Horizontal Ground Heat Exchanger Research Articles

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.

Thermal and economic performance of horizontal ground source heat pump systems with different flowrate control methods

The horizontal ground heat exchanger (HGHE) integrated with the heat pump unit can be defined as the horizontal GSHP system. Most horizontal GSHP systems are always operating at a rated flowrate in the secondary loop, resulting in the electricity waste of circulation pump when the system operates in partial load. This study aimed to compare the thermal and economic performance of horizontal GSHP systems with different flowrate control approaches. Two novel variable flowrate regulation methods, namely the segmented and time-varying flowrate control (SFC and TFC) method, together with the traditional constant flowrate control (CFC) method, were employed in the secondary loop of system. 3D numerical model of the HGHE considering the isotropic porous soils and varying ambient environmental conditions was built and coupled with the model of heat pump to analyze the heating and economic efficiency of the horizontal GSHP system. Considering that the buried depth of HGHE, H, may vary in different conditions, the impact of flowrate control methods in various H was analyzed. The results reveal that compared with the CFC and SFC method, the variation range of the fluid temperature difference between the inlet and outlet of the HGHE, ΔTf, and system COP, COPsys, are smallest and the level of ΔTf and COPsys are greatest for the TFC method. The system operation costs and electricity charges proportion of water pump can decrease by up to 9.80% and 47.26% as the control method varies from CFC to TFC, but the differences between the variable flow control technologies are not significant. The discrepancy of COPsys and operation costs between the constant and variable flowrate control methods decrease by 39.13% and 48.72% with the increases of H. This study highlights the significance of using variable flowrate regulation methods in the secondary loop of the horizontal GSHP system to improve thermal and economic performance.

A fast numerical approach for the horizontal ground heat exchanger

A fast thermal response numerical (TRN) approach for horizontal ground heat exchanger (HGHE) was developed and verified. The shape equivalence method was used to process the shapes of pipe cross section and elbows, reducing the meshing difficulty and the grid quantity of HGHE. A series of geometric equivalent quantization values were presented. Meanwhile, a semi-analytical equation was derived to reduce the sensitivity of the TRN model to the time step. The simulation results show the calculation speed of the TRN model is 38.98 times that of the Fluent model in the same case. Furthermore, long-term operations of HGHE were simulated, revealing that the soil around HGHE has good heat recovery capacity.

Design strategy and techno-economic optimization for hybrid ground heat exchangers of ground source heat pump system

This study aimed to formulate an accurate design strategy for hybrid ground heat exchanger (HyGHE) of ground source heat pump system and optimize HyGHE size based on techno-economic analysis. First, to analyze the thermal performance of the HyGHE system, a detailed heat transfer numerical model of borehole heat exchanger and horizontal ground heat exchanger was proposed. Then, a novel techno-economic model was employed to assess the economic efficiency of the foregoing system. Subsequently, the design optimization model of the HyGHE system was developed to determine the optimal shave factor (α and β), building load distribution scheme, and HyGHE size. Finally, sensitivity analyses were implemented to determine the optimal design under different conditions. The results reveal that the costs of borehole drilling, pipe, and equipment account for approximately 80% of total capital investments of the HyGHE system. The system initial costs increase with α and can reach 91,385.11 CNY when α = 1.00. In contrast, its annual operational costs first decrease and then increase with increasing α. For the fundamental case, the thermal and economic performance levels of the HyGHE system improve with decreasing α. The optimal design is more sensitive to the unit prices of borehole drilling, pipe, and excavation.

Thermo-Economic Analysis of Near-Surface Geothermal Energy Considering Heat and Cold Supply within a Low-Temperature District Heating Network

This study evaluates low-temperature district heating (LTDH) networks with different geothermal heat sources under thermo-economic criteria. In particular, the heat and cold supply of modern neighbourhoods are taken into account in a dynamic simulation model built on the modelling language Modelica. Both horizontal and vertical ground heat exchangers (GHE) were investigated in respect to the load profiles of the consumers, depending on dimension as well as location. The selected base case represents a LTDH network near Stuttgart (Germany). The corresponding results of an annual simulation show that a horizontal GHE is suitable for pure heat supply and can reduce costs by up to 12% compared to a vertical system. This economic advantage remains when the cooling demand is considered. Subsequently, a variation of the system location was carried out. It is shown that horizontal GHEs operate more economically in northern regions, whereas vertical ones are more advantageous in regions with increased cooling demand. For both cases, possible savings of between 3.0% and 4.2% resulted from the simulations. The heating-to-cooling demand ratio was used as a first decision criteria to weigh-up between the two systems. Vertical GHEs were more economical than horizontal systems as soon as the ratio dropped below 1.5.

