Ground Thermal Conductivity Research Articles

Numerical Analysis of Minimum Ground Temperature for Heat Extraction in Horizontal Ground Heat Exchangers

In this work, numerical simulation calculations were performed to investigate the minimum ground temperature that occurs when extracting thermal energy in a horizontal ground heat exchanger system in the Central European climate. The influence of ground thermal conductivity, heat flux extracted from the ground, periodic interruptions in the operation of the heat exchanger, periodic supply of heat energy to the ground, relative humidity of the ambient air, evaporation rate coefficient, and convective heat transfer coefficient on the ground minimum temperature were investigated. Based on the simulation, it was found that the high value of ground thermal conductivity favorably affects the operation of the installation with a ground heat exchanger. Both the reduction of the maximum heat flux taken from the ground, as well as periodic interruptions in the operation of the exchanger effectively protects the ground against excessive cooling. Further, it was found that heat supply to the ground in summer only slightly raises its minimum temperature, as well as the decrease of the relative humidity of the ambient air and evaporation rate coefficient. The change of the convective heat transfer coefficient has no significant impact on the minimum annual ground temperature.

Simulation of thermal influence between independent geothermal boreholes in densely populated areas

• The analytical model is tailored to hydraulically independent geothermal boreholes. • Thermal influence can be detected even in case of a few, far apart installations. • New installations can be strongly affected by older neighbouring installations. • Similar systems can perform differently due to neighbouring installations. Ground Source Heat Pumps (GSHPs) connected to Borehole Heat Exchangers (BHEs) are a fast-growing technology for thermally efficient buildings. Therefore, areas with several independent GSHP installations close to each other are becoming more and more common. To guarantee an optimal operation of these systems, it is necessary to design them considering the influence of the neighbouring installations. However, a tailored model for this scope has not been found in the literature. In this paper, we aim at filling this gap by proposing and validating a methodology to calculate the thermal influence between neighbouring independent boreholes. It is based on the Finite Line Source (FLS) model and prescribes novel boundary conditions, tailored to hydraulically independent boreholes. The methodology allows to prescribe different thermal loads to different BHEs and imposes uniform temperature boundary condition on each borehole wall. We also show how to implement and apply the model. Our application shows a thermal influence of up to 1.5 K during the lifetime of a GSHP and of up to 0.8 K during the first year of operation in an area with a relatively low number of installations, underlying the importance of considering the thermal influence and the usefulness of our proposed model. Finally, a sensitivity study on the ground thermal conductivity shows the importance of a correct estimation of this property for accurate simulation results.

Ground thermal conductivity estimation using the thermal response test with a horizontal ground heat exchanger

The thermal response test (TRT) is a highly developed and widespread method for measuring ground thermal conductivity using ground source heat exchangers (GHEs). However, its application is mostly limited to vertical ground heat exchangers (VGHEs), with very few previous studies on TRT with horizontal configurations. Therefore, the objective of this article is to evaluate the applicability of TRT for estimating the ground thermal conductivity with horizontal ground heat exchangers (HGHEs) at shallow depths. The short duration of the TRT prevents thermal interference between the ground surface and the horizontal heat exchanger, allowing the use of the same equation to calculate the ground thermal conductivity with VGHEs and HGHEs. This advantage is not presented when evaluating the long-term performance of GHEs, a procedure that is generally more complex in horizontal systems. For the test, the HGHE consisted of a straight HDPE pipe arranged in a U-shape (depth 2.5 m, length 36 m). The effective ground thermal conductivity obtained with the TRT has a value of 0.72W/m·K. This result is in good agreement with the thermal conductivity values of sandy loam, the type of soil at the test site.

