Buried Heat Exchanger Research Articles

Numerical simulation of temperature field around buried pipes of ground source heat pumps based on mathematical models

The author established a physical and mathematical model for the heat exchange of a ground source heat pump buried heat exchanger under the co-operation of heat and seepage, including the soil and fluid inside the pipe surrounding the heat exchanger. Using ANSYS finite element APDL language for programming, based on the line heat source model, simulate the temperature field around the vertical double U-tube underground heat exchanger, the effects of soil thermophysical properties, temperature outside the pipe, soil type and backfill material on soil temperature field were obtained through simulation analysis. The experimental results indicate that, the changes in soil temperature are also significant with different backfill materials. Therefore, it is necessary to conduct serious research and optimization on backfill materials, develop new types of backfill materials, improve backfill construction techniques, and conduct in-depth research by combining theoretical analysis with practical engineering to ultimately find efficient and economical backfill materials. The change in equivalent pipe diameter has little effect on soil temperature, and the linear heat source model is used for calculation without causing significant errors. It has been proven that the soil itself has strong resilience and has reference value for the design of buried heat exchangers in practical engineering.

Thermal recovery test for determining the thermal conductivity of the soil

This paper presents a new in-situ experimental procedure for determining a thermal conductivity of soil, as one of the thermophysical properties necessary for dimensioning vertical buried heat exchangers of geothermal heat pumps. The proposed method, called the thermal recovery test, is based on the assumption that there is a direct analogy between the hydrodynamic process of filling (recovery) the well with water from a porous aquifer after its partial or complete discharging and the process of establishment of thermal equilibrium in an infinite medium previously disturbed by a line heat sink (source). Apart from presentation the theoretical background of these two phenomena, the display of the identical theoretical logarithmic character of the change of hydrostatic pressure and soil temperature, description of the experimental procedure, the experimental results of three performed experiments are also presented. Additionally, results for the experimentally obtained values of the thermal conductivity of the soil are compared with those obtained from the thermal response test, and the advantages and drawbacks of the new thermal recovery test method are analyzed.

A review on earth tube heat exchanger in heating and cooling systems

About 40% of the primary energy in the building sector is consumed for heating and cooling applications, where it started by relying on the use of sustainable energy sources through using an Earth tube heat exchanger (ETHE). ETHE is apparatus that works to transfer heat from ambient air to underground and vice versa which is used in heating and cooling applications. ETHE depends on undisturbed subsoil temperature, where the underground temperature doesn't change through the year at a certain depth. Ambient air passes through the buried heat exchanger in the ground and then cools down in summer and warms in winter. The good performance of this system depends on design efficiency. This paper reviews the experimental and numerical researches of ETHE which focuses on its design. The use of geothermal energy through ETHE leads to meeting energy demand and preserving the environment by reducing carbon dioxide emissions, which in turn affects the ozone.

Examining heat transfer of a buried pipe heat exchanger in arch tunnel lining with protection engineering

Cooling towers and internal reservoirs are two conventional heat transfer methods used in protection engineering. However, they have the drawbacks of easy exposure and limited capacity. Although the traditional ground source heat pump system can become a new heat transfer method, its high installation cost and space requirements are obstacles to the development of this technology. To address this problem, the heat exchange pipe of the ground source heat pump system being directly laid in the tunnel lining was proposed in this study. A heat transfer performance test platform for a lined buried pipe heat exchanger based on an arch tunnel was built, and the water velocity, wind velocity, and temperature of each measuring point were recorded under appropriate working conditions. The effects of the inlet water temperature, inlet water velocity, ventilation velocity, and initial soil temperature on the heat transfer performance were analyzed. The results have shown that the heat transfer performance of the tunnel reached its maximum when system operation commenced and then decreased gradually with time, and the rate of decrease gradually slowed. Along the radial direction of the tunnel, the shorter the distance to the buried heat exchanger, the greater the increase in soil temperature, and the shorter the time for the temperature to reach the peak value. As the distance increased, the temperature gradient gradually decreased. Along the circumferential direction of the tunnel, the lining temperature on the inlet side of the buried heat exchanger was higher than that on the outlet side. The temperature distribution on both sides of the symmetrical surface of the tunnel was not uniform, resulting in additional temperature stress in the horizontal direction. The time required for the heat transfer capacity of the tunnel to return to its initial value was approximately eight times that of the system. Compared with the ventilation and water velocities, the inlet water temperature and initial soil temperature had a greater impact on heat transfer. The heat-transfer law of an arch tunnel differs from that of a circular tunnel. There were some deviations when comparing the experimental data of the circular tunnel with those of the actual arch tunnel.

