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Building concentric tubes is one of biggest practical challenges in the construction of freeze-pipes of selective artificial ground freezing (S-AGF) applications for underground mines. In this study, the influence of tubes eccentricity on phase-front expansion (i.e., expansion of the frozen body) and energy consumption of S-AGF systems is analyzed. A 1 + 1D semi-conjugate model that solves two-phase transient energy conservation equation is derived based on the enthalpy method. The 1 + 1D model is first validated against experimental data and then verified with a fully conjugate model from our previous work. After that, the 1 + 1D model is extended to a field-scale of typical underground mines to examine the effect of freeze-pipe eccentricity. The results show that concentric freeze-pipes form the desired frozen ground volume 17% faster than eccentric freeze-pipes. Also, the geometrical profile of the phase-transition front of the frozen ground is found to be significantly influenced by the freeze-pipe eccentricity. Furthermore, in the passive zone, where S-AGF coolants are isolated from the ground to reduce energy consumption, freeze-pipe eccentricity can increase the coolant heat gain by 20%. This percentage can increase up to 200% if radiation heat transfer is minimized.

Solidification consists of three stages at macroscale: subcooling, freezing and cooling. Classical two-phase Stefan problems describe freezing (or melting) phenomenon initially not at the fusion temperature. Since these problems only define subcooling and freezing stages, an extension to characterize the cooling stage is required to complete solidification. However, the moving boundary in solid-liquid interface is highly nonlinear, and thus exact solution is restricted to certain domains and boundary conditions. It is therefore vital to develop approximate analytical solutions based on physically tangible assumptions, like a small Stefan number. This paper proposes an asymptotic solution for a Stefan-like problem subject to a convective boundary for outward solidification in a hollow cylinder. By assuming a small Stefan number, three temporal regimes and four spatial layers are considered in the asymptotic analysis. The results are compared with numerical method. Further, effects of Biot numbers are also investigated regarding interface motion and temperature profile.

The artificial ground freezing (AGF) systems are designed to operate continuously for an extended period of time to control the groundwater seepage and to strengthen the groundwater structure surrounding excavation areas. This mode of operation requires a massive amount of energy to sustain the thickness of the frozen body. Therefore, it is of great interest to propose new concepts to reduce energy consumption while providing sufficient structural stability and safe operation. This paper discusses the principle of the freezing on demand (FoD) by means of experiment and mathematical model. A lab-scale rig that mimics the AGF process is conceived and developed. The setup is equipped with more than 80 thermocouples, flow-meters, and advanced instrumentation system to analyze the performance of the AGF process under the FoD concept. A mathematical model has been derived, validated, and utilized to simulate the transient FoD concept. The results suggest that the overall energy saving notably depends on the coolant’s temperature; the energy saving increases while decreasing the coolant inlet temperature. Moreover, applying the FoD concept in an AGF system leads to a significant drop in energy consumption. Keywords: Artificial ground freezing, Freezing on demand, Energy saving, Conjugate heat transfer, Porous media

Building concentric tubes is one of biggest practical challenges in the construction of freeze-pipes of artificial ground freezing (AGF) applications for deep underground mines. In this study, the influence of tubes eccentricity on phase-front expansion (i.e., expansion of the frozen body) and energy consumption of AGF systems is analyzed. A 1+1D semi-conjugate model that solves two-phase transient energy conservation equation is derived. The model is firstly validated against experimental data and then verified with a fully-conjugate model from the literature. After that, the model is extended to a field scale of typical deep underground mines to study freeze-pipe eccentricity. The results show that an eccentric freeze pipe can reduce the phase-front expansion by around 25%, as compared with a concentric one. Also, the geometrical profile of the phase-front is significantly influenced by the freeze-pipe eccentricity. Furthermore, in the passive zone, where AGF coolants are isolated from the ground to reduce energy consumption, freeze pipe eccentricity can increase the coolant heat gain by 10%. This percentage can increase up to 200% if radiation heat transfer is minimized.

Artificial ground freezing (AGF) has historically been used to stabilize underground structure. Numerical methods generally require high computational power to be applicable in practice. Therefore, it is of interest to develop accurate and reliable analytical frameworks for minimizing computational cost. This paper proposes a singular perturbation solution for a two-phase Stefan problem that describes outward solidification in AGF. Specifically, the singular perturbation method separates two distinct temporal scales to capture the subcooling and freezing stages in the ground. The ground was considered as a porous medium with volume-averaged thermophysical properties. Further, Stefan number was assumed to be small, and effects of a few site-dependent parameters were investigated. The analytical solution was verified by numerical results and found to have similar conclusions yet with much lesser computational cost. Keywords: artificial ground freezing, Stefan-like problems, singular perturbation, porous media, outward solidification.

