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Highlights • This paper provides a proof-of-concept to a newly proposed virtual borehole. • The Virtual Borehole (VB) is proposed for calibrating thermal response test units. • The VB mimics the thermal behavior of a real borehole installed in the ground. • A commercial thermal response test unit is successfully calibrated using the VB. Abstract This paper presents the development of a virtual borehole (VB) used to calibrate the ground thermal conductivity obtained from thermal response test (TRT) units. The VB is composed of an aboveground plate heat exchanger and chiller unit carefully controlled to mimic the thermal behavior of the ground by reproducing the time evolution of the mean fluid temperature for a user-selected ground thermal conductivity. During calibration, TRT units are connected to the VB just like if they were connected to a real borehole. The various components of the VB are described including the characterization of the heat exchanger, the implementation of a resistance-capacitance (RC) borehole model, and the required control algorithm. The VB concept is successfully tested by comparing the results obtained on a real borehole to those given by the VB for given conditions. An uncertainty analysis reveals that the ground thermal conductivity set by the VB is accurate to within ± 2.7%. The usefulness of the VB is then demonstrated by calibrating a commercially available TRT unit for two ground thermal conductivities, 1.0 and 3.0 W m−1 K−1. Results of this calibration indicate that the TRT unit evaluates ground thermal conductivities of 1.02 and 3.18 W m−1 K−1, respectively. [ABSTRACT FROM AUTHOR]

An accurate and reliable 3D engineering geological model (expressed herein as a 3D litho-stratigraphic model) is essential for designing slopes, underground structures, and tunnels to reduce the risk of failure. Due to budget constraints, subsurface investigations with traditional methods (e.g., boreholes and geophysical prospecting) are often limited in quantity. Insufficient investigation often leads to inadequate litho-stratigraphic data that can degrade the reliability of the developed 3D litho-stratigraphic models. To this end, LiDAR, which can collect topographic data more efficiently and economically, may complement traditional geological investigations for developing litho-stratigraphic models. However, the task of integrating the subsurface geological data and LiDAR data to create 3D litho-stratigraphic models for engineering applications is challenging. This paper proposes a methodology that combines LiDAR-derived geological data (Type I data herein) and borehole data (Type II data herein) to develop realistic 3D litho-stratigraphic models. To demonstrate the methodology, we took a 1.2 km × 1.2 km area with sedimentary rocks in northern Taiwan as the study area. Under the assumption that the strata are parallel, we first fitted Type I data to a surface regression model (a polynomial mathematical model). Then, we incorporated Type II data into the regressive process to develop the desired 3D model. Comparing the former model (based on Type I data) with the latter model (based on Type I + II data) through a Root-Mean-Squared-Error (RMSE) analysis, it was found that the RMSE of the latter model decreased significantly, indicating a decrease in the error between the developed litho-stratigraphic model and the 'ground truth.' To verify this finding, we compared the newly developed 3D model with the 'known' geological cross-section of the Qidu Tunnel constructed nearby. The results indicated that the distribution of the primary sandstone strata uncovered from the previous Qidu Tunnel project and those projected by the newly developed 3D model was highly consistent. Thus, this study demonstrates that LiDAR data combined with a few borehole data has the potential to create reliable and accurate 3D geological models, providing an effective solution for obtaining 3D geological information at the scale of engineering applications with limited boreholes. Finally, the proposed modeling approach is further demonstrated through two hypothetical engineering application cases. • 3D litho-stratigraphic modeling using LiDAR and limited borehole data. • Modeling in a dip-slope terrain was verified through ground truth along a tunnel. • Combination of LiDAR and borehole data can greatly improve the model reliability. • Precise borehole data and extensive range of LiDAR data complement each other. [ABSTRACT FROM AUTHOR]

