Borehole Heat Exchangers Research Articles

Evaluating Long-Term Performance of a Residential Ground-Source Heat Pump System under Climate Change in Cold and Warm Cities of Japan

A residential ground-source heat pump system often requires a long payback time to recover the capital cost. Long-term uncertainty in such a system’s performance increases as the climate changes. This study compares 20-years hourly heating/cooling demands of a typical residence in the present (2000–2020) and in the future (2076–2095) for two locations in Japan. This study also calculated soil temperatures as heat sources through 1D heat-transfer simulation based on the A1B climate scenario in the Intergovernmental Panel on Climate Change’s Special Report. System performance and simple payback times were compared in one cold and one warm city in Japan (Sapporo and Tokyo, respectively). Soil temperatures at a middle depth of a borehole heat exchanger were predicted to increase in the future by ~1 °C, with insignificant effects on a borehole heat exchanger. Seasonal performance factors increased in Sapporo because thermal demands would be kept even in the future, but decreased in Tokyo, which has a higher ratio of the energy used in operating the system in cooling mode compared with its small heating demand. The simple payback time was estimated at 16.2 and >20 years in Sapporo and Tokyo, respectively, both in the present and future, with the constant energy prices. If oil and gas prices doubled, the payback time would be halved in Sapporo to 8.4 years but remain around 20 years or more in Tokyo.

Investigating the Influence of Groundwater Flow and Charge Cycle Duration on Deep Borehole Heat Exchangers for Heat Extraction and Borehole Thermal Energy Storage

Decarbonisation of heat is essential to meeting net zero carbon targets; however, fluctuating renewable resources, such as wind or solar, may not meet peak periods of demand. Therefore, methods of underground thermal energy storage can aid in storing heat in low demand periods to be exploited when required. Borehole thermal energy storage (BTES) is an important technology in storing surplus heat and the efficiency of such systems can be strongly influenced by groundwater flow. In this paper, the effect of groundwater flow on a single deep borehole heat exchanger (DBHEs) was modelled using OpenGeoSys (OGS) software to test the impact of varying regional Darcy velocities on the performance of heat extraction and BTES. It is anticipated that infrastructure such as ex-geothermal exploration or oil and gas development wells approaching the end of life could be repurposed. These systems may encounter fluid flow in the subsurface and the impact of this on single well deep BTES has not previously been investigated. Higher groundwater velocities can increase the performance of a DBHE operating to extract heat only for a heating season of 6 months. This is due to the reduced cooling of rocks in proximity to the DBHE as groundwater flow replenishes heat which has been removed from the rock volume around the borehole (this can also be equivalently thought of as “coolth” being transported away from the DBHE in a thermal plume). When testing varying Darcy velocities with other parameters for a DBHE of 920 m length in rock of thermal conductivity 2.55 W/(m·K), it was observed that rocks with larger Darcy velocity (1e-6 m/s) can increase the thermal output by up to 28 kW in comparison to when there is no groundwater flow. In contrast, groundwater flow inhibits single well deep BTES as it depletes the thermal store, reducing storage efficiency by up to 13% in comparison to models with no advective heat transfer in the subsurface. The highest Darcy velocity of 1e-6 m/s was shown to most influence heat extraction and BTES; however, the likelihood of this occurring regionally, and at depth of around or over 1 km is unlikely. This study also tested varying temporal resolutions of charge and cyclicity. Shorter charge periods allow a greater recovery of heat (c. 34% heat injected recovered for 1 month charge, as opposed to <17% for 6 months charge).

