Horizontal Ground Heat Exchanger Research Articles

Optimization of double layered horizontal directional drilled ground heat exchangers by water injection into the borehole

Ground Heat Exchangers (GHEs) drilled using Horizontal Directional Drilling (HDD) technology have been proven to be a cost-efficient way of installing ground source heat pump systems (GSHP). They can also be installed in places where conventional horizontal ground heat exchanger (HGHE) would not be possible e.g., due to lack of land.In this research, a double layered horizontal borehole was installed as a GHE in Saga City, Japan. The two holes had a diameter of 114.3 mm and a length of 59 m and 56 m, respectively, while reaching depths of 5 m and 9.5 m. A polyethylene pipe with an outer diameter of 60 mm was inserted as a borehole heat exchanger (BHE). In March 2022, a two days thermal response test (TRT) showed the influence of rainfall on the system. A numerical model developed in FEFLOW based on the GHE dimension and the region's geology was validated using the measured temperatures of the heat medium at the return point and outlet of the GHE during the TRT. The validated model was then used to carry out sensitivity analysis examining different conditions of water injection into the borehole. The result of the sensitivity analysis showed that water injection into the borehole can improve the performance of the system and reduce the temperature change of the heat medium. Varying the injection rate can improve performance to a certain degree and the injection point has a strong influence on the performance. The injection temperature was also found to play a role in the performance, but even high temperatures in the case of heating showed significant improvement. Lastly, the influence of permeability was investigated and showed that impermeable layers are more favorable for injection compared to permeable layers.

Thermal performance of three horizontal ground heat exchanger systems: comparison of linear-loop, spiral-coil and slinky-coil arrangements

Ground source heat pumps (GSHPs) are highly efficient for heating and cooling buildings. Among the different types of ground source heat pumps, horizontal ground heat exchangers (HGHEs) gaining popularity due to their cost-effectiveness, ease of installation and high regeneration capacity. However, studies on the interaction and influence of the local environment with the HGHE and the thermal performance influenced by the arrangement and performance of the HGHE are insufficient. This study uses a three-dimensional numerical model to simulate three types of HGHEs, including linear-loop, spiral-coil, and slinky-coil arrangements. Instead of just one unit, three units are connected in series or parallel. The model considers the energy balance and the heat transfer and exchange processes between the heat exchanger and the soil. Simulation of cooling and heating for 1 year revealed that the slinky-coil arrangement is the most affected by heat fluxes at the ground surface, because of its maximum length, which leads to its lowest coefficient of performance (COP) value. The thermal performance of linear-loop and spiral-coil arrangements is similar, but considering the material cost, the linear-coil arrangement has the lowest installation cost and operating cost for 30 years.

Temperature Field Modelas a Factor to Improve a Horizontal Ground Heat Exchanger of a Heat Pump System: Review of Scientific Approaches and the Content of the Analytical Model

The article is devoted to the research of issues related to the organization of control systems for heat pump installations (HPS). HPS is used to use low-potential energy of the environment in the heat supply of buildings. HPS with horizontal ground heat exchangers (HGHE) are of great importance for improving energy efficiency. The efficiency and reliability of HPS with HGHE strongly depends on the state of the soil temperature field. A large number of scientific publications are devoted to modeling soil temperature fields forHGHEparameterdetermination; a significant part of them is devoted to the study of vertical ground heat exchangers. Scientific developments presented in foreign publications on modeling the temperature field of the HGHE location environment are focused on the use of HPS in the cold supply mode and/or in the heat supply mode under conditions of a slight decrease in temperature, which excludes soil freezing. The aim of the study was to apply the existing methods of the analytical theory of thermal conductivity to develop a soil-environment TP model for the placement of a HGHE HPS; the developed model is intended for engineering calculations of the main design HGHEparameters; the model is also applicable to determine the annual specific heat flow as an important parameter in the optimal control system of the HPI. The development of the model is based on the condition of a two-element structure of the soil massif: under the HGHE and above the HGHE; the target parameter is the temperature of the soil mass, depending on the coordinate and time, disclosed through the indicator 'relative temperature rise'; in accordance with the provisions of the thermal conductivitytheory, the relative excess temperature is given for the cases of a semi-infinite body (soil mass under the HGHE) and a slab (soil mass above the HGHE). The proposed model is implemented on the example of two constituent entities of the Russian Federation. The results obtained made it possible to determine the minimum guaranteed possibility of using HPS for heat supply in these climatic conditions. The model presented in the article can be used as a context for further studies of the main parameters of HGHE HPS operation and optimization of HPS operation modes.

