The sandstones of the Middle and Upper Buntsandstein are suitable host rocks for the installation of shallow geothermal systems due to their high thermal conductivities typically ranging between 1.9 to 4.6 W m^-1 K^-1 (Verein Deutscher Ingenieure 2010). Knowledge of the effective thermal conductivity is crucial to efficiently dimension borehole heat exchanger (BHE) systems. The standard method for determining effective thermal conductivities at a site is the thermal response test (TRT). However, thermal conductivity can also be analysed in the laboratory from core samples. In addition, various prediction models for the estimation of thermal conductivity based on lithological properties, such as porosity, exist. In this study, depth-specific thermal conductivities of sandstone samples of the Upper and Middle Buntsandstein are comprehensively analysed by applying different methods. First, thermal conductivities of about 140 core samples are analysed in the laboratory, and relations between material properties, such as porosity and mineralogy, and thermal conductivity are investigated. Furthermore, common prediction models are applied, and in addition, the measured and estimated thermal conductivities are compared to the effective thermal conductivities evaluated with an enhanced thermal response test (ETRT). The average effective thermal conductivity analysed with the ETRT is 4.7 W m^-1 K^-1, while the thermal conductivities analysed in the laboratory on saturated core samples range between 2.7 to 6.4 W m^-1 K^-1 with an average value of 4.6 W m^-1 K^-1. The best estimate from the prediction models is achieved by the Voigt-Reuss-Hill model with an average error of 13 % and a maximum error of 26 %. Overall, prediction models that assume a random distribution of solid and fluid components can achieve reliable estimates of the thermal conductivity of the sandstone. Thus, the results demonstrate that laboratory analyses can provide representative values of the effective thermal conductivities at a site with negligible or low groundwater flow. However, we also show that in order to achieve a representative value a sufficient number of samples has to be analysed, which entails high expenses for laboratory analyses.

Verein Deutscher Ingeniere. VDI 4640, part 1: Thermal use of the underground, fundamentals, approvals, environmental aspects, 2010.

How to cite: Albers, A., Koch, F., Menberg, K., Steger, H., Fliegauf, C., Schindler, L., Wilke, S., Zorn, R., and Blum, P.: Measuring the thermal conductivity of sandstones, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14738, https://doi.org/10.5194/egusphere-egu24-14738, 2024.

Most shallow geothermal systems harness renewable energy from the ground by circulating a heat-carrier fluid through borehole heat exchangers (BHEs). While these systems have emerged as promising contributors to the ongoing energy transition, open questions regarding their durability and performance in practical applications remain. Critical factors are the uncertainty in the long-term heating/cooling demand as well as the uncertainty in the evolution of the ground thermal conditions. To enhance the environmental and economic appeal of such systems that should be operated for decades, optimal control procedures that adjust the heat extraction during the course of operation have been proposed. In this study, we introduce an improved sequential simulation-optimization procedure to tune the efficiency of BHEs exposed to transient groundwater flow conditions. The variability of groundwater flow not only perturbs subsurface heat transfer but also impairs the predictability through standard BHE modeling tools. For example, the well-established moving finite line source (MFLS) formulation offers no capabilities to represent transient trends in advective heat transport associated with groundwater flow. We restructure the MFLS by a time-varying groundwater flow term that ensures compliance with thermal equilibrium assumptions. This modified variant is validated with a numerical model and then interfaced with a linear programming algorithm to optimize the operation of the BHE array. To examine the efficacy of the proposed procedure, a synthetic field containing ten BHEs operating in the heating mode is implemented. Three distinct groundwater fluctuation scenarios, with monthly resolutions over a ten-year operational lifespan, are considered. The groundwater flow dynamic exhibits variations in an increasing, decreasing, and periodic manner. As the objective, local cooling is to be minimized. This is achieved by determining the monthly optimal load pattern for individual BHEs. The proposed methodology employs a cost function to minimize the maximum temperature change over two distinct temporal terms. The first term considers the entire operational lifetime, while the second term focuses on a forthcoming 12-month horizon.
The proposed methodology does not only achieve optimal BHE operation, but it also facilitates calibrating unknown or transient model parameters. This is demonstrated for the groundwater flow velocity that is estimated by solving a nonlinear least-squares problem using the trust-region-reflective algorithm. The benefit of sequential learning is compared with results obtained by optimization without any model calibration. This calibration-optimization routine, informed by transient temperature changes in the simulated ground, outperforms sequential optimization that relies on non-tunable model parameters.

How to cite: Soltan Mohammadi, H., Ringel, L. M., Bott, C., and Bayer, P.: Tuning the performance of a borehole heat exchanger array in response to transient hydrogeological conditions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5378, https://doi.org/10.5194/egusphere-egu24-5378, 2024.

