Coaxial Borehole Heat Exchanger Research Articles

Study on improving heat extraction capacity of borehole heat exchanger by forced convection of groundwater

Borehole Heat Exchangers (BHEs) has the advantage of "extracting heat without extracting groundwater". Its main drawback is low heat extraction power. Aiming at developing medium-deep hydrothermal geothermal resources, a Branch Well Reinforced Coaxial Borehole Heat Exchanger (BWR-CBHE) is proposed firstly. An assisted circulation loop formed by the wellbore and geothermal reservoir is created through drilling branch well and installing electric submersible pump (ESP). By driving the forced convection of groundwater within the assisted circulation loop, heat extraction capacity of the CBHE is enhanced. Then, a numerical model of the BWR-CBHE is established. The main circulation of the working fluid within the CBHE is coupled with the assisted circulation of groundwater, and the heat extraction performance of the BWR-CBHE is calculated. Under the conditions studied in this paper, the net heat extraction power (after deducting the energy consumption of the ESP) of the BWR-CBHE is 6.09 times higher than that of conventional CBHEs. Finally, impacts of parameters, including working fluid, branch well and ESP, on the heat extraction capacity of the BWR-CBHE are discussed. The heat extraction power of the BWR-CBHE is increased significantly without extracting groundwater, which provides a new approach for the development of BHEs.

A numerical study on the sustainability and efficiency of deep coaxial borehole heat exchanger systems in the cold region of northeast China

Numerical simulations utilizing OpenGeoSys have been conducted on a deep coaxial borehole heat exchanger situated in the Songliao Basin. The findings reveal that the outlet temperature exhibits a downward trend as circulation flow increases, while it rises with an increase in inlet temperature. Additionally, the heat transfer power escalates with higher flow rates but diminishes as inlet temperature rises. Notably, the long-term operation results in a reduction of outlet temperature and heat transfer power. After 30 years, the decline of them is approximately 0.3 °C and 8 kW under current situation. Furthermore, long-term operation induces a decrease in formation temperature, predominantly around the vicinity of well, where the maximum temperature decline is nearly 30 °C. The reduction in formation temperature is directly proportional to the increase in flow rate and inversely proportional to inlet temperature. Within the depth range of well, the maximum influence radius expands in correlation with both the depth and duration of mining, while remaining relatively stable with variations in flow rate and inlet temperature. Following three decades of extraction, the influence radius is approximately 95m. The results provide vital scientific insights for the retrofitting of abandoned wells in Northeast China into geothermal heat exchange wells for building heating purposes.

Harnessing the heat below: Efficacy of closed-loop systems in the cooper basin, Australia

Transitioning to renewable energy is vital for combating climate change and reducing CO2 emissions. Geothermal energy, sourced from the earth's crust, presents a promising alternative. While shallow geothermal energy extraction grows steadily, tapping into deep Hot Dry Rock reservoirs poses challenges. Sustainable utilization of deep geothermal energy is crucial for large-scale electricity generation. Australia's Habanero enhanced geothermal system (EGS) projects faced obstacles due to complex fracture networks. Borehole heat exchangers (BHEs) offer a solution, circulating a working fluid without direct contact with geofluids. This study assesses coaxial BHEs' feasibility in the Habanero reservoir using a 2D axisymmetric model (2D-AM), comparing it with a simpler 1D depth-integrated model (1D-DIAM). Both models yield similar outcomes. Additionally, results indicated that the production temperature of the coaxial BHE exponentially decreased from 250 °C to 100 °C within 10 h, rendering energy recovery for electricity generation unsuitable after 10 h of operation. The study proposes an energy recovery cycle of 10 h followed by a 110-h shutdown. Injection temperatures and flow rates significantly impact production efficiency, while steel casing's thermal conductivity has minimal influence on heat exchanger performance.

