Underground Heat Research Articles

Assessing coupling between soil temperature and potential air temperature using PALM-4U: implications for idealized scenarios

Abstract. Underground heat extremes, amplified by factors such as underground infrastructure or poorly adjusted geothermal systems, have long been discussed in the geosciences. However, there is little emphasis on the exchange between these subsurface heat extremes and the atmosphere. To address the issue, this study investigates the impact of varying soil temperatures on potential air temperatures in an idealized domain using the turbulence- and building-resolving large-eddy-simulation urban microclimate model PALM-4U (Parallelized Large-Eddy Simulation Model for Urban Applications). This involves two steps. First, we test if and how idealized domains can be simulated, and second, the coupling between surface and subsurface energy fluxes, or rather temperatures in air and soil, is in focus. We develop several scenarios, distinguishing between cyclic and Dirichlet/radiation boundary conditions along the x axis, between summer and winter, and between various land cover types. Our results demonstrate that cyclic boundary conditions induce modifications in potential air temperatures due to changes in soil temperature. The magnitude of the impact varies with respect to the tested land covers, which primarily affect absolute temperatures. The time of day and season have a larger influence on the magnitude of the modifications. A 5 K increase in subsurface temperatures at 2 m depth results in a maximum increase of 0.38 K in near-surface potential air temperatures during winter between 09:00 and 10:00 local time after 3 d of simulation. When soil temperatures are decreased, we find predominantly inverse patterns. The least influence is found during summer at 09:00, when elevated soil temperatures increase potential air temperatures by only 0.02 K over short and tall grass and by 0.18 K over bare soil. When using Dirichlet/radiation boundary conditions, the atmosphere cannot develop freely, and changing soil temperatures do not impact potential air temperatures. These results help enhance our understanding of the coupling between soil and atmospheric temperatures and also provide recommendations for the “simulatability” of idealized but reality-oriented scenarios in PALM-4U. This is one of the first studies to demonstrate that heat and cold sources in the soil can affect atmospheric parameters.

Ensemble-based assisted history matching of DTS monitoring data in HT-ATES systems: application to a real-life case study

Underground heat storage is an important element in accelerating the energy transition. It can significantly contribute to CO2 emission reduction and cost savings since it is one of the cheapest forms of energy storage and it enables the seasonal storage of large energy surpluses from sustainable sources, e.g. wind, sun, geothermal. Numerical models are used for the prediction of thermal behavior important in establishing the high efficiency of the high temperature aquifer thermal energy storage (HT-ATES) systems. However, the lack of exact knowledge of the subsurface conditions introduces modeling uncertainty. It is therefore important to employ approaches that reduce subsurface uncertainty. History matching is a methodology where the numerical models are updated to match historical observations that will in turn not only increase understanding of the subsurface but also improve accuracy of the model predictability of future behavior. In this research, models of the first large-scale operational HT-ATES system in Middenmeer, the Netherlands, were used to evaluate the thermal evolution in the storage aquifer and the over- and underburden clay layers. The HT-ATES system, consisting of a hot and warm well, with a monitoring well inbetween, became operational in the summer of 2021. The extensive monitoring program implemented for the first few operational years provided an opportunity to study the performance of such a system from an environmental and operational point of view. A state-of-the-art assisted history matching approach was applied to the first storage cycle, using a coupling between history matching software and the thermal flow simulator. This approach was compared to a more traditional single-model manual history matching method. Rock properties of the aquifer and over- and underburden layers were updated in the randomly generated prior ensemble of models to fit the simulated temperature evolution measured down the monitoring well with the distributed temperature sensing (DTS) data. The observations gathered during the second year of operations were used to validate the accuracy of the prediction capabilities of the updated models. The obtained results indicate the value of history matching to improve understanding of the subsurface conditions for HT-ATES systems and obtain models with better predictability of the future behavior of heat in the storage reservoir and overburden formations. Such improved models are instrumental in providing engineers with a better quantitative grip on the environmentally responsible storage potential and heat deliverability of the target storage site, which is important to achieve cost-effective site-specific design (e.g. number of wells, well placement) and performing operational strategies (e.g. injection/production rates and temperatures) for new HT-ATES systems. Moreover, the benefits of the assisted history matching approach over manual method are highlighted and both approaches are validated where the assisted history matching method produced more accurate predictions than the manual approach.

