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This paper presents a new in-situ experimental procedure for determining a thermal conductivity of soil, as one of the thermophysical properties necessary for dimensioning vertical buried heat exchangers of geothermal heat pumps. The proposed method, called the thermal recovery test, is based on the assumption that there is a direct analogy between the hydrodynamic process of filling (recovery) the well with water from a porous aquifer after its partial or complete discharging and the process of establishment of thermal equilibrium in an infinite medium previously disturbed by a line heat sink (source). Apart from presentation the theoretical background of these two phenomena, the display of the identical theoretical logarithmic character of the change of hydrostatic pressure and soil temperature, description of the experimental procedure, the experimental results of three performed experiments are also presented. Additionally, results for the experimentally obtained values of the thermal conductivity of the soil are compared with those obtained from the thermal response test, and the advantages and drawbacks of the new thermal recovery test method are analyzed. [ABSTRACT FROM AUTHOR]

The design process of a borehole heat exchanger (BHE) requires knowledge of building thermal loads, the expected heat pump's COP and the ground's thermophysical properties. The thermal response test (TRT) is a common experimental technique for estimating the ground's thermal conductivity and borehole thermal resistance. In classic TRT, a constant heat transfer rate is provided above ground to the carrier fluid that circulates continuously inside a pilot BHE. The average fluid temperature is measured, and from its time-dependent evolution, it is possible to infer both the thermal resistance of the BHE and the thermal conductivity of the ground. The present paper investigates the possibility of a new approach for TRT with the continuous injection of heat directly into the BHE's grouting by means of electrical resistance imparted along the entire BHE's length, while local (along the depth) temperature measurements are acquired. This DTRT (distributed TRT) approach has seldom been applied and, in most applications, circulating hot fluid and optical fibers are used to infer depth-related temperatures. The distributed measurements allow the detection of thermal ground anomalies along the heat exchanger and even the presence of aquifer layers. The present paper investigates the new EDDTRT (electric depth-distributed TRT, under patenting) approach based on traditional instruments (e.g., RTD) or one-wire digital sensors. The accuracy of the proposed method is numerically assessed by Comsol Multiphysics simulations. The analysis of the data obtained from the "virtual" EDDTRT confirms the possibility of estimating within 10% accuracy both thermal ground and grout conductivities. [ABSTRACT FROM AUTHOR]

Ground source heat pump (GSHP) systems depend on the capacity for heat transfer between the system and the ground, and it is good practice to carry out an in situ thermal response test (TRT) to determine the undisturbed ground temperature, the thermal conductivity of the ground, and the thermal resistance of the borehole. Conventionally, a TRT is undertaken in a replica borehole heat exchanger (BHE); however, alternative methods have been developed that can provide continuous depth-resolved temperature recordings. The enhanced TRT (ETRT) uses a hybrid cable system which incorporates a resistance heating wire to provide a linear heat source and a fibre optic cable to measure the temperature along the length of the borehole. In this paper, a case study is presented in which a TRT and ETRT were carried out in the same BHE to evaluate its thermal response and estimate the thermal characteristics of the ground. After a brief introduction of both methods and their interpretation, a comparison between them is presented regarding their advantages and disadvantages using the results of the performed tests, which revealed an 8% difference in the soil thermal conductivity values, averaged over the length of the BHE.

Developing shallow geothermal energy with borehole heat exchangers helps energy supply and CO2 emission reduction. This paper comprehensively investigates the influencing factors on heat transfer efficiency of a single borehole heat exchanger based on the field investigations in Linqu County, Shandong Province, China. We built a three-dimensional numerical model validated by a thermal response test. We systemically investigated the influence of groundwater seepage, circulating water flow rate, pipes spacing and length, the operation interval, and the annual operation mode on the heat transfer efficiency. The results show that the heat transfer efficiency decreases with long-term operation due to surrounding soil’s cold/heat accumulation. The groundwater seepage increases the heat transfer efficiency. The heat transfer efficiency increases with the pipe length, inlet and outlet pipes spacing, and operation interval. It decreases with the circulating water flow rate. The annual operation mode is of greatest importance in the heat transfer efficiency among the studied influencing factors. Compared to working only in winter, the average heat transfer efficiency coefficient increased by 0.152 when working in winter and summer. The results provide a reference for the practical installation and optimization of the borehole heat exchanger for sustainable utilization of shallow geothermal energy.

Determination of ground thermal properties is the prerequisites for the design of ground-coupled heat pump systems (GCHPs), and it is crucial for evaluating the thermal performance of borehole heat exchangers (BHEs). These parameters are usually obtained by in-situ thermal response test (TRT) based on infinite line source model (ILSM). Though the effect of the groundwater flow on the estimation of ground thermal parameters is considered in homogenous ILSM, the estimated deviation can be enlarged as seepage condition varies based on the BHE model in practical geological stratification. Based on the developed numerical layered seepage BHE model (NLSBM), this paper evaluates the effects of seepage location and seepage velocity on estimated accuracy of borehole thermal resistance and ground thermal conductivity. Relative error (RE) between effective thermal conductivity λ eff and the thickness-weighted thermal conductivity λ TW will be up to 30.5% with the increase of the thickness of the seepage layer. The relative error between λ eff and λ TW increases from 5.3% to 93.5% when seepage velocity changes from 1 × 10−6 m/s to 1 × 10−4 m/s. The minimum RE between borehole thermal resistance R b , NLSBM and effective borehole thermal resistance R b , eff is still up to 37% when seepage locates in the 5th layer. With the increase of the seepage velocity, the RE between R b , eff and R b , NLSBM is enlarged, and the minimum RE is up to 29.4% corresponding to the lowest seepage velocity of 1 × 10−4 m/s. The change of seepage velocity or seepage location has little effect on R b , NLSBM , and the highest RE is only 0.2%. [ABSTRACT FROM AUTHOR]

The characteristics and distribution of the thermal and physical properties of Quaternary strata and red-bed rock in the shallow 200 m of Hefei City were revealed by a comparative analysis of laboratory and field thermal response tests in this study. The results show that the thermal conductivity of the Quaternary and red beds in the strata with depths shallower than 200 m in Hefei increased from 2.1~2.8 W/(m·K). There are two closed-loop peak areas (2.8 W/(m·K)) in the northwest and southeast of Hefei City. The volumetric heat capacity increased from 2.1~2.7 MJ/(m³ ·K) gradually from northwest to southeast, which is consistent with the direction of groundwater runoff. The average value of thermal conductivity in different strata was as follows: Quaternary System (1.56 W/(m·K)) < Cretaceous System (1.69 W/(m·K))

