Geothermal Heat Exchanger Research Articles

Progress and challenges of helical-shaped geothermal heat exchangers

Among various types of renewable energy, geothermal energy is recognized as an effective method for supplying thermal energy. Ground heat exchangers, as the main part of a geothermal energy system, are utilized for the extraction of the heat from the ground. Helical-shaped geothermal heat exchangers are very popular in this field as they need less land space compared to the other traditional straight ones. They have simple assembly and a high density of coils in their configuration. Considerable efforts have been done on the development of this type of geothermal heat exchanger. However, this topic has not been subject to a review. To address this issue, we present an overview of the potentials and challenges of helical-shaped geothermal heat exchangers in this study. The environmental and economic aspects, recent progress about the numerical simulations, soil features, different types and arrangements, and geometrical parameters for this type of heat exchangers are investigated. The installation cost is a critical challenge in the practical applications of these exchangers. However, the previous studies are mostly focused on the technical evaluation and optimization of the thermal performance of this type of heat exchanger, while little attention is paid to their installation costs. It is essential to understand the potential environmental impacts of each renewable energy technology to have a correct evaluation of the system. The life cycle assessment can be used as a proper method to assess the environmental issues of the helical-shaped geothermal heat exchanger in the studies.

Performance characterization of novel caisson-based thermal storage for ground source heat pumps

To avoid the catastrophic results of climate change, a major shift towards clean, sustainable and renewable energy technologies is inevitable. Since buildings are capable of on-site thermal energy generation and exhibiting predictive patterns of heating and cooling, many attempts are currently underway to incorporate more renewable energy (e.g., geothermal heating and cooling) systems in buildings. Subsurface geothermal resources represent a great potential for direct use of energy; put another way, the planet is a six sextillion (1021) metric ton battery that is continually being replenished by solar radiation, lightning, and heat from its deep-down molten core. Despite the enormous energy potential, geothermal systems are not adopted widely due to three main reasons: high drilling costs, energy imbalance in the ground, and lack of drilling space. To address these challenges, a novel foundation-based geothermal heat exchanger system incorporating phase change material (PCM) has been designed. The proposed technology, utilizing a foundation caisson, reduces the construction and installation costs by integrating energy and structural systems together into a single ground installation. The present study aims to characterize the thermal performance of the proposed system through a numerical model that is validated and calibrated using the experimental data generated from a demonstration site that hosts a foundation caisson. Efficacy of the use of PCM on improving the thermal performance of the system is characterized in terms of energy savings and greenhouse gas (GHG) emission reduction. Sensitivity studies demonstrate improvement in the performance of the caisson with the use of PCM with increased thermal conductivity. Also, as the seasonal heat injection to extraction ratio deviates further away from unity, the PCM helps improve the thermal performance as evidenced through reduced primary energy consumption.

Characterization of scaling material obtained from the geothermal power plant of the Balmatt site, Mol

Geothermal energy is a renewable energy source by which heat is extracted from the subsurface. Due to changes in pressure and temperature and chemical reactions when the geothermal fluid is pumped through the wells and surface installations, scales can be formed. These scales can accumulate in different locations of the geothermal system and/or parts of the surface installation and cause clogging of the wells, pumps, pipes, heat exchangers and/or filter units. Consequently, the efficiency of the geothermal system can decrease. To obtain insight into the formation of scales in the surface installation of the Balmatt geothermal site, heat exchanger and filters scaling material was sampled after a circulation time of 240 h and 46 h respectively. This material was chemically, mineralogically and radiologically characterized. The scaling material found in the heat exchanger consists mainly of galena (PbS). Minor amounts of challacolloite (KPb2Cl5) and laurionite (PbClOH) were also observed. Additionally, in the filter scaling material carbonates such as witherite (BaCO3), siderite (FeCO3) and possibly rhodocrosite (MnCO3) are present as well, together with an amorphous phase and elemental copper. Salt minerals such as sylvite (KCl), halite (NaCl) and challacolloite (KPb2Cl5) and iron oxides such as hematite (Fe2O3) were also observed in the scaling material. However, these minerals are thought to be formed during drying of the scalings after sampling. Together with these minerals, radionuclides such as 226Ra, 210Pb and 210Po are observed. 226Ra seems to predominantly co-precipitated with and/or sorb on witherite and hematite, while 210Pb and 210Po are rather accumulated in galena and laurionite. Equilibrium calculations, using the PhreeqC software were performed to gain insight into the geochemical processes. The calculations indicate that galena is one of the main oversaturated minerals. Precipitation of native copper and carbonates are also observed and supported by the equilibrium calculations. The formation of these precipitates is probably a result of electrochemical corrosion of carbon steel and/or corrosion by dissolved CO2.

