Borehole Heat Exchangers Research Articles

Shallow geothermal potential and numerical modelling of the geo-exchange for a sustainable post-earthquake building reconstruction (Potenza River valley, Marche Region, Central Italy)

The southern part of the Marche Region (Italy) was hit in 2016 by dramatic seismic events that caused damages and fatalities. After these events, the national and local administrations started a development plan to improve the energy efficiency of the restored or reconstructed buildings. Shallow geothermal energy, consisting of closed-loop Borehole Heat Exchangers (BHEs) coupled with heat pumps, could represent a first-order renewable choice for indoor air conditioning (heating and cooling) systems. The feasibility and potential of the BHEs also match this technology's null visual outdoor impact which is paramount in preserving the landscape of the earthquake-affected areas. However, the exploitation, sustainability, and correct design of these systems require detailed knowledge of the ground's geological, hydrogeological, and thermophysical properties. In this framework, the present study was carried out to map and test, through the G.POT algorithm, the shallow geothermal potential and sustainability of the Potenza River valley through the publicly available data from the Italian Seismic Microzonation studies — an extensive database of geognostic drillings and other geological and geophysical surveys. This step allowed us to assign the main thermal parameters (thermal conductivity, volumetric heat capacity, specific heat extracted) to each geological layer and averaged them over the first 100 m (i.e., a typical depth for BHEs). All the data were then interpolated to produce geothermal thematic maps to visualize and compare the potential and adequacy of the territory for the installation of closed-loop BHEs. Finally, finite elements numerical models (FEFLOW® software) were developed in a balanced mode through the annual heating and cooling cycles to verify the sustainability of this renewable concerning the thermal impact induced to the ground by the BHEs during a long-term (20 years) operation.

Performance and economic analysis for optimal length of borehole heat exchanger considering effects of groundwater

This study introduces a method for designing borehole heat exchangers (BHEs) that considers the system performance and cost according to the design parameters. A cost-performance factor for the BHE length (CPL) is also proposed to determine the optimal BHE length that balances the initial and operating costs. The long-term performance of ground source heat pump (GSHP) systems was analyzed in a single-family house in Seoul, South Korea. The ground thermal properties applied to this study are determined based on the analysis results of the thermal response test. The groundwater velocity is estimated by the apparent effective thermal conductivity which is overestimated in the TRT analysis. By varying the borehole length and design parameters, the changes in the ground temperature and system performance were calculated along with the initial and operating costs. As a result, applying the minimum CPL in each case, the optimal BHE lengths were determined to be 150 m for Case 1 and 140 m for Cases 2 and 3. It could reduce the −645.7, −757.5 and −807.9 USD with maintaining the high system performance, compared with ASHP for 20 years of the operating system.

Achieving a mitigation of ground thermal imbalance and size-reduction of borehole heat exchangers by connecting a hybrid heat source in series

Recent studies have highlighted the potential of hybrid ground source heat pump (HyGSHP) systems for enhancing the coefficients of performance (COPs) of heat pumps and reducing the total required length of borehole heat exchangers (BHEs) when compared with ground source heat pump (GSHP) systems. However, previous studies were predominantly focused on the advantages of parallel-connected HyGSHP systems; the detailed analysis and quantitative interpretation of series-connected HyGSHP systems have not been reported owing to the dearth of analysis methods available for such a configuration. To address this, this study proposes the implementation of serial HyGSHP systems with permanent-dewatering (PD) modules covering 10% of the building peak load as a hybrid heat source. To evaluate the long-term energy efficiency of the proposed HyGSHP systems and the reduction in the total required length of BHEs, an optimization method coupled with a comprehensive simulation tool is employed. The results show that although the total required length of BHEs is reduced by half, the proposed serial-connected HyGSHP cases effectively fulfill the building load demand, similar to the conventional GSHP cases, even with higher COPs in certain cases. This improvement can be attributed to the decreasing temperature of the source-side heat carrier fluid facilitated by the serial-connected PD module with BHEs, which considerably mitigate the imbalance between the ground heat injection and extraction.

