The geothermal storage concept involves a modified form of the multi-stage stimulation system. In this process, horizontally oriented layers or fissures in hard rock are penetrated by vertical or sloping boreholes. The layers and/or fissures form the geothermal storage, through which the hot water flows and releases the heat.

Heat can be stored in or extracted from the geothermal store as required. The planned geothermal store is divided into an above-ground and an underground part. The essential components of the underground heat store are the horizontal elements of the reservoir and the boreholes that pass through the individual reservoir elements and connect to them.

Geo-Energie Suisse is planning a geothermal energy storage pilot project in Bern on behalf of Energie Wasser Bern (ewb).

In summer, the geothermal storage is charged with waste heat from the Forsthaus energy centre (charging cycle). In a closed circuit, hot water is injected through the central main borehole into the permeable sandstones of the Lower Freshwater Molasse (the so-called channel belt sandstones) and is returned cooled through the secondary boreholes. The heat spreads relatively quickly along the permeable sandstones; in the low-permeability marl and clay rocks, however, the heat spreads relatively slowly.

In winter, the geothermal reservoir is discharged (discharge cycle). To do this, the water flow direction is reversed. The heat previously stored in the form of hot water is pumped to the surface from the main well. In the energy centre, the heat is transferred to a heat exchanger and thus recovered. The cooled water is then returned to the underground via the secondary well(s).

The monitoring well is used to monitor the temperature and water chemistry. For this purpose, it is equipped with temperature sensors and a multi-packer system that allows water samples to be taken from specific sandstone layers of the Lower Freshwater Molasse.