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Energy storage in the rock environment

A number of techniques for the storage of thermal energy have been developed. Different temperature ranges and different media are used, though water is the most common. For underground storage of surplus electrical energy in the form of thermal energy and waste heat etc. use can be made of the rock environment which provides good insulation as well as thermal capacity that guarantees reversible usage of the stored energy at times of high demand.

Research on energy storage being carried out by the Czech Geological Survey in frame of the government long-term strategic energy plan to the year 2030 is concerned with two principal themes:

Reversible storage of energy in a rock massif (TAČR TA0120348)

The aim of this investigation is to develop a procedure for storing excess thermal and electrical energy directly in the rock environment within the territory of the Czech Republic, e.g. in the vicinity of power generation and transmission plants. Information about the geology and physical properties of different rock massifs in the Czech Republic is being gathered systematically for this purpose. The response of different plutonic, sedimentary, volcanic and metamorphic rocks to repeated thermal loading up to temperatures as high as 360° C is being tested in the laboratory by petrographic observations and measurements of petrophysical properties. The most suitable rocks for thermal energy storage in a model “energy well” are being identified so that the results of this research can be used for commercial purposes in future.

Research on a Thermally Loaded Rock - Perspectives for Underground Thermal Energy Storage (Projekt FR-TI 3/325)

The aim of this project is to test the behavior of the intrusive rock exposed to thermal loading in the Josef mine at Mokrsko. The mine site is situated 30 km south of Prague. Here, 100 m below the surface, heating equipment has been installed directly within the rock mass of the Sázava tonalite. This is used to heat the rock mass over an extended period of time. The artificially created geothermal anomaly reaching a temperature up to 95°C is monitored using a relatively dense net of boreholes. The boreholes contain a number of instruments for measuring a range of different parameters used to construct a thermodynamic model of the behaviour of the rock mass during the cycles of thermal loading (more information about this project can be found on www.geology.cz/mokrsko). The three-dimensional design of this experiment enables the influence of controlled heating up and cooling down of the rock mass to be measured precisely over time. The anisotropy in the rate at which heat spreads in the rock mass can be determined, and the balance between input and output during reversible cycles can be modelled. The experiment is located at depths reached by drill holes used to obtain geothermal energy by heat pumps.

Structural model of the surroundings of the experimental site

Structural model of the surroundings of the experimental site. (Author: David Čížek)

The Mokrsko experiment is of great practical significance because it enables the thermodynamic conditions in the rock environment to be mapped over a short time period, or even on a seasonal basis if reheating of the rock mass is carried out using different sources of thermal energy. Commercial application of the results of this project depends on defining the parameters that determine the economic balance between extraction and storage of heat in the rock environment. This requires a precise understanding of the petrophysical properties of the rock. These are governed by the geometry of the network of joints and fractures and the intrinsic anisotropy and mineralogy of the rock that affect the rates and amounts of thermal energy that can be input and output from a given rock storage. The depth of storage must also be considered in relation to the optimum long-term life of storage in the vicinity of commercial and industrial buildings.

Experimental locality situation and experiment instrumentation in the Josef tunnel near Mokrsko

Experimental locality situation and experiment instrumentation in the Josef tunnel near Mokrsko in the frame of the project “Research on a Thermally Loaded Rock”. The picture documents the locality before an isolation of the large-scale heating borehole and surrounding monitoring boreholes. (Author: Jan Franěk)

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Czech Geological Survey
Klárov 131/3
118 21 Praha 1
phone: +420257089447
fax: +420 257 531 376
jan.franek@geologycz