Performance of vacuum-insulated central pipes for deep borehole heat exchangers in geothermal systems

Abstract Geothermal energy is considered a promising future energy prospect, with the geothermal well outlet temperature being one of the important parameters affecting possible utilization options. For ground source heat pump applications or direct district heating, using lower temperatures can be acceptable. However, efficient electricity production requires a higher enthalpy gradient, which cannot be achieved without high temperature at the wellhead. The selection of the dry co-axial close-loop deep borehole systems (DBHE) may be, in some cases, very beneficial. The operating performance of co-axial DBHE can be optimized if the undesired heat transfer between the central pipe and annular fluid zones is minimized. Therefore, the operational performance of such a system depends strongly on the high thermal resistance of the central pipe. The most common option would be a low thermal conductivity material, such as high-density polyethylene (HDPE). In addition, vacuum-insulated tubing (VIT) used as the central pipe could be considered. The article presents results from the study aimed at the comparison of the homogeneous central pipe made of HDPE material and the gap-insulated central pipe. In the study, various air pressure levels as well as variations of surface emissivity were examined to reveal the effect on the heat transfer between the fluid channels. The simulation has been performed using a new purposely developed WellTH simulation software. A coaxial heat exchanger system using a VIT outperforms significantly the heat exchanger with an HDPE for deep geothermal wells. However, this advantage diminishes for shallow wells and therefore this tendency should be considered in the design stage.

A semi-analytical temperature solution for multi-segment deep coaxial borehole heat exchangers

A semi-analytical and a finite-difference scheme are presented for the simulation of temperature and the heat transfer in a multi-segment coaxial borehole heat exchanger. The single-segment solution on closed-form is extended to a semi-analytical multi-segment solution, where each segment may have unique properties. These properties are such as different casings, widths of the annulus, radius of the inner tubing, material properties, rock properties and geothermal gradients. The multi-segment model is a simple and powerful alternative to numerical methods for simulating a complex coaxial borehole heat exchanger with a constant flow rate. It is demonstrated with a deep coaxial borehole heat exchanger made of three different segments. The analytical and semi-analytical models are validated by comparison with numerical solutions obtained with an upstream finite difference scheme. The match between the solutions is excellent. The solution on a closed-form is used to study the temperature difference between the outlet and the inlet regarding two dimensionless numbers. It is found that the maximum temperature difference occurs when the dimensionless heat transfer coefficient for the casing-rock is much larger than one. A second necessary condition is that the dimensionless heat transfer coefficient for the insulator between the inner tube and the annulus must be much less than one. The power leakage from the inner tubing to the annulus is also at a maximum under these conditions.

Efficiency analyze Borehole Heat Exchangers (BHEs) of the research geothermal polygon placed at VŠB-Technical University of Ostrava

Among renewable energies, low-temperature geothermal energy is gaining importance due the different applications that are being accomplished within residential sector. Forecasts suggest a “promising future” for this energy source. A study carried out during the realization and operation of the Research Polygons placed at VSB – Technical University of Ostrava. New University Hall is heated up by the heat pumps. This object is the biggest building in the Czech Republic and in the Central Europe which uses this type of the system. Monitoring boreholes are situated in the area of heat pumps boreholes. They are equipped with heat sensors which monitoring temperature changes in circulations systems and in the rock massive during heating (accumulation of heat that comes from air conditioning in summer) and during cooling (pumping of heat from the rock massive by heat pumps in winter). The main goal is to efficiency analyze Borehole Heat Exchangers (BHEs) of the Research geothermal Polygons

Analysis and optimization of a medium-depth ground source heat pump heating systems with heat storage and borehole heat exchangers

