Borehole Thermal Resistance Research Articles

Analysis on Thermal Performance of Ground Heat Exchanger According to Design Type Based on Thermal Response Test

A ground source heat pump (GSHP) system has higher performance than air source heat pump system due to the use of more efficient ground heat source. However, the GSHP system performance depends on ground thermal properties and groundwater conditions. There are many studies on the improvement of GSHP system by developing ground heat exchanger (GHX) and heat exchange method. Several studies have suggested methods to improve heat exchange rate for the development of GHX. However, few real-scale experimental studies have quantitatively analyzed their performance using the same ground conditions. Therefore, the objective of this study was to evaluate the thermal performance of various pipe types of GHX by the thermal response test (TRT) under the same field and test conditions. Four kinds of GHX (HDPE type, HDPE-nano type, spiral fin type, and coaxial type) were constructed in the same site. Inlet and outlet temperatures of GHXs and effective thermal conductivity were measured through the TRT. In addition, the borehole thermal resistance was calculated to comparatively analyze the correlation of the heat exchange performance with each GHX. Result of the TRT revealed that averages effective thermal conductivities of HDPE type, HDPE-nano, spiral fin type, and coaxial type GHX were 2.25 W/m·K, 2.34 W/m·K, 2.55 W/m·K, and 2.16 W/m·K, respectively. In the result, it was found that the average borehole thermal resistance can be an important factor in TRT, but the effect of increased thermal conductivity of pipe material itself was not significant.

Constant-temperature thermal response test (TRT) with both heat injection and extraction for ground source heat pump systems: Methodology and a case study

Abstract An accurate estimate of site-specific underground thermal properties from thermal response tests (TRTs) is vital to the efficient and sustainable use of ground source heat pump (GSHP) systems. During a conventional TRT, the ground is perturbed by a heat injection and the response is measured in time. Despite the simplicity of the concept, conventional TRT fails to take into account the ground thermal response to heat extraction, which may affect the reliability of the thermal properties derived. To address the problem, an improved TRT based on both heat injection and extraction was proposed in this study. The improvements were demonstrated with an in-situ test carried out in Taiyuan, China. The measurement results showed that the ground thermal conductivity in Taiyuan was 1.56 W/(m K) and the borehole thermal resistance was 0.22 (m K)/W. The results also showed that, in the case of Taiyuan, ground thermal conductivities derived solely from heat injection test and heat extraction test were, respectively, 7.6% higher and 8.3% lower than that from the improved TRT. Thus, compared with the conventional TRT, the improved TRT consisting of both heat injection and extraction tests can provide test data more accurately reflect the annual performance of GSHP systems.

A Borehole Thermal Resistance Correlation for a Single Vertical DX U-Tube in Geothermal Energy Application

The ground source coupled heat pump is considered as one of the most important technologies utilized in the field of sustainable energy. The borehole thermal resistance has a great impact on the total thermal resistance between the fluid that flows inside the buried U-tube and ground. This issue is related directly to the heat transfer efficiency of the ground part of the heat pump and the effective coefficient of performance of the heat pump system. The capability of the ground heat exchanger to dissipate or absorb energy to or from the ground determines the size and geometry configuration of these heat exchangers. The present research represents a model for the prediction of the borehole thermal resistance of a ground heat exchanger. The U-tube heat source or sink was replaced by a single equivalent concentric tube in the borehole possessing equal thermal resistance as that of the original U-tube heat exchanger. The model was applied for four different U-tube/borehole configurations, the test U-tubes were (9.52) mm, (12.7) mm, (15.88) mm, and (19.05) mm for a borehole to U-tube diameter ratios range of (3.94) to (7.88). The correlation showed a nonlinear dependency for the equivalent tube diameter and hence the thermal resistance of the filling on the U-tube diameter. It has also shown that for the same U-tube/borehole configuration, increasing of the U-tube legs spacing reduces the thermal resistance and approaching a minimum as the tube legs are located close to the borehole wall. Further, for the same borehole size, the thermal resistance exhibited a decrease as the U-tube size was increased and vice versa. At a borehole size of (75) mm, shank spacing to tube diameter ratio of (2), and a grout thermal conductivity of (0.78) W/m.K, the borehole total thermal resistance of the (9.52) mm U-tube size was higher than that of the (19.05) mm by (74)%. The model revealed that the grout thermal conductivity plays an important role in the thermal resistance assessment; the latter showed a decrease as the thermal conductivity increases to the highest test value of 1.9 W/m.K. The predicted thermal resistance was compared with other available correlations in the open literature and found to be consistent in the data trend and magnitudes with acceptable margin.

