Ground Heat Exchanger Research Articles

Numerical Study on Efficiency and Applicability of Prefabricated Modular Ground Heat Exchangers in Shallow Depth

This study developed and validated a CFD model to evaluate the performance of modular ground heat exchangers (GHEs) compared to vertical closed-loop GHEs. The reliability of the model was confirmed through comparison with field experiment data, with RMSE values of 0.35°C and 0.27°C for inlet and outlet temperatures, respectively. Additionally, the performance of modular GHEs installed at shallow depths was evaluated under various external environmental conditions, revealing a temperature difference of approximately 7°C between the intermediate and cooling periods. A comparison of the performance of modular and vertical GHEs of the same heat exchanger length showed that, when a constant heat load of 2 kW was maintained for 100 hours, the temperature in the vertical GHE increased to 32.8°C, while the modular GHE reached 38.5°C. Despite the high thermal conductivity resulting from ground heat storage effects, the modular GHE exhibited a greater temperature increase due to its shallow installation depth compared to the vertical GHE. On the other hand, as the thermal conductivity of the ground decreased, the temperature difference between the two systems also decreased. These results suggest that modular GHEs can be a cost-effective and efficient solution, particularly in regions with low ground thermal conductivity.

Numerical Performance Analysis of Modified Vertical U-tube Ground Heat Exchangers by Adjusting the Impact of Leg Spacing

Geothermal energy is a reliable and renewable form of energy that offers steady cooling and heating of buildings without relying on sunlight or wind, avoiding price fluctuations. Ground heat exchangers (GHEs) are designed to utilize geothermal energy for building heating and cooling. The present study conducted a numerical investigation on the thermal performance of Modified Vertical U-tube GHEs. Enhancement of the thermal performance of various configurations of GHEs by adjusting the leg spacing and incorporating spiral configuration of U-tube GHEs. The upper section's leg spacing varies to reduce thermal interference and the lower section's leg spacing is kept constant. Some designs incorporated a combination of spiral and straight U-tubes. All GHE models were considered a 20-meter depth. Performances of these GHEs are evaluated with polyethylene, high-density polyethylene, and low-density polyethylene tubes and boreholes filled with silica sand. The surrounding soil consisted of clay. In some configurations, the lower section was modeled with spiral geometry after 10 meters in depth. The design parameters for the spiral section were 70 mm and 100 mm diameter. The simulations were conducted using the ANSYS FLUENT 22.2 software program for cooling operation. The outlet temperature was lower for spiral configurations than for the other configurations after being simulated for 72 hours. Compared to the traditional U-tube configuration, the heat transfer rate achieved by each arrangement is significantly higher. Results indicated that the combination of spiral shape with traditional shape led to a 31.2% increase in average heat transfer rate. Leg spacing variation opens up the opportunity to combine multiple geometries in a single borehole, also providing the solution to reduce the thermal interference between the inlet and outlet pipe.

Heat transfer of mid-deep ground source heat pump for crude oil gathering and transportation

Wax precipitation in crude oil gathering and transportation (COG&T) causes substantial decrease in oil flowability and thus blockages of pipelines, presenting high safety risks and dramatic economic loss. Conventional oil/gas blending and low-temperature transport are inefficient and energy-intensive. Geothermal energy released from abandoned oil wells may be used as renewable heat source, but remains unexplored. This study focused on feasibility of utilizing geothermal energy from abandoned oil wells by mid-deep ground source heat pump (MD-GSHP), so as to alleviate wax precipitation for short-distance COG&T. Numerical models of ground heat exchanger (GHE) were established by using finite difference method (FDM) based on validation of engineered examples. The results demonstrated that, to enhance geothermal extraction, it was essential to increase fluid flow rate, lower inlet temperature, use materials with high thermal resistance, and optimize the pipe dimensions. Additionally, the impacts of operational parameters of GHE and COG&T subsystems on MD-GSHP system performances were also analyzed. Increasing inlet temperature by 8 °C resulted in 39.5 % reduction in system load and 89.4 % increase in coefficient of performance (COP). This study provides a proof-in-concept demonstration for extracting geothermal energy from abandoned wells by MD-GSHP to mitigate wax precipitation in COG&T project.