Introduction: Geothermal heat pump systems GSHPs use the Earth’s subsurface as a heat source in winter and heat sink in summer. A GSHP system integrated with vertical HDPE U-loops for transferring the heat to and from the ground is called a closed-loop ground heat exchanger (CLGHE) system placed by drilling the boreholes. In this study, the relationship between borehole diameter, thermal resistance, and the optimum loop length of CLGHE
systems is investigated.
Methods: Utilizing the standard thermal line source equation, analysis of borehole thermal resistivity bR variations across a spectrum of borehole diameter d sizes (ranging from 80-180 mm) has been performed. The soil thermal properties like conductivity λ are considered as 1.5 W/mK , initial ground temperature is considered to be 25⁰C. The analysis is extended to evaluate the influence of borehole diameter sizes on the optimal length of geothermal single U-loops using Ground Loop Design (GLD) software. This comprehensive assessment incorporated critical factors such as soil thermal properties, heat transfer fluid characteristics, and U-loop pipe attributes which has been replicated from the analytical study for a heat load of 10MWh.
Results: The outcomes of our simulations revealed notable correlations: larger boreholes consistently demonstrated increased thermal resistance analytically. It has also been observed through the GLD simulations that the loop length increases as the borehole diameter increases for example: when the diameter increases from 80mm to 100mm the loop length increases from 57.6 to 56.5m for a geothermal grid of 2x2 rectangular configuration for the same soil properties and thermal loading conditions. The observed relationship holds implications for optimizing system design and performance, particularly in diverse soil conditions. In conclusion, our study contributes valuable insights into the thermal efficiency of GSHP systems, emphasizing the importance of borehole diameter in influencing overall energy transfer capabilities. These findings provide a foundation for further research and practical applications for the CLGHE systems.

How to cite: Roy, S., Chakraborty, T., and Shahu, J. T.: The Impact of Borehole Diameter on Loop Length and ThermalResistivity in Closed-Loop Ground Heat Exchanger System, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21884, https://doi.org/10.5194/egusphere-egu24-21884, 2024.

Shallow geothermal energy technologies have seen significant development over past decades and the number of ground-source heat pump (GSHP) installations has seen an increasing trend worldwide. Importantly, a plethora of scientific research has explored performance, mechanical stability, optimisation, and innovative approaches to utilise the ground as a source or sink of thermal energy. Despite this, however, there is often a gap between the designed performance of a system and the realised in-situ operation. Such a gap can result from a lack of information, such as imprecise ground profile/thermal properties, changes in the environment and conditions, such as in building usage due to changes in behavioural patterns, as well as the fact that GSHP design is typically undertaken in isolation of other system elements, e.g., by using only estimates of the heating and cooling demands.

Real-time monitoring and appropriate smart control methods can help bridge this discrepancy, alleviating potential issues. Heat pumps and building management systems typically record useful data, such as the temperature of the fluid in the ground loop, constituting valuable sources of information on the current system performance at a given moment. However, using these data to predict how the system will perform in the future, and thus inform system operation, is not trivial. A field that can help tackle this problem is data science, which uses data-driven artificial intelligence (AI) approaches to provide useful insights from data and has had tremendous advancements in recent years. It is therefore expected that statistical AI approaches can be used with data from GSHP systems meaningfully, to inform the efficient operation of the system.

This research focuses on a case study in Cambridge, UK, where a GSHP system, with capacity of 320 kW and a ground loop with 24 160-m deep boreholes, provides heating and cooling for a recently constructed university building. The data recorded from this system is utilised together with a combination of AI methods, such as gradient boosting and deep neural network predictive models, and finite element modelling to assess how well the ground loop performance can be predicted in real time and the implications this can have on the performance of the system. Thus, this work demonstrates how GSHP data can be leveraged to make useful decisions based on updated information and thereby ensure that a GSHP system performs efficiently throughout its lifetime.

How to cite: Makasis, N., Kreitmair, M., Zhuang, C., Menberg, K., Choi, W., and Choudhary, R.: A case study on the use of AI methods towards bridging the gap between design and operation of ground-source heat pump systems., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16828, https://doi.org/10.5194/egusphere-egu24-16828, 2024.

In recent decades, there has been a notable rise in the utilization of the subsurface as a source for heating and cooling through shallow geothermal installations. This trend is exemplified by the emergence of Energy Geostructures (EGs), an innovative technology that not only provides structural support to the ground or buildings but also facilitates the exchange of heat with the subsurface. EGs offer versatile applications for various energy needs, including their integration with ground source heat pumps for space heating, cooling, and domestic hot water provision.

Energy Quay Walls (EQWs) represent a promising type of energy Geostructures that demonstrate a unique ability to exchange heat with both the surrounding soil and open water. Despite the considerable potential for energy efficiency that EQWs hold, their limited implementation underscores the need for extensive research to comprehend their thermal behavior.

This study conducts an in-depth examination of EQWs, employing two Finite Element numerical models to reconstruct the undisturbed temperature profile within the soil and conduct a detailed 3D analysis of heat exchange processes in an EQW application. These models are validated using real data obtained from a full-scale test in Delft, The Netherlands. The results emphasize the significance of transitioning from laminar to turbulent flow regimes within the heat exchanger pipes, showcasing improved energy extraction efficiency, particularly from the open water layer.

