The absence of an instantaneous local thermal equilibrium between rock and fluid has been observed multiple times in geoscientific, and especially geothermal, applications. The alternative, describing heat transfer between phases explicitly following Newton’s law of cooling, has been found essential to describe various processes, such as thermal stresses around wellbores or thermal breakthrough curves. While heat transfer founds growing interest in the geoscientific community, parameterization of the heat transfer coefficient and experimental data is still ambiguous. On the other hand, heat transfer has been studied extensively for decades in mechanical engineering for applications such as heat exchangers, chemical catalytic reactors, and thermal insulation.

In this work, the lessons learned about heat transfer from mechanical engineering are presented and tested for validity and transferability to geoscientific applications. This includes the differentiation of several heat transfer mechanisms, various scale-dependent types of thermal non-equilibrium, and useful equations for the heat transfer coefficient. As will be pointed out, there are several key findings from mechanical engineering applications, that can be applied to geosciences almost directly. There are a few results that would require further verification, and there are substantial conceptual differences in the heat transfer between mechanical engineering and geosciences. This work will provide present key takeaways from mechanical engineering on heat transfer with a direct influence on geothermal projects and will point out future directions of required research in geosciences.

How to cite: Heinze, T.: Heat transfer in pores and fractures – lessons learned from mechanical engineering, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4563, https://doi.org/10.5194/egusphere-egu23-4563, 2023.

In installations with multiple borehole heat exchangers (BHEs), neighboring BHEs commonly influence each other. Imbalanced heat extraction and operation over many years thus causes growing interference and a pronounced thermal anomaly in the ground. This may not only have detrimental environmental effects but hamper the long-term efficiency of the entire system. As a remedy, concepts for adjusting the geometric layout and individual heat extraction loads of BHEs to a given site condition and heat demand have been presented. However, the more critical question is how to obtain an optimal and well-controlled system during the entire lifespan that often covers decades. In most of the optimization concepts that have been developed so far, the optimal load distribution is calculated on the basis of an estimated fixed heating demand. It is clear that this is not a realistic assumption due to various reasons such as changing weather conditions or a shift in consumption behavior. In our presentation, a more flexible combined simulation-optimization is introduced that takes into account a fluctuating heat demand at a monthly resolution. Here, in case of a deviation from the predefined heating demand, the BHE loads are revised and adjusted to the new conditions. Another aspect that can be considered in this sequential updating procedure is the inaccuracy of the model predicting the heat transport mechanisms in the subsurface. The new sequential optimization allows accepting a margin of uncertainty in subsurface thermal responses and taking into account thermal conditions in the ground that differ from what has been initially predicted. In our work, for demonstration, a heat demand profile with a series of monthly variations as well as a range of uncertainties in the measured temperatures of some individual boreholes in a BHE field is chosen. The optimization problem is solved efficiently by linear programming. To demonstrate the capability of the sequential procedure, fields with different densities and configurations of BHEs are studied.

How to cite: Soltan Mohammadi, H., Ringel, L. M., and Bayer, P.: Sequential simulation-optimization of a borehole heat exchanger field considering variability in heating demand and subsurface thermal responses, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3243, https://doi.org/10.5194/egusphere-egu23-3243, 2023.

The consequences of climate change and the urgent need in the EU to produce our self-clean energy from renewable resources are rapidly changing the rules. To meet the goal to produce our energy with 45% from the renewable resource for 2030, the diversification of the renewable resources to use to produce our energy is compulsory, and not only from solar and wind. Renewable energy hybrid systems are proven to be the most energy efficient and emissions savers as they take most of the natural resources intermittently, which is usually a drawback for a single renewable energy system.

Spain (along with Portugal) accounts for the highest yearly radiation rates, which energy is mainly used to produce electricity. Moreover, in Spain, 31% of the total energy is consumed by industry eminently consumed in Industrial parks, considered energy-consumers hotspots. Therefore, a hybrid PV-geothermal system is proposed as a more efficient system to produce heat and cold from renewables for the industry, both for industrial processes and heating and cooling. However, Spain shows very different orography and climate conditions along its territory, provoking different performance ratios of the proposed hybrid systems. Among the most influential factors affecting these systems' environmental and economic performance are the yearly solar irradiation, the energy demand, and the geothermal resource potential that differ in a spatial component.

In this work, a Multicriteria Decision Making (MCDM) tool is created and applied to spatially assess the performance of these systems and identify the optimal zones for PV-geothermal hybrid facilities based on the main spatial environmental factors. The tool is applied in high energy industrial demand areas from the new "Heat map of Spain" (Ecology Transition and demographic challenge Ministry of Spain) and is developed in a GIS environment. The main results expected are the performance indicators of PV-geothermal systems in Spain for the industrial sectors and the selection of the optimal zones of these systems in Spain based on the energy demand, solar irradiation, and geothermal potential.

