Geothermal Heat Exchanger Research Articles

An analytical heat transfer model for the mid-deep U-shaped borehole heat exchanger considering groundwater seepage

Ground source heat pump systems consisting of deep boreholes and heat pumps can be used for heating buildings. It is known as an effective way to utilize geothermal resources, however, there is a problem with high development costs. Therefore, it is important to establish a precise heat transfer model to develop a high-efficiency geothermal heat exchanger to reduce the development cost of mid-deep geothermal energy. Most of the existing heat transfer models for deeply buried pipes only consider thermal conductivity and treat the geotechnical soil as a homogeneous medium, or do not consider the stratification and groundwater seepage phenomena, which make the models inaccurate. This study established an analytical heat transfer model considering geotechnical stratification and seepage developed for the U-shaped deep borehole heat exchanger (U-DBHE), and the analytical solution for temperature response is obtained. Using the developed analytical model, we plotted the trends that reveal the relationship between the thermal performance of U-DBHE and various parameters and explored the influence of seepage phenomena on heat extraction performance. The model can also serve as a reference for the development of heat transfer models for buried pipe heat exchangers with groundwater seepage. Given that the proposed analytical model is time-consuming and stable, and can be an efficient tool for U-DBHE design and optimization.

Inverse heat conduction to model and optimise a geothermal field

Geothermal heat exchanger fields are complex systems that exploit the soil as a heat reservoir for space heating and cooling. They consist of several heat exchangers conveniently arranged in a given soil portion. The design of heat exchanger fields is a key phase to ensure the long-term sustainability of such renewable energy systems. This task requires modelling the relevant processes in the system, i.e., the heat transfer within and outside the exchangers. We propose a mathematical model for the study of the heat conduction into the soil that considers the presence of the exchangers. This problem is formulated and solved with an analytical approach. Some numerical experiments are used to show the effectiveness of the proposed method through a comparison with a reference approximation procedure, based on a finite difference method. Moreover, the obtained analytical solution is used in an optimisation procedure to compute the best position of the exchangers by minimising the adverse effect of neighbouring devices. The obtained results are promising and show that the proposed procedure can be exploited as an effective tool in the design of geothermal systems.

Potential of CO2 Emission Reduction via Application of Geothermal Heat Exchanger and Passive Cooling in Residential Sector under Polish Climatic Conditions

The article summarizes the results of the 25-year time horizon performance analysis of the ground source heat pump that serves as a heat source in a detached house in the climatic conditions that prevail in Wrocław, Poland. The main aim is to assess the potential of ground regeneration and reduction of CO2 emission by passive cooling application. The study adds value to similar research conducted worldwide for various conditions. The behavior of the lower source of the heat pump was simulated using EED software. The ground and borehole properties, heat pump characteristics, heating and cooling load, as well as the energy demand for domestic hot water preparation have been used as input data. Based on the brine temperatures for all analyzed cases including the ground with lower and higher values of conductivity and heat capacity, the borehole filler of inferior and superior thermal properties, and the passive cooling option turned on and off, the seasonal efficiencies of the heat pump have been calculated. The energy and emission savings calculations are based on the values obtained. The application of passive cooling reduces the brine temperature drop by 0.5 K to over 1.0 K in consecutive years in the analyzed cases and the thermal imbalance by 65.0% to 65.9%. Electric energy savings for heating and domestic hot water preparation reach 4.5%, but the greatest advantage of the system is the possibility of almost emission-free colling the living spaces which allows reducing around 33.7 GWh of electric energy and 1186–1830 kg of CO2 emission for cooling.

Laboratory assessment of corrosion rate of carbon steel ground heat exchangers

Abstract. The materials used in the manufacture of geothermal heat exchangers for shallow geothermal applications play an important role in the overall system performance, especially if grout is not being used to seal the boreholes in which the heat exchanger is installed. The subject of this study is the durability evaluation of a vertical coaxial ground heat exchanger made of steel that is coupled directly to the ground. This solution minimizes the thermal resistance between the heat exchanger and the ground, but presents the important drawback of removing any protection toward the surrounding environment Among the materials proposed for manufacturing such vertical geothermal heat exchanger, carbon steel is suitable and have potential, due to its low cost and high thermal conductivity. The main disadvantage of this material is that it is strongly subject to corrosive attack, according to the chemo-physical properties of the underground. This study investigated the corrosion behaviour of carbon steel used in an experimental underground heat exchanger and assessed its durability over time. Corrosion rate of steel samples were measured in the laboratory by weight loss method after exposure over a specified period in a selected ground medium. Different ground conditions were tested, resulting in different densities and moisture contents of ground samples collected on the field. Based on the results, the corrosion rate of carbon steel is evaluated as a function of water content and rate of ground compaction. This information has allowed to advance more accurate quantitative forecast of the expected operational life of installed geothermal exchangers and their safety over time.

