Double U-tube Research Articles

Analysis of the Combined Effect of Major Influencing Parameters for Designing High-Performance Single (sBHE) and Double (dBHE) U-Tube Borehole Heat Exchangers

In this paper, a comprehensive analysis of the combined effect of major influencing parameters on heat transfer in a single U-tube BHE (sBHE) and a double U-tube BHE (dBHE) with two independent circuits was performed by using a validated numerical heat transfer model. Geometrical parameters, such as shank spacing (with maximum, average, and minimum values), borehole diameter (large, medium, and small borehole sizes), and borehole depth (shallow, average, and deep borehole depths) as well as the thermal conductivity of soil and grout, which ranges from minimum to high values, were considered. The combined impact of these parameters was included under the following four major cases: (1) the combined effect of borehole depth, borehole size, and shank spacing; (2) the combined effect of borehole depth and soil and grout thermal conductivity; (3) the combined effect of soil and grout thermal conductivity and borehole size; and (4) the combined effect of soil thermal conductivity, borehole size, and shank spacing. Each of these major cases has nine different design options for both sBHEs and dBHEs. A series of results of heat transfer per unit borehole depth were generated for all the considered various cases. With the given parameters, the BHE case that provides the highest heat transfer among the various cases of sBHEs and dBHEs were obtained.

Design consideration of a filled layer U-pipe evacuated tubular solar collector with flat diffuse reflector: energy and exergy analysis

A lack of research about critical design factors, notably the center-to-center tube spacing-to-tube diameter ratio (C/D) and the relationship between length and diameter, is conspicuously evident in the context of U-pipe evacuated tubular solar collectors (ETSCs) employing the filled-layer approach. Furthermore, using a U-pipe configuration in ETSCs coupled with a flat diffuse reflector (FDR) remains conspicuously absent from the scientific literature as a potential enhancement strategy. This study evaluates the influence of collector tube length, diameter and the C/D on output parameters, including T out , η En and η Ex . The findings reveal that while η En experiences a decrease with increasing C/D, the η Ex does not reach its minimum value when C/D was three due to the corresponding increase in outlet temperature. Furthermore, introducing the FDR into the filled U-pipe evacuated tubular solar collector enhances daily η En and η Ex by 12.91% and 0.65%, respectively. Moreover, a suggestion of the length and diameters of a filled type boosted with FDR is L = 2000 mm for Dg/Dp = 47/37 mm and L = 1800 mm when Dg/Dp = 58/47 mm, which has the maximum η Ex per day of 2.87% and 1.87%, respectively. However, if the objective is to maximise η En per day, lengths of L = 1450 mm for Dg/Dp = 47/37 mm or L = 1200 mm for Dg/Dp = 58/47 mm would be preferable. Abbreviations: ESC: evacuated solar collector; ETC: evacuated tubular collector; DUFET: filled-type evacuated tube with double U-tubes; ETSC: evacuated tubular solar collector; FDR: flat diffuse reflector; C/D: center-to-center tube spacing to the collector's tube diameter (cover tube); Dg/Dp : diameter of cover tube and diameter of absorber tube

Comparison of the thermal and hydraulic performance of single U-tube, double U-tube and coaxial medium-to-deep borehole heat exchangers

