Performance Of Ground Heat Exchangers Research Articles

Evaluating tidal impact on ground source heat exchanger performance under fluctuating groundwater levels

Coastal regions undergo dynamic changes due to tidal forces, posing distinct challenges for groundwater management and utilization of ground source heat pump (GSHP) systems. This study examines the impact of tidal fluctuations on groundwater levels and the performance of ground heat exchangers (GHEs), specifically focusing on the suitability of horizontal directional drilling (HDD) technology in high-tidal zones. Itoman City in Okinawa Prefecture, Southern Japan, serves as an ideal location for this study due to its complex coastal hydrogeological conditions and increasing interest in renewable energy solutions. Employing a comprehensive approach involving field experiments, FEFLOW software simulations, and model validation, we aim to elucidate the intricate relationship between tidal dynamics, groundwater behavior, and GHE performance. Furthermore, our investigation assesses the effectiveness of HDD-GHEs installed in highly tidal-affected zones to determine the optimal installation depth for this type of GHE in coastal regions. According to our research, the layer at a depth of 30 m is most affected by tidal influence and fluctuations in groundwater levels in coastal regions. This layer is the most suitable for installing horizontal HDD-GHEs, with an average heat exchange rate of about 89 W/m. This rate is approximately four and two times higher than the depths above and below this layer, respectively, due to groundwater saturation and flow in the formation. Furthermore, we noticed that the heat exchange rate increases from the top to the bottom of this layer, but the extent of this increase diminishes as the depth increases. This is because deeper depths are less affected by tidal fluctuations in groundwater levels.

Study on heat transfer performance of a ground heat exchanger under different heat transfer mechanisms

Taking a ground source heat pump operating in summer as a case study, the heat transfer performance of a ground heat exchanger was studied under different heat transfer mechanisms, considering stratified soil conditions and only considering thermal conduction. It was concluded that when the aquifer was located in the lower layer of stratified soil, the heat transfer performance of the ground heat exchanger is improved by 4.07%–37.24% in the most favourable case and by 4.11%–51.3% in the least favourable case when water migration in unsaturated soil and seepage in saturated soil are incorporated. The influence of aquifer position on the heat transfer performance of the vertical ground heat exchanger was also analysed. The results show that the heat transfer performance of the ground heat exchanger is improved by 1.05%–2.0% and 0.35%–1.61%, respectively, when the aquifer is located in the upper and middle layers of stratified soil rather than in the lower layer of stratified soil. Using the results of this paper to revise some existing studies that do not consider complex soil conditions, more accurate and practical conclusions can be obtained.

Influences of seasonal changes of the ground temperature on the performance of ground heat exchangers embedded in diaphragm walls: A cold climate case from North China

Compared with ground heat exchangers (GHEs) in typical vertical boreholes with depth of 100–120 m, GHEs embedded in the diaphragm wall of buildings are more easily affected by periodic changes of the shallow ground temperature. Here we numerically solve the heat transfer model of the diaphragm wall with GHEs considering actual ground temperature boundary conditions in cold climate region, North China, and investigate the influences of seasonal variations of the ground temperature on heat transfer performance of GHEs. Results show that due to the above seasonal effect, the heat exchange performance of GHEs may decrease to different extent especially in relatively shallow diaphragm walls, thereby causing non-negligible design deviations. Under the present geometric and climate conditions, as the depth of GHEs increases, the maximum thermal influence distance from the diaphragm wall tends to increase during the cooling season and decrease during the heating season, respectively. The optimal spacing between GHE groups ranges from 0.5 to 0.9 m, and though a high construction cost, a smaller spacing is more favorable for improving the overall energy efficiency. The present work is expected to provide useful references for optimal design and performance prediction of GHEs embedded in diaphragm walls with similar climate conditions.

Experimental investigations on the thermal performance of a novel ground heat exchanger under the synergistic effects of shape-stabilized phase change material and nanofluid

Ground source heat pump (GSHP) is known as the most promising green energy utilization technology in the 21st century. However, the heat transfer efficiency of GSHP systems cannot be significantly improved owing to the limitations of heat transfer fluid and surrounding backfill material, which has become a major obstacle to the widespread application of the system. In this paper, efforts had been made to enhance the heat transfer performance of the ground heat exchanger (GHE) under the joint actions of structure and material. For this purpose, an experimental platform for horizontal spiral-coil GHE was built to study the synergistic effects of using shape-stabilized phase change material (SSPCM) as backfilling and CuO/water nanofluid as the heat transfer fluid on thermal performance of the GHE. The results showed that the heat transfer amount increased by 69.9% and the thermal resistance decreased by 81.77% under the synergistic effects of SSPCM and nanofluid. The ground thermal influence radius with SSPCM backfill was about 80% of that with sand backfill. Nanofluid and SSPCM promote and reinforce each other. The performance improvement effect was more significant under the synergistic effects of SSPCM and nanofluid. The findings of this study can help designer to develop high-efficiency GHE.