• A novel fiber Bragg Grating (FBG) based extensometer working on the magnetostatic interaction. • Borehole extensometer for ground settlement monitoring. • Real-time monitoring and data collection. Monitoring for sub-surface settlement in deep excavation construction and tunneling works will provide an early indication of potential settlement or sinkhole formation at the ground surface. Currently, rod and magnetic extensometers may be used to monitor sub-surface settlement, with the latter most commonly used. In this paper, a novel Fiber Bragg Grating (FBG) based extensometer is proposed working on the magnetostatic interaction between a ring magnet and a disc magnet. The proposed FBG extensometer can be automated for continuous monitoring, as compared with the commonly used magnetic extensometer which monitors manually at prescribed intervals. Multiple units of FBG extensometer can be connected together and easily installed in a deep borehole. The number of FBG extensometer used is not limited by the borehole size, as compared with the rod extensometer where a maximum of 3 tips per borehole is typically specified. The proposed extensometer with current instrumentation gives a measurement range of 80 mm with a measurement resolution of 0.65 mm/με. [ABSTRACT FROM AUTHOR]

Field characterization in oil industry is a challenging task which aims to determine or estimate some of the petrophysical properties of a reservoir. These properties are expected to be later used in a non-deterministic workflow (which might depend on subjective interpretations carried out by each petrophysicist) whose aim is to answer questions such as whether exploiting the reservoir is feasible or not, how much hydrocarbon can be extracted from it and whether the geology of the reservoir will stand the stresses of exploitation without collapsing. Three properties stand-out amongst all the rest, as they are closely correlated to the amount of hydrocarbon present in the reservoir and also to the feasibility of its exploitation: permeability, porosity, and lithology. Obtaining reliable and robust measurements of these properties requires extracting core samples from the reservoir, which is, however, a very resource-consuming task. Hence, it is common to use other tools (such as wirelogging tools or borehole imagers) to extract several other properties from the reservoir, in an attempt to obtain information that might help experts to estimate these properties on those intervals at which no core sample could be extracted. In this context, estimating lithofacies helps petrophysicists to automatize the process of identifying the lithology of the reservoir. Previous work (Basu et al., 2002; Chai et al., 2009; Linek et al., 2007; Newberry et al., 2004; Hall et al., 1996) used a considerable amount of wirelog data and borehole image logs to induce the lithology of the reservoir. In the work presented in this paper, we introduce a new methodology for automatic reservoir lithofacies identification. Our model relies only on ultrasonic and microresistivity borehole image logs as inputs, which are characterized by a deep residual convolutional network which then infers the probability of each sample to be classified as each type of previously defined lithofacies classes. The method presented in this work was tested on a carbonate well from the Brazilian pre-salt oilfields, and it allowed us to obtain an average classification accuracy of 81.45% and an average area under the ROC curve of 92.70% for all classes, for the blind test sample. • Lithological facies classification with an accuracy greater than 81 % and an average area under the ROC greater than 92 %. • Borehole images used as exclusive inputs for characterization purposes (no extra wirelog used). • Deep learning techniques (deep residual convolutional neural networks) applied to reservoir data. • Use of local neighbourhood features instead of single-points to retain geological spatial correlation from images. • Discussion on quantifiable correlation between borehole images and lithofacies. [ABSTRACT FROM AUTHOR]

Abstract Knowledge of the in situ stress state of rock mass is fundamental for engineering, geological and geophysical applications. In situ stress state determination requires in principle the evaluation of the three principal stresses and the related principal directions, but it is widely recognized in the literature that the maximum horizontal stress is the most difficult component to accurately estimate. In the context of borehole methods, this paper proposes a step-by-step analytical procedure to estimate some bounds to the maximum horizontal stress, starting from a geomechanical description of the rock and relying on information generally available in the engineering practice. The procedure is divided in substeps, each one requiring additional information about the mechanical properties of the rock and on the geometrical properties of the failed portion of rock: more information available implies a lower uncertainty on in situ stress estimate. Furthermore, since the proposed procedure is analytical, it allows a complete and very easy implementation in a spreadsheet. The aim of the work is thus to provide a rigourous but simple analytical tool that can be used in engineering practicte to estimate some bounds to the maximum horizontal in situ stress state. The approach is finally validated by means of both numerical simulations, performed with a sophisticated numerical tool, and experimental field data coming from the literature. Highlights • Determination of in situ stress state is crucial for any underground application in many fields of engineering. • Borehole methods have been accepted to be one of the most reliable method to estimate in situ stress state. • A step-by-step analytical procedure is presented to estimate the in-situ stress state relying on drilled borehole information. • Any step of the procedure reduces of the bounds of the maximum horizontal stress, at the cost of more information required. • The procedure is validated by means of both simulations with advanced numerical methods and in situ literature data. [ABSTRACT FROM AUTHOR]