Comparison of Two Laboratory Methods for Measuring Soil Critical Temperature

Abstract Coupled heat and moisture transfer above a critical temperature is important in various geothermal applications (e.g., geothermal heat exchangers, buried power cables, and energy piles) because of its impact on nonsteady soil thermal properties. In this study, temperature and moisture content of seven sandy soils were measured using two laboratory approaches—vertical and radial—to evaluate the critical temperatures. The vertical approach measured temperature and moisture data from top to bottom of a soil column, and the radial method obtained the measurements from center to outside of the specimen. To validate the test results, computational modeling was also carried out in MATLAB. Although the results obtained from radial testing included rapid changes in temperature gradients near the boundaries between dry and wet zones, ambiguous temperature profiles (i.e., gradual gradients over the entire range) were observed in vertical testing possibly because of horizontal boundary heat losses. In contrast to the gradual temperature gradients, moisture content profiles showed significant variations near the transitional regime, thus the critical temperatures were determined based on both temperature and moisture profiles. Average critical temperature for seven soils was 45.8°C, and the critical temperature was dependent on the void ratio. As void ratio decreased, critical temperature increased with a corresponding decrease in the size of the dry zone. The critical temperatures obtained from radial testing were higher (up to 5.2°C higher) and took more time to reach steady-state condition (up to 9 h longer) than those obtained from vertical testing. Because there are limited data and methodology in published studies that discuss a standard methodology to measure soil critical temperature, a consistent, universally accepted, procedural-based method is needed for the measurement or determination of the critical temperature.

Enhanced multipole method for the transient thermal response of slender geothermal boreholes

During the design stages of a new geothermal heat exchanger special care is devoted to the peak heating and cooling demands of the building. These lead to extreme temperatures in the heat carrying liquid that may cause unwanted material damages in the installation. The short duration of these peak demands, of a few days at most, require the inclusion of the thermal inertia of the heat carrying liquid, the grout, and the ground located close to the boreholes in the theoretical model used to predict the thermal response of the geothermal heat exchanger. By expanding the grout and ground temperatures in terms of conveniently chosen multipoles, the present work develops such a model. The resulting enhanced multipole method, meant originally for peak heating and cooling demands, is also valid in the limit of slowly varying heat injection rates in which it delivers the same results as the classical multipole method developed by Bennet, Claesson, and Hellström.

Passive air-conditioning of domestic using a hybrid system of solar updraft tower and a geothermal in Tikrit city-Iraq

In constructing a hybrid solar updraft tower with an underground heat exchanger, this study provides a theoretical and experimental investigation. A valuable function that lowers energy consumption in the case of domestic buildings is the Underground Tube Heat Exchanger. The submerged underground heat exchanger tube for heating in winter season and cooling in summer and the air is to be supplied by the influence of solar updraft tower to the building. The findings show that at 3 m depth from the earth, which is called buried depth at a 152 mm outer diameter, the maximum efficiency of the underground heat exchanger system. In comparison, with rising velocity of air, the temperature of the outlet air from the ground heat exchanger increases, varying from (22.2- 28 °C) and also increases with increasing pipe diameter and decreases with increasing tube length. The difference between the findings of the theoretical and the experimental tests was 1.8 %. The combined geothermal heat exchanger and solar updraft tower system is used as an air conditioning system and was satisfactory efficient for thermal building loads.