Experimental investigation on thermal conductivity of tar-rich coal under the simulated in-situ conditions

The thermal conductivity of tar-rich coal is an important parameter for optimizing the underground heating method. In this study, the thermal conductivity of tar-rich coal was measured in an experimental system in which the in-situ conditions can be simulated. The pore-fracture structure of tar-rich coal samples under different pyrolysis conditions was reconstructed through the micro computed tomography to reveal the reason for the thermal conductivity variation. The moisture, pyrolysis temperature, and pore-fracture structure play the major role in the thermal conductivity of tar-rich coal at different temperature stages during the in-situ pyrolysis. At 300–500 ℃, the thermal conductivity undergoes different trends for various in-situ stresses. The drop trends of the thermal conductivity occur under the C3.1&A2.6 MPa (representing a confining stress of 3.1 MPa and an axial stress of 2.6 MPa) and C4.8&A4.0 MPa. By contrast, the elevated trends in the thermal conductivity occur at C6.4&A5.3 MPa, C7.7&A6.4 MPa, and C9.3&A7.8 MPa. At 300–500 ℃, the tremendous volatiles of tar-rich coal are released, which not only extends the pore-fracture structure but also weakens the strength of the coal matrix. The thermal conductivity is decreased from 0.0544 W/(m⋅K) to 0.0298 W/(m⋅K) at 300–500 ℃ as the volume fraction of pore-fracture in tar-rich coal increases from 18.29 % to 53.16 % under C4.8&A4.0 MPa. However, the volume fraction of pore-fracture in tar-rich coal is reduced from 53.16 % to 20.24 % at C4.8&A4.0 to C9.3&A7.8 MPa under 500 ℃, demonstrating the compressed pore-fracture structure of tar-rich coal. The in-situ stress exceeds the strength of the coal matrix, inhibiting the pore-fracture expansion and thus increasing the thermal conductivity. Besides, the fitting equations of the thermal conductivity varying with temperature and in-situ stress are achieved. The variation mechanism of thermal conductivity can guide the underground heating method of coal in applications.

Harnessing geothermal energy in Hungary

This paper provides a summary of the current status of geothermal energy utilization in Hungary, with a brief reference to geothermal potential. Approximately 1000 thermal wells are in operation, producing from two principal aquifers: carbonate aquifers (predominantly Mesozoic) and Upper Miocene siliciclastic aquifers. The utilization of geothermal energy in Hungary commenced with balneological exploitation, with documented evidence dating back to the Roman era. Of particular note is the city of Budapest, which has the distinction of being home to 16 thermal spas, a fact that has led to its designation as the spa capital of Europe. In the Hungarian portion of the Pannonian Superbasin, 26 municipalities have implemented geothermal district heating systems. Furthermore, geothermal energy plays a significant role in the agricultural sector, with approximately 300 thermal wells currently in operation. Nevertheless, the generation of geothermal electricity is still in its infancy, with only one operational geothermal power plant. However, based on the exploration permits submitted, there are significant prospects for further development. The same is true for ground source heat pumps, where there is considerable potential for growth, particularly in comparison with our neighbouring country's utilization. Recent research directions are promising, ranging from the use of geothermal energy from abandoned hydrocarbon wells as deep borehole heat exchangers, to the potential for extracting critical materials from thermal water, to exploring the possibility of underground heat storage. The favourable geothermal conditions in Hungary offer the potential to achieve up to 15–20 times the current production of 6.5 PJ, assuming the application of existing technologies. This presents a significant opportunity for the transition to a more sustainable energy source, particularly in the context of heating.

Comparison between new enhanced thermal response test methods for underground heat exchanger sizing

For the efficient design and implementation of a Ground Source Heat Pump (GSHP) system, the local subsoil stands as the core element. Alongside the conventional Thermal Response Test (TRT), recent research has developed improved approaches that garner more detailed information about ground thermal properties. One such technique is the fiber optic-based distributed thermal sensing. It relies on copper wires to thermally stimulate the ground, while optical fibers collect temperature variations over time along the cable. Another pioneering technology, the enhanced GEOsniff (produced by enOware GmbH), enables high-resolution, spatially-distributed representation of subsoil thermal properties along the Borehole Heat Exchanger (BHE) via wireless data transmission. This study compares and discusses data acquired through these two innovative techniques at the new campus for the humanities of the University of Padova, situated in Northern Italy's Eastern Po river plain. The findings are further juxtaposed with conventional TRT results, in terms of thermal conductivity and borehole thermal resistance. The thermal conductivity vertical profiles are also compared with direct measurements conducted on samples. These advanced techniques show promise in aiding the optimization of borehole length design, particularly in geological settings of heightened complexity.

