By integrating heat exchange pipes into the segments of a shield tunnel, the tunnel not only serves as a structural load-bearing element but also extracts shallow geothermal energy from the surrounding environment, providing a sustainable energy source for building heating and cooling. This study integrates heat exchange pipes into shield tunnel segments, enabling energy tunnel segments (ETS) to extract shallow geothermal energy while serving structural functions for sustainable building heating and cooling. A coupled 3D thermo-hydraulic finite element model of ETS in a cross-river double-layer shield tunnel is developed to assess long-term heat transfer performance under various parameters and connection configurations. The results show that overlying water's thermal buffering stabilizes soil temperature and reduces seasonal power fluctuations. Higher seepage velocity increases power fluctuations in cross-river tunnels but reduces them in mountain tunnels. Water-soil interface geometry has little impact at low seepage velocity but becomes critical as seepage increases, with water's high heat capacity mitigating thermal disturbance. Parameter analysis finds inlet temperature difference, flow rate, pipe conductivity, fireproof board thickness, and segment number (at constant flow) significantly enhance heat exchange with stable gains; airflow velocity, water head, porosity, pipe diameter, and segment spacing have smaller or nonlinear effects. Optimizing connection configurations by adding series branches to parallel setups (PCE-S, PCU-S) improves flow distribution and reduces interference, with PCE-S showing superior long-term performance and lower COP degradation after five years. This study provides an understanding of the thermohydraulic behavior of ETS in complex cross-river tunnel environments, offering guidance for the design and optimization of sustainable energy tunnel systems.

Energy tunnels, which integrate underground construction with geothermal utilization, provide a promising approach for urban renewable energy development. However, in shielded energy tunnels, the asymmetry of heat exchanger arrangement and tunnel lining heat transfer conditions results in significant circumferential non-uniformity, and the heat flux dynamically varies with operating conditions, making traditional models based on constant heat flux and axisymmetric assumptions inadequate. This study proposes a segmental analytical heat transfer model for shielded energy tunnels, accounting for variable heat flux and asymmetric bidirectional heat transfer at the tunnel lining inner and outer surfaces. Under the assumption of uniform axial temperature, the model analytically predicts fluid temperature along the flow direction and the surrounding medium temperature field, and can be extended to multi-layer linings via superposition, enabling refined characterization of temperature and heat transfer in multi-ring energy tunnels. Validation against summer and winter field data shows that the root mean square errors of fluid temperature predictions are 0.40 degrees C in summer and 0.48 degrees C in winter, demonstrating high accuracy. Parametric analysis indicates that increasing inlet temperature or flow rate enhances circumferential heat flux non-uniformity and lining surface temperature differences, which may induce thermal stress. Increasing heat exchanger thermal conductivity improves overall heat transfer but has limited impact on circumferential non-uniformity. Multi-loop operation analysis shows that series operation maximizes heat transfer, achieving 1.52 % higher performance than parallel operation in summer and 2.76 % higher in winter. Parallel operation achieves higher system efficiency but limited heat transfer, while hybrid operation balances efficiency and heat transfer, offering the best overall performance. The proposed model overcomes limitations of existing analytical studies assuming constant heat flux and single-ring structures, providing a refined tool to describe circumferential non-uniformity and multi-ring effects, and offers a theoretical basis for optimizing energy tunnel design and thermo-mechanical coupling studies.

To enhance the thermo-mechanical performance of phase-change energy piles (PEP), this study proposes a topology-optimized phase-change energy pile (TPEP). A multi-objective optimization framework integrating topology optimization algorithms is employed to identify the optimal zones of thermal storage composite concrete (TSCC). Numerical models are developed to investigate the effects of U-pipe configurations, groundwater seepage, and TSCC distribution across the array of TPEPs under different weights (w1-w4). Results indicate that S (w1 = -0.5, w2 = 0.5) and R (w1 = 0.5, w2 = -0.5) enhance heat storage and heat release, respectively. With fewer U-pipes, moderate thermal loads, and short-term operation, S maximizes heat storage while minimizing soil disturbance; with more U-pipes, higher loads, and long-term operation, R provides superior sustained heat release. Groundwater seepage modifies optimal TSCC zones, with S-W and R-W enhancing heat storage and release, though S-W is less effective than S when maximizing heat exchange power. Stress optimization indicates that S-S (w1 = 0.2, w4 = 0.8) minimizes total stress but reduces heat exchange, whereas S-G (w1 = 0.2, w3 = 0.8) balances stress mitigation with high heat transfer. Across TPEP arrays, central TSCC concentration enhances heat storage, outermost concentration enhances heat release, and seepage promotes more uniform distribution.