How to cite: Ramos Escudero, A., García Gil, I., Molina Garcia, A., and García Cascales, M. S.: A multi-criteria decision-making tool to identify optimal locations for PV-geothermal systems for Heating and cooling in industrial areas., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1566, https://doi.org/10.5194/egusphere-egu23-1566, 2023.

Water from abandoned, flooded mines can form an excellent low-enthalpy, renewable long-term heat source, provided it is managed properly. Its sustainability, however, is only as good as its proper management. The poor understanding of the condition of the mine, post-closure makes the investment in these projects risky compared to other alternatives. Our modelling allows us to explore uncertainties and reduce a variety of project risks.

By combining numerical and analytical methods with digitised legacy mine data, we developed a tool to estimate the variations in the abstraction water temperature over the lifetime of a project. We couple the heat transfer approximation method originally proposed by Rodriguez and Diaz (2009) to that of flow in a pipe network as described by Todini and Pilati (1987). We refine the original heat transfer approximation by accounting for a flow regime specific heat transfer coefficient between the rock mass and the water, as prescribed by Loredo et al. (2017). We also develop a novel weighting function to account for the interference between adjacent mine galleries.

This method is applied to investigate the scenario in which multiple users will extract heat from the same mine water block. We investigate the interference resulting from heat extraction at multiple locations, using a mine system from the North East of England as a study case. The results of this study provide constraints on the maximum mine water extraction rates and proximity of the different users. The poorly constrained connectivity (through mine shafts, connecting roadways or porous flow) between mine workings from different coal seams is shown to be one of the most significant uncertainties in assessing the feasibility of a mine system as a sustainable heat source.

References:

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

How to cite: van Hunen, J., Mouli-Castillo, J., Sweeney, A., Chapman, S., Adams, C., and Townsend, D.: Geothermal heat extraction from abandoned mines, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7458, https://doi.org/10.5194/egusphere-egu23-7458, 2023.

There is an increasing interest in the use of the thermal energy within abandoned, flooded coal mines within the UK, which offers a potential seen as a low-carbon source that could support the decarbonizing of heating and contribute to the nation’s 2050 net-zero emission goal. With about 78% of UK dwellings currently using natural gas to fuel central heating, residential and commercial heating is responsible for 23% of the countries carbon emissions. Whilst all the underground coal mines are now closed, about 25% of the population still live above legacy mine workings, a proportion of which remains in deprived rural mining areas and is prone to be affected by energy poverty. Using an open-loop heat pump mine-water heating system, heat could be harnessed from the 12-20°C mine-water filling the underground mining voids to provide an economic low carbon heat resource that could directly benefit the local population, provided that the heat extracted does not exceed the heat in place.

Whilst the potential of flooded mine workings to provide sustainable heat energy has been extensively investigated, only a limited number of mine-water heating system are currently operating worldwide, such as Heerlen in the Netherlands. In order to aid the development of the resource in the UK, a better understanding of the sustainability and thermal footprint of heat extraction is required. However, generating an optimal production scenario through numerical modelling requires a thorough understanding of the geometry of mine workings. This is generally highly complex and subject to numerous uncertainties, due to the long history of mining, poor documentation of the mine workings and the inability to characterize the current state of the workings. Hence, no standard modelling approach to quantify the potential thermal resource of abandoned mine workings has yet been developed. In order to develop such a tool, it is essential to quantify the effects and uncertainties linked to the choice of a modelling approach, to the mine geometry or to the values of rock properties.  

Here, we focus the analysis on the relative importance of geometrical features in controlling the dynamic heat recharge and extraction rate from pillar-and-stall and longwall mines, using different modelling approaches. We show that the volume of the mining zone and the permeability contrasts between the caved and fracture zone are key controls on the thermal output and that equivalent porous models can reasonably reproduce the power output of more detailed models. A combination of georeferenced mining data, monitoring temperature, hydraulic data, and a range of typical rock property values for the coal measures is then used to develop a conceptual model of the Bilston Glen mine in the UK and provide a first assessment of its static heat potential, accounting for the uncertainty in the mined volume. Calibrated numerical models are finally developed and compared to the analytical solutions to get insights into the dynamic heat recharge of the system in the long-term, and support the development of a generic conceptual tool for the assessment of sustainable rate of heat extraction from mine workings.

