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Summary: Due to their relatively high thermal effectiveness and ease‐of‐use, direct contact heat exchange units are a favorable choice in heat recovery applications. Today, spray‐based direct heat exchangers are commonly used for a variety of applications, including evaporative cooling and heat recovery. Yet in spite of their versatility, these heat exchangers are not well‐understood, mainly due to their complex liquid/gas interactions, which may deem poorly‐designed systems as undesirable and infeasible. Spray nozzle height and type (hollow and full cone), droplet diameter distribution, inlet air velocity and humidity, and water spray velocity are key parameters required to establish a clear understanding of direct heat exchange performance. To better understand such a multivariable phenomenon, this article offers a comprehensive study of the impact of these key factors. The main purpose of this study is to evaluate the validity of two different methods proposed for evaluating the performance of heat and mass transfer in large‐scale direct contact heat exchangers, namely one‐dimensional and three‐dimensional approaches. The results of the present study show that while the one‐dimensional approach is able to capture the experimental data within 10% of agreement with experiment, the three‐dimensional approach reduces this error to 5%. In addition, this study aims to provide a framework to conduct future parametric studies with the goal of highlighting key performance factors understanding of which are a necessity for improved design of direct contact heat exchangers used in mine waste heat recovery applications. Novelty Statement: To investigate the performance of an industrial‐scale direct (spray) heat exchange system with a vertical configuration for application in mine waste heat recovery, two different models, namely; '1‐D' and '3‐D', are developed.Accuracy of two models in predicting heat transfer is evaluated through comparison with experimental data.A series of parametric studies are conducted to examine the impact of various operating parameters on the performance of heat exchange system. [ABSTRACT FROM AUTHOR]

With the increasing engineering applications of geothermal borehole heat exchangers (BHEs), accurate and reliable mathematical models can help advance their thermal design and operations. In this study, an analytical model with a time-dependent heat flux boundary condition on the borehole wall is developed, capable of predicting the thermal performance of single, double, and multiple closed-loop BHEs, with an emphasis on solar- and waste-heat-assisted geothermal borehole systems (S-GBS and W-GBS) for energy storage. This analytical framework begins with a one-dimensional transient heat conduction problem subjected to a time-dependent heat flux for a single borehole. The single borehole scenario is then extended to multiple boreholes by exploiting lines of symmetry (or thermal superposition). A final expression of the temperature distribution along the center line is attained for single, double, and multiple boreholes, which is verified with a two-dimensional finite-element numerical model (less than 0.7% mean absolute deviation) and uses much lesser computational power and time. The analytical solution is also validated against a field-scale experiment from the literature regarding the borehole and ground temperatures at different time frames, with an absolute error below 6.3%. Further, the thermal performance of S-GBS and W-GBS is compared for a 3-by-3 borehole configuration using the analytical model to ensure its versatility in thermal energy storage. It is concluded that our proposed analytical framework can rapidly evaluate closed-loop geothermal BHEs, regardless of the numbers of boreholes and the type of the heat flux on the borehole wall. [ABSTRACT FROM AUTHOR]

Summary: The geothermal potential available from deep underground mines has yet to be utilized. However, stope‐coupled heat exchangers (SCHE) are aiming to take advantage of the unused low‐grade geothermal energy. Backfilled stopes provide a unique opportunity to install nonlinear heat exchangers, as the geometry is not limited to the shape of a borehole. Helical pipes deliver superior fluid mixing and heat exchange compared to straight pipes, due to the effect of the secondary flow within the helical pipe. The helical closed‐loop geothermal heat exchanger enables the backfilled stopes of the mine to be repurposed as thermal energy storage units. This article delves into the experimental results from a unique state‐of‐the‐art laboratory scale helical closed‐loop heat exchanger with varying thermophysical parameters. Additionally, a novel conjugate numerical model is developed and its results are validated against the base case of the experimental studies. Additionally, the numerical model is validated in a spatial‐temporal sense with thermocouple data from the experimental rig. The numerical model is also applied to a helical SCHE situated within a backfilled stope for the first time. The results of the numerical model suggest that the pumping rate through the SCHE has a significant effect on the heat exchange rate and the overall energy transfer between the SCHE and the backfill. Additionally, the temperature contours from the numerical model suggest that a decreased pitch/helical diameter will increase the storage capacity of the helical SCHE. Overall, an average of 2.5 MW can be stored over the first 4 days of geothermal charging with the investigated full‐scale SCHE, boasting a pseudo‐steady‐state storage rate of 1.7 MW. [ABSTRACT FROM AUTHOR]

