Geothermal Heat Exchanger Research Articles

A combined moving boundary and discretized approach for dynamic modeling and simulation of geothermal heat pump systems

The major goal of this study is to demonstrate the feasibility of the dynamic modeling and simulation of both conventional and direct exchange geothermal heat pump applications particularly with regard to performance evaluations. A direct exchange geothermal heat pump system is a geothermal heat pump system in which the refrigerant circulates through tubing placed in the ground. In this research, dynamic models for the simulation of geothermal heat pump systems with the working fluid propane for the application in building-scale energy systems have been developed based on the Moving Boundary approach and the challenges of dynamically evaluating these kind of energy systems have been hereby met. Additionally, the performance of both horizontal (slinky-coil) and vertical (borehole) geothermal heat exchangers, which have been modeled based on the Conventional Discretized approach, for such systems has been evaluated. Since these two modeling approaches are not compatible with one another, the coupling of the moving boundary model with the discretized model is therefore another challenge and of great importance. On the basis of a case study, the complete conventional and direct exchange geothermal heat pump systems with vertical and horizontal heat exchangers have been simulated and energetically and exergetically analyzed.

Natural convection of third-grade non-Newtonian fluid flow in a porous medium with heat source: Analytical solution

Heat transfer by natural convection occurs in many physical and engineering applications. Governing equations of these problems are non-linear and need special methods for being solved. This paper aims to conduct an analytical analysis on the natural convection of a non-Newtonian fluid flow between two infinite vertical flat plates within a porous medium considering a variable heat source. The addition of porous medium and heat source marks the difference between the present work and previous researches. In the first phase of the current analysis, the governing equations including partial differential equations (PDE) are turned into ordinary differential equations (ODE) utilizing the similarity solution. Afterwards, a system of differential equations is solved using the Least Square Method (LSM), and reliable functions for the temperature and velocity distributions are presented. In order to investigate the accuracy of this method, the governing equations are also solved using numerical solutions. Proper agreement is observed between the analytical and numerical results. Regarding the very small errors observed in the results yielded by the LSM method, it can be concluded that this method is an efficient and reliable approach for solving non-linear ordinary differential equations. Finally, the effects of two main parameters, namely porosity and heat source parameters, are meticulously discussed. It is shown that as the value of the heat source parameter increases, the values of velocity and temperature decrease. Also, the results indicate that by enhancing the porosity parameter the flow velocity decreases, whereas there is no change in temperature. The results could be helpful for systems such as geothermal systems, heat exchangers, petroleum reservoirs and nuclear waste repositories in which natural convection is important.

Enhanced efficiency of photovoltaic panels by integrating a spray cooling system with shallow geothermal energy heat exchanger

In this study, a cooling system with shallow-geothermal energy is experimentally investigated to mitigate the photovoltaic panel conversion efficiency decline problem, and a mathematical model is built for predicting the system performance. This cooling system cools the panel by spraying water onto the reverse of the panel, and returns the water to the tank. To enhance the cooling capacity, the recycled water is collected in a U-shaped borehole heat exchanger (UBHE), which is installed in an existing well, and the water exchanges heat with shallow-geothermal energy. Finally, the panel is again sprayed with water to cool it. The experiments are comprised of three stages: The first involved the panel operating without cooling system. The second involved the panel operating with a cooling system but without a UBHE. The third is the cooling system operating with a UBHE. The experimental results and the mathematical models show the same trend: The cooling system can improve the panel conversion efficiency; moreover, as the temperatures and the number of panels increase, the benefit becomes obvious. For a plant factory powered by panels, for example, this cooling system can improve the efficiency by 14.3%, and its equipment costs recovered in 8.7 years with this system.

Improvement of a direct solar dryer performance using a geothermal water heat exchanger as supplementary energetic supply. An experimental investigation and simulation study

aiming to improve the thermal performance of a direct solar dryer, a new technique of heat supply was proposed and investigated using a double level tubular heat exchanger with geothermal water, constructed in LENREZA laboratory. The heat exchanger was placed deliberately above the absorber plate facing the holes so as to enable the air penetration. The temperature of the circulating water was set to 70 °C analogically to that of geothermal water in southern regions of Algeria. The experiment was carried out in LENREZA laboratory, (laboratory of development of new and renewable energy in arid and Saharan zones), University of Ouargla, Algeria. Moreover, numerical simulation was carried out and results were compared with the experimental. The findings showed that the integration of heat exchanger improves significantly the solar dryer performance. By using heat exchanger, the lowest drying air temperature was found to be 46 °C, while the highest one was 58 °C. After sunset and throughout the night, the drying air temperature remains important and almost steady with an average value of 46 °C. The integration of the heat exchanger ensures the continuity of drying process at the night and even during cloudy days.

