Geothermal Heat Exchanger Research Articles

Analyzing the effectiveness of heat transfer in a horizontal geothermal heat exchanger using numerical methods

Analyzing the effectiveness of heat transfer in a horizontal geothermal heat exchanger using numerical methods

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.

Sustainable reuse of oil and gas wells for geothermal energy production: Numerical analysis of deep closed loop solutions in Italy

This study assesses the feasibility of repurposing abandoned oil and gas wells in Italy for geothermal energy production employing a geothermal closed-loop system. A systematic methodology was developed, beginning with raw data collection and progressing to numerical simulations using COMSOL Multiphysics to model a U-shaped deep closed-loop geothermal heat exchanger. The analysis relied on a public database of wells drilled in Italy since the mid-20th century. The Horner plot correction method was applied to measured temperature data to obtain accurate geothermal gradients across Italy, which were then used as input parameters for a numerical sensitivity analysis. The results highlight the critical role of the geothermal gradient and heat carrier fluid flow rate in determining system performance. Regions in Italy with geothermal gradients exceeding 40 °C/km, particularly in the Tyrrhenian area, were identified as having high potential for this technology. A preliminary analysis of a virtual Organic Rankine Cycle (ORC) system estimated power production of 73 kW, with an efficiency of 11.66 % after 25 years of operation under optimal conditions (5 l/s flow rate, 60 °C/km geothermal gradient, and 70 °C evaporation temperature).

Composites with Synergistically Enhanced Thermodynamic and Mechanical Properties for Geothermal Heat Exchangers

Geothermal energy, derived from the radioactive decay of elements within the Earth's core, is a renewable and clean energy source. The extraction of geothermal energy primarily depends on ground heat exchangers. To address the issue of incompatibility between corrosion resistance, high thermal conductivity, and mechanical performance in current ground heat exchangers, this study develops composites (PEPOE/SCA@SiC) composed of high‐density polyethylene (HDPE) modified with toughener polyethylene‐octene copolymer elastomer (POE) and silicon carbide (SiC) crystals modified with γ‐aminopropyltriethoxysilane (SCA). HDPE offers excellent corrosion resistance, SiC crystals provide efficient phonon transport, and SCA improves the interface compatibility of the composite. The thermal conductivity of PEPOE/SCA@SiC reaches 3.83 W m−1K−1, with a maximum tensile strength of 20.3 MPa, a tensile elastic modulus of up to 498 MPa, and a maximum bending stress of 22.2 MPa with a bending elastic modulus of 561 MPa. Corrosion resistance tests indicate no signs of corrosion in PEPOE/SCA@SiC when exposed to geothermal simulation fluids at 90 °C for 30 days. This study demonstrates that PEPOE/SCA@SiC composites have substantial potential for application in ground heat exchangers, offering promising prospects for advancing sustainable use of geothermal energy and environmental protection.

Determination of the Temperature Development in a Borehole Heat Exchanger Field Using Distributed Temperature Sensing

The use of geothermal borehole heat exchangers (BHEs) in combination with ground-source heat pumps represents an important part of shallow geothermal energy production, which is already used worldwide and becoming more and more important. Different measurement techniques are available to examine a BHE field while it is in operation. In this study, a field with 54 BHEs up to a depth of 120 m below ground level was analyzed using fiber optic cables. A distributed temperature sensing (DTS) concept was developed by equipping several BHEs with dual-ended hybrid cables. The individual fiber optics were collected in a distributor shaft, and multiple measurements were carried out during active and inactive operation of the field. The field trial was carried out on a converted, partly retrofitted, residential complex, “Lagarde Campus”, in Bamberg, Upper Franconia, Germany. Groundwater and lithological changes are visible in the depth-resolved temperature profiles throughout the whole BHE field.

Numerical investigation of the geometric parameters effect of helical blades installed on horizontal geo heat exchanger

In this study, we investigate the enhancement of horizontal geothermal heat exchangers equipped with helical fins on the pipe's exterior and internally ribbed turbulators. Our approach focuses on the interplay between geometry and thermal efficiency through innovative design modifications. Utilizing the finite element method, three-dimensional numerical simulations assessed the effects of varying geometric parameters such as the diameter and thickness of the fins. Our findings indicate significant increases in heat transfer efficiency with the addition of helical fins; specifically, increasing the fin diameter from 5 mm to 10 mm results in a 15 % increase in the heat transfer rate, while doubling the fin thickness from 2 mm to 4 mm enhances the rate by 10 %. These improvements are due to the expanded surface area facilitating greater heat exchange. Optimization using the desirability function approach yielded models with high performance, achieving desirability scores of 0.9879 for outlet temperature and 0.9534 for the heat transfer coefficient. This reflects the effective tuning of geometric parameters to maximize thermal performance. The study also introduces two predictive mathematical models for the outlet temperature and convective heat transfer coefficient of the U-shaped pipe equipped with these enhancements. These models, derived from extensive numerical data, provide practical tools for future design and operational applications of geothermal heat exchangers. This research advances the design and operational efficiency of geothermal heat exchange systems, establishing new benchmarks for thermal efficiency in the field with actionable insights and robust mathematical tools.