Energy performance assessment of thermo-active micro-piles via numerical modeling and statistical analysis

Energy micro-piles (EMPs) constitute a promising emerging technology, able to provide both energy and structural retrofitting to existing buildings. Although a number of studies have been published on the energy performance of standard energy piles (EPs), bespoke analysis is required for EMPs due to their different geometry and peculiar design and site constraints. In this work, a parametric study is carried out by means of numerical Finite Element simulations complemented by statistical analysis, aimed at identifying the dominant parameters in maximizing EMP thermal efficiency. Two categories of parameters, namely design-dependent and site-dependent ones, are separately considered to provide guidance for practitioners on both detailed geothermal sizing and overall feasibility assessment. The parameter space is efficiently explored resorting to Taguchi Experimental Design statistical tools. Results show that different design criteria to those for EPs should be used for EMPs. Notably for the latter, contrary to the former, the diameter of heat exchanger pipes emerges as one of the most important factors promoting thermal efficiency, while being the single easiest parameter to engineer. On the other hand, while maximizing the number of U-pipes and pile diameter is crucial for EPs, it does not impact the energy performance of EMPs, due to geometry constraints and the expected occurrence of thermal interferences. Among site-dependent factors, a large ground thermal conductivity is confirmed as an important feature to ensure a high energy performance, while interesting insights are obtained about the role of basement thermal insulation in the long-term EMP thermal output.

Experimental and numerical investigation of the influence of groundwater flow on the borehole heat exchanger performance: A case study from Tangshan, China

In this paper, field test was conducted in Tangshan, China, combining pumping test, tracer test and thermal response test, to investigate the effect of groundwater flow on the thermal performance of borehole heat exchanger (BHE). The thermo-physical parameters of the collected samples were measured in laboratory. The influence of groundwater is quantitatively evaluated through numerical simulation. Several potential ways are also explored to enhance the role of groundwater in BHE. Test results indicated that ground’s thermal conductivity and heat transfer rate increased with an increase of groundwater velocity. Measured ground thermal conductivity in laboratory are smaller than those made in situ. Simulation results showed that the heat transfer effect of BHE is almost not improved by groundwater under natural condition, unless the thermal conductivity of the borehole and groundwater velocity are improved simultaneously. With the increase of groundwater velocity, the land occupation of group BHEs will be reduced. Most of the injected heat and cold are carried away by groundwater seepage during operation of BHE. Artificial transformation of local formation is also needed to increase groundwater velocity and aquifer thickness.

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.

A numerical investigation to analyze the effect of changing ambient conditions on accurate and stable identification of thermal conductivity of ground source heat pump system using thermal response test

This study aimed to investigate the effect of changing ambient conditions on the accuracy and stability of ground thermal conductivity, λs, using the thermal response test (TRT). First, a detailed numerical model considering the heat fluxes on the field surface of TRT was developed. Then, the numerical model was compared with that with the adiabatic surface boundary in two representative test seasons. After that, the impacts of three parameters associated with the surface heat fluxes, including the surface absorptivity, evaporation coefficient, and surface infrared emissivity on the λs were analyzed. Lastly, statistical analyses with various ambient conditions were employed to evaluate the degree of estimated λs deviating from its actual value in different specifications of TRT and actual ground thermal conductivities. Results show that the surface absorptivity can be generally seen as the most significant contributory factor to the uncertainty of λs. The λs is overestimated and the uncertainty of TRT is aggravated by considering the changing ambient conditions, especially when the depth of drilling test well is short and the actual λs is high. For such conditions, the heat insulation on the test site surface is highly recommended considering the acceptable accuracy of λs with the adiabatic boundary.

