Borehole Field Research Articles

Laboratory and numerical study on innovative grouting materials applicable to borehole heat exchangers (BHE) and borehole thermal energy storage (BTES) systems

In this study, a laboratory-scale prototype of a borehole field has been designed and built to assess various innovative grouting products in a fully controlled environment. Three novel grout formulations are developed and evaluated: enhanced grout, a mixture of enhanced grout and microencapsulated phase change material, and a mixture of enhanced grout and shape stabilized phase change material. The objective is to evaluate the enhancement in their thermal properties (i.e., thermal conductivity and thermal energy storage capacity) compared to those using a commercial reference grout. Besides, three-dimensional numerical modeling is performed to provide a better understanding of the heat transfer and phase transition inside and outside the grout columns and to study the capability of the developed grouts to be used in a borehole heat exchanger or as borehole thermal energy storage system. To the best of the authors' knowledge, there have been just a few numerical studies on using phase change materials inside borehole heat exchangers to assess thermal energy storage applications. The experimental and numerical results showed much higher efficiency of the grout developed with a high thermal conductivity than the reference grout in terms of heat transfer in both the grout column and the surrounding sand. Furthermore, the results indicated the noticeable influence of the microencapsulated phase change material's presence in the grout formulation in terms of heat absorption/storage during the phase transition (from solid to liquid). However, it is concluded that reengineering shape stabilized phase change material should be conducted to make it more appropriate for thermal energy storage applications.

Long-term performance investigation of a GSHP with actual size energy pile with PCM

• Long term Performance investigation of ground source heat pump system. • Energy pile coupled with phase change materials under an actual building load. • Optimization of Phase change materials’ location inside the energy pile concrete shell. • Effect of utilizing multiple PCM melting temperature inside a single energy pile. The use of geothermal energy has increased significantly (90 time) since 1995. Among these increases, Ground Source Heat Pumps (GSHP) has contributed by 40 times in an effort to reduce the burning of fossil fuels and contribute to the reduction of green house gas emissions. The space requirements and high initial cost of borehole fields hinder the widespread use of GSHPs. The use of foundation piles, used as a ground heat exchanger, has been proposed to overcome these limitations. Research has shown that foundation piles have a lower depth and smaller spacing compared to boreholes. Phase Change Materials (PCMs) have been proposed as a potential solution to increase the storage capacity and decrease the thermal radius of energy piles. In the current study, a 3-D finite element model was developed to study the effects of PCMs on the performance of energy piles against a real building load for a complete year, while integrating an actual heat pump (HP) performance curve into the numerical model. The effects of the PCM location and melting range were also investigated. The study revealed a 5.2% enhancement in the Coefficient of Performance (COP) during the melting of the PCM, and a negative effect of up to 1.8% during the completely solid-state. Locating the PCM cylinders inside of the concrete shell led to better performance compared to when the PCM cylinders were located outside of the concrete shell. The best locations were found to be between the center of he pile and the U-loop. The PCM melting range of (4–6)°C was better than the melting range of (1–3)°C for the current study load. Lastly, the use of multiple PCM melting temperatures for a given design was investigated. The results revealed that the use of multiple PCM melting temperatures led to a performance enhancement of up to 26%.

Thermal interactions among vertical geothermal borehole fields

Borehole heat exchanger applications are growing every day – so pronouncedly that many borehole fields will likely operate in dense installation areas in the foreseeable future. In these conditions, thermal interactions will have the potential to develop between neighboring fields, with detrimental effects for the performance and efficiency of such installations in the absence of appropriate designs strategies. Currently, scarce knowledge is accessible on thermal interaction effects among borehole heat exchanger fields and a few approaches allow to effectively consider such effects in the design of these heat exchangers. This paper expands the limited competence on thermal interaction effects among borehole heat exchanger fields and presents and validates a methodology to facilitate the design of such heat exchangers. With reference to a virtual operation of multiple vertical borehole fields in the Loop district of Chicago, USA, the work highlights significant thermal interactions among neighboring borehole fields, responsible for temperature drops of up to 5.9 °C during geothermal operations lasting 50 years. The proposed methodology allows correcting the design of borehole fields to avoid overexploitation of the geothermal resource and the occurrence of litigation cases between geothermal users, representing a powerful approach to cope with thermal interaction effects among geothermal boreholes at scale.