A composite analytical model to predict the thermal performance of borehole ground heat exchangers within stratified ground

The thermal performance of borehole ground heat exchangers is of significance to the efficacy of the ground source heat supply systems. This paper presents a new composite analytical model to evaluate the thermal performance of borehole ground heat exchangers within stratified ground. The model integrates critical factors influencing heat transfer, encompassing ground stratification, thermal interactions among boreholes, variability of heat flux along borehole depth, and flexible layouts of boreholes. It enables efficient computation of thermal data pertinent to heat transfer both inside and outside the boreholes during operation. Implemented in the TRNSYS platform, the model offers versatile applicability to ground-source heat pump systems, borehole thermal energy storage, and district heating and cooling networks. Model validations were carried out through four distinct experiments, covering varied borehole quantities and ground conditions, to substantiate its accuracy. Employing this model, a case study further investigated the thermal performance of a borehole field with 16 boreholes in a three-layer ground (comprising backfill soil, clay, and fine sand) extending to a depth of 63 m. Over a 60-day warm water injection period at 30 °C, the effects of borehole layout and ground stratification on the thermal performance of individual and collective boreholes were analysed. Key findings in this analysis include: (1) Across the square, L-shape, and linear layouts, the impact of layout on the thermal performance of borehole fields diminishes as the spacing increases from 1 m, 2 m, and 3 m, becoming negligible at 4-m spacing, and the positions of the most and least affected boreholes by thermal interferences vary by layout; (2) Designing a GSHP system in stratified ground using a homogenous ground model with equivalent thermal properties would overestimate the heat injection performance of the borehole field and lead to undersizing of the borehole ground heat exchangers; and (3) ground stratification should be considered when estimating the thermal performance of a borehole field, particularly when the boreholes are sparsely arranged, e.g., with spacing of 3 m or more.

Development of a doubly-fed compound control for hybrid ground source heat pump systems in hot summer and cold winter areas

In hot summer and cold winter areas, the impact of soil thermal accumulation is significant even for hybrid ground source heat pump systems (HGSHPs), especially after a long-term operation. To alleviate the energy performance degradation caused by soil temperature rise, a detailed temperature field heat transfer model of ground heat exchangers (GHXs) has been established and a doubly-fed compound control for HGSHPs is developed and integrated into a central controller module. The central controller can automatically control the loop flow distribution of units and achieve group activation of GHXs based on the load sequences forecasting feedforward and system variables feedback processes. Five control modes are included in the central controller, and the energy performances and soil temperature rise distribution of the HGSHPs with different control modes for fifteen years are compared. The results indicated that the HGSHPs performs best in terms of energy performance and soil thermal alleviation with Control Unit 13. Compared to the Normal control mode, the mean water temperature of GHXs, energy consumption of the chiller and heat pump decreased by 1.6 °C, 5.9 %, and 15.5 %, respectively, and the units COP increased by 11.3 %.

Heat Transfer Performance Factors in a Vertical Ground Heat Exchanger for a Geothermal Heat Pump System

Ground heat pump systems (GHPSs) are esteemed for their high efficiency within renewable energy technologies, providing effective solutions for heating and cooling requirements. These GHPSs operate by utilizing the relatively constant temperature of the Earth’s subsurface as a thermal source or sink. This feature allows them to perform greater energy transfer than traditional heating and cooling systems (i.e., heating, ventilation, and air conditioning (HVAC)). The GHPSs represent a sustainable and cost-effective temperature-regulating solution in diverse applications. The ground heat exchanger (GHE) technology is well known, with extensive research and development conducted in recent decades significantly advancing its applications. Improving GHE performance factors is vital for enhancing heat transfer efficiency and overall GHPS performance. Therefore, this paper provides a comprehensive review of research on various factors affecting GHE performance, such as soil thermal properties, backfill material properties, borehole depth, spacing, U-tube pipe properties, and heat carrier fluid type and velocity. It also discusses their impact on heat transfer efficiency and proposes optimal solutions for improving GHE performance.