Design optimization of the borehole system for a plus-Energy kindergarten in Oslo, Norway

ABSTRACTThis paper presents the case study of a newly constructed 1600 m2 kindergarten building in Oslo, Norway. The building has been designed within the framework of the Norwegian Research Council Project LowEx, which aims at engineering solutions to achieve a seasonal coefficient of performance (SCOP) of 6–10 for heating, a seasonal energy efficiency ratio (SEER) of 80–100 for cooling, and an 80% reduction in the purchased electric energy for heating and cooling of the buildings. Several architectural and technical measures have been implemented in the case study building to meet these requirements. This paper first provides an account of the design measures implemented in the building to achieve the ambitious energy performance targets. It then focuses on the design of the ground source heating and cooling system for the building and presents the preliminary design of the borehole system to provide low-temperature heating and high-temperature cooling to the kindergarten. The possibility of improving the borehole system design by optimizing the solar heat gains through the building envelope to balance the ground thermal loads is explored next. Finally, the effect of uncertainties in the design input values of ground thermal conductivity, effective borehole thermal resistance, and undisturbed ground temperature on the final design of the borehole system is evaluated.

Thermo-hydraulic performance of the U-tube borehole heat exchanger with a novel oval cross-section: Numerical approach

In this paper, a novel U-tube Borehole Heat Exchanger (BHX) with an oval cross-section is proposed for improving the performance of Ground Source Heat Pump systems (GSHP). To study the thermo-hydraulic performance of Borehole Heat Exchanger (BHX) with an oval U-tube: A three-dimensional, symmetric and unsteady state multi-physics Computational Fluid Dynamic (CFD) simulations are carried out by ANSYS FLUENT environment. The conjugate heat transfer and fluid flow are simulated. Borehole thermal resistance, fluid inlet and outlet temperatures, fluid temperature profile along pipe length and pressure drop inside the tube are calculated and analyzed. The oval shape dimensions are adapted, then a comparison between the circle and the oval cross-sections is carried out; four different cases are investigated. As a result, the CFD results show a good agreement with their experimental pairs via Thermal Response Test (TRT) test which was done during September 2017 by our laboratory members. Besides, the results show that the oval shape enhances the heat transfer process and achieves the lowest borehole thermal resistance of 0.125 m K/W, also, it decreases the thermal resistance by 18.47% compared with the circular one. Besides, the oval shape shows a pressure drop of 32% less than the circular tube. Consequently, the oval shape could enhance the coefficient of performance (COP) of GSHP, decrease the borehole length, decrease the operating pressure and cut down initial and operating costs. Also, it fit easily inside tight borehole, so it is preferable in high-densely urban areas.

Thermophysical parameters from laboratory measurements and in-situ tests in borehole heat exchangers