Obtaining accurate geotechnical thermal parameters is essential for the efficient use of geothermal energy because their accuracy directly affects the economy and reliability of a heat pump system. When determining geotechnical thermal property parameters using thermal response tests (TRTs), voltage fluctuations, uncertain test duration, and discard time are often encountered. Based on the linear source theory, the Fourier number (Fo) was selected as the reference standard in this study to accurately determine the most appropriate data discard time and test duration. Then, to obtain an accurate initial ground temperature (T 0), a new cycling without power method was proposed and demonstrated to be accurate and feasible. The errors caused by voltage fluctuations during the test were handled by using data segmentation to select the best-fitted data for solving the thermal property parameters, with a good fitting accuracy (R 2) of 0.996 and root mean square error (RMSE) as low as 0.006. The most accurate value of the comprehensive geotechnical thermal conductivity (λ s) was determined to be 1.88 W/(m·°C). The thermal resistance (R b) was determined to be 0.14 °C/W by combining T 0 and parameters from model fitting. In conclusion, the test process and data processing should be considered to improve the accuracy of the TRT. [ABSTRACT FROM AUTHOR]

The design process of a borehole heat exchanger (BHE) requires knowledge of building thermal loads, the expected heat pump’s COP and the ground’s thermophysical properties. The thermal response test (TRT) is a common experimental technique for estimating the ground’s thermal conductivity and borehole thermal resistance. In classic TRT, a constant heat transfer rate is provided above ground to the carrier fluid that circulates continuously inside a pilot BHE. The average fluid temperature is measured, and from its time-dependent evolution, it is possible to infer both the thermal resistance of the BHE and the thermal conductivity of the ground. The present paper investigates the possibility of a new approach for TRT with the continuous injection of heat directly into the BHE’s grouting by means of electrical resistance imparted along the entire BHE’s length, while local (along the depth) temperature measurements are acquired. This DTRT (distributed TRT) approach has seldom been applied and, in most applications, circulating hot fluid and optical fibers are used to infer depth-related temperatures. The distributed measurements allow the detection of thermal ground anomalies along the heat exchanger and even the presence of aquifer layers. The present paper investigates the new EDDTRT (electric depth-distributed TRT, under patenting) approach based on traditional instruments (e.g., RTD) or one-wire digital sensors. The accuracy of the proposed method is numerically assessed by Comsol Multiphysics simulations. The analysis of the data obtained from the “virtual” EDDTRT confirms the possibility of estimating within 10% accuracy both thermal ground and grout conductivities.

Fractures are developed in carbonate rock areas, and the fracture water flow significantly influences the heat exchange between buried pipes and the rock mass by induing heat convection, providing the carbonate rock area a strong heat exchange capacity and preferable conditions for shallow geothermal development and utilization. In this paper, the calculation method of heat exchange capacity of buried pipes based on fracture distribution characteristics is proposed and deduced, featuring such advantages as quick speed and low cost. Taking an actual project in carbonate rock area as an example, the heat exchange capacity of buried pipes was obtained by the following two methods: in-situ thermal response test and calculation based on fracture distribution characteristics. In the thermal response test, the initial ground temperatures of the two test holes were 15.18 °C and 12.72 °C. By fitting the linear equation of time and average temperature with a linear thermal source model, the heat exchange capacities were 57.21 W/m and 58.22 W/m, the thermal conductivities were 3.56 W/(m·K) and 2.32 W/(m·K), the thermal diffusivities were 1.71 × 10−6 m2/s and 1.12 × 10−6 m2/s, and the volume specific heat capacity was 2.08 × 106 J (m3·K). The test results indicated that the thermal property parameters of rock and soil mass were higher than those of other areas, with obvious wide-range distribution characteristics. Through the statistical analysis of outcrop fracture characteristics, combined with the cube law to calculate the fracture water flow and convective heat transfer, an alternative method for the calculation and optimization of buried pipe heat transfer in fractured rock mass area is also proposed in this paper. According to the measured fracture distribution characteristics of the field outcrop, the heat exchange capacities of the two holes were 57.26 W/m and 58.56 W/m, which were basically consistent with the thermal response test values and verified the reliability of the calculation method of heat exchange capacity of buried pipes based on fracture distribution characteristics. [ABSTRACT FROM AUTHOR]

The use of renewable energies, and of geothermal energy in particular, is increasingly being applied in Germany and Europe for the development of new residential districts. The use of geothermal borehole heat exchangers (BHE), in combination with ground-source heat pumps (GSHP), represents an important part of shallow geothermal systems, which are used, among other systems, in urban areas due to their small space requirements. Over the course of planning BHE systems, performance must be determined via the parameters of thermal conductivity, thermal capacity, undisturbed ground temperature, and borehole thermal resistance. These can be identified by the experimental approach known as thermal response testing (TRT). The thermal parameters change due to the influences of the seasonal temperature fluctuations that take place in the ground. In this paper, a pilot double-U BHE heat exchanger field with a depth of 120 m was investigated from this perspective. TRT was carried out using monthly measurements taken over the period of one year using an electrically powered mobile TRT device. The evaluation of the individual tests was carried out using the line-source, moving-line-source, and cylinder-source theories. Our results show that the season in which TRT was implemented had an influence on the determined thermal parameters, with better thermal conditions being obtained in winter months. This is especially visible for thermal conductivity, with monthly deviations of 0.1 W/(m∙K), independent of the evaluation approaches used. [ABSTRACT FROM AUTHOR]

Geothermal energy exhibits considerable development potential in space heating. Shallow geothermal energy stored in the soil in the form of low-grade energy is mainly extracted via the ground source heat pump (GSHP) system. GSHP systems use the subsoil as a heat source, typically involving a vertical borehole heat exchanger (BHE) to extract heat from the formation. Accurate measurement of the thermal properties of the formation is very important for the design of BHEs. At present, the most common and effective method to measure the thermal conductivity of the formation in the field is the thermal response test (TRT). However, the test conditions (heat load, test time) during the thermal response test can impact the test results. Therefore, in this study, a borehole with a depth of 130 m was evaluated in the field. The TRT module and the distributed thermal response test (DTRT) module based on distributed optical fiber temperature sensor (DOFTS) technology were used to monitor the test with different working conditions in real-time. In the field tests, geothermal conditions and the evolution of the formation temperature with time and depth were determined. Based on the test results under different heat loads and test times, the influence of the test conditions on the thermal conductivity results was analyzed and described. A constant temperature zone was located at a depth from 25 m to 50 m, and an increasing temperature zone was located at a depth from 50 m to 130 m, with a geothermal gradient of 3 °C/100 m. The results showed that the heat load slightly influenced the thermal conductivity test results. At the initial stage of the test, the temperature significantly increased from 0 to 12 h. After reaching the quasi-stable state, the test time slightly influenced the thermal conductivity test results. The characteristics of the formation thermal recovery stage after the test stage were studied. The heat load decreased, which could shorten the time for the formation to recover the initial temperature. The results could provide a basis for the optimization of thermal response test conditions. [ABSTRACT FROM AUTHOR]

