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The Double U-pipe ground heat exchanger, known for its simple process, cost-effectiveness, high heat exchange efficiency, and low thermal resistance, remains the predominant type of ground heat exchanger in today's shallow geothermal energy development and utilization. In recent years, significant research has focused on the factors influencing heat transfer and the heat exchange performance of Double U-pipe ground heat exchangers through experimental testing methods. However, studies that integrate numerical simulation with in situ testing have been less common. Utilizing the cylindrical heat source model theory and the results of regional in situ thermal response tests, this paper develops a Double U-pipe ground heat transfer model by establishing physical, mathematical, and heat transfer geometric models. It evaluates the effects of varying inlet temperatures, flow rates, and initial ground temperatures on heat exchange efficiency under heating conditions. The results confirm the accuracy of the Double U-pipe ground heat exchanger model based on in situ testing. They indicate that increasing the temperature differential between the inlet and initial temperatures, raising the initial ground temperature, and moderately enhancing the flow rate can improve the system's heat exchange efficiency. [ABSTRACT FROM AUTHOR]

AbstractThe present study employs equilibrium molecular dynamics simulation based on the Green-Kubo approach to investigate the thermal conductivity of liquid water at the nanoscale. Specifically, the discussion centers around the influence of the size and surface wettability of two-dimensional nanochannels on the confined water thermal conductivity. The simulation results reveal a significant impact of nanochannel size on the thermal conductivity of confined water, with a decrease in channel spacing leading to a reduction in thermal conductivity below its bulk value. However, the thermal conductivity of water confined within a surface is not influenced by the water surface wettability. Additionally, the study investigates the influence of water thermal conductivity on droplet formation. The results indicate that incorporating changes in droplet thermal conductivity during the nucleation process can partially address the issue of overestimating the heat transfer coefficient for dropwise condensation. [ABSTRACT FROM AUTHOR]

The development of a novel method for modelling fluid flow and heat transfer in naturally fractured geothermal reservoirs represents a significant advancement in geothermal energy research. This Study presents a hybrid approach, which combines discrete fracture and single continuum techniques, to effectively capture the complex interactions between fluid flow and heat transfer in geothermal fractured reservoirs. In addition, the incorporation of the local thermal nonequilibrium method for simulating heat transmission accounts for the disparities in temperature between the rock matrix and the fluid, providing a more realistic representation of heat transfer processes. The study also presents a fully coupled thermo-poro-elastic framework that integrates fluid flow and heat transfer to comprehensively evaluate reservoir responses to injection/production scenarios. This coupled approach allows for the prediction of changes in reservoir properties, such as permeability and porosity, under varying fluid pressure and temperature conditions. The application of the proposed model to evaluate a geothermal reservoir's long-term response to injection/production scenarios provides valuable insights into the reservoir's behaviour and potential energy production capacity. The sensitivity analysis further enhances the model's utility by identifying the key reservoir parameters that significantly influence the thermal depletion of the reservoir. Overall, this novel modelling approach holds promise for improving the understanding and management of naturally fractured geothermal reservoirs, contributing to the optimization of geothermal energy extraction strategies. [ABSTRACT FROM AUTHOR]

The high‐voltage electric pulse fracturing (HVEPF) technology represents a novel and highly promising approach in rock fracturing. The investigation of thermal damage inflicted upon rocks by high‐voltage electrical pulses under multi‐physical field coupling is of great significance in the development of deep geothermal energy. This study establishes a damage model for rocks under electric fragmentation conditions by integrating electric field, heat transfer field, and solid mechanics field. Based on the developed damage model, the insulating properties, temperature variations, and forms of damage of rocks during electric fracturing are explored. Subsequently, the influence of voltage on rock damage status is investigated. The findings reveal that damage to the rock does not occur immediately after electrical breakdown; rather, it increases with the growth of current and temperature within the breakdown channel. Initial damage occurs at the ends of the breakdown channel, followed closely by damage in the central region of the channel. The predominant form of damage in rocks is tensile failure, with shear failure playing a secondary role, and the volume of damage increases with voltage. These results elucidate the characteristics of rock damage during electric fracturing, providing valuable insights for the engineering application of electric fracturing techniques. [ABSTRACT FROM AUTHOR]

This paper focuses on the study of the evolutionary mechanism governing the temperature field of geothermal reservoir under low-temperature tailwater reinjection conditions, which is crucial for the sustainable geothermal energy management. With advancing exploitation of geothermal resources deepens, precise understanding of this mechanism becomes paramount for devising effective reinjection strategies, optimizing reservoir utilization, and bolstering the economic viability of geothermal energy development. The article presents a comprehensive review of temperature field evolution across diverse heterogeneous thermal reservoirs under low-temperature tailwater reinjection conditions, and analyzes key factors influencing this evolution. It evaluates existing research methods, highlighting their strengths and limitations. The study identifies gaps in the application of rock seepage and heat transfer theories on a large scale, alongside the need for enhanced accuracy in field test results, particularly regarding computational efficiency of fractured thermal reservoir models under multi-well reinjection conditions. To address these shortcomings, the study proposes conducting large-scale rock seepage and heat transfer experiments, coupled with multi-tracer techniques for field testing, aimed at optimizing fractured thermal reservoir models' computational efficiency under multi-well reinjection conditions. Additionally, it suggests integrating deep learning methods into research endeavors. These initiatives are of significance in deepening the understanding of the evolution process of the temperature field in deep thermal reservoirs and enhancing the sustainability of deep geothermal resource development. [ABSTRACT FROM AUTHOR]

The use of geothermal energy is spreading globally due to its many advantages, especially for heating and cooling. The correct design of a geothermal system requires knowledge of the parameters of the subsoil rocks, and particularly the thermal conductivity (k), which is the intrinsic ability of a material to transfer thermal energy as a result of a temperature gradient. A thermal conductivity map of the geological formations is time-consuming to produce, but can be of great help when selecting the location of a low-enthalpy geothermal installation, resulting in significant savings and an increase in the efficiency of that installation. The preferred option for determining k is an in situ thermal response test, but laboratory methods may be an alternative if it is not available or affordable. In this work, the needle thermal probe method has been used to measure the k of representative outcropping rocks in Oviedo (NW Spain), since it allows to obtain a rapid determination, its cost is comparatively low and it can be implemented in a portable device. 162 measurements have been carried out on a total of 27 samples, ranging from 0.2 (clay) to 5.4 W m−1 K−1 (quartzite). A relationship has been found between the k of the rocks and their characteristics, such as mineralogy, anisotropy or geological age and a thermal conductivity map was created. [ABSTRACT FROM AUTHOR]

As a renewable energy source, geothermal energy has been widely used to provide space heating and cooling for buildings. The thermal performance of ground heat exchanger (GHE) is significant for the operating efficiency of the ground source heat pump (GSHP) systems. This paper presents a comprehensive review of developments and advances of three kinds of GHE, including vertical borehole GHE (VBGHE), Pile GHE (PGHE), and deep borehole GHE (DBGHE) which are currently popular in larger GSHP systems. Firstly, analytical models proposed to analyze heat transfer process of VBGHE with different geological conditions are summarized, such as homogenous or heterogeneous ground, with or without groundwater advection. Numerical and short-time step models and measures to improve GHE thermal performance are also reviewed. Secondly, a summary of research advances in PGHE is provided, which includes the heat transfer models of PGHE, the effects of geometric structure, operation modes, pile spacing, use of phase change material (PCM), thermal properties of PCM, thermo-mechanical behavior and/or thermal performance of PGHE. The effects of groundwater flow direction and velocity on PGHE are also summarized in brief. Lastly, models of three kinds of DBGHEs, i.e., deep coaxial GHE (DCGHE), deep U-bend GHE (DUGHE) and super-long gravity heat pipe (SLGHP), are reviewed. The physical bases of the different analytical models are elaborated and also their advantages and disadvantages are described. Advances in numerical modelling and improving numerical model calculation speed of DCBHE, DCBHE array, and DUBHE are summarized. The review provides a meaningful reference for the further study of GHEs. [ABSTRACT FROM AUTHOR]

