Borehole Heat Exchangers Research Articles

Operational strategies to alleviate thermal impacts of the large-scale borehole heat exchanger array in Beijing Daxing Airport

Large-scale ground source heat pump (GSHP) systems are increasingly used for space heating and cooling. In comparison with smaller ones, large GSHP systems are often coupled with much more borehole heat exchangers (BHEs). Because of the intense thermal interactions between BHEs, they are more susceptible to significant ground temperature changes. Meanwhile, they possess the advantage that their operational strategies can be applied with a high degree of freedom, which presents chances to alleviate intense thermal interactions. In this study, we used a new performance indicator to access the effectiveness of GSHP operational strategies on alleviating thermal anomalies. The Daxing Airport GSHP system, contains 10,497 BHEs and is the largest in the world; therefore, it was selected as the test case for performance enhancement through operational strategies. We established a 2D model to predict ground temperature changes during the 50-year operation of the BHEs. First, it was revealed that the most severe thermal anomalies in the study area mainly occurred both within and between the BHE arrays, which should be mitigated. To alleviate the thermal anomalies caused by the thermal interactions of BHEs, operational strategies were applied by adjusting the cooling/heating starting sequence, setting time-dependent thermal loads, and reallocating thermal loads according to the position of the BHEs. Our study demonstrates that only the operation strategy that adjusts the cooling/heating starting sequence is beneficial for different BHE layouts, while the operational strategy that reallocates the thermal loads depending on BHEs position may be only effective for specific BHE layouts. In addition, our new performance indicator can be used to evaluate the effectiveness of the operational strategies and determine the spacing of adjacent BHE arrays. Therefore, it benefits the operation management of BHE array and design of BHE layout, and further guarantees the sustainable operation of the GSHP system.

Evaluation of modified Hantush’s function: A tool for parametric study with the finite line-source model

A solution to the three-dimensional finite line-source (FLS) model for borehole heat exchangers (BHEs) is identical to Hantush’s M function, commonly used in groundwater pumping modeling. A modified Hantush function E appears in the steady-state solution to the moving finite line-source (MFLS) model for borehole heat exchangers (BHEs) with groundwater flow that takes into account heat conduction and advection by Péclet number. An exact series representation is obtained for this two-parametric function E, and from it two-types of approximate expressions for the E are derived. One is suitable for small values of a parameter Pe and the other for its large values. High accuracy of the approximate formulae is verified by comparison with the exact values provided by modified Hantush’s function E in those validity domains.These analytical expressions can be used for designing complex borehole configurations and may be useful in steady-state analysis of partially penetrated wells (PPWs) in a leaky aquifer.

Comparison and integration of simulation models for horizontal connection pipes in geothermal bore fields

The heat transfer along horizontal connection pipes in geothermal bore fields can have significant effects and should not be neglected. As practical and design-related applications require simple and efficient models, we investigate suitability of different models for the first time within this context. Three ground and three pipe models of different complexity are studied. All model combinations are coupled with a fixed ground load boundary condition on one side and a borehole heat exchanger (BHE) model on the other side. Models are tested under a variety of realistic conditions to evaluate performance. The investigations show that all investigated pipe models are equally suitable for the application. For the ground models, the horizontal finite line source model and the numerical 2D model produce identical results for homogeneous ground properties. The soil resistance model neglects the temperature accumulation in the ground and thus leads to considerable deviations and should be avoided. Based on the findings, we propose a computationally efficient approach using a novel combination of established simple steady-state models for the BHE and connection pipes. In the selected example scenario, the consideration of a 30 m connection pipe attached to the BHE leads to an increase in the BHE load by 40% for the heating case and a reduction in the BHE load by 5% for the cooling case.

