Special Issue "Geothermal Energy: Utilization and Technology 2018"

A special issue of Energies (ISSN 1996-1073).

Deadline for manuscript submissions: closed (31 August 2018).

Special Issue Editors

Prof. Dr. P.G. Pathegama Ranjith
Website
Guest Editor
Deep Earth Energy Laboratory, Department of Civil Engineering, Monash University, Melbourne, Australia
Interests: geomechanics; CO2 sequestration; shale gas; coal seam gas; geotherml energy
Special Issues and Collections in MDPI journals
Prof. Dr. Sheng-Qi Yang
Website
Guest Editor
State Key Laboratory for Geomechanics and Deep Underground Engineering, School of Mechanics and Civil Engineering, China University of Mining and Technology (CUMT), XuZhou 221116, China
Interests: crack evolution behavior and mechanism of rock containing pre-existing fissures; strength, deformation, seepage and damage failure behavior of deep rock; rock time-dependent experimental and model mechanics under complex stress state; CO2 geological sequestration under saline environment; large deformation and control mechanism of deep tunnels or roadways; high temperature behavior and permeability evolution of rocks related to geothermal energy and ncluear waste disposion project

Special Issue Information

Dear Colleagues,

Geothermal energy is a very attractive form of renewable energy. In comparison to fossil fuel, it is more environmentally friendly because of less CO2 emission and less damage to the environment. Geothermal energy has traditionally utilised hot aquifers with significant permeability to allow liberal flow of heated waters. In recent years, attention has turned to exploitation of dry (hot) rocks for geothermal energy by circulation of fluids through a ‘closed’ fracture network produced by hydraulic stimulation of the hot rock. However, commercial exploitation of this renewable resource is currently met with limited success either due to loss circulation of the injected fluid and/or inefficient extraction of heat from the rock mass. This proposal calls for papers in the areas of new sciences developed to enhance the recovery process of heat from deep geothermal reservoirs as well its utilisations.

We will, therefore, especially welcome submissions on the following topics:

  • Conventional shallow geothermal systems
  • Deep Hydrothermal systems
  • Hot dry rocks
  • Heat pumps
  • Constitutive modelling and numerical methods
  • Challenges in deep drilling in hot rocks
  • Reservoir geomechanics, and wellbore and drilling mechanics
  • Use of new circulations fluids such as Carbon dioxide
  • Numerical modelling of coupled thermal-mechanical-fluid flow systems
  • Techniques to enhance heat recoveries, such as new fracking and innovations in well      drilling
  • Associated environmental issues
  • Coupled thermo-hydro-chemical-mechanical processes
  • Experimental works on soil and rocks at medium to high temperatue
  • Case studies of international interest
Prof. Dr. Ranjith Pathegama Gamage
Prof. Dr. Sheng-Qi Yang
Guest Editors

Manuscript Submission Information

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Keywords

  • hot dry rocks
  • shallow geothermal
  • conventional geothermal
  • deep geothermal
  • renewable energy

Published Papers (20 papers)