Особливості і розвиток технологій виявлення витоків трубопроводів тепло- та водопостачання в умовах зношеності та мілітарних впливів

The article is dedicated to analyzing the current state of underground heat and water supply pipelines in Ukraine, particularly under conditions of significant wear and tear and military impacts. The challenges associated with the operational detection of hidden leaks are exami¬ned, as well as the capabilities of known methods for identifying damaged sections and leak locations. The development of acoustic and correlation leak detectors is discussed as a promi¬sing direction for improving pipeline diagnostics in war conditions and amid network dete¬rioration. Several innovations are proposed, including a new parametric correlation method, spatial-frequency selection of useful leak signals, and other advancements to enhance the diag¬nostics of critical infrastructure. These measures aim to improve the effectiveness of leak de¬tection, ensure the continuity of essential services, and strengthen the resilience of the energy system in the face of current challenges.

Evaluation of Electrochemical Properties and Life Prediction of Sensor Wire in Leak Detection Systems of Underground Heating Pipelines

In this study, we investigated the electrochemical properties and lifespan of the NiCr (NiCr 8020) sensor wire of a resistance leaking detection (LD) system to detect pipe corrosion and leakage in an actual district heating (DH) system. The temperature and applied stress of the sensor wire during the actual operation of the resistance LD system of the DH system were derived through simulations and calculations. The anodic dissolution of the sensor wire was accelerated with the increased temperature and the applied current. The corrosion type changed from localized corrosion, such as pitting, to uniform corrosion. The applied stress caused ductile fracture of the thinned sensor wire by anodic dissolution. In conclusion, we confirmed that in the resistance LD system of a DH system, where current and stress are applied at high temperatures, the sensor wire becomes thin due to the anodic dissolution and subsequent ductile fracture. In addition, the lifespan of the sensor wire was derived according to the resistance level measured in the resistance LD system of the DH system. Our findings contribute to preventing failure and improving the reliability of the resistance LD systems of DH systems.

Validation of an Enhanced Drinking Water Temperature Model during Distribution

Drinking water temperatures are expected to increase in the Netherlands due to climate change and the installation of district heating networks as part of the energy transition. To determine effective measures to prevent undesirable temperature increases in drinking water, a model was developed. This model describes the temperature in the drinking water distribution network as a result of the transfer of heat from the climate and above and underground heat sources through the soil. The model consists of two coupled applications. The extended soil temperature model (STM+) describes the soil temperatures using a two-dimensional finite element method that includes a drinking water pipe and two hot water pipes coupled with a micrometeorology model. The extended water temperature model (WTM+) describes the drinking water temperature as a function of the surrounding soil temperature (the boundary temperature resulting from the STM+), the thermal sphere of influence where the drinking water temperature influences the soil temperature, and the hydraulics in the drinking water network. Both models are validated with field measurements. This study describes the WTM+. Previous models did not consider the cooling effect of the drinking water on the surrounding soil, which led to an overestimation of the boundary temperature and how quickly the drinking water temperature reaches this boundary temperature. The field measurements show the improved accuracy of the WTM+ when considering one to two times the radius of the drinking water pipe as the thermal sphere of influence around the pipe.

Experimental research on the heat balance of independent heat extraction-release double helix energy pile

The use of spiral buried pipe of ground source heat pump in civil defense engineering shows great potential, exploring its complex heat transfer features and thermal balance through experimental approaches is essential. This paper utilizes an independent heat extraction-release double helix energy pile to manage condensation heat discharge in underground projects. An experimental platform was established to compare three scenarios: pure heat release, centralized heat extraction, and uniform heat extraction. The pure heat release mode resulted in substantial soil heat accumulation, whereas the centralized heat extraction and uniform heat extraction demonstrated significant reductions in average temperature rise, with decreases of 24.7 % and 31.3 % near the buried pipes, and 27.3 % and 46.3 % in the central regions. The heat-extracting pipe under centralized heat extraction and uniform heat extraction conditions remove 18.7 MJ and 20.3 MJ of accumulated heat, amounting to 10.3 % and 11.2 % of the total heat dissipation, respectively, effectively mitigating the thermal accumulation in the soil during operation. Besides, the outlet temperature of the heat-extracting pipe is predominantly influenced by the highest temperature in the heat exchange zone, lowering the inlet temperature effectively increases the temperature difference between the inlet and outlet, enhancing the amount of heat removed. The accumulation of underground heat can enhance the heat extraction performance of the pipes, and increasing the water flow rate of the heat-releasing pipe positively impacts the heat extraction capacity. After soil temperatures rise, running the heat-extracting pipe with cooler inlet water and a moderately higher flow rate is advised for extracting the accumulated heat.