Thermal performance evaluation of a vertical closed-loop ground heat exchanger according to rock type in Korea

Geothermal heat pump systems have a variety of uses including air conditioning and heating for buildings and greenhouses, hot water supplies, and crop drying applications. The installation environment and conditions of operation can have a large influence on the performance of the system and need to be considered. In this study, we applied the line-source method to different types of rocks and borehole depths in order to analyze the characteristics of effective thermal conductivity. We conducted thermal performance tests for a geothermal heat exchanger in accordance with initial ignoring times (5, 7, 9, and 12 h) and analyzed the stability of effective thermal conductivity and the relationship between the heat injection and slope of k in λeff = Q/4πHK according to variations in appropriate heat injection levels. The analysis of effective thermal conductivity according to the rock type and borehole depth for 40 boreholes showed that the value of effective thermal conductivity was generally high at the borehole depth of 200 m. This seemed to be due to the increase of underground space for energy storage. Consequently, we determined that the aquifer had the largest effect on the effective thermal conductivity.

Investigating the effects of geometric parameters on finned conical helical geothermal heat exchanger and its energy extraction capability

With the onset of the warm and cold season, the temperature inside the building goes far away from the human comfort condition, so an optimal energy source is needed to reach the desired temperature. Naturally, the earth is a massive source of heat that is easily accessible under the buildings, yards, and squares of the city; therefore, the geothermal heat exchangers can be used in depth of the ground to drain the heat from the earth. In optimizing heat transfer equipment to achieve higher energy efficiency, the focus is on reducing the size of the heat exchanger on the one hand and increasing the intensity of heat transfer area on the other hand. Helical heat exchangers are compact and can be used in geothermal systems to receive energy. In this paper, a conical coil tube geothermal heat exchanger is simulated according to the environmental conditions of Tehran, the capital city of Iran, and the potential of using such system is examined by changing the geometric parameters and operational requirements. The coil diameter, coil pitch and the angle of conic of the helical tube are varied for the heat exchanger buried at a depth of 3m from the earth surface. The soil temperature profile is obtained from the air temperature data Synoptic Meteorological Station. Design parameters are coil diameter, coil pitch, cone angle, length to width ratio of the fin, number of fins and Reynolds number. Taguchi algorithm is applied to find the best geometric parameters. Finally, different volume fractions of Al2O3 nanoparticles are added to the optimized geometry to enhance the heat transfer rate. Results indicate that the optimized geometry considering thermal-hydraulic performance in the range of this study is θ=0°, Dc=1000 mm, Pc=60 mm, Re=3000, R=1 and Fn=6 for cone angle, coil diameter, coil pitch, Reynolds number, length to width ratio of fin and number of fins respectively. Changing the objective to increase heat transfer (Nusselt number) the optimized values alter to θ=3.14°, Dc=1000 mm,Pc=80 mm, Re=4993, R=1.5 and Fn=6. Heat flux on the surface of the tube increases to 18% for 0.5% volume fraction of nanoparticles.

Hybrid semi analytical method for geothermal U shaped heat exchanger

In this paper, convective-conductive heat transfer processes between the all components of geothermal borehole heat exchanger (BHE) such as pipe-in, pipe-out and grout have been studied. A set of coupled partial differential equations subjected to general initial and boundary conditions are governed to this problem. Modified Homotopy perturbation method (MHPM) is a hybrid of discrete Fourier Transform, spectral analysis and Homotopy Perturbation Method that is developed in order to solve the partial differential equations of geothermal borehole systems. The results of MHPM are compared with the numerical solution for validation and, furthermore, the effectiveness and accuracy of the applied method have been shown.