How to cite: Receveur, M., McDermott, C., Gilfillan, S., Fraser-Harris, G., and Watson, I.: Is commercially viable heat extraction from legacy mine workings in the UK sustainable without dynamic heat recharge?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9432, https://doi.org/10.5194/egusphere-egu23-9432, 2023.

The promotion of Energy Geostructures (EGs) is strongly related to the use of renewable and clean energy resources for the heating and cooling of buildings. They couple the structural role of geostructures with the exploitation of Low Enthalpy Geothermal Energy (LEGE). During their operation, EGs are continuously subjected to thermal variations, due to the heat exchange between the soil and heat transfer fluid circulating in the pipes inserted in the structure. This can lead to an impact on the mechanical response of the structure, and the role of the soil-structure interface takes on relevance in this operation. Nevertheless, experimental results deriving from the literature on the Thermo-Mechanical (TM) soil-structure interface behavior suggest that the effect of temperature on the shear resistance is quite limited, in the case of interaction with a building material such as concrete, especially for coarse-grained soils. The case of fine-grained soils is more complex: some studies suggest an enhancement of the interface shear strength, showing an increase of adhesion or a slight increase in friction angle at the interface during heating; while other studies show no significant variations of the interface behavior with thermal cycles. Such differences are likely due to the multitude of experimental configurations, development protocols, and composition of the samples used during tests. With the aim of better understanding this controversial framework on the interface behavior, a modified device for direct shear tests was developed at the Laboratory of Geotechnical Engineering of the University of Perugia: starting from the conventional direct shear apparatus, this has been equipped with a heating cement plate, where a thermal resistance and a temperature probe for continuous temperature control have been integrated. The first tests on silty sand reconstituted samples have shown that the thermal effects at the interface are limited to a decrease in shear strength of less than 3%.

How to cite: Lupattelli, A., Salciarini, D., Brunelli, B., and Cattoni, E.: Thermally-controlled direct shear tests at the soil-concrete interface, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12666, https://doi.org/10.5194/egusphere-egu23-12666, 2023.

Ground Heat Exchanger (GHE) used with heat pumps have the potential to lower energy consumption and greenhouse gas emissions. In particular, Standing Column Well (SCW), which is a category of GHE that uses groundwater has the heat transfer medium, is more compact and less expensive than conventional closed-loop GHEs. SCWs are usually coupled with an injection well to enhance the advection process and the overall efficiency during peak power period. Due to the sediment load present in groundwater, water reinjection tends to clog the aquifer, making it less permeable, and in the worst case, leading to undesirable overflows. To avoid this problem, the use of a sedimentation tank placed before the injection well is investigated. To assess the feasibility of this solution, a fully coupled numerical model has been developed based on standard SCW conditions and on laboratory analysis performed on sediments. The fluid dynamics and settling processes have been coupled and simulated with the Mixture model at a constant temperature. Results show that a conventional sedimentation tank can reduce the sediment concentration in the groundwater returned to the aquifer for different flow rate conditions. As the temperature has a major impact on the sedimentation process, a coupled Heat transfer Mixture model simulation was developed to analyse the response of the tank efficiency for a range of typical SCW operating conditions.

How to cite: Hernandez, D., Pasquier, P., and Guibault, F.: Numerical simulation of a sedimentation tank applied to an open loop Ground Heat Exchanger system, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15458, https://doi.org/10.5194/egusphere-egu23-15458, 2023.

Shallow open-loop Aquifer Thermal Energy Storage (ATES) systems have been adopted by three large adjacent buildings in the centre of Brussels. The doublets of pumping and reinjection wells of two administrative buildings are located in a shallow aquifer made of Cenozoic mixed sandy and silty sublayers and operations started in 2014 and 2017. A third ATES system located in the underlying deep aquifer made of Palaeozoic fractured phyllites and quartzites, was started recently (2020) to provide the needed heating and cooling power to a large multi-service building. Groundwater levels variations in these two aquifer systems are different and pumping tests performed in the upper aquifer system have shown no impact on the groundwater levels in the Palaeozoic bedrock aquifer. After being calibrated on groundwater flow conditions in both aquifers, a 3D hydrogeological model using Feflow© was developed to simulate the cumulative effect of the three geothermal installations in the two exploited aquifers.

In terms of heat interactions, a previous model has shown how the thermal imbalance of the ATES system started in 2014 was jeopardising the thermal state of the upper aquifer (Bulté et al. 2021). Here, interactions with the third ATES system located in the deep aquifer are studied and modelled with different operational scenarios. Even though hydraulic interactions between the two aquifers are very limited, heat exchanges occur between the two aquifers, through an aquitard formed by low permeability Cretaceous base deposits and the weathered top of the bedrock.