Studying the air flow heating process in the vertical type ground heat exchanger

Studying the air flow heating process in the vertical type ground heat exchanger

Dynamically coupled modelling of ground source heat pump systems considering groundwater flow and unbalanced seasonal building thermal loads

The ground source heat pump (GSHP) system is a promising technology to exploit shallow geothermal energy via ground heat exchangers, GHEs (e.g., deep boreholes or piles) for space heating or cooling. Although GSHP systems have been widely used in temperate regions owing to their environmentally clean characteristics and excellent energy-saving performance, their use in cooling-dominated areas, such as subtropical or tropical regions (e.g., Hong Kong or Singapore), remains limited. The operation of GSHP systems in these regions may be subjected to unbalanced seasonal thermal loads on buildings and induce heat accumulation in soils surrounding GHEs, leading to potential deterioration in the long-term performance (e.g., coefficient of performance, COP) of GSHP systems. To properly evaluate the long-term COP of GSHP systems in cooling-dominated areas, a dynamically coupled simulation approach is proposed in this study. The proposed method integrates building thermal loads with ground heat transfer under groundwater seepage flow, within a unified framework. The effectiveness of this approach is demonstrated through a case study where, in the absence of groundwater seepage flow, the soil temperature increased by 17.2 °C. In contrast, when groundwater seepage flow was considered, the GSHP system not only maintained a high long-term cooling COP but also achieved up to 30 % electricity savings. These findings demonstrate the feasibility and sustainability of applying GSHP systems in cooling-dominated areas with the aid of groundwater seepage flow.

Assessing the impacts of air-sealing on the sizing, operation, and economic feasibility of ground-source heat pumps for electrifying single-family houses in the US

According to recent studies and reports, in single-family houses (SFHs), air-sealing can significantly lower the thermal loads for space heating and cooling. Thus, air-sealing in SFHs could reduce the required size and cost of ground source heat pump (GSHP) systems for electrifying SFHs. This study investigated the costs and benefits of integrating air-sealing with GSHPs for retrofitting existing SFHs when compared with air-source heat pumps. A whole building energy simulation tool integrated with an advanced design tool for modeling ground heat exchangers was used to calculate changes in required GSHP capacity, total borehole length, and building energy consumption with and without air-sealing in SFHs in 16 US climatic regions. The results from this study showed that reducing outdoor air infiltration from 0.8 air changes per hour (ACH) to the minimum ventilation requirement (0.35 ACH) can significantly reduce borehole length (up to 55%), GSHP capacity (up to 48%), and total heating electricity reduction, especially in cold climates (up to 44%). The results also showed that for airtight homes (0.03 ACH infiltration) with a direct outdoor air system, the minimum required borehole length, GSHP capacity, and total heating electricity consumption can be reduced up to 70%, 68%, and 67%, respectively, when compared with SFHs with 0.8 ACH infiltration. Moreover, the life cycle cost analysis showed that air-sealing in conjunction with a GSHP is more profitable than replacing the existing system with an air-source heat pump, even without any incentives for most climatic regions in the US (except for some hot regions).

Composites with Synergistically Enhanced Thermodynamic and Mechanical Properties for Geothermal Heat Exchangers

Geothermal energy, derived from the radioactive decay of elements within the Earth's core, is a renewable and clean energy source. The extraction of geothermal energy primarily depends on ground heat exchangers. To address the issue of incompatibility between corrosion resistance, high thermal conductivity, and mechanical performance in current ground heat exchangers, this study develops composites (PEPOE/SCA@SiC) composed of high‐density polyethylene (HDPE) modified with toughener polyethylene‐octene copolymer elastomer (POE) and silicon carbide (SiC) crystals modified with γ‐aminopropyltriethoxysilane (SCA). HDPE offers excellent corrosion resistance, SiC crystals provide efficient phonon transport, and SCA improves the interface compatibility of the composite. The thermal conductivity of PEPOE/SCA@SiC reaches 3.83 W m−1K−1, with a maximum tensile strength of 20.3 MPa, a tensile elastic modulus of up to 498 MPa, and a maximum bending stress of 22.2 MPa with a bending elastic modulus of 561 MPa. Corrosion resistance tests indicate no signs of corrosion in PEPOE/SCA@SiC when exposed to geothermal simulation fluids at 90 °C for 30 days. This study demonstrates that PEPOE/SCA@SiC composites have substantial potential for application in ground heat exchangers, offering promising prospects for advancing sustainable use of geothermal energy and environmental protection.