The medium-depth ground source heat pump (MD-GSHP) heating technology is considered a clean energy solution with the potential to mitigate air pollution issues attributed to winter heating in northern China. However, it currently suffers from problems such as a mismatch between heat supply and demand and a lack of optimal operation. To overcome these limitations, this study focuses on a 3000 m2 residential building in Xi’an and implements two configurations: coupling MD-GSHP heating systems with heat storage tank and shallow borehole heat exchanger (BHE). Additionally, an optimization strategy based on the improved particle swarm algorithm is proposed, integrating chaos optimization and dynamic parameters. Finally, noteworthy outcomes were revealed by optimizing the MD-GSHP heating systems under different objective functions. In system coupled with shallow BHE, the variation trend of flow in the medium-depth BHE was similar to that of heating load when the heating cost was taken as the objective function. After the optimization, heating cost reduced by 10.02%, while system coefficient of performance (COP) and medium-depth geothermal utilization coefficient increased by 10.43% and 2.00%, respectively. For system coupled with heat storage tank, the optimized flow exhibited a generally decreasing-increasing-decreasing trend, with the peak preceding the heating load curve. This resulted in a 12.82% reduction in heating cost, along with increases of 10.35% and 1.96% in system COP and the medium-depth geothermal utilization coefficient, respectively. Comprehensive consideration of heating cost and system COP suggested a favorable weight of 2.8 for continuous system operation. In conclusion, when prioritizing heating cost, the system coupled with heat storage tank yields greater economic benefits compared to the system coupled with shallow BHE. The findings of this study can provide a guidance for the applications of the MD-GSHP heating technology.

Comparative analysis of ground thermal conductivity and thermal resistance of borehole heat exchanger in different geological layered sequence

Borehole heat exchanger usually passes through many geological layers of the different thermo-physical properties from one layer to another in practical project of ground-coupled heat pump system. Considering the effect of heterogeneity in geological structures on heat transfer of borehole heat exchanger, it is necessary for in-situ thermal response test analysis to investigate the effect of geological layered sequence on ground thermal conductivity and borehole thermal resistance. Firstly, based on the validated numerical layered borehole heat exchanger model and numerical homogenous borehole heat exchanger model, the simulated distributions of water temperatures in U-pipe and heat flux along the depth in thermal response test are respectively compared and analysed. Then, the effective thermal conductivity and effective borehole thermal resistance estimated by the infinite line source model are respectively obtained based on the simulated thermal response tests carried out in the borehole heat exchanger installed in the six types of geological layered structures. The borehole thermal resistances in the two numerical model were respectively calculated by integrating the local thermal resistance along the depth. Finally, the ground thermal conductivities and borehole thermal resistances are compared in six types of geological layered structures. The results show that the maximum relative error of 7 % between the effective thermal conductivity and the thickness-weighted thermal conductivity in the numerical layered borehole heat exchanger model is located in the sequence of S-L-C due to BOTTOM 60m-thickness clay with lowest thermal conductivity. The relative error between effective borehole thermal resistance and borehole thermal resistance in the numerical layered borehole heat exchanger model are 3.5 %∼7.9 % in the six types of layered geological structures, and the maximum relative error between the two borehole thermal resistances corresponding to the two numerical models is only 1.1 %, which proves that effect of geological layered sequence on the deviation is limited in the two numerical models.

Optimization of double layered horizontal directional drilled ground heat exchangers by water injection into the borehole

Ground Heat Exchangers (GHEs) drilled using Horizontal Directional Drilling (HDD) technology have been proven to be a cost-efficient way of installing ground source heat pump systems (GSHP). They can also be installed in places where conventional horizontal ground heat exchanger (HGHE) would not be possible e.g., due to lack of land.In this research, a double layered horizontal borehole was installed as a GHE in Saga City, Japan. The two holes had a diameter of 114.3 mm and a length of 59 m and 56 m, respectively, while reaching depths of 5 m and 9.5 m. A polyethylene pipe with an outer diameter of 60 mm was inserted as a borehole heat exchanger (BHE). In March 2022, a two days thermal response test (TRT) showed the influence of rainfall on the system. A numerical model developed in FEFLOW based on the GHE dimension and the region's geology was validated using the measured temperatures of the heat medium at the return point and outlet of the GHE during the TRT. The validated model was then used to carry out sensitivity analysis examining different conditions of water injection into the borehole. The result of the sensitivity analysis showed that water injection into the borehole can improve the performance of the system and reduce the temperature change of the heat medium. Varying the injection rate can improve performance to a certain degree and the injection point has a strong influence on the performance. The injection temperature was also found to play a role in the performance, but even high temperatures in the case of heating showed significant improvement. Lastly, the influence of permeability was investigated and showed that impermeable layers are more favorable for injection compared to permeable layers.