Double-pipe helical heat exchangers are integral to contemporary mechanical refrigeration equipment. Modification of flow geometry has been widely adopted to enhance heat transfer performance of a heat exchanger. The objective of this study is to numerically investigate heat transfer and entropy generation in a double pipe helical heat exchanger with various cross-sections. A computational model for laminar convective heat transfer was developed and validated against the results from previously published literature. To capture entropy generation, the entropy balance equation for open system is adopted. Effect of inner pipe Dean number, inner pipe and annulus inlet mass flow rate ratio, eccentricity, and flow configuration (co-flow and counter-flow) were examined and discussed in light of computational results. To ensure fair comparison, the considered geometries have same inner pipe cross-section area, same annulus cross-section area, and same outer surface area of inner pipe. The results suggest that square cross-section offers best performance in term of heat transfer, pressure drop and entropy generation. In addition, concentric configuration is more appropriate for low flow rate application while eccentric outer configuration is more suitable for high flow rate application. [ABSTRACT FROM AUTHOR]

Conjugate heat transfer of liquid-gas fin and tube heat exchangers is widely used in industry. However, heat transfer characteristics of such system is difficult to predict as the limiting heat rate can be from either side. This paper aims to quantify the conjugate heat transfer performance of fin and tube heat exchangers via mathematical modeling. Three-dimensional conjugate fluid flow and heat transfer model is developed and validated against the state-of-the-art experimental data and existing analytical correlations. Turbulent k-ε model has been employed for the fluid flow and heat transfer modeling. Statistical method, i.e. data reduction and multivariate nonlinear regression techniques, is implemented to analyse the results and to quantify the interaction between parameters. Wide range of parametric studies and simulations is further carried out to evaluate the significance of design, geometrical and operating parameters. According to the results of the study, larger fin length factor and fin pitch and the smaller tube diameter are in favor of fin and tube heat exchangers within the Reynolds number range of 3000–12,000. Finally, a novel conjugate heat transfer correlation for liquid-gas finned tube heat exchangers is proposed to assist engineers for practical designs and applications. The results suggest that our new correlation gives rise to a more accurate conjugate heat transfer prediction as compared to that of traditional non-conjugate counterpart. • A Conjugate heat transfer model of liquid-gas finned tube heat exchanger in turbulent regime is developed using CFD method. • A novel non-dimensional heat transfer correlation for finned tube heat exchanger is proposed. • Thermal performance of finned tube heat exchangers with the various geometric and operating parameters is analyzed. • Finned tube heat exchanger has a better thermal performance with the larger fin pitch and length factor and smaller tube diameter. [ABSTRACT FROM AUTHOR]

• A numerical model to assess the performance of mine exhaust heat recovery system. • Model comparison and validation carried out based on existing literature. • Thermally coupled and decoupled heat transfer models are compared. • Hourly ambient air temperature data is used to estimate energy and carbon savings. • Impacts of heat exchangers effectiveness and mine intake air flow rate are studied. Mine air conditioning is a common practice where liners and other shaft units should be protected against freezing temperatures in cold regions. In conventional mine intake air conditioning applications, heating is usually performed with natural gas burners, accounting intense energy consumption and emissions. This fossil fuel dependency issue can be effectively mitigated with a heat recovery and reutilization technique called mine exhaust heat recovery. Mine exhaust heat recovery systems can ensure large savings with minimum fossil burning at relatively lower operating costs. Present paper investigates the applicability of a novel numerical heat transfer model to assess the performance of mine exhaust recovery systems with comparably higher precision. Unlike other work done in the field, this work also considers the coupling effect between the heat exchangers involved. Accordingly, the operational significance and the impact of thermal coupling in the model is highlighted in detail with an emphasis on the project financial feasibility. Furthermore, a parametric study is also presented identifying the impact of different parameters i.e. heat exchanger effectiveness and airflow rate on savings for a real-life Canadian underground mine example. [ABSTRACT FROM AUTHOR]

Remote communities that have limited or no access to the power grid commonly employ diesel generators for communal electricity provision. Nearly 65% of the overall thermal energy input of diesel generators is wasted through exhaust and other mechanical components such as water-jackets, intercoolers, aftercoolers, and friction. If recovered, this waste heat could help address the energy demands of such communities. A viable solution would be to recover this heat and use it for direct heating applications, as conversion to mechanical power comes with significant efficiency losses. Despite a few examples of waste heat recovery from water-jackets during winter, this valuable thermal energy is often discarded into the atmosphere during the summer season. However, seasonal thermal energy storage techniques can mitigate this issue with reliable performance. Storing the recovered heat from diesel generators during low heat demand periods and reusing it when the demand peaks can be a promising alternative. At this point, seasonal thermal storage in shallow geothermal reserves can be an economically feasible method. This paper proposes the novel concept of coupling the heat recovery unit of diesel generators to a borehole seasonal thermal storage system to store discarded heat during summer and provide upgraded heat when required during the winter season on a cold, remote Canadian community. The performance of the proposed ground-coupled thermal storage system is investigated by developing a Computational Fluid Dynamics and Heat Transfer model. [ABSTRACT FROM AUTHOR]

