Performance Of Borehole Heat Exchanger Research Articles

Comparison of simulation tools for optimizing borehole heat exchanger field operation

Model predictive control (MPC) is a promising approach for optimizing the performance of borehole heat exchangers (BHEs) in ground-source heat pump systems. The central element of MPC is the forward model that predicts the thermal dynamics in the ground. In this work, we validate the prediction accuracy of four BHE modeling approaches against real-world measurement data across various operational events and timescales. We simulate the fluid temperature leaving a BHE using a fully discretized 3-D numerical model, a resistance–capacitance model, a g-function model, and a hybrid model. The simulated temperatures are compared to measured temperatures using three validation metrics that quantify temperature offset, noise, and accuracy. The main reason for a mismatch between measured and modeled temperatures is a temperature offset of the simulated temperature. To remove this effect, the models were calibrated for their most sensitive parameter, the ground temperature, and their prediction accuracy over 4 years was evaluated. Thereby, model calibration seems to be a viable solution to account for an unknown load history. The results show that the resistance–capacitance model provides decent predictions in the short term and the g-function model in the long term. However, both models are strongly dependent on accurate calibration. The hybrid model provides the most accurate short and long-term predictions and is less dependent on calibration. Still, its integration into optimization syntax poses challenges compared to the other models. Although not yet applied in model predictive control, the hybrid model stands out as a promising choice for optimizing BHE field operations across various timescales.

Assessing the impact of borehole field data on AI-based deep learning models for heating and cooling prediction

Sensor readings play a critical role in the prediction performance of AI-based data-driven fault-detection and diagnostics for borehole heat exchangers in heating and cooling systems. The accuracy and algorithm selection of individual deep neural networks are well-investigated research topics but the impact of quality of sensors data on algorithm selection and its further impact on prediction performance has not been investigated and quantified for geothermal heating/cooling systems. The objective of the current work is to investigate the impact of data quality, available as measured time-series data from the borehole field, on the prediction accuracy of the deep neural networks-based models. In the current work four different data sets (D-1, D-2, D-3, D-4) are used for data-driven modeling using four different deep machine learning approaches, i.e., LSTM, BD-LSTM, GRU and CNN. The developed AI models have been trained, validated, and tested using the real-field data sets with same number of independent input parameters i.e., inlet temperature and mass flow rate to predict the fluid outlet temperature. The validated AI models are used to predict the performance of borehole heat exchanger to meet the heating and cooling loads. The prediction results show that the BD-LSTM outperforms other deep neural networks in terms of accuracy. With the data sets D-1, D-2, D-3 and D-4 it was observed that BD-LSTM had lowest MAPE of 2.99 %, 0.22 %, 6.39 % and 2.21 % respectively. Several experiments show that the BD-LSTM and LSTM models are better than the other used approaches in terms of prediction accuracy, but larger number of trainable parameters results in more computation time than CNN and GRU respectively. More data points with higher sampling frequency, results in more accurate prediction as depicted by the results from D-2 and D-4. Although, the results showed that the data collection interval is crucial, but different test runs indicate that avoiding the loss of time-series data during the cleaning process is equally important to accurately capture the Seasonality and Trend of the heating/cooling data. This is realized in case of D-1 where almost 58 % of the values were removed as outliers and these missing values were needed to be approximated which also affected the model prediction accuracy. The presented analysis highlights that the LSTM based DNN model has the capability to predict borehole heat exchanger performance accurately and the predictions are highly dependent on data frequency, the quality as well as data quantity from geothermal installations used to train the model.

