Research on a Thermally Loaded Rock -Perspectives of Underground Thermal Energy Storage

The research of thermally loaded rocks and UTES were examined in a geological environment at temperatures about 90 °C. Long-term in-situ experiment was placed in the SP-47 adit in the Underground Research Laboratory Josef, Mokrsko, Central Bohemia. The elevated temperature reached distance 3 m far from a heater. At the end of the cooling the rock massif was cooled to the natural temperature (9 °C). The maximal efficiency was 23 %. Fluctuations in heating intensity induce rapid increase / relaxation of stress and strain in the rock massif. The extent of induced stress reaches farther than the extent of elevated temperature. Thermally induced strains are almost completely reversible. Only a surface of the rock may occur irreversible changes. Significant thermally induced stress is of the same order of magnitude as tensile strength of the bulk rock massif. Thermal load of the rock mass has a measurable effect on the hydraulic permeability of the rock environment and composition of the percolating underground water. Chemical composition of groundwater was influenced by the presence of Na-silicate based geopolymer and by the ambient properties (elevated temperature, evaporation, contact with air etc.) Hydraulic properties were influences by thermal stress and relaxation during the heating/cooling cycles.