A field test of medium-depth geothermal heat pump system for heating in severely cold region

Due to the worldwide attention of fossil fuel consumption and environment pollution, geothermal is recognized to be amongst the prospering alternative energy. Previous researches mainly focused on the shallow-depth geothermal heat pump systems (SD-GHPs), while medium-depth geothermal heat pump system (MD-GHPs) lacked of the experimental support especially in severely cold region. A field test of MD-GHPs using deep borehole heat exchanger (DBHE) was conducted and verified in terms of temperature variation, heat exchange and COP. The indoor temperature remained 20 °C around even though the outdoor temperature was as low as −25.10 °C. The water supply and return water temperature on the customer side performed steadfast heating characteristics that the averages were 35.13 °C and 30.97 °C. Moreover, flow rate and borehole depth are the essential influencing factors on DBHE outlet temperatures. This suggests that lifting flow rates is a fantastic option when only considering the enhancement of the thermal extraction. The DBHE average heat exchange per meter is 1000/1200 m for 61.1/84.9 W/m and 2000 m is 152.6 W/m respectively. Besides, the average heat exchange of DBHEs for working days was 485.4 kW while during Chinese New Year Holidays was 388.6 kW. Correspondingly, the average COPs for MD-GHPs ground side were 4.5 and 5.2, respectively. Results show that MD-GHPs can be extended to severely cold regions and obtain superior performance.

Effects of climate change on long-term building heating performance of medium-deep borehole heat exchanger coupled heat pump

The medium-deep borehole heat exchanger (MDBHE) coupled heat pump (HP) is a promising building heating solution for carbon reduction. Existing studies claim that the MDBHE-HP suffers a bottleneck challenge of long-term heating performance degradation because of the heating extraction attenuation of the MDBHE, which largely hinders the practical applications of the MDBHE-HP. This study innovatively proposes and justifies that climate change positively compromises the heating extraction attenuation of the MDBHE and significantly improves the long-term building heating performance of the MDBHE-HP. Climate change reduces the building heating load, which mitigates the heating extraction attenuation of the MDBHE, thereby reducing the unmet level of the building heating load. The mitigated heating extraction attenuation enhances the coefficient of performance of the MDBHE-HP, thereby reducing the heating energy consumption. Case studies of 30-year operation in the severe cold and cold zones of China with different geological conditions show that while the heating extraction attenuation of the MDBHE increases the heating energy consumption by 5.0% – 8.7%, climate change reduces the heating energy consumption by 9.2% – 17.0%. Moreover, climate change reduces the unmet level of the building heating load by 70.5% – 100%. Therefore, the positive effects of climate change on the long-term building heating performance of MDBHE-HP should be considered, which contributes to promoting the practical applications of the MDBHE-HP for low-carbon building heating.

Estimation method of layered ground thermal conductivity for U-tube BHE based on the quasi-3D model

The distribution of ground thermal conductivity (λs) is a significant consideration for the design of ground source heat pump systems. This paper proposed a new method to estimate the layered λs for U-tube borehole heat exchanger (BHE) using particle swarm optimization (PSO) and distributed thermal response test (DTRT) data. Firstly, a quasi-3D heat transfer model considering geothermal gradient for U-tube BHE had been built to simulate fluid temperature profiles. And the experiment in DTRT was performed using a 45 m sandbox. Then, the layered λs were estimated through PSO and DTRT data based on the established model. The results showed that the convergent λs distribution could be reliably obtained by short-term DTRT. Furthermore, the estimation results are independent of the number of layers and data points. The mean difference in λs distribution between 9 layers and 45 layers is 0.007 W/(m∙K) and the layered λs also could be obtained when the number of layers is more than the number of temperature data points, and the root mean square error between 30 and 220 points is less than 0.3 W/(m∙K).