The single-well closed-loop heat extraction technology can realize efficient exploitation of mid-to-deep geothermal resources. A two-dimension heat transfer model is established. The effects of flow rate, inlet temperature, length and diameter of insulated tubing on heat extraction power and outlet temperature were investigated by the two-way absolute grey correlation analysis method. Results indicate that thermal power increases with the increase of displacement and length of insulated tubing, and decreases with the increase of injection temperature, outer diameter of insulated tubing and inner diameter of insulated tubing. [ABSTRACT FROM AUTHOR]

In the field of scientific exploration, this study delves into the captivating realm of ground heat exchangers (GHE) by investigating the impact of rectangular protrusions strategically incorporated at the base of these systems. As vital components within geothermal systems, GHEs are responsible for efficiently transferring of thermal energy between the earth and the working fluid. However, through the ingenious integration of rectangular protuberances, the convective heat transfer dynamics within GHEs can be skillfully manipulated, potentially leading to remarkable enhancements in the heat exchange process. The maximum percentage increase in the heat transfer rate reached 23% at a distance of 2,500 meters, emphasizing the importance of strategic positioning of the protrusions. The primary objective of this research endeavor is to comprehensively examine the effects of these protrusions on the extracted temperature of the injected working fluid, thus yielding invaluable insights for optimizing GHE performance and advancing of geothermal energy utilization. [ABSTRACT FROM AUTHOR]

Copyright of Coal Geology & Exploration is the property of Xian Research Institute of China Coal Research Institute and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Coal Geology & Exploration is the property of Xian Research Institute of China Coal Research Institute and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This study presents a novel methodology for determining the apparent thermal diffusivity of subsoil in situ, employing two heat transfer models within the subsurface: one method is based on heat conduction caused by air temperature oscillations, while the other considers heat transmission via both conduction and convection due to groundwater flow. Differential equations were solved, and non-linear regression analysis was employed. This method has direct applications in various engineering and environmental domains, such as underground transmission lines, oil and gas pipelines, radioactive waste management, and geothermal systems, especially in the context of implementing horizontal geothermal collectors (HGC). The apparent thermal diffusivity value of 1.514 × 10−6 m2 s−1, within a 95% confidence interval spanning 1.512 × 10−6 m2 s−1 and 1.516 × 10−6 m2 s−1, was obtained from the section between 1.67 and 3.86 m depth in a research borehole located in Asturias, Northern Spain, using twenty-one temperature sensors. The method allowed for the calculation of the subsoil's apparent thermal diffusivity up to a depth of 14.55 m. [ABSTRACT FROM AUTHOR]

Shallow geothermal energy usually uses underground buried pipes to achieve the purpose of extracting heat while storing cold in winter and extracting cold while storing heat in summer. However, the heat transfer mechanism under the alternate operation of heat–cold extraction in winter and summer under multiple heat exchanger groups is still worth studying. Based on the constructed flow and heat transfer model in pipelines and reservoirs, this study first analyzes the temperature field evolution of a shallow buried pipe system (SBPS) under the alternate operation of heat–cold extraction, and then discusses the heat transfer performance under different pipeline flow rates, pipeline wall thermal conductivity, heat injection durations, numbers of heat exchanger groups, and flows of underground fluid. The results show that the continuous alternating process of heat–cold extraction has a promoting effect on the temperature increase or decrease in the next operating cycle due to the low- or high-temperature zone produced in the previous operating cycle. As the number of multiple heat exchanger groups increases, the heat transfer efficiency of the SBPS significantly improves. With a rise in the groundwater flow velocity, the heat transfer efficiency first decreases and then increases. [ABSTRACT FROM AUTHOR]

Geothermal energy is a kind of green and renewable energy. Conventionally, ground source heat pumps can be used to harvest geothermal energy from the subsurface. To reduce the initial investment, a good solution is to use tunnel linings as heat exchangers to extract/dump heat. This special infrastructure is called an energy tunnel. In addition to the thermal performance, the impact of pipe network configuration on thermal efficiency is still challenging in the design of energy tunnels. To solve this problem, this study makes the first attempt to carry out research on the optimization of pipe circuits in energy tunnels by a series of numerical analyses. A fully coupled thermo-hydraulic 3D finite element model is established to investigate the response of tunnel-soil interaction under cyclical thermal loading (initial soil temperature varies from 8 ℃ to 18 ℃), as well as the thermal transient interactions among air, absorber pipe, tunnel linings and ground, to quantify the amount of useful heat that can be extracted from the tunnel and the ground. On the other hand, the influence of 3 various heat-carrying pipes layout is also investigated. It is found that higher heat transfer efficiency can be obtained when the entrance and exit of pipelines are located below the tunnel in the study. The spatial location of pipelines will also affect the exchanged heat output. [ABSTRACT FROM AUTHOR]

To understand the heat transfer performance of the middle-deep U-bend geothermal well, the 2500 m deep U-bend geothermal well was taken as the research object at the Hebei University of Engineering. This study established a mathematical and physical model of heat transfer in a U-bend geothermal, and the influence of geothermal gradient and lithologic changes on heat transfer performance was considered. The established model was verified based on the experimental data. Numerical simulation results show that a reasonable reduction of inlet temperature can increase thermal power. The thermal power of the geothermal well will increase by about 40 kW for every 1°C reduction of inlet temperature. The thermal power can be increased with a reasonable increase in the flow rate of the circulating fluid, but the increment of the thermal power decreases gradually. The simulation results show that the thermal power and outlet temperature of a deep U-bend geothermal well can be effectively improved by appropriately reducing the inlet temperature, increasing the flow rate, increasing the collection length, increasing the well depth, increasing the insulation length of the vertical well section and using low thermal conductivity insulation materials. The research results from this study could provide a reference for the heat transfer system optimization of the middle-deep U-bend geothermal well. [ABSTRACT FROM AUTHOR]