DETERMINATION OF GROUNDWATER POTENTIAL AREAS IN KADUNA METROPOLIS, KADUNA, NIGERIA

Groundwater is one of the major resources important for sustainable growth and development. Proper management of the resources is therefore important to meet all the water requirements for human existence to continue. In this study, an investigation is made to explore for groundwater potential areas of Kaduna Metropolis, Kaduna State using remote sensing and GIS technique. The data used for the study are as follows; Landsat 8 (OLI) 30m, Aster data (30m), Geologic map, Soil map and Topographic map was used to generate the factors such as; Land use/cover, Slope, Geomorphology, Geologic rock types, soil textures and drainage density. All the factors and its attributes were weighted and rated respectively and were classified according to their importance to groundwater occurrence using the modified DRASTIC model and the weighted overlay technique was used to create a groundwater potential map of the metropolis. Groundwater potential map were classified into four categories that best describes the potential of each area. These classes are; Poor (21%), Moderate (23%), Good (26%) and Very Good (30.2%) groundwater potential which was found to be concentrated in the central part of the metropolis. This result was verified against existing borehole data and field observations to validate the accuracy and was found to be 76% accurate. The study demonstrated the effectiveness of Remote Sensing and GIS as an effective tool in delineation and identification of groundwater potential areas.

Performance Evaluation and Control Scenarios for Targeted Heat Injection and Extraction in an Existing Geothermal Borehole Field in Norway

Performance Evaluation and Control Scenarios for Targeted Heat Injection and Extraction in an Existing Geothermal Borehole Field in Norway

Different Approaches for Evaluation and Modeling of the Effective Thermal Resistance of Groundwater-Filled Boreholes

Groundwater-filled boreholes are a common solution in Scandinavian installations of ground source heat pumps (GSHP) due to the particular hydro-geological conditions with existing bedrock, and groundwater levels close to the surface. Different studies have highlighted the advantage of water-filled boreholes compared with their grouted counterparts since the natural convection of water within the borehole tends to decrease the effective thermal resistance Rb*. In this study, several methods are proposed for the evaluation and modeling of the effective thermal resistance of groundwater-filled boreholes. They are based on distributed temperature sensing (DTS) measurements of six representative boreholes within the irregular 74-single-U 300 m-deep borehole field of Aalto New Campus Complex (ANCC). These methods are compared with the recently developed correlations for groundwater-filled boreholes, which are implemented within the python-based simulation toolbox Pygfunction. The results from the enhanced Pygfunction simulation with daily update of Rb* show very good agreement with the measured mean fluid temperature of the first 39 months of system operation (March 2018–May 2021). It is observed that in real operation the effective thermal resistance Rb* can vary significantly, and therefore it is concluded that the update of Rb* is crucial for a reliable long-term simulation of groundwater-filled boreholes.

A novel three-dimensional implicit numerical model of a borehole field heat exchanger that accounts for seasonal fluctuations of the soil temperature

This study proposes a new three-dimensional implicit numerical model to simulate a borehole field heat exchanger that accounts for seasonal fluctuations of the soil temperature. The model uses an alternating-direction implicit finite difference approach to perform the simulation. The effect of seasonal changes in the soil temperature is considered using an internal heat source term in the governing equations. The model is validated against several published in-situ tests as well as a laboratory experimental study and the results obtained from a TRNSYS version of the duct storage (DST) model, demonstrating an average error of less than 3%.The validated model is then applied to analyse the impact of accounting for seasonal soil temperature fluctuations at various borehole lengths and climate zones. This analysis compares the results obtained from a model that uses constant temperature boundary conditions against those from a model that contains the internal heat source term and uses adiabatic boundary conditions. A significant discrepancy between the outputs of the two models has been observed for shallow borehole heat exchangers (H < 60 m) and at locations with a high annual amplitude of the surface temperature variations (ΔTa > 5°C). Thus, the assumption of constant temperature at the ground surface is only appropriate for deep boreholes (H > 60 m) or at locations with a relatively low annual amplitude of the surface temperature variations (ΔTa < 5°C).

