Vertical Ground Heat Exchanger Research Articles

Investigation of the Long-Term Performance of Waste Backfill Materials of High Thermal Conductivity in Vertical Ground Heat Exchangers

Investigation of the Long-Term Performance of Waste Backfill Materials of High Thermal Conductivity in Vertical Ground Heat Exchangers

Heating test of a silty sand in a short cylinder with a vertical hydronic grouted heating pipe

Ground heat exchangers (GHEs) consist of HDPE pipes embedded in the ground to exchange thermal energy with the surrounding soils. Vertical GHEs are backfilled with grout materials to seal the soil boreholes, which are critical to the heat transfer at the pipe and soil interface. In this study, a short vertical soil cylinder was developed to measure the heat transfer process at the soil and pipe interface. The soil test cylinder consists of a 30 cm tall acrylic cylinder with a vertical 2.5 cm diameter (ID) HDPE pipe at its center. A grout of 85% bentonite and 15% graphite was compacted at 36% moisture content around the HDPE with a 1.4 cm radial thickness. A silty sand was chosen as the testing soil and compacted into the cylinder at 9% moisture and a dry density of 1.26 g/cm3. Moisture, temperature, and heat flux sensors were installed along the radial direction to measure the heat transfer process. The HDPE pipe was connected to a temperature-controlled water bath for a heating source at the specified constant temperature. The heating test of the silty sand with and without grout was compared. The thermal contact resistance at the pipe-grout interface was determined. The results demonstrated the impact of thermal grout on the heat transfer of GHEs, and the findings will help improve its design.

A Simplified Numerical Model as a Design Tool for Vertical Single U-Tube Ground Heat Exchangers

A Simplified Numerical Model as a Design Tool for Vertical Single U-Tube Ground Heat Exchangers

The influence of borehole lengths on a numerical model of a double-tube vertical ground heat exchanger

GHEs are underground pipes that transmit heat between the earth and pipes. ANSYS Fluent was used to numerically investigate the thermal and hydraulic performance of a vertical twin tube ground-linked heat exchanger in a pile foundation in cooling mode. The CFD analysis uses three borehole length models—M1-10 m, M2-15 m, and M3-20 m—with flow rates from 1-100 lit/min and Reynolds numbers from 162 to 1625. The numerical results show that the average heat transfer rate of the Model M1 was 13% higher than that of the Model M2, 32% higher than that of the Model M3, and 95% higher than that of the Model M2. The pressure drop was minimal for 1 to 5 lit/min (laminar flow condition) but very high for 10 to 100 lit/min. The bore cavity length increased the pressure drop and decreased the average output temperature. These two-tube vertical GHEs with optimized length and selection of pipe materials can improve natural convection heat transfer by contacting ground waters.

Comparative Analysis of Methods for Predicting Brine Temperature in Vertical Ground Heat Exchanger—A Case Study

This research was carried out to compare selected forecasting methods, such as the following: Artificial Neural Networks (ANNs), Classification and Regression Trees (CARTs), Chi-squared Automatic Interaction Detector (CHAID), Fuzzy Logic Toolbox (FUZZY), Multivariant Adaptive Regression Splines (MARSs), Regression Trees (RTs), Rough Set Theory (RST), and Support Regression Trees (SRTs), in the context of determining the temperature of brine from vertical ground heat exchangers used by a heat pump heating system. The subject of the analysis was a public building located in Poland, in a temperate continental climate zone. The results of this study indicate that the models based on Rough Set Theory (RST) and Artificial Neural Networks (ANNs) achieved the highest accuracy in predicting brine temperature, with the choice of the preferred method depending on the input variables used for modeling. Using three independent variables (mean outdoor air temperature, month of the heating season, mean solar irradiance), Rough Set Theory (RST) was one of the best models, for which the evaluation rates were as follows: CV RMSE 21.6%, MAE 0.3 °C, MAPE 14.3%, MBE 3.1%, and R2 0.96. By including an additional variable (brine flow rate), Artificial Neural Networks (ANNs) achieved the most accurate predictions. They had the following evaluation rates: CV RMSE 4.6%, MAE 0.05 °C, MAPE 1.7%, MBE 0.4%, and R2 0.99.

