Ground Temperature Distribution Research Articles

Influence of heat storage on performance of multi-borehole mid-deep borehole heat exchangers

Storing heat underground is an effective method to solve the problem of apparent ground temperature decrease due to long-term operation of a multi-borehole mid-deep borehole heat exchanger (MMBHE), which will cause heating capacity decrease and even heating failure. To understand the influence of heat storage on the MMBHE operation performance, based on the proposed model considering heat storage and extraction, this research comprehensively analyzes the influence of heat storage on the MMBHE heat extract capacity, ground temperature distribution, fluid temperature variation along flow path. The influential factors analyzed include borehole layout, borehole amount, borehole spacing, and borehole depth. The results indicate that adopting heat storage in non-heating seasons significantly improves the ground temperature and the heat extraction capacity of the MMBHE. The MMBHE can extract more heat with heat storage than that without heat storage when the heat storage fluid temperature is higher than the critical heat storage temperature which is much less the borehole bottom temperature. As the case discussed in this paper, the mean heat extraction capacity with heat storage is 24.25 % higher than that without heat storage. As the borehole depth increases, there is a substantial increase in the heat extraction capacity of the MMBHE with no heat storage, and the heat storage effect is weakened when heat storage adopted in non-heating period. Less borehole number and larger borehole spacing make larger mean borehole heat extraction capacity of an MMBHE with no heat storage. However, when the borehole layout, borehole amount, borehole spacing, and borehole depth change, fluctuation of the mean borehole heat extraction capacity with heat storage is much smaller compared to those with no heat storage. When the borehole spacing increases from 25 m to 45 m with no heat storage, the mean heat extraction capacity of the MBHE is increased by 27.67 kW, while that is 11.88 kW with heat storage. This research can provide some inherent understanding of the heat storage on MMBHE performance and reference for designing MMBHE with heat storage systems.

Investigation on grinding temperature characteristics of CF/PEEK and material removal mechanism under temperature treatments

Carbon fiber reinforced thermoplastic (CFRTP) composites grinding temperatures could even exceed the glass transition temperature (Tg) of the material due to heat accumulation, combined with the unknown material removal mechanism, resulting in machining defects. To investigate the grinding temperature distribution and maximum temperatures of carbon fiber-reinforced poly-ether-ether-ketone (CF/PEEK) composites, as a typical kind of CFRTP, an analytical temperature model considering the anisotropy was proposed based on the moving heat source theory, verified by the experimental results measured by the infrared camera. Further, the material removal mechanism under temperature treatments (below and above Tg) was also studied. In high temperatures (above Tg), there were many defects on surfaces after grinding, including resin smearing, severe deflection, fiber distortion, and grinding wheel clogging. Therefore, to reduce the grinding temperature below Tg, the vortex tube cooling-assisted grinding method was proposed in the research. The proposed method could reduce the grinding temperature in the grinding area, suppress resin smearing, improve the surface quality, enhance the elastic modulus and nano-hardness of the workpiece, and reduce the clogging of grinding wheels. This study provided a better understanding of the CF/PEEK grinding temperature characteristics and suggested a method to improve the grinding quality.

Application of the cooling measures in the highway roadbed in permafrost regions of the Qinghai-Tibet Plateau

Permafrost in the Qinghai-Tibet Plateau (QTP) accounts for a total area of 1.05 × 106 km2. Many highway engineerings have been through the permafrost regions of the QTP, including the Qingkang Highway (QKH), Qinghai-Tibet Highway (QTH), and Gonghe-Yushu expressway (GYE), and the Qinghai-Tibet expressway (QTE) will be constructed in the future. Under the action of roadbed construction and global warming, permafrost under roadbed is degraded and causes serious pavement damage. Although many useful cooling measures have been applied to prevent permafrost degradation, their application presents significant challenges on account of the stronger endothermic action of asphalt pavement. This paper summarizes the working principle of existing cooling measures, and ascertains the relationship between pavement damage and ground temperature distribution of permafrost under roadbed. Based on the relationship, geotemperature distribution criteria and requirements for the cooling measures are proposed, and some suitable composite structures and measures are suggested. The results reported in this paper can provide a valuable guidance for the design and construction of the QTE in the future.