Performance Study of Ground Heat Exchanger Based on Thermal Conductivity of Hybrid Soil

The need for renewable energy sources has grown as the world has significantly changed, and fossil fuels have been used extensively in global perspective. As a result, renewable energy has replaced fossil fuels in many places around the world because it is better for the environment. Geothermal energy is the most efficient way to heat and cool space from all renewable energy sources. Geothermal renewable energy, in particular from ground heat exchangers (GHE), has enormous potential for use in construction (building). In order for the GHE to function appropriately (efficiently), it is critical (better) to have sufficient quantities of backfilling material. This material is used to fill the gap between the soil surrounding. The heat transfer rate from the air to the soil, which is controlled by thermal characteristics of the soil near the GHE pipe, determines the thermal performance of the GHE. Using some backfilling materials that have been thermally improved, the thermal properties of the soil around the GHE pipe can be improved as well. Therefore, the current study examines the impact of hybrid soils without moisture on the GHE performance. The hybrid soils comprise of two components: native soil and bentonite. A thermal property analyzer was used to measure the thermal conductivity of the hybrid soil. According to the study, compared to other grain sizes, native soil with a grain size of 2.0 to 2.5mm has the highest thermal conductivity value at 20% bentonite, which is 0.331 W/m.K. The effectiveness of the GHE system was assessed using a mathematical model, demonstrating that the GHE has significantly reduces temperatures along pipes with length of 0 to 16m. In a nutshell, once the thermal conductivity of hybrid soil increases, the performance of GHE will improve.

Investigation on local geothermal energy attenuation after long-term operation of ground heat exchanger with considering aquifer effect

The long-term operation of deep buried ground heat exchangers (GHE) for heat extraction would attenuate the local geothermal field. The variation of geothermal energy in the affected area over time is a subject worthy of dedicated study. In this paper, the heat transfer between GHE and surrounding geological strata including aquifuge and aquifer layers is modeled and numerically solved to investigate the variations of GHE performance and attenuation behaviors of ground temperature field. The results show that the reduction ratios of GHE performance in the aquifer and aquifuge layers between 1st and 8th years are 1.3% and 8.9%, respectively, due to the convective heat transfer induced by groundwater in the aquifer layer. The analysis of local temperature variation implies that the groundwater flow leads to the circumferential asymmetry of ground temperature reduction in aquifer layer, which also affects the temperature distribution in geological layers adjacent to the aquifer. The worst ground temperature recovery point appears on the borehole wall in the aquifuge layer, while it appears at a fixed position downstream in the aquifer layer. Taking the ground transient and initial temperature difference greater than 0.5°C as the threshold to characterize the attenuation region, the geothermal field attenuation area of the eighth year in the aquifer is about 4.5 times that of the first year, while it is about 5.1 ∼ 5.5 times that of the first year in the aquifuge layers. The analysis of variations of GHE performance and ground temperature field quantitatively evaluates geothermal attenuation behaviors in aquifer and aquifuge layers subject to the long-term operation of GHEs.

An experimental investigation on performance of microencapsulated phase change material slurry in ground heat exchanger

As a promising alternative to conventional heat transfer fluids, microencapsulated phase change material slurry (MPCS) was applied in ground heat exchanger (GHE) in this paper. A sandbox system embedded with horizontal serpentine GHE was installed and MPCS using N-eicosane (C20H42) as phase change material is prepared. To explore the merits of MPCS, a comparison study between performance of water and MPCS was conducted. In addition, the enhanced heat transfer mechanism of MPCS were studied by comparing performance of MPCS with different inlet temperature and particle concentration. The experimental results indicate that the heat transfer rate and pumping consumption of 25 wt% MPCS with inlet temperature of 30 °C are 10.5 and 2.8 times that of water, respectively. The heat load-to-pumping consumption ratio has an enhancement of 3.73 times in comparison to water. Obviously, considering the phase transition process, MPCS can efficiently improve thermal performance of GHE. However, in the non-phase transition temperature range, the performance of MPCS is even worse than that of water. The comprehensive performance factor of 15 wt% MPCS and 25 wt% MPCS with inlet temperature of 38 °C declined by 45.6% and 36.9% in comparison with pure water.

