Design Of Ground Heat Exchanger Research Articles

Effects by geothermal gradient on thermal response tests for boreholes

A thermal response test (TRT) on a borehole provides information needed in the design of ground heat exchangers for ground source heat pump systems. Conventional analysis of TRT data sets uses the late-time period to estimate the ground thermal conductivity and the effective borehole resistance. During this quasi-steady-state period, the transient average fluid temperature has a logarithmic linear trend, which is consistent with conventional heat transfer models that assume a uniform undisturbed ground temperature. As the depth of the borehole increases the undisturbed ground temperature increases with depth following the geothermal gradient. This paper focuses on how the geothermal gradient affects a TRT. As the geothermal gradient increases, the required time to reach the conventional logarithmic linear trend may increase beyond any practical testing duration in coaxial boreholes. Then, conventional analysis methods based on a quasi-steady-state period incorrectly estimate the ground thermal conductivity. The magnitude of this effect is controlled through a dimensionless parameter |qratio|, which includes the geothermal gradient and other test parameters. In place of conventional models, a transient model that includes the geothermal gradient and fluid thermal storage can estimate the ground thermal conductivity and local borehole resistance within the typical TRT testing duration.

Simplified water-source heat pump models for predicting heat extraction and rejection

Simplified heat pump models are needed for the design of ground heat exchangers used with ground-source heat pump systems. Design engineers commonly have only manufacturer's data with which to construct a heat pump model that can predict heat extraction and heat rejection given building heating and cooling loads and take account of the entering fluid temperatures to the heat pump. Earlier work has used simple polynomials to predict the ratio of heat extraction to heating and heat rejection to cooling. Unfortunately, manufacturers' catalog data do not always provide sufficient support to calculate the polynomial coefficients. Therefore, in the absence of experimental data, simplified and acceptably accurate models are needed to exploit a limited number of operating points.This paper presents a review of available manufacturers' data in North American and European markets. The available data vary from country to country, depending on the standards in use in each country. In particular, data availability is poorer for the European heat pumps compared to North American heat pumps.Then, a range of models is investigated, characterizing the results by the required inputs and the root mean square error and recommendations are provided. Models are validated against catalog and experimental data.

Oscillatory thermal response tests to estimate the ground thermal diffusivity

Thermal Response Tests (TRT) have been extensively studied in the past decades and is an important area of research belonging to ground heat exchanger design. The most important thermal property inferred in a TRT is the equivalent subsurface thermal conductivity. Previous studies were presented where the thermal diffusivity was estimated but with limited success. In this paper, a TRT with an oscillatory excitation (OTRT) is used to obtain a better uncoupling between thermal conductivity found from constant heat injection and thermal diffusivity. The method was already used in the past with a simplified ground model. It is presented here with a better model that takes into account the ground and the borehole thermal response. The method is based on the concept of concentric composite cylinder where the equivalent radius is used for the U-tube arrangement. Results obtained with this method were successfully correlated to those obtained with numerical simulations and the method was then used to analyze an experimental field test.

Generation of gridded temperature map of constant-temperature layer based on meteorological data for shallow geothermal applications

The energy exchange between the subsurface and the infrastructure may serve as an effective way to reduce energy consumption in space heating. In China, the concepts of climate battery and shallow geothermal have obtained growing attentions. The depth and the temperature of the constant-temperature layer (CTL), also called neutral zone, in the ground are significant factors during the design modeling. Unfortunately, the lack of knowledge in these site-dependent variables often leads to conservative design of ground heat exchangers and thus limits their implementations due to the high upfront cost. This study presents an improved analytical model based on the subsurface energy balance equation, which can be used to estimate the temperature and depth of CTL based on the meteorological and geological datasets. The model was first validated against the temperature records from 77 shallow geothermal case studies in China. With the development of remote sensing technology and agricultural dataset, the spatial mapping of CTL was then generated based on the variability of the geological and meteorological parameters. Finally, the simplifications were made to the proposed model when there is no available meteorological data and proven as sufficiently robust in results delivery. By performing this model at the initial stage of the engineering practice, the long-term heat exchange potential of the geology can be anticipated with higher fidelity, and the suitability of a certain region to be treated as a climate battery may be determined without costly drilling efforts.