Decarbonisation of heat is essential in curbing carbon dioxide emissions and can be achieved through the use of geothermal systems. Recently, single-well, closed-loop, deep borehole heat exchangers, using a coaxial design, have become the focus of attention, partly due to the potential to repurpose existing infrastructure (such as oil and gas wells); however, few have investigated the potential for other types of heat exchanger for middle-deep geothermal systems. Therefore, in this study, a comprehensive numerical analysis was undertaken using OpenGeoSys software to investigate the thermal and hydraulic performance of coaxial, single U-tube and double U-tube middle-deep borehole heat exchangers (MDBHEs). The purpose of this paper is to test the maximum operational depth for each type of pipe configuration as few wells have been completed to depths exceeding 500 m using single/double U-tube configurations. The best performing MDBHEs should minimise parasitic and hydraulic losses, whilst maximising thermal output. Furthermore, ground sourced heat pumps require electricity; therefore, at times where electricity prices are high (and drilling costs can be minimised) it may be more beneficial to utilise MDBHEs to encounter greater temperatures.Results indicate that coaxial MDBHEs provide the best performance in terms of specific heat extraction and lowest pressure/parasitic losses. Double U-tube MDBHEs can provide a similar thermal performance to the coaxial design, but have significantly greater hydraulic pressure losses across all simulations, which translates to greater parasitic pumping power costs. Single U-tube MDBHEs demonstrate the poorest performance in terms of heat extraction and pressure losses. At the end of the 25-year base case scenario for a 800 m MDBHE, coaxial, U-tube and double U-tube configurations, all with a fluid circulation rate of 5 L/s, provided specific heat extraction rates of 39.1 W/m, 32.8 W/m, and 36.0 W/m, respectively, with the fluid inlet temperature set as a constant of 5 °C. For these simulations, pressure losses were estimated as 85 kPa (coaxial), 1.46 MPa (single U-tube) and 423 kPa (double U-tube)—the single U-tube value being close to the nominal 16 bar (1.6 Mpa) pressure rating of SDR11 high density polyethylene pipe. Further parametric analysis was also undertaken, investigating depth, flow rate, rock thermal conductivity, pipe diameter and shank spacing.

Use of a residential building’s foundation as Energy Geo-Structures: A case study in the Mediterranean environment

Geothermal energy, a type of renewable energy obtained from the Earth, can be used by Ground Source Heat Pumps (GSHPs) for space heating and cooling. GSHP systems extract/reject heat from/into the earth via a network of pipes or tubes known as Ground Heat Exchangers (GHEs),. GHEs can be horizontal or vertical and come in a variety of forms, including slinky loops and vertical spiral loops for horizontal configurations, and single U-tube, double U-tube in series or parallel, coaxial pipe, and spiral/helical pipe for vertical configurations. Although GSHPs exhibit a higher performance than conventional Air Source Heat Pumps (ASHPs), the systems are less appealing as investments due to their longer payback times and greater upfront costs. A competent design of the GSHP system with the appropriate GHE sizing, including the case of employing the GHEs in the structures foundations, can minimize the initial cost. Such structures are referred to as Thermo-Active Structure systems or Energy Geo-Structures (EGS), which have already found applications as energy piles, barrette piles, diaphragm walls, shallow foundations, retaining walls, embankments, and tunnel linings [1, 2, 3].
 Residential buildings in Cyprus have a higher cooling than heating demand, with an extended payback period for the investment, especially if the system is solely utilized for heating [4]. EGS coupled with a GSHP could offer a substitute to reduce the system’s initial expenses and make it more attractive as an investment. EGS have not yet seen applications in Cyprus, therefore an initial computational analysis utilizing the COMSOL software and relevant data is taken into consideration. To determine the heating and cooling loads, a three-bedroom, two-storey house with a total floor space of 190 m2, with nearly Zero Energy Building (nZEB) characteristics, is used as the typical case for a single detached residential building in Cyprus. The TRNSYS software was used to calculate the heating and cooling loads. In order to analyze the rejected/absorbed energy from the earth, typical foundations were constructed as full sized models in COMSOL (Figure 1). With the exception of pipes, where a 1D solution is employed, the convection diffusion equation for heat transfer is utilized with 3D conservation of heat transfer for an incompressible fluid. A detailed methodology can be found in Aresti et al. [5].
 For the COMSOL compuations, the site ground temperature characteristics and the local ambient temperature were taken into account. Based on the provided loads, the inlet and outlet temperatures were hourly calculated for the highest demand months: February for winter – heating, and July for summer – cooling.
 To assess the potential of using the foundation bed as an alternative to energy piles, a comparison on the performance of the two cases is presented in Figure 2 for both summer and winter conditions. The foundation bed is seen to work in a steady manner, which is highly desired in a system. However, it performs less well, with a lower COP than the energy piles system, which exhibits greater COP values than the foundation bed system both in winter and summer. It remains that both systems exhibit high enough COPs and nearly steady conditions. The COPs vary between 4.4 and 4.8. A yearly simulation (not shown here) yielded a lowest COP value of 4.5.
 The systems were also assessed economically by being compared to a convectional ASHP system. The economic study demonstrated that the suggested foundation systems had relatively short payback times, making them a plausible alternative. More detailed discussion on the results can be found in Aresti et al. [5]. As a future objective, an environmental and Life Cycle Analysis investigation, similar to the one performed by Aresti et al. [6], could provide a deeper insight of the plausibility of using the building foundation as EGS.