Abstract Borehole drilling and cone penetration test (CPT) are frequently employed site investigation methods for identifying subsurface stratification. However, these two methods have their respective pros and cons, and their corresponding soil type classification protocols are different. Therefore, an approach that can jointly interpret raw data from both investigation methods and provide unified soil classification results is in great demand. Motivated by the aforementioned point, this paper presents a novel semi-supervised clustering based stratification identification approach using information from both boreholes and CPT logs. The proposed approach is established on a hidden Markov random field (HMRF) framework so that the supervision constraints could be introduced by using borehole data during the clustering of CPT sounding samples. Further, the presented approach employs a Monte Carlo Expectation Maximization (MCEM) algorithm to perform the clustering process, which enables estimating the subsurface stratification in a probabilistic manner. The performances of the proposed approach are evaluated using real-world site investigation data. The test results indicate that the proposed approach is effective and robust for identifying subsurface stratification. Highlights • A stratification interpretation approach considering both CPT and borehole data is presented. • Considering constraints derived from borehole data can improve the accuracy of obtained stratification. • The presented method can interpret stratification using borehole and CPT data under a unified soil classification system. • The presented method can interpret stratification at multiple boreholes and CPTs locations at a practical site. [ABSTRACT FROM AUTHOR]

Abstract Rock permeability and porosity are some of the most important features to be determined during the exploitation of a certain hydrocarbon reservoir, as these physical properties help petrophysicists understand the most likely distribution and/or presence of oil/gas reservoirs, as well as decide whether that certain field might be exploitable or not. Despite their importance, in order to obtain permeability and porosity values it is usually required to run tests on borehole core samples that are either destructives or that need to be done under very controlled conditions, which makes them very expensive and time consuming. In the last decades, some authors have been able to estimate both these properties measurements from well-logging reservoir curves –such as Gamma-Ray response (GR), Bulk Density (RHOB) or Neutron Porosity (NPHI)–. In this study, we present a novel method that can be used to estimate formation permeability and porosity from borehole image logs –instead of simple logs– using deep autoencoders. The data used in this work belongs was extracted from a Brazilian pre-salt carbonate well. First, we use a 3–level stacked autoencoder to extract internal features from both ultrasonic and microresistivity images, thus characterizing the features contained inside these images. Afterwards, we use a support vector machine regressor (SVM) to link the encoded features extracted from the autoencoders for each type of data (ultrasonic and microresistivity borehole images) to the petrophysical measurements logs. Using this method we were able to achieve a correlation coefficient between original and estimated validation samples of R 2 = 96.30 % for effective porosity logs and R 2 = 96.06 % for permeability logs, and a normalized squared mean error of N M R S E = 7.30 % and N M R S E = 5.51 % , respectively, all results in the blind test sample. Highlights • Permeability and E_ective Porosity wireline logs estimation with over 96% accuracy. • Use of Borehole Image Data Logs (Ultrasonic and Microresistivity) as only source of information for the petrophysical logs estimation. • Application of Deep Learning methods to borehole well data (Stacked Autoencoders). • The use of sliding window to extract hidden features from the Borehole Images allow us to preserve the information of the geological structures on the surroundings of each sample. • The correlation between Ultrasonic and Resistivity Image Data Logs and petrophysical logs (Porosity and Permeability) is shown to be objective in this paper, rather than something subjective or intuitive (as it used to be in the past, when petrophysicists could not quantify objectively the amount of information from these data sources that should be taken into account). [ABSTRACT FROM AUTHOR]

The objective of this paper is to examine the factors affecting the sizing and positioning of Ground Heat Exchangers (GHEs) in Cyprus. This is achieved through the investigation of the influence of the temperature, thermal conductivity, specific heat and density of the ground as well as pipe diameter on the performance of GHEs using computer software modelling in conjunction with test data. Also, the long term temperature variation of the ground around the boreholes is examined since this affects the positioning of the GHEs. Because of the large number of parameters involved in the design the desired result can be achieved in various ways by considering the specific parameters. The results of the simulations have shown that, generally speaking, the island of Cyprus is suitable for geothermal heat pumps. [ABSTRACT FROM AUTHOR]