Numerical assessment on the thermal imbalance of multiple ground heat exchangers connected in parallel

• Numerical approach considering heat pump and water separator/collector was built. • Self-adjusting of GHE loads is beneficial to the thermal balance of borehole field. • Heat accumulation is overestimated ignoring the interconnections among GHEs. • Effects of load ratios, borehole layouts and scales were investigated. Thermal imbalance is a common problem in ground source heat pump (GSHP) systems, especially those with large scale multiple ground heat exchangers (GHEs). It usually happens due to the imbalance between the accumulated heat extracted from and injected into the soil and can lead to heat or cold accumulation, cooling or heating performance deterioration and even system failure. In most projects, the GHEs are not independent of each other, but are connected in parallel as an entirety. Therefore, the interconnections among each GHE within the borehole field should be considered. In this study, a numerical approach considering models of the heat pump and the water separator/collector was established for the thermal imbalance evaluation of multiple GHEs connected in parallel, and it was validated by an in-situ test of a single GHE. The necessity of introducing such models was discussed, and the effects of load ratios, borehole layouts, and scales were also investigated. Results show that when both the heat pump and the water separator/collector were considered, the temperature distribution of the borehole field was always uniform during the five-year operation with the highest temperature unevenness value of 0.37 °C, proving that the self-adjusting mechanism of the GHE load (i.e., the GHE load distribution of the borehole field is able to be optimized automatically according to the heat transfer condition of each GHE) is beneficial to maintaining the thermal balance of the borehole field. However, when the interconnections among the GHEs are ignored, the thermal imbalance of the borehole field is overestimated, in particular, under conditions with large load ratios. With the increase in the field scale, the temperature unevenness value of the borehole field first increases and then decreases whether in square or regular hexagon layouts. The numerical approach in this study is helpful to correctly evaluate the thermal imbalance of the borehole field.

Energy, economic and emission assessment of a solar assisted shallow earth borehole field heat pump system for domestic space heating in a north European climate

The performance of a solar assisted ground source heat pump (SAGSHP) system was evaluated and compared with a conventional gas boiler system using simulation, for a central England location. The earth energy bank was the long-term heat store element of the system and consisted of a very shallow field (1.5 m deep) of borehole heat exchangers (BHE). The mathematical model of the system was formulated, and parametric analyses were carried out by varying the number of BHEs and their spacing. The energy performance was expressed using four energy metrics, while its economy and CO2e emissions were compared with a natural gas boiler (NGB) system via the life cycle cost method and the fractional CO2e savings, respectively. The system can be energy self-sufficient by installing 40 BHEs at 1.25 m spacing or with 32 BHEs at 1.5 m spacing. The NGB system appears more economic than the SAGSHP system, due to low natural gas prices, the high price of the imported electricity, the low price of the exported electricity and the higher capital cost of the SAGSHP system. However, the SAGSHP system was found to have net negative carbon emissions, in contrast to the high positive emissions of the NGB system.