Laboratory and in-situ tests were carried out with the aim of investigating underground thermal properties and mechanisms of heat transfer of a pilot ground-source heat pump system consisting of two borehole heat exchangers. Samples of grouts used for filling the boreholes and of the main lithotypes penetrated by drilling were analysed in the lab for assessing their thermal properties. Grouts with different mechanical characteristics and similar mineral composition were used in the two holes. The grout thermal conductivity ranged from 1.65 to 2.13 W m−1 K−1 and thermal diffusivity from 0.61 to 0.80 μm2 s−1. Thermophysical measurements on the lithotypes showed the largest thermal conductivity (2.66 W m−1 K−1) in sandstones, whereas pelite denoted the lowest value (1.82 W m−1 K−1). The average thermal diffusivity was 0.73 μm2 s−1. The in-situ experiments included thermal logs, to evaluate the undisturbed underground temperature, and thermal response tests, which were interpreted with the moving line-source model to infer the underground thermal conductivity, the borehole thermal resistance and the groundwater velocity. The inferred thermal conductivity varied from 2.09 to 2.48 W m−1 K−1 and thermal resistance from 0.188 to 0.193 m K−1 W−1. Values of Darcy velocity ranged from 5 to 9 × 10−7 m s−1. After the heat injection tests, the temperature variation with time was recorded at 20-m-depth intervals in both holes. The temperature-time series were used to assess the thermal conductivity variation with depth. In both boreholes, thermal conductivity was little variable from 20 to 80 m depth (2.14–2.34 W m−1 K−1) and it increased (2.49–2.68 W m−1 K−1) at the hole bottom. The average thermal conductivity for the two boreholes was quite similar (2.29–2.32 W m−1 K−1) and consistent with the values obtained with the moving line-source model. The results obtained with this approach were also consistent with the laboratory measurements and gave estimates of thermal conductivity at different depths.

Bayesian inference of structural error in inverse models of thermal response tests

For the design of ground-source heat pumps (GSHPs), two design parameters, namely the ground thermal conductivity and borehole thermal resistance are estimated by interpreting thermal response test (TRT) data using a physical model. In most cases, the parameters are fitted to the measured data assuming that the chosen model can fully reproduce the actual physical response. However, two significant sources of error make the estimation uncertain: random error from experiments and structural bias error that describes the discrepancy between the model and actual physical phenomena. Generally, these two error sources are not evaluated separately. As a result, the suitability of selected models to correctly infer parameters from TRTs are not well understood. In this study, the Bayesian calibration framework proposed by Kennedy and O’Hagan is employed to estimate the GSHP design parameters and quantify the random and structural errors in the inference. The calibration framework enables us to examine structural errors in the commonly used infinite line source model arising due to the conditions in which the TRT takes place. Two in situ TRT datasets were used: TRT1, influenced by contextual disturbances from the outdoor environment, and TRT2, influenced by a strong groundwater flow caused by heavy rainfall. We show that the Bayesian calibration framework is able to quantify the structural errors in the TRT interpretation and therefore can yield more accurate estimates of design parameters with full quantification of uncertainties.

An hourly simulation method for the energy performance of an office building served by a ground-coupled heat pump system

Abstract Ground heat exchangers are key component of ground-coupled heat pump systems, and their thermal response is therefore very important for ground-coupled heat pump system design and operation. This paper proposes a new hourly simulation method, and uses it to study the performance improvement potential for the ground-coupled heat pump system. First, with an effective U-pipe shank spacing determined by the calculated and measured borehole thermal resistance, a reasonable and accurate fluid temperature prediction method is developed, and the hourly energy performance simulation method is also proposed accordingly with the Fast Fourier Transform superposition algorithm. This hourly simulation method is validated using experimental data collected from a well-designed ground-coupled heat pump experiment platform, which shows that the maximum absolute error for the predicted fluid temperature is smaller than 1.04 °C. Second, using the proposed hourly simulation method, a framework for the energy performance simulation of an office building served by the ground-coupled heat pump system is developed. Impact factors on ground-coupled heat pump system performance are systematically analyzed using this simulation method, and the results show that performance can be improved with shorter operation schedules and lower heat fluxes.

Determination of ground thermal properties for energy piles by thermal response tests

Thermal properties of ground heat exchanger (GHE) such as effective thermal conductivity and borehole thermal resistance are commonly measured in the field by thermal response tests (TRTs). TRT has been proved to be a consolidated method to determine thermal properties of traditional borehole heat exchangers (BHEs). However, there is still lack of data for adopting TRT on energy piles with often a large diameter and deficiency in validation of TRT results with geological materials. In this study, ground thermal properties for typical configured GHEs of energy piles are investigated. Three TRTs are conducted and the obtained results are analyzed. Effective thermal conductivity, λeff, of the ground derived by following the traditional linear source model shows large deviation as compared to the thermal conductivity of the geological materials. In order to determine λeff properly, the linear source model is modified and an equivalent radius, req, of energy piles is considered. The λeff estimated by the modified model shows a good agreement with thermal conductivity of the in situ geological materials. In addition, there has been no obvious correlation between borehole thermal resistances and thermal efficiency due to heat transport of energy piles that depends not only by borehole thermal resistance but also by the pile’s diameter and ground conditions. The findings drawn from this study indicate that the modified model is reasonable and useful in determining thermal properties of energy piles.