A hybrid analytical-numerical model for predicting the performance of the Horizontal Ground Heat Exchangers

Horizontal Ground Heat Exchangers (HGHE) as a means of exploiting geothermal energy has come to the fore for a few decades. Various analytical and Computational Fluid Dynamics (CFD) methods have been proposed to predict the performance of the HGHEs. The available analytical approaches are fast; however, they are based on various simplifications and assumptions, affecting their accuracy. On the other hand, CFD methods are more accurate, but their computational cost is a burden. Therefore there is an acute need for an accurate and fast method for predicting the long-term performance of HGHEs. To this aim, this study puts forward a novel hybrid analytical-numerical model for predicting the performance of HGHEs. The proposed approach is based on Duhamel's theorem, modified infinite line source method, and superposition principles that allow fast and straightforward simulation. The thermal interference between adjacent pipes is also considered via the superposition method. The computational time and performance of the proposed approach are evaluated via several benchmark cases and are compared with the results of a three-dimensional CFD model. The results depicted that the present model can predict well the transient performance of linear HGHEs with various pipe arrangements. Moreover, according to the obtained results, the proposed semi-analytical method is up to 50 times faster than the CFD approach.

Experimental study of horizontal ground heat exchangers embedded in the backfilled mine stopes

Abundant geothermal resources are stored in underground mines in China. In order to realize the green cooperative development of deep mineral resources and geothermal resources, the heat storage functional backfill technology can be used to form artificial thermal reservoir, and the geothermal energy can be extracted through the buried pipes, so as to expand the auxiliary value and reduce the cost of backfill mining. It's important to investigate the heat storage/release behavior of the backfill body with buried pipes and its influencing factors for improving the geothermal energy utilization efficiency. In view of the limitations of field experiments, a scale model test platform for backfill coupled heat exchanger of mine was established in this paper. According to the heat transfer differential equations, the similarity criterion number and similarity constant that should be satisfied between the model and the prototype were deduced, and the effects of different pipe arragement, flow rate of heat-carrying fluid, surrounding rock temperature on the heat transfer performance of the backfill coupled heat exchanger were studied. Based on the heat balance, the total energy efficiency of the backfill coupled heat exchanger system was calculated. The experimental results showed that with the increase of the distance between pipes, the pipe length, the flow rate and the surrounding rock temperature, the heat extraction and water outlet temperature of the system all increased. With the increase of the velocity of heat-carrying fluid, the water outlet temperature decreased. The increase in the number of pipes in the backfill body and the decrease in the distance between pipes lead to the increase in thermal interference and thus the decrease in heat extraction, but the decrease value is less than the increase value of heat charging caused by the increase in pipe length (serpentine) and flow rate (double U-type). The total energy efficiency of the system was the highest, which was 47.9%, when the serpentine pipe arrangement was adopted, the working fluid velocity was 0.7 m/s, and the surrounding rock temperature was 35°C. According to the scale model, 1.06 × 109 kJ of heat can be obtained from the mines a year by applying BCHE system in engineering practice.

Experimental and numerical studies on the performance of horizontal U-type and spiral-coil-type ground heat exchangers considering economic aspects

The benefits of the use of horizontal U-type and spiral-coil-type ground heat exchangers (GHE) were investigated and compared based on heat transfer performance and economic analysis. The feasibility of a novel backfill material (controlled low-strength material) for a horizontal ground heat exchanger was also assessed. A numerical analysis model was developed to simulate the heat transfer in a horizontal ground source heat exchanger and calculate the annual heat exchange using 100 case studies with different GHE types, backfill materials, fluid velocities, and installation depths. A thermal response test was conducted using a spiral-coil GHE backfilled with sand and a controlled low-strength material to validate the numerical model. The economic feasibility of each case was investigated using analytical tools including the annual heat exchange, payback period, and internal rate of return. The results indicate that, although a spiral-coil-type GHE has 30% higher total initial installation costs, its high annual heat exchange (twice that of U-type) results in a shorter payback period (12.7 years compared with 8.4 years for U-type) and a higher internal rate of return (12.0% compared with 6.9% for U-type). The use of a controlled low-strength material leads to a slight increase in the initial investment; however, as expected, its high heat exchange capacity shows better long-term economic performance than a ground heat exchanger backfilled with normal soil.

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.