Defining the Shallow Geothermal Heat-Exchange Potential for a Lower Fluvial Plain of the Central Apennines: The Metauro Valley (Marche Region, Italy)

In this work we assessed the shallow geothermal heat-exchange potential of a fluvial plain of the Central Apennines, the lower Metauro Valley, where about 90,000 people live. Publicly available geognostic drilling data from the Italian Seismic Microzonation studies have been exploited together with hydrogeological and thermophysical properties of the main geological formations of the area. These data have been averaged over the firsts 100 m of subsoil to define the thermal conductivity, the specific heat extraction rates of the ground and to establish the geothermal potential of the area (expressed in MWh y−1). The investigation revealed that the heat-exchange potential is mainly controlled by the bedrock lithotypes and the saturated conditions of the sedimentary infill. A general increase in thermal conductivity, specific heat extraction and geothermal potential have been mapped moving from the coast, where higher sedimentary infill thicknesses have been found, towards the inland where the carbonate bedrock approaches the surface. The geothermal potential of the investigated lower Metauro Valley is mostly between ~9.0 and ~10 MWh y−1 and the average depth to be drilled to supply a standard domestic power demand of 4.0 kW is ~96 m (ranging from 82 to 125 m all over the valley). This investigation emphasizes that the Seismic Microzonation studies represent a huge database to be exploited for the best assessment of the shallow geothermal potential throughout the Italian regions, which can be addressed by the implementation of heating and cooling through vertical closed-loop borehole heat exchanger systems coupled with geothermal heat pumps.

Use of vertical geothermal heat exchanger with nanofluid for heat supply systems

Future district heating systems need to be more flexible and move towards a carbon-neutral generation. Compression heat pumps are foreseen to be promising future tools for district heating systems in EU countries. The paper presents the simulation data or a vertical double-pipe geothermal heat exchanger with ethylene glycol-Al2O3 as a working fluid. To simulate the operation of the heat exchanger, the STAR-CCM + CFD application was used. It has been determined the optimal volume particle concentrations of nanofluid and enhance of heat exchanger performance.

Thermal performance comparison of standing column well and cross-mixing balancing well heat exchangers

A study was attempted to change the existing operation method of operating two adjacent SCW underground heat exchangers, each with a single well. SCW-type geothermal heat exchanger allows up to 20 % bleed discharge in the event of a maximum load, which is thought to be an unreasonable use of groundwater. In this study, the purpose of this study was to improve the coefficient of performance (COP) of geothermal system by fundamentally preventing underground water discharge and keeping the temperature of the underground heat exchanger constant by organizing the abandoned bleed discharge water by two balanced well cross-mixing methods. The existing SCW heat exchange system and the balancing well cross-mixed heat exchange system are cooled through the test target installed at the site. As a result of the heating operation, the measured performance coefficient was 23 % for cooling operation and 12 % for heating operation. In addition, when operating with a balanced well cross-mixed heat exchange system, it was confirmed that the initial temperature of the underground was constant with a standard deviation of 0.08 to 0.12°C.

The Effects of Internal Heat Generation or Absorption on Mixed Convection in a Lid-Driven Rectangular Cavity using Finite Volume Method

Mixed convection heat transfer in cavities is a significant phenomenon in numerous engineering fields, such as nuclear reactors, solar energy storage, and heat exchangers. Despite acknowledging that a square is a basic shape found in these systems, not all the figures are geometrical. Less attention was given to the rectangle cavity even though it could be found in these systems. Various internal reactions could occur inside the systems, especially in geothermal heat exchangers. Therefore, this research aims to analyze the effect of internal heat generation or absorption in a two-dimensional (2D) horizontal cavity to the fluid flow and heat transfer process numerically. The vertical walls are well insulated. Meanwhile, the top and bottom walls are kept at and , respectively, where . The top wall moves at a constant speed from left to right. The finite volume method (FEM) and SIMPLE algorithm are employed to discretize the governing equations. Next, the algebraic equations are solved iteratively using the tri-diagonal matrix algorithm (TDMA). The influences of heat generation or absorption parameters are investigated in terms of the flow, heat transfer, and Nusselt number. The numerical results are plotted in the form of streamlines and isotherms. It is found that the presence of heat generation or absorption has a significant effect on the fluid flow and heat transfer process in the horizontal cavity. Overall, for internal heat generation, the heat transfer rate decreases, while the opposite pattern can be observed for the case of internal heat absorption. However, for Ri = 10.0, as the heat generation's value increases from 2 to 4, the heat transfer rate is the same.