Thermal response analysis of a medium-deep coaxial borehole heat exchanger by circulating CO2

Geothermal energy is abundant and renewable. It is of great interest to extract geothermal energy efficiently and sustainably. In this paper, a composite coiled tubing (CCT) is put forward and a coaxial borehole heat exchanger (CBHE) technology is utilized to extract heat from the medium-deep geothermal reservoir using phase-change CO2 as the working fluid. CCT has all the advantages of coiled tubing, and thermal insulation could be realized more easily. Considering the transient heat transfer of the formation, a mathematical model of the CBHE system is established and validated by two field tests. Then, both the phase change process of CO2 in the CCT and the long-term heat extraction performance are predicted. Effects of key parameters on the heat output are discussed. It is shown that extracting medium-deep geothermal energy by the phase-change CO2 is realized, and the latent heat of vaporization accounts for a high proportion of the heat carried by production CO2. The larger the mass flow rate is, the more heat could be carried out. But the larger flow rate would lead to the lower temperature of the production CO2 and the more frictional loss. The results provide a reference for exploiting the medium-deep geothermal energy sustainably.

Energy analysis of an ultra-low temperature district heating and cooling system with coaxial borehole heat exchangers

The decarbonisation of society requires developing new environmentally friendly solutions for heating and cooling. Ultra-low temperature district heating and cooling (ULTDHC) allows operation with energy sources and systems hard to integrate into previous generations of district networks, making this technology more efficient and sustainable. This work analyses a novel integration of coaxial borehole heat exchangers (CBHEs) to control the temperature of the network, drastically reducing operating costs and emissions compared to existing systems. The concept is analysed based on single and double-loop networks. As a case analysis, it is studied to a ULTDH located in the south of Spain with annual heating and cooling demands of 1045 MWh and 416 MWh, respectively. The single-loop configuration requires more 100 m depth CBHEs connected, 47, whereas the double-loop layout, with a more efficient thermal balance, requires 42. In contrast, it has higher values of annualised costs. The sensitivity analysis of the main parameters proves the system is robust and effective in compensating for external variations. It operates more efficiently in warm/hot regions, increasing the possible applications and implantation expansion. The proposal is useful for new systems and for adapting the existing ones.

Study on long-term performance sustainability of medium deep borehole heat exchanger based on simplified one-dimensional well model

Heating buildings and residences is a low-cost and high-efficiency renewable energy utilization method through the coupling of medium deep borehole heat exchanger and ground source heat pump systems. Since the performance of the medium deep borehole heat exchanger system will gradually decrease during long-term operation, its sustainability becomes a key issue in current research. In this paper, a comprehensive numerical model with a simplified one-dimensional well is established using COMSOL Multiphysics by considering the influence of factors including layered strata, groundwater flow, thermophysical properties, and geothermal gradient. The rationality of the model is verified by the thermal response test data of the medium deep borehole heat exchanger system located in Daxing District, Beijing, China. The outlet temperature of the medium deep borehole heat exchanger system declined by 0.48 °C (3.1%) next 20 years of continuous operation. The area greatly affected by heat transfer process is basically within 20 m around the borehole. In the fifth, tenth, fifteenth, and twentieth years, the heat transfer impact scope is about 43 m, 56 m, 74 m, and 84 m, respectively. This study can provide a reference for the performance sustainability of long-term operation of medium deep borehole heat exchanger systems.

Collective behavior of boreholes and its optimization to maximize BTES performance

Borehole layout strongly affects the behavior of borehole heat exchangers (BHEs) and changes the performance of a borehole thermal energy storage (BTES). This study investigates the existence and importance of the optimum collective behavior of BHEs to maximize the performance of BTES. Charge benefit ratio, storage efficiency and configurational benefit factor are proposed as performance indicators for better and finer performance evaluations of BTES systems. A small-scale BTES consisting of ten boreholes arranged on a concentric double-ring layout is considered as an application. Performance variations with the inner and the outer radii of the borehole field are analyzed for the first five years of operation. The temperature fields of different configurations show the transition from collective to individual behavior of boreholes, which leads to an optimal radial configuration maximizing the performance indicators. It is seen that the indicators strongly depend on both inner and outer radii and they reach their maximums for the same distinct radial configuration. The optimum arrangement can almost double the thermal performance indicators. It is thus of great importance to optimize collective behavior to maximize the usage of stored thermal energy. The results are qualitatively general and represent the common behavioral patterns of BTES systems.