Experimental investigation and numerical modeling of an innovative horizontal coaxial ground heat exchanger (HCGHE) for geothermal heat pump applications

Geothermal is an ideal renewable energy source for building heating and cooling via a ground source heat pump (GSHP) system. However, the high initial cost of installing the ground heat exchanger limits its adoption, especially among owners of residential and smaller non-residential buildings. This study aims to develop a new methodology for installing an innovative horizontal coaxial ground heat exchanger (HCGHE) and to investigate its thermal performance in GSHP applications through field testing and numerical simulation. Horizontal directional drilling and a new manifold design were used to install the new HCGHE, significantly reducing costs by minimizing time, materials, labor, and landscape disruption. The transient numerical model, developed in Ansys Fluent using the finite volume method, was validated against experimental data and employed to predict temperature distributions in the supply pipe, return pipe, and surrounding soil. The study determined the heat transfer rate per unit borehole length and entire thermal resistance of the HCGHE to vary from 23.0 to 35.7 W/m and 0.28 to 0.74 K·m/W in winter, and from 24.3 to 37.4 W/m and 0.12 to 0.52 K·m/W in summer. The short-term soil temperature distributions in both the radial and longitudinal directions of the HCGHE are modeled using second-order polynomial equations, with R2 values ranging from 0.994 to near unity, while the long-term soil temperature distributions follow a power law. This research not only confirmed that the innovative HCGHE design and installation can achieve good thermal performance but also provided valuable insights into continual design improvement. The innovative HCGHE-based GSHP system provides an efficient and sustainable solution for heating and cooling buildings by harnessing the earth’s natural heat, offering the advantages of simplified installation and reduced initial investment.

Performance analysis of a solar-assisted ground source heat pump with a single vertical U-tube ground heat exchanger

Owing to higher heat loads than cooling loads in cold climates, the performance of ground source heat pumps (GSHPs) can significantly degrade over time, leading to eventual system failure for systems not properly sized or when there is insufficient regeneration. Solar-assisted ground source heat pump (SAGSHP) systems are receiving increasing interest as a means of enhancing heating performance and maintaining ground thermal balance in cold regions. However, a critical gap remains in the literature: no study has comprehensively analyzed configurations of SAGSHP systems to enhance their overall performance considering long-term performance and realistic operating conditions. For this purpose, a comprehensive investigation of the long-term performance of single U-tube and double U-tube ground heat exchangers (GHEs) for solar-assisted ground source heat pump systems (SAGSHPs) was carried out and compared with the conventional GSHP. The effect of collector tilt angles (0–90°) on the performance of the single U-tube SAGSHP was also examined. A finite volume computational fluid dynamic model was developed and coupled with an actual building energy load profile and a solar irradiation profile based on Calgary’s weather conditions using a novel custom-coded thermodynamic model. Results show that the yearly average heating coefficient of performance (COP) increases by a factor of 1.21 and 1.18 for the single U-tube SAGSHP and double U-tube SAGSHP, respectively, when compared with the conventional GSHP. However, the average annual cooling COP reduces by 0.65 for the single U-tube and 0.72 for the double U-tube SAGSHPs. Moreover, considering the effect of tilt angle on the single U-tube SAGSHP, it is possible to improve the cooling COP when the collector tilt angle is 90°, at the expense of lower heating COP. However, the highest heating COP and annual energy savings are achieved with a tilt angle of 45°. This study provides practical insight into SAGSHP performance enhancements in cold climates.

Performance of thermally enhanced backfill material in vertical ground heat exchangers under continuous operation in a cooling season

Backfill materials in vertical ground heat exchangers significantly affect the heat-transfer efficiency of ground-source heat pump systems. In this study, the use of graphite improving the thermal conductivity of backfill materials is investigated via laboratory tests, and its feasibility is verified via in-situ thermal response testing. Based on the results, the relationship between the coefficient of performance (COP) and thermally enhanced backfill materials is built through a numerical model. The variation of COP under different thermal-physical soil conditions and backfill materials with enhanced thermal conductivities is investigated; then a method for selecting backfill materials is proposed by matching thermal properties of backfill material and surrounding soil. The results indicate that when the soil thermal conductivity is at a relatively low level of 1.0 W/(m·K), the highest COP improvement ratios range between 7.99 % and 16.13 % during the 90 days operation However, the COP improvement ratio decreases gradually as the soil thermal conductivity increases, and the COP improvement ratios decrease to less than 1.0 % as the soil thermal conductivity increases to 5.0 W/(m·K). Parameter-design charts are made to intuitively illustrate the COP variation. The result of contributions of influencing parameters shows that high-velocity groundwater seepage contributes positively to COP improvement, followed by thermal conductivity of soil, and then that of backfill material during a cooling season.