The purpose of the present work was to determine the thermal performance of borehole heat exchangers, considering the influences of their geometric configurations and the thermophysical properties of the soil, grout and pipe wall material. A three-dimensional model was developed for the heat and mass transfer in soil (a porous medium) and grout, together with one-dimensional conductive heat transfer through the pipe walls and one-dimensional convective heat transfer of the heat transfer fluid circulating in the pipes. An algorithm was developed to solve the mathematical equations of the model. The COMSOL Multiphysics software was used to implement the algorithm and perform the numerical simulations. An apparatus was designed, installed and tested to implement the thermal response test (TRT) method. Two wells of depth 50 m were drilled in the Almaty region in Kazakhstan. Gravel and till/loam were mainly found, which are in accordance with the stratigraphic map of the local geological data. In each well, two borehole heat exchangers were installed, which were an integral part of the ground source heat pump. The TRT measurements were conducted using one borehole heat exchanger in one well and the data were obtained. The present TRT data were found to be in good agreement with those available in literature. The numerical results of the model agreed well with the present TRT data, with the root-mean-square-deviation within 0.184 °C. The TRT data, together with the predictions of the line-source analytical model, were utilized to determine the soil thermal conductivity ( λ g = 2.35 W/m K) and the thermal resistance of the borehole heat exchanger from the heat transfer fluid to the soil ( R b = 0.20 m K/W). The model was then used to predict the efficiencies of the borehole heat exchangers with various geometric configurations and dimensions. The simulation results show that the spiral borehole heat exchanger extracts the highest amount of heat, followed by the multi-tube, double U-type parallel, double U-type cross and single U-type. It is also found that the spiral configuration can save 34.6% drilling depth compared with the conventional single U-type one, suggesting that the spiral configuration is the best one in terms of the depth and the maximum heat extracted. The simulation results showed that (i) more heat was extracted with a higher thermal conductivity of grout material, in the range of 0.5–3.3 W/m K; (ii) the extracted heat remained unchanged for a thermal conductivity of pipe material higher than 2.0 W/m K (experiments in the range of 0.24–0.42 W/m K); (iii) the extracted heat remained unchanged for a volumetric flow rate of water higher than 1.0 m3/h (experimental flow rate 0.6 m3/h); and (iv) the heat extracted by the borehole heat exchanger increased with an increase in the thermal conductivity of the soil (experiments in the range of 0.4–6.0 W/m K). The numerical tool developed, the TRT data and simulation results obtained from the present work are of great value for design and optimization of borehole heat exchangers as well as studying other important factors such as the heat transfer performance during charging/discharging, freezing factor and thermal interference. [ABSTRACT FROM AUTHOR]

To design an efficient ground source heat pump (GSHP) system, it is important to accurately measure the thermophysical parameters of the geotechnical layer. In the current study, a borehole is tested in detail using a combined thermal response test system (CTRTS) based on a distributed optical fiber temperature sensor (DOFTS) and a laboratory test. Real-time monitoring of the stratum temperature according to depth and operation time and the geothermal profile and thermal conductivity of each stratum are obtained. The results show that the undisturbed ground temperature is 10.0 °C, and the formation temperature field within 130 m can be divided into variable temperature formation, constant temperature formation (9.13 °C), and warming formation (geothermal gradient is 3.0 °C/100 m). The comprehensive thermal conductivity of the region is 1.862 W/m·K. From top to bottom, the average thermal conductivity of silty clay, mudstone, argillaceous siltstone, and mudstone is 1.631 W/m·K, 1.888 W/m·K, 1.862 W/m·K, and 2.144 W/m·K, respectively. By comparing the measurement results, the accuracy and effectiveness of the CTRTS are verified. Therefore, it is recommended to use the thermal conductivity obtained by the CTRTS to optimize the design of the borehole heat exchanger (BHE). This study provides a case for establishing a standard distributed thermal response test (DTRT). [ABSTRACT FROM AUTHOR]

The high initial investment of borehole heat exchanger (BHE) prevents the popularization and application of ground-coupled heat pump system (GCHPs). As the premise of designing BHE, ground thermal properties are usually obtained using thermal response test (TRT), and parameter estimation method are applied based on homogeneous BHE model independent of geological stratification. In this paper, a multi-parameter estimation method using Bat algorithm (BA) is firstly proposed to obtain the layered rock-soil thermal properties based on the numerical layered BHE model, considering the characteristics of geological stratification. Then, a TRT experimental platform of three-layer BHE is developed to measure thermal responses of different stratified subsurface. Finally, the six thermal properties are obtained by the proposed multi-parameter estimation method based on BA, and their accuracy is validated using the experimental data in TRT. The five temperatures obtained from the numerical layered BHE model using the estimated results are compared with the corresponding experimental data, the results show that the maximum error of the inlet and outlet water temperatures of the BHE is respectively 3.32 % and 3.59 %, and it is 3.31 %, 2.38 % and 2.7 % for the coarse sand, fine sand and brown soil, respectively. [ABSTRACT FROM AUTHOR]

