Lateral groundwater-river exchanges estimation via actively heated fiber optics based thermal response test

This study investigates the feasibility and performance of integrating shallow geothermal energy systems into the structural foundations of high-rise buildings with developed underground parts. A comprehensive experimental program was conducted, including field measurements of soil thermal properties and laboratory testing of heat exchange elements. Thermal response tests using a 100-meter geothermal probe demonstrated effective ground heat extraction, with the maximum temperature drop in the active zone reaching 6.8°C and an 83% temperature recovery within 3 months. Energy piles with diameters of 0.8 and 1.2 meters exhibited heat outputs of 4.6 and 7.2 kW, respectively, confirming that larger surface areas enhance thermal capacity. The ground source heat pump system operated with an average coefficient of performance of 4.21 during heating and 3.82 during cooling, achieving up to 98% of the projected thermal load. Numerical simulations confirmed the experimental findings, indicating an annual heating energy yield of approximately 2450 MWh. The results validate the integration of geothermal systems into foundation structures as an efficient and reliable approach to reducing energy consumption and enhancing sustainability in high-density urban development.

On the use of thermal response tests for deep geothermal exploration in urban areas: A case study made on the Greater Montréal (Canada)

The Thermal Response Test – abbreviated as TRT – is a standardized procedure for geothermal heat pump systems with closed-loop heat exchangers. The test enables the determination of key physical properties of the ground – thermal conductivity and borehole thermal resistance – which are essential for the correct sizing of the ground heat exchanger system, ensuring it meets the requirements of the building’s HVAC system. Additionally, the TRT provides valuable information regarding the technical and financial effort required for the drilling operation.

Introduction of the Excitation-Modulated Thermal Response Test (EM-TRT) for the improved Evaluation of Vertical Ground Heat Exchangers

Abstract The paper deals with the so-called house demonstration unit that was built during the international student competition Solar Decathlon 21/22 in Wuppertal and is now part of the Living Lab there. The design was focused on minimizing energy consumption, using renewable energy sources, preferring environmentally friendly and recycled materials while maintaining high quality of the indoor environment. Participation in the Living Lab allows for long-term monitoring of its actual performance using built-in sensors and other measuring devices. Thermal transmittances of the external wall and the roof were successfully compared with calculations. The co-heating test and alternating underfloor heating tests allowed the heat transfer coefficient to be determined and compared with the calculation value. In parallel, a dynamic thermal response test was carried out and compared with the simulation results.

In cold regions, performance reduction in a Ground-Coupled Heat Pump (GSHP) system has been frequently reported. Many operational strategies have been adopted to mitigate such an undesirable phenomenon. However, these strategies have limited effects because the specific heat rate of Borehole Heat Exchangers (BHEs) is usually treated as constant. In this study, eight BHEs were installed in typical loess areas in Northwestern China to investigate how borehole depth affects its thermal performance. Thermal response tests (TRTs) showed that deeper boreholes led to a higher fluid outlet temperature. Compared to 150 m and 100 m boreholes, the energy coefficient factor (η) for a 200 m borehole increased by 18.02% and 45.0%, respectively. Numerical simulation also confirmed that deeper BHEs perform better. In addition, the initial ground temperature influences the thermal performance sensitively, but in the opposite way for heating and cooling modes. These findings offer valuable insights for installing GSHP systems to achieve sustainable and high thermal performance in cold regions.

In order to develop and analyse ground heat collectors, experimental plants with reliable measurement data are needed. Here, an experimental plant with a planar ground heat collector installed vertically in a trench is presented. The ground heat collector is extensively equipped with measurement technology installed to track its thermal behaviour and that of the surrounding ground. This includes 3 different types of temperature measurements as well as the determination of the volumetric water content and the bulk electrical conductivity of the subsurface near the collector. The description also includes detailed information on the geometry of the experimental plant and the sensors installed, the data acquisition systems used and information on measurement accuracy and calibration procedures. Measurement data collected by these sensors during a thermal response test on the planar ground heat collector are described.

It is well known that the operation of a Borehole Heat Exchanger (BHE) can thermally induce groundwater convection in aquifers, enhancing the thermal performance of the BHE. However, the effect on the thermal performance of BHEs installed in low-permeable rock formations remains unclear. In this study, two BHEs were installed in a silty sandstone formation, one backfilled with high-permeable materials and the other grouted with sand–bentonite slurry. A Thermal Response Test (TRT) showed that the fluid outlet temperature of the high-permeable-material backfilled BHE was about 2.5 °C lower than that of the BHE refilled with sand–bentonite slurry, implying a higher thermal efficiency. The interpreted borehole thermal parameters also show a lower borehole thermal resistance in the high-permeable-material backfilled BHE. Physical model tests reveal that groundwater convective flow was induced in the high-permeable-material backfilled BHE. A test of BHEs with different borehole diameters shows that the larger the borehole diameter, the higher the thermal efficiency is. Thus, the thermal performance enhancement was attributed to two factors. First, the induced groundwater flow accelerates heat transfer by convection. Additionally, the increment of the thermal volumetric capacity of the groundwater stored inside a high-permeable-material refilled borehole stabilized the borehole’s temperature, which is key to sustaining high thermal efficiency in a BHE. The thermal performance enhancement demonstrated here shows potential for reducing reliance on fossil-fuel-based energy resources in challenging geological settings, thereby contributing to developing more sustainable geothermal energy solutions. Further validation in diverse field conditions is recommended to generalize these findings.