Geologic and thermal conductivity analysis based on geophysical test and combined modeling

Strata thermal conductivity (λe) influences efficiency of medium-depth ground heat exchangers. However, a systematic approach to the acquisition of λe is still lacking. Hence, stratigraphic heat conduction was analyzed using geophysical test in Shenyang, combined with composite modeling. Cenozoic strata (0–730m) exhibited an average λe of 1.32 W·m−1·K−1 due to higher porosity and mud content; Archaean (730–2500m) strata displayed a higher λe of 2.80 W·m−1·K−1, due to dense quartz sandstone with lower porosity and mud content. The CBHE in Shenyang can achieve excellent heat extraction relying on good thermal conduction. Spearman correlation revealed strong negative correlations between λe and acoustic time difference, permeability, and porosity, while positive correlations were observed with depth, resistivity, wave speed, and rock skeleton. Higher rock skeleton thermal conductivity intensified the decrement effect of mud content on λe, while higher mud content diminished the enhancement effect of rock skeleton thermal conductivity. As porosity increased, the decline in λe slowed, particularly pronounced at higher saturation levels. λe exhibited a gradual decrease with temperature, with a more notable change rate observed at lower porosity levels. Principles for enhancing thermal conduction were elucidated through a three-phase analysis. These findings provide foundation for the study and design of medium-depth boreholes.

Comparison of the Energy Contributions of Different Types of Ground Heat Exchangers Related to Cost in a Working Ground Source Heat Pump System

Geothermal systems face adoption challenges due to their high initial investment cost. Accurate cost analyses and a more precise understanding of updated prices could assist geothermal industry projects in obtaining investment financing and better money management with the right equipment. As the cost of geothermal installations can vary widely depending on case and location, it seems essential to clarify the factors and parameters that determine the cost of the system. These include the type of loop system, the ground conditions, the type of heat pump, the system size, and the geographical location. The scope of this study is to compare the operation of various types of ground heat exchangers (GHEs) present in a Ground Source Heat Pump (GSHP) system installed in the coastal area of the Mediterranean climate zone of Cyprus. The highlight of this work is that it presents real installation cost data as well as recorded total energy contributed by the GHEs to the GSHP system of a HP cooling and heating capacities of 101 kW and 117 kW, respectively. The input contribution from the GHEs to the HP is 85,650 kWh (308,340 MJ) in summer and 25,880 kWh (93,168 MJ) in winter. It is shown that, among the three groups of GHEs investigated, the open-well GHE complex has the lowest cost per kWh ratio (0.32 EUR/kWh), followed by the vertical GHE complex (1.05 EUR/kWh), and lastly by the helical coil GHE (2.77 EUR/kWh). This clearly suggests that when underground water is available, the open-well GHE is much more favorable than other GHE types.

Research on the heat transfer model of double U-pipe ground heat exchanger based on in-situ testing

Research on the heat transfer model of double U-pipe ground heat exchanger based on in-situ testing

Analytical model development for vertical medium/deep ground heat exchangers with U-tube(s)

The U-shaped ground heat exchanger (UGHE) offers an effective solution for building heating and cooling purposes. In the practical applications of vertical UGHEs, the borehole wall temperature varies along the borehole depth due to the uneven initial ground temperature and transient heat transfer process. The assumption of a uniform temperature along the borehole wall in the existing quasi-3D model of UGHE causes a non-negligible error in the calculation of fluid temperature, especially for the cases of shallow-medium boreholes. Thus, two new analytical models are established to calculate the fluid temperatures for the single U-shaped GHE (1-UGHE) and double U-shaped GHE (2-UGHE), considering the depth-dependent borehole wall temperatures. The convolution theorem is employed to solve the mathematical problem of the nonlinear condition on the borehole wall, and the Laplace transformation is used for getting the final temperature expressions. The new analytical models are compared and verified by the on-site experiments. The results show that the two models exhibit promising prediction accuracies and similar variation trends in fluid temperatures. Then, parametrical and economical analyses for 1−/2- UGHEs have been carried out. Finally, the geographical thermal potentials of 1−/2- UGHEs across China have been comparatively investigated and the corresponding thermal map-based application potential recommendations have been given.