Fast segregation of thermal response functions in short-term for vertical ground heat exchangers

A detailed understanding of the short-term performance of the ground heat exchanger (GHE) is a major concern in the design of ground-source heat pumps (GSHP), as it has a significant impact in the efficiency and costs of the facility. The use of numerical models or artificial intelligence techniques may help in the short-term, although they are generally very time-consuming, becoming unpractical in numerous cases. The aim of this work is to obtain the thermal response factors according to the GHE, segregating the transient borehole behavior from the soil effect, based on a previous study which applies experimental data from thermal response tests (TRT) to the finite line source (FLS) model. The proposed method implies an extensive study of the fluid temperature as well as the development of new auxiliary thermal resistances related to the borehole and fluid, avoiding the implementation of numerical models. This procedure has been validated with the temperature outcomes of a 3D numerical model, obtaining low deviations, into a range of ±0.1 K, and mean absolute deviations below 0.01 K.

Investigation on the influence of mixed borehole depths in vertical ground heat exchanger systems

Most studies about vertical borehole heat exchangers (BHEs) tended to focus on optimizing total borehole length with all boreholes having the same depth. This is mainly because most of the vertical BHEs design tools do not have the feature of designing mixed borehole depths at the same bore field. Because thermal saturation occurs over time at the center of the field due to thermal interaction between the confined boreholes in the interior region, reducing depth of these less efficient boreholes is a desired goal. This paper studies the effects of mixed borehole depths where shorter boreholes are designed to be in the interior region, and deeper ones in the perimeter. The python package pygfunction has been adapted in this paper to generate the thermal response factor of a bore field with mixed borehole depths and to simulate its performance in difference design scenarios. The results reveal that total BHE-length could be decreased by up to 10 % in some cases of mixed borehole depths compared with the case of same-depth boreholes. However, since the total BHE-length could increase in few cases, careful examination is crucial when applying mixed borehole depths.

Vertical coaxial ground heat exchangers with teeth and grooves on their outer pipes; a hydrothermal investigation

The negative characteristics of fossil fuel consumption make it necessary to consider renewable energy sources that have lower costs and also justify increased environmental consequences. In this study, the performance of a vertical coaxial ground heat exchanger, as a renewable energy system, with teeth and grooves on its outer pipe is investigated. This study aims to investigate the effect of important parameters, including shapes of the grooves and teeth, internal and external pipe materials, heat transfer, and heat losses on the performance of coaxial heat exchangers. Simulation is performed for the heat absorption mode of operation. The SST k-ω turbulence model is applied for the simulation of the turbulent flow. The results are presented for the specific temperature/soil conditions in this study. By using the teeth and grooves with different geometries on the surface of the outer pipe, the heat transfer inside the heat exchanger is improved. With triangular teeth and grooves with depths of 5 mm, the best thermal performance among different types of geometry is obtained and the fluid inlet and outlet temperature difference has increased by 6.5 %. The highest pressure drop of 10.9 kPa occurs in the heat exchangers with triangular teeth and grooves with a depth of 5 mm. Values of local Nusselt numbers in the annular region of the heat exchanger have been calculated at different depths.

Multi-Criteria Study on Ground Source Heat Pump with Different Types of Heat Exchangers

Heat pumps are currently one of the most frequently applied heat sources in residential buildings. Ground source heat pumps are more reliable than air source heat pumps in terms of energy efficiency, especially in colder climates. However, they are more expensive and involve increased material inputs; therefore, multi-criteria analyses taking into account environmental and economic aspects seem necessary for the green design of these systems. The aim of this work was to analyze the environmental and economic impacts of the ground source heat pump providing heating for a family house located in eastern Poland, cooperating with three types of ground heat exchangers (each in two sizing options): helix, vertical and horizontal. The multi-criteria analysis was based on the life cycle assessment methodology using IMPACT 2002+ and life cycle costs methods. The lowest environmental impact was reported for the variants with vertical ground heat exchangers, mainly due to their high efficiency in the operation stage. On the other hand, the lowest economic impact was observed for the horizontal heat exchangers, which are not demanding in terms of material and construction costs. Final recommendations based on multi-criteria analysis propose the vertical probes as a sustainable solution, with a weighted sum indicator in the range 0.085–0.297 on 0–1 scale.