Heat extraction analyses and energy consumption characteristics of novel designs of geothermal borehole heat exchangers with elliptic and oval double U-tube structures

The borehole heat exchanger (BHE) is an efficient and economic tool for extracting geothermal energy within ground source heat pumps (GSHPs). The layout and cross-section tube configuration of the BHE is one of the crucial bottlenecks that strongly influenced the effectiveness of the BHEs, which recently attracted numerous attention.Hence, the interest in retrofitting cost-efficient configurations of BHEs is recently regarded as an improvement pathfor maximizing geothermal energy production. In this study, novel geothermal borehole heat exchangers with elliptic and oval double U-tube (2U) configurations are investigated and compared to a traditional circular 2U BHE.This paper explains the development of traditional circular 2U-tube BHEs that makes an optimally designed model for maximizing the heat extraction rates and minimizing the energy consumption characteristics and BHE number. A 3-D numerical model was developed and validated by relevant field experiments (Tianjin geothermal project). The numerical study was performed on four cases corresponding to different widths of elliptic and oval 2U BHEs configurations. Furthermore, a sensitivity analysis was performed to parametrically investigate the impacts of key design and operational parameters on heat extraction performance. Additionally, a 3-D numerical model coupled with a finite line heat source model was also developed to perform case studies for energy consumption analysis for two goals of optimization. The results show that the oval 2U BHE (type II) outperformed the other investigated 2U BHE configurations, which increased the maximal and mean heat transfer rate by up to 54.0% and 12.30%, respectively, compared to circular 2U BHE. While the elliptic 2U-tube BHE (type II) was minimal pressure drop and energy consumption rates, which showed a pressure drop of 36.20% lower than the circular 2U BHE. In the case study findings, the utilization of elliptic 2U-tube BHE (type II) can not only reduce the BHE number by 6.70% but also minimize the energy consumption of the water pump and the entire system by 36.9% and 8.2%, compared to that of traditional circular 2U-tube BHE, respectively.

Vertical ground heat exchanger pressure loss – Experimental comparisons and calculation procedures

Vertical ground heat exchangers (GHEs) used with ground source heat pump systems have been constructed in a variety of configurations including single U-tubes, double U-tubes, quadruple U-tubes, and coaxial-type. Selections of vertical GHE other than single U-tubes are often motivated by a desire to decrease the borehole thermal resistance and therefore reduce the required amount of drilling and/or improve the thermal performance of the system. As with conventional heat exchanger design, the hydraulic performance – i.e. how much pressure loss is incurred – is also very important to the overall system performance. Yet, relatively little attention has been paid to this aspect of the design.In this paper, we present experimental measurements of pressure loss versus flow for four types of vertical GHE in a 200 m deep borehole: a single U-tube, double U-tubes connected in parallel, and in series, a coaxial heat exchanger, and a single U-tube with internal heat transfer enhancing ridges. Procedures for calculating the pressure loss in these configurations are tested. While standard procedures are adequate for the smooth U-tubes, the internally-ridged U-tube required a new correlation. For the coaxial heat exchanger, seemingly minor fittings that intruded into the annulus were shown to be important for calculating the pressure loss. Finally, the effects of using fiber optic cable assemblies inserted into the pipe are investigated and correlations for the excess pressure loss are given.

3D Numerical Modeling to Evaluate the Thermal Performance of Single and Double U-tube Ground-coupled Heat Pump

The heat transfer rate and borehole design represent great challenges to the thermal equipment designer of the ground-coupled heat pump. The present model represents a mathematical and numerical technique implemented to tackle such a problem. A thermal assessment was established to estimate the total energy dissipated to the ground zone for a heat pump utilized for cooling purposes in the summer season. Comsol Multiphysics 5.4 software was used to build a 3-dimensional model to assess the thermal performance of single and double U-tube boreholes that circulate water as a thermal transfer medium. The (Heat Transfer) module has been implemented for this investigation under the (Stationary) study option. The model couples both heat conduction in solids, including tube metal, grout, and soil regions, and thermal medium fluid flow inside the U-tubes. The numerical solutions were compared for both heat exchangers at fixed borehole geometry, diameter, and depth and constant operating conditions in a steady-state mode. The double U-tube heat exchanger was tested in the parallel circuiting orientation of the U-tubes. The total mean resistance of the single U-tube borehole was higher than the half-loading double U-tube heat exchangers by 14.6%. The results also revealed that the heat transfer rate enhancement for the double U-tube was in the range of 10–14% when operating at the same fluid mass flow rate and inlet temperature for a given borehole design. Doi: 10.28991/HIJ-2022-03-02-01 Full Text: PDF