Numerical analysis of ground heat exchange by horizontal ground heat exchangers and a prediction of temperature effects on a subsurface ecosystem

Numerical analysis of ground heat exchange by horizontal ground heat exchangers and a prediction of temperature effects on a subsurface ecosystem

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

Thermodynamic Evaluation of a Direct Expansion Ground-Sourced Heat Pump with Horizontal Ground Heat Exchangers Using Advanced Exergy Analysis

This paper deals with the advanced exergetic analysis of a horizontal direct-expansion ground sourced CO2 heat pump operating in a transcritical cycle. The cycle is thermodynamically modeled in Engineering Equation Solver (EES) considering the pressure drops in both high and low temperature heat exchangers, and the system is to provide a fixed heating load. Conventional exergy analysis orderly suggests a compressor, expansion valve, gas cooler and ground heat exchanger to be considered for system improvement, while tracing exergy destruction of all components in detail demonstrates true improvement potential of each and all components and the system as a whole and offers a different order. Advanced exergy analysis points out that the compressor is directly and indirectly responsible for 56% of the overall exergy destruction generated in the cycle, confirming the detrimental role of this component in the system. The second influential component is recognized to be a ground heat exchanger accounting for 20% exergy destruction of the compressor as well as submitting 89% avoidability in its own exergy destruction, and expansion valve proves to be the last option for system improvement according to this analysis.

Utilization of Thermally Activated Building System with Horizontal Ground Heat Exchanger Considering the Weather Conditions

The thermally activated building system (TABS) can reduce the peak load by integrating with the ground heat exchangers. When integrated, the cost of groundwork and stability of the ground temperature would counteract because the weather conditions would influence the ground temperature in shallow depth. However, previous studies on TABS assumed constant ground temperatures such as average outdoor air temperature. In this study, ground temperatures in different depths are simulated for their detailed investigations, and simulated results of ground temperature were applied to building energy simulations for observing the load-handled ratio (LHR), representing the peak load reduction by TABS evaluated in various weather conditions. Simulation results of ground temperatures from 1 m to 39 m depths show that the temperature stabilized at 2 m to 11 m depths depending on the characteristics of the outdoor air temperature. LHR increased as the ground depth increased because the ground temperature at shallow depths increased during peak hours. Ground depths of 8 m were found ideal for maintaining consistent LHR for all weather conditions. Detailed observation of ground temperature and its effect on LHR in various weather conditions can help system engineers design and operate the TABS with the ground system.

An experimental work on the performance of a solar-assisted ground-coupled heat pump using a horizontal ground heat exchanger

Solar Assisted Ground Coupled Heat Pumps are thoroughly investigated today. Although the best method to extract heat from the earth is by using vertical ground heat exchangers, they are expensive. Horizontal ground heat exchangers are cheaper but the downside is that they are not so efficient. The main objective of this study is to find efficient, alternative methods to assist the horizontal ground heat exchanger in order to decrease the ground freezing period and increase its performances. For that, a Solar Assisted Ground Coupled Heat Pump system was established, using two solar assistance sub-systems (solar thermal panels and solar photovoltaic panels). A third heat source was considered, consisting in the residual heat lost from the building's heated basement. The results indicated that using auxiliary heat sources eliminated the ground freezing period. Although heat transfer into the Horizontal Ground Heat Exchanger increased the Seasonal Performance Factor by approximative 15% for the season peak, a high amount of heat was lost in the process. This study also addresses possible methods to reduce the heat loss. Overall, the results indicated better performance for the Horizontal Ground Heat Exchanger when transferring heat into it.

An all-in-one machine coupled with a horizontal ground heat exchanger for the air-conditioning of a residential building

Heating, ventilation and air conditioning (HVAC) systems are considered of great importance in the building sector, as they represent a significant part of the energy demand in buildings. Moreover, they are deeply related to indoor environmental quality, which strongly affects occupants' quality of life. This paper presents the analysis of a compact all-in-one and plug-and-play machine providing heating, cooling and controlled mechanical ventilation to a low energy residential building. In particular, the system is composed of an air-to-air heat pump coupled to a heat recovery unit which can pre-heat the fresh air and contribute to domestic hot water production. In the present work, an additional heat exchanger embedded in the building's foundations has been included in the system to pre-heat or pre-cool the outdoor air before it enters the heat recovery unit. The compact energy system has been mathematically modelled, and its operations have been monitored in a real residential building recently constructed in Udine (North-East of Italy). The annual energy performance of the all-in-one machine has been obtained, and the system layouts with and without ground heat exchangers have been compared. The use of the ground heat exchanger helps to decrease the primary energy consumption and the operating costs (in heating, 14% and 11% respectively), improving, at the same time, the thermal comfort conditions inside the building.