Closed loop low enthalpy geothermal systems for underground heat exchange: public records, available data, implications for planning

[Article in Italian] Impianti geotermici a bassa entalpia a circuito chiuso per il geoscambio nel sottosuolo: registri pubblici, dati disponibili, implicazioni per la programmazione

Analysis of thermotechnical characteristics of the thermoelectric film heating system

Modern world trends in increasing the energy efficiency of heat supply systems, in particular, are aimed at decentralizing the supply of heat to consumers, and also involve the use of low-temperature heating systems. The Institute of Technical Thermophysics of the National Academy of Sciences of Ukraine together with the specialists of the Department of Thermal and Alternative Energy of the National Technical University of Ukraine “Ihor Sikorskyi Kyiv Polytechnic Institute” carried out scientific research and analysis of the main thermal characteristics of the electrothermal film heating system (model ZHDM-WM-220-180 /225/270) and accessories for it manufactured by ZHONGHUI (HEILONGJIANG) UNDERGROUND HEATING CORPORATION (People's Republic of China). The results of the study of the main thermal technical characteristics of the thermoelectrical film heating system are given. The analysis of the design of the heat-dissipating mat showed relatively high reliability of its components and acceptable operational characteristics for various objects of the housing and communal economy.

ROGER: A method for determining the geothermal potential in urban areas

Shallow geothermal energy is increasingly adopted for heating and cooling purposes because of the short pay-back time of initial installation investments. As a result, a relevant concentration of Ground Heat Exchangers is being experienced in urban areas. Planning issues thus arise to manage interferences and optimize the use of underground heat resources without depletion, harm to the environment nor efficiency losses on heat pumps or plant oversizing. This study provides a rational approach to optimise geothermal resources based on the use of Geographic Information Systems and transient 3D Thermo-Hydro numerical models. An optimised semi-analytical formula for the assessment of Borehole Heat Exchangers geothermal potential in hydrodynamic conditions is developed through a parametric numerical study. The long-term performances of BHE subjected to groundwater velocity in the range of 0 to 1 m/day were analysed with multiple aquifer thermal parameters. This analytical expression allows a fast and accurate assessment of the potential even in large areas without leading to excessively conservative evaluations. This may serve designers in the preliminary sizing of installations and city planners in the development of appropriate policies for the promotion and management of shallow geothermal resources. An example of the application to the central district of the city of Turin (Italy) is also shown.

Heat flow density measurement during non-destructive testing

The object of the study is the development of a device capable of accurately and reliably measuring heat flux density in various environments. The development of a heat flux density meter designed for non-destructive analysis of thermal processes in various fields of application is presented. The developed device is intended for evaluating the thermal insulation condition of underground pipelines. The functionality of the heat flow device relies on comparing standard temperature values with experimental ones measured on the soil surface. To ensure accurate and reliable measurement of heat flux density, the basis is a thermoelectric battery converter, which uses the auxiliary wall method. The heat flow density measuring device is constructed in the shape of a restricted cylinder, with one base serving as the working surface, while the second base establishes thermal contact with the body at ambient temperature. Embedded heaters enable the generation of heat flow through the thermoelectric sensor in directions perpendicular to its base. For calibrating the heat flux device, experiments were conducted using a standard copper-constantan calibration table. Temperature increments were determined from thermo electromotive force, and tests were performed on an existing heating network. The conducted measurements validate the fundamental feasibility of employing the proposed device for implementing the non-destructive thermal testing method on underground heating mains. The results of the experiment can be used not only for research, but also for monitoring and regulating processes in various fields of science and technology. The developed heat flux meter promises a significant contribution to the development of modern methods for analyzing thermal processes. The dimensions of the thermoelectric battery converter are also determined and the coefficient (kq) should be in the range from 4.0 to 12.0 W/(m2⋅mV), and the electrical resistance should be in the range of 12–20 kOhm