Experiment and simulation research on heat exchange of different types of underground heat exchangers

Compared with traditional bored pipe heat exchanger geothermal collection technology, the continuously optimized energy pile technology is more efficient and economical in the application of building energy-saving projects. According to the structural characteristics of three different underground heat exchangers of deep buried pipe energy pile, inside buried pipe energy pile and traditional bored and buried pipe heat exchanger, we analyze the heat transfer characteristics through field test and numerical simulation. The results show that at the same depth, the heat exchange and heat exchange efficiency of single deep buried pipe energy pile are higher than those of traditional bored buried heat exchanger. Under the same pile length, the heat exchange rate of single deep buried pipe energy pile is higher than that of the inside buried pipe energy pile. The high thermal conductivity of the pile foundation can significantly improve the heat exchange effect of the heat exchanger. The well-pile section heat exchange ratio of the deep buried pipe energy pile reaches 1.95. The installation of the deep well can not only reduce the thermal interference caused by the pile foundation thermal accumulation, but also compensate for the adverse effect of the small heat exchange tube spacing on the heat exchange. By optimizing the construction process of the heat exchanger and reducing the thermal interference effect between the heat exchange tubes, the overall heat exchange effect of the heat exchanger will be effectively improved. <fig fig-type="abstract-image" id="F1" orientation="portrait" position="float"><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="SP.J.1249-2022-39-1-20/48BEB0D3-18F9-4a9d-93C3-E7253C6DB4C2-F001.jpg"/></fig>

Enhanced Heat Transfer Performance of a New Horizontal Buried Tube Heat Exchanger

Horizontal buried heat exchangers cost less than vertical buried heat exchangers, but they need more land area. In order to reduce the required land area of horizontal buried tube heat exchanger, a finned strengthened heat exchanger pipe was designed to enhance the heat transfer performance, and a three-dimensional research model of shallow geothermal utilization system was established in Chengdu area . In this paper, the optimal shape and arrangement of fins are investigated, and the effects of inlet velocity, soil type, ambient temperature and intermittent operation on the enhanced heat transfer performance are studied. The results show that the enhanced heat transfer performance of rectangular fins is the best in the case of horizontal arrangement. The working fluid flow rate should not exceed 0.3m/s.Soil type has a great influence on heat transfer performance. The heat transfer of rock and soil is the largest, the heat transfer of clay is the smallest, but the strengthening effect of clay is the best. The longer the system runs, the obvious the influence of ambient temperature is, and the heat transfer efficiency increases by about 4% when the ambient temperature drops by 5K.

Research on Influencing Factors of Heat Transfer Performance of Vertical Single U-Tube for Ground Source Heat Pump

The heat transfer performance of the buried heat exchanger is a key factor affecting the efficiency of the ground source heat pump. In this paper, the heat transfer model of the vertical single U-type heat exchanger is first established under the condition of variable heat flow to make it closer to the actual operating conditions, for simplicity The calculation process and the calculation time are saved. The boundary element method is used to analyze the heat resistance of the buried heat exchanger, and then the numerical simulation software is compiled to simulate the heat transfer performance of the buried heat exchanger. The results of the study show that the soil temperature changes within a range of 1.5m from the buried tube heat exchanger. The hole spacing of the buried tube heat exchanger cannot be less than 4m in practical applications; when the soil thermal conductivity increases by 25%, The heat transfer per unit depth of the buried tube heat exchanger increases by 14.8%; When the diameter of the drill hole is increased from 160mm to 300mm, the heat transfer per unit depth of the hole increases by 8.2%.

Slagging behavior and mechanism of high-sodium–chlorine coal combustion in a full-scale circulating fluidized bed boiler

The contents of chlorine and sodium in Xinjiang Shaerhu (SEH) coal are extremely high, leading to severe slagging. In this paper, the slag was sampled from a circulating fluidized bed (CFB) boiler purely burning SEH coal, to analyze the slagging mechanism based on the characterization of morphology and composition. The results show a three-layer structure for the slag sampled from the buried heat-exchanger in the dense-phase zone of the CFB boiler. The inner layer close to the heat-exchanger is NaCl, which enhances the adhesion of ash particles, while the middle layer and the outer layer are mainly composed of Ca2Al2SiO7 and other Si–Al materials. In comparison, the slag sampled from the refractory wall shows a molten state without a layered structure and mainly composed of NaCl, NaAlSiO4, Ca2Al2SiO7, and CaSiO3. The effect of mixing bed material, on the ash melting and release of chlorine and sodium was further conducted, which indicates that the mixing of bed material has no significant effect on the release of chlorine(Cl) and sodium(Na) but highly affects the melting temperature and compositions. The ash fusion temperature reaches the lowest with a 50% mixing ratio of bed material, which is 120 °C lower than that of SEH coal ash. This study can provide better guidance for controlling severe slagging, from the combustion of high Na and Cl coal in industrial furnaces.

