David Bertermann

Laboratory device to analyse the impact of soil properties on electrical and thermal conductivity

Abstract Gathering information about soil properties in an efficient way is essential for many soil applications also for very shallow geothermal systems (e.g. collector systems or heat baskets). In the field, electrical resistivity tomogramphy measurements enable non-invasive and extensive analyses regarding the determination of soil properties. For a better understanding of measured electrical resistivity values in relation to soil properties within this study, a laboratory setup was developed. The structure of this laboratory setup is geared to gather electrical resistivity or rather electrical conductivity values which are directly comparable to data measured in the field. Within this setup grain size distribution, moisture content, and bulk density, which are the most important soil parameters affecting the electrical resistivity, can be adjusted. In terms of a better estimation of the geothermal capability of soil, thermal conductivity measurements were also implemented within the laboratory test sequence. The generated data reveals the serious influence of the water content and also provides a huge impact of the bulk density on the electrical as well as on the thermal conductivity. Furthermore, different behaviour patterns of electrical and thermal conductivity in their particular relation to the different soil parameters could be identified.

Soil thermal behavior in different moisture condition: an overview of ITER project from laboratory to field test monitoring

The thermal properties of soils can be considered one of the most important parameters for many engineering projects designing. In detail, the thermal conductivity plays a fundamental role when dimensioning ground heat exchangers, especially very shallow geothermal (VSG) systems, interesting the first 2xa0m of depth from the ground level. However, the determination of heat transfer in soils is difficult to estimate, because depends on several factors, including, among others, particle size, density, water content, mineralogy composition, ground temperature, organic matter. The performance of a VSG system, as horizontal collectors or special forms, is strongly correlated to the kind of sediment at disposal and suddenly decreases in case of dry-unsaturated conditions in the surrounding soil. Therefore, a better knowledge of the relationship between thermal conductivity and water content is required for understanding the VSG systems behavior in saturated and unsaturated conditions. Key challenge of ITER project, funded by European Union, is to understand how to enhance the heat transfer of the sediments surrounding the pipes, taking into account the interactions between the soil, the horizontal heat exchangers and the surrounding environment. In detail, changes of soil moisture content in the same climatic conditions and under the same thermal stress for five different soil mixtures have been monitored in the ITER test site. The relationship with precipitation and natural/induced ground temperature variations, reaching also water freezing point, are here discussed.

Bulk density and water content-dependent electrical resistivity analyses of different soil classes on a laboratory scale

The goal of this study was to achieve relations between electrical conductivity and the most influential soil properties which should be transferable to natural field conditions. Electrical conductivity of soil is mainly affected by soil moisture. This, for many soil applications like determination of thermal soil properties, crucial relation is focused within this article. Electrical conductivity was measured in dependency of bulk density and water content by applying a laboratory setup which enables a comparison to natural in-situ conditions. For this purpose, four different pressure loads and up to 12 saturation steps were performed on the textural soil types of sand, silt loam and clay. A comparison between electrical conductivity and saturated pore volume confirms the impact of soil moisture. For the final analysis natural conditions were specified using field capacity ranges and defined bulk densities. With these predetermined ranges of water content and bulk density a healthy regulation for defining natural conditions has been constructed. Due to this generated constraint a soil texture independent relation between electrical conductivity and soil moisture content with a correlation coefficient of r2xa0=xa00.95 has been developed. This correlation was also implemented within the developed measurement tool of the GeoSurf project for determining the thermal potential of soil.

European project "Cheap-GSHPs": installation and monitoring of newly designed helicoidal ground source heat exchanger on the German test site

Nowadays, the energy price fluctuations and the economic crisis are jeopardizing the development and diffusion of renewable technologies and sources. With the aim of both reducing the overall costs of shallow geothermal systems and improving their installation safety, an European project has took place recently, under the Horizon 2020 EU Framework Programme for Research and Innovation. The acronym of the mentioned project is Cheap-GSHPs, meaning “cheap and efficient application of reliable ground source heat exchangers and pumps”; the Cheap-GSHPs project involves 17 partners among 9 European countries such as Belgium, France, Germany, Greece, Ireland, Italy, Romania, Spain and Switzerland. In order to achieve the planned targets, a holistic approach is adopted, where all involved elements that take part of shallow geothermal activities are here integrated. In order to reduce the specific costs of geothermal installations, some newly designed geometries of heat basket-type ground source heat exchanger (GSHE) are modified drastically to receive a better performance of the geothermal installation. Within the sector of very shallow geothermal systems, these new developments are also tested on six representative demonstration sites around Europe. At the German test site in Northern Bavaria, four heat basket-type GSHEs are installed and equipped with certain monitoring systems (moisture, two different temperature sensors) and various backfilling materials of different grain size classes. The different installations will be tested for 12xa0months to evaluate the best combination of the newly designed heat basket-type GSHE and corresponding backfilling material mixture.

Thermal properties variations in unconsolidated material for very shallow geothermal application (ITER project)

Abstract In engineering, agricultural and meteorological project design, sediment thermal properties are highly important parameters, and thermal conductivity plays a fundamental role when dimensioning ground heat exchangers, especially in very shallow geothermal systems. Herein, the first 2 m of depth from surface is of critical importance. However, the heat transfer determination in unconsolidated material is difficult to estimate, as it depends on several factors, including particle size, bulk density, water content, mineralogy composition and ground temperature. The performance of a very shallow geothermal system, as a horizontal collector or heat basket, is strongly correlated to the type of sediment at disposal and rapidly decreases in the case of dry-unsaturated conditions. The available experimental data are often scattered, incomplete and do not fully support thermo-active ground structure modeling. The ITER project, funded by the European Union, contributes to a better knowledge of the relationship between thermal conductivity and water content, required for understanding the very shallow geothermal systems behaviour in saturated and unsaturated conditions. So as to enhance the performance of horizontal geothermal heat exchangers, thermally enhanced backfilling material were tested in the laboratory, and an overview of physical-thermal properties variations under several moisture and load conditions for different mixtures of natural material was here presented.