A performance of a heat pump system connected a new conic helicoidal geothermal heat exchanger for a greenhouse heating in the north of Tunisia

The aim of this experimental study is made to examine the performance of a new conic helicoidal geothermal heat exchanger (CHGHE) for greenhouse heating. The main advantage of this geometry of heat exchangers type is to reduce the used area and the operating cost compared to horizontal and vertical, respectively. An experimental study is composed of CHGHE implanted in 3 m depth connected to a geothermal heat pump which is connected to a ceiling panel installed into a greenhouse. This ceiling unit is constituted of an exchangers suspended in internal air and others placed on the ground. This conditioning system allowed a recover the energy in excess into the greenhouse during the day by the exchangers placed on the ground, or, this energy stored is brought back to heat the air in the greenhouse by the suspended exchangers during the night. The experimental results showed that the recovered heat rate from the ground by the CHGHE is about 4.7 kW. However, they are between 12 kW and 10 kW in into greenhouse. The coefficient of the performance of the heat pump (COPhp) and the overall system (COPsys) were found to be 3.93 and 2.64 respectively. The geothermal system insure a quantity of heat equal to 692.208 kW witch correspond to a temperature increasing of 3 °C under greenhouse for an optimal water flow rate of 0.6 kg/s.

Open‐loop geothermal heat exchanger system for heating and cooling of the Sport arena in Skopje

AbstractThe alternative energy sources are essential for the sustainable development and protection of the environment. In this context, the renewable energy usage had become the new engineering paradigm. The direct use of geothermal energy for cooling and heating is simple and efficient alterative which is widely available and without any restrictions or conditions. Nowadays, more and more buildings use the geothermal energy satisfying more than 80% of the total energy demand for heating and cooling. Most popular are the geothermal heat exchanger systems with deep installations due to their efficiency and cost‐effectiveness. Such an example is the geothermal system of the new Sport arena build in Skopje with open‐loop geothermal heat exchangers is classified as energy efficient facility under class. The primary fluid‐water is exhilarated from the five 30 m geothermal exploitation wells, which through heat pumps is brought to the accumulation chamber inside the building bellow the main court where the heat is stored. Hence, the system is planned to operate in night mode during the cheaper energy tariff used by energy pumps, later to use the stored heat or cold for daily needs. A significant role in the system plays the controlling system with three different management levels.

Simulation of thermo-mechanical performance of pile geothermal heat exchanger (PGHE) considering temperature-depend interface behavior

Pile geothermal heat exchanger (PGHE) has attracted great interests in recent years, but some new challenges have emerged with its application, especially in understanding its thermo-mechanical behaviors. In this paper, based on the experimental data from a modified direct shear test, a finite element simulation model is developed to investigate the thermo-mechanical behavior of PGHE. The simulation model has been verified by an in-suit test. The influence of interface behavior, thermal loads, and soil properties on the PGHE’s thermo-mechanical behavior has been investigated. The results show that the changes in contact force and friction coefficient has to be considered in a comprehensive way in estimating the influence of thermal load on the bearing capacity of PGHE. Compared with the results without thermal loads, bearing capacity of PGHE shows a decreasing ratio of 8.7%, and an increasing ratio of heating is found to be 13.2%. In addition, the simulation results suggest that without head load imposed, at a certain depth, the axial stress has a linear relationship with the change of temperature, but when a head load is imposed, the linear relationship is only separately valid in each temperature region (heating or cooling). The thermo-mechanical performance of PGHE should be fully considered during the design stage, and this paper has the certain actual reference significance to engineering applications.

In-situ response test of various borehole depths and heat injection rates at standing column well geothermal heat exchanger systems

In this study, we analyzed the thermal conductivity characteristics of a well-type underground heat exchanger with respect to the amount of injected heat per meter. Although a constant amount of heat was injected into the system, we compared and analyzed data for different borehole depths. We compared three different injection heat values (250, 300, and 350 W/m). We also analyzed the effective thermal conductivity with respect to different initial ignoring times (2 h, 3 h, and 5 h) based on different rock types and borehole depths. We did not observe a linear relationship between the rock types (alluvium, gneiss, and granite) and the borehole depth. The results suggest that different rock types have different thermal conductivity characteristics and the variation in the results is likely caused by groundwater, which affects the thermal conductivity measurements. For future geothermal heat exchanger designs, it is imperative to select a specific value for heat flow and an appropriate injection heat value depending on the rock type.

