Geothermics最新文献

Underground power cable systems (UPCSs) are generally buried close to the ground surface, exposing them to significant influences from ambient air and ground temperatures, which can affect heat dissipation and thermal efficiency. This study compares the heat transfer performance of UPCS with different cable bedding materials at critical current carrying capacity, considering the effects of ambient air and ground temperatures on system performance. The findings indicate that current carrying capacity decreases with higher ground temperatures, and that the critical ampacity leading to maximum cable temperature in UPCS is significantly influenced by actual ambient air and ground temperatures, rather than standard reference values like 20 °C. The newly developed cable bedding material, prepacked aggregate concrete (PAC), to enhance heat dissipation efficiency and prevent cable overheating is also proposed. Experimentally, PAC, with a higher thermal conductivity of 2.094 W/(m·K) versus 1.365 W/(m·K) for sand, lowers the maximum cable temperature to 70.6 °C, compared to 77.6 °C for sand under critical conditions. Moreover, the analytical solutions for ground temperature distribution models as boundary conditions are also highlighted, in which steady-state ground temperature analysis at the relevant depth may impact the accuracy of cable temperature predictions related to UPCS operation for both the system itself and the surrounding earth materials.

Internal flows (downflow and upflow) occur in geothermal wells because of different factors that affect the pressure and temperature measurements used to characterise a geothermal reservoir. In extreme cases, an internal flow can cool a hot reservoir if a sufficient flow at a low temperature is present. This work examines the internal flows, analysing two geothermal well tests from El Salvador that show downflow. We employed two approaches to model the downflow effect. Although the internal flows affect the pressure and temperature measurements, the model approaches can effectively simulate the downflow, capturing the complex interaction in the well and formation. The pressure response is more perceptible to detect a downflow because it generates instant perturbations. The temperature response experiences less noticeable perturbations because of downflows, being more appreciable by the end of the temperature derivative.

Generally, the exploitation of a high-temperature geothermal resource mainly includes heat extraction from the thermal reservoir, geothermal fluid transportation in the wellbore and energy conversion in the power generation system. As the intermediate link, the flow behaviour of geothermal fluid inside the wellbore plays a significant role in ensuring the efficient operation of an enhanced geothermal system (EGS) for the development of hot dry rock geothermal resource. In the present work, to explore the effect of the wellbore flow behaviour on the wellhead performance, a transient two-phase flow wellbore simulator is developed based on the finite element method. In the mathematical model, the fluid pressure, velocity, and enthalpy are selected as the primary variables. Shi's drift velocity model is employed to represent the velocity slip between the liquid and vapour phases, and heat transfer between wellbore and formation is described by an analytical approach. The feasibility and reliability of the presented simulator is validated with an analytical solution, numerical solutions and logging data. The novelty of present work lies in addressing the shortcomings of current studies that use single-phase flow models to estimate the wellbore productivity. The application of deliverability equation can link the wellbore with the heat extraction system of the hot dry rocks, thereby achieving integrated and efficient operation strategy management of EGS reservoir and wellbore. According to the geological conditions of Qiabuqia geothermal field in China, a comprehensive analysis is conducted on the discharge test and sensitive parameters. The results demonstrate that the decrease of fluid pressure is the root cause of flashing of the high-temperature geothermal fluid. Wellhead pressure, bottom-hole temperature and wellbore inner diameter have a significant effect on the flow behaviour of the geothermal fluid. The predicted power generation of Qiabuqia geothermal field is about 4.8 MW.

Cement pastes of different compositions were hydrated in a geothermal solution with the simultaneous action of high temperature (150 °C) and pressure (18 MPa) for 7 days. The influence of solution on phase composition and compressive strength of pastes was investigated based on the comparison with hydration in water and using TGA, XRD, FTIR, ²⁹Si, and ²⁷Al NMR. The geothermal solution accelerated hydration and pozzolanic reactions. A higher amount of more thermally stable products was formed. Undesired transformation to crystalline α-C₂SH in the samples with the highest C/S ratio was restricted. The uptake of carbonates from the solution led to an increased amount of calcite. As a result, increased compressive strength values were determined. Preferential incorporation of Al³⁺ into the structure of C-(A-)S-H or hydrogrossular phases depended on the composition of cement pastes.