Optimizing vertical ground heat exchanger modelling through GPU-accelerated computation strategies

Enhancing Vertical Ground Heat Exchanger (VGHE) modelling for efficient Vertical Ground-Coupled Heat Pump (VGCHP) systems, renowned for their eco-friendly operation, has garnered significant attention. Optimally designing and sustaining the long-term efficiency of VGCHP systems necessitates rigorous, protracted VGHE modelling, a process fraught with computational intensity. To surmount this challenge, this contribution introduces an accelerated VGHE modelling paradigm grounded in Graphics Processing Unit (GPU) processing. This entails employing the Finite Line Source (FLS) model based on Fast Fourier Transform (FFT). To ameliorate computational demands, the processing capabilities of both Central Processing Units (CPUs) and GPUs are harnessed, leveraging CPU- and GPU-based FFT and inverse FFT (IFFT). Hybrid and non-hybrid processing modes are explored, differentiating between CPU- and GPU-based FFT in non-hybrid scenarios, while hybrid approaches combine both for distinct simulation periods. By scrutinizing two processing systems with varying specifications, diverse CPU- and GPU-based FFT combinations are assessed. Findings unequivocally underscore the superiority of CPU-based FFT for initial years, succeeded by GPU-based FFT for ensuing years, in terms of computational efficiency. The efficacy of the hybrid computation approach is demonstrated through a reduction of total computational time in 20-year VGCHP operation simulations by up to 33.5 % compared to conventional CPU-based FFT methods.

Cooling Potential Evaluation of a Ground Heat Exchanger in a Tropical Climate: A Case Study of an Office Building

The main implications of the hot-humid climate characteristics stipulate the need for cooling indoor environments. Ground thermal characteristics can be exploited in the search for sustainable ways to cool the indoor environment. Thus, this work aims to assess the cooling potential of the ground thermal characteristics through a vertical ground heat exchanger (VGHE) in a tropical climate such as Panama City via dynamic simulation. This potential is evaluated in a two-stories office building complying with Panama’s building regulation requirements, where a VGHE system is designed to cover the cooling needs. Since underground temperature data are unavailable, parametric and sensitivity analyses are carried out. For sizing the VGHE, a vertical ground system is chosen since this allows a better configuration in the installation for a building with large cooling demands and eliminates the need for large amounts of land. A difference of 33.52% (4481.4 kWh) in the electricity consumption for cooling was obtained compared to a conventional air conditioning system, which translates into a 770.80$ saving (0.172$/kWh). These results confirm that the system studied for air conditioning is feasible since it could provide positive results in thermal comfort and energy savings for the user.

Study on configurational operation strategy of ground heat exchangers under the effect of groundwater flow

In large-scale ground source heat pump (GSHP) systems, the accumulation of thermal energy resulting from seasonal load imbalances has emerged as a significant impediment. The implementation of a rational operation strategy can effectively mitigate heat accumulation in soil. This study investigates the impact of the configurational operation strategy for large-scale vertical ground heat exchanger (GHE) on the spatial distribution of ground temperature under the influence of groundwater flow. The internal heat source model is proposed to describe heat transfer process of GHE. Sandbox experiment is conducted to verify the model with maximum deviation of 3 %. The validated model is then utilized to simulate soil temperature profile in the large-scale GHE field over a 10-year operational period, considering a matrix of three configuration strategies and three operation strategies. The results shows that trapezoidal configurations combined with operation Strategy 3 has a minimal temperature increment of 2.9 °C in soil temperature. The contribution of this study lies in provision of a methodology for mitigating thermal accumulation in large-scale GHE field.

Simulation of borehole thermal energy storage (BTES) systems using simplified methods

This paper presents two complementary approaches for simulating the thermal performance of borehole thermal energy storage (BTES) systems. The first approach uses the concepts of heat exchange and storage efficiencies as a function of the state-of-charge of the BTES. The second method employs a technique similar to thermal response factors used to model bore fields in ground-source heat pump systems. It utilizes non-dimensional average storage temperatures and bore field thermal resistances of the BTES over time. The two methods rely on numerical experiments to obtain the required performance curves. As shown through examples, the two approaches are relatively simple to use, and it is shown that they are in good agreement with the well-known duct ground storage model. In one of the examples, it is suggested that the design of a BTES could be performed using three successive thermal pulses, similar to the approach used to determine the length of vertical ground heat exchangers in a ground-source heat pump systems.

Topology optimization of geothermal bore fields using the method of moving asymptotes

A new method is proposed to optimize the configurations of vertical ground heat exchangers using topology optimization and the method of moving asymptotes. The problem formulation minimizes the required number of boreholes of a given length with simultaneous constraints on the entering fluid temperature in cooling and heating mode. The method relies on the adaptation of the alternative ASHRAE design method and a new analytical formulation for the calculation of -functions based on a boundary condition of equal average fluid temperature. The method is applied to large bore fields consisting of more than a hundred boreholes. The paper also studies the effect of different types of domains and ground load profiles on the optimized configurations. It is shown that the shape of the domain has minimal impact on the optimized configurations. A higher density of boreholes on the perimeter of the domain compared to the center is recommended in the case where one operation mode is dominant. A uniform configuration is better suited when the load profile is balanced.