Mathematical Modelling and Operational Analysis of Combined Vertical–Horizontal Heat Exchanger for Shallow Geothermal Energy Application in Cooling Mode

Geothermal heat exchangers (GHEs) represent a buried pipe system, which can be utilised to harness renewable thermal energy stored in the ground to improve the efficiency of heating and cooling systems. Two basic arrangements of GHEs have been widely used: vertical and horizontal. Vertical GHEs generally have a better performance in comparison with the horizontal arrangement, and these systems are particularly suitable for confined spaces. Nevertheless, the main technical challenge associated with GHEs, for either the vertical or the horizontal arrangement, is the performance deterioration associated with an increase in the operation times during summer or winter seasons. In this paper, a combined horizontal-vertical GHE arrangement is proposed to address the current challenges. The combined GHE arrangement can be operated in five different modes, corresponding to different thermal loading conditions. These five operation modes of the combined GHE are analysed based on the transient finite difference models previously developed for the horizontal and vertical arrangements. The simulation results reveal that for the single operation mode (horizontal or vertical only), the vertical GHE performs better than the horizontal GHE due to relatively stable ground temperature deep down. While, for the combined operation mode, the series operations (horizontal to vertical or vertical to horizontal) of the GHE are superior to the split mode. It is found that the effect of the fluid mass flow rate ratio is trivial on the heat dissipation of the split mode GHE. The highest heat transfer rate in the split flow operational mode is rendered by the ratio of the mass flow rate of 40% horizontal and 60% vertical. In addition, the climate condition has more effect on GHE’s performance and the increase of the fluid flow rate it can enhance the amount of energy released by the GHE.

Optimization and energy assessment of geothermal heat exchangers for different circulating fluids

One significant obstacle to the adoption of geothermal heat pump (GHP) technology is the installation costs of geothermal heat exchangers (GHE). Cost reduction through optimization of system parameter offers the potential for increased applications. In the current work, five major parameters are considered: length, radius, well numbers, the flow discharge inside the pipe, and the pipe's external radius for optimization using a genetic algorithm (GA) for a residential building in hot climatic conditions. In addition, system optimization is critical in determining values of design parameters for assessing the impact different circulating fluids on the energy consumption of GHP. A ten-year simulation is undertaken to evaluate the capacity of various circulating fluids and their effects on energy consumption reduction. The simulation shows a significant decrease in energy consumption based on varying levels of Ethylene glycol, Methanol, Potassium acetate, Sodium chloride, Freezium™ compared to pure Water in the GHP. The COP of the GHP system is also calculated with different circulating fluids. In addition, the circulating fluid with the highest performance loss during ten years of operation is identified. Based on the results, Ethylene glycol is selected as the preferred solution for use in the GHP. In the present study, we have also established the optimum configuration of GHEs according to a reliable evolutionary algorithm for investigating the effect of various circulating fluids on the system's energy consumption.

Energy and economic evaluation of a solar assisted ground source heat pump system for a north Mediterranean city