Reduced order 1 + 3D numerical model for evaluating the performance of solar borehole thermal energy storage systems

This study presents a computationally efficient numerical model for solving the fluid flow and heat transfer phenomena in co-axial boreholes heat exchangers arranged on a rectangular grid (N × N) drill hole pattern. Such systems of boreholes are an integral component of a Solar-Borehole Thermal Energy Storage System for seasonal thermal storage application. The numerical model couples a one-dimensional fluid flow and convective heat transfer model in the heat exchanger with a three-dimensional model of conductive heat transfer in the strata. In addition to solving mass and heat transfer in borehole systems, it integrates a solar thermal collector system and building dynamic thermal load to design a Solar- Borehole Thermal Energy Storage system for a 182-unit residential apartment building. Simulation also considers and evaluates the conductive, convective, and radiative thermal losses of the system. The model is validated using experimental data from a coaxial borehole heat exchanger and compared to the results of a commercial finite volume solver for multiple co-axial boreholes. The computational cost of developed numerical model is reduced by a factor of 194 compared to a commercial finite volume solver. Solar- Borehole Thermal Energy Storage system simulation is performed for a period of five years considering hourly fluctuations in solar irradiance and building dynamic thermal energy demand. Parametric study is performed on depth and spacing of boreholes, and mass flowrate in the solar thermal collectors. It is found that a system of 100 boreholes, each 100 m deep, linked to 674 solar thermal collectors can provide the heating for the building with a total thermal demand of 5600 GJ per year.

Application of a thermal transient subsurface model to a coaxial borehole heat exchanger system

We present an application of a thermal transient model to a coaxial borehole heat exchanger system. We compare two numerical methods. First, a model with a prescribed formation temperature (PFT); secondly, a method with a modeled formation temperature (MFT). In this comparison, several parameters are analyzed, such as the transient temperature profiles, the heat flux along the wellbore, the overall heat transfer rate, the thermal conductivity of the formation, and the type of flow inside the pipe and annulus — laminar or turbulent. The description of the system by the MFT method is more physically consistent. Then we proceed validating this method against two experimental setups, thereby showing good agreement. We perform a sensitivity analysis to the MFT method, varying the direction of the flow, regular and reversed, and the center tube material, with a high (steel) or low (polyethylene) thermal conductivity. It is shown that the reverse circulation has a better heat extraction, while regular flow performs better in the case of heat injection. For the center tube material, polyethylene shows a better thermal performance when compared to steel.

A spectral method aimed at explaining the role of the heat transfer rate when the Infinite Line Source model is applied to Thermal Response Test analyses

The present paper introduces new concepts related to the modeling of vertical Borehole Heat Exchangers for Ground Coupled Heat Pump applications. A sensitivity analysis on how specific parameters affect the ground thermal conductivity kgr estimation when the Infinite Line Source model is used to interpret a Thermal Response Test has been performed. The study has been conducted considering shallow and deep BHEs, with and without geothermal gradient, and for homogeneous and stratified ground thermal conductivities. The qratio parameter scales the external heat rate to a natural heat rate associated with the geothermal gradient. The effect of qratio on the TRT analysis has been related to a specific dimensionless g-transfer function called g0, which incorporates the geothermal gradient. Three in-house built Fortran90 codes implementing the finite-difference models related to coaxial, single and double U-BHE geometries are exploited to evaluate the dimensionless g-transfer functions related to each fluid volume. A spectral method aimed to reconstruct the fluid temperature profiles by superposing two separated convolutions in the time domain exploiting the Fast Fourier Transform leads to considering qratio as the dominant parameter when the ILS model is used to estimate kgr. In the case of a single-layered subsurface, qratio >>1 guarantees the correct ILS-based kgr estimation for any BHE geometry. In the coaxial center-pipe inlet case with a single-layered subsurface and qratio<1, the ILS-based kgr estimation when the g0-function is taken into account can differ by -14% from the correct ILS-based kgr estimation without taking into account the g0-function. In the case of a multilayered subsurface, the qratio parameter indicates when the effective kgr estimated by the ILS model departs from the weighted-thickness average. A departure of 10% occurs for qratio between 2 and 2.5 for the coaxial center-pipe inlet cases considered and the departure increases with decreasing qratio.