Modelling permafrost distribution using the temperature at top of permafrost model in the boreal forest environment of Whatì, NT

Current permafrost models in Canadian boreal forests are generally of low spatial resolution as they cover regional or continental scales. This study aims to understand the viability of creating a temperature at the top of permafrost (TTOP) model on a local scale in the boreal wetland environment of Whatì, Northwest Territories from short-term field-collected temperature data. The model utilizes independent variables of vegetation, topographic position index, and elevation, with the dependent variables being ground surface temperature collected from 60 ground temperature nodes and 1.5 m air temperature collected from 10 temperature stations. In doing this, the study investigates the relationship vegetation and disturbance have on ground temperature and permafrost distribution. The model predicts that 31% of the ground is underlain by permafrost, based on a mean annual temperature at TTOP of <0 °C. This model shows an accuracy of 62.5% when compared to cryotic assessment sites (CAS). Most inaccuracies, showing the limitations of the TTOP model, came from peat plateaus that had been burned in the most recent forest fire in 2014. These resulted in out-of-equilibrium permafrost and climatic conditions that TTOP cannot handle well. Commonly, permafrost mapping places Whatì in the extensive discontinuous zone, estimating that between 50% and 90% of the ground is underlain by permafrost. The study shows that a climatically driven TTOP model calibrated with CAS can be used to illustrate ground temperature heterogeneity from short-term data in boreal forest wetland environments. However, this approach likely underestimates permafrost extent and is perhaps not the best-suited modelling choice for near-surface permafrost, which is currently out of equilibrium with the current climate.

Study on long-term operation characteristics of the medium-deep ground source heat pump system with solar heat storage

Long-term heating operation of the medium-deep ground source heat pump (MGSHP) system will cause the underground temperature around the mid-deep borehole heat exchanger (MBHE) significantly lower than its initial value, which will reduce the operating efficiency of the MGSHP system. In some cases, the temperature of the circulating water in MBHE even is lower than the lowest value required by the heat pump unit, which will lead to operation failure of the unit. Attenuating the ground temperature decrease by storing heat underground during non-heating seasons is an effect way to maintain the heat extraction capacity of the MBHE. In this paper, the influence of storing solar energy underground during non-heating seasons on the long-term operating characteristics of the MGSHP system is studied. The MBHE heat transfer model which considers the heat storage process is developed and embedded in the software of TRNSYS. Based on the numerical simulation model of solar assisted mid-deep ground source heat pump (SAMGSHP) system built by TRNSYS, the long-term characteristics including the variations of the fluid temperature of the MBHE, the ground temperature distribution and the COP of the heat pump unit of the MGSHP system with heat storage during non-heating seasons are analyzed and compared with the case without heat storage. The results show that storing solar energy underground during non-heating seasons can effectively improve the ground temperature, significantly slow down the decline of the circulating water temperature of the MBHE in heating seasons and apparently increase the COP of the heat pump unit. The results can serve as a reference for engineering applications of MGSHP systems, especially those cannot completely meet the original design heating load.

Rainwater: Harvesting and Storage through a Flexible Storage System to Enhance Agricultural Resilience

Many climatic variables are projected to occur with more intense and frequent extreme events, possibly unpredictable patterns and negative feedback loops with other environmental processes. Agriculture has faced uncertainty regarding ground temperature and rainfall distribution during the last few years, making water availability one of the major concerns for farm management. In this scenario, rainwater harvesting could represent a powerful tool to mitigate this problem, and consequently, the research community has been fostering new technical solutions. On the other hand, a few studies on agronomic assessment of rainwater harvesting systems are present in scientific literature. The present study reports preliminary data of a long-term study on a Flexible Water Storage System (FWSS) evaluating the possibility of enhancing agriculture systems resilience, shifting from rainfed production to irrigated agriculture relying on excessive rainfall, collectible from extreme events. The idea of intercepting excess rainfall, which is generally lost, thanks to an innovative water harvesting system, and using it to mitigate drought stress for crops is in line with sustainable approaches aiming to improve the resilience of agricultural systems. The results highlighted that the system studied could potentially collect an annual average of 831.7 m3 of water, mitigating the excess of water in the ditch that can potentially cause flooding and storing fresh water to provide irrigation during dry periods.