Electrical discharge drilling is a promising method that allows for the reduction of the costs of well drilling in hard rocks including the energy costs. This paper describes experimental drilling of a well in granite. The well has the diameter of 340 mm and the depth of 15 m, and transformer oil was used as the drilling fluid. In order to study the characteristics of the drilling process and drill string stability we used two drill bits with the interelectrode gaps of 40 mm and 70 mm and applied pulse voltage of positive and negative polarity to the drill bit electrodes. The results are as follows. Increasing the inter-electrode gap has allowed us to reduce the energy costs while increasing the drilling rate, but it has slightly reduced the lifetime of the drill string insulation. The polarity of the voltage pulse has no effect on the penetration rate. At the same time, in the case of negative pulse polarity, the electrical stability of the drill string insulation is higher than that for the positive one. For the 70 mm inter-electrode drill gap, the specific energy consumption and energy input per unit length of the inter-electrode gap are lower while the performance of a single pulse is greater. In this paper we are also discussing the peculiarities and prospects of industrial implementation of electrical discharge drilling. In addition to increasing the overall reliability of the drill string and pulse power source, it is necessary to find suitable insulation material with high flashover resistance in water-based mud. [ABSTRACT FROM AUTHOR]

The aim of this paper is to show a practical way of estimating the thermal ground properties, namely the ground thermal conductivity, and in particular the thermal diffusivity and the volumetric heat capacity in a reliable manner, for sizing Ground Heat Exchangers (GHEs). A well-known thermal model, proposed by Blackwell in 1954, is applied and is validated both in the heating mode and in the cooling mode, using a GHE as a probe. The value of the thermal conductivity can be easily determined by the model but the procedure also requires knowledge of the ground specific heat capacity and density, which are normally deduced from the (non-accurate) geological data of the site. In addition to the above, the thermal model is also solved analytically –based on the actual parameters used in the experiment–leading to the computation of the ground thermal diffusivity, the volumetric heat capacity and the thermal resistance of the GHE. The possible errors and drawbacks of the whole method are then discussed and finally a complete set of guidelines is provided to the field Engineer for estimating the ground thermal properties from a single test, rendering the use of the geological data of the side unnecessary. [ABSTRACT FROM AUTHOR]

This paper proposes an analytical solution, using the combined Laplace–Fourier integral transform technique, for a borehole drilled in transversely isotropic porous medium and subjected to a fluid discharge over a finite length of its surface. Especially, the coupled boundary condition between the total radial stress and injection-induced pore pressure at the borehole surface is addressed in a rigorous fashion, which leads essentially to a set of dual integral equations that can be solved through standard numerical procedure. The study focuses on the calculation of stress fields around the borehole with particular attention given to the time-dependent effective tangential stress and pore pressure distributions. Numerical solutions are presented for verification with those recently derived for the limiting case of an isotropic medium and, more importantly, to investigate the influences of material anisotropy on the stress responses of the borehole and porous medium. [ABSTRACT FROM AUTHOR]

Deep, investigation boreholes for suitable location of radioactive repositories and holes with larger diameter for storing such waste at great depth must be effectively isolated from the biosphere. A method for sealing boreholes by use of dense smectite clay where the rock is low-permeable and has only very fine fractures, and by concrete cast where the holes intersect water-bearing fracture zones, has been used for sealing of up to 500 m deep exploration boreholes in Sweden and Finland. The paper describes the practical conditions for construction of seals, and the processes that lead to the required tightness of the clay and concrete materials. Application of this technique to even deeper boreholes requires use of mud instead of water for stabilization and for moderating the rate of maturation. Placeability requires that the maturation of the initially unsaturated clay is neither too fast nor too slow and means of controlling the rate are described. The proposed concrete material has a very low cement content, and talc as fluidizer, which gives slow but ultimately very significant strengthening and low solubility of the cement reaction products. At a few kilometer depth the temperature can be more than 60 °C, which has to be sustained by the sealing materials, and this makes saponite and mixed-layer smectite/illite clay possible alternatives to pure smectites. The concept is judged to be applicable also to sealing of abandoned boreholes used for fracking. [ABSTRACT FROM AUTHOR]