Model-based predictive control to minimize primary energy use in a solar district heating system with seasonal thermal energy storage

This paper investigates the development and assessment of a model-based predictive control strategy for the district heating system at the Drake Landing Solar Community (DLSC), in Okotoks (Alberta, Canada). Thermal energy is collected by solar thermal collectors and stored seasonally by means of a borehole field. Two water tanks are used as short-term storage, acting as a central unit connecting solar collectors, long-term storage and a district loop. The DLSC has succeeded in using solar energy collected during the summer to provide nearly all the heating needs of this 52-home community in winter, with solar fractions consistently over 90%. The proposed predictive control strategy aims to minimize primary energy consumption while maintaining the same solar fraction. This simulation study –based on model calibrated with on-site measurements– focuses on the optimization of circulation pump speed to manage energy exchange between long-term and short-term storage systems. Minimizing pumping electricity use is a critical aspect of the community environmental impact, since fossil-fuel thermal plants are prevailing in Alberta. Simulation results indicate that the proposed strategy would save, on an annual basis, about 47% of total pump electricity use. This would result in savings in terms of cost (38%) and greenhouse gas emissions (32%).

First Italian TRT database and significance of the geological setting evaluation in borehole heat exchanger sizing

The correct sizing of the borehole field in closed-loop Ground Source Heat Pump (GSHP) systems requires accurate value of the ground thermal properties, especially the thermal conductivity. One of two methods are generally applied to evaluate this parameter: the Thermal Response Test (TRT) and databases reported in guidelines or Standards for lithologies identified in the local stratigraphy.This paper presents a database of more than 100 Thermal Response Tests performed in Italy. The equivalent thermal conductivities derived from the TRT outputs are here directly compared with the value estimated through knowledge of the local geological stratigraphy. The obtained results are analyzed in terms of geological setting. A sensitivity analysis on the borefield design has been conducted to evaluate the effects of the equivalent thermal conductivity estimation error on the calculation of the required total borehole length. The borehole length, in turn, strongly affects the initial investment costs as well as the operating conditions of the heat pump over the long term.The obtained results, in some cases significant, highlight the importance of performing the TRT not only when the thermal capacity of the GSHP is high but also in the case of strong geological uncertainty, or in particular geological settings such as high plain areas, alpine valley floors and rocky environments. In the other cases as low plain areas, this database can provide an initial estimate of the range of the expected equivalent thermal conductivity value; therefore, it can be useful for designers of both GSHP systems and other applications where the knowledge of the underground thermal behavior is necessary.

Recent Developments in the Design of Vertical Borehole Ground Heat Exchangers for Cost Reduction and Thermal Energy Storage

AbstractGround source (geothermal) heat pumps (GSHPs) can meet the thermal demands of buildings in an energy-efficient manner. The current high installation costs and long payback period limit the attractiveness of GSHP installation in the United States. Vertical borehole ground heat exchangers (VBGHEs), which are commonly used in GSHP systems, contribute most to the cost premium of GSHPs. Reducing the cost of VBGHEs could help increase market penetration of GSHP systems. This paper reviews recent developments for VBGHEs, including improvements in borehole heat transfer and borehole field layout, integration with thermal energy storage, and new design tools. Improvements in the borehole design and materials are more likely to be justified when the ground has high thermal conductivity. Integrating thermal energy storage can provide additional value to the GSHP system, especially when flexible electric demand at buildings becomes more valuable. Advanced design tools for VBGHEs that account for the thermal response of irregularly shaped borehole fields and that are more closely integrated with whole-building energy simulation programs may facilitate more innovations and optimization of GSHP system designs.

Underground heat mine – potential for large scale production and storage of thermal energy

We have studied possibilities to utilize the 1.4 km deep Pyhäsalmi mine as a source of geothermal energy. Measured thermal conductivities of the Precambrian (1.9 Ga) crystalline bedrock hosting the massive Cu-Zn sulphide deposit vary between 2.5 and 3.5 W/m·K, depending on the rock type, and the bedrock temperature increases with depth by 12–14 K/km. The design concept for heat utilization studied in this project is based on an underground borehole field exchanging heat between the bedrock and the water circulation loop, which then transfers thermal energy to the heat pumps. In the borehole heat exchanger (BHE) performance evaluations, we focused on a coaxial configuration, in which formation water circulates down in contact with borehole wall and returns inside a pipe. The borehole field modelled has a hemispherical geometry, so that all boreholes start from a central hall at the bottom of the mine (where the temperature is 21 °C) and would be directed radially outwards. Our reference case consists of 136 boreholes having length of 300 m, thus covering a rock volume of 57 million cubic meters. Simulations indicate that this BHE fields can sustain heating power of 1 MW for at least 44 years, but the average temperature of the borehole field would decrease by about 10 degrees. Thermal power can be increased substantially by increasing the borehole density, but the lifetime of the BHE field decreases.

