Ground-coupled Heat Exchanger Research Articles

Study of Thermal Inertia in the Subsoil Adjacent to a Civil Engineering Laboratory for a Ground-Coupled Heat Exchanger

Study of Thermal Inertia in the Subsoil Adjacent to a Civil Engineering Laboratory for a Ground-Coupled Heat Exchanger

The thermal performance of ground-coupled heat exchanger with different geometrical shapes – a numerical study

ABSTRACT The thermal performance of different cross-sectional areas of a U-shape ground-coupled heat exchanger (GCHE) is numerically investigated for summer and winter conditions in the present paper. The four different cross-sectional tubes, viz. square, circular, triangular, and rectangular, of fixed hydraulic diameter and length are considered. Numerical simulations are carried out for different velocities ranging from 2 m/s − 5 m/s using K-epsilon turbulent model. It is found that the rectangular cross-sectional area GCHE provides a more stable air temperature for both summer and winter conditioning, and a maximum temperature drop/rise is observed at a shorter length in this condition. The temperature drop of 14.2 K is observed at a length of 12.6 m in the case of GCHE with rectangular cross-section, while the same drop is reported at 19.2 m, 17.4 m, and 16.5 m for circular, square, and triangular cross-sectional areas, respectively. A maximum temperature rise of 8.3 K is reported at 14.8 m, 20.7 m, 18.2 m, and 17.5 m in rectangular, circular, square, and triangular cross-sectional GCHE for heating conditions. Therefore, the rectangular cross-section GCHE may be preferred for space constraints. The net temperature difference is found to be dependent on inlet air temperature. It increases with an increase in the inlet air temperature for summer cooling conditions and reverses in winter conditions. The inlet velocity greatly influences the thermal performance of GCHE, and optimum performance is found at a lower velocity of 2 m/s due to the more settling time. The effectiveness is found to increase initially along the length, and it attains a maximum value at a shorter length in the case of rectangular cross-section compared to other sections. The maximum effectiveness is achieved between 14 m to 15 m length of the pipe for the case of rectangular cross-sectional GCHE.

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.

A novel radiative sky cooling-assisted ground-coupled heat exchanger system to improve thermal and energy efficiency for buildings in hot and humid regions

Thermal efficiency degradation of the ground heat exchanger (GHE), caused by the imbalance between heat extraction and rejection, is a severe issue that largely restrains GHE from wider application in hot and humid regions. This study proposed a novel hybrid ground-coupled heat exchanger (HGCHE) system by combining the coaxial ground heat exchangers (GHE) with radiative sky cooling radiators (RSCR) to overcome the heat imbalance so as to improve the thermal efficiency of coaxial GHE. Also, the RSCR, providing sub-ambient cooling under sunlight without extra energy input, was developed as the heat rejector, with consideration of the spectral selectivity of the coated radiative cooling material. Moreover, a case study on a building in Hong Kong was conducted, and the cooling capacity of the RSCR assisted-GCHE system was investigated. The result shows that the RSCR can achieve daytime sky cooling, and the yearly accumulated daytime cooling power can reach up to 127.5 kWh/m2, 31.0% of the total cooling power in the first year. The RSCR has the best cooling performance in January, due to the low solar radiation, ambient air temperature and precipitable water vapor. The thermal efficiency of the RSCR assisted-GCHE system can be improved by 6.6% compared with the GCHE without a heat rejector. The cooling capacity of coaxial GHE is proportional to the mass flow rate and borehole depth, neglecting the ground thermal gradient. The findings of this proposed HGCHE system could provide useful engineering guidance for integrating passive radiative cooling technology into space cooling, serving for low-energy buildings, especially in hot and humid regions.