Investigating the thermal behaviour of geothermal pavements using Thermal Response Test (TRT)

Geothermal pavements represents a novel approach to shallow geothermal energy applications, in which horizontal ground heat exchangers are implemented within the pavement structure instead of traditional purpose-built trenches, thus decreasing capital costs. This study aims to better understand the feasibility and potential of these systems, by investigating the thermal response of the ground in a full scale geothermal pavements system, the first of its kind in Australasia. For this purpose, a fully instrumented geothermal pavements segment, measuring 20 m × 10 m, was constructed in the city of Adelaide, Australia. The geothermal pavement was subjected to Thermal Response Testing (TRT) and numerical modelling is adopted herein and validated using TRT data to further understand the ground response. In addition to providing insights on the capacity of energy provision for geothermal pavements, this work also introduces and discusses various methods of TRT data analysis for the geothermal pavement, specifically for obtaining the effective ground thermal conductivity, a key parameter in shallow geothermal design. The results indicate that from the considered methods, the conventional semi-log method can lead to overestimation of thermal conductivity, the guarded hot plate model tends to underestimate its value and detailed numerical modelling is most accurate, but computationally more expensive. The results also show that the radius of influence of the geothermal pavement in the examined case is close to 0.5 m and even though the geothermal pavement is considerably affected by the ambient temperature, it can be a viable solution for heating and cooling purposes, showing a heat exchange of 50.2 W/m2 and a rate per length of the pipe of 25 W/m in the case analysed here.

Corrigendum to “On the ground thermal conductivity estimation with coaxial borehole heat exchangers according to different undisturbed ground temperature profiles” [Appl. Therm. Eng. 173 (2020) 115198

Corrigendum to “On the ground thermal conductivity estimation with coaxial borehole heat exchangers according to different undisturbed ground temperature profiles” [Appl. Therm. Eng. 173 (2020) 115198

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.

Effect of environmental thermal disturbance on effective thermal conductivity of ground source heat pump system

Ground thermal conductivity, λs, is one of the important design factors for the ground source heat pump system. This study aimed to investigate the effect of environmental thermal disturbance on the λs estimation during the thermal response test (TRT), which is interpreted by the infinite line source model. For this purpose, a detailed numerical model considering the heat exchange between the circulating fluid inside the above-ground pipe of TRT rig and the outdoor environment in two representative seasons was developed. The numerical model was compared with the model with the adiabatic boundary assigned to the outer surface of above-ground pipe to evaluate the fluctuations of λs on account of the atmospheric disturbance. The parametric and sensitivity analyses were carried out to quantify the impacts of TRT seasons and above-ground pipe parameters including the surface absorptivity, thickness, and thermal conductivity of insulation material, and the length of the above-ground pipe on the λs and corresponding design borehole length. An approach of prolonging the TRT duration was proposed to impair the disturbance when the pipe is not insulated. Results show that λs fluctuates strongly and is largely misestimated considering the atmospheric disturbance, especially when TRT duration is limited. The λs varies widely as the length of above-ground pipe, insulation surface absorptivity, and insulation thermal conductivity increase, insulation thickness decreases, and TRT starts in summer. The pipe length is the most significant factor to the uncertainty of λs. The test time is recommended to last at least 3 days in summer and 3.6 days in winter when the pipe is not wrapped by a layer of insulation material. This study quantitatively assesses the impacts of environmental thermal disturbance during the TRT and gives guidance to carry out TRT properly.

Usage of the net present value-at-risk to design ground-coupled heat pump systems under uncertain scenarios

Hybrid ground-coupled heat pump systems can efficiently heat and cool buildings by exchanging heat with geological materials. Their sizing is, however, complex and their financial profitability is hard to establish during the design phase due to uncertainties that taint important design parameters. Historically, impacts of uncertainties are assessed after sizing completion using sensitivity analyses. Unfortunately, these analyses cannot mitigate design risks, should the consequences of uncertainties be significant. Here, we show how the net present value-at-risk, a stochastic financial indicator inspired by a metric used in the financial sector, can weave the impacts of uncertainties throughout the design phase. The net present value-at-risk is compared to the net present value in a case study that considers uncertainty of construction costs, building’s heat load, energy tariffs and ground thermal conductivity. Results show that this financial indicator, although it comes at a higher computational price than traditional net present value indicator, leads to shorter payback periods, greatly reduces the risks of unforeseen financial losses and does not require further sensitivity analysis. By describing uncertain parameters with statistical distributions during sizing, the proposed designs are more conservative, still efficient and financially viable while being shielded from worst case scenarios.

