introduction.

Coupled thermo-hydro-mechanical modeling on geothermal doublet subject to seasonal exploitation and storage

Combining the conventional high-temperature aquifer thermal energy storage system with the common geothermal reservoir development system is a potential alternative to increase energy access, but effects of natural parameters like fracture aperture and reservoir permeability and development parameters like flow rate and injection temperature on the efficiency of energy exploitation and storage have seldomly been investigated. In this study, three assessment parameters and three evaluation criteria were defined to evaluate the sustainability and efficiency of deep geothermal reservoirs with seasonal energy exploitation and storage, and a robust approach of optimizing the development parameters was proposed by combining numerical reservoir model, experimental design and surrogate model, and multiple response optimization method. Based on a case study of the geothermal doublet in Tsinghua University, China, the robustness and reasonability of the developed optimization method were demonstrated. The results show that the optimal development parameters can not only maximize the total recoverable energy and effective recovery efficiency, but also satisfy the requirements of sustainably developing deep geothermal reservoirs. The recoverable heat energy can increase about 20% by adding artificial thermal storage into geothermal reservoir development. Therefore, it is strongly recommended to add energy storage into the future geothermal reservoir development system.

The Influence of Regional Groundwater Flow and a Neighbouring River on the Behaviour of an Aquifer Thermal Energy Storage System

One promising solution for mitigating CO2 emissions in arid regions is to use Aquifer Thermal Energy Storage (ATES) systems in cooling and heating systems. However, ATES systems need to be subjected to geohydrological investigations before their installation to ensure high performance. Two geohydrological properties are considered: regional groundwater flow and the influence of neighbouring rivers. This study considers a hypothetical ATES system within the city of Hilla, Iraq. MODFLOW 6.1 software was used to simulate the influence of the two properties. The simulation tested two locations situated at 75 m and 300 m from the river. Each location was explored using three flow rates: 10 m3/d, 50 m3/d, and 100 m3/d. The results indicate that the temperature change in the warm and cold wells increases proportionally with time of operation and rate of flow. For example, the temperature of the middle layer (for 10 m3/d operation) changes from 29 °C (after one year) to 34 °C (after twenty years operation), while it changes from 34 °C (one year) to 35 °C (twenty years) under 100 m3/d operation. Another result is that the available regional groundwater flow has a negligible influence on the storage system, while the neighbouring river has a high influence on the stored energy when the distance between them is 75 m or less. The paper recommends the installation of ATES systems at least 300 m from the bank of a river.

Risk of surface movements and reservoir deformation for high-temperature aquifer thermal energy storage (HT-ATES)

High-temperature aquifer thermal energy storage (HT-ATES) systems are designed for seasonal storage of large amounts of thermal energy to meet the demand of industrial processes or district heating systems at high temperatures (> 100 °C). The resulting high injection temperatures or pressures induce thermo- and poroelastic stress changes around the injection well. This study estimates the impact of stress changes in the reservoir on ground surface deformation and evaluates the corresponding risk. Using a simplified coupled thermo-hydraulic-mechanical (THM) model of the planned DeepStor demonstrator in the depleted Leopoldshafen oil field (Upper Rhine Graben, Germany), we show that reservoir heating is associated with stress changes of up to 6 MPa, which can cause vertical displacements at reservoir depth in the order of 10–3 m in the immediate vicinity of the hot injection well. Both the stress changes and the resulting displacements in the reservoir are dominated by thermoelasticity, which is responsible for up to 90% of the latter. Uplift at the surface, on the contrary, is primarily controlled by poroelasticity with by two orders of magnitude attenuated displacements of << 10–3 m. Our calculations further show that the reservoir depth, elastic modulus, and injection/production rates are the dominant controlling parameters for the uplift, showing variations of up to two order of magnitudes between shallower reservoirs with low elastic moduli and deeper and more competent reservoirs. In addition, our findings demonstrate that the cyclic operation of HT-ATES systems reduces the potential for uplift compared to the continuous injection and production of conventional geothermal doublets, hydrocarbon production, or CO2 storage. Consequently, at realistic production and injection rates and targeting reservoirs at depths of at least several hundred meters, the risk of ground surface movement associated with HT-ATES operations in depleted oil fields in, e.g., the Upper Rhine Graben is negligible.