Numerical modelling of an advective geothermal system in the Central Andes: Sustainable utilization through borehole heat exchangers

The utilization of low-enthalpy geothermal resources emerges as a promising solution for district heating in the Central Andes region. Focused on the Pismanta field, a representative example of the prevalent advective geothermal systems in the area, this study investigates into the intricate dynamics of the geothermal reservoir. Situated within a fault-controlled basin, the Pismanta field undergoes deep infiltration of meteoric water, leading to peak temperatures of up to 90 °C. Numerical simulations conducted at varying scales yield critical insights into the characteristics, efficient exploitation, and potential repercussions of energy extraction from the reservoir. A comprehensive approach is adopted to facilitate the optimal management of this geothermal resource and a strategic closed-loop system comprising 10 shallow interconnected borehole heat exchangers (BHE) is proposed. This recommendation takes into account the specific basin geometry and the intricate interplay of pressures within the confined geothermal reservoir. Extensive simulations over a 10-year period show the sensitivity of the system to operating flow rates. By employing the suggested configuration, it is anticipated that a maximum thermal power extraction of 1 MW can be achieved. The circulating water within the system is projected to maintain a temperature of approximately 26 °C.

Exploiting heat gains along horizontal connection pipes in existing borehole heat exchanger fields

This study investigates three options for utilising the additional capacity of borehole heat exchanger (BHE) fields through gains along horizontal connection pipes, whose contribution is routinely ignored. The analysis considers thermal load profiles with different heating to cooling ratios and finds that the effect of horizontal pipes becomes more significant with unbalanced loads. The study explores the potential of extended operation, increased loads and an optimised operating strategy to exploit the idle capacity gain from the horizontal connection pipes. It is shown that for the scenarios investigated, the BHE field operational time can be extended by more than 25 years without violating the critical fluid temperatures in the design phase. Alternatively, the thermal load can be increased by up to 26%. In addition, the study highlights the potential of an optimised operating strategy involving adjustments to the number of BHEs operated to reduce power consumption and therefore reducing operating costs in existing systems by utilising heat gains from the horizontal connection pipes.

Thermal performance of medium-deep U-type borehole heat exchanger based on a novel numerical model considering groundwater seepage

The medium and deep U-type borehole heat exchanger (MDUBHE) system is a novel technology to extract deep geothermal energy for building heating. The thermal performance of MDUBHE is influenced by multiple factors, where groundwater seepage is one of the most significant factors. To our best knowledge, the effects of groundwater seepage on MDUBHE are still unclear. In this paper, a novel numerical heat transfer model for MDUBHE considering groundwater seepage is proposed, and validated by CFD simulation and experimental data, with root mean square errors of 0.5 °C and 1.8 °C, respectively. Based on the novel numerical model, the effects of groundwater seepage on thermal performance of MDUBHE and thermal recovery characteristics of rock-soil are investigated. The results show that the computational efficiency of the novel numerical model is improved by two orders of magnitude compared with CFD software in the same case. The heat extraction rate of MDUBHE gradually increases with increase of seepage velocity. The heat extraction rate is improved by 13.63 % when the seepage velocity is 5.0 × 10−7 m/s. Furthermore, groundwater seepage can improve the thermal recovery rate of rock-soil. Compared with no groundwater seepage, the thermal recovery rate is improved by 7.60 % when the seepage velocity is 1.0 × 10−7 m/s. This study is significant for revealing the effect of groundwater seepage on the MDUBHE system.