Performance evaluation of a new type of horizontal ground heat ex-changer: Coil-Column System (CCS)

Ground source heat pump system is well-known as an environmentally friendly and high-efficiency technology for heating and cooling the building. The horizontal ground heat exchanger (GHE) has been paid more attention to in recent years as it has a low construction cost and easy installation [1]. However, the drawback of horizontal GHE is the requirement of a large installation area [2]. In this study, the performance of a new GHE type, named Coil-Column System (CCS), is evaluated. CCS is expected to increase the total heat exchange rate of the GHE, which will result in reducing the required installation area.
 Firstly, the numerical analysis was conducted using COMSOL Multiphysics to investigate the heat exchange performance of the CCS, and previous exchanger types (e.g., spiral-coil type and straight-line ground heat exchanger (Figure 1(a)). The numerical model was validated using an in-door thermal response test (TRT) in the mock-up steel box with a dimension of 5m × 1m × 1m (Figure (1(b)) [2]. Afterward, the parametric study was conducted to evaluate the effect of the pitch of the coil and installation depth on the heat transfer efficiency of the CCS. Finally, the feasibility of the CCS was comprehensively accessed through the heat transfer performance and economic parameters (internal rate of return, payback period).
 Strong agreement between the outlet fluid temperature demonstrates that the proposed numerical model is suitable to simulate the heat exchange in the ground heat exchanger (Figure 1(c)). The comparison results of different GHE types indicate that the heat exchange rate of CCS is double that of the spiral-coil and even three times higher than that of the straight-line type (U-type, conventional type) (Figure 1(d)). The parametric study shows that a higher heat exchange rate is observed at a shorter pitch of CCS. However, the heat exchange rate of CCS is almost independent on the pitch after 168 h. This is attributed to the reduction in the temperature gradient of soil temperature and the inlet fluid temperature and thermal interference at the narrow space between pipes. In addition, an increase in installation depth results in an increase in the performance of CCS for both the short term and long term because the deeper CCS has a higher difference between fluid temperature and soil temperature.
 It should be noted that using the shorter pitch (longer pipe length) or increasing the installation depth causes a significant increase in the material cost, and excavation cost, respectively. The economic analysis results of CCS indicate that the installation depth of 4 m and the pitch of 0.15 m has the highest internal rate of return and shortest payback period since they can compromise between the material cost and heat transfer performance. Furthermore, although the investment cost (construction cost) of a CCS is significantly higher than that of the straight-line type and spiral coil type, its high heat exchange performance results in a shorter payback period (5.2 years compared with 12.0 years for straight-line and 7.0 year for spiral-coil) and higher internal rate of return (15.6% compared with 7.3% for straight-line and 13.9% for spiral-coil [2]) (Table 1). Based on the heat exchange efficiency and economic aspects, it is feasible to use the CCS as a new type of horizontal GHE to increase the total heat exchange capacity or reduce the area required for the installation and expect to spread the use of geothermal energy. In further study, the factors that may influence the performance of the CCS (i.e., weather change, rainfall infiltration, soil-atmosphere interaction) will be evaluated.

Performance Assessment of Horizontal Ground Heat Exchangers under a Greenhouse in Quebec, Canada

Among the various approaches to agriculture, urban greenhouse farming has gained attention for its ability to address food security and disruptions to global food supply chains. However, the increasing impact of climate change and global warming necessitates sustainable methods for heating and cooling these greenhouses. In this study, we focused on the potential of slinky-coil horizontal ground heat exchangers (HGHEs) to meet the energy demands of urban greenhouses, assuming they are installed beneath the greenhouse to optimize space utilization. Climate data, an energy consumption profile for a greenhouse being designed in La Pocatière (Québec, Canada) and in-situ ground thermal properties assessments were used to build numerical models using FEFLOW and to evaluate the performance of the HGHEs simulated. Four scenarios were simulated and compared to a base case, considering the greenhouse’s maintenance of a constant temperature above an HGHE limited to the greenhouse’s dimensions. Our findings reveal that a minimum of 7.1% and 26.5% of the total heating and cooling loads of a small greenhouse (133 m2 area) can be covered by HGHEs installed at a 1.5 m depth when there is no greenhouse above. When installed under a greenhouse with a constant inside temperature of 21 °C, the coverage for heating loads increases to 22.8%, while cooling loads decrease to 24.2%. Sensitivity analysis demonstrates that the constant temperature in the greenhouse reduces the system’s reliance on surface temperature fluctuations for both heating and cooling, albeit with reduced efficiency for cooling.