A method for predicting the ambient temperature distribution of high-temperature tunnels and influencing factors analyze

In high-temperature tunnels, it is difficult to predict the temperature distribution, and the effectiveness of comprehensive cooling measures is not yet clear. This study proposed a highly accurate tunnel environmental temperature prediction model using FDM and FEM and considering the heat transferred from rock, construction machinery, and personnel. The effects of air volume, air temperature, and insulating layer on tunnel ambient temperature were investigated. The following are the key findings: (1) The distribution of the environment temperature and the lining surface temperature of the two tunnels align with the ground temperature distribution. (2) The prediction model using FDM + FNM closely matches the actual monitoring data, with a maximum absolute error of only 2.52 °C. (3) Increasing air volume or decreasing the temperature of the wind can reduce the temperature of the environment, which has no effect on the temperature distribution of tunnels. (4) The thermal insulating layer impedes ground temperature heat transfer and alters the ambient temperature distribution. The distribution of ambient temperature tends to align as the thickness of the thermal insulating layer grows, while cooling efficiency drops when the thickness is greater than 6 cm.

Study on configurational operation strategy of ground heat exchangers under the effect of groundwater flow

In large-scale ground source heat pump (GSHP) systems, the accumulation of thermal energy resulting from seasonal load imbalances has emerged as a significant impediment. The implementation of a rational operation strategy can effectively mitigate heat accumulation in soil. This study investigates the impact of the configurational operation strategy for large-scale vertical ground heat exchanger (GHE) on the spatial distribution of ground temperature under the influence of groundwater flow. The internal heat source model is proposed to describe heat transfer process of GHE. Sandbox experiment is conducted to verify the model with maximum deviation of 3 %. The validated model is then utilized to simulate soil temperature profile in the large-scale GHE field over a 10-year operational period, considering a matrix of three configuration strategies and three operation strategies. The results shows that trapezoidal configurations combined with operation Strategy 3 has a minimal temperature increment of 2.9 °C in soil temperature. The contribution of this study lies in provision of a methodology for mitigating thermal accumulation in large-scale GHE field.

Distribution of Ground Temperature in the Geothermal Manifestation Area of Lompio, Donggala Regency, Central Sulawesi Province

The distribution of non-volcanic geothermal energy is widely found on the island of Sulawesi, one of which is in Lompio Village, Sirenja District, Donggala Regency, Central Sulawesi Province. It is important to study the geothermal potential in the village in order to learn how the geothermal system works. This study aims to identify the distribution pattern of temperature distribution in shallow soil depths as a manifestation of the geothermal system below the surface and its relation to geothermal manifestations that appear on the surface. This research was carried out through a series of field observations and direct soil temperature measurements in the study area using a thermometer. The results show that the distribution pattern of land temperature distribution in the high-temperature zone is associated with the presence of geothermal manifestation points on the surface (hot springs). The association of temperature and manifestation is possible because the presence of hot springs is an indication of a weak zone for the release of hydrothermal fluid from the geothermal system, which is correlated with the geological structure in the research area.

Evaluation indices of the temperature difference of subgrade and the optimization of mitigation measures in cold regions

With the construction and operation of railways in cold regions, the asymmetric deformation of subgrades due to the difference in the transverse ground temperature has become a prominent issue. A comprehensive evaluation of the transverse ground temperature difference and investigation of the corresponding mitigation measures should be conducted to avoid or minimize the damage resulting from this difference, thereby improving subgrade stability and reducing deformation. In this study, the time history variations in the homogeneity and symmetry indices of the ground temperature at typical instances that reflect the spatial and temporal changes in the temperature difference of the subgrade were proposed as evaluation indices. The feasibility of these evaluation indices was verified through numerical models with different types of anti-frost berms. Subsequently, the numerical models were used to analyze the ground temperature evaluation indices of a subgrade with expanded polystyrene (EPS) insulation board and polyurethane (PU) insulation board at different locations. Additionally, the performances of each mitigation measure in eliminating or reducing the ground temperature difference were assessed and compared. The results show that all the mitigation measures could improve the homogeneity and symmetry of the ground temperature distribution. The maximum mitigation rates for the homogeneity and symmetry are 97.87% and 45.90%, respectively. This study provides a comprehensive evaluation method for the temperature difference of subgrades constructed in cold regions and a theoretical reference for the selection of anti-frost measures in the design, operation, and maintenance of subgrades in cold regions.