Heat transfer performance of energy pile and borehole heat exchanger: A comparative study

Geothermal structures are widely utilized to satisfy the heating and cooling demands of buildings, providing a promising alternative to the escalating scarcity of energy. As two types of geothermal heat exchangers, energy piles and borehole heat exchangers exhibit different efficiencies in extracting geothermal energy. This paper examines the thermal performance of an energy pile and a borehole heat exchanger under heating and cooling conditions through field tests, introducing the heat exchange rate per unit pipe length (q0) as a metric. Furthermore, a three-dimensional heat transfer numerical model was developed and validated. The parametric analysis was subsequently conducted to investigate the sensitivity of heat exchange efficiency to five parameters, including flow rate, inlet temperature, pipe diameter, soil thermal conductivity, and soil temperature. The results showed that the heat transfer efficiency of the borehole heat exchanger was 42.2 % and 48.7 % lower than that of the energy pile under the heating and cooling conditions, respectively. Moreover, the heat transfer efficiency of the energy pile and borehole heat exchanger is most sensitive to inlet temperature, with the latter showing greater sensitivity to pipe diameter. This study provides valuable insights into optimizing the design and operation of energy piles and borehole heat exchangers to meet the growing energy demands of buildings.

Understanding the impact of flow mechanisms in unsaturated layer on heat transfer near a partially submerged geothermal heat exchanger

Heat transfer near geothermal heat exchangers in the presence of groundwater flow has been extensively studied over the past years. However, previous studies often overlooked the presence of an unsaturated layer in the shallow subsurface soil, or disregarded phase change and fluid flow within this layer. In this study, a comprehensive hydro-thermal model is developed to evaluate heat transfer near a partially submerged geothermal borehole in groundwater flow while incorporating phase change and liquid and vapor flow in the unsaturated layer. The effect of unsaturated soil hydraulic analysis on heat transfer and energy efficiency of the geothermal piles is explored under varying soil permeabilities, air-entry suctions, solid thermal conductivities, and soil types. Consideration of water content variation and unsaturated fluid flow in the model enhances heat injection rate by up to 11.23 % in highly permeable soils. However, in scenarios with higher solid thermal conductivity, or lower air-entry suction values, the heat injection rate reduces by as much as 16.13 % due to unsaturated soil hydraulic analysis. Furthermore, it is found that consideration of unsaturated soil hydraulic analysis is of greater importance in Bonny silt than sandy soil, lowering the heat injection rate by 8.53 % and 2.53 %, respectively.

Effects of longitudinal ventilation and GHEs on geothermal energy extraction and HRC in high geothermal tunnels

With the excavation of deep tunnels, the problem of high geothermal heat is becoming more and more prominent. However, high geothermal heat can be utilized as a renewable energy source. In order to effectively reduce thermal damage and utilize geothermal energy, it is necessary to study the effects of longitudinal ventilation and ground heat exchanges (GHEs) on the heat-regulating circle of the surrounding rock. In this study, an experimental platform was designed and constructed in a laboratory to test different geothermal heat exchanger flow rates and ventilation air velocities and to monitor the heat extraction from the geothermal heat exchanger, the ventilation heat extraction, and the temperature field of the surrounding rock. The experimental results show that when the temperature field tends to be stabilized, the flow rate and ventilation velocity of the GHEs are closely related to the temperature distribution of the surrounding rock. The initial heat storage and heat replenishment capacity of the enclosing rock have a great influence on the heat extraction efficiency. Ventilation has a great influence on the heat extraction efficiency of GHEs, and the optimal wind speed range is 1.5∼2 m/s considering the cooling and heat extraction efficiency. When the wind speed is less than 1.5 m/s, the heat extraction efficiency of GHEs can be improved, and the total heat transfer can be increased. When the wind speed is greater than 1.5 m/s, the situation is reversed. The thermal entropy theory for circular tunnels is not fully applicable to arched tunnels containing GHEs.