Semi-analytical and numerical modeling of U-bend deep borehole heat exchanger

U-bend deep borehole heat exchanger (DBHE) is a new promising kind of ground heat exchanger, but nowadays there are lack of accurate efficient models to simulate U-bend DBHE. This paper has developed a semi-analytical model of U-bend DBHE, which considers the unsteady heat transfer of borehole, heterogenous soil thermal properties and geothermal gradient. A 2D numerical model and an analytical model based on infinite line-source theory are also developed, and the three models are compared with experimental data for different boundary conditions. Based on the soil thermal properties estimated by logging data and rock identification, the three models have large errors when compared with experimental data. Based on a soil thermal conductivity λs estimated by matching experimental data, the three models match better with experimental data, and it is inferred that λs should be measured by thermal response test instead of rock identification. The semi-analytical model and 2D numerical model match very well for all the time, but the analytical model has large differences with 2D numerical model, which is because the analytical model ignores borehole heat capacity and time-varying heat flows at the borehole wall. This paper could provide a cost-effective semi-analytical model and some advices for U-bend DBHE.

Assessing the Impact of a Shallow Geothermal System Operation through Multi-Layer Temperature Monitoring in a Mediterranean Climate

Shallow Geothermal Energy (SGE) exchanges heat with the ground. In continuous, long-term operation, the initial temperature field can be disturbed, and subsurface thermal changes can be developed. In this paper, the thermal impact of a SGE system under a Mediterranean climate is handled. Temperature monitoring was conducted on 15 investigation boreholes equipped with a total of 92 thermal sensors placed at specific depths. Investigation boreholes were drilled 1–2 m from SGE system borehole heat exchangers installed in a university building. The analysis handles a one-year monitoring period of SGE system operation. Temperature depth profiles, reaching up to 140 m depth, were registered with a 10 min time step, resulting in a large amount of data. Ground thermal conductivity was estimated experimentally and semi-empirically, allowing us to obtain, using a numerical model, the initial undisturbed ground temperature profiles and compare them with the monitored values. Climate data were recorded by the university meteorological station. Globally, the measured and computed data were coherent, and a non-negligible impact of the SGE system operation in the first year was observed. The building orientation as well as the nearby departments had significant impacts on the shallow ground temperature. Maximum ground temperature changes observed at depths higher than 10–20 m, ranging from 2 to 3 °C as observed in different boreholes, indicate that the system is operating efficiently.

Properly shortening design time scale of medium-deep borehole heat exchanger for high building heating performances with high computational efficiency

The medium-deep borehole heat exchanger (MDBHE) is a clean solution for building heating with renewable geothermal energy. The long-term performance evaluation with the design time scale same as the life span of the MDBHE is generally adopted by the MDBHE design for high heating performances. However, the long-term performance evaluation largely increases the computational load of the MDBHE design, resulting in practical inconvenience. This study proposes the idea that properly shortening the design time scale could maintain the information of the long-term performance evaluation for the MDBHE design with high heating performances while reducing the computational load. The idea is verified by the MDBHE design for building heating in the severe cold zone of China. The multiple design parameters of the MDBHE are simultaneously optimized considering climate change. The results show that under different geological conditions, economic conditions, and system safety requirements, compared with the life span of the MDBHE of 30–50 years, the design time scale properly shortened to 10 years achieves similar high heating performances regarding the energy consumption, cost, and system safety, and reduces the computational load by 66.1 %–79.3 %. This study contributes to high computational-efficient and high-performance design of the MDBHE for building heating.