Multi-segmented tube design and multi-objective optimization of deep coaxial borehole heat exchanger

Deep coaxial borehole heat exchanger (DCBHE) is a key component to extract medium-deep geothermal energy for low-carbon building heating. It is a convention that both center tube and annuals tube are respectively using a single material. However, a basic idea behind this study is using non-linear design to ask for performance improvement. It is assumed that non-uniform tube design pattern can meet different heat exchanger demand at different depth under a geothermal gradient dominated underground environment. The whole study is to prove this conjecture and make it useful for engineering application. A new numerical model is put forward to considered multi-segmented structure in heat transfer computation and a nondominated sorting genetic algorithm (NSGA-Ⅱ) is adopted in optimization process. The results of this study show that the optimized designed case can increase outlet temperature by 3 °C while reducing investment of 10000RMB. In addition, a home-made software is developed to perform the thermal and economic evaluation of DCBHE as well as automatically optimization of this multi-segmented structure. This study can provide a new model, algorithm, results and software tool for better utilizations of deep geothermal energy.

Heat transfer performance analysis of medium - deep coaxial borehole heat exchanger (CBHE): combination of heat extraction operation test and numerical simulation

Abstract Geothermal energy is a clean and renewable energy source that can effectively reduce carbon emissions. Coaxial borehole heat exchanger (CBHE) is a crucial component of the geothermal mining system. To understand the heat transfer mechanism of a CBHE and to design its construction, this paper analyzes the geotechnical thermal response characteristics of CBHE under constant power conditions. COMSOL was used to simulate the performance of the heat exchanger during intermittent operation in both summer and winter. The results indicate that the inlet water temperature and the thermal conductivity of the grouting material have a weak impact on the energy efficiency coefficient of the heat exchanger. The energy efficiency coefficient can increase by about 22% when the inner pipe's thermal conductivity is lower. The temperature difference between the medium flowing into the inner pipe at the bottom of a shallow cased buried pipe and the medium flowing out of the inner pipe defines the thermal short circuit value. The influence on the thermal short circuit value is on the order of the inner pipe thermal conductivity, grouting material thermal conductivity, and the inlet water temperature. The fluctuation of soil temperature in the summer is greater than that in the winter, the soil temperature of shallow-buried pipes is easier to recover in the winter. The characteristic law obtained in this paper provides a valuable reference for the scientific design of CBHE.

Hellfire Exploration: the origins of ground source heat in early mining technology

The ground source heat pump (GSHP) was first used in 1862, for freezing ground in connection with sinking a shaft in Swansea, UK. It was subsequently developed in Germany in 1882-83 into the “Poetsch process” for freezing ground during construction of mine shafts. The Poetsch process was an indirect closed loop GSHP system, circulating a chilled brine from a heat pump around a network of coaxial borehole heat exchangers. These early systems typically employed ammonia as a refrigerant and a calcium or magnesium chloride solution as the brine. Such a system was used in 1904-1906 to sink the shafts of Dawdon Colliery, Co. Durham, UK through water-bearing Permian strata. Also, around 1904, the Newcastle-based turbine pioneer, Charles Parsons, suggested that such a GSHP system could transport heat to the surface during the construction of a 12 mile deep “Hellfire Exploration” shaft, that could potentially access geothermal power.

Numerical Simulation of Geothermal Energy Development at Mount Meager and Its Impact on In Situ Thermal Stress

The Meager Mountain Geothermal Project stands as one of the pioneering geothermal energy initiatives in its early stages of resource development. Despite its abundant geothermal heat resources, no prior studies have systematically evaluated the potential of implementing coaxial borehole heat exchangers on site. This study addresses this research gap by presenting a comprehensive heat transfer model for an underground closed-loop geothermal system utilizing a single coaxial well. Finite element analysis incorporated fluid and solid heat transfer, as well as solid mechanics. The results obtained facilitated the construction of the temperature and thermal stress profiles induced by the cooling effects resulting from years of heat extraction. After 25 years of operation, the outlet temperature has reached approximately 74 °C, and the maximum radial tensile thermal stress amounts to ~47 MPa. Furthermore, the analysis demonstrates that higher fluid velocities contribute to more perturbed temperature and stress distributions. The study attained maximum thermal and electric power outputs of 208 kW and 17 kW, respectively. This research also underscores the significant impact of geothermal gradient and well length on BHE design, with longer wells yielding more power, especially at higher injection velocities.

