Search Results (337)

The Yishu Fault Zone (YSFZ) is located in a key tectonic transition zone shaped by the interaction between the Pacific and Tethyan tectonic domains in eastern China. Despite sparse geothermal borehole coverage across this region, the deeply incised fault structures create favorable hydrogeological prerequisites for fault-mediated subsurface heat migration and hydrothermal fluid circulation. This study integrates hydrochemistry, multi-isotope tracing (δD, δ¹⁸O, ⁸⁷Sr/⁸⁶Sr, and δ³⁴S), multi-mineral equilibrium modeling, and silica–enthalpy mixing analysis to constrain the evolution process and genetic mechanism of geothermal groundwater in the middle segment of the YSFZ. The geothermal groundwater displays weakly alkaline to alkaline properties and in situ temperatures of 35.2~75.0 °C, which is characterized by the HCO₃–Na, SO₄–Na, and Cl·SO₄–Na type. Stable isotope signatures demonstrate that the geothermal groundwater is recharged by the atmospheric precipitation with elevations of 822~1274 m. The hydrochemical evolution of the geothermal waters is governed by silicate weathering, evaporite dissolution, and cation exchange. The ⁸⁷Sr/⁸⁶Sr ratios indicate mixed solute contributions from silicate and evaporite lithologies, whereas the δ³⁴S signatures suggest that SO₄²⁻ is predominantly derived from gypsum dissolution. Two distinct hydrochemical evolution patterns can be identified in the study area. Samples GG1 and GG5 are characterized by HCO₃⁻ enrichment, whereas GG2, GG3, and GG4 exhibit enrichment in Na⁺ and SO₄²⁻. Reservoir temperatures estimated using multi-mineral equilibrium geothermometry range from 56.7 °C to 92.1 °C, with circulation depths of 1648~2304 m and cold-water mixing ratios of 48%~69%. The results of this study provide geochemical evidence for hidden geothermal resource exploration in deep fault zones. Full article

Offshore geothermal energy and the reuse of decommissioned oil and gas platforms are emerging as linked pathways for reducing the carbon intensity of marine energy supply while extending the value of mature offshore assets. This review examines offshore geothermal development from a full-chain perspective that connects resource assessment, platform and wellbore reuse, heat extraction, medium- and low-temperature conversion, multi-energy coupling, techno-economic evaluation and environmental risk management. The paper first clarifies the resource logic of offshore geothermal systems, especially sedimentary-basin resources that spatially overlap with mature petroleum provinces. It then analyzes two principal engineering routes: the reuse of existing offshore platforms as energy hubs and the reutilization of abandoned wells as open-loop or closed-loop heat-extraction systems. The review finds that platform and wellbore reuse can reduce drilling demand, shorten offshore construction cycles and lower life-cycle environmental burdens, but engineering feasibility remains constrained by wellbore integrity, thermal losses, corrosion and scaling, platform life extension, regulatory liability and the limited availability of field-scale demonstration data. Coupling geothermal energy with offshore wind power, hydrogen production, OTEC and desalination can improve system stability and equipment utilization; however, standardized assessment boundaries and comparable cost models are still insufficient. Future research should focus on resource-engineering-economic integrated assessment, standardized reuse packages, long-term offshore reliability databases, corrosion-resistant material systems, auditable TEA/LCA models and risk-based regulatory frameworks. This review provides a technical basis for offshore geothermal pilot projects and for the low-carbon transformation of offshore oil and gas infrastructure. Full article

To tackle the intermittency of renewable energy and realize the repurposing of abandoned oil and gas wells, this study proposes a compressed air energy storage (CAES)-based cogeneration system integrated with geothermal energy and abandoned oil and gas wells, and conducts a five-dimensional comprehensive analysis covering exergy, exergoeconomic, exergoenvironmental, economic and environmental performance. The optimal operating parameters are determined as air compressed to 200 bar, an ORC turbine inlet pressure of 16 bar and an inlet temperature of 110 °C. The system’s annual total power generation is 2,971,416.5 kWh during low-power daytime operation, and 20,131,785 kWh during high-power nighttime operation. Compared with conventional CAES systems, the proposed system reduces total exergy destruction by 4121.35 kW and increases exergy efficiency from 48.49% to 63.38%. Coolers, geothermal heat exchangers and compressors are the main sources of exergy destruction cost and capital investment, while COM1, HE1 and HOT1 are the key components causing environmental impacts. The system realizes cogeneration of power, hydrogen and pure water, with a static payback period of about 5.4 years and significantly reduced TEWI value at elevated turbine inlet pressure. This system achieves multi-objective synergies in energy efficiency, economy and environment, providing a feasible scheme for the green repurposing of abandoned oil and gas wells and cascaded utilization of renewable energy. Full article