Analysis of thermal response tests on boreholes with controlled inlet temperature versus controlled heat input rate

Thermal response tests (TRT) are performed on borehole heat exchangers to estimate design properties needed when the boreholes are coupled to heat pumps. In conventional TRTs the heat injection or extraction rate from an external heater or heat pump is controlled to be nearly constant. More recently some investigators have conducted TRTs while controlling the inlet fluid temperature to be nearly constant. This paper develops computationally efficient heat transfer models that match measured transient temperature curves throughout the entire duration for both types of tests. The models include variable heat rates, thermal storage of the borehole fluid and the different properties between the grout and ground. The models are combined with parameter estimation methods and applied to both types of data sets from the same borehole. An uncertainty analysis indicates the models estimate ground thermal conductivity and borehole thermal resistance with approximately the same uncertainty for both types of tests.

Nationwide Determination of Required Total Lengths of Multiple Borehole Heat Exchangers under Variable Climate and Geology in Japan

This study determined the required lengths of borehole heat exchangers (BHEs) in ground-source heat pump systems for heating/cooling a building (with 300 m2 of floor area) across Japan’s four main islands through a simulation approach. Hourly thermal loads were estimated in 10 km gridded cells based on the outside temperature and humidity. Three-dimensional estimates of ground thermal conductivity from our previous study at the depths of the BHEs were used. A 5-year system operation was simulated in a total of 4059 cells with 81 combinations of individual lengths and total numbers of BHEs to determine the shortest total length required to achieve sustainable use and targeted performance. The optimal combination of individual length and total number varied regionally due to climate conditions and locally among adjacent cells due to geological conditions. The total required lengths ranged widely from 78 to 1782 m. However, the lengths were less than 400 m in 85% of the cells. Additionally, cost-effectiveness in 69% of the cells was shown by reducing the total lengths to half or less of those in the practical method. The reduction could potentially increase the feasibility of heat pump system use in Japan. The total lengths were dependent on the heating/cooling loads approximately as secondary-polynomial functions, but the relations with the ground thermal conductivity were not clear.

Evaluating Variability of Ground Thermal Conductivity within a Steep Site by History Matching Underground Distributed Temperatures from Thermal Response Tests

The variability of ground thermal conductivity, based on underground conditions, is often ignored during the design of ground-source heat pump systems. This study shows a field evidence of such site-scale variations through thermal response tests in eight borehole heat exchangers aligned at a site on a terrace along the foothills of mountains in northern Japan. Conventional analysis of the overall ground thermal conductivity along the total installation length finds that the value at one borehole heat exchanger is 2.5 times that at the other seven boreholes. History matching analysis of underground distributed temperature measurements generates vertical partial ground thermal conductivity data for four depth layers. Based on the moving line heat source theory, the partial values are generally within a narrow range expected for gravel deposits. Darcy velocities of groundwater are estimated to be 74–204 m/y at the borehole with high conductivity, increasing in the shallow layers above a depth of 41 m. In contrast, the velocities at the other seven boreholes are one-to-two orders of magnitude smaller with no trend. These high and low velocity values are considered for the topography and permeability. However, the relatively slow groundwater velocities might not apparently increase the partial conductivity.

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.

Comparison of influencing factors and level optimization for heating through deep-buried pipe based on Taguchi method

For building heating systems with a deep-buried coaxial double-pipe, its heat transfer performance was studied using three evaluation parameters in five heating periods under five factors and the corresponding four levels. The three evaluation parameters are time-averaged heat transfer per meter QL-a, time-averaged performance parameter COP-a of heat pump unit, and five-year decay rate of heat exchange rate Rd-5. The five factors are buried pipe depth, water flow rate, inlet water temperature, geothermal gradient, and thermal conductivity of ground. The levels for each factor were selected in combination with practical problems. Sixteen sets of orthogonal simulation conditions (L1645) with five factors and four levels were designed following the Taguchi method. Using signal-to-noise ratio and analysis of variance, QL-a, COP-a, and Rd-5 were evaluated under various factors and levels. The optimal combination of various factors for the best heat transfer performance of buried pipe and the contribution rate of each factor to the heat transfer performance of buried pipe were obtained.