The simulation results show that despite the unbalanced ATES system affecting mainly the shallow groundwater conditions, an adjacent but deeper ATES system can operate without significant interactions. Acquisition of additional measured data (i.e., piezometric heads, groundwater temperatures, detailed pumping, injection flow rate, etc.) will be crucial to improve the reliability of the simulated results for different operational scenarios. This will be particularly useful for the future management of the three ATES systems in order to avoid losses in both efficiency and durability.

This work was mainly conducted in the frame of the Master thesis of Caroline De Paoli. This was done with the partial support of the MUSE project—Managing Urban Shallow geothermal Energy. MUSE has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 731166 under the terms of GeoERA program—ERA-NET Cofund Action.

Bulté, M.; Duren, T.; Bouhon, O.; Petitclerc, E.; Agniel, M.; Dassargues, A. Numerical modeling of the interference of thermally unbalanced Aquifer Thermal Energy Storage systems in Brussels (Belgium). Energies 2021, 14, 6241. Special Issue on Geothermal Systems, https://doi.org/10.3390/en14196241

How to cite: Dassargues, A., De Paoli, C., Orban, P., Agniel, M., Petitclerc, E., and Duren, T.: Predicting interactions between three neighbor open-loop Aquifer Thermal Energy Storage (ATES) systems in two overlaying aquifers in Brussels (Belgium), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2371, https://doi.org/10.5194/egusphere-egu23-2371, 2023.

The Member States of the European Union pledged to reduce greenhouse gas emissions by 80-95% by 2050. Shallow geothermal systems might substantially contribute by providing heating and cooling in a sustainable way through seasonally storing heat and cold in the shallow ground (<200m). When the minimum yield to install a cost-effective aquifer thermal energy storage (ATES) system cannot be met, borehole thermal energy storage (BTES), relying mostly on the thermal conductivity of the ground, is proposed. However, for large-scale applications, this requires the installation of hundreds of boreholes which entails a large cost and high disturbance of the underground. In such cases, ATES systems can nevertheless become interesting. In this contribution, we present a case study performed on a Ghent University campus, where the feasibility of ATES in an area with a low transmissivity was determined. The maximum yield of the aquifer was estimated  at 5 m³/h through pumping tests. Although this low yield was attributed to the fine grain size of the aquifer, membrane filtering index tests and long-term injection tests revealed that the clogging risk was limited. A groundwater model was used to optimize the well placement while limiting the risk of interactions between the wells resulting in a thermal breakthrough or flooding at the surface. It was shown that a well arrangement in a checkerboard pattern was most effective to reach these objectives. Hence, for large-scale projects, a minimal CO₂ output might be reached using a (more cost-effective) ATES system even in low permeable sediments.

How to cite: Tas, L., Simpson, D., and Hermans, T.: Assessing the potential of low transmissivity aquifers for ATES systems: a case study in Flanders (Belgium), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1455, https://doi.org/10.5194/egusphere-egu23-1455, 2023.

The geothermal heat available in urban environments is becoming increasingly interesting for its possible use as geothermal resource available at shallow depth beneath cities.

This heat is mainly composed of the natural thermal background regime of the area of interest, the anthropogenic effect due to the urbanization process and the contribution due to the global warming. To be able to differentiate between these contributions provides a better understanding on how the urban heat resource could be used and managed.

In detail, global climate warming has a direct consequence in the increase of surface air temperature over time affecting, at local scale, the heat energy exchanges between the air-ground interface. The air temperature fluctuations can penetrate several meters deep into the subsurface raising the ground surface temperatures and, consequently, the mean annual temperature of shallow aquifers. A detailed analysis of temporal series of air, ground and shallow depth ground temperature data is expected to show this behaviour.

The city of Padova and its surroundings, located in the north-east of Italy, were selected as case study area. Here, starting from a comprehensive analysis of surface air temperature variation in the last 60 years, the local climate change was tracked. The correspondence between increase temperature on surface and underground was researched using at first underground temperature profile data collected in surface groundwater wells in urban and agricultural areas. The preliminary results confirm the influence of air temperature variation, affected by global warming, on shallow depth ground. Next step consists in the analysis of underground temperature profile variations over approximately 100 m depth in a geothermal borehole installed in the city centre, to assess the contribution of the climatic temperature to the background thermal regime and anthropogenic heat effect.

How to cite: Di Sipio, E., Cenni, N., and Galgaro, A.: Climate change and subsurface urban heat island: ground surface, well temperature and surface air temperatures correlation in the city of Padova, Italy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14638, https://doi.org/10.5194/egusphere-egu23-14638, 2023.