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22 pages, 5628 KiB

Open AccessArticle

A Practicable Guideline for Predicting the Thermal Conductivity of Unconsolidated Soils

by David Bertermann, Mario Rammler, Mark Wernsdorfer and Hannes Hagenauer

Soil Syst. 2024, 8(2), 47; https://doi.org/10.3390/soilsystems8020047 - 18 Apr 2024

Cited by 2 | Viewed by 2749

For large infrastructure projects, such as high-voltage underground cables or for evaluating the very shallow geothermal potential (vSGP) of small-scale horizontal geothermal systems, large-scale geothermal collector systems (LSCs), and fifth generation low temperature district heating and cooling networks (5GDHC), the thermal conductivity (λ) of the subsurface is a decisive soil parameter in terms of dimensioning and design. In the planning phase, when direct measurements of the thermal conductivity are not yet available or possible, λ must therefore often be estimated. Various empirical literature models can be used for this purpose, based on the knowledge of bulk density, moisture content, and grain size distribution. In this study, selected models were validated using 59 series of thermal conductivity measurements performed on soil samples taken from different sites in Germany. By considering different soil texture and moisture categories, a practicable guideline in the form of a decision tree, employed by empirical models to calculate the thermal conductivity of unconsolidated soils, was developed. The Hu et al. (2001) model showed the smallest deviations from the measured values for clayey and silty soils, with an RMSE value of 0.20 W/(m∙K). The Markert et al. (2017) model was determined to be the best-fitting model for sandy soils, with an RMSE value of 0.29 W/(m∙K). Full article

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21 pages, 2677 KiB

Open AccessArticle

Influence of Groundwater on the Very Shallow Geothermal Potential (vSGP) in the Area of a Large-Scale Geothermal Collector System (LSC)

by Mario Rammler, Robin Zeh and David Bertermann

Geosciences 2023, 13(8), 251; https://doi.org/10.3390/geosciences13080251 - 19 Aug 2023

Cited by 2 | Viewed by 1733

Abstract

The water balance in the very shallow subsurface can be influenced by capillary rise due to a high groundwater table. Since moisture content is an important factor for the thermal conductivity of soils, this can also have an influence on the very shallow geothermal potential (vSGP). For this reason, the effect of spatial and seasonal variations in groundwater tables on moisture content in essential depth layers was investigated at a large-scale geothermal collector system (LSC) in Bad Nauheim, Germany. Quasi-one-dimensional simulations using the FEFLOW^® finite-element simulation system were employed to determine site-dependent and seasonally varying moisture contents, from which thermal conductivities were derived. The model setup was previously validated based on recorded moisture contents. The simulations resulted in groundwater-related maximum seasonal and spatial differences in thermal conductivity of 0.14 W/(m∙K) in the LSC area. Larger differences of up to 0.21 W/(m∙K) resulted for different soil textures at the same depth due to different thermal properties. The results indicate that an efficient design of LSCs requires a sufficiently detailed subsurface exploration to account for small-scale variations in grain size distribution and groundwater level. Full article

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21 pages, 3772 KiB

Open AccessArticle

Comparison of Measured and Derived Thermal Conductivities in the Unsaturated Soil Zone of a Large-Scale Geothermal Collector System (LSC)