Long-term heat storage opportunities of renewable energy for district heating networks

This study compares different thermal heat storage solutions existing in the market, fuelled with energy from different renewable energy sources with a focus on integrating thermal heat storage into the district heating grid. The paper is based on a case in the municipality of Silkeborg, Denmark, which has the largest solar thermal panel plant in Northern Europe. A theoretical approach was used to compare with assumed excess power from wind and solar in the DK1 area, with Silkeborg's allocated excess power at 0.01%. This yielded overall efficiencies between η = 0.739–0.765 and η = 0.864–0.895 for the Silkeborg solar thermal plant. Four different thermal heat storage solutions were compared: tank thermal energy storage, pit energy storage, aquifer thermal energy storage and borehole energy storage (BTES). The analysis showed that, of the four solutions compared, BTES was the best for storing thermal energy for a longer period of time, with the lowest heat loss rate of 0.6% and the highest efficiency of up to 89.5%. However, some complications make it difficult to establish a BTES storage solution, since it is very much dependent on earth conditions and initial capital.

City-scale heating and cooling with aquifer thermal energy storage (ATES)

Sustainable and climate-friendly space heating and cooling is of great importance for the energy transition. Compared to conventional energy sources, Aquifer Thermal Energy Storage (ATES) systems can significantly reduce greenhouse gas emissions from space heating and cooling. Hence, the objective of this study is to quantify the technical potential of shallow low-temperature ATES systems in terms of reclaimable energy in the city of Freiburg im Breisgau, Germany. Based on 3D heat transport modeling, heating and cooling power densities are determined for different ATES configurations located in an unconsolidated gravel aquifer of varying hydrogeological subsurface characteristics. High groundwater flow velocities of up to 13 m d−1 cause high storage energy loss and thus limit power densities to a maximum of 3.2 W m−2. Nevertheless, comparison of these power densities with the existing thermal energy demands shows that ATES systems can achieve substantial heating and cooling supply rates. This is especially true for the cooling demand, for which a full supply by ATES is determined for 92% of all residential buildings in the study area. For ATES heating alone, potential greenhouse gas emission savings of up to about 70,000 tCO2eq a−1 are calculated, which equals about 40% of the current greenhouse gas emissions caused by space and water heating in the study areas’ residential building stock. The modeling approach proposed in this study can also be applied in other regions with similar hydrogeological conditions to obtain estimations of local ATES supply rates and support city-scale energy planning.

Thermal performance and analysis of high-temperature aquifer thermal energy storage based on a practical project

Geothermal heating technology based on high-temperature aquifer thermal energy storage (HT-ATES) is one of important development directions of geothermal multi-energy complementary and integrated energy system. In present article, thermal performance of HT-ATES based on a practical project is first proposed, and a long-term cycle of HT-ATES is constructed and verified. The heating up storage and isothermal heat storage of HT-ATES are experimentally conducted. The different standing time schemes causing mutation of the heat storage mechanism have been implemented to identify the optimal scheme. And then the outlet water temperature, heat storage efficiency is further calculated and analyzed under the same and different conditions of heat storage capacity. Compared with original heat production, the outlet water temperature is promoted and the single well heat storage efficiency can reach >80 % after 9 years of heat storage. The heat storage flow rate decreases or the heat storage temperature increases can prevent heat dissipation to the surroundings, so that obtaining high outlet water temperature and heat storage efficiency under the persistent annual heat storage capacity. Results also show that the outlet water temperature and heat storage efficiency are promoted when the heat storage temperature increases and heat storage flow rate decreases.

Heat losses in ATES systems: The impact of processes, storage geometry and temperature