Research on mine heat exchange system and its heat exchange performance prediction

As a clean and renewable energy source, mine geothermal energy holds immense potential for application. To better utilize this resource, a buried tube ground-source heat pump system has been proposed for mine geothermal energy development. A mathematical model for the mine geothermal heat exchange system has been established. Firstly, the experiment of constant heat flow and heat transfer is conducted to verify the applicability of the mathematical model and to explore the temperature response law of single pipe fluid and rock and soil. Subsequently, the influence of ten single-tube factors on the heat transfer performance of the mine geothermal heat exchange system is studied using the single-factor analysis method. The sensitivity of the single tube factor is analyzed using the range analysis method to obtain six main factors. The impact of the group tube and time factors on the heat transfer performance is qualitatively analyzed. Finally, the orthogonal experiment and the non-linear regression method are employed to derive a prediction expression for the average output temperature of the mine geothermal heat exchange system by considering the single tube, group tube, and time factors. The research results enable the quantitative prediction of heat exchange performance and guide the design and optimization of mine geothermal heat exchange systems.

Numerical analysis of heat gain to geothermal heat exchangers with moisture evaporation in the structure

Relevance. Provisions on the need to transition to alternative energy sources and energy security, reduction of environmental emissions and energy conservation in various industries can largely rely on the use of geothermal energy. A separate area in the field of modeling geothermal technologies is the study of thermal regimes of geothermal heat exchangers. The need to ensure reliable thermal contact between the geothermal heat exchanger and its environment leads to the use of a variety of backfills, including pre-moistened ones. Aim. Study of the intensification of heat supply to geothermal heat exchangers, taking into account the influence of moisture evaporation in its backfill, its characteristics and operating conditions of the systems under consideration. Methods. The solution to the problem was obtained by the finite difference method. An implicit difference scheme and sweep algorithm were used. The coordinate step ranged from 1 to 10 mm. Results and conclusions. Study was carried out for six months of continuous operation of a geothermal heat exchanger. When analyzing the thermal conditions of the system under consideration, the main attention was paid to the study of the intensification of heat supply to the geothermal heat exchanger, taking into account the influence of moisture evaporation in its backfill, its characteristics and operating conditions of the systems under consideration. It has been established that an increase in the humidity of the sand backfill of a geothermal heat exchanger leads to a growth in heat inflows by 3.2–7.8%. A significant influence of the unsteadiness of transfer processes on the intensification of heat transfer in the system under consideration was revealed. It was shown that the increase in heat flow to the geothermal heat exchanger due to the presence of evaporation is about 22%. The conclusion is substantiated that when choosing an option for regulating the thermal conditions of geothermal heat exchangers, the volumetric humidity of the sand backfill should be primarily changed.

Enhancing geothermal heat pump efficiency with fin creation and microencapsulated PCM: A numerical study

The main cause of global warming is the emission of carbon dioxide from fossil fuels. Several methods to reduce CO2 emissions have been explored to ensure energy conservation. Many researchers have focused on renewable energy sources for this reason. Ground-source heat pumps (GSHPs) have emerged as a popular alternative in various sectors that utilize shallow geothermal energy sources for heating and cooling. This study presents a novel approach to enhancing the efficiency of geothermal heat exchangers, a critical component of GSHPs, through numerical simulation using computational fluid dynamics (CFD) with Ansys Fluent software. The research investigates the impact of adding four models of spiral fins to the exterior body of the heat exchanger: single spiral and double spiral configurations with pitches of 60 mm and 40 mm, respectively. Additionally, the study explores the effect of incorporating microencapsulated phase change materials (PCMs) with volume fractions of 0 %, 10 %, 30 %, and 50 % into the backfill material surrounding the heat exchanger. The results demonstrate that the double spiral fins with a 40 mm pitch significantly enhance thermal efficiency by reducing the output temperature by up to 1 °C compared to the model without fins. Furthermore, the addition of 50 % microencapsulated PCM by volume to the backfill material can further decrease the output temperature by up to 4 °K. The combined use of double spiral fins and microencapsulated PCMs results in a 7.5 % improvement in the efficiency of the heat exchanger and the coefficient of performance of the heat pump. This study highlights the potential of innovative design modifications and material enhancements to significantly improve GSHP performance for more efficient and sustainable heating and cooling solutions.