For the air-water source coupling heat pump (A-WSCHP) system, the water supply temperature of the intermediate loop (T_WS-INT) directly affects the performance of the two heat pump units, which makes the system characteristics change. In this study, an A-WSCHP system located in Taiyuan, China, was experimentally studied. According to the experimental results, the effects of different intermediate loop water supply temperatures on system coefficient of performance (COP), energy consumption, and equipment characteristics under low-temperature heating conditions were analyzed. The results show that the performance of the A-WSCHP system is strongly consistent with that of the air source heat pump (ASHP) unit, as the T_WS-INT changes in the range of 25.0 ∼35.0 ℃. With the decrease of the T_WS-INT, the power consumption of the system is greatly reduced, the COP is obviously improved, and the number of starts and stops of the ASHP unit is reduced, which is more beneficial to extending the service life of equipment and improving the stability of the system.

Limiting global temperature rise to between 1.5 and 2 °C will likely require widespread deployment of carbon dioxide removal (CDR) to offset sectors with hard-to-abate emissions. As financial resources for decarbonization are finite, strategic deployment of CDR technologies is essential for maximizing atmospheric CO₂ reductions. Carbon capture and sequestration (CCS), using either direct air capture (DACCS) or bioenergy (BECCS) technologies has a particular synergy with geothermal electricity generation. This is because expensive geothermal infrastructure can be leveraged to transport dissolved CO₂ for storage in subsurface reservoirs.

Here, we present a techno-economic comparison of renewable electricity generation coupled with either BECCS or DACCS at high-temperature, low-gas hydrothermal systems. We use a systems model that quantifies energy, carbon and financial flows through a generic hybrid power plant. At a CO₂ market price of $100/tonne, the geothermal-BECCS system has a lower median cost of electricity generation ($88/MWh) than geothermal-DACCS ($181/MWh) and conventional geothermal ($89/MWh).

Geothermal-BECCS also had the lowest costs of overall emissions abatement, $122/tCO₂, accounting for carbon removal and assuming displacement of fossil-fuel generation. Abatement costs are even lower, $45/tCO₂, for BECCS retrofit of existing geothermal plants, owing to discounted costs of pre-existing injection wells, steam fields, and plant equipment.

For a case study based on a geothermal field in New Zealand's Taupō Volcanic Zone (TVZ), we determined that achieving CDR rates of 1 MtCO₂/year via new geothermal-BECCS builds would require 62 standard geothermal wells and 790 kt/year of feedstock and result in 511 MWe in installed capacity. In contrast, geothermal-DACCS would need 49 wells and no external fuel source to achieve 1 MtCO₂/year scale but result in only 190 MWe in installed capacity. Both pathways are calculated to require similar upfront investment costs at $2.2 billion and $2.3 billion for geothermal-BECCS and geothermal-DACCS respectively.

Although geothermal-DACCS removes CO₂ at high rates, its high parasitic load increases the overall decarbonization cost ($187/tCO₂). In contrast, when biomass hybridization is considered, geothermal-BECCS has a lower cost of emissions abatement and produces 20 % more electricity than the benchmark geothermal plant. We conclude that this increase in electricity production makes geothermal-BECCS the more cost-effective geothermal-based CDR configuration. Finally, we argue that revenues from net-negative CO₂ emissions and increased power production make geothermal-CDR a cost-competitive decarbonization technology.

Aquifer thermal energy storage (ATES) is attained by storing thermal energy in aquifers, using the groundwater as a carrier for the heat. Hence, in ATES systems, the background groundwater flow velocity may affect the efficiency if a significant amount of stored heat is moved away from the storage well by advection. This paper presents an alternative solution to the typical “pump and dump” open-loop shallow geothermal system configuration using the ATES concept with a reversed extraction-injection well scheme. This particular placement is able to increase the energy efficiency of a conventional open-loop system while reducing the thermal impact downstream the system.