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Research

Open AccessArticle
Application of Borehole Thermal Energy Storage in Waste Heat Recovery from Diesel Generators in Remote Cold Climate Locations
Energies 2019, 12(4), 656; https://doi.org/10.3390/en12040656 - 18 Feb 2019
Cited by 5
Abstract
Remote communities that have limited or no access to the power grid commonly employ diesel generators for communal electricity provision. Nearly 65% of the overall thermal energy input of diesel generators is wasted through exhaust and other mechanical components such as water-jackets, intercoolers, [...] Read more.
Remote communities that have limited or no access to the power grid commonly employ diesel generators for communal electricity provision. Nearly 65% of the overall thermal energy input of diesel generators is wasted through exhaust and other mechanical components such as water-jackets, intercoolers, aftercoolers, and friction. If recovered, this waste heat could help address the energy demands of such communities. A viable solution would be to recover this heat and use it for direct heating applications, as conversion to mechanical power comes with significant efficiency losses. Despite a few examples of waste heat recovery from water-jackets during winter, this valuable thermal energy is often discarded into the atmosphere during the summer season. However, seasonal thermal energy storage techniques can mitigate this issue with reliable performance. Storing the recovered heat from diesel generators during low heat demand periods and reusing it when the demand peaks can be a promising alternative. At this point, seasonal thermal storage in shallow geothermal reserves can be an economically feasible method. This paper proposes the novel concept of coupling the heat recovery unit of diesel generators to a borehole seasonal thermal storage system to store discarded heat during summer and provide upgraded heat when required during the winter season on a cold, remote Canadian community. The performance of the proposed ground-coupled thermal storage system is investigated by developing a Computational Fluid Dynamics and Heat Transfer model. Full article
(This article belongs to the Special Issue Geothermal Energy: Utilization and Technology 2018)
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Open AccessArticle
Development of a Multi-Well Pairing System for Groundwater Heat Pump Systems
Energies 2018, 11(12), 3485; https://doi.org/10.3390/en11123485 - 13 Dec 2018
Cited by 2
Abstract
Groundwater heat pump systems (GWHPs) can achieve higher coefficient of performance (COP) than air-source heat pump systems by using the relatively stable temperature of groundwater. Among GWHPs, multi-well systems have lower initial investment costs than conventional closed-loop geothermal systems, because they typically require [...] Read more.
Groundwater heat pump systems (GWHPs) can achieve higher coefficient of performance (COP) than air-source heat pump systems by using the relatively stable temperature of groundwater. Among GWHPs, multi-well systems have lower initial investment costs than conventional closed-loop geothermal systems, because they typically require installation of fewer boreholes for the same building load. However, the performance of GWHPs depends significantly on the groundwater properties, such as groundwater temperature, permeability and water quality. Moreover, pumping and injecting of groundwater during long-term operation may lead to problems such as overflow or clogging of the wells. In order to ensure reliable energy from ground sources, the development of sustainable operation methods for multi-well systems is essential for preventing overflow and well clogging. In this study, we have developed a pairing technology that connects the injection and supply wells through a spillway. This pairing technology can be used to control groundwater levels in wells and can be sustainably operated. To accurately estimate the performance of a multi-well system with the proposed pairing technology, the heating and cooling performance of the developed system was compared to that of a standing column well (SCW) system in a field-scale experiment. Furthermore, the effects of the multi-well pairing system on groundwater levels in the injection well were analyzed by numerical simulation. Moreover, in order to decide the required conditions of the multi-well pairing system, case studies were conducted under various hydraulic conductivity and pumping conditions. Full article
(This article belongs to the Special Issue Geothermal Energy: Utilization and Technology 2018)
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Open AccessArticle
Distributed Thermal Response Tests Using a Heating Cable and Fiber Optic Temperature Sensing
Energies 2018, 11(11), 3059; https://doi.org/10.3390/en11113059 - 07 Nov 2018
Cited by 8
Abstract
Thermal response tests are used to assess the subsurface thermal conductivity to design ground-coupled heat pump systems. Conventional tests are cumbersome and require a source of high power to heat water circulating in a pilot ground heat exchanger. An alternative test method using [...] Read more.
Thermal response tests are used to assess the subsurface thermal conductivity to design ground-coupled heat pump systems. Conventional tests are cumbersome and require a source of high power to heat water circulating in a pilot ground heat exchanger. An alternative test method using heating cable was verified in the field as an option to conduct this heat injection experiment with a low power source and a compact equipment. Two thermal response tests using heating cable sections and a continuous heating cable were performed in two experimental heat exchangers on different sites in Canada and France. The temperature evolution during the tests was monitored using submersible sensors and fiber optic distributed temperature sensing. Free convection that can occur in the pipe of the heat exchanger was evaluated using the Rayleigh number stability criterion. The finite and infinite line source equations were used to reproduce temperature variations along the heating cable sections and continuous heating cable, respectively. The thermal conductivity profile of each site was inferred and the uncertainly of the test was evaluated. A mean thermal conductivity 15% higher than that revealed with the conventional test was estimated with heating cable sections. The thermal conductivity evaluated using the continuous heating cable corresponds to the value estimated during the conventional test. The average uncertainly associated with the heating cable section test was 15.18%, while an uncertainty of 2.14% was estimated for the test with the continuous heating cable. According to the Rayleigh number stability criterion, significant free convection can occur during the heat injection period when heating cable sections are used. The continuous heating cable with a low power source is a promising method to perform thermal response tests and further tests could be carried out in deep boreholes to verify its applicability. Full article