Numerical Investigation of the Long-Term Load Shifting Behaviors within the Borehole Heat Exchanger Array System

In the process of development and utilization of a large-scale borehole heat exchanger (BHE) array system, the phenomenon of load shifting within BHE array can be observed. In this paper, OpenGeoSys software coupled with TESPy toolkit is used to establish a comprehensive numerical model of BHE system (without depicting the heat pump part), and the behaviors of load shifting between BHEs with different design parameters are studied. The results show that the outlet temperature of single BHE and BHE array is generally rising, and the soil temperature around the BHE has accumulated unbalanced heat. The soil temperature near the BHEs array fluctuates more obviously than the single BHE system, and the distribution is uneven. At the end of the 15th year, the soil temperature near the center BHE increased by 2 °C compared with the initial soil temperature, which was more favorable in winter, but was not conducive to the performance improvement in summer. Further analysis by changing the inter-borehole spacing shows that with the increase of the inter-borehole spacing, the load shifting behaviors are gradually weakened, and the maximum shifted load of the central BHE is linear with the change of the inter-borehole spacing. After changing the layout methods, we observe that the more intensive the layout is, the more load shifting behavior is, and the unbalanced rate of soil temperature distribution around the linear layout is lower than other layouts. With the increase in the number of BHEs, the load shifting behaviors are further enhanced. By analyzing the proportion of shifted load amount relative to the average value, it is found that the system will take a longer time to reach heat balance with the increase of BHEs’ number. A shutdown of part of BHEs for a certain period of time will help to improve the long-term operational efficiency of the large-scale shallow ground source heat pump (GSHP) system.

Thermophysical and Heat Transfer Characteristics Based on Thermal Response and Thermal Recovery Test of a U-Pipe Borehole Heat Exchanger

Thermophysical and Heat Transfer Characteristics Based on Thermal Response and Thermal Recovery Test of a U-Pipe Borehole Heat Exchanger

Unlocking deep geothermal energy in the UK using borehole heat exchangers

In the wake of COP 26, the international community is aiming to reduce carbon emissions by adopting alternative and renewable energy sources. Deep geothermal energy can help to achieve this as it represents a low carbon‐emitting energy resource that can provide a constant base load of energy. In the United Kingdom, the development of deep geothermal has been limited due to high geological uncertainty and risk. Past exploration has focused on hot sedimentary aquifers and hot dry‐rock granites, with limited success. To mitigate risk and extract heat with a lower reliance on geological properties, such as permeability, new development methods have been conceived using deep borehole heat exchangers, where fluid is circulated in a closed‐loop system. Feasibility studies have been undertaken through modelling of deep borehole heat exchangers with the hope that these novel technologies can be used to exploit geothermal energy.

Long-Term Investigation of a Borehole Heat Exchanger (BHE) Array with the Influence of Same Inlet Temperature (SIT) and Same Heat Load (SHL) Boundaries

Borehole heat exchanger (BHE) array is a key element in the ground source heat pump system (GSHPS). The BHE array is traditionally simplified by assuming the same heat exchange rate for each BHE, which normally is not correspondent to the real applications. In this investigation, a TRT was firstly conducted for a site located in Shanxi Province (China) to get the local ground thermal properties. A numerical simulation framework was next verified by the TRT results. Subsequently, the ground thermal properties obtained by the TRT were used in the numerical simulation model to investigate the influence of two operation strategies on the ground temperature development of the surrounding soil and the BHE performance. The results showed that both ground temperature and BHE array performance decreased during the 8-year operation period of the BHE array in the heating mode. The ground temperature around the center BHE was lower in the same heat load (SHL) scenario (same heat load for the BHEs in the array) than in the same inlet temperature (SIT) scenario (same carrying fluid inlet temperature for the BHEs in the array), and this difference can be 2 °C in the 7th year. The annual average outlet temperature in the SHL scenario was also lower than that in the SIT scenario, especially in the long term. The investigation shows that SIT working strategy superiors the SHL strategy, providing a reference for the upcoming investigation on the design aspect of the GSHPS.

Numerical modeling of the effects of the radial and axial location of added micro-encapsulated phase change materials in vertical borehole heat exchangers

A numerical model of a vertical borehole heat exchanger (BHE) with micro-encapsulated phase-change material (PCM) was used to examine the thermal effect of the radial and axial location of the added PCM within the borehole. In the radial direction, PCM could be placed at the center of the borehole, at the borehole periphery, or interspersed within the grout. In the axial direction, PCM could be concentrated either in the top, bottom, or mid portions of the borehole grout, or again interspersed evenly. PCM could also be added to the borehole heat transfer fluid. Each location has its own thermal, and therefore economic, advantages and disadvantages, and thus this present work investigated these effects toward decreasing borehole length. Analysis of results showed that the best radial location for the PCM is intermixed within the grout, even though that configuration has a disadvantage of increasing the overall borehole thermal resistance. In the axial direction, it was found that the thermal behavior of the PCM varied over the borehole length, and thus adding the PCM-grout mixture to only the bottom half-depth of the borehole reduced the thermal benefit by only about 19%. Thus, it is concluded that strategic placement of PCM within borehole grout can significantly improve the economics of its use in vertical GCHP systems, but at current costs of PCM, its use in borehole grouts remains an economic challenge.