The solar district heating system combined with borehole thermal energy storage (BTES) in the Drake Landing Solar Community (DLSC) has managed to provide 97% of the community's annual space heating demand with solar power. Following such exceptional results, the focus of this paper is to analyse the influence of location changes on the DLSC. A model of the community is created with TRNSYS and simulations are carried out in five different locations: Helsinki [FI], Hohhot [CN], Dublin [IE], Oviedo [ES] and Perpignan [FR]. To fulfil the specific needs of each location, adaptive measures are taken by modifying key parameters of the system's original design. Results show that insulating the houses and using lower temperature heating systems can significantly increase the system's solar fraction (SF). Despite increasing SF, the use of lower temperature heating systems in well-insulated homes may result in a decrease in the BTES's efficiency (η_BTES), which is also dependent on local ground properties. In hot climates, the BTES may be omitted due to high levels of winter solar radiation. Very high SFs are found to be possible in all studied locations (>95%), as long as appropriate modifications are made to the original design when necessary. [ABSTRACT FROM AUTHOR]

This paper describes a mathematical method for estimating static formation temperatures using a function based on extrapolation of the measured data in the thermal recovery process of geothermal and petroleum wells. This method uses the least squares fit of a rational polynomial model to describe the transient temperature data and also provides an analytical expression of thermal recovery processes. This expression gives the asymptotic value for final temperature. We then apply this method for estimating static formation temperatures of a geothermal and petroleum drilled well. In this work we have developed the equations of least squares fit of rational polynomial function and used the equation to estimate the uncertainties of its coefficients. This fitting procedure allows us to obtain a mathematical model that represents the thermal recovery process and to extrapolate the value of final temperature. This is a practical method to estimate static formation temperature, having the advantages that the thermal properties of the drilling mud and its formation are not required; it can also be used in the geothermal and petroleum industries. [ABSTRACT FROM AUTHOR]

The design of a borehole field should be based on a long-term simulation of its thermal response for the intended energy loads. A well-known method to evaluate the response is based on a pre-calculated dimensionless function, the g -function. When calculating g -functions, there are two commonly used approaches for treating the boundary condition at the borehole wall: a constant heat flux at every instant of time, or a uniform temperature at a constant total heat flow to the borehole field. This paper is focused on a new approach to model the thermal process of borehole fields; in particular with a precise representation of a uniform temperature boundary condition at the borehole wall. The main purpose of this model is to be used as a research tool to either generate g -functions for particular cases or handle situations that cannot be addressed by others methods. First, the almost constant temperature along the borehole heat exchanger in operation requires a boundary condition of essentially isothermal boreholes along the depth. In a common case, the borehole heat exchangers are connected in parallel, thus all boreholes should have the same temperature. Also, the total heat flow to the borehole field should be constant over time. For this purpose, a numerical model in which the boreholes are filled with a hypothetical highly conductive material has been built, reproducing the isothermal condition. By thermally interconnecting the boreholes, the equal temperature condition is satisfied. Finally, the specified total heat flow is fed into one spot at the highly conductive material. The model is validated by generating g -functions of some simple borehole field configurations. The g -functions present, in general, a good agreement with the existing solutions for a similar boundary condition. Moreover, the model is also tested against real experimental data from a 2 × 3 borehole field at an office building. The simulated daily fluid temperatures are compared with measured daily fluid temperatures for the sixth year of operation. The simulated values present, in general, a good agreement with the measured data. The results show that there are no significant differences with regard to the boundary conditions at the borehole wall, which for this specific case is due to the fact that the system is thermally balanced. [ABSTRACT FROM AUTHOR]

Heating and cooling of a building is a major energy demand and a source of CO 2 emission in the atmosphere. Using geothermal energy in the form of borehole ground coupled heat exchanger (GHX) can be an effective and economical alternative. To make the GHX design quick and easy to do in its preliminary stage, a simple and easily implemented method is needed. This paper synthesizes relations between the GHX spacing, diameter, soil thermal conductivity and peak heating/cooling energy output for a typical building foundation in Louisiana by proposing a graph method. One set of design graphs were developed for northern Louisiana, where subsurface soils are mainly stiff clays and dense sands. The GHX boreholes were designed without any tedious calculation by using these graphs. The GHX solutions from the developed graph-method were reliable and accurate, and agreed fairly well with the results from the industry-popular software packages GLD 2012 and GLHEPRO for a house cooling example that was selected in Ruston, Louisiana. [ABSTRACT FROM AUTHOR]