Considering buried depth for vertical borehole heat exchangers in a borehole field with groundwater flow—An extended solution

The prediction of ground temperature is often necessary for the design and simulation of a borehole heat exchanger (BHE) system in its service life (which usually spans decades). A moving finite line source solution (MFLSD) calculates the ground temperature while considering the buried depth and groundwater flow for a single borehole. However, a BHE system normally comprises a borehole field containing multiple vertical boreholes in commercial (non-residential) applications to satisfy the intended heat demand of the project. Coupled with a spatial superpostion scheme, the MFLSD solution was extended from one single borehole to a borehole field. The MFLSD solution for borehole fields was numerically verified with an equivalent three-dimensional (3D) finite-element (FE) model developed in COMSOL Multiphysics. Then a sensitivity analysis was carried out to evaluate the effect of the buried depth on the heat transfer in borehole fields with groundwater flow. Results indicate that the negligence of buried depth leads to an over-prediction about the exploitable heat. This over-prediction is exacerbated by an increase in the number of boreholes in borehole fields, or by a decrease in the Peclet number. Considering the combined effect of buried depth and groundwater flow in a multiple borehole scenario, the extended MFLSD solution can help in a BHE system design for commercial applications.

Research on heat transfer characteristics and borehole field layout of ground heat exchangers to alleviate thermal accumulation with groundwater advection

The heat transfer performance of ground heat exchanger is significant to the ground source heat pump. The soil thermal parameters, the groundwater advection and the different borehole field layout are important factors to affect the performance of ground heat exchanger. Therefore, the influence of groundwater advection velocity, soil physical parameters and different borehole field layout on the soil temperature distribution and evolution around the ground heat exchanger has been analyzed and studied with unbalanced seasonal thermal load based on the moving finite line heat source model with groundwater advection. The results show that the heat accumulation is easy to occur as the time develops when the groundwater advection is taken into consideration. No matter what type of geological condition is employed, a reasonable borehole field layout can effectively avoid heat accumulation problems under the condition of keeping the same amount boreholes without changing the original ground area. The borehole field layout dispersed along the center line of the advection and concentrated in the advection downstream relatively can effectively reduce the heat accumulation.

Modelling and validation of a new hybrid scheme for predicting the performance of U-pipe borehole heat exchangers during distributed thermal response test experiments

The reliable design of Ground Coupled Heat Pumps (GCHP) requires accurate models for both predicting the long term behavior of the borehole (BHE) field and their sub-hourly response during the thermal response test experiments. Several literature models have been proposed in the past to cope with this goal: resistance/capacitance (RC) models, finite difference (FD) thermal descriptions of both inner BHE and surrounding ground, FEM models based on commercial software packages. The present model, as Fortran code, is a hybrid approach that employs an RC scheme (to single or double U pipes) BHEs and solves the full Fourier equation in the (even layered) ground volume. The fluid vertical energy transfers are treated with an upwind scheme. New thermal parameters have been introduced and used as indicators of the physical and geometrical conditions of the U-BHE. Crucial of the RC part is the proper estimation of local resistances and the correct position of the inner BHE thermal capacitance, to be located somewhere in between the pipe surface and the BHE periphery. The extensive validation with experimental TRT data allowed the model to be refined in order to provide very close agreement among predictions and measurements (root mean square error within 0.17 °C).

