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Thermal energy is one of the most widely encountered energy forms in our daily life. To ensure efficient utilization and conversion of this energy, the balance between supply and demand needs to be maintained. For this purpose, thermal energy storage is required. There are various thermal energy storage systems available; one of the most basic is sensible thermal energy storage which includes rock thermal energy storage (RTES). This rock‐based energy storage has recently gained significant attention due to its capability to hold large amounts of thermal energy, relatively simple storage mechanism and low cost of storage medium. Accordingly, numerous studies have been conducted to elucidate the basic flow and heat transfer mechanism and to evaluate the performance of this energy storage. The major technical challenges hindering the wide adoption of this technology are the enormous pressure drop across the storage and nonoptimal heat transfer from the heat transfer fluid to the storage medium and vice versa. These issues will directly and indirectly affect the overall cost (capital, operational, and maintenance costs) of the system. To eliminate this issue and assist further development of this technology, it is crucial to compile and extract important findings from these previous studies, identify the challenge and research gap, and draw guidelines for the upcoming research and development. At the moment, this kind of compilation does not exist. Hence, this paper is prepared with the primary objective to comprehensively review the current technology and development of RTES and to propose a potential way forward based on the pain point identified. Discussion on the nontechnical aspect such as policy and regulations as well as community awareness will also be outlined and discussed. [ABSTRACT FROM AUTHOR]

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]

Characterization of flow through packed bed of spheres with large particle diameters (O ~ m) is important in industrial applications, such as mining, geothermal, oil and gas, and construction industries. In this study, a three-dimensional pore-scale mathematical model of packed bed of spheres with large particle diameters considering uniform and non-uniform particle size distributions is developed and validated against experimental data. It was found that the available correlations fail to predict flow behavior in packed bed with larger particle sizes. Hence, Ergun and Forchheimer theories along with pore-scale mathematical model are utilized to simulate and develop permeability (Carman-Kozeny) and inertial resistance (Forchheimer) correlations which are valid for flow through packed bed of spheres with large particle sizes, i.e. sphere diameter from 0.04 to 1.2 m and porosity between 0.2 and 0.7. The developed correlation can be used in either analytical or volume-averaged computational models which is useful for industrial/practical. Unlabelled Image • Pore-scale model of flow through packed bed of spheres with large particle diameters. • Model is validated with experimental data. • Available correlations fail to predict flow behavior in packed bed with larger particles. • New correlation is proposed for Ergun and Forchheimer parameters; valid for particle size 0.04–1.2 m, porosity 0.2–0.7. • Permeability and inertial resistance are functions of porosity and particle size. [ABSTRACT FROM AUTHOR]

Underground mining is among the most energy-intensive industries and ventilation comprises a significant portion of the energy demands of this important industry. Using the vast volume of broken rock, left in a decommissioned mine pit, as a thermal energy storage mass has enormous potential to lower ventilation-related energy costs in deep underground mines. This approach facilitates moderating seasonal air temperature variations. Seasonal thermal energy storage is a cost-effective solution to improve cooling and heating process efficiencies, thereby reducing associated costs. Temperature gradients observed in the proposed storage system suggest the presence of a natural convection heat transfer mechanism that is buoyancy-driven. The effect of natural convection and a variety of heat transfer mechanisms were modeled and simulation results and field-data measurements were compared. The conjugate heat transfer and fluid flow model that was developed considers the porous rock mass in the rock-pit along with the air (i.e. fluid) blanketing the top surface. The effects of rock size, permeability and porosity were studied. It was observed that, for the range of porosities (from 0.45 to 0.20), these parameters have a small effect on the outlet air temperature and the performance of thermal storage phenomenon. The novel model compares forced (from ventilation fan) and natural (result of buoyancy) convection. Further, it incorporates the effect of design factors, such as air trench positions and flow rate of ventilated air, on energy savings. [ABSTRACT FROM AUTHOR]

• Two green heating options are proposed for a residential apartment of 826 units. • Solar-Borehole Thermal Energy Storage system vs Sewage Heat Recovery system. • Technical, economic, and environmental analysis are performed. • Comparison is made with natural gas heating system and payback period is computed. • Cost of natural gas and electricity has significant impact on preference. The extensive use of fossil fuels for space and water heating in buildings is raising CO 2 emissions into the atmosphere. To address this concern, renewable and alternative energy sources should replace conventional energy options. This paper examines the technical feasibility, economic viability, and environmental impact of substituting conventional fuel with renewable and alternative sources for space heating and hot water purposes. Two different choices are offered to replace a natural gas-fired boiler in two residential buildings with 826 apartment units: Solar-Borehole Thermal Energy Storage System and Sewage Heat Recovery System. Solar-Borehole system couples the solar thermal collectors to boreholes, storing excess energy generated by solar collectors into the boreholes and supplying it to buildings when the energy demand is high and the solar energy is not sufficient. In a sewage system, low-grade thermal energy from the sewage beneath the building's site is upgraded and supplied to buildings via heat pumps. Arguments for and against each of the heating options are discussed. Proposed systems are designed using state-of-the-art technology, and their capital and operating costs are quantified. Both of these newly proposed systems are compared against a natural gas heating system to determine energy cost savings. To make the study location independent, additional calculations are undertaken to compute energy cost savings for changing electricity and natural gas pricing rates. The findings show that both proposed technologies are technically possible to execute on the studied construction site. While the Solar-Borehole system outperforms Sewage Heat Recovery technology in the examined location, the financial viability of other sites is determined by the difference in the unit cost of energy from electricity and natural gas. The sewage system surpasses the Solar-Borehole system in areas where natural gas is more expensive. Additionally, project execution can result in yearly CO 2 emission reductions of 1.1 million kg. This study can aid in understanding the technical and economic issues of transitioning multi-family apartment complexes from convectional heating to a renewable and alternative energy option. [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]

