Efficient and inexpensive energy storage systems undoubtedly play a significant role in the modern sustainable energy strategy. The thermal energy storage, defined as the temporary storage of thermal energy at high or low temperatures, appears to be the most appropriate method for correcting the mismatch occurring sometimes between the energy supply and demand. In the Underground Research Laboratory Josef in central Europe, granitic rock is being studied as a host environment of the underground thermal energy storage recently. In-situ experiment has been set up to evaluate influence of cyclic heating up to 95°C and cooling on the thermo-hydro-mechanical and chemical characteristics of the rock. Long-term in-situ heating experiment was designed to describe changes in geomechanical, chemical, petrological and hydrogeological properties of granitoids during and after repeated heating and cooling cycles. Duration of one full cycle of heating-cooling lasts nine months.

Specific objectives are to determine and verify:

  • the extent and range of possible influence of thermal stress on the structure of granitic rocks,

  • the possibility of changes in hydraulic, hydrochemical, petrographic and geotechnical parameters of rocks in relation to the distance from the heat source,

  • evaluation of effective rock thermal parameters and correlation with numerical models involving temperature, stress, strain and hydrodynamic simulations.

Monitoring boreholes are used for application of non-destructive measuring methods, particularly for the monitoring of hydraulic (water pressure and permeability), geotechnical (stress and strain meters), thermodynamic (temperature) and microseismic data. Results indicate very rapid increase / relaxation of stress and strain in the rock induced by fluctuations in heating intensity. The induced strain is almost completely reversible, only near the free surface of the rock irreversible changes may occur. Thermal load of the rock mass has also a measurable effect on hydraulic permeability of the rock environment. Moreover, present results suggest also important impact of rock heating to intensive growth of specific microorganisms, which may significantly influence future underground industrial applications.

Experiment results will also be fully utilized in the future in a topic different from the present research, i.e. in the issue solving the repositories of spent nuclear fuel in deep geological structures.

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