Assessment of alumina/water nanofluid in a glazed tube and sheet photovoltaic/thermal system with geothermal cooling

This article presents evaluation of alumina/water nanofluid (NF) in a glazed tube and sheet photovoltaic/thermal (PV/T) system with ground-coupled heat exchanger (GCHE) in semi-arid region of Pilani, Rajasthan (India). Conventionally in the literature, the cooling performance of NF is reported compared to the normal fluid cooling under equal Reynolds number (Re) comparison criterion. However, according to many researchers, equal pumping power-based evaluation is more pragmatic and is explored here. The first law of thermodynamics analysis is invoked to appropriately assess the replacement of water with NF in the PV/T system coupled with GCHE. Nanoparticle concentration is varied from 0.5% to 6% by volume fraction and the Re from 3000 to 9000. Temperature of PV panel, temperature difference in PV/T outlet and inlet, electrical efficiency, thermal efficiency and Bejan number in PV as well as in GCHE are computed using a validated mathematical model for both equal Re and equal pumping power evaluation criteria. The performance of PV/T system with NF is found superior under equal Re comparison. The improvement observed are reduction in PV panel temperature by 2 °C, reduction in temperature difference in PV/T outlet and inlet by 6 °C, rise in electrical efficiency by 0.1% and a rise in thermal efficiency by 4%. However, the performance of PV/T system with NF is found inferior under equal pumping power evaluation criterion.

A Simplified Thermal Design Model for a Single Vertical U-Tube Borehole Utilized in Ground-Coupled Source Heat Pumps

The present work dealt with the thermal assessments of the ground-coupled heat pump heat exchangers utilized in the field of geothermal energy source. A model was performed to predict the overall thermal resistance of a single vertical heat exchanger embedded in the borehole. The philosophy of the U-tube replacement with a single equivalent tube size was implemented with new development. Four tube sizes were used to build several borehole geometry configurations, they were (9.53) mm, (12.7) mm, (15.88) mm, and (19.05) mm accommodated in borehole sizes of (65) mm, (75) mm, (90) mm, and (100) mm respectively. Twelve borehole geometry configurations were examined as DX condensers circulate R410A refrigerant. These geometry assemblies produced a range of (0.29-0.57) for tube spacing to borehole ratio tested at (0.73) W/m K to (1.9) W/m K filling thermal conductivity range. The results of the present correlation showed good agreement with previously published correlations in the open literature. A mean temperature difference between the condensed vapor refrigerant and soil was assumed to have existed as (14) °C. Increasing the tube spacing from (2) to (3) times the tube diameter exhibited an augmentation in the heat loading of the borehole. This rise in the heat loadings of the U-tube was (8-10) % and (13-17) % for the geometry configurations of (9.53) mm and (12.7) mm tube sizes respectively. The tube diameter has also shown its importance in the thermal process of the borehole. At (75) mm borehole size and tube spacing of (2) times tube outside diameter, the predicted borehole thermal resistance for (9.53) mm tube diameter was higher than that of (19.05) mm one by (78-80) % for the test range of grout thermal conductivity.

Experimental investigation of a ground-coupled heat exchanger combined with an air conditioner for southern India

Increasing the energy ratio of an air conditioner that has a cooled air condenser is a difficult problem, particularly in areas where the temperature and humidity are very high. The efficiency of air conditioners based on standard vapor compression technology can be enhanced by combining active cooling technologies with less energy-intensive inactive cooling technologies. In this study, the cooling performance of combined cooling systems has been experimentally explored. Using four different modes of a hybrid ground-coupled heat exchanger (GCHE), an analysis was carried out to enhance the energy ratio of the active cooling system connected to the GCHE. This was achieved by using the fact that the GCHE outlet air temperature is lower than the ambient temperature and can directly supply space heating or cooling to the condenser tubes of conventional air conditioners. It was observed that with the use of a GCHE for the condenser tubes, the energy consumption of a traditional window air conditioner was reduced by 16.18%.

Assessment of Reducing Pollutant Emissions in Selected Heating and Ventilation Systems in Single-Family Houses

The article presents the results of a study aiming to select the optimal source of heat for a newly designed single-family home. Commercial software was used to compare heating and ventilation systems involving a bituminous coal boiler, a condensing gas boiler, a biomass boiler, a heat pump with water and glycol as heat transfer media. The effectiveness of natural ventilation, mechanical ventilation with a ground-coupled heat exchanger, and solar heater panels for water heating were evaluated. The analysis was based on the annual demand for useful energy, final energy, and non-renewable primary energy in view of the pollution output of the evaluated heating systems. The analysis revealed that the heat pump with water and glycol was the optimal solution. However, the performance of the heat pump in real-life conditions was below its maximum theoretical efficiency. The biomass boiler contributed to the highest reduction in pollutant emissions (according to Intergovernmental Panel on Climate Change Change guidelines, carbon dioxide emissions have zero value), but it was characterized by the highest demand for final energy. Mechanical ventilation with heat recovery was required in all analyzed systems to achieve optimal results. The introduction of mechanical ventilation decreased the demand for final energy by 10% to around 40% relative to the corresponding heating systems with natural ventilation.