Explicit Multipole Formulas for Calculating Thermal Resistance of Single U-Tube Ground Heat Exchangers

Borehole thermal resistance is both an important design parameter and a key performance characteristic of a ground heat exchanger. Another quantity that is particularly important for ground heat exchangers is the internal thermal resistance between the heat exchanger pipes. Both these resistances can be calculated to a high degree of accuracy by means of the well-known multipole method. However, the multipole method has a fairly intricate mathematical algorithm and is thus not trivial to implement. Consequently, there is considerable interest in developing explicit formulas for calculating borehole resistances. This paper presents derivation and solutions of newly derived second-order and higher-order multipole formulas for calculating borehole thermal resistance and total internal thermal resistance of single U-tube ground heat exchangers. A new and simple form of the first-order multipole formula is also presented. The accuracy of the presented formulas is established by comparing them to the original multipole method. The superiority of the new higher-order multipole formulas over the existing formulas is also demonstrated.

A Study on the Effects of Design Parameters of Vertical Ground Heat Exchanger on the Borehole Thermal Resistance

A Study on the Effects of Design Parameters of Vertical Ground Heat Exchanger on the Borehole Thermal Resistance

Evaluation of the Internal and Borehole Resistances during Thermal Response Tests and Impact on Ground Heat Exchanger Design

The main parameters evaluated with a conventional thermal response test (TRT) are the subsurface thermal conductivity surrounding the borehole and the effective borehole thermal resistance, when averaging the inlet and outlet temperature of a ground heat exchanger with the arithmetic mean. This effective resistance depends on two resistances: the 2D borehole resistance (Rb) and the 2D internal resistance (Ra) which is associated to the short-circuit effect between pipes in the borehole. This paper presents a field method to evaluate these two components separately. Two approaches are proposed. In the first case, the temperature at the bottom of the borehole is measured at the same time as the inlet and outlet temperatures as done in a conventional TRT. In the second case, different flow rates are used during the experiment to infer the internal resistance. Both approaches assumed a predefined temperature profile inside the borehole. The methods were applied to real experimental tests and compared with numerical simulations. Interesting results were found by comparison with theoretical resistances calculated with the multipole method. The motivation for this work is evidenced by analyzing the impact of the internal resistance on a typical geothermal system design. It is shown to be important to know both resistance components to predict the variation of the effective resistance when the flow rate and the height of the boreholes are changed during the design process.

Numerical Simulation of Soil Thermal Response Test with Thermal-dissipation Corrected Model

Based on the duct ground heat storage model on TRNSYS software, a thermal-dissipation-corrected transient model which takes the heat dissipation from ground and testing tube surfaces into consideration is established. An experimental platform is built for in-situ thermal response test (in-situ TRT) in Shandong Province, China. The presented model is verified by in-situ TRT with similar inlet and outlet temperatures of borehole heat exchanger (BHE). Furthermore, the key parameters, such as injected heat power, circulation flowrate, etc. are analyzed to study the influences on identified soil thermal conductivity, borehole thermal resistance and heat flow per unit length of BHE. It is showed that test duration has the largest impact on identified soil thermal conductivity, followed by injected heat power, abandoned initial hours, the circulation flowrate and backfill material conductivity; injected heat power has the largest influence on heat flow per unit length of BHE.