The experimental and computational analyses of a 500 m deep coaxial borehole heat exchanger system for geothermal power generation are studied in this paper. The experiments are carried out on a high temperature resourced well with an average thermal gradient of 0.38 °C/m. The experiment lasts 456 h, and the findings are described in terms of fluid inlet and outlet temperatures, subsurface temperature distribution profiles over time, and flowrate. The in-situ ground temperature distribution profile is measured using Distributed Temperature Sensing System for different hours of the experiment. A detailed, three-dimensional, unsteady-state, finite volume-based computational model has been developed, which solves conjugate fluid-flow and heat transfer phenomena. The simulation outcomes are compared to the results of the experiments. With validated numerical model, to determine the circle of influence, temperature profile of the ground is observed at various radial distances from the borehole. A parametric study is performed by varying inlet temperature of the fluid, mass flow rate, and the thermal conductivity of the ground. The numerical model developed also predicts the temperature recovery behavior of the ground. The results indicate that the average output thermal power for 456 h from the geothermal system ranges from 172 kW to 262 kW based on operating condition chosen and the total thermal energy generated changes between 82 MWh and 194 MWh from a single borehole. After the extraction is terminated, around 86% of the initial ground temperature is restored after 456 h. • Field-test experiment was performed for 456 h in a fresh drilled well. • 3D conjugate numerical model was developed and validated. • Parametric study was performed for different operating conditions. • Sensitivity analysis was conducted to study the degree of influence. • Temperature recovery period was predicted with the developed numerical model. [ABSTRACT FROM AUTHOR]

• A zero-carbon heating system for remote underground mines has been established. • A rock-pile seasonal thermal energy storage has been incorporated with exhaust heat recovery system. • A valid analytical model has been presented showing the techno-economic feasibility. • Financial analysis has been conducted; the proposed model has a promising payback period. Millions of liters of diesel or propane are burnt every year in remote mines for pre-heating the intake air, leaving a monumental size of carbon footprint every year. A novel zero-carbon heating model for remote underground mine has been investigated. Both the technical and financial feasibility of integrating a validated seasonal thermal energy storage with an exhaust heat recovery system in the proposed model have been investigated. An analytical model has been developed and inspected along with different set-point temperatures, heat exchanger effectiveness, and vulnerability of fossil fuel price to judge the performance of this integrated approach. Energy savings by the exhaust heat recovery system and the seasonal thermal energy storage have been enumerated separately, and based on that, a rockpile-based seasonal thermal energy storage has been sized reasonably throughout the study. The system reaches the breakeven point in 2.6–4.8 years, depending on the operating conditions. The life cycle assessment compares the amount of carbon emission from different available heating systems for remote mines with the proposed integrated heating system. The present system provides a typical remote underground mining operation with an opportunity to eliminate the entire 15–23 million kilograms of carbon emission for heating purposes. [ABSTRACT FROM AUTHOR]

This study introduces a novel concept of semi closed-loop double-pipe heat exchangers with a controlled fractured zone at the well bottom to improve the geothermal heat extraction. The proposed model is benefitted from the reliable design of a double-pipe and the higher heat extraction rate of open geothermal systems. A conjugate mathematical model is developed and validated to simulate geothermal heat extraction, while the fractured zone is calculated by implementing the fractal theory. The results suggest that the proposed model can significantly improve the heat extraction rate and thermal power by up to more than 90%, with an additional 28% of pumping power. Several operating parameters of the fractured zone are also evaluated, indicating that the increase of the heat extraction rate compared to the closed-loop model was within the range of 48%–144% based on its design. Overall, this novel design shows the potential to improve the thermal and hydraulic performances of the conventional closed-loop geothermal system. [ABSTRACT FROM AUTHOR]

• Heat transfer modeling in a coaxial heat exchanger by developing an efficient code. • A coupled solar thermal system with a building load demand is used in the model. • The model is validated with both experimental and fully conjugate numerical models. • The code is 430 times faster than the fully conjugate model. • The model is employed to scale up a solar-borehole thermal energy storage system. In cold climates, a borehole thermal storage system is key to supplying renewable energy year-round for heating applications. For such systems, coaxial borehole heat exchangers are getting more attention in recent years due to their superiority over other types of borehole heat exchangers. As such, more research is needed to understand the heat transfer mechanism and enhance the heat transfer process. In this study, an efficient reduced-order numerical code is developed to solve the heat transfer phenomenon in the coaxial borehole heat exchanger system for application in solar borehole thermal energy storage system. This concept assists in transferring heat between the subsurface and the Heat Transfer Fluid. In addition, the numerical solution integrates the building thermal load, thermal energy originated from the solar collector system, and heat loss to the adjacent strata and atmosphere. The developed numerical model is validated against the field-test experimental data performed with a coaxial borehole heat exchanger. The accuracy and computational speed of the model are compared its corresponding three-dimensional full-scale finite volume based model. Finally, the verified numerical code is employed to estimate the efficiency of solar-borehole thermal energy storage system for a two multi-family residential building in Ontario, Canada. The proposed model offers a novel holistic approach for estimation of the solar heat collection, geothermal heat storage/extraction, and heat loss phenomenon in a solar-BTES system accurately and efficiently. Moreover, it can serve as the basis to design solar-borehole energy storage systems of any size and at any location. [ABSTRACT FROM AUTHOR]