Numerical simulation and artificial neural network prediction of hydrodynamic and heat transfer in a geothermal heat exchanger to obtain the optimal diameter of tubes with the lowest entropy using water and Al2O3/water nanofluid

Since the world is facing an energy crisis, and the emission of carbon-based fuel combustion is causing global warming, more research has been devoted to studying renewable energies. Due to the broad use of geothermal heat exchangers in central air-conditioning systems and also as a direct source of energy, in the present study, we have conducted a simulation of a geothermal heat exchanger and investigated the optimal diameter and nanoparticle concentration in order to minimize entropy generation. The fluid flow is turbulent, and SST K−ε is utilized as the turbulence model. The results show that as the thermal resistance of the inner wall increases, the outlet temperature rises. The effects of thermal resistance and diameter are thoroughly investigated, and the results have been used for selecting the least entropy generation. The results showed that the diameter in which the entropy is minimized increases by increasing the inner wall's thermal resistance. The best case is high thermal resistance in the outer walls and meager resistance in the inner walls. Using inner walls with high thermal resistance can decrease the adverse effects of entropy generation in large diameters. Also, the addition of nanoparticles is investigated, and the average Nusselt number and entropy generation, respectively, increased and decreased by almost 10%. Artificial neural network models are proposed to predict Nusselt number and entropy generation based on numerical results. The models are able to achieve MAE of lower than 3% and R2 higher than 0.95.

Study on Geothermal Heat Exchangers With Nanofluids Containing Ceramic Nanoparticles

Abstract A geothermal heat exchanger (GHE) uses geothermal energy for heating or cooling residential places during winter or summer. Two different designs of GHEs, the straight pipe and coiled pipe designs, are evaluated in this study, and the effect of nanofluids as the working fluid is investigated. For this purpose, a mathematical model is developed, validated, and used to predict the temperature gain, heat gain, exergy gain, and pressure loss of the working fluid for different concentrations of additive ceramic nanoparticles of aluminum oxide (Al2O3) and magnesium oxide (MgO) in the working fluid. It is shown that the coiled pipe design has a better performance compared to the straight pipe design for GHEs. It is also shown how the temperature, heat gain, and exergy gain change with increasing the additive nanoparticles into the base fluid, which is water, while the pressure loss does not change significantly. The temperature gain increases about 60% when the volume fraction of nanoparticles in the base fluid reaches 2%. This also helps to improve the natural circulation of working fluid and the GHE may not need a circulating pump to run at low flowrates. It is also shown that the additive MgO nanoparticles are more effective than Al2O3 nanoparticles to improve the GHE performance.

Tikhonov regularization stabilizes multi-parameter estimation of geothermal heat exchangers

Parameter estimation from thermal response tests (TRTs) becomes unreliable when testing time reduces or the number of estimated parameters increases because of low identifiability and ill-posed mathematical feature. To overcome this challenge, this paper reports an inversion algorithm integrating a short-time temperature response model and the zero-order Tikhonov regularization strategy. We applied the algorithm to a reference sandbox dataset and examined four scenarios: simultaneous estimation of four, five, six, or seven parameters of U-shaped geothermal heat exchangers. The preliminary results indicate that the Tikhonov regularization can improve the accuracy and precision of the nonlinear multi-parameter estimation of ground heat exchangers for both long (>48 h) and short (<48 h) tests. The improved performance is contributed to the short-time model, which enables the short-time high-sensitivity data to be useable, and the regularization, which stabilizes the iterative optimization-solving procedure.