Evaluation of heating capacity of shallow ground heat exchangers in different Locations– A case study of two administrative districts of Xi'an

As a case study, two administrative districts of Xi'an are selected. Eight sites are drilled for shallow ground heat exchangers (SGHEs), and thermal response tests (TRTs) are conducted on the ground to investigate the heating capacity of SGHEs in different locations. The vertical distribution of initial ground temperature is identified using a temperature sensing system in TRTs. Simultaneously, the real-time inlet and outlet water temperatures of SGHEs are monitored by controlling heating power and buried pipe flow rate to be constant, and the thermophysical parameters of the ground are derived. A three-dimensional full-scale numerical model for buried pipe heat transfer is developed based on the results of TRTs. The heating capacity distribution of SGHEs is determined by clarifying the characteristics of buried pipe heat transfer at various locations and depths. The results showed that the maximum difference in geothermal gradient and ground thermal conductivity is 51.2 °C/km and 0.51 W/(m·K) respectively, indicating that the ground parameters vary significantly within the chosen places. The heat transfer intensity of SGHEs is directly related to buried pipe depth and mean circulating water temperature at the same locations. The same prediction formula can describe the heat transfer intensity of SGHEs at various locations.

Study on temperature field and influencing factors of the high geothermal tunnel with extra-long one-end construction ventilation

The increasing number of deep-buried high geothermal tunnels with extra-long one-end ventilation distance makes it critical to develop an accurate and efficient computational model to predict the ambient temperature of such tunnels. A numerical model capable of quickly calculating the longitudinal temperature field of the tunnel construction environment has been established by combining the finite difference method and finite element software. The model, which accounts for the initial ground temperature distribution as well as the heat exchanges at the working face, agrees well with the field monitoring results of the Dechang Tunnel. The research results show that the air temperature during tunnel construction varies as follows: stable heating section in the duct → rapid heating section near the working face → slow heating section of the return air → temperature drop section of the return air. The temperature of the characteristic sections (duct outlet, working face, and tunnel maximum temperature section) all show a quadratic function upward with construction length (L). These also show an exponential trend with ventilation volume (v), insulated duct thickness (t1), and tunnel insulation layer thickness (t2). To achieve a temperature change slope greater than 0.5 in the working face of the Dechang tunnel, the recommended criteria include a ventilation volume below 110 m3/s, an insulated duct thickness below 3 cm, and a tunnel insulation layer thickness below 7 cm. The research results can provide a basis for ventilation and heat insulation design of the extra-long high geothermal tunnel.

Ground temperature distribution and evolution of Lanzhou subway in the Chinese Loess Plateau

The temperature field of the subway stratum is a consequential basis for adjusting the thermal environment in the subway. In this paper, the initial ground temperature of Lanzhou subway in the Loess Plateau was measured, and the temperature field evolution of the surrounding rock in the subway tunnel was explored under operational conditions. The results show that in the initial ground temperature field, the annual variable temperature layer of the subway is located at the surface to the depth of 12 m, with a sinusoidal distribution and a phase lag phenomenon. The lag time of the initial ground temperature at 11 m depth is about 80 days compared with the initial ground temperature at 1 m depth. The annual constant temperature layer is located below the depth of 12 m with a temperature of 15.2 °C. Moreover, the air temperature has the greatest influence on the heat exchange between the air and surrounding rock. The heat flux is negatively correlated with the heat exchange time, while positively correlated with the low water content and air temperature. However, the tunnel shape and high water content have little effect on the variation in heat flux, which is less than 0.1 W/m2.