A Novel Analytical-ANN Hybrid Model for Borehole Heat Exchanger

Optimizing the operation of ground source heat pumps requires simulation of both short-term and long-term response of the borehole heat exchanger. However, the current physical and neural network based models are not suited to handle the large range of time scales, especially for large borehole fields. In this study, we present a hybrid model for long-term simulation of BHE with high resolution in time. The model uses an analytical model with low time resolution to guide an artificial neural network model with high time resolution. We trained, tuned, and tested the hybrid model using measured data from a ground source heat pump in real operation. The performance of the hybrid model is compared with an analytical model, a calibrated analytical model, and three different types of neural network models. The hybrid model has a relative RMSE of 6% for the testing period compared to 22%, 14%, and 12% respectively for the analytical model, the calibrated analytical model, and the best of the three investigated neural network models. The hybrid model also has a reasonable computational time and was also found to be robust with regard to the model parameters used by the analytical model.

Modelling of a Large Vertical Ground Heat Exchanger Integrated with a Heat Pump for Building Energy Simulation in China

Vertical ground coupled heat pump (GCHP) systems used in buildings in China can help to increase the share of renewable energy and improve the energy efficiency. A building energy simulation (BES) tool including a vertical GCHP model is beneficial for analysing the system performance under dynamic building loads. This paper presents a large-scale vertical ground heat exchanger model integrated to a heat pump model in the BES tool COMFIE, in a Matlab-Delphi co-environment. This model was applied to a residential building block with a 26 × 26 borehole field in Wuhan, China. The heat pump performance improvement was investigated for different design parameters. For half-hourly simulation of 1 year, the specific calculation time of the proposed GCHP model is 4 seconds out of 8 minutes for the whole building. The proposed model can be used as a design aid to optimise the system performance.

Granite Exploration by using Electrical Resistivity Imaging (ERI): A Case Study in Johor

Electrical Resistivity Imaging (ERI) is a tool used in near surface geophysical surveys by flowing an electric current through electrodes that were injected into the ground. The usage of electrical resistivity imaging (ERI) method dominated by geophysicist has increased tremendously in geotechnical engineering application owing to the efficiency and effectiveness of the method in term of time, cost and also data coverage. The survey performed with respect to the particular reference to ERI in determining the granite rock underneath the ground. There were seven (7) lines of ERI performed at the study area by using ABEM Terrameter LS 2 set of equipment based on Schlumberger protocol. Six (6) boreholes were also drilled to obtain engineering properties of rock at the study area. In order to develop the relationship between resistivity and engineering properties, a comparison between borehole field test result and the resistivity value were made. Results from the ERI indicated the presence of zones with high resistivity values identified as overburden soil, fractured granite and solid granite. The findings of this study also showed that the electrical resistivity imaging coupled with borehole drillings were applicable tools for the determination of the granite rock underneath the ground via subsurface profiles and such as depth of overburden soil and engineering properties of soil. In conclusion, ERI is a method that does not disturb the structure or the function of the soil which successfully became the best method in exploring the granite rock.

Extending capabilities of Thermal Response Tests in vertical ground heat exchangers: An experiment-based local short-time temperature response factor

This work presents a fast and practical method to determine a characteristic function that predicts a local ground response of vertical ground heat exchangers (GHE) in a short term, which is mandatory when intermittent operation modes are modelled. It expands the use of a Thermal Response Test (TRT) that is recommended previous to the design of a borehole field into a vertical Ground Source Heat Pump (GSHP). These tests are used to provide some ground thermal characteristics, such as ground conductivity or borehole effective thermal resistance. The same device can also be used to measure the undisturbed ground temperature along depth. From the measurements, it has been determined a temperature response factor function that characterizes the ground behavior in a short term as a consequence of a heat pulse. This function is then included into a finite line-source model to simulate the average temperature of the fluid that flows into the borehole pipes along time. In order to validate this method, several additional tests have been performed by the same device used for TRT’s, with intermittent operation modes. For each test, both experimental and simulated average fluid temperatures have been compared. Results present an excellent accuracy; thus, they demonstrate de effectiveness of the method, as well as other advantages: i) fast and accurate; ii) prevents high precomputing times; iii) several uncertainties from measurements disappear when it is used.