Borehole heat exchanger with nanofluids as heat carrier

This paper presents numerical study on the use of nanofluids to replace conventional ethylene glycol/water mixture as heat carrier in a BoreHole Heat Exchanger. Nanofluids contain suspended metallic nanoparticles: increasing their concentration, in comparison to the base fluid, the thermal conductivity increases and the volumetric heat capacity decreases. The first effect is positive for the reduction of borehole thermal resistance, since it causes the grow of fluid convective heat transfer coefficient, while the second one is detrimental, due that it decreases the heat transfer between fluid and borehole wall.A numerical model based on energy and momentum balances is used to evaluate which is the best nanofluid with low nanoparticles volumetric concentration (0.1%–1%) that ensures the highest decreases of borehole thermal resistance and minimum increases of pressure drop among silver, copper, aluminium, alumina, copper oxide, graphite and silicon oxide. Moreover, a simple economic analysis was done.Results show that copper is characterized by highest borehole thermal resistance reduction, that reaches the value of about 3.8%, in comparison to that of base fluid, when nanoparticles volumetric concentration is 1%, but also the second one highest pressure drop. In this case, the cost of copper-based nanofluid is about 10€m−1, i.e. about 12% of total cost of BoreHole Heat Exchange – Ground Source Heat Pump system.

Bayesian inference for thermal response test parameter estimation and uncertainty assessment

The effective ground thermal conductivity and borehole thermal resistance constitute information needed to design a ground-source heat pump (GSHP). In situ thermal response tests (TRTs) are considered reliable to obtain these parameters, but interpreting TRT data by a deterministic approach may result in significant uncertainties in the estimates. In light of the impact of the two parameters on GSHP applications, the quantification of uncertainties is necessary. For this purpose, in this study, we develop a stochastic method based on Bayesian inference to estimate the two parameters and associated uncertainties. Numerically generated noisy TRT data and reference sandbox TRT data were used to verify the proposed method. The posterior probability density functions obtained were used to extract the point estimates of the parameters and their credible intervals. Following its verification, the proposed method was applied to in situ TRT data, and the relationship between test time and estimation accuracy was examined. The minimum TRT time of 36 h recommended by ASHRAE produced an uncertainty of ∼±21% for effective thermal conductivity. However, the uncertainty of estimation decreased exponentially with increasing TRT time, and was ±8.3% after a TRT time of 54 h, lower than the generally acceptable range of uncertainty of ±10%. Based on the obtained results, a minimum TRT time of 50 h is suggested and that of 72 h is expected to produce sufficiently accurate estimates for most cases.

Correlations to determine the mean fluid temperature of double U-tube borehole heat exchangers with a typical geometry

In the evaluation of thermal response tests and in the dynamic simulation of ground-coupled heat pumps, the mean temperature Tm of the working fluid in a borehole heat exchanger is usually approximated by the arithmetic mean of inlet and outlet temperature. In thermal response tests, this approximation causes an overestimation of the thermal resistance of the heat exchanger. In the dynamic simulation of ground-coupled heat pumps, this approximation introduces an error in the evaluation of the outlet temperature from the ground heat exchangers. In this paper, by means of 3D finite element simulations, we provide tables of a dimensionless coefficient that allows an immediate evaluation of Tm in any working condition, with reference to double U-tube borehole heat exchangers with a typical geometry. These tables allow a more accurate estimation of the borehole thermal resistance by thermal response tests and a more accurate evaluation of the outlet temperature in dynamic simulations of ground-coupled heat pump systems. Criteria for the extension of the results to other geometries are also provided.

Validation of borehole heat exchanger models against multi-flow rate thermal response tests

A recently developed vertical borehole ground heat exchanger model that accounts for transit time effects and time-varying short-circuiting heat transfer has been validated against two multi-flow-rate thermal response tests (MFR-TRT). The MFR-TRT, when performed with a wide range of flow rates, results in significant changes in the borehole thermal resistance, the borehole internal thermal resistance, and the short-circuiting heat transfer between the two legs of a single U-tube. The model accounts for short-circuiting by an analytically computed weighting factor that is used to determine the mean fluid temperature. The weighting factor portion of the model can be readily utilized in other ground heat exchanger models that currently rely on a simple mean fluid temperature. Use of the weighting factor is shown to give significantly better estimations of entering and exiting fluid temperature than using the simple mean fluid temperature. The new model is also compared to an alternative approach − using an effective borehole thermal resistance. While both the effective borehole thermal resistance model and the weighting factor give quite good results a few hours after a step change in flow rate, the weighting factor model gives much better results in the first few hours after a step change in flow rate.