Experimental study on convective heat transfer of an open-loop borehole heat exchanger

Open-loop borehole heat exchanger (OBHE) is a single well geothermal heat exchanger with an open-loop structure that can realize the geothermal energy extraction without mining the geothermal water. In this paper, a sandbox experiment is designed to simulate the convective heat transfer process in the reservoir area of OBHE. The mechanism of convective heat transfer in the reservoir area is studied, and the key factors that affect the convection heat transfer intensity are analyzed. The results show that the convection heat transfer of OBHE in the reservoir area is affected by both the driving effect of fluid flow inside the screen tube and the buoyancy effect. In the forward flow mode, the two effects have the opposite direction. While in the backward mode, the two effects have the same direction. The backward flow mode is more conducive to convective heat transfer. In addition, many factors influencing significantly the convective heat transfer of OBHE include inlet temperature, inlet flow rate, reservoir temperature, fluid flow direction and inner tube diameter.

A novel sustainable cooling system in a tunnel with high geotemperature: Concept and thermal performance

An increasing number of tunnels pass through the anomaly geothermal zones and inevitably encounter thermal engineering geological problems during tunnel operation. In order to efficiently eliminate the thermal damage, an efficient and energy-saving cooling system composed of borehole heat exchangers (BHEs) and thermal insulation layer in a high geotemperature tunnel is proposed. Taking Sangzhuling tunnel as an example, a 3D numerical model is established for analyzing the thermal performance of the cooling system, and is validated by the field measured data. The cooling effect of the cooling system in the high-geotemperature tunnels is proved to be remarkable in comparison with the tunnel having thermal insulation layer. Moreover, the effects of design parameters on the thermal performance are revealed during a 180-day heat extraction. The results show that a lower inlet temperature contributes to a shorter drop to the target tunnel ambient temperature. The effect of the cooling system is significantly improved as the borehole depth increases within a limited extent because the cooling effect cannot be infinitely improved by increasing the borehole depth. The thermal insulation layer plays a beneficial role in reducing the maximum ambient temperature and an adverse effect on the temperature reduction rate. These findings contribute to preliminary thermal performance estimation of the BHE cooling system in high-geotemperature tunnels without large-scale simulation.

Influences of stratified ground thermophysical properties on the performance and thermal response test of deep coaxial borehole heat exchanger

ABSTRACT Deep coaxial borehole heat exchanger (DCBHE) is a promising kind of ground heat exchanger for geothermal exploitation. In this study, effects of stratified ground thermophysical properties (GTP) are considered, and then an improved semi-analytical heat transfer model is developed and validated to analyze the DCBHE performance. Then, influences of stratified GTP on the DCBHE performance are investigated. Finally, thermal response test (TRT) of DCBHE with stratified GTP is simulated by a numerical model, and by using the simulated TRT data, the improved model is combined with a parameter estimation method to identify the effective values of GTP, which are compared with actual values. The results show that stratified ground thermal conductivities have an important influence on the DCBHE performance, and that stratified volumetric heat capacities of ground have a small influence on the DCBHE performance. The DCBHE performances based on the effective values and actual values of GTP match well with each other for different cases and geothermal gradients, indicating that for stratified thermal conductivities or volumetric heat capacities of ground, the effective GTP are reliable and can be used for accurately predicting the DCBHE performance.