Cooling magma plutons and intrusions are the heat sources for hydrothermal systems in volcanic settings. To explain system longevity and observed heat transfer at rates higher than those explained by pure conduction, the concept of fluid convection in fractures that deepen because of thermal rock contraction has been proposed as a heat‐source mechanism. While recent numerical studies have supported this half a century old hypothesis, understanding of the various regimes where convective downward migration of fractures can be an effective mechanism for heat transfer is lacking. Using a numerical model for fluid flow and fracture propagation in thermo‐poroelastic media, we investigate scenarios for which convective downward migration of fractures may occur. Our results support convective downward migration of fractures as a possible mechanism for development of hydrothermal systems, both for settings within active zones of volcanism and spreading and, under favorable conditions, in older crust away from such zones. Plain Language Summary: Geothermal energy is transferred through and stored in the rock and fluids of the Earth's crust. If temperature increases sufficiently with depth and the crust provides sufficient pathways for water to flow through, colder water sinks and percolates downward, gets heated at depth and then rises due to its lower density at higher temperature. This creates a hydrothermal circulation system that transports heat from the deep crust to shallower depths from where it can be produced. Wells drilled into these systems produce hot water and/or steam for direct heat utilization or electricity production. To understand the renewability of hydrothermal systems, we need to understand how heat is transferred deep in the crust. A hypothesis has been proposed, suggesting that fractures, propagating downwards because of contraction of the water‐cooled surrounding rock, are central to maintaining the heat transfer from the deep crust. Our computer simulations corroborate this hypothesis. Based on settings found in Iceland, we show how fluid flow and propagation of fractures can be important for development of hydrothermal systems both in active regions of volcanism and, under favorable conditions, also in older crust away from such regions. The latter results are important for the identification of hidden geothermal systems. Key Points: Numerical modeling supports convective downward migration of fractures as a source mechanism for hydrothermal systemsFluid flow, fracture opening and propagation in a thermo‐poroelastic rock mass are simulated in different geological settings in the crustCrustal stresses are key to understanding whether a hydrothermal system can evolve in regions away from active zones of volcanism [ABSTRACT FROM AUTHOR]

Dry hot rock geothermal resources by virtue of its wide distribution, large reserves, clean and low-carbon, stable, high utilization rate, and other characteristics have been widely used. The enhanced geothermal system (EGS) is the most efficient approach for harnessing and exploiting geothermal energy from hot, arid rock formations. To investigate the impact of varying parameters on heat recovery in EGS operations, we employed the COMSOL numerical simulation software to construct a seepage heat transfer model for fractured rock masses. Essential parameters and boundary conditions were established, followed by conducting numerical simulations. Through the numerical simulation results, the temporal and spatial changes of coupling effects among seepage field, stress field, and temperature field in fractured rock mass were analyzed. We investigated the impact of water injection temperature, injection-production pressure difference, injection flow rate, and initial reservoir temperature on the heat transfer process. The findings indicate that raising the water injection temperature and injection-production pressure difference can enhance the reservoir's heat recovery capability. However, it may also accelerate thermal breakthrough and reduce the system's operational lifespan. The higher injection flow rate can improve the heat recovery efficiency. However, too large injection flow can cause problems in other aspects of the reservoir; increasing reservoir temperature leads to higher production temperatures, which can potentially result in dynamic catastrophes. Therefore, while ensuring the heat recovery efficiency of the system, the operation life of the system can be extended by adjusting the water injection temperature in stages, setting a reasonable injection and production pressure difference, and selecting an appropriate injection flow rate, so as to achieve the purpose of EGS optimization. [ABSTRACT FROM AUTHOR]

Fracture networks within hot dry rock (HDR) geothermal reservoirs are complex, and heat extraction via water injection is thus a coupled process of heat-fluid-solid multifield. In this paper, utilizing the theory of normally distributed random functions, we develop a corresponding pre-processing subprogram to establish a discrete network model of complex fracture distribution in HDR reservoirs; then construct a heat-fluid-solid finite element model for heat extraction via water injection and compare the numerical solution with the analytical solution of the one-dimensional non-isothermal consolidation problem for verification. The numerical simulation results show that the main factors affecting the heat extraction efficiency of HDR reservoirs include fracture width, fracture density, fracture permeability, and matrix permeability. When a HDR reservoir is injected with water for heat extraction, there is a certain threshold value of these influential parameters, beyond which the outlet temperature drops significantly, resulting in an obvious thermal breakthrough. When injecting water for heat extraction, the values of these parameters should be controlled and kept at a reasonable level, otherwise, the HDR reservoir may enter a thermal breakthrough stage in advance, which is not conducive for long-period heat extraction. Influenced by the random distribution of complex fractures, the leading edge of the cold front may present an irregular distribution. During the process of heat extraction, the stress gradually changes from a compressional state to a tensile state, which induces further damage to the HDR reservoir. [ABSTRACT FROM AUTHOR]

Hybrid nanofluids have better thermal conductivity and heat transfer properties than common fluids and currently have good prospects for applications in enhanced geothermal development. In this paper, a convective heat transfer experimental platform was built independently to study the convective heat transfer capability of Al2O3–SiO2/water hybrid nanofluid. The effect of different parameters on the enhanced heat transfer capability of the hybrid nanofluid was investigated, and the stability of the hybrid nanofluid under flow heat transfer conditions was evaluated. The following research results were obtained. Compared to water and single nanofluid, Al2O3–SiO2/water hybrid nanofluid has better enhanced heat transfer capability. The coaxial casing with "outside in and inside out" is more energy-efficient compared with the U-shaped heat exchanger tube. For Al2O3–SiO2/water hybrid nanofluid, there was an optimal particle concentration and ratio to optimize the convective heat transfer capability: a particle concentration of 0.10 wt % and a particle ratio of 5:5. The heat extraction of the fluid was negatively correlated with the inlet temperature and positively correlated with the flow rate. However, after the flow rate was higher than a certain extent, the heat exchange energy efficiency dropped significantly. The stability of Al2O3–SiO2/water hybrid nanofluid was well maintained in the flow heat exchange process. And the higher the flow rate, the more stable the particles are. However, the enhancement effect of macroscopic flow on the particle suspension stability would be weakened by too high fluid temperature. [ABSTRACT FROM AUTHOR]

In thermal convection of water, the movement of water as a whole includes movement of not only heated water but also the motion of the water that surrounds it. It is conceivable that confusion between the movement of water and the transfer of heat is caused by not paying attention to the movement of water around the heated water. In this study, we created a teaching tool that can observe time-dependent changes of both temperature and flow velocity distributions simultaneously and clarify the movement of water around heated water. In addition, the case of a cooling experiment was similarly clarified. Two particular flows were observed in the region of water around heated water: the "movement of water that rises with the upward flow generated from the heating part" and the "movement of flowing water such that it fills the area where there was a heating part-generated upward flow." Similarly, in the case of cooling, the "movement of water around cooled water" was revealed. By using this teaching tool in the unit "Heat Transfer in Matter" of fourth-grade elementary science, it is expected that confusion between heat transfer and movement of water can be prevented. [ABSTRACT FROM AUTHOR]

The ground‐source heat pump heating and cooling system is currently the most important development and utilization method of shallow geothermal resources, but its key component is the ground heat exchanger system. Herein, a new type of ground heat exchanger, the soil‐cement energy pile, is proposed. The similarity model comparison experiments of the soil‐cement energy piles and the soil energy pile in summer and winter were conducted respectively, and the 3D numerical models of heat transfer of the two kinds of energy piles are established, their related thermal properties are verified. Then the heat transfer calculation methods of the soil‐cement energy pile are presented and validated. The research has shown that the soil‐cement energy pile has good thermodynamic performance, which effectively reduces the largest thermal resistance in the heat transfer system of the energy pile, the thermal resistance of pile body material, and the heat transfer per linear meter of the soil‐cement energy pile is 21.20%–55.60% higher than that of the soil energy pile. The soil‐cement energy pile is of great significance to improve the thermal efficiency of the ground‐source heat pump system and can reduces the overall project cost. [ABSTRACT FROM AUTHOR]