Heat transfer performance of energy piles in seasonally frozen soil areas

Energy piles constitute a combination of ground source heat pump and building pile foundation technology. Energy piles can not only exchange heat with soil and extract geothermal energy but can also bear upper loads. However, due to the complex geological and climatic conditions in seasonally frozen soil areas, research on the heat transfer performance of energy piles in these areas is limited. According to the characteristics of the ground temperature distribution in seasonally frozen soil areas, the soil region can be divided into frozen and nonfrozen layers. First, a laboratory model test was carried out to investigate the heat transfer performance in these two layers. Then, a three-dimensional heat transfer model of an energy pile suitable for seasonally frozen soil areas was established. The actual working conditions of the real-scale model were simulated, and parametric analysis was then conducted. The results indicated that in annual energy pile operation, the energy pile maximum heat exchange amount per unit length can reach 125 W/m. In the performed parametric analysis, the pile length, concrete thermal conductivity, fluid volumetric flowrate and pipe type impose the greatest influence on the energy pile heat transfer performance, while the pile diameter and concrete cover thickness impose relatively little influence. Moreover, the influence of the pipe diameter should be further studied.

Study on heat transfer characteristics of the deep-buried ground heat exchanger under different multi-pipe layouts

The pipe group layout and buried pipe spacing in large-scale applications are of great significance for guiding the design of pipe group systems and improving the utilization potential of regional geothermal resources. This work takes a deep-buried coaxial double-pipe heating project at a depth of 2,539 m in Xi'an as an example. It establishes three-dimensional full-scale numerical models that couple the internal and external heat transfer with a multi-pipe system based on the measured ground temperature distribution, ground thermophysical parameters, and an in-situ experiment. The multi-pipe models are divided into two categories based on the pipe layout's minimum basic unit: the straight-line and the equilateral triangle models. Both model categories contain three independent deep-buried coaxial double-pipes, and each model involves four spacings of buried pipes (5 m, 10 m, 15 m, and 20 m). Based on the established models, the heat transfer characteristics of buried pipes under different pipe layouts and buried pipe spacing are studied, and the influences of ground thermal conductivity on the heat transfer in multiple pipes are discussed. Consequently, this study provides a scientific reference for the design of pipe groups for large-scale utilization of geothermal energy in medium-deep layers.

Influence of groundwater flow on the ground heat exchanger performance and ground temperature distributions: A comprehensive review of analytical, numerical and experimental studies

The performance of the ground heat exchanger (GHE) possesses a pivotal role in the geothermal exploitation. In recent years, a growing body of literature that recognizes the importance of groundwater flow has been presented. This article reviewed the literature dealing with the impact of groundwater flow on the GHE performance and ground temperature distribution, which were classified by analytical, numerical and experimental approaches. The characteristics of the three approaches were summarized respectively. Various GHE configurations, including U-tube, double U-tube, spiral tube, tunnel lining, horizontal spiral GHEs and GHE groups, were involved in the discussion. It was found that the groundwater flow is beneficial to the ground temperature recovery and improves the GHE heat exchange rate. The effective thermal conductivities of rocks and soils, and borehole resistances, are also affected by groundwater flow. The impact extent depends mainly on the thickness, depth and flow rate of the aquifer layer. The discussion on the collected literature demonstrated that ignoring the groundwater flow would cause non-negligible prediction errors in the evaluation of GHEs. Based on the analysis of the characteristics and applicabilities of the discussed models and methods, this review is expected to provide a more comprehensive reference for promoting the utilization of geothermal energy by GHEs.

An analytical model for heat transfer characteristics of a deep-buried U-bend pipe and its heat transfer performance under different deflecting angles

This study develops an analytical model for calculating heat transfer characteristics of deep-buried U-bend pipes. The model considers actual thermophysical parameters and a non-uniform ground temperature distribution around the buried pipes. To discuss the analytical model's applicability in both short- and long-term heat transfer operations, the study combines a 72-h field experiment and a 2904-h numerical simulation, verifying the analytical model. The field experiment was conducted on a project comprising deep-buried U-bend pipes at a 2781 m depth in Xi'an and served as a basis for establishing the numerical model. The experimental values and analytical solutions of buried pipes' heat transfer intensity within 72 h are compared, revealing a 10.47 kW time-weighted average of the absolute differences in 8–72 h period. The buried pipes' heat transfer intensity of both the analytical and the numerical solutions was tracked over 2904 h, yielding a 10.58 kW time-weighted average of absolute differences between the analytical and the numerical solutions over a period of 8–2904 h. The established analytical model enables the discussion of the heat transfer performance of U-bend buried pipes at different deflecting angles and promotes providing the design suggestions for deflecting angles in practical engineering.