Modeling and techno-economic comparison of two types of coaxial with double U-tube ground heat exchangers

Among different types of ground heat exchangers (GHEs), multi-external chamber coaxial GHE is recently a promising design due to its geometrical characteristics of concentric tube-in-tube configuration and maximal heat exchange performance. Nevertheless, the characterization and commercialization of multi-external chamber coaxial GHEs need a deep understanding of the effect of different opt-geometrical and operational parameters on their thermo-economic performance. Therefore, accurate numerical modeling of the transient heat transfer proficiencies and the techno-economic feasibility of the multi-external chamber coaxial GHE and coaxial GHE compared to conventional double U-tube GHE is crucial to promise an optimal design of ground-coupled heat pump systems. This study introduces a comprehensive systematic and techno-economic analysis of the transient heat transfer characteristics of coaxial and multi-external-chamber coaxial GHEs compared to conventional double U-tube GHE. A three-dimensional numerical model was adopted and validated by field experiments. Moreover, a sensitivity analysis was conducted to investigate the impacts of several key operating and design parameters on heat transfer performance under heating and cooling modes conditions. Furthermore, various case studies, along with a comprehensive analysis of multi-objective optimization for minimization of the GHE number and energy consumption, were also carried out. In addition, a techno-economic analysis was also carried out using the total annual cost method to assess the economic viability of the proposed GHE designs. The results reveal that, compared to double U-tube GHE, coaxial GHE has the best thermal performance with maximum and average heat transfer rates of 127.54% and 17.67%, while the multi-external-chamber coaxial GHE is estimated to be 62.38% and 11.72%, respectively. Also, the corresponding total thermal resistance of coaxial GHE is the lowest by 14.86%, and for multi-external-chamber coaxial, it is 10.30%; also coaxial GHE has the lowest pressure drop by 76.45%, followed by multi-external-chamber coaxial GHE with 57.18%. In the case studies, replacing double U-tube GHE with coaxial or multi-external-chamber coaxial GHEs, the GHE number can be decreased by 13.3% for both the designs, or the energy consumption of the water pump by 33.91% and 30.97%, and the energy consumption of the entire system by 17.21% and 10.54%, respectively. The techno-economic analysis results indicate that replacing double U-tube GHE with coaxial GHE is the most feasible alternative, which can decrease the total annual cost by 13.58%, followed by multi-external-chamber coaxial GHE by 6.77%. Further discussion for the deep applications was performed, which concluded the optimal GHE design. The findings of this paper can be used as a guide for future research on GHEs for potential applications of ground-coupled heat pump systems.

Experimental investigation on the use of CuO/water nanofluid in horizontal spiral-coil ground heat exchanger

The thermal performance of a ground heat exchanger (GHE) largely depends on the heat transfer fluid circulating in the system. In this study, a laboratory test apparatus for a horizontal spiral-coil ground heat exchanger (HSGHE) was designed and constructed to investigate the effects of different volume fractions of CuO/water nanofluid on the energy efficiency of the HSGHE. The results showed that, at a volumetric fraction of 1%, the CuO/water nanofluid led to a 9.4% increase in the heat exchange rate of the HSGHE, compared to water. In addition, the heat transfer enhancement effect at a lower volume fraction of the CuO/water nanofluid was greater than the effect of its viscosity on the performance of the HSGHE. The CuO/water nanofluid with a volumetric fraction of 1% was not recommended for use in the HSGHE owing to its performance efficiency coefficient (PEC) of less than 1. The optimal volume fraction was 0.5%, at which the PEC was the highest (1.025). Therefore, it was determined that the utilization of a low volume fraction of CuO/water nanofluid in HSGHE provides energy efficiency enhancements independent of the GHE design.

A new design of ground heat exchanger with insulation plate for effectively geothermal management

This paper proposes a new design of the ground heat exchanger (GHE) with the insulation plate based on the numerical approach. The heat transfer problem is solved in non-dimensional fashion under the important effects of many characteristic parameters, such as Reynolds number (Re), Prandtl number (Pr), pipe width, pipe depth, particularly the thermal conductivity (TC) and the dimensions of insulation plate. The numerical model has a good validation due to its excellent agreement with the results obtained in the literature. It was found that the temperature at the outlet (Tout*) is non–linearly proportional to Re and approaches a minimum value for a particular Re. Furthermore, the optimal design point where Tout* is minimal moves toward the low Re when Pr increases. The Tout* also decreases with increasing the pipe depth, however, its variation becomes smaller and flatter when the pipe width increases simultaneously. Additionally, the thermal performance increases significantly with decreasing the TC of insulation plate. The Tout* is up to 15 % lower than that without an insulation plate. In particular, the new design of the GHE prototypes with the insulation plate is first proposed. This idea can be developed not only for the U–shaped pipe, but also for the other GHE prototypes that are frequently used in practice, for example, the double U–tube, the W–shaped tube, and the spiral tube. The results obtained are very useful to effectively improve the thermal performance of the GHEs applied in the geothermal management.