(This article belongs to the Special Issue Geothermal Energy: Utilization and Technology 2018)
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Open AccessArticle
Estimation of Cement Thermal Properties through the Three-Phase Model with Application to Geothermal Wells
Energies 2018, 11(10), 2839; https://doi.org/10.3390/en11102839 - 20 Oct 2018
Cited by 4
Abstract
Geothermal energy has been used by mankind since ancient times. Given the limited geographical distribution of the most favorable resources, exploration efforts have more recently focused on unconventional geothermal systems targeting greater depths to reach sufficient temperatures. In these systems, geothermal well performance [...] Read more.
Geothermal energy has been used by mankind since ancient times. Given the limited geographical distribution of the most favorable resources, exploration efforts have more recently focused on unconventional geothermal systems targeting greater depths to reach sufficient temperatures. In these systems, geothermal well performance relies on efficient heat transfer between the working fluid, which is pumped from surface, and the underground rock. Most of the wells designed for such environments require that the casing strings used throughout the well construction process be cemented in place. The overall heat transfer around the wellbore may be optimized through accurate selection of cement recipes. This paper presents the application of a three-phase analytical model to estimate the cement thermal properties. The results show that cement recipes can be designed to enhance or minimize heat transfer around wellbore, extending the application of geothermal exploitation. Full article
(This article belongs to the Special Issue Geothermal Energy: Utilization and Technology 2018)
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Open AccessArticle
Investigation of Thermal Stress of Cement Sheath for Geothermal Wells during Fracturing
Energies 2018, 11(10), 2581; https://doi.org/10.3390/en11102581 - 27 Sep 2018
Cited by 6
Abstract
Geothermal energy development has increasingly been studied in recently decades because of its renewable and sustainable features. It can be divided into two categories: traditional geothermal (hydrothermal) systems and enhanced geothermal systems (EGS) based on the type of exploitation. The hot dry rock [...] Read more.
Geothermal energy development has increasingly been studied in recently decades because of its renewable and sustainable features. It can be divided into two categories: traditional geothermal (hydrothermal) systems and enhanced geothermal systems (EGS) based on the type of exploitation. The hot dry rock (HDR) in the EGS incorporates about 80% of all thermal energy, and its value is about 100–1000 times that of fossil energy. It is pivotal for geothermal wells to improve the flow conductivity of the HDR mass, enhance the communication area of natural fractures, and constitute the fracture network between injection and production wells by hydraulic treatments. While the wellbore temperature significantly decreases because of fracturing, fluid injection will induce additional thermal stresses in the cement sheath, which will aggravate its failure. Considering the radial nonuniform temperature change, this paper proposes a new thermal stress model for a casing-cement sheath-formation combined system for geothermal wells during fracturing based on elastic mechanics and thermodynamics theory. This model is solved by the Gaussian main elimination method. Based on the analytical model, the thermal stresses of cement sheath have been analyzed. The effects of the main influencing parameters on thermal stresses have also been investigated. Results show that the radial and axial tensile thermal stresses are both obviously larger than tangential tensile thermal stress. The maximum radial and axial thermal stresses always occur at the casing interface while the location of the maximum tangential thermal stress varies. Generally, thermal stresses are more likely to induce radial and axial micro cracks in the cement sheath, and the cement sheath will fail more easily at the casing interface in fracturing geothermal wells. For integrity protection of the cement sheath, a proper decrease of casing wall thickness, casing linear thermal expansion coefficient, cement sheath elasticity modulus, and an increase of the fracturing fluid temperature has been suggested. Full article
(This article belongs to the Special Issue Geothermal Energy: Utilization and Technology 2018)
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Open AccessFeature PaperArticle
Numerical Modeling of Dynamic Behavior and Steering Ability of a Bottom Hole Assembly with a Bent-Housing Positive Displacement Motor Under Rotary Drilling Conditions
Energies 2018, 11(10), 2568; https://doi.org/10.3390/en11102568 - 26 Sep 2018
Cited by 7
Abstract
Fully rotary drilling is one of many useful technologies used for the exploitation of petroleum and geothermal resources, but fully rotating drill-strings are extremely complicated. Therefore, according to the Hamilton principle, a non-linear coupled bottom hole assembly (BHA)-bit-formation-wellbore model is proposed for BHAs [...] Read more.