Study on influence of geotechnical characteristics on heat transfer performance of borehole heat exchanger

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

Influencing factors analysis for the long-term thermal performance of medium and deep U-type borehole heat exchanger system

The medium and deep U-type borehole heat exchanger (MDUBHE) system has recently received extensive attention for heating buildings. The MDUBHE system could operate with low energy efficiency or even shut down due to the improper design influencing factors. Therefore, studying the influencing factors is a great need to ensure the stable and efficient operation of MDUBHE system. In this paper, a transient heat transfer model for the MDUBHE system is proposed, and its accuracy is validated using experimental data. The impacts of influencing factors on the performance of MDUBHE system during long-term operations are quantified. The results show that the buried pipe depth and geothermal gradient significantly impact the MDUBHE system's performance, while the pipe specifications have less impact on it. Furthermore, after 15 years of operation, when the inlet water temperature increases from 4.5 °C to 24.5 °C, the heat extraction rate decreases by 43.4%, and the energy consumption decreases by 52.7%. The thermal conductivity of grout increases from 2.2 W/(m ∙ K) to 2.7 W/(m ∙ K), while the heat extraction rate increases by only 0.7%, which is not significant to improve the performance of MDUBHE. This study has significant guidance for the engineering practice of MDUBHE system.

Repurposing a Geothermal Exploration Well as a Deep Borehole Heat Exchanger: Understanding Long-Term Effects of Lithological Layering, Flow Direction, and Circulation Flow Rate

In the drive to achieve net-zero carbon emissions, decarbonisation of heating is essential. This can be facilitated by geothermal energy, but drilling geothermal wells is associated with high risks and costs. The use of preexisting wells (e.g., exhausted hydrocarbon wells or failed geothermal exploration boreholes) offsets this cost while potentially turning liabilities into assets. The Newcastle Science Central Deep Geothermal Borehole (NSCDGB) is a geothermal exploration well that was drilled to target the Carboniferous Fell Sandstone Formation at 1418.5 to 1795 m depth. However, low hydraulic conductivities prevented the development as a conventional “wet” geothermal abstraction well; therefore, new alternative methods of development are being explored. This work investigates the repurposing of the NSCDGB as a deep borehole heat exchanger (DBHE), focusing on the sustainable operation of the system in the long term by employing a constant heat load designed to contribute to local buildings or a heat network. Numerical modelling was undertaken by using OpenGeoSys software to analyse the thermal and hydraulic performance of the system. Both homogeneous and heterogeneous models were developed to compare the influence of lithological layering in contrast to a homogeneous (nonstratified) subsurface geological model. Results from homogeneous simulations modelling the DBHE to a depth of 922 m show that a 50-kW heat load can be supported for a lifetime of 25 years. This corresponds to a 65-kW building load when coupled to a heat pump with a coefficient of performance of 4.33. Thus, the DBHE could meet up to 72% of the heat demand of the adjacent urban sciences building. Rather than being a purely hypothetical case study, this work considers a real existing borehole, adjacent to a building cluster which could make use of the geothermal heat. Heterogeneity, which has been considered for the first time at the NSCDGB site, exhibits a minor impact in comparison to homogeneous simulation results. Flow direction and mass flow rate also exhibited small effects on the system performance, whereas if the exploration well could be repurposed to increased depths, the heat load could be increased. This is the first study of a coaxial DBHE at the NSCDGB site considering long-term effects of mass flow rate, heterogeneity, and flow direction. The study evaluates the feasibility of repurposing an exploratory geothermal well in the UK as a DBHE that can be used as a low-carbon heat source for space heating, thus converting liabilities into potential “green energy” assets.