The technical and economic success of an Aquifer Thermal Energy Storage (ATES) system depends strongly on its thermal recovery efficiency, i.e. the ratio of the amount of energy that is recovered to the energy that was injected. Typically, conduction most strongly determines the thermal recovery efficiency of ATES systems at low storage temperatures (<25 °C), while the impact of buoyancy-driven flow can lead to high additional heat losses at high storage temperatures (>50 °C). To date, however, it is unclear how the relative contribution of these processes and mechanical dispersion to heat losses across a broad temperature range is affected by their interaction for the wide range of storage conditions that can be encountered in practice. Since such process-based insights are important to predict ATES performance and support the design phase, numerical thermo-hydraulic ATES simulations were conducted for a wide range of realistic operational storage conditions ([15–90 °C], [50,000–1,000,000 m3/year]) and hydrogeological conditions (aquifer thickness, horizontal hydraulic conductivity, anisotropy). The simulated heat loss fractions of all scenarios were evaluated with respect to analytical solutions to assess the contribution of the individual heat loss processes. Results show that the wide range of heat losses (10–80 % in the 5th year) is the result of varying contributions of conduction, dispersion and buoyancy-driven flow, which are largely determined by the geometry of the storage volume (ratio of screen length / thermal radius, L/Rth) and the potential for buoyancy-driven flow (q0) as affected by the storage temperature and hydraulic conductivity of the aquifer. For ATES systems where conduction dominates the heat losses, a L/Rth ratio of 2 minimizes the thermal area over volume ratio (A/V) and resulting heat losses for a given storage volume. In contrast however, the impact of dispersion decreases with L/Rth and particularly for ATES systems with a high potential for buoyancy-driven flow (q0 > 0.05 m/d), increasingly smaller L/Rth ratios (<1) strongly reduce the heat losses due to tilting. Overall, the results of this study support the assessment of thermal recovery efficiencies for particular aquifer and storage conditions, thereby aiding the optimization of initial ATES designs.

Optimizing scenarios of a deep geothermal aquifer storage in the southern Upper Rhine Graben

Based on a newly developed geological 3D reservoir model for the demonstration site of the ‘Freiburger Bucht’ in the Upper Rhine Graben (SW Germany), geothermal development and realization concepts of an aquifer thermal energy storage (ATES) in the Buntsandstein aquifer were elaborated and energetically evaluated by numerical modeling. The thermal–hydraulic coupled modeling was performed with the FE-software OpenGeoSys and COMSOL. For this purpose, the geological model was converted into a numerical model and calibrated by local and regional, hydrogeological and geothermal measured values. A detailed study based on two-phase storage-heating cycles per year with constant injection temperature on the ‘hot side’ of the ATES, different volumetric flow rates, and temperature spreads was performed to quantify possible storage capacities, energies, and efficiencies. The calculated efficiency of the cyclic storage operation in this study, averaged over 10 storage heating cycles, are between 50 and 85%, depending on flow rate and temperature spread. The efficiency of the individual storage heating cycles increases from year to year in all scenarios considered, as the ‘hot side’ of the storage heats up in the long term. To increase ATES’ efficiency, also horizontal wells were integrated into the numerical model and the results were compared with those of inclined wells.

Sustainability assessment of hydrochemical and microbiological disturbances in shallow aquifer by thermal utilization

The use of geothermal systems with shallow groundwater to achieve global carbon neutrality has increased substantially. However, they not only have hydraulic and thermal effects on the environment, but also induce hydrochemical and microbial changes that can worsen the sustainability of water resources. This study focused on evaluating groundwater mixing and pollution in a surrounding shallow aquifer induced by operation of a groundwater heat pump (GWHP) system. To explain these phenomena, hydrogeochemistry, multiple isotopes (O, H, and Sr), Rn, and microbial data were combined with geochemical modeling of the test system. Few studies have been conducted on field measurements using these combined approaches. The hydrogeochemical and microbial data showed specific characteristics accompanying the thermal use of groundwater. The calculated saturation index values and PHREEQC modeling results indicated a high probability of clogging effects in wells located near the boundary of the thermal plume. There was also a decrease in the microbial diversity index. Rn and stable isotopes indicated active mixing between circulating water and ambient groundwater along the main groundwater flow direction during the operation. This study demonstrated that subsurface hydrological and microbial environments, such as those of the GWHP system, responded to impacts of moderate intensity. Because the applied data were able to derive possible environmental and operational problems of shallow geothermal systems, hydrological and microbiological signatures should be considered for sustainable management of groundwater in geothermal sites. Our study can have implications for energy reuse from storage within an aquifer, such as aquifer thermal energy storage (ATES) based on field tests, using combined analysis.