A novel study on influence of ground surface boundary conditions on thermal performance of vertical U-shaped ground heat exchanger

To better understand the influence of ground surface boundary conditions (GSBC) on the thermal performances of U-shaped ground heat exchangers (UGHE), this study proposes a numerical model with one energy balance boundary condition (EBBC) considering the spectrally selective properties of ground. This study comprehensively discusses the effects of operation and design parameters on the thermal responses of various GSBCs and UGHE. Additionally, the geographical performances of the EBBC are compared. The main results indicate that implementing EBBC in UGHE reduces prediction errors for water and borehole wall temperatures, with a maximum decrease of 1.70 °C and 1.78 °C, respectively. Regarding the average water temperature, the variation induced by different GSBCs can be reduced by 51.0% as the borehole depth increases from 50 to 125 m. Similarly, the difference induced by different GSBCs can be reduced by 22.7%, with ground thermal conductivity decreasing from 3.0 to 1.5 W/(m·K). Based on the geographical analysis, the convective GSBC can be imposed on UGHE for engineering approximation in only temperate and tropical regions. For other climate regions, the use of EBBC is recommended. Finally, the sensitivity analysis results indicate that borehole depth is the most important dependent variable, followed by ground conductivity and volumetric heat capacity.

Application of a Model Based on Rough Set Theory (RST) for Estimating the Temperature of Brine from Vertical Ground Heat Exchangers (VGHE) Operated with a Heat Pump—A Case Study

This work presents the results of a study that used a model based on rough set theory (RST) to assess the brine temperature of vertical ground heat exchangers (VGHEs) to feed heat pumps (HP). The purpose of this research was to replace costly brine temperature measurements with a more efficient approach. The object of this study was a public utility building located in Poland in a temperate continental climate. The building is equipped with a heating system using a brine–water HP installation with a total capacity of 234.4 kW, where the lower heat source consists of 52 vertical ground probes with a total length of 5200 m. The research was conducted during the heating season of 2018/2019. Based on the data, the heat energy production was determined, and the efficiency of the system was assessed. To predict the brine temperature from the lower heat source, a model based on RST was applied, which allows for the analysis of general, uncertain, and imprecise data. Weather data, such as air temperature, solar radiation intensity, degree days of the heating season, and thermal energy consumption in the building, were used for the analysis. The constructed model was tested on a test dataset. This model achieved good results with a Mean Absolute Percentage Error (MAPE) of 12.2%, a Coefficient of Variation Root Mean Square Error (CV RMSE) of 14.76%, a Mean Bias Error (MBE) of −1.3%, and an R-squared (R2) value of 0.98, indicating its usefulness in estimating brine temperature. These studies suggest that the described method can be useful in other buildings with HP systems and may contribute to improving the efficiency and safety of these systems.

Long-term performance prediction of ground source heat pump system based on co-simulation and artificial neural network

Ground source heat pump (GSHP) systems are efficient and widely used form of shallow geothermal energy utilization. In practical application, GSHP systems face the problem in predicting the long-term performance and energy efficiency variations in GSHP systems in regions with thermal imbalances. In this study, a coupling heat transfer model of vertical ground heat exchanger (GHE), heat pump, and building load was established to achieve refined simulation calculations of the long-term operation of GSHP systems. Parametric modeling was conducted by employing the co-simulation of COMSOL and MATLAB, enabling dynamic simulations of 600 sets of GSHP system models throughout the entire year. A database was established by extracting the numerical results, and a prediction model was developed using artificial neural network (ANN) methodology to learn from the database. The model predicted the ground temperature, seasonal energy efficiency ratio (SEER), seasonal coefficient of performance (SCOP), annual electricity consumption, and annual energy savings of the GSHP system and the prediction model was valid. The results were used to estimate the ground temperature, coefficient of performance (COP), payback period for static investment, and carbon dioxide emission reduction. Finally, the prediction model was applied to a GSHP system engineering project in Yancheng, China, forecasting the operation performance of the system for 10 years. The accuracy of ANN in predicting 10 years of long-term performance is verified.