Influence of groundwater flow on the ground heat exchanger performance and ground temperature distributions: A comprehensive review of analytical, numerical and experimental studies

The performance of the ground heat exchanger (GHE) possesses a pivotal role in the geothermal exploitation. In recent years, a growing body of literature that recognizes the importance of groundwater flow has been presented. This article reviewed the literature dealing with the impact of groundwater flow on the GHE performance and ground temperature distribution, which were classified by analytical, numerical and experimental approaches. The characteristics of the three approaches were summarized respectively. Various GHE configurations, including U-tube, double U-tube, spiral tube, tunnel lining, horizontal spiral GHEs and GHE groups, were involved in the discussion. It was found that the groundwater flow is beneficial to the ground temperature recovery and improves the GHE heat exchange rate. The effective thermal conductivities of rocks and soils, and borehole resistances, are also affected by groundwater flow. The impact extent depends mainly on the thickness, depth and flow rate of the aquifer layer. The discussion on the collected literature demonstrated that ignoring the groundwater flow would cause non-negligible prediction errors in the evaluation of GHEs. Based on the analysis of the characteristics and applicabilities of the discussed models and methods, this review is expected to provide a more comprehensive reference for promoting the utilization of geothermal energy by GHEs.

An Analytical Model for the Thermal Assessment of a Vertical Double U-Tube Ground-Coupled Heat Pump System in Steady-State Conditions

An analytical model was built to predict the thermal resistance of a vertical double U-tube ground-coupled heat pump that operates under steady-state conditions. It included a geometry obstruction factor for heat transfer throughout the backfill medium due to the presence of the second loop. The verification of the model was achieved by the implementation of five different borehole configurations and a comparison with other correlations in the available literature. The model considered a U-tube spacing range between (2) and (4) times the U-tube outside diameter producing a geometry configuration factor range of (0.29–0.6). The results of the model were utilized for the assessment of the DX ground heat exchanger coupled heat pump system. For similar geometrical configurations, the borehole thermal resistance experienced a decrease as the geometry factor increased. The single U-tube borehole thermal resistance was higher than that of the double U-tube heat exchanger by (10–27)% for the examined geometry configurations. The borehole thermal resistance at tube spacing of twice the tube diameter was higher than the predicted value at the triple diameter and fell in the range of (18–34)%.

Analytic models and effective resistances for coaxial ground heat exchangers

Ground heat exchangers (GHE) have been extensively studied in the past decades in order to optimize the heat exchange with the ground in particular when they are coupled with a building equipped with heat pumps. Ground heat exchangers can be categorized in several ways: vertical, horizontal or even pile heat exchangers. In the special case of vertical heat exchangers, the single U-tube and double U-tube are the most common configurations but coaxial boreholes have drawn attention in recent years. This paper provides some new insight on this configuration based on analytical models that were validated with numerical simulations. The outcome of the study gives new comprehension on the effective borehole resistance associated with coaxial configurations in ground heat exchangers design.

A 3-Dimensional Numerical Thermal Analysis for A Vertical Double U-Tube Ground-Coupled Heat Pump