Influence of eccentricity on the thermal performance of pipe-in-pipe heat exchanger utilized for geothermal heating, an experimental study

In the past few decades, pipe-in-pipe coaxial borehole heat exchangers (BHEs) have become increasingly widespread for building heating through the usage of medium-deep geothermal energy. Enhancing the heat transfer efficiency of BHEs is crucial in order to achieve higher efficiency in utilizing geothermal energy. This study introduced a novel consideration regarding the eccentric inner pipe as a technique to enhance the thermal performance of the pipe-in-pipe BHE. The impact of eccentricity varying from 0 to 0.9 on the BHE performance was experimentally explored to establish the correlation for Nusselt number. The convective heat transfer coefficients were obtained based on 480 sets of test results conducted under varied heating powers (200 – 4500 W/m2) and flow rates (0.16 – 1.00 m3/h). The results demonstrated that in comparison to the coaxial BHE, the eccentricity leads to a considerable enhancement in the heat transfer coefficient, ranging from 20 % to 35 %. The thermal performance is higher when the eccentricity is between 0.4 and 0.6, while the pressure loss can be decreased by 33 % − 54 %. The research results are expected to provide a theoretical basis to improve the geothermal energy utilization efficiency without improving the cost of BHE systems.

Heat extraction from abandoned petroleum wells utilizing coaxial borehole heat exchanger in Ordos basin, China

To extract geothermal energy from abandoned petroleum wells in Ordos basin, China, a coaxial deep borehole heat exchanger (DBHE) is studied through a 3D numerical model. Factors, such as thermophysical properties, geological characteristics and operational conditions, are considered. Given the geothermal gradients and rock/soil properties, numerical results suggest that a sustainable heat extraction rate can be achieved for building heating purpose in long term; after 20 years’ operation, with an injection rate of 30 m3/h at 20 °C and wellbore length being 2350 m, the outlet temperature is around 24 °C, the heat load per unit depth ranges from 89.2 W/m to 67.7 W/m. Through comparison between on-site tests and numerical modeling, it is suggested that factors like well integrity, operational conditions and insulation between the inflow and outflow pipe should be taken into account. Numerical modeling on variation of well depth is performed; results show that both the wellbore length of 1800 m and 2350 m could satisfy a heat extraction load of 80 W/m, and the greater the wellbore length, the larger the outlet temperature. According to results from a DBHE array model, the general 200 m–300 m wellbore interval would not cause thermal interaction.

A semi-analytical temperature solution for multi-segment deep coaxial borehole heat exchangers

A semi-analytical and a finite-difference scheme are presented for the simulation of temperature and the heat transfer in a multi-segment coaxial borehole heat exchanger. The single-segment solution on closed-form is extended to a semi-analytical multi-segment solution, where each segment may have unique properties. These properties are such as different casings, widths of the annulus, radius of the inner tubing, material properties, rock properties and geothermal gradients. The multi-segment model is a simple and powerful alternative to numerical methods for simulating a complex coaxial borehole heat exchanger with a constant flow rate. It is demonstrated with a deep coaxial borehole heat exchanger made of three different segments. The analytical and semi-analytical models are validated by comparison with numerical solutions obtained with an upstream finite difference scheme. The match between the solutions is excellent. The solution on a closed-form is used to study the temperature difference between the outlet and the inlet regarding two dimensionless numbers. It is found that the maximum temperature difference occurs when the dimensionless heat transfer coefficient for the casing-rock is much larger than one. A second necessary condition is that the dimensionless heat transfer coefficient for the insulator between the inner tube and the annulus must be much less than one. The power leakage from the inner tubing to the annulus is also at a maximum under these conditions.