This study presents a novel thermal response testing (TRT) approach to minimize test duration, labor, and associated costs. The proposed method, Oscillatory Thermal Response Test (OTRT), employs an oscillating heat flux using a sinusoidal wave with specific frequency and amplitude characteristics. A custom-designed TRT apparatus was developed to regulate the heating injection pattern for precise control. In Sapporo City, Hokkaido Prefecture, and Kai City, Yamanashi Prefecture, Japan, two in-situ OTRTs were conducted. The temperature profiles of the fluid and undisturbed soil were measured through optical fiber cables integrated within the U-tube legs. A novel analytical method was devised to filter, smooth, and fit the recorded data, enabling the determination of the response temperature amplitudes at the borehole inlet and outlet. Also, the temperature time derivative method calculates the effective thermal conductivity in the early 3 h. Comparative analysis with Normal Thermal Response Tests (NTRTs) demonstrated the accuracy and validity of the OTRT approach. Significant conclusions from this study include a remarkable reduction in TRT duration by over 95 %, with a high accuracy of 92 % achieved using only the first 3 h of testing. The proposed method exhibited a Root Mean Square Error (RMSE) of 0.1 W/(m·K) compared to NTRTs. To establish a strong correlation between the Amplitude Ratio and Effective Thermal Conductivity, further experimental studies are essential to address the limitations of the current study. These studies should explore a variety of geological and hydrological conditions, borehole diameters, type of heat exchanger, diameter of U-tubem grouting material, combined with numerical analysis, to enhance our understanding of this relationship. • Successful development and testing of a new Oscillatory Thermal Response Test apparatus in Sapporo and Kai cities, Japan. • Advanced analysis techniques ensure the precise determination of response temperature amplitudes and thermal conductivity. • Innovative Oscillatory Thermal Response Test reduces test time and costs by over 95 %. • OTRT achieves over 92 % accuracy in early thermal conductivity measurement compared with NTRT. • The temperature derivative is used to calculate the thermal conductivity. [ABSTRACT FROM AUTHOR]

The technology proposed in this study aims to improve the performance characteristics and coefficient of performance (COP) of a geothermal system by fundamentally preventing underground water discharge and maintaining a constant temperature of the underground heat exchanger composed of bleed discharge water that utilizes two balancing wells using cross-mixing methods. The results of measurements using this technique show that the change in operation from a regular SCW-type heat exchange system to a well-intersected heat exchange system not only improves the COP of geothermal heating and cooling systems, but also ensures a stable supply of geothermal sources. This result is useful for essentially eliminating the ultimately wasted bleed water.

Southeast Asian countries have experienced fast economic growth for several decades. This rapid growth has been accompanied by an increased energy demand per capita. Ground source heat pumps could be a solution to improve energy efficiency. However, there are several concerns with the introduction of this technology. Unlike in four-season countries, where a seasonal balance between cooling and heating demand exists, the use of ground source heat pumps in Southeast Asia is primarily for cooling. The temperature difference between the ground and air is also small. In this paper, we evaluate the first thermal response test results ever performed in Southeast Asia at sites in Bangkok and Hanoi. Borehole heat exchanger models were established on the basis of the apparent thermal conductivities according to the thermal response test results. The borehole heat exchanger models were then used to evaluate the five-year performance of a ground source heat pump in a typical small-sized detached office building under specific climatic conditions. The increasing borehole heat exchanger temperature is unavoidable because of the absence of heat extraction. Consequently, the coefficient of performance decreasing, along with the increasing power consumption, over time. Yet, the results indicating that the ground source heat pump can still provide good thermal performance. We also evaluated the effect of the number of boreholes on the ground source heat pump performance. The results indicate that the maximum coefficient of performance can be relatively improved by 16.1% and 14.9% in Bangkok and Hanoi, respectively, using 10 instead of 6 boreholes.

This paper presents an advanced borehole heat exchanger that has been developed in order to apply a ground source heat pump to a volcanic island where the existing borehole heat exchangers are inapplicable by local ordinance. The advanced borehole heat exchanger was fabricated and installed at a verification-test site to evaluate its heat capacity in terms of refrigeration ton (RT). The proposed heat exchanger was also compared with the conventional heat exchanger that was made of high-density polyethylene (HDPE) heat exchanger. The thermal response test was carried out by flowing water at various temperatures into the heat exchangers at the fixed flow rate of 180 L/min. The results revealed that the maximum heat capacity for the developed heat exchanger was measured at 63.9 kW, which is 160% higher than that of the high-density polyethylene heat exchanger (39.9 kW). It was also found that the developed HX has the highest heat gain achieving 94 kW as compare to 21 kW for high-density polyethylene-Hx.

The soil heat transfer parameters constitute the key information required for designing ground source heat pump (GSHP). Due to the simplification of the common analytical models, traditional methods are difficult to achieve accurate identification of soil thermal parameters and seepage velocity simultaneously. In this work, a high-precision method to simultaneously identify soil thermal conductivity, volumetric heat capacity, and seepage velocity based on deep neural network is proposed. Through the inversed orthogonal method, the training and validation samples are obtained from a large number of thermal response tests (TRTs) on a full-scale simulation platform. The accuracy of this method was verified by comparing identification results with the true values. Meanwhile, the uncertainty of identification results under different noise conditions was quantified, and the impact of test duration was discussed. The results showed that when the maximum random noise is 0.1 °C, the identification errors of the thermal conductivity, volumetric heat capacity, and seepage velocity are only 1.14 %, −4.34 %, and −3.08 %, respectively. The identification reliability can be improved by obtaining the average value of the results under multiple tests and extending the test duration. When the test duration increased from 50 to 100 h, the uncertainty of the identified parameters reduced by 54.57 %, 48.41 %, and 65.70 %, respectively. [ABSTRACT FROM AUTHOR]

• 3D analytical model for planar trench collectors for dimensioning purposes. • The model comprises the finite plane source, seasonal fluctuations and the collector resistance. • The analytical model is verified with numerical simulations and validated with experimental measurements. • computationally optimised solution of the finite plane source. Vertical planar installations in shallow ground are uncommon technological variants for geothermal heat supply. Still, they are of increasing interest when depth and space restrictions do not allow the drilling of boreholes or the installation of horizontal collectors. This work is dedicated to plate-shaped closed-loop heat exchangers that are installed in trenches at a few meters depth. A novel three-dimensional analytical model is presented that accounts for the thermal properties as well as seasonal temperature variation in the ground. The model represents the trench collector as a finite plane source with a specific thermal resistance to simulate the mean temperature of the circulating heat carrier fluid. Both, a detailed dimensional analysis and a successful comparison to numerical simulation are presented. Even though only conductive heat transport is simulated, the model could be validated to the conditions observed at an experimental field site with a trench collector installed at Biberach, Germany. The presented analytical method can serve as an ideal tool for the fast dimensioning of vertical planar heat collectors in practice, and it represents a fundamental framework for the integration of advection or latent heat transfer in frozen ground. [Display omitted] [ABSTRACT FROM AUTHOR]

This research assesses how groundwater flow induced by tide influences the performance of a borehole heat exchanger (BHE) using a case study in Guayaquil, Ecuador. The Thermal Response Test (TRT) was applied to determine the thermal properties of the ground. The experimental results of TRT showed periodic fluctuations in the response, presumably induced by the tide. An explanation of the effects of the tide on thermal performance was carried out systematically. Later, an analytical solution was proposed to estimate the phreatic level near the shore. A geometric mean model then predicted the thermal properties of the ground, given the groundwater level fluctuations.Consequently, a relationship between the effective thermal capacity of the soil and the phreatic level was found. Results show that thermal diffusivity is overpredicted by 50% and thermal conductivity by 8.8%. Finally, we evaluated the heat dissipation of a borehole heat exchanger using a TRNSYS simulation, revealing an overestimation of 12% in the heat rate capacity of the BHE when ignoring tidal effects. This overestimation shows how using standard thermal properties measurement methodologies may affect the design process resulting in substantial inaccuracies in performance and implementation costs.