Numerical Investigation on Energy Efficiency of Heat Pump with Tunnel Lining Ground Heat Exchangers under Building Cooling

For mountain tunnels, ground heat exchangers can be integrated into the tunnel lining to extract geothermal energy for building heating and cooling via a heat pump. In recent decades, many researchers only focused on the thermal performance of tunnel lining Ground Heat Exchangers (GHEs), ignoring the energy efficiency of the heat pump. A numerical model combining the tunnel lining GHEs and heat pump was established to investigate the energy efficiency of the heat pump. The inlet temperature of an absorber pipe was coupled with the cooling load of GHEs in the numerical model, and the numerical results were calibrated using the in situ test data. The energy efficiency ratio (EER) of the heat pump was calculated based on the correlation of the outlet temperature and EER. The heat pump energy efficiencies under different pipe layout types, pipe pitches and pipe lengths were evaluated. The coupling effect of ventilation and groundwater flow on the energy efficiency of heat pump was investigated. The results demonstrate that (i) the absorber pipes arranged along the axial direction of the tunnel have a greater EER than those arranged along the cross direction; (ii) the EER increases exponentially with increasing absorber pipe pitch and length (the influence of the pipe pitch and length on the growth rate of EER fades gradually as wind speed and groundwater flow rate increase); (iii) the influence of groundwater conditions on the energy efficiency of heat pumps is more obvious compared with ventilation conditions. Moreover, abundant groundwater may lead to a negative effect of ventilation on the heat pump energy efficiency. Hence, the coupling effect of ventilation and groundwater flow needs to be considered for the tunnel lining GHEs design.

Numerical investigation of energy tunnel lining ground heat exchangers in a mountain environment

The energy tunnel lining ground heat exchangers (GHEs) is a new technology to extract geothermal energy. At present, the researches on the feasibility of energy tunnel lining GHEs design are rare. In order to evaluate the feasibility of energy tunnel lining GHEs design in a mountain environment, a 3D numerical model was developed to simulate the transient heat transfer of energy tunnel lining GHEs. In this model, the inlet temperature varied with time based on the building refrigeration load, the numerical results were verified with the field monitoring data. According to the relationship between outlet temperature and the energy efficiency ratio (EER) of heat pump, the numerical results of outlet temperature were used to calculate the EER. Then, the energy efficiency of energy tunnel lining GHEs was evaluated. The results show that the design of the energy tunnel lining GHEs with a pipe pitch of 0.6 m and a pipe length of 329.4 m is feasible. Under continuous operation mode, the energy efficiency ratio reaches 4.44 and the refrigeration power reaches 175 kW in summer.

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.

Experimental and numerical investigations on the thermal behavior of ground heat exchanger in stratified soils across unsaturated and saturated layers

This paper presents a model that takes moisture migration (in unsaturated soil), the presence of groundwater seepage (in saturated soil), and soil stratification into consideration simultaneously. The thermal behavior of a ground heat exchanger field located in stratified soils across unsaturated and saturated layers is also investigated, experimentally and numerically. Simulated temperature data at three monitoring points based on a 3-dimensional finite volume numerical model were compared with the experimental data. The agreement between the simulated data and experimental data proves that the numerical model considering soil stratification and ground seepage has good accuracy. Based on the proposed model, the influencing factors of soil thermal conductivity, soil porosity, seepage velocity, daily operation time, inlet water temperature, and velocity on the heat transfer rate of a 3 × 3 bore field were investigated. In general, the heat transfer rate of the bore field located in the stratified subsurface increases as the soil thermal conductivity, seepage velocity, inlet fluid temperature, and velocity increase. Conversely, it decreases as soil porosity and daily operation time increase. This study achieves elaborate ground heat exchanger design and simulation, and consequently help promote a regulatory framework for design method formulation with soil stratification and groundwater flow.

