Environmentally compatible use of geothermal heat storage. Determination and evaluation of thermal changes in groundwater

To reduce fossil energy consumption, underground thermal energy storage (UTES), also called geothermal heat storage (GTS), is a building block in the transformation of heating and cooling. Such storage facilities are to be fed by renewable energy sources and other waste heat. The groundwater is thermally influenced. The thermal impact space of UTES has been systematically investigated and illustrated with numerical simulations for seasonal buffering and heat storage, building air-conditioning and use of excess electricity (power-to-heat). Effects of temperature change on hydro- and geochemical processes and groundwater ecology were investigated by including extensively researched literature. In oxygen-rich (oxic) freshwater aquifers in unconsolidated rocks, the high temperature sensitivity of groundwater fauna necessitates tighter temperature thresholds for their precautionary protection and maintenance of their ecosystem services to keep impacts minor. Less restrictive temperature thresholds can be derived for low-oxygen (anoxic) aquifers, where a microbiome can adapt more flexibly to changes in groundwater temperature and properties. With the derived thermal thresholds of insignificance, sustainable groundwater management is also possible with geothermal heat storage. The examples shown make it easier for involved planners and experts in the fields of geology, hydrogeology, groundwater ecology, geothermal energy as well as in authorities to estimate the thermal insignificance. Thermal insignificance means that the use of geothermal reservoirs has no adverse environmental impact on groundwater.