• Quasi-steady-state model of deep borehole handles geothermal gradient. • Geothermal gradient may delay or prevent development of quasi-steady-state period. • Geothermal gradient affects TRT estimates of borehole and ground properties. • Choice of fluid inlet in coaxial borehole changes geothermal gradient effects. • As parameter | q ratio | decreases to 1 or less, models must include geothermal gradient. A thermal response test (TRT) on a borehole provides information needed in the design of ground heat exchangers for ground source heat pump systems. Conventional analysis of TRT data sets uses the late-time period to estimate the ground thermal conductivity and the effective borehole resistance. During this quasi-steady-state period, the transient average fluid temperature has a logarithmic linear trend, which is consistent with conventional heat transfer models that assume a uniform undisturbed ground temperature. As the depth of the borehole increases the undisturbed ground temperature increases with depth following the geothermal gradient. This paper focuses on how the geothermal gradient affects a TRT. As the geothermal gradient increases, the required time to reach the conventional logarithmic linear trend may increase beyond any practical testing duration in coaxial boreholes. Then, conventional analysis methods based on a quasi-steady-state period incorrectly estimate the ground thermal conductivity. The magnitude of this effect is controlled through a dimensionless parameter | q ratio |, which includes the geothermal gradient and other test parameters. In place of conventional models, a transient model that includes the geothermal gradient and fluid thermal storage can estimate the ground thermal conductivity and local borehole resistance within the typical TRT testing duration. [ABSTRACT FROM AUTHOR]

The ground thermal properties are the basic parameters for the design of borehole heat exchanger (BHE) in ground–coupled heat pump system (GCHPs), and their accuracy directly affects the economy and reliability of the heat pump system. In traditional design, ground is usually regarded as an isotropic homogeneous medium, and its thermal properties are obtained by solving the inverse heat transfer problem in BHE through in-situ thermal response test (TRT). In fact, different geological layers can be observed along the depth of BHE, and thermal physical properties of each layer are different based on the ground geological conditions. In order to investigate the effect of geological stratification on estimated accuracy of ground thermal parameters in TRT, a validated numerical layered BHE model (NLBM) is presented to simulated TRT, and the fluid temperature response is used to estimate effective ground thermal conductivity and borehole thermal resistance based on line source model (LSM). Secondly, the distributions of fluid temperature in U-pipe and heat flux of BHE along the depth are compared and analyzed based on the NLBM and the numerical homogeneous BHE model (NHBM). At last, borehole thermal resistance from the NLBM and estimated borehole thermal resistance from LSM are compared. The results show the maximum heat flux in the 3rd layer of the NLBM is 21.1% higher than that of NHBM, and the minimum heat flux in the 1st layer is reduced by 46.9% in the 100 h duration. The estimated λ LSM using the fluid temperature responses in the NLBM is 1.3% higher than the thickness-weighted thermal conductivity in the NLBM. The minimum relative error between R LSM and R b is still up to 10.45% in the duration of 100 h even though extending the duration is helpful to improve the estimated accuracy of R LSM. • A numerical layered BHE model is proposed and validated by the experiment. • The distributions of fluid temperatures are compared from the two models. • The differences of heat flux are illustrated in the two models. • Effect of geological stratification on estimated accuracy is investigated in TRT. [ABSTRACT FROM AUTHOR]

Obtaining soil thermophysical parameters is the premise for design ground heat exchanger in ground source heat pump system, but it may not be accurately determined due to the limitations of the analytical models. In this paper, artificial neural network (ANN) is used to directly establish the mapping relationship between temperature response and soil thermophysical parameters, and the identification accuracy of traditional method and ANN under different measurement errors is compared. In addition, Kalman filter and fitting regression are used to remove the interference noise. The results show that the identification accuracy and stability of the traditional method are relatively weak affected by temperature measurement error, but the identification accuracy is limited. The maximum deviation errors of thermal conductivity and volumetric heat capacity are 10.68% and 18.42%, respectively, and no matter which kind of noise reduction method cannot improve the identification accuracy. The identification stability of ANN is relatively greatly affected by temperature measurement error, but the identification accuracy is high. The maximum deviation errors of the two parameters are 10.05% and 5.4%, respectively. Through the logarithmic function fitting of noise date can further improve the identification accuracy and stability, the maximum deviation errors are only 2.12% and 3.65%. • A way to determine soil thermal parameters based on neural network is proposed. • This way can establish link between temperature response and unknow parameters. • Different temperature measurement errors on identification accuracy are analyzed. • The identification accuracy of volume heat capacity by the new way is improved. • Logarithmic fitting of the noise data can further improve identification accuracy. [ABSTRACT FROM AUTHOR]

The technology proposed in this study aims to improve the performance characteristics and coefficient of performance (COP) of a geothermal system by fundamentally preventing underground water discharge and maintaining a constant temperature of the underground heat exchanger composed of bleed discharge water that utilizes two balancing wells using cross-mixing methods. The results of the cooling and heating operations of the existing standing column well (SCW) and the balancing well cross-combined heat exchange systems show that the measured COP increases by 23% during the cooling operation and 12% during the heating operation. When operating with a balanced well-cross-mixed heat exchange system, the initial temperature of the underground is constant with a small standard deviation of 0.08–0.12 °C. The results of the measured values using this technology show that, if the operational method is changed from the ordinary SCW-type heat exchange system to a balanced well-intersected heat exchange system, the COP of the cooling and the heating system using geothermal heat not only improves but also ensures a stable supply of geothermal sources by maintaining a constant initial temperature. This finding is useful to fundamentally eliminate the ultimately wasted bleed water. [ABSTRACT FROM AUTHOR]

Use of ground source heat pumps has increased significantly in recent years for space heating and cooling of residential houses and commercial buildings, in both heating (i.e., cold region) and cooling (i.e., warm region) dominated climates, due to its low carbon footprint. Ground source heat pumps exploit the passive energy storage capacity of the ground for heating and cooling of buildings. The main focus of this paper is to critically review how different construction and operation parameters (e.g., pipe configuration, pipe diameter, grout, heat injection rate, and volumetric flow rate) have an impact on the thermal efficiency of the vertical ground heat exchanger (VGHE) in a ground source heat pump (GSHP) system. The published literatures indicate that thermal performance of VGHEs increases with an increase of borehole diameter and/or pipe diameter. These literatures show that the borehole thermal resistance of VGHEs decreases within a range of 9% to 52% due to pipe configurations and grout materials. Furthermore, this paper also identifies the scope to increase the thermal efficiency of VGHE. The authors conclude that in order to enhance the heat transfer rate in VGHE, any attempt to increase the surface area of the pipe configuration would likely be an effective solution.