In this study, a PVT based solar assisted ground source heat pump (SAGSHP) system was investigated regarding its energy performance and cost-effectiveness for the city of Thessaloniki (Greece). The SAGSHP system was set to cover the space heating and domestic hot water needs for a low-rise dwelling. A mathematic model formulated in TRNSYS was used with the aim to carry out parametric analysis by varying the number of the PVTs. It is worth noting that, two of the most important components of the employed model, the PVT collector and the geothermal heat exchanger, have already been validated via experimental data. Simulations were conducted and through the results seven energy metrics were estimated, with the objective to examine the system’s energy performance from various perspectives. The SAGSHP system with 16 PVTs was found capable to covering 73% of the heating load and to generate 1.22 times more electricity than that consumed by the system. The electricity yield of PVTs was not affected throughout the parametric analysis, and the maximum specific productivity was estimated at 301.5 kWhe PVT−1 per year. The results suggest that a SAGSHP system equipped with about 14 PVTs can reach energy self-sufficiency. As regards the economics of the SAGSHP system, this was compared with a natural gas boiler system via two methods, the life cycle cost (LCC) and the life cycle savings (LCS). A sensitivity analysis with major economic parameters of the systems was carried out. It was found that the cost-effectiveness of the SAGSHP system is influenced mainly by its capital cost and by the price of the natural gas. Systems equipped with less than 12 PVTs can be cost-competitive by subsiding from 8% up to 42% of their capital cost. Also, system with more than 12 collectors were found of more benefit than the smaller ones, in the case where feed-in-tariff schemes are applied, or the bank loan’s interest rate is low. It can be concluded that, the proposed system can be an attractive monetary solution for covering the heating load in comparable dwellings with a similar climate to Thessaloniki.

Conjugated Numerical Approach for Modelling DBHE in High Geothermal Gradient Environments

Geothermal is a renewable energy source that can be untapped through various subsurface technologies. Closed geothermal well solutions, such as deep geothermal heat exchangers (DBHEs), consist of circulating a working fluid to recover the available heat, with less dependency on the local geological settings than conventional geothermal systems. This paper emphasizes a double numerical method to strengthen the assessment of DBHE performances. A computational fluid dynamics (CFD) commercial software and the 1D coupled wellbore-reservoir geothermal simulator T2Well have been used to investigate the heat transfer and fluid flow in a vertical DBHE in high geothermal gradient environments. The use of constant water properties to investigate the energy produced from DBHEs can lead to underestimating the overall heat transfer at high temperature and low mass flow rate. 2D axisymmetric CFD modelling improves the understanding of the return flow at the bottom of the DBHE, readjusting and better estimating the pressures losses commonly obtained with 1D modelling. This paper highlights the existence of convective cells located at the bottom of the DBHE internal tubing, with no significant effects due to the increase of injected water flow. Both codes are shown to constrain the numerical limitations to access the true potential of geothermal heat extraction from DBHEs in high geothermal gradient environments and demonstrate that they can be used for geothermal energy engineering applications.

Balancing Well 교차혼합 지중열교환기의 스마트 냉난방 히트펌프 시스템의 성능평가에 관한 연구

This study performed a single hole operation method using a balancing well-cross-mixed underground heat exchanger, and conducted thermal performance studies of an SCW-type underground heat exchanger using a two-well. The study attempted to change the existing operating method of the two adjacent SCW underground heat exchangers with one ball each. The SCW-type geothermal heat exchanger is considered to enable up to 20% of bleed discharge at maximum load, which makes groundwater usage unequal. The efficiency factor of the geothermal system was improved by constructing the discharged water by cross-mixing two balancing wells to prevent the discharge of groundwater sources and keep the temperature of the underground heat exchanger constant. As a result of the cooling and heating operation with the existing SCW heat exchange system and the balancing well-cross-mixed heat exchange system, the measured performance coefficient improved by 23% and 12% in cooling and heating operations, respectively. In addition, when operating with a balanced cross-mixing heat exchange system, it has been confirmed that the initial basement temperature is constant with a standard deviation of 0.08 to 0.12°C.

Modeling the thermal response of geothermal boreholes during peak heating and cooling demands

Designing a geothermal heat exchanger requires special care with peak heating and cooling demands of the building as material damages in the installation can occur due to the extreme temperatures reached by the heat carrying liquid. These peak demands last a few days at most and require the inclusion of the thermal inertia of the heat carrying liquid, the grout, and the ground located close to the boreholes in the theoretical model used to predict the thermal response of the geothermal heat exchanger. The present work develops such a model by expanding the grout and ground temperatures in terms of conveniently chosen multipoles.