Fully rotary drilling is one of many useful technologies used for the exploitation of petroleum and geothermal resources, but fully rotating drill-strings are extremely complicated. Therefore, according to the Hamilton principle, a non-linear coupled bottom hole assembly (BHA)-bit-formation-wellbore model is proposed for BHAs with bent-housing positive displacement motor using the finite element method to investigate the dynamic behavior and steering ability under fully rotary drilling. The impact force, acceleration, axial loading, torque, and dynamic stress were simulated, and factors influencing the dynamic steering forces were investigated. The results indicate that the impact force, acceleration, axial loading, torque, and dynamic stress under fully rotary drilling are much higher than under conventional drilling. The numerical simulation and field test in well B confirmed that the rotation of the drill-string is conducive to the hold-on of the deviation angle. With the increase in the weight-on-bit, bend angle, and stabilizer height, the deflecting force on a drill bit increases. Conversely, with the increase in stabilizer diameter, the deflecting force on the drill bit decreases; the lower the deflecting force, the better the effectiveness of hold-on. With increasing deviation angle, the deflecting force on the drill bit first decreases and then increases. The present model can provide a theoretical basis for wellbore trajectory control and optimization design of BHA. Full article
(This article belongs to the Special Issue Geothermal Energy: Utilization and Technology 2018)
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Open AccessFeature PaperArticle
Comparison of Mechanical Behavior and Acoustic Emission Characteristics of Three Thermally-Damaged Rocks
Energies 2018, 11(9), 2350; https://doi.org/10.3390/en11092350 - 06 Sep 2018
Cited by 11
Abstract
High temperature treatment has a significant influence on the mechanical behavior and the associated microcracking characteristic of rocks. A good understanding of the thermal damage effects on rock behavior is helpful for design and stability evaluation of engineering structures in the geothermal field. [...] Read more.
High temperature treatment has a significant influence on the mechanical behavior and the associated microcracking characteristic of rocks. A good understanding of the thermal damage effects on rock behavior is helpful for design and stability evaluation of engineering structures in the geothermal field. This paper studies the mechanical behavior and the acoustic emission (AE) characteristic of three typical rocks (i.e., sedimentary, metamorphic, and igneous), with an emphasis on how the difference in rock type (i.e., porosity and mineralogical composition) affects the rock behavior in response to thermal damage. Compression tests are carried out on rock specimens which are thermally damaged and AE monitoring is conducted during the compression tests. The mechanical properties including P-wave velocity, compressive strength, and Young’s modulus for the three rocks are found to generally show a decreasing trend as the temperature applied to the rock increases. However, these mechanical properties for quartz sandstone first increase to a certain extent and then decrease as the treatment temperature increases, which is mainly attributed to the high porosity of quartz sandstone. The results obtained from stress–strain curve, failure mode, and AE characteristic also show that the failure of quartz-rich rock (i.e., quartz sandstone and granite) is more brittle when compared with that of calcite-rich rock (i.e., marble). However, the ductility is enhanced to some extent as the treatment temperature increases for all the three examined rocks. Due to high brittleness of quartz sandstone and granite, more AE activities can be detected during loading and the recorded AE activities mostly accumulate when the stress approaches the peak strength, which is quite different from the results of marble. Full article
(This article belongs to the Special Issue Geothermal Energy: Utilization and Technology 2018)
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Open AccessArticle
Assessment of Energetic, Economic and Environmental Performance of Ground-Coupled Heat Pumps
Energies 2018, 11(8), 1941; https://doi.org/10.3390/en11081941 - 26 Jul 2018
Cited by 17
Abstract
Ground-coupled heat pumps (GCHPs) have a great potential for reducing the cost and climate change impact of building heating, cooling, and domestic hot water (DHW). The high installation cost is a major barrier to their diffusion but, under certain conditions (climate, building use, [...] Read more.
Ground-coupled heat pumps (GCHPs) have a great potential for reducing the cost and climate change impact of building heating, cooling, and domestic hot water (DHW). The high installation cost is a major barrier to their diffusion but, under certain conditions (climate, building use, alternative fuels, etc.), the investment can be profitable in the long term. We present a comprehensive modeling study on GCHPs, performed with the dynamic energy simulation software TRNSYS, reproducing the operating conditions of three building types (residential, office, and hotel), with two insulation levels of the building envelope (poor/good), with the climate conditions of six European cities. Simulation results highlight the driving variables for heating/cooling peak loads and yearly demand, which are the input to assess economic performance and environmental benefits of GCHPs. We found that, in Italy, GCHPs are able to reduce CO2 emissions up to 216 g CO2/year per euro spent. However, payback times are still quite high, i.e., from 8 to 20 years. This performance can be improved by changing taxation on gas and electricity and using hybrid systems, adding a fossil-fuel boiler to cover peak heating loads, thus reducing the overall installation cost compared to full-load sized GCHP systems. Full article
(This article belongs to the Special Issue Geothermal Energy: Utilization and Technology 2018)
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Open AccessArticle
Three-Dimensional Numerical Investigation of Coupled Flow-Stress-Damage Failure Process in Heterogeneous Poroelastic Rocks
Energies 2018, 11(8), 1923; https://doi.org/10.3390/en11081923 - 24 Jul 2018
Cited by 3
Abstract
The failure mechanism of heterogeneous rocks (geological materials), especially under hydraulic conditions, is important in geological engineering. The coupled mechanism of flow-stress-damage should be determined for the stability of rock mass engineering under triaxial stress states. Based on poroelasticity and damage theory, a [...] Read more.
