Characterization of a Lignite-rich Subsurface for Shallow Geothermal Applications

Knowledge of subsurface thermal properties, particularly thermal conductivity, is essential for the effective design and operation of shallow geothermal energy systems. This study presents the results of a geological and thermal characterization conducted at a borehole heat exchanger field used for shallow geothermal heating and cooling of a non-residential building southwest of Leipzig, Germany. The subsurface is characterized by a pronounced heterogeneity, including lignite-rich layers. The presence of these units leads to a strong vertical variability in thermal conductivity, posing challenges for conventional geothermal site characterization.

To address this, different in situ and laboratory-based measurement techniques with differing spatial resolutions and support volumes were applied to assess the vertical distribution of thermal conductivity and subsurface heat transport properties. The results obtained from the various methods were systematically compared and evaluated. Additionally, laboratory analyses of lignite samples were performed to better quantify the influence of the organic-rich layers on thermal properties at the site.