The ground heat exchanger plays a major role in the thermal performance and economic optimization of the ground-coupled heat pump. The present study focuses on the effect of the borehole size and the grout and soil thermal properties on the thermal assessment of these heat exchangers. A double U-tube heat exchanger was studied numerically by the COMSOL Multiphysics 5.4 software in a 3-dimensional discretization model. The double U-tube was circuited as a parallel flow arrangement and situated in a parallel configuration (PFPD) deep in the borehole. The grout and ground thermal conductivities were selected in the range of (0.73-2.0) W/m.K and (1.24-2.8) W/m.K respectively. The results revealed that the ground thermal conductivity showed a more pronounced influence on the thermal performance of the ground heat exchanger and with less extent for the grouting one. Increasing the grout filling thermal conductivity from (0.73) W/m.K to (2.0) W/m.K at a fixed ground thermal conductivity of (2.4) W/m.K has augmented the heat transfer rate by (10) %. The heat transfer rate of the ground heat exchanger exhibited marked enhancement as much as double when the ground thermal conductivity was increased from (1.24) W/m.K to (2.8) W/m.K at fixed grout thermal conductivity range of (0.78-2.0) W/m.K. It has been verified that increasing the borehole size has a negligible effect on the ground heat exchanger thermal performance when a grout with a high thermal conductivity was utilized in the ranged of examined configurations. The steady-state numerical analysis model outcomes of the present work could be implemented for the preliminary borehole design for a ground heat exchanger.

Comprehensive simulation based thermal performance comparison between single and double U-tube borehole heat exchanger and sensitivity analysis

In the ground coupled heat pump system that delivers space heating and cooling to the buildings, often questions are raised as to which borehole heat exchanger configuration (single or double U-tube) provides higher thermal performance. To answer this question, in this paper, detailed comparative transient heat transfer analysis between single and double U-tube borehole heat exchanger (BHE) with two independent circuits (that can operate with the same or different mass flowrate and inlet fluid temperature) is conducted. Validated numerical heat transfer model was applied to perform the dynamic simulation and analysis. The two borehole heat exchangers are compared from the perspectives of their thermal performance in the heating and cooling mode of operation. Detailed sensitivity analysis of the BHE has also been performed by investigating the impact of the important borehole parameters, thermal properties and working fluids on the thermal performance of the double U-tube BHE. The simulation result revealed that the heat injected and extracted by the double U-tube BHE at steady state is 21.8 (77%) and 10.2 W/m (71.8%) more than that of the single U-tube BHE respectively. The simulation results also indicate that the thermal effectiveness of the single and double U-tube BHE at steady state are 0.24 and 0.31 respectively. The single U-tube BHE showed higher borehole thermal resistance than that of the double U-tube BHE with values of 0.47 and 0.31 m.K/W, respectively. The results of the study also revealed that among the studied working fluids, CaCl2 (25%) solution showed better thermal performance than other anti-freeze solutions with thermal effectiveness of 35.6%. The simulation result of this work is crucial as an input for the optimization studies made to obtain a single set of optimal borehole paramaters and thermal properties that results in the best performance of BHE coupled to ground source heat pump system designed to provide space heating and cooling of buildings.

Study on the Coupled Heat Transfer Model Based on Groundwater Advection and Axial Heat Conduction for the Double U-Tube Vertical Borehole Heat Exchanger

In this paper, a dynamic heat transfer model for the vertical double U-tube borehole heat exchanger (BHE) was developed to comprehensively address the coupled heat transfer between the in-tube fluid and the soil with groundwater advection. A new concept of the heat transfer effectiveness was also proposed to evaluate the BHE heat exchange performance together with the index of the heat transfer rate. The moving finite line heat source model was selected for heat transfer outside the borehole and the steady-state model for inside the borehole. The data obtained in an on-site thermal response test were used to validate the physical model of the BHE. Then, the effects of soil type, groundwater advection velocity, inlet water flow rate, and temperature on the outlet water temperature of BHE were explored. Results show that ignoring the effects of groundwater advection in sand gravel may lead to deviation in the heat transfer rate of up to 38.9% of the ground loop design. The groundwater advection fosters the heat transfer of BHE. An increase in advection velocity may also help to shorten the time which takes the surrounding soil to reach a stable temperature. The mass flow rate of the inlet water to the BHE should be more than 0.5 kg·s−1 but should not exceed a certain upper limit under the practical engineering applications with common scale BHE. The efficiency of the heat transfer of the double U-tube BHE was determined jointly by factors such as the soil’s physical properties and the groundwater advection velocity.