Vertical ground heat exchanger parameter characterization through a compound design of experiments

Vertical ground heat exchangers (VGHE) are the most extended elements in geothermal utilization systems. For design and optimization purposes, the precise determination of several thermal properties is of utmost importance. While soil properties can be accurately obtained from simple mathematical models and short identification procedures, the grout characterization and additional uncertainties are usually identified from time-consuming and complex models. In order to optimize and simplify this process, without loss of accuracy, the present work proposes the use of design of experiments (DoEs) for that estimation. Thus, a multi-physic 3D model that replicates the geothermal heat exchanger (GHE) environment has been employed to develop a fractional factorial design of experiments, obtaining a set of output errors by comparison with experimental data. A response surface methodology (RSM) has been used to conduct second-order linear regression fittings, that approximate the error functions to obtain the parameter values through the minima of such surfaces. Overall mean errors lower than 0.01 °C are obtained, which provide excellent agreement in modelling the experimental test. Finally, an additional full-scale experiment has been simulated in order to validate the numerical DoE approach. Consequently, the proposed methodology has been demonstrated to accurately predict the grout thermal parameters.

Characteristics and energy performance of novel MicroPCM C50 energy pile in cooling mode

Energy piles utilize geothermal energy via integrated geothermal heat exchangers (GHEs), which can sustain structural loads while also harvesting geothermal energy to meet the building's heating and cooling requirements. In the study, we report a systematic investigation of the Microencapsulated Phase Change Material (MicroPCM) C50 energy pile, from characterizing the material properties of the MicroPCM C50 concrete to analyzing the energy performance of the MicroPCM C50 based energy piles. Through integrating the MicroPCM into structural concrete, MicroPCM C50 concrete is manufactured and its properties, such as mechanical compressive strength and thermal conductivity, are characterized. With measured material properties implemented into a numerical model of the MicroPCM C50 energy pile, the energy performance when the energy pile works in the cooling mode is assessed. Our results suggest that (1) incorporating MicroPCM into C50 concrete specimens (up to 5 wt%), will dramatically reduce the compressive strength of C50 concrete; (2) Thermal conductivity and heat capacity of C50 concrete specimens are improved as mass fraction increases from 1 to 3 wt%, while reduced as mass fraction increases from 3 to 5 wt% (maximum at 3 wt%); (3) When considering both the mechanical and thermal performance of the energy pile, the energy pile with W shape GHE and 1 wt% MicroPCM addition is the best option for this scenario, followed by the energy pile with U shape GHE and 1 wt% MicroPCM, which will collect around 5% more geothermal energy, compared to normal concrete energy pile, and meanwhile retain a high level of mechanical strength. Our results will provide support for the design and optimization of the MicroPCM C50 energy piles toward real-world applications.

Theoretical-Experimental Analysis of the Performance of Geothermal Heat Pumps for Air Conditioning Greenhouses in Arid Zones

This study shows the results of a simulation tool using the TRNSyS 2017 simulator, validated with experimental data from a greenhouse in an arid zone in northwestern Mexico. Additionally, experimental data on the performance of geothermal heat pumps are shown during the year 2020 in heating and cooling mode. With this information, an average deviation of the simulator for the outlet fluid temperature of the geothermal heat exchanger (GHE) of 2.77% and an average deviation of the coefficient of performance in cooling mode (EER) of the geothermal heat pump (GHP) of 3.7% was obtained. In the experimental study, it was observed that in the last 2 weeks of July and the first 2 weeks of August, the subsoil is saturated, which causes a decrease in the thermal inertia of the GHE. During the experimental study, it was possible to determine that the flow indicated in the GHE to obtain the highest performance of the GHP system in greenhouses in arid zones corresponds to 1 GPM, obtaining an EER of 3.24.

Geothermal deep closed-loop heat exchangers: A novel technical potential evaluation to answer the power and heat demands

This paper investigates and optimises the thermal performance of deep closed-loop heat exchanger (DCHE) systems by applying a computational numerical approach. The investigated DCHE configuration accounts for two deep vertical boreholes, an injection and a production well, connected by a horizontal borehole at depth and an insulated pipeline at the surface, establishing an effective closed-loop system. First, a parametric sensitivity study explores the effects of the environmental, design and operating variables on the production temperature. The simulation uses realistic geological and geothermal conditions, depths, circulation rates and injection temperatures. Two complex numerical models are then solved for site-specific DCHEs in different geological scenarios: a foreland basin and a convergent margin hosting low-to-intermediate and high-temperature geothermal resources, respectively. Production temperatures beyond 40–60 °C and 100 °C, sustainable for both heat and electric power generation, are obtained, depending on the geothermal conditions and closed-loop dimensions. Furthermore, circulation rates of 0.02–0.04 m3 s−1 are cost-effective, and the system's efficiency and sustainability increase when a fluctuating and periodic heat extraction strategy is employed. When efficiently operated, DCHEs are a viable solution for renewable energy production and should be integrated into the local heat market and distribution network infrastructure. Field of researchEarth and Planetary Sciences; Energy; Environmental Sciences