Analysis on influencing factors of heat transfer characteristics of vertical borehole heat exchanger under stratified soil conditions

In recent years, factors influencing the heat transfer characteristics of the vertical borehole heat exchanger (BHE) is a hot topic in the research of the ground source heat pump (GSHP) systems. Given the complexities involved in environmental situations with respect to groundwater seepage in saturated soil and moisture migration in unsaturated soil, this paper examined the effects of various influencing factors on the heat transfer characteristics of the vertical BHE under stratified soil conditions, using theoretical research, experimental research, and numerical simulations. Subsequently, through altering five influencing factors—groundwater seepage velocity, porosity, saturation, thermal conductivity, and initial soil temperature—the working conditions of the vertical BHE under stratified soil conditions were established, and the heat transfer rate was calculated. The results showed that the weight of the influencing factors on the heat transfer characteristics of the vertical BHE had a descending order: thermal conductivity, groundwater seepage velocity, porosity, saturation, and initial soil temperature. These results can provide an important reference for further optimizing future BHE designs.

CuO/water and Al2O3/water nanofluids as working fluid in an abandoned oil well to improve thermal performance in the seawater desalination process

BackgroundAbandoned Oil/gas wells could be employed as heat sources for various technical processes such as seawater desalination. MethodsIn this study, the thermal performance of an abandoned oil well is enhanced by the addition of nano Cu and Al2O3 particles into the working fluid (water) and by using two distinct thermal insulations in the coaxial double-pipe borehole heat exchanger to decrease heat losses of the heated working fluid. Diligent simulation of the process was carried out using the k-ω SST turbulence model and sweep meshing method. The effect of Cu/water and Al2O3/water nanofluid concentrations on extracted fluid temperature was investigated. Significant FindingThe results revealed that 0.5 wt.% of Cu/water and Al2O3/water as working fluid at a flow rate of 2 kg/s raised the extracted fluid temperature by 14% and 12.9% compared to pure water, respectively. Also, SiO2/GO composite thermal nano-insulation could improve heat loss by 7.4% compared to polystyrene thermal insulation. Furthermore, 806 m3/day of potable water was produced by a multi-effect distillation modified with flash-boxes desalination, where brines were reused as a secondary feed for potable water production. Noticeable increase achieved compared to conventional multi-effect distillation systems.

Enhancement of shallow ground heat exchanger with phase change material

Heat pumps perform better when coupled with ground as thermal source than with air. In literature, several studies and applications suggest and analyse the use of phase change materials (PCMs) coupled with single or double U-tube vertical borehole heat exchangers (BHEs). Usually, PCMs are mixed with the grouting material during the installation. An alternative solution to vertical BHEs is the use of horizontal ground heat exchangers (HGHEs). The present work investigates the possibility of coupling PCMs with a flat-panel HGHE installed inside a trench 2 m under the ground surface. The study analyses the case in which PCMs are adjacent to the HGHE, taking a cue from alternative coupling technologies which have PCMs added to the backfilling material of the trench where the HGHE is installed. The analysis has been conducted with COMSOL software tool. A simulation model of the system was developed to carry out a parametric analysis. The objective of the simulations is the investigation of the thermal behaviour of the HGHE patent pending coupled with PCMs under cycles of operation which represent how the heat pump could work in GSHP system. The results show the meaningful difference of using the PCM in direct contact with the HGHE.

Effect of horizontal connection pipes on operation of borehole heat exchangers under different climatic conditions

We investigate the effect of horizontal connection pipes in shallow geothermal systems attached to a borehole field for the 25 most representative climate zones across the World. For this we use a comparable 30m connection pipe buried at 1m depth connected to a 100m deep borehole heat exchanger (BHE) as the unit representation of the whole system for all locations. The system is considered monovalent with weather dependent loads derived from local air temperatures. The connection pipes and BHE are modeled using thermal resistances and response functions. In the investigated reference scenario, the consideration of the connection pipes leads to an average reduction in BHE load of 34% in tropical climates, 19% in temperate climates, 11% in arid and 4% in continental climates, leading to a significant drilling cost reduction. Exceptions were the cities of Moscow and Heihe both located in continental climates and under severely colder conditions, where the BHE load increased by 13% and 72% due to consideration of the horizontal connection pipes, leading to a reduced efficiency or the need for additional drilling only for these cases.