Fractures are developed in carbonate rock areas, and the fracture water flow significantly influences the heat exchange between buried pipes and the rock mass by induing heat convection, providing the carbonate rock area a strong heat exchange capacity and preferable conditions for shallow geothermal development and utilization. In this paper, the calculation method of heat exchange capacity of buried pipes based on fracture distribution characteristics is proposed and deduced, featuring such advantages as quick speed and low cost. Taking an actual project in carbonate rock area as an example, the heat exchange capacity of buried pipes was obtained by the following two methods: in-situ thermal response test and calculation based on fracture distribution characteristics. In the thermal response test, the initial ground temperatures of the two test holes were 15.18 °C and 12.72 °C. By fitting the linear equation of time and average temperature with a linear thermal source model, the heat exchange capacities were 57.21 W/m and 58.22 W/m, the thermal conductivities were 3.56 W/(m·K) and 2.32 W/(m·K), the thermal diffusivities were 1.71 × 10−6 m2/s and 1.12 × 10−6 m2/s, and the volume specific heat capacity was 2.08 × 106 J (m3·K). The test results indicated that the thermal property parameters of rock and soil mass were higher than those of other areas, with obvious wide-range distribution characteristics. Through the statistical analysis of outcrop fracture characteristics, combined with the cube law to calculate the fracture water flow and convective heat transfer, an alternative method for the calculation and optimization of buried pipe heat transfer in fractured rock mass area is also proposed in this paper. According to the measured fracture distribution characteristics of the field outcrop, the heat exchange capacities of the two holes were 57.26 W/m and 58.56 W/m, which were basically consistent with the thermal response test values and verified the reliability of the calculation method of heat exchange capacity of buried pipes based on fracture distribution characteristics.

The use of renewable energies, and of geothermal energy in particular, is increasingly being applied in Germany and Europe for the development of new residential districts. The use of geothermal borehole heat exchangers (BHE), in combination with ground-source heat pumps (GSHP), represents an important part of shallow geothermal systems, which are used, among other systems, in urban areas due to their small space requirements. Over the course of planning BHE systems, performance must be determined via the parameters of thermal conductivity, thermal capacity, undisturbed ground temperature, and borehole thermal resistance. These can be identified by the experimental approach known as thermal response testing (TRT). The thermal parameters change due to the influences of the seasonal temperature fluctuations that take place in the ground. In this paper, a pilot double-U BHE heat exchanger field with a depth of 120 m was investigated from this perspective. TRT was carried out using monthly measurements taken over the period of one year using an electrically powered mobile TRT device. The evaluation of the individual tests was carried out using the line-source, moving-line-source, and cylinder-source theories. Our results show that the season in which TRT was implemented had an influence on the determined thermal parameters, with better thermal conditions being obtained in winter months. This is especially visible for thermal conductivity, with monthly deviations of 0.1 W/(m∙K), independent of the evaluation approaches used.

The purpose of the present work was to determine the thermal performance of borehole heat exchangers, considering the influences of their geometric configurations and the thermophysical properties of the soil, grout and pipe wall material. A three-dimensional model was developed for the heat and mass transfer in soil (a porous medium) and grout, together with one-dimensional conductive heat transfer through the pipe walls and one-dimensional convective heat transfer of the heat transfer fluid circulating in the pipes. An algorithm was developed to solve the mathematical equations of the model. The COMSOL Multiphysics software was used to implement the algorithm and perform the numerical simulations. An apparatus was designed, installed and tested to implement the thermal response test (TRT) method. Two wells of depth 50 m were drilled in the Almaty region in Kazakhstan. Gravel and till/loam were mainly found, which are in accordance with the stratigraphic map of the local geological data. In each well, two borehole heat exchangers were installed, which were an integral part of the ground source heat pump. The TRT measurements were conducted using one borehole heat exchanger in one well and the data were obtained. The present TRT data were found to be in good agreement with those available in literature. The numerical results of the model agreed well with the present TRT data, with the root-mean-square-deviation within 0.184 °C. The TRT data, together with the predictions of the line-source analytical model, were utilized to determine the soil thermal conductivity (λg = 2.35 W/m K) and the thermal resistance of the borehole heat exchanger from the heat transfer fluid to the soil (Rb = 0.20 m K/W). The model was then used to predict the efficiencies of the borehole heat exchangers with various geometric configurations and dimensions. The simulation results show that the spiral borehole heat exchanger extracts the highest amount of heat, followed by the multi-tube, double U-type parallel, double U-type cross and single U-type. It is also found that the spiral configuration can save 34.6% drilling depth compared with the conventional single U-type one, suggesting that the spiral configuration is the best one in terms of the depth and the maximum heat extracted. The simulation results showed that (i) more heat was extracted with a higher thermal conductivity of grout material, in the range of 0.5–3.3 W/m K; (ii) the extracted heat remained unchanged for a thermal conductivity of pipe material higher than 2.0 W/m K (experiments in the range of 0.24–0.42 W/m K); (iii) the extracted heat remained unchanged for a volumetric flow rate of water higher than 1.0 m3/h (experimental flow rate 0.6 m3/h); and (iv) the heat extracted by the borehole heat exchanger increased with an increase in the thermal conductivity of the soil (experiments in the range of 0.4–6.0 W/m K). The numerical tool developed, the TRT data and simulation results obtained from the present work are of great value for design and optimization of borehole heat exchangers as well as studying other important factors such as the heat transfer performance during charging/discharging, freezing factor and thermal interference.