Study on Zoning Effect of Shallow Geothermal Energy Suitability Based on Structural Matrix Method

Geothermal energy is a clean, renewable resource that can be harnessed to help China achieve its twofold carbon goal and advance its energy security policy. Shallow geothermal, middle-deep geothermal, and dry hot rock geothermal all belong to the clean energy. The thermal energy held in rock, soil, groundwater, and surface water up to a depth of 200 metres is referred to as the shallow geothermal variety, which is characterized by temperatures below 25∘C. Its use is mostly dependent on ground source heat pump technology. The ground source heat pump ground pipe heat exchange system, the ground source heat pump groundwater heat exchange system, and the ground source heat pump surface water heat exchange system are all included in the shallow geothermal heat exchange mechanism. The structural matrix approach is used in this study to determine if the groundwater heat exchange system for the shallow geothermal ground source heat pump in Sanmenxia’s urban setting is appropriate. The results showed that: the most influential factor was hydrogeological conditions, followed by hydrodynamic and water chemistry conditions. Aquifer recharge capacity and water capacity was the necessary condition of suitability partition. Studied hydrodynamic field and chemical field, analyzed aquifer thickness and groundwater flow under current conditions and determined the appropriate zoning, proposed groundwater aquifers and recharge horizon.

Numerical Investigation of the Coaxial Geothermal Heat Exchanger Performance

Space heating and cooling using geothermal heat exchangers is a promising environmentally friendly green energy solution. Modeling these energy storage systems is crucial for optimizing their design and operation. In this context, the present study consists of numerically investigating the effects of various physical properties, including thermal conductivity, density, and specific heat capacity of each material, as well as flow velocity, on the process of heat transfer in vertical geothermal heat exchangers using coaxial pipes to optimize their energy performance. Numerical simulations were carried out using Gambit-Fluent software. Different materials that make up the coaxial heat exchanger structure studied were tested to highlight their effects on the progress of heat flux and temperature. Thermal and fluid mechanics aspects were also studied. At the end of this study, a comparative analysis was carried out using the U-tube geothermal heat exchanger. The results indicate that the heat exchanger using a coaxial tube demonstrates superior thermal efficiency compared to the U-tube configuration. It has been found that using a low velocity with an appropriate selection of tube, grout, and soil materials results in enhanced dynamic exchanges, thereby enhancing the thermal efficiency of the geothermal exchanger.

Analysis of a domestic air heat pump integrated with an air-geothermal heat exchanger in real operating conditions: The case study of a single-family building

The work presents an in-depth analysis of the real performance of an air heat pump unit installed in a single-family building and quantitatively determines the reduction in energy consumption achieved by the integration of an air-geothermal heat exchanger. The case study is a single-family building located in northern Italy. It is equipped with a heating, ventilation, and air conditioning (HVAC) system consisting of an air-geothermal heat exchanger coupled with a compact air source heat pump supported by an electric heater installed in a thermal storage and devoted to cover the peak demand. The main objectives of the work are to analyse the HVAC performance under real operating conditions compared to the design values and to highlight the existence of factors that affect this performance. The analysis was carried out by developing an accurate model of the heat pump system based on the real operating data acquired by the on-board monitoring system. The results confirmed the influence of the air-inlet conditions (temperature and humidity) and of the control strategy (more specifically, the ventilation flow rate) on the unit performance in both the heating and cooling modes. In particular, the analysis showed that minimising indoor ventilation in the case of low outdoor temperatures is of paramount relevance not only because it reduces the indoor heat demand, but also because it maintains the unit efficiency close to the design values. Furthermore, the installation of a geothermal heat exchanger guarantees a reduction in electricity consumption of approximately 30%.

Use of electrical resistivity tomography measurements for investigation of different grouting materials for very shallow geothermal applications within varying seasonal conditions; Applied on a geothermal earth-air heat exchanger system

To achieve the current climate targets, heat pump systems like very shallow geothermal applications are gaining ground. However, the dimensioning of these ground coupled systems is soil-dependent and thus subject to significant differences in efficiency. Furthermore, the thermal properties of the grouting materials are essential for heat transfer in the direct vicinity of the geothermal application. To provide a non-invasive assessment of relevant soil and grouting characteristics, electrical resistivity tomography (ERT) measurements were performed.In this case, the surrounding of an Earth Air-Heat Exchanger – system (EAHE) with different grouting materials (fine sand and fine sand with bentonite) was investigated. For investigation the electrical resistivity (ER) of initial and modified soil conditions were measured regarding soil texture and moisture content. Thereby, the different grouting materials are clearly identified. Thus, based on the ERT measurements, a characterisation of key soil and grouting material properties for applications focusing on heat transfer in soil has been carried out.Furthermore, the depth of soil disturbance due to the EAHE installation could have been traced.Due to a monitoring over different season (February and May) changes in raw ER data could have been ascribed to temperature changes.