Moreover, the study underscores the critical role of open water movement in the energy extraction process. This finding emphasizes the dynamic nature of open water in contributing to the overall effectiveness of EQWs as an EG. The study's outcomes provide important considerations for optimizing the design and implementation of EQWs, pointing towards the benefits of promoting turbulent flow regimes within the heat exchanger pipes and emphasizing the advantages of harnessing energy from actively moving open water layers. As the implementation of EQWs continues to expand, these insights contribute to advancing our understanding and enhancing the energy efficiency of this innovative technology. 

How to cite: Gerola, M., Cecinato, F., Haasnoot, J. K., and Vardon, P. J.: Optimizing Energy Quay Walls: Insights from Comprehensive Thermal Analysis and Turbulent Flow Enhancement, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11181, https://doi.org/10.5194/egusphere-egu24-11181, 2024.

Displacement cast in situ Fundex (DCSF) energy piles are a new type of energy pile with the advantages of convenient construction, simple manufacturing, low vibration, and low noise and cost. This work focuses on the response of a stand-alone DCSF energy pile under different mechanical loads simultaneously with the operation of a ground source heat pump through a series of full-scale field tests. After applying an axial load on the pile head (60% of the bearing capacity), the pile was subjected to ten thermal cycles. The effects of mechanical load and the impact of temperature on the mechanical capacity of the DCSF energy pile were investigated. 

The results indicate that the cyclic thermal loadings induce a progressive increase in the compressive stress of piles. Furthermore, a residual compressive stress was observed and attributed to the drag-down effects of the surrounding soil.

During cooling phase, the tensile stress induced by thermal load decreased drastically due to the shrinkage of the near soils, leading to an insignificant effect on the energy pile during cooling. The maximum thermo-mechanical axial compressive stress in the foundations was approximately 1.55 MPa, well within structural limits and not expected to affect the building.

Progressive pile head displacement was observed indicating that thermal creep can affect pile head displacement at higher working loads.

How to cite: Rafai, M., salciarini, D., and Vardon2, P. J.: Thermo-mechanical response of a cast in situ displacement energy pile, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7965, https://doi.org/10.5194/egusphere-egu24-7965, 2024.

Standing column wells (SCW) are ground heat exchangers that recirculate groundwater in a deep uncased borehole and “bleed” only a fraction of the pumped water during peak demand periods to boost advective heat transfer. While operational feedback collected from numerous systems in the northeastern United States is available and provides general design guidelines (Orio et al., 2005; 2006), practitioners have been slow to embrace SCWs outside their area of emergence. This reluctance can be explained in part by a lack of awareness, as well as lingering concerns about groundwater chemistry and the reliability of these systems in diverse climatic and geological settings. In this context, the present work presents a case study of a demonstration SCW system that was retrofitted in a school near Montreal, Québec, Canada, with the aim of sharing the knowledge gained during the design and commissioning phases.

The demonstration system’s design relied on an early exploratory phase, which included an exploratory drilling, a thermal response test, a pumping test, and groundwater analyses. These field operations first uncovered the presence of a highly productive sandstone aquifer, which 1) halted drilling early at 133 m due to the elevated water pressure, and 2) had a strong influence on the thermal response test’s results due to the high efficiency of advective heat transfer, even in the absence of bleed. Accordingly, the development and calibration of an advanced coupled thermo-hydrogeological numerical model was deemed necessary to evaluate the proper sizing of the ground heat exchanger.

Following design and construction, a review of the available data was conducted to evaluate the SCW system’s general performance metrics. This exercise first demonstrated its overall efficiency, which reduced drilling lengths by approximately 73%, construction time by 52%, and initial costs by 37% compared to conventional closed-loop boreholes. It was also found that the SCWs were able to sustain building loads over 200 W/m and to reduce peak electrical power demand by 71% compared to electric resistance heating, this on the coldest winter day when the air temperature was -26 °C. Monitoring of the pressure losses through the plate heat exchanger and step-drawdown tests did not indicate any immediate groundwater quality concerns. On the other hand, the elevated energy consumption of the pumping equipment affected the system’s seasonal performance factor, and a few operational issues related to corroded probes and inefficient control sequences compromised energy and financial savings and had to be resolved.

In conclusion, the results of this case study demonstrate the strong potential of SCWs for reducing the environmental and economic costs of heating operations in cold climates. The importance of conducting an early exploratory phase was emphasized, as well as the potentially significant impact of productive aquifers and groundwater flow on field testing, design studies and overall performance metrics. Lastly, it also became evident that careful selection of the pumping equipment and control sequences, as well as post-commissioning efforts, were necessary to ensure the optimal operation of this innovative technology.

How to cite: Beaudry, G., Pasquier, P., Faucher, J., Tonellato, G., and Kummert, M.: Standing column wells in cold climates: a case study in a highly productive aquifer, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20924, https://doi.org/10.5194/egusphere-egu24-20924, 2024.

Ground-coupled heat pump systems exchange heat between a building and the surrounding ground. To simulate a ground heat exchanger (GHE), a transfer function is commonly used. This function represents the ground's capacity to exchange energy. Its use allows for the simulation of the ground's response to different heat load patterns. The heat transfer process is affected by key factors such as the ground's thermal properties and the groundwater flow. Accurate evaluation is critical for designing the GHE field to meet the thermal needs of a building for heating and cooling.