The failure mechanism of heterogeneous rocks (geological materials), especially under hydraulic conditions, is important in geological engineering. The coupled mechanism of flow-stress-damage should be determined for the stability of rock mass engineering under triaxial stress states. Based on poroelasticity and damage theory, a three-dimensional coupled model of the flow-stress-damage failure process is studied, focusing mainly on the coupled characteristics of permeability evolution and damage in nonhomogeneous rocks. The influences of numerous mesoscale mechanical and hydraulic properties, including homogeneity, residual strength coefficient, loading rates, and strength criteria, on the macro mechanical response are analyzed. Results reveal that the stress sensitive factor and damage coefficient are key variables for controlling the progress of permeability evolution, and these can reflect the hydraulic properties under pre-peak and post-peak separately. Moreover, several experiments are conducted to evaluate the method in terms of permeability evolution and failure process and to verify the proposed two-stage permeability evolution model. This model can be used to illustrate the failure mechanics under hydraulic conditions and match different rock types. The relation of permeability with strain can also help confirm appropriate rock mass hydraulic parameters, thereby enhancing our understanding of the coupled failure mechanism in rock mass engineering. Full article
(This article belongs to the Special Issue Geothermal Energy: Utilization and Technology 2018)
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Open AccessArticle
Influence of Temperature on the Microstructure Deterioration of Sandstone
Energies 2018, 11(7), 1753; https://doi.org/10.3390/en11071753 - 04 Jul 2018
Cited by 11
Abstract
Macroscopic properties of sandstone are commonly attributed to the degradation of its microstructure during heating treatment processes. However, few previous studies have focused on comprehensive observations on how the microstructure of sandstone changes with temperature. In this study, a kind of sandstone containing [...] Read more.
Macroscopic properties of sandstone are commonly attributed to the degradation of its microstructure during heating treatment processes. However, few previous studies have focused on comprehensive observations on how the microstructure of sandstone changes with temperature. In this study, a kind of sandstone containing quartz, albite, calcite, and laumontite (little), was collected from Linyi (Shandong Province, China) to observe the microstructure degradation changes with temperature by X-ray diffraction (XRD), Scanning electron microscopy (SEM) and thermo-gravimetric analyses (TGA). Firstly, 10 groups of sandstone samples were heated from 25 °C to 900 °C. Then, some core micro-parameters including lattice constant, full width at half maximum (FWHM), micro-strain, dislocation density, TGA curve changes and failure characteristic of the mineral were analyzed comprehensively. Finally, the underlying mechanism causing the microscopic thermal damage at different temperature intervals was also discussed. The results showed that: (1) quartz, the framework component of this sandstone, underwent an α- to β-phase change over the temperature range from 400 °C to 600 °C. This phenomenon caused the lattice constant, micro-strain, dislocation density and TGA curve to decrease sharply during this interval, leading to the microstructure deterioration of sandstone; (2) calcite underwent a decomposition reaction between 600 °C and 800 °C, and resulted in the XRD pattern peak, lattice constant, micro-strain and TGA curve dropping continuously. It destroyed further the internal microstructure of sandstone and produced numerous inter-granular cracks around quartz crystals; (3) further examination found that the decomposition reactions of minerals presented non-synchronized characteristics due to the different sensitivities of minerals to temperature, which led to thermal stress, thermal fracturing of minerals, and thermal reactions happening in different temperature intervals. Full article
(This article belongs to the Special Issue Geothermal Energy: Utilization and Technology 2018)
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Open AccessArticle
Numerical Analysis of Heat and Gas Transfer Characteristics during Heat Injection Processes Based on a Thermo-Hydro-Mechanical Model
Energies 2018, 11(7), 1722; https://doi.org/10.3390/en11071722 - 01 Jul 2018
Cited by 4
Abstract
Heat injection is an important artificial technique, which can significantly enhance the extraction efficiency of coal seam gas (CSG) and reduce the outburst risk caused by CSG. Although heat injection has been comprehensively investigated, the effect of temperature on the coal–gas interactions in [...] Read more.
Heat injection is an important artificial technique, which can significantly enhance the extraction efficiency of coal seam gas (CSG) and reduce the outburst risk caused by CSG. Although heat injection has been comprehensively investigated, the effect of temperature on the coal–gas interactions in CSG extraction is still not clear. In this study, a thermo-hydro-mechanical model was developed considering the expansion of coal mass and the change of adsorption capacity induced by heat injection. Subsequently, the reliability of the model was verified through a comparison with other theoretical models and field data. Finally, a numerical simulation and parameter analysis of the heat injection process were performed and compared with the traditional gas extraction method. The simulation results show that heat injection can significantly increase the gas production rate and cumulative gas production through the gas desorption and the permeability increase. The gas content in the coal seam dramatically decreases in the vicinity of the production and heat injection wells under the condition of heat injection, which greatly accelerates the gas drainage. The coal deformation caused by thermal-induced gas desorption has a more dominant effect on the porosity than other factors, i.e., pore pressure, thermal strain and compressive strain. Full article
(This article belongs to the Special Issue Geothermal Energy: Utilization and Technology 2018)
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Open AccessArticle
The Integration of 3D Modeling and Simulation to Determine the Energy Potential of Low-Temperature Geothermal Systems in the Pisa (Italy) Sedimentary Plain
Energies 2018, 11(6), 1591; https://doi.org/10.3390/en11061591 - 18 Jun 2018