Development of a novel cement-based grout with enhanced thermal and sealing performance for borehole heat exchangers

Filling a borehole with proper grout is important for maintaining the safety and performance of borehole heat exchangers (BHEs). The current grouts have problems of large shrinkage and low thermal conductivity, which may restrain BHE efficiency and pose safety risks. To address this, this study explores solutions to enhance the thermal and sealing performance of cement-based grout, specifically for BHE applications. A novel grout, composed of ordinary Portland cement (OPC)-calcium sulfoaluminate (CSA) cement-gypsum ternary system and graphite, was developed and thoroughly examined. The developed grout exhibited good flowability and volumetric stability, meeting the required compressive strength in one day. It displayed high thermal conductivity, ranging from 1.606 to 2.504 W/mK at oven-dry conditions and 2.173 to 3.532 W/mK at saturated conditions. At last, numerical modeling was conducted to compare the thermal performance of boreholes using the novel grout developed in this study and a conventional grout. Numerical models prove that using developed grout can improve the thermal performance of boreholes and decrease the borehole resistance, thus lessening the required borehole length, further reducing the costs and carbon emissions of borehole construction. Overall, the developed grouts showcased enhanced thermal and sealing performance. They can be applied in BHE systems not only to enhance operational efficiency and safety, but also to reduce costs and carbon footprints.

Investigating the impact of borehole field data’s input parameters on the forecasting accuracy of multivariate hybrid deep learning models for heating and cooling

Heat pump systems coupled with Borehole heat exchanger (BHEx) are crucial for the efficient and cost-effective heating and cooling of buildings. Accurate performance forecasting of BHEx can help to reduce energy consumption, optimize system design, improve system reliability and ultimately contribute to the sustainability of the built environment. The performance of deep neural networks, including hybrid deep learning algorithms, is dependent on the number and true identification of input parameters used to train the model. While research on the accuracy and selection of individual deep neural networks is well-investigated, the impact of selected input parameters on hybrid algorithm selection and its subsequent influence on multi-step forecasting performance has not been quantified specifically for geothermal heating/cooling systems. The focus of the current work is to investigate and quantify this impact on multivariate hybrid deep learning algorithms for forecasting borehole heat exchanger outlet temperature. In the current work the developed hybrid AI models (LSTM-CNN, GRU-CNN, CNN-LSTM, CNN-GRU) have been trained, validated, and tested using the measured borehole-field data with variable number of input parameters (3,4,5 and 6). The validated AI models are used to forecast 24 h ahead performance of borehole heat exchanger to meet the heating and cooling loads. The most essential input parameters for the data-driven AI models are determined via an importance-level analysis by using Explainable AI techniques of Shapley Additive Explanations (SHAP). The findings show that increasing the number of input parameters has the potential to enhance the multi-step forecasting accuracy of the examined deep learning models. However, it is crucial to note that there is a point where including additional input parameters may not lead to further improvements. Therefore, it is essential to identify the optimal number of input parameters that can maximize the accuracy of the hybrid deep learning models. Moreover, the analysis highlights that depending on the architecture of specific model layers, it is possible to eliminate redundant weather sensors associated with time variables without compromising the accuracy of the hybrid deep learning models.

Meso-scale investigation of the effects of groundwater seepage on the thermal performance of borehole heat exchangers

In view of the Net Zero Initiative, geothermal energy has been widely used. The borehole heat exchanger is a device that extracts geothermal heat from a borehole. Understanding the thermal performance of it is crucial for the analysis and design of a ground source heat pump system. In this paper, a novel methodology is proposed to evaluate the thermal performance of borehole heat exchangers based on a meso-scale model. With the images showing the meso-structure of the geomaterial, a method to quantify grain ellipticity, grain roughness, porosity and grading is proposed, with which the meso-structure of the geomaterial is reconstructed in a finite-element model. The Monte-Carlo simulation is then used in conjunction with the finite-element method to calculate the thermal release coefficient of borehole heat exchangers probabilistically. After validating the proposed methodology using experimental results, the effects of a list of factors on the thermal performance of borehole heat exchangers are investigated. The results show that grain ellipticity and roughness significantly affect the performance of borehole heat exchangers. The present study contributes to the field by quantifying the interactive effects of groundwater seepage and the physical characteristics of geomaterials and providing recommendations on the design of borehole heat exchangers.