The open-source Python package Geoloop introduces a novel, semi-analytical model for predicting the performance of deep (>500 m depth) vertical borehole heat exchangers (BHEs), with a focus on capturing depth-dependent variations in subsurface thermal properties, i.e., geothermal gradient and thermal conductivity. Conventional computationally efficient semi-analytical models based on load-aggregation of g-functions often assume uniform subsurface thermal properties. Geoloop addresses this gap by implementing a vertically stacked approach, allowing for realistic simulation of depth-variability in both the subsurface and borehole material properties. The model is benchmarked in the shallow domain against standard depth-uniform g-function implementations (up to 100 m depth) and for deeper conditions with a numerical finite volume model, demonstrating strong agreement and validating its accuracy and efficiency. Simulations for typical Dutch conditions show that deeper BHEs (up to 2000 m) can achieve significantly higher thermal power supply than shallower systems, and results in terms of resulting inlet/outlet temperatures for given heat extraction rates can strongly deviate (>4 °C) from results obtained by depth-uniform assumptions in thermal properties. Application of the model to the Dutch context reveals a non-linear increase in heat extraction potential with depth, surpassing values assumed in common practice by Dutch industry. The results highlight the importance of considering local geological heterogeneity and depth-dependent properties for accurate deep borehole heat exchanger (BHE) performance assessment and system optimization. Geoloop thus offers a robust, versatile platform for advancing the design and analysis of deep vertical BHE systems. Full article

Engineered geothermal system (EGS) cross-well flow of 30 L/s producing heat at a rate of Q~20 MW for 30 days was achieved by the UtahForge project in 2024. The cross-well flow doublet measured ℓ~400 m in length at L~100 m vertical offset. A first-order question is how sustainable the doublet’s 20 MW heat extraction is. Where once the answer would be framed in terms of pipe-like cubic-law flow along stress-aligned fault-scale planar heat exchange surfaces, UtahForge flow data rule out this heat exchange picture. The EGS flow data indicate aquifer-like volumetric cross-well flow with heat exchange at the grain scale. More specifically, the EGS flow data indicate no cross-well flow for a dozen hydrofrack attempts, while the 30 L/s flow occurred when the 400 m doublet wells were rendered effectively open to the crustal formation by drilling out all hydrofrack gear. An essential further observation is that the producer well flowed at only 70% of the injector rate: 30% of injected fluid was lost to flow heterogeneity in the cross-well volume. A four-step deconstruction of these observations explicitly characterizes the flow heterogeneous volume: (i) flow stimulation of the cross-well volume, (ii)wellbore-centric flow in/out of cross-well volume along the 400 m open well reach, (iii) heat advection in the cross-well volume, and (iv) sustainability-specific heat conduction into the cross-well volume. EGS stimulation process step (i) is attested by microseismic emissions (Meqs) registered on downhole sensors. Meq size and spatial correlations in turn reflect the flow heterogeneity of the cross-well volume. EGS step (iv), crustal heat conduction sustainability, is approximated by assuming radial heat energy extraction at rate Q/ℓ by a central line-sink of radius R < L/2. The line-sink analytic solution yields heat reservoir sustainability of ~3–10 years. Greater sustainability at Q/ℓ rate requires larger cross-well offsets L. The intimate relation between fluid flow and seismic emissions enables downhole seismic sensor data to image EGS flow stimulation activity. The future of EGS heat extraction depends to a large degree on feasible sizes of cross-well offset L in the flow-heterogeneous crust. Full article