This study presents an experimental investigation to determine the heat transfer enhancement in a novel heat exchanger known as the "Helicoidal Square-Shaped Heat Exchanger" with and without using a nanofluid. The experiments were performed for the range of Reynolds numbers from 4400 to 8000, using nanofluid (Al2O3-pure water) at the concentrations 0.1, 0.25, and 0.5%. This experimental investigation found that the heat transfer ratio is improved by increasing the nanofluid concentrations and the flow Reynolds number. The highest value of the heat transfer ratio was at Re = 8000, and 0.5% concentration of nanofluids. The corresponding increment in the heat transfer rate was 13.46 %, the heat transfer coefficient augmented by 9.64 %, and the Nusselt number improved by 10.43% compared to the results obtained experimentally with distilled water. The results obtained for the distilled water were verified with the Dittus-Boelter equation and numerical simulation. In addition, all obtained experimental data were compared with the CFD simulation. The use of nanofluids for heat transfer enhancement has a wide range of applications. Therefore, the presented results suggest using Al2O3-water nanofluids to improve the efficiency of many renewable energy plants, including solar and geothermal energy systems. [ABSTRACT FROM AUTHOR]

The energy replenishment and heat convection induced by fracture water flowing through the rock mass impact the shallow geothermal energy occurrence, transfer and storage mechanisms in it. In this article, a suitability evaluation and categorization system is proposed by including judgement indexes that are more closely aligned with the actual hydrogeological conditions in fracture developed regions; an assessment approach of regional shallow geothermal energy is proposed by coupling the influences of fracture water into the calculation methods of geothermal capacity, thermal balance and heat transfer rate. Finally, by taking two typical fracture aperture distributions as examples, the impacts of fracture water on the investigation and evaluation of shallow geothermal energy are quantitatively analyzed. Although the fracture apertures only share 1.68% and 0.98% of the total length of a borehole, respectively, in the two examples, the fracture water convection contributes up to 11.01% and 6.81% of the total heat transfer rate; and the energy replenishment potential brought by the fracture water is equivalent to the total heat extraction of 262 boreholes. A single wide aperture fracture can dominate the aforementioned impacts. The research results can support more accurate evaluation and efficient recovery of shallow geothermal energy in fracture developed regions. [ABSTRACT FROM AUTHOR]

Deep geothermal reservoirs have great potential for exploitation and are characterized by high temperatures, high stress, and strong heterogeneity. However, these reservoirs contain widely and continuously distributed dominant flow channels with high permeability, predisposing these reservoirs to the formation of dominant flow, which notably decreases the efficiency of heat extraction. Focusing on the dominant flow in fractures, this study provides a definite concept, systematically reviews current studies, and puts forward suggestions for future research. It is expected that this study will serve as a reference for the sustainable, high-quality development of deep geothermal resources. [ABSTRACT FROM AUTHOR]

After the closure of an underground mine, tunnels commonly become flooded and can be potentially exploited for its geothermal resource. Developing new methods for quantifying the energy content of this reservoir with a complex geometry is important to assess the economic value of its heat resource. The objective of this paper is to provide a first-order assessment of the geothermal potential of an underground mine using the Con Mine near Yellowknife, Northwest Territories, Canada, as an example. Field data allowed the development of an improved analytical approach to evaluate the geothermal potential of the Con Mine based on a thermal power budget. The sources considered in the thermal power budget are the thermal inertia of the water in the mine and the surrounding host rock, and the water that infiltrates the mine from nearby lakes. Analytical calculations are based on the finite linear heat source equation, considering cooled or heated water in the mine tunnels as heat sources or sinks. The geometry of the underground mine is consequently simplified to a linear heat source of equivalent length and fixed radius. The Thiem equation is used to calculate the water seeping into the mine. Results obtained with this approach and considering the operation of a geothermal heat pump system indicate a total energy of 17,520 MWh y−1 and 8234 MWh y−1 that can be delivered to buildings for heating and cooling purposes, respectively. This new approach provides an improvement to the volumetric method commonly used to assess geothermal resources. [ABSTRACT FROM AUTHOR]

An advanced enhanced geothermal system (EGS) is capable of unlocking many thousands of megawatts of power in hot dry rocks, and the fracture configuration is a critical factor influencing its efficiency. A flyover-crossing fracture (FCF) configuration for EGS was proposed in this paper, to enhance heat transfer between the working fluid and reservoir and thus the productivity of EGS. A 3D transient thermal-hydraulic model was established to analyze the heat extraction of the FCF-EGS, and the effects of intersection angles of the FCF on the heat and mass transfer as well as the productivity of the EGS were examined. The FCF-EGS was found to bring higher production temperature and thermal power output than the conventional double-horizontal-well EGS during a 30-year heat recovery period, and the heat extraction capacity of the FCF-EGS was found to increase with the intersection angle. It was also found that the FCF with a 90°cross-angle has the best heat extraction performance, with the output thermal power increased by 5.12% compared to the conventional double-horizontal-well EGS, due to the enhancement of heat transfer between the working fluid and rock. Although this fracture system requires more investment in fracturing and larger pressure in production, the increase in heat production far outweighs this input. This study provides a novel potential fracture configuration for exploiting HDR geothermal energy. [ABSTRACT FROM AUTHOR]

The ground heat exchanger (GHE) is one of the main forms of geothermal energy utilization in middle-deep strata and typically includes coaxial-type deep-buried GHE (C-DGHE) and U-type deep-buried GHE (U-DGHE). This study compares the heat transfer characteristics of C-DGHE and U-DGHE. A field experiment was carried out on C-DGHE and U-DGHE at 2539 and 2781 m depths, respectively, in a housing estate in Xi'an. Simultaneously, the full-scale numerical models of both C-DGHE and U-DGHE at 2781 m were established based on the field logging data. A short-term experiment (72 h) and a long-term heat transfer simulation (five heating periods) serve to comprehensively compare the heat transfer performances of the two deep-buried pipe types. Results demonstrate that the heat transfer amount per unit meter of C-DGHE and U-DGHE were 132.18–145.91 W/m and 111.15–140.76 W/m respectively in this study, which means C-DGHE is superior to U-DGHE under similar boundary conditions when only heat transfer in buried pipes is considered. However, when heat transfer in buried pipes and water pump power consumption are considered simultaneously, a critical G (G C was ∼8.63 kg/s in this study) makes the comprehensive heat transfer performance of U-DGHE better than that of C-DGHE. [ABSTRACT FROM AUTHOR]

The transition from fossil energy to clean energy is an ongoing trend. Because geothermal energy is buried beneath oil and gas wells, it is desirable to convert abandoned oil and gas wells to geothermal energy wells. The candidate wells can be dry holes in oil and gas exploration or end-of-life oil and gas wells in depleted oil and gas reservoirs. There is a knowledge gap to fill between the oil and gas wells and geothermal wells in the well conversion engineering, that is, factors affecting the performance of the geothermal wells are not fully understood. This work investigated the factors affecting the temperature of produced water of geothermal energy wells converted from abandoned oil and gas wells. Both vertical and horizontal well options were considered. The result of the field case study using the data for a well in the Songliao Basin of Northeastern China shows that, without pipe insulation, the temperature of the returned water is very close to that of the injected water, regardless of vertical or horizontal wells. With pipe insulation, the temperature of the returned water in the horizontal well is higher than that in the vertical well. The temperature of the returned water declines quickly as the thermal conductivity of pipe insulation increases in the low-thermal conductivity region. The temperature of the returned water in horizontal wells is affected by the horizontal hole section length for heat transfer. But this effect levels off after about 1,000 m of horizontal hole section is reached, meaning that 1,000 m of horizontal hole section is adequate for heat transfer from the geothermal zone to the injected water. This paper provides an analytical method for the technical feasibility assessment of converting abandoned oil and gas wells to geothermal energy wells. [ABSTRACT FROM AUTHOR]