Temperature Field Modelas a Factor to Improve a Horizontal Ground Heat Exchanger of a Heat Pump System: Review of Scientific Approaches and the Content of the Analytical Model

The article is devoted to the research of issues related to the organization of control systems for heat pump installations (HPS). HPS is used to use low-potential energy of the environment in the heat supply of buildings. HPS with horizontal ground heat exchangers (HGHE) are of great importance for improving energy efficiency. The efficiency and reliability of HPS with HGHE strongly depends on the state of the soil temperature field. A large number of scientific publications are devoted to modeling soil temperature fields forHGHEparameterdetermination; a significant part of them is devoted to the study of vertical ground heat exchangers. Scientific developments presented in foreign publications on modeling the temperature field of the HGHE location environment are focused on the use of HPS in the cold supply mode and/or in the heat supply mode under conditions of a slight decrease in temperature, which excludes soil freezing. The aim of the study was to apply the existing methods of the analytical theory of thermal conductivity to develop a soil-environment TP model for the placement of a HGHE HPS; the developed model is intended for engineering calculations of the main design HGHEparameters; the model is also applicable to determine the annual specific heat flow as an important parameter in the optimal control system of the HPI. The development of the model is based on the condition of a two-element structure of the soil massif: under the HGHE and above the HGHE; the target parameter is the temperature of the soil mass, depending on the coordinate and time, disclosed through the indicator 'relative temperature rise'; in accordance with the provisions of the thermal conductivitytheory, the relative excess temperature is given for the cases of a semi-infinite body (soil mass under the HGHE) and a slab (soil mass above the HGHE). The proposed model is implemented on the example of two constituent entities of the Russian Federation. The results obtained made it possible to determine the minimum guaranteed possibility of using HPS for heat supply in these climatic conditions. The model presented in the article can be used as a context for further studies of the main parameters of HGHE HPS operation and optimization of HPS operation modes.

Study on heat transfer performance of a ground heat exchanger under different heat transfer mechanisms

Taking a ground source heat pump operating in summer as a case study, the heat transfer performance of a ground heat exchanger was studied under different heat transfer mechanisms, considering stratified soil conditions and only considering thermal conduction. It was concluded that when the aquifer was located in the lower layer of stratified soil, the heat transfer performance of the ground heat exchanger is improved by 4.07%–37.24% in the most favourable case and by 4.11%–51.3% in the least favourable case when water migration in unsaturated soil and seepage in saturated soil are incorporated. The influence of aquifer position on the heat transfer performance of the vertical ground heat exchanger was also analysed. The results show that the heat transfer performance of the ground heat exchanger is improved by 1.05%–2.0% and 0.35%–1.61%, respectively, when the aquifer is located in the upper and middle layers of stratified soil rather than in the lower layer of stratified soil. Using the results of this paper to revise some existing studies that do not consider complex soil conditions, more accurate and practical conclusions can be obtained.

Preparation of wells using an electrohydraulic drill and modeling of heat transfer processes in heat transfer elements of a heat pump

The article discusses the use of innovative electrohydraulic technology in the preparation of vertical ground heat exchangers to create heat supply using heat pumps. The discharge energy in the operating range varied E=900÷2600 J. The most economical mode of operation of the electrohydraulic installation is carried out with the following parameters: discharge voltage 30÷51 kV, capacitor bank capacity 2 µF and interelectrode distance 8÷15 mm. It is determined that the optimal energy of destruction of natural materials depends on their thickness. Based on the research, the limits of the electrophysical parameters of the approach were established, at which the concentrated destruction of hard rocks – the main stones ‒ begins. The effective amount of energy for the destruction of stones with a thickness of 53 mm is 900 J. For stones with a thickness of 78 mm, there should be 2600 J. If to increase the energy of the discharges to 1837 J, the process of complete destruction of the stones is underway. Using an electro hydraulic drill, we have prepared vertical wells up to 25 m deep. In the future, we have installed special temperature sensors along the length of the pipes, inside the well. Using a software product (Temp Keeper), data from temperature sensors are received in real time. This program allows to visually monitor temperature changes occurring at a local point. Temperature and time dependences are also obtained at coolant velocities of 0.2÷0.4 m/s. A significant decrease in the temperature in the well and at the outlet was observed at a coolant velocity of 0.35 m/s for one hour. The study conducted on the basis of the data obtained enables to achieve higher optimal drilling performance compared to conventional installations