Analytical study of integrating downhole thermoelectric power generation with a coaxial borehole heat exchanger in geothermal wells

Geothermal power generation employing Organic Rankine Cycle (ORC) technology is a widely acknowledged and conventional approach for harnessing geothermal energy. In an innovative advancement, we propose a novel design integrating downhole thermoelectric power generation with a coaxial borehole heat exchanger. This design aims to enhance the efficiency and sustainability of geothermal energy utilization. In this innovative design, the geothermal well is divided into two distinct sections: a power generation section and a heat exchanging section, achieved through the implementation of a packer positioned from the uppermost part of the targeted zone. The process involves the injection of cold fluid downhole via an insulated pipe. Subsequently, a portion of the injected fluid is directed to flow in reverse within the casing-tubing annulus above the packer, while another portion circulates into the casing-tubing annulus below the packer before ascending through the tubing. This dual flow mechanism establishes distinct cold and hot sources for the thermoelectric generator, a key feature facilitated by this innovative design. Analytical models detailing of downhole temperature distribution for thermoelectric power have been meticulously developed. A comprehensive case study, focusing on a geothermal well with 3000 m length of power generation section and 500 m heat exchanging section, has been conducted. The results indicate that a significant generating capacity could be achieved with a higher wellhead temperature, and the payback period under different carbon tax scenarios is about 6–8 year. Furthermore, the effects of injection rate, fluid diversion ratio, and casing-tubing configuration on power performance and thermal-electricity efficiency are also discussed. This method not only enables the concurrent harvesting of geothermal energy and power generation but also operates consistently throughout the year. The results thus emphasize the viability and economic feasibility of the proposed approach.

Study on long-term operation characteristics of the medium-deep ground source heat pump system with solar heat storage

Long-term heating operation of the medium-deep ground source heat pump (MGSHP) system will cause the underground temperature around the mid-deep borehole heat exchanger (MBHE) significantly lower than its initial value, which will reduce the operating efficiency of the MGSHP system. In some cases, the temperature of the circulating water in MBHE even is lower than the lowest value required by the heat pump unit, which will lead to operation failure of the unit. Attenuating the ground temperature decrease by storing heat underground during non-heating seasons is an effect way to maintain the heat extraction capacity of the MBHE. In this paper, the influence of storing solar energy underground during non-heating seasons on the long-term operating characteristics of the MGSHP system is studied. The MBHE heat transfer model which considers the heat storage process is developed and embedded in the software of TRNSYS. Based on the numerical simulation model of solar assisted mid-deep ground source heat pump (SAMGSHP) system built by TRNSYS, the long-term characteristics including the variations of the fluid temperature of the MBHE, the ground temperature distribution and the COP of the heat pump unit of the MGSHP system with heat storage during non-heating seasons are analyzed and compared with the case without heat storage. The results show that storing solar energy underground during non-heating seasons can effectively improve the ground temperature, significantly slow down the decline of the circulating water temperature of the MBHE in heating seasons and apparently increase the COP of the heat pump unit. The results can serve as a reference for engineering applications of MGSHP systems, especially those cannot completely meet the original design heating load.

Evaluating an enhanced thermal response test (ETRT) with high groundwater flow

Enhanced thermal response tests (ETRT) enable the evaluation of depth-specific effective thermal conductivities. Groundwater flow can significantly influence the interpretation of ETRT results. Hence, this study aims to critically evaluate an ETRT with high groundwater flow (> 0.2 m d−1). Different approaches in determining the specific heat load of an ETRT are compared. The results show that assuming constant electrical resistance of the heating cable with time can account for an inaccuracy of 12% in the determination of effective thermal conductivities. Adjusting the specific heat loads along the borehole heat exchanger (BHE) depth, the specific heat loads vary within 3%. Applying the infinite line source model (ILS) and Péclet number analysis, a depth–average hydraulic conductivity is estimated to be 3.1 × 10–3 m s−1, thereby, confirming the results of a pumping test of a previous study. For high Darcy velocities (> 0.6 m d−1), the uncertainty is higher due to experimental limitations in ensuring a sufficient temperature increase for the evaluation (ΔT > 0.6 K). In these depths, the convergence criterion of Δλeff/λeff < 0.05/20 h for the ILS sequential forward evaluation cannot be achieved. Thus, it can be concluded that time-averaging of the heat load by monitoring voltage and current during ETRT is essential. Therefore, the specific heat load adjustment along the heating cable is recommended. To improve the estimation of depth-specific effective conductivities with high groundwater flow and to reduce the sensitivity towards temperature fluctuations (ΔT ~ 0.1 K), measures for applying higher specific heat loads during the ETRT are essential, such as actions against overheating of the cable outside the BHE.