Effects of aquifers on soil temperature distribution around deep buried heat exchanger

Deep buried heat exchanger has been increasingly applied into the building heat source through the utilization of geothermal energy. During the geothermal system design process, the aquifer is an important factor that affects the overall performance prediction. The flow of underground water at different positions changes the temperature distribution of the soils around the buried pipe and then influences the heat exchange at different depths. Consequently, the variation of the temperature field causes the thermal radius of the heat exchanger no longer regularly distributed, which affects the determination of the well spacing. With the aims of improving the accuracy of performance estimation, the aquifer effect is carried out by this study. The solution to the moving line source theory is used to acquire the temperature response at different positions. The overall effect of the underground water with different velocities on the performance is evaluated. The soil temperature distribution in the heat exchange zone with aquifers at different depths is studied in detail. The predicted thermal influence radius at different depths with various groundwater velocities and soil thermal properties showed that the temperature distribution around the buried heat exchanger is largely influenced by the groundwater movement. Under the simulation condition, the thermal radius increases from less than 10 m for pure conduction condition to a maximum value of 25.3 m with groundwater flow. The aquifer enhances the heat absorption of the heat exchanger, but the high flow rate also leads to a large thermal radius. The obtained thermal influence radius under different conditions can provide guidance for the design of the geothermal system.

Enhancing the Performance of Dry Air-Cooled Systems with Wet Geothermal Cooling in Saudi Arabia

Reducing the temperature of the working fluid in the electric generation systems (gas turbine air inlet as well as steam from the steam turbines) requires an effective cooling medium The present work is a part of a research project conducted in the Islamic University of Madinah, Saudi Arabia, to present reasonable solutions to pre-cool the inflow air to the air-cooled condensers in electric generation systems in KSA, when the ambient air temperature increases above the design air inlet temperature. An extensive amount of measurements has been carried out to investigate the effects of soil parameters on a buried earth-air heat exchanger. Among many parameters, the current research focused on investigating the effects of soil moisture contents and degree of compaction-represented by relative density-on the thermal performance of the buried heat exchanger. Results of the study show that; an average daily basis, a significant induced air temperature drop of 23.5 °C is achieved at the soil moisture content of 30 % and soil compaction of 91.17 %. Furthermore, the effectiveness of considered BEAHE is increased by about 33.34 % when the soil compaction is increased from 26.47 % to 91.17 %.

Thermal performance predictions and tests of a novel type of flat plate solar thermal collectors by integrating with a freeze tolerance solution

A novel design concept of Flat Plate Solar Collectors (FPSCs) is conceived and developed by integrating with a freeze-tolerant (so-called ‘ice immune’) solution using flexible silicone tubing. It is intended to directly run water in the solar thermal systems with the FPSCs instead of using expensive anti-freeze fluids, and to remove secondary heat transfer facilities (e.g. an extra tank with a buried heat exchanger). Successful development of such kind of solar thermal collectors will enable a reduction of installed cost of conventional solar thermal systems without needing secondary heat transfer facilities. In the prophase design, thermal performances of FPSCs with two configurations, i.e. the serpentine tube type and the header riser type, were predicted based on the collector lumped thermal capacitance model alongside CFD (Computational Fluid Dynamics) calculations. Then two prototypes of FPSCs with the ice-immune silicone tubing (one AES serpentine tube type, one modified Chinese micro-heat-pipe-array panel) were made to determine the collector performance and compared to an original AES solar keymark reference panel via experimental tests. The results show that the Chinese micro-heat-pipe-array panel performs better than the AES header riser solar keymark panel in terms of flow rate per m2 collector aperture area, while the AES serpentine tube panel with silicone tubing performs somewhat lower than the solar keymark with Tm∗≤0.035 and better than the solar keymark when Tm∗>0.035. The serpentine tube panel and the Chinese micro-heat-pipe-array panel both integrated with silicone tubing for freeze tolerance are proven to be effective as the modification doesn’t compromise the collector thermal performance markedly.

