Thermal Conductivity Of Backfill Research Articles

Effect of gradation on the thermal conductivities of backfill materials of ground source heat pump based on loess and iron tailings

The addition of iron tailings to backfill soil of ground source heat pump (GSHP) systems can efficiently improve backfill thermal conductivity. The engineering properties of iron tailings and soil are significantly affected by particle gradation. Therefore, the effect of gradation on the thermal conductivity of backfill material composed of iron tailing and loess was investigated. The thermal conductivities of natural (bad) gradation and manmade (good) gradation materials with different iron tailings content, water content, and dry density were tested and analyzed. Results showed that the thermal conductivity of manmade gradation materials was usually higher than that of natural gradation materials. For the same water content and dry density, the influence of gradation on thermal conductivity decreased with increasing water content. For saturated material, the maximum difference in thermal conductivity caused by gradation ranged from 39.1% to 8.6% compared to dry material. The maximum increases in thermal conductivity caused by gradation were approximately 0.12–0.24 W/(m·K). The maximum increase percentage of thermal conductivity increases with decreasing water content, and the value range is 28.4–60.5%. The empirical formula for thermal conductivity was established based on the water saturation. The influence of gradation on formula parameters is analyzed. Finally, suggestions for gradation selection in practical applications of GSHP backfill materials are given.

Energy performance of seasonal thermal energy storage in underground backfilled stopes of coal mines

Thermal storage of the energy is essential for district heating systems to mitigate intermittency related issues. The extensive cavities created after extraction of ores/coal in mines could provide a unique opportunity for storage and extraction of thermal energy. This paper shows that underground backfilled stopes present a viable potential for being retrofitted to thermal energy storage batteries. The present study evaluates the energy performance of thermal energy storage in underground backfilled stopes by installing heat exchange tubes prior to backfill placement. A numerical fluid mechanics and heat transfer model for backfill stopes was developed in Fluent to investigate the heat exchange inside the tubes as well as the heat transfer happening within the backfill during the heat storage and extraction process. The proposed model is compared against an analytical heat transfer model to determine its reliability and accuracy. The results obtained from the validated numerical model showed that energy storage system in a typical underground backfilled stope of coal mines can provide about 23 GWh of thermal energy storage capacity with an average recovery rate of 60% during extraction cycles. Accordingly, several sensitivity analyses were conducted to define the optimum center-to-center distance for the pipe installation to attain the best system performance. Through a series of parametric studies, it was found that thermal conductivity of backfill and rock, circulating fluid mass flow rate, initial rock temperature and thermal saturation time constitute the most significant influencing parameters on energy storage performance of such applications. It can be concluded that thermal energy storage systems in backfilled stopes are relatively more techno-economically feasible, compared to conventional borehole energy storage systems, in terms of their significantly lower drilling/installation costs and higher hosting temperature.

PSO of power cable performance in complex surroundings

Underground cable performance indices such as maximum cable temperature and ampacity are non-continuous functions of the configuration parameters such as depth and width of various trench layers. In this respect, existing traditional gradient-type methods cannot be used to optimise such performance indices. This study presents an efficient methodology for optimising power cable thermal performance with respect to configuration parameters involving cable spacing, depth of burial and size of backfill. The new methodology integrates the powerful features of the finite elements (FEs) technique coupled with the flexibility and effectiveness of the particle swarm optimisation (PSO) algorithm in order to handle various geometrical parameters in the complex surrounding operating environment. The introduced methodology is tested using a commercial FE simulation package used in conjunction with developed PSO code. The integrated methodology can be employed to minimise the maximum cable temperature, minimise installation cost or maximise cable ampacity. Practical applications are presented for 15 kV cables, which demonstrate the usefulness and versatility of the presented methodology. Notable improvements have been achieved by optimising the cable trench configuration parameters. For example, the cable ampacity was maximised, optimising the cable spacing, barrier depth and backfill thermal conductivity, which resulted in an appreciable increase of 4.5%.

Thermal analysis of underground power cable system

The paper presents the application of Finite Element Method in thermal analysis of underground power cable system. The computations were performed for power cables buried in-line in the ground at a depth of 2 meters. The developed mathematical model allows determining the two-dimensional temperature distribution in the soil, thermal backfill and power cables. The simulations studied the effect of soil and cable backfill thermal conductivity on the maximum temperature of the cable conductor. Also, the effect of cable diameter on the temperature of cable core was studied. Numerical analyses were performed based on a program written in MATLAB.

The effect of soil and cable backfill thermal conductivity on the temperature distribution in underground cable system

The paper presents a mathematical model of heat transfer in the underground cable system. The computations were performed for flat formation of power cables buried in the ground at a depth of 2 meters. The model allows determining the two-dimensional temperature distribution in the soil, thermal backfill and power cables. The simulations studied the effect of soil thermal conductivity on the maximum temperature of the cable conductor. Furthermore, the effect of thermal backfill soil conductivity on the cable conductor temperature was studied. Numerical analyses were performed based on a program written in MATLAB.

An Experimental Study on the Effective Parameters of Thermal Conductivity of Mine Backfill

This paper investigates the thermal conductivity of cemented backfill. Two major measurement methods, namely, steady state and unsteady state, are compared. Effect of backfill design parameters such as pulp density and binder content, along with sample homogeneity and sample size, are studied. Thermal conductivity of backfill with and without sodium silicate additive, as a new binder, is investigated. A novel method for interpreting thermal conductivity of backfill is presented by introducing backfill as a multiphase porous material for which the effects of physical parameters, such as degree of saturation, porosity, and thermal conductivity of solid particles, are studied. The results of this research help to better understand the thermal interaction of backfill with the surrounding rock mass and therefore contribute to progressive improvement of backfill technology.

Factors influencing ampacity and temperature of underground power cables

This paper presents the factors that influence ampacity and temperature rise of three-phase, single-core 33- and 500-kV XLPE underground cables (UGC) using CYMCAP software. These factors are conductor cross-sectional area, soil thermal resistivity, cable burial depth, cable separation, sheath bonding, bedding and backfill heights and thermal conductivities, nearby parallel heat source, formation of dry zone, loss tangent and segmented conductors. Results reveal that increasing the separation distance between phases gives higher ampacity, contrary to the burial depth. The rate of conductor temperature reduction due to the increase in the bedding thermal conductivity is more pronounced than that achieved by increasing backfill thermal conductivity. Furthermore, increasing the native thermal conductivity and/or the maximum conductor temperature increases the UGC ampacity and consequently increases the induced sheath voltage. Sheath losses are significant in transmission UGC where the load currents are always high. High conductor temperature and hence degradation rate is expected for UGC carrying currents of highly fluctuating loads. UGC must be derated as they age (increasing loss tangent), or when dry zones are formed around them, or when a nearby parallel heat source. Finally, it is found that the increase in the number of conductor segments nonlinearly increases the UGC ampacity.

Thermal analysis of power cables in multi-layered soil. II. Practical considerations

For Pt.I, see ibid., vol.8, no.3, p.761-771, 1993. A practical study is presented of the heat dissipated from the cable in the surrounding soil using the finite difference method as well as the energy conservation principle proposed by the authors. The trench dimensions, soil properties and environmental conditions are varied to simulate the actual field situation. The backfill thermal conductivity, the width of the trench, and the cable size are among the main parameters that influence heat dissipation from the cable. >