Modern data centers are playing a pivotal role in the global economic situation. Unlike high-quality source of waste heat, it is challenging to recover the decentralized and low-quality waste heat sourced from data centers due to numerous technological and economic hurdles. As such, it is of the utmost importance to explore possible pathways to maximize the energy efficiency of the data centers and to utilize their heat recovery. Absorption chiller systems are a promising technology for the recovery of waste heat at ultra-low temperatures. In fact, the low temperature heat discharged from data centers cannot be retrieved with conventional heat recovery systems. Therefore, the present study investigated feasibility of waste heat recovery from data centers using an absorption chiller system, with the ultimate goal of electrical energy production. To fulfill this objective, a techno-economic assessment of heat recovery using absorption chiller (AC) technique for the data centers with power consumption range of 4.5 to 13.5 MW is performed. The proposed AC system enables saving electricity for the value of 4,340,000 kWh/year and 13,025,000 kWh/year leading to an annual reduction of 3068 and 9208 tons CO2 equivalent of greenhouse gas (GHG) emissions, respectively. The results of this study suggest an optimum change in the design of the data center while reducing the payback period for the investors. [ABSTRACT FROM AUTHOR]

• A rock-pile waste heat storage is proposed, and its corresponding numerical model is developed. • Both fluid flow and heat transfer are validated against experimental data. • Local Thermal Equilibrium and Non-Equilibrium approaches are compared. • Parametric studies are performed on thermo-physical properties. • Payback time analysis is performed, and the system is economically justified. Remote communities in arctic climates are solely dependent on diesel generators for continuous power supply due to their detached loci from national power gridlines or natural gas pipelines. Moreover, to get along with the harsh, long winters, these communities directly or indirectly depend on fossil-fuel based heating systems. Due to the lower efficiency of the conventional diesel engines, these generators discard a significant amount of heat through the exhaust. Occasionally, during the wintertime, this waste heat from the exhaust is used directly as a heat source after recovery. However, during summer, when the heating demand is lower or nil, this heat is commonly discarded, and its energy potential wasted. A rock-pile based seasonal thermal energy storage is a viable solution that can sustainably resolve this issue. This paper presents a validated numerical study that focuses on the coupling of a waste heat recovery system to a diesel exhaust stream offering a performance assessment of the proposed rock-pile waste heat storage system in a remote, off-grid community located in northern Canada. It examines the impacts of local thermal equilibrium versus non-equilibrium approaches as well as temperature-dependent properties, variation of air mass flow rate, particle size and finally the thermo-physical properties on the proposed model. The presented results show that air should be treated as an ideal gas and that the local thermal non-equilibrium approach should be used for its considerably higher accuracy. Techno-economic assessment of the system resulted in a relatively short payback period of less than six years. [ABSTRACT FROM AUTHOR]

Deep underground mining is highly energy intensive due to the need to overcome high pressure rise required by ventilation fans, high cooling load in summer due to rise in rock temperature and the auto-compression effect, and heating requirement in winter. Rising energy costs have led the mining industry to look for alternatives in energy-efficient systems to reduce the operating costs as well as to reduce the carbon footprint. This paper addresses the challenge by utilizing naturally available renewable energy source from seasonal cycle for heating and cooling of underground mines: heat in the summer is stored in the rock-pit to be used for heating in winter, and the “cold” energy in winter is captured within the rock-pit for cooling during summer. A three-dimensional unsteady local thermal non-equilibrium model is developed to evaluate thermal storage and heat transfer between ventilation air and rock-pit. The results suggest that the seasonal thermal energy storage of rock-pit is able to assist thermal management in underground mine and to reduce energy consumption for winter heating and summer cooling. The ventilation air temperature is about 15–20 °C higher/lower as compared to ambient temperature in winter/summer, respectively. Clearly, this shows potential application of large scale seasonal thermal energy storage systems in mining industry. [ABSTRACT FROM AUTHOR]