Geothermal energy exhibits considerable development potential in space heating. Shallow geothermal energy stored in the soil in the form of low-grade energy is mainly extracted via the ground source heat pump (GSHP) system. GSHP systems use the subsoil as a heat source, typically involving a vertical borehole heat exchanger (BHE) to extract heat from the formation. Accurate measurement of the thermal properties of the formation is very important for the design of BHEs. At present, the most common and effective method to measure the thermal conductivity of the formation in the field is the thermal response test (TRT). However, the test conditions (heat load, test time) during the thermal response test can impact the test results. Therefore, in this study, a borehole with a depth of 130 m was evaluated in the field. The TRT module and the distributed thermal response test (DTRT) module based on distributed optical fiber temperature sensor (DOFTS) technology were used to monitor the test with different working conditions in real-time. In the field tests, geothermal conditions and the evolution of the formation temperature with time and depth were determined. Based on the test results under different heat loads and test times, the influence of the test conditions on the thermal conductivity results was analyzed and described. A constant temperature zone was located at a depth from 25 m to 50 m, and an increasing temperature zone was located at a depth from 50 m to 130 m, with a geothermal gradient of 3 °C/100 m. The results showed that the heat load slightly influenced the thermal conductivity test results. At the initial stage of the test, the temperature significantly increased from 0 to 12 h. After reaching the quasi-stable state, the test time slightly influenced the thermal conductivity test results. The characteristics of the formation thermal recovery stage after the test stage were studied. The heat load decreased, which could shorten the time for the formation to recover the initial temperature. The results could provide a basis for the optimization of thermal response test conditions.

Ground source heat pump systems (GSHPs) for soil heat utilization have been widely concerned in the world due to their energy saving, high efficiency, and no pollutant emission. The thermal parameters of the local soil must be known first when designing GSHPs, and the in-situ thermal response test (TRT) is currently the conventional method for determining their values. Given this, this paper deals with the related contents of parameter identification based on TRT from four aspects. First, various modeling methods for ground heat exchanger (GHE) and inversion algorithms are introduced and compared. Next, many factors that influence the identification accuracy are summarized, including test conditions, external disturbance, groundwater seepage and subsurface natural convection. Then, the negative influence of correlation between parameters on identification results is analyzed, and the research progress of the uncertainty assessment of identification results is introduced, which can improve the reliability of the results. Furthermore, the characteristics of various advanced TRT setups are also summarized. Finally, several imperfects and objectives for improving the reliability of parameters identification are discussed. This work provides useful information for improving the identification accuracy of soil thermal parameters so that the GHSPs can be designed more reliably. • Various modeling methods for GHE, inversion algorithms, and many influence factors on identification accuracy are reviewed. • The negative influence of correlation between parameters on identification results is analyzed. • The research progress of uncertainty assessment methods is introduced. • The characteristics of various advanced TRT setups are summarized and compared. • Several unsolved problems needed to strengthen the applicability of TRT are discussed. [ABSTRACT FROM AUTHOR]

Thermal properties of rock-soil are of great importance for the design of borehole heat exchangers (BHEs). In the process of testing thermal properties of rock-soil through an in-situ thermal response test (TRT), groundwater seepage directly affects the evaluation of thermal properties in TRTs and leads to a varied arrangement and engineering costs of BHEs. Therefore, comprehensive hydrogeological surveys of groundwater and TRTs to determine geothermal properties are necessary. In this study, we made a preliminary attempt to use hydrogeological surveys and correlated TRTs to investigate the heat exchange efficiency of a single U-tube BHE. The geological conditions were used to predict and analyze the groundwater form, groundwater depth, and flow trajectory in the test area based on the natural electric field frequency selection method. Using a comprehensive dataset, the groundwater velocity was determined with ANSYS. The results showed that the initial temperature of rock-soil was 21.8 °C and that the comprehensive thermal conductivity of rock-soil was 1.942 W/(m·°C). Confined and phreatic aquifers existed in the test area, and the depth of the confined aquifer was approximately 70 m. The groundwater velocity in the test area was considered to be approximately 2–2.5 m/d. The results of this study confirm that TRTs combined with groundwater surveys can improve the test accuracy and consequently provide guidance for engineering practices in ground-coupled heat pump systems.

To design an efficient ground source heat pump (GSHP) system, it is important to accurately measure the thermophysical parameters of the geotechnical layer. In the current study, a borehole is tested in detail using a combined thermal response test system (CTRTS) based on a distributed optical fiber temperature sensor (DOFTS) and a laboratory test. Real-time monitoring of the stratum temperature according to depth and operation time and the geothermal profile and thermal conductivity of each stratum are obtained. The results show that the undisturbed ground temperature is 10.0 °C, and the formation temperature field within 130 m can be divided into variable temperature formation, constant temperature formation (9.13 °C), and warming formation (geothermal gradient is 3.0 °C/100 m). The comprehensive thermal conductivity of the region is 1.862 W/m·K. From top to bottom, the average thermal conductivity of silty clay, mudstone, argillaceous siltstone, and mudstone is 1.631 W/m·K, 1.888 W/m·K, 1.862 W/m·K, and 2.144 W/m·K, respectively. By comparing the measurement results, the accuracy and effectiveness of the CTRTS are verified. Therefore, it is recommended to use the thermal conductivity obtained by the CTRTS to optimize the design of the borehole heat exchanger (BHE). This study provides a case for establishing a standard distributed thermal response test (DTRT).

The thermal conductivity of soil at different depths is a key parameter for the evaluation of shallow geothermal energy. A thermal response test based on the actively heated optical cable (ATRT) is one of the effective ways to obtain the in-situ distributed thermal conductivity. Compared with traditional thermal response test (TRT) and distributed thermal response test (DTRT), ATRT has better test efficiency. One of the key issues that determines the error and the effect of ATRT is how to effectively lay a heat source that satisfies the assumption in the line source model. Using the finite element numerical simulation software ComsolMultiphysics, a two-dimensional finite element heat transfer model of porous media was established, and we explored the influence of U-shaped internal heating cable's spacing on the calculated thermal conductivity. The results show that the thermal response process of ATRT can be divided into three stages, namely the cable-influence stage, the grout-influence stage, and the soil-influence stage. U-shaped laying of the cable will increase the heating time when the soil influence stage is stable, which is not conductive to improve the efficiency of ATRT. Due to the influence of simplified calculation, the thermal conductivity calculated from the test results of the U-shaped optical cable has a peak value significantly higher than the thermal conductivity of the soil at the initial stage of heating. With the increase of heating duration, the temperature rise rate under the condition of U-shaped optical cable layout gradually approaches the ideal line heat source temperature rise rate, the calculation result is closer to the true value, and reducing the spacing can speed up this process. As the spacing increases, the recommended heating duration will increase rapidly in order to control the error within 10%. Therefore, to improve the test efficiency, the distance between the U-shaped optical cables should be minimized in the Held. [ABSTRACT FROM AUTHOR]

