| Abstract |
In the course of the growing decentralized utilization of geothermal energy, many shallow geothermal installations will be constructed, in particular in footings/foundations and paved surfaces. Most of these installations are located on and in partially saturated soils, which due to the presence of water and air in variable proportions, form a three-phase system. By using backfill materials, which usually stand in direct thermal and hydraulic contact to the surrounding soil, a huge increase in heat exchange efficiency can often be achieved. Thermal interference in these backfill materials and soils leads to changes in the water content and, consequently, to altered hydraulic and thermal properties. The changes of these properties in such porous three-component systems are more difficult to predict than in pure porous two-component systems, such as solid and water or solid and air because the additional third substance interacts with the two others. To provide technically and economically efficient heat input or heat withdrawal (e.g. by a seasonal heat storage installation), modelling of hydraulic and thermal properties of the backfill material and soil requires the input of reliable thermal and hydraulic parameters. Two experiments allowing for simultaneous determination of several hydraulic and thermal parameters at variable water contents of a sample were developed accordingly. From the data, interrelation between thermal conductivity and water content, as well as interrelation between thermal conductivity and water retention characteristics are derived. The retention characteristics of backfill material or soil are determined in an evaporation test, while unsaturated hydraulic conductivity and thermal conductivity are measured simultaneously as a function of the water content. Thermal conductivity is determined using a full space line source embedded in the sample. In this experiment, primarily fine clastic, undisturbed soil samples or backfill materials are characterized during dewatering. Sandy soils have been studied in a columnar experiment using frequency domain reflectometry sensors to determine the water content, tensiometers and thermal conductivity sensors at different levels of the sample. The soil samples in the column can be irrigated from top and bottom. These tests currently represent a variety of experiments carried out to generate a statistically representative number of measurements on different soil types. To implement these data into finite-element-modelling, it is required to describe them by mathematical expressions representing the interdependence of water retention, unsaturated hydraulic and thermal conductivity. Various mathematical models are developed on the basis of these data and validated against the growing data set. |