Performance evaluation of a novel horizontal ground heat exchanger: Coil-column system

Horizontal ground heat exchangers (GHEs), which are used for heating/cooling buildings, have garnered significant attention in recent years owing to their low construction cost and ease of installation. However, they require a large installation area due to their low efficiency. In this study, a new horizontal GHE type, named a coil-column system (CCS), is proposed, and its feasibility is investigated. CCS is expected to increase the total heat exchange rate of the GHE, thus reducing the required installation area. First, the heat exchange performance of the CCS is investigated and compared with existing exchangers using COMSOL Multiphysics. The numerical model was validated using an in-door thermal response test in a 5 m × 1 m × 1 m mock-up steel box. Afterward, a parametric study was conducted to evaluate the effect of coil pitch, installation depth, and space between the coil-column on the heat transfer efficiency of the CCS. Finally, the feasibility of the CCS was comprehensively evaluated by considering the heat transfer performance and economic parameters. From the results, it was found that the heat exchange rate of CCS was double and triple that of the spiral-coil and the straight-line types. The parametric study reveals that the pitch between the coil and the space between the coil-column affect the short- and long-term heat exchange performance of CCS, whereas the installation depth influences both short- and long-term performances. The economic analysis of the CCS indicates that at an installation depth of 4 m, a space between the coil-column of 0.2 m, and a pitch of 0.15–0.2 m have the highest internal rate of return because they offer a trade-off between the construction cost and heat transfer performance. Furthermore, using a controlled low-strength material as a thermal-enhanced grouting material increased the annual heat exchange rate by 25% and resulted in a notable increase in the internal rate of return (19.5% compared with 15.6% when using soil).

Enhancement of shallow ground heat exchanger with phase change material

Heat pumps perform better when coupled with ground as thermal source than with air. In literature, several studies and applications suggest and analyse the use of phase change materials (PCMs) coupled with single or double U-tube vertical borehole heat exchangers (BHEs). Usually, PCMs are mixed with the grouting material during the installation. An alternative solution to vertical BHEs is the use of horizontal ground heat exchangers (HGHEs). The present work investigates the possibility of coupling PCMs with a flat-panel HGHE installed inside a trench 2 m under the ground surface. The study analyses the case in which PCMs are adjacent to the HGHE, taking a cue from alternative coupling technologies which have PCMs added to the backfilling material of the trench where the HGHE is installed. The analysis has been conducted with COMSOL software tool. A simulation model of the system was developed to carry out a parametric analysis. The objective of the simulations is the investigation of the thermal behaviour of the HGHE patent pending coupled with PCMs under cycles of operation which represent how the heat pump could work in GSHP system. The results show the meaningful difference of using the PCM in direct contact with the HGHE.

A comprehensive study of Fin-Assisted horizontal ground heat exchanger for enhancing the heat transfer performance

A horizontal ground heat exchanger installation is less expensive than a vertical ground heat exchanger, but more land is required. The system’s required pipe length and land area can be decreased by improving the system. On the other side, adding fins is one strategy to increase heat transfer. In this paper, cylindrical fins are explored in horizontal ground heat exchangers for the first time to improve heat transfer and, the overall efficiency of the ground-source heat pump. These fins were examined by varying parameters such as length, diameter, position, and material. The heat transfer rate changes with and without fins were also investigated as soil properties changed. The heat transfer simulations in cooling mode for a 1D-3D model using COMSOL Multiphysics revealed that changing the fin diameter directly affects the outlet temperature and, the non-isothermal pipe flow (niofl) is used for the pipe. There is minimaldifference between improving and increasing the heat transfer rate when the fin length is increased to more than 1 m. Moreover, the distance between the ground heat exchanger and the fin is critical; if it exceeds 5 cm, it loses some effectiveness, although it is still useful for soil recovery. In general, the fin increases the soil contact area, and a fin of length one meter can increase heat transfer per unit of pipe length up to 20.7 %. Comparing horizontal and vertical ground heat exchangers in finned and non-finned modes revealed that both modes increase performance with the fin. Still, the horizontal pipe increases performance by about 3 % more than the vertical spiral with the same number of fins.