A three-dimensional full-scale model for predicting the performance of deep-buried ground heat exchanger considering groundwater flow

It is critical to precisely determine the performance of deep-buried ground heat exchangers involving different geological layers in engineering projects of the geothermal energy utilization. Compared to the existing approaches solving only conduction between the ground heat exchanger and the surrounding ground based on the two-dimensional model, this paper established a three-dimensional full-scale coaxial ground heat exchanger model containing both aquifer and aquifuge layers involving groundwater effect. The convective heat transfer in pipes was calculated using the empirical correlation of Nusselt number to improve the computation efficiency. The groundwater velocity in the energy governing equation was assumed to be constant and along with the horizontal direction to further simplify the computation. The developed method was applied to a 2800 m and 2600 m depth ground heat exchangers with different geological conditions, and was validated by the field test and published data. The relative errors are less than 6.9%. The thermal influence ranges of the aquifer layer and the aquifuge layer close to aquifer are asymmetric about the circumferential direction. The results demonstrated that the three-dimensional full-scale model is necessary for accurately evaluating the performance of ground heat exchangers when there is groundwater migration.

Comparison of influence factors on horizontal ground heat exchanger performance through numerical simulation and gray correlation analysis

Horizontal ground heat exchanger (HGHE) is a cost effective method to exploit geothermal energy due to its shallow installation depth, which meanwhile causes its performance sensitive to climatic conditions. However, quantitative effects of climatic conditions on the HGHE performance still remain unclear. Influence degrees of key factors on the HGHE performance are not yet studied. Therefore, we do not know which factors should be priorities for the HGHE design. To this end, influence degrees of all key factors, including injection mass flow rate, operation modes, heat storage, subsurface water flow, soil thermal conductivity, installation depth and especially atmospheric conditions (precipitation and air temperature), on the HGHE performance are comprehensively analyzed and compared through numerical simulation and gray correlation analysis. Results indicated that non-consideration of climatic conditions could underestimate HGHE outlet temperature in summer and winter. The intermittent operation mode showed much better performance than continuous one. Increasing installation depth and improving grout thermal conductivity were both beneficial for HGHE performance. The most significant impact factors on the HGHE performance were groundwater flow velocity > circulation flow rate > precipitation > air temperature. Based on simulation results, subsurface water flow, injection mass flow rate and climatic conditions should be priorities for the optimal design of HGHE.

Thermal performance of water‐based brines as a heat carrier for ground heat exchanger under severely cold environments

AbstractThe aim of the present study is to investigate the performance of water‐based brine solutions in the ground heat exchanger (GHX) for space heating applications under severely cold climate conditions. The conventional ethylene glycol (EG) and propylene glycol (PG)‐based water solutions have been considered for investigation. For the closed‐loop vertical U‐tube GHX, a mathematically built model is developed for ambient temperature below 0°C. The thermal performance analysis of GHX is carried out for the variation in fluid flow rate and brine solutions for different mixture percentages of water in EG and PG. The percentage of the mixture and fluid flow rate varied from 25% to 60% and 0.2 to 0.6 LPS, respectively. GHX performance parameters, namely convective coefficient, pressure drop, borehole resistance, and GHX effectiveness are determined for both EG and PG brines. Better performance of GHX is noticed with EG brines as compared to PG brines for similar operating conditions. The drop in convective coefficient from 22% to 44% in the case of PG brines as compared to EG brines with the increase in mixture percentage from 25% to 60%, for the same operating conditions. The pressure drops decreased from 15% to 74% for EG mixture as compared to PG mixture at considered flow rate and mixture percentage. The effectiveness and overall borehole resistance were higher for EG as compared to the PG solution.

A comprehensive investigation on U-tube ground heat exchanger performance considering insulation layer in deep geothermal system

This study established a full-size coupling model of unsteady flow and heat transfer between U-tube and ground considering the insulation layer, which was solved by the finite volume method. The model's reliability was validated through experimental data of geothermal U-tube in Xi'an. The maximum relative error of this model was 6.46% in 2–6 h, and the average relative error was 0.51% in 7–72 h. This model performed better than the other two models published. According to the outlet temperature, thermal power, quantity of heat, and the temperature distribution of the U-tube at different insulation depths, the optimal insulation depth was the intersection of the ground temperature and the temperature curve in the tube and the insulation depth was 1615.85 m. Analyzed the influence of flow rate and other parameters on outlet temperature and thermal power, the sensitivity of each factor to heat extraction power was obtained by orthogonal test. The results showed that the flow rate was the most sensitive factor to thermal power, and the percentage loss of water and the thermal conductivity of rock were generally sensitive to thermal power. This study could provide a theoretical reference for developing and utilizing u-tube geothermal in deep layers.