A new analytical model for transient heat transfer modeling of geothermal monotube heat exchangers

In this study, we present a new analytical model for the analysis of transient heat transfer (the model is solved after Laplace transformation) in geothermal monotube heat exchangers (GHE). The model accounts for various factors, including the heat exchanger's geometric parameters, the nature of the fluid (air and water), flow characteristics (laminar and turbulent), and heat transfer boundary conditions. Initially, the model's performance is validated by comparing its results with those obtained from computational fluid dynamics simulations using OpenFOAM. This validation process ensures the reliability and accuracy of the model. Additionally, an analysis of the heat transfer mechanisms and key modeling parameters identified from the comparison is provided. In the subsequent section, the analysis is extended by comparing the proposed model to four existing analytical models available in the literature. These reference models employ distinct approaches to simulate heat transfer to the ground. By examining a range of cases, the performance of the model is systematically evaluated against these conventional methods. This work closes by an analysis of the analytical models and the impact on the accuracy of the results highlighting the advantages of the proposed method in geothermal applications.

Investigation of the operating characteristic of a demand-controlled 368 m deep CO2 thermosyphon geothermal borehole heat exchanger for building heating

This paper examines a 368 m deep geothermal probe for heating an existing building. The geothermal probe based on the thermosyphon principle works with CO2. The working fluid evaporates and condenses in the probe so that no pump is required for transportation. The geothermal probe has an output of up to 22 kW at an underground temperature of up to 18°C. The demand-controlled use of the geothermal probe was investigated and the installed heat pump was optimized for its use. The average COP of the last heating season was between 2,3 and 4,6. For the second heating season the temperature of the soil around the probe is also measured via a distributed temperature sensing system, that covers the whole depth of the probe. It has been shown that the ground temperature only cools down to a certain point and does not fall below this point after a short regeneration period. It was shown that the heat pump with geothermal probe operates at constant temperatures at the evaporator compared to air heat pumps, even at cold outside temperatures.

Energy demand parametric analysis and geothermal heat exchanger design applied to a nearly zero energy PV building in northern Italy

Nearly-Zero-Energy and Zero-Emission Buildings are one fundamental part of the world strategies addressed at mitigating the global warming trend and coping with the goal of setting 1.5 °C the planet temperature increase with respect to the pre-industrial conditions. The present paper refers to a specific case study, the Smart Energy Building (SEB) located in the Savona Campus of the University of Genoa. The SEB is a very innovative building for both the envelope (ventilated highly insulated facades) and the energy systems (including a ground coupled heat pump and an exhaust air-to-air heat pump for building ventilation); the building is equipped also with a PV module field on the rooftop for electricity production. The present paper first evaluates the heating and cooling loads of the building by means of an EnergyPlus model and analyses the impact of different control strategies related to the temperature setpoints. In particular, case #1 refers to a minimum temperature to be assured by heating only in winter and to a maximum one to be avoided by cooling only during summer; on the other hand, in case #2 the temperature inside the building is controlled within a defined range (both heating and cooling modes) all year long. From EnergyPlus simulations, the hourly heat loads have been averaged as monthly values and employed for the calculation of the required overall length of the ground heat exchangers of the geothermal heat pump of the SEB building. The ground-side analysis is performed by applying the Authors’ ASHRAE-Tp8 method, in its recently released web version. For the BHE (borehole heat exchanger) calculations, a parametric analysis has been performed in terms of ground thermal conductivities. Finally, the PV electrical production has been estimated by EnergyPlus simulations and compared with the corresponding measured one, thus showing the net energy zero behaviour of the present building.

Investigating the coupling effect of ventilation and GHEs on temperature distribution and heat transfer characteristics

Tunnel lining geothermal heat exchangers (GHEs) offer numerous advantages, including small area, cost-effectiveness and high heat exchange efficiency. It is a good equipment to utilize geothermal energy in tunnels. However, few research has been conducted on the combined impact of tunnel lining GHEs and ventilation on the temperature field of the surrounding rock. In this study, laboratory test model of combined impact of heat transfer between ventilation and GHEs was established, the effects of different flow rates and wind speeds on temperature field and heat exchange characteristics of the rock were investigated, and the changes in heat transfer under different operational conditions were analyzed. The findings reveal that tunnel ventilation initially enhances the heat transfer efficiency of GHEs, but subsequently decreases the overall heat transfer. Moreover, higher wind speeds exacerbate this negative impact. The radius of the heating ring decreases with increased heat transfer, and the temperature of the surrounding rock rises with heightened heat exchange at same locations within the heating ring. There are reasonable ranges of wind speed and GHEs flow rate, which are 1.5 m/s–2 m/s for wind speed and 0.4 m/s–0.6 m/s for flow rate, respectively. These results are important for the evaluation of thermal effect of tunnel lining GHEs and ventilation design.