Examining thermal conductivities of shallow subsurface materials for ground source heat pump utilization in the Pannonian part of Croatia
Thermogeological and hydrogeological properties of the shallow subsurface in the Croatian part of the Pannonian Basin System (PBS) were investigated in the context of its utilization for ground source heat pump (GSHP) systems installation. Detailed borehole core determinations, sampling and thermal conductivity measurements were conducted for seven boreholes at four locations in diverse geological settings. Results of 418 measurements conducted on 82 sediment and rock samples were compiled and analysed in the context of existing collections of thermal conductivities from Germany and Switzerland which are frequently utilized for borehole heat exchanger (BHE) dimensioning in other countries. Discrepancies have been detected between thermal conductivity data from different geographical regions and geological settings. These results imply that local data sets should be further developed to enable accurate BHE sizing. In four of the boreholes thermal response tests (TRTs) were also conducted so the results were compared to direct measurements on samples. The comparison demonstrated that in the environments with significant groundwater flow (coarser-grained sediments) the laboratory measurements systematically underestimated natural-scale thermal conductivities, while the discrepancies were low in finer-grained sedimentary environments. That is why in the presence of significant groundwater flow the TRTs are recommended for all installations, while at locations with low flow velocities the analogy to values cited in data collections can be used for small-scale installations. It is important to have the collection of measurements for local materials in cases when thermal response testing would not be economically feasible. The values reported in this paper can be a guideline for proper GSHP system dimensioning in order to achieve better performance, especially of small-scale systems in Croatia and other geologically comparable localities in the PBS area.
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