Due to climate-induced variations, changes in the water content of the soil occur, and consequently its hydraulic and thermal properties change as well. Bedding materials for buried energy cables or geothermal installations are usually in capillary contact to the surrounding soil. Thus, they are subject to seasonal changes in the water content caused by natural climatic variations. For the design and construction of structures using thermal energy or dissipating waste heat in the shallow ground, it is important to be able to make reliable forecasts of the thermal and hydraulic properties of partially saturated bedding materials and soils. For this purpose, the climate-related changes of the thermal and hydraulic properties have to be investigated. In this technical note, an experimental setup is described, allowing for the simultaneous measurement of several hydraulic and thermal parameters during dewatering of a specimen. From the data, interrelations between thermal conductivity and water content, water retention characteristics and unsaturated hydraulic conductivity can be derived. This is why, a common evaporation test device was extended using a thermal needle probe. This setup enabled the examination of disturbed and undisturbed soils. A further method, which is also presented in this paper, provides the possibility of investigating cementitious bedding materials in this evaporation test for the first time. This experiment takes only a few days for cementitious bedding materials as well as for undisturbed soil samples. Thus, with only one single experiment, all the basic parameters required for unsaturated hydraulic modeling can be identified. Since thermal conductivity is measured along with the hydraulic parameters, it is possible to assign a value to the thermal conductivity for many hydraulic states of the soil or bedding material.
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.
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.
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