Modeling the thermal interaction of geothermal boreholes with aquifers using asymptotic expansion techniques

To design an optimally-sized geothermal heat exchanger it is crucial to accurately forecast its thermal response over the whole lifetime of the building. Since the presence of aquifers highly affects the heat exchange between geothermal boreholes and the ground, both heat transfer mechanisms, conduction and convection, must be taken into account in the ground. In the present work the existence of large disparities in time and length scales in the problem is exploited using asymptotic expansion techniques to develop a theoretical model capable of taking into account both aforementioned heat transfer mechanisms for any Peclet number of the groundwater flow.

Smart Heating and Cooling Heat Pump System by Standing Column Well and Cross-Mixing Balancing Well Heat Exchangers

Standing column well (SCW) geothermal heat exchanger permits a bleeding discharge of less than 20% in the event of a maximum load, which is an inappropriate method of using underground water. In this study, the existing operational method of two adjacent SCW geothermal heat exchangers, each with a single well, was modified. This technology aims to improve the coefficient of performance (COP) of the geothermal system by fundamentally preventing underground water discharge and maintaining a constant temperature of the underground heat exchanger. To curb the bleed water discharge, two balancing wells of cross-mixing methods were employed. The result of the cooling and heating operations with the existing SCW heat exchange system and the balancing well cross-combined heat exchange system showed that the measured COP increases by 23% and 12% during the cooling and heating operations, respectively. When operating with a balanced well cross-mixed heat exchange system, the initial temperature of the underground was constant with a small standard deviation of 0.08–0.12 °C.

Environmental and Economic Impact of the Antifreeze Agents in Geothermal Heat Exchangers

Borehole heat exchangers (BHEs) generally employ water-antifreeze solutions to allow working fluid temperatures to fall below 0 °C. However, some local regulations have forbidden antifreeze additives (even non-toxic ones) to avoid groundwater pollution in case of pipe leakage. This paper presents a techno-economic and environmental analysis of four different fluids: propylene glycol at 25% and 33% weight concentrations, calcium chloride at 20% weight concentration (CaCl2 20%), and pure water. Thermal loads from 36 case studies in six different climate zones are used to perform BHE sizing and compare the abovementioned fluids from the economic, operational, and environmental points of view. The economic analysis and the carbon footprint assessment are performed on a life cycle of 25 years considering the installation (BHE drilling, fluid) and operation (heat pump and ground-side circulation pump energy demand, fluid replacement) of the simulated GSHPs. Results highlight that using pure water as a heat carrier fluid is convenient for cooling-dominated buildings but, for heating-dominated buildings, this choice leads to a noticeable increase of the BHE needed length which is not compensated by the lower operational costs. On the other hand, avoiding the use of antifreeze additives generally leads to a reduction of the lifetime carbon footprint, with a few exceptions in very cold climates. CaCl2 20% proves to be a good choice in most cases, both from the economic and the environmental points of view, as it allows a strong reduction of the installed BHE length in cold climates with a low additional cost and carbon footprint.

Extension of Winkler’s solution to non-isothermal conditions for capturing the behaviour of plane geostructures subjected to thermal and mechanical actions

Before this study, no analytical models had been made available for describing the behaviour of plane geostructures subjected to thermal and mechanical actions. This knowledge gap notably represented a limitation for investigations about the behaviour of so-called energy geostructures, which are subjected to the considered actions due to their geothermal heat exchanger and structural support roles. In this study, the first analytical model that allows describing the behaviour of plane geostructures subjected to thermal and mechanical actions is presented. This model extends Winkler’s solution to non-isothermal conditions for quantifying the effects of temperature variations, axial loads, transversal loads and bending moments applied to plane geostructures resting on an elastic soil mass. The model is applied to the analysis of an elementary unit represented by a single beam as well as to more complex plane geostructures using the superposition principle. The obtained results are compared with predictions deriving from more rigorous yet time-consuming numerical analyses, showing close agreement. This result makes the developed analytical model a useful tool for scientific and engineering purposes, paving the way for future developments in this scope.