The transfer function of the GHE represents its heat transfer over a time range from seconds to years of operation. The long-term component is usually defined by an analytical model, while the short-term component requires a more complex model to be evaluated. Indeed, analytical models do not accurately represent the transient heat transfer effects that occur in the borehole materials and heat transfer fluid. To avoid using thermal transfer models, the data from a thermal response test (TRT) can be used to retrieve an experimental transfer function. This study aims to outline the different applications of a deconvolution algorithm to retrieve a short-term transfer function from experimental TRT data.

Applying a deconvolution algorithm to the outlet temperature signal of a TRT allows for estimation of the short-term response of the GHE without the need for a defined thermal model. The algorithm can iterate on the transfer function form, enabling accurate reconstruction of experimental temperature. One advantage of this method is that it only requires experimental data from a TRT to construct the transfer function.

The methodology is applied to tests with constant and varying operating conditions, allowing to obtain one or more transfer functions depending on the number of operating conditions. Additionally, a deconvolution algorithm can be utilized to interpret distributed thermal response tests, helping in the identification of geological layers with the best thermal properties. This can assist system designers to reduce drilling costs for systems with multiple boreholes. Results present transfer functions that are smooth and close to ones obtained from a more advanced numerical model. Additionally, they can reconstruct experimental temperature precisely. It is worth noting that the transfer function curve is affected by groundwater flow, with larger flows resulting in a decrease in the curve for similar operating conditions.

In conclusion, this research demonstrates the diverse applications of a deconvolution algorithm in interpreting a thermal response test across various geological settings, groundwater flow rates, operating conditions, and types of GHE. This leads to the estimation of a short-term transfer function, which can be used either to compute thermal parameters or validate various heating loads on the GHE through implementation in a simulation algorithm.

How to cite: Dion, G. and Pasquier, P.: Experimental transfer function of ground heat exchanger from thermal response tests using a deconvolution approach, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2003, https://doi.org/10.5194/egusphere-egu24-2003, 2024.

Due to their potential for reducing energy consumption and greenhouse gas emissions, ground heat exchangers (GHE) coupled with heat pumps are now more commonplace. In urban areas, interference between neighboring GHEs can be a cause for concern, as it can affect average ground temperature and the long-term efficiency of heat pumps. Standing column wells, which use groundwater as a heat transfer fluid, are particularly suited to urban contexts as they do not require a productive aquifer or large areas of land. Unlike open-loop systems, groundwater is mainly recirculated in SCWs. However, to improve their performance, a fraction of the recirculation flow can be diverted to an injection well (IW), enabling the development of a thermal plume around the SCW and IW. Therefore, the delineation of the thermal plume for SCW systems is important to prevent environmental disturbances and maintain their thermal efficiency. A case study is conducted on a real system consisting of five SCWs and one IW installed in a productive fractured aquifer in the city of Mirabel, Canada. Using a 25-day hydraulic tomography, geophysical logs, drilling reports and thermal profiles, a 3D numerical model coupling heat transfer and groundwater flow was developed and calibrated. Using this calibrated model, numerical simulations were used to generate a 3D thermal plume and assess the environmental impact of the SCW system. The results indicate that the impact of SCW on this particular aquifer is limited, and that the system alone can be operated for 10 years without significant loss of efficiency or environmental impact.

How to cite: Champagne-Péladeau, L. and Pasquier, P.: Delineation of the thermal plume associated with a standing column well system, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11234, https://doi.org/10.5194/egusphere-egu24-11234, 2024.

A rapid decarbonisation of the heating and cooling sector requires accelerated retrofitting of buildings, increased use of renewable energy and waste heat utilisation (European Commission, 2016). From 2023, the German Heat Planning Act will legally mandate this development for cities and municipalities, with a special obligation to utilise unavoidable waste heat (BMWSB, 2023). In the transformation of future heating and cooling networks, a synergy of different energy sources is crucial, whose complementary strengths and weaknesses must be coordinated. A holistic concept for the future heating and cooling supply at campus level has been developed in cooperation with various disciplines from the fields of energy technology, building energy technology and engineering geology at the TU Berlin, using the university campus in Berlin-Charlottenburg as a example. Key components of the concept include load shifting between buildings, heat recovery from data centres and process cooling, expansion of a cooling network and the use of chillers as heat pumps during the heating season (Stanica et al, 2022).

The project also investigated the use of a seasonal geothermal energy storage system. The hydrogeological and urban planning parameters support the use of a borehole thermal energy storage (BTES). The subsurface is characterised by glacial fluvial sands and clays from the last ice ages, which have high thermal conductivity and capacity. The groundwater temperature is approximately 12°C and the subsurface has a very low groundwater flow velocity, which favours a high efficiency of the BTES. On the North Campus there is a 12,900 m² open space that can accommodate about 130 heat exchangers and has a heat storage capacity of about 1.2 GWh/a.