• Field test on a double-layered horizontal directional drilled ground heat exchanger. • Numerical modeling was used to investigate the effect of water injection. • Water injection improves the performance of ground heat exchanger. • Injection rate, place, temperature, and the permeability affect the performance. 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. [ABSTRACT FROM AUTHOR]

The Adapted Conductivity Trilobular (ACT) design, comprising an up-flow central pipe and three satellite downward pipes, represents a novel configuration of a borehole heat exchanger. The novelty lies not only in the geometry of the heat exchanger, but also in the use of materials perfectly adapted to the thermal use for which they have been specified. The central pipe is constructed of a composite bilayer material with very low thermal conductivity, whereas the satellite pipes are based on a novel highly conductive plastic material. The primary objective behind this design was to simplify installation, by using essentially the same drilling and grouting methods used for conventional single-U or double-U borehole heat exchangers. In response to the rising demand for more efficient and affordable ground heat exchangers to be connected to Ground Source Heat Pump (GSHP) systems, this innovation was one of the promising outcomes of the GEOCOND European project. To assess ACT performance, the Geothermal Laboratory on the campus of the Universitat Politècnica de Valencia conducted a number of thermal response tests (TRTs), the experimental results of which are collected and analyzed in this paper. Additionally, a modified version of the Composite Two Region Line Source modeling scheme is described and used to assess the experimental data. A total of four demonstration boreholes – two single-U baseline and two ACT boreholes – have been tested. The experimental investigation using the C2RLS methodology reveals that the ACT configuration has a significantly lower thermal than the typical BHE arrangement and allows a greater specific heat injection rate. Specifically, a borehole resistance drop from 0.149 (m.K)/W to 0.07 (m.K)/W allowing higher injection rates to be achieved without an unsustainable increase in ground temperature. These findings may have significant implications for the development of more efficient and cost-effective GSHP systems. • New borehole heat exchanger: Adapted Conductivity Trilobular design (ACT). • Novel trilobular design enhances borehole exchanger efficiency. • Central and satellite plastic pipes combine different thermal conductivities. • Thermal resistance is significantly below standard borehole exchangers. • Allows a 50% boost in specific heat injection rate. [ABSTRACT FROM AUTHOR]

The vertical borehole ground heat exchange performance is still the issue for the engineers installing vertical borehole ground plants using ground-coupled heat pumps. Besides geological and climate change challenges, they face the extraordinary heat transfer process between the circulating fluid and the ground surrounding the U-tube and interactions of vertical boreholes. This paper describes the technique to evaluate the grout, soil thermal parameters and borehole thermal resistance simultaneously using the particle swarm optimization algorithm. The reference thermal response data set from the sandbox laboratory was used for the analysis. A thermal response test (TRT) was made, including the different temperature response functions, for a few time scales. The estimates and errors of the grout, soil thermal conductivity and borehole resistivity were presented and compared with the results of the laboratory experiment and researchers. The target functions, in our case root mean square error values, were less than 0.034 for all analysis cases. The calculation algorithm was written using the Matlab 2016 program and could be easily expanded by increasing the number of target functions and evaluation algorithms. The presented TRT data analysis will increase the knowledge about the vertical borehole ground heat exchange design.

Investigating the constructions of borehole heat exchangers with high efficiency (unit heat transfer between the heat carrier and ground) is important. One of the means to improve efficiency is the use of the most efficient construction of the borehole heat exchanger. The paper describes research on borehole heat exchangers’ thermal efficiency, which is mainly characterized by parameters obtained from a thermal response test: effective thermal conductivity and borehole thermal resistivity. The borehole heat exchangers of the Laboratory of Geoenergetics in Poland were studied. Based on thermal response test interpretation and empirical equations, one of which is proprietary, the heat transfer is calculated independent of the duration of the thermal response test. Other conditions for using borehole heat exchangers in downtowns are discussed. The research aims to determine the best borehole heat exchanger design from five basic possibilities studied. A lack of unequivocal statements regarding this matter in the literature was observed. The influence of the interpretation method on the research results is determined. A single U-tube system filled with gravel is shown to be the most advantageous design by a very small margin. The applied interpretation methods, however, confirm the hitherto ambiguity in the selection of the best construction. The maximum heat carrier temperature at the end of thermal response tests was 32 °C for a geological profile mostly made up of clay (low thermal conductivity) and 23 °C for Carpathian flysch (sandstones and shales, with a higher value of conductivity).

Summary: Borehole thermal resistance and ground thermal properties (thermal conductivity and heat capacity) are the key parameters to implement the ground source heat pump (GSHP), usually obtained by thermal response test. In this study, a novel sequential parameters estimation method for the above three parameters is proposed, and the sensitivity analysis by using a special correlation method is performed to decide the best estimation sequences. At first, the Spearman partial rank correlation coefficient was used to represent the correlation between the estimated thermal properties and fluid temperature for the line source model (ILS), then the estimation sequence for the three parameters could be determined by the correlation results. Lastly, with the estimation step, Monte Carlo method was adopted to determine the parameters replacing conventional iterative algorithms. In addition, the effect of value bounds and initial inputs as well as random samples was investigated. The results showed that compared to the other estimation steps, the estimation sequence following borehole resistance firstly, then thermal conductivity, heat capacity lastly could get the best precision with 4.5%, 0.4%, 1% respectively. Specially, the estimation precision for ground heat capacity could be promoted by the sequential estimation. Also, the effect of value bounds on estimation precision was nearly eliminated by the proposed method. [ABSTRACT FROM AUTHOR]

Standing column well (SCW) geothermal heat exchanger permits a bleeding discharge of less than 20% in the event of a maximum load, which is an inappropriate method of using underground water. In this study, the existing operational method of two adjacent SCW geothermal heat exchangers, each with a single well, was modified. This technology aims to improve the coefficient of performance (COP) of the geothermal system by fundamentally preventing underground water discharge and maintaining a constant temperature of the underground heat exchanger. To curb the bleed water discharge, two balancing wells of cross-mixing methods were employed. The result of the cooling and heating operations with the existing SCW heat exchange system and the balancing well cross-combined heat exchange system showed that the measured COP increases by 23% and 12% during the cooling and heating operations, respectively. When operating with a balanced well cross-mixed heat exchange system, the initial temperature of the underground was constant with a small standard deviation of 0.08–0.12 °C. [ABSTRACT FROM AUTHOR]