A new semi-analytical solution addressing varying heat transfer rates for U-shaped vertical borehole heat exchangers in multilayered ground

Existing analytical solutions for vertical borehole heat exchangers (BHEs) in a multilayered ground normally require a prescribed heat transfer rate at the borehole wall, such as a constant heat transfer rate. However, in reality, the heat transfer rate between a BHE and its surrounding ground varies along the length of the borehole. To account for this variation, this study proposed a new semi-analytical solution named the “MVQ solution”—Multilayered ground with Varying heat transfer rates (Q). The MVQ solution was first verified with an equivalent three-dimensional (3-D) finite-element (FE) model. The results showed that the MVQ solution had a mean absolute percent error (MAPE) of less than 2.50% for the average borehole wall temperature. In comparison, the conventional constant heat transfer rate solution was found to potentially overpredict the temperature by up to 50.82%. Lastly, a parameter study revealed that the inhomogeneity of ground thermal properties strengthened the heat transfer rate variation along the borehole length. Thus, the MVQ solution is indispensable for thermal performance predictions of BHEs in the multilayered ground, and it can improve the design of BHE systems in shallow geothermal applications.

Comprehensive thermal performance analysis and optimization study on U-type deep borehole ground source heat pump systems based on a new analytical model

U-type deep borehole heat exchangers (UDBHEs) can provide low-carbon heating for buildings utilizing medium-deep geothermal energy. This study presents a new analytical model based on the Segmented Finite Line Source (SFLS) method to investigate the heat transfer process of a UDBHE. The developed model is verified by comparing with experimental results. Subsequently, the soil and fluid temperature are analyzed, and the influences of design and operation parameters on UDBHE performance are investigated. The results indicate that the heat extraction capacity increases as the higher thermal conductivity of backfill material, pipe diameter and flow rate. We identify a linear correlation between the horizontal or vertical borehole length and the heat extraction rate. In addition, an integrated model for the optimization study on the U-type deep borehole ground source heat pump (UDBGSHP) system is presented, which comprehensively considers the dynamic heating loads of the buildings and the dynamic heat transfer process in the UDBHEs. With the control method, the electricity consumption of the heat pump decreases from 5110 kWh to 4812 kWh within a 72-h operation. This work can offer guidance for the design and optimization of UDBGSHP systems.

Numerical Simulation Study of Geothermal Energy Extraction in Medium-deep Formation

Geothermal energy is a kind of renewable energy with rich content, relatively low cost and wide distribution. China is rich in geothermal reserves and urgently needs efficient heat extraction technology. Closed cycle heat extraction technology is a new geothermal development model that is suitable for medium-deep geothermal reservoir and has high heat exchange efficiency without taking water. In this system, the circulating working fluid is pumped into the annulus from the ground, and the heat is extracted from the formation through heat conduction, and then returned to the ground through the central pipe. In this paper, the heat recovery capacity of the heat extraction system is numerically simulated. The research results show that the coaxial borehole heat exchanger technology has a good heat recovery capacity, and the designed thermal insulation structure has a positive effect on heat extraction and heating, and the heat extraction 100 days can stably produce 360 Kw of heat.

Numerical investigation of vertical borehole heat exchanger heat transfer under coupled conditions of groundwater seepage and layered subsurface

Vertical borehole heat exchangers (VBHEs) have been considered to be one of the best sustainable geotechnical technologies for building heating or cooling. In most of the current numerical models for the analysis of VBHE heat transfer, the assumption of subsurface homogeneity is considered. However, the actual subsurface presents a layered structure, and some aquifer groups have groundwater seepage. An improved numerical model is proposed that takes into account not only the coupled condition of groundwater seepage and the layered subsurface but also the heat transfer of VBHEs at different locations, including inside the borehole, borehole wall and outside the borehole. The heat transfer characteristic at these different locations under groundwater seepage and layered subsurface coupled condition is investigated. Moreover, the model is applied to investigate the thermal behaviors of VBHE arrays in two typical arrangements (matrix and staggered arrangement). Furthermore, the influence of groundwater seepage in different layers on the heat transfer ability of the VBHE is discussed. The results demonstrate that the seepage can effectively improve the average heat transfer performance of VBHEs array, and the relative improvement rate is about 62.0%, Compared with the matrix arrangement, the average heat flow of staggered arrangement is about 0.09 kW higher, and the relative increase rate is about 3.3%.