A novel approach to the evaluation of contact thermal resistance at soil-structure interfaces

Considering the need to reduce our dependence on unsustainable energy sources and reducing the carbon footprint associated with building climate control, shallow geothermal energy represents an attractive sustainable technology for providing renewable heating and cooling. The temperature field generated around ground-coupled heat exchangers, and thus their energy efficiency, fundamentally depends on the heat transfer mechanism and the thermal properties of the materials involved. While the thermal properties of materials that make up the system can be defined with some certainty, little is known about the impact of contact thermal resistance at the soil-structure interface. Contact thermal resistance will reduce heat exchange efficiency and increase mechanical impacts associated with temperature changes within energy geo-structures. This paper describes a laboratory test method to quantify the contact thermal resistance of soil-concrete interfaces. The methodology is first evaluated using numerical analysis, and then validated against a test using a limestone aggregate concrete and fine, silica sand at differing levels of compaction.

Assessment of options to reduce pollutant emissions in single-family houses in north-eastern Poland

The article presents the results of a study aiming to select the optimal source of heat for a newly designed single-family home. Commercial software was used to compare heating and ventilation systems involving a bituminous coal boiler, a condensing gas boiler, a biomass boiler, and a heat pump with water and glycol as heat transfer media. The effectiveness of natural ventilation, mechanical ventilation with a ground-coupled heat exchanger, and solar heater panels (flat and tubular) for water heating was evaluated. The analysis was based on the annual demand for useful energy, final energy and non-renewable primary energy in view of the pollution output of the evaluated heating systems. The analysis revealed that the heat pump with water and glycol as heat transfer media was the optimal solution. However, the performance of the heat pump in real-life conditions was below its maximum theoretical efficiency. The biomass boiler contributed to the highest reduction in pollutant emissions, but it was characterized by the highest demand for final energy. Mechanical ventilation with heat recovery was required in all analyzed systems to achieve the optimal results. Laboratory analyses confirmed the high efficiency of the tube heat exchanger in winter.

Numerical analysis of a heat exchanger pile response in a Brazilian subtropical region

Ground-coupled heat exchanger systems have been used as acclimatization systems for residential and commercial buildings in many countries. Brazil is the ninth largest consumer of electrical energy in the world, for this reason, local researchers are investigating the use of the energy piles in order to reduce the consumption of electricity. Thermal response tests have been carried out on a heat exchanger pile at the geotechnical experimental site of the University of São Paulo in São Carlos city, a region of subtropical climate. Simultaneously, using the thermal properties obtained in these tests, numerical analysis has been performed to investigate the heat exchange performance of energy piles installed in this site. For this numerical analysis, the effect of soil and concrete properties, pile geometry and the flow rate on the pile thermal response were evaluated. The current paper presents the results obtained by the analysis of 67 models tested to found an optimal configuration of an energy pile through the software COMSOL, using Heat Transfer and Non-Isothermal Pipe Flow modules. From this work, it was observed that the optimal configuration was obtained for a turbulent flow condition in piles in the heat exchanger pipes.

An analytical model for transient heat transfer in ground-coupled heat exchangers of closed-loop geothermal systems