In-situ investigation on the effects of groundwater flows around bore-hole heat exchangers

Although vertical Borehole Heat Exchangers (BHE) is a booming technology for both cooling and heating buildings, several improvements could still be proposed in the dimensioning of such systems. Nowadays, most of the dimensioning methods consider only radial conductive heat flux around BHE using a homogeneous ground thermal conductivity determined from thermal response tests (TRT) or tables. Impacts of groundwater flows on the heat refurbishment around BHE are generally not explicitely considered. 
 Many numerical or analytical studies have investigated and quantified the positive impact of groundwater flows on the performance of BHE [4]. However, those results are rarely compared and validated with in-situ temperature measurements around BHE. Such measurements require the installation of temperature sensors in the ground around BHE.
 In this work, an experimental platform composed of 4 vertical BHE drilled at depths of 85 m has been exploited. The 4 vertical BHE cross a succession of horizontal geological layers (Fig. 1). The study focuses on the heat transfers in a 30-m thick sand unconfined aquifer layer, whose 17 m are saturated. Each BHE is equipped with PT100 (installed at the extremities of the unconfined aquifer and just below the groundwater table level). Based on the expected direction of groundwater fluxes, the upstream BHE is thermally activated with a pre-determined heat injection and duration. The temperature evolution is recorded by means of PT100 sensors in the activated BHE and in the three non-activated BHEs. Groudwater velocity in the upper part of the aquifer is characterized through a non-convential tracer test [1] performed in a piezometer drilled at the center of the 4 BHE (v= 7 10-7 m/s).
 A clear impact on the groundwater flows on the temperature field in the aquifer around the activated BHE is observed. To quantify the heat transfers in the ground around the activated BHE, a methodology was developed to infer the hydro-geothermal parameters of the ground, namely the intrinsic thermal conductivity, the volumetric heat capacity and groundwater velocity and direction. From an analytical solution considering conductive, advective and dispersive heat transfers [2], the hydro-geothermal parameters of the ground are obtained by fitting the measured to the predicted temperatures evolution (Fig. 2 – Table 1). The obtained hydro-geothermal parameters demonstrate (i) a groundwater velocity in the upper part consistent with the value measured in-situ, (ii) the important role of the saturated aquifer that significantly enhances the apparent thermal conductivity of the ground and, in case of groundwater flows, induces an anisotropic propagation of the temperature plume [3](Fig.3), as well as (iii) the non-uniform groundwater flows along the saturated part of the aquifer (Table 1).

Thermodynamic Performance Analysis of High Thermal Conductivity Materials in Borehole Heat Exchangers in the European Climate

While heat pumps have been acknowledged as a key enabling technology to achieve Net Zero goals, their uptake is limited by their performance and cost. In this paper, a simulation-based study is conducted to analyse the performance of ground source heat pumps (GSHPs) utilising high thermal conductivity materials for the borehole heat exchanger (BHE) pipe (1 W/mK) and grouting (3 W/mK) developed in the GEOCOND project. Exergy analysis is conducted to account for energy quantity and quality with a focus on BHE performance. An annual hourly simulation was performed using DesignBuilder V5.4 and Earth Energy Designer (EED4) for representative cool and hot locations in Europe—Stockholm and Valencia, respectively. For a constant BHE length, the results for Stockholm show that the high conductivity materials result in an increase of about 13% BHE exergy extraction compared to the standard grout and pipe, but no such improvement was observed for Valencia. The difference between outdoor temperature and its dynamic variation from the indoor setpoint is identified as a key factor in the overall GSHP exergetic performance. In future research, we propose a thorough life cycle analysis across diverse locations and varying indoor comfort criteria to pinpoint areas where the high thermal conductivity material can enable cost-effective, sustainable heating and cooling.

