The operation of underground energy storage systems is based on the high heat capacity of the soil and the high thermal insulation that it provides. Consequently, the soil or the underground aquifer can accept large amounts of heat without requiring large installations. The energy storage takes place both during the winter and summer and is available when needed, so that nothing goes to waste. The underground energy storage is like the heating of an enormous potato, which even when cut into pieces after many months, it will still keep warm at its core. Underground Thermal Energy Storage is particularly advantageous in plants (buildings or other facilities) that reject heat or seek high cooling loads and have thermal requirements during winter. The energy storage technology is another form of shallow geothermal energy use, since the principle similar to the shallow geothermal systems for heating and cooling. Heat exchange is succeeded either by recirculating the water solution into boreholes or by pumping and reinjecting the pure groundwater of an underground aquifer. The underground thermal energy storage is considered as a renewable energy source, since no conventional fuels are used. Furthermore, UTES is abundant and requires little amount of electricity. As a result the need for fossil fuels is eliminated and the CO2 and NOx emissions are considerably reduced in the primary sector. The applicable types of underground thermal energy storage (UTES) store heat in aquifers (ATES-Aquifer Thermal Energy Storage) and in the solid ground through boreholes (Borehole Thermal Energy Storage). BTES is the predominant form of UTES due to its simple operation and reliability and no need of specific control, compared to the ATES which has complex design and requires special attention during operation. Regardless of the applied system it can be combined with geothermal heat pumps. |