Copyright of Hydrogeology & Engineering Geology / Shuiwendizhi Gongchengdizhi is the property of Hydrogeology & Engineering Geology Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The ultra-long gravity heat pipe has a long heat transfer distance and narrow working fluid flow channel within its tube. Due to these unique design features, the vapor–liquid counter-flow and heat transfer characteristics of these heat pipes are more complex than those found in conventional-size heat pipes. This paper innovatively proposes the design of a segmented visualization window structure of an ultra-long gravity heat pipe, which successfully overcomes the challenge of visualizing the internal flow during operations. A visualization experimental platform, measuring 40 m in height with an inner diameter of 7 mm and the aspect ratio up to 5714, was built to investigate the evolving characteristics of two-phase flows with an increasing heat input and the impact of the phase change flow characteristics on the thermal performance of ultra-long gravity heat pipes. The results obtained can provide guidance for the development of the internal structure of ultra-long gravity heat pipes that are being applied in exploiting geothermal energy. The results show that, at low heat input (200 W, 250 W), there are separate flow paths between the condensate return and the steam, but the high hydrostatic pressure due to the height of the liquid injection results in the presence of an unsaturated working fluid with a higher temperature in the liquid pool area, which has a lower evaporation rate, limiting the heat transfer through the heat pipe. It is found that if increasing the heat input up to 300 W, the evaporative phase change in the heating section becomes intense and stable. At the same time, despite the intermittent formation of liquid columns in the adiabatic section due to the vapor–liquid rolls, which increases the resistance to the vapor–liquid counter-flow, the liquid columns are blocked for a short period of time, and the path of steam rises and the condensate return is smooth, which does not seriously affect the steam condensation and liquid return evaporation. At this point, the overall temperature of the heat pipe is evenly distributed along the tube and the heat transfer performance is optimal. When the heat input further increases (350 W, 400 W), a large amount of condensate is trapped in the upper part of the adiabatic section and the condensing section by long liquid plugs for a long time. At this point, the condensate flow back to the heating section is significantly reduced, and the steam is seriously prevented from entering the condensation section, resulting in a significant increase in the temperature gradient between the lower part of the evaporating section and the upper part of the adiabatic section and deterioration of the heat transfer performance. [ABSTRACT FROM AUTHOR]

The current work reports a numerical investigation of the water produced and thermal performance of a solar still (SS). Using a SS for desalination is a proposal for low-income remote communities needing potable water. The study deals with the SS under five different concentrations of salt (0, 5, 10, 20, and 35 g/kg). Previous experimental results reported in the literature indicate that the increase in salinity leads to a decrease in productivity, so phase change material (PCM) was added under the water basin to counter the reduction. The mathematical model and numerical methodology were validated by comparing them with experimental results reported in the literature. The relative difference between temperatures was less than 2%, and for water production, it was less than 3.5%. The present mathematical model has the novelty of utilizing the water properties as a function of temperature and salt concentration, contrary to other models that use pure water properties. The results show that daily productivity decreases when the salinity increases from 0 to 35 g/kg. For each case, the time evolution of hourly and cumulate productivity is presented, as well as water temperature and the temperature difference between water and glass. Also, the behavior of heat flux between water and PCM is analyzed. The overall efficiency is calculated for all the cases. [ABSTRACT FROM AUTHOR]

Open-loop borehole heat exchanger (OBHE) is a single well geothermal heat exchanger with an open-loop structure that can realize the geothermal energy extraction without mining the geothermal water. In this paper, a sandbox experiment is designed to simulate the convective heat transfer process in the reservoir area of OBHE. The mechanism of convective heat transfer in the reservoir area is studied, and the key factors that affect the convection heat transfer intensity are analyzed. The results show that the convection heat transfer of OBHE in the reservoir area is affected by both the driving effect of fluid flow inside the screen tube and the buoyancy effect. In the forward flow mode, the two effects have the opposite direction. While in the backward mode, the two effects have the same direction. The backward flow mode is more conducive to convective heat transfer. In addition, many factors influencing significantly the convective heat transfer of OBHE include inlet temperature, inlet flow rate, reservoir temperature, fluid flow direction and inner tube diameter. [ABSTRACT FROM AUTHOR]

The Eastern Helan Mountain can be the largest concentrated and contiguous producing areas for the wine grape in China. Specifically, the wine grape planting area reached 388.7 square kilometers in Ningxia by the end of 2022, accounting for nearly 1/3 of the nation, and the comprehensive output value exceeded 30 billion Yuan. However, the extremely low temperatures (down to -30.0℃) in winter in the eastern part of the Helan Mountains can lead to the occurrence of overwintering frost damage, which greatly limits the sustainable development of Ningxia's grape industry. In this study, a gravity heat pipe system was designed using shallow groundwater geothermal energy for the soil warming in the root zone of grapes. Numerical simulation was also carried out to analyze the influence range and distribution of the gravity heat pipe temperature field under different spacing. A series of field experiments were used to verify the working performance of the gravity heat pipe and the warming characteristics of the soil. The results show that the soil temperature field was evenly spread around the gravity heat pipe. The temperature field around the heat pipe was divided into four regions, according to the temperature level: the temperature range was 5.0-8.50℃ high-temperature affected region â… , the temperature range was 0-5.0℃ medium-temperature affected region â…¡, the temperature range was -2.57-0 ℃ low-temperature affected region â…¢, and the temperature field was not affected region â…£. The highest temperature (8.50 ℃) appeared in the temperature field area â…  condensation section pipe wall, whereas, the lowest temperature (-2.57 ℃) was in the temperature field unaffected area â…£. In addition, the diameter of the medium temperature affected area of the heat pipe in the soil was 30.8 cm horizontally and 32.8 cm vertically, where the grape roots were not affected by frost. There were evenly distributed temperature fields around the heat pipes partially overlapped and the temperature fields under the conditions of 10 cm burial depth and 15 cm spacing between adjacent heat pipes. The soil temperature increased by 7.0 ℃ on average in the experimental group during the test period, compared with the control group, indicating the outstanding increase in temperature. The maximum temperature difference was 3.5 ℃ between the evaporation and condensation sections during the normal operation of the gravity heat pipe. The maximum temperature differences were 1.0 and 0.2 ℃ within the evaporation section, and within the adiabatic section, respectively, indicating the excellent isothermal distribution. The average start-up and operating temperature difference of the heat pipe were 5.4 and 2.9 ℃, respectively. The clean and renewable shallow geothermal energy was taken as a source of heating energy in the root zone of grapes, which was in line with the current green development concept. The finding can provide the theoretical basis and technical support to treat the frost damage in the root zone of grapes. In addition, a way was proposed to realize the development and utilization of shallow geothermal energy using gravity heat pipes. The Eastern Helan Mountain can be the largest concentrated and contiguous producing areas for the wine grape in China. Specifically, the wine grape planting area reached 388.7 square kilometers in Ningxia by the end of 2022, accounting for nearly 1/3 of the nation, and the comprehensive output value exceeded 30 billion Yuan. However, the extremely low temperatures (down to -30.0℃) in winter in the eastern part of the Helan Mountains can lead to the occurrence of overwintering frost damage, which greatly limits the sustainable development of Ningxia's grape industry. In this study, a gravity heat pipe system was designed using shallow groundwater geothermal energy for the soil warming in the root zone of grapes. Numerical simulation was also carried out to analyze the influence range and distribution of the gravity heat pipe temperature field under different spacing. A series of field experiments were used to verify the working performance of the gravity heat pipe and the warming characteristics of the soil. The results show that the soil temperature field was evenly spread around the gravity heat pipe. The temperature field around the heat pipe was divided into four regions, according to the temperature level: the temperature range was 5.0-8.50℃ high-temperature affected region Ⅰ, the temperature range was 0-5.0℃ medium-temperature affected region Ⅱ, the temperature range was -2.57-0 ℃ low-temperature affected region Ⅲ, and the temperature field was not affected region Ⅳ. The highest temperature (8.50 ℃) appeared in the temperature field area Ⅰ condensation section pipe wall, whereas, the lowest temperature (-2.57 ℃) was in the temperature field unaffected area Ⅳ. In addition, the diameter of the medium temperature affected area of the heat pipe in the soil was 30.8 cm horizontally and 32.8 cm vertically, where the grape roots were not affected by frost. There were evenly distributed temperature fields around the heat pipes partially overlapped and the temperature fields under the conditions of 10 cm burial depth and 15 cm spacing between adjacent heat pipes. The soil temperature increased by 7.0 ℃ on average in the experimental group during the test period, compared with the control group, indicating the outstanding increase in temperature. The maximum temperature difference was 3.5 ℃ between the evaporation and condensation sections during the normal operation of the gravity heat pipe. The maximum temperature differences were 1.0 and 0.2 ℃ within the evaporation section, and within the adiabatic section, respectively, indicating the excellent isothermal distribution. The average start-up and operating temperature difference of the heat pipe were 5.4 and 2.9 ℃, respectively. The clean and renewable shallow geothermal energy was taken as a source of heating energy in the root zone of grapes, which was in line with the current green development concept. The finding can provide the theoretical basis and technical support to treat the frost damage in the root zone of grapes. In addition, a way was proposed to realize the development and utilization of shallow geothermal energy using gravity heat pipes. [ABSTRACT FROM AUTHOR]