Improved Analysis of Borehole Heat Exchanger Performance

This paper provides recommendations for improved analyses of the performance of ground-coupled heat pumps. Most research on ground-coupled heat pumps focuses on improving the performance of borehole heat exchangers (BHE) and reducing system costs. However, the potential improvements are mainly assessed at the BHE level rather than considering the entire system incorporating a heat pump, circulation pump, and building needs. This paper shows that such an approach can be misleading, and improvements in BHE are significantly overestimated if the operation of the entire system is not simulated. For instance, improvements in pipe thermal conductivity (from 0.4 to 3 W/(m K)) result in a 7.57% improvement in BHE performance when simulating only a BHE (constant inlet temperature assumed). However, a more realistic simulation of the entire system shows that improvements at a system level are only 0.15%. Other important simulation aspects are also investigated, focusing on different choices regarding the sensitivity analysis method, flow condition type, and optimization strategy. The results suggest that modifications to individual BHE parameters have a limited impact on the overall system performance, while modifying all parameters simultaneously can lead to more significant reductions in total system energy consumption (6% in this study). Furthermore, the research also shows that the potential savings in investment costs (by reducing the borehole depth) outweigh potential savings in operational costs.

Numerical analysis of borehole heat exchanger using a single shape-stabilized phase change material in heating and cooling seasons

Borehole heat exchanger (BHE) plays a key role in ground source heat pump (GHSP) and underground thermal energy storage for heating and cooling, which can backfill with phase change material (PCM) to enhance its thermal performance. In this study, a novel PCM-grouted BHE which can run in both heating and cooling seasons by using a single shape-stabilized PCM (SSPCM) is presented and investigated, and the phase change hysteresis (PCH) of SSPCM is detected and applied to achieve the all-seasons run. A heat transfer model considering PCH for U-type BHE was established to evaluate the thermal performance of the novel BHE. And four kinds of backfill materials were concerned and compared to investigate the effect of initial temperature, and pipe spacing on the performance of the BHE and its characteristics of intermittent operation. The results show that octadecane SSPCM with PCH showed the best comprehensive performance in the two seasons with an energy storage rate of 68.9 W/m in heating and 75.9 W/m in cooling, presenting 21.4 % higher than BHE with conventional backfill material. Mean COP during cooling can be improved by 3.61 % when the thermal conductivity increases from 0.5 W/(m·K) to 2 W/(m·K). It also reveals that the selection of PCM for different regions depends on the ground initial temperature.

Long-Term Investigation of a Borehole Heat Exchanger (BHE) Array with the Influence of Same Inlet Temperature (SIT) and Same Heat Load (SHL) Boundaries

Borehole heat exchanger (BHE) array is a key element in the ground source heat pump system (GSHPS). The BHE array is traditionally simplified by assuming the same heat exchange rate for each BHE, which normally is not correspondent to the real applications. In this investigation, a TRT was firstly conducted for a site located in Shanxi Province (China) to get the local ground thermal properties. A numerical simulation framework was next verified by the TRT results. Subsequently, the ground thermal properties obtained by the TRT were used in the numerical simulation model to investigate the influence of two operation strategies on the ground temperature development of the surrounding soil and the BHE performance. The results showed that both ground temperature and BHE array performance decreased during the 8-year operation period of the BHE array in the heating mode. The ground temperature around the center BHE was lower in the same heat load (SHL) scenario (same heat load for the BHEs in the array) than in the same inlet temperature (SIT) scenario (same carrying fluid inlet temperature for the BHEs in the array), and this difference can be 2 °C in the 7th year. The annual average outlet temperature in the SHL scenario was also lower than that in the SIT scenario, especially in the long term. The investigation shows that SIT working strategy superiors the SHL strategy, providing a reference for the upcoming investigation on the design aspect of the GSHPS.

Study on influence of geotechnical characteristics on heat transfer performance of borehole heat exchanger