Thermal conductivity is a key input parameter in geotechnical and shallow geothermal engineering, directly influencing the design, efficiency, and long-term performance of ground heat exchangers, energy piles, and ground-source heat pump systems. Reliable parameterisation of this property in sandy soils remains challenging due to nonlinear interactions between water content, bulk density, and soil structure. This study develops a machine-learning-based workflow for robust parameterisation of thermal conductivity in quartz-rich sands using a large, internally consistent laboratory dataset comprising 1716 samples, including 1455 moist measurements used for modelling, obtained from nationwide site investigations. Air-dry specimens were identified as laboratory-induced drying states and excluded to restrict the analysis to hydro-mechanical conditions representative of typical shallow subsurface environments. Several regression algorithms representing different modelling strategies were evaluated within a unified and reproducible framework and benchmarked against selected classical empirical formulations. Model performance was assessed using standard accuracy metrics together with diagnostics describing the functional stability of predicted thermal-conductivity surfaces. The results reveal a systematic trade-off between predictive accuracy and functional consistency, indicating that models optimised for accuracy may produce functionally unstable and less suitable parameterisations for engineering applications. Accuracy-optimised models frequently produce locally irregular parameter fields, whereas more strongly regularised models yield smoother and physically more coherent response surfaces. The proposed workflow supports reliable thermal-property parameterisation for geotechnical design and shallow geothermal modelling. Full article

Measuring soil moisture is crucial for optimizing agricultural irrigation, but also, from an energy efficiency standpoint, for the proper design of very-low-enthalpy geothermal energy (VLEGE) facilities. VLEGE represents a renewable energy resource with great potential for residential and industrial applications, as it can provide heating and cooling with high energy efficiency and minimal environmental impact. Soil moisture plays a decisive role in the thermal performance of VLEGE facilities, where small variations in water content can significantly alter the thermal conductivity of the soil and, consequently, the efficiency of their horizontal heat exchangers. This paper presents a low-cost capacitive soil moisture sensor featuring optimized interdigitated electrodes and a controlled dielectric coating that ensures mechanical and electrical stability in subsurface environments. The novelty of this work lies in the validated integration of optimized IDE design, dielectric protection, embedded capacitance acquisition, and gravimetric calibration into a low-cost soil water content measurement device for environmental, agricultural, and VLEGE applications. The developed system converts capacitance variations into direct estimates of soil water content through an integrated microcontroller-based signal-conditioning stage. The developed device is robust, reliable, and readily reproducible. Furthermore, given its low cost (around €50 if manufactured manually; mass-produced it would be much cheaper) and its excellent sensitivity and precision, it is ideal for setting up continuous monitoring networks, even for domestic applications, both in VLEGE installations and in other application domains, such as agriculture and environmental monitoring, where soil moisture measurement is a crucial parameter. This work contributes to the development of more efficient and accessible solutions for harnessing geothermal energy, particularly in installations where dynamic tracking of soil moisture is essential to ensure stable long-term performance. Full article

Geothermal energy is a clean, renewable, and baseload-stable resource of strategic importance for carbon neutrality. Hot dry rock (HDR) reservoirs are characterized by high temperatures, great depths, and abundant reserves. However, their extremely low natural permeability requires artificial fracturing to establish effective heat exchange networks. Conventional hydraulic fracturing in enhanced geothermal systems (EGS) faces major challenges under HDR conditions, including excessive water consumption, strong water–rock interactions, and elevated induced seismicity risks, limiting its engineering applicability. Waterless or low-water fracturing technologies offer alternative stimulation pathways due to their distinctive physicochemical properties. Existing reviews have mainly addressed individual aspects, such as specific fracturing media or proppant transport, without systematically integrating recent advances in supercritical CO₂ fracturing, foam fracturing, liquid nitrogen fracturing, and hybrid-fluid fracturing technologies, or comprehensively evaluating their engineering implications. This review systematically analyzed the fracturing mechanisms, heat exchange performance, environmental risks, and HDR-specific engineering challenges of these technologies. Results indicate that waterless/low-water fracturing technologies enhance heat extraction efficiency by generating complex fracture networks while mitigating seismic and reservoir damage risks. However, large-scale application requires further advances in the high-temperature stability of fracturing media, material durability, multiphase flow control, and field validation. Full article