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.

Hybrid thermosyphons have been installed in several permafrost protection applications due to their ability to operate continuously irrespective of seasonal temperature variations. In winter seasons, the thermosyphon operates passively by transferring energy between the ground and cold ambient air; while in warmer/summer seasons, an active refrigeration plant is used as a substitute for colder climate to extract the heat and freeze the ground. This study presents a novel conjugate mathematical model of hybrid thermosyphons based on thermal resistance networks, coupled with transient two-phase artificial ground freezing heat flow based on the enthalpy method. The model is validated against laboratory experimental data from literature and field test data from the Giant Mine in Yellowknife, Canada. Various design and operating parameters are investigated with the aim to maximizing ground heat extraction while minimizing energy consumption. The results indicate that active refrigeration substantially accelerates the formation of the desired frozen ground volume. After a certain time, passive cooling mode can be continuously adopted to reduce the energy consumption of refrigeration plants while maintaining the desired frozen ground thickness. Finally, the model can be used to assist engineers and practitioners to optimize the design of hybrid thermosyphon for permafrost protection or other ground freezing applications.

Selective artificial ground freezing (S-AGF) applications usually extend to very deep levels (more than 400 meters); numerical modeling of such large AGF applications encounters two main issues: (i) Predicting the variable heat extraction capacity along the freeze-pipe depth and (ii) the extremely long computational time. In this paper, we develop novel semi-conjugate reduced-order models that accurately predict heat extraction along the freeze-pipe while substantially reducing the computational time. In regards to the thermal modeling novelty, the freeze-pipe boundary condition of S-AGF is mathematically derived considering the development of the coolant flow temperature and boundary layer. As for the computational novelty, fast semi-conjugate reduced-order algorithms are developed for S-AGF, with the optional incorporation of analytical solutions and spatial correction. The models are validated against experimental data and verified with established fully-conjugate models. The thermal results demonstrate that the phase transition front profile of the frozen ground is primarily shaped by the thermal development of the flow. On the other hand, the computational results reveal that the computational time of the reduced-order algorithms is decreased by more than 99%, as compared with the established models. In short, our proposed reduced-order models are proven to be reliable and computationally efficient, which shows potential for practical field application.

In some artificial ground freezing (AGF) applications of civil or mining projects, only particular parts of the ground need to be frozen. Selective artificial ground freezing (S-AGF) is an AGF technique where a specific portion of the freeze pipe is insulated, usually by an air gap, to prevent any undesirable ground freezing. In this study, a laboratory scale experimental setup that mimics actual S-AGF systems has been established. The experimental rig is built in a fully controlled environment and equipped with advanced instrumentation. Additionally, a mathematical model coupling the flow of the coolant, air, and porous ground is developed by solving the conservation equations of mass, momentum, and energy. The mathematical model is then validated against the experimental measurements of the ground and outlet coolant temperatures. Moreover, the natural convection inside the air gap is further validated at higher Rayleigh numbers with an experimental study from the literature. The results indicate that the energy consumption of AGF plants can be significantly reduced by applying the S-AGF concept, as compared to convectional AGF systems. Also, it is found that the optimum air gap thickness, L opt , relates to the cavity height, H, and Rayleigh number, Ra H , as L opt ~ 2 HRa H - 1 / 4 .

In this work, an optimisation analysis is carried out in order to estimate the optimal power generation expansion strategy, using sustainable power generation technologies for the United Arab Emirates (UAE). Eight alternative configurations of sustainable power generation systems, such as, the integration of carbon capture and storage (CCS) technologies and solar-based power generation systems are examined and compared with the business as usual (BAU) scenario for a range of natural gas price. Based on the input data and assumptions made, simulations indicate that the BAU scenario (i.e. expansion of the power system using conventional power generation technologies) is the least cost option. However, in the case UAE will move towards the use of sustainable power generation technologies, for the reduction of carbon dioxide emissions, the natural gas combined cycle technology integrated with CCS systems and the use of concentrated solar power systems with 24/7 operation are the most suitable alternative tech...