Cited by 2
Abstract
Shallow, low-temperature geothermal resources can significantly reduce the environmental impact of heating and cooling. Based on a replicable standard workflow for three-dimensional (3D) geothermal modeling, an approach to the assessment of geothermal energy potential is proposed and applied to the young sedimentary basin [...] Read more.
Shallow, low-temperature geothermal resources can significantly reduce the environmental impact of heating and cooling. Based on a replicable standard workflow for three-dimensional (3D) geothermal modeling, an approach to the assessment of geothermal energy potential is proposed and applied to the young sedimentary basin of Pisa (north Tuscany, Italy), starting from the development of a geothermal geodatabase, with collated geological, stratigraphic, hydrogeological, geophysical and thermal data. The contents of the spatial database are integrated and processed using software for geological and geothermal modeling. The models are calibrated using borehole data. Model outputs are visualized as three-dimensional reconstructions of the subsoil units, their volumes and depths, the hydrogeological framework, and the distribution of subsoil temperatures and geothermal properties. The resulting deep knowledge of subsoil geology would facilitate the deployment of geothermal heat pump technology, site selection for well doublets (for open-loop systems), or vertical heat exchangers (for closed-loop systems). The reconstructed geological–hydrogeological models and the geothermal numerical simulations performed help to define the limits of sustainable utilization of an area’s geothermal potential. Full article
(This article belongs to the Special Issue Geothermal Energy: Utilization and Technology 2018)
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Open AccessArticle
Geothermal-Related Thermo-Elastic Fracture Analysis by Numerical Manifold Method
Energies 2018, 11(6), 1380; https://doi.org/10.3390/en11061380 - 29 May 2018
Cited by 6
Abstract
One significant factor influencing geothermal energy exploitation is the variation of the mechanical properties of rock in high temperature environments. Since rock is typically a heterogeneous granular material, thermal fracturing frequently occurs in the rock when the ambient temperature changes, which can greatly [...] Read more.
One significant factor influencing geothermal energy exploitation is the variation of the mechanical properties of rock in high temperature environments. Since rock is typically a heterogeneous granular material, thermal fracturing frequently occurs in the rock when the ambient temperature changes, which can greatly influence the geothermal energy exploitation. A numerical method based on the numerical manifold method (NMM) is developed in this study to simulate the thermo-elastic fracturing of rocklike granular materials. The Voronoi tessellation is incorporated into the pre-processor of NMM to represent the grain structure. A contact-based heat transfer model is developed to reflect heat interaction among grains. Based on the model, the transient thermal conduction algorithm for granular materials is established. To simulate the cohesion effects among grains and the fracturing process between grains, a damage-based contact fracture model is developed to improve the contact algorithm of NMM. In the developed numerical method, the heat interaction among grains as well as the heat transfer inside each solid grain are both simulated. Additionally, as damage evolution and fracturing at grain interfaces are also considered, the developed numerical method is applicable to simulate the geothermal-related thermal fracturing process. Full article
(This article belongs to the Special Issue Geothermal Energy: Utilization and Technology 2018)
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Open AccessFeature PaperArticle
An Influence of Thermally-Induced Micro-Cracking under Cooling Treatments: Mechanical Characteristics of Australian Granite
Energies 2018, 11(6), 1338; https://doi.org/10.3390/en11061338 - 24 May 2018
Cited by 32
Abstract
The aim of this study is to characterise the changes in mechanical properties and to provide a comprehensive micro-structural analysis of Harcourt granite over different pre-heating temperatures under two cooling treatments (1) rapid and (2) slow cooling. A series of uniaxial compression tests [...] Read more.
The aim of this study is to characterise the changes in mechanical properties and to provide a comprehensive micro-structural analysis of Harcourt granite over different pre-heating temperatures under two cooling treatments (1) rapid and (2) slow cooling. A series of uniaxial compression tests was conducted to evaluate the mechanical properties of granite specimens subjected to pre-heating to temperatures ranging from 25–1000 °C under both cooling conditions. An acoustic emission (AE) system was incorporated to identify the fracture propagation stress thresholds. Furthermore, the effect of loading and unloading behaviour on the elastic properties of Harcourt granite was evaluated at two locations prior to failure: (1) crack initiation and (2) crack damage. Scanning electron microscopy (SEM) analyses were conducted on heat-treated thin rock slices to observe the crack/fracture patterns and to quantify the extent of micro-cracking during intense heating followed by cooling. The results revealed that the thermal field induced in the Harcourt granite pore structure during heating up to 100 °C followed by cooling causes cracks to close, resulting in increased mechanical characteristics, in particular, material stiffness and strength. Thereafter, a decline in mechanical properties occurs with the increase of pre-heating temperatures from 100 °C to 800 °C. However, the thermal deterioration under rapid cooling is much higher than that under slow cooling, because rapid cooling appears to produce a significant amount of micro-cracking due to the irreversible thermal shock induced. Multiple stages of loading and unloading prior to failure degrade the elastic properties of Harcourt granite due to the damage accumulated through the coalescence of micro-cracks induced during compression loading. However, this degradation is insignificant for pre-heating temperatures over 400 °C, since the specimens are already damaged due to excessive thermal deterioration. Moreover, unloading after crack initiation tends to cause insignificant irreversible strains, whereas significant permanent strains occur during unloading after crack damage, and this appears to increase with the increase of pre-heating temperature over 400 °C. Full article
