New Methods for Determining the Thermophysical and Hydraulical Properties of Unsaturated and Unconsolidated Rocks

ABSTRACT The heat conductivity and diffusivity as well as the hydraulic properties of unconsolidated rocks are important parameters to quantify the conductive and convective heat transfer in the subsurface. Depending on the type and way of installation of an underground heat exchanger system the involved soil undergoes compaction, change in saturation e.g.. The most soil properties will be changed somehow, just the grain size distribution remains constant. In the operation phase the temperature and saturation are variable in time and space. The change in the ratios of the gas phase and water phase in the subsoil affects the themo-physical performance. The hydraulic conductivity of soils is a non-linear function on the water content. The heat capacity of unconsolidated rocks can be calculated from the heat capacities of the individual components and their volume fractions. In contrast, there is no linear dependence of the thermal conductivity and the water content. Established computational models provide approximations for the thermal conductivity as a function of water content and other constraints such as temperature. However, for the additional determination of convective transport behavior, the unsaturated hydraulic conductivity and water retention function, largely dependent on the tortuosity of the pore space, have to be determined simultaneously. Here, for the integrated study of soil mechanics, hydraulic and geothermal properties of unconsolidated rocks, a heat and temperature conductivity meter has been developed and patented. The device allows measurements of samples either under constant pressure of up to 7.6 Mpa which means soil compaction can be varied stepwise or it can be driven at a constant sample volume. The treatment temperatures of the soil samples can be varied from -10 to +80 ° C. Additionally to the parameters such as temperature, pressure, volume and water content, the capillary tension is recorded during the measurement. For the simultaneous study of water transport characteristics and the unsaturated conductivity of undisturbed unconsolidated rock samples, an evaporation test has been developed. It is equipped with a full-space line source to determine the thermal conductivity. This allows the simultaneous measurement of thermal conductivity, water retention characteristics and hydraulic conductivity of a soil sample up to the air entry point of the ceramic tensiometer cap (approx. -780 hPa). The functionality of the equipment and methods has been validated and the devices were used for soil investigation in numerous projects. Determination of the design parameters of shallow geothermal systems and of the heat transfer of burried cables is thepurpose of the methods presented in this study. The data compile mathematical models for the thermo-physical parameters of soils. A mathematical function for calculating the thermal conductivity in dependence of the capillary tension is introduced here and the test results of geotechnical and geothermal soil parameters of sand, clay and silt are presented

Heat Dissipation in Variable Underground Power Cable Beddings: Experiences from a Real Scale Field Experiment

To prevent accelerated thermal aging or insulation faults in cable systems due to overheating, the current carrying capacity is usually limited by specific conductor temperatures. As the heat produced during the operation of underground cables has to be dissipated to the environment, the actual current carrying capacity of a power cable system is primarily dependent on the thermal properties of the surrounding porous bedding material and soil. To investigate the heat dissipation processes around buried power cables of real scale and with realistic electric loading, a field experiment consisting of a main field with various cable configurations, laid in four different bedding materials, and a side field with additional cable trenches for thermally enhanced bedding materials and protection pipe systems was planned and constructed. The experimental results present the strong influences of the different bedding materials on the maximum cable ampacity. Alongside the importance of the basic thermal properties, the influence of the bedding’s hydraulic properties, especially on the drying and rewetting effects, were observed. Furthermore, an increase in ampacity between 25% and 35% was determined for a cable system in a duct filled with an artificial grouting material compared to a common air-filled ducted system

Experiment for validation of numerical models of coupled heat and mass transfer around energy cables

The increasing decentralization of electrical energy production as well as an increasing number of fluctuating regenerative energy sources require significant investments in grid expansion. Among other assessments, an exact prediction of the thermal behavior of the cable environment is necessary to be able to design cable routes both technically and economically sufficient. To investigate the coupled thermal and hydraulic processes around a cable-like heat source with high temporal and spatial resolution under controlled boundary conditions, a cylindrical laboratory test was designed and experiments with two soils conducted. The data collected can be used to validate models of coupled heat and mass transfer around power cables. Within this study, the experimental data was compared with a modified model approach that is based on experimentally determined input data for the thermal and hydraulic properties of the examined soils. Although overall good agreement in the temperature field around the central heat source was observed, differences in the spatial distribution of the dry-out zone near the heat source led to some shift between the measured and simulated temperatures

Supplementary Information to ‘Experiment for Validation of Numerical Models of Coupled Heat and Mass Transfer around Energy Cables'

The increasing decentralization of electrical energy production as well as an increasing number of fluctuating regenerative energy sources require significant investments in grid expansion. Among other assessments, an exact prediction of the thermal behavior of the cable environment is necessary to be able to design cable routes both technically and economically sufficient. To investigate the coupled thermal and hydraulic processes around a cable like heat source with high temporal and spatial resolution at defined boundary conditions, a cylindrical laboratory test was designed and experiments with two soils conducted by Verschaffel-Drefke et al. (2021). The data collected can be used to validate models of coupled heat and mass transfer around power cables. Both the experimentally determined soil properties and the measured data of the three conducted test series are provided

Supplementary Information to ‘Heat Dissipation in variable Underground Power Cable Beddings: Experiences from a Real Scale Field Experiment'

The increasing decentralization of electrical energy production as well as an increasing number of fluctuating regenerative energy sources require significant investments in grid expansion. To prevent accelerated thermal aging or insulation faults in cable systems due to overheating, the current carrying capacity is usually limited by specific conductor temperatures. As the heat produced during the operation of underground cables has to be dissipated to the environment, the actual current carrying capacity of a power cable system is primarily dependent on the thermal properties of the surrounding porous bedding material and soil. To investigate the heat dissipation processes around buried power cables in real scale and with realistic electric loading, a field experiment consisting of a main field with various cable configurations, laid in four different bedding materials, and a side field with additional cable trenches for thermal enhanced bedding materials and protection pipe systems was planned and constructed by Verschaffel-Drefke et al. (2021). Both the experimentally determined bedding properties and the presented data of the conducted experiments are provided