Analytical study of integrating downhole thermoelectric power generation with a coaxial borehole heat exchanger in geothermal wells

Geothermal power generation employing Organic Rankine Cycle (ORC) technology is a widely acknowledged and conventional approach for harnessing geothermal energy. In an innovative advancement, we propose a novel design integrating downhole thermoelectric power generation with a coaxial borehole heat exchanger. This design aims to enhance the efficiency and sustainability of geothermal energy utilization. In this innovative design, the geothermal well is divided into two distinct sections: a power generation section and a heat exchanging section, achieved through the implementation of a packer positioned from the uppermost part of the targeted zone. The process involves the injection of cold fluid downhole via an insulated pipe. Subsequently, a portion of the injected fluid is directed to flow in reverse within the casing-tubing annulus above the packer, while another portion circulates into the casing-tubing annulus below the packer before ascending through the tubing. This dual flow mechanism establishes distinct cold and hot sources for the thermoelectric generator, a key feature facilitated by this innovative design. Analytical models detailing of downhole temperature distribution for thermoelectric power have been meticulously developed. A comprehensive case study, focusing on a geothermal well with 3000 m length of power generation section and 500 m heat exchanging section, has been conducted. The results indicate that a significant generating capacity could be achieved with a higher wellhead temperature, and the payback period under different carbon tax scenarios is about 6–8 year. Furthermore, the effects of injection rate, fluid diversion ratio, and casing-tubing configuration on power performance and thermal-electricity efficiency are also discussed. This method not only enables the concurrent harvesting of geothermal energy and power generation but also operates consistently throughout the year. The results thus emphasize the viability and economic feasibility of the proposed approach.

Performance comparison of U-tube and coaxial borehole heat exchangers with numerical simulations

It is well known that geothermal heat is a virtually unlimited energy source beneath the ground, which is formed by the radioactive decay of naturally unstable elements. However geothermal energy has been utilized since the ancient ages, it is still a mission for humanity – even in the times of energy crisis with high prices – to find sustainable solutions for high-efficiency heat energy production. Apart from the conventional hydrothermal wells and production/injection systems, an increasing attention is drawn into deep coaxial heat exchangers and heat pumps, utilized in a single wellbore. Optimization of these systems need to be done with an increased care, however it is possible with results obtained from a set of numerical simulations. The present paper investigates hypothetical, single deep “U”, “double-U” and coaxial arrangements under different flow rates, where the main target was to analyze efficiency of each model, with short time thermal response simulations.

Improved modeling analysis on heat transfer performance of deep coaxial borehole heat exchanger with different operation modes

The deep coaxial borehole heat exchanger (DCBHE) is a highly promising technology which has garnered significant attention. However, there has been a notable scarcity of studies focusing on the physical and mathematical models essential for accurately describing the heat transfer processes involving the DCBHE, well, and rock. This study addresses this gap by employing an improved numerical heat transfer method through programming to conduct an in-depth analysis of the heat transfer performance. The model is solved using the finite volume method (FVM) and validated using experimental data from an existing literature. The simplified effect of pipes and filling material on the heat transfer performance of the DCBHE are analyzed. Furthermore, the characteristics of outlet temperature, heat extraction power, change rate, reduction degree of DCBHE with different operation modes are also analyzed, considering both the heat extraction of one season and 20 years. The results indicate that the simplification of the outer pipe and filling material as thermal resistances is unreasonable while the simplification of inner pipe is acceptable. The maximum outlet water temperature decreases year by year, as the rock-soil temperature fails to recover to its initial level. The water outlet temperature decreases with the run stop ratio while the heat extraction increases. Moreover, the operation mode of 2:2 is recommended both in one heat extraction season and 20 years operation.

Multi-factor optimization of thermo-economic performance of coaxial borehole heat exchanger geothermal system based on Levelized Cost of Energy analysis