Field measurements and numerical investigation on heat transfer characteristics and long-term performance of deep borehole heat exchangers

The coaxial deep borehole heat exchanger (DBHE) is widely applied in geothermal energy to extract heat from the ground for the building heating in the winter. The heat transfer characteristics and long-term performance of DBHE are of importance for the application. This work measured the heating season operation data of a middle-deep (2500 m) geothermal heating system from 2020 to 2021 in Xi'an, Shaanxi Province, China. And then a full-scale coaxial DBHE numerical model was developed based on the field measurement data. The influences of the material, the structure and the operation factors on the heat extraction were investigated for the DBHE. Moreover, the long-term performance of DBHE was evaluated. The results show that DBHE has the promising heat extraction with an average power of 300 kW. The decrease of the inner pipe's thermal conductivity and the increase of the backfill material's thermal conductivity can improve the heat extraction power of DBHE. The intermittent operation strategy can increase the outlet-water temperature of DBHE by 23.55%. After a non-heating season, the ground temperature can recover. In the ten-year operation, DBHE has little effect on the temperature of the surrounding rock and soil, and the outlet-water temperature of DBHE only attenuated by 3.68%.

Effect of seepage condition in geological stratification on thermal response test analysis of borehole heat exchanger

Determination of ground thermal properties is the prerequisites for the design of ground-coupled heat pump systems (GCHPs), and it is crucial for evaluating the thermal performance of borehole heat exchangers (BHEs). These parameters are usually obtained by in-situ thermal response test (TRT) based on infinite line source model (ILSM). Though the effect of the groundwater flow on the estimation of ground thermal parameters is considered in homogenous ILSM, the estimated deviation can be enlarged as seepage condition varies based on the BHE model in practical geological stratification. Based on the developed numerical layered seepage BHE model (NLSBM), this paper evaluates the effects of seepage location and seepage velocity on estimated accuracy of borehole thermal resistance and ground thermal conductivity. Relative error (RE) between effective thermal conductivity λeff and the thickness-weighted thermal conductivity λTW will be up to 30.5% with the increase of the thickness of the seepage layer. The relative error between λeff and λTW increases from 5.3% to 93.5% when seepage velocity changes from 1 × 10−6 m/s to 1 × 10−4 m/s. The minimum RE between borehole thermal resistance Rb,NLSBM and effective borehole thermal resistance Rb,eff is still up to 37% when seepage locates in the 5th layer. With the increase of the seepage velocity, the RE between Rb,eff and Rb,NLSBM is enlarged, and the minimum RE is up to 29.4% corresponding to the lowest seepage velocity of 1 × 10−4 m/s. The change of seepage velocity or seepage location has little effect on Rb,NLSBM, and the highest RE is only 0.2%.

Improving LEED-certified building loads on borehole heat exchangers by coupling subsurface variables

To evaluate the performance of a ground source heat pump (GSHP) system, it is necessary to assess the subsurface thermal regimen around the borehole heat exchanger (BHE). This paper presented the preliminary results of an analysis of a newly constructed GSHP system for the Campus Instructional Facility at the University of Illinois at Urbana-Champaign, which will be a Platinum-level Leadership in Energy and Environmental Design (LEED)-certified building. Energy modeling of the entire building was performed in DesignBuilder software by considering all the high-efficiency thermal-insulated building subsystems. A numerical model for a single BHE constructed in a heterogeneous interlayered geology was first validated with data from a distributed thermal response test. The numerical model was then used to simulate the ground thermal response over both the short term (1 to 3 months) and long term (1 year). The geological and hydrological impacts and the benefits of the LEED-certified building load were evaluated. The results showed that under the fixed length of the BHE, the modest heating and cooling demand of the Campus Instructional Facility would result in reduced temperature fluctuations in the U-bend pipe at the outflow, by a maximum 8.91 °C, and would raise the coefficient of performance of the heat pump by 26.8 % when operating in extreme weather conditions and 7.4 % during July. The presence of an external heat source underground—a segment of the campus steam tunnel—raised the local ground temperature by 1.99 °C, providing slightly improvement in the performance of the BHE during the heating season and a similar drop in performance during the cooling season. This study validated the insignificance of the local groundwater velocity (2.07 × 10-7 m/s) on the thermal behavior of the BHE. The proposed model setup and simplifications could be applied to a large-scale borefield analysis at any site within a similar geological context. The proven enhanced efficiency of the BHE may also facilitate future implementation of GSHPs in green buildings.