Analytical and numerical solutions of radially symmetric aquifer thermal energy storage problems

Abstract Aquifer thermal energy storage (ATES) systems offer reduced energy costs, lower carbon emissions, and increased energy resilience. The feasibility, however, depends on several factors and usually require optimization. We study an ATES system with injection and extraction wells (cf. graphical abstract). The purpose of the investigation was to calculate the recovery factor of an ATES system with a cyclic repetition of injection and pumping. In the paper, we discuss analytical and numerical radial solutions of differential equations for heat transport in water-saturated porous media. A similar solution was obtained for a 2-D-horizontal confined aquifer with a constant radial flow. Numerical solutions were derived by using a high-resolution Lagrangian approach suppressing spurious oscillations and artificial dispersion. The numerical solution and the analytical solutions give consistent results and match each other well. The solutions describe instantaneous and delayed heat transfer between fluid and solid, as well as time-varying water flow. In hydrological terms, these solutions are relevant for a wide range of problems where groundwater reservoirs are utilized for extraction and storage (namely, irrigation; water supply; geothermal extraction).

Prediction of Dynamic Temperature and Thermal Front in a Multi-Aquifer Thermal Energy Storage System with Reinjection

The accurate temperature and thermal front prediction in aquifer thermal energy storage systems during reinjection are crucial for optimal management and sustainable utilization. In this paper, a novel two-way fully coupled thermo–hydro model was developed to investigate the dynamic thermal performance and fronts for multiple aquifer thermal energy storage systems. The model was validated using a typical model, and the evolution characteristics of wellbore temperature before and after the breakthrough of the hydraulic front and thermal front were deeply studied. Sensitivity analysis was conducted to delineate the influence of various reservoir and reinjection factors on the thermal extraction temperature (TET). The results revealed that thermal conductivity significantly impacts the thermal extraction rate among the various reservoir factors. In contrast, volumetric heat capacity has the weakest influence and negatively correlates with the TET. Concerning the reinjection factors, the effect of the reinjection volume rate on the TET was significantly more significant than the reinjection temperature. Furthermore, the correlation between the TET and different properties was observed to be seriously affected by the exploitation period. The coupled model presented in this study offers insight into designing the exploitation scheme in deep reservoirs and geothermal resources.

Dimensionless Thermal Efficiency Analysis for Aquifer Thermal Energy Storage

AbstractSeasonal warm and cold water storage in groundwater aquifers is a cost‐effective renewable energy technology for indoor heating and cooling. Simple dimensionless analytical solutions for the thermal recovery efficiency of Aquifer Thermal Energy Storage (ATES) systems are derived, subject to heat losses caused by thermal diffusion and mechanical dispersion. The analytical solutions pertain to transient pumping rates and storage durations, and multiple cycles of operation, and are applicable to various well configurations and thermal plume geometries. Heat losses to confining layers, its implications for optimizing plume geometries and aspect ratios, and heat spreading due to free convection are also discussed. This provides a general tool for broad and rapid assessment of aquifers and ATES systems. We show that if heat exchange with the confining layers is negligible, the thermal recovery efficiency of thermal plumes with cylindrical geometry is independent of the aquifer porosity and heat capacity C0. Therefore, if mechanical dispersion is negligible as is often the case, the only aquifer property that affects the recovery efficiency is the aquifer thermal conductivity. The field‐scale aquifer thermal conductivity λ can therefore be inferred from the recovery efficiency of a push‐pull heat recovery test. Remarkably, an increase in C0 could either increase, decrease, or not affect the recovery efficiency, depending on the thermal plume geometry. Hence, the analytical solutions reveal that the recovery efficiency is affected by complex interactions between the thermal plume geometry and aquifer properties. Approximate analytical solutions for subsurface temperature profiles over an entire ATES cycle are also derived.

History of the hydrogeochemical study of groundwater in the Netherlands and the research motives

AbstractAn overview is presented of research on the hydrogeochemical aspects of groundwater resources in the Netherlands conducted since the early nineteenth century. The earliest studies investigated groundwater as a resource for drinking water. The first systematic, national study was in 1868 and was motivated by the cholera epidemics at that time. At the beginning of the twentieth century, research for drinking water production was institutionalised at national level. Since the 1960s, the range of organisations involved in hydrogeochemical research has broadened. Societal motives are also identified: shallow, biogenic methane as fossil fuel (already researched since the 1890s); groundwater contamination; freshening/salinisation of aquifers; ecohydrology and nature conservation; aquifer thermal energy storage; national and regional groundwater monitoring for policy evaluation; impact of climate change and weather variability; and occurrence of brackish groundwater and brines in the deeper subsurface. The last-mentioned has been driven by a series of motives ranging from water supply for recreational spas and mineral water production to subsurface disposal of radioactive waste. There have been two major scientific drivers: the introduction of techniques for using isotopes as tracers, and geochemical computer modelling. Another recent development has been the increasing capabilities in analytical chemistry in relation to the contamination of groundwater with emerging pollutants. Many of the motives for research emerged in the 1980s. Overall, the societal and associated technical motives turn out to be more important than the scientific motives for hydrogeochemical research on groundwater in the Netherlands. Once a research motive has emerged, it commonly tends to remain.