ABSTRACT The global energy demand has put forward new requirements for building cooling and heating. As a renewable energy, shallow geothermal resources have attracted much attention. Ground source heat pumps are an effective utilization form of geothermal resources, most of which use U-shaped borehole heat exchangers to exchange heat with rock and soil mass. Therefore, the thermal physical parameters of rock-soil and the stratum structure will affect the heat transfer performance of the borehole heat exchanger. In this paper, a sandbox test bench and a 3D single U numerical model are constructed for the main geological conditions in karst areas to explore the heat transfer law of borehole heat exchanger under special geological structure. The results indicate that: (1) The thermal conductivity of limestone is higher than that of dolomite and sandstone, and the limestone stratum has strong heat transfer capacity and the fastest heat recovery rate. In order to avoid the thermal accumulation caused by the long-term operation of the system, the recovery time of stratum temperature field shall be kept at 1:1 ratio to the operation time of the system; (2) The average heat exchange of the three layers is 22.6 W/m, which is better than that of the double layers 21.4 W/m and the single layer 18.4 W/m. The heat transfer capacity of layered strata is better than that of single-layer strata, and they have layered characteristics; (3) The heat transfer capacity of borehole heat exchanger increases with the increase in thermal conductivity along the vertical direction of the formation. Each increase of 0.4 W/(m·K) in the thermal conductivity of the lower rock-soil increases the heat exchange per unit well depth by about 6 W/m; (4) The calculation equation of heat transfer under multi-layer stratum structure is proposed by using the calculation method of comprehensive thermal conductivity.

Effect of seepage condition in geological stratification on thermal response test analysis of borehole heat exchanger

Determination of ground thermal properties is the prerequisites for the design of ground-coupled heat pump systems (GCHPs), and it is crucial for evaluating the thermal performance of borehole heat exchangers (BHEs). These parameters are usually obtained by in-situ thermal response test (TRT) based on infinite line source model (ILSM). Though the effect of the groundwater flow on the estimation of ground thermal parameters is considered in homogenous ILSM, the estimated deviation can be enlarged as seepage condition varies based on the BHE model in practical geological stratification. Based on the developed numerical layered seepage BHE model (NLSBM), this paper evaluates the effects of seepage location and seepage velocity on estimated accuracy of borehole thermal resistance and ground thermal conductivity. Relative error (RE) between effective thermal conductivity λeff and the thickness-weighted thermal conductivity λTW will be up to 30.5% with the increase of the thickness of the seepage layer. The relative error between λeff and λTW increases from 5.3% to 93.5% when seepage velocity changes from 1 × 10−6 m/s to 1 × 10−4 m/s. The minimum RE between borehole thermal resistance Rb,NLSBM and effective borehole thermal resistance Rb,eff is still up to 37% when seepage locates in the 5th layer. With the increase of the seepage velocity, the RE between Rb,eff and Rb,NLSBM is enlarged, and the minimum RE is up to 29.4% corresponding to the lowest seepage velocity of 1 × 10−4 m/s. The change of seepage velocity or seepage location has little effect on Rb,NLSBM, and the highest RE is only 0.2%.

Thermal Response Measurement and Performance Evaluation of Borehole Heat Exchangers: A Case Study in Kazakhstan

The purpose of the present work was to determine the thermal performance of borehole heat exchangers, considering the influences of their geometric configurations and the thermophysical properties of the soil, grout and pipe wall material. A three-dimensional model was developed for the heat and mass transfer in soil (a porous medium) and grout, together with one-dimensional conductive heat transfer through the pipe walls and one-dimensional convective heat transfer of the heat transfer fluid circulating in the pipes. An algorithm was developed to solve the mathematical equations of the model. The COMSOL Multiphysics software was used to implement the algorithm and perform the numerical simulations. An apparatus was designed, installed and tested to implement the thermal response test (TRT) method. Two wells of depth 50 m were drilled in the Almaty region in Kazakhstan. Gravel and till/loam were mainly found, which are in accordance with the stratigraphic map of the local geological data. In each well, two borehole heat exchangers were installed, which were an integral part of the ground source heat pump. The TRT measurements were conducted using one borehole heat exchanger in one well and the data were obtained. The present TRT data were found to be in good agreement with those available in literature. The numerical results of the model agreed well with the present TRT data, with the root-mean-square-deviation within 0.184 °C. The TRT data, together with the predictions of the line-source analytical model, were utilized to determine the soil thermal conductivity (λg = 2.35 W/m K) and the thermal resistance of the borehole heat exchanger from the heat transfer fluid to the soil (Rb = 0.20 m K/W). The model was then used to predict the efficiencies of the borehole heat exchangers with various geometric configurations and dimensions. The simulation results show that the spiral borehole heat exchanger extracts the highest amount of heat, followed by the multi-tube, double U-type parallel, double U-type cross and single U-type. It is also found that the spiral configuration can save 34.6% drilling depth compared with the conventional single U-type one, suggesting that the spiral configuration is the best one in terms of the depth and the maximum heat extracted. The simulation results showed that (i) more heat was extracted with a higher thermal conductivity of grout material, in the range of 0.5–3.3 W/m K; (ii) the extracted heat remained unchanged for a thermal conductivity of pipe material higher than 2.0 W/m K (experiments in the range of 0.24–0.42 W/m K); (iii) the extracted heat remained unchanged for a volumetric flow rate of water higher than 1.0 m3/h (experimental flow rate 0.6 m3/h); and (iv) the heat extracted by the borehole heat exchanger increased with an increase in the thermal conductivity of the soil (experiments in the range of 0.4–6.0 W/m K). The numerical tool developed, the TRT data and simulation results obtained from the present work are of great value for design and optimization of borehole heat exchangers as well as studying other important factors such as the heat transfer performance during charging/discharging, freezing factor and thermal interference.