In this paper, an experimental investigation was conducted to study the heat transfer and flow characteristics of latent functional thermal fluid (LFTF) in a microchannel liquid-cooling plate with different pin fins (triangular, cylindrical, and cubic). A water-based microencapsulated phase change materials suspension (MPCS) was prepared and used as the LFTF. The effects of mass concentration, Reynolds number, and pin fin structure of microencapsulated phase change material (MicroEPCM) on heat transfer and flow performance of MPCS were investigated. The results indicated that the thermal resistance and heat transfer coefficient of MPCS were significantly better than those of water. Among the three types of pin fin liquid-cooling plates, the cubic pin fin plate has the highest friction factor, while the cylindrical pin fin plate has the lowest friction factor. When the Reynolds number is 400, the comprehensive evaluation coefficients for 2.5 wt% MPCS and 5 wt% MPCS in the cylindrical pin fin liquid-cooling plate were 1.83 and 2.56, respectively. These findings demonstrated that MPCS exhibits higher convective heat transfer performance compared to water. The pin fin structures enhance the comprehensive heat transfer performance of the fluid, with the cylindrical pin fin structure offering the best overall performance. The results presented in this paper would help understanding heat transfer and flow performance of LFTF in microchannel and for designing microchannel liquid-cooling plates.

The formation of financial risk involves the interaction of many factors, showing a highly nonlinear characteristic relationship. In this study, an intelligent financial risk assessment system based on deep learning (DL) is constructed to solve the key problems of the traditional pre-alarm mechanism, such as insufficient identification accuracy and poor timeliness. This model gives full play to the advantages of Convolutional Neural Network (CNN) in complex feature extraction, and automatically captures the deep-seated related features in financial data through end-to-end learning. The research shows that, compared with traditional methods, this system shows excellent prediction performance on three independent data sets, while maintaining high operational efficiency. This innovative scheme, which integrates the functions of intelligent pre-alarm and dynamic assessment, provides an accurate and efficient decision support tool for modern enterprise risk management. The theoretical contribution and practical value of this study are mainly reflected in: first, a new paradigm of financial risk analysis is put forward; second, an intelligent risk control system with practical application potential is developed.

This study addresses the issue of performance degradation in medium-deep ground source heat pump (MD-GSHP) systems caused by thermal imbalance during over prolonged periods of operation. A hybrid solution is proposed, integrating pile foundation ground heat exchangers (GHE) with medium-deep borehole heat exchangers (MDBHE) to meet the demands of space heating and cooling. Dynamic simulation models for the MD-GSHP system, pile foundation hybrid system, and solar-coupled MD-GSHP system were developed on the TRNSYS platform. Through a ten-year simulation and multi-dimensional comparison, the collaborative energy supply patterns between pile foundation GHEs and MDBHEs were analyzed. Results show the pile foundation hybrid system manages about 30 % of peak heating load and 45 % of cooling load, effectively reducing MDBHE heat extraction. After ten years, its inlet water temperature increased by 48.55 % and 11.61 %, and outlet water temperature by 31.92 % and 9.14 %, compared to the MD-GSHP and solar-coupled systems, respectively. The pile foundation hybrid system's average coefficient of performance (COP) was 3.73, improving by 11.53 % and 8.85 % over the MD-GSHP and solarcoupled systems, with energy consumption reduced by 10.11 % and 9.20 %. Annual operating cost was 61,100 CNY, saving 10.15 % and 9.21 %. Coal consumption was 408.27 tons, with energy savings consistent with the above reductions. This study confirms the effectiveness of the "shallow peak shaving" and "deep base load complementary" strategy, demonstrating its potential to provide an efficient, stable, and economically viable pathway for sustainable building energy systems under cold-climate conditions.