The cooling demand for the North Campus is approximately 4.7 GWh/a. The cooling systems are to be supported by free cooling at outdoor temperatures below 0°C and by the BTES at temperatures above 0°C. The special point here is that the so-called regeneration of the storage in summer is realised by the existing waste heat on the campus and not by solar thermal energy. This allows the available roof space for photovoltaic systems to be used to generate additional electricity for the operation of the individual systems. To operate sustainably and efficiently, the chillers require an inlet temperature between 6 and 12°C.

The dynamic interaction between the BTES and the associated heating and cooling system has a significant impact on the efficiency of such underground storage system. In order to optimise the design of the BTES and to ensure sustainable operation of all components, all systems were considered holistically and coupled with each other. The results of this study show that utilising the maximum amount of free space as storage in combination with the other cooling sources is not efficient. The storage capacity for the BTES is about 500 MWh/a. The use of the BTES has significantly increased the use of waste heat and reduced electricity consumption for the chillers by 200 MWh/a. In addition, 200 t of CO₂ per year will be saved, which corresponds to a further increase of 20 %.

How to cite: Schumann, F., Konenenko, N., and Fernandez-Steeger, T.: Integration of a seasonal borehole thermal energy storage into a transformed cooling network , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18386, https://doi.org/10.5194/egusphere-egu24-18386, 2024.

In urban areas, increased thermal use of the subsurface, including infrastructure development (e.g., tunnels and underground car parks) and adaptation strategies (e.g., more frequent thermal use of aquifers for "cooling" purposes or increased implementation of the sponge city concept), associated with global warming will inevitably increase urban groundwater temperatures. Likewise, anthropogenic adaptation strategies could have a greater impact than climate change itself.

In scope of the presentation strategies for the thermal management of urban groundwater resources in northwestern Switzerland are presented by discussing climate change, thermal potentials, and opportunities for adaptation measures. In particular, there are opportunities related to unused anthropogenic waste heat, especially in the subsurface of urban areas, and the energy potential that could be tapped through suitable construction measures.

How to cite: Epting, J.: Thermal management of urban groundwater resources - climate change, thermal potentials and opportunities, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21533, https://doi.org/10.5194/egusphere-egu24-21533, 2024.

Aquifer thermal energy storage (ATES) is a promising technology for sustainable and climate-friendly space heating and cooling which can contribute to lower greenhouse gas (GHG) emissions. Using 3D heat transport models, this study quantifies the technical potential of shallow low-temperature ATES in the city of Freiburg, Germany. The numerical models consider various ATES configurations and different hydrogeological subsurface characteristics relevant for the study area. Based on the modeling results, spatially resolved ATES power densities for heating and cooling are determined and compared to the space heating and cooling energy demands. High ambient groundwater flow velocities of up to 13 m d^-1 cause relatively high storage energy losses resulting in maximum ATES power densities of 3.2 W m^-2. Until now, these still reveal substantial heating and cooling energy supply rates achievable by ATES systems. While heating supply rates of larger than 60 % are determined for about 50 % of all residential buildings in the study area, the cooling energy demand could be supplied entirely by ATES systems for 92 % of the buildings. In addition, ATES heating alone could result in greenhouse gas emission savings of up to about 70,000 tCO_2eq a^‑1. The proposed modeling approach in this study can also be applied in other urban areas with similar hydrogeological conditions to obtain estimations of local ATES supply rates and support city-scale energy planning for heating and cooling.

How to cite: Blum, P., Lee, H., Menberg, K., and Stemmle, R.: Heating and cooling with aquifer thermal energy storage (ATES) in cities, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16517, https://doi.org/10.5194/egusphere-egu24-16517, 2024.

Aquifer Thermal Energy Storage (ATES) can store and supply a high capacity of seasonal heating and cooling. Well-balanced and well-designed ATES systems provide a very efficient energy storage, up to around 70-90% and thereby play an important role in providing a sustainable, and low-carbon solution for heating and cooling. Factors affecting the efficiency and capacity of ATES deployments are mainly subsurface properties and related design decisions. We set up a framework to test the impact of a wide range subsurface and design parameters to deduce their impact on ATES system performance. Aquifer thickness, lateral permeability and permeability anisotropy are considered as main aquifer properties, and cool-warm well lateral spacing and vertical offsetting of cool and warm plumes as main design decisions. The thickness of the injection and production interval can be dictated by aquifer permeability variations and/or be chosen by varying screen length.  

The parameters listed above are combined into dimensionless numbers, such as effective aspect ratio of the system and effective lateral spacing of wells to summarize and group different aquifers and possible ATES deployment designs. For a wide range of effective aspect ratios and effective well spacing ATES system behaviour is predicted by flow simulation and key performance indicators computed, including thermal efficiency, CO₂ savings, cool/warm plume sizes and stored energy density. The first two indicate the potential expected capacity of ATES systems. The latter provide recommendations for best use of land area, especially if multiple ATES systems are planned in areas with high concentration of cooling and heating demand. Given heating and cooling demand, specific aquifer conditions and land area available for use, the predicted behaviour metrics will help design optimal ATES deployments and show the potential for energy savings across multiple different settings.