Technical Performance Comparison between U-Shaped and Deep Borehole Heat Exchangers

The geothermal industry is fronted by a fundamental decade to grow and become an energy supplier in transitioning to a sustainable energy system. The introduction of Closed-Loop Geothermal energy systems (CLG) can overcome the negative social response and increase the attractiveness of geothermal developments. The present work aims to investigate and compare the performance of CLG systems. For the comparison, the case study of Campi Flegrei was chosen. The maximum depth was fixed at 2000 m, and the two configurations were set up to analyse the performance and evaluate the best operational configuration. Both CLG configurations showed decay in the output temperature of the working fluid during the production time. For a U-shaped design, it is possible to find a working condition that allows constant thermal power over time. The DBHE specific power was always more significant, up to 350 kW/m, compared to the U-shaped, which attained a maximum of 300 W/m (15%). The comparison with Beckers et al. analysis highlights the similarity of our results with their base case. The consideration of the CLG system’s length is related to the heat exchange and investment costs. For longer exchangers, there are higher investments and lower specific power.

A New Type in TRNSYS 18 for Simulation of Borehole Heat Exchangers Affected by Different Groundwater Flow Velocities

Heating ventilating air-conditioning (HVAC) systems have been increasingly widespread in Italy: they can exploit renewable energies, are energy efficient systems, do not directly consume fossil fuels, and in the post-pandemic era, have also been subject to incentive processes by the Italian government. In South Tyrol, subject to harsh climates in both the winter and summer seasons, ground-source heat pump (GSHP) systems can be an excellent solution for the air conditioning of buildings. Unfortunately, too often, the design of HVAC systems with borehole heat exchangers (BHEs) is not adequate, and therefore, an innovative and expeditious numerical solution is proposed. A new numerical element (named Type285), written in Fortran code, was developed for TRNSYS 18 and able to implement the main features of BHEs and the surrounding aquifer. Type285 was compared with numerical models present in the literature (using hydrogeological software such as MODFLOW) and validated with the experimental data. The demonstration of the exchanged energy increase between the BHE and subsoil due to the increase in the groundwater flow velocity was carried out and evaluated. The choice to simulate BHE in TRNSYS using Type285 can be a fast and advantageous solution for HVAC system design.

Thermal recovery characteristics of rock and soil around medium and deep U-type borehole heat exchanger

Medium and deep U-type borehole heat exchanger (MDUBHE) coupled with ground source heat pumps is a novel technique to extract geothermal energy from deep rock and soil. The thermal recovery characteristics of rock and soil are critical for the sustainable exploitation of deep geothermal energy. However, the thermal recovery characteristics of rock and soil around MDUBHE are unclear. In this study, a coupled heat transfer model considering multilayer properties is established to reveal the thermal recovery characteristics of rock and soil. The root mean square errors (RMSE) of this model are 1.8 °C and 0.3 °C compared with the experimental and CFD simulation data, respectively. The temperature distribution of rock and soil under short-term and long-term operations is investigated. The results show that the thermal recovery rate of deep rock and soil is over 91.2% after one heating-recovery cycle (1 year). The thermal decay rate of rock and soil after 15 years of operation is less than 9.3%. Furthermore, the thermal effect zone of MDUBHE increases faster early and slower later. The thermal effect zones of the descending pipe are 38.2 m, 74.3 m, 109.1 m, and 132.2 m in the 1st, 5th, 10th, and 15th years, respectively. This study could promote the stable utilization of deep geothermal energy with MDUBHE system.