Vertical borehole heat exchangers are widely used as sustainable and reliable tools to effectively extract thermal energy from the ground. For an effectual design and implementation of these borehole heat exchangers, it is essential to have an in-depth understanding of heat transfer within the boreholes and the geothermal reservoir. While in situ tests and numerical heat transfer analyses are proven to provide reliable results, they usually need highly compatible hardware along with expensive software and expertise acquisition. The present study presents a ‘one-dimensional’ analytical heat transfer representation of borehole geothermal heat exchangers which can amenably predict the performance of these systems. For validation of the proposed model, several grid sizes are selected to represent a wide range of typical application scenarios. Selected scenarios are run to test the capability of the proposed analytical model versus its numerical counterpart. These scenarios are examined under two distinct test cases. In the first case, the proposed analytical solution is tested in estimating the borehole wall temperatures of single and multiple borehole arrangements in a time scale ranging from hours to years. The results obtained in this case are validated with two-dimensional numerical models with a constant flux boundary condition on the borehole wall. On the other hand, a second round of tests are focused on estimating the fluid outlet temperatures by introducing an actual fluid flow in the proposed three-dimensional, U-tube, ground-coupled heat exchanger scenarios within similar transient time-scale. In this sense, single borehole, double borehole, quadruple borehole and ‘N × N’ borehole scenarios; the latter representing very large grids, are numerically modeled and compared with the proposed analytical model. This comparison study shows the amenability of the proposed analytical model in predicting system performance with high precision. Numerical validations conducted with both two-dimensional and three-dimensional transient models have shown that the proposed unidimensional analytical solution is capable to estimate system performance in an effective manner. Overall, the proposed analytical solution is proven to provide robust results in prediction of transient heat transfer in single and multiple ground-coupled heat exchangers both in short and long term with high accuracy.

Experiments Using Capillary Mat as Ground Heat Exchanger for Ground Source Heat Pump Heating Application

The cooling and heating of spaces are among the largest sources for household’s energy demand. Ground Source Heat Pump (GSHP) is a promising technology to reduce the energy for cooling and heating purposes. However, the major obstacle hindering the utilization of this technology is the high initial cost, especially for the installation of ground coupled heat exchanger. The horizontal closed-loop system offers lower installation cost, as it requires no vertical borehole construction. Instead, the heat exchangers can be installed in shallow trenches that may be excavated, by small excavator or even by human labor. This paper presents the comparison of two different heat exchangers, namely, the capillary mat and the widely used slinky pipe. Both heat exchangers are connected to a heat pump, where continuous heating tests were carried out for 165 hours (~7 days) for each configuration. The purpose of this research is to show the performance of capillary mat in comparison to slinky pipe. Despite during the entire test for capillary mat required 6% higher electricity consumption, compared to slinky heat exchanger, the results still suggest the potential use of capillary mat as alternative to slinky heat exchanger. Additionally, the results also highlight the high hydraulic resistance of installed capillary mat heat exchangers may become the major disadvantage of the capillary mat.

THE USAGE OF GROUND-COUPLED HEAT EXCHANGERS IN SUBTROPICAL CLIMATES

This study aims to identify the feasibility of using a ground-coupled heat exchanger system for air conditioning in subtropical regions. Geothermal systems are used effectively in cold climates, but little explored in hot and humid climate regions, although they show potential for passive air-conditioning environments. The tubes were installed in Southern Brazil, in Foz do Iguaçu, Paraná, which has temperatures of 0ºC in the winter and 40ºC in the summer. The methodology for the research was organized as follows: bibliographic research and geological characterization of the site, system sizing, installation of 22 meters of a 300mm PVC tube, 4 meters depth, and subsequently measurement and data collection on temperatures with use HOBO U10-001 Data Logger. The research was held from March to October 2016. The results showed that at external temperatures of 30ºC, the system was able to cool on average 6ºC, reaching peaks of 7ºC in the temperature of air exit to the environment. At temperatures below 6ºC, the system was able to heat on average 10ºC, with peaks of up to 11ºC. Based on the results, it is concluded that a buried pipe system is a viable alternative for passive climate control in subtropical climates

Experimental Investigation of a Ground Coupled Heat Exchanger in Khulna, Bangladesh

Experimental Investigation of a Ground Coupled Heat Exchanger in Khulna, Bangladesh

A Review of Underground Soil and Night Sky as Passive Heat Sink: Design Configurations and Models

Energy consumption for heating and cooling constitute the majority of the energy use for building loads. Using passive cooling systems to reduce the energy consumption or to make the process more efficient can be very beneficial. Ground coupled heat exchangers and night sky radiative cooling systems have been used for centuries to achieve cooling and ice making. Ground coupled heat exchangers use the temperature difference between underground soil and ambient air or water for heat transfer between the soil and the fluid passing through buried pipes. Night sky radiative cooling takes advantage of the night sky as the coldest heat sink available for heat transfer with any surface. Use of these simple designs with the modern cooling/heating systems has the potential for a major impact on the heating and cooling needs. This review paper describes the various designs, configurations and applications of these systems as well as determining the parameters that impact their performance.