Convective transport from geothermal borehole heat exchangers embedded in a fluid-saturated porous medium

Multi-dimensional natural convection and mixed convection have been studied numerically for the vertical borehole, a heated cylinder characterized by length Lb and diameter D, of a ground source heat pump in a fluid-saturated porous medium. The effect of natural convection over a range of ground properties characterized by Rayleigh number (RaL) is found to be significant and deviates strongly from boundary layer solutions for Rayleigh number less than 106, due to the flow turning required by the ground surface. For instance, the natural convection heat flux deviates from boundary layer prediction by over 20% for RaL = 104. Only at Rayleigh numbers of less than 2 × 103 can the heat transfer be treated as simple conduction, with the deviation less than approximately 5%, for fluid-saturated soil properties. The presence of groundwater flow, due to a hydraulic gradient, produces mixed convection leading to a convective heat transfer depending also upon Peclet number (PeD). A sufficient range of properties are examined such that the multi-dimensional mixed convection is shown to be captured between asymptotes to natural and forced convection. The dimensionless group (Lb/D)PeD/RaL is identified and shown to parameterize the variation between these two limits, with values less than 1.4 asymptoting to natural convection and values greater than 2.9 asymptoting to forced convection. Finally, a correlation encompassing all three regimes, natural, forced and mixed, is proposed.

Design studies on horizontal and vertical configuration of ground earth heat exchanger

In this work, user friendly software is described for the easy design of the GEHE. There are different procedures for designing the geothermal heat exchanger. Based on the different ways to extract the energy and the configurations of the GEHE system several methods are used for the design of the system. The software developed is based on two of such methods which are explained along with their applications in this paper. Results of the software along with some images of the software is also presented in this work which is compared by the help of a system which is installed in the library building of Deenbandhu Chhotu Ram University of Science and technology, Murthal along with the results of the software are compared with various scholarly articles published by different researchers based on different experimental setups or the simulation results for both the horizontal and vertical systems. A proposed system for the vertical type GEHE which can be installed in DCRUST Murthal is also presented in this paper. The results show that the methods used in the designing of the GEHE in the software gives very close values, i.e., the percentage average deviations are in the range of 0.5 to 6% to the actual system for the cases considered in this work.

Design Framework and Laboratory Experiments for Helix and Slinky Type Ground Source Heat Exchangers for Retrofitting Projects

The focus of the experimental work was on shallow spiral geothermal heat exchanger configurations. Real-scale experiments were carried out for vertically oriented spiral collectors (helix) in sand and soil. One objective was to develop a measurement concept in laboratory environment to create a framework for a validated database. This database serves as the basis for further and new development of engineering design tools. To achieve the highest possible data-point density in the observed environment, temperature sensors and a fiber-optic temperature measurement system (DTS) were used. Soil probes were taken in situ before and after the measurements and analyzed at a thermophysical laboratory to determine material properties. The heat flow was controlled by an electric heating cable, which was installed in the form of a spiral-shaped heat exchanger in a 1 m³ container. To guarantee constant boundary conditions, the measurements were carried out in a climate chamber at a defined ambient temperature. The evaluation of the transient response behavior is spatially resolved. The results are coordinate-based temperature points, which describe temperature gradients in all axes of the container over time, which are combined with known soil properties. The collected data was used to develop computational fluid dynamic (CFD) models, which are used to extend the variety of geometry and soil configurations for developing new design tools.

European and municipal scale drillability maps: A tool to identify the most suitable techniques to install borehole heat exchangers (BHE) probes

The most suitable drilling technology for shallow geothermal applications in a given geological context plays a key role in the techno-economic evaluation of shallow geothermal solutions. The installation costs are one of the main constraints to the wider application of shallow geothermal heat exchangers and are due mainly to the drilling time and fees.This paper defines drillability as a tool to select the most suitable drilling technique for borehole heat exchanger installations in a given geological and hydrogeological setting. Drillability maps at European and municipal scale are proposed as a guideline for drillers and ground source heat pump designers. The former summarizes the most suitable drilling technology based on lithology and borehole heat exchangers type, whilst the latter provides an insight in the ground thermal properties and the installation timing/costs. In the GEO4CIVHIC Project, a database correlating drilling technique to rock types, time needed to drill a borehole at 100 m depth and drilling costs was created. Local drillability features are compared to in situ real data in four test sites. Two out of four test sites, completed at the time of writing, shows a good agreement between local maps and real site conditions, validating the innovative methodology proposed.