Estimating the Subsurface Thermal Conductivity and Its Uncertainty for Shallow Geothermal Energy Use—A Workflow and Geoportal Based on Publicly Available Data

Ground-source heat pumps with borehole heat exchangers (BHE) are an efficient and sustainable option to heat and cool buildings. The design and performance of BHEs strongly depend on the thermal conductivity of the subsurface. Thus, the first step in BHE planning is often assisted by a map representing the thermal conductivity of a region created from existing data. Such estimates have high uncertainty, which is rarely quantified. In addition, different methods for estimating thermal conductivity are used, for example, by the German federal states, resulting in incomparable estimates. To enable a consistent thermal conductivity estimation across state or country borders, we present a workflow for automatically estimating the thermal conductivity and its uncertainty up to user-defined BHE lengths. Two methods, which assess the thermal conductivity on different scales, are developed. Both methods are (1) based on subsurface data types which are publicly available as open-web services, and (2) account for thermal conductivity uncertainty by estimating its lowest, mean, and maximum values. The first method uses raster data, e.g., of surface geology and depth to groundwater table, and provides a large-scale estimate of the thermal conductivity, with high uncertainty. The second method improves the estimation for a small, user-defined target area by calculating the thermal conductivity based on the available borehole data in that area. The presented approach’s novelty is a web-based geodata infrastructure that seamlessly connects data provision and calculation processes, with a geoportal as its central user interface. To demonstrate the approach, we use data from the federal state of Hamburg and compare the results of two target areas with the thermal conductivity estimation by the Geological Survey of Hamburg. Depending on the selected region, differences between the two estimates can be considerable (up to 1.2 W m−1 K−1). The differences are primarily due to the selection of the thermal property database and the consideration of wet and dry rock. The results emphasize the importance of considering and communicating uncertainty in geothermal potential estimates.

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.

Dynamic and Static Investigation of Ground Heat Exchangers Equipped with Internal and External Fins

Using fins on the inner and outer surfaces of pipes is one method to improve the heat transfer rate of ground heat exchangers (GHEs), thereby reducing the borehole depth and construction and operation costs. Results of 3D numerical studies of simple and finned U-tubes with outer and inner fins are evaluated for GHEs under similar physical conditions. Dynamic and static simulations show the effects of longitudinal fins on the thermal performance of borehole heat exchangers (BHEs) and heat transfer rate between circulating fluid and soil around pipes, while the dynamic tests include short timescale and frequency response tests. The results indicate that the maximum fluid temperature change is about 2.9% in the external finned pipe and 11.3% in the internal finned pipe compared to the finless pipe. The effects of the inlet velocity on temperature profiles, the patterns of the velocity and temperature contours due to the borehole curvature and the response times of the systems under various frequencies are also investigated in detail.