This work experimentally investigates the behavior of a new indoor air conditioning system based on the application of Peltier cells in a Horizontal Air–Ground Heat Exchanger (HAGHE). To this end, a laboratory-scale prototype focusing exclusively on the terminal section of the system was developed and tested under controlled conditions. A series of configurations was tested, each representing an evolution of the previous one. The results highlight the strong dependence of system performance on airflow velocity, applied voltage, and heat dissipation effectiveness, demonstrating both the potential and the critical limitations of the proposed configurations. The most promising results were obtained in the advanced (fourth and fifth) configurations, yielding average temperature increases of approximately +1.9 °C on the hot flow and decreases ranging from −1.0 °C to −1.7 °C on the cold flow at moderate total voltages (40–50 V) and higher airflow velocities (0.5–0.6 m/s). In line with the principles of the circular economy, the prototype was constructed using recycled materials, including plastic pipes and Peltier cells recovered from discarded devices. Full article

Compared with previous analytical designs for deep UBHE, the present study is new in three aspects: (1) a segmented FLS model combined with the virtual heat source method is applied to the full U-shaped path (injection, horizontal, and production wells) in a unified formulation; (2) equivalent thermal conductivity is introduced to account for groundwater seepage in porous media, avoiding the need for separate CFD or coupled numerical solvers; (3) the relationship between production well depth and the maximum effective insulation length is quantified and discussed. Deep U-shaped borehole heat-exchangers (UBHE) systems boast high heat-exchange efficiency, yet most analytical models are too simplistic, causing inaccuracies. This study proposes a segmented finite line source (FLS) model for UBHE using the virtual heat source method. Introducing equivalent thermal conductivity (k_equ), it treats rock-soil as a groundwater-saturated porous medium, coupling seepage’s dynamic heat-transfer impact. By comparing the simulation results of the same type of research within 720 h, the average temperature difference between the models was found to be 1.31 °C, with an error rate of 5.31%, which is 40.87 percentage points lower than the existing achievements, thereby demonstrating the accuracy of this model. In addition, based on this model, the influence trends of five main factors such as seepage velocity and geothermal gradient on the system’s heat exchange were drawn and analyzed. Among them, the laying length of the insulation layer was analyzed in detail. The results show that its maximum laying length should be in line with the depth node where reverse heat exchange occurs with the production well. Under the set conditions of this study, when the depth of the production well is 2500 m, the maximum laying length of the insulation layer is 1900 m. Full article

Global climate change has intensified the need for clean and stable energy sources. Geothermal energy, with its consistent availability, is crucial for the transition to renewable energy systems. This study aims to numerically evaluate the enhancement of heat extraction in a mid-deep coaxial geothermal heat exchanger (GHE) when using water-based Al₂O₃ and SiO₂ nanofluids. A comprehensive 1D pipe flow- and 3D subsurface heat transfer-coupled model was developed and validated against field experimental data. The results demonstrate that the nanofluids significantly enhanced heat extraction. The water–SiO₂ nanofluid achieved the highest outlet temperature, exceeding pure water by approximately 0.2 °C after 2000 h. A lower inlet temperature of 5 °C increased heat extraction by 88.57% compared to 25 °C, despite a lower outlet temperature. The thermal influence radius expanded from <2 m at 300 h to ~6 m at 1800 h. This study provides quantitative insights and a validated framework for optimizing GHE performance through nanofluid selection and operational control. Full article

Heat pumps are an essential part of decarbonization of buildings, providing efficient and sustainable heat. Analyzing the future contribution of decentralized heat pumps requires an estimation of the potential of local environmental heat sources at high spatial resolution. These heat sources are mainly ambient heat and shallow geothermal heat, and the analysis needs to account for the space available around the building, noise immissions due to the heat pump, and heat exchange rate with the ground. So far, no consistent analysis of available heat sources for both residential and non-residential buildings is publicly available. In this study, we present a GIS-based methodology developed for estimating the maximum achievable heating capacities with heat pumps and publish them at building level. It can be shown that buildings in rural areas have a median air-source heat pump potential of 30 to 85 kW, while the median in urban areas is roughly 100 kW, with quieter options up to 300 kW. This shows that even in big cities, heat pumps have potential. In most regions, the potential for air-source heat pumps is higher than ground-source heat pumps. This study lays the foundation for investigating the competitiveness of heat pumps compared to other heating options. Full article