Sustainability study of medium-deep geothermal buried pipe for heat supply

In order to study the temperature change trend of the surrounding geotechnical soil during the operation and thermal recovery of the medium-deep geothermal buried pipe and the influence of the geotechnical soil on the operational stability of the vertical buried pipe after thermal recovery. Based on the data of geological stratum in Guanzhong area and the actual engineering application of medium-deep geothermal buried pipe heating system in Xi’an New Area, the influence law of medium-deep geothermal buried pipe heat exchanger on surrounding geotechnical soil is simulated and analyzed by FLUENT software. The results show that: after four months of heating operation, in the upper layer of the geotechnical soil, the reverse heat exchange zone appears due to the higher fluid temperature; in the lower layer of the geotechnical soil, the temperature decreases more with the increase of depth and shows a linear increase in the depth direction; without considering the groundwater seepage, after eight months of thermal recovery of the geotechnical soil after heating, the maximum temperature difference after recovery is 3.02 ℃, and the average temperature difference after recovery is 1.30 ℃ The maximum temperature difference after recovery was 3.02 ℃ and the average temperature difference after recovery was 1.30 ℃. The geotechnical thermal recovery temperature difference has no significant effect on the long-term operation of the buried pipe, and it can be operated continuously and stably for a long time. Practice shows that due to the influence of various factors such as stratigraphic structure, stratigraphic pressure, radioactive decay and stratigraphic thermal conductivity, the actual stratigraphic temperature below 2000m recovers rapidly without significant temperature decay, fully reflecting the characteristics of the Earth’s constant temperature body.

Numerical simulation of a combination of a new solar ventilator and geothermal heat exchanger for natural ventilation and space cooling

Numerical simulation of a combination of a new solar ventilator and geothermal heat exchanger for natural ventilation and space cooling

Thermal and hydraulic analysis of a novel double-pipe geothermal heat exchanger with a controlled fractured zone at the well bottom

This study introduces a novel concept of semi closed-loop double-pipe heat exchangers with a controlled fractured zone at the well bottom to improve the geothermal heat extraction. The proposed model is benefitted from the reliable design of a double-pipe and the higher heat extraction rate of open geothermal systems. A conjugate mathematical model is developed and validated to simulate geothermal heat extraction, while the fractured zone is calculated by implementing the fractal theory. The results suggest that the proposed model can significantly improve the heat extraction rate and thermal power by up to more than 90%, with an additional 28% of pumping power. Several operating parameters of the fractured zone are also evaluated, indicating that the increase of the heat extraction rate compared to the closed-loop model was within the range of 48%–144% based on its design. Overall, this novel design shows the potential to improve the thermal and hydraulic performances of the conventional closed-loop geothermal system.

Geothermal Deep Closed-Loop Heat Exchangers: A Novel Technical Potential Evaluation to Answer the Power and Heat Demands

Geothermal Deep Closed-Loop Heat Exchangers: A Novel Technical Potential Evaluation to Answer the Power and Heat Demands

Selection of backfill grout for shallow geothermal systems: Materials investigation and thermo-physical analysis

A main aspect of the geothermal well design is the selection of grouts backfilling the space between a geothermal probe and the surrounding ground, taking into consideration the mineralogical, thermo–physical, mechanical and flowability properties. The EU GEO4CIVHIC project aimed at developing efficient and cost-effective geothermal systems suitable for the air-conditioning refurbishment of urban buildings. A selection of different grouts was performed to ensure the whole efficiency of shallow geothermal heat exchangers designed within the project. Experimental tests were executed to identify flowable grouts with the best possible combination of different material properties, while also fulfilling a competitive cost target set forth in the preliminary project cost analysis. Grout acceptance criteria were defined: a working time of at least 4 h, a water bleeding target of 2% and a volumetric shrinkage upper bound of 4%, a thermal conductivity in water saturated conditions of at least 1.4 to 1.5 Wm−1K−1, a compressive strength range of 1 to 2.1 MPa, a flow time range of 60 ± 15 s. Two out of six commercial grouts were selected for installations onsite.