The paper makes heat transfer analysis on borehole ground heat exchangers (GHEs) of ground-coupled heat pump system, for one thing, the thermal transmission on two cases including sing U-tube and double U-tubes inside borehole are illustrated, for another, the heat transfer characteristics outside borehole are presented based on the mathematical model, it is meaningful to obtain the analytical solution of temperature response in the underground medium. The relevant analysis lays a firm foundation for the optimal design of borehole GHEs because it can provide theoretical guidance for the calculation and design. In addition, some factors which can exert influences on the heat exchange performance of GHEs are pointed out, such as the arrangement of boreholes GHEs, the type of circulating liquid and so on. Afterwards the impacts degrees of these factors are studied according to simulation. It is necessary to optimize the design of borehole GHEs because in such a way can the size of GHEs be reduced so that the initial cost of the system is lowered, this is favourable for the application of ground-coupled heat pump technology. [ABSTRACT FROM AUTHOR]

• A design methodology for downscaled borehole fields (lab-scale) is proposed. • Analytical invariance requirements in form of dimensionless numbers are derived. • Convection as top boundary condition needs to be considered for short boreholes. • Identification of potential systematic errors and practical constraints. • Numerical simulations of borehole fields with different spacings. This paper presents a methodology for designing Laboratory Borehole Storages (LABS) intended to generate reference Thermal Response Functions (TRFs) for model validation. The design method is based on analytically-derived invariance requirements demanding the conservation of the Fourier and Biot numbers. Accordingly, convective boundary conditions (BCs) need to be up-scaled when downscaling the borehole field, especially for short boreholes. Indeed, numerical simulations show that natural convection as top BC leads to TRF values more than 14 % higher than a Dirichlet BC. In addition, this BC effect is proposed as a possible explanation for previously reported differences between experimental and analytical results. Finally, the numerical simulations are used to find suitable size – height and radius of twice the borehole length–and test durations for the LABS. [ABSTRACT FROM AUTHOR]

Karst cavities at or near pile locations pose a substantial danger to underground construction projects. To locate karst cavities at pile locations, advance drilling boreholes are commonly used, but a cavity can be missed due to its limited size. In this paper, we present a new borehole-based technology employing multifrequency acoustic wave (MFAW) detection of karst cavities near drilling boreholes. First, we developed an MFAW detector and signal processing software. The high- and low-frequency transmitters operate in the frequency range of 0.05 kHz to 10 kHz and the range of 1 kHz to 50 kHz, respectively. Next, we evaluated the performance of the prototype in a controlled borehole model. Lastly, the MFAW detection system was tested at a field site. A nearby borehole core was used to verify the MFAW detection results. The testing of the prototype detection system revealed the following findings. (1) The integrity of the rock surrounding the borehole can be assessed by the energy of the high-frequency zero-offset data. The zero-offset section has a high correlation with the energy of the high-frequency detection signal. The rock is intact when the high-frequency detection signal is strong. The rock is fragmented when the test signal exhibits amplitude variance over a short-range away from the borehole. The test signal becomes weak when the borehole meets a karst cavity filled with sand, clay or water. A color bar showing the signal amplitude can represent the integrity of the borehole wall more accurately than a core sample. (2) A karst cavity near the borehole can be detected by low-frequency waves. The low-frequency acoustic detection wave field includes a reflected guided wave from the interface that the borehole meets, the wave in the borehole, and the effective reflected body wave from the karst cavity. The integrity of the rock around the borehole wall obtained by high-frequency acoustic detection can help identify the reflected guided wave. Median filtering suppresses the strong amplitude borehole wave and helps reveal the weaker reflected waves. The reflected wave from the karst cavity near the borehole can be extracted after filtering. (3) The azimuth of the cavity is identified by the different detection recordings of the azimuthal receivers. Reflected wave amplitudes obtained by the azimuthal receivers facing the karst cavity are greater than those obtained by the receivers facing away from the karst cavity. The azimuthal angle can be obtained by the three-dimensional electric compass in the downhole tool. The MFAW detection system is shown to be an effective tool for investigating the geological environments at pile locations in detailed geo-investigation periods. • A new technology employing multi-frequency acoustic wave detection of karst cavities near boreholes is provided. • The integrity of rock surrounding the borehole can be assessed by the zero-offset image of the high-frequency signal. • A karst cavity near the borehole can be detected by low-frequency waves. • The azimuth of the cavity is identified by the different detection recordings of the azimuthal receivers. [ABSTRACT FROM AUTHOR]