A methodology for lithology-based thermal conductivities at a regional scale for shallow geothermal energy – Application to the Brussels-Capital Region

A methodology for the determination of lithology-based thermal conductivities at a regional scale is proposed, assigning a saturated and unsaturated thermal conductivity to each stratigraphic unit encountered in the region. Such a methodology is paramount for GIS-supported mapping of shallow geothermal energy at a regional scale. The analysis is primarily based on the interpretation of thermal response tests (TRT), assuming that the thermal conductivity determined during TRT is a thickness-weighted average of the individual thermal conductivity of each stratigraphic unit constituting the ground along a ground heat exchanger (GHE). Enhanced thermal response tests, reference geological material-based thermal conductivities and laboratory optical scanning tests achieved on remolded specimen from drilling cuttings are used to validate the results. The relevance of the methodology is illustrated through its application to the Brussels-Capital Region (Belgium), and consistent saturated and unsaturated thermal conductivities are obtained for each stratigraphic unit. An uncertainty analysis on the thermal conductivity is proposed, and its impact on the design of GHE is discussed. In most cases, the relative error on the ground thermal conductivity is lower than 10 %, and its impact on GHE length remains limited.

Application and Design Aspects of Ground Heat Exchangers

With the constant increase in energy demand, using renewable energy has become a priority. Geothermal energy is a widely available, constant source of renewable energy that has shown great potential as an alternative source of energy in achieving global energy sustainability and environment protection. When exploiting geothermal energy, whether is for heating or cooling buildings or generating electricity, a ground heat exchanger (GHE) is the most important component, whose performance can be easily improved by following the latest design aspects. This article focuses on the application of different types of GHEs with attention directed to deep vertical borehole heat exchangers and direct expansion systems, which were not dealt with in detail in recent reviews. The article gives a review of the most recent advances in design aspects of GHE, namely pipe arrangement, materials, and working fluids. The influence of the main design parameters on the performance of horizontal, vertical, and shallow GHEs is discussed together with commonly used performance indicators for the evaluation of GHE. A survey of the available literature shows that thermal performance is mostly a point of interest, while hydraulic and/or economic performance is often not addressed, potentially resulting in non-optimal GHE design.

Proposition of Design Capacity of Borehole Heat Exchangers for Use in the Schematic-Design Stage

This study proposes a simple ground heat exchanger design capacity that is applicable in the schematic-design stage for several configurations used for borehole heat exchangers (BHEs). Three configurations—single, compact, and irregular types—were selected, and the heat transfer rate per unit BHE was calculated considering heat interference. In a case study with a typical configuration and general range of ground thermal conductivity, the BHE heat transfer rate of the compact configuration decreased owing to heat interference as the number of BHEs increased. However, with respect to the irregular configuration, the heat transfer rate increased as the same number increased. This was attributed to the relatively large increment rate of the distance between the boreholes in the irregular configurations, making the heat recovery factor more dominant than the heat interference. The results show that the average heat transfer rate values per BHE applicable to each configuration type in the schematic-design stage were 12.1 kW for the single configuration, 5.8 kW for the compact type, and 10.3 kW for the irregular configuration. However, owing to the large range of results for each case study, the error needs to be reduced by maximally utilizing the information available at the schematic-design stage.

Structure optimization design of ground heat exchanger by topology method to mitigate the geothermal imbalance

Intensive thermal interference between adjacent buried pipes and heat buildup around the surrounding soil lead to the deterioration of thermal performance of ground heat exchanger (GHE). The study aims at designing an optimized structure for GHE to accelerate releasing rate of the accumulating heat and mitigate the degree of thermal interference. In this paper, the topology optimization method is proposed to the design of highly conductive GHE and the optimization result is obtained. Moreover, the rules of generation and evolution of optimization results are studied by exploring the influences of volume fraction and different boundary conditions. Finally, a comparison study between thermal performance of optimized GHE and conventional serpentine GHE is carried out. Taking the minimum dissipation of heat transport potential capacity as the optimization objective, the topology optimization result presents a symmetrical tree-shaped structure. The tree-structure GHE yields a lower standard deviation of soil temperature and a smaller maximum soil temperature. Compared with conventional serpentine GHE, the tree-structure GHE can efficiently accelerate the releasing heat rate and improve heat transfer performance by about 25.3%, which provides a new way to design GHE to avoid heat accumulation.