In this paper, an experimental investigation was arranged to study the heat transfer and flow characteristics of latent functionally thermal fluid (LFTF) in circular tubes with different fins (smooth tube, straight finned tube, and cross finned tube). The water-based microencapsulated phase change materials suspension (MPCS) as LFTF was prepared. The effects of microencapsulated phase change materials (MicroEPCM) mass concentration, inlet temperature, heat flux, and circular tube structure on heat transfer and flow performance of MPCS were investigated. The results indicated that the convective heat transfer capacity of MPCS was significantly better than that of water under the same experimental conditions. The optimal inlet temperature and heat flux were 29 degrees C and 4301.48 W/m2, respectively. Among the three types of circular tube structures, the cross-fin tube exhibited the best enhanced heat transfer capability but also resulted in the highest pressure drop loss. When the heat exchange section was a smooth circular tube, the comprehensive evaluation coefficients of 5 wt% MPCS and 10 wt% MPCS were approximately 1.3 and 1.5 times higher than that of water, respectively. This demonstrated that MicroEPCM can enhance the overall heat transfer capability. This study investigated the heat transfer and flow characteristics of LFTF in circular tubes, contributing to the expansion of its practical applications electronic device heat dissipation and battery thermal management.

Beishan granite, China's candidate host rock for high-level radioactive waste (HLW) disposal, experiences alterations in strength and failure characteristics under prolonged thermo-mechanical coupling conditions, impacting the long-term stability evaluation. In this study, the strength and failure behavior of Beishan granite specimens, which were heated to 25, 200, 300, 400, 500, and 600 ℃, and subsequently cooled to room temperature, were investigated by triaxial tests under confining pressures of 5, 15, and 25 MPa. The results indicated that the triaxial compression strength (TCS) exhibited non-monotonic strength variation with temperature. Below 300 ℃, TCS increases by 10.2-14.7% through crack closure from differential thermal expansion (quartz α = 11 × 10⁻⁶/℃ and feldspar α = 5 × 10⁻⁶/℃) and evaporation-induced effective stress enhancement. Beyond 400 ℃, TCS shows an average decline of 14.27%, primarily governed by intergranular cracking as expansion stresses exceed intergranular bonding forces. Acoustic emission monitoring revealed that crack propagation transitions from distributed micro-fracturing to localized macro-cracking as the temperature exceeds 400 °C. Moreover, the strength degradation rate with increasing temperature of Beishan granite declines from 19.4% to 9% with increasing confining pressure (5 to 25 MPa), demonstrating that elevated in-situ stresses (representing greater disposal depths) effectively suppress thermal damage. These findings establish a critical temperature threshold and depth-compensation principle, providing thermo-mechanical design criteria for HLW repository engineering.

This study proposes a topology-optimized phase-change energy wall (TPEW) enhanced with PCM-enhanced concrete (PEC) and develops a coupled three-dimensional thermo-hydro-mechanical finite element model to systematically analyze the heat exchange performance and thermo-mechanical coupling characteristics of TPEWs under various optimization strategies and boundary conditions. Results show that TP-G and TP-T distinctly affect PEC zones depending on pipe configurations: for DW 1U-pipe, TP-G shifts PEC toward the soil, while TP-T shifts it toward the pipe; for DW 2U-pipe, TP-G forms X-shaped PEC, while TP-T forms droplet-shaped PEC near the pipes; for PW 1U-pipe, TP-G distributes PEC horizontally, while TP-T distributes it vertically; for PW 3U-pipe, TP-G forms I-shaped PEC, while TP-T creates six isolated zones. Under seepage, TP-T shifts PEC downstream as seepage velocity v(w) increases, whereas under airflow, TP-T shifts PEC toward the soil-side as airflow velocity v(a) increases. Temperature-dominated topology (TP-T) enhances heat exchange power, gradient-dominated topology (TP-G) reduces thermal stress and thermal interference TP-T further improves power. Compared with conventional center-backfilled (CB), the maximum power increases of TP-T under pure soil, seepage, and air boundary conditions are 25.7%, 6.2% and 20.5%, respectively. Under pure soil, seepage, and air boundary conditions, TP-TI further enhances power for larger pipe configurations, achieving increases over TP-T of 12.6%, 19.5%, and 5.4%, and reduces heated zones by up to 22%. In summary, the topology-optimized PEC zones effectively enhance power, reduce thermal stress and long-term stability, providing practical guidance for the design and optimization of phase-change energy walls under various pipe configurations and boundary conditions.