The performance of a Horizontal Ground Heat Exchanger (HGHE) under the seasonal ground energy storage behaviour

A well-known backfill soil was considered to be used as the backfill substitutive material. The hydrothermal properties of the backfill material were estimated in laboratory and then injected in a numerical framework considering the atmosphere-soil-HGHE interaction. Numerical simulations were performed for a HGHE installed in the compacted backfill soil and the local materials. Two heat storage scenarios at three different installation depths were also investigated. The results show that an inlet fluid temperature of 50°C in summer increases highly the system performance (13.7% to 41.4%) while the improvement is less significant (0% to 4.8%) for the ambient inlet temperature scenario. A deeper installation depth increases also the system performance.

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

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

Parametric optimization of a spiral ground heat exchanger by response surface methodology and multi-objective genetic algorithm

Horizontal spiral-coil type ground heat exchanger (HSGHE) has been increasingly used in ground source heat pump systems owing to its high heat transfer performance and low cost. This study proposes a method to optimize the design and operation parameters based on the combination of response surface methodology (RSM) and multi-objective genetic algorithm (MOGA). First, a 3D numerical model simulating the thermal–hydraulic characteristics of the HSGHE was built and the reliability of the model was validated through an indoor test rig. Subsequently, some parametric studies were performed to analyze the degree of influences that each input parameter on the outputs. On this basis, the Box-Behnken design method and RSM were used to establish response surface models between the input parameters (spiral diameter, pitch, and fluid velocity) and objective functions (net heat exchange rate and thermal performance capability) and determined the interactions between them. The results showed that the spiral diameter had the most significant influence, followed by the pitch, and fluid velocity. Finally, based on the developed models, the MOGA employing the Pareto optimum solutions were used to optimize the design and operation parameters and determine their optimal combination: spiral diameter of 0.4 m, pitch of 0.1 m, and fluid velocity of 0.4 m/s. The findings of this study may assist designers in the development of high efficiency ground heat exchangers.

Evaluating the long-term sustainability of geothermal energy utilization from deep coal mines

Deep underground mines offer a great potential as low-enthalpy geothermal resource to be adopted in direct regional heating. In this paper, a parallel horizontal ground heat exchangers (pHGHE) system is introduced to utilize the geothermal resource in deep coal mines, however, the sustainability of the system in a long-run performance has not been well addressed in the previous study. This work presented a quantitative evaluation on the long-term sustainability of pHGHE system installed in backfilled stopes of deep coal mines. A novel numerical model is applied to characterize the heat transfer occurring between the horizontal pipes and backfill stopes. The proposed model was first verified to an analytical solution to determine the model reliability and accuracy. The validated numerical model was further applied to a typical underground backfilled stope of Anju coal mine in China. The results demonstrated that the maximum sustainable specific heat extraction rate must not exceed 40 W/m to maintain a 50-year exploitation period. A total thermal capacity of 17 GWh could be realized during each extraction cycle under the current conditions, which could provide heating to more than 170,000 m2 of residential building area. Accordingly, sensitivity analysis was performed to identify the key parameters influencing system sustainability. In addition, the effects of groundwater flow on the thermal interaction and long-term sustainability of the pHGHE systems were examined. Groundwater flow can be beneficial to the long-term sustainability of the system.This work confirms the feasibility and sustainability of geothermal resource utilization in backfill stopes of underground coal mines. The insights gained in this study can provide technical guidance for the design of geothermal systems in similar coal mines.