Badanie efektu dławienia w pożarze tunelu

As per the emergency ventilation strategy, air velocity of 3 m/s in case of the longitudinal ventilation is sufficient for smoke control in all fire conditions. Numerical experiments were carried out with FDS software to estimate the numerical value of the critical velocity. Numerical models were realized in 0-6% slope tunnels with a 1% step for 5, 10, 20, 30 and 50 MW fires for four types of fuel: gasoline, diesel fuel, oil and firewood. The paper notes that the dynamic pressure induced by a strong fire is much higher than the static pressure of tunnel jet fans. As a result, following the algebraic summation of positively-directed ventilation flows and the negatively-directed flows induced by fire, an intense back layering occurs, which casts doubt on the suitability of the specified emergency ventilation strategy when designing the fire ventilation. The critical ventilation speed of 3 m/s cannot cope with the traction caused by fire, expressed by the ascending movement of the high-temperature and low-density combustion products. The paper discusses the numerical modelling results with an adiabatic underground heat exchange model and presents typical tunnel fire modelling plans, which correspond to an inclined tunnel for ascending and descending ventilation flows as well as a horizontal tunnel. The article gives the regularities obtained by the numerical models of changes in the variables of average air temperature and density, average carbon monoxide, average carbon dioxide and soot concentrations. According to the emergency ventilation strategy, critical velocity is an important value and a major determinant of back layering prevention in sloping tunnels. Although many papers have been devoted to this problem, the obtained results differ much. The present paper shows that strong fires induce much greater dynamic pressures than the static pressures of the tunnel jet fans are. Consequently, the flows caused by these forces, as they move in different directions, following their algebraic summation, cause a strong back-layering in case of positive ventilation flows, i.e., when the ventilation flow is descending and the fire seat is found at a lower point compared to the air supply portal. The new results can be used to develop fire ventilation plans as well as life-saving and emergency control solutions in the operating tunnels for personnel and rescuers.

Configuration method for medium-deep ground source heat pump system considering renewable energy consumption and smart grid interaction

Medium-deep ground source heat pump (GSHP) systems have the advantages of low carbon, high heat exchange intensity, good thermal storage capacity, and intermittent operation characteristics, which are conducive to renewable energy consumption and grid demand response. The source-side parameters affect the configuration of underground borehole engineering, heat pump units, and other equipment. However, current research mainly focuses on the underground heat exchanger modeling, underground heat exchange, and system operation control, while there are few studies on multi-factor ground source-side parameter design and optimal configuration. In this study, the indicator analyzing the unsteady features and thermal balance state of medium-deep geothermal resources was introduced, and the source-side water temperatures were optimized based on the dynamic characteristics of the geothermal and building sides. A simulation configuration methodology for the source-side flow rate and storage capacity was developed considering the electricity prices, life-cycle cost (LCC), and grid interaction. The optimization of the ground source-side design temperature, flow rate, and capacity parameters of residential and office building heating scenarios were analyzed in northern China, respectively. The results showed that the borehole inlet and outlet temperature can be designed to 5.5 °C/17.5 °C under the annual sustainable state, and the optimal flowrate can be set to 65.77 m3/h, to achieve the lowest cost and highest heating season guarantee rate for the residential scenario. For the office building with optimal system and storage capacity, the average annual LCC is 192,000 yuan/year, with the cumulative peak shaving of 55.8 %, and an increase of 103.3MWh renewable electricity consumption.

Coupled system for underground heating exchange and solar heat-humidity regulation in greenhouse: Experimental study and simulation analysis

In winter, greenhouses located in mid to high latitudes are required a function of dehumidification and warming due to the low outdoor temperature. Focusing on small spires greenhouses, this study regulates the low temperature and high humidity environment inside the greenhouse and proposes a new-type of greenhouse heat and humidity regulation system by combining composite solid desiccant dehumidification with ground heat exchange system. The greenhouse internal environment is warmed and dehumidified at night by utilizing the adsorption effect of the compound dehumidifier and the sensible heat exchange between the heat exchange system in the ground and the greenhouse. Solar thermal energy is collected during the day using multi-curved tank collector (MTC) for thermal storage in the geothermal system and desiccant desorption. The rationality of pipeline layout is verified through CFD simulation, and the indoor temperature and humidity distribution during the system operation is explored. The experimental results show that the moisture adsorption capacity of the composite desiccant SG-CaCl2 for 10h is 0.511 g/g. The optimal regeneration temperature is 80–100 °C. This system can reduce the nighttime relative humidity of the greenhouse in winter from 96.5 % to 83.6 %, and the average indoor air temperature after warming is 12.8 °C. The heat exchange efficiency of the underground heating exchange system is 0.38. During the desorption phase on daytime, the maximum outlet temperature of MTC is 103.8 °C, and the average heat collection efficiency is 0.42. The system can effectively regulate the air environment in the greenhouse to the appropriate zone for crop growth by combining solar thermal collector technology with recyclable solid dehumidification technology.