An evaluation for spiral coil type earth-air heat exchanger at different climate conditions

ABSTRACT The experiments have proven that the temperature level of subsoils in the summer and winter is, respectively, lower and higher than the ambient air temperature. The buried tubes in the soil can use this potential to cool or heat the ambient air. The spiral tube heat exchanger is a good choice for earth-air heat exchangers as they are buried in a fixed buried depth, and they serve higher heat transfer coefficient due to the existence of secondary flow in the cross section of tube. In this research, the spiral tube, in an earth-air heat exchanger, has been analyzed, and the performance has been evaluated in different eras and areas of use. Cities have been selected under different climatic conditions in Iran and thus the system had been simulated in both heating and cooling modes. Furthermore, the effects of various factors, such as the depth level of buried heat exchanger in soil, the velocity and the temperature of inlet air on the exhaust air temperature, as well as the thermal load absorbed by the heat exchanger, are all being investigated. Higher buried depth of the heat exchanger increases the transferred heat, while the increase in the inlet air velocity reduces the heat exchange between the soil and the air. Tabriz which is a city located in a semi-arid climate in Iran, serves the best heating and cooling loads; whereas Babolsar another city in Iran which has a humid subtropical climate, indicates the poorest performance of all in comparison to other cities and climate levels in Iran.

A New Analytical Model for the Estimation of Three-phase Rock-soil Thermal Conductivity for Geothermal Utilization

As a kind of clean energy resource, geothermal energy is widely utilized in many fields, especially for heating and ventilation through heat exchangers buried underground. The use of geothermal heat exchange system will contribute to energy saving as well as building sustainable, therefore it has become more and more popular in recent years. Rock-soil thermal conductivity plays a noticeable role in the performance of ground buried heat exchanger. In the present study, a new analytical model was proposed to describe the spatial structure of the multiphase rock-soil and relative position of solid, liquid and gas phase. Through analyzing the relationship among different phases and coding FORTRAN program, the model structure parameters can be obtained for the calculation of parallel thermal resistances. The expression of the thermal conductivity derived from the model was then applied to obtain the thermal conductivity of the Tripoli sand from North Africa, before the results were compared with previous tests. After comparative analysis, the newly proposed model in this study was proved accurate in predicting the thermal conductivity of the chosen soil with around 20% averaged relative error, which will contribute to the prediction of rock-soil thermal properties as well as the design of ground buried heat exchanger.

The performance of a buried heat exchanger system for PV panel cooling under elevated air temperatures

Performance of photovoltaic (PV) panels relies on the surrounding working temperature. In hot climatic regions most of the incident solar radiation is dissipated as accumulating heat on the PV panel causing adversely affect on PV performance due to elevated temperatures. An effective cooling technique is needed in order to keep the performance of PV module at acceptable level. In the current paper, the viability of a newly PV cooling system based on integrating PV panel with a buried heat exchanger (BHE) has been experimentally investigated. To prove the idea, the atmospheric air with various elevated temperatures 35 °C, 40 °C and 45 °C and flow rates 0.0228 m3/s, 0.0248 m3/s, 0.0268 m3/s and 0.0288 m3/s is forced to the (BHE) and the exit pre-cooled air is then simultaneously introduced to regulate the temperature of the PV back surface. The accomplished experiments revealed that integrating (BHE) to the PV panel is a promising active cooling system that can effectively regulate the operating temperature and consequently improve the PV electrical conversion efficiency. Empirical correlations for the PV module electrical and thermal efficiency as a function of the ambient air temperature (ºC), solar irradiance (W/m2), and air flow rate (m3/s) are proposed in a good agreement with the experimental results.

Numerical investigation of the backfilling material thermal conductivity impact on the heat transfer performance of the buried pipe heat exchanger

Based on the in-site thermal response experiment of a shallow geothermal energy plant in Guiyang, a model characterizing the heat transfer process between well-backfill material-ground material of a buried double vertical U-tube underground heat transfer system is established. Moreover, the influence of the thermal conductivity of the backfill material on the heat exchange efficiency for the summer condition is investigated. Results show that the maximum error between the outlet water temperatures calculated from model and obtained from field testing is less than 1.7%, which verifies the correctness of the model. When the thermal conductivity of the backfill is low, the heat in the buried heat exchanger is basically limited in the domain of the borehole. Furthermore, it is observed that the heat transfer efficiency increases, as the thermal conductivity increases. When the thermal conductivity of the backfill is close to that of the surrounding rock, the heat transfer efficiency increases in a faster rate. However, when the thermal conductivity of the backfill further increases, the heat transfer efficiency insignificantly increases. It is found that higher thermal conductivity of the backfill material has limited improvement on the heat exchange capacity of the buried pipe.