This study investigates the effects of different sealing materials, steel lining and Fiber-Reinforced Polymer (FRP), on the thermodynamic properties of hydrogen storage in lining rock caverns (LRCs). A thermodynamic coupling equation for hydrogen storage in LRCs was developed, and its validity was confirmed by comparison with simulations and data from the Huntorf power station. The leakage rate, temperature changes, and stress distribution of steel and FRP sealing layers with varying thicknesses were analyzed over a 30-day period. The results show that the leakage rate of the FRP layer decreases significantly with thickness, with a peak leakage of 1.3 g/s for the 10 mm layer, stabilizing at around 0.1 g/s for the 100 mm layer. Temperature fluctuations within the cavern are periodic, with more pronounced changes in the FRP layer as its thickness increases. Steel lining experiences minimal temperature changes. Additionally, increasing the FRP thickness results in larger fluctuations in the principal stress of both the sealing and lining layers, impacting the surrounding rock mass. The FRP lining experiences higher tensile stress, while the steel lining shows smaller changes. A thicker FRP layer also increases normal and shear stresses in the surrounding rock, potentially affecting cavern stability. These findings emphasize the importance of selecting appropriate sealing materials and thicknesses to improve the efficiency and stability of hydrogen storage systems.

The hot and toxic smoke is a major reason for deaths and injuries in tunnel fire hazards, therefore, it is of vital importance for safe evacuation to effectively control the smoke. This paper proposed a smoke control strategy, i.e., completing smoke extraction, and developed the design criterion of exhaust rate based on small-scale experiments and theoretical analysis. The heat release rate (HRR), damper length, and interval were considered. Experimental results showed the critical exhaust rate for completing smoke extraction rose with the increase in HRR and declined with a growing damper interval. Besides, it first rapidly decreased and then turned to be smooth with rising damper length. Subsequently, the ratio of the suction force, Fd to the force, Fs was adopted to determine the completing smoke extraction using force analysis. The results illustrated that the HRR and damper interval barely affect the relative magnitude between the suction force, Fd, and the force, Fs. It was linearly dependent on the dimensionless damper length and then exponentially grew. The critical length of the damper was 0.1 m. Finally, a prediction model was established, and the evaluated results deviated from the experimental data within 15

The expansion of megacities and the need to reduce carbon emissions have prompted the exploration of urban underground space as an important option. However, the lack of quantitative analysis on the low carbon effect of underground space hinders its further development. This study aims to address this gap by proposing a method to quantify the low carbon capacity and carbon emissions of urban underground space. The dynamic relationship between these factors under different development intensities is also investigated. A case study was conducted in the Nanjing Xinjiekou area, using data on underground space development since 2006. The results indicate that the low carbon capacity of underground space is lower than the carbon emission in the initial stage. However, over time, the net CO2 emission gradually decreases and eventually reaches zero carbon emission. The time to achieve zero carbon emission depends on the intensity of underground development and socioeconomic factors. When ignoring socioeconomic impact, a higher intensity of underground space corresponds to a faster time to reach zero carbon emissions. However, excessively high levels of development can result in a high vacancy rate, which negatively affects the low carbon capacity and delays the achievement of zero carbon emissions. Therefore, an optimal development intensity of underground space can be determined to minimize the time required to achieve zero carbon emission. Furthermore, a quantitative relationship between the optimal development intensity and urban economic indicator (i.e. civilian motor vehicles in this study) was established in this study based on zero carbon emission analysis. These findings provide valuable insights for achieving faster carbon neutrality in underground space development and inform future city planning strategies.

In this paper, a finite element numerical model of thermal-hydro-mechanical of energy piles under multi-layer geological conditions was established, and field tests of ultra-long energy pile (1000-mm-diameter, 44-m-long) were carried out to reveal the temperature distribution and mechanical properties of energy pile under typical working conditions. Based on the analytical results, a softening shear model of the energy–soil interface under the condition of large shear displacement was proposed with the load transfer method, and the reliability of the model was verified. The model can simulate the shear–displacement relationship of the pile–soil interface under different geological conditions.