Techno ‐economic analysis of Al 2 O 3 / CuO nanofluid applied in various horizontal ground heat exchangers

A ground source heat pump (GSHP) is widely regarded as a promising technology for building heating, ventilation, and air conditioning because of its higher energy efficiency. Nanofluids (NFs) have also received extensive attention due to their excellent heat transfer characteristics. In this paper, CuO/Al2O3 particles mixed with propylene glycol/water (PG/W, volume ratio: 60/40) are applied to GSHPs as a heat transfer medium. The combination of different pipe types and different concentrations of NFs is studied by comparing the coefficient of performances (COPs) and outlet temperatures. The results show that the 2% CuO NF combined with the spiral type offers the best performance with a 28% rise in the outlet temperature in the pipe and an average COP value of 4.6. When the concentration of Al2O3 NF is 2%, the combined effect of the meander type is the best. The optimal combination of CuO (2%) and spiral type is used for economic analysis by comparing with the performance of natural gas boilers, air source pumps, and GSHPs under similar conditions. The levelized cost of heat analysis results indicate that the horizontal GSHP system coupled with NF can maximize heat exchange efficiency and mitigate environmental effects, but the cost is still high as ¥ 1355/MWh ($212.46/MWh).

Performance evaluation of different configurations of solar humidification-dehumidification desalination system with subsurface condenser

The solar humidification-dehumidification desalination system with a subsurface condenser is a promising renewable energy-based desalination system. This desalination system is particularly suitable for condensation irrigation in greenhouses, especially where conventional energy sources are limited. A computational model of the system comprising a horizontal solar film evaporator and a subsurface condenser is developed. A previously proposed hybrid analytical–numerical model for horizontal ground heat exchangers is modified for the subsurface condenser. An experimental setup of the subsurface condenser is also built and used to verify the developed model of the subsurface condenser. The developed model is then used to evaluate the long-term performance of several closed-loop configurations of the system and compare it with an open-loop system. It is demonstrated that the average daily water yield and Gained Output Ratio (GOR) of the closed-loop system are 115 % and 127 % higher than the open-loop system, respectively; however, the ground temperature increases significantly, which can lead to decreased respiration and root growth. According to the simulation results, using the air-cooled condenser, internal heat exchanger, and the hybrid system with both the air-cooled condenser and internal heat exchanger enhances the freshwater produced and GOR by 6.0 %, 6.3 %, and 10.8 %, respectively. The hybrid system outperforms other configurations and leads to only a 2.0 °C increase in the ground temperature over ten years of the system operation.

Heat transfer optimization in a horizontal helical-shaped ground heat exchanger through response surface methodology

Various combinations of objective functions and design variables have been considered in several optimization studies of ground heat exchangers (GHEs). In this study, the optimum values of the coil pitch and coil diameter of a helical-shaped GHE were determined to maximize its convection heat transfer rate. Central composite design was used to determine the coil pitch and coil diameter dimensions to be used in GHE modeling. Nine models were generated in Fusion360 which were then exported to ANSYS Fluent for CFD simulation. Then, a second-order model of the convective heat transfer rate per unit length as a function of the coil pitch and coil diameter was generated using the response surface methodology. For the design space considered (coil pitch: 600 to 1200 mm; coil diameter: 600 to 1200mm), the highest heat transfer rate was achieved at a coil diameter of 600mm and at a coil pitch of 1200mm. Furthermore, this study showed that if both the coil pitch and coil diameter are considered simultaneously in the optimization of the heat transfer rate of a helical-shaped GHE, then coil pitch would have a directly proportional relationship with the heat transfer rate, while coil diameter would have an inversely proportional relationship.

Numerical Investigation on Auxiliary Heat Sources for Horizontal Ground Heat Exchangers

Human interference with the Earth’s climate cannot be ignored any longer. Renewable energy sources need utmost attention in all energy sectors. For buildings, geothermal energy for heating, cooling, and domestic hot water is a sustainable solution. Horizontal ground heat exchangers (HGHE) demonstrate promising results with low installation costs. Research is focused on increasing their thermal performances by structural improvements and ground thermal proprieties improvements, with little research on using auxiliary heat to increase their performances. A numerical model for an HGHE was established to investigate the effects of auxiliary heat sources on the performances of the HGHE. The results demonstrate that heat transfer into the HGHE increases the overall ground temperature at the end of the heating season by 138.50% compared with no heat transfer from auxiliary heat sources. The ground freezing period decreased by approximately 24.74% by having a heated basement, approximately 40.20% by transferring heat with solar thermal panels, and approximately 62.88% by using both auxiliary heat sources. The difference between the undisturbed ground temperature and the ground temperature with no auxiliary heat sources at the end of the season was 3.45 °C. The difference between the undisturbed ground temperature and the ground temperature with all auxiliary heat sources resulted in 0.92 °C.