A Numerical Study on the Impact of Grouting Material on Borehole Heat Exchangers Performance in Aquifers

U-pipes for ground source heat pump (GSHP) installations are generally inserted in vertical boreholes back-filled with pumpable grouts. Grout thermal conductivity is a crucial parameter, dominating the borehole thermal resistance and impacting the heat exchanger efficiency. In order to seal the borehole and prevent leakages of the heat carrier fluid, grouting materials are also hydraulically impermeable, so that groundwater flow inside the borehole is inhibited. The influence of groundwater flow on the borehole heat exchangers (BHE) performance has recently been highlighted by several authors. However groundwater impact and grouting materials influence are usually evaluated separately, disregarding any combined effect. Therefore simulation is used to investigate the role of the thermal and hydraulic conductivities of the grout when the BHE operates in an aquifer with a relevant groundwater flow. Here 3 main cases for a single U-pipe in a sandy aquifer are compared. In Case 1 the borehole is back-filled with the surrounding soil formation, while a thermally enhanced grout and a low thermal conductivity grout are considered in Case 2 and Case 3 respectively. Simulations are carried out maintaining the inlet temperature constant in order to reproduce the yearly operation of the GSHP system. For each of the 3 cases three different groundwater flow velocities are considered. The results show that a high thermal conductivity grout further enhances the effects of a significant groundwater flow. The conditions when neglecting the grout material in the numerical model does not lead to relevant errors are also identified.

An Experimental Study on the Thermal Performance Evaluation of SCW Ground Heat Exchanger

In recent years, application of the standing column well (SCW) ground heat exchanger (GHX) has been noticeably increased as a heat transfer mechanism of ground source heat pump (GSHP) systems with its high heat capacity and efficiency. Determination of the ground thermal properties is an important task for sizing and estimating cost of the GHX. In this study, an in situ thermal response test (TRT) is applied to the thermal performance evaluation of SCW. Two SCWs with different design configurations are installed in sequence to evaluate their effects on the thermal performance of SCW using a single borehole. A line source method is used to derive the effective thermal conductivity and borehole thermal resistance. Effects of operating parameters are also investigated including bleed, heat injection rate, flow rate and filler height. Results show that the effective thermal conductivity of top drawn SCW (Type A) is 11.7% higher than that of bottom drawn SCW (Type B) and of operating parameters tested bleed is the most significant one for the improvement of the thermal performance (40.4% enhanced in thermal conductivity with 10.9% bleed).

Impact of grout thermal conductivity on the long-term efficiency of the ground-source heat pump system

Quality cementation of a borehole heat exchanger is often considered to be important parameter when analysing long-term heat pump efficiency. Implementing low conductivity grout leads to an increase of borehole thermal resistance and poor heat transfer. This paper analyses one real retrofit project which comprises of 16 borehole heat exchangers with thermal enhanced grout completion. Novel approach of testing the borehole heat exchanger was implemented, so called Steady-State Thermal Response Step Testing, which incorporate series of power steps to determine borehole capacity at steady-state heat transfer conditions and thermogeological properties of ground. Paper objective was to quantify what is real benefit by implementing more expensive thermal enhanced grout, compared to traditional bentonite, silica-sand grouts. Simulation of borehole temperatures and coefficient of performance (COP) changes over time was carried out with Ground Loop Design(GLD), taking into account four different grout mixes. Resultsshown that in thermogeological environment with mediocre thermal conductivity, distinction in COP was modest for each of the four analysed grouting material during 30 years. Comparing investment costs with savings in electricity for each grout mix case, conclusion is that there is no real benefit of implementing enhanced grout in mediocre thermal conductivity environment such as marly-clays.