Changes in precipitation and air temperature contribute comparably to permafrost degradation in a warmer climate

Surface energy budgets of high-latitude permafrost systems are poorly represented in Earth system models (ESMs), yet permafrost is rapidly degrading and these dynamics are critical to future carbon-climate feedback predictions. A potentially important factor in permafrost degradation neglected so far by ESMs is heat transfer from precipitation, although increases in soil temperature and thaw depth have been observed following increases in precipitation. Using observations and a mechanistic ecosystem model, we show here that increases in precipitation hasten active layer development beyond that caused by surface air warming across the North Slope of Alaska (NSA) under recent and 21st century climate (RCP8.5). Modeled active layer depth (ALD) in simulations that allow precipitation heat transfer agreed very well with observations from 28 Circumpolar Active Layer Monitoring sites (R2 = 0.63; RMSE = 10 cm). Simulations that ignored precipitation heat transfer resulted in lower spatially-averaged soil temperatures and a 39 cm shallower ALD by 2100 across the NSA. The results from our sensitivity analysis show that projected increases in 21st century precipitation deepen the active layer by enhancing precipitation heat transfer and ground thermal conductivity, suggesting that precipitation is as important an environmental control on permafrost degradation as surface air temperature. We conclude that ESMs that do not account for precipitation heat transfer likely underestimate ALD rates of change, and thus likely predict biased ecosystem responses.

A novel model and heat extraction capacity of mid-deep buried U-bend pipe ground heat exchangers

This study presents a novel simple model that could quickly simulate the heat transfer of mid-deep buried U-bend pipe ground heat exchangers (MBUGHEs). This model simplified a MBUGHE into three independent heat transfer sections: the descending, horizontal, and ascending sections. It was assumed that there was no thermal interference among the different sections because the horizontal pipe was usually very deep and long enough, which meant that each section could be solved separately in independent cylindrical coordinates. CFD software and experimental data were used to validate the established model. The results showed that the model calculation results had high reliability. The model was used to analyze the MBUGHE heat extraction characteristics. The results indicated that the heat extraction capacity increased with the increase of the ground thermal conductivity, ground volumetric heat capacity, buried pipe depth, and horizontal pipe length. The results also showed that the MBUGHE heat extraction capacity had a significant decrease in the first several years, but the decrease was gradually reduced with the increase of the operation time and the decrease became approximately stable after 15 years.

Significance evaluation of performance factors on horizontal spiral-coil ground heat exchangers

Ground-coupled heat pump systems using horizontal spiral-coil ground heat exchangers (HSGHE) are being utilized for an increasing number of applications due to the efficiency in terms of thermal performance. To optimize design accuracy and thermal performance, significant design factors of HSGHEs should be identified. Thus, this study investigates the effects of influential factors on the thermal performance of HSGHEs through sensitivity analysis. Numerical models were established with respect to the operation of HSGHEs using the finite element method. Variance-based sensitivity analysis was conducted using the numerical results to quantitatively evaluate the overall effects, which include the main effects as well as interaction effects between the design factors. Furthermore, case studies of two different regions were applied to the sensitivity analysis to compare the effects of regional climate factors on the order of significance of the input factors. As a result, ground thermal conductivity, installation depth, and inlet fluid temperature were determined as major design factors. On the other hand, the order of significance of the major factors was found to be dependent on regional climate factors. This result implies that it is necessary to consider different critical factors of HSGHE design according to the ambient conditions of the installation region.