Field-scale experimental and numerical analysis of a downhole coaxial heat exchanger for geothermal energy production

The experimental and computational analyses of a 500 m deep coaxial borehole heat exchanger system for geothermal power generation are studied in this paper. The experiments are carried out on a high temperature resourced well with an average thermal gradient of 0.38 °C/m. The experiment lasts 456 h, and the findings are described in terms of fluid inlet and outlet temperatures, subsurface temperature distribution profiles over time, and flowrate. The in-situ ground temperature distribution profile is measured using Distributed Temperature Sensing System for different hours of the experiment. A detailed, three-dimensional, unsteady-state, finite volume-based computational model has been developed, which solves conjugate fluid-flow and heat transfer phenomena. The simulation outcomes are compared to the results of the experiments. With validated numerical model, to determine the circle of influence, temperature profile of the ground is observed at various radial distances from the borehole. A parametric study is performed by varying inlet temperature of the fluid, mass flow rate, and the thermal conductivity of the ground. The numerical model developed also predicts the temperature recovery behavior of the ground. The results indicate that the average output thermal power for 456 h from the geothermal system ranges from 172 kW to 262 kW based on operating condition chosen and the total thermal energy generated changes between 82 MWh and 194 MWh from a single borehole. After the extraction is terminated, around 86% of the initial ground temperature is restored after 456 h.

Evaluation on the stability of expressway embankment combined with L-shaped thermosyphons and insulation boards in warm and ice-rich permafrost regions

The embankment construction in permafrost regions changes the energy balance on the surface, resulting in the degradation of the underlying permafrost and instability in the embankment structure. Considering the strict evenness requirement for an expressway, large deformation and instability in expressway embankment cannot be allowed. Accordingly, in this study, a composite measure combined with two-phase closed thermosyphons (TPCTs) and insulation boards is proposed for stabilizing embankments. Moreover, a full-scale experimental expressway embankment is constructed in the Qinghai–Tibet Plateau (QTP) to analyze and investigate the stability of the composite embankment. The experimental results show that the composite measure significantly cools the permafrost under the embankment center and shady slope, and reliably alleviates the embankment deformation and deformation difference. However, the special ground temperature distribution around the TPCTs causes the asymmetric thermal regime and longitudinal cracks in the composite embankment. Additional reinforcement measures should be developed to alleviate the asymmetric thermal regime and prevent such longitudinal cracks. Moreover, timely repair of existing cracks and taking waterproofing measures in advance are favorable for embankment stability.

A Thermal Effect Model for the Impact of Vertical Groundwater Migration on Temperature Distribution of Layered Rock Mass and Its Application

On the basis of the one-dimensional heat conduction–convection equation, a thermal effect model for vertical groundwater migration in the stratified rock mass was established, the equations for temperature distribution in layered strata were deduced, and the impacts of the vertical seepage velocity of groundwater and the thermal conductivity of surrounding rocks on the temperature field distribution in layered strata were analyzed. The proposed model was employed to identify the thermal convection and conduction regions at two temperature-measuring boreholes in coal mines, and the vertical migration velocity of groundwater was obtained through reverse calculation. The results show that the vertical temperature distribution of the layered rock mass is subject to the migration of the geothermal water; the temperature curve of the layered formation is convex when the geothermal water travels upward, but concave when the water moves downward. The temperature distribution in the stratified rock mass is also subject to the thermal conductivity of the rock mass; greater thermal conductivity of the rock mass leads to a larger temperature difference among regions of the rock mass, while weaker thermal conductivity results in a smaller temperature difference. A greater velocity of the vertical migration of geothermal water within the surrounding rock leads to a larger curvature of the temperature curve. The model was applied to a study case, which showed that the model could appropriately describe the variation pattern of the ground temperature in the stratified rock mass, and a comparison between the modeling result and the measured ground temperature distribution revealed a high goodness of fit of the model with the actual situation.