Analysis of geothermal heat recovery from abandoned coal mine water for clean heating and cooling: A case from Shandong, China

Exploiting heat from abandoned mine water using heat pumps is attracting increasing attention worldwide. China has a large number of abandoned coal mines, but there are few applications of geothermal heat recovery from mine water. Here, we report a new case from Shandong, North China. Based on actual geological and underground space structures, numerical analysis is performed to study the flow and heat transfer in the complex connection network among roadways. For given temperature differences on the source side, the underground heat transfer is greatly influenced by flow rates in roadways. Under the acceptable performances and long-term stability of heat pumps, the maximum heat transfer capacity of roadways reaches 697.3 kW at the flow rate of 100 m3/h, with the power per unit length of 151.3 W/m, indicating a promising heat recovery potential. When switching between the extraction and injection well, both fluid directions and the distribution of flow rates in roadways change obviously, thereby affecting the underground heat transfer. Besides dominant flow paths, there exist weak flow paths, which can be identified by numerical analysis, and necessary blocking can be applied to optimize the flow and heat transfer. Excessively long horizontal straight roadways for heat exchange should be avoided.

Prediction and optimization of heat extraction in ground source heat pump system using surrogate model

Ground source heat pump (GSHP) system, which extracts underground heat for building heating and cooling, uses renewable energy to decrease CO2 emissions for the goals of carbon peaking and carbon neutrality. Hence, the prediction and optimization work is significate especially for existing GSHP system. This study focuses on the prediction of heat extraction and optimization of operational strategy based on a three-story house installed with GSHP system located in Cleveland, Ohio, USA. Firstly, the numerical model is built to simulate operation of ground heat exchanger (GHE). Then, Latin hypercube sampling (LHS) is employed to generate different parameter combinations that are calculated to get the heat extraction results for training XGBoost-based surrogate model. Finally, the particle swarm optimization (PSO) is used to realize the optimization of operational strategy. The results indicate that the numerical model can simulate the GHE well, and the accuracy of surrogate model can reach 0.990. The COP of GSHP system is maximized to protect the soil heat balance and reduce the electrical energy consumption. The optimization method also makes it possible to evaluate the heat extraction capability, and prepare for the extra heat demand in advance.

Thermodynamic analysis of pavement roads underground heat harvesting through water pipelines for effective management of urban heat island effect integrated to space cooling applications

Urban heat island effect drastically affects the climate by approximately increasing the surroundings temperature by 2°C, thus, rises the energy demand for space cooling. As roads/asphalt pavements makes 20–30% of urban areas that are fully exposed to solar radiations, this unused energy can be utilized by laying down a network of underground cross-linked polyethylene (PEX) pipes full of heat transfer fluids. In this research, the cold-water from a reservoir is pumped to various pipeline configurations to gain effective heat from the pavements/roads. This water circulation absorbs the heat, increasing the temperature (10–25°C) of cold water, also decreasing the pavement temperature by about 15°C. Furthermore, a solar collector is attached to increase temperature of circulated water to be used by low-grade temperature applications. Here, double effect absorption cooling (DEAC) and ejector cooling systems have been analysed thermodynamically for efficient space cooling application. The overall energy and exergy efficiency of DEAC integrated system is calculated to be 40.79% and 5.08%, respectively. The rate of cooling obtained by the evaporator is 6.75 kW, when rate of heat supplied is 4.3 kW. For the ejector cooling integrated system, the overall energy and exergy efficiencies are recorded as 15.27% and 3.38%, respectively. The rate of effective cooling through ejector cooling system is 2.7 kW.

Thermal Properties of Backfill Material for Underground Heat Exchange Applications

We are investigating the effect of the thermal properties of backfill material on the efficiency of underground thermal energy systems. We have used aluminum to increase the thermal properties of clay-bentonite and to investigate its potential as a backfill material in underground heat exchange applications. The measurements of thermal properties were made at room temperature using the Transient Plane Source (TPS) technique. The measurements were based on an infinite homogeneous medium using different composites of aluminum and clays in amounts of aluminum concentration ranging from 6 to 25 percent of the sample’s weight. The results confirmed an overall relative increase of 170% and 135% in the thermal conductivity and diffusivity, respectively as the Al content increased by 25%. The estimated average of the thermal transmittance (U-value ) of these samples was in the range from 0.40 to 1.1 W/m2K, which was directly proportional to the relative increase in the measured properties. Our findings can be applied to underground thermal energy systems, ground source heat pumps, and solid dielectric underground transmission lines.