ROGER: A method for determining the geothermal potential in urban areas

Shallow geothermal energy is increasingly adopted for heating and cooling purposes because of the short pay-back time of initial installation investments. As a result, a relevant concentration of Ground Heat Exchangers is being experienced in urban areas. Planning issues thus arise to manage interferences and optimize the use of underground heat resources without depletion, harm to the environment nor efficiency losses on heat pumps or plant oversizing. This study provides a rational approach to optimise geothermal resources based on the use of Geographic Information Systems and transient 3D Thermo-Hydro numerical models. An optimised semi-analytical formula for the assessment of Borehole Heat Exchangers geothermal potential in hydrodynamic conditions is developed through a parametric numerical study. The long-term performances of BHE subjected to groundwater velocity in the range of 0 to 1 m/day were analysed with multiple aquifer thermal parameters. This analytical expression allows a fast and accurate assessment of the potential even in large areas without leading to excessively conservative evaluations. This may serve designers in the preliminary sizing of installations and city planners in the development of appropriate policies for the promotion and management of shallow geothermal resources. An example of the application to the central district of the city of Turin (Italy) is also shown.

Multi-objective optimization and long-time simulation of a multi-borehole ground heat exchanger system

Multi-objective optimization and CFD simulation are conducted to optimize the design of a multi-borehole ground heat exchanger (GHE) system and assess its long-time performance. The multi-objective optimization is performed to minimize the entropy generation number (EGN) and total cost rate by using various evolutionary algorithms, including NSGA-II, GDE-3, MOEA/D, PESA-II, SPEA-II, and SMPSO. NSGA-II and GDE-3 algorithms perform best in obtaining Pareto optimal solutions. Three prominent points on the NSGA-II Pareto frontier, representing the results of single-objective thermodynamic, single-objective economic, and multi-objective optimizations, are simulated in three dimensions over three months. The trends of EGN variations extracted from the transient CFD simulation agree well with those from the steady analytical model. The EGN obtained from multi-objective optimization is 58.8% lower than the EGN obtained using single-objective economic optimization and 1.9 times higher than that calculated from single-objective thermodynamic optimization. Likewise, the total cost rate obtained from multi-objective optimization is 64.4% lower than the value obtained from single-objective thermodynamic optimization and four times higher than that calculated using single-objective economic optimization. The proposed optimization approach can be reliably applied to improve the design of multi-borehole GHE systems.

The role of pit lake thermal dynamics on the thermal performance of ground heat exchangers

The addition of ground heat exchangers (GHEs) to a pit lake’s basin has the potential for abundant, clean and renewable geothermal energy extraction using shallow geothermal systems. Basin-embedded GHEs avoid direct interaction with mine water, which has been shown to impact efficiency and longevity in mine open-loop geothermal systems negatively. The now accelerated closure of open-pit coal mines presents itself as an opportunity to use this technology. However, no guidelines currently exist for designing or operating GHEs embedded in the sediment of water bodies. Furthermore, the two-way coupling between the complex annual thermal fluid dynamics that lakes are naturally subjected to and heat fluxes on the sediments and the GHE system has not been explored. In this study, we develop and validate finite element models to assess the relevance of lake thermal stratification in the performance of a geothermal system embedded in water bodies basins, e.g., on open-pit mine closures, under temperate residential thermal loads. The results show that the pit lake’s role as a thermal sink improves significantly when the lake’s thermal dynamics are accounted for, with an increase of up to 292% in the lake’s available energy budget. A minor variation in energy budget (~8%) was found whether the lake is modelled explicitly or simplified as a transient Dirichlet temperature boundary condition. This small difference vanishes if horizontal circulation along the lake is considered, highlighting the lake’s thermal energy potential. Finally, the impact on the GHE Coefficient of Performance (COP) is evaluated, with a maximum of ~15% difference among all cases.