The medium-deep coaxial borehole heat exchanger (CBHE) has been demonstrated to exhibit excellent heating performance. To further investigate the efficient heat transfer characteristics of novel inner-tube materials, a numerical simulation approach based on OpenGeoSys was employed. This approach was used to systematically examine and compare the thermal performance of novel and conventional pipes under various operational scenarios during both short- and long-term operation. The results show that the novel geothermal pipe was found to possess superior thermal insulation performance over the conventional pipe. During operation, an increase in thermal resistance of 0.5 m·K/W (approximately a 30.5% improvement) and a higher pressure drop of 0.4 MPa (approximately a 12.3% rise) were observed. This contributed to an outlet temperature approximately 1 °C higher (approximately a 2.5% improvement) and a temperature drop 2.5 °C lower (approximately a 19.2% reduction). Over a 20-year simulation, the novel pipe achieved a heat exchange capacity approximately 3.5 kW greater than that of the conventional pipe (approximately a 1.5% improvement). Full article

Geothermal energy assumes an increasingly crucial role in advancing carbon neutrality. However, heat transfer calculations for shallow ground-heat exchangers (GHE) face challenges, including large computational loads for pipe arrays and insufficient long-term operational analysis. This study proposes two key innovations: first, the introduction of the Response Factor Method (RFM), which accelerates long-term heat-transfer calculations by constructing a coefficient matrix library; second, a dimension-reduction algorithm for large-scale pipe arrays (LADR), balancing simulation speed and accuracy. The simulation model is developed and validated experimentally, with the simulated outlet temperature showing a 0.2% average relative error compared to measured values, with a 20-times speed-up of simulation time compared to the original method. Moreover, the LADR can realize a reduction in calculation load into only two or three boreholes while the neglectable errors do not affect numerical results. The study found that heat extraction increases linearly with borehole depth, but with diminishing returns. Increasing pipe diameter and spacing enhances heat extraction, while overloading reduces reliability. Intermittent operation significantly boosts the load-bearing capacity of individual pipes. The thermal effect radius during the transitional period is larger than that during the heating/cooling periods. We observed and explained the ground heat accumulation in a thermally balanced system for the first time. Additionally, there are differences in thermal performance at different borehole locations within the array, along with a load transfer effect. This research provides valuable insights for optimizing shallow GSHPs. Full article

Urban shallow geothermal systems are increasingly adopted for low-carbon heating and cooling, yet their performance and environmental impact depend on vertical heat transfer processes that are often simplified, particularly across the unsaturated zone that links the urban surface and groundwater. This study quantifies the buffering role of the unsaturated zone and assesses how its explicit representation affects predicted geothermal thermal impacts in an urban alluvial aquifer. We combine multi-depth temperature observations from instrumented piezometers and thermocouple arrays in the Zaragoza alluvial aquifer (NE Spain) with a three-dimensional transient groundwater-flow and heat-transport model implemented in FEFLOW. Model performance was evaluated by comparing simulated temperature profiles against field observations at −2 m, −5 m, and the water table, yielding root mean square errors (RMSE) of 1.24 °C, 0.58 °C, and 0.42 °C, respectively. Scenario simulations show strong damping and phase delay of seasonal signals through the unsaturated zone and indicate that surface heat exchange controls shallow thermal amplitudes (up to approximately 10 °C at approximately 1 m). Simplified configurations that neglect the unsaturated zone and/or surface heat transfer bias impact assessments by increasing simulated aquifer warming (up to 1 °C at the end of summer injection periods) and altering plume intensity and geometry (plume extents on the order of 80 m laterally in the analyzed configuration). These results underline that urban geothermal assessments require field-constrained representations of unsaturated-zone heat transfer and realistic surface boundary conditions to support sustainable subsurface energy planning. Full article