In this study, the fullerenes have been inserted into water base-fluid to investigate the atomic and thermal behavior of nanofluid and hybridnanofluid as heat transfer fluid. This choice derives from low cast and high thermal stability of this nanostructure. Our computational results from Molecular Dynamics (MD) simulations indicate that the addition of nanoparticles with 4% atomic ratio produced an appreciable effect on the nanofluid. The maximum value of density, velocity, and temperature profile have reached to 0.029 atom/(cubic°A), 0.005 °A/ps, and 321 oC. Its thermal conductivity would increase to 0.82 W/mK Heat flux reached to 2019 kW/m2 after t= 10 ns. Also, the aggregation phenomenon detected after t= 5.84 ns. This hybridnanofluid has been used to enhance the energy efficiency of the heat exchangers at high temperature for the nuclear industry applications for the first time. Numerically, by the temperature increase of nanofluid structure to 625.15 K, thermal power of nanofluid reached to 3881 MW. The thermal performance of hybridnanofluid can be improved by more than 30% by adding concentration of fullerene and doped fullerene at 4 vol%. [ABSTRACT FROM AUTHOR]

The aim of this research is to develop a fractional supported thermal model for studying the features of modified hybrid nanofluid endorsed by uniformly accelerating plate. The novel impact of this work is observing the comparative thermal enhancement of water base fluid by utilizing four types of nanoparticles. The silver, copper, aluminum oxide and titanium oxide nanomaterials are utilized to present the comparative thermal aspect of modified hybrid nanofluid model. Moreover, the inclined direction of magnetic impact is treated. The second-grade nonlinear model is used to explore the base fluid properties. The fractional model is first attained into dimensionless form. The fractional computations with employing the Prabhakar fractional mathematical definitions are reported. The motivations for suggesting the Prabhakar algorithm are justified as this fractional algorithm contains modern definitions without any restriction of singularities. The verification of model is accomplished after simulating the comparison task with already performed studies. The physical dynamic and thermal enhancement of transportation phenomenon is performed for specific range of flow parameters like 0. 1 ≤ α ≤ 0. 7 , 0. 1 ≤ β ≤ 0. 7 , 0. 1 ≤ γ ≤ 0. 7 , 0. 7 ≤ Pr ≤ 4. 5 , 0. 0 1 ≤ φ ≤ 0. 0 4 , 0. 8 ≤ Sc ≤ 2. 6 , 1. 7 ≤ Gr ≤ 3. 7 , 1. 5 ≤ Gm ≤ 3. 2 and π 2 ≤ 1 ≤ π 6 Based on the computational model, it is concluded that the thermal transportation phenomenon is more impressive for water-based titanium oxide nanoparticles. The temperature profile rises due to factional parameter for both copper–water- and sliver — water-based hybrid nanofluid suspension. [ABSTRACT FROM AUTHOR]

The heat exchanger can use shallow geothermal energy to provide regional heating and cooling demand in winter and summer. In this paper, a large-scale public building is taken as the example, and the energy system in the building is taken as the research object. Firstly, through the collection of geothermal drilling geographic information and geothermal data, the geothermal reserves and geothermal recoverable resources are evaluated. Secondly, the cooling and heating demand of the building is calculated by using HVAC simulation software. Then, the heat transfer capacity of a single pile is evaluated and the lay-out scheme of the underground heat exchange pile foundation of the building is given. The actual heating effect of the heat exchange pile foundation system is explored, and the heat transfer characteristics of heat exchange pile foundation under different working conditions are compared and analyzed. Finally, reasonable suggestions for the arrangement of heat exchange pile foundation are given. [ABSTRACT FROM AUTHOR]

As a kind of clean and renewable energy, shallow geothermal energy has the characteristics of wide distribution and huge reserves. The use of pile foundation heat exchange for building heating is a new technology to realize the effective utilization of geothermal energy. Improving the mechanical properties of the pile body material and improving the heat exchange efficiency of the pile foundation concrete will effectively promote the application of concrete in building structures. In this paper, the effect of steel fiber content on the thermal conductivity and mechanical strength of concrete is discussed by carrying out physical and mechanical properties test of concrete material of pile body. It is proposed to incorporate steel fibers into the concrete of the pile body to improve its thermal conductivity, thereby improving its heat transfer efficiency per unit time. At the same time, the role of steel fiber in enhancing heat transfer performance in pile material is analyzed from the microscopic mechanism, which provides theoretical support for the optimization process of concrete ratio of pile foundation in practical engineering. [ABSTRACT FROM AUTHOR]

The global energy demand has put forward new requirements for building cooling and heating. As a renewable energy, shallow geothermal resources have attracted much attention. Ground source heat pumps are an effective utilization form of geothermal resources, most of which use U-shaped borehole heat exchangers to exchange heat with rock and soil mass. Therefore, the thermal physical parameters of rock-soil and the stratum structure will affect the heat transfer performance of the borehole heat exchanger. In this paper, a sandbox test bench and a 3D single U numerical model are constructed for the main geological conditions in karst areas to explore the heat transfer law of borehole heat exchanger under special geological structure. The results indicate that: (1) The thermal conductivity of limestone is higher than that of dolomite and sandstone, and the limestone stratum has strong heat transfer capacity and the fastest heat recovery rate. In order to avoid the thermal accumulation caused by the long-term operation of the system, the recovery time of stratum temperature field shall be kept at 1:1 ratio to the operation time of the system; (2) The average heat exchange of the three layers is 22.6 W/m, which is better than that of the double layers 21.4 W/m and the single layer 18.4 W/m. The heat transfer capacity of layered strata is better than that of single-layer strata, and they have layered characteristics; (3) The heat transfer capacity of borehole heat exchanger increases with the increase in thermal conductivity along the vertical direction of the formation. Each increase of 0.4 W/(m·K) in the thermal conductivity of the lower rock-soil increases the heat exchange per unit well depth by about 6 W/m; (4) The calculation equation of heat transfer under multi-layer stratum structure is proposed by using the calculation method of comprehensive thermal conductivity. [ABSTRACT FROM AUTHOR]