Numerical Investigation of the Physical Properties Effect on the Thermal Performance of a Vertical Geothermal Heat Exchanger

Numerical Investigation of the Physical Properties Effect on the Thermal Performance of a Vertical Geothermal Heat Exchanger

Numerical Investigation of the Physical Properties Effect on the Thermal Performance of a Vertical Geothermal Heat Exchanger

Low-temperature geothermal energy is a promising technique for heating and cooling residential and commercial premises, especially since it is one of the green energy solutions that respect the environment. The principle of this technique is based on thermal exchange between the heat pump and the basement using a vertically buried heat exchanger. This is usually made of a U-shaped tube inserted vertically in a borehole made in the ground and filled with a filler material. The purpose of the present study is to vary the different construction materials of the U-tube, the filling material and the soil, in order to obtain the most energy-efficient parameters. The evolution of temperature and heat flux as a function of time has been highlighted for different combinations. Knowing that an experimental study requires a considerable monetary fund, the present model has been validated using previously literature results. Recommendations on the choice of different materials of the geothermal heat exchanger are proclaimed at the end of this work.

Numerical Simulation of Nanofluid Suspensions in a Geothermal Heat Exchanger

It has been shown that using nanofluids as heat carrier fluids enhances the conductive and convective heat transfer of geothermal heat exchangers. In this paper, we study the stability of nanofluids in a geothermal exchanger by numerically simulating nanoparticle sedimentation during a shut-down process. The nanofluid suspension is modeled as a non-linear complex fluid; the nanoparticle migration is modeled by a particle flux model, which includes the effects of Brownian motion, gravity, turbulent eddy diffusivity, etc. The numerical results indicate that when the fluid is static, the nanoparticle accumulation appears to be near the bottom borehole after many hours of sedimentation. The accumulated particles can be removed by the fluid flow at a relatively high velocity. These observations indicate good suspension stability of the nanofluids, ensuring the operational reliability of the heat exchanger. The numerical results also indicate that a pulsed flow and optimized geometry of the bottom borehole can potentially improve the suspension stability of the nanofluids further.

Thermal properties variations in unconsolidated material for very shallow geothermal application (ITER project)

Abstract In engineering, agricultural and meteorological project design, sediment thermal properties are highly important parameters, and thermal conductivity plays a fundamental role when dimensioning ground heat exchangers, especially in very shallow geothermal systems. Herein, the first 2 m of depth from surface is of critical importance. However, the heat transfer determination in unconsolidated material is difficult to estimate, as it depends on several factors, including particle size, bulk density, water content, mineralogy composition and ground temperature. The performance of a very shallow geothermal system, as a horizontal collector or heat basket, is strongly correlated to the type of sediment at disposal and rapidly decreases in the case of dry-unsaturated conditions. The available experimental data are often scattered, incomplete and do not fully support thermo-active ground structure modeling. The ITER project, funded by the European Union, contributes to a better knowledge of the relationship between thermal conductivity and water content, required for understanding the very shallow geothermal systems behaviour in saturated and unsaturated conditions. So as to enhance the performance of horizontal geothermal heat exchangers, thermally enhanced backfilling material were tested in the laboratory, and an overview of physical-thermal properties variations under several moisture and load conditions for different mixtures of natural material was here presented.

Mathematical model to analyze the flow and heat transfer problem in U-shaped geothermal exchangers

In this study, we propose a mathematical model for U-shaped geothermal heat exchangers based on the unsteady Navier–Stokes problem. In the numerical solution of this problem, we divide the exchanger into two computational domains: rectilinear pipes where the temperature field is computed analytically, and a U-curved pipe where solutions for both the flow and heat exchange are calculated using a numerical procedure based on the Galerkin finite elements method. The results of some numerical simulations are provided and used to study the performance of geothermal exchangers by assessing the effective energy produced. We also present a validation analysis based on experimental measurements obtained from a real geothermal exchanger.