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.

Comparative study on the thermal performance and economic efficiency of vertical and horizontal ground heat exchangers

The ground-coupled heat pump is a shallow geothermal exploitation method taking soil as the thermal energy source. The ground heat exchanger is an important component of this system, which includes vertical or horizontal configurations. However, to the best of our knowledge, few studies exist involving the comparison of thermal performances and installation costs of two heat exchanger types considering the influence of ground climate, which makes the selection of heat exchanger configuration challenging for a specific field application. Hence, a 3-dimensional numerical model considering the variations of atmospheric conditions and soil water content is constructed in this paper. Based on this model, the thermal performances and economical efficiencies of vertical and horizontal ground heat exchangers are compared. The results indicate that the thermal performance difference between the two heat exchangers is greater in winter than in summer. The thermal performance is hardly influenced by the injection mass flow rate, while it is considerably affected by the length of heat exchanger. The thermal power rises linearly with the increase in heat exchanger length, and the increment of the vertical ground heat exchanger is higher. In addition, when the heat exchanger length is shorter than 40 m, the installation cost and thereby the total cost of the horizontal ground heat exchanger is considerably higher. With regard to both the thermal performance and economic efficiency, a vertical ground heat exchanger is only recommended when installing a single shallow ground heat exchanger. Cited as: Cui, Q., Shi, Y., Zhang, Y., Wu, R., Jiao, Y. Comparative study on the thermal performance and economic efficiency of vertical and horizontal ground heat exchangers. Advances in Geo-Energy Research, 2023, 7(1): 7-19. https://doi.org/10.46690/ager.2023.01.02

Investigation of the performance of ground-coupled heat exchanger technology for tempering air

A horizontal ground heat exchanger has been applied as a simpler sustainability measure in buildings compared to its vertical counterpart, making it more suitable for residential application. A lack of contextual scientific findings within the specific construction culture has precluded its widespread application in the developing world. In this study, an experimental and simulation investigation was carried out on the thermal performance of an air-based horizontal ground-coupled heat exchanger buried 3 m below the ground. The study was performed in the tropical climate of Mauritius with a focus on space cooling. The ground temperature and air temperature inside the pipeline at several locations of the installation was measured. A CFD simulation model was developed and calibrated against the experimental data, which allowed further analyses on the influence of system parameters on performance. The study allowed to confirm the performance of the technology for application as a sustainability measure in the local construction industry and to identify practical challenge that need to be addressed. A drop in temperature of up to 5C was achieved at 2.3 m/s and 8C at 4 m/s. The latter result holds promise to achieve thermal comfort by achieving indoor air temperature of 27 C or lower when ambient air is at 33-34C during typical summer periods.

Study on thermal performances of a horizontal ground heat exchanger geothermal system with different configurations and arrangements

Horizontal ground heat exchanger (HGHE) is a low-cost shallow geothermal exploitation method compared with vertical ones, but it needs more land area. Optimizing geometrical configurations and arrangements of ground heat exchangers is desirable to enhance heat exchange per unit land area. However, a comprehensive study on different HGHE configurations and arrangements considering atmosphere-soil-HGHE interaction is still inadequate. To this end, thermal performances of four configurations, including slinky, spiral, multi-tube with serial connection and tiled tubes, are compared based on a 3D numerical model for HGHE considering atmosphere conditions variation and humidity migration. Effects of main geometrical parameters on HGHE performance and economical efficiency are investigated for these four configurations. Finally, different tube arrangements of multi-tube HGHE are designed and their thermal performances are analyzed. Results indicated that longer HGHE tube length has better thermal performance but lower economical efficiency, thus being not advisable for the HGHE design. Among these four configurations, multi-tube with serial connection shows the best thermal performance. Increasing the tube spacing is beneficial for reducing the tube mutual interference and enhance the heat exchange per unit land area, which is recommendable. Results are expected to provide meaningful suggestions for the HGHE design.