(This article belongs to the Special Issue Geothermal Energy: Utilization and Technology 2018)
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Open AccessArticle
Economic and Environmental Analysis of Different District Heating Systems Aided by Geothermal Energy
Energies 2018, 11(5), 1265; https://doi.org/10.3390/en11051265 - 15 May 2018
Cited by 5
Abstract
As a renewable energy source, geothermal energy can provide base-load power supply both for electricity and direct uses, such as space heating. Regarding this last use, in the present study, district heating systems aided by geothermal energy, the so-called geothermal district heating systems, [...] Read more.
As a renewable energy source, geothermal energy can provide base-load power supply both for electricity and direct uses, such as space heating. Regarding this last use, in the present study, district heating systems aided by geothermal energy, the so-called geothermal district heating systems, are studied. Thus, three different options of a geothermal district heating system are evaluated and compared in terms of environmental and economic aspects with a traditional fossil installation. Calculations were carried out from a particular study case, a set of buildings located Province of León in the north of Spain. From real data of each of the assumptions considered, an exhaustive comparison among the different scenarios studied, was thoroughly made. Results revealed the most suitable option from an economic point of view but always considering the environmental impacts of each one. In this regard, the assumption of a district heating system totally supplied by geothermal energy clearly stands out from the rest of options. Thus, the manuscript main objective is to emphasise the advantages of these systems as they constitute the ideal solution from both the economic and environmental parameters analysed. Full article
(This article belongs to the Special Issue Geothermal Energy: Utilization and Technology 2018)
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Open AccessArticle
Investigation of Flow and Heat Transfer Characteristics in Fractured Granite
Energies 2018, 11(5), 1228; https://doi.org/10.3390/en11051228 - 11 May 2018
Cited by 3
Abstract
Hydraulic and heat transfer properties of artificially fractured rocks are the key issues for efficient exploitation of geothermal energy in fractured reservoirs and it has been studied by many previous researchers. However, the fluid temperature evolution along the flow path and rock temperature [...] Read more.
Hydraulic and heat transfer properties of artificially fractured rocks are the key issues for efficient exploitation of geothermal energy in fractured reservoirs and it has been studied by many previous researchers. However, the fluid temperature evolution along the flow path and rock temperature changes was rarely considered. This study investigated flow and heat transfer characteristics of two sets of fractured granite samples each with a single fissure. The samples were collected from a geothermal reservoir of Gonghe basin in Qinghai province in China. The results show that the larger area ratio, the higher hydraulic conductivity exhibited. Hydraulic conductivity of fractured rock masses is positively proportional to injection pressure, but inversely proportional with both confining pressure and temperature. In order to analyze heat transfer during the flow process, temperature distribution along the flow path in a fracture was monitored. The temperature of the fluid was determined to increase with distance from the flowing inlet. Increasing the temperature of the rock or decreasing the injection pressure will raise the temperature at the same location. Furthermore, in order to understand the heat transfer in rock mass, temperature distribution was observed by using an infrared thermal camera. Finally, the energy exchange efficiency during the flowing process was examined. The energy exchange rate increases continuously with the rock temperature, with an effective stress ratio of 1:2. Full article
(This article belongs to the Special Issue Geothermal Energy: Utilization and Technology 2018)
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Open AccessFeature PaperArticle
Soil Thermal Balance Analysis for a Ground Source Heat Pump System in a Hot-Summer and Cold-Winter Region
Energies 2018, 11(5), 1206; https://doi.org/10.3390/en11051206 - 09 May 2018
Cited by 10
Abstract
As a renewable and high energy efficiency technology providing air conditioning and domestic hot water, the ground source heat pump system (GSHPS) has been extensively used worldwide in recent years. Compared with conventional systems, GSHPSs with heat recovery reject less heat into the [...] Read more.
As a renewable and high energy efficiency technology providing air conditioning and domestic hot water, the ground source heat pump system (GSHPS) has been extensively used worldwide in recent years. Compared with conventional systems, GSHPSs with heat recovery reject less heat into the soil and extract more heat from it, which can help reduce soil thermal imbalance in hot-summer and cold-winter regions. In this paper, conventional GSHPS, and GSHPS with different heat recovery ratios, in a typical city were compared based on thermal imbalance ratios, average soil temperatures and soil temperature increases. The transient system simulation software was used to simulate the operation performance of GSHPS. The thermal imbalance ratio and soil temperature decreased with increasing heat recovery ratio. After 20 years of operation, the soil thermal imbalance ratios of the GSHPS were 29.2%, 21.1%, 16%, and 5.2%, and the soil temperature rises were 8.78 °C, 5.25 °C, 3.44 °C, and 0.34 °C, while the heat recovery ratios were 0, 18%, 30% and 53%, respectively. Consequently, a GSHPS with heat recovery is a potentially efficient and economical approach for buildings in hot-summer and cold-winter regions. Full article
(This article belongs to the Special Issue Geothermal Energy: Utilization and Technology 2018)
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Open AccessArticle