The deep coaxial borehole heat exchangers (BHEs) are commonly employed for geothermal heating purpose. In this investigation, using the established thermo-economic model and fast algorithm for heat extraction prediction, the response surface approach was adopted for investigating the thermo-economic performance and optimizing the design parameters of the coaxial BHE system. The results show that the heat exchange rate is enhanced from 235 kW to 1,061 kW when the depth varies between 2,000 m and 4,000 m. A change in flow rate from 20 m3/h to 80 m3/h yields a 30% augmentation in heat exchange rate. However, increasing the depth will cause extra drilling costs. Extremely large or small flow rates and pipe diameter ratios result in additional equipment and operating costs. The system's levelized cost of energy (LCOE) is contingent upon the interaction among the depth, flow rate, and diameter ratio. The optimal design parameters to meet the minimum LCOE are 3912.8 m depth, 34.32 m3/h flow rate and 0.64 pipe diameter ratio. The optimized LCOE is $ 7.84/GJ with an improvement of 22.02–63.02%, demonstrating a competitive thermo-economic performance to traditional energy sources. This research will contribute to the parameter design of BHE and effective utilization of geothermal energy.

Thermal Performance Analysis and Multi-Factor Optimization of Middle–Deep Coaxial Borehole Heat Exchanger System for Low-Carbon Building Heating

Ground-source heat pumps with deep borehole heat exchangers can fully utilize deep geothermal energy, effectively reducing the consumption of non-renewable energy for building air conditioning and achieving energy conservation and emissions reduction goals. However, the middle–deep coaxial borehole heat exchange (MDBHE) development is insufficient, and there is currently a lack of definitive guidelines for system optimal design and operation. This paper firstly establishes an effective and efficient system model and examines nine important parameters related to the design and operation of the MDBHE using a single-factor analysis. Thereafter, we compare and analyze the impact of different parameters through an orthogonal experimentation method. The findings reveal that the three most significant factors are borehole depth, inlet temperature, and mass flow rate, in descending order of importance. In addition, in terms of operation mode, this paper makes a comparative analysis of the operation of the MDBHE in variable flow mode and constant flow mode. The results showed that the average energy consumption of the pump in the variable flow mode decreased by 9.6%, and the surrounding ground temperature recovered at a faster rate.

Research on the Heat Extraction Performance Optimization of Spiral Fin Coaxial Borehole Heat Exchanger Based on GA–BPNN–QLMPA

Geothermal energy, a renewable energy source with enormous reserves independent of the external environment, is essential for reducing carbon emissions. Spiral fin coaxial borehole heat exchanger (SFCBHE) is vital for geothermal energy extraction. Its heat extraction performance requires further improvements for efficient performance that consider the structural sizes and installation positions of the SFCBHE and the nonlinear coupling with respect to several factors. The heat extraction performance of SFCBHE is optimized using a combination of genetic algorithm–back-propagation neural network (GA–BPNN) and the Q-learning-based marine predator algorithm (QLMPA). This study analyzes and compares the effects of geothermal energy extraction of smooth pipe TY-1, structure before optimization TY-2, and optimized structure TY-3. Following optimization with GA–BPNN–QLMPA, the heat extraction performance of TY-3 is enhanced by 30.8% and 23.6%, respectively. The temperature of maximum extraction is improved by 26.8 K and 24.0 K, respectively. The power of maximum heat extraction is increased by 148.2% and 109.5%, respectively. The optimization method can quickly and accurately determine the heat extraction performance for different structural sizes and installation positions of the SFCBHE. These findings are crucial for developing high-performance SFCBHE and efficiently using geothermal energy.

Heat extraction calculations for deep coaxial borehole heat exchangers: matrix analytical approach

SUMMARY Deep boreholes represent a source of clean energy. Therefore, effective calculations of potential extraction of heat from boreholes for realistic models of the Earth’s crust with variable thermal conductivity and diffusivity are needed. We deal with heat extraction in a quasi-steady state from coaxial boreholes where downward and upward flows of pumped fluid (water) are separated by an inner pipe and connected only at the bottom. We first obtain theoretical estimates of heat extraction for a thermally isolated inner pipe and a model of the ground with constant thermal diffusivity and conductivity. Then, we develop a new analytical matrix method for a general layered ground model that enables us to include depth-dependent ground properties as well as heat exchange between the downward and upward flows of fluid in the borehole. Our straightforward and fast approach is thus suitable for various parametric studies or as a tool for benchmarks of numerical software. A key role in heat extraction from coaxial boreholes is played by the inner-pipe thermal resistance. We apply our method to the parametric study showing the dependence of pumped water temperature and total heat extraction from the borehole on realistic borehole geometries under different amounts of water pumping. The calculations are performed for a 3 km deep borehole as the representative of present deep boreholes used for extraction of geothermal energy and for a 10 km deep borehole. Drilling of such a superdeep borehole has just started in China and our results demonstrate potential limits of geothermal energy extraction from such great depths.