Thermal transport investigation in Al2O3–H2O and Al2O3–H2O over sensor surface is significant from medical sciences, biological, and chemical engineering. Therefore, the analysis is conducted to investigate the thermal transport against the preeminent parameters. The nanofluid models are transformed in self-similar version via similarity transformations and then treated numerically. It is perceived that the velocity drops against higher values of ϕ. The thermal behavior of the nanofluids enhances for multiple permeable parameter and volume fraction. It is also perceived that thermal performance in γAl2O3–H2O prevailed throughout the analysis. Therefore, these nanofluids are good conductor and better for aforementioned applications. Moreover, comparative analysis proved the authenticity of the analysis γ. [ABSTRACT FROM AUTHOR]

The heat and moisture transfer through dry and wet fabric made up of hydrophobic fibers was modeled and experimentally verified. In the modeling, the governing equations are obtained with consideration of the energy balance and the mass balance of the liquid water, where the specific initial and boundary conditions were set according to measurement of the cooling property of fabric. By comparing temperature changes at the fabric inner surface, the model validity was confirmed by the agreements between the experimental results and numerical solutions. Thus, this simulation was able to predict the cooling performance of fabrics. The effects of internal and external factors on heating/cooling performances of fabrics were analyzed by this model. With an increasing evaporation rate, thermal conductivity and water content of fabric, the cooling capability of fabric was improved. Surrounding conditions with lower temperature, lower relative humidity and higher air velocity demonstrated a positive effect on fabric thermal and moisture transfer performances, which benefits the thermal comfort of the human body after excessive sweating. [ABSTRACT FROM AUTHOR]

In the era of high energy demand caused by rapid population growth, renewable energy is required in the energy mix for the sustainability of both human and industrial development. This paper investigates the potential of extracting heat energy from abandoned oil and gas wells for electricity production. Four wells located in the Niger Delta were considered. Each well has its properties and steam flow rates. The geothermal reservoir was modeled as a liquid-vapor-dominated system at in situ reservoir conditions. The fluid temperature in the reservoir was determined to be 170oC in the reservoir. Equations describing geothermal resource potentials, energy flow in a wellbore, and gross electrical power production using turbines were presented. A Single-flash turbine system was used because steam was produced in the reservoir. MATLAB software was used in the simulation of the heat transfer process. The results showed that the geothermal energy corresponding to the reservoir considered was 2.45 x 1012 Joules. The electricity production result showed that there was a reduction in gross electrical power output due to an increase in sink temperature, while the relationship between gross electrical power output and separator temperature was such that there was an initial increase in gross electrical power output due to an increase in separator pressure until an optimum point was reached wherein further increase in separator pressure progressively decreased the value of the gross electrical power output. The optimum separator temperatures for well 1, well 2, well 3, and well 4 were 120oC, 120oC, 110oC, and 105oC respectively at a sink temperature of 45oC. Because of the high reservoir pressure used, the separator pressure was higher than the actual reservoir pressure. Proper understanding of heat flow which enables accurate estimation of heat production is necessary to maximize heat extraction from the reservoir. The simulation conducted reveals that geothermal resources holds great potentials for electrical energy generation and should be put to developmental use in Nigeria as a means to increase the total power generation and increase the portion of renewables in Nigeria energy value chain. [ABSTRACT FROM AUTHOR]

Using super-long gravity heat pipes (SLGHPs) to exploit deep-earth geothermal energy has indicated its technical superiority and viability; yet there is no practical application so far. The present work reports an SLGHP geothermal space heating system constructed in Taiyuan, China, which has two SLGHPs (2020 m and 2180 m long, respectively) in combination with a single heat pump. The two wells are located only ∼30 m apart, and the well-log temperature at 2000 m depth is around 63 ° C for both. Temperatures measured by the optical fiber arranged along the SLGHP outer wall show remarkable uniformity, indicating the good performance of SLGHPs. Testing with the SLGHP system finds that the slightly longer SLGHP extracts 70 % more heat than the other one. Analyses reveal that the heat transfer in the geothermal formations surrounding the shorter SLGHP approximately follows the heat conduction regime while the convection of groundwater contributes less; the high yield of the slightly longer SLGHP is due to an interesting downhole heat transfer enhancement mechanism arising from the interlayer crossflow of groundwater. Further, a numerical model is developed to predict the SLGHP geothermal system's performance in 120-days' operation. It is found that this system can output 1 MW of heat, sufficing space heating of ∼25,000 m2 buildings. The simulated 20-years' operation indicates the thermal output degradation is 20.4 % for the shorter SLGHP, whereas it is only 6.8 % for the slightly longer SLGHP; the downhole groundwater crossflow lowers the SLGHP system degradation rate. • Super-long gravity heat pipe is practically used for geothermal exploitation. • A geothermal space heating system of two ∼2000 m long heat pipes is constructed. • Optical fiber temperature monitoring corroborates the heat pipes' good performance. • Performance-enhancement relying on interlayer groundwater crossflow is revealed. • The system can output 1 MW heat, sufficing space heating of ∼2.5 × 104 m2 buildings. [ABSTRACT FROM AUTHOR]

Deep-buried pipe-group heat transfer is a primary form of utilizing geothermal energy in the middle and deep ground layers. This study introduces a methodology to simplify the heat transfer process of a pipe-group into a single borehole within a finite computational domain. The proposed methodology allows for rapid and accurate assessment of the heat transfer performance of the pipe-group with coaxial deep-ground heat exchangers (C-DGHEs). The feasibility of this simplified method is supported by numerical validation. Consequently, a 20-year numerical computation is performed for pipe-group models with spacings of 15, 20, 30, and 40 m, alongside a comparative study on the heat transfer of a single borehole C-DGHE with a computational domain radius of 200 m. For a pipe-group spacing of 15 m, the results demonstrated that the heat transfer boundary is exceeded in the first year of heat extraction but stabilized into a steady decline. For a spacing of 40 m, the decrease in heat transfer over 20 years was less than 15 % compared to the single C-DGHE. This research effectively evaluates the heat transfer performance of pipe groups, providing theoretical support and technical guidance to develop and promote the applications of deep geothermal energy. • A simplified pipe-group of C-DGHE heat transfer algorithm was proposed. • Quasi-3D full-size numerical models for pipe-group were established. • Heat transfer characteristics of five pipe spacing in pipe-group were analyzed. • Variation rule of ground temperature under different pipe spacings was expounded. [ABSTRACT FROM AUTHOR]

Exploiting heat from abandoned mine water using heat pumps is attracting increasing attention worldwide. China has a large number of abandoned coal mines, but there are few applications of geothermal heat recovery from mine water. Here, we report a new case from Shandong, North China. Based on actual geological and underground space structures, numerical analysis is performed to study the flow and heat transfer in the complex connection network among roadways. For given temperature differences on the source side, the underground heat transfer is greatly influenced by flow rates in roadways. Under the acceptable performances and long-term stability of heat pumps, the maximum heat transfer capacity of roadways reaches 697.3 kW at the flow rate of 100 m3/h, with the power per unit length of 151.3 W/m, indicating a promising heat recovery potential. When switching between the extraction and injection well, both fluid directions and the distribution of flow rates in roadways change obviously, thereby affecting the underground heat transfer. Besides dominant flow paths, there exist weak flow paths, which can be identified by numerical analysis, and necessary blocking can be applied to optimize the flow and heat transfer. Excessively long horizontal straight roadways for heat exchange should be avoided. [ABSTRACT FROM AUTHOR]