An Experiment on Heat Recovery Performance Improvements in Well-Water Heat-Pump Systems for a Traditional Japanese House
Energies 2018, 11(5), 1023; https://doi.org/10.3390/en11051023 - 24 Apr 2018
Cited by 1
Abstract
Concerns about resource depletion have prompted several countries to promote the usage of renewable energy, such as underground heat. In Japan, underground heat-pump technology has begun to be utilized in large-scale office buildings; however, several economic problems are observed to still exist, such [...] Read more.
Concerns about resource depletion have prompted several countries to promote the usage of renewable energy, such as underground heat. In Japan, underground heat-pump technology has begun to be utilized in large-scale office buildings; however, several economic problems are observed to still exist, such as high initial costs that include drilling requirements. Further, most of the traditional dwellings “Kyo-machiya” in Kyoto, Japan have a shallow well. This study intends to propose an effective ground-source heat-pump system using the well water from a “Kyo-machiya” home that does not contain any drilling works. In previous research, it was depicted that the well-water temperature decreases as the heat pump (HP) is operated and that the heat extraction efficiency steadily becomes lower. In this study, an experiment is conducted to improve efficiency using a drainage pump. Based on the experimental results, the effect of efficiency improvement and the increase in the electric power consumption of the drainage pump are examined. It is indicated that short-time drainage could help to improve efficiency without consuming excessive energy. Thus, continuous use of the heat pump becomes possible. Full article
(This article belongs to the Special Issue Geothermal Energy: Utilization and Technology 2018)
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Open AccessArticle
The Effects of NaCl Concentration and Confining Pressure on Mechanical and Acoustic Behaviors of Brine-Saturated Sandstone
Energies 2018, 11(2), 385; https://doi.org/10.3390/en11020385 - 07 Feb 2018
Cited by 16
Abstract
To better understand the mechanical behavior of rock with brine saturation, conventional triaxial experiments were carried out on sandstone for a range of confining pressures (0–60 MPa) and NaCl concentrations (0–30%). As the confining pressure and NaCl concentration increased, the triaxial compressive strength, [...] Read more.
To better understand the mechanical behavior of rock with brine saturation, conventional triaxial experiments were carried out on sandstone for a range of confining pressures (0–60 MPa) and NaCl concentrations (0–30%). As the confining pressure and NaCl concentration increased, the triaxial compressive strength, crack damage threshold, Young’s modulus, cohesion, and internal friction angle all increased. Real-time ultrasonic wave and acoustic emission (AE) techniques were used to obtain the relationship between acoustic behavior and stress level during the whole triaxial compression process. During the whole deformation process, the evolution of P-wave velocity and accumulated AE count could be divided into four phases. The microstructural characteristics of brine-saturated sandstone, before and after loading, indicated that the strength enhancement mechanism may be attributed to an increase in inter-particle friction resulting from salt crystallisation around the points of contact. The angle of friction increased by more than 86% at maximum NaCl concentration compared to that for distilled water. The NaCl deposition in the pore space resulted in nonlinear strength increases for the brine-saturated sandstone specimens with increasing salinity. The present study is expected to improve the knowledge of the strength and failure mechanisms of sedimentary rock in deep saline aquifers. Full article
(This article belongs to the Special Issue Geothermal Energy: Utilization and Technology 2018)
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Open AccessArticle
Multi-Step Loading Creep Behavior of Red Sandstone after Thermal Treatments and a Creep Damage Model
Energies 2018, 11(1), 212; https://doi.org/10.3390/en11010212 - 16 Jan 2018
Cited by 10
Abstract
Triaxial compressive creep tests were conducted on red sandstones after different thermal treatments. Subsequently, the thermal influence on the axial, lateral and volumetric creep curves under various stress levels was analyzed. The results show that both the instantaneous and time-based deformation behaviors depended [...] Read more.
Triaxial compressive creep tests were conducted on red sandstones after different thermal treatments. Subsequently, the thermal influence on the axial, lateral and volumetric creep curves under various stress levels was analyzed. The results show that both the instantaneous and time-based deformation behaviors depended largely on the stress and temperature conditions. The instant axial strain increases linearly with increasing deviator stress and the instant deformation modulus decreases non-linearly with temperature. An interesting phenomenon was observed whereby the lateral creep strain had an apparent linear correlation with the axial creep strain. Furthermore, the fitting lines’ slopes of lateral and axial creep strain increase gradually with the increasing deviator stress at identical temperature and first decreases and then increases as temperature is elevated. Then, on the basis of the Burgers creep model and the concept of strain energy, a creep damage model implemented in FLAC3D (Fast Lagrangian Analysis of Continua 3D) is presented, and this model was able to describe the entire creep process completely including primary creep stage, secondary creep stage, and tertiary creep stage comparing with the experimental and theoretical results based on test data and numerical calculations. The influence of two damage parameters on creep curves and the thermal influence on creep parameters are subsequently discussed. Under the same stress level, the parameters K, GM and GK and ηK of creep model decrease with temperature, while the parameter ηM first augments as temperature rise to 300 °C and then decreases as temperature at above 300 °C. The higher is the temperature, the smaller the critical stress ratio (CSR). Full article
(This article belongs to the Special Issue Geothermal Energy: Utilization and Technology 2018)
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