Numerical investigation of the deep borehole heat exchanger in medium-depth geothermal heat pump system for building space heating

The deep borehole heat exchanger (DBHE) is a vital part in medium-depth geothermal heat pump system (MD-GHPs). In comparison to other forms of heat exchangers, the DBHE has the following outstanding advantages: the small footprints, the little damage to rock as well as the high efficiency. However, numerous DBHE models were fixed variable values such as flow rate or inlet fluid temperature and there were few studies combining the user heating load variation with the temperature fluctuation on the DBHE side. In order to fill this gap, a DBHE heat transfer model with dynamic building heating load constraint is developed. In this heat transfer model, the load boundary condition is the hourly flow rate and the difference in inlet and outlet water temperature, and the outlet water temperature is obtained through iteration. In addition, this model is validated by a building space heating experiment data in Harbin and guaranteed its correctness. The average heat exchange per meter at the coldest heating moment also ensures the viability of MD-GHPs, which is 141.5 W/m. A series of typical coordinates are used to analyze the effect of DBHE on the surrounding rock, and the concept of DBHE recommended insulation length are proposed. One single DBHE with 2000 m burial depth possesses a temperature effect radius of 13.0 m for one heating period. Besides, some critical conclusions are as follows: For the structural parameter, the recommended insulation length is 178 m. For the operating parameter, there is a minimum value for the total power consumption and 18 m3·h−1 is the recommended optimal flow rate in this situation. For the geological parameter, raising the geothermal gradient can dramatically improve inlet fluid temperature (IFT) and outlet fluid temperature (OFT), and enhance the ability of DBHE to carry the building heating load. When maintaining the same IFT at the coldest moment, the geothermal gradient of 0.04℃·m−1 is 1.33 times more capable of carrying loads than the 0.03℃·m−1. This paper can provide basis for a series of DBHE thermal performance in practical space heating engineering applications of MD-GHPs.

NUMERICAL ANALYSIS OF BOREHOLE HEAT EXCHANGER PERFORMANCE IN SHALLOW GRAVEL AQUIFERS AND CLAY-DOMINATED SOIL

The performance of geothermal heat extraction in shallow aquifers depends on both Borehole Heat Exchanger (BHE) and soil or aquifer properties. In this work, an analysis of the thermal yield of a shallow geothermal reservoir was made numerically with the finite element method used to simulate heat and mass transfer in the three-dimensional reservoir. The main parameters for analysis which have been considered are the geometry and physical parameters of the BHE and grout, as well as aquifer matrix and groundwater fluid. Physical parameters are thermal conductivity, flow conductivity, expansion coefficient, porosity, volumetric heat capacity, anisotropy and dispersivity. The numerical tests have been performed in single BHE line source configuration representing numerically modelled thermal response test for the estimation of sustainable heat extraction. The domain size was a 100x100 meter rectangle with a depth of 200 meters. Three main lithological configurations have been modelled: gravel aquifer with low and high convection of groundwater fluid, as well as a shallow geothermal reservoir dominated by clay material without convection. For selected cases, the analysis for temporal and spatial discretization was also made. Three-dimensional transient modelling was made in FEFLOW® software with pre- and post-processing done in user-defined Python scripts. The results show the most influential parameters to be considered when setting up the real case simulation of geothermal heating and cooling, as well as optimal temporal and spatial discretization set-up with respect to expected thermal gradients in the reservoir.