Results indicate that ATES’ bidirectional subsurface thermal storage nearly always produces more energy than unidirectional open-loop systems, even when thermal recovery is low. Only when cold and warm plumes are placed side-by-side, closer than half thermal radius apart, negative thermal efficiency occurs, and more energy is put in the system than extracted. However, placing cold and warm plumes at very small spacing is still efficient, when plumes are also offset vertically, and similar thermal behaviour as large lateral well spacing is achieved.

How to cite: Jacquemyn, C. and Jackson, M. D.: Impact of aquifer properties, well spacing and vertical offsetting of warm and cool plumes on ATES systems., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6390, https://doi.org/10.5194/egusphere-egu24-6390, 2024.

The increasing energy prices and technological progress in ground source heat pumps (GSHP) in recent years has substantially propelled the use of shallow geothermal energy in Germany. With the swiftly growing numbers of installed GSHP in urban areas, the optimal and sustainable use of shallow geothermal resources requires a thorough analysis and understanding of the key components and physical processes involved in the underground at different spatial and time scales. In particular, the question of possible thermal and/or thermo-hydraulic interaction of neighboring small-power (<30kW) geothermal facilities typical of single-family houses is most relevant. We address this question within the framework of the ongoing joint research project WärmeGut by studying the efficiency of shallow geothermal heat recovery in terms of avoiding negative thermal or thermo-hydraulic interferences between neighboring installations.  

Using 3D finite-element numerical modelling and simulation, this work focuses on the impact that variably saturated flow and seasonably varying underground temperature have on several optimized pipe assemblage designs at different scales. Specially, we consider different shallowest-depth geothermal heat collector patterns and different soil thermal and hydrogeological properties. Taking into account the impact that varying saturation has on the soil thermal properties, we vary the location, depth, arrangement, pipes layout, and operational schemes to elucidate the controlling factors on the optimized sustainable use of shallowest-depth geothermal resources.

Employing COMSOL Multiphysics, we conduct a series of simulations intended to systematically analyze the complex thermo-hydraulic interaction between neighboring shallow geothermal installations under varying climatological conditions. Since there is no requirement by the German state geological surveys to provide any detailed modelling on the performance and thermal impact of small-power (<30kW) shallow geothermal facilities, we illustrate our simulation results in detail for a large range of parameter variation. We present in this work our most recent results.

This work is conducted within the joint research project WärmeGut "Flankierung des Erdwärmepumpen-Rollouts für die Wärmewende durch eine bundesweite, einheitliche Bereitstellung von Geoinformationen zur oberflächennahen Geothermie in Deutschland" and is financed by the German Federal Ministry for Economic Affairs and Climate Action (FKZ: 03EE4046B).

How to cite: Meneses Rioseco, E., Ravidà, D. C. G., Dussel, M., and Moeck, I. S.: Optimization of shallowest-depth geothermal heat collectors for sustainable energy production under variable saturation conditions and seasonal temperature fluctuations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19709, https://doi.org/10.5194/egusphere-egu24-19709, 2024.

Precise prediction of heat transport in porous media holds crucial significance in Earth Sciences for diverse applications, ranging from the design of geothermal systems to utilizing heat as a tracer in aquifers. Traditionally, the description of heat transport has been simplified by assuming local thermal equilibrium (LTE), where the temperature within the fluid and solid phases in the representative elementary volume is presumed to reach an instant equilibrium. In reality, assuming two distinct phases coupled by heat transfer across their surface area describes the complete physics and is referred to as the local thermal non-equilibrium (LTNE) model. While earlier research delved into the theoretical aspects of LTNE effects, a notable gap exists due to the absence of experimental data to elucidate the heat transport mechanism in porous aquifers containing natural grains. To address this gap and investigate LTNE on a granular scale, we conducted systematic flow-through experiments employing porous media containing glass spheres with distinct grain sizes of 5, 10, 15, 20, 25 and 30 mm. Each sphere contained a small temperature sensor for the solid temperature, accompanied by sensors in the adjacent pore space to measure the fluid temperature. Our findings revealed that the temperature difference between two phases grows with increasing grain size and flow velocity ranging from 9 to 61 m d^-1, thereby highlighting qualitative LTNE effects in relation to grain size and flow velocity. To further enhance our understanding, we used a numerical model to investigate the heat transfer coefficient, fitting the LTNE model to the experimental data. These simulation results indicated evidence of non-uniform flow which we included into our model to estimate its effects on heat transport. This comprehensive approach contributes valuable insights into the intricate interplay of LTNE effects, grain sizes, and flow velocity, advancing our understanding of heat transport in natural porous media.

How to cite: Lee, H., Gossler, M., Zosseder, K., Blum, P., Bayer, P., and Rau, G.: Local thermal non-equilibrium (LTNE) effects revealed through porous media heat transport experiment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6026, https://doi.org/10.5194/egusphere-egu24-6026, 2024.