Gravity-driven flow in a cross-bedded porous rock

Many geological layers include cross bedding, which leads to different values for permeability along and across the bedding planes. We explore how such cross bedding impacts buoyancy-driven flow through an inclined aquifer. For each bedding angle and ratio of the permeability along and across the bedding, a free buoyancy-driven plume rises at a particular angle to the horizontal. If the angle of inclination of the aquifer to the horizontal is smaller than this angle, then the plume rises along the upper boundary, otherwise, somewhat surprisingly, the buoyant plume rises along the lower boundary of the aquifer. We present new laboratory experiments to support these predictions. We also test a model for the effective permeabilities which control the speed and the rate of spread of the plume along one or other boundary of the aquifer. We consider the impact of our results for modelling geological storage of ${\rm CO}_2$ or aquifer thermal energy storage.

Numerical Investigation and Optimization of a District-Scale Groundwater Heat Pump System

Groundwater heat pump (GWHP) systems are acknowledged as renewable and sustainable energy sources that can effectively fulfill the heating and cooling requirements of buildings on a district level. These systems harness geothermal sources available at shallow depths. To ensure the long-term sustainability of the system, the thermally used water is generally reinjected into the aquifer, creating a thermal plume starting from the injection well. Over time, this thermal plume may reach the abstraction well in the long term, potentially leading to a reduction in system efficiency. The operation types have a significant impact on this matter, and their effects have not been extensively studied in the existing literature. Therefore, this study aims to determine the optimal operating configurations for the Northern Gateway Heat Network, a GWHP system established in Colchester, UK. In this study, four distinct operation types are considered: (1) continuous heating (actual system), (2) heating and recovery, (3) heating and cooling, and (4) aquifer thermal energy storage (ATES). The results indicate that ATES operation yields the highest thermal energy output due to its ability to benefit from stored energy from the previous operation. However, implementing the ATES system may encounter challenges due to factors such as well development, hydraulic conductivity, and hydraulic gradient. On the other hand, implementing heating and cooling operations does not require additional considerations and offers not only free cooling to buildings but also a delay in thermal feedback time.

Best practices for characterization of High Temperature-Aquifer Thermal Energy Storage (HT-ATES) potential using well tests in Berlin (Germany) as an example

Application of Aquifer Thermal Energy Storage with High Temperatures (HT-ATES) ranging from 60–90 °C is a promising technique to store large amounts of energy in urban areas. However, these areas typically lack information on hydrogeological and thermal parameters of the subsurface to determine the potential for energy storage. Moreover, conventional exploration methods as pumping tests do not account for the variation in density caused by the high temperature gradients or changes in salinity as encountered in HT-ATES operation. The objective of this study is therefore to develop best practices for characterizing the hydrogeological and thermal properties of groundwater wells and their surrounding formation that determine the potential performance of HT-ATES-systems. In addition to conventional pumping tests, a set of Push–Pull tracer Tests (PPTs) with cold and hot water are proposed and scrutinized using Berlin as case study. There, the research well Gt BChb 1/2015, which is characterized by a reservoir temperature of 17 °C at a depth between 220 und 230 m below ground surface was tested.In 2017, seven Slug-Withdrawal Tests (SWTs), a Step-Rate-Test (SRT), a production tests, and two Push–Pull tracer Tests (PPTs) with hot and cold water were performed during a period of 40 days. These tests were accompanied by Distributed-Temperature-Sensing (DTS) monitoring. The temperature measurements provide indications of injection areas based on the warmback period during a PPT with 81 °C hot water.The determined aquifer transmissibility T=3.2×10−5m2/s, the related Productivity Index (PI=2.0m3/(h bar)), and maximum flow rates of about 5m3/h indicate that the aquifer has potential for HT-ATES. However, the PPT and the DTS monitoring revealed cross flow between the target aquifer and an overlying aquifer. Thus, a new well with a design avoiding cross flow is required to utilize the aquifer’s energy storage potential. A set of best practices for characterizing HT-ATES potential was derived from the experiences in this study.