Research on the heat storage characteristic of deep borehole heat exchangers under intermittent operation mode: Simulation analysis and comparative study

Deep borehole heat exchanger (DBHE) extracts heat from mid-deep geothermal energy through heat transfer process. As DBHE commonly applies coaxial borehole heat exchangers with depth of 2–3 km, the volume reaches more than 30 m3, similar to the small heat tank underground. When the heat pump system turns off, the water in DBHE still extracts heat from the high-temperature ground and stores heat. This kind of heat storage characteristic of DBHE was studied in this paper with simulation analysis. Benefiting from the heat storage characteristic, the average heat extraction rate (Qr) of DBHE under intermittent operation is 8.6%–64.7% higher than Qr under continuous operation, with run-stop ratio decreasing from 24 to 0 to 8–16 per day. Then the influence of operation modes, parameters and thermo-physical properties of DBHE on the heat storage characteristic were studied quantitatively, proving the wide-range regulation ability of Qr. Therefore, the regulation ability of DBHE matches the variation of space heating load very well, and also matches the production rules of clean electric power like photovoltaic and wind power. Hence, the DBHE could further participate in the demand response of electrical power system, and absorb clean electric power.

Study on effects of multi-climatic parameters on performance of ground source heat pump through coaxial borehole heat exchanger

ABSTRACT The accurate calculation for heat flux of borehole heat exchanger (BHE) is of great importance for design of ground source heat pump (GSHP); the climatic conditions influence GSHP performance through GHE. In this study, a 2-D heat transfer model for coaxial BHE was built to investigate the effects of multi-climatic parameters on BHE heat transfer and GSHP performance. The outlet temperature, ground temperature profiles and COP of GSHP system for the cooling period were concerned. The effects of climatic conditions under different borehole depths, ground thermal conductivities and flow rates were studied. The studies showed that the outlet temperature from BHE increased about 10% and system COP declined about 5% in cooling mode with considering the effect of climatic conditions. The comparison indicates that the effects of climatic conditions on coaxial BHE are stronger than that on U-tube BHE. Generally, the climatic conditions influence BHE heat transfer mainly through the shallow ground and the influences strengthen with time. The effects can be more significant with a larger ground thermal conductivity, but it weaken with increase of borehole depth. Also, the effects of climatic conditions on BHE and GSHP seem to be independent of flow rate.

Analytical model of heat transfer of U-shaped borehole heat exchanger under convolution theory and its optimal heat extraction analysis

Middle deep borehole heat exchanger (DBHE) has become widely known as one of the main ways of geothermal energy development and utilization, and the use of middle DBHE system for heating buildings has received much attention. More accurate analytical models for a coaxial borehole heat exchanger (CBHE) have been built using the convolution theorem, however, a U-shaped deep borehole heat exchanger (U-DBHE) has not been reported. Therefore, in this paper, to obtain more accurate fluid temperature distribution, an analytical model of middle DBHE considering geological stratification phenomenon is developed by using convolution theorem. The relative error of the analytical model is verified within 5% by numerical simulation. At the same time, the interaction between the three factors was analyzed and studied by response surface method: mass flow, inlet temperature and geothermal gradient on heat extraction and heat loss rate. The results reveal that for thermal extraction, the significance levels of the three factors are from large to small: geothermal gradient > inlet temperature > mass flow, for heat loss rate, the two factors of flow rate and inlet temperature are significant, and the impact of geothermal gradient is not significant. In this engineering project, when the inlet water temperature is 10 °C, the flow rate is 26.65 kg/s and the geothermal gradient is 4.5 °C/hm, there is a maximum heat extraction of 2424.16 kW.