Heat Exchangers for Utilization of the Heat of High-Temperature Geothermal Brines

The basic component of two-circuit geothermal systems is the heat exchanger. When used in geothermal power systems, conventional shell-and-tube and plate heat exchangers cause problems related to the cleaning of the latter from salt-deposition and corrosion products. Their lifetime does not exceed, as a rule, 1 year. To utilize the heat of high-temperature geothermal brines, a heat exchanger of the “tube-in-tube” type is proposed. A heat exchanger of this design has been operated for several years in Ternair geothermal steam field; in this heat exchanger, the thermal potential of the saline thermal water is transferred to the fresh water of the secondary circuit of the heating system for apartment houses. The reduction in the weight and size characteristics of the heat exchangers is a topical problem that can be solved with the help of heat transfer enhancers. To enhance the heat transfer process in the heat exchanger, longitudinal ribbing of the heat exchange surface is proposed. The increase in the heat exchange surface from the heat carrier side by ribbing results in an increase in the amount of the heat transferred from the heating agent. The heat exchanger is easy to manufacture and is assembled out of components comprised of two concentrically positioned tubes of a definite length, 3–6 m, serially connected with each other. The method for calculation of the impact of the number and the size of the longitudinal ribs on the heat transfer in the well heat exchanger is presented and a criterion for the selection of the optimal number and design parameters of the ribs is formulated. To prevent the corrosion and salt deposition in the heat exchanger, the use of an effective OEDFK (oxyethylidenediphosphonic acid) agent is proposed. This agent has a long-lasting corrosion-inhibiting and antiscaling effect, which is explained by the formation of a strongly adhesive chelate layer difficult to wash off the surface. The passivating OEDFK layer is restored by periodical pulsed introduction of the agent solution into the brine at the heat exchanger inlet.

Multi-dimensional simulation of underground subway spaces coupled with geoenergy systems

Old and deep subway lines suffer from overheating problems, particularly during summer, which is detrimental for passenger comfort and health. Geothermal systems could serve as one of the potential energy efficient cooling solutions, compared to energy intensive conventional cooling. The waste heat of the subway tunnel can be harnessed, to provide heating to residential and commercial blocks above the tunnels. This paper presents a multi-scale co-simulation framework for quantifying the amount of useful heat that can be extracted from overheated underground subway tunnels using geothermal heat exchangers. The co-simulation is applied and tested on a representative section of the London Underground's Central Line. The Central Line is modelled using a 1D heat and mass transfer model. The geothermal system, on the other hand, is represented using a 3D finite element model. The 1D and 3D models are co-simulated, using the subway tunnel's outer wall temperatures as boundary conditions. The model is run parametrically to identify the best arrangement and depth of geothermal heat exchangers for extracting excess heat from subway tunnels. Results show that the depth of 15 m. below the tunnel is sufficient for vertical closed loop heat exchangers to yield temperature drop of 4C in the subway tunnel and platforms. Partially insulated boreholes, alternating between extracting and injecting heat into the soil, are also assessed for their potential to provide heating and cooling demand simultaneously and improve the overall geothermal system efficiency. The heat extracted along a representative section of the tunnels is compared to the heating demand of the buildings above ground.

Dynamic modelling and simulation of a slinky-coil horizontal ground heat exchanger using Modelica

Ground source heat pumps are attractive alternatives for both conventional heating and cooling systems because of their higher energy efficiencies. A ground source heat pump system consists of a conventional heat pump coupled with a ground heat exchanger where water or a water-antifreeze mixture exchanges heat with the ground. Several major design options are available for ground heat exchangers, which are classified by layout to open and closed loops. Closed loop tubing can be installed horizontally as a loop field in trenches or vertically as a series of long U-shapes in wells. Slinky-coil horizontal ground heat exchangers are a compromise between vertical ground heat exchangers and conventional horizontal ground heat exchangers. They combine the relatively small installation cost of conventional horizontal ground heat exchangers and the high heat exchange capacity of vertical ground heat exchangers. These traits make a slinky-coil horizontal ground heat exchanger a suitable alternative for space heating in areas with limitations on land space usage. However, caused by a lack of appropriate dimensioning tools, these systems are often undersized or overdesigned. In this research, the feasibility of modelling and simulation of slinky-coil geothermal heat exchangers in the modelling language Modelica has been demonstrated. A model of a slinky-coil horizontal ground heat exchanger has been developed to simulate the performance of such systems. The model includes a ground heat exchanger and a soil cell. The ground heat exchanger has been modelled in two levels of detail, one with coil-like pipes and one with thin plates instead of pipes. In order to be able to assess the validity of the model, it has been developed based on a field test in Japan, and simulated under the same conditions. Reasonably good agreement has been obtained between the numerical predictions and published experimental measurements, which demonstrates the reliability and accuracy of the model.

Determination parameters of geothermal heat-exchangers for biological treatment facilities

Determination parameters of geothermal heat-exchangers for biological treatment facilities