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

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

Experimental study on the thermal imbalance and soil temperature recovery performance of horizontal stainless-steel ground heat exchanger

Soil thermal imbalance has become a common problem for ground source heat pump (GSHP) system, leading to the fluctuation of ground temperature and decline of heat transfer efficiency. However, compared with vertical ground heat exchanger (GHE), there is a lack of research and evaluation on the soil thermal imbalance for horizontal GHE. Therefore, in this paper, to investigate the extent of soil thermal imbalance and its effects on performance of GSHP system, a sandbox experiment embedded with horizontal stainless-steel GHE was implemented. Various operation conditions were designed to explore effects of intermittent time, water content (rainfall) and buried depth of pipes on soil temperature distribution and restoration performance. The experimental data was validated by a numerical simulation. The results indicated that 5-day intermittent operation of GSHP leads to an increase of 6.41 ℃ in soil temperature. After 2-day natural recovery, soil temperature recovery rate can only reach 0.72. Increasing soil moisture content and intermittent control are effective solutions to mitigate thermal accumulation and improve thermal performance. The heat transfer performance of GHE with an intermittent ratio of 6:18 which GSHP system runs for 6 h and stops for 18 h during a day is 33% higher than that with the intermittent ratio of 14:10 and 10:14. The higher the water content, the lower the soil temperature fluctuation and the stronger the soil temperature recovery capacity. This can be attributed to the increase of soil thermal conductivity and capacity, while the decrease of thermal diffusivity.

Thermal performance of ground heat exchanger under winter conditions

The use of buried pipe ground source heat pump technology can provide a green and environmentally friendly solution for the winter bridge snow melting project. However, the factors affecting the heat transfer efficiency of buried pipes in winter conditions are still unclear. For the single U-bend buried pipe heat exchanger, numerical calculations were preformed to study the effects of different inlet temperatures, backfill material ratio, hole depth, and spacing on the heat extraction capacity and the surrounding soil temperature distribution. Combined with an actual bridge heating project, three single heat exchanger models with different buried depths and three heat exchanger group models with different spacing were established. The results show that when the system runs for 48 hours, the heat transfer power at 2 ℃ will increase by about 82% compared with that at 8 ℃. Reducing the content of bentonite in the backfill is beneficial to heat transfer. As the heat exchanger depth increases, the heat extraction capacity per linear meter decreases. The expansion of the heat exchanger spacing can reduce the thermal interference effect between each other. When running 8 hours, the heat transfer power at an interval of 4 m is increased by 5.4% compared with that at the interval of 3 m, and the heat transfer power at an interval of 5 m is increased by 1.9% compared with that at the interval of 4 m.

Study on Influence Factors of Horizontal Ground Heat Exchanger Performance Considering Climatic Conditions and Groundwater Flow

Study on Influence Factors of Horizontal Ground Heat Exchanger Performance Considering Climatic Conditions and Groundwater Flow

Numerical approach for sizing vertical ground heat exchangers based on constant design load and desired outlet temperature

Vertical ground heat exchangers (GHE), as part of a ground source heat pump system, have been utilized for energy extraction/injection from/to the ground, i.e. a renewable source of energy, in the heating/cooling months to reduce building energy utilization. The current available GHE sizing methods are mostly based on several over-simplified one or two dimensional analyses, which cannot predict its thermal behavior with the accurate and realistic boundary conditions or in series configurations. In this study, thermal performance of vertical GHEs is numerically investigated at a given constant load through solving full transient 3D Navier-Stokes equations inside the U-tubes and the energy equation all over the computational domain, i.e. both fluid and solid parts. There are three interconnected parameters, namely GHE load, the heat transfer fluid outlet temperature, and the GHE depth that influence its thermal performance and consequently, the required investment cost. Therefore, the thermal performance of GHEs is computed for different constant loads at different depths, and it is shown that at each constant load, there is a trade-off between the investment cost and the thermal performance. Furthermore, the GHEs thermal performance in parallel and series configuration is investigated at a fixed depth and different loads. In light of the obtained data, the required size of the GHEs for a test building with an imbalanced energy extraction/injection rate is derived and the thermal performance of the proposed cluster of GHEs is numerically simulated for 20 years.