Effect of one-dimensional simplification of ground heat exchanger on prediction errors of numerical thermal response test

Assessing the variation in the thermal conductivity of heterogeneous rock materials can be critical when upscaling models to simulate geothermal system operation, especially for petrothermal systems, where conduction dominates over convection. This study’s objective was to evaluate heterogeneity effects when assessing the thermal conductivity of geological materials, in this case, metamorphic rocks from Kuujjuaq (Canada), where the installation of a ground-coupled heat pump system is expected. Four core samples of gneissic rocks were analyzed in detail and compared to results obtained from a thermal response test. Thermal conductivity measurements in dry conditions were performed on the cylindrical surface of the samples with an optical thermal conductivity scanner. The 2D thermal conductivity spatial distribution was obtained by ordinary kriging interpolation method and used for numerical modeling to simulate steady-state conductive heat transfer along the sample vertical direction. Then, the effective thermal conductivity was computed according to Fourier’s law, using the simulated temperature to investigate the effect of scale variation with the heterogeneity. Results indicate the importance of distinguishing between the sample section’s effective thermal conductivity and local average thermal conductivity. Significant scale effects were identified with a variation ratio comprised between −10% and +16% when varying the length of the sample section. The representative elementary volume for the effective thermal conductivity was determined equivalent to half of the sample length. This volume gave a thermal conductivity that is equal to the harmonic mean of the laboratory-assessed values with a relative error <5%. A comparison between the in situ and laboratory-assessed thermal conductivity indicates that the thermal conductivity inferred from the thermal response test is adequate for sizing a geothermal system, assuming a range of variability equivalent to 1.5 times its standard deviation.

The central role of heating and cooling in energy transition has been recognised in recent years, especially with geopolitical developments since February 2022 which demand an acceleration in deploying local energy sources to increase the resilience of the energy sector. Geothermal energy is a promising and vital option to optimize heating and cooling systems, promoting sustainability of urban environments. To this end, a proper design is of paramount importance to guarantee the energy performance of the whole system. This work deals with the optimization of the technical and geometrical characteristics of borehole heat exchangers (BHEs) as part of a shallow geothermal plant that is assumed to be integrated in an already operating gas-fired DH grid. Thermal performances of three different configurations were analysed according to the geological information that revealed an aquifer at −36 m overlying a poorly permeable marly succession. Numerical simulations validated the geological, hydrogeological, and thermo-physical models by back-analysing the experimental results of a thermal response test (TRT) on a pilot 150 m deep BHE. Five-year simulations were then performed to compare 150 m and 36 m polyethylene 2U, and 36 m steel coaxial BHEs. The coaxial configuration shows the best performance both in terms of specific power (74.51 W/m) and borehole thermal resistance (0.02 mK/W). Outcomes of the study confirm that coupling the best geological and technical parameters ensure the best energy performance and economic sustainability.

Abstract Triassic sandstones of the Middle and Upper Buntsandstein are highly suitable for ground source heat pump (GSHP) systems. Thus, knowledge of their thermal properties, which can be measured or estimated by theoretical models, is crucial. However, the transferability of estimated thermal conductivities to the field scale has not yet been thoroughly examined. Therefore, in this study, the thermal and lithological properties of 156 core samples from a borehole in the Buntsandstein are analysed in the laboratory. Various theoretical models are applied and compared to the laboratory-derived thermal conductivities. The best agreement is achieved with the Voigt-Reuss-Hill model with an average thermal conductivity of 4.5 W m−1 K−1 and an RMSE of 0.7 W m−1 K−1 (T = 20 °C). The results of this model are compared to depth-specific, effective thermal conductivities from an enhanced thermal response test (ETRT). These effective thermal conductivities range between 2.3 and 6.1 W m−1 K−1 with an average of 4.7 W m−1 K−1. We demonstrate that some theoretical models can provide an initial estimation of the effective thermal conductivity of sandstones when groundwater flow is negligible. However, the accuracy of the estimation is limited by sample quantity and model assumptions.