Growing demand for space in urban areas is accelerating the utilisation of the shallow subsurface for residential and commercial spaces, transport systems, industrial processes, and energy applications. The associated underground infrastructure can act as a source and sink of heat within the subsurface, altering the ambient temperature of the soil. The extent and magnitude of such a temperature anomaly is affected by the type and density of the structures in the ground, the environmental hydraulic conditions governing groundwater flow, and the geological properties impacting conductive heat transfer. As a result, temperature distributions will vary spatially beneath a given city, thereby also affecting the geothermal potential available for heating and cooling. Knowledge on where the greatest geothermal potential lies within a city can be crucial for large-scale planning of geothermal systems, and incorporating these within building and district-level systems.

We present a methodology to map the geothermal potential under cities, extending on a recently developed and published large-scale subsurface temperature modelling methodology, which statistically identifies commonalities in how natural and anthropogenic features affect subsurface heat transfer and creates subsurface archetypes. The extension entails incorporating ground heat exchanging structures, e.g. boreholes, within the archetypes to further subdivide existing thermal archetypes into geothermal archetypes and thus enable comparison of the relative performance of a ground heat exchanger in different areas of a city. The methodology is applied to the city of Cambridge, UK to generate a map of geothermal potential. Additionally, the demand for the city, computed using field data, is mapped to produce a demand-to-capacity map of the city. By exploring different exploitation scenarios of the geothermal potential, i.e. first-come-first-served vs. co-ordinated, we further determine the ability of the resulting deployment patterns to meet likely changes in future demand under the effects of climate change.

How to cite: Kreitmair, M., Makasis, N., Chen, K., Choudhary, R., and Soga, K.: Demonstration of a city-scale geothermal resource assessment using a statistical archetypes-based approach, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16846, https://doi.org/10.5194/egusphere-egu24-16846, 2024.

The Climate Change (Emissions Reduction Targets) (Scotland) Act 2019 demands a net zero economy in Scotland by 2045, yet in the same year over three quarters of domestic heating was met by natural gas. A novel method and dynamic map resource is developed to visualise low enthalpy geothermal potential for space heating on a community scale using Ground Source Heat Pumps (GSHPs) and District Heat Networks (DHNs). This resource is intended to provide a screening tool that enables communities and policy makers to effectively reduce carbon emissions by aiding early-stage decision making and understanding of geothermal potential within the context of their communities. 

ArcGIS software is used to infer geothermal potential in 49,768km² of superficial deposits (64% of total land area), in Scotland, using a Favourability Index (FI) and a 1km² grid. Cells are assigned an FI value (0.0 - 5.0) using ten metrics based on key criteria: 1) deposit coverage, 2) thickness, 3) aquifer productivity, 4) temperature, 5) ground conditions, 6) heat demand, 7) protected land. Map resources developed show lowland areas generally exhibit more favourable conditions particularly within the Midland Valley, and settlements predominantly lie in high favourability areas. ~60% of the population is identified as living in areas where further investigations into community scale GSHPs is warranted, suggesting that the thermal resource held in unconsolidated sediments has significant potential to decarbonise the Scottish heating sector.

How to cite: Roberts, T. A., Hartley, A., and Bond, C.: Visualising low enthalpy geothermal favourability in Scotland: A map-based screening tool for community scale open-loop ground source heat pumps in superficial aquifers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15820, https://doi.org/10.5194/egusphere-egu24-15820, 2024.

Increasing urbanization puts pressure on urban subsurface temperatures and the impact on groundwater quality and human health. However, consequences of the subsurface urban heat island phenomenon, i.e. heat anomalies due to the urban lifestyle, on groundwater ecosystems have rarely been addressed to date. Of particular interest are microorganisms, who are omnipresent in groundwater and intimately involved in the cycling of carbon and nutrients, the (im)mobilization of metals, the natural attenuation of contaminants, and providing essential ecosystem services. In the framework of the research project ‘Heat below the City’, 150 groundwater wells located in the city of Vienna were sampled twice, once in spring and once in autumn 2021. A multitude of physical-chemical and biological parameters of the groundwater samples were analyzed, including the characterization of the microbial community via 16S rDNA amplicon sequencing. Groundwater temperature, combined with other stressors, such as organic and inorganic pollutants, as well as lack of dissolved oxygen, was hypothesized to prominently impact the composition, diversity and activity of microbial communities. The results revealed a complex interplay of hydrogeological and physico-chemical conditions and microbial community parameters. Microbial diversity and activity showed both increasing and decreasing trends with increasing groundwater temperature, depending on the hydrogeological aquifer type. The dominant microbial taxa were not directly impacted by the observed temperature gradients. The number of heat sources in the vicinity of a sampling well explained microbial community composition better, than any specific heat source alone. Current attempts to explore urban groundwater microbial communities of Berlin and Munich are being pursued, with the aim to improve the fundamental understanding of the relationship between hydrogeology and pressures from urbanization on the groundwater microbiome.

How to cite: Cukusic, A., Karwautz, C., Englisch, C., Kaminsky, E., Steiner, C., Stumpp, C., and Griebler, C.: Is temperature a key driver of microbial community composition in urban shallow groundwater? – A case study from Vienna, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16880, https://doi.org/10.5194/egusphere-egu24-16880, 2024.