A new approach to modelling of seasonal soil temperature fluctuations and their impact on the performance of a shallow borehole heat exchanger

A borehole heat exchanger (BHE) could be used in heating and cooling systems as the ground temperature is normally higher than the average air temperature in winter and lower in summer. This difference in the temperature can be used to improve the efficiency of air conditioning systems (in summer) and heat pumps (in winter). The performance of a BHE is influenced by soil temperature that may have significant seasonal changes. These changes were largely ignored in previous BHE modelling approaches being a source for large errors in theoretical predictions. This paper presents a new internal source term approach to account for the (seasonal) fluctuation in soil temperature on a shallow BHE's performance. The proposed model is able to consider the heat flow in multi-ground-layers. The governing equations of the model are solved by using explicit finite different method. The model is validated using experimental data. The simulated soil temperatures with and without the internal heat source term showed that the maximum relative errors are 3.7% and 14% respectively. The use of high thermal conductivity grout resulted in a decrease in the borehole thermal resistance. The outlet fluid temperature under the continuous operation mode is higher than that operated under the intermittent mode.

Numerical Study on the Thermal Performance of a Single U-Tube Borehole Heat Exchanger Using Nano-Enhanced Phase Change Materials

To investigate the impacts of using nano-enhanced phase change materials on the thermal performance of a borehole heat exchanger in the summer season, a three-dimensional numerical model of a borehole heat exchanger is created in the present work. Seven nanoparticles including Cu, CuO, Al2O3, TiO2, SiO2, multi-wall carbon nanotube, and graphene are added to the Paraffin. Considering the highest melting rate and lowest outlet temperature, the selected nano-enhanced phase change material is evaluated in terms of volume fraction (0.05, 0.10, 0.15, 0.20) and then the shape (sphere, brick, cylinder, platelet, blade) of its nanoparticles. Based on the results, the Paraffin containing Cu and SiO2 nanoparticles are found to be the best and worst ones in thermal performance improvement, respectively. Moreover, it is indicated that the increase in the volume fraction of Cu nanoparticles could enhance markedly the melting rate, being 0.20 the most favorable value which increased up to 55% the thermal conductivity of the nano-enhanced phase change material compared to the pure phase change material. Furthermore, the blade shape is by far the most appropriate shape of the Cu nanoparticles by considering about 85% melting of the nano-enhanced phase change material.

Simulation of temperature and moisture fields around a single borehole ground heat exchanger: effects of moisture migration and groundwater seepage

This paper aims to investigate the effects of moisture migration and groundwater seepage on the heat transfer capacity of ground heat exchangers in stratified soils. A three-dimensional unsteady groundwater flow and heat transport model was established using finite volume method. Sixteen cases with different model considerations and initial soil conditions were simulated based on the proposed model. A group of 8 cases considering only transverse moisture migration and another group considering both transverse and longitudinal moisture migration were compared. The heat and moisture fields after 30 days of operation reveal that considering the change of saturation caused by vertical moisture transfer, the soil temperature field will be affected, but borehole outlet temperature was less influenced. The absolute value of outlet temperature difference between corresponding cases in the two groups is only about 0.2 °C. The position of groundwater seepage and arrangement of unsaturated soil layers with different degrees of saturation on heat transfer capacity of vertical ground heat exchanger were further explored. The results show that the longitudinal moisture migration would be made more influential by the existence of seepage layer, because the average relative deviation of inlet and outlet temperature difference between the corresponding cases of Group 1 and Group 2 was 1.34% when setting seepage layer and was 0.44% when without seepage layer. Heat transfer performance of borehole heat exchanger is also affected by the location of seepage layer. The average relative deviation of inlet and outlet temperature difference between the reference case and cases with seepage in the top, middle and bottom layers is 34.18%, 25.08% and 16.82%, respectively. The arrangement of unsaturated soil layers also has a certain effect. When the soil layer with low degree of saturation is located in the upper layer of soil, heat transfer capacity is better.

Laboratory investigation on the use of thermally enhanced phase change material to improve the performance of borehole heat exchangers for ground source heat pumps

Laboratory investigation on the use of thermally enhanced phase change material to improve the performance of borehole heat exchangers for ground source heat pumps