An experimental investigation of CuO/water nanofluid heat transfer in geothermal heat exchanger

The energy efficiency of geothermal heat exchanger (GHE) systems can be significantly impacted by the thermal performance of heat transfer fluid inside. In the present study, nanofluid (CuO/water) was experimentally used in the GHE to investigate its thermal performance as circuit fluid. The main part of experimental system consisted of a 0.8 m × 0.5 m × 0.58 m box, two double U-tubes, thermocouples and thermal resistors. A three-dimensional discrete phase model was built to simulate the flow process. By using nanofluid, the heat transfer rate and pumping power consumption of the GHE system increased by 39.84% and 16.75%. Moreover, the heat load-to-pumping power ratio had an enhancement of 20.2%. Furthermore, the previous literature showed that the nanofluids, which had a significant effect on the heat transfer of other types of heat exchangers, had an insignificant effect (less than 5%) on the energy efficiency of GHEs. The simulation result showed that the special structure of traditional GHEs may be the main reason for the lower possibility of collision between nanoparticles and the insignificant effect of nanofluid on its heat transfer enhancement. Therefore, the length of every straight tube segment should be restricted to achieve the better thermal performance of GHEs using nanofluids as heat transfer fluid.

Effects of the Circuit Arrangement on the Thermal Performance of Double U-Tube Ground Heat Exchangers

Given that the issue of variations in geometrical parameters of the borehole heat exchanger (BHE) revolves around the phenomenon of thermal resistance, a thorough understanding of these parameters is beneficial in enhancing thermal performance of BHEs. The present study seeks to identify relative changes in the thermal performance of double U-tube BHEs triggered by alterations in circuit arrangements, as well as the shank spacing and the borehole length. The thermal performance of double U-tube BHEs with different configurations is comprehensively analyzed through a 3D transient numerical code developed by means of the finite element method. The sensitivity of each circuit configuration in terms of the thermal performance to variations of the borehole length and shank spacing is investigated. The impact of the thermal interference between flowing legs, namely thermal short-circuiting, on parameters affecting the borehole thermal resistance is addressed. Furthermore, the energy exchange characteristics for different circuit configurations are quantified by introducing the thermal effectiveness coefficient. The results indicate that the borehole length is more influential than shank spacing in increasing the discrepancy between thermal performances of different circuit configurations. It is shown that deviation of the averaged-over-the-depth mean fluid temperature from the arithmetic mean of the inlet and outlet temperatures is more critical for lower shank spacings and higher borehole lengths.

GES-CAL: A new computer program for the design of closed-loop geothermal energy systems

The purpose of this paper is to present a new tool developed for the calculation and design of shallow closed-loop geothermal systems. Most of the available geothermal computer programs only allow to consider vertical heat exchangers configurations (i.e. single or double-U tubes), being the horizontal and helical designs excluded. As an attempt to fill this gap, GES–CAL tool, presented here, is capable of providing the complete design of all the most common configurations used in low enthalpy geothermal systems. This software was initially developed for its implementation in the region of Ávila (Spain), including the most relevant results of previous author’s researches in this area. Throughout this work, the new software is deeply described and implemented in the calculation of three different study cases. Results of GES–CAL are complementary compared with the ones obtained from the most used geothermal software, EED (Earth Energy Designer). From the analysis of these results, it was possible to conclude that GES–CAL tool constitutes an optimal solution for planning a shallow geothermal system, but especially for those installations placed in the region of Ávila. In this area, the well field can be designed in more precise way which results in lower drilling lengths and, hence, lower initial investments. The conclusions of this work indicate that GES–CAL offers remarkable advantages such as the automatic calculation of the space energy demand, the inclusion of all the heat exchanger configurations and an economic and environmental evaluation of the final geothermal solution.

Multi-external-chamber coaxial borehole heat exchanger: Dynamic heat transfer and energy consumption analysis

The borehole heat exchanger (BHE) is a key component of a ground-coupled heat pump (GCHP) system, and its heat transfer performance strongly influences the entire system. This study aims to evaluate the dynamic heat transfer characteristics of a multi-external-chamber coaxial (MECC) BHE, and compared to a conventional double U-tube (2U) BHE. A three-dimensional numerical model was established and validated by a field experiment. The effects of several key design and operating parameters on heat transfer were investigated, and a sensitivity analysis was performed. The effect of groundwater flow on heat transfer was investigated using a modified three dimensional model. Case studies including comprehensive energy consumption analysis were carried out for two optimization objectives, based on the three-dimensional model coupled with a pulse-load finite line source model. The results indicate that the maximum and average heat transfer rate of the MECC BHE can be increased by 65.2% and 10.32% compared with the 2U BHE. An optimized design for deep application was performed in which the optimal flow direction and optimal insulation length ratio were included. For case studies conducted with 2U BHE, applying MECC BHE instead can reduce the BHE number by 13.3%, or the energy consumption of the water pump and the entire system by 28.16% and 9.66%, respectively.