Study on Temperature Field of Buried Heat Exchanger in a Residential District

The long-term stable operation of soil source heat pump system depends on the recovery characteristics of soil temperature field. Using DEST-h model of Hefei a high-rise residential buildings and FLUENT software to simulate software of soil source heat pump are analyzed through the summer and autumn season recovery after the change of soil temperature field around the buried pipe. It is pointed out that the intermittent operation of the compensation tower soil-coupled heat pump is beneficial to promote the recovery of soil temperature field. It has certain engineering application value for the improvement of soil source heat pump performance in hot summer and cold winter area.

Numerical Investigation of the Physical Properties Effect on the Thermal Performance of a Vertical Geothermal Heat Exchanger

Low-temperature geothermal energy is a promising technique for heating and cooling residential and commercial premises, especially since it is one of the green energy solutions that respect the environment. The principle of this technique is based on thermal exchange between the heat pump and the basement using a vertically buried heat exchanger. This is usually made of a U-shaped tube inserted vertically in a borehole made in the ground and filled with a filler material. The purpose of the present study is to vary the different construction materials of the U-tube, the filling material and the soil, in order to obtain the most energy-efficient parameters. The evolution of temperature and heat flux as a function of time has been highlighted for different combinations. Knowing that an experimental study requires a considerable monetary fund, the present model has been validated using previously literature results. Recommendations on the choice of different materials of the geothermal heat exchanger are proclaimed at the end of this work.

Study on accurate identification of soil thermal properties under different experimental parameters

The thermal properties of soil are vital in designing the length of buried heat exchangers. Based on a matured experimental technique commonly used for thermal response test in situ, a simulation model was built in this paper to determine the thermal properties of soil. Several sets of thermal response tests were carried out using the simulation model under different experimental conditions. The line heat source model (LHSM) and the column heat source model (CHSM) were applied to identify the thermal properties of soil. In order to analyze the variation of the identification error of the thermal properties under different experimental conditions, the identification results were then compared with the original setting values. The results showed that both identification methods could produce some errors in the identification of soil thermal and physical parameters under different experimental parameters. However, the accuracy of identification could be improved by selecting an appropriate test time. The best test time was duration to determine the thermal properties of soil was defined as an optimum length of time when the identification error was minimized. The best test time duration varied with different experimental parameters and different identification models. It is found that the best test time duration of the column heat source model was shorter than the one of the line source model. Giving longer lead time before counting the test time duration, increasing the thermal conductivity of the backfill material and heat injection, or reducing the circulating fluid flow rate, could effectively shorten the best test time duration.

Heat Transfer Visualization of the Application of a Cooling Pipe in Sand with Flowing Pore Water

AbstractThe authors introduce an experimental method that focuses on the flow-thermal relation and the visual observation of heat distribution around a cooling pipe in sand under the flow condition. This experiment aims to understand the effect of seepage flow on the heat transfer behavior of sand under cooling and freezing conditions due to a vertically buried heat exchanger pipe. This method combines a flow test, cooling or freezing processes, and the visual observations of temperature profiles with saturated Toyoura sand as the heat transfer medium. A vertical copper pipe inserted into the sand medium is used as the thermal exchanger, while an infrared thermograph is used to measure and visualize the surface-temperature profile of the specimen. The heat transfer distribution behavior is investigated by conducting a series of cooling and freezing experiments on the specimens with variation of the flow rate condition, initial condition, and pipe depth. The thermograph observation results show how water f...

One underground heat exchanger for multiple heat pumps

A heat pump that is coupled thermally with the ground extracts heat from the soil in winter, and discharges heat in the summer. The coupling is made through a buried heat exchanger. In this paper we explore the idea of using a single heat exchanger that serves more than one heat pump. Each heat pump draws its mass flow rate from the heat exchanger and, in addition, a background flow rate circulates permanently through the heat exchanger. The places where the heat pumps are connected to the exchanger vary. The objective of the design is to select the configuration of the multi-component system such that the total enthalpy flow rate delivered to the heat pumps is larger and the total pumping power is smaller. The paper documents the effect of geometry (the connections) and the relative sizes (mass flow rates of heat pumps) on the total enthalpy flow rate. The paper shows the parametric domain in which the design with a single heat exchanger is superior in comparison with the classical design where each heat pump is connected to its own heat exchanger.