A two-step parameter estimation method for estimating soil thermal properties of coaxial ground heat exchangers

Soil thermal properties are important parameters to design the coaxial ground heat exchanger (GHE). This paper adopts an accurate semi-analytical model to analyse the sensitivities of soil thermal properties on the performance and parameter estimation of coaxial GHE, and then proposes a two-step parameter estimation method by using early-time thermal response test (TRT) data to estimate the soil volumetric heat capacity and using long-time TRT data to estimate the soil thermal conductivity, and finally verifies the proposed method by comparison with another parameter estimation method and experimental data. The main findings are as follows: there is strong correlation between soil thermal conductivity and coaxial GHE performance, and the correlation between soil volumetric heat capacity and coaxial GHE performance for the early 10 hours is stronger than that after 10 hours; long-time TRT data can lead to more accurate estimation of soil thermal conductivity than early-time TRT data, and early-time TRT data can lead to more accurate estimation of soil volumetric heat capacity than long-time TRT data; the soil thermal properties (especially volumetric heat capacity) estimated by the proposed method have higher precision than those by another method, which uses long-time TRT data to estimate the soil thermal properties.

Study on Influence Factors of Horizontal Ground Heat Exchanger Performance Considering Climatic Conditions and Groundwater Flow

Horizontal ground heat exchanger (HGHE) is a cost effective method to exploit geothermal energy due to its shallow installation depth, which meanwhile causes its performance sensitive to the climatic conditions and groundwater flow. However, effects of key factors on HGHE performance when considering climate and groundwater flow still remain unclear, which is vital for HGHE optimal design. To this end, based on a 3D numerical model considering atmosphere-HGHE-ground interaction for a HGHE, this paper comprehensively investigated effects of operation modes, heat storage, precipitation, air temperature, groundwater flow velocity, ground thermal properties, circulation flow rate and installation depth on HGHE performances. The influence degrees of these factors were compared through the gray correlation analysis. The results indicated that non-consideration of climatic conditions could underestimate HGHE outlet temperature in summer and winter. The intermittent operation mode showed much better performance than continuous one. Increasing installation depth and improving grout thermal conductivity were both beneficial for HGHE performance. The gray correlation analysis demonstrated that the most significant impact factors on HGHE performance were groundwater flow velocity > circulation flow rate > precipitation > air temperature. Consequently, it is necessary to consider groundwater flow and climatic conditions for the precise evaluation of HGHE performance.

Impact of soil model complexity on the long-term thermal performance prediction of a new shallow bore ground heat exchanger

Shallow bore ground heat exchangers have been investigated in recent years because they have the potential to reduce the initial cost of the ground source heat pump system. Correctly modeling the transient heat transfer between the ground heat exchanger and the surrounding soil is essential for predicting a ground heat exchanger's performance. Simplification of the soil model could increase computation speed but sacrifice accuracy. This study investigates the impact of the soil model complexity on the annual performance prediction of a new shallow bore ground heat exchanger, the underground thermal battery (UTB). A simple 1D soil model and a more detailed 2D soil model were implemented, and they were validated against a 3D soil model. The resulting predictions of the UTB’s response from the 1D and 2D models to a given thermal load in different climates were compared. The results show that the root mean square differences between the hourly temperatures of the UTB during a year predicted using the two soil models range from 1.17 °C to 3.39 °C. As a result, the difference in the annual power consumption of the heat pump was between 0.7% and 3.4%. Furthermore, the dimensions of the UTB affected its performance, and a longer UTB was less sensitive to the soil models.

Investigation of pipe materials and thermal conductivity of soil on the performance of ground heat exchanger operating under Malaysia climate

In nature, renewable energy is inherently free and must be implemented. The use of air conditioning and refrigerants that affect global warming is a serious issue. In building applications, renewable energy from the geothermal source, namely ground heat exchanger (GHE), has great potential. The main concept of GHE is utilizing the ground as an infinite thermal reservoir for cooling and heating to the fluid medium. In the GHE system, the air is used as a fluid medium of work. Because of the temperature difference between the air and underground temperature, the air cools in summer and gets heated in winter. In this present work, a study has been conducted to investigate the effect of pipe materials and thermal conductivity of soil on the performance of the GHE. The study acknowledges that the pipe materials do not give a significant effect on the performance of the GHE. Therefore, the lower thermal conductivity of pipe materials with low cost can be used in GHE implementation. The study also revealed that the range of thermal conductivity of soil which gives good ground heat exchanger performance is between 1.5 to 5 W/m·K. Besides, the length of the pipe was reduced from 25 to 10 meters.