• Thermal properties differences on both sides of the source are fully considered. • The classical models are deduced via special cases of the proposed model. • The whole heat transfer period inside the pile is divided into two stages. • The measures of heat transfer enhancement are proposed. This paper presents a new cylindrical source heat transfer model for simulating the influence of the thermal properties difference between the borehole and pile ground heat exchangers and the surrounding soil directly. Based on the energy conservation law, the model proposes a heat transfer equation that heat transfers from the heat source to both sides, and the analytic solution in Cartesian coordinate system is obtained via Laplace transform and inverse transformation. Because special cases of the analytical solution can be deduced to the classical models, the proposed model can be considered a unified model of the classical ones. The heat transfer analysis using this model shows that the thermal properties of the materials inside and outside the heat source have a greater impact on the initial period of heat transfer. Furthermore, the proposed model enables more accurate energy consumption analysis and energy pile design. Base on the model, to improve the heat transfer efficiency, the following measures can be adopted for the materials inside and outside the heat source. The moisture content of the soil outside the heat source can be increased, optimally keeping the soil saturated. Inside the heat source, on the premise of satisfying the requirements of mechanical properties, the thermal conductivity can be increased, whereas the volumetric heat capacity and diameter can be decreased. [ABSTRACT FROM AUTHOR]

• Field tests of a pavement solar collector connected to boreholes. • Highly efficient pavement solar collector with efficiency of 42%. • Numerical investigation of the effect of albedo, flow rate and pipe spacing. • First time report of surface condensation on pavement solar collectors. Solar energy is a renewable energy source that is globally available. To utilise this type of energy, novel technologies are developed across the globe. Among these, the pavement solar collector (PSC) technology has a considerable potential. A PSC consist of pipes, that are embedded into the upper pavement layers through which fluid is circulated. Solar radiation heats the pavement surface, and the absorbed heat is transferred to the circulating fluid. One applications is to use the heat for recharging shallow geothermal boreholes with solar energy during summer, thereby reducing the electricity consumption of ground source heat pumps during winter. In Scandinavia, however, the knowledge on the PSC application is limited. The Swedish transport administration has therefore established a field station to gain more insight on the PSC in Scandinavia. This paper reports the findings of the investigation from the summer of 2018 and how the efficiency of the PSC is affected by altering the albedo, fluid flow rate, and pipe spacing. The measured harvested energy is 245 kWh/m2 with a solar efficiency of 42%. It is found that by altering the albedo and flow rate, the efficiency could be enhanced by up to 49%. This high efficiency achieved in this study is dependent on the short pipe spacing of 5 cm and results in surface condensation on several occasions. Condensation on PSC has not been reported previously but should not pose a risk to road traffic because surface temperatures are above freezing. [ABSTRACT FROM AUTHOR]

• A tool for calculating borehole resistance for groundwater filled boreholes. • Increased accuracy compared to estimating an effective filling conductivity. • Increased accuracy of early stage design. One of the challenges of designing ground source heat pumps systems is the calculation of the effective thermal borehole resistance which affects the heat extraction rate and thereby the required length of the borehole. Calculating effective borehole resistance is especially demanding when the boreholes are filled with groundwater. The natural convection in the groundwater affects the heat transport from the borehole heat exchanger to the borehole wall. The objective of this paper is to present the results of the modified Pygfunction software in which recently developed correlations for calculating influence of the natural convection in boreholes has been incorporated. The calculated results were validated by using results of thermal response test (TRT) from four adjacent groundwater filled borehole in the central parts of Sweden. The numerical results reveal that the original and unmodified Pygfunction can be used for accurate calculation of fluid temperatures in borehole heat exchangers by using an effective thermal conductivity of the filling material. However, that requires access to thermal response test data. The modified software calculates the effective borehole resistance with a deviation of about 15–30% compared to the effective borehole resistance from TRT. This deviation is an improvement compared with the standard assumption of neglecting natural convection in boreholes. However, for large installations with more than 4000 m of borehole heat exchangers a thermal response test is still recommended in order to ensure a more correct sizing of geothermal system. [ABSTRACT FROM AUTHOR]