by Mario Rammler, Hans Schwarz, Jan Wagner and David Bertermann

Energies 2023, 16(3), 1195; https://doi.org/10.3390/en16031195 - 21 Jan 2023

Cited by 5 | Viewed by 2153

Abstract

The design, energetic performance, and thermal impact of large-scale geothermal collector systems (LSCs) are dependent on the thermal conductivity of unsaturated soils (λ). The aim of this study was to investigate the benefits of two different λ measurement methods using single-needle sensor measuring devices on a laboratory scale. Since large-scale determinations are required in the context of LSCs, the potential for deriving λ from electrical resistivity tomography measurements (ERTs) was also examined. Using two approaches—the continuous evaporation method and the punctual method—thermal conductivities of soil samples from Bad Nauheim (Germany) were measured. The results were compared with averaged λ derived from three ERT sections. With the evaporation method, significant bulk density changes were observed during the experimental procedure, which were caused by the clay content and the use of repacked samples. The punctual method ensures a sufficiently constant bulk density during the measurements, but only provides a small number of measurement points. The thermal conductivities derived from ERTs show largely minor deviations from the laboratory measurements on average. If further research confirms the results of this study, ERTs could provide a non-invasive and unelaborate thermal exploration of the subsurface in the context of large-scale infrastructure projects such as LSCs. Full article

(This article belongs to the Special Issue Shallow Geothermal Energy in Densely Inhabited Areas)

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19 pages, 2865 KiB

Open AccessArticle

Suitability of Screened Monitoring Wells for Temperature Measurements Regarding Large-Scale Geothermal Collector Systems

by David Bertermann and Mario Rammler

Geosciences 2022, 12(4), 162; https://doi.org/10.3390/geosciences12040162 - 4 Apr 2022

Cited by 5 | Viewed by 2572

Abstract

Groundwater temperature (GWT) is usually measured using screened monitoring wells (MWs). The aim of this study was to investigate whether MWs are suitable for monitoring the effects of large-scale geothermal collector systems (LSCs) on GWT, focusing on possible vertical flows within the MWs due to both natural and forced convection. Comparative temperature depth profiles were therefore recorded over a period of nine months in both shallow MWs and in small-diameter, non-screened temperature monitoring stations (TMSs), each of which was installed in a single borehole. Particularly high temperature deviations were measured in MWs in the upper part of the water column where the GWT reached up to 1.8 K warmer than in the surrounding subsurface. These deviations correlate unambiguously with the prevailing positive thermal gradients and are caused by thermal convection. Where forced convection occurred, the GWT was measured to be up to 0.8 K colder. Potential temperature deviations must be considered when monitoring very shallow GWT as thermal gradients can be particularly high in these zones. For monitoring concepts of LSCs, a combination of MW and TMS is proposed for GWT measurements decoupled by the effects of convection and in order to enable further investigations such as pumping tests. Full article

(This article belongs to the Section Hydrogeology)

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18 pages, 4279 KiB

Open AccessReview

Large-Scale Geothermal Collector Systems for 5th Generation District Heating and Cooling Networks

by Robin Zeh, Björn Ohlsen, David Philipp, David Bertermann, Tim Kotz, Nikola Jocić and Volker Stockinger

Sustainability 2021, 13(11), 6035; https://doi.org/10.3390/su13116035 - 27 May 2021

Cited by 46 | Viewed by 6399

Abstract

Low temperature district heating and cooling networks (5GDHC) in combination with very shallow geothermal energy potentials enable the complete renewable heating and cooling supply of settlements up to entire city districts. With the help of 5GDHC, heating and cooling can be distributed at a low temperature level with almost no distribution losses and made useable to consumers via decentralized heat pumps (HP). Numerous renewable heat sources, from wastewater heat exchangers and low-temperature industrial waste heat to borehole heat exchangers and large-scale geothermal collector systems (LSC), can be used for these networks. The use of large-scale geothermal collector systems also offers the opportunity to shift heating and cooling loads seasonally, contributing to flexibility in the heating network. In addition, the soil can be cooled below freezing point due to the strong regeneration caused by the solar irradiation. Multilayer geothermal collector systems can be used to deliberately generate excessive cooling of individual areas in order to provide cooling energy for residential buildings, office complexes or industrial applications. Planning these systems requires expertise and understanding regarding the interaction of these technologies in the overall system. This paper provides a summary of experience in planning 5GDHC with large-scale geothermal collector systems as well as other renewable heat sources. Full article

(This article belongs to the Special Issue A Pathway to Renewable Energy Promotion for Achieving Low-Carbon Energy Networks: Electricity, District Heating and District Cooling Networks)

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