Thermal energy storage is a key measure to ensure the efficient and sustainable operation of medium-deep geothermal heat exchange systems. To address the performance and efficiency issues associated with the standalone application of natural recovery and artificial heat storage, this study combines numerical simulation with practical projects to explore the synergistic effects of these two recovery methods. Based on the numerical heat transfer model of a medium-deep coaxial borehole heat exchanger, the study designs a collaborative mode of natural recovery and artificial heat storage to analyze the impact of different combinations on system performance. By comparing the effects of various combinations on heat extraction performance, thermal storage performance, and system efficiency, the optimal synergistic mode of the two heat recovery measures is identified. The results indicate that under the conditions of a flow rate of 10 kg/s, a temperature of 70 degrees C, and a storage period of 4 months, the "4 + 4+heating" mode achieves optimal heat extraction and storage performance. Furthermore, the study suggests that during transitional seasons, when the duration of artificial heat storage is limited, it is beneficial to prioritize natural recovery before proceeding with artificial heat storage to optimize system performance. These findings provide important theoretical foundations and practical guidance for the efficient and stable operation of medium-deep geothermal heat exchange systems and the design of heat recovery strategies.

To better understand fluid migration in fractured granite joints, an improved middle axis (IMA) method and 3D laser scanning are employed to accurately measure the aperture and roughness of real rock fractures, respectively. The Monte Carlo method and the principle of fractional Brownian motion (FBM) are used to generate fracture models based on the actual fracture structures observed in the field, overcoming the limitations of assuming random distributions in existing modeling methods. Furthermore, the lattice Boltzmann method (LBM) is used to investigate the effect of various fracture distributions on fluid seepage characteristics. The results showed that the distribution of aperture and roughness within fractures significantly influences fracture seepage flow, including fluid velocity and pressure distributions. Abrupt changes in roughness reduce the effective cross-sectional area of fluid flow, increase the hydraulic slope drop, and induce local vortices, leading to elevated fluid frictional resistance and energy losses. The permeability of fractured rock mass is influenced by the nonlinear interaction between aperture and roughness, with this effect being particularly strong in small apertures. As the aperture increases, the impact of roughness decreases, and the aperture becomes the dominant factor. When the ratio of average aperture geometry to maximum roughness undulation exceeds 10, the influence of roughness on fluid seepage is minimal, and the aperture dominates flow characteristics.

The space of production well plays a crucial role in the heat extraction performance of the Enhanced Geothermal System (EGS), which have the potential to meet the growing global energy demand as a reliable energy source. Nonetheless, there has been insufficient research and attention focused on comprehending the impact of the space of production well on the efficiency and effectiveness of EGS. In this work, a series of numerical simulations were conducted to assess the impact of the space of production well on heat extraction efficiency in EGS. Three different cases were considered: Case 50 with 50 m production well spacing, Case 100 with 100 m production well spacing, and Case 150 with 150 m production well spacing. At the X-Y plane and Y-Z plane, the simulation results indicated that there were slight differences in temperature variation among the Case 50, Case 100 and Case 150. And the cooling area decreasing as the spacing of production wells decreased. Moreover, the delivery of cooling water via the injection well and its subsequent distribution to various reference points lead to a decline in temperature at each point, albeit with varying degrees of variation. Besides, the established efficiency (ef) for Case 50 is smaller than the Case 100 and Case 150 during the last 15 years. These findings contribute valuable insights to the exploration and exploitation of EGS systems and can serve as a guide for further research in this field.

A detailed and primordial derivative process of the classical Maxwell problem applied to thermal conductivity was presented in this study, aiming to offer a foundation for further research on various correlations predicting the effective thermal conductivity (ETC) of porous-composite media, especially for the unifying equation with five fundamental structural models. This process of theoretical analysis is, in essence, solving the Laplace's Equation for temperature distribution in a spherical coordinate under certain initial and boundary as well as other assumed conditions. The particular solution processes also overcome various degrees of shortcomings existed in previous literatures in this field, e.g. problems of nondegenerate, insufficient boundary conditions and many tiny mistakes (typos or misprints). We hope this commentary letter can maintain the rigor of academic research in a rightful and suitable form, and prevent any potential confusion for future readers in this field.

Thermal response tests employing resistively heated methods (TRT-RH) have shown advanced performance in evaluating ground properties, while the effect of various TRT-RH setups is still vague in practice. This study examines the effect of different TRT-RH setups based on numerically generated data and field data. As a result, the various setups alter the heat transfer and change the test time required to meet the ideal linear heat source (ILHS) assumption. Time thresholds that indicate TRT-RH are positively related to eccentric distance, negatively related to borehole radius, and positively related to grout thermal conductivity difference. The distance-based general sensitivity analysis result shows that the radius of the borehole or trench, which separates the heat source and the ground, has intensive sensitivity to the test effectivity. This study presents the standard setup strategy for TRT-RH to measure the thermal and hydraulic properties based on the ILHS assumption.