The short-term performance of ground heat exchangers (GHEs) is crucial for the optimal design of ground-source heat pumps (GSHPs), enhancing their contribution to sustainable energy solutions. Local short-time thermal response functions, or short-time g-functions (STGFs) derived from thermal response tests (TRTs), are of great interest for predicting the heat exchange due to their fast and simple applicability. The aim of this work is to perform a sensitivity analysis to assess the impact of thermal parameter variability and TRT operating conditions on the accuracy of the average fluid temperature (Tf) predictions obtained through a local STGF. First, the uncertainties associated with the borehole thermal resistance (Rb), transmitted from the soil volumetric heat capacity (CS) or some models dependent on GHE characteristics, such as the Zeng model, were found to have a low impact in Tf resulting in long-term deviations of ±0.2 K. Second, several TRTs were carried out on the same borehole, changing input parameters such as the volumetric flow rate and heat injection rate, in order to obtain their corresponding STGF. Validation results showed that each Tf profile consistently aligned well with experimental data when applying intermittent heat rate pulses (being the most unfavorable scenario), implying deviations of ±0.2 K, despite the variabilities in soil conductivity (λS), soil volumetric heat capacity (CS), and borehole thermal resistance (Rb).

Analysis of the performance and interpretation of thermal response tests for four boreholes of various depths in the town of Dźwirzyno (Zachodniopomorskie Voivodeship)

Influence of borehole characteristics on thermal response test analysis using analytical models

This study presents the implementation of a new Type for TRNSYS based on the integral exponential function, which is fundamental in subsurface heat transfer modeling. This function can be expressed as a series expansion, and the developed Type enables the representation of multiple terms in this expansion. To validate its accuracy, the model’s results are compared with experimental data from two geothermal boreholes located in Asturias, Spain: Q-Thermie-Uniovi 1 and Q-Thermie-Uniovi 2. These boreholes, spaced 7.3 m apart and each 48 m deep, were used in a thermal response test, where Q-Thermie-Uniovi 1 underwent thermal injection while temperature measurements were recorded in Q-Thermie-Uniovi 2 over a period of three months. This study evaluates how the number of terms in the series expansion influences the model’s accuracy and demonstrates that the newly developed Type effectively replicates experimental data. This work provides a robust tool for optimizing the design of geothermal systems and enhancing the understanding of subsurface heat transfer dynamics.

The thermal properties of grouting materials characterise the heat transfer around borehole heat exchangers (BHE). However, these properties are typically determined in the laboratory. Thus, this study aims to assess the properties of grouting materials in the field. Two BHE grouted with two different grouting materials within unsaturated loess and limestone were excavated up to a depth of 15 m. Collected field samples show higher thermal conductivities by 13% (W/S = 0.3) and 35% (W/S = 0.8) than laboratory samples of the same material. These differences in thermal properties are mainly related to the filtration of the grouting suspension. In addition, with a short-time enhanced thermal response test (ETRT), 17% lower in-situ thermal conductivities are determined than in comparison with the field samples. The deviations are attributed to the geometry of the borehole, the trajectory of the BHE pipes and the heating cable. Thereby, this study shows the limitations when transferring laboratory-derived properties to a field site and emphasises the importance of considering site conditions, such as geology and hydrogeology.

Shallow geothermal energy (SGE) has a wide range of applications in the field of building cooling and heating. Ground source heat pump (GSHP) system is a technology to extract SGE. The design of borehole heat exchanger (BHE) has a great impact on heat transfer performance and investment cost, so it is important to accurately measure the thermal conductivity of rock and soil. Therefore, this study conducted field in-situ thermal response test (TRT) and laboratory sample test based on distributed optical fiber temperature sensor (DOFTS) in LY research area of Changchun, Northeast China. After comparing the differences and analyzing the reasons, an in-situ thermal conductivity prediction model was established based on artificial neural network (ANN) algorithm to predict in-situ thermal conductivity based on basic physical property parameters of laboratory tests. This model is used to supplement the layered thermal conductivity lacking in the CY study area. The results show that the distributed thermal conductivity can be obtained and the layered thermal conductivity can be calculated by improved combined thermal response test (ICTRT). The average layer thermal conductivity of laboratory test is about 12.2% lower than that of field test, but the thermal conductivity of the two test methods has the same variation trend along the depth. The thermal conductivity of rock mass is positively correlated with water content, negatively correlated with porosity and positively correlated with density. The result error of the in-situ thermal conductivity prediction model established by calculation is mainly within ± 5%, which is reliable and accurate. This model is used to supplement the layered thermal conductivity of the CY01 test hole. The research results can provide a new way to determine the thermal conductivity in SGE exploration.

A pilot borehole heat exchanger field in Egypt: Subsoil investigation, thermal response tests and sensitivity analysis