Evaluation of utilizing horizontal directional drilling technology for ground source heat pumps

The drilling cost of Ground Heat Exchangers (GHEs), which increases the capital cost, is one of the main obstacles to their widespread development. Horizontal Directional Drilling (HDD) is a lower cost no-dig drilling technology commonly used for drilling tunnels or for the installation of urban infrastructure such as pipes, cables, etc., using the prescribed underground path. Thus, using HDD technology for horizontal GHE (HGHE) is expected to improve the competitiveness of ground source heat pump (GSHP) systems, since its drilling cost and the requirement of surface land area are significantly smaller than those of conventional excavation.In this research, a horizontal U-shaped hole was drilled as an experimental GHE in Saga City, western Japan. This hole with diameter of 230 mm, length 63 m, and a maximum depth of 8 m was used to place a polyethylene pipe with a 52 mm ID as GHE, inside. The thermal response test (TRT) with a heat load of 35−45 W/m effectively demonstrated the good heat exchange ability of the HGHE due to the larger heat exchange surface and the lack of thermal interference between pipes. The developed numerical model in FEFLOW and based on GHE design as well as the field’s geological condition were validated through the history matching of the heat medium temperatures measured during the TRT. This model was then used to carry out a sensitivity analysis under different design conditions. The results showed that the heat exchange rates were significantly enhanced in larger diameter of GHEs, while the unit heat exchange rates declined in longer lengths of GHE. The deeper installation of GHE was found to be insignificant due to the delayed seasonal change of the ground temperature, not to mention the consequent higher drilling cost. In addition, the effect of groundwater flow indicated that faster groundwater velocities, and in perpendicular direction, show the maximum system improvement.Finally, an economic and comparative study was performed to calculate and compare the merits of HDD with equivalent conventional GSHPs including single and double U-tube, coaxial and slinky GHE. The results of economic analysis showed that HDD technology can reduce GHSP cost up to a maximum of 78 %.

Simulation-Based Comparison Between the Thermal Behavior of Coaxial and Double U-Tube Borehole Heat Exchangers

In this study, the thermal behavior of the coaxial and double U borehole heat exchangers was investigated using numerical simulations in both the long- and short-term. As a reference for borehole heat exchanger specifications, the existing coaxial and double U probes of a geothermal heat pump installed within the Horizon 2020 research project named “Cheap GSHPs” were considered. Nine years of simulations revealed that when borehole heat exchangers are subjected to a balanced thermal load, and intermittent operating modes of the ground source heat pump system are set, the coaxial pipes’ configuration provides better thermal performance due to the higher thermal capacitance of the heat-carrier fluid and the lower borehole thermal resistance. The analysis was conducted considering two different types of ground with different thermal conductivity values. As result, the more conductive ground type highlights the higher yield of the coaxial probe.

A field study on the application of distributed temperature sensing technology in thermal response tests for borehole heat exchangers

Although the enhanced thermal response test (ETRT) method has been used to determine the distribution of ground temperatures and effective thermal conductivities, there are a number of obstacles which limit the wide application of this technology in the discipline of geoengineering. In this literature, four aspects of ETRT technology were investigated: (a) acquisition of ground temperature, (b) installation of the heat exchange tubes, (c) optimization of the monitoring positions, and (d) the difference in thermal conductivity obtained by the ETRT and numerical simulation. To investigate these issues, a field trial was carried out in Heze, Shandong Province, China, and the corresponding numerical models were built. The results demonstrate that: (i) the conventional methods that infer undisturbed ground temperature using water in tubes have large errors, whereas the distributed temperature sensing (DTS) technique enables the measurement of precise temperature profiles; (ii) the thermal conductivity measured using double U-tubes reflects the soil thermal property more accurately than that for a single U-tube; (iii) it is more reasonable to install optical fibers outside the U-tube sidewall than inside the observation tube; and (iv) it is essential to quantitatively consider various interface thermal impedance when estimating ground thermal conductivities using numerical simulation.