The uni-directional ATES pumping scheme compensates the heat transport by groundwater flow extracting the groundwater from the downstream well and re-injecting back in the upstream well. This research presents a numerical feasibility study and sensitivity analysis of the effects of the well spacing, pumping scheme and groundwater flow velocity on the efficiency of a uni-directional ATES. Optimal combinations are suggested to ensure the maximum re-capture by the downstream well of the heat injected in the upstream well in the previous season and subject to thermal transport by advection, with a maximum heat recovery between 55 and 75 % depending on the conditions. The results of the modelling analysis showed that the optimal inter-well distance depends on the groundwater flow velocity and the total annual storage volume. This paper also demonstrates the mitigation effect of the thermal perturbation downstream of a uni-directional ATES compared to a conventional open-loop scheme.

The understanding of heat transport in fractures is crucial for mining geothermal systems. Studies of heat transport in natural fractures at scales comprised between those of laboratory experiments and those of field tracer tests are seldom. To bridge the gap, a joint surface with characteristic plumose was scanned in the field using LiDAR technology. The scanned surface was used to build a numerical model of mode 1 fracture. Fluid flow and heat transport were modeled solving the steady-state Stokes equation and assuming Fourier transport, respectively. We considered three different fracture apertures and varied systematically roughness in order to investigate the impact of plumose on fluid and heat transport. The 3D velocity flow fields were characterized by mean hydraulic aperture and by statistics on the directional components of the velocity vector. The method of temporal moments was used to extract first and second moments from temperature breakthrough curves. Heat transport parameters (local and macroscopic) were calculated from first and second moments.

We show that hydraulic aperture and the longitudinal component of the velocity vector decrease with increasing roughness. The local variation of heat transport parameters is controlled by fracture roughness. For the macroscopic transport parameters, several transport regimes were identified. At low fracture aperture (i.e. 1 mm), conductive regime dominates heat transport in agreement with low Péclet numbers. In this case, fracture roughness affects the transport parameters via the loss of hydraulic aperture. With higher aperture (i.e. 3 mm) geometrical dispersion regime is dominant, roughness controlling the amplitude of transport parameters. At 5 mm aperture, transition from geometrical to Taylor dispersion occurs and the roughness tends to decrease dispersion and dispersivity according to the mean flow velocity.

Shallow geothermal energy is increasingly adopted for heating and cooling purposes because of the short pay-back time of initial installation investments. As a result, a relevant concentration of Ground Heat Exchangers is being experienced in urban areas. Planning issues thus arise to manage interferences and optimize the use of underground heat resources without depletion, harm to the environment nor efficiency losses on heat pumps or plant oversizing. This study provides a rational approach to optimise geothermal resources based on the use of Geographic Information Systems and transient 3D Thermo-Hydro numerical models. An optimised semi-analytical formula for the assessment of Borehole Heat Exchangers geothermal potential in hydrodynamic conditions is developed through a parametric numerical study. The long-term performances of BHE subjected to groundwater velocity in the range of 0 to 1 m/day were analysed with multiple aquifer thermal parameters. This analytical expression allows a fast and accurate assessment of the potential even in large areas without leading to excessively conservative evaluations. This may serve designers in the preliminary sizing of installations and city planners in the development of appropriate policies for the promotion and management of shallow geothermal resources. An example of the application to the central district of the city of Turin (Italy) is also shown.

Evaluating the thermo-mechanical response of rocks under high temperature treatments is crucial for various engineering geology projects. Current predictions of rock thermo-mechanical response rely on simplistic mathematical fittings treating temperature as a reduction factor, while existing machine learning algorithms often present practical challenges due to their black-box solutions. In this study, highly practical grey-box solutions, utilizing gene expression programming (GEP) are proposed for forecasting rock strength following high-temperature treatments. The dataset, comprising temperature, rock type, rock density, sample size, crack damage stress, confining pressure, and elastic modulus, serves as input parameters, with rock strength from triaxial compression tests as the output. Three grey-box solutions (mathematical formulations) based on distinct input parameter sets are proposed, all demonstrating excellent accuracy with high R²-values (R² > 0.95) and low error values across both the training and testing phases. Feature importance analysis highlights crack damage stress, confining pressure, and elastic modulus as statistically significant parameters influencing the strength of rocks subjected to high temperatures. External validation of the proposed models indicates strong generalization capabilities, underscoring their ability to perform well beyond the training data. Furthermore, a monotonicity study demonstrates that the proposed models align with the expected physical processes. The proposed formulations offer valuable field implications, effectively addressing the limitations of labor-intensive and costly laboratory processes for evaluating rock thermo-mechanical responses.