Dynamic thermal analysis of a residential ground-source heat pump

We present the dynamic thermal analysis of a residential ground-source heat pump (GSHP) with the following objectives: (1) to present a comprehensive GSHP model accounting for the dynamic behavior of a vapor compression unit, lumped serviced space, and the ground heat exchanger (GHE); (2) to identify a set of GHE temperature, mass flow rate, and evaporator area fraction (α) improving the time-averaged coefficient of performance (COP‾) while retaining thermal comfort in the serviced space; and (3) to propose a GHE design satisfying the desired design and operating conditions determined in (2). For a 4-ton residential GSHP considered herein, we verified the existence of a critical GHE mass flow rate of 1kg/s after which the COP‾ hardly varied. Furthermore, we observed a design trade-off between cooling and heating modes wherein a smaller α was preferred in cooling mode while the heating mode favored a higher α. Cooling COP‾ was sensitivity to both α and GHE temperature (TB), whereas that in heating mode was insensitive to TB as long as both α and Lt were sufficiently high. The optimal α impartial to both modes was 0.29 and the peak COP‾=5.49 and 3.6 in cooling and heating mode, 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 %.

Experimental investigation on using Al2O3/ethylene glycol-water nano-fluid in different types of horizontal ground heat exchangers

This study presents the experimental results for the first time use of Al2O3/ethylene glycol-water (EG-water) nano-fluid (NF) in ground heat exchangers (GHE) within the ground source heat pump (GSHP) system which is prepared with 8 nm γ-Al2O3 nanoparticles. A test room with an area of 21 m2 was prepared in the cold climate region (39°42′24.8″ N, 37°02′01.5″E) of Turkey, and GSHP system was installed into the room. Horizontal GHE designs are preferred as GHE design in order to maintain cost and efficiency balance. Although numerical studies on the use of NF-assisted fluids in GSHP systems are guiding in the literature, these studies have not been supported experimentally. In this study to see performance of NFs in real environment conditions Al2O3 nanoparticles were examined experimentally in horizontal U-type GHE and spiral GHE by adding different volume concentrations (0.1%, 0.2%) to EG-water (25–75%) base fluid. Our results showed that, with a volume concentration of 0.1%, NF provided a 19% and 22% increase compared to EG-water in U-type GHE and spiral GHE, respectively, concerning GHE output temperatures. In addition, the coefficient of performance (COP) of the system using NF with a volume concentration of 0.1% showed a higher performance of 2.5% in U-type GHE and 3% in spiral GHE compared to EG-water base fluid. On the other hand, in both GHE designs, the NF with a volume concentration of 0.2% did not achieve any performance increase. The NF with a volume concentration of 0.1%, which is the most successful variation of the U-type GHE and spiral GHE systems, showed a 2.7% higher COP value than the spiral GHE in U-type GHE. As a result, it has been determined that the use of low concentration of NF in GHEs for the GSHP system provides performance improvements independent of the GHE design.

A novel shallow bore ground heat exchanger for ground source heat pump applications—Model development and validation

A novel ground heat exchanger design, the Underground Thermal Battery (UTB), was proposed as a low-cost alternative to the conventional vertical bore ground heat exchanger (VBGHE) for the application of ground source heat pumps (GSHPs). The UTB is designed to be installed in a shallow borehole (less than 6 m deep) and thus could cost much less than the conventional VBGHE, which usually is installed in vertical bores 60 m deep. By utilizing natural convection of water and phase change materials, the UTB tempers its temperature change in response to thermal loads, which helps improve the efficiency of GSHPs. A one-dimensional (1D) model of the UTB has been developed and validated with the measured performance data of a small-scale UTB, as well as the simulation results of a more detailed three dimensional (3D) numerical model, which accounts for both heat transfer and fluid dynamics in the UTB. This 1D model is computationally much more efficient than the 3D model and thus can simulate the annual performance of the UTB with acceptable computation time. The 1D model has been used to evaluate the performance of the UTB so that it can be compared with the conventional VBGHE.

A three-dimensional numerical investigation of a novel shallow bore ground heat exchanger integrated with phase change material

The heat transfer performance of an innovative design of ground heat exchanger, called underground thermal battery (UTB), was investigated in this paper. The UTB is designed to be installed in a shallow subsurface of the ground (less than 20 ft or 6 m below the grade), which is much shallower than the vertical bores (deeper than 200 ft or 60 m) required for installing conventional vertical bore ground heat exchangers (VBGHE). The UTB consists of a water-filled tank that also contains a phase change material (PCM), which impart the UTB a large thermal storage capacity. A three-dimensional (3D) numerical model has been developed to evaluate the performance of the UTB and to compare it with a conventional VBGHE. The simulation results indicated that a UTB can better regulate the entering water temperature of a ground source heat pump (GSHP) for more efficient operation than a conventional VBGHE under short-term high thermal load conditions. In addition, the UTB can make more effective use of PCMs compared with using PCMs to replace the conventional grouting material in a VBGHE.