Study on Application Potential of Seasonal Thermal Energy Storage-Hybrid Ground Source Heat Pump in Taiwan—Taking Experiments in Tainan as Examples

Ground source heat pumps (GSHPs) are widely used in building energy conservation in many countries and regions. However, they are rarely seen in Taiwan. The main reason is the extraordinary imbalance between the heating load and cooling load of buildings in Taiwan. Hybrid ground source heat pump (HGSHP) is a hybridization of a traditional GSHP system, and can effectively balance the heat injected into and extracted from the ground over an annual cycle. This study focuses on the application of seasonal thermal energy storage HGSHP (STES-HGSHP). Based on the data of six experiments in Tainan, Taiwan, this study finds out the ways to make the process of cold energy storage run with high efficiency, including (1) increasing the flow rate in the ground coupled heat exchanger (GCHE); (2) using double-U GCHE instead of single-U GCHE; (3) starting the process of cold energy storage at the time with low wet bulb temperature; (4) storing more cold energy than necessary in order to lower the ground temperature. Finally, by analyzing the level of wet bulb temperature in winter, this study confirms that the application of STES-HGSHP has great potential in Tainan.

Использование дополнительного охлаждения масляных трансформаторов при совместной работе термоэлектрических преобразователей и грунтовых теплообменников

Transformers belong to the main equipment of power plants, raising, lowering and distribution substations, as well as various types of converter devices. In the process of operating oil transformers, damage or defects can occur, the timely detection of which enables taking measures to prevent their furtherance and to keep the equipment intact. Severe consequences are caused by damage to the windings and main insulation of the transformers as it might bring about malfunctions in the cooling system. Existing cooling devices are not always efficient. We have developed a complementary cooling system for power transformers with thermoelectric converters. The operating principle of the device is based on the following: at night, when the ambient temperature is the lowest, water ice is produced the melting of which keeps the transformer oil cold in the hottest daytime. Two operating states of the developed device are described. We also herein present a mathematical description of the water cooling process. We have analyzed the results of studies consisting in evaluating the alteration in the heat flow drawn by vertical ground-coupled heat exchangers. The results of the research show it is possible to use the system for thermoelectric module- and VGHE-enabled liquid cooling of power oil transformers that will allow to draw up to 50% of heat at peak thermal loads, whereas the transformer power is expected to increase by 25–40% depending on the specifications.

Heat transfer performance of buried extremely long ground-coupled heat exchangers with concentric pipes

This study focuses on the heat transfer between the vertical concentric tubes buried underground and the medium-deep rock-soil geothermal energy. The numerical simulation based on an in-house FORTRAN computation program was performed to analyze the mechanism of heat exchange underground. The program was shown to predict the test results of outlet water temperature and heat gain very well under the corresponding operating conditions when engineering data of an actual geothermal well were obtained to validate the numerical scheme. The effects of inlet water temperature and flow rate on the heat exchanger performance were also analyzed in detail. The comparison indicated that the present numerical scheme is able to predict the heat transfer performance of extremely long ground-coupled heat exchanger subject to various geothermal conditions.

Investigation of shallow geothermal potentials for different types of ground source heat pump systems (GSHP) of Wuhan city in China

Abstract To ensure techno-economically suitable installation of ground source heat pump (GSHP) systems, thermal and hydrogeological properties of the subsoil need to be investigated. In this paper, the geothermal potential for three types of GSHP installations in the urban area of Wuhan city is assessed based on preliminary geological investigations. The potential for shallow geothermal energy is evaluated for surface water heat pump systems (SWHP), groundwater heat pump systems (GWHP) and ground coupled heat exchanger heat pump systems (GCHP). The mapped shallow geothermal potentials provide essential information for the installation of GSHPs and for the management of geothermal resources of Wuhan city. Furthermore, the heat transfer rates for some typical configured borehole heat exchanger (BHE) are tested by field Thermal Response Tests (TRT). In order to understand the techno-economic feasibility of the GSHPs, different types of the installed systems are measured and analyzed.