The thermoroad combines a 5th generation district heating and cooling (5GDHC) grid with a sustainable urban drainage system (SUDS) by utilizing the roadbed for surface water retardation and as a geothermal energy source. The porous roadbed is hydraulically separated from the surrounding soil by a bentonite membrane and is able retain approximately 150 mm of infiltrating surface water when fully drained. Water is drained to the roadbed through drain grates and the roadbed is then drained to the sewer by embedded drainage pipes. The drainage pipes connect to a water brake that restricts maximum water flow to the sewer to 0.78 l/s. Therefore, water is prevented from overloading the sewer in case of extreme precipitation. Instead, the water accumulates in the roadbed and is safely drained to the sewer later. 1200 m of geothermal piping is embedded in the roadbed, separated in two groups on the manifold. A further 3 borehole heat exchangers have been established serving as backup and heat sink for cooling in the summer. A single 100 m long 40 mm 1U pipe has been placed below the central wastewater pipe at a depth of roughly 2.5 m, to harness the waste heat. Once commissioned, 12 single family houses are supplied with surface water management in addition to heating and cooling by ground source heat pumps connected the 5GDHC grid. The system is fully monitored, with energy meters on the brine side of the heat pumps and on the geothermal sources in addition to compiled weather station data from the field site. Model analysis of operational data from the first prototype of the thermoroad, shows that the energy extraction from the geothermal pipes in the roadbed is increased by 56% from the drainage of surface water. The thermoroad is an example of integration of the energy and water sectors where synergies are created. The project consortium has built the second prototype of the thermoroad in full scale and for real consumers near Horsens in central Jutland, Denmark. The thermoroad is commissioned in February 2024. We present the engineering approach behind the thermoroad and the first experience with commissioning of the system.

How to cite: Poulsen, S. E., Nielsen, K. B., and Tordrup, K. W.: The thermoroad - full-scale demonstration of geothermal 5th generation district heating and cooling combined with sustainable urban drainage, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15138, https://doi.org/10.5194/egusphere-egu24-15138, 2024.

Flooded disused mines have significant potential to supply clean heating and cooling and for seasonal storage in areas that could continue benefiting from the mines after closure. A proved technology, a more widespread deployment of mine water geothermal development is hampered by technical, socio-economical, and regulatory challenges. Among the technical challenges, the long-term system behaviour is an uncertain but fundamental element, regarding both the groundwater flow and heat distribution in the subsurface aquifer and the optimal performance of the geothermal installation and its components. A further development of mine water geothermal requires information and data from pilot, commercial and research installations to improve the knowledge about these complex systems, understand the interaction with the surrounding environment and learn from the experiences towards a more optimal design and construction of the geothermal infrastructure.

The UK Geoenergy Observatory (UKGEOS) in Glasgow was built between 2019 and 2023 as an at-scale research facility to study mine water geothermal. The observatory includes five boreholes drilled and screened into two levels of mine workings, four of them equipped with pumps and valves to allow for multiple configurations of abstraction and reinjection with operational pumping rates up to 12 l/s. The mine water boreholes are also equipped with hybrid fibre-optic cables for distributed temperature sensing (DTS) and electrical resistivity tomography (ERT) sensor arrays. The monitoring capabilities are complimented with an additional non-screened mine borehole, also equipped with DTS and ERT, and five environmental boreholes screened into the bedrock and the superficial aquifer to monitor the hydrogeological and thermal responses in the surrounding aquifers. The geothermal installation includes a sealed pipe between the abstraction/reinjection boreholes, three heat exchangers that can be used independently to test their performance, and a 200-kW heat pump/chiller. The system is equipped with sensors in the geothermal pipe circuit, the wellhead and downhole for high temporal resolution monitoring of hydraulic and thermal changes during the use of the Observatory and under natural conditions.

In this work we present results from some of the first geothermal tests performed in the Observatory in 2023. These include abstraction-reinjection in both heating and cooling modes with multiple configurations and variable flow rates and reinjection temperatures taking advantage of the capabilities of the Observatory. The datasets have been processed and examined with the support of numerical modelling. The analysis of hydraulic and thermal data from the multiple sensors in the mine and monitoring boreholes, the DTS and ERT, and the geothermal installation before and after heat exchange and reinjection have provided further insights about short- and long-term responses of the system. The observations show the different temporal and spatial scales of the hydraulic and thermal responses to the use of the geothermal infrastructure that constitute valuable information for the design of new geothermal installations in disused mines. The Observatory is now operative and open to academic and research projects aiming to understand better mine water systems.

How to cite: Gonzalez Quiros, A., Monaghan, A., Starcher, V., Walker-Verkuil, K., Boon, D., Wilkinson, P., Al-Jawad, J., Receveur, M., MacAllister, D. J., and Kuras, O.: Supporting deployment of mine water geothermal in disused coalfields with high-resolution datasets from a highly instrumented geoenergy observatory, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16026, https://doi.org/10.5194/egusphere-egu24-16026, 2024.