Deep geothermal energy has tremendous potential for decarbonizing the heating sector. However, one common obstacle can be the mismatch between geologically attractive regions in the countryside and urban areas with a high heat demand density, which are therefore attractive for district heating systems. In the last years, an increasing number of regions consider the transport of geothermal heat into urban clusters. One example of such a region is the South German Molasse Basin in Upper Bavaria. However, such heat transport pipelines come along with massive upfront investment costs due to the required large pipe diameter and insulation thickness. While the classic concept foresees the use of water as a heat carrier in such long-distance heat transportation pipelines, CO 2 can be an attractive alternative. This study investigates the thermo-economic performance of CO 2 as a heat transport carrier for a potential long-distance heat transmission pipeline with a length of 20 km, which could connect a planned geothermal project in the South of Munich with the existing district heating network of Munich. The results of the base case scenario demonstrate that for both heat carrier options water and CO 2 rather low LCOH for the transport of the heat can be achieved. • A Bavarian case study for long-distance heat transport has been analysed. • CO 2 and water-based systems LCOH are compared to identify the most attractive one. • The cost of the system appears to be dominated by the heat exchangers cost. • The LCOHs of the solutions are comparable but CO 2 -based system is more flexible. [ABSTRACT FROM AUTHOR]

• A three-dimensional unsteady state mathematical model of the multi-field coupling of backfill body, tube and fracture was established. • A test bench was designed and built to verify the three-dimensional coupling model. • The effects of a single fracture position, aperture and fractured depth on the heat extraction performance of a full-size BFHEs unit in the fully dry or fully saturated conditions were investigated. • The combined effects of multiple fractures on the performance of a BFHES unit were discussed. Thermal hazards in deep mines can be mined synergistically with the deposit as associated geothermal energy resources. Backfill heat exchangers (BFHEs) as a combination of mine backfilling and ground heat exchanger technology are one of the effective methods to realize ore deposit-geothermal energy synergy mining. However, in the complex environment of deep mines, BFHEs will inevitably suffer from fracture damage during long-term heat storage/release cycle operation, which will affect their working performance. To address this problem, this paper established a scaled-down experimental setup for fractured BFHEs units, and used the experimental data to verify the accuracy of the established three-dimensional transient heat transfer mathematical model of the BFHEs unit. Numerical simulation methods were used to investigate the effects of the position, aperture and fractured depth of a single fracture on the heat extraction performance of the BFHEs unit under fully dry or fully saturated working conditions. Significant differences were found in the effect of different fracture positions on the heat extraction capacity of the BFHEs unit. The extraction of geothermal heat by the BFHEs unit was favored when it was in the middle of two tubes, while the opposite was true for other positions. The effect of fracture on the heat extraction capacity of the BFHEs unit increased linearly with fracture aperture. The effect of the fractured depth was relatively greater in the 1/2–3/4 H b range. The presence of groundwater significantly reduced the effect of the fracture. The effect of the fracture on the heat extraction capacity of the BFHEs unit was reduced by more than 80 % by changing the operating conditions from fully dry to fully saturated condition. The overall effect of a fracture on the heat extraction capacity of the BFHEs unit was not significant, with a 5 mm wide fracture having an effect of no more than 2.5 %, even under the fully dry condition. However, there was a combined effect of multiple fractures, and for a deposit size of 900 × 7 × 80 m, ten 2 mm apertures fractures would results in a maximum reduction of geothermal extraction from BFHEs by 8527 GJ in 10 years, which was equivalent to the heat load of a 13,753 m2 building for one winter in the cold climate of China. [ABSTRACT FROM AUTHOR]

Shallow geothermal energy is a clean and effective form of energy that can overcome the problems associated with the depletion of carbon-based energy carbon emissions. Due to the special hydrogeological conditions in karst regions, the heat transfer between heat exchange boreholes and the ground formation is a complicated, multi-physical process. The abundant groundwater flow plays an important role in the heat transfer process, and even presents an opportunity to mitigate the heat imbalance during the long term operation of ground-coupled heat pumps (GCHP). In this study, both laboratorial experiments and numerical simulations were performed to analyze the mechanism that shows how fracture water impacts on heat capacity and the thermal imbalance of the energy storage rock mass. The results showed that the overall temperature fluctuation of the rock mass was reduced by the fracture water, and the temperature curve with time became gentler, which means in practice that the heat imbalance in the rock mass could be delayed. However, the temperature contour map showed that the impact of the fracture water flow was constrained in the nearby areas and decreased obviously with distance. The temperature field was also dragged along the direction of the fracture water flow. During the shutdown period, the fracture water significantly enhanced the thermal recovery ability of the rock mass. The results will assist in further understanding the mechanism of heat transfer and energy balance in a rock mass with fracture water flow. It is proposed that the U pipes should be located at zones with abundant fracture water if the construction condition permits. U pipes that are near the fractures should share more of the load or a denser layout could be possible as their heat transfer capacity is improved by the water flow. [ABSTRACT FROM AUTHOR]

The study of heat transfer in ground heat exchangers (GHEs) considering the fluid advection inside the pipes; the heat transfer between the fluid and the ground through the grout material; and the thermal interaction between GHEs is a challenging task. The present paper presents a new semi-analytical method that takes into account the aforementioned effects to consider both the short- to long-term effects of GHEs. The heat transfer between the fluid and grout was studied by a transient multipole expansion considering time-dependent fluid temperatures and an advection model for the pipes obtained from an energy balance on the heat carrier fluid. Thermal interactions were analyzed using an equivalent borehole method while penalizing the transient multipole expansion to include thermal interaction effects. Validation of the short-term predictions was performed by comparing the proposed model to experimental data found in the literature and to an FEA model. The proposed model was then compared with a FEA model in long-term simulations of a geothermal field comprised of 24 GHEs for multi-annual simulation. The method resulted in substantial reduction in computational time while preserving good accuracy when compared with the FEA model. [ABSTRACT FROM AUTHOR]

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Human interference with the Earth's climate cannot be ignored any longer. Renewable energy sources need utmost attention in all energy sectors. For buildings, geothermal energy for heating, cooling, and domestic hot water is a sustainable solution. Horizontal ground heat exchangers (HGHE) demonstrate promising results with low installation costs. Research is focused on increasing their thermal performances by structural improvements and ground thermal proprieties improvements, with little research on using auxiliary heat to increase their performances. A numerical model for an HGHE was established to investigate the effects of auxiliary heat sources on the performances of the HGHE. The results demonstrate that heat transfer into the HGHE increases the overall ground temperature at the end of the heating season by 138.50% compared with no heat transfer from auxiliary heat sources. The ground freezing period decreased by approximately 24.74% by having a heated basement, approximately 40.20% by transferring heat with solar thermal panels, and approximately 62.88% by using both auxiliary heat sources. The difference between the undisturbed ground temperature and the ground temperature with no auxiliary heat sources at the end of the season was 3.45 °C. The difference between the undisturbed ground temperature and the